Outline the calculation steps of a time domain response function method.
- A given heat transfer equation in the time domain is transformed into a subsidiary equation in an imaginary space using a Laplace transform;
- this subsidiary equation is solved while imposing a unit boundary condition – this gives a Unit Response Function (URF);
- actual boundary conditions (BC) are resolved into equivalent triangular approximations;
- this BC representation is multiplied by the URF to give the specific system response; and
- responses from different BC/URF pairs are added to give the overall response.
What is the principal difference between the time and frequency domain variants of the response function method?
Time domain approximates continuous phenomena such as flux and temperature as a series of triangular pulses. Frequency domain approximates continuous phenomena as a series of sine waves of increasing frequency and reducing amplitude.
Identify 3 principal assumptions underlying the Admittance Method and indicate how these assumptions will reduce accuracy.
- Boundary condition is mean + 24 hour harmonic. This means that some climate time series might be poorly represented and that casual gains might be ‘clipped’.
- Problem parameters (e.g. heat transfer coefficients, material properties etc) are time-invariant where in reality they will vary as a function of temperature and moisture content. This will lead to an under- or over-estimation of heat flow depending on the departure of the system from the standard conditions implicit in the assumed parameter values.
- Solar gain is handled by solar gain factors that embody assumptions on shortwave radiation penetration through windows and intra-zone distribution. This means that the complex shading devices and non-orthogonal room geometries are crudely represented.
Identify the three response factors as used in the Admittance Method and state the principal energy flow to which each relates.
Admittance: ratio of the swing in energy entering an internal surface to the corresponding temperature swing at the environmental point – surface convection.
Decrement: ratio of the cyclic flux transmission through a construction to the steady state flux transmission – transient conduction.
Surface Factor: ratio of the cyclic heat flux re-admitted from a surface to the total cyclic flux absorbed – solar radiation.
Describe the influence of the surface admittance factor on a room’s response to a temperature fluctuation.
Materials with low admittance values (if located at the innermost position of a wall) cannot readily absorb any fluctuation in room temperature (insulation products generally have lower admittance values). Such designs will tend to overheat.
