Hydrogen losses in fueling station operation

Journal of Cleaner Production xxx (xxxx) xxx

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Hydrogen losses in fueling station operation Matteo Genovese a, *, David Blekhman b, Michael Dray c, Petronilla Fragiacomo a a

Department of Mechanical, Energy and Management Engineering, University of Calabria, Arcavacata di Rende, 87036, Cosenza, Italy Department of Technology, Hydrogen Research and Fueling Facility, California State University Los Angeles, Los Angeles, 90032, CA, USA c Hydrogen Research and Fueling Facility, Los Angeles, 90032, CA, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2019 Received in revised form 30 October 2019 Accepted 10 November 2019 Available online xxx

This paper describes an engineering approach for hydrogen accounting from production to dispensing at Cal State Los Angeles Hydrogen Research and Fueling Facility, equipped with an electrolyzer for on-site production. Particularly, the accounting process has been carried out by taking an in-depth look at current practices and databases, and its analysis has been based on data from quarterly reports of hydrogen production and dispensing related parameters and from its data acquisition system. It is described how the station addressed this analysis investigating several station areas. Following there is an assessment of the possible critical points, flanked by considerations, calculations, mathematical modeling and analysis of the database. The analysis led to a marked improvement on the station operation know-how. For most of the analyzed months the current average percentage of losses was found to be between 2 and 10%, whereas before it was between 30 and 35%. This current range of percentage (2e10)% includes all experiments done, defueling the buffer tanks, rebooting the dispenser, venting the lines, the uncertainty of the mass flow meter inside the dispenser (±5%) and the inherent uncertainty of Faraday’s law for hydrogen production estimate. Among all areas analyzed, maintenance activities revealed themselves as the most critical ones, leading to data mismatching in hydrogen accounting. The paper aims also to provide a guideline with recommended practices based on our experience, deduced for station operators and builders, including several steps for leakage monitoring, prevention, and troubleshooting. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Sandro Nizetic Keywords: Hydrogen station Recommended practices Hydrogen assessment Hydrogen accounting Leakage prevention

1. Introduction 1.1. Context and background The hydrogen economy is becoming a reality, and green mobility is part of its branches. Fuel cell electric vehicles (FCEVs) are a clean technology, virtually with zero emissions. Their operation can strongly improve air quality with an 84% of emission reduction (Ahmadi, 2019), with a global warming potential 45% lower than internal combustion engine vehicles operated with conventional fuels (Rosenfeld et al., 2019). To help the sustainability of the hydrogen highway, refueling infrastructure development is a key element. Many states all around the world have focused part of their national energy plan on helping the “chicken and egg” problem on the spreading out of

* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Genovese).

hydrogen stations (Brey et al., 2017). Concerning this hot topic, several studies have been carried out, giving important contribu€ gel et al. (2018) conducted a perception analysis based on a tion. Bo survey on seven states in the European Union, emphasizing how an effort on increasing public awareness with dissemination and information campaigns. Wulf et al. (2018) proposed a life cycle assessment on three different hydrogen supply chain: liquid, gaseous (compressed in trailers) and pipelines/caverns, addressing the hydrogen delivery problem. A roll-up assessment has been conducted by Iordache et al. (2017). In their work, they proposed 15 year-based scenarios for the spreading out of the hydrogen refueling infrastructures in the European Union. Important insights have been given also on the economic aspect. As a way of example, Acar and Dincer (2019) provided a comprehensive assessment of several hydrogen production technologies, including energy per€ formance and environmental impact. El-Emam and Ozcan (2019) investigated the recent technology enhancement on the hydrogen production options. They provided also a future expectation analysis for costs and technical aspects. Blazquez-Diaz (2019) modeled the operation of a hydrogen station in order to execute a cost

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Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

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analysis for a cost-effective design, in terms of storage size and number of tanks. These elements (storage banks), with the high pressure compressors, are the most expensive equipment in the station layout. In their work, Mayyas and Mann (2019) identified them as the “key cost contributors” in their manufacturing competitiveness investigation on hydrogen fueling stations. Da sar et al. (2019) presented a comprehensive overview on Silva Ce the Brazilian hydrogen productive chain, its weakness and challenges, as well as strengths, providing guidance and potential future directions. Despite these barriers, all around the world FCEVs and their refueling infrastructures are being deployed and installed. Japan H2 Mobility is planning to build 80 stations by 2021, with a strong and strategic alliance between automakers and manufactures/suppliers (Potter and Graham, 2019). As a matter of facts, Tokio Olympic Games will be supported by hydrogen vehicles and hydrogenpowered villages (Popov and Baldynov, 2018). Europe is also investing in hydrogen infrastructures and their network (Ortiz Cebolla and Navas, 2019). National Organization of Hydrogen and Fuel Cell Technology (NOW) in Germany is teaming up a 50 stations program and strategic policies have been proposed (Budde and Konrad, 2019). Norway and Netherlands are commissioning more hydrogen stations installations (Ulleberg and Hancke, 2019) and UK Government is accelerating more funding to boost hydrogen economy (Hacking et al., 2019), as well as Iceland (Shafiei et al., 2017). California is the frontrunner in hydrogen infrastructure expansion and spreading out, with 39 open retail stations already in operation and more than 6000 of FCEVs sold and on the road (Xiong et al., 2019). The roll-up of these technologies is an on-going process, but their operation in a large scale is still unclear. As described above, pilot projects have been deployed, but few deep investigations on their technical operation have been published. In fact, hydrogen stations are a complex multi-component system, and even though their deployment is providing the keystone for hydrogen mobility for both light-duty (Hussain and Dincer, 2010) and heavy-duty vehicles (Lee et al., 2018), their management and operation are delicate, and they need to be done properly. It is well known that hydrogen has a lot of potentials, but it still the lightest chemical element and potential losses could easily occur. Considering that hydrogen dispensing is mostly the only source of revenue for this kind of infrastructures, losses over the plant could affect the business model (Melaina and Penev, 2013). Not accounting venting, components replacement or logging each maintenance event could be a problem, while their recording could allow a data analysis from which interesting and preventive activities could be promoted and extrapolated. A more intense research effort on hydrogen station installations and operation can address this gap, allowing research and industry to glimpse and face problems associated with off scheduled maintenance, and the integration of all associated components. To address this research gap, this paper proposes an engineering approach for hydrogen accounting from production to dispensing at Cal State LA hydrogen station, equipped with an electrolyzer for on-site production. As a novel way to share lessons learned, the hydrogen assessment has been carried out by taking an in-depth look at current practices and databases, and its analysis has been based on data reports by means of a data acquisition system. In additions, the paper contributes to hydrogen station operation know-how by sharing issues and critical points, supported by calculations, mathematical modeling and database analysis.

1.2. Literature review Hydrogen industries are living an exponential growth (Bezdek, 2019), and more than 300 hydrogen stations are operating worldwide. In their report (Research and Market, 2019), the authors compared the previous forecast for hydrogen infrastructures with the actual market trend, noticed a slight downward trajectory. Even if they positively describe how different states are showing greater interest in the hydrogen economy and the latter is picking up, they reported also how some government offices did not find an easy way to respect the infrastructures network deployment roadmap or how low hydrogen demand badly affected the business development, leading them to shut down. It is therefore evident how important it could be to share the experience or recommended practices for hydrogen stations operation. Serdaroglu et al. (2015) investigated how the weather and environmental conditions could affect an open-space located hydrogen storage, analyzing temperature and pressure fluctuations also with the presence of shading and under the effect of the operating pressure, tanks geometry and material properties. It is interesting to notice how these effects have been assessed also considering maintenance events and station downtime, pointing out their importance for a hydrogen infrastructure operation. Rothuizen and Rokni (2014) recommended an optimal number of main storage tanks in order to minimize the energy consumption for a hydrogen infrastructure. In their study, they took into account several parameters related to the tanks, as their operating pressure, volume and number. They found out how the compressors count for the highest rate of required energy during a complete dispensing cycle. In their work, Genovese et al. (2018) provided technical guidelines on the minimum number of buffer tanks needed to ensure pulsation-free operation in a station adopting booster compressors for a direct refueling process. Bensmann et al. (2016) investigated a system based on an electrolyzer, compressor and hydrogen storage for power-to-gas application. They analyzed the operating pressure of the electrolyzer with the goal to optimize the plant configuration and reduce losses and energy consumption. They advised keeping this pressure under 20 bar as the optimal range of operation. Elgowainy et al. (2017) provided the needed technical features for a pre-cooling unit in a hydrogen station, after investigating its integration taking into account costs and energy demand. Elgowainy et al. (2014) introduced a disruptive algorithm and method called “high-pressure tube-trailer consolidation” which is a smart strategy to reduce the compression cost and increase the station daily-capacity, allowing more back-to-back fueling processes. The station layout strongly depends on the hydrogen delivery model (if in gaseous or liquid form). As a consequence, different safety measurements have to be taken according to the delivery model. Liquid hydrogen has to be kept in cryogenic tanks maintaining a temperature of 20 K in a pressure range of 0.6 MPae35 MPa. Insulation is a key element, and boil-off phenomena can be frequent. Indeed it is crucial to control and to monitor the pressure levels, as well as to install venting valves to avoid explosions (Sørensen and Spazzafumo, 2018). In fact, evaporation phenomena generates quantities of gaseous hydrogen, increasing the pressure within the tank. If the pressures is not released, leaks and thus potential ignitions could occur in presence of static electricity or of condensed air. If liquid hydrogen vessels happen to have a leak, strict measurements must be taken. Since liquid hydrogen has a very low temperature level, a leak can cause hypothermia and frostbite. If air leakages into the tank, moisture will be introduced that may create ice damaging the lines, valves

Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

M. Genovese et al. / Journal of Cleaner Production xxx (xxxx) xxx

and instruments. For short distances, the most preferred delivery model in the recent state of the art is the compressed hydrogen within trailers at a pressure of 70/80 MPa. Despite the bulkiness of such model, it seems to be a more cost effective solution than liquefaction/vaporization cycles (Sørensen and Spazzafumo, 2018). If a leak occurs, hydrogen will disperse rapidly in air. All pressure vessels have to be operated in accordance to several standards and codes. According to the hydrogen gas cycling test, tank must be filled and emptied without deterioration up to 1000 cycles (Frischauf, 2016). Another smart option for the hydrogen economy transition is the on-site production with water electrolyzers. Their installation overcomes the frequent need for external storage trailers, liquid or gaseous, competing with tanker trucks delivering hydrogen with bulk high-pressured gaseous tanks or cryogenic energy-intensive vessels. With this configuration, safety issues are strongly reduced in the supply chain, affording reliability and avoiding failures, cracking and energy inefficiencies during the delivery. A technical analysis has been carried out by Bauer et al. (2019), considering both liquid and gaseous station configurations. The authors found out how liquid storage could lead to less energy consumption since a gaseous station needs high-pressure storage which influences the design and the layout. As already mentioned above, when hydrogen is stored in its liquid form, cryogenic tanks are needed, but transfer and boil-off losses can easily occur, and for small tanks as those used for automotive applications the evaporation rates can reach values of 1% of hydrogen lost per day (Barbir, 2013). In the analysis carried out by Petitpas et al. (2018), the authors consider all the leaks related to several steps involved in liquid hydrogen production, delivering, storage, pumping and dispensing, describing lessons learned during experimental activities. If the station is supplied with gaseous hydrogen, possible configurations could be by means of delivered trailers or through an on-site system for production, as steam reforming plant or a water electrolyzer. In the first scenario, station layout is simpler than with an on-site unit for production, where more issues could occur due to the complexity of the whole system. However, an onsite electrolyzer could potentially set the emissions to zero, and for refueling capacities that exceed 1000 kg/day, the presence of onsite electrolysis could accelerate access to the market and reduce supply costs, as analyzed by Reddi et al. (2017). On-site hydrogen refueling stations are contributing to the spreading out of hydrogen economy and its related technology, through a low-carbon supply chain when they rely on hydrogen green production. Nistor et al. (2016) investigated the performance of two configurations for an on-site hydrogen station, considering a PEM and an Alkaline electrolyzer, adopting a technical and economic perspective to explore the feasibility of these systems and basing their analysis on data from a pilot project in the United Kingdom. Their results demonstrated how green hydrogen is becoming a potential substitute to carbon-based fuels, even if they promoted as future works the embedding of the hydrogen demand side in the model to achieve a more realistic scenario. In this perspective, investigations on hydrogen losses from production to dispensing is an intermediate step between the production and the demand side which can improve the analysis carried out in literature and adding a glimpse of issues that can occur and affect the business development of a hydrogen station. This topic is not widely investigated by the scientific community, but its great importance can be understood from the few scientific articles the literature offers. In their review, Kurtz et al. (2019) analyzed all the features and challenges which are limiting the scaling up and the spreading out of these hydrogen infrastructures. Among them, the lack of strong reliability is affecting maintenance costs by performing several unscheduled caused also by learning failures, and indeed hydrogen

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leaks. In their work, Lipman et al. (2013) reported seven lessons learned from the operation of the hydrogen facility located at the University of California in Berkeley. They marked how the station team has to carefully plan and schedule maintenance activities, considering that they could help in reducing station downtime. Monitoring the infrastructure by mean of data analysis could also prevent unexpected failures or issues and reveal performance degradation. Stolzenburg et al. (2009) described the operation of nine hydrogen infrastructures involved in the Clean Urban Transport for Europe project, equipped with 27 hydrogen-powered buses. The infrastructures presented different design and layout: 6 of them have an on-site production unit (whose 4 are with water electrolyzer and 2 with a steam methane reformer) and 3 of them with external storage (2 with gaseous hydrogen and one with liquid). Among the several parameters and performance analyzed, the authors focused also on the specific hydrogen losses and overall losses. They reported that about 80 tons out of 274.5 tons of hydrogen were lost during the operational phase of all the nine stations, with a range in between 7% and 46% of specific losses. The major events which caused these main leaks have been hydrogen contamination which occurred because of failures or oil spills in the compressors, problems in metering and hydrogen purging, venting and background leaks. Three stations equipped with on-site electrolysis showed values among 7e9% of specific hydrogen losses, and only one 20% because of the long downtime period. They reported also an average of 5% of hydrogen used for purification regeneration. The only station with liquid storage in London showed the highest value of 69%, due to frequently evaporations and boil-off phenomena caused by low demand and long standby periods. In steam-reformers based stations about 15% of hydrogen is normally used for hydrogen purification. All these studies reported above are representative cases of potential possibilities of how to use station data and data acquisition system for pointing out, addressing and mitigating some of the major risks that could occur during infrastructure operation. Few of them are focusing on hydrogen accounting and hydrogen losses. Recently, an accident happened on June 1, 2019, at a hydrogen chemical plant in Santa Clara, California. A liquid hydrogen tanker truck started to leak after being fueled, resulting in a fire and then in an explosion. The Santa Clara facility is one of the main hydrogen supplier of the Northern California, in the San Francisco Bay Area. Its downtime period affected the Northern Californian hydrogen station network for three months, disrupting the hydrogen economy and the supply chain. In an independent way, an accident in a hydrogen station near Oslo, in Norway, occurred in June 2019. A hydrogen leak from the high-pressure storage caused an explosion. Also Gye et al. (2019), in their risk analysis for an urban hydrogen station, marked how leaks from the dispensing equipment and the trailers are the main critical ones. A comprehensive database with the hydrogen related accidents can be found in the Hydrogen Tool Portal (“Hydrogen Tool Portal”, 2019). It is clear how literature review offers few lesson learned on hydrogen station operation and assessment. Shared experience and lessons learned are critical and it could strongly avoid explosions or dangerous situations as the ones reported above. In fact, it would help industries to reduce risks and to improve the sustainable scaling up of these technologies. To address this research gap, the present work provides a hydrogen assessment at a refueling station, describing its research, engineering approach and sharing lessons learned at Cal State LA Hydrogen Research and Fueling Facility. As a novelty, the accounting process has been carried out by taking an in-depth look at current practices and database, and its analysis has been based on data from quarterly, monthly and daily reports of hydrogen

Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

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production and dispensing related parameters and from its data acquisition system. It is pointed out how the station addressed this analysis investigating several station areas. The station evaluation is based on the standard operation and on the maintenance operation. The station standard operation is analyzed during the standby period, during the production process and during the dispensing process. Maintenance operation focuses on four main actions that frequently occurred: lost dispensing communication, aborted refueling process, leak detection and purification/purging process. The analysis lead to a comprehensive assessment of the critical points, supported by considerations, calculations, mathematical modeling and analysis of the database. Since the on-site analysis led to a marked improvement on the station operation know-how, the present paper provides also a guideline with recommended practices based on station real operation and experience, deduced for station operators and builders, including several steps for leakage monitoring, prevention, and troubleshooting. 2. Hydrogen Research and Fueling Facility at California State University, Los Angeles Hydrogen Research and Fueling Facility is a hydrogen fueling station located at California State University in Los Angeles, and it has been the first station to be certified to sell commercial hydrogen on a per kilogram basis and the largest US university hydrogen station when it started its operation (Mesa et al., 2014). This facility is a living lab for students and faculties at the Campus and a research facility serving as a proving ground for components, systems, data acquisition, and control testing. Being the center of an international hydrogen network and hosting international researchers and scholars, the station is considered to be a laboratory that provides a venue for groundbreaking innovations and collaboration with partners. The facility is also a cornerstone of Cal State LA’s Sustainable Energy and Transportation Technology Program. A more detailed description has been reported in our previous work (Genovese et al. (2018)). It is probably the only attended station in LA County, helping drivers in becoming familiar with fuel cell electric vehicles, hydrogen economy and environmental benefits. It is also a connector station, close to the highway, for the strategic planning of hydrogen stations network and logistic. The station is becoming a key element to achieve green campus mobility (hydrogen vehicles and hydrogen buses), being the "refueling core" of a shared mobility program with hydrogen vehicles for students and faculties. Briefly, the station deploys hydrogen with the capacity to produce and dispense 60 kg/day, sufficient to fuel 15e20 vehicles. The station utilizes a water electrolyzer, a low-pressure compressor, and two high-pressure compressors, enabling 350 bar, 700 bar fueling, and 60 kg of hydrogen ground storage. The station is gridtied with certified 100% renewable power. Fig. 1 shows the station diagram. The facility could be considered to be composed of three main areas: hydrogen production, hydrogen dispensing and station control room. The hydrogen production phase contains all the converters and the energy supply, the demineralized water supply, an electrolysis-based hydrogen generator from Hydrogenics, a PDC Inc. diaphragm compressor and the main ground storage. All the components related to the production have an own pipeline, not affecting the dispensing line, which helps to avoid long downtime period in case of electrolyzer stand-by period or maintenance activities. The hydrogen dispensing area is composed of:
a low pressure buffer tank, whose goal is to keep stable the compressors suction pressure;


two hydrogen booster compressors manufactured by HydroPac;
four high pressure buffer tanks to smooth hydrogen vibrations and assuring pulsation-free operation;
a glycol-based hydrogen chiller;
a double side hydrogen fuel dispenser, by Quantum, which can enable both 350 and 700 bar fueling processes. Following SAE J2601 protocol tables, fueling time is typically in the range of 7e15 min, depending on ambient temperature, vehicle initial pressure, and hydrogen chiller temperature. Some components are also equipped with additional ancillary systems, as instances for pneumatic operation, cooling units, filters, safety valves, and water treatment. Although the dispenser contains the user interface hardware (card reader, keypad, screen), it does not provide the control system for this hardware. That control system is located remotely from the dispenser and it provides the go-no-go information to start refueling. The core of the management of the station is the control room, where the station operators can monitor all the station parameters and they can view real-time trends inside the station and during the refueling process. This real-time feature is unique to this station. It allows a glimpse of possible malfunctions in real time and it allows operators to have full command of the station, in terms of safety, performance, and operation. Considering that most of the existing stations are not attended, they do not have on-site operators and thus there is not an immediate control of the station itself. For the control of the station, a Supervisory Control and Data Acquisition (SCADA) System is used, and it enables monitoring and the issuing of process commands, such as controller and set point changes. All information related to the production, compression, chilling and dispensing processes is routed through the computer for supervisory control. Through the control room, the station team has the option to operate with two different decision modes: cascade recharge and vehicle fill. A third decision mode is represented by a manual control diagnostics. The Human Man Interface (HMI) enables the operator to start and stop equipment, to monitor safety features and to control the operation of compressors, motors, and valves. The station operating status, the individual equipment parameters, and valve status are monitored. The equipment control can be operated from the utility room or the field. There is also the possibility of remote control. 3. Planning assessment/evaluation e identifying discrepancy sources Intending to plan and to describe the hydrogen assessment carried out in this work, the adopted approach investigated two different levels and topics, as shown in Fig. 2. This section will stay on a high-level description, discussing generalities and leaving the focus on details in the following sections. First, the research activity focused on the applied methodology and instruments involved. Some errors or data mismatching could be caused by non-calibration or metering problems, or they could be in the range of the accuracy of the meters. Thus, data acquisition system structure and recording have been analyzed, to have a deeper understanding of the data collection. Different frequencies of collection (quarterly, monthly, daily analysis) have been investigated, in order to achieve a better overview of the assessment and to extrapolate more information as much as possible. In fact, the station database contains all the necessary information to calculate and to analyze the performance and the different hydrogen flow within and outside the station. To cut off some potential data missing values between the PLC and the SCADA system, different types of data collection have been analyzed, considering electronic

Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

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Fig. 1. Station diagram.

databases and the station manual logbook. Once the instruments have been checked and the methodology has been approached, station evaluation has been performed, distinguishing on station standard operation (which includes stand-by period), production phase and dispensing process, and maintenance activities. Interesting discoveries have been found, and the following sections will provide more details about the activities and the critical points, flanked by considerations, calculations, mathematical modeling, experimental observations, and actions.

3.1. Methodology and instruments 3.1.1. Measurement instruments description To enable effective data collection on the station performance, a significant number of sensors and meters have been installed at the station. A custom-designed software package is utilized for data collection and reporting to the National Renewable Energy Laboratory (NREL). It is possible to monitor in real time the majority of the measured quantities at the station. As data have been collected and analyzed, the station hardware and software have been gradually upgraded for optimization and other technical/safety enhancements. The software SQL Server Database is used to manage and to store the data. Reports are generated based on a time schedule or when desired. The necessity to understand whether these data reflected realistic information has led the station team to investigate in detail how the database works and how it saves the data. All data acquired from the station are stored in five databases in SQL («1sData», «10sData», «MinuteData», «FuelingData» and «FuelingReport»).

The station is also equipped with three mass flow meters located within the system, as shown in Fig. 1:
A mass flow meter is located after the electrolyzer. It is manufactured by Sage Metering, Inc. Its task is measuring the hydrogen coming from the electrolyzer after the purity process.
A mass flow meter is located immediately after the main storage tanks. Its task is to measure how much hydrogen is compressed by the boosters. It can be used also as a comparison tool for the hydrogen dispensed. It is manufactured by Rheonik, and it is a Coriolis flow meter.
A mass flow meter is located inside the dispenser case. Its task is to measure how much hydrogen is dispensed. It is approved, certified and sealed by the Weight and Measurement California Office. It is manufactured by Rheonik, and it is a Coriolis flow meter. The accuracy of the Rheonik meters is about ± 5% of the reading, while the accuracy of the Sage meter is ± 1%. To assure the correct operation of the meters, it was decided to check their calibration or at least their zeroing. The manufacturers were contacted for specifications and guidelines and for in-situ calibration check. Due to a different calibration reference condition, the station operators were not able to calibrate in-situ the meter: the reference pressure used by the manufacturer was 1.2 MPa. It corresponds to 20% more than the circuit pressure at the outlet of the electrolyzer. The company suggested sending the meter back for a re-calibration in their test bench. Since the reliability of the other meters was not practical, the

Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

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Fig. 2. Hydrogen assessment approach.

only meter used to carry out the described analysis was the existing Rheonik meter inside the dispenser. Concerning the hydrogen production, it is not directly measured, because of the lack of calibration regarding the Sage meter. The production is estimated through electrolyzer supplier data about Faraday’s law, using the current of the cell stacks within the electrolyzer. Concerning the pressure meters installed over the station, those used for the analysis carried out in this work are related to the hydrogen main storage tanks. Their pressure gauges are

manufactured by McDaniel Controls and their accuracy is 1%. The pressure transmitters are by Noshok, and they have an accuracy of ±0.25% of the full scale, which is 55.15 MPa (8000 psi). Table 1 summarizes the measurement instruments specs, adding the working pressure of line they are related to.

3.1.2. Hydrogen accounting - data mismatching The data acquisition system of the Cal State LA Hydrogen Fueling Station allows the generation of automatic quarterly reports. They

Table 1 Devices specifications. Device

Model

Accuracy

Pressure Level in the line

Mass Flow Meter Mass Flow Meter Mass Flow Meter Pressure Gauges Pressure Transmitters

Sage Metering Inc. Rheonik Rheonik McDaniel Controls Noshok

±1% of the reading ±5% of the reading ±5% of the reading ±1% of the reading ±0.25% of the full scale

Up Up Up Up Up

to to to to to

1.0 MPa 45.0 MPa 80.0 MPa 45.0 MPa 45.0 MPa

Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266

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contain all the necessary information to calculate and analyze the performance of the station, the amount of hydrogen produced, the amount of hydrogen compressed at high pressure and the amount of hydrogen dispensed. To test how efficiently the station is performing, the simplest operation is to compare the amount of hydrogen produced and the quantity of hydrogen dispensed. The analysis included data from 2014 to 2016. When the reports were generated, the station data showed a consistent discrepancy between the hydrogen produced and the hydrogen dispensed. For a summary reason, only 2016 has been reported, since in 2014 and 2015 station dealt with different tests needed for the start-up and certifications. The specific hydrogen losses have been calculated with Eq. (1).

Specific hydrogen lossesð%Þ ¼

H2p  H2d H2p

(1)

where:
H2p is the amount of hydrogen produced during the month (or the quarter) considered for the analysis.
H2d is the amount of hydrogen dispensed during the month (or the quarter) considered for the analysis. Part of the discrepancy could also be affected by overproduction in the range of time antecedent to the one when the analysis has been performed. As an instance, for a monthly analysis, the data acquisition system does not take into account if at the beginning of the month the main storage tanks were filled. The amount of hydrogen needed to fill the tank at the beginning of the month depends on the history of the previous month and the hydrogen dispensed in the previous month. This amount of hydrogen is not produced for there has been a demand for dispensing in the current month, but for the demand of the previous month. Considering this amount of fuel as the demand for the actual month can occur in data mismatching and wrong interpretation, leading to a bigger discrepancy during the current month between production and dispensing. If the accounting starts with half a container or with a deficit, it will affect the current situation, while the analysis performed in this section regards a starting point that should occur with a full container. In this perspective, analysis with different frequencies (quarterly, monthly and daily) helps in extrapolating the right information and achieving a better data understanding. The 2016 situation of the station showed a maximum

Fig. 3. Cal state LA Hydrogen Station, 2016 monthly production VS monthly dispensing.

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discrepancy approximately 50% comparing the several months and 40% comparing the quarters. Skipping the month of July (that presents the maximum value of the year), the other months have almost the same average percentage (30%). Fig. 3 above shows a 30% discrepancy during a normal month. This delta results in a considerable loss of hydrogen during each month: 30% of the monthly production is equal to 60 kg of gaseous hydrogen lost every month. The Cal State LA Hydrogen Fueling Station team is always at work: in the control room, they monitor continuously the parameters of the station and periodically they perform the equipment checks. Being compressed at high pressures, the gaseous hydrogen venting is very noisy and it is easily recognized by technicians working in this field. The station operators have developed a certain sensitivity to understand if something is going on with strange noises, leaks, and malfunctions. It is well known that the flammability makes hydrogen difficult to deal with, and special measures must be taken when handling it. Hydrogen burns with a light blue color flame that is invisible to the naked eyes in daylight. Hence detectors should be deployed to identify the invisible flame. Cal State Hydrogen Station is equipped with hydrogen gas sensors and flame detectors: no one of them detected flame or high gas concentration during the station operation. They are periodically tested and certified. It is worthy to note that small leaks at connections are common and may occur at any of the piping connections. Small connection leaks are considered the most plausible under normal conditions, and they would release very small quantities of hydrogen. Medium-sized leaks from the fill connection and manual valve stems are possible but they are mitigated. Regular leak checks at maintenance intervals should detect leaks early. Large leaks from a catastrophic failure of the storage vessel and components such as the pressure-building regulator are very unlikely without outside forces acting on the component. Piping components are purchased and installed according to the standards and safety codes and they are rated for the environment (pressure and temperature) in which they are applied. 3.2. Station evaluation 3.2.1. Station standard operation To differentiate possible losses that can occur when the equipment is in operation or motion and possible losses that can occur when the station is in standby mode, three separate subsections have been created in the discussion, dealing with station stand-by period, production operation and dispensing process. 3.2.1.1. Stand-by. In this work, the standby period of the station is defined as a period where there are no working components, compression, production or dispensing. The station can be represented by this situation when the main storage tanks are full and nobody is trying to fuel an FCEV. In this case, the electrolyzer and the compressors are not supposed to be in operation. Another similar situation could be during the weekend when the station is closed to the public and fueling processes are very few. Both of these main cases have been considered. If there is a leak in the circuit during a standby period, hydrogen will vent until the line will achieve an equilibrium with the atmospheric pressure. It is necessary to investigate if this amount of hydrogen can be negated and not considered or whether its contribution is substantial. As a test, the hydrogen mass held inside the pipelines between the electrolyzer and the main storage tanks has been calculated. Considering a length of 20 m and an internal diameter of 10 mm, the volume of the pipelines results to be 0.0157 m3. Between the electrolyzer and the main storage tanks, the low-pressure compressor (PDC) is present. It increases the pressure from 1 MPa

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up to 40 MPa. It is conservative to consider all the volume in this circuit at the same average pressure of 40 MPa, with the temperature set at 30  C. Under these conditions and after calculations, 0.025 kg of hydrogen are present in this line. Using the same size for pipelines and considering the high-pressure line between the main storage tanks and the dispenser (with a hypothesis of average pressure of 80 MPa), 0.042 kg of hydrogen are confined within the piping. It means that if the lines are emptied every day because of the losses, at the end of the month the station is going to lose only 1.8 kg. Particular attention has been paid to the Cal State LA Hydrogen Fueling Station vessels and storage tanks. Component failures in the gas storage and filling station areas can create leaks. Small gas leaks at valve stems or connectors are more likely and they would result in the release of modest quantities of hydrogen. Breakaway fittings appear to be a special situation: wear and defects may render the system non-performing as the intended designed and leaks can occur. The main storage tanks contain about 60 kg of hydrogen. The data acquisition system stores in the database some quantities relative to the main vessels, such as ambient temperature and pressure of each tank. Through them, the hydrogen mass contained in each tank has been extrapolated using the real gas equation provided by NIST (Zheng et al., 2010). Eight weekends without fueling processes have been analyzed, from Friday to Sunday, considering a recorded data at 11:59 p.m. each day. The idea was to monitor the amount of hydrogen present in the tanks three consecutive days when the station has not been operated. For this reason, weekends without fuel dispensing have been analyzed. The amount of hydrogen gas in the tanks depends on tank temperature-pressure correlation, and it is also affected by the ambient temperature value and heat transfer phenomena. Pressure data analysis reveals a maximum pressure excursion of 0.178 MPa and a minimum excursion of 0.186 MPa, whose values are very close to the range of the pressure transducer reading error (±0.25% of the full scale). No leaks could be observed from pressure data. Assuming this reading error constant with ambient temperature variation, fluctuations in hydrogen mass calculations are attributed to variation in temperature and instrument reading errors, where even the thermal mass of tanks could play a transient role. Therefore these fluctuations could be just within instrument error and they are not related to hydrogen losses. After comparing multiple statistics on the weekends, as shown in Fig. 4, a percentage threshold value of 1% has been assigned as maximum hydrogen

mass fluctuation. If the daily fluctuation exceeds this value, it means that the variation is not caused by the change of temperature, but from possible losses in the tanks or from the valves of the tanks. The mass difference in percentage results to be always under 1%. Only one day the percentage occurs to be about 1%, but on that day also the delta in temperature was high and almost 2%, so it is an outlier point. In total, it can be assumed that no leaks were coming from major reservoirs during the standby period of the station. 3.2.1.2. Production. While during the standby period, when a leak occurs, the hydrogen present in the pipes pours out to reach equilibrium with the atmospheric pressure, during the production the compressors in the circuit are in operation. This implies that the pressure is virtually guaranteed by the compressor. The loss allows the leakage of hydrogen resulting in a pressure drop, but the compressor immediately takes to raise the pressure to the required value. It is a dynamic situation and no longer stationary. A small and simple model was developed using the Matlab/Simulink software to obtain a rough estimate of how much hydrogen can be lost from the pipelines between the electrolyzer and the low-pressure compressor during the phase of production. The leaking hole is considered as an orifice. In Fig. 5 the orifice model is represented. The behavior of the orifice can be described through Eq. (2) (Nakayama and Boucher, 1998).

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C m_ leak ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffi,ε,A, 2 Dp,r1 yCgeneral ,A, 2 Dp,r1 1  b4

(2)

For this model, the area of the leaking hole was considered as a square with a side equal to 0.1 mm. For a leak, it is a considerable size. Since the area of the leak is smaller than the diameter of the piping, a substantial restriction is expected. In order to represent this situation, the value of the Cgeneral is set to 0.1. The compressor is modeled with the hypothesis of constant suction pressure (1 MPa). This implies that the volumetric flow rate that the compressor manages to dispose of is constant. The mass flow rate depends on the compressor suction pressure, while the discharge pressure has been imposed equal to the pressure in the main storage tank. Through pressure and temperature, the density needed is calculated for the compressor mass flow rate.

rinlet ¼



pinlet

Rg T 1 þ a pinlet T



(3)

As the input of the dynamic model, it is possible to insert the value of the initial pressure inside the tank, to calculate the initial mass within it. The tank pressure is then calculated through the hydrogen real equation of state. In Table 2 the input data window for the model is shown. The

Fig. 4. Extrapolation of hydrogen mass values in main storage tanks.

Fig. 5. Orifice model (Lau, 2008).

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M. Genovese et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 2 Input data in Matlab/Simulink model. Temperature

308.15

K

Compressor Pressure Discharge Atmospheric Pressure Tank Initial Pressure A Cgeneral

40 101325 30 0.01 0.1 1.9155E-06 0.767 4124.3 0.0022

MPa Pa MPa mm2 e K/Pa m3 J/kg K m3/min

a

Tank Volume Hydrogen Gas Constant Compressor Flow Rate

amount of hydrogen leaked calculated by the model is approximately 0.2 kg per each tank per day. Considering the presence of three tanks, five working days per week and 4 weeks per month, the station can lose 12 kg of hydrogen per month. This result strongly depends on the area of the leak, and the value used in the model is considerable and so conservative. However, hydrogen leaks during the gas production could be consistent and on-site investigations are needed to check and verify real leaks presence. 3.2.1.3. Dispensing. Dispensing activity is related to the station high-pressure zone, characterized by booster compressors, lowpressure buffer tank, high-pressure buffer tanks, and a hydrogen dispenser. The low-pressure buffer tank and four high-pressure buffer tanks contain significantly lower masses than the main storage tanks. Their operation is not equipped with electronic pressure transmitters, but on-site observations have been performed through the pressure gauge indicators. Pressure trends and variations in the high buffer tanks have been analyzed during a refueling process in previous work (Genovese et al. (2018)), and no losses were observed during the experimental activities, given that the model represented with a good approximation the trend of the pressure in the high-pressure line. Instead, in the experimental activities associated with this work, on-site investigations were conducted during the same weekends analyzed for the main storage tanks (8 weekends). Changes in pressures were detected to be within 1% of the accuracy of the pressure gauges. As described for the main storage tanks in the previous section, no consistent hydrogen mass variations have been observed. As for the production, during a fueling process, the situation is completely different from the operations during the standby period of the station. The model described for the production phase above has been applied also for the high-pressure line, and the conclusion is that it is easier to lose hydrogen during the phase of production or dispensing than during the standby period. Under this consideration, it was imperative to approach and address the problem by on-site checking the working conditions of all associated equipment. These activities have been performed through the drawing up of leak check-lists, described below. 3.2.2. Maintenance operation During station operation, maintenance activities are extremely important, to ensure safety, proper activation and functioning of the equipment. Many activities are planned and recommended by manufacturers, such as regular and preventive maintenance. With more frequency, many activities have been carried out beside the scheduled maintenance, due to unforeseen events or out-ofstandard operations. In both cases, the station also utilizes a logbook. It is a record of important events in the management and operation of the station, such as fueling processes or maintenance, and as an essential log it is filled in several times daily. Moreover,

9

Cal State LA Hydrogen Fueling State is equipped with a data acquisition system. SQL Server Database is the software used to manage and to store the data. The comparison between the production and the dispensing is performed using the data from the reports. The necessity to understand whether these data reflected realistic information has led us to investigate in detail how the database works and how it saves the data, by extrapolating daily data from these databases in order to understand comprehensively the flow of hydrogen within the station and to the outside of the station. 3.2.2.1. Lost communications. A daily analysis was performed, from May 2016 to September 2016. The use of the logbook as a comparison was thought to be an excellent data validation tool: every fueling process is recorded in the logbook with the amount of hydrogen dispensed to the vehicle and displayed by the dispenser window. Considering, for instance, a monthly time range, 8 out of 30 days appeared to not have data recorded related to dispensing and production, since the station does not have a weekend work schedule and during weekend-days, it is unattended. For 19 days, and the percentage difference between hydrogen dispensed and saved in the recorded and hydrogen amount recorded in the logbook does not match and it shows always a constant value of percentage discrepancy (about 6.9%). This percentage value described above has been found common to all months. After some investigations, this discrepancy has been related to lost communication between the database and the mass flow regulator. In general, the value of the hydrogen dispensed is stored in the database only when the fueling process ends. In this exact moment, the value is recorded into the database and the IR (infrared) system of the vehicle closes the entrance to the tank and the PLC commands the station to stop the fueling process. Every system, and above all the air compressor system, has a delay time, due to the inertia of system. A pneumatic delay is defined as the time required by the mechanical components to perform the requested command: overcoming the pressure in a fixed volume in order to change its configuration. During this span of time, the flow meter inside the dispenser continues to measure the mass flow rate passed. It will stop just when the air compressor system completes its task. This amount does not go into the vehicle, rather it vents. The amount of vented hydrogen and hydrogen discrepancy between database and logbook depends on the distance between the mass flow regulator and the dispenser, and on the delay time related to the pneumatic actuation system. In conclusion of this analysis, periodic comparison with the logbook is required to avoid missing data and wrong perceived information of dispensing. 3.2.2.2. Aborted refueling process, software, and dispenser rebooting. To ensure good performance in time and safety during a fueling process, the Society of Automotive Engineers released the SAE J2601 fueling protocol standard and guidelines, introducing APRR (Average Pressure Ramp Rate) tables for the station to abide by. Measuring the actual ambient temperature, the station accesses the tables and commands a target APRR to be achieved. There is a tolerance in deviating from it, and this deviation is defined through two APRR corridors, the lower and the upper ones (Schneider et al., 2014). During its operation and refueling processes, Cal State LA hydrogen infrastructure uses and meets through its station control procedures the SAE J2601 e 2014 pressure APRR corridors. During “communication” fueling, the State of Charge (SOC) becomes the main parameter of evaluation, while during a “non-communication” fueling the station ensures that the pressure target is met. Two days in a monthly analysis presented a percentage difference of about 10e12% between database and logbook. It has been

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found a particular correlation: when percentage value appears to be about 10e12%, it is because of in that day a greater number of aborted fueling processes have occurred, caused above all by extreme weather conditions or lower cooling block performance, which led the process to fall out the J2601 pressure corridors and interrupted the refueling process. Under this scenario, in order to achieve a consistent value of the state of charge or to meet the pressure target, station operators or vehicle drivers have to try more times starting again the refueling process. For each refueling process, as explained before, a small amount of hydrogen is not recorded. If for the same car more refueling processes are needed, the absolute amounts of hydrogen unrecorded have greater weight as a percentage than a single vehicle filling. Another discovery has been found through this investigation: every time the software is rebooted because of a loop or malfunction during a fueling process, the amount of hydrogen dispensed during that fueling process is not saved and recorded into the database. It gets lost. At the same time, whenever the station has a lack of communication from the PLC, or a reboot of the dispenser, for safety reasons, the vent valves open, and the entire line associated with the refueling will lose pressure and hydrogen. In that line, there is also a low-pressure buffer tank, to stabilize the suction pressure of booster compressors. If its manual valve is not closed before rebooting, all the hydrogen present in it will be lost. These situations occurred three times in a month analysis and led to a percentage of the discrepancy between 26 and 86%.

3.2.2.3. Leak detection. Like other volatile fuels, hydrogen needs to be handled carefully. As a light gas, hydrogen disperses into the air rapidly. Hence it should be constantly checked and monitored for any leaks during its transfer from one stage or component to another. The piping should be properly maintained and the threading of their seated positions is imperative to prevent any leaks. Due to foregoing reasoning, not only initial testing but also periodical inspection is required. The results of the dynamic model in leak simulation during the station operation led the station team to look for a method to check the station and to find leaks. The method had to be systematic and effective, intending to ensure a future guideline and for possible programmed operation checks. The best solution seemed to be drafting checklists concerning the three main areas where it is most likely that leaks have been determined to occur. The three main analyzed areas were essentially the high-pressure zone, the low-pressure zone, and all tasks related to the station operation. The station Piping and Instrumentation Diagrams have been studied in detail in order to recognize and to find all the valves and possible sources of leaks. Within the checklists, the same nomenclature was adopted as in the equipment engineering sheets of the station. This allows an immediate cross-reference in performing the checklists operation: the station team can move through the station with the engineering flow-sheets. In this manner, he can easily recognize all fittings, valves or equipment and perform tightness testing or leak testing. The three main analyzed areas were essentially the highpressure zone, the low-pressure zone, and all tasks related to the station operation. During checking for possible leaks, the station team used a hydrogen bubble detector and indicators (water with soap). Four minor leaks have been found:
PDC check valve.
PDC Pressure gauge.
DBB valve between the Main Storage Tanks and the LowPressure Buffer Tank.
Booster compressor cylinder.

The station technicians repair the leaking equipment, but the difference between production and dispensing still existed. 3.2.2.4. Purification and purging. Other possible sources of hydrogen leakage are the gas purification process after the production phase and the hydrogen purge. The electrolyzer process ensures highly pure hydrogen, through a first catalytic conversion stage and a second drying step. During this process, a certain percentage of hydrogen is converted to water or water vapor, and it is not delivered to the low-pressure compressor for the filling of the main storage tanks. According to private communication with Hydrogenics, this percent can be neglected. The hydrogen purge is an operation phase following the nitrogen purge. The nitrogen purge is recommended before maintenance operation and after maintenance operation. The purpose of the hydrogen purge is to remove the nitrogen from the hydrogen gas system and to replace it with hydrogen. The Hydrogen purge can only be activated after a successful nitrogen purge. Apart from some operational parameters, the hydrogen purge is functionally identical to hydrogen production. However, the produced gas is vented to the atmosphere instead of being released to the user line and being delivered to the main storage tanks. The amount of hydrogen used for this purification step is contained in a volume of 2000 L of water, which is 2 m3. Considering that the hydrogen purge is performed with a pressure of 10 bar and a temperature of 20  C, the hydrogen density under these condikg tions is equal to 0.85 m 3 . This means that the hydrogen mass used for every hydrogen purge is about 1.7 kg per process. The amount of hydrogen lost per month for this procedure depends on how many times the hydrogen purge is performed during the month. It is heavily dependent on maintenance operations, reboots or troubleshooting. In the logbook, only the day when the hydrogen purge is performed is reported, but not how many times the procedure is performed on that day. In order include anyway this slice of hydrogen lost in the leak analysis, it was decided to consider a hydrogen average value used for purging and to relate it in percent with an average value of monthly production of hydrogen. Based on visual experience, on average, about 10 purge operations are performed during a month. So, on average, 17 kg of hydrogen are used monthly for purging. By comparing these values, it can be concluded that, on average, 10% of the production is lost during the purging process. This percentage is strongly influenced by the low demand for hydrogen. If the use of hydrogen was greater, these hydrogen aliquots lost in these phases would have a much lower influence and the percentage would be much lower. 4. Analysis of results During the experimental activities related to this work, several topics have been investigated in order to perform a hydrogen assessment and a specific hydrogen loss analysis. Possible causes of inaccuracy in data or potential hydrogen losses have been studied:
Instruments calibration and accuracy;
Data acquisition system analysis: frequency and types;
Station standard operation: stand-by period, production phase and dispensing process.
Maintenance operation and drafting of checklists;
Database analysis; After the comparison between the logbook and database data, the discrepancy, between hydrogen produced and hydrogen dispensed, decreases greatly. Table 3 shows the final data of the analysis comprehensive of the average percent of hydrogen lost during the purification process in the electrolyzer. In Fig. 6 a

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Table 3 Hydrogen Assessment, current situation after the analysis. Month

Hydrogen Produced [kg]

Hydrogen Dispensed [kg]

Specific Hydrogen Losses [-]

May June July August September October November

191.9 140.2 180.1 132.6 117.7 172.7 190.2

184.3 121.2 109.5 115.9 112.3 155.6 185.3

3.9 13.6 39.2 12.5 4.6 9.9 2.6

Fig. 6. Hydrogen assessment and error.

comparison between the previous situation and the current situation is shown. July should be left out because it is a singularity due to various maintenance operations and equipment testing. More hydrogen purges were performed during that month. It should be remembered that the amount of hydrogen considered in this analysis for purging the electrolyzer is simply an average value. For May, June, August, September, and November, the current average percentage of losses is found to be between 2 and 10%, whereas before it was between 30 and 35%. This current range of percentage (2e10)% includes all experiments done, defueling the buffer tanks, rebooting the dispenser, venting the lines, the uncertainty of the mass flow meter inside the dispenser (±5%) and the inherent uncertainty of Faraday’s law for hydrogen production estimate. Considering the results of the analysis, Table 4 reports the

degree of importance that was discovered for each topic analyzed for the Cal State LA hydrogen fueling station. Hydrogen assessment analysis has to be carried out by mean of a proper methodology and care. As a way to begin the analysis, a check on hydrogen storage level could help in avoiding data mismatching and wrong interpretation, leading to a bigger discrepancy during the current month between production and dispensing. Being aware of how the data acquisition system is working, communicating and recording data is a key element, and to avoid data missing values due to electronic problems, a periodic comparison between a logbook and the database is strongly advised. Performing data monitoring and analysis with different frequencies could also lead to a deeper understanding of the phenomena involved and potential issues that are occurring. This work advises a quarter analysis, accompanied by a monthly and a daily one when it is necessary. Leaks during the standby period of the station can be overlooked, while potential losses during the normal station operation should be considered. Maintenance activities are the most critical ones, and periodical checklists can be performed to minimize leakages or to prevent failures and leaks. To carry out this analysis, hydrogen production has been estimated through the Faraday equation, and the hydrogen dispensed has been measured with a Rheonik mass flow meter. However, the utilization of the other 2 m is strongly advised, in order to improve data comparison. A meter right after the electrolyzer (Sage Meter at Cal State LA Station) could ascertain a clear perception of the hydrogen going to the low-pressure compressor after the drying process. Knowing this value means to know not the gross production, but the net generation. This concept depends on how accurate the electrolyzer company is in estimating the production through the current and the percentage of hydrogen lost during the drying process. Another very useful meter could be a meter right before the booster compressors (Rheonik meter at Cal State LA Station) which can measure how much hydrogen leaves the main storage tanks to transfer to the vehicle. If some high-pressure buffer tanks are part of the station layout, as in the station analyzed in this paper, it is important to have them almost full to compare the

Table 4 Leak Analysis, final results. Action

Level of Importance Low

Main storage tanks full at the beginning of the month Comparison between Logbook and Database Different frequencies in data collection and comparison Leaks during the standby period of the station Leaks during station operation Leaks from the storage tanks Maintenance Activities Periodic use of the checklists Use the Sage Meter after the electrolyzer Use the Rheonik Meter located after the main storage tanks

Medium

High ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓

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Rheonik measured values with the hydrogen dispensed. Recommendations for an accurate comparison between « hydrogen produced and hydrogen dispensed» and specific hydrogen losses are:
Perform regular and periodic controls through checklists.
When the dispenser is rebooted, close the manual valve of the low-pressure buffer tank: every time that this procedure is negated, 0.5 kg of hydrogen get lost and vents.
Use the Data acquisition system to improve the station.
Upgrade the station with welded joints along the stretches where it is not necessary to disassemble for maintenance of the piping.
Ensure that personnel will be able to easily reach joints and fittings (to check for leaks).
Be sure that at the beginning of the month all the main storage tanks are almost full to avoid poor comparisons and misunderstandings during the reading of the data.
Compare periodically the Logbook and the Database.
Record in the logbook how many times the hydrogen purge procedure is performed, and not just the day when it is performed.
Use more hydrogen mass flow meters to improve data collection and to increase analysis reliability; a meter close to the electrolyzer and a meter right before the hydrogen boosters are strongly advised;

5. Conclusions This paper described an engineering approach for hydrogen accounting from production to dispensing at Cal State LA hydrogen station, equipped with an electrolyzer for on-site production. Particularly, the accounting process has been carried out by taking an in-depth look at current practices and database, and its analysis has been based on data from daily, monthly and quarterly reports of hydrogen production and dispensing related parameters and from its data acquisition system. It described how the station addressed this analysis investigating several station areas, carrying out an assessment of the possible critical points, flanked by considerations, calculations, mathematical modeling and analysis of the database. The analysis focused on 2016 station data, which showed an inordinately large discrepancy that the station displayed between the amount of hydrogen produced and the quantity of hydrogen dispensed, of about 35%. In order to carry out a hydrogen assessment to investigate specific hydrogen losses at the station, several topics have been investigated. In fact, the adopted approach investigated two different levels and main classes, “Methodology and Instruments” and “Station Evaluation”. Among the several activities, the key elements which have been analyzed are the instruments calibration, measurement, and accuracy, performing a database analysis through the data acquisition system. Data collection has been executed with a different frequency (daily, monthly and quarterly), comparing both the electronic database and the station logbook. Station standard operation has been investigated, focusing on stand-by periods, production phase and dispensing processes. Drafting of checklists helped in finding potential sources of leaks. Among all areas analyzed, maintenance activities revealed themselves as the most critical ones, leading to data mismatching in hydrogen accounting. The analysis led to a marked improvement on the station operation know-how. In fact, for most of the analyzed months (May, June, August, September, and November), the current average

percentage of losses was found to be between 2 and 10%, whereas before it was between 30 and 35%. This current range of percentage (2e10)% includes all experiments done, defueling the buffer tanks, rebooting the dispenser, venting the lines, the uncertainty of the mass flow meter inside the dispenser (±5%) and the inherent uncertainty of Faraday’s law for hydrogen production estimate. The results found out during this research activity provide a guideline with recommended practices based on the authors’ experience, deduced for station operators and builders, including several steps for leakage monitoring, prevention, and troubleshooting. It is in the authors’ programs and future works to use the assessment results, the estimated errors and the described lesson learned, to discuss how leaks and maintenance activities could economically affect a hydrogen station and its business case. Data will be used to describe how external factors, such as nearby station fuel deficiency, failures or technical issues can positively affect the hydrogen dispensed, increasing the hydrogen demand. Another future work will be a comprehensive description of the Cal State LA Hydrogen Station fueling distribution, in order to share with the scientific community interesting data on several back-toback hydrogen vehicle fueling.

Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This research was supported by a U.S. Department of Energy grant (DE-EE0005890) and in part by the National Science Foundation CREST Center for Energy and Sustainability funding (NSF1547723). The research was also supported by the grant PON RI 2014e2020 for Innovative Industrial PhD (CUP H25D18000120006 and Code DOT1305040), funded by the European Union and the Italian Ministry of Education, University and Research (MIUR). The authors also thank Crystal Xie of Crystallogy Consulting for the operations support to the hydrogen facility.

Nomenclature % A APRR C Cgeneral m_ p PDC Rg T

Percentage [-] Leakage area [mm2] Average pressure ramp rate [MPa/min] Discharge coefficient [-] General flow coefficient [-] Mass flow rate[kg/s] Pressure [Pa]

Diaphragm gas compressor company Hydrogen gas constant, 4124:3½J =ðkg ,KÞ Temperature [K]

Greek

a Dp b ε

r

Coefficient for Hydrogen Equation of state,1:9155  106 ½K =Pa Difference between the pressure upstream and downstream the leaking hole [Pa] Diameter ratio [-] Expansion coefficient[-] Density [kg/m3 

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Subscript 1 2 D Leak p

Related to parameter upstream the leaking hole Related to parameter downstream the leaking hole Dispensed Related to leakage Produced

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Please cite this article as: Genovese, M et al., Hydrogen losses in fueling station operation, Journal of Cleaner Production, https://doi.org/10.1016/ j.jclepro.2019.119266