Hydrogen Storage – The Silver Bullet?

There has been recent speculation around the use of hydrogen storage as a cure for the downside of some renewable energies – the mismatch between supply and demand. For example, solar energy is usually most abundant through the middle of the day, yet in many situations this can be when demand for electricity is relatively low.  In this type of situation hydrogen storage could act like a battery, storing energy when there is more supply than demand, and then later using that hydrogen in a fuel cell to generate electricity when demand exceeds supply.

The New Zealand Government is fully behind the use of Hydrogen to help meet New Zealand’s energy demand with a recent agreement with a Japanese firm Obayashi Corporation to build a pilot plant to produce hydrogen, utilising geothermal energy in Taupō. Additionally, the Port of Auckland already has plans underway to build New Zealand’s first hydrogen fuel station.

For the use of hydrogen for electricity storage the concept works by using excess grid electricity to split water into hydrogen and oxygen. The hydrogen is then stored at very high pressures or in other forms. When electricity is required this hydrogen can be injected into a fuel cell where it produces electricity. This process is very clean, only producing water vapour as a by-product. Effectively the inputs and outputs of the process are identical.

This all sounds great, but the issue is that it’s horribly inefficient. Splitting the water into hydrogen and oxygen typically has an efficiency of 70 to 80% in terms of stored energy in the hydrogen. But then you also have efficiency losses when you use the hydrogen. Oxidising hydrogen in a fuel cell only converts about 50 to 60% of the stored energy to electricity, while the remainder of the energy ends up as heat. This means that unless you have a cogeneration plant that can make use of the heat, you’re only getting about 0.40 kWh of electricity back for every 1 kWh you put into the system. This doesn’t bode well as a large-scale solution.

However, we’ve come across an interesting alternative. A Santa Barbara based Energy start-up, ARES received funding to bring a 19th century solution to a 21st century problem. By harnessing the power of gravity, the company can store 12.5 MWh of electricity by utilising electric motors to move 9,600 tonnes of rock and concrete filled railcars up a 650 m high hill. When demand is low these railcars are winched up the hill using electric motors, and when demand is high they are released down the hill producing electricity in a similar way to regenerative braking.

According to ARES on a very large scale the cost of install would be only USD 168 /kWh (NZD 247 /kWh) of storage, with an efficiency of up to 78%, which seems like a great option for the grid. As a comparison, the 129 MWh Tesla battery project recently installed in South Australia cost AUD 66 Million (NZD 77 Million), so about NZD 543 /kWh of storage.

A slightly more novel solution developed in the office think-tank was that a 28 m diameter lead ball being rolled up and down the 300 m Port Hills would achieve something similar and would have the bonus of looking fairly spectacular.