Past Project: Solar Hot Water System

On the living room sofa; cloudy, 53.1 F, 0526

Using electricity to heat water is fairly expensive, as I discovered when we received our first electric bill. Solar hot water systems harvest energy directly from the sun and are an efficient, inexpensive way to save money.

Bolting down solar collector to roof

My friend, Jay and I installed these solar hot water collectors in August 2007. They are Model AE-40 collectors made by Alternative Energy in Jacksonville, Florida. As the name implies, they have about 40 square feet of collector area.

Collectors installed, tilted 40 degrees for better efficiency

System Design

This is a drain back system that uses water as a heat transfer fluid. The differential controller monitors the temperature of the bottom of the storage tank and the top of the solar collector. When the differential temperature is greater than 15 degrees F, the pumps turn on. Pumps move the water through a heat exchanger thence up to the collector removing the heat back to the drain back tank. A second, smaller pump moves water from bottom of the storage tank through the heat exchanger thence back into the top of the storage tank. The drain back tank acts as a reservoir. When the large pump shuts off, the collectors empty out thus preventing freeze damage.

The system diagram:

System Diagram, drain back solar hot water system
System Diagram, Drain back solar hot water system

Cost and savings

The total cost of the system was right around $5,000.00. I received a Federal Income tax incentive of $1,500.00 and a New York State Income tax incentive of $1,250.00 making the net cost $2,250.00.

It reduced our electric use by approximately 3,500 KWh per year or 45.5 MWh over thirteen years. Electricity rates fluctuate over time however, the average rate over the last thirteen years is about 15.5 cents per KWh (includes delivery charge). That is a $7,050.00 reduction in my electricity bill and is a net savings of $4,800.00. Thus this solar thermal system has paid for itself thrice over.

The US Energy Information Agency notes that each KWh generated in the United States produces 0.99 pounds of CO2 emissions. A reduction of 45.5 MWh represents a 22.5 ton reduction of CO2 emissions.