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I have a heat pump which moves heat from outside to inside. I also have a solar heating array that collects radiant heat from the sun. Given that my goal is to heat the inside (with minimal energy consumption from the heat pump), my question is: Is it better to heat the air inside with the solar array or to heat the heat pump's evaporator outside?

Breaking this down, it's a question of whether it's better to use these sources in parallel or in series. In parallel, the solar array heats the inside directly and the heat pump heats the inside directly. In series, the solar array heats the evaporator and the heat pump heats the inside directly.

Some other relevant details, the evaporator exchanges heat by convection and is limited to a minimum temperature of 0°C. This is because any colder and you risk freezing the convector, thus stopping convection. The environment can get as low as 3°C at night, meaning heat is exchanged across a minimum of a 3°C difference.

Assumptions for this, the heat pump can move 5kW of heat across a 12.8°C difference (8.3-21.1°C), consuming 2kW. The indoor heat loss is (let's say) 5kW for a 15°C temperature difference (5-20°C). The solar array acts as a heat reservoir with 10kWh stored at 40°C (5°C to 40°C). Finally, let's assume the heating time is 10 hours.

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    $\begingroup$ So, have you done an energy balance for the possibilities at various ambient temperatures? Not cfd, but just 3 different static points... $\endgroup$
    – Solar Mike
    Jan 2 '18 at 22:07
  • $\begingroup$ Not yet. I'm looking to see if there were things I haven't considered before doing an energy balance. I'll likely assume all components are linear, but I suspect that, energetically, they are equivalent. In series, the heat pump pulls from a hotter source. In parallel, it has less load. At the same time, it could be that parallel is better, much like how two heat pumps in series uses more power than two in parallel for the same load. $\endgroup$
    – Hari Ganti
    Jan 2 '18 at 22:13
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    $\begingroup$ You “suspect” - best prove it so you don’t waste time and effort yours and others. $\endgroup$
    – Solar Mike
    Jan 2 '18 at 22:15
  • $\begingroup$ Like I said, I want to see what I might be missing. You don't have to "waste" your time if you don't want to. $\endgroup$
    – Hari Ganti
    Jan 2 '18 at 22:24
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    $\begingroup$ No, for starters, use up solar hydronic at night/morning to heat house. If, during the day, you max out your storage, begin sending excess to house. Solar operates at pretty high temps, like you have to take care not to boil over. I ran a hotel system at about 195 degrees f year round (in the Florida Keys). You can exchange to a HWH loop before servicing the HVAC system. High available temperatures generally mean it's a bad idea to use them to front-end a heat pump. But it might be worth a look at assisting with the HP deicing cycle. After all, the control for deicing is built in to the HP. $\endgroup$
    – Phil Sweet
    Jan 3 '18 at 4:20
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Assuming you don't have solar panels on your heating array, it would be more efficient to heat the load directly as you're only moving the heat once, so you wouldn't lose as much in the process.

However if you have solar panels, these work more efficiently at lower temperatures. Depending on your climate, you might be able to move more heat from the solar array into the heat pump evaporator than you would be able to move indoors. This could mean you are able to cool the panels a little more, making them generate slightly more power. (You'd have to check this though, if you're getting down to 3°C you probably don't need any panel cooling!)

You don't say where the heat pump evaporator is located, but in some places it is located underground, below the frost line. If this is the case, then pumping heat from the solar array to the underground evaporator will also allow some level of heat storage by heating up the ground, which will then be accessible to the heat pump overnight, improving efficiency.

You will likely also be able to get away with a simpler system if you move heat from the solar array to the heat pump evaporator, as you won't have to worry about maintaining a comfortable temperature and can just pump everything. The heat pump won't complain if it gets too warm, unlike the indoor occupants.

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    $\begingroup$ Well, now the interesting bit is that the solar array is a cogenerating array, so the heat it produces is a byproduct of PV electricity generation. PV panels pretty much always need cooling since they can easily get up to 30+°C in an ambient 0°C with 800W/m^2 irradiance. My heat pump is air sourced, but I am planning to use concrete for heat storage. $\endgroup$
    – Hari Ganti
    Jan 8 '18 at 2:43
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    $\begingroup$ Sounds like pumping heat into the concrete will give you more benefit then, so you can use that heat overnight which you probably couldn't if you were pumping from the solar array directly indoors. $\endgroup$
    – Malvineous
    Jan 8 '18 at 6:41

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