Use of solar energy for cooling purposes

Use of solar energy for cooling purposes

Use of Solar Energy for Cooling Purposes* H. Tabor National Physical Laboratory of Israel, Jerusalem, Israel HE use of solar energy for cooling purpo...

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Use of Solar Energy for Cooling Purposes* H. Tabor National Physical Laboratory of Israel, Jerusalem, Israel

HE use of solar energy for cooling purposes-whether of food or of dwellings--has a two-fold attraction: a - - T h e demand for cooling is generally greatest at times of maximum solar intensity--in contrast to the use of solar energy for winter heating of dwellings. b - - T h e cooling of food and of man is more important in hot than in cold regions: refrigeration and air conbitioning that might otherwise be considered luxuries may, in fact, become necessities for the development of hot underdeveloped countries. It is thus not surprising that on the subject of solar cooling studies are being conducted in various research institutes. The use of conventional electrically operated compression cooling machines--with the electric power provided by a solar power plant--need not concern us here because this is entirely a problem of solar power. Also, for this discussion it should be remembered that any solar cooling system comprises two parts: the cooling machine or mechanism and the solar heat source to operate it. A number of heat-operated cooling and air-conditioning systems are already known such as absorption systems, dehumidification systems, jet ejectors and the like, which are conventionally operated by sources of heat such as gas or kerosene (occasionally electricity). It is hardly to be expected, therefore, that solar scientists will suddenly produce new devices of this kind that are vastly superior to those already produced in a competitive technological world. Rather, the problem is that, because solar heat has certain limitations compared with fuel--intcrmittency, low energy concentration, difficulty of control etc.--the solar operated cooling device may have to be specially designed to suit the characteristics of solar heat collectors. As an example of the difference between a solar cooling device and a fuel operated device we consider an absorption cooling unit. According to the system chosen and the type of cooling required (e.g. air conditioning or ice production) the energy input temperature may be 60, 90, or 120 degrees C. As far as fuel heating is concerned these various temperatures do not create any problems: but when solar heat is used there is a profound difference: while 60 degrees C ('.an

T

*. General. report on papers given in Session_, III D of the Umted Nations Conference on New Energy Sources, Rome, Italy, August, 1961.

136

readily be obtained from a flat-plate solar collector, the higher temperatures mean lower collector efficiencies or more sophisticated (and hence more expensive) collectors. If, therefore, we recognise that a solar cooling device can be considerably less efficient, but never more efficient, than a fuel operated device, we have a measure of the limits that can be approached but not exceeded in solar cooling devices. For the source of heat we require some form of solar collector and must borrow from the experience gained in other aspects of solar energy utilisation such as solar water heating and solar power. TECHNICAL

AND THEORETICAL ASPECTS

The heat source

Experience already gained in solar collectors shows that : l--The collection efficiency of solar collectors decreases with increase in temperature of the output. 2--Present-day flat-plate collectors are satisfactory for temperatures up to about 70-80 degrees C under favorabh' climatic conditions. For example, consider a typical fiat plate collector under bright sunshine conditions. Assumptions: Single water-white cover glass; selective absorber emissivity 0.12, absorptivity 0.92; 5 cm rear insulation; large area collector--no edge losses; 10 percent losses in extraction, shading, dirt etc.; mean solar intensity during sunshine hours 200 Btu per hr per square foot or 74 kcals per hr per square meter. Efficiency is for "boiling" i.e. where whole collector is approx, at the output temperature 1, 3. The daily collection efficiency E has been computed as follows: E = 0.71 - 0.003AT°F = 0.71 -- 0.0054AT°C where AT is the temperature rise above ambient. Thus for a temperature rise of 39 degrees C the efficiency is down to 50 percent: for a rise of 70 degrees C the efficiency is down to 33 percent. These results are for a good collector in clear strong sunlight: many practical collectors give much poorer results. 3--Focussing collectors that track the sun can give much higher temperatures. However it is extremely unlikely that a tracking collector would be acceptable for domestic needs, except for rather primitive applications and where the collection time is short. (Paper S/82 deals with a domestic solar refrigerator where regeneration is effected during a period of about two hours using a paraboloidal mirror). 4--Cylindrical parabolic mirrors can give temperatures between those obtained from fiat-plate collectors and those obtained by fully sun-tracking focussing collectors. If the Solar Energy

optical concentration is limited to about three, no daily adjustment is needed--for about seven or eight hours of collection-though seasonal adjustment of tilt is required 3 and collection efficiencies of the order of 40 percent can be obtained for temperature rises of 100-130 degrees C above ambient. A higher optical concentration, giving better efficiency at the same temperature (or permitting the omission of a glass cover on the receiver), is possible if one is prepared to make periodic adjustments of the mirror during the day. In paper S/109 such a system is used for ice making: the tilt of the mirror must be adjusted five to six times on the solstice days, less on other days and not at all on the equinox days. 5--The problem of elevated temperatures is less important in cooling machines than in solar power units (where the higher the temperature the better), but it is, nevertheless significant, particularly as the domestic user of a solar device will demand a simpler system than, say, the operator of a solar power unit supplying a village. 6--Solar collectors are not as cheap as one would like and much work still has to be done to decrease their first cost and increase their longevity.

The Cooling Device A solar cooling device will be similar to any other heat-operated cooling device except for the added limitations imposed b y the solar source of heat. H e a t operated coolers are of two classes: the absorption cycle and other cycles related to it and the jet compression cycle. The absorption cycle. This is a well known and technically successful system and is exemplified b y the units built by Electrolux, Servel (Arkla) and others. Large commercial units and small units giving subzero temperatures (i.e. for refrigeration) usually use water as the absorber and a m m o n i a as the refrigerant. For air conditioning i.e. above-zero temperature, the lithium bromide-water combination is popular. In all absorption cycles, heat at a temperature Tg, which is the generator temperature, is supplied while heat is extracted at the evaporator where "cold" is produced at a temperature T~. H e a t is rejected from the system to the surroundings (outside the space to be cooled) from a condenser at temperature T~ and from an absorber, at temperature Ta. T~ and Tc are about equal, (and will be referred to as T~) being a few degrees above ambient for air-cooled condensers and absorbers (and being a few degrees above local wetbulb temperature for water-cooled systems). Two i m p o r t a n t facts emerge from thermodynamic considerations of absorption cycle, which are quite independent of the system or materials used. 1--The amount of heat supplied at the hot end will always be more than the amount of "cold" produced: in an ideal system these quantities approach equality. As the coefficient of performance, COP, of a cooling machine is defined as the ratio of calories extracted at the cold end to the calories of heat supplied, the COP is always less than unity. 2--The temperature "lift" i.e. Tg - T~ is, in an ideal system, approximately equal to the temperature "depression" i.e. T~ - T~. In real systems the lift is always a little greater

Vol. 6, No. 4, 1962

than the depression. (Examples are given in paper S/37 where for T~ = 55 degrees F and T~ = 85 degrees F, i.e., depression of 30 degrees F, To = 118 degrees F i.e. the lift is 33 degrees: for Tc = 90 degrees F the depression is 35 degrees and the lift 45 degrees F: for Tc = 100 degrees F the depression is 40 degrees F and the lift is 55 degrees F). These facts apply to "single-stage" systems. Multiple stage systems can give different results: higher COP's can be obtained at the expense of greatly elevated generator temperatures. While some experiments are being pursued on such systems, no commercial units are presently built, using multi-stage systems because of the increased complexity. With the generator temperature at about 180 degrees C for a two stage system and a COP about twice the usual value, it is doubtful if it could be justified for solar operation unless solar calories were so much more expensive than fuel calories that heat economy might become the key factor in over-all cost. These two facts are vital in appreciating the problem of solar cooling. The first fact indicates the minimum quantity of heat to be supplied from the collector for a given refrigeration effect. While the ideal COP can approach unity, real values are about 0.7 for large industrial plants, falling to about 0.4 for small units

(S/82). The second fact fixes the operating temperature for the generator and hence of the collector. For example, normal air-conditioning practice calls for an evaporator temperature of 4 to 5 degrees C (40 degrees F) in order to be able to cool the air down with a small heat transfer surface on the evaporator. If the ambient temperature in the summer, is, say, 30 degrees C--and it could easily be more--thus making Tc about 35 degrees C, the depression is 35 - 5 = 30 degrees C: the lift will therefore be in the region of 40 to 45 degrees C making the generator temperature 75 to 80 degrees C: the collector will be a little hotter. This is rather high for a fiat-plate collector. This point has been appreciated in paper S/37 where the authors make the evaporator temperature 55 degrees F instead of 40 degrees F--thereby necessitating an increase in the size of the evaporator surface--but reducing the generator temperature: i.e. for Tc = 95 degrees F (35 degrees C) the depression is 40 degrees F and the lift 55 degrees F making Tg = 140 degrees F (50 degrees C) which is a reasonable temperature for a fiat-plate collector. As the lift is a little greater than the depression, a rise of 1 degree in Tc results in a rise of about 2½ degrees at the generator. The authors of paper S/27 have, of course, used water cooling to get the condenser temperature to 95 degrees F or even lower: were air-cooling used Tc would shoot up to well over i00 degrees 17 in the summer thereby requiring generator and collector temperatures considerably higher than 140 degrees F. Thus, by using water cooling, 137

which may not be too popular in some areas, and the highest possible evaporator temperature, the authors have been able to consider flat-plate collectors where otherwise these could not efficiently have been used. The relationship of temperature lift to temperature depression makes it virtually impossible to operate a refrigerating cycle (i.e. subzero temperatures) with a fiat-plate collector. Thus the papers dealing with solar refrigerators, (S/82, S/70, S/109), all indicate focussing collectors for obtaining subzero temperatures. A full discussion of all types of cooling considered is not possible here and reference should be made to the original papers. As is to be expected, every system has advantages and disadvantages. The lithium bromidewater system is probably the best for air conditioning as it is a well tried system. But it cannot provide subzero temperatures and is therefore inapplicable for ice making and food refrigeration. The water-ammonia system is established in this field and for large plants gives a good COP. However in commercial small units i.e. domestic refrigerators, the COP is much lower. (To avoid the use of mechanical pumps the PlattensMunther system used on small equipment incorporates an inert gas which inevitably reduces the efficiency.) Hence there is room for further research taking into account that in a solar unit it is the total capital cost (cooler plus collector) that must be minimised. Thus Prof. Daniels has started some studies on the combination of sodium thiocyanate as absorber and ammonia as the refrigerant instead of water and ammonia. The possible advantages are that lower pressures result-permitting a lighter structure--and, as the absorber does not vaporize, the rectification needed in waterammonia systems to prevent distilling over of water, is eliminated. The earliest absorption machines were intermittent in operation. The refrigerant was first boiled out of the generator and condensed. Then it was expanded in the evaporator and re-absorbed to return the generator to its original condition. Intermittent behavior was not viewed kindly by the user so continuous cycles were developed whereby regeneration goes on simultaneously with the other processes and there is no interruption of the cooling effect. Solar radiation is itself intermittent and it was thus natural to consider, once again, intermittent absorption cycles. These are well described in paper S/82 and the simplest refrigerator comprises two spherical containers at the ends of a connecting tube, and containing an absorber-refrigerant solution. For a period of two hours one sphere, containing all the solution, is heated in the focus of a solar mirror, the ammonia being distilled over into the other sphere. This sphere is then put in the cold box and the evaporation of the ammonia on its way back to being absorbed in the first 138

sphere (now at ambient temperatures) produces the desired cooling effect which can last most of the re~ maining 22 hours of the day. The COP is rather low-about 0.4--because there are no heat-recovery exchangers used. The unit is rather heavy to manipulate. With some improvement in performance and reduction in internal pressure and weight, such a system might be quite attractive for underdeveloped countries where the manual manipulation, including periodic adjustment of the solar mirror during regenerating, would be acceptable. The mirror, which has about 40 percent heating efficiency, can probably also be improved. The continuous cycles are, of course, more complicated and one problem is the circulation of the working fluids. When electricity is available small pumps are used, otherwise the system has to pump itself by some form of thermopump. This is not possible with all systems and constitutes a further limitation in completely solar-operated absorption machines. So far we have discussed only closed cycles i.e. where no component in the cycle, other than heat, is consumed and such cycles are essential for refrigeration. For air conditioning, when the "refrigerant" is water, open cycles are sometimes used i.e. water is added or subtracted from the atmosphere. Such systems are often far simpler than closed systems but the COP is low principally because the regeneration process involves heating a great mass of air as well as the absorber and the water vapour. One example in this class is paper S/88, which describes what superficially appears to be a simple system of cooling a dwelling. It is an open intermittent cycle using absorbent material in the walls of the dwelling, but the COP is 0.10 to 0.15 so that the heat supply required for operation is large. Jet Cooling. High-pressure steam is sometimes used for air-conditioning machines: by means of a jet pump a suction is caused in an evaporator and cooling occurs. In effect, this is a compressor-type cooling machine, the mechanical compressor being replaced by a jet pump. The wear and tear of mechanical systems is avoided. The mechanical efficiency of a jet pump increases with the pressure of the steam, i.e. with the temperature, but is far below that of conventional heat. engines and pumps, and the performance of jet cooling machines appears to be satisfactory only in large sizes. (Steam jet pmnps cannot be used to produce subzero temperatures so that a Freon jet pump would be necessary for refrigeration.) Jet pumps are used in powerstation equipment where the emergent steam is used in the process: the mechanical output is low but the thermal efficiency is virtually tO0 percent: only temperature is lost. Because of the rather high steam temperatures required for efficient operation, the system has not attracted much attention in solar energy research centrcs

Solar Energy

although Ward 4 believes that a reasonable system, with 180 degrees F at the collector, can be designed for an evaporator temperature of 60 degrees F with an over-all performance coefficient, including the collector, of 0.3. This is an interesting line of research and should be examined further.

Economic Aspects Conventional air conditioning is not cheap and there is no reason to expect that solar air conditioning would be vastly cheaper. Domestic refrigeration being a smaller item, the cost factor is less significant. Solar cooling is similar to other forms of heatoperated cooling except that the source of heat is from a solar collector while the cooling machine will, in general, be somewhat less efficient and perhaps more expensive than a fuel-operated machine. If, therefore, we assume that, as the result of further research, the solar-operated cooling machine can be made for the same cost and having the same COP as a conventional machine, the comparative economics of cooling resolves itself into the cost of solar calories as compared with calories from fuel. Let Q~ = total solar radiation falling on one square meter of collector, during the summer season. (Kcals per square meter, season) Ec = collector efficiency C = cost per square meter of collector US $. J~ = annual charges on capital (amortisation and interest). Then the annual cost P~ of 108 calories of solar heat is: CJo

P~=~X

10~($)

If we assume that fuel with a calorific value of H Kcals per ton, costing $ K per ton, is burned in a furnace with an efficiency E/, then the cost P : of 106 fuel calories is K P: = ~ X 10~ Solar heat is thus equal in cost, to fuel heat (assuming identical maintenance costs) when P : = P~ i.e. when the fuel cost is K = HECJ~/Q,

Inserting some representative values gives a picture of the chances of economic viability. Assume H = l07 kcals per ton. E~ = 0.5. This is on the high side. E: = 0.7. This is low for very large units but high for small ones. C = $20 per square meter. This is lower than any present-day collectors but is a fair target price. J~ = 0.103. Assumes 15 years life and 6 percent interest rate and sinking fund. (See S/54). Q, = l06 kcals per square meter, season. A sunny climate will give about twice this value for the whole year. Then K = $28.80 per ton. Vol. 6, No..~, 1962

This is about twice the cost of imported fuel oil on a seashore. However, many small consumers in isolated places pay up to three times this price for fuel oil or its calorific equivalent delivered. The sample representative values are optimistic. Flat-plate collectors cost about $60 per square meter according to Sheridan (S/39, type B) which might fall to about $46 if built as part of a roof. Ashar and Reti (S/37) estimate $24 per square meter for fiat collectors built in India. Focussing collectors are at present more expensive than fiat-plate collectors, but there are hopes that a price of $20 per square meter will be attained by the use of plastics. In this case a 15-year lifetime is unrealistic, six years probably being more reasonable and this makes ,L = 0.203 or almost twice the assumed value. The value for Q~ depends on the local climate and in particular on the length of the cooling season. For the case of the intermittent solar refrigerator using only two hours of sunshine a day Q, would be only 0.6 X 108 keals per square meter even for 365 days' use a year in a clear sky. The over-all picture is, however, encouraging. Wherever fuel is scarce or expensive the solar collector has a chance to compete in the supply of low-temperature calories. In highly developed areas with cheap fuels, solar calories are presently non-competitive.

CLIMATISATION OF DWELLINGS Circumstances may permit, climatisation in a cheaper and simpler manner than by heat-operated machines, whether fuel or solar. Further, a comprehensive use of all climatic factors is logical. Thus, for example, hot sunny desert climates often have cold nights and clear skies. The cold nights can he used to cool the building by storing night cold. The clear skies can be used to cool a building by outgoing radiation from roof and walls to the cold sky. Where the climate is hot and dry, evaporative cooling will in general reduce the temperature to a comfortable level and if the increased humidity causes discomfort the cool humidified air can be used to cool dry air through a heat-exchange surface. Of course evaporative cooling systems (tan t:e expensive in the use of water. Finally it is a problem for the architect to design the dwelling so as to obtain the best compromise indoor climate with the minimum of auxiliary aids. French scientists, thinking in terms of Saharan conditions, have been active in this field (S/76, S/64, S/111). Paper S/39, from Australia, pays much attention to the house design and considers a solar absorption machine for areas too moist to use evaporative cooling. Paper S / l l l gives an interesting account of how to exploit night radiation to cool a roof and cause cold air to descend into the dwelling, while "windows" can be •;sed either for heating purposes (by incident solar 139

radiation) or for cooling by radiation. The selectivity of special surfaces can also be exploited. Selective "hot" surfaces are those which absorb solar radiation but emit little heat, and selective "cold" surfaces those which being white, absorb little sunshine but may approximate to black-body thermal emitters. Paper S/76 deals with evaporative cooling in a Saharan climate but the cold humidifed air passes inside the hollow walls of the dwelling thereby cooling the interior of the dwelling without adding moisture. The authors correctly point the need to have low heat capacity walls in this special case in contrast to the more usual system, as given in S/39, of having a high wall heat capacity to even out day and night temperature variations. It is suggested that the movement of the air (,.an be caused by solar energy falling on the blackened walls and roof producing a chimney effect although it is admitted that if air filters are employed a mechanical fan is needed. (S/88 reports the solar chimney effect in another experimental house as being very small.) A full analysis may show that, where electricity is ~vailable, the solar chimney might be an expensive way of moving air compared with a small fan. Paper S/64 is a special case of climatisation, i.e. of a hot-house wherein water trays under the roof permit solar distillation and control of the micro-climate in the hot-house. This paper should prove of great, interest to farmers trying to raise special crops in a too-sunny climate. Economic analysis of these various systems is not presented and much work still has to be done in this field to determine technical practicability and economic justification. CONCLUSIONS Solar cooling for food and dwellings is attractive because of the extra value of cooling in sunny climates and the correlation between cooling load and solar intensity. Of the types of heat-operated cooling devices considered, the most promising are the closed-cycle absorption machines. These should follow conventional design but be modified to suit solar heat sources, in particular to have the lowest possible supply temperature. For air-conditioning the supply temperature can be sufficiently low (60 degrees C) to permit use of flat-plate collectors: refrigerating units for food preservation or ice-making require higher source temperatures, thus necessitating some form of focussing collector which, apart from the cost., involves more manipulation. Open-cycle absorption systems require more input calories and are thus not attractive although their first cost, apart from the collectors, may be low. Much work still has to be done to establish 140

the best absorption cycles for solar operation but considerable progress has been made. Jet-pump cooling is ~ possible alternative to the absorption machine and may be cheaper and simpler to construct although the heat consumption is likely to be large, thereby calling for large collectors, unless the jet pump is skillfully designed and the supply temperature can be kept up. Careful studies and experiments are required to determine whether this relatively untried system is a valid alternative to the absorption systems. Assuming that cooling machines adapted for operation by solar collectors will ultimately be about as efficient as similar machines operated by fuel, the solaroperated system cannot at present compete with fuel systems where fuel costs correspond to the normal world price for fuel, but may compete in remote areas where fuel costs are many times the world price. Under these circumstances solar air conditioning may not be far from practical realization. A promising development, particularly suitable for more primitive communities in hot areas, is the intermittent absorption cycle domestic refrigerator regenerated by a solar mirror. The improvements needed are a lower pressure system to permit a lighter construction (and reduce the danger of bursting by overheating); ~n improvement in the COP and a more efficient mirror for regeneration. (The mirror can be used for other purposes, such as solar cooking, when not being used for regeneration). Certain auxiliary cooling methods for dwellings should not be overlooked as an aid to solar cooling or even by themselves. These include evaporative cooling in hot dry climates (assuming adequate water supplies) and radiation cooling at night in clear-sky regions, while sound design principles in the dwelling construction itself are important.

(Following the plenary discussion of the papers in Session I I I D, Dr. Tabor prepared the following additional pertinent remarks as a supplement to the rapporteur report. Editor.) We should have no illusions. Solar energy will not solve all the problems, even not most of them, but only some of them, even with new breakthroughs. The fundamental limitations of the amount of sunshine in a given place and of thermodynamics are unaltered by any technical breakthrough: only the regions of viable applicability are increased. We must take every area separately: consider its problems and then see if solar energy can help. For example, one delegate spoke of the need of solar energy air conditioning in industries in hot climates to increase productivity. Another pointed out that in such cases there was competition from electricity, so solar Solar Energy

energy was not practical. Yet another said that there w a s electricity in the factory but not enough and the authorities would not agree to its use for such "luxuries" as air conditioning. Clearly these three delegates were referring to three different places. Thus it is extremely dangerous to generalize. We can divide the field into cooling and refrigeration for the preservation of food (this includes ice-making): and the cooling of dwellings (air conditioning).

Refrigeration Research shows that by using solar energy more progress was recorded in intermittent machines than in continuous machines. This is to be expected from the intermittent nature of sunshine: one matches the device to the solar cycle. There were reasonable prospects of producing a very simple household device for cooling a food container which required some manipulations (S/82). But more research was needed to reduce the internal pressures and thus reduce the weight and cost of the device. Another successful solar device manufactured ice at a price competitive with ice delivered from a distant central large ice-making plant. [This is an example of a general philosophical principle in solar energy utilization, that because solar energy is distributed free whereas other forms of energy involve distribution cost, solar energy should in general be harnessed at the point of use.] The important question arose as to what was more important: domestic units or village units. The general feeling was that, at least in the initial stages, we should concentrate on village or community units unless the dwellings arc so spread out that village units would not be practical. The reasons favoring community units are numerous and rather convincing, being technical, economic, social and anthropological: 1. A large unit is usually more efficient and hence relatively cheaper. 2. It is easier to maintain a technical device at the communal level. :3. A community will often have the money to buy a machine whereas the individual householders cannot. 4. Refrigeration, unlike cooking, is something quite new, and the introduction of, not an alternative device to that with which the people are accustomed, but a completely new device, will meet with resistance. There is, in many cases, a strong prejudice against eating any food which has been kept more than one day (and the hygenie reasons for this prejudice are obvious)--the food is said to be "dead". To persuade an individual that a strange new device will allow him to keep his food for long periods means a long tough educational process. Vol. 6, No. 4, 196~

A farmer or trader will learn much faster. One delegate pointed out that in his country there was an overproduction of food but that what was not sold on market day was thrown away: the wastage was so high that they actually had to import food. Another referred to price variations in a common commodity of 12:1 simply because there were no food preservation facilities. It was further pointed out that many new states have central governments with policies designed to strengthen local agriculture and these governments would support the purchase of refrigeration equipm e n t - - a n d particularly solar equipment if practical-for village, farm and market use. One delegate stated clearly that further development was needed: that this could not be done in the new countries and that if a manufacturer from an advanced country were able to supply the proper equipment, he would find a market.

Air Conditioning On the question of air conditioning (i.e., cooling of dwellings) it is axiomatic that air conditioning is never cheap and there is no reason to believe that solar air conditioning will be any cheaper. Thus there was almost unanimous agreement that we should not interest ourselves, at this stage, in air conditioning of dwellings in the underdeveloped countries. It was further pointed out that in many cases the climate in the areas under review was not as bad as generally believed (with of course many exceptions), and that what was needed was to pay more attention to the design of the dwelling so that no mechanical aids (or only the very simplest) would be needed. We can often learn a great deal from the building traditions of people who have lived in a region a long time. The addition of a little knowledge of the physics of heat transfer would be helpful. However, the cooling or air-conditioning of public buildings, schools, hospitals and factories is quite another matter. Of course in many such cases electricity or fuel for absorption machines would be available and competitive with solar cooling. There were in fact, no examples or papers in the Conference on solar cooling of such public dwellings but it seems that it would be practical (though not cheap) in areas where other fuels were expensive.

Summarizing There is a need for refrigeration for food preservation and the accent should be on village or communal units first. In air conditioning, this should be limited to public buildings and more attention paid to simple methods of improving individual dwellings by intelligent construction. 141

W h a t of t h e f u t u r e ? I a m q u i t e o p t i m i s t i c here, if m o r e research is done in the a d v a n c e d countries. A simple i n t e r m i t t e n t h o u s e h o l d u n i t should be quite feasible: larger i n t e r m i t t e n t u n i t s for c o m m u n a l llse seem quite p r a c t i c a l a n d are d e v e l o p e d sufficiently t o p e r m i t t h e i r being t a k e n up b y i n d u s t r i a l concerns. T h e a p p r o x i m a t e s y n c h r o n i z a t i o n of cooling d e m a n d a n d solar r a d i a t i o n s u p p l y is obvious. So I feel t h a t t h i s whole a r e a h a s a f u t u r e if we p a y p r o p e r care to e x a c t l y w h a t is r e q u i r e d a n d where.

1.

2. 3. 4.

REFERENCES Tabor, H. "Solar Energy Collector Design" Bull. Res. Council of Israel 5C No. l, pp. 5-27 (Nov. 1955) Reprinted in Transactions of International Conference on The Use of Solar Energy. Vol. II Part I Section A pp. 1-23. Tabor, H. Bull. Research Council of Israel, Vol. 6C, No. 3 Aug. 1958 pp. 155-175. "Radiation, Convection and Conduction Coefficients in Solar Collectors". Tabor, H. "Solar Energy" Vol. II, No. 3-4, July-Oct. 1958 "Stationary Mirror Systems for Solar Collectors". Ward, G. T. "Possibilities for the Utilisation of Solar Energy in Underdeveloped Rural Areas" F.A.O. Publica-

To

142

Our

Reviewers--A

tion Informal Working Bulletin No. 16 (Agricultural Engi-

neering) (1960).

P a p e r s C o n t r i b u t e d to t h e Session on Agenda Item III. D S/82. R. Chung and J. Duffle, "Cooling with solar energy" S/70. T. Oniga, "Unit~ frigorifique ~ absorption avec r4flecteur conoidal fixe" S/109. F. Trombe and M. Fo~x, "Bilan dconomique de la fabrication de glace avec un appareil ~ absorption utilisant le soleil comme moyen de ehauffage" S/37. N. Ashar and A. Reti, "Engineering and economic study of the use of solar energy especially for space cooling in India and Pakistan" S/39. N. Sheridan, "Prospects for solar air conditioning in Australia" S/64 and S/64 rev. F. Trombe and M. Fo~×, "Utilisation de l'6nergie solaire pour la rdalisation siinultan~e de la distillation de l'eau saum~tre et la climatisation des serres en zones arides" S/76. E. Crausse and H. Gachon, "Etude d'une maison solaire saharienne" S/88. S. Adler, G, Levite and H. Tabor, "The Altenkirch solarcooled house" S / l l l . F. Trombe and H. La Blanchetais, "Principes de climatisation des maisons dans les pays ~ ciel clair"

Word

of Appreciation

A vital b u t u n s e e n role i n a n y s u b s t a n t i a l p r o f e s s i o n a l j o u r n a l is p l a y e d b y t h e r e v i e w ers o f t h e t e c h n i c a l p a p e r s s u b m i t t e d for p u b l i c a t i o n . It is by t h e i r efforts t h a t t h e readers of the journal can have confidence in the soundness of the papers. The reviewers p r o v i d e t h e a n c h o r t o q u a l i t y t h a t is t h e most valuable asset of a technical journal. T h e a s s i g n m e n t g i v e n t h e r e v i e w e r is t i m e c o n s u m i n g , a n d h i s o n l y r e w a r d is t h a t h e has made a contribution to the advancement o f h i s p r o f e s s i o n . S e l d o m is a r e v i e w e r i d e n t i fied e v e n t o t h e a u t h o r o f t h e p a p e r h e

examines. More often than not his criticism is c o n s t r u c t i v e a n d r e s u l t s in a n i m p r o v e ment of the paper. Occasionally, on his r e c o m m e n d a t i o n , a p a p e r is r e j e c t e d . All p a p e r s s u b m i t t e d for p u b l i c a t i o n in SOLAR E N E R G Y s i n c e t h e b e g i n n i n g o f t h i s y e a r h a v e b e e n r e v i e w e d by a t l e a s t o n e person particularly qualified to examine the technical content of that paper. Those who h a v e r e s p o n d e d t h u s far t h i s year t o o u r r e q u e s t for t h i s i m p o r t a n t s e r v i c e - - s o m e o f them more than once--are named below To them our thanks. The Editor.

WM. M. CONN, C o n s u l t a n t A. J. DRUMMOND, E p p l e y L a b o r a t o r i e s , Inc. J. A. DUFFIE, S o l a r E n e r g y L a b o r a t o r y , U n i v e r s i t y of W i s c o n s i n COL DUWEZ, C a l i f o r n i a I n s t i t u t e of T e c h n o l o g y FRANK E. EDLIN, E. I. d u P o n t de N e m o u r s & Co. ERICH A. FARBER, U n i v e r s i t y of F l o r i d a PETER GLASER, A r t h u r D. L i t t l e , Inc. R. E. HENDERSON, G e n e r a l M o t o r s Corp. HAROLD HEYWOOD, T h e W o o l r i c h P o l y t e c h n i c HOYT C. HOTTEL, M a s s a c h u s e t t s I n s t i t u t e of T e c h nology DAVID S. JENKINS, C o n s u l t a n t RICHARD C. JORDAN, U n i v e r s i t y of M i n n e s o t a

PAUL D. JOSE M. L. KASTENS, U n i o n C a r b i d e I n t e r n a t i o n a l c. F. KETTLEBOROUGH, U n i v e r s i t y of A u c k l a n d ~v. ~v. KELLOGG, T h e R A N D C o r p o r a t i o n GEORGE O. G. LOF, C o n s u l t a n t RONALD LYON, S t a n f o r d R e s e a r c h I n s t i t u t e T. H. MACDONALD, U. S. W e a t h e r B u r e a u RUDOLPH MARCUS, S t a n f o r d R e s e a r c h I n s t i t u t e MORTON B. PRINCE, E l e c t r o - O p t i c a l S y s t e m s EDWARD SPEYER, A m e r i c a n M a c h i n e a n d F o u n d r y HARRY TABOR, N a t i o n a l P h y s i c a l L a b o r a t o r y of Israel MARIA TELKES, C o n s u l t a n t JOHN YELLOTT, J o h n Y e l l o t t E n g i n e e r i n g Associates

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