Heat Pumps Explained
What is a Heat Pump?
A heat pump is just a refrigerator in reverse. In a refrigerator, we extract heat from something we want to cool and reject that heat to the environment. In a heat pump we extract heat from the environment and add that heat to something we want to heat. The components required to do this are identical; two heat exchangers (called an evaporator and a condenser), a compressor and an expansion valve as shown in Figure 1.
Figure 1. The heat pump cycle
Heat pumps have been around for as long as we have had refrigeration. If you have an air-conditioner that you use for heating in winter – also known as a reverse-cycle air-conditioner – then you are using a heat pump: The air-conditioner is extracting heat from the cool outside air and bringing it into the house via the heat pump cycle.
Key applications for heat pumps include:
- Air-conditioning: As noted above, an air-conditioner working reverse cycle mode is just a heat pump.
- Water heating: Heat pumps can be used for heating water for space heating or for domestic hot water heating. In Europe, the use of heat pumps to heat water for in-slab heating is very popular; some of these use the ground, rather than the air, as the heat source. Heat pump driven domestic hot water has been available in Australia for over 30 years.
- Industrial heat recovery: In industrial processes, heat pumps can be used to recycle lower temperature waste heat up to useful, higher temperature heat. Heat pumps can also be used in drying processes.
In this series of articles we’ll explore heat pump technology in more detail with a particular focus on carbon dioxide heat pumps, which are set to have a revolutionary impact on heat pump applications.
Nerd Space: What happens inside a heat pump?
Let’s start at the evaporator. The evaporator is typically a heat exchanger between the outside air and the refrigerant. The refrigerant enters the evaporator as a low pressure liquid. The boiling point of this liquid is cooler than the temperature of whatever we are extracting heat from – typically outside air. So as the outside air passes through the heat exchanger it makes the refrigerant evaporate. The energy required to evaporate the refrigerant is known as latent heat, and is all absorbed at one temperature, being the boiling point temperature – just like water boils at a fixed temperature of 100°C under normal conditions.
The refrigerant leaves the evaporator as a low pressure gas.
This low pressure gas enters the compressor where – as the name suggests – it is compressed. The compression process turns the cool, low pressure gas from the evaporator into a hot, high pressure gas. This gas enters the condenser.
The condenser is a heat exchanger between whatever it is we want to heat (which might be water or air) and the refrigerant. Because the refrigerant is under pressure, its boiling point temperature is higher than it was in the evaporator. Meantime, the water or air that we are heating is cooler than the refrigerant boiling point. This causes the refrigerant to condense and become liquid, releasing the latent energy to whatever we are heating.
The refrigerant leaves the condenser as a warm liquid. It then passes through the expansion valve where the pressure drops and as a result the refrigerant vaporises.
The magic of this process is that we transfer heat from a cold object to a hotter – something that can only happen because of the mechanical work input at the compressor.
Heat Pump Efficiency
Normally when we think of efficiency, we think of a percentage, i.e. of the energy we put into an appliance, only some of it provides a useful output and some is wasted. Thus we talk about a boiler being 90% efficient – because only 90% of the fuel energy input ends up as useful output heat, with the rest being lost via the flue or via radiant and convective losses from the boiler.
By contrast, heat pumps seemingly undertake the impossible: you get more heating out than the energy you put in. This is possible because we are using energy to move heat – rather than converting the energy directly to heat. As a result the apparent efficiency in terms of heat output is greater than 100%. This is shown in Figure 2.
Figure 2. The heat pump cycle.
The ratio of electrical energy input to heat output is called the coefficient of performance or COP; the higher the COP, the more efficient the heat pump. For the heat pump in Figure 2, the COP is 4 units of heat output divided by one unit of electricity input, i.e. a COP of 4.
The COP can be maximised by careful design of the heat pump (efficient compressor, fans) and the use of a thermodynamically appropriate refrigerant.
The key external factor affecting both the COP and capacity of a heat pump is the temperature difference between the evaporator and the condenser. The narrower this temperature difference, the easier it is to transfer the heat and so the more heat we can transfer for every unit of energy input. This means that in a space heating application, for instance, the heat pump will be very efficient at mild temperatures but less efficient when it’s really cold.
Nerd Space: What’s so good about CO2 in heating?
The conventional refrigeration cycle operates between two phase-change processes: An evaporator in which liquid refrigerant evaporates, creating a cooling effect, and a condenser where gaseous refrigerant condenses, creating a heating effect. In each case what is transferred is known as latent heat. Just as water boils at a fixed temperature of 100degC and ice melts at a fixed temperature of 0°C – both phase change processes occur at fixed temperature. This characteristic is both useful and limiting, particularly if you want to generate very high condensing temperatures. CO2 has a particular property that at high (but achievable) pressures it enters a state known as “supercritical” which means that it no longer has a phase change between gas and liquid and thus has no fixed temperature latent heat transfer. This means that instead of transferring a large amount of heat at, say, 50°C, it transfers a comparable amount of heat, but across a wide range of temperatures from 90°C down to 10-20°C (and lower). This normal “sensible” heat transfer matches exactly the sort of heat transfer we want to achieve when heating water from cold to hot. As a result, supercritical CO2 pumps are perfectly matched to high temperature rise heating applications like domestic hot water.
Note that because the CO2 doesn’t condense when it transfers heat, the heat exchanger normally known as the condenser is called a gas cooler in CO2 heat pump systems.
All Change: New refrigerants in heat pumps
The refrigerant market has been subject to significant changes over the past 40 years because of concern about the environmental impacts of refrigerants.
The Montreal Protocol (1987) initiated the phase out of CFC (chlorofluorocarbon) refrigerants such as R11 and R12 because of the massive damage these were doing to the ozone layer in the atmosphere.
The second phase of this phase out is drawing to a close currently, removing many of the refrigerants that were brought in to replace CFCs, being the HCFCs (hydrochlorofluorocarbons) such as R22 and R123. These chemicals, while of far lesser ozone depletion potential than the CFCs, still damage the ozone layer. Production of these refrigerants has essentially ceased, and it will become harder over the next few years to source replacement refrigerant for systems using these chemicals.
In 2016 the international community agreed to phase down the next group of refrigerants, being the HFCs (hydrofluorocarbons) such as R134a. These refrigerants have zero ozone depletion potential but unfortunately are significant contributors to global warming – one kg of R134a has the global warming potential of 1,430kg of CO2; a massive impact. The phase down is scheduled to occur between 2018 and 2036 by which Australia has committed to reduce its production and imports of bottled HFCs by 85% by 2036, as shown in Figure 3.
Figure 3. Australia’s HFC phase down schedule.
While 2036 may seem a long way away, the fairly linear nature of the phase down means that refrigerant equipment being specified today is already at risk of a reduced lifespan due to the reduced availability and doubtless increased cost of HFC refrigerants in 10-15 years’ time: HFC imports and production will be over 60% lower in 2028 than in 2018.
It is one of the truisms of refrigeration that every phase of new refrigerants has been accompanied by a boost of innovation that expands the efficiency and applications of the technology in new and often not entirely expected ways. The current wave of new refrigerants in response to the phase down of HFCs – and the need to develop refrigerants with low global warming potential – has propelled CO2 into the fray as a major new refrigerant. CO2 has zero ozone depletion potential and, by definition, a global warming potential of 1.
The thermodynamic characteristics of CO2 make it particularly suitable for use in heat pump applications, operating at far higher temperatures than has been feasible with HFC heat pump technology. A second aspect of CO2 as a heat pump is that it works best where the heating process occurs over a large temperature range, such as is the case with domestic hot water, where water has to be heated from below 20°C to above 60°C. Converse this means that CO2 heat pumps are not a natural fit for conventional space heating hot water systems operating across a 20°C temperature difference, unless the water entering the heat pump is quite cool, i.e. 20-30°C.
In the final article in this series, we will look at the Eco-Cute CO2 heat pumps as an example of how these performance and can be applied in practice.
Nerd Space: Maximum efficiency of a heat pump
Heat pumps have so many counterintuitive features – an apparent efficiency greater than one, transferring heat from a cold object to a hot object – because they are playing with the second law of thermodynamics. Most sensible people don’t want spend much time thinking about the second law of thermodynamics, and it is generally the preserve of bespectacled, crazy-haired physicists (like the author). For those interested, the maximum efficiency of a conventional heat pump is described using the Carnot efficiency equation:
|COPmax =||Tcond (K)|
|Tcond (K) – Tevap (K)|
It can be seen from the format of the equation that the larger the temperature difference is between evaporator and condenser, the lower the maximum COP. If we take an evaporator at 10°C (283K) and a condenser at 60°C (333K) the theoretical maximum COP is 6.7, which is well ahead of anything we are achieving today, so there is clearly scope for the technology to become even more efficient in the future.
CO2 heat pumps in practice
CO2 heat pumps are rapidly becoming available is a wide variety of configurations and from a wide range of suppliers. In this final article, we use the Eco-Cute CO2 heat pumps available from Automatic Heating to demonstrate the key performance features of CO2 heat pumps.
The COP of CO2 heat pumps is higher than for HFC heat pumps in applications where a high temperature difference is required between the inlet and outlet of the water being heated. This is illustrated in Figure 4.
Figure 4. Efficiency of CO2 heat pump as a function of ambient temperature (domestic hot water application).
The figure demonstrates a number of generic features of CO2 heat pumps:
- The efficiency is routinely higher than equivalent HFC heat pumps in domestic hot water application.
- The increase in COP is roughly fixed across non-freezing ambient temperatures. This means that the relative efficiency benefit increases as the ambient temperature drops towards zero.
- The maximum hot water temperature achieved by the CO2 system is higher than that for the HFC system.
Similar effects are visible in the capacity curve of CO2 heat pumps as a function of ambient temperature, as illustrated in Figure 5.
Figure 5. Heating capacity of CO2 heat pump as a function of ambient temperature (domestic hot water application)
For Australian conditions, one is typically designing for minimum ambient temperatures between minus 2°C and 10°C. Looking at the figures, it can be seen that the oversizing required for the CO2 heat pump to compensate for the low temperature performance is far smaller than it is for the HFC heat pump.
HFC heat pump technologies are generally limited to maximum temperatures in the region of 45-55°C. CO2 heat pumps can work at temperatures of up to 90°C, while maintaining a high COP, as shown in Figure 6.
Figure 6. Effect of outlet temperature on CO2 heat pump performance (domestic hot water application)
Greenhouse Gas Benefits
Comparing a heat pump to a condensing boiler, the degree of greenhouse gas benefits depends on the ratio of the greenhouse gas coefficients of electricity and gas. Using the current greenhouse gas coefficients for Australia, the threshold COPs for a greenhouse benefit are as shown in Figure 7 . It can be seen that in domestic hot water applications, a CO2 heat pump at a notional COP of 4 is beneficial in all states other than Victoria; as the grid decarbonises, this situation will improve further.
Figure 7. Threshold COPs for greenhouse emissions benefit for heat pumps (2017 Emission factors used)
Refer to our CO2 Heat Pump Packages page for more information how CO2 Heat Pumps can save on energy costs.