The photovoltaic solar cell

The silicon photovoltaic (PV) solar cell51 consists of a thin slice of silicon into which appropriate impurities have been introduced to create what is known as a p-n junction. The most efficient cells are sophisticated constructions using crystalline silicon as the basic material; they possess efficiencies for conversion of solar energy into electricity typically of 15% to 20%; experimental cells have been produced with efficiencies well over 20%. Single crystal silicon is less convenient for mass production than amorphous silicon (for which the conversion efficiency is around 10%), which can be deposited in a continuous process onto thin films. Other alloys (such as cadmium telluride and copper indium diselenide) with similar photovoltaic properties can also be deposited in this way and, because they have higher efficiencies than amorphous silicon, are likely to compete with silicon for the thin-film mar-ket.52 However, since typically about half the cost of a solar PV installation is installation cost, the high efficiency of single crystal silicon, which means a smaller size, remains an important factor. A number of new PV materials or devices are also under intensive investigation some of which are beginning to compete in terms of efficiency or cost.

Cost is of critical importance if PV solar cells are going to make a significant contribution to energy supply. This has been coming down rapidly. More efficient methods and larger-scale production are bringing the cost of solar electricity down to levels where it can compete with other sources. The decline in cost with increase in installed capacity over the last 25 years and a projection for the next 5 years is illustrated in Figure 11.18.

20-i

2002

2007

2012

£ 1 10 100 1000 10 000 100 000 Cumulative installed capacity (in MW)

Figure 11.18 The increase in installed capacity and the falling cost of PV modules over the last 25 years and (dashed) projected into the future (data from 1982-2002 from Shell Renewables and from 2002-2007 from Energy Technology Perspectives, International Energy Agency 2008).

In CSP, to produce a sufficiently high temperature at the boiler, the solar energy has to be concentrated using mirrors (Figure 11.17). One arrangement employs trough-shaped mirrors aligned east-west which focus the Sun on to an insulated black absorbing tube running the length of the mirror. A number of such installations have been built, particularly in the USA, where such solar thermal installations provide over 350 MW of commercial electricity. Developments that are currently being pursued are of integration of solar and fossil fuel heat sources in combined cycle operation to enable continuous electricity provision throughout the day, and, in arid areas, of co-generation of power and heat for desalination for the delivery of fresh water.

Energy from the Sun: solar photovoltaics

Turning now to photovoltaic sources of electricity, solar panels on spacecraft have provided electrical power from the earliest days of space research 50 years ago. They now appear in a host of different ways in everyday life; for instance, as power sources for small calculators or watches or for lighting of public areas in remote places. Their efficiency for conversion of solar energy into electrical energy is now generally between 10% and 20%. A panel of cells of area 1 m 2 facing full sunlight will therefore deliver between 100 and 200 W of electrical power. A cost-effective way of mounting PV modules is on the surface of manufactured items or built structures rather than as free-standing arrays. In the fast-growing building-integrated-PV (BIPV) sector, the PV façade replaces and avoids the cost of conventional cladding. Installed on rooftops in cities, they provide a way for city dwellers to contribute renewably to their energy needs. Japan was the first to encourage rooftop solar installations and by 2000 had installed 320 MW capacity. Germany and the USA followed with large rooftop programmes, Germany with a target of 100 000 roofs that was met by 2003 and the USA with a target by 2010 of 1 million roofs. The cost of energy from solar cells has reduced dramatically over the past 20 years (see box) so that they can now be employed for a wide range of applications and providing the fall continues can also begin to contribute to the large-scale generation of electricity.

Solar energy schemes can be highly versatile in size or application. Small PV installations can provide local sources of electricity in rural areas especially in developing countries. About a third of the world's population have no access to electricity from a central source. Their predominant need is for small amounts of power for lighting, radio and television, refrigerators and air conditioning and for pumping water. The cost of PV installations for these purposes is now competitive with other means of generation (such as diesel units). Over the 20 years to the year 2000, over 1 million 'solar home systems' and 'solar

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