The promise of solar energy

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Solar energy is the main renewable energy resource throughout the world. Other renewable energy sources, e.g. biomass energy and wind energy, are derived directly from it. It is an abundant energy source. Our planet receives from the sun the equivalent of 15 000 times the energy consumed in the world, but this energy is diffuse and intermittent.

The power received at noon with no cloud cover is about 1 kW/m2. Given the day-night intermittence and weather fluctuations, the energy received in 24 h is between 2 kWh/m2 and 3 kWh/m2 in northern Europe, and between 4 kWh/m2 and 6 kWh/m2 in southern Europe or between the tropics.

Solar energy can be captured as either heat or electricity using the photovoltaic effect.

There is considerable scope for the development of low-temperature thermal solar energy in the short term. Heat is supplied by solar sensors consisting of a black absorbent surface which transfers the heat to a heat exchange fluid, generally a mixture of water and glycol to prevent the possibility of freezing. A glazed surface is fitted over the absorbent surface to block the infrared radiation re-emitted.

Selective coatings such as chromium oxide deposited on copper are used to reach temperatures of 70-90 °C, in order to limit the re-emission of infrared radiation. To reach temperatures of more than 100 ° C, sensors under vacuum are necessary, in particular to supply absorption solar air conditioning systems [64, 65].

In the housing sector, thermal solar energy is used mainly to provide sanitary hot water. It may also be used to cater for a certain proportion of heating requirements. For these applications, flat sensors have efficiencies in the region of 50%. An area of about 4 m2 of sensors is required to meet the hot water requirements of a family of four [64].

High-temperature thermal solar energy to produce electricity, requiring concentration sensors, is not yet profitable and its future prospects are still a subject of debate.

Photovoltaic electricity generates high hopes, although it is not yet directly competitive with the electricity produced in the current power stations. Consequently, the only present applications concern isolated sites for the supply of relatively reduced levels of power. Significant progress has nevertheless been observed. As with wind power, incentive policies can be set up to offset the cost of developments on a larger scale. In particular, the possibility of exporting some of the electricity produced to the electricity grid, at a sufficiently attractive purchase price, should promote deployment of photovoltaic panels in the housing sector. As mentioned in Chapter 5, integration of solar panels in the housing sector could revolutionise the future design of buildings.

Photovoltaic cells are assembled in modules. After a significant drop, the average cost of the production modules is currently tending to stagnate or even increase slightly due to a shortage of silicon, further to a rapid increase in demand. The cost of a system connected to the grid is about D 5/Wp3. For a system equipped with battery storage, this cost is between D 6/Wp and D 8/Wp. The cost of the kWh produced is between D 0.25 and D 0.5 without storage and between D1 and D 1.5 with storage [61].

The global installed power increased from 20MWp in 1985 to 37500MWp in 2005 [60]. Current forecasts predict an installed power of 66 400 MWp in 2020.

The market is currently dominated [80% share) by mono- or multi-crystalline silicon cells. We can expect further progress in the field of photovoltaic cells, especially by reducing the thickness of the silicon layers and by mass production. Thin layers are produced using multicrystalline silicon deposited on various substrates (10-40 mm layers) or amorphous silicon (1 mm layers).

3 Wp, watt of peak power. The peak power is the maximum power which can be delivered when solar radiation is at a maximum.

However, while the conversion efficiency of monocrystalline silicon cells is approximately 16-17%, the figure drops below 10% with multi-crystalline silicon thin layers. The efficiency of amorphous silicon cells is even less, which accounts for the fact that their market share has become very low. Mixed structures combining crystalline and amorphous silicon seem to offer a potentially interesting option.

Studies are being conducted on other materials such as copper-indium diselenide, from the family of chalcopyrites, which offers the possibility of high efficiencies, nearly 20%. Semiconducting organic materials may also be used. They are easy to implement; they can be used in the form of flexible sheets but, for the time being, their efficiency is at best about 5% and their lifetime is insufficient [62-64].

As prices continue to drop, photovoltaic electricity production should eventually become competitive.

Before we can expect to see widespread development in the use of photovoltaic cells, progress is required in the field of energy storage to overcome the problem of intermittent energy supply.

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