evacuatedtube

An evacuated tube is a type of solar thermal collector designed to absorb solar energy and convert it to heat with minimal heat loss. It consists of a glass vacuum seal surrounding an inner absorber tube. The vacuum reduces heat transfer by convection and conduction, allowing for higher operating temperatures and improved performance in cool or cloudy conditions.

Construction and variants: Most modules are composed of multiple glass tubes, each with an outer evacuated glass envelope and an inner absorber tube coated with a selective surface to maximize solar absorption while minimizing radiative losses. There are two common configurations: heat-pipe and direct-flow. In heat-pipe designs, a small copper heat pipe within the inner tube transfers heat to a header connected to the storage system. In direct-flow designs, the heat transfer fluid circulates directly through the inner tube. The outer glass tube is evacuated to maintain the near-vacuum insulation.

Applications and performance: Evacuated-tube collectors are widely used for domestic hot water and space heating systems, particularly in colder climates. They can reach higher temperatures and maintain performance with low solar irradiance than flat-plate collectors. They are modular, allow flexible installation on roofs or façades, and are compatible with storage tanks and solar-assisted heating systems.

Advantages and considerations: Advantages include higher efficiency under low sun, high-temperature output, and suitability for compact vertical layouts. Limitations include higher cost than flat-plate collectors, greater fragility due to glass components and vacuum seals, and potential for tube breakage from hail or impact. Maintenance focuses on avoiding vacuum loss and ensuring good reflective coatings.

History: Development of modern evacuated-tube collectors accelerated in the late 20th century, with ongoing improvements in coatings, tube designs, and integrated heat-transfer technologies.