Heat Pumps Today

15 Peter Jackson, Controls Systems Engineer for Op-Tec Systems Ltd and multiple awards-winner for innovation in refrigeration and heat pump technologies looks at the debates on the efficacy of air source heat pumps for domestic heating. Much Ado About Heating There is much pressure on builders and consumers to reduce the carbon footprint of domestic dwellings. The following is intended to stimulate debate and creativity in solving some of the issues in the move toward low carbon heating. As a replacement for gas boilers the air-source heat pump (ASHP) is the main means of fulfilling the heating need, but the ASHP is just one part the solution. There are technologies in the heat transfer, energy storage, electricity supply and controls industries that could be integrated, and if done successfully the whole will be greater than the sum of the parts. The heat pump paradox: When you need it most it gives you least, and to make sure it gives you enough in the worst-case scenario, you have to over-size it. The worst-case scenario would be an air source heat pump operating in the depths of winter. Being expected to produce domestic hot water at 60°C and heat a dwelling simultaneously is asking too much of the simple ASHP. It can be shown to work, but the plant will be uneconomically oversized, it will be working at the limit of its operating envelope, there will be frequent periods without heat due to defrosts, e ciency will be at its lowest and the demand for, and hence the price of, electricity will be at its highest. These downsides are what the heat-pump user will remember long after the glossy brochures have been recycled. The air-source heat pump needs additional resources, such as a Ground Source Heat Pump (GSHP) using a shallow (<300m) borehole to transfer heat MUGSHOT "THE HEAT PUMP PARADOX: WHEN YOU NEED IT MOST IF GIVES YOU LEAST…" CMYK / .ai CMYK / .ai CMYK / .ai www.acrjournal.uk/heat-pumps / .ai CM / . i CM / . i O P I N I O N from underground. If ground can o er a higher-grade source of heat than ambient air, then to increase both e ciency and e cacy the ground should be the preferred heat source. The challenge then is that as surely as digging out coal, or pumping oil out of the ground, the energy source will be depleted over time unless there is local volcanic activity, which brings its own unique set of problems. There is still the issue of high electricity price, but the plant could be more economically sized and e ciency for at least the first year will far exceed what ASHP’s could achieve. In summer, domestic hot water is the dominant but lesser heating demand, ambient air temperature is high, photo- voltaic panels and wind turbines are productive, and on the smart grid electricity demand and hence the price is low, frequently to the point of negative tari s (suppliers paying consumers to use electricity). Back to the heat pump paradox: When you need it least is when it gives you most, and output from a heat pump sized for winter operation far exceeds the summer requirement. Using the excess capacity when the air heat source is good, coe cient of performance is high, electricity is plentiful and cheap, then the ground can be used as an e cient heat sink to store cheap, or even free energy into the vicinity of the borehole. Summer is the perfect opportunity to deposit heat into this energy bank for use in winter. Multi-source, multi-sink heat pump technology is well proven in hundreds of commercial and retail premises, and the principles hold good to integrate ASHP and GSHP into one refrigeration circuit to fulfil a domestic role. Smart Grid technology is also well proven and can feed electricity price and availability data into controllers equipped to use it. Logical steps would be to integrate air source and ground resource heat pumps, to capitalise on the highest CoP identified by smart grid pricing to store energy, and use the stored energy to enhance heating capacity when air source e cacy reduces in winter. An intelligent controller can optimise heat transfer and storage while minimising expenditure on electricity. It can also monitor the heat transfer between air- resource and heating load, or between air-resource and ground-resource, or between ground-resource and heating load. It can learn to optimise energy usage, automatically selecting the most appropriate operating mode to maximise the annual e ciency of not just an intelligent heat pump installation, but wise heat pump operation.