It is a popular misconception that hydrogen is a fuel. It is not. It is an "energy carrier", meaning that hydrogen must be generated using a primary fuel, and so can store some of that fuel's initial energy, albeit encountering energy losses along the way. On first glance, hydrogen appears as a holy grail to the green environmental cause. It is claimed to be a "clean fuel" (mindful that it is not in fact a fuel), since as used at source, the only product of its "combustion" is water - clean enough to drink as it drips out of the exhaust pipe of an "eco-car", powered by hydrogen as it combines with oxygen in a fuel cell. This mode of combustion is preferred to using hydrogen actually in an internal combustion engine, as the efficiency is greater, and more of the energy stored in hydrogen is thereby extracted from it in terms of "miles per gallon" of fuel. As I have already noted, hydrogen has to be made in the first instance, and the majority of world hydrogen - used mainly for industrial processes, including combination with nitrogen to make ammonia and hence artificial fertilizers - is produced from natural gas, by a process called "reforming".
One is reminded of the "reformation" of the monasteries, in that a new social order emerged from it - in the present context, to feed a rising population that currently stands at 6.5 billion, about 4 billion of whom would not be alive without the agency of synthetic fertilizers to feed them. To reform natural gas (methane), it is mixed with steam and blasted through a high temperature furnace, when it reacts according to the equation:
CH4 + 2H2O ---> CO2 + 4H2.
Therefore, the process still produces CO2 in contravention of the green dream, unless it is separated and "sequestered" in some way, itself an energy needful step, rather than being vented into the atmosphere. Actually, converting natural gas to hydrogen is worse in terms of CO2 emissions than simply burning it directly, since (fossil) fuel is required to heat the furnace and an overall 10% more CO2 is produced. Since carbon sequestration (pumping the gas underground into worked gas-wells or aquifers or liquifying it for disposal in the deep ocean) might consume up to 30% of the energy ultimately delivered from methane via conversion to hydrogen, we are perhaps 40% down on the deal overall.
Ideally, we want to avoid producing CO2, in order to cut our greenhouse gas emissions and so we would prefer not to use natural gas at all, since whichever way it is used, CO2 is an unavoidable end product. So, as an alternative, we need to consider how feasible it is to produce our hydrogen requirements using renewably generated electricity which can be used to manufacture hydrogen by the electrolysis of water. Now, this is clean. The process simply involves splitting water (H2O) into hydrogen and oxygen: 2H2O ---> 2H2 + O2. If the electrical energy required for this is produced, e.g. using wind-power, the whole process is sustainable since it uses a renewable source of energy, e.g. the wind, which will never run out, and it is perfectly clean ("green") both on the side of feedstock (H2O) and product (H2 + O2). The oxygen could be used to treat sewage effluent, for example, so overall electrolysis is environmentally benign, even though it is an energetically inefficient process compared to gas reforming, and loses 30% of the energy originally available in electricity; however, costing in CO2 sequestration, the two methods carry similar energy losses.
If we were to produce enough hydrogen to supplant all the hydrocarbon fuels currently used for transportation, how might we go about it? In other words, what scale of generating capacity would we need? To answer this it is perhaps helpful to look at some figures. The following refer to the U.K. only, for the year 2003:
Road Transport 42 Million Tonnes (oil equivalent)
Aviation 11.9 Million Tonnes (oil equivalent)
Rail 0.3 Million Tonnes (oil equivalent)
Total 54.2 Million Tonnes (oil equivalent).
I find it highly significant - and a real eye-opener - that just over one fifth (22%) of the total national fuel budget is taken by aviation, and that this amount is double the share this sector had in 1970. To arrive at some meaningful quantities, we need to convert the mass of fuel used into units of generating capacity in MW. So, here goes:
1 Tonne (eq) oil = 42 GJ; 1kWh = 3.6 MJ. Hence, 1 Tonne (eq) of oil = 11,667 kWh, and so 54.2 Mt of it = 6.32 x 10*11 kWh.
Now that is the amount used over a year = 8760 hours.
Therefore, 6.32 x 10*11/8760 = 7.21 x 10*7 kW = 7.21 x 10*4 MW = 72,100 MW.
Although we are principally concerned with renewables here, I shall consider nuclear too since it is often referred to as a potential source of electricity for hydrogen production, and how the capacity we have arrived at might be met by (i) nuclear in comparison with (ii) wind energy.
(i) Nuclear. Sizewell B is rated at 1188 MW of electricity generating power. Therefore, we would need 72,100/1188 = 61 new Sizewell B capacity reactors, and that is on top of the 30 or so new reactors we will need to replace those due for decommissioning by 2025. As I have commented previously, the uranium fuel required to fuel them is in finite supply and we can expect "peak uranium" at some point, beyond which more energy is used in milling and extracting the increasingly poor uranium ore than is got back in terms of it providing nuclear power. Thus, the 90 or so new nuclear reactors would need to be "Fast Breeders" which produce plutonium, then the available uranium fuel would last for hundreds of years. It would not secure security of supply, since uranium is imported from Canada now (and possibly from Australia or Kazakhstan in the future?), but it would curb our CO2 emissions significantly, mainly through the more efficient use of the uranium fuel, since the majority isotope (238) could be used rather than being a surplus waste product.
(ii) Wind Energy. Generating 72,100 MW of energy via wind turbines would require 72,100/0.2 = 360,000 MW at full capacity, where 0.2 is the "capacity factor", the amount of energy that could realistically be extracted from a wind turbine over time. This translates into 721,000 turbines rated at 0.5 MW, located on mainland sites or around 180,000 2 MW turbines situated on off-shore wind farms. The far taller (ca 80 meter high) structures required for 2 MW turbines, with their longer rotor blades would need to be located away from mainland sites as otherwise many people would find them objectionable - an eyesore, and noisy, if they lived close to them. It is not clear where so many 180,000 2 MW turbines might be situated around the coasts of these islands without them becoming an obstruction to shipping! Add into all of this daunting engineering feats, the construction of a vast infrastructure required to store, transport and supply hydrogen as a fuel, and the whole enterprise on any significant scale appears doomed. We could not implement anything on this scale in the short term ( by 2020, as is thought necessary to curb the U.K.'s CO2 emissions before it is catastrophically too late to prevent climate change), if ever. I call again that we limit our demands on transportation by acting as locally as possible, so cutting back on probably 90% of the current energy requirement for this sector which currently uses 25% of the nation's total energy budget to fuel it.
No comments:
Post a Comment