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 (next cycle) regenerated from that mass by the secondary medium. The thermal mass can be the wall material of the flow ducts or a porous medium, through which alternating the primary and the secondary flow is 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 as the heat has to diffuse in and out of the intermediate storage material, hereby loosing exergy as the heat front in the flow direction is flattened and not all heat can be regenerated. Further the intermediate storage material (mostly 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.
To obtain the required effectiveness only a recuperator can be used.
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