the roofs (which save 2% of the country’s fuel bill) and take up no land area. These rooftops could accommodate ten times this area of collectors or PV panels if a local use could be found for the heat collected or, in the case of PV panels, if the electricity could be coupled into the grid. The design of buildings to exploit solar radiation to reduce consumption of fuels for heating and lighting is another example of overcoming the solar ratio limitation in part: more space must be left between the buildings than would otherwise be the case. The dream of each household generating its own electricity needs from the sun is still a long way off. This is not just a question of cost (currently between 50-100 US cents /kWh) but of statistical load sharing: batteries could help on short-term storage, but seasonal storage would be very difficult. Much fossil fuel is consumed for the propulsion of vehicles. Power for vehicles could come from the sun using bio-coverters producing oils directly — such as the copafera plant proposed by Prof. Calvin — but the net conversion efficiencies are extremely low, thus severely limiting this approach. Alternatively, electric vehicles could be driven with electricity produced from the sun by any of the proposed methods, with the advantage that this integrates with non-solar sources of electricity. So far we have discussed limitations of availability of solar energy. But costs are also a limitation. The simplest, high-efficiency thermal converters — for such purposes as water heating — are currently economically viable in sunny areas. As one moves to less sunny areas and to more sophisticated collector systems, the economic viability decreases. In particular, the sophisticated tracking systems may involve high maintenance costs and at least two recent reports from the field on tracking trough-type collector systems have shown that the value of the energy produced did not cover the cost of maintenance! From this we have to pursue the philosophy that the sophistication should be in the thinking, not in the hardware. (A classical example of this is the salt-gradient solar pond where the scientific background and engineering planning are both highly sophisticated, leading to a relatively simple low-cost collector system). The limitations of solar energy availability mentioned above do not provide any excuse for not exploiting solar energy wherever possible, any more than Carnot’s restrictive law prevents engineers burning millions of tons of fuel every year to produce power in a highly ‘inefficient’ process! In particular, the high solar ratio in the developing world indicates that, in many areas, there will be ample solar radiation (provided these areas are not in too much of a hurry to catch up — in energy consumption — with the industrialized world). Since the areas of high solar ratio are those with low energy consumption, an important field for R and D is that of energy transport over long distances. High- temperature dissociation of chemicals to form gases that can be transported by pipeline and re-combined at the far end, is being studied as a possible means of low-cost energy transport. The limitation in insolation availability point to two lines of approach, apart from a constant effort to raise conversion efficiencies: 1. To find ways of using part of the 2/3 of total solar radiation that falls on the oceans. OTEC (ocean temperature energy conversion) is one example, but the difficulties are considerable. Other examples are extension of marine agriculture and the concept of underwater habitations to reduce the congestion on the land areas. 2. To go into outer space to collect solar radiation that would not, otherwise, reach
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