As we plan for the clean, non-petroleum-fuelled automobile and truck fleet of the future, we envision a propulsion technology portfolio that includes biofuel powered, electric drive, and hydrogen fuel cell vehicles (FCV).
The last of these is perhaps the most technically challenging, but also the most attractive technology in terms of its ability to dramatically decrease oil consumption, CO2 greenhouse gas emissions, and tail pipe pollution. However, hydrogen is not an energy source – it is an energy carrier. And to fully realise its benefits, we must produce it not from fossil sources, but from renewable energy.
The world produces huge quantities of hydrogen today for industrial and commercial purposes, probably in excess of 50 million tonnes/year. But most of that production is fossil-energy based, either from reforming natural gas, or electrolysis using electricity produced from coal, natural gas, petroleum, or nuclear.
Renewables on the other hand are a desired energy source for hydrogen production due to their diversity, regionality, abundance, and potential for sustainability. That being asserted, there are many challenges to producing Hydrogen from renewables – and perhaps the major one is reducing the cost to be competitive with gasoline and diesel.
Renewable hydrogen can be produced in several ways:
- Electrolysis – splitting water into hydrogen and oxygen using electricity from one of the many renewable sources;
- Biomass conversion – via either thermochemical or biochemical conversion to intermediate products that can then be separated or reformed to hydrogen; or fermentation techniques that produce hydrogen directly;
- Solar conversion – by either thermolysis, using solar-generated heat for high temperature chemical cycle hydrogen production or photolysis, in which solar photons are used in biological or electrochemical systems to produce hydrogen directly.
The order above is, in general, also representative of the technological maturity of these pathways, and thus roughly the chronological order in which we might expect to see them commercially available.
Figure 1 provides an overview of the various options.
There is a substantial worldwide business in producing electrolysers, and building electrolysis facilities for hydrogen production. The challenges for transportation-ready renewable hydrogen are both in cost, and in understanding the logistics and economics of large central production plants versus smaller distributed facilities located nearer the vehicle users.
A 100% efficient electrolyser requires 39 kWh of electricity to produce 1 kg of hydrogen. The devices today require as much as 48 kWh/kg. So, if electricity costs are 0.05 US$/kWh, the power cost for the electrolysis process alone is 2.40 US$/kg of hydrogen. (NB: In the USA, average residential electricity cost is approximately 0.10 US$/kWh and industrial 0.06 US$/kWh). Capital costs for an electrolysis facility can be a huge factor, and for smaller installations can actually become the predominant cost factor.
One advantage of electrolysis is that it is capable of producing high purity hydrogen (>99.999%), which is good for FCVs, whose fuel cells will, at least initially, be susceptible to contaminants and will require ultra-high hydrogen purity.
The worldwide electricity production potential from renewables is staggering. If addressed and utilised aggressively, there is sufficient resource to support not only large inputs to the electrical grids across the planet, but also significant hydrogen production. As an example, by itself the available wind power resource in the USA is estimated to be more than 2,800 GW (today, total US electricity generation capacity is roughly 1,100 GW), enough to produce over 150 billion kg/year of hydrogen, which exceeds the US gasoline quantity consumed annually in terms of energy equivalency.
Several renewables-to-hydrogen electrolysis test projects are underway in the USA and worldwide. At the US National Renewable Energy Laboratory (NREL) in Colorado, a partnership between NREL and the local utility, Xcel Energy, has resulted in a pilot scale project using wind and PV (see figure 2 and case study – ‘renewables to hydrogen’). The hydrogen is stored, then used to fuel NREL's Mercedes Benz F-Cell FCV, or converted into electricity for injection back onto the grid during times of peak electrical loads.
In the 1920s and 1930s, MW-scale alkaline electrolysers were built next to hydroelectric facilities in several locations around the world. So, we know how to do renewable hydrogen through electrolysis, have done it in the past, and now need to overcome the relatively modest technical and economic barriers to renewable hydrogen electrolysis for future transportation needs.
Because biomass is our only renewable source of hydrocarbons, conversion of a small portion of the planet's huge biomass resource to fuels is an important option for our transportation needs. Hydrogen can be produced from this renewable feedstock. A recent US National Research Council (NRC) report (Transitions to Alternative Transportation Technologies: A Focus on Hydrogen, July 2008) asserts that centralised production of hydrogen from biomass gasification is the renewable pathway that has the highest likelihood of commercial viability in the 2015-3035 timeframe.
Biomass-to-hydrogen is complex, not only because of the technical details of the conversion processes themselves, but also because of the many process types that could be employed. The conversion type with the most potential for large-scale centralised production, as pointed out in the NRC report, is gasification, which in itself is but one of several technologies available within the larger category called thermochemical conversion.
Gasification – whether steam, air/oxygen, catalytic, or indirect – involves subjecting the biomass to elevated temperatures and pressures in order to reduce the organic materials to hydrogen and carbon monoxide/dioxide gases (along with varying quantities of undesirable solid and gaseous byproducts). From there, the hydrogen can be separated out by membrane, chemical, or catalytic steps. Technoeconomic analyses indicate that gasification biorefineries may have to be large to be economically feasible, which means significant capital investment as well as a broad feedstock production and delivery infrastructure to supply each installation.
A second thermochemical option is to convert the biomass to a bio-oil via thermal decomposition known as fast pyrolysis, followed by catalytic steam reforming of the liquid (or its vapours) to hydrogen. An advantage of this approach is that the bio-oil, as an intermediate product, has a higher energy density than the biomass feedstock and can more easily be transported. This technique may prove to be applicable to smaller, distributed biorefineries, whereas the gasification process described above may cater to the large, centralised installations.
Biochemical conversion of biomass to hydrogen also presents several possible pathways. Ethanol produced from lignocellulosic materials could be further reformed to hydrogen, as could other biofuels or intermediate products of various biochemical routes Certain regional implications, feedstock types, or end-use requirements might make this a viable, if not a widespread, option.
More interesting perhaps is dark fermentation, a process that uses anaerobic microorganisms to produce hydrogen directly, much in the way that bacteria or yeast can produce ethanol via fermentation. Such organisms might be enhanced to better perform the hydrogen production task. They typically need to start with glucose, so the cellulosic ethanol pretreatment and hydrolysis techniques that are being developed now to break down cellulose into glucose would also be required for the dark fermentation pathway.
Perhaps the most intriguing options, with huge potential but requiring more development time, are solar conversion techniques. These are thermolysis and photolysis, and are shown on the far left and far right of figure 1, respectively.
Thermolysis involves using the heat produced from concentrated solar power (CSP) to drive one of many thermochemical reactions (hundreds of which are known) that can produce hydrogen, or to drive electrolysis at very high temperatures for more efficient water decomposition.
Photolysis may be the ultimate “holy grail” for hydrogen production, using solar photons to produce hydrogen directly via biological or electrochemical systems. Photobiological methods use photosynthetic organisms such as some cyanobacteria and green algae to photoproduce hydrogen – no carbon-based molecules are needed in the process. Much work is still needed to optimise the processes within the organisms, and numerous engineering challenges need to be met to develop large hydrogen generation photobiological systems.
Photoelectrochemical photolysis involves the disassociation of water into hydrogen and oxygen directly at the surface of a semiconductor through the irradiation the semiconductor by solar photons. This can be thought of as electrolysis without the electrolyser, because the photovoltaic semiconductor material acts as a catalyst to produce hydrogen directly at the semiconductor and water interface. A major hurdle is finding a semiconductor material that has the right photoelectrochemical properties, while being economical and robust enough to withstand the severe chemical and physical environment.
Is there enough?
Can renewables really produce enough hydrogen to make a difference? Figure 2 answers this question for the USA, providing a county-by-county indication of the hydrogen potential from solar, wind, and biomass – compared to gasoline consumption in the US alone.
Only those in blue could produce less hydrogen than their equivalent gasoline use, and those in green approach the capability of 1,000 times more hydrogen than their own needs. The few counties that fall short are typically surrounded by others with an abundance. Even though the US has a significant renewable resource, a global analyses might be expected to provide similar results.
So, with all these options for renewable hydrogen production and the significant, diverse renewable energy resources upon which we might draw worldwide, where do we stand in terms of the research and development (R&D) needed to address the challenges?
The answer is not clear, largely due to the confusing global energy picture and recent economic downturn, combined with an apparent emphasis on nearer-term solutions being evaluated to reduce CO2 emissions and stem global warming. Some speculate that there is less enthusiasm for hydrogen than we have seen in recent years, while others offer that hydrogen and fuel cells need to find their place in the portfolio of future transportation propulsion options.
In the USA, the end of the President's Hydrogen Fuel Initiative (in which President Bush pledged US$1.2 billion to hydrogen and fuel cell R&D during fiscal years 2003 through 2008) has had a budgetary effect on hydrogen R&D in general, and on renewable production R&D in particular. Whereas the US Department of Energy's hydrogen R&D budget had been climbing to more than US$280 million in 2008, the Department's fiscal year 2009 request showed a decrease, including a zeroing of applied R&D funding for hydrogen production (which had been US$40 million in 2008). The rationale was that at least one production pathway had made the US$2-US$3/kg cost goal – albeit via a non-renewable pathway known as distributed steam reforming of natural gas – and that funding was being focused on hydrogen storage and fuel cell R&D, where there is greater immediate need.
Internationally, renewable hydrogen production R&D efforts continue. The long-standing Hydrogen Implementing Agreement of the International Energy Agency (IEA), in place since 1977, continues to work toward renewable hydrogen production. And many individual countries, including Japan, Australia, Iceland, to name just a few, continue to pursue renewable hydrogen options. Also, the more recent International Partnership for the Hydrogen Economy (IPHE) links many nations in collaborative efforts.
If we are able to transcend the “chicken-and-egg” problem with respect to FCVs and the supporting hydrogen production and distribution infrastructure, we should be able to make the technical and business case for renewable hydrogen as the energy carrier for our clean vehicles of the future. And, we should be able to do that, not by the 2040-2050 timeframe as some suggest, but in the nearer term in order to offer a renewable, sustainable, and clean transportation option to our future global portfolio.
|About the author
|Dale Gardner is the associate laboratory director for Renewable Fuels & Vehicle Systems at the National Renewable Energy Laboratory (NREL), a US Department of Energy laboratory located in Golden, Colorado, USA. His research, development, and demonstration technology portfolio at the lab includes biofuels, hydrogen and fuel cells, and advanced vehicle technologies.