Thursday, February 11, 2010

Hydrogen Fuel Cells vs Batteries

So while diligently working to catalog every episode of Top Gear I saw their review of the Honda FCX Clarity in series 12 episode 7.  To summarize they loved it.  The Clarity is a hydrogen fuel cell powered vehicle.  This began a lengthy bout of research into hydrogen fuel cells, which I had pretty much ignored.  Instead of giving a clear and concise list of pros and cons let me instead launch into a rambling, probably error filled, lecture on energy which no one will ever read.

When you read about replacements for fossil fuels you often come across something explaining the difference between an energy source and an energy store.  The laws of thermodynamics state that energy can never be created or destroyed.  Rather it can only be transformed from one state to another.  So gasoline isn't an energy source, because there is no such thing as an energy source.  Where does the energy that powers your gasoline car today come from you ask?  The big bang flung matter apart, giving it gravitational potential energy (GPE), the same as water you pump up into a tower has.  It takes energy to pump the water up, and when it comes down it will release that energy.  The spread out matter (mostly hydrogen) then clumped up and formed balls of gas.  The compression of gravity increased the temperature (total heat energy remains constant but since it's in a smaller space the average energy goes up, temperature is average heat energy).  At extremely high temperature hydrogen atoms undergo nuclear fusion.

Nuclear fusion releases some of the energy stored in matter itself (all matter is simply very dense energy, the conversion factor being E=mc2).  That energy radiates away in all directions, and a tiny portion warms the surface of the Earth.  Plants use that energy to separate carbon from oxygen.  The energy is now stored as carbon in the plants.  The plants eventually die and most of that stored energy (carbon) is used by microorganisms for power and released back into the air as CO2.  However, sometimes the plant is sealed up and nothing is able to use its carbon.  After millions of years of this we get large stores of carbon (energy) underground in the forms of coal and oil.

Oxygen is highly reactive, and will bond with carbon or hydrogen readily.  Thus oxygen typically doesn't exist by itself, when it does it's a sign that it is being constantly replenished by some process (like life).  When oxygen combines with carbon it forms CO2, and when it combines with hydrogen it forms H2O.  Both those reactions are exothermic, which means they give off heat (energy), as opposed to endothermic, which would mean they absorb heat (energy).  So when you burn either carbon (coal, oil, wood) or hydrogen all you are doing is combining it with oxygen in a reaction that gives off energy.  The carbon or hydrogen remains (bonded to oxygen) and can be separated in a reaction that will consume energy.  In theory the energy needed to separate them, and the energy gained by combining them is the same.  In practice you will lose some energy in both processes, so there will be a net loss.

So as you can see gasoline is simply storing the energy of the sun, which is itself storing GPE from the big bang.  Why then do people say that fossil fuels are energy sources instead of energy stores?  Because we don't have go through the process of storing the energy as carbon, the work was done over millions of years by plants.  So practically carbon is an energy source.

You will often hear people state that hydrogen is the most common element in the universe.  This is true, but also very unimportant.  First off the Earth is very much a closed system, isolated from the universe.  It doesn't much matter if there is an abundance of hydrogen in the Sun, since it is very unlikely we'll be harvesting hydrogen directly from the Sun any time soon.  Hydrogen is only the 10th most common element in Earth's crust.  This too is irrelevant though.  We are in no danger of running out of any element ever.  With the exception of nuclear reactions the elements are never changed, rather they are simply organized differently.  What matters is that while hydrogen is the 10th most common element in the Earth's crust virtually all of it is bonded to other elements, largely oxygen (and confusingly for this greatly simplified lesson carbon).  As we know from above bonded hydrogen is worthless.  We won't be running out of carbon or hydrogen any time soon.  What we are running out of is free carbon (as in unbonded), and there is virtually no free hydrogen on Earth.

Now that we have all the unnecessary theory out of the way what does this mean in practice?  Well if we want to get power we have a couple options, get it from the (running out) free carbon, get it from the Sun (via solar or wind), get it from residual heat from the formation of the earth (geothermal), or from nuclear reactions.  This is all well and good for stationary things like houses that can be wired up to a massive grid and provided power from whatever source we want.  However, for mobile things like cars and planes we must store energy from one of those sources, or use the already conveniently stored carbon energy.

There are many ways of storing energy, but most aren't that great.  In fact we don't have any method that can beat carbon in terms of density, ease of use, practicality, and cost.  A kg of gasoline holds 44 MJ of energy, and occupies about 1.3 liters of volume.  It can be stored easily, and to use it you simply provide an initial source of heat.  Contrast this with hydrogen, which holds 142 MJ per kg, which at first seems much better.  The problem is that a kg of hydrogen at normal temperature and pressure will fill 11 cubic meters which is roughly the size of a small car.  In order to store a reasonably amount of hydrogen you have to either pressurize it, or liquify it.  This brings up two problems.  First pressurizing things is notoriously energy intensive (ask your fridge), and second storing pressurized things requires strong heavy containers.  To be fair the energy used to pressurize the gas could perhaps be partly recovered, that would add complexity and cost though.

The other main way of storing energy is batteries.  Most people are aware of the problems with using batteries in cars.  Mainly they are heavy, expensive, and take a long time to charge.  I feel obliged to point out there are also mechanical ways of storing energy, which are actually some of the best ways for many applications.  Power plants pump water up to store energy, and flywheels have a lot of potential.  However, our subject is cars, and they are unpractical for use in cars.

We've broken the problem into two parts.  First power generation (fossil, nuclear, solar, geothermal), then power storage (carbon, hydrogen, battery).  These two parts are not quite as independent as they may seem at first.  Carbon storage effectively mean gasoline, which doesn't require generation.  While it will have some extraction costs which could be viewed as generation costs, they are largely dependent on whatever main generation method we use.  Battery storage requires electrical energy.  We already have a grid for providing electrical energy almost everywhere.  This grid would be inadequate to provide all the power needed for transportation, but any change will require investments in upgrades.  The main issue is that electric cars will require largely constant electrical energy.  While people will tend to recharge over night, they will also likely want to recharge while at work, during the day.  Switching to battery stored electrical energy for transportation will likely require an increase in constant generation capacity equal to the power demand of transportation.

Hydrogen storage has the advantage here.  The hydrogen would be created on site at power plants, and once created can be stored indefinitely without loss.  Because of this it could be created at night using excess capacity.  Indeed it could be used as a buffer so that a power plant always ran at capacity.  When the grid power demands went up hydrogen production would go down, and when the grid demands lowered the production would increase.  Hydrogen production also lends itself to variable generation (solar/wind), which is otherwise difficult to utilize in the grid.

The main negative hydrogen faces is that it is much less efficient than direct electrical energy.  In a battery stored energy system the only inefficiencies that matter are transmission losses, and then charging and battery losses.  These are already well understood as we have been using batteries powered by the grid for some time.  Efficiency of power generation is the same for both, and so irrelevant.  Wikipedia claims 86% efficiency from grid to wheels using batteries.  Hydrogen on the other hand has numerous steps and inefficiencies, some of which are difficult to predict and estimate.  First hydrogen must be generated, then compressed, then transported, then finally used in a fuel cell.  Wikipedia lists 70% for generation, and 40% for fuel cell, and a total grid to wheel efficiency of 25%.  Using these numbers for every 4 joules generated only 1 joule will end up at the wheels in a hydrogen powered car, a battery powered car would get 3.44 joules.

Here are the pros and cons or batteries vs fuel cells (you should probably skip down to this point instead of wasting your time reading all that):
3.44 times as efficient as fuel cells.
Potential for break through to eliminate many of the cons.
Heavier than fuel cells (even with a pressure vessel to hold hydrogen).
Very slow to recharge.

Fuel Cells:
Similar usability to gas.
Fast to refuel.
Flexible generation lends itself to take advantage of otherwise unusable generation capacity.
Weight comparable to gas means it could be used in planes as well.
Dealing with a high pressure gas or liquid will add complexity.
Many steps leads to inefficiency.

1 comment:

  1. I think that even the Hydrogen and Lithium Ion batteries will also need an upgrade in the near future. Algae batteries, and paper thin batteries, and even nuclear batteries have been on the rise and will be making an appearance in car manufacturing too.