Two main types of heat exchangers exist, differing in using an intermediate storage or direct transfer of heat.
In a regenerator, heat from the primary medium is first stored in a thermal mass and later on (during the next cycle) regenerated from that mass by the secondary medium. The thermal mass can either be the wall material of the flow ducts or a porous medium, through which the primary and the secondary flow is alternately led.
In a recuperator, both media are separated by a wall through which heat is transferred directly.
Further split systems are used in which an intermediate medium carries heat from the primary medium in a first heat exchanger to the secondary medium in a second heat exchanger.
The regenerator has a serious disadvantage since the heat has to be diffused in and out of the intermediate storage material, hereby loosing exergy as the heat front in the flow direction is smoothed out and not all of the heat can be regenerated. Additionally, the intermediate storage material (generally the wall of the flow ducts) needs to be a good heat conductor for the storage function, which causes high heat conduction in the flow direction, inducing a considerable loss of effectiveness (<<90%).
In a recuperator, the only fundamental loss is the heat conduction through the wall in the flow direction, which, however, can be reduced to less than a per mille by using material with low heat conductivity like plastics.
Only a recuperator can be used for obtaining the required effectiveness.
If the information here above caught your interest and you want to know more, please fill in our contact form.