Recent studies show that many occupants of air-conditioned buildings are not satisfied with the thermal environment even though the environment meets the currently followed thermal comfort standards. This cannot be justified in view of the large amount of energy spent in maintaining the thermal environment. Studies suggest that a major reason for the poor satisfaction with thermal environment is the lack of individual and personal control over the environment and the designing of indoor spaces for a theoretical, average occupant, instead of actual individuals. Hence, it is suggested that building environment control systems be designed by incorporating personalized control systems for greater satisfaction at much less energy consumption. A simple three-level approach is suggested to achieve this goal, which can lead to a sustainable thermal comfort system.
Owing to changing lifestyles, human beings are found to spend increasingly more and more of their lives indoors. Studies show that now people spend nearly 90% of their lives inside some kind of building or the other . Considering this fact that most of our lives are spent inside buildings, it should come as no surprise that buildings account for 38% of equivalent greenhouse gas emissions and 40% of global end energy usage . And in buildings, a considerable amount of energy is expended to keep people thermally comfortable. For example, in India, 31% of the energy budget in commercial buildings is for air conditioning and ventilation . With such large amounts of energy being used, it would at least be considered worthwhile to find if occupants were really being kept comfortable. Unfortunately, several studies show that this is not always the case. An overview of thousands of surveys filled by occupants in office spaces, collected over the last 20 years, showed that around 39% of occupants are dissatisfied with the temperature in their workspace . With such surprisingly widespread levels of dissatisfaction with indoor thermal conditions in spite of spending so much energy, it is only appropriate that the thermal comfort of occupants should be examined from a fresh perspective. This is especially important for countries like India that are aspiring to provide comfortable dwellings to billions of their populace. At present, the market penetration of air conditioning is still very low in India, but it is rising fast as shown by the sales of air conditioners and air conditioning systems . In fact, even with such low penetration levels, the not-too infrequent power blackouts that are seen during the peak of summer months are partly attributed to the increased use of air conditioners in urban areas. Hence, the added burden a further rise in energy-intensive air conditioners will place on Indian energy sector cannot be overstated. And even more unfortunate is the fact that such a rapid rise in air conditioning is not likely to satisfy the occupant needs completely, as gathered from the existing evidence.
Thermal Comfort in Built Environment
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) defines thermal comfort as “a condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation” . Since it is difficult to assess and quantify the mental conditions of occupants, designers need to rely on thermal comfort standards and guidelines to make sure that the indoors provide comfortable living and working environment. In this, they are guided by international and local standards, such as ASHRAE Standard 55, European Standard EN15251, or National Building Codes of India. For most of the standards worldwide, comfort in air-conditioned spaces is based on the predicted mean vote (PMV) model . The PMV model has been in use for more than four decades now. The PMV value provides an average thermal sensation vote of occupants on a seven-point scale ( 3 to +3), with the thermal sensation ranging from cold ( 3 ) to hot (+3), with neutral (0) in the middle. Achieving neutral vote is generally considered as the goal for thermal comfort design. The PMV calculated based on indoor thermal conditions, occupant clothing, and activity is used to calculate an index called predicted percentage dissatisfied (PPD), which represents the percentage of occupants that are likely to be dissatisfied with the existing thermal conditions. Figure 1 shows the variation of PPD with PMV. As shown in the figure, according to the PMV–PPD model, the lowest value of PPD that can be achieved at neutral condition (PMV = 0) is 5%.
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