To evaluate the effects of thermal mass on building energy performance, the author conducted an investigation on a rectangular-shaped open-spaced building model using quantities research method and simulation analysis. A 10-story building, similar to the U.S. Department of Energy’s (DOE) benchmark commercial building model, was selected as the base case model for the study (DOE, 2008). It should be noted that the main focus of this research has been to address the impact of eTM on building energy consumptions in various climates, not necessarily the building’s practical design.
The wall thickness was chosen to evaluate the impact of eTM on energy performance. The annual heating and cooling energy use and the combination of these two parameters were selected as the measurement parameters of the building’s energy consumption. The heating and cooling loads were also measured to determine the extent of eTM effects on the size of HVAC systems. Heating and cooling requirements were represented by the amount of gas and electricity consumption per Kilo British Thermal Unit (kBtu), respectively. Also, the unit of kBtu/hour was used to measure the heating and cooling loads. The energy simulation was conducted through the Energy Plus computer program using Design Builder as the interface software. In addition, eight different locations in the U.S. representing eight different climate zones were chosen, as follows:
Miami, FL: 1A, very hot and humid
Phoenix, AZ: 2B, hot and dry
San Francisco, CA: 3C, warm and marine
Albuquerque, NM: 4B, mixed-dry
Chicago, IL: 5A, cool-humid
Minneapolis, MN: 6A, cool-marine
Duluth, MN: 7, cold-dry
Fairbanks, AK: 8, very cold
Table 1 describes the architectural, structural, and mechanical characteristics of the base case model for the residential building, including U-values, set-point temperatures, occupancy and activity schedules, and internal loads. These specifications were determined on the basis of the DOE benchmark model and the International Energy Conservation Code as implemented by the Design Builder and Energy Plus programs.
Each floor of the building consists of two main thermal zones: (1) the core circulation area, and (2) the open space around the core area. To assess the impact of eTM within variable wall constructions, four different wall thicknesses were chosen: 20 cm, 30 cm, 30 cm and 50 cm. However, the concrete slab thickness of 15 cm and the window-to-wall ratio of 30% remain constant for all cases. To ensure that the principal element of the building façade that delays heat transfer is the concrete wall and not the windows, single-pane glass was selected for the windows instead of double-pane. To study the operation of HVAC systems (VAV with terminal re-heat), an open office space was chosen to represent the primary usage of this building type for HVAC operation. To strictly focus on the role of eTM in improving building energy performance without any interference from thermal insulation, the code-required thermal insulation was excluded from the wall assembly.
It is known that extra layers of materials can lead to greater heat resistance (lesser heat transmittance), which can affect the building’s energy use. To minimize (neutralize) such effects and solely assess the impact of thermal mass property of concrete material while increasing the thermal mass wall thickness, a parallel study was conducted using identical case studies as the control group. The only difference was the type of material used, which is changed from concrete to lightweight metallic cladding. presumably, with little to zero thermal mass properties as compared with the concrete material. To ensure similar heat resistance/transmittance between the thermal mass and “non-thermal mass” (control) cases, the thickness of metallic cladding was modified to obtain the similar U-value as the corresponding concrete wall scenarios. For instance, an 20 cm concrete wall has the U-value of 3.18 (W/m2 K). The thickness of 4.19 cm is chosen for the control wall to achieve the similar U-value. Table 2 shows all equivalent wall thicknesses for the lightweight metallic cladding.
Eventually, by comparing the heating and cooling requirements of cases studies, which are expected to vary, the effects of thermal mass and non-thermal mass walls on building energy use will be determined. In addition, the decrease and/or possible increase of energy use in various climates as the result of modifying wall thicknesses can be explained which, in turn, could lead to discovering the optimal wall thickness for each climate zone.
Although this building model is based upon the U.S. DOE benchmark commercial building, the specifications of the model are applicable to many large scale buildings in most developing countries. In fact, the use of concrete as structure and even curtain wall may be even more common in developing countries that it is in the U.S. Ghoreishi and Ali (2011) showed that in most developing countries, concrete is the most commonly used structural material, which, in turn, would make its thermal mass characteristics widely available to utilize. Therefore, the results of this research can be used by designers and engineers in most developing countries who would like to see the benefits of concrete thermal mass being utilized for energy conservation purposes.
Figure 1 demonstrates the heating energy demands in a residential building given four different wall thicknesses and in eight different climate zones as previously discussed.
Table 3 demonstrates the percentage reduction of heating energies for four wall thickness scenarios in various locations, comparing each case against the scenario where, presumably, there is no thermal mass.
As shown in Table 3, there is a direct positive correlation between the presence of eTM and the reduction of the building’s heating requirements for all locations. The reductions resulting from eTM ranges from 1% to 37% depending upon the climatic conditions. In comparison, the 20 cm thick wall appears to have, on average, the biggest reductions across the board. Moreover, hot climates such as Miami and Phoenix are shown to have the most heating energy reductions. It should be noted that, given the small impact of heating demands on building energy use in these climates, the overall energy savings may not be very tangible. However, San Francisco and Albuquerque, with significant heating degree days, show a heating energy reduction of up to 15%.
As shown in Figure 2, the cooling energies of residential building scenarios were evaluated in the same U.S. climates given their different eTM wall thicknesses and non-thermal mass arrangements representing different degrees of thermal mass in building envelope.
Table 4 demonstrates percentage reduction and/or increase of cooling energy use for all four wall thickness scenarios comparing different thermal and non-thermal mass cases.
As shown in Table 4, despite a few cases where the presence of the eTM is shown to have led to a cooling energy increase, thermal mass has been able to reduce cooling demands ranging from 2% to 40 %. The 30 cm wall thickness shows the best energy reduction performance in almost all locations across the board. In fact, the increase of eTM beyond 30 cm shows an energy reduction, which can be attributed to the phenomenon of “diminishing returns,” where added thermal mass may not translate into greater reductions of energy consumption. San Francisco and Duluth demonstrate the largest energy reduction by up to 40%.
To evaluate the overall energy performance of buildings, total heating and cooling requirements of the different scenarios were studied. Figure 3 shows the results of building energy consumption for different climate zones based on wall thickness and eTM cases.
Table 5 indicates the percentage reduction and/or increase of total energy use for all wall thickness scenarios comparing different eTM cases.
As demonstrated in Table 5, the building’s total energy performance significantly improved for most cases in the presence of eTM as compared to that of non-eTM walls. San Francisco and Albuquerque show the largest overall energy reductions ranging from 4 to 11%. Except for San Francisco, the increase of wall thickness from 20 cm to 50 cm does not appear to have considerably changed the trend of energy reductions.
Heating and cooling loads are used to size and design a building’s primary mechanical systems such as chillers and boilers. In addition to heating and cooling energy demands, this study evaluated the effect of eTM and non-eTM walls on heating loads as shown in Figure 4.
Table 6 demonstrates the effect of thermal mass cases on the heating loads. Except for a few cases, heating loads are reduced, ranging from 1% to 44% lower in various locations. On average, hot climates show a larger reduction of heating loads in the presence of eTM. The 30 cm wall once again is demonstrating the larger reductions of heating loads in almost all climates as compared to other wall thicknesses. Albuquerque and Minneapolis show a rather consistent increase of heating loads because of eTM increase.
Figure 5 shows the effects on cooling loads in various climate zones with and without eTM wall assemblies.
Table 7 indicates the extent of cooling load changes due to eTM effects. Except for Duluth, other locations show considerable cooling load reductions. As a matter of fact, San Francisco and Fairbanks show a reduction of cooling loads of up to 28%, which is significant especially for large scale projects. The 20 cm wall seems to have the best performance in terms of reducing cooling loads in comparison with other case studies, with the exception of in Fairbanks.