[Greenbuilding] is it ever sensible to use PV to heat water?

[larencorie at axilar.net]

From: Carmine Vasile <gfx-ch at msn.com>

> Since a PV system is nearly 100% efficient for electric
> space & water heating,

Hi Carmine;

Though I think you may mean electric resistance (not PVs)
PVs, depending on the type, are more like 10% efficient, not 100%

> We live in an all-electric house and use about 20 MWh/yr.

Yikes!  That is about 17 time as much as I use, and I don't
use any fossil fuel either, in a very cold climate (that
includes for lawn care and clearing snow in a very snowy
climate). You use nearly double what the average American
household uses. Surely you have an interest in reducing that.

> Our average monthly bill is $237 -- which is a
> lot less than my neighbor's oil + electric bill.

Think about how nice it would be to only have
to pay about $200 less than that, every month.
IIRC you do not live in a very cold climate.

> 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.

Why don't you work on a little energy efficiency?
It should be easy to reduce your usage by 50-80%
Your household is virtually hemoraging energy. It
is bound to cause Summer overheating.   That rate
of usage is equal to about 7800BTU/hr, about 1-1/2
time as much heat as runnng the largest plug-in
electric heater 24/7/52

From: molasses at q.com

> 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.

Hi Kat;

  What about simple, low-cost Solar space heating?
What is your South wall like? There is an inherent
flaw in the strategy of waiting for something to
change in your financial life, instead of making
it change.  For most houses there are many, very
cost-effective things that can be done now, that
will save as much as they cost, in a fairly short
time.   In fact, had you ask your question, and
done something in the Fall, you could have had
your money back already (possibly more than just
one time, for some energy improvements).  Most
houses are very leaky. In general, caulk is a
real good investment, and it is so cheap that
you don't have to wait to have the money for
it.   You can get a very functional caulk gun
at many dollar stores, and for two or three
dollars a tube caulk can eliminate a lot of
uncomfortable and expensive drafts.  It is
also healthier for your house, to not have
your warm humid interior air condensing its
moisture inside your walls.  You might also
make window insulation, or add some insul-
-ation to a poorly insulated attic.  There
are also freee tricks, like learning to live
comfortably with the thermostat lower in
Winter, ort without AC in Summer.  Dressing
for the season saves money.   Another trick
is to heat a two-liter soft drink bottle,
filled with water, in the microwave, and
using it as a bedwarmer, or against your
body, while lounging on the couch.  The
list goes on. What is important is that
these initial saving will improve you
financial circumstances, and can be used
to indirectly fund your future Solar
electric system.  Think of your energy
efficiency improvement as investments,
and plan accordingly. Energy = Money.
Sure, there is a whole lot more to it
than that. But, when it comes to
financing a system, it is all about
Energy = Money. You can finance your
system with your savings from energy
improvements that you can afford......now!

First, do the thing that will return
your money fast. Get them woking before
you consider tying your money up for year,
in slow payback strategies. You live in a
relatively overcast climate, with relatively
cheap electricity. PVs probably should be
near the bottom of your to-do list.

> Here is the situation with my house:

Basically old.....probably would gain a lot from
insulating and sealing.

> Gas water heater currently in basement
> Get rid of natural gas (personal preference)

Bad idea at this point, since natural gas is currently cheap.
Remember, as pure as your heart may be, you can do no good
without the finacial means.   So, do not make the tragic
mistake of crippling your self finacially. Do the things
that will strengthen your financial position, like buying
and using a caulk gun. Unfortunately, it is easiest to
find air leaks in cold weather, when you can feel the
cold, or even the air flow of the stronger stack effect,
than it is this time of year..

> 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

  If you have someone who is handy (you?) you might make
a batch type Solar water heater Tank in an insulated glazed
box). Your average January low is a couple degrees above
freezing, so with multiple glazings, good insulation, and
closing an insulating lid, you should be in the positive,
only using a little heat tape for freeze protection. You
might even build it from mostly salvaged materials.

> Don't want a tankless electric because they're not
> very efficient

You definitely do not want one of those.

> Put 20 gal electric tank on wall above washer/dryer

Wash in cold water......no water to heat, at all.
No water heater to buy.

> 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

Keep your present heater. Maybe add an insulation blanket.

> or a solar water heating system.

 Maybe a batch heater....do a net search.
There is a free book download about them
on Gary's www.builditsolar.com

> Given my situation, I think PV would make more sense
> than solar water heating.

And buying a jumbo jet makes more sense than sprouting wings.
However, there are plenty of energy improvements that make
several times more sense, than either of those systems.

>>From that I can maybe derive a few rules of thumb:
> 1) solar access

What about your South wall?

> 2) storage space

For simple Solar air heating the mass of your
rooms and furnishings will serve as storage.

> 3) money (I can do this plan in stages

Yes, but start with energy conserving.
How much electricity do you use in an average month?

>- electric water heater now, PV array later)

How do you heat the house?
What makes you think that an electric tank heater
will save you anything?  If not, then invest your
money in improvements that will payback quickly,
so that you can get your money back, to reinvest,
while the original improvements continue their savings.

> 4) occupant usage patterns

Yep....how many in your household?

> From: "Reuben Deumling" <9watts at gmail.com>

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

Hi Reuben;

  Though the bigger issue (than overcast) is your
relatively short heating season, you can still get
good payback from site built wall mounted Solar air
heaters, which could deliver about 100,000BTU/sqft
per heating season.  At around $3/sqft material
costs, the payback would be fast.

 Laren Corie-
 Natural Solar Building Design Since 1975
 www.ThermalAttic.com

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