Performance Of Heat Pump System Research Articles

Viability of an Open-Loop Heat Pump Drying System in South African Climatic Conditions

Drying agricultural produce consumes a considerable amount of energy. As an energy-efficient system, a heat pump can improve the energy efficiency of the drying process and hence reduce the energy consumption, especially in South Africa, where both sub-tropical and temperate weather conditions dominate. The objective of this research is to experimentally investigate the impacts of weather conditions on the operational conditions and thermal performance of an open-loop air-source heat pump drying system. The experimental investigation was conducted in a climate chamber where the climate conditions were simulated from −10 °C to 20 °C with an interval of 10 °C for the typical temperature range of the harvesting season in South Africa. The findings indicate that ambient temperatures have a significant impact on both the operating conditions and thermal performance of an open-loop heat pump system; the change in ambient temperatures from −10 °C to 20 °C leads to a 141.6% improvement in the suction pressure, a 214.2% increase in the discharge pressure, and 30.1% increase in the compression ratio, as well as a consequent increase of 130.6% in the refrigerant mass flow rate (from 0.0067 to 0.0155 kg/s), resulting in a corresponding increase in the coefficient of performance (COP) of the heat pump drying system by about 42.1%. Therefore, this study suggests that, while using an open-loop air-source heat pump drying system utilising R134a refrigerant is feasible in South Africa, it may be practically limited to regions with warm climates or during warmer seasons.

Research on refrigerant charge determination under different compressor speed and its effects on the performance of transcritical CO2 air-conditioning heat pump system in electric vehicle

CO2 is assumed to be one of the most potential refrigerant alternatives for electric vehicles for its excellent properties. However, the charge determination of CO2 in current studies remain controversial. In this study, a transcritical CO2 air-conditioning heat pump system was established and experimentally tested to analyze the optimal charge amount. Based on two conflicting methods of charge determining proposed by the preceding research, this paper substantiated the existing controversy and subsequently proposed a more comprehensive method. The effects of different refrigerant charge on the system characteristics were investigated. The influence of the compressor speed on the optimal refrigerant charge and system characteristics was also analyzed. It was found that the optimal charge plateau occurred from refrigerant of 500 g–580 g at the compressor speed of 3000 r·min−1. However, the optimal charge declined with the increment of compressor speed from 3000 r·min−1 to 4500 r·min−1. Among three models, Hughmark's model was proved to be the most appropriate for the theoretical calculation of optimal charge within an error of 6.09%. Further study illustrates that refrigerant mass in high-pressure pipe and intermediate heat exchanger/accumulator (IHX/A) accounted for the main proportion about 52.6%–55.14%.

Enhancement of the Power-to-Heat Energy Conversion Process of a Thermal Energy Storage Cycle through the use of a Thermoelectric Heat Pump

The principal strategy for achieving a neutral climate entails enhancing the share of renewable energies in the energy mix, in conjunction with promoting innovation in efficient technologies. Thermal energy storage systems have the potential to efficiently handle the intermittent nature of renewable energy sources. Furthermore, these systems can effectively handle shifts in both heat and electrical demand. Thus, efficient power-to-heat technologies are needed to boost thermal energy storage. This manuscript explores the potential of utilising a thermoelectric heat pump system in conjunction with electric resistances for charging a thermal energy storage. In order to achieve elevated temperatures, the thermoelectric system integrates thermoelectric heat pump blocks in a two-stage configuration. Air has been employed as a heat transfer medium for sensible heat storage. Higher airflow rates improve the performance of thermoelectric heat pump system. Moreover, its impact on the optimal voltage supply of the thermoelectric system is observed when it is combined with an electric resistance to achieve elevated temperatures. In comparison to the basic charging process that solely relies on the electric resistance of a thermal energy storage at 120 °C, a significant 30 % increase in power-to-heat energy conversion has been achieved by including the thermoelectric heat pump system. In fact, it efficiently elevates the temperature from the initial ambient temperature of 25 °C to a remarkable 113.1 °C, achieving a coefficient of performance of 1.35 with an airflow rate of 23 m3/h. Therefore, the use of this technology to enhance a complete process of storing excess renewable energy in the form of heat for subsequent use in both heat and electricity through a combined heat and power cycle is demonstrated.

Performance improvement of vapor compression heat pump with superhydrophobic finned-tube evaporator

Residential buildings frequently employ vapor compression heat pump for winter heating. Its advantages over electric and coal-fired heating include great energy effectiveness and low emissions. However, the outside evaporator's frosting negatively impacts the performance of heat pump systems in low-temperature and high-humidity environments. To ameliorate the frosting problem, a finned-tube heat exchanger with superhydrophobic surface wettability prepared by the wet chemical etching method was applied to a vapor compression heat pump in this study. The performance of a commercial evaporator unit and the superhydrophobic evaporator unit under five frosting conditions was investigated through experiments. The maximum heat transfer rate of the superhydrophobic heat exchanger unit was increased by 17%–44% compared with that of the commercial unit. The COP improvement of the superhydrophobic unit reached 11.2%, 14.3%, 18.8%, 20.9% and 31.9%, respectively. The total mass of frost formation of the superhydrophobic unit experienced a 59.2%–76.3% decrease. The operation time of the superhydrophobic evaporator unit before defrosting determination was significantly extended, reducing the number of defrosting times during a long-time operation. This study proves the advantages of superhydrophobic finned-tube evaporators used in heat pumps in terms of heat transfer performance and frost suppression performance, promoting the application of superhydrophobic finned-tube evaporators in heat pump air-conditioning systems.

A predictive model of long-term performance assessment of Ground Source Heat Pump (GSHP) systems in Japanese regions

This study predicts the long-term performance of ground source heat pump (GSHP) systems in different climatic regions of Japan. The research model calculates the dynamic heat load of a typical Japanese building in various cities using their meteorological data, considering different schedules for heating and cooling seasons. The model aims to analyze the performance degradation of the GSHP system after 15 years of operation due to the imbalanced heating and cooling loads. Additionally, it compares ground temperature changes, coefficient of performance (COP) variations, and annual operation times based on different thermal imbalance ratios (TIR) for the case studies. The results demonstrate that TIR serves as an appropriate factor for comprehensively evaluating system aspects, including GHE length, ground temperature, and COP degradation, across various climates. It was found that ground temperatures increased by a maximum of up to 4 °C in cooling-dominated regions due to higher thermal imbalance accumulative loads, while it decreased to around 1 °C in heating-dominated regions. The model also revealed that the COP decreased in the dominant operation for each case study due to induced thermal imbalance and soil temperature change. The thermal imbalance caused the system cooling COP decreased by 0.015/year in the worst case (Okinawa City) and 0.006/year in average for other cases.

Development and performance evaluation of intelligent algorithm for optimal control of a hybrid heat pump system during the cooling season

The purpose of this study is to develop a control method for a hybrid heat pump system based on an artificial neural network (ANN) to reduce energy use and create a more comfortable thermal environment. The proposed optimal control method uses an ANN-based predictive model to predict the heat storage tank and indoor temperature during the cooling period and controls the flow rate of the circulation pump on the heat source and load side of the system. The performance of the predictive model for the heat storage tank temperature (R2 = 0.9988; coefficient of variation of the root mean square error [CV(RMSE)] = 1.06 %; normalized mean bias error [NMBE] = 0.16 %; and mean absolute error [MAE] = 0.09℃) and the indoor temperature (R2 = 0.9893; CV(RMSE) = 1.66 %; NMBE = 0.16 %; and MAE = 0.15℃) was excellent. The temperature control of the heat storage tank using the optimal algorithm exhibited an improvement of 18.39 % for the CV(RMSE), 3.10 % for the NMBE, and 1.31 °C for MAE compared with rule-based. For the indoor temperature, the optimal algorithm improved the CV(RMSE) by 1.30 %, NMBE by 0.42 %, and MAE by 0.29℃ compared to rule-based. The energy use was reduced by 52.85 % for the entire system using the optimal control method compared with the existing control strategy under similar outdoor conditions. Using the proposed control method, it is thus possible to improve thermal comfort and reduce carbon emissions in the building sector by improving the control and energy performance of hybrid heat pump systems.

Research on the influence of underground seepage on regional heat accumulation of ground source heat pump heat exchanger

During the long-term operation of the ground source heat pump system, the temperature imbalance of the rock and soil around the heat exchanger results in the deterioration of the performance of the ground source heat pump system. Through previous projects, it has been found that in the area of underground seepage speed, it is helpful to alleviate the heat or cold accumulation of rock and soil mass around the ground source heat pump system. Based on this, two test sites in Zhangqiu District, Jinan City, Shandong Province, were selected to conduct pumping and irrigation tests to change the underground seepage velocity, determine the flow velocity through isotopic tracer tests, obtain soil thermal physical parameters and simulate heat accumulation through thermal response tests, and establish long-term numerical simulation through Opengeosys to synthesize the conclusion that the seepage velocity increases. The more efficiently the temperature of the rock and soil mass is restored to the initial condition, the more efficiently the heat exchange performance of the buried pipe heat exchanger and the surrounding rock and soil mass is maintained, and the efficient operation of the ground source heat pump system in winter and summer is ensured.

Seasonal performance of an energy pile heat pump system and prediction of building thermal load

Ground source heat pumps (GSHPs) are widely used for cooling and heating buildings, which can reduce the burning of fossil fuels and contribute to the reduction of carbon emissions. Energy piles, as underground heat exchangers, have advantages over conventional buried borehole pipes, saving drilling costs and accelerating the construction period. However, few studies have focused on the actual operation of an energy pile heat pump system, and the prediction of the building thermal load can reduce the instrument arrangement and test recording time. Hence, a heat pump system was built in this study to serve a single lab using energy piles with a cap structure as the energy supply, and summer cooling and winter heating tests were conducted. Under the test conditions in this study, the GSHP had a faster start-up speed and less power consumption than the air source heat pump (ASHP), and the average coefficient of performance (COP) of the energy pile heat pump system was 3.78, which is 40.0 ∼ 51.2% higher than that of the corresponding ASHP (the normative reference COP is 2.50 ∼ 2.70). The methods of higher-order surface fitting and an artificial neural network model were both suitable for predicting building thermal load.

Assessment of exergy-based performance and sustainability indicators of a sewagewater source heat pump system

Assessment of exergy-based performance and sustainability indicators of a sewagewater source heat pump system

Assessment of exergy-based performance and sustainability indicators of a sewage water source heat pump system

Assessment of exergy-based performance and sustainability indicators of a sewage water source heat pump system

Solar photovoltaic/thermal (PVT) technology collectors and free cooling in ground source heat pump systems

Ground source heat pump (GSHP) systems offer a low carbon heating and cooling solution for the decarbonization of buildings. As global temperatures rise, the cooling requirements of buildings will grow, even in regions where cooling systems have been historically uncommon due to their colder climate, such as Sweden. The combination of free cooling (FC) with GSHPs seems like a natural way to meet the increasing cooling needs, since the heat extracted from the building during the summer months can be injected into the ground to potentially regenerate the borehole field and enhance heat pump performance. However, a technology that is generally integrated with GSHP systems for borehole regeneration are photovoltaic/thermal collectors. This study investigates the performance of a ground source heat pump system with free cooling for a multi-family building in Stockholm, Sweden, and the interference on the free cooling capabilities of the system when photovoltaic/thermal collectors are present. The results demonstrate that the integration of PVT and FC not only maintains the cooling supply but also enhances heat pump performance, all the while reducing borehole length and land area requirements.

Life cycle energy, economic, and environmental analysis for the direct-expansion photovoltaic-thermal heat pump system in China

The photovoltaic-thermal heat pump system, which integrates multiple energy technologies, can meet the building's diverse energy needs. This study evaluates the energy, economic, and environmental performance of a direct-expansion photovoltaic-thermal heat pump system for domestic hot water supply through a life cycle approach. The annual thermal and electrical outputs are estimated. On this basis, economic analysis is conducted with indicators of life cycle cost, levelized cost of heat, and investment payback time. Moreover, a detailed life cycle assessment is performed to evaluate the environmental impact, following three methods: cumulative energy demand, IPCC 2021 global warming potential, and ReCiPe 2016 Endpoint. Results show that the proposed system is highly effective, with a 9.67% increase in annual electricity output over traditional photovoltaic systems and a self-sufficient rate of 88.21%. The system also has a significantly lower environmental impact than conventional heating systems, with a life cycle environmental impact of only 3–5% compared to electric boilers. The environmental payback time ranges from 0.33 to 5.18 years, according to the three methods used in the study, which is shorter than the investment payback time of 2.49–5.21 years, suggesting the need for cost reduction for future promotion. Moreover, electricity consumption during the operation stage accounts for 32–52% of the total environmental impacts. Thus, decarbonization of the power grid and improvements in system efficiency can significantly reduce the system's environmental footprint. Overall, the photovoltaic-thermal heat pump system is an intriguing strategy for building energy conservation, offering high energy efficiency, acceptable costs, and low environmental impact.

Heating performance and optimization of ice source heat pump system with supercooled water

An ice source heat pump, which can extract latent heat from the icy water, has a better system performance in low-temperature areas. Cosiderring the disadvantange of ice source heat pump using static ice-making method or scraping ice-making method, an ice source heat pump based on supercooled water method is proposed and its heating performance is measured in the study. Then, a system model is developed to study and optimize the system performance. It concludes: (1) The ice source heat pump system COP decreases with the increasing supercooled water flow rate. The system COP reaches about 3.3 when the condensation temperature is 42 °C. (2) The proper range of supercooled degrees in ice source heat pump is mainly from 2 °C to 3 °C. (3) The rank of local exergy loss rate in ice source heat pump system is listed as follows: compressor > condenser > valve > evaporator > pump. The exergy loss proportion of compressor and condenser is about 65%. (4) The system COP and exergy efficiency of ice source heat pump increase approximately linearly with the increase of capacity. The optimal supercooled degree mainly depends on the rate of change of the compressor power and the pump power concerning the supercooled degree. This work provides insight into the performance of ice source heat pump with supercooled method and guides the design of ice source heat pump system.

An experimental study on energy-storage based defrosting performance of an air source heat pump system with a micro-channel heat exchanger as outdoor coil

For air source heat pump systems used for indoor space heating in cold areas, when the surface temperature of the outdoor coil is lower than the dew point temperature of the air and the freezing point temperature of the water, the metal surface of the outdoor coil will frost. The formed frost layer affects the normal operation performance of the air source heat pump system and reduces the energy efficiency, so the periodic reverse cycle defrosting becomes very necessary. Since the studies on defrosting performances of air source heat pump systems with micro-channel heat exchangers as outdoor coils were insufficient, especially for those coupled with energy storage technology, this study investigates the energy-storage based heating and defrosting performances of an air source heat pump system with a micro-channel heat exchanger as its outdoor coil. Based on the experimental results, the defrosting efficiency of the experimental air source heat pump system was quantitatively evaluated. The results show that the defrosting energy amount from the compressor decreased by 55.8%, and the defrosting energy consumption for evaporating frost and heating ambient air decreased by 25.0% and 64.9% when the energy storage unit was employed. Besides, the defrosting period decreased by 43.9%, and the defrosting efficiency increased by 7.72% when the energy storage unit was employed. The results of this study are meaningful for the optimization of the air source heat pump system used in cold regions.

Analysis of the performance and operation of a photovoltaic-battery heat pump system based on field measurement data

Photovoltaic-heat pump (PV-HP) combinations with battery and energy management systems are becoming increasingly popular due to their ability to increase the autarchy and utilization of self-generated PV electricity. This trend is driven by the ongoing electrification of the heating sector and the growing disparity between growing electricity costs and reducing feed-in tariffs in Germany. Smart control strategies can be employed to control and optimize the heat pump operation to achieve higher self-consumption of PV electricity. This work presents the evaluation results of a smart-grid ready controlled PV-HP-battery system in a single-family household in Germany, using 1-minute-high-resolution field measurement data. Within 12 months evaluation period, a self-consumption of 43 % was determined. The solar fraction of the HP amounts to 36 %, enabled also due to higher set temperatures for space heating and domestic hot water production. Accordingly, the SPF decreases by 4.0 % the space heating and by 5.7 % in the domestic hot water mode. The combined seasonal performance factor for the heat pump system increases from 4.2 to 6.7, when only considering the electricity taken from the grid and disregarding the locally generated electricity supplied from photovoltaic and battery units.

Numerical investigation on operational performance of seawater-source heat pump system coupled with capillary-box heat exchangers

In view of the energy-saving potential of the seawater-source heat pump system (SWHPs) coupled with capillary-box heat exchangers (CBHEs), the operational performances of this system were investigated on various temporal scales, including annually, monthly, and daily. An overall system numerical model was developed by integrating the CBHEs, heat pump and dynamic building loads model. The results indicated that the annual COPh was 3.4. In the coldest month, the monthly COPh was 3.3, and the daily COPh was mainly determined by the daily heating demand. The corresponding annual COPc was 4.8. In the hottest month, the monthly COPc was up to 5.0, and the daily COPc was more sensitive to seawater temperature fluctuations. On a typical summer day, the operating time of the heat pump with an hourly capacity ratio above 50 % was about 2.7 times that of a typical winter day. Due to the effective removal of the heat discharged from the CBHEs in the seabed, the impact of heat accumulation on the performance of this system can be neglected in the long-term operation. This study provided practical implications for the design and application of the CBHEs-coupled SWHPs.

New formulations and experimental validation of non-stationary convolutions for the fast simulation of time-variant flowrates in ground heat exchangers

Flowrate control can have a significant positive impact on the thermal performance and economic profitability of ground-source heat pump systems. Including dynamic advective processes in the design phase, however, remains a challenging task, as few computationally efficient modeling tools allow for their adequate and accurate representation. The present work addresses this issue by presenting new formulations of non-stationary convolutions, an efficient simulation algorithm that relies on the theory of linear time-variant systems for predicting the thermal response of a ground heat exchanger to both dynamic heat loads and flow rates. First, the new original formulations are presented, which include (1) a simple time-domain expression and (2) a fast frequency-domain expression. Then, the efficiency and validity of the new formulations are verified using experimental multi-flowrate thermal response tests involving dynamic circulation, pumping and bleed flow rates in closed-loop and standing column well ground heat exchangers. Results show that the new formulations can reproduce the outlet fluid temperature of both experimental test cases with good accuracy ( MAE = 0.06 ∘ C and 0.26 ∘ C , respectively). At last, the high efficiency of the new frequency-domain expression is demonstrated, with the computing times ( 0.04 s and 0.01 s ) being 100 and 8 times faster than the original formulation in both scenarios.

Influence of composition of binary mixed working fluids on the performance of cascade heat pump

The optimized design of the cascade heat pump facilitates the recovery and reuse of waste heat in printing and dyeing wastewater. This paper constructs a cascade heat pump system with two-stage cycles sampling the same binary mixed working fluids. The effects of three kinds of combinations of working fluids, including R134a/R152a, R134a/R22, and R152a/R22, on the COP and heating power, are discussed through the thermodynamic cycle process simulation when the component ratio varies. It is found that, for the R134a/R152a mixed working fluids, the system performance can be adjusted approximately linearly by changing the component ratio. For the two mixed working fluids of R134a/R22 and R152a/R22, the performance of the cascade heat pump system is closer to the case of using pure R134a and R152a as corresponding working fluids. The system’s heating capacity is significantly reduced, compared with the case of pure R22 employed. Besides, with the component ratio of working fluid increasing, the COP and heating power change with opposite trends.

How to improve the energy performance of mid-deep geothermal heat pump systems: Optimization of heat pump, system configuration and control strategy

Mid-deep borehole heat exchangers (MDBHEs) extract heat from geothermal energy with depth of 2–3 km, and provide high-temperature heat source for heat pump heating system. To improve the energy performance, this paper conducted optimization analysis on heat pump, system configuration and control strategy. Firstly, the energy efficiency and regulation characteristic of magnetic bearing variable-speed centrifugal heat pump (MBC-HP) was examined through field tests and simulation analysis. Results showed that MBC-HP performed efficiently with wide-range variation of heating load and compression ratio. And average COP of MBC-HP reached 6.21 during heating season. Besides, as evaporator-side water temperature difference (dTe) reached about 10 °C, two-stage MBC-HPs connected in series was raised to fit the large dTe, and the average COP of heat pump was improved to 6.97. Furthermore, the evaporator-side water flow rate (Ge) affected the energy efficiency of heat pump system and heat transfer performance of MDBHEs significantly. Thus the water pumps were recommended to be connected in parallel and then connected with heat pumps. Then the numbers and frequency of water pumps should be regulated to maintain dTe about 9.0 °C. Compared with control strategy of constant Ge, the COPs of heat pump system increased to 5.28, showing 18.7 % improvement.

Performance of an automotive reversible CO2 heat pump system during periodic frosting-defrosting cycles

Electric vehicles (EVs) have seen rapid growth in the global market share due to the demand for low-emission commuting. However, EVs face the challenge of range anxiety in cold weather due to the capacity degradation of lithium-ion batteries and the increased demands of cabin and battery heating. Heat pump (HP) technology improves the heating efficiency compared to resistive heating, but in cold and humid weather, frost can grow on the outdoor evaporator and reduce the efficiency of the HP system. In this paper, experiments are carried out to assess the impacts of ambient temperature and relative humidity (RH) on the capacity and efficiency of a transcritical CO2 HP system during the continuous frosting-defrosting cycles under some challenging operating conditions (ambient temperature below 0 °C and RH over 80%). Moreover, the mass of frost accumulated during the frosting period, and that of water retained, drained, and vaporized during the defrosting period are measured and analyzed. The results show that when the ambient temperature decreases from 0 to −10 °C, the peak value of the heating capacity in the first frosting or HP mode period reduces from 4.85 to 3.54 kW, and the coefficient of performance (COPhp) drops from 1.61 to 1.47, but the operating time of the first frosting period significantly increases from 32 to 108 min. Similarly, lowering RH from 90% to 80% at 0 °C also helps to extend the operating time of the frosting period in one frosting-defrosting cycle from 32 to 84 min. In addition, with the current commonly used defrost initiation criterion, which is the air-side pressure drop across the outdoor evaporator increasing by five times, the heating capacity and system efficiency drop by less than 10% at the end of one frosting period. This indicates the potential for a longer operating time of the HP system which could reduce the undesirable impact on the passengers’ comfort during the reversed cycle defrosting.