A mini-heliostat was installed to check the tracking and control system. Pointing to where the focal beam that is refl ected from this single heliostat is concentrated, is Thomas Roos with students from the universities of Stellenbosch and Johannesburg.
A visualisation of the tower of the CSIR concentrating solar research facility. (Courtesy of AL Badenhorst of s fr architects)
The CSIR is in the process of creating a concentrating solar thermal power station as a small scale research facility. Successful, timely completion may result in a world first: pressurised thermal storage that will allow the gas turbine generator to run when the sun is not shining. Further, unlike competing concentrating solar technologies, this plant will consume no water in the power cycle.
For some of the components of the solar thermal research facility, existing technologies are customised, while some components have to be designed and built at the CSIR. The systems and their integration are modelled to ensure optimal functioning before verifying results through experiments.
The South African situation
Thomas Roos from the CSIR explains, “South Africa faces numerous energy-related challenges. These challenges are about providing access to electricity to all; securing energy supply in the short and the long term; reducing carbon dioxide emissions; and ensuring that current and future electricity and fuel production does not aggravate water security or availability of arable land. This has led us to investigate concentrating solar power (CSP).
“In CSP, sunlight collected over a large area is focused onto a much reduced area using mirror assemblies that move to track the sun. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine that is connected to an electrical power generator.
“The use of a CSP will mean that South Africa’s abundance of sunshine is used, greatly reducing carbon dioxide emissions. The choice of a gas turbine-driven system means no water is used either for cooling or for the power cycle; and that both the infrastructure costs and the cost of the electricity generated are lower than competing steam-turbine driven systems.”
In addition, the power can be generated from distributed energy plants, which can be built faster and closer to municipal centres than large centralised power stations. The use of thermal storage allows excess solar heat to be harvested and stored for later use when the sun is not shining (it is cheaper to store heat than it is to store electricity).
Various components required
The construction of a solar thermal research facility requires the design and integration of numerous components. A heliostat collects solar radiation, and a heliostat field with numerous heliostats replicates this effect - concentrating it through angling to a single, common receiver. This solar receiver uses concentrated solar flux to raise the gas turbine compressor delivery temperature to 800-1000 ⁰C. The other components are a gas turbine, modified for solar operation; a tower for the receiver and the turbine; and a high temperature pressurised thermal storage system.
The team relies heavily on techniques such as sophisticated mathematical modelling and optical ray tracing to ensure that every heliostat is angled and positioned perfectly for every changing moment during the day, and to prevent what is referred to as ‘spillage’, in other words, the maximum radiation in the focal spot not being captured by the receiver.
It also had to be tested for environmental durability, ensuring that it can withstand hailstones and strong winds. In addition to writing an algorithm to predict wind load for every one of the many positions of the heliostats, another algorithm predicts the sun’s positions and the range of movement required by the heliostat to be optimally positioned at all times. The heliostat design specifi es low-cost components and simple systems.
Design of the heliostat field
To capitalise on existing knowledge, the German Aerospace Centre, DLR, was called upon to design the layout of the heliostat field. This requires the optimisation of the tower height and the individual positions of the many heliostats in the field to minimise mutual shading and blocking, maximising heat collected throughout the year and minimising the combined heliostat and tower infrastructure costs.
Shading of a heliostat occurs when another heliostat, moving to track the sun, obstructs incoming sunlight at some time of the day; by contrast blocking occurs when sunlight reflected by the heliostat is obstructed from reaching the target. The site’s contours add another level of complexity. Furthermore, the design must guarantee enough power during sunshine hours to simultaneously drive the gas turbine and to build sufficient heat reserve in the storage system.
A gas turbine capable of running on natural gas (which is used commercially in other parts of the world) was commissioned. Rated at 100kW, the turbine is large enough to behave in a manner representative of much larger gas turbine plants and so prove the concept, yet small enough to greatly reduce the capital outlay for the plant. While the turbine can run on solar and diesel in a hybrid model, CSIR researchers hope to operate the system on solar and biogas from sewage, replacing the diesel. The design of the tower, which houses the thermal storage system and the receiver, is in progress. This design has to heed aspects such as the weight of storage to enable running for several hours when the sun is not shining.