SMITHSONIAN MISCELLANEOUS COLLECTIONSVOLUME 98, NUMBER 5
UTILIZING HEAT FROM THE SUN(With Four Plates)
BYC. G. ABBOTSecretary, Smithsonian Institutionf^%m\M wsriri
(Publication 3530)
CITY OF WASHINGTONPUBLISHED BY THE SMITHSONIAN INSTITUTIONMARCH 30, 1939

SMITHSONIAN MISCELLANEOUS COLLECTIONSVOLUME 98, NUMBER 5
UTILIZING HEAT FROM THE SUN(With Four Plates)
BYC. G. ABBOTSecretary, Smithsonian Institution
(Publication 3530)
CITY OF WASHINGTONPUBLISHED BY THE SMITHSONIAN INSTITUTIONMARCH 30, 1939
ZU £orb QBaltimote tyveeeBALTIMORE, MD., U. S. A.
UTILIZING HEAT FROM THE SUNBy C. G. ABBOT,Secretary, Smithsonian Institution(With Four Plates)In large regions lying in the low latitudes desert conditions prevail.Day after day the sun glares down, occasionally dimmed a little bycirrus clouds, or perhaps by a few heaping cumuli. In such regions80 percent or even more of the hours of daylight would be usefulfor furnishing solar heating.QUANTITY OF SOLAR ENERGYThe quantity of energy available from solar radiation under suchconditions as I have pictured is immense. As I shall show in whatfollows, we may count on the possibility of converting 15 percent ofthe energy of such solar rays as are intercepted by our devices intomechanical work. Assuming that to avoid appreciable losses throughshading one unit by another, and to allow plenty of room for otherpurposes, only one-tenth of the area available is actually covered byheat collectors, and further allowing for night and cloudy weather,still the State of New Mexico could supply from solar radiation overten trillion horsepower-hours per year of mechanical power, whichcompares with the power possibilities of all coal, oil, and water atpresent used annually for heat, light, and power combined in theUnited States.INTERMITTENCE AND STORAGE OF SOLAR POWERLike hydroelectric power, solar power demands no continuingexpense other than for care and interest on the investment. Unfor-tunately, however, solar power is subject to the drawback that itceases during night hours, and when the beam is intercepted by clouds.There are certain uses, such as pumping water for irrigation forinstance, where this intermittence is no serious objection. But formost purposes power must be available at all times. Hence to becomea great industrial factor solar power demands the association ofstorage of energy, either as heat, or in chemical, electrical, orSmithsonian Miscellaneous Collections, Vol. 98. No. 5.
2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98
mechanical forms. As efficient production of solar power must never-theless be the first step, I shall postpone considerations of storage forthe present.WATER DISTILLING AND COOKING BY SOLAR HEATINGThere are other uses for solar heating which do not so much involvestorage. Among them are the distillation of water and the cookingof food. It is stated that in Bermuda the past year was so deficientin rainfall that fresh water was imported from New York, althoughthe Atlantic ocean is all around. In some of our western States thewater in many parts is too alkaline for drinking or for storagebatteries. In Florida the water in some localities stinks with hydrogensulphide. Under such conditions as these the solar distilling deviceshould be useful. As for cooking, though not practical in cloudyregions, it is easy to provide heat storage for 24 hours, and with asolar cooking device the ovens may be kept at baking temperaturesfor weeks and months continuously in the more cloudless parts ofour country.RECENT COMMERCIAL PRODUCTS A BOON TO SOLAR DEVICESThe cheap production of efficient solar heat devices has awaitedthe commercial development of aluminum products, now so plenti-fully used in the industries, and the common use of vacuum devices,which came with the incandescent lamp and radio industries. For-merly, inventors relied on glass mirrors which were heavy, costly,and not durable. It is now possible to purchase the so-called "Alcoa"in thin sheets, which readily take the curvature of a suitable cradleform without previous shaping. This material reflects over 80 percentof solar radiation, and may be used for years without dimming. Asthe loss of heat in the boiler at the focus of a solar-radiation applianceis the great obstacle to be provided against, the possibility of makingcheap glass jackets enclosing high vacua like thermos bottles is theother great improvement which has become practicable in recent times.MECHANISM FOR FOLLOWING THE SUNThe daily march of the sun through the sky, and its yearly marchfrom north to south, must be considered. If one uses a sphericalboiler and a circular mirror, he must allow for both of these apparentmotions as Eneas did at the ostrich farm, Pasadena, about 1905. If,however, one uses a tubular boiler parallel to the axis of the earth,
NO. 5 UTILIZING HEAT FROM THE SUN ABBOT 3the yearly adjustment is unnecessary and the daily motion can beallowed for by merely rotating the parabolic cylindric ray-concen-trating mirror about an axis coincident with that of the boiler tubeat the rate of 15 ° per hour. In this arrangement the boiler tube maybe firmly fixed. This, with the simplicity of the mechanical drivingof the mirror, renders this arrangement preferable to all others formost purposes. The mirror may be driven by clockwork, or, ifelectric facilities are available, still better by a 60-cycle synchronousmotor through a worm and wheel.I have used both types of driving. Our solar cooker on MountWilson, having been built long ago, is cumbersome. It has a heavymirror, 8 feet wide by 12 feet long. I attached to the mirror a groovedwheel 30 inches in diameter coaxial with the lower trunnion of themirror. A steel wire in the groove of the wheel supported a weightof about 200 pounds, sufficient to rotate the mirror toward the west.Through a second steel wire wound in the groove in the oppositesense, the weight also drove a clockwork. This clockwork trainended in a flyvane. A long hand rotated with the central shaft andonce in each revolution was stopped by a displaceable pin. The longhand would make a full rotation in about 3 minutes. A commonalarm clock was provided with a wheel of 12 pins on the back ofits hour shaft, and these pins, acting through a lever escapementdisplaced the stop-pin once each 5 minutes. Hence the mirror movedintermittently as governed by the alarm clock, and was never morethan 1 minute from its proper position to focus sun rays upon theheater tube. Still simpler clockwork contrivances may be used todrive smaller mirrors for solar heating devices.DOMESTIC WATER HEATERSThose who have visited Florida or southern California may knowof the roof water heaters which are used considerably for providinghot water for bath and other household purposes. A shallow depres-sion is let into the south roof exposure, and lined with blackenedsheet metal. Therein is supported a blackened grid of pipes like asteam radiator. The boxlike depression is covered tightly with glasswindows. Water circulates through the piping, and thence to areservoir at a higher level within the house. Such a system acts bygravity like the water heater system of a cook stove. If the reservoiris well insulated from heat losses and the location is relatively cloud-less and never freezing, such a system is found to be very usefulfor furnishing hot water both day and night, without maintenance
4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98
cost or attention. While on Mount Wilson, several years ago, Ibought 200 feet of black garden hose. I coiled 150 feet of it in aflat coil upon a wooden X, and carried it up the ladder to the southside of the cottage roof. The other 50 feet I connected to the waterhydrant in the yard and to a spigot in the bathtub. By this simplearrangement we could draw 5 gallons of very hot water each halfhour on every sunny day. SOLAR COOKERSWhen we attempt cooking by sun heating we require temperaturesfar above the boiling point of water. Hence some other liquid of amuch higher boiling point is desirable as a heat conveyor, otherwisehigh pressures and evaporation would be met with. In our cooker onMount Wilson I used engine cylinder oil within a blackened metaltube in the focus of the mirror. About 60 gallons of this oil wereemployed in the system, so that there was a large capacity for heat,and cooking could be done by night as well as by day. However, itrequired about 2 days of sun to get the system heated initially, forowing to nearby trees there were only 7 hours per clay of sunshine.In recent installations I have preferred to use "Arochlor," a nearlyblack liquid product of the Monsanto Chemical Company. I havemade this liquid almost completely absorptive of sun-rays by addinga small amount of lampblack in suspension therein. While enginecylinder oil chars somewhat, and evaporates considerably at 210° C,
"Arochlor" does not boil below 350 C, and evaporates scarcely anyat lower temperatures. This liquid, being highly absorptive, may beused directly in the vacuum-jacketed glass focus tube. Circulationmay be provided by bringing back from the oven sheath a small metalor glass tube within the focus tube to near its lower end. Such afocus tube passes freely through the hollow trunnion at the upperend of the mirror, and is sealed by a well-designed stuffing box tothe metal sheath which encloses the oven. According as one wishesfor a quickly heating oven, or on the other hand for one to remainhot through temporary cloudiness and the night hours, the ovensheath contains little or much of the liquid. This part of the systemmay be surrounded by a thick layer of glass wool for insulation,leaving, of course, means for reaching the oven door.In another embodiment of the cooking device, I have sealed theglass vacuum-jacketed focus tube to a vertical cylindrical glass jarto contain the liquid. Within the liquid is an inner glass jar used asthe oven. The oven is approached from above with food to be cooked.


NO. 5 UTILIZING HEAT FROM THE SUN ABBOT 5In this embodiment the outer of these two glass cylinders may beitself surrounded by evacuated space. This makes a very beautifuland highly efficient, quickly heated oven of small capacity. For alarge installation it is better not to use the liquid directly as theabsorbing medium, but to contain it in a blackened copper tube, itselfsurrounded by a vacuum jacket of glass. This arrangement lendsitself to a more robust connection of the heater tube to the ovenjacket. Liquid may then be supplied to give a large capacity for heatand to heat a plurality of ovens.To fix approximately our ideas of the size of an outfit for solarcooking, I give the following figures. In clear sky conditions onemay depend on from 1.2 to 1.4 calories per square centimeter perminute of energy in the solar beam. Using the lower of these figureswe have still to encounter the following losses. Mirror reflection82 percent, vacuum jacket transmission, if direct to liquid, about89 percent, if through a blackened metal tube to liquid about 80 per-cent. Hence there remains about 0.79 to 0.87 calory per squarecentimeter per minute. The maximum temperature which a mirrorwill maintain in an oven depends on the rate of loss of heat. Thetime required to approach that temperature depends on the capacityof the oven and its surroundings for heat. These variables I cannot,of course, predict without specifications. But it may safely be saidthat, with good design, a mirror of 4x8 feet surface will keep twoovens of ordinary size hot enough to bake biscuits well, by night aswell as by day, in any fairly cloudless regions in the temperate zones.TOY SOLAR COOKER
I have constructed a toy cooker with a mirror surface of 15x20inches to warm an oven 3^ inches square, 2^ inches high, and insulatedby 3 inches in thickness of glass wool. It requires about an hour toheat the oven to about 130 C. above surrounding temperatures, andthe oven bakes cakes 3 inches square very nicely in a half hour.SOLAR WATER DISTILLINGDistillation of water may be very efficiently done with solar heating.The arrangement of the mirror is similar to that just described forcooking purposes. In this case, however, the elongated vacuumjacket, like a thermos tube except that it is not silvered within, issupported in the focus of the mirror with its open end at the bottom,and its closed end extending a foot or more above the top of themirror, which rotates on rollers bearing the hollow trunnions of the
6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98
mirror. In the case of the cooker, and also of the power flash boiler,soon to be described, the absorber of rays is made as small in diameteras possible in order to reduce heat losses, so that the temperaturemay run high. In the solar water distiller, however, the temperaturecannot exceed the boiling point of water. With a vacuum jacketsurrounding the focus tube, heat losses at that temperature are smallper unit area. Hence the focus tube is made much larger in diameterin (jrder to provide freer escape for steam. This requires a largervacuum jacket than in the devices for cooking and for power.I pour the water to be distilled into a vessel supported behind themirror and nearly at the level of the upper end of the mirror. A longsnout runs from the bottom of the water vessel down behind andparallel to the mirror, and, bending at right angles, comes up to jointhe focus tube of copper, which is blackened outside to absorb solarrays. Thus the water flows by gravity from the vessel to an equalheight within the focus tube. Within this snout and focus tube is asmaller tube for steam. It extends from above the level of the waterin the vessel to above the level of the mirror in the focus tube. It isopen to the atmosphere above the vessel, and open to steam abovethe water in the focus tube. A branch leaves the steam tube at itslowest point, and passes sealed through the wall of the snout, so thatdistilled water may drop from the steam tube into a receptacleunderneath.Only one difficulty is met with in this device. The steam must becaused to escape by such a protected orifice that the surging, boilingwater within the focus tube does not ever reach that orifice to minglewith the condensed steam. This is accomplished by a series ofumbrella diaphragms along the upper part of the steam tube, and byusing a diminished orifice, well shielded by a cap.The efficiency of the device is very high. The steam being con-densed by flowing through the entering water, that water reaches thelower end of the boiler tube at almost boiling temperature. Thus itis only the latent heat of steam that must be provided by solarradiation, and not the heat required to raise water to boiling. Inexperiments made in Florida in March 1938, the stinking water ofArcadia was distilled to perfect purity and odorlessness. Distillationcommenced within 5 minutes after the sun came out of a cloud. Amirror of 1 1 square feet of surface distilled between 2 and 3 gallonsof water, entirely automatically in one cloudless day.
NO. 5 UTILIZING HEAT FROM THE SUN ABBOTTHE SOLAR FLASH BOILER FOR POWERSince cumulus clouds are apt to obscure the sun occasionally inregions suited to solar power production, the flash boiler, rather thanthe boiler of large heat capacity, is indicated. For if it takes an houror more to raise the desired steam pressure, many days will be wastedwhen the sky about the sun is clear one half of the time. Accordinglymy efforts in recent months have been directed toward the develop-
Fig. i.—Diagram of solar flash boiler showing water injection governeddirectly by steam pressure. In a later model the water injection is governed bythe temperature expansion of the boiler tube.
ment of the automatic flash-boiler solar engine. That is, an engine ofa single tube boiler protected from heat losses by an elongated glassvacuum jacket, and fed by a current of water automatically graduatedin flow by. the temperature of the boiler. The device is intended toraise full steam pressure within 5 minutes after solar exposure.Should the sun enter a cloud, the water supply is immediately cut off.Should the steam pressure rise above the desired maximum, the watersupply is increased. Thus the boiler is fully automatic, and it takesadvantage of all the clear sky which comes between clouds.
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98FLASH-BOILER DEVICES
It would be convenient if it were practicable to have the glassvacuum jacket open at both ends so that water could flow in at thebottom and go out as steam at the top. But the unequal linear expan-sion of the inner and outer tubes of the glass jacket is difficult toallow for in a permanent sealage. Accordingly, I have preferred tomake the vacuum jacket, surrounding the boiler tube, like an elongatedthermos bottle with open end up. This requires the water tube enter-ing at the top to pass through the steam to the lower end of the boiler.I introduce two metallic tubes sealed upon the water tube within theboiler, called, respectively, the spreader tube and the vacuum jackettube. The spreader tube encloses the water tube in the lower two-thirds of the length of the boiler tube, and forces the water to circu-late in a thin layer against the inner wall of the boiler tube, so as tobe most favorably situated to burst into steam. The vacuum jackettube is sealed upon the water tube in the upper one-third of the lengthof the boiler tube, so as to reduce the tendency of the entering waterto cool the superheated steam in the upper part of the boiler tube.AUTOMATIC REGULATION OF FLOW OF BOILER WATER
I accomplish automatic regulation of the water supply as follows
:
A pump is provided whose stroke is continuously adjustable betweenthe limits zero and the greatest required. The essence of this regula-tion consists in an eccentric pin forming part of a shaft driven bythe same small synchronous motor that rotates the mirror. One endof the pin is coaxial with the shaft bearings, but the other end revolvesin a small orbit. The shaft carrying the pin is mounted in a carriage,displaceable longitudinally, so that according to its longitudinal posi-tion the pin gives more or less throw to the pitman that works thepump.To govern the position of the carriage I impart motion by a screw,driven by a tiny direct-current motor operated by dry cells. Theoperation of this motor forward or backward is governed by a suitablemultiple contact switch. The switch is operated by a lever systemworked by the differential expansion between the boiler tube and aninvar tape attached to the lower end of the boiler tube. Hence thetemperature of the boiler, which is the index of the prevailing steampressure, governs the position of the carriage. There is mountedupon this carriage the uniformly rotating eccentric pin, and this inturn governs the stroke of the pump which forces water into the boiler.
SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98, NO. 5, PL. 3
1. SOLAR WATER DISTILLER AS USED IN FLORIDA 1938
2. Solar Boiler for y2 Horsepower Engine as exhibited toInternational power Congress at Washington 1936
SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98, NO. 5, PL. 4
1. Solar Flash Boiler ys Horsepower capacity as Used inFlorida 1938
2. Driving Mechanism for the Mirror and Water Injector forthe Solar Flash Boiler
NO. 5 UTILIZING HEAT FROM THE SUN ABBOT 9EFFICIENCY OF SOLAR FLASH BOILERRegarding the efficiency of conversion of solar energy into mechani-cal power by the flash boiler, the following computations are pertinent
:
Efficiency of Solar Flash BoilerA. Efficiency of the boiler, assumed temperature ioo° C.
:
Mirror reflection 82 percentTransmission by vacuum jacket 85Absorption by boiler tube 95Loss of heat through the jacket 10Boiler efficiency 0.82 X 0.85 X 0.95 X 0.90 = 60B. Thermodynamic factor for perfect engine
:
Assumed temperature of condenser 30° C.Efficiency factor- I9°
,
—^-^- 34-5 percent190 +273C. Mechanical efficiency of engine is assumed to be 75 percent.D. Final result. Efficiency of conversion of solar to mechanicalenergy
:
Factor = 0.60 X 0.345 X 0.75 = 15.5 percentCOMPETITION OF SOLAR POWER AGAINST COAL, OIL, AND WATER POWERIn the experiments of most earlier inventors, the protection of theboiler tube by a vacuum jacket was not practicable, the cheap butaccurate construction of the mirror to give high reflection with per-manency was not feasible, and the simplest arrangement to followthe sun was not generally made use of. Consequently, the cost wasup and the efficiency was down. Hence these earlier devices werequite unable to compete with power from coal or water under mostconditions. With the high efficiency and great simplicity of the presentflash boiler scheme, I compute that power can be had from the sunat not exceeding 0.5 cent per horsepower-hour, and still give a goodreturn on the investment.WILLSIE AND BOYLE EXPERIMENTS
I wish, however, to refer to one of the earlier inventions in whichefficiency was sacrificed for cheapness. It was that of Willsie andBoyle who installed a solar power plant at Needles, Ariz., about 1910.Their scheme comprised a large, shallow black-bottomed pond whereinthe water attained temperatures considerably below the boiling point.This heat they used to drive a sulphur dioxide engine, cooled by theevaporation of water. They claimed that their device was able tocompete with coal in that locality, although both its boiler efficiencyand its thermodynamic efficiency were low. It appears not to havecome into much commercial use, however.
IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98STORAGE OF SOLAR HEAT OR POWER
I will now consider briefly some suggestions relating to the storageof heat or of power from the sun. As everyone knows, heat is proneto dissipate itself. There are no insulators against heat conductioncomparable in efficiency to those which prevent the flow of electricity.My friend Dr. Cottrell, however, proposed to me a scheme which maybe worth a trial. He suggests a silo-shaped, cement-lined pit in theground, filled nearly to the top with dry coarse sand, and roofed over.Above the sand lies a layer of perhaps 10 feet of glass wool, such asis used for roof insulation. A pipe leading from the solar heater tothe center of the upper surface of the sand has an appropriate net-work of branch pipes covering the surface. A similar network at thebottom of the pile leads to an outlet pipe, and thence back to theheater. An automatic pump which runs only while the focus tube is hot,draws hot air through the solar heater into the top of the sand. Owingto the notoriously bad conductivity of dry sand, and the high degreeof protection from upward convection and conduction offered by thethick layer of glass wool, the sand pile receives the heat, and keepsit in a horizontal layer. The heated layer gradually works down, till,if the storage operation is very long-continued, the whole sand pilebecomes of nearly as high temperature as the air in the focus tubeitself. With a sand silo of sufficient capacity, Dr. Cottrell thinks theefficiency would be so high that when the heat was drawn away,perhaps months later, by reversing the circulation of air, the airwould come away from the top of the sand very nearly as hot as itformerly entered. No one has tried this interesting scheme, but itwould be desirable to do so. Should it succeed, it might show theway to use the heat of summer to warm one's house in winter.Electric storage batteries are so well known that it is unnecessaryto point out that solar power may be conserved thereby for night use.It is the cost which shades this proposal.Chemical storage might be clone by electrolyzing water, and savingthe hydrogen to be burned in air with boilers to generate steam. Thisinvolves the problem of successful use of hydrogen as a steaming fuel.Mechanical storage could be accomplished by pumping water to ahigh level reservoir, to be used in a hydroelectric plant later. Thisalso looks costly, and difficult except in hilly country.Possibly best of all would be a heat storage within a pressure tankfilled with water, and surrounded by a thick envelope of glass wool.The water, heated far above the boiling point, would supply steamfor hours of cloudiness or nigrit.
NO. 5 UTILIZING HEAT FROM THE SUN ABBOT IICOMMERCIAL USE AND COST OF SOLAR HEATING
It is probable that so long as coal is cheap and abundant there willbe no extensive use of solar power. However, small installations, in2- to 5-horsepower units, may become profitable under favorableconditions. Solar heat has already been used successfully for refri-geration, and possibly might be combined with a heating system forconditioning the air in ranch propositions in cloudless regions. Theclassic use of solar power is, of course, for irrigation, and here, asremarked above, the problem of storage is not important. It is con-ceivable that great reservoirs might be pumped full of water from
.rivers or lakes by solar power in dry years to irrigate land whenrains fail.As remarked above, both solar cooking and solar distilling of non-potable water are practical and efficient propositions, which it is likelywill be in common use before very long if the necessary outfits canbe produced at attractive prices. The cost of solar devices, as of allother products, depends greatly on the volume of sales. These devices,however, as compared with automobiles are extremely simple. Thoughit might cost prohibitively to produce them singly, I think not ifproduced in thousands.