Measuring Solar Thermal Energy

Active solar thermal energy is captured with systems and methods that use the sun's energy to heat a fluid.  This fluid can be air, water, glycol, antifreeze or a refrigerant that changes between liquid and gaseous states.   Various types solar collectors can be used to heat this fluid which is then circulated by convection, fans or pumps to transfer it from the point of capture to the point of use.

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Passive solar, on the other hand, refers to the direct absorpsion of solar power energy by a material that has thermal mass which can store it such as a tile or concrete floor.

Solar power panels capture and convert the sun's energy directly into electricity.  Our discussion here will focus on active solar thermal energy systems and how to measure their solar energy efficiency. The various types of solar collectors we discuss can be indexed directly from the following links:

Flat Plate Collectors  l   Evacuated Tubes  l   Low Temp Panels  l   Measuring Kwh  l   Measuring Btu's

Common methods of extracting active solar thermal energy for residential use include flat plate collectors, evacuated tubes, solar energy water heater panels and integrated storage collector panels.  Other methods such as parabolic troughs, power towers, or solar powered absorpsion chillers are considered too complex and expensive to deliver solar energy for homes.

Before delving into quantifying solar thermal energy output, lets examine the different types of residential solar collector panels.  For a more in-depth discussion of solar thermal systems and how they work check out the details at Sunup-Solar Power.com or Easy Solar Options.com.

Flat Plate Collectors

Flat plate collectors have been the mainstay of solar heating for decades. They operate by using flow tubes to circulate water or antifreeze over a dark, insulated absorber plate enclosed in a glazed box.  

Direct systems circulate the actual water to be heated through the collector.   Indirect systems circulate antifreeze or glycol through the collector and transfer the heat to the water with a heat exchanger.  Indirect systems can operate year round in climates with sub-freezing temperatures.

When measuring the true output of an active solar thermal energy collector, power used for pumps, fans and controls must be considered.  These kilowatt-hours used must be converted to Btu's and subtracted from the total energy output unless the circulation system is powered by a photo voltaic array.

Evacuated Tubes

Evacuated tubes take the flat plate concept and extend it to a higher level of solar energy efficiency.  They produce higher temperatures and can be used in both residential and commercial applications.

Inside the collector panel evacuated tubes are tied into a header pipe.   Each tube contains a small copper pipe which is enclosed in a double-walled glass tube.  The space between the glass tubes is evacuated to create a vacuum.   This vacuum serves as an insulation barrier, just like a Thermos bottle, minimizing heat loss and increasing efficiency.

The inner glass tube contains a black absorption layer to collect heat from the suns rays.  This heat is transferred to the fluid circulating inside the copper pipe.  Water is heated directly or antifreeze/glycol is heated and, in turn, heats water through a heat exchanger.

Since evacuated tubes are round they can capture more of the suns energy throughout the day than flat panels.  Flat panels face the sun directly at noon but are at some lesser angle of incidence throughout the rest of the day.  Round evacuated tubes expose the same amount of absorption area to the sun from early morning to late afternoon increasing efficiency.

Low Temperature Panels

Some solar heating applications do not produce high temperature output.   Swimming pool solar energy water heaters, for example, are designed to heat large volumes of water to no more than 90 degrees F.  They are simply extruded, black polypropylene panels that absorb solar power energy without glazing or metal piping.   Circulation is powered by the existing pool pump.

Another type of low temperature panel is called the integrated storage collector.   It is a self-contained unit that consists of a storage tank and glazed collector.   Limiting output temperatures to no more then than 100 degrees F. these panels can be used to heat swimming pools or send warm water to supplement a solar energy water heater.   Since water is circulated directly through the collector and tank, these units must be drained during the months of sub-freezing temperatures.

Measuring Kwh Output

Measuring the output of a solar thermal system can be done in one of two ways.  The first method is to determine the amount kilowatt-hours saved and the second is to measure the actual Btu's produced.   We can match system results either way by knowing the following conversion formulas:

1 Btuh = 0.000293 Kilowatt-hour or

1 Kilowatt-hour = 3,413 Btuh

If your solar thermal energy system is supplementing an electrical appliance such as a hot water heater, its output can be measured deductively by using load profiling or sub-metering techniques or a circuit level home energy monitor system.

Regardless of technique, the strategy is to first measure the amount of electricity required to run the system without any solar thermal energy input.  Once you have a baseline established, activate the solar thermal energy system and log the energy baseline again.  Subtract the new baseline from the old baseline to determine the output the system.

As a minimum, we recommend using use at least two weeks of data to establish each baseline under similar weather conditions.  A month is even better.  Take the total energy used and divide by the number of days the measurements were taken. This gives you an average daily consumption for determining the net difference.  

To verify your calculations multiply the daily net difference by 30 days and compare it with your electric bill.  Are your calculations reflected with a similar drop in consumption on the bill?  Apply the cost per kilowatt-hour to the net difference to determine your monthly savings.   Divide this savings into the cost of the solar thermal energy system to find out how many months it will take to recover your investment.

Measuring Btu Output

If your solar thermal energy system supplements a gas water heater or heats your pool, output will need to be measured in British Thermal Units or Btu's.  You may recall that a Btu is the amount of heat required to raise one pound of water one degree Fahrenheit.  When expressed as Btu per hour, or Btuh, it becomes an energy unit of measure.  Btuh's can be converted to kilowatt-hours, or visa versa, as noted above.

Btuh measurement must take into account initial temperature, final temperature, flow rate and specific heat of the liquid.  Here is a diagram showing how this would be set up on a flat panel solar collector.

The initial temperature is for the cold water entering the collector and final temperature is for the warm water leaving the collector.   The flow rate through the collector is expressed in gallons per minute (GPM) and the specific heat of water is unity or one.  If a different fluid is used then its specific heat would be applied.

Mathematically this heat flow can be described as follows.  The first step is to convert the flow rate from GPM to Lbs/Hr:

Flow Rate (GPM) x 60 Min/Hr x 8.3 Lbs/Gal = Flow Rate (Lbs/Hr.)

Next, apply the change in temperature to the flow rate:

Flow Rate (Lbs/Hr.) x (Final Temp - Initial Temp) x Specific Heat =

Heat Flow (Btuh)

Lets look at an example.  If our solar collector raised the temperature from 90 to 110 degrees F. and maintained a flow rate of 2 gallons per minute, how many Btuh would it produce?

2 GPM x 60 Min/Hr x 8.3 Lbs/Gal = 996 Lbs/Hr

996 Lbs/Hr x (110-90) x 1.0 = 19,920 Btuh

What would be the equivalent savings on our electric bill?  Simply apply the conversion formula from above to our example:

                     19,920 Btuh     = 5.84 Kwh
3,413 Btuh/Kwh

Multiplying these kilowatt-hours times an electric rate of ten cents per kilowatt-hour indicates the solar collector would save about 58 cents per hour when fully utilized.  However, this savings must be tempered with piping heat losses, outside air temperature, sun angle, cloud cover and the intermittent on/off cycling of the water heater element.

If you are interested in measuring the performance of your solar thermal energy collector, please visit our page that shows you how to build a simple Btu meter for about $100 to $125.   This meter captures inlet temperature, outlet temperature and flow rate.   Readings are taken manually and tallied in a spreadsheet for analysis.  Btu loggers are available that capture and record this information automatically but will cost much more.