RENEWABLE ENERGY REVIEWS

In my thirty years of experience with solar water heating systems, I have often seen design strategies that were doomed to fall short. One such strategy that has popped up over and over again involves the use of multiple residential-sized hot water tanks in an effort to provide thermal storage for large commercial installations like hotels and hospitals.
Why would some professional solar installers be tempted to use five 120 gallon tanks instead of one 600 gallon tankc
One possible answer is fairly straightforward: 120 gallon tanks are readily available at any plumbing supply house and 600 gallon tanks are not.
Another answer could be related to the perception of cost. Residential water heaters come in many sizes, from 15 gallons up to 120 gallons. They all are rated to withstand water pressure up to 125 psi, after which a pop off valve opens and relieves the pressure.
Millions of water heaters are made each year and the specifications for manufacturing, testing and safety have been standardized for many years. With this kind of volume, the prices are relatively low.
When a pressurized vessel goes above 120 gallons, it falls under the boiler code regulations which are much more stringent than those for residential water heaters. A 600 gallon pressure vessel can cost up to 5 times what a string of water heaters cost. Such a vessel may even require additional insurance coverage.
In other words, the added cost and regulations can make using a boiler code pressure vessel an expensive pain in the neck.
There is a more important question to be asked: Is a series of 120 gallon residential tanks better than a single non-pressurized commercial tankc The answer is a resounding no!
Multiple tanks loose on every count when performance is the subject. First, a string of tanks is much less efficient in storing thermal energy. Their surface area is much greater than one tank of the same volume. The greater surface area loses much more energy through the insulation compared to a single tank with the same insulation. So, efficiency goes down dramatically.
Second, a string of tanks requires a great deal of piping between the tanks. This costs extra money and adds even more to the thermal losses of the system.
Remember, a solar system can't be turned back on when the heat runs low. It can only generate energy when the sun is shining. Every Btu lost from the system is gone forever. Therefore, maximizing the storage efficiency is critical for optimal performance.
Third, all the piping between tanks to make the array act like one big tank will never achieve the efficiency of one big tank. Multiple tanks can have temperature differences between the different tanks, because the piping and water flow cannot be perfect over the whole array. Hot and cold spots are a sign of poor efficiency. One big tank eliminates all the crisscross piping. There are no connection irregularities, and it takes up less space in the equipment room.
Most importantly, ongoing maintenance of multiple tanks is a much more difficult task. For every tank added to the system, several new points of failure over the coming years of operation arise. Troubleshooting for a specific problem within a multiple tank configuration becomes a nightmare. I've seen it happen too many times to mention.
In summary, we see that using a string of 120 gallon tanks for solar thermal storage is intended to avoid the perceived higher cost and boiler code regulations that come with a bigger pressurized single tank. However, this convenience comes with a significant loss of performance and increased maintenance issues.
Suppose that all the cost negatives of a bigger tank could be overcome. Suppose a single non-pressurized commercial-grade tank could be designed that cost less than multiple 120 gallon tanks. A system that could be sized from 15 gallons to 5000 gallons and more, that didn't have to meet any boiler codes.
Imagine also that all the operating components, such as heat exchangers, pumps, and controls could be factory built into this system, so that none of that work would have to be done on the job site. Such a system would be factory designed so that all components worked perfectly together, where high craftsmanship and quality control could be applied before it was ever shipped to the job site.
In 1978, I was pondering this question: How can I put all the solar engineering into a factory built product that is simply installed (not assembled) at the site just like an appliance.
The answer was to redesign of the entire solar water heating concept. Forget pressurized tanks that had to meet boiler codes. Remove unnecessary heat exchangers, pumps, and controls between the collectors and the tank. Eliminate the need for antifreeze and all its additional baggage. Minimize maintenance!
The result was an approximate 20% increase in efficiency over competing systems and up to 30% longer life. The product that came from the effort is called the Fluid Handling System: more than just a tank.
Since then, thousands of these commercial solar hot water tanks have been manufactured, many of which are still running after 30 years of minimal maintenance!

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Solar Hot Water Systems

Source: http://www.articlesnatch.com/Article/Commercial-Solar-Hot-Water-Systems---Increased-Value-Via-Optimal-Tank-Performance/2001880