Design masterclass 2 Thermal response Hot topic Masterclass Professor Doug King
In his second masterclass, Professor Doug King looks at how the natural thermal response of buildings can influence the design and control of HVAC systems
W
e think we know what thermal mass is all about. After all, we use it to night-cool low-energy buildings. But how many of us stop to consider how a building’s
thermal response will impact on the building services systems that we subsequently design? If we don’t consider the thermal response as part
of our system design, then we may get into trouble. The natural response of the building fabric can have a fundamental effect on the operation of HVAC systems, especially when these are close coupled to the mass, such as in underfloor heating or TermoDeck ventilation. An easy way to understand the periodic thermal
response of a building is to consider an electronic analogy: a simple circuit with a resistor and capacitor (RC) in series (see Figure 1) behaves in the same way as the thermal mass of materials in a building. In the analogy, the resistance to thermal conduction in a material is represented by the resistor and its heat capacity by the capacitor. A step change to the input voltage (switch) causes
current to flow through the resistor, building up charge on the capacitor. Thus, the voltage across the capacitor rises over time until it reaches the input voltage, whilst the current reduces as the inverse. Exactly the same can be observed if we change the temperature at the surface of a material. Initially, heat flows into the material and its temperature rises until it reaches equilibrium with the source. Now, by adjusting the values of resistance and capacitance, we can tune the response, or time constant, of the RC circuit relative to the periodicity of the change in input (Figure 2). As we increase the
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time constant, the output waveform tends towards a saw-tooth, smoothing out the transients and delaying the occurrence of peak amplitude. The circuit is now acting as an integrator: its output is approximately proportional to the time since the last change in input, whilst its peak value is the amplitude of the input change. Eventually, when the time constant is long enough,
the capacitor does not become fully charged before starting to discharge again and the output becomes attenuated in amplitude. At this point the circuit is acting as a low pass filter, attenuating transients whose frequency is less than the response frequency of the
An experiment to classify the thermal response of a TermoDeck slab being conducted at the University of Bath
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Figure 1: The simple RC circuit can be used as an analogy to thermal response in a building. When the switch is closed, current flows through the resistor to charge the capacitor. The Time Constant T = RC is the time for Vc to rise to (1-e-1)Vs
August 2010 CIBSE Journal 47
Doug King
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