I wrote this article for Power & Beyond Magazine; you can see it on their website here.
Green hydrogen has great potential for zero-carbon energy storage in applications like power grid balancing. This article discusses the technologies involved and the barriers to overcome for ensuring full commercial success.
Comprising only one electron and one proton, hydrogen is the simplest and most abundant element on earth – and it can store or deliver a massive amount of clean energy. This makes it a possibly attractive green alternative to battery-based energy storage in applications like grid balancing, which buffers intermittent renewable energy sources to meet energy consumers’ real-time requirements.
However, the gas rarely exists by itself in nature and must be produced from compounds using electrochemical extraction processes.
This creates an issue; while hydrogen handles energy cleanly, its method of production is not necessarily green. So, to fulfil hydrogen’s attractive potential for ‘deep green’ energy storage, we must firstly be sure that any production methods used are truly carbon-free, and secondly, understand and address the challenges associated with harnessing hydrogen on a commercial scale.
This article looks at the issues involved. We discuss what is actually meant by ‘green’ hydrogen, and how it can be used in grid balancing applications. Then we look at electrolyzers and fuel cells, which are the key components of such systems. We conclude by reviewing the challenges facing these components and their integration into grid balancing systems.
A rainbow of hydrogen colors
Green hydrogen is just one (albeit the cleanest) of a rainbow of hydrogen colors as understood within the industry. They reflect the various ways the gas is produced.
Green hydrogen: Produced using renewable energy sources like wind or solar power. As the only type produced in a climate-neutral manner, it plays a vital role in global efforts to achieve net-zero emissions by 2050.
Blue hydrogen: Blue hydrogen is derived from natural gas (methane). It’s created through a process called steam reforming, where the carbon generated is captured and stored underground using industrial carbon capture and storage (CCS).
Grey hydrogen: The most common form of the gas, and produced from natural gas (methane) without carbon capture. As a result, it has higher emissions than green or blue hydrogen
Brown hydrogen: Produced from coal, and the least environmentally friendly option due to direct CO₂ emissions during production.
Other hydrogen colors, including turquoise, pink, and yellow, also exist.
Green hydrogen is typically produced by electrolysis, in which electricity separates H2O water molecules into hydrogen and oxygen. If the electricity comes from a renewable source such as solar or wind power, the hydrogen produced can be considered as green.
Emissions from green hydrogen production can be as little as 43 gCO2e/kgH2 (e = equivalent) of hydrogen produced using electrolysis. In any case, they cannot exceed 3.4 kgCO2e/kgH2 to comply with the EU’s carbon intensity ceiling for a ‘green’ definition.
To provide context, one kg of hydrogen provides as much energy as one gallon of gasoline, which produces 9.3 kg/CO2 during combustion.
However, costs are a barrier. Without pricing in carbon emissions, grey hydrogen is inexpensive, costing €1 to €2 per kilogram. By contrast, green hydrogen costs €3 to €8/kg in some regions, depending on the availability of abundant, low-cost renewable resources.
Yet production costs will decrease over time, due to continuously falling renewable energy production costs, economies of scale, lessons from projects underway and technological advances. As a result, green hydrogen will become more economical. And, as it becomes more attractive, it will increasingly be used in applications from hydrogen vehicles to power grid energy balancing.
GREEN HYDROGEN
Efficient Electrolysis through Comprehensive Power Conversion Solutions
Energy balancing is essential because solar and wind power, while established among the most promising renewable energy technologies, are disadvantaged by their unpredictability and because installations’ peak output rarely coincides with times of peak demand.
Hydrogen can address these issues by helping to balance fluctuations in renewable power supply and demand, supporting grid stability and enhancing the integration of intermittent renewable energy sources. It does so by pairing electrolyzers and fuel cells. During times of excess renewable energy production, the electrolyzers act as a load on the grid; they use the renewable grid power to extract and store hydrogen from water. When demand outstrips supply, the stored hydrogen is fed to fuel cells which generate electricity for feeding back into the grid.
Below, we take a closer look at how electrolyzers and fuel cells work, and then at some of the issues associated with integrating them into grid networks.
Electrolyzers
Electrolyzers comprise an anode and a cathode separated by an electrolyte. Electrolyzers function in various ways, mainly governed by the type of electrolyte material involved and the ionic species it conducts.
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.
Alkaline electrolyzers operate via 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 have been commercially available for many years. Newer approaches using solid alkaline exchange membranes (AEM) as the electrolyte are showing promise on the lab scale.
Electrolysis is a leading hydrogen production route to achieving the U.S. Department of Energy’s Hydrogen Energy Earthshot’s goal of reducing the cost of clean hydrogen by 80 % to $1 per 1 kilogram in 1 decade ("1 1 1") since its launch on June 7, 2021. Hydrogen produced via electrolysis can result in zero greenhouse gas emissions, if the electrolysis process uses electricity from renewable sources.
Fuel cells
Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes - a negative electrode (or anode) and a positive electrode (or cathode) - sandwiched around an electrolyte.
A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a polymer electrolyte membrane (PEM) fuel cell, a catalyst separates hydrogen atoms into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they reunite with oxygen and the electrons to produce water and heat.
Note that in a fuel cell, the anode is negative relative to the cathode, which is opposite to the polarization of an electrolytic cell. This is because the anode emits electrons into the external circuit, while the cathode receives them from the circuit.
PEM cells operate at relatively low temperatures and can quickly vary their output to meet shifting power demands and can be used for stationary power production. Their fast response also makes them ideal for grid support services.
Other fuel cell types include direct-methanol, alkaline, phosphoric acid, molten carbonate, and solid oxide, used variously for portable and stationary power applications.
As fuel cells produce heat as well as electricity, combined heat and power fuel cells are of interest for fulfilling not only electrical but also heating needs, including hot water and space heating, in houses and other buildings. Total efficiencies as high as 90 % are possible.
Regenerative (or reversible) fuel cells are an emerging technology of particular interest to the application we have been discussing – power grid balancing – because they can
not only produce electricity from hydrogen and oxygen but can also be reversed and powered with electricity to produce hydrogen and oxygen.
Challenges of hydrogen-based grid energy management systems
After green hydrogen is produced by electrolysis, it must be stored, possibly transported, and converted back to electricity by fuel cells. All of these stages have issues that must be addressed to achieve commercially-attractive deployment of green hydrogen for grid balancing applications.
Production: Electrolysis has an efficiency of around 60-80 % by calorific value. The commercialization and large-scale deployment challenges of electrolysis include a need for improved overall energy efficiency and additional onsite compressors. The low lifetime of electrolyzers, currently below five years, is insufficient.
Storage: Hydrogen is stored typically by three methods: compression, cooling, or hybrid. Material based hydrogen storage is also being developed in the form of solids, liquids, or surface-based materials. Hydrogen can be stored on-site or in bulk for production plants and end-use applications, and bulk storage is used in large-scale geographical hydrogen storage systems such as salt caverns, abandoned mines and similar locations. However, there are some storage challenges:
High energy requirement in compressed hydrogen storage, due to low specific gravity.
Temperature and pressure requirements while storing hydrogen in solid form.
Design aspects, legal issues, social concerns, and high cost.
Low durability of materials (fiber, metals, polymers etc.) for storage, and potential chemical reactions raise safety concerns.
Bulk storage at geographic features may contaminate the hydrogen, creating the need for further purification before end-use.
Transport and distribution: The electrolyzers used in grid balancing applications may be located remotely from their client fuel cells; this calls for transportation and distribution of the gas.
Common transport and distribution methods include pipelines, high-pressure tube trailers, and liquified hydrogen tankers. Pipelines are currently the least expensive way and are already in use near large refineries and chemical plants. Liquified hydrogen tankers transport hydrogen that has been cooled into liquid form. This increases the density of distributed hydrogen, making it more efficient for transportation than high-pressure tube trailers. However, if the delivery and consumption rates are not matched, the compressed hydrogen will evaporate causing significant losses and ineffective utilization. The challenges in transport and distribution of hydrogen are as follows:
The existing hydrogen transportation pipeline infrastructure is not sufficient to meet future demand.
Existing natural gas pipelines cannot be directly used for hydrogen due to embrittlement. Mixing hydrogen with natural gas, although an option, significantly affects pipeline life even at 5 % concentration by volume.
Fluctuations in temperature during fast transfers of compressed hydrogen have to be controlled optimally to minimize losses and prevent thermal instability.
Alternate ways for transporting hydrogen, for example using liquid organic materials as hydrogen carriers, are being researched to enable a low-cost high energy density transfer of hydrogen.
Fuel cells: As the last link in the hydrogen-based grid balancing chain, fuel cells also create challenges when being considered for large-scale commercialization. Efficiency, degradation issues, durability, resiliency, size as well as power, and current densities of fuel cells, all need improvement.
Also, fuel cell systems are highly complex, especially relating to thermal and water management, purification, and humidification. These issues are exacerbated by insufficient monitoring of the system’s performance and state of health. Additionally, hydrogen fuel cells have a short lifespan and need frequent replacement.
Conclusion
The above shows the challenges of using hydrogen for grid balancing, and ensuring that it’s green. However, there is huge motivation to see it fulfil its potential, and new developments are continuously being announced. For example, Australian company Hysata claims they have developed a capillary-fed electrolyzer which achieves 95 % efficiency. Meanwhile, researchers at Tohoku University have developed a novel catalyst model which sets new standards in fuel cell technology .
There are larger drivers on the world stage as well, including the World Economic Forum; they have launched their Clean Hydrogen Project Accelerator initiative as part of their Transitioning Industrial Clusters platform, to find ways to accelerate its adoption.
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