TomatoTomahto wrote: ↑
Mon Apr 02, 2018 7:06 am
madbrain, jdb, others interested in solar and batteries, I will be starting a thread soonish (thanks Elon) about our new house. We have a large solar field installed, but the utility is the battery, and I am wondering about converting to heat pumps (we are near Boston), adding batteries, etc. I am wondering about the economics of re-doing the energy profile of the house.
Heat Pumps. Tricky in New England. Gas in NE is expensive or unavailable, but electricity is also very expensive. A storage battery is unlikely to have anything like enough energy to keep your home warm, unless you are super insulated.
has the conversion factors for energy content. You need to know (because USA uses BTU) that 1 kwhr = 3,412 BTU.
If the new home is well insulated, HP will work. Then the question is Geothermal (Ground Source) v. Air Source:
- GSHP costs a lot to install - big issues there. If you have room for a trench, then it's probably doable for a predictable cost (relative did that on a farm in Ontario v. electric/ propane heating, and it was good payback - less than 7 years). Coefficient of Performance (COP) should average 4.0 or higher
- ASHP a lot less expensive (you can get a sound baffle if noise is an issue w neighbours) - the good Japanese ones should get you near 4.0, but it will fall off below 32F/ 0 degrees C
Probably, if you have access to natural gas:
- NG will be cheaper even if NE prices
- it will be a wash on emissions because NE electricity is not particularly clean (as the nukes shut down) and the marginal kwhr is likely to be imported from another electricity grid, and thus be emissions intensive (coal fired)
If you don't have NG then HPs are probably better than propane or oil fired solutions. Not necessarily cheaper at today's prices, but in the round environmentally cleaner.
HPs run best long and slow. They don't put out the heat output of a gas/ propane/ oil furnace-- they don't heat the house up quickly. Thus, you need a well insulated house, and you run the HP all the time.
Your net metering/ Feed In Tariff also comes into play. What rate are you getting for exporting to the grid? Because you will be generating electricity at mid day, when it's less valuable, and needing it post sundown, when it's very expensive. On the other hand, if your house is well insulated, you can "store" heat via the HP-- heat the house up in the day, then cut the HP in the crucial hours for the grid (sundown to about 9-930 pm).
You can see by the graph below that you would have to store a *lot* of power for a long time to meet your December/ January heating needs, when solar insolation is so low in those months. It's possible to get a solar thermal heatstore (basically a big water tank, or air in gravel, highly insulated) but it would be a *big* tank.
Biomass (wood) is of course an option BUT 1. it smells nice, but it's not clean (in fact the air pollution is quite significant, to the extent places with temperature inversions like Aspen CO have banned new installations; if you have neighbours with asthma, they will not thank you) ; 2. cost of so doing varies a lot-- if you can store green wood, it's going to be a lot less than if you have to buy dried wood. In any case it will vary a lot by locality even within NE. Given the Black Soot Problem (subject of much scientific debate) I am not at all sure burning biomass is environmentally friendly.
Apparently, the annual electric cost to the house is $0, and I've heard that there is quite a net surplus. In 2017, this is what the monitoring software tells me about production. In any case, I don't think charging my Tesla will swing the needle into the negative
Perhaps this can be an inducement to get my wife to order a Tesla also.
Say your household electricity consumption is 6,000 kwhr pa (about 40-50% of the average US home with AC). In NE, something like 6 peak kw solar array should handle that.
Your household heating consumption is likely to be something like say 100k BTU ie c 29k kwhr pa*. Half that if excellent insulated. That is an absolutely huge solar array. Even if you average COP 4.0, that's 7,500 kwhr pa or another peak kw array of 7.5 kw additional.
You'd have to add your EVs on top of that. Over to you how much they burn.
In a household with one EV, I'd want the second vehicle to be petrol-engined or PHEV. Thus covering the 2 eventualities (not enough juice, not enough gas).
* I am hand waving on that number, you'd have to track it more closely. Also estimating you at 1 peak kw = 1000 kwhr pa. In London, which has a higher latitude and probably more cloud, it's about 850 kwhr for a south facing array with no shade.