Characteristics Analysis and Operation Methods of Solar Energy - Ground Source Heat Pump Air Conditioning System

1 Introduction With the economic development and scientific and technological progress, energy and the environment have become two outstanding social problems in today's world. The energy currently consumed by human society, including construction energy, is mainly petrochemical energy such as coal, oil and natural gas. These energy sources have limited resources and can not be regenerated and will eventually be depleted. And traditional energy, like coal, oil, etc., can cause serious pollution to the environment. According to the statistics, the total energy utilization rate of China's energy system was only 30% in the 1980s, which is 20% lower than the advanced level in other countries [1]. And with the continuous improvement of living standards, people are increasingly demanding on the comfort of their living environment, resulting in an ever-increasing consumption of building energy. According to the statistics, building energy consumption accounts for 30% to 50% of the total social energy consumption. , So the application of solar-ground source heat pump air conditioning system in buildings will become one of the important measures for sustainable utilization of energy and energy conservation.

2 ground source heat pump air conditioning system
2.1 ground source heat pump overview

Ground-Source Heat Pumps is an energy-efficient device that uses both hot and cold ground-floor geothermal resources, including groundwater, soil or surface water. During summer cooling, the earth serves as a heat rejection site and discharges indoor heat and compressor energy into the ground through the buried coil. The heat is then dissipated through the thermal conductivity of the soil and the migration of moisture in the soil. Winter heating, the earth as a low-temperature heat pump heat pump through the coil to obtain soil heat for indoor heating. Both heat exchangers can be used as both condenser and evaporator, but different functions due to different seasons. It can be seen that in geothermal heat pump air-conditioning system, since the heat removed from the earth in winter can be compensated in summer, the heat of the earth can be basically balanced.

2.2 ground source heat pump air conditioning system research status quo

Ground-source heat pump history can be traced back to a patent in Switzerland in 1912, and the true significance of commercial applications of ground-source heat pump only 10 years of history. In the United States, as of 1985 there were 14,000 ground-source heat pumps in the country, up from 45,000 in 1997 and 400,000 already so far, with steady growth of 10% annually. Ground-source heat pump systems in commercial buildings in the United States accounted for 19% of the total air-conditioners in 1998 [2].

In recent years, the ground-source heat pump industry in our country has started to take off and its momentum of development is promising. Tianjin University, Tsinghua University, Harbin Institute of Technology and Qingdao Institute of Construction and other institutions of ground source heat pump, especially ground source heat pump have carried out more in-depth research work for the ground source heat pump in our country's development laid the theoretical foundation.

2.3 ground source heat pump air conditioning system features

2.3.1 Ground source heat pump air conditioning system advantages

As the ground source heat pump air conditioning system to take a special heat exchange method, it has an air source heat pump incomparable advantages: (1) energy efficient: As the ground source temperature remains constant throughout the year, and the winter temperature is higher than the ambient air temperature, summer Than the ambient air temperature is low, coupled with the heat capacity of water, heat pump and high efficiency. According to the EPA of the United States Environmental Protection Agency, the design and installation of a good ground source heat pump can save 30-40% of the operating costs of the heating and cooling air conditioner on average. (2) Significant environmental benefits: When the ground source heat pump system is heating in winter, No need for boilers or additional auxiliary heater, and ground source heat pump will be cold or heat into the earth, the human environment will not cause damage; (3) low operating costs: high efficiency of the system, the compressor low (4) low maintenance costs: a simple system makes the ground source heat pump system without special care, without the need for frequent maintenance; (5) to save space: due to ground source heat pump air conditioning system to take Heat or heat are dependent on the part of the underground coil, the ground does not affect the landscaping of landscaping; and does not require cooling towers and other outdoor heat exchange equipment to improve the building's external image; (6) Selection of a wide range of cold and heat source: Yes Use of solar energy, soil, groundwater and surface water resources, according to the actual situation of selection of cold and heat sources; (7) system is stable and reliable, a multi-purpose machine: Temperature is relatively constant, making the heat pump operation more reliable and stable, but also ensure the system's efficiency and economy; and ground source heat pump system can be a multi-purpose machine, heating, air conditioning, but also for domestic hot water.

2.3.2 ground source heat pump air conditioning system problems

Although the ground source heat pump air conditioning system has been gradually applied to engineering practice, but because of this system technology is not perfect, so there are still some problems to be solved: (1) equipment capacity problems: due to the soil thermal conductivity is small, Buried heat exchanger area will be relatively large, and due to specific conditions in various regions of the restrictions, alone rely on ground source heat pump air conditioning system can not meet the design requirements; (2) the initial investment is larger: groundwater heat pump initial investment not only includes traditional air conditioning The system needs the pipeline and equipment on the ground investment, but also includes buried coil investment, buried coil laying investment, etc .; (3) soil temperature field recovery problem: with the ground source heat pump air conditioning system for heating or for Cold run time extension, the temperature field within the soil can not be restored for a long time, the system will not be able to meet the building's cooling and heating load requirements; especially for large and small heating and cooling load in the region, taking too much heat from the ground or to Underground heat too much, perennial operation will have an impact on the underground temperature field; (4) Maintenance inconvenience: once the buried heat exchanger leaks, it is difficult to dimension Repair; (5) design issues: Due to the current ground heat exchanger underground heat exchanger heat transfer model has not yet formed a mature theory, the various agencies on the underground part of the heat transfer mechanism is still in the discussion stage, so in the buried plate The design of the pipe mainly depends on the empirical data, but due to the different geological conditions and the distribution of the underground temperature in each area, there is no reliable theoretical basis for the design. (6) Pollution problem: Geothermal resources mainly include groundwater, soil or surface water Etc., the pollution will be more or less exerted on them, especially the pollution of groundwater bodies is more prominent. Once the groundwater body is polluted, the recovery period will be relatively long (according to different regions and different geological conditions, the recovery period after stopping the pollution is about In 1 to 2 years or more).

3 solar - ground source heat pump air conditioning system
3.1 Ground source heat pump air-conditioning system to introduce solar energy as a necessity of heat source

In the northern part of our country, the heat load of buildings in winter is larger, so the system design is mainly based on heat load. If the full use of ground source heat pump for heating, then the geothermal heat exchangers and units of the initial investment is relatively high, continuous operation is also less efficient. In summer operation, the unit capacity is too large, resulting in waste. And because these areas take more heat from the earth in winter than in summer, the long-term operation will cause the temperature of the earth to decrease, the COP value of the heat pump system will be relatively low, and the system can not meet the design requirements. Therefore, the energy saving effect of the heat pump can not be demonstrated. The use of solar energy as an auxiliary heat source, so that the heat pump system can be designed according to summer conditions, by the solar collector to bear part of the heat load, which will greatly reduce the initial investment in the ground source.

3.2 solar energy as a viable source of heat

Our country has a wealth of solar energy resources (see Table I). According to statistics, the total amount of solar radiation received by China on land each year equals to 2.4 trillion tons of standard coal. The annual sunshine time in 2/3 of the total area in the country exceeds 2000 hours, especially in some areas in the northwest over 3000 hours. This is the case in heat pump systems The use of solar energy provides a valuable resource. And solar energy is inexhaustible green energy, is not controlled and monopolized by anyone, its use is also more flexible, the scale can be large or small. However, solar energy also has two main disadvantages: first, low energy density; and second, its intensity can not be maintained under the influence of various factors (season, location, climate, etc.). These two major shortcomings greatly limit the effective use of solar energy. However, with the continuous improvement of science and technology, people will continue to use solar energy technology maturely, so that solar energy will be gradually applied to heating and cooling systems in buildings to improve human living environment instead of the current primary petrochemical energy Become one of the important energy sources of this century.

Solar energy resources in China Table 1

3.3 solar - ground source heat pump air conditioning system operation mode

Solar energy as an auxiliary heat source and ground source heat pump joint operation, there are two main modes of operation: series and parallel mode. (1) In-line system: In this system, the heat collected by the solar collector is stored in the heat storage water tank, and the hot water in the water tank is used to increase the temperature of the medium entering the evaporator inlet to provide the whole System COP value. (2) parallel system: This system is the solar heating system and ground source heat pump system alternately. When the temperature of the solar collector is high, the heat of the collector can be transferred to the ground for storage, so that the soil temperature field can be quickly restored and the heat collection efficiency can be improved. When the solar heating system can not meet the requirements of buildings Demand (cloudy or night), you can use ground source heat pump system heating.

3.4 solar - ground source heat pump air conditioning system features

Solar and ground source heat pump systems operate jointly, with great flexibility, to make up for the lack of heat pumps alone. All year round can be used to improve the device utilization factor. Although the total amount of solar energy is very large, its energy density is low. If the solar heating system is used alone, it will inevitably cause a large area of ​​the collector to collect heat. Meanwhile, the intensity of solar radiation varies with the season, morning and evening, day and night There are also intermittent and unsteady issues with solar energy utilization, all of which can be overcome by the combined operation of the GSHP system.

The solar-ground source heat pump air conditioning system has the following advantages: (1) when the solar heat collector is used to heat the heat source, the evaporation temperature of the unit is increased, so that the power consumption of the heat pump compressor is reduced and the operating cost is saved; (2) Nighttime operation of solar collectors can be used as auxiliary cooling equipment, thereby reducing the amount of heat to the ground in summer, so that the ground temperature remained stable in a few years to ensure that the unit running at high efficiency; (3) In winter operation due to evaporation temperature (4) In the system design, make the ground source heat pump system can be designed according to the summer conditions, thus reducing the capacity of the underground heat exchanger, reducing the increase of the user side outlet or air outlet temperature, comfort; Ground source heat pump investment in the underground part.

3.5 solar - ground source heat pump air conditioning system to be solved

At present, there are still some problems in the application of solar-ground source heat pump air-conditioning system: (1) Domestic and international researches on solar-ground source heat pump system are relatively few, and the reliability of the system operation and the best solar energy and ground source (2) The energy density of the sun and the low heat flux of the soil, so the required area of ​​heat exchangers and solar collectors is large, and the application depends on the environmental conditions of the building and the load of the building (3) The study of solar-ground-source heat pump system has not yet formed a complete set of theoretical system, the application of basic data is not enough, it can not provide sufficient theoretical basis for the application of solar energy-ground source heat pump.

4 Conclusions From the above analysis, we can see that solar energy is rich in resources and clean and pollution-free. It belongs to both primary energy and renewable energy. Although its energy density is low and its energy supply is random and intermittent, the utilization of solar energy is limited. However, With the continuous improvement of technical conditions, some problems existing in the current solar-ground source heat pump air conditioning system will be gradually solved, and its theory will be continuously improved. As a result, the solar energy will be used as an auxiliary heat source in air conditioning, refrigeration and heating, etc. Broad prospects for development. And our country is in the more advantageous area of ​​using solar energy, so long as it has a certain technical level and necessary capital investment, it can be used freely. Therefore, the use of solar energy - ground source heat pump technology will not only greatly reduce the consumption of China's petrochemical energy, China's energy structure is optimized to achieve sustainable development plan; and will be to solve our energy and environmental issues, one of the important measures, Will be the 21st century HVAC industry's mainstream direction.

5 References
[1] Any in. Energy Engineering Management. Beijing: China Electric Power Press, 2004

[2] Zhang Qunli, Wang Jin. Fluid Machinery, 2003, Volume 31 No. 5: 50 ~ 54

[3] Bi Yuehong, Chen Lingen. Refrigeration, 1999, Volume 18, No. 3: 25 ~ 28

[4] Qu Yunxia, ​​Fang Zhaodong, Zhang Linhua, etc. Renewable Energy, 2003, No. 1: 8 ~ 10

Types of

Area

Annual sunshine hours (h)

Solar energy accepted (kcal / cm 2. Year)

The equivalent standard coal (kg / m 2. Years)

1

Northern Ningxia, northern Gansu, southeast Xinjiang, western Qinghai, western Tibet

2800 ~ 3300

160 ~ 240

225 ~ 335

2

Northwest Hebei, northern Shanxi, Inner Mongolia, southern Ningxia, central Gansu, eastern Qinghai, southeastern Tibet, southern Xinjiang

3000 ~ 3200

140 ~ 160

200 ~ 225

3

Shandong, Henan, southeastern Hebei, southern Shanxi, northern Xinjiang, Jilin, Liaoning, Yunnan, northern Shaanxi, southeastern Gansu, southern Guangdong, southern Fujian, northern Jiangsu, northern Anhui, Taiwan, Beijing, Tianjin

2200 ~ 3000

120 ~ 140

170 ~ 200

4

Hunan, Guangxi, Jiangxi, Zhejiang, Hubei, northern Fujian, northern Guangdong, southern Shaanxi, southern Jiangsu, southern Anhui, Heilongjiang, Shanghai

1400 ~ 2200

100 ~ 120

140 ~ 170

5

Sichuan, Guizhou

1000 ~ 1400

80 ~ 100

115 ~ 140

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