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Hydrogen Venting
Basics
SOME BASIC PRINCIPLES OF HYDROGEN VENTILATION
The most reliable and Fail-Safe way to ventilate hydrogen from a battery shelter is by using the principles of natural convection, taking advantage of the natural property of light-density gas to rise upward, and heavier-density gas to sink downward. Under most common conditions, Hydrogen has a lighter density than air and tends to rise upward when in contact with air. Warmer air is less dense than cooler air, and so warm air tends to push upwards when in contact with cooler air.
These natural principles can be put to good use by the proper use of vents, ducts, heat exchangers and other components. But only the right combination of components will provide effective and fail-safe hydrogen ventilation. Just as a bowl will hold water when right side up, but will not when upside down, a hydrogen ventilation system will trap hydrogen unexpectedly if the geometry is arranged upside down or backwards.
The ideal hydrogen ventilation system should react only to the presence of hydrogen, and not provide constant ventilation air when it is not needed, so as to avoid the extra heating and cooling problems that this would create. It should not require a fan or other mechanical or electrical components that are prone to random failures. It should not react to air temperature, only the presence of hydrogen.
The following pages show five different configurations intended to ventilate hydrogen from a battery box, but only one is not fatally flawed.
Example 1 shows how a Fan can be used, but in the inevitable event of mechanical and electrical failure, ventilation would cease and the box would become a hydrogen trap.
Example 2 shows how High and Low vents can be used, but this is not very well regulated, allowing too much outside air into the battery shelter, creating extreme temperatures and creating a hydrogen trap at times when the inside is cooler than the outside.
Example 3 shows how a small chimney (called a riser tube) can be used with a low vent to trap warm air in the battery box. Unfortunately this system acts as a very effective and dangerous hydrogen trap as well.
Example 4 shows how a riser tube and a high vent can be used, but this will only ventilate effectively when the inside is warmer than the outside. At other times it, too, will become a dangerous hydrogen trap.
Example 5 shows that when the chimney is reversed (now called a dip tube), and high vents and a heat exchanger are properly arranged, that hydrogen is ventilated without any of the drawbacks or failures of the other 4 examples.
The Zomeworks patented H2Vent™ passive hydrogen ventilation systems are designed according to the principles shown in Example 5. This system is available on all Cool Cell™ passive temperature regulating battery enclosures and is the only system that cannot fail and become a trap for hydrogen. The H2Vent™ passive hydrogen ventilation systems can also be designed into other battery charging cabinets, vaults, rooms or shelters. For more information, see the H2Vent™ brochure or web page or contact Zomeworks Corporation.
EXAMPLE 1 - HYDROGEN VENTILATION USING
A FAN
FIGURE 1 – Hydrogen Ventilation Using an Exhaust Fan and Fresh Air Vent.
A battery enclosure using fan ventilation as shown in Figure 1, must be designed to provide enough fresh air to dilute the hydrogen to a safe level even during runaway battery charger conditions. When a constant fan speed is used, this results in excessive amounts of outside air constantly entering the fresh air vent. When the temperature is too hot or too cold outside, the batteries could be damaged or compromised by the constant exposure to extreme temperatures.
To maintain a safe concentration of hydrogen, the fan must supply 25 times the volume of hydrogen generated under the worst possible conditions. A bank of batteries using a 1 KW charger that fails to shut off can generate 3.4 liters per minute of hydrogen, requiring 85 liters per minute of fresh air to prevent an explosion. This amount of continuous ventilation virtually eliminates the effectiveness of any insulation that may exist in the walls and roof of the enclosure.
The fan is subject to eventual mechanical or electrical failure, and so are any sensors or other fan controls in such a system. It is always possible for the ventilation fan system to fail during thermal runaway conditions. If this does happen, explosive conditions will be the result.
EXAMPLE 2 - HYDROGEN VENTILATION USING HIGH AND LOW VENTS
A battery enclosure using high and low vents is shown in Figure 2A & 2B. This type of ventilation system takes advantage of the natural property of light-density gas to rise upward, and heavier-density gas to sink downward. Under most common conditions, Hydrogen has a lighter density than air and tends to rise upward when in contact with air. Warmer air is less dense than cooler air, and so warm air tends to push upwards when in contact with cooler air.
The ventilation system shown in Example 2 is driven more by air temperature differences than by hydrogen concentration, and can therefore cause unwanted temperature extremes to occur in the battery box by providing too much fresh air when it is not needed.
FIGURE 2A – Hydrogen Ventilation Using High and Low Vents. Cold Day Ventilation by Convection.
Figure 2A shows that on a cool day, the lighter warm air and lighter Hydrogen will rise together out the high vent, drawing fresh cool air in through the low vent. Both the temperature of the warm air and the presence of hydrogen will drive the ventilation rate. Increases in either the temperature or the concentration of hydrogen inside the box will increase the flow of fresh air. Under these conditions, this vent system may work quite well.
FIGURE 2B – Hydrogen Ventilation Using High and Low Vents, Warm Day Hydrogen Trap.
Figure 2B show that on a warm day, when the inside temperature
is cooler than outside, the direction of air flowing through the vents can
reverse. When this happens, warm air trying to enter the top vent pushes back
the Hydrogen trying to rise out the same vent, causing the hydrogen to stagnate
and collect inside the box. The box becomes a hydrogen trap and diffusion soon
mixes the hydrogen with the air throughout the box. Under the right conditions,
this will cause explosive levels to be reached inside the box.
EXAMPLE 3 - HYDROGEN VENTILATION USING LOW VENTS AND RISER TUBE
A battery enclosure using low vents and a riser tube is shown in Figure 3A & 3B. This type of ventilation system makes an attempt to use the properties of natural convection as described in Example 2. Placement of the inlet and outlet vent tubes at the same height (low on the wall) prevents some of the unwanted thermal convection described in Example 2 which causes too much air flow and excessive temperature extremes inside the box.
Since Hydrogen tends to rise inside the box, this system works against the natural forces at work in the battery box. A high vent is really needed to effectively remove hydrogen from the top of the box by natural convection.
FIGURE 3A – Hydrogen Ventilation Using Low Vents and Riser Tube, Cold Day Convection Trap.
Figure 3A shows that on a cool day, the lighter warm air and lighter Hydrogen will rise together to the top of the box and be trapped there creating a very dangerous explosion hazard. Natural diffusion of the hydrogen will cause it to mix with the air throughout the box. Neither the warm air nor the light-density hydrogen gas will sink down the riser tube by natural convection.
FIGURE 3B – Hydrogen Ventilation Using Low Vents and Riser Tube, Warm Day Hydrogen Trap.
Figure 3B shows that on a warm day, when the inside temperature is cooler than outside, air flow through the vents may occur, driven entirely by the temperature difference between the inside and outside air. The Hydrogen will tend to rise and be trapped in the top of the box. Natural diffusion will spread the hydrogen throughout the box and the airflow on a warm day may dilute it, but not with any reliability or efficiency. Under the right conditions, this system can cause explosive levels to be reached inside the box.
EXAMPLE 4 - HYDROGEN VENTILATION USING HIGH VENT AND LOW VENT WITH RISER TUBE
A battery enclosure using a high vent and a riser tube is shown in Figure 4A & 4B. This type of ventilation system also attempts to use the properties of natural convection as described in Example 2. The use of a riser tube helps to prevent some of the unwanted thermal convection described in Example 2 which can cause too much air flow and excessive temperature extremes inside the box.
Figure 4A – Hydrogen Ventilation Using High Vent and Low Vents with Riser Tube, Cold Day.
Figure 4A shows that on a cool day, the lighter warm air and lighter Hydrogen will rise together to the top of the box and exit from the high vent. This will happen only if there is a wide enough temperature difference between the warm and cold air to draw heavier cool air up the riser tube, into the box.
Figure 4B – Hydrogen Ventilation Using High Vent and Low Vents with Riser Tube, Warm Day.
Figure 4B shows that on a warm day, when the inside temperature is cooler than outside, air flow through the vents will stagnate, allowing hydrogen to collect at the top of the box. Warm air trys to rise into the top vent, blocking the hydrogen from coming out. The Hydrogen is too light to sink down the riser tube, and most of the cool air in the box is trapped below the top of the riser tube preventing any significant reverse airflow. When these stagnation conditions occur in conjunction with a battery charger in thermal runaway, explosive levels are likely to build up inside the box.
EXAMPLE 5 – HYDROGEN VENTILATION USING HIGH VENTS, DIP TUBE AND HEAT EXCHANGER
A battery enclosure using high vents, a riser tube and a heat exchanger is shown in Figure 5. This type of ventilation system makes complete use the properties of natural convection as described in Example 2. Placement of the inlet and outlet vent tubes at the same height (high on the wall) prevents some of the unwanted thermal convection described in Example 2 which causes too much air flow and excessive temperature extremes inside the box. A heat exchanger transfers heat from the warmer vent tube to the cooler vent tube, reducing the temperature difference between the tubes, which reduces thermal convention even more. Since there is no height difference between the outlets, and very little temperature difference, airflow driven by thermal convention is virtually eliminated. Only the presence of light-density hydrogen-rich-air forces air out of the box, drawing fresh air in the dip tube. The airflow is proportional to the concentration of hydrogen, and the more hydrogen is generated, the more ventilation occurs regardless of air temperature.
FIGURE 5 – Hydrogen Ventilation Using High Vents, Dip Tube and Heat Exchanger.
All the features described in EXAMPLE 5 are available at CONVECTION H2VENT™ from ZOMEWORKS CORPORATION (U.S. Patent# 5660587).
