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. *5 I I 1 1 /5k Hi 1 \ 1 1 / \ i 1 \ \ *L w \ I 1 r- Vs ii \ 1 • . . ..< : V A^ \ i \ \ 1 i HI \ ^ s i , / H s\ \ \ \ ' I V IV \ . v k. ' \ \ \ \ \ \ I A Y \ \, N t lf\\ V s \ V N t i f \ i ■ | s ^ •> 1 I b * * J: N, w I A s ^J ^ > \ ^ § \f fi \ — >5 i 1 tY* ibil ty ; 1/ ' t* V vi ^SKyl I I ^^ ^ ^ sil m~ L A v. I I 300 400 500 & >» >» «. Si .^ ^ ^ S^Vfc S» § l 750 1000 1250 1500 1750 2000 2250 Wa/e Length j MM 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] Solar Energy for Heating Water H PQ < CO pa DQ M o ^> S o 6 ss 02 t) 03 CD r/l © H «H M ?-t QQ 3 2; CO ri 2 P) M O a? 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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 o &H a S3 •-( ft fc w K W Cm ft w ft W O S! as & GO « 33 Q s ?a 03 QQ ft H < O 1 o w 2 *« Ed W a w w 33 W &, &< Eh < Eh w o o Eh O w K Eh Ratio of total radiation absorbed by glass- covered black surface to depleted direct noon radiation^ CO CM OOO 05 05 05 00 o COtNN OOOON — UJ05 t—CD IO »Qt)< CO CON o CM — »- d Additional diffuse radiation received by sloped absorber, per cent OOOO CO oo OOOO OO NNN 1- t~ ~ CO w* Proportion of depleted direct noon radiation absorbed by glass- covered black surface, J degrees i«-«l< CM O oo oo 00 oo ooi»* OOCM CO tfilO WN OO -if CO CM O *< CO CM CM O CO CM -l •^ o O Ratio of direct radiation absorbed by black surface to incident direct radiation ( 7 ) «o CO CO CO CO 05 05 05 05 COlOKS 05 05 05 "5 -fi -*i 05 05 05 CO"-- OS 05 05 00 CO CO oo OOOO N 05NO CO -> s o o 03 -u C CD A CO A 7& bfi C A A O CD 3 -d CD 03 ai - CD a a 3 "3 V 91 2 a co o3 A • . A 5 oi< o a C3 CD J3 +s o CB a °35 o O s a a a 1 a a « 3 -3 a 2 3 _ and f col f col «« 3 es O O O "7S rmul hose hose O 01 o In o -*-' +-* -t-> cc ^ ^ >» >5 O o "° -^ XI J • ^ T3 T3 Tl *» "5 .2 .* ,o 8 3 fl N. i-l 3 a & § | a a >,|| 3 3 n O O "" « o o o o o "S .9 .2 .2 .2 « n ffl » g S 3 S M M OD O P>h Pm P=< fe c3 * * -»— ++ •- s * * * 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 /40 /30 ^ Z^O \ \ ! ! | | so 70 o > \ • • o ' , • a o i 1 • CO O 1 « cN o o q No No 4-ooJ o o O < ' -//or/. • 1 • • !** CD • < .^O CO p_o_\ o / £ ° l S/Opea £(7/ cZay /e/n/> f de/77/>. ZOO'/ \ i • • o > o o o o V. ■+- + -H ■-+-+- +--K.. • • 4 • • > 1 ON I o Sf o a SZopeo' £a/?Zc, ^ i"" + *-t» cZay Se/7?/?. 7S'/~, s//y/$/ tv/ 1 • • • CO . i K o + s. • o > o o 1 • > o { ° 1 • • • »• 1 o so 1 • ! • o O • • • • < • • 1 • /O /S ZO 25 (?t/0/7///j/ o/ iva/er c/rc7^/7 J (raZZo/ps SO SS - /07 7" *o— t) O-o. \ 1 1 r ~ s T r • i \ A r / e a/'r te/7?p 94°, c \ \ \j \ \ \ \ > X s #e/>fc ce/776 '/?/! tv ■atA f. i ~* ^ t* nfi tvacer ac o-^u /it W- ■-\--\-\ " /O /S 20 ZS SO (?c/o/7t/£y of water c/raiv/?, aa//o/?s JS 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. /40 \ /30 | r I Q //O \ t SO 70 0*^^ <*< y ^ — ^Sf ^ ft A/'r , fe/r?/. t a<* 5^7/ JZ'f /n/rt//r>(/m /y// V/7 t <9 te^M. 4 t\. X.J i 1 Xv S. 4 i ■ J 'rtco/ VMO wa £er £e/v, o. 6. 9'f /o 20 /OO 30 40 SO 60 70 SO 90 Qc/a/7£/£i/ of woter a'ran'/J /ro/r> 3, SO-ya//o/7 So/? As, jn?//o/?s //O /?o 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 /■/O i t I* ! f I i ! i /JO /eo //O /oo 90 so 7S ■ 1 **t r.r n °S/S 1 4 i W//e/ 1 / //raw/7 & '/ 8 oo /i.m. oe/>/. /a 'OH//? r> a :/t a at SOC AM r.Se, / ^ S o 4 S o 6 O ? o <9 O 9 O /c ■>o // 'O /£ (?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> /> £/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&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 / /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' *** '*>* //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