Adaptive Thermal Comfort in Sustainable Cooling Solutions


The introduction of air-conditioning in tropical countries paved the way for more comfortable offices and homes during harsh outdoors, significantly improving productivity and health. Singapore’s First Prime Minister Lee Kuan Yew once stated that air conditioning was a significant factor in driving Singapore’s success and transformation from a developing country into a developed country.

However, the benefits of air conditioning have come at a huge cost to the environment. Air conditioners are energy intensive appliances and use refrigerants that have high Global Warming Potential. Air conditioners have helped resolve thermal conditions but have created multiple other challenges too. Alternative cooling strategies have not been given enough consideration. Energy consumption for space cooling in non-OECD countries alone has risen by 50% in just five years, from 555 TWh in 2010 to almost 833 TWH in 2015, and is expected to grow at a much faster rate.

There is no doubt that thermal comfort is essential for a productive and healthy environment, whether at work or at home. But is air conditioning the only way to achieve thermal comfort? Will all employees or residents only be productive or comfortable between the narrow band of 21-23 oC, irrespective of outdoor conditions?

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation. Thermal comfort gets determined by physiological, psychological and environmental conditions; environmental factors such as air temperature, relative humidity and air motion; and personal factors such as metabolic rate and clothing.

Adaptive thermal comfort theory indicates that human comfort depends upon historic and relevant outdoor conditions and one’s ability to control the immediate environment. It suggests that humans adapt to a wider range of thermal conditions. This could have a tremendous impact on comfort conditions in buildings, and hence on energy consumption.

People in warm, tropical countries need a different range of thermal comfort conditions than those in temperate climates. Those permanently living in conditioned spaces have a narrower band of comfort as compared to people living in naturally ventilated buildings. People have higher expectations of thermal conditions when they are in an air conditioned environment, but their expectations do not remain high when they are in naturally ventilated buildings. Two independent studies in India and Japan indicated that close to 80% of occupants are comfortable with indoor operating temperatures of 24-28 oC in these countries. This thermal band becomes wider in the presence of air motion around occupants. This is much higher than the thermal comfort range defined for air conditioned buildings by ASHRAE 55 – 2010, followed more in north American countries.

In 2005 Japan launched “Cool Biz”, an initiative to reduce electricity consumption and CO2 emissions due to air conditioning. Cool Biz urged businesses and government offices to set the AC thermostat to 28 oC and asked office workers to discard their suits and ties, and opt for light summer clothing to keep themselves comfortable at a higher AC set point. Following the 2011 earthquake and tsunami, Japan launched “Super Cool Biz” to further cut down power consumption and reduce emissions. Japan’s Ministry of Environment reported an estimated 1.56 million tons’ reduction in CO2 emissions in 2011. “Cool Biz” also resulted in lower electricity expenditure.

The case for “Cool Biz” in India is so much stronger, given the power deficit. Reducing electricity demand would help accelerate ‘Power for All’. India can rely on its own thermal comfort model, developed considering India’s climate, social and cultural context. Deployment of adaptive thermal comfort models could pave the way for affordable and sustainable cooling.

Designing and operating buildings for the thermal comfort range of 24-28 oC opens up adoption of a range of low energy cooling solutions, instead of relying solely on conventional compressor based air conditioning. These include non-vapour compression cooling technologies, the use of natural ventilation (NV) and mixed mode (MM) operation, i.e. using a combination of air cooling systems and natural ventilation, in cooler months or times of the day. Deployment of low energy cooling systems can be supported by building envelope design using high performance wall and roofing materials, glazing coupled with operable windows; and, encouraging climate-appropriate attire and behaviour change to combat heat.

Thermal Comfort in India

India’s current air conditioning footprint is much smaller than that of the USA and China, with household penetration of air conditioners in 2011 estimated at 5%. However, the sales of room air conditioners have been increasing 10-15% annually leading to an estimated household penetration of 47% by 2030. Add to that the cooling demand of commercial buildings and MSME/light industry, which is also growing. Relying solely on current design practices, referring existing codes & standards, technologies and solutions to meet this demand, would result in an estimated peak demand of 150GW (30% of total peak demand) by 2030.

Offices, homes and institutions in India need vastly improved Indoor Environment Quality (IEQ) for better productivity and health, and thermal comfort is one of the critical parameters of IEQ. With a push for ‘Make in India’ one cannot ignore the need for IEQ standards and thermal comfort in MSME units as well. In their report Heat Stress and its effect on Psychological Aspects of Workers in India (2016), Dheeraj Sharma and Rajesh Chandwani of IIM Ahmedabad studied 175 units across the country, concluding that productivity increased by 12% in “cooler” workplaces, signalling the possibility of even more cooling demand. There is a silver lining though, with the authors indicating that in India “cooler” means indoor temperatures of 25-30 oC.

Providing thermally comfortable living and working spaces is also critical to combating heat-related sickness and death. A study by Michael Greenstone of the Energy Policy Institute at the University of Chicago indicated that heat-related deaths in the US declined by more than 70% from 1960 to 2004, primarily due to the adoption of air conditioning[1].

With a better understanding of the thermal comfort range for Indians, India can tackle heat stress in a more affordable and sustainable way by designing and operating buildings for improved IEQ and thermal comfort.  Research conducted by Richard De Dear, Gail Brager, Fergus Nicol and M. A. Humphreys in tropical countries has led ASHRAE to acknowledge the role of acclimatization in people’s thermal comfort preferences.

In developing the India Model for Adaptive Comfort (IMAC)[2] researchers at CEPT University conducted field studies covering office buildings in 5 cities in each climate zone. The buildings were classified as Naturally Ventilated (NV), Air conditioned (AC) and Mixed Mode (MM), i.e. naturally ventilated during some seasons and air conditioned during others. The study included a Thermal Comfort Survey (TCS) of 6330 occupants. A key finding of this study is that occupants in Indian offices are more adaptive and tolerant of warmer temperatures. The IMAC model for naturally ventilated buildings indicated the acceptable range of indoor operating temperature as 19.6-28.5 oC across the year depending upon seasonal outdoor conditions. At the same time, in mixed mode buildings range is 21.5-28.7 oC. This means that Indians can be comfortable in wider thermal comfort band.  The study also found that even in fully air conditioned buildings the acceptable range for indoor operating temperature is 23.5-25.5 oC +/- 2 oC, higher than the 22.5 oC +/- 1 oC “Class A” spec (ASHRAE 55-2010) spec that commercial air conditioned buildings in India operate at all year round. The study points out that the average energy consumption per unit area due to HVAC operation is 120-290 kWh/m2/year, and based on modelling studies, the expected reduction in Energy Performance Index (EPI) for HVAC per degree increase in thermostat setting is 5-6%.

These results emphasise that

  • Air conditioning is not the only way to achieve thermal comfort in India’s work places, homes, institutions and recreational buildings.
  • Buildings can be designed and operated to exploit natural ventilation and mixed mode operations to meet occupants’ thermal comfort and other IEQ requirements.
  • Awareness campaigns need to be conducted to maintain indoor temperatures in accordance with adaptive thermal comfort theory (IMAC in case of India) in all air-conditioned buildings. Broadly it translates to an indoor temperature range of 24-28°C, this range can be extended in presence of air motion provided by fans.

Fairconditioning has been running a campaign on Corporate Thermal Comfort Policy, to encourage corporate offices and industry associates to establish AC thermostat settings and dress codes policies to enable occupants to operate at the highest possible operating temperatures without compromising on the comfort of its occupants. The Fairconditioning team is currently piloting a centrally air-conditioned 4-storey commercial building consisting of 75,000 square foot built-up-space with an average occupancy of 1500 employees at ICICI’s Thane (Maharashtra) Call Centre. ICICI already has a regulation to set all ACs at 24 °C. The team has found out from preliminary analyses that raising the set point temperature to 26 °C could lead to reduction of annual energy consumption of the building from approximately 1 million kWh per year to approximately 0.87 million kWh i.e. 13 % energy reduction.

Keeping Cool in India

The Sustainable and Smart Space Cooling Coalition believes in LBNL’s Lean, Mean, Green strategy for sustainable & smart space cooling, i.e. reduce cooling loads, promote use of the most energy-efficient solutions, and power cooling with green fuels.

Identifying and adopting the optimal thermal comfort range for India would facilitate better building design and more efficient building operations, while accommodating occupants’ thermal comfort and improving productivity and health.

Now is the time to incorporate adaptive thermal comfort standards, along with IEQ, to develop cooling solutions with smaller energy and GHG footprints. With a significant increase in the building stock and AC sales every year, each year of delay in moving to sustainable and smart space cooling solutions would be costly in terms reaching India’s goals on Power for All and climate change.

It is suggested that India’s building codes, NBC and ECBC 2016, be updated to incorporate a comfort range in accordance with IMAC, with well laid out rules and guidelines on incorporating energy-efficient building design. The National Building Code (NBC) draft revision of December 2015 has set the cooling degree day reference temperature at 23 oC. This can be set higher and other IEQ parameters such as lighting, air quality and acoustics can be included in the code.

It’s essential to develop programs to underpin the adoption of adaptive thermal comfort theory, such as Fairconditioning’s workshops in which they have already trained many students, architects and engineers. Further, mandating annual energy consumption reporting for commercial and public buildings, including specific consumption for space cooling, would help in verifying the impact of implementing adaptive thermal comfort guidelines.

Finally, messaging on adaptive thermal comfort should be included in campaigns on combating heat: Emphasise the effectiveness of setting the thermostat at 24-28 oC, as well as alternative ways to stay cool, e.g. natural ventilation, clothing, food habits, etc.

This newsletter on Adaptive Thermal Comfort was compiled by the AEEE team with inputs from coalition partners CEPT and Fairconditioning, and from research reports and news articles.

[1] Adapting to Climate Change: The Remarkable Decline in the U.S. Temperature-Mortality Relationship Over the 20th Century, 2015

[2] Field studies of thermal comfort across multiple climate zones for the subcontinent: India Model for Adaptive Comfort (IMAC), 2016