Direct Ground Cooling Systems for Office Buildings

Doctoral thesis, 2021

The solving of crucial global energy challenges hinges on improving energy efficiency in building energy systems. Accomplishing energy-efficiency targets often entails incorporating sustainable energy sources into the energy supply system. Direct ground cooling systems (DGCSs) are among the most sustainable technologies for comfort cooling in office buildings. With this technology, cooling is provided by the circulation of the working fluid through the ground heat exchangers. This technology is mostly used in cold climates where the underground temperature is low, and the building cooling loads are low enough to be offset by the ground cooling. Using only a modest amount of electricity to drive the circulation pumps, this technology is incredibly energy efficient. However, designing DGCSs presents some unique challenges, and only a handful of studies on this subject are available.

This work aims to develop knowledge about comfort cooling for office buildings using DGCSs and expand upon design and operation practices for this technology. The findings presented in this work are based on experimental and simulation results. The experimental results build upon existing knowledge for operating cooling systems and substantiate new operation methods for the DGCSs. The experimental results are also used to develop and validate simulations. The simulation results facilitate investigating the short- and long-term thermal and energy performance of the DGCSs for various design circumstances.

The borehole system design is usually performed independently from the building energy system design. In view of this work’s findings, considering the whole system (borehole, control system, terminal units) can enhance the design. A sub-system’s input design requirements can be aligned with the corresponding output of other sub-systems in a comprehensive design approach.

This work demonstrates and quantifies that terminal units with slow response, such as thermally active building systems (TABS), can smooth out the daily peak heat rejection loads to the ground, resulting in shorter boreholes. Thus, the ground system can be much smaller than required for fast-response terminal units, such as active chilled beams.

This work analyses different operation practices for DGCSs. The results suggest that allowing the room temperature to rise somewhat during the “on-peak” cooling loads can reduce the ground heat rejection loads, for which shorter boreholes can be designed. If combined with precooling the space during the “off-peak” cooling loads, a further reduction in the ground loads is yielded.

This work also investigates the design and application of the DGCSs in existing office buildings. A systematic approach is provided to evaluate the influence of common renovation parameters on the design and energy performance of a DGCS. The systematic approach includes a step-by-step methodology to explain how sensitive the subsequent system design might be to the variations in the renovation parameters. Furthermore, the results quantify the potential electricity savings by using the DGCS instead of a chiller.