Storing heat...

[Externet]

Hi.

Picture a bunch of solar panels on a roof to generate 3KW electricity.   The purpose, to heat a dwelling, nothing else.  No connection to mains, no inverter to supply appliances nor lighting.  Just heat.

Powering a heater during the daylight hours, warming the dwelling plus heating up something that will release warmth at night time.  A water reservoir works well absorbing/releasing heat, but needs piping implemented toward radiators like in some very old houses.

Barrels of rocks heated by electric elements can warm air passing trough them to release warm air at nights into existing HVAC ducts.  What if the rocks are replaced by iron scrap, like engine blocks in barrels ?  What convenient dry material would better retain heat per volume ?

[npts2020]

A typical method is to heat water in an insulated tank, then circulate the water through spaces where heating is desired. This can even be done with a passive solar system (you would definitely want back-up heating with that, tho)

[Halc]

My parents had an old solar heater that worked exactly that way. No electricity was generated. When it was sunny, water would circulate through the panels which focused sunlight onto the pipes.  The water was stored in a tank which fed into the main water heater, reducing the effort to heat it to full temperature. They had a forced-air heating system for the house, so it was never used for winter home heating.

As for barrels of rocks, rocks have far higher specific heat than metal, so you'd not want to replace them. I've seen active solar houses, and most of the heat sinks were rocks/concrete.

[studiot]

13 hours ago, npts2020 said:

A typical method is to heat water in an insulated tank, then circulate the water through spaces where heating is desired. This can even be done with a passive solar system (you would definitely want back-up heating with that, tho)

I can assure you from practical experience that this method doesn't work.

An indirect heat exhanger is necessary.

[Enthalpy]

Passing over electricity is the wrong start to heat from sunlight.

Melting paraffins are the best heat stores at identical mass and near-best at identical volume, and they do it at a useful temperature.

But is the volume the true goal? Running multiple long air pipes in the ground stores heat in soil, soil costs nothing, and this provides cool air in summer too.

[Externet]

Thank you.

Electric powered hot water reservoir circulating into a radiator inserted in a dwelling HVAC ducting for heating in winter.  No other use.  Over half the power generated during sunlight hours to be used for night time circulation.  [3Kw of zero cost panels array on hand  ]  

[Ken Fabian]

2 hours ago, Enthalpy said:

Running multiple long air pipes in the ground stores heat in soil, soil costs nothing, and this provides cool air in summer too.

Ground source heat pumps - probably the most efficient heating with respect to energy input -  use liquid, usually glycol/water but not include (as far as I know) any extra insulated liquid storage and do require some electricity. Of all ways to use solar energy to heat a home, using it to run a GSHP looks the best to me, potentially improved with relatively small battery capacity. You still need appropriate location.

 I had a question about using the ground as heat storage some time back ie reversed flow for summer cooling adding heat to the ground the pipework is laid in, and how effective that might be storing heat that can be drawn on over winter, arising out of my understanding that if the distance within the ground is sufficient it can be considered insulating. I would expect some priming with heat would be involved and there would continue to be loss, but some proportion of that heat would persist. How much? I don't think it got a satisfactory answer - or at least any answers based in expertise or experience.  With purposeful addition of solar heating... ?

Related are claims that 1 - 2 m deep trenches around home foundation perimeter, filled with insulation - an in ground heat barrier - work well to make the ground underneath a heat store and that insulating all the way underneath that earth (full excavation) is not necessary. Sorry, no references for that. 

I have also heard of PV fitted homes using daytime excess in resistance water heaters, just because they can. Presumably doing that with (air source) heat pump hot water systems instead of resistance type would be much more efficient and perhaps be sufficient for a large water tank for home heating. I have also heard of PV and battery fitted (and insulated) off-grid homes in milder climates being able to run air source heat pump heating (aka reverse cycle A/C). Battery capacity needs to be sufficient - expensive but costs are coming down. I think the one example I read about faced very high grid connection costs and it offered a viable off grid alternative.

[Enthalpy]

If the figure-of-merit really is the energy stored at identical volume, and electricity must be involved, a battery is better than heat storage.

But I'd no go through electricity, as the conversion from sunlight is expensive and inefficient.

[studiot]

5 hours ago, Ken Fabian said:

in milder climates

Lots of combinations work well in milder climates.

The real point is that users want/need the heat and power mostly in the winter and also the dark.

[Ken Fabian]

Ground source heat pumps apparently do work effectively including in cold climate. Air source heat pumps work best in milder climates. Both deliver a lot more heat than active energy (electricity) input. The former uses electricity only to move the working fluid and fans to push air through heat exchangers - ie a small amount of electricity goes a long way. ie it should be possible to use solar power to operate a GSHP for effective heating even though it would be insufficient for direct heating without that heat stored in the ground. Insufficient in colder climates to run air source heat pumps, sure but they are effective enough in milder climates.

The effectiveness of combining GSHP with enhanced heating of the ground - by using it for summer cooling or purposefully adding extra heat, say by passive solar - should make it give back more heat (be more efficient) during winter. Like all options good design of the system and the home is essential.

[Externet]

12 hours ago, Enthalpy said:

...and electricity must be involved, a battery is better than heat storage...

Doesn't the inclusion of batteries introduce more losses in charging, discharging, maintenance, shorter life and wallet losses ? 

 Seems that if what is wanted is heat; then make heat directly from photovoltaics.  Heat leaks/losses of a hot water reservoir are still providing heat inside the dwelling as intended.

========================================================================================

Ken :  Important charts if this is what you are after :  Deeper than 10m, soil temperature is stable; and temperatures likely to find at certain dates by depth in order to harvest its heat (or cold)

 

[Ken Fabian]

20 minutes ago, Externet said:

Doesn't the inclusion of batteries introduce more losses in charging, discharging, maintenance, shorter life and wallet losses ? 

Not necessarily - the energy gains from using heat pumps are significant. Like I said, the electricity requirements for running GSHP is very low compared to the heat it delivers - 1/3 to 1/5th? But GSHP could run during daytime alone and still keep a home warm - and even if a separate hot fluid store might help it is probably still better to heat the mass of the building itself and use it as the heat store. Inclusion of batteries and the cost effectiveness would include other considerations - installed just for running heating would be unusual. Our grid connected solar + batteries are not a financial gain but nor are they adding greatly to our overall electricity costs, but grid reliability is an issue and they cope with power outages easily. No tossing out thawed freezers of food either - a financial plus as well as avoided inconvenience.

I also note that passive solar hot water systems around here are being displaced by air source heat pump hot water systems, including for homes with solar electricity. Because lots of homes have PV (it is no longer expensive) that most days produce more power than is used it is actually cheaper to install and run ASHP hot water systems that run off the PV than install passive solar. Even if heating water at night via battery stored solar electricity - not the most efficient use but it may be needed sometimes - the batteries we have at our home (modest in size) would support that and most times still have plenty left for cooking breakfast (on our old tech, inefficient electric stove). The cost difference may be the material costs - lots of copper and or stainless steel in passive solar HW? They don't last forever either.

[studiot]

11 hours ago, Ken Fabian said:

Not necessarily - the energy gains from using heat pumps are significant.

Can be significant, not are significant.

 

read this realistic american article.

http://daytonthermalinspection.com/2017/09/27/the-tale-of-two-comforts-complexities-of-heat-pumps/

 

 

12 hours ago, Externet said:

Doesn't the inclusion of batteries introduce more losses in charging, discharging, maintenance, shorter life and wallet losses ? 

 Seems that if what is wanted is heat; then make heat directly from photovoltaics.  Heat leaks/losses of a hot water reservoir are still providing heat inside the dwelling as intended.

One  other point to consider with solar PV is the replacement cost of storage batteries, they are not cheap.

But batteries are essential in most PV systems as the panel output is low voltage and thus suffers high relative resistive losses if used for directly for heating as it must be high current.

One possible future use of PV that occurs to me which makes a lot of sense, but has received little if any coverage would be charging the family electric car.
After all, electric cars are the coming thing.

 

 

[Ken Fabian]

11 hours ago, studiot said:

But batteries are essential in most PV systems as the panel output is low voltage and thus suffers high relative resistive losses if used for directly for heating as it must be high current.

The panels on my roof are strung together in series at 400V DC - batteries running at ~48V, returning 240V AC to home - and running an old style resistance stove at night and early morning without issue. Not all hotplates + grill + oven at once - we tend not to waste nighttime power - it would need a higher power inverter, but those are available - a decade ago they barely existed as mass market items. I think that the system can be set up to be capable of electrically heating things. 

But I didn't suggest direct heating (just noted use of excess solar for resistance heating of hot water), I suggested Ground Source Heat Pumps run by solar with daytime use extended by inclusion of batteries. I don't claim it would be cheaper than gas or oil burning (at least not with climate externalities/Social Cost of Carbon excluded) but believe it would work. In a well designed home. Insulation and draft proofing absolutely required and solar panels may need to be on walls if there is a lot of lingering snow

 

11 hours ago, studiot said:

read this realistic american article.

http://daytonthermalinspection.com/2017/09/27/the-tale-of-two-comforts-complexities-of-heat-pumps/

That article is about Air Source aka reverse cycle A/C, not Ground Source. I note that I noted above that Air Source/reverse cycle is less suited to cold climates but that GSHP appear to work okay.

Externet, thanks for those graphs.

On 2/9/2021 at 9:00 AM, Externet said:

Ken :  Important charts if this is what you are after

Edited February 9, 2021 by Ken Fabian

[Enthalpy]

I understand photovoltaics as the conversion to electricity first which is a waster of money and efficiency if heating is the goal. In that case, battery. Tesla claim >94% on a cycle and guarantee (by replacing for free) 10 years operation.

But if sunlight shall heat a storage medium, I prefer this over a battery, sure.

The only decision is whether the heat storage medium shall be hot, then smaller but with an oriented sunlight concentrator, or warm, then bulky but with a static collector, say on the roof's slope.

[studiot]

On 2/9/2021 at 10:03 PM, Ken Fabian said:

The panels on my roof are strung together in series at 400V DC - batteries running at ~48V, returning 240V AC to home - and running an old style resistance stove at night and early morning without issue. Not all hotplates + grill + oven at once - we tend not to waste nighttime power - it would need a higher power inverter, but those are available - a decade ago they barely existed as mass market items. I think that the system can be set up to be capable of electrically heating things. 

But I didn't suggest direct heating (just noted use of excess solar for resistance heating of hot water), I suggested Ground Source Heat Pumps run by solar with daytime use extended by inclusion of batteries. I don't claim it would be cheaper than gas or oil burning (at least not with climate externalities/Social Cost of Carbon excluded) but believe it would work. In a well designed home. Insulation and draft proofing absolutely required and solar panels may need to be on walls if there is a lot of lingering snow

But I said, and you even quoted, the panel voltage is low.

Sure you can string together many panels in series, but then the maximum current you can supply is the current available from a single panel.
And if you want to charge batteries then it is current you want, not voltage (the hint is in the word charge).
Furthermore a series connection means that you are at risk of loosing your supply (by sod's law at the most inconvenient moment)   -  just likwe old fashioned christmas tree lights.

 

Yes, ground source have advantages over air source, but they also have disadvantages.

Firstly how many people in blocks of flats, especially high rise could employ them ?
Bear in mind that by far the greatest % of folks live in cities  - even in Australia - and I understand the trend is increasing not decreasing.

Ground source installations require quite a lot more capital investment, usually twice as much on average.

Thirdly, by their very buried nature, pipework will be much more expensive to repair or replace.

So there you have it.
There is no perfect system.

 

[J.C.MacSwell]

On 1/7/2021 at 1:30 PM, Halc said:

My parents had an old solar heater that worked exactly that way. No electricity was generated. When it was sunny, water would circulate through the panels which focused sunlight onto the pipes.  The water was stored in a tank which fed into the main water heater, reducing the effort to heat it to full temperature. They had a forced-air heating system for the house, so it was never used for winter home heating.

As for barrels of rocks, rocks have far higher specific heat than metal, so you'd not want to replace them. I've seen active solar houses, and most of the heat sinks were rocks/concrete.

Steel is a lot more dense than rocks, so the heat capacity is greater despite the lower specific heat. 

[studiot]

26 minutes ago, J.C.MacSwell said:

Steel is a lot more dense than rocks, so the heat capacity is greater despite the lower specific heat. 

Wish things were this easy.

Yes, steel is approx 3.3 times as dense an concrete but its specific heat is about half that of concrete.

All this means is that to store a given amount of heat the mass of steel requires is about 2/3 the mass of concrete per degree storage temperature.

Or that you have to heat the concrete to a higher temperature for equal masses of steel and concrete to store the same amount of heat.

However there is another factor in play.
 

The heat transfer coefficients (both internal and external) are much higher for steel so you would loose heat more quickly from the steel, perhaps more quickly than you want to in a poorly designed heat store.

[J.C.MacSwell]

17 minutes ago, studiot said:

 

The heat transfer coefficients (both internal and external) are much higher for steel so you would loose heat more quickly from the steel, perhaps more quickly than you want to in a poorly designed heat store.

That's generally an advantage in a well designed heat store. It will absorb or release it's heat more readily and more efficiently (less air/fluid flow and time required all other things being equal). It depends on what you want to do. Steel certainly isn't a good insulator if that's what you're getting at.

My statement is that easy. It's just a statement of fact. You may not want to replace rocks or concrete with steel, for a number of reasons, but the statement stands. It's just a fact to be considered. 

You can store more heat capacity at a given temperature within a given volume with steel than concrete, and store more heat capacity with concrete in a given mass.

It's not rocket surgery.

44 minutes ago, studiot said:

Wish things were this easy.

Yes, steel is approx 3.3 times as dense an concrete but its specific heat is about half that of concrete.

All this means is that to store a given amount of heat the mass of steel requires is about 2/3 the mass of concrete per degree storage temperature.

Or that you have to heat the concrete to a higher temperature for equal masses of steel and concrete to store the same amount of heat.

However there is another factor in play.
 

The heat transfer coefficients (both internal and external) are much higher for steel so you would loose heat more quickly from the steel, perhaps more quickly than you want to in a poorly designed heat store.

I think you meant volume there, correct?

[Ken Fabian]

13 hours ago, studiot said:

But I said, and you even quoted, the panel voltage is low.

Sure you can string together many panels in series, but then the maximum current you can supply is the current available from a single panel.
And if you want to charge batteries then it is current you want, not voltage (the hint is in the word charge).

But they DO charge the batteries, with enough left over that my home still sends 4x more electricity to the grid than it uses. They run electric kettle and stove, that have substantial loads. With limitations, they run A/C - full power on sunny hot days and some hours at lower power running from batteries. Some reverse cycle heating on the same basis. I would like better batteries - of course - but I note that even a decade ago the grid connected solar+battery system I have was not even a readily available option let alone reasonably priced enough that no substantial increase in costs for electricity over the life of the system has been incurred.

 I don't really get your argument about solar energy being lesser than other electricity; a 1 GW solar farm can deliver 1 GW of power that is indistinguishable from 1 GW from a coal plant - except probably more precise voltage and frequency. Yes, not all the time - but as experience in Australia has shown, heatwaves and coal and gas burning power plants are a failure inducing combination; the contributions from a great many rooftops have more than once kept the whole system going when the "reliables" failed to work as and when required.

It is a system with many elements that support each other - cheap daytime solar power displaces coal and gas power and up to a point - that is itself a moving target - can be accommodated and utilised effectively - and reduce costs. Australia's electricity market operator thinks above 70% wind and solar within 5 years is possible (although political will to change market rules to enable it is lacking) and above 90% within a decade without loss of system reliability. It is also a system in transition; the hypothetical 100% right now option is not an option, just an argument. More RE is coming on line and the issues that arise are both foreseeable and do not need to be fixed in any 100% right now way. If and when real rather than imagined constraints on how much emerge, we will take stock and, if necessary, change course; even legislated targets get unlegislated when sufficient problems emerge.

As for heating a home - sure, it is simpler and no doubt cheaper (externalised climate costs not counted) to connect to the grid and draw the power needed as needed but as a hypothetical (which I suspect do exist as actual working systems) solar power and GSHP's offer a way to use solar to heat a home in a cold climate. It does get used on (under) larger, multistory building too. Where I suspect the deeper ground does take up and store heat as a key element, from using it for cooling over summer; these tend to be deep 'borehole' types that do not rely on seasonal ground heating.

 

Edited February 15, 2021 by Ken Fabian

[Externet]

Thanks, gentlemen.  Revisiting the basics of the thread;

- The photovoltaic generation (~3KW) to be used solely for dwelling heating day and night, no cooking, inverters, lights, grid feed... If appears makes no money sense; OK, that is not a purpose of the discussion.

- The preference to not use batteries is because of added cost, maintenance, fumes, weight, and they are not storing heat.  And if lately is sunny and batteries got full charge, today is not cold enough, why 'waste' the irradiation as batteries cannot store more beyond full charge state ?  An extra barrel of water 'inline' or raising rocks a few degrees more is still in reach instead.

- The material to store heat from electric heating elements in adequate barrels can be stones, concrete chunks, oils, iron scraps, water.  Which would store the most heat per volume regardless of convenience or cost ?  To deliver heated air, heated fluid to proper ducting.  Fancy state-changing chemical salts are beyond consideration to keep simplicity.   10m underground depth geothermal storage is not preferred when an ample basement is available.

 

[Ken Fabian]

6 hours ago, Externet said:

Revisiting the basics of the thread;

If you are using PV - ie beginning with solar electricity rather than directly solar heated water (or air or other fluid) - then I would still be looking at including some kind of heat pumps to get extra heat gain if possible. And I would still look at making the home itself or under it the primary heat store and rely on thermal inertia to carry through nights. Absent any use of heat pumps and using only direct resistance heating I suppose a large insulated water tank would retain the heat and underfloor pipework could do the heating. It could be set up to run by convection. But 3KW for a few hours each day is not a lot of direct heating. Draft proof and insulate first, to have any chance.