According to Jeff Wisnia :
I wonder if it would be cost effective for some company to produce a
heat exchanger combined with a small circulating fan for recovering that
heat? The fan could be switched on by a thermostat so that it only ran
when the hot air from the dryer warmed the unit up.

I've been doing some research on this, and it appears that the
technology just seems to have not produced something economically
viable yet for residential class dryers. For multiple commercial
dryers (ie: laundramats) certainly, but not a single relatively
little used unit.

Coping with the lint is perhaps the major factor, otherwise,
standard heat recovery ventilators (ie: those in R2000 homes
are typically $500-$900) would be common already for this purpose.

The cheapest heat reclamation units I've seen suitable for dryers
are on the order of $3K at a bare minimum, and routine service
schedules (lint cleanout etc.) is apparently important. Significant
effectiveness and a reasonably short repayment period in a commercial
setting (especially when you share them amongst several units), but
not useful in a residence.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.