On the path to net-zero: long-term energy storage

Our transition to a low-carbon society has intensified the need for variable, weather-dependent renewable energy. This poses a range of new challenges for the UK and global power grids. The intermittent nature of VRE creates an imbalance between supply and demand for energy, as times of generation rarely coincide with periods of consumption. To ensure that generation satisfies demand with growing inputs from VRE future energy grids will need much greater balancing. Variation management strategies can be based on installing more VRE (excess capacity) in different regions, building new transmission lines, or providing additional energy storage. Storage requirements for a system vary and depend on the fraction of energy being supplied by VRE, dynamics of the system and degree of response of the other flexibility measures. The need for storage can be minimized by:

• Optimizing the ratio of wind to solar generation. As wind and solar production trends are complementary on a daily and seasonal basis an optimum ratio, for example, could lead to a storage size of 1.5x the monthly demand, whereas a 100% wind-only scenario would lead to 2.7x.
• Generating excess power to supply enough energy during low resource times. The trade-off for stability through this step is an additional CAPEX and a greater probability of curtailment.
• Improving storage efficiency. Balancing capacity is directly linked to storage efficiency. For ideal 100% efficient storage, the balancing requirement can be about 5% of the annual power demand, whereas a 60% efficiency would make it unfeasible (>100%). As a result, less efficient storage must be compensated with additional generation capacity.
• Upgrading the transmission gird. VRE systems without storage or transmission need 100% excess production at European level and almost 60% at a global level. Optimal expansion of transmission may reduce these values and the need for storage.
• Implementing demand-side measures. This affects the time-related component and can be in direct competition with storage. It enables shifting the peaks in the load aiming to make it more stable and match the generation curve.

Taking into account the above, several pathways to a low carbon economy have been formulated (Ofgem, IEA, IMechE, CBI, Shell, UKERC, DECC, Committee on Climate Change etc). Each pathway leads to a different energy and storage mix scenario for the energy systems to reach net-zero greenhouse gas emissions in 2050. The conclusion is that with capacity factors of VRE installations between 20% to 50% only a little storage is needed. A fraction of VREs between 50% and 80% can be reached with long term storage (LTS) durations of circa 10 h. And that seasonal storage devices are likely to be required once more than 80% of the electricity demand is met by VRE. Using the US grid as an example, a graphical representation of a high-level relationship between the overall storage duration needed to satisfy demand and the fraction of annual energy from VRE is plotted below.

 Figure 1. Semi-quantitative Overview of the Maximum Duration of Electricity Storage Needed to Ensure Demand Is Met at All Times versus the Fraction of Annual Energy from Variable Generators, Such as Wind and Solar.

UK in Q1 2020 reached a level of 32% of annual electricity from VER. At this level, the grid requires very little support from storage and most of the need to manage VRE intermittency on timescales from hour to day can be met with batteries. Batteries are emerging as one of the key solutions for the successful incorporation of high levels of VRE in power systems around the world. The price of lithium-ion batteries has fallen by around 80% over the last five years increasing the competitiveness and wider adoption of the batteries in grid storage. The larger challenge, as the proportion of VRE in the UK, grows to 50-65% in 2030, and reach three quarters (76%) by 2050 will be to smooth out variability in renewable output on timescales of days and weeks. In 2050 the UK will require more than 30 GW of storage capacity to shore the UK against prolonged windless and sunless periods. This can be met with the available energy storage technologies shown in the graph below. There are various options covering timescales from seconds to months that can help meet the demand for short and long term storage. Grid-scale LTS solutions are presented in Figure1 and located in the top right section of the graph. These fall into two categories - mechanical and chemical. Mechanical energy storage includes pumped hydro (PHS) and compressed air energy storage (CAES). Chemical storage of energy includes hydrogen (H2) and synthetic natural gas (SNG). So far, pumping water up the hill is the most efficient storage solution. PHS has outperformed other technologies and has become the world's largest, most mature, cost-effective and most efficient battery technology. It accounts for over 94% of installed energy storage capacity, well ahead of lithium-ion and other battery types. 

Pumped hydro storage

PHS stores energy by using off-peak electricity to pump water from a lower reservoir to an upper reservoir. It restores energy by allowing the water to flow back through turbines to produce power. Pumped hydro takes advantage of the efficiency of converting electricity to mechanical motion by means of an electric motor, and converting it back again using a generator. Round-trip efficiencies can be as high as 85%. In terms of energy storage, it’s one of the best. If we look at other storage options: thermal storage, electromechanical, electrochemical it can be noticed that the PHS dominates, it is way more efficient than the other options, there are simply no large-scale long-term alternatives as efficient as pumped hydro. PHS is expected to grow in the coming decades as this technology can greatly reduce grid balancing requirements. The161 GW of installed PHS capacity worldwide is set to double by 2050. Europe alone expects to see 191 GW of hydropower capacity operating on EU grids in 2050 compared to the existing 45 GW. In the UK, pumped hydro is also the main storage solution providing some 78% of grid storage. The installed energy storage capacity is 4.7 GW, including 2.8 GW pumped hydro. There are four main schemes in the United Kingdom (Dinorwig, Cruachan, Ffestiniog, Foyers) with the youngest Dinorwig in Wales (1.7GW) operating since the 1980s. Although no new schemes have been commissioned since then a range of suitable sites have been found by investors where additional PSH capacity could be built, some estimate as much as 15 GW of PHS potential in the UK is waiting to be tapped.

Because PHS is the one technology we do have available now that we know works, is efficient and lasts a long time there has been a resurgence in its popularity. The issue is that it is difficult to build new large scale pumped-hydro storage plants, due to the permitting implications of large water-based infrastructure, relatively high up-front costs, long project lead times, lengthy lifespans (>50 years), and long project payback periods. But if officials and investors pursued pumped hydro now, it could be ready to provide 100-day energy storage by the time renewables own 80% of the grid. Additionally, new small-scale hydropower projects, including community-led projects, can be developed across the UK to balance intermittency on shorter timescales. With changing VRE levels a more distributed approach to new PHS can be applied. The increase in VRE results in many pumped storage assets no longer working on daily cycles. Rather, pumped storage assets have greatly improved ramping and often run up to 60 times a day. This calls for an innovative approach and new PHS solutions. Teams of innovators and experts around the world are working on developing the next generation of hydro plants that are much more affordable and simpler to implement than the existing ones. Innovators are being incentivised through government grants and prizes. For example, in the US the Department of Energy (DOE) has set up a FAST prize to challenge competitors to cultivate new ideas, designs, and strategies to accelerate PSH development and reduce the deployment time of new plants. Winning concepts include steel dams, tunnel-boring machines for underground PSH, engineering innovations and 1-10 MW modular “closed-loop” plants using big water tanks, with no need for natural reservoirs. Small modular pump hydro systems are emerging as a tempting alternative to the classic PHS solution. 

Compressed air energy storage (CAES)

Presently, however, hydropower plants still have many challenges. As PHS take twice as long to permit as other energy sources including solar, wind, or natural gas projects, and time is money, engineers look into other less efficient alternatives to fill the energy storage gap, such as CAES and Power to Gas (P2G). Compressed air energy storage (CAES), stores energy either in an underground structure or an above-ground system, by running electric motors to compress air and then releasing it through a turbine to generate energy. Unfortunately, large-scale CAES plants are very energy inefficient. Compressing and decompressing air introduces energy losses, resulting in an electric-to-electric efficiency of only 40-52%, compared to 70-85% for PHS. Worldwide, innovators and engineers are working to improve the round-trip efficiency and lower capital cost of the process. For example, Magnum is currently developing the Magnum CAES Project with two CAES facilities in operation one in the US and the second in Germany. Their approach based on isothermal CAES aims to improve the process efficiency and cost. Isothermal CAES requires heat to be removed and added continuously, but in principle, it can be 100% efficient. Another project - Deep Blue led by a team at UC Santa Barbara is putting this concept to test. It is an ocean assisted isothermal modular energy storage solution. It pumps air into containers positioned on the ocean bed. The cooling is provided by the ocean making it more cost-effective and efficient. Globally, there are other technologically innovative projects under development that have the potential to revolutionize grid-scale energy storage if given the right support and funding. 

Power-to-gas (P2G) storage

Power-to-gas (P2G) storage is the second most viable alternative to pumped hydro for handling long-term fluctuations of renewable energy on a scale. P2G refers to a method of converting electrical energy into chemical energy via gas production. The efficiency of this technology is smaller than that of PHS and Lithium-ion batteries. However, it remains an important technology because it allows large amounts of energy to be stored over longer periods of time. The P2G uses renewable or excess electricity to break water into hydrogen and oxygen in a process called electrolysis (Power-to-Hydrogen). This hydrogen can be used directly as a final energy carrier or it can further be converted into methane (or “synthetic natural gas”). It is possible to store either hydrogen or methane until their energy is required, at which point the energy can be recovered in a fuel cell (for hydrogen) or by combustion (for methane). Unfortunately, neither electrolysis nor methanation is yet close to cost-competitive. These conversion processes are extremely difficult and expensive to operate in intermittent mode. They have low “round-trip efficiency” of storing and then re-generating electricity: an estimated 34-44% for the hydrogen pathway and 30-38% for the methane pathway. For all its challenges the P2G is a strong solution to transforming and storing renewable power. The commercialisation of power-to-gas technologies is at a relatively early stage of growth, with a limited number of pilot and demonstration plants in operation or under development.  

Energy storage future 

Energy storage is one of the most important issues in the energy market – it has the ability to determine the speed, scale and cost of the energy transformation. In addition to other technologies, such as interconnection and flexible generation, energy storage is needed to incorporate more renewables into the system and allow the decarbonization of the grid. The role of storage will become more important with increasing levels of VRE. For long terms storage, the UK will require technologies that are efficient and more cost-effective than batteries to meet its 30WG storage needs in 2050. While there is a wide variety of available energy storage solutions, each has associated major technical challenges that will require innovative approach and research to resolve. Therefore, an increased investment effort should be directed not only on batteries and short-term storage but also on LTS, as it is a core component of our existing and future energy systems. While startups routinely create products that break down barriers they are usually geared towards investor-driven goals and work to appease venture capitalists. It is not enough to rely on venture capital to fund new technologies. Public financing and resources are also required to help this segment of the energy industry expand. Grants and prizes, along with conventional venture and start-up financing methods, are required to facilitate the integration of renewable resources to the gird.

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