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.

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.

Two systems like what? Please clarify.

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.

@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.

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.

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.

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.

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.

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.