Calculation of transient heat transfer through the envelope of an underground cavern using Z-transfer coefficient method

► Z-transfer function is used for calculate heat flow through the envelope of caverns. ► Thermal zones generated by annual and daily temperature wave are different. ► The Z-transfer coefficients of annual and daily thermal zones are also different. ► Boundary of thermal zone is defined by frequency characteristic of transfer function. ► Total heat flow is a superposition of the annual and daily heat flow waves. A Z-transfer function method is proposed to calculate transient heat flow through the envelope of an underground cavern in this paper. The envelope of an underground cavern has great heat capacity and high thermal inertia. In order to obtain the correct Z-transfer coefficients, indoor air temperature is expressed as a superposition of mean annual temperature, annual temperature wave and daily temperature wave. Corresponding to the three types of temperatures, three heat flows are calculated separately, and then are superposed. The thermal zones formed by annual temperature wave and daily temperature wave are different; thus, the ratio of amplitude frequency characteristics of the transfer functions is proposed to define the boundaries of the two different thermal systems. After transforming an underground cavern into an equivalent cylinder, the Z-transfer function coefficients of two thermal systems are calculated. Case study shows that heat fluxes at interior surface calculated by Z-transfer coefficient method have good agreement with those by numerical finite difference method, and the deviation between the results from two methods is only 1% of that from numerical method. However, the computation time of Z-transfer coefficient method is less than 1% of that of numerical method.