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About Troughs

Parabolic troughs are the only commercially available solar concentrator that can be used to deliver high temperature thermal energy.

Parabolic troughs are the most utilitarian of solar collectors in terms of the markets they can serve. Troughs can deliver heat at temperatures ranging from 100 - 560 F (40 - 290 C) for applications such as hot water, space heating, air-conditioning, steam generation, industrial process heating, desalination and power generation.

It is a principle of geometry that a parabolic reflector pointed at the sun will reflect parallel rays of light to the focal point of the parabola. A parabolic trough is a one-dimensional parabola that focuses solar energy onto a line. Physically, this line is a pipe with a flowing liquid inside that absorbs the heat transmitted through the pipe wall and delivers it to the thermal load.

A trough captures sunlight over a large aperture area and concentrates this energy onto a much small receiver area, multiplying the intensity of the sun by a concentration ratio in the range of 30-80. It is the process of concentration that allows troughs to delivery high temperature thermal energy. However, to achieve such concentration, a trough tracks the sun in one axis continually throughout the day. The required tracking accuracy is within a fraction of a degree.

Establishing the concentration ratio is the major tradeoff in designing a trough concentrator. The goal is to balance the interception of solar energy at the receiver against heat losses from the receiver. The larger the absorber diameter the greater the heat loss from the absorber area. However, the absorber must be large enough to intercept most of the sunlight reflected from the mirror. This intercept is affected by factors such as the accuracy of the parabola, the size of the solar disk (the sun is not a point source), the quality of the reflector, the accuracy of collector tracking and location of the receiver with respect to the true focal point.

To maximize the sunlight incident on the absorber, the reflectance of the parabolic reflector must be as high as possible. Aluminum or silver reflectors are used. Silver has the higher reflectance, but is harder to protect against the corrosive effects of the outdoor environment. It is also important to keep the reflectors clean since dirt will degrade the reflectance of light from the parabola.

The receiver of a trough concentrator is typically a metal absorber surrounded by a glass tube. The absorber is coated with a selective surface. This is a surface that has a high absorptance for incoming light in the visible range, and a low emittance (or radiative loss) in the infrared wavelength. The surrounding glass insulates the pipe from the effects of the wind and greatly reduces convective and conductive heat loss. The gap between the absorber tube and the inside of the glass is sized to minimize heat loss across the air gap. Glass is also a radiation barrier to infrared light so it reduces heat loss due to radiation. Since the light from the parabola must first pass through the glass before it hits the absorber, the glass is a source of optical inefficiency since some light is reflected from the inside and outside glass/air surfaces, and absorbed in the glass itself. The negative effect of the glass tube is reduced by coating it with an anti-reflective surface to minimize optical losses due to reflectance.

Taking all these factors into account, the peak optical efficiency of a parabolic trough is in the range of 70-80%. Since thermal losses from the receiver are relatively small and increase only moderately as operating temperatures increase, at peak conditions, a trough can be expected to deliver 60+% of the radiation incident on the collector even when taking into account heat losses in the solar field piping.

Parabolic troughs are highly modular. Troughs are aimed at commercial and industrial markets, but they can be configured in any reasonable collector area to meet the desired load. Though east-west or north-south orientation of the collector axis is typically specified for year-round or summer-peaking loads, respectively, troughs can actually be oriented in any direction. The arrangement of troughs in parallel rows simplifies system design and field layout, and minimizes interconnecting piping. Some rough models that can be mounted on the ground or on a roof.

Tracking of a parabolic trough involves fixed costs associated with the drive and control system. In large systems for commercial and industrial applications, costs for the drive and control system are relatively less pronounced and the cost of the collectors dominates the overall system cost. Materials are a major component of collector costs.  Compared to a flat plate collector, a parabolic trough module is 3 to 4 times less weight, and consequently large trough systems are less costly that equivalent flat plate or evacuated tube collector installations.

Though the tracking of troughs involves more maintenance compared to flat plate and evacuated tube collectors, the cost of electricity to power trough systems is less because pumping power to circulate the collector fluid is reduced several times. Importantly, troughs can meet temperature demands for energy far beyond the capabilities of none tracking collectors.