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ALL you need to know about domestic hot water systems
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There are only four terms you need to understand: direct, indirect, passive, and active I'll take each of these terms and make it real simple for you. The easiest terms to understand are active and passive.
Direct solar systems heat the water that is consumed or stored in the water heater. If the water from the storage tank or water-heater is heated in the solar collector, it is a direct system. Indirect systems have a heat exchanger separating the solar collector from the storage tank.
If a solar water heating system has a circulation pump to transfer heat from the collector to the solar storage tank, it is an active system. If the system has no pump or control system to transfer the heat to the storage tank, it is a passive system.
Passive systems use natural forces like thermosyphon (cold water is dense and heavier then hot water) or batch/bulk storage. Thermosyphon action is where the cold water forces the hot water out of the collector into a storage tank located above the collector. The tank can be located on the roof or in the attic, maintaining the low profile of
the solar water heating systems appearance on the roof. Thermosyphon systems are the most popular system design used outside of the United States. Travel anywhere in the Caribbean, Middle East, Mediterranean, Australia or Asia and you'll see the common thermosyphon system. These are all areas where solar water heating has been long considered the only option for heating domestic hot water in homes and small businesses like restaurants and hotels. Americans have a problem seeing the tank on the roof so the solar water heating industry has developed and is working on systems that install the storage tanks into the attics or located on areas of the roof not visible from the front of the house.
The simplest solar water heater is the batch or bulk storage system that combines the solar collector and the storage into one unit. These systems have integrated collector storage (ICS) and have a higher profile on the roof or are set down into the roof to conceal the size of the system. These systems should only be used in the warmest climates due to their associated heat loses at night, heat losses are especially high in areas with cool clear nights like the desert that will allow heat stored in the system to reradiate back to the sky. The hot water generated by ICS systems should be used during the afternoon and evenings so the stored hot water is consumed or transferred into a separate storage tank in the house before dark.
Direct solar systems heat the water that is consumed or stored in the water heater. If the water from the storage tank or water-heater is heated in the solar collector, it is a direct system. Active and passive systems including all of the ICS systems are or can be direct solar water heaters. Direct systems should be limited to warm climates or those areas that experience only a couple of freezing days per year. They are intended to be located in areas that have occasional freezes in the winter, not sustained days or long, clear nights below 40 degrees Fahrenheit.
The choice of a solar water heating system is quite simple. The most important factor is the climate where the system is to be installed. If you live anywhere that experiences regular seasonal freezing weather, you need to have an indirect system. Most of the U.S., except parts of Florida and California, need indirect systems. Indirect solar water heating systems have a heat exchanger in them to transfer the heat energy from the collector to the water heater. Heat transfer fluid if left in the solar collectors during freezing conditions will damage the collector unless it is protected with anti-freeze. That is not to say all indirect systems use antifreeze. Some systems have a drain-back design that allows all the heat exchange fluid to drain out of the solar collector into a reservoir when the pump stops circulating. Pumps should not run at night, only during the day when there is heat in the solar collector.
What about days when it's overcast or there isn't enough seasonal sun to heat the water up to the desired temperature? This is something very important you need to think about. What is the typical water heating option in the area? Electricity, natural gas, LPG or propane -- all of the conventional water heating systems -- can be used as the back-up heating fuel. Electricity is the easiest; most solar storage tanks have an electric element that heats only a portion of the storage tank when necessary to increase the temperature for delivery into the house.
If you use gas, other then a demand heater, you must use a two-tank system. One tank for the solar heated water and the other a standard gas water heater. Because a typical gas water-heater has the burner located at the bottom of the tank, whenever hot water is used in the house, cold water entering the tank turns the burner on and it heat the whole tank. To use a solar system with gas as a back up, you must have a separate solar storage tank that has the connection for cold water entering the system. The solar storage tank will supply preheated water to the typical gas back-up heater. This way the gas burner will only run when necessary to boost the temperature of the preheated water for delivery into the house.
Many active solar water-heating systems use a photovoltaic module to generate the electricity to run the pump. This is one of the better uses for PV; the current generated by the PV panel controls the speed of the pump that has a direct relationship to the amount of heat energy being generated by the solar thermal collector. Consumers can have PV, along with a solar water heater. Now that is really cool! Water Pumping is one of the most cost-effective PV applications.
by William T. Guiney
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