Presentation on theme: "Dr. Congxiao Shang Room No.: 01 37P Section 3: Thermal Comfort ENV-2D02 (2006):Energy Conservation – power point versions of lectures."— Presentation transcript:
Dr. Congxiao Shang Room No.: 01 37P Section 3: Thermal Comfort ENV-2D02 (2006):Energy Conservation – power point versions of lectures
3.1 Introduction Main objective: To examine whether an environment is thermally comfortable at minimum energy consumption. No two people will react the same in a given environment If all individual have the same clothing that when the temperature is at the optimum: 2.5% --- too cold; 2.5% ---- too hot Voting is normally done on the ASHRAE scale ranging from -3 for too cold to +3 for too hot People who vote with values +2 are those who are at the extremes Extremes
3.1 Introduction 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. Extremes Main objective: To examine whether an environment is thermally comfortable at minimum energy consumption.
3.1 Introduction Bear in mind: for every 1 o C, 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 ! ! ! ! !
3.2 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 H Heat lost through respiratio n R 1, Exhaust air is warmer than air taken in, AND 2, it is more moist - latent heat of evaporation Heat lost through evaporation from skin (sweat) E So net heat generated by body is (Q) from metabolic Q = H - R - E
3.2 Thermal Comfort Theory 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: Net heat generated by body is (Q) Q = H - R - E = Qr + Qc If: Q > Qr + Qc Q < Qr + Qc
3.3 Factors affecting thermal comfort a) The air temperature b) The mean radiant temperature c) The relative humidity d) The level of clothing e) The activity level (Kcal.hr -1 m -2) f) The air velocity
3.3 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.
3.3 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
3.4 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: 8 o C The external wall: 18 o C. The air-temperature: 20 o C. What is AMRT? 3m 2m
3.4 Calculation of average mean radiation temperature double glazing 3m 2m How about with double glazing? The internal surface temperature of the windows would rise to around 14 o C and the AMRT is19.44 o C, rather than o C From the previous example, a rise of 1 o C in AMRT Rise of Mean Vote by around 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.
3.5 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 individuals Predicted Mean Vote (PMV) 1, Computer prediction with Fangers Equations 2, Manual use of the charts CLO level from thermal comfort clothing chart Their activity level Air temperature Humidity Wind speed Mean radiant temperture A controlled environment
3.5 Computation of thermal comfort level Charts and tables for PMV at specific conditions were produced according to both experimental data and Fangers Equations 1) 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 and air 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 1 o C 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.
3.5 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. Window N ºC 17ºC (19.5ºC) 18.5ºC 3 Internal 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, °C on the other sides of the room, and - 20°C in a similar south facing room. An office worker wearing clothing with a CLO value of 1.0 and has an activity level of 60 kcal hr -1 m -2 complains of being too cold. What action would you take ?
3.5 Thermal Comfort - Example Window N ºC 17ºC (19.5ºC) 18.5ºC 3 Air Temp., °C PMV The same people at different activity levels may feel comfortable at different temp. The corrections for MRT difference: At 50 kcal hr -1 m -2 correction is At 100 kcal hr -1 m -2 correction is Estimate Predicted Mean Vote (PMV); - Consider Strategies At 60 kcal hr-1m-2
3.5 Thermal Comfort - Example Air Temp., °C PMV Known values: Air Temperature, ºC By linear interpolation: Predicted Mean Vote at: 19.5 °C = ( ) * [ (- 0.75)] /(20-18) = PMV ? 21 A B C C B At 60 kcal hr-1m-2
3.5 Thermal Comfort - Example Activity Level, KcalHr -1 m -2 MRT Correction ? The same people at different activity levels may feel comfortable at different temp. The corrections for MRT difference: At 50 kcal hr -1 m -2 correction is At 100 kcal hr -1 m -2 correction is Hence, the correction at 60 kcal hr -1 m -2 : 0.12 – (60-50) x [( ) / (100-50)] = 0.11
3.5 Thermal Comfort - Example Window N ºC 17ºC (19.5ºC) 18.5ºC 3 Hence: ACTION: -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.) In North facing Office near window PMV = * ( ) = on other side of office PMV = * ( ) = in south facing office PMV = * ( ) =
80 Kcal.hr-1m-2 for a person who is standing.
3.6 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 Fangers. 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
Correction charts for the increment of PMV for each 1% variation in humidity from 50%.
Correction charts for the increment of predicted mean vote for each 1 o C variation of mean radiant temperature from the air (dry-bulb) temperature.
PMV (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
Note: Thermal comfort- Clothing ( CLO ) – CLO is a CLOthing insulation unit (I cl ); – 1 clo = m 2 °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.