Energy Efficiency Of Heat Pump Research Articles

Experimental Studies and Performance Characteristics Analysis of a Variable-Volume Heat Pump in a Ventilation System

Air-to-air heat pumps are used in today’s ventilation systems increasingly often as they provide heating and cooling for buildings. The energy transformation modes of these units are subject to constant change due to the varying outdoor air state, including temperature and humidity. When choosing how to operate and control energy transformers, it is important to be able to adapt effectively to the changing outside air conditions. Nowadays, modern commercial heat pumps offer two levels of control flexibility: a compressor with a variable speed and an electronic expansion valve. This combination of control elements has boosted the seasonal energy efficiency of heat pumps. For a long time, cycle control possibilities have been dominated by electronic controls. The authors of this paper aim to present an additional element to the traditional heat pump controls, which provides a third level of control over the cycle. To achieve the objective, experimental investigations of a heat pump integrated into a ventilation unit have been carried out under real-life conditions. The experiments involved varying the operating modes of the unit by adjusting the compressor speed, the position of the expansion valve, and the volume of the system loop. The study examined the performance characteristics of the heat pump and found that the performance of a variable-volume heat pump is comparable to that of a conventionally operated typical constant-volume heat pump system. In addition, the study found that by adding a third level of volume control to the active heating circuit, in combination with conventional controls, the heat pump’s heat output range could be extended by 69.62%. The study determined the variation of the heat pump cycle in the p-h diagram with the variation of the loop volume. The benefits and drawbacks of a heat pump with a variable-volume loop are discussed in this study.

Performance evaluation of a novel humidification-dehumidification desalination system operated by a heat pump

Decentralized and small-scale desalination technologies can effectively address the limitations associated with large-scale desalination systems and contribute to solving the issue of fresh water scarcity. However, low energy efficiency remains a primary obstacle in their widespread application. To tackle this challenge, this paper introduces a novel heat pump humidification-dehumidification desalination system, in which humid air from the dehumidifier enters the heat regenerator and heat pump evaporator in sequence, enabling deep dehumidification. Subsequently, the cold air is reheated by the heat regenerator and subcooler before entering the humidifier. This sequential process can increase not only the energy efficiency of heat pump but also the fresh water productivity. Models are proposed and calculated for the theoretical evaluation of the novel system. Firstly, heat pump design parameters are optimized. Subsequently, performances of the novel system under different operation conditions are compared with those of the conventional system. The results indicate that optimizing heat pump system design parameters can significantly improve system performance. When compared to the conventional system, the novel system exhibits significant improvements. It achieves a maximum fresh water productivity of 39.73kg/h, representing an increase of 12.83%, and a maximum gain output ratio of 6.52, which is 13.39% higher. The lowest fresh water production cost is 9.52 $/m3, also a reduction of 5.15%. Additionally, both the novel and conventional systems possess optimal feed temperature and mass flow rate ratio. At last, the optimal performances of the novel system are compared with the reported results, which further validates the thermodynamic and economic advantages of the novel system.

Chemisorption heat pump governed by asynchronous start-stop method for stable heat output

To guarantee clean heating of buildings in cold regions, an off-peak electricity driven chemisorption heat pump employing SrCl2/sulfurized expanded graphite composite sorbent is developed. However, the heat output temperature of chemisorption heat pump fluctuates greatly, up to 30 °C, which seriously restricts its popularization and application. In this paper, the above-mentioned problems are solved from two aspects of sorption bed structure design and system operation strategy. The sorption bed utilized for thermal energy storage is especially composed of several unit reactors, and the asynchronous start-stop control method of unit reactors is innovatively proposed. By controlling their start and stop time, the temperature fluctuation range of hot water that the heat pump outputs is significantly reduced. Simultaneously, to improve the energy efficiency of heat pump, small temperature difference fan coil units are adopted indoors. The results indicate that when ten reactors operate asynchronously, the output temperature of hot water fluctuates by as low as 1.4 °C. With four reactors in asynchronous operation, the temperature fluctuation range can be reduced to 3 °C, and 35 °C hot water enters the small temperature difference fan coil units to release heat to the indoor space. Additionally, the heating performance of the chemisorption heat pump is relatively stable in cold regions. When the evaporating temperature decreases from 5 °C to −15 °C, its energy efficiency is almost constant, about 1.47. Eventually, this chemisorption heat pump can shift electricity from peak periods to off-peak periods, and also saves 0.7 to 6.3 yuan per day in space heating cost in comparison to the conventional vapor-compression one.

A specially-designed test platform and method to study the operation performance of medium-depth geothermal heat pump systems (MD-GHPs) in newly-constructed project

Medium-depth geothermal heat pump systems (MD-GHPs) use deep borehole ground heat exchangers (DBHEs) to extract heat from the medium-depth geothermal energy with depth of 2–3 km. To study the local geothermal condition and operation performance of MD-GHPs, so as to guide the system design and operation control, a test platform and method are raised and applied in a newly-constructed project, where a DBHE with depth of 2750 m is constructed. Results show that the maximum heat extraction rate of DBHE reaches 450 kW with inlet and outlet water temperature of 5.0 ℃ and 18.9 ℃, and the maximum heat supply rate of heat pump reaches 600 kW with supply and return water temperature of 51.8 ℃ and 40.6 ℃ under continuous operation mode. Then the energy efficiency of heat pump (COP) can still remain higher than 3.60 with renewable energy utilization rate higher than 72.2 % in this maximum load condition. Besides, the DBHE still extracts heat from outside ground to store heat when the heat pump system shuts off, thus the transient maximum heat extraction rate could reach 600 kW or even higher under intermittent operation mode. Finally, the optimization direction of DBHE and heat pump systems are analyzed, so as to improve the overall operation performance of MD-GHPs and achieve better energy conservation effect.

Preliminary design method for absorber pipe length of tunnel lining ground heat exchangers based on energy efficiency of heat pump

Many ongoing tunnel projects provide a favorable opportunity for the investigation and application of tunnel lining ground heat exchangers (GHEs). Tunnel lining GHEs can be connected to a heat pump to extract geothermal energy for heating and cooling buildings. Numerous studies have focused on the thermal performance of tunnel lining GHEs; however, the studies on the interaction between heat pumps and tunnel lining GHEs are relatively rare. In this study, a coupled heat transfer model of heat pumps and tunnel lining GHEs was proposed and then calibrated based on in situ test results. The model was used to evaluate the energy efficiency of a heat pump with tunnel lining GHEs under different conditions. The results show that the energy efficiency ratio (EER) increases exponentially with the absorber pipe length and thermal conductivity of the surrounding rock. The EER is governed by the convection heat transfer coefficient, which varies exponentially; meanwhile, the EER decreases approximately linearly with the annual average air temperature in the tunnel. Different types of heat pumps affect the EER significantly, and the EER of a Type-3 heat pump is higher than that of a Type-1 heat pump by 27.1%. Based on the aforementioned results, an empirical formula for the EER and absorber pipe length was established. Moreover, a preliminary design method for the absorber pipe length based on this empirical formula was developed. The method was employed to determine the appropriate absorber pipe length for the tunnel lining GHEs in the Shapu tunnel in Shenzhen, China. Finally, groups of absorber pipe layouts with a pipe spacing of 0.5 m, area of 135 m2, and length of 293.5 m were preliminarily determined.

Optimization on annual energy efficiency of heat pumps based on maximum solving of definition functions with multi constraints

Air conditioners is expected to be the second largest source of growth in global electricity demand. To promote electrification, heat pump air conditioner is encouraged to replace natural gas furnace. Improving heat pump’s energy efficiency can help reduce global energy consumption and greenhouse gas emissions. Annual performance factor is one of the key metrics to evaluate heat pump systems’ energy performance. Seeking annual performance factor maximum belongs to an optimization problem with multi constraints. However, due to the nested variables, continuous non-differentiable and piecewise form of annual performance factor definition functions, the optimized objective function is usually based on the simplification with large errors. Meanwhile, the multi constraint boundaries are either ignored or considered but through complicated and high-cost tests. In this research, the variable iteration logic of the annual performance factor definition functions is sorted firstly. Then the above exact mathematical definitions are programmed as the objective functions without any simplification. Enumeration method is used to establish the annual performance factor maximum solution algorithm. More importantly, taking a 3.5 kW cooling capacity mini-split heat pump prototype as an example, energy performance tests are conducted to fit the multi constraints, namely using less global parameters to cover more local parameters, which greatly simplifies the test procedures and cost. Then, using our proposed method, the maximum annual performance factor is predicted and validated by experiments. The prediction accuracy is 99.6%, with a 38.4% improvement compared to the best reported value from literature. Based on the recommended variable values, the prototype’s annual performance factor achieves the maximum with a 7.71% improvement from the baseline. The proposed method can save time and experimental resources in the product development stage compared with the traditional trial and error method.

Numerical Investigation on Energy Efficiency of Heat Pump with Tunnel Lining Ground Heat Exchangers under Building Cooling

For mountain tunnels, ground heat exchangers can be integrated into the tunnel lining to extract geothermal energy for building heating and cooling via a heat pump. In recent decades, many researchers only focused on the thermal performance of tunnel lining Ground Heat Exchangers (GHEs), ignoring the energy efficiency of the heat pump. A numerical model combining the tunnel lining GHEs and heat pump was established to investigate the energy efficiency of the heat pump. The inlet temperature of an absorber pipe was coupled with the cooling load of GHEs in the numerical model, and the numerical results were calibrated using the in situ test data. The energy efficiency ratio (EER) of the heat pump was calculated based on the correlation of the outlet temperature and EER. The heat pump energy efficiencies under different pipe layout types, pipe pitches and pipe lengths were evaluated. The coupling effect of ventilation and groundwater flow on the energy efficiency of heat pump was investigated. The results demonstrate that (i) the absorber pipes arranged along the axial direction of the tunnel have a greater EER than those arranged along the cross direction; (ii) the EER increases exponentially with increasing absorber pipe pitch and length (the influence of the pipe pitch and length on the growth rate of EER fades gradually as wind speed and groundwater flow rate increase); (iii) the influence of groundwater conditions on the energy efficiency of heat pumps is more obvious compared with ventilation conditions. Moreover, abundant groundwater may lead to a negative effect of ventilation on the heat pump energy efficiency. Hence, the coupling effect of ventilation and groundwater flow needs to be considered for the tunnel lining GHEs design.

Scientific and technological progress and future perspectives of the solar assisted heat pump (SAHP) system

Among all available solar technologies, solar-assisted heat pump (SAHP) is the most popular technology for low- and medium-temperature applications, i.e., drying, space heating, and water heating. This paper presents a critical review of the SAHP, leading to identification of the key technological challenges with the current SAHP technology, including (1) poor energy performance at low ambient temperatures; (2) difficulty in coupling solar thermal panels with heat pumps to achieve increased operational time and reduced electrical energy usage; (3) mismatch between heat demand of the building and heat supply of the SAHP system; and (4) lack of multi-functional modelling tool. A recently developed SAHP technology is introduced, which can effectively tackle the challenges with the current SAHPs, thus achieving enhanced solar efficiency, improved heat pump energy efficiency, and fast responsive time to the heat demand of buildings. The future research directions of the SAHP were identified: (1) multi-source heat pump; (2) heat pump defrosting capacity using waste heat and renewable energy; (3) advanced heat storage and exchange unit (HSEU) technology; and (4) advanced machine learning and multi-objective evolutionary optimisation models for performance prediction and optimisation of the SAHP using big datasets.

Energy Efficiency of Heat Pumps Heating Systems at Subsoil Waters for South-East Regions of Europe

Energy Efficiency of Heat Pumps Heating Systems at Subsoil Waters for South-East Regions of Europe

Study on energy efficiency evaluation and influencing factors of ground source thermal energy management system operation

The paper combines the testing work of ground source heat pump systems, discusses the installation, use methods and test errors of various testing instruments, combines the field test experience, gives the main points of ground source heat pump energy efficiency testing, and according to the problems encountered during the test. The faults with high frequency are analyzed, and the methods of fault diagnosis and troubleshooting are given. This paper uses the ground-source heat pump experimental platform to set up multiple sets of experimental schemes to study the impact of factors such as pipe diameter, single and double U, and flow rate on system energy efficiency. At the same time, it combines engineering cases to ?large flow small temperature difference?, ?imbalance? phenomenon is analyzed, and feasible solutions and suggestions are given. By analyzing the energy efficiency and benefits of actual engineering of multiple ground source heat pump systems, the paper concludes that ground source heat pump systems have a wider application prospect in a certain area of Hubei.

Dynamic coupling method between air-source heat pumps and buildings in China’s hot-summer/cold-winter zone

The existing selection methods for air-source heat pumps often result in insufficient heating effects and high electricity consumption in winter, in China's hot-summer/cold-winter zone. Because the existing basis for selection are diverse and difficult to convert for comparison, selection methods are not connected with actual dynamic load demand. This paper presents a dynamic coupling selection method for air-source heat pumps based on load balance. Dynamic output capacity models and energy efficiency models for air source heat pumps, dynamic load demand models for buildings, and dynamic coupling models are established to interpret the relationships between heat pumps and buildings. This paper puts forward the stable operation conditions, the output capacity, and the energy efficiency of heat pumps under the most unfavorable operation conditions, in order to select units. Our study finds that the lowest coefficient of performance is 1.12, which is only 43% of the rated value in the region. Heat pumps with higher performance generated excessive cooling capacity relative to load demands of buildings. Hence, the dynamic coupling method is a simple and engineering tool for optimizing entire air-source heat pump energy systems in buildings by matching the performance surfaces of heat pumps and buildings. It is a general method which can be applied by users to directly compare different units in buildings with higher energy efficiency and operation reliability, and also to guide manufacturers with a performance surface to complement existing indexes. This method has great significance for selection optimization and promotion of air-source heat pumps.

Heat pump energy efficiency monitoring and diagnostic system

Heat pump energy efficiency monitoring and diagnostic system

Optimization of deterministic controls for a cooling radiant wall coupled to a PV array

Thermally activated building systems (TABS) can work as thermal energy storage (TES) systems, which are useful in shifting the energy use of space cooling and heating in buildings. The present study analyses and optimizes simple deterministic control concepts for radiant wall supplied by a heat pump for cooling purposes. First, the “solar” concept was studied, which was focused on exploiting the output of a photovoltaic (PV) array. Secondly, a “peak load shifting” concept exploiting the low electricity cost and high heat pump energy efficiency during night periods was evaluated. The results showed that the “solar” concept saved between 57% and 95% in comparison to a conventional control in different PV installed capacities. Moreover, the optimized “peak load shifting” concept had lower operation cost than the conventional control with most of the PV configurations proposed. Therefore, the study showed that the investment in the PV array was fully harnessed only with specific controls. Furthermore, the “solar” control concepts were found to help achieving the goals of net-zero energy buildings by maximising self-consumption of renewable energies in the building, as well as reducing the total imported/exported energy.

Assessing the validity of European labels for energy efficiency of heat pumps

The European Union implemented Ecodesign and Labelling Directives to support the market diffusion of energy efficient products. Accurate signals for consumers on energy efficiency (EE) are essential, as disinformation might lead to sub-optimal market allocations. Considering complex devices such as heat pumps (HPs), a conflict between simplicity of calculation on the one hand and accuracy on the other hand arises. For this reason, main differences on EE between real working conditions and test procedures carried out according to regulations are examined within this study: Firstly, the most important deviations between the test procedure and the current state of the art are presented. Secondly, their influence on the validity of HP labels is investigated using spreadsheet calculations and a MODELICA simulation model. The results indicate that the omission of important influence factors – such as local conditions and the applied control strategy – in the regulations leads to significant differences between reality and labelling. The band of uncertainty found within this study covers high deviations of + 80% to − 24% from the label value. Therefore, we provide several recommendations to mitigate these deviations and to optimize the information content of the label. Among these are the implementation of a higher spatial resolution of climate conditions, the consideration of higher insulation standards, and the inclusion of effects caused by price-driven controls of the HP unit.

Research of waste heat energy efficiency for absorption heat pump recycling thermal power plant circulating water

The waste heat energy efficiency for absorption heat pump recycling thermal power plant circulating water has been analyzed. After the operation of heat pump, the influences on power generation and heat generation of unit were taken into account. In the light of the characteristics of heat pump in different operation stages, the energy efficiency of heat pump was evaluated comprehensively on both sides of benefits belonging to electricity and benefits belonging to heat, which adopted the method of contrast test. Thus, the reference of energy efficiency for same type projects was provided.

Radial piston expander as a throttling valve in a heat pump: Focus on the 2-phase expansion

Two-phase expansion devices are a practicable solution for improving the energy efficiency of heat pumps. The study of the fluid behavior in such machines has not yet been investigated in detail. In the present study, a radial piston expander was used in a heat pump operating with R134a in place of the throttling valve. Special attention was paid to investigate both numerically and experimentally the behavior of the fluid inside a cylinder within a full cycle of the engine, at different inlet thermodynamic conditions and rotational speeds. The measured indicated cycles were compared with those predicted by a quasi-steady model and a rather large difference between them was noticed. Therefore, a non-isentropic model to predict the in-cylinder pressure trend during the expansion phase at closed valves, developed on the basis of a model proposed for water at atmospheric conditions, was applied. The results achieved with the improved non isentropic model showed a significantly higher accuracy in predicting the experimental results along the expansion phase at closed valves, thus filling the gap registered with the full isentropic models, which lack in this part of the cycle.

Assessing Energy Efficiency of Compression Heat Pumps in Drying Processes when Zeotropic Hydrocarbon Mixtures are Used as Working Agents

Presents the results of studies of innovative materials in the field of renewable energy.The paper proposes a design and a formula for assessing energy efficiency of the heat pump air dryer, which uses zeotropic hydrocarbon mixtures of saturated hydrocarbons as a working agent and applies the principle of a counter-current heat exchanger with a variable temperature of both the working and the drying agents. Energy efficiency of the heat pump is achieved by means of obtaining a greater part of heat from renewable energy sources, in this case by cooling the air and condensing the water vapors in the heat pump. A conducted analysis identified correlations in establishing the marginal real coefficient of performance of the compression heat pump dryer running on zeotropic hydrocarbon mixtures and operating a cycle with variable temperatures of both the working and the drying agent in the evaporator and the condenser of the heat pump. According to the established correlations, the marginal real coefficient of performance of the compression heat pump dryers running on zeotropic hydrocarbon mixtures of 40 mol% of R600a and 60 mol% of R601 is 1.92 times higher than that of the same dryers running on only R600 (n-butane).

Study on the Influence of the Operation Parameters to the Ground Source Heat Pump Energy Efficiency Ratio

This paper is based on 30 ground source heat pump projects measured data which is comparative analyzed, using SPSS statistical software for modeling and find out several key independent variables that influence the system COP and the linear relationship between them. Verify this model in order to lay a foundation for unified evaluation criterion for the ground source heat pump energy efficiency ratio.

Energy Efficiency Testing of Ground Source Heat Pump System and Discussion of Fault Diagnosis

In order to improve the scientificity and accuracy of ground source heat pump energy efficiency testing, the paper discusses the principles and methods of energy efficiency testing, the requirements of the testing instruments' selection and installation and the arrangement of measuring points.The paper shows how to diagnosis instruments and process system failure according to the concrete situation on site,and points out the methods of systematic troubleshooting and fault elimination.

Energy performance of air-to-water and water-to-water heat pumps in hotel applications

We present work on measurement of the energy performance of heat pumps for hotel operations in subtropical climates. Two city hotels in Hong Kong were investigated. The first case was an application of an air-to-water heat pump to provide heating for an outdoor swimming pool during the heating season. The second case was the installation of three water-to-water heat pumps to complement an existing boiler system for hot water supply. The heating energy output and corresponding electricity use were measured. The heat pump energy efficiency was evaluated in terms of the coefficient of performance (COP), defined as the heating energy output to the electrical energy use. The air-to-water heat pump provided 49.1 MW h heating while consuming 24.6 MW h electricity during the 6 1 2 -month heating season from mid-October to April. For the water-to-water heat pumps, the estimated annual heating output and the electricity use were 952 and 544 MW h, respectively. It was found that the heat pumps generally operated in a COP range of 1.5–2.4, and the payback period was about two years, which was considered financially attractive.