Static Approach to Thermal Comfort

3.5Adaptive Approach to Thermal Comfort

The adaptive comfort model takes into account the thermal sensation of the occupants, different actions in order to adapt to the (changing) thermal environment (e.g. change of clothes, opening windows) as well as variable expectations with respect to outdoor and indoor climate, seeking for a "customary" temperature. The underlying assumption is that people are able to act as "meters" of their environment and that perceived discomfort is a trigger for behavioral responses to the thermal environment. Although these behavioral phenomena cannot be described theoretically yet in full detail a model was derived from results of field studies, which represents limits for the operative temperature as a function of the outdoor temperature. This simplified approach also avoids difficulties occurring with the assumption of appropriate clo- and met-values, as it has to be done with the PMV approach. They are included in the resulting accepted temperature as part of the adaptation.

ROOM temperature versus running mean ambient air temperature

EN ISO 15251:2007-8 evaluates the operative room temperature in relation to the running mean of the ambient air temperature (Figure 2 ). Again, the temperature range defining thermal comfort in summer correlates with user satisfaction: ±2.0 °C, ±3.0 °C and ±4.0 °C (classes I, II and III). The different ranges refer to the categories defined in the standard (category I: less than 6% dissatisfied, category II: less than 10% dissatisfied, category III: less than 15% dissatisfied, category IV: more than 15% dissatisfied - based on occupants' expectation with regard to indoor climate).

Figure 2 shows these operative temperature limits for non-mechanically cooled buildings. The outdoor temperature has to be calculated as a weighted running mean value referring to the idea that most recent experience (last 1 to 7 days) might be more important for the "thermal memory". The running mean ambient air temperature () is given as function of the running mean ambient air temperature of the previous days () and the daily mean ambient air temperature of the previous days () with .

ISSO74:2005 describes thermal comfort in naturally ventilated buildings and, therefore, considers the thermal adaptation. The comfortable room temperature responds to the running mean ambient air temperature of the last three days using the same formula as DIN EN 15251 but with another reference temperature (Figure 2 ).

The ISSO 74 algorithm is based on a meta-analysis of existing models.

Room temperature versus monthly mean ambient air temperature

ASHRAE 55 defines thermal comfort for naturally ventilated buildings with reference to the monthly mean ambient air temperature for the adaptive model, as this is generally available from meteorological stations (Figure 2 ).

The tolerance range is determined in dependence on occupant satisfaction, namely ±2.5 °C for 90% acceptance and ±3.5 °C for 80% acceptance.

Table 2 Summary of standards and guidelines of thermal comfort.

AT THE END OF THE DOCUMENT

Most relevant static and adaptive comfort models with the respective reference value for ambient air temperature are illustrated in Figure 2 – adaptive: ASHRAE-55, EN 15251, ISSO 74 and static: DIN 1946, ISO 7730, and VDI 4706. Thermal comfort results are presented for 8 European buildings: hourly measure operative room temperature is plotted versus the specific ambient air temperature. Obviously, the defined comfort requirements for the winter season –running mean ambient air temperature below 15 °C– are similar for all comfort models. Room temperatures are limited to a range of 19 to 24 °C.

On the contrary, comfort requirements differ significantly for the summer season, especially for the adaptive and static comfort models. Considering the static comfort models, the defined set points for room temperature range between 20 and 26 °C (comfort class II). For the adaptive models, the room temperature set points increases with higher ambient air temperatures. Although thermal comfort results differ significantly between the European buildings, most of the buildings comply with the temperature requirements of the adaptive models. This does not hold true for the static comfort models, where temperature limits are often violated.

Figure 2 presents the thermal footprint of the European buildings: thermal comfort is evaluated for all six comfort models considering the upper temperature limits during the summer season. Obviously, different comfort results are achieved depending on the comfort model applied. For example for the non-residential building in Denmark: thermal comfort is in compliance with class II during 95 % of the occupancy considering the adaptive comfort models and with class II considering the PMV comfort model. For the VDI and DIN 1946 model, operative room temperatures are higher than permitted during the entire summer season. The same conclusion is derived for the Finish non-residential building. The building achieves thermal comfort according to class I and II considering all comfort models applied, except the VDI model.

In conclusion, comfort models defined in various standards and guidelines results in different conclusions. In some cases, results differ considerably, in particular for the adaptive versus the static approach. Therefore, it is important to develop a classification of cooling concepts and the corresponding thermal comfort models.

Figure 2 Models for the evaluation of thermal comfort: ASHRAE 55, DIN 1946, EN 15251-Adaptiv, EN 15251-PMV, ISSO 74 and VDI4706 (from upper left) for 8 low energy buildings in Europe.