X-ray is an important tool for charactering and analyzing materials. However, current X-ray generation is cost with low efficiency. For X-ray tube, which is mostly used in laboratories, only has an energy usage of 1% with all other energy dissipated into tremendous heats, and it needs continuous cool water flows to cool down the cathode. It generates X-ray by the bremsstrahlung of high energy electrons bombarding on the cathode target, the bremsstrahlung would contain X-ray with sufficiently high energy of the electrons. But most part of the electron energy becomes heats. In order to generate X-ray more cheaply with higher efficiency, methods about reusing the released heats during the working of the X-ray tube are brought up. Mimicking the photovoltaic effect, nonequilibrium carriers could also be injected via thermion emission when heating a metal, such injection is same to that of photonic injection which produces electromotive in a photovoltaic cell. In a photovoltaic cell the electron-hole pair generated by incident photons are nonequilibrium carriers that causes electromotive, while the thermion emission creates such electron-hole pairs via thermal excitation. Connecting metals suitable for thermion emission from the cathode into the p-n junction so that thermions as nonequilibrium carriers can be well injected into the p-n junction when the metals are heated by the cathode, with Thomson effect which enhances such injection, a thermal voltaic cell can be constructed and it can produce electricity only by heating the metals outside. Applying such thermal voltaic cell into current X-ray tube, it would produce electricity while absorbing the tremendous heats emerges when X-ray tube is working. Water flows are still used to control the temperature, but letting them boiling to keep the cathode at a temperature best for thermal voltaic cell, and the vapor may be used to drive a mini thermal power plant. In this way, the energy usage could be modified to a higher proportion. Stepwise up-conversion is possible to generate X-ray more cheaply but there are no suitable materials so far.