The water heated by the PEX tubing is passed through a heat exchanger coil in a domestic water preheat tank. The domestic water is preheated in this tank before it goes on to the regular water heating tank.
No real data is given on performance, and I suspect that for many climates, this is a three season heater at best, but it does offer an interesting alternative to solar, and it keeps the external look of the roof unchanged.
One article suggests that the cost of a 40 ft section of the PEX ridge vent assembly might be around $300, and that a full kit including the preheat tank with heat exchanger might be about $3100.
Here are some thoughts about DIY implementation of the ridge vent design that could (I think) be done for less than $1000 -- maybe substantially less.
Thanks very much to Chris for finding this and suggesting it!