Design, Construction and Operation of a Site-Built PV/Hot Air Hybrid Energy System Richard Komp and Terry Reeser Abstract For a passive dwelling in Louisville, Kentucky, we have developed and are constructing a linear concentrator array. It is built into the roof structure of an attached sunspace that uses natural convection to extract excess heat from the fin module assemblies and deliver that heat to the home in the winter. In the summer, the heat is exhausted from clerestory windows, creating a draft of cooler air into the lower part of the building. The 17m x 2.5m array containing 72 fins with compound curved collectors of 2/1 ratio arranged vertically at a slope angle equal to the 38¡ latitude of Louisville. It was constructed at the site using 100 mm round single crystal photovoltaic cells imbedded in silicone resin onto aluminum fins for good heat transfer. The entire array will have a rated output of 2.5 kiloWatts. The power is fed to a 3500 Ampere-hour 12 VDC storage battery bank and can be delivered either directly to the low voltage lights and appliances in the home or to a 1200 Watt Heart Interface inverter for conversion to 120 vac. Uses of the power include running an IBM PC and a small plastic injection molding machine as an income producing cottage industry. This remote site has no utility power available. Introduction and Objectives Photovoltaic modules are becoming an attractive alternative source of electric power for remote homes but the high cost of the systems is delaying the implementation of this new technology. Since the main cost of the modules is that of the solar cells, concentrator designs that increase the output of each cell are a desirable way to decrease the cost per watt. Linear concentrators can operate at a ratio of up to 2/1 without need of any tracking arrangements. Care must be taken in the design to arrange for dissipation of the heat generated within the cells. Vertically oriented, passively cooled photovoltaic fins with linear curved concentrators were incorporated into the design of a new solar home being constructed by Terry Reeser near Louisville, Kentucky. Although the site is near a major urban area, it is located almost a kilometer from the nearest utility line. Installing conventional power would have been expensive. The first step in the design of such a system is a calculation of the expected power needs; Table 1 shows the expected and actual load requirements of the remote home. INSERT TABLE 1 PV HOME SYSTEM SIZING In addition to being a dwelling, the structure also serves as the base for two cottage industries. The computer is used to develop record keeping programs for a chain of video tape rental stores; the programs being sent through a telephone connection to the stores. There is also a small plastic injection molding machine to make small parts that are sold by mail order. The molding machine actually draws 900 watts but has a 1/3 time duty cycle. The use of the plastic molder is expected to grow so it was decided to expand the size of the photovoltaic array to furnish more energy than currently needed. Photovoltaic System Design The dwelling structure is a large enclosure built around a pre-existing house trailer. The entire south facing side of the structure is a two story greenhouse/sunspace with a 17 meter long by 2.5 meter high sloping roof used for mounting the hybrid photovoltaic fins. Figure 1 shows a cross section of the structure. We used a unique photovoltaic/hot air hybrid array with linear curved concentrating reflectors arranged vertically instead of the more normal horizontal placement. Komp (1985) found that instead of the more normal ratio of 2 to 1 or less, the efficiency penalty for a vertical system is only 5% in the summer and less in the winter when the hours of useful sun light are short. INSERTFigure 1 72 fins, each 2.5 meters long would just fit into the available roof space; 5 fins in each of 14 (1.2 meter) spaces between the roof rafters with 2 extra fins at the east end of the area. Figure 2 shows a cross section of a pair of fins and reflectors. The vertical oriented fins and reflectors form sloping channels for the air to rise as it is heated into the clerestory . The large contact surface area insures good heat transfer between the fins and the air. In the winter the warm air is drawn down to floor level by means of a PV powered fan. In the summer, the hot air exits through the open upper windows and the draft draws cooler air from the ground level into the dwelling. "The Solar Electric Home" (Davidson and Komp, 1983) contains practical details on the sizing and installation of this type of home PV array. INSERTFigure 2 System Construction and Assembly 100mm diameter round single crystal cells were soldered into long strings. After being tested, the strings were laminated onto the u-shaped aluminum channels using two part catalyzed silicone resin and polyester cloth between the cells and the aluminum to insure a good thermal contact and excellent electrical insulation. Details of the array construction are given in "Practical Photovoltaics" (Komp, 1981). A rather complex wiring scheme allowed 42 strings of 36 cells each to occupy the 72 fins. Table 2 shows the expected output of the system when completed. The KWh per month expected from the array is larger than now needed but growth in the use of the plastic molding machine is anticipated. INSERTTable 2 The 12V electric power from the system is fed through a 42 line fuze array to the battery bank through two pair of 000 buss cables. Thirty-two 6V golf cart storage batteries are series/parallel wired into two separate 12V banks to facilitate later conversion to a split 24V system as the power needs increase. Right now, charge and load control are done manually by monitoring the battery voltage and specific gravity. The state of charge on such a large system changes very slowly, necessitating a decision only every two or three days. All the lighting circuits are fed 12 VDC directly from the battery bank. This takes advantage of the greater efficiency of low voltage lighting and the better performance of high frequency fluorescent lamp ballasts. The stereo system, water pump and many other appliances are also DC powered to reduce the load on the inverter. The ac loads are fed from a Heart Interface 1200W inverter; its 3500W surge capacity allows for the starting current of larger motors. The ac wiring meets the usual US code requirements; the DC wiring is similar in the use of normal ac wire, switches and outlet boxes except that "auto cigarette lighter sockets" are used as outlets. The house trailer inside the greenhouse shell is now being modified and partially dismantled for increased interior space and as this progresses, the permanent DC wiring is being completed utilizing short runs of #12 or #10 wire to insure a low voltage drop. All DC wire connections should be soldered for low wiring resistance. Operation Experience to Date Since the photovoltaic array is just now being finished and only a few rows of completed reflectors have been installed, the data on power output is incomplete. However, the partial array has been furnishing electric power to the dwelling since occupation last winter. During the winter a back-up gasoline powered generator was used three times to recharge the batteries but since half of the fins in the system have been installed in March, this has been unnecessary. Some measurements have been made on both the electrical and heat output of the completed section of the array. At noon (local sun time) on a hot summer day with slightly hazy sun conditions (700W/m sun intensity) a single string of cells produced 1.3A without and 2.0A with reflectors (short circuit current). At an outside air temperature of 33¡C and 31¡C inside the greenhouse, the air exiting from the top of the reflector air passages was 50¡C, a 19¡C rise in air temperature for this thermosyphon system. The final reflector fins should be in place by the end of September and accurate instrumentation of the system should produce more complete data on the operation of this unique installation. System Cost The total cost of the installed PV hybrid system was less than $9,000 giving a cost of only $3.60 per peak watt, but this cost is unrealistically low since the cells were purchased surplus from a solar company shut down by its oil company parent and all the module assembly work was done on site. These costs, however, are for the complete installation including the inverter and battery bank and even including a normal labor rate, the system still is cost effective compared to the alternative of bringing a utility line from the nearest existing pole. Future plans include the installation of a wind generator for winter operation when long cloudy but windy periods are common. A combined PV/wind hybrid system is cost effective in this part of the US compared to either source of power alone. Future reports will detail the operating experience of this unusual system, believed to be the largest photovoltaic installation to date in Kentucky. References Davidson, J. and R. Komp (1983). "The Solar Electric Home". Aatec, Ann Arbor. Chap. 4, Chap. 5. Komp, R. (1981). "Practical Photovoltaics". Aatec, Ann Arbor, pp. 65-88. Komp, R. (1985). Field Experience and Performance Evaluation of a Novel Photovoltaic-Thermal Hybrid Solar Energy Collector. INTERSOL 85 PROCEEDINGS, Vol. 3, pp. 1748-1752. Richard Komp works with SunWatt Corporation, RFD Box 751, Addison, ME 04606, or telephone: 207-497-2204.