Heat pumps, the central theme of the recent IIR Congress
An overview of the research carried out in the field of heat pumps, with the aim of achieving greater flexibility and increased energy efficiency, in particular through to the use of thermal energy storage.
No fewer than 115 papers presented at the IIR International Congress of Refrigeration in August 2023 in Paris were devoted to heat pumps. This means that for the first time in an IIR congress, this theme, attached to IIR Commission D2 “Heat pumps”, was the dominant theme compared to all the other areas covered by the ten IIR commissions.
This figure also reflects the vigour of research and development work currently being carried out in a context where, according to the IEA (1), the number of heat pumps installed worldwide is expected to increase from 180 million in 2020 to around 600 million in 2030, in order to achieve the goal of net zero emissions by 2050.
At the 2023 IIR congress, several papers associated thermal energy storage (TES) with heat pumps, and this combination of technologies is proving very promising in many applications.
Heat pumps are considered a key solution for decarbonising the heating sector due to their energy-saving capabilities and low carbon emissions (2). (653) However, the electricity required by heat pumps may add burden to the electricity grid during peaks in heat demand (3). This could be a significant barrier to the wider use of heat pumps to replace the current fossil fuel boilers in buildings. (0886) To cater for these fluctuations in electricity demand, the integration of thermal energy storage (TES) for demand side management is seen as an effective solution from both an environmental and economic perspective (4).
TES systems are classified into three types: sensible heat storage, latent heat storage, and thermochemical storage (5). Several application examples using these types of storage were presented at the IIR Congress.
Regarding latent heat storage, M. J. Huang et al. (3) described the technical and economic performances of a heating and hot water production system by air source heat pump using a composite Phase Change Material (PCM) with expanded graphite mixed with copper powder as the thermal mass for the underfloor heating structure.
For their part, S. Yagnamurthy et al. (4) have highlighted the potential of thermochemical storage, which is considered to have energy densities 8 to 20 times greater than thermal storage of sensible and latent heat. They explored the potential benefits of integrating NaOH-based aqueous thermochemical storage into a heat pump system for domestic heating applications. Three integration configurations were studied for a typical building in the United Kingdom. It was found that the cascade configuration with the thermochemical storage discharging the heat into the room and the heat pump connected to the ambient air offered the highest potential savings in terms of cost - up to 16% - and an annual reduction in electricity consumption of more than 40% compared to the configuration without thermal storage.
M. Essadik et al (2) have introduced the concept of the “flexible heat pump”. This is a heat pump system that integrates thermal storage into the refrigerant cycle to theoretically improve performance by up to 20%. Of the various types of heat pumps, air source are the most commonly used due to their cost effectiveness and ease of installation. However, one of the main challenges they face is the formation of frost on the evaporator at low ambient temperatures and in high humidity conditions, which affects performance by reducing heat transfer and heating power of the heat pump. The flexible heat pump provides continuous heating while defrosting the evaporator by subcooling the refrigerant, using thermal storage as an alternative heat source. For a heat pump with a heating capacity of 3.6 kW and a condensing temperature of 35°C, when defrosting is activated, it is theoretically possible to save a further 5.6% on compressor electrical consumption compared to a heat pump using reverse-cycle defrosting.
E. Zanetti et al. pointed out that the main disadvantage of air-source heat pumps is their poor performance when the air temperature is low and this occurs in periods when the heating demand is higher. To overcome this problem, they presented a prototype dual-source air/solar heat pump with CO2 as the refrigerant. The heat pump produces hot water using air or solar radiation as the low-temperature heat source. When operating with air, a finned coil heat exchanger is used as an evaporator, while photovoltaic-thermal (PV-T) solar collectors are used as evaporators to harness solar radiation. In PV-T collectors, part of the incident solar radiation is converted into electrical energy by the PV modules and part is converted into heat to evaporate the CO2. When operating in solar source mode, if the compressor speed is equal to 40% of the maximum speed and under a solar irradiation of 900 Wm-2, the coefficient of performance using PV-T collectors as evaporator is approximately 20% higher than using air as the heat source.
(2) Essadik M. et al. Study of the defrosting operation of the flexible heat pump cycle. Download in FRIDOC (free for IIR standard and corporate members).
(3) Huang M. J. et al. Technical and economic performance of air-source heat pump for building heat supply with advanced pcm thermal storage. Download in FRIDOC (free for standard and corporate members)
(4) Yagnamurthy S. et al. Potential assessment of heat pumps fully integrated with thermal storage for domestic heating applications. Download in FRIDOC (free for standard and corporate members)
(5) Govindasamy P. K. et al. Energy storage using reversible heat pump and Organic Rankine Cycle. Download in FRIDOC (free for standard and corporate members)
(6) Zanetti et al. CO2 dual source solar assisted heat pump with PV-T evaporators: performance and control of low pressure. Download in FRIDOC (free for standard and corporate members)