From Envirowiki

A team from the Worcester Polytechnic Institute, in Massachusetts, USA, have come up with a proof of concept for gathering solar thermal energy from asphalt, and converting it into usable energy^[1]. The technology may be able to be retrofitted to existing roads and carparks, and since it's a solar thermal technology, it should be able to generate electricity for some time after it's shaded (either by cars, or the sun going down.

If it works, the technology will produce no greenhouse gas while operating, however asphalt is a fossil fuel (albeit rarely used as a fuel), and digging it up and putting it on the earth's surface is ultimately going to add some carbon to the bio-cycle, and perhaps the atmosphere.

[edit] References

1. ↑ "Alternative Energy Hits the Road (media release)". Worcester Polytechnic Institute (2008-08-11). Retrieved on 2008-08-19.