Comments on: For Coffee Drinkers… http://301south.net/2009/12/for-coffee-drinkers/ (an exercise for the reader) Sat, 17 Jul 2010 22:11:53 -0700 http://wordpress.org/?v=2.8.4 hourly 1 By: brandon http://301south.net/2009/12/for-coffee-drinkers/comment-page-1/#comment-153 brandon Fri, 18 Dec 2009 02:42:58 +0000 http://301south.net/?p=140#comment-153 <a href="#comment-152" rel="nofollow">@brandon</a> I used this for my anecdotal numbers: http://www.standardsasap.org/documents/WH_Q&A_Aug_2009_final.pdf It seems like "Energy Factor" or E.F. that they use to rate water heaters is the efficiency of transferring heat to the water; but the linked document discusses energy efficiency at the <i>point of use</i> and says it's more like 43% for most water heaters. @brandon
I used this for my anecdotal numbers:
http://www.standardsasap.org/documents/WH_Q&A_Aug_2009_final.pdf

It seems like “Energy Factor” or E.F. that they use to rate water heaters is the efficiency of transferring heat to the water; but the linked document discusses energy efficiency at the point of use and says it’s more like 43% for most water heaters.

]]> By: brandon http://301south.net/2009/12/for-coffee-drinkers/comment-page-1/#comment-152 brandon Fri, 18 Dec 2009 02:24:30 +0000 http://301south.net/?p=140#comment-152 <a href="#comment-150" rel="nofollow">@ Tobin Fricke</a> sigh... hot showers are my one pernicious indulgence. well, hot showers and sleeping in. and today I had a hamburger.. So I have many indulgences. Anecdotally, gas-fired heaters are about 80% efficient (heat loss out the exhaust). Electric heaters are themselves 100% efficient, although as you say, electricity generation is only maybe 35%. Generally, electricity should not be used for heat unless it's running a heat pump, which can multiply efficiency by 2 or 3 (or more) as long as it's not too cold out. [heat pumps basically air-condition the outside]. Gas-powered water heaters are a different story, since instead of just making heat they have to transfer the heat to the water. If the water is stored in a tank, then it basically just fritters away energy. Again anecdotally, tank gas water heaters are about 50% efficient. Tankless (on-demand) models are more like 75%. If we back-of-the-envelope it, say a gas water heater is 50% efficient, the temperature change of the water is 40C, the flow is 2.5gpm or 9.5 liters per minute ... and this isn't a proper life-cycle assessment, of course ... just the heating of the water would take about 3.2 MJ worth of natural gas per minute of shower, not counting the energy to pump the water in from Colorado. In contrast, driving a car (well, my car) at 70mph I get 30mpg or 4.3 MJ of gasoline per mile. That gives an equivalency of the better part of a mile of driving per minute of shower. @ Tobin Fricke
sigh…

hot showers are my one pernicious indulgence. well, hot showers and sleeping in. and today I had a hamburger..

So I have many indulgences.

Anecdotally, gas-fired heaters are about 80% efficient (heat loss out the exhaust). Electric heaters are themselves 100% efficient, although as you say, electricity generation is only maybe 35%. Generally, electricity should not be used for heat unless it’s running a heat pump, which can multiply efficiency by 2 or 3 (or more) as long as it’s not too cold out. [heat pumps basically air-condition the outside].

Gas-powered water heaters are a different story, since instead of just making heat they have to transfer the heat to the water. If the water is stored in a tank, then it basically just fritters away energy. Again anecdotally, tank gas water heaters are about 50% efficient. Tankless (on-demand) models are more like 75%.

If we back-of-the-envelope it, say a gas water heater is 50% efficient, the temperature change of the water is 40C, the flow is 2.5gpm or 9.5 liters per minute … and this isn’t a proper life-cycle assessment, of course … just the heating of the water would take about 3.2 MJ worth of natural gas per minute of shower, not counting the energy to pump the water in from Colorado.

In contrast, driving a car (well, my car) at 70mph I get 30mpg or 4.3 MJ of gasoline per mile.

That gives an equivalency of the better part of a mile of driving per minute of shower.

]]> By: Tobin Fricke http://301south.net/2009/12/for-coffee-drinkers/comment-page-1/#comment-150 Tobin Fricke Thu, 17 Dec 2009 21:01:12 +0000 http://301south.net/?p=140#comment-150 Thanks for the link - I'm one of those people who needles anyone who reaches for the disposable (styrofoam!) cup at work. I wonder how the efficiency of an American-style automatic dishwasher compares? Also, how does the energy consumption of a typical American-style hot shower compare to other energy uses? I bet it's huge. (How many miles driving is equivalent to a 20-minute hot shower, etc?) How does electric water heating compare to gas heating? I'm rather aghast at the concept of electric <i>heating</i>, since (I think!) conversion of fuel to heat can be nearly 100% efficient, but converting fuel to power to heat is limited by the carnot efficiency (etc). Thanks for the link – I’m one of those people who needles anyone who reaches for the disposable (styrofoam!) cup at work. I wonder how the efficiency of an American-style automatic dishwasher compares?

Also, how does the energy consumption of a typical American-style hot shower compare to other energy uses? I bet it’s huge. (How many miles driving is equivalent to a 20-minute hot shower, etc?)

How does electric water heating compare to gas heating? I’m rather aghast at the concept of electric heating, since (I think!) conversion of fuel to heat can be nearly 100% efficient, but converting fuel to power to heat is limited by the carnot efficiency (etc).

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