ShareHigher AC setting temperatures to reduce thermal discomfort and energy demand in warm countries
Using data from four cities in India, Philippines, and Thailand, a recent study found that occupants experience thermal discomfort from unnecessarily low air conditioning (AC) temperature settings. Warmer indoor temperature settings would lower overall discomfort as well as reduce cooling energy demand.
A recent study looked into the issue of thermal comfort associated with cooling demand in warm countries. [1] Thermal comfort is most commonly defined as the expression of satisfaction and acceptability of the thermal environment experienced which is represented by a comfortable state of mind. Several international standards exist to assess thermal comfort, such as ISO (International Organization for Standardization) 7730, ASHRAE Standard 55, and EN (European Standard) 16798-1:2019.
The authors used the ASHRAE Global Thermal Comfort Database II, a collection of thermal comfort studies representing over 30,000 responses from occupants of air-conditioned buildings in cities around the world. For the purpose of their study, they retrieved data for four cities: Bangalore (India), Bangkok (Thailand), Delhi (India), and Makati (Philippines). The data included only office settings.
Thermal comfort assessment involves examining the physical, personal, and subjective metrics of a building’s occupants in an enclosed space.
Physical metrics include the occupant-associated air temperature, mean radiant temperature, air velocity, relative humidity, while personal metrics are the insulation value of clothing and the metabolic activity level. Physical and personal metrics are correlated with subjective metrics such as the thermal sensation vote (TSV), which is the occupants’ individual perception of their thermal environment on a scale ranging from cold (−3) to neutral (0) to hot (+3). The PMV (predicted mean vote) is used to identify the mean thermal sensation of a group of building occupants through a range of physical and personal variables on the same scale as the TSV. Consequently, predictions (i.e. PMV) can be compared with observations in real buildings (i.e. TSV).
In the published study, the authors found that the mean TSV observed in the data was lower than zero, suggesting the presence of cold thermal discomfort in these warm climates. The true extent of cold, compared to hot, discomfort was estimated by looking at the proportion of votes (TSV and PMV distribution) falling below −1 and exceeding +1. The comfort standards mandate overall discomfort to be less than a threshold of about 80 % for normal office conditions. For the four cities, the authors observed, on average, 70 % of the votes falling into cold discomfort compared to only 24% for hot discomfort.
Moreover, regression analysis of the comfort votes suggests that, on average, the occupants are likely to prefer indoor temperatures that are 2.2 K above those currently supplied. Using computer simulations, the authors found that elevated indoor temperature settings would drive down average cooling demand by around 8% and peak demand by about 6%, per K increase in set point temperatures. Findings for each city revealed an 8.0% decrease in energy demand for cooling per K increase in building set point temperature for the Bangalore group. Reduction rates for Bangkok, Delhi, and Makati were 10.5 % per K, 9.1 % per K, and 10.0 % per K, respectively. Furthermore, elevated indoor temperature settings can also improve the productivity and wellbeing of office buildings occupants, which is significantly affected by cold thermal discomfort.
Such findings support the relevance of including temperature settings alongside minimum energy performance standards (MEPS) for ACs in order to significantly reduce energy demand. According to the IEA, when combined with energy-efficient equipment, even low-tech solutions such as increasing the indoor temperature setting by 1°C (either through behavioural change or thermostatic controls), could contribute to further potential energy savings. [2] For instance, the Indian government has imposed a default temperature setting of 24 °C for room air conditioners, a measure in line with the country’s updated energy performance standards. [3]
Sources:
[1] Alnuaimi, A.N.; Natarajan, S. The Energy Cost of Cold Thermal Discomfort in the Global South. Buildings 2020, 10, 93. https://doi.org/10.3390/buildings10050093
[2] IEA (2018), The Future of Cooling, IEA, Paris https://www.iea.org/reports/the-future-of-cooling
[3] IIR news: https://iifiir.org/en/news/the-indian-government-implements-a-default-temperature-setting-on-room-air-conditioners