The Challenges Of Renewables And Energy Storage

Article by Ollie Boyacigiller, VIth Form Student, King Edward VI College, Totnes

One of the main challenges facing renewable energy sources currently is the need to be able to balance the energy flows between demand and generation to keep the electricity system working within its physical limits. 

 Mains electricity in the UK runs at a frequency of 50Hz. If this rises or falls by as little as 1% it can cause major damage to equipment and infrastructure. For this reason, the electrical frequency is intensely monitored by the National Grid to ensure it remains constant (or at least it only fluctuates between the regulated limits). 

If the total amount of generated energy in the grid increases relative to demand, so does the frequency of the grid, and, conversely, if the total amount of relative generated energy decreases then so does the frequency. 

For this reason, it's very important to be able to control the amount of energy being fed into the grid. This is where the issues with renewable energy sources arise. 

With traditional energy sources such as gas, coal or oil-fired power stations, the amount of energy produced could be easily altered by increasing or decreasing the amount of fuel being fed into the power station. In other words, with these sources of power, National Grid can easily keep the frequency of electricity constant by altering the power production of a small number of generation plants to make sure the amount of power produced always matches demand. 

Renewable energy generation is often referred to as “distributed energy” because typically the generation locations are distributed across an electricity network widely as opposed to the more concentrated locations of a few fossil fuel generation power stations historically.

This physical distribution of renewable generation sources means that it is difficult to control their output dynamically at any moment in time. 

In addition, renewables produce power “intermittently” i.e., we do not know necessarily when or how much power they will produce due to the fluctuating levels of wind & / or sun from moment to moment, day to day. 

The combination of these two factors of locational distribution and intermittency can wreak havoc with the frequency of the electrical Grid if not properly managed. 

One solution is to try to manage each renewable generation source, lowering their output when there is too much generation. However, given the number of installations and their physical distribution this is a very difficult prospect.

The alternative is to manage demand to balance the supply and keep frequency at acceptable levels. 

This is where the need for energy storage comes into play.

Energy storage, such as batteries, can be used to alter the overall demand depending on the generation supplied. For example, at times of high energy production (high supply) storage systems can be “charged up”, increasing demand such that it balances supply. When production is low, this stored energy can be discharged, increasing supply so that it matches demand. 

Storage is also increasingly important if we are to move towards achieving a 100% renewable energy Grid. When renewables are responsible for producing all, or most of, our power, their intermittency will become a much greater issue – imagine days or weeks of windless, cloudy days. 

To make a fully renewable energy Grid viable we must have a large amount of “long duration” storage, which will be able to power the grid for long periods of time when no or little renewable electricity is being produced. This will enable us to store all of the electricity when supply heavily outweighs demand (such as sunny, windy days), so we have a vast supply of electricity during long periods of low production (cloudy, non-windy days).

In a study, referred to in the recent Economist Technology Quarterly on the “Energy Transition” (see https://www.economist.com/technology-quarterly/2022/06/23/electrifying-everything-does-not-solve-the-climate-crisis-but-it-is-a-great-start) , undertaken by the Long Duration Energy Storage (LDES) Council (https://www.ldescouncil.com/) , the most cost effective path to a world with net-zero emissions by 2040 was modelled. The study found that the world needed between 1.5 – 2.5 Terra Watts (TW) of power and 85-140 Terra Watt Hours (TWh) of energy storage. This is an approximate duration of 70 hours i.e., 140 TWh / 2 TW = 70. To put this in context, The Economist notes that the US total generating capacity is currently 1.1 TW and 140 TWh is about 5% of the EU’s annual electricity consumption – big numbers yet achievable with the right investment now.

However, it should be noted that both long and short  duration storage solutions are now completely essential to the National Grid. 

As a result, this has caused a great deal of innovation, with new methods of energy storage currently being designed and implemented. 

One of the most common forms of these energy storage systems is known as pumped-storage hydropower (PSH).  PSH involves 2 water reservoirs of different elevations.  When supply of energy is greater than demand, the excess supply is used to pump water uphill to the higher reservoir – providing an immediate balancing mechanism. When demand for energy is greater than supply this water can be allowed to flow to the lower-level reservoir, producing electricity to enable supply to catch up with demand. This arrangement can store vast quantities of energy, which can then be converted back into electrical energy fairly quickly making PSH exceptionally useful as a form of energy storage for the National Grid. 

Unfortunately, PSH does have its downsides. Creating the 2 reservoirs involves flooding large areas of land, which destroys habitats and can have major impacts on local ecosystems. They are also very hard to set up for a few reasons: they're extremely costly, construction takes years and they require very specific conditions meaning there are only a few suitable locations for them.

Another solution is chemical storage, also known as battery storage. As a result of the huge developments in the electric vehicle market, the main chemistry that predominates currently is lithium-ion. The major advantage of lithium-ion batteries is the sheer speed at which they can release the stored chemical energy. Almost momentarily, chemical energy can be either stored or discharged to achieve the same balancing outcome as PSH. 

The critical advantage of battery storage versus PSH is that it can be done on a relatively small scale, meaning it's a viable solution for millions of homes and businesses globally. 

However, there are downsides to lithium-ion batteries.

1. Degradation: This applies to all batteries that store energy chemically. Unfortunately, their capacity to store energy degrades over time. In other words, they are able to store less and less energy over time, becoming much less useful. 

2. Supply of Lithium: While there is no specific physical shortage of lithium, the extraction process is complicated meaning a new lithium mining project can take upwards of 10 years to set up and many of the sources of lithium are in politically unstable regions. So while demand continues to skyrocket, due mostly to the boom in EV sales, very little can be done to increase supply. This has caused prices to skyrocket, increasing over 400% in the last year.

3. Duration: While lithium-ion batteries (and other chemistries) are effective for small scale energy solutions (usually domestic) their short duration (ranging from 30 mins to 10 hours) limits their utility as a larger scale solution.

However, a major report by the Department of Energy (DoE) in the United States (US) showed that from a pure financial perspective Lithium-ion is leading the pack for non-Grid scale, lower duration technologies if you analyse the Levelised Cost of Energy Supply (LCoS) between it and other technologies. The LCoS attempts to provide a cost per kWh of energy provided that takes into account the capital cost and running costs, including degradation. This is hown in figure 1 below.

Figure 1: LCOS Analysis by DoE (up to 100MW, ref: https://www.pnnl.gov/lcos-estimates) 

What is clear from figure 1 is that when you get up to Grid scale batteries (100MW+) and durations that are required to balance the Grid i.e. > 24 hours, Lithium-ion does not become the most favourable and it is replaced by Compressed Air Energy Storage (CAES), PSH and gravitational systems.

With CAES, excess generated electricity is used to run a compressor which produces heated compressed air, which is then stored under pressure (usually underground). When there is excess demand and electricity is needed, the pressurised air is heated causing it to expand, which turns a turbine to produce electricity. This solution can store huge amounts of electricity for a long period of time rendering it very useful as a storage system for the National Grids and is financially viable as shown in figure 1 above. However, although it does have low overall life-cycle costs once built, CAES has high start-up costs which require large capital financing.  The larger systems are generally built over a large salt cavern (which are rare) which is used to store the pressurised air as they are naturally hermetic, and the pressurised salt seals any cracks that form.  Otherwise, if salt caverns are not available then a huge space must be mined out (which further increases costs).

In conclusion, there is a huge requirement of energy storage, and the different forms will provide solutions across the range of requirements with lithium-ion leading the charge for lower duration requirements and CAES for multi-day durations. Whatever happens, the race to zero will provide an amazing opportunity for innovation and commercial solutions globally over the coming decade(s).