31.20 1999 ASHRAE Applications Handbook (SI) are the two-pipe arrangement used with chain trenchers and the four- or six-pipe arrangements placed in trenches made with a wide backhoe bucket. An overlapping spiral configuration shown in Figure 22, has also been used with some success. However, it requires special attention during the backfilling process to ensure soil fills all the pockets formed by the overlapping pipe. Large quantities of water must be added to compact the soil around the overlapping pipes. The back- filling must be performed in stages to guarantee complete filling around the pipes and good soil contact. The high pipe density (up to 10 m of pipe per linear metre of trench) may cause problems in pro- longed extreme weather conditions, either from soil dry out during cooling or from freezing during heating. The extra time needed to backfill and the extra pipe length required make spiral configurations nearly as expensive to install as straight pipe configurations. However, the reduced land area needed Fig. 20 Approximate Groundwater Temperatures (°C) in the United States Fig. 21 Horizontal Ground Loop Configurations Fig. 22 General Layout of a Spiral Earth Coil Geothermal Energy 31.21 for the more compact design may permit their use on smaller resi- dential lots. The spiral pipe configuration laid flat in a horizontal pit arrangement is used commonly in the northern midwest part of the United States, where sandy soil causes trenches to collapse. A large open pit is excavated by a bulldozer, and then the overlapping pipes laid flat on the bottom of the pit. The bulldozer is also used to cover the pipe; the pipes should not be run over with the bulldozer tread. Most horizontal loop installations place flow loops in a parallel rather than a single (series) loop to reduce pumping power (Figure 23). Parallel loops may require slightly more pipe, but may use smaller pipe and thus have smaller internal volumes requiring less antifreeze (if needed). Also, the smaller pipe is typically much cheaper for a given length, so total pipe cost is less for parallel loops. An added benefit is that parallel loops can be flushed out with a smaller purge pump than would be required for a larger single-pipe loop. A disadvantage of parallel loops is the potential for unequal flow in the loops and thus reduced heat exchange efficiency. The time required to install a horizontal loop is not much differ- ent from that for a vertical system. For the arrangements described above, a two-person crew can typically install the ground loop for an average house in a single day. While not restricted to single-family residential applications, horizontal loops are rarely used in larger commercial buildings due to the land area that is required. Even if the land adjacent to the building were initially available, installation of a horizontal loop could prevent any future construction above the loop field, tying up a considerable investment in vacant land. Placement of horizontal loops under parking lots may have a negative impact on the effec- tiveness of the ground loop due to the greater surface heat exchange. Soil characteristics are an important concern for any ground loop design. With horizontal loops, the soil type can be more easily deter- mined because the excavated soil can be inspected and tested. EPRI et al. (1989) compiled a list of criteria and simple test procedures that can be used to classify soil and rock adequately enough for hor- izontal ground loop design. Leaks in the heat-fused plastic pipe are rare when attention is paid to pipe cleanliness and proper fusion techniques. Should a leak occur, it is usually best to try to isolate the leaking parallel loop and abandon it in place. The time and effort required to find the source of the leak usually far outweighs the cost of replacing the defective loop. Because the loss of as little as 1 L of water from the system will cause it to shut down, leaks cannot be located by looking for wet soil, as is commonly done with water lines. GROUNDWATER HEAT PUMPS A groundwater heat pump system (GWHP) removes groundwa- ter from a well and delivers it to a heat pump (or an intermediate heat exchanger) to serve as a heat source or sink. Both unitary or central plant designs are used. In the unitary type, a large number of small water-to-air heat pumps are distributed throughout the build- ing. The central plant design uses one or a small number of large- capacity chillers supplying hot and chilled water to a two- or four- pipe distribution system. Regardless of the type of equipment installed in the building, the specific components for handling the groundwater are similar. The primary items include (1) the wells (supply and, if required, injec- tion), (2) well pump, and (3) groundwater heat exchanger. The spe- cifics of these items are discussed in the section on Direct-Use Systems. In addition to those comments, the following consider- ations apply. Groundwater Flow Requirements Generally, the greater the groundwater flow, the better the per- formance (COP or EER) of the heat pumps. However, increasing heat pump performance can be compromised quickly by well pump power at high groundwater flow rates. For this reason, optimum groundwater flow should be based on electrical power requirements of the well pump, heat pumps, and circulating pump. Optimum groundwater flow (for minimum system energy consumption) is a function of groundwater temperature, well pump pressure, heat exchanger design, loop pump power and heat pump performance. For moderate-efficiency heat pumps (COP of 4), efficient loop pump design (0.016 W/W), and a heat exchanger approach of 1.5 K, Figure 24 provides curves for two different groundwater tem- peratures (21°C and 10°C) and two well pump pressures (300 and 900 kPa). Although the four curves show a clear optimum flow, sometimes operating at a lower groundwater flow reduces the well/pump cap- ital cost and reduces the problem of fluid disposal. These consider- ations are highly project specific, but do afford the designer some latitude in flow selection. Well Pumps Submersible pumps have not performed well in higher-tempera- ture, direct-use projects. However, in a normal groundwater temper- ature as encountered in heat pump applications, the submersible pump is a cost-effective option. The low temperature eliminates the need to specify an industrial design for the motor/protector, thereby greatly reducing the first cost relative to direct-use. Caution should still be exercised for wells that are expected to produce moderate Fig. 23 Parallel and Series Ground Loop Configurations Geothermal Energy 31.23 2. Chiller capacity is controlled by the heating water (condenser) loop temperature, and the groundwater flow through the chilled water exchanger is controlled by chilled water temperature. For buildings with a significant heating load, the former may be more attractive, while the latter may be appropriate for conventional building in moderate-to-warm climates. SURFACE WATER HEAT PUMPS Surface water bodies can be very good heat sources and sinks if properly used. In some cases, lakes can be the very best water sup- ply for cooling. A variety of water circulation designs are possible and several of the more common are presented. In a closed-loop system, a water-to-air heat pump is linked to a submerged coil. Heat is exchanged to (cooling mode) or from (heat- ing mode) the lake by the fluid (usually a water-antifreeze mixture) circulating inside the coil. The heat pump transfers heat to or from the air in the building. In an open-loop system, water is pumped from the lake through a heat exchanger and returned to the lake some distance from the point at which it was removed. The pump can be located either slightly above or submerged below the lake water level. For heat pump operation in the heating mode, this type is restricted to warmer climates; water temperature must remain above at least 5.5°C. Thermal stratification of water often keeps large quantities of cold water undisturbed near the bottom of deep lakes. This water is cold enough to adequately cool buildings by simply being circulated through heat exchangers. A heat pump is not needed for cooling, and energy use is substantially reduced. Closed-loop coils may also be used in colder lakes. Heating can be provided by a separate source or with heat pumps in the heating mode. Precooling or supplemental total cooling are also permitted when water tempera- ture is between 10 and 15°C. Heat Transfer in Lakes Heat is transferred to lakes by three primary modes: radiant energy from the sun, convective heat transfer from the surrounding air (when the air temperature is greater than the water temperature), and conduction from the ground. Solar radiation, which can exceed 950 W per square metre of lake area, is the dominant heating mech- anism, but it occurs primarily in the upper portion of the lake unless the lake is very clear. About 40% of the solar radiation is absorbed at the surface (Pezent and Kavanaugh 1990). Approximately 93% of the remaining energy is absorbed at depths visible to the human eye. Convection transfers heat to the lake when the lake surface tem- perature is lower than the air temperature. Wind speed increases the rate at which heat is transferred to the lake, but maximum heat gain by convection is usually only 10 to 20% of maximum solar heat gain. The conduction gain from the ground is even less than convec- tion gain (Pezent and Kavanaugh 1990). Cooling of lakes is accomplished primarily by evaporative heat transfer at the surface. Convective cooling or heating in warmer months will contribute only a small percentage of the total because of the relatively small temperature difference between the air and the lake surface temperature. Back radiation typically occurs at night when the sky is clear, and can account for significant amount of cooling. The relatively warm water surface will radiate heat to the cooler sky. For example, on a clear night, a cooling rate of up to 160 W/m 2 from a lake 14 K warmer than the sky. The last major mode of heat transfer, conduction to the ground, does not play a major role in lake cooling (Pezent and Kavanaugh 1990). To put these heat transfer rates in perspective, consider a 4000 m 2 lake that is used in connection with a 35 kW heat pump. In the cooling mode, the unit will reject approximately 44 kW to the lake. This is 11 W/m 2 , or approximately 1% of the maximum heat gain from solar radiation in the summer. In the winter, a 35 kW heat pump would absorb only about 26 kW, or 6.5 W/m 2 , from the lake. Thermal Patterns in Lakes The maximum density of water occurs at 4.0°C, not at the freezing point of 0°C. This phenomenon, in combination with the normal modes of heat transfer to and from lakes, produces temperature pro- files advantageous to efficient heat pump operation. In the winter, the coldest water is at the surface. It tends to remain at the surface and freeze. The bottom of a deep lake stays 3 to 5 K warmer than the sur- face. This condition is referred to as winter stagnation. The warmer water is a better heat source than the colder water at the surface. Fig. 25 Central Plant Groundwater Systems 31.24 1999 ASHRAE Applications Handbook (SI) As spring approaches, surface water warms until the temperature approaches the maximum density point of 4.0°C. The winter strati- fication becomes unstable and circulation loops begin to develop from top to bottom. This condition of spring overturn (Peirce 1964) causes the lake temperature to become fairly uniform. Later in the spring as the water temperatures rise above 7°C, the circulation loops are in the upper portion of the lake. This pattern continues throughout the summer. The upper portion of the lake remains relatively warm, with evaporation cooling the lake and solar radiation warming it. The lower portion (hypolimnion) of the lake remains cold because most radiation is absorbed in the upper zone. Circulation loops do not penetrate to the lower zone and con- duction to the ground is quite small. The result is that in deeper lakes with small or medium inflows, the upper zone is 21 to 32°C, the lower zone is 4 to 13°C, and the intermediate zone (thermocline) has a sharp change in temperature within a small change in depth. This condition is referred to as summer stagnation. As fall begins, the water surface begins to cool by radiation and evaporation. With the approach of winter, the upper portion begins to cool towards the freezing point and the lower levels approach the maximum density temperature of 4.0°C. An ideal temperature ver- sus depth chart is shown in Figure 26 for each of the four seasons (Peirce 1964). Many lakes do exhibit near-ideal temperature profiles. However, a variety of circumstances can disrupt the profile. These characteristics include (1) high inflow/outflow rates, (2) insufficient depth for strat- ification, (3) level fluctuation, (4) wind, and (5) lack of enough cold weather to establish sufficient amounts of cold water necessary for summer stratification. Therefore, a thermal survey of the lake should be conducted or existing surveys of similar lakes in similar geo- graphic locations should be consulted. Closed-Loop Lake Water Heat Pump The closed-loop lake water heat pump shown in Figure 17 has several advantages over the open-loop. One advantage is the reduced fouling resulting from the circulation of clean water (or water-antifreeze solution) through the heat pump. A second advan- tage is the reduced pumping power requirement. This results from the absence of an elevation head from the lake surface to the heat pumps. A third advantage of a closed-loop is that it is the only type recommended if a lake temperature below 4°C is possible. The out- let temperature of the fluid will be about 3 K below that of the inlet at a flow of 54 mL/s per kilowatt. Frosting will occur on the heat exchanger surfaces when the bulk water temperature is in the 1 to 3°C range. A closed-loop system has several disadvantages. Performance of the heat pump lowers slightly because the circulation fluid temper- ature drops 2 to 7 K below the lake temperature. A second disadvan- tage is the possibility of damage to coils located in public lakes. Thermally fused polyethylene loops are much more resistant to damage than copper, glued plastic (PVC), or tubing with band- clamped joints. The third possible disadvantage is fouling on the outside of the lake coil—particularly in murky lakes or where coils are located on or near the lake bottom. Polyethylene (PE 3408) is recommended for all intake piping. All connections must be either thermally socket fused or butt fused. These plastic pipes should also have protection from UV radiation, especially when near the surface. Polyvinyl chloride (PVC) pipe and plastic pipe with band-clamped joints is not recommended. The piping networks of closed-loop systems resemble those used in ground-coupled heat pump systems. Both a large-diameter header between the heat pump and lake coil and several parallel loops of piping in the lake are required. The loops are spread out to limit thermal interference, hot spots, and cold pockets. While this Fig. 26 Idealized Diagram of Annual Cycle of Thermal Stratification in Lakes Fig. 27 Closed Loop Lake Coil in Bundles (Kavanaugh 1991) Geothermal Energy 31.25 layout is preferred in terms of performance, installation is more time consuming. Many contractors simply unbind plastic pipe coils and submerged them in a loose bundle. Some compensation for thermal interference is obtained by making the bundled coils longer than the spread coils. A diagram of this type of installation is shown in Figure 27. Copper coils have also been used successfully. Copper tubes have a very high thermal conductivity, so coils only one-fourth to one-third the length of plastic coils are required. However, copper pipe does not have the durability of PE 3408 or polybutylene, and if the possibility of fouling exists, coils must be significantly longer. Antifreeze Requirements Closed loop horizontal and surface water heat exchanger sys- tems will often require an antifreeze be added to the circulating water in locations with significant heating seasons. Antifreeze may not be needed in a comparable vertical borehole heat exchanger since the deep ground temperature will be essentially constant. At a depth of 2 m, a typical value for horizontal heat exchangers, the ground temperature varies by approximately ±5 K. Even if the mean ground temperature were 15°C in late winter, the natural ground temperature would drop to 10°C. The heat extraction process would lower the temperature even further around the heat exchanger pipes, probably by an additional 5 K or more. Even with good heat transfer to the circulating water, the entering water temperature (leaving the ground heat exchanger) would be around 5°C. Lakes which freeze at the surface in the winter approach 4°C at the bottom, yielding nearly the same margin of safety against freezing of the circulating fluid. An additional 5 K temperature difference is usually needed in the heat pump’s refrigerant-to-water heat exchanger to transfer the heat to the refrigerant. Having a refrigerant-to-water coil surface temperature below the freezing point of water risks the possibility of growing a layer of ice on the water side of the heat exchanger. In the best case, icing of the coil would restrict and may eventually block the flow of water and cause a shutdown. In the worst case, the ice could burst the tubing in the coil and require a major service expense. Several factors must be considered when selecting an antifreeze for a ground loop heat exchanger. The most important consider- ations are: (1) impact on system life cycle cost, (2) corrosivity, (3) leakage, (4) health risks, (5) fire risks, (6) environmental risks from spills or disposal, and (7) risk of future use (will the antifreeze be acceptable over the life of the system). A study by Heinonen et al. (1997) of six antifreezes against these seven criteria is summarized in Table 9. No single material satisfies all criteria. Methanol and eth- anol have good viscosity characteristics at low temperatures, yield- ing lower than average pumping power requirements. However, they both pose a significant fire hazard when in concentrated forms. Methanol is also toxic, eliminating it from consideration in areas that require non-toxic antifreeze to be used. Propylene glycol had no major concerns, with only leakage and pumping power require- ments prompting minor concerns. Potassium acetate, calcium mag- nesium acetate (CMA), and urea have favorable environmental and safety performance; but they are all subject to significant leakage problems, which has limited their use in the past. REFERENCES Anderson, K.E. 1984. Water well handbook, Missouri Water Well and Pump Contractors Association, Belle, MD. Austin, J.C. 1978. A low temperature geothermal space heating demonstra- tion project. Geothermal Resources Council Transactions 2(2). Bullard, E. 1973. Basic theories (Geothermal energy; Review of research and development). UNESCO, Paris. Caneta Research. 1995. Commercial/institutional ground-source heat pump engineering manual. ASHRAE, Atlanta. CSA. 1993. Design and construction of earth energy heat pump systems for commercial and institutional buildings. Standard C447-93. Canadian Standards Association, Rexdale, ON. Campbell, M.D. and J.H. Lehr. 1973. Water well technology. McGraw-Hill, New York. Carslaw, H.S. and J.C. Jaeger. 1947. Heat conduction in solids. Claremore Press, Oxford. Chandler, R.V. 1987. Alabama streams, lakes, springs and ground waters for use in heating and cooling. Bulletin 129. Geological Survey of Alabama, Tuscaloosa, AL. Christen, J.E. 1977. Central cooling—Absorption chillers. Oak Ridge National Laboratories, Oak Ridge, TN. Combs, J., J.K. Applegate, R.O. Fournier, C.A. Swanberg, and D. Nielson. 1980. Exploration, confirmation and evaluation of the resource. In Spe- cial Report No. 7, Direct utilization of geothermal energy: Technical handbook. Geothermal Resources Council. Cosner, S.R. and J.A. Apps. 1978. A compilation of data on fluids from geo- thermal resources in the United States. DOE Report LBL-5936. Lawrence Berkeley Laboratory, Berkeley, CA. Table 9 Suitability of Selected GCHP Antifreeze Solutions Category Methanol Ethanol Propy- lene Glycol Potas- sium Acetate CMA Urea Life cycle cost *** *** ** 1 ** 1 ** 1 *** Corrosion ** 2 ** 3 *** ** ** 4 * 5 Leakage *** ** 6 ** 6 * 7 * 8 * 9 Health hazard risk * 10,11 ** 10,12 *** 10 *** 10 *** 10 *** 10 Fire risk * 13 * 13 *** 14 *** *** *** Environmental risk ** 15 ** 15 *** ** 15 ** 15 *** Risk of future use * 16 ** 17 *** ** 18 ** 19 ** 19 Key: * Potential problems, caution in use required ** Minor potential for problems *** Little or no potential for problems Category Notes Life cycle cost 1. Higher than average installation and energy costs. Corrosion 2. High black iron and cast iron corrosion rates. 3. High black iron and cast iron, copper and copper alloy corrosion rates. 4. Medium black iron, copper and copper alloy corrosion rates. 5. Medium black iron, high cast iron, and extremely high copper and copper alloy corrosion rates. Leakage 6. Minor leakage observed. 7. Moderate leakage observed. Extensive leakage reported in installed systems. 8. Moderate leakage observed. 9. Massive leakage observed. Health risk 10. Protective measures required with use. See MSDS. 11. Prolonged exposure can cause headaches, nau- sea, vomiting, dizziness, blindness, liver dam- age, and death. Use of proper equipment and procedures reduces risk significantly. 12. Confirmed human carcinogen. Fire Risk 13. Pure fluid only. Little risk when diluted with water in antifreeze. 14. Very minor potential for pure fluid fire at ele- vated temperatures. Environmental risk 15. Water pollution risk. Risk of future use 16. Toxicity and fire concerns. Prohibited in some locations. 17. Toxicity, fire and environmental concerns. 18. Potential leakage concerns. 19. Not currently used as GSHP antifreeze solution. May be difficult to obtain approval for use. Source: Heinonen and Tapscott (1996) 31.26 1999 ASHRAE Applications Handbook (SI) Culver, G.G. and G.M. Reistad. 1978. Evaluation and design of downhole heat exchangers for direct applications. DOE Report No. RLO-2429-7. Di Pippo, R. 1988. Industrial developments in geothermal power produc- tion. Geothermal Resources Council Bulletin 17(5). Efrid, K.D. and G.E. Moeller. 1978 Electrochemical characteristics of 304 and 316 stainless steels in fresh water as functions of chloride concentra- tion and temperature. Paper 87, Corrosion/78, Houston, TX. EPRI. 1989. Soil and rock classification for the design of ground-coupled heat pump systems. International Ground Source Heat Pump Associa- tion, Stillwater, OK. Electric Power Research Institute, National Rural Electric Cooperative Association, Oklahoma State University. Ellis, P. 1989. Materials selection guidelines. Geothermal Direct Use Engi- neering and Design Guidebook Ch. 8. Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR. Ellis, P. and C. Smith. 1983. Addendum to material selection guidelines for geothermal energy utilization systems. Radian Corporation, Austin, TX. Ellis, P.F. and M.F. Conover. 1981. Material selection guidelines for geother- mal energy utilization systems. DOE Report RA/27026-1, Radian Cor- poration, Austin, TX. EPA. 1975. Manual of water well construction practices. EPA-570/9-75- 001. U.S. Environmental Protection Agency, Washington, D.C. Eskilson, P. 1987. Thermal analysis of heat extraction boreholes. University of Lund, Sweden. Gudmundsson, J.S. 1985. Direct uses of geothermal energy in 1984. Geo- thermal Resources Council Proceedings, 1985 International Symposium on Geothermal Energy, International Volume, Davis, CA. Hackett, G. and J. H. Lehr. 1985. Iron bacteria occurrence problems and control methods in water wells. National Water Well Association, Wor- thington, OH. Heinonen, E.W. And R.E. Tapscott. 1996. Assessment of anti-freeze solu- tions for ground-source heat pump systems. New Mexico Engineering Research Institute for ASHRAE RP-863. ASHRAE. Heinonen, E.W., R.E. Tapscott, M.W. Wildin, and A.N. Beall. 1997. Assess- ment of anti-freeze solutions for ground-source heat pump systems. ASHRAE Research Report 90BRP. Ingersoll, L.R. and A.C. Zobel. 1954. Heat conduction with engineering and geological application, 2nd ed. McGraw-Hill, New York. Interagency Geothermal Coordinating Council. Geothermal energy, research, development and demonstration program. DOE Report RA- 0050, IGCC-5. U.S. Department of Energy, Washington, D.C. Kavanaugh, S.P. 1985. Simulation and experimental verification of a verti- cal ground-coupled heat pump system. Ph.D. thesis. Oklahoma State University, Stillwater, OK. Kavanaugh, S.P. 1991. Ground and water source heat pumps. Oklahoma State University, Stillwater, OK. Kavanaugh, S.P. 1992. Ground-coupled heat pumps for commercial build- ing. ASHRAE Journal 34(9):30-37. Kavanaugh, S.P. and M.C. Pezent. 1990. Lake water applications of water- to-air heat pumps. ASHRAE Transactions 96(1):813-20. Kavanaugh, S.P. and K. Rafferty. 1997. Ground-source heat pumps— Design of geothermal systems for commercial and institutional build- ings. ASHRAE, Atlanta. Kindle, C.H. and E.M. Woodruff. 1981. Techniques for geothermal liquid sampling and analysis. Battelle Pacific Northwest Laboratory, Richland, WA . Lienau, P.J. 1979. Materials performance study of the OIT geothermal heat- ing system. Geo-Heat Utilization Center Quarterly Bulletin, Oregon Institute of Technology, Klamath Falls, OR. Lienau, P.J., G.G. Culver and J.W. Lund. 1988. Geothermal direct use devel- opments in the United States. Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR. Lund, J.W., P.J. Lienau, G.G. Culver and C.H. Higbee, C.V. 1976. Klamath Falls geothermal heating district. Geothermal Resources Council Trans- actions 3. Lunis, B. 1989. Environmental considerations. Geothermal direct use engi- neering and design guidebook, Ch. 20. Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR. Mitchell, D.A. 1980. Performance of typical HVAC materials in two geo- thermal heating systems. ASHRAE Transactions 86(1):763-68. Muffler, L.J.P., ed. 1979. Assessment of geothermal Resources of the United States—1978. U.S. Geological Survey Circular No. 790. Nichols, C.R. 1978. Direct utilization of geothermal energy: DOE’s resource assessment program. Direct Utilization of Geothermal Energy: A Sym- posium. Geothermal Resources Council. OSU. 1988a. Closed-loop/ground-source heat pump systems installation guide. International Ground Source Heat Pump Association, Oklahoma State University, Stillwater, OK. OSU. 1988b. Closed loop ground source heat pump systems. Oklahoma State University, Stillwater, OK. Peirce, L.B. 1964. Reservoir temperatures in north central alabama. Geolog- ical Survey of Alabama Bulletin 8. Tuscaloosa, AL. Pezent, M.C. and S.P. Kavanaugh. 1990. Development and verification of a thermal model of lakes used with water-source heat pumps. ASHRAE Transactions 96(1). Rafferty, K. 1989a. A materials and equipment review of selected U.S. geo- thermal district heating systems. Oregon Institute of Technology, Geo- Heat Center, Klamath Falls, OR. Rafferty, K. 1989b. Absorption refrigeration. Geothermal direct use engi- neering and design guidebook, Ch. 14. Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR. Reistad, G.M., G.G. Culver, and M. Fukuda. 1979. Downhole heat exchang- ers for geothermal systems: Performance, economics and applicability. ASHRAE Transactions 85(1):929-39. Roscoe Moss Company. 1985. The engineers manual for water well design. Roscoe Moss Company, Los Angeles, CA. Stiger, S., J. Renner, and G. Culver. 1989. Well testing and reservoir evalu- ation. Geothermal and direct use engineering and design guidebook, Ch. 7. Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR. Svec, O. J. 1990. Spiral ground heat exchangers for heat pump applications. Proceedings of 3rd IEA Heat Pump Conference. Pergamon Press, Tokyo. UOP. 1975. Ground water and wells. Johnson Division, UOP Inc., St. Paul, MN. BIBLIOGRAPHY Allen, E. 1980. Preliminary inventory of western U.S. cities with proximate hydrothermal potential. Eliot Allen and Associates, Salem, OR. Anderson, D.A. and J.W. Lund, eds. 1980. Direct utilization of geothermal energy: Technical handbook. Geothermal Resources Council Special Report No. 7. Caneta Research. 1995. Operating experiences with commercial ground- source heat pumps. ASHRAE Research Project 863. CHAPTER 32 SOLAR ENERGY USE Quality and Quantity of Solar Energy 32.1 Solar Energy Collection 32.6 Heat Storage 32.11 Water Heating 32.11 Components 32.14 Cooling by Solar Energy 32.16 Cooling by Nocturnal Radiation and Evaporation 32.16 Solar Heating and Cooling Systems 32.17 Sizing Solar Heating and Cooling Systems— Energy Requirements 32.19 Installation Guidelines 32.23 Design, Installation, and Operation Checklist 32.25 Photovoltaic Applications 32.26 HE major obstacles encountered in solar heating and cooling Tare economic—the equipment needed to collect and store solar energy is high in cost. In some cases, the cost of the solar equipment is greater than the resulting savings in fuel costs. Some of the prob- lems inherent in the nature of solar radiation include: • It is relatively low in intensity, rarely exceeding 950 W/m 2 . Con- sequently, when large amounts of energy are needed, large collec- tors must be used. • It is intermittent because of the variation in solar radiation inten- sity from zero at sunrise to a maximum at noon and back to zero at sunset. Some means of energy storage must be provided at night and during periods of low solar radiation. • It is subject to unpredictable interruptions because of clouds, rain, snow, hail, or dust. Systems should make maximum use of the solar energy input by effectively using the energy at the lowest temperatures possible. QUALITY AND QUANTITY OF SOLAR ENERGY Solar Constant Solar energy approaches the earth as electromagnetic radiation, with wavelengths ranging from 0.1 µm (X rays) to 100 m (radio waves). The earth maintains a thermal equilibrium between the annual input of shortwave radiation (0.3 to 2.0 µm) from the sun and the outward flux of longwave radiation (3.0 to 30 µm). Only a lim- ited band need be considered in terrestrial applications, because 99% of the sun’s radiant energy has wavelengths between 0.28 and 4.96 µm. The current value of the solar constant (which is defined as the intensity of solar radiation on a surface normal to the sun’s rays, just beyond the earth’s atmosphere at the average earth-sun distance) is 1367 W/m 2 . The section on Determining Incident Solar Flux in Chapter 29 of the 1997 ASHRAE Handbook—Fundamentals has further information on this topic. Solar Angles The axis about which the earth rotates is tilted at an angle of 23.45° to the plane of the earth’s orbital plane and the sun’s equator. The earth’s tilted axis results in a day-by-day variation of the angle between the earth-sun line and the earth’s equatorial plane, called the solar declination δ. This angle varies with the date, as shown in Table 1 for the year 1964 and in Table 2 for 1977. For other dates, the declination may be estimated by the following equation: (1) where N = year day, with January 1 = 1. For values of N, see Tables 1 and 2. The relationship between δ and the date varies to an insignificant degree. The daily change in the declination is the primary reason for the changing seasons, with their variation in the distribution of solar radiation over the earth’s surface and the varying number of hours of daylight and darkness. The earth’s rotation causes the sun’s apparent motion (Figure 1). The position of the sun can be defined in terms of its altitude β above the horizon (angle HOQ) and its azimuth φ, measured as angle HOS in the horizontal plane. At solar noon, the sun is exactly on the meridian, which contains the south-north line. Consequently, the solar azimuth φ is 0°. The noon altitude β N is given by the following equation as (2) where LAT = latitude. Because the earth’s daily rotation and its annual orbit around the sun are regular and predictable, the solar altitude and azimuth may be readily calculated for any desired time of day when the latitude, longitude, and date (declination) are specified. Apparent solar time (AST) must be used, expressed in terms of the hour angle H, where (3) Solar Time Apparent solar time (AST) generally differs from local standard time (LST) or daylight saving time (DST), and the difference can be significant, particularly when DST is in effect. Because the sun The preparation of this chapter is assigned to TC 6.7, Solar Energy Utiliza- tion. δ 23.45 sin 360° 284 N+()365⁄[]= Fig. 1 Apparent Daily Path of the Sun Showing Solar Altitude (β) and Solar Azimuth (φ) β N 90° LAT δ+–= H number of hours from solar noon()15°×= number of minutes from solar noon()4⁄= Solar Energy Use 32.3 To determine θ, the surface azimuth ψ and the surface-solar azimuth γ must be known. The surface azimuth (angle POS in Fig- ure 2) is the angle between the south-north line SO and the normal PO to the intersection of the irradiated surface with the horizontal plane, shown as line OM. The surface-solar azimuth, angle HOP, is designated by γ and is the angular difference between the solar azimuth φ and the surface azimuth ψ. For surfaces facing east of south, γ = φ − ψ in the morning and γ = φ + ψ in the afternoon. For surfaces facing west of south, γ = φ + ψ in the morning and γ = φ − ψ in the afternoon. For south-facing surfaces, ψ = 0°, so γ = φ for all conditions. The angles δ, β, and φ are always positive. For a surface with a tilt angle Σ (measured from the horizontal), the angle of incidence θ between the direct solar beam and the nor- mal to the surface (angle QOP ′ in Figure 2) is given by: (8) For vertical surfaces, Σ = 90°, cos Σ = 0, and sin Σ = 1.0, so Equa- tion (8) becomes (9) For horizontal surfaces, Σ = 0°, sin Σ = 0, and cos Σ = 1.0, so Equation (8) leads to (10) Example 2. Find θ for a south-facing surface tilted upward 30° from the horizontal at 40° north latitude at 4:00 P.M., AST, on August 21. Solution: From Equation (3), at 4:00 P.M. on August 21, From Table 1, From Equation (5), From Equation (6), The surface faces south, so φ = γ. From Equation (8), ASHRAE Standard 93, Methods of Testing to Determine the Thermal Performance of Solar Collectors, provides tabulated values of q for horizontal and vertical surfaces and for south-facing sur- faces tilted upward at angles equal to the latitude minus 10°, the lat- itude, the latitude plus 10°, and the latitude plus 20°. These tables cover the latitudes from 24° to 64° north, in 8° intervals. Solar Spectrum Beyond the earth’s atmosphere, the effective black body temper- ature of the sun is 5760 K. The maximum spectral intensity occurs at 0.48 µm in the green portion of the visible spectrum (Figure 3). Thekaekara (1973) presents tables and charts of the sun’s extrater- restrial spectral irradiance from 0.120 to 100 µm, the range in which most of the sun’s radiant energy is contained. The ultraviolet portion of the spectrum below 0.40 µm contains 8.73% of the total, another 38.15% is contained in the visible region between 0.40 and 0.70 µm, and the infrared region contains the remaining 53.12%. Solar Radiation at the Earth’s Surface In passing through the earth’s atmosphere, some of the sun’s direct radiation I D is scattered by nitrogen, oxygen, and other molecules, which are small compared to the wavelengths of the radiation; and by aerosols, water droplets, dust, and other particles with diameters comparable to the wavelengths (Gates 1966). This Fig. 2 Solar Angles with Respect to a Tilted Surface cos θ cos β cos γ sin Σ sin β cos Σ+= cos θ cos β cos γ= θ H 90°β–= Fig. 3 Spectral Solar Irradiation at Sea Level for Air-Mass = 1.0 H 415 °× 60 ° == δ 12.1 ° = sin β cos 40° cos 12.1° cos 60° sin 40° sin 12.1°+= β 30.6°= sin φ cos 12.1° sin 60° cos 30.6° ⁄ = φ 79.7°= cos θ cos 30.6° cos 79.7° sin 30° sin 30.6° cos 30°+= θ 58.8°= 32.4 1999 ASHRAE Applications Handbook (SI) scattered radiation causes the sky to appear blue on clear days, and some of it reaches the earth as diffuse radiation I. Attenuation of the solar rays is also caused by absorption, first by the ozone in the outer atmosphere, which causes a sharp cutoff at 0.29 µm of the ultraviolet radiation reaching the earth’s surface. In the longer wavelengths, there are a series of absorption bands caused by water vapor, carbon dioxide, and ozone. The total amount of attenuation at any given location is determined by (1) the length of the atmospheric path through which the rays traverse and (2) the composition of the atmosphere. The path length is expressed in terms of the air mass m, which is the ratio of the mass of atmosphere in the actual earth-sun path to the mass that would exist if the sun were directly overhead at sea level (m = 1.0). For all practical purposes, at sea level, m = 1.0/sin β. Beyond the earth’s atmosphere, m = 0. Prior to 1967, solar radiation data was based on an assumed solar constant of 1324 W/m 2 and on a standard sea level atmosphere containing the equivalent depth of 2.8 mm of ozone, 20 mm of pre- cipitable moisture, and 300 dust particles per cubic centimeter. Threlkeld and Jordan (1958) considered the wide variation of water vapor in the atmosphere above the United States at any given time, and particularly the seasonal variation, which finds three times as much moisture in the atmosphere in midsummer as in December, January, and February. The basic atmosphere was assumed to be at sea level barometric pressure, with 2.5 mm of ozone, 200 dust particles per cm 3 , and an actual precipitable moisture content that varied throughout the year from 8 mm in midwinter to 28 mm in mid-July. Figure 4 shows the variation of the direct normal irradi- ation with solar altitude, as estimated for clear atmospheres and for an atmosphere with variable moisture content. Stephenson (1967) showed that the intensity of the direct normal irradiation I DN at the earth’s surface on a clear day can be estimated by the following equation: (11) where A, the apparent extraterrestrial irradiation at m = 0, and B, the atmospheric extinction coefficient, are functions of the date and take into account the seasonal variation of the earth-sun distance and the air’s water vapor content. The values of the parameters A and B given in Table 1 were selected so that the resulting value of I DN would be in close agree- ment with the Threlkeld and Jordan (1958) values on average cloud- less days. The values of I DN given in Tables 15 through 21 in Chapter 29 of the 1997 ASHRAE Handbook—Fundamentals, were obtained by using Equation (11) and data from Table 1. The values of the solar altitude β and the solar azimuth φ may be obtained from Equations (5) and (6). Because local values of atmospheric water content and elevation can vary markedly from the sea level average, the concept of clear- ness number was introduced to express the ratio between the actual clear-day direct irradiation intensity at a specific location and the intensity calculated for the standard atmosphere for the same loca- tion and date. Figure 5 shows the Threlkeld-Jordan map of winter and summer clearness numbers for the continental United States. Irradiation val- ues should be adjusted by the clearness numbers applicable to each particular location. Design Values of Total Solar Irradiation The total solar irradiation I tθ of a terrestrial surface of any orien- tation and tilt with an incident angle θ is the sum of the direct com- ponent I DN cos θ plus the diffuse component I dθ coming from the sky plus whatever amount of reflected shortwave radiation I r may reach the surface from the earth or from adjacent surfaces: (12) The diffuse component is difficult to estimate because of its non- directional nature and its wide variations. Figure 4 shows typical values of diffuse irradiation of horizontal and vertical surfaces. For clear days, Threlkeld (1963) has derived a dimensionless parameter (designated as C in Table 1), which depends on the dust and mois- ture content of the atmosphere and thus varies throughout the year: (13) where I dH is the diffuse radiation falling on a horizontal surface under a cloudless sky. Fig. 4 Variation with Solar Altitude and Time of Year for Direct Normal Irradiation I DN Ae B– β sin ⁄ = Fig. 5 Clearness Numbers for the United States I t θ I DN cos θ I d θ I r ++= CI dH I DN ⁄= [...]... Density °C kg/m3 °C kg/m3 °C kg/m3 0 1 2 3 4 5 6 999 .87 999.93 999.97 999.99 1000.00 999.99 999.97 7 8 9 10 11 12 13 999.93 999 .88 999 .81 999.73 999.63 999.52 999.40 14 15 16 17 18 19 20 999.27 999.13 9 98. 97 9 98. 80 9 98. 62 9 98. 43 9 98. 23 create an inlet Froude number of 1.0 or less However, values up to 2.0 have been successfully applied (Yoo et al 1 986 ) The inlet Froude number Fr is defined as Q Fr =... Black Paint Transmittance Incident Angle, Deg Single Glazing Double Glazing Absorptance for Flat Black Paint 0 10 20 30 40 50 60 70 80 90 0 .87 0 .87 0 .87 0 .87 0 .86 0 .84 0.79 0. 68 0.42 0.00 0.77 0.77 0.77 0.76 0.75 0.73 0.67 0.53 0.25 0.00 0.96 0.96 0.96 0.95 0.94 0.92 0 .88 0 .82 0.67 0.00 incurring excessive costs for labor or materials Materials most frequently used for collector plates are copper, aluminum,... Diamond, S.C and J.G Avery 1 986 Active solar energy system design, installation and maintenance: Technical applications manual LA-UR86-4175 HUD 1 980 Installation guidelines for solar DHW systems in one- and twofamily dwellings U.S Department of Housing and Urban Development, 2nd ed (May) Knapp, C.L., T.L Stoffel, and S.D Whitaker 1 980 Insolation data manual SERI/SP-755- 789 Solar Energy Research Institute,... Institute 1 981 Solar design workbook—Solar federal buildings program SERI/SP-62-3 08 U.S Department of Energy and Los Alamos Scientific Laboratory Solar Energy Research Institute 1 981 Solar radiation energy resource atlas of the United States SERI/SP642-1037 Golden, CO Solar Environmental Engineering Co., Inc 1 981 Solar domestic hot water system inspection and performance evaluation handbook SERI/SP 981 89-1B... Equation (8) gives cos θ = = = θ = cos 70.6° cos 0° sin 30° + sin 70.6° cos 30° ( 0.332 ) ( 1 ) MP ( 0.5 ) + ( 0.943 ) ( 0 .86 6 ) 0. 983 10.6° From Table 1, A = 1 085 W/m2, B = 0.207, and C = 0.136 Using Equation (11), I DN = 1 085 ⁄ e – 0.207 ⁄ sin 70.6° = 87 1 W ⁄ m 2 Combining Equations (14) and (15) gives Fig 11 Variation of Absorptance and Transmittance with Incident Angle I tθ = 0.136 × 87 1 ( 1 +... determine the thermal performance of solar domestic water heating systems Standard 95-1 981 (Reaffirmed 1996) ASHRAE 1 988 Active solar heating systems design manual ASHRAE 1 989 Methods of testing to determine the thermal performance of unglazed flat-plate liquid-type solar collectors Standard 96-1 980 (Reaffirmed 1 989 ) ASHRAE 1991 Active solar heating systems installation manual Bennett, I 1965 Monthly... Cheremisinoff, eds 1 980 Solar energy technology handbook, Part B: Application, systems design and economics Marcel Dekker, Inc., New York DOE 1978a DOE facilities solar design handbook DOE/AD-0006/1 U.S Department of Energy DOE 1978b SOLCOST—Solar hot water handbook; A simplified design method for sizing and costing residential and commercial solar service hot water systems, 3rd ed DOE/CS-0042/2 U.S Department... 22(3):269 -82 Klein, S.A., W.A Beckman, J.A Duffie 1976 TRNSYS—A transient simulation program ASHRAE Transactions 82 (1):623-33 Kutscher, C.F 1996 Proceedings of the 19th World Energy Engineering Congress Atlanta, GA LBL 1 981 DOE-2 Reference Manual Version 2.1A Los Alamos Scientific Laboratory, Report LA-7 689 -M, Version 2.1A Report LBL -87 06 Rev 2, Lawrence Berkeley Laboratory, May Lister, L and T Newell 1 989 ... SERI/SP 981 89-1B Solar Energy Research Institute, Golden, CO CHAPTER 33 THERMAL STORAGE Economics 33.2 APPLICATIONS 33.3 Off-Peak Air Conditioning 33.5 Storage for Retrofit Applications 33 .8 Industrial/Process Cooling 33 .8 Off-Peak Heating 33 .8 Other Applications 33.9 STORAGE TECHNOLOGIES 33.10 Water Storage 33.10 Ice Storage and Other... collector performance evaluation Solar Energy 18( 5):451 32. 28 1999 ASHRAE Applications Handbook (SI) Stephenson, D.G 1967 Tables of solar altitude and azimuth; Intensity and solar heat gain tables Technical Paper No 243, Division of Building Research, National Research Council of Canada, Ottawa Svard, C.D., J.W Mitchell, and W.A Beckman 1 981 Design procedure and applications of solar-assisted series heat . Glazing 0 0 .87 0.77 0.96 10 0 .87 0.77 0.96 20 0 .87 0.77 0.96 30 0 .87 0.76 0.95 40 0 .86 0.75 0.94 50 0 .84 0.73 0.92 60 0.79 0.67 0 .88 70 0. 68 0.53 0 .82 80 0.42 0.25 0.67 90 0.00 0.00 0.00 32.10 1999. 0.5 () 0.943 () 0 .86 6 () += 0. 983 = θ 10.6°= I DN 1 085 e 0.207– sin 70.6°⁄ 87 1 W m 2 ⁄ = ⁄ = I tθ 0.136 87 1 1 cos 30+ () 2 111 W m 2 ⁄ = ⁄× = I tθ 87 1 cos 10.6 111+ 967 W m 2 ⁄ == q u 967 0 .87 0.96 ×() 7.3. of typical HVAC materials in two geo- thermal heating systems. ASHRAE Transactions 86 (1):763- 68. Muffler, L.J.P., ed. 1979. Assessment of geothermal Resources of the United States—19 78. U.S. Geological