Weak current College】 【heat pump ground heat exchangers review and improvement of heat transfer models --- Power By】 【China power house network.

Buried pipe ground source heat pump technology is currently the leading edge of air conditioning in the field of research. Buried pipe heat pump system with a high coefficient of performance, energy efficiency, renewable energy, environmental effects, the advantages of simple systems, in Europe and more widely applied, but in our country is still in its infancy, there are only very few underground pipe heat pump unit run successfully. China's vast, rich of surface layer (usually less than 400m) geothermal resources, therefore the effective use of shallow geothermal resources to overcome the traditional heat pump technology limitations and shortcomings, is very meaningful and practical value. Currently buried pipe heat pump is not like the rapid development of air-source heat pump that, in addition to the underground tube heat pump of high initial investments and to the requirements of rock and soil, do not exclude still lack reliable system design and simulation tools. Cane and calculation considers the current Forgas through North America's underground pipe heat pump works instance ground heat exchangers of the tube length is greater than the actual rate of 10% to 30% [1], which makes the short term recovery funds more impossible, is not conducive to the underground tube heat pump in the development and promotion.

Ground heat exchangers and rock steady-state heat transfer between right and wrong, and heat transfer in an infinite area, the process is very complicated, many influencing factors. From the existing literature, on the ground heat exchanger for heat transfer mechanism analysis focused on research and on the application stage, the soil body temperature field of study is based on heat conduction theory, consider groundwater movement of heat transfer and heat transfer mechanism, and so on very few, and domestic and foreign literature indicates the horizontal flow of groundwater on the soil body heat transfer processes have great impact [25]. Therefore, in order to understand how to set ground heat exchangers after the original characteristics of the underground environment, on heat conduction and groundwater movement of ground heat exchangers heat transfer mechanism is necessary for research.

1 ground heat exchangers of heat transfer models

Affect the buried pipe heat pump system performance factors are, including groundwater flow, backfill material performance, heat exchanger around the possibility of transformation, as well as along the length changes of physical properties of rock, and so on, how to improve ground heat exchangers-heat transfer model, make it a better simulation of ground heat exchangers of real heat situation, determine the best ground heat exchangers is the size of the development and promotion of underground pipe key ground source heat pumps.

Buried pipe system at present is still in the research phase, it has always been buried pipe heat pump technical difficulties, the existing underground pipe heat pump design methods are mostly based on a ground heat exchangers. Ground heat exchangers in general there are three forms, namely vertical buried, horizontal pipe and spiral pipe. Horizontal tube usually shallow burial, technical requirements, the initial investment is often less than the vertical buried; but because of the horizontal tube heat exchanger capacity is often less than the vertical buried, but laying area, excavation volume, sometimes it may not be the economy. According to the laying of differently, vertical buried usually has a u-tube and pipe two, and buried pipe heat pump project commonly used u-tube heat exchanger, although tube heat exchangers heat better than u-tube heat exchanger, but because of its initial investment, application there is very little, only used for shallow buried way.

Existing ground heat exchangers design software based primarily on line heat source theory, cylinder heat source theory [68], energy balance theory [915], establishing control equation. In the design of ground heat exchangers to consider long-running after ground heat exchangers, heat, heat imbalance caused by soil body temperature raise or lower the temperature of the analytical method for the ability to easily and quickly get long run results and personable, but if you consider the inlet and outlet water temperature, water flow rate, the geological layer and fill with affected analytics solution is difficult, therefore, you must make some necessary simplification, such as u-tube is equivalent to an equivalent single-tube to use column source theory, or as an infinite line heat source to use line heat source theory, etc. For long-running, these simplified little effect on the results, but for short run is not, this time using numerical solution more efficient. So there are some model considering the numerical and analytical methods.

Uses the parse method of calculation models mainly: Ingersoll model [1617], IGSHPA model [18], Hart and Couvillion model, Kavanaugh model [19] and Mei model [20], you can reference [21].

Design by numerical method to calculate the model mainly has following several. 1) Mei and Emerson heat transfer model and calculation methods [22]

Mei and Emerson developed an applicable to horizontal pipelines, considered around a mathematical model of permafrost. The mathematical model using finite difference method three one-dimensional PDE, describes around, tundra and distal region of the radiation heat transfer process. In addition, and on this basis, additional fluid along on tube length direction of one-dimensional heat transfer equation, a quasi two-dimensional model. The model for the wall, permafrost areas with different time step, for pipe fluid and non-permafrost areas with much more time step. Mei and Emerson provides analog values and 48d experimental values of comparison results.

2) Eskilson heat transfer model and calculation methods

Eskilson takes one dimensional a temperature response factor g function to simulate the underground tube heat exchangers of temperature field. The underground tube time varying thermal flow into the step function, then the step function together to strike the rock area of temperature field.

3) Nwwa (nationalwaterwellassociation) model and calculation methods

The model is in Kelvin line heat equation analysis solutions established on the basis of the soil layer, thereby determining the temperature field heat exchanger size. This method is also a kind of common ground heat exchangers calculation method, it can directly give the average within the heat exchanger and a temperature of fluidSuperposition method simulation of heat pump intermittent running [24].

4) Glhepro and Gchpcalc model

Glhepro model is in Sweden Lund University of heat transfer models based on analysis, may 1-year or multi-year to design vertical heat exchangers length. This heat transfer analysis is not only suitable for groundwater movement, there is no balance of thermal inhalation or release. While it is under design Gchpcalc model under the soil body heat absorbed or released to calculate the length of the heat exchanger [25].

5) Muraya model and calculation methods [25]

Muraya use a dynamic, two-dimensional finite element model of U-tube feet hot interference problems between. The model by trying to define heat exchanger efficiency, based on the soil body composition and backfill soil characteristics, 2-pin spacing, distal and tube temperature and the thermal diffusivity to quantify this type of interference problems. The model has been used constant heat flux, temperature two conditions of heat source theory analysis method of authentication. Using this model, you can calculate depending on tube geometry of synthetic heat transfer efficiency and impact of backfill.