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 lifted 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 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. 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 electromechanical 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.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.