Evacuated-tube solar collectors tend to be more efficient but also more expensive. They are used more frequently for commercial applications in India. A collector is made up of several pipes in parallel rows that are connected at the top. The tubes consist of a vacuum with a pipe running through the middle containing the working fluid. The water moves up and down and along through the series of pipes to exit the system at a significantly higher temperature. The heat loss is significantly reduced thanks to high negative pressure in the glass tubes. They can be deployed nearly horizontally on flat roofs.
