Effect of the ground properties on the fluid temperature of Geothermal Heat Exchangers

Abstract

Ground heat pumps (GHP) are very efficient devices that use the ground in order to operate. GHPs reach a high coefficient of performance (COP), which is between 3 and 6 as opposed to 1.75–2.5 for air-source heat pumps. Actual COP of a geothermal system that includes the power required to circulate the fluid through the ground heat exchangers (GHE) can be lower than 2.5. The setup costs are higher than for conventional systems, but the difference is usually paid back in energy savings in 3–10 years. The ground heat exchangers are made of polyethylene or polypropylene tubes transferring heat from or to the ground from the heat pump. This study describes the modeling, based on the convection-diffusion equation, of two types of GHEs, vertical or horizontal, and examines the effect of the properties of the ground on the fluid temperature circulating in them. For this purpose two simulation models were constructed one for each type of HE and the FLEXPDE simulation software was used for running the models. The large variety of ground types of Cyprus, ranging from volcanic to sedimentary, were used for measuring the thermal properties and initial temperature. It is shown that a nearly linear relation exists between the the initial ground temperature and the mean fluid temperature of a vertical HE, under the conditions examined. Usually it is mentioned that a vertical HE is more efficient than a horizontal HE because, in the cooling mode, the ground temperature below about 10 m is always cooler that the top layer temperature, which means that the horizontal HE is embedded in hotter ground. The results here show that this is not always so: a horizontal HE will produce a hotter outlet fluid than a vertical one if the center to center (c-c) tube distance is kept equal to that of the vertical HE. If the c-c distance is increased enough then the horizontal HE may produce a lower temperature than the vertical one. This can be done easily because the width of the trench that the horizontal HE is placed in can be increased as it is on the surface, while the borehole diameter cannot. It is also shown that there is a critical c-c distance of the horizontal HE, in the examined case being about 0.7 m, after which no observable effect will result on the mean fluid temperature. Finally, a comparison is made between various layer materials in order to study the effect of the ground type on vertical and horizontal HEs. For this purpose the thermal diffusivity of the ground is varied and is shown that diffusivities above 16 m2 /s are not important for the vertical HE. Below that value diffusivity is very important because the lower the value the greater the temperature of the HE. It is observed that an increase of temperature of about 6.5°C corresponds to a diffusivity value of 3.4 m2 /s. For the horizontal HE the critical diffusivity value is about 9 m2 /s, while a lower value of 3.4 m2 /s will produce a temperature difference, in the mean fluid, of about 3.5°C.