EbeneTechniken-ThermKomfort/en
Aus IBPSA-Germany
thermal comfort (comfort models)
Assessment of temperatures/indoor climate/thermal comfort
The current state of physiologists' knowledge tells us, that human sensitivity of too warm or too cold (thermal uncomfortableness or thermal discomfort) does not only depend on the indoor temperatures, but also on further parameters of a room (e. g. air speed, humidity, etc.) as well as on the state of the human being itself (e. g. activity, clothing, physical condition, etc.). Thus the indoor climate depends on several parameters, which are divided into six basic parameters and additional parameters.
This is why a complete description or assessment of the temperatures/thermal comfort/indoor climate/ should take into account all relevant values. For this purpose corresponding properties (thermal index of the relevant climate range) are available. Besides these adequate classification of indoor climate more simple criteria for assessing thermal comfort (which neglect certain important influences) do still exist - probably by historical reasons.
On the other hand, recent research about the human sense of indoor climate resulted in highly complex human comfort models, which consists of several parts of the body; these models are able to model physiological processes within the body (e. g. blood circulation).
Climate ranges and correlating thermal index
Meanwhile it has been agreed worldwide that the indoor climate is to be classified into three ranges(ambient climata according to EN ISO 11399), which are basing on a thermal balance of human body all:
- cold (the thermal balance is negative, too much heat is extracted from the body): thermal index = IREQ index according to ISO/TR 11079 (requirements on the thermal insulation of the body)
- moderate (the thermal balance is in an equilibrium nearly): thermal index = PMW/PPD index according to EN ISO 7730
- warm (the thermal balance is positive, too much heat is injected to the body): thermal index = thermal load according to EN ISO 7933
Thermal well-being - thermal comfort
Basically thermal comfort only can be achieved, if the thermal balance of the body is in - or nearly in - an equilibrium. Thus only thermal neutral indoor climata are defined as the comfort zone. Concerning thermal comfort, all parameters are divided into criteria, which are corresponding with the whole body (thermal balance of the body), and criteria, which are focusing on individual parts of the body (local discomfort).
In EN ISO 7730, three classes of thermal comfort (category A/B/C) are defined, which include both kinds of comfort criteria and which might be interpreted as follows:
- category A: good/very good
- category B: middle/suboptimal
- category C: moderate/small
- (category D: ... lower ...)
Heat balance of the whole body – PMV/PPD index
The “Predicted Mean Vote” (PMV) or “Predicted Percentage Dissatisfied” (PPD), basing on the work of Fanger and defined in EN ISO 7730, can be regarded as the adequate thermal index for thermal neutral indoor climate.
By the way it might be interesting, that even with an optimal indoor climate always some people remain complaining about the temperatures. These people show a feeling of temperatures, which differs from the feeling of most other people; this is a fact and it reflects the individuality of humans.
The PMV/PPD index depends on six basic parameters; four of these parameters are properties of the room and two of them refer to the human body.
Parameters relating to the room:
- air temperature resp. dry bulb temperature
- mean radiation temperature resp. surface temperatures weighted by area
- humidity of air
- air speed
Parameters relating to the human body:
- the body’s heat production as a function of activity
- insulation value for clothing
Apart from this six basic parameters, the feeling of thermal comfort is also influenced by additional parameters (e. g. thermal regulation of the body, acclimatization, healthyness, etc.).
Experience has shown that the PMV/PPD index is dominated by the air temperature as well as by the radiation temperature very often. This is why the influence of these two temperatures is often summarized in form of the operative temperature (also resultant temperature, felt temperature, room temperature). It includes the convective heat exchange of the body with the surrounding indoor air as well as the heat exchange of the body via thermal radiation with the surrounding surfaces.
Especially the thermal radiation exchange might play an important role concerning thermal comfort in buildings with excessive glass surfaces: During summer, the glass panes heat up by absorption of the incident solar radiation and thus works like a "radiant heating"; in winter, the glass panes cool down more than the walls and thus produce a "cold radiation" that – despite the heated indoor air – can be felt as quite uncomfortable.
Local discomfort - DR and PD Index
However, it turned out that people do not only complain about thermal discomfort if the heat balance of the body is not in an equilibrium, but also if there occur specific local phenomena. For this reason, additional criteria of local discomfort are defined in DIN EN ISO 7730.
Draught effects / DR index
Vertical gradient of air temperature
Asymmetric thermal radiation
Too cold or too warm floor
Differences between mechanical chilled rooms and rooms without cooling system
Recent results from extensive surveys of office users in the United States of America (ASHRAE-Standard 55), Denmark and also in the Federal Republic of Germany (ProKlimA study) show that the individual assessment of thermal comfort in summer depends on the fact whether a room has a cooling system or not! The conform results of the surveys concerning rooms with and without cooling system can be summarized as follows:
Rooms with a cooling system (mechanical cooling)
If rooms are chilled mechanically (e. g. by a ventilation system with pre-cooled supply air, recirculation air cooling, fan coils, cooling blankets, etc.) peoples assessment of indoor climate are conform with the predictions of ISO 7730 (Fanger).
Rooms without active cooling system
For rooms without an active cooling system – as often can be seen in office buildings in Germany and other European countries – a significant discrepancy occurs between the predictions by ISO 7730 and the results of the extensive surveys.
This makes evident that ISO 7730 (Fanger) only is valid for cooled rooms.
In the meantime this most interesting acknowledgement has also influenced the guidelines rsp. standards of the Energy Performance of Buildings Directive (EPBD) of the European Union (EU). Within the scope of a EU mandate, the CEN organization has published a new standard, which definitely allows higher operative temperatures for rooms without cooling. Similar to ASHRAE standard 55 it allows higher summer indoor temperatures for rooms without air-conditioning.
For rooms with passive climate concepts (i.e. without air-conditioning or cooling system) the new EN 15251 recommends maximum operative temperatures in summer, which depend on the average ambient air temperature measured for several days before. The thermal comfort according to DIN EN 15251 is categorized into classes I, II and III. During longer periods of hot weather with high ambient temperatures (like e.g. in the summer of 2003 or June/July 2006) EN 15251 accepts operative temperatures in rooms with natural ventilation, which might rise up to 29° to 31°C – depending on the category. The historical meaning of this new standard EN 15251 is that for the first time an official standard is available in Germany, which indicates a limitation of summer indoor temperatures for rooms with natural ventilation only (rooms without any air-conditioning).
Simple assessment criteria of the thermal comfort
For most building projects, the climate range of thermal neutrality is valid. However, experience shows that the – comparatively young – adequate thermal index (PMV or PPD index) to assess thermal comfort has not yet gained much acceptance in the design teams. Thus most of the time designer fall back on more simple assessment criteria for thermal comfort, which still are used in older, but nevertheless official German standards and guidelines.
Mean air temperature
For the assessment of thermal comfort by the air temperature only, the mean air temperature, averaged over the entire room, is being used. Thus the distribution of air temperature in the room, which does exist in reality, is neglected and substituted by the mean air temperature, which represents all the air in the room.
The main disadvantage of this simplified evaluation of thermal comfort by mean air temperature is that the effect of surface temperatures onto the human body by thermal radiation is not taken into account. In practice problems occur, if surface temperatures differ considerably from the mean air temperature, or - in other words: thermal radiation shows an important influence on thermal comfort - e.g. at
- cool glass surfaces in winter
- hot internal sun screen on warm and sunny days
- strongly heated glass roof in summer
- ... etc.
Because of this weakness - which is quite well-known among experts meanwhile - most of the standards and guidelines were adapted to the operative temperature, which includes thermal radiation. Despite of that there still exist some standards and guidelines in Germany, which classify the thermal comfort by the mean air temperature, e.g.:
- VDI 2078 (1996): The cooling load calculation, which is based on a simple, steady-state energy balance of the room; thus the current dimensioning of air-conitioning in Germany base on a maximum design indoor air temperature (in most times 26°C).
- Workplace guidelines ASR 6 (May 2001): For thermally comfortable workplaces the ASR demands (original citation from the ASR6, chapter 3, section 3.3): “The air temperature at workplaces should not exceed + 26°C. If the ambient air temperature is above that, the indoor air temperature might be higher exceptionally.”
In order for a better assessment of thermal comfort, a continued developement of these two, elder standards resp. guidelines is recommended so that – analogous to most other standards – at least the operative temperature is used for a recommendation or evaluation (or even better: the thermal index PMV or PPD).
At present, at least VDI 2078 is being revised.
Mean air and mean radiation temperature: operative temperature
The operative temperature consists of the mean air temperature (averaged air temperature in a room, see above) and the mean radiation temperature (see below).
More precisively the mean radiation temperature depends on the position into the room (this can be shown by the method of "view factors" – a purely geometric function). The mean radiation temperature measured next to a large, heated glazing clearly shows higher values than at a position in room depth. The “visible room fraction” of to the hot glass façade is significantly higher than at a position in room depth.
The calculation of such spacial effects by zonal simulations requires a detailed geometrical room model, for which the corresponding view factors – for the position of interest in the room - must be determined. This spacial influence is being neglected by using the simple mean radiation temperature, which is determinded according to the ratio of the components of the entire room surface.
Detailed models for assessment of thermal comfort
In the scientific world and other industries (but definetely not in building industry) more detailled models of the human body are used for assessing thermal comfort.
- Existing models ? Model of Fialla..?
- How is the thermal comfort evaluated through that?
- Status quo of research ?
