Conventional radioisotope thermoelectric generators (RTGs) used to power spacecraft use a radioactive material whose radiation is used to heat a block of material and then converted to electricity using a thermocouple. Thermocouples are very inefficient and their replacement with TPV could offer significant improvements in efficiency and thus require a smaller and lighter RTG for any given mission. Experimental systems developed by Emcore (a multi-junction solar cell provider), Creare, Oak Ridge and NASA 's Glenn Research Center demonstrated 15 to 20% efficiency. A similar concept was developed by the University of Houston which reached 30% efficiency, a 3 to 4-fold improvement over existing systems.