Evacuated Tube Solar Hot Water Collection Systems are among the most widely used out there. With the ability to customize the sizings of the system, it's important to know how to calculate the requirements and be able to size a system to fit your needs.
The first place to begin is with your average consumption. It's important to know whether you need to heat 10 gallons or 100. Figuring out your consumption will give you the basis for your hot water heating system. For the purpose of this example, we will figure out a household average of 30 gallons per person per day.
Incoming water temperature is important, as the colder the water gets the more heat you need to increase the temperature to it's appropriate level. On average, water from a well or city source is ~ 50 degrees f. If we want to bring this water up to 120 degrees f, we need to raise it by ~ 70 degrees. This is where BTU count is important. BTU stands for British Thermal Unit, which is the measurement of the amount of energy it requires to raise 1 pound of water 1 degree. This means we have 30 gallons of water per person, at 50 degrees that we want to raise up to 120 degrees. One gallon of water is about 8 pounds, which gives us 240 pounds of water per person.
240 pounds of water x 70 degrees of temperature increase = 16,800 BTU.
By this calculation, for each person in the house you should be generating no less than 16,800 BTU of heat collection for your Hot Water System.
A Solar Collector Tube can collect ~220 BTU/hour. 10-tubes would thus generate 2200 BTU every hour. On an 8-hour average, that gives us a potential 17,600 BTU/day. This is the amount of heat collected at the solar tube array. To maximize the amount of heat your tank receives, you need to minimize heat loss over distance, or increase the amount of heat you collect to compensate for the loss. Insulation goes a long way in a Solar Hot Water system.
Topics such as how to pump water in an off grid system, water storage practices and dealing with waste and grey water.
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