Optimization of ground-coupled systems for heating and cooling of buildings
Licentiate thesis, 2005
The use of ground-coupled systems in commercial buildings offers economic as well as environmental advantages. When both heating and cooling is required, a ground-coupled system can function both as a heat source and a heat sink. At the same time, it can also serve as a short term as well as a seasonal storage.
The main goal of this thesis is to show the effect of different building load patterns and different ground system designs and configurations on the overall efficiency. The results of this work can be used for technical as well as economical optimization.
The methodology used was to study by simulation the influence of the heating and cooling load characteristics of the building (load pattern), the size of the ground system (depth and number of the boreholes) and the geometry of the ground system (configuration, i.e. positioning of the boreholes). To facilitate the simulations, simple models of the heat pump and cooling system heat exchanger were derived. These models, together with data for different building load profiles and ground system configurations, were used to provide inputs to the ground system design software EED. The models developed in this work together with the design software, were applied in a case study (Astronomihuset, Lund). Simulated and measured data agree reasonably well.
As a result of this work, three key numbers were introduced to characterize the system of building-heat pump-ground system. Firstly, the load factor describes the relation between heating and cooling loads in the building. Secondly the specific borehole load is an indicator of the effect of the building loads on the borehole. Thirdly, the geometry factor is a measure of the potential of the ground system for heat exchange with the ambient ground and/or as a storage.
Examination of simulation results with different configurations using the same specific borehole load showed that the brine temperature differences are not very large. On the other hand, changes in specific load have a large influence. Furthermore, the simulations show that a system with balanced heating and cooling and storage is more sensitive to the accuracy of the assumed load pattern than is a system for heating or cooling only. A general observation is that with a dominant heating or a dominant cooling load in the building, a linear borehole geometry is preferable. With a balanced heating and cooling situation, however, a rectangular layout with storage capability may be advantageous.