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!

Dusty PV

Energy is of vital importance in today's world, but creating sustainable energy sources is even more important for the future sustainability of Earth. When assessing sustainable energy sources, renewables come to mind. But they always come with a warning label. There are those who will always criticise renewables for being inefficient and costly. There is one notable issue with renewables that people always reference - a lack of reliable renewable resources. If the sun doesn't shine or the wind doesn't blow, they are useless (that isn't strictly true as solar can still produce energy with little sunlight). But if you're lucky enough to live in a sunny, windy, hydrologically and geologically active, that tends not to be an issue.

In sunny countries, solar makes an ideal renewable energy. However dust can be an issue as it settles on the surface of PV modules, covering the surface and reducing the amount of solar radiation hitting the surface. This is a real issue for sunny desert countries, but what are the implications? Well I suppose I wouldn't be writing a blog or asking the question if I hadn't found a rather nice recent article that looks at the issue!

Zarei and Abdolzadeh (2016) modelled the thermal and optical impact of dust on solar PV panels. The authors compared and validated their model with literature values. They modelled the amount of "6.44 μm sized mono-disperse dust particles" which withstand average angles of solar PV installation. At a 30° tilted dusty PV cell, Zarei and Abdolzadeh (2016) found the maximum power with 0.224 mg/cm2 of dust (the amount of dust that tends to stick to the module at 30°) is 13.53% lower than a dust-free solar PV cell (figure 1).

Figure 1: maximum power output of PV cells at different dust densities (Source)

This information is very useful, especially in modelling the effectiveness of solar PV cells in dusty countries. Furthermore combined with average local dust or sand sizes, the model can be localised to find the maximum electrical output of dusty solar PVs. Finally, in national and international energy models, this information is vital for recreating realistic and accurate understandings of renewable energy production.

Resources are vital, but our over reliance on energy is incredible. For a sustainable future, we need to rely on clean and un-exploited sources of fuel. These small studies can help our fuller understanding of global energy consumption and potentially calculate and reduce the impact of climate change!

The glimmer of hope in COP21

In light of COP21 and this blog's (slowly developing) aims to investigate resource utilisation with regards energy production, it makes sense to look at renewables! Essentially, renewables can provide countries with unlimited, free electricity, if the conditions are correct. Therefore, it shouldn't be surprising to know that sunnier countries have pledged high levels of solar energy investment to contribute towards their carbon targets.

Morocco has made great strives towards reducing their reliance on coal. In 2011, Morocco was heavily reliant (47%) on coal for energy production. However, this has considerably reduced from 2001 (77%). Furthermore, they have made great steps through commitments at COP21, agreeing to reduce GHG emissions by 32% by 2030 through 50% renewable electricity generation by 2025 and reduced energy consumption of 15% by 2030. Finally, and most significantly, they have offered to host COP22, solidifying their drive and dedication to meeting their climate objectives.

To do this, Morocco requires support. The UN Green Climate Fund can be of some help; however investment by private companies is becoming increasingly significant in efforts. Saudi investment has been important in the Arab region.

The shining hope in COP21 (Source)

A solar thermal plant planned to open next month will aim to eventually supply energy for 20 hours a day from energy collected from the sun and thermally-stored in liquid salt. The potential renewable generation capacity could provide export capacity to Europe, instead of the heavy importation of electricity from Spain.

Another great blog post has questions the hopefulness of COP21, citing Morocco and Ethiopia as leaders in setting targets internationally. I agree that more needs to be done - but maybe the baby steps will eventually make more of an impact in the future? I think there is some hope from COP21, even if it is only the smaller and less developed countries providing that hope.