Renewable Solutions: Novel Ideas

As previously discussed, energy storage for renewables is important, especially when relying on them for a greater proportion of global energy production. The innate unpredictability of natural renewable energy resources (wind, sun, river discharge, tides) makes energy storage facilities vital for supplying constant levels of electricity to our constant (exploitative) demand.

The most wide spread and popular energy storage technologies include (taken from Perna et al., 2015):

• Electrochemical batteries
• Supercapacitors
• Thermal-storage materials
• Flywheels
• Pumped hydro reservoirs
• Superconducting magnetic energy storage
• Chemical (hydrogen, synthetic natural gas, etc.) storage
• Compressed air energy storage (CAES and ACAES)

Each method is suited to different applications and vary in stored capacity and efficiency, and discharge rate. Electrochemical batteries can be highly efficient and store large amounts of energy, but have limited life cycles and discharge stored electricity at a slow rate (Perna et al., 2015). CAES systems similarly have a high efficiency, but contrastingly have longer life cycles and can store a varied amount of electricity, depending on the built capacity. Hydrogen-based energy storage systems (water electrolysis) tend to have lower efficiencies, but has a high storage of energy per mass, whilst having a long life cycle (Perna et al. 2015). However, this post will look at some of the more interesting solutions published recently!

Perna (et al., 2015) undertook an interesting study looking at how hydrogen-based energy storage systems could be integrated with biomass powerplants, making the biomass production more efficient. Overall, electrical efficiencies of the integrated systems ranged between 40-43%. To me this is incredibly low, but compared against incineration or biomass gasification (which has an efficiency of 20-24%), efficiency levels look great! Furthermore, integration provides a demand for electricity when consumer demand is low, reducing electrical fluctuations across the supply networks and improving reliability!

Another unique and new means of storing energy is the use of using liquid carbon dioxide. Wang (et al., 2015) investigated different systems by which to pressurise liquid carbon dioxide. During off-peak or low demand periods, liquid carbon dioxide is pumped from one tank to another through a series of compressors (consuming excess power). When additional power is required, the pressurised liquid carbon dioxide is released through turbines to generate electricity. Heat exchangers are cooled using oil, and the heated oil is used as a secondary source of electrical generation, heating water to turn turbines. 

Figure 1: Comparison of methods of energy storage with
RTE (Round Trip Efficiency - total efficiency) and EVR (energy:volume ratio) (Wang et al., 2015)

Wang's paper explores a number of schematics, suggesting that the improvement of thermal energy storage (heated oil) can improve overall efficiency (RTE) to 56.7%. Furthermore, the energy to volume ratio (EVR) is a reasonable 36.kWh/m3, making liquid carbon dioxide energy storage a more efficient (in terms of volume to energy ratio) means of storing energy compared to CAES and Pumped hydroelectric reservoirs (Figure 1).

When combined with storage mediums, renewables can be very useful. A mixture of various renewable resources combined with storage capacities mitigates reliability issues. To wrap up this post, I have found an exciting article which provides a model for a completely renewable-powered city.

Richardson and Harvey (2015) have modelled the renewable potential surrounding Ontario, Canada, investigating what would be required to move from conventional fuels to a fully renewable system which includes pumped hydroelectrical and battery storage. The model results aren't particularly detailed, as rough estimates are used based on existing literature or known specifications. However they optimistically conclude that Ontario could move towards a renewable-based electrical generation system which is reliable and a renewable-fuelled city "can be maintained without excessive generation costs". The idea, they explain, is technically feasible, however there are issues surrounding potential demand fluctuations with electrification of transportation, which could prove to be problematic.

The results from all 3 studies are optimistic. There are an abundance of methods and means to cope and sustain our excessive demands for energy, and when scaled up to a city-wide model, the level of technology we currently are at seems to prove that we can indeed live sustainably (whist still exploiting the abundance of energy)!
As a side note, Richardson and Harvey do note that a change in behaviours would probably help to make renewable-based cities a reality for more parts of the world. I completely agree with this, but that conversation is for another blog!

Renewable Solutions: Compressed Air

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!