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Solar Thermal as a pre-heater for a heat-pump

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Over on the Say Hello topic I asked whether solar thermal should continue to be used as an input to a DHW cylinder, of if it's better to re-configure it as a pre-heater to a heat-pump when one gets installed.

To which @chris-a replied "that's an interesting question. We're not set up for it to input to the heating circuit, so for us it will remain for hot water. The advantage to that is that it can raise the DHW temperature far hotter than would be efficient for the heat pump (we have a thermostatic mixing valve to prevent scalds, so a hotter tank just means more hot water). How the heat pump would react to having the heating circuit raised to above its flow design temperature I'm not sure! When the conditions are right, it can dump a LOT of heat into the cylinder. Sadly for heating, during winter when you need it most, it will do less. But of course one needs DHW all year round, so perhaps that's another argument in its favour."

and @derek-m commented "My research indicates that solar thermal is up to 4 times more efficient than solar PV, so you could get the same amount of energy from 1/4 the roof space. We have solar PV, which provides the vast majority of our hot water from Spring through to Autumn and also assist the gas boiler in producing hot water in the Winter. So very similar to your solar thermal, but with the added advantage that it has reduced our electrical consumption by 50% over a full year."

I certainly agree with Derek. The max possible efficiency of a solar PV panel made with silicon is 23%. That's entirely due to chemistry of the silicon atom and the two electrons in its 'outer shell'.

Contrast that with about 80% efficiency from a solar thermal vacuum-tube array under similar direct sunlight.

The previous comment posted by @chris-a is based on the assumption that he has a binary choice of whether to continue using the existing solar thermal input in a DHW cylinder once a heat-pump is installed. I'm not convinced that it is efficient or effective to do so.

Consider the following simplistic diagram:

HeatPump4 andStore

In operation, the heat-pump maintains a 5degC differential between the cold-return and the warm-flow pipes.

If the radiators call for heat, then the 3-port valve rotates such that the return water is at 30degC, for example.

If the DHW cylinder calls for heat, then the return might be at 42degC.

Q1: Does the heat pump's internal logic successfully cope with switching from one circuit to another when they are delivering such different temperatures?

HeatPump4 andStoreB

Suppose the design of your choice of heat-pump is such that it has two separate valves; one for CH and the other for DHW.

Q2: What happens if both circuits call for heat at the same time?

a: The return to the ASHP settles at the midway point (36degC), to which the heat-pump adds 5degC, making the output 41degC. So  it now sends water back to the DHW cylinder 1degC cooler than it left!

b: The ASHP may have temperature sensors on the two cylinders in addition to those internal to it. So it now tries to send water back to each cylinder at a temperature warmer than its thermostat. At that point the heat-pump no longer maintains the 5degC differential, and its efficiency plummets.

 

I feel there are too many unknowns.

Until you know the logic of how your installer is expected by the manufacturer to place the valves and sensors, you don't know where you can best deploy a solar-thermal array. The answer can be different according to the choice of heat-pump, its controller and the temperature sensors.

Interestingly, there was a field trial funded 100% by BEIS in which 250 homes in SE England had their existing gas boiler CH completely removed and an ASHP installed. Those who had existing solar-thermal were told that it would be reconnected back into the new system.

In fact, none of them were reconnected. No ASHP manufacturer seemed to have any 'approved method' by which a solar-thermal could operate alongside their system. 🤔 

This topic was modified 2 years ago by Transparent

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To answer the questions posed above from the knowledge I have gained from reading quite a number of manufacturers manuals.

Q1

The heat pump controller sets the desired water flow temperature as calculated by the control algorithms located within, so if the system is running in CH mode with a desired leaving water temperature (LWT) of say 40C, then if the LWT is too low then the compressor will work harder to raise the temperature and if the LWT is too high then the compressor will slow down or may even stop.

The required Delta T between LWT and RWT is set within the controller, and can quite often be adjusted within specified limits, so it is not set in tablets of stone. Lets think about what happens if the above system is running happily with an LWT of 40C and a RWT of 35C, then someone increases the desired indoor temperature from 20C to 21C. The controller may calculate a new required LWT of say 42C, so speeds up the compressor to produce warmer water, but the RWT coming back from the heat emitters is still at 35C, so we now have a Delta T of 7C. As the now 42C water works its way through the heat emitters, the RWT starts to increase and may eventually reach 37C, and thus provide the desired Delta T. In practice the controller is continually measuring the various temperatures and adjusting the operation of the system to try to achieve the desired temperatures.

When the controller switches from CH to DHW production it merely increases the LWT to a higher value, which causes the heat pump to work harder. Switching from DHW to CH would normally have the reverse effect.

Q2

Because the LWT required for DHW is normally higher than that for CH it is not possible or desirable for a heat pump to try to do both at the same time, which is why a 3-way valve if often used to divert the warm water from the heat pump one way or the other. If two individual valves have been installed rather than a 3-way valve, then an interposing relay would be used to prevent both valves being opened at the same time.


   
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That's a better explanation than mine @derek-m

I'm still thinking through the situation which @chris-a raised, in which he wishes to retain an existing solar-thermal input to the DHW cylinder whilst having an ASHP installed.

Is there possibly a need to have two different time/temperature settings applied to DHW storage from the HP controller:

a: daytime, when sunshine might allow the solar-thermal to heat the water; eg between 9-5 don't start HP unless temp below 42degC

b: evening; 50degC

Such a controller may not exist. My suggestion is entirely hypothetical.

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@transparent I don't have time for a long reply, but just to say that this is very much a live question for me, and one that I/my plumber will be discussing with our heat pump people (GES) on Wednesday, when we plan to commission the ASHP.

My heating engineer suggested a scheme whereby the heat pump does not supply DHW whilst the solar thermal pump is running (which only happens when the rooftop panels are got enough to provide heat to the bottom of the tank), via some sort of simple on/off cut-out. Once the solar is running it will take over, and when it stops the heat pump will top up as necessary.

This prioritizes efficiency over hot water availability, but should ensure that the solar thermal gets a good "crack of the whip." What I'm not sure is how possible it is with our ASHP setup.

I'll post more when I learn the result of the discussion! One thing to note: the solar heats the bottom of the tank, and can far exceed the temperature threshold of the HP, so it's possible that on sunny days the HP will just not be running that much/at all anyway: the top of the tank will stay hot.


   
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@chris-a

You will no doubt be able to set a schedule, within the heat pump controller, as to when DHW heating is to be allowed, so you could just schedule it off during daylight hours. The only problem with this arrangement would be if there is little or no solar thermal energy to heat the water during the day, though I do believe some controllers have the facility to manually override the schedule and start DHW heating.

Even when switched on, DHW heating by the heat pump will only take place if the cylinder temperature falls below the specified level. So if the water in the cylinder has already been heated by solar thermal, then the heat pump will not try to do so.

If DHW heating by the heat pump is allowed throughout the whole day, then the worst that could happen is that you have both the heat pump and solar thermal heating the water at the same time, which would of course heat the water quicker, and switch the heat pump off sooner. By setting the desired water temperature produced by the solar thermal between 60C and 65C, and the heat pump control below this value, then not only will the heat pump switch off first, but it will also reduce the necessity for running a legionella cycle.

This post was modified 2 years ago by Derek M

   
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@derek-m very valid points. I'm tempted to set the heat pump DHW to 12:30-4:30, as we're on Octopus Go which gives (very) cheap electricity during those hours. If the tank is still got from yesterday, then it won't kick in, but will ensure there is hot water first thing. Then the solar gets the day to do what it can, with a boost if necessary in the evening.


   
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Posted by: @transparent

Over on the Say Hello topic I asked whether solar thermal should continue to be used as an input to a DHW cylinder, of if it's better to re-configure it as a pre-heater to a heat-pump when one gets installed.

To which @chris-a replied "that's an interesting question. We're not set up for it to input to the heating circuit, so for us it will remain for hot water. The advantage to that is that it can raise the DHW temperature far hotter than would be efficient for the heat pump (we have a thermostatic mixing valve to prevent scalds, so a hotter tank just means more hot water). How the heat pump would react to having the heating circuit raised to above its flow design temperature I'm not sure! When the conditions are right, it can dump a LOT of heat into the cylinder. Sadly for heating, during winter when you need it most, it will do less. But of course one needs DHW all year round, so perhaps that's another argument in its favour."

and @derek-m commented "My research indicates that solar thermal is up to 4 times more efficient than solar PV, so you could get the same amount of energy from 1/4 the roof space. We have solar PV, which provides the vast majority of our hot water from Spring through to Autumn and also assist the gas boiler in producing hot water in the Winter. So very similar to your solar thermal, but with the added advantage that it has reduced our electrical consumption by 50% over a full year."

I certainly agree with Derek. The max possible efficiency of a solar PV panel made with silicon is 23%. That's entirely due to chemistry of the silicon atom and the two electrons in its 'outer shell'.

Contrast that with about 80% efficiency from a solar thermal vacuum-tube array under similar direct sunlight.

The previous comment posted by @chris-a is based on the assumption that he has a binary choice of whether to continue using the existing solar thermal input in a DHW cylinder once a heat-pump is installed. I'm not convinced that it is efficient or effective to do so.

Consider the following simplistic diagram:

HeatPump4 andStore

In operation, the heat-pump maintains a 5degC differential between the cold-return and the warm-flow pipes.

If the radiators call for heat, then the 3-port valve rotates such that the return water is at 30degC, for example.

If the DHW cylinder calls for heat, then the return might be at 42degC.

Q1: Does the heat pump's internal logic successfully cope with switching from one circuit to another when they are delivering such different temperatures?

HeatPump4 andStoreB

Suppose the design of your choice of heat-pump is such that it has two separate valves; one for CH and the other for DHW.

Q2: What happens if both circuits call for heat at the same time?

a: The return to the ASHP settles at the midway point (36degC), to which the heat-pump adds 5degC, making the output 41degC. So  it now sends water back to the DHW cylinder 1degC cooler than it left!

b: The ASHP may have temperature sensors on the two cylinders in addition to those internal to it. So it now tries to send water back to each cylinder at a temperature warmer than its thermostat. At that point the heat-pump no longer maintains the 5degC differential, and its efficiency plummets.

 

I feel there are too many unknowns.

Until you know the logic of how your installer is expected by the manufacturer to place the valves and sensors, you don't know where you can best deploy a solar-thermal array. The answer can be different according to the choice of heat-pump, its controller and the temperature sensors.

Interestingly, there was a field trial funded 100% by BEIS in which 250 homes in SE England had their existing gas boiler CH completely removed and an ASHP installed. Those who had existing solar-thermal were told that it would be reconnected back into the new system.

In fact, none of them were reconnected. No ASHP manufacturer seemed to have any 'approved method' by which a solar-thermal could operate alongside their system. 🤔 

Looking at the above diagrams, I don't think that there will be many systems with a thermal store, particularly with a heating coil directly connected to the heat pump. Having basically what is a heat exchanger is not very efficient, since the LWT from the heat pump would probably need to be 5C higher than feeding the heat emitters directly from the heat pump. A possible better arrangement would be to have the water flowing from the heat pump passing directly through the thermal store to the heat emitters, and have the heating coil within the thermal store fed from the solar thermal system via a 3-way valve. In that way the solar thermal could heat the DHW cylinder and then when that is up to temperature could divert the heat energy to the thermal store, for immediate use or to be used later.

An alternative method would be to replace the thermal store with a small heat exchanger, either in the pipework between the heat pump and the heat emitters, or in the return pipework to the heat pump. On one side of the exchanger would be the water from the heat pump and on the other side the water from the solar thermal system. This would allow any excess heat energy from the solar thermal system to support the heat pump, but would not allow for energy to be stored for later use.

An idea I have been thinking about is to use stored solar thermal energy to preheat the air going into the ASHP. For maximum efficiency a heat pump needs to extract as much energy from the ambient air as possible, to reduce the amount required from the electrical supply. At an outside air temperature of 10C an ASHP will probably obtain 3kW of energy from the air for each 1kW from the electrical supply, giving a COP of 4. By the time the outside air has fallen to 0C, it is probably only obtaining 2kW from the air for each 1kW from the electrical supply, giving a COP of 3. If it were possible to heat the air flowing through the heat pump back to 10C, then it would provide approximately 33% more energy.

I'm afraid I don't have a decent drawing package, so you will need to imagine the arrangement from the description below.

One or more solar thermal panels would be mounted on a suitable framework at ground level at an angle of 10 degrees from vertical, leaning in a northerly direction. This would provide maximum output during the Winter period when on the shortest day the Sun only rises 13 degrees above the horizon. The panel(s) would need be sited near to the heat pump and in direct sunlight.

A well insulated thermal store, of suitable size, would need to be positioned either behind or to the side of the panel(s), which would then be connected to the panel(s) by a pipe at the bottom and a pipe containing an electrically controlled valve at the top. Two temperature sensors would be required along with a differential amplifier, one sensor to monitor the temperature in the heat store and one to measure the temperature at the pipe at the top of the solar thermal panel(s). As the water in the panel(s) is heated by solar energy, it will reach a point where it is warmer than that in the thermal store. This will be sensed by the differential amplifier and the valve will be opened, which will hopefully allow the warmer, less dense, water to flow from the panel(s), to be replaced by cooler, more dense water from the thermal store. In this way it should be possible to store energy during sunny days. As the sunlight reduces there will be a point where the water is no longer being heated and the valve will be closed.

Two additional pipes would be connected to the thermal store, again the one at the top containing an electrical valve, and would be used to supply a heat exchanger (think car radiator or similar) sited within the air intake of the heat pump. A further temperature sensor measuring the intake temperature would also be required. The system would hopefully operate in the following manner. As the outside air temperature falls to a preset value of say 7C to 8C, the electrical valve would be opened to allow the warm water to flow from the top of the heat store into the heat exchanger, which in turn would provide heat energy to heat the air flowing into the heat pump. By providing energy from the heat store it should be possible to improve the overall efficiency of an ASHP system during the colder, less efficient, nighttime periods.

Obviously there will be days when there is little or no solar energy available, but as they say 'every little helps'.

I estimate that a DIY system with one solar panel would cost in the region of £1000, but since there are very few moving parts it would require little maintenance, but of course would require a suitable quantity of antifreeze.

You may think how can a small quantity of water heat a large volume of air, but since the thermal capacity of water is approximately 3500 times that of the same volume of air, it may be possible.

Possibly a University could try building such a system and evaluate the feasibility of my suggestion.


   
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Posted by: @derek-m

An idea I have been thinking about is to use stored solar thermal energy to preheat the air going into the ASHP.

Taking this to the extreme, one could eliminate the intermediate air and use a water source heat pump, with the water/antifreeze mixture heated up by an array of heat collectors. Admittedly this would work best during the daytime, although some sort of heat store might make an appearance.

As an aside, isn't solar thermal best used to heat already warm water up to the highest temperature? My limited understanding is that vacuum collectors are effective even on cloudy days. Solar thermal has the potential to reach very high temperatures given low flow rates, unlike heat pumps which work most efficiently with small temperature differences.


   
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Posted by: @chickenbig
Posted by: @derek-m

An idea I have been thinking about is to use stored solar thermal energy to preheat the air going into the ASHP.

Taking this to the extreme, one could eliminate the intermediate air and use a water source heat pump, with the water/antifreeze mixture heated up by an array of heat collectors. Admittedly this would work best during the daytime, although some sort of heat store might make an appearance.

As an aside, isn't solar thermal best used to heat already warm water up to the highest temperature? My limited understanding is that vacuum collectors are effective even on cloudy days. Solar thermal has the potential to reach very high temperatures given low flow rates, unlike heat pumps which work most efficiently with small temperature differences.

My idea was to try to develop a system that is an add-on, and does not require any form of modification to the heat pump itself. The objective was also to store energy during the warmer period of the day, when the heat pump is already operating in a more efficient manner, such that the stored energy could then be used during the colder nighttime period to help improve the overall efficiency of the system.

During days when there is little solar thermal energy produced, the lowest temperature to which the water in the heat exchanger would fall would be the ambient air temperature. In a system where the Evaporator is actually in contact with the water supply, the temperature could fall much lower.

If you could develop a water based system that has a constant energy source then if may be a more efficient option.

Don't let me put anyone off from proposing any ideas that could be investigated.


   
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Posted by: @derek-m

If you could develop a water based system that has a constant energy source then if may be a more efficient option.

If I had the time then I'd love to head in that direction. But I haven't yet set a date when I will want to install a GSHP, and it's the use of a thermal-solar pre-heater for that technology which interests me.

Returning now to the situation for @chris-a and the imminent meeting with the prospective installer:

Posted by: @chris-a

My heating engineer suggested a scheme whereby the heat pump does not supply DHW whilst the solar thermal pump is running (which only happens when the rooftop panels are got enough to provide heat to the bottom of the tank), via some sort of simple on/off cut-out. Once the solar is running it will take over, and when it stops the heat pump will top up as necessary.

This suggestion is heading in the right direction. However a simple switch over to heat DHW from the ASHP is too primitive.

  1. Once the transfer to ASHP has occurred, any subsequent heat-input from the solar-thermal that afternoon will be ignored/discarded
  2. The efficiency of a HP is proportional to the number of one/off cycles it performs per hour. With diligence, an installer may be able to reduce this to 3/hr (20-min intervals). So neither do you want a system which quickly switches back to solar-thermal if the sun comes out again

Here's a graph from a local substation which supports single-phase, community-owned GSHPs. Their expertise has reduced 'cycling' to 25mins.

GSHPcycling

Let's just step back a moment and compare the type of energy input which is available from a renewable source, such as solar, with that from a man-made device (boiler or heat-pump). The lower curve shows the effect on water temperature in the DHW cylinder.

ThermalRiseCurve

We don't want to switch the heat-source from thermal-solar to a heat-pump when output is lowered due to an hour or so of cloud-cover.

My proposed solution is to develop a new device which I'll call a Hold-Off Controller (HOC)

The HOC at its most basic is a time delay. It holds off the temperature sensor on the DHW cylinder from switching across to the heat-pump.

A more sophisticated HOC might have live weather data input, analysing the probability of further sunshine later in the day.

This post was modified 2 years ago 3 times by Transparent

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@transparent

GSHP's are more efficient than ASHP's, certainly during the heating season, because they have a more constant energy source at about 8C to 10C, whilst the temperature of the energy source to an ASHP is much more variable. My idea to try to make an ASHP operate more like a GSHP.

You could use what I have suggested with a GSHP, but replace the car radiator type heat exchanger with a plate heat exchanger. The GSHP fluid flowing through one side and the solar thermal system supplying the other side.

I think you will find that the efficiency of a heat pump is more to do with keeping the LWT as low as possible to meet the heat demand, which in turn reduces the on/off cycling of the system, rather than the other way around.

Probably the easiest way for Chris-A to control his system would be to set the heat pump to heat the DHW to say 45C and the solar thermal system to heat the water to 60C. When no solar thermal is available then the heat pump would heat the water to 45C and then switch off, any solar thermal will take the temperature above 45C and keep the heat pump off. Unless you use a great deal of hot water and there is a period of low solar thermal production, the heat pump will probably not be required.


   
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Posted by: @derek-m

@transparent

Probably the easiest way for Chris-A to control his system would be to set the heat pump to heat the DHW to say 45C and the solar thermal system to heat the water to 60C. When no solar thermal is available then the heat pump would heat the water to 45C and then switch off, any solar thermal will take the temperature above 45C and keep the heat pump off. Unless you use a great deal of hot water and there is a period of low solar thermal production, the heat pump will probably not be required.

Just wanted to give some feedback based on the first week's operation, during which time we had mostly cloudy weather with some sunny intervals. The heat pump has hardly been running - it kicked in for the first time in days this morning after I had a shower.

The design of the system, after consulting with the heat pump engineer, was to keep it as simple as possible. The solar and heat pump run independently of each other. However, as you said, the heat pump threshold has been set low (45C). Also, the solar coil is at the bottom of the DHW cylinder, whereas the heat pump sensor and coil is in the top half. In practice, this has meant that, unless we run out of hot water at the top of the tank, the heat pump will stay off. In the meantime, the solar thermal gets the chance to heat the bottom of the cylinder with whatever sun is going. It can take the cylinder way higher than 45 degrees on a sunny day, which will prevent the heat pump running for at least 24 hours, given our tank size and water usage. If it does take the cylinder up above 60C then (1) the Legionnaire's timer on the heat pump is reset, so we're not wasting energy doing that (2)The thermostatic mixing valve on the hot water ensures that we use it slower, as it's mixed down to 45C before use. This gives the solar more time to find some sun! Even if it doesn't succeed in getting the water above 45 degrees, it still pre-warms it to some extent, giving the heat pump less work to do.

I'm really happy with the way it's running. It seems to be effectively using the design and temperature of the hot water cylinder to communicate, and even in not-very-sunny conditions has been keeping us in hot water whilst running the heat pump sparingly (the heating's still off at the moment). If there's one alteration I would wish for, it's a time-based setting so if the heat pump needs to heat the water it only does so overnight when we have cheap electricity. We could run out of hot water that way, but it would be cheaper. However, the way it's working at present, this doesn't feel very urgent. I'll see if I change my mind in winter!

All the best,

Chris.


   
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