Water-to-air-heat exchanger and indirect evaporative cooling in buildings with green roofs

- Green roofs may be problematic in climates with high cooling loads when they remain too warm over night.
- Water-to-air heat exchangers may provide an additional cooling strategy for buildings with green roofs.
- An investigation of the coupling of green roofs with water-to-air heat exchangers is performed.
- Test cells using different water-to-air heat exchangers have been monitored in a hot and dry climate for over a year.
- The additional benefits of evaporative cooling to the water heat sink tank are assessed.

Green roofs have been proposed for energy saving purposes in many countries with different climatic conditions. However, the energy saving potential of green roofs depends on several aspects, such as the climate characteristics or the building loads. For this reason, the authors have been working on ways to modify the thermodynamic behavior of green roofs through passive low energy systems operating according to rules based on the relationships between the indoor and outdoor temperatures. This paper discusses the improvements in indoor thermal comfort which can be obtained by adopting water-to-air heat exchangers and indirect evaporative and radiant cooling strategies in buildings with green roofs. The study specifically looks at the effect of combining a simple evaporative/radiant system that cools the water pond where the water-to-air heat exchange occurs. Ad-hoc built test cells were investigated in southern California for over a year. Overall, the water-to-air heat exchangers proved to cool the indoor air in the test cells by almost 10 °C when the exterior temperatures were above 35 °C. In this system, the heat from the interior of the cells that could not be absorbed by the green roof, was transferred to the coupled water sink, and then dissipated into the atmosphere. This study shows that the benefits of the water-to-air heat exchange and of the evaporative cooling system are promising, while the water consumption is limited. Finally, the experimental investigation summarizes the benefits of combining green roofs and evaporative and radiant cooling of a water-to-air heat exchanger as a solution for building cooling and proposes simple equations to anticipate their temperature cooling effects.