Perspectives on Heat Pumps

A publication by the Energy Saving Trust, The heat is on: phase 2, provides a useful introduction to heat pumps [1]. Its focus is on the performance of heat pumps installed in UK homes during the years 2010 and 2013, both air-source and ground-source types. An earlier trial (phase 1) demonstrated poor performance in some installations, and remedial action was taken prior to phase 2. Several measures of heat pump efficiency - Seasonal Performance Factors - are discussed. For the phase 2 trials the metric SPFh4 was chosen; the calculation of this SPF includes “the electricity supplied to the heat pump, all fans or pumps and electricity delivered to any incorporated auxiliary or immersion heater”.

38 heat pumps were used in phase 2, with average SPFh4 figures of 2.45 for air- source heat pumps and 2.82 for ground-source heat pumps. However, the performance range was wide, with SPFh4 values from 2.0 to 3.6 for ASHPs, and 1.6 to 3.8 for GSHPs. Consequently several of the sites fell below the threshold at which they would be deemed renewable by the EU Renewable Energy Sources Directive. Installer practice and consumer behaviour are considered to be important explanatory factors. 

The report concludes that well installed and operated heat pumps can perform to a very high standard in UK homes. Customers were generally satisfied but required more information in order to achieve the best performance from the equipment.

A 2019 Joint Research Centre report [2] places importance on the role of heat pumps in the future of the European heating sector, claiming that if “all current fossil-fuelled heat generation technologies were replaced by heat pumps overnight the combined emissions of the heat and power sector would be reduced by 16%”, and citing a scenario in which heat pump penetration increases strongly until 2050, representing 6.8% of the total electricity demand. It claims to see “a clear trend in all member states towards heat pumps beating gas boilers, if not already the case”.

Figure 31 on p.27shows that average heat pump performance has improved significantly between 2001 and 2015, but this is followed by a section on the challenges to using heat pumps. It notes that the benefits will be realised “only if the power sector is decarbonised in an affordable way”, and that transferring heat demand to the power system will cause increased peaks during winter, combining with other factors to create a “higher overall weather dependency”. Another challenge is linked to affordability since costs of heat pumps are higher than those of conventional heat generating technologies. Installation can be challenging in urban areas where living space is restricted, and buildings equipped with heat pumps need to be well insulated due to the nature of heat pump operation. Its efficiency is inversely proportional to the temperature difference between the desired set-point temperature and the temperature of the low grade reservoir (typically ambient air or the ground); the higher the difference the lower the efficiency. Hence for efficiency, heat from the pump should be supplied to the building at relatively low temperature, and when the ambient temperature is also low (e.g. around minus 10°C) there may be very low efficiency.The report contains an extensive literature review, and some of the papers mentioned will be briefly discussed below.

Chaudry et al. [3] explore the risks and uncertainties associated with the transition to a low carbon heat system in the UK. Heat demands are at present predominantly met by natural gas fired boilers, a consequence of the ready availability of this energy source. In reviewing the current status of heat supply technology, the authors note that the “UK’s building stock presents unique challenges in retrofitting new heat supply technologies” and quote an earlier writer:  ‘…conversion of the UK housing stock to electric heating would be “at best, extremely difficult, and, more likely, infeasible”.’ While recognising the potential key role of heat pumps in decarbonising space and water heating demands, the paper points out that in contrast to other parts of Europe, in the UK they are still a relatively new technology. While likely to be less affected by fuel cost uncertainty than gas heating, the latter has the advantage of lower capital cost. The impact of seasonal performance factors is analysed from the viewpoints of levelised costs, annual running costs and carbon abatement cost, taking a range of SPF values from 1.2 to 4 for ASHPs, and 1.5 to 5 for GSHPs (pp. 631-2).

Raghavan et al. [4] investigate scenarios for the decarbonisation of residential water heating in California. They consider options including advanced heat pump technology and solar thermal with heat pump back up. Prospective energy factors are compared over the years 2016 -2050 for heating methods using natural gas, electric resistance, and heat pumps, the latter having assumed energy factors in the range 2 to 3.5. The study includes the global warming potential of the refrigerants used in heat pumps, with data on leakage. A typical GWP of about 1430 is cited in 2016, but the use of CO2 as a refrigerant (with a GWP of 1) is anticipated in advanced heat pumps. The study concludes that an 80% reduction in 2050 emissions relative to 1990 is technically possible, but points to the desirability of achieving a 25% reduction in hot water usage, which could help lower the life cycle costs of adopting more efficient and higher priced heat pump technologies.

The previous paper mentions the issue of peak grid demand, which is central to Baeten et al. [5]. The writers propose that rather than creating grid congestion, the use of heat pumps to reduce greenhouse gas emissions could aid in reducing peak loads on the electricity grid. This can be achieved through using a thermal energy storage tank in the heating system to give added flexibility. They describe a predictive control strategy which takes into account the user’s energy cost, the environmental impact of energy use and the impact of expanding the electricity generation capacity. This strategy was used in a large scale case study, and shown to reduce the required peak load capacity substantially, shifting energy to base generating plants. A resulting small increase in costs for the consumer results, and heat pump owners should be encouraged to participate when peak shifting is needed by remunerating them for their additional expenses.

Felten and Weber [6] explore similar issues in a paper which describes a systems model for a floor-heated building with heat supplied by means of a heat pump combined with thermal energy storage. Model behaviour is compared with results obtained from trials in an actual building over a nine month period. Assessment of results focussed on the potential reductions in electricity consumption, electricity-cost savings, economic feasibility and load shifting ability of flexible heat pumps. The authors conclude that “very few set-ups and conditions qualify as being economically feasible”, although in some cases “flexible heat-pump operation may become economically viable in the intermediate future.” They discuss the part played by the heat capacity of the building, noting that using it “has been observed to yield higher operational cost savings”. However, in “no circumstances are investments in additional storage volumes … economically feasible.” The authors claim that having “analyzed the most common heat-pump type in Germany, results obtained in this study demonstrate that long-term expectations concerning the penetration of distributed demand-side management by heat pumps shall be regarded in detail and with some skepticism.”

The impact on both heat pumps and electric vehicles on low voltage residential distribution networks is the subject of a study by Shao et al. [7]. The authors consider a situation in which every household in a Danish representative urban residential LV distribution network has both a HP and an EV, and they analyse three different control strategies. The first is uncontrolled operation, following the users’ energy demands, the second allows the network operators to control HP operation under conditions of grid constraint, but not EV charging, and the third allows network operators to control both HPs and EV charging under these circumstances. Details of the system models are given, and it is assumed that there will be periods when ambient temperatures are too low to allow efficient HP operation. In the particular distribution network chosen, both the second and third control strategies are shown in the simulation to adequately control grid congestion without inconveniencing users. The authors point out that at temperatures below the minimum used in the study, falling HP performance might not be accommodated.

A wider view of research in the flexible coupling of power and heat sectors, renewable energy integration and decarbonisation is provided by Bloess et al. [8]. The paper provides an extensive literature review of model-based approaches to the subject, including work specific to heat pump performance. In their conclusions the authors note that many of the studies they cite see a central role for heat pumps, whether decentralized or connected to district heating grids, because of their beneficial effects on system costs, renewable energy integration, and decarbonisation.

References

[1]     The heat is on: phase 2 heat pump field trials phase 2

          Energy Saving Trust

https://energysavingtrust.org.uk/sites/default/files/reports/TheHeatisOnweb(1).pdf

[2]     Decarbonising the EU heating sector

Integration of the power and heating sector

Joint Research Centre EU 2019

Kavvadias K., Jiménez-Navarro J.P., Thomassen G.

https://pdfs.semanticscholar.org/8f19/c30e0a0674f5e692ae42c1e32325445f9908.pdf

[3] Chaudry, Modassar, Muditha Abeysekera, Seyed Hamid Reza Hosseini, Nick Jenkins, and Jianzhong Wu. 2015.  Uncertainties in Decarbonising Heat in the UK.  Energy Policy 87:623–40.

https://www.sciencedirect.com/science/article/pii/S0301421515300306

[4] Raghavan, Shuba V., Max Wei, and Daniel M. Kammen. 2017. Scenarios to Decarbonize Residential Water Heating in California. Energy Policy 109(May):441–51.

https://escholarship.org/content/qt8px3t10b/qt8px3t10b.pdf

[5] Baeten, Brecht, Frederik Rogiers, and Lieve Helsen. 2017.

Reduction of Heat Pump Induced Peak Electricity Use and Required Generation Capacity through Thermal Energy Storage and Demand Response.        Applied Energy 195:184–95.

https://www.sciencedirect.com/science/article/abs/pii/S0306261917302854

[6] Felten, Björn and Christoph Weber. 2018.

The Value(s) of Flexible Heat Pumps – Assessment of Technical and Economic Conditions.  Applied Energy 228(June):1292–1319.

https://www.sciencedirect.com/science/article/abs/pii/S0306261918309000

[7] Shao, Nan, Shi You, Helena Segerberg, and D. K. Kgs Lyngby. 2013. Integration of 100 % Heat Pumps and Electric Vehicles in the Low Voltage Distribution Network : A Danish Case Study.  3 Microgen.

https://backend.orbit.dtu.dk/ws/portalfiles/portal/54539959/INTEGRATION.pdf

[8] Bloess, Andreas, Wolf-Peter Schill, and Alexander Zerrahn. 2018. Power-to-Heat for Renewable Energy Integration: A Review of Technologies, Modeling Approaches, and Flexibility Potentials. Applied Energy 212:1611–26.

https://www.sciencedirect.com/science/article/pii/S0306261917317889