Solar Power » Technologies

Concentrated Solar Thermal Systems

CST power plants (often referred to as CSP) produce heat from sunlight, which is subsequently converted into electricity. A number of technologies have been developed to concentrate and collect sunlight. The collected sunlight creates heat, which in turn is used to drive a turbine and generator as in a conventional power station.

Stand-Alone Solar Rankine System

CST Rankine System


In a stand-alone configuration, a heat transfer fluid is heated as it circulates through the receivers in the collectors. It runs trhough a heat exchange to generate high-pressure heated steam. The steam is then fed into a separate circle to drive a conventional steam turbine. The spent steam from the turbine is condensed into liquid ready to be re-heated in the steam generator to complete the circle.

Storage Tank

The system becomes more 'despatchable" with the addition of a storage tank.

  • Indirect Storage: Most storage mediums are heated indirectly through the heat transfer fluid. Known storage mediums include concrete and molten sand. To extract the heat on demand, the process is reversed through the heat exchanger. At AndaSol CST power plant, a mixture of 25,000t of sodium and potassium nitrate is heated to 384°C, providing the plant with over 6 hours of storage.
  • Direct Storage: Alternatively, steam could be stored directly in a latent heat storage. Due to both high temperature and pressure involved, this comes at a high cost and is therefore only practical as a buffer storage for peak power.


Most CST power plants operate according to the Rankine cycle, a thermodynamic cycle that uses steam as a working fluid at temperatures up to 655°C. Required cooling comes usually from water. The exception is the parabolic dish.

Heat Transfer Fluid

The heat transfer fluid carries the heat to the storage tank and the steam generator. Good fluids have a low viscosity (i.e. flow easily) and thermal capacity. Otherwise, the choice of fluid also depends on outside temperature (need low freezing point in cold climates), and concentration ratio, as high concentration may require high boiling point.

  • Aromatic Oils: The most widely used HTF is a hydrocarbon oil, which has a wider liquid temperature range than water, but a lower thermal capacity and higher viscosity. The oil is not as toxic as others and is classified as non-hazardous by U.S. standards, but care needs to be taken and spills must be adequately decontaminated.
  • Water: Water is an excellent medium in terms of thermal capacity and viscosity. Direct steam generation would save cost for the HTF-cycle and heat exchanger, enhancing efficiency by 15 - 20%. However, due to the lower boiling point of water, the solar collectors need some modification to cope with higher pressure and lower viscosity. The Plataforma Solar de Almeria has a 1.3MW test loop for direct stream generation.

Hybrid System

CST Hybrid System

Fossil Fuel Backup

Though CST plants can operate at their full rated electric power for up to 12 hours a day in clear and sunny locations, most CST plants include a fossil-fuelled backup capability that can be used to supplement the solar output during periods of low solar readiation.

This backup can either be a facility that heats up the heat transfer fluid or an additional, fossil-fuel fired steam boiler.

Power plants in California have been connected to the grid for years, and use up to 25% natural gas in order to cover sunless periods.

Integrated Solar Combined Cycle System (ISCC)

CST Integrated Solar Combined Cycle SystemThe ISCC is a very promising design. It integrates a CST plant with a combined cycle gas turbine.

Combined Cycle System

The gas turbine in a combined cycle system operates at an input temperature range from 900°C - 1,400°C while the output ranges from 450°C to 650°C. This high output temperature makes the combination with a Rankine cycle as the second cycle very appealing. To achieve this, the heat from the gas turbine is fed into the waste heat recover system, which produces high-pressure steam (420°C - 580°C) for the steam cycle.

Integration of CST

To increase the power output from the second cycle, solar heat is used either to produce high-pressure steam fed into the waste heat recovery system or to produce additional low-pressure steam for the Rankine turbine.

The ISCC design is particularly promising, as many combined-cycle plants will be built regardless, though their efficiency can be greatly augmented by the use of solar thermal heat.


The first such project is being built in Algeria with a total plant capacity of 150MW, 20MW of which will be supplied by a solar thermal trough system. It will have 180,000m² of solar collectors.
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