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Hydrogen

1 Introduction

Hydrogen is a flammable gas when its concentration rises above 4% Vol in Air. Furthermore hydrogen is invisible, odorless and tasteless which makes it necessary to use a sensor for detection of gas. Hydrogen is a promising potential fuel for cars, buses, and other vehicle and can be converted into electricity in fuel cells. It also is already used in medicine and space exploration as well as in the production of industrial chemicals and food products. Users of fuel cell or other applications using hydrogen want to avoid the risk of an explosion caused by hydrogen leakage. They need a reliable sensor technology which detects a leakage of hydrogen quickly and securely to protect people and machinery. Hydrogen sensor is the ideal solution for the detection of hydrogen, not only in portable, mobile applications but also stationary applications.

1.1 Background

Many concentration sensors have been designed and tested in the past. One of the most popular concentration sensors is a 'catalytic combustible' or 'hot wire' sensor. It consists of two beads of resistive elements arranged in a Wheatstone bridge configuration. The changing of resistance is depending on concentration of gas. This hot wire technique to measure binary gas concentrations was conducted by Adler (1971). He found that the heat loss from a hot-wire sensor is a function of four properties which are temperature, pressure, velocity and composition.

The second hydrogen sensor is called the MOS (metal oxide semiconductor) sensor, which consists of mixed iron, zinc and tin oxides heated to 150-350°C. Its conductivity is caused by shortages of oxygen. This reaction change material resistance by modifying the number of oxygen shortages. The material is heated, similar to the catalytic pearls, but the measurement is different. The resistance variation of the material itself is measured and not that of the heating element. This resistance variation is connected to hydrogen concentration by a non-linear correlation.

A third sensor is FED Field effect Gas Sensor. This sensor type is based on a metal oxide field effect transistor. Hydrogen diffuses into the transistor and this change the potential of the transconductance in an electrical connection. Although this sensor can be very sensitivity, it tends to saturate at low levels of hydrogen, making for it unsuitable for explosive limit detection.

A forth sensor is resistive Palladium Sensor. This consists of a catalytic active Palladium surface. Hydrogen is adsorbed, dissociated to hydrogen atoms and generates palladium hydride, which has a higher electrical resistance as the pure palladium. This resistance change is measured and linked to hydrogen concentration by a linear correlation. This sensor however suffers from the fact that the magnitude of the change in resistance is typically small, on the order of 1-10% in pure hydrogen, and can be temperature sensitive. The small signal level can present a problem in electrically noisy environments.

1.2 Aim of the project

The aim of this project is to estimate the relationship between the sensor voltage and sensor varying hydrogen concentration by using an analytical and numerical method. This project is focused on the aspirating concentration probe and understanding of its performance and relationship. Also it will be observed that the measurement of the concentration of hydrogen in reacting and non reacting flow. This results will be compared with the experimental results (from the reference) in order to improve the accuracy of the sensor. Then the sensitivity of the sensor will be investigated and the percentage of error in this sensor due to i.e. fluctuation by the pressure will be calculated.

2. Theory of the Aspirating probe operation

(Theory and equations)The method of measuring the concentration of a binary gas mixture is based on measuring the change in heat loss of a hot-wire probe placed downstream of a sonic nozzle in a small chamber.

3. Principle of the sensor

The sensor consists of 4 systems. Concentration probe system, Data acquisition system and discharge system.

Once the binary gas mixture passes through the hot wire inside of the aspirating probe, this will be results of the changing resistance of the hot wire due to the heat transfer between the wire and gas mixture. The aspirating probe is connected to the vacuum suction pump in order to achieve a choked flow. The Wheatstone bridge circuit will convert this resistance change into the electrical signal. Then this electrical signal will be amplified by amplifier. The bridge output voltage is fed to a control unit containing a high-gain feedback amplifier whose output is the bridge excitation. The amplifier sees any bridge-unbalance voltage as error signal and causes its output to change until the error signal is zero.

It is assumed that a steady and choked flow field with mixture ratios of hydrogen and other gases (air or CO) variable from 100 percent other gases to 100 percent of hydrogen can be created in this project.

4. Design of the Hydrogen sensor

4.1 Aspirating probe design

One of the advantages associated with the aspirating probe are a high frequency response, good spatial resolution, small sensitivity to flow angle, long life and a reduction in the effects of probe blockage.

The design of an aspirating probe is largely a matter of balancing a variety of conflicting design requirement to obtain the greatest measurement accuracy. The probe had to be very short as well as being small as possible to obtain optimization of probe design. This wire (which has 0.5mm diameter and 5mm length) located in the downstream of the hole in probe. Any further decrease in the wire diameter would give too high a chance of wire breakage. To maintained at a constant temperature well above the fluid temperature by constant temperature anemometer (CTA). In this project, it was predicted that the temperature difference between wire and the fluid is 80°C. The probe is connected to a vacuum pump via a surge tank in order make a choked flow rate. This is because of the critical pressure ratio across the hot wire exceeds the critical pressure ratio. As a result, the velocity across the hot wire depends only on the stagnation conditions of the mixture.

4.2 Wheatstone bridge circuit

One way to measure or calculate the resistance can be place a resistor in series with a power supply, finding the potential difference across the resistor then dividing by the current through the resistor. However, there are certain difficulties and inaccuracies for some application. Because of this reason, the sensor is connected into a Wheatstone-bridge circuit to which sufficient excitation is provided to cause self-heating in the element. The definition of the Wheatstone bridge circuit is a resistance measuring technique that uses a meter to detect when the voltage across that meter is zero.

One of the advantages of the Wheatstone bridge circuit is the accuracy. Since the accuracy of the measurement depends on the accuracy of the known resistor, it is relatively easy to have these resistances accurately know.

There are two types of operating the modes for heated element which are the bridge circuit of constant- temperature and constant-current supply. The former has an unheated temperature-compensation sensor connected into the bridge arm adjacent to the arm constituted by the heated sensor. As the sensor cools due to the concentration or other factors, its resistance change causes a bridge unbalance.

In this project, the bridge of the constant-current obtains its excitation from a constant-current supply. The current is adjusted so that the sensor is heated to a temperature optimized for a given application. The resistance of wire can be found from the graph that resistance versus sensor temperature. An increase the concentration of hydrogen will causes the cooling the wire increasingly. The resulting resistance change causes a bridge unbalance and commensurate changes in the bridge output voltage.

During this project, in order to predict the sensor response the resistance of wire must be calculated. This is done by using the relationship between the sensor resistance and temperature.

The relationship between sensor resistance and temperature is proportional. Thus, the equation of sensor resistance is where the resistance of wire and is the fluid temperature can be calculated since the temperature of fluid is know.

5. Procedure

An analytical and numerical method has been used to estimate the relationship between the sensor response and hydrogen concentration. The programming language MATLAB is used throughout the project for programming and obtaining graphical presentation of the results.

The primary part of this project is determining the sensor response with varying the concentration of hydrogen. This sensor should be used for any binary mixtures. Also the measurement of hydrogen concentration in reacting and also in non-reacting flows should be considered.

5.1 Concentration of Hydrogen in the non reacting flows

The sensor response for different concentration of Hydrogen in air should be determined. Also the aspirating probe temperature and pressure response is needed to calibrate. Therefore the two characteristics needed to be evaluated and understood.

5.1.1 Pressure response

The response of the sensor to a step change in pressure is evaluated. After obtaining the graphical presentation of the results, the effect of this pressure change is evaluated and discussed.

5.1.2 Temperature response

The response of the sensor to a step change in temperature is evaluated. After obtaining the graphical presentation of the results, the effect of heat transfer on the data is evaluated and discussed.

5.1.3 Validation

Since the results such as correlation between sensor voltage and concentration are presented and predicted they should be compared with experimental values and the pattern of obtained results in order to improve accuracy of the sensor.

5.1.4 The sensor response for different composition

This sensor can be used for any binary gases. Then the sensor response for different concentration of hydrogen in carbon dioxide is predicted and this result is compared with the previous results in order to see effect of different gases which has different thermal conductivity for sensor response.

5.2 Concentration of Hydrogen in the reacting flows. (Complete Reaction)

Consider the chemical reaction between the water and carbon monoxide, the product come out from this reaction will be Hydrogen and Carbon dioxide. Again the sensor response with varying the concentration of hydrogen in is determined. Assume that in these reacting flows the complete reaction is occurs.

5.2.1 Validation

Since the results such as correlation between sensor voltage and concentration are presented and examined they should be compared with reference values and the pattern of obtained results in order to improve accuracy of the sensor.

5.3 Concentration of Hydrogen in the reacting flows. (Incomplete Reaction)

Consider the chemical reaction between the water and carbon monoxide, the product come out from this reaction will be Hydrogen and Carbon dioxide. However because of the incomplete reaction, the CO still remain in products. Again the sensor response with varying the concentration of hydrogen in and is determined.

5.3.1 Comparing the sensor response for complete reaction and incomplete reaction.

Different chemical energy will be released from the reaction and due to this the sensor response will be different. Therefore, the effect of the CO will be evaluated and discussed.

5.4 Sensitivity

Since the sensitivity is the prominent characteristics of interest for sensor applications the effect of the fluctuation of pressure will be tested. Using this result, the percentage of error in measuring the concentration of Hydrogen can be determined.

6. Results and Discussion

6.1 Concentration of Hydrogen in the non reacting flows.

Aspirating probe consists basically of a thin heated wire whose cooling due to fluid is indicative of mass flow rate. The cooling is detected in the form of a change in wire resistance. When the binary gas inject to the probe, molecular of gas will observe the energy from the wire and this causes the cooling effect. The amount of energy loss is depend on the concentration of gas and also different amount of energy will be observed in binary gas mixture since the sensor is based on fact that the thermal conductivity of different gases is different.

6.1.1 Pressure response

It can be observed that the sensor response for concentration of hydrogen have been shifted upwards by increasing the pressure level. This is because of movement of molecular of hydrogen is rapidly increased due to high pressure. This leads to the mass flow rate in the probe increases then the Reynolds number increases. More molecular will observe the energy from the wire in other words the heat transfer rate increases. Then the cooling effect will occur and therefore the voltage that required maintaining the constant temperature on the wire will be increased.

6.1.2 Temperature response

Voltage Output vs the concentration of the hydrogen with varying the temperature.

Similar curve is observed from which is varying the temperature level. However, in this case, the sensor response for the concentration of hydrogen has been shifted downward while temperature is increased. The cooling effect is reduced since the temperature difference between the wire and flows have been reduced. Therefore less voltage is required to maintain the constant temperature on the wire.

After presenting the sensor response for different concentration of hydrogen, pressure and temperature, the voltage output of the aspirating probe for different molar fractions of hydrogen has been examined and compared with the experimental results (which is from the other reference).

6.2 Concentration of Hydrogen in the reacting flows. (Complete Reaction)

This project also considers the measurement of hydrogen concentration in reacting flows. In this section, the reaction of the carbon monoxide and water is used and results of this reaction carbon dioxide and hydrogen come out as product of reaction. Assume that complete reaction occur in react flow. Then chemical equation can be written as sssume that the volume fraction of and varied 0 to 1 using this relationship, the concentration of hydrogen can be measured in react flow.

6.2.1 Validation (Comparing with the reference results)

After presenting the sensor response for different concentration of hydrogen in react flow, the voltage output of the aspirating probe for different molar fractions of hydrogen has been examined and compared with the experimental results (which is from the other reference).

6.3 Concentration of Hydrogen in the reacting flows. (Incomplete Reaction)

Again, consider the measurement of hydrogen concentration in reacting flows. In this section, the reaction of the carbon monoxide and water is used and results of this reaction carbon dioxide and hydrogen come out as product of reaction. Assume that incomplete reaction occur in react flow. Then chemical equation can be written as where x is volume fraction of gas. Assume that x=0.2 in this case.

Because of the incomplete reaction between the carbon dioxide and hydrogen, CO is still remaining in the product as results of the reaction. Using this relationship, the concentration of hydrogen can be measured in react flow.

Sensor response (Voltage Output) of the aspirating probe for different concentration levels of hydrogen in carbon dioxide and carbon monoxide shows that high concentration of the hydrogen present high voltage compare to the low concentration as it was expected.

6.3.1 Comparing the sensor response for complete reaction and incomplete reaction.

The amount of different sensor response represents the effect of the CO, carbon monoxide. Since the flow is considered as incomplete react flow, the carbon monoxide is still remained in product of reaction of water and Carbon monoxide. Thus, measuring the concentration of hydrogen in 3 gases is considered.

Because of the effect of the CO in incomplete reaction, there is more heat transfer from the wire. Also since the amount of chemical energy released from the complete reaction is greater than incomplete reaction, the high temperature of gases in complete reaction can be expected. It means that more voltage output is needs in order to maintain the constant wire temperature.

The effects of the CO is not large compared with the complete reaction where volume fraction of CO is 0. Obviously, increasing the volume fraction of CO will lead the increasing the voltage output due to the cooling effect. The maximum difference of voltage output between complete and incomplete is around 0.1V. This is because of the thermal conductivity of carbon dioxide and carbon monoxide is quite similar. It means that the amount of heat transfer is similar too.

6.4 Sensitivity

Sensitivity is the prominent characteristics of interest for sensor applications. For life safety scenarios, early detection is critical to damage or leakage prevention and personnel survival.

The purpose of this section is the optimizing the sensor sensitivity. If sensor is too sensitivity then it can be interference by the even noise. It will increase the measurement error. Likewise, if sensor has low sensitivity then it will not measure the concentration of hydrogen properly which might be cause of exploration by leakage of hydrogen.

6.4.1 The sensor response at different amplitude of fluctuation

As previously stated, the aspirating probe mainly concerned with the heat transfer which has function of four properties such as temperature, pressure, velocity and composition; these are the factor that can effect to the sensitivity of the sensor. Also, the unsteady condition being measure and unsteady heat transfer through the probe, the process may have been influenced by unknown complication of hot wire use in gas mixture. In this project, assume that the pressure can effect the sensitivity of the sensor strongly rather than other factors.

The sinusoidal is the most common fluctuation form in control system. The range of the amplitude A, is in between 0.1 and 0.3.

The probe response to variation in flow angle was analyzed by MATLAB. It can be found that the incidence effects were negligible 20°. The maximum errors found within ±90° were less than 6%, showing excellent angle insensitivity. The fluctuation error is very small (i.e. range of error is 1.2% to 5.4%) . It means that even there is interference by high pressure which cause of the vibrating aspirating probe or creating noise, the sensor response for this error is not that. Therefore this sensor has reasonable accuracy for measuring the concentration of the hydrogen.

6.4.2 Percentage of error in concentration of hydrogen

Using the maximum fluctuation error from the Figure.16 the percentage of error in concentration of hydrogen can be found. The voltage output against error in concentration of hydrogen. It can be seen that the maximum error can get in the concentration of hydrogen is 0.2%. It means that this hydrogen sensor is very accurate.

There is number of possible sources of error in this sensor. For example, the flow field is assumed to be uniform over the probe cross-sectional area, which may not be the case in reality. The flow field entering the aspirating probe is non uniform hence the flow properties seen by each wire are different. This kind of errors is very difficult to quantify since they are very dependent upon the properties of the flow field. However this can be overcome by reducing the size of the aspirating probe. This will make the probe blockage is much smaller than for probes of similar size and the effects will be expected to be small.

Conclusion

This project has shown that the measurement of the concentration of hydrogen in analytically and numerically. It has excellent sensor response result for varying with the concentration of hydrogen in reacting and non reacting flows. Also calibration of the sensor response for varying the total temperature and pressure has shown. Therefore the relationship between the sensor response for pressure and temperature are understood. Comparing this result with the reference results (experimental) has been done in order to develop the sensor more accurate. Although, experimental and this project has been done in different condition, the prediction of the sensor response for measurement has similar pattern as experiment. This means that this project has been done very successfully. The sensitivity for fluctuation by pressure has been determined using the theoretical predictions in order to improve the accuracy of the sensor. The application of this sensor to measurement of concentration of hydrogen in binary gas mixture is suitable for measuring the leakage of hydrogen in fuel cell or rocket fuel tank.

Recommendation

This project proved to be a useful instrument, but like most instruments, there is room for improvement. Suggestions for improving these projects are:

Time delay in the fluid convection between the flows and wire.
Three-dimensional effects on the binary gas mixture.
Further study the effects of back pressure on aspirating probe.

Because the time response of the hot-wire system is very small and this only depends on the time for gas mixture passing through the distance from the probe entry to the wire. There might be time of the diffusion of the reactant from the gas mixture to the hot wire and also for the kinetic reaction. This time delay will be very useful for engineer in order to find out the leakage of hydrogen.

In this project, the uniform flow is assumed. However in reality, in terms of gas dynamics, they move non uniformly. As it mentions previously, it might be causes of the error in the sensor response. Further study in this field will be very useful to minimize the error in the sensor.

Also, in this project, the choked flow is made by suction vacuum pump. Because of choked flow the velocity across the hot wire depends only on the stagnation conditions of the mixture. However, when flow is not choked then the velocity will be effect by the back pressure on aspirating probe. Further development of this will be very useful for developed the sensor response.

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