[at-l] Confounded Carnots! :)

[icw@xxxxxxxxxxx (Clark Wright)]

What I want to know is what is the Carnot Efficiency of my stomach
eating little debbies, versus cheese whiz?  Then we can divide those
efficiencies by weight to arrive at the best Carnot Efficiency per ounce
foods as metabolized by the average human body - now THAT would be some
useful Trail info for our 2003 friends! :)

thru-thinker

J Bryan Kramer wrote:

> OK time for a Thermo talk: the theoretical efficiency of anything is known
> as the Carnot Efficiency and is calculated as
>
>  Eff= (Temp(hot) - Temp(cold))/Temp(hot)
>
>  with the Temperature in degrees K. So for a power plant assuming a
> Temp(hot) of about 900 deg K (that's the superheated steam) and a Temp(cold)
> of about 300 K (in the condenser) you get
>
> eff=(900-300)/900 = 67%
>
> The efficency that gets thrown around all the time is percent of theoretical
> efficiency so a 95% efficient power plant would be (95/100)*67 = 64%
>
> Getting the steam temp higher or the condenser temp lower boosts efficiency
> but there are thermodynamic problems with doing much of that since steam
> isn't a theoretically perfect fluid. For example if you chill water below
> 270 or so K you get ice which is hard to pump.
>
>  But in general power plant efficiencies run in the 60% range, cars are in
> the 20-30% range. So whenever you see an efficiency of 90+ % you can
> instantly know that they are talking about percent of theoretical or Carnot
> efficiency since no real world device gets there. Electical devices
> themselves are highly efficient but you have to include the efficiency with
> which the electricity was produced that is needed to run them, so a 90%
> efficient motor is running on 60% efficient electricity.
>
> Bryan
>
> "Si vis pacem para bellum"
>
>
>>-----Original Message-----
>>From: Bob C. [mailto:ellen@clinic.net]
>>Sent: Thursday, January 30, 2003 18:12
>>To: Jim and/or Ginny Owen
>>Cc: jbryankramer@msn.com; at-l@backcountry.net; amberchime@comcast.net
>>Subject: Re[2]: [at-l] majority of Americans want more wilderness
>>
>>
>>
>>>"... we were hitting on 93-96% theoretical efficiencies back in the early
>>>60's."
>>>
>>I don't know what theories Jim's sources used to measure
>>electrical generating
>>efficiency. But I sat through scores of hours at hearings on the
>>licensing of
>>electrical power plants in which the impact of cooling devices
>>were the major
>>issue. The "experts" all seemed to think that 70 percent of the
>>energy in the
>>fuel escaped into the environment one way or another and only 30 percent
>>actually turned the turbines that generated electricity.
>>
>>Gas turbines can achieve efficiencies much higher efficiencies
>>than this. They
>>use a totally different technology. But power plants burning coal
>>or oil, or
>>using nuclear fuel to generate steam to generate electricity ,
>>i.e. most of the
>>electricity used in this country, cannot. At least I reported
>>this as a fact for
>>30 years or more and never once did an applicant dispute these
>>basic facts.
>>
>>It's the lost energy from power plants that make electrical generation so
>>expensive and why heating houses with electricity is never as
>>cost effective as
>>burning the same fuel in a home furnace or boiler. The oil burner
>>in my basement
>>claims 92 percent efficiency. If the power plant could achieve
>>this efficiency
>>everyone would heat by electricity.
>>
>>Weary
>>
>>
>>
>>
>>
>
>
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