geothermal solar thermal storage

I wonder if a phase-change substance could be used, like in hand warmers. No need to insulate, just hit it when you need the heat.

Cheers

"Hit it"??? What does that mean??? Even phase change material will be at an elevated temperature and require insulation. It may not need to be 80

won't do you much good for heating.

The entropy increase involved in taking something very hot and using it to heat something only mildly hot is fairly heartbreaking. Folks have been using solar collectors to heat pits full of rocks since the '70s at least, but so much of the energy goes into heating the storage through a useless temperature range that it's a mugs game.

Having said that, a bit of rock storage could warm up the outside end of your heat pump quite usefully.

Cheers

Phil Hobbs

I'm not sure what you are referring to here. You do realize this is about using the Sun to heat your home, right? Can't get much hotter than that an d yet heating your bath water with it rather than some arbitrary piece of r eal estate is a zero entropy change.

Eh? What is useless? You mean because the rocks start at something well b elow your room temperature? In a system that heats all winter with stored heat that is a one time effort. The storage system never gets below a usef ul temperature unless it is undersized which would be a bad idea.

???

I think he is referring to an active thermal battery, ie a material that stores energy molecularly, which can be triggered to store or release the energy thermally, maybe with an electric field etc.

That could be activated in portions to get rid of most of the insulation problem etc.

I was considering a crazier idea before, to use stainless steel drop pipes instead of poly tubing, and use a solar concentrator instead of solar collector panels, and then melt the rock under the ground, thats about as far as I got :)

cheers, Jamie

Not likely that it would ever need to be repaired, it is the same construction as bore hole geothermal which last 50+ years or more, also if the thermal battery is under the basement, then the heat can be stored at a depth so that it begins to radiate through the basement floor in winter as well, ie lagging 6months summer to winter.

I agree, but also a lower temperature of the thermal battery to improve efficiency I think, potentially this would require a heat pump at some point through the winter, which could bring the thermal battery temperature below room temperature. Phil Hobbs mentioned using a heat pump with a thermal battery. That could also be an alternative to gas backup heat. A reversible heat pump could also be used before winter to increase the thermal battery temperature ~10C more etc.

Solar collectors seem overpriced, I think a rectangular tall vertical wall would be cheaper than roof mounted commercial ones, the sun angle would not be as good especially in summer though.

cheers, Jamie

nd

at an elevated temperature and require insulation. It may not need to be

t won't do you much good for heating.

That would be ice-nine, or something like it - which is to say, science fic tion.

In places that use air conditioning, the solar collectors could be dual purpose, ie designed to have cold air flow through them time, and then cool the ground (ie a thermal battery for winter house heating and a thermal sink for summer house cooling).

cheers, Jamie

nd

at an elevated temperature and require insulation. It may not need to be

t won't do you much good for heating.

That would be a chemical reaction and I've not see any that weren't primary batteries, i.e. non-rechargeable. Otherwise the storage is either based o n using specific heat to store energy at increasing temperatures or phase c hange to store heat at a given temperature that is higher than the temperat ure required.

The system discussed above uses specific heat of the earth to store energy.

Melt rock??? Yeah, that's gonna take a lot of mirrors. I believe there's a few you can get cheap at Crescent Dunes.

wiki page and a 10 year report:

Eg hydrated copper sulphate (blue crystals) can be converted to anhydrous copper sulphate (white crystals) by heating it (which produces water vapour). the crystals can be stored cold if kept dry. reintroduction of water gives a release of heat. (11.3kJ/mol) - it's not very efficient: less than 6% just accounting for boiling the water.

There are probably better reactions than this, it's just one I remeber from high school.

just use a longer drill, you'll find molten rock eventually :)

I'm thinking or the supersaturated solution packs you get for camping or in my case, cold funerals. They're sealed packs of solution, heating them dissolves the solid in the liquid. Then they are allowed to cool and you can store them for ever without insulation in this 'charged' state. When you 'hit them' - a physical shock from flexing a metal disk

- the solid crystallises out giving heat. They are, or were, quite common.

Of course, you'd use a heat exchanger as 'charged' packets cooled to pre-heat 'discharged' packets. And I'm not literally suggesting thousands of individual packets, though you would need some way of stopping the phase change from spreading more than required.

While Googling for you, I came across this...

...damn, too late again!

nd

at an elevated temperature and require insulation. It may not need to be

t won't do you much good for heating.

n.

Not sure why you say googling for me. I've never seen these hand warmers a nd wouldn't know what to look for. It's great that they figured this out. I'm wondering if they are truly practical or not. There's a lot of detail s that need to be right to get the efficiency right, etc. At one point the y talk about twice the temperature of boiling water! But this isn't a scie ntific article so you have to take that sort of stuff with a grain of salt.

I noticed somethings about the Drake Landing project. The efficiency of th e underground storage is only around 50% if I am reading the data correctly . So only 50% of the energy pumped into the ground ever comes back out.

I expect the efficiency of the phase change of the crystals would be better ... but there are other issues that affect the efficiency. The process of heating the solid to form the gel will have losses depending on how hot it needs to be. The solid is heated to form the gel, then if the storage is v ery long the specific heat energy will be lost from cooling. How much that is depends on the temperature required to form the gel. But 50% isn't a d ifficult target to meet I expect.

The intended application for this is diurnal storage which it would seem to be a good match for.

Yeah, heat is the least useful form of energy. I don't know what the numbers look like, but for all the trouble of heat capture.. I wonder if solar PV would make more sense.

Yup, If we'd had force air heating in my house I would have looked into a heat pump with some ground water (or dirt) to source and sink the heat from. (When the last oil furnace crapped out.)

George H.

No, because the heat was free; the issue is the amount of excavation to install the heat exchangers. The heat exchanger gets you the heat until the end of winter, at which point the mass can be at ambient temperature (zero loss regardless of insulation), so initial heat loss can be high and it'll still work out.

In economic terms, a layer of clay to inhibit water movement might be more important than insulation. The dirt-layer thickness multiplies the R-value at no great cost; heck, the core of this planet is hot enough to melt iron, but our feet aren't burning.

e seasons by six months (or 180 degrees) which is exactly what you want.

loss.

The heat is very much not free. To capture the heat requires the collector s which cost money, real money. In fact, as much as that they use roof spa ce which is also limited. Notice the garage roofs with the collectors are very tall to provide the right angle for the collectors. They have maxed o ut the garages and would have needed to add more to the homes with the same odd looking roof line.

(zero loss

ill work out.

Only if you have enough heat going into the reservoir to compensate for the heat loss in storage. The data they provide seems to show a 50% efficienc y of the ground reservoir. If it were better they could have saved money o n the collectors or provided more heat to the homes requiring less gas to b e burned.

e important than

cost; heck, the core of this

There's some difference between the miles of earth between us and the mantl e and few feet of soil around the bore hole thermal storage. The plan is t o heat the soil around the pipes reversibly. The lack of good insulation r esults in much less reversible heating of a large amount of soil around the reservoir, lost heat. I'm not sure how much good insulation around the co re would do really. When you are talking about months of storage, I'm not sure increasing the insulation by R-12 will really accomplish much. The gr ound "insulates" by establishing a thermal gradient once the surrounding so il is also heated. That's where the other 50% of the heat goes.

It was interesting to see that they learned a lot about managing the coolan t flow to save on electricity costs of running the pumps. I believe they r educed it by a factor of 2 over the course of 10 years. There's a lot of i nteresting data there.

and

end

be at an elevated temperature and require insulation. It may not need to b

it won't do you much good for heating.

at

on

mary batteries, i.e. non-rechargeable. Otherwise the storage is either bas ed on using specific heat to store energy at increasing temperatures or pha se change to store heat at a given temperature that is higher than the temp erature required.

rgy.

claims "In 2012 the installation achieved a world record solar fraction of 97%; th at is, providing that amount of the community's heating requirements with s olar energy over a one-year time span.[5][6]"

even though there are several other places running on 100% solar heat

Hi,

The 97% is from 2011-2012, in 2015-2016 they had a solar fraction of 1. (page 5 of the pdf).

It is a bit silly to design the system capacity so close to the usage, ideally it would have capacity always a bit greater than usage.

cheers, Jamie

d, and

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l be at an elevated temperature and require insulation. It may not need to

nt it won't do you much good for heating.

that

tion

rimary batteries, i.e. non-rechargeable. Otherwise the storage is either b ased on using specific heat to store energy at increasing temperatures or p hase change to store heat at a given temperature that is higher than the te mperature required.

nergy.

; that is, providing that amount of the community's heating requirements wi th solar energy over a one-year time span.[5][6]"

And how would you come up with that number??? It is very hard to design so mething that will always do the job. It might cost a lot more as well. Yo u won't even be able to save on installing the gas backup since you will st ill need that for breakdowns. Then there is the issue of the gas company n ot being happy with a customer how uses gas once in 10 years.

It seems very practical to use some other heat source for 10% of your heati ng needs routinely and saving a lot of money on overdesign. I wish they ha d used electric as backup. Eventually that will be carbon free which is wh ere we need to be.