Geocooling Handbook - Cooling of Buildings using Vertical Borehole Heat Exchangers

Pahud, Daniel and Belliardi, Marco (2011) Geocooling Handbook - Cooling of Buildings using Vertical Borehole Heat Exchangers. Project Report UNSPECIFIED

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Geocooling systems with borehole heat exchangers integrated in office buildings are examined. The analysis is focused on the thermal interaction between the building and the ground coupled system, heating and cooling distribution included. This study takes advantage of previous studies and simulation tools made in the field of borehole heat exchanger applications. A low energy office building is defined and used as a reference. A detailed methodology is established for building analysis and geocooling system sizing. Building design as well as weather data are studied, using climate data of both north and south side of the Alps. Sensitivity to the main sizing parameters of the ground coupled system is analysed. Definitions of sizing keys and capacity values for heating and geocooling are proposed to analyse the results of all simulations. They provide simple and fast design guidelines for a pre-sizing of a geocooling system. The building envelope has a strong influence on the geocooling system sizing. Most of the studied cases showed that triple instead of double glazing windows enables to halve the total borehole length. Prerequisites to the feasibility and efficiency of a geocooling system are a low energy building concept, solar protections complying to the requirements of Swiss norm SIA 382/1, internal heat gains that do not considerably exceed standard values of a typical office building and so on. These building prerequisites normally make possible to heat and cool the building with active concrete plates. Dehumidification, if necessary, is limited to the minimum indispensable for indoor air quality purpose. In this case the ventilation system is only used to provide latent cooling energy and is not coupled to the ground system. Active concrete plates are the most suitable distribution system to minimize both heating and cooling annual distributed energies. Their auto-regulating properties are a key factor to avoid heating and cooling conflict during midseason. Allowing for high cooling distribution temperatures (greater than 20°C), they are also most suitable for a geocooling application. One of the most important parameter on the total borehole length is the ground recharge ratio. Best geocooling systems are obtained at a seasonal ground recharge ratio of about 50%. Good systems are obtained with a ratio lying between 40 and 80%. Assuming a ground thermal conductivity of 2 W/(mK) and borehole heat exchangers placed under the building, typical sizing keys are: Heating criterion: 25 – 35 W/m relatively to the design heat extraction rate. Geocooling criterion: 30 – 50 W/m relatively to the maximum distributed cooling power. Another key parameter that conditions the geocooling potential is the temperature difference between design forward fluid temperature in the cooling distribution and the initial ground temperature. The higher the design forward cooling temperature the greater the cooling potential. A capacity value is defined on the basis of the design key to take into account the available temperature difference. A similar approach is also applied to the heating design key. Capacity values that derive from the typical sizing keys are: Heating criterion: 2 – 3 W/m/K relatively to the design heat extraction rate. Geocooling criterion: 6 – 7 W/m/K relatively to the maximum distributed cooling power. Some examples are calculated to illustrate the use of the pre-sizing design rules. However the available simple design rules do not substitute a proper system simulation. It is important to have the possibility to simulate, as no general design rules can be stated. As a by-product of this research project, the COOLSIM2 tool is available for ulterior studies of such geocooling applications.

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