The solar energy group of the Renewable Energy Research Centre, Vietnam, has designed an efficient solar water heater. The system consists of three collector-cum-storage tanks with a total capacity of 90 litres. Inside the tank, a metal plate is linked under the solar energy absorber surface. The water-heating mechanism in this is based on thermosyphon effect (with solar radiation, the space between the absorber and the metal decreases, making the water rise and fall alternatingly). A convection current within the tank then occurs resulting in the mixing of hot and cold water and thus warming all the water.