Investigating control strategies for a domestic low-temperature heat pump heating system

Despite energy conservation regulations and efforts for improving HVAC operations, numerous domestic buildings do not perform energy efficiently and many times the indoor environment is far away from specified comfort levels. Especially in houses served from low-temperature heating systems the low ability of the heating system to respond to fast changing thermal loads is common. In such cases, the implementation of new, sophisticated controls is an important issue. In this study, we use a reference model of a domestic low temperature heat pump heating system developed in TRNSYS–EES and analyse its operation. Several methods of control strategies have been applied for specified time periods in order to keep the comfort within reasonable ranges. Prognostic climatic control and increased ventilation rates when required are some of these methods. The results depict the influence of the control method on the indoor temperature and the comfort indexes of PMV and PPD. The highest indoor temperature difference for a chosen day reaches 4 °C when there is no shading and when there is internal shading with the option of applying prognostic climatic control. Generally, the findings highlight the importance of dynamics in controlling functions and the difficulty of incorporating in models unpredictable factors as the solar radiation.     

Exhaust air heat recovery in buildings

The technique of heat recovery from ventilation air in dwellings started in Sweden in late 1979. This was due to an energy crisis and new building codes. The competing heat recovery system, air to air heat exchangers, had a firm grip on the market. Today the situation is on the contrary. Almost all new single family houses are equipped with exhaust air heat pumps. This paper describes the development of the market in Sweden and Germany and also the different techniques of supplementary heating due to national differences in electricity prices. Germany has a situation very similar to Sweden concerning new building codes concerning the allowable energy use for space heating. Starting in 1976 and continued from 1982 to 1995, the building code has prescribed tighter and more insulated houses. The new building code for the year 2000 contains requirements for well insulated and tight buildings so the energy demand for heating from ventilation air tends to reach about 60% of the total annual energy demand for the building. Under these circumstances new buildings must have ventilation systems with heat recovery. Different means of heat recovery from the ventilation system, and the benefit for the environment, by using heat pumps are described. The German market for heat recovery systems is approx. 5–10.000 units/year. Most important for the efficiency of a ventilation system is to maintain the quality criterias concerning:equipmentplanning, installation, taking into operationoperation.VEW ENERGIE AG has accomplished a field survey of 60 units from 1994 to 1996. As the result was not statistically sufficient, the field survey is followed by an investigation into air quality and reliability.     

空气源户式机组地板采暖系统应用研究

为了研究空气源热泵冷热水机组作为地板采暖的热源的运行特性,以一围护结构保温良好的房间为研究对象,分别测试了热泵机组采用低水温进行供暖运行时的能效、在高低温的运行环境下供暖房间内温度的变化。结果表明,在保温性能良好的建筑,采用热泵机组以35℃的低供水温配合高效地暖盘管进行地板供暖,既能保证室内的舒适性,又具有良好的节能效果。得出用于地板采暖的空气源热泵机组,其控制开停机的温差需要比传统的空气源热泵机组大,在低温环境下,机组性能衰减的特性对室内达到相同供暖效果所需的时间影响不大,为设备、工程设计提供参考。     

“Heat pumps — status and trends” in Asia and the Pacific

In Asian and Pacific regions, economic growth in the last decade has propelled the use of air-conditioners for space cooling along with the use of reversible heat pumps for year round space conditioning. This has led to the rapid increase of electricity demand for air conditioning in summer. To cope with the increasing power demand and the requirement for efficient energy use for space conditioning, governments and energy supply utilities have encouraged effective use and leveling of power load using a heat pump with thermal storage systems and gas cooling systems, by enacting financial and promotional supports. Status and trends of heat pumps in Asian and Pacific regions, related to the use of heat pumps for space heating and cooling were surveyed from the view points of climate, energy consumption, technologies, markets and promotion measures.     

Heat pumps in the USA

First commercially introduced in the 1930s, heat pumps are now used to heat, or heat and cool, approximately 30% of all new buildings, both residential and commercial, in the USA. Their use is considerably lower for domestic water and industrial process heating, and is still at the pioneering stage for district heating and cooling. Widespread installation of air conditioning, prevalent use of air rather than water for heat distribution, and generally lower energy prices result in major differences in heat pump acceptance compared to that in European countries. This Paper surveys the current status of heat pumps in the USA and factors influencing their future.     

Regional report Europe: “heat pumps — status and trends”

By 1997 about 90 million heat pumps have been installed worldwide, only less than 5% are located in Europe, historically the cradle of this “thermodynamic heating and cooling process”. The majority of the approximately 4 million installed heat pumps are imported reversible air-to-air systems in southern Europe and only 30% represent the typical European-made heating only electric driven compression systems for space and water heating in buildings in central and northern Europe. The first and second oil crises has been the main cause for a first European heat pump “boom” at the end of the seventies. Consequently the following drop in energy prices negatively influenced the market in some countries. The new renaissance in Europe in the middle of the nineties was initiated by the understanding of sustainable development for a more efficient energy use and the related protection of the environment.     

Experimental study of sensible heat recovery of heat pump during heating and ventilation

Indoor space requires heating, cooling and ventilating for maintaining human occupant space to a comfortable level. Heat pump system is now widely used since it has the capabilities of providing both cooling and heating with a single unit. Ventilation, which exhausts the contaminated indoor air and brings in the fresh outdoor air is essential for maintaining pleasant indoor air quality. Ventilation, however, causes energy loss since air-conditioning is necessary to change the state of outdoor air to that of indoor. When outdoor air is introduced into the interior space, it must be cooled or heated to bring it to the indoor space condition. In this work, three methods of recovering sensible heat during heating and ventilation process of heat pump have been studied experimentally. Those methods are by a separate sensible heat exchanger, introduction of indoor air to the evaporator (single heat recovery), and finally a combination of fore-mentioned two methods (double heat recovery). An air-source heat pump system with none, single and double heat recovery capabilities has been built and tested in two constant-temperature and constant-humidity thermal chambers that simulate the indoor and outdoor environments. From the experiment performed under standard heating condition with a ventilation ratio of 23.1%, coefficient of performance for none, sensible heat exchanger, single and double heat recoveries were 2.88, 3.20, 3.18 and 3.28, respectively. Double heat recovery heat pump that has the ventilation and double heat recovery functions integrated into a single unit showed the best COP performance.     

Heating performance enhancement of a CO2 heat pump system recovering stack exhaust thermal energy in fuel cell vehicles

A CO₂ heat pump system using recovered heat from the stack coolant was provided for use in fuel cell vehicles, where the high temperature heat source like in internal combustion engine vehicles is not available. The refrigerant loop consists of an electric drive compressor, a cabin heater, an outdoor evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the heat pump system were investigated and analyzed by experiments. The results of heating experiments were discussed for the purpose of the development and efficiency improvement of a CO₂ heat pump system, when recovering stack exhaust heat in fuel cell vehicles. A heater core using stack coolant was placed upstream of a cabin heater to preheat incoming air to the cabin heater. The performance of the heat pump system with heater core was compared with that of the conventional heating system with heater core and that of the heat pump system without heater core, and the heat pump system with heater core showed the best performance of the selected heating systems. Furthermore, the coolant to air heat pump system with heater core showed a significantly better performance than the air to air heat pump system with heater core.     

地板辐射间歇供冷暖—置换通风在办公楼中的适用性分析

在分析办公楼类建筑采暖空调负荷特性的基础上,阐述了传统采暖空调系统的弊端.得出以水源热泵为冷热源,地板辐射间歇供冷暖加置换通风系统以其节能、舒适,便于控制等特点,在办公楼中有很强的适用性.以邯郸市某办公楼为实验对象,分析了系统的舒适性及节能性,提出了此系统在我国有着广泛的应用前景.     

Residential CO2 heat pump system for combined space heating and hot water heating

A theoretical and experimental study has been carried out for a residential brine-to-water CO₂ heat pump system for combined space heating and hot water heating. A 6.5 kW prototype heat pump unit was constructed and extensively tested in order to document the performance and to study component and system behaviour over a wide range of operating conditions. The CO₂ heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW.

The CO₂ heat pump was tested in three different modes: space heating only, DHW heating only and simultaneous space heating and DHW heating. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35 °C, and the set-point temperature for the DHW was 60, 70 or 80 °C. Most tests were carried out at an evaporation temperature of −5 °C, and the average city water temperature was 6.5 °C. The experimental results proved that a brine-to-water CO₂ heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the-art brine-to-water heat pump systems as long as: (1) the heating demand for hot water production constitutes at least 25% of the total annual heating demand of the residence, (2) the return temperature in the space heating system is about 30 °C or lower, (3) the city water temperature is about 10 °C or lower and (4) the exergy losses in the DHW tank are small.     

A novel parallel-type hybrid-power gas engine-driven heat pump system

This paper presents a novel concept to integrate a heat pump system and a power system which form a hybrid-power gas engine-driven heat pump (HPGHP) system. The power system of the HPGHP system includes an engine, a motor, a set of battery packs, a continuous variable transmission device and a power-control module. The engine in the power system is capable of operating constantly with high thermal efficiency and low emissions during the four different operating modes: for operating mode A, the ICE powers directly to match the compressor's demand load by throttling the natural-gas flow or adjusting the speed of the ICE, correspondingly the battery packs are disengaged and the ICE operates alone; for operating mode B, the ICE operates in the unique condition with the lowest fuel consumption ratio, meanwhile, the battery packs discharge to provide the supplementary power by the power-control module; for operating mode C, the ICE operates in the unique condition with the lowest fuel consumption ratio, and the redundant power provided by the ICE is converted by the motor to charge the battery packs, here, the motor is used as a generator; for operating mode D, the ICE is disengaged and the battery pack is used alone. Simulation results of the power system showed that for a conventional gas engine-driven heat pump (GHP) system the maximum and minimum thermal efficiencies of the power system are 33% and 22%, respectively; compared with the conventional GHP system, the power system in the novel HPGHP system has superior performance with the maximum and minimum thermal efficiencies of 37% and 27%, respectively.     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.     

A study on the performance of multi-stage heat pumps using mixturesEtude sur la performance de pompes à chaleur multi-étagées utilisant des mélanges

Multi-stage heat pumps composed of a condenser, evaporator, compressor, suction line heat exchanger, and low and/or high stage economizers are studied by computer simulation. Their thermodynamic performance and design options are examined for various working fluids. In the simulation, HCFC22/HCFC142b and HFC134a are studied as an interim and long term alternatives for CFC12 while HFC32/HFC134a and HFC125/HFC134a are studied as long term alternatives for HCFC22. The results indicate that the three-stage super heat pump with appropriate mixtures is up to 27.3% more energy efficient than the conventional single-stage system with pure fluids. While many factors contribute to the performance increase of a super heat pump, the most important factor is found to be the temperature matching between the secondary heat transfer fluid and refrigerant mixture, which is followed by the use of a low stage economizer and suction line heat exchanger. The contribution resulting from the use of a high stage economizer, however, is not significant. With the suction line heat exchanger, the system efficiency increases more with the fluids of larger molar liquid specific heats. From the view point of volumetric capacity and energy efficiency, a 40%HCFC22/60%HCFC142b mixture is proposed as an interim alternative for CFC12 while a 25%HFC32/75%HFC134a mixture is proposed as a long term alternative for HCFC22.     

成都地区毛细管采暖系统运行特性与热舒适性分析

对成都地区一户式空气源热泵毛细管地板辐射采暖系统运行工况进行测试,测得了间歇运行模式下毛细管地板辐射采暖运行特性、规律以及室内热湿环境参数,并以所测参数为基础,对毛细管辐射采暖的热舒适性从一般热舒适性指标和局部热舒适性指标两方面进行分析。研究结果为该地区毛细管辐射采暖系统的推广提供了技术数据,并为该机组的进一步优化提供了参考。     

Heat sources — technology, economy and environment

Heat sources for heat pumps in buildings as well as in industry are discussed. Furthermore, some environmental aspects concerning choice of heat source are highlighted. Only systems for heat pumping are included, i.e. air-conditioning types which can also partly work as heat pumps are excluded. Recent heat pump installations in Sweden are mainly in small systems. Ambient air, exhaust air, soil and rock are the most common heat source types. Data on COP, investment costs, working fluids, present Swedish market etc. for these types of heat sources are given. Data on industrial heat pump installations in some countries and distribution of these according to heat pump type and industry sector as well as heat source temperatures are reported. Process integration aspects when choosing heat source size and temperature are also discussed as well as the relation between these parameters and the choice of heat pump type. Finally, the influence on the economy of the heat source temperature is presented. The cost-effectiveness of heat pumps for reducing CO₂ emissions compared with other heating technologies is discussed. The main results are: (1) heat pumps can in many cases in the future contribute to a reduction of CO₂; (2) there is a rather big difference, for larger systems a major difference, between the water-based and the ambient air-based heat pumps in terms of efficient reduction of greenhouse gas emissions.     

热泵型房间空调器在中国的热冷比研究第1部分:典型城市住宅建筑的需求热冷比

明确热泵空调器的热冷比概念，阐明空调器热冷比HCRs与空调房间需求热冷比HCRD的关系，提出HCRD的计算方法；采用模拟分析方法研究位于哈尔滨、北京、上海、深圳四城市的一套典型住宅在不同围护结构、在不同朝向、不同空调使用时段条件下的全年负荷特性，并根据热泵空调器的变工况性能特点，给出了各地节能建筑与普通建筑的HCRD范围及分布。     

Experimental study on the performance of a simultaneous heating and cooling multi-heat pump with the variation of operation mode

The cooling load in the winter season becomes significant in commercial buildings and hotels because of the wide usage of office equipment and improved wall insulation. In this study, a simultaneous heating and cooling multi-heat pump having four indoor units and an outdoor unit was designed and tested in five operation modes: cooling-only, heating-only, cooling-main, heating-main, and entire heat recovery. The performance of the system with R410a was optimized by adjusting the system's control parameters. In the cooling-main mode, the rate of the bypass flow to the heating-operated indoor unit was optimized by controlling the EEV opening of the outdoor unit. In the heating-main mode, the mass flow rate to the cooling-operated indoor unit was optimized by adjusting the EEV opening in the outdoor unit. In the entire heat recovery mode, the compressor speed was controlled to improve the system COP with appropriate heating and cooling capacities.     

Frost retardation of an air-source heat pump by the hot gas bypass method

This study is concerned with a hot gas (refrigerant) bypass method to retard the formation and propagation of frost in an air-source heat pump. The feasibility of the hot gas bypass method was investigated experimentally and the method's performance is compared with that of a normal, 1.12 kW capacity air-source heat pump system with no defrost equipment such as an electric resistance heater. Results indicate that the hot gas bypass method is useful for retarding the formation and growth of frost at the outdoor coil. The best performance is shown under a bypass refrigerant flow rate of 0.2 kg/min (20% of the whole system refrigerant flow rate). During 210 min of heat pump operation, the hot gas bypass method improved COP and heating capacity at an average of 8.5% and 5.7%, respectively, relative to the normal system.     

The application of photo-coupler for frost detecting in an air-source heat pump

This experimental study is carried out to investigate reliability and effectiveness of a new method of using photo-coupler for detecting frost formation in an air source heat pump, and further to determine the most efficient initiation point of the defrost cycle. This new method of using photo-coupler as a frost sensing device is evaluated by comparing its performance with conventional time control defrost system in which defrost cycle is set to start at predetermined interval, e.g. about at every 1–1.5 h. Results indicate that overall heating capacity of photo-coupler detection method (case IV) is 5.5% higher than that of time control method. It is also shown that for maximum efficiency the defrost cycle must be initiated before the frost build-up area exceeds 45% of total front surface of the outdoor coil.     

Efficiency of free cooling using latent heat storage integrated into the ventilation system of a low energy building

This article presents the results of an investigation into the free cooling efficiency in a heavyweight and lightweight low energy building using a mechanical ventilation system with two latent heat thermal energy storages (LHTESs), one for cooling the fresh supply air and the other for cooling the re-circulated indoor air. Both LHTESs contain sphere encapsulated PCM (paraffin RT20). Using a developed and experimentally verified numerical model of the LHTES, the temperature response functions, based on the heat storage size, the air flow rates and the PCM's thermal properties, are established in the form of a Fourier series and empirical equations and used in the TRNSYS building thermal response model. Several mechanical ventilation, night cooling and free cooling operation modes were analysed and compared. It was found that the free cooling technique enables a reduction in the size of the mechanical ventilation system, provides more favourable temperatures and therefore enables better thermal comfort conditions, and in our studied case also fresh air for the occupants.