Date of Award
Doctor of Philosophy
Department of Process, Energy and Transport Engineering
Dr. Michael D. Murphy
Dr. Paul D. O' Sullivan
Overheating in non-residential nearly zero energy buildings (nZEBs) presents a major risk to the well-being, productivity and thermal satisfaction of the people who occupy them. As global temperatures are expected to increase in the future, the overheating risk and cooling demand is also expected to increase. Therefore, low cost and low energy solutions that are able to maintain thermally comfortable conditions without the need for mechanical systems will be vital in decarbonising the building stock and ensuring that buildings are comfortable and remain low carbon now and in the future. Passive cooling has long been championed as a low cost and low energy cooling solution, that when designed and incorporated into buildings correctly, can result in high levels of both comfort and energy performance. However, passive cooling techniques like natural ventilation (NV) need further demonstration to support their incorporation into the dominant design of nZEBs, in particular in non-residential buildings. The overall aim of this research was to determine what potential exists for the use of controlled passive cooling systems in non-residential nZEBs without compromising on thermal comfort performance. The thermal comfort performance of passively cooled non-residential nZEBs was assessed via, 1) a detailed thermal comfort field study of the actual performance of unique nZEB retrofit test-bed building which has a novel multiconfiguration slotted louvre natural ventilation system, 2) the calibration and validation of detailed whole building energy model for said application, 3) a comparative analysis and approach comparison of different occupancy schedules and opening control assumptions, and 4) a simulation-based study of different passive control strategies for maritime and continental climates in external conditions now, and in potential extreme conditions in 2050.
In the first part of this PhD study, the thermal comfort performance of an nZEB test-bed building was measured using both subjective and objective methods, in a thermal comfort field study. The study was designed to assess the performance of the buildings passive NV system (system had a combination of automated openings and purpose provide openings) in response to an overheating scenario during shoulder seasons. Based on the responses of 35 study participants it was found that the use of openings above head height, that had a proportion of net openable area to floor area (POP) of 1.1%, provided the best response to overheating without overcooling. The study also indicated that the effective temperature (ET) index correlated best with the mean thermal sensation votes (MTSV) of study participants (R^= 0.71), with indoor relative humidity (RH) (R^ = 0.65) and air temperature (R^ = 0.56) having the strongest correlations to MTSV of all measured physical internal environmental parameters.
In the second part of this PhD study, a detailed whole building energy model (WBEM) of the nZEB test-bed was made, calibrated under winter, summer and shoulder season conditions, and validated in shoulder season conditions. The part of this PhD study was designed to assess the limits of accuracy of the detailed WBEM for predicting indoor air temperature and RH for the nZEB application. This calibration process utilised a large amount of measured data as inputs into the model and resulted in the model being able to predict indoor air temperature and RH with a low level of root mean squared error (RMSE) (RMSE: Temperatureand RH <19%).
In the fourth and final part of this PhD study, a simulation based study was designed to assess the performance of various passive control strategies in different climates for external conditions currently and in an extreme future climate presented in 2050. The strategies considered combinations of day-time ventilation (D), night-time ventilation (N), external solar shading (S), and the limitations of external RH (R), as well as rigid (non-adaptive set-points) and adaptive set-points (A). The study aimed to determine which control strategies resulted in optimal trade-offs between thermal comfort performance and the potential need for mechanical energy. The results demonstrated that passive cooling systems are a viable solution at maintaining comfortable conditions for over 90% of the occupied hours, however discomfort risks are climate dependant. For maritime climates, overcooling risk was seen as the biggest challenge and it was found that this could be eliminated by using an adaptive control strategy (A_D). For continental climates, overheating was a significant risk. Currently, advanced passive cooling systems that utilise a combination of day-time and night-time ventilation and solar shading (DNS, A_DNS) are capable of maintaining comfortable conditions for over 95% of the occupied hours. However, the results presented here show that the number of occupied hours where overheating is a risk will increase by 4% to 14% from now to 2050 (depending on what passive control strategy is used). The use of the advanced passive cooling systems indicated was found to be capable of reducing the need for mechanical cooling in 2050 by 88% to 92% when compared to a typical day-time ventilation strategy.
Combining the results from all part of this thesis, it was determined that a large potential exists to control and maintain comfortable conditions passively in non-residential nZEBs, by using controlled passive cooling systems. Although this potential is undoubtedly linked to the potential of climate, controlled passive cooling has the potential to satisfy comfort requirements for greater than 90% of the occupied year if multiple passive cooling techniques are combined.
O'Donovan, Adam, "Passive Cooling in Non-Residential Nearly Zero Energy Buildings: A Thermal Comfort Analysis Based on Measurement, Modelling and Simulation" (2019). Theses [online].
Available at: https://sword.cit.ie/allthe/301