|Fig. 1: CSP systems: Parabolic Trough (top), Power Tower (middle), Parabolic Dish (bottom). (Courtesy of the US Department of Energy) |
Solar energy is the most abundant renewable energy source. Everyday the earth receives energy from the sun enough to meet our electricity demand for 30 years.  Unlike photovoltaic solar cells, concentrating solar power (CSP) uses mirrors to focus sunlight to generate heat. The heat is carried by a heat transfer fluid (HTF) to run steam turbines for generating electricity. Developing countries represent the biggest growth market for this technology.
Three main types of CSP systems can be identified. Parabolic Trough systems use mirrors in the form of troughs to focus energy on a fluid carrying receiver tube located at the mirror's focal point. The HTF typically synthetic oil, molten salt, or steam circulates in the tubes before passing through heat exchangers to produce steam. Solar troughs are considered the most mature and commercially proven of the CSP technologies.  In a Power Tower system many individual mirrors called heliostats are used to track the sun and reflect its light onto a receiver mounted on top of a tall tower. Power towers are thought to have greater potential for wide-scale implementation because of their higher thermal conversion efficiency and greater stored energy densities.  Parabolic Dish or dish engine uses mirrors shaped as a dish to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish center. A Stirling engine is typically used to convert thermal energy to mechanical power for electricity generation.  Dish systems are generally between 10 kW and 25 kW in size and are considered highly modular allowing single or multiple deployment for small-scale or large-scale utility applications. 
CSP is a proven energy technology. The first CSP plants were built in California (SEGS) and continue to operate today. In US alone there is 1176 MW of utility-scale solar power operational as of January 2012. About 43% of this capacity is provided by CSP of which almost all come from trough technology.  Around the world, the total power produced by CSP has reached 2.5 GW. Plants currently under construction will generate an additional 2 GW.  Since these systems look like most power plants, common existing equipment can be used in their construction which would bring cost down. They can also be integrated with fossil plants to create hybrid power plants.
CSP has a small carbon footprint. One meta-analysis study estimated the life cycle GHG emissions from parabolic trough CSP after harmonization to be 69 g CO2 equivalent per kWh and for power tower CSP to be 25 g CO2 equivalent per kWh.  Compared to other clean energy sources that are intermittent, CSP is capable of producing energy around the clock by storing energy during the day and releasing it to the generator at night or in any weather. Thermal energy storage (TES) enables CSP to generate electricity well into the dark when electricity is more expensive. Existing CSP facilities have capacity factors around 25% and 50% depending on whether or not they have TES. The first utility-scale plants with storage are now operating in Spain (Andasol 1-3). 
Another advantage of concentrating solar technology is its relatively high energy density which requires less land per MW among the renewables.  For example, SEGS 3-9 cover more than 1500 acres with a capacity of 310 MW averaging 4.84 acres per MW. This is compared to 6.86 acres per MW for Solana station in Spain which includes TES.  Compared to PV which relies on scarce materials for manufacturing, CSP plants are made from low-cost readily available materials.
Solar-to-electric efficiency for CSP systems ranges between 15-25%.  Compared to other CSP technologies, parabolic dish conversion efficiencies are the highest, reaching over 30% of peak efficiency.  Overall efficiency is composed of collection and conversion components. Conversion efficiency improves with higher operating temperature, therefore, working fluids capable of running at higher temperatures have to be employed. Using a single fluid for heat transfer and storage is also desirable to eliminate the need for heat exchangers. 
Like other thermal power generation plants, CSP requires water for cooling. The amount of water consumption can be a concern given the arid nature of environments that are suitable for this technology. CSP water requirements are relatively high at about 3 liters per kWh compared to about 2 liters per kWh and 0.8 liters per kWh for coal and combined-cycle natural gas plants respectively.  Instead, Dry cooling can be used however at the cost of reduced production and increased cost. One study estimated that dry-cooling can reduce life-cycle water consumption by 77% while increasing GHG emissions by 8%.  The dry-cooling performance penalty of Power Towers can be smaller given their higher efficiency. 
Current CSP technologies cost around $0.10 -$0.12 per kWh of solar power. Department of Energy has established SunShot Initiative which sets a cost goal of $0.06 per kWh by 2020.  Technological improvements and financial incentives are expected to allow concentrated solar power to reach this goal in this decade.
© Yashar Rajavi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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