Environmental Engineering Reference
In-Depth Information
installation. The suggestion is that every house or small neighborhood in a village
might have its own energy supply. It appears that a fuel cell would be used to turn
the stored hydrogen into electricity when needed. A small-scale solution applied to
billions of homes potentially becomes a large-scale solution. The suggested analogy
may be the mobile cellular phone, available on an individual basis to billions of
people. It is reported (http://en.wikipedia.org/wiki/Mobile_phone) that in 2010
the number of subscriptions to mobile phones was 4.6 billion. Nocera advocates
bottom-up solutions of the projected energy shortage (http://techtv.mit.edu/
videos/633-daniel-nocera-describes-new-process-for-storing-solar-energy) and is
critical of the conventional power grid, which is prone to system-wide failure and
does not reach locations where as many as 1.4 billion people live. Fuel cells, which
turn hydrogen into electricity, are high technology and not inexpensive. Conven-
tional methods for storing hydrogen range from high-pressure tanks to liquid
tanks, but convenient small-scale methods are still in development, as we will
discuss next.
The publicity on the arti
cial leaf seems to emphasize the small-scale, hand-
held device, but to be useful it would seem that a collection area perhaps 10m
2
,
about 10.3 feet on a side, might be minimum useful area for a roof. If electrical
ef
ciency is 10%, on a day/night average sun power of 200W/m
2
,this
would average 200 watts on a continuing basis, while the peak power would be
1000W/m
2
10m
2
1000W. According to the estimate above of 5.5%
conversion of sunlight energy 200W/m
2
into its hydrogen equivalent, this would
give an average power about 110 watts in hydrogen equivalent. To apply the rule-
of-thumb cost of $1/W peak power, at peak power 1000W the cost estimate is
$1000 for the rooftop array. That is a minimum for the cost of the solar cells alone,
ignoring the water circulation, hydrogen collection, and fuel cell aspects. If the
system were allowed to accumulate hydrogen over a day, the hydrogen energy
would be 110 watts
0.1
¼
9.5MJ, or about 3.6 kWh. The nominal value
of the energy at 14c/kWh is then $0.5 per day, or $184 per year. On this basis, it
would take 5.4 years to pay back the assumed $1000 investment.
While the emphasis of Nocera [113] (http://techtv.mit.edu/videos/633-daniel-
nocera-describes-new-process-for-storing-solar-energy) is on small-scale applications,
where the storage of the resulting hydrogen seems problematic, on a large-scale
photovoltaic installation arti
cial leaf cells might also be useful. Assuming the cells
can automatically switch from delivering power to an electrical load to producing
and collecting hydrogen based on the load demand, a large-scale facility could afford
even a liquefying system to store hydrogen produced when the electrical demand
is small.
24
3600 J
¼
9.5
Hydrogen Fuel Cell Status
A fuel cell takes hydrogen gas as input and produces electricity, with ef
ciency in
the range of 50
-
70%, greater than the internal combustion engine,
30
-
35%.