WO2015047092A1

WO2015047092A1 - Kinetic energy storage system

Info

Publication number: WO2015047092A1
Application number: PCT/NL2014/050661
Authority: WO
Inventors: Wouter ENGELS
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2013-09-26
Filing date: 2014-09-26
Publication date: 2015-04-02

Abstract

Energy storage system comprising a mass body and energy conversion means arranged to convert electrical energy into kinetic energy of the mass body. The mass body comprises a one or morecarriages (5), and the energy storage system further comprises a track (2) in the form of a continuous loop supporting the one or more carriages (5) in an operational velocity range.

Description

Kinetic energy storage system

Field of the invention

The present invention relates to an energy storage system comprising a mass body and energy conversion means arranged to convert electrical energy into kinetic energy of the mass body.

Prior art

International patent publication WO2012/155170 discloses an energy storage system based on kinetic energy, including a flywheel having a substantially cylindrical flywheel portion, and a housing defining a substantially cylindrical cavity configured so as to receive the substantially cylindrical flywheel portion. The flywheel is rotatable in the cavity about a central axis of the flywheel, and an energy exchange device is configured to convert between electrical energy and kinetic energy associated with rotation of the flywheel. The system may also have a magnetic restraint arranged to exert a magnetic restraining force on the flywheel in a direction towards the central axis. Fly-wheels have a limited diameter and are driven at a very rotational speeds. Due to the required centripetal force, very high demands on the material must be made, which makes it expensive. The idea here is to use a large amount of relatively cheap material.

Further systems for storing large amounts of energy have been proposed. The Energy Island, using the potential energy of water in an artificial lake and hydro-power as conversion means, would require a large area and hence can only be built at specific locations in e.g. the North Sea. The use of heat to store energy, either without (pumped heat electricity storage) or with a phase transition (molten salt thermal storage) has also been proposed. This is not really feasible on a significant network-utility scale.

Compressed air energy storage (CAES) stores energy by compressing air to high pressures. The round trip efficiency (state-of-the-art) can be in the order 60-70%. By storing the air underground in geologically suitable locations, this technology is currently the most economical energy storage method, but is deemed economically unfeasible. Battery storage is more expensive and not considered feasible for large scale storage. A further technique has been presented with the acronym MAPS (MAglev Potential energy Storage), and is an idea where a Maglev train is used to ferry concrete weights up and down an incline on a mountain. One could say this is similar in essence to pumped hydro-energy and still requires a mountainous area.

Summary of the invention

The present invention seeks to provide an energy storage system, which is capable of storing a very high amount of energy, e.g. generated by sustainable energy sources (wind, solar), for use at a later time when demand is high or production is low.

According to the present invention, an energy storage system according to the preamble defined above is provided, wherein the mass body comprises one or more carriages, and the energy storage system further comprises a track in the form of a continuous loop supporting the one or more carriages in an operational velocity range. Such an energy storage system allows to implement a capacity sufficiently large to e.g. store energy from sustainable sources during low demand periods.

In specific embodiments the present invention relates to the use of a chain or train of one or more heavy carriages in a closed-loop or circular set-up that are accelerated and decelerated for the storage and retrieval of (electrical) energy. The invention makes use of maglev technology and vacuum to minimize losses in further embodiments. The invention can thus be summarized as the use of maglev supported trains for energy storage. The main benefit of this invention is the capacity to store significant amounts of electrical energy during longer periods of time. Storage capacity will reduce the required reserve capacity in the grid and may reduce network capacity in the electrical system. Relative to an energy-island using hydro-energy for storage, the present invention embodiments can be built anywhere and multiple times, opening the possibility of (some) economies of scale. Furthermore, a reduced impact on environment is obtained. Relative to compressed air energy storage (CAES) faster startup rates are provided, in the order of a second (as compared to start-up times in the order of 20 minutes for CAES), as well as lower round trip losses.

Short description of drawings

The present invention will be discussed in more detail below, using a number of exemplary embodiments, with reference to the attached drawings, in which Fig. 1 shows a top view of an exemplary track wherein the present invention is embodied;

Fig. 2 shows a partial sectional view of the track of Fig. 1 with a chain of carriages;

Fig. 3 shows a cross sectional view of a further embodiment of the energy storage system; and

Fig. 4 shows a top view of a further embodiment of a track for the present energy storage system.

Detailed description of exemplary embodiments

Large amounts of renewable power generation will be implemented in the electrical system by 2030-2050. This will require storage of electric energy. The technical issue to be solved is to store a large amount of energy (>100MW) during some period (up to days) economically in locations where no mountains are available to create dams and artificial lakes. The round trip efficiency should be high, costs low and the solution should support the electrical grid in both short and long term.

A solution may be found in using the kinetic energy of a heavy object moving at high speed to store (electrical) energy. The idea is to accelerate a large mass shaped somewhat like a train in an (underground) ring shaped tunnel to a very high velocity (e.g. 500 m/s). To limit losses, this mass would be supported with magnetic fields to eliminate rolling friction losses and to have a lowered air pressure in the tunnel to reduce drag. This mass would then be accelerated or decelerated depending on energy demand and supply. This acceleration and deceleration can happen on the basis of linear induction motors. Initial calculations show that such a kinetic energy storage system is cheaper than other energy storage options.

Grid support and grid balancing can be used to displace peak-capacity power plants, reduce start-up cost for conventional power plants and to reduce peak loading on the (national and transnational) grid. Delayed transport (e.g. store solar energy produced in Spain during the day in Spain, transport by night to Northern Europe and use it there the next day) can be used to make maximum use of and reduce the required maximum strength of the international grid. Furthermore, energy storage in case of excessive (wind) energy production can be accommodated. The various embodiments of the kinetic energy storage system according to the present invention will be described below. Fig. 1 shows a top view of a first exemplary track wherein the present invention is embodied, and Fig. 2 shows a partial sectional view of the track of Fig. 1 with a chain of carriages. The energy storage system comprises a mass body and energy conversion means arranged to convert electrical energy into kinetic energy of the mass body. The mass body comprises one or more carriages 5, or even a plurality of carriages 5. In a further embodiment some or all of the one or more carriages are linked together to form a chain (or train) of carriages 5. The energy storage system further comprises a track 2 in the form of a continuous loop supporting the chain of carriages 5 in an operational velocity range (i.e. the range in which electrical energy is converted into kinetic energy of the chain of carriages 5 and vice versa). The track 2 as shown in Fig. 1 is e.g. implemented as a tunnel in which the chain of carriages 5 is running during operation.

As will be clarified in further detail, the energy storage system is provided with a subsystem for guiding the one or more carriages 5 in (or on) the track, which in one embodiment is a very low friction supporting system 4 e.g. in the form of a magnetic levitation system. Such a supporting system provides for a friction which is much smaller than a normal train-rail system and even smaller than an air-bearing system.

As shown in Fig. 2, when more than one carriage 5 is present, the individual carriages 5 are linked together using a flexible joint 6, e.g. a pivoting joint 6. This allows the carriages 5, which in itself are of a stiff material, to move as a chain in the ring-like track 2, and also allow further movements between adjacent carriages 5 to cater for dynamic effects in the energy storage system, e.g. small changes in distance between adjacent carriages. This is an advantage over flywheel systems which are known for small scale energy storage, where the flywheel is arranged as a single solid material ring.

The train like carriage 5 implementation of the mass body also allows to use a very heavy material as a load inside each carriage, e.g. in the form of stone material or (wet) sand (order of 2000kg/m³), which is needed to obtain a sufficiently high total mass of the mass body in the present invention embodiments (see below for exemplary calculations).

Fig. 3 shows a cross sectional view of a further embodiment of the energy storage system, where the track 2 is implemented and formed inside a tunnel 3, which is e.g. provided below ground level in a rock formation 3a. This allows a relatively easy implementation as tunnel building is a well known art. The rock formation 3a (or other underground material depending on the local geology) provides for the support and rigidity needed for the tunnel 3 and track 2. The tunnel 3 (and hence track 2) can be provided with a substantially horizontal and level orientation, reducing building costs and also eliminating the need for any vertical structure in other energy storage systems (hydro-power, Maglev Potential energy Storage (MAPS), etc.). The track 2 may be provided as a 'normal' tunneled track 2, i.e. wherein the track (such as rails) are provided on the lower part of the tunnel. However, as in the present invention embodiments high speeds are obtained, it is advantageous to have the track 2 moved more upwardly towards the outside perimeter of the tunnel 3. Also it would be conceivable to have a multi-rail system as implementation of the track 2, with rails in the lower, outer and even upper part of the inside surface of the tunnel 3. Especially when using a maglev system, multiple rails may be used to form the track 2 for the one or more carriages 5.

In the embodiment shown in Fig. 3, the energy conversion means are combined with the low friction supporting system in the form of a magnetic levitation system 4. A linear induction motor 4a is positioned parallel to the track 2, and associated

components 4b are provided on the one or more carriages 5. Various implementation variants may be considered, although having the more costly and sensitive components installed as fixed non-moving parts on the track side is of course more advantageous. In a further exemplary embodiment, the linear induction motor 4a, and optionally also the components 4b on the one or more carriages 5, are implemented using superconducting elements.

In the embodiment of Fig. 3, each of the one or more carriages 5 is provided with a low friction outer surface 5b, which reduces the friction losses of the energy storage system, in particular of the one or more carriages 5 when moving, especially at high speeds. To further reduce the friction losses, the tunnel may be kept under lowered atmospheric conditions, i.e. a medium or even low vacuum environment.

Fig. 4 shows a top view of a further embodiment of a track 2 for the present energy storage system. In one embodiment, the track 2 has a plurality of sections 2a-2d, wherein one or more of the plurality of sections 2a-2d has a radius r of at least 1000m, e.g. 2500 m. This will limit the centrifugal forces during operation of the energy storage system, yet provide a practical and realizable implementation thereof. Also, as opposed to the embodiment of the track 2 shown in Fig. 1, the track of the Fig. 4 embodiment has a plurality of sections 2a-2d which comprise one or more straight sections 2b, 2d. This can lengthen the track distance in an easy manner, and thus additional length of the chain of carriages 5 and the total mass thereof, and thus the energy storage capacity of the entire system. The required space for the track 2 can then also be minimized, allowing easier and more cost-effective realization of the energy storage system. Furthermore, the straight sections 2b, 2d allow for an easier implementation of accelerating the one or more carriages 5, especially if one or several linked carriage trains are on the track 2.

In one embodiment, a length of the one or more carriages 5 corresponds to a length of the track 2. This embodiment would resemble a conventional flywheel as far as possible, as the entire track 2 (and tunnel 3) is 'filled' with mass. Alternatively, a length of the one or more carriages 5 is smaller than a length of the track 2. This embodiment would allow a more robust control of the moving mass body, and also would allow to have two or more 'trains' or chains of carriages 5 on the track 2.

In the following, calculations are made for design parameters of an exemplary embodiment of the energy storage system according to the present invention.

An assumption is made that storage is needed for 20 000 MWh (=72 TJ), which is roughly similar to the capacity of an energy island using hydro-power energy conversion.

For a kinetic energy storage system, assuming a maximum operational speed of 500 m/s, a moving mass is needed of 576x10⁶ kg = 576000 ton. This results in an energy density of 125 kJ/kg, as compared to typical flywheel values at 360-500 kJ/kg, lead-acid batter value of 170 kJ/kg. For hydropower, this energy density corresponds to a height difference of 12.7 km.

At a density of 2000 kg/m³ (high density material 5a, such as stone or wet sand), 288 000 m³ of material is needed. For comparison: the lake of the energy-island displaces a volume of 1.6xl0⁹ m³, or 1.6xl0¹² kg.

Assuming a tunnel with a radius of 2500 m (diameter of 5 km), a train occupying the entire length of the tunnel would be 15.7 km long. The cross section of the tunnel would have a surface area of 18.3 m² and a cross-section diameter of 4.8 m. At this velocity and radius of the tunnel, the centrifugal acceleration would be (V²/r)= 100 m/s² which is about 10 g. Going slower would reduce this centrifugal acceleration more than enlarging the diameter, but would increase the required mass quadratic.

It is of course desired that there is a high efficiency in the cycle. Losses are assumed to mainly arise from friction of the air and with the support of the chain of carriages. For comparison, the efficiency of pumped-hydro systems is about 81%.

Assuming <2% energy loss per day (per kg that is 2500 J per day of: 0.029 W, Loss force (P/V) per kg is 0.58 mN), and assuming that the friction is distributed as 1/3 for air drag and 2/3 for support friction, would provide the following calculations.

Drag is mainly the so called 'skin' friction:

_ 1 where p is the density of the medium in which the chain of carriages 5 moves, V is the velocity of the chain of carriages 5, A_s is the surface area of the 'skin' of the chain of carriages 5 and C_d is the friction coefficient.

The friction coefficient C_d is dependent on the state of the flow and the Reynolds number. Assume: C_d = 0.0019 (which is typical for a laminar flow, and a Reynolds number of 5x10⁵).

Skin friction per meter train would then be: πϋ

Mass per meter train: 1/4πϋ² x 2000

Skin drag is: 0.5 x 500² p DC_d

Force per mass: 0.098 p = 0.19 mN

The pressure in the tunnel 2 would have to be about 1.9 g/m³, which is about l/500th of one atmosphere. This is considered a 'medium' vacuum.

For the support friction an assumption can be made that the losses behave comparable to normal 'rolling' friction, where friction force = c*normal force.

Normal force per kg at 100 m/s² is 100 N, thus the friction coefficient c should be 3.8xl0^"6. For comparison: Steel on steel rolling (i.e. a normal train) has a rolling resistance coefficient of 3.0xl0^"4, air bearings have a rolling resistance coefficient in the order of lxlO^"5. The required friction coefficient c is thus a factor 3 smaller than that of an air bearing, which can e.g. be provided by using a magnetic levitation system 4, well known from e.g. high speed Maglev trains. The losses due to support are likely to be higher than those due to skin friction. When looking at trend and scaling, the following observations can be made. When the maximum operational velocity increases (the radius of the tunnel ring, density and the total stored energy remain the same), the loss forces must be reduced to get a similar amount of loss/unit energy, because a larger distance is travelled. The total mass reduces as velocity squared, but the required centripetal acceleration increases quadratic. The normal force/unit energy remains the same, the support friction/unit energy remains the same, energy loss/day/unit energy due to this friction therefore increases with velocity (due to the higher distance travelled). The skin friction/unit energy increases as velocity increases, mass decreases quadratic, the cross section diameter of the train reduces as 1/V. The surface area also decreases as 1/V, thus friction/unit energy increases with V. To compensate, the air pressure could be reduced even further. Thus, the losses/unit energy increases (linearly) with speed, and the required mass reduces quadratic.

In the present invention embodiments, operational use can thus be envisaged in an embodiment where the operational velocity range of the one or more carriages 5 is from 0 to at least 500 m/s. In a further embodiment, a minimum operational speed can be implemented, e.g. at least 100 m/s, to ensure continued proper operation of the magnetic levitation system 4. The total mass of the one or more carriages 5 is in a further embodiment at least 100 million kg, e.g. more than 500 million kg. This allows to have an operational capacity of the energy storage system of about 20000 MWh as shown by the above calculations.

By having the structure of the various embodiments as described herein, known building techniques can be used to build an actual system at economic costs. The building cost can be recuperated already based on e.g. the energy price variation during daily or longer periods (i.e. store when high production is available (low cost per MWh) and provide energy to grid when demand is high (high cost per MWh).

Cost can be further controlled by building the energy storage system in a controllable climate, no transport of people is needed, and it could be built outside of built-up and populated areas. The area occupied by the train of carriages 5 (if built underground) can also be used for other purposes, closer to existing grid.

The present invention embodiments have been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1. Energy storage system comprising a mass body and energy conversion means arranged to convert electrical energy into kinetic energy of the mass body,

wherein the mass body comprises one or more carriages (5), and the energy storage system further comprises a track (2) in the form of a continuous loop supporting the one or more carriages (5) in an operational velocity range.

2. Energy storage system according to claim 1, wherein the one or more carriages (5) are supported in the track (2) using a very low friction supporting system (4), e.g. a magnetic levitation system.

3. Energy storage system according to claim 1 or 2, wherein the one or morecarriages (5) are linked together using a flexible joint (6).

4. Energy storage system according to any one of claims 1-3, wherein the one or more carriages (5) is loaded with high density material (5a).

5. Energy storage system according to any one of claims 1-4, wherein the energy conversion means comprise a linear induction motor (4a) positioned parallel to the track, with associated components (4b) provided on the one or more carriages (5).

6. Energy storage system according to any one of claims 1-5, wherein the track (2) has a substantially horizontal, level orientation.

7. Energy storage system according to any one of claims 1-6, wherein the track (2) is formed inside a tunnel (3).

8. Energy storage system according to claim 7, wherein the tunnel (3) is kept under lowered atmospheric pressure conditions.

9. Energy storage system according to any one of claims 1-8, wherein the one or more carriages (5) are provided with a low friction outer surface (5b).

10. Energy storage system according to any one of claims 1-9, wherein the track (2) has a plurality of sections (2a-2d), wherein one or more of the plurality of sections (2a- 2d) has a radius (r) of at least 1000m, e.g. 2500 m.

11. Energy storage system according to any one of claims 1-10, wherein the track (2) has a plurality of sections (2a-2d), wherein the plurality of sections (2a-2d) comprise one or more straight sections (2b, 2d).

12. Energy storage system according to any one of claims 1-11, wherein a length of the one or more carriages (5) corresponds to a length of the track (2).

13. Energy storage system according to any one of claims 1-11, wherein a length of the one or more carriages (5) is smaller than a length of the track (2).

14. Energy storage system according to any one of claims 1-13, wherein the operational velocity range of the one or more carriages (5) is from 0 to at least 500m/s.

15. Energy storage system according to any one of claims 1-14, wherein the total mass of the one or more carriages (5) is at least 100 million kg, e.g. more than 500 million kg.

16. Energy storage system according to any one of claims 1-16, wherein some or all of the one or more carriages are linked together to form a chain of carriages (5)