There is a company called Power Chips which claims being able to convert heat to energy by a so-called Avto Effect (see Avto Metals) – all owned by a cooperation called Borealis.

That Avto Effect is based on the fact, that an excited electron (heat) that hits a wall with gaps finer of its own wavelength must release its energy to exist (some quantum effects stuff). Please read the details on their web page.

The same should be possible with photons (they’ve got a Photon Power site, now a little bit defunct). X-ray photons would require much finer gaps, so x-rays would be still a challange.