Solar Thermal Electricity Technologies
It is a long road we have embarked on, but Abengoa has spearheaded work in the field of CSP which has benefitted millions around the world, from Spain to Morocco to USA and South Africa, by supplying clean and secure energy. Today we will have a look at the journey taken so far, and examine some of our achievements in the area of Solar Thermal Electricity (STE) technologies.
Solar Thermal Energy (STE) technologies can be classified into solar Parabolic Trough Collector (PTC), Central Receiver System (CRS) or tower system, linear Fresnel, and parabolic dish. Of all these technologies available today, the CRS is moving to the forefront and it might become the technology of choice.
As shown below, a typical central receiver system, also known as solar tower power, consists of three major subsystems, namely the heliostat ﬁeld, the receiver and the power conversion system. The solar ﬁeld consists of numerous computer-controlled mirrors that track the sun individually in two axes and reﬂect the solar radiation onto the receiver located on the top of the tower. The receiver absorbs the heliostat reﬂected solar radiation and converts it into heat at high temperature levels. Depending on the receiver design and the heat transfer ﬂuid nature, the upper working temperatures can rangefrom 250 ºC to 1000 ºC. A power conversion system is used to shift thermal energy into electricity in the same way as conventional power plants.
Thermal energy storage can solve the mismatch between solar energy supply and electricity demand, providing a distinct advantage to STE plants compared to other renewable energies, like wind or photovoltaic.
Two commercial plants that use steam accumulator thermal energy storage are PS10 and PS20, both located in Spain – owned by Atlantica Yield and designed and built by Abengoa- and which started commercial operation back in 2007 and 2009, respectively. Not only were they the first two commercial solar towers in the world, but also the starting point for the operation of direct steam technology.
Second generation of STE direct steam towers use superheated steam that uses a second receiver, its main function being to re-heat the steam produced by the first receiver (evaporator), enabling higher temperatures to be reached. The live steam, which feeds the turbine, can reach a temperature of 540 ºC and 130 bars of pressure, increasing the efficiency of the power cycle by 30% compared to its forerunner PS20.
If we are to mention other fluids for second generation of tower, then we should highlight the molten-salt tower (MST). This uses a mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3), known as Solar Salt as heat transfer and storage medium. A tubular-type receiver mounted on top of a tower where the reflected solar energy from a heliostat field heats the molten salt receiver; molten salts are heated from 290°C to 565°C and then enter into the hot thermal storage tank. Hot salts are then pumped from the storage system to generate steam within a molten salt steam generator in the power block, where the molten salts are used to generate steam that is fed into a conventional reheat steam turbine to produce electricity. The cooled salt is returned through the thermal storage system to the receiver.
The thermal storage system buffers the molten salt steam generator from solar transients and also supplies energy during periods of no insolation, at night or on partly cloudy days due to the fact that the salt remains in a single liquid phase throughout the process, and because of its relatively high heat capacity.
This technology allows the use of direct storage system, which in turn enables the discharge at nominal conditions, therefore increasing the average efficiency of the plant.
Regarding the evolution of commercial tower plants in Abengoa, Khi Solar One is a 50 MWe tower second direct steam generation plant which uses steam accumulator thermal energy storage. It islocated in Upington, South Africaand was designed and built by Abengoa – commercial operation began at the beginning of 2016, and prior to this a landmark was reached when a thermal solar tower plant was in operation for a 24 consecutive hour period for the very first time in the history of Africa..It has a 205 m tower height and uses 4120 heliostats of 140 m2 each. Khi Solar One has a storage capacity of around 2 hours using 19 steam accumulator tanks that allows storing the saturated steam generated in the evaporators, feeding the turbine and generating electricity even when there is no sun. Today, Khi Solar One is owned by Atlantica Yield.
Additionally, Abengoa have new commercial projects with molten salt tower in development.
In conclusion, we can say that the technology that Abengoa applied first is now used worldwide. The evolution and transformation of STE technologies in general will continue to depend on their ability to deliver secure renewable energy sustainably and economically. Meanwhile, the company will continue to search for improvements in developing solar plants around the world.