Solar heat pump technology and its heating application

Keywords: solar heat pump technical features COP energy saving

0 Preface

With economic development and technological progress, energy and the environment are two prominent social problems in the world. This has prompted people to become more aware of the importance of energy to mankind and pay more and more attention to solar energy utilization and energy-saving heat pump technology. At present, the thermal utilization of solar energy in our country is mainly concentrated in passive solar heating and water heaters to provide domestic hot water, while the active solar heating system development and utilization is relatively backward. The use of energy-saving device - the heat pump and solar thermal equipment, thermal storage system connected to the system, not only can effectively overcome the thinning and intermittency of solar energy itself, but also to achieve high energy savings and reduce environmental pollution purposes, Has great potential for development and application. Heat pump technology is a very good energy-saving air-conditioning cooling and heating technology, is the use of a small amount of high-grade electric energy as the driving energy, low-temperature heat from low-temperature heat source efficient absorption of heat, and its transmission to high temperature heat pump to achieve the purpose of heat , So that the energy with lower energy coefficient is energy with high energy coefficient (saving high-grade energy), that is, the technology of improving energy grade. With the increasing demand for hot water for domestic use, the intermittent characteristics of solar energy is difficult to meet the supply of heat around the clock. To solve this problem, heat pump technology and solar energy utilization is undoubtedly a good choice.

A heat pump basic principles and types

Heat pump is a reverse use of the refrigerator, refrigerator and the difference is only the working temperature range. Heat pump system works as shown in Figure 1 [1] [2]. Evaporator after absorbing heat, the working fluid of high temperature and low pressure superheated gas in the compressor after adiabatic compression into a high temperature and pressure of the gas, the condenser pressure condensate to a low temperature and pressure liquid (release of refrigerant gasification heat, And condensate water heat exchange, so that the condensate water is heated to hot water for users to use), the liquid refrigerant and then through the pressure reducer valve into low-temperature and low pressure liquid heat, into the evaporator heat pressure constant pressure heat absorption and evaporation To overheat the steam to complete a cycle. So cycle back and forth, will continue to heat the heat transfer to the condensate.

According to the first law of thermodynamics, there is: Qg = Qd + A. According to the second law of thermodynamics, the electric power A consumed by the compressor plays a compensating role so that the refrigerant can continue to absorb heat from the low-temperature environment (Qd) High temperature exotherm (Qg), cycle again and again. Therefore, the energy consumption of the compressor is an important technical and economic indicator. Generally, the energy efficiency of the device is measured by the coefficient of performance (COP), which is defined as:

COP = Qg / A = (Qd + A) / A = 1 + Qd / A

Obviously, the heat pump COP is always greater than one. Therefore, the heat pump is an efficient energy-saving device and an important way to implement building energy saving in the field of refrigeration and air conditioning. It plays a positive role in saving conventional energy, alleviating air pollution and greenhouse effect.

All types of heat pump evaporation and condensation have two temperature levels, the use of expansion valves or capillary pressure relief throttling refrigerant, but the pressure increases in different forms, mainly mechanical compression, thermal compression and steam jet compression. Among them, the mechanical compression heat pump, also known as electric heat pump, has been widely used in building heating and air conditioning, occupy the leading position in the heat pump market; heat compression heat pump, including absorption and absorption of two types, including water - lithium bromide absorption And ammonia - water absorption of the hot water unit has gradually embarked on the road to commercial development, and adsorption heat pump is currently in the research and development stage, we must also overcome the intermittent operation and system performance and low cold-to-heavy ratio and other issues in order to Truly applied to reality. According to the different forms of heat source, heat pump can be divided into air source heat pump, water source heat pump, ground source heat pump and solar heat pump. Foreign literature usually groundwater heat pump, surface water heat pump and ground source heat pump collectively referred to as ground source heat pump.

2 solar heat pump technology and its characteristics

Solar heat pump generally refers to the use of solar energy as a heat pump heat pump system, different from the solar photovoltaic or thermal power-driven heat pump unit. It combines heat pump technology and solar thermal technology, which can improve the efficiency of solar collectors and heat pump system performance. Collector absorbed heat as a low-temperature heat pump heat source in the rainy days, straight-expanded solar heat pump into an air-source heat pump, non-direct-expansion solar heat pump heating system as an auxiliary heat source. Therefore, it can work 24 hours a day, providing hot water or heat.

2.1 solar heat pump classification

According to the solar collector and heat pump evaporator combination, can be divided into direct-expansion (direct-expansion soalar assisted heat pump, DX-SAHP) and non-direct expansion. In direct expansion systems, solar collectors and heat pump evaporators combine into one, ie the refrigerant is evaporated directly by absorbing solar radiation in the solar collector (Figure 2). In non-direct expansion systems, the solar collector is separate from the heat pump evaporator and absorbs solar energy in the heat collector through the heat collecting medium (typically water, air, antifreeze solution) and transfers the heat to the evaporator in the evaporator Or pass the heat directly to the air or water that needs to be warmed up. According to the connection form of solar collector loop and heat pump cycle, non-direct expansion system can be further divided into series type, parallel type and dual heat source type. Series refers to the collector loop and the heat pump loop through the evaporator to be connected in series, the evaporator heat from the solar heat collection loop all the heat (Figure 3); parallel refers to the solar collector loop and The former is generally used to preheat the latter heating object or the latter as an auxiliary heat source (as shown in Fig. 4). The dual heat source type and the tandem type are basically the same except that the evaporator can simultaneously utilize the heat source including solar energy , Including two low-temperature heat sources (Figure 5) [3].

2.2 solar heat pump technical characteristics

Solar energy heat pump solar energy technology and heat pump technology combine with the following aspects of the technical features [4]:

1) Compared with the traditional direct solar heating system, the biggest advantage of the solar heat pump is that the simple structure of the collector can be used, and the heat collecting cost is very low. In a direct expansion system, the operating temperature of the solar collector is the same as the heat pump evaporation temperature, and is close to the outdoor temperature. In a non-direct expansion system, the solar heat collection loop is often used as a low-temperature heat source of an evaporator, Medium temperature is usually 20 ℃ ~ 30 ℃, so the collector heat loss is very small, the collector efficiency is also increased accordingly. Studies have shown that the collector efficiency is as high as 60% ~ 80%, even in a non-cold area with a simple and inexpensive conventional flat-plate collector, and even a bare-chip, non-insulated bare-plate collector is acceptable .

2) Due to the shortcomings of low density, intermittency and instability of solar energy, conventional solar heating systems often require large collector and heat storage devices with corresponding auxiliary heat sources, which not only causes the initial investment of the system High, but also larger area of ​​the collector is difficult to layout. Solar heat pump based on energy-saving heat pump and the efficiency of the collector, under the same heat load conditions, solar collector heat collector area required for heat storage and volume are much smaller than the conventional system, making the system more Compact, more flexible layout.

3) In the case of solar radiation is good, the solar heat pump can often get higher than the air-source heat pump evaporation temperature, which has a higher coefficient of heating performance (COP can reach more than 4), and the heating performance by the outdoor temperature The impact of the decline is smaller.

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, cool in summer, and hot water 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) Non-direct expansion system has the advantages of various forms, flexible arrangement and wide application range, suitable for central heating, air conditioning and hot water supply 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 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 for buildings, with the development of the city and the improvement of people's living standards, "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. Due to the shortcomings of low density, intermittent and unstable solar energy, the conventional solar heating system is difficult to meet the "all weather" requirements. In order to meet the "all weather" requirement, conventional methods use electric heating or gas heating as an auxiliary heat source , But it is easy to cause safety problems and consume a large amount of high-quality energy. The use of solar heating system can solve the "all-weather" problem. At present, China's solar heat pump should be mainly in public buildings, for example, Beijing Olympic Village and Olympic venues for domestic hot water and heating energy are used solar 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 coal-dominated energy structure and lower fuel prices in western China will inevitably 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 architectural appearance, to be 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.

references

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