Passive and Low Energy Cooling of Office Buildings

In this work, the cooling performance of night ventilation systems and different earth heat exchangetechnologies were experimentally analysed in three office buildings in Southern Germany. One of the firstpassive energy standard office buildings in Europe was extensively monitored over a three year period toanalyse the summer performance of a highly insulated and well shaded building in which night coolingventilation was based on stack effect and cross ventilation. This was combined with a mechanical ventilationsystem incorporating a ground coupled heat exchanger to supply daytime fresh air.For comparison, an energy analysis was made of a mechanically driven exhaust air night ventilation systemand a supply and exhaust air system in the other two buildings. The first of these was a passively cooledrefurbished building in Tbingen which utilises mechanical night ventilation to effectively discharge ceilingswith phase change material. In addition, fresh air cooling is achieved using a horizontal brine-earth heatexchanger. The second comparison building was a low energy office building in Freiburg which uses waterbased ground coupled heat exchangers for fresh air cooling and an exhaust air ventilation system for nightcooling.During a typical German summer, in which the number of hours that the ambient air temperature exceeds25 C is less than 160, the passively ventilated building performed excellently, even with relatively highinternal heat loads of 200 to 400 Wh.m-2.day-1. However, when the ambient air temperature was significantlyhigher, such as in the summer of 2003 (i.e. 3 K higher than the average summer temperature), nearly 10% ofall office hours recorded room air temperatures above 26 C. In the case of the two buildings that were nightcooled by mechanical ventilation, cooling performance was limited by the rather low air exchange rate of2 h-1. This resulted in overnight room temperature reductions of just 2 - 3 K during hot summer nights. Alsothe coefficient of performance (COP) was relatively low for this approach at between 4 to 6.All the earth heat exchangers showed excellent energy performances with COP's between 20 and 50.However, due to the limited fresh air volume flow in such buildings, the earth heat exchanger only removeda small part of the total load.Key words: passive cooling, mechanical night ventilation, ground coupled heat exchangers, summerbuilding performance, passive building standard, phase change cooling.