Thread: MBTA Bus & BRT
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Old 12-29-2017, 09:25 PM   #456
Joel N. Weber II
Senior Member
Join Date: Aug 2015
Posts: 587
Re: MBTA Bus & BRT

Originally Posted by Arlington View Post
Which is 2.3x more heat than you get with resistance heat (COP =~ 1) and therefore even at -4F, cuts in half the energy drawn from the battery to do any heating. And by ~20F you're back to COP = 3.

For the cost of a valve and a bit of electronics, next gen e-buses can reverse the operation of the A/Cs already onboard and can radically improve their cold weather performance, particularly if some of that pumped heat is directed to keeping the battery in warmish (40F ~ 60F?) (which increases its usable capacity).
I certainly don't disagree that using a heat pump is desirable even at -4F when the COP is only 2.3.

The other challenge with heating in extreme cold is that if the energy requirement for the heating / cooling is basically proportional to the difference between inside and outside temperature, and if you want an inside temperature around 72F, then an outside temperature of 99F gives you a 27F difference, and a -8F outside temperature gives you an 80F difference, which suggests the worst case winter BTU/h requirement may be roughly 3x the summer requirement (although I'm ignoring the possibility that some energy might be expended on dehumidification in the summer). If you're serious about using the heat pump to minimize the battery draw in Boston's coldest weather, you might want a much larger heat pump than you'd want if you were just going to use it for cooling in the summer. (And this is somewhat climate dependent: this imbalance between worst case heating and worst case cooling probably doesn't look the same in Arizona or southern California.)
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