The difference to the MC Solar is its tube regenerator. It is a usual
tube for a household drain (of a sink)(50mm diameter)
Inside the tube there is usual steel wool or comparable material as the
regenerator. The amount of the steel wool needed is hard to tell,
so it has to be adjusted by test runs. So keep the tube removable to fill
in more OR less of steel wool for your test runs.
The shape of the polyurethane part of the displacer is formed that way, that
it has a hollow for the air flow (deepest point about 6mm).
The top of the displacer has to have an elliptic shaped hollow near the plastic
tube area (see TOP VIEW, showing the
edges of that hollow). This hollow is necessary to enhance the air flow,
too.
Instead of the plush regenerator there has to be a small-as-possible-gap
all around the displacer (to the wall) or a rubber sealing
(from an old bicycle tire) has to be mounted here. There is some polyurethane
foam that expands when being heated, so it's better to
have a 2mm-smaller-width polyurethane foam and an only 0.5mm gap at the plywood
(to the walls).
Anyway the tube version of the MC Solar need more effort to build. Its advantage
is, that you can adjust the regenerator better to
the optimal working mode and (although I have no records about it) the tube
regenerator will have a longer duration.
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