Cooling design day generation methods' and risk levels' impact on the design capacities and risk of thermal discomfort in a cold climate

Meteorological data is needed to determine the size of the cooling/heating systems. A design day is commonly used for cooling. ISO 15927-2 and the ASHRAE handbook provide comprehensive guidelines for generating design day weather data with different risk levels. However, the impact of different risk levels and design day generation methods on thermal discomfort has been rarely quantified. This study integrated design day analysis with long-term thermal comfort simulations to assess the cooling system design capacities obtained using design days generated by the aforementioned methods and different risk levels in the temperate and mild climate of Estonia. A generic open-plan office was used for sizing cooling units and subsequently for thermal comfort assessment based on long-term historical data. The ASHRAE Fundamentals method performed better in Estonia's climate, whereas ISO 15927-2 needs further development to avoid reaching controversial results among risk levels. Solar radiation and internal heat gains contributed most to the space cooling capacity, and the differences between the design capacities for the risk levels were negligible. The space cooling temperature setpoint of 25 °C was rarely or never exceeded by more than 0.5 °C in long-term simulations, except in the North-oriented zones. Therefore, a risk level of 10% was recommended for sizing space cooling without significantly increasing the risk of thermal discomfort. Ventilation supply air cooling capacity depends more on the enthalpy of outdoor air, and therefore, a risk level of 5% is recommended.