An Innovative Method to Enable Stable, High Efficient, Evaporation in Thin Volume ORC Evaporator Modules

orc2019 Tracking Number 106

Over the last decade flow boiling in thin spaces has gained attention due to the developments on Waste Heat Recovery (WHR) systems in the industry. For example, WHR systems for heavy duty vehicles are explored and WHR opportunities for hybrid and fuel cell powered vehicles are identified as well. The elevated fluid temperatures, commonly dumped to the environment, are now used in an ORC to create a pressurised vapour which drives the expander. This expander provides mechanical or electrical energy. The more heat extracted, the more vapour massflow created, the more energy the expander can realise [1]. A counterflow heat exchanger is known to be the most efficient heat exchanger type [2]. However, due to boiling explosions occurring in small diameter/ thin volume channels, most evaporators employ of some kind of crossflow heat exchanger layout. Our investigations [3] on boiling explosions in small diameter/ thin volume channels has given us understanding on the origin of the explosive vapour bubble growth, see Figure 1. This knowledge has led to the insights to control them properly and even use the phenomenon to our advantage. The control over the boiling explosions has led to the elimination of the large pressure fluctuations causing the unwanted fluid backflow. Additionally the fast liquid propulsion through the evaporator is annihilated, providing enough time to fully evaporate. Currently, the above mentioned knowledge is used to realise numerical models describing the flow boiling heat transfer in our innovative structure. Furthermore, heat transfer models on the hot fluid side (eg. Flue gasses) incorporating fin structures are developed. Combining these models, amongst others, allows us to propose evaporators based on a counterflow heat exchanger layout, see Figure 2. Further optimisation studies show a potential vapour massflow gain of about 25% compared to currently conventional crossflow evaporators. Additionally, the counterflow principle allows more compactness and/or significant pressure drop reductions.