By Wiley Freeman, Installer for Alternative Power Enterprises
This system was installed in November, 2011. It was put in during the original construction of the home and uses four Sunda Seido-1 16 tube collectors sloped at about 60 degrees and facing just east of south. This gives the collectors good morning sun exposure as the sun is blocked somewhat by the hillside to the west in the late afternoon.
Even though the evacuated tubes shown in this picture are covered in frost the temperature on the inside of the collector was over 100 degrees at the time this photo was taken.
The solar is used to heat domestic water throughout the year and help with the space heating as well, a system known as a Combi solar thermal system.
Below is a piping diagram that shows the major components and how they are linked together to send solar heat to the Domestic Hot Water (DHW) tank and the house heating system. It also shows the backup boiler.
The system is a “drainback” system, which, means that the heat transfer fluid that moves through the solar collectors is pure water. All the piping to and from the collectors on the roof is sloped so that when the solar pump shuts off the water drains back into the big solar storage tank and the water cannot freeze. The drainback feature is also good for high limit protection. If the solar tank gets to its maximum temperature of 175 degrees the solar pump will shut down and the water will drain back so that the collectors do not overheat the tank.
The big storage tank is made out of stainless steel and insulated with four inches of rigid foam insulation. The Taco iSeries mixing valve ensures that the temperature of the water going to the heating system from the solar tank is adjusted to match the heating requirements according to the outdoor temperature.
The pumps in this system are controlled by a combi system solar differential controller.
Performance of the System
This system has been up and running since November, and according to the installer should be doing everything that it was designed to do. Often, how well a solar hot water system is performing is based on how well the installer says it’s working. For the homeowner it is not often easy to tell how much work the solar is really doing without paying very close attention its daily function and having a keen sense of how the system operates. Touching pipes to see if they’re hot and checking tank temperatures from time to time is usually all a home owner has to go by. This system is equipped with a datalogger, so let’s see what the numbers say about the system performance so far.
Using this chart we can witness how much energy is being produced by the solar and also how much energy is being pulled from the tank to feed the heating system.
Let’s take a look at day 3 in the chart above. Point A shows the tank temperature at the beginning of the day, 80 degrees. Throughout this sunny day the tank gets heated up to point B or about 124 degrees. Once the sun goes down we can see a little bit of heat loss from the tank, the small decline at the top of the plateau between point B and C. At point C the house’s heating system calls for heat and the tank begins to send its heat to the floor. Roughly 4 hours later the tank can no longer heat the house and the tank’s temperature is at 80 degrees again.
Now let’s see if we can pull some numbers from this. The tank has a capacity of about 355 gallons, but it is only filled with water to about the 340 gallon level. To find the amount of energy in this tank we look at the weight of the water and the temperature differential. The equation used is,
1 BTU = the amount of energy needed to raise 1 LB of water 1 Degree Fahrenheit.
In the tank we have 340 gallons which weigh 8.33 lbs each so a total of 2832 lbs. The chart shows that the tank started at 80 degrees and got to 124. This is a difference of 44 degrees. Plugging these numbers into the above equation shows us:
340gal. x 8.33lbs/gal. x 44 degree temperature differential = 124,616 BTUs
On a sunny day in January this system generates about 124,616 BTUs or about 1 ¼ Therms or 36.5 KWH. Natural gas is typically sold in units of Therms and electricity is sold in KWH, so this gives you an idea of the value of these BTUs. By the way, sun energy isn’t sold—it’s free.
The chart demonstrates that, minus a little heat loss, roughly the same amount of BTUs went into the heating system as well. This took place later in the evening when the house called for heat.
The house’s natural gas boiler will also send heat to the house, so if the solar can’t keep up this appliance acts as a backup. Another thing to note is that the temperature of the water going through the floor is between 80 and 120 degrees. These low temperatures are able to heat the house’s floor because of the house’s high mass and closely spaced PEX tubing in the floor. Whereas most houses that rely exclusively on a boiler have tubing spaced at 12” or more this house has a pipe spacing of 6”. This allows the house to be heated with lower water temperatures, which allows the solar to do more work, save more energy and save more money.
Installing contractors:
Solar: Alternative Power Enterprises www.alternative-power.com
Heating system: GT Mechanical (970)-209-6423, gtmmech@hotmail.com
