Hydrogen vs. Batteries

With dwindling oil and natural reserves, rising prices, and increasing atmospheric concentrations of carbon dioxide, the future of the automobile industry will have to involve a type of fuel besides gasoline.

I've always envisioned electric cars run on batteries. These batteries would be charged from the existing power grid (with increased transmission lines to deal with the additional load). The electricity could be generated by renewable and sustainable energy sources such as wind, and solar power.

However, it was recently pointed out to me that hydrogen might be a more viable option because it has a much larger energy density than batteries. This is certainly true by mass (hydrogen has 39.7 kWh/kg vs. about 0.2 kWh/kg for a battery). Energy density by mass is important in powering cars because lighter cars take less power and can go faster (although if a car is too light it can be dangerous). Hydrogen's energy density by volume is 100x less than that of batteries when uncompressed. However, current hydrogen cars use tanks compressed to 5,000 or 10,000 psi which brings hydrogen's energy density up to 1.1-2.2 kWh/L vs. the 0.3 kWh/L for batteries.


So at first glance, hydrogen seems like a lot better way of storing energy in a car than a battery, but I wondered if this would still be true if you took into account the efficiency of running a car on hydrogen instead of electricity, the weight of the hydrogen tanks, or the energy it takes to make all this hydrogen. Hydrogen storage would be a major problem if hydrogen is to be produced outside the car and held at fueling stations so many are proposing a "regenerative" hydrogen fuel cell mechanism which uses solar or wind power to create hydrogen on the vehicle via electrolysis and later consume it as fuel. I decided to calculate the "true" efficiencies of five systems: 1. Hydrogen at 5000 psi reacted in a PEM fuel cell to form electricity used to power an
electric motor
2. Hydrogen at 5000 psi combusted in an internal combustion engine
3. Electricity stored in a Lithium-ion polymer battery and then used to power an
electric motor
4. Hydrogen at 5000 psi formed via electrolysis in, reacted in a PEM fuel cell to create
electricity used to power an electric motor
5. Hydrogen at 5000 psi formed via electrolysis in and then combusted in an internal
combustion engine

I chose a PEM fuel cell because they currently have the best power to weight and volume ratios. Low temperature fuel cells like the PEM currently need platinum to catalyze the reduction reaction. I've heard that there is not enough platinum on earth to use these fuel cells in cars - although some researchers in Australia claim that they can use gortex as a substitue for platinum. I chose a lithium-ion polymer battery because they are one of the lightest batteries around and have high efficiencies. Both of these technologies are priced competitively in relation to other fuel cells and batteries respectively.

The true efficiencies of the first three systems are plotted below both by volume and by mass.













While only considering the energy efficiency of the car, hydrogen has a higher energy density than batteries both on a mass and a volume basis. Even combusting hydrogen in an internal engine would be better than using batteries.

The true efficiencies of the systems 3-5 are plotted below both by volume and by mass.













If hydrogen would need to be produced and compressed on board, using batteries has a higher energy density than combusting hydrogen. Although creating and reacting hydrogen in a fuel cell still has a higher energy density by volume than using Li-polymer batteries, Li-Polymer batteries have a higher energy density by weight, which is arguably more important in automobiles. For these calculations I didn't consider the volume of the gas compressor, or the weight and volume of the PV cells/wind power

So what does this mean for the future? Well as with most things in life, financial cosiderations are a key player in decision making. However, since I'm not really interested in money, I'm didn't consider that. From my non-monetary analysis, I've reached the following conclusions:

1. Using hydrogen as fuel is only a viable option if we are going to invest in a complete hydrogen economy with fueling stations, hydrogen production plants, wind and solar energy to provide the electricity for hydrolysis, etc. Creating a cars which produce hydrogen onboard is not a viable option unless the weight of fuel cells, compressors, and gas storage tanks decrease sufficiently, or if the hydrogen system is sufficiently cheaper than a battery system. Of course there are also safety issues associated with sitting on a 5000 psi tank of hydrogen.

2. If we are not going to invest in a complete hydrogen economy, it is energetically best to store electricity directly in batteries. This electricity could be created onboard or at a power plant whichever is more cost effective. There are also safety issues associated the production and disposal of massive amounts of batteries full of corrosive, toxic materials.

Personally, I think that it is not realistic to think that hydrogen production on a mass scale with hydrogen stations etc. will exist in the near future. That would require large scale orchestration and high capital costs, which, considering the state of the economy, is not likely. There would need to be large infrastructure changes for electric cars as well. More solar and wind farms to generate electricity, as well as more transmission lines to transport this electricity are both important. President Obama announced that all three of these things are part of the recovery package that was recently passed, so we are well our way.

It is important that our options which offer small scale changes and transition stages. A small scale change leading to hydrogen cars would be to make hydrogen cars which make their own hydrogen on board. However, I have shown that it is currently better to use batteries for this purpose. Since this is the case, electric cars are more likely to initially take off than hydrogen cars, I believe that electric cars are the way of the future.