Although the potential of solar thermal collector systems is enormous, their contribution to the global primary energy supply is still rather low. Assuming 70 million m2 of installed glazed collectors by the end of 2001 and an average heat production of 600 kWh/m2, the total global solar thermal heat production was about 150 PJ. This is equal to 0.04 per cent of the global primary energy demand. China is by far the world's largest solar water heater manufacturer and user. By the end of 2002, the accumulated installed area of solar domestic hot water systems in China was about 40 million m2. Greece had a noticeable installed area per capita of 264 m2 per 1000 inhabitants by 2001. Projecting these per capita numbers to the global population, the total installed area would increase more than 20 times. Table 1.15 shows the number of installations of the countries with the most significant solar collector markets.
Due to its frequent non-commercial use in developing countries, solid biomass is by far the largest renewable energy source, representing 10.4 per cent of world total primary energy supply, or 77.4 per cent of global renewables supply in 2001. Africa covers about 50 per cent of its energy supply by biomass; some countries such as Mozambique or Tanzania have even higher figures of more than 90 per cent. High growth rates have been experienced by non-solid biomass combustible renewables and waste, such as renewable municipal solid waste, biogas and liquid biomass. This segment grew annually on average at 7.6 per cent since 1990. These growth rates are expected to continue even in industrial countries.
Geothermal energy is also used for heat production. However, the global share is relatively low. In the long term, hydrogen produced by renewable energy sources could achieve a significant share. However, since production costs of hydrogen are very high, it will take several decades to have a noticeable impact on world energy supply.
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