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PARHyS System: a new approach to H2 concentration measurements in the subsurface

Maria Rosanne *1, Fabian Rupin 1, Louis Gorintin 1, Julio Aguilar 1, Hélène Buee 1,  Julien Werly 1, Olivier Lhote 2, Xi Yao 3 

1 Lab Nanotechnology, Sensors & Wireless, ENGIE-CRIGEN, France, 2 Key program natural H2, ENGIE research, France, 3 New Business factory, ENGIE, France 


Hydrogen is a hot topic on the energy domain and key component of the energy transition movement associated to the climate change mitigations initiatives. Natural emanations of hydrogen coming out of the ground have been detected in several places in the world. However, this molecular hydrogen (hereafter hydrogen or H2) is extremely diffuse and requires dedicated instrumentation to be detected. 

At present, to quantify emanations, current measuring instruments (such as the GA500) allow only one-off measures with their own uncertainties.  

However, emanations may depend on many factors and may not be daily or regular. This is why it is important to be able to monitor them continuously over a sufficiently long period. So, ENGIE has developed a technology for continuous hydrogen measurement: PARHyS system enable to analyze gases pumped in the soil autonomously in real time over a long period. 

Parhys system 


The core of the PARHyS system is composed of several parts integrated in a casing (Figure 1): 

  • A soil gas sampling unit/chamber composed of an a hydrogen solid electrochemical sensor associated with humidity and temperature compensating sensors and including a peristaltic pump (up to 500 ml/min) to draw gas from the ground. 
  • A measurement acquisition system composed of a radio frequency transmitter, allowing data acquisition from the sensors via a LoRa concentrator.  
  • Two interchangeable batteries with an autonomy up to 1 year.

Figure 1: Description of the different elements integrated in PARHyS system. 


The on-site H2 concentration measurement and data acquisition method are schematized on Figure 2. The gas in the subsurface at 0.8m depth is removed with a peristaltic pump at a given flow rate and passes into the sampling tube through the 2 mm diameter holes on 15 cm at the end of the tube. The aspired gas is collected in the sampling gas unit where the sensors for measuring H2 concentration, temperature and relative humidity are installed. After passing through the sensors, the gas diffuses outside the chamber without being pumped and is then released into the atmosphere. Measurements are made in automatic mode every hour during 2 minutes: only the maximum H2 value for each of these measurements is considered. Then data are recovered by Bluetooth communication or by using a radio frequency transmitter via a LoRa concentrator. The data should then be collected on any remote location thanks to a satellite connection. 

 Figure 2: Diagram of the operating principle of the PARHyS system. 

Hydrogen sensor characteristics 

The hydrogen solid electrochemical sensor utilized in PARHyS system is a commercially available 3-electrode model with a platinum-based sensitive electrode. 

The value of a gas sensor depends on four properties: selectivity (detection of a specific gas), sensitivity, response time and sensor lifetime. 

The manufacturer’s technical specifications of the selected hydrogen sensor for PARHyS system indicates a fast measured response time (T90 < 30 s) and a sensitivity down to 2 ppm. 

In standard sensing environments (-40 to 50 °C, 10 to 95 % RH, 800 to 1200hPa), the expected life time is up to 5 years. 

However, lifetime can be limited by several factors including the extreme temperature and pressures, low humidity environment and involvement of toxic gases. 

Traditionally, the current generated by sensing (electrochemical) processes is used to reflect the concentrations of hydrogen gas (sensitivity). However, sensitivity of the selected hydrogen sensor for PARHyS system is dependent on cross sensitivity to carbon monoxide (CO) and temperature (and possibly humidity) changes.  

To reduce the effect of temperature on hydrogen concentration, the equation that characterizes this dependence is integrated into the sensor algorithm. 

Hydrogen sensor testing 

In order to validate the hydrogen sensors performance, tests were carried out by a third party laboratory which characterized 3 PARHyS systems with hydrogen standard concentrations (100 and 800 ppm) under external controlled temperature and humidity (climatic chamber). The H2 sensor concentration range is from 10 to 1000 ppm with a maximum overload of 2000 ppm. 

Both, dry gas and wet gas were considered. 

The results show a full-scale dispersion of 0.3 for the temperature range between 5 and 45°C and a relative humidity between 35 and 75% and were not-temperature dependent (Figure 3). 

Additionally, there is no interference to CH4 and CO2 on H2 concentration measurement and no obvious correlation between H2 concentration and gas relative humidity from 35 and 75%HR as expected according to the manufacturer’s specifications (Figure 4). 

Figure 3H2 concentrations measured by the 3 sensors tested by a third party laboratory as a function of temperature when a standard gas (dry and wet) at 800 ppm is injected into the PARHyS system. 

Figure 4: H2 concentrations measured by the 3 sensors tested by a third party laboratory as a function of relative humidity when a standard wet gas at 800 ppm is injected into the PARHyS system. 

Field tests monitoring in Brazil 

In Sao Francisco basin there are numerous fairy circles which have confirmed hydrogen emissionsBefore placing PARHyS systems, H2 measurement campaigns with GA5000 were carried out on 29 structures located in several areas of Minas Generais (GEO4U, Alain Prinzhofer): Corinto, Pirapora, Jequitaí, Ponto Chique, São Romão, Brasília de Minas, Bonfinópolis, Santa Fé, Brasilândia e Morada Nova de Minas. 

To understand occurring hydrogen emanations and better quantify them to estimate the potential of this new energy source, ENGIE has installed a hundred PARHyS systems network on two fairy circles (called Campinas and Baru), located in the south of the Saõ Franciscõ basin (Figure 5). The monitoring in the Campinas structure was done for 8 months and in Baru for 6 months. 

The surface area of the structures Campinas and Baru are respectively approximately equal to 300 000 m2 and 418 200 m2. 

 Figure 5: Aerial view of PARHyS systems deployment (in yellow) on two fairy Circles  (Campinas and Baru) in the São Francisco basin, Brazil. 

Figures 6 and 7 show an example of the evolution over time of the H2 concentration associated with gas temperature and gas relative humidity respectively extracted from one PARHyS system located in Baru. 

During the monitoring campaign (from September 2019 to May 2020), the air temperature varied between 20°C (evening) and 36°C (day). However, as the Parhys systems were exposed to sunlight during the day, the temperature variations measured could vary from 22°C to 50°C, as can be seen in Figure 7. The measured relative humidity values could also be equal to 100%.  

Such temperature and humidity values deviate from the measuring range of the sensors. However, results indicate that H2 concentration is relatively independent on gas temperature and relative humidity, demonstrating that the algorithm implemented in PARHyS system to mitigate the effects of temperature is relatively effective and allows us to conclude that H2 emanations are not constant over time but appear as intermittent pulses. 

Figure 6Example of the evolution over time of the H2 concentration measured by a PARHyS system located in Baru and compared with gas temperature. 

Figure 7: Example of the evolution over time of the H2 concentration measured by a PARHyS system located in Baru and compared with gas relative humidity. 

Conclusions & perspectives 

The first of its kind remote and autonomous continuous field measurements over a period of 8 months was carried out in Brazil, where the sensors were exposed to extreme weather conditions, shows good robustness and highlight potential ways to optimize PARHyS system.  

This world first demonstration of hydrogen system confirm the added value of these sensors. 

Thus, some of the objectives of the new PARHyS version underway is to increase the accuracy and reliability of the sensors and to implement remote control (man-machine interface). In the long term, it could be considered, upon customer request, to replace the H2 sensor by other gas sensors (He, CH4, CO2, or other gases at convenience). 

ENGIE has planned new measurement campaigns in Brazil and other regions of the world with PARHyS systems networks to monitor H2 emissions to determine the prospective areas. The furniture of these set of sensors associated with digital services is now available for commercial collaboration. 

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