How much PV do I need if I want to get rid of the wood stove

[georgepds]

I’ve had my new chevy volt for about a year, and I estimate I’m about net zero for both the house and car( the credit a year ago was similar to the credit today, ~$1400). Alas that does not include the 2 chords of wood I buy each year to heat

So, realizing I’m getting older, I asked myself how much more PV would I have to add to heat the house (without wood). I use ~2 chord of wood per year with the new Woodstock progress Hybrid. Working through the numbers I figure I need to deliver ~10 MWH to heat the house through the season (5 MWh/chord)

Up here in New England, I get about 1.2 MWH for each Kw of PV installed.. If I just used a resistance heater, I’d need 8.3 kW of PV. But wait, I use a split duct heat pump with an average COP of 3, so that reduces the size of the needed pv to about 2.8 kw.

I’m building the frame for the new PV this fall


Calculation notes :

wood 1 chord
energy /chord 20 Mbtu
kwh/mbtu 293
energy/chord (kwh) 5860 kwh
stove efficiency 85%
energy to house 4981
mwh 5.0 Mwh

PV system 1 kw
mwh/year/kw 1.2
mwh 1.2 MWh
COP 0f HP 3
total MWH 3.6 MWH
PV needed to replace a chord 1.4 kw

chords 2
MWH needed 2.8 MWH

 

[jebatty]

I've had thoughts like you. It's a "win" on ease, convenience and saving of labor, as well as eliminating risk of injury in c/s/s wood, and probably eliminating some risk related to fire. Wood is "free" for me as it comes from our own land. I will continue to keep this in mind as age takes its toll on me in ability to process wood.

 

[EatenByLimestone]

I think I'd keep the stove around must in case...

 

[georgepds]

I think I'd keep the stove around must in case...

Oh.. the stove will never go...

For one, nothing else feels as warm...More of a sizing exercise to see if I could do it with a reasonable amount of pv

 

[peakbagger]

Not easy to answer.( I expect you know some of this already but I write the post in case someone else is thinking the same thing) I use a nominal 1 ton mini split for exclusive shoulder season heat in Northern NH. I ran it exclusively until early December last year. I am heating 952 square foot main floor and a 433 foot second floor with one unit. I expect if I installed a smaller additional unit in the second floor things would be more efficient. The house is 1987 vintage 6" walls with some energy upgrades. One thing to note, there is no radiant component of heating from a heat pump compared to wood stove. Most folks will heat the house warmer with heat pump as they can go sit in front of woodstove and soak up the radiant heat. For reference I have 4.7 KW of PV and this summer generated about 2MW hrs of surplus for this winter. Some of this will get eaten up by household use being over my generation. I would guess 1.5 MWhr for heating.

I find that the minisplit really starts to loose ground any temp below 20 F, its still putting out heat down to -10 but the output goes down and so is the air temp. The COP takes a nose dive as the temps drop and around -10 its darn close to 1.0 plus its defrosting frequently. Thus depending on where you are in New England, a mini split is not the total solution. If you want to spend the bucks, a ground source heat pump will be able to heat your home if sized properly as its looking at ground temp not air temp (thus the COP stays higher). Ground source units installed are usually 20 K up but its very dependent on the soils and you need a hot air system. The rule of thumb is 300 feet of well for one ton of heat and most homes would need 3 tons minimum. I have a basement and at some point I need to put heat into it to keep the pipes from freezing and my floors warm.

Before you plan out a system you need to research your utilities net metering options. The only way PV makes sense is to have a good net metering option where you store the excess summer generation for use in the winter. That's utility specific. NH has a good program but it may be going away. Other utilities do a yearly "reset" where you can store excess power but on a certain date like January 1st, they either write you check for your surplus at some low industrial rate and start from zero or they just take the surplus. Obviously that makes a big difference so you need to find out what how the local utilities plan works and design around it. Don't even think about not using net metering as PV output is low in the winter when you need it most. The PVS are more efficient in cold temps but the short days really cut the overall output plus a couple of days of dreariness happens far more often in the winter.

Assuming you have good net metering deal I would suggest doing two spreadsheets, one would be for ground source heat pump for all winter and the other would be minisplit down to 20 degrees and resistance heat down below this temp. You can get historical temp data for you region which will allow you to estimate how many hours of each. The trade off is high up front cost for ground source but lower electrical usage. The PV/resistance heat is lower capital cost but bigger PV array which may offset the ground source unit. To further complicate things is you need to see if you can sell SRECs which are pieces of paper representing renewable generation. In Mass they were selling for over $400 a MWhr of production while NH is $50. Thus in a state where you can sell SRECs your income stream goes up the more the PV generate..

I think you are going to come to the conclusion that a small woodstove in combination with a mini split and PV is the best fit but all the other factors can really swing things. I burned about 2.5 cords last year in the boiler. I can live with that but if it was 6 or 8 like my neighbor I would be drilling soon.

 

[begreen]

What state are you in? Are you far enough south so that a mini-split system could cover the major portion of your heating needs?

 

[georgepds]

I'm in Massachusetts..I have PV ( ~4.5 kw), I get SRECs, and already have a air-air split duct heat pump (Fujitsu RLS12h) . I just use the Fujitsu in the shoulder season, and as backup if I can't get to the stove. As the winter progresses, I rely more on the wood stove. Massachusetts is a net-metering state, with SRECs, and kwh allocation that roll over (forever, or at least, for the last 3 years)

I wanted to see just how much I'd have to increase the system size to cover " those last 2 chords of wood". I'd rely on the utility to load shift from summer to winter ( as it does now). It's not much... 2.8 kw of PV, rougly half the size of the existing system

The biggest problem is the very cold days, and the fact that the mini-split just does not feel that warm (the radiant heat problem)

As to SRECs, I'm under the original program , which currently yields ~$400/rec. The addition would come under the current program, ~$300/ rec

I can't go ground source because my lot size is wayyyyyyy too small

re"Are you far enough south so that a mini-split system could cover the major portion of your heating needs?" That was the point of the calculation, I know my needs are 2 chords, which translates to a 2.8 kw system here ( using an air to air heat pump)

 

[EatenByLimestone]

Propane space heating might be a viable option too. It would function much like a woodstove.

 

[georgepds]

re "I expect if I installed a smaller additional unit in the second floor things would be more efficient."

That's what I think I'd have to do. There is a 1 ton on the first floor.. I'd probably need a 1 ton on the second floor.

The house was "designed" to be heated by wood ( open plan, minisplit, BR,LR,Kitchen and bath on first floor).

 

[peakbagger]

Heating two floors with one minisplit is a compromise, half the second floor is unfinished so the heat has to go up a stair way into a small common room on the head of the stairs with doors heading off to unheated spaces and then into the office. I have to heat the downstairs warmer to heat the upstairs. As I work from home I would really be better off replacing an existing older 9000 btu split ac unit in the office and leave it to heat just the office and turn down the rest of the house for cold weather.

BTW You can go ground source, the systems installed up north are mostly drilled wells in granite. Expensive but the ground temp is higher the deeper you go. The shallow systems composed of coils don't do as well up here as ground temp at typical burial depth is relatively shallow so the temps are lower. There really isn't a difference in pumping head loss between the two methods as the loop is full of full of water so its just friction loss. They grout the loop into the well casing to increase heat transfer.

 

[iamlucky13]

I find a lot of mixed info about the real economics of ground source heat pumps. If the install cost were lower, they'd probably make sense, but it's not clear they can beat mini-splits because the up-front cost is so high, and mini-splits have gotten pretty efficient.

George, you said your system is split ducted. I'm not quite clear what you mean.

Is it a small ducted system built around the condensor from a mini-split? Or a regular split-system ducted (large condenser paired with a traditional furnace air handler)?

Do you know the BTU/hour output and HSPF? Can I ask what your electric rate is?

I assume you mean you want to heat the house while achieving annual net zero bill, not heat the house directly from the output of your solar panels or go off-grid, right? The latter options are not really practical.

 

[georgepds]

Small Split duct.. condenser outsde heat exchanger inside
Hspf 12
seer 24
12 kbtu/hr

Electric rate changes summer to winter $0.15 to$0.20 / kwh

Searching for net zero annually

 

[DBoon]

I can't go ground source because my lot size is wayyyyyyy too small

You can go vertical wells for this - more cost, but effective. I'm just finishing a vertical well install in Central NY.

There is a 1 ton on the first floor.. I'd probably need a 1 ton on the second floor.

I'd probably put them both on the first floor if you are primarily going to use them for heating and the floor plan is open. If the bedrooms are on the 2nd floor and doors are left mostly open, you may find this is the most effective way to heat the house. If you want to cool with it as well, then the 2nd floor is where it would go.

I think your calcs are pretty good. For my ground-source system heating a 1600 sq ft house that will be remodeled to be fairly energy efficient (21,000 BTU loss / hour at 0 degree F outside temperature) in an 8000 heating degree day climate (probably 30% colder than yours), I figured I would need another 4 MWh each year, and in my cloudy climate, that is about a 4kW array addition (to add to my 5.3kW array).

I'd go with the mini-split in MA if you like the one you have. I have a Fujitsi RLS15H and I am amazed at what that can do below 0 degrees. You get 80% of the benefit for 20% of the cost with the mini-split. I wanted the ground-source since we are remodeling for hydronic heat with cast iron radiators. The mini-split will augment that ground-source and cool in the summer once we are done.

 

[iamlucky13]

Small Split duct.. condenser outsde heat exchanger inside
Hspf 12
seer 24
12 kbtu/hr

Electric rate changes summer to winter $0.15 to $0.20 / kwh

Searching for net zero annually

Ok, took a moment to run my own numbers and got the same number you did if I use the same inputs, so your method is on target.

Depending on a few assumptions, you might be able to get away with as little as 2 kW extra solar. For a couple reasons, including those peakbagger mentioned about decreased efficiency at lower temperatures, I'd probably still stick with 2.8 kW if you want to be fairly confident in maintaining net zero.

I'm not sure if your net metering is billed based on your surplus or deficit at year end, or on a monthly value accrual, but that summer vs. winter split will hurt somewhat if it's the latter case, because you'll be using the heat during your low solar production times. If it's a monthly value accrual, that extra 3.6 MWh you use during the winter will presumably cost you $720, but when you make it back up in the summer, it will only earn you $540 back (rough initial estimate - actually a bit more since some of the extra production will still be in the winter).

 

[woodgeek]

Depending on your location, I would assume a seasonal average COP, or SCOP of 2.5 or so. And if you used electric backup some when it gets really cold, you might bump it down to more like 2 or so.

Still, saving 50% is pretty good.

 

[OhioBurner©]

I suppose if you're paying premium prices for New England firewood it might be cost effective in the long run, but with a majority of my wood being free I can't imagine replacing it with pricey solar. But in the long run sure would save a ton of work!

 

[begreen]

How much AC do you use? Solar could be paying for that.

 

[peakbagger]

Solar is nice as I don't need to be there to heat the house. With the recent cold snap I fired the boiler early, its fine for about 2 to 3 days but nice to have a backup for when I go away.

 

[georgepds]

Not so much price as my ability. I'm mid 60's and wood was a lot easier 20 years ago. I figure it will be a lot harder if I ever reach my mid 80's.

I could always pay for the propane or electric, but , hey.. the sun comes up every day.

ROI on PV in Ma is huge.. used to be ~30%, and is now ~14% with the SREC 2 program

 

[woodgeek]

Do keep in mind that net metering arrangements on solar are likely to change long term, making the 'grid battery' that is now free have a seasonal cost. That is, if PV penetration gets high in New England, summertime elec rates will be very cheap (more hours of daylight AND clear weather AND low demand/AC). So, there will be a pressure to not pay too much for summer production. Those PV panels (in New England) in the cloudy winter will have meager production, so the utility and its costs will revert to conventional power, peakers and imports, which will likely cost more.

Of course, none of this is a problem at low PV penetration, but in a hypothetical future banking summer electrons for winter use might get an associated cost that is currently not there, and this effect will vary in different regions of the country, but probably be most severe in the NE.

 

[jebatty]

... assuming that grid level storage technology fails to advance. And that utility efforts to increase rates is not met by individual customers, residential and commercial, going off-grid as storage technology becomes economical on a less than grid level based in part on the rising costs of grid supplied electricity. Disruptive forces will force movement to a new equilibrium.

 

[woodgeek]

I hear you Jim, but we are talking about two different things. Grid storage, either utility owned or residential WILL be deployed as renewable energy ramps up, and its costs will fall. BUT, it will only be good for diurnal storage for overnight or a couple days. Storing 0.5 to 2 days usage costs a lot less than storing a whole season's worth of energy.

You will probably be able to add a battery to a PV system and go off-grid, and depending on your local climate, you might be ok, or you might be 'fat' in the summer (more power than you can store/need) and then struggle to meet your needs in the winter. So in New England I don't see someone buying a future 8th generation Tesla powerwall, hooking their PV to it, going off grid and then heating their house all winter with minisplits. AC and summertime, sure. Go off grid with fossil or wood heat, and oversize your PV 2X for winter electrical loads, ok. But oversizing your PV and buying the storage will always cost more than staying on grid.

I don't think we will see a lot of 'grid defection' in New England....more likely is that PV keeps getting cheaper, and the net metering arrangement keeps getting less profitable, and people muddle through. Residential storage (used on grid, not off) might help some people realize more value from their system (or provide useful backup, etc), but most people will still want that grid connection as seasonal backup IMO.

 

[jebatty]

I understand your point, and yet I foresee potential radical changes. Plus change from political power. What I would not like to see is treatment of electrical utilities the same as a police or fire department, as a road or highway authority, or as a public water/sewer utility, with forced tax support and no option out. A rate structure more reflective of actual costs/benefits could be helpful, provided that it would not drive low income families into a deeper pit.

 

[georgepds]

Fwiiw. There are already winter/summer rates here (can't remember exact figures.. something like $0.20 per kwh summer $0.30 winter)

I accumulate in summer and spend in winter..that said..I have a$1400 credit built up over the years, which I'll spend eventually..or give it away

I won't go off grid.. it works too well. I just think the electric company has to join the new millennium and prepare a bit for distributed generation.

In the meantime I'm happy to exploit current net metering rules..if they change, tant pis

 

[peakbagger]

Fwiiw. There are already winter/summer rates here (can't remember exact figures.. something like $0.20 per kwh summer $0.30 winter)

I accumulate in summer and spend in winter..that said..I have a$1400 credit built up over the years, which I'll spend eventually..or give it away

I won't go off grid.. it works too well. I just think the electric company has to join the new millennium and prepare a bit for distributed generation.

In the meantime I'm happy to exploit current net metering rules..if they change, tant pis

Our utility in NH and our legislature to date have backed grandfathering the net metering rate for early adopters. The initial net metering program was capped at 1% of the states power generation. When that cap was hit, a second round was created with the potential to "sunset" pure net metering. I like the approach that the early adopters that went out on limb and spent more out of pocket will in theory be rewarded in the long term.

The reality is nothing is safe when the legislature is in session .