Inter-annual optimization of ventilation strategies in cruise ships: Integrating thermal comfort and infection risk mitigation across seasonal variations

Indoor ventilation optimization is important for air quality, thermal comfort, and airborne transmissions of infectious droplets, but research on cruise ships is scarce, especially across whole year seasons. How to weigh air quality and human comfort is an essential and practical issue. We utilize numerical simulations verified by field experiments to examine the thermal comfort and infection risk under natural ventilation and air-conditioning ventilation in various seasons. The scientific problem of this study is the proper utilization of natural and mechanical ventilation at the inter-annual scale to provide a good environment for passengers in the transportation environment. The effect of window opening configurations and ambient wind directions on natural ventilation has been explored. Varying filtration efficiencies are considered for air conditioners. The Monte Carlo method and dose-response model are adopted to quantify infection risk (IR). 
Results reveal that surrounding turbulent airflows create positive pressure on the windward side ship's surface and negative pressure on the leeward side. Compared to driving following wind and against the wind, the best ventilation in the cabin occurs during side wind, due to the large windward area with an air change rate per hour (ACH) above 61.78 h^-1. In spring and fall, opening all side windows provides good thermal comfort (-1<PMV<+1) and maintains low IR (median below 0.61%, 95% CI: 0.27% to 2.00%). However, thermal comfort decreases and mechanical ventilation is required in summer and winter. When using mechanical ventilation, comfort is improved (PMV=-0.21). However, the median IR reaches 6.14% (95% CI: 5.71% to 7.87%) under recirculating air conditioners. With filtration efficiency increasing to 30%, the median IR decreases to 0.79% (95% CI: 0.82% to 1.02%). Threshold analysis indicates that a filtration efficiency of 14.06% is the threshold to decrease IR effectively. To ensure human thermal comfort and infection risk, windows can be opened for natural ventilation when the temperature is suitable. However, if mechanical ventilation is required, an air conditioner system with a filtering and sterilization function must be adopted.