Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 24

Most evacuated tubes on the market today use a heat pipe to absorb and transfer the solar heat. A heat pipe is a sealed hollow tube that is filled with a small amount of fluid. This fluid can be water, an alcohol/water mix, ammonia or some sort of proprietary blend. A vacuum is then created inside the pipe and it is sealed. When the atmospheric pressure is reduced inside the pipe, the vaporization point of the liquid is lowered so that when the pipe gets hot the liquid vaporizes at a lower-than-normal temperature. When a liquid changes state to a gas, a lot of energy is absorbed. When a gas condenses back to a liquid, a lot of energy is released. This energy transfer when a material changes state is called latent heat, and because so much energy is transferred when the state is changed this makes heat pipes very efficient at moving heat.

The heat pipe is attached to the absorber plate and placed inside the vacuum tube and then extends out the top. The absorber plate construction and coating materials that are used in evacuated tube collectors are the same as those used in flat plate collectors. When sunlight strikes the plate, the pipe is heated and the liquid inside it evaporates. The hot vapor then rises to a heat exchanger in a manifold located along the top of the tubes. On the other side of this heat exchanger is the solar fluid, which absorbs the heat and circulates throughout the system. As the solar fluid cools the vapor, it condenses and drops back down into the pipe. Some models of evacuated tube collectors have an automatic overheat protection built into the heat pipe using a bi-metal switch. When this switch gets too hot, risking overheating the system, it changes shape, causing flow in the heat pipe to stop, interrupting the transfer of heat to the solar fluid.

Collector configurations that don’t use a heat pipe are called direct-flow or flow-through evacuated tubes. Almost all direct-flow systems use a double-tube configuration, so there isn’t a metal/glass connection involved. The riser tube containing the solar fluid passes down along the absorber plate, makes a U-turn and comes back out the same end it entered. Early evacuated tube versions passed the solar fluid directly through the tube, going in one end and out the other. This configuration was problematic because it required two seals, and they consistently lost their vacuum. Direct-flow configurations are typically more efficient than those using a heat pipe because the solar fluid is in direct contact with the absorber plate, and you eliminate any losses in the heat transfer process from the absorber plate to the heat pipe.