Passive Hydrogen Ventilation Systems

(Available exclusively with Cool Cell™)*

Without proper ventilation, an outdoor battery enclosure can accumulate explosive concentrations of hydrogen. The explosive power of hydrogen rich air can blow out the doors, walls, and roof of an enclosure. No company that deploys outdoor battery enclosures in public places can afford to ignore the risk of inadequate hydrogen ventilation.

Batteries in enclosures can go into thermal runaway, which very rapidly produces hydrogen, whenever several conditions are simultaneously present.

1. The batteries are exposed to high temperatures inside the enclosure (above 100 deg. F) for a sustained period, either because the enclosure is unprotected, or because the cooling system is inadequate or has failed.
2. The batteries continue to be charged at high voltage and current because there is no temperature compensation in the battery charging circuit, or because said circuit has failed.
3. Batteries have aged and become dehydrated.

Solar heat loading will cause the internal temperature of a thermally unprotected enclosure to go above 100 deg. F in most of the country during much of the year. According to Bellcore, internal enclosure temperatures will increase to between 21 deg. F and 27 deg. F above the external daily high temperature, depending on the U. S. location. Thus, if the external daily high temperature reaches only 80 deg. F, then the internal temperature of an unprotected enclosure will surely rise above 100 deg. F, setting the stage for thermal runaway.

Temperature compensation in battery charging circuits is a relatively new technology. Where it has been utilized, it has proven to be generally effective, but, as with any system subjected to environmental extremes, not completely reliable. And, in most cases, a potential failure is not easily detectable until it has already occurred. Although thermal runaway remains relatively rare, it occurs with sufficient regularity to be of grave concern.

Once thermal runaway has begun, hydrogen can accumulate in an unventilated enclosure to a concentration above the lower explosive limit (4%) in as little as 35 minutes. While the explosive effect of such a concentration is small, if hydrogen continues to be released, it can accumulate to far more dangerous levels over just a few hours. The concentration can keep rising to the point that ignition will completely destroy the enclosure and its contents (above 10%). Given the large number of enclosures in populated areas, the potential for collateral damage to people and property is unacceptable.

High and low vents are unreliable and hazardous. One expects them to expel lighter hydrogen rich air through the high vent while fresh air flows in through the low vent. This will only take place when the hydrogen rich air is as warm or warmer than the outside air. When the hydrogen rich air is colder than the outside air, the flow is stifled and the hydrogen can easily reach explosive concentrations. High and low vents also admit hot air during hot weather and cold air during cold weather, eliminating the advantage of an insulated battery enclosure.

All Cool Cell™ passive temperature regulating battery enclosures are fitted with one of two patented methods of hydrogen ventilation. These are the Convection H2 Vent™ (U.S. Patent No. 5660587) and the Diffusion H2 Vent™ (U.S. Patent No. 5603656).

Generally, the Convection H2 Vent™ is used in applications where multiple strings of batteries are housed in the Cool Cell™, providing the potential for a rapid accumulation of a large amount of hydrogen. The Diffusion H2 Vent™ is installed in Cool Cells™ designed for fewer, smaller batteries.

Like the Cool Cell™ itself, both H2 Vents™ are completely passive devices. There are no sensors, fans, or other electrical or moving parts. Both H2 Vents™ are virtually failure proof, a critical specification in this application.

The Convection H2 Vent consists of two tube openings at the top of one side of the Cool Cell™. These openings are at the same level, a few inches apart, and of the same diameter. The rear tube extends through the sidewall to the opposite side of the enclosure, and then drops to within a few inches of the floor of the Cool Cell™. The adjacent forward tube extends only a few inches into the Cool Cell™.

The exterior of the openings is fitted with wind and rain guards, and with heavy-duty mesh to prevent intrusion by large insects or other animals. There is nothing to impede the flow of air into or out of the H2 Vent tubes.

This patented ventilation design allows hydrogen to easily escape from the Cool Cell™. Hydrogen rich air is buoyant; it rises. When hydrogen is released by the batteries, it mixes and hydrogen rich air finds its way to the short tube opening, and escapes to the outside. Fresh air is drawn into the rear tube opening, through the horizontal heat exchanger, down the dip tube to the bottom of the Cool Cell™.

The Convection H2 Vent maintains hydrogen concentrations at or below 85% of the lower explosive limit when hydrogen is introduced into a Cool Cell™ at the rate of 200 ml per minute per 48 volt, 125 ampere hour string of VRLA batteries. As hydrogen generation increases, the convection rate accelerates, which in turn reduces the concentration to a steady state flow below the lower explosive limit. As the Cool Cell™ is scaled up for larger battery complements, the Convection H2 Vent is scaled up accordingly.

The horizontal placement of the Convection H2 Vent openings virtually eliminates thermal convection. Comparative tests have shown that the Convection H2 Vent lowers the cooling efficiency of the Cool Cell™ by less than 2 deg. F. The safety assurance provided by the Convection H2 Vent is well worth the small loss of cooling efficiency.

The Diffusion H2 Vent™

The Diffusion H2 Vent operates on a different principle than the Convection H2 Vent. It consists of a panel of high-density fiberboard inserted into the door or wall of the Cool Cell™. The panel is sized for the number and size of batteries housed within the Cool Cell™.

As with the Convection H2 Vent, the exterior face of the Diffusion H2 Vent panel is fitted with a wind and rain guard, and with heavy-duty mesh to prevent intrusion by large insects or other animals.

When hydrogen is present in a Cool Cell™ fitted with a Diffusion H2 Vent, it passes through the fiberboard almost as if it were a direct opening to the outside. This is because the hydrogen molecules are much smaller than the passageways through the densely packed fibers.

Hydrogen acts like water, and air like molasses trying to pass through the same sieve; the passage of air is blocked in both directions. The penetration of warm air from outside the Cool Cell™ is also prevented.

Passive Design - The Ultimate in Reliability
Both the Convection and Diffusion H2 Vents are completely passive in design. This results in the highest degree of reliability, a critical specification in this safety related application. Simply put, if hydrogen becomes present in a Cool Cell™ equipped with either of our H2 Vents, it will escape.

At Zomeworks, our philosophy is to accomplish a task with no external power or moving parts wherever possible. Cool Cells™ are outdoor enclosures, and must be able to function in a wide variety of hostile environments. By keeping our designs simple, our products can operate reliably across a wide range of environmental parameters.