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

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 in a solar power station. This is commonly referred to as Concentrating Solar Power (CSP). Large electricity generating plants using imaging concentrating collectors have been constructed in dry climates around the world with great success. A lot of research is being done in this field, and we can expect this technology to grow significantly in the near future.

      Figure 3.6: Concentrating collector

      Although there have been attempts to use imaging collectors in domestic water heating systems, no successful or durable products have been developed. Problems were encountered with the tracking mechanisms, the precision needed in the mechanisms and the durability of the reflectors and linkages. Recently a couple of firms have been releasing imaging collectors for small commercial and industrial use. Given the poor track record of imaging collectors, we are still unsure whether this second attempt will be a success. They are particularly vulnerable to snow and ice buildup in cold climates.

Pool Collectors

      The single largest application of active solar heating systems in the world is in heating swimming pools. Special collectors have been developed for heating seasonal swimming pools: they are unglazed and made of a special copolymer plastic. Most are made of proprietary materials containing UV stabilizers to prolong their lifespan. The collectors don’t have to be glazed because they are used only when it is warm outside, and they are heating water to a relatively low temperature that is just slightly above ambient. These collectors cannot withstand freezing conditions.

      Figure 3.7: Pool collector

      Figure 3.8: Web-and-tube configuration

      The absorber plate in copolymer collectors is an extruded mat that has many waterways, or risers, that are set close together. Many designs have been produced, but the design that has proven most successful is a web-and-tube configuration. With this design, the tubes are separated by a small web so they are not touching one another. Much like copper absorber plates, these collectors have a header running along the top and bottom of the absorber. The best designs, like web-and-tube absorbers, are made so that each riser is individually attached to the header, which allows the least back pressure within the collector and allows individual connections to be repaired, if needed. Because the risers are situated close together, this type of collector has a large wetted surface, adding to its efficiency. In fact, these collectors can outperform glazed collectors in seasonal pool heating applications.

      Solar pool heating systems that use unglazed plastic collectors are direct systems in which the pool water is circulated through the collectors. This is the most efficient configuration as there are no heat exchanger losses. Most pool water contains additives such as chlorine, which is highly corrosive. This corrosive property of pool water makes copper absorbers or copper pipes noncompatible. This is one of the reasons the collectors are made of plastic, which is not affected by chlorine. The same holds true for the piping, which is typically PVC.

      Plastic pool heating collectors are typically mounted flat on a roof. The collectors are held in place with a set of straps that go over the collectors but are not actually attached to them. The straps are often plastic-coated stainless steel and are threaded through special clips that are bolted to the roof. This method of holding down the collectors allows them to expand and contract on the roof without binding and makes them easy to install.

Comparing Solar Water Heating Collectors

      People often ask us, “What is the best type of collector?” We would like to be able to give you an easy answer, but unfortunately there isn’t one. We have found that many factors come into play when it is time for you to select a collector. It depends on the type of application, the climate of the installation, the amount of room available for the collector array, the budget for the project and other reasons too numerous to mention. What is more important than trying to find the “best” collector is being able to match the characteristics of the collector to the job that needs