Using water as a dynamic heat sink/source

[fcb]

What about two IBC's.  One heavily insulated (hot water store) and one exposed to the subsoil.

If you can get the solar collector to work without a pump (?) then you have a tank of warm/hot water for night/winter use and a cold tank for driving some sort of aircon in the summer.

Just how hot do typical solar collectors get?

[Marco]

The vacuum ones can heat it to boiling, the problem is they are expensive ... and an immersion heater running from PV power can get pretty hot as well, it can even do it on diffuse light.

Without an extensive pipe network below the frostline or boreholes it's unrealistic to expect to be able to dump/extract significant heat to/from the soil. Just the contact surface of the IBC is going to be way yoo slow.

PS. I wonder if a lot of houses in a dense urban area start using ground source heatpumps if the frostline will drop significantly.

[Someone]

But, the critical point, is that the system effectiveness, in terms of $ per kWh of heat, is driven by non physical metrics, broadly the fact that our electricity cost is not fixed, but varies by time of day, so by being able to regulate when i take power from the grid, means i can also leverage those differences in cost to add value to my system.   A simple AC electrical immersion heater can again be very simply and cheaply added, whcih can be used to "charge" the store during times of cheap electricity, perhaps even when the price is actually negative!

Also consider that the thermal mass could be more effective if inside the houses insulation shell. Cost effective solutions can even be found around "resizing" (to apparently ridiculous extents) normal components of the heating and cooling systems.

[Someone]

PS. I wonder if a lot of houses in a dense urban area start using ground source heatpumps if the frostline will drop significantly.

There are some good works on that topic, an easy one for example:
https://www.withouthotair.com/c21/page_152.shtml

[ogden]

You don't understand the core idea about using heat pump. You don't use it to store energy somewhere (soil).

Right. Geothermal heat pump (GHP) do not store energy. Then there is ground source heat pump (GSHP) which does exactly that -  freeze cool-down (big enough) land mass during winter in hope that it will be heated during summer. I would avoid installing GSHP because you may end-up with permafrost under your harden and little to no heat during winter.

[max_torque]

There's actually another interesting possibility that using a Heat pump as a T/T converter (Temperature to Temperature) opens up, and that is to run the solar hot water collection loop at a mean temperature BELOW ambient!  The problem with solar hot water panels is that because they conventionally need to be above ambient, they must be well insulated to avoid most of the thermal radiation energy from simply "leaking" out into the surrounding colder environment. 
Realistically, you want an outlet water temp of lets say 60 degC, which is hotter by at least 30 degrees than most ambient environments.

But with a heat pump, you can cool the entry temperature of the water going into the solar panels to below ambient, far below, and now, not only do you collect practically 100% of the solar radiation (only reflection now counts as a loss) but you also actually gain heat from the environment!

This means cheap panels, and very high efficiency, and the ability to collect heat even on a cloudy day, where conventional solar hot water panels cannot, because there heat loss exceeds the incoming radative gain.

[Marco]

It doesn't get you anything, there is never any reason to pump something below ambient in heating. Yes, you're also heating something up with that pumping .... but you could just pump against ambient instead and be more efficient. While the solar collector could take water at ambient and get more useful energy relative to ambient, which you have at infinite capacity.

Air ambient is always there, nothing below it makes sense. A heat pump is not a heat engine, being able to get larger temperature differences between fluids might be potential energy, but it's not energy you can use.

[ogden]

Air ambient is always there, nothing below it makes sense.

Really? How do you think air source heat pumps are able to operate then? 

[Marco]

Heatpumps are non intuitive

I guess you could just use a water to water heatpump and use the output of the primary side of the heatpump as the input of the solar collector, and connect the output of the solar collector as the input of the primary side of the heatpump ... if the collector can't raise the temperature above ambient though it will not get you anything.

It would be more effective to heat the primary side water to ambient with a simple fin radiator, and only then heat the refrigerant up from ambient with the collector.

[ogden]

It would be more effective to heat the primary side water to ambient with a simple fin radiator, and only then heat the refrigerant up from ambient with the collector.

Good point. AFAIK most solar hot water collectors are used without heat pumps, they usually are just (pre)heaters for boilers. When it is about heating then again - during time/season when you can extract actual heat from solar collector, most likely you don't need any heating at all. You rather need cooling. In result solar-assisted heat pump installations are not that popular and usually of industrial scale.

[max_torque]

It doesn't get you anything, there is never any reason to pump something below ambient in heating.

Oh dear, i hope you don't need to use physics for your day job...   


The beauty of storing heat energy at a constant temperature is that you can manage the losses, because those loses are driven by the thermal graident to the ambient environment.

For example, i have two cups of water on my desk, in a room at 25 degC. The water in one cup is at 50 degC, the other is at 25 degC.. Which cup looses more energy to it's environment ?

Solar hot water heating panels normally have to tread a fine line, they have to allow as much solar radiation into the coolant medium, but let as little heat energy escape.   Up on your roof it's probably cold, and windy, if the solar radiation heats the water in the panel to say 50degC, then that's significantly higher than the environment, so no matter how good the insulation is, some of the heat energy gathered from the radiation is lost.

Now consider a panel where the water temperature in the panel is not allowed to get above ambient.  What are the loses now?  Can you see the advantage?

Of course, the trade off becomes a balance of the CoP of your thermal converter (heat pump) vs the delta T of the system. The higher the deltaT, the lower the CoP (RE: carnot cycle efficiency).

This is what i shall run some simulation to characterise, as there is plenty of data on practicalable heat pump CoP vs deltaT with different refrigerants.

The single benefit of a water storage system is however cost, or lack of it.  Using a thermal temperature converter allows you to utilise low level heat, and that means cheap storage.  My 1000 litre ICB, with a working temperature range of +- 25degC from ambient contains the same amount of energy as a big Tesla Battery (~100kWh) and yet the plastic tank costs around $100 and the water in it is effectively free!   It also can be charged and discharged an infinite amount of times, and the charging rate, thanks to the higher specific heat capacity of water is actually pretty good too

[Marco]

The beauty of storing heat energy

Heat is energy, but it's not necessarily useful energy.

For example, i have two cups of water on my desk, in a room at 25 degC. The water in one cup is at 50 degC, the other is at 25 degC.. Which cup looses more energy to it's environment ?

The one which has something to lose.

Now consider a panel where the water temperature in the panel is not allowed to get above ambient.  What are the loses now?  Can you see the advantage?

Lets say for a moment the collector heats it up to exactly ambient temperature, what according to you would be optimal ... what did that get you? You don't need to use a solar collector for what a radiator can do, in fact you shouldn't because a solar collector is designed to be a terrible radiator. When there is no sunshine the radiator will still heat it up to ambient, but a good solar collector will do nothing. The radiator always works, the collector doesn't.

The collector is for heating above ambient.

[richard.cs]

Or to put it another way, you already have an infinite heat source/sink at ambient temperature, which is cheap to couple into with a conventional refrigerant:air heat exchanger. Taking heat from a solar collector at ambient temperature has no advantages* over taking the same amount of heat from the ambient air. If you run the solar collector below ambient then you'll be pumping heat against a bigger temperature differential and your COP will fall and you'll have to put more energy in to move that heat than if you took it from ambient.

It's a valid point that using a solar collector to feed a heat pump allows you to benefit from a collector too cool to usefully heat water, but this is only true if your solar collector is still above ambient so that it improves your COP compared to pumping form ambient. e.g. in a 10'C ambient, pumping heat from a 40'C collector to heat domestic water to 60'C is a good idea, but only because it improves your achievable COP (the theoretical max goes from 7 to 17 but you won't get near that in practice, looking at some graphs online you might go from 4 to 12 which is still a big benefit).

*well, one maybe, it won't ice up.

[max_torque]

As you rightly spotted, the ability to avoid freezing is significant, but the other advantages are:

1) increase the operating temperature range of my coolant store.  This basically gets you a "bigger" storage system because you can swing its temperature a greater amount.   

2) It enables a much cheaper heat source.   To pull say 5kW continuously from the air, and avoid freezing in the UK especially is difficult. Air has a low specific heat capacity, and contains a lot of moisture.   Using 1 tonne of water as a dynamic store and a cheap solar array (operating close to ambient it can be very cheap indeed, think coiled up, black painted rubber hose pipe! In winter, where air temps are typical around 6 t0 8 degC, that becomes particularly problematic.

3) for a lot of a typical year in the UK, if i want my house at say 18degC, then i'm going to need to be heating it in the evening and overnight, and cooling it during the day, that can be done without any external heat source using a big enough storage tank, by just pushing low level heat backwards and forwards.


You point about the balance between thermal gradient and CoP is very valid, as i previously mentioned, hence the plan to do some simulation with realistic heat pump numbers to see if a "sweet spot" exists, and what sort of control algorythm would be needed to optimise the system for the lowest energy input 

As i said originally, the primary aim to to be able to use "cheap rate" electricity (or 'lecy from a local solar PV array, that may have an erratic output) to replace my current energy supplied by my gas boiler.  To do that with a purely electric system is very expensive, because batteries are expensive, but water tanks and water are not.......

[Marco]

2) It enables a much cheaper heat source.

An unreliable heat-source.

To pull say 5kW continuously from the air, and avoid freezing in the UK especially is difficult. Air has a low specific heat capacity, and contains a lot of moisture.

The ice has a lot higher moisture density, which is to say it's far faster to melt than to accumulate.

3) for a lot of a typical year in the UK, if i want my house at say 18degC, then i'm going to need to be heating it in the evening and overnight, and cooling it during the day, that can be done without any external heat source using a big enough storage tank, by just pushing low level heat backwards and forwards.

Nah, an IBC is days worth of storage ... you don't want to be pushing it this way and that all the time, any time you miss the mark (all the time) you waste a ton of energy. I think a large tank for heating and a smaller tank for cooling makes more sense, most of the time when you're cooling you'll have PV electricity after all.

If I were going to do it, I'd get something like a Panasonic air-to-water TCAP monoblock. Connect the IBC and a smaller tank as buffers. Throw immersion heaters into the IBC. Have an electric boiler for hot shower/tap water and route inlet water through a heat exchanger in the IBC (saves you significant on the amount of electricity used by the boiler). Or just use an existing gas boiler if you have it, but still with the heat exchanger.

I think for the most part you could let the standard controller just do its own thing. Just need to add some extra control to switch the buffer tank when you switch to cooling and turn the immersion heaters on when you want to use excess power. Maybe the heatpump will need some additional duty cycle limiting though with such a huge buffer, going to take a while to get up to temperature.

Of course you're only going to save money if you count your time as free

[tszaboo]

1000 kg of water is a cube only one meter on a side.  Not an unmanageable amount.  I expect insulating it will be interesting.

Im thinking a block work "bund" lined with 100mm of foil faced PIR insulation, surrounding a 1000 litre IBC.

U values of around 0.022 W/mK , roughly 6m^2, say average 50 degC delta to ambeint, so that's  a pretty low heat loss. Of course, air drafts and the necessary pipe peircings will reduce that insulation performance significantly, but at a first look it doesn't look too hard to do.

You need almost a cubic meter if isolation for that, and that isolation has almost zero weight. Ground water can actually push objects, like empty tanks out of the ground, and this could easily happen to this. Or you dont make it waterproof, and then it will be soaked in water, and U value is off.