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!
What a fascinating and novel idea, Louis! I've never heard of this before, as it doesn't usually crop up in a 'renewable' list of energies. Despite the downsides of reliance on natural gas and reliability, are there any issues with the cost of setting these up? Also, why do you think this has yet to become a mainstream technology in operation?
ReplyDeleteAs stored energy systems are not renewable themselves, I suppose they are not deemed renewable energy schemes.
DeleteTo answer both questions at once, I believe the limiting factor of these systems is cost, which is why they have yet to be taken up into mainstream energy systems. For example, salt mine storage CAES systems cost $6-$10 per kWh (https://www.princeton.edu/pei/energy/publications/texts/SuccarWilliams_PEI_CAES_2008April8.pdf). To put that in perspective, wind power on average costs about $0.5 per kWh (http://www.ewea.org/fileadmin/files/library/publications/reports/Economics_of_Wind_Energy.pdf) and (in 2011 (average costs)), nuclear cost $0.021 kWh, Oil cost $0.2156 kWh, coal costs $0.0323 kWh, natural gas cost $0.0451 kWh (http://instituteforenergyresearch.org/analysis/electric-generating-costs-a-primer/).
Overall, costs are considerably higher, but with added investment and uptake, costs can be reduced and the process can be made more reliable and suitable.
There are alternative stored energy options such as hydrogen (splitting water into hydrogen and oxygen with excess power and then burning the hydrogen) and hydroelectric (pumping water back upstream to go through the hydroelectric power plant again). Many are experimental, but the solutions exist!
Hi Louis, cracking blog. I was already aware of issues surrounding fluctuations in electrical power output especially for wind power - within the UK, just like for sailing there is always too much or not enough wind!! However, I didn't realise that technologies for improving reliability and storage were becoming increasingly available/ around for quite some time.
ReplyDeleteMy question is the same as Katy's - why haven't these technologies become common place yet? Do costs make them unrealistic?
Furthermore, despite the Paris agreement arguably sending a strong global signal as to the beginning of the end of the fossil fuel era and renewed investment in renewables, is ACAES really capable filling the gap left by traditional, non-environmentally energy sources which must be phased out to reach the Paris agreements goal of zero net carbon emissions in the second half of the century? If so is there any indication of the timescales required to make these technologies serious contenders?
Fluctuation in electrical power input
Thanks Alex! The costs I feel play a considerable part in making these sorts of solutions unrealistic. Not only are they 'new' technologies which adds to the cost (not new per say, but new as they haven't really taken off or developed into mainstream energy systems), but they also require deep mine shafts to store the compressed air. There are alternative schemes to pump compressed air into porous rocks, but this could start to go back to the fracking argument and hinder the progress of CAES systems.
DeleteI feel ACAES systems could fill the gap, but as they are an additional infrastructural cost, and renewables are relatively difficult to get installed and funded to begin with, the overall chance of these projects being installed is minimal (at least in the short term). However, as they are being discussed in the literature, I am hopeful that they might make a reappearance! But if I'm honest, I have no idea - sorry!
This is really interesting! I think while CAES has obviously shown a lot of promise, it would be best to move towards an energy storage solution that doesn't have large pollution potential and that doesn't rely on the fossil fuel markets. This is because I believe renewable energy has to grow completely independently of fossil fuels for it to really be useful in the long run and be able to be competitive (take Elon Musk's solar city for example, which provides solar energy AND runs completely on renewable energy. But the progress of CAES into ACAES provides a lot of hope for this!
ReplyDeleteI agree entirely Kaitlin. This is why ACAES proves to be an interesting solution. The key issue with both methods is the geographical restrictions of both, as they have to be near large geological caverns (salt mines, for example) to be able to store the vast quantities of air. My newest post looking at other stored energy methods gives an example of two non-geographically restricted methods which could prove more useful or efficient, especially when it comes to city-wide integration!
DeleteI am highly dubious of the claims in efficiency that Brown makes and would love to know more about where his claim of 'up to 90% efficiency' comes from... I've looked on his website but couldn't find any further information on it. I would imagine once heat losses from compressing the air are taken into account and inefficiencies in the compressor, as well as leakages in the system are considered the recoverable energy would be much less than 90% of starting energy? No need for response if you know no more than I do on this one... Just thought I'd raise my view
ReplyDeleteHi Robert. The 90% efficiency claim comes from this sentence in the linked news article:
Delete"LightSail said that the thermal efficiency of compressed air energy storage systems can be improved from 60% to 90% using the startup’s alternative concept" (http://cleantechnica.com/2013/02/21/lightsail-gets-5-5-million-for-compressed-air-energy-storage/#gsc.tab=0).
I do however agree that the figure may be a bit questionable, other sources cite a much lower efficiency rating for CAES systems (See the table in my next blog post: http://theearthisneverenough.blogspot.com/2016/01/renewable-solutions-novel-ideas.html). Furthermore, the figure is cited from the company's website. However I feel they would have data to back up this claim, and therefore am inclined to believe that they have results of 90% efficiency. Thanks for your comment Robert!