6.3 Solar electricity

Solar photovoltaic (PV) electricity

Solar energy can be harnessed directly to produce electricity using solar photovoltaic (PV) cells. They are made of specially prepared layers of semiconducting materials (usually silicon) that generate electricity when photons of sunlight fall upon them. Where silicon is used, it must be of an extremely high (99.9999%) purity. Since each cell only produces a low voltage, they are normally produced in ‘modules’ or ‘panels’ containing a large number of cells. These can range in size up to a metre square. Arrays of PV modules can be mounted on the roofs of domestic, commercial or industrial buildings, usually providing only some of their electricity needs. However, since 2005, an increasing proportion of installations have been in large grid-connected ‘solar farms’. Although their use has raised concerns about conflicts of land use for food production.

Large-scale production of PV panels has been under way since the 1990s and volume manufacture has resulted in falling prices. A critical factor has been the setting up, since 2005, of very large panel manufacturing plants in China and Taiwan. In 2023, approximately 95% of solar modules and their components came from Asia, primarily from China. China produced about 80% of PV modules and controlled more than 95% of the market for components such as silicon ingots and wafers (Fraunhofer, 2024).

The cost of electricity from large solar PV farms has now fallen to the point where it is highly competitive with that from fossil-fuelled generation (even in the UK). The result has been an extraordinary global growth in PV electricity generation right around the world. Between 2010 and 2023 it increased by a factor of nearly 50, an annual compound growth rate of 35% (Energy Institute, 2024). In 2023, PV electricity supplied over 5% of world electricity demand, including almost 5% of that in the UK.

Silicon PV panels have a limited efficiency, only turning about 17% of the incident light into electricity, so large areas are required to produce appreciable quantities of electricity. Considerable ingenuity is being used to finding suitable spaces, such as the large PV array on the roof of London’s Blackfriars railway station (Figure 20).

The world’s latest large-scale solar PV projects are in desert areas and on an extraordinary scale. In India, the Bhadla solar park in the Rajasthan desert covers 56 km², and has an installed generating capacity of over 2200 MW. Similar sized projects are under construction in China, Egypt and the United Arab Emirates. PV is also very useful on a much smaller scale, particularly when coupled with reliable rechargeable batteries. ‘Off-grid’ PV can supply electric light to many regions of the world beyond the reach of electricity grids, such as sub-Saharan Africa.

Concentrated solar power (CSP)

PV supplies the bulk of the world’s solar electricity. However, in many sunny countries, the Sun’s rays are strong enough to allow the production of high-temperature steam using arrays of concentrating mirrors. This can then power steam turbines for electricity generation. Such concentrated solar power (CSP) systems using parabolic mirrors have been operational in California since the 1980s and are now regarded as a ‘mature technology’. More recently, large ‘power tower’ systems using steerable arrays of flat mirrors called heliostats have been built in the USA, Spain, the Middle East and China (see Figures 21 and 22).

The large size of the systems (10 MW or more) is a result of the use of ‘standard’ steam turbines, as manufactured for fossil-fuelled industrial power plants. CSP requires clear skies to operate, unlike the rival photovoltaic panels which can also produce electricity under cloudy conditions. However, CSP plants, being thermal, have a certain flexibility of operation. The most important addition is heat storage using a molten salt such as sodium or potassium nitrate. This potentially allows continuous solar-powered electricity generation right through the day and night.

Future growth

Electricity from CSP is more expensive than that from PV, so most of the future solar growth is likely to be in PV. Projections of future PV deployment see continued rapid growth.

The International Energy Agency (IEA) in its 2023 World Energy Outlook suggests that world PV electricity might increase ninefold from its 2022 value by 2040 (IEA, 2023). It could then be supplying over a quarter of the world’s electricity. However, meeting a ‘Net Zero by 2050’ target requires even faster growth. That shown in Figure 17 requires that the annual amount of electricity generated globally by both solar PV cells and CSP plants should increase by a factor of almost 20 by 2040. Globally, these systems could cover about 70 000 km², equivalent to about one third of the area of the UK.