Solar Water Heating--Revised & Expanded Edition. Bob Ramlow

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СКАЧАТЬ 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.

       Other Characteristics

      Some evacuated tubes use a getter to ensure that all of the gasses have been removed from inside the tubes. Commonly made of pure barium, the getter will absorb any residual gasses left over after the vacuum has been drawn. The barium will leave a shiny silver coating on the bottom of the tube. If the tube loses its vacuum, this barium coating will be exposed to oxygen and will become white or foggy, clearly indicating that the vacuum has failed and the tube needs to be replaced.

      Since the tubes themselves are not structural components of the system, a rack is usually constructed to hold them in place. There is typically a bottom rail that all of the tubes can rest in and be secured. The top of the tubes enter into the manifold and are secured there. Most mounting kits come with bracing members to connect the bottom rail to the manifold. This assembly can either be flush mounted to the roof or raised using struts. Because the tubes are removable from the manifold, they can be installed after the rack has been constructed. Evacuated tubes that use a heat pipe must always be mounted at a pitch of at least 25 degrees to ensure that the evaporation/condensation cycle will flow.

ICS Collectors

      ICS stands for Integral Collector Storage. In an ICS unit, the solar hot-water-storage tank is the solar absorber. The tank (or tanks) is mounted in an insulated box with glazing on one side and is painted black or coated with a selective surface. The sun shines through the glazing and hits the black tank, warming the water inside the tank. Some models feature a single large tank (30 to 50 gallons) while others feature a number of metal tubes plumbed in series (30- to 50-gallon total capacity). The single tanks are typically made of steel, and the tubes are typically made of copper. These collectors weight 275 to 450 pounds when full, so wherever they are mounted, the structure has to be strong enough to carry this significant weight.

      Figure 3.3: Tank type ICS collector

      Figure 3.4: Tube type ICS collector

      ICS collectors are widely used around the world in climates that never experience freezing conditions. They work great, given their climatic restrictions. They are a direct type of system, as the water you use is actually heated in the collector. They do not suffer from hard-water problems nearly as much as do flooded collectors (collectors with water in them at all times) because all waterways in the collector are very large and also because the collector never gets hot enough to precipitate minerals out of solution, and their simplicity makes them a very affordable choice.

      ICS collectors are also used in seasonal applications such as campgrounds and summer homes, where they are used only during the warm months of the year and are drained before freezing conditions occur. Bob has sold many of these collectors with excellent results.

      As mentioned above, these collectors are heavy, so if they will be mounted on a roof, make sure that roof is strong enough to hold their weight. You may have to reinforce an existing roof to make it acceptable. These collectors should always be tilted so they will properly drain.

      The tube type of ICS collector will outperform the tank type because more surface area is exposed to the sun. Another advantage of the tube type is that its profile is much smaller, which affects aesthetics. On the other hand, tube type collectors cool off more quickly at night because of their larger surface area. On cool nights, the water stored in these collectors will cool off, so they lose efficiency. You can maximize this kind of system’s efficiency by using as much hot water as you can during the day and early evening hours.

      Another kind of collector system often classified as an ICS system is called a thermosiphon type system. These use a flat plate or evacuated tube collector mounted directly to a storage tank that is located directly above and attached to the collector. They use a heat transfer fluid that flows through the collector or uses a heat pipe to deliver heat from the collector to the storage tank. These systems are very popular in southern Europe, Asia and areas of Australia where freezing conditions never occur. These systems do not lose heat during the evening hours as quickly as the tank type collectors do because the storage tank is better insulated.

      Figure 3.5: Thermosiphon type ICS collector

      All ICS systems are plumbed in series, with the backup water heater acting as a pre-heater. Very often they can provide 100 percent of the daily domestic hot water when installed in hot, sunny climates or during the summer months when the sun shines most days.

Concentrating Collectors

      Concentrating collectors use a reflective parabolic-shaped surface to reflect and concentrate the sun’s energy to a focal point, where the absorber is located. Concentrating collectors use only direct sunlight; most other types of collectors can also use indirect sunlight. Concentrating collectors come in two types: imaging reflectors and non-imaging reflectors. A non-imaging reflector collector looks much like a flat plate collector, as the mirrors and receivers are mounted in a glazed frame of similar dimensions to flat plate collectors. Non-imaging reflectors do not track the sun but remain stationary while still retaining the ability to focus the sun’s energy on the receiver as the sun moves across the sky. These collectors make up a very small percentage of the total collector market. To work effectively, imaging reflectors must track the sun. These collectors can achieve very high temperatures because the diffuse solar resource is concentrated on a small area. In fact, the hottest temperatures ever measured on the earth’s surface have been at the focal point of a massive concentrating solar collector. This is sort of like starting a fire with a magnifying glass on a sunny day.

      Imaging collectors have been used to make steam that spins an electric generator СКАЧАТЬ