Parabolic Trough Power Plant System Technology
A parabolic trough solar power plant uses a large field of collectors to supply thermal energy to a conventional power plant. Because they use conventional power cycles, parabolic trough power plants can be hybridized—other fuels can be used to back up the solar power. Like all power cycles, trough power plants also need a cooling system to transfer waste heat to the environment.
Parabolic trough power plant technologies include:
The SEGS IV parabolic trough power plant in Kramer Junction, California.
Credit: Sandia National Laboratories
Power Cycles
There are a number of different power cycles that can be used for parabolic trough power plants. And there are a number of options for how to integrate solar energy into the power cycle.
Steam Rankine Cycle
All of the SEGS (solar electric generating system) plants and most new projects are planning to use steam Rankine power cycles. These power plants have power cycles very similar to those used for many coal, nuclear, or natural gas-fired steam power plants.
The 80-MWe SEGS plants use a regenerative reheat steam turbine cycle that has a gross steam cycle efficiency approaching 38% with high-pressure steam conditions of 100bar, and 370°C. The power cycle uses a solar steam generator in place of the conventional boiler fired by natural gas, coal, or waste heat from nuclear fission. Otherwise the power cycle is very similar with the following components:
For standard overhaul, workers remove a 30-MWe steam turbine from the SEGS V power plant in Kramer Junction, California.
- A surface condenser
- Multiple low-pressure and high-pressure feedwater heaters
- Deaerator
- Wet cooling towers.
Solar energy is used to generate the high-pressure steam and also to reheat the steam. The solar field (or thermal energy storage system) supplies the hot, heat transfer fluid (HTF) to the power plant. The heat transfer fluid passes through a series of shell-in-tube heat exchangers to generate the high-pressure steam that runs the Rankine steam turbine. The cold heat transfer fluid is then returned to the solar field (or thermal energy storage system).
Organic Rankine Cycle
The organic Rankine cycles (ORCs) use an organic fluid—such as butane or pentane—instead of water, like a steam Rankine cycle.
This single-stage, 5-MWe organic Rankine cycle turbine operated on butane at the Mammoth Hot Springs geothermal power plant in California.
Organic Rankine cycles are also typically much simpler in design. They are often used for applications with a lower resource temperature, such as for geothermal power plants. Also, many organic Rankine cycles operate at lower pressures, which reduces the capital cost of components.
For small power plants—ranging in size from 100 kWe to 10 MWe—the organic Rankine cycle has some advantages. One advantage is that many of the working fluids in organic Rankine cycle systems can be condensed at or above atmospheric pressures. This eliminates the need for maintaining a vacuum in the condenser.
For more information, see our publications on parabolic trough organic Rankine cycles.
Combined-Cycle Systems
It's possible to integrate solar steam into the Rankine bottoming cycle of a combined-cycle parabolic trough power plant. This type of plant is called an integrated solar combined cycle system (ISCCS).
A combined-cycle system uses solar heat for steam generation and gas turbine waste heat for preheating/superheating the steam. It can approximately double steam turbine capacity. However, when solar energy isn't available, the steam turbine must run at part load, which reduces efficiency.
Adding thermal energy storage could help double the solar contribution. And the cost for increasing steam turbine size for a combined-cycle power plant is substantially lower than the cost of a stand-alone Rankine cycle power plant.
Several new projects using an integrated solar combined cycle system are under development.
Figure 1. ISCCS process flow schematic. Credit: FlagSol
For more information, see our publications on integrated solar combined cycle systems.
Fossil-Fired (Hybrid) Backup
Because parabolic trough power plants use conventional power cycle technologies, fossil-fired boilers or heaters usually can be integrated to enable power plant operation at full-rated output during periods of low solar radiation, such as overcast days and at night.
Most existing parabolic trough power plants have hybrid backup capability. They can operate using 100% solar input, 100% natural gas input, or any combination in between. Typically the fossil backup efficiency is much lower than for a modern combined-cycle power plant. So the fossil fuel is typically only used for backup during the utility's peak demand periods.
Although the SEGS (solar electric generating system) plants initially operated 25% of the time on fossil fuel, currently they only produce a few percent of their annual output from natural gas.
Direct Steam Generation
Another option under consideration for future parabolic trough plants is the possibility of generating steam directly in the solar field. This eliminates the need for an intermediate heat transfer fluid and steam-generation heat exchangers. It also should allow the solar field to operate at higher temperatures, resulting in higher power cycle efficiencies and lower fluid pumping parasitics.
Ceimat and DLR (German Aerospace Center) are currently testing direct steam generation (DSG) at the Plataforma Solar de Almeria in Spain. They must address a number of technical issues. But direct steam generation is still one of the most promising opportunities for future cost reductions.
For more information, see our publications on parabolic trough direct steam generation.
Wet and Dry Cooling
Historically, parabolic trough power plants have used wet cooling towers. But now they can be designed to use dry cooling technology for reducing water consumption. Utilization of dry cooling usually only requires a modest increase in electricity cost.
The SEGS (solar electric generating system) plants use approximately 800-1000 gallons of water per MWh generated. With wet cooling, the cooling tower represents approximately 90% of a Rankine parabolic trough power plant's raw water consumption. The other 10% of water consumption includes the steam cycle makeup cycle (8%) and mirror washing (2%).
For more information, see our publications on parabolic trough power plant wet, dry, and hybrid cooling.
Operation and Maintenance
The high-pressure demineralized water system—called Mr. Twister—has sprayers that spin as they move down when washing the mirrors.
The operation and maintenance (O&M) of a parabolic trough power plant is very similar to conventional steam power plants that cycle on a daily basis.
Parabolic trough power plants typically require the same staffing and labor skills to operate and maintain them 24-hours per day. However, they require additional O&M requirements to maintain the solar fields.
Initial plants required a substantial number of mechanics, welders, and electricians to maintain immature solar technology. Modern parabolic trough solar technology is much more robust and requires minimal preventive or corrective maintenance. The one exception is mirror washing.
Experience has shown that solar field mirrors must be washed frequently during the summer. But the increase in solar output pays for the cost of labor and water. Current power plants may wash mirrors weekly during the peak solar times of the year. It's usually only necessary every few months during the winter.
For more information, see our publications on parabolic trough power plant operation and maintenance.
