GEOTHERMAL ENERGY SYSTEM

TECHNICAL FIELD

The present invention relates to an extension of a geothermal energy system and to a method of operating a geothermal energy system.

PRIOR ART

One of society’s biggest challenges is global climate change. To limit the global mean temperature rise, a reduction of the amount of carbon dioxide (CO ₂ ) emitted to the atmosphere is required.

Renewable wind and solar energy has enormous resource potential and would present an alternative to traditional carbon-emitting power plants. However, the storage of wind and solar energy for later use is a problem which is not yet satisfactorily solved.

To decarbonize existing fossil-fueled power plants or other industrial processes that emit CO ₂ to the atmosphere, CO ₂ emissions can be captured, transported, typically in a pipeline, to a storage site, and then injected into a subsurface reservoir. This process is called Carbon Capture and Storage (CCS). CCS reduces the emission of CO ₂ into the atmosphere from sources such as fossil fuel power systems, cement factories, biofuel refineries or from other large CO ₂ point sources by permanently storing the CO ₂ underground in deep storage formations. These storage formations are located typically in excess of 800 meters underground to ensure the CO ₂ is in a super-critical state, which both improves storage safety and maximizes storage volumes. Due to the depth of the storage formation, and given that the temperature inside Earth typically increases with depth, the average reservoir temperatures are greater than the temperatures of the injected CO ₂ and can be significantly greater than the surface temperature. This allows the injected CO ₂ to extract heat from the reservoir located at depth. This heat extraction process has led to the proposal of geothermal energy systems which can be combined with CCS. Such systems use the CO2 as a heat extraction fluid and operate as a Carbon Capture Utilization and Storage (CCUS) system. An example of such a geothermal energy system is the CCh-Plume Geothermal (CPG) system.

The CPG system operates by producing hot CO2, which is geothermally heated in the storage formation, here called reservoir, to the surface for electric power and/or heat generation. The produced CO2 is then reinjected into the reservoir, in a cold dense state, allowing the injected CO2 to extract heat from the reservoir. The reservoir is usually a natural high-permeability sedimentary basin with a large storage volume. The CPG system therefore extracts geothermal energy.

US 8 316 955 discloses such a CPG system. This geothermal energy generation system comprises an underground reservoir located under a caprock and at least one injection well for injecting a non- water based working fluid, such as CO2, into the reservoir. The reservoir has a first temperature and the working fluid, when injected, has a second temperature being below the first temperature. Exposure of the working fluid to the first temperature produces heated working fluid capable of entering a production well. Thermal energy contained in the heated working fluid and pressure can be converted to electricity, heat or combinations thereof, in an energy converting apparatus. Such underground reservoirs are usually located at 2 to 4 km depth.

The following papers also describe the CPG system:

Randolph, J.B. and Saar, M.O. (2001).“Combining geothermal energy capture with geologic carbon dioxide sequestration.” Geophysical Research Letters, 38, L10401.

- Adams, B.M., Kuehn, T.H., Bielicki, J.M., Randolph, J.B., and Saar, M.O. (2014). “On the importance of the thermosiphon effect in CPG (CO2 Plume Geothermal) power systems.” Energy, doi.org/10.1016/j.energy.2014.03.032, 69, 409-418.

- Adams, B.M., Kuehn, T.H., Bielicki, J.M., Randolph, J.B., and Saar, M.O. (2015). “A comparison of electric power output of CO2 Plume Geothermal (CPG) and brine geothermal systems for varying reservoir conditions.” Applied Energy, 140, 365- Garapati, N., Randolph, J.B., Saar, M.O. (2015).“Brine displacement by CO2, energy extraction rates, and lifespan of a CCk-limited CO2 Plume Geothermal (CPG) system with a horizontal production well.” Geothermics, 55, 182-194.

US 9 739 509 B2 refers to a geothermal energy system in naturally permeable reservoirs which uses water as primary storage medium. Multiple reservoirs are used to time-shift parasitic loads of a power generation cycle, wherein some of the reservoirs are located deep and are hot and some of the reservoirs are ponds used for water or brine and they are located at the surface.

In addition, there exists another approach of CO2 reduction, which is described in US 6 668 554 Bl. It uses CO2 in an enhanced geothermal system (EGS), which is also known as hot dry rock (HDR) or petrothermal system. In EGS, the permeability of the underground formation is artificially created by hydraulic stimulation, such as hydro - fracturing and/or hydro-shearing. This results in very limited porosity and permeability along fractures and not in a large-scale, overall highly permeable reservoir that accommodates pervasive fluid flow everywhere.

A main difference between CPG and EGS is the reservoir, which is naturally permeable and large for CPG and artificially generated, for example through fracturing, and small for EGS. Another difference can be found in the amount of CO2 that circulates through the underground formation and in how pervasive the flow is and therefore how large the rock- fluid heat exchange is. The“heat-mining” efficiency of the CPG system is substantially higher than the one of the EGS system.

Fleming, M.R., Adams, B.M., Randolph, J.B., Ogland-Hand, J.D., Kuehn, T.H., Buscheck, T.A., Bielicki, J.M., and Saar, M.O. (2018),“High Efficiency and Large-scale Subsurface Energy Storage with CO2”. Proceedings of the 43 ^rd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, February 12-14, 2018, suggest a further developed system based on the above mentioned CPG technology. This expanded system uses a second reservoir located at a minor depth of 800 m to 2 km, preferably of about 1.5 km underground. The principle of the expanded system is shown in figure 1 and described later in this text in more detail. During the energy storage mode (also called“charging”) CO2 is cooled and pumped from the shallow reservoir to the deep reservoir. During the electricity generation mode (also called“discharging”) the high-pressure and geothermally heated CO2 in the deep reservoir is released and rises typically buoyantly (but could also be pumped) in the production well to the surface, where the CO2 is expanded in a turbine, then cooled to increase its density by a cooling tower and then injected into the shallow reservoir. Injection into the shallow reservoir could be augmented by pumping. Within the shallow reservoir, the CO2 is in a super-critical state, as that increases subsurface CO2 storage safety, since the CO2 is less likely to boil, fracture the sealing formation and leak upwards.

The shallow reservoir enables storing the CO2. The combined use of an underground deep reservoir and an underground shallow reservoir temporally separates energy generation and energy storage. The shallow reservoir stores the CO2 in an intermediate state after it is expanded in the turbine but before most of the parasitic cooling and pump loads. Later, during the energy storage mode, CO2 is re-extracted from the shallow reservoir, then cooled and/or compressed/pumped, consuming energy, before it is injected into the deep reservoir.

This expanded system therefore uses the deep reservoir and the shallow reservoir to store and discharge energy like a battery. The system is termed an earth battery extension (EBE).

However, this system, using two reservoirs at different depths, encounters two main problems:

The shallow reservoir at 800 m to 2 km, preferably at about 1.5 km depth, has a significant pressure of more than 10 MPa, requiring about 8 MPa at the injection wellhead at the surface to reach this downhole pressure. The increase of the CO2 from about 8 MPA to about 10 MPa within the injection well occurs due to the density-driven pressure increase which results from decrease in potential energy. This results in a pressure on the downstream side of the turbine (i.e. the CO2 turbine- back-pressure) of about 8 MPa as well. This is quite large, given that the upstream pressure of the turbine typically has a pressure of about 12 MPa, resulting in a pressure drop (in this example) of only about 4 MPa. As the power generated by the turbine is linearly proportional to the pressure difference across it, this limits the amount of electric power that can be generated, compared to having much lower turbine back-pressures.

The shallow reservoir has to meet several conditions:

o It has to be significantly permeable to enable CO2 injection and retrieval o It has to have a low- to zero-permeability caprock to prevent upwards leakage of the CO2 from the reservoir to the Earth surface or to near the Earth surface.

o Given typical hydrostatic pressures within the Earth, it has to be at least approximately 800 m deep so that the CO2 pressure is higher than its critical pressure and so it is in a super-critical state. A super-critical state is necessary mainly for safety reasons, but at the same time it has to be as shallow as possible to have as low of a hydrostatic pressure as possible to result in as low turbine back-pressure as possible

o It should have a thickness of a few tens of meters as particularly large or small thicknesses can cause problems with the combined injection- production well system in the shallow reservoir and the exchange of CO2 between this well system and the shallow reservoir.

It may be difficult to find locations fulfilling these conditions while having at a deeper depth a natural storage formation suitable to form the deep reservoir.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve the magnitude of power generated during the generation mode of geothermal energy storage systems, especially of the energy storage system, referred to above as the Earth battery extension (EBE).

According to one embodiment of the invention an extension of a geothermal energy system is provided solving the problems of the system comprising a shallow reservoir. The geothermal energy system comprises a cycle with at least one injection well for accessing, from the Earth’s surface or from near to the Earth’s surface, with at least one reservoir located underground and with at least one production well forming an outlet of the reservoir to the Earth’s surface or near to the Earth’s surface. The working fluid can flow within the cycle, the working fluid passing through the reservoir in a first state. The inventive extension comprises at least one energy converting device bringing at least one part of the working fluid flowing out of the reservoir from the first state into a second state and the extension further comprising at least one storage connected with the at least one energy converting device and storing the at least one part of the working fluid in this second state at close-to-atmospheric pressure and/or in a sub-critical state, wherein the storage releases the stored at least one part of the working fluid into the reservoir at discretion.

Close-to-atmosphere pressure is preferably a pressure below atmospheric pressure but it can also be a pressure above atmospheric pressure which is still much lower than the pressure at the land surface that is required to inject the CO2 in a subsurface shallow reservoir. The pressure usually needed for injection in a subsurface shallow reservoir located at 800 m to 2 km below surface is about 8 MPa.

A reservoir is a geologic formation underground or an engineered structure underground that can hold a fluid and preferably has a permeability of at least 10 ¹⁶ m ² . Herein, a reservoir is assumed to have a fluid pressure equivalent to the hydrostatic pressure unless otherwise noted. The hydrostatic pressure is the pressure at the bottom of a column of static fluid, which may be liquid water containing dissolved solids, affecting the fluid’s density, of a height equivalent to the depth of the reservoir. The reservoir pressure may change by 100% or more depending on the local geology, for example, in geo-pressured reservoirs, the pressure can increase by a factor of three or more. Conversely, a sealed, gas- filled reservoir can have its pressure reduced to near-atmospheric pressure.

A caprock is a geologic formation or an engineered structure with sufficiently low permeability, preferably below 10 ¹⁶ m ² , that keeps a fluid within a reservoir.

A storage is a naturally occurring or human made device which may store a volume of fluid. A fluid is a medium which can flow. It can especially be a gas, a liquid and combinations of one or multiple gases and/or liquids.

A well in the meaning of this description is any kind of natural or human made access or pathway, which connects a reservoir located under the Earth’s surface with the Earth’s surface or near the Earth’s surface. A well can have horizontal components, in addition to vertical components.

The surface of the Earth can also be under water, i.e. under the ocean, i.e., submarine, or under a lake, pond, etc.

An energy converting device converts the energy of a medium. It can thereby change the aggregation states of the medium but it can thereby also just lower or increase its temperature and/or pressure. Preferably, energy converting devices are turbines. We sometimes write here“energy generation” or“energy consumption”, thereby referring only to its conversion to or from electricity for purposes of emphasis, as energy is principally conserved.

In this text, compressors mean also pumps and pumps mean also compressors where technically applicable.

In this context, a battery is any device that can store energy (charging the battery) and can release that energy (discharging the battery).

By storing at least one part of the working fluid in a storage and by releasing the stored at least one part of the working fluid into the reservoir at discretion, the system can be used as a battery, which can be charged and discharged independently of the use of the geothermal energy system, especially when the geothermal energy system is part of a power plant. Energy can be stored (charging the battery) when not required and energy can be discharged from the battery when energy is needed, for example, by the power grid. The inventive system can therefore be used as both a geothermal energy system and/or an energy storage system. By storing the at least one part of the working fluid at close-to-atmospheric pressure, a larger pressure drop of the working fluid can be used within the energy converting device, especially within a turbine of an electricity plant. This results in more power generation during the power generation mode of the earth battery system than the earth battery system with the shallow reservoir. Peak power demands can be fulfilled with the same deep reservoir by using the inventive extension instead of the basic CPG system or the earth battery system with the shallow reservoir. The storage therefore enables a significantly larger pressure drop across the turbine compared with the system using an underground shallow reservoir. Preferably, the turbine back pressure is reduced to be below about 8 MPa.

The turbine and related electric power output can be significantly increased. Compared with the above described earth battery system using a shallow reservoir the electricity generation rate during the power generation mode can be increased by about four times depending on the embodiments of the invention used. The extension according to the invention is therefore an improved EBE which can, for example, handle peak power demands, for example, from the power grid, or to provide ancillary power grid services.

Preferably, the geothermal energy system is a CPG system as described above. The extension can be part of the CPG system and it can use the same energy converting devices as the CPG system. Preferably, the extension uses its own energy converting devices.

In a preferred embodiment, the working fluid is a non-water based fluid. Preferably, the working fluid is CO ₂ fluid or a CO ₂ based fluid.

The extension can use own wells to connect the storage with the underground reservoir. Preferably, the extension uses the at least one injection well and the at least one production well of the basic geothermal energy system.

Preferably the extension is“shut on and off’ independently of the basic use of a basic part geothermal energy system, wherein the basic part preferably produces electricity. The extension can therefore be used as an EBE which helps to operate a geothermal power plant in the most profit-orientated way. This will be explained later in this text in more detail.

Preferably, the underground reservoir holds the working fluid in a super-critical state. The reservoir is preferably a deep reservoir, i.e. at a depth of preferably about 1 to 5 km, more preferably of about 2.5 km, below the Earth surface.

Preferably, the storage holds the at least one part of the working fluid in a sub-critical state. This reduces the risks of storage. When the working fluid is a CO2 based fluid, the fluid will be in the sub-critical state in the storage at a close-to-atmospheric pressure.

In a first embodiment, the working fluid is still stored underground, but in a different storage than the above mentioned shallow reservoir. In this case, it still has to be closed by a caprock to avoid leaking of the working fluid or it is a man-made storage just located underground. However, its location shall enable storage of the working fluid in a sub- critical and/or close-to-atmospheric pressure state. For a typical reservoir at hydrostatic pressure, the maximum depth is 800 m to maintain the CO2 at a pressure less than its critical pressure of 7.4 MPa. However, if the reservoir has the necessary geology, its pressure may be substantially lower than the hydrostatic pressure. In these cases, it may be possible to have a storage with a substantial depth, potentially even at a depth larger than the depth of the deep reservoir, while still maintaining the CO2 below its critical pressure. Thus, the required depths of the underground reservoir and the underground storage are different for different working fluids and in different geologic settings, depending on the local pressure and temperature conditions at depth. However, the safety of such an underground storage may be reduced, compared to the storage located at the surface.

Therefore, in preferred embodiments, the inventive system comprises a low-pressure storage, i.e. a close-to-atmosphere pressure storage, at or close to the Earth surface. Not only the safety is improved but there is another advantage as well. As the low-pressure storage, such as a gasometer, is a largely engineered device, the low-pressure storage can be built and placed anywhere on the Earth’s surface. Since only a deep reservoir has to be found underground and no shallow reservoir nearby is required, the likelihood of finding suitable locations for such a system is significantly increased. The low-pressure storage used has preferably a volume of several hundred thousand cubic meters or several millions of cubic meters, when the pressure is below atmospheric pressure and can be smaller when the pressure is about or slightly higher than atmospheric pressure. The deep underground reservoir is preferably a reservoir as described in the publications of the CPG system mentioned above. Most preferably, it is a natural reservoir.

The use of a low-pressure storage at or near the Earth surface instead of a shallow reservoir underground is not a mere exchange of storage means. The state of the working fluid to be stored has to be considered: it is in a high pressure and super-critical-state in the shallow reservoir underground and in a low pressure and sub-critical state in the surface, or near surface, storage. The physical properties are different in these two states, such as the density, the dynamic viscosity and the specific heat capacity. The deep reservoir depth, the geothermal temperature gradient, the enthalpy drop across the turbine, the expansion and compression, the cooling and heating in the production and the injection wells, the deep reservoir permeability, the deep reservoir porosity, the working fluid saturation in the deep reservoir, the thickness and the lateral extent of the deep reservoir, and the CO ₂ injection rates into the deep reservoir have to be considered as well. In addition, a large mass of the working fluid in the gas phase needs to be stored at near-atmospheric pressure. For example, at atmospheric pressure, CO ₂ has low density of about 2 kg/m ³ and thus requires a very large volume of preferably several hundred thousand cubic meters or several millions of cubic meters. Storing CO ₂ at the surface in high-pressure tanks would reduce the volume needed but such tanks would require quite thick walls, while still having to be quite large, making them expensive. In addition, storing CO ₂ at a high pressure would increase the turbine backpressure and would therefore reduce the power generated, compared to storing the CO ₂ at close-to-atmospheric pressure.

As mentioned above, the at least one energy converting device of the extension is preferably at least one turbine. It can be a single-stage or a multi-stage turbine. In preferred embodiments, the extension further comprises at least one compressor arranged in a line between the storage and the reservoir, the working fluid having to pass the at least one compressor when flowing from the storage into the reservoir. The compressor can be a single-stage or a multi-stage compressor. In preferred embodiments, at least one cooling unit is arranged in a line between the storage and the reservoir, the working fluid having to pass the at least one cooling unit when flowing from the storage into the reservoir. Preferably, the at least one cooling unit is arranged down-flow of the at least one compressor.

In preferred embodiments, at least one cooling unit and at least one energy converting device are present. However, in some embodiments, the cooling unit is optional and/or a pump is added after the cooling unit.

In a preferred embodiment, multiple stages in at least one of the one or more compressors and the one or more turbines are used. Preferably the one or more turbines and the one or more compressors comprise multiple stages. A multiple stages turbine with inter-stage heating decreases the density and increases the power output compared with a single-stage turbine. The reason for this is that the work generated is inversely proportional to the fluid density. Using multiple stages with inter-stage cooling in the compressor decreases the compressor work required. In some embodiments the heat sink is further used as an additional energy source. In such embodiments, the heat injection preferably comes from an external process that requires a heat sink. In these embodiments, preferably single-stage turbines are used, since it results in the lowest temperature heat sink. More useful work, or exergy, may be accomplished when the heat sink temperature is minimized. In embodiments where the turbine power output should be maximized, multi-stage turbines are preferably used. In embodiments where compressor power should be reduced at the cost of increased machinery, multi-stage compressors are preferably used.

Preferably the at least one energy converting device comprises a heater, for example a sublimator, transferring a substance, such as CO2, by sublimation from the solid to the gas state. The heater preferably acts as a heat sink.

This is a further advantage of the inventive system claimed: If the heat sink is part of the inventive system it can be used as an energy source as well. The explanation will be given with the example of a CCk-based working fluid. By reducing the CO2 pressure to close-to- atmosphere pressure just before and in the low-pressure storage, the CO2 expands and reduces its temperature down to about -78° C, forming a mass composition of about 70% CO ₂ gas and about 30% solid CO ₂ , i.e. dry ice. The CO ₂ needs to be completely in gaseous form to be compressed during the low-pressure storage mode. Therefore, the -30% solid CO ₂ at about -78° C needs to be transferred to gas, for example by sublimation. As the dry ice is transferred to its gaseous state, which is an endothermic process, energy is required. This energy can be taken directly from the environment, for example by using ambient air. However, this would be wasting this heat sink. It is therefore advisable to use the heat sink to cool an external process or generating electricity.

In a first embodiment, the heat sink can be used to chill water or some other fluid to cool nearby buildings or industrial processes.

In a second embodiment, the heat sink can be used to generate electricity or perform some other work, for example as the cold end of a heat engine, for example to generate electricity.

In a third embodiment, the heat sink is used in a cryogenic CO ₂ capture system to condense gaseous CO ₂ to liquid CO ₂ . This is a well-known form of so-called“direct (air) CO ₂ capture”, where the CO ₂ is typically captured from air, hereafter assumed as an example, but could be captured from any fluid mixture. However, in the state of the art, this type of direct air CO ₂ capture (DACC) is normally energetically and thus economically not of interest, as such extreme cooling requires large amounts of energy, typically generating more CO ₂ . In the inventive system, the extreme cold of -78°C may be viewed as a byproduct. As the inventive system works by having a large, deep geological reservoir to store large amounts of CO ₂ underground anyway, it provides a CO ₂ storage location for the CO ₂ coming from a DACC addition to the inventive system using a surface storage, representing an entire DACC and storage (DACCS) system. Therefore, the inventive system, using a surface low-pressure working fluid storage instead of a shallow subsurface reservoir, can, after some initial filling with externally provided CO ₂ , capture its own additional CO ₂ directly from the air anywhere on Earth and store this additional CO ₂ , together with the initially, externally provided CO ₂ , underground in the deep reservoir. This contributes to the reduction of CO ₂ in the atmosphere and helps to reduce global warming. This is claimed herein as a separate invention. In this third embodiment, the inventive system can start with a fairly small size and a limited amount of CO2 that can be brought-in by train, truck, pipeline, ship or any other appropriate means. Once the system is primed with a, for its initial size, sufficient amount of subsurface CO2, the DACCS system can continuously add CO2 and store it in the original deep reservoir. Once the additional CO2, captured typically from the air but could be captured from some other gas mixture, reaches a sufficient amount, a larger portion of the deep reservoir can be used for storage or an additional deep reservoir can be used for storage. The inventive system can therefore increase in size which facilitates financing the system in stages, i.e. not all up front, which is an important advantage when realizing the inventive system.

The EBE with the low pressure storage is preferably an extension of a basic CPG power plant using a deep reservoir, although they do not necessarily have to be extensions but can form the CPG power plant themselves. Thus, a basic CPG power plant can be financed, built and operated first without a connected Earth battery extension (EBE). Later, if desired, the EBE can be added, possibly financed by the revenue from the basic CPG system. Once the CPG power plant and the EBE are combined, they can be operated independently from each other even though their components are physically tightly connected with each other. This enables at least the following operations modes, which correspond to times 1, 2, and 3 in figure 7:

1) Continuous, full-capacity operation of the CPG power plant:

Geothermal energy is, like wind and solar energy, considered to have no fuel cost once the power plant is built. Geothermal energy replenishes itself at a given location on Earth, and globally, the use of Earth’s geothermal energy does not reduce Earth’s geothermal energy resource in any significant way, so that geothermal energy is indeed also a renewable energy resource, just like solar and wind energy. However, contrary to wind and solar energy, geothermal energy is available almost at any time. The CPG power plant can therefore continuously generate electricity without requiring energy storage to do so. This constitutes a baseload energy resource.

2) Energy storage with the EBE:

When there is excess power, for example because of reduced demand on the power grid or because of an increased wind or solar or other energy supply, the excess power can be stored in the EBE. Additionally, CPG power may also be stored in the EBE if it is not needed elsewhere. This can be financially interesting because at times of excess power, electricity prices are low. Thus with the EBE system, the energy can be generated cheaply by the CPG system and/or purchased cheaply from the power grid, stored and then provided to the power grid later (in operational mode 3) when electricity prices have increased again, i.e. electricity arbitrage.

3) Peaking power dispatch from the EBE:

When power demand on the power grid, or locally, exceeds power supply by the power generators, such as the CPG power plant, the EBE can provide peaking power and/or ancillary services. This can stabilize the power grid. This is not only important for society but also commercially interesting, as peaking power and ancillary services are typically financially compensated very well.

The combination of the CPG power plant with an EBE can therefore provide a continuously working power plant using the geothermal energy of the deep reservoir and, at the same time, but independently from this first use, use the deep reservoir in combination with a second storage means as storage of the energy of the working fluid, i.e. as a battery extension. Using a storage means, which keeps the working fluid at low pressure and/or in a sub-critical state, increases the power output of the EBE system during the battery discharge mode and at the same time improves the possibilities to find appropriate locations on Earth to build such a combined system, especially when the low- pressure storage means are located at or near the Earth surface and/or they are man-made. In addition, the EBE, using such a low-pressure storage means, can use the heat sink provided by such a system for cooling of buildings or industrial processes or for cryogenic direct air (or other fluid) CO2 capture (DACC) which reduces the CO2 in the atmosphere and enables additional electricity generation, as stated in more detail above.

The EBE can also be used without the basic CPG power plant but still with the deep reservoir as a stand-alone geothermal energy storage system. The inventive extension then also forms the geothermal energy system, i.e. in the claim language used, the extension and the geothermal system are the same. An inventive method for operating a geothermal energy system, especially a geothermal energy storage system, as described above comprises the steps of

- injecting a working fluid into an underground reservoir located at a first depth, wherein the working fluid is geothermally heated in the underground reservoir,

- guiding the working fluid from the underground reservoir located at a first depth into an energy converting device at or near the surface of the Earth,

- storing the working fluid in a storage at a close-to-atmosphere pressure and/or in a sub- critical state,

- releasing the stored working fluid to the underground reservoir located at a first depth at discretion.

Further embodiments of the invention are laid down in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

Figure 1 schematically shows a geothermal energy generation system with an underground Earth battery extension (EBE) according to an embodiment according to the state of the art;

Figure 2 shows a pressure-enthalpy diagram of carbon dioxide of the geothermal energy generation system with an underground Earth battery extension (EBE) according to figure 1 ;

Figure 3 a schematically shows a geothermal energy generation system with a surface

Earth battery extension (EBE) according to a first embodiment according to the invention;

Figure 3b shows a variant of the first embodiment according to figure 3 a; Figure 4 shows a pressure-enthalpy diagram of carbon dioxide of the geothermal energy generation system according to figures 3a and 3b;

Figure 5 a schematically shows a geothermal energy generation system with a surface

Earth battery extension (EBE) according to the invention in a second embodiment according to the invention;

Figure 5b shows a variant of the first embodiment according to figure 5 a;

Figure 6 shows a pressure-enthalpy diagram of carbon dioxide of the geothermal energy generation system according to figures 5 a and 5b and

Figure 7 shows three time periods ti, t2, t3 of the systems according to figures 1, 3a,

3b, 5a, and 5b.

DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments described herein are based on a working fluid which mainly contains carbon dioxide (CO2) or is largely or only CO2. The teaching can also be applied to other suitable working fluids. In addition, please note that the underground reservoirs described may contain additional fluids, especially native fluids such as water, brine, oil, gas or mixtures and/or combinations thereof.

Figure 1 shows a CPG system and an EBE system with a shallow underground reservoir as suggested in the state of the art. The EBE system is marked with reference number 6. The principle of the technique is explained below based on this embodiment, wherein below features are mentioned which may not already have been disclosed to the public. Not yet disclosed features or features which may have been disclosed within a grace period shall therefore be considered as being part of the invention claimed as well and they may be claimed in this application in subsequently filed claims as well.

The system comprises a working fluid sequestration component A located underground and a geothermal energy generation component B located at or near the Earth surface. The land surface or Earth surface is marked in the figures with the reference number 1. The working fluid sequestration component A is a geological fluid sequestration or storage.

In a first embodiment, the energy generated is thermal energy, i.e. heat. In a second embodiment the energy generated is electricity. The two embodiments can also be combined, so that heat and electricity are generated. The system can also be used to operate a separate power cycle or to generate energy in a different way. Examples hereto are given in US 8 316 955 B2 and can be applied to the systems described herein as well.

The working fluid can be any suitable fluid. It is preferably not water based, but can contain some amount of water. The working fluid can also contain solids, in dissolved or non-dissolved form. However, it should be capable of absorbing thermal energy from its surroundings and of further releasing the thermal energy. Most preferably, the working fluid mainly consists of CO2. The working fluid, especially within the underground CO2 plume, can contain an amount of native fluid as well. Details about the working fluid are disclosed in detail in US 8 316 955 B2.

The working fluid, especially CO2, is preferably a waste stream from a power plant, such as a fossil fuel power plant. Examples are coal plants, natural gas plants and the like. The working fluid, typically CO2, can however be obtained from any kind of process that produces it, typically as a byproduct, such as from a biofuel refinery, cement or steel manufacturers, chemical industry, etc.

In preferred embodiments, the working fluid is capable of being transported via any suitable means, such as a pipe, a truck, a ship, or by rail, over a desired distance. In figure 1, an inlet line 9 with a first one-way valve 90 is shown to visualize the charging of the system, namely a deep reservoir 2.

In some embodiments, the working fluid provided by the fluid source can be used“as it is”. In other embodiments, the fluid source provides a pre-working fluid which is further processed before it is used as working fluid in the inventive system. This is also described in detail in US 8 316 955 B2. Preferably, the geothermal energy generation component B is located at a site above the working fluid sequestration component A. However, the working fluid can also be guided through a pipeline or by other appropriate means to a geothermal energy generation component B located at a distance of the working fluid sequestration component A.

The working fluid sequestration component A comprises at least one deep reservoir 2 in whichever geometry, which is covered by at least one first caprock 20 or other fluid trapping formation in whichever geometry. The reservoir 2 has, due to the geothermal heat, a natural temperature which is higher than the temperature of the working fluid injected into this deep reservoir 2. Preferably the deep reservoir 2 is located at least 800 m, preferably at least 2 km beneath the Earth’s surface 1, and most preferably it is located about 2 km to about 4 km beneath the Earth surface

Preferably, the at least one deep reservoir 2 in whichever geometry, is part of a sedimentary basin. The formation can be natural or man-made. The deep reservoir 2 does not have to be a hot dry rock reservoir. It does just have to be sufficiently permeable, preferably larger than 10 ¹⁶ m ² , to allow preferably multidirectional routes for dispersion or flow of working fluid at preferably relatively high rates, including lateral dispersion or flow.

The at least one first caprock 20 is preferably a geologic feature, or other fluid-trapping feature in whichever geometry, having a very low permeability, preferably smaller than about 10 ¹⁶ m ² . The at least one first caprock 20 acts as a barrier for the working fluid contained in the deep reservoir 20 below. The at least one first caprock 20 can be porous and/or permeable as well. As long as the permeability of the one first caprock 20 is sufficiently low, the fluid will move so slowly it will be essentially trapped on the timescales of relevance here.

These kind of deep reservoirs, the corresponding caprocks, and the layer(s) above the caprock(s) are described in detail in US 8 316 955 B2 and the teaching applies to the systems described herein as well. At least one injection well 21 leads from the Earth surface 1 through the first caprock 20 into the at least one deep reservoir 2. At least one production well 22 leads from the at least one deep reservoir 2 through the first caprock 20 to the surface 1. The injection well 21 and the production well 22 extend preferably in vertical direction and they are preferably rectilinear. Other orientations and bendings of the wells are possible as well, including inclined and horizontal portions of the wells.

The working fluid within the deep reservoir 2 flows to the production well 22. When the working fluid is CO2 or mainly CO2, it forms a so-called CO2 plume when it is released from the injection well 21 into the deep reservoir 2. The CO2 then flows through openings, such as pores, fractures, fissures, faults, conduits, caverns, and drill holes, present or generated in the deep reservoir 2. Due to increased temperature within the deep reservoir compared to the Earth surface and compared to the temperature of the injected CO2, and due to the thermal exchange with the material of the deep reservoir 2, the CO2 eventually becomes hot CO2 before entering the production well 22. The CO2 may enter the production well 22 together with reservoir-native fluids as well as dissolved and/or undissolved minerals in the fluids. Native fluids (e.g. water) can also dissolve into, and thus be present in, the CO2 that enters the production well 22.

The surface geothermal energy generation component B can comprise a CPG expansion unit. In this embodiment, the expansion unit is a CPG turbine 40 of an electricity plant. The working fluid, in this example the hot CO2, or mostly CO2, leaving the production well 22, is directly expanded in the turbine 40. The energy converted into electric energy is marked with wi. The working fluid is then subsequently typically cooled in a CPG cooling/condensing unit 41 following the turbine 40. The CPG cooling/condensing unit 41 is for example wet cooling towers. Exhaust, i.e. heat, is released in the CPG cooling/condensing unit 41 and the density of the working fluid, here the CO2, is increased. The parasitic electric energy used is marked with W2 (outward-pointing arrows are used in the drawings for both energy generators and energy consumers so that energy consumed is a negative value - see tables) and the heat, i.e. the exhaust, is marked with q2.

After the cooling process, the working fluid may be compressed using a pump or compressor 5. This pump or compressor 5 is optional. The energy used is marked with W3. It then may be further compressed down the injection well 21 to the deep reservoir 2. In the deep reservoir 2 the cold, dense working fluid extracts heat as the working fluid moves away from the injection well 21 towards the production well 22.

This CPG power generation cycle 70 is marked in the figures with reference number 70. It is a substantially closed system, wherein virtually no CO2 is lost to the atmosphere and CO2 only enters the system at one or more one-way valves 90, as described above. A one way valve 43 connects this working fluid stream with the injection well.

Details about the working fluid within this CPG power generation cycle 70 are described in US 8 316 955 and they apply to this system as well.

The working fluid sequestration component A comprises at least one second reservoir, herein called shallow reservoir 3. This at least one second reservoir is located at a minor depth, preferably above or near the deep reservoir 2. The shallow reservoir 3 is covered by at least one second caprock 30 as well. The shallow reservoir 3 and the second caprock 30 have preferably characteristics and attributes of the same group as described above with reference to the first (i.e. deep) reservoir 2 and the first caprock 20.

However, the temperature within the shallow reservoir 3 is usually lower than the temperature in the deep reservoir 2. The deep reservoir is hot, preferably hotter than 100°C, and the shallow reservoir is cold near the surface temperature, preferably colder than 50°C.

In addition, the volume of the shallow reservoir 3 is usually smaller than the volume of the deep reservoir. The shallow reservoir 3 is preferably located 800 m to 2 km, preferably about 1.5 km, below the Earth’s surface 1. At least one well 31 leads from the Earth surface 1 through the second caprock 30 into the shallow reservoir 3. This well 31 can act both as a fluid, such as CO2 or mostly CO2, injection and production well at different times, which is sometimes referred to as a“huff-and-puff’ well. This is shown in figure 1. In other embodiments, separate wells 31 for fluid injection and production are present. More than two wells can be used as well. An EBE discharging line 71 connects the production well 22 with an EBE turbine 60 during the EBE power generation mode. The EBE turbine 60 is preferably followed by a first isolation valve 62. The first isolation valve 62 separates and connects the turbine outlet with the well 31 leading into the shallow reservoir 3.

Preferably, a second isolation valve 63 connects the well with a working fluid (e.g. CO2) EBE charging line 72 or separates the well 31 from this line 72, depending on the state of the second separation valve 63. Preferably, either the EBE discharging line 71 or the EBE charging line 72 is connected with the shallow reservoir 3 and the other line is closed.

A third isolation valve 42 is present which enables the closing and opening of the CPG power generation cycle 70 between the production well 22 and the CPG turbine 40. A fourth isolation valve 65 opens and closes the EBE discharging line 71 between the production well 22 and the EBE turbine 60. The third and the fourth isolation valves, 42 and 65, respectively, can be open at the same time or they can be opened alternatively, depending on how the overall system is to be operated.

A second one-way valve 43 arranged in the CPG power generation cycle 70 and a third one-way valve 66 arranged in the EBE charging line 72 prevent a back flow of the working fluid.

The EBE charging line 72 leads to the working fluid injection well 21, wherein an EBE cooling/condensing unit 61, for example wet cooling and/or condensing towers, are located between the second isolation valve 63 and the injection well 21. Preferably, a throttle valve 64 is arranged in the EBE charging line 72 between the EBE cooling/condensing unit 61 and the working fluid injection well 21. Instead of a throttle valve 64, a pump or compressor may be used as well.

In this embodiment, the same working fluid injection well 21 and the same working fluid production well 22 are used for the EBE system as for the CPG system. However, there may also be multiple wells, wherein, depending on the embodiments, some or all of the wells are used by both systems and some or all of the wells are used by one of the systems only. The hot working fluid (e.g. CO2, used hereafter as an example), when leaving the deep reservoir 2 buoyantly (and/or due to pumping) through the production well 22, is directly expanded in the EBE turbine 60 to produce power. The energy converted to electrical energy is marked with w _a . No additional cooling means are needed in this embodiment. However, in other embodiments, depending on the fluid production pressure, temperature, and mass flowrate, some cooling may be necessary after the EBE turbine 60 to increase the density to increase the downhole pressure when the fluid is injected into the shallow reservoir 3. The CO2 is typically stored in a super-critical state, as that increases the CO2 storage safety.

The working fluid, now having increased density due to the cooling, can be injected into the shallow reservoir using only the gravitational compression in the vertical part of the well 31. Alternatively, the working fluid may not be cooled but instead pumped into the shallow reservoir, even though this is not preferred. The shallow reservoir 3 stores the working fluid until the end of the electric (and/or heat) power generation mode. After the EBE power generation mode, the first isolation valve 62 will be closed.

In the EBE energy storage mode, the second isolation valve 63 is opened and the working fluid stored in the shallow reservoir 3 is brought through well 31 to the EBE cooling/condensing unit 61. It is cooled there and/or compressed using a pump or compressor, thereby consuming energy. The electric energy used for cooling is marked with reference W _b (outward-pointing arrows are used in the drawings for both energy generators and energy consumers so that energy consumed is a negative value - see tables), the outgoing heat with reference qb. Wb is the parasitic electric energy needed to reject the heat, for example, to operate one or more fans. In this example, the parasitic electric energy is assumed to be 3% of the heat rejected, but it can vary. The working fluid is then injected back into the deep reservoir 2 through the vertical injection well 21, preferably without using a further pump or compressor. Any energy used during this here- described EBE energy storage mode that results in the injection of the working fluid from the shallow reservoir 3 into the deep reservoir 2 constitutes the energy storage component of the EBE, as surplus (i.e. in that moment not needed) energy from elsewhere (e.g. the power grid) is used for this. The shallow reservoir 3 therefore stores the CO2 in an intermediate state after it passes through the EBE turbine during the EBE discharging mode, but before the parasitic loads during the EBE charging mode, separating the components which generate and consume energy. The cycle described above is a Rankine power cycle fueled by geothermal energy. The system typically generates more net energy to, for example, the power grid than it consumes from, for example, the power grid, due to the addition of geothermal energy from the deep reservoir 2 during the EBE charging mode. The following table 1 shows the specific (i.e. per unit mass) electric energy w and heat energy q for the CPG system and the EBE system according to this embodiment using the shallow reservoir 3. Multiplying these energy per unit mass values by mass flowrates yields power.

The following table 2 shows the pressure P [MPa], the temperature T [C], the specific enthalpy h [kJ/kg], the specific entropy s [kJ/kg-C], the quality x [dim] and the phase of the CCh-based working fluid at the points ©, ©, ©, ® , © and © of the CPG cycle and at the points (D, (b), © and (d) of the EBE cycle.

Table 2

Figure 2 shows the P-h diagram, i.e. the pressure as a function of the specific enthalpy at these different points in the two cycles. The CPG cycle is marked with a solid line, the EBE cycle with broken lines, wherein the EBE discharging line 71 is dashed and the EBE charging line 72 is dotted.

For the example given, the following was found:

WCPG, net = 12 kj/kg for the CPG cycle

WEBE, peaking = 10 kj/kg for the EBE cycle in the discharging mode and

WEBE, storage = -3 kj/kg in the storage mode of the EBE cycle.

As before, w is the energy per unit mass. Multiplying w by a mass flowrate will give the power.

The calculation was made with the assumption that the deep reservoir is located at 2.5 km depth and the shallow reservoir is located at 1.0 km depth in a region which exhibits a geothermal temperature gradient that is typical for Earth’s continental crust, such as approximately 35° C/km. Typically, these regions have not traditionally been considered for geothermal development. The difference between reservoir injection and production pressure is assumed to be 2 MPa, which is approximately the pressure difference expected for a 320 kg/s CO2 flow through a 300 m thick, 50 mD, pure CO2, inverted 5-spot reservoir at the given depth and temperature.

Figure 3a shows an embodiment of the invention claimed. The inventive extension is marked with reference number 6 again. The system still uses the CPG system with the CPG power generation cycle 70 with the CPG turbine 40, the CPG cooling/condensing unit 41, the pump or compressor 5 and the deep reservoir 2. The isolation valves, the throttle valves, and the one-way valves are present as well. The above description applies to this embodiment as well. The same elements are marked with the same reference numbers. The same applies for figures 3b, 5a, and 5b.

In this embodiment, instead of the underground working fluid (e.g. CO2) storage in the form of the shallow reservoir 3, the system comprises fluid storage means located at or near the Earth surface. These fluid storage means are used as a surface battery connected to the CPG cycle 70 with an EBE discharging line 71 and an EBE charging line 72.

In the example shown in figure 3 a, the EBE discharging line 71 starts after the CPG turbine 40 and it leads to the EBE turbine 60 and through the throttle valve 64 to a heater 80. The heater 80 is followed by the first isolation valve 62 and by at least one low pressure storage 81 , preferably by a gasometer. The low pressure storage 81 is therefore located at or near the Earth surface. A gasometer within this description is any kind of suitable container which is capable of maintaining the stored fluid close to atmospheric pressure even when the amount of stored fluid increases or decreases within a certain range.

The heater is used to phase change the approximately 30% solid CO2, i.e., dry ice, to gas. This can be done before the working fluid is stored in the low pressure storage 81 and/or it could be heated while in the low pressure storage 81.

An EBE charging line 72 connects the low pressure storage 81 with the injection well 21. After the low pressure storage 81, but before a compressor 82, the second isolation valve 63 is located. The compressor 82 is followed by the EBE cooling/condensing unit 61 and by an EBE pump or compressor 83. The EBE cooling/condensing unit 61 is already described in the embodiment comprising a shallow reservoir.

The low pressure storage 81 is preferably one or more surface or near-surface tank which can hold large volumes of near-atmospheric-pressure gas. The volume of the tank is preferably more than 100’ 000 m ³ . Multiple tanks may be used. Such tanks, that can store increasing amounts of gas at close-to-atmospheric pressures, are usually called “gasometers”. The one or more low pressure storage 81 holds the CO2 close-to- atmospheric pressures and therefore in a sub-critical state. The turbine back-pressure can therefore be decreased from about 8 MPa to about 0.6 MPa, enabling significantly larger pressure drops in the EBE turbine 60. Therefore, the pressure drop across the EBE turbine 60 increases from about 4 MPa to about 11 MPa in this example. This increase in pressure drop across the EBE turbine 60 increases the EBE turbine 60 energy output w _a’ by about 450% (i.e. by a factor of about 4.5). This is a significant increase in turbine and related electric energy output, compared to using a shallow underground reservoir 3 as in the state of the art.

In preferred embodiments, at least one of the compressor 82 of the EBE charging line 72, the EBE pump compressor 83 and the EBE cooling/condensing unit 61 is connected to a wind and/or solar power plants and/or any other power plant or power generator in order to store some or all of the excess power from such a power plant during the EBE charging mode and dispatch it later as peaking power during the EBE discharge mode. In figure 3a, a wind power plant is shown as an example power plant and marked with reference sign W. This additional power plant is shown in figure 3 a only. However, such power plants can also be present and connected to the inventive system as described in the other embodiments, especially in the embodiments according to figures 3b, 5a, and 5b as well.

The electric energy per unit mass generated and needed as well as the heat energy per unit mass generated are listed for figure 3 a in the following table 3:

Table 4 shows the states at the different locations marked in figures 3a and 3b.

Table 4

The basis for the calculation was the same as with the embodiment having a shallow reservoir. I.e. the deep reservoir is located at a depth of 2.5 km in a geothermally not particularly active region. Figure 4 shows the P-h diagram, i.e. the pressure as a function of the specific enthalpy at these different points in the two cycles. The CPG cycle is marked with a solid line, the EBE cycle with a broken line, wherein the EBE discharging line is shown with a dashed line and the EBE charging line with a dotted line.

For the example given the following was found:

WCPG, net = 12 kj/kg for the CPG cycle

WEBE, peaking = 57 kj/kg for the EBE cycle in the discharging mode and

WEBE, storage = -257 kj/kg in the storage mode of the EBE cycle.

Additionally, in this example, -116 kj/kg of heating is required at -78°C, i.e., this is a heat sink, providing -78°C cooling to some external process. This lowers the ratio of peaking energy to storage energy to 0.22. Should a heat sink at this low temperature not be required, a higher ratio of peaking energy to storage energy may be obtained by using the next embodiment of figures 5a and 5b.

The embodiment according to figure 3b shows a variant of the system according to figure 3a. In figure 3b, the EBE discharging line 71 starts before the CPG turbine 40 and working fluid passing the EBE discharging line 71 does therefore not pass this CPG turbine 40 in the same working process. In this variant of figure 3b, the EBE turbine 60 can be bigger than the corresponding turbine 60 in the embodiment according to figure 3 a. While the CPG turbine 40 of figure 3a is working in the normal power generation cycle of the CPG system as well as during the EBE discharging mode of the system, the CPG turbine 40 of figure 3b is only working in the CPG power generation mode of the system.

The P-h diagram shown in figure 4 is applicable to the variant according to figure 3b as well.

Figure 5a shows a further embodiment of the invention claimed. The inventive extension is marked with reference number 6 again. The CPG cycle is still the same as in the two embodiments described above. The EBE cycle corresponds to the embodiment according to figure 3, wherein instead of a heater and a single-stage turbine 60, a multi-stage turbine 600 with inter-stage heating is used and instead of the single-stage compressor 82 and a large post-compression cooling/condensing 61, a multi-stage compressor 820 with inter stage cooling and a smaller post-compression cooler/condenser 61 are used.

The energy per unit mass generated and consumed as well as the heat energy per unit mass produced for figure 5a is listed in the following table 5 :

Table 6 shows the states at the different locations marked in figures 5a and 5b.

Table 6

The basis for the calculation was the same as with the embodiments described above.

Figure 6 shows the P-h diagram, i.e. the pressure as a function of the specific enthalpy at these different points in the two cycles corresponding to figures 5a and 5b. The CPG cycle is marked with a solid line, the EBE cycle with a broken line, wherein the EBE discharging line is shown with a dashed line and the EBE charging line with a dotted line.

For the example given the following was found:

WCPG, net = 12 kj/kg for the CPG cycle

WEBE, peaking = 169 kj/kg for the EBE in the discharging mode and

WEBE, storage = -213 kj/kg in the storage mode of the EBE. In the embodiment according to figure 5 a, the EBE discharging line 71 starts after the CPG turbine 40. In the variant according to figure 5b, the EBE discharging line 71 starts before the CPG turbine 40. The multi-stage turbine 600 of figure 5b can therefore be bigger than the one of figure 5a.

Figure 7 shows the three time periods ti, t2, t3 of the systems according to figures 1, 3a, 3b, 5 a, and 5b. As can be seen by comparison of the different tables and figures 2, 4, 6, and 7, the embodiments using a low-pressure storage for the Earth battery extension (EBE) at or near the Earth surface with the working fluid being in a sub-critical state and/or near atmospheric pressure have a far better performance than the one using a shallow reservoir with the working fluid being in a super-critical state and/or at significantly higher than atmospheric pressure in the shallow reservoir.

Figures 3a and 3b illustrate embodiments, where a lower peaking energy to storage energy ratio of 0.22 is obtained, but with a large, low-temperature heat sink. The exergy per unit mass, or the useful energy per unit mass that could be generated in these examples, was 69 kJ/kg. Conversely, if a low-temperature heat sink is not needed, multi-stage turbines and/or compressors can be used as shown in figures 5a and 5b. In these cases, the peaking energy to storage energy ratio is increased to 0.72. However, then the heat sink is at a much higher temperature, and thus its exergy is reduced to 21 kJ/kg.

LIST OF REFERENCE SIGNS

Earth surface 64 throttle valve

65 fourth isolation valve deep reservoir 66 third one-way valve first caprock

injection well 70 CPG power generation cycle production well 71 EBE discharging line

72 EBE charging line shallow reservoir

second caprock 80 heater (i.e. a heat sink) working fluid charging and 81 low pressure storage discharging well to/from the 82 compressor

shallow reservoir 820 multi-stage compressor with inter-stage cooling

CPG turbine 83 EBE pump

CPG cooling/condensing unit

third isolation valve 9 inlet line

second one-way valve 90 first one-way valve

A working fluid sequestration

CPG pump component

B geothermal energy generation

Earth battery extension (EBE) component

EBE turbine

EBE multi-stage turbine with W wind power plant, inter-stage heating representing any power EBE cooling/condensing unit generator (e.g. solar) or first isolation valve power grid external to the second isolation valve CPG and/or EBE system