Critical evaluation of analytical methods for thermally activated building systems

This study critically reviews and evaluates state-of-the-art analytical models describing the heat transfer process in a thermally activated building system (TABS) that aims to reduce use of energy for space conditioning by using low-exergy renewable energy. An effective deployment of this system depends on the development of control strategies to address the slow response time resulting from the high thermal inertia inherent to the system that, in turn, requires an in-depth knowledge of the heat conduction in a thermally massive slab. In this study, numerical simulations validated by experiments are conducted to provide data for model verification. The resultant temperature in the analytical model is the superposition of the steady-state temperature, the transient temperature as derived from inhomogeneous initial values, and the transient temperature as derived from 1D heat conduction from the pipes to the slab surface. Results show that the maximum relative deviation between the steady-state analytical and numerical solutions is only 0.65%. This study amends the reviewed transient solution by adding critical coefficients to address the dependency of the solution on the initial values. Numerical simulations show that the assumption of 1D heat conduction is invalid and that heat conducts radically from the pipes into the slab. The existing transient solution yields unrealistic fast heat conduction from the pipes to the slab surface. The assumption of 2D heat transfer in the slab is valid and leads to a small error of 0.4 °C as compared to the 3D full-scale simulation. This study provides a foundation for the rigorous assessment of future TABS control strategies.