This invention relates to an energy system. The invention also relates to an energy converter and to a mobile unit comprising such an energy converter.

BACKGROUND OF THE INVENTION

Three megatrends characterize the energy situation of the world. Firstly, the global energy consumption shows a steep growth rate due to development of dense populated areas of the world (Asia, Africa and South-America). Secondly, we are running out of easily available hydrocarbon fuel like oil and gas. Thirdly, accelerated use of fossil fuel is polluting our atmosphere with carbon dioxide (CO2) possibly causing global warming and increased ecological instability. These megatrends, put together, represent a huge challenge for our civilisation in general. Our policy and decision makers: politicians, scientists and leaders in corporations struggle to find a sustainable way further.

To reduce the amounts of CO2 released it has been proposed various methods for capturing or recycling CO2. An example of such a method is separation and final storage of CO ₂ below ground. Another example is recycling and reformation of CO ₂ into methane (CH ₄ ) as proposed in e.g. WO 2008/054230 and "Advanced materials for global carbon dioxide recycling", Hashimoto et al., Materials Science and Engineering A304-306 (2001 ) 88-96.

In the concept proposed by Hashimoto et al. electricity is produced by solar cells in desert areas. At coasts close to the desert, the electricity is used for hydrogen production by seawater electrolysis (water splitting) and the hydrogen produced is used for CH ₄ production by reacting hydrogen with CO ₂ . The CH ₄ produced is liquefied and transported by ships to energy consuming, more populated, areas where CH ₄ is used as a fuel and allowed to react with air in regular combustion systems. The CO2 produced in the combustion is separated from other combustion products (mainly water and nitrogen oxides) and recovered, liquefied and transported back to the CH ₄ production site at the coast close to the desert. This way CO ₂ can be globally recycled. Hashimoto et al. focus mainly on the problems associated with seawater electrolysis and the catalysts for CO ₂ conversion and claims that these problems are solved.

Concepts of the Hashimoto-type seem not yet to have been implemented on the large scale they are intended for. A reason for this might be that such large projects are not easy to start. Other reasons may be of technical nature, for instance that the problems related to large-scale seawater electrolysis, CO ₂ conversion and/or CO ₂ recovering have turned out to be greater than expected. There is still a need for sustainable energy converting methods, systems and devices that do not release net amounts of CO ₂ into the atmosphere.

SUMMARY OF THE INVENTION

An object of this invention is to provide a sustainable energy system that exhibit an improved possibility of being realized compared to the systems previously presented. This object is achieved by the system defined by the technical features contained in independent claim 1. Another object is to provide an energy converter suitable for use with such a system. This object is achieved by the energy converter defined by the technical features contained in independent claim 7. The dependent claims contain advantageous embodiments, further developments and variants of the invention.

The invention concerns an energy system, comprising a first energy converting plant located at a first geographical position and a second energy converting plant located at a second geographical position, wherein the first energy converting plant is configured to produce hydrocarbon (HC) and oxygen (O2) from carbon dioxide (CO2) and water (H ₂ 0) using energy obtained from a non-fossil energy source, wherein the second energy converting plant is configured to produce power, electricity and/or heat by reacting HC and O ₂ such as to form CO2 and H2O, and wherein said energy system further comprises a transporting system configured to transport the HC produced in the first energy converting plant to the second energy converting plant and to transport the CO2 produced in the second energy converting plant to the first energy converting plant. The inventive energy system is characterized in that the first energy converting plant is configured to extract the O2 produced and that the transporting system is configured to transport said O2 to the second energy converting plant. Besides re-circulation of CO2 such an energy system enables combustion of the HC in pure 0 ₂ that has been produced in the first plant. Thus, the oxygen that conventionally is considered only as a bi-product of the first plant (or, as in the case of Hashimoto et al., is not paid any attention to at all) is used as a main reactant in the second plant.

Combustion of HC in pure 0 ₂ , in contrast to combustion of HC in air that contains only around 20% 0 ₂ and 80% nitrogen gas (N ₂ ), provides for a high efficiency of combustion and no production of nitrogen oxides (NO _x ) compounds. Besides being corrosive, NO _x -compounds together with non- oxidized nitrogen gas contaminate the CO2 produced. Thus, combustion of HC in pure oxygen produces relatively pure CO ₂ that easily can be recovered and fed/transported back to the first energy converting plant in a partly closed energy system. By properly positioning the first and second energy converting plants, i.e. by properly selecting the first and second geographical positions, an energy system is provided that makes use of non-fossil or renewable energy that is available at a first deserted location, e.g. by using sun light in a desert or heat from geothermal sources, and that provides useful energy (heat, electricity etc) at a second densely populated location using HC as energy carrier and using system generated pure O2 to achieve an efficient combustion and simplified recovering of C0 ₂ .

The distance between the first and the second geographical positions is typically at least 1000 km but shorter distances are conceivable depending on climate, distribution of population, the magnitude of energy supply and demand, topography between the geographical positions, etc. If the distance is very short it is likely that the non-fossil energy source could be used in a more direct way instead. A minimum distance may be around 100 km.

The term non-fossil energy source refers to a source that does not make use of a fossil fuel, such as coal, petroleum or natural gas. Main non-fossil energy sources are hydroelectric, nuclear, geothermal, solar, tide, wave or wind, or rely on burning of wood or waste. Renewable energy is meant to be energy which comes from natural resources such as sunlight, wind, rain, waves, tides, and geothermal heat, which are renewable (naturally replenished). A non-renewable resource is a natural resource which cannot be produced, grown, generated, or used on a scale which can sustain its consumption rate. These resources often exist in a fixed amount, or are consumed much faster than nature can create them. Fossil fuels (such as coal, petroleum and natural gas) and nuclear power (uranium) are examples. If the primary goal of using the inventive system is to reduce the atmospheric emissions of CO2 it is possible to use nuclear power as the energy source.

In an embodiment of the invention the second energy converting plant is configured to extract the H ₂ 0 produced and the transporting system is configured to transport said H ₂ 0 to the first energy converting plant. This creates a completely closed energy system - only primary, non-fossil energy is fed to the system (at the first plant) and only useful output energy is fed out from the system (at the second plant). The chemical products of the first plant form reactants in the second plant and vice versa. This system may be noted Closed Loop Carbon Capture and Recycle (CL CCR). This is a particular embodiment of Sanner cycle energy system described in the Norwegian patent application NO20101664 Sanner cycle energy system .

Besides the elimination of environmental emissions, this embodiment of the invention has the advantage that water suitable for HC-production is fed to the geographical location of the first energy plant which makes it possible to avoid the complicated processing of salty seawater and which is necessary if there is no water source at all present at that location.

In an embodiment of the invention the first energy converting plant comprises a first reaction unit configured to split water into hydrogen (H ₂ ) and oxygen (O ₂ ) using energy obtained from said source, and a second reaction unit configured to produce hydrocarbon (HC) and water (H ₂ O) by reacting carbon dioxide (CO ₂ ) with the H ₂ produced in the first reaction unit.

In an embodiment of the invention the non-fossil energy source is a renewable energy source, such as solar radiation, wind, flowing water or geothermal heat.

In an embodiment of the invention the HC is methane, methanol or ethanol. In an embodiment of the invention the transporting system comprises ships and/or pipelines.

The invention also concerns an energy converter comprising: a reaction unit configured to react oxygen (0 ₂ ) and hydrocarbon (HC) such as to produce carbon dioxide (C0 ₂ ) and water (H ₂ 0) as well as output energy, such as heat and/or electricity; a first supplying arrangement configured to supply the reaction unit with HC; and a second supplying arrangement configured to supply the reaction unit with O2 that is substantially free from nitrogen, wherein the energy converter comprises a collecting unit configured to collect the CO2 produced in the reaction unit. The inventive energy converter is characterized in that it comprises a tank member having a first and a second compartment separated by a flexible wall, wherein the first compartment forms a HC supply tank that forms part of the first supplying arrangement, wherein the second compartment forms the CO2 collecting unit, wherein the tank member is provided with a filling and discharge arrangement for filling HC into the first compartment and for discharging CO2 from the second compartment, and wherein the filling and discharge arrangement is arranged such as to allow discharging of CO2 when the first compartment is refilled with HC. Such an energy converter provides for CO2 recycling and is suitable for use with an energy system of the above type in a stationary application or arranged on a mobile unit, such as a ship or a vehicle, for propelling the mobile unit. That the separating wall of the tank member is flexible means that it is movable and/or is at least partly made of a flexible material, which makes it capable of adjusting the volume ratio between the two compartments such as to equalize, or at least decrease the difference between, the pressure in the two compartments. Together with the filling and discharge arrangement, such a tank member saves space and enables efficient refilling while at the same time allows easy and efficient discharging, collecting and recovering of CO2.

The HC and oxygen are preferably taken from an energy system of the above type. The carbon dioxide recovered from the second compartment is preferably also fed to such a system. In an embodiment of the invention the filling and discharge arrangement comprises a first valve member for filling HC and a second valve member for discharging C0 ₂ . In an embodiment of the invention the flexible wall is movable.

In an embodiment of the invention the flexible wall is at least partly made of flexible material. In an embodiment of the invention a separation unit for separating C0 ₂ and H ₂ 0 is arranged between the reaction unit and the C0 ₂ -collecting unit, i.e. downstream the reaction unit and upstream the C0 ₂ -collecting unit.

In an embodiment of the invention the energy converter comprises a collecting unit configured to collect H ₂ 0 produced in the reaction unit.

In an embodiment of the invention the energy converter comprises a second tank member forming part of the second supplying arrangement, said second tank member intended to form a supply of substantially nitrogen-free 0 ₂ .

In an embodiment of the invention the reaction unit comprises a fuel cell or a combustion engine.

In an embodiment of the invention the energy converter comprises a regulating unit for regulating the supply of HC and 0 ₂ to the reaction unit.

In an embodiment of the invention the hydrocarbon is methane (CH ₄ ).

The invention also refers to a mobile unit, such as a ship or a land vehicle, comprising an energy converter of the above type, wherein the energy converter is arranged for propulsion of said mobile unit. BRIEF DESCRIPTION OF DRAWINGS

In the description of the invention given below reference is made to the following figure, in which:

Figure 1 shows, in a schematic view, an embodiment of the inventive energy system, and

Figure 2 shows, in a schematic view, an embodiment of the inventive energy converter.

Figure 3 shows the pressure drop in 1000 km pipeline against pipeline diameter for transport of hydrogen (800 MW heat transport at 10 to 30 bar total pressure).

Figure 4 shows the pressure drop in 1000 km pipeline against pipeline diameter from transport of oxygen (800 MV heat transport at 10 to 30 bar total pressure).

Figure 5 shows the principle of the inventive cycle indicating recovery of energy at different temperature levels.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

The closed energy system

Figure 1 shows, in a schematic view, an example of an energy system 1 according to the invention. The system 1 comprises a first energy converting plant 10 located at a first geographical position and a second energy converting plant 20 located at a second geographical position. The first energy converting plant 10 is configured to produce hydrocarbon (HC), in this case methane (CH ₄ ), and oxygen (O ₂ ) from carbon dioxide (CO ₂ ) and water (H ₂ O) using energy E _in (arrow 3) obtained from a non-fossil energy source. For this purpose, the first plant 10 comprises a first reaction unit 11 configured to split water into hydrogen (H ₂ ) and oxygen (O ₂ ) using said energy Ε, _η 3. The first plant 10 further comprises a second reaction unit 12 configured to produce HC, in this case methane, and water by reacting CO ₂ with the H ₂ produced in the first reaction unit 11. The first geographical position is in this example a desert area with a great surplus of energy in the form of heat and light. Thus, the energy source used for obtaining E _in is solar radiation.

The chemical reaction in the first reaction unit 11 can be written: Ε, _η + 4H ₂ 0 → 4H ₂ + 20 ₂ , and the chemical reaction in the second reaction unit 12 can be written: C0 ₂ + 4H ₂ → CH ₄ + 2H ₂ 0. Thus, the resulting chemical reaction in the first energy converting plant 10 can be written as: E, _n + C0 ₂ + 2H ₂ 0→ CH ₄ + 20 ₂ . The first and second reaction units 11 , 12 are connected to each other such that hydrogen can be fed from the first unit 11 to the second unit 12 and such that water, and preferably also heat, produced in the second unit 12 can be fed to the first unit 11. Plants, reaction units, equipment etc. for carrying out the reactions in the first plant 10 are known as such to the person skilled in the art.

The second energy converting plant 20 is configured to produce output energy E _ou t (arrow 23) in the form of power, electricity and/or heat by reacting HC and purified 0 ₂ , i.e. not air but oxygen substantially free from nitrogen (N ₂ ), such as to form C0 ₂ and H ₂ 0. For this purpose the second plant 20 comprises a reaction unit in the form of an oxyfuel combustion unit 21. Such a combustion unit is also, as such, known to the person skilled in the art. The resulting chemical reaction in the second energy converting plant 20 can be written as: CH ₄ + 20 ₂ → C0 ₂ + 2H ₂ 0 + E _ou t, i.e. in principal the resulting chemical reaction in the first energy converting plant 10 in reverse.

The second geographical position is in this example a densely populated area with a great demand of energy in the form of heat and electricity. Both plants 11 , 12 are configured to extract and separate the resulting chemical compounds produced at each plant.

The energy system 1 further comprises a transporting system 30 configured to transport the methane and oxygen produced in the first energy converting plant 10 to the second energy converting plant and to transport the carbon dioxide and water produced in the second energy converting plant 20 to the first energy converting plant 10. Arrowed lines 31-34 denote transporting means that, for instance, may be pipelines and/or ships. Land vehicles and aircraft are also possible as alternatives or complements.

This means that the chemical products of first plant 10 can be used as reactants at the second plant 20, and vice versa. The energy system 1 thereby forms a completely closed system which may be denoted Closed Loop Carbon Capture and Recycle (CL CCR). Only primary, non-fossil and renewable, energy is fed to the system 1 (at the first plant 10) and only useful output energy is fed out from system (at the second plant 20).

The system can be said to charged at the first plant 10 - renewable energy upload (RE Upload) - and to be discharged at the second plant 20.

It is possible to set up a model for an energy value chain collecting renewable energy from multiple distant sources, convert this to the storable and movable energy carrier, methane (CH ₄ ), transport this to the markets, and deliver the energy to the customers as power and heat. This can be done by using water splitting: 4H ₂ O + Energy -> 4H ₂ + 2O ₂ ; and the Sabatier process, an exothermic reaction transforming hydrogen and carbon dioxide to methane and water: CO2 + 4H ₂ -> CH ₄ + 2H ₂ O + Heat. The system 1 has a feedback loop for transporting the discharged energy carrier (CO ₂ ), back to the renewable energy area for re-charging. The physical system consists of renewable energy collectors, industrial plants and energy transportation carriers (pipes and ships). These are existing components used today in different industrial context. Put together in a complete system, this represents a Renewable "closed loop" Carbon Capture and Recycling (CCR) value chain, with no chemical emission to the environment (zero CO2). By using the nature's carbon cycle as a roadmap, a closed industrial loop for the global energy value chain can be set up, resulting in no atmospheric consequences or imbalances. By converting renewable energy, produced at distant sites, to methane (CH ₄ ) - the main component in natural gas, a flexible proven energy carrier is provided for all types of renewable energy (solar / wind / hydro /wave / geothermal etc.). The return cycle with CO2 and H ₂ 0 (if not present at renewable site) is required to get the loop fully closed. Ship transport may be required between nodes in the network, where pipes are not feasible. The ships could then carry the discharged components back when picking up new energy loads. Thereby, the return transport do not necessary load heavily to the business case (especially taken into consideration that C0 ₂ -removal might represent an income flow).

By using energy collected with renewable energy collectors, combined with input of H ₂ 0 and CO ₂ , we will have a Power Gas Plant (the first energy converter 10) instead of a Gas Power Plant (such as the second energy converter 20). This represents "The art of running as Gas Power Plant in reverse", and could be called reversed combustion.

The proven success of wood, coal, oil and gas, as energy providers for the world, is not only about availability, it's also about collectability, movability and storage ability. For instance, there has always been more solar power available in the world than hydrocarbon resources. But, until now there has been technically and economically more feasible to collect energy from the concentrated fossil energy reservoirs, - energy ready to be transported and stored down the value chain. With fast development of technology for e.g. mirror-based solar parks in the desert (...or floating windmill parks, or geothermal parks ...) combined with water-splitting and methanization, the Renewable "closed loop" CCR value chain could at a certain point in the future become competitive.

Many different renewable energy sources can be utilized as the energy provider for the water splitting and methanization, such as solar, wind, water, wave, tide, salt energy.

In the system 1 the renewable energy is captured where the natural conditions is optimal. This will be e.g. solar energy in the equator close deserts, wind parks in rough climate, geothermal parks at the hot spots of the earth, or distant hydropower stations far away from cities.

The energy is converted into chemical energy by charging CO2 to CH ₄ by water splitting and methanization. The fuel elements CH ₄ and 2Ο2 balance the type and number of atoms (and thereby the mass) of the recycling elements C0 ₂ and 2H ₂ 0. Cooling can be exchanged between input and output component regarding LNG loading operations.

At the consumption site, i.e. at the second plant 20, the methane is combusted with concentrated 0 ₂ (oxyfuel). Heat and electrical energy is produced in the power plants. In this system the power plants will have no emission to the environment; - the resulting C0 ₂ and water (H ₂ 0) will be kept in the loop and transported back to the "recharging site". The inventive energy system 1 is not limited by the embodiment described above but can be modified in various ways within the scope of the claims. For instance, the carbon dioxide and water produced in the second energy converting plant 20 do not necessarily have to be separated at the second plant 20 but can be transported before separation.

Further, it is not necessary to transport water from the second to the first plant but this is an advantage if there is no water present at the first geographical position or if only salty seawater, that complicates the process considerably, is present.

Moreover, the HC may be other than CH ₄ , e.g. methanol (CH ₃ OH) or ethanol (C ₂ H ₅ OH). If so, some of the chemical reactions involved will be slightly different from what is described above. For instance, if the hydrocarbon is methanol the second reaction unit 12 would be a methanolization unit where the main chemical reaction can be written: CO ₂ + 3H ₂ → CH ₃ OH + H2O. The resulting chemical reaction of such a first energy converting plant 10 can be written as: E _in + C0 ₂ + 2H ₂ 0→ CH ₃ OH + 3/2Ο2. At the second plant of such a methanol system, the reaction can be written: CH ₃ OH + 3/2Ο ₂ → CO2 + 2H ₂ 0 + E _o u _t . Thus, the compounds re-circulated between the two plants in an energy system using methanol as HC will be the same as if methane is used. The same holds also for e.g. ethanol.

Further, it is not necessary to use H ₂ to produce HC and oxygen from carbon dioxide and water. This may instead be achieved in a one-step process using heat and/or electricity obtained from the non-fossil source in a more direct way (together with suitable equipment, catalysts etc.). Thus, it is not necessary that the second energy converting plant 10 comprises any water splitting reaction unit 11.

Heat exchangers may be included in the system to transfer heat between incoming and outgoing flows at either or both plants. This is advantageous both for heating the incoming reactants and for cooling the products that are to be transported to the other plant, possibly in liquid form.

The first and second plants may operate intermittently and with a varying production rate to account for e.g. variations in the inflow of non-fossil energy at the first plant or variations in the demand for output power at the second plant. To allow for such varying operation the compounds can be accumulated in the transporting system. The energy system can include more than one first energy converting plant, i.e. more than one plant of the charging type, and/or more than one second energy converting plant, i.e. more than one plant of the discharging type. Thus, the transporting system is not limited only to a direct transport between the first and second plants; this transport may be carried out via other first and/or other plants in the system. Similarly, a single plant can include more than one reaction unit working in parallel. Such an energy system could be noted Sanner cycle network, described in the Norwegian patent application NO20101664 Sanner cycle energy system.

The closed energy converter or closed engine

All components in the system 1 (value chain) are preferably driven by energy captured by the renewable energy collectors. This means that e.g. LNG (liquid natural gas)-terminals and gas transportation ships will combust some of the CH ₄ - 0 ₂ and produce C0 ₂ - H ₂ 0. Using special engines or energy converters, this CO ₂ and H ₂ O will not be emitted to the environment. All matter will be fed back into the value chain. Theoretically, no atoms will escape. Equipment with this capability could be defined to be "CL CCR- certified".

The same principle as described above can be used in an energy converter in the form of a (closed) engine that may be a stationary unit or a mobile unit, i.e. it can, for instance, be used to propel ships used in the transporting system of the inventive system 1 but also to propel other boats, land vehicles or aircraft.

Figure 2 shows, in a schematic view, an embodiment of an energy converter 50 according to the invention. The energy converter 50 comprises a reaction unit 70 configured to react oxygen (O ₂ ) and hydrocarbon (HC), in this example methane (CH ₄ ), such as to produce carbon dioxide (CO2) and water (H ₂ 0) as well as output energy E _out . (indicated by arrow 71 ), such as heat and/or electricity. The reaction unit 70 may be e.g. a fuel cell or a combustion engine, which are well known as such for a person skilled in the art.

The energy converter 50 further comprises a first supplying arrangement configured to supply the reaction unit 70 with methane and a second supplying arrangement configured to supply the reaction unit 70 with 0 ₂ that is substantially free from nitrogen. The latter means that air, which contains around 80% nitrogen, is not fed to the reaction unit. In this example the oxygen is substantially pure.

A collecting unit 62 configured to collect the CO2 produced in the reaction unit 70 is arranged downstream of the reaction unit 70.

The energy converter 50 further comprises a tank member 60 having a first and a second compartment 61 , 62 separated by a flexible wall 65. The first compartment 61 forms a methane supply tank that in turn forms part of the first supplying arrangement. The second compartment 62 forms the C02 collecting unit. That the separating wall 65 is flexible means that is capable of adjusting the volume ratio between the two compartments such as to equalize, or at least decrease the difference between, the pressure of methane and CO2 in the two compartments. The flexibility of the wall 65 is indicated with a partly dotted line.

The tank member 60 is provided with a filling and discharge arrangement in the form of first and second valve members 63, 64 for filling methane into the first compartment 61 and for discharging C0 ₂ from the second compartment 62. The filling and discharge arrangement is, in combination with the flexible wall 65, arranged such that C0 ₂ present in the second compartment 62 is forced out through the second valve member 64 when the first compartment 61 is refilled with HC. The discharged C0 ₂ can thereby easily be recovered in a pure form and, for instance, be recycled in an energy system according to what is described above. The energy converter 50 further comprises a second tank member 80 intended to contain substantially pure oxygen, which second tank member 80 forms part of the second supplying arrangement. Since pure oxygen, and not air, is fed to the reaction unit 70 the products formed in the reaction unit 70 are in principle only water and carbon dioxide. Using air as an oxygen source, nitrogen oxides (NO _x ) are produced in significant amounts, which nitrogen oxides contaminate the CO ₂ and cause corrosion problems. A regulating unit 65 is arranged to regulate the flow of methane and oxygen to the reaction unit 70.

A separating, condensing unit 75 is arranged downstream of the reaction unit 70 for separating gaseous CO ₂ from liquid water. The separated water is collected in a water collecting unit 90.

Thus, methane and oxygen (in pure form) are fed in a controlled manner to the reaction unit 70 where the two compounds react and produce water, carbon dioxide and output energy E _ou t- As described above, the water and the carbon dioxide are fed to the separating unit 75 from which the carbon dioxide is further fed to the carbon dioxide collecting unit/second compartment 62.

The methane and oxygen used for refilling the first and second tank members 60, 80 are preferably taken from an energy system of the above described type.

The inventive energy converter 50 is not limited by the embodiment described above but can be modified in various ways within the scope of the claims. For instance, the hydrocarbon used may be other than methane (see also above). Moreover, it is not necessary to separate water from the carbon oxide before it enters the second compartment 62. However, to simplify handling and recycling of the carbon dioxide it is an advantage if it is purified before it is feed into the tank member 60.

Further, although the water may be released to the environment instead of being collected in the water collecting unit 90, the collected water, which will be very pure, can be used in an advantageous way. It can, for instance, be transferred (back) to the energy system 1 described above or be used for making drinking water on a mobile unit using the energy converter for propulsion.

Besides methane, methanol and ethanol, examples of useful hydrocarbons (HC) in both embodiments described above are ethane, propane, butane, pentane, gasoline, biogasoline, butanol and diesel.

The invention shows a surprisingly good potential regarding efficiency as shown in the following modelling example:

- The calculations are based on the simplest type of flow: 2H ₂ 0 + Energy <=> 2H ₂ + 0 ₂ .

- Losses in conversion of thermal energy to hydrogen or electric power are not included in the energy upload section.

- Ambient temperature: 20 °C (gases and water are cooled to 20 °C during transport)

- Heat recovered in streams down to 50 °C (heat content between 20 and 50 °C lost)

- Transport distance: 1000 km

- Low gas velocity in pipeline: 2-4 m/s

- Pressure in UPLOAD unit is higher than the pressure drop in the pipelines as a minimum

- Temperature difference in heat exchangers: Minimum 30 °C.

- No leakage of hydrogen or oxygen during the transport. The inventive cycle was simulated using Aspen Plus V7.1 process modelling tool. Sufficient heat and/or electric power (HEAT-POW) are supplied to the UPLOAD unit (assumed to be operated at elevated pressure) to split water molecule into hydrogen and oxygen gases. Warm produced gas (H1-H2 and H1-O2) are used to pre-heat the inlet water stream (WATER) in H1-COLD. It is assumed that the produced gases are cooled to 50 °C upstream the pipelines. Water enter the H1-COLD heat exchanger at 20 °C. SEP simulates the separation of hydrogen and oxygen, but this separation process will be part of the UPLOAD unit if use of a water electrolyser unit are assumed. Other options may exist, but this is not part of this evaluation. Hydrogen and oxygen is transported through 1000 km pipelines. In order to reduce pressure drop to below 5 bar the gas velocity should be around 2 m/s for oxygen and around 3.5 m/s for hydrogen (see below).

Hydrogen is assumed combusted with oxygen in the unit COMB. This may generate a very high temperature (3000-4000 °C) and the reaction chamber must be cooled to a reasonable temperature by an appropriate cooling medium. This cooling medium may be part of a power generating cycle.

In principle a significant amount of the heat (84-85%) that are released can be recovered at a very high temperature (>1000-2000 °C). Since pure hydrogen and oxygen are used in the combustor, heat of condensation of the resulting steam can also be recovered. The heat level depends on the pressure in the condensator, but can typically be recovered between 100 and 200 °C in the COND unit.

The pressure in the cycle is generated by means of a pump. Condensed water is pumped back to the UPLOAD unit through a 1000 km pipeline. In order to minimise pressure drop in pipelines a rather low gas and water velocity must be used. This of cause may result in rather large pipeline diameters. The model indicates higher pressure drop for transport of oxygen than hydrogen in the proposed cycle indicating that the oxygen pipeline should at least have the same diameter as the hydrogen pipeline (figure 3 and 4). This also indicates that the gas velocity in the oxygen pipeline must be lower than in the hydrogen pipeline to avoid recompression.

Figure 5 shows the principle of the cycle indicating that heat may be recovered at two different levels; High (H) and low (L). The low level heat can be recovered from condensation of steam and from cooling of condensate from the condensation temperature down to 50 °C. The condensation temperature depends on the pressure in the condensator unit. Simulation runs at different pressures between 20 and 95 bara in the UPLOAD unit gives about the same total efficiency. The only difference is that a higher pressure will allow recovery of the vapour condensation heat at a higher temperature. E.g. with a operating pressure in the UPLOAD unit around 20 bar the pressure in the condensator will be about 13.5 bara resulting in a condensation temperature of about 190 °C. At 60 bara the condensation temperature will be about 275 °C. There will be a minor amount of heat that can be recovered between the condensation temperature and 50 °C since there is more heat in the water stream than can be used for pre-heating the oxygen and hydrogen feed gases. In a real process flow chart the GASHEAT unit used will be split in two heat exchangers.

The table below indicates the main efficiency numbers for the proposed cycle. Base case is 100% input as heat and electric power (W _in + 0, _η ). Table 1 : Main efficiency numbers (estimated at 20 bara in UPLOAD unit)

Fuel cells may convert hydrogen to electric power with a theoretical efficiency of about 80% if pure oxygen is used as oxidant. The efficiency of a fuel cell (and lifetime) is dependent on the amount of power drawn from it. As a general rule, the more power (current) drawn, the lower the efficiency and operating at maximum 50-60 % efficiency is a more likely scenario. Similar to a combustion system using pure oxygen and hydrogen, recovery of most of the heat of condensation will be feasible in a fuel cell system.

In an optimized power cycle around 50-60% of the uploaded heat may be converted to electric power and 49-39% may be used for heating purposes. Modelling of different power generation options will be needed to evaluate this further. However, since oxygen and hydrogen can be transported with only minor losses (1 % if no leakage of gases are assumed) this type of energy will be as valuable (and even more) as e.g. natural gas.

Assuming input of 1000 MW (heat and/or electric power) in the UPLOAD unit the following pipeline diameters and velocities will represent a feasible operation (Table 2). Total pressure drop at 20 bara cycle inlet pressure was estimated to 1 1 bar and assuming 95 bara in the UPLOAD unit will give a total pressure drop of about 35 bara. The pump duty (100 -300 kW) is not very significant and some higher pressure drop will be feasible. Especially the water pipeline can be made smaller and several pump station can be included along the line if needed. It is important to avoid gas compression in the system to achieve a very high efficiency, but high pipeline costs may favour some recompression of gases and water even though this will reduce the overall efficiency.

Table 2: Pipeline diameter assuming 2 m/s (O ₂ ) and 3.5 m/s (H ₂ ) gas velocities at different pressure

Assuming the high pressure case (95 bara), scale up to e.g. the capacity of Langeled (31.5 GW) will need 5 hydrogen pipelines each with a diameter of about e.g. 1.6 m or 10 pipelines each with a diameter of about 1.1 m. The same number of pipelines (and dimensions) will be needed for transport of oxygen 1000 km. Reduced distance will increase the capacity in the pipelines.

A long pipeline will have a significant buffer capacity. A 1000 km pipeline with a diameter of 0.63 m will store 311725 m ³ gas. If inlet pipeline pressure is 95 bara and exit pressure is 86 bara and production of hydrogen is stopped for 16 hours the pipeline pressure will drop to about 70-73 bara if it is a continuous consumption of hydrogen of about 1 GW. A continuous consumption of 1 GW will in this case need an UPLOAD capacity of 3 GW for 8 hours (e.g. solar based energy upload).

It is demonstrated by modelling that the proposed (H2O, O ₂ , H ₂ ) cycle can be operated with a very high efficiency assuming that:

- produced oxygen and hydrogen are transported through (separate) pipelines (e.g. 1000 km) without leakage of the gases - transported oxygen (pure) are used in the combustion process instead of air

- losses in conversion of thermal energy to hydrogen or electric power are not included (cost of hydrogen production are mainly CAPEX dependent) Around 84-85% of energy in produced hydrogen (based on High Heating Value) can be recovered at a very high temperature (> 1000 °C). Efficient use of this heat is possible (e.g. for generation of electric power). Around 12% of available energy in the transported hydrogen can be recovered at a temperature around 100 to 275 °C (useful for e.g. district heating). This depends on the cycle operating pressure. In addition about 2% of the heat will be available for heating purposes down to about 50 °C indicating a heat transport cycle efficiency of around 99%.

Conversion of thermal energy to either hydrogen or electric power will generate a certain amount of "waste" heat. Some of this heat may be reused either by heat-exchanging with inlet streams to the process or it can be used for low temperature heating purposes. How much of this energy that will be utilized will always be a question of investment costs verses cost of the energy. In the UPLOAD section of this cycle energy cost is assumed to be close to zero (stranded heat) and investments in capturing and conversion of the heat will be the main cost factor. The efficiency in conversion of heat to hydrogen and oxygen is thus not very important if the investments costs are moderate. Estimation of an efficiency number including losses during hydrogen production is less relevant without including capital costs for this stage.

In the other end of this cycle where hydrogen is burned with oxygen an efficient use of the fuel is very important. Co-transport of oxygen secures a very efficient use of the transported hydrogen fuel, but cost of the oxygen pipeline must be evaluated against the value of heat that can be utilised and potential increased efficiency in conversion of hydrogen to electric power. The low pipeline velocities will require quite large pipeline diameters that can cause high costs. Leakage of hydrogen during such a long distance may also be a challenge. The main challenge will be cost efficient conversion of e.g. solar heat to hydrogen and oxygen.

Theoretically the main advantages of this cycle are:

- Energy (in form of e.g. hydrogen and oxygen) can be transported with minimal losses in pipelines assuming low gas velocities (2-4 m/s). Total loss of recoverable heat is only 1-3% (depending on ambient temperature and possible use of heat between 50 and 100 °C).

- Uploaded heat in form of e.g. hydrogen can be converted to electric power with about the same efficiency as e.g. use of natural gas.

- If both oxygen and hydrogen are transported to the energy conversion plant, heat of water condensation (about 15% of available energy content in hydrogen) can be used for heating purposes (100 - 275 °C). If air are used in the combustion this will be manly lost.

- A long pipeline will have a significant buffer capacity. A 1000 km pipeline with a diameter of 0.63 m will store 311725 m ³ gas. If production of hydrogen is stopped for 16 hours the pipeline exit pressure will only drop from about 86 to 70-73 bara if it is a continuous consumption of hydrogen of about 1 GW.