The energy consumption globally has been increasing drastically the past decades, mainly due to the population growth and the industrial and technological progress. In order to address this issue, the European Union has launched several directives to decrease energy use, increase energy efficiency and increase use of renewable energy sources. The aim is that by 2020 all new buildings should be nearly zero-energy buildings. A solution that could contribute to this is the combination of photovoltaic panels for the production of electricity and phase change material (PCM) for the reduction of peak cooling demand.
In the present simulation study, the coupling of nighttime radiative cooling with PCM for cooling an office room was investigated. For cooling water through nighttime radiative cooling two types of solar panels were utilized, an unglazed solar collector and photovoltaic/thermal (PV/T) panels. Apart from cold water for space cooling, the installation was capable of providing domestic hot water from both types of panels and electricity from the PV/Ts. This system was simulated for the period from 1st of May until 30th of September, under the weather conditions of Copenhagen (Denmark), Milan (Italy) and Athens (Greece).
In Athens and Milan the operative temperature was within the range of Category III of EN 15251 (23 – 26oC, 73.4 – 78.8oF) for 81% and 83% of the occupancy period respectively, while in Copenhagen it was within the range only for 63%. Furthermore, the percentage of PCM used at the end of the occupancy period was 86%, 81% and 80% for Copenhagen, Milan and Athens, respectively. Nighttime radiative cooling provided for Copenhagen 61%, for Milan 36% and for Athens 14% of the cooling energy required for discharging the PCM. Furthermore, the average cooling power per unit area provided by the PV/T panels was 43 W/m2 for Copenhagen, while for Milan and Athens it was 36 W/m2 and 34 W/m2, respectively. The cooling power of the unglazed solar collector was negligible. Finally, the total electricity produced in Copenhagen for the simulated period was 371 kWh, while for Milan and Athens it was 380 and 439 kWh, respectively.
It was concluded that the nighttime radiative cooling can be a satisfying solution for providing space cooling to office buildings. The performance of the installation could be improved by implementing a solar shading system and a more precise control strategy.
