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Monday, November 24, 2003

a crazy step toward a hydrogen economy

i’ll be the first to admit this is way outside my realm, but since i’ve read and seen a few things about the “hydrogen economy” lately, i thought i’d apply a few braincells and bring this up so it could be shot down by someone smarter than myself.

i’ll leave it to others to explain the advantages of a hydrogen economy.

the question that got my attention is the “tipping point” – we need to figure out how to transition to a hydrogen-based economy. and, since i’m slightly crazy, i recognized an existing infrastructure that might (and i stress might) make things go a little easier – the natural gas distribution infrastucture.

there is a huge installed base of equipment that uses natural gas, and converting the distribution system to hydrogen is going to leave all this equipment without a functional fuel source. that’s much too expensive to propose (at least until there’s a fuel crisis). to make this more economically feasible i’m thinking, perhaps, it would be possible to keep the appliances in place for the rest of their “natural” lives with a little help from a black box – a methane re-composing black box.

the short version

distribute hydrogen gas via existing natural gas pipelines. build a box to ease the pain.

the long version

there exists an infrastructure of pipelines dedicated to the transmission of natural gas to individual buildings and households. this proposal would convert this existing infrastructure to the transmission of hydrogen gas.

once hydrogen gas is available at an individual building, it can be used for electrical generation (fuel cell), heating (as a byproduct of fuel cell operation and/or as a combustion process), hydrogen-vehicle fueling, even refrigeration and cooling (metal-hydride refrigeration cycle).

at the terminal end (in or near each building), the hydrogen supply would feed a combined household appliance that provides several “end use” functions from the hydrogen supply. this appliance would serve as a use meter, contain fuel cells for local power generation, operate as a heater (for water or space), and as a methane re-composer to provide a natural-gas-like fuel supply to run existing natural gas appliances with little or no modification.

wherefrom hydrogen?

two general processes are available for the production of hydrogen to feed the network – methane decomposition (CH4 => C + 2H2) and electrolysis of water (H2O => O + H2). there may eventually be other technologies – biological processes or something else – but this is a start.

methane decomposition
the head-end of natural gas pipeline network has a ready supply of natural gas (95% methane). with this methane supply, a large volume of hydrogen gas can be produced in a single location, and by-products (carbon) sequestered efficiently at the source. combined with methane re-composition at the household, the total atmospheric carbon burden should be significantly reduced (maybe even net-negative).

water electrolysis
water is also generally available at distribution head-ends, and has no carbon byproducts. companies have demonstrated hydrogen production by electrolysis at a cost of 3.9kWh/m3.

both processes have byproducts, which the utility will have to handle, or, preferably, sell in some form. it is technically feasible to “make hydrogen” at the utility end of the pipe. important to the utility is the potential advantage in environmental impact (convert several thousand homes to hydrogen in your service area at a time), and hydrogen provides a very-low-transmission-loss mechanism of moving power to the consumer.

creating a win-win-win-win-win

at the terminal end (household), the utility installs black boxes that provide new hydrogen-based services, and keep the existing household running. on the consumer-side of the box, these become available (perhaps in a modular as-needed form):

re-composed methane
for appliances (stoves, household heat, etc) that are designed for natural gas, re-composing methane at the household could make the house a net-consumer of carbon. more interesting, by making a natural gas-like fuel available at the house after the natural-gas infrastructure is converted, it dramatically reduces the total cost of conversion.

electricity – low voltage dc
more and more appliances are designed to operate on low-voltage dc power (witness the proliferation of wall-warts and transformers in your own house). with local power generation, a common dc supply for the house or building becomes practical. this could eliminate several conversion losses – from dc (at the fuel cell) to ac (household current) to dc (at the wall wart) – to provide more efficient power for modern electronic equipment that doesn’t require ac power.

this does, of course, demand development of a standardized dc outlet and dc-to-dc converters (which should be both smaller and more efficient than their ac-to-dc cousins). as a spinoff, even if this doesn’t prove economically viable, i would personally welcome a standardized dc power source. something a little more functional than a wall-wart or a bastardized car cigarette lighter.

electricity – standard voltage ac
of course, many household appliances do still require or function best with alternating current, so there’s a good reason to make this flavor of electricty available. this proposal provides a significant boost to net efficiency by eliminating long-haul transmission and high-voltage conversions in the grid. any surplus power generated could be sold back to the grid to offset operating costs.

heat
fuel cells produce heat as well as electricity, which can be used to provide hot water or space heat. depending on the design, this could be a “pre-heater” that reduces the burden on standard (electrical or natural gas-burning) heating equipment, or serve as a complete replacement.

hydrogen
if you can find something to do with hydrogen, it is available at your home. this could be used to fuel hydrogen-powered vehicles at the residence or serve in metal-hydride refrigeration equipment. i’m not sure i’d get too involved with rocket-engine design in my kitchen, but, the hydrogen is there.

does the math work?

in the united states, the average annual consumption is something on the order of 13,000kWh of electricity and 2500m3 of natural gas. this suggests that a household converted entirely to a hydrogen supply would require about 8600 m3 of hydogen gas (H2 energy content of 3kWh/m3 at 50% efficiency) for electricity plus 5000m3 of H2 for methane re-composition per year. ignoring gains in efficiency, this means delivering 13,600m3 of hydrogen where 2500m3 of natural gas is being delivered now.

based on the 3.9kWh/m3 electroysis figure, each household would then consume 13,600 * 3.9kWh = 53,040kWh per year at the point of generation.

i don’t have handy sources to determine if this several-fold increase in energy consumption is fully offset by the other factors. someone with their fingers on the figures will have to tackle this part for me… how big is the gap, really?

what else is wrong?

methane production from carbon dioxide and hydrogen (2H2 + CO2 => CH4 + O2) is demonstrated in biological systems (methanobacterium ruminantium), but i can’t say if it’s scale-efficient or commercially viable.

is the pipeline/distribution infrastructure for natural gas compatible with hydrogen gas? if the existing infrastructure can’t be used as-is (or converted very cheaply), then there isn’t any point in going much further. i know there are some issues with iron-hydrogen reactions, but i don’t know if the materials used in natural gas distribution are affected.

can the natural gas infrastructure handle the increased volume of gas? hydrogen is the most efficient fuel available by weight, but at atmospheric pressure it occupies a huge volume. if this is going to make pipes burst, the whole thing is a non-starter.

can a “smellable” chemical be added that won’t spoil fuel-cell operation? much like natural gas, some human-compatible way of detecting leaks is an important safety issue. both natural gas and hydrogen are odorless, but i don’t know if the “gas leak smell” will ruin catalysts or otherwise impede operation of fuel cells. if it does, is there an alternative? if not, we’re done.

i’m sure i missed a few deal-breakers, but…

i’m done now, tell me i’m crazy

it was just a thought… please proceed to shoot it down.

update (2004.12.06): stephen gloor has dropped in with a link to his variation on this theme. it’s worth checking out. while there are a few key differences, he presents some of the science that underlies both.

posted by roj at 2:27 am