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All solar collector and boiler tank systems that I can find on the market use a liquid to circulate the heat between solar collector panels (or PVT panels) and boiler tanks. Is there a reason why they do not use expandable gas and a compressor instead?

I was looking for an air conditioning system that would not waste excessive heat during the summer, or at least use it for preheating tap water.

This is what I came up with:

Combined heating cooling system

Since I'm a software engineer, this idea will evidently be full of "bugs" that need to be ironed out. But let’s start with the first most obvious question: Why don't boilers use gas instead of water (or even worse glycerine where freezing may be an issue)?

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    $\begingroup$ They make something like this. Look up Daikin Templifier. It's a heat pump that takes waste heat from HVAC systems and heats water. $\endgroup$
    – user71659
    Commented Jan 2, 2023 at 22:17
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    $\begingroup$ Note those links have nothing to do with solar panels -- in fact to incorporate solar into such a system, you generally want to use PVT (which you misused in your question -- solar heat collectors are not based on PVT) to supply the electrical power demand of the heat pump(s). The COP of the heat pump compensates for PVT inefficiencies and the electrical output is flexible to also supply other electric loads. $\endgroup$
    – Ben Voigt
    Commented Jan 3, 2023 at 18:25
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    $\begingroup$ rather my comment meant PV, and PV/T in the question is correct. $\endgroup$
    – Ben Voigt
    Commented Jan 3, 2023 at 19:08
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    $\begingroup$ Back in the 1980s, there WERE systems that used waste heat from A/C condensers to heat water (my house, built in 1981, had one). The thing is, hot Freon was always hotter than the desired minimum temperature for hot water, never got hot enough to literally BOIL water, and there were few legal limits to how hot "hot" water was allowed to BE, so it was almost entirely a win-win situation... you could pump random water from the tank to a simple heat exchange, and it only helped the efficiency of both. (continued) $\endgroup$
    – Bitbang3r
    Commented Jan 5, 2023 at 16:49
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    $\begingroup$ (continued) With R410a, things get ugly. R410a doesn't (usually) run as hot a Freon... so it's entirely possible that naively pumping "hot" water from the tank to a heat exchange could hit you with a "one-two" deathblow... cooling the water you already spent money to heat, and ADDING heat to the refrigerant you're trying to COOL. You could come up with a more complex system (say, only routing cold incoming water to the heat exchange on its way to the tank), but that raises the price... and new r410a systems don't last nearly as long as older systems did, so you have less time to break even $\endgroup$
    – Bitbang3r
    Commented Jan 5, 2023 at 16:49

6 Answers 6

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Heat pumps work well to extract heat from one zone and discharge it into a zone at a higher temperature. The output of solar water-heating panels is generally much higher than the boiler tank temperature so a heat pump is not required and the added complexity adds cost, reliability factors and running cost.

Put simply, a heat pump is not required as the source is hotter than the load so the simplest and most reliable solution is to let the heat "run downhill" from the hot water to the cooler tank.

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    $\begingroup$ OTOH, one could make the solar panel much smaller and thus easier to install, even at less than optimal positions / angles and still generate enough heat. $\endgroup$
    – Philipp Imhof
    Commented Jan 2, 2023 at 21:04
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    $\begingroup$ @PhilippImhof, why do you think that? You can't take more heat out of the panel than it collects from the sun and that's determined by insolence and surface area. $\endgroup$
    – Transistor
    Commented Jan 2, 2023 at 21:27
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    $\begingroup$ I think @PhilippImhof is correct, if the temperature difference is greater, the panel will absorb more energy, when the panel is already saturated with heat it will not absorb as much. From the answers so far I gather that there is no concern about the boiler pipes designed for different pressure, corrosion issues or anything like that (that's more like what I was expecting, but am glad not to see). $\endgroup$
    – Louis Somers
    Commented Jan 2, 2023 at 23:25
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    $\begingroup$ @LouisSomers I once visited a guy who exploited this by having a huge tank of relatively cold water which was connected to the solar panels. He used a heat pump to pull heat from this large tank into a boiler he used for heating, showering and so on. The water in the large tank could become very cold, which increased the efficiency of the panels. The funny thing is that often the panels produced some energy even during the night because the ingress water was much colder than the air outside :-) $\endgroup$
    – JohnEye
    Commented Jan 3, 2023 at 11:26
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    $\begingroup$ @PhilippImhof When pumping heat from 18°C air, very large volumes of air are used (which circulate with fans). I you were pumping the heat from the solar panel that are initially at 30°C in the absence of sun, their temperature would drop very quickly and pumping would soon be inefficient. A much larger panel surface would be needed to avoid the steep drop (with the associatde cost to install them). $\endgroup$
    – PierU
    Commented Jan 5, 2023 at 14:26
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The transmission of heat by phase change (liquid to gas and gas to liquid) IS used in some solar collectors.

These are the evacuated tube type and are the "Rolls Royce" equivalent when compared to flat plate collectors. They will collect more energy in the extreme conditions compared to any other collector, BUT they do cost more.

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    $\begingroup$ Do you have an example? I cannot find one where the gas flows through the whole system including the boiler tank. With the HeatPipe version, the gas stays in the glass tube and there is an extra heat exchanger above the glass pipe that transfers the heat to water or glycol which in turn goes to the boiler tank. But maybe you have an example of one that uses gas through the whole system? I would like to know if there are any extra requirements for the boiler or the gas with respect to corrosion and/or pressure. $\endgroup$
    – Louis Somers
    Commented Jan 2, 2023 at 23:58
  • $\begingroup$ Some evacuated tubes indeed use a phase changing material, but most of them simply use a copper absorber in order to heat water, usually compressed in order to reach temperatures higher than 100°C while still liquid. You might want to add a diagram showing the type of collector you have in mind. $\endgroup$
    – Eric Duminil
    Commented Jan 4, 2023 at 17:21
  • $\begingroup$ @EricDuminil do a search for evacuated tube collectors and flat plate collectors - so man y examples of both which is why I mentioned both in my answer. $\endgroup$
    – Solar Mike
    Commented Jan 4, 2023 at 18:11
  • $\begingroup$ My point is that many evacuated tube collectors only use liquid water, and nothing else. The way your answer is written makes it sound like every evacuated tube collectors use phase changes, which isn't the case. I'm not talking about flat plate collectors at all. $\endgroup$
    – Eric Duminil
    Commented Jan 4, 2023 at 19:11
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You can't tie these two systems together because the demands on them are pretty much independent of one another. They each need to operate according to their own run schedule. The HVAC system runs based on demand since it has about 5 minutes of "storage" in terms of thermal mass vs temperature dead zone. The solar water heater runs when there is a positive temp difference and can be designed with storage to suit the occasion. Temperature mixing valves let you run the storage tanks 60 degrees hotter than the burner cut-in temps.

There are some niche cases where both sides of a heat pump can be used. Desalinators can mop up a lot of random heat to good effect. Paired hot tub and plunge tanks are another.

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  • $\begingroup$ I agree that decoupling the systems will decrease complexity allot. Some are relatively easy like simply switching the fan of the indoor airco unit off to reduce its effect on the room temperature (even if the gas keeps flowing through it). Others are more challenging. During the night, we may be heating the solar panels or cooling the boiler tank, so yes, the system would need a few more diverter valves and brains. But we live in an age of mobile phones, so we should be able to crate a network of intelligently coupled thermal harvesters and consumers... I'll be back to the drawing board... $\endgroup$
    – Louis Somers
    Commented Jan 3, 2023 at 0:43
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    $\begingroup$ @LouisSomers Air conditioners work efficiently because the refrigerants undergo phase changes, which occur at specific pressure-temperature points, and depend strongly on the actual refrigerant chemical(s). These temperatures may not be what you want, e.g. too cold for domestic hot water. Further, throw in things like flammability, ozone depletion, global warming, toxicity, mass flow, glide, etc. and it becomes a very non-trivial problem. Any system isn't going to be practical without considering the behavior of refrigerants. $\endgroup$
    – user71659
    Commented Jan 3, 2023 at 2:20
  • $\begingroup$ @user71659 Thanks I think I get the picture now. The boiling point of the liquid will kind of form a brick-wall limit as to how hot it can get under the working pressure. And the ideal boiling point differs for each application. Thanks for pointing this out. So in order to combine the systems we would need additional heat exchangers making it even more complex and expensive... O, well, it's better to fail in design stage than after an expensive experiment :-) $\endgroup$
    – Louis Somers
    Commented Jan 3, 2023 at 10:05
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I'm not quite sure what you meant by your question - about boilers using gas vs water. Are we talking about refrigerants being liquid or gas at a particular point in the cycle? circulating hot water for indirect water heaters? burning natural gas?

I'm actually tinkering with a similar concept - trying to use both ends of heat pump along with seasonal energy storage - (looking at insulating a tall, well-insulated tank to store cold, warm and hot water at varying proportions depending on season - making use of water's relatively decent insulation via thermaclines). I'm modifying a hot-water heat pump to replace the fan coil with a flat-plate exchanger to make it a water-water heat pump with a relatively small refrigerant circuit - and using plain water for everything else - heating, cooling, and wasting excess heat/absorbing heat as needed via an outdoor fan-coil using water. Fortunately, I'm at a frost-free location, but would think this could be adapted with sufficient storage if temperatures are often well above freezing for periods of time in winter. Quite a bit of work ahead still but happy to share more if there is interest (perhaps a different forum).

I don't think there is much to gain from adding a thermal plate to a PV panel - depending on the local climate and working fluid you would be dealing with a lot of connections and extra refrigerant (a hazardous material under pressure) + complexity and extra controls. But ignoring all that - in the circuit you provided, if you're using both the hot and cold ends of the heat-pump, it would add a bit more superheat on the compressor input which should raise its efficiency a bit to reach a high outlet temp. Running the panels a bit cooler also increases their efficiency. However, if you're mostly using AC with little hot water, you want the condensor as cold as possible for best efficiency - so you'd want to disable this much of the time.

Alternatively your PV-thermal-plates could run an additional loop of water/antifreeze - perhaps connected to a coil at the bottom of that tank to preheat incoming cold water. I would design that tank to directly circulate water via inlets/outlets at different levels, instead of being static with many different coils. You'd need a coil at the top for potable hot water - and also consider how to deal with any parts of the system that might freeze.

Note that your "condensor" is in the wrong place. It isn't a discrete piece of hardware, just the hot heat-exchanger on the output of the compressor (the outdoor fan-coil in a typical AC unit). The expansion valve keeps this side at high pressure, allowing the refrigerant to liquefy (condense) at a high temperature as it heats its environment. For example, if this heats your hot water, then as the water heats up, the refrigerant will end up being pumped to higher and higher pressures to reach a higher temperature, generally at lower efficiency and higher system stress. The system will have enough overall refrigerant to operate of the range of temperatures it is expected to manage.

Have a look at a heat pump cycle on a refrigerant enthalpy pressure chart to get a better idea at what happens during the 4 portions of the cycle.

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I was looking for an air conditioning system that would not waste excessive heat during the summer, or at least use it for preheating tap water.

A "[Heat Pump Water Heater][1]" works exactly like that. It uses a heat pump to transfer ambient heat to a hot water tank, cooling the room in the process. It might not be so great in the winter, for most people.

If you search for "solar air conditioner" (look at the pictures) you will find some products that use a solar thermal collector to warm up the outdoor unit of a heat pump that is used for heating, thus improving its efficiency. I don't know how well they work, but in principle they transfer heat from solar collector panels, using heat pump technology, as you asked.

As Ben Voigt pointed out in a comment, a pure PV panel that produces electricity that is used to heat up water via a heat pump approaches the efficiency of a thermal solar water heater and is more flexible because you can distribute the electricity to the usage you need, according to the season, or even lend it out to the grid. From what I read, current PV panels have efficiency of about 20% and heat pumps have efficiency 400% (COP=4) therefore a PV heat pump should reach 80% efficiency (transfer 0.8 kWh of heat to water from 1 kWh of received solar radiation). Where I live, pure thermal-solar water heaters are very common, and therefore much cheaper to buy and install -- about 1000 EUR, than a combined PV + heat-pump system. Having bought one myself, I realize that I throw away heat in the summer by not using enough hot water. The U.S. seems to have bypassed thermal solar water heaters and going directly to PV panels.

A Photovoltaic/thermal hybrid solar collector could add even more water-heating capacity, but it would suffer from the same excess heat problem in the summer that thermal solar water heaters have. [1]: https://www.energy.gov/energysaver/heat-pump-water-heaters

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Heat pumps don't magically generate extra heat from a source, they just move it around, specifically we use them to move heat energy in the direction opposite to where it would normally move (from cold to hot).

When used for heating, they rely on the almost unlimited amount of heat available in the air as a source. They concentrate that heat into a small amount of fluid at a higher temperature. The total energy hasn't changed, just that some of it is now inside your house.

Solar thermal panels are a limited source of heat energy that's already hot enough. If you added a heat pump, you'd get a smaller amount of fluid at a higher temperature but the same amount of energy.

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  • $\begingroup$ Will the higher temperature difference (by cooling the panel) not lead to the panel absorbing more heat from the sun? PVT panels are not as hot as evacuated tube panels. Insulated ones can reach 60 Celsius and the cheap ones that clamp behind a regular solar panel only reach about 40 Celsius (per panel, usually they string up quite a few in series to get higher temps). As a bonus they boost the electrical performance during hot summer days by cooling the panel. I would think that a higher temperature difference should boost the performance on both sides, and reduce the number of panels needed. $\endgroup$
    – Louis Somers
    Commented Jan 5, 2023 at 11:52
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    $\begingroup$ @LouisSomers - a heat pump won't cool the panel any more than effectively than running the fluid through your hot water tank and radiators. You only need a heat pump when you want to move heat from cold to hot. If the panels are hot and the house is cold, the heat will flow perfectly well on it's own. $\endgroup$
    – Robin Bennett
    Commented Jan 5, 2023 at 17:29

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