Gas behind the scenes

By the original Public Utility Regulatory Policies Act (PURPA) mandates, no more than 25% of the energy input to solar energy systems could come from non-renewable sources, but an excuse was found to allow a higher percentage at the Solar Energy Generating System (SEGS) in the Mojave Desert.

The heat-transfer fluid called therminol freezes at 55 degrees F (13 degrees C), so it requires heating at night, even in the Mojave Desert. They use natural gas to do so.  This heating was considered to be “process heat,” not subject to the 25% limitation.  In fact, 38% of the heat comes from natural gas, and the overall thermal efficiency is an unimpressive 25.6%.

The eight separate parabolic reflector facilities produced 857 GWh of electricity in 2000, with the emission of 144 tonnes of NOx and 190,000 tonnes of CO2,  amounting, respectively, to 223 tonnes of CO2 per GWh (gigawatt-hour) and 0.168 tonnes of NOx per GWh.  

These figures are better than those the generic natural gas plant, 519 and 1.30 respectively.  For almost 17¢/kWh wholesale price, there ought to be some advantage, but even this one is overrated, because a modern combined-cycle power plant burning natural gas and produces only 382 tonnes of CO2 per GWh, and 0.046 tonnes of NOX per GWh.

That is, the solar unit produces about three-and-a-half times as much NOX as the co-generation power plant for every GWh of energy generated.