operations from the raw material to the completed array, possibly in part similar to integrated circuit fabrication. A number of ideas have been advanced for individual process steps which could be important to the establishment of such integrated processing. Foremost among these are methods for obtaining ribbons or sheets of single crystal or large crystal silicon by (1) edge-defined film growth [42], (2) dendritic growth [43], (3) rolling silicon [44], and (4) casting sheets with subsequent recrystallization through heated or molten zones [45]. Once such single or large crystal sheets are available, integrated solar arrays could be formed from them by possibly as few as 7 operations, e.g., vapor deposition, heating, and ion implantation in a continuous, flow through process. Optical filters with a comparable number of such operations are now quoted at $0.40/ft.2. Another approach consists of automated assembly of small silicon spheres, produced by a modified shot tower method, into a large plastic encapsulated assembly. All of these approaches require experimental verification of technical feasibility. Several cost analyses have been provided which show possibility of reaching the economic goal of generating cost-competitive electric power [42,46]. Silicon itself is the second most abundant element in the earth's crust and is produced in the U.S. at the annual rate of 66,000 tons in metallurgical purity at $600/ton [46], which is a sufficiently low basic material cost. Thin film Cu2 S-CdS solar cells have been prepared in pilot line quantities (total 10 kw) for over 12 years, with efficiencies of 4 to 6 percent for terrestrial applications, and over 8 percent reported on development models [47]. The processes used for their fabrication appear readily amenable to low cost mass production 56-516 O 75 Pt. 1C - 27 methods. In the past, the thin film Cu2 S-CdS solar cells have been plagued by degradation and low yields for long life [48]. Interest in the U.S. has been reawakened for terrestrial applications, and recently reported process changes are hoped to result in long life arrays [47,49]. A cost estimate for a system using Cu, S-CdS solar cells and including power conditioning, system protection, installation, and a 15% annual cost of capital, taxes, and maintenance, has arrived at a cost of 2 to 44/kwh of generated electricity [50]. Other semiconductor materials have not been sufficiently developed to be of interest for extensive solar energy applications at this time but research into new methods of solar energy conversion should be initiated [51]. Current R&D is primarily directed towards increasing size and decreasing weight of spacecraft power systems, increasing resistance against radiation in space, and increasing the efficiency of silicon solar cells towards 20 percent in space. This work is supported by NASA and the U.S. Air Force at a combined annual level of approximately $3 million. NSF under the RANN program and NASA under its Applications program have an interest in terrestrial applications, and are beginning to support R&D towards low cost photovoltaic approaches. During the last year, one company has started to develop reduced cost photovoltaic systems in an effort to expand the market in "packaged power" as a step towards large scale terrestrial applications of photovoltaic systems. Also, a group of companies has pooled capabilities to develop the concept of obtaining power from space. Photovoltaic Systems on Buildings CONCEPT DESCRIPTION The application of photovoltaic arrays on buildings locates the generator at the place of the load, reducing the need for energy transmission and distribution with the associated losses and costs. The system thus matches the distributed nature of solar energy to the distributed pattern of energy consumption. In this approach, photovoltaic arrays are mounted on buildings or form part of their structures, the latter presenting economic and possibly aesthetic advantages. The basic installation does not differ from that of solar thermal collectors. A particularly attractive approach is to combine the photovoltaic array with a flat plate thermal collector [52,53], since most buildings require both thermal energy (hot water supply, space heating, absorption refrigeration, air conditioning, low temperature process heat, etc.) and electrical energy (lighting, motive power, electronics, high temperature process heat, etc.). In this case which is illustrated in Figure 13 the absorber surface of the thermal collector is formed by the solar array which converts a portion of the incoming solar energy into electrical energy, and permits collection of about 50% of the remaining energy in the form of heat. Although some of the efficiency of the solar array is traded off for thermal energy collection, this combination system provides advantages: up to 60 percent of the available solar energy can be utilized; ⚫ the thermal collector uses the same land area as occupied by the buildings; and ⚫ the components fulfill several functions, yielding a more cost effective system. As in all terrestrial solar energy utilization systems, energy storage is required. In general it is considered uneconomical to provide local storage capacity for more than an average day's requirement [6]. Thus, auxiliary energy will be needed at times. The overall system concept shown in Figure 14 includes several subsystems: solar array, electrical power conditioning, energy storage, auxiliary energy, and possibly power line interfaces. Storage can be provided by batteries, electrolysis cell-fuel cell systems, flywheels, etc. [54,55,56]. Benefits of the use of photovoltaic systems on buildings are: • Minimal effect on the ecology through use of land areas already being used for other purposes. About three times the present average household consumption of electric power can be collected from average-size family residences, even in the northeastern U.S. |