Heat Pump System Research Articles

Comprehensive performance evaluation on a transcritical CO2 ejector-expansion heat pump system

In order to reduce irreversible loss and recover expansion work for improving automotive thermal system performance, this study experimentally evaluated the performances and characteristics of a CO2 ejector-expansion (EJE) heat pump system for electric vehicles under typical cooling and heating operating modes, examined the adaptability of the EJE system with a fixed-dimension ejector, and compared the EJE system with the basic-regenerative (REG) and vapor-injection (INJ) systems in the − 30 °C to 50 °C range. The results show that the increasing electronic expansion valve (EXV) opening leads to an increasing entrainment ratio and a decreasing lift ratio in both two modes. The maximum COP is 2.00 at 35 °C/27 °C (out-cabin/in-cabin air temperature) and 1.85 at 0 °C/20 °C. The COP improvement reaches a maximum of 21.6 % and 31.0 % at the ejector design temperature of 45 °C, compared to the REG and INJ systems, respectively. However, the heating performance of the ejector designed at a high temperature significantly decreases as the ambient temperature goes down. At − 30 °C/20 °C, the EJE system performs worse with a COP degradation of 6.1 % compared to the REG system.

Optimization of Geothermal Heat Pump Systems for Sustainable Urban Development in Southeast Asia

This study examines the optimization of ground source heat pump (GSHP) systems and energy piles for sustainable urban development, focusing on Southeast Asia. GSHPs, which utilize geothermal energy for indoor HVAC needs, offer a sustainable alternative to traditional systems by utilizing consistent subsurface temperatures for heating and cooling. The study highlights the importance of understanding thermal movement within the soil, especially in soft marine clays prevalent in Southeast Asia, to improve GSHP system efficiency. Using a one-dimensional finite difference model, the study examines the effects of soil thermal conductivity and density on system performance. The results show that GSHP systems, especially when integrated with energy piles, significantly reduce electricity consumption and greenhouse gas emissions, underscoring their potential to mitigate the urban heat island effect in densely populated areas. Despite challenges posed by the region’s hot and humid climate, which could affect long-term effectiveness, the study highlights the need for further study, including field experiments and advanced modeling techniques, to optimize GSHP configurations and fully exploit geothermal energy in urban environments. The study’s insights into soil thermal dynamics and system design optimization contribute to advancing sustainable urban infrastructure development.

Multi-objective optimization and evaluation of the building-integrated photovoltaic/thermal-energy pile ground source heat pump system

Building integrated photovoltaic thermal (BIPV/T)-energy pile ground source heat pump (GSHP) system effectively maintains the soil thermal balance and improves the photovoltaic efficiency by recovering the waste solar heat from the BIPV/T collector to charge the ground. However, due to the strict carbon emission restriction and economic consideration for system application, multi-objective optimization should be implemented for the system design to further enhance its overall performance. Therefore, a complete optimization framework is proposed in this paper, which is based on the NSGA-II algorithm and functions by coupling TRNSYS and jEPlus+EA. Using the proposed optimization frame, a comprehensive optimization is conducted for the system, which optimizes five main design variables and adopts the total power consumption (TPC), life cycle climate performance (LCCP), and life cycle cost (LCC) to reflect the system's energetic, environmental, and economic performances. Seven optimization cases are included in the optimization project to identify design schemes that meet all possible proposed performance requirements for the system application. The optimized results show that the system optimization improves its performance greatly compared to the design condition, with a range of 38.03% ∼ 50.05% for the TPC, 24.52% ∼ 25.86% for LCCP, and 56.74% ∼ 61.66% for LCC. The energetic, environmental and economic performances of the system can be maximized by minimizing the TPC to 357.29 MWh in Case 1, LCCP to 596.15 tCO2 in Case 2, and LCC to 17.03 × 104 CNY in Case 3. By breaking down the objective functions, it is found that the heat pump unit holds paramount importance in all aspects of system performance, and the BIPV/T collector proves advantageous in enhancing the system's long-term performance due to its massive electricity generation. Accordingly, one important finding is that it is advisable to incorporate as many BIPV/T collectors as feasible while maintaining soil heat balance. The optimal energy pile number varies among the seven optimization cases, resulting in different heating capacities for the heat pump unit. Research outcomes in this work offer references for designing the energy-pile GSHP system to enhance the penetration of renewable energy into buildings.

Cost-Effective Control of Hybrid Ground Source Heat Pump (GSHP) System Coupled with District Heating

Hybrid ground source heat pump systems (GSHP) offer energy flexibility in operation. For hybrid GSHP systems coupled with district heating, limited studies investigated control strategies for reducing system energy costs from the perspective of building owners. This study proposed a cost-effective control strategy for a hybrid GSHP system integrated with district heating, investigating how power limits of district heating/GSHP, COP value for control (COPctrl), and control time horizon impact the system annual energy cost, CO2 emissions, and long-term borehole heat exchanger system performance. The simulations were performed using the dynamic building simulation tool IDA ICE 4.8. The results indicate that to realize both the energy cost savings and the long-term operation safety, it is essential to limit the heating power of district heating/GSHP and select an appropriate COPctrl. The control time horizon insignificantly affected the annual energy cost and long-term borehole heat exchanger system performance. The recommended COPctrl was 3.6, which is near the GSHP seasonal performance factor. Eventually, the cost-effective control reduced the system’s annual energy cost by 2.2% compared to the GSHP-prioritized control. However, the proposed control increased the CO2 emissions of the hybrid GSHP system due to the higher CO2 emissions from district heating.

Experimental and simulation study on the performance of a solar assisted multi-source heat pump drying system in Zhengzhou area

Zhengzhou of Henan province is an important agricultural production base. To improve the performance of the combined drying system and promote the development of agricultural products in Zhengzhou area, a solar assisted multi-source heat pump (SMSHP) drying system was proposed based on a traditional solar assisted air source heat pump (SASHP) drying system. The simulation models were developed for the air source heat pump (ASHP), SASHP, and SMSHP drying system respectively. The coefficient of performance, energy consumption and specific moisture extraction rate of the drying systems under different seasons were analyzed. Moreover, to validate the simulation models, an experimental platform of the combined drying system was designed and set up, and the experimental tests were carried out on the ASHP, SASHP, and SMSHP drying system. The simulation results revealed that the SMSHP drying system had lower energy consumption, higher coefficient of performance and specific moisture extraction rate than that of the SASHP drying system. The most significant performance enhancement was observed in spring, where the SMSHP system demonstrated a 22.69 % increase in coefficient of performance, a 19.2 % increase in specific moisture extraction rate, and a 16.11 % decrease in energy consumption compared to the SASHP drying system. Comparing with the experimental data, the simulation model errors of the energy consumption, coefficient of performance, and specific moisture extraction rate of the SMSHP drying system were 3.19 %, 0.28 %, and 2.27 %, respectively. Therefore, the accuracy of the simulation results was confirmed. To better calculate the heat provided by the solar collection system, the concept of the tank heating guarantee rate was first introduced, and it was found to be more scientific than the solar energy guarantee rate. At last, The economic and environmental benefit of the drying systems were analyzed during the life cycle, and the SMSHP drying system exhibited better comprehensive performance than the SASHP drying system. This paper provides valuable insights into the comprehensive utilization of solar energy and the control logic of solar assisted air source heat pump combined drying system under different seasons.

Global selection appraisal study for heat pump system of electric vehicle based on energetic, economic, and environmental analysis

The development of electric vehicles (EVs) exhibits rapid and remarkable progress nowadays, serving as a crucial route to accomplish the target of mitigating greenhouse gas emissions. As an integral part of the thermal management system oriented toward electric vehicles, the heat pump air conditioning system for electric vehicles is the result of a comprehensive choice that trades off the cooling and heating performance, environmental performance and economic cost. Particularly, different regions around the world suffer varying cooling and heating challenges due to the complicated climatic characteristics. Thus the most suitable refrigerant and system cycle structure may differ. This paper focuses on evaluating both the refrigerants and cycle structures to screen the most suitable choice. According to the climate conditions of different cities, the annual energy consumption, life cycle climate performance, and economic cost of the basic system (Base), two-stage compression system (TSC,IC), and vapor injection (VI) system with CO2, R134a, and R1234yf refrigerants respectively, are quantitatively analyzed and evaluated. Subsequently, through comparative analysis, a comprehensive selection map for heat pump systems in electric vehicles worldwide is developed and the most suitable heat pump air conditioning system for each cites is determined. The results can provide a selection reference and decision-making for the air conditioning system of electric vehicles from regional considerations. It was found that the CO2 HPACVI was recommended for cold regions to meet both environmental and economic requirements. In warm region, the R1234yf HPACBase system was recommended to be used. For regions transitioning from cold to warm climates, the R1234yf HPACVI system was suggested. In hot region, the R1234yf AC system was recommended.

Performance evaluation of underground thermal storage integrated dual-source heat pump systems

The increasing demand for electricity stresses the existing electric grids. Buildings consume 73% of all U.S. electricity and are responsible for 30% of U.S. greenhouse gas emissions. Integrating thermal energy storage (TES) in building heating/cooling systems, which consume considerable electricity, can mitigate the challenges to electric grids. This study reports on a novel thermal energy storage device integrated heat pump system to reshape the building electricity demand profile while maintaining thermal comfort. The annual performance of the proposed system has been evaluated through a dynamic system simulation with high fidelity in the Modelica platform. The dynamic model of the novel hybrid component named ‘dual purpose underground thermal battery’ was developed and validated. It was then incorporated into the system model. Given a time-of-use tariff, a rule-based control strategy was designed to shift the electric demand and switch the heat pump source for a typical single-family house in different climate zones of the United States. The system performance of the new TES-integrated dual-source heat pump was compared with that of a conventional air-source heat pump system. The results indicate that the proposed system can reduce the annual HVAC electricity cost by up to 52% while saving 45.2% on electricity consumption. In the Northern areas, the annual peak load of the HVAC system can be reduced by 64.9%. However, this reduction is less in the Southern areas as the system’s higher efficiency in winter dominates the overall energy-saving potential.

Electrification of residential and commercial buildings integrated with hybrid renewable energy systems: A techno-economic analysis

This study assesses the techno-economic viability of city-scale building electrification, comparing a base case scenario with conventional HVAC systems to an electrified scenario with heat pump systems. EnergyPlus simulations reveal a 28% increase in annual electricity consumption in the electrified scenario, shifting the peak consumption from summer to winter with a 49% surge. Hybrid renewable energy systems (HRESs) are then evaluated using HOMER Pro, considering renewable penetration from 0% to 100%. The net present cost (NPC) of the base case and electrified scenarios range from $10.34 to $24.38 billion and $8.79 to $50.31 billion, respectively. Sensitivity analysis explores the impact of carbon tax, fuel price, and renewable subsidies on HRESs in the electrified scenario. Results indicate that applying a carbon tax up to $120/tonne of CO2 increases the optimum renewable fraction from 5% to 26%, while higher fuel prices and renewable subsidies further enhance the feasibility by shifting the optimum renewable fraction to 60% and 54%, respectively, demonstrating the interplay between economic factors and renewable integration in electrified urban contexts.

Research on quantitative assessment method of multi-time scale regulation capability of heat source tower heat pump for grid regulation demand

Abstract In the context of power saving, the increasing energy consumption of air conditioning and the limited energy demand has become an urgent problem. Therefore, according to the feature that the accuracy of grid load forecasting improves gradually with the shortening of time scale and the flexible intra-day regulation characteristics of heat pump units, a multi-timescale regulation model of a heat pump system is developed. Aiming for the lowest operating costs and best performance, three system scheduling scenarios were developed: day-ahead, mid-day, and real-time, respectively. Through the coordination of multiple time scales, it is ensured that the output of the system meets the requirements of the grid. Taking the Wuhan area with four heat source tower units as an example, a simulation experiment comparing the performance of the multi-timescale scheduling method with the day-ahead scheduling method was conducted in the same system, which showed that compared to the day-ahead scheduling method, the multi-timescale scheduling method can reduce the system energy consumption by 13.1% and improve the system performance factor by 12.4%.

Performance study of air source heat pump system applying two types of distributors

Abstract Air source heat pump heating is the most effective way to solve heating problems, and gas-liquid two-phase refrigerant in the uniform distribution and pressure drop problem of finned heat exchanger branch tube are the main factors affecting the heat transfer ability. To study the effects of two types of manifolds applied in air-source heat pumps on the system performance and pressure drop when the system is in operation, an engineering-based test system for the actual operation of air-source heat pumps was constructed. The study shows that the difference between the venturi distributor and the rectifier nozzle distributor in the impact on the system performance is relatively small, while the rectifier nozzle distributor compared to the venturi distributor in the pressure drop on the effect is obvious. The evaporator pressure drop will be transferred to the distributor part of the reduction of pressure drop losses in the evaporator on the impact of heat transfer performance. This paper summarizes the performance study of two types of distributors applied to air source heat pumps, which provides a meaningful reference for similar studies.

Study on a high-performance heat-driven thermoacoustic heat pump

Heat-driven thermoacoustic heat pump (HDTAHP) is a promising heating technology with high reliability. Current HDTAHP face the challenge of being relatively inefficient and not capable of further improvement. This study aims to address this problem by introducing an innovative energy matching tube (EMT) into the conventional thermoacoustic heat pump to realize more reasonable acoustic power matching between the engine and heat pump units, further greatly improve the system efficiency. The key reason for the limited performance of HDTAHP is first explored and lies in the constraints that exist between temperatures. Through Sage modeling and calculations, the comparison between the proposed system and the conventional thermoacoustic heat pump system verifies that the introduced EMT can dramatically increase the system performance. At a heating temperature of 700 K, the overall Coefficient of Performance (COPh) of the system is improved by 43.7 % compared to the conventional thermoacoustic heat pump without EMT. The performance under different working temperatures is then investigated and demonstrated a strong adaptability of the proposed system under different heating temperature objectives. The comparison with existing thermoacoustic-Stirling heat pumps further indicate the high-performance improvement by EMT and promising potential of thermoacoustic heat pumps in domestic heating domain.

To Zone or Not to Zone When Upgrading a Wet Heating System from Gas to Heat Pump for Maximum Climate Impact: A UK View

Domestic heating systems across northern Europe are responsible for a substantial fraction of their countries’ carbon footprints. In the UK, the vast majority of home space heating is via natural gas boilers with ‘wet’ hydronic radiator systems. Most of those use TRVs (thermostatic radiator valves) for micro-zoning, to avoid overheating, improve comfort and save energy. To meet Net Zero targets, 20 million such UK gas systems may be retrofitted with heat pumps. Heat pump system designers and installers are cautious about retaining TRVs in such systems in part because of worries that TRV temperature setbacks that lower heat demand may raise heat pump electricity demand in a “bad setback effect”, thus wasting energy. This paper presents a new view of heat pump control and provides the first exploration of this issue through the development of a simple physics-based model. The model tests an installation industry claim about the negative effect of TRVs, and finds that though real it should not apply to typical UK retrofits with weather compensation. The energy efficiency benefits of TRVs for older and partly occupied homes, and to keep bedrooms cooler, remain valid. Comfort-seeking householders and installers should know that setting ‘stiff’ temperature regulation may invoke the bad setback effect and cost dearly in energy and carbon footprint.

Heat transfer performance and optimal design of shallow coaxial ground heat exchangers

The heat transfer performance of coaxial ground heat exchangers (GHEs) can be strongly influenced by their structure parameters and improved by the optimal design. To improve the heat transfer efficiency, this study proposed a coaxial GHE with large pipe diameters and thick inner pipe insulation for shallow ground source heat pump systems, which had heat transfer rates of more than 120 W/m in 12 h during both heat injection and extraction experiments. The validated three-dimensional CFD model was used to numerically investigate the influence of various structural parameters on the heat transfer efficiency considering the thermal short-circuiting effect. The thermal short-circuiting effect could be alleviated with thicker inner pipe insulation, while the decreasing annular space occupied by the insulation would reduce the borehole heat transfer rates, thus, an inner pipe with a 35 mm-thick insulation obtained the optimal heat transfer rate. The inner pipe diameter increase contributed to a more severe thermal short-circuiting and poor heat transfer performance. Increasing the outer pipe diameter improved the heat transfer efficiency and an optimal outer pipe diameter of 155 mm existed when considering the heat transfer performance and its efficiency. Therefore, increasing the inner pipe insulation thickness and outer pipe diameters can be regarded as an efficient method to improve the thermal performance of coaxial GHEs.

Comparative analysis of energy efficiency for three heating and cooling supply schemes in a region with hot summers and cold winters in a chemical industrial park

Building energy consumption in China accounts for 45% of the total national energy consumption, with air conditioning energy consumption representing approximately two-thirds of that. Therefore, energy efficiency in buildings is of utmost importance. This study focuses on a chemical industrial park located along the Fujiang River and compares three heating and cooling supply schemes: the river water source heat pump system, which utilizes river water as the heat source and heat sink; the water cooling unit and boiler system, which uses water-cooled electric compression chillers for cooling and an oil-fired boiler system for heating; and the split air conditioning and gas water heater scheme, which relies on refrigerants such as fluorine-containing compounds for cooling and a gas water heater for heating. By calculating the energy consumption of the above three schemes and conducting a comparative analysis, it is found that the river water source heat pump system exhibits significantly higher energy efficiency throughout the year compared to the water cooling unit and boiler system and the split air conditioning and gas water heater scheme. This highlights the notable energy efficiency advantage of the river water source heat pump system.

Experimental study on performance and compressor characteristics of transcritical CO2 heat pump system

The accelerated advancement of the economy has led to a heightened focus on the issue of energy conservation and reduced consumption within heat pump systems. An experimental investigation was conducted to analyze the impact of discharge pressure and outlet water temperature on the performance of the transcritical CO2 single-stage heat pump system and vapor injection heat pump system. The experimental results show that the heating capacity first increases and then tends to be constant or even slightly decreases with the increase of discharge pressure in single-stage heat pump system. When subjected to the same operating conditions, the coefficient of performance of vapor injection heat pump system exceeds that of single-stage heat pump system. Compared with single-stage heat pump system, the performance coefficient of vapor injection heat pump system has increased by an average of 6.21% and the heating capacity has increased by an average of 9.54%. Additionally, the volumetric efficiency of the compressor substantially decreases when producing high-temperature hot water. Semi-empirical models for evaluating the compressor performance in both single-stage heat pump system and vapor injection pump system are established through the parameterization of the volumetric efficiency with respect to the pressure ratio and compressor speed. These models effectively forecast compressor performance.

Investigation by monitoring and numerical simulation of the performance of a ground-source heat pump system with and without regeneration in different configurations and operating modes

This paper is focused on restoring the thermal imbalance of the ground caused by conventional GCHP systems through seasonal and continuous regeneration using a geothermal-solar facility with a vertical GCHP combined with photovoltaic (PV) and PV-thermal (PV/T) panels in a heating-dominated climate, located in Timisoara, Romania. Therefore, the performance of a GCHP system integrated into the heat supply system of an experimental office, operating in single and double U-tube borehole heat exchanger (BHE) configuration, with regeneration in the summer months by injecting heat into the ground, is experimentally evaluated. Exploratory research has indicated an increase in the average temperature of the working fluid by 2.4% and an improvement in system performance when using heat injection (seasonal performance factor (SPF) by 3.5% and 6.6% higher and CO2 emission by 1.2% and 3.8% lower in the single and double U-tube BHE configurations, respectively). Additionally, numerical simulations with Polysun software are also used to test the performances of the hybrid GCHP-PV/T system with continuous regeneration throughout the year by PV/T panels and of a conventional GCHP integrated into the heating and DHW production systems for a single-family building. The simulations were experimentally validated through measurements on the geothermal-solar facility for heating and DHW production in the experimental office. The simulation results show that the SPFHP-PV/T values of the hybrid GCHP-PV/T system in the single and double U-tube BHE configurations, equal to 6.00 and 5.79, respectively, are 32% and 34% higher than the SPFHP corresponding to the conventional GCHP system, equal to 4.56 and 4.32, respectively. In addition, the SPFHP-PV/T in the single U-tube BHE configuration is 3.6% higher than in the double U-tube BHE configuration for the GCHP combined with the PV/T array (31.8 m2).

A visual defrosting control method for air source heat pump system based on machine vision

Frosting on the coil of the air source heat pump (ASHP) significantly affects its performance. An accurate defrosting control method is essential for the ASHP unit. Here, a novel visual defrosting control method based on machine vision is proposed. The frosting degree of the unit is accurately obtained by frosting image analysis with digital image processing to achieve on-demand defrosting. Fractal dimension, frosting degree coefficient, and single-circuit frosting evenness value are introduced to evaluate the frosting degree of coil images. The image acquisition point is determined with confusion matrix. Based on this, the logical framework of the proposed method is constructed, in which the defrosting starting and termination conditions are set as the moment when the frosting degree coefficient is equal to 0.9 and 0, respectively. A multi-variable model of fractal dimension is fitted, which improves the generalization of the proposed method. Finally, comparative experiments are conducted between the proposed method and the traditional temperature-time (T-T) method in typical cases. The results indicate that the COP and total heating capacity increase by 11.8 %, 10.6 % and 7.4 %, 6.6 %, respectively, and the defrosting frequency is reduced by 75 %, which demonstrates dominant advantages of the proposed method in accuracy, energy-saving, and indoor thermal comfort management.

Experimental and simulation study on a new gradient waste heat recovery system with a wider application range

This paper proposes a cascade sustainable waste heat recovery technology with low power consumption and the ability to recover waste heat in a wider temperature range, namely, the Absorption-Compression Coupled Heat Pump System. Experiments and simulations were conducted to reveal the effects of different factors on system performance. The results demonstrate that the coefficient of performance and exergy efficiency of the new system increase with an increase in generator outlet water temperature. The coefficient of performance of the new system decreases with an increase in supply heating water inlet water temperature, while the exergy efficiency increases with an increase in supply heating water inlet water temperature. As intermediate temperature increases, the coefficient of performance of new system shows a trend of first increasing and then decreasing, the total heating shows a trend of first increasing and then stabilizing, and the exergy efficiency shows a decreasing trend. Compared to single-effect absorption heat pump system, the new system can recover heat at lower heat source temperatures, and compared to compression heat pump system, it can reduce electricity consumption. Finally, by comparing with experimental data, it was found that the proposed model has good reliability, with a comprehensive error within ±7 %.

Performance Analysis of a New Cascade Waste Heat Recovery System With a Wider Range of Applications

Abstract This paper proposes a new cascade waste heat recovery system with low power consumption and the ability to recover waste heat in a wider temperature range, namely, the absorption–compression coupled heat pump system. Simulations were conducted to reveal the effects of generator inlet and outlet water temperatures, supply hot water inlet and outlet water temperatures, and intermediate temperature on system performance. The results demonstrate that the coefficient of performance and exergy efficiency of the new system increase with an increase in generator outlet water temperature. The coefficient of performance and exergy efficiency of the new system decrease with an increase in supply heating water outlet water temperature. The coefficient of performance of the new system decreases with an increase in supply heating water inlet water temperature, while the exergy efficiency increases with an increase in supply heating water inlet water temperature. As the intermediate temperature increases, the coefficient of performance of the new system shows an increasing trend, and the exergy efficiency shows a decreasing trend. In addition, compared to the single-effect absorption heat pump system, the new system can recover heat at lower heat source temperatures, and compared to the compression heat pump system, it can reduce electricity consumption.

Analysis of a domestic air heat pump integrated with an air-geothermal heat exchanger in real operating conditions: The case study of a single-family building

The work presents an in-depth analysis of the real performance of an air heat pump unit installed in a single-family building and quantitatively determines the reduction in energy consumption achieved by the integration of an air-geothermal heat exchanger. The case study is a single-family building located in northern Italy. It is equipped with a heating, ventilation, and air conditioning (HVAC) system consisting of an air-geothermal heat exchanger coupled with a compact air source heat pump supported by an electric heater installed in a thermal storage and devoted to cover the peak demand. The main objectives of the work are to analyse the HVAC performance under real operating conditions compared to the design values and to highlight the existence of factors that affect this performance. The analysis was carried out by developing an accurate model of the heat pump system based on the real operating data acquired by the on-board monitoring system. The results confirmed the influence of the air-inlet conditions (temperature and humidity) and of the control strategy (more specifically, the ventilation flow rate) on the unit performance in both the heating and cooling modes. In particular, the analysis showed that minimising indoor ventilation in the case of low outdoor temperatures is of paramount relevance not only because it reduces the indoor heat demand, but also because it maintains the unit efficiency close to the design values. Furthermore, the installation of a geothermal heat exchanger guarantees a reduction in electricity consumption of approximately 30%.