Abstract
Reflective roof membranes have been shown to lower air conditioning costs by reducing the influx of thermal energy through a roof assembly and into the building. However, there have been studies suggesting that reflective roofs in cold climates give rise to increased winter heating costs, due to the lack of absorption of solar thermal energy. Such studies could be used to justify continued use of dark, absorptive roof membranes even in areas that are striving to reduce urban heat island effects and/or lower contributions to global warming. In a prior modeling study, by this author and others, based on gas heat, it was found that reflective roofing provides net annual energy savings so long as air conditioning was used. Studies by others have suggested that when electric heat is used, the winter heating cost savings associated with non-reflective roofing outweigh summer air conditioning cost savings with reflective roofing. However, these studies did not take into account electric demand charges. Therefore, this present study modeled the energy efficiency of commercial buildings in order to compare the effect of gas versus electric heat with varying levels of demand charge included, on the net energy efficiency. Four different levels of demand charges were compared, along with three levels of solar reflectance for thirteen cities located throughout the US. In every studied case, when gas heat was assumed, net annual energy savings were predicted for reflective membranes. For electric heat, net energy savings were achieved in most cases even when demand charges were zero. In three northernmost cities, this was the case provided that demand charges exceeded a relatively small minimum. This finding suggests that reflective roofing provides for net energy efficiency improvements in most US cities and all cities when demand charges exceed USD 6.25 Therefore, efforts by cities to encourage reflective roofing as part of urban heat island effect mitigation programs should not be reduced.
Highlights
Taylor and Hartwig, using CoolCalc and CoolCalcPeak published by Oak Ridge National Laboratory (ORNL), showed that net energy costs would be reduced throughout the US by using cool roofing [3]
Savings are lower compared to the cases with gas heat but are still projected to occur so long as demand charges are above a minimum
The modeling study, done using 2017 energy costs, has confirmed previous work showing that energy savings are projected to occur for all US cities included in this study, so long as gas heat is used, after conversion to reflective roofing membranes
Summary
The use of reflective roofing has been recognized as an adaptive response to global warming [1]. While cool roofs can raise the surface albedo of urban areas and have been encouraged by states and cities as part of urban heat island mitigation and global warming reduction efforts (see, for example, Gaffin et al [4]), they have been shown to lower air conditioning loads on buildings, thereby saving energy in both North American [5,6] and European studies [7]. Hosseini and Akbari modeled the behavior of buildings in four cold-climate cities in North America: Anchorage, AK, Milwaukee, WI, Montreal, QC, and Toronto, ON [8] They predicted net annual energy cost savings for cool roof use for these four northern cities. Taylor and Hartwig, using CoolCalc and CoolCalcPeak published by Oak Ridge National Laboratory (ORNL) (part of the US DOE), showed that net energy costs would be reduced throughout the US by using cool roofing [3] They assumed air conditioning (electric) and gas heat.
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