SOLAR TODAY Blog


>> Waste heat and thermal pollution
April 16, 2008, 5:27 pm
Filed under: Utilities

By Seth Masia
SOLAR TODAY

On March 14, NPR’s Science Friday did an hour on concentrating solar power, with Mark Mehos (NREL), Fred Morse (Abengoa) and David Mills (Ausra). A listener called in with a question: What’s the effect of excess heat bounced back from the mirrors into the atmosphere?

The answer is that there’s none. The solar array creates no new heat — it simply intercepts some of the sun’s heat and converts it to other forms.

Waste heat is in fact a legitimate issue for most forms of electrical power. The typical coal, gas or nuclear-powered electric generating station is only about 35% efficient, which means that most of the energy produced goes out not as electricity but as hot water dumped into a river, lake or ocean, or as hot water vapor pumped into the air. Worldwide, this year, it appears that electrical plants will dump the heat equivalent of 2.4 trillion watts into the air and groundwater every year, over and above any heat retained through greenhouse gas emissions.

I talked to Chuck Kutscher of NREL about thermal pollution from power plants. He calculates that powerplant thermal pollution amounts to about 4% of the heating effect due to greenhouse gases.

“That’s just my back-of-the-envelope calculation and, to be honest, it’s a bit larger term than I expected,” Kutscher said. “That is, it’s not completely negligible.”

Concentrating solar power (CSP) does require large flows of of cooling air or water to condense the steam exiting the turbine, and that can constitute a local water-use issue. The discharged heat is not, however, waste heat, in the sense that it’s converted solar radiation that would have entered the biosphere anyway.

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3 Comments so far
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Dear Solar Today folks:

The blog looks great!

Comment by Carol Gulyas

I am a science teacher at a private high school in NH. My students are currently working on a project to design and create model structures that would reduce or completely inhibit the return of warmed water from power plants back to the enviroment, with a focus on re-using the water for a productive purpose. Your sight has been useful to gather info. Any other ideas or resources you can suggest for us? Thanks.
Traci Dorsey

Comment by Traci Dorsey

Good project! Let me first suggest that returning water directly to the river, unpolluted and at its original temperature, is in itself a useful purpose. The point is to draw as much heat from the water as possible and use that heat for some useful purpose. The question is, what the limit of heat transfer — that is, at what point in the heat-recovery process does it become thermodynamically or economically inefficient to recover more, and how much warmer than ambient is the cooling discharge at that point?

Comment by Seth Masia




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