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Linear Electric Machine-Based Gravity Energy Storage for Wind Farm Integration

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Linear Electric Machine-Based Gravity Energy Storage for Wind Farm Integration Linear Electric Machine-Based Gravity Energy Storage for Wind Farm Integration Christoff D. Botha and Maarten J. Kamper Department of Electrical and Electronic Engineering Stellenbosch University Stellenbosch, South Africa [email protected], [email protected] Abstract—In this paper an above-ground, dry gravity energy support, long-term bulk storage and other grid support services storage system to help integrate wind energy sources into the such as upgrade deferral [5]. energy mix, is described and developed. Using the principle of There are numerous energy storage technologies, making gravitational potential energy and a single piston example, multi- piston shafts and multi-shaft systems are proposed. From this it useful to divide them into different categories. These tech- analysis, some of the basic characteristics of the system, such nologies can be classified either by the service they provide as round trip efficiency and energy density, is derived. Using a or the method by which the energy is stored, i.e. the form in generic wind farm and available literature, the paper discusses which the energy is stored. The from of energy storage can how the system can be constructed and used to help integrate be divided as either mechanical, chemical, electrochemical, wind farms with an electrical grid, while also demonstrating the comparatively small surface area that the storage system electrical or thermal energy storage, while the type of service requires. they provide can be divided into three overarching categories, Index Terms—Energy storage, wind farms, renewable energy i.e. bulk storage, distributed storage and power quality services sources, gravitational potential energy, linear electric machines [8]. Storage systems can be further evaluated by comparing their technical characteristics, with some of the important I. INTRODUCTION characteristics being the system’s energy and power rating, Renewable energy has grown tremendously since the start of round-trip efficiency and cost, usually determined using a cost the century, supplying 26 % of global electricity production metric such as levelised cost of storage (LCOS). in 2018 [1]. A total of 1246 GW of renewable energy (not The most commonly used utility-scale storage system is including pumped hydroelectricity) was installed by the end pumped hydroelectric energy storage (PHES), with batteries of 2018, with wind power contributing 591 GW thereof [1]. and flywheel storage systems being popular choices for storage Similarly, wind power represents 52 % of the renewable power systems installed alongside wind farms [3], [4], [9], [10]. Some supply of South Africa [2]. other technologies include hydrogen storage, supercapacitors, With wind power set to increase its share of power gener- superconducting magnetic energy storage, gravitational poten- ation responsibilities further, integrating the additional wind tial energy storage methods and compressed air schemes. power capacity presents many challenges due to the inherent The focus of this paper is on a specific gravitational po- variability of wind. This not only introduces uncertainty in the tential energy storage system, developing the energy storage availability of wind, but also short term fluctuations [3]. concept proposed in [8] further. The paper is divided up This uncertainty and variability results in power quality into the following sections: Section II describes some of problems, with ancillary services needed to provide frequency the existing and proposed gravity energy storage methods, response, power smoothing and peak shaving, among others with Section III further explaining and describing the specific [4]. The need to ensure adequate grid flexibility and reliability storage method being considered in this paper. Section IV with the growing participation of wind power has renewed the discusses how energy storage technologies are used to help search for technologies that can assist with grid integration, integrate wind farms into the grid, with Section V detailing such as new forecasting tools, demand-side control and energy a design for the LEM-GES. Lastly, some conclusions and storage [5]. recommendations are given in Section VI. Energy storage specifically has received renewed attention II. GRAVITY ENERGY STORAGE TECHNOLOGIES because of its wide range of applications [6], [7], from improving power quality, providing voltage and frequency The use of gravitational potential energy as a method of energy storage is an old concept. PHES, which is currently The research presented in this paper is financially sponsored by the Centre the most used and mature energy storage method [11], is a for Renewable and Sustainable Energy Studies (CRSES) in Stellenbosch, form of gravitational energy storage. A number of variations South Africa. on traditional PHES concepts have been proposed since its inception in the 1890s, including the use of old mineshafts 978-1-7281-4162-6/20/$31.00 ©2020 IEEE for underground PHES (UPHES) and various piston-based pumped storage technologies [12]–[14]. PHES provides ob- vious advantages, e.g. scalability, commercial viability and E = mgh; (1) high round trip efficiencies [15], but it also has some distinct disadvantages. Gravitational pull is relatively weak and the with E being the potential energy in Joule, m the mass of density of water is low, thus PHES requires either a large the object in kilogram, h the height of the system in meters head or large volume of water with which to store a significant and g the gravitational constant (9.81 m=s2). With 1 kWh amount of energy. Site selection criteria is also very strict, as being equal to 3.6 x 106 Joule, it is clear from (1) that a large a potential site has to have sufficient water supply, the correct mass or height is needed to be able to store even a few MWh topography, economic feasibility and social acceptability [15]. of power, as can be seen by Gravitricity’s proposed storage It is only recently that waterless forms of gravity energy system being only a few MWh despite a piston mass of 3000 storage have also gained some traction, as can be seen by the tonne and a height difference of 1500 m. This highlights two proposed systems from ARES LLC [16] and Gravitricity [17]. problems when designing any type of gravity energy storage Advanced rail energy storage (ARES) uses rail road shuttles system, namely how to achieve a sufficiently large object mass to move large blocks of concrete, typically between 45–64 t and system height so that enough energy can be stored for the each, uphill to charge the system and let the concrete blocks intended energy storage application. descend under gravity when discharging the system [16]. ARES has a round-trip efficiency of 80 %, a proposed lifetime A. Piston Description of 40 years and no standby storage losses. ARES currently has their first commercial project underway in Nevada, a 50 MW, LEM technologies directly convert electrical energy to linear 12.5 MWh system. The aim is to build systems with a power movement and vice versa, without the need for intermediary rating ranging from 100 to 3000 MW and energy ratings up systems such as gearboxes or wire ropes. LEMs are commonly to 6 GWh. used in wave energy converter systems, magnetically levitated Gravitricity proposes vertically raising and lowering a large (MAGLEV) trains and ropeless elevator systems [19]–[21]. mass down an abandoned mineshaft [17]. The system uses a The LEM-GES system utilises the ropeless operation en- piston (the large mass being hoisted) with a mass up to 3000 abled by the use of LEM technology, by foregoing the use of tonne over a distance of 1500 m, hoisted with a specialised a single, large piston in favour of multiple, smaller pistons. system that is similar to those found in existing deep shaft Each piston takes the form of a rectangular prism, with the mines. This system has a claimed round-trip efficiency be- LEM attached to the four sides of the prism, as shown in Fig. tween 80 - 90 % and a system lifetime of 50 years, with a 1, with lp being the piston length and wp being the piston rated power of up to 40 MW and an energy rating of a few width. MWh. Gravitricity has received funding to build a 250 kWh LEMs can be obtained by ’cutting and rolling out’ the prototype [18]. corresponding rotary machine and can be classified according The authors of [8] also use the idea of moving a piston to topologies or the shape of the LEM, i.e. flat or tubular. vertically to store energy, however without the use of the [8], [19]. Similar to a rotary machine, it has one side with more conventional mining hoist system. Instead, the piston armature windings, which is referred to as the primary. The forms part of a linear electric machine (LEM), which is used other side is called the secondary, or translator, and relates to to move the piston. This enables the system to operate without the rotor of a rotary machine. LEMs can have any combination the need for wire ropes, allowing for the use of multi-piston of permanent magnets and windings on the primary, secondary system, improving the storage system in numerous ways, not or both. least of which is an increase in storage capacity. The authors For an application such as the LEM-GES, where the also proposed that the LEM-based gravity energy storage secondary needs to cover the system height, it would be (LEM-GES) system could be constructed in two different prohibitively expensive to have any active material (copper ways. One method would be to place the system in an old or permanent magnets) placed on it. Thus, any suitable LEM mineshaft, thereby ensuring that the system has a large height topology for the LEM-GES would require a passive secondary, difference and can store a significant amount of energy.
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