What Is The Coefficient Of Performance OF A Heat Pump?
One of the big questions of the day, as the UK plans its strategy for decarbonising heat, is how to compare the performance of different technological solutions.
Heat pumps work by transferring heat energy from the source (usually the air, the ground or water) to a working fluid, which is then compressed to a high pressure using electricity. The high heat energy in this working fluid is then passed to heating elements – eg radiators or floor heating. If you want to know more about how they work, watch our video explainer on YouTube.
The performance of a heat pump is dependent on a few factors including:
-The type of working fluid used. At Clade we prefer to use CO2 because it allows for smaller pipework and can deliver higher temperatures (comparable to the temperature of a traditional gas boiler.)
-The source temperature
-The quality of plant and installation.
There are two main ways we can judge the performance of a heat pump: Coefficient Of Performance (COP) and Seasonal Performance Factor (SPF).
The Coefficient Of Performance Of A Heat Pump
The COP is the ratio of heating capacity and total power input. A COP of 2.5 would mean, for example, that at a steady state you generate 2.5 kW of heating capacity by using 1 kW of electric power (here we assume that the heat pump is driven by an electric motor, which is usually the case).
The first thing to say is that, in almost all scenarios, heat pumps are more efficient at generating heat than traditional heating technologies such as natural gas boilers. Whereas a natural gas boiler could be expected to achieve a COP of c. 0.9, the COP of a heat pump may vary between 1.5 to 4 (this range is wider for air-source systems and more narrow for ground- or water-source systems, whose temperature is more stable). Heat pumps, however, are driven by electric energy and thus the COP should be multiplied by the primary energy factor (PEF). Taking this into account, heat pumps can still be 20-30 % more efficient than gas boilers.
Depending on the weather, the COP of a heat pump can rise to around 4. At Clade, we typically see COPs in our heat pumps between 3 and 4, sometimes even higher depending on the conditions. Ours are particularly high performing due to our 35 years of experience manufacturing heat pumps and refrigeration equipment, and our use of CO2 as the working fluid. CO2 has excellent heat transfer properties and can provide high grade heat and higher COPs than other types of working fluids such as synthetic gas. This means that a CO2 heat pump could provide domestic heating (for hot water and radiators) at up to 85 degrees C while maintaining a high COP.
The Seasonal Performance Factor Of A Heat Pump
Because heat pumps rely on heat energy in the external source (the air, the ground or a water source) their COP is dependent partly on external temperatures.
Seasonal Performance Factor (SPF) takes that into account by summing up the heat that is generated per year and dividing it by the total electric energy use. This calculation for air-source systems is done by dividing the total ambient temperature range into sub-intervals and defining the duration of each. Obviously, two identical systems will perform differently in two different climate areas.
If one designs an air-source heat pump for monovalent use (the heat pump covers 100 % of the heat needed i.e. no additional heat source is used), the lowest ambient temperature and highest supply water temperature has to be assumed in order to make sure that the heat pump will supply sufficient heat at worst-case conditions. As the ambient temperature increases so does the compressor capacity and therefore the system design must include sufficient capacity control so that the compressors are not cycling extremely. CO2 heat pumps like the ones we build at Clade can operate at extremely low ambient temperatures at which other technologies are “lost”. This is due to the fact that CO2 has a higher pressure as compared to other refrigerants and therefore the compressor envelopes are much wider. However, for extremely low ambient temperatures two-stage compression systems may be needed.
Heat pumps should be expected to generate SPFs of 2.5 or higher, which means a year-round COP of 2.5 or higher. Indeed, if a heat pump’s SPF is lower than 2.5 it isn’t eligible for the Renewable Heat Incentive under EU law.
There are two main ways to measure the performance of a heat pump – coefficient of performance, and seasonal performance factor. Against traditional heating systems, heat pumps always outperform them in both of these measures.
This is what makes heat pumps such an attractive technology for replacing carbon-emitting heating systems. They’re available now, at scale, and they perform extremely well all year round with low operational expenditure required. Other renewable technologies can produce higher COPs at certain times of the year (for example, a solar thermal system could generate a COP of 70+ on a sunny day) but are not practical as a year-round solution.