thermodynamics

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My commentary on a question about Venus

Note: I wrote the following at the request of climate blogger "Science of Doom" - go there to read the full article in context! Also, please make comments over there, not here, thanks.

[The following is in regard to models of the high temperature of the surface of Venus which ascribe that high temperature essentially to the high atmospheric pressure on that planet, without properly recognizing the role of infrared absorption by greenhouse gases. In particular, Leonard Weinstein, in previous discussions there, had essentially claimed that even if all solar radiation were absorbed by a thin high layer in Venus' atmosphere at a relatively cool temperature, the surface temperature would still be very high. The following is why I disagree.]

The problems with combined heat and power (CHP critique part 3)

"Combined Heat and Power" (CHP) or "cogeneration" systems for producing both heat and electric power are generally mature and really can reduce emissions of CO2 compared to other fossil-fuel technologies. But there are two problems with typical discussion of CHP:

(1) Fossil-fuel-based CHP cannot be a long-term solution on climate or energy because they still burn fossil fuels, and therefore still emit a lot of CO2. Reducing that by 20% or even 50% is not enough; we need to take steps that over the next 30-40 years will bring fossil CO2 emissions close to 0.

(2) Efficiency claims for CHP systems are frequently greatly overstated. Heat is lower-quality energy than electricity, and only at high temperatures does it become close to comparable. Efficiency claims for CHP systems that use high-temperature heat are not so far off, but CHP systems that make use of low-temperature waste heat have much lower thermodynamic efficiencies than usually claimed.

The inflated efficiency claims often lead to assertions that CHP is the "largest" or one of the largest potential solutions. But the number of applications that require high-temperature heat where CHP efficiency really is quite high are limited. And the modest efficiency gains with low-temperature waste heat use, which could be much more widely applied, don't lead to very much improvement in overall energy use. The combining of heat and power production in CHP systems can reduce our fossil CO2 emissions by a few percent, but much more than that is needed in coming decades.

The problem with heating "efficiency" (CHP critique part 2)

My criticism of various claims about "Combined Heat and Power" (CHP) or cogeneration systems was something I had thought I could keep brief, since the issue was intuitively clear to me. But explaining my understanding of the subject in straightforward non-scientific language has proven trickier than I'd expected. This post constitutes part 2, and will cover problems with many claims of "efficiency" in heating systems; part 3 will be more specifically focused on the several different types of CHP, what is good about them, and what is frequently over-hyped.

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