Solar thermal collectors and applications
Parabolic trough collector systems
Several parabolic trough solar thermal systems have been build and operated throughout the world. Most of these systems provide process steam to industry. They displace fossil fuels such as oil or natural gas as the energy source for producing steam. These systems incorporate fields of PTC having aperture areas from 500 to 5000 m2. Most of these systems however supply industrial process steam from 150 to 200 °C.
The most current example of power production using parabolic trough is the nine commercial solar energy generating systems (SEGS). The total installed capacity of SEGS is 354 MW and are designed, installed and operated in the Mojave Desert of Southern California. These plants are based on large parabolic trough concentrators providing steam to Rankine power plants. The first of these plants is a 14 MW electric (MWe) plant, the next six are 30MWe plants, and the two latest are 80 MWe [65].
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The plants can supply peaking power, using solely solar energy, solely natural gas, or a combination of the two, regardless of time or weather, within the constraint of the annual limit on gas use. The most critical time for power generation and delivery, and the time in which the selling price of the power per kW h is highest. This is between noon
and 6 p. m. in the months from June to September. Operating strategy is designed to maximise solar energy use. Natural gas is used to provide power during cloudy periods. The turbine-generator efficiency is best at full load, therefore the use of natural gas supplement to allow full-load operation maximises plant output.
A schematic of a typical plant is shown in Fig. 43. As it can be seen the solar and natural gas loops are in parallel to allow operation with either or both of the energy resources. The plants do not have energy storage facilities. The major components in the systems are the collectors, the fluid transfer pumps, the power generation system, the natural gas auxiliary subsystem, and the controls.
A synthetic heat transfer fluid is heated in the collectors and is piped to the solar steam generator and superheater where it generates the steam which drives the turbine. Reliable high-temperature circulating pumps are critical to the success of the plants, and significant engineering effort has gone into assuring that pumps will stand the high fluid temperatures and temperature cycling. The normal temperature of the fluid returned to the collector field is 304 °C and that leaving the field is 390 °C. Experience indicates that availability of the collector fields is about 99% [97].
The reflectors are made of black-silvered, low-iron float - glass panels which are shaped over parabolic forms. Metallic and lacquer protective coatings are applied to the back of the silvered surface, and no measurable degradation of the reflective material has been observed [97]. The glass is mounted on truss structures, with the position of large arrays of modules adjusted by hydraulic drive motors. The reflectance of the mirrors is 0.94 when clean. Maintenance of high reflectance is critical to plant operation. With a total of 2.32 X 106 m2 of mirror area, mechanised equipment has been developed for cleaning the reflectors, which is done regularly at intervals of about 2 weeks.
The receivers are 70 mm diameter steel tubes with cement selective surfaces surrounded by a vacuum glass jacket in order to minimise heat losses. The selective surfaces have an absorptance of 0.96 and an emittance of
0. 19 at 350 °C.
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The collectors rotate about horizontal north-south axes, an arrangement which results in slightly less energy incident on them over the year but favours summertime operation when peak power is needed and its sale brings the greatest revenue. Tracking of the collectors is controlled by a system that utilise an optical system to focus radiation on two light- sensitive sensors. Any imbalance of radiation falling on the sensors causes corrections in the positioning of the collectors. There is a sensor and controller on each collector assembly, the resolution of the sensor is 0.5°.
A promising new configuration that combined SEGS parabolic-trough technology with a gas-turbine combined - cycle power plant is conceived to meet utility needs for continuous operation and peaking power with minimal environmental damage. Such a hybrid combined-cycle plant uses the solar field as the evaporation stage of an integrated system, with the gas-turbine exhaust being recycled for superheating and preheating, thus, the solar field serves as the boiler in an otherwise conventional combined-cycle plant. This approach has several advantages:
1. The direct steam generation system can take advantage of the steam turbine, generator, and other facilities of the combined-cycle plant at a modest increase in capital cost.
2. Adding the direct steam generation facility requires no additional operators or electrical interconnection equipment.
3. Thermodynamic efficiencies are maximized because steam is evaporated outside the waste-heat recovery system; only the remaining thermal-heat exchange processes take place in the recovery heat exchanger. Thus, higher working-steam conditions can be achieved for the same degree of heat use which increases overall cycle efficiency.
This new configuration is preferable from the perspective of the second law of thermodynamics because the solar field reduces the production of entropy in the system.
