Tim Mitchell, Sales Director of Klima-Therm says it pays to consider natural refrigerants in heat pump applications
Heat pumps are set to play a significant role in the decarbonisation of heat. However, as they are rolled out on a larger scale, the problem of harmful refrigerants leaking into the atmosphere becomes more significant.
Carbon dioxide (CO2) can be viewed as both hero and a villain in relation to the environment. On the one hand, this ubiquitous chemical compound is the most noteworthy long-lived greenhouse gas in our planet’s atmosphere and, therefore, a big contributor to global warming. On the other, using CO2 as a natural refrigerant in air conditioning systems can help save energy and reduce the carbon emissions that increase the likelihood of global warming.
Liquid and solid CO2 are significant refrigerants, Solid carbon dioxide – also known as ‘dry ice’ – is particularly useful in the food and drink sector or in medical applications (such as vaccine distribution) for small shipments where refrigeration equipment is impractical.
Liquid CO2, also known as R744, was often used as a refrigerant before the discovery of R12 (freon), whose use was banned in 1996 because of its harmful effects on the ozone layer. However, over time, it fell out of use.
Correct design and application is critical
Now, CO2 is enjoying something of a renaissance because it is less damaging to the environment than many alternative refrigerants currently available. Indeed, its environmental advantages could make it the future working fluid to replace HFC coolants in a host of cooling applications including supermarkets, offices, hotels, shops and healthcare facilities.
However, there remains resistance among some designers to using CO2 as a refrigerant because it operates at high pressures (up to 130 bar in some scenarios) and therefore requires specially designed high pressure components. Also because of the operational risks with these higher pressures, CO2-based systems can be more complex to ensure the safety of the overall system.
On top of this, specialist training is required to safely handle, service, maintain and install CO2 systems.
Furthermore, CO2 heat pumps generally demand large temperature differences in the fluid used to remove heat from the heat pump. This is fine if the heat pump is used to create hot water for consumption, perhaps for domestic, sanitary or process hot water, but for a space heating system, this could require a significant reconfiguration of the building heat distribution system with all its attendant disruption and expense.
Nonetheless, in the right applications, using CO2 as a refrigerant in heat pumps does have significant benefits. Research and product development continue with heat pumps suitable for a huge range of applications, from individual dwellings to multi-megawatt heat networks.
Indeed, there is increasing interest and development going into heat pumps using CO2 as a refrigerant, according to an influential report published in 2020.
The path to a zero-carbon future
The London Energy Transformation Initiative (LETI) is a network of more than 1,000 built environment professionals that are working together to put the UK on the path to a zero-carbon future. The voluntary group is made up of developers, engineers, housing associations, architects, planners, academics, sustainability professionals, contractors and facilities managers.
The ‘LETI Climate Emergency Design Guide – How new buildings can meet UK climate change targets’, says: “Besides having zero Ozone Depletion Potential (ODP), CO2 also has an ultra-low GWP of 1 (unlike most currently used refrigerants, the most common of which typically have GWPs in the range of 1400 to 2100), is non-flammable (unlike almost all other natural refrigerants) and is non-toxic (unlike ammonia, which is often used as a refrigerant in large scale industrial applications).
“But perhaps of most interest in the context of Future Heat for buildings is the ability of CO2 heat pumps to deliver domestic hot water (DHW) temperatures at relatively high efficiencies compared with most currently available heat pumps.”
CO2 as a green refrigerant
The environmental benefits of CO2 as a refrigerant are compelling. They include:
- Low global warming potential (GWP) – CO2 has a GWP of 1; this compares with a GWP of 675 for R32 refrigerant (which is often perceived to be the most practical and efficient cooling option for heat pumps).
- Since CO2 is a naturally occurring substance, it forms part of natural biogeochemical cycles and does not, therefore, create persistent waste in the atmosphere, water or biosphere.
- Non-ozone depleting
- CO2 is neither poisonous nor flammable, is relatively cheap, and is sustainable and future-proof. It is also far easier and cheaper to dispose of than manmade refrigerants.
Selecting the right heat pump
Environmentally responsible cooling and heating involves using natural refrigerants in combination with energy efficient appliances and buildings. Chillers and heat pumps can cause both direct and indirect emissions. Direct emissions result from the release of refrigerants which have a significant greenhouse effect. Indirect emissions are caused by the energy consumption of the appliances in operation.
Using natural refrigerants can reduce both types of emission. However, it is not just the refrigerant that matters when it comes to selecting a heat pump. The right technology is also crucial. That’s why it pays to specify a high efficiency unit such as Enerblue’s HP90/HP90W heat pump.
With a heating capacity of 14.5 to 133.2kW, this CO2 refrigerant gas-based range is designed for most commercial/industrial sectors and can heat water to temperatures up to 90 dec C with an external temperature down to -20 deg C.
It boasts a host of features and benefits including semi-hermetic reciprocating compressors, axial fans, and the ability to provide total cooling recovery whilst heating. This leads to high thermodynamic performance and zero impact on the environment for an excellent future-proof solution for both new and retrofit buildings.
Available in several configurations – including water to water, air to water and air to water with cool recovery – the HP90/HP90W heat pump is flexible enough to suit most applications.
The working principle involves a series of storage tanks that ensure hot water stratification. As the user starts consuming the hot water, the storage tanks start to empty, with the unit only running if necessary to top up. When the heat pump senses that the temperature in the coolest tank is low enough, it starts and the tanks are refilled again. This process can be timed to take account of cheaper off-peak electricity tariffs or greener electricity supplies.
For example, in a hotel application, the heat pump system will typically have two main peak demands each day – one in the morning when guests wake up and take a shower and another in the evening when they return to the hotel and get ready for dinner. The hot water storage is refilled at night, often taking advantage of favourable tariffs.