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

Licentiate thesis, 2019

Direct-ground cooling systems are defined as systems in which the ground is used as the only source for cooling mainly in commercial buildings. These systems benefit from exchanging heat with the ground, of which its temperature is basically constant below a certain depth year around. Since electricity demand of these systems is only about driving the circulation pumps, the direct-ground cooling systems are among the most environmentally sustainable and energy efficient systems available for cooling buildings.

This thesis is undertaken with a two-fold aim: presenting the design parameters of the ground-coupled systems, and evaluating the methods for controlling the cooling capacity of the direct-ground cooling systems. A comprehensive literature review has been performed on three main design parameters for the ground cooling systems, including ground thermal properties, borehole thermal resistance and building thermal load. All these parameters have been investigated regarding their influence on the energy demand of the system. The literature survey has been further extended to the terminal units operating with high-temperature chilled water, as they are suitable indoor heat terminal units for the direct-ground cooling application. The most common high temperature cooling terminal units have been studied regarding their working temperature levels and cooling capacities.

Control methods for direct-ground cooling systems is the second major aspect studied in the present work. Two control methods, supply temperature control method and flow rate control method, have been applied to a ground-coupled ceiling cooling panel system and a fan-coil unit in laboratory settings. The experiments have been conducted in an office-scaled test room under different thermal indoor climates and heat gains. 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, and the temperature levels of the ground. A high flow rate in the ground loop or in the building loop will not enhance the cooling capacity of the terminal units, but only caused increase in the energy use of the circulation pump. On the other hand, too low flow rate in the building loop increases the condensation risk on the pipes. This is because the supply water temperature in the building loop became closer to the ground temperature which is below the dew point of the space.