Is hydrogen any good for storing renewable energy?

In my earlier essay “The Wicked Battle Over Hydrogen”, I highlighted the fact that hydrogen is not a source of energy but rather a substance for storing energy. In this essay, I will describe how to use hydrogen for energy storage and what it takes to do so.

The energy in fossil fuels is solar energy captured hundreds of million years ago by algae and bacteria. The energy in fossil fuels pumped out of the ground is ready for release by burning. Not so with hydrogen. Until recently, there were no known hydrogen deposits. It was as recent as 2012 when geologists confirmed that the gas emanated by a well in Mali was 98% hydrogen. Now, geologists suspect that water seeping deep into the earth’s mantle oxidises hot iron deposits, thereby releasing hydrogen. That hydrogen may accumulate under impermeable layers. The question is whether it is profitable to exploit such deposits. Until it is, we need to obtain molecular hydrogen (H2) by removing hydrogen atoms from hydrogen-containing molecules, such as water or hydrocarbons. Breaking down molecules containing hydrogen requires energy. We can recover part of that energy later by letting hydrogen react with oxygen (burn it) or by converting it to electricity in an electrochemical reaction (fuel cell).

Hydrogen is an essential input to some large-scale industrial processes. Worldwide, hydrogen consumption is around 100 Mt (million tonnes) a year. Over half of the consumption is for producing ammonia for fertilisers needed to feed the world population. Petroleum refining consumes another quarter. The rest goes into the production of steel and other chemicals, such as methanol.

Making hydrogen
The predominant method of hydrogen production is by steam reformation of natural gas (which is mostly methane). Steam reformation consists of heating natural gas and steam (water) at high pressure to produce hydrogen and carbon monoxide. The carbon monoxide further reacts with steam to produce CO2 and more hydrogen. Hydrogen obtained from steam reformation of natural gas is classified as grey hydrogen because of the CO2 emitted in its production.

An alternative, CO2emission-free, way of producing hydrogen that is splitting water (H2O) into oxygen and hydrogen by electrolysis. Such hydrogen is {\em green} hydrogen provided that the electric energy for the electrolysis comes from renewable sources. Producing hydrogen by electrolysis of water with solar electricity is a CO2 emission-free way to store renewable energy.

How much energy can we store in hydrogen?
When one kilogram of hydrogen reacts (burns) with oxygen to produce water, it releases 143MJ (Mega Joule) of energy (heat). Thus, hydrogen stores 143MJ per kilogram. We say that the specific energy content (energy per unit mass) of hydrogen is 143MJ/kg.

The specific energy of hydrogen is nearly three times the specific energy of the common fossil fuels (natural gas, gasoline, kerosene or diesel). With the same mass of hydrogen in the tank, we could drive, without any CO2 emission, three times the distance that we would drive with natural gas. Hydrogen looks like an ideal fuel once we have in the tank.

What does it take to put hydrogen in the tank?
An average internal combustion engine (ICE) car can travel 500 km on a full fifty-litre tank of fuel. Let’s calculate how much hydrogen we need to put in the tanks to drive 500 km in a hydrogen-powered vehicle.

We could adapt the motor of a conventional ICE car to burn hydrogen, but an electric vehicle (EV) makes more efficient use of energy. To power an EV with hydrogen, we need a fuel cell to convert the chemical energy stored in hydrogen into electric energy. The conversion efficiency of a fuel cell is around 60%, and electric motors are over 90% efficient in converting electric energy into mechanical energy.

Manufacturers claim an energy consumption per 100 km of around 16 KWh for an average battery EV (BEV). 16 KWh is 57.6 MJ. By using a fuel cell instead of a battery we need more energy. With a 60% efficient fuel cell we need 57.6 MJ/0.6 = 96 MJ to ravel 100 km in the fuel cell EV (FCEV). For 500 km we need 480 MJ, which equals the energy content of 3.4 kg of hydrogen. The hydrogen tank of the FCEV has to hold at least 3.4 kg of hydrogen.

The first hurdle to putting 3.5 kg of hydrogen in a tank is that molecular hydrogen (H2) is a gas at the standard temperature and pressure (STP) of 0oC and atmospheric pressure at sea level. One kg of hydrogen gas at STP takes up a volume of 11,200 litres (11.2 cubic metres).

To drive the 500 km we need 38 cubic metres of hydrogen gas! Storing and moving such large volumes of gas is impractical. To carry the 38 m3 would require a trailer with the width of the car (1.8m), 2 m high, and 10.6 m long — hardly a practical proposition. So, let us try to compress the hydrogen to fit into a typical 50l tank of a petrol car. To fit into a 50l tank, we need to compress the hydrogen to 50/38000 = 1/760 of its original volume, which takes a pressure of 760 atmospheres (remember the law of ideal gasses). For comparison, this pressure is 25 times the test pressure of 30 atmospheres (bars) for a propane gas steel bottle. The hydrogen tank has to be much, much stronger. We can build high pressure tanks. Tanks for FCEVs made out of carbon fibre composite that can store around 4 kg of hydrogen and withstand a pressure of 700 atmospheres weigh around 100kg. Although such containers are heavier, bigger and more expensive than the 50l petrol tank, they still fit into an FCEV.

An alternative way to reduce the volume of a gas is to cool it until it condenses into a liquid. For natural gas, this is a proven method. By cooling methane at atmospheric pressure to -162 degrees Celsius, it turns into a liquid that occupies around 2 litres per kilogram. Refrigerated Liquefied Natural Gas (LNG) is routinely shipped around the world in thermally insulated containers near atmospheric pressure. To do the same with hydrogen gas, one must cool it to 20.28 K (−252.87oC). Once cooled, it can be stored for some time at atmospheric pressure in highly thermally insulated containers.

The liquefaction temperature for hydrogen is about 100 oC lower than the liquefaction temperature of methane and only 20 oC above absolute zero. Liquefying hydrogen consumes a lot more energy than compression. In practice, it takes more than 40Mj/kg, or nearly a third of its specific energy, to liquefy hydrogen. Furthermore, there are inevitable evaporation losses from liquid hydrogen stored at atmospheric pressure in highly thermally isolated containers. Thus, storing liquid hydrogen in an FCEV looks highly impractical.


Recovering the stored energy

So far, we have found that we can recover up to 60% of the energy stored in the hydrogen fuel once it is in the tank. We still need to think about how to get the hydrogen from the production site into the tank. First, we need a refilling station capable of filling hundreds of vehicles. We can avoid transporting the gas to the refilling station by installing electrolysers that produce the hydrogen on-site and dispense it. The refilling station will not need a large storage tank, but it will need a compressor. The electrolyser needs to be powered by renewable energy (solar or wind), and so must the compressor. Electrolysers and hydrogen compressors both have efficiencies of around 70%. Thus, there are non-recoverable energy losses throughout the process of storing and recovering the energy. From the energy needed to run the electrolyser, we only store 70% in the hydrogen. The compressor leaves us with another 70% of that. Finally, the fuel cell gives us only 60% back. Overall, we recovered 0.7 X 0.7 X 0.6 = 0.3 times the energy put in. To store the 480MJ that we need to drive 500 km, we need to use 480/0.3 = 1600MJ of solar electricity. Last but not least, each step costs money.

A state-of-the-art battery gives back over 95% of the energy consumed in charging it. So, why would one use hydrogen if one can use a battery at a lower cost to store renewable energy? Why go through the trouble of spending renewable electricity to make, store, and transport hydrogen to later convert it back to electricity, losing 70% of the energy in the process, when we can use renewable electricity directly with little loss?

We must conclude that storing renewable electric energy in green hydrogen may only be helpful in situations where battery storage or other storage solutions are insufficient or impractical, as in aviation.

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