I'm hearing a lot about "Passive House" designs these days. Is this the same as a passive solar house design?
What a good question. That's like saying to a wine lover, "Red and white taste the same, right?" Passive House and the Passive Solar houses of the 70s and 80s are not the same thing.
Passive Solar houses rely on optimal orientation, large thermal masses for the collection of solar energy, and climates with dramatic diurnal swings. The caveat to the approach is that we all don't have ideal exposures, and identical climates, not to mention that the physical reality stands that whatever lets the heat in, also consequently lets it out.
Passive House is an approach to energy efficient home design and construction that relies on a superefficient envelope, essentially minimizing losses to optimize one's gains (like having body heat, cooking, and drying clothes). With a superefficient envelope, you get to keep that heat.
There is a component of optimizing solar gains to Passive House; it shares this in common with the solar movement, but its primary focus is conservation and minimizing of energy demands. The Passive House approach yields (in any climate with any exposure, I might add) 75%-90% more efficient structures; combine this with the other elements necessary to a Passive House, like ample fresh air delivery and high-performing windows, and you end up with a substantially more comfortable, affordable and healthy house.