Engineers are obsessed with energy efficiency, and people would love to cut the costs of heating and hot water preparation.
Over the years, heat pumps have proven to be very cost effective and energy efficient when it comes to heating a home and preparing hot drinking water. The number of heat pumps is rising, and efficiencies (COP) are rising, too. There are some specifics to consider, though. With rare exceptions, heat pumps generally provide lower water temperatures than traditional oil/gas/firewood boilers, typically ranging from 55 to
60 degrees C. This means that in order to be able to heat a home, larger heating bodies (radiators)are required than when heating with a boiler. Underfloor heating is
the preferred solution in many cases as it offers the largest possible heating area.
There is another thing to consider, though. With all the heat pumps, and especially with the air to water ones, efficiency drops when the source (air, soil, water) is colder. In the same situation, they also generate less heat. To be specific, air to water heat pumps perform the worst when heat is needed most. Soil and water source heat pumps tend to have a time lag, because soil doesn’t cool down as fast as air. But eventually, it does, especially towards the end
Solar systems share the same problem with heat pumps, and it is
even more pronounced. Their heat output during cold, foggy winter days is low. That’s when your home needs the most heat to keep you warm and comfortable. The problem is not insurmountable - you install a larger array of dolar panels and a heat storage tank and you need another heat source to cover the coldest days. The same applies to heating and hot water preparation.
Now, here’s an idea: a combination of a solar system with a heat pump. Is such a combination possible? Two
heat sources that both offer low heating costs. Sounds great. It sure is possible, in different variants:
- solar panels provide hot drinking water, heat pump provides heat
for space heating
- solar panels provide hot drinking water and space heating support and heat pump provides the rest of space heating
- other variants, used more rarely.
But there are a few questions regarding effectiveness of such combinations.
First and foremost, both systems provide the least heat when it’s most needed. This is especially true for solar systems and air source heat pumps. 0 + 0 is still close to 0.
Second, let’s suppose it’s early spring and the sun is shining, but heating is still needed. The solar system provides so much heat that the temperature of water inside the hot-water tank rises to 70 degrees C. In this case, the heat pump’s circulator pump better not try to run, because the heat pump would get those 70 degrees C on its return and that would cause a high pressure switch to shut it down (which will probably require a service intervention). Not that this isn’t preventable, but it should be considered, raising cost and adding complexity to such a system.
Third, the summer. There’s lots of sun, and the temperatures of air, soil and water are warm. Solar panels perform very well and heat large quantity of water to hot temperatures. Sometimes too hot. If the system is designed to work reasonably well in winter, it will probably produce way too much heat in summer. It can be used to shower more frequently, but still, there is too much heat which is thrown away. Also, steam may form inside pipes, which isn’t a desired effect.
Now to the heat pumps. Again, air source heat pumps are the ones who suffer the most, as the air gets warmer than soil or water. Generally, with increasing source temperature, COP gets higher and heat output also rises. Sometimes to the point where the heat exchanger in heat tank cannot transfer such a quantity, causing the heat pump’s compressor to stop. In order to protect the compressor, it is only restarted a few minutes later. Meantime, electric heater kicks in, consuming large amount of electricity when the heat pump should perform best.
Another problem is heat pumps’ upper source temperature limit. When air reaches over 35 degrees C, heat pumps stop working because of high refrigerant pressures.
But of course, combining solar panels and a heat pump you won’t run in such trouble, because in summer only solar panels should more than suffice.
Which brings us to the financial aspect of solar panels and heat pump combination. Firstly, such a system is costly to build. It is basically two costly heating systems combined with hot water tanks with two spirals instead of one plus some additional controls.
And the running costs? Solar uses little electrical power to operate. It is basically one circulating pump, and with modern, low energy pumps, that makes for very low operating cost. But, during winter, additional energy is needed to heat drinking and heating water.
Heat pumps use more electric power to operate, but require less (or zero) additional heat than solar systems. Also, in summer, COP numbers can be very high, and combined with the fact that a household generally requires a reasonably small amount of heat for hot drinking water (at least in comparison with heating water), it is fair to pose a question whether it is sensible (cost wise) to actually combine a heat pump with a solar system.
Lastly, there’s the complexity aspect. Adding complexity means:
- adding investment cost (pumps, valves, electronic controls, installation)
- adding user frustration (switching between systems, telling what works and what does not, poor automation user interfaces made for technicians instead of users)
- using more room
- increasing the risk of mechanical/electrical failure or leakage, also increasing inspection and servicing costs
- increasing running costs as additional pumps and motorized valves use electricity.
The last point here is quite interesting. Both heat pumps and solar systems perform best with small heating loads, like in passive and low-energy houses, for example. In such cases, electric power needed to drive a heat pump is very low, maybe 300-600 W. Adding a single additional circulator pump, which might use some 20-40 W of electric power, lowers the overall system efficiency significantly. So it is again fair to ask if it is reasonable to add complexity to the system by combining these two systems.