Improved thermoelectric (FE) efficiency of AlxIn1-xN/GaN heterostructure (HS) requires high Seebeck coefficient (S) and electrical conductivity (a); but small thermal conductivity (k). Built-in polarization (BIP) electric field of AlxIn1-xN/GaN HS enhances S and a. In this work theoretically BIP effect on k of HS is explored. It is found that thermal conductivities k and k(p) (in absence and presence of BIP, respectively) vs temperature show a crossover at a temperature T-p. Below T-p, k(p) is lower than k due to negative thermal expansion causing dominance of spontaneous polarization (sp) over piezoelectric polarization (pz); while above T-p, k(p) is higher than k due to positive thermal expansion causing dominance of pz over sp. This change in polarization with temperature signifies poyroelectric behavior with T-p as transition temperature between primary and secondary pyroelectricity because above T-p thermal expansion takes place which is reason of secondary pyroelectricity. For x = 0.1, 0.2, 0.4 and 0.5, T-p are close to 200, 210, 300 and 360 K, respectively. T-p can be raised above room temperature by changing x and interfacial strain judiciously. Theoretical findings are made comparisons with results available in the current literature. Thus, TE efficiency of HS is improved by polarization field below T-p; making it suitable simultaneously for pyroelectric sensors and TE module for maximum power production.