Field

[0001] The invention relates to a grid energy firming process, in particular to a process for the grid energy firming of chemical energy, to a system for grid energy firming and to the use of the system in the reversible storage and release of energy.

Background

[0002] Grid energy firming and storage is essential to the constant provision of electricity to any large-scale electrical power grid. In order for the grid to supply power at constant voltage, supply of energy into the grid and demand for energy from the grid must be closely matched at all times. Demand for energy from the grid varies according to daily and seasonal cycles and while it can be predicted to a large degree, it cannot be predicted with complete certainty. Supply of energy into the grid results from the sum of supplied energy from the power suppliers attached to the grid. Some of these power sources can be considered pretty much constant (e.g. nuclear - it's either on or its off - or to a lesser extent coal-fired power). Other sources can be turned on and off more rapidly (e.g. gas- fired power) or even instantly (e.g. pump storage). Still other sources such as wind, solar, wave and tidal (which all happen to be renewable sources) are variable. While to a large extent tidal provides a predictable flow of power, the other renewable sources vary in the amount of power that they supply at any given time depending on the weather conditions.

[0003] The requirement to match supply and demand results in a need to store energy when supply exceeds demand and release energy from those stores when demand exceeds supply. This issue is becoming increasingly prominent as the proportion of energy from renewable sources (which also happen to be intermittent sources) increases. The increase in the proportion of renewables on the grid is because of concerns about climate change and the need to move towards decarbonising our economy. So renewable, in attempting to address one problem (climate change) create another problem (requirement for more energy storage capacity).

[0004] The existing paradigm relating to energy storage is to try to store excess energy from renewable sources - that is to make the energy supply from intermittent sources more constant. By storing 'excess' renewable energy at times of over-supply and releasing it when there is excess demand, renewable (plus storage) can provide a flow of power that better matches demand.

[0005] Conventional energy storage on the grid is typically gravitational pump storage. At times of excess electricity on the grid, it is used to pump water from a low reservoir to a high reservoir. When demand exceeds supply, the process is reversed, with water being allowed to flow from the high reservoir to the low reservoir driving a turbine and generating the electricity required to balance the grid. This process, while simple, is expensive to run - there are 'roundtrip' efficiency losses and the low energy density of storing energy in pumped water necessitates very large volumes to be pumped and associated high capital costs.

[0006] Grid energy firming systems therefore effectively provide for a load levelling of power into the grid, such that a constant supply can be provided.

[0007] Some chemical storage techniques are known, including battery storage and the storage of hydrogen. However, these methods are generally both costly, and in particular in the case of batteries, prone to poisoning of the system which requires regular maintenance and replacement. Therefore there remains a need for a grid energy firming system which is reliable, flexible, carbon neutral or carbon negative, inexpensive and can be rapidly implemented.

[0008] The invention is intended to overcome or ameliorate at least some aspects of these problems.

Summary

[0009] Accordingly, in a first aspect of the invention there is provided a process for the grid energy firming of chemical energy comprising:

a) forming a first carbon-containing product through an endothermic reaction; b) storing the first product; and

c) exothermically reacting the first product to form a second carbon- containing product of greater thermodynamic stability when electricity is needed.

[0010] It will be understood that whilst the invention is primarily for grid energy firming, and so this terminology is used, it could also, in some cases be applied to grid energy storage. [0011] As the first product is formed through an endothermic reaction, the first product is generally thermodynamically unstable. This provides for a system where the first product readily reacts to form the second product, releasing the energy stored through the formation of the chemical bonds in the endothermic reaction, and so the system offers the benefit of reversible storage and release of the energy on demand. In addition, the use of an endothermic reaction provides for a system wherein the first product is energy enriched compared to the materials from which it was formed, because energy must be supplied to the system in order that the first product be formed. This energy will generally be heat energy, which is converted to chemical energy through the bond formation between the carbon and other atoms in the first product. There is therefore provided a system which offers grid storage of energy in an "energy enriched" form, as the process of the invention does not simply harvest the energy from carbon-containing molecules as reaction products of exothermic reactions, but deliberately creates high energy carbon-containing molecules for storage and later release of the energy.

[0012] Further, the selection of carbon-containing products is advantageous as bonds to carbon are stable, yet easy to break under moderate reaction conditions, allowing for stable storage of the product, yet rapid release of the energy when this is needed, ensuring that the flow of energy to the grid is continuous, even during the transition between the primary energy source (for instance a variable energy source) and the grid storage.

[0013] Currently, excess electricity from variable energy sources is generally converted into kinetic energy (driving the pumps to push the water uphill) and then into gravitational potential energy. When the energy is released from storage the process is reversed - gravitational potential energy is converted to kinetic energy as the water falls, driving a turbine and is turned into electrical energy.

[0014] In the invention, it will generally be the case that heat energy is used to drive endothermic reactions thereby transforming heat energy into chemical energy. That chemical energy can then be stored (with far higher energy density than any gravitational pump storage method). When energy is required, the chemical energy is converted (through a fuel cell) into electrical energy.

[0015] The key difference is that known processes take energy in a high grade form (electricity) and generally convert it into lower-grade forms of energy (such as kinetic energy in pump storage). The process above takes fairly low grade energy (heat) and transforms it into chemical energy. As heat is very difficult to store, this conversion also provides for easy storage of the energy.

[0016] This makes both variable and base-load power sources more flexible and so it then becomes possible to increase the proportion of intermittent (renewable) sources on the grid. In one example of the invention, the grid could take as much or as little energy from variable power sources as happened to be flowing at that time and then balance supply with demand through use of the process claimed, the chemical energy being topped up by supply of heat energy from conventional sources when demand is lower than supply.

[0017] As used herein the terms "first product" and "first carbon-containing product" are intended to be used interchangeably. Similarly, the terms "second product" and "second carbon-containing product" are intended to have the same meaning.

[0018] It will often be the case that the first carbon-containing product be a simple low molecular weight molecule as the synthesis of these is generally facile, and such products are easy to store, often being gases at room temperature. The gas can therefore be compressed for storage, although other methods such as further chemical conversion are also possible.

[0019] Often, the first carbon-containing product is selected from carbon monoxide, carbon dioxide, methane, and combinations thereof. Combinations may be used in cases where more than one reaction is being carried out whether simultaneously or sequentially in step c) of the process, or because the reaction selected produces more than one product from the list provided above. It will generally be the case that a single reaction will be used, as this reduces the need to separate reaction products prior to storage. However, multiple reactions may be used, and the "first products" created may be stored together, or separated. Separation has the advantage that the products cannot react with one another during storage.

[0020] The second product will be a product derivable from the first product, and will typically be more thermodynamically stable than the first product, as a result of the release in energy required for transfer to the grid system. As used herein the terms "greater thermodynamic stability" and "more thermodynamic stability" are relative to the thermodynamic stability of the first product, whatever this may be. There is intended to be a measurable difference in the stability, such that the energy release from the exothermic reaction forming the second product is enough to warrant the selection of the reaction for storage. For instance, the change in enthalpy in the reaction, or reactions, of step c) may be in the range -50 to -1000 kJmol ^"1 , often in the range -200 to -900 kJmol ^"1 , in many cases in the range -300 to -600 kJmol ^"1 .

[0021] Often the second product will be selected from carbon dioxide, carbon and combinations thereof. Where multiple reactions are occurring, it may be that these products can be recycled and used as starting materials for the generation of the first product. Where recycling is not required and the second product is carbon dioxide, the carbon dioxide produced will typically be sufficiently pure (less than 1% impurities, for instance, in the range 0% to 1%, often in the range 0.0001% to 1%, or 0.01% to 0.5% impurities), to be directly sequestered. This provides for a method of "clean" energy storage, as the carbon dioxide produced is not released to the environment, and so does not contribute to the green house gases in the atmosphere. The carbon will typically be sold for use in alternative industries. It will often be the case that the second product will either not be recycled or will only partially be recycled, so that the system is not a closed loop system. This is particularly the case where the second product is carbon dioxide, as it is desirable to create a carbon neutral or carbon negative power generation system through sequestration of the carbon dioxide produced. This provides for a system where carbon is being consumed across the two reaction steps.

[0022] In view of the above, step a) of the process will often comprise a reaction selected from:

C0 ₂ + C→ 2CO,

C0 ₂ + CH ₄ → CO + 2H ₂ ,

CH ₄ + H ₂ 0→ CO + 3H ₂ ,

2C + 2H ₂ 0→ CH ₄ + C0 ₂ , and combinations thereof.

Most often, step a) will comprise the reverse Boudouard reaction, C0 ₂ + C— » 2CO. This reaction is a reversible endothermic reaction, requiring 172.5 kJmol ^"1 to drive the reaction to form carbon monoxide. Often, the reverse Boudouard reaction will be the primary, or even the only, reaction used in step a).

[0023] It should be noted that as used herein references to the energies required to drive the reaction, or which are released from the reactions, as cited in kJmol ^"1 are the enthalpy calculations at standard temperature and pressure. [0024] The heat may be generated by a dedicated power plant, the sole purpose being to store energy for release to the grid at times of high demand, and/or to smooth power output from the variable energy source. Any dedicated power plant would generally be using a constant fuel source, which could include one or more of nuclear fuel, fossil fuel, biomass, hydroelectricity and geothermal power. Often, the fuel source for a dedicated power plant would be selected from nuclear fuel, biomass or hydroelectricity, as these are clean forms of energy, often nuclear fuel will be used as nuclear power plants are not particularly limited by location, and offer a well tested and efficient method of electricity generation.

[0025] Alternatively, the heat may be generated from one or more variable energy sources, typically through the conversion of electricity to heat in one of a range of conventional manners. As used herein, the term "variable energy source" and similar terms are intended to mean sources of energy which are not constant, for instance because they are affected by their environments. Such sources include photovoltaic generation (variable because the energy can only be generated when there is sufficient ambient light), wind power (variable because the force of the wind is not constant), tidal power (variable with the ebb and flow of the tide), and wave power (variable as wave generation is related to the force of the wind, and the tide).

[0026] As a further alternative, a proportion of the energy generated by a conventional, for instance nuclear, power plant, may be diverted to such energy storage, for instance the heat that would otherwise be lost to the environment. This arrangement could be used to improve the overall efficiency of the power plant.

[0027] If the temperature of reaction drops, the equilibrium in reverse Boudouard reactions shifts to the left, and the carbon monoxide reverts to carbon dioxide. For this reason, it is desirable to maintain high operating temperatures (which favour the forward reaction as shown) where the reverse Boudouard reaction is being used.

[0028] Often the source of carbon will be coke, as this is generally the most cost effective source of carbon available, although other carbon sources may also be used. The coke will often be heated to a temperature in the range 800 - 1400K, on many occasions in the range 950 - 1250K, or in the range 1000 - 1250K. At these temperatures the equilibrium for the reaction is to the right, strongly favouring the formation of carbon monoxide.

[0029] Often, step c) of the process will comprise a reaction selected from: 2CO→ C0 ₂ + C,

CO + H ₂ 0→ C0 ₂ + H ₂ ,

CH4 + 2Ο2→ CO2 + 2H2O, and combinations thereof.

Most often, step c) will comprise the Boudouard reaction (2CO— » CO2 + C), in particular where the reaction of step a) is the reverse Boudouard reaction, as these reactions are complementary. Often, the Boudouard reaction will be the primary, or even the only, reaction used in step c).

[0030] It could therefore be said that the process comprises the steps of:

a) forming carbon monoxide as the first product from carbon dioxide and carbon in the reaction: CO2 + C→ 2CO;

b) storing the carbon monoxide; and

c) exothermically reacting the carbon monoxide to form carbon dioxide as the second product in the reaction: 2CO— » CO2 + C, when electricity demand is high.

Often the endothermic reaction of step a) is driven with heat generated from one or more variable energy sources; although alternatively the reaction of step a) may be driven by heat generated from a constant fuel source, as described above.

[0031] It may be the case that the carbon dioxide for use in the reaction: CO2 + C— » 2CO is generated through calcination of a metal carbonate. Often the metal carbonate will comprise calcium carbonate, magnesium carbonate or a combination thereof. Where calcium carbonate is used, this will often be in the form of limestone; where magnesium carbonate is used, this will often be in the form of magnesite as this is the dehydrated form of magnesium carbonate. With hydrated magnesium carbonates, the water of hydration, typically released as steam, must be separated from the carbon dioxide increasing system complexity and costs. Where calcium carbonate is used, the reaction will follow the formula:

CaCOs→ CaO + C0 ₂

The calcination reaction is endothermic, for calcium carbonate requiring 178 kJmol ^"1 to drive the reaction. Temperatures will generally also be as described above, so calcination can occur at temperatures in the range 800 - 1400K, on many occasions in the range 950 - 1250K, or in the range 1000 - 1250K. In some examples, this reaction could be driven using energy derived from a variable power source, or the dedicated constant power source described above, providing for a process comprising an additional step before step a), in which carbon dioxide for use in step a) is generated from the calcination of a metal oxide.

[0032] The storage of the first product is an important aspect of the invention. The removal of the reaction product once formed, and retention for future release of the chemical energy it contains, is the feature which allows the combination of reactions described herein to be used as a system for grid energy firming. Many reactions, such as the Boudouard reaction are in equilibrium, or can form reaction loops where through a chain of reactions the products are cycled until one or more products are consumed, for instance by the removal of carbon dioxide as a second product during sequestration. An aspect of the process of the invention is the temporal shift in these reactions, wherein a key reaction product, the first product, is removed from the system, only to be released when the energy stored within the molecule is required to smooth the energy supply during a dip in production from a variable energy source, or when energy demand is high.

[0033] In a second aspect of the invention there is provided a grid energy firming system comprising a reactor for forming a first carbon-containing product and/or a second carbon- containing product in fluid communication with a storage vessel for the first carbon- containing product. Often the system will comprise a first reactor for forming the first product, and second reactor for forming the second product; allowing separation of the reaction processes and easy removal of the first product for storage. In such cases, both reactors are generally in fluid communication with the storage vessel, although it may be that the storage vessel is in fluid communication with the first reactor during formation of the first product, and in fluid communication with the second reactor during formation of the second product.

[0034] In many cases, the second reactor will comprise a fuel cell, the fuel cell will often be a high temperature fuel cell, such as a solid oxide fuel cell (SOFC), although other high temperature fuel cells may also be used, including molten carbonate fuel cells (MCFC). The fuel cells used may be adapted so that carbon monoxide is the primary source of fuel, carbon monoxide may be the only source of fuel.

[0035] As used herein, the term "high temperature fuel cell" is intended to mean fuel cells with operating temperatures in the range 900 - 1250K, often in the range 950 - 1150K. In general usage, this term would often include operational temperatures as low as 700K, however, it is generally the case that in order to maintain the equilibrium in an endothermic reaction to the right (for instance, the reverse Boudouard reaction, favouring the production of carbon monoxide), higher temperatures will be used in the fuel cells used in the system and process of the invention. Often SOFC's will be used as these are resilient to carbon monoxide poisoning and so easily adapted for use in systems where the first product is carbon monoxide. Where the typical operating temperature of the fuel cell selected for use in the process of the invention is lower than 900K, the temperature can be increased. This may be through combustion of some of the first product to release it's energy, or through the use of heat derived from the external energy source, be this the variable energy source that the grid energy firming is intended to complement, or a dedicated energy source for the creation of the chemical energy to be stored in the systems of the invention.

[0036] The fuel cells may be arranged to multiply their energy production using typical methods such as the formation of fuel cell stacks or placing multiple fuel cells in series, as would be known to the person skilled in the art.

[0037] Storage will typically be for a time period in the range 30 minutes to 7 days, or 72 hours or 48 hours, often 1 to 24 hours. Storage may be using conventional methods, such as gas holders, or chemical methods such as the formation of complexes of the first product. An example of this could be the storage of the first product as a metal complex. For instance, carbon monoxide could be stored as a metal carbonyl complex, such as chromium hexacarbonyl (Cr(CO) ₆ ). The conversion of a gaseous first product to a metal complex solidifies the product, compacting it and allowing storage of the product in a much smaller volume than if stored as a gas, even if the gas is compressed. This provides for large scale production and storage of the first product, without the need for huge storage facilities for the gas, and without the problems of gas storage, such as leakage and often flammability. However, in some systems storage as a gas may be preferred as conversion to a solid, for instance to a metal complex, requires an additional reaction step which may require energy input, and can result in loss of product even high yielding reactions generally to not result in a 100% conversion of starting material to product. Where storage as a gas is required, the first reactor is often in fluid communication with the gas storage vessel, for instance the gas holder, during the formation of the first product; the second reactor is in fluid communication with the gas storage vessel during formation of the second product. Where conversion of the first product to a solid is intended, the first and second vessels will generally be in communication with a secondary reaction vessel, in which the formation of the solid, for instance the metal complex occurs. In such cases, the secondary reaction vessel may be in fluid communication with the first reactor during formation of the first product, with the second reactor during formation of the second product, and with a storage vessel once conversion from a gas to solid has been carried out. Alternatively, the secondary reaction vessel may also be the storage vessel, to minimise loss of product through transfer within the system. Further, it may be that the first/second reaction vessels and the storage/secondary reaction vessels are not in fluid communication during formation of the first and/or second products, but only after reaction is complete to allow flow of the product from one chamber to another. This may be desirable for batch formation processes.

[0038] In a third aspect of the invention there is provided the use of the system according to the second aspect of the invention, in the reversible storage and release of chemical energy to a grid energy system.

[0039] There is therefore provided a grid energy firming system comprising a first reactor for forming carbon monoxide from carbon dioxide and carbon in the reverse Boudouard reaction; a secondary reaction vessel in fluid communication with the first reactor into which the carbon monoxide flows on formation; a second reactor for forming carbon dioxide and carbon from carbon monoxide in the Boudouard reaction, the second reactor being in fluid communication with the secondary reaction vessel during formation of the second product, and a heat exchanger. The secondary reaction vessel being adapted to store carbon monoxide within a metal carbonyl complex, such as chromium hexacarbonyl.

[0040] Unless otherwise stated each of the integers described in the invention may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

[0041] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0042] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about".

Brief Description of the Drawings

[0043] In order that the present invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

[0044] Figure 1 is a schematic representation of one example of the invention.

Detailed Description

[0045] Figure 1 shows one example of the flow of materials through a system of the invention. In this example, the first product is carbon monoxide produced from carbon dioxide and coke in the reverse Boudouard reaction (C0 ₂ + C— » 2CO). The carbon monoxide is stored until energy firming is required, when it is oxidised in a fuel cell stack 5 to produce a second product, carbon dioxide in the Boudouard reaction (2CO— » CO2 + C). In this example, the carbon dioxide produced as the second product is sequestered using known techniques.

[0046] Carbon dioxide is transferred to a carbon reactor 10 where it is reduced with coke. The reduction process is an endothermic process which is driven using waste heat from a conventional power plant. The carbon monoxide produced is then stored in storage vessel 15 until energy release is required.

[0047] When grid energy firming is needed, the carbon monoxide is passed into the fuel cell stack 5 and oxidised to produce carbon dioxide and electricity.

[0048] It will be understood that hydrocarbons may be used in place of coke. In these examples, the carbon reactor 10 produces syngas, and oxidation of the syngas in the fuel cell stack 5 produces carbon dioxide and steam. Separation of carbon dioxide and steam occurs by condensation of the steam using a condenser (not shown).

[0049] Figure 2 shows the operation of the fuel cells 5, in this example the fuel cells 5 are solid oxide fuel cells 5, operating at a temperature of 1200K (±5 OK). Oxygen is drawn from air, and separated by migration through the fuel cells 5. The carbon dioxide produced is of sufficient purity that it can be directly sequestered, but at the point of exit from the fuel cells 5, the carbon dioxide is at a temperature of around 1200K (±5 OK). To ensure that the maximum benefit is obtained from this heat energy, prior to sequestration the carbon dioxide is passed through a heat exchanger (not shown), the heat being used to heat the carbon reactor 10. Fans are used to move the gases around the system.

[0050] The invention therefore provides a process and system for grid energy firming through energy storage in chemical bonds for rapid release through fuel cell oxidation when energy is required.

[0051] It should be appreciated that the processes and systems of the invention are capable of being incorporated in the form of a variety of embodiments, only a few of which have been illustrated and described above.