The April 2003 issue of Wired has an article written by Peter Schwartz and Doug Randall advocating an accelerated conversion to a hydrogen economy. After discussing the problems inherent to storing hydrogen in gaseous and liquid forms they argue that solid materials as hydrogen sponges will be the best long term solution.
In the long run, the most promising approach is to fill the tank with a solid material that soaks up hydrogen like a sponge at fill-up and releases it during drive time. Currently, the options include lithium hydride, sodium borohydride, and an emerging class of ultraporous nanotech materials. Unlike gaseous hydrogen, these substances can pack a lot of power into a small space of arbitrary shape. And unlike liquid hydrogen, they can be kept at room temperature. On the other hand, energy is required to infuse the solid medium with hydrogen, and in some cases very high temperatures are required to get the fuel back out, exacting a huge toll in efficiency. Also, filling the tank can take far more time than pumping gasoline. Government money could bridge the gap between today's experiments and a viable solution.
But will the problems involved in solid hydrogen storage be any more tractable and yield to any better solution than the problems with gaseous or liquid storage? Will the solid material needed to store the hydrogen weigh so much as to make it weigh as much as a battery which would contain the same amount of energy? The authors provide no indication as to why their preferred approach will turn out to be so advantageous.
The bigger problem with the article is that it does not explain why the use of hydrogen will allow us to reduce and eventually eliminate the use of fossil fuels. Hydrogen is not a source of energy. It would be more accurate to say that hydrogen is a way to store, transport, and use energy. Therefore it competes with other forms of stored energy. In cars and other vehicles hydrogen could be burned in fuel cells. But energy is needed to produce the hydrogen in the first place. To be a better automotive fuel hydrogen would somehow have to reduce the total usage of fossil fuels and do that better than other approaches that could be pursued.
Fossil fuels are a major source of energy today. Fossil fuels could be converted to hydrogen. But hydrogen advocates have not made a clear case for why hydrogen as an intermediate storage and end use form of energy is a more efficient way to use fossil fuels. There are too many unsolved problems and questions. Again, hydrogen does not really compete against other types of originating fuels. Rather, it relies on other types of originating fuels because it has to be produced using these other fuels.
If hydrogen is produced from electricity then the electricity must first be generated. But most electricity is generated by burning coal or natural gas. Hydro and nuclear also produce small fractions of the total electric supply. We've pretty much harnessed the available hydroelectric sources and hydroelectric is a pretty small fraction of total electric generation. The other big current alternative is nuclear energy. But for electricity generation nuclear power costs more than burning fossil fuels. There is no big economic incentive on a global scale to drive the building of massive numbers of nuclear power stations to cause a conversion to a nuclear-hydrogen economy. Also, widespread use of nuclear power on a global scale would so increase the availability of enriched uranium and plutonium that it would cause unacceptable risks of nuclear and radiological weapons proliferation.
The economic case for the use of nuclear power looks even worse than current fossil fuel prices suggest. The marginal cost of oil production (in some fields it is about $3/barrel) in the Middle East is much lower than current oil prices. Therefore nuclear power can not displace the use of Middle Eastern fossil fuels unless nuclear power becomes much cheaper than it is now.
Fossil fuels could be used to generate hydrogen. Would this be a more efficient way to use fossil fuels for transportation purposes? Keep in mind that each step in the use of hydrogen would produce an energy loss. The efficiency of the energy conversion of fossil fuels to hydrogen would be less than 100%. The hydrogen could then be piped (or driven) to what are now gasoline stations. If liquid hydrogen was used in cars then the hydrogen would have to be cooled first to liquid form. To keep it cool would require a great deal of insulation and probably additional cooling on-going. Therefore a car just sitting in a parking lot would consume energy at some low rate. As the Wired article points out, even a solid storage method may require energy usage in order to get the hydrogen into the solid and to get it back out again. Meanwhile, there are an assortment of ways to make the old internal combustion vehicle more fuel efficient. Therefore hydrogen is not just competing against today's internal combustion engine transportation systems. It is also competing against tomorrow's.
Hydrogen would most likely propel vehicles by being burned in a fuel cell. In theory fuel cells are a more efficient means of converting a liquid or gaseous fuel to mechanical power than the internal combustion engine. But hydrogen is not the only energy form that can be burned in fuel cells. There are fuel cell designs that will burn methane gas for instance. In fact, due to the greater efficiency of fuel cells for the conversion of fosil fuels to electricity fuel cells will become widely used for electric power generation from fossil fuels before they become used in transportation.
Is hydrogen the only viable candidate as an energy storage form to replace gasoline and diesel fuel in vehicles? In a word, no. Lead acid batteries have an energy storage density of 35 Watt Hours per kilogram. This leads to electric cars that weigh too much and have too short a range between recharges. MIT professor Donald R. Sadoway believes lithium polymer batteries can be developed that will have over an order of magnitude greater energy density than lead acid batteries.
Niels Bohr, the Danish physicist and Nobel Laureate, once cautioned that prediction is always dangerous, especially when it is about the future. With this disclaimer, then, we speculate on what is in store for rechargeable lithium batteries. In the near term, expect the push for all-solid- state, flexible, thin-film batteries to continue. This is driven by the desire to maximize the electrode–electrolyte inter-facial area while minimizing diffusion distances within the electrodes themselves, in order to combine high capacity with high rate capability. Recent results from our laboratory indicate that in a multi-layer configuration comprising an anode of metallic lithium, a solid polymer electrolyte, and a cathode of dense, thin-film vanadium oxide, it is possible to construct a battery with projected values of specific energy exceeding 400 Wh/kg (700 Wh/l) and specific power exceeding 600 W/kg (1000 W/l).10,11 Another trend is distributed power sources as opposed to a single central power supply. This allows for miniaturization (e.g., the microbattery). Expect also the integration of energy generation with energy storage, for example, a multilayer laminate comprising a photo-voltaic charger and a rechargeable battery. Ultimately, if scientific discoveries prove to be scalable and cost-effective, we should witness the large-scale adoption of electric vehicles.
When the cost of photovoltaics is lowered far enough to compete with fossil fuels then a combination of photovoltaics and lithium polymer batteries may well be the combination of technologies that will lead to the phase-out of the use of fossil fuels as vehicle power sources.
The article co-authored by Donald Sadoway and Anne Mayes is from the August 2002 issue of MRS Bulletin dedicated to lithium batteries.
|Share |||Randall Parker, 2003 March 24 02:45 AM Energy Tech|