The dialectics of thermal comfort

Transcription

1 The dialectics of thermal comfort Fergus Nicol Inaugural lecture 19th Feb 03 Buildings, people and temperature This talk is about the relationship between people and buildings: how a building affects people and is affected by them. Also the role of the climate, both indoors and out, culture, economics and so on. The study is multi-disciplinary involving the disciplines of physiology, physics, psychophysics, building science, social science, meteorology among many Buildings, people and temperature These disciplines will each have a different model of the building-occupant system. Most models are multi-disciplinary. Each model has its own logic and framework but there will be parallels between them. The complexity of the system suggests a dialectical approach We share with other mammals the mechanisms of temperature regulation - shivering, sweating, changing the distribution of blood between the body s peripheral circulation and the deeper organs. But we also use clothing and shelter and burn fuel to warm and cool us. The use of these cultural mechanisms to control our temperature has made it possible for our species to survive in almost all climates, but it has also created new kinds of vulnerability. Our body temperature now depends on the price of clothing or fuel, whether we control our own furnaces or have them set by landlords, whether we work indoors or outdoors, our freedom to avoid or leave places with stressful temperature regimes... Thus our temperature regime is not a simple consequence of thermal needs but rather a product of social and economic conditions R Levins & R Lewontin, The Dialectical Biologist Dialectical approach History and context I will try to apply the dialectical approach to our science of thermal comfort bearing in mind the prescriptions: Historicity - historical and scientific context Interconnection Heterogeneity Interpenetration of opposites Integration - synthesis The context of the study of thermal comfort is the multi-billion dollar air conditioning (AC) industry (turnover $28bn in equipment alone) The need to define comfortable environments arose from AC industry 1

2 History Before air conditioning buildings were built on the experience of countless builders of the past, experience being passed down the generations Some of the most beautiful and comfortable buildings were built by people who had no science of temperature or heat flow Yazd, Iran History Two other recent influences exist: Concerns about the energy use of buildings Linked to this is the need to use the scientific method to inform the design of buildings, in particular those without air conditioning which are acknowledged to use less energy Yazd, Iran Proportions of Fossil Fuel Use in Developed Economies Transport 25% Buildings 50% Proportions of CO 2 -Production in air conditioned office buildings Heating & Hot Water 32% Cooling 13% Pumps & Fans % Industry 25% Courtesy Max Fordham Courtesy Max Fordham Lighting 21% Catering 4% 2

3 The cost of energy The cost of comfort I don t need to dwell on the environmental consequences of the use of fossil fuel: climate change, rising seas, pollution There is also a social cost: most of the trouble spots in the world are associated with oil production: Nigeria, Venezuela and of course the Middle East. The cost of the Great Power(s) policing these areas must be added to the cost of extraction Much of the environmental, social and political cost of oil extraction to the inhabitants of oil-producing regions can therefore be related back, through the oil and air-conditioning industries, to the pursuit of thermal comfort in the buildings of the developed world The cost of comfort What is Thermal Comfort? Whilst we cannot (should not) insist that everyone is uncomfortable, nevertheless the particular form which comfort takes in an AC building uses lots of energy and yet has only been seen as necessary relatively recently There is a global imperative to reduce the use of energy in buildings That state of mind which expresses satisfaction with the thermal environment ASHRAE Scale Bedford scale +3 Hot 7 Much too warm +2 Warm 6 Too warm +1 slightly warm 5 Comfortably warm 0 Neutral 4 Comfortable neither cool nor warm -1 slightly cool 3 Comfortably cool -2 Cool 2 Too cool -3 Cold 1 Much too cool Called the Comfort Vote The science of thermal comfort The science of thermal comfort What is the origin of the science of thermal comfort? Air conditioning allowed people to choose indoor climate Provision of AC became an industry Comfort is its product A definition of comfort was needed in terms of the controllable physical environment Creating thermal comfort for man is a primary purpose of the heating and air conditioning industry, and this has had a radical influence... on the whole building industry thermal comfort is the product which is produced and sold to the customer PO Fanger, Thermal Comfort, 1970 pp14,15 3

4 The science of thermal comfort Thermal Comfort Models By defining the problem in terms of heating and ventilation we define the solution in those terms as well Better (or more comfortable) buildings are not the required solution but buildings which will supply an indoor environment - usually an indoor temperature - which will fall within expected norms This meant developing models based on physics and physiology to define a psychological phenomenon (a state of mind ) The best known of these, based on a heat balance model is the Predicted Mean Vote or PMV which predicts the average comfort vote of people on the basis of the temperature, humidity air speed, and the amount of clothing worn by and the activity of occupants Thermal Comfort Models PMV is based on a determinist scientific model which has been calibrated by experiments in special laboratories climate chambers where the environment can be controlled by the experimenter In climate chambers the subjects are divorced from the real variable world in which we all live Heat Exchange of the Body with the Environment 1 kw/m 2 Short Wave Radiation from the Sun Wet Air Dry Air EVAPORATION RESPIRATION RADIATION 35ºC 25ºC CONVECTION Courtesy Max Fordham Thermal comfort standards Thermal comfort standards Type of Clothing Activity Category Operative Temperature Mean Air Velocity Building/ Cooling Heating Cooling Heating Cooling Heating Space Season Season season Season season season (summer) (winter) (summer) (winter) (summer) (winter) Clo Clo met C C ms -1 ms -1 Office A 24.5 ± ± B 24.5 ± ± C 24.5 ± ± Cafeteria/ A 23.5 ± ± Restaurant B 23.5 ± ± C 23.5 ± ± Department A 23.0 ± ± Store B 23.0 ± ± C 23.0 ± ± The US Government allows the AC industry (ASHRAE) to set standards for indoor temperatures in buildings Note that a more closely controlled indoor climate (i.e. one which needs more AC equipment and almost certainly uses more energy) is graded higher, yet there is little justification for close control From Olesen and Parsons, Energy and Buildings 34(6) 4

5 Thermal comfort models Because of the ability to define comfort in AC buildings it is also necessary to have a means of measuring the comfort of the environment provided by buildings which are not AC A common method for defining comfort is by the statistical analysis of results from surveys in the field Lisbon, Portugal Lisbon, Portugal Field studies Thermal comfort models The environment is measured using instrumentation (air temperature, radiant temperature, humidity and air movement) the clothing worn by subjects and their activity are noted They are asked for their comfort vote and often whether they would prefer a warmer or a cooler environment. A third method of approaching comfort in buildings is the Post occupancy survey used to great effect by e.g. Adrian Leaman and Bill Bordass Without even measuring indoor conditions, they have been able to identify the key variables effecting comfort and productivity in buildings including the availability of controls and management style Interconnectivity: Field surveys Field surveys are carried out amongst subjects who are going about their normal everyday business, and dressed to suit themselves (or their boss) Such surveys provide the facts which a successful thermal comfort model should explain Comfort temperature Results from field surveys Mean temperature experienced From Nicol & Humphreys Energy and Buildings 34 (6) Europe Pakistan Humphreys 5

6 Results from field surveys Results from field surveys People adapt to the average conditions they experience In free-running (unheated or cooled) buildings the indoor temperature tracks outdoor temperature, this means comfort temperatures track outdoor temperatures In heated or cooled buildings the comfort temperature is largely de-coupled from the outdoor temperature Neutral or comfort temperature o C AC buildings, line B B 34 Free-running buildings, line A Monthly mean outdoor temperature o C Tn = To From Humphreys Building Research and Practice (1978) 6(2) A The relationships have not changed much between the 1970s results of Humphreys and the 1990s results from the ASHRAE database Similar results from buildings throughout Europe (SCATS) from a mixture of conditioned and free-running buildings TC TRM80 mean outdoor air temp (C) mean outdoor air temp (C) Humphreys and Nicol, 00, ASHRAE Transactions, 6(2) McCartney & Nicol (02) Energy and Buildings 34(6) Design of free-running buildings Using meteorological records the linear relationship between comfort temperature and outdoor temperature can be used to predict the likely comfort temperature in free-running buildings Possible approaches to passive design - which use no energy - can be evaluated using the relationship: Design of free-running buildings The comfort temperature in the winter is according to local custom Nicol graph for Tehran jan feb mar apr may jun jul aug sep oct nov dec Nicol and Humphreys 02 Energy and Buildings 34(6) Tomin Tomax Tom Tcomf 6

7 Implications of field surveys The empirical approach based on the results of field surveys is known as the adaptive approach This suggests a variable indoor temperature set-point related to the outdoor temperature, meaning: Naturally ventilated (= low energy) buildings are more likely to be acceptable AC buildings use less energy Interconnectivity: Field surveys The indoor climate in a building or other environment influences the behaviour of the building occupant(s) according to the adaptive principle: If a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort. Human occupants are not simply acted upon the environment but play a part in deciding it - becoming both the subject and the object Interconnectivity: Field surveys The passive view of the human occupants of buildings typified by the diagram in slide 22 (heat exchange of the human body ) is no longer a viable model We must deal with occupants as inter-active beings who can make their own decisions: Picture: Sab Ventris Thermal Comfort Models Changing ourselves The behaviour of occupants can be of two types basic types, they can: 1 adjust their own characteristics so that they are comfortable in the existing environment or 2 adjust the environment to suit their needs The following pictures show the changes which Pakistani office workers make to their clothing - and to a lesser extent their posture - to cope with indoor temperatures which can be as hot as 35 o C in the summer and as cold as 15 o C in winter. 7

8 Summer in Saidu Sharif, Pakistan (photo M Humpheys) Winter in Saidu Sharif, Pakistan (photo M Humpheys) Our knowledge of the language of comfort makes it easy to guess which is in the hotter and which in the colder environment! From an idea of Maria Kessler, apologies to Claire Bretecher Changing ourselves As the temperature changes so the level of clothing, the air movement (which can cool the body by convection and/or evaporation of sweat) and the moisture of the skin will change. It is also probable that people are less active in the heat, but because metabolic rate is measured by activity this is not clear Personal variables Indoor temperature Clothing insulation Air velocity Metabolic rate Skin moisture Data from Pakistan Proportion of subjects comfortable The result of these actions is shown in this graph of the level of discomfort at different indoor temperatures among office workers in Pakistan Little discomfort Mean indoor temperature o C Nicol, Raja, Allauddin & Jamy (1999) Energy and Buildings (3) 8

9 Constraints Time and thermal comfort There are often constraints - social or managerial - on the use of clothing Control over air movement is related to the availability of fans, openable windows etc (see below) The acceptability of skin moisture is also cultural, people in hot humid climates often take it as natural, and may dislike a dry skin Results from surveys suggest that reactions to changes in the outdoor climate take time to be reflected in people s clothing and comfort temperature Analysis suggests that they are best related to people s recent experience which can be expressed by the running mean of the outdoor temperature Running mean temperature The rate at which the comfort temperature changes is related to the running mean of the outdoor temperature (red line) Temperature Outdoor temperatures in Oxford for June/July /1/96 6/11/96 6/21/96 7/1/96 7/11/96 7/21/96 daily mean To Trm33 Trm80 Tr0.90 Trm99 Interconnectivity: controls Negative feedback helps explain the order in the response to indoor temperatures It also arises in the use occupants make of the controls available to them in a building The way occupants use controls cannot be predicted exactly but is a stochastic (probabilistic) process driven by the efforts of the occupants to avoid discomfort Occupants open windows to improve the temperature in the room or increase air movement Likewise curtains or blinds are used to cut solar gain Lights, switched on to alleviate glare, will increase the thermal load and the temperature % running 100% 80% 60% 40% % Use of fans as a function of outdoor temperature in Pakistan 0% Mean outdoor temperature Fans are available in almost all Pakistani offices this graph shows the proportion in use a different temperatures 9

10 % running 100% 80% 60% 40% % Use of fans as a function of outdoor temperature in Pakistan Probability control is in use is given by: p = e (a+bto) /(1+e (a+bto) ) a and b are determined by regression analysis % running 100% 80% 60% 40% Use Fans of fans UK Europe Pakistan 0% Mean outdoor temperature A curve of p on T o can then be drawn to show the probability that a control is being used % 0% Mean outdoor temperature Details: Nicol, J.F. (01) 7th international IBPSA conference, Rio Use of Windows windows Use of Blinds blinds/curtains or 100% 100% 80% 80% % open 60% 40% UK Europe Pakistan % drawn 60% 40% UK Europe % % 0% Mean outdoor temperature 0% mean outdoor temperature The use of curtains is better related to the external illuminance: Use of Heating heating Blinds in use vs illuminance 100% 100.0% 80% Proportion of blinds drawn 80.0% 60.0% 40.0%.0% Actual P predicted P % on 60% 40% % UK Europe Pakistan 0.0% Log (base 10) of external illuminance 0% mean outdoor temperature From data of Yannick Sutter, ENTPE, Lyons 10

11 Interconnectivity: controls Behavioural use of controls links the physiology/psychology of the body and the physics of the building as such it is a key link in the dynamic interaction between buildings and their occupants Use of controls is a key element in linking dynamic simulations of the human body and the simulation of buildings. Adaptive opportunity Baker and Standeven suggested that the adaptive opportunity (meaning the availability of controls) in a building will influence the ability of the occupants to remain comfortable This has been shown to be the case (though it is difficult to quantify) and fits with the findings of Leaman and Bordass that people are more forgiving of buildings which offer more control Constraints There are of course constraints on the use of controls: Are they available? Do they interfere with each other s use (blinds which snag on windows )? Does their use introduce other problems (street noise through the open window)? In multi-occupied spaces do people disagree about their use? Constraints In a study of noise in offices in Pakistan and UK people were asked if they would have preferred to have the window open, and those who said yes were asked why they had not done so. Reason for not opening Pakistan UK (Summer) Noise 28% 56% Air quality % % Other people 25% 40% Air movement &/or 26% 2% Temperature Nicol, Wilson & Dubiel, CIBSE national Conference 1997 Comfort is achieved by the occupants adapting to the building Occupant Or by the occupants adapting the building to suit them Building Cautionary note: Whilst essentially a negative feed-back system aimed at avoiding discomfort, the behaviour resulting from discomfort can lead to a positive feed-back in energy use: Air-conditioning This has to be done within the climatic, social, economic and cultural context of the whole system Global warming Energy use 11

12 In theory there should be no contradiction between the different models of thermal comfort In fact predictions of comfort using the determinist model represented by PMV shows consistent biases when compared with the actual comfort votes in field studies, especially in variable conditions indoor comfort temperature Tcomf ( o C) buildings with natural ventilation 27 (OBS) comfort temp 26 (FITTED TO OBS) adaptive model (PREDICTED) lab-based PMV model mean outdoor air temperature Ta,out ( o C) From dedear & Brager, Energy & Buildings 34 (6) bias (pmv-ashrae vote) NV.4.2 PPD (ISO 77) -3 N = AC APD(votes -3,-2,2,3) pmv (ashrae scale units) PMV From Humphreys & Nicol, Energy & Buildings 34(6) From Humphreys & Nicol, Energy & Buildings 34(6) The initial contradiction is between the definition of comfort which is psychological (that state of mind..) and the determinist development of a steady-state model based on physics and physiology There is also an internal contradiction in the definition of PMV (see Humphreys & Nicol (1996) Proc. CIBSE conference) PMV ignores the part played by social constraints and time in the interactions between people and their environment because it is based on three-hour climate chamber experiments. Although it includes all thermal variables, in reality one variable (temperature) is generally used as the control 12

13 But this is also a problem for the adaptive model which predicts comfort temperatures in buildings though, based on field studies, it does include the effect of variations which occur in posture, clothing, activity, use of controls etc. An adaptive comfort temperature is also a moving target making it difficult to define and to evaluate In effect the adaptive model - if it exists - is an empirical black box in which interactions are assumed but not always quantified and the range of conditions covered is limited to those which occur in the field This applies also to the results of post occupancy surveys One responsibility we have, as researchers, is to explain and generalise the processes which go to make up these black boxes so that the lessons of field surveys are reflected in the models we develop pooh New approaches New approaches Empirical results can be used to develop new standards for indoor climate in buildings, but they will always suffer from the limitations of the comfort surveys on whch they are based New approaches are needed based on recent research - or inspired by it. New whole-body dynamic simulations and more sophisticated clothing models render obsolete the simple steady-state assumptions of existing guidelines (PMV) Data available from field surveys could enable thermal modelers and building simulators to develop stochastic algorithms for likelihood that various thermal controls (windows, fans etc) are in use 13

14 New approaches New approaches At present these dynamic simulations tend to make simplified assumptions about systems outside their own confines Thus the physiological models assume a constant or slowly changing environment Dynamic building simulations assume best practice use of controls and a simple steady state comfort model (PMV) Comfort is achieved by the occupants adapting to the building Occupant simulation Empirical results Or by the occupants adapting the building to suit them Building simulation This has to be done within the climatic, social, economic and cultural context of the whole system New approaches: problems Conclusion Existing simulations will themselves be changed by the need to introduce variability A full dynamic human-building system may be difficult to use in practice: 1 A large number of input parameters may increase the error in the prediction as each one has its own errors 2 Formulaic errors can also be cumulative I have tried to show that determinist science, whilst it can explain parts of the system which is our thermal interaction with buildings, does not explain the whole. The scientific principles of dialectical materialism can help us to understand how the whole of the system interacts with the parts - and the parts with the whole Conclusion Conclusion Thermal comfort is a psychological state which expresses satisfaction with the thermal environment connected to our thermal physiology The science of thermal comfort was intended to provide information on the indoor climates which are consistent with thermal comfort Concerns about the energy use by buildings and the best way to design of buildings without air conditioning has reawakened interest in the results of field surveys in occupied buildings These show people to be in a constant dynamic interaction with the buildings they inhabit 14

15 Conclusion Conclusion There is a conflict between our desire for comfort and the actual state of our environment which leads us to take actions which are part of the system itself The interaction applies in both directions: people are not just acted upon by the environment but they act upon it to suit their needs Stasis or equilibrium is not the natural way as is assumed by most determinist models based on heat balance Change is the natural state of affairs: equilibrium will only occur in special conditions of balance between positive and negative feedback Conclusion Lessons for research The order in the empirical results presented suggests that scientific analysis is possible The disorder in the results suggests that scientific analysis is needed but any analysis needs to take account not only of the building and its occupants but also of the climatic, social, economic and cultural context of the whole system The lessons for the future of thermal comfort research are that context (both internal and external) is an important concern and whilst aspects of the system may be taken as constant or given for the purposes of a particular investigation, this assumption should always be borne in mind in interpreting results Lessons for simulation Lessons for design Behaviour in buildings is not random but directed by people s desire to avoid discomfort The stochastic nature of people s use of buildings, and of the outdoor conditions suggest not exact solutions, but stochastic ones, not a single but a distribution of outcomes - we can be precise about the past but the future is fuzzy! The lessons for the design of buildings is that change and contradiction are a part of the comfort equation - as they are in any design process. Change, and the ability of occupants to use their natural desire and ability control their environment must be an essential ingredient of the design process 15

16 Air-conditioning! No war for oil 16