

Friday, June 20, 2008

Is Hydrogen Really Green?

The hydrogen economy seems to be full of promise. A high energy fuel that burns almost completely clean (other than water and a bit of nitrous oxide) and can be used in internal combustion, gas turbine and external combustion (sterling cycle) engines as well as fuel cells. However, there are no free lunches, let’s examine what is really involved in the production, transportation and use of hydrogen as a fuel.

Since there is relatively no free hydrogen in nature, hydrogen has to be produced through electro-chemical or catalytic means. The most common means of hydrogen production is the electrolysis of common water, you take two water molecules (2-H2O) and combine them with a jolt of electricity and you get 2 hydrogen molecules (2H2) and one oxygen molecule (O2). In a loss-less system you could then take the hydrogen and recombine it with the oxygen and get back the energy used to separate the molecules back in the form of heat, (the reaction is exothermic, as in gives off heat) or in the case of a fuel cell electricity, and have a waste product of pure water. However, there is no such thing as a loss-less conversion going either way so it takes more energy to produce hydrogen gas than you get from burning it or using it to produce electricity in a fuel cell.

Hydrogen is much less dense (in liquid form) than gasoline, this means that while hydrogen provides more btu of energy per pound than gasoline, a pound of hydrogen takes up more volume. This density difference means that to take advantage of the increased BTU per pound you have to burn or convert a larger volume of hydrogen. How about a 60 gallon tank for your SUV? Hydrogen also must be kept compressed and/or insulated to prevent losses. Hydrogen tends to cause hydrogen embrittlement of most metals so long term storage is also an issue. Liquid natural gas lines could possibly be used to transport the liquefied hydrogen, however, better insulation and the embrittlement issues would have to be addressed before the existing infrastructure could be used safely.

One promising hydrogen storage technology uses metal hydrides such as zirconium hydride that allow storage of hydrogen in interstitial sites in the crystal lattice, some tests show that storage densities exceeding liquid densities by several fold are possible. The use of metal hydride storage would put the fuel tank back at the current size in your SUV and would provide added safety since the hydrogen is not in liquid or gas form.

It should be obvious that using fossil fuels such as natural gas, oil, or coal to generate electricity to create hydrogen would be a huge mistake. The amount of carbon dioxide (CO2), the major greenhouse gas, which would be created from using any fossil fuels to produce hydrogen would cause far more damage from CO2 emissions than any benefits gained from using the hydrogen thus produced. However, it waits to be seen if some enterprising third-world country (no doubt financed by mainline energy companies) doesn’t use massive coal burning to produce hydrogen for sale to the more industrialized countries.

This leaves us with wind, solar, tidal or nuclear power to provide the needed energy to produce hydrogen in sufficient quantities to make it a viable energy source. What are the economic considerations of each of these energy sources?

Use of wind power

On the surface wind power looks good. You put up a tower (or two, or a hundred) with a wind generator and get electricity and dump the electricity into an electrolytic cell that produces hydrogen. Of course some of the electricity needs to go into compressors to store the gaseous hydrogen, some needs to go into pumping and purifying the water being fed into the electrolytic cell. At current manufacturing costs electricity from wind runs 4-6 cents per kilowatt hour, when the wind blows, it isn’t raining to hard, freezing or being repaired because of lightening strikes. Wind also ties up huge amounts of real estate, causes noise pollution and has reliability issues. Plus to produce the nearly 250 gigawatts of energy needed to produce the amount of hydrogen gas to support just the needs of the USA to replace fossil fuels in transportation alone would require 12,500 2 megawatt wind turbines and a mere 5000 of the new 5 megawatt mega-turbines. Want one over the top of your house?

Use of Solar Power

Let’s look at solar power, it is quiet, produces no waste, perfect right? Not quite. Even if the new high efficiency cells pan out where we double or quadruple the efficiency of existing cells by using new technologies (to 40-60 percent conversion efficiency) we will still only have a cost of 8-10 cents per kw-H, more expensive than wind generation technology. At a solar constant of 1,395 watts per acre and a 60% conversion efficiency yields a back of the envelope calculation of 837 watts per acre. To provide the 250 gigawatts using solar we would need to clear 30 million acres of land and put in high efficiency solar panels, how does that grab you?

Use of Nuclear Power

Nuclear power can produce immense amounts of energy while requiring only small amounts of space. The 250 gigawatts required for the hydrogen gas production would require 150-160 new reactors to be built. At 100 acres per plant this is only 16,000 acres of land, as a comparison, Ted Turner’s ranch properties are estimated at over 1.9 million acres. As to the nuclear waste issues, the new designs for reactors promise to reduce waste and make better use of recycling of fuel. This won’t completely eliminate the nuclear waste problem but may make it more manageable. At 11.1 to 14.5 cents per kw-H it is currently one of the more expensive options until you consider the environmental costs of other technologies. However, using the heat from the nuclear process to facilitate steam methane reforming, biomass gasification or coal gasification to produce hydrogen more efficiently we could reduce the cost, increase the output and reduce the number of needed nuclear plants thus further mitigating the nuclear waste problem.


So, before we all leap upon the hydrogen technology bandwagon we need to step back and examine the real costs and the real technologies needed to make it a reality. Is hydrogen really a green technology? As with all things technology the answer is a fully qualified maybe.

1 comment:

Richard said...

Hi Mike - I used to be 100% anti nuclear, but that was way back when the nuclear was (let's be honest) there to produce bomb material, and safety was, it seemed, not a big priority (Chernobyl, fires at Sellarfield, etc.).

Time moves on, and things change, and now providing energy cheaply is definitely a major problem for the world, as China and the Indian sub-continent begin to want lots of energy and NOW!

Nuclear is all we have right now, and, as long as we are careful, it's going to be a lot safer than warring over oil, food, land, etc.