There are a wide range of technologies which can be used to store energy for an electrical grid, each with their own benefits and drawbacks. For the purpose of this proposal, only energy storage technologies plausible for the site and situation presented by Kinlochleven were considered. Therefore, promising solutions such as pumped hydro were excluded, while flywheels and supercapacitors were discarded from further consideration at an early stage.
Batteries operate by converting electrical energy to chemical energy when charging, and the reverse while discharging. This is achieved by electrodes which exchange charged ions and electrons between an electrolyte and the electrical circuit [31]. Battery technology has advanced significantly in recent years as electrical energy storage has become increasingly important with the increase of portable consumer electronics as well as electric vehicles. A variety of battery types exist, most notably are: Lead acid, Lithium ion, Nickel Cadmium, and Nickel metal hydride. A summary of the characteristics of the four battery types considered is shown in the table below.
| Lead Acid | Nickel Cadmium | Nickel Metal Hydride | Lithium Ion | |
|---|---|---|---|---|
| Energy (Wh/kg) | 20-40 | 40-60 | 50-70 | 150-250 |
| Power (W/kg) | 5-200 | 10-150 | 10-100 | 150-500 |
| Cycles (1000s) | 1-5 | 1-3 | 1-3 | 1-20 |
| Energy Efficiency (%) | 60-90 | 80 | 80 | 90-98 |
| Temperature range (°C) | -10-50 | -20-45 | -20-45 | -20-50 |
| State of Charge Range (%) | 0-100 | 0-100 | 0-100 | 20-90 |
Flow batteries consist of two tanks of electrolyte separated by an ion/proton exchange membrane. Energy is stored by oxidising the electrolyte on one side and reducing the electrolyte on the other side, therefore increasing the potential difference between the liquids. Hence, energy is released by reversing the process [33].
Flow batteries have a number of desirable qualities: they are easily scalable as storage capacity is proportional to tank size, they are relatively low cost for large scale applications, they do not lose capacity from deep discharging, have a relatively long lifespan, and have negligible self-discharge rates [32]. An issue with flow batteries is that they have extremely low energy density compared to traditional batteries. The operation of flow batteries is also complex due to the pumps and piping required for them to function [33].
In a typical fuel cell system, the reactants (most commonly hydrogen and oxygen) are stored separately from the reaction site. The reactants are pumped into the fuel cell where they react and generate electricity. In a hydrogen-oxygen fuel cell, the biproduct from this process is water. Conversely, to store energy, water can be split into hydrogen and oxygen gases by electrolysis [33].
Hydrogen based fuel cells have a number of positive attributes, notably: a high energy density, negligible self-discharge rate, and they do not release toxic pollutants [32]. Fuel cells are also modular, hence are easily scalable. Drawbacks of hydrogen fuel cells are that they suffer from poor round trip efficiency and are still expensive for energy storage [33].
Based on the characteristics of the various storage types discussed, it was decided that a lithium ion battery bank should be implemented. This was chosen due to the high round-trip efficiency of lithium ion batteries and their relatively long lifespans. In the future, flow batteries may offer a more cost effective solution due to the scalability, however, currently they are not widely implemented.
To increase the lifespan of lithium ion batteries, it is typical to keep them at a state of charge (SOC) between 20 – 90% [32]. Hence, the storage capacity used for sizing calculations should be 70% of the rated capacity.
The rate at which the battery can charge and be discharged must also be considered when designing a battery system. A C-rate of 1C means that the battery could be charged from 0% to 100% SOC in 1 hour. Therefore, a C-rate of 2C would fully charge the battery in 30 minutes [32]. For the battery calculations performed, the C-rate did not affect the sizing, therefore, a relatively low C-rate of 0.25C was chosen.