(Tianjin University Geothermal Research and Training Center)
The annual JSRAE annual meeting was held in Okayama University, Japan in June 2002. The Department of Heat Transfer, Faculty of Engineering, Okayama University is the organizer of this conference. As a doctoral student who is about to graduate from this department, I have the honor to help organize and attend this meeting. A total of 173 papers were read out, including 18 papers related to geothermal energy, accounting for 10% of the total papers. The contents of the conference paper mainly include: ① Overview of the development of ground source heat pump; ② Related basic and soft science research; (3) Groundwater, pile and other ground source heat pumps and air conditioning systems; (4) using geothermal energy for road snow melting; ⑤ Ground source cold storage and soil freezing.
1 development and review of ground source heat pump
Professor Nokeze, President of Hokkaido University, quoted J.W.Lund's data in Review of the Development of Ground Source Heat Pumps in the World and Japan, and analyzed the development status and market characteristics of ground source heat pumps in the United States, Europe and Australia. According to the data in 2000, the United States is the most popular country for ground source heat pumps, with a national installation load of 4.8 million kilowatts, which is equivalent to installing 400,000 units 12 kilowatts, accounting for about 68% of the total installed capacity in the world. Among them, the vertical underground heat exchanger is the most, accounting for 46%; Horizontal type accounts for about 38%; There are not many open heat exchangers, accounting for only 15%.
Although the electricity price in Japan is 4-5 times that of diesel and much higher than that in Sweden (1.8 times), the total installation load in Sweden is nearly 100 times that of Japan, and most ground source heat pump systems are both hot water and hot water. In Austria, over 70% of the more than 2,000 heat pumps installed in 2000 only use ground source as heat source. In Japan, ground-source heat pump has attracted attention after 1990. It is mainly used for air conditioning and road snow melting in Shikoku Island and Kyushu. Recently, the Ministry of Land and Resources of Japan set up a road snow melting project in its northeast, focusing on the scientific and technological strength of the Environmental Industry Research Institute. Since the implementation of the project, it has received good results.
As a volcanic island country with the second economic strength, Japan is rich in geothermal resources, but its development and utilization lags behind many European countries and even some other developing countries in Asia. This fact has attracted the attention of many Japanese scholars and institutions. It is predicted that in the next few years, Japan may invest a lot of money in geothermal utilization.
Related basic research and soft science research
If the research of ground source heat pump is attributed to the simple heat conduction problem of semi-infinite solid linear source, the earliest research can be traced back at least half a century ago. In fact, ground source heat pump is a complex problem involving many disciplines. One of the difficulties is how to determine the boundary conditions of the surface. On this issue, scholars such as Hironobu Ono of Osaka University put forward a mathematical and physical description model to describe the equilibrium relationship among surface water, steam and heat, and it is in good agreement with the measured results. The model mainly considers solar radiation, wind speed, temperature and humidity of ambient air.
Another interesting research is to use the data of land and resources to carry out a research paper on thermal storage and recovery of regional groundwater layer. In this paper, Sapporo, Hokkaido is taken as the research object, and the technical feasibility of using thermal storage to take heat from the groundwater layer in metropolis is studied by using the relevant underground and aboveground data of Sapporo. This research result can understand the regional distribution of heat storage and consumption from a macro perspective, thus providing the scale of ground source heat pump and providing theoretical basis and countermeasures for controlling urban heat island phenomenon. The author believes that strictly speaking, this is a complex dynamic simulation problem, and it is also a subject that government agencies must involve.
3. Groundwater, piles and other ground source heat pumps and air conditioning systems.
Due to the many advantages of ground source heat pump, some small and medium-sized companies in Japan began to organize the development and research of ground source heat pump system. Although it started late, it has already shown its potential. At this meeting, Zeneral Heat Pump Co., Ltd. in Nagoya, Japan cooperated with JMC Geothermal Engineering Co., Ltd. in Tokyo to install nearly 90 horsepower ground source heat pump units in Japan and Changchun in northeast China. See table 1.
Table 1 Units installed by Zeneng Heat Pump Co., Ltd.
Air conditioning, hot water. 1 horsepower = 0.7457 kw.
The 50 HP unit installed in Changchun consists of 5 units 10 HP. In order to facilitate the experiment, the ground source heat exchanger is 16 tubes, with a length of 100 meters, different pipe diameters and different materials. The COP of the unit is 2.7 ~ 3.3, and the heating temperature is about 40℃. The average length of heat exchanger tubes is about 30 watts/meter.
Pile-type ground source heat pump system has dual functions of heat source or cold source and solid foundation of building. The Department of Engineering of Fukui University has carried out numerical simulation and experimental research on yi system, which is the first attempt in this field in Japan. The numerical simulation calculation of 70 piles (underground heat exchangers) with an area of 3693m2 shows that the heating load can reach 437.9GJ, and the cooling load can also reach nearly 300GJ. This is equivalent to the thermal load of each foundation pile of 4 1.08MJ/ day. The COP value can reach 3.60 ~ 4. 14. The air conditioning system is shown in figure 1. Economically, the ratio of initial investment of 50 tons to initial investment of air source heat pump system is less than 15%. The total investment of pile foundation type is 6,543.8+07.2 million yen, and that of air source type is about 654.38+05150,000 yen. Although the experiment didn't go smoothly, the analysis of the experimental data shows that compared with the air-source air-conditioning system, the energy-saving effect of 12. 1% can be improved in refrigeration and about 26.7% in heating.
Using ground source heat pump to clear road snow is an early geothermal research project in Japan. Mr. Kenji Morita of Ibaraki Industrial and Commercial Research Institute is a famous research scholar in this field. Recently, researchers from private companies have also begun to intervene, which may be related to the road privatization policy recently implemented in Japan. In the cold areas in the north, there are many traffic accidents caused by snow, and they often mainly appear in sharp turns (Figure 2).
Therefore, it is necessary to use ground source heat pump snow melting system in some key places. In order to improve work efficiency in winter, the same system can be used to collect solar radiation heat energy on roads and store it underground in summer (Figure 3). The unit with heating capacity of 56 kW can melt the road surface with an area of 332m2, which is equivalent to 170W/m2. If the effective length of each underground heat exchanger is 15 1m, the required quantity is related to the temperature of the fluid returning to the underground heat exchanger, and the lower the temperature, the less the required quantity (Figure 4).
Figure 1 Pile-based Ground Source Heat Pump System
Fig. 2 Road snow melting system using ground source heat pump.
Fig. 3 Winter snow melting operation mode (I) and summer heat storage mode.
Fig. 4 the relationship between the inlet temperature of underground heat exchanger and the number of heat exchangers and COP.
4 ground source cold storage and soil freezing
Because there is still water in the soil, it can undergo phase change solidification and store cold at low temperature. The latent heat of phase change accounts for a large proportion of the total storage capacity, so the water content determines the storage capacity to some extent. Osaka Seiko Co., Ltd. conducted experimental and theoretical research on the cold storage mechanism of soil with volume moisture content of 0.6m3/m3. Include refrigerant inlet temperature, buried pipe arrangement, etc. In general, the heat recovery coefficient can reach more than 80%. According to the simulation results, practical application examples can be predicted: cooling load: 9 ~105 kj/h; Heat recovery per meter of pipeline length: 630kJ/mh. Then the predicted values of circle radius r, root number n and pipe length are used for the required pipe group (Table 2).
Table 2 Range Radius and Tube Group Length
Frozen soil process is harmful to buildings or roads in most cases. However, adopting appropriate technical conditions can bring its advantages into play. Ito A, Department of Environmental Engineering, Osaka Shennan University, put forward the idea of strengthening foundation by using frozen soil technology. An exhaust pipe for pumping water vapor is added around the frozen soil layer. The exhaust pipe placed at the periphery can prevent the water vapor from continuously condensing to the frozen soil layer during the frozen soil process, so as to achieve the dynamic balance of mass transfer, while the internal water vapor exhaust pipe can play the role of partial exhaust when the construction is about to complete natural melting. The author points out that this scheme can also be applied to the treatment of underground pollution, because with the flow of water vapor, the concentration of pollutants will also gather, thus achieving the purpose of recovery and removal. Although the changes of mass transfer and internal pressure are given, the description of heat transfer process during phase change is not given.
5 Conclusion and discussion
In this paper, the research progress of ground source heat pump in Japan was summarized at the Japan Refrigeration and Air Conditioning Conference last year. Although it partly reflects the research and utilization of ground source heat pump in Japan, it reflects the frontier direction of this field in Japan to some extent. Using ground source as heat source or cold source of heat pump has more advantages than air source. The author summarizes the main advantages of using ground (ground) source air conditioning system as follows:
(1) The operation is stable. From the stability condition of air conditioning heat source, the vertical buried tube heat exchanger is better than the horizontal buried tube heat exchanger. Soil burial is better than lake and other open areas. Air source heat pump is generally difficult to start below-15℃, while the outlet temperature of ground source underground heat exchanger is generally higher than this temperature.
(2) Seasonal heat storage and cold storage can be used.
(3) Compared with air source heat pump, it has higher COP.
Many application examples are introduced above, and some empirical data or design models are given. But the author reminds readers that it is best not to copy these data. As mentioned in the related basic and soft science research section above, the surface boundary condition, as one of the basic application and simulation conditions, is still under study. Reasonable design depends on consideration of many factors. Designers need not be discouraged, because many heat or substance transfer processes have their limits or limitations under certain conditions. As long as they understand the corresponding restrictions under this condition, they will have the initiative in design. This is the so-called design standard problem, and it is also an urgent problem to be demonstrated and established in China.