All solar heating collectors have a negatively sloping temperature vs efficiency curve. The negative efficiency means that as the fluid temperature increases, the power output decreases

Solar Vacuum tubes have been around since the early 1900s’.However, their use in solar heating is relatively new. When dealing with a vacuum tube solar heating system, one must be very cautious as the temperatures of vacuum tubes versus flat panel collectors can be as much as two times higher so overheating needs to be addressed properly to avoid damage and possible bodily harm.