Materials for external walls
All potential materials of k and CV within the aforementioned ranges were computed in BuildingEnergy as the external or internal walls. The room was assumed to locate in Hefei, China, where the cooling/summer season is from June 15th to September 5th and the heating/winter season is from December 5th to March 5th of the following year. The climate data used in BuildingEnergy were the typical yearly meteorological data provided by the Chinese Architecture-specific Meteorological Data Sets for Thermal Environment Analysis. The thicknesses of the external and internal walls were set as 240 and 100 mm, respectively, and other wall thickness can be equivalently converted into these values through a treatment described in the supplementary information. Owing to such a treatment, the conclusions from the fixed thicknesses will be universal for all values of thicknesses.
Figure 1 shows the energy consumption contours for the external walls made of different materials, in which the materials of the internal walls are fixed as common bricks. The thermophysical properties of the brick are provided in Table 1. As Fig. 1 depicts, both the thermal conductivity and volumetric heat capacity of the external-wall materials exert a significant impact on the energy performance, and the energy consumption varies extensively along with k and CV. A value of zero can be achieved for an extremely low k, due to the absence of a window and internal heat source.
For the summer application (Fig. 1(a)), generally, either a decrease in conductivity or an increase in volumetric heat capacity of the materials causes a reduction in cooling energy consumption of the room. A low k and a high CV imply a small thermal diffusivity α, which is defined as k/CV or k/(ρcp). α affects the transient thermal conduction process through a wall: in materials of a small α, heat transfers sluggishly, and thus the outdoor environment has a smaller influence on the indoor environment than the situation with materials of a great α. In addition to retarding the thermal conduction within the wall through a small α, a low k also contributes to blocking the heat transfer across the boundary of the external wall. If k is low enough, the heat may rarely reach the indoor-side surface from the outdoor environment, so CV cannot exert its effect on the heat transfer process across the interior. As a consequence, when k is lower than 0.25 W/(m·K) in Fig. 1(a), the contour lines are nearly horizontal, implying that CV has a negligible effect on the energy performance and that a low k takes priority over a great CV.
As k increases, the slopes of the contour lines also increase, namely, the significance of CV is increasing. When k is higher than 3.0 W/(m·K), the lines are nearly vertical, which means that the energy performance is almost exclusively affected by CV. Such a phenomenon may be explicated from the lumped capacitance approximation. When this approximation holds, i.e., the assumption of a uniform temperature distribution within the solid is reasonable, the temperature gradients within the solid may be neglected, therefore the change in thermal conductivity has an insignificant influence on the heat conduction. Basically, lumped capacitance approximation is satisfied for a situation that the resistance to conduction within the solid is much less than the resistance to convection between the surface and the fluid24. In our case, if k is high enough, the wall may behave as a lumped capacitance solid, making the energy performance influenced individually by CV.
For the winter application (Fig. 1(b)), the general tendency of how the material properties affect the energy performance is consistent with that in summer, but the slopes of the contour lines are almost zero when CV ≳ 2000 kJ/(m3·K), indicating that CV has a limited influence in winter.
Some typical building materials, whose properties are presented in Table 1, are also plotted in Fig. 1. When made of one of these materials, the corresponding external wall is diverse in energy performance. The trend is, ordinarily, that the energy consumption reduces with the decreasing conductivity. For close values of k (the granite and marble, for example), the energy consumption is determined by CV: the material with a higher CV leads to a lower energy consumption.
As mentioned above, the energy performance in Fig. 1 was discussed with a fixed thickness of the walls. In practical situations, the thickness with the same energy performance may also be a reference parameter. Figure 2 illustrates the comparisons of thickness and mass for some typical materials, whose cooling energy performances approximate that of a 240 mm brick wall. The thickness of the polystyrene is just 2% of the marble and 7.5% of the brick. Furthermore, the mass per unit wall area of the polystyrene wall is much smaller than those of the other materials due to a low density of the polystyrene. A small mass per unit area means a lower construction cost, and a smaller thickness results in larger net area. Therefore, an external wall made of light insulation materials, like polystyrene, will be recommendable in buildings following an improvement in mechanical strength.
Materials for internal walls
Now we consider the energy performance of the internal-wall materials. Similar contour map are presented in Fig. 3, in which the external-wall materials are common bricks. It can be observed that the energy consumption decreases as k increases when k≲ 0.5 W/(m·K). A high k facilitates the thermal conduction. In summer, for example, the temperature of the indoor-side surface can be lowered through the conduction of some heat into the interior of the wall, resulting in a reduction in energy consumption for cooling (as Eq. (8) in the supplementary information explicates). For materials of k higher than 0.5 W/(m·K), the contour lines are vertical so the energy performance is influenced exclusively by the volumetric heat capacity. The increase in CV causes the reduction in both cooling and heating energy consumptions. Regarding the materials in Table 1, the reinforced concrete, whose volumetric heat capacity is the highest, is the best candidate for the internal-wall material.
Note that when k and CV vary, the energy consumption varies from 7.2 to 8.3 kWh/m2 in summer, and the range is 35.88 ~ 36.28 kWh/m2 in winter. Nevertheless, the corresponding ranges in Fig. 1 are 0 ~ 22.5 and 0 ~ 87.2 kWh/m2. The much broader ranges imply a more significant role of the external wall in the energy performance, meanwhile, a greater potential for improvement.
Thermal conductivity and volumetric heat capacity are inherent thermophysical properties of a material. Nonetheless, materials are embodied in some building components, such as a wall, a window, a floor, etc. For this reason, engineers prefer employing the parameters which are able to describe a whole component to particular materials. Overall heat transfer coefficient, also termed U-value, and total heat capacity are customarily used to characterize the thermal insulation performance and heat storage capacity of a wall, respectively. With the analysis elaborated in the supplementary information, the requirements for the wall materials may also be stated as the demand for the wall as a whole component, which can be summarized as: the total heat capacities of both the external and internal walls should be high, and the U-value of the external wall should be low.
Effects of windows and internal heat gains
As previously declared, we have ignored the potential influence of the window until now. Here Fig. 4(a,b) depict the performances of a room having a window. The single-glazed window, which locates in the center of the external wall, has a size of 1.5 × 1.5 m2 and a solar transmittance of 77%. Comparing the situations with and without a window, it is discovered that the presence of the window raises the cooling energy consumption, but does not change the trend of how the wall materials influence the energy performance. Due to the absence of a window, the lowest energy consumption that can be obtained through the improvement in the external wall is zero in Fig. 1(a), while the corresponding value with a window is 11.4 kWh/m2 in Fig. 4(a). The gap between the lower limits is generated by the transparent part of the envelope, i.e., the window, and may be filled by the continuous development of the windows, revealing that a high-performance building envelope should be achieved by the simultaneous improvements in the transparent and opaque parts.
To further generalize the results, internal heat gains were also considered in the room with a window to simulate a more realistic situation. The heat gain from the occupants and equipment is taken to be 4.3 W per unit of floor area, and that from the lighting is 3.5 W per unit of floor area while the lights are on, from 18:00 until 22:00 each day. The results are plotted in Fig. 4(c,d), which illustrate that the consideration of internal heat gains does not change the general rules of the influence of wall materials on the energy performance. The effects of other configurations of the room on the general rules, e.g., the orientation, the room size, were also proved to be negligible, and the details can be seen in the supplementary information.
Effects of climate conditions
The foregoing discussions were established for the city of Hefei, which has a climate of hot summer and cold winter. To examine the effect of climates, Fig. 5 shows the situations for Beijing with a cold climate, and Guangzhou with a climate of hot summer and warm winter. A heating period is absent for Guangzhou due to the fact that the average temperature of the coldest month is still 14 °C. The trends of the influence of the material properties on the energy consumption are utterly the same as those in Hefei, which means that these trends are independent from climates. The only difference occurs in the energy consumption ranges: rooms in Guangzhou exhibit a higher cooling consumption than those in Hefei, and rooms in Beijing have a higher heating consumption. The results for more extreme climates are presented in the supplementary information, and the general trends are still consistent.
Because hypersonic vehicles face environments with extremely high temperatures in which the vehicles׳ outer shells experience single-sided heating and the temperature varies nonlinearly over time, a self-designed radiation and one-sided heating test apparatus is established in this paper, which can perform the thermal insulation performance test for lightweight ceramic thermal protection materials of hypersonic aircraft in extremely high-temperature/oxidation environments up to 1700 °C. Meanwhile, experimental research on thermal insulation performance of a lightweight ceramic material specimen and a novel ceramic-nanomaterial hybrid structure in high-temperature/oxidation environments up to 1700 °C was carried out. Different materials alone and in combination were analyzed and compared to select the most effective insulation scheme. It is found that the thermal insulation performance of the ceramic-nanomaterial hybrid thermal protection structure can be increased approximately 50% compared with that of the lightweight ceramic material specimen. Additionally, non-linear time-varying thermal environments with extremely high temperatures were generated and the thermal insulation performance tests were performed in this paper. The present work provides an important test method for the thermal protection design of hypersonic aircraft through the establishment and application of an extreme high-temperature/oxidation test system which can create one-sided heating, time-varying thermal environments.