Hydrogen Leakage Sensing and Control: (Review)

Hydrogen is the smallest chemical element found in all organic materials and fossil fuels; however, it’s not available in nature in its pure molecular form. H2, in pure molecular form is the lightest gas, colorless, odorless, and nontoxic. It is also considered as a clean fuel because when it is burned there are no emissions besides water vapor [1]. Pure hydrogen is widely used as a clean source of energy in many applications, such as space exploration, chemical industries, and in transportation using fuel cells to provide the vehicles with electrical energy and reduces the huge dependence on traditional fossil fuels [2]. By Comparison with traditional fuels, hydrogen has three times the energy exists in one gasoline mass unit, also very wide flammable range (4-75 vol. %), seven times wider than methane, with very low ignition energy (0.02mJ). Therefore, it is a very important issue to ensure the safety in utilization of H2 sensing and detection of H2 leakage, and infrastructure should be designed and prepared carefully [3], storage and transportation of H2 should be considered as well. Consequently, accurate, sensitive, rapid response, wide measuring range and other parameters of the Received: September 25, 2019


Introduction
Hydrogen is the smallest chemical element found in all organic materials and fossil fuels; however, it's not available in nature in its pure molecular form. H 2 , in pure molecular form is the lightest gas, colorless, odorless, and nontoxic. It is also considered as a clean fuel because when it is burned there are no emissions besides water vapor [1]. Pure hydrogen is widely used as a clean source of energy in many applications, such as space exploration, chemical industries, and in transportation using fuel cells to provide the vehicles with electrical energy and reduces the huge dependence on traditional fossil fuels [2]. By Comparison with traditional fuels, hydrogen has three times the energy exists in one gasoline mass unit, also very wide flammable range (4-75 vol. %), seven times wider than methane, with very low ignition energy (0.02mJ). Therefore, it is a very important issue to ensure the safety in utilization of H 2 sensing and detection of H 2 leakage, and infrastructure should be designed and prepared carefully [3], storage and transportation of H 2 should be considered as well. Consequently, accurate, sensitive, rapid response, wide measuring range and other parameters of the H 2 sensor have great importance in recent studies and attractive for researchers [4].
Hydrogen energy is expected to be the best alternative for fossil fuels taking into consideration leakage of hydrogen. Nowadays, some hydrogen vehicles have already been produced but unfortunately, because of risks of H2 leakage, it is still not ready to be used widely as a conventional fuel. Leakage of H 2 is serious problem because, the resulted gas mixture of H 2 with air is flammable in wide range comparing to gasoline mixture and may cause fire or explosion as the upper and lower flammability limits of hydrogen with other common fuels shown in Table 1. The leakage of H 2 can be caused by several factors [5] such as, H 2 embrittlement, that caused by long term contact between the container materials and H 2 , which later can cause cracking and leakage [6]. Moreover, the broad usage of H 2 in industrial applications increases the possibilities of accidental leakage such as in storage, production, and transportation fields. The source shape of leakage is the key factor which affects the leakage characterization and has been studied by many researchers. There are two classes of H 2 leakage; a rapid leakage causing combustion which can modeled by Classic turbulent jet flame, and slow unignited leakage that is very attractive for researchers [4,[7][8][9]. K Ramamurthi et al. [7] studied the cloud mixture formation from H 2 and air resulted in an open area from leakage at different temperature and the effects of atmospheric conditions on the dispersion of H 2 using Gaussian model. K. Ramamurthi et al. [7] found that a flammable mixture cloud can be created in stable atmospheric conditions, such as low wind speeds and temperature, in winter clear sky. While S Vudumu et al. [8] studied the flammability limits and transient behavior of the mixture of H 2 with air. They studied two conditions, sudden release at the bottom of cylinder, and the jet release using software simulation.

Main Properties of Hydrogen
In the nature, Hydrogen is the most abundant chemical element, and can be produced from many resources such as water, biomass, coal, and a natural gas as a feed stock, using different sources of energy, including renewable energy, such as solar, wind, and nuclear energy. Thus, a lot of countries in the world will have the opportunity to produce this fuel locally. Otherwise, hydrogen is environmentally friendly source of energy, because the only byproducts of using this fuel are water, heat, and electricity, also as it is replaces the fossil fuels, greenhouse emissions will be decreased and the quality of the air will be improved.

Comparing of Safety Properties of Hydrogen with other Fuels
For more than 40 years, H 2 has widely been used in industry, and as fuel for space exploration. During that, infrastructure has been developed to produce, store, and transport H 2 safely. Hydrogen like other flammable gases should be handled carefully for safety requirements [10]. The main advantages and disadvantages of hydrogen with respect to other most commonly used fuels are listed in Table 2. Hydrogen is the lightest gas (14 times lighter than air) and the smallest molecule; therefore, it has the greatest tendency to release [11]. A hydrogen leakage can be more dangerous than other gases and its detection becomes more challenging [3]. H 2 diffusivity is rapid as 3.8 times faster than NG (natural gas); when H 2 released, it is quickly dilute and at rise speed two times faster than He (helium), six times faster than NG which is 20m/s as mentioned in  Because of the absence of carbon and the existence of water vapor which is absorbing heat when hydrogen burns, hydrogen burns have considerably less radiant heat in respect to a hydrocarbon fire.
As the flame emits low levels of heat, the risk of secondary fires is lower. This fact has a great impact to rescue workers and the public. And like any other flammable fuel, hydrogen may combust.
But its buoyancy, diffusivity and small molecular size make it hard to enclose and form a combustible mixture. Hydrogen fire to start; needs an adequate concentration of hydrogen, the presence of an ignition source, and the right amount of oxidizer (such as oxygen) all at the same time. Hydrogen has a wide flammability range (4-74% in air) and very low energy required to ignite hydrogen (0.02mJ).
However, at concentrations below 10% the ignition energy required is high, similar to the energy required to ignite NG (natural gas) and gasoline in their respective flammability ranges, making hydrogen realistically more difficult to ignite near the lower flammability limit. On the other hand, if the hydrogen concentration increased toward the stoichiometric mixture (most easily ignited mixture) of 29% hydrogen (in air), the ignition energy drops to about one fifteenth of that required to ignite NG (or one tenth for gasoline).
An explosion cannot occur in any tank or container that contains only H 2 [12]. Hydrogen can be explosive at 18.3-59% concentrations. Although the range is wide, it is important to realize that gasoline can be more dangerous than hydrogen since the possibility of gasoline explosion is at much lower concentrations,  Measuring range 0-4 vol% H2 in air;

Stationary:
Measuring range Up to 1 vol% H2 in air (alarm limit) Lower detection limit <0. i. High sensitivity.
ii. Fast response.
iii. Wide dynamic range.
iv. High selectivity and immunity to common interferences (such as humidity and existence of other gases).
v. The sensor must be very stable.
vi. Long lifetime with acceptable low false positive responses.
vii. Operation near room temperatures.
viii. For widespread commercial acceptance sensors, low cost sensor networks are essential.

Traditional Principles for Hydrogen Sensing
Thermal conductivity is the most extended measuring principle for H 2 and then GC (Gas chromatography) is the second most applied which is widely used for H 2 sensing and detection in the industry [18]. Process MS such as GC must be safety certified. MS requires special shelters for air-conditioned, safety monitors, many gases for calibration (e.g., fragmentation gases), and frequent maintenance (weekly) by a skilled operator, Similar to GC, the MS also requires extensive sample systems and continuous flow for sample gases that add cost and make it more complex to carry out.
A laser gas analyzer based on laser light which absorbed by

Main Types of Hydrogen Sensors
Several types of hydrogen sensors working by different mechanisms become available recently, most of them use palladium to hold H 2 . A reliable sensor should have some characteristics such as high sensitivity, linear response in wide range of concentrations, good stability, and low response and recovery times [19]. Another critical factor to be considered is Low power consumption.
Selecting a proper material is the first step in order to achieve these characteristics and reach high performance. In addition, morphology, structure, and chemical composition have a significant role on the operation of this material. Moreover, one of the most common disadvantages of these sensors is reliability degradation due to aging factors. For example, many changes can occur in Pdsemiconductor sensors that are exposed to room air such as loss of sensitivity, with a lengthened response time. Aging can also change particle morphology, chemical composition and interfacial structure. Aging depends basically on the environmental storage, operating conditions and on how well the sensor materials are isolated from the environment [20].  [15].

Electrochemical Sensors
Electrochemical sensors (EC), detect H 2 depending on a specific electrochemical reaction between the cathode and the anode [22,23], hydrogen is oxidized on the surface of an electrode coated with a catalyst, such as platinum [24]. The resulting electrical signal is either a current or a voltage correlated to the concentration of H2 which can be correlated by a nonlinear relationship with the concentration. Usually the reaction at the cathode is the reduction of oxygen, which must be present for correct operation of this type of sensors. Amperometric or potentiometric sensors [25,26], which based on electrochemical reaction produces an electrical current/ voltage proportional to the concentration of the target gas, by oxidation process which includes the electrons transfer between the electrode and the molecule. A porous membrane which is a gas barrier of diffusion is incorporated at the gas-electrode interface so that the process is controlled by diffusion as shown in Figure 1 resulting in an electrical current proportional to gas concentration.

Figure 1:
Electrochemical sensor with membrane for gas transport, and metal housing to prevent electrolyte leakage [15].
Amperometry devices are commercially available from many manufacturers, they are small, and have good sensitivity and a broad linear range [1]. The sensors are stable with lifetimes of up to two years, which is significantly less than the DOE target of 10 years.
Limitations include a restricted temperature range due to a liquid is another disadvantage of classic MOX [28][29][30][31]. Figure 2 shows the schematic diagram of MOX sensor [3]. The performance of a MOX sensor can be permanently corrupted when exposed to certain silicon compounds (which may include common industrial sealants and lubricants) [32][33][34][35][36][37][38][39][40]. C Hsu et al. [51], developed and investigated an interesting transistor-based hydrogen detection system, Experimentally. And they found that even at a low hydrogen concentration, the studied sensor still has a high sensitivity at room temperature. Also, a simple hydrogen sensing system with a LED array (light emitting diode) and buzzer is designed and established. The proposed system could display and alarm the hydrogen concentration quickly. Based on a micro-controller, the studied hydrogen sensing system is shown to be of low cost, fast response, portable, and easy operations.

Catalytic Gas Sensors (Cgs): Catalytic sensors consist of two
thin platinum wires each embedded in a ceramic bead (pellistor) and connected to each other in a Wheatstone bridge [52][53][54].
One pellistor is coated with a catalyst material which selectively catalyses the oxidation of hydrogen as shown in Figure 3. The  technology for fabrication [16].

Thermal Conductivity Sensors: Thermal conductivity (TC)
sensors [55] based on a temperature change of an electrically heated sensing element following exposure to a specific gas as shown in Figure 4. The TC sensor is not heated to a temperature that induces combustion, but only to a temperature where the resistance of the sensing element deviates from the linear limit of observed. In addition to the input power, which is controlled by the TC control circuit, the thermal conductivity of the surrounding gas affects the shape of the I-V curve [57][58][59][60].
Some manufacturers claim a 0-100% range for hydrogen; more recently, devices for lower concentration ranges relevant to safety have appeared on the market. Thermal conductivity sensors are stable devices; they are less liable to contamination. They tend to be non-selective, but by using special coatings vendors have reported improved selectivity. Since thermal transport between a solid and a gas will be dependent on the temperature, density, and composition of the gas, these sensors are affected by environmental parameters such as temperature, pressure and humidity. As with many technologies the environmental effects can be compensated, but this requires independent measurement for ambient conditions. Operation of a TC sensor requires less power than that required for catalytic combustion. The TC sensor is applicable in MEMS technology [61,62], which reduces the power requirements.
Also results in rapid response times less than second. of material, including a hydrogen-sensitive chemical (tungsten or molybdenum oxide) that undergoes a color change when exposed to hydrogen. The colorimetric agent is very selective to hydrogen.
Also the tip is coated with a hydrogen-permeable protection layer to protect the sensor from chemical poisons. As with all fiber optics systems, remote operation is achieved.
These sensors also have a good sensitivity to hydrogen, simple operating mechanism, long-term stability, and reversibility where oxygen is required. Conversely, the response time is relatively slow, and although the sensor provides quantitative data, its accuracy is less than the DOE targets. Several fiber optic sensors based on palladium films also have been developed, but many of them are not yet commercially available. Colorimetric indicators are being developed for hydrogen. Such systems do not require electronic circuitry for detection. The indicator also can be incorporated into an electronic platform, however. These systems are passive and undergo a spontaneous color change when exposed to hydrogen.
Reversible and irreversible formulations have been developed.
The active material can be incorporated easily into a low-cost H 2 indicating paint. The material is stable environmentally and its process is not affected by temperature, pressure, or relative humidity significantly. This system is low cost and easily implemented; though, quantization is not achieved [64][65][66][67]. The usage of electricity in presence of hydrogen is always a matter of concern taking into account possible arcing. With such limitations and possible hazards for other detection techniques, optical sensor has gained importance in safety applications [68].

Palladium Film and Palladium-Alloy Films
The unique and highly selective permeability of hydrogen into Palladium (Pd) has led to Pd-film technology to be applied to several types of H 2 sensors. One basic technology is the Pd film and Pd alloy films [69,70]. The film resistance changes with adsorption c) Palladium optical sensors, consisting of optical active material coated by a layer of Pd, transforms the concentration of H2 to an optical signal [64]. d) Palladium Meso-wire (nanowire) and nano-particle sensors [74]. e) Palladium Nanotube and Nano-clusters sensors [75].
Palladium films also can be applied to other sensor types, for example coating mechanical devices such as Surface Acoustic Wave sensor (SAW) or Quartz Crystal Microbalance sensor (QCM) to achieve good selectivity. Basically, adsorption of H 2 changes the mass of the Pd film so changes the resonant frequency of the mechanical sensor. Small shift in frequency can be accurately measured, so it is excellent for detection lower limit [76,77].

Combined Technology Sensors: Combined sensor (COMB)
is a combination of different detection technologies in one sensor [78], such as, catalytic and thermal conductivity.  are used in and around such facilities. As hydrogen sensors and solid-state sensors become more reliable, and if their cost is reduced they will replace the common flammable gas sensors. Such storage tanks are constantly boiling-off H 2 , otherwise the pressure within the tank will rise well above the nominal 10-15 psig. Therefore, the uses of accurate and precise hydrogen sensors to report the changing partial pressure are very important.

Hydrogen Fuel Cells and Automobiles
Hydrogen cell has also to be monitored to prevent possible explosions as the concentration of hydrogen is in the range 0-4% H 2 at oxygen-reduced mixture. Hydrogen leak sensor is also required in the fuel cell ventilation areas [75]. An accurate leak detector should be able to detect 0-4% H 2 in air backgrounds without any cross-sensitivity to other gases such as, methane (CH 4 ), carbon monoxide (CO), carbon dioxide (CO 2 ), and nitrous oxides (NO, NO 2 ).
The major requirement for a reliable hydrogen sensor in the fuel cell environment is to hold up 100% condensing humidity.
Most of the fuel cells have rich humidity environment and the sensor needs to operate continuously in this environment. In some cases, the H 2 sensor can also be operated at very low temperatures (as low as −40°C). So, the sensor also needs to reach ambient tempera-

Hydrogen Safety and Control
Safety is considered as one of the most significant issues that the promotion of hydrogen use has to face. More utilization of hydrogen fuel will increase the possibility of accidents in the H 2 facilities or infrastructure. Therefore, various safety issues have been discussed in the use and handling of H 2 . These issues include hydrogen permeation, embrittlement, leakage, ignition and explosion. Research on H 2 leakage is mainly important for preventing accidents and setting safety guidelines and standards for hydrogen usage due to embrittlement, bad sealing, human errors and/or system degradation. So, to ensure the safety in this field, the development of risk management plane or strategies for H 2 leakage is required. As the sensing of H 2 is very important in this system as discussed previously, the control system and actuators that take the decision to do suitable action to the risk level based on the measured H 2 concentration and environmental conditions are also significant parts in such system.
Recently, a lot of researchers are focusing on issues related for hydrogen leakage. These include a liquid H2 jet flow through a nozzle [90], transient distribution in a vertical cylinder [91], as well as ventilation and dispersion in a garage [92,93], a tunnel [94], and a domestic room [95]. As found in these studies, hydrogen distribution and accumulation in highly confined spaces are very dangerous. Regardless of the little guidance of risk developed for such scenarios. For this circumstance, Matsuura et al. [96] proposed ways of reducing the risk by changing the vent positions, conditions and its relationship with the outside environment, and how this affects the distribution of H 2 . Matsuura et al. [96] proposed an innovative adaptive algorithm for sensing leakage of H 2 in a partially open space. Matsuura [96] also observed the effects of ventilation system on H 2 distributed and accumulation based on natural ventilation (NV) and forced ventilation (FV). Usually FV is expected to be more rapid than NV. However, using NV or FV depending on the geometrical configuration of a ventilated space [96], the sensing algorithm was proposed for NV only [96]. Other actions were taken place mainly in storage facilities of H 2 or fuel cell vehicles is shutting down the supply system by using controllable valve on H 2 cylinders.

Conclusion
The accurate hydrogen detection is critical requirement in several industries for everyday safety and process improvement because of the growing needs for hydrogen fuel. And from this review work the main points to be concluded are: a) The user should carefully consider the application and environmental parameters before selecting an optimum hydrogen sensor. Real-time and in-line H 2 sensors are in immediate need for the process industry as alternative to analytical instruments such as gas chromatography.
b) H 2 sensitivity and selectivity is very important to prevent false alarms for almost any applications.

c)
There is several technologies that have been developed and manufactured for the detection of hydrogen gas. Just some of these are well-established commercially and for years. g) When designing a system or facilities that use hydrogen, it's very important to take into consideration natural ventilation and/or forced ventilation to reduce the risk of H 2 accumulation and ignition or explosion as well as automatic controlled detection system for leakage.
h) The most common control actions that take place when hydrogen leakage happens is shutting down the source of hydrogen and activate forced ventilation system automatically.