Field Measurements of Thermal Comfort Conditions in Buildings with Radiant Surface Cooling Systems

Transcription

1 Field Measurements of Thermal Comfort Conditions in Buildings with Radiant Surface Cooling Systems MICHELE DE CARLI, PhD Student, Dip. di Fisica Tecnica, University of Padova, Italy BJARNE W. OLESEN, Head of Research and Development, Wirsbo-VELTA GmbH &Co. KG, Norderstedt, Germany ABSTRACT The performance of radiant heating systems (floor heating, ceiling panels) is relatively well documented. Water based systems, where pipes are embedded in the building structure, are now being increasingly used for cooling purposes. Several theoretical studies based on the use of computer simulations have been published, but very little is reported on the actual performance in existing buildings. There are still uncertainties on how such systems operate and on how well the space temperatures under varying external and internal load is kept within the comfort range. To study this aspect, field measurements of thermal comfort conditions were made in several buildings with radiant surface cooling systems. The systems comprise floor cooling, wall cooling and cooling with pipes embedded in the concrete slabs between each floor in a multi storey building. Long term measurements of operative, air, surface, system and external temperatures have been carried out. The analysis of the data shows that, for the major part of the time of occupancy, the operative temperature is inside the comfort range. The analysis has been lead for different classes of comfort according to existing standards. The data show an increase in space temperature during the day, which is counterbalanced by a corresponding decrease during the night. This study shows that hydronic radiant cooling systems in many buildings are an interesting alternative to full air conditioning systems, for obtaining acceptable indoor thermal environments during summer. 1. INTRODUCTION In the eighties and nineties the market for radiant cooling panels increased significantly in middle Europe. From the middle of the nineties interest in radiant floor cooling started and from the end of the nineties the so called active thermal slab have been installed in several multi-storey buildings. Up to now papers presenting theoretical studies of this new technology have been published (Brunello et al. 21, Hauser et al. 2, Simmonds et al. 2, Koschenz, Meierhans and Olesen 1999, Meierhans 1996). At the beginning of 21 more than 6 buildings with the active thermal slab technique are in operation or being constructed in Germany. Most of them are office buildings with a floor area between 25 m 2 and 4. m 2 but also other type of buildings like museums, hospitals, schools are build with embedded cooling systems. Clima 2/Napoli 21 World Congress - Napoli (I), September 21

2 During the last three years temperature measurements on some buildings have been made in order to investigate the performance of different radiant cooling systems. In the present paper measurements from three buildings are presented. 2. METHOD In monitoring existing buildings the evaluation of energy consumption, operating conditions of the air conditioning system and thermal comfort satisfaction of the occupants should be investigated simultaneously. Measurement and evaluation of the thermal comfort conditions are therefore focussed. Evaluation criteria from different national and international standards are used. 2.1 Thermal comfort evaluation Comfort requirements can limit the capacity of the radiant heating and cooling systems. On the basis of the international standards and guidelines (ISO , CR , DIN 1946 Part 2) the comfort range for operative temperatures is between 2 C and 24 C, for people with sedentary activity level (1.2 met) in heating conditions (winter), with clothing thermal resistance equal to 1. clo. In cooling conditions the comfort range, for an activity level of 1.2 met and a clothing thermal resistance of.5 clo, is 23 C to 26 C. These ranges are based on a predicted percentage of dissatisfied (PPD) below 1% and a predicted mean vote (PMV) between.5 and +.5. For radiant systems it is very important to refer to the operative temperature, both for evaluating the comfort conditions and the performance of the system itself. Figure 1 - Requirements in the German Standard DIN 1946-Part2. Horizontal Dashed Zone: Cool Range. Crossed Dashed Zone: Comfort Range. Vertical Dashed Zone: Warm Range. Thermal active slab systems are using the thermal storage of the concrete slabs to reduce peak loads and transfer some of the cooling to outside the time of occupancy. This has a dynamic effect on the thermal characteristics of the indoor environment, i.e. the operative temperature will often increase during daytime. In the German standard DIN1946 the space

3 temperature may increase to 27 C with outside temperatures of 32 C (see figure 1) In order to maintain the comfort conditions in the range above mentioned there is a need to know the external and internal loads and the capacity control of the thermal active slab. In EN ISO 773 and CR 1752 the comfort criteria for the temperature level inside is given as a range for PMV or PPD. Depending on economy and outside climate not all countries will design for the same comfort level. Also depending on the type of building there may be acceptance to design for different levels of comfort. Therefore CR 1752 and a proposed revision of ISO 773 introduces three classes as shown in Table 1. The standards give values for the comfort conditions in steady state conditions, but according to Knudsen (1989) the PMV-PPD index can be used as long as the rate of temperature change is lower than 5 K per hour. Table 1: Classes of Thermal Comfort (CR 1752) Class Comfort requirements range PPD PMV Winter 1. clo 1.2 met Summer.5 clo 1.2 met [%] [/] [ C] [ C] A < < PMV < B < < PMV < C < <PMV < In many cases it may be too costly to keep the temperature levels in buildings always within the specified comfort ranges. There should be some allowance for a limited time to exceed the specified range. There is therefore a need to evaluate the number of hours the comfort conditions are exceeded This can be done by a simple addition of the numbers of hours where the comfort range is exceeded. But it is more severe if the range is exceeded with 3 K for one hour compared to 1 K for one hour. Therefore another method is to weigh the hours exceeded with the degrees it has been exceeded. In this way a kind of degree-hours is calculated. An example is shown in Table 2. A third method is based on using the PPD value instead of temperature for weighting the number of hours exceeded. For this purpose a weighting factor wf has been proposed, which can be expressed as: PPDPMVactual wf = (1) PPDPMV Limit where PPD PMVactual is the instantaneous value in which the PPD exceeds the limit PPD PMV Limit which depends on the class of comfort. The values of PPD PMVactual can concern cool sensation in the heating period and warm feeling in the cooling season. The single weighting factors are then multiplied by the time step (expressed in fractions of hour) in which the phenomenon is observed. The total amount of the factors calculated in this way is defined as weighting time [h] which can be calculated as: WT warm = wf time for PMV > PMV limit (2) and WT cold = wf time for PMV < PMV limit (3)

4 An example of weighting factor values can be seen in Table 2 for the summer (cooling) comfort range corresponding to class B. It can be seen that the weighted hours according to the PPD-method are higher than after the temperature method. Table 2. Example of Weighting Factors for Degree-hour Calculations wf(t) and for Weighting Factors According to the PPD Method wf(ppd). The values refer to Class B (Table 1) Comfort Range During Summer Conditions (1.2met,.5 clo, Relative Air Velocity <.1m/s, Relative Humidity ~5%) [ C] wf(t) wf(ppd) Cool Neutral Warm Measurements Measurements have been made with different temperature probes, whose specifications are reported in Table 3. For operative temperature three different probes have been used. The first probe is a temperature sensor shaped like an ellipsoid (5mm x 12mm) which will directly measure the operative temperature (Olesen 1982, ISO 7726). The second temperature sensor II is a globe thermometer with 4 mm diameter, which due to the size according to ISO 7726 specifications will measure the average value between air- and mean radiant temperature at low air velocities.. The third probe III is a half globe thermometer with 4 mm diameter which has been installed on the surfaces opposite to the external walls. In this way the sensor will be exposed to a radiant heat exchange with all surfaces except the backwall. The probes II and III were transferring the temperatures to a-logger via radio frequency. Table 3. Specifications of the Measurement Probes Probe Type Accuracy K Resolution K Building Operative temperature I Ellipsoide.5.1 1, 2, 3 Operative temperature II Globethermometer (4 mm).5.1 2, 3 Operative Temperatur III Half globethermometer (4 mm) Air temperature I Pt , 2, 3 Air II NTC sensor Dew point temperature Chilled mirror.5.1 1, 2, 3 3. RESULTS AND DISCUSSION The measurements were made in three office buildings with different type of surface cooling systems. Building 1 in Bregenz, Austria has a combination of wall-floor-ceiling heating-cooling system. Building 2. in Halle, Germany, has a floor heating-cooling system Building 3 in Stuttgart, Germany has an active thermal slab system. The analysis of the comfort conditions in the buildings has been made both for the whole day and for the working time only, during the whole measuring period. This has been defined

5 as typical office hours, i.e. between 8: and 18: from Monday till Friday. In this paper the data of the working time only are reported. During the measurements no subjective evaluations from the occupants were collected. 3.1 Building 1 This is a two storeys building in a very light construction. In order to limit the indoor operative temperatures floor heating/cooling was installed in the offices and in the hallway also a wall heating/cooling system (Figure 2). The cooling is supplied from a ground heat exchanger, where pipes are embedded in the foundations, which is more or less surrounded by ground water. This allows to achieve a supply water temperature at 16 C. The offices are all located in west direction with external sun screens. There is a ventilation system which provides a primary air. Sky-light Window Room 44 Pipes Hall Window Room 34 Steel support Figure 2 - Construction of Building 1.

6 Figure 3 - Operative Trends in One Typical Warm Week. Figure 4 - Trends for Office 34 in One Typical Warm Week. Three offices and the hallway have been investigated for one month (between July and August 1998). The operative temperatures were measured by means of an elliptical probe (Table 3). The external temperature has been provided by the meteorological institute. A sample of measured operative temperatures is shown in Figure3: Even if the outside temperature is higher than 3 C the inside temperature is always below 26 C. For the same

7 week detailed information can be seen in Figure 4, in which the floor and the air temperature are also shown. Table 4 shows that the operative temperature in the offices is for 95% of the total working hours between 21 C and 25 C and the temperatures increases above 26 C only in the hall way. The temperature sensor in the hallway is exposed, around noon, to the direct sun radiation coming from the glazed ceiling. Table 5 shows that the operative temperature during a working day are in most cases below 4 C, with the exception of the hall way and office 44. By evaluation according to the German standard (Table 6) the operative temperatures in the offices are for more than the 5% of the time in the optimal comfort range between 22 C and 26 C and about 4% in the cool range between 2 C and 22 C. The weighted-hours on the warm side are zero for the offices and only 14 h (wf(ppd)) and 6 h (wf(t)) for the hallway. The weighted-hours for the cool side is however high. The tuning of the control system was not completed at the time of the measurements and the excess of low temperature could be avoided by stopping the cooling earlier in the night. Table 4. Percentage of Operative Distribution During Working Time range [K] Hallway Room 34 Room 44 Room 414 Total for rooms 34, 44, 414 < >3. Table 5. Percentage of the Operative Change Distribution During a Working Day change during a working day [K] Hallway Room 34 Room 44 Room 414 Total for rooms 34, 44, 414 < > Table 6. Percentage of Operative Distribution According to DIN1946 range [K] Hallway Room 34 Room 44 Room 414 Total for rooms 34, 44, 414 < >

8 Table 7. Percentage of Operative Distribution According to CR1752 During Working Time change during a day [K] Hallway Room 34 Room 44 Room 414 Total for rooms 34, 44, 414 Class Class Class Class Class A B C A B C A B C A B C A B C Cool Comfort Warm Table 8. Weighted Time Values for the Working Period for Class B Hallway Room 34 Room 44 Room 414 WT (T) cool [h] warm 6. WT (PPD) cool [h] warm 14.1 Number of hours The results show that even in a very light building it is possible with a surface cooling system to keep the room temperatures from getting too high. 3.2 Building 2 This rectangular building (11.5 m by 6 m) is equipped by floor heating/cooling. In this building the control of the floor heating cooling system is separated in an east and a west zone. The supply water temperature was regulated according to the outside temperature. For the cooling the supply temperature was limited according to the dew point measured in an east oriented office for the east zone and in a west oriented office for the west zone. The reception is south oriented, with full glazed facades in east, south and west directions and with a glass ceiling. Only the ceiling and south facades provided with internal sun screens. The s in the offices are shielded from sun radiation by means of internal or external screens. The building has operable s and no mechanical ventilation. Measurements was made from middle June to middle August in 2. Operative temperature has been measured by means of a half globe thermometer (Table 3) in eleven rooms except the reception where a globe thermometer (Table 3) was located. A sample of temperature trends can be seen in Figure 5. More detailed measurement has been done for a room on the east side where the reference sensor for the control was installed., The operative temperature was here also measured by means of an elliptical probe (Table 3). For this room the trend of relevant parameters is shown in Figure 6. The outside temperature has been measured on the east side by means of a PT1. For some hours of the day this sensor was exposed to sun radiation, which can be seen from some high temperatures in Figure 6.

9 Figure 5 - Sample of Trend for the Building 2. Figure 6 - Sample of Trend for the Building 2. Tables 9-12 show the temperature distribution in temperature classes according to relevant standards. Table 13 shows the weighted hours according to the two methods.

10 As it can be noticed from Table 9 to 13, the reception temperature is high for many hours, because this is a glazed atrium which is internally shielded from sun radiation only on the south side; on west and east side the reception has no sun screen. On the other hand the office rooms are below the comfort range for long periods. The data in figure 6 (from the reference room) show also relatively low temperatures. This is because this room was not occupied during the measuring period.i.e. no internal loads from people and equipment. From the measured water and floor temperature in figure 6 it is seen that on several days ( June 27- July 3, and July25 the cooling was not in operation. Even in the period June 27 to June 3 heating was provided. This is caused by the central control which according to outside temperature and internal temperature in the reference room turns on and off the heating and cooling. This means for these periods the reception was also not provided with cooling even if the room temperatures were high. The control system has the capability of using also an individual room control. This is used for heating operation but not for cooling, which meant all offices would get cooling if the reference room required cooling. Instead the local control should have been used so that in rooms, where the temperatures was low enough the room control would shut of the water flow to this room. Looking at Table 1 it can be observed that for most of the working days the temperature range is inside an acceptable range of 4 K. Table 9. Percentage of Operative Distribution During Working Time range [K] Reception East West Inside Outside All except reception < > change during a day [K] Table 1. Percentage of Operative Change Distribution During a Working Day Reception East West Inside Outside All except reception < > It is anyway interesting to notice that the rooms with external screens present values of inside temperature lower than the ones with internal screens. Also the temperature change (Table 1) during a day is lower in rooms with external screens. This shows how important the position of the sun screen is, especially for surface cooling systems characterised by a limited cooling capacity.

11 As far as the weighted hours are concerned, the offices have acceptable values for the warm side, but the 919 h for the reception is much too high. Table 11. Percentage of Operative Distribution According to DIN1946 During Working Time change during a Reception East West Inside Outside All except reception day [K] < > Table 12. Percentage of Operative Distribution According to CR1752 During a Working Day change during a Reception East West Inside Outside All except reception day [K] Class Class Class Class Class Class A B C A B C A B C A B C A B C A B C Cool Comfort Warm Table 13. Weighted Time Values for the Working Period for Class B Reception West side Internal West side External East side External WT (T) cool [h] warm WT (PPD) cool [h] warm Number of hours The results from this building show the importance of optimising the control setting also after the building is in operation. The results also show that in fully glassed spaces like the reception an outside sun screen is needed. For the offices the temperatures were in most cases below the upper level of the comfort range, but did in several cases go below the comfort range. 3.3 Building 3 The thermal plant is based on active thermal slab system. A displacement ventilation system has been equipped and the s are operable. In this building 65 m 2 of active thermal slab system are installed. The building has raised floors for installation of cables, only a limited area with suspended ceiling and operable s. The ventilation is provided by displacement system. Measurements have been conducted in two open space offices on the 4 th and. All measurements have been done by means of the globe thermometer probes Table 3) with exception of the middle place on the measured during the 1999 campaign; here the operative temperature was measured by means of the elliptic sensor (Table 3). Measurements have been made during August 1999 and from middle June till middle October 2. In figure

12 7 and 8 samples of trends can be seen. The outside temperature has been measured by means of a probe which was not adequately screened by direct sun radiation. This explains some of the very high temperature values in the diagrams. The water temperatures during the 2 measurements were in the range C. Figure 7 - Sample of Trend for the Building 3 Figure 8 - Sample of Trend for the Building 3

13 From Table 14 it appears that around 95% of the total working time operative temperatures are between 21 C and 26 C. From Table 15 it can be noticed that the change of temperature during working time is for all the offices below 4 C with exception of the office placed at the west oriented which exceeds 4 C during only 4% of the working hours. From Figure 8 and the Tables it can be seen that temperatures on the are higher than the on 4 th floor. This is because the two floors are connected by an opened stair way in the middle of the landscape offices. Therefore some of the convective part of the internal loads on the 4 th floor (people, equipment, sun) will rise to the and increase the internal load there. The weighting factors for these offices do not exceed 15 h if they are evaluated with reference to the temperature and 5 h if they are evaluated by means of the PPD index. range [K] Table 14. Percentage of Operative Distribution During Working Time 4 th floor 4 th floor meeting middle room west south west east all offices except middle < > Table 15. Percentage of Operative Change Distribution During in a Working Day 4 th floor 4 th floor meeting middle room west south west east change during a day [K] all offices except middle < > Table 16. Percentage of Operative Distribution According to DIN1946 During Working Time change during a day [K] 4 th floor west 4 th floor south west east middle meeting room all offices except middle < >

14 Table 17. Percentage of Operative Distribution According to CR1752 During Working Time 4 th floor west 4 th floor south west east middle meeting room change during a day [K] all offices except middle Class Class Class Class Class Class Class A B C A B C A B C A B C A B C A B C A B C Cool Comfort Warm Table 18. Weighted Time Values for the Working Period for Class B 4 th floor west 4 th floor south west east middle meeting room WT (T) cool [h] warm WT (PPD) cool [h] warm Number of hours It can be seen that in some days the temperature in the morning is on the cool side. As no subjective evaluations was made it is not possible to determine if this caused a real comfort problem or not. The low temperatures could probably be avoided by decreasing the number of hours cooling operation during the night. Using evaluation after existing standards (ISO 773 and CR 1752) will result in a significant time below the comfort ranges. 6. CONCLUSIONS The three examples show that surface heating and cooling of office buildings is not only theory, but will also work in practice. In all three buildings the measured operative temperatures were only during few hours above the comfort range (> 26 C) specified in existing standards even at outside temperatures above 3 C. Only exception was full glassed reception in one of the buildings, which only had internal sun screens in the ceiling and towards the south. The operative temperature variation during a working day was in most cases below 3-4 K. In all three buildings the operative temperature was in the morning at several work places on the cool side. As the subjective evaluation is missing it is not possible to determined if this caused a real comfort problem. Using existing international standards, as ISO 773 or CR 1752, these spaces would be judged as cool during several days. This could be avoided by decreasing the hours of cooling during the night. The measurements which in some of the buildings lasted several months has also been evaluated using weighted-hours according to how much and how long the temperatures were below or above the comfort ranges listed in existing standards. These calculations show relative few weighted hours on the warm side, but a significant number of weighted hours on the cool side. By revision of the international standards it must be carefully discussed, how to make long term evaluations. It seems not appropriate to use the same clothing level for the whole summer period may to September.

15 REFERENCES Brunello, P., Di Gennaro, G., De Carli, M., Zecchin, R. (21) Mathematical modelling of radiant heating and cooling with massive thermal slab, Clima 2, Napoli (I). CR 1752(1998): Ventilation for Buildings: Design Criteria for the Indoor environment, CEN, Brussels. DIN 1946 Raumlufttechnik Teil 2,1994.Berlin: Deutsches Institut für Normung. Hauser,G., Kempkes, Ch., Olesen, B. W. (2), Computer Simulation of the Performance of a Hydronic Heating and Cooling System with Pipes Embedded into the Concrete Slab between Each Floor. ASHRAE Winter Meeting, Dallas, 5-9 February 2. ISO 7726, 1998: Ergonomics of the thermal environment Instruments for measuring physical quantities. International Organization for Standardization, Geneva, Switzerland. ISO 773: Moderate thermal environments Determination of the PMV and PPD indices and specification of the conditions for thermal comfort. Juli Knudsen, H. N. et. al. (1989), Thermal Comfort in Passive Solar Buildings. Technical University of Denmark. Koschenz, M., Lehmann, B.: Thermoaktive Bauteilsysteme tabs. Buch, ISBN Meierhans, R. A. (1996), Room air conditionning by means of overnight cooling of the concrete ceiling. ASHRAE Trans. V. 12, Pt. 2. Meierhans, R.A. and Olesen, B. W. (1999), Betonkernaktivierung, Book, 67 pg. ISBN Oesterle, E., Koenigsdorff, R.: Thermische Aktivierung von Bauteilen zum Heizen und Kühlen von Gewerbebauten. Klimatechnik HLH. Olesen, B. W. (1997a), Possibilities and Limitations of Radiant Floor cooling, ASHRAE Trans. V.13, Pt.1. Olesen, B. W. (1997b), Flächen-Heizung/Kühlung Einsatzbereiche Fußboden, Wandund Decken-Systeme. 19. Internationaler Velta-Kongreß, St. Christoph/Tirol. Olesen, B.W., Michel, E., Bonnefoi, F., De Carli, M. 2. Heat Exchange Coefficient Between Floor Surface and Space by Floor Cooling: Theory or a Question of Definition. ASHRAE Trans. Part 1. Pinck: Kühlen im Betrieb. Thermoaktive Decken im Bürohaus BMG Ariola GmbH. Beratende Ingenieure, März 1998, Springer-VDI-Verlag. Simmonds, P., Gaw, w., Holst, S., Reuss, S., Using Radiant Cooled Floors to Condition Large Spaces and Maintain Comfort Conditions, ASHRAE Trans. 2, Part 1 (in print). Thiel, P. (1999), Untersuchung zur Bauteilkühlung. DKV-Tagung, November 1999.