Linear Concentrator Systems
Introduction
Linear concentrating collector fields consist of a large number of collectors in parallel rows that are typically aligned in a north-south orientation to maximize both annual and summertime energy collection. With a single-axis sun-tracking system, this configuration enables the mirrors to track the sun from east to west during the day, ensuring that the sun reflects continuously onto the receiver tubes. Parabolic Trough Systems
The predominant CSP systems currently in operation in the United States are linear concentrators using parabolic trough collectors. In such a system, the receiver tube is positioned along the focal line of each parabola-shaped reflector. The tube is fixed to the mirror structure and the heated fluid—either a heat-transfer fluid or water/steam—flows through and out of the field of solar mirrors to where it is used to create steam (or, for the case of a water/steam receiver, it is sent directly to the turbine).
Currently, the largest individual trough systems generate 80 megawatts of electricity. However, individual systems being developed will generate 250 megawatts. In addition, individual systems can be collocated in power parks. This capacity would be constrained only by transmission capacity and availability of contiguous land area. Illustration of a linear concentrator power plant using parabolic trough collectors. Sunlight is shown reflecting off the parabolic troughs and onto the receivers positioned along the focal line of each trough. The hot heat-transfer fluid (shown as red) exiting the receivers flows to a turbine, generating electricity that is fed into the power grid. The cool heat-transfer fluid (shown as blue) exiting the turbine flows into a steam condenser to be further cooled and sent back into the solar field. Thermal storage tanks are shown between the turbine and the steam condenser.
A linear concentrator power plant using parabolic trough collectors.
Trough designs can incorporate thermal storage. In such systems, the collector field is oversized to heat a storage system during the day that can be used in the evening or during cloudy weather to generate additional steam to produce electricity. Parabolic trough plants can also be designed as hybrids, meaning that they use fossil fuel to supplement the solar output during periods of low solar radiation. In such a design, a natural-gas-fired heater or gas-steam boiler/reheater is used. In the future, troughs may be integrated with existing or new combined-cycle natural-gas- and coal-fired plants.
Linear Fresnel Reflector Systems
A second linear concentrator technology is the linear Fresnel reflector system. Flat or slightly curved mirrors mounted on trackers on the ground are configured to reflect sunlight onto a receiver tube fixed in space above these mirrors. A small parabolic mirror is sometimes added atop the receiver to further focus the sunlight. Illustration of a linear concentrator power plant using linear Fresnel collectors. Sunlight is shown reflecting off the mirrors on the ground and onto receivers located above the mirrors. The hot heat-transfer fluid (shown as red) exiting the receivers flows to a turbine, generating electricity that is fed into the power grid. The cool heat-transfer fluid (shown as blue) exiting the turbine flows into a steam condenser to be further cooled and sent back into the solar field.