batteries – Cabot Institute for the Environment blog

Hydrogen: where is low-carbon fuel most useful for decarbonisation?

Posted on November 10, 2020March 31, 2023 by janet.crompton

Is hydrogen the lifeblood of a low-carbon future, or an overhyped distraction from real solutions? One thing is certain – the coal, oil and natural gas which currently power much of daily life must be phased out within coming decades. From the cars we drive to the energy that heats our homes, these fossil fuels are deeply embedded in society and the global economy. But is the best solution in all cases to swap them with hydrogen – a fuel which only produces water vapour, and not CO₂, when burned?

Answering that question are six experts in engineering, physics and chemistry.

Road and rail

Hu Li, Associate Professor of Energy Engineering, University of Leeds

Transport became the UK’s largest source of greenhouse gas emissions in 2016, contributing about 28% of the country’s total.

Replacing the internal combustion engines of passenger cars and light-duty vehicles with batteries could accelerate the process of decarbonising road transport, but electrification isn’t such a good option for heavy-duty vehicles such as lorries and buses. Compared to gasoline and diesel fuels, the energy density (measured in megajoules per kilogram) of a battery is just 1%. For a 40-tonne truck, just over four tonnes of lithium-ion battery cells are needed for a range of 800 kilometres, compared to just 220 kilograms of diesel.

With the UK government set to ban fossil fuel vehicles from 2035, hydrogen fuel cells could do much of the heavy lifting in decarbonising freight and public transport, where 80% of hydrogen demand in transport is likely to come from.

A fuel cell generates electricity through a chemical reaction between the stored hydrogen and oxygen, producing water and hot air as a byproduct. Vehicles powered by hydrogen fuel cells have a similar driving range and can be refuelled about as quickly as internal combustion engine vehicles, another reason they’re useful for long-haul and heavy-duty transport.

Hydrogen fuel can be transported as liquid or compressed gas by existing natural gas pipelines, which will save millions on infrastructure and speed up its deployment. Even existing internal combustion engines can use hydrogen, but there are problems with fuel injection, reduced power output, onboard storage and emissions of nitrogen oxides (NOₓ), which can react in the lower atmosphere to form ozone – a greenhouse gas. The goal should be to eventually replace internal combustion engines with hydrogen fuel cells in vehicles that are too large for lithium-ion batteries. But in the meantime, blending with other fuels or using a diesel-hydrogen hybrid could help lower emissions.

It’s very important to consider where the hydrogen comes from though. Hydrogen can be produced by splitting water with electricity in a process called electrolysis. If the electricity was generated by renewable sources such as solar and wind, the resulting fuel is called green hydrogen. It can be used in the form of compressed gas or liquid and converted to methane, methanol, ammonia and other synthetic liquid fuels.

But nearly all of the 27 terawatt-hours (TWh) of hydrogen currently used in the UK is produced by reforming fossil fuels, which generates nine tonnes of CO₂ for every tonne of hydrogen. This is currently the cheapest option, though some experts predict that green hydrogen will be cost-competitive by 2030. In the meantime, governments will need to ramp up the production of vehicles with hydrogen fuel cells and storage tanks and build lots of refuelling points.

Hydrogen can play a key role in decarbonising rail travel too, alongside other low-carbon fuels, such as biofuels. In the UK, 6,049 kilometres of mainline routes run on electricity – that’s 38% of the total. Trains powered by hydrogen fuel cells offer a zero-emission alternative to diesel trains.

The Coradia iLint, which entered commercial service in Germany in 2018, is the world’s first hydrogen-powered train. The UK recently launched mainline testing of its own hydrogen-powered train, though the UK trial aims to retrofit existing diesel trains rather than design and build entirely new ones.

Aviation

Valeska Ting, Professor of Smart Nanomaterials, University of Bristol

Of all of the sectors that we need to decarbonise, air travel is perhaps the most challenging. While cars and boats can realistically switch to batteries or hybrid technologies, the sheer weight of even the lightest batteries makes long-haul electric air travel tricky.

Single-seat concept planes such as the Solar Impulse generate their energy from the sun, but they can’t generate enough based on the efficiency of current solar cells alone so must also use batteries. Other alternatives include synthetic fuels or biofuels, but these could just defer or reduce carbon emissions, rather than eliminate them altogether, as a carbon-free fuel like green hydrogen could.

Hydrogen is extremely light and contains three times more energy per kilogram than jet fuel, which is why it’s traditionally used to power rockets. Companies including Airbus are already developing commercial zero-emission aircraft that run on hydrogen. This involves a radical redesign of their fleet to accommodate liquid hydrogen fuel tanks.

Three aeroplanes of different designs fly in formation.

An artist’s impression of what hydrogen-powered commercial flight might look like.
Airbus

There are some technical challenges though. Hydrogen is a gas at room temperature, so very low temperatures and special equipment are needed to store it as a liquid. That means more weight, and subsequently, more fuel. However, research we’re doing at the Bristol Composites Institute is helping with the design of lightweight aircraft components made out of composite materials. We’re also looking at nanoporous materials that behave like molecular sponges, spontaneously absorbing and storing hydrogen at high densities for onboard hydrogen storage in future aircraft designs.

France and Germany are investing billions in hydrogen-powered passenger aircraft. But while the development of these new aircraft by industry continues apace, international airports will also need to rapidly invest in infrastructure to store and deliver liquid hydrogen to refuel them. There’s a risk that fleets of hydrogen aeroplanes could take off before there’s a sufficient fuel supply chain to sustain them.

Heating

Tom Baxter, Honorary Senior Lecturer in Chemical Engineering, University of Aberdeen & Ernst Worrell, Professor of Energy, Resources and Technological Change, Utrecht University

If the All Party Parliamentary Group on Hydrogen’s recommendations are taken up, the UK government is likely to support hydrogen as a replacement fuel for heating buildings in its next white paper. The other option for decarbonising Britain’s gas heating network is electricity. So which is likely to be a better choice – a hydrogen boiler in every home or an electric heat pump?

First there’s the price of fuel to consider. When hydrogen is generated through electrolysis, between 30-40% of the original electric energy is lost. One kilowatt-hour (kWh) of electricity in a heat pump may generate 3-5 kWh of heat, while the same kWh of electricity gets you only 0.6-0.7 kWh of heat with a hydrogen-fuelled boiler. This means that generating enough hydrogen fuel to heat a home will require electricity generated from four times as many turbines and solar panels than a heat pump. Because heat pumps need so much less energy overall to supply the same amount of heat, the need for large amounts of stored green energy on standby is much less. Even reducing these losses with more advanced technology, hydrogen will remain relatively expensive, both in terms of energy and money.

So using hydrogen to heat homes isn’t cheap for consumers. Granted, there is a higher upfront cost for installing an electric heat pump. That could be a serious drawback for cash-strapped households, though heat pumps heat a property using around a quarter of the energy of hydrogen. In time, lower fuel bills would more than cover the installation cost.

A large fan unit sits outside an apartment building. — Heat pumps, like this one, are a better bet for decarbonising heating.
Klikkipetra/Shutterstock

Replacing natural gas with hydrogen in the UK’s heating network isn’t likely to be simple either. Per volume, the energy density of hydrogen gas is about one-third that of natural gas, so converting to hydrogen will not only require new boilers, but also investment in grids to increase how much fuel they can deliver. The very small size of hydrogen molecules mean they’re much more prone to leaking than natural gas molecules. Ensuring that the existing gas distribution system is fit for hydrogen could prove quite costly.

In high-density housing in inner cities, district heating systems – which distribute waste heat from power plants and factories into homes – could be a better bet in a warming climate, as, like heat pumps, they can cool homes as well as heat them.

Above all, this stresses the importance of energy efficiency, what the International Energy Agency calls the first fuel in buildings. Retrofitting buildings with insulation to make them energy efficient and switching boilers for heat pumps is the most promising route for the vast majority of buildings. Hydrogen should be reserved for applications where there are few or no alternatives. Space heating of homes and buildings, except for limited applications like in particularly old homes, is not one of them.

Electricity and energy storage

Petra de Jongh, Professor of Catalysts and Energy Storage Materials, Utrecht University

Fossil fuels have some features that seem impossible to beat. They’re packed full of energy, they’re easy to burn and they’re compatible with most engines and generators. Producing electricity using gas, oil, or coal is cheap, and offers complete certainty about, and control over, the amount of electricity you get at any point in time.

Meanwhile, how much wind or solar electricity we can generate isn’t something that we enjoy a lot of control over. It’s difficult to even adequately predict when the sun will shine or the wind will blow, so renewable power output fluctuates. Electricity grids can only tolerate a limited amount of fluctuation, so being able to store excess electricity for later is key to switching from fossil fuels.

Hydrogen seems ideally suited to meet this challenge. Compared to batteries, the storage capacity of hydrogen is unlimited – the electrolyser which produces it from water never fills up. Hydrogen can be converted back into electricity using a fuel cell too, though quite a bit of energy is lost in the process.

Unfortunately, hydrogen is the lightest gas and so it’s difficult to store and transport it. It can be liquefied or stored at very high pressures. But then there’s the cost – green hydrogen is still two to three times more expensive than that produced from natural gas, and the costs are even higher if an electrolyser is only used intermittently. Ideally, we could let hydrogen react with CO₂, either captured from the air or taken from flue gases, to produce renewable liquid fuels that are carbon-neutral, an option that we’re investigating at the Debye Institute at Utrecht University.

Heavy industry

Stephen Carr, Lecturer in Energy Physics, University of South Wales

Industry is the second most polluting sector in the UK after transport, accounting for 21% of the UK’s total carbon emissions. A large proportion of these emissions come from processes involving heat, whether it’s firing a kiln to very high temperatures to produce cement or generating steam to use in an oven making food. Most of this heat is currently generated using natural gas, which will need to be swapped out with a zero-carbon fuel, or electricity.

A worker in silver, protective gear stokes a furnace spewing molten metal. — Furnaces in the steel industry are generally powered by fossil fuels.
Rocharibeiro/Shutterstock

Let’s look in depth at one industry: ceramics manufacturing. Here, high-temperature direct heating is required, where the flame or hot gases touch the material being heated. Natural gas-fired burners are currently used for this. Biomass can generate zero-carbon heat, but biomass supplies are limited and aren’t best suited to use in direct heating. Using an electric kiln would be efficient, but it would entail an overhaul of existing equipment. Generating electricity has a comparably high cost too.

Swapping natural gas with hydrogen in burners could be cheaper overall, and would require only slight changes to equipment. The Committee on Climate Change, which advises the UK government, reports that 90 TWh of industrial fossil fuel energy per year (equivalent to the total annual consumption of Wales) could be replaced with hydrogen by 2040. Hydrogen will be the cheapest option in most cases, while for 15 TWh of industrial fossil fuel energy, hydrogen is the only suitable alternative.

Hydrogen is already used in industrial processes such as oil refining, where it’s used to react with and remove unwanted sulphur compounds. Since most hydrogen currently used in the UK is derived from fossil fuels, it will be necessary to ramp up renewable energy capacity to deliver truly green hydrogen before it can replace the high-carbon fuels powering industrial processes.

The same rule applies to each of these sectors – hydrogen is only as green as the process that produced it. Green hydrogen will be part of the solution in combination with other technologies and measures, including lithium-ion batteries, and energy efficiency. But the low-carbon fuel will be most useful in decarbonising the niches that are currently difficult for electrification to reach, such as heavy-duty vehicles and industrial furnaces.

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This blog is written by Cabot Institute member Valeska Ting, Professor of Smart Nanomaterials, University of Bristol, Tom Baxter, Honorary Senior Lecturer in Chemical Engineering, University of Aberdeen; Ernst Worrell, Professor of Energy, Resources and Technological Change, Utrecht University; Hu Li, Associate Professor of Energy Engineering, University of Leeds; Petra E. de Jongh, Professor of Catalysts and Energy Storage Materials, Utrecht University; and Stephen Carr, Lecturer in Energy Physics, University of South Wales.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The new carbon economy – transforming waste into a resource

Posted on October 16, 2018April 13, 2023 by janet.crompton

As part of Green Great Britain Week, supported by BEIS, we are posting a series of blogs throughout the week highlighting what work is going on at the University of Bristol’s Cabot Institute for the Environment to help provide up to date climate science, technology and solutions for government and industry. We will also be highlighting some of the big sustainability actions happening across the University and local community in order to do our part to mitigate the negative effects of global warming. Today our blog will look at ‘Technologies of the future: clean growth and innovation’.

On Monday 8 October 2018, the IPCC released a special report which calls upon world governments to enact policies which will limit global warming to 1.5°C compared with pre-industrial levels, failure to do so will drastically increase the probability of ecosystem collapses, extreme weather events and complete melting of Arctic sea ice. Success will require “rapid and far-reaching” actions in the way we live, move, produce and consume.

So, what comes to mind when you hear carbon dioxide – a greenhouse gas? A waste product? You’re not wrong to think that given the predicament that our planet faces, but this article is going to tell the other side of the story which you already know but is often forgotten.

For over a billion years, carbon dioxide has been trapped and transformed, almost miraculously, into an innumerable, rich and complex family of organic molecules and materials by photosynthetic organisms. Without this process, life as we know simply would not have evolved. Look around you, – I dare say that the story of carbon dioxide is weaved, one way or another into all the objects you see around you in this moment. Whether it’s the carbon atoms within the material itself – or that old fossilised sourced of carbon was used to smelt, melt or fabricate it.

The great growth and development of the last two centuries has been defined by humanity’s use of fossilised carbon which drove the first and second industrial revolutions. But now – the limitations of those very revolutions are staring us in the face and a new revolution is already underway, albeit it quietly.

An industrial revolution is said to occur when there is a step change in three forms of technology, Information, Transport and Energy. The step change that I will discuss here is the use of carbon dioxide coupled with renewable energy systems to deliver a circular carbon economy that aims to be sustainable, carbon neutral at worst and carbon negative at best. This burgeoning field comes under the name carbon capture and utilisation (CCU). CCU, represents a broad range of chemical processes that will most directly impact energy storage and generation and the production of chemical commodities including plastics and building aggregates such as limestone.

In our research we are developing catalysts made of metal nanoparticles to activate and react CO2 to form chemicals such as carbon monoxide (CO), formic acid, methanol and acetate. They be simple molecules – but they have significant industrial relevance, are made on vast scales, are energy intensive to produce, and all originate in some way from coal. The methods that we are investigating while being more technically challenging, consume just three inputs – CO2, water and an electrical current. We use a device called an electrolyser, it uses electricity to break chemical bonds and form new ones. The catalyst sits on the electrodes. At the anode, water is broken into positively charged hydrogen ions called protons and oxygen, while at the opposite electrode, the cathode, CO2 reacts with the protons, H+, to form new molecules. It sounds simple but encouraging CO2 to react is not easy, compared to most molecules, CO2 is a stubborn reactant. It needs the right environment and some energy such as heat, electricity or light to activate it to form products of higher energy content. The chemicals that can be produced by this process are industrially significant, they are used in chemical synthesis, as solvents, reactants and many other things. CO for example can be built up to form cleaner burning petroleum/diesel-like fuels, oils, lubricants and other products derived by the petrochemical industry.

Formic acid and methanol may be used to generate energy, they can be oxidised back to CO2 and H2O using a device called a fuel cell to deliver electricity efficiently without combustion. One day we could see electrically driven cars not powered by batteries or compressed hydrogen but by methanol which has a higher volumetric energy density than both batteries and hydrogen. Batteries are heavy, too short-lived and use high quantities of low abundance metals such as lithium and cobalt – meaning their supply chains could suffer critical issues in the future. While the compression of hydrogen is an energy intensive process which poses greater safety challenges.

However, there are still many hurdles to overcome. I recently went to the Joint European Summer School on Fuel Cell, Electrolyser and Battery Technologies. There I learned about the technical and economic challenges from an academic and industrial perspective. In an introductory lecture, Jens Oluf Jensen was asked “When will we run out of fossil fuels?”, his answer “Not soon enough!”. An obvious answer but there is something I wish to unpick. The task for scientists is not just to make technologies like CO2 capture, CO2 conversion and fuel cells practical – which I would argue is already the case for some renewable technological processes. The greatest challenge is to make them cost competitive with their oil-based equivalents. A gamechanger in this field will be the day that politicians enact policies which incorporate the cost to the environment in the price of energy and materials derived from fossil fuels, and even go so far as to subsidise the cost of energy and materials-based on their ability to avoid or trap carbon dioxide.

Even without such political input there is still hope as we’ve seen the cost of solar and wind drop dramatically, lower than some fossil fuel-based power sources and only with limited government support. Already there are companies springing up in the CCU sector. Companies like Climeworks and Carbon Engineering are demonstrating technology that can trap CO2 using a process known as Direct Air Capture (DAC). Carbon Engineering is going even further and developing a technology they call Air to Fuels™. They use CO2 from the air, hydrogen split from water and clean electricity to generate synthetic transportation fuels such as gasoline, diesel or jet fuel. You may question why we should need these fuels given the rise of battery powered vehicles but a better solution for fuelling heavy goods vehicles, cargo ships and long-haul flights is at the very least a decade way.

In 1975, Primo Levi wrote a story about a carbon dioxide molecule and he said in relation to photosynthesis “dear colleagues, when we learn to do likewise we will be sicut Deus [like God], and we will have also solved the problem of hunger in the world.”. The circular carbon economy may still be in its infancy, but the seeds have sprouted. Unlike the first and second industrial revolution, the 3rd industrial revolution will not be dependent on one single energy source but will be a highly interdependent network of technologies that support and complement each other in the aim of sustainability, just like nature itself.

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This blog is written by Cabot Institute member Gaël Gobaille-Shaw, University of Bristol School of Chemistry. He is currently designing new electrocatalysts for the conversion of CO2 to liquid fuels.
For updates on this work, follow @CatalysisCDT @Gael_Gobaille and @UoB_Electrochem on Twitter. Follow #GreenGB for updates on the Green Great Britain Week.

Gael Gobaille-Shaw

Read other blogs in this Green Great Britain Week series:
1. Just the tip of the iceberg: Climate research at the Bristol Glaciology Centre
2. Monitoring greenhouse gas emissions: Now more important than ever?
3. Digital future of renewable energy
4. The new carbon economy – transforming waste into a resource
5. Systems thinking: 5 ways to be a more sustainable university
6. Local students + local communities = action on the local environment