Hi,
I finished up the first cut at one version of a heat exchanger to recover heat from the dryer exhaust, and did some testing on it.
The heat exchanger is made from some used twinwall polycarbonate glazing I had around. Its a stackup of 14 sheets of the twinwall with spaces between each twinwall sheet. The dryer air travels up through the spaces and the room air travels down through the twin wall -- a counter flow HX.
As it is, it manages to extract energy equal to about 32% of the electricity that goes into the dryer.
I see some opportunity for changes that might make it do a lot better. The biggest one would be to get it to the point where it condenses out a good fraction of the water in the dryer exhaust stream. This would not only recover the heat that went into vaporizing that water in the dryer, but it (I think) would allow you to vent an electric dryer inside without concerns about humidity. If you include all the energy sinks and sources (including your furnace having to heat air the dryer pulls into the house), this appears to me to reduce the dryer energy use by about 80%!
There is a full description of the heat exchanger and results from a test on it here:
Testing a Prototype Dryer Heat Recovery Heat Exchanger
Would appreciate any comments or suggestions or pointing out any errors.
Gary