My last few blogs will revolve around the topic of making renewables more suitable for the world. One of the largest issues that renewables face is they only work when the wind blows or when the sun shines (or when earthquakes occur...!). This is fantastic for sunny countries, where this can work to power high energy-demand installations with few issues. However, for countries where it isn't always sunny or windy, renewables are used to top up fossil-fuel generation, as their unreliability is too great to make them a dominant source of electrical generation.
This is where stored energy comes into play! Storing energy during off-peak or excess supply periods provides a more reliable and constant supply of electrical energy to renewable-dominant countries during periods of low supply or high demand. Storing energy on large scales is known to be inefficient, hence why power stations are switched on and off to meet demands of energy (Steadman, 2013). However more efficient means of storing energy is being developed.
(There are a vast number of other electrical storage systems. A good summary of the literature was conducted by Chen (et al, 2009). Newer technologies, such as hydrogen storage are not included in the review, but are an important technology that is efficient and has lots of potential (Schiller, 2014).)
Compressed air storage uses off-peak excess electricity to power air compressors. The air is compressed into large vessels or geological formations, such as old mineshafts, mixed with natural gas, and then released to generate electricity through thermoexpanders (Pendick, 2007). The mixing with natural gas increases the efficiency of electrical generation. There is one down side to the Compressed Air Energy Storage (CAES) method: there is still a pollution aspect. CAES is predicted to be approximately 60-90% efficient, depending on methods used (Brown, 2013).
Two examples of CAES in operation are: Huntorf in Germany, built in 1978 and has a capacity of 290 MW and facility in McIntosh, Alabama, USA, built in 1991 and has a capacity of 110 MW, with both facilities using salt mineshafts as a means of storing the air (Succar and Williams, 2008). Both facilities run efficiently (~60% efficiency) and prove that this is a suitable low-cost energy storage technology. However, CAES, as briefly mentioned previously, has a pollution element, and requires natural gas. It has previously been found that CAES makes wind power less profitable and is heavily reliant on fossil fuel markets (Greenblatt, et al., 2007).
The solution: adiabatic CAES (ACAES). Now I would attempt to explain this, but there is a video with a far better explanation available... so let's rely on that instead:
To summarise, the heat energy is used and conserved, negating the need for natural gas to be used in the thermoexpanders to regain the stored electrical energy.
So, how do we understand which is best? Well of course I would not ask that question if I did not already know!
Boumana (et al., 2015) recently published a model examining the lifetime (from mining material out of the ground to make metals, etc., to the decommissioning of the facilities) environmental impacts of both CAES systems. They found that the most significant environmental impacts are from the natural gas consumption (CAES) and thermal-storage tanks construction (ACAES) (insulation, considerable amount of plumbing work, and overall construction). However, overall, ACAES is deemed to be the least environmentally impactful and cheaper in the long term, due to not requiring a constant natural gas consumption.
So, what should we take home from this? Renewables can be unreliable, but their reliability can be improved!
