# Is it better to heat a heat pump evaporator or the load directly?

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.

• So, have you done an energy balance for the possibilities at various ambient temperatures? Not cfd, but just 3 different static points... Commented Jan 2, 2018 at 22:07
• 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.
– Hari
Commented Jan 2, 2018 at 22:13
• You “suspect” - best prove it so you don’t waste time and effort yours and others. Commented Jan 2, 2018 at 22:15
• 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.
– Hari
Commented Jan 2, 2018 at 22:24
• 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. Commented Jan 3, 2018 at 4:20