... ,Solar vacuum tube for solar energy water heater 2,ISO,CE 3,Available in various size 4.5days delivery 5.OEM available 1, Vacuum tube ... the space between the two glass tubes to form a vacuum, so it eliminates heat loss.7, This why ... in low temperature and low sunlight medium. MODELITEMSGB(single target)ConfigurationDouble-layer concentric boronsillicate glass tubesGlass thermal expansion coefficient3. ...