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Thermal mass and heat capacity: What is their mysterious connection?
In building design, thermal mass is an important physical property that determines how the flow of thermal energy in a building affects its temperature changes. However, there seems to be no consensus among experts regarding the definition of thermal mass. Many scholars regard it as synonymous with heat capacity, which is an object's ability to store thermal energy, while others describe thermal mass in conjunction with its volume and volumetric heat capacity. This inconsistency has led to thermal mass being considered pseudoscience in the eyes of some scholars. This article will explore the connection between thermal mass and heat capacity and their application in building design.
Thermal mass is usually represented by the symbol
Cth, and its SI unit isJ/KorJ/°C(both are equivalent effect).
The relationship between heat Q and thermal mass can be defined as: Q=CthΔT, where Q represents the heat energy transferred, and < strong>ΔTis the change in temperature. This means that when heat enters an object, the thermal mass determines how much the object's temperature rises.
For example, if 250 Joules of heat are added to a copper gear with a thermal mass of 38.46 Joules/°C, its temperature will rise by 6.50°C. This uniformity of matter makes the calculation of thermal mass relatively simple, using only the mass of the object and its specific heat capacity.
The difficulty of calculating heat capacity increases in buildings composed of multiple materials. Typically this requires summing the heat capacities of each pure material, or making direct measurements of the object as a whole. This makes thermal mass analysis in multi-material composites more complex.
"Thermal mass depends on the relationship between the material's specific heat capacity, density, thickness and thermal conductivity."
In the wider architectural context, the impact of thermal mass is seen as having a direct link to comfort. If the outside temperature changes frequently, adding thermal mass may improve comfort for a certain period of time. However, thermal mass can be a liability when the space is used intermittently. With the continuous flow of heat, the effect of the thermal mass is barely noticeable. Therefore, heat capacity calculations are usually not included in engineering calculations.
"In the United States and Canada, building heating and cooling equipment is usually planned according to the Air Conditioning Contractors Association Manual J (Manual J), but this process does not consider the impact of heat capacity."
According to Manual J, a building's heating and cooling equipment needs to be measured in detail based on details such as the size, structure, insulation, air tightness, equipment characteristics and user loads of the house. While heat capacity is assumed during this process, equipment is designed primarily to maintain a comfortable indoor environment at extreme temperatures.
The mysterious connection between thermal mass and heat capacity, especially in the context of sustainability and comfort in building design, remains an area worthy of further research. As building technology advances, how to better utilize thermal mass and its associated properties may become key to future designs. When considering thermal mass, should we re-evaluate its potential to reduce energy consumption and maintain comfort?
