Wednesday, March 14, 2012

3 Years of Solar Hot Water Data

Here is some of the most comprehensive data on solar hot water performance you will find anywhere:  3 years of daily records for my 80 gallon Reynolds solar hot water system.  It has performed remarkably well, year round.  The number of days it did not run at all is surprisingly small and consistent:  53 days in 2009, 49 days in 2010 and 48 days in 2011. The number of days it reached 100% of capacity is 120 days in 2009, 181 days in 2010, and 190 days in 2011. The number of hours it ran is a little less consistent:  1990 hours in 2009,  1888 hours in 2010 and 1651 hours in 2011.  While it may seem odd that 2011 has both the fewest hours and the most 100% days, it is because the system needed to run less, as the storage tank was frequently still hot from the day before.  Unfortunately, I don't have an hour meter on the electric heating elements, or a water meter to measure actual use.  I estimate that we use between 30 and 40 gallons of hot water each day.  Since the system uses only 175 watts when running, it is using less than $50/year in electricity, while saving over $400.  The system has paid off the initial investment, and is now giving me a return in excess of 30%.  
         This is one form of renewable energy that is truly competitive with conventional sources.  Here are the reasons why:
            1:  The systems are uncomplicated.  Two panels with antifreeze circulating in them, a heat exchanger, a pair of small pumps, and a standard 80 electric hot water heater tank.
            2: They are efficient.  Hot water panels are approximately 40% -60% efficient vs.  14% for solar electric panels.
             3: The systems are reliable.  My panels, heat exchanger and pumps are all 28 years old, with plenty of life left in them.
            4:  Unlike solar or wind electric, the energy is easily stored for later use.  It usually takes 3 overcast days for my system to go cold.  In 2011, there were only 23 days where the system was below 90 degrees. Even 90 degrees provides a significant savings, as my electric heater does not have to heat 60 degree well water. Heating 90 degree water reduces my electric consumption by 30% vs. heating 60 degree water.

In 2009, the system made 63% of our hot water
It 2010, it made 77%
In 2011, a record 80%
Finally, some suggestions for those considering installing a system.  If your latitude is in the 40's, angle your panels to at least 45 degrees.  Why?  Because if you have them at a lower angle, they will overheat in the summer, and under-perform in the winter.  Ideally, the system is running through most of a hot summer day, which prevents the panels from overheating.  When the antifreeze is not circulating, the panels can easily reach 220 Fahrenheit, and the antifreeze will begin to break down and become acidic. Not good for the pipes.  An angle of 45 degrees or higher will reduce the amount of direct sun exposure in the summer, while still providing plenty of hot water.

The 45 degree angle helps in the winter, when the sun is low in the sky.  The panels will capture a good amount of winter sun.  Even in my worst winter, 2009, the system was making 40% of my hot water.  Another advantage is that snow readily slides off the panels.  I rarely lost more than a day due to snow coverage.  If it was a sunny day, I would often turn on the pumps manually to defrost the panels. It wasn't long before the snow slid off and the panels were capturing more energy than the defrost mode used.  This method worked with as much as 8 inches of snow on the panels.

I believe that for most people, evacuated tube collectors are unnecessary and simple flat panels are a better choice. Here my explanation why.

This is a picture of the panels on the day after a snowstorm that deposited about 12 cm of snow on the panels. It is 10:45 AM, and the outside temperature is -12c.  A small area of the panels was exposed by the wind, and that was enough to begin to warm the panels the panels to 38c. The pumps started up, and soon the entire system was at 38c.  That rapidly melted the remainder of the snow.  By 11:40, the panels were 80% clear, and the system temperature was now 57c.
These pictures also illustrate why I believe that solar panels should be installed at a minimum angle of 45 degrees in northern climates.  The snow readily slides off, and the panels are well positioned to capture the winter sun.  They produce more than enough hot water in the summer, even though the sun is then higher than 45 degrees.

In only 3 hours, the system had heated  300 liters of water from 23c to 43c, while the outside temperature never exceeded -10c, and the winds averaged 22kph. 

Unfortunately I have no comparable data on the performance of evacuated tube collectors under these conditions, but this performance is very good, especially considering these panels are now 29 years old.  It is unlikely that I will ever have to replace these panels, for given their current condition, I estimate their working life to be approximately 50 years.  But if I did replace them, I would stay with flat panels

Upper right. Discharge temperature from 300 liter storage tank.

Lower right. Return temperature of antifreeze in panel loop.  This temperature is after the heat exchanger. The water entering the exchanger from the panels is about 58c.