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Introduction to GEOTABS: combining thermally activated building systems (TABS) with ground coupled heat pumps (GCHP) and passive heat exchangers

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GEOTABS operation modes ["Advanced system design and operation of GEOTABS buildings", REHVA, 2013]

GEOTABS is an acronym for the combination of geothermal heat pumps with thermally activated building systems (TABS).  It is an innovative clean technology for energy efficient and healthy buildings which has several advantages; the radiant heating and cooling system has already proven to be one of the most comfortable ways to condition indoor spaces, especially multi-storey offices.

 

Due to the large emission surfaces, relatively low fluid temperatures can be used for heating and relatively high ones for cooling of buildings. Supply temperatures close to the desired indoor temperature allow the implementation of geothermal systems to become highly efficient. The ground can work alternately as heat source and heat sink to allow underground thermal energy storage.

 

As the figure shows, GEOTABS generally has three operation modes.

  • Heating mode. The low temperature ground source needs to be upgraded through heat pumps to obtain the required supply temperatures.
  • Passive cooling mode. The ground temperature is mostly sufficiently low to obtain the required supply temperatures by direct use of the ground; no heat pumps need to be implemented and singular passive cooling is possible in most cases.
  • Active cooling mode. Reversible heat pumps (called chillers in case of cooling) can be implemented to achieve lower supply temperatures.

 

Since the temperature differences between the heat source and heat sink (i.e. the TABS) are relatively low, the use of Ground Coupled Heat Pumps to upgrade the low-grade energy source is also highly efficient. If electricity, either from the grid or produced on site e.g. by a photovoltaic system, is the only source of energy needed to feed the heat pump, no Greenhouse Gases are produced in the building by the GEOTABS.

 

The thermal inertia of the emission system allows load shifting by thermal energy storage in the structural elements. Intelligent management of these systems use the load shifting ability as an advantage to stabilize the electric grid and match supply peaks with demands in order to develop towards smart electric grids.

 

GEOTABS is compatible with the Trias Energetica strategy (elaborated by Duijvestein at the TU Delft):

  • Reducing energy demand. Besides the fact that GEOTABS buildings are generally well insulated, the radiant emission system (TABS) reduces required ambient temperatures in the building to achieve optimal comfort conditions, having a reducing effect on the energy demand.
  • Using sustainable sources of energy instead of fossil fuels. GEOTABS aims at an increased use of low-grade energy sources (i.e. low exergy systems) on one hand and upgrading low or moderate temperature sources on the other hand. The advantage of seasonal thermal underground storage is integrated in the concept’s efficiency as well.
  • Using fossil energy as efficiently as possible. In practice GEOTABS still requires a secondary, more flexible emission system to compensate the base system's limited ability of providing fast responses to changing set points. However, by using fossil energy as clean and efficiently as possible (e.g. heating with low water temperatures), and implementing an advanced control that limits its operation as much as possible and gives priority to the GEOTABS component (e.g. by prediction), the hybrid solution is altogether optimizing the energy sustainability of the building.  
Introduction to GEOTABS: combining thermally activated building systems (TABS) with ground coupled heat pumps (GCHP) and passive heat exchangers