CSP development

Photovoltaics (PV) is the dominant solar technology with more than 12 GW installed in 2010 alone, but PV (not to mention wind power) has inherent limitations of intermittency. This gives CSP a distinct advantage, and current advances in heat transfer and storage could increase the implementation of CSP as a significant renewable resource.

Introduction to CSP :

For concentrating solar power (CSP) there are four different technology approaches – parabolic trough, power tower, linear reflector, and sterling dish . Currently, more than 90% of installations use parabolic troughs to generate electricity.

The main advantage of CSP against other sources of renewable energy such as PV or wind is the capability to provide dispatchable power – by storing solar energy in thermal reservoirs and releasing it as and when it's needed i.e. during periods of peak power demand, during cloudy weather or even at night. So storage can eliminate intermittency as well as extend energy production past sun-set. Another advantage to CSP is that it can be used as part of a hybrid energy source, in that a regular gas fired plant can be used to heat the HTF/TES materials (see below) in the event of solar down time. This configuration is much like the early hybrid cars.

Despite these advantages however, CSP-derived electricity is expensive in comparison with other technologies. The key to CSP's commercial success remains in developing an economical and effective energy storage capability.

There are two intertwined technology paths for CSP which both need to be advanced – the solar collection technology and the heat conduction technology. In this article we will address the latter.

Developments in heat transfer and storage materials

Heat conduction technology :

Except for sterling dish technology, CSP needs a Heat Transfer Fluid (HTF), and in some cases a Thermal Energy Storage (TES) medium. The HTF and TES materials are the interface between the solar energy input and the power block.

From a commercial perspective HTF and TES are at very early development stages. Although they can function adequately, the current HTFs suffer from significant shortcomings. As far as the TES elements, this is even more challenging, with only marginal industrial activities underway.

But there is now a strong push to extend the capabilities of HTF/TES, and the results could help enable the acceleration of CSP beyond the 500 MW of current installations. Activity for both remains very much an R&D effort, especially for TES. But there is now an increased emphasis to address both the limitations of current HTF/TES, as well as to develop advanced elements. Groundbreaking research is looking at HTF and TES – for example how nanostructures can be used to tailor the fluids' thermo-physical properties?

New funding is also being directed towards a broad range of near-term improvements (as well as long-range R&D).

The key considerations for improvement of HTF/TES materials include the ability to vary the operational temperatures; the range of useful temperatures; the heat capacitance; and – very importantly – the cost. The latter is essential in the world of solar energy. The rapid implementation of PV in the last year has been accelerated by a drop in PV prices. It is essential for CSP to achieve some of the same cost reductions .

Current limitations to HTF/TES :

Up to now, CSP plants have used synthetic oils as heat transfer fluids and molten salts for thermal energy storage (molten salt can also be an HTF). .

However there are significant limitations which impact the overall cost of CSP electricity. The most proven and commonly used CSP technology is parabolic trough technology, which absorbs solar radiation and reaches temperatures of around 700°F (371°C). In the heat exchanger, water is preheated, evaporated, and superheated into steam, which runs a steam turbine. The water is cooled, condensed, and reused in the heat exchangers. Most of these plants have little or no storage and use oils which are flammable. Operating at higher temperatures enables higher power cycle efficiencies to be achieved .

The organic oil-based HTF currently tend to break down at high temperatures (around 400°C), which prevents solar thermal plants from running at maximum efficiency. Inorganic materials such as salts, on the other hand, maintain stability at high temperatures, but then solidify easily at temperatures as high as 230°C – a problem because when the sun drops in the desert, so does the temperature.