Investigation and optimisation of heat storage tanks for low-flow SDHW systems

This thesis, ‘Investigation and optimisation of heat storage tanks for low-flow SDHW systems’, describes a study of the heat transfer and flow structure in vertical mantle heat exchangers for low-flow Solar Domestic Hot Water (SDHW) systems. The heat storage is a key component in SDHW systems and the vertical mantle heat exchanger is one of the most promising heat storage designs for low-flow SDHW systems. The study was carried out using a combination of experimental and numerical methods.

Thermal experiments of mantle heat exchangers with different mantle inlet designs showed that the mantle inlet port with advantage can be located a distance from the top of the mantle. Consequently, the mantle heat exchangers marketed today can be improved by changing the mantle inlet position.

The heat transfer and flow structure in mantle heat exchangers are rather complex and the thermal experiments were followed by investigations by means of advanced experimental and numerical techniques such as Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD). Using a transparent glass mantle tank, experimental flow visualisation was carried out with a PIV system. The flow structures inside the mantle and inside the tank were visualised and then compared with the flow structures predicted by CFD-models. The investigations showed that the CFD-models were able to model the flow in the mantle and in the tank correctly. The CFD-models were also validated by means of thermal experiments with a steel mantle tank.

With the verified CFD-models, a parameter analysis was carried out for differently designed mantle heat exchangers for different typical conditions to reveal how the mantle tank parameters influence the flow structure and heat transfer in mantle heat exchangers. The heat transfer in the mantle near the mantle inlet port showed to be in the mixed convection regime, and as the distance from the inlet increased, natural convection started to dominate. The heat transfer between the tank wall and the domestic water in the tank is governed by natural convection. Dimensionless heat transfer theory was applied, and Nusselt number correlations for the heat transfer in vertical mantle heat exchangers were developed, based on the CFD-analysis.

The CFD-calculations and PIV measurements revealed that thermal stratification is built up in the inner tank above the mantle due to natural convection flow along the tank wall. Based on CFD-calculations, a method was developed for determining the heat transfer caused by the natural convection flow inside the tank. Furthermore, a method was developed for determining the mixing inside the mantle due to the mantle inlet jet.

The developed heat transfer correlations, the method for determining the heat transfer in the inner tank caused by natural convection and the method for determining the mixing in the mantle were implemented in a simulation program for SDHW systems, MantlSim. The simulation program predicts the yearly thermal performance of low-flow SDHW systems based on mantle tanks. MantlSim was verified and afterwards used as a tool for heat storage design analysis. The heat storage design analysis showed that vertical mantle heat exchangers could be designed in a better way than done today.