Monday, March 8, 2010

Solar Thermal in the Desert - A Design Optimization Problem

Solar thermal power is an efficient, well proven method for generating power from sunlight by concentrating sun light in order to heat up a working fluid that, in turn is used to generate steam to run turbines. You need to put these things where there is a lot of sun. Such places are usually hot and dry... e.g. deserts.

But, running a power plant typically requires significant amounts of water to run and cool the turbines.
This 2002 report summary puts the number for wet cooling at around 15,000 gal / MWh.
Fortunately, there are technologies to use much less water: 200 - 250 gal / MWh, a >90% drop. However these technologies are:
  • Expensive: ~7x to 17x the wet cooling system cost 
  • Less efficient: 
    • power output must be reduced at times when the ambient temperature exceeds the design temperature (to avoid damaging the turbines) 
    • Output of the heat cycle drops when the output temperature is high.
So, in this context, when you hear about a project like Ivanpah, a 400MW solar thermal plant to be put in the CA desert, near (~60miles from) Death Valley National Park, you should be wondering where the water will come from.

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Fortunately, BrightSource Energy is using dry cooling so that they "Will use 100 acre feet [~32 million gal] per year, the equivalent of 300 homes’ annual water usage."  If every power plant did this, following the logic of my previous post,  it would:
  • easily beat the ~20% improvement you'd need to be equivalent to all buildings being LEED 3.0 for water efficiency
  • would be enough to meet the ~46% improvement you'd need to replace all the water used domestically and industrially.
So at least you know where much of the $1.4B in loan guarantees from the DOE must be going and it's a good thing.