Conference Agenda

COP28 pledged to triple the world's renewable energy capacity by 2030 and to double global energy-saving efforts over the same period. For energy efficiency, examples include an over 30 times increase in electric and hybrid light vehicle use, a doubling of building renovation, a nearly 10 times increase in smart meters, and a nearly 10 times increase in the number of heat pump installations. Domestic and commercial installation processes are well established and yet would always still benefit from system and component improvements, including new and or natural working fluids to address the per- and polyfluoroalkyl substances (known as PFAS) challenges. It is with PFAS that a concern is raised. The International Energy Agency (IEA) Heat Pump Technologies Annex 58, addressing High Temperature Heat Pumps details the significant research and commercial effort to deliver heat pumps in that heat from 100⁰C to 200⁰C and above. There are significant proportions of these systems, both existing and emerging, that use PFAS refrigerants. Therefore, what working fluids can we utilise to match the operating temperatures of IEA HPT Annex 58, while maintaining or indeed increasing efficiency and while increasing the operating temperature range. A natural working fluid with a high critical temperature is of course water (R718). Its large vapour volume is of course one of its disadvantages but there appears to be routes to its successful deployment. In investigating multistage compression for a heat source e.g., from waste heat from industry at for example 90⁰C to attaining 300⁰C, a single stage lift will have a very poor coefficient of performance (COP). Multistage compression can deliver much higher COPs e.g., approving 4.0, if single compressor can operate with up to 7 injection ports along its compression path and initiate for example, 30K temperature lifts during each stage. Experience has shown both flash tank heat pumps and injection port positioning increase COP with a slight loss in capacity as less refrigerant reaches the evaporator. Furthermore, the system increases in complexity with additional flash tanks, control valves, injection ports and no doubt a management system. So, while multistage compression may be theoretically very efficient, in practice, achieving 300⁰C may need some support. The support may appear from the adsorption pair of a heat transformer utilising water and carbon. A thermal transformer requires pressure and instead of the 55 Bar to attain 300⁰C in a vapour compression heat pump (single or multistage), a compressor delivering 17 bar R718 vapour to a thermal transformer, could realise a reasonable system COP of about 2.7, with the complexity of multi-stage compression that may be challenging to implement. Therefore, this paper will illustrate the modelling and design aspects to achieve the higher COP multi-stage system, while also presenting a lower complexity hybrid heat pump system with a lower COP but with a potentially lower capital cost.

Heat pumps have been sought as a promising technology for air and water heating processes in buildings. Several OEMs have been commercializing heat pumps with varying capacities; However, the application has been limited to lower temperatures (less than 70 C). With the recently growing interest in replacing gas-fired equipment for buildings and industrial processes, a new class of heat pumps is gaining substantial interest where the target temperatures are higher than the conventional heat pump technology (greater than 90 C). The current study is focused on this class of heat pumps and aims to provide a holistic overview of state-of-the-art technology while highlighting major challenges and opportunities. The discussion will be focused on the availability of technology to enable higher sink temperatures, deployment-based applications, waste heat recovery, and process integration and controls.

R-1336mzz(Z) is a hydrofluorolefin (HFO) refrigerant with a high critical temperature of 171.4 °C and has therefore prospects for application in high-temperature heat pumps (HTHP). The refrigerant has a saturation pressure below ambient at room temperature, causing concern about non-condensable gases (NCGs) like ambient air coming into the system during servicing or subatmospheric pressures in the heat pump. This study experimentally tests for negative effects of growing levels of air in the system using a HTHP with internal heat exchanger (IHX), suction gas accumulator and liquid receiver. Tests are conducted at air pressures of 5, 15.3 and 25.6 mbar resembling air mass fractions between 0.004% and 0.02%. Characteristic data (i.e., COP, Pressures, discharge temperature) was collected at two different operating conditions for all three NCG levels. No effect of the NCG was found in the performance tests. Additional dew point temperature measurements also did not show any effect of the NCG. An increase of the resting pressure was measurable and is in line with the expected effects of NCGs in a vapor compression system. It is concluded that the maximum tested level of NCGs was insufficient to alter the overall system performance.

High temperature heat pumps are gaining significant interest in recent years for decarbonizing industrial energy processes and in more recent developments they have been shown to reach up to 200°C of delivery temperature. For such heat pumps, several potentially suitable refrigerants have been considered, among them zeotropic mixtures are particularly interesting owing to their characteristic temperature glide that can match the temperature profile of the heat sink and heat source. If the temperature glide in source and sink can be set flexibly, the full potential of zeotropic mixtures can be exploited. A zeotropic water-ammonia mixture is considered in this analysis, given its significant potential in terms of thermodynamic performance and favorable environmental impact. In this work the effect of the mixture composition adjustment in the evaporator to increase the waste heat recovery is analyzed. A quasi-two stage vapour compression heat pump with the addition of a flash tank is considered. It separates the liquid and vapor phases of the outlet stream from the condenser with the aim of creating streams with different composition. A part of the liquid outlet will be mixed with the vapor outlet to adjust the composition of the working fluid passing through the evaporator and the quality of the injection stream into the compressor. To investigate the performance of this modified system, a sensitivity analysis is performed to identify the optimal liquid split ratio, the flash tank pressure and injection pressure in terms of COP and heat recovery at the evaporator. It was observed that, compared to a non-modified quasi-two stage heat pump, the composition adjusted cycle achieves a COP gain of ~5% and an ~11% rise in evaporator heat gain. Additionally, a general trend of higher COP corresponding to a higher intermediate pressure at the flash was also observed.

A large share of industrial process heat demand is required as steam. One possibility for steam generation is to evaporate water against condensing refrigerant in a heat pump and compress it with a separate compressor to the desired final pressure. This work aims to demonstrate this process with a 70-kW butane (R600) heat pump and a small-scale centrifugal compressor for steam recompression. The high-temperature heat pump was built with a 6-cylinder reciprocating compressor, flat plate heat exchangers as the evaporator and condenser, and a tube-in-tube internal heat exchanger. The study shows transient data for the transition from water heating to steam generation. Furthermore, data from a parametric study on the steam generation rate was collected from 42 to 80 kg/h. The compressor frequency had to increase linearly, while the COP was between 1.75 and 2 throughout the parametric study. Focus is on the flat-plate condenser where refrigerant desuperheats, condenses, and subcools on one side. In contrast, on the other side, subcooled water is heated to saturation temperature, evaporates, and is superheated. The heat transfer rate at 80 kg/h was approx. 67 kW with a difference of the refrigerant condensing and water evaporation temperature of 17 K and a pressure drop of 0.12 bar for the given condenser.

Industrial processes require high temperature heat. Traditionally, the high temperature heat is generated by fossil fuels. Once the industrial process is realized, a large amount of this heat is wasted. High temperature heat pumps (HTHP) are an effective technology for generation of the process heat and recovery of the waste heat. This paper investigates the performance of HTHP, optimally integrated with PV/T, waste heat recovery, and energy storage in industrial multi-energy systems. Because of the time-varying waste heat (heat source) and process heat (heat demand), as well as the intermittency of renewable energy and integration of energy storage, dynamic models of HTHP are required for performance simulation of industrial multi-energy systems. The dynamic models are developed in Matlab using a component-oriented approach. The compressors are modeled with performance characteristics data. The heat exchangers (evaporators and condensers) are modeled using the lumped parameter and variational approach. The systems work with different refrigerants which properties are calculated using Coolprop. Assessment of the HTHP design and operation performance characteristics is presented. The influence of different operating conditions on the performance characteristics is investigated. The results show high energy efficiency and high operational flexibility of HTHP in industrial multi-energy systems. High COP (5-12) of HTHP is obtained depending on temperature lift, compressor efficiency, evaporator and condenser operating conditions, and heat pump cycle. A HTHP based on a cascade system topped with a mechanical vapor compression system for steam generation is proposed for application in the dairy industry. The HTHP is further integrated with energy storage for the temporal decoupling of energy flows. The share of renewable energy to drive the HTHP is increased when integrated with PV/T, waste heat utilization, and energy storage technologies. The HTHP achieves efficient, clean, and sustainable electrification of the industrial thermal sector and represents an important asset of distributed integrated multi-energy systems.