Hydrogen: A Potential Solution for Energy Storage

What is hydrogen?

Hydrogen is the first element of the periodic table. It is the simplest, lightest and most abundant substance found in the universe. Hydrogen atoms can’t exist freely on their own and hence hydrogen is always available in the form of a compound formed by bonding of hydrogen atoms with atoms of another element. Hydrogen has been in use in many industrial applications. It has already been a key component of chemical industrial processes, the hydrogenation of vegetable oils, and the steel industry. Hydrogen, used so far for industrial applications, is produced mainly from natural gas. Since natural gas is not in unlimited stock and producing hydrogen from natural gas involves carbon footprints, other options for producing hydrogen are in exploration. Hydrogen is the most abundantly available on the earth in water. Breaking the molecular bonds of water, i.e., hydrolysis can produce pure hydrogen. Water is available in abundance on earth and producing hydrogen from water does not leave carbon footprints. However, the process of generation of electricity used for producing hydrogen may not be free of carbon footprints.

Classification of Hydrogen based on its Carbon Footprints

a.  Green Hydrogen: 

When the electrical energy generated from renewable sources like wind/water/solar is used to power electrolysis for producing hydrogen, the hydrogen so produced can be stored in tanks and fed into gas grid, which is called green hydrogen. Green hydrogen is a solution for cutting our carbon footprints.

b.  Blue hydrogen

When natural gas is mixed with hot steam and a catalyst, a chemical reaction takes place, creating hydrogen and carbon monoxide. Then water is added to that mixture, which turns carbon monoxide into carbon dioxide and more hydrogen is produced. This process of producing hydrogen from natural gas is called steam methane reforming. If the carbon dioxide emissions are then captured and stored underground, the process is carbon-neutral, and the resulting hydrogen is called “blue hydrogen”. But owing to fugitive leaks of methane from the drilling, extraction and transportation process, natural gas production inevitably results in methane emissions. Methane does not last in the atmosphere as long as carbon dioxide, but it is much more potent as a greenhouse gas. Over 100 years, one ton of methane can be considered an equivalent to 28 to 36 tons of carbon dioxide, according to IEA.
c.  Grey hydrogen

When hydrogen is produced from natural gas, forming like blue hydrogen, but without any efforts to capture carbon dioxide byproducts, it is called “grey hydrogen” and it leaves a carbon footprint.

d. Pink hydrogen

When electrolysis for producing hydrogen is powered by nuclear energy, the produced hydrogen is called “Pink hydrogen”. It does not have carbon footprints but creates radioactive wastes which must be stored safely for thousands of years.

e. Yellow hydrogen

When grid electricity is used to power electrolysis for producing hydrogen, the hydrogen so produced is called “yellow hydrogen”. The carbon emissions in the production process vary greatly depending on the sources powering the grid.

f. Turquoise hydrogen: 

When hydrogen is produced from methane pyrolysis, i.e., splitting methane into hydrogen and solid carbon with heat in reactors or blast furnaces, hydrogen so produced is called “turquoise hydrogen”. Turquoise hydrogen is still in its nascent stages of being commercialized, and its climate-conscious value depends on powering the pyrolysis with clean energy and storing the physical carbon.

Hydrogen and Energy Link

Energy is consumed for producing Hydrogen.

The process of producing hydrogen involves breaking the molecular bonds of compound containing hydrogen (such as water), consumes energy because energy is required to break those molecular bonds. This means energy is an essential raw material in the production of hydrogen.

Energy is produced from combustion of Hydrogen.

On the other hand, when hydrogen is burnt, energy is generated in the form of heat with water as a byproduct. This means energy can be generated from hydrogen by burning it and this process of generating heat energy from hydrogen is free from carbon footprints. So, there is a possibility that if the process of generating electricity to be used for producing hydrogen is free of carbon footprints, the whole process of transforming the hydrogen so produced in the useful format of energy is totally free of carbon footprints. This presents hydrogen as one of the potential energy sources that could help reduce carbon emissions and eventually slow down global warming. 

Energy Value of Hydrogen by weight and by Volume
Gravimetric Energy Density of Hydrogen

The energy content of hydrogen is described by its (lower and higher) heating value. The lower gravimetric heating value of hydrogen can be expressed as 33.33 kWh/kg and the higher gravimetric heating value is 39.39 kWh/kg. For practical purposes, 33 kWh/kg can be used. The energy of 1 kg of hydrogen is equivalent to 3.3 kg of gasoline (based on the lower heating value).

Volumetric Energy Density of Hydrogen 

The lower volumetric heating value of hydrogen can be expressed as 2.8 to 3.00 kWh/Nm³ and the higher volumetric heating value is 3.54 kWh/Nm³. The unit Nm³ refers to Normal Cubic Meters i.e., Cubic meters at normal temperature (293-degree Kelvin) and pressure (1 atmospheric pressure). The lower heating value of 3 kWh/Nm³ is usually used if the hydrogen is not burnt directly. Hence, a practical medium value to keep in mind is roughly 3 kWh/Nm³ or 3 Wh/l. 
The energy content of 1 Nm³ (=1000 NL) hydrogen gas is equivalent to 0.36 L gasoline; 1 L liquid hydrogen is equivalent to 0.27 L gasoline. 

Uses of Hydrogen 
Traditional Uses in Industries

Hydrogen has been in use for a long time in various industrial applications, including chemical industrial processes and in the steel industry. Specific industrial applications of hydrogen include:
a.  In the synthesis of ammonia to manufacture nitrogenous fertilizers.

b.  In the hydrogenation of unsaturated vegetable oils for manufacturing vanaspati ghee or fat for the kitchen.

c.  In manufacturing of many useful organic compounds, including methanol.

d.  In preparing important chemical agent like hydrogen chloride (HCl).

e.  In metallurgical processes for extracting metals from many metal oxides. Often hydrogen-based chemicals are used in these processes.

Hydrogen used for the above purposes is produced from natural gas. Producing hydrogen from natural gas is cheaper than producing pure hydrogen from water but natural gas is not only getting scarce, it leaves carbon footprints too. So current use of hydrogen in industrial applications may not be sustainable in the long term as well as involves carbon footprints. Production and use of clean hydrogen in those industrial processes are critical to reducing carbon emissions.

Uses of Hydrogen in Energy Sector: New Paradigm

Considering that hydrogen may be a viable option for transforming and storing energy and that such energy use may combat climate change, the use of pure hydrogen as a source of energy in the industrial, transport and domestic sectors has become a topic of interest to all.
a.  Hydrogen is seen as a promising means of transformation, transition, and transportation of energy.

b.  Hydrogen is also seen as a promising means for storing surplus energy in the grid as well as energy generated from intermittent sources.

c.  Hydrogen can be used as fuel for the transport sector. Although non-carbon emitting electric storage batteries are found suitable for personal vehicles, hydrogen may be an alternative to electric batteries for heavy trucks and airplanes that contribute a lot to global warming and batteries are not suitable for them. The use of hydrogen for personal vehicles also cannot be ignored.

d.  Hydrogen is also seen as a promising source of energy for the kitchen, i.e., for cooking.
Countries like Nepal which have abundant sources of clean and renewable energy, but no fossil fuels are spending huge amounts of their hard-earned foreign currency on the import of fossil fuels for industrial, transportation and domestic energy requirements. Storing domestically produced electrical energy in the form of hydrogen and then reusing it as fuel when required can substitute imported fuels from industrial, transport and household uses.
Hence, irrespective of the debate on the demerits of low efficiency and high costs, most countries around the world are engaged in partnerships to advance the hydrogen industry and technology for its use in various sectors. Countries have largely accepted that the development and use of hydrogen for energy generation will be a game changer for the clean energy transition. Apart from being a clean energy source, hydrogen’s big advantage is its versatility as an energy source. 

Other Side of the Coin 

The main drawback of hydrogen is that it is expensive. Producing hydrogen from natural gas costs about $1.50 per kilogram, whereas clean hydrogen costs about $5 per kilogram and the energy equivalent of 1 kg of hydrogen is 33 kWh. This means 1 kWh of energy from hydrogen costs about NRs 20. The US Department of Energy has launched a program called the Hydrogen Shot, which aims to reduce the cost of clean hydrogen to $1 per kilogram in one decade. The strategy to down the price may include improving the efficiency, durability, and manufacturing volume of the electrolyzer as well as improving pyrolysis, which generates solid carbon, not carbon dioxide, as a byproduct. Driving down the price of clean hydrogen would be a huge step toward solving climate change.

Some environmentalists argue that with an electric storage battery, we get back 90 percent of what we put in but with hydrogen, in the best case we get back only 37 percent of what we put in and 63 percent of it, is lost. With such a huge loss, hydrogen cannot be a commercially practical option. Similar arguments were floated against solar power that initially had a price between 30 to 40 USC, but now the prices are downsized to around 5 USC presenting solar power as competitive to power from other sources. 

Connect and Synergy with Renewable Power Generation 

Pure hydrogen can be produced via electrolysis of water at any moment the power is available for electrolysis. This process of hydrogen production via electrolysis offers opportunities for synergy with dynamic and intermittent power generation from some renewable energy technologies. For example, the inherent variability of wind is an impediment to the effective use of wind power even though the cost of wind power has continued to drop. Electric power generation at a wind farm can be integrated with hydrogen fuel. It might allow flexibility to shift production to best match resource availability with system operational needs and market factors. Also, it is possible to use the excess electricity to produce hydrogen through electrolysis in times of excess electricity production from wind farms instead of curtailing the electricity, as is commonly done.

Normal grid electricity is not the ideal source of electricity for electrolysis because:

(i) Most of the grid electricity is generated using technologies that result in greenhouse gas emissions and are energy intensive.

(ii) The price of direct grid electricity may not be suitable for producing hydrogen.

But electricity generation using renewable or nuclear energy technologies, either separate from the grid or as a growing portion of the grid mix, is a possible option to overcome these limitations for hydrogen production via electrolysis.
 

Hydrogen Production: Electrolysis
Why is electrolysis considered as the pathway for hydrogen production?

The US Department of Energy has launched the Hydrogen Energy Earth Shot program with the goal of reducing the cost of clean hydrogen by 80 percent to $1 per kg in a decade. Electrolysis is a leading hydrogen production pathway to achieve this goal. Hydrogen produced via electrolysis incurs zero greenhouse gas emissions, depending on the source of the electricity used. The source of the required electricity—including its cost and efficiency, as well as emissions resulting from electricity generation—must be considered when evaluating the benefits and economic viability of hydrogen production via electrolysis. Hydrogen production via electrolysis is being pursued for renewable (wind, solar, hydro, and geothermal) and nuclear energy options. These hydrogen production pathways result in virtually zero greenhouse gas and criteria pollutant emissions; however, the production cost needs to be decreased significantly to be competitive with more mature carbon-based pathways such as natural gas reforming.

What is Electrolysis and how hydrogen is produced?

Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production.

Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in different ways, mainly due to the different types of electrolyte material involved and the ionic species it conducts. Different types of electrolyzers being considered are:

a.  Polymer Electrolyte Membrane (PEM) Electrolyzer

In a polymer electrolyte membrane (PEM) electrolyzer, the electrolyte is a solid specialty plastic material. Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons). The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode. At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas. Anode Reaction: 2H2O → O2 + 4H+ + 4e- Cathode Reaction: 4H+ + 4e- → 2H2
 

b. Alkaline Electrolyzers

Alkaline electrolyzers operate via the transport of hydroxide ions (OH-) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte has been commercially available for many years. Newer approaches using solid alkaline exchange membranes (AEM) as the electrolyte are showing promise on the lab scale.
c. Solid Oxide Electrolyzers

Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions (O2-) at elevated temperatures, generate hydrogen in different ways. Steaming at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit.

Solid oxide electrolyzers must operate at temperatures high enough for the solid oxide membranes to function properly (about 700°–800°C, compared to PEM electrolyzers, which operate at 70°–90°C, and commercial alkaline electrolyzers, which typically operate at less than 100°C). Advanced lab-scale solid oxide electrolyzers based on proton-conducting ceramic electrolytes are entitled for lowering the operating temperature to 500°–600°C. The solid oxide electrolyzers can effectively use heat available at these elevated temperatures (from various sources, including nuclear energy) to decrease the amount of electrical energy needed to produce hydrogen from water.

Foreseen Challenges and Research Focuses

The Price 

The cost per kg of hydrogen at present is approximately US$5. Considering this and the further losses, using hydrogen as a fuel is not commercially viable. But considering the environmental and other advantages of hydrogen as a fuel for countries like Nepal that import fossil fuels, the development of hydrogen as a fuel cannot be ignored. Hence, the current price of pure hydrogen is a primary challenge as well as it calls for more research. 
The Hydrogen Shot program of the US Department of Energy has a clean hydrogen cost target of $1/kg H2 by 2030 (and an interim target of $2/kg H2 by 2025) through an improved understanding of performance, cost, and durability trade-offs of electrolyzer systems under predicted future dynamic operating modes using CO2-free electricity. This also involves: 

a)   Reducing the capital cost of the electrolyzer unit and the balance of the system. 

b)   Improving energy efficiency for converting electricity to hydrogen over a wide range of operating conditions and increasing understanding of electrolyzer cell and stack degradation processes, thereby developing mitigation strategies to increase operational life.

Storage 
Hydrogen has a gravimetric energy density of about 33 kWh/kg. The gravimetric energy density presents hydrogen as an efficient carrier for storing and transporting energy, surpassing the energy storage capacity of Li-ion batteries. But it has a very low volumetric energy density of 2.8 Wh/l, which poses a major challenge in its efficient storage and transportation. Since a large volume of hydrogen shall be required for little energy value, large size of storage tanks is a major challenge. 

Physical-based methods of storing and transporting hydrogen include compressed hydrogen, Cold/Cryo-compressed and liquid hydrogen. Similarly, material-based methods include liquid organic and metal hydrides.

i.  Compressed Hydrogen 

Compressed hydrogen is the most common mechanical storage method, as the technology and costs are well-known for compressed storage. But for its low volumetric energy density of 870 Wh/l at 370 bars and 1400 Wh/l at 700 bars; it is not efficient as the compressing consumes 15-16% energy. Besides this, special tanks are needed to endure compression and meet safety requirements and possibilities of gas leakage exist. 

ii.  Liquified hydrogen 

Liquified hydrogen has higher volumetric energy density and high purity, but the equipment is costly and energy-intensive that requires 11.9 -15 kWh/kg of H2 resulting in a current liquification cost of 2.5-3 $ per Kg of LH. High boil-off losses can consume up to 40 percent of available energy during storage and transportation and handling. Requirement of sophisticated tanks and facilities to maintain -253 degrees centigrade temperature makes storage difficult. Absence of safety standards and regulations impedes the development of infrastructures for liquified hydrogen.

iii.  Liquid Organic Hydrogen Carriers (LOHC) 

LOHC is based on the absorption of hydrogen by organic compound and release through a chemical reaction. It has high volumetric energy density; higher stability and safety as LOHC do not require high pressure and extremely high temperature; has compatibility with existing infrastructures like pipeline and tanks to reduce the cost; can facilitate prolonged storage and long-distance transportation without energy loss and energy can be released in a controlled manner at time and location of need. But LOHC is inefficient and may need purification steps to increase the costs.

iv. Metal hydrides

Metal hydrides’ hydrogen storage method involves forming a chemical compound between hydrogen and metal. It has a good volumetric capacity of up to 18 wt. percent of hydrogen that makes it suitable for onboard applications. However, hydrogen release temperature is high in the range of 100 to 400-degree centigrade. Promising light metal hydrides have high gravimetric storage capacity and can release hydrogen at low temperatures. But maintaining their efficiency at a commercial scale and reducing the cost of metal hydride production and implementation are major challenges.

Metal hydrides and LOHC have promising applications in various industrial sectors, including energy storage in residential and industrial settings as well as power sources for Unmanned Arial Vehicles (UAVs like drones). Thus, metal hydrides can become a feasible option for transportation and portable applications. In conclusion, hydrogen has the potential to revolutionize the storage and transportation of energy.

25 k W PEM electrolyzer

Hydrogen as Cooking Energy

Nepal is known to be an agrarian country with the majority of the population living in rural areas. But according to preliminary reports of Census 2021, 67 percent of the Nepalese population is now living in metropolitan, sub-metropolitan and municipal (urban and semi-urban) areas. This is a major demographic shift showing a trend of fast, aggressive migration from rural to urban areas. 

Nepal has huge hydro and solar potential as source of clean energy but lacks fossil and mineral fuels. Most of the domestic energy especially for rural kitchens comes from biomass, whereas urban and sub-urban kitchens depend on imported petroleum products (LPG). A recent census has indicated a demographic change in the country with around 67 percent of the Nepalese population now living in urban and semi-urban areas and depending upon imported LPG for their kitchens. Imported LPG is making penetration aggressively in rural areas also. For that reason, it is important to know if hydrogen gas can be used in kitchens as fuel for cooking like LPG. If it is proven to be technically and commercially viable option, it could be a game changer for countries like Nepal that do not have natural gas reserves and spend a major portion of their hard-earned foreign reserves for importing petroleum-based fuels on cooking, transport and other industries.

Since hydrogen produces heat energy when it is burnt and 33 kWh energy can be availed from 1 kg of hydrogen, there is a possibility to use it in the kitchen as cooking fuel. Research and development works are ongoing to make it technically and commercially possible by overcoming the following challenges:

a. Hydrogen has a very low volumetric energy density, i.e., a large volume of hydrogen at NTP is required for a small quantum of energy. This requires very large cylinders for hydrogen storage in the kitchen. Gas cylinders and technology to store large volumes of hydrogen in cylinders like LPG or oxygen cylinders will have to be developed. 

b. High-efficiency stoves with special burners suitable for burning hydrogen need to be developed.

c.  Suitable arrangement will have to be provided in the stove to regulate and control the flow of hydrogen to the burners so that kitchen personnel finds hydrogen-based cooking like LPG-based cooking.

d.  A dependable safety system, including flash arrestors, will have to be provided at a suitable place as close to the stove burner as possible.

e.  The flame of burning hydrogen is colorless and not visible to the cook in the light although it is visible in the dark. Some arrangements will have to be made so that the flame is clearly visible and is attractive like in an LPG stove.

f.  The cost of hydrogen will have to be comparable to other fuels in the affordable range of consumers.

Gas stoves with burners suitable to burn hydrogen have been developed that look very similar to LPG stoves. Small-sized hydrogen gas cylinders with a limited volume of hydrogen have been used for hydrogen burning in these special cook stoves for experimental purposes. Hydrogen gas has been burnt successfully to generate heat in these stoves on an experimental basis. But to make it a practical option for the kitchen, all the above-mentioned challenges must be overcome through further research and development.

But it is the impression that at the current stage of development of hydrogen-based cooking, 100 percent hydrogen for cooking purposes is theoretically possible, but there are practical problems. If we mix hydrogen with natural gas in a definite proportion and compress this mixture, not only the gas cylinders can be downsized, but the burning flame will also be attractively visible in light. We can start with a mixture of 20 percent hydrogen and 80 percent natural gas and then slowly increased hydrogen portion as the technology will keep on improving. Ultimately, an optimal proportion of a mix of hydrogen and natural gas can be identified for commercial use. Even if the proportion is 50-50, the country can make a huge savings on its foreign currency reserve. The clean Hydrogen production process available at the current stage costs $5/kg and there are losses too. Using hydrogen for cooking is a costly affair (USC 15 per kWh) for now.  But if the electricity used in electrolyzer is off-peak surplus of the grid or intermittent renewable sources, the cost may be lowered to make it competitive. So, the efficiency of hydrogen production, storage, and transportation process as well as the use of surplus energy can make it a commercially viable option for the kitchen. 

What is possible now in Nepal?

Hydrogen production and storage through grid electricity is not a commercially viable solution as of now in Nepal due to the tariff rate of grid electricity supplied by the utility. Similarly, standalone hydro, solar or wind power plants linked with hydrogen plant is also not a commercially viable option to recover the cost of the power plant and hydrogen plant. There are small, mini, or micro hydropower plants owned and operated by the community in off-grid mode in remote localities. Since the local households do not have a demand for electricity during off-peak hours of the day, the generating capacity during off-peak hours is not utilized. If this off-peak generating capacity of the power plant is utilized to produce hydrogen and supply this hydrogen to the local community as fuel for their kitchen, it could not only earn palpable benefits to the local community but would manifest hydrogen gas as a dependable fuel for our kitchen. The community can install an electrolyzer of suitable capacity, say 25 kW, operate a power plant during off peak hours also and power the electrolyzer with this off-peak electricity. Thus, the cost of powering the electrolyzer will be the bare minimum. The produced hydrogen may be stored and used by locals in their kitchens on a daily basis so that storing hydrogen is not a big deal.

Mr. Bhat is the former DMD of Nepal Electricity Authority

This article is taken from Urja Khabar bi-annual Journal Publish on 16th June, 2023