The Hybrid Approach
In part one, we considered the challenges and limitations of an Air Source Heat Pump (ASHP) only system, with particular focus on the problems commercial organisations faced when retrofitting existing properties with new heating and hot water applications. In this concluding part, we look at the advantages of adopting a hybrid system approach based on ASHP technology…
A hybrid approach where an ASHP is deployed in a packaged combination with a gas boiler and control system presents an attractive alternative, retaining the element of gas boiler technology that customers are comfortable with. Plus, it also offers better compatibility with existing heating distribution systems and thermal demands of higher heat loss buildings meaning less adaptation is required. There are also technical advantages, such as the ability to optimise heat pump efficiency and switching to the gas boiler at times of network peak.
The facility of two heat sources to meet the demands for space heating and/or hot water is especially relevant for the commercial sector where bespoke system design is often required to meet the particular needs of a project, such as applications with a high heat loss. In this case, the gas boiler can be operated to meet peak demands on the coldest days, allowing the heat pump to be reduced in size compared to the capacity of a pure electric heat pump system.
Installing a heat pump alongside an existing gas boiler, together with a control system also makes sense in retrofit installations, especially, in applications where a relatively new boiler has been installed, which should be highly efficient, and which can be retained for peak heating loads. The key challenge technically is to ensure that the control system for the ASHP and existing boiler operate together efficiently.
In such cases, given that the ASHP does not replace an existing heating system, the driver for installing the system is largely to reduce running costs and make quick gains towards improving environmental performance.
Hybrid systems based around an ASHP are likely to require some system refurbishment in many retrofit installations in order to ensure that a substantial proportion of the annual demand is met by the heat pump (though this is likely to be lower than a pure electric system). Even so, when including the cost of a gas boiler replacement, the cost of refurbishing heating systems for the installation of a hybrid system should be lower than in the case of a single heat pump system. This is due to the reduced heat pump capacity requirement since the boiler can provide higher flow temperatures to meet peak heat demands. When comparing the cost of a heating system refurbishment opting to install a hybrid system versus a ‘pure’ ASHP system a reduction in comparative costs of as much as 50% could be achieved (Source: Frontier Economics).
Once installed, levels of carbon savings are generally slightly higher when allowing for hybrid solutions – suggesting that up until 2030 hybrid solutions could be consistent with meeting carbon targets. Although the average cost-effectiveness of carbon abatement is somewhat lower than in the scenarios which exclude hybrids. These savings are estimated based on comparison with a standalone ASHP, assuming that a hybrid system will use a smaller heat pump with a capacity reduced by as much as one third. For a hybrid ASHP system, expectations will be for the heat pump to meet as much as 75% of the annual heat load, the remainder being met by a gas boiler. This delivers similar operating costs and comparable CO and CO₂ savings at current grid carbon intensity (the reduced heat pump coverage of the overall thermal demand can be compensated by the ability to run the heat pump at closer to optimum efficiency).
Whilst the long-term use of hybrid systems may be perceived as not fully consistent with meeting carbon targets and they can equally be limited by space requirements and noise issues that also affect standalone ASHP installation, there remains a strong argument for their use across the commercial sector.
In the long term, hybrid systems should fall behind pure electric systems in terms of carbon benefits as the grid decarbonises and may become less cost-effective if volumes of gas supplied for the heating drop. But looking out to 2050, innovations in the provision of hydrogen and green gas, using extant infrastructure which currently supports 85% of UK heating, means hybrid systems may prove to be a defining low carbon option. One that provides the means to support the very particular, practical needs of the commercial market with versatile, cost-effective systems, all without sacrificing the drive to lower emissions as part of the process of achieving net-zero.