Solar Hot Water Performance Measurement Part 2

   With almost two years worth of data from the solar hot water system, now might be a good time to review how well it is working.

   The solar hot water system is run in series with a traditional oil furnace so that the better the solar system works, the less oil is consumed.  For nearly three years now, the run time of the furnace has been measured, a time span that includes one year before the installation of the solar hot water system and two years since its installation.
  
    The furnace is monitored using a Hobo U9-004 motor on/off data logger.  The logger comes with a magnetic back that allows it to be attached to the furnace fuel pump motor.  A sensor inside the logger detects the varying magnetic fields that leak from the AC motor when it is running and creates a log entry every time the motor turns on or off.  This data has to be processed with an Excel spreadsheet to compute "duty cycle", i.e. what fraction of the time the motor is on during the day.

    One last trick for this measurement is the knowing that the burner nozzle dispenses oil at a fixed rate.  Conveniently, my burner nozzle outputs 1 gallon per hour.  So if my furnace runs for 1 hour per day or a 4% duty cycle (1/24), then 1 gallon of fuel will be consumed per day.  At about $4/gallon, that would work out to about $4/day.  So on to the data.

 The graph above shows a full year's worth of data (365 data points) from the 2010 to 2011 heating season.  There are a few things worthy of note. First is that during the heating season, there is a lot of day to day variation in the percentage of time that the furnace has to run in response to day to day changes in weather.  In contrast, the fuel consumption in the summer is more consistent as it only varies with the amount of hot water used and idle losses in the furnace.

   The discussion of the oil consumption is facilitated by constructing a simplified model of the furnace usage and fitting curves based on that model.  The first assumption is that there are two seasons for a furnace, Winter and Summer.  During the winter, the furnace usage depends on 1) home heating load, 2) hot water usage, and 3) idle losses.  During the summer, the furnace usage only depends on the last two of these items.  The next assumption is that the winter usage can be represented by a simple parabolic curve fit and the summer by a constant value.  These lines are shown on the graph.

 The graph above shows three years worth of data, and the impact of the solar hot water heater can easily be seen by comparing the summers.  The summers of 2012 and 2013 show far lower furnace duty cycle  than in 2011.   Ideally, the consumption would be zero in the summer, but unfortunately, the way the furnace controls are set up, the furnace works to keep itself at 140°F all year long whether it needs to be on or not.  This might be fixable in the future with a "furnace bypass" system.

   Also note in the graph that there is significant variation during the winter from one year to the next.  This is most likely due to the intensity of the winter weather varying significantly from one year to the next.   The impact of the solar hot water system in the winter is likely to be small (due to less sun) and difficult to measure due to the fact that most of furnace run time is going into heating the home.

   The graph above shows the same data with a different vertical scale (gallons of oil consumed per day).  In the summer of 2011, about 1.13 gallons/day is consumed.  However, in the summer of 2012, only about 0.47 gallons per day are consumed due to the addition of the solar hot water system.  Over the roughly 150 days of the summer, this resulted in 177 gallons consumed in 2011 versus 71 gallons in 2012.

    This amounts to approximately 100 gallons of oil saved in the summer alone.  At $4/gallon, this means there is about a $400/year savings just based on the summer data.  Now given the over $8000 out-of-pocket expenses I have laid out for the solar hot water system, it might take a long time to break-even on my investment.  But there are a couple of bright spots.  Firstly, there is likely some additional savings in the winter.  Secondly, I should be able to put a furnace-bypass system in place now to further reduce oil consumption in the summer.  Thirdly, it seems likely that oil prices will rise in the future and that would reduce payback time still more.

   The relatively small financial saving here are sobering.  However, solar power systems are too cool to be judged solely on the dollars.  There is great personal satisfaction from heating your home water using the only the sunlight that falls on your property versus from oil being pumped up out of the ground in the middle east and shipped halfway around the world and then burned in your furnace creating local pollution.