18 May 2017

K.1.9.1 The Potential and Challenges of Solar Boosted Heat Pumps for Domestic Hot Water Heating

The use of air-source heat pumps (HPs) for domestic hot water production is well established. These approaches may not be optimum as cold outdoor temperatures can lower overall capacity and COP. An alternative approach is to use a solar-thermal collector to boost the HP’s evaporator temperature (and energy input) during cold ambient periods. The HP also cools the solar absorber, reduces heat losses and increases collection efficiency. Various system configurations have been proposed for solar boosted HPs including: “direct” or “indirect”; “series” or “parallel”; and dual-source. To date, the most successful designs use simple, unglazed solar thermal panels that act as an air-source during low-sunlight conditions. These simple solar absorbers reduce costs but have limited solar-boosting capability at low ambient temperatures. The use of high-performance solar panels (with glazed and insulated absorbers), however, limits a unit’s “non-solar”, air-source capacity; reducing their benefit. Consequently, new systems are being developed that include dual- or tri-mode solar collectors that act as efficient solar- or air-source evaporators and may even include photovoltaic/thermal absorbers. Combined with new system configurations and components (e.g., new variable speed, high-efficiency compressors), fully integrated, high performance, solar/HP hybrid water heaters are possible. This paper describes new approaches to solar boosted heat pumps and discusses their technical potential and challenges.