4) As solar energy is ubiquitous and inexhaustible, the solar heat pump has a wide range of applications, which are not limited by the local water source conditions and geological conditions, and hardly affect the natural living environment.
5) Solar heat pump, like other types of heat pump, also has the advantage of "one machine with multiple functions", that is, it can be heated in winter and cool in summer, and domestic hot water can be provided throughout the year. Due to the heat storage system in the solar heat pump system, nighttime valley power can be utilized for cold storage operation during the daytime for cooling in the daytime. It is not only cheaper to operate, but also contributes to power peak shifting.
6) Direct-expansion solar heat pumps are generally suitable for small heating systems, such as domestic water heaters and heating and air conditioning systems, taking into account refrigerant charge and leakage issues. It is characterized by small heat collecting area, compact system, high heat collecting efficiency and heat pump performance, good adaptability and high automatic control, especially for producing hot water. The utility model has the advantages of high efficiency and energy saving, convenient installation and all-weather. The cost and air source Heat pump water heaters quite superior performance.
7) having a non-linear expansion systems in various forms, layout flexibility, wide range of applications, etc., suitable for central heating, air conditioning and for hot water systems . Easy to integrate with the building.
3 solar heat pump water heater research status quo
As early as the early 1950s, Jodan and Therkeld, pioneers of solar thermal utilization, pointed to the advantages of solar heat pumps in improving solar collector efficiency and heat pump system performance. Subsequently, Japan, the United States, Sweden, Australia and other developed countries have invested a lot of manpower and material resources to carry out in-depth research and development of solar heat pumps in many places around the implementation of a number of solar heat pump demonstration projects such as hotels, residential, schools, hospitals , Library and swimming pool, made some economic benefits and good social benefits. Today's energy and environmental issues are increasingly serious, solar heat pump because of its significant energy-saving and environment-friendly, has been more and more attention. In recent years, developing countries such as Turkey and Indonesia have also done a lot of research on solar heat pump [5] [6] . In the industrial development, the United States Solar King series of solar heat pump heating equipment and Australia's Quantum series of solar heat pump water heaters, etc. are more typical product examples.
China's solar heat pump research started late, the literature and reports are in more than ten years. Tianjin University, Southeast University, Qingdao Institute of Construction and Engineering, Shanghai Jiao Tong University has conducted experiments and theoretical studies on solar heat pump, and achieved some results. The experimental research and theoretical analysis of tandem solar heat pump hot water supply system in Tianjin University show that the system can operate reliably throughout the year and provide users with hot water at 50 ℃. The COP reaches 2.64 ~ 2.85 (winter) and 2.61 ~ 3.5 (Summer) [7] . Qingdao Institute of Architecture has carried out an experimental study on the tandem solar heat pump heating system. The system has the ability of multi-function adjustment. The heat pump unit works stably during winter heat pump heating, with an average COP value of 2.71, which has obvious energy-saving effect [8] . Shanghai Jiao Tong University conducted a pilot study on a direct-expansion solar heat pump water heater that supplies 150L domestic hot water at 45 to 50 ° C throughout the day and consumes about 1 kW · h (summer) to 2 kW · h (winter) of electricity daily, Its split structure is particularly suitable for high-rise or multi-storey buildings. In addition, the water heater can work normally on rainy days and its working form is changed into an air source heat pump [9] .
4 solar heat pump and building applications
In recent years, with the development of solar energy and energy-saving requirements of buildings, with the development of the city and the improvement of living standards of the people, "solar energy and building integration" and "heating all-weather" have become important topics for solar thermal utilization in our country. "Solar energy and building integration" is the solar energy products as building components to install, make it organically combine to meet architectural aesthetics requirements, and as much as possible the use of new energy sources such as solar energy and renewable energy alternative to conventional energy to reduce building energy consumption on the conventional Energy dependence, reducing the proportion of building energy consumption in China's total energy consumption, and improve the conventional energy efficiency. As solar heat pump with a collection of high efficiency, high coefficient of heating performance, the form of diverse, flexible layout, a multi-purpose machine, the application of a wide range of advantages, can better meet the "solar and building integration" requirements. Since solar have the disadvantage of low density, intermittent and stability, the conventional solar heating systems to satisfy the "all-weather" requirements, to meet the "all-weather" requirement, a conventional method is adopted electric heating or gas heating is an auxiliary However, it is easy to cause safety problems and consume a large amount of high-quality energy. However, solar heating system can solve the problem of "all weather". At present, solar energy heat pump primarily in the public buildings, such as the Beijing Olympic Village and Olympic venues heating and domestic hot water are powered by solar energy heat pump heating system.
5 solar heat pump technology problems
China's solar heat pump development and application of some problems still exist:
1) Investment economy. Energy structure and fuel prices have a direct impact on the economics of solar heat pumps. For example, the energy structure dominated by coal in western China and lower fuel prices will surely affect the market competitiveness of solar heat pumps. At the same time, high initial investment in solar heat pump system is also one of the important factors affecting its economy.
2) performance and reliability. Various types of solar heat pump performance needs to be improved, to make the matching between components to achieve the best investment and operating efficiency, the system design and architectural design should be combined, it is necessary to consider the system performance but also consider the construction of aesthetics, to implement intelligent Control, which requires all professionals, people in all fields work together and cooperate with each other.
3) The public lacks sufficient knowledge and understanding of this technology. At present, the main obstacles to the application of solar heat pump in our country are the high initial investment of the system and the lack of sufficient understanding and understanding of this technology by the government, architectural designers and the general public. Through the joint efforts of government departments, research institutes and engineering and technical personnel, learn from the successful experience of other countries, China's solar heat pump will be faster to promote and develop.
6 Conclusion
Solar energy is the main source of all energy on earth, it is an inexhaustible and clean source of pollution and is also the most promising energy source for mankind in the 21st century. China's vast territory, annual sunshine time greater than 2000h area accounts for about 2/3 of the country's land area, in the more favorable use of solar energy within the region. Solar heat pump technology is an organic combination of solar thermal technology and heat pump technology. It has the advantages of high heat collection efficiency, high coefficient of heating performance, diverse forms, flexible layout, multiple use of a machine and wide range of applications. It can better solve the problems of " Building integration "and" all-weather "issues. Solar heat pump technology will occupy an important position in the utilization of solar energy, has a broad development prospects.
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