Underground buried heat exchanger is a key component of ground source heat pump system. Whether the form of selection is reasonable and whether the design is correct is related to whether the whole ground source heat pump system can meet the requirements and normal use. At present, there are two main forms of ground source heat pump underground heat pipe heat exchangers, namely horizontal buried pipes and vertical buried pipes.
1, horizontal buried pipe
The horizontal buried pipe mainly has the form of single groove single pipe, single groove double pipe, single groove two layer double pipe, single groove two layer four pipe, single groove two layer six pipe, etc. [1], because the lower layer pipe of the multilayer buried pipe is in A more stable temperature field, the heat exchange efficiency is better than a single layer, and the floor space is small, so the application of multi-layer tubes is more. In recent years, two horizontal buried pipes have been newly developed in foreign countries, one is a flat curved tube and the other is a spiral tube. They have the advantage of shortening the length of the trench and increasing the length of the tube that can be buried. The embedding of pipelines depends on the geotechnical conditions, and trenching or large-area excavation methods can be adopted. The optimum depth of the single-layer pipe is 0.8-1.0m, and the double-layer pipe is 1.2-1.9m, but it should be buried below the local freezing line in any case.
Because the buried depth of the horizontal pipe is shallow, the performance of the buried heat exchanger is not as good as that of the vertical buried pipe, and the construction site takes up a large space. In actual use, the single layer and the multiple layers are often matched with each other; the spiral pipe is superior to the straight pipe. But it is not easy to construct. Because the shallow buried horizontal pipe is greatly affected by the ground temperature, the underground rock has good heat balance in winter and summer, so it is suitable for single season use (such as Europe only for winter heating and domestic hot water supply), which is very good for users of winter and summer heating and cooling system. less.
2, vertical buried pipe
According to the different types of buried pipes, there are generally single U-shaped tubes, double U-shaped tubes, small-diameter spiral coils and large-diameter spiral coils, vertical cylindrical tubes, spider tubes, casing tubes, etc.; The difference is divided into shallow burial (≤30m), medium burial (31~80m) and deep burial (>80m). The most commonly used U-tubes, sleeves and single-tube types are briefly described below.
1) U-shaped pipe type is to install U-shaped pipe in the bored pipe well. The general pipe diameter is 100-150mm, the well depth is 10~200m, and the U-shaped pipe diameter is generally below φ50mm (mainly the flow should not be too large). Because of its simple construction, good heat transfer performance, high pressure, less pipe joints, and less leakage, it is currently the most widely used. For example, Stokes University in California supplies ground-source heat pump systems for 480,000 m2 air-conditioned buildings. There are 390 underground buried pipes with a depth of more than 120 m. According to reports, the use of such ground-source heat pump systems can save various kinds of air conditioners every year. The cost is $455,000, of which the energy cost is $330,000, the power saving is 25%, and the fuel cost is 70%.
Some foreign projects bundle U-shaped tubes on the steel mesh frame of the pile foundation, and then water the concrete without occupying the ground. For example, a ground source heat pump system in a factory in Switzerland absorbs heat or cold from 600 pile foundations and is used for heating and cooling of 20,000 square meters of buildings.
2) The outer diameter of the tube-type heat exchanger is generally 100 to 200 mm, and the inner tube is φ15 to φ25 mm. Since the heat exchange area between the outer wall of the pipe and the rock is increased, the heat exchange capacity per unit depth is high. According to the test results of the literature [2], the heat transfer efficiency is 16.7% higher than that of the U-shaped pipe. The disadvantage is that the diameter of the casing and the diameter of the borehole are large, the lower tube is more difficult, and the initial investment is higher than the U-shaped tube. It is not easy to handle at the end of the casing and the inlet and outlet of the inner pipe, and it is easy to leak. Therefore, it is suitable for vertical buried pipes with depth ≤30m. It is prudent to use this form for burying. In order to prevent water leakage, the end portion of the casing end should be manufactured by the factory and installed on site to ensure the tightness.
3) Single-tube type is often called “hot well†in foreign countries. It is mainly used as a heat pump system for groundwater as a heat source. Generally speaking, this type of investment is less. The installation method is that the steel casing is used as a jacket above the groundwater level, and the diameter and the aperture are the same, and the typical aperture is 150 mm. Below the groundwater level is a natural hole with no facilities. There is an outlet pipe in the hole to supply water to the heat pump unit, and the return water naturally discharges or returns to the tube well. This method is greatly restricted by groundwater resources and relevant national policies and regulations.
This article is reproduced from Langfang Zhongyang Insulation Materials Co., Ltd.: http://
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