1、Y. Zou is an associate professor in the School of Environmental Science and Engineering, Donghua University, Shanghai, P.R. China. H. G. Yu is a master student in the School of Environmental Science and Engineering, Donghua University, Shanghai, P.R. China. Y.Liu is a research fellow at Dianwei ltd,
2、 Shanghai, P.R. China. In-Situ Measurement of Building Thermal Resistance with a Plane Heater Haigang Yu Yun Liu Yue Zou, PhD, PE Associate Member ASHRAE ABSTRACT In this paper, a new methodology for the field measurement of thermal resistance of building envelope is proposed. A plane heater such as
3、 electric blanket is applied to heat one side of the test wall and heat flux transducers are fixed on the other side of wall to measure the heat flux through the measurement section. This could reduce the equipment size significantly and make the installation more easily compared to the traditional
4、hot-box method but still can create the enough temperature difference and one-direction heat flux to calculate the thermal resistance of wall in a relatively short period. For one of the most common type of wall used in southern China, extensive In-situ measurements and numerical simulations were ca
5、rried out to identify the influence of plane heater size to the accuracy of thermal resistance measurement. The evaluated thermal resistance values can be considered as or at least close to the true value when the heating sizes are larger than 1mx1m(3.3 ft x3.3 ft), but these values are always overe
6、stimated and deviate rapidly from the true value when heating sizes are less than 1mx1m(3.3 ft x3.3 ft). However, it is possible to amend those overestimated values by the curve developed in this paper. BACKGROUND As the worlds most populous country with a fast-growing economy, China consumed about
7、28% of all her energy in building operation in 2013 and this ratio will increase up to 35% in 2020. Therefore, it is very important for China to know the thermal resistance and other properties of building envelope for evaluating the energy efficiency of building. This parameter of building material
8、s can be tested accurately under laboratory conditions such as steady-state, one-dimensional heat transfer without consideration of convection and radiation; however, these conditions may be “unrealistic” for an actual building. If the building thermal performance needs to be precisely defined, in-s
9、itu measurement must be made. The fluctuating ambient temperature, multi-layer construction, variable moisture content and material dimension make the measurement of in-service building thermal performance difficult. According to the ASTM (the American Society for Testing and Materials) standards, a
10、 complete building thermal resistance includes the following parts: calibration of probes such as temperature sensors and HFTs (heat flux transducers); suitable venues for measuring heat flow; field measurement lasting 3 to 7 days and calculation of thermal resistance. There are two main techniques
11、to measure the thermal resistance of an existing building: hot-box method and heat flux meter method. During the hot-box test, a relatively big (about 2.5 m x 2.5 m (8.2 ft x 8.2 ft) or larger) area of a clear wall is used to evaluate the thermal resistance. The climatic and metering chambers of the
12、 apparatus allow the temperature of both sides of the specimen wall to be kept as constant as possible to generate a stable and detectable heat flux through the test wall. The cooling or heating of the wall surfaces is usually achieved by the air circulating in both chambers. The uncertainty of this
13、 method is reported as between 1 and 10 percents (ASTM 2011). The heat flux meter method may not be as accurate as the hot-box method, but the heat flux meter method is more commonly used in China for simpleness and convenience. Only temperature sensors and HFTs are needed in this method to record t
14、he outside and inside wall surface temperatures and the “natural” heat flux through the wall. Without the help of the device, for example, “hot-box” to keep the one-direction heat flux in the wall, the heat flow meter method is always carried out in the heating season. In Northern China, the heating
15、 period is more than 3 months and the average temperature difference between indoor and outdoor is easily over 10 degrees. The higher the temperature difference is, the more precise the evaluated thermal resistance and also the shorter the test period are needed. In Southern China, the heating seaso
16、n is much shorter, especially for the South China Sea coastal cities such as Guangzhou and Fuzhou. In Guangzhou, the mean temperature in the “coldest” January during 1971 to 2009 was around 14C (57.2F). It is even hard to find the indoor air temperature consistently higher than the outdoor temperatu
17、re for 3-5 days during the “winter”. Guangzhous muggy summer lasts from April to September. During these six months, the monsoon brings over 80% of annual rainfall that is about 1750 mm (69 in.). Almost everyday will meet some sharp downpours even ferocious thunderstorms with quick stop. 3-5 days fi
18、eld measurements of building envelope under “natural” condition could be also unrealistic during this period. In this study, a new methodology for field measurement of thermal resistance of building envelope situated in a subtropical region is studied. The system takes less space and is removed more
19、 easily compared to the traditional hot-box method but still can heat one side of a wall to create enough temperature difference and one-direction heat flux to calculate the thermal resistance of the wall in a relatively short period. OUTLINE OF TEST SYSTEM The entire test system layout is given in
20、Figure 1. It consists of one heating element including surface temperature sensors, one heat flux receiver element including HFTs and surface temperature sensors. All the temperature and heat flux data are sent to a multi-channel data logger with the sampling rate up to 1Hz. Figure 1 Sketch of Appar
21、atus for On-site Wall Thermal Resistance Measurements. The primary purpose of the heating element is to warm one side of the test wall and make enough temperature difference for measurements. The plane heater could be either electrothermal film or electric blanket. 2 mm (0.08 in.) Wall Extruded Poly
22、styrene Board Plane Heater Aluminum Sheet HFT with Temperature Sensor,sej jTqTemperature Sensor sijTData Acquisition System aluminum foil or sheet is covered on the plane heater to obtain uniform temperature distribution. Two temperature sensors are mounted on the central section of the aluminum she
23、et with 2 mm (0.08 in.) soft foam glue. All the surface temperature sensors in this study are foil-type Pt100 with dimension of 11x30x0.13 mm (0.43x1.18x0.005 in.), 0.2 mm (0.008 in.) copper foils are used on the sensor surfaces to protect the temperature sensors and give a good contact with the tes
24、t wall. The plane heater is insulated with a 46mm (1.81 in.) extruded polystyrene board (XPS) to make sure the heat transmission through the wall. A heat flux transducer is fixed on the cold side of the test wall, in the center of the corresponding heating section in the “warm” side of the wall to r
25、eceive the heat flux through the wall. The size of the heat flux transducers used is about 50x100 mm (1.97x3.94 in.) with sensitivity about 0.05mV/(W/m2) (0.538 mV/(W/ft2) based on the technical data provided by the manufacturer. One most frequently used heat flux transducer is calibrated twice duri
26、ng the whole test period, using the standard guarded hot plate apparatus by an independent test laboratory. The total measurement accuracy of this transducer is estimated within 4% reading values. One surface temperature sensor with copper protective layer is also coated on the upper part of the hea
27、t flux transducer to measure the temperature on the cold side of the wall. After a sufficient history of measurement, the thermal resistance of the test wall can be calculated with the Average Method (ISO 1994): 11()nsij sejjnjjTTRq=(1) Where: R = thermal resistance of the test wall sijT = interior
28、surface temperature of the test wall at the j moment sejT = exterior surface temperature of the test wall at the j moment jq = density of heat flux rate Compared to the traditional “air circulation heating method” in the Hot-Box, the plane heater, for example, only 0.5 mm electrothermal film, is emp
29、loyed to warm one wall surface. This can decrease equipment size significantly and make installation more easily. Since the temperature sensors are only insulated from the aluminum sheet by a 2 mm (0.08 in.) soft foam glue, one preliminary test was carried out to make sure the temperature sensors me
30、asure the wall surface temperature instead of the aluminum sheet temperature: the heating element was suddenly moved away but the temperature sensors were still kept on the wall, there was almost no deviation of temperature data. The key point for the thermal resistance test is to keep “one-dimensio
31、nal” heat transmission in the measurement section as far as possible. The heat flux parallel to the wall surface should be less than 4% of the perpendicular one in this section (Buratti et al 2003). Therefore, a large heating area is always desirable in order to minimize flank loss, but a large heat
32、ing area could be a disaster for in-situ measurement. A practical compromise must be reached and the optimal plane heater size needs to be determined. OPTIMIZATION OF PLANE HEATER SIZE In-situ measurements were conducted on two interior walls of a typical Southern China university building from Marc
33、h to July, 2014. Actually, the exterior wall of this building has almost the same structure as that of these two interior walls. However, over 50% of window-wall ratio makes it very difficult to find a big enough exterior wall to test. Wall No. 1 is located on the third floor between two graduated s
34、tudent laboratories. Wall No. 2 is located on the first floor between one equipment room and corridor. In most of the time during the test period, the air conditioning system was off and about one third of the windows were opened. The duration of each test ranged from 4-5 days. The variation of indo
35、or air temperature in the test rooms during each measurement period could be between 6 C (10.8F) and 8 C (14.4F). Wall No. 1 was 240 mm (9.45 in.) thick and composed of sintered porous clay bricks, dimension of 90x115 mm (3.54x4.53 in.) and thickness of 200 mm (7.87 in.), the wall was plastered and
36、painted on both sides, the thickness of the plaster and paint is 20 mm (0.79 in.) for each side. The construction of Wall No. 2 is the same as that of Wall No. 1 but with an extra white ceramic tile on one side, the thickness of ceramic tile is about 4 mm (0.16 in.). And the total thickness of the w
37、all is 244 mm (9.61 in.). This type of wall is also widely used as the exterior wall in Southern China. In order to investigate the effects of the heating area on the thermal resistance measurement, a commercially available soft electric blanket with 7mm (0.28 in.) thick was chosen as the heating el
38、ement. The original size of this electric blanket was 1.2x1.5 m (3.94x4.92 ft) and the total heating capacity was 100 watts, which means 55.6 W/m2 (5.16 W/ft2), A 2 mm (0.08 in.) aluminum foil was glued on the electric blanket to uniform the surface temperature distribution. As shown in Figure 2(a),
39、 for a selected heating area, for example, 1x1 m (3.28x3.28 ft), just put the electric blanket on 46 mm (1.81 in.) hard XPS board with the corresponding size and stuck the undesired blanket part on the backside of XPS board. 5 temperature sensors were applied to measure the warm-side wall surface: o
40、ne was located on the center of the heating section and the others were cross-layout with a 150 mm (5.9 in.) distance from the central one. The measurement results indicated that the temperature difference among these 5 locations was always less than 1C. The net weight of the whole heating element w
41、as less than 5 kg (11 lb). On the other side of the wall, two HFTs with dimension of 50x100 mm (1.97x3.94 in.) were fixed on the center of the corresponding heating area to receive the heat flux through the wall. There were also 3 temperature sensors employed to measure the cold-side wall surface te
42、mperature and check the temperature uniformity. (a) (b) Figure 2 Heating Element 1x1m (3.28x3.28 ft) (a) and Heat Flux Receiver Element (b) for Wall No. 1 EXPERIMENTAL RESULTS Wall No. 1 6 heating sizes were studied for Wall No. 1: 1.2x1.5 m (3.94x4.92 ft), 1x1 m (3.28x3.28 ft), 0.7x 7 m (2.3x2.3 ft
43、), 0.6x0.6 m (1.97x1.97 ft), 0.5x0.5 m (1.64x1.64 ft) and 0.45x0.45 m (1.48x1.48 ft). To keep consistence, the measurement positions for different heating sizes were identical. The test for each size took about 4-5 days to finish the measurement. Before changing to a new size, the heating element wa
44、s moved away and the test wall could cool down for at least the same length of time for heating to avoid the influences from the last test. Figure 3 demonstrates the mean temperatures on both sides of the measurement section when the heating area was 1x1 m (3.28x3.28 ft). The air temperatures in bot
45、h rooms were also recorded. Due to many opened windows in both rooms, two room air temperatures were very close and varied with the same periodicity as the outside air. The plane heater warmed the wall surface from 18C (64.4F) to about 40 C (104F) and became relatively stable in 3 days. The small pe
46、riodic fluctuation after 3 days was probably caused by the heat transmission through XPS board to unsteady room air. Due to the directly local heat exchange phenomena between wall and air, the wall surface temperature in the heat flux transducer side is more unstable than that in the warm side. The
47、temperature difference of the measurement section and the consequent heat flux rate are presented in Figure 4. The periodic trends of both parameters are almost the same. 05101520253035404517:00:5521:30:5502:00:5506:30:5511:00:5515:30:5520:00:5500:30:5505:00:5509:30:5514:00:5518:30:5523:00:5503:30:5
48、508:00:5512:30:5517:00:5521:30:5502:00:5506:30:5511:00:5515:30:5520:00:5500:30:5505:00:5509:30:5514:00:5518:30:5523:00:55TimeT( C)32425262728292102112T(F)mean heating side temperature mean HFT side temperatureroom 1 air temperature room 2 air temperatureFigure 3 Mean Surface Temperatures on Measurem
49、ent Section of Test Wall No. 1 under 1x1 m (3.28x3.28 ft) Plane Heater Condition and Air Temperatures in both Rooms 0510152025303540455017:00:5522:00:5503:00:5508:00:5513:00:5518:00:5523:00:5504:00:5509:00:5514:00:5519:00:5500:00:5505:00:5510:00:5515:00:5520:00:5501:00:5506:00:5511:00:5516:00:5521:00:5502:00:5507:00:5512:00:5517:00:5522:00:55TimeHeatflux(w/m2)024681012141618T(C)heat fluxtemperature difference0815.9Heat flux(Btu/hrft2)032.416.2T(F)Figure 4 Temperature Differences between both Sides and Heat Flux of Test Wall No. 1 under 1x1m (3.28x3.28 ft)
