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

Transparent · 2022-08-27T19:46:25Z

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: 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? 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. 🤔

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Mars

(@editor)

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22/09/2022 9:06 am

@chris-a, thanks for sharing and it’s all looking very positive. Keep us updated as temperatures start to drop. I’ll be great to get an update in December.

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Derek M

(@derek-m)

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22/09/2022 1:57 pm

@chris-a

Thanks for the update, it is always very useful to get feedback, whether suggestions work or do not. Dependent upon the capabilities of your heat pump controller, it may be possible, during the Winter period, to set the DHW heating temperature to say 50C or 55C during the cheap overnight period and then drop it back to 45C during the rest of the day.

This would help keep the cost down, but would of course mean the heat pump would be working harder during the colder overnight period when it will be less efficient.

This is one of the problems with both water heating and home heating using ASHP's, in that weather conditions can vary quite considerably from day to day, so settings that work well one day are far from ideal the following day. I suppose it is a matter of finding the settings which give the most consistent results during the different periods of the year, so you may need Spring settings, Summer settings, Autumn settings and Winter settings.

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Ken Bone

(@ken-bone)

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20/02/2023 9:45 pm

Have installed two systems like this for clients already.

Heat pump installer

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Derek M

(@derek-m)

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20/02/2023 9:49 pm

Posted by: @ken-bone

↑

Have installed two systems like this for clients already.

Two systems like what? Please clarify.

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Ken Bone

(@ken-bone)

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Joined: 4 years ago

Posts: 20

20/02/2023 10:25 pm

Posted by: @derek-m

↑

Posted by: @ken-bone

↑

Have installed two systems like this for clients already.

Two systems like what? Please clarify.

I have had the pleasure in working with clients who believed in my proposed setup, (which is not new).

So the system consisted of heat pump and system upgrade, new dual coil solar/heat pump hot water cylinder.

1/ ST array wall mounted facing South/East.

2/ ST array facing South on pitched roof.

Another buffer the would be heating with low grade ST in the winter, once it hits a certain threshold it can be used to preheat the heat pump return.

Heat pump installer

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Johnmo

(@johnmo)

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Posts: 424

20/07/2023 7:48 pm

Posted by: @transparent

↑

configure it as a pre-heater to a heat-pump when one gets installed

I am currently doing a project where the ST is directly plumbed into the return piping to the ASHP via a close coupled tee. The ASHP does DHW and UFH, with UFH flow temps typically around 30, sometimes higher or lower depending on OAT. The aim is to add temperature to the return flow when solar is available, through a close coupled tee.

The solar system is a drain back with PHE.

Basically I will run the DHW via a thermostat and timer and the solar thermal has a legionella program, so will run at the same time as the DHW call for heat. This will increase the return to the heat pump up to 60 degs, the hope is the heat pump will stop adding heat but carry on circulation until thermostat is made.

UFH will be similar to the DHW, but will be limited to 35 degrees, via the normal setting within the controller. A temperature probe downstream of the CCT will be linked into the ST controller.

Hoping the low temperature requirements will improve the yield from the panel. I have a heat meter installed to get an idea of efficiency and how much energy savings are available.

Maxa i32V5 6kW ASHP (heat and cooling)
6.5kW PV
13.5kW GivEnergy AIO Battery.

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