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!