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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.

Hari
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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.

Malvineous
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