Building overcooling in warm climates

Abstract

Building construction and operations accounted for 36% of the share of global final energy use and 39% of energy-related CO2 emissions. 20% of worldwide building primary energy use and 8% of carbon emissions from energy demand are attributable to space cooling. In warmer climates, space cooling is the largest and fastest-growing building end-use. Most built environment growth will occur in warmer regions. Therefore worldwide primary annual energy demand for space cooling is predicted to increase by three folds by 2050 from current levels. This expected increase in cooling energy usage is partly driven by "overcooling," which generates uncomfortably low temperatures. This thesis studies overcooling's impact on warm-climate building occupants and performance.

Primarily, a definition of building overcooling is developed in this thesis to define, detect, and determine this phenomenon as it relates to thermal comfort in buildings. Overcooling, thermal comfort, the built environment, and building performance are reviewed systematically in the literature. The review explores existing definitions and develops the overcooling definition. Then, global thermal comfort data is analysed using the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Thermal Comfort Database II, which encompasses 27 nations and over 90,000 occupant responses. The database is used to evaluate the developed overcooling definition's capacity to detect overcooling and the deviation from prescribed comfort temperatures in warmer climates.

Further, the relationship between current thermal comfort requirements and the notion of overcooling is examined by analysing existing field data sets for warm climates. If uncomfortable indoor conditions are a direct result of the application and adoption of international comfort standards, this provides the rationale for investigating a localised concept of comfort that results in the least discomfort. The thermal comfort metric used in current field data for warm climates is studied to establish its efficacy. The predicted and actual occupant thermal sensations are examined to understand the difference between standard-driven comfort and desired comfort. The discrepancy between observed indoor temperatures and calculated neutral temperatures based on occupant responses is examined to assess the change in direction and degree of prescribed thermal conditions against the actual desired conditions. Comparing actual occupant response to PMV in thermal comfort studies demonstrates the disjuncture between standardized comfort from desired occupant comfort in warmer climates.

Furthermore, this thesis analyses overcooling's effects on cooling energy consumption in warm-climate buildings. Energy simulations using observed and comfort temperature setpoints from field data for warm climates are used to analyse cooling energy demand on site. Using models of prototype buildings, the EnergyPlus simulation engine estimates the cooling energy consumption between desired and standard-driven comfort setpoint temperatures. Initial simulations evaluate each location's comfort setpoint temperatures and energy demand. Simulations of varying building features, such as facade glazing ratio, heat loss parameter, and obstacles that reflect desired and standardized comfort setpoint temperatures using random sampling are evaluated. An average decrease in site cooling energy demand of roughly 7% per 1°C increase in setpoint temperature is observed across warm-climate field data sets.

Finally, as a real-world case study, Qatar, a hot desert environment that is wealthy in fossil fuels and is rapidly urbanising is selected as an ideal case study for exploring the comfort and energy impacts of overcooling. Qatar's hot climate requires active cooling to maintain building comfort. Qatar's low energy costs increase demand for HVAC cooling, resulting in significant per capita carbon emissions. Occupant and building field data is collected during the summer season across eight office buildings in Qatar. The data is analysed to determine occupant overcooling and desired comfort levels across a range of indoor temperatures. Energy simulations are utilized to analyse cooling demand over the building's operational range and design characteristics. The balance between comfort and energy is investigated by comparing occupant cold discomfort to site cooling energy demand savings. Findings suggest over 30% of occupants are uncomfortably cold, and an increase of 2°C from existing setpoint temperatures would improve occupant comfort and building energy.

Date of Award	29 Mar 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Sukumar Natarajan (Supervisor) & Tristan Kershaw (Supervisor)