Tuesday, February 8, 2011

Solar Hot Water System Design

The solar hot water system will be the first improvement I will attempt to our current residence. This post presents the sizing of the system.

First parameter to consider in the number of occupants of the dwelling. We are currently 7 persons living in the house. We will soon be 4 only. Average occupancy in the future will likely be between 4 and 7 people. This is a 5 bedroom house so the system will be sized for 6 adults.

Solar Collectors.
Rule of thumb is:

  • 20sqft for each of the first two occupants
  • 12-14sqft for each additional occupant
  • 80% efficiency of the home made panels

[ Reference ]

This gives an area of 110 to 120 sqft for 6 people, and 95 to 102 sqft for 5 people.

We will use solar absorber plates from [ Sunraysolar ], which provides several dimensions:

  • 4FT * 8FT = $275. 3 panels provide 96sqft ($825), 4 panels 128sqft ($1100)
  • 4FT * 10FT = $322. 2 panels provide 80sqft ($644), 3 panels 120sqft ($966)

Three 4*8 is just enough for 5 people at a cost of $825, while three 4*10 is enough for 6 people, at $966. Although the 10FT panels are a better choice for area, the 8FT will be easier to build using 4' * 8' OSB boards.
Sunraysolar sells separately the fin tubes. Building the absorber using fin tubes costs about half, but requires soldering the fin tubes to the copper header. This also allows to build the absorber to the exact dimensions allowed to fit the site, so I may go this route.
At this time, I will assume the collector will use three 4X8 FT absorber plates.
The solar absorber plates will give a higher efficiency to the panels. The 80% figure assumes pex tubing in the collector, while I will be using copper pipes with aluminum spreaders. Although the collector is slightly under-sized, the higher efficiency should compensate.

Storage Tank.
There are different rules for tank sizing. I used the following website:
[ Reference ].
A rule of thumb is 10 to 15 gallons per person per day, or about 75 gallons for 6 persons.
Another rule of thumb is 2.5 gallons per sqft of collector area, which is 300 gallons for our 120sqft collector. This represents 4 days of storage for 6 persons, a good feature to have in our cloudy climate.
A 300 gallon tank will approximately be 3.5 ft cube.
Silicon solar (reference for the collector area) gives a rule of thumb that results in a smaller tank size. I don't think there is a drawback in having a bigger tank, except that it will take longer to heat the water after a series of cloudy days.
The tank will be build with 4FT sections of 2"*4". With the added insulation, the capacity should be slightly less than 300 gallons.

It will be a drainback system, so there is a head to account for in sizing the pump.
Solar absorber flow rate = 1.3GPM per absorber = 3.9 GPM total ~ 240 GPH ~ 900 LPH
The head depends on the location of the absorber. I have three locations in mind at the moment, on the roof (head ~ 25FT), against the garage wall (head ~ 18FT) or against the electric fence (head ~ 12FT). Eventually, there will be a collector in each of these three locations. The pump for each location will differ, due to the different heads. That means that there will eventually be three pumps. To reduce the number of inlets and outlets, all three pumps will be inline pumps, located outside of the tank.
I will assume at this time that I will chose the lowest head, 12FT. Now I need to find a pump that has 240GPM flow rate and a maximum lift of at least 12FT.

The major components are now defined:

  • A 8FT * 12FT solar collector.
  • A 300 gallons storage tank.
  • A 240 GPH Pump, 12FT lift min.

With those values in hand, I can now make a better decision as to where to locate each element of the system.


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