Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
heat, and the modern high-tech methods, such as sputtering, physical vapor deposition (PVD), black chrome or black crystal, can reduce the emittance levels to 5 to 10 percent.
Evacuated Tube Collectors
When you look at a flat plate collector you will see that the sides and back are well insulated. However, there is no way to insulate that large pane of glass because it would block all the solar radiation. This obvious limitation has led to the development of evacuated tube collectors. Inventors sought a way to permit the transmittance of solar radiation while still insulating.
Evacuated tube collectors are constructed of a series of glass tubes. Each tube is made of annealed borosilicate (Pyrex) glass and has an absorber plate within the tube. During the manufacturing process, a vacuum is created inside the glass tube. The absence of air in the tube creates excellent insulation, allowing higher temperatures to be achieved at the absorber plate by minimizing heat losses. Air is the medium in which convective heat is transferred. If all the air is removed from the tube, this method of heat movement is interrupted.
Because evacuated tube collectors are able to minimize heat losses they are able to achieve higher temperatures than other collector types. This can be an advantage or a disadvantage depending on how the system has been designed and/or what climate the collector is installed in. For applications that require high temperatures, such as solar cooling, evacuated tubes are the most common choice. However, care must be taken when planning and installing these collectors to ensure that the fluid does not overheat and boil during periods of stagnation or when the load on the system is low. Whenever the sun is out, you need to ensure that a fluid is flowing through the system to prevent overheating.
Evacuated tube collectors vary widely in how they are constructed and how they heat a fluid. The principal distinctions are how many layers of glass they have and whether they heat the solar fluid directly or use a heat pipe.
Figure 3.2: Evacuated tube
Glazing
The collectors can be constructed of either a single or double tube of glass. In single-tube versions, the absorber plate and riser tube rest directly inside the vacuum. The configuration means that the solar radiation has to pass through only one layer of glass, but it requires a seal where the riser tube leaves the vacuum. This seal needs to be able to withstand the high temperatures experienced within the vacuum tube, but it must also remain pliable enough to withstand the expansion and contraction of the copper pipe extending out the end as it heats and cools.
Double-tube versions go by many names, Sydney tubes, Dewar tubes or Twin tubes, but all versions are principally the same. A tube is placed inside a larger tube, and the two are sealed together at the ends. The vacuum is then drawn into the space between the layers of glass. The absorber plate is either coated on the underside of the inside layer glass or placed within the vacuumed space. In the latter case, the heat will need to transfer through the inner layer of glass. Having the double tube eliminates the metal/glass connection, which may extend the longevity of the vacuum but requires energy transfer through two layers of glass.
Heat Transfer
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.
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