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Solar Panels, Electrolysis, and Feasibility
Name: Jon
Status: Other
Grade: Other
Location: MI
Country: United States
Date: April 2006
Question:
Energy: the feasibility of a vehicle with solar
panels used for electrolysis in separating water into hydrogen
for fuel. Say, drive to work and panels sit in sun all day
producing hydrogen. Would hydrogen production be fast enough to
supply daily energy needs?
Replies:
At present, the direct use of hydrogen as "the perfect fuel" is
far from practical reality for a number of reasons.
1. The cost and energy required to produce hydrogen exceeds the
energy of combustion of hydrogen in air.
2. Solar panels are not very efficient. They convert only a few
percent of the sunlight into electricity. Existing solar panels are
too large to produce an adequate amount of electricity. Direct
conversion of sunlight to electricity is more efficient than electrolysis.
3. There are not many parts of the surface of the earth where the
sun shines bright all day long -- but you being from Michigan do not
need to be reminded of that.
4. The hydrogen / air combustion temperature is high enough to
produce nitrogen oxides, so that pollution problem would not be eliminated.
5. The hydrogen / air reaction speed is so very fast, a different
sort of engine than a conventional internal combustion engine would
likely have to be used -- possibly a Sterling engine -- but that
would have to be developed.
6. Storing hydrogen safely in a vehicle in case of an automobile
accident is a difficult issue that has not been solved.
7. One very important issue (my opinion) not being adequately
addressed by hydrogen fuel advocates, and possibly the most
important issue, is that water vapor is the most efficient of all
the "greenhouse gas". It has the highest concentration in the
atmosphere of all the "greenhouse gases" by far. It is essentially
transparent to ultraviolet radiation. It absorbs far more infrared
radiation than carbon dioxide or hydrocarbons. It has three very
intense infrared absorptions, a rich overtone/combination spectrum
in the near infrared, and a prolific pure rotational spectrum in the
far infrared. In contrast, carbon dioxide has only two infrared
absorptions, fewer infrared active absorptions in the near infrared,
and no pure rotational spectrum at all, since it does not have a
permanent dipole moment, which is required for a molecule to have a
pure rotational spectrum.
8. The effects of adding large amounts of additional water into the
atmosphere are not known.
9. Electrolysis, in particular, has a very fundamental thermodynamic
problem. Think about the process. Water is converted to hydrogen by
some energy source. This conversion is at best 100% efficient and
free. The hydrogen is burned to produce work. The efficiency of the
conversion of heat to work is absolutely limited by the Second Law :
Efficiency = (Thot -Tcold) / Thot, where Thot is the hot temperature
of any theoretical engine and Tcold is the ambient temperature.
Efficiencies seldom exceed 50-70%. The exhaust is liquid water. So
what has been done is to convert water at ambient temperature to
water at ambient temperature. That's a closed cycle -- an identity
-- water-to-water. And there is an energy cost somewhere in between.
Hydrogen is far from the "perfect fuel" some dream of. The more
dramatic positive impact on the "greenhouse effect" is to take
vehicles off the road by moving "work" nearer to "home" and by
various transportation forms such as "light rail" to move
substantially more people from place to place with very little
increase in the use of energy. But these are sociological and
demographic issues which are far more difficult problems than
chemistry and physics. It is clear this lesson has not been
learned, because we at least in the U.S. continue to subsidize
"sprawl" with longer wider highways from where people work to where they live.
Vince Calder
Hi, John. Presuming the array is large enough, this is theoretically
possible, but let me raise some points:
First, let us consider that the amount of energy required to power a
vehicle (200 to 300 Watt-hours per mile) would require several hundred
square feet of solar panels. It is not feasible to carry them along on
a vehicle. They could be set up in a stationary location, however, and
used to store energy (either in the electricity grid, in batteries, or
by cracking hydrogen) for later refueling/recharging.
The setup you propose (presuming we use a stationary array) converts
solar energy (about 25% efficient) to potential chemical energy
(hydrogen), which has to be compressed and stored, and delivered at
high pressure into a storage vessel on the vehicle. Then it is either
1) converted it to mechanical energy by burning it in an internal
combustion engine (about 30% efficient) or converted to electricity in
a fuel cell (about 70% efficient). If you use the fuel cell, then you
have another 80 to 90% efficiency factor added into the mix by going
through a controller for an electric motor.
Handling hydrogen requires a filter, dryer, and a compressor that uses
electricity, further taking away from the overall conversion
efficiency.
Each energy conversion involves losses that makes the required array
larger. There are also equipment issues with handling the hydrogen in
such a way as to minimize loss of the gas.
How far are you driving everyday? A typical commute is less than 50
miles a day total. In this case, it is simpler, cheaper, and more
efficient overall to use an electric vehicle (www.evalbum.com showcases
hundreds) for the commute. The array can then be placed on a residence
and used to generate electricity to run the home and recharge the
vehicle in a few hours after you get home. If the array is connected
in a "batteryless grid-tie" arrangement, then you are essentially using
the electricity grid as a storage unit, reducing system cost. If you
have excess solar array capacity, it is used to power your home. A
"battery grid-tie" arrangement will keep the home powered during
outages as well. Some people even take advantage of peak and off peak
rate differences, selling power back at peak rates and charging during
off peak rates.
The particular advantage of this arrangement is that it is available
using technology that is easily available to the general public at
reasonable cost. Adding hydrogen cracking, compression, storage,
transfer equipment, and a fuel cell into the mix increases cost by
orders of magnitude, if you can actually lay your hands on the fuel
cell.
So it is possible, but not practical or feasible as you describe. by
modifying the setup to use a stationary array and equipment, it could
be made to work more readily.
David Brandt
I suppose that for some people, such a vehicle could be possible.
Unfortunately, there are other factors that would come into
play. How far does the person drive to work? How is the traffic on
thier drive? Even their driving habits can play a huge role.
(aggressive drivers burn more fuel) Lastly, Unless you live in a
particularly arid climate, the weather may also come out to get
you. After all, who wants to be stuck at work, just because it got
cloudy and they're out of fuel because of it?
Weather and traffic can be mitigated to some extent, by providing
the vehicle with a larger fuel tank, (thus ensuring an ample
reserve), although such a vehicle would still probrably need some
external means of fueling.
Ryan Belscamper
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Update: June 2012
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