A review of heat pump systems for heating and cooling of buildings in China in the last decade

Heat pump technology fully shows the principle of energy recycling in terms of Heating, Ventilating and Air Conditioning (HVAC). It avoids unipolarity of energy using in the conventional HVAC system. Heat pumps use high-grade energy as a driving energy, recovering and upgrading low-grade energy for avail, like a pump. Because heat source used in HVAC usually is low temperature heat, heat pump systems adopted in HVAC will help improve heating performance coefficient. Therefore, HVAC is one of ideal users of heat pump applications, and thus high-grade energy used in HVAC can be replaced with a large number of low-temperature renewable energy. Through the heat pump technology, natural low-grade energy stored in the soil, water, air or waste heat from variant industries and daily lives, is supplied for building cooling/heating and hot water serving. Therefore, vast applications and developments of heat pump technology are presented in HVAC in China, and some progresses are achieved in the system innovation, experimental research, product development and engineering application, etc. This paper reviews the progress of researches, applications and development in the field of heat pumps for building cooling/heating in China since the 21st century.     

Comparison of heat pump performance using fin-and-tube and microchannel heat exchangers under frost conditions

Vapor compression heat pumps are drawing more attention in energy saving applications. Microchannel heat exchangers can provide higher performance via less core volume and reduce system refrigerant charge, but little is known about their performance in heat pump systems under frosting conditions. In this study, the system performance of a commercial heat pump using microchannel heat exchangers as evaporator is compared with that using conventional finned-tube heat exchangers numerically and experimentally. The microchannel and finned-tube heat pump system models used for comparison of the microchannel and finned-tube evaporator performance under frosting conditions were developed, considering the effect of maldistribution on both refrigerant and air sides. The quasi-steady-state modeling results are in reasonable agreement with the test data under frost conditions. The refrigerant-side maldistribution is found remarkable impact on the microchannel heat pump system performance under the frost conditions. Parametric study on the fan speed and the fin density under frost conditions are conducted as well to figure out the best trade-off in the design of frost tolerant evaporators.     

Optimization of hybrid – ground coupled and air source – heat pump systems in combination with thermal storage

Ground coupled heat pumps are attractive solutions for cooling and heating commercial buildings due to their high efficiency and their reduced environmental impact. Two possible ideas to improve the efficiency of these systems are decoupling energy generation from energy distribution and combining different HVAC systems. Based on these two ideas, we present several HVAC configurations which combine the following equipments: a ground coupled heat pump, an air to water heat pump and a thermal storage device. These HVAC configurations are linked to an office building in a cooling dominated area in order to evaluate in these conditions the total electrical consumption of each configuration to obtain which one satisfy the thermal demand more efficiently. The results of our simulations show that the electrical energy consumption obtained when the system employs a suitable configuration is of around the 60% compared with an HVAC system driven by an air to water heat pump and around the 82% compared with an HVAC system driven by a ground coupled heat pump.     

Study on hybrid ground-coupled heat pump system for air-conditioning in hot-weather areas like Hong Kong

The ground-coupled heat pump (GCHP) system is becoming attractive for air-conditioning in some moderate-weather regions due to its high energy efficiency and reliable operation capability. However, when the technology is used in buildings where there is only cooling load in hot-weather areas like Hong Kong, the heat rejected into the ground by the GCHP systems will accumulate around the ground heat exchangers (GHE). This heat accumulation will result in degradation of system performance and increment of system operating costs. This problem can be resolved by using the hybrid ground-coupled heat pump (HGCHP) system, which uses supplemental heat rejecters to reject the accumulated heat. This paper presents a practical hourly simulation model of the HGCHP system by modeling the heat transfer process of the system’s main components. The computer program based on this hourly simulation model can be used to calculate the hour-by-hour operation data of the HGCHP system. As a case study, both a HGCHP system and a traditional GCHP system are designed for a hypothetic private residential building located in Hong Kong, and the economic comparisons are conducted between these two types of systems. The simulation results show that the HGCHP system can effectively solve the heat accumulation problem and reduce both the initial costs and operating costs of the air-conditioning system in the building.     

A cost‐effective operating strategy to reduce energy consumption in a HVAC system

The operation of the building heating, ventilating, and air conditioning (HVAC) system is a critical activity in terms of optimizing the building's energy consumption, ensuring the occupants' comfort, and preserving air quality. The performance of HVAC systems can be improved through optimized supervisory control strategies. Set points can be adjusted by the optimized supervisor to improve the operating efficiency. This paper presents a cost‐effective building operating strategy to reduce energy costs associated with the operation of the HVAC system. The strategy determines the set points of local‐loop controllers used in a multi‐zone HVAC system. The controller set points include the supply air temperature, the supply duct static pressure, and the chilled water supply temperature. The variation of zone air temperatures around the set point is also considered. The strategy provides proper set points to controllers for minimum energy use while maintaining the required thermal comfort. The proposed technology is computationally simple and suitable for online implementation; it requires access to some data that are already measured and therefore available in most existing building energy management and control systems. The strategy is evaluated for a case study in an existing variable air volume system. The results show that the proposed strategy may be an excellent means of reducing utility costs associated with maintaining or improving indoor environmental conditions. It may reduce energy consumption by about 11% when compared with the actual strategy applied on the investigated existing system. Copyright © 2007 John Wiley & Sons, Ltd.     

A study on the optimized control strategies of geothermal heat pump system and absorption chiller‐heater

The world is becoming increasingly interested in renewable energy including geothermal energy. The utilization of geothermal systems is currently low because geothermal systems and existing source systems are used independently, but the supply rate of a geothermal system is increasing. Therefore, suggesting efficient operation plans and evaluations of the energy consumption and efficiency of a geothermal system is needed. This paper reports the results of a field study and survey of the present applications and operation conditions of a geothermal system. In addition, this paper proposes an efficient operation strategy for a geothermal system and compares this operation strategy with an existing operation strategy through simulation. The problems of existing operation condition were found out through a field study, and alternatives were proposed. The improvements were evaluated using the transient systems simulation program. And it would be possible for the reduction of the energy consumption through the comparative analysis of equipment efficiency and energy consumption. The result of analyzing the proposed combination header method through simulations compared with existing operation conditions can increase the use of geothermal systems, but the combined cooling and hot water of a geothermal heat pump and existing thermal source system reduced the efficiency of the heat pump. As a result of simulation on individual load‐sharing method, efficiency of geothermal system is increasing compared with the combination header method. This method was especially made to separate geothermal system's water loop and existing thermal source system's water loop. Copyright © 2013 John Wiley & Sons, Ltd.     

Vertical-borehole ground-coupled heat pumps: A review of models and systems

A large number of ground-coupled heat pump (GCHP) systems have been used in residential and commercial buildings throughout the world due to the attractive advantages of high efficiency and environmental friendliness. This paper gives a detailed literature review of the research and developments of the vertical-borehole GCHP technology for applications in air-conditioning. A general introduction on the ground source heat pump system and its development is briefly presented first. Then, the most typical simulation models of the vertical ground heat exchangers currently available are summarized in detail including the heat transfer processes outside and inside the boreholes. The various design/simulation programs for vertical GCHP systems primarily based on the typical simulation models are also reviewed in this paper. Finally, the various hybrid GCHP systems for cooling or heating-dominated buildings are well described. It is found that the GCHP technology can be used both in cold and hot weather areas and the energy saving potential is significant.     

Analysis of rankine cycle heat pump driers

We present a simulation analysis of closed Rankine cycle convective heat pump driers in which the product air recirculation ratin within the chamber is high. The configurations include driers with both external and internal refrigerant evaporators. The effects of modifications to basic designs are examined and real plant details are added in successive stages. Throughout the study the efficiency of each system is related to the second law loss mechanisms discussed by Carrington and Baines (1988). A detailed audit of exergy losses is presented for two driers in particular situations using data based on operational timber driers. The simulation results illustrate the need for the drier control strategy to take advantage of the performance characteristics of the heat pump drier. In addition, it is important to carefully match fans and heat exchangers to the drier requirements because the fan power will often be comparable with the compressor input.     

Combination of ground source heat pumps with chemical dehumidification of air

In the present paper the possible synergies provided by the combination of an underground thermal energy storage (UTES) system with a desiccant based air handling unit (AHU) are analysed. Differently from the conventional solutions, the summer humidity control is obtained here by chemical dehumidification of the ventilation airstream performed by liquid desiccants in a packed column. Being the water temperature of the boreholes heat exchangers generally suitable to meet the sensible load without any integration with the chillers, the plant can operate in a complete free-cooling mode. In winter, the main benefits are due to the higher temperature level at which the UTES works and to the AHU configuration allowing sensible and latent heat recovery. For the same reasons, the required UTES size is sensibly smaller, reducing in this way not only the operation but above all the investment costs. The UTES system competitiveness is then increased. The described solution is investigated by a computer simulation referring to a modern office building in the climate of northern Italy and its performance has been compared to a traditional HVAC plant and to a traditional ground source heat pump (GSHP) system. Finally, some economic evaluations are reported, showing the competitiveness of the proposed configuration.     

Refrigeration systems with minimum charge of refrigerant

Concern for the environmental effects of HFC-refrigerants as well as the use of flammable refrigerants has resulted in a need of decreasing the refrigerant charge in refrigeration and heat pump systems. This paper discusses the possibility of such reductions, both at the systems- and the component level. It is shown that a move towards indirect systems, using secondary refrigerants, on both the cold and the hot side of the system may result in considerable reduction of charge. However, this reduction may come at the cost of slightly reduced system performance, which in itself is detrimental from an environmental point of view. At the component level, it may be shown that the main contents of refrigerant is usually contained in the heat exchangers. By selecting compact designs the charge may be reduced to extremely low levels. Specifically, mini-channel heat exchangers can be used for reaching low charge. With proper selection of heat exchangers, the system performance should not be influenced by the reduction of charge. For indirect systems, the amount of refrigerant solved in the compressor oil may be comparable to the amount in the (compact) heat exchangers. A possible solution to reduce this amount is to use compressors with less oil. With components selected for minimum charge, the system design may be different than what is usual. Instead of a high pressure receiver and a thermostatic expansion valve, a capillary tube may be used in combination with a minimal low pressure receiver, similar to the system design used in household refrigerators.     

A distributed model of a space heat pump under transient conditions

A prototype heat pump was designed and tested, as means of active thermal management for electronics packages to be used on stratospheric balloon missions. The evaporator worked as a cold plate to absorb heat dissipated by the electronics, while the condenser rejected heat primarily by radiation to the rarified environment. To predict the transient performance of the heat pump under varying environmental temperature and cooling load conditions, a dynamic model of the heat pump is created with a graphical user interface (GUI). The simulation of the evaporator and condenser are fully transient and the components are segmented, whereas the compressor and expansion device are lumped models and assumed to be at quasi-steady state. A detailed model for the mass and energy conservation in the two heat exchangers is presented. The spatial and temporal variation of temperature and mass flow rate in the heat exchangers are predicted. Several types of transient conditions such as step changes of the space temperature and cooling load, system start-up, shutdown, and cycling, are studied. The space temperature, cooling load, compressor power, mass flow rates of the compressor and expansion device, pressures and refrigerant charges of the condenser and evaporator, and temperature distribution in the heat exchangers are dynamically displayed on the GUI. The simulation results are compared with experimental data for step changes in the cooling load and show good agreement in terms of trends. Copyright © 2002 John Wiley & Sons, Ltd.     

Multi‐fault detection and diagnosis of HVAC systems: an experimental study

The objective of this study is to detect faults due to multiple element failures in HVAC systems occurring concurrently. To classify and detect single as well as multiple faults, measurements were made of supply air temperature, OA‐damper position, supply fan pressure, indoor temperature and airflow rate in a variable air volume heating ventilating and air conditioning test facility. Experimental results show that three types of patterns emerge in the analysis of multi‐fault problems. To solve the multi‐fault problem, a new strategy based on pattern classification and the use of residual ratios is presented. It is shown that the residual ratio can be used to diagnose and accurately identify and detect multiple‐faults occurring in HVAC systems. Copyright © 2005 John Wiley & Sons, Ltd.     

A modified zone model for estimating equivalent room thermal capacity

The zone model has been widely applied in control analysis of heating, ventilation and air conditioning (HVAC) systems to achieve a high building efficiency. This paper proposed a modified zone model which is much simpler in the HVAC system simulation and has the similar accuracy to the complicated simulation model. The proposed model took into consideration the effect of envelop heat reservoir on the room indoor temperature by introducing the thermal admittance of the inner surfaces of the building enclosure. The thermal admittance for the building enclosure was developed based on the building thermal network analytical theory and transfer function method. The efficacy of the proposed model was demonstrated by comparing it with the complicated model — heat balance method (HTB2 program). The predicted results from the proposed model well agreed with those from the complicated simulation. The proposed model can then make the HVAC system dynamic simulation much faster and more acceptable for control design due to its simplicity and efficiency.     

Performance analysis of a heat pump assisted drying system

The paper describes a simulation model developed to predict the performance of drying systems assisted by vapour-compression heat pumps. The heat is used to preheat the air stream before it enters the drying chamber. Energy consumption is thus reduced, as the heat pump is capable of delivering more energy as heat than it in fact consumes as input work. Ambient air provides the heat source. A computer program, based on simplified modelling of components (compressor, heat exchangers and drying chamber) has been developed. Results have been produced for a typical application, revealing that a considerable reduction in energy consumption can be obtained with the use of a heat pump. The effect of air flow rate on system performance is also studied.     

Natural convection heat exchangers in solar water heating systems: Theory and experiment

Natural convection heat exchangers can be used in solar hot water systems to replace the pump on the tank side of the exchanger. There is currently no experimentally verified way of designing this type of heat exchanger. An experimental apparatus to test natural convection heat exchangers was built and an extensive set of measured data obtained on two different exchangers sized for low-flow stratified tank system. Two theoretical models for the exchanger are presented: a finite-volume primitive variable numerical solution of the fundamental laminar equations of fluid motion and a laminar forced-convection-based solution method. Comparison of the model predictions with the experimental data showed good agreement when the modified Rayleigh number is less than about 400. The poor agreement under other conditions was attributed to turbulence and recirculation neither of which was accounted for in the models.     

Optimizing a heat pump for heating purposes

A computer simulation routine is used to analyse the economic effects of varying the heat exchangers and fans of a 3 hp commercial (base-line) air-air heat pump, on the assumption that it is used only for heating purposes. Both long-term (life-cycle costing) and short-term (pay-back time) economic measures are considered. the results indicate that there is economic justification for substantial increases in the capacity of the heat exchangers of the base-line unit. Such an optimized heating-only heat pump has technical and energetic advantages over the base-line unit.     

Preliminary investigation of a transcritical CO2 heat pump driven by a solar‐powered CO2 Rankine cycle

In this paper, a transcritical carbon dioxide heat pump system driven by solar‐owered CO₂ Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar‐powered heat pump cycle using CO₂ as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO₂ heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO₂ heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO₂ flow rate of two sub‐cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.     

中央空调系统变频节电及余热回收技术

运用中央空调系统热负载关系图,分析了压缩机、循环水泵、空调箱三大电耗部分的节能原理、潜力及国外最新发展动态,介绍了中央空调变频闭环节能系统和余热回收系统,提出了饭店业制冷、供热系统能源循环利用的节能新模式。     

Numerical study of horizontal ground heat exchangers for design optimization

Despite ground source heat pump has been proven as highly efficient, high initial cost discourages homeowners and small-medium enterprises to opt for such systems. Horizontal ground heat exchangers offer relatively low-cost solution that may help promoting these systems usage worldwide. This study examines ways to optimize the designs for horizontal ground heat exchangers by using different layouts and pipe materials. CFD simulation of three dimensional models was performed to achieve this objective. All cases tested are able to yield comparable heat exchange rate for an equal trench length. However, the effective period differs one from the other. Additional initial and overhead costs are worthy as slinky ground heat exchangers prolongs heat transfer process when compared against straight configuration. Pipe materials with superior thermal conductivity also promote longer high efficiency operation. An improvement of 16% is reported when copper pipe is used instead of the conventional HDPE pipes. Effective period can be extended by 14% when ground heat exchangers are installed in vertical orientation. Thermal interference in slinky configuration is prevalent during initial operation. In a long run, the effect is observed to be minimal except in vertical orientation. However, it is avoidable beforehand at design stage.     

Development and optimization of artificial neural network algorithms for the prediction of building specific local temperature for HVAC control

This research accounts for the outcome of a major cloud-based smart dual fuel switching system (SDFSS) project, which is a dual-fuel integrated hybrid heating, ventilation, and air conditioning (HVAC) system in residential homes. The SDFSS was developed to enable optimized, flexible, and cost-effective switching between the natural gas furnace and electric air source heat pump (ASHP). In order to meet the optimal energy consumption requirements in the house and provide thermal comfort for the residents, various high-quality sensors and meters were installed to record multiple data points inside and outside the house. The performance of the system was monitored in the long term, which is a common practice in energy monitoring projects. Outdoor temperature data plays the most crucial role in operating HVAC systems and also is a key variable in the decision-making algorithm of the SDFSS controller. Therefore, this study introduces an innovative and unique approach to obtain the outdoor temperature that could potentially replace high precision sensors with a data-driven model utilizing weather station data at a time resolution of 2 minutes and 1 hour. In this work, a series of artificial neural network algorithms were developed, optimized, and implemented to predict the outdoor temperature with an average of 0.99 coefficient of correlation (R), 1.011 mean absolute error (MAE), and 1.315 root mean square error (RMSE). It has been demonstrated that the developed ANN is a reliable and powerful tool in predicting outdoor temperature. Thus, the proposed model is strongly suggested to be implemented as an alternative to temperature sensors in hybrid energy systems or similar systems requiring accurate ambient temperature measurements.