Assuming that the heat is taken from outside air, then for conventional hot air drying here in New England during winter one would have to have a heat pump that could extract heat from air between 30F and -20F and release it at dryer air temperatures -- more than 100F. Accomplishing this might require a two stage heat pump with different refrigerants to have any hope of having a COP greater than 1. Note cost and complexity of heat pumps and double them. At a COP of 1, one might as well just use direct electric heating. In summer, swimming pool heat pumps can be efficient, but they are taking heat from moderate exterior air and warming the pool to a modestly higher differential temperature.

There was a thread here some months ago about having a heat pump that could work in Maine winters to heat a house. I think the conclusion was that it was marginally doable with the latest designs. Now take that house heat and increase its temperature by the same amount again for the dryer. Using a deep well for a heat source might be efficient, ignoring the cost of the well, but I think the market would be small.

I can't comment on condensing dryers, but if they have the drying capacity of my Miele Optima, I might as well just hang the wet clothes on an interior clothesline and use a floor fan. Some humidity might be expected.