Concentrating Solar Power (CSP) plants use mirrors to concentrate sunlight onto a receiver, which collects and transfers the solar energy to a heat transfer fluid that can be used to supply heat for end-use applications or to generate electricity through conventional steam turbines.
Large CSP plants can be equipped with a heat storage system to allow for heat supply or electricity generation at night or when the sky is cloudy.
In Concentrating Solar Power (CSP) plants, mirrors concentrate sunlight and produce heat and steam to generate electricity via a conventional thermodynamic cycle. Unlike solar photovoltaics (PV), CSP uses only the direct component (DNI) of sunlight and provides heat and power only in regions with high DNI (i.e. Sun Belt regions like North Africa, the Middle East, the southwestern United States and southern Europe).
CSP plants can be equipped with a heat storage system to generate electricity even under cloudy skies or after sunset. Thermal storage can significantly increase the capacity factor and dispatch ability of CSP compared with PV and wind energy.
It can also facilitate grid integration and competitiveness. In sunny, arid regions, CSP can also be used for water desalination.
The CSP technology includes four variants, namely Parabolic Trough (PT), Fresnel Reflector (FR), Solar Tower (ST) and Solar Dish (SD). While PT and FR plants concentrate the sun's rays on a focal line and reach maximum operating temperatures between 300-550°C, ST and SD plants focus the sunlight on a single focal point and can reach higher temperatures. PT is currently the most mature and dominant CSP technology. In PT plants, synthetic oil, steam or molten salt are used to transfer the solar heat to a steam generator, and molten salt is used for thermal storage. Among other CSP variants, ST is presently under commercial demonstration, while FR and SD are less mature.
Commercial PT plants in operation have capacities between 14-80 MWe. They reach a maximum operating temperature of 390°C, which is limited by a thermal degradation of the synthetic oil used as the heat transfer fluid. The efficiency (i.e. the ratio of electricity generated to the solar energy input) is about 14-16% and the capacity factor is on the order of 25-30%, depending on the location. Some PT and ST plants have molten salt thermal storage systems with storage capacities of 6-15 hours, which increase the plant capacity factors to over 40% and 70%, respectively. Two plants (i.e. a 5-MW PT plant in Italy and a 20-MW ST plant in Spain) are currently testing the use of high-temperature (550°C) molten salt for heat transfer and storage purposes.
This option is expected to significantly improve the CSP performance and storage capacity. The available operational experience suggests that PT plants have a lifetime of more than 30 years. In the ST plants, steam (direct steam generation) and compressed gasses can also be used as alternative heat transfer fluids, and significant potential exists to improve performance (i.e. temperature and efficiency). The cost of CSP plants is still high in comparison with conventional power plants and other renewable technologies.
The International Energy Agency (IEA) estimates a current investment cost for CSP plants between USD 4,200-8,500 per kW, depending on local conditions, DNI, the presence of thermal storage and – last but not least – the maturity level of the project (i.e. pilot, demonstration or commercial). Recent estimates by the International Renewable Energy Agency (IRENA) suggest upfront investment costs of between USD 5,500-8,000 per kW for PT plants with no storage and costs between USD 7,500-8,500 per kW for PT plants with six hours of storage. ST plants are usually designed with high storage capacity.
Concentrated Solar Power (CSP) plants use mirrors to concentrate the sun's rays and produce heat for electricity generation via a conventional thermodynamic cycle. Unlike solar photovoltaics (PV), CSP uses only the direct component of sunlight (DNI) and can provide carbon-free heat and power only in regions with high DNI (i.e. Sun Belt regions). These include the Middle East and North Africa (MENA), South Africa, the southwestern United States, Mexico, Chile, Peru, Australia, India, Western China, southern Europe and Turkey.
While CSP plants produce primarily electricity, they also produce high-temperature heat that can be used for industrial processes, space heating (and cooling), as well as heat-based water desalination processes. Desalination is particularly important in the sunny (and often arid) regions where CSP plants are often installed.
Technology Advances and Cost Reductions
In many countries, research and industry are committed to improve CSP performance and reduce its costs. Important drivers for cost reduction include:
● Technology advances of components and systems;
● Advanced thermal storage;
● Increased plant size and economies of scale; and
● Industrial learning in component production.
Technical advances, such as high-reflectivity mirrors with reduced maintenance needs, apply to all CSP technologies while others focus on specific CSP variants.
A detailed analysis of potential technical advances and associated cost reductions for each CSP technology has been carried out by AT-Kearney and ESTELA (ATKearney, 2010).
Advanced thermal storage systems include:
● Lithium-based molten salts with high operation temperatures and lower freezing points;
● Concrete or refractory materials at 400–500ºC with modular storage capacity and low cost (USD 40/kWh);
● Phase-change systems based on Na- or K-nitrates to be used in combination with DSG; and
● Cheaper storage tanks (e.g. single thermocline tanks), with reduced (30%) volume and cost in comparison with the current two-tank systems.
An increased plant size reduces the costs associated with conventional components and systems, such as power block and balance of plant rather than the cost of the solar field, which depends primarily on industrial learning and large-scale production of components.