Installation of large collector arrays presents specific piping problems. This section examines issues related to the installation of pipes, supports, and insulation; pumps; valves; and instrumentation. Generally, the plumbing involved in solar energy systems is conventional, except in cases where a toxic or non-potable heat transfer fluid is circulated in the collector loop. A general guide is that the less complex the system is, the more trouble free its operation will be.
5.9.1 Pipes, supports, and insulation
The material of a solar energy system piping may be copper, galvanized steel, stainless steel, or plastic. All pipes are suitable for normal solar system operation except plastic piping, which is used only for low-temperature systems, such as swimming pool heating. Another problem related to plastic piping is its high coefficient of expansion, which is 3–10 times as high as that for copper pipes and causes deformation at high temperatures. Piping that carries potable water may be copper, galvanized steel, or stainless steel. Untreated steel pipes should not be used because they corrode rapidly.
System piping should be compatible with the collector piping material to avoid galvanic corrosion; for example, if the collector piping is copper the system piping should also be copper. If dissimilar metals must be joined, dielectric couplings must be used.
Pipes can be joined with a number of different methods, such as threaded, flared compression, hard soldered, and brazed. The method adopted also depends on the type of piping used; for example, a threaded connection is not suitable for copper piping but is the preferred method for steel pipes.
Pipes are usually installed on roofs; therefore, the piping layout should be designed in such a way as to allow expansion and contraction, have the minimum roof penetration, and keep the roof integrity and weatherability. A way to estimate the amount of expansion is indicated earlier in this chapter; the supports selected for the installation, however, have to allow for the free movement of the pipes to avoid deformation. An easy way to account for the expansion–contraction problem is to penetrate the roof at about the center of the solar array and allow for two equal lengths of loops on each side of the penetration point. If the pipes must be supported on the roof, this must be done in a way so as not to penetrate the weatherproof roof membrane. For this purpose, concrete pads can be constructed on which the pipe supports can be fitted.
Another important issue related to the installation of collector array piping is the pipe insulation. Insulation must be selected to have adequate R value to minimize heat losses. Other issues to be considered are insulation availability and workability, and because the insulation is exposed to the weather, it must have a high UV durability and low permeability by water. The last factors are usually obtained by installing a suitable protection of the insulation, such as aluminum waterproofing. Areas that require special attention in applying the waterproofing are joints between collectors and piping, pipe tees and elbows, and special places where valves and sensors protrude through the waterproofing. The types of insulation that can be used are glass fiber, rigid foam, and flexible foam.
5.9.2 Pumps
For solar energy systems, centrifugal pumps and circulators are used. Circulators are suitable for small domestic-size systems. Construction materials for solar system pumps depend on the particular application and fluid used in the circuit. Potable water and drain-down systems require pumps made from bronze, at least for the parts of the pump in contact with the water. Pumps should also be selected to be able to work at the operating temperature of the system.
5.9.3 Valves
Special attention must be paid to the proper selection and location of valves in solar energy systems. Careful selection and installation of a sufficient number of valves are required so that the system performs satisfactorily and is accessible for maintenance procedures. Using too many valves, however, should be avoided to reduce cost and pressure drop. The various types of valves required in these systems are isolation valves, balancing valves, relief valves, check valves, pressure-reducing valves, air vents, and drain valves. These are described briefly here:
• Isolation valves. Isolation or shutoff valves are usually gate of quarter-turn ball valves. These should be installed in such a way so as to permit certain components to be serviced without having to drain and refill the whole system. Special attention is required so as not to install isolation valves in a way that would isolate collectors from pressure relief valves.
• Balancing valves. Balancing or flow-regulating valves are used in multi-row installations to balance the flow in the various rows and ensure that all rows received the required quantity of flow. As already seen in this chapter, the use of these valves is imperative in direct return systems (see Section 5.4.2). The adjustment of these valves is done during commissioning of the system. For this purpose, flow rate or pressure may need to be measured for each row, so the system must have provisions for these measurements. After the balancing valves are adjusted, their setting must be locked to avoid accidental modification. The easiest way to do this is to remove the valve handle.
• Relief valves. Pressure safety or relief valves are designed to allow escape of water or heat transfer fluid from the system when the maximum working pressure of the system is reached. In this way, the system is protected from high pressure. This valve incorporates a spring, which keeps the valve closed. When the pressure of the circuit fluid exceeds the spring stiffness, the valve is opened (valve stem is lifted from its base) and allows a small quantity of the circulating fluid to escape so as to relieve the pressure. Two types of relief valves are available: the adjustable type and the preset type. The preset type comes in a number of relief pressure settings, whereas the adjustable type needs pressure testing to adjust the valve spring stiffness to the required relief pressure. The relief valve may be installed anywhere along the closed loop system. Attention should be paid to the fact that the discharge of such a valve will be very hot or even in a steam state, so the outlet should be piped to a drain or container. The latter is preferred because it gives an indication to the service personnel that the valve opened and they should look for possible causes or problems. The use of a tank is also preferred in systems with antifreeze, because the fluid is collected in the tank.
• Check valves. Check valves are designed to allow flow to pass in only one direction. In doing so, flow reversal is avoided. This valve comes in a number of variations, such as the swing valve and the spring-loaded valve. Swing valves require very little pressure difference to operate but are not suitable for vertical piping, whereas spring-loaded valves need more pressure difference to operate but can be installed anywhere in the circuit.
• Pressure-reducing valves. Pressure-reducing valves are used to reduce the pressure of make-up city water to protect the system from overpressure. These valves should be installed together with a check valve to avoid feeding the city circuit with water or antifreeze solution from the solar energy system.
• Automatic air vents. Automatic air vents are special valves used to allow air to escape from the system during fill-up. They are also used to eliminate air in a closed circuit system. This valve should be installed at the highest point of the collector circuit. Automatic air vent valves are of the float type, where water or the circulating fluid keeps the valve closed by forcing a bronze empty ball against the valve opening due to buoyancy. When air passes through the valve, the bronze empty ball is lowered because of its weight and allows the air to escape.
• Drain valves. Drain valves are used in drain-down systems. These are electro-mechanical devices, also called solenoid valves, that keep the valve closed as long as power is connected to the valve (normally open valves). When the valve is de-energized, a compression spring opens the valve and allows the drain of the system.
5.9.4 Instrumentation
Instrumentation used in solar energy systems varies from very simple temperature and pressure indicators, energy meters, and visual monitors to data collection and storage systems. It is generally preferable to have some kind of data collection to be able to monitor the actual energy collected from the solar energy system.
Visual monitors are used to provide instantaneous readings of various system parameters, such as temperatures and pressures at various locations in the system. Sometimes, these are equipped with a data storage. Energy meters monitor and report the time-integrated quantity of energy passing through a pair of pipes. This is done by measuring the flow rate and the temperature difference in the two pipes. Most energy meters must be read manually, but some provide an output to a recorder.
Automatic recording of data from a number of sensors in a system is the most versatile but also the most expensive system. This requires an electrical connection from the various sensors to a central recorder. Some recorders also allow processing of the data. More details on these systems are given in Chapter 4, Section 4.11. Nowadays, systems are available that collect and display results online on the Internet. These are very helpful in monitoring the state of the system, although they add to the total system cost. In countries where schemes such as the guaranteed solar results operate, where the solar energy system provider guarantees that the system will provide a certain amount of energy for a number of years, however, this is a must.
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