Harnessing the power of the sun

Posted on: 01 September 2014
By: UP
Institution: University of Pretoria
Research Area: Mechanical Engineering

The idea of concentrating the rays of the sun is of course not new. Archimedes, probably better known for his discovery of buoyancy and why things float in water, devised a weapon that used the sun’s rays. During the siege of Syracuse a couple of centuries BCE (before the Common Era), he aimed a group of mirrors at enemy ships and destroyed them with fire. CSP uses the same principle to focus the sun’s rays, not to cause a fire, but to heat a fluid, called the heat transfer fluid, to store and transport thermal energy.

Different CSP technologies exist, with parabolic troughs being the oldest and most established. For the troughs, often more than a kilometre long, the sun is focused onto a line at the focal point of the parabolic-shaped reflector. The working fluid is commonly a synthetic oil, although new research is investigating other liquids that are more environmentally friendly. Another line-focus CSP concept uses the idea of a Fresnel lens, based on the work of Augustin-Jean Fresnel, a French engineer, in the 1800s. This CSP technology is called a Linear Fresnel Reflector and is being investigated by Prof Craig and his group. Basically, each mirror in a series of long, flat mirrors is aimed at a downwards-facing receiver located above it, and, remaining focused on the receiver, tracks the sun throughout the day.
 

As shown in the figure, the mirrors are located on a horizontal plane and reflect the solar rays onto the cavity receiver. This receiver has a glass cover like a glass greenhouse to trap the heat from the sun. Actually, it lets the sun’s energy through but blocks some of the energy radiated back from the hot surfaces inside the cavity, like the pipes. The research that Prof Craig and postgraduate students Mohammad Moghimi Ardekani (doctoral) and Ansuya Rungasamy (master’s) are performing together with some undergraduate students, is aimed at improving the efficiency of the transfer of heat from the sun’s rays to the liquid inside the pipes. For this they are using computer simulations called computational fluid dynamics or CFD. These computer programmes solve the equations describing how fluids flow and how heat is transferred through radiation, conduction and convection. Coupled with optimisation software, these simulations allow the researchers to seek the best geometry for the cavity and mirror array as well as to investigate different coatings on the cavity surfaces that will soak up as much of the sun’s heat as possible without giving too much of it back to the surrounding air.

 

Another CSP technology that is used for large-scale power production is called a solar power tower. The largest plant of this type in the world is located at Ivanpah in California and can produce close to 400 MW. For this technology, a central receiver is located on a tower in the middle of a field of mirrors called heliostats. Each heliostat tracks the sun individually, much like Archimedes’ sun weapon. Because this means that there is a point focus rather than a line focus as described above for the parabolic troughs and Linear Fresnel Reflectors, much higher temperatures are reached, easily in excess of a thousand degrees. Designing a receiver that can firstly withstand these temperatures without melting, and that can secondly take this heat away to a steam plant where electricity is generated without radiating too much of it back to the surrounding air, is very challenging. On this, Prof Craig and his group are collaborating with researchers at Stellenbosch University in the Solar Thermal Energy Research Group or STERG. Innovative concepts of central cavity receivers are being investigated using CFD and then optimised to maximise the optical and thermal efficiency of these central receivers. The modelling of the sun’s rays and discovering the ways they are absorbed, reflected and transmitted are particularly challenging. Novel ways are being researched in how the CFD software can simulate the radiative properties of the receiver surfaces for different temperatures and different wavelengths of the sun’s energy. Other optical software, like SolTrace from the US National Renewable Energy Laboratory, is also linked with the CFD software to provide the thermal energy of the sun as a boundary condition. Participating in this research are Mohammed Moghimi Ardekani (doctoral), Logan Page (doctoral) and Justin Marsberg (master’s).


The heliostats or mirrors that make up the combined reflector in these solar tower plants can number in the hundreds of thousands and their cost could be half that of the initial costs of such a solar plant. Obviously, if each heliostat can be made lighter and cheaper, the overall savings could be significant. As these heliostats or mirrors are located outside and are exposed to a variety of environmental conditions, like high winds, hail storms, and dust storms, (depending on where they are located) they need to be designed to withstand these conditions for the duration of the solar plant’s life, often in excess of 20 years. Along with several undergraduate students, three of Prof Craig’s postgraduate students, Pierre Poulain (doctoral), Dawie Marais (master’s) and Gerrie Delport (master’s), are using both CFD and Finite Element Analysis software to investigate heliostat designs that are lightweight, strong and cheaper to build while able to withstand wind loads and other environmental conditions. On this, they are also collaborating with Stellenbosch University and researchers at the CSIR.

Along with the computer simulations being done of these interesting CSP topics, plans are underway to establish experimental research facilities where these designs can be evaluated. Several UP campus buildings are being evaluated as candidates for such facilities. These would not only provide much needed laboratory space in the form of roof-top labs, but they could also assist in reducing the carbon footprint of the University by providing thermal and electric energy to the individual buildings in a renewable way.