Energy Storage Systems

Energy storage is the capture of energy produced at one time for use at a later time. A device that stores energy is sometimes called an accumulator or battery. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped. As new technologies and uses increase the value of energy, means of storing excess and waste energy will become increasingly important.

Energy storage systems provide a wide array of technological approaches to managing our power supply in order to create a more resilient energy infrastructure and bring cost savings to utilities and consumers. To help understand the diverse approaches currently being deployed around the world, we have divided them into six main categories:
• Solid State Batteries - a range of electrochemical storage solutions, including advanced chemistry batteries and capacitors
• Flow Batteries - batteries where the energy is stored directly in the electrolyte solution for longer cycle life, and quick response times
• Flywheels - mechanical devices that harness rotational energy to deliver instantaneous electricity
• Compressed Air Energy Storage - utilizing compressed air to create a potent energy reserve
• Thermal - capturing heat and cold to create energy on demand
• Pumped Hydro-Power - creating large-scale reservoirs of energy with water


A number of very different methods exist to store “electric energy,” some of which are listed in Table 1. Only two of those shown actually store the energy in electric form: super-capacitors and SMES (Superconducting Magnetic Energy Storage which keep the energy as electric charge or magnetic fields respectively. Batteries actually store the energy in a chemical form, but the natural operation of the battery converts the power to direct current electric power upon being provided with a pathway for the power to flow.
Energy storage systems include various means of storing and recovering energy for later use. Electric energy storage systems accept and return the stored energy as electric power, although they may store the energy in another form. Non-electric energy storage units store the energy in some other form. Despite this, they may be of interest to electric utilities and power engineers since they can materially affect the shape of daily demand curves
Mechanical storage includes several types of flywheels, compressed air, and pumped hydro systems. The last two are practical and widely used on a system (100 MW peak capacity or larger) scale.
Thermal storage systems use electricity to heat a liquid to very high temperatures and then use that, via a heat exchanger, to heat steam to drive a steam turbine generator or a sterling cycle generator


Batteries are a chemical form of energy storage. Chemical energy can be defined as the energy stored in atoms and molecules that can be released during chemical reactions. The next few sections of this white paper are dedicated to chemical energy storage, including batteries—conventional battery technology and flow batteries, and ends with electrochemical capacitors. These sections will elucidate how different battery technologies and electrochemical capacitors work, the pros and cons of each, and the current direction of R & D.

Battery Energy Storage (BES)
Battery energy storage (BES) is a technology most Americans are familiar with in their everyday life. The major function of a battery is to convert between electrical and chemical energy. The basic schematic of how BES functions can be found in Figure 3.2 below. Batteries consist of multiple electrochemical cells that are connected either in series or in parallel. Within each cell there is an anode and a cathode, as well as an electrolyte. Anodes connect to the negative end of the battery where electrical current enters during discharge. Cathodes connect to the positive end of the battery where electrical current leaves during discharge. The electrolyte, which contains electrically charged particles/ions, can be solid, liquid, or viscous in state. While a battery is discharging, both the anodes and the cathodes of each cell undergo an electrochemical reaction.


Mechanical energy is the energy of an object due to its position or motion. When an object has the ability to do work due to its position it is said to have potential energy. Kinetic energy, in contrast, exists due to an object’s ability to perform work due to its movement. Mechanical energy storage is the means of stockpiling that energy until it is needed in the future. The next three sections will discuss three major types of mechanical energy storage: compressed air energy storage (CAES), pumped hydroelectric energy storage (PHES), flywheels, and gravitational storage. Each is better suited for a particular application and the Port may be able to incorporate multiple types of mechanical energy storage into its future planning.


Hydrogen Energy Storage
The first process is to produce hydrogen from electricity or sunlight. This is commonly done through water electrolysis, which separates the hydrogen from oxygen atoms, but also can be done via other processes such as gasification or photo conversion. After electrolysis, the hydrogen is then compressed to high pressure and stored in high pressure containers or pipelines. Hydrogen can also be stored in “hydrogen absorbent” materials at low pressure, but this storage requires some energy to capture and release the fuel. Conversion of hydrogen gas back to electricity is typically done either via fuel cell or internal combustion. Fuel cells have a higher efficiency than combustion processes so they are often the preferred method. Fuel cells vary in their choice of electrolyte and fuel1. However, not all fuel cells require electrolyte. For example, proton exchange membrane (PEM) fuel cells utilize a proton exchange membrane in replacement of an electrolyte. Fuel cells work by taking hydrogen and oxygen, which is supplied from the air, and converting the two reactants to electricity, water, and heat. Hydrogen and oxygen are funneled into the fuel cell continuously and are not intended to be stored within a fuel cell in the same way batteries store energy.