Variation in Heat gain through the roof of a building due to changes in Solar reflectance. Neville Selby ( B. Eng )

Transcription

1 Variation in Heat gain through the roof of a building due to changes in Solar reflectance. Neville Selby ( B. Eng ) Sept. 2006

2 Abstract! Work carried out to measure the solar reflectance of Solacoat Insulating Paint, and commercially available coloured steel roofing materials, has demonstrated significant reductions in heat flow downwards into a building if the roof is coated with Solacoat.! Energy savings due to a reduction in air conditioning load associated with the downward heat flow from the roof are swn to be as high as 33%. For an average use, this could result in the saving of about 900 units of electricity (kwh) over a 12 month period and reduction of about 900 Kg of green use gas being discharged to the atmosphere.! An effective R Factor for Solacoat has been demonstrated. A dwelling with a gypsum plasterboard ceiling and an above ceiling airspace, when treated with Solacoat on the roof surface, will have an R factor ( the resistance to downward heat flow into the building) exceeding 2.2 witut the need for under roof insulation. The R Factor of 2.2 is a mandatory requirement for new dwellings in northern regions by the Building Control Board of Australia

3 Introduction. Recent work has been carried out by Dr. Harry Suehrcke (PhD) of James Cook University, in association with Neville Selby (B Eng). of Townsville to quantify the effects of varying Solar Reflectance of roofing materials. This work was undertaken to demonstrate that an effective R Factor could be assigned to Solar Reflective paint, and to quantify the heat flow downwards into building for various types and colours of roofing materials. The findings has resulted in the preparation of two papers. These are titled 1. Solar Reflectance of White Solacoat Insulating Paint and other common Roofing Materials This details the metd used to measure the Solar Reflectance of roofing material coated with Solacoat heat reflective paint, and other common types of roofing material. The results of the tests carried out are tabulated to provide a measure of heat absorption. 2.. Effect of Solar Reflectance on the Heat Gain through the Roof of a Building. This paper develops a mathematical formula to describe the downward heat flow into a building, and employs this formula together with a range of other data available to evolve an equivalent R factor for Solacoat reflective paint. It also quantifies the amount of heat flowing downwards into a building, and demonstrates reductions to air conditioning heat load by increasing the reflectivity of the roof surface. This report is a précis of the above papers, which are being published. The full text can be made available on request. Solar Reflectance. The results of paper No. 1 are given in the following table. The derivation of the final figures can be understood by reading a complete copy of the paper. A statistical analysis of the data has enabled a level of uncertainty of + / to be assigned to these figures. The reflectance figure for Solacoat paint is a close match to the figure of 0.80 obtained by Dr. Bell at the Queensland Institute of Technology. The test samples were standard flat metal sheet roofing material. The Solacoat sample was hand painted, and the coloured sheeting was commercially available Colourbond. Test Sample Average Reflectance Absorption White painted Solacoat White Colorbond (Surfmist) Mid Green Colorbond (Wilderness Green) Dark Grey Colorbond (Woodland Grey) New Zincalume coated Sheet Red Terracotta Tiles It will be recognized that the above figures were derived from measurements carried out on newly prepared and unweathered surfaces. Paper No. 2 has introduced the effects of aging on the surface absorption, and a degradation figure of 0.17 was derived for Solacoat weathering. This was deduced from very limited sampling, one of which was a roof next to a cement plant where dust

4 build up was significant and abnormal. Other tests suggest that a reasonable figure for degradation after some years of application would be in the order of This would give a figure for absorption of a weathered Solacoat surface of about Obviously weathering will take place on all roof surfaces, causing similar variations in solar absorption. Tests on White Colorbond swed an increase in absorption due to weathering by 0.11, giving a solar absorption of the weathered material of about 0.50 (up from 0.39). The effects of weathering can be negated to some extent by washing the roof with a high pressure cleaner, since much of the degradation will be due to dust and dirt build up. The paper does not make specific reference to Galvabond or Zincalume coated steel sheeting. Tests on new sheeting of this type, gives a absorption figure of 0.38, wever this material rapidly weathers, even after a few months. The only test carried out on this type of sheeting gave an absorption of 0.66 (a degradation of 0.28) This type of surface has different reflectance characteristics to a painted surface, so that about 2/3 of the incident energy will be flow into the building from a weathered Zincalume roof. Steady State Heat Flow Equation. Dr. Suehrcke has derived an equation for the steady state heat flow into a building from external causes. This includes heat flow due to outside ambient temperature and the heat absorbed on the surface due to solar radiation. The full derivation of this formula is available in paper No. 2 detailed above. The formula is expressed as q1 = U [ (Ta Ti ) +!G ] (1) * where q1 = rate of heat flow per sq. metre from the illuminated roof to the inside U = the overall heat transfer coefficient between the ambient and inside [W/m 2.K] Note that 1 = U hi and 1 = R (the thermal resistance) U Ta = Ambient (outside) Temperature [ 0 C] Ti = Inside Temperature [ 0 C] = Outside heat transfer coefficient between roof and ambient (including radiation heat loss to sky) [W/m 2.K] h1 = Heat transfer coefficient between the roof surface and the building inside [W/m 2.K]! = Absorptance to Solar Radiation G = Solar radiation per unit area [ Watts / m 2 ] K = Absolute temperature ( o Kelvin) This equation (1)* has been employed by Dr. Suehrcke, to define an expression for building heat gain. Daytime heat gain into a building = U [ "T +! G ] #t daylight (2) * "T = Average daily temperature difference between inside and outside #t = daylight time length in seconds. A full derivation of this formula is detailed in paper No. 2

5 Physical Data. To enable a comparison to be made and demonstrate the benefits, which can be gained by applying Solacoat to a roofing surface, some typical figures need to be assigned to the symbols in the heat transfer equation. If a comparison is based on an average weathered roof, if data in the above chart for absorption are considered, figures between about 0.5 and 0.9 are evident. It would therefore be reasonable to assign an average solar absorption to a roof of about 0.7. This compares with the figure for weathered Solacoat of around Using figures available from the Bureau of Meteorology, average daytime temperatures in northern Australia are about 30.5 o C. If it is assumed that the design temperature for t climate air conditioning is 24 o C, a figure for "T would be about 6.5 o K. Obviously for other climatic conditions where summer temperatures may reach 38 o or more, the figure for "T will be correspondingly larger if the building is air conditioned. This type of climate would likely have cooler winter periods so that average daily temperatures over 12 months could be similar to the 30.5 o figure. Average daily radiation figures for areas in northern Australia have been derived from published data. A figure of 500 W / m 2 for a 12 ur day is typical. The downward heat flow through a roof will depend on the heat loss coefficient to ambient,. The higher the wind speed over the surface, the higher the heat loss coefficient. Available data, suggests a heat flow transfer coefficient, including wind and sky radiation exchange, in the order of 25 W / m 2.K. Downward heat flow due to Solar Radiation and Temperature difference Using the above physical data, the following figures can be assigned : "T = 6.5! = 0.7 for an average roof and 0.35 for a roof coated with Solacoat = 25 W / m 2.K G = 500 watts / m 2 Using the equation (2)* above, the downward heat flow due to radiation =!G / equates to - for an average roof x 500 / 25 = 14 for a Solacoat roof x 500 / 25 = 7 Heat flow due to temperature difference "T = 6.5 o It is evident that Solacoat can reduce the heat flow due to radiation by a factor of 2 or more. For an average roof heat flow due to radiation is more than twice that due to outside to inside temperature difference. The Building code of Australia, 2005 requires a total R Value (= 1/U) for the roof of 2.2 m 2.K / W.

6 If equation (2)* is applied to a typical domestic dwelling of 200 m 2 roof area, with a solar absorption of 0.7, and an R value of 2.2, the heat flux will be 9.3 W / m 2 (= 1 / 2.2 [ ]) This will translate to an average rate of heat flow from the roof of 1.86 KW if the inside of the building is kept to 24 o C. Over a 12 ur period, this results in a heat gain of MJ (Megajoules) For a typical air conditioner with a COP of 3, this translates into an energy consumption of 26.8 MJ, equivalent to 7.44 kwh. (units of electricity). If Solacoat is applied to the same roof, the heat flux will reduce to 6.14 W / m 2 (= 1 /2.2{ [) Over a 12 ur period, heat flow will now be 1.23 KW, and over a 12 ur period heat gain would be 53.1 MJ, giving an energy consumption of 17.7 MJ. This is equivalent to 4.91 kwh. ( the result is a saving of about 33% ) Effective R Factor Solacoat reduces energy intake by reflecting part of the incident radiation from the sun back into the sky. As such, reflective coatings do not attenuate the rate of heat flow into a building in the same way as insulating materials with a physical R Factor. It is possible, wever to calculate an effective R factor for a new Solacoat treated roof by finding the value of physical roof insulation needed below a Solacoat treated roof which will give the same downward heat flow as would be achieved by a roof with known absorption and standard insulation thickness. For purposes of comparison, an average roof is used with an absorption figure of 0.7. Using equation (2)* and equate the downward heat flow for the new Solacoat roof to the downward heat flow for the reference roof, then - Downward heat flow U ["T +!G ] = Uref ["T +!ref G ] If Rref (= 1/ Uref ) = 2.2 m 2.K/W as required by BCA 2005 for new uses, and other figures are tse previously used for downward heat flow calculations then U [ x 500 / 25 ] = 1 /2.2 [ ] U x 11.1 = 9.36 Therefore a Solacoat roof will have U = and R = 1.19 The Building Code of Australia 2005 (see sections J1.2 and J1.3) states that the R value for a roof with air space and gypsum plasterboard ceiling is 1.45 m 2.K / W for an unventilated ceiling, and 1.63m 2.K / W for a ventilated ceiling space. Even with an unventilated ceiling space, it is evident that the application of Solacoat provides a total R factor of 2.64 which exceeds the 2.2 mandatory R value requirements of BCA 2005 Conclusions. " Energy savings associated with the application of Solacoat to an existing roof have been demonstrated and quantified. The results obtained were for an Average roof, and a typical set of circumstances. Variables include inside to outside temperature difference, solar radiation level, roof reflectance, insulation R Factor, wind speed etc. Given a known set of conditions a specific energy saving calculation can be performed. Because the solar

7 absorption of Solacoat will always be lower than any other normal roof surface, some downward reduction in heat flow will always be evident. This will result in lower air conditioning energy demand or lower internal temperature in the building. " For a typical dwelling in northern Australia, an energy saving due to reduction in air conditioning load of about 33% has been demonstrated. A reduction of 2.5 units of electricity per day (at 15c / unit) over 12 months relates to an energy saving of 912 kwh, and a cost saving of $137. This would result in the reduction of about 900 Kg of greenuse gas being discharged to the atmosphere. It suld be noted that this saving applies only to downward energy into the building from the roof. Obviously many other factors contribute to total air conditioning load. This would include building orientation, wall shading, size and positioning of windows, roof overhang etc. Even if the total air conditioning load is doubled due to other factors, the application of Solacoat to the roof can still result in an overall saving of 16% or more. " In a large building such as a spping center where the outside walls are of small area in comparison to the roof, the largest contributor of energy flowing into the building will be the roof. Energy saving under these circumstances, can be expected to be in the 20 30% range for the physical conditions stated. " Dr. Suehrcke has demonstrated that an effective R Factor can be assigned to a Solacoat surface when a comparison is made to a roof with known solar absorption. It has been swn that the application of Solacoat to a roof with airspace and a gypsum plasterboard ceiling, can obviate the need to fit any other physical insulation barrier, while still meeting the requirements of the Building Code of Australia. " It is obvious that the retrofit of Solacoat to the roof of a building can achieve the same results as applying an under roof insulation barrier. " The information used in the two papers listed in the introduction, is to some extent based on data available for northern areas of Australia, and is averaged for year round conditions. Results equally apply to other areas during the summer months where temperatures may exceed tse averages which apply in the tropics. During the cooler parts of the year, where some heating may be necessary, Solacoat does not attenuate the upward flow of heat from the building, and a physical insulation barrier could be required to minimize heat loss. " In northern areas where heating is not required, the use of an added roof insulation barrier can be counter productive, as it will slow down the rate of heat loss to the night sky after sundown. The building will therefore take longer to cool down. Solacoat actually accelerated the rejection of heat from the building at night. References References for information detailed on this report are the two papers listed in the introduction. These in turn, detail a further list of references to support the derivation of the results and conclusions of the two papers. These will be made available on request.