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From an indoor air environment perspective: evaluation of low-exergy systems in office buildings!
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August 14, 2012

From an indoor air environment perspective: evaluation of low-exergy systems in office buildings!

Healthy Buildings Conference, July 11, 2012, Brisbane, Australia

Low exergy heating and cooling systems allow the use of low valued energy, which is (easily) delivered by sustainable energy sources (e.g. by using heat pumps, solar collectors, either separate or linked to waste heat, energy storage etc.). Low-exergy system is able to provide heating and cooling in a building where the same radiant system can be used for heating with low temperature water in winter and for cooling with high temperature water in summer. This solution ensures a comfortable indoor climate without the risk of local discomfort. Practical measurements of indoor climate parameters in comparison with the requirements of the international standards can help to assess the low-exergy systems in buildings with glass façade.

Emilia Durisova from the Slovak University of Technology in Slovakia presented her research carried out in three buildings with different types of low-exergy systems in Slovakia (location Bratislava: average day temperature in winter -2°C and in summer 30-34°C). All three buildings have glass façades and are used for commercial purposes. The first building (I.) has a radiant heating and cooling provided through the metal panels made of copper pipes (8 and 12 mm diameters), built in the suspended ceiling. The second building (II.) has capillary pipes (diameter 5 mm) connected to a heat pump and these pipes are embedded in the additional surface layer of the ceiling next to the concrete panels ensuring thermal storage in the slabs. The third building (III.) has a system of pipes (diameter 20 mm) embedded in  massive concrete slabs with a heat pump as a source of cooling and heating.

All kinds of field measurements were carried out in buildings in short and long term sessions in different seasons along with CFD simulation. Air temperature in the building (III.) varied greatly in winter period although in summer period proved to be more stable. The average temperature in all buildings was around 24°C which corresponds to category II in standard EN 15 251. Measurement results of relative humidity show the RH in winter between 20-50% and in summer between 60-70% corresponding to the requirements of ISO 7730 (30-70%). Short term measurements indicate that the cooling system in buildings (I. and II.) provides good cooling (category I). Nevertheless, buildings with glass façades might have higher air velocity of cooling air from ventilation system during hot summer days. The air velocity in occupied zone was between 0.05-0.08 m/s and this poses no significant risk of draught and falls into the category A or B. The CFD simulation proved that one of the advantages of low-exergy systems is the lower air velocities, i.e. below 0.15 m/s.

Low-exergy system provides good heating and cooling in buildings with glass façades, where the attention must be paid to the significant risk of local thermal discomfort. This system ensures that the temperature is stable and there is no draught. This reduces the possibility of the negative effect of a glass façade. The indoor thermal comfort can be easily affected by outdoor climate (risk of overheating by sun or by insufficient controls of building systems) and so it may have a negative impact on changes in the indoor environment of buildings. Low temperature radiant systems provide comfortable environment for the occupants and these systems can operate effectively, especially in the summer period.

Photo by Petra Vladykova