Figure 2.4: Prognosis published 1975 for cost of solar cells The solar cell in itself is very thin (the active layer approximately 0.01 mm) From a material's point of view it is thus reasonable to suppose that the costs could be reduced very drastically. A solar cell produced today needs approximately 12 years to repay the energy used in manufacturing it (30) - i.e. produce the amount of energy used during the production process from extraction of silicon from sand to the finished product. A reduction of the 18 energy needed by a factor of 10 is expected (30). It would then take 1 2 years for the solar cell to deliver as much energy as has been used in producing it (24). A solar cell needs to be mounted on some support. In the OTA-study (31) the costs for mounting the cells have been indicated. These are based on studies by General Electric and Westinghouse. Installation of solar cells on roofs is said to cost approximately 40 Sw. Crs/m2. At the same time the cost for roofing is saved. This is in the same order of magnitude. Mounting solar cells on roofs and walls would mean limited extra cost for installation. Basically only the cost for the solar cell itself need be included in the calculation. Solar panels could also be mounted over parking lots etc. The cost for surface and pillars is given as approximately 60 Sw. Crs/m2 of covered area which gives approximately 100 Sw. Crs/m2 including installation. Electricity from solar cells must be stored in some way in order to get correspondence in time between demand and production. There is need for both shorttime storage (in the order of 24 hours) and for storage over longer periods. For shorter periods water power, pump storage or batteries can be used. The need for storage over longer periods is caused by the fact that insolation is greatest during the summer months whereas demand for electricity is greatest during the winter (cf fig. 2.3). Of the energy produced by solar cells at most 35 - 40% needs seasonal storage (34). This storage can in principle be done in the same way as short-term storage. An important difference however is that the number of storage cycles (recharging decharging) per year is very small with seasonal storage. Using the same methods as for short-term storage thus becomes expensive. If cheap hydrogen storage can be developed electrical energy could be stored in this form. If not, it would seem most appropriate to try to store it in the form of solid or fluid fuels. Hydrogen and oxygen could be produced from water by 19 electrolysis. From these components, just as from biomass, methanol can be produced. This is simple to store and can, e.g. via fuel cells, be used to produce electricity and heat. The figures 35 - 40% seasonal storage refer to a case where the energy from solar cells must be distributed over the year in accordance with the demand for electricity. However, since both wind power and the contribution by co-generation are greatest during the winter the need for storage capacity is smaller. Development is going on along several lines to catch and store solar energy, e.g. direct production of hydrogen or some other chemical compound, which can be stored (8, 24, 35). This work may produce methods that are more interesting than solar cells of the type discussed above. Solar heating of buildings can be done in several ways. The simplest is to utilize the radiation from the Sun coming through tne windows. By orienting buildings and their windows so that solar radiation can be utilized significant quantities of solar energy can be obtained. This presupposes in most cases individual thermostats in each room. A first measure to use solar heat is therefore systematically to orient new buildings towards the south. We do not think that this poses a great problem in most cases, but it probably needs specific directives in building codes and norms. The extra energy supplied per flat in this way is in the order of some 1,000 kWh annually and is thus considerable. In the examples given below this form of using solar energy is not included. Other methods are based on active systems for catching the Sun's energy. In principle completely self-supplying buildings are possible. The amount of radiant energy per year is sufficient for providing the necessary energy with a reasonable area of solar collector. 20 Production and storage of heat from solar radiation is an existing and functioning technology. The problem is that the heat must be stored from summer to winter, cf fig. 2.5. Low cost systems are at the moment at an experimental stage (8, 24, 37). Several development projects exist both in Sweden and elsewhere. These include both design of solar collectors and storage in the ground, in large water recervoirs, in molten salts (utilizing the heat of melting) in distillation storage etc. (8, 24, 37). Figure 2.5: Schematic illustration of the variation in insolation in relation to the need for heating of a house on the latitude of Stockholm (60°N). The need and possibility for storage are indicated (28). With the use of large heat storage (equivalent to the energy needs of some dozen houses) the cost per stored unit of energy can be cut, and the heat losses reduced in seasonal storage. AB Atomenergi has proposed a combination of solar collectors and heat storage for a group of houses (cf fig. 2.6). The storage is a large hole in the ground which has been water-proofed with concrete. Insulation on top is with floating blocks of plastic. On top of these blocks solar collectors and mirrors are placed (38). The State Power Board (39) has constructed a similar example. Both find that storage common to several houses already today seems to be economically viable and advocate the building of test facilities. 21 One company, Sunvex AB in Växjö, has a grant application with the Council for Energy Production Research for a plant and claim that with present energy prices the economy of their plant is only some tenths of percent worse than present competing alternatives. The installation utilizes parabolic conduit pipes, that track the Sun during the day and give a water temperature of 130°C. The high temperature produced makes this method interesting also for existing district heating systems. Figure 2.6: In a centralized system the solar collectors can be placed near the storage and/or on the individual houses. The storage tank, dam or underground cavity is connected to the buildings with a culvert system of the same type as a conventional district heating system (28). Active solar heating systems will have heat storage capacity for periods of days to weeks (for evening out differences in insolation between days) (37). In this way 40 - 75% of the heating needs (including hot water) can be met. The rest can either be met by storing energy in some form from summer to winter (cf above) or by using electricity or fuel (supplementary heating). If there is heat storage capacity it is not necessary to supply the supplementary heat when the need for heat is greatest. It can instead be supplied via the heat storage. This means that the demands for high |