Concentrating solar power (CSP) systems concentrate the energy from the sun for electricity production. This is done by heating a fluid which is then used to raise steam for a conventional turbine  for on and off-grid  electricity provision. These systems can also provide heat, either at high temperatures directly for chemical reactions, e.g., chemical processing, or as a by-product for desalination plants or cooling systems, depending on requirements.
They can be located in deserts or any high insolation  area. The size of the area needed for the mirrors varies according to the output required and the type of system. It has been calculated by Trieb et al. (2006) that if 0.5% of the world’s deserts were used for this technology, it would supply all the world’s electricity requirements by 2050.
Solar thermal can integrate well with conventional power generation equipment and advanced technology. When combined with storage  the power availability is suitable for grid markets with good load matching in demand in the summer in countries with high temparatures. Conventional materials are used with modular construction. There are three basic designs for CSP, but all use mirrors to concentrate the energy from the sun onto a receptor vessel which may contain a liquid or gas which is heated and then used to power a steam turbine (Dell and Rand, 2004).
A simple parabolic dish focuses the sun’s energy onto a thermal receiver mounted at the focal point of the dish. Temperatures greater than 1000°C can be reached. Due to its limited size the output from one dish is about 25 kWe at maximum. Another type is the ‘central receiver’ type or solar tower which has thousands of mirrors able to track the sun and these are arranged round a central tall tower. A heat transfer fluid (such as a molten salt, air, water/steam, liquid sodium) flows through the receiver collecting the heat. The temperatures involved are in the region of 300-1000°C. This is then used to make steam to generate electricity with power outputs in the range 30-200 MWe. If air is used at 1000°C then it can be used directly in a gas turbine (60% efficiency) replacing natural gas. The third type of arrangement is the parabolic trough principle. The parabolic trough mirror tracks the sun and may be up to 100 m long. At the focus of the mirror is a heat pipe which carries away the heat produced. The temperature range is lower at 200-400°C and arranging the troughs in rows allows a flexible power output ranging from 30 to 350 MWe.
Both the parabolic trough and the power tower designs allow thermal storage facilities so that electricity can be supplied in the evening as well. This is shown in Figure 1 below.
The heat from the system can also be used in a combined heat and power mode, in a desalination plant, and for solar-assisted air conditioning. Such multiple applications could increase the efficiency of CSP plants to a level of 80-90%.
The Parabolic Trough technology has been pursued in Europe and the southwestern USA as will be described below. The most famous example is the 354 Mwe Solar Energy Generating Systems (SEGS) plant  in California which has been operating since the 1980s. In case of insufficient sunshine, back up support is provided by gas fired burners. The plant has 2 million m2 of mirrors (see Figure 2).
Within Europe, CSP technology has been applied in Italy and Spain. The Italian Government funds the Archimedes Project  in Sicily, which is a solar plant integrated with an existing gas-fired combined cycle power station plant using new designs of mirrors, tubes and molten salt for heat transfer. In Spain CSP is supported by, among others, a feed-in tariff (increased from € 0.12 to 0.18/kWh) which has led to more than 12 new 50-MWe projects. In Spain, the first two commercial solar tower CSP plants  were constructed with a capacity of 10-20 MW. Three 10 kW parabolic dish plants have been in operation in Spain since 1992.
The parabolic trough technology and specifically the Integrated Solar Combined Cycle (ISCC) hybrid projects have been either planned or implemented in Algeria, Egypt, Iran, India, Mexico and Morocco (see examples below). However, national governments had to fund the projects with help from international donor agencies and development banks. Projects have been delayed in India and Egypt and Mexico. In South Africa a decision has been taken to use molten salt solar tower technology with the intention of building a series of 100-MW commercial towers.
The parabolic dish technology is a flexible system which is applicable either as a stand alone, off grid unit, or as a larger dish park with many units connected to the grid. The single unit has a capacity of 25 kWe and smaller units of 10 kWe capacity are being designed. These units have a high conversion efficiency of over 30% and can be linked to form a hybrid plant in line with the other designs. Operational experience has been gained with demonstration projects, though reliability is an issue and costs still need to be lowered. The main problems are the price of the mirrors when there is no large-scale production to lower costs, and the fact that the greatest resources are in the southern European countries and round the Mediterranean, North Africa and the Middle East, Australia and Southern parts of USA.
This means that in many cases there will be a requirement for long distance transport of the solar thermal electricity through high-voltage direct current (HVDC) links, which have a lower transport loss (3% per 1000 km). The normal high-voltage alternating current (HVAC) links can be used for shorter distances. For example, the Trans-Mediterranean Renewable Energy Cooperation  TREC-EUMENA project  has suggested to build a new HVDC electricity grid ‘highway’ to allow easy, efficient transport of electricity from a variety of alternative sources within the EU and from Northern Africa and the Middle East.
Another operational issue is the choice of the fluid as heat transfer medium. The oil currently used for this in the commercial plants in California  is not optimal and the goal is to reduce losses in heat transfer within the absorber tubes by applying direct steam production. This could of course be used in a gas turbine directly.
Finally, the Global Market Initiative (GMI) , launched in 2003 to expedite the building of 5000 MWe of CSP systems worldwide by 2013, has formulated a strategy to overcome barriers to CSP development and transfer:
- Mandatory national targets for CSP (and renewables) by 2010 and beyond;
- Specific feed-in tariffs for CSP, such as those in Germany and Spain for renewables, are required to provide a stable investment context;
- Leadership by multilateral institutions as well as programmes under GEF, UNEP and UNDP; and
- Regional programmes such as that by TREC with HVDC lines and region-specific strategies.
Other activities include research on new combinations and materials to increase efficiency and cost effectiveness, and scaling up systems in size with demonstration funding. GEF (2005) also suggest that a critical mass of manufacturing capability is required (in 2004-5, there were only 300 MW of firm projects where several thousand MW are required for a viable commercial competitiveness).
The solar thermal technology has been demonstrated to work. The TREC-EUMENA project  claims that the technology is capable of supplying the EU, Mediterranean and North African region’s energy needs by 2050 with only 0.3% coverage of the region’s deserts with the additional advantages of desalinated water and cooling as by products of the process.
The parabolic trough CSP technology is for electricity grid connection and is not new. The trough configuration is the most advanced of CSP systems with 354 MWe installed in Southern California  in the Mojave Desert, which has been operational since the 1980s. Other parabolic trough systems are being built in Nevada (USA), southern Spain and Australia. The system is commercially available with an operating temperature potential up to 500 0C with a net plant efficiency of 14%. Investment and operating costs have been commercially proven. It has the best land-use factor of all the designs, lowest material demand and storage capability. Apart from continuing development of new materials for mirrors and heat transfer, etc., the technology has become standard (ENTTRANS, 2008). The application of the technology tends to be mainly carried out by developed country partnerships, rather than local developing country companies. Countries which have shown an interest in this technology include Australia, Germany, Greece, Israel, Morocco, Russian Federation, Spain, Switzerland, and the USA.
The central receiver (solar tower) technology is also used for grid electricity supply and operates at a higher temperature than the trough. The largest plant built to date has a capacitiy of 10 MWe. Though not yet commercially available, it has good prospects for high conversion efficiencies with temperatures above 1000 0C (565 0C proven). Storage at high temperatures and hybrid systems with other renewables or fossil generation are possible. Commercial operation still needs to be demonstrated (ENTTRANS, 2008).
Research projects for CSP development and transfer
In terms of research activities to accelerate CSP development and transfer a range of activities take place, a number of which are explained below.
The European Parabolic Trough R&D programme (EuroTrough)  consortium, funded by the EU, focuses on improving the structural design and optical accuracy of CSP systems.Prototypes have been successfully demonstrated and were added to the SEGS plant in California in 2003 and have been in continuous operation. Funding has been made available by the German SKAL ET consortium and the German Ministry of Environment. The EU also sponsors the Direct Solar Steam (DISS)  andIntegration of DSG Technology for Electricity Production (INDITEP) programmes to achieve direct steam generation within absorber tubes. DISTOR is concerned with a phase change storage medium for direct steam systems. The TREC initiative promotes CSP and a regional co-operation on CSP as described above.
The IEA manages a collaborative programme called SolarPACES , which is an international co-operative organisation bringing together teams of national experts from around the world to focus on the development and marketing of CSP systems.
The GEF and national governments have facilitated parabolic trough technology and specifically the Integrated Solar Combined Cycle plant  (ISCC) hybrid projects in Algeria, Egypt Iran India Mexico and Morocco. Algeria has set up its own national programme without GEF support for the promotion of renewable energy sources and proposes a 150 MW ISCC plant. Mexico is planning a 250 MWe gas fired CC plant with solar fields of 25MWe with a grant from GEF for the solar part. In Morocco, the African Development bank with the GEF has helped to fund a 220MW ISCC plant due to start operation in 2008. In Egypt, the National Renewable Energy Agency has commissioned a national utility-owned project comprising a solar plant and an ISCC to be operational by 2008.
The economic and market potentials would increase with large scale mirror production, which would lower capital costs (see also below). The technical potential of solar thermal systems is large and Brakmann et al. (2005) estimate that by 2040 5% of the world’s electricity needs could be covered by solar thermal power.
Whether the technical potential of CSP can be realised partly depends on policy measures to support CSP development and transfer (Brakmann et al., 2005). For example, the EU Directive  sets a target of doubling the share of renewable energy sources from 6 to 12% by 2010. The USA has Renewable Portfolio Standards  to increase the contribution of renewable power.
Mainly through funding from GEF and other international organisations, private companies and national governments the following countries have become involved in developing solar thermal power plants based on parabolic trough design: Algeria, Egypt, India, Iran, Mexico, and Morocco. Other countries interested or actively pursuing CSP are Jordan and South Africa. Brakmann et al. (2005) point out that GMI recommends that developing countries not interconnected to industrialised countries in Europe should have preferential financing with grants or soft loans, etc.
In terms of socio-economic benefits, CSP technology projects could result in local revenues from the electricity supply and from the possible desalination plants, as well as the possibility of other economic activities contributing to the local economy. Additional opportunities can arise from the configuration of the plant as the mirrors are very large and create shaded areas underneath, which can be used for horticulture irrigated by desalinated water generated by the plants. The cold water produced can also be used for air conditioning. Finally, Brakmann et al. (2005) expect that CSP project could result directly and indirectly in 54,000 jobs worldwide by 2025.
Environmental impacts in terms of flora and fauna damage and biodiversity degradation are low. In addition, CSP systems do not emit pollutants. As the amount of desert area involved is relatively low and would not have been used otherwise, ecological impacts would be low.
When CSP electricity production capacity replaces capacity based on fossil fuels, greenhouse gas (GHG) emission reductions could be achieved, although GHG emissions related to manufacturing and construction of the system and its components need to be subtracted (the latter emissions amount to approximately 0.010 to 0.015 kt/kWh (ENTTRANS, 2008).
For calculation of GHG emission reduction of a CSP project, it is recommended to apply the Approved Consolidated Methodology ACM0002 , which has been developed for CSP projects under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html 
There are wide variations in assessments of costs of electricity production and capital costs of CSP plants depending on assumptions and state of development of the different designs. Brakmann et al. (2005) report that the costs of parabolic trough CSP dropped from USD 4,000/kWe to under USD 3,000/kWe due to scaling up from 30 to 80 MWe units from 1984 to 1991. Currently, this price has fallen to € 210/m2 and is expected to fall to € 110-130/m2 in the longer term (ENTTRANS, 2008). Developing countries would be expected to be 15% cheaper. In 1999, the World Bank assessment of installed capital costs were € 3,000-2,4440/kWe for 30-200 MWe Rankine cycle plants  and € 1,080/kWe for a 130 MWe hybrid Integrated Solar Combined Cycle plant  (ISCC) with 30 MWe equivalent solar capacity.
Generation costs range from € 0.10/kWh to less than € 0.07/kWh for ISCC plants. Brakmann et al. (2005) estimate that electricity costs from trough plants could fall to USDcents 7-8/kWh in the medium term compared to the current USDcents 14-17/kWh from the 354 MW installed in the USA. There are conflicting estimates of the future costs and market penetration, but Central receivers or solar tower systems are not so well developed for commercial use yet (Brakmann et al., 2005). The costs of transmission through HVDC lines could be relatively low and combinations of CSP with desalination and air conditioning bring the costs further down.
For the solar tower concept the costs are still high and no generation costs for commercial plants are available. For the new solar tower projects in Spain the capital costs are expected to be in the order of € 2,700/kWe and electricity costs between € 0.14 - 0.20/kWh.
Brakmann, G., Aringhoff, R., Geyer, M. and Teske, S., 2005, Concentrated Solar thermal power Now! 
Dell, R.M. and Rand, D.A.J., 2004. Clean Energy, RSC Clean Technology Monographs .
ENTTRANS, 2008. Sustainable, Low-Carbon Technologies for Potential Use under the CDM – A description of their environmental, economic, and energy aspects , Groningen, the Netherlands.
GEF, 2005. Assessment of World bank/GEF strategy for the market development of concentrating Solar Thermal Power , GEF/C.25/Inf11.
Trieb, F., KAbarti, M., Bennouna, A., El Nokraschy, H., Hassan, S., Hasni, T., El Bassam, N. and Satoguina, H., 2006. Trans-Mediterranean Interconnection for Concentrating Solar Power , Report by DLR for Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Germany.