High Temperature Heat Pump Research Articles

Study on the leakage in water-injected twin-screw steam compressor

Steam compressor shows great energy-saving potential in mechanical vapor compression and high-temperature heat pump systems. However, leakage remains a huge problem for compressor performance improvement and design optimization. In this article, a thermodynamic model with detailed leakage flow model was developed for the working process in water-injected twin-screw compressor. Altogether, leakage in different paths and heat and mass transfer between injected liquid and compressed vapor were considered. Compared with experimental results, the accuracy of predicted efficiency is within ± 5% for all tested operating conditions. In the validated model, leakage flow in five leakage paths and their effects on compressor efficiency was investigated. Results indicate that leakage can reduce volumetric efficiency by up to 50.2%, while the injected water liquid vaporization can improve volumetric efficiency by 7.9% at 5000 r·min−1. The leakage through contact line (CL), tip sealing (TS) line, and discharge end face (DE) show the most important impact on compressor efficiency, while the leakage through blow hole (BH) and suction end face (SE) has little impact on compressor efficiency. Compared with zero leakage under certain conditions, the reduction of volumetric efficiency caused by CL, TS line, and DE can reach up to 22.4%, 20.4%, and 12.7%, respectively. Reducing clearances of the above three leakage paths can efficiently improve the performance of steam compressor, especially for CL.

Life cycle techno-enviro-economic assessment of dual-temperature evaporation transcritical CO2 high-temperature heat pump systems for industrial waste heat recovery

Replacing fuel-fired boilers by using efficient heat pump plants to recover industrial waste heat is an effective solution to achieve the “dual carbon” target. Three novel transcritical CO2 high-temperature heat pump systems (Ej-Evap2-A, Ej-Evap2-B, and Ej-Evap2-C) are proposed in this study, by introducing the technique of dual-temperature evaporation realized with an ejector for cascade heat absorption from the heat source. Considering the application in the scenario of industry requirement of hot water heating, the life cycle performances of the new proposed heat pump systems and fuel-fired boilers are comprehensively studied from the perspectives of energetic, emissions, and economic. A sensitivity analysis about the new configuration heat pump system is also conducted considering the variation in electricity and coal price. The results demonstrate there exists an optimum discharge pressure that maximizes the coefficient of performance (COP). Ej-Evap2-C shows a maximum COP of 4.85, which is 14.40% higher than the baseline CO2 heat pump system (Base), and the exergy efficiency of Ej-Evap2-C is 7.86–15.19% higher than that of Base. Among the eight heating methods including coal-fired boilers (CFB), gas-fired boilers (GFB), electric heating boiler (EHB) and five kinds of CO2 heat pump systems, Ej-Evap2-C shows the least pollutant emissions and life cycle cost. Furthermore, Ej-Evap2-C has the shortest payback period of fewer than 7 years compared with the CFB. The dual-temperature evaporation CO2 high-temperature heat pump is promising to substitute traditional fuel-fired boilers to generate high-temperature fluid in the future.

Improved Energy Efficiency of Power‐to‐X Processes Using Heat Pumps Towards Mobility Sector Defossilization

AbstractCombining Power‐to‐X (PtX) processes with high temperature heat pumps (HTHP) can significantly increase their energy efficiency. Evaluating the example of oxymethylene ethers (OME) production from H2 via H2O electrolysis and captured CO2 from air shows that by upgrading waste heat streams using HTHP, a process overall energy efficiency of higher than 61 % can be achieved compared to 30 % in conventional integrated processes. Thereby, the waste heat stream from H2O electrolysis already covers the low temperature heat demand for CO2 capture via direct air capture, not only for OME but also for various PtX products. Importantly, a significant lever for the energy efficiency enhancement is the consideration of other low temperature heat‐demanding sectors. High overall process energy efficiencies and the electrification of the industry are key aspects towards a sustainable mobility sector.

Assessment of the off-design performance of a solar thermally-integrated pumped-thermal energy storage

Long-duration storage (i.e., 6 + h) could foster the power sector transitions towards renewable energy sources. Pumped thermal energy Storage (PTES) recently gained much attention. Thermally-Integrated PTES (TI-PTES) exploiting additional thermal energy sources to boost electric roundtrip efficiency are promising and can exploit low-temperature heat sources, including solar thermal energy. Since the solar resource availability substantially varies along the year, assessing the yearly system performance is mandatory to estimate the solar TI-PTES practical potential. The paper studies a solar TI-PTES plant based on low-concentration Solar Thermal Collectors (STC), a High-Temperature Heat Pump (HTHP), and an Organic Rankine Cycle (ORC). In the paper, the HTHP and ORC off-design performance are mapped for a set of representative conditions through an off-design model. Such maps are used to simulate the system operation when coupled with a solar thermal energy production profile for some periods considered representative of the whole year. Results show that the average round trip efficiency is always between 0.85 and 0.87, and the system can operate in various conditions, even in the cold seasons. While the system performance is almost constant over the year, the operating charging and discharging hours and the stored energy are more than doubled from January to July.

Graphic analysis of energy and exergy combined systems of solar collector and high-temperature heat pump

Graphic analysis of energy and exergy combined systems of solar collector and high-temperature heat pump

Bottom-up assessment of industrial heat pump applications in U.S. Food manufacturing

Decarbonization of the industrial process heat supply through electrification could contribute significantly to climate change mitigation efforts. In the U.S. industry, thermal processes accounted for more than two-thirds of the total final energy demand in 2021. Cross-cutting electrification technologies like industrial heat pumps are suitable for the process heat supply to several industrial unit operations in a sustainable way while also improving overall energy efficiency. This study employs a bottom-up approach to investigate the techno-enviro-economic potentials of deploying high-temperature and steam-generating heat pumps in the major U.S. food manufacturing sectors in different timeframes. The results show that the annual technical potential energy and CO2 savings by electrifying process heat supply are 325 PJ (or approximately 20% of the total final energy demand in U.S. food manufacturing) and 31 MtCO2 (equivalent to the annual CO2 emissions from over 6 million cars in the U.S.) in 2050, respectively; however, these incur additional costs in each sector. Although there may be individual cost-effective opportunities for electrifying heat supply in specific industrial sites, the overall costs are estimated to be high in the food sectors due to the large disparity between electricity and natural gas prices and low heat source temperatures. To overcome the identified techno-economic barriers, comprehensive action plans for different stakeholders are needed. This study provides novel insights that should inform policymakers’ and executives’ decisions about the electrification of the current and future U.S. industrial heat supply in relevant industrial sectors.

The techno-economic integrability of high-temperature heat pumps for decarbonizing process heat in the food and beverages industry

High-temperature heat pumps (HTHPs) are an emerging technology to improve overall process efficiency and reduce energy demand while enabling a switch from fossil fuels to renewable electricity. New industrial HTHP technologies aim to achieve an output heat temperature of 250 °C, suitable for decarbonising the food and beverages industry considering its temperature requirements of <250 °C. Here, we employ a bottom-up approach to investigate the techno-economic feasibility of integrating new HTHP technologies into heat processes of the German food and beverages industry and estimate emissions reduction potentials under waste heat scenarios. Our results indicate that the new HTHP technologies could meet 12 TWh of process heat demand in the German food and beverages industry and cut emissions by 9% considering Germany's current electricity fuel mix. A modest carbon tax of 38 €/t CO2 eq. or higher makes the HTHPs cost-competitive with an optimised fossil fuel-based alternative.

Life cycle performance evaluation of cascade-heating high temperature heat pump system for waste heat utilization: Energy consumption, emissions and financial analyses

Inspired by the principle of cascade energy utilization, the concept of cascade-heating is introduced, and five layouts of cascade-heating high temperature heat pump (HTHP) systems are proposed and optimized to utilize the waste heat. The energy consumption, carbon and pollutant emissions, life cycle cost (LCC) and payback period (PBP) models are established. Meanwhile, detailed comparisons are made with conventional HTHPs and boilers. The results indicate it is more positive for cascade-heating HTHPs in terms of energy consumption, carbon and pollutant emissions, LCC and PBP. The primary energy consumption of cascade-heating water cooled saturated suction system (CWSAS) is declined by 1.8–22.8% and 3.4–19.2% in contrast to conventional HTHPs and traditional boilers, respectively. The life cycle carbon emissions of CWSAS are reduced by 1.7–6.1% and 76.5% compared to conventional HTHPs and electric boiler. The pollutant emissions can also be prominently declined by adopting cascade-heating HTHPs. For life cycle economic performance, LCC of cascade-heating flash tank superheated suction system (CFSUS) can be reduced by 4.5–18.3% compared with traditional HTHPs. Furthermore, PBP of CFSUS is the lowest of 3.1 years, and 15.4–65.9% shorter than traditional HTHPs. CWSAS is the most recommended configuration on account of its remarkable overall energy, environment and economic performances.

Experimental speed of sound for cis-1,3,3,3-tetrafluoropropene (R1234ze(Z)) and hexafluoropropene (R1216) in gaseous phase

cis-1,3,3,3-Tetrafluoropropene (R1234ze(Z)) and hexafluoropropylene (R1216) are considered the alternative low-global-warming-potential working fluids due to their good performance in high-temperature heat pumps and organic Rankine cycle (ORC) systems. Thermodynamic data with high precision are required for performance analysis and industrial application, among which speed of sound data are essential for designing the system components and deriving other thermodynamic properties. In this work, we measured the speed of sound in the gaseous phase of R1234ze(Z) and R1216 using a fixed-path cylindrical acoustic resonator. The experimental temperature and pressure ranges of R1234ze(Z) are (303.15 to 363.15) K and (129 to 730) kPa, respectively; and those for R1216 are (293.15 to 363.15) K and (130 to 1001) kPa. The ideal-gas heat capacity and the virial coefficients up to 3th order were calculated from the experimental data. The uncertainties of the measured and derived properties are exhaustively analyzed and calculated. The combined expanded relative uncertainties (k=2) of the measured sound of speed are 2.8×10−4 for R1234ze(Z) and 2.4×10−4 for R1216. The combined expanded relative uncertainties (k=2) of the derived ideal-gas heat capacity are 0.018 for R1234ze(Z) and 0.010 for R1216. The combined expanded relative uncertainties (k=2) of the second density virial coefficient are 0.033 for R1234ze(Z) and 0.013 for R1216.

A high-temperature heat pump for compressed heat energy storage applications: Design, modeling, and performance

The current paper presents the design and performance of a high-temperature heat pump (HTHP) integrated in an innovative, sensible, and latent heat storage system. The HTHP has been designed to work between a heat source from 40 to 100 °C and a heat sink above 130 °C. An initial refrigerant analysis has revealed that R-1233zd(E) is the best candidate to meet the required performance and environmental considerations. The first part of this paper deals with the sizing and selection of the main components while discussing the challenges and working limits. A numerical model is also presented and validated. The second part of the paper is dedicated to develop parametric studies and performance maps under different operating conditions. The results show that the current HTHP, at a source temperature of 80 °C, consumes from 3.23 to 9.88 kW by varying the compressor’s speed from 500 to 1500 rpm. Heat production is achieved in the form of latent heat (7.40 to 21.59 kW) and sensible heat (from 6.35 to 17.94 kW). The heating coefficient of performance (COPHTHP) is around 4.

Theoretical Performance Analysis of an Ejector Enhanced High-Temperature Heat Pump with Dual-Pressure Condensation and Evaporation

Theoretical Performance Analysis of an Ejector Enhanced High-Temperature Heat Pump with Dual-Pressure Condensation and Evaporation

Investigation on water injection characteristics and its influence on the performance of twin-screw steam compressor

Water injected twin-screw steam compressor has great energy-saving potential in mechanical vapor compression, high temperature heat pump and steam production applications. To investigate the water injection characteristics and its influence on compressor performance under different operating conditions, a detailed thermodynamic model was developed for the working process. Heat and mass transfer between injected water and compressed vapor were considered, together with leakage of two-phase flow and power consumption of excess liquid water. Comparison between simulated results and experimental results show reasonable agreement with an error of ±5%. With the validated model, the effects of operating parameters on compressor performance and optimum injected water flowrate were investigated. Results proves water injection to be an efficient approach to improve compressor performance if properly applied. Compared with no water injection, the volumetric efficiency and isentropic efficiency with injection can be promoted by 11.9% and 17.2% respectively. As the injected water flowrate increases, the compressor efficiency firstly increases to a maximum value and then reduces when the injected flowrate is superabundant. The optimum injected water flowrate is dominated by rotating speed and nearly proportional to the rotating speed, while the optimum mass ratio of the injected water to the compressed vapor varies between 8.9% and 10.4%.

Integration of desiccant wheels and high-temperature heat pumps with milk spray dryers

Heat pump technologies are promising to decarbonise industrial drying processes by improving energy efficiency and shifting the energy source from fossil fuels to renewable-sourced electricity. While desiccant materials have proved effective in improving drying efficiency for dryers at temperatures < 120 °C, this paper focuses on the performance of desiccant wheel assisted high temperature heat pump (HTHP) technology to heat the dryer inlet air up to 150 °C using heat recovered from the dryer exhaust. Thermodynamic models of heat pump cycles were established to evaluate the performance of all the HTHP components. A one-dimensional finite difference model was used to simulate the dehumidification of exhaust air through the evaporator of the HTHP. The performance of the desiccant wheel was numerically simulated considering only the gas-side heat and mass transfer resistance. When compared with configurations of a simple HTHP, the desiccant wheel assisted HTHP requires a much smaller evaporator size (50–60 % less than the simple HTHP), similar to the HTHP with a heat recuperator. Its improvement in overall system coefficient of performance (COP), exergy efficiency and the drying capacity when applied to a spray dryer is marginal when compared to the model accuracy. However, comparisons between two cases with different ambient conditions (15 °C and 60 % RH vs 25 °C and 80 % RH) show that such improvement by the desiccant wheel assisted HTHP develops with increasing humidity in the incoming drying air.

Thermodynamic analysis of subcritical High-Temperature heat pump using low GWP Refrigerants: A theoretical evaluation

An Industrial heat pump recovers low-grade waste heat from the industrial processes and upgrades it into usable heat, obviating the need for conventional boilers. The subcritical technique of high-temperature heat pumps (HTHP) has proven to be an environmentally friendly solution for a wide variety of industrial processes requiring temperatures between 100 °C and 150 °C. The purpose of this paper is to determine the feasibility of a high-temperature heat pump operating over a wide temperature range and utilising an eco-friendly working fluid other than the relatively high GWP R245fa. Environmental and thermodynamic characteristics of R1233zd(E), R1336mzz(Z) and R601 are evaluated and compared to R245fa. A steady-state thermodynamic model of a single-stage HTHP with an additional internal heat exchanger was developed to determine system operational constraints and assess the cycle’s energetic and exergetic performance. The minimum degree of superheating for dry refrigerants is mapped to assess the specific performance at different temperature lifts. The evaporator and condenser heat transfer coefficients are computed and theoretically analysed. The energetic findings confirmed that using R1233zd(E), R1336mzz(Z), and R601 as working fluids enhanced the coefficient of performance (COP) by up to 8.32 %, 11.68 %, and 19.61 %, respectively, in comparison to R245fa. According to the exergetic results, the compressor followed by the expansion process has the greatest thermal losses and therefore has the most potential area for future improvement. Furthermore, R1336mzz(Z) and R601 require larger compressor sizes when compared to R1233zd(E) due to their lower volumetric heat capacity value. Based on the environmental analysis of direct and indirect emissions, utilising low GWP refrigerant results in a significant reduction in the total environmental impact value when compared to using R245fa.

Reversible high-temperature heat pump/ORC for waste heat recovery in various ships: A techno-economic assessment

An alternative waste heat recovery solution for ships is examined, relying on a reversible unit. This unit switches its operation between an organic Rankine cycle (ORC) for electricity production and a high-temperature heat pump (HTHP) for steam generation, using the refrigerant R1233zd(E), with the compressor/expander operating with a similar volume ratio that favors the performance. The waste heat of the engine's cooling water is exploited, when the ship is both at sea and at port, increasing the capacity factor. The performance of each mode has been examined at reference conditions in relation to the heat exchangers' sizing for the same compressor/expander, showing that the evaporator should be as large as possible to maximize the efficiency. The costs have been then assessed and compared with HTHP-only and ORC-only solutions, leading to the cost-optimal sizing that minimizes the discounted payback period (PBP). The reversible unit has been then up-scaled to match the steam needs and waste heat availability of several ship types, calculating the net fuel savings and PBP. The latter becomes much shorter than the one of the individual solutions, and ranges from 3.5 to 5 years, reduced to less than 4 years in case the fuel prices increase by 20%.

Energy, exergy, economic and environmental analyses and optimization of a novel vapor injection autocascade heat pump for high-temperature water heating

Aiming to develop a high-efficiency and low-cost high-temperature heat pump with large temperature lift ability, a novel vapor injection autocascade heat pump for high-temperature water heating is proposed in this paper. The proposed system utilizes the vapor injection technique to improve the limited performance of the conventional autocascade system and can achieve outstanding heating performance with over 100 °C heating temperature. The energy, exergy, economic and environmental performance of the proposed system are investigated by simulation method and compared with those of the conventional autocascade heat pump. The proposed system averagely improves the heating coefficient of performance and capacity by 51.8% and 104.3% over the conventional system when the evaporating temperature ranges from 0 °C to 20 °C, respectively. The exergy analysis indicates that the excellent exergy performance of the proposed system mainly benefits from the irreversible loss reductions in the throttling valves and cascade condensers. The proposed system possesses the lowest energy cost except for the coal-fired boiler, and produces the minimum pollutant emission except for the gas-fired boiler. The heating coefficient of performance of the proposed system is improved by 12.4% ∼ 14.0% through single-objective optimization. The optimal coefficient of performance and levelized cost of energy are 2.59 and 0.070 $ kWh−1, respectively.

Investigation of cascade high temperature heat pump optimal design theory based on experiment supporting multi-objective optimization

Cascade high temperature heat pump has been drawing wide attention due to its impressive application prospects to upgrade the low temperature heat. However, the relatively high initial cost has been hindering its application. Therefore, a refined design theory considering both thermodynamic performance and economic performance has been proposed. Herein, multi-objective optimization model with the improved non-dominated sorting genetic algorithm was adopted to determine the key design parameters: evaporation temperature and condensation temperature. The cost of per exergy output and initial cost of per heat capacity were cited as objective functions. Based on the Pareto fronts and the technique for order preference by similarity to an ideal solution method, the most favorable evaporation temperature and condensation temperature were determined. For the working condition of constant heat source temperature and different output water temperatures, the optimal evaporation and condensation temperatures were 15.27 °C and 101.93 °C, 13.06 °C and 111.77 °C, 11.44 °C and 121.58 °C, which denoted that the optimal condensation temperature increased along with the output water temperature but the differences between them were kept with a rather small range. For the working condition of constant output water temperature and different heat source temperatures, the optimal evaporation temperature and condensation temperature were 11.21 °C, 11.44 °C, 11.95 °C and 121.8 °C, 121.58 °C, 122.03 °C, respectively. Conclusion that optimal design evaporation temperature and condensation temperature were barely influenced by the heat source temperature could be drawn.

Assessment of Deep and Shallow Geothermal Resources and Measurement of Waste Heat Potentials from Industrial Processes for Supplying Renewable Heat for Industry and Urban Quarters

Deep geothermal resources for heat supply and waste heat potentials were assessed and measured for a high-temperature dairy plant. For the industrial waste heat, a borehole heat exchanger (BHE) seasonal storage was configured and simulated after an extensive investigation of shallow geothermal resources. We developed a concept for the subsequent use of the residual and waste heat from the plant in a low-temperature heating and cooling (LTHC) grid for the neighbouring former military camp “Martinek-Kaserne” with a future use as mixed-use urban quarter were investigated in two projects. The modelling of the deep geothermal resources showed that of the three potential reservoirs one is most feasible for geothermal heat supply with temperatures between 129 and 146 °C, which could be used with a high-temperature heat pump for process heat. The waste heat in all sub-processes of the dairy plant were measured over 18 months to identify the most suitable waste heat streams with regard to temperature and continuity. The results showed that 25 % of the waste heat from a sub-process of the plant (fresh products logistics) is sufficient to provide heat for the adjacent LTHC grid with a total energy demand of 3428 MWh per year. The simulation of the BHE field resulted in 96 BHE with 180 m depth for a dis-/charging capacity of 643.7 MWh and 20 decentral heat pumps in the buildings. The BHE field operates quite balanced with only 12.8 MWh of difference in the annual balance. The results of the feasibility study for deep and shallow geothermal resources, and the assessment of the industrial waste heat show that the whole cascade of high-temperature heat for industry to low-temperature heat for the LTHC grid could be realized at the investigated site.

Theoretical Study of HFO1336mzz(Z)/HFO1224yd(Z) Refrigeran t Mixtures for High-temperature Heat Pump

Background: The global energy consumption problem is becoming more serious, therefore, it is a hot trend to improve energy efficiency and choose environmentally friendly alternative working fluids. Objective: The main thermophysical parameters of the refrigerant mixture, which blended HCFO 1224yd(Z) and HFO1336mzz(Z) in the high-temperature heat pump cycle, were theoretically studied to replace HFC245fa. The theoretical cycle model of refrigerant mixtures of high-temperature heat pumps was established. Methods: In this paper, a theoretical cycle model of the mixed refrigerant high-temperature heat pump is established, and the cycle performance of the mixed refrigerant in the high-temperature heat pump system under different working conditions is compared. Results: It was found that HFO1336mzz(Z) could reduce the exhaust temperature of the compressor and increase the heat pump coefficient COPh, and HCFO1224yd(Z) could improve the problem of excessive suction specific volume in high-temperature heat pump systems. Conclusion: The result shows that the refrigerant mixture with 0.6/0.4 HCFO1224yd(Z)/HFO1336 mzz(Z) has the best cycle performance when the condensing temperature is 150 °C, and the cycle temperature rise ranges from 50 to 70°C.When the cycle temperature rise is 70°C, and the condensing temperature ranges from 110 °C to 150°C, the coefficient of performance value of the refrigerant mixture with 0.2/0.8 HCFO1224yd(Z)/HFO1336mzz(Z) reaches the maximum of 3.11.

Integration of Heat Pumps in Buildings and District Heating Systems—Evaluation on a Building and Energy System Level

The use of heat pumps in buildings is one of the best and often the only option for the decarbonization of the building stock. District heating seems a promising solution in urban areas and in existing buildings when the use of heat pumps is restricted and also technically and economically challenging (source exploitation, space restrictions, sound emissions, etc.). Heat pumps can be integrated in various ways in buildings and district heating systems: large central high-temperature heat pumps in district heating, medium-size heat pumps block- or building-wise or small heat pumps decentral apartment-wise. The best option depends on the individual district heating CO2 emissions and the electricity mix as well as on the perspective of the building owner versus that one of the district heating system and its future development. Austrian examples of district heating systems and different variants of integrating heat pumps are investigated in a comprehensive way by means of an energetic and environmental simulation-based analysis. This assessment includes a detailed investigation of the capabilities of the booster heat pump to increase the PV own-consumption and is also expanded to include various scenarios for the development of the electricity mix and the decarbonisation of district heating.