heat pipes to preheat hot water tank?

[Cleanriver]

Hi ECO DIYers,

I have a Geospring hybrid hot water heater. But in the endless pursuit of efficiency, I am thinking about using an assembly of "heat pipes" on my roof to pre-heat the tank water....assuming that will reduce water heating costs.

Although I have re-plumbed my entire house as the old copper sprung leaks, I am not strong on the integration of an outside heat source into an existing system.

I would appreciate any of the following:

1. Links to sites with schematics, and pitfalls
2. Your own experiences
3. Advice as to whether this is best bang for buck, or would you do something else?
4. Heat pipe sources, pref in US.

Background:
- location N. Va
- strong S exposure on "hip" roof, but not as spacious as the non-hip sides.

ANY advice gratefully appreciated!

Cleanriver