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Active Solar Water Heater Systems

Tanks

Solar Water Heater Storage Tanks

Stainless Steel

Stainless steel water storage tanks are the best for solar water heater applications. They can withstand the high pressure and excessive stresses put on them by constant heating and cooling throughout the day from the solar water heater. Stainless steel offers excellent corrosion resistance in various water conditions. The only week point of a stainless steel tank is at the seams of the welds. In order to protect the welds, we utilize magnesium anode rods which will corrode first before the stainless steel or any points on the welds can corrode. If these rods are replaced on time, the tank can easily last for a very long time, very possibly a lifetime.

Vitreous Enamel Tanks

VE water tanks are made of carbons steel and utilize a glass lining to protect the metal against corrosion when exposed to water or air. Typical home water heaters are VE style tanks and are generally preferred as they are cheaper than stainless steel. However, VE will fail when a tank reaches temperatures of 70°C or more due to excessive expansion and contraction. The VE will crack, causing the carbon steel tank to become exposed and susceptible to corrosion. Since such high temperatures are common in solar applications, VE tanks need to be specially designed for solar applications. Usually the glass lining is made to be much thicker so that cracking is less likely under the extreme temperature variations.

Using existing Water Storage Tanks

It is possible to use a tank which does not have a built-in heat exchanger. In order to do so, a plate heat exchanger and an extra pump will be needed. Tees will need to be installed on the water ports of the tank to check the temperature of the water in the tank. When the collector is hotter than the tank, the solar circulation pump and water tank pump circulate simultaneously through the plate heat exchanger. Heat exchanges from the glycol loop to the water and then returns to the tank.

Although it is possible to utilize a traditional tank for solar water heating as shown above, Duda Energy does not recommend doing so. While some costs can be saved on the installation, there are many other disadvantages which greatly outweigh those low cost savings. If a home owner already has a new tank that he wants to use because it is new, he should consider selling it to recoup its value or keep it as a backup instead.

Disadvantages of using standard water heating tanks in solar applications:

  1. Additional cost for a circulation pump and plate heat exchanger
  2. more electricity usage for the 2nd pump
  3. More heat loss through additional piping
  4. The tank is evenly heated since there is only one cold water in and one hot water outlet to utilize the heating loop. Rather than the hot water on the top and the cold water on the bottom, with solar applying only to the cold water, the entire tank is heated evenly through the day. This means that in order to have adequately heated water readily available, the water will need to be pre-heated by the electrical element and the solar heat will only be supplemental above that. If the preheating is not done, then hot water can only be used when the sun has completely heated the entire tank to the minimal operating temperature. As hot water is used, the cold will mix with the hot and bring down the overall temperature (unless used only at night when there is no heating) Only with nighttime usage and with non domestic water heating (home heating) applications is this not an issue.
  5. The maximum temperature that can be safely obtained is 70°C.
  6. When comparing a Duda Solar tank to a typical water heater tank, the Duda Solar tank preserves heat much better (Higher R-Value)

On-Demand Water Heating

Tankless water heaters, or on-demand water heaters work very well with solar water heater systems. The solar water heater tank stores the "pre-heated" water. When hot water is used, it passes from the tank, through the tankless heater and to the domestic supply. If the water is above the setting of the on-demand heater, it will not turn on and simply let it pass through. If the water is not quite hot enough, the on-demand will turn on and boost the temperature up to the required temperature. The combination of a solar water heater system with an on-demand heating system gives free heat and without any loss to backup heat which is generated, since the backup heat is generated on demand, rather than within the tank.

Tanks

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