The utilization of participating gases and long-wave thermal radiation in a passive cooling skylight

The motivation for this thesis was to study how the greenhouse effect, which controls the temperature of our planet, could be put to use for future energy needs. The main objective of this work was to identify and quantify the possibilities of utilizing a controlled greenhouse effect inside a multiple-glass window.

The way so-called participating gases influence the heat transfer between space and earth is here harnessed to generate consumer energy or lower the need for energy use. The intention is to control the radiative heat flow from ground level buildings or other objects to the sky by using a gas that absorb and emit thermal radiation. The gas is contained in a skylight where it acts as the working fluid in a heat exchanger that “connects” the room with the sky. This requires that the skylight is constructed from special materials and that the skylight design is suitable for building applications. Therefore, a cooling or an insulating effect can be produced to the space located below the skylight. This implies that the sky should not only be seen as a potential source of energy in the form of the direct sunlight but also as a low-temperature reservoir of cooling. The cooling effect is obtained through radiative heat exchange between a radiator, located on the surface of the earth, and cold air masses situated above this radiator.

As the cooling demand for residential houses in Finland is around 5 W/m², a radiative cooling skylight as tested in this work could fulfill the cooling demand of a 20 m² room. Moreover, the need for cooling is growing. Thus, radiative cooling provides a viable option to decrease the cooling demand of a building.

The annual variations and the cumulative frequency distribution for varying radiator temperatures were investigated. These were based on weather measurements procured by the Finnish Meteorological Institute. Based on the weather measurements in Helsinki, a radiator working at room temperature can emit a heat flow of 100 W/m² during 70% of the time.

The radiative cooling and insulating performance for the proposed skylight was first assessed for typical summer and winter conditions in Finland using resistance networks and, later, with simulations. The reason for this was that the radiative and convective heat transfer networks described physically very different processes, and therefore results were inadequate. The thesis presents results from simulation work done with the modeling software COMSOL. Results from the first simulations showed that 117 W/m² of cooling can be achieved with the proposed system at average summer conditions in Helsinki using CO₂ as the gas in the skylight (compared to 15 W/m² when using air). The same skylight in insulation mode showed transfers of only 88 W/m² heat under the same conditions with CO₂ (compared to 19 W/m² when using air). Polyethylene was used as the window material for containing the gas inside the skylight.

A further evolvement of the skylights radiative resistance network model was presented. This updated model allowed the wavelength to be divided into four different band sections. By this, it was possible to compare the radiative cooling possibilities of various gases. In the first resistance networks and simulations, the gases’ absorptance had no wavelength dependency. Therefore, the calculations were expanded to include wavelength dependency. The initial gas, CO₂, was confirmed to give limited results at ambient pressures, and an increase in pressure would increase material costs and decrease the transmittance of the window used. The gas should have a high transparency in the visible wavelength range, and a high absorptance in the atmospheric window spectral range (8 – 13 µm). Furthermore the gases’ health, safety, and environmental impact were assessed.

New materials studied were pentafluoroethane, a hydrofluorocarbon gas, and Zinc sulfide as window material. Zinc sulfide is transparent to both visible light and long-wave heat radiation. Changes to the mechanical design enable the skylight to be built to different dimensions by re-dimensioning the window to an optimal size, and then modularizing it to a shutter-type skylight. This new design was optimized for both summer and winter use. When in summer mode, the skylight’s cooling capacity is maximized, corresponding to as large a heat transfer through the skylight as possible. While for winter use, the insulation property is maximized, corresponding to a minimal amount of heat transferred through the skylight. For the radiatively cooled skylight, the gas thickness between the enclosing windows must be long enough so that the gas can emit enough heat radiation. The gas thickness must also be long enough so that the designed convective cooling will take place while, on the other hand, when in insulating mode small enough to prevent a significant convective heat transfer.

The effects of air, CO₂, and pentafluoroethane were studied in a set of outdoor experiments under different conditions, showing that indeed temperatures below the ambient can be reached in the skylight. Computational fluid dynamics simulations based on temperature measurements from these experiments imply that a controllable cooling effect of 100 W/m² can be attained with the designed device during summer. The cooling effect in the current setup is limited to nighttime due to the influx of sunlight during the day.

Since the amount of cooling needed for refrigeration and air conditioning is expected to increase significantly in the near future, so will the amount of energy required to produce it. However, by radiative cooling, less or no external energy input is needed for this.

This thesis concludes that skylights which have so far brought light and a sensation of space, could be used to actively decrease a building's energy use for heating or cooling. During a clear night, a passive cooling effect of 100 W/m² is certainly achievable, but the amount of cooling is, of course, weather dependent.