Solar Thermal Heater: Molten Salt & Synthetic Oil

I came across a useful article on solar thermal power plants by MIT's Technology Review. It summarizes efforts of large energy companies to harness solar energy by capturing the energy through evaporation of a suitable working fluid in a Rankine cycle. The resulting steam from the evaporation is used to drive turbine blades, which then converts the steam's energy into electricity.

Siemens wants to use a mixture of molten potassium & sodium nitrate (molten salt) for the solar thermal power plant it designs and manufactures. The problem is that this molten salt freezes at around 220 °C. Kilometer long pipes used for this large scale power plant can be clogged by solid chunks since it is also difficult to control the pipe temperature to be always above the freezing point 24/7. Heat loss - radiative and convective - can be significant and freezing risk is real. A large scale power plant would require a heat storage to regulate the molten salt temperature.

This limitation also means that molten salt is not a good material for small scale power plant. The daily fluctuations of solar intensity makes molten salt unattractive as a result.

Solar thermal energy conversion method is more promising than solar photovoltaic since photovoltaic researchers still muse about 14 cents/kWh price, while solar thermal technology already has 13-20 cents/kWh pricing.

Another material used for solar thermal power plant is synthetic oil. The problem here is the temperature cannot exceed 390 °C since the oil will break down.