In parabolic trough collector, long, U-curved mirrors focus the rays of the sun into an absorber pipe. The mirrors track the sun on one linear axis from north to south during the day. The pipe is seated above the mirror in the center along the focal line and has a heat-absorbent medium (mineral oil, synthetic oil, molten salt etc.) running in it. The sun’s energy heats up the oil, which carries the energy to the water in a boiler heat exchanger, reaching a temperature of about 400°C. The heat is transferred into the water, producing steam to drive turbine. A study supported by Japanese government found an annually-averaged collector efficiency using supercritical CO2 as the working fluid, higher than water/vapor.