Heat Pump Applications Research Articles

Computational multiphase mixture simulations of a two-phase R-744 ejector geometry in transcritical R-744 heat pump applications

Purpose Two-phase R-744 ejectors are critical components enabling energy recovery in R-744 heat pump and refrigeration systems, but despite their simple geometry, the flow physics involve complex multiphase mixing phenomena that need to be well-quantified for component and overall system improvement. This study aims to report on multiphase mixture simulations for a specific two-phase R-744 ejector with supercritical inlet conditions at the motive inlet side. Design/methodology/approach Four different operating conditions, which have motive inlet pressure range of 90.1 bar–101.1 bar, are selected from an existing experimental data set. A two-phase thermodynamic equilibrium (TPTE) model is used, where the fluid properties are described by a thermodynamic look-up table. Findings The results show that the TPTE model overpredicts mass flow rates at the motive inlet, resulting in a relative error ranging from 15.6% to 21.7%. For the mass flow rate at the suction inlet, the relative errors are found less than 1.5% for three cases, while the last case has an error of 12.4%. The maximum deviation of the mass entrainment ratio is found to be 8.0% between the TPTE model and the experimental data. Ejector efficiency ranges from 25.4% to 28.0%. A higher pressure difference between the ejector outlet and the diverging nozzle exit provides greater pressure lift. Research limitations/implications Based on the results, near future efforts will be to optimize estimation errors while enabling more detailed field analysis of pressure, density, temperature and enthalpy in the computational domain. Originality/value The authors have two main original contributions: 1) the presented thermodynamic look-up table is unique and provides unique computation for the real-scale ejector domain. It was created by the authors and has not been applied before as far as we know. 2) To the best of the authors’ knowledge, this study is the first study that applies the STAR-CCM+ multiphase mixture model for R-744 mixture phenomena in heat pumps and refrigeration systems.

Application of heat pump assisted solar evacuated tube water heater operated within passive solar house in severe cold region

Replace coal with solar energy to low-cost heating in severe cold regions can improve indoor air quality, but indoor thermal comfort is not fully guaranteed. Therefore, a dual-source heat pump (DSHP) assisted solar evacuated tube water heater heating system was used in a house with sunspace to meet the heating stable and cleaning. TRNSYS model verified by experiment was used to research performance and economy of system. Results found the higher ambient temperature and solar radiation, the more heat from sunspace, solar assisted heat pump (SAHP) and solar water heater, conversely, the longer heating time of air-source heat pump (ASHP). During heating season, solar fraction of heating was 51.6 %, solar heat collection efficiency was 46.0 %, EER was 3.35, COP were 2.81 (ASHP) and 3.65 (SAHP). Heating time proportion of ASHP, SAHP and solar water heating were 15.0 %, 40.3 %, 44.7 %, ASHP heating was accounting for 22 % and 50 % in November and January. The system, sunspace supplied 16707.1, 2307.6 kW h of heat for room, solar effective heat collection was 10107.0 kW h, power consumption was 5164.7 kW h, CO2 emission reduction was 14232.4 kg. The dynamic payback period of system was 6.78 years, as will be shortened with support of clean energy heating policy.

Power Converter Topologies for Heat Pumps Powered by Renewable Energy Sources: A Literature Review

Heat pumps (HPs) have become pivotal for heating and cooling applications, serving as sustainable energy solutions. Coupled with renewable energy sources (RES) to run the compressor, which is the major energy-consuming component, they contribute to eco-conscious practices. Notably, their adaptability to be supplied by either alternating (AC) or direct (DC) currents, facilitated through converters, makes them more flexible for versatile renewable energy (RE) applications. This paper presents a comprehensive review of converter topologies employed in various HP applications. The review begins by exploring previous applied photovoltaic (PV)-HP projects, focusing on the gaps in the literature concerning the employed converter topologies. Additionally, the review extends to include a broader examination of the converter topologies that could be employed on the source and load sides of a system powered by a mix of renewable energy sources, such as photovoltaics (PV), wind turbines (WTs), and energy storage systems (ESS), and analyzes their strengths and weaknesses. Special emphasis is given to understanding the various topologies of the power electronics converters in the context of HP applications. Finally, the paper concludes with a summary of the literature gaps, challenges, and directions for future research.

Heat Pump-Assisted extractive Pressure-Swing distillation with integrated feed Preconcentration/Solvent recovery for Isopropanol-Benzene-Water separation

Recently, heat-integrated pressure-swing distillation (HIPSD) has been explored for recovering isopropanol and benzene from wastewater, but the process remains highly energy-intensive. Given the dilute nature of the feed, intensified extractive distillation with an integrated preconcentration column (IEDP) is more suitable. However, previous research has often overlooked the critical role of pressure and lacked rigorous optimization of operating pressure in such systems. In this article, we developed novel extractive pressure-swing distillation with integrated feed preconcentration/solvent recovery column (IEPSD) by introducing a pressure-swing configuration and incorporate energy-saving technologies to achieve more sustainable and cost-effective separation processes. First, thermodynamic analysis is conducted to explore the effect of pressure on extractive pressure-swing distillation. Then, a parallel genetic algorithm is applied for rigorous optimization, followed by the application of heat integration and heat pump. The IEPSD is superior to IED in terms of total annual cost (TAC) and CO2 emission. With the inclusion of energy-saving technologies, IEDPHI and IEPSDHI further reduced TAC by 5.29% and 7.40%, and cut CO2 emissions by 25.18% and 24.03%, respectively, compared to their base processes. The use of a heat pump is particularly advantageous for IEPSD due to its lower boiling temperatures under vacuum pressure, leading to the development of IEPSDHIHP, which offered an additional 22.36% CO2 reduction and marginally lower TAC than IEPSDHI. Ultimately, IEPSDHIHP proves to be the best option, offering a 51.15% TAC reduction and 68.42% CO2 emissions reduction compared to HIPSD. In summary, the developed IED and IEPSD processes, especially with heat integration and heat pump technologies, offer significant economic and environmental advantages over conventional HIPSD.

Experimental investigation and numerical modeling of an innovative horizontal coaxial ground heat exchanger (HCGHE) for geothermal heat pump applications

Geothermal is an ideal renewable energy source for building heating and cooling via a ground source heat pump (GSHP) system. However, the high initial cost of installing the ground heat exchanger limits its adoption, especially among owners of residential and smaller non-residential buildings. This study aims to develop a new methodology for installing an innovative horizontal coaxial ground heat exchanger (HCGHE) and to investigate its thermal performance in GSHP applications through field testing and numerical simulation. Horizontal directional drilling and a new manifold design were used to install the new HCGHE, significantly reducing costs by minimizing time, materials, labor, and landscape disruption. The transient numerical model, developed in Ansys Fluent using the finite volume method, was validated against experimental data and employed to predict temperature distributions in the supply pipe, return pipe, and surrounding soil. The study determined the heat transfer rate per unit borehole length and entire thermal resistance of the HCGHE to vary from 23.0 to 35.7 W/m and 0.28 to 0.74 K·m/W in winter, and from 24.3 to 37.4 W/m and 0.12 to 0.52 K·m/W in summer. The short-term soil temperature distributions in both the radial and longitudinal directions of the HCGHE are modeled using second-order polynomial equations, with R2 values ranging from 0.994 to near unity, while the long-term soil temperature distributions follow a power law. This research not only confirmed that the innovative HCGHE design and installation can achieve good thermal performance but also provided valuable insights into continual design improvement. The innovative HCGHE-based GSHP system provides an efficient and sustainable solution for heating and cooling buildings by harnessing the earth’s natural heat, offering the advantages of simplified installation and reduced initial investment.

Heating Performance and Energy Efficiency Analysis of Air-Source Heat Pumps in Public Buildings Across Different Climate Zonings

Article Heating Performance and Energy Efficiency Analysis of Air-Source Heat Pumps in Public Buildings Across Different Climate Zonings Junbao Fan 1, Yilin Liu 1,*, Jing Ma 2, Ying Cao 1,2, Zhibin Zhang 1, Xin Cui 1 and Liwen Jin 1,* 1 School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi’an 710049, China 2 China Architecture Design and Research Group, Beijing 100044, China * Correspondence: ylliu@xjtu.edu.cn (Y.L.); lwjin@xjtu.edu.cn (L.J.) Received: 11 July 2024; Revised: 3 September 2024; Accepted: 5 September 2024; Published: 20 September 2024 Abstract: Public buildings exhibit the highest operational energy consumption and contribute the most to carbon emissions compared to other types of building. The electrification of energy terminals in public buildings is crucial for the energy conservation and emission reduction, especially the energy-saving retrofitting of heating systems. Given the significant impact of climate on the performance of air-source heat pumps, this study explored the performance and energy efficiency of air-source heat pump systems in public buildings across different climate zonings. Using Design Builder software, the physical models of three types of public buildings (commercial, hotel, and office) were constructed, and the annual variations in the building load was analyzed. Considering the effects of defrosting and low-temperature conditions, an air-source heat pump heating system models were developed using TRNSYS software. The simulation results showed that the average COP of the heat pump system on the coldest day in Harbin, Beijing, and Shanghai were 1.7, 2.46 and 2.49, respectively. Moreover, the analysis factor correlation analysis reveals that the COP of the heat pump system is positively correlated with the dry-bulb temperature, negatively correlated with the building load. Surprisingly, the COP is not affected by the types of public buildings. The findings of this study are expected to provide valuable guidance for the application and regulation of air-source heat pumps in the public buildings.

Application of a semi-empirical modelling approach to a Two-Stage Rotary CO2 compressor

2-stage Rotary CO2 compressors are currently in use in refrigeration and heat pump applications, including occasionally including vapour injection. The need of correctly assessing the compressor’s efficiency by means of a computationally fast model arises for cycle performance prediction and optimization. In response, an innovation on an existing semi-empirical modelling approach has been introduced in this paper to describe 2-stage Rotary compressors. The model has been validated with experimental data of a CO2 compressor available in open literature, giving maximum errors of around ±3% for mass flow and ±1.82% for power prediction. Calibration of a simplified 1-stage model is also performed to analyse to which extent it is necessary to complexify the model, and to identify key or neglectable modelling elements. It is found that doing this increases the error of the compressor performance prediction. Model analysis has been done to identify the inefficiencies introduced by each physical effect considered.

Investigation on ejector design for CO2 heat pump applications us-ing Dymola

In this paper, the Dymola modelling tool is used to study the influence of ejector design onto the whole heat pump cycle working with carbon dioxide. The cycle is built using the components provided by the TIL Modelica library. It is found that the ejector models in TIL are quite limited, namely by their inability to properly capture the on-design plateau and rapid decrease in performance in off-design operation. Therefore, an in-house state-of-the-art ejector model, originally developed in Python, is implemented as a Dymola object. This model is then calibrated onto CO2 experimental data. The operation of a simple CO2 heat pump system is investigated, with focus on the ejector sizing at fixed geometry. It is found that there exists an ejector size that maximises the COP of the cycle. Furthermore, critical ejector pressure is not reached at this optimum COP point; the ejector is operating well under the on-design regime.

Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications

Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications

Selection of working fluids and heat pump cycles at high temperatures: Creating a concise technology portfolio

As global efforts to address climate change intensify, industrial processes are of the highest interest due to their substantial contributions to greenhouse gas emissions and use of fossil energy. The deployment of high-temperature heat pumps (HTHPs) presents a significant potential for mitigating emissions and decreasing primary energy consumption in industrial applications. However, for widespread adoption, further technological development is crucial to enhance the technology's availability at higher supply temperatures. This study is dedicated to optimising HTHPs by exploring the intricate interplay between working fluids and heat pump cycle design, with the goal of establishing a technology portfolio that guarantees high performance across a wide range of high-temperature conditions. The research evaluates 16 working fluids in 10 HTHP configurations across a broad spectrum of high-temperature conditions, with source temperatures from 0 °C to 100 °C and temperature lifts from 20 K to 150 K. A polynomial estimating the Lorenz efficiency was proposed based on these configurations and temperature conditions. A portfolio of heat pumps using either isobutane, isopentane, or hexane, in a one-stage cycle layout with a suction line heat exchanger, a two-stage cycle layout with an open intercooler using R-718, and cascade configurations using these working fluids, consistently demonstrated the highest Lorenz efficiency for 95% of the evaluated temperature conditions. Including ammonia and carbon dioxide elevated this to 98% of temperature conditions, improving particularly at large sink temperature glides. While underscoring the necessity for an approach considering both COP and production costs, the study highlights the COP as the most important aspect. The marginal improvement observed by including HFOs highlights the potential of a natural working-fluid-based portfolio to achieve high performance, instilling optimism for the future of sustainable high-temperature heat pump applications.

Experimental investigation and industrial application of a cascade air-source high temperature heat pump

For industrial thermal scenarios where there is no waste heat to recycle, the Air-source High Temperature Heat Pump (ASHTHP) is the only choice to replace the traditional industrial boiler and decrease energy consumption. A cascade ASHTHP has been designed and developed to support 125 °C hot water, which utilizes R410A and R245fa as working mediums. It is not only theoretically simulated and experimentally investigated, but also applied in an electroplating factory in Ningbo, China to provide 120 °C hot water. When the ambient temperature is 20 °C and the output temperature is 125 °C, the experimental heating capacity of the unit is 121.5 kW with a COP of 1.71. During the experimental investigation, the heating capacity attenuation remains within 19.25 %. Through the comparison and verification of the experimental data, the system simulation results have high accuracy with the error controlled within 5 %. When used in the electroplating factory to provide hot water at 120 °C, the cascade ASHTHP units can provide a heating capacity of 480 kW with a COP of 1.8 at the ambient temperature of around 25 °C, which results in annual savings of approximately $123,200 and a payback period of 1.65 years.

Experimental analysis and investigation of desiccant coated heat exchanger applications involving condensation and sorption mechanisms

This study focuses on the application of desiccant coated heat exchangers (DCHEs) in conditions where condensation occurs below the dew point temperature. Previous experimental studies on DCHEs and DCHPs have predominantly examined the sorption mechanism above the dew point, leaving a gap regarding material selection and system design in the presence of condensation. To address this, a guide is introduced that combines the understanding of condensation and sorption processes for desiccant applications. The guide is applied to evaluate the suitability of a superabsorbent-LiCl (SAP-LiCl) composite desiccant for desiccant coated heat pump (DCHP) applications. The stability of the desiccant in the presence of condensed water droplets is validated. Additionally, an experimental investigation explores the relationship between sorption and condensation mechanisms, identifying the transitional coolant temperature at which dehumidification shifts from condensation-dominant to sorption-dominant. By incrementally adjusting the coolant temperature in 5 °C steps, the transitional temperature of the SAP-LiCl based DCHE is determined to be approximately 15 °C under the target operation scenario, rendering its suitability for additional below-dew-point applications. The study reveals optimal operational parameters for DCHE, demonstrating a potential energy saving of 20 %. Based on these findings, a DCHP prototype is constructed to assess the feasibility of DCHE in real-world applications. The transient behaviors of this prototype are analyzed during each operating cycle, yielding a coefficient of performance of 4.97 and a cooling capacity of 1.5 kW under baseline operating conditions.

Raising the Drying Unit for Fruits and Vegetables Energy Efficiency by Application of Thermoelectric Heat Pump

Drying food stuffs and other materials belongs to one of the most commonly used feedstock processing techniques, featuring rather high energy consumption. The major disadvantage of conventional electric convective-type household dryers is substantial thermal energy emission into the environment with a wet exhaust, worked-out drying agent. Among other principal disadvantages common to all dryers of this type, the following have to be mentioned: spatial inhomogeneity of heating a product under processing and that of drying agent distribution due to its temperature reduction and relative humidity growth as it moves upwards. A block diagram and a breadboard model of a convective-type thermoelectric dryer employing a thermoelectric heat pump have been designed. In our approach, a product is treated with the help of a drying agent (normally, heated air) with partial exhaust-air recirculation and heat recovery. Laboratory studies of the drying process have been carried out using apple fruits as a test material in order to evaluate the power consumed for evaporation of 1 kg of water in the newly developed convective-type thermoelectric drying unit. Physical parameters of apple fruits before and after drying both in the thermoelectric drying unit and in a conventional series-produced convective-type domestic dryer have been reported. The energy efficiency of the newly designed drying unit has been compared with that of some series-produced samples. It has been found out that, unlike conventional convective-type dryers, the breadboard model of the developed thermoelectric drying unit features a smoother product drying process owing to the presence of side air channels and more effective drying agent path organization in the processing chamber. This conclusion was supported by the results of the carried out tests. Application of thermoelectric heat pumps with the function of the exhaust drying agent heat recovery will make it possible to reduce the drying agent heater installed capacity and the power consumed by the newly designed convective-type thermoelectric drying unit by up to 20% in the course of the drying process, compared to series-produced household convective-type dryers.

High-temperature heat pumps: Fundamentals, modelling approaches and applications

In March 2023, the European Parliament and Council reached a consensus to increase the binding renewable energy target (RES) to a minimum of 42.5 % by 2030, effectively doubling the proportion of RES from the 2020 baseline. This significant development aligns the EU more closely with the objectives of the European Green Deal and the REPowerEU initiative. High-temperature heat pumps (HTHP), due to their appropriateness for industrial-scale applications, integrate perfectly within this progressive trajectory. They enable waste heat generated by various production processes to be recovered (temperatures typically ranges from around 50 °C–100 °C) and subsequent use at temperatures above 100 °C, thus reducing the consumption of fossil fuels and greenhouse gas emissions. The high operating temperatures and pressures of HTHPs are challenging. They require an in-depth analysis of the system processes involved, taking into account refrigerants, efficient heat pump cycles and key components. One possibility for a preliminary analysis of vapour compression HTHP system performance without incurring the costs associated with manufacturing and testing the device is modelling. However, there is no comprehensive review of research work on possible software. Commonly, the researchers report on one chosen methodology and the tools used. This paper provides a comprehensive review of modelling approaches. It also discusses aspects related to the principles of operation, refrigerants and system components. Additionally, the paper presents an overview of vapour compression heat pump applications in various sectors. The literature review conducted indicates the need for further research and development of HTHP covering not only technological aspects but also software development.

Some Features Relating to Application of Heat Pumps and Electric Heating in District Heating Systems

Some Features Relating to Application of Heat Pumps and Electric Heating in District Heating Systems

Experimental and chemical kinetics study on the flammability limit of CO2/C3H6 mixture working fluid at variable initial temperature

CO2/C3H6 mixture has a wide range of applications in refrigeration systems, heat pumps and Organic Rankine Cycle (ORC) systems, but its flammability safety needs to be considered. An experimental and computational investigation has been conducted on the determination of the flammability limits of CO2/C3H6 and on the understanding of associated limit phenomena in general. We tested the flammable zone of CO2/C3H6 at variable initial temperature. The accuracy of the prediction model of calculation adiabatic flame temperature were verified based on the experimental data. Meanwhile, a high-speed camera was used to record the process of CO2 suppression of flame propagation near flammability limit. Furthermore, based on kinetic numerical calculation, the effect of CO2 chemical and thermal on the flammability limit of C3H6 were analyzed. The results show that the thermal effect of CO2 is greater than that of chemical effect near the flammability limit. The theory of chemical reactivity inhibition reduces the yield and molar ratio of key radicals. The addition of CO2 can inhibit the activity of the dominant chain and increase the activity of the dominant chain termination reaction in the global combustion kinetics. Results of this study provide theoretical guidance for the safe application of CO2/C3H6 mixture.

Barocaloric effect in a Gay-Berne liquid crystal.

A coarse-grained molecular Monte Carlo simulation of the barocaloric effect in a model Gay-Berne liquid crystal is presented, following the so-called indirect approach wherein the caloric response is obtained from thermodynamic arguments applied to the simulated equilibrium relation between pressure, density, and temperature. From simulation, the magnitude of the effect across the isotropic-nematic and nematic-smectic phase transitions is estimated, together with the heat exchanged outside these transitions, in order to assess its potential for novel cooling and heat pumping applications. Under adiabatic conditions, pressure-induced phase transitions are predicted to result in a temperature variation of a few degrees Kelvin. As a by-product of the simulation, an approximate partial phase diagram of the Gay-Berne fluid under study is also obtained.

Cooling and heating innovations: exploring the diverse applications of heat pumps

Heat pumps are versatile and energy efficient devices that play a crucial role in modern heating, cooling, and refrigeration applications. This paper provides a concise overview of the diverse applications and benefits of heat pumps across residential, commercial, industrial, and transportation sectors. The paper discusses the principles of heat pump operation, emphasizing their capability to transfer heat from one location to another using thermodynamic processes. The work highlights key applications such as residential heating, ventilation, and air conditioning systems, commercial refrigeration, hot water heating, process cooling, and renewable energy integration. The energy efficiency and environmental benefits of heat pumps are also emphasized, showcasing their potential to reduce carbon emissions and contribute to sustainable energy practices. By understanding the broad scope of heat pump applications outlined in this work, researchers, engineers, policymakers, and industry stakeholders can gain insights into the significance of heat pump technology in advancing energy efficiency and addressing climate change challenges.

4E assessment of ejector-enhanced R290 heat pump cycle with a sub-cooler for cold region applications

Currently, the worldwide implementation of the Montreal Protocol has accelerated the development of low GWP refrigerants. R290 as an alternative refrigerant with low GWP has great advantages over R134a for heat pump applications. However, the use of expansion valve throttling in the heat pump cycle has a large irreversible loss. Therefore, an ejector-enhanced sub-cooler vapor injection heat pump cycle (ESVIC) with R290 for water heating is proposed in this paper to improve the energy efficiency by applying ejector. The thermodynamic models are established based on the 4E (energy, exergy, economic and environmental) analysis to compare its performance with the basic ejector-enhanced heat pump cycle (BEEC) and the sub-cooler vapor injection heat pump cycle (SVIC). The energy analysis results show that compared with the BEEC and SVIC, the volumetric heating capacity of the ESVIC is improved by 42.0 % and 21.2 %, and the heating coefficient of performance (COPh) of the ESVIC is improved by 27.6 % and 5.3 %, respectively. The exergy analysis results indicate that in BEEC, SVIC and ESVIC the exergy destruction of the expansion valves is 0.3 %, 13.5 % and 0.8 %, respectively. It is indicated that the adoption of ejector can significantly decrease the exergy destruction of expansion valves. Moreover, the environmental analysis results reveal that the carbon emissions of ESVIC is 21.5 % and 5.0 % lower than those of BEEC and SVIC, respectively. Besides, for the ESVIC system, the carbon emissions of the life cycle climate performance when using R290 are 9.9 % lower than that of using R134a, indicating that the R290 is a potential substitute for R134a in heat pump applications. The economic analysis results demonstrate that the unit exergy production costs of ESVIC is 21.6 % and 5.1 % lower than BEEC and SVIC, respectively.

Experimental investigation of thermophysical properties of propylene glycol based secondary fluids for ground source heat pumps and indirect refrigeration systems

More than 1.7 million ground source heat pumps are currently in operation in the European Union and a large majority of systems is using propylene glycol as secondary fluid. In this study the thermophysical properties of propylene glycol solutions with specific freezing points between −5 and −50 °C are investigated at low temperatures of interest for refrigeration and heat pump applications. The experimentally determined data are compared to a large number of data from the literature. It has been found that the freezing point results for all concentrations were in very good agreement with both Melinder (2010) and ASHRAE (2021) data. Moreover, the literature data and present study for concentrations above 40 wt-% have different density curve slopes compared to ASHRAE data. The viscosity results for concentrations below 40 wt-% were up to 21.5 % higher than ASHRAE data and the viscosity results for solutions with concentrations above 40 wt-% were lower by up to 9.7 %. The thermal conductivity results for concentrations above 40 wt-% were 10 % higher than ASHRAE (2021) data. The experimental specific heat capacity curves and other studies do not have the same linear trend as Melinder (2010) and ASHRAE (2021) data. This difference indicates that currently available ASHRAE data have been indirectly computed from thermal conductivity data or that they have not been fully experimentally validated. Finally, more studies are required to investigate concentrations below 25 wt-% to identify solutions with higher specific heat capacity than water that could be of interest for specific applications.