Performance improvement by using dual heaters in a storage-type domestic electric water-heater

In many designs of storage-type domestic electric water-heaters (EWHs) the internal heating elements are mounted at the bottom of the tank. Energy-utilization efficiencies of such EWHs are not always high since the whole tank of water is heated for even a quick shower, where the hot-water requirement is only a small fraction of the total tank capacity. In this study, performance of employing a secondary heating-element near the top part of a standard-size storage tank was experimentally investigated for energy conservation. Data were obtained for two draw-off rates of 5 and 10 L/min, and by locating a standard heating-element at three different positions; mounted vertically at the bottom and horizontally on the lateral surface 380 and 600 mm from the bottom surface. It is found that, with the heater located on the lateral surface of the storage tank, only the water above the heater can be heated while the water below the heater remains almost unaffected by the heating process. For the heater located at a height of 600 mm from the bottom, 85% of the stored energy can be utilized to supply almost 50 L of warm water, which is enough for one person to take a shower. Then, it is possible to design a tank with dual heaters, giving the users the chance of switching between the elements depending on the amount of hot-water required. This will facilitate the rational use of energy in domestic hot water preparation. Considering that the extra cost of producing an EWH with an auxiliary heating element is less than US$50, the application of dual heaters is worth considering by the manufacturers.