Water Heater Design Using Solar Technology
Solar technology offers the most economical and environmentally friendly approach for water heating for domestic consumption. On a typical sunny day in southern California or in similar regions around the world, solar radiation provides energy of more than 200 BTU per hour per square foot, representing a significant source of energy at no cost. Currently, high-technology solar collector design offers operating
efficiencies exceeding 70 percent . This translates into a solar energy of 6400 BTU per hour available from a 4 ft x 8 ft solar collector. Assuming a solar collector efficiency of 70 percent, an absorbed energy of 4480 BTU/hour is available from a 4 ft x 8 ft solar panel, which amounts to 1.31 kWh of electrical power available, because 3413 BTU is equal to 1 kWh. To recover this amount of electrical energy in a typical installation requires a 100-W circulation pump.
It is important to point out that water heating for household use requires an enclosed unit to insulate it against heat loss due to large temperature differences between the ambient air and the solar panels or collectors, which could be as much as 100掳F particularly in the winter season. Physical layout of a commercial water heater  for domestic use is shown in Figure 7.1. The 4 ft by 8 ft solar collector appears to be most satisfactory for small households and is widely available in the market for home use. However, for large households two or three solar panels may be required, as illustrated in Figure 7.1. Solar panels or collectors that are 4 ft by 10 ft are commercially available, which offer better economical operation. However, they are proportionately larger and heavier compared to 4 ft by 8 ft solar collectors and could increase the installation costs. The most appropriate and ideal location for the solar collector is the roof of the house or building.