A new parameter ε (0≤ε≤1), called coefficient of performance, is introduced, whose value quantifies the suitability of the distribution of resistive and capacitive layers in a multi-layer wall, within problems concerning thermal building-plant interaction in case of external sinusoidal or impulsive thermal variations. After giving an integral expression for the calculation of ε, the coefficient’s of performance properties are discussed; simple relations allowing to evaluate the value of ε in case of walls made of two materials only, I.e. of a capacitive layer followed by a resistive one (two-component walls), are also deduced. The developed analysis shows that the optimal ideal wall, to which the maximum value of ε corresponds (ε=1), is the lumped-parameter, three-layer, symmetrical one, obtained by lumping all capacity in the midplane of the wall and by distributing the resistance, split up into two equal slabs, on the faces. Finally, the coefficient’s of performance values for different twocomponent walls, realized with materials commonly used in building, have been calculated.
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