Presentation on theme: "Section 3: Thermal Comfort"— Presentation transcript:
1Section 3: Thermal Comfort Dr. Congxiao ShangRoom No.: 01 37PENV-2D02 (2006):Energy Conservation – power point versions of lectures
23.1 IntroductionMain objective: To examine whether an environment is thermally comfortable at minimum energy consumption.No two people will react the same in a given environmentIf all individual have the same clothing that when the temperature is at the optimum: 2.5% --- too cold; 2.5% ---- too hotVoting is normally done on the ASHRAE scale ranging from -3 for too cold to +3 for too hotPeople who vote with values <-2 or > +2 are those who are at the extremesExtremes
33.1 IntroductionMain objective: To examine whether an environment is thermally comfortable at minimum energy consumption.The number of people voting at particular values follows a Gaussian (normal) distribution, which has its peak at a mean vote of zero at an optimum setting.If we respond only to complaints from those who are feeling too cold or too hot then we are likely to find that more people will be dissatisfied as the curve will be shifted to hotter or colder end.people who are too cold complain more actively than the other way, with the consequence that the temperature is often kept unnecessarily high.ExtremesExtremes
43.1 Introduction! ! ! ! !Bear in mind: for every 1oC, the energy requirement rises by 8-10% (in the UK).So: We need to be objective in any response to complaints by investigating the overall situation - not just the immediate problem of one or two complainants
53.2 Thermal Comfort Theory The perception of thermal comfort for an individual depends on maintaining a balance between heat produced by body and heat losses:Heat generated by body, depending on metabolic rate HHeat lost through respiration R1, Exhaust air is warmer than air taken in, AND2, it is more moist - latent heat of evaporationHeat lost through evaporation from skin (sweat) ESo net heat generated by body is (Q) from metabolicQ = H R E
63.2 Thermal Comfort Theory Net heat generated by body is (Q)To maintain a balance, this heat of value, Q, must be “removed” by radiation (Qr) and convection (Qc) from the clothing in an actual environment, as follows:Q = H - R - E = Qr + QcIf: Q > Qr + QcQ < Qr + Qc
73.3 Factors affecting thermal comfort a) The air temperatureb) The mean radiant temperaturec) The relative humidityd) The level of clothinge) The activity level (Kcal.hr-1m-2)f) The air velocity
83.3 Factors affecting thermal comfort At a specific area, the mean radiant temperature within the area is measured with a Globe Thermometer and is related to the exchange of heat between a person and his/her surroundings.However within a building:We should consider the position you are in the room for measurement,Different walls have different surface temperatures, e.g. internal walls, external walls, windows etc. The mean radiant temperature is different at different points for measurements in the building.
93.3 Factors affecting thermal comfort At a specific area, the mean radiant temperature within the area is measured with a Globe Thermometer and is related to the exchange of heat between a person and his/her surroundings.However within a building:As an approximation, the AVERAGE MEAN radiant temperature within a room may be estimated by the following ( surface areas x surface temperatures)total surface area
103.4 Calculation of average mean radiant temperature It is actually a mean temporal, mean spatial MRT!Example:Office Size: 3m x 3m x 3m (typical of UEA).Windows: 2m high and full width of one wall.Internal surface temperature of windows: 8oCThe external wall: 18oC.The air-temperature: 20oC.3m2mWhat is AMRT?
113.4 Calculation of average mean radiation temperature How about with double glazing?The internal surface temperature of the windows would rise to around 14oC and the AMRT is19.44oC, rather than 18.56oCFrom the previous example, a rise of 1oC in AMRT Rise of Mean Vote by around 0.11.Therefore, There will be improvement through double glazing in this room. Alternatively we could reduce the air temperature slightly to get the same equivalent comfort level.double glazing3m2m
123.5 Computation of thermal comfort level Actual Thermal Comfort Votes– For the analysis of a particular environment. Thermal comfort experiments need to be done at particular conditions for a large number of individualsPredicted Mean Vote (PMV)1, Computer prediction with Fanger’s Equations2, Manual use of the chartsCLO level from thermal comfort clothing chartTheir activity levelAir temperatureHumidityWind speedMean radiant tempertureA controlled environment
133.5 Computation of thermal comfort level Charts and tables for PMV at specific conditions were produced according to both experimental data and Fanger’s Equations1) select table for appropriate metabolic rate, indicated by activity level.2) select appropriate clothing level sub-table.3) now read of vote value corresponding to air (dry-bulb) temperature andair velocity (m/s) corresponding to wind speed.Now how to obtain the PMV via the tables for other environments?5) Use the Humidity correction chart which gives the correction for each 1% variation in humidity from 50%6) Repeat for the Mean Radiant Temperature Correction chart i.e. for each 1oC that the MRT differs from the air (dry bulb) temperature.7) Apply these corrections to the basic value to obtain the corrected PMV (predicted Mean Vote).8) Use further chart to estimate proportion of people likely to be dissatisfied with thermal environment.
143.5 Thermal Comfort - Example An office is 3m x 3m x 3m high with an external wall which is has a large 2m high window on the full width of the external wall, facing North.NInternal air in the building: T = 19.5°C;air velocity = 0.The mean radiant T at a worker's desk:- 17°C near the external wall,- 18.5°C on the other sides of the room, and- 20°C in a similar south facing room.(19.5ºC)Window231An office worker wearing clothing with a CLO value of 1.0 and has an activity level of 60 kcal hr-1m-2 complains of being too cold.18.5ºC17ºC320ºCWhat action would you take ?3
153.5 Thermal Comfort - Example - Estimate Predicted Mean Vote (PMV);Consider StrategiesNAt 60 kcal hr-1m-2(19.5ºC)Window2Air Temp., °CPMV18-0.7520-0.323The same people at different activity levels may feel comfortable at different temp.The corrections for MRT difference:At 50 kcal hr-1m-2 correction isAt 100 kcal hr-1m-2 correction is118.5ºC17ºC320ºC3
163.5 Thermal Comfort - Example Known values:Air Temp., °CPMV18-0.7520-0.32At 60 kcal hr-1m-2By linear interpolation:PMV18201919.5-0.75-0.32?21ABCC’B’Air Temperature, ºCPredicted Mean Vote at: 19.5 °C =( ) * [ (-0.75)] /(20-18) =
173.5 Thermal Comfort - Example The same people at different activity levels may feel comfortable at different temp.The corrections for MRT difference:At 50 kcal hr-1m-2 correction isAt 100 kcal hr-1m-2 correction isActivity Level, KcalHr-1m-2CorrectionMRT10060500.060.12?Hence, the correction at 60 kcal hr-1m-2 :0.12 – (60-50) x [( ) / (100-50)]= 0.11
183.5 Thermal Comfort - Example Hence:In North facing Office near windowPMV = * ( ) =on other side of officePMV = * ( ) =in south facing officePMV = * ( ) =N(19.5ºC)Window2ACTION:Moving the desk to the other side of the room or better still to a south facing office.By complaining, an office worker must be voting a PMV < -2, and yet the above PMV is only slightly negative, suggesting that the person always feels cold and should be encouraged to wear an extra sweater.( Increasing the CLO value to 1.25 will increase the vote by about 0.3.)3118.5ºC17ºC320ºC3
213.6 Thermal comfort summery Thermal Comfort measurements may be used to assess a given environment and are a useful additional aspect of Energy Management.The level of comfort may be predicted using Fanger's Equations, however, you should note the following:It is difficult to accurately assess metabolic rate, and there is a tendency to underestimate value for people who are seated unless they have been in the particular Environment and at the particular activity level for at least an hour.Fanger's Theory strictly applies only to individuals having the same clothing, but taking the mean values of a large number of votes should give the same as Fanger’s.If actual votes are available, then we can still use Thermal Comfort Tables made under standard conditions, or the computer to assess the effects of changes in the Environmental Conditions on the mean VOTE.Rarely is actual thermal comfort data used in Energy Management Decisions - responses are usually made for those who feel too cold without identifying the real problem
22Correction charts for the increment of PMV for each 1% variation in humidity from 50%.
23Correction charts for the increment of predicted mean vote for each 1oC variation of mean radiant temperature from the air (dry-bulb) temperature.
24PMV (predicted mean vote) It represents on a thermal sensation scale of the mean vote of a large population of people exposed to a certain environment;It is derived from the physics of heat transfer combined with an empirical fit to sensation;It establishes a thermal strain based on steady-state heat transfer between the body and the environment and assigns a comfort vote to that amount of strain.The PMV equation for thermal comfort is an empirical equation for predicting the mean vote on a ordinal category rating scale of thermal comfort of a population of people (not required here).PMV > 0 towards hot discomfort; PMV < 0 towards cold discomfort
25Note: Thermal comfort- Clothing (“CLO”) “CLO” is a CLOthing insulation unit (Icl); 1 clo = m2 °C/W; Lowest clo value is 0 (naked body) ; Highest practical clo value = 4 clo (Eskimo clothing, fur pants, coat, hood, gloves etc.) ; Summer clothing ~ 0.6 clo ; Winter clothing ~ 1 clo .