The Importance of Duty Cycle in Solar Thermal Systems

In a recent letter from Kevin about single tank vs two tank solar hot water systems, he brings up a very important concept that we didn’t cover in the previous discussion. It is known as “duty cycle”, or the load vs time.

To start this off, let’s go back to Kevin’s comments about single tank systems.

“Well, in an 80 gallon tank, let’s say the burner has heated all 80 gallons by 6 am.  Now everyone in the household takes their morning shower and uses 40 gallons.  Since the thermostat is high up in the tank, it doesn’t see the new cold water that is now in the bottom of the tank.  The burner doesn’t come on, and solar will heat up the bottom of the tank, and if there is excess solar heat for the day, it rises up and helps keep the burner off all night as well.”

Here, Kevin is talking about the time of day that hot water is used and the time of day that solar will add energy.  He says [paraphrasing] the family gets up, takes showers in the morning, eats breakfast, turns on the dishwasher and goes off to school or work.  The single tank solar thermal system was heated over night with conventional energy, which is now gone, so the solar system can take over during the day and heat the tank back up.

Well and good.  But suppose, like mine, the family takes all their showers at night, and eats cereal and toast for breakfast which doesn’t require cooking or running the dishwasher until evening.  Now we have the load reversed from above.  Most of the energy is used in the evening, not the morning.  If we have a working family, we can expect most of the energy to be used at night for laundry, dishwashing,  maybe showers.   On the weekend, hot water might be used all day.

If conventional energy has heated the water in the single tank solar hot water system, the solar has nothing to do.  It will probably add a little energy to the tank during the day, but its capability has been crippled by a hot water heater that stays hot all day.  This doesn’t happen in a two tank solar thermal system. No conventional energy is put into the solar storage, none.  It is not “artificially” heated.   When the family uses the energy the night before, it stays gone until the next morning.  When the sun comes out the next day, the solar tank is “cold”.  That gives optimum efficiency to the solar hot water system.

But, lets expand on this idea.  If there is a home maker working all day cleaning dishes and washing clothes, then the “load profile” is intermediate between the “morning” family and the “evening” family.

When energy is used during the day, an interesting thing happens.  While the solar thermal system is putting energy into the solar tank, it is also being taken right back out.  People have called me before and complained that their solar tank doesn’t go above 90 to 100ºF during the day.  I ask if they are using hot water during the day.  If the answer is yes, then I tell them that if the temperature went to 120 or 140ºF, then something would be broken.  They are taking the heat out as it is being put in.  When the load is in sync with the solar day, then the system has the highest efficiency because the average solar tank temperature is less and the collectors are more efficient at lower temperatures.

This is a hard concept to understand. Most people think that heat (i.e. energy) = temperature. Temperature is half the equation.   A good example is the little pocket torches, or jeweler’s torches, that reach 5000ºF.  But the energy is so small that it would not heat a pot of water.

Suppose, in the winter the cold water coming into a water heater is 40ºF, and the water is heated to 140ºF.  I am using round numbers to make the math easy.  That is a 100ºF rise in temperature. Ignoring the density change of water with temperature, the amount of energy needed to heat the water from 40 to 90ºF (half way) is the same as the energy to heat the water from 90 to 140ºF (the other half). So, a solar thermal system that heated water to 90ºF would put 50% of the energy needed into the water on that day.  The conventional system, gas or electric, would put in the other 50% of the energy needed.  We always want the solar system to do the colder “bottom part” of the heating, where it is most efficient.

On many commercial jobs, such as a laundry, hospital/nursing home, or cafeteria, the load is constant throughout the day and the solar energy is used as it is being captured.  We can then account for some of the load as representing increased storage capacity.  Think about it.  If we were to double the volume of the solar hot water system, the same amount of input energy would only raise the temperature half as much.  If we reverse our thinking and have a solar hot water system that takes the energy out as fast as we put it in, then that is equivalent to more storage volume.

When is this concept useful?  Mainly when there is no space for a proper size tank.  A smaller tank might do just as well as a “right size” tank in such circumstances.  When doing the F-Chart energy contribution calculation, we can pick an appropriate storage volume, which might not match the tank volume.

Now the $64,000 question: How do we know how much extra storage credit to give to the day load profile?  The answer is – drum roll – I don’t know!  I mentioned in a previous blog that solar engineering is both an art and a science.  This is where experience and insight comes in to play.  Every job has to be judged on its own merit.  With the gallons per day load in front of me, I might pick 25% of that as added storage.  If the customer wobbles around in duty cycle from day to day, I might be more conservative and pick 10%.  Or, I might give it no credit at all.

Solar professionals have to make these decisions daily, and we don’t have a lot of data on the results of these decisions.  As we move into this new era of solar development, the one thing that has changed the most over previous eras is digital computation.  We now have sophisticated Btu meter systems that can measure the energy actually going into the load.  In a few years, we will have some real data to talk about for solar hot water systems.

Let me know your thoughts!
Dr. Ben