REFRIGERANTS | CO 2 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 adoption 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 35C. The oversizing of plant, pumps, pipes and heat emitters is widespread. 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 oversizing 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 overcome easily. Higher temperature/pressure The higher temperatures, pressures and complexity inside a CO2 heat pump require special components, and experienced design and manufacturing, to ensure safety and efficiency. For most users or building operators, this should never be a concern, because 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, more than 800kW a separate evaporator and compressor will be necessary. In this case, specialists must be engaged to complete the installation and certify in accordance with the Pressure Equipment Directive. 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 A HFC-based heat pump installed now would require replacement or modification before its end of life, increasing the total cost of ownership Figure 4: Suction pipe diameter required for CO2 which has greater density is approximately half that of R-404A to be replaced well before their end of life. The Montreal Protocol and the Kigali Amendment 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 phaseout process. A HFC-based heat pump installed now is likely to require replacement or modification before its end of life, increasing the total cost of ownership. As part of an organisations carbon footprint disclosure, it must account for all F-Gas fugitive emissions. That is, the quantity of HFC, HFO and hydrocarbon lost from their systems each year because of 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, so is 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, because the temperature difference between flow and return is larger. 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 the regulatory burden and additional cost will ramp up. Heat pumps are widely 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. CO2s thermodynamic properties, as laid out above, mean 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 significantly lower than the alternatives. While, in some instances, there will be challenges in the deployment of CO2 heat pumps, these can be overcome with good engineering practice and an experienced, qualified manufacturing and maintenance partner. CJ GBOR BSZRMNYI, engineering manager and thermodynamics leader at Clade 72 November 2022 www.cibsejournal.com CIBSE Nov 22 pp67-68, 70, 72 CO2 paper.indd 72 21/10/2022 18:57