I2 + SO2 + 2H2O → 2HI + H2SO4 (120 oC)
H2SO4 → SO2 + H2O + 0.5 O2 (850 oC)
2HI → I2 + H2 (300 - 450 oC)
As you can see the Sulfur and Iodine are recoverable, making this a recyclable process. The thermal efficiency of this cycle for nuclear power plants can be higher than the thermal efficiency of electricity production combined with electrolysis. For example, this detailed study found an ideal efficiency of 51 % (possibility higher with membranes) and an actual efficiency of 36 %. Such a reactor should still be capable of converting the remaining thermal power to electricity at around 30 % efficiency. Hence a thermochemical hydrogen reactor could potentially output 50 % hydrogen, 15 % electricity, and 35 % waste heat.
Hydrogen produced from nuclear power is a popular topic with Hydrogen economy advocates such as Dr. Ballard. However, there is a key drawback to using nuclear thermal energy for the Sulfur-Iodine process: most nuclear reactors simply don't operate at 850 oC. New reactors with novel cooling systems such as helium or sodium are necessary to produce the requisite temperatures.
While these numbers for the Sulfur-Iodine cycle may look impressive, I don't think hydrogen advocates are doing themselves any favours in pushing for new, untested nuclear technology. In reality, it represents another huge capital cost on the 'bridge' to a hydrogen economy.
The three main nails in the coffin of the hydrogen economy are:
- Well-to-wheel efficiency
- Financial potential gap
Well-to-wheel efficiency is a major drawback of hydrogen. Principally, hydrogen does not reduce our energy consumption because its overall efficiency is similar to that of spark ignition gasoline vehicles. A diesel-hybrid, on the other hand, can increase efficiency by a factor of three. Hydrogen advocates often gloss over this problem, choosing to examine the high tank-to-wheel efficiency of fuel cells rather than the low well-to-tank efficiency of hydrogen production. I would submit, however, that as a portable, carbon-neutral energy source hydrogen is inferior to biofuels. After all, if we want hydrogen as a portable energy source, and we don't feel electricity storage is up to the task, why shouldn't we compare the energy return on energy investment (ERORI) of hydrogen to biofuels like corn-based ethanol or biodiesel?
Let's take David Pimetal of Cornell's study of ethanol production. This is widely regarded as one of the most hostile treatments of ethanol around. Even for corn, one of the worst sources of biofuel available, Pimetal finds an ERORI of 71 %. If every Joule of input into the production of corn-based ethanol was electricity, it would still have a superior ERORI to electrolysis-produced hydrogen. There's nowhere to go but up for biofuels, while hydrogen is stuck stationary due to the laws of thermodynamics.
Storage is a well known problem. Solid-state methods are generally no better than batteries; pressurized storage is potentially dangerous and consumes energy; cryogenic storage consumes a huge proportion of the heating value of hydrogen for the cooling process. Carbon-fuels -- whether from renewable sources or not -- have a much higher energy density while electricity can lay claim to far superior return rates (in excess of 90 % for Li-ion).
The financial potential gap is the chicken and egg paradox of infrastructure. No one can afford to build hydrogen fuel infrastructure until they have a market, and no consumer can afford to buy a hydrogen powered car without the supporting fuel infrastructure. This is in sharp contrast to incremental technologies like the hybrid. The hybrid has several paths where it can make incremental improvements in fuel economy and carbon economy while slowly evolving the existing infrastructure. It can improve the ancillary equipment, by transforming from mechanical belts and transmissions to electrical systems. It can enlarge the battery capacity, becoming a plug-in hybrid with increasing all-electric range with every generation. It can switch from spark-plug ignition to compression ignition (i.e. diesel). It can incorporate biofuels such as ethanol, biogas, or biodiesel into its tanks in increasing proportions. A fuel economy advances, it will become practical to advance from E5 to E20 to E40. Essentially, while the hydrogen economy faces a fiscal cliff, the hybrid only has to climb a set of stairs.