The tubes in a solar collecter are filed with?

The tubes in a solar collector are typically filled with a heat transfer fluid, which can be water, glycol, or a mixture of both. This fluid absorbs heat from the sun and transfers it to a storage system or directly to a heating application. In some designs, evacuated tubes may contain a vacuum to enhance thermal insulation and efficiency. Overall, the choice of fluid and design optimizes the collector's ability to harness solar energy effectively.