The third type of solar thermal power unit is the solar dish. A solar dish is more accurately a parabolic mirror, at the centre of which is placed a small heat collector and electricity generator. The reflector tracks the sun and focuses its energy onto the collector.
Unlike the two preceding technologies which are being developed for utility scale generation, the solar disk will always be a relatively small-scale
electricity plant. Those currently being tested have diameters of between 5 and 15 m and outputs of 5-50 kW. Larger dishes seem possible and there is a plan to build one with a capacity of greater than 1 MW, but even so utility multi-megawatt capacities can only be achieved by installing large numbers of individual units.
The two key components of a dish system are the parabolic reflector and the heat engine. Since the reflector must track the sun, a tracking system must also be included. Reflectors can be made using traditional glass-based techniques but these are very heavy and new, lighter fabrication methods are needed to bring down costs.
The most popular type of engine for use with a solar dish is a sterling engine. This is a piston engine (see Chapter 6) but a piston engine in which the pistons are part of a completely closed system. The energy source, heat, is applied externally. Consequently this is perfectly suited to solar dish applications.
The solar dish is the most efficient of all the solar thermal technologies. The best recorded solar-to-electrical conversion efficiency is 30%, but the Stirling engine is theoretically capable of 40% efficiency. This is of importance because of the area needed for a solar power plant. While parabolic trough systems require 2.2-3.4 ha for each megawatt of generating capacity, solar dishes need 1.2-1.6 ha.4
Solar dishes are currently expensive but costs can be reduced significantly. However they are unlikely to be a cost effective as the solar tower. Their main use is likely to be for stand-alone remote generation where their high efficiency and reliability could eventually challenge that of the solar cell, the solar device currently used most widely for such applications.
The solar photovoltaic device, more commonly known as the solar cell, exploits a completely different means of converting sunlight into electricity. This depends on the physical characteristics of materials called semiconductors.
The solar cell is a solid-state device which shares a heritage with the diode, the transistor and the microchip. It was developed in the Bell Laboratories in the early 1950s and soon found action in the US space programme. Today it remains the most widely used means of providing electric power to satellites and space vehicles.
Solar cells began to be used for terrestrial applications during the 1980s, mainly in remote locations where reliable power was needed without regular human intervention. As costs began to fall (although they remained extremely expensive) their use was extended to a wider range of applications. From 1990 onwards, grid-connected solar cells began to appear in domestic and some commercial applications. This usage continued to expand during the first years of the twenty-first century.
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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.