| Abstract |
This work focuses on the compact geothermal heat exchangers. Currently, the main geothermal exchangers found on the market are of two types: vertical or horizontal: ? Horizontal ground heat exchangers are buried at a depth of about 1.5 to 2m, their implementation is therefore simple and inexpensive. However, the exchange area necessary is 1.5 to 2 times the surface to be heated, which restricts the use of this technique in many cases. ? The vertical ground heat exchangers are buried at about a hundred meters depth; they do not require large areas of land but are expensive, especially for drilling part. Compact geothermal heat exchangers are a good alternative to expand the use of geothermal energy in building systems. They only need 5 meters deep drilling (not too expensive) and an area of 10 to 20 m² for each exchanger. There is a real need from industrials that use such technology. The objective of this project is to optimise this new kind of compact geothermal exchangers to capture a maximum energy from soil. Three axes are developed for numerical simulation part. First, the dynamic of the system is optimised; followed by a focus on the geometry to finally test dynamic and geometry all together. A measured campaign will take place in the first quarter of 2010 which allow comparing the calculation with the different dynamics. Usual numerical models of compact geothermal heat exchangers use a simplified approach with a continuous solicitation during the heating season. The innovative part of this study is to explore short thermal solicitations. Developing a new model is necessary to take into account shorter time step (around one hour). Regarding the time response of the system, it is necessary to simulate the whole system taking into account the heat exchange in the soil, to the energy needs of the building and behaviour of the heat pumps. Initially, a 2D approach representing an existing compact spiral coil heat exchanger coupled with a heat pump was performed with the ANSYS/FLUENT software to validate all of the heat transfer fluid flowing through the coils (soil-fluid, fluid-heat pump). This model will be further refined across time and coupled with energy needs of the building. The soil is represented by a 50 meters long and 20 meters depth rectangle. Its temperature is imposed at the edges over time and depth using the following equation: T(x,t) = T0 e- ? 2ax cos ?t - ? 2a x ?? ? ?? ? x: depth [m] t: time [s] ?: thermal diffusivity of soil [m²/s] The compact geothermal exchanger is modelled by a series of 29 coils of 2.5 cm diameter. The hypothesis of the coils was made to simplify the geometry and can thus make a calculation in two-dimensional axi-symmetric. Initially, the temperature of the water in the coils is equal to the soil temperature (realistic assumption throughout the period when the heat pump doesn’t work). The heat pump is represented using a linear function from the manufacturer’s data that links inlet temperature to outlet temperature of the ground heat pump. Thus the temperature of the first coil is imposed by the heat pump, the software then calculates the heat flow exchanged between the fluid and the soil. Compact geothermal exchangers are subjected to variable thermal duration and intensity in order to identify and characterize the cycles of charging and discharging heat from the ground near to them. |