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 compressor energy consumption is an important technical and economic indicators, the general coefficient of performance (coefficientofperformance, referred to as COP) to measure the energy efficiency of the device, 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 dominant 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 sorassheathepump, 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 heat collection loop and heat pump cycle through the evaporator to be connected in series, the heat from the evaporator all from the solar collector loop to absorb heat (shown in 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 heat sources 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.
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