On or about January 7, 2011 my Solar Hot Water (SHW) system was activated. The system consists of three 4′ x 7′ flat-plate solar collectors and an electric backup, 120-gallon water storage tank with a single heat exchanger.
The panels are located on the southwest roof of my home. The tank is in the basement. The tank and panels are connected by two insulated flexible stainless steel pipes. One is for hot fluid off the roof, the other returns the cool fluid to the roof. The fluid is a mixture of water and propylene glycol (also known as antifreeze). The solar panels, water storage tank, circulator, and electronic controller were manufactured by Schuco of Germany. (To understand how a solar hot water system works, see the first footnote.)
Since my company sells and installs solar hot water and solar electric systems, I felt it would make sense to add some monitoring so we could see what the system was doing and validate what we tell our customers. To monitor the electricity input for the electric heater and circulator pump, we installed a traditional glass-front kilowatt-hour meter. To monitor the sun’s input, we installed a SunReports monitoring system. The kilowatt-hour meter measures electrical energy added to the system and the SunReports system logs roof-to-tank inlet temperature, tank-to-roof outlet temperature, and circulator flow rate. Using these three measurements, we can compute the sun’s energy contribution to the tank.
Shortly after finishing the installation, it snowed – a lot. Within two weeks, we received about 3 feet of snow. My panels were buried. To make matters worse, the northeastern winds blew much of the snow on the northeast side of my home over the peak on to the southwest side – further burying my panels. Recognizing the risks, I decided to wait for the snow to melt. Unfortunately, my roof (recently replaced) is covered with EnergyStar rated shingles. These shingles are light in color and highly reflective (high albedo) therefore, they don’t heat up the way traditional dark shingles do. This meant that the snow melt was really slow – and making matters worse, it was a particularly cold January.
Finally I gave up waiting. I had one of my crew members bring me a ladder and the snow rake and I manually cleared the panels of snow (See snow clearing warnings below). The clearing day was February 6, 2011, one month after the system go-live date.
The system collected measurable amounts of solar energy for 25 of the next 34 days. However, most of the days the system collected energy for less than one hour. In those 34 days, the system collected and stored 139,848 BTUs* of thermal energy.
What’s this got to do with our oil addiction? Quick question: How much usable energy can be extracted from a gallon of oil? Quick answer: Depending on the “quality” of the oil we can expect about 138,000 BTUs of heat energy when we burn a gallon of oil.
Yes, that’s right. It took one winter month for my solar hot water system to collect the equivalent amount of energy stored in a gallon of heating oil. How big is a gallon of oil: There are 231 cubic inches in a gallon. One gallon is a cube of 6.1″ x 6.1″ x 6.1″. Oil is portable, relatively cheap, easy to burn. No wonder we haven’t broken the oil habit yet.
But I keep trying.
-Mark
How Solar Hot Water Systems Work
Antifreeze is heated in the solar collectors on the roof. When the antifreeze temperature exceeds the tank water temperature by more than ten degrees (F) the pump controller turns on the circulator pump. The heated antifreeze is driven to a heat exchanger in the bottom of the water storage tank where it transfers its energy to the stored water. When the temperature difference between the roof and the tank water drops to five degrees, the controller shuts off the circulator pump.
Clearing Snow from Solar Panels
Don’t do it.
I’m not going to tell you how I did it so you can’t blame me when you get hurt.
BTU
The amount of energy required to raise one pound of water one degree Fahrenheit.