Saturday, November 29, 2014

Power to Gas

Scaling up clean renewable energy systems will generate more electricity than we need. The Centre for Alternative Technology (CAT) have put forward proposals [1] which would generate about 1,160 TWh of electricity in an average year. The average total demand would only be around 770 TWh per year. However, the problem is that the electricity isn't necessarily generated when we need it. There will be a mismatch between supply and demand, with both large surpluses and shortfalls.

The key is to develop large amounts of energy storage which can be saved until needed. At the moment technologies like fly wheels, compressed air, batteries and pumped hydro plants can't store enough energy to keep the lights on when there's reduced amounts of renewable electricity. For example, in 2010 there was little wind but a high demand for electricity due to the cold weather.

The solution is to develop a technology called Power to Gas or P2G.


Hydrogen
Hydrogen can be made by the electrolysis of water – splitting H2O into hydrogen (H) and oxygen (O) using electricity. Electrolysers can use electricity at times when there is abundant surplus of electricity, to create hydrogen gas for storage. In principle, hydrogen can be stored and then used directly to produce electricity using gas turbines or fuel cells. However, hydrogen is a very light gas that needs to be highly compressed for storage. Itis also quite explosive and can even corrode metal. It is possible to store relatively large amounts of hydrogen (a few 100 GWh) over long periods of time, for example in salt caverns. However, compared to natural gas (primarily methane), hydrogen is difficult to store and transport and there is almost no existing infrastructure suitable for it.

Biogas & Synthetic Gas
Biogas and synthetic gas are both produced from renewable sources. Biogas, a mixture of methane and carbon dioxide, can be produced by anaerobic digestion (AD) – the decomposition of biomass (for example, grass, animal manure or food waste) in an oxygen-free environment. Carbon neutral synthetic gas is made via the Sabatier process. Here, hydrogen (made by electrolysis) and carbon dioxide (from burning biomass, or from biogas) are combined to produce methane. Methane is easier to store than hydrogen. The Sabatier process can be seen as ‘upgrading’ hydrogen to a gas that is easier to handle. The process of using electricity to produce gaseous fuel is sometimes referred to as ‘power to gas’ (GridGas, 2012).

Methane gas is also the primary component of today’s fossil fuel natural gas. The methane in biogas and synthetic gas can be stored in very large quantities just as natural gas is currently. The UK today has a highly developed gas infrastructure that includes storage facilities, such as the Rough gas store off the coast of Yorkshire, which has a capacity of 35,000 GWh. However, methane is a powerful greenhouse gas, so it is very important that any escaping from pipelines or storage is kept to a minimum.

Biogas and synthetic gas, once stored, can be burned in power stations (again, like natural gas today) to provide energy when electricity supply from renewable sources is insufficient to meet demand. Gas power stations burning biogas or synthetic gas can be flexible – we can turn them on or off quickly. We can use them as ‘back up’ generation to meet demand when electricity supplies from variable renewables fall short. They can also supply industry for very energy intensive processes which would be difficult to run on electricity.

It is important to remember that burning methane is only carbon neutral when it is produced using biomass and/or renewable electricity. When methane gas is produced from biomass, the amount of CO2 released by burning it is reabsorbed when new biomass plants are grown, resulting in no net increase of GHGs in the atmosphere. Synthetic gas is carbon neutral when the hydrogen used is produced using renewable electricity, and the CO2 used is from non-fossil fuel sources (like
biomass).

The processes involved in creating a significant biogas and synthetic gas back up system have many losses associated with them. As energy is converted between forms (electricity and biomass to gas, and back to electricity), we lose energy in the process – about 50%. However, the ability to store energy in this way forms an integral part of an energy system powered by renewables, and is a good way of using electricity which would otherwise be surplus to requirements.

Who is making the technology happen?

A Sheffield company called ITM Power [2] is developing and installing technology to make Power 2 gas a reality.

References:
[1] http://www.zerocarbonbritain.com/images/pdfs/ZCBrtflo-res.pdf for the full report
[2] http://www.itm-power.com/energy-storage/power-to-gas-energy-storage-solution/

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