Great discussion. The kicker in all of this is the darn lint. It complicates things and can be a fire hazard. I saw a
on the use of the mylar (shiny plastic) dryer vent tubing. They suggested the use of either rigid or flexible aluminum venting material.
Regarding heat exchangers, I think that any design entails hundreds of small cross section channels is doomed to failure because they will fill with lint and lint is a good insulator/poor heat transferer and a fire hazard.
I think that a better design than the original cylindrical with fins would be simply a large flat relatively thin box where the dryer vent goes in the bottom and out the top. Fashion it so that one can remove the front panel easily for cleaning.
I also like the idea of making a heat exchanger and heating the input air from the output air. This is the concept of
countercurrent exchange. One could have a large flat box sandwiched against a large flat box. One could put fins in the boxes that span the common wall, increasing transfer of hot to the cold but not interfering too much with cleaning.
Lastly, all these ideas have to compete with the spin dryer. I am not sure any of them can. The most efficient system seems to be ultra-spinning the washed clothes and then running them very lightly through the dryer (or even better hanging them) and forgetting about trying to grab heat from the dryer exhaust.