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.     

Enhanced CANDU reactor with heat upgrade for combined power and hydrogen production

Nuclear energy is considered a key alternative to overcome the environmental issues caused by fossil fuels. It offers opportunities with an improved operating efficiency and safety for producing power, synthetic fuels, delivering process heat and for multigeneration applications. The high-temperature nuclear reactors, although possess great potential for integration with thermochemical water-splitting cycles for hydrogen production, are not yet commercially established. Current nuclear reactor designs providing heat at relatively low temperature can be utilized to produce hydrogen using thermochemical cycles if the temperature of their thermal heat is increased. In this paper, a hybrid chemical-mechanical heat pump system is proposed for upgrading the heat of the Enhanced CANDU (EC6) reactor design to the quality required for the copper-chlorine (Cu–Cl) hybrid thermochemical water splitting cycle operating at 550–600 °C. A modification to the heat pump is proposed to bring the heat to temperature higher than 650 °C with operating coefficient of performance estimated as 0.65.     

Thermodynamic analysis of a hybrid geothermal heat pump system

A thermodynamic analysis of a hybrid geothermal heat pump system is carried out. Mass, energy, and exergy balances are applied to the system, which has a cooling tower as a heat rejection unit, and system performance is evaluated in terms of coefficient of performance and exergy efficiency. The heating coefficient of performance for the overall system is found to be 5.34, while the corresponding exergy efficiency is 63.4%. The effect of ambient temperature on the exergy destruction and exergy efficiency is investigated for the system components. The results indicate that the performance of hybrid geothermal heat pump systems is superior to air-source heat pumps.     

The potential for heat pumps in drying and dehumidification systems I: Theoretical considerations

Drying is one of the most energy intensive unit operations. In many applications the drying temperatures required are low enough to make the inclusion of a heat pump in the system worthy of consideration. Five drying/dehumidification systems, including three with heat pumps, have been compared theoretically on the basis of specific power consumption (SPC), (i.e. the energy supplied per unit of moisture condensed) and primary energy consumption (PEC) which is (SPC) divided by the efficiency of primary energy conversion. The efficiency of each system is improved as the relative humidity of the air leaving the dryer is increased. The optimum is, however, very flat and a heat pump should be advantageous when a minimum relative humidity of 30 per cent is acceptable within the drying chamber. A closed cycle dryer is shown to be the most advantageous but requires careful matching.     

A critical review on design aspects and developmental status of metal hydride based thermal machines

Over two decades, research in the field of metal hydride based thermal machines has gained immense attention by the researchers of different fields. Because of its capability to store large volume of hydrogen per unit mass at near ambient condition, its utilization has been spread in numerous applications such as energy storage and other biological, chemical, aerospace and nuclear applications. Though there have been several review reports published on metal hydride based hydrogen storage, but the present work is focused on the thermal management issues and worldwide developmental status of various metal hydride based thermal machines such as thermal energy storage, heat upgradation, heat pump, cooling system, and hydrogen purification and compression. With a brief discussion about the basic understanding of metal hydride alloy formation, this paper also covers screening of metal hydride alloys, design considerations and evolution of different reactor geometries for various metal hydride based thermal management systems. The review also addresses the benefit of coupling of a metal hydride based hydrogen energy system with a conventional thermal system in order to a produce hybrid system with much higher performance and almost zero environmental pollution.     

Transcritical carbon dioxide heat pump systems: A review

Carbon dioxide is a safe, economic and environmentally sustainable refrigerant which can be used in heat pump and refrigeration systems. Research into the performance and benefits of a transcritical heat pump cycle using carbon dioxide began in the early 1990s. Theoretical and experimental research, as well as commercial system development, has improved transcritical system performance to a level similar to that of conventional heat pump systems. This paper presents an overview of transcritical carbon dioxide heat pump systems. The paper begins with a summary of carbon dioxide's use as a refrigerant and the distinctions of the transcritical cycle, followed by a numerical analysis of transcritical cycle performance. The study will then present a review of research on transcritical carbon dioxide heat pump systems, which covers system components, configurations and modifications and how these factors affect overall system performance.     

An iterative code to investigate heat pump performance improvement by exhaust gases heat recovery

The race between the development of technologies and energy demand has drawn the guidelines of energy strategies for the next two decades. Indeed, the governmental organizations as well as the private sectors are spending huge effort to come up with new adequate strategies that allow to decrease energy consumption. Having said that, heat pump becomes an essential system in our daily life not only in residential building but also in hospital, industrial and touristic building. Nonetheless, (HP)s have very high energy consumption rate. Thus, and to be in line with the new trends in energy strategies, it is convenient to find new methods to enhance the performance of heat pump in order to reduce energy consumption. In this frame, the present paper suggests an approach to enhance the performance the heat pumps using the heat recovery from generators. For this purpose, an in-house code is developed allowing to simulate two new proposed systems (condenser upstream exhaust gases heat recovery system (CU-EGHRS) and condenser downstream exhaust gases heat recovery system (CD-EGHRS). It has been shown that the increase in the performance of the heat pump depends on the capacity of the generator. Also, the CD-EGHRS is shown to be the best. For instance, in the case of a 15 kVA generator, the enhancement could reach 42% for the CD-EGHRS. This enhancement increases to 5640% in the case of a 180 kVA generator.     

Exergetic performance evaluation of heat pump systems having various heat sources

In this study heat pump systems having different heat sources were investigated experimentally. Solar‐assisted heat pump (SAHP), ground source heat pump (GSHP) and air source heat pump (ASHP) systems for domestic heating were tested. Additionally, their combination systems, such as solar‐assisted‐ground source heat pump (SAGSHP), solar‐assisted‐air source heat pump (SAASHP) and ground–air source heat pump (GSASHP) were tested. All the heat pump systems were designed and constructed in a test room with 60 m² floor area in Firat University, Elazig (38.41°N, 39.14°E), Turkey. In evaluating the efficiency of heat pump systems, the most commonly used measure is the energy or the first law efficiency, which is modified to a coefficient of performance for heat pump systems. However, for indicating the possibilities for thermodynamic improvement, inadequate energy analysis and exergy analysis are needed. This study presents an exergetic evaluation of SAHP, GSHP and ASHP and their combination systems. The exergy losses in each of the components of the heat pump systems are determined for average values of experimentally measured parameters. Exergy efficiency in each of the components of the heat pump systems is also determined to assess their performances. The coefficient of performance (COP) of the SAHP, GSHP and ASHP were obtained as 2.95, 2.44 and 2.33, whereas the exergy losses of the refrigerant subsystems were found to be 1.342, 1.705 and 1.942 kW, respectively. The COP of SAGSHP, SAASHP and GSASHP as multiple source heat pump systems were also determined to be 3.36, 2.90 and 2.14, whereas the exergy losses of the refrigerant subsystems were approximately 2.13, 2.996 and 3.113 kW, respectively. In addition, multiple source heat pump systems were compared with single source heat pump systems on the basis of the COP. Exergetic performance coefficient (EPC) is introduced and is applied to the heat pump systems having various heat sources. The results imply that the functional forms of the EPC and first law efficiency are different. Results show that Ex_loss,total becomes a minimum value when EPC has a maximum value. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance analysis of a novel dual-nozzle ejector enhanced cycle for solar assisted air-source heat pump systems

In this study, a novel dual-nozzle ejector enhanced vapor-compression cycle (DEVC) for solar assisted air-source heat pump systems is proposed. In DEVC, the use of the dual-nozzle ejector for recovering the expansion losses is a very promising approach to improve the cycle performance. A mathematical model of the DEVC is developed to predict its performance under specified operating conditions. The simulation results indicate that for the range of given operating conditions, the coefficient of performance (COP) and the volumetric heating capacity of the novel cycle using refrigerant R410A are theoretically improved by 4.60–34.03% and 7.81–51.95% over conventional ejector enhanced vapor-compression cycle (CEVC), respectively. The results imply that the solar-air source heat pump systems could take advantage of the best features of the DEVC. The potential use of DEVC therefore deserves further experimental validation. It is expected that this new cycle will be beneficial to developing dual-source coupled heat pump applications.     

Improving heat recovery using retrofitted heat pump in air handling unit with energy wheel

The world is facing a challenge to reduce energy use to meet the environmental goals set for the future. One factor that has a great impact on the energy performance of buildings is the ventilation losses. To handle these losses, heat recovery systems with rotating heat exchanger are often implemented. These systems have been shown to recover about 60–70% of the energy in the exhaust air on an annual basis.After a heat recovery system is installed it is hard to improve the efficiency of the installed recovery system with an acceptable economic payback period. In the present paper one way to improve the energy performance of a building with this type of heat recovery system by the use of a heat pump is investigated by simulations in TrnSys.The heat pump system is arranged so that the evaporator is connected to a heat exchanger mounted in the exhaust airstream after the energy wheel, and the condenser of the heat pump is mounted so that the temperature of return water from the heating coil is increased.The simulations show that there is a possibility to increase the heat recovery rate of the air handling unit in a significant way by retrofitting a heat pump to the system.     

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.     

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.     

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.     

Performance modeling of air cycle heat pump water heater in cold climate

Air (reverse Brayton) cycle has promising features in cold climate heat pump applications. In this study, an air cycle heat pump water heater (ACHPWH) simulation model considering the off-design performance of components was developed and validated with experimental data from literature. With this model, the performance of ACHPWH was numerically compared with two typical vapor compression heat pump water heaters (VCHPWH) under two different heating schemes, namely instantaneous heating and recirculation heating. For instantaneous heating, the COP of ACHPWH is comparable to that of VCHPWH when supplying high temperature water or operating at low ambient temperature. A significant improvement on annual performance would be achieved as well if higher efficient compressor and expander were applied in ACHPWH system. For recirculation heating, although the COP gap got larger, ACHPWH would save plenty of heating time when operating at low ambient temperature.     

Thermodynamic analysis of the reverse Joule–Brayton cycle heat pump for domestic heating

The paper presents an analysis of the effects of irreversibility on the performance of a reverse Joule–Brayton cycle heat pump for domestic heating applications. Both the simple and recuperated (regenerative) cycle are considered at a variety of operating conditions corresponding to traditional (radiator) heating systems and low-temperature underfloor heating. For conditions representative of typical central heating in the UK, the simple cycle has a low work ratio and so very high compression and expansion efficiencies and low pressure losses are required to obtain a worthwhile COP. An approximate analysis suggests that these low loss levels would not necessarily be impossible to achieve, but further investigation is required, particularly regarding irreversible heat transfer to and from cylinder walls. In principle, recuperation improves the cycle work ratio, thereby making it less susceptible to losses, but in practice this advantage is compromised when realistic values of recuperator effectiveness are considered.     

Heat pump assisted continuous drying part 1: Simulation model

The addition of a heat pump to a conventional hot air drying system can enhance the efficiency of drying. To investigate the performance of heat pump assisted continuous drying system a detailed simulation model has been developed. The system modelled consists of a vapour compression heat pump coupled to a continuous cross-flow dryer. The model takes account of detailed heat and mass transfer phenomena taking place in each component of the system and can be used to investigate many different system configurations.     

Correlations for cost of ground‐source heat pumps and for the effect of temperature on their performance

Ground source heat pump systems are becoming more and more popular, even though their high initial cost is an obstacle to their wider penetration of the heating and cooling market. The purchase of the heat pump itself is one of the dominant costs, and the heat pump selection also influences the operation costs through its coefficient of performance (COP) value. However, few studies are available on this topic. Based on 23 water–water heat pump models available on the market, a correlation was developed to estimate their purchase cost as a function of the nominal cooling load of the heat pump. These heat pumps can be used in geothermal applications as well as in other heating, ventilating, air conditioning and refrigeration (HVAC&R) systems. The correlation is valid for a nominal cooling load between 20 and 841 kW. The nominal COP of the heat pumps was found to have virtually no effect on their purchase costs. Also, two correlations were developed to relate variations of cooling power and COP to the temperature levels on both sides of the heat pump. The heating mode is also considered. The correlations are useful to estimate the required nominal size of a heat pump given design operating conditions and to optimize ground source heat pump systems from a techno‐economical standpoint. Copyright © 2014 John Wiley & Sons, Ltd.     

采用不同工质的高温复合热泵开水器特性研究

热泵开水器具有较高的能源利用效率,是公共场所电加热开水装置的理想替代品。从提高能源效率和一机两用的角度,构建了一种高温复合热泵开水器系统。建立了系统热力学模型,选R236fa、R245fa、R365mfc、R245ca、RC318和R236ea等6种较高临界温度的制冷工质,通过能量分析和[火用]分析的方法,探讨了不同制冷工质对高温复合热泵开水器系统性能的影响。研究结果表明:R245fa作为工质的高温复合热泵开水器系统具有最佳的性能,而以RC318作为工质的系统性能最差。在给定工况下,R245fa作为工质系统制热性能系数(COP_h)为2.47,而其制冷性能系数(COP_c)为3.37,[火用]损失和[火用]效率分别为9.47 kW和49.07%;与R245fa相比,RC318作为工质系统的总能耗增加了39.53%。     

CO2跨临界循环地源热泵的研究

给出了CO2跨临界循环地源热泵的系统流程，并在考虑输气系数和绝热效率的基础上，与R22和R134a等进行了循环性能比较。结果表明，用于需要较高供水温度的空调系统或热水供应系统时，CO2可具有和常规工质相当的性能。同时对于一特定的CO2地源热泵，分析了在热水流量和热水温度变化时的运行特性，并讨论了CO2地源热泵容量调节的方法。     

油田污水源热泵的用能分析与经济性评价

介绍了胜利油田东二联污水源热泵原油加热试验项目的工艺流程,并进行了用能分析和经济性评价。结果表明,热泵系统的制热系数和一次能源利用率较高,分别为6.35和1.83;热泵系统可用能利用率很低,只有27.07%;燃油价格越高,采用加热炉加热的效益越差,采用污水源热泵加热方式的经济性越好;污水温度越高,热泵制热系数越高,制热效果越好,耗电越少。