Using water as a dynamic heat sink/source

[max_torque]

Has anyone considered using a volume of water (well water/gylcol mix) as a dynamic heat sink for a system using a heat pump?

A 1000 litre IBC is cheap, could be buried in a pit, insulated with some normal insulation, and with a heat pump arranged to be able to heat or cool that volume, would be a significant energy store, enabling dynamic load shedding espically wrt to cheap grid energy / solar power?

1000 litres of 70/30 water glycol is 1057 kg, has a specific heat of 3.74 kJ/kg.K, and with a heat pump using a refrigerant to coolant exhangers to avoid icing (no ambient air containing moisture to freeze up) and probably avoiding boiling too (makes system simpler as it won't have to be a pressure vessel) could probably operate with a 90 degC deltaT if necessary on that coolant store. That means around 100kWh of energy can be dynamically exchanged.

I live the UK, where we tend to have cold nights, and warmer days, and our solar load is very peaky, and our grid electricity costs are highly industry demand driven, and recently have been going negative overnight!

So, in the winter, when i say want to heat my house for 6 hours in the evening (4pm to 10pm) i can use a cheap solar hot water array on the roof to heat the coolant store, and if that is insufficient (which it will be for most of the winter) then i can turn on electric immersion heating during cheap rate electricity the night before.

In summer, if i want to cool my house (increasingly required as our summers get hotter) then i can simply reject heat from the house into the coolant store, and over night, when it's cooler, use a simple cheap air/water radiator to reject that heat to ambient air, over a longer period.


This, at first glance seems a sensible system, and relatively cheap, using a heat pump to move heat around, and cheap components that could certainly be sourced (sic) second hand in order to build it?


By leveraging a heat pump, the energy i ultimately pull from the grid can be around 3 times lower than the heating/ cooling power, and of course, it would be possible to move that to a solar electricity based system eventually....

Have i missed something important here?  I'd have to run a simulation over a typical UK year to see how it stacks up i guess   

[Marco]

Don't see how solar collectors are ever worth it any more, just put down more PV.

Any heatpump system will have a thermal buffer to prevent cycling ... just making that huge with insulated IBCs is certainly an option. Given that you don't have net metering it might often make sense to run the heatpump and buffer heat/cold rather than feed in.

The only problem I foresee is how to automate it, given that Smart Export Guarantee requires a certified system ... will there be certified systems to dynamically optimize between thermal storage and feeding in?

[MarkF]

Ever hear of Geothermal Heat Pumps? 

[Marco]

Ever hear of Geothermal Heat Pumps?

Aquifer availability isn't exactly universal. Using insulated pools just under the surface instead is quite common.

[coppice]

Ever hear of Geothermal Heat Pumps?

Aquifer availability isn't exactly universal. Using insulated pools just under the surface instead is quite common.

What is the relevance of aquifers?

[Marco]

What is the relevance of aquifers?

Needs less boreholes. If you need a ****ton of boreholes to store heat directly in small regions near the boreholes I'm not sure it still makes sense to store thermal energy underground, otherwise why do they do the insulated pool systems?

[coppice]

What is the relevance of aquifers?

Needs less boreholes. If you need a ****ton of boreholes to store heat directly in small regions near the boreholes I'm not sure it still makes sense to store thermal energy underground, otherwise why do they do the insulated pool systems?

Most people don't do use insulated pool systems. Most people do what is in the image a few messages back. They put a network of tubes 1m to 2m below the garden. Soil insulation is sufficient that 1m down the temperature hardly changes between summer and winter. So, you can fairly efficiently push heat into that soil as you cool the house in the summer, and then draw it back when you need to warm the house in winter.

[jc101]

A long time ago now I worked for years in the Sinclair Research building (Ex Clive Sinclair, Spectrum computers etc.).  For it's time it had an advanced heating / cooling system. 

In the basement was a huge water tank, overnight that would be heated by a heat pump that took water from a bore hole (it used to be a mineral water bottling plant back in the day) to store in the tank.  That heat was then used to heat the building. The bore hole water was a steady 12c all year round, and when it worked it was great.  In the summer, the bore hole water could be pumped up to an exchanger on the roof to cool the air in the building.

Sadly, many years of building alterations and no-one understanding how it all worked meant it was eventually removed.  The basement was turned into offices, and I believe is now a room full of UPS gear.

We used to get letters from all over the world asking for help with old Sinclair computer gear, if we could help we did reply.

Needles to say, we had a banister around the stairs when I was there.... 

[Marco]

They put a network of tubes 1m to 2m below the garden.

Can you show any such system which actually charges the soil to a proper high temperature (lets say 20 degrees above the natural soil temperature at that depth) and show it retaining that heat for significant time when there is no load?

AFAICS storage in these systems is mostly just a legacy use for surplus heat from solar collectors, for which even an extremely lossy storage is better than nothing. With PV getting ever cheaper and storage competing with feed-in, the storage needs to be somewhat low loss.

[coppice]

They put a network of tubes 1m to 2m below the garden.

Can you show any such system which actually charges the soil to a proper high temperature (lets say 20 degrees above the natural soil temperature at that depth) and show it retaining that heat for significant time when there is no load?

AFAICS storage in these systems is mostly just a legacy use for surplus heat from solar collectors, for which even an extremely lossy storage is better than nothing. With PV getting ever cheaper and storage competing with feed-in, the storage needs to be somewhat low loss.

It was good enough for rich people to have summer ice cream in medieval times.

[Marco]

It was good enough for rich people to have summer ice cream in medieval times.

Yet they were doing it wrong ... wood, straw and air were much better insulators than soil.

[max_torque]

Ever hear of Geothermal Heat Pumps? 

(Attachment Link)

of course.

But what i am suggesting is subtly different! I am not using the water as an energy source or sink, i am using the water as a buffer, so that water, unlike a GSHP, needs to be well insulated from its surroundings!

GSHP pushes heat into, or pulls heat from the earth and rock.   My system will take heat from other sources, and simply use the water as a very mobile, very high specific heat "battery" in effect.  The idea relies on the fact that we don't have the heating constantly, tend to want more heat at certain times a day, and that thanks to industries running mostly during the day, we have a net energy surplus overnight, meaning energy costs can invert (ie they PAY YOU to use electricity!)

I could cheaply put in say 20 kW of mains electrical immersion heaters, and charge my liquid heat battery, making money as i do so, and then slowly deplete that energy as an when i need it to heat my house, or warm water etc

[max_torque]

[With PV getting ever cheaper and storage competing with feed-in, the storage needs to be somewhat low loss.

Which of course drives one towards a larger reservor, because the surface area to volume ratio improves continuously as the size of that reserve increases!

[duak]

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

I looked into doing something like this in 2007 when energy prices were high.   There were at least two German companies providing parts for systems like this: Stiebel-Eltron and Viessmann.  I also remember that Glauber salts were also used to store heat because it changes phase around 30 deg C.

In the end, I went with a high efficiency water heater, furnace and heat pump.  The climate here in Vancouver, Canada is similar to that of the UK.  On the other hand, we have cheaper electricity and natural gas.  If energy prices were higher or if there were shortages, it would change the calculus to adding solar collection and storage.  BTW, we had a major failure in one of the natural gas pipelines recently.  People were asked to reduce gas useage.  I was able to run the entire air and water heating system on electricity with a few simple changes.

About the Sinclair building.  One of my employers started as a small company in a building very much like Sinclair's. It used a pond as a heat store and had water pumps in the basement and heat exchangers for the HVAC system on the 5th floor.  As far as I know, it used common parts and controls and was reasonably reliable.  Like anything that mixes water, metal and electricity there's trouble sooner or later.  eg., autos, washing machines, etc.  It would be interesting to see how effective and efficient it was vsi-a-vis a standard HVAC system.

The building was designed and built in the early 80s for the provincial government to act as an incubator for entrepreneurs.    The company expanded and ended up taking over the entire building.  It turned out the design wasn't right for entrepreneurs because they usually wanted their own entrances.  The building also got a bad reputation because a number of the tenants were considered to be running scams.  It became a location for filming various TV series because it looked futuristic.

[jc101]

The Sinclair building was very efficient for it's day.  The problem was internal walls were thrown up which caused airflow / circulation problems.  That started the downward spiral as successive teams tried to fix it without really understanding how it was originally designed, and people moving the walls about.  After all, it was a home grown design by Clive himself.

The McLaren Technology Centre ( https://en.wikipedia.org/wiki/McLaren_Technology_Centre ) uses the artificial lake to cool the building and the wind tunnel.  It's a really impressive building to visit and have a poke about.  The F1 cars are built in what looks like a sterile workshop, everything is pure white, if *anything* dripped or fell of it would stand out a mile.

Rather Tham 1m3 of water, their lake is 50,000m3, but is only used for cooling.

[max_torque]

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.

[Marco]

warm water

I'm not sure that makes sense. For legionella reasons you want your hot water circuit for tap/showers to be much hotter than temperatures which can still provide all the heat for your central heating (with low temperature radiators or floor heating).

[NiHaoMike]

What does really well for thermal storage is air conditioning using ice, since then you can take advantage of the phase change effect and end up with similar energy density to using batteries, at a much lower cost.

I'm not sure that makes sense. For legionella reasons you want your hot water circuit for tap/showers to be much hotter than temperatures which can still provide all the heat for your central heating (with low temperature radiators or floor heating).

Largely a non concern with chlorinated city water (actually, it's becoming more common to use chloramine which is more effective than just chlorine), but even if it is, it can be solved pretty easily with a heat exchanger so the tank water is separate from potable water. Most likely some sort of heat exchanger will be the most economical choice anyways so the tank doesn't have to handle the full pressure.

[max_torque]

The important point is that the bulk store is

1) An energy store, not a temperature store

and

2) Is never used for end users, so can contain corrsiion and biological inhibitors as necessary


The key tech is the heat pump that sits between the various bits of the system and "converts" between the different temperatures of those systems.


For example, i can use a roof mounted solar water heater, to lift the bulk heat store by say 20 degrees.  That's too little to use directly as hot water for the end user, (heating or washing) but the heat pump can leverage the heat in the heat store and convert it to a more useful temperature in another part of the system.

[Marco]

I think heatpumps will just circulate water from the buffer tank through central heating directly if it's hotter than their max return temperature. If so you only really need a heat exchanger to heat up the "cold" water input for a boiler, the IBC would be the heatpump buffer. Although to be able to switch over between heating and cooling temporarily without dumping all the storage energy it might be better to have a separate smaller buffer so you can decouple the IBC for a bit.

I just don't see how solar collectors make sense any more, any room they take would be better occupied by PV.

[wraper]

They put a network of tubes 1m to 2m below the garden.

Can you show any such system which actually charges the soil to a proper high temperature (lets say 20 degrees above the natural soil temperature at that depth) and show it retaining that heat for significant time when there is no load?

AFAICS storage in these systems is mostly just a legacy use for surplus heat from solar collectors, for which even an extremely lossy storage is better than nothing. With PV getting ever cheaper and storage competing with feed-in, the storage needs to be somewhat low loss.

Why would you want to "charge" the soil? What you want to avoid is that soil temperature changes significantly (heat pump efficiency drops) and increase heat exchanger area if that happens. Earth is not storage, it's a heatsink with stable temperature and basically infinite thermal capacity.

[Marco]

Why would you want to "charge" the soil?

Because a heat pump still takes power ... a charged thermal storage allows you to do heating/cooling with only the pump consuming power.

[wraper]

Why would you want to "charge" the soil?

Because a heat pump still takes power ... a charged thermal storage allows you to do heating/cooling with only the pump consuming power.

You don't understand the core idea about using heat pump. You don't use it to store energy somewhere (soil). If soil temperature changes, it's unwelcome side effect. Air heat pump works the same, just less efficient as heat pump needs to deal with much larger temperature difference.

[Marco]

You don't understand the core idea about using heat pump.

Most heat pumps have small storage buffers to prevent frequent cycling ... so energy storage is a standard feature. OP wants to have a little more storage so he can turn on the heat pump (or even an electric heater) when electricity is cheap and it can carry heat across a couple days.

Under ground storage systems are being used too (ATES/BTES). But it's expensive and I doubt the storage efficiency is very good compared to having massive insulated water tanks.

[max_torque]

A "heat pump" is basically the same in the thermal environment as a DC/DC converter using an inductor is in the electrical environment.  The inductance in a DC/DC allows one to move current against the normal direction of potential, the phase change of the working fluid in the heat pump system allows one to move heat against the normal temperature gradient.

How you use either is not fixed.

In my case, a heat pump would allow me to collect a large amount of "Low level" heat energy, energy at too low a temperature to be useful, and then once i have cheaply stored that energy, to at a later time access it, and "concentrate it".

This means i can use very cheap heat sources,like solar water heater (which can be as cheap as a length of hose black hose pipe coiled on the flat roof of my workshop) to charge my heat store.  It also allows me to potentially pull my bulk store BELOW ambient temperature, and so be able to access a greater total amount of energy from that store (like being able to discharge a capacitor to a negatoive voltage).

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!