Suhendri, Suhendri
(2023)
A study on innovative ways of incorporating radiative cooling with solar energy application in buildings.
PhD thesis, University of Nottingham.
Abstract
Solar energy technologies for sustainable building heating have been widely established. But most populated regions on earth need both heating and cooling for their buildings. Providing both heating and cooling sustainably for building in one device is particularly challenging since, like solar technologies, the established passive cooling technologies only serve one function. In the recently revived passive cooling mechanism, namely radiative sky cooling or radiative cooling, one may find a solution to tackle this problem. This is possible because the radiative cooling mechanism is a kind of the reverse of solar absorption.
Research on radiative cooling itself is rejuvenated after the breakthrough in passive daytime radiative cooling almost a decade ago. Since then, researchers in the field have been pursuing the real-world application of radiative cooling, besides also enhancing the optical and mechanical properties of the material. Interested in tackling the above mentioned problem and motivated by the revival of the radiative cooling research field, this thesis aims to find innovative ways to use radiative cooling in buildings. Along with radiative cooling as the passive cooling mechanism, the passive heating mechanism investigated in this thesis comes from the sun. Hence, the research investigates strategies to effectively combine radiative cooling with solar energy applications in buildings.
Reviewing previous research on similar topics, there are at least three common themes of how radiative cooling can be combined with solar technologies. Firstly, the radiative cooling mechanism can be used to enhance the performance of existing solar technologies. Secondly, radiative cooling can be used to extend the working time of solar technologies. Also, still related to the second theme, radiative cooling can alternatingly work with solar energy technologies to provide cooling and heating in the respective seasons.
This thesis offers four novel strategies to combine the two technologies in diverse ways. Computational fluid dynamics (CFD) simulations and experiments were conducted to evaluate the proposed concepts. The discussion on the proposed concepts is presented from a fully passive strategy such as radiative cooling combination with solar chimney ventilation to a fully active strategy such as its combination with photovoltaics/thermal module.
The first proposed design is a fully passive solar chimney-radiative cooling (SC-RC) ventilation, as discussed in Chapter 3. The SC-RC ventilation can increase the average daily ventilation flow rate of a conventional solar chimney to 1.2 air change per hour, an increase of 0.2 air change per hour. Also, the SC-RC ventilation can cool the inlet air to be more than 3 °C lower than the room air temperature.
Modified from the SC-RC ventilation strategy to accommodate passive heating into the system and make it a multi-seasonal device, the second proposed design employs a switchable absorber/emitter surface on the ventilation air channel. Unlike the previous SC-RC ventilation that is fully passive, this so-called Switchable SC-RC ventilation also introduces a fan to deliver the ventilation. In the annual term, the Switchable SCRC ventilation can reduce the total unmet hours, and thus provide a more thermally comfortable room than the conventional ventilation. Annually, the unmet hours of the Switchable SC-RC ventilation case are 429 hours fewer than the base case.
Furthermore, the third design proposed to combine a solar air heater with radiative cooling. Changes were made in the spectral properties of a solar absorber and the cover of the collector to allow more thermal emission in the atmospheric window band. This so-called photothermal-radiative cooling (PT-RC) collector collects heating during the day and cooling during the night. When installed as part of an HVAC system, the fully active device has the potential to save up to 32.7% of energy from heating and cooling in the Mediterranean climate.
Moreover, the PT-RC collector design can be more developed into a PVT-RC collector. The difference between them is if the PT-RC collector is a modified solar collector, the PVT-RC, however, is a modified PV module. Hence, the PVT-RC has one additional function which is to generate electricity. Also, the PVT-RC design uses two fluids (i.e., water and air) to collect the heating and cooling energy. The bi-fluid PVT-RC collector is estimated to have a total efficiency of 72% in the daytime. In optimal conditions at night, the bi-fluid PVT-RC collector can provide up to 45 W/m2 cooling power.
To conclude, the proposed radiative cooling combination designs have been shown capable of improving the performance and/or extending the working time of the solar technologies, hence saving energy from heating and cooling. Also, how radiative cooling is integrated with solar energy technologies can be done in a fully passive or fully active way. Nonetheless, further studies still need to be done especially for studying the use of thermal storage mechanisms or for appraising the economic viability of the designs.
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