Some of the key advantages of flywheel energy storage are low maintenance, long life (some flywheels are capable of well over 100,000 full depth of discharge cycles and the newest configurations are capable of even more than that, greater than 175,000 full depth of discharge cycles), and negligible environmental impact. Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics.

Typically, users of high-speed flywheels must choose between two types of rims: solid steel or carbon composite. The choice of rim material will determine the system cost, weight, size, and performance. Composite rims are both lighter and stronger than steel, which means that they can achieve much higher rotational speeds. The amount of energy that can be stored in a flywheel is a function of the square of the RPM making higher rotational speeds desirable. Currently, high-power flywheels are used in many aerospace and UPS applications. Today 2 kW/6 kWh systems are being used in telecommunications applications. For utility-scale storage a ‘flywheel farm’ approach can be used to store megawatts of electricity for applications needing minutes of discharge duration.

How Flywheel Energy Storage Systems Work

Flywheel energy storage systems (FESS) employ kinetic energy stored in a rotating mass with very low frictional losses. Electric energy input accelerates the mass to speed via an integrated motor-generator. The energy is discharged by drawing down the kinetic energy using the same motor-generator. The amount of energy that can be stored is proportional to the object’s moment of inertia times the square of its angular velocity. To optimize the energy-to-mass ratio, the flywheel must spin at the maximum possible speed. Rapidly rotating objects are subject to significant centrifugal forces however, while dense materials can store more energy, they are also subject to higher centrifugal force and thus may be more prone to failure at lower rotational speeds than low-density materials. Therefore, tensile strength is more important than the density of the material. Low-speed flywheels are built with steel and rotate at rates up to 10,000 PRM.

More advanced FESS achieve attractive energy density, high efficiency and low standby losses (over periods of many minutes to several hours) by employing four key features: 1) rotating mass made of fiber glass resins or polymer materials with a high strength-to-weight ratio, 2) a mass that operates in a vacuum to minimize aerodynamic drag, 3) mass that rotates at high frequency, and 4) air or magnetic suppression bearing technology to accommodate high rotational speed. Advanced FESS operate at a rotational frequency in excess of 100,000 RPM with tip speeds in excess of 1000 m/s. FESS are best used for high power, low energy applications that require many cycles.

Additionally, they have several advantages over chemical energy storage. They have high energy density and substantial durability which allows them to be cycled frequently with no impact to performance. They also have very fast response and ramp rates. In fact, they can go from full discharge to full charge within a few seconds or less. Flywheel energy storage systems (FESS) are increasingly important to high power, relatively low energy applications. They are especially attractive for applications requiring frequent cycling given that they incur limited life reduction if used extensively (i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycle).

FESS are especially well-suited to several applications including electric service power quality and reliability, ride-through while gen-sets start-up for longer term backup, area regulation, fast area regulation and frequency response. FESS may also be valuable as a subsystem in hybrid vehicles that stop and start frequently as a component of track-side or on-board regenerative braking systems

How Compressed Air Energy Storage Works

Compressed air energy storage (CAES) plants are largely equivalent to pumped-hydro power plants in terms of their applications. But, instead of pumping water from a lower to an upper pond during periods of excess power, in a CAES plant, ambient air or another gas is compressed and stored under pressure in an underground cavern or container. When electricity is required, the pressurized air is heated and expanded in an expansion turbine driving a generator for power production.

The special thing about compressed air storage is that the air heats up strongly when being compressed from atmospheric pressure to a storage pressure of approx. 1,015 psia (70 bar). Standard multistage air compressors use inter- and after-coolers to reduce discharge temperatures to 300/350°F (149/177°C) and cavern injection air temperature reduced to 110/120°F (43/49°C). The heat of compression therefore is extracted during the compression process or removed by an intermediate cooler. The loss of this heat energy then has be compensated for during the expansion turbine power generation phase by heating the high pressure air in combustors using natural gas fuel, or alternatively using the heat of a combustion gas turbine exhaust in a recuperator to heat the incoming air before the expansion cycle. Alternatively the heat of compression can be thermally stored before entering the cavern and used for adiabatic expansion extracting heat from the thermal storage system.

Diabatic CAES Method

Two existing commercial scale CAES plants in Huntorf, Germany, and in McIntosh, Alabama, USA, as well as all the proposed designs foreseeable future are based on the diabatic method. In principle, these plants are essentially just conventional gas turbines, but where the compression of the combustion air is separated from and independent to the actual gas turbine process. This gives rise to the two main benefits of this method.

Because the compression stage normally uses up about 2/3 of the turbine capacity, the CAES turbine – unhindered by the compression work – can generate 3 times the output for the same natural gas input. This reduces the specific gas consumption and slashes the associated carbon dioxide emissions by around 40 to 60%, depending on whether the waste heat is used to warm up the air in a recuperator. The power-to-power efficiency is approx. 42% without and 55% with waste heat utilization.

Instead of compressing the air with valuable gas, lower cost excess energy can be used during off peak periods or excess renewable energy in excess of local energy demand.

The aforementioned plants both use single-shaft machines where the compressor-motor/ generator-gas turbine are both located on the same shaft and are coupled via a gear box. In other conceptual CAES plant designs, the motor-compressor unit and the turbine-generator unit will be mechanically decoupled. This makes it possible to expand the plant modularly with respect to the permissible input power and the output power. Using conventional gas turbine exhaust heat energy for the purposes of heating the high-pressure air before expansion in an air bottoming cycle allows for CAES plants of variable sizes based on cavern storage volume and pressure.

Adiabatic Method

Much higher efficiencies of up to 70% can be achieved if the heat of compression is recovered and used to reheat the compressed air during turbine operations because there is no longer any need to burn extra natural gas to warm the decompressed air.

Storage Options

Independent of the selected method, very large volume storage sites are required because of the low storage density. Preferable locations are in artificially constructed salt caverns in deep salt formations. Salt caverns are characterized by several positive properties: high flexibility, no pressure losses within the storage repository, and no reaction with the oxygen in the air and the salt host rock. If no suitable salt formations are present, it is also possible to use natural aquifers – however, tests have to be carried out first to determine whether the oxygen reacts with the rock and with any microorganisms in the aquifer rock formation, which could lead to oxygen depletion or the blockage of the pore spaces in the reservoir. Depleted natural gas fields are also being investigated for compressed air storage; in addition to the depletion and blockage issues mentioned above, the mixing of residual hydrocarbons with compressed air will have to be considered.

CAES power plants are a realistic alternative to pumped-hydro power plants. The capex and opex for the already operating diabatic plants are competitive.

How Isothermal Compressed Air Energy Storage Works

Controlling the pressure-volume (P-V) curve during compression and expansion is the key to efficient CAES.

Rather than employing numerous stages to compress, cool, heat and expand the air, isothermal CAES technologies attempt to achieve true isothermal compression and expansion in situ, yielding improved round-trip efficiency and lower capital costs. In principle it also negates the need to store the heat of compression by some secondary means (e.g., oil).

Challenges

Isothermal CAES is technologically challenging since it requires heat to be removed continuously from the air during the compression cycle and added continuously during expansion to maintain an isothermal process. Heat transfer occurs at a rate proportional to the temperature gradient multiplied by surface area of contact; therefore, to transfer heat at a high rate with a minimal temperature difference one requires a very large surface area of contact.

Although there are currently no commercial Isothermal CAES implementations, several possible solutions have been proposed based upon reciprocating machinery. One method is to spray fine droplets of water inside the piston during compression. The high surface area of the water droplets coupled with the high heat capacity of water relative to air means that the temperature stays approximately constant within the piston – the water is removed and either discarded or stored and the cycle repeats. A similar process occurs during expansion.

The companies developing Isothermal CAES quote a potential round-trip efficiency of 70-80%.

The technology compresses and expands gas near-isothermally over a wide pressure range, namely from atmospheric pressure (0 psig) to a maximum of about 2,500 psig. This large operating pressure range, along with the isothermal gas expansion (allowing for recovery of heat not achieved with adiabatic expansion), achieves an approximately 7x reduction in storage cost as compared to classical CAES in vessels.