SOLAR POWER: In depth

(Source: Engineer)A MATTER OF FOCUS

The focusing apparatus - known as the concentrator - is key to CSP technology Concentrating solar power (CSP) is a thermal technology; that is, it generates electricity by boiling water, then forcing the high-temperature, high-pressure steam through a series of turbines. In this sense, it is analogous to every baseline electricity generation technology. Rather than burning fossil fuel or tapping the heat of a nuclear reaction, it uses the heat of the sun as it hits the Earth's surface.

In order to do this, it has to focus the sun's rays. Rather than heating water directly, it focuses solar energy onto a working fluid with a high specific heat capacity. This runs in a circuit, with the hot fluid generating steam by passing its energy on to water in a heat exchanger to send steam to the turbines. The focusing apparatus - the concentrator - can be a mirror or a lens, although mirrors are generally preferred.

There are three types of concentrator arrangement. The most established is a linear trough with a parabolic cross-section; the working fluid runs in a pipe at the focus of the parabola. A variation on this uses a linear Fresnel lens - a flat sheet of glass etched with a pattern which focuses light passing through it as though it were a full-thickness converging lens - with the working fluid running below the lens. These linear concentrators have been in use for around 25 years.

The second-generation technology is a point-focused system, with a field of mirrors reflecting energy to the top of a 'power tower' where the working fluid is heated. The heat exchangers sit at the base of the tower, with the turbines sited adjacent. Several plants of this type are now in operation in southern Spain.

The third type of system is known as point-focused distributed. The concentrators are dishes, aiming the sun's heat at the focus of the parabolic dish. This raises the possibility of installing a Stirling engine - a simple closed-circuit heat engine, which converts heat directly into mechanical work and then electricity - at the focal point. Currently, only one dish-based installation is working on a commercial basis - a 500kW facility near Phoenix, Arizona. Several experimental installations are also operating in Spain and south-western US.

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