Most of the solar power systems currently in existence are solar thermal systems, which capture the sun's warmth to heat water. Solar thermal energy has obvious potential in warm climates; in Cyprus, for example, the majority of homes and businesses have solar thermal panels fitted to their roofs. Solar thermal energy can also be focused on a heat exchanger and converted into a heat engine to produce electric power or applied to other industrial processes.
Solar cells, also known as PV cells, are devices that use the photovoltaic effect of semiconductors to generate electricity from sunlight. While solar cells have been used for many years in low-power devices such as calculators, their use on a larger scale has been limited because of high manufacturing costs and concerns over cost efficiency.
Nonetheless, while solar PV remains relatively expensive compared with conventional electricity, declining manufacturing costs are expanding the range of cost-effective uses. Japan, Germany, Spain, the US and Australia have all begun to aggressively push ahead with solar PV. Taxes, subsidies and building regulations are deployed to encourage homes and businesses to use solar PV plates to generate their own electricity, with the result that in many cases systems are able to pay for themselves in five to 10 years.
However, a shortage of refined silicon between 2004 and 2005 hampered the growth of the industry. Silicon producers have since begun to recognise the market's needs and increased production, suggesting that PV may finally play a major role in countries' renewable electricity profiles. This belief has gained impetus from the emergence of China as a major producer of PV. Ray Noble, commercial manager at BP Solar in the UK, has said that the timescale for getting to the $1-per-watt mark has been significantly reduced by developments in China.
The Chinese government has stated it wants 10% of electricity and 5% of primary energy to come from renewables by 2010, rising to 20% and 15% by 2020. The targets for PV are 450MW by 2010 and 8GW by 2020 - which, if achieved, would put China ahead of the world in PV use. Shanghai has announced a 100,000 PV roof programme to install 70MW of PV by 2010. Dongtan, the world's first 'sustainable city', will feature both solar water heating and building-integrated PV.
But some in the Chinese government are understood to regard PV as too expensive, and many believe that PV will take off in China only if and when the government imposes a feed-in tariff, which would oblige utilities to buy PV electricity at a set price.
Wind is the fastest-growing renewable energy technology. Installed capacity across the world has risen at an annual growth rate of nearly 30% since the early 1990s, and long term the technical potential of wind energy is thought to be several times current production global energy demand. Most wind farms are onshore, though there is arguably greater potential for offshore developments, given the vast amount of space and higher wind speeds at sea. Wind power is used in large-scale wind farms for national electrical grids, as well as in small individual turbines for providing electricity to rural residences or grid-isolated locations. At the end of 2006, wind still produced less than 1% of the world's electricity use, though it accounts for 18% in Denmark, 9% in Spain and 7% in Germany.
Because wind speeds fluctuate in all places across the globe, power from a wind facility cannot be guaranteed unless it is combined with other energy sources or storage systems, meaning that it must be connected to the grid. Some estimates suggest that 1,000MW of conventional wind generation capacity can be relied on for just 333MW of continuous power. This may change with improved technology, but for the foreseeable future wind power will be effective only when it is incorporated with other power sources or a storage facility.
The viability of a business generating its own electricity through on-site turbines will be heavily dependent on its location. But in some countries energy suppliers are offering their clients the opportunity to source some or all of their energy needs through wind power. Gordon McGregor, group environmental director of Scottish Power, which has developed extensive wind power capacity, says: "We offer green energy contracts to business customers, ranging from 100% to 10%. It costs more than other forms of energy, but firms in the UK that buy renewable energy from their suppliers are exempt from the climate change levy, which makes it cost-effective."