Mechanical Energy Storage Systems (MESS)

1.1 Kinetic Energy Storage Systems: Flywheel Energy Storage (FES)

   A flywheel is a device which is used to store rotational energy (kinetic energy) [Beacon Power, 2012]. This device was commonly used in combination with a motor/generator to supply emergency power in the case of an interruption of the primary source. The original disadvantages of flywheels were their high initial costs (increasing the running energy costs) and long and frequent periods of repair compared to static systems (such as batteries). However, technology developments have enabled the emergence of a new type of flywheels, such as the “Beacon Power Flywheel (BPF)”. The main principle of operation of the BPF is that it uses a vacuum chamber and magnetic bearing to house the rotors, minimising losses and wear as a consequence [DTI, 2006]; a motor/generator is mounted on the shaft of the rotors [Rounds and Peek, 2009; Beacon Power, 2012]. Fig. 1 shows a general topology of a FES system, however, the technology and diagram of BPF is a bit more complicated than that shown in Fig. 1; see [Beacon Power, 2012] for more detailed information.

Fig. 1: A general topology of a FES system (adapted from [Beacon Power, 2012]).

 

 The energy stored in a rotating flywheel can be calculated as follows [DTI, 2006]:

Where:

 Kinetic energy [Joules]

Moment of inertia of the rotor [ kg*(m^2)]

Rotational rotor speed [rad/s]

   Therefore, the storage capacity in a rotor can be increased by incrementing its inertia or its rotational speed. However, increasing the mass (in order to increase the inertia) might provide difficulties in the installation [DTI, 2006]. Therefore, the main objective of “Beacon Power”  focuses on taking full advantage of the rotational speed (i.e. increasing it).

    Referring to the Fig. 1, at the time of charging, the flywheel’s motor acts as a load drawing power from the system to increase the speed of the rotor (power in). At the time of discharging the motor is changed to a generator mode and the rotor kinetic energy is used to drive the generator; the electrical energy obtained is supplied back into the network (power out) [Rounds and Peek, 2009; Beacon Power, 2012].

   The Beacon Power Flywheels (BPF’s) are classified as high–speed flywheels. Currently, the “Beacon technology” allows the rotors to be operated at speeds between 8,000 and 16,000 rpm [Rounds and Peek, 2009] and the estimated life of these flywheels is around of 20 years [DTI, 2006]. The efficiency estimated for a flywheel system is about 90% [PIER, 2011].

 1.2 Pumped Hydro Storage (PHS)

   Fig.2 shows a general topology of a Pumped Hydro Storage (PHS) system. These types of storage system collect the water from creeks and/or rivers and store it in an upper reservoir (commonly under off–peak demand). Therefore, the energy is stored as potential energy. The potential energy is transformed back into electrical energy in the same way as is done in traditional hydro power plants: the water goes through a turbine which is connected to a generator. The energy stored is most commonly used to meet peak demands [Wagner and Mathur, 2011]. The 1,730 MW Dinorwig pumped hydro storage system in the UK is an example of a large scale application of this type of energy storage systems [DTI, 2006].

   This energy storage process implies some losses. Electrical power is needed to pump the water into the upper reservoir; therefore, losses due to electrical and mechanical equipment are present. When the water flows down, again losses are present as well. About the 20% of the energy used in the whole process is lost, and about the 80% of the energy is recovered [Wagner and Mathur, 2011]. Also, this type of energy storage system needs long construction times and high capital costs for both the plant and transmission lines (since it depends on specific geographic locations which are commonly far away from consumption centres) [PIER, 2011].

Fig. 2: A general topology of a PHS system (adapted from
[Wagner and Mathur, 2011]).

   However, PHS seems to be the best choice of energy storage since, similar to common hydro power stations, it has the ability for almost instantaneous starting and stopping. Therefore, this type of energy storage is a good choice to compensate peak demands. Also, its operation, generation cost and maintenance are lower than other type of energy storage. It also has a high efficiency (about 80%) [Wagner and Mathur, 2011] compared with other energy storage technology. To see a list of the largest PHS systems in the world, visit [Carmona Sanchez, 20015] or [Wagner and Mathur, 2011].

1.3 Compressed Air Energy Storage (CAES)

   The common way of working in this type of energy storage system is to store air during off–peak hours through the use of compressors. Then, during the peak hours, the compressed air is usually used in combination with a modified gas turbine to generate electricity [DTI, 2006]. Electrical power generated by Wind energy could also be used to compress the air when generation exceeds demand [DTI, 2006]. Due to the large amount of air required, CAES systems are commonly composed of an underground cavern or chamber. Such a cavern is used to store compressed air, see Fig. 3.

Fig. 3: A general topology of a CAES system (adapted from [PIER, 2011]). 

   CAES systems were first developed for combustion turbine systems [DTI, 2006]. This type of energy storage system has a low efficiency (about 50%) and also it requires specific geographic locations for large–scale storage since man–made tanks are only suitable for small–scale storage [PIER, 2011].

References:

[Beacon Power, 2012] Beacon Power, LLC: http://www.beaconpower.com/, accessed: 04/12/2012.

[Carmona Sanchez, 2015] J. Carmona Sanchez. “A Smart Adaptive Load for Power-Frequency Support Applications”. PhD Thesis, Power Conversion Group, The University of Manchester, UK, December 2015. Available at: https://www.escholar.manchester.ac.uk/uk-ac-man-scw:300748

[DTI, 2006] Department of Trade and Industry  (DTI). “Electrical Energy Storage Systems – A mission to the USA”. Report of a DTI GLOBAL WATCH MISSION, December 2006.

[PIER, 2011] Public Interest Energy Research (PIER) Program. “2020 Strategic Analysis of Energy Storage in California”. Final Project Report. California Institute for Energy and Environment – University of California: California Energy Commission. November 2011.

[Rounds and Peek, 2009] Robert Rounds and Georgianne H. Peek. “Design & Development of a 20–MW Flywheel–based Freqeuncy Regulation Power Plant: A Study for the DOE Energy Storage Program”. SANDIA REPORT SAND2008–8229, Unlimited Release, Printed January 2009.

[Wagner and Mathur, 2011] Herman–Josef Wagner and Jyotirmay Mathur. Introduction to Hydro Energy Systems: Basics, Technology and Operation. Springer–Verlag Berlin Heidelberg 2011.