Differences between revisions 3 and 4
⇤ ← Revision 3 as of 2016-04-28 17:27:28 →
Size: 797
Editor: KeithLofstrom
Comment:
← Revision 4 as of 2016-04-28 17:28:01 → ⇥
Size: 793
Editor: KeithLofstrom
Comment:
Deletions are marked like this. Additions are marked like this.
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. $0.40 per full bathtub, $160/year for a bath every night		  * $0.40 per full bathtub, $160/year for a bath every night
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. $0.93 per full bathtub, $340/year for a bath every night		  * $0.93 per full bathtub, $340/year for a bath every night
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 * $0.35 per full bathtub, $130/year for a bath every night
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 . $0.35 per full bathtub, $130/year for a bath every night

A Bathtub of Water

  • Bathtub holds 9 cubic feet, 0.25 cubic meters, 250 kilograms

  • person displaces 50 kg? -> 200 kg of heated water

  • Inlet water 10C, tub water 40C

  • 30C * 2e5 g * 4.184 J/calorie -> 25 MJ

  • Gas is $1 per therm, or $1 per 63 MJ at 60% heating efficiency
  • $0.40 per full bathtub, $160/year for a bath every night

Electric Resistance

  • $0.12 per 3.6 MJ, 90% efficiency -> $1 per 27 MJ

  • $0.93 per full bathtub, $340/year for a bath every night

Electric Heat Pump

  • $0.12 per 3.6 MJ, 240% efficiency -> $1 per 72 MJ

  • $0.35 per full bathtub, $130/year for a bath every night

  • est $2000 purchase and install plus one service call -> 67 year payback

  • If electric rates rise because of renewables, no savings at all

Bathtub (last edited 2016-04-29 05:31:00 by KeithLofstrom)