Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 258

2006). Therefore, solar-based hydrogen production with water is widely viewed as a
very promising option for a sustainable future.
Even in the conventional fossil fuel industry, hydrogen has a major role in the
upgrading of petroleum products. Also, hydrogen is a necessity for the production
of fertilizers in the agricultural industry. Currently, oil upgrading and fertilizer pro-
duction account for about 50% and 40% of the hydrogen consumption, respectively
(Dalcor, 2005; Freedonia, 2010; Kramer, 2005). The rising need of hydrogen by mod-
ern agriculture and petroleum products will strongly advance the hydrogen economy
(Forsberg, 2002; Naterer et al., 2008). However, the major hydrogen production meth-
ods of today are generally not “clean'' because more than 95% of the global hydrogen is
produced from fossil fuels, i.e., 48% from steam methane reforming (SMR), 30% from
refinery/chemical off-gases, and 18% from coal gasification (IEA, 2010; NYSERDA,
2010). Water electrolysis accounts for less than 4%. Even this 4% is not fully clean
because the electricity used for hydrogen production is not fully generated from clean
fuels. The usage of fossil fuels to produce hydrogen generates large amounts of green-
house gases. Therefore, the future hydrogen production pathway from solar-based
water splitting is a promising solution.
Another option for reducing the depletion rate of our planet's fossil fuel reserves is
to recycle the CO 2 emissions with solar energy. This may improve the renewability of
limited fossil fuels and at the same time store the solar energy and minimize the impact
of intermittency of the sunlight. In addition to the renewable benefit, the CO 2 recycling
is also a safer measure compared with the geological sequestration of CO 2 into deep
oceans or geological formations, because the sequestrated CO 2 has many unpredictable
risks such as leakage of CO 2 back to the atmosphere and the change of ocean water
properties (Yang, 2011; Little et al., 2010; Spicer, 2007). Even though care is taken
to identify the appropriate geological areas for the storage of CO 2 , there is always a
likelihood of leakage due to different reasons such as an earthquake. The liquefied
CO 2 on the basin of the ocean floor may have a much higher CO 2 concentration than
normal levels. This makes it difficult for some ocean organisms to survive near the
ocean basin and as a result, the whole ecosystem is disturbed. As for deep ground
sequestration, the leaked CO 2 may mix with groundwater and consequently make the
water toxic and unsuitable for human consumption. Although deep saline reservoirs
potentially have a large capacity to store CO 2 , high pressure CO 2 can significantly
acidify the fluids in the reservoir and dissolve minerals such as calcium carbonate.
As a consequence, the permeability is increased which could allow CO 2 -rich fluids to
escape the reservoir along new pathways and contaminate aquifers used for drinking
water (Kharaka et al., 2006).
Since CO 2 alone is not a fuel and most fuels and organic compounds comprise
hydrogen, so hydrogen is a necessity in the conversion of CO 2 into other useful fuels
and organic compounds such as syngas, methanol, and dimethyl ether. However, nowa-
days more than 96% of the world's hydrogen is produced from fossil fuels through
processes such as steam-methane reforming (SMR) and coal gasification with steam
(IEA, 2012). As discussed previously, these hydrogen production processes are deplet-
ing the fossil fuel reserves and emitting large amounts of pollutants and greenhouse
gases. Hence, to find renewable and low-cost methods of producing hydrogen in large
capacities is also critical to the CO 2 recycling.