non-battery solar energy storage?

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Suit

Member
Oct 26, 2014
9
CT
Are there any non-battery means of storing excess energy produced by solar electric panels? My utility pays back less than 20% for solar supply of what they charge for use, and batteries are very expensive -- takes many years to pay back the return on investment for batteries. I am looking for a way to offset energy use throughout the dark hours. Maybe feed a heat pump that heats hot water that could then be stored for use by my heating/domestic hot water systems throughout the evening hours to reduce fuel oil use? Seems like there must be something cheaper than batteries that isn't going to wear out... even a tower with a motor and pulley and weight to power a generator on the way down or something. In the warmer months I can heat the pool and using the air conditioners during the day will mostly use up the solar capacity-- maybe the simplest thing is to put in some electric heat pumps to put the excess heat into the house during the day in the cooling months?
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,420
South Puget Sound, WA
A pump system that lifts water to a big upper cistern during the day that drains into the lower cistern at night while powering a generator?
 

peakbagger

Minister of Fire
Jul 11, 2008
6,588
Northern NH
Nothing commercial for residential. Look up the back issues for Home Power magazine. Over the years there were home brew systems on occasion.

There is a firm in Australia who claims it is coming out with a home hydrogen based storage system. It converts excess solar to hydrogen and then combusts it when power is needed. Its definitely "bleeding edge" technology that may not make it to market.

Yes there are mechanical concepts out there. If you just want to store heat, heat up a storage tank with a resistance heater. That is what us folks with thermal storage do with their wood boiler. Storing mechanical energy is also possible, pumped hydro is possible if you have the elevation. Lifting concrete blocks is also possible but it comes down to energy density and cost. Used batteries are the lowest cost method. Plenty of folks home brewing battery packs out of used reconfigured auto battery packs. Just be careful, folks are also burning their homes down by home brewing batteries. Lithium chemistry is far less forgiving than lead acid, you need a Battery Management System and that adds cost and complexity.

Rarely do the economics make sense. I picked up one of the DC Solar Trailers with two large forklift batteries at a substantial discount to what they cost to build. I will keep the batteries alive as long as they last but doubt that I will replace them when they die (hopefully in about 10 years).
 

EbS-P

Minister of Fire
Jan 19, 2019
1,451
SE North Carolina
Maybe feed a heat pump that heats hot water that could then be stored for use by my heating/domestic hot water systems throughout the evening hours to reduce fuel oil use?
Heat pump water heater on a timer. (I have an 80 gallon not on a timer and love it) An Electric car that you can charge when the sun is out. If you do that the Tesla power wall might make sense. It’s really hard to store energy not in batteries. You would probably be money ahead to improve the insulation and air sealing over a battery. a mini split heat pump on a programable thermostat. But a small Mitsubishi will probably run $5,000 or more. It’s going to take a long time to break even. And here is the thing, Peak sun is 10-2. Peak cooling is about 4-10. Peak heating Always ends before 10am. So the temp swings to make use of the excess electricity are going to be very noticeable. Put as much on timers as you can.

weights and water pump ideas are possible but the cost will never save you money. Some utilities use lakes at different elevations to store excess generation. hydro generators are used as pumps to pump water up to the higher lake. Then they let I’d flow back down during peak demand times. I done see how you could make that work on a single house scale to save money .
 

Corey

Minister of Fire
Nov 19, 2005
2,603
Midwest
Guess it depends a lot on what you need/want out of the system. If you need heat or cold, then the heat pump you mention might be a good option... in cool months you could heat the house toasty warm before the sun sets, then 'coast' on that thermal charge through much of the night. Reverse in the summer - cool to nice frosty temps, then coast through the night on that. A step up would be making hot water or chilled water (or better yet, ice) for additional thermal storage.

If you need electricity back out, the weight could work, but you need a lot of weight moving a lot of distance. Plus wear/tear on mechanical parts to lift the weight, plus a motor/generator, so that eats a lot of profit/savings. Pumped hydro could be an option, but you need very specific geography... mainly a big hill with a reservoir on top and bottom.

Other minor things could be effective like timing all appliances ...washer, dryer, dishwasher, etc to run at your peak production times.

If you really got crazy, possibly electrolyze water to hydrogen, then burn that back to heat or electricity...though again, fairly complex, somewhat dangerous and lots of 'moving parts' to eat into any savings.

Guess you could also 'cheat' a bit on buying a battery - if it happened to be in an electric car. You might get some sort of rebate, plus the battery could be transportation and energy storage.

Discussion of 'thermal battery'

Solar Hydrogen
 

woodgeek

Minister of Fire
Jan 27, 2008
4,401
SE PA
I like the HWH on a timer idea. That would work great.

But what the heck is wrong with batteries? Sure lead acid will cost you too much per kWh, often pencilling out to $0.50 per kWh delivered. Lithium could be better, but existing systems are also spendy.

If you had a geo system, you could hack your controller and pump heat into the earth, to improve winter COP.

Just buy an EV. and drive it. Cheaper than gasoline by far.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,420
South Puget Sound, WA
Agreed, our car is our solar battery, though with covid we have been driving less in general. The thermal battery idea in the second video is less interesting where air conditioning needs are low. What does he do during the heating season? Does he heat the house to 78º at night for thermal storage?

After watching the hydrogen video my first thought was, who could afford this? My second thought was that now after 15 yrs, the serious maintenance and costs will be kicking in. My head aches at the complexity of the systems he will need to service and manage.
 
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EbS-P

Minister of Fire
Jan 19, 2019
1,451
SE North Carolina
How much excess generating capacity do you have. HPWH run less than 1kw. Can you monitor your solar production and usage in near real time?
 

peakbagger

Minister of Fire
Jul 11, 2008
6,588
Northern NH
The few hydrogen systems I have heard of were subsidized by industrial customers. The equipment and installation cost would be well above any economic payback. Hydrogen is a PITA to deal with, its a small molecule and likes to leak. All the piping is usually stainless with ferrule fittings. The electrolyzer and fuel cell are both prone to contamination. My former employer built a few hydrogen fueling stations and they were constantly having to fix them.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,195
Grande Prairie, Alberta, Canada
Batteries seem to be the only commercially available and cost effective option for residential use.

I have though contemplated using a type of pumped hydro system using a deep water well and a large above ground water tank, many of our water wells around here the water table is 200-300ft deep, which creates a substantial potential energy difference between ground level and the water level of the well. A pump would be needed to lift the water up, and some kind of downhole turbine/pelton wheel would be required at the water level in the well to recapture the energy of the water returning to the well.

However, such as system does pose a great possibility of contaminating the aquifer from the water being returned to the well. I don't think this would even be legal here due to this risk.
 

blades

Minister of Fire
Nov 23, 2008
3,679
WI, Leroy
Some where along the line there has to be someone/co playing with capacitors. for the purpose of storing electrical energy. Not that big caps are cheap But the watt is ), and the circuitry for a slow drain at a useful power level is a bit daunting.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,420
South Puget Sound, WA
Caps are useful for short term bursts of power, like in a car, but a battery is better suited for the steady drain in a home application. There was talk several years ago of developing large, super flywheels for energy storage, but I haven't heard anything on that score for years.
 

peakbagger

Minister of Fire
Jul 11, 2008
6,588
Northern NH
Beacon Power built a few flywheel storage devices which were subsidized. They wen bankrupt in 2011.
 

semipro

Minister of Fire
Jan 12, 2009
4,077
SW Virginia

semipro

Minister of Fire
Jan 12, 2009
4,077
SW Virginia
My utility pays back less than 20% for solar supply of what they charge for use, and batteries are very expensive
It appears that Net Metering is available in CT. Why not take advantage of that?
Add a HPWH if needed to take advantage of some of your electricity production excess if needed.
 

Suit

Member
Oct 26, 2014
9
CT
It appears that Net Metering is available in CT. Why not take advantage of that?
Add a HPWH if needed to take advantage of some of your electricity production excess if needed.
Semipro your comment was really helpful! I think I misunderstood how the electric company credits for supplied energy -- now I see it looks like they subtract the kwh supplied to the grid from the kwh consumed from the grid before charging all the per kwh rates and fees. I think before I was looking at a different rate for what they actually pay out at end of year if I've supplied more than I consumed. I guess net metering makes the power company like a massive battery as long as there's no power outage.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,195
Grande Prairie, Alberta, Canada
Semipro your comment was really helpful! I think I misunderstood how the electric company credits for supplied energy -- now I see it looks like they subtract the kwh supplied to the grid from the kwh consumed from the grid before charging all the per kwh rates and fees. I think before I was looking at a different rate for what they actually pay out at end of year if I've supplied more than I consumed. I guess net metering makes the power company like a massive battery as long as there's no power outage.

Your original question was a valid one though, not all jurisdictions (including mine) have net-metering, and if residential scale battery storage decreases substantially in price there is a market for it.

For example in the summer I pay 39cents/kwh for electricity and get back 25cents/kwh when I export. In winter I pay 20cents/kwh and get back 6cents/kwh when I export. Essentially I pay a 14cent/kwh transmission fee on import that I don't get credit for when I export. Our jurisdiction calls this system "Net-Billing".

For instances like mine a small battery could make sense if sold for a low enough cost to cover night time consumption and help with self consumption on cloudy days, and would show up as a lower electric bill. I suspect that as renewable buildout increases utilities will move away from net-metering to another system, because there are costs associated with utilities acting as a battery for homeowners and these costs will be passed on.
 

Suit

Member
Oct 26, 2014
9
CT
Your original question was a valid one though, not all jurisdictions (including mine) have net-metering, and if residential scale battery storage decreases substantially in price there is a market for it.

For example in the summer I pay 39cents/kwh for electricity and get back 25cents/kwh when I export. In winter I pay 20cents/kwh and get back 6cents/kwh when I export. Essentially I pay a 14cent/kwh transmission fee on import that I don't get credit for when I export. Our jurisdiction calls this system "Net-Billing".

For instances like mine a small battery could make sense if sold for a low enough cost to cover night time consumption and help with self consumption on cloudy days, and would show up as a lower electric bill. I suspect that as renewable buildout increases utilities will move away from net-metering to another system, because there are costs associated with utilities acting as a battery for homeowners and these costs will be passed on.
I see these batteries from China on eBay, LiFePO4, Felicity Solar, some down below $250/kwh, and wonder if they're worth a gamble. I thought batteries cost more like $1000/kwh when I first posted.
 

peakbagger

Minister of Fire
Jul 11, 2008
6,588
Northern NH
Every utility may have different rate programs for "net metering" many states have laws requiring "net metering" but they vary from state to state. Most have limits on how much power can be interconnected to the grid, some limit the total wattage of the panels that can be connected and some limit the total rated wattage of the inverters that can be connected to the grid. There is a difference in that if the limit is inverter rated, the rating of the panels can exceed the inverter capacity. This is frequently done to increase the output of the system during less than optimal conditions. This can lead to "clipping" where some output is lost on sunny optimal days around noon but it gets made up by more overall production. the other reason its done is state rebate programs typically pay a certain amount per watt so the rebate is higher. Some installers really push that to get a higher rebate.

The other big difference in net metering programs is how the utility deals with excess generation. Ideally any excess stays tied to the system forever. Other less desirable net metering programs have a reset date where any excess power at that date is either taken with no compensation or taken and paid for at some low rate. The reset date is also critical for planning in those states, Solar systems generate more power in the summer than winter and power usage is higher in the winter in many climates (hot weather states may be the exception). With no reset date the ideal is to size a system to generate enough extra power in the summer to coast through the winter. If there is mandatory reset date and it happens on January first than any extra power generated in the summer gets taken or sold on January 1st leaving the owner with no surplus for most of the winter and spring.

With respect to cheap chinese batteries, these may be "seconds" or just plain low grade cells. Lithium based batteries have a high power density and if the battery has an internal fault, it will burn/melt which will usually ignite adjacent cells. It best not to install them in living spaces as if they burn, it will probably take the building with it. I think most codes requite installation outside living space, Even if it doesnt light the building on fire, it will put out a large volume of toxic smoke. The typical approach to putting a lithium battery fire out is to apply a large volume of water for for 3 days until its inert. In either case it can be a big impact to a homeowner and the homeowner had best make sure they have insurance paid up to cover it. A home brew battery may not be covered as the insurance company has no one to sue to recover the damage. There are more than few home brew home battery builders that have had big fires and for the ones who stick with it they build a battery shed. I have a large industrial Tesla installation going in and the fire department requires a large buffer space on all sides so the fire department can keep the surrounding buildings from burning. Its important to note that Tesla has no published recommendations on how to deal with what left after a fire. Its hazardous waste and mostly likely will need to packed in drums and shipped to a landfill that accept hazardous waste at a hefty cost. My guess is at some point as batteries get more popular for homes that insurance companies my require extra cost riders to cover batteries.
 

semipro

Minister of Fire
Jan 12, 2009
4,077
SW Virginia
Semipro your comment was really helpful! I think I misunderstood how the electric company credits for supplied energy -- now I see it looks like they subtract the kwh supplied to the grid from the kwh consumed from the grid before charging all the per kwh rates and fees. I think before I was looking at a different rate for what they actually pay out at end of year if I've supplied more than I consumed. I guess net metering makes the power company like a massive battery as long as there's no power outage.
In Virginia, you can either get credit for the excess at a 1:1 ratio and use it or lose it within a year or you can get paid for the excess at a rate much lower than retail. I think this is a fairly typical net metering policy.
As mentioned by others, the challenge is to right-size your system so you're not giving away excess to the utility (although I figure the more green energy on the grid the better).
 

peakbagger

Minister of Fire
Jul 11, 2008
6,588
Northern NH
NH has a perpetual net metering, no reset date . I try to keep 1000 KW in the "bank". There is also a small SREC market that just about covers my monthly fee from the utility for being connected to the grid.
 

hansenjw

New Member
Dec 14, 2020
41
Madison, WI
Passive solar houses incorporate large amounts of thermal mass into the structure, in the form of either exposed concrete slabs or large central concrete columns in the house. It occurred to me that you could run radiant floor tubes through that, and the depending on the season, use all of yours excess solar to either over heat or over cool it, so that it would be a heat battery throughout the night.