Efficient use of coolants in heat exchangers for hydrogen tank filling

The purpose of this report is to give some basic facts about heat exchangers and to summarize a series of fluid heat transfer simulations done on a simple counter-flow concentric tube heat exchangers that can be used for hydrogen gas filling systems. The gas and coolant mass flow-rates as well as pressure and temperatures were chosen to be in a range suitable for filling medium sized fuel tanks at hydrogen flow-rates of 1 - 15 kg/min. The possibilities when using some typical coolants, such as water and ethylene glycol-water mixtures, are presented. Effects that limit the efficient heat transfer between hydrogen gas and coolant will be discussed. The appendices give some tables for typical values of heat transfer coefficients as well as the minimum cooling power and the flow-rate needed to fill a hydrogen fuel tank of medium size in a fast and safe way.