Proper Hydrogen Venting - Avoid Stack Effect or Pay for more Heat!
Aerodynamics states that hot air rises. It's simple to understand, and the basis for much in the physical world; from weather pattern to HVAC this rule dominates study. As such, many have studied stack effect, or chimney effect, the aerodynamics and forces created in a tube, when there are different temperatures on either end of the pipe. The image to the right shows a normally operational chimney pushing air up through the stack, which cools as it rises.
Modern batteries, being installed indoors, for maximum operating power, experience some of these temperature differentials as the days transpire. Either it is cold outside, and the battery box is warm. Or it is hot outside, and the battery box is in an air conditioned space. Therefore, the temperature differentials we see in the stack to the right actually occur naturally within your battery boxes, as you cool or heat your home. During the extreme seasons, summer and winter specifically, these temperature differentials are at their greates, and the potential for heat loss is at it's height.
This effort has been taken to show you how just a 2" vent can cost you THOUSANDS!!! Install your battery box properly or prepare to pay far more than you thought!!!
For this example we will scrutinize a 2 inch PVC vent system, 14' tall, with conventional downpipe, and updraft pipe, separated.
Both the input and output flow pipes are 2".
This computes to a .043 sq ft opening to the world.
Math explained:
Area of a circle = PI x R2
Pi x R2 x 2 pipes for opening size = (3.1416 x ((2" x .5)2)) x 2 pipes) = 6.283 sq inches
Square inches to sq feet by dividing by 144.
Result = .0436 square feet.
Now we need an equation, and some temperatures to see what can happen here...
Outside Temp 25° F
Inside Temp 65° F
And since it's freezing, the air is pretty dry.
Our equation looks like this:
Q = stack effect draft/draught flow rate, m3/s
A = flow area, m2
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 9.81 m/s2
h = height or distance, m
Ti = average inside temperature, K
To = outside air temperature, K
Obviously those units are a bit different, so we convert them to U.S. customary units:
Q = stack effect draft/draught flow rate, ft3/s
A = area, ft2
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 32.17 ft/s2
h = height or distance, ft
Ti = average inside temperature, °R
To = outside air temperature, °R
When we do the math, we find that this little setup moves over 28 Cubic Feet of Air Per Minute.
When you spent all that time on insulation, windows, heating, etc. that is BAD!!!
The loss of heat equates to over 1230 BTU/hr.
A heater pushing 360 watts would be required to compensate for that.
To run that heater from batteries would require another 32AH per hour, so 787AH each day.
Assuming you wanted the battery bank to last, you would only want to discharge to 50%, so a 1575 AH battery bank increase would be required.
To power that battery bank, an increase of solar panels, to the tune of 3150 Watts in a 6+ hour sun zone, would be required to charge it back up.
Assuming you need a 2" outflow pipe, put a 1/2 inch copper pipe inside a 2" copper pipe.
The choice of metal vent pipes, instead of PVC makes them a heat sink (metals are good heat conductors).
To install it, remember that H2 goes up.
Attach the 2" copper pipe to the top of the box, and send it vertically to the vent opening.
(bends of 30 - 45° are ok, but less is more).
Next, bring the 1/2" copper pipe through the 2" pipe to the bottom of the box.
Attach interior vent pipe at top of the vent opening, and even turn down to avoid rain.
The resulting battery and power savings are astounding...
97% of the negative problems with the non heatsinked Vent System are eliminated.
Sq ft opening size; 0.0013 sq ft. (33%)
Air moved 0.882973551 CFM (3.1%)
H2 that can escape: 26.48 CFM (93.75%)
Heat Loss: 38.4 btu/hr (3.1%)
The metal vent pipe's cost is offset by the savings of the heat sink venting system.
Static installation, no moving parts, and natural forces as power.
BD Batteries - A Broomfield-Designers Company
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Aerodynamics states that hot air rises. It's simple to understand, and the basis for much in the physical world; from weather pattern to HVAC this rule dominates study. As such, many have studied stack effect, or chimney effect, the aerodynamics and forces created in a tube, when there are different temperatures on either end of the pipe. The image to the right shows a normally operational chimney pushing air up through the stack, which cools as it rises. 
