CoP of non-heatpump ground source heating and cooling

[Lilly_Pines]

I'm looking to use ground source non-heatpump heating/cooling to pre-condition ventilation air before the MVHR, but I'm having a hard time finding data to base performance calculations on. The idea is to have one or more boreholes (e.g. geothermal pile foundations) with fluid circulating through them, then going through a water-to-air heat exchanger to heat intake air in winter and cool+dehumidify it in summer, without incurring the losses of a heat pump in between, as this should result in vastly higher return on the energy expended. However, I can't find how much higher the effective CoP would be, only vague estimates like "around 30-100".

 

I'm comfortable doing the math on fans, filters and ducts to minimise pressure loss and fan power, but haven't had luck on things like "if I have X m3/h of air and Y l/min of water going through this heat exchanger, and their entry temperatures are Z and W, how much will their temperatures change and how much pressure loss will be incurred in each". Does anyone here know how to find the relevant information and equations?

[JohnMo]

If you can get a free heat source why waste it heating the air, which is already get pre heated by the outgoing air.  Can you not use it directly for UFH heat source?

 

Or are you looking at 8 to 10 degrees temperature source?

[Lilly_Pines]

Just now, JohnMo said:

If you can get a free heat source why waste it heating the air, which is already get pre heated by the outgoing air.  Can you not use it directly for UFH heat source?

 

Or are you looking at 8 to 10 degrees temperature source?

Yes, the idea is to get 0c air to 10c and 25c air to 15c before it goes to the MVHR (in winter) or bypasses it (in summer).

 

With a 87% efficient MVHR and 20c indoor temperature the incoming air would be 18.7c instead of 17.4c in winter, cutting ventilation heat losses in half. Which is kind of a big deal if you're going for multiple air changes an hour. In summer the incoming air would be below the desired indoor temperature, helping compensate for heat gains and high air temperatures without using compressor-based (significantly less efficient) air conditioning.

The raw borehole fluid would not be suitable for UFH as it would be cooling the house down, not heating it up. Using a GSHP would be possible, but running the compressor on the refrigerant consumes electricity and limits the achievable CoP, so I'd prefer to use it only for purposes that really need the temperature differential (DHW and backup heating when heat gains and intake air preconditioning aren't enough).

[SteamyTea]

Start here:

 

https://www.engineeringtoolbox.com/heat-transfer-coefficients-exchangers-d_450.html

 

then move to this:

 

https://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html

 

Then ask yourself why it is not done everywhere, successfully.

 

[ProDave]

I would be interested to hear your location and how deep the borehole has to be to deliver water at your expected temperature.

[Temp]

Sorry for the poor quality sketch but my understanding is that mvhr heat exchangers work something like this..

 

To make much difference the incoming air would have to be preheated so that the 18C incoming air is hotter. But if it goes over 20C the heat exchanger will work in reverse heating the outgoing air. 

 

Am I missing something?

[Lilly_Pines]

10 minutes ago, ProDave said:

I would be interested to hear your location and how deep the borehole has to be to deliver water at your expected temperature.

The mean annual ground temperature in the UK is around 9-11c and once you reach 10m depth there is very little change throughout the year.

 

12 minutes ago, SteamyTea said:

Start here:

 

https://www.engineeringtoolbox.com/heat-transfer-coefficients-exchangers-d_450.html

 

then move to this:

 

https://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html

 

Then ask yourself why it is not done everywhere, successfully.

 

At 600 m³/h getting air from 0c to 8c is roughly 1.6kW. The heat transfer coefficients for air-to-water are listed as 600-750 W/m²K, or 4,800-6,000 W/m² at a 8c temperature difference, thus indicating that a heat exchanger of roughly 0.3 m² would be required, which seems perfectly viable.

 

I found an earlier discussion, with some numbers giving a CoP estimate of about 40. Looking at the product in question, the heat exchanger seems excessively small resulting in unnecessary resistance to airflow and I'm also questioning if the brine pumping could be more efficient. However, even a CoP of 40 is massively higher than can be achieved with an ASHP which means a lot more bang for the buck, and a lot more nonsense in the building (can I cool a greenhouse with this? numbers say yes) before building control crucifies me. The possibility of using the ground as the only source of cooling also opens up the prospect of avoiding the hardware of an ASHP/whatever for space cooling and just plugging a DHW GSHP into the same boreholes. A GSHP also seems easier to soundproof than the outdoor unit of an ASHP buzzing somewhere on the property.

 

The reason this isn't done more often is that it requires the groundworks of a GSHP which makes it prohibitively expensive compared to the gain in most cases, but if you're a weirdo wanting to do silly stuff and willing to DIY the hardware it starts to seem a lot more attractive to me.

[Lilly_Pines]

57 minutes ago, Temp said:

Sorry for the poor quality sketch but my understanding is that mvhr heat exchangers work something like this..

 

 

To make much difference the incoming air would have to be preheated so that the 18C incoming air is hotter. But if it goes over 20C the heat exchanger will work in reverse heating the outgoing air. 

 

Am I missing something?

If the incoming air starts at 10c instead of 0c it arrives at 19c instead of 18c. Not a huge difference until you crank up the ventilation rate to silly levels because you like fresh air even in winter, and enthalpy cores that exchange moisture as well as heat have lower efficiency to begin with.

[saveasteading]

Interesting.

What are the ground conditions?

If rock or clay you don't get much recovery from the surrounding ground once you have taken the heat out of it.

Looking at heating only, for simplicity.

 

You need a pump to circulate the tempered fluid.

I have gathered from too many visits to caves and mines that the ambient temperature is about 16C.

Therefore, assuming the source is constant,  you will draw your brine/ammonia at somewhere under 16C, then have to pass it over your additional MVHR, using a fan, to bring the incoming temperature up.

 

I have heard much worse ideas.

 

My gut feeling, with no maths, is that it would marginally effective in use, but horrendously expensive in outlay. And that only if you had warm rock to start with, or porous ground with a water flow through it to provide fresh energy.

 

I tried very hard to justify ground source heating, speaking to the industry. It could never be justified. Where it was used was with government grants to make it anywhere close to  air source.

The last discussion I had with the GS industry acknowledge that you had to replace the heat in the ground in the summer.

 

I say spend a tenth of the cost on better construction and insulation. Not so much fun, but it will work.

Optimising construction details can be fun too.

 

 

[saveasteading]

13 minutes ago, Lilly_Pines said:

enthalpy cores

Tech term of the day award.

[Temp]

We have our mvhr system up fairly high. We regularly dry washing on a rack in the main bathroom which doesn't need/have a seperate extractor fan. I doubt it would be high enough to move heat around the building though. 

 

I did find this document with some info on testing a pair of boreholes. Gives figures for the temperature uplift and power extracted but I've no idea if they are typical..

 

ground_loops_testing_report.PDF

[Lilly_Pines]

10 hours ago, saveasteading said:

Interesting.

What are the ground conditions?

If rock or clay you don't get much recovery from the surrounding ground once you have taken the heat out of it.

Looking at heating only, for simplicity.

 

You need a pump to circulate the tempered fluid.

I have gathered from too many visits to caves and mines that the ambient temperature is about 16C.

Therefore, assuming the source is constant,  you will draw your brine/ammonia at somewhere under 16C, then have to pass it over your additional MVHR, using a fan, to bring the incoming temperature up.

 

I have heard much worse ideas.

 

My gut feeling, with no maths, is that it would marginally effective in use, but horrendously expensive in outlay. And that only if you had warm rock to start with, or porous ground with a water flow through it to provide fresh energy.

 

I tried very hard to justify ground source heating, speaking to the industry. It could never be justified. Where it was used was with government grants to make it anywhere close to  air source.

The last discussion I had with the GS industry acknowledge that you had to replace the heat in the ground in the summer.

 

I say spend a tenth of the cost on better construction and insulation. Not so much fun, but it will work.

Optimising construction details can be fun too.

 

 

I don't have a plot yet so ground conditions are entirely hypothetical.

 

Looking at heating only is probably not a good way to analyse this as the main benefit is intended to be for cooling, and the heating (of ventilation air and DHW) is mainly to balance out the incoming heat from summer cooling. If the balance works out right and the ground has enough storage capacity for year-round use there would be absolutely no reliance on energy flows with the surrounding ground, as the heat that goes in in summer comes out in winter (if anything, my intuition suggests that the ground is more likely to get too hot than too cold if used like this).

 

MVHR is already pretty much non-negotiable because I want serious amounts of fresh air even in the winter, and I don't think solar & internal heat gains are anywhere near sufficient to compensate.

 

As far as outlay is concerned, in the ideal case a number of shallower boreholes that could be drilled DIY with cheap equipment would suffice. If it can be combined with pile foundations (which seems highly compatible with a post&beam frame) the outlay would not only funge against ASHP cooling but also conventional foundations. If geothermal pile foundations permit building the house closer to mature trees etc. without harming either the house or the trees the aesthetic value of that also counts.

 

Better construction and insulation may be easier said than done. I'm planning on overshooting the permitted glazing areas of Approved Document O by a factor of 2 or so, and east/west-facing windows are hard to shade with anything other than mature deciduous trees. A back-of-the-envelope calculation for a 100m² house with 40m² of windows at G=0.5 suggests I'd be getting 40kWh (minus shading) of solar gains per day in June, while heat gains through the walls and roof would only be around 10kWh per day at a constant outdoor temperature of 30c with U=0.2. Conductive heat through windows at U=0.6 for the entire window would be 6kWh per those same 24 hours. The windows are already calculated as about as good as they get, and improving the walls and roof from 0.2 to 0.1 would only reduce the total heat gains from 56kWh/day to 51kWh/day at zero shading and 36kWh/day to 31kWh/day at 50% shading. The real numbers would be even worse because solar gain during heatwaves is greater than the monthly average which contains cloudy and rainy days.

 

If incoming ventilation air is cooled from 30c to 15c, while the indoor temperature is 20c, 600m³/h is sufficient to produce around 20-25kWh of effective cooling per day; doubling the ventilation rate seems like it would be approximately sufficient. This cooling could be achieved by simply rigging a balanced pair of fans to bypass the MVHR and spread the cool air using the same ducts. Pressure losses in the ducts would be greater than when the MVHR is used but bypassing the core helps, and the ducts would obviously be sized to accommodate the need. If I can convince building control that this is still a flavour of mechanical ventilation (which counts as a passive method) instead of air-conditioning (which is to be avoided where possible) passing Approved Document O should be a non-issue. If building control considers the pre-conditioning to be "active", being able to show ridiculous CoP numbers should still be helpful in getting away with the natural lighting I want.

10 hours ago, Temp said:

We have our mvhr system up fairly high. We regularly dry washing on a rack in the main bathroom which doesn't need/have a seperate extractor fan. I doubt it would be high enough to move heat around the building though. 

 

I did find this document with some info on testing a pair of boreholes. Gives figures for the temperature uplift and power extracted but I've no idea if they are typical..

 

ground_loops_testing_report.PDF 2.32 MB · 0 downloads

Those numbers look a bit worrying, and I'm not fully sure if they're representative considering what I'm seeing elsewhere (e.g. "20m of borehole for 1kW of power" for GSHPs, which a 2W/mK would indicate a 25K temperature difference or a -15c working fluid), but ground conditions vary and could probably easily explain this. In a real application I'd obviously be doing a proper survey or at least an empirical experiment before making a system load-bearing.

[Lilly_Pines]

Insofar as the diurnal buffering goes, the thermal mass* required seems achievable. If we want to buffer 50kWh/day at a temperature change of no more than 5K, and the house might have 500m² of exposed surface area of floors, walls, ceilings etc., it's 100Wh/m² requiring a usable heat capacity of 20Wh/m²K or 72kJ/m²K. Most building materials are more or less 1kJ/kgK meaning that a useful thermal mass of 72kg/m² would suffice. Some materials like wood are better but their low heat conductivity means only the surface participates, some, like granite, are worse but their high density and conductivity make it less of an issue. Looking at clay plaster in particular, its higher heat capacity of 1.3kJ/kgK means only 55kg/m² is needed, and at a density of 1.6kg/dm³ this means 35mm layers on exposed walls. A similar calculation for floor and ceiling materials shows that many materials commonly used for thermal mass would provide adequate heat capacity.

 

Another option to help with the cooling would be to circulate the cool water through the floors, walls and ceilings like reverse UFH. This has the additional benefit of not requiring forced air movement beyond what is necessary for fresh air, and low-temperature surfaces make high-temperature air significantly easier to cope with. With a bit more complexity the same pipes could be used for low-temperature heating in the winter (e.g. 23c) to improve radiant comfort without wasting heat on air that will soon be exhausted anyway.

 

* Yes, "thermal mass" or perhaps "exposed/usable heat capacity"; decrement delay doesn't apply when practically all of the heat is coming through the windows.

[SteamyTea]

I think I am slowly seeing where you are coming from now.

You can calculate the fan power to move 600 m³.h^-1 here.

 

https://www.engineeringtoolbox.com/fans-efficiency-power-consumption-d_197.html

 

That will be your energy input.

Then if you know the range of heating and cooling ∆Ts, you can work out the energy transferred at your flow rates.

CoP is not the usual term used in the UK for forced air systems, but no reason it should not be used.

 

[JohnMo]

Bit baffled why you need (from your figures) 6 ACH. We have a fresh air environment with under 0.5 ACH.

 

Think you are making things way more complex than needed, for little or no real life benifit.

 

Your house will be in very noisy and full of big pipes.  Think you could run a split Aircon unit for less than the cost of running an MVHR unit at 600m3/h, plus the extra brine pumps etc.

 

A 2kW A2A heat A+++ rated, at 30 degC outside temp would give you up to 35kWh of cooling per day at an EER of 9, increasing to an EER of 14 at 25 degrees outside air temp.

 

With an EER of 9 generating 1.47kW of cooling power, is 163W.

 

600 m3/h at an installed SPF,with duct losses is circa 0.8 W/l/s. 

 

600 m3/h is 166l/s, 0.8x166= 132W.  Plus pumps etc digging boreholes etc.

 

Little or no saving, same would be true in the winter.

[Lilly_Pines]

21 minutes ago, JohnMo said:

Bit baffled why you need (from your figures) 6 ACH. We have a fresh air environment with under 0.5 ACH.

 

Think you are making things way more complex than needed, for little or no real life benifit.

 

Your house will be in very noisy and full of big pipes.  Think you could run a split Aircon unit for less than the cost of running an MVHR unit at 600m3/h, plus the extra brine pumps etc.

 

A 2kW A2A heat A+++ rated, at 30 degC outside temp would give you up to 35kWh of cooling per day at an EER of 9, increasing to an EER of 14 at 25 degrees outside air temp.

 

With an EER of 9 generating 1.47kW of cooling power, is 163W.

 

600 m3/h at an installed SPF,with duct losses is circa 0.8 W/l/s. 

 

600 m3/h is 166l/s, 0.8x166= 132W.  Plus pumps etc digging boreholes etc.

 

Little or no saving, same would be true in the winter.

Assuming a house with 100m² of area and 300m² of conditioned volume, 600m³/h is 2 ACH. A human needs roughly 200m³/h of fresh air for a steady-state CO2 concentration of 100ppm above atmospheric levels. With atmospheric CO2 going up for the foreseeable future, this seems like a prudent precaution when 1000ppm has a noticeable impact on cognitive performance compared to 600ppm. 2 ACH in the example house is sufficient to achieve truly fresh air (which I'll define as a CO2 rise of 100ppm or less) for 3 people or a lesser standard of 200ppm rise for 6 people. The large exchange rate is also desirable for controlling dust when combined with filters at exhaust vents.

 

Pipes are only required to connect the MVHR units (e.g. Brink Flair 300 Enthalpie x3) to the intake and exhaust, and to the air distribution system inside the house. If all internal structures are inside the conditioned and airtight envelope, and built of good materials that don't release nasty stuff, there is very little obstacle to using spaces within the house (like closets and other built-in furniture) to form parts of the ducting. This, in conjunction with commercial duct silencers, also avoids ventilation noise and crosstalk between rooms. A2A heat pumps still require air distribution inside the house and the most logical location inside the intake ducting of the ventilation system effectively just replaces the borehole with an ASHP outdoor unit as the source of cool fluid.

 

Also, can you give me an example model of an A2A heat pump that gives 9-14 EER? Search engines are being really unhelpful here.

23 minutes ago, SteamyTea said:

I think I am slowly seeing where you are coming from now.

You can calculate the fan power to move 600 m³.h^-1 here.

 

https://www.engineeringtoolbox.com/fans-efficiency-power-consumption-d_197.html

 

That will be your energy input.

Then if you know the range of heating and cooling ∆Ts, you can work out the energy transferred at your flow rates.

CoP is not the usual term used in the UK for forced air systems, but no reason it should not be used.

 

Assuming a total pressure loss of 100Pa (a bit challenging, but achievable with low flow rates compared to duct sizes) the ideal power consumption of 600 m³/h would be 17W. Using two EBM Papst R3G220-RD53-03 fans in series (intake & exhaust, each seeing 50Pa) I'm seeing roughly 50W combined and a sound power level of 57dB(A) each. According to its PHI certificate Brink Flair 300 Enthalpie has a specific electric power of 0.21Wh/m³ at roughly the same operating parameters, resulting in 120W consumption which is not too terrible. Operating it without its built-in filters and using significantly larger and less restrictive (e.g. 605x605mm) filters should help a bit.

 

If I use the fan-only numbers, and give the brine pump another 50W to be conservative, that's 100W steady-state, for a cooling power of maybe 3kW at 30c->15c or a CoP of 30 for the intake air (MVHR would only get the intake temperature down to something like 22 and further active cooling of the air indoors would be necessary). On further thoughts increasing the ventilation rate to cool the house down is nonsensical so additional cooling beyond that should be distributed from hydronic pipes in the structure (ceilings are the best place for them, as they have the least obstructions; I'm planning an open grid of exposed beams and joists with e.g. terracotta tiles between them, so anything that needs fastening to the ceiling has an obvious and visible structure to attach to so the pipes wouldn't be in any danger).

[SteamyTea]

3 minutes ago, Lilly_Pines said:

ceilings are the best place for them, as they have the least obstructions; I'm planning an open grid of exposed beams and joists with e.g. terracotta tiles between them, so anything that needs fastening to the ceiling has an obvious and visible structure to attach to so the pipes wouldn't be in any danger

Why not make your own MVHR unit. I did.

Basically just a simple plate with controlled airflow either side.

Then pump air through it.

Greater the area, and you will have about 80m² of area, the more power can be transferred.

The room inlet and outlet vents can be hidden in shadow gaps in the wall.

Also has the advantage that any ceiling lighting, and you are having a stupid amount of lighting (get your eyes checked for cataracts, I did, and now only use a 3W LED in an 18m² room) puts the thermal energy directly into the heat exchanger.

[JohnMo]

A2A and everything ventilation, euro certification database.

 

https://www.eurovent-certification.com/

 

The one I quoted was a Mitsubishi heavy industries A+++ rated for heating and cooling.  Model number is on my other computer.

 

[saveasteading]

3 hours ago, Lilly_Pines said:

ground conditions are entirely hypothetical.

Not generally the best type. Or perhaps perfect when simply hypothesising.

For cooling, the ground type is equally important, as the surface of the borehole has to absorb the heat and then conduct it quickly away.

 

Here is my suggestion for cooling. 

Construct a long length of drain pipe at 1m deep. This can be a long run or snaking. Perforated land drain in gravel will increase the heat exchange, but would have to be above the water table.

End pops up into fresh air, some distance away and you suck in air to replace/force out the used air.

If possible this pipe will be in a shaded area, as the sun's heat will penetrate 1m.  But even better if the pipe, and especially the open end, are in woodland, where the trees have done a lot of work in keeping the temperature down.

Thinking more, with the woodland source (easy when all hypothetical) you don't need the long run of pipe.

Simpler, and will work. How much pipe and how strong a fan I don't know. Best have the fan a distance from the building and ventilated so that it's energy doesn't enter your system.

 

In a very simple way I have done this for a sports hall. with fan ventilation on one end and louvres on the other, there is constant movement of heat outwards. By extracting at the sunny end , the air from the shaded end is brought in, at significantly lower temperature.

[Lilly_Pines]

10 minutes ago, SteamyTea said:

Why not make your own MVHR unit. I did.

Basically just a simple plate with controlled airflow either side.

Then pump air through it.

Greater the area, and you will have about 80m² of area, the more power can be transferred.

The room inlet and outlet vents can be hidden in shadow gaps in the wall.

Also has the advantage that any ceiling lighting, and you are having a stupid amount of lighting (get your eyes checked for cataracts, I did, and now only use a 3W LED in an 18m² room) puts the thermal energy directly into the heat exchanger.

Building an enthalpy exchanger with high efficiency seems like one of those tasks that, while doable, are sufficiently annoying that I'm happy to leave it to people who do it as a job. Agreed on the shadow gaps, although they need to work with the aesthetic.

 

The reason for the stupid amount of lighting is that I have seasonal affective disorder which means I get dysfunctionally depressed when in inadequate light. If I don't have my greehouse lamp (315W CMH, roughly 30k lumens) on it noticeably affects my mood even with the ~20k lumens remaining in the living room.

[saveasteading]

I  was once working with a company re methane from waste; a proper boffin.

I asked how best to reclaim the heat from cold-stores which is chucked out into the world, and wasted.

I didn't know I was talking abut an enthalpy exchanger, but I was.

He said that even with that high temperature of air, and similarly with cooling water in power stations, it is difficult to reclaim even 5% of the energy in a useful manner, and it isn't worth it.

[SteamyTea]

36 minutes ago, saveasteading said:

didn't know I was talking abut an enthalpy exchanger, but I was.

How timely.

I just drive past a large, new, cold store, was wondering if there was merit in secondary cooling of the exterior. Goes in with something that @JohnMo asked about evaporation losses from walls (still not looked into it).

 

Maybe a new thread when the sun sets for winter, and the rain is lashing down.

[JohnMo]

Mitsubishi SRK20ZSX-W was the A2A I referred too.

[saveasteading]

1 hour ago, SteamyTea said:

secondary cooling of the exterior

An additional white or metallic sheet of cladding, facing south, on a spacer system, would reflect most of the sun, absorb the rest and ventilate it away. But an extra 25mm of pir in the wall would be cheaper, and it is already 200mm if memory serves.

In reality the first of these was what we did if involved in the spec decisions...build a lightly insulated shed, then cold store inside it. I think this works better than using the cold store panel as the weather wall.

[Lilly_Pines]

4 minutes ago, saveasteading said:

Not generally the best type. Or perhaps perfect when simply hypothesising.

For cooling, the ground type is equally important, as the surface of the borehole has to absorb the heat and then conduct it quickly away.

 

Here is my suggestion for cooling. 

Construct a long length of drain pipe at 1m deep. This can be a long run or snaking. Perforated land drain in gravel will increase the heat exchange, but would have to be above the water table.

End pops up into fresh air, some distance away and you suck in air to replace/force out the used air.

If possible this pipe will be in a shaded area, as the sun's heat will penetrate 1m.  But even better if the pipe, and especially the open end, are in woodland, where the trees have done a lot of work in keeping the temperature down.

Thinking more, with the woodland source (easy when all hypothetical) you don't need the long run of pipe.

Simpler, and will work. How much pipe and how strong a fan I don't know. Best have the fan a distance from the building and ventilated so that it's energy doesn't enter your system.

 

In a very simple way I have done this for a sports hall. with fan ventilation on one end and louvres on the other, there is constant movement of heat outwards. By extracting at the sunny end , the air from the shaded end is brought in, at significantly lower temperature.

The problem with running ventilation pipe in the ground is that 1. it requires a lot more groundworks than a vertical hole 2. you need to pay extra attention to drainage and lining materials because water will condense in the pipe and you don't want it to become a problem 3. large amounts of air require a large number of parallel pipes to reduce pressure loss which increases the ground disturbance. Using a liquid for the heat transfer is supposed to avoid these drawbacks, and at least in my impression is often suggested as an alternative when someone brings up ground tubes.

 

Looking deeper into the matter, this paper gives some useful numbers on heat extraction rates, temperatures, etc.

 

In 11c limestone, brine coming in at 1c and leaving at 3c can extract between 30-40W/m. Assuming that a halved temperature delta means halved heat extraction, that would be brine coming in at 16c, leaving at 15c, extracting 15-20W/m. For 3kW of cooling a borehole length of 150-200m would be indicated. Assuming thermal piles on the corners and middle of long sides of a 5x10m house the required length would be 25-35m per pile modulo differences in ground characteristics compared to limestone.