Why Use CO2 In A Heat Pump? //
This paper is intended to set out the technical basis for CO2 (R744) as the preferred working fluid for heat pumps.
The vapour-compression cycle, as used in both heat pumps and refrigerators, has an intertwined history, with refrigeration being the dominant service provided to date.
Therefore, the term “working fluid” and “refrigeration cycle” has become commonplace. In order that this paper reflects the changing market “working fluid” and “heat pump cycle” will be used.
Simple comparisons between CO2 and other working fluids such as HFCs can be misleading because its low critical temperature either requires differences in system design, such as the use of cascade systems, or to transcritical operation.
As a result, like-for-like comparisons are not easy to make. This paper will attempt to set out the reasons why CO2 should be the working fluid of choice for heat pumps in many applications, giving comparisons where possible.
History of CO2
CO2 was one of the very first working fluids to be used in a heat pump cycle.
Subsequently, synthetic working fluids were invented and employed which are designed specifically for refrigeration. These synthetic fluids have very high GWP (Global Warming Potential; a measure of the effect on global warming measured in CO2 equivalence) and have high embedded carbon in production, but they are ideal for refrigeration.
Over the last 10 years CO2 has come back as the working fluid of choice for many applications because it has excellent heat transfer properties and engineering standards have advanced sufficiently to make it safe and economic. The first CO2 specific parts were developed in the 1990s but it took several years for them to become commonly available. However, CO2 technologies are now considered as ‘standard’ and the component availability is much wider and is still developing in line with the market.
The Thermodynamic Properties Of CO2
There are two key values for a working fluid that give rise to its behaviour; the critical point and the triple point.
The triple point is the only temperature and pressure where all three phases will exist. It is unique to a substance and can be used to identify it. The critical point is the highest temperature and pressure at which a pure material can exist in vapor/liquid equilibrium.
When compared to other working fluids, CO2 has a high triple point and a low critical point, as shown opposite.
At atmospheric pressure solid CO2 transforms directly to gas – something you might recognise from the dry ice effect in theatre and concerts. One of the complications with CO2 as a working fluid is that the reverse is also true and unskilled technicians can cause solid CO2 to deposit if the pressure is not controlled properly during a service.
The critical point occurs at 31°C. Above this point the CO2 is a supercritical fluid. This means that there is no phase change when heat is removed from the transcritical fluid – we call this gas cooling. In a heat pump system, supercritical CO2 will not condense until the pressure has dropped below the critical pressure.
No other commonly used working fluid has such a low critical temperature. The advantages this offers are discussed in the following section.
CO2 As A Viable Working Fluid
CO2 has an exceptionally low GWP of 1, compared to other working fluids such as R410A (HFC) or R448A (HFO) – see table below.
Every system will lose some of its gas each year from service activities or from leakage between components. Manufacturers and those working with plant take considerable precautions to prevent loss, but a 15% loss is not unusual. This loss of gas leads to fugitive emissions which must be accounted for in carbon footprint calculations. An example comparison is given below.
F-gas regulations have been steadily reducing the quantity of high GWP gases available and some of the worst are now completely banned. This trend is expected to continue in order to reduce fugitive emissions and emissions created during the manufacture of synthetic gases. Not only are they being regulated out but there have been significant price rises, in some cases by over 300 %, as supplies become restricted.
There is no prospect of a regulatory change which could phase out or prevent the use of CO2 and supply is plentiful. Therefore, there is no stranded asset or cost increase risk associated with its adoption as a working fluid.
It should be noted that some working fluids (e.g. HFOs) are in fact blends of different chemicals. Losses from HFO blends can result in the need to replace the entire volume as it is impossible to determine which component chemicals have been lost. This results in a much higher loss of high GWP fluid. This does not apply to CO2.
|Working fluid||GWP||CO2e for annual losses*|
|R410A (HFC)||2088||110 tons|
|R448A (HFO)||1387||74 tons|
|*Calculation based on like for like volume of 354kg|
CO2 is non-flammable and non-toxic
CO2 is non-corrosive, non-toxic and non-flammable. It is a stable molecule which does not decompose either in the system or when accidentally released. In comparison, a recent study by the University of New South Wales in Sydney, Australia, suggests that elevated levels of high-GWP HFC-23 (R23) in the atmosphere could be linked to the uptake of HFO1234ze, which the study says produces R23 as it decomposes in the atmosphere.
R23 has a GWP of 14800 which makes it amongst the worst gases for climate change. CO2 is an asphyxiant in large concentrations therefore the use of detectors in confined spaces is normal. As the gas is heavier than air it drops to the floor where detectors should be placed. The best placement for a CO2 heat pump is outside where the gas disperses naturally in the highly unlikely event of a severe leak.
CO2 allows for higher temperatures
The index of compression is very high for CO2, so the discharge temperature is higher than for the HFCs. Index of compression, also known as the polytropic exponent, is 1.289 for CO2 and only 1.005 for R404A. A polytropic process is a thermodynamic process that obeys the relation:
Where p is the pressure, v is specific volume, n is the polytropic index, and C is a constant. The polytropic process equation can describe multiple expansion and compression processes which include heat transfer. The value of n is different in different thermodynamic processes. The polytropic index is a measure of the work done by the system. If you have a value for n, then you can determine the heat of compression by the equation below. It is common to look at the measured suction, discharge temperatures and compression ratios and determine the polytropic index.
T2 / T1 = (p2 / p1) [(n – 1)/n] where T is the thermodynamic temperature
Where the numbers 1 and 2 denote the states at the beginning and end of the compression process. It can be seen that a higher value of n gives a higher differential in temperatures, thus CO2 has a greater temperature difference than HFCs. This means CO2 can deliver useful temperatures for heating applications whilst drawing heat from air at normal ambient temperature ranges, all year round. The performance difference can be seen on the graph below where CO2 is in green against a typical synthetic fluid in blue, vs. COP.
CO2 systems producing high water temperatures have a higher efficiency than systems with other working fluids. However, there are many other factors in heat pump design which also influence this. Different manufacturers will control the heat pump using proprietary algorithms and careful component selection resulting in performance differences.
CO2 has Greater Density
CO2 is more dense when compared to other working fluids. This means that all the pipework, number of compressors, components and rack size in general is smaller. For example, the required suction pipe cross-section area for CO2 is approximately half that required for R404A (for the same volumetric capacity). This is especially valid for large capacity systems that would require too many compressors and much larger diameter pipework if designed for HFC/HFOs.
Building systems engineering
CO2 is a great option for a heat pump working fluid however it faces two significant challenges from the buildings sector:
1) the building system return temperatures, and;
Traditionally heating systems have been designed for high temperature delivery, with a moderate difference between flow and return temperatures. This system design parameter exists in much of our building stock – even the emergence of gas condensing boilers has had limited impact on this. Any heat pump requires a different approach to design and commissioned temperatures. CO2 heat pumps can deliver high flow temperatures which are a better match for existing buildings but require a low return water temperature of around 35°C.
The over sizing of plant, pumps, pipes and heat emitters is endemic. Resilience and reliability are often cited as reasons, but the cause is also a lack of data driven design and poor design capability. Oversizing leads to inefficient and malfunctioning systems of which there are many examples. The marginal cost of over sizing heat pumps is significantly greater than for boilers.
This reinforces the need for a better engineering approach. With good engineering practice both these challenges are easily overcome.
The higher temperatures, pressures and complexity inside a CO2 heat pump require special components and experienced design & manufacturing to ensure safety and efficiency. For most users or building operators this should never be a concern as the CO2 heat pump comes as a single sealed unit – only trained service personnel need ever touch the CO2 side. On larger heat pumps, say over 800 kW, a separate evaporator and compressor will be necessary. In this case specialists must be engaged to complete the installation and certify against the Pressure Equipment Directive (PED).
Energy transition risk
The next energy transition is just starting and will be a period of rapid change in all respects including technologically, socially and regulatory. It will result in islanded assets that will have to be replaced well before their end of life. Commercial gas boilers being installed now are a great example.
The Montreal protocol and the Kigali Amendment to the Montreal Protocol set out the reduction of harmful high-GWP refrigerants. Over the lifetime of a commercial heat pump, which should be around 20 years, it is expected that regulations will accelerate the phase out process. An HFC based heat pump installed now would either require replacement or modification before its end of life, increasing the total cost of ownership.
As part of an organisation’s carbon footprint disclosure it must account for all F-gas fugitive emissions. That is the quantity of HFC, HFO and HC lost from their systems each year due to leaks or service work. High-GWP working fluids will result in a higher disclosure, affecting organisational carbon reduction plans and public disclosure. CO2 has a low GWP and is not subject to the Montreal protocol and is therefore a safe technology to use when considering transition risk.
Good system match
The high temperatures generated on the flow side and the low return water requirements make CO2 a good match for future building systems. Whole heating system efficiency is improved when using a CO2 heat pump, as the temperature difference between flow and return is larger. Now boilers are no longer part of heating systems there is no point in generating heat, sending it out to the building then bring it back in a hot return pipe. Pipes that are sized for an average flow rather than peak combined with two port control and variable speed pumps are robust design approaches, which feature in good district heating design where performance is directly linked to commercial success.
Preparation for the energy transition is a huge challenge for all organisations. Selecting the right heat technology will have enormous long-term effects as penalties for emissions, both in terms of regulatory burden and additional? cost, will ramp up.
Heat pumps are widely, almost unanimously, seen as the most likely replacement technology for current heating systems. The use of CO2 as the working fluid in these heat pumps has, thanks to technological and engineering advancement, now become possible. Due to CO2’s thermodynamic properties as laid out above, its performance is much greater than that of other commonly used working fluids. Overall, the total cost of ownership of a CO2 heat pump is likely to be much, much lower than the alternatives.
While in some instances there will be challenges in the deployment of CO2 heat pumps, these challenges can be overcome with good engineering practice and an experienced, qualified manufacturing and maintenance partner.
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