Direct-Ground Cooling Systems for Office Buildings Design and Control Considerations

The direct-ground cooling systems are defined as systems in which the ground is used as the only source for cooling the buildings. Such systems benefit from exchanging heat with the ground, of which its temperature is basically constant below a certain depth year round. As the electricity demand of such systems is only for driving the circulation pumps, the direct-ground cooling systems are the most environmentally sustainable and energy efficient systems available for cooling buildings. A comprehensive literature review has been done for the three main parameters viz. for the ground cooling systems, including ground thermal properties, borehole thermal resistance and building thermal load. All of these parameters have been investigated regarding their influence on the energy demand of the system. The terminal units operate with high-temperature chilled water, as they are suitable indoor heat terminal units for the direct-ground cooling application. The high temperature cooling terminal units have been studied regarding their working temperature levels and cooling capacities. Two control methods viz. supply temperature control method and flow rate control method, have been applied for direct-ground cooling systems to a ground-coupled ceiling cooling panel system and a fan-coil unit and the results have shown that the design of the control system shall be done in relation to the flow rate limits in the building and ground loops as well as the temperature levels of the ground. A high flow rate of fluid in the ground loop or in the building loop will not enhance the cooling capacity of the terminal units, but only increase in the energy use of the circulation pump. In contrast, the very low flow rate in the building loop increases the condensation risk on the pipes because the supply water temperature in the building loop becomes closer to the ground temperature which is below the dew point of the space.