The average annual operating cost for a standard hot water heater is $500 or higher.  This is for a 50 gallon water heater with a life expectancy of 8 to 12 years.  A solar hot water heater, on the other hand requires an annual operating cost of only $50, has a capacity of 80-120 gallons and has a life expectancy of 15 to 30 years (SolarDirect.com). 

The payback for a solar hot water system is approximately 7 years.  This is due in part to the tax credit that is available from the government.  This tax credit is for 30% of the system cost and eligibility has been extended to the year 2016.  The system cost in a new home or existing is approximately the same.  The cost for an 80 gallon tank using 1 to 2 solar panels is approximately $5400 and the cost for a 120 gallon tank using 2 to 3 panels is approximately $7000(MissouriSolarLiving.com).  

Another payback from solar hot water heat is the environmental benefit.  The use of solar power reduces the use of fossil fuels for heating purposes.  Carbon dioxide emissions can be reduced by approximately 2000 lbs per year for each solar hot water heater.  Nitrous oxide emissions can be reduced by approximately 1 lb per year for each solar hot water heater. 

Theory of Solar Heating and Terms

Solar Constant - The rate of solar energy hitting the earth’s atmosphere assuming no loss is 429.2 BTU’s per hour per square foot.  This energy if collected for three hours without any inefficiency could power the entire earth for one year. 

Insolation - Since the earth’s atmosphere filters out much of the sun’s energy, the most efficient we can be inside the atmosphere is about 70% or 320 BTU’s per hour per square foot. 

Wavelength Conversion - Solar radiation is a stream of short wavelengths that becomes thermal energy when it is absorbed.  The thermal energy is composed of a new stream that is wavelengths that are 10 times longer. 

Greenhouse Effect - Many materials like the atmosphere will allow the passage of the shorter wavelengths of light but will not allow the returning longer wavelengths of heat to exit.  This “Greenhouse effect” is used in many solar collectors to maximize thermal energy.  

Black Body - Materials that are good at absorbing radiant energy and re-radiating that energy are called black bodies.  The most efficient black bodies can re-radiate up to 96% of the energy that arrives. 

Absorber - An absorber is that portion of the solar collector that contains the black body.  It is usually a flat black surface with a high absorbance rate.  It is responsible for converting the radiant energy into heat. 

Flat Plate Collector - This is the most efficient type of collector in areas where the temperature can get below freezing.  It is made of an absorber, heat exchanger and also includes insulation, glazing, plumbing and the casing to enclose the entire assembly.  These collectors are usually set up facing to the South and tilted at an angle to catch the most amount of radiant energy.  The other advantage to Flat Plate Collectors is that they can absorb diffuse radiation that is produced on cloudy days. 

Convection - This is the basic form of heat transfer that occurs in a solar collector.  A cool liquid or gas is heated in the collector and then moves away to be replaced by more cool material.  This simple process allows for the continual heating of water during times when radiant energy is available (SolarExpert.com/Heat-theory). 

Basic System Operation

The solar hot water system can be either a one-tank or two-tank system.   

A One-tank system has one tank that is heated from the solar system as well as the conventional heating system. 

Two-tank systems have one tank that only heats water using the solar heat system.  This water is then transferred into a second tank that can be heated using a conventional heater if needed.

In an existing home scenario, there is a cost saving option that is available.  In certain circumstances, the existing water heater can be integrated into the solar hot water system.  The cost savings in utilizing the existing hot water heater can be as high as $800.  There are a few basic requirements that are necessary:

Existing electric hot water heaters can be utilized in a one tank system to work with a solar hot water system if thery are less than 5 years old and are at least 80 gallons.  The reason not to use electric hot water heaters older than 5 years is because they will have deposits in the bottom of them that can cause problems in the functioning of the solar hot water system.  If they are smaller than 80 gallons, they do not have enough capacity to be retrofitted to include the solar system.  With the electric hot water heater, the tank will be retrofitted to act as the storage tank and the back up heat source in one. 

Existing gas hot water heaters can be used as the back up in a two-tank solar hot water system.  A separate solar storage tank is installed and then feeds in to the cold water supply of the gas back up heater.  The back up heater will only ignite if the heated water from the solar tank cannot keep up with demand.  The problem with the two-tank system is that it requires additional space that some people may not have in their basements.  The gas hot water heater can be an older heater and can be as little as 50 gallons since it is only being used as the backup.

In warm climates, the cold water is transferred to the roof using a pump and lines to the solar collectors on the roof.  The water is heated and then is returned to the storage tank.  The water from this solar tank is now the supply water that goes into the conventional water heater portion of the system.   

In colder climates, a non-freezing transfer fluid is pumped to the roof and passes through the solar collectors.  It returns to the water heater and then gives off heat through a heat exchanger inside the water heater (EnergySavers.gov).   

The conventional portion of both systems will only ignite if the solar portion cannot keep up with the demand for hot water.  Most solar systems will provide 70% of the required hot water.  The burner element in the conventional portion of the heater will take care of heating the other 30% of the water (SolarExpert.com). 

Conclusion