This is a lengthy article, which will be published in the next issue of the journal "Science Progress". A quick summary can be found in the article immediately following this one.
The
Problem of Energy.
The world population of 7
billion humans uses energy at a mean rate of 16 TW (16 terawatts). This amounts
to an annual 504 EJ (504 exajoules = 5.04 x 1020 J), and is provided
from fossil fuels (oil, gas and coal), plus nuclear and hydro (hydroelectric)
power along with all other forms of renewable energy1. The breakdown
of these various contributions is given in Table 1. Around one third of the total
energy used by humans on Earth is provided by crude oil, and close to one
quarter each by natural gas and coal, although the amount of coal being burned
is rising, particularly in China. Nuclear and hydro-power each contribute
around 6% of the total energy mix, while the combination of renewable energy
from all sources, wind, wave, geothermal, wood, solar etc. amounts to just a
little above 1%. In 2004, humanity used 471 EJ of energy2, and while
the relative proportion of each contributing energy source has remained
modestly constant, it is clear that at a rise to 504 EJ in 2010, the demand
placed upon these energy sources is rising relentlessly. This follows not
merely a growing human population per se,
but an increasingly affluent industrialised consumer society. We need to
consider two aspects: firstly, that the CO2 produced by burning
fossil fuels is believed to contribute to global-warming and this may lead to
unwelcome or even catastrophic changes to the global climate3. Secondly,
and more immediately, the fossil fuels and uranium too (for nuclear power) are
in finite supply, and there is compelling evidence that each source will meet
its own production peak within the next two decades. Most vulnerable appears to
be crude oil (petroleum), world supplies of which are predicted to peak (“peak
oil”) probably during the next 5 years4. Irrespective of the exact
timing of peak oil, there are salient predictions (see below) that a gap will
emerge in the supply of oil against demand for it, from the end of this year
(2012), rising to a shortfall of 10 million barrels a day by 20155.
This situation has been termed “gap oil”6, and can only be
exacerbated by peak oil, when supply must draw-down against rising demand, thus
enlarging the gap from both sides. Thus, in order to curb carbon-emissions and
to extend limited resources, alternative and ideally renewable sources of
energy are needed.
Liquid
Fuels and Transportation.
A simple comparison of the
energy content delivered from different energy sources, as is made in Table 1,
is somewhat misleading, since it seems to imply that if the production of one
of them begins to fail, it can be readily substituted by another. The issue of
transportation is a singular example where this is not the case, since
practically all the vehicles used in the world – cars, lorries, buses, trains,
ships and planes - have been engineered to run on liquid fuels that are refined
from crude oil. Therefore the likely consequences of “peak oil”, with dwindling
supplies and escalating costs of liquid fuels, are very serious. Ward's, the
U.S. based publisher, estimated that as of 2010 there were 1.015 billion motor
vehicles in use in the world7. This figure represents the number of
cars, light, medium and heavy duty trucks, and buses, but does not include
off-road vehicles or heavy construction equipment. Between 1950 and 1970, the world
vehicle population doubled roughly every 10 years, passing the 250 million mark
in 1970, and exceeding 500 million in 1986. It has been estimated that the
world's road transportation fleet will reach 2 billion by 2020, of which at
least 50% will be cars. China’s and India’s automobile fleets are expected to
grow at an annual rate of around 7 or 8%, while in the United States, it will
be under 1% a year, and around 1 to 2% in Western Europe, but this depends
tacitly on finding an expanding liquid fuel supply, and it is this which is at
issue. Indeed, the International Energy Agency (IEA)
has issued a report8 to the effect that a shortfall in oil
production of 64 million barrels a day (mbd) can be expected by 2030, which
represents a loss of 62% of the world supply of conventional crude oil,
currently 84 mbd, assuming a demand by 2030 of 96 mbd, a figure significantly
downgraded from prior estimates by the IEA of 120 - 130 mbd. At a mean decline
rate of 2.9 mbd/year (-3.4%/year) this value accords closely with the
prediction in a recent U.S. Army report5 that there will be a
deficiency of 10 mbd by 2015, following a loss of any spare capacity for crude
oil against demand for it by the end of this year (2012).
While
it is possible to run cars and other road vehicles on electricity, provided
either from batteries or hydrogen/fuel cells, actually converting their number
substantially to these alternative energy carriers (neither electrons nor
hydrogen being primary fuels, i.e. they must be created from primary sources)
would be such a considerable undertaking that the scheme is not feasible.
Vehicles can be adapted to run on gas but a peak in natural gas production is
expected within twenty years, following oil, and converting them all would take
many decades, so this is no solution either. It is, therefore, a new source of
liquid fuels that must be sought, since they would be far more compatible with
a transportation fleet and distribution infrastructure designed for liquid
petroleum fuels. These, ideally, should be “carbon-neutral” in order to reduce
carbon emissions, a condition which certainly does not apply4 to
coal-to-liquids processing (with probably twice the carbon emissions overall
that are incurred in the production and burning of diesel or petrol derived
from petroleum), nor gas-to-liquids either, unless the CO2 is
captured and stored in some way that prevents it escaping into the atmosphere.
Biofuels are attended by a number of vexed issues6: the competition
for arable land between growing crops for fuel or crops for food, the increased
amount of freshwater required to grow fuel crops above that already needed for
agriculture, the clearing of rainforest to produce high-energy fuel-crops, e.g.
palm-oil, and the fundamental EROEI which for bioethanol may only marginally
exceed the overall energy costs of its production, or in some cases not quite break-even.
Clearly, some other strategy is necessary.
Table 1. Fuel type: Average
power in TW (1012 W) Energy/year
in EJ (1018 J) (2010 figures), (Data from reference 1).
Oil: 5.4 TW (169 EJ)
Gas: 3.8 TW (120 EJ)
Coal: 4.7 TW (149 EJ)
Hydroelectric: 0.8 TW (26
EJ)
Nuclear: 1.0 TW (33 EJ)
Geothermal, wind, solar,
wood: 0.2 TW (7 EJ).
Total: 16.0
TW (504 EJ) (2010).
Solar
Energy.
Figure 1 summarises the
quantity and fate of solar radiation striking the top of the earth’s atmosphere2.
We see that 52 PW (1015 W) is reflected back into space (i.e. 30% of
the total). Thus, in outer space, there is more solar energy available to be
collected, which has prompted potential schemes to launch photovoltaic arrays
into space on satellites2, with which to capture the sun’s energy
and then beam it back to earth in the form of microwaves for terrestrial
applications. At the top of the atmosphere, with the sun directly overhead, the
radiation flux provides around 1.4 kW/m2 of energy, the “solar
constant”2. Since the total amount of energy1 (oil, gas,
coal, nuclear, hydro, everything) used on earth by humans amounts to a power of
16 TW, at 174 PW, the amount of radiation striking the exposed hemisphere of
the earth is well over 10,000 times that. So if we could capture even a small
amount of this bounty, and convert it into useable energy, the imminent energy
crisis could be averted. Since the production of solar energy (and renewable
energy generally, other than hydropower) has an intermittent quality – the sun
doesn’t always shine, or not to the same extent throughout the day, and not at
all at night – some method of storing it is essential to its use as a serious
energy source, to supplant fossil fuels, and the creation by its means of a
chemical fuel (solar fuel) would be the best means to achieve this.
Furthermore, and as noted earlier, if liquid fuels could be produced in
quantity, they would be entirely compatible with the present world
transportation fleet based on liquid petroleum fuels, and a distribution
network designed to supply the latter.
Means
for capturing solar energy2.
Solar energy can be captured
by various means, which boil-down to either collecting heat directly from the
sun’s rays, or using the wavelengths of the solar spectrum to grow biomass
through photosynthesis, or to excite electrons in photovoltaic materials to
produce electricity.
(1) Direct heating systems2
à on-roof
water heating systems; solar furnaces; concentrating
solar thermal power (CSTP) plants etc.
(2) Photosynthesis9
(PS) Ã
creates biomass, and a total of 200 EJ of fuel (Shell estimate) could be
produced by hydrothermal conversion. PS is 12% efficient as a theoretical
maximum, but most plants give 0.1 - 6%. Growing biofuel crops also suffers from
competition with food crops for fertile land, hence if we turned all the
available arable land in the U.K. over to biofuel crops (i.e. grew no food at
all) we could only match ca 17% of
our fuel from rapeseed/biodiesel or ca
50% from sugar beet/ethanol as is currently made from crude oil. These
estimates assume that all vehicles are first converted to diesel engines, which
are more efficient in terms of tank-to-wheel miles than spark-ignition engines,
which burn petrol, by about 40%.
(3) Photovoltaics2,10.
In essence, light photons knock electrons into higher energy states in
semi-conductor materials such as silicon and cause conduction (electron
diffusion), generating electricity. The photo-active material needs a band-gap
of < 3.2 eV (i.e. at the edge of the visible spectrum, 400 nm), and ideally
down to around 1.0 eV (1250 nm) - which is the “near” IR region. The actual
recovered efficiencies for single-junction silicon cells are now approaching
the theoretical efficiency of 33%, which is way ahead of that for
photosynthesis (6%). In principle, quantum dot (QD) cells might achieve an
efficiency of 42% or even 65% as a result of multiple exciton generation2,
although the record to date is a rather more modest 6%11.
Fuels
Made Using Concentrating Solar Thermal Power12,13.
Those “fuels” that have been most
pursued using various solar methods are H2 and CO, and while each
may indeed be used either in fuel cells or combustion engines, a mixture of
them (known as synthesis gas or syngas), can be converted to liquid fuels, e.g.
methanol, or hydrocarbons, using the Fischer Tropsch process4. The
latter involves a series of chemical reactions that lead to a mixture of
hydrocarbons (CnH(2n+2)) (equation 1):
(2n+1) H2 + n CO
→ CnH(2n+2) + n H2O (1),
where 'n' is a positive integer. It is
mainly straight-chain alkanes that are formed and so the product is quite
suitable as a diesel fuel. As we now discuss, there are various routes to
forming H2, but CO can be formed essentially either through steam
reforming4 carbon-containing materials (methane, oil, coal, biomass)
– which also yields H2 - or by dry reforming14 with CO2.
Now the latter lends the possibility that CO2, either as a product
of steam reforming natural gas or as captured from power stations, might be
intercepted from being emitted into the atmosphere and instead converted to
useful fuels, in substitution for those presently refined from petroleum.
H2 by direct thermolysis of
H2O.
Although it is
conceptually the simplest process, the thermal dissociation of H2O
to
H2 and
O2 is difficult to achieve in practice, since very high temperatures
are required to attain a reasonable degree of dissociation (e.g. 2725°C for 64%
dissociation at atmospheric pressure12), and it is necessary to
separate the H2 from O2 to obviate the build-up of an
explosive mixture. Ceramic membranes, such as those made from zirconia, have
been tested but even these usually fail to withstand the thermal shocks
incurred when exposed to high-flux solar radiation. Rapid-quench methods, in
which a cold gas is expanded in a nozzle, or a solar-irradiated target is
immersed in liquid water, are straightforward but cause a fall in the energy
efficiency which is compounded by significant energy losses by radiation from
the extremely hot reactor. Furthermore, the gas mixture is explosive. There are
water-splitting multi-step thermochemical cycles which may be performed at
significantly lower temperatures (below 950 oC), expected to be
available in the future either from concentrating solar thermal power or from
very high temperature nuclear reactors (VHTR), and which further avoid the
necessity to separate H2 from O2.The leading candidates
are a three-step sulphur iodine cycle (equations 2 - 4)13 based on
the thermal decomposition of sulphuric acid at 850°C and a four-step UT-3 cycle
(equations 5 - 8)13 based on the hydrolysis of calcium bromide and
iron bromide at 750°C and 600°C, respectively. It is proposed that, in the
longer run, a complete
substitution of fossil fuels by solar H2 might be achieved, while the
decarbonization of fossil fuels in the medium term creates a link between today’s
fossil-fuel-based technology and the solar chemical technology of tomorrow.
2H2SO4 → 2SO2 + 2H2O +O2 850 oC (2)
2HI
→ H2 + I2 300
oC (3)
I2
+ SO2 + 2H2O → 2HI + H2SO4 100 oC (4)
2Br2
+ 2CaO → 2CaBr2 + O2 600
oC (5)
3FeBr2
+ 4H2O → Fe3O4 + 6HBr + H2 600 oC (6)
CaBr2
+ H2O → CaO + 2HBr 750
oC (7)
Fe3O4
+ 8HBr → Br2 + 3FeBr2 + 4H2O 300 oC (8)
H2 by
decarbonization of fossil fuels.
There are
principally three solar thermochemical processes for H2 production
using fossil fuels, namely: cracking, reforming, and gasification. In solar cracking, natural
gas, oil, or other hydrocarbons are decomposed thermally into H2 and
carbon. The carbon can either be sequestered to avoid the release of CO2,
or used for other purposes. Steam-reforming of natural gas or oil, and
steam-gasification of coal and other solid
carbonaceous materials furnishes syngas, which may be converted to liquid
hydrocarbon fuels, vide supra. The
chemical properties of syngas accord principally to its differing content of H2,
CO, and CO2. The CO content can be reduced in favour of H2
via a catalysed reaction with steam, known as the water-gas shift reaction
(equation 9):
CO(g) + H2O(g)
→ CO2(g) + H2(g) (9)
CH4(g)
+ CO2(g) → 2CO(g) + 2H2(g) (10)
The product, CO2,
can be separated from H2, e.g. by using the pressure-swing adsorption
technique, which is a method for the separation of a gas from
a mixture of gases under pressure, according to particular molecular
characteristics and affinity for an adsorbent material (e.g. through
differential molecular electric quadrupole moments). Syngas may also be produced by the solar dry
reforming of CH4 with CO2 (equation 10) and transported
to locations to be used as a fuel directly. Stored solar energy is released by the
reverse exothermic reaction in the form of heat, which can be used for
generating electricity via a Rankine cycle. [The Rankine cycle
generates about 90% of all electrical power used throughout the world,
including that from virtually all solar thermal, biomass, fossil fuel and
nuclear power plants. Heat is supplied externally to a closed loop, which
normally uses water. The cycle also provides the fundamental thermodynamic basis
of the steam engine]. Similarly, ammonia
(NH3) can be used in a chemical heat pipe to store and transport
solar energy. In one system developed by the Australian National University
(ANU), ammonia is dissociated in an energy storing (endothermic) chemical reactor
as it absorbs solar thermal energy. At some subsequent place and time, the
reaction products of H2 and nitrogen (N2) undergo an
exothermic reaction in which ammonia is re-formed, releasing heat. An
industrial demonstration plant has been announced using four of ANU’s 400 m2
parabolic dishes15.
Solar steam gasification.
The steam-gasification of coal and oil shale has been investigated
using concentrated solar energy. A conceptual design of a solar reactor for the
gasification of carbonaceous materials was created using
optical fibres to direct the solar radiation into the
reaction chamber. In the project SYNPET15 (2003–2009), a
solar thermal technology for the steam-gasification of
petcoke particles was developed, using a 10 kWth (th = thermal) solar
reactor, directly exposed to concentrated solar radiation, with a continuous
gas-particle vortex flow confined to a cavity receiver. A scale-up of the
vortex solar reactor to 500 kWth at the Plataforma Solar de AlmerÃa
is currently being undertaken15.
Catalysed thermal methods17 for the
production of H2, CO and CH4 from H2O and CO2.
The cerium(IV) oxide–cerium(III) oxide cycle (CeO2/Ce2O3
cycle) is a two-step thermochemical process based on cerium(IV) oxide/cerium(III)
oxide, and is normally employed for hydrogen production. One advantage of this approach is that the H2
and O2 are generated in two distinct steps, and so it is unnecessary
to separate the components of a high-temperature gas. The process constitutes a
redox system:
Dissociation: 2CeO2
→ Ce2O3 + 0.5 O2 (11)
Hydrolysis: Ce2O3
+ H2O → 2CeO2 + H2 (12)
In the first step (equation 11), which
is endothermic, cerium(IV) oxide is dissociated thermally, under an inert gas
atmosphere at high temperatures, into cerium(III) oxide and oxygen. In the
second step (equation 12), which is exothermic, cerium(III) oxide reacts at
lower temperatures with water to produce hydrogen and regenerate cerium(IV)
oxide. The strategy is also applicable to the dissociation of CO2 (equations
13 - 14):
Dissociation: 2CeO2
→ Ce2O3 + 0.5 O2 (13)
Hydrolysis: Ce2O3
+ CO2 → 2CeO2 + CO (14)
If both H2O and CO2
are fed into a suitable solar thermal reactor containing a ceria catalyst, which
is cycled within the temperature range, 800 oC - 1500 oC,
syngas is produced. Although the yields are low, with only around 0.7 - 0.8% of
the solar thermal energy being harnessed by the fuel, it is thought this is
only a limitation of the scale and design of the system rather than of the
underlying chemistry. Most of the energy is lost as heat through the wall of
the reactor, or by the re-radiation of sunlight back through the aperture of
the device. But the researchers are confident that efficiency rates of up to
19% can be achieved through better insulation and smaller apertures. Such efficiency
rates, they say, could make for a viable commercial device. It is worth noting
that of various oxide materials that might be employed as redox catalysts for
splitting water or carbon dioxide, ferrite-based oxides show fairly slow
reaction rates, whereas oxides such as ZnO and SnO2 sublime during
the decomposition stage and require rapid quenching of gaseous products to
avoid recombination. In contrast, ceria is stable to volatility and to
sintering (which reduces the surface area and activity), and shows relatively
rapid reaction rates. This is thought to be due partly to the presence of
nonstoichiometric oxidized and reduced
phases, a high oxygen diffusion rate, and to its thermal structural stability.
By impregnating a samarium/ceria catalyst with nickel11, methane was
produced, shown to arise from catalytic hydrogenation of carbon particles
formed on the surface of the metal by decomposition of CO2.
Fuels Produced by “Artificial Photosynthesis”.
I have some issue with the
accuracy of the term “Artificial Photosynthesis”17, but it is snappy
and sounds very “green” so I think it will stick. As a reminder, natural
photosynthesis9 is a “water-splitting” process by which green plants
and green algae fix CO2 from the atmosphere to build carbohydrate,
and grow. The rate of energy capture by photosynthesis is immense, at
approximately 100 TW, which is about six times the entire power
consumption of human civilization. Photosynthesis is also the sole source of carbon
in all life on Earth, and converts around 370 billion tonnes of CO2 into
250 billion tonnes of biomass each year, in which is stored almost 4,000 EJ
worth of energy. The counterpart
product is O2, and hence photosynthesis is the origin of virtually
all atmospheric oxygen, with a roughly 50:50 contribution made by land-based
plants and by oceanic phytoplankton. The process may be summarised as follows:
nCO2 + nH2O
+ hν → (CH2O)n + nO2
Production of ONE O2
molecule requires the transfer of FOUR electrons:
2H2O → O2
+ 4e- + 4H+
And FOUR electrons are
required to reduce ONE CO2 molecule:
CO2 + 4e-
+ 4H+ → (CH2O) + H2O
Mechanism.
The energy resulting from
light adsorption by the chlorophyll photocatalyst is transferred to a
manganese-protein complex called “Photosystem II”, which oxidises water:
H2O – e-
→ [H2O+•] → HO• + H+
2 HO• → H2O2
H2O2 – e-
→ [H2O2+•] → HOO• + H+
2 HOO• → O2 + H2O2
In a broad analogy with this, the
label “artificial photosynthesis” is commonly used to describe any scheme for
capturing and storing the energy from sunlight in the chemical bonds of a material
that might be used as a fuel (a solar fuel). Photocatalytic water splitting
converts water into protons (leading to H2) and O2, and
provides a major research topic in the field. Another important area is “light-driven
carbon dioxide reduction” which aims to replicate carbon fixation by natural
photosynthesis, and might provide a carbon-mitigation strategy, along with the
creation of fuels independent of petroleum and natural gas. In the broad
classification of “artificial photosynthesis”, accepting that light can be
harvested using external PV cells and converted to electrons, we give due mention
to electrochemical processes, which may be photo-assisted or direct. Thus,
although the primary reduction product of CO2 is CO, a direct photoelectrochemical
conversion of carbon dioxide and water to methanol using a p-type semiconductor
(GaP) electrode has been reported with faradaic efficiencies of 88 – 100%. The
process appears highly selective, since other reduction products such as formic
acid and formaldehyde were not detected, nor hydrogen.18 The direct
electrochemical reduction of CO2 has mainly been thwarted by the
impractically high overpotentials required to drive the process. In contrast, a
direct conversion of CO2 to CO has been achieved at overpotentials
of less than 0.2 volt. The medium uses an ionic liquid
(1-ethyl-3-methylimidazolium tetrafluoroborate, EMIM-BF4) as its
electrolyte, which is thought to lower the energy of the CO2−•
intermediate through the formation of a complex of the type “EMIM+-CO2−•”.19
As pointed out previously20, for ionic liquid applications to be
used in earnest, they would need to be synthesised on a large scale, and from
molecules derived from crude oil. This case is no exception, if it is to
provide a source of syngas and hence liquid hydrocarbon fuels, in a quantity
that in any way matches the expected loss of petroleum and fuels derived from
it. Dye-sensitized solar cells2 (DSSC) have been used in H2
production. A DSSC is a kind of thin-film cell in which a semiconductor,
normally TiO2, along with a coating of an organic or inorganic dye
to act as a photosensitizer is coated on the anode, while a platinum catalyst
is present at the cathode, where H2 is generated. Thin-film cells
offer the considerable advantage2,10 over conventional solar cells
that perhaps only 1/100th the amount of conductive material is
required in their fabrication, and is thought to be critical both to the rate
and scale at which PV technology can be implemented. TiO2 is the
photocatalytic semiconductor material most studied as a potential solar
water-splitting catalyst, and yet despite 4 decades of research the process
remains inefficient and economically unsound, due mainly to rapid recombination
of holes and electrons, the rapid reconversion of oxygen and hydrogen back to
water and poor activation of TiO2 by visible light. The hole-charge
recombination can be inhibited by loading the particles at their surface with
metals, most commonly platinum, but its limited world supply and high cost
imposes a severe limitation on the technology, which is has to be said remains
rather inefficient in any case. The implantation of high-energy transition
metal ions (accelerated by high voltage) has been shown to modify the
electronic structure of TiO2, so that its photo-response is shifted
into the visible region (up to 600 nm), which may prove useful in the
development of the “second generation photocatalyst”21.
Nocera and his co-workers have
developed what they describe as an “artificial leaf”22. In the
overall strategy, sunlight can be captured and converted to electricity using a
silicon solar PV cell, but the crucial advance lies in the design of the
electrode surface to provide a highly efficient anode electrocatalyst (Oxygen
Evolving Electrode, OEE) for use in the electrolysis of water employing
inexpensive materials. An indium tin oxide (ITO) electrode was immersed in
water containing Co2+ cations and potassium phosphate (Pi). By
application of a voltage to the electrode, cobalt, potassium, and phosphate
accumulated on its surface, to form the catalytically active phase, where water
is oxidised yielding O2 and protons18. By reduction of
these protons, H2 might result, using a suitable cathode, which
originally was made from platinum. During the process, the cobalt-based phase
decomposes, but is regenerated (“self-heals”) by cobalt, potassium and phosphate
being adsorbed from the solution. To reduce the protons to H2, a
cathode made form cheap, readily available (“Earth Abundant”) materials rather
than precious metals is required, if a serious scale-up of the technology is to
be feasible. To this end, a Co/borate catalyst was deposited on the surface of
a triple-junction, amorphous silicon PV semiconductor (which was in contact
with a stainless steel plate, to act as the anode (OEE), while a ternary
Ni/Mo/Zn alloy was employed as the cathode (Hydrogen Evolving Electrode, HEE)23.
The silicon was passivated (protected) by an ITO layer, which protects it from
reactive oxidising species formed at the anode, e.g. HO• radicals. The two
electrodes were employed in two configurations: (1) in which they were connected
by a wire, and (2) in a wireless mode, where the Ni/Mo/Zn was deposited
directly onto the stainless steel backing of the silicon wafer. The
efficiencies of the two devices are 4.7% for a wired configuration (1) and 2.5%
for the wireless arrangement (2). It is thought that the efficiency of (2) is
reduced by the relatively greater distance that the protons must travel to the
cathode from the front face of the anode, which imposes substantial ohmic
losses in the wireless cell, in comparison with the 1 mm gap between the two
electrodes in the wired cell (1). It is proposed that this technology could be
adapted from a panel geometry to one based on (nano)particles free in
solution/suspension22. However, in the in situ arrangements described, as opposed to standard
electrolysers where H2 and O2 are generated in different
chambers of the device, explosive mixtures of H2/O2 would
be produced, and need to be separated probably using some form of membrane
technology.
Nocera
envisages a widescale future application of such devices for energy production
say at the level of individual homes, with H2 acting as an energy
storage medium so that power can be provided even at night (using a fuel cell)
when the sun is not shining. He sees the most fruitful regions of the world,
for the technology to be developed, as the “non-legacy”(i.e. developing)
nations, which are less entrenched by tradition and vested interest in
centralised, large-scale power production from fossil fuels and nuclear, than
is the case in the “legacy” (developed) nations. Thus, remote communities could
be provided with electricity at a local level. In October 2010, Nocera signed
with the Tata Group of India to commercialize his research, a country in which
there could be great demand for such decentralized, “personal” energy
production, which he sees as key to the future of humanity. As we note later,
due to the failing supply of crude oil and the absence of other fuels on a
matching scale to run the global transportation network, a relocalisation of
human civilization and its societies appears inevitable, as access to cheap and
extensive personalised transport (mainly cars) will be prohibited both by cost
and actual fuel shortages. In such localised communities, small-scale power
generation will be necessary, and perhaps Nocera’s invention (or something like
it) might contribute to a de-industrializing West, along with sustaining the
existing non-legacy nations. However, many daunting challenges remain,
including the instability of the cells which lose their activity over periods
of hours and that there is a world shortage of indium24 (for ITO),
which it is thought may run-out within 5 years, due to other pre-existing
demands on it, e.g. for thin-film solar cells and light-emitting diodes (LEDs).
Thus, any substantive technology is probably many decades away, along with the
putative hydrogen economy itself, if that will ever arise in the full-ascension
that some envisage. Meanwhile, we will need to address a more immediate and
drastic failing of our energy supply, particularly that from crude oil and
natural gas, most likely by transforming from a global village to a globe of
villages.
Liquid Solar Fuels Through Industrial
Photosynthesis25.
It has been estimated that to match
the entirety of 16 TW of energy, as used by humans on Earth, in the form of
hydrogen would require splitting some 18 billion tonnes (18 km3) of
water annually, using photoelectrochemical methods or other means, e.g.
concentrating solar thermal power, all as yet to be fully developed let alone
proliferated on this scale. However, there is an established solar technology
already available to us, namely photosynthesis. The basis of photosynthesis, as
it occurs in Nature, is outlined in the previous section, which captures
solar-energy at a rate of around 100 TW. Of the resulting 4,000 EJ worth of
stored energy, clearly only a fairly small proportion might be sensibly
harvested, and a deliberate growing of particular energy crops is necessary,
much as we grow crops for food, while allowing animals to graze on available
common land and set-aside pasture. There is, as already noted, competition
between the use of arable land for food crops or fuel crops, and to place this
into perspective, if in the UK we turned over all our crop-land to growing
sugar beet for bioethanol production, and grew no food at all, we might match
just half of our national fuel demand as is currently met from crude oil. If we
chose rapeseed for the same purpose, a mere one sixth of that demand could be
supplied in the form of biodiesel. There is the further issue of the freshwater
demand, of which agriculture already struggles to secure enough to meet its
needs, and in a sustainable picture of the future, supplies of water appear
uncertain against the countenance of climate change. It is in the light of
these considerations that algae/algal fuels have begun to look very appealing6,
especially given the claimed very high yields that can be obtained per hectare
as compared say with rapeseed and biodiesel. From a survey26 of the
results from different studies, it has been estimated that between 40 – 90
tonnes of biodiesel/hectare can be produced from algae. However, the yields
reported have often been extrapolated from growing algae over far smaller areas
than a hectare, and there seem to be problems encountered when actual scale-ups
are attempted. To date, no one has yet succeeded in producing fuel commercially
and at scale, and indeed many small firms that started out to do this have
stopped trading, one of which being the MIT spin-off Greenfuel Technologies
which closed in 2009 after receiving £44 million ($70 million) of investment to build its own
mini-algae plant. The algae programme of the UK’s Carbon Trust has been
scrapped in the wake of the government austerity measures, as it attempts to
cut the massive debt incurred in having to bale-out the banks after the 2008
economic crash. One problem is that the algae cannot be grown very densely,
because those closest to the surface screen the sunlight from reaching those
below. Another difficulty is to actually extract the oil from algae, and it
appears that producing highly oil-yielding strains en mass is more difficult than previously thought. There are other
means to process algae than by oil extraction/transesterification to biodiesel,
including hydrothermal liquefaction26, where the water is not
removed (a highly energy intensive process) but the material is heated wet
under pressure (perhaps with a catalyst) to decompose it into liquid
hydrocarbon fuels and methane. Conventional algae production can be combined
with water clean-up strategies6,24,26, to remove N and P from
agricultural run-off water and sewage effluent, both to prevent eutrophication
(nutrient build-up in water), which causes algal blooms, and to conserve the
precious resource of phosphate, since a peak in world phosphate rock production
is expected around 203024. “Peak phosphate” is connected to “peak
oil” since phosphate is mined using oil-powered machinery, and in the absence
of sufficient phosphorus, we will be unable to feed the rising global human
population, since modern industrialised farming depends on heavy inputs of
phosphate, along with nitrogen fertilizers. Pesticides, too, derived chemically
from crude oil, are essential, along with oil-refined fuels for farm machinery.
It is, nonetheless, doubtful that the world’s liquid transportation fuel
requirements can be met through standard methods of algae cultivation entirely,
though fuel production on a smaller scale seems thus feasible. I gave an analogy
for the latter as growing algae in a “village pond” for use by a community of
limited numbers.
Noting the troubles and limitations of
growing algae by conventional means and producing fuel from it on a grand
scale, a breakthough does appear possible through the use of genetically
modified cyanobacteria. The company, Joule Unlimited, Inc., has proposed a new
high-productivity solar-to-fuels platform that uses direct product synthesis
and continuous production25. The process is claimed to produce
15,000 gallons (US) of diesel/acre/year, to be compared with 3,000 gallons of
biodiesel/acre/year. Now, this is hydrocarbon diesel that is being produced,
with an energy content of (based on the alkane, heptadecane, C17H36)
47.2 MJ/kg, as opposed to 41 MJ/Kg for biodiesel. Noting that the respective
densities of these fuels are 777 kg/m3 and 890 kg/m3,
this amounts to 110.7 tonnes/hectare/year compared with 25.4
tonnes/hectare/year for the diesel and biodiesel. When the differential energy
content of the fuel is taken into account, it can be seen that the direct,
continuous process is almost exactly five times as productive in its energy
yield/hectare than the conventional batch method. Through advances in genome
engineering, solar energy capture and bioprocessing by the organism, a
photosynthetic efficiency of 7.2% is obtained, to be compared with the
theoretical limit of 12.1% of available sunlight at the ground being used for
photosynthesis. The process is innately less complex in terms of avoiding much
of the downstream processing attendant to conventional algae-to biodiesel
conversion, and the product, being hydrocarbon diesel, is completely compatible
(fungible) with the existing engine and liquid fuel distribution
infrastructure. It is stressed that the “platform converts sunlight and waste
CO2 (at a concentration some 50 – 100 times that in the atmosphere,
in a closed system) directly into liquid fuels in a continuous process that is
not limited by costly biomass intermediates, processing or the use of natural
resources. According to the sales-pitch: “this platform can yield renewable
diesel fuel in unprecedented volumes with a fraction of the land use incurred
by current methods, leapfrogging biomass-dependent approaches and eliminating
the economic and environmental disadvantages of fossil fuels.” The technology
does indeed sound promising, but it is going to have to be proliferated on a
massive scale across the world, and rapidly, if it is to cope year on year with
a failing conventional oil supply5,8 of 2.9 mbd/year. This means
that by 2015 – a mere 3 years time –
daily production by such unconventional means needs to be around 10 million
barrels5, and 64 million barrels8 by 2030, to offset the
fall in conventional crude oil production and meet a projected increase in
demand from the current 84 mbd to 96 mbd. It is worth noting that this amounts
to a very modest anticipated growth of 0.8%/year. Previous estimates for growth
during this period were up to 3%/year, reaching a production of 130 mbd, but
this has been reckoned-down significantly.
Overall
summary and outlook.
In conclusion, we are faced with an
overall serious energy problem, and most pressingly the challenge of how to
fill the enlarging hole created by a declining production of conventional crude
oil. It appears almost certain that there will be profound efforts made in
obtaining “unconventional oil” from shale and in liberating gas from various
geological formations by “fracking”; the production of “synthetic crude” from
tar-sands will doubtless increase too. Noting that world light crude oil
production peaked in 2005, it is increasingly the heavy oils, e.g. from the
Orinoco Belt in Venezuela, that will need to be recovered and processed,
requiring the building of a new swathe of oil refineries that can handle this
kind of material. Thus, not only are supplies of conventional crude oil going
to fall, but what is recovered will be increasingly difficult to process. How
difficult it is to produce an energy resource is usually expressed by the Energy
Returned on Energy Invested (EROEI). Thus in the halcyon days of the Texan
“giant gushers”, 100 barrels of crude oil could be recovered using the energy
equivalent to that contained in one barrel of crude oil, which gives an EROEI =
100. The figure has fallen since then, and presently EROEIs in the range 11 -18
are obtained for North Sea (Brent Crude) oil, and as low as 3 – 5 for heavy oil
and tar sands “oil”.
Oil shale should be distinguished from shale oil, though both are misnomers. Oil shale contains no oil, as such, but a solid primordial material called kerogen, which must be thermally cracked to obtain a liquid that resembles crude oil. The production and use
of oil-shale is hardly environmentally “clean”, taking account of its carbon emissions (both
in the retorting of shale and in burning the final fuel) and large water demand
(3 – 10 barrels of water to produce each barrel of oil), and as yet there is no serious commercial production. Nontheless, becasue vast resources of oil-shale can be claimed, it is trumpeted in
some quarters that the US will become self-sufficient in “oil” by 2020. In contrast, shale oil (the correct term is tight oil) is actual crude oil (petroleum), but is trapped within impermeable rock. The rock is broken open using hydraulic fracturing ("fracking"), which allows the oil to flow out, whereupon it may be refined in the normal way. Current
US production of shale-oil is around 0.5 mbd, and which is predicted to rise to 3 mbd
by 2020. However, this must be gauged against a loss of conventional oil by 27 mbd
across the world. Can solar fuels fill the gap? As we have seen, much of the
solar fuel technology is very much at the research stage. Most of what is
ongoing aims to produce H2, but even if half the “new” platinum
recovered annually were used to fabricate fuel cells, only something like 1% of
the billion road vehicles currently in existence could be so substituted by
“hydrogen cars” over the next 10 years. Hence, a global transportation network
based on hydrogen/fuel cells, let alone a full-scale solar hydrogen economy, is
a pipe-dream. If hydrogen can be made renewably on the grand scale, as an energy
carrier (it is not really a fuel, since it must be created from primary energy
sources), it will probably need to be used by combustion. The fabrication of
electric cars runs into similar resource difficulties, especially in terms of
rare earth metals, and so a strategy based on liquid fuels would seem most
sensible. Liquid fuels are furthermore entirely compatible with the prevailing
transportation infrastructure, in regard to the distribution of fuels and their
deployment in internal combustion engines. The Fischer-Tropsch (FT) process is
a well-established technology for converting syngas to liquid hydrocarbons, but
the means to obtain H2 + CO on a large scale without using fossil
fuels is not. Even when (or if) those clean technologies based on artificial
photosynthesis are developed, a whole new generation and scale of FT plants
will need to be installed, which at the level envisaged would take decades. Any
such timescale must be judged against that for the depletion of conventional
crude oil. Of those approaches considered here for the production of liquid
fuels, the use of genetically engineered cyanobacteria looks the most
promising, but even so, meeting the global demand for them seems to be a bridge
too far. Producing millions of electric cars is just not a practical
proposition, and the only realistic means to move people around in number using
electrical power is with light railway and tram systems. The notion of
personalised transport will be relegated to history by massive fuel prices, and
an absence of any cheaper “car ownership” option. Our global civilization is
underpinned almost entirely by crude oil – as refined into liquid fuels for
transporting people and consumer goods around nations; for growing and
distributing food; for mining coal, shale and all kinds of minerals, including
metallic ores and rock phosphate for agriculture; and as a raw feedstock for
the chemical industry, to make pharmaceuticals and to support healthcare. If
our stalwart “black gold” is set to abandon us over the next few decades, and
it is not possible on that same timescale to produce alternative liquid fuels –
“the supply side” - we can only address the problem from the demand side. This
means a substantial curbing of transportation and a relocalisation of society,
to become more locally sufficient, e.g. in food production, at the community
level. Such are the aims of the “Transition Town” movement27. It is
likely that energy production will become increasingly decentralized, and done
at the smaller scale, to power such communities. Fuel too, e.g. for local
agriculture, might be produced from algae at least on a regional scale, as
integrated with water treatment schemes6,24,26 to conserve the
resource of phosphate, and to avert algal blooms. Methods of regenerative agriculture,
including permaculture, provide means to food production that demand far less
in their input of fuels, fertilizers and pesticides, and actually rebuild the
carbon content of soil. It is thought that 40% of anthropogenic CO2
emissions might be sequestered in soil using no-till practices, if practiced
across all the Earth’s 3.5 billion acres (14 million km2) of arable
land28. Solar energy may also be harvested usefully and directly in
the form of heat2 (rather than converting it to a fuel), at greater
efficiency than through PV, using concentrating solar thermal power plants,
roof-based water heating systems, solar cookers, solar stills and water
sterilization units, and homes especially designed to absorb and retain thermal
energy. Though the foreseeable transition to a lower energy and more localised
way of life is unequivocally daunting, we should remain optimistic.
“We
act as though comfort and luxury were the chief requirements of life,
when all that we need to make us really happy is something
to be enthusiastic about.” – Charles Kingsley (1819 – 1875).
References.
(1) BP Statistic Review of World
Energy 2011. http://www.bp.com/liveassets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2011.pdf
(2) Rhodes, C.J. (2010), Sci. Prog. 93, 37.
(3)http://www.ipcc.ch/publications_and_data/publications_and_data_figures_and_tables.shtml#.T3BFgdWjmeF
(4) Rhodes, C.J. (2008), Sci. Prog. 91, 317.
(5) http://www.marketoracle.co.uk/Article33300.html
(6) Rhodes, C.J. (2009), Sci. Prog. 92, 39.
(7) http://en.wikipedia.org/wiki/Motor_vehicle
(8) http://www.independent.com/news/2012/mar/18/toward-energy-literacy/
(9) Rhodes, C.J. (2011), Sci. Prog., 94, 339.
(10) McIntyre, R.A. (2010), Sci. Prog. 93, 361.
(11) Tang, J. et al. (2011), Nature Materials, 10, 765.
(12) Steinfeld, A. (2005), Solar Energy, 78, 603.
(13)http://www.solaritaly.enea.it/Documentazione/Solar%20Thermal%20Energy%20Production.pdf
(14)https://netfiles.uiuc.edu/mragheb/www/NPRE%20498ES%20Energy%20Storage%20Systems/Carbon%20Dioxide%20Reforming.pdf
(15) http://www.solarpaces.org/Library/docs/Solar_Fuels.pdf
(16) Chueh, W.C. and Haile, S.M.
(2009), ChemSusChem. 8, 735.
(17) Styring, S. (2012), Faraday Discussions, 155, 357.
(18) Barton, E.E., Rampala, D.M. and
Bocarsly, A.B. (2008), J. Am. Chem. Soc.130, 6342.
(19) Rosen, B.A. et al. (2011), Science 334, 643.
(20) Rhodes, C.J. (2011), Sci. Prog. 94, 211.
(21) Anpo, M. (2004), Bull. Chem. Soc. Jpn. 77, 1427.
(22) Reece, S.Y. et al. (2011), Science 334, 645.
(23) Pijpers, J.J.H. et al. (2011) PNAS 108,10056.
(24) Rhodes, C.J. (2011) Sci. Prog. 94, 323.
(25) Robertson, D.E. (2011) Photosynth. Res. 107, 269.
(26) Rhodes, C.J. in Algal Fuels: Phycology, Geology,
Biophotonics, Genomics and Nanotechnology, J.Seckbach (ed.), Springer,
Dordrecht, in press.
(27) http://scitizen.com/future-energies/transition-town-reading-_a-14-3716.html
(28) http://www.rodaleinstitute.org/files/Rodale_Research_Paper-07_30_08.pdf
