Sensible heat storage

The sensible heat storage is one efficient way of storing thermal energy in order to the conservation and utilization of the thermal energy and can be used to store and/or deliver energy in many processes such as:  solar collector systems, domestic cogeneration, exhaust gases from domestic burners and then use this energy in domestic heating and domestic sanitary hot water. One way to improve its efficiency is simulation using numerical methods to predict their behavior and take advantage in the development of consumer applications. Through the development of numerical simulation and the development of codes different situations can be raised and solved experimentally and then confronted with the reality to get a faster progress in their practical applications.

In the CTTC, numerical and experimental tools are developing for the study and optimisation of sensible heat accumulation systems, focusing on the analysis of a specific sensible heat accumulator based on a cylindrical tank with an internal (straight, coiled) pipe.

Fig. 1: Possible different configurations of heat storage system.
The numerical simulation has been performed using a high-level platform that links two codes to solve the system. Solving at different scales and linking the three different elements (fluid inside tank, tube and in-tube fluid) which constitute the system. The fluid inside the tank is solved with a CFD&HT (Computational Fluid Dynamics & Heat Transfer) software called TermoFluids and the fluid flow inside the pipe is solved considering a quasi-homogenous fully-implicit one-dimensional model, where the governing equations (momentum, continuity, and energy) are discretized along the whole tube domain. The resolution of the pipe wall in a detailed way couples both fluid solvers. A specific effort has been devoted to assure good coupling of heat transfer between in-tank and in-tube fluids.



Fig. 2: Views of the unstructured mesh used for the fluid inside the tank.

An experimental unit has been built to study sensible heat accumulator systems (Fig. 3 and 4). The unit is conceived to test the validity of the numerical models studying the thermal and fluid dynamic behaviour of each element of the accumulator system. The experimental unit is thus designed to supply hot and cold water to a sensible heat storage device (test section) and to study its performance accumulating and delivering energy.

Fig. 3: Schematic representation of the experimental unit of the CTTC.
Fig. 4: Detail view of the experimental unit.

Fig. 4 - Detail view of the experimental unit.

Fig. 5: On the left, heat extraction process: energy recovered for different flow rates. On the right, heat loss to environment during storage period: available energy in the device with time.

Fig 6: Non-dimensional in-tank water temperature for different instants of time (s).

Fig. 7: Heat extraction process. On the left, evolution of non-dimensional temperature of the in-tank water at different positions, where NT is the number of turns in the coil. On the right, evolution of the delivered energy by the device.

References:

S. Torras, O. Lehmkuhl, J. Rigola and A. Oliva. Numerical simulation of heat storage for domestic applications. Proceedings of the 23rd IIR International Congress of Refrigeration. (2011).

S.Torras, C. Oliet, O. Lehmkuhl and J. Rigola. Numerical simulation and experimental validation of sensible heat accumulators oriented to zero energy buildings. Proceedings of the CLIMA 2013 - 11th REHVA World Congress & 8th International Conference on IAQVEC.(2013)