Thermal Comfort; Operative Temperature in the Sun Ida Bryn (Ph.d) Marit Smidsrød (MSc) Erichsen&Horgen AS, Postboks 4464 Nydalen, 0403 Oslo, E-mail: ihb@erichsen-horgen.no, telephone: 47 22026333, telefax: 47 22026390. 1. INTRODUCTION Thermal comfort is an important issue in the indoor environment. The operative temperature is one of the main parameters that describe thermal comfort. The operative temperature is normally calculated as described Thermal comfort Analysis and applications in environmental engineering, P.O Fanger. In common practice today the operative temperature is measured and calculated for a location in the shade. Short wave radiation on the body due to the sun is not included. This paper proposes a method to include direct solar radiation in the evaluation of thermal comfort. 2. MEASURED THERMAL COMFORT IN AN OFFICE A south facing office located in Oslo was chosen for measurement of thermal comfort with different shading devices. The office was equipped for one person and a 100 W heater simulated the person. The work place in the office is shown in figure 1. An operative temperature sensor is located at the desk in front of the PC. This location is close to where the person is located. In the back of the room where there is no sun we both measure the air and the operative temperature. Figure 1 South facing office at SIEMENS Linderud. Measurement of operative temperature.
The office has 11 m 2 floor area and 3.6 m 2 glazing fazing south. The windows has clear double glazing with U-value: 2.7 W/m 2, g-value 0,76 and light transmission 80%. One external and internal shading are installed. Measurements were performed in the office on sunny days without shading, with internal and external shadings. It is electrically heated and mechanically ventilated. Operative temperature at the workplace and in the back of the room were measured together with outdoor air-, ventilation inlet air and room air temperature and outdoor solar radiation. Temperature in south facing office at SIEMENS Linderud 12.08.00, Sun, No shading 35,00 30,00 25,00 Temperature, C 20,00 15,00 10,00 5,00 0,00 Room temperature Supply air temperature Outdoor air temperature Operative temperature in the sun Operative temperature in the shade 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time,h Figure 2 Temperatures in the office with no shading. The temperatures are measured in the office with no shading in the middle of august. As we see the operative temperature at the workplace goes up to 31 o C. This will cause severe discomfort. At the same time the operative temperature in the shade is maximum 22.5 o C and maximum air temperature is 22 o C. When calculating thermal comfort only the operative temperatures in the shade is calculated in the existing calculation methods and simulation programs. By using common practice today the planners would calculate the operative temperature to be 22.5 o C and the air temperature 22 o C which mean it would be a very good indoor climate. The operative temperature at the workplace in figure 2 shows the reality. The operative temperature at the workplace is 8.5 o C higher than would be calculated. This also shows that the common practice today often may lead to poor thermal comfort. We will here present a method to overcome this gap between theory and reality.
Temperatures in south facing office at SIEMENS Linderud 09.09.00 Sun, Outdoor blinds lamellae 45 o 35,00 30,00 25,00 Temperature, C 20,00 15,00 10,00 5,00 0,00 Room temperature Supply air temperature Outdoor air temperature Operative temperature in the sun Operative temperature in the shade 0 5 10 15 20 Time, h Figur 3 Temperatures in the office with outdoor shading with slats at 45 o
35,00 Temperatures in south facing office at SIEMENS Linderud 09.09.00 Sun, Indoor blinds, lamellae 45 o 30,00 25,00 Temperature, C 20,00 15,00 10,00 5,00 0,00 Room temperature Supply air temperature Outdoor air temperature Operative temperature in the sun Operative temperature in the shade 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time, h Figur 4 Temperatures in the office with indoor shading and lamellae slope at 45 o. In Figure 3 we see the operative temperatures and the air temperature in the room is measured on a clear sunny day the 09.09.00. During this day the outdoor shading was used. These measurements shows that the operative temperature at the workplace will be 23 o C while the operative temperature in the shade and air is equal and 22.5 o C. The difference between the operative temperature at the workplace and in the back of the room is now only 0.5 o C. The reason for this is the difference in direct solar radiation at the workplace for the two cases. Figure 4 shows that the operative temperature at the workplace is 1 o C higher than the operative temperature in the shade with interior blinds. This means that we for this situation has a slightly higher direct solar radiation than the case with the outside shading 3. CALCULATON OF OPERATIVE TEMPERATURE IN THE SUN In this project we were interested in finding a way to calculate the operative temperature in the sun. If we could calculate this temperature it will be easy to say something about the indoor environment when no shading or curtains is used. P.O. Fanger describes a calculation of the mean radiant temperature for a person who is affected by a high-intensity radiant source. In our case the sun is a high intensity radiant source. We can then use his equation to calculate the mean radiant temperature with influence of radiation: where, T mrt 4 ( T + ( const f q ) 0, 25 = [1] umrt p ir sun
T mrt total mean radiation temperature included sun radiation [K] T umrt radiation temperature without sun contribution [K] const 1/(0,97* ) 5,77*10-8, Stephan Boltzmans constant [W/m C 4 ] f p projected area factor Absorption factor q sun can be found like this: where q I sin h sun = k [2] I h Global horizontal radiation [W/m ] Angel of incidence [ ] i shade factor or direct solar transmittance When the mean radiation temperature is found, t mrt is used to calculate the operative temperature in the sun, by using this relation: where, t optsun ta + tmrt = [3] 2 t a ambient air temperature [ C] Equation [3] is valid when the air velocity is below 0,4 m/s and when the mean radiation temperature is below 50 C. 4. EXAMPLE In the calculation of the mean radiant temperature from Fanger, equation [1] we had to make some assumptions. The projected area factor, f p, is an area factor. Fanger describes, in his book, this factor with relation to a seated or standing person. In our case we have a globe, and an area factor for a globe will be f p = A s /A k. Where A s area of a circle A k area of a globe The equation that is valid for this case is A s 4 d 2 = [4] 2 A k = d [5] This results in an area factor for a globe at 0.25. If we uses data from Fanger's diagram for seated person, for an altitude at 60 and an azimuth at 0 we get a projected area factor at
0.26. We decided to use the area factor for a globe in our calculations, so we where nearest to our measurements. The absorption was set to 0.85, since a black globe was used to measure. In our project we measured the global horizontal radiant, Ih, from the sun. The measurements were worked through 12.08.00. We measured I h = 889 W/m at 2:00 pm. When we calculated the theoretic sun we got I h = 681 W/m. The measured value here is over the theoretic possible value and we suspect that our measurements are too high. We assume the reason for this can be the placing of our instrument. It may have been exposed for considerable reflections, which lead to higher values of radiant than expected from the sun. For this reason we have used theoretic sun in our calculations. f p = 0,26 = 0,85 i = 0,69 (direct solar transmission of glass) Measured Measured Calculated Calculated Calculated Calculated Calculated Calculated Measured Air Operative I h, global Incidence Solar Radiant Mean radiant Opt temp Opt temp Temperat Temperat horizontal Angle radiation temperature temperatur in the sun in the sun ure ure rad. e 2*T opt_shade from (1) from (3) - T a T a T opt_shade q sun T umrt T mrt T opt_sun T opt_sun Time C C W/m W/m C C C C 1.00 pm 21,2 22,0 730 57 872 22,9 42,8 32,0 31,1 2.00 pm 22,2 22,7 681 68 736 23,2 40,2 31,2 30,7 At the hours 1:00 PM and 2:00 PM, when the weather was bright, the measurements and the theoretic calculations are quite similar. The measured temperature is slightly lower than the calulated. This may be caused by inexact direct solar transmission of the glass as the data used was taken from a catalogue. This leads us to believe that this method can be used to calculate the operative temperature in the sun. It also shows that operative temperature in the shade is not a correct measure for thermal comfort. To calculate the operative temperature in the sun we first calculate the operative temperature in the shade with a building simulation program where we use the g-value for the facade component. Then we calculate the operative temperature in the sun with the method described here where we also need the direct solar transmission. We therefore also need to know the direct solar radiation to compare to facade products. We suggest that the method should be verified in laboratory tests where data on glazing is available and surroundings are controlled. 5. CONCLUSION We wanted to find a method to calculate the operative temperature in the sun. By using Fanger's theory about high-intensity radiant source we have found a method to calculate the mean radiant temperature for persons exposed to high radiance. In our calculations we saw
that the calculated and the measured operative temperature was similar. This shows that this method can be used for calculating the operative temperature in the sun. The results also show that operative temperature in the shade is insufficient to determine thermal comfort. We need also to calculate the operative temperature in the sun. The g-value is insufficient to describe the solar properties of a facade product. Direct solar transmission for the glazing and shading together is also necessary. Since our measurement of the global horizontal radiant is not reliable there will be necessary with a control of the method with new measurements. New measurements are planned in the near future that will be used to support the method described in this note. 6. REFERENCES P.O. Fanger. 1972. Thermal comfort Analysis and applications in environmental engineering. McGraw-Hill Book Company.