Measuring Performance of the Solar Hot Water System

    "So how is your solar hot water heater working out for you?" I am often asked.  This simple question often has me struggling for a simple answer.   For my solar electric panels, I can tell people that the panels make more electricity in the summer than I use and I get a ZERO dollar electric bill about six months of the year.  This seems to satisfy most people.  So why can't I say something similar about my solar hot water panels?  In one word, it's instrumentation.
   Solar electric panels typical record the number of KiloWatt Hours (KWH) produced either daily, weekly, or cumulatively (most likely all of the above).  Solar hot water panels, in contrast, record absolutely nothing.  The differential temperature controller turns the pump on and off as needed, but does not record anything.
     It is technically possible to record the amount of energy produced by the solar hot water panels.  Caleffi makes the WMZ-G1 Energy Heat Meter (257202A $250).  But to use it, you must also buy the Vortex Flow Sensor (NA15015 $117) and the Relative Pressure Sensor (NA15014 $90), both of which also measure temperature.  And if you want the data stored, you must also get the DL2 data logger (257201A $410).   All of which ends up being about $1000 after getting all the fittings to plumb it into your system.  As an Engineer, I love data, but adding $1000 to the price of my system just to get data, seems a little steep.

   Having forgone the expensive option during installation, I put a simple data logger on my 2012 Christmas wish list which my loving wife picked up for me (nothing says you care more than a data logger for Christmas).  This beauty was an Onset Computing Hobo U12-006 four channel 12-bit recorder ($113) capable of recording 43,000 data points all while being powered with a single CR2032 watch battery which lasts more than one year.  Complementing the data logger was a TMC6-HD temperature sensor ($39) which was installed in the bottom of the 80 gallon hot water tank.

 

Unfortunately, the data logger cannot record the amount of energy produced, but it can give some very useful insight into the performance of the solar hot water system.  At this point in time, I have a full year and a half worth of data recorded that can give some graphical feedback on the performance of the system.

A Day In The Life

   The graph below shows a typical 24 hour cycle for the solar hot water system.  Starting at midnight, the bottom of the tank is at 92.7°F showing that there is very little heat left in the tank, although the top of the tank is probably considerably hotter and that is what really counts.

   By around 7 am, the temperature has slowly dropped to 91.0°F due to a small loss of heat from the well insulated stainless steel tank.  Then someone wakes up to take a shower.

    By 7:30 am the temperature in the tank has dropped to 86.0°F as a result of the shower.  Most likely, the hot water from the tank was not hot enough for a shower, and some supplemental heating via the oil furnace was needed.  Nevertheless, the water was at least partially heated by the sun.

    Then the sun gets down to business.  Between 9 am and 3 pm, the 80 gallon tank is heated to 158.8°F.  This is pretty impressive with water more than hot enough for a shower.  Over the course of the afternoon, the temperature of the tank drops to 147°F likely due to some incidental use of water (hand washing, dishwasher, clothes washer, etc) and heat traveling up to the top of the water tank leaving the colder water at the bottom.

   From about 7pm to 9pm, the water temperature drifts slowly downward from 147 to 145.3°F likely due to losses through the insulated tank walls or conduction out through the connected copper pipes.

    At about 9:30pm a second family member takes a shower dropping the temperature at the bottom of the tank from 145.3 to 128.7°F.

    Starting at midnight, the cycle will repeat itself the next day in the same way, as long as the sun is shining.

A Year and a Half In The Life

 The graph below shows about 70,000 data points for the temperature at the bottom of the tank.  Remember from the graph above, that each day the tank cycles from cold to hot to cold again as the water is heated up and then used in showers.  So within a one day cycle it is possible for the measurement to be as low as 60°F and as high as 180°F (which is the maximum setting before the heat is sent to the heat dump).

    Concentrate of the lower part of the graph for a moment.  The horizontal axis starts at January 2012, passes through a full year to January 2013 and on for an additional half year until July 2013.  It is clear that in the winter, the incoming water temperature is about 50°F.  In the spring and the fall, the incoming water temperature increases to about 60°F.  In the middle of the summer of 2013, the temperature seldom falls below 80°F.  However, it would seem unlike that the incoming water temperature is so high.  Instead, the more likely explanation is that not all of the 80 gallons of hot water in the tank is needed in the summer and some residual heat is left in the tank on most summer days.

      Now concentrate of the upper part of the graph.  In January, the water is only heated up to about 75°F by the sun.  If you are a glass is half empty sort of person, it would seem that the solar hot water system is "doing nothing".  But in fact, the water is being slightly "pre-heated" before the fossil fuel furnace has to heat it the rest of the way to 140°F.  So some solar heating is better than none.  

     Starting some time in March or April, more serious water heating begins with temperatures pushing to the 140 to 150°F range.  Also notice that sometimes the sun does not shine for several days and the water temperature remains at 60°F.

    In the middle of the summer, the peak temperatures for the day are often above 170°F supplying ample hot water for showers.

    While this data is interesting, it really doesn't supply a quick answer to the question of "how is your solar hot water system working?"  But at least it shows that the systems is working, and, in the summers at least, working quite well.