Keywords: solar assisted heat pump household central air-conditioning solar-air dual heat source heat pump solar energy
0 Introduction
With the appearance of houses and villas with an area of ​​more than 100m 2 , and the increasing demand for air quality in air-conditioned rooms, research and development of a residential central air-conditioning system with excellent economic and environmental benefits Central air conditioning) is imminent. At the same time, research and development and utilization of new energy sources have become a common strategic goal of energy research and development in various countries in the world. Since the energy crisis of the 1970s, solar energy has gradually become the focus of domestic and foreign research as a new energy source available. Recent research shows that by 2050, nuclear energy will take the first place and solar energy will take the second place. By the end of the 21st century, solar energy will take the place of nuclear energy. Solar energy attracts people's attention because of its inexhaustible, safe, no need to transport, clean and pollution-free. As the solar energy is greatly affected by the seasons and the weather and the heat flux density is low, various forms of direct solar heat utilization systems are limited in application. With the improvement of living standards, heat users are also demanding more and more heat. Some limitations of solar energy use are increasingly apparent: (1) In countries and regions where the solar irradiation time is low, their application is greatly affected Limit; (2) The increase of plate surface temperature during daytime will lead to the decrease of heat collection efficiency; (3) Continuous heating for 24 hours can not be achieved when there is not enough solar radiation at night or overcast and rainy days. If auxiliary heating mode is adopted, But also consume a large amount of other energy sources; (4) longer heating cycle; (5) the traditional solar collector and the building is not easy to combine, to a certain extent, affect the appearance of the building; (6) conventional solar water heater in the room Top water tank, at low temperatures at night, water tanks and collectors heat to the outside world caused a lot of heat loss. In order to overcome the above problems in the utilization of solar energy, people have also proposed the use of a solar heating system as a heat source for a vapor compression heat pump system. The biggest problem in the practical application of vapor compression heat pump is that the efficiency of the heat pump system is relatively low when the temperature of winter is very low. However, the heat collector in the solar thermal system has higher heat collection efficiency at low temperature, while the heat pump system has a higher system efficiency when its evaporation temperature is higher, then a solar heating system may be considered as a heat source of the heat pump system. This will not only overcome the problem of solar heating system but also solve the problem of heat pump system in winter low efficiency. Solar heat pump system due to the use of solar energy has the role of energy saving and rapid development [1-2] .
1 solar heat pump system type
Solar thermal technology and heat pump technology with a very flexible combination of system forms are also diverse, generally divided into solar-driven heat pump and solar assisted heat pump two categories. Solar-driven heat pump mainly refers to the solar photovoltaic or thermoelectric-driven compression heat pump and solar radiation heat directly driven absorption, adsorption, injection and chemical heat pump. Most of these heat pumps to achieve the main purpose of solar refrigeration and air conditioning, the general requirements of the solar collector temperature is high, and the prevalence of bulky, high cost, low efficiency and other issues, more difficult to achieve miniaturization and commercial development. Solar assisted heat pump usually refers to the system as a solar heat pump auxiliary heat pump system, including a separate auxiliary heat pump and solar radiation heat energy as a heat pump evaporator. Most of these heat pumps to heat-based, involving building heating, domestic hot water supply and industrial heat and other applications, the solar collector temperature is not high, but also has a flexible and diverse system forms, reasonable economic and technical performance and good Commercial practical prospects. According to the different heat-collecting medium, the solar assisted heat pump can be divided into direct expansion and non-direct expansion of the two categories. In the direct-expansion system, the refrigerant absorbs heat and evaporates directly in the solar heat collector / evaporator as a solar heat-collecting medium, and then releases the condensation heat to the heated object through a heat pump cycle, as shown in FIG. 1 (a). Such a system has great potential for miniaturization and commercialization. However, solar radiation conditions are subject to change due to geographical latitude, seasonal change, diurnal alternation and various complex weather factors, and the unstable working conditions will inevitably lead to System performance fluctuations. In non-direct expansion systems, solar collectors usually use fluids such as water, air or antifreeze solution to absorb heat in the solar collector and then transfer the heat directly to the heating object or heat pump as a heat source of the evaporator Heat up and then heat the object. According to the different forms of connection between solar heat collection cycle and heat pump cycle, the non-direct-expansion solar assisted heat pump can be divided into three basic forms: tandem type, parallel type and dual heat source type, as shown in Figure 1 (b), (c), (D) below. Series refers to the solar collector cycle and the heat pump cycle through the evaporator to be connected in series, the evaporator heat all from the heat cycle to absorb the heat; parallel refers to the solar collector cycle and heat pump cycle independent of each other, which only as the former can not Auxiliary heat source to meet the heat demand; dual heat source and series are basically the same, except that the heat pump cycle includes two evaporators, which can simultaneously utilize two low temperature heat sources including solar energy or both. Dual-heat source solar assisted heat pump due to the use of two kinds of low-temperature heat sources, including solar energy or both complement each other to make the system has better stability. In addition, heat storage devices can be added to the system to reduce the rated capacity of the heat pump units, reduce system operating costs, increase solar energy dependency (defined as the effective heat gain from the sun over the required heat load), and in summer Solar-free storage operation to meet the needs of room air conditioning.
At the same time, due to the solar radiation has discontinuities, large fluctuations and other shortcomings, and changes in solar radiation intensity will inevitably lead to heat pump system performance fluctuations. Therefore, how to make full use of solar energy and ensure the stability and reliability of the system is an important issue that must be solved in practical application of solar heat pump system. At the same time, commercial split air conditioners and household electric water heaters, a wide range of applications, so that the consumption of household electrical energy increased significantly. Therefore, consider the solar heat pump and hot water supply joint operation, instead of split air conditioners and electric water heaters. As a result, solar energy can be utilized to meet user requirements for air conditioning and domestic hot water, thereby reducing the power consumption cost of the user.
(A) direct expansion; (b) tandem; (c) parallel; (d) double heat source Figure 1 Solar assisted heat pump type schematic
2 solar-air dual heat source heat pump system design and analysis
Solar-air dual heat source heat pump and hot water system is based on the double-heat source solar assisted heat pump on the basis of improved and improved. It is suitable for residential or villas with an area of ​​more than 100m 2 and can be used for cooling in summer, heating in winter and living hot water throughout the year. The system solves the common solar energy heat pump system due to changes in solar radiation intensity of the system operating performance fluctuations in large problems, it also has the characteristics of environmental protection and energy saving and stable operation, as a residential central air conditioning system, a new program has obvious advantages, Great application and promotion value. At present, there have been many reports on the research of solar assisted heat pump [3-5] , but there is no report about the application of such solar-air heat pump and hot water system at home and abroad.
1- evaporator (condenser); 2- chilled water circulating pump; 3- user fan coil; 4- collector circulating pump; 5- solar collector;
6-temperature sensor, 7 cold water inlet, 8 hot water outlet, 9 hot water tank, 10 hot water circulating pump, 11 electric heater,
12 heat exchanger 13 throttling element 14 air-conditioner priority switching valve 15 four-way switching valve 16 controller 17 compressor 18 condenser (evaporator)
Figure 2 solar - air double heat source heat pump and hot water system diagram
Solar - air double heat source heat pump and hot water system shown in Figure 2. The system mainly consists of two subsystems: solar collector cycle system, heat pump air-conditioning system, heat pump unit mainly includes evaporator, condenser, four-way valve, compressor, expansion components, air conditioning water circulating pump and end fan coil, Evaporator and condenser rely on four-way reversing valve in the refrigerant loop to achieve winter and summer interchange. Solar collector cycle system includes solar collectors, collector circulating pump, hot water tank and other components. In order to strengthen the stability of the system operation, an increase of electric heaters and heat exchangers. The entire system relies on temperature sensors, air conditioning priority switching valves and controllers to achieve operational control. The heat pump system has two evaporators, one with air as the heat source and the other with the heat source heated by the solar heat source. Solar - air dual heat source heat pump and hot water system to achieve the overall summer cooling, heating in winter, year round to provide domestic hot water. Heat pump unit is preferred for air conditioning. According to the solar radiation conditions and air conditioning load changes are mainly the following modes: 1) In the winter heating season, when the solar radiation intensity is large enough, do not need to turn on the heat pump, direct use of solar energy to meet the requirements; when the solar radiation intensity is small So that when the water temperature in the water tank is very low, the heat pump is turned on to work with air as a heat source. When the intensity of the solar radiation is in between, the heat pump is operated with the hot water heated by the solar water in the water tank as a heat source ; When both solar and heat pumps can not meet the hot water supply requirements, start electric water heater in the hot water tank to assist the heating; 2) In the summer cooling season, turn on the air conditioning priority switching valve, heat pump for cooling to meet the room Comfort requirements, the use of solar water heaters to provide hot water; 3) in the spring and autumn transition season without air conditioning load, the air conditioning system can run in the new way for indoor ventilation, improve indoor air quality. Domestic hot water is provided by the solar collector system, when insufficient heat pump system is started to supplement. The general principle of system operation control is: give priority to the use of solar energy to meet the heating, hot water for domestic demand, start the heat pump cycle when insufficient. When the solar energy and heat pump cycle can not meet the demand for heat, start the electric heater auxiliary heating. This makes full use of the solar energy to achieve the purpose of economy, environmental protection, but also to ensure the stability of the system to fully meet the user's air conditioning for domestic hot water needs.
3 system applications need to solve the problems and means
Before the actual application of the system, the following major issues need to be solved:
1) To study how the internal and external characteristic parameters of the system affect the operating characteristics of the system;
2) Under the circumstance that solar radiation intensity, air conditioning load and domestic hot water demand change, how to make full use of solar energy and ensure the stability of system operation when operating the system.
It can be solved by combining theory and experiment:
1) On the basis of designing and establishing the system, by measuring the operating performance of the system in one year and combining with the established system simulation model, the internal characteristic parameters of the system (including solar heat collecting structure, evaporator and condenser, compressor capacity , Storage tank volume, refrigerant properties, etc.) and external characteristics (such as solar radiation intensity, ambient temperature, outdoor wind speed, water temperature, etc.) on the operating characteristics of the system are analyzed.
2) The dynamic simulation model of solar-air heat pump and hot water system is established. The actual operating parameters are used as input variables, and the simulation results are compared with the measured data. A comprehensive analysis of the system internal parameters and external parameters of the system operating characteristics of the impact;
3) On this basis, the solar-air heat source heat pump and hot water system optimization design to enhance the system's energy efficiency and operational stability, promote the practical process of the system.
4 Conclusion
Based on the dual-heat-source solar assisted heat pump, this paper presents a solar-air heat source heat pump and hot water system, which is suitable for residential central air-conditioning systems in residential or villas with area over 100 m 2 , At the same time throughout the year to provide users with hot water. The system has the advantage of making full use of the solar energy and ensuring the stability and continuity of the system operation.