[Greenbuilding] Yes if you install a ground array and have lots of money.RE: is it ever sensible to use PV to heat water?

[Carmine Vasile]

Kat: No need to have "rules of thumb" or Reuben's "instinct" ; simply total your energy usage from your fuel & electric utility bills and convert to kWh/yr less fuel wasted. Since a PV system is nearly 100% efficient for electric space & water heating, you can estimate the PV array size. If your roof is too small, install a ground array in your backyard.    We live in an all-electric house and use about 20 MWh/yr. Our average monthly bill is $237 -- which is a lot less than my neighbor's oil + electric bill.    Unfortunately, our local utility will not offer rebates for a ground-mounted 20  MWy PV array and our roof is too small to fit 20 MHw worth of PV panels.  Carmine   

Date: Thu, 15 Mar 2012 15:57:55 -0400
From: molasses at q.com
To: greenbuilding at lists.bioenergylists.org
Subject: Re: [Greenbuilding] is it ever sensible to use PV to heat water?



I think that my house would be a good candidate for PV  instead of solar water heating, and someday when I have the money it's what I plan to do.  Here is the situation with my house:

Portland, Oregon
700 SF one-level home w/ 200 SF unfinished, uninsulated basement open to vented crawlspace
Urban area with one plane of gable roof south-facing; deciduous trees in neighbor's yard partially shade roof during the summer
Poorest zip code in Portland - 100K of upgrades to this house would not make financial sense
Nowhere near enough roof area (or money) to put a solar array on the roof that would take care of entire (modest) annual electrical load of occupants, though we'd come a lot closer to covering summer loads with a PV array than higher winter loads
Heavy cloud cover for weeks just when we want the most hot water (winter)
Gas water heater currently in basement
Get rid of natural gas (personal preference)
No room for large storage tank (unless it's in the basement) (solar systems require rather large tanks, yes? The ones I remember seeing have 2 tanks, both 60 gal or so)
Don't want a tankless electric because they're not very efficient (? if I'm remembering right - perhaps this will change by the time I have the money)
Put 20 gal electric tank on wall above washer/dryer (storage space is at a premium!) on switch/timer, set temp high and use mixing valves to prevent scalding.  Leave off most of the time, turn on 45 min before showering.  Timer keeps WH on for 75 min (?).

I'll be able to afford the change in water heaters LONG before I would ever (perhaps never) be able to afford a PV array or a solar water heating system.  Given my situation, I think PV would make more sense than solar water heating.

>From that I can maybe derive a few rules of thumb:
1) solar access
2) storage space
3) money (I can do this plan in stages - electric water heater now, PV array later)
4) occupant usage patterns
5) ???

-Kat

From: "Reuben Deumling" <9watts at gmail.com>
To: "Greenbuilding" <greenbuilding at lists.bioenergylists.org>
Sent: Thursday, March 15, 2012 11:55:15 AM
Subject: [Greenbuilding] is it ever sensible to use PV to heat water?

My instinct says 'rarely.' but in the following case I wonder:

two person household; very frugal with hot water usage; already flip breaker on electric tank water heater off most of the time

planning on adding a PV system that will produce more than they use in the summer (minimum system size requirements for grid connected subsidies.... you know). 


Neither solar PV or solar thermal contribute much of anything in the winter here (cloudy PNW)

Are there rules of thumb?

Thanks.


 		 	   		  
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