excess PV capacity used for heating

[tuulikki]

Hi everyone, first post here though I've read many threads in the past.

We're looking at having a roof-mount solar system installed. Our home is located in the Boston MA area and is heated by one-pipe steam, powered by a new natural gas boiler. I'm trying to do the mental arithmetic around what the natural gas savings would be if we have more electricity generation capacity than we need and use some of it to power space heaters, for example as extra heat in a bathroom or in the basement.

Don't laugh--I realize it's probably not the best idea, I'm just trying to get the numbers to show me how bad an idea it is and so far haven't been able to set up the problem correctly.

I've read here about some of the ideas of using excess electricity as a preheater for HW but we're not up to any hacking right now.

Numbers that may be useful:
--Total natural gas usage last year (all for heat/HW): 926 therms, at about 80 cents a therm.
--tentatively looking at 4.000 kW DC system, expect it to generate ~4700 kWh AC annually. (House is good but not great re tilt, azimuth, shade.)
--our annual electricity usage is ~4300 kWh.

I'm trying to figure out what else we can do to minimize our CO2 footprint; initially (pre-new gas boiler) had hoped for geothermal but $$ didn't work at all, so we stuck with steam.

Thanks for any thoughts. I have to go now but will try to write out my calculations later and maybe someone can figure out what I'm messing up.

[randen]

If you might have been following Xringers posts on his operating an A7 heat-pump hot water heater which he was operating with solar PV panels. It is truly amazing this time in our lives that these technologies are avalible. Although you have passed on the geothermal you may be able to power some of the efficient air to air heat-pumps like Xringers Sanyos. During the sunny winter weather you may be able to heat your home for nothing. They would no doubt perform well for air-conditioning during the summer. Long hot sunny days powering the air-conditioning. Nice.

The initial outlay is a big consideration but the cost of natural gas and electricity will only move in one direction, up

Randen

[tuulikki]

An air to air heat-pump is a great idea. I'll look at what kind of power that takes. Trying to right-size the upcoming solar power set-up without going overboard. Thanks!

[Xringer]

My daughter lives in Burlington and uses gas heat. She says it's pretty cheap..
I read out my power use for the last heating season (which was on the mild side)
and my power usage wasn't very high.

Here's our full power use. If you use 458 kWh (June) as the base-line (no heating or cooling), anything above that is from running the Sanyo ASHPs..


During January we burned up $115.60 of NStar power (Measured with a TED)..
During that same month, our neighbors (with similar houses) burned oil..
My guess of their cost was $300 to $500 to the oil man..

WOBURN WEATHER CENTER - WU Station History

Our hot water ASHP project this summer is doing okay so far.
The A7 Airtap is using 15 to 17 cents per day right now. (When on-grid)
That's way better than $2 a day using oil heated hot water..

[tuulikki]

---ETA: found a mistake. Left this post uncorrected, but read following posts below--

I've been doing a lot of math, and it looks like this whole idea may work! Many thanks to Xringer for his usage data and the detailed thread about heat pumps.

Comparing his house to our house:

For last year, we used 926 therms of natural gas. This breaks down into ~20 therms per month for hot water and the rest for heat (so ~690 therms for heat).

We also used 4280 kWh of electricity. Some of that was powering window A/C units.

Xringer used a total of 8195 kWh of electricity. He estimates his house baseline at his June usage (458 kWh for 30 days). Scaled up for a year, this gives 5572 kWh.

His heat & A/C via heat pump is therefore 8195 - 5572 = 2623 kWh.

We are located very close to each other, and both homes experienced the same mild winter last year, so heat usage is based on the same outside temperature data.

Now to check how heat production compares:

1 therm produces 10e5 BTU.
690 therm produces 6.90e7 BTU.

1kWh produces 3414 BTU.
2623 kWh produces 8.95e6 BTU.
but it's via a heat pump, so multiply by the HSPF factor ~9
gives 8.08e7 BTU
so we're in the same ballpark!

Cost savings for gas: 690 therm x $0.80 /therm = $552 / year.
Cost of heat pump system like Xringers: $1600 to $2000 plus installation.
Still working on cost of solar system.

If we can produce enough electricity to power the heat pump at this level, we would save (690 therms)(13.5 lb CO2/therm) = 9315 lb CO2. More CO2 than we use with our current driving.

We would have steam heat powered by natural gas as our back-up system and as our hot water system (they're tied together, same boiler heats water which is stored in a SuperStor).

Heat pump would be installed as to be approximating a second heat/cool zone, probably indoor unit would be on the second floor.

Not taken into account:
--boiler efficiency (high 80's or low 90's, can't remember --it's whatever the best available is for steam)
--some of our electricity usage is for A/C
--difference in house size
--difference in insulation

Conclusion: I'm going to talk to the solar contractor about what our maximum production capacity would be given roof size and shade factors.

[Xringer]

"Xringer used a total of 8195 kWh of electricity. He estimates his house baseline at his June usage (458 kWh for 30 days). Scaled up for a year, this gives 5572 kWh.

His heat & A/C via heat pump is therefore 8195 - 5572 = 2623 kWh."


Wow, @ 16.33 cents/kWh $428.33 ($35.69 per month) for heating and cooling with the Sanyo Mini-Splits..
That $428 would buy about 110 gallons of heating oil..
IIRC, that's about what we used to burn on cold months in the BS days.. (BS is Before Sanyo)

[tuulikki]

In discussing my results with a friend, it has come to light that there is an error in one of the equations I am using. Led astray by the internet--imagine!

In calculating heat production by the heat pump, I was using this equation:

<copied from site>
HSPF = hs / Pws 3413 *
where*
hs = heat produced during the season (Btu)
Pws = elctrical power consumed during the season (kWh)
<end copy>

which I found here:

(can't do link--too new)
source is
engineeringtoolbox dot com
section is heat pump efficiency ratings

I have realized their equation is incorrect. It should be:

HSPF = hs / Pws 1000

(the 1000 is to convert kW to watts)

One source for correction:
‪Construction Calculations Manual‬,
by Sidney M. Levy.
On page 651, Google book search shows

HSPF = (BTU of heat produced over the heating season)/(watt-hrs of electricity used over the season)

Update: 10/5/12
engineeringtoolbox site has now corrected their formula.

[tuulikki]

Redoing the math since my original analysis was based on an incorrect conversion for heat production by the heat pump. Again, many thanks to Xringer for his usage data and the detailed thread about heat pumps.

Comparing his house to our house:

For last year, we used 926 therms of natural gas. This breaks down into ~20 therms per month for hot water and the rest for heat (so ~690 therms for heat).

We also used 4280 kWh of electricity. Some of that was powering window A/C units.

Xringer used a total of 8195 kWh of electricity. He estimates his house baseline at his June usage (458 kWh for 30 days). Scaled up for a year, this gives 5572 kWh.

His heat & A/C via heat pump is therefore 8195 - 5572 = 2623 kWh.

We are located very close to each other, and both homes experienced the same mild winter last year, so heat usage is based on the same outside temperature data.

Now to check how heat production compares:

1 therm produces 10e5 BTU.
690 therm produces 6.90e7 BTU.

For heat pump,
HSPF = (seasonal heat production in BTU) / (watts-hrs used through season)

or

(seasonal heat production in BTU) = HSPF * (watts-hrs used through season)

Xringer's heat pump made: (HSPF = 9) * (2623 kWh) * (1000watts/kW)
= 2.36e7 BTU

So between 1/3 and 1/2 the heat we used.

Guesstimating heat pump would offset 1/2 the heat we use:

Cost savings for gas: 345 therm x $0.80 /therm = $276 / year.
Cost of heat pump system like Xringers: $1600 to $2000 plus installation.
Still working on cost of solar system.

If we can produce enough electricity to power the heat pump at this level, we would save (345 therms)(13.5 lb CO2/therm) = 4657 lb CO2.

Not taken into account:
--boiler efficiency (high 80's or low 90's, can't remember --it's whatever the best available is for steam)--but thinking about this factor is why I guesstimated 1/2 heat production
--some of our electricity usage is for A/C
--some of Xringer's electricity usage is for A/C
--difference in house size
--difference in insulation
--correct HSPF to use for this application. There are ways to adjust HSPF for colder climates, but my system would not necessarily use the heat pump during those very cold times, so it's not clear to me that I should use these adjustments (and thus I haven't).

We would have steam heat powered by natural gas as our back-up system and as our hot water system (they're tied together, same boiler heats water which is stored in a SuperStor).

Heat pump would be installed as to be approximating a second heat/cool zone, probably indoor unit would be on the second floor.

Conclusion: still thinking about it!
I want to run a carbon comparison too.

[Blue Bomber Man]

If you are grid tied and receive payment for your excess capacity I wouldnt use the electricity for heat, you are still providing the carbon benefit to society. Your natural gas heater produces energy more efficiently then the way grid usually makes it and you would actually be sacrificing net carbon footprint by producing heat with your solar panels.

If you wish to start offsetting heat I would suggest building solar heat capacity in addition to your PV, or even use solar heat instead of PV.

The carbon and monetary payback of solar heat imho is much faster then PV. builditsolar's website has many interesting ideas that may provide a solution for your heating needs.

Good luck =)

PS: If you dont want to do the solar heat, i still strongly recommend not using the extra juice for electric heaters

[MN Renovator]

If you do use your solar PV for heat, I wouldn't do it using anything other than either the most efficient mini-split you can buy or a ground source heat pump.

I also wouldn't do it if you are burning 926 therms of natural gas. I'm down at 167 therms over the past 12 months and I'm considering using a heat pump but since I'm in a climate to where I'd still need backup heat since we have plenty of overnight lows below 0f/-20c I'm hesitant to do it, especially when my gas costs ~65 cents/therm. $108.55 in gas is cheaper than the $127.80 but disconnecting the service would cost me more money than running the most efficient mini-split around at 11 cents/kwh because I'd still need backup heat and the initial cost of the mini-split is also on top of all of that. Granted I could enjoy the 25 SEER air conditioning and the lower cost it provides along with the higher efficiency heating when its warmer and sunny out but it would take outdoor temperatures colder than what is efficient before I'd really use it all that much which is the catch.

Best thing you can do is improve the building envelope to get you well under the 926 therms heat load usage before you consider heating electrically.

When I build my next house I'm planning to build to a heat load of under 5000BTU/hr at design temperature and I'll use the most efficient mini-split heat pump that I can buy that includes a strip heater for when it can't keep up(Daikin sells these). 5000BTU/hr is the same as single 1500 watt space heater heating the whole house at design temperature. I'll skip the natural gas bill, skip the cost of buying a furnace and I'll have solar PV which will be cheaper than the cost of the natural gas service fees to heat the house and provide hot water on an annual basis. Insulation will be 8-12 inches thick to accomplish this depending on the size of the house.