In search of the battery holy grail

There is a multitude of options for storing energy that may help overcome the variability of wind and solar. But none is a clear winner by itself and they all require improvement.

As discussed in the Climate Spectator article ‘Wind and solar are not enough’, it is incredibly important we achieve greater progress in technologies that would help complement the short-term variability of wind and solar. Both solar PV and wind technologies can play a valuable role in displacing generation from fossil fuels (the overall quantum of electricity produced by fossil fuels) but they aren’t so good at displacing the capacity from fossil fuels (the potential to produce electricity on demand at any time).  

A key thing to point out is that this does not mean we need a ‘baseload’ technology. Baseload does not equate to a reliable supply of electricity when we need it, it means a constant supply of electricity – there is a difference.  Even if wind and solar PV output vary, provided there are other technologies available that can fill the gaps between electricity demand and supply from wind and solar PV, we can achieve reliable supply. 

And the problem essentially comes down to energy storage. Fossil fuels’ primary advantage over wind and solar stems from being a form of energy that is highly concentrated and easily stored for use when required. 

The chart of the week below from CSIRO provides a very neat way of categorising the various energy storage options. 

On the vertical column it provides the cost per MWh for each of the options, taking into account both the cost of the equipment/battery and the operating cost. But from what I could gather it doesn’t incorporate the cost of the energy involved in ‘charging’ the technology, which would add considerably to the cost if it came from renewable energy (but not for biomass). On the horizontal axis it provides the discharge time. This gives you a feel for how long the storage option could supply energy after it has been ‘fully charged’.

2030 forecast of the cost and discharge time for different forms of energy storage

Source: CSIRO (2012)

To put the cost numbers into context, the current wholesale cost of electricity in the NEM is about $55 per MWh, although during peak periods – which is when storage would be most in demand – it’s around $80 to $120 on average. Most of the storage options in the chart would be delivering energy into the wholesale market because they would be located some distance from end customers. For example pumped hydro needs to be located in mountainous areas. 

However for the smaller, more portable battery technologies (lead-acid and lithium-ion) which can be located in homes and businesses, they’d be in competition with delivered electricity, which will be closer to $250-$600 (depending on the extent to which peak network constraints are reflected in prices).

Pumped hydro, compressed air ('CAES' in the chart) and concentrated solar thermal with molten salt with 6 to over 24 hours worth of output are the prime candidates for filling in fluctuations within the period of a day. This would cover things like wind speed dropping away or to fill in the peak demand period from 3pm to 8pm when the sun’s power drops away. However their discharge time is not ideal for covering extended multi-day periods of low wind speeds or solar radiation. 

This is where biomass becomes important because it can be converted into forms very similar to fossil fuels. As compressed pellets or torrefied pellets it is not dissimilar to coal. It can also be converted to gas made predominantly of methane or syngas which is similar to natural gas. And lastly it can be converted into liquids with similar properties to oil. 

These can be used in power stations just like those employed for fossil fuels and operate as a form of readily controlled (dispatchable) generation.  However the amount of biomass available at suitable cost is substantially lower than the wind and solar resource, and insufficient to meet all our needs, so it is well suited to providing a back-up role.

Also, provided the periods of extended low solar radiation and wind were reasonably infrequent, then there is no reason why we couldn’t continue to employ fossil fuels in conjunction with biomass, while still meeting aggressive emission reduction targets. To save on costs it may well be worthwhile maintaining existing coal power stations in an operational state, but converting them to be able to operate on high levels of biomass (for example the British Drax power station is looking to convert to as much as 100 per cent biomass).

Then there are the lithium ion and lead acid battery technologies which have quite high costs and small discharge times but are still potentially useful because of their smaller size. This enables them to be located on customer premises and therefore minimise the need for network augmentations.

Distribution networks tend to be characterised by capacity constraints during a short three to five hour period. So the discharge times of these batteries are a reasonably good match and could work well in concert with solar PV. 

So each storage option could conceivably have a role to play. But once you add the cost of renewable energy to 'charge-up' these storage options there is a major competitiveness gap still to bridge.

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