Charging the Battery, or Not So Much

It should have been a simple task. Get a solar PV panel, charge controller, and a lead-acid battery to support the electronics on my solar hot water system, but it wasn't .

The solar PV system on solar thermal hot water heater has never worked properly. The system struggles to charge the batteries which typically end up being dead by the time I get home from work in the evening. I know the batteries work properly, because if I charge them on an external charger, everything works fine. The batteries have more than enough energy to run the differential controller all night.  

Some Background
  Not wanting to plug my new solar powered solar hot water system into mains power, I purchased two 20 watt solar PV panels and mounted them next to the solar hot water panels.  But for the solar PV panels to do their job, they need both a charge controller and a battery.
 
   For the charge controller, I found this 10 AMP CMP12/24 unit on ebay.  Sure it is an inexpensive Chinese made unit, but none of the brand name people had much in the way of small charge controllers.  It seems to do the job. 

   The charge controller is intended to be paired with one or two 12 volt lead acid batteries.   In my experience, these batteries only last about 4 years, so I wanted to try something different.

    Eneloop NiMH batteries have been serving me well in other applications, so I thought I would give them a try in this application.  The question was, how many batteries would I need to simulate the 24 volts of two lead acid batteries.  Since these batteries are typically listed as 1.2 volt, simple math show 20 batteries × 1.2 volts = 24 volts.  Perfect, right?  perhaps not as we shall see.

Since the beginning of operation of solar hot water system, there has been a problem with startup in the morning.  There didn't seem to be enough energy in the batteries to have the controller up and running all night long.  Therefore in the morning, the exhausted battery pack couldn't open the zone valves.  Typically the differential controller would try to open zone valve, but as soon as current started to flow from the batteries, the battery pack voltage would dip below 21 volts and the charge control would kill the power to the load.  This setup a cycle that could repeat about every 20 seconds for quite a long time, where the valves would start to open and then the power would get cut..

Collecting the data.
     As is often the case in problem solving, the first thing that needed to happen was to collect data so the problem could be understood.  I have previously used the Hobo U12-4 data logger to record temperature data, and the same system is capable of recording voltages.  Unfortunately, the Hobo data logger can only record voltages between 0 and 2.5 V.  To record higher voltages required a special cable to be made with a 2.5 mm stereo plug and a voltage divider circuit between the center pin and the shield.  The voltage divider used a 1Kohm and a 22 Kohm resistor.  This resulted in a gain of 23× and made the range expand to between 0 and 57.5 volts.   This is a larger range then needed, but those resistors were readily available.

   One side note about the Hobo U12-4 is that it forces all the grounds to be the same for all the four data channels it is acquiring data from.  This was a bit of a problem for my application.  I intended to record 1) the solar PV voltage, 2) the battery voltage, and 3) the load voltage.  Unfortunately that did not work with this system as forcing the grounds to be the same for those three items made the charge controller malfunction.  So, for my data collecting, I settled on only collecting the load voltage.


On the worst day
After recording data for several days, a pattern emerged.  On the worst days the load voltage looked like the following graph.

In the early morning hours, the voltage was switched off because it was below the 21 volt minimum.  In fact, the voltage was probably below 10 volts.  The pack was well and truly drained.  Starting about 7:00 in the morning, light began to fall on the panels.  Unfortunately, the battery voltage was so low, the charge controller tries to put the system into 12 volt mode which makes it impossible for the valves to open because they require 24 volts.
   The charge controller repeatedly turns the voltage on and off as the battery voltage fluctuates as the differential controller starts to turn on and draw loads.  This continues until about 11:00 when enough solar flux lands on the solar panels and somehow the charge controller decides that this is a 24 volt battery pack after all.
    Throughout the day, 27.5 volts is applied to the battery pack to charge it, perhaps ending at 5:00 pm.  From that time on, the differential controller draws about 1.5 watts from the pack which result in the battery voltage dropping slowly.  Around approximately 8:00 at night, it looks like one of the cells suddenly drops voltage significantly.  Later, at approximately 9:00 at night, another cell voltage collapses and the voltage drops below 21 Volts.
    From that point on, there is repeated cycling.  When the voltage drops below 21 volts, the charge controller cuts the power to the load.  With the load removed, the battery voltage recovers a little and once above 23 volts, the charge controller restores the voltage to the load.  This quickly drops the battery voltage back below 21 volts and the load is cutoff from the batteries, and the cycle continues.

On the most days
Most days didn't see the charge controller automatically switch to 12 volt mode as can be seen in the graph below.

 But in every other regard, the behavior is similar with a difficult cyclical startup in the morning and a cyclical end to the day.

 
So what is the problem
  One of the more troubling aspects of the data is that only 27.5 volts is applied to the pack during the day.  This amounts to (27.5V/20cells) = 1.38 volts per cell.  This should be enough to charge the batteries, but I have seen my smart charger apply 1.4V or more to these cells to properly charge them.  So perhaps a little more voltage per cell would be useful.

   However, there isn't any charging voltage adjustment on the low priced charge controller that I'm using.  An alternative is to use fewer batteries.
    Given how the battery pack is constructed, it would be useful to have a "dummy" battery that is just a shorted cell.  Fortunately I found some on ebay and purchased them as shown in the picture below.

So what is the solution
I decided to try 19 eneloop batteries and one dummy battery.  That would give (27.5V/19cell=) 1.45 volts/cell, which is significantly more than the 1.38 volts/cell previously available.  The results were immediate and dramatic.

After a day to let the batteries charge, for the first time, the battery pack was able to run the differential controller all night long and into the next day as can be seen in the graph.  The battery voltage never dipped down below 24 volts which is well above the 21 volt cutoff for the charge controller.  The difficult startup seen most mornings, with the cyclical behavior, was eliminated and it looks like the valves opened cleanly on the first time (although that behavior was not recorded).

Into the future
The only remaining question is can the batteries be maintained over, say, a week of cloudy days and then startup smoothly when the sun returns.  One possible solution to that problem would be a PV panel cut-off.  This would be a circuit that would cut power to the load unless the sun is shining brightly on the solar PV panels and producing at least some voltage (e.g. 24 volt).  This would prevent the drain on the battery in the evening and could make it possible for the battery pack to stay charged almost indefinitely.

Update:  Sure enough, after a couple of back to back cloudy days, the batteries were exhausted and power to the differential controller was cut by the charge controller.

 

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.

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.

To Float, or Not To Float, that is the question for the Hydrometer

As it is now the middle of the winter, and getting very cold, concern naturally arises about the quality of the anti-freeze in the solar hot water system.  It has been over a year in service and the hot summer temperatures might have degraded the anti-freeze properties of the propylene glycol/water mixture.  If cold temperature outside causes the solar working fluid to freeze, it will ruin some very expensive solar panels.

The tool for checking out the quality of the anti-freeze is called a hydrometer.  This uses the specific gravity of the working fluid as an indicator of the level of freeze protection.

The most common hydrometers are used for testing the anti-freeze in car radiators.  However, those hydrometers will not work for the solar panels.  Cars use highly toxic ethylene glycol which has a different specific gravity than the non-toxic propylene glycol used in the solar panels.  The hydrometer must say on it that it is for propylene glycol or the reading will be wrong.

As a good starting point for any shopping these days, I surfed on over to Amazon.com.  There I found the Thexton 107 Cold-Chek for about $15 with mostly 5 star reviews.

 The basic working principle is easy enough, suck up some fluid into the device and see how many disks float.  If one disk floats, the fluid is good to +20°F, if all five disks floats, the fluid is good to -50°F.  To get started, I tried some water as shown on the left side of the picture below.  As expected, none of the disks floated indicating no freeze protection at all.
 

Next I removed a small amount of solar working fluid from the fill/drain port on the solar hot water system.  As can be seen on the right side of the image above, all the disks float and I should be good to -50°F which is very reassuring .

I will try to remember to come back and check this again next year.

One slight problem with the hydrometer was that it was easy to get false readings.  Air bubbles can attach themselves to the disks and cause them to float.  This made my measurements on the water totally unreliable.  After playing with it for a while, I discovered the vigorously tapping the side of the plastic housing caused the air bubbles to rise and then the disks would give a true reading.  I learned that a disk floating may be an error, but if you get it to sink, that is the correct reading.

Next post, I will talk about my worryingly low pressure in the solar working fluid.