UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
SOLAR ENERGY AND
ITS USE FOR HEATING WATER
IN CALIFORNIA
F. A. BROOKS
BULLETIN 602
NOVEMBER, 1936
UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA
CONTENTS
PAGE
Availability of solar energy 3
Nature of solar radiation reaching the
earth's surface 4
Atmospheric depletion of solar energy. ... 4
Direct solar radiation intensities at Fresno 6
Solar energy received on a horizontal sur-
face at the ground 6
Number of days of sunshine in different
parts of California 12
Ten-year records of average sunshine,
frost dates, and seasonal temperatures 13
Absorption of solar energy 13
Solar-energy absorptivity of various sur-
faces and surface emissivities at ordi-
nary temperatures 17
The effect of angle of incidence on the
amount of light transmitted by glass
and absorbed by a black surface 20
The solar water heater 23
General characteristics of solar-energy ab-
sorbers 23
Exposed-tank solar water heater 24
Enclosed multiple-tank solar water heater 24
Single-pipe absorber with storage tank. . . 26
Multiple-pipe absorber with storage tank 20
Ideal thin, flat-tank solar-energy
absorber 26
Nonf reeze solar water heaters 27
Steam generators and high-temperature
absorbers 28
Hot- water demand 28
Temperature of hot water needed for vari-
ous domestic purposes 28
Quantity of hot water needed for various
purposes 28
Quantity and temperature of hot water
needed for farm dairies 30
Experimental investigations 31
Heat output of thin flat-tank absorber ... 31
Performance characteristics of round-
tank absorbers 32
Performance characteristics of enclosed
multiple-tank absorbers 35
Thermosiphon circulation and temper-
atures in pipe absorber with storage
tank 37
PAGE
Relation between rate of circulation and
temperature rise in different pipe ab-
sorbers 39
Size of absorber in relation to quantity of
water to be heated 41
Length of pipe run for satisfactory thermo-
siphon performance 41
Use and construction of solar water-heater
systems 45
Combination of solar heater and furnace or
range water coil 45
Location and connection of solar absorber 45
Safe piping practice for furnace or range
coil 45
Installation of extra emergency side-arm
heater 47
Combination of solar heater with automatic
water heater 48
Two-tank combination of solar and auto-
matic water heater 48
Storage-tank capacity of the auxiliary
automatic water heater 48
Single-tank combinations of solar and
automatic water heaters 48
Solar tank heater to reduce operating cost
of automatic heater 51
Nonfreezing solar-energy absorbers 52
Commercial nonfreeze type solar water
heater 53
Separate fluids for nonfreezing solar-
energy absorber 54
Construction of solar energy absorbers 54
Construction procedure for a built-in ab-
sorber box on a new roof 55
Removable glazed cover 55
Parallel- pipe absorber coils 57
Methods of obtaining greater heat output
from limited absorber area 59
Initial cost and carrying charges of solar
water heaters 60
Cost of commercial solar water heaters ... 60
Cost of common pipe-coil solar water
heater 61
Cost of solar absorber tank heaters 61
Summary of use and construction 61
Literature cited 62
SOLAR ENERGY AND ITS USE
FOR HEATING WATER IN CALIFORNIA 1
F. A. BROOKS 2
Practically all the energy on the earth's surface has sunshine as its
primary source. Hydroelectric energy is obtainable because sunshine
promotes evaporation of moisture ; great masses of air containing this
water vapor circulate so that water from low levels is carried back to
higher levels, constantly replenishing the lakes and rivers. Wood and
agricultural products used for fuel cannot grow without sunshine, and
coal and petroleum are the concentrated carbon products of age-old
plant and marine life that used the sunshine of past geologic time.
Direct use of solar energy as heat is now being made by several thou-
sand solar water heaters in California. Successful use depends, of course,
on the number of sunshine days in different parts of the state. Maps on
pages 10 and 11 indicate the general availability of sunshine in Cali-
fornia. There are two common types of solar water heaters and several
methods of combining the solar heater with other water-heating systems.
Recommendations for installations and construction of solar water
heaters are to be found on pages 45 and 54.
Investigations and experiments concerning water temperatures and
the rate of heating water in different solar-heater systems are described
on pages 31 to 43. The results of the experiments are incorporated in the
recommendations mentioned above.
The technical nature and availability of solar energy, discussed on
pages 4 to 22, may not be of interest to the home owner but must be con-
sidered in studying the theory of solar water heaters. The information
collected in this first section may be of use also to agricultural scientists
concerned with radiation, evaporation, and plant growth.
AVAILABILITY OF SOLAR ENERGY
The quantity of solar energy reaching the earth's surface is so great
that there need never be any fear of lack of energy for the earth as a
whole. Every square mile of ground in California receives during each
clear summer day about as much energy as can be produced by all the
power plants of one of the largest electric utility systems in the state.
1 Received for publication June 22, 1936.
2 Associate Professor of Agricultural Engineering and Associate Agricultural En-
gineer in the Experiment Station.
[3]
4 University of California — Experiment Station
Unfortunately, this vast amount of solar energy is not easily utilized for
power ; but for heat, part of it is readily available during the daytime
in clear weather.
The amount of radiant energy coming from the sun is almost con-
stant — 7.15 B.t.u. 3 per square foot per minute — on a surface perpen-
dicular to the sun's rays and outside the earth's atmosphere/ 04 Reflec-
tion and scattering of the sun's rays by water, dust, and gas molecules
decrease the amount of solar energy entering the lower atmosphere.
Absorption by water vapor, ozone, and carbon dioxide gases in the at-
mosphere further diminishes the solar radiation reaching the earth's
surface.
NATURE OF SOLAR RADIATION REACHING THE EARTH'S SURFACE
Curve I of figure 1 (22) shows the energy distribution with respect to
wave length in the normal solar spectrum outside the atmosphere, and
curves II to V inclusive show the energy distribution after average de-
pletion due to scattering by the dust and gases of the atmosphere. Curve
VI indicates the response curve of the eye, yellow light being the most
visible. Curve VII shows the diffuse energy received from the sky from
the scattering of the direct rays by water, dust, and gas molecules.
Without this scattering the sky would be dark, and the stars visible in
the daytime. In general, more than one-sixth of the total energy reach-
ing sea level is this diffuse radiation of predominantly short-wave length.
On a horizontal surface at sea level the diffuse radiation is relatively
constant throughout the greater part of the daytime, ranging from
about 0.4 to 0.6 B.t.u. per square foot per minute.
Observations by Coblentz and Kahler (10> near Washington, D. C, show
that the total solar energy received at sea level may vary even on clear
days more than 25 per cent (from 3.64 to 4.86 B.t.u. per square foot per
minute) at noontime in September. Of this the ultraviolet part (300
to 389 m/x) 5 of the radiation ranges from 1 to 2 per cent of the total
energy. The visible component (389 to 750 imx), averaging about 47 per
cent of the total energy, ranges from 40 to 52 per cent, while the infrared
(750 to 3,000 rn.fi) averages about 52 per cent and ranges from 59 to 46
per cent of the total energy.
Atmospheric Depletion of Solar Energy. — The large variation in
energy received on the earth's surface on clear days at the same time of
3 B.t.u. or British thermal unit = quantity of heat required to raise the tempera-
ture of 1 pound of water 1 degree F.
4 Superscript numbers in parentheses refer to "Literature Cited" at the end of
this bulletin.
5 m/x or millimicron = lx 10~ 9 meter.
Bul. 602]
Solar Energy for Heating Water
year indicates great differences in the composition of the atmosphere
and in the quantity of suspended particles of smoke or dust. The absorp-
tive effect of heavy smoke over large industrial cities can be judged by
the simultaneous observations (24) showing that the intensity of sunshine
in Chicago may be only 55 per cent of that at Madison, Wisconsin. This
turbidity indicates approximately a half ton of soot in the air per square
mile.
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Fig. 1. — Solar radiant energy distribution :
I, Normal solar-energy curve outside the atmosphere.
II, Solar-energy curve; solar altitude at 65°.
III, Solar-energy curve; solar altitude at 30°.
IV, Solar-energy curve; solar altitude at 19.3°.
V, Solar-energy curve; solar altitude at 11.3°.
VI, Relative visibility of radiation.
VII, Skylight-energy curve, Mount Wilson, California.
Except during great dust storms, the depletion due to dust in the
atmosphere is less obvious than that due to smoke. The usual fine at-
mospheric dust, when considered as including smoke, haze, and liquid
particles, can be estimated as causing about 10 per cent depletion' 24 ' at
midday and, of course, more as the sun approaches the horizon when the
sun's rays pass more obliquely through air for a greater distance. The
dense dust cloud that passed over Washington, D. C, on May 11, 1934,
was over a mile thick and contained about 100 tons (17) of dust per square
6 University of California — Experiment Station
mile, reducing the solar radiation received to one-fourth its usual value.
In Wisconsin on May 10, when the dust storm covered a large section
of the Middle West, one could see the sun shining faintly, and the bright-
ness was less than 1 per cent (14) of its normal value.
Direct Solar Radiation Intensities at Fresno. — Variation in atmos-
pheric depletion caused by the difference in length of path of the sun's
rays through the air at different times of day can be judged by the
observations in table 1, made at Fresno, California, (26) and from curves
II, III, IV, and V of figure 1. In December the more sloping rays of the
winter sun have a longer air path, but at the same time the earth is
nearer the sun and the solar radiation is nearly 7 per cent more intense.
The winter atmosphere, furthermore, is more clear in the interior valleys
of California than the summer. These two effects serve to minimize the
difference between summer and winter total radiation intensities, and
at noon on clear winter days the energy received on a perpendicular
surface is approximately 80 per cent of the yearly maximum. The
observations recorded in table 1 show that the March intensity in Cali-
fornia is 9 per cent higher than the corresponding October figure be-
cause of different atmospheric conditions in the spring and fall. These
are direct radiation observations and if increased for diffuse sky radia-
tion, according to the Mt. Whitney ratio, indicate that the total energy
received on a perpendicular surface at noon is approximately 5.5 and
6.0 B.t.u. per sq. ft. min. in October and March respectively. Comparable
July and December figures of 5.4 and 4.8 can be deduced from the noon
maximums in figure 2. Hence, for practical purposes, except in the
winter time, the total perpendicular noon radiation can be taken as
about 5.5 B.t.u. per sq. ft. min. A safe assumption for the 6 or 8-hour
daily heating period is 5 B.t.u. per sq. ft. min. total solar energy imping-
ing on a surface perpendicular to the sun's rays.
The insolation at right angles to the sun's rays is of interest in plant
transpiration and photosynthesis, but for soil heating and water-surface
evaporation the energy received on a horizontal surface is more impor-
tant. The essential difference is that the reception area of a horizontal
surface is less than that of a normal surface in proportion to the cosine
of the angle of incidence (or the sine of the sun's altitude above the
horizon).
Solar Energy Received on a Horizontal Surface at the Ground. — The
regular Weather Bureau pyrheliometers measure the insolation on a
horizontal surface inside a spherical glass. The readings, being based
on a calibration with an uncovered master pyrheliometer, represent the
energy received outside the instrument. Figure 2 shows the average
Bul. 602]
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daily record for clear weather at Fresno in July and December, 1933.
The relation of the horizontal receiving surface to a normal surface is,
of course, zero at sunset and sunrise. It is 0.97 at noon in July and 0.50
at noon in December. The most important seasonal variation is the
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number of hours of sunshine — nearly 15 in midsummer, but only a
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The area under the curves represents the total energy received per
day. This is the value plotted as the ordinate of figure 3. Any cloudiness
would greatly reduce the total daily energy received, as is seen in the
published records (35) giving the average daily totals per week. Figure 3
shows the normal annual curves for Fresno, Riverside, and La Jolla. As
Bul. 602]
Solar Energy for Heating Water
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Fig. 4. — Approximate number of clear days plus half the number of partly
cloudy days per year. The unshaded area has the equivalent of about nine
months of sunshine days or more per year. (Based on relief map copyrighted by
H. A. Sedelmeyer.)
Bul. 602]
Solar Energy for Heating Water
11
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Fig. 5. — Approximate average number of clear days plus half the number of
partly cloudy days between latest and earliest frosts. The unshaded area has the
equivalent of about seven months or more of sunshine days during the normal
growing season. (Based on map coyprighted by H. A. Sedelmeyer.)
12 University of California — Experiment Station
the Fresno curve is an eight-year average, its smoothness hides large
annual departures (20 to 30 per cent reductions to be expected, espe-
cially in the spring) when the usual cloudiness may come in one week or
another. The Riverside record is for only two years, and the five-year
La Jolla record is not consecutive. As shown in figure 3, although Fresno
receives more sunshine for eight months, Riverside and La Jolla both
enjoy much more winter sun. The Fresno winter minimum total energy
actually received is only % the summer maximum, although the clear-
day record of figure 2 shows the winter energy 37 per cent of the sum-
mer. These figures indicate that average winter cloudiness deprives
Fresno of more than half the energy that would be available in Decem-
ber to a locality free from clouds.
NUMBER OF DAYS OF SUNSHINE IN DIFFERENT PARTS
OF CALIFORNIA
The Monthly Weather Review™ reports the number of clear, partly
cloudy, and cloudy days for nearly 200 California localities. As only
nine stations report the percentage of possible sunshine, for a general
understanding one must use the cloudiness reports. The number of clear
days at any one station does not represent fairly the total available sun-
shine because of a possible large number of partly cloudy days. By
definition 6 "partly cloudy" indicates sunshine, on the average, for about
half the day. In estimating, therefore, the total number of days of avail-
able sunshine, one may reasonably include half the number of "partly
cloudy" days with the number of "clear" days. This arbitrary interpre-
tation of the reports of the past ten years shows a consistent distribution
of available sunshine throughout the state. As figure 4 shows, almost all
the major agricultural areas of the state have the equivalent of nine
months of sunshine or more per year. Large variations occur annually,
and there are pronounced local differences. Table 2, giving the ten-year
figures for a large selected group of stations, shows the general effect
of topography and water on the weather. More variation is seen in figure
4 from 204 sunshine days per year at Santa Ana to 310 at Corona, 20
miles away across the Santa Ana mountains ; and from 215 days at San
Francisco to 302 days at Alvarado, only 23 miles away across the San
Francisco Bay.
Nearly 300 sunshine days or more per year are reported at many other
stations besides Corona and Alvarado, namely, Barrett Dam, Blythe,
Fairmont, Fontana, Greenland Ranch, Helm, Hollister, Imperial,
Lancha Plana, Napa, Oakdale, Ojai, Santa Barbara, Trona, Watson ville,
6 Clear = sun obscured for to 0.3 of the day; partly cloudy = obscured for 0.4 to
0.7 of the day; cloudy = obscured for 0.8 to the whole day.
Bul. 602] Solar Energy for Heating Water 13
and Yorba Linda. As this list shows, large areas of California enjoy
the equivalent of ten months of sunshine annually.
Ten-Year Records of Average Sunshine, Frost Dates, and Seasonal
Temperatures. — Winter sunshine is not, however, so effective as sum-
mer radiation ; and the ordinary solar water heater may freeze in cold
weather. The cloudy days occur, furthermore, mostly in the cold months
of November, December, January, and February, so that the sunshine
record of figure 5, taken for the normal growing season from average
latest spring frost to average earliest fall frost, shows the number of
days of most effective insolation. It is to be noted again that the major
agricultural areas have the equivalent of seven months or more of sun-
shine between frosts. The relation between total days and sunshine days
between frosts can be found by comparing columns 12 and 13, table 2.
Frost dates observed at stations on tops of city buildings are not compa-
rable with observations at ground level in open country. Maps of frost
dates and growing seasons are given in the United States Department of
Agriculture Climatological Data. m)
The average temperature data, by seasons (table 2, columns 14, 15,
16, and 17) are useful in estimating the relative rates of heat loss of
sunshine-absorbing surfaces exposed to the air. The seasonal vapor
pressures (13) of a few stations (columns 18, 19, 20, and 21 ) are of interest,
for water vapor is largely responsible for the decrease in solar radiation
penetrating the atmosphere. The relative humidities (columns 22, 23,
24, and 25) are included because of their importance in air-condition-
ing. The values reported at 5 a.m. and 5 p.m. happen to represent
approximately the maximum and minimum relative humidities re-
spectively.
ABSORPTION OF SOLAR ENERGY
Surfaces exposed to the sunlight reflect, transmit, or absorb the incident
short-wave solar radiation (fig. 1 and table 1). Exposed surfaces also
emit long-wave radiation to the sky or to surrounding surfaces. The
absorption and emission of radiant energy depends upon the surface
characteristics of the material, the relation of the radiating surface to
its surroundings, and the temperatures of the sending and receiving
surfaces. The temperature of the sun's surface (more than 11,000° F (4) )
is so great that ordinary temperature changes of receiving surfaces on
the earth do not affect the incoming radiation rate. Although reradia-
tion from the earth to the sky varies considerably with the temperature,
at night with a clear sky it can be assumed to be approximately 0.5 B.t.u.
per minute per square foot for a black body.
14
University of California — Experiment Station
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Average relative hu-
midity, per cent
(upper figure, 5 a.m. ;
lower figure, 5 p.m.)
Sept.
Oct.
Nov.
June
July
Aug.
Mar.
Apr.
May
23
Dec.
Jan.
Feb.
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Average vapor pres-
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Oct.
Nov.
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June
July
Aug.
20
Mar.
Apr.
May
Dec.
Jan.
Feb.
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Sept.
Oct.
Nov.
June
July
Aug.
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Mar.
Apr.
May
15
Dec.
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modified by the reflectivity of the exposed surface and hence the absorp-
tivities given in table 3 show large characteristic differences. Coefficients
for clouds, ground surfaces, and fields are usually given in terms of
albedo, namely "the ratio of the intensity of the radiation diffusely re-
flected from the surface of the earth to the intensity of that received
by it." ) Therefore, some of the absorption factors given in table 3 are
determined from 1.00 minus the observed albedo. The cooling effect of
out-going radiation also depends upon the surface absorption or emis-
sion characteristic. The emissivities given in table 3 for a wave length
corresponding to the usual outdoor temperature of the emitting surface
are, therefore, different from the short-wave absorptivities. Although
data on field plants and various soils are, unfortunately, meager, there
is sufficient information to establish the approximate heat balance in
such phenomena as the cooled air on the leeward side of an alfalfa field
in midsummer, when the sunshine, although intense, does not furnish
all the heat of vaporization utilized in transpiration.
Observations of fresh and of soiled snow showing that the soiling
effect quadruples the solar energy absorbed led in Russia (1) to spreading
coal dust over the snow, about 100 pounds per acre. In this way the
spring melting is advanced and permits an early growing season.
Table 3 gives the coefficients of various surfaces for short-wave (0.6/x)
absorption and for long- wave (9.3/u) outgoing radiation. In general the
short-wave absorption varies roughly as the visual darkness of the sur-
face. Long-wave emissivity cannot be judged visually. Common build-
ing materials, paints, and roofs (except asbestos and metals) have
nearly perfect emitting surfaces for long-wave radiation. Many com-
mon materials exhibit wide differences in short-wave absorption and
long-wave emission. Finished plaster absorbs only 35 per cent of the
impinging solar radiation and yet is 93 per cent effective as a long-wave
radiator. Whitewashing further reduces the solar-energy absorption ;
and is used in Egypt and Arabia for keeping buildings as cool as pos-
sible. In other words, white acts out-of-doors as a one-way heat valve
because it reflects most of the sunshine and yet at night readily emits
ordinary heat waves to the cold sky. However, new galvanized iron, for
example, has a solar absorptivity of 0.66 and a long-wave emissivity of
0.23, indicating high daytime heating and relatively low cooling power
by radiation. Polished metals, particularly aluminum, act as radiation
shields, being excellent reflectors and also poor emitters. Such properties
are valuable for minimizing temperature fluctuations.
18
University of California — Experiment Station
TABLE 3
Solar Energy Absorptivity of Various Surfaces and Surface Emissivities at
Ordinary Temperatures
Materials
Standards
" Hohlraum," theoretical perfectly'black body
Black silk velvet (minimum reflector)
Magnesium oxide (MgO; standard maximum white) . .
Deposited silver (optical reflector) fresh, untarnished.
Mirror, silver-backed glass
Mirror, mercury-backed glass
Meteorological
Cloud surface
Water
Wet surfaces
Frosted surfaces, 0.004 to 0.008 inch thick
Snow, fresh, bright, sparkling (maximum reflection). .
Snow , soiled
Ice
Earth's surface as a whole, average cloud cover. . . .
Earth's surface as a whole, land and sea, no clouds .
Ground and pavements
Soil, surface
Soil, brown, dry
Soil, brown, wet
Sand, Maine, yellow, white grains of many kinds. .
Sand, Florida, very white
Gravel
Granite
Sandstone
Limestone
Granolith pavement
Concrete
Asphalt pavement, dust-free
Vegetation
Grass, dead turf
Grass, dead, wet (after rain, no sun)
Grass, 80-90 per cent new green
Grass, 80-90 per cent new (in sunshine after rain) . .
Grass, fresh, dry
Leaves, green
Leaves, early summer, high water content
Leaves , late summer . after dry period
Vegetable mold
Building material, roofing, etc.
Sawdust
Wood, planed oak
Paper, white
Cotton cloth, white handkerchief
Artificial leather, black
Rubber
Felt, black
Felt, roofing, bituminous
Felt, roofing, aluminized
Asbestos cement board, white
Short-wave
absorption
00
99
025
07
12
20
22, 0.26
90f (vert.)
18
54
57
83
68
84
75
60
55
54-0.76
33-0.50
83
65
93
81, 0.82
85
77
67
67-0.75
75
81t
71t
2S
5b
90
88
38
59
Long-wave
emission
1.00
0.97
0.01
0.914, 0.965
0.985
0.985
0.74
0.914-0.965
0.38
0.52
0.28
0.44
0.97
0.64
0.75
0.90
0.95
0.90-0.95
0.96
Refer-
ences*
19, 16
SO
1H
16
16
8,26
28,8$
83
83
21,16
21
83
8
16
19
23
16
16
15
7, 15
7
7
15
16,8
16
21
21
21
21,33
23
16
23
23
15
15
15
15
16
9
82
16
16
16
16,28
* Numbers in the reference column refer to "Literature Cited" p. 62.
t Includes transmissivity.
Bul. 602]
Solar Energy for Heating Water
19
TABLE 3 — Continued
Materials
Building material, roofing, etc. — Continued
Asbestos felt (white impregnated covering for corrugated
iron roofing)
Roll roofing, green
Slate, dark-gray
Gypsum 0.02 inch thick on smooth plate
Plaster, finished
Bricks, Gault, cream
Bricks, sand-lime, red
Glass
Paints
Whitewash
Enamel, ceramic white
Porcelain enamel on steel plate, white
Porcelain enamel on steel plate, green
Bright aluminum, 2 coats
Fine bronze
Bronze with 2 coats varnish
White
Gloss- white
Cream
Light-yellow
Light-blue
Medium-blue
Light-green
Dark-green
Red
Lampblack
Lacquer, black shiny
Graphite
Metals
Aluminum sputtered (optical reflector) with oxid zed film
Aluminum, Alcoa lighting sheet, specular finish, weather-
proof
Aluminum foil
Aluminum foil with coat of linseed oil
Aluminum, polished
Aluminum, oxidized
Aluminum, commercial polished sheet
Duralumin
Brass, polished
Brass, as rolled
Brass, dull
Brass, oxidized
Chromium
Copper, polished
Copper, rolled, tarnished
Copper, black oxidized
Galvanized iron, new
Galvanized iron, oxidized
Galvanized iron, very dirty
Galvanized iron, white washed
Iron, pure, polished
Iron, cast, oxidized
Iron, rusted
Short-wave
absorption
0.25(approx.)
0.88
0.89
0.35
0.38
0.72
0.92f
0.22-0.25
0.34-0.40
0.76
0.35-0.54
0.11-0.18
0.35
0.23-0.26
0.35
0.39
0.64
0.52-0.53
0.88
0.87
0.98-0.97
0.78
0.11
0.18-0.20
0.26
0.53
0.49
0.18
0.64
0.65
0.91
0.22
0.45
Long-wave
emission
0.50(approx.)
0.91-0.97
0.90
0.93
0.93
0.90-0.95
0.90
0.90
0.28-0.45
0.51
0.88
0.95 (approx.)
0.95 (approx.)
0.92-0.96
0.96
0.96
0.82
0.41
0.08
0.56
0.04-0.05
0.11
0.20-0.25
0.05
0.07
0.28
0.61
0.08
0.04
0.64
0.78
0.23
0.28
0.06
0.63-0.98
0.62-0.69
Refer-
ences*
9
16,31,8
15
82
15
15
15,28
15
16,27
31
SO, 32
16
27, 7, 9
15
15
2, 7
19
2, 19, 18
19
2,16
31
16
28
28
15
15
8
15
31
15
31
15
15
7, 15
7, 15
15
16,28
28
16
16
15
15
15
* Numbers in the reference column refer to "Literature Cited" p. 62.
t Includes transmissivity.
X Letter from Howard M. Flye, of the Aluminum Company of America, Feb. 25, 1936.
20
University of California — Experiment Station
TABLE 3— Concluded
Materials
Metals — Continued
Iron, rolled oxidized
Steel, sheet insulation
Steel, as rolled
Steel, oxidized
Lead, oxidized
Lead, old roofing
Magnesium
Tinned steel plate, bright .
Zinc, pure, polished
Zinc, polished, oxidized. .
Short-wave
absorption
0.79
0.30
0.46
Long-wave
emission
0.66
0.28
0.65-0.82
0.79
0.28-0.43
0.07
0.05-0.09
0.02
0.28
Refer-
ences*
15
37
31 . 32
15
15
16
15
31
15
15
* Numbers in the reference column refer to "Literature Cited" p. 62.
Glass is opaque to long-wave radiation and therefore as an emitter of
ordinary heat waves acts in the same way as a painted surface. Sun-
shine or short-wave radiation is readily transmitted, only a few per
cent being absorbed by the glass itself. In the sunshine, glass acts as a
one-way heat valve : the solar radiation enters with little loss, but the
covered surface heated bv the sunshine emits long-wave radiation which
cannot pass out through glass except by conduction after being recon-
verted to sensible heat in absorption by the glass. Practically, the trap-
ping of solar heat by glass is opposite to the cooling effect of white
surfaces, although for a different reason, namely transmission and
opaqueness instead of reflection and emission for short and long-wave
radiation respectively.
The Effect of Angle of Incidence on the Amount of Light Trans-
mitted by Glass and Absorbed by a Black Surface. — The total solar
energjr received by fixed glass-covered absorbers such as greenhouses,
cold frames, and solar water heaters is considerably less than the total
radiation impinging on a surface perpendicular to the direct rays at
noon. The difference can be ascribed partly to greater absorption by the
glass, but is due mostly to the smaller portion of rays intercepted on a
fixed surface and to the increasing reflection from the glass cover and
the absorbing surface as the angle of incidence increases.
In table 4, columns 3 to 7 show these effects of angle of incidence;
column 9 shows the Fresno average for atmospheric depletion in March
and September due to the length of air path at different times of day
as deduced from table 1. The diffuse radiation from the sky (fig. 1,
curve VII) is not affected by the angle of incidence, but suffers a larger
transmission loss in the glass due to the greater opacity of glass to very
short wave-length light. This loss varies greatly with different kinds of
glass. Assuming a 25 per cent loss, a fixed absorber exposed to 0.8 of the
Bul. 602]
Solar Energy for Heating Water
21
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984 81
965 92
939 69
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642 79
573 58
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422 62
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258 82
173 65
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March and September
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Noon
11:40 a.m
11:20 a.m
11:00 a.m
10:40 a.m
10:20 a.m
10:00 a.m
9:40 a.m
9:20 a.m
9:00 a.m
8:40 a.m
8:20 a.m
8:00 a.m
7:40 a.m
7:20 a.m
7:00 a.m
6:40 a.m
6:20 a.m
6:00 a.m
Angle of in-
cidence, de-
grees from
perpendicu-
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22 University of California — Experiment Station
sky would receive 60 per cent of the diffuse radiation. Column 11, table
4, shows the approximate addition due to diffuse radiation; column
12 the approximate total energy transmitted through the glass absorbed
by a black surface underneath. The corresponding external noon radia-
tion would be 1.00 for direct radiation plus 0.10 for diffuse radiation,
giving a maximum of 1.10 total impinging short-wave energy.
At 60° or 2 hours before sunset a fixed glass-covered absorber can
receive or less than % the total noon maximum, although the bright-
1.10
ness of the direct sunshine has decreased only to %. Horizontal ab-
sorbers have lower midday ratios but differ little from sloped absorbers
as the sun approaches the horizon.
If column 12 is multiplied by the observed noon maximum direct in-
tensity (as indicated in table 1), the approximate short-wave input can
be calculated, assuming no dust on the glass or absorber surface. Not
all the energy received can be converted immediately into useful heat.
There are large heat losses from the glass and structure when the latter
are hotter than the outside air, and heat absorption by the structure and
insulation delays and flattens the useful-heat curve in comparison with
the input (fig. 12).
Bul. 602] Solar Energy for Heating Water 23
THE SOLAR WATER HEATER
Many persons living in the interior valleys of California, where daily
sunshine is dependable for six or seven consecutive months, have found
solar heaters satisfactory and economical for supplying hot water. Al-
though many heaters are homemade there are also a large number of
commercial manufacture.
The average household needs much water at temperatures of 100°
to 140° F, and more would be used if the cost of heating could be re-
duced. Dairies, schools, and manufacturing plants also use much water
heated to temperatures well below that obtainable with fixed solar
heaters.
The University of California receives numerous inquiries regarding
the construction and performance of solar heaters. Earlier studies of
the problem of securing satisfactory performance at low cost with this
type of equipment were made in 1927 by Farrall, as reported in Bulle-
tin 469. 7 Increasing use of solar water-tank heaters, together with numer-
ous difficulties with thermosiphon circulation in pipe absorbers, and
growing interest in the use of solar heat absorbers in combination with
automatic water heaters led to the experiments and investigations re-
ported herein.
GENERAL CHARACTERISTICS OF SOLAR-ENERGY ABSORBERS
Solar energy is used for many different purposes, for each of which a
characteristic type of apparatus has been developed. The "burning-
glass" is an ancient method of converging the sun's rays to obtain tem-
peratures high enough to start combustion. The modern apparatus for
producing extremely high temperatures, as developed by the Zeiss
Works in Germany, includes a 100-inch searchlight mirror which con-
verges the sun's rays to % inch where a temperature of 6,300° F is
reached in about 30 seconds. (11) This pure thermal energy is useful for
melting solids in a vacuum. Large parabolic cylindrical reflectors are
used to generate steam for a pumping plant in Egypt where the high
cost of fuel justifies the alternative extra expense of a solar-power plant.
In 1913 this solar-power plant produced 12 pounds of steam per hour
per 100 square feet of interception area. (5) At the other extreme a de-
hydrator operator near Davis, California, draws his replacement air
through about 200 feet of idle 8-inch irrigation pipe laid in the sun, to
reduce his heating cost.
Apparatus for utilizing solar energy for heating water to the moder-
7 Farrall, A. W. The solar heater. California Agr. Exp. Sta. Bul. 469:1-30. 1929.
24
University of California — Experiment Station
ate temperatures required for domestic use has proved economical in
comparison with other heating methods in California, and many differ-
ent types of solar water heaters have been developed.
Exposed-Tank Solar Water Heater. — Simple, bare water boilers
mounted outdoors where they will not be shaded have long been used
during the summer for furnishing late afternoon hot showers. The tanks
are usually horizontal or vertical, but experiments described later indi-
Fig. 6. — Exposed bare tank suitable for heating water for late
afternoon shower baths on hot, clear days.
cate that sloped mounting (fig. 6) is more effective. These exposed tanks
cool nearly to air temperature at night and are useless before noon.
Enclosed Multiple-Tank Solar Water Heater. — An improvement of
the inexpensive exposed tank heater is seen in figure 7. From several
tanks enclosed in an insulated, glass-covered box, a large supply of
water above 120° F can be obtained in the afternoon. This system might
be used for general domestic hot water if the clothes could be washed in
the late afternoon when the water is hottest. During the night the water
cools off so rapidly that morning temperatures are too low for clothes
washing, though it yet might serve for all the other needs. The sunshine
falling in the box space between the tanks and on each side indirectly
furnishes extra heat for the tanks by convection of hot air. Although the
daytime thermal efficiency of the glass-covered tank heaters is high, the
large losses at night make the 24-hour efficiency less than for pipe ab-
sorbers with insulated storage tanks.
Bul. 602]
Solar Energy for Heating Water
25
Fig. 7. — Triple-tank absorber in insulated box with hotbed sash cover.
Fig. 8. — Single-pipe solar-energy absorber for small installations. A
black background and proper length of box as seen in figure 9 are rec-
ommended instead of the experimental reflector-bottom shown in this
photograph.
26
University op California — Experiment Station
Single-Pipe Absorber with Storage Tank. — The usual "solar heater"
consists of a flat, glass-covered, zigzag pipe-coil absorber connected for
thermosiphon circulation with an insulated storage tank (fig. 8) . In this
system when the storage tank is above the absorber there is no appre-
ciable reverse circulation at night, and the high daytime temperature is
conserved by the tank insulation so that temperatures over 140° F are
available at all times if the system is properly designed.
Multiple-Pipe Absorber with Storage Tank. — When the required ab-
sorber area is too large for a single zigzag pipe, the flow resistance can
be decreased by installing several pipes in parallel (fig. 9). The heat
Fig. 9. — Multiple-pipe solar-energy absorber, showing branch-tee method
of connecting parallel pipes.
transfer operation is more effective than for the single-pipe absorber of
the same area because with the faster flow the temperature rise will be
less and the heat losses from the absorber lower.
Ideal Thin, Flat-Tank Solar-Energy Absorber. — Although some of
the solar energy falling in the space between pipes can be utilized indi-
rectly by convection of hot air, or by conduction through a cement bed,
or by extended fin surface, the ideal absorber has a continuous black
surface covering a thin sheet of water. This type is seen in the center of
figure 11. The inherent disadvantage of the flat tank is its inability to
withstand even low water pressures without an expensive construction
using heavy plate and many stay bolts. Its high efficiency, due to mini-
Bul. 602]
Solar Energy for Heating Water
27
mum heat losses, is usually not economical in comparison with a larger,
less efficient pipe absorber.
Nonfreeze Solar Water Heaters. — When it is desirable to operate a
solar water heater on bright days during the winter, the danger of
bursting absorber pipes during cold nights can be avoided by separating
Fig. 10. — Dr. Abbot and his solar power unit
for high temperatures. The parabolic mirror ap-
pears black because of reflecting the central heat-
absorbing pipe.
the absorber circulating fluid from the usable water in the storage tank,
and using a nonfreezing solution in the absorber circuit. Figure 23
shows a commercial storage tank with separate heating-fluid jacket
which is the only significant difference from the ordinary system of pipe-
coil absorber with circulation directly through the storage tank. Stand-
ard tanks with internal heating coils or with external heat exchangers
might also be used.
28 University of California — Experiment Station
Steam Generators and High-Temperature Absorbers. — Figure 10
shows a parabolic reflector-type, high-temperature, solar-energy ab-
sorber developed by Dr. C. G. Abbot. (fl) The patented unit shown devel-
ops about Yq horsepower and can be used for steam generation or, with a
special circulating fluid, can be used for heating baking ovens to tem-
peratures of 350° to 400° F. This type of absorber is also suitable for
using solar energy for heat-operated absorption refrigerators. Both the
two-hour and the continuous heater refrigerators require a minimum
working fluid temperature of 250° F, and 325° F is desirable for freez-
ing ice cubes. The reflector used by Dr. Abbot is Alcoa lighting sheet,
specular finish, No. 24 gauge, 24 inches x 72 inches, costing about 30
cents per square foot in small lots. The two concentric glass tubes with
vacuum between them are on the reflector axis fixed parallel to the
earth's axis of rotation. The reflector is rotated about this axis following
the sun so that the sun's rays are always concentrated on the central
tube.
HOT-WATER DEMAND
The decision as to the kind, size, and system of solar water heater to be
installed depends upon several interrelated factors. The nature of the
hot-water demand is, of course, the primary question. This demand in-
volves temperature, quantity, and time of day ; it varies widely in indi-
vidual cases, because, of differences in personal habits, plumbing facili-
ties, and the relative expense of heating water.
Temperature of Hot Water Needed for Various Domestic Purposes. —
The desirable temperature of water used for various purposes is known
within reasonably close limits. The temperature data in table 5 were
observed in common domestic practice. The water for a hot shower is
definitely too hot at 105° F and verges on the cool at about 90° F. Dish-
water at 120° F requires the use of a mop because it is too hot to keep
the hands in ; and although dishes can be scrubbed clean in cold water
every housewife would object to dishwater below 105° F. Assuming the
usual temperature of the supply water to be 60° to 70° F, the difference
in the amount of heating required to obtain the upper and lower limits
of temperature is only about 25 per cent.
Quantity of Hot Water Needed for Various Purposes. — The variation
in quantity of water used, however, is so great that no average assump-
tion can be considered narrowly. If unlimited inexpensive hot water is
readily available at the turn of a faucet, its use will be almost extrava-
gant; the water will be left running while one is doing short chores,
tubs will be filled to overflowing, showers will be run to heat bathrooms,
and so on. If, on the other hand, a person must wait while water is being
Bul. 602] Solar Energy for Heating Water 29
heated or must make an effort to obtain hot water or finds that hot water
is expensive, he naturally will use a minimum. The quantity of hot water
obtainable from a solar heater varies with the amount of sunshine avail-
able ; and to make up a deficiency by starting an auxiliary heater in-
volves a time delay, personal effort, and expense so that the natural use
tends to follow the available supply. This elasticity of demand greatly
extends the period of usefulness of solar water heaters both before the
long cloudless summer and afterward into the autumn, often until a
water coil in the range or furnace can be depended upon during the win-
ter (fig. 19).
Despite the inherent variations in personal habits one must assume
some average hot-water demand in order to estimate the size of water
heater that will give adequate service without being unduly large.
For late afternoon hot showers for field workers, a temperature of
102° F is needed, and a minimum quantity of 12 to 15 gallons per person.
This is usually obtained by simple water tanks exposed to the sun
(fig. 6).
For general domestic purposes the average rural demand is usually
considered approximately 40 gallons of hot and cold water together per
person per day, of which one-third is assumed to be heated. The Amer-
ican Society of Heating and Ventilating Engineers recommends an esti-
mate of 40 gallons of hot water per person per day for apartment houses.
This difference of 3 to 1 is due largely to the convenient availability of
hot water in apartment houses equipped with steam boilers operated by
a janitor.
The data in table 5 were obtained by J. R. Tavernetti from a test in-
stallation of a low-wattage electric water heater for the California Com-
mittee on the Relation of Electricity to Agriculture. These metered
observations covering the daily springtime routine for a family of two
adults, two small children, and a baby give a direct intermediate exam-
ple of hot-water use when an adequate supply is always available from a
solar heater operated in series with an automatic electric water heater,
which was considered expensive to operate. All figures include the quan-
tity wasted in warming approximately 40 feet of cold pipe.
If the family is considered as equivalent to four or five adults, the
average total daily hot-water demand is approximately 20 to 25 gallons
per person. This figure, though much greater than the accepted rural
average, is only 50 to 60 per cent of the demand recommended by the
American Society of Heating and Ventilating Engineers for apartment
houses. Nevertheless, the observed total coincides with the general rec-
ommendations of a manufacturer of commercial solar water heaters.
30
University of California — Experiment Station
Recognizing that the figures in table 5 represent neither a scant}'
nor an extravagant use of hot water, one notes that 58% gallons of
hot water is needed in the first 3 hours on wash days (3 times a week) ;
then only 14% gallons is used in the following 5% hours, with no de-
mand at all for the next 3% hours. Then another heavy demand of 49%
gallons of hot water comes with the evening baths, usually within 3
hours. Obviously, the solar heater cannot meet the first demand of 60
TABLE 5
Daily Hot-Water Demand for a Family of Five
Time
7:00 a.m
8:00-8:30 a.m
8:30-9:15 a.m
9:30 a.m
9:15-10:00 a.m
10:00 a.m
10:15-11:30 a.m
11:30 a.m.-3:30p.m
7:00-7:30 p. m
7:30-8:30 p.m
8:30 p.m.-6:30 a.m.
Use
Shaving and incidental
Clothes washing, one machineful re-used.
Dishwashing
Washing clothes by hand in sink
Four trays rinse water
Baby's bath
Incidental
Washing dinner dishes and incidental. . . .
Two children's baths
Washing supper dishes
Two adults' baths
Temperature,
degrees F
100
128
128
120
100
100
125
92
125*
95
Quantity of hot
water, gallons
4, plus cold blend
14
12
Z l A, plus cold blend
22, plus cold blend
3, plus cold blend
3
im
25, plus cold blend
7
17H. plus cold blend
Wash-day total 122H gallons hot water
Average daily total 96 gallons hot water
* An electric dishwasher requires 150° water to sterilize, but water 160° F or hotter causes trouble. The
total quantity including the preliminary spray (waiting for water to run hot) averages 5 or 6 gallons per
charge. Usually an additional 3 gallons is used simultaneously for washing pans and for incidental cleaning -
up.
gallons early in the morning without an insulated tank to keep hot the
water heated the previous day. Since, furthermore, the evening baths
will be taken from this storage of hot water needed in the morning, the
storage tank should be large enough, as a general rule, to hold the entire
day's requirement.
Quantity and Temperature of Hot Water Needed for Farm Dairies. —
For farm dairies (up to 60 cows) the hot-water demand is approxi-
mately 10 gallons twice a day at 120° F for washing utensils ; but steam
(about 35 pounds per day) is also required for sterilization; and both,
of course, are needed every day regardless of the weather. Steam can be
generated by specially designed solar heaters (fig. 10), but since an
auxiliary boiler is required for cloudy days, the investment in a special
solar heater for steam generation would be warranted only if the cost of
fuel is exceptionally high. Dairies producing milk to be used for manu-
facturing purposes can manage with hot water alone if at 180° F. This
temperature is obtainable with well-insulated double-glass pipe absorb-
ers and might be cheaper than the fuel used in the auxiliary heater.
Bul. 602]
Solar Energy for Heating Water
31
EXPERIMENTAL INVESTIGATIONS
The useful heat output of a fixed solar-energy absorber depends pri-
marily upon the short-wave energy received, which changes rapidly
during the day and differs from day to day because the intensity of the
sunshine varies with the season and with the character of the atmos-
phere. Second, the useful heat output is affected by the heat losses of
the absorber, which vary with temperature and wind. No single-day
Fig. 11. — Three experimental solar water heaters with elevated storage tank.
At the left is the enclosed triple-tank heater shown in figure 7. In the center is
the thin flat-tank absorber used for reference. At the right is a pipe-coil absorber
(also shown in figure 8) with connection at A from the bottom of the storage
tank. Above the pipe-absorber outlet B there is an insulated vertical riser to C,
where it turns to enter the top of the storage tank.
figure or curve, therefore, can be a standard. In order to compare the
results of different experiments on different days, three different types
of heaters were built by Charles Barbee and H. D. Lewis (fig. 11) . Simul-
taneous observations were made throughout the late summer and fall
and are discussed in the following sections.
Heat output of Thin Flat-Tank Absorber. — Figure 12 indicates the
useful heat obtainable on clear days near the end of the usual solar-
heater season. Midsummer values would be much higher. The data for
this curve were obtained with water constantly flowing through a flat,
thin tank 20.9 square feet in area with a single-glass cover, as shown in
the center of figure 11. The rate of flow was maintained constant at
32
University of California — Experiment Station
about 1 quart per minute by gravity from the float chamber seen in fig-
ure 11, above and to the left of C. The discharge was into an open funnel,
visible at the upper left-hand corner of the flat absorber. A noon tem-
perature rise of about 40° F above the inlet temperature of about 80°
F occurred, and the average absorber temperature was somewhat above
average air temperature, as would be the case in a solar water heater.
The total useful heat obtained per day, as indicated by the area under
6 JM. 7
o °
/o
// /Voo/f / 2
r//7?e of c/oy
S fi/W.
Fig. 12. — Useful heat output of thin flat-tank absorber on clear days,
September 23-27, 1935.
the curve of figure 12, was 1,360 B.t.u. per square foot. There are large
heat losses from the hot absorber box and considerable heat absorption
by the insulation. The latter, though not a true loss, delays and lowers
the peak of the curve in relation to the theoretical input (col. 12, table 4) .
Performance Characteristics of Bound-Tank Absorbers. — The tem-
perature rise in round-tank absorbers differs from that in thin flat-tank
absorbers mainly because of the large water quantity associated with a
Bul. 602]
Solar Energy for Heating Water
33
given absorber area. The common 30-gallon hot-water boiler 1 foot in
diameter by 5 feet long has such a poor ratio of area to volume that the
water does not warm rapidly. Since larger-capacity tanks have even
poorer ratios of area to volume, and smaller diameter would be special,
this study by H. D. Lewis was confined to the regular 30-gallon tanks.
Figure 13 shows the performance of exposed horizontal and sloped
tanks which on calm days furnish enough hot water for two or three hot
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Fig. 13. — Temperature of hot water drawn at 4:00 p.m. from simple uncovered
30-gallon tank in July; temperature readings were taken of each gallon as drawn.
showers at 102° F. The horizontal tank on days with average air-day
temperatures of 100° F furnished 20 per cent less hot water than the
sloped tank even on days 10° colder. The horizontal tank is less efficient
because all the sunshine falls on the hottest part of the tank. If the tank
is sloped (fig. 6), the sun shines on the lower, cold end as well as on the
upper hot end. In this case much of the colder water is heated directly
at low temperature and with small heat loss. In all cases the average
water temperature reached its maximum before 4 p.m.
34
University of California — Experiment Station
The lowest curve (fig. 13) indicating the heat output on a cold sum-
mer day, shows the need for protecting the tank against cold air and
wind ; but even so there is ample water for one hot shower.
The addition of a simple glass cover supported by an inverted V
frame so that the glass formed a coop over the sloped tank resulted in
much better performance (fig. 14). The water was drawn three hours
/fO
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40
Fig. 14. — Temperature of hot water drawn at 7:15 p.m. from glass-covered, 30-
gallon tank without insulation, July 17 and 18, 1935; temperature readings were
taken of each gallon as drawn.
later in figure 14 than in figure 13, during which time the air tempera-
ture dropped 16° F.
When the quantity of hot water available from a 30-gallon tank heater
is not sufficient but the characteristic temperature performance is satis-
factory, a larger quantity is obtained by using several tanks in parallel,
with all the cold-water inlets connected together in one direction and all
the hot outlets connected together in the opposite direction. The tanks
should be spaced well apart to avoid the shading of one by another in
the early morning and late afternoon.
Bul. 602]
Solar Energy for Heating Water
35
The tanks in figure 7 were on 24-inch centers, and the inside width of
the box was 8 feet.
Performance Characteristics of Enclosed Multiple-Tank Absorbers. —
Multiple 30-gallon tanks are easily enclosed in an insulated box and
covered with regular hotbed sash. Although temperatures over 140° F
are obtainable in the late afternoon, the morning temperature cannot
be expected to exceed 110°, which is too cold for washing clothes. This
system (fig. 7) has, however, the advantage of simplicity, high daytime
efficiency, and self-storage, and is nonfreezing in most of the agricul-
tural areas of California.
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Fig. 15. — Temperature of water drawn from enclosed triple-tank absorber, after
one day's heating, at 4:15 p.m. and 8:30 a.m., September 11 and 12, 1935; tempera-
ture readings were taken of each 5 gallons as drawn.
Figure 15 shows the performance of three enclosed tanks connected
in parallel. The tanks had been filled at 8 :30 a.m. with cold water (67°
F) . The hot-water output per tank in the insulated box at 4 :15 p.m. on
September 12 is better than that from the exposed tank in July (fig. 13) .
Furthermore, the exposed tank cooled nearly to air temperature every
night ; and although the water temperature in the enclosed tank dropped
25° to 35°, this drop was less than one-half the difference between the
evening hot-water temperatures and the morning air temperatures. It is
unfortunate that usual methods of insulation will not preserve a tern-
36
University of California — Experiment Station
perature the next morning at 8 :30 a.m. high enough for efficient clothes
washing.
If the three tanks are connected so that cold water enters the two out-
side tanks and hot water is drawn from the center tank alone, 30 gallons
of water drawn at night does not lower the temperature of the center
tank appreciably, but during the night the hotter center tank cools more
rapidly. Figure 16 shows that this method of connecting the three tanks
/so
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(?c/0/7t/£y of ivo/er draw/? from 3, SO-00//0/7 £a/?As, £o//o/7s
Fig. 16. — Temperature of water drawn, 30 gallons at night and 60 the next morn-
ing, from enclosed triple-tank absorber with parallel connection and with cold water
connected to two outside tanks and hot water drawn from the center tank ; tempera-
ture readings were taken of each 5 gallons as drawn.
gives no higher morning temperatures than would be expected from the
regular parallel connection. In case of frequent daytime use of hot
water there may be some advantage in such a connection, in that it would
avoid mixing cold tap water in the center tank, and some saving might
be made if the absorber box were partitioned to isolate the hot tank.
The data for figures 15 and 16 were obtained with tin-plate reflectors
under each tank so curved that all the incident light was thrown onto
the tanks. The entire box appeared black from all angles except at the
east and west edges. In November these reflectors were removed, and the
entire box was painted black. The useful heat output with or without
Bul. 602] Solar Energy for Heating Water 37
reflectors was 724 B.t.u. per clay per square foot of glass area. The
amount of solar energy received by the thin, flat absorber, was 1,020
B.t.u. during the day the triple tanks had reflectors and 1,003 B.t.u.
per sq. ft. during the day the triple-tank box was plain black. This indi-
cates no advantage in using tin-plate reflectors in an insulated box. The
general results of all comparisons between the enclosed triple tanks and
the ideal thin, flat-tank absorber indicate an approximate relative effi-
ciency of 70 to 75 per cent for daytime heating.
Thermo siphon Circulation and Temperatures in Pipe Absorber with
Storage Tank. — The faults of the round-tank absorbers — namely, small
absorption area in proportion to the tank capacity, and large nocturnal
losses — are remedied by separating the storage tank from the absorber
area. The absorber area can then be designed to satisfy the heat require-
ments independently of tank size ; and the storage tank, being separate,
is easily insulated to minimize heat losses.
The separation of absorber and storage tank requires some means of
heat transfer from absorber to tank during the day and the prevention
of heat loss from tank to absorber during the night. This transfer can be
accomplished positively by forced circulation, using a positive pump
that is operated only during the heating period. For domestic installa-
tions this system is objectionable because of expense, leaky packing-
glands, and the need for mechanical or electrical power not directly
available from sunshine.
Solar heat itself is, however, available upon absorption by the water
for producing thermosiphon circulation if the piping system from tank
bottom to absorber and back to tank top is properly designed. Water
warmed in the absorber becomes of lower density than the colder water
in the pipe from the tank bottom, which will flow into the absorber and
push the heated water into the top of the tank. The force available for
this circulation is proportional to the difference in density of the hot and
the cold water (table 7).
If the absorber is well below the tank, it becomes a low, cold pocket at
night and thermosiphon circulation ceases. The night losses are thus con-
fined to the escape of heat through the tank insulation and to the cool-
ing of the absorber unit, which contains but little water.
The rise in temperature of the water in the absorber pipe due to the
sun's heating depends primarily upon the length of time the water re-
mains in the absorber pipe. These factors act together so that an auto-
matic balance exists between the pipe friction and the force available
from the difference in water density due to heating. If the sunshine sud-
denly becomes more intense, creating an excess in temperature differen-
38
University of California — Experiment Station
tial, the increased difference in water density provides more force to
make the water flow faster; and then it will be in the absorber for a
shorter time and will be warmed to a lower degree, thus balancing 1 any
temporary temperature excess.
Figure 17 shows the observed temperature differentials and rates of
circulation for a five-pipe absorber connected to a storage tank with the
a AM 9
// A/OO/7 /
A/ov./S
4 S 6 S /O M/0/7/fAt £ 4
Time of day
6AM.r a 9
/Vov. M
Fig. 17. — Observed temperatures and thermosiphon circulation in a 5 -pipe absorber
connected with a large storage tank, November 13 and 14, 1935, under a clear sky
and light wind.
hot circulation inlet 7% feet above the center of the absorber. The rate
of flow, curve A, was determined by the deflection of a vane suspended
in a horizontal glass tube, previously calibrated.
Circulation begins slowly at 9 o'clock, gradually pushing the cold
water in the vertical riser into the tank. Then suddenly, an hour later,
a surge occurs when the hot water, too long in the absorber, fills the
vertical riser, while the very cold water due to night cooling still fills
the vertical part of the pipe from the tank to the bottom of the absorber ;
thus a maximum temperature differential is provided. This surge quickly
dies away as unheated water rushing through the absorber fills the
vertical riser while hot water from the tank is drawn into the vertical
drop ; thus a temporary minimum temperature difference is created. An-
Bul. 602] Solar Energy for Heating Water 39
other smaller surge occurs later for similar reasons, and then, except for
a minor surge at 11 :15, the circulation builds up steadily to a maximum
at noon. It gradually falls off thereafter as the tank water warms up
and the solar intensity decreases. The flow stops rather suddenly when
the absorber becomes shaded. Because of nocturnal cooling of the ver-
tical riser which enters the tank 4 feet above the pipe to the bottom
of the absorber, slight reverse circulation occurs, reaching a maximum
about sunrise.
The temperatures of the water leaving and entering the absorber are
shown by curves B and C respectively. These reflect the flow surges
previously described. The temperature difference between B and C
does not indicate properly the driving force for the circulation, because
of the importance of the water density in the vertical riser shown in
figure 11 from points B to C and in the vertical part of the cold pipe
from tank bottom to pipe-absorber bottom at point A of figure 11. The
relation observed in figure 17 between the outlet temperature B and
tank-center temperature D shows the mutual dependence of the two.
This 120-gallon storage should have had an absorber nearly three times
as large as the experimental 40.3 square feet, so that the daily rise in
tank temperature would be much steeper. Then B and C would also rise
much more sharply during the day because the water will not flow from
the absorber into the tank unless the absorber is hotter than the tank.
Practically, the tank temperature D shown in figure 17 is what might
be expected in a full-sized installation when about 100 gallons of hot
water is gradually drawn during the day.
The temperatures inside the absorber box at the top and at the bottom
are included in figure 17 to indicate the heat transfer by air convection
from the black bottom to the pipes. The minimum box temperature was
4° F below minimum air temperature, and the glass was frosted at
sunrise although the pipe did not quite reach freezing. This absorber
was exposed during the entire winter of 1935-36 at Davis and did not
burst although a minimum air temperature of 25° F was recorded. This,
however, cannot be considered safe practice.
Relation between Bate of Circulation and Temperature Rise in Differ-
ent Pipe Absorbers. — The rate of heat input to the storage tank is in-
fluenced by the temperature of the tank water, because a high inlet
temperature means greater losses from the absorber box. This change,
however, is not so important in limiting the maximum storage-tank
temperature during nonuse of hot water as is the continual heat loss 24
hours a day from the hot tank and absorber to the colder air and sur-
roundings.
40
University of California — Experiment Station
The automatic balance between rate of thermosiphon circulation and
temperature rise in the absorber is distinctive for different types of
absorbers (table 6). No corrections have been applied to reduce the
observations to comparable exposed areas because it is difficult to pre-
dict what portion of the increased heat input with larger area would
result in larger temperature rise and what portion in more rapid cir-
culation.
TABLE 6
Observed Noon Average Rates of Flow and Temperature Eise for Different
Flat Absorbers
Absorber
Length of
single pipe,
feet
Glass
area,
sq. ft.
Rate of
free flow, gal.
per min.
Temperature
rise in
absorber, °F
Heat ab-
sorbed, B.t.u.
persq.ft.min.
Date
Storage tank kept cold at tap water temperature
Flat-tank .
Five-pipe.
One-pipe.
40
170
20 9
40 3
36
80
65
20
12.5
21
53
4
2.8
2 4
Sept. 2
Nov. 5
Sept. 23
Storage tank initially hot and allowed to rise in temperature
Flat-tank .
Five-pipe .
One-pipe.
0.78
80
20
10
15
41
3.1
2 5
1.9
Sept. 1
Nov. 13
Sept. 18,
25, 26
Common pipe absorbers, utilizing by convectional heat transfer part
of the solar energy falling in the space between the pipes, are slightly
less efficient than round-tank absorbers, capturing about 60 to 70 per
cent as much energy per square foot of glass as the ideal thin, flat-tank
absorber. Therefore pipe absorbers should be about 50 per cent greater
in glass area than flat absorbers ; but this larger size is usually less costly
than the thin, flat-tank type.
If the space between the pipes is filled with concrete up to half the
pipe diameter, there is considerable gain in useful heat by conduction —
sometimes as high as 20 per cent — making the filled multiple-pipe
absorber possibly 80 per cent as efficient as the flat-tank.
A flat reflector of tin-plate under black pipes was found to be only 86
per cent as efficient as a black tar-paper bottom at the time of year when
the absorber was perpendicular to the sun's rays at noon. At other sea-
sons a flat reflector would be more useful but probably not superior to
a black bottom.
The use of a double glass to improve the thermal insulation of the
absorber box is not justified for moderate temperatures, since the de-
Bui>. 602] Solar Energy for Heating Water 41
crease in radiation received due to the extra glass is usually greater than
the reduction of heat loss. With high temperatures or where strong
winds are prevalent the double glass is often justified.
Size of Absorber in Relation to Quantity of Water to be Heated. —
The average daily heat absorption on clear days in September can be
judged from the curve of figure 12 to be approximately 1,400 B.t.u. per
square foot of glass for a flat-tank absorber. Such an absorber, although
most efficient, is not practical because of its tendency to deform when
subjected to internal pressure. A larger-area pipe absorber is more
economical, and curves in figure 18 show a simultaneous daily heating
by pipe absorbers of approximately 1,000 B.t.u. per square foot of glass.
If a supply water temperature of 65° F is assumed, each gallon of hot
water will require about 700 B.t.u. for heating. The night losses are
about 100 B.t.u. per gallon of high-temperature water, making a total
daily requirement of about 800 B.t.u. per gallon. As some allowance must
be made for dust on the glass and for desired operation when the sky is
not entirely clear, an arbitrary figure of 1 square foot of pipe-absorber
area per gallon of hot water can be assumed adequate for satisfactory
solar-heater operation for seven or eight months of the year in the cen-
tral valleys of California.
LENGTH OF PIPE RUN FOR SATISFACTORY THERMO SIPHON
PERFORMANCE
There is considerable difference in hot- water temperature at the top of
the tank when using single zigzag and when using parallel pipe ab-
sorbers, especially over a short heating period. With the single-pipe
zigzag small quantities of water hot enough for the small demands are
available long before the whole tank temperature builds up with the
rapid-flow type. Figure 18 shows the storage-tank temperatures reached
in one day's heating, contrasting the large-flow, low-degree rise of the
five-parallel-pipe absorber with the small-flow, high-degree rise of the
single zigzag. The single-pipe curve is as observed September 25 and
27 when hot water was drawn from the top of the tank while new cold
water ran in at the bottom. Although some mixing occurs, the curve has
a significant shape. The five-pipe data observed November 6 was cor-
rected by the ratio of heat absorbed in the triple-tank heaters on the
same days and also reduced to equivalent glass area. Since this absorber
area is about one-third that recommended for the 120-gallon storage
tank used, all the temperatures are far below normal practice and do
not indicate much difference in overall efficiency. The lower-tempera-
ture system is more efficient.
42
University of California — Experiment Station
Referring to table 6 and considering the effect of doubling or trebling
the absorber area to suit the usual domestic hot-water demand, we see
that the temperature in a single zigzag pipe two or three times as long
as the 170-foot experimental system would produce an excessive tem-
perature rise of the order of 100° F and consequently large absorber
so
jS/A
ft* /
v'pe
ss
so
s
p//>e±
//? />
?ra//6
7 y *
%
*\
jl> 40
1
\
1
\
\\
N 3S
K
1
\\
^ OS*
s_
v \
-^
s
^
/O fO SO tfO SO 60 70 SO 90 /OO //O /20 /JO
Water draws? from /ZO-~yaA sfarajfa /a/? A, ya//o/73
MO
Fig. 18. — Water temperature rise after one day's heating by single pipe and by
5-pipe absorbers connected to an oversized storage tank ; readings were taken of each
5 gallons as drawn.
losses to the colder air. If the storage-tank water was initially hot, such
a temperature rise would produce boiling in the absorber coils, which
is very objectionable. The single zigzag pipe is therefore suitable only
for the small 40-gallon systems, and for most solar water-heater sys-
tems several pipes should be used in parallel.
Probably at noon a 30° minimum temperature rise is desirable. This
could be obtained with single %-inch pipes about 70 to 100 feet long
when the absorber discharges into the storage tank 7% feet above the
Bui* 602] SOLAR ENERGY FOR HEATING WATER 43
center of the pipe absorber coils and the pipes to and from the storage
tank are l 1 /^ inch for 3 or 4 parallel pipes and 1% for 5 or 6 parallel
pipes. Large variations will occur, determined by the number of return
bends, the height of storage-tank connections, and the weather condi-
tions. When this arbitrary criterion is applied to pipe absorbers con-
taining 4 or 5 lineal feet of %-inch pipe per square foot of glass area
and per gallon of storage-tank capacity, with the bottom of the tank 2%
feet above the center of the absorber, a 45-gallon system should have
three or four pipes in parallel ; a 60-gallon system, five or six pipes. In
larger systems the storage tank may be raised to obtain greater thermal
head. Each pipe run should be about 10 or 15 feet per foot of effective
riser height from center of absorber to discharge into storage tank.
In many houses there is no room in the attic above the top of the
absorber coils for the storage tank. In such cases a false chimney can be
built around the storage tank above the roof. If the storage tank is in-
stalled level with the pipe absorber, a thermal valve is required to pre-
vent reverse circulation at night. The low head available for producing
thermosiphon circulation calls for larger connecting pipes and also more
paralleling in the absorber. An alternative is to use multiple-tank ab-
sorbers previously described.
Calculations, following the procedure given in the Chemical Engi-
neers' Handbook, (29) indicate rates of flow approximately twice that ob-
served by means of the vane meter. Special tests at night, when all
temperatures were within 1° F and flow was obtained by differential
elevation of open inlet and discharge reservoirs, showed the resistance
to be higher than calculated and in agreement with the vane-meter
thermal-head observations. Comprehensive viscous-flow tests to deter-
mine losses in various fittings and short pipe have not been run. Since
the rate of flow observed in all the foregoing tests was well within the
viscous or laminar-flow region, the resistance to flow was proportional
to the viscosity of the water, which, at various temperatures, is given in
table 7. The change of viscosity with temperature is rapid, the value
near the boiling point being only one-sixth of the value near freezing.
44
University of California — Experiment Station
TABLE 7
Density, Viscosity, and Kinematic Viscosity of Water
1
t/sns/ty
//scos/ry"
fWe/yfyi)
KMMAT/C
v/scos/ry
1
fes
0£//s/ry
(USe/g/M)
i//scos/ry
(We/gM
/CMMA//C
V/SCOS//Y
1
pws/ry
We/f/ti)
v/scos/ry
(We/gAt)
KMMAr/C
1//SC0S/7Y
°F
W/A/fl*
Mj/6/ffsec.
rjftt/sec
"f
*j /6/W.
7/,/6/r/sec
r t f£*/rec
"f
w,3/r/ j
d, ft//? see
Vj/yfeec
32
62. 4/8 40
000/2042
/929X/OS
76
77
62.232 78
62 243 S3
0.000608/
000600S
0-977x/o- s
096SX/O*
720
72/
722
67 7/280
67. 6974/
6/ 68/39
0- OOO 3 762
■ OOO 3727
0. OOO 3692
0.6/OX/OS
0.6O4
0S99X/0*
33
34
33
62. 420S7
62. 422 40
62. 423 SO
.OOZ/8/3
.OOZ/SSO
. O0//368
/893
73S7
782/
78
79
80
62.234 99
62. 22S82
62. 2/6 47
0OOS93O
.OOOS8S6
. OO0S784
0-9S3
0-94/
0.93O
723
724
723
6766622
676S04O
6/634 4S
.OOOS6S8
■ OOO 36 2 S
. OOO 339/
0S93
OSS8
OS83
36
37
38
62-42S07
62.42S 9/
62. 42643
.00/ //S7
.O0/09S9
. 00/0763
/787
/7S6
/ 724
8/
82
83
62206 94
62/9724
62/8736
■ 00OS7//
. OOOS64S
■ OOOSS77
9/9
0-908
0.897
726
727
728
676/836
6/.602 /3
6/ 383 76
■0OO3SS9
■OOOSS27
. 0OO349S
OS 76
0372
0S67
39
40
4/
62. 42664
62 426 S4
62 426/4
■00/OS70
.00/0383
OO0/O2O6
7693
/663
763Sx/O s
84
8S
36
62/77 30
62. 76708
62. /S6 69
■ OOOSS/O
. OOOS444
OOO 3381
886
0-876
O- 966 x 70S
729
730
73/
67S69 23
67SS260
67 333 3/
■OOOJ464
■0OO3433
0.0OO34O3
OS63
0SS8
0SS3X/O*
43
43
44
62 42343
62. 424 44
62. 423 /S
.00/0OJ3
. 00OS863
.0009697
/■607
/■SSO
7 SS3
37
88
89
62/46/2
62 /3S 40
62/243/
OOO 33/7
■ O0OS2S6
. OOOS/96
3S6
O 846
836
732
/33
/34
67S/S93
67 SO/ 97
67 40493
OOO 33 73
■ OOO 3344
.0OO33/S
0S48
0.344
0339
4S
46
4/
62 42/ SS
62. 4/9 74
62. 4/762
.00O9S34
. 0OOS37S
.0009222
7S27
7S02
7 477
90
9/
92
62. //3 43
62/02 23
62O9O0S
.00OS/37
.OOO SO 78
.0OOSO2/
327
0-8/8
0-809
733
736
737
6/467 73
6 V. 4S0 3/
67 433/2
.00O3286
.O0O32S8
0003230
0.S3S
0-33O
OS26
48
49
SO
62-4/S 23
62 4/2 37
62.409 6S
0009O7S
.0008930
0. OOO 8787
74S4
/.43/
7408x/0~ s
93
94
S3
62 079 32
62.067 63
62. OSS 79
. OOO 4 96 S
. OOO 4909
OOO 48 S 5
0.8OO
0-79/
0782 x/O*
738
739
/40
67 4/S63
67 398 OS
67 38037
. OO032O3
. OOO S/ 76
0. OOO 3 /SO
0S22
0.3/7
0S/3X/O J
S/
S3
S3
62 406 46
62 403 0/
62 39932
■ 0OO86SO
■ 000 8 S 76
. OOO 8383
7386
/ 36S
7344
S6
97
98
62.04380
62-03/ 66
62. 0/937
■ 0OO48O3
.00047S/
■ OOO 4698
0-774
0-766
07S8
/43
/46
749
6/ 326 4
6/27/3
6/2/49
. 0OO3O72
. OOO 2998
O- OOO 2926
0SO/
0-489
0-478X/O-*
S4
SS
se
62. 39338
62-39/ /9
62 386 77
. 0008237
■ 0008/32
.00080/0
7 323
7 303
7284
99
/OO
/O/
62. 0O6S2
67 99433
6/ 98/ 60
.OOO 4642
.0OO4S98
■ 0004S49
O 7SO
0-742
734
732
735
738
67 /S78
6/ /OOO
6/ 04/ 4
.OOO 28 S3
. OOO 2792
OOO 2729
467
0-437
0-447x/O' J
S7
S3
SS
62. 3S2 /2
62-37723
62.372/2
■ 000 7092
. 00O 7776
O-0OO 7663
726S
7247
/■229x/0' s
702
703
/04
6/ 968 72
67 SSS 7/
67. 942 36
■ 000 4S02
■ 0OO44S4
OOO 44 08
7£6
O 7/9
0- 7/2 x 70*
76/
764
767
60.98/ 7
60- 92 /O
60- 839/
■ 0002668
■ OOO 2609
0OOO2SS3
0437
0.428
0.4/9x70'*
60
6/
62
62 366 78
62- 36/ 22
62.3SS4S
■ OOO 7SS3
■ 00O 744S
■ OOO 7339
72//
7794
7/77
70S
706
/07
6/929 2S
67 9/S 78
67.902/7
OOO 4362
■ OOO 43/8
■ OOO 4274
07O4
0697
069O
770
773
776
60- 7963
60- 7326
60. 668/
■0002498
. OOO 2446
O OOO 2396
0.4/7
0-4O3
39Sx/O~ J
63
64
63
62.349 4S
62 34324
62 336 82
■ 000723S
. 000 7//9
.000 7036
7760
7/42
7/29
/OS
709
//O
67 8884/
67 374 S/
6/ 660 44
. 00O423O
.0004/88
■ OOO 4/46
0-683
O 677
0-67O
779
782
783
60-6O2 7
60- 336 S
60-4696
■ OOO 2347
. 0OO23OO
00OO22S4
0.387
0380
0.373x/O' i
66
67
68
62 330/8
62. 32334
62. 3/ 6 29
■ OOO 6939
. 000 68 4 S
0OOO67S3
/.//3
7098
7084x70*
///
//2
//3
6/ #4622
6/ 03/8S
6/3/736
.0OO4/O6
.OOO 406 S
O OOO 4024
0-664
0-637
06S/X/O*
788
79/
/94
60 40/ 6
60 3328
60.2629
. 00022/0
.0002/68
0- 000 2/27
0-366
0-339
O.S33
69
70
7/
62 3090S
62 30/60
62293 /S
■0OO6663
■ 0OO6S7S
■ 0OO6488
7069
/OSS
7 04/
//4
//S
//6
67 802 76
67 738 OS
67 77324
.OOO 3984
■OOO 3946
■OOO 3908
64S
0.639
0633
/97
200
203
60- /92 4
60/2/ 3
60. 0494
.0002087
.0OO2O48
0- OOO 20/2
0-347
0-34/
0.33Sx/O J
72
73
74
62-286 /O
62- 278 O 6
62- 26983
. 0OO64O3
■ OOO 632/
■ 0006239
/■028
70/S
/O02
7/7
//8
//9
67 733 30
67 743 2S
6/ 723 08
■ OOO 38 7/
. O003S34
. 0OOS797
06 27
0-62/
0-6/3
206
209
2/2
39. 976 7
S9. 9030
39- 828 4
.000/976
.000/94/
000O/907
0329
324
0-3/9x/O' S
7S
62 26/40
0.0006/SS
0-989 X/0' s \
De/rs/tjr 7s 7/7der/>o/a7ec7 at J co.
nveriea 1 So £/ig//4A &/?/£s 7ro/??
fat".
17/sc
St.
Cr/t. rat/
■)s//y /we/0>
/> £</. 7933
?S, /928 1/
tt) /s //>/e.
M />27 it*
rpo/atec/ a
7{//£//>//er o
■7O0iser£ec/
7 6242664.
to £~/7g//J
•a 4/ &'6
V/<$/79SJ70C
A 0s//'£s A
0067/97/
)
'o/n S/n/Mjo/770/7 PAys/ca/ 7^3/es,
/og- 6. S27330/-/O)
Bul. 602] Solar Energy for Heating Water 45
USE AND CONSTRUCTION OF SOLAR WATER-HEATER
SYSTEMS
An adequately designed, properly installed solar-heater system will
furnish hot water continually throughout the summer in the central
valleys of California without an auxiliary heater. The use of an auxiliary
heater can be avoided also in the spring or fall if the hot-water demand
can be adjusted to the sunshine periods. In the winter, however, some
auxiliary water-heating system is desirable. The solar water heater can
be used also with an automatic water heater to insure a continual supply
of hot water regardless of the weather but at minimum heating cost.
COMBINATION OF SOLAR HEATER AND FURNACE OR RANGE
WATER COIL
In the most economical water-heating system (fig. 19), the solar heater
is depended upon from early spring to late fall ; then, during the winter,
the furnace coil or water-back in the range is used for obtaining hot
water. If the furnace or range is in daily use, usually the solar heater
is shut off and drained to avoid freezing.
Location and Connection of Solar Absorber. — The solar absorber is
most conveniently placed on a roof sloping south and in front of attic
windows so that the glass cover can be readily cleaned with a hose (at-
tached to the storage-tank drain) or with a mop. The usual absorber
construction is similar to that of a skylight, but with a special provision
for an insulated pipe outlet at the top that permits a continuous rise in
the pipe to the storage tank. This requirement usually interferes with
standard flashing practice. A simple method is shown in figure 19 ; the
hot pipe is brought through the end of the absorber above the roof
surface and turned to enter the attic through a separate hole higher in
the roof, the pipe being insulated and enclosed in a lead sheath under
which the rain water can pass from around the top of the absorber. The
flashing around the lead sheath where it pierces the roof is separate
from the flashing around the absorber box. In an alternative method
(fig. 20), the hot pipe leaves the top edge of the absorber through a
special box (shown next to the dormer-window wall) running from the
roof slope out to the absorber frame, with the roof flashing arranged to
drain each way from this obstruction along the top edge of the absorber.
Safe Piping Practice for Furnace or Range Coil. — Valves between the
tank and the cold-water supply line and between the auxiliary heaters
and the tank are intentionally omitted to avoid danger of bursting. If
46
University of California — Experiment Station
a valve were provided in the supply line to the hot-water tank, the tank
might be drained to avoid freezing ; and later the range or furnace might
be thoughtlessly started with all valves closed and result in an explosion.
A single valve at the meter, shutting off all the water, is not objection-
able, for the cold-water supply is always turned on before one thinks of
starting a hot-water heater. If hot-water shut-off valves are to be in-
W#/er co/7 //? ft/r/?<7ce,
/"/re/) /ace j or r<7/7&<?.
Fig. 19. — Solar absorber and furnace coil with single storage tank.
stalled, recommended dairy sterilizer connection practice should be
followed, which specifies a check valve in a by-pass around each shut-off
valve so that in case of higher pressures in heaters the water can back
up in the supply line even though the manually operated valves are
closed. This by-pass and check-valve system permitting reverse flow is
alternative to the pressure relief valve, which might not operate prop-
erly after long periods of nonuse. By putting a union in the cold-water
Bul. 602] Solar Energy for Heating Water 47
line to the solar-heater storage tank near the auxiliary heater, one can
easily connect the cold water supply across direct to the auxiliary heater
when for any reason the solar storage tank must be removed from the
line and the hot-water system promptly restored to operation.
Valves between the solar absorber and the tank are necessary to per-
mit draining the absorber in cold weather without disturbing the rest
of the hot-water system. These valves do not create a danger from over-
heating because the heat losses from very hot absorbers usually balance
the solar-energy input at a temperature below the boiling point. If the
absorber coils are filled with water that is confined by closed valves,
solid expansion by heating might produce serious breaks if there is no
entrained air ; but this is not so dangerous as the explosion of a closed
dry boiler heated by a fire.
"Whenever the water freezes in the pipes between the storage tank and
furnace or range coil, there is another serious danger of explosion when
a fire is started. The use of tees instead of elbows at the upper connec-
tions to heating coils makes it easy to inspect by loosening the plugs
and draining enough water to notice whether the pressure remains con-
stant.
With gravity-flow water-supply systems, the highest pipe in the
solar-heater system must be considerably lower than the bottom of the
water-supply tank to insure full lines for thermosiphon circulation.
Installation of Extra Emergency Side-Arm Heater. — The combina-
tion of solar heater and furnace or range coil works well in the Sacra-
mento and San Joaquin valleys during the summer and the winter.
During a few days in the spring and fall the sunshine is not sufficient
to produce water temperatures high enough for clothes washing. On such
occasions washing is postponed until the morning following a bright
day, or else an emergency side-arm heater (fig. 19) is used.
An extra heater of this type when connected in the furnace coil line
often leads to unsatisfactory operation of both heating coils unless the
hot-water pipe from each heater is carried separately into the storage
tank or connected together very near the tank. This high connection
satisfies the requirement that the thermal density differential head in
each heater riser be greater in proportion to the total circulation head
than the ratio of heater and riser-flow friction to the friction in the
complete circuit. One should remember that the flow friction in the
heating coil often increases rapidly because of scale formation, thus
requiring a higher connection point.
48 University of California — Experiment Station
COMBINATION OF SOLAR HEATER WITH AUTOMATIC
WATER HEATER
When a continual supply of hot water is essential during cloudy weather
and the use of a furnace or range coil is not desirable, an automatic
water heater can be installed without sacrificing the advantages of a
solar heater.
Two-Tank Combination of Solar and Automatic Water Heater. — The
combination of solar heater and auxiliary heater can be made very
simply if the tank for the auxiliary heater is separate from the tank
for the solar heater. With two tanks both the solar heater and the auxil-
iary heater can work simultaneously, and an automatic oil, gas, or
electric water heater will function only when the water coming from
the solar-heater tank is not up to thermostat temperature.
When the solar heater is connected in series with an automatic auxil-
iary heater (fig. 20), the hot water drawn from the regular heater is
replaced by water from the top of the solar-heater tank. On clear days
this water will be hot enough already ; but if on cloudy days it is colder
than the thermostat setting, the auxiliary heater will operate to make
up the difference. This combination system insures an adequate supply
of hot water, and the housewife will never be bothered by finding the
water occasionally only lukewarm.
Storage-Tank Capacity of the Auxiliary Automatic Water Heater. —
For the irrangement shown in figure 20, the tank size of the automatic
heater need only be adequate to meet sudden demands, though the daily
rate of heating water must, of course, be sufficient to provide all the hot
water when there is a long period of cloudy weather. According to the
demand figures in table 5, if the auxiliary heater is on c<" itinuously for
24 hours it must be able to heat at least 5 gallons per ho . to provide
120 gallons per day. (Assuming a cold-water inlet of ^5° F and a
thermostat setting of 145° calling for an 80° rise, a minimum of 3,320
B.t.u. per hour is required. This amounts to only 1 kw. per hour or 4 to
5 cubic feet of natural gas per hour, according to the efficiency of the
heater.) If 5 gallons of hot water per hour can be obtained from the
heater and 55 gallons is needed within 2 hours, the minimum automatic
storage tank size is approximately 45 gallons for the example of small-
family routine given above. This example indicates that the automatic
heater storage-tank capacity should be about one-half the average daily
hot-water demand.
Single-Tank Combinations of Solar and Automatic Water Heaters. —
Since two tanks and a connecting line have greater heat losses than a
Bui* 602]
Solar Energy for Heating Water
40
single larger tank, the thermally ideal combination system is to connect
a regular automatic external heater to the top half of a solar-heater
storage tank, which is large enough for one day's hot-water demand.
The auxiliary heater can be located downstairs (fig. 19) if the thermo-
stat is attached to the center of the storage tank and operates the aux-
So/ar /peat storage ta/?A
iv/'t/? ta/?A Sot to/7? at>ove
of adsorder.
tfega/ar water 4ea£er
/acatea" w/?ere co/7ire/7/e/?t,
nearest /?ot water /at/cets.
Fig. 20. — Combined solar and automatic water heater with separate storage tanks.
iliary heater by remote control. The solar absorber connection to the
bottom half still makes use of the entire tank, but in cloudy weather
only the top half will function as a separate automatic storage heater.
The connection of the solar absorber to the bottom half of the tank, as
shown by full lines in figure 21, is objectionable only in that the maxi-
mum solar-heat temperature is not available in small quantities at the
top of the tank. This objection is important only when the auxiliary
50
University of California — Experiment Station
heater is not in use. The thermal efficiency of the center-inlet-connection
type is the same as the top-connection type because all heat absorbed is
available in reducing the demand on the automatic heater. It would be
a mistake to use an auxiliary heater to warm the entire solar-heat storage
tank, because circulation through the solar-heat absorber does not start
Fig. 21. — Solar absorber and automatic water heater with single storage tank.
until the absorber is hotter than the tank. In cold weather the absorber
may never reach thermostat-control temperature but yet may be much
hotter than the cold-water supply, which it can warm up when the con-
nections are made as shown in figure 21.
When this single-tank system is to be used with a gas, oil, or coal water
heater which should be shut off in good solar-heater weather, the con-
nection of the hot-water pipe from the absorber should go to the top of
tank as indicated by the dotted line at A. Both connections can be made
in parallel, and the top valve opened when the auxiliary heater is not
Bul. 602]
Solar Energy for Heating Water
51
to be used. There is no loss in efficiency in having both valves open except
when the auxiliary heater is in operation .
The main objection to this system, which is useful in the winter, is
the danger of freezing the pipe-absorber coils. The simplest way to avoid
freezing is to drain the solar absorber on cold nights.
Fig. 22. — Solar double-tank heater combined with automatic water heater.
Solar Tank Heater to Reduce Operating Cost of Automatic Heater. —
When an automatic water heater is in daily use and the installation of
an adequate solar water-heater system is not feasible, one may utilize
solar heat to reduce the operating cost of an automatic water heater by
installing a simple tank heater (fig. 7) on the cold-water line between
the supply and the automatic heater (fig. 22). This system has the ad-
vantages of low cost, simplicity, high daytime efficiency, self -storage,
and of being nonfreezing. Two 30-gallon boilers and two 3x6 foot hot-
bed sash cost less than $25 and will absorb at least 20,000 B.t.u. per day
52
University of California — Experiment Station
for about eight months ; and with an electric water heater on the 1%
cent rate per kw.-hr. they will give a net saving of about $15 a year. This
combination avoids the main fault of low morning water temperature
in the tank absorbers because the water solar-heated the previous day
is drawn into the automatic-heater storage tank when the evening baths
are taken and does not reradiate during the night. Any type of aux-
iliary heater can be used — electric, gas, oil, or coal — that has automatic
temperature controls.
£x/><7/?s/ort fonA a/to?
f/7/er for no/?- freeze / /<?£/ /c/
rr- -761-
r-
/Vo/7- freeze jec/cef
Fig. 23. — Commercial type nonfreezing solar-heater-system tank.
NONFREEZING SOLAR-ENERGY ABSORBERS
A solar heater used in combination with an automatic heater can be
operated to advantage throughout the winter if the danger of pipe ab-
sorber freezing can be avoided. There is not much danger of freezing
during ordinary nocturnal radiation frosts on calm, clear nights when
the air temperature remains only a few degrees below 32° F for a short
time (fig. 17) because of the high heat capacity of the pipe, water, and
insulation. Freezing temperatures with wind, however, are almost sure
to do damage unless there is considerable reverse circulation at night
drawing warm tank water through the cold pipes. Such reverse circu-
lation occurring with low-level storage tanks is ordinarily very objec-
Bul. 602]
Solar Energy for Heating Water
53
tionable in wasting heat previously absorbed. Usually the ordinary pipe
absorber must be drained during periods of freezing temperatures if
damage is to be avoided. If frost warnings are not received regularly,
it is good practice to drain the absorber as soon as the weather is cold
enough for house heating.
TABLE 8
Physical Properties of Water, Alcohol Solution, Brine, and Light Spray Oil
Item
9
10
11
12
13
14
15
16
17
18
19
20
Fluid characteristics, with unit or base
Freezing point, °F
Pour point, °F
Specific gravity, at 68° F
A.P.I, gravity, degrees A.P.I
Density, pounds per cu. ft. at 95° F
Thermal volumetric expansion per °F at 95° F
Thermal density change, pounds per cu. ft. °F
Viscosity, pounds per ft. sec. at 95° F
Say bolt viscosity, seconds at 100° F
„, . , a ... density change
Thermosiphon flow criterion,
viscosity X24.7
Specific heat at 95° F.
Heat-flow criterion,
"^Heat-flow criterion .
items 3X10X11
0.997
Thermal conductivity
B.t.u. per sq. ft.
°F per ft.
v (Thermal conductivity) 4
Solar-heater criterion (28 ^
0.4278
Boiling point, °F at 1 atmosphere. . . .
Flash point, °F at 1 atmosphere
Fire point, °F at 1 atmosphere
Burning point, °F at 1 atmosphere. . .
Water
32
0.9983
62.056
0.000193
-0.012
0.00048
1.00
0.997
1.00
1 00
346
4278
1.00
212
Alcohol,
15 per cent
solution*
20
0.971
60.3
0.000176
— 011
0.00079
55
947
0.50
87
0.310
3918
0.80
190
Sodium
chloride, t
24 per cent
solution
1
1.18
73.2
0.000247
-0.018
0.00087
0.84
0.80
79
0.95
0.284
3653
0.82
224
No. 1
tank-mix
spray oil
below
0.865
32
53.2
0.0004
-0.022
0036 to
0.0053
45 to 55
0.24 to 0.17
0.46
08
0.60
0.086
0.1405
0.20
500(approx.)
270
290
500
* By weight.
t Should be normalized for use in black iron pipe with about \i per cent sodium chromate (Na2Cr20?)
plus enough lye (NaOH) to turn pink litmus paper blue.
The covered tank absorber (fig. 7) is practically nonfreezing in the
valley and coastal areas because of the large heat content of the water
and the large volume of the tanks, which are very difficult to freeze
solid. The connecting pipes are, of course, susceptible to freezing and
should be well insulated.
Commercial Nonfreeze Type Solar Water Heater. — The usual com-
mercial type solar heater is of the type in which a nonfreezing solution
is used in a separate circulating system (fig. 23). In one of these de-
signs <12) the absorber fluid flows through the jacket space around the
storage tank and does not mix with the usable hot water. This circulat-
ing fluid merely transfers heat from the pipe absorber to the storage
54 University of California — Experiment Station
tank, and being separate from the usable water in the storage tank it
can be any suitable nonfreezing solution. Standard tanks with internal
coils but without stratification shields are obtainable from plumbing-
supply houses. These can be connected (fig. 23) with an expansion
chamber for the separate nonfreeze circulating fluid, but will not give
high-temperature water quickly.
Separate Fluids for Nonfreezing Solar-Energy Absorbers. — Various
ingredients can be added to water to lower its freezing point. The most
common solutions are alcohol mixtures and brines. Other fluids not
having the freezing characteristic of water might also be used, notably
oil. Addition of alcohol to water is the simplest nonfreeze expedient ;
but as the alcohol vaporizes rather easily in a solar heater and the solu-
tion weakens, some addition should be made every fall. Various brine
solutions, widely used industrially for refrigeration, are recommended
for nonfreeze solutions where the operator is familiar with the operating
technique to avoid corrosion in black iron pipe. The lightest grade of
highly refined spray oil is the most suitable of the petroleum products,
but does not circulate readily under thermal density differential head.
Table 8 indicates the important characteristics of these three nonfreeze
fluids and of water. As shown in item 16 light spray oil is only one-fifth
as effective as water as a thermosiphon heat-transfer medium. This
means that the absorber when filled with oil would operate at much
higher temperature than when filled with water and would be less effec-
tive because of large absorber heat losses unless forced circulation is
used.
CONSTRUCTION OF SOLAR ENERGY ABSORBERS
Essential features of solar absorbers are the absorber pipes or tanks,
the insulated box, and the glass cover. The angle of slope toward the
south is not very important except that for winter operation the slope
should favor the winter sun, which at noon is only about 30° above the
horizon in California. Most solar absorbers are placed on sloping roofs
and it is much simpler to keep the same slope than to provide a special
slope for the absorber. In case of new construction, seasonal solar heaters
should be placed on roofs of % to % pitch, and all-year absorbers on
% or %-pitch roofs.
The construction details for the insulated box should conform with
regular structural practice, and many different designs will give ex-
cellent results if the simple requirements are met. Separate boxes
mounted in the yard where there is no shade or on a pergola do not
differ much in structure from the common roof box.
Bul. 602] Solar Energy for Heating Water 55
Construction Procedure for a Built-in Absorber Box on a New Roof. —
When absorbers are built right into the roof structure the simplest
procedure (fig. 24) is to (1) sheathe solid the underside of the rafters;
(2) fill the rafter space with bulk insulating material such as redwood
bark fiber, mineral wool, processed rice hulls, or the equivalent; (3)
solid-sheathe on top of the rafters the same as for regular roofing; (4)
frame in the sides of absorber box; (5) lay galvanized iron pan with
edges turned up and bottom soldered and provided with screened drain
fos6e/7 cfotfrt w/£6 /po screws /o
/>er/n/£ occos/orto/ re/noro/ for c/eo/7/'/70'
j/rto'er s/o'e of y/oss.
ft/£er/7o£/ve ra66e£ec/ sasA 6or
f/n/Y/ec/ fro/77 e"x£"). Ofjfe/7 useo*
for t/rt/x/tt/ecf o/oz/r>y of &ree/7
Aoi/ses. fte jf/oss /s /?e/o' //?
pos/^/o/7 />y /eocY cY//>s.
Fig. 24. — Construction plan of pipe-absorber roof box.
pipe to carry off possible leaks from pipe joints or broken glass; (6)
nail on furring strips to support pipe coils or tanks; (7) install pipe
coils, being sure of a continuous rise from bottom to top; (8) flash
around the outside; (9) paint black all over; and (10) glaze in accord-
ance with regular skylight practice.
Removable Glazed Cover. — The glazed frames must be removable for
spring cleaning of the underside of the glass and for servicing the pipes
or tanks in case of trouble. It is also desirable to place the solar heater
in front of an attic window to facilitate cleaning of the outside surface,
which in some localities should be done as often as once a month.
The shape of the absorber depends upon the glass sash used. Hotbed
sash 3x6 feet is the cheapest but has considerable wood area ; four sash
would be needed for a 60-gallon system. If the absorber box can be long,
single-light window sash 18 x 48 inches can be used, twelve being re-
quired for a 60-gallon system. In this case the puttying at the bottom
must be brought up on the glass far enough to drain off the water when
the window sash is in its sloped position. In all cases the pipe must rise
56
UJ
University of California— Experiment Station
Fig. 25.— Absorber piping diagram for two pipes in parallel.
3
Wen/on *^ c/! ^ *&£ **"' *Pi f'*'** 1 .
Fig. 26.— Absorber piping diagram for three pipes in parallel.
Bul. 602]
Solar Energy for Heating Water
D7
continuously from the drain to the storage tank. Any dip in the absorber
coils will form an air pocket and seriously interfere with the thermo-
siphon circulation.
Parallel-Pipe Absorber Coils. — The common pipe absorber is a zigzag
of %-inch pipe, and this is suitable for small solar heaters. If the total
length of pipe in the single-coil type is so long in comparison with the
effective riser height that thermosiphon flow is unduly restricted and
provides only very small quantities of excessively hot water, the solar-
Fig. 27. — Absorber piping diagram for four pipes in parallel.
absorber heat losses are large because of operating at unnecessarily high
temperature. One can then improve the efficiency by paralleling the
absorber pipes to reduce the length of each line and also to divide the
flow into multiple paths.
Figures 25, 26, and 27 show the fittings and pipe lengths for two-,
three-, and four-parallel-pipe absorbers. The combination of fittings
shown gives the closest pipe spacing possible and a practical covering of
absorber area. The methods shown of making the terminal connections
also permit the use of all equal-length pipe runs from end to end. In the
two and three-pipe systems one union is located at the opposite end of
the box, top and bottom, to preserve the equal-length feature of the
58
University of California — Experiment Station
main pipe lengths. The diagrams, though not complete, give the unit-
section dimensions so that the full installation can be easily sketched.
Such pipe systems have, of course, considerable flexibility : the unit sec-
tions are easily stretched out or compressed a little, and ordinary inac-
curacies in pipe lengths will give no trouble.
Figures 28 and 29 show the five- and six-parallel-pipe absorbers using
the standard branch-tee connection for the terminals. Complicated sys-
it
o
J /4 '/>'/>« •)
•O V/%*7
ZJ> £
O
o
1 E
MeJii//n retvr/J ienc/
1 c
Q
3 C
Q
1 L
Cj
3 C
o-
3 L
O
1 c
o
L_
o
1 L
M^sfe^ 4 *s*o<c* J#»** *"<*■*'*** */*»<» *** to"***" *»■ fj
3 I
3 C
Q
1 c
• 4'sAor/
=©
^
3 c
3 C
3 C
=0
o
D^
3| s
■s
3t
1
&d
3
H
Fig. 28. — Absorber piping for five pipes in parallel using branch-tee terminals.
terns of separate pipe fittings might be used, but the 1% inlet and
outlet fitting is too large to maintain the close pipe spacing shown in
figures 25, 26, and 27. Branch tees were commonly used for industrial
wall pipe radiators, but because of the present preference for cast radi-
ator sections these branch tees are not always carried in stock by plumb-
ing-supply houses. They are, however, readily obtainable on order and
give a much neater pipe absorber. With the branch tees the two pipes
connecting at the ends must be shorter than all the rest to allow for the
unions.
Bul. (502]
Solar Energy for Heating Water
59
Methods of Obtaining Greater Heat Output from Limited Absorber
Area. — Usually the most economical method of obtaining greater heat
output is to increase the absorber area and extend the simple pipe coils.
When, however, the available space for a solar absorber is limited, the
equivalent of about 20 per cent greater absorber area can be obtained
by embedding the ordinary pipe coil to half -pipe diameter in cement
M pipes /except 4 as voSeJl snncJrw M , <?<>' *** '*>* //w*f /**? '* ^ *>*
Fig. 29. — Absorber piping for six pipes in parallel using branch-tee terminals.
mortar. This arrangement provides a better means of transfer for the
heat generated in the space between the pipes to the pipes themselves.
A suitable concrete mixture is made of 1 part cement and 4 parts sand.
After the cement has dried, the whole surface should be painted black.
Cast wall-radiator sections would be much superior to pipe absorbers
because the reduced open space and the depth of the section will catch
60 University of California — Experiment Station
almost all the direct sunshine. The objection to extra weight is not se-
rious, and their use is recommended if enough sections for 80 per cent
of the designed pipe-absorber area happen to be available at the same
cost as pipe and fittings. A radiator section cracked by freezing would
of course be more expensive to repair than a cracked branch tee, and
both these are much more expensive and cause more delay than the
repair of a burst pipe or fitting.
Thin flat tanks covering the entire absorber area need have only 70
per cent of the area specified for ordinary pipe coils. This type of equip-
ment can be made to order and might prove advantageous for nonfreeze-
type solar absorbers in which the circulating fluid is not subject to high
water pressure. The proper thickness of metal and the number of stay-
bolts required depend upon the size of the absorber units and the height
of the overflow pipe.
INITIAL COST AND CARRYING CHARGES OF SOLAR WATER HEATERS
The sunshine is free, and every surface exposed to the sun is heated at
no cost, but to obtain useful energy for heating water some apparatus
is necessary and this usually requires an expenditure which connotes an
interest charge. Then, too, the apparatus deteriorates because of rusting
and exposure, so that some annual depreciation charge also should be
considered. There are operation costs, even though very low, because of
the care required for cleaning the glass and replacing occasional broken
panes.
The heat output of a solar water heater cannot be specified exactly in
comparison with auxiliary heaters because the auxiliary heater is oper-
ated only when needed, whereas the solar heater, operating whenever
there is sunshine, is of value only when used.
In many places, such as isolated cabins or houses where the fire hazard
of auxiliary water heaters is objectionable, the relative cost of solar heat-
ers is not important. In other cases where full automatic water heaters
are needed anyway, the carrying charges of a solar absorber must be
compared with the bare cost of fuel or electricity it saves.
Cost of Commercial Solar Water Heaters. — The nonfreeze commer-
cial solar water heater with special tank is sold at the factory for about
$3 per gallon capacity. The installed prices, including insulation, extra
pipe and fittings, labor, and the like are about $5 per gallon. The small
systems have a slightly higher cost than these figures.
By assuming a useful life of eighteen years and servicing costs for
renewal of alcohol and repairs at 4 cents per gallon capacity a year, the
total annual cost will average about 47 cents per gallon capacity a year.
Bui* 602] SOLAR ENERGY FOR HEATING WATER 61
Each gallon capacity represents a free heat absorption of about 1,000
B.t.u. per day, and the nonfreeze type can be assumed to be fully oper-
ative for about 270 days a year. These assumptions indicate a solar heat
cost per 1,000 B.t.u. of about % cent, which is equivalent to an elec-
tricity rate of 6 mills per kw.-hr. when fully utilized.
Cost of Common Pipe-Coil Solar Water Heater. — A standard pipe-
coil solar heater system (figs. 19 and 20) can be installed for about $3
per gallon capacity. By assuming for this type a useful life of about
fifteen years and 210 days per year of full operation, the solar heat
costs about % cent per 1,000 B.t.u., which is about equal to the cost when
using natural gas in the Sacramento Valley.
If the solar-heater system is installed by the owner and the cost con-
sidered is that for materials only, these annual carrying charges might
be halved, in which case the solar heater would compete economically
with the manual fuel-oil heater, which is bothersome and a fire hazard.
Cost of Solar Aosoroer Tank Heaters. — The 30-gallon range boilers,
hotbed sash, and insulation can be obtained for about 60 cents per gal-
lon for the enclosed tank heater shown in figure 7. If fully utilized, this
installation furnishes hot water at a cost of about %5 cent per gallon.
The least expensive system, the exposed second-hand tank (fig. 6) for
afternoon showers, heats water on sunshiny days for no appreciable
cost.
SUMMARY OF USE AND CONSTRUCTION
Enclosed 30-gallon hot-water boilers with glass covers can be used as
solar heaters without pipe-absorber coils and will furnish two or three
hot showers per tank in the late afternoon or evening of bright sunshiny
days. These tank absorbers do not keep their high temperatures over-
night and are not a satisfactory means of obtaining hot water for wash-
ing clothes.
The glass area of the ordinary pipe-coil absorber should be about as
large in square feet as the number of gallons of storage-tank capacity.
The %-inch pipes are conveniently spaced about 2% or 3 inches center
to center and usually should be arranged in parallel circuits to avoid
excessive temperature rise. The length of single pipe of about 70 to 100
feet when the absorber discharges into the storage tank about 7 feet
above the center of the absorber gives over 30° F temperature rise, which
is adequate. When the tank inlet is lower, the single-pipe length should
be reduced in proportion.
The insulated storage tank used with regular pipe-coil absorbers
should have a capacity equal to the whole day's hot water demand be-
cause about half of the hot water is often used after sunset and about
62 University of California — Experiment Station
half is often needed early in the morning before the sunshine has time
to heat much water.
To insure a constant supply of hot water regardless of the weather,
the hot outlet pipe from the solar-heater storage tank can be connected
to the cold inlet of an automatic auxiliary heater. Then if the solar-
heated water is not up to thermostat-control temperature the automatic
heater will operate to raise the temperature to the desired point. When
there is good sunshine the water entering the automatic heater when-
ever a faucet is opened will already be hot enough, and the auxiliary
heater need not operate. With such a combination system the housewife
will never be bothered by lukewarm water, yet will save heating ex-
pense when the sun shines.
LITERATURE CITED
1 Anonymous.
1934. Items. Science 80(2078) Supplement p. 7.
2 Anonymous.
1935. Increased light values with white paint. Indus, and Engin. Chem., News
ed. 13(19) :397.
3 Abbot, Charles G.
1922. The reflecting power of clouds. Smithsn. Inst. Astrophys. Observ.
Astrophys. Ann. 4:375-381. (See specifically p. 379.)
4 Abbot, Charles G.
1929. The sun. 433 p. (See specifically pages 116, 298.) D. Appleton & Co.,
New York.
5 Abbot, Charles G.
1934. How the sun warms the earth. In: Smithsn. Inst. Ann. Kept. 1933:1-
476. (See specifically p. 157.)
6 Abbot, Charles G.
1936. Solar energy now caught with 15% efficiency. Science News Letter
Jan. 11, 1936. p. 23.
7 Building Eesearch Board.
1933. [Gt. Brit.] Dept. Sci. and Indus. Research, Bldg. Research Bd. Rept.
1932:88,90.
8 Callender, J. H.
1934. Aluminum foil for insulation. Architectural Forum 60(1) :68.
9 Coblentz, W. W., and C. W. Hughes.
1925. Emissive tests of paints for decreasing or increasing heat radiation
from surfaces. U. S. Dept. Com., Bur. Standards Technol. Papers 18
(254):177.
10 Coblentz, W. W., and H. Kahler.
1921. A new spectropyrheliometer and measurements of the component radia-
tions from the sun. . . . U. S. Dept. Com., Bur. Standards Sci. Papers
16(378) :240-41.
Bul. 602] Solar Energy for Heating Water G^
11 Cohn, Willi M.
1934. New means of producing extremely high temperatures. Glass Indus.
15(7): 149-50.
12 Day and Night Water Heater Company.
1932. Day and night solar water heater. (Advertising circular.) Day and
Night Water Heater Co., Ltd., Monrovia, California.
13 Day, Preston C.
1917. Eelative humidities and vapor pressures over the United States. U. S.
Mo. Weather Eev. Sup. 6:25, 59.
14 Elvey, C. T. [Yerkes Observatory.]
1934. Some observations of the sun through a dust storm. U. S. Mo. Weather
Eev. 62(6) :201-2.
15 Fishendon, M., and O. A. Saunders.
1932. The calculation of heat transmission. 280 p. (See specifically p. 17-22.)
His Majesty's Stationery Office, London.
i6 Powle, Frederick E.
1933. Smithsonian physical tables. 8th revised ed. 682 p. (See specifically p.
378, 381.) Smithsonian Institution, Washington, D. C.
17 Hand, Irving F.
1934. The character and magnitude of the dense dust-cloud which passed over
Washington, D. C, May 11, 1934. U. S. Mo. Weather Eev. 62(5) :156.
is HOTTLE, HOYT C.
1934. Eadiant heat transmission. In: Chemical Engineers' Handbook. 2,609
p. (See specifically p. 883.) McGraw-Hill Book Co., New York.
19 International Critical Tables.
1929. Table 5. Albedo; white light. In: International Critical Tables, vol. 5,
470 p. (See specifically p. 262.) McGraw-Hill Book Co., New York.
20 Jones, H. Spencer.
1934. Aluminum surfaced mirrors. Nature 133(3363) :552. London.
21 Kalitin, N. N.
1930. The measurements of the albedo of a snow cover. U. S. Mo. Weather
Eev. 58:59-61.
22 Kimball, H. H.
1924. Eecords of total solar radiation intensity and their relation to daylight
intensity. U. S. Mo. Weather Eev. 52:474, 478. Figs. 1, 4.
23 Kimball, H. H.
1926. [Eeview of] A. Angstrom: The albedo of various surfaces of ground.
U. S. Mo. Weather Eev. 54:453.
- 4 Kimball, H. H.
1928. The distribution of energy in the visible spectrum of sunlight, sky-
light and total daylight. International Illuminating Congress [Saranae
Inn, New York] Paper No. 12" 4 :10.
20 Kimball, H. H.
1929. [Eeview of] G. C. Simpson: Distribution of terrestrial radiation. U. S.
Mo. Weather Eev. 57:340.
64 University of California — Experiment Station
26 Kimball, H. H.
1929. Solar observations. U. S. Mo. Weather Rev. 57:26-27.
27 King, W. J.
1932. The basic laws and data of heat transmission. Mech. Engin. [New York]
54(7) :496.
28 McAdams, Wm. H.
1933. Heat transmission. 383 p. (See specifically p. 45-49 and p. 210, equation
43.) McGraw-Hill Book Co., New York.
-'" Perry, John H.
1934. Flow in pipes, ducts and channels. In: Chemical Engineers' Handbook.
2,609 p. (See specifically p. 736-740.) McGraw-Hill Book Co., New
York.
30 Priest, Irwin G.
1935. The Priest-Lange reflectometer applied to nearly white porcelain enam-
els. U. S. Dept. Com., Bur. Standards Jour. Research 15:544.
31 Schack, Alfred.
1933. Industrial heat transfer. 371 p. (See specifically p. 354-6.) John Wiley
& Sons, Inc., New York.
32 Schmidt, E.
1927. Warmestrahlung technischer Oberflachen bei gewohnlicher Temperatur.
Beihefte zum Gesundheits-Ingenieur. Reihe 1, Heft 20:1-21. R. Olden-
bourg, Miinchen and Berlin.
33 Schmidt, E.
1934. Thermal radiation of water and ice and of frosted and wet surfaces.
Ref rig. Engin. 28 (3 ) : 152, 156.
34 United States Department of Agriculture, Weather Bureau.
1925-34. Climatological data. U. S. Mo. Weather Rev. 53-62.
3G United States Department of Agriculture, Weather Bureau.
1929-35. Solar observations. U. S. Weather Rev. 57-63.
36 United States Department of Agriculture, Weather Bureau.
1933. [Maps of frost dates and growing season.] U. S. Dept. Agr. Climatolog-
ical Data, California section 37:40, 48, 56, 64, 71.
37 Van Dusen, M. S.
1934. Sheet steel as insulation. Refrig. Engin. 28(3) :152.
15m 10, '36