The principle here is that a 240-litre tank is heated by flat plate solar collectors supplemented by a gas burner within the body of the cylinder. A heat exchanger coil in the upper half of the tank heats the central heating circuit. A production solar-gas combination boiler with 2.83 m2 of solar collectors produces 4GJ/year;3 a 5.4 m2 collector supplies 5.64GJ/year. A test involving 25 units showed boiler efficiency of 90.1%. On average, consumers saved 650 m3 of natural gas compared with conventional systems. At 2000 prices this gave a payback time of 13 years; at today's prices it will be much shorter. Another factor to consider is that it would be carbon neutral if biogas were employed (see Fig. 2.8).
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.