Concentrated solar power (CSP) uses mirrors, or sometimes lenses, to concentrate sunlight onto a smaller area so that it generates a higher temperature. It is most commonly used to produce steam, which is then used to generate electricity in a conventional steam turbine. It may also be used for industrial process heat.
The efficiency of CSP depends on the optical efficiency of the collector mirrors and efficiency with which the receiver converts incident light into heat. If the CSP system is used to generate electricity using a steam turbine, the total system efficiency will depend on the thermodynamic efficiency of converting heat into mechanical work and the electrical generator efficiency.
The concentration factor is the ratio between the area of sunlight collected and area of the solar receiver onto which it is focused. The concentration factor is one of the critical parameters affecting the efficiency of a CSP system. A higher concentration factor produces higher temperatures,which in turn means greater thermodynamic efficiency for steam turbine power generation, as limited by the Carnot efficiency. However, as temperature increases, the thermal radiation properties of the solar receiver change, making it less efficient at absorbing the solar radiation. There is, therefore, an optimum temperature for a given configuration. The range of applications for industrial process heat is also limited by the temperature that can be achieved. Greater temperatures may enable a range of industrial processes to directly use solar energy, with the huge potential to reduce greenhouse gas (GHG) emissions.
The Promise of High Temperature CSP
Current commercial CSP installations typically have concentration factors of up to 500, operate at temperatures of less than 800 °C and have maximum conversion efficiencies of less than 35 percent. However, theoretical calculations show that efficiency increases with the concentration factor and current systems do not reach optimal temperatures. A further reason to increase operating temperatures is that it enables a greater amount of thermal energy to be stored in thermal accumulators. These are large insulated tanks that store the hot heat transfer fluid so that it can be used to generate steam for electricity generation or process heat when required.
Perhaps the most well-established high-concentration factor design is the Big Dish. It has been in development since the 1970s, initially by the National University of Australia and then commercially by Wizard Power and Sunrise CSP. The Big Dish is a 500 m2 parabolic dish that concentrates energy onto a point receiver held by a tripod. It has a concentration factor of more than 2,000 and can be configured for temperatures of between 500 °C and 1,700 °C. However, this design has proven difficult to manufacture and operate economically.
Heliogen recently received attention for achieving a temperature of over 1,000 °C. It claims that is a record. According to Bill Gates, who has invested in Heliogen, “At that temperature, Heliogen can replace the use of fossil fuels in critical industrial processes, including the production of cement, steel and petrochemicals. Industrial processes like those used to make cement, steel and other materials are responsible for more than a fifth of all emissions… (Heliogen’s) capacity to achieve the high temperatures required for these processes is a promising development in the quest to one day replaces fossil fuel.”
Process heat for cement production is an interesting area for reducing CO2 emissions. Currently, 8 percent of global CO2 emissions result from the production of cement. However, not all of that is energy related. Cement is produced by heating limestone, clay and other materials in rotating kilns at approximately 1,450 °C. This causes calcination, in which the material is split into calcium oxide and carbon dioxide. The resulting grey balls are known as clinker and ground with gypsum and limestone to produce concrete. Heating the kilns results in about 40 percent of the total CO2 emissions. Almost 10 percent is caused by transport, quarrying and grinding. More than 50 percent of the carbon is released by the limestone during the calcination process, a component that cannot be reduced by changing the energy source. The potential for CSP to reduce cement’s CO2 emissions is, therefore,significant but sometimes overstated.
Perhaps the most exciting prospect for high-temperature CSP is that it could enable the direct production of hydrogen from solar energy. This could immediately reduce the emissions caused by nitrate fertilizer production. In the long term, it offers the tantalizing possibility of a hydrogen economy in which solar energy can be easily stored and transported to power vehicles, heat homes and generate electricity on demand.
In fact, a test plant has been using CSP to generate hydrogen since 2008. The Hydrosol project uses an array of flat mirrors to concentrate 750 kW of solar radiation into a thermo-chemical redox reactor. The process has two stages. In the first stage, a redox material, such as nickel ferrite or cerium oxide, is heated to 1,400 °C causing it to release its oxygen, or chemically reduce. In the second stage, water vapor is passed over the reduced material at a temperature of 800-1000 °C. The previously reduced material is re-oxidized,taking oxygen from the water and leaving hydrogen gas, which flows out of the reactor. The metal oxide remains in the reactor, and the cycle is repeated.
In theory, a perfectly aligned CSP redox reactor can achieve a total efficiency of approximately 45 percent. That’s the efficiency of converting solar radiation into chemical energy in the form of hydrogen. It may sound much less efficient than pumped water storage, however, once the efficiency of converting solar radiation into electricity is also taken into account, producing hydrogen starts to look more competitive. There are many other advantages of hydrogen, such as being able to be situated anywhere, easy transportation using pipelines and a dense energy store for vehicles.
Concentrated solar power systems with high concentration factors and operating temperatures have huge potential. They already provide efficient zero emissions electricity generation. They can also easily be combined with thermal storage to deliver base load or dispatchable power, something few renewable sources can achieve. The potential to use the high-temperature heat directly in industrial processes also has enormous potential while direct production of synthetic fuels could be truly game changing. I will be going into more detail on each of these areas in future articles.