6 OG Lorand R A. TYBOUT are competitive with oil in Miami In most loca tions, solar heat can be delivered for combined use at lower cost than for use during only one part of the year in Santa Maria, however, solar energy for heating can be provided at considerably lower unit cost than for combined use because the cooling demand is so small that amortization of the $1000 surcharge results in a cost increase. It should be realized that complete design optimization requires minimizing annual cost of delivered solar energy plus auxiliary, not solar alone. Thus, if auxiliary energy is more expensive than solar, least annual total costs will be realized with a design providing more solar at high cost, than does the design for cheapest solar energy only. Specifically. total annual costs will be minimized by use of the design for which the last increment of solar heat is provided at a unit cost equal to the unit cost of auxiliary heat. The greater the cost of auxiliary energy. the larger will be the fraction of solar energy use in the optimized system. These evaluations thus require consideration of conventional energy costs in each location examined. The significance of these results with respect to the prospects for use of solar heating and cooling in the United States is heightened by the impending shortages and cost increases of oil and gas. A doubling of natural gas prices within ten years has been frequently forecasted. A change of this magnitude. possibly accompanied by economies in manufacture of solar energy systems, will make solar heating and cooling competitive with fuel in almost all U.S. locations. Acknowledgement--The authors express their apprecia tion to Resources for the Future. Inc.. Washington, D.C. and the Ohio State University for support of the research on which this paper is based. REFERENCES 1. R. A. Tybout and G. O. G. Löf. Solar house heating. Natural Res. J. 10(2), 268–326 (1970). 2. G. O. G. Lof and R. A. Tyhout. Cost of house heating with solar energy. Solar Energy 14(3), 253–278 (1973). 3. G. O. G. Lof and R. A. Tybout. A model for optimizing solar heating design. Paper presented November 1972. at Meeting of American Society of Mechanical Engineers, proposed for publication in Engng J. Power. 4. 1. J. Wagner. Gas versus oil for space heating. Gus Age p. 22 (January, 1966). Résumé Une analyse étendue de chauffage des habitations en utilizant l'énergie solaire dans une Le modèle mathématique de ces systemes a été modifié afin d'inclure les unités réfrigérant du type Ou a trouvé pour la majorité de cas que le systeme complexe était plus économique que le chauffage sol, de telle sorte que certains endroits où le chauffage solaire seul n'était pas rentable. devenaient intéressant quand le systeme complexe était considéré. La communication contient la description des méthodes de calcul et des résultats concernant les projets et l'analyse des coûts. Resumen-Los autores han publicado recientemente un análisis extenso que llevaron a cabo de la calefacción por energía solar de las viviendas en ocho ciudades de los Estados Unidos. En cada ciudad se optimizó el diseño de calefacción solar, y se determinó el coste de la calefacción solar. Se ha modificado el modelo matemático de estos sistemas para incluir las unidades refrigerantes de absorción del tipo bromuro litico actuadas por el calor. Se hicieron aproximadamente un centenar de análisis del rendimiento horario de refrigeración (y de calefacción) en estas ciudades durante un año Se reoptimizaron los diseños para minimizar los costes globales anuales de la energia para la refrigeración y la calefacción de las viviendas, así como para el suministro de agua caliente. Se hallo que el sistema combinado resulta más economico que la calefacción por si sola en la mayoría de los lugares, y algunos lugares donde no interesaha la calefacción solar han optado por el sistema combinado. El artículo contiene una descripcion del metodo de calculo y los resultados de los analisis de los diseños y los costes b. Article by Richard A. Mirth, "The Sun Can Heat Our Homes - Even in the North," the Northern Engineer, Fall 1974 by Richard A. Mirth. The Sun Can Heat Our Homes-- Even at high latitudes, there are several ways to use solar energy effectively. Among the most attractive of these is a flat-plate collector of solar energy. Coupled with a heat storage reservoir, such collectors can make a significant contribution to heating a northern home. The idea of harnessing the sun's energy to do work has intrigued man for centuries. In the first several decades of this century, and especially in the 1950's, a certain amount of sporadic effort was expended toward developing methods to use the sun to heat homes and to run small electrical appliances. For the most part, however, "solar energy" was usually thought of as being impractical as a large scale energy source and the machines which were developed were considered scientific curiosities. This is no longer true. The advent of world-wide energy shortages this past year has brought about increasing interest in the possibilities of using sunshine as a major energy source. At present, the technology for implementing large-scale solar converters exists, and year by year the cost effectiveness of such converters can be expected to improve. One generally thinks that solar energy would be a realistic energy source at mox rately low latitude locations which receive many hours of sunshine, such as the deserts in the southwestern United States. For, as one goes farther and farther north from the equatorial zones, the amount of possible sunshine deposited on a horizontal surface diminishes in intensity because of the lower solar angle (Strock, 1959). Also, the short daylight hours during the northern winters make solar energy least beneficial when energy requirements (for home heating, especially) are most intense. Yet, for a portion of the year, the northern latitudes enjoy an abundance of that solar energy could make in the There are a number of ideas for using solar energy. These include fuel generation electricity generation and low temperaby photosynthesis or other processes, ture heat generation for space heating and cooling. Fuel Generation Fuels can be generated from solar energy by photosynthesis and other methods (Thomas, 1973; Solar Energy Evaluation Group Report, 1973). sunlight. At the Arctic Circle, there are Direct burning of plants produced by photosynthesis can be used to produce heat and power. To employ this method, a way is needed to stimulate photosyn thesis and the amount of solar energy fixed by plants. Presently, about 1 per cent of the solar energy is converted to potential fuel by photosynthesis. At this rate, one acre would produce about 230 Btu per year (Thomas, 1973). Fuels can also be produced by plants that photosynthetically produce hydrogen methane (Solar Energy Evaluation Group Report, 1973). Photochemical systems can also generate hydrogen from water by electrolysis. The electric current or TABLE 1 SOLAR ENERGY RECEIVED ON A HORIZONTAL SURFACE AT VARIOUS LATITUDES 323 384 468 546 531 485 547 522 435 400 necessary for this operation could be generated by solar energy. Such methods as these need considerably more research. Electricity Generation Solar energy can generate electricity using normal thermal generation tech niques or photovoltaic cells. The heat level required for electric generation 15 greater than 400°F (S.E.E.G.R., 1973). This range of tem peratures can be achieved by focusing sunlight or by using a selective collector that absorbs light in the visible range but does not radiate it in the infrared range. Parabolic and linear focused collectors can easily attain light concentrations fifty times greater than normal (Pope et al., 1972) Partial cylinder collectors can achieve temperatures of 900°F. Focusing collectors have several drawbacks: they need to track the sun and are unable to collect diffuse light. Diffuse light can account for up to 33 per cent of the solar energy in an area such as Boston, Massachusetts (S.E.E.G.R., 1973). Selec sophisticated tive collectors require materials and entail greater expense. Photovoltaic cells are presently in use in many space applications. The most common types are made of silicone copper cadmium sulfide and are 10 to 12 per cent efficient. Thomas (1973) con. cludes that a 100 mile by 100 mile desert area of such cells could produce our entire 1985 electricity requirement. Other proposals involve an array of cells in space that produce electricity and trans mit the energy to earth via microwave (Bergstrom, 1971) The costs are esti mated as comparable to nuclear power generation. The principal drawbacks at present are the high cost of cell manufacture and the energy required to produce the cells. Currently, the energy required to produce a cell equals approximately 40 years power production from the cell (S.E.E.G.R., 1973). Additional research is being conducted to improve the cost and efficiency of the photovoltaic cell. Low Temperature Heat Collectors Photothermal conversion, which converts solar photons to heat, can also be used in a relatively low heat range. Simple flat plate collectors of one to three glass panes, an air space, a black surface laid over fluid conducting tubes and insulated backing can achieve water temperatures of 100 to 200°F (Donovan et al, 1972) Such collectors are similar to the high temperature selective collectors except they do not retain as much of the infrared radiation. However, because they operate at a lower temperature, they have lower heat losses. The heat from the collector can be stored by passing air through loosely packed rocks, by holding water in insulated tanks or by using the latent heat of salts (Thomas, 1973; S.E.E.G.R., 1973). The collectors can be tilted to achieve an optimum angle with the sun's rays. Collector efficiency can range from 0 to 100 per cent depending on the tem perature, absorber, and insulation (S.E.E.G.R., 1973). One collector, operating at 212° F and using a selective absorber in a vacuum tube, achieved a 62 per cent efficiency. A 50 per cent efficiency is shown as typical of a 150° F flat plate collector (Stromberg). This flat greenhouse type collector is adequate for space and water heating. Summary The flat plate solar energy collector offers the best energy collection method for home heating. The remaining techniques need additional development to prove technology, to improve efficiency, and to lower costs. Collectors of the flat plate type are used to heat over 20 buildings in the United States (Thomas, 1973). To use solar heat in addition to supplemental conventional heat is less costly than electric heat in most of the contiguous United States and is cheaper than gas or oil in favored areas (Donovan et al., 1972, Thomas, 1973). FACTORS AFFECTING SOLAR ENERGY INTENSITY According to Johnson and Hartman (1971) and Strock (1959), the primary factors affecting the amount of solar energy deposited on a surface are the geographic latitude and a group of local conditions. Latitude The latitude of a solar collector has a pronounced effect on the collection of radiation. Table 1 indicates the variation in the average solar energy received on a horizontal surface at various latitudes. The principal reason for the variation is the angle between the sun's rays and a perpendicular to the horizontal surface (incidence angle). In the high latitudes, the sun does not get directly overhead. The maximum rise, in degrees, above the horizon can be calculated from the equation, 113.5 latitude (Johnson and Hartman, 1971). The sun, if it strikes on an angle from the perpendicular, deposits less energy than if it struck perpendicular to the horizontal; therefore, the time of day affects the amount of solar energy collected by a fixed surface. The latitude also influences the duration of sunshine in any day and influences its distribution with the seasons. Table 2, derived by Hartman and Johnson, indicates the relationship of sunlight in winter (21 Location and SOLAR ENERGY RECEIVED ON HORIZONTAL, VERTICAL SOUTH-FACING, AND VERTICAL EAST-OR WEST-FACING PLANES FOR SELECTED LOCATIONS* Average Daily Solar Radiation by Month and National Climatic Center 1971 Annual Summary: Annette, Barrow, Bethel, Fairbanks. S is the ratio of recorded hours of sunshine to possible hours of sunshine (Strock, 1959). The albedo of the surface is affected by color and texture. New snow reflects approximately 90 per cent, while bare ground reflects only about 10 per cent of the radiation falling on it. Summary It is important to tilt a solar collector to achieve an optimum angle with the sun. A nearly vertical face will enhance the energy that is collected when the sun remains near the horizon as it does in high latitudes. There is very little chance of controlling the local conditions governing solar energy intensity; however, they should certainly be considered when designing a solar collector for a particular site. Snow near vertical collector panels increases the amount of energy available for collection. SOLAR ENERGY AVAILABLE Table 3 shows the solar energy received at seven locations from Spokane, Washington to Point Barrow, Alaska. The National Weather Service considers the accuracy of some of the solar radiation data of its network to be questionable, but the single source is the best available. The data are daily averages in langleys (gm calories/square centimeter). Multiplying langleys by 3.68 converts them to Btu/square foot. The data were in terms of the radiation on a horizontal surface. Using data from Hartman and Johnson, a multiplier was calculated to yield values for south and east or west facing planes. Data Variations The radiation on a horizontal surface for the equinox months of March and September with increasing decreases higher latitudes. Radiation on the south, east, or west-facing plane exceeds that on the horizontal plane for the same months at higher latitudes, because of the sun's low angular altitude and its near per pendicular incidence on the vertical planes. In June, the radiation on a horizontal plane does not diminish with increasing latitude because of the longer hours of daylight which make up for the low elevation of the sun. Nevertheless, a large difference between winter and summer radiation is most evident at Point Barrow where the sun disappears entirely for nearly three months. CONTRIBUTION OF SOLAR ENERGY TO HOME HEATING The heat required depends on the home size, its construction type, its in. sulation, the air leakage, and the number of doors and windows in it. Configuration Figure 1 shows a home consisting of an 8 inch thick dome, 36 feet in diameter, constructed from styrofoam planks welded together by heat. The technology is well proven and domes of up to 204 feet in diameter have been built. This structure has approximately 1940 square feet of usable floor space with a ceiling height of at least 6 feet. There is additional desk or storage area near the walls of the second floor. Such a structure presents a minimum of exterior surface, is well insulated and is structurally sound. The exterior can be coated with concrete, epoxy coatings or merely painted. The interior can be made fire retardant by using plaster or spray-on materials. Doors and windows can be cut as desired, but openings are held to a minimum: five 4-foot diameter windows, and two doors. The doors would be en |