Concentrated Solar Thermal Versus Concentrated PV

A Hawaiian based concentrated solar thermal company called Sopogy has been making headlines recently with it's trough based system being used by Sempra Energy utility Southern California Gas Company (SoCal Gas) in combination with a thermal air conditioning unit to demonstrate the power of a solar air-conditioning demonstration system.

The small-scale solar thermal startup has substantial investor backing from the likes of eBay founder Pierre Omidyar, Ohana Holdings so we thought it was worth a closer look at the fairly simply constructed solar thermal collectors to see what all the fuss is about.

According to Sopogy's own application notes the systems is not very efficient. They specify a 40% loss from solar to thermal and a further 80% loss thermal to electric. That gives an overall electric generation system efficiency of just 12%. This means that a single 5.57m2 trough collector generates just 512 watts of electrical power or $0.25 worth per day. Of course a solar thermal A/C system avoids the electric generation stage by making use of the thermal energy which is 60% energy efficient.



A good comparison is between concentrated solar thermal and concentrated Photo voltaic such as those built by Solar Systems in Australia. The 15 meter diameter CS500 parabolic dish PV system generates over 36 kw of electrical power at an energy efficiency of 36.5% using just 0.23m² of PV solar cell material. The concentrating dish uses a similar principle to Sopogy's trough with cheap material used for the reflective surface to concentrate the sun by 500x. This dramatically reduce system costs compared with a flat plate PV system of the same power that require approximately 350m² of expensive PV cells. The surface area of PV material used in the concentrated PV system is 1/1000 that of flat-plate PV systems.

While the dish PV system generates electricity at 36.5% (compared to 12% for Sopogy) much of the work in developing this system has been to remove the remaining 63% of the sun's energy from the back of the PV cell to keep it within it's operating range of 60C, no mean feat when you're dealing with temperatures at the focal point that can melt steel.

At present this huge amount of removed thermal energy is not used for co-generation and is either run through a cooling system or ground sunk. Since the CS500 systems waste is equal to the entire thermal output of the Sopogy collector, Australian based Solar Systems have the potential to run thermal A/C and-or organic rankine cycle power generation, which is basically an A/C run in reverse, in addition to the PV generated electrical output.