【 Weak current College 】 solar heat pump principle and technical analysis

Heat pump technology is a new energy-efficient refrigeration and heating technology, for a long time mainly applied in the field of heating and air conditioning of buildings. Because of the heat pump for heating in energy conservation and environmental protection aspects of good performance, the hot water supply systems are increasingly used as a heat source heat pump equipment [2]. With outdoor air for air-source heat pump heat source, the structure is simple and does not require a dedicated room, convenient installation, in sanitary hot water supply, an irreplaceable advantage, apart from the major air-source heat pump hot water system, now has more than one brand of small household air-source heat pump water heater also put on the market. But one of the main air-source heat pump is heating and heating performance coefficient with the outdoor air temperature lower and lower, so its use is affected by ambient temperature restrictions generally applies to the minimum temperature-10 ° c above the area [3].

The heat pump and solar domestic hot water, combined with supply at home and abroad for a lot of research in this field, there are two ways, one is directly to the air-source heat pumps as a solar energy system for auxiliary heating equipment, another is the use of solar energy for hot water and low temperature heat source or the solar collector as heat pump evaporator of solar assisted heat pump systems. The former mainly for solar direct heating for air-source heat pumps as a supplement to address the issue of continuity of solar heating, but still cannot get rid of the ambient temperature of the heat pump heating performance impact; the latter entirely to solar energy as heat pump heat sources, greatly improving the efficiency of the use of solar energy, solar energy resource shortage still need to add other auxiliary heat source, and the heat pump capacity affected by solar heat, smaller scale in General.

In a large, solar central heating system, air source heat pump is an ideal auxiliary heating equipment, in order to improve the air source heat pump for heating low temperature environment to run performance, expanding its use of regional, domestic and international solar heat pump advanced experience in the study, we developed a suitable low-temperature environment of solar-heat pump Central hot water system. The system uses a novel use of low temperature solar assisted air-source heat pump and solar thermal systems, solar and heat pump for auxiliary heating source, maximize the utilization of solar energy, rainy weather and winter temperature lower solar hot water supply is insufficient resources guaranteed rate, do 24 x 7 x 365 to supply hot water.

1 solar-heat pump consists of the central heating system
1.1 solar-heat pump hot water system basic composition of the Central
Solar-heat pump hot water system of the main central components for solar collectors and solar assisted air-source heat pump heating, other auxiliary devices and conventional Central hot water system, including solar heating hot water circulating pump, pump, heat exchanger, water tank and controller, etc.

1.2 solar assisted air-source heat pump heating
1.2.1 solar assisted air-source heat pump heating works
For the air source heat pump in low-temperature environment in efficient, stable, reliable operation, many scientific research institutes at home and abroad and the production companies; development and improvement, can be summarized in three ways. One is to rely on external auxiliary heat source to increase the heat pump heating performance at low temperature, such as through electric heating to improve heat pump heating water temperature, the use of auxiliary heating burner heat exchanger, laying around in the compressor phase change material to increase low-temperature heating operation output and so on; the second is through improving refrigerant circulation system to improve low-temperature heat pump heating performance, such as a two-stage compression of air-source heat pump, located in the middle of the air-source heat pump air circuit; the third is the frequency system, low-temperature conditions to let the compressor speed work to increase the amount of refrigerant cycle, while at the same time the compressor working cavity. spraying fluid to prevent it from overheating, so that the heat pump can run properly.
Solar assisted air-source heat pump heating is based on the first way, as shown in Figure 2. In the evaporator unit has increased on a secondary heat exchanger. Heat pumps at low temperature under heating runs, the higher the ambient temperature of the solar hot water flowing through the secondary heat exchanger, and enters the evaporator in outdoor air heat exchange to increase its temperature, so that the refrigerant at a relatively high environment evaporation heat absorption and increases evaporation temperature, improve the working conditions of the compressor.

2. solar assisted air-source heat pump schematic heating
1.2.2 solar assisted air-source heat pump heating performance characteristics [7]
With normal air-source heat pumps, solar-assisted air-source heat pump heating in running under low temperature conditions have the following several obvious characteristics:

(1) COP significantly improve
In the same environment temperature, solar assisted heat of vaporization temperature refrigerant system, and unit heating performance coefficient compared with ordinary air source heat pump unit has dramatically improved, heat pump heating performance coefficient with the evaporation temperature changes as shown in Figure 2.

(2) prevent the evaporator frosting and defrosting time reduction
Since the heating effect of auxiliary heat source, improving the air enters the evaporator temperature to reduce the possibility of frost, which prevents the evaporator surface frosting to maintain high heat transfer efficiency, at the same time, the unit of frequency and duration of defrosting also greatly reduced, can save a lot of energy and heat pump units continuous uninterrupted operation.

(3) the improvement of air conditioning compressor working environment, to extend the service life of the unit
In the ambient temperature is low, the air conditioning compressor of compression, the compressor's rapid rise in temperature often exceed exhaust compressor allows the scope of work, causing the compressor frequent stops and does not work, if this situation persists, it will damage the overall performance of the compressor, reducing the service life of air conditioning equipment. Through solar energy as an auxiliary heat source system evaporating temperature, indirectly improve the compressor working environment, not only solved the compressor in the outside-temperature environment is not working properly, and you can make the entire heat pump units effectively extend the service life.

1.2.3 solar assisted heat exchanger design
Secondary heat exchanger is located in the heat pump evaporator, either forAs the heat pump unit of a part with a heat pump units production, the use of synchronous design and the evaporator the same overall dimensions and material of finned tube heat exchanger. Secondary heat exchanger for heat transfer area, air through the temperature rise and heat pump evaporator spacing should be based on solar collector can provide auxiliary heat, solar, water temperature, ambient temperature and heat pump evaporator temperature, exhaust volume and other parameters for the design calculations.

1.3 solar collectors
At present in the solar water heating projects commonly used solar collectors mainly in flat-plate solar collector, all-glass vacuum tube heat exchanger, u-shaped pipe vacuum tube heat exchanger, heat pipe vacuum tube collectors and circulations vacuum tube collector five [8]. For a comparison of annual use large solar hot water system, called central solar collector should have a certain pressure capability, high thermal efficiency, smaller set of pipes, frost resistance, easy to maintain. In a solar collector, all-glass vacuum tube collector although heat set high thermal efficiency, market share, but because it cannot be confined to run, easy to crack, not suitable for use in large solar hot water system, the majority are to be used as a solar water heater hot parts of the set. The other four are metal heat absorption of solar collector, you can run under pressure, suitable for large solar hot water engineering applications.


Flat-plate collector is applied earlier a solar thermal plant, which is also the World Solar market leading products, widely used in various areas of low-temperature hot water heating, but with the vacuum tube solar collector, subject to the limitations of its own structure, heat efficiency in the collection do not have the advantage on, winterizing and collector performance affected by seasonal and environmental impact, mainly in the southern winter temperature higher application, in the North in the cold regions of poor winter running effect, not recommended for application in large hot water project.
U-tube vacuum tube heat exchanger, heat pipe vacuum tube collectors and circulations vacuum tube collectors are all-glass vacuum tube collector developed on the basis of the product, the common features have relatively high thermal efficiency of the set to metal as receiver, you can run, but under pressure from the set thermal efficiency, leakproof, anti-fouling, durability, security, reliability, installation and maintenance difficulty in evaluation, heat pipe vacuum tube heat exchanger is the most appropriate at central hot water supply system in the solar collector type, u-tube vacuum tube collectors and circulations vacuum tube collector. Heat pipe vacuum tube collectors use heat pipe heat, dry tube connection, running water, a small heat capacity, heat transfer, thermal shock resistant to frost, and pressure, thermal insulation, no leakage, easy maintenance, etc., u-tube vacuum tube collectors and circulations collectors use vacuum tube concentric directly on refrigerants, in addition to the operating temperature is high, bearing capacity and good thermal shock performance characteristics, the thermal efficiency is higher than the set of collectors and can be installed horizontally, simplify mounting bracket, reduce installation area, avoid collector affect architectural appearance, in terms of solar energy and building combined with strong adaptability, but its specific heat pipe vacuum tube Setup collectors are more complicated, the interface, hidden with leakage in the operation of the system maintenance cost is relatively high.

2 solar-heat pump Central hot water system works
Solar and solar-assisted air-source heat pump combination heated as central hot water system of a heat source, its purpose is to learn from each other, that the two complement each other as a spare, in sunny time priority use solar heating hot water, solar collectors produce low temperature hot water as heating solar assisted air-source heat pump in auxiliary heating source, thus improving heat pump operation, improve its heating performance. This combination forms, both in the relatively stable and efficient conditions, ensure the system 24/7/365 of sanitary hot water supply. Air-source heat pump heating process essentially on the air in the promotion of solar energy utilization, according to the characteristics of the heat pump, in the hot water system is running, the heat pump unit as auxiliary heat source runs the supply of heat, only a small part from power, solar energy, heat pumps, central water heating system boosts solar energy utilization, reducing primary energy consumption.

Solar-heat pumps run Central hot water system in the following four conditions:
(1) solar heating hot water supply
In the majority of Rizhao good sunny day, the system works by this condition, the work of the solar circulation pump by the system controller according to the solar collector and water tank temperature control, continuous use of collectors collecting heat through intermediate heat exchanger to the hot water tank.

(2) solar assisted heat pump heating domestic hot water
When cloudy or cloudy weather, solar collector temperature is lower than the temperature of the hot water tank to direct heating domestic hot water, heat pump start, use air as heat source for heating domestic hot water in the hot water tank. In the autumn and winter season, when the ambient temperature is lower than the economic operation of the heat pump, the temperature of the heat pump system efficiency decreased and the evaporator surface frosting, auxiliary heating, heat pump, circulating start higher than the ambient temperature low temperature solar hot water into heat pump units within the secondary heat exchanger, air preheating through, so that the heat pump efficiency, and cut to prevent evaporator frosting to savings in power consumption of heat pump unit.

(3) solar and heat pump units while heating domestic hot water
In sunny sunshine well if the hot water system of heat loss is greater than the solar thermal system for efficient heating capacity or solar collectors for a smaller number, do not meet the hot water system with heat demand, solar and heat pump hot water system at the same time working to district heating.

(4) heat pump unit direct heating domestic hot water
In continuous rain and snow weather, hot water system energy consumption entirely by air source heat pump unit. At this point, the solar system is in standby, the heat pump to work alone on the hot water tank heating.

3 solar-heat pump hot water system design center
3.1 solar assisted air-source heat pump heating power ofR > solar-heat pump Central hot water system, solar-assisted air-source heat pump heating in fine weather as solar thermal systems auxiliary heat source equipment, solar energy resources is low or rainy weather as the main heat source system guarantees the normal supply of hot water, heating power should follow the design of the entire water system for determining the thermal load. For full-time Central hot water system, the heat pump unit power in accordance with the hot water system design hourly heat load determined that for part-time Central hot water system, the power of the heat pump unit shall be based on the maximum consumption of water, hot water tank, heating time and other parameters to determine the specific see documents [9]. Heat pump rated heating power of not less than central hot water system of comparison of load, in the winter, cold regions may be appropriate to increase the unit model, make it as far as possible during the day temperatures higher hours running, in a relatively short period of time to meet the needs of the thermal system.

3.2 solar collector area of
Solar-heat pump Central hot water system, solar collector area should be based on the hot water system heat load or according to the actual situation to determine the amount of solar heat, and as a fundamental basis of calculation of the project is located to the area of solar collector average daily effective heat to determine solar collector of the mounting area.
Hot water project, solar collectors in General is a fixed point of installation, its effective per unit area, more calories are subject to seasonal changes and every day changes in solar irradiation intensity, and is not a fixed value. The main factors influencing the collector installation angle, system operating conditions, location of meteorological parameters and sun exposure. Different collector types have different sets of thermal efficiency, also valid heat, so in practice generally based collector manufacturers offer collectors set performance parameters, such as thermal and solar irradiation data analysis, annual average.

3.3 solar collector system forms


For solar-heat pump, central water heating system solar system for hot water heating as well as the main heat source and as a heat pump in auxiliary heating source, and you should be able to withstand low environmental temperature, so should be used in closed systems, the system cycle refrigerants used antifreeze solution.

4 solar-heat pump hot water system of the central significance of the
4.1 operational reliability analysis
As solar-heat pump hot water system's main central components of solar and air-source heat pumps are sophisticated energy saving and environmental protection products. Solar domestic hot water system at the scale of use already has 20 years of history, extensive application of air-source heat pumps have decades of history [10], solar heat pumps, solar hot water system of the Central and the organic combination of air-source heat pump technology, without affecting their original function, making it significantly improves operational efficiency in order to ensure the system is stable and reliable operation, saving hot water system of conventional energy consumption.

4.2 energy efficiency analysis
According to China's northern most region of solar irradiation information, in accordance with the sanitary hot water system average heat loss and solar collectors to determine the average have thermal solar hot water system of collector area, solar-thermal pump Central hot water system, solar direct heating hot water system as a whole can meet 60-80% of the heat demand, and the remaining 20 — 40% thermal heating by solar assisted air-source heat pump supply, heat pumps can be up to the average COP, i.e. its 3.0 supply more than 65% of calories come from the collector cannot directly use solar and air heat energy. In the entire system running, collectors absorb solar energy utilization close to 100%, auxiliary heating power consumption only system total energy consumption 7 — 14%, compared with conventional sources of energy-saving hot water system can at least 85%.
By the above analysis shows that solar-heat pump Central hot water system is a reliable, environmentally friendly energy saving hot water system, the system uses only a small amount of solar energy and energy, on the environment without any pollution. Minimum temperature during the day-15 ºc, solar irradiation good in most parts of our country can be applied. Commission and hot water supply, winter heating and run-time savings over 60% of conventional energy sources, in most of the cold winter and hot summer, this system can also be building heating and air conditioning system with maximum energy saving effect.

5, the project application
In 2004, in solar-heat pump hot water system as the Central Academy of Olympic technology project "the Olympic village and the Olympic Stadium of solar heat pump Central hot water system demonstration of" project of the main research content, in Beijing, a training Hall yuetan Sports Center comprehensive demonstration project, the main function for domestic hot water supply, concentrated affluence calorie for auxiliary heating of swimming pools. The roof level installation mining 800m2 circulations vacuum tube solar collectors, solar-assisted air-source heat pump heating output power 300kW. In December 2004 has been to try to run the debugging phase, according to preliminary data, the system in the winter sunshine solar collector system meet the system design of 90% of the quantity of heat, rainy weather heat pump running smoothly, using 10 ℃ temperature solar hot-water heat pump in auxiliary heating source, heating performance significantly increased. According to the weather conditions in Beijing and solar irradiation data, the system, integrated energy saving rate can be expected by more than 90% of the target.