Hydrogen could become the energy carrier that stores renewable power ready for the driver’s foot to go down on the accelerator. There are still many problems to overcome, but also new solutions on the horizon, says Jon Herbert.
Hydrogen is in the news for more than one good reason. It has long been seen as a potentially carbon-free — or low-carbon — popular energy source of the future. Despite many years of development delays, it could put its motive power behind transport if technical hurdles can finally be resolved.
A more fundamental aspect of hydrogen is, however, becoming newsworthy. Hydrogen is not a primary energy source; it is better defined as an “energy carrier”. Because of its propensity to combine easily with other elements, the smallest molecule in the Periodic Table is not found in freely available concentrations in nature. Hydrogen has to be “made”.
This might at first seem a major disadvantage. Hydrogen is produced commercially as the by-product of other industrial chemical processes and, as with all such processes, there are inevitable losses in efficiency.
However, hydrogen could begin to score if it can be produced economically by splitting water through hydrolysis using renewable energy. Then, its role as an energy-carrier would become a virtue as a store of sustainable power. A similar case has been made in parallel for nuclear energy.
One problem with many forms of renewable energy — wind, wave and tidal — is the concept of “wrong-time” power. There is no guarantee that nature will supply energy at exactly the time that we want to us it. As an extreme example, if the wind blows strongly during Christmas Night when industry is closed down, electrical energy that exceeds demand may be wasted.
Being able to “package” power into a product that has the versatility of a fossil fuel could open up new transport opportunities, which, with the exception of railway electrification, by definition work independently of the grid.
Right time for wrong-time
Solutions for wrong-time energy have made news recently. The Institution of Mechanical Engineers is impressed by a new liquid air technique which, it estimates, could be up to 70% efficient. The concept is that wrong-time energy is used to remove CO2 and water vapour (which would otherwise freeze) and then chill the remaining air — mostly nitrogen — to -190°C.
Although this involves energy input, the key to the process is a phase change from gas to liquid. The small volume of liquid air that results can be held in large vacuum flasks.
When power demand rises, cold liquid is warmed back to atmospheric temperature. Crucially, as it does so, the reverse phase change produces an expanding gas stream (air) that is strong enough to turn generating turbines. Power is generated with no combustion.
While this is only some 25% efficient, two further factors push up the efficiency. Firstly, low-grade heat normally lost to atmosphere from neighbouring industrial plant could be used to boost thermal expansion. Secondly, cold waste air from the chilling process is passed through gravel tanks that absorb and hold the coolness.
The net effect is a 70% conversion efficiency that compares well to the 80% that can now be achieved from advanced battery designs. In addition, the technology will only involve well-tried and tested standard industrial components.
With pumped hydro-power and high-voltage distribution networks the only practical energy storage systems presently at work in the UK, the Department of Energy and Climate Change (DECC) expects to announce incentives for energy storage innovation in the near future.
Hydrogen is likely to be a less efficient, if more convenient, way to store wrong-time energy. While already well-established in static power plants, its potential mobility is a key attraction. Meanwhile, developments on the horizon could make it relatively cheap and copious. Its future depends on economics. A new hydrogen filling station, opened recently on a service area near Swindon, could mark the beginning of the UK’s first “hydrogen highway” centred on the M4.
As fuel for the internal combustion engine, it has twice the efficiency of hydrocarbons, but there are better ways to harness hydrogen. Fuel cells combine hydrogen with atmospheric oxygen very efficiently to create an electrical current, with pure water as the only by-product. This opens the door for increasingly efficient hybrid motive combinations, which are explained below.
However, the demanding technology required, which has been under development for years, has to be perfected for the road. To date, the overall manufacture and transition is quite energy inefficient, with as little as 25% left available for traction.
As a result, development has centred on reducing the energy needed to isolate hydrogen from water, natural gas or biomass, compressing or liquefying the light gas, shipping it to the end user and finally converting it back into useful energy through fuel cells. The options are bulk manufacture and an extended nationwide distribution network, as with the supply of petrol and diesel.
There are several ways, however, in which hydrogen could be introduced into the mass transport system. One is to manufacture hydrogen on-board the vehicle during travel. The other is to refine the technology for hydrogen to be produced in bulk for easy, safe vending at service areas.
The first successful large-scale introduction of hydrogen could come about via a halfway house: hybrids. The concept of hybrids is to combine a combustion engine with an electric motor, plus regenerative braking to recover kinetic energy during deceleration, which is turned back into electrical energy and stored in an on-board battery. Hydrogen helps in two ways.
- Hydrogen produced by converting a small proportion of hydrocarbons via a reformer — or fuel processor — can be used to significantly improve the combustion of hydrocarbons, thus leading to a smaller carbon footprint through lower petrol consumption.
- Alternatively, hydrogen can be used via a fuel cell to provide the electricity that powers the electric motor to give an extra zero-carbon boost to the internal combustion engine when needed. At low revs, which half of the system kicks in depends on how much power has been stored in the battery.
A fuel cell/electric motor combination is two to three times more efficient than an internal combustion engine alone but has a low power-to-weight ratio.
Until renewable or nuclear-derived hydrogen is produced en masse by water electrolysis, the next best option could be the steam reforming of natural gas at filling stations, avoiding the need for a bulk hydrogen delivery infrastructure.
Alternatively, steam reforming of natural gas, oil or coal could also take place at centralised plants for distribution by pipeline, canister, tanker or novel delivery scheme. Extremely pure hydrogen is needed for fuel cells.
However, providing hydrogen by reforming hydrocarbons is not a zero-emissions solution, with emissions in the range of 150 to 300 grams of carbon dioxide per mile. Tests show that 11.9kg of CO2 is produced for every kilogram of hydrogen. In the future, in-situ steam reforming coupled with yet-to-be-developed carbon capture and storage technology might be sustainable.
It is possible that a feedstock such as biomass could also be sustainable. An alternative might be to produce methanol — using atmospheric carbon — as a carrier. This would be easier to transport and store and has a higher energy density than hydrogen.
There are, however, more challenges ahead before hydrogen can be put on the roads. Tankering petrol around the Britain’s roads sounds hazardous but results in very few lethal accidents. Calling into a gas station for a hydrogen top up has to be equally uneventful. Remember the Hindenburg?
Hydrogen stores less energy by weight and volume than hydrocarbons and is less passive to store. Liquefied hydrogen only exists under pressure. Nanotube technology has been considered, in which the busy molecules of hydrogen are stored less actively in a micro nano-matrix. Hydrogen can also be stored in a solid state as sodium borohydride, a chemical created from borax. As sodium borohydride releases its hydrogen, it turns back into borax, and vice versa, indefinitely.
Original equipment manufacturers and energy supply companies have examined many potential systems. In some cases they have walked away having decided that the commercial risks are too large. There are also further practical hurdles. When the driver’s foot hits the pedal, hydrogen must provide instant acceleration, which could mean operating temperatures of 3000°C.
Microbes to the rescue
There may be other sustainable options. Ammonia created by bonding hydrogen with atmospheric nitrogen is seen as a viable energy carrier that is easier to liquefy, transport and use as a fuel. A theoretical alternative to a hydrogen economy might be a methanol or ethanol economy that makes liquid fuels from carbon dioxide, including the CO2 from fossil-fuel burning power plants.
Researchers are also focusing on an “electron economy”, the shortest and most economical way of transferring green energy to consumers, which has been proposed as the most efficient way forward without wasteful energy conversions. This would mean electricity becoming the prime energy carrier.
However, limitless hydrogen could be harvested from self-powering cells fuelled by bacteria, according to US research from Pennsylvania State University. This breakthrough would mean that no external source of electricity is required. At present, costs are too high for any commercial application, but that is the nature of progress.
Naturally-occurring bacteria are used in the microbial fuel cell to release electrons and generate electricity as they break down organic matter. Releasing protons that combine with these electrons, the microbial electrolysis cell could be the technical advance needed to provide bulk cheap hydrogen. Add fresh water, sea water and dividing membranes in a “reverse electrodialysis” process, and even the small amount of additional power needed to energise the system becomes unnecessary.
A small voltage could be created in what would be known as a microbial reverse-electrodialysis electrolysis cell using stacks of membranes to harvest energy created from the movement of charged atoms from saltwater to freshwater.
Researchers plan to produce larger cells that could bring the high production costs of hydrogen down and reduce environmental concerns.
The ultimate prize would be to treat wastewater — a benefit in itself — and produce hydrogen without any renewable, nuclear or grid energy input.
Who says there is no such thing as a free lunch?
First published by Croner-i on 11 Dec 2014