GB2506652A

GB2506652A - An energy storage system and a method of storing energy

Info

Publication number: GB2506652A
Application number: GB1217871.1A
Authority: GB
Inventors: Ha Kan Fredrik Serck
Original assignee: CASU VASU AS
Current assignee: CASU VASU AS
Priority date: 2012-10-05
Filing date: 2012-10-05
Publication date: 2014-04-09
Also published as: WO2014053662A1; GB201217871D0

Abstract

An energy storage system is described comprising a closed gas circuit which comprises an expansion chamber 2, a compression chamber 3 coupled to the expansion chamber, a compressor for compressing gas from the expansion chamber as it is delivered to the compression chamber, a turbine 5a for recovering energy stored by the gas in the compression chamber as the gas is released back into the expansion chamber and a generator coupled to the turbine 5b for generating electrical energy from the recovered energy. The compression chamber is located within the expansion chamber, so that heat generated as the gas is compressed is conducted through the wall of the compression chamber and into the expansion chamber directly. The system can be powered by solar energy or any other form of renewable energy, with the energy being stored where it is produced and where the electrical energy is to be consumed.

Description

Energy Storage System and Method of Storing Energy The present invention relates to an energy storage system and a method of storing energy, for example, using the system. In particular it relates to a system that receives electrical energy and stores the energy through compressing a gas. The present invention also extends to the associated method of storing energy in an energy storage system.

With the recent improvements in local electricity generation, it is becoming more popular for households to generate their own electricity during the day from solar or wind power and store it for use during the night. Typically the energy is stored using batteries so that it can power electrical loads such as lighting.

Batteries have a high energy storage density and offer reasonably high efficiency in terms of the energy storage and release cycle. However, they also have a number of disadvantages. For example, batteries contain chemicals that are harmful to the environment and a battery can only be charged a certain number of times before it will need to be replaced.

Alternative energy storage systems to batteries are known. For example, it is known that inputted electrical energy can be stored by compressing a gas, releasing the energy later, as required, by using the expansion of the higher pressure gas to drive a turbine and generate electricity. The apparatus typically comprises two or more pressure vessels connected by pipes, the gas being compressed by a compressor into a chamber of one vessel on one side of the system, and released into a chamber of a vessel on the other side of the system via a turbine. The compression and expansion chambers are located in separate pressure vessels.

Some energy storage systems based on this principle are described in US-A-2011/0204064. This document describes how certain gas-based, energy storage systems can achieve energy densities that compare well to lead-acid batteries. However, the reality is that without complicated systems to control the operations, the storage and release cycle is generally much less efficient than for batteries.

It is well known that when a gas is compressed, its temperature increases.

As the gas is released from a compression chamber and allowed to expand again, its temperature will decrease.

It is also known, for example, from US-A-201 1/0204064, that if the compression stage is performed isothermally, it will reduce the amount of work required to compress the gas and make the process more efficient. A number of solutions have been proposed to reduce the temperature of the gas as it is compressed, for example, a water spray may be introduced to transfer heat out of the gas in the compression chamber.

US-A-201 1/0204064 also suggests that near iso-thermal performance can be achieved by operating the process slowly so that there is time for the heat to conduct through the walls of the chambers, e.g., to flow out of the compression chamber or to flow into the expansion chamber. However difficulties are seen in terms of the scalability of such an arrangement.

Another problem with the prior art gas-based energy storage systems is that they are often quite complex and expensive to install, and so they do not offer a proper turn-key solution.

It would be desirable to provide an improved energy storage system which can reduce the temperature during the compression cycle while avoiding some of the complications associated with the prior art arrangements.

According to the present invention there is provided an energy storage system, the system comprising a closed gas circuit which comprises: an expansion chamber; a compression chamber coupled to the expansion chamber; a compressor for compressing gas from the expansion chamber as it is delivered to the compression chamber; a turbine for recovering energy stored by the gas in the compression chamber as the gas is released back into the expansion chamber; and a generator coupled to the turbine for generating electrical energy from the recovered energy, characterised in that the compression chamber is located within the expansion chamber.

An advantage of this system is that it offers a simple solution for removing the heat produced during compression. The heat generated as the gas is compressed is conducted through the wall of the compression chamber and into the expansion chamber directly. This reduces the work required to compress the gas because the temperature in the compression chamber is reduced. Also since the heat passes directly into the expansion chamber it compensates for the reduction in temperature in that chamber. This further reduces the work required to extract the gas from the expansion chamber because the temperature in that part of the system will be higher than if the heat from the compression chamber had been kept separate.

Thus preferably the entire surface area of the compression chamber can act as a route for conducting the heat to the expansion chamber.

Viewed from another aspect, the present invention can also be seen to provide a method of storing energy, wherein during storing of the energy, heat passes directly through a wall of a compression chamber into an expansion chamber. In particular, the present invention provides a method of operating an energy storage system, preferably the energy storage system as described above, where the compression chamber is located within the expansion temperature, wherein during the storing of the energy, heat passes directly through a wall of a compression chamber into an expansion chamber. In this way, the heat from the compression chamber is able to compensate substantially for a drop in temperature of the gas within the expansion chamber as the pressure of the gas in the expansion chamber is reduced.

The compression chamber may be constructed from a metallic material, such as stainless steel, which allows heat to be conducted through the wall of the pressure chamber easily.

However, non-metallic materials are becoming more popular for these types of devices and in practice may allow the heat to transfer satisfactorily, as the compression cycle takes place slowly over several hours. Consequently non-metallic materials such as fibre reinforced resin materials could also be used for the compression chamber.

A perceived downside of the energy storage system of the present invention compared to batteries is that the poor efficiency of the energy storage cycle makes the system look undesirable. A battery, for example, will be able to store and release the energy on demand with losses of less than about 10%, i.e., efficiency of about 9Q% or more. By contrast, the energy storage and release cycle in the present invention, while more efficient than many of the prior art compressed gas systems, may still lose up to half of the electrical energy that was input originally into the system.

However, a number of economic factors have influenced the potential of this new, gas-based, energy storage system. On the one hand solar energy from photo-voltaic cells has become much cheaper as the costs of silicon cell production have dropped significantly in recent years. Also local generation of electricity close to where it is to be used is now becoming more popular and is often encouraged for new developments, so that wherever possible they can be off-grid or classified as zero-carbon. In addition the power consumption of many electrical appliances and loads have become less, for example, the power consumption of lighting has reduced dramatically through the use of LEDs.

Indeed the costs of energy production using solar cells has dropped to a point where the comparative lack of efficiency for the energy recovery is no longer considered a problem on the cost viability of the complete energy storage installation, particularly when the reduced power consumption of certain loads such as lighting is taken into account.

Preferably the gas in the energy storage system is nitrogen. Nitrogen has the advantage that it is cheap and non-polluting. In addition, nitrogen is essentially moisture free, avoiding the problems this moisture can induce through freezing and corrosion. Moreover nitrogen has a very low freezing point itself which avoids any problems it could have with freezing in cold remote environments, particularly when the gas is released back through the turbine into the expansion chamber and additional cooling results from the expansion of the gas. Other gases could also be used where appropriate.

The energy storage system is based on a closed gas circuit. In other words the overall volume of gas in the entire system remains constant and does not vary as it is passed from the expansion chamber into the compression chamber and back again.

Thus the system can be seen to comprise a single pressure vessel that defines internally an expansion chamber, where during the storing of energy, the gas will be at a lower pressure, and housed within that pressure vessel there are one or more compression chambers where the gas will be at a higher pressure, the one or more compression chambers being in fluid communication with the expansion chamber via a connecting fluid circuit that contains a compressor and a turbine. The connecting fluid circuit may comprise two sections; one for the compression flow and one for the expansion flow. More preferably the fluid circuit comprises only one section that is used for both the compression flow and the expansion flow.

The energy storage system is preferably pressurised at an overall pressure (or this could be referred to as a "neutral" pressure, i.e., when the pressure in the compression chamber equals the pressure in the expansion chamber) of about 10 bar (1 MPa) or higher. More preferably this overall (or neutral) pressure is 20 bar (2MPa) or more, more preferably still it is 30 bar (3MPa) or more, and most preferably it is 40 bar (4MPa) or higher.

Preferably the energy storage system is configured so that it can be buried in the ground, close to the energy supply (e.g., the solar panels, wind turbine or other source of electricity) and also close to the energy load (e.g., the building or lamp post). In this way, one or more units of the energy storage system can be provided locally, where they are needed, allowing loads such as domestic or public lighting, for example, in remote areas, to be supplied with electricity without needing a connection to the electricity grid. Such energy storage units may, of course, also be used where there is already a connection to the grid as a way to supplement or replace the existing grid electricity usage.

A preferred arrangement is where a pressure vessel, which internally defines the expansion chamber, also houses a plurality of smaller pressure vessels defining the compression chambers. In one embodiment where the expansion chamber is generally cylindrical having domed ends, the compression chambers, which may also be of similar cylindrical shape, are arranged in a ring within the expansion chamber. In another embodiment, the expansion chamber is provided by a spherical pressure vessel and the compression chambers may comprise smaller spherical or other shaped pressure vessels within.

Both the compressor and turbine are also preferably housed within the pressure vessel that forms the expansion chamber. In one embodiment the compressor and turbine are provided by the same piece of equipment, which acts as a compressor when operated in one direction and as a turbine when operated in the other direction. The compressor and turbine should be optimised as far as possible to operate at low speeds, since preferably the compressor/turbine will be receiving electrical energy from solar panels and/or other energy sources over a period of many hours during the day, and then it will need to be switched over to release this stored energy over an extended period, allowing lighting and other electrical loads to be powered for as long as possible during the night.

The pressure vessel forming the expansion chamber can be made in two (or more) parts to facilitate the assembly of the system, with the compressor/turbine and the one or more compression chambers housed within the expansion chamber.

By providing the components within the pressure vessel that forms the expansion chamber, it also provides a neat and compact, turn-key product that can be installed easily where it is needed, e.g., where the energy is consumed, either singularly or as part of a rack of energy storage units.

Certain preferred embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which: Fig. 1 illustrates a preferred energy storage system; Fig. 2 shows a preferred energy storage system comprising a plurality of compression chambers located within an expansion chamber; Fig. 3 illustrates units of the energy storage system being used to supply electricity for domestic and street lighting; and Fig. 4 illustrates a schematic representation of a closed fluid circuit of the energy storage system.

A preferred energy storage system is illustrated in Fig. 1. It comprises a pressure vessel 1, the internal surface of which defines an expansion chamber 2 (which we will refer to in the following text as the "Vasu"). This Vasu 2 contains a gas which will normally be at a pressure lower than atmospheric when energy is being stored. Housed within the pressure vessel 1 is a compression chamber 3 (which we will refer to in the following text as the "Casu") which is able to store the gas at a higher pressure than the Vasu 2 when energy is being stored.

The energy storage system is a closed gas system, in that the volume of gas within the pressure vessel 1 remains constant. Inside the pressure vessel 1, the gas can distribute itself between the Vasu 2 and the Casu 3. A port (not shown) can be provided to introduce the gas initially.

In this embodiment the gas is nitrogen rather than air, since that will mean it will be dry, thereby avoiding problems with moisture in the gas freezing, and inert, so that it will not create a hazard in the event that gas is released.

Thus, in comparison with an air-filled system, the pressure vessel 1 will contain gas which is at least 95 wt.% nitrogen, more preferably still at least 97 wt.% nitrogen, and most preferably it will contain 99 wt.% or more nitrogen. The nitrogen does not need to be a high purity source.

The Casu 3 is less than half the volume of the Vasu 2, e.g., preferably about a third or less of the volume of the Vasu 2, more preferably around a quarter or fifth of the volume. The actual comparative volumes will depend on the maximum pressures that are desired within the chambers.

An example is a Vasu 2 of 800 litres holding a Casu 3 of 200 litres (i.e., the pressure vessel is of the order of 1000 litres to house the Casu 3). A system of this size would take up around 1.3m3 when in the form of a spherical pressure vessel, which would make it an appropriate size for storing in the basement of a house or perhaps underground near a lamp post, i.e., close to where the energy is to be consumed. The Casu 3 and Vasu 2 of the present invention may, of course, be other sizes as desired for the electrical load that the system needs to supply.

As energy is stored, the pressure in the Casu 3 will begin to rise and the pressure in the Vasu 2 begin to fall. The pressure in the Casu 3 when the maximum level of energy in the storage system has been reached might be in the range of 100-300 bar (10-30 MPa), more preferably in the range of 150-250 bar (15-25 MPa) and most preferably around 200 bar (20 MPa).

Thus, by way of example, when the maximum of energy is stored, this could be when the Casu 3 reaches a pressure of around 200 bar (20 MPa) while the Vasu 2 drops to around 50 mbar (5kPa) or less.

During recovery of the stored energy, the pressure in the Casu 3 will fall as the pressure in the Vasu 2 rises. When all the energy stored in the system has been recovered, the pressures in the Casu 3 and Vasu 2 will equalise.

Preferably in this "neutral" position, the Casu 3 and the Vasu 2 are both under an overall pressure of 10 bar (1 MPa) or more. More preferably the pressure is 20 bar (2 MPa) or higher, for example 40 bar (4 MPa).

The pressure vessel 1 providing the Vasu 2 may be made of a suitable metal or other material such as a fibre reinforced plastic, and is preferably provided with insulation (not shown), either externally or internally. The Vasu 2 can be of any shape but is preferably a generally cylindrical shape as shown, comprising a substantially cylindrical body and domed ends, or it may be any other shape such as a spherical vessel. It can be arranged vertically as shown in the figure, horizontally or in any other orientation. It may be of any suitable size as appropriate.

The Casu 3 is housed within the Vasu 2. The Casu 3 may be made of metal or other similar heat conductive material but equally other more insulative materials like fibre reinforced plastics can be used because the heat conduction does not need to be rapid. It further needs to be able to withstand the higher pressure of the gas within the Casu 3 when the maximum amount of energy is stored. Again it can be any shape and orientation, but preferably it is a similar shape to the Vasu 2 (e.g., cylindrical or spherical, etc.) and a similar orientation.

The Vasu 2 may be formed from two or more parts in order to allow the installation of the Casu 3 within.

Also within the pressure vessel 1 there is provided a fluid path to complete the closed gas circuit, the fluid path allowing the gas to flow from the Vasu 2 to the Casu 3 during the energy storage part of the cycle, and vice versa, to return from the Casu 3 to the Vasu 2 when the energy is being recovered.

In the embodiment shown the fluid path is split into two sections 4a, 4b. The gas flow from the Vasu 2 to the Casu 3, as the gas is being compressed and energy is being stored, is driven by a compressor 5a as the gas passes through the first section 4a of the fluid path. The flow from the Casu 3 to the Vasu 2 as the gas is being released into the Vasu 2, passes through a turbine Sb in a second section 4b of the fluid path, in order to recover the stored energy.

However, embodiments are also envisaged where there is instead a common fluid path for both directions of flow to and from the Casu 3, and indeed this may be the more preferred configuration because of its simplicity. Thus, the compressor 5a and turbine 5b can be the same component, with the component being operated in different directions to perform the compression or electrical energy generation operations.

The turbine 5b is used to generate electricity and may either incorporate an integral electrical generator or the generator could be a separate component that is driven mechanically by the turbine 5b.

While it is preferred to house these components within the pressure vessel 1, in some instances, it may be desirable to have one or more of the compressor 5a, turbine 5b and generator (if not already integral with the turbine) located external to the pressure vessel 1.

The compressor 5a may be driven by any suitable renewable energy source, but preferably it is driven by an array of solar panels and/or a wind turbine or other electrical energy source.

Fig. 2 illustrates a modified embodiment in which the pressure vessel 1 contains a plurality of Casus 3. In the figure, the Vasu 2 is indicated by an "A" and four of the Casus 3 are indicated by a "B". The Casus 3 may be arranged in a ring within the Vasu 2 or in any other desired configuration.

Such an arrangement of multiple Casus 3 also increases the surface area to volume ratio of the Casu 3.

Fig. S illustrates an example of a complete power system where solar panels and/or wind turbines are used to power the compressor 5a. As the price of solar cells has dropped considerably, the energy generation capacity can exceed the normal daily load required by the user without this having a significant impact on the overall cost of the installation. Thus an array of energy storage system units may be provided according to the desired power load. As described by way of example above, each unit may be of the order of 1 to 2rn3, most preferably 1 3m3, and the array may be located beneath the ground, in the basement of a building or some other such location close to where the energy is to be consumed.

As examples of the possibilities of the system, during the day, ten to fourteen hours of charging may be possible at, say, up to 300 watts per panel, and this may input around 2 kWh into the energy storage system as an average across the day. This can be used to generate pressures in the Casus 3 close to 200 bar (20 MPa). The corresponding pressure in the Vasu 2 may have dropped to below 0.2 bar (0.02 MPa), for example, 50 mbar (5kPa). By releasing the compressed gas from the Casu 3 into the Vasu 2 via a turbine and generator, it may be possible to recover around 1 kWh or more of electrical energy during the night. Additional units may be provided, for example a rack of, say, thirty two 1.3m3 energy storage systems could be coupled with a 100 m2 of photovoltaic panels to achieve in excess of 20 kWh in an average daylight cycle, which will meet most household needs.

Fig. 4 illustrates schematically an example of the closed gas circuit. The Vasu 2 is coupled to the Casu 3 by a first section 4a of a fluid path that extends through a pair of isolation valves 6 and a compressor 5a. The return section 4b of the fluid path couples the Casu 3 to the Vasu 2 and extends through a pair of isolation valves 6 and a generator Sb. In the preferred arrangement where the compressor Sa and generator Sb are the same component, only one of section would be present providing both fluid paths. Also only one pair of isolation valves 6 would be needed.

The present invention offers many advantages because it stores the energy where it is produced, and most importantly where the electrical energy is to be consumed. This makes the system particularly suitable for remote applications where there is no electricity grid supply. For example, the invention can be used for domestic power in remote locations, e.g., cabins and the like. However, it is also suitable for domestic loads where there is already a connection to the grid. The "free" electrical energy stored in the system may be used to supplement "bought" electrical energy from the grid or even replace it. It is also suitable to power road or path lighting, with one or more energy storage system units being used at locations spaced along the road or path to power one or more lights, providing savings through not having to install electrical power lines to such electrical loads.

Claims

Claims: 1. An energy storage system, the system comprising a closed gas circuit which comprises: an expansion chamber; a compression chamber coupled to the expansion chamber; a compressor for compressing gas from the expansion chamber as it is delivered to the compression chamber; a turbine for recovering energy stored by the gas in the compression chamber as the gas is released back into the expansion chamber; and a generator coupled to the turbine for generating electrical energy from the recovered energy, characterised in that the compression chamber is located within the expansion chamber.
2. A system as claimed in claim 1, wherein the gas contained in the closed gas circuit is at least 95 wt.% nitrogen.
3. A system as claimed in claim 1 or 2, wherein the gas contained in the closed gas circuit is substantially free of water vapour.
4. A system as claimed in claim 1, 2 or 3, wherein the compression chamber is coupled to the expansion chamber by a fluid path which includes the turbine for driving the generator.
5. A system as claimed in claim 4, wherein the fluid path further includes the compressor.
6. A system as claimed in claim 4 or 5, wherein the turbine is provided by the compressor being operated in a reverse direction.
7. A system as claimed in any preceding claim, wherein the compression chamber is free of thermal insulation.
8. A system as claimed in any preceding claim, wherein the expansion chamber houses a plurality of compression chambers.
9. A system as claimed in claim 8, wherein the compression chambers are arranged in a ring within the expansion chamber.
10. A system as claimed in claim 8 or 9, wherein three or more compression chambers are fed gas from the expansion chamber by a single common compressor.
11. A method of storing energy in an energy storage system as claimed in any preceding claim, wherein during storing of the energy, heat passes directly through a wall of the compression chamber into the expansion chamber.
12. A method as claimed in claim 11, wherein during storing of the energy, the heat from the compression chamber compensates substantially for a drop in temperature of the gas within the expansion chamber as the pressure of the gas in the expansion chamber is reduced.
13. A method as claimed in claim 11 or 12, wherein the method includes pressurising the system to an overall pressure of 10 bar (1 MPa) or more when the pressures inside the compression chamber and expansion chamber are equal, more preferably 20 bar (2MPa) or more.
14. A method as claimed in any of claims 11 to 13, wherein energy is stored in the system by drawing gas from the expansion chamber to compress a remainder of the gas within the compression chamber up to a pressure of at least 150 bar (15 MPa), more preferably 175 bar (17.5 MPa).
15. An energy storage system substantially as hereinbefore described with reference to the accompanying drawings.
16. A method of storing energy substantially as hereinbefore described with reference to the accompanying drawings.