WO2014042580A1

WO2014042580A1 - Method for improving the performance of thermodynamic cycles

Info

Publication number: WO2014042580A1
Application number: PCT/SE2013/051059
Authority: WO
Inventors: Esko Ahlbom; Joachim KARTHÄUSER; Thomas ÖSTRÖM; Olle BERGSTRÖM
Original assignee: Climeon Ab
Priority date: 2012-09-12
Filing date: 2013-09-11
Publication date: 2014-03-20

Abstract

A method for improving the performance of thermodynamic cycles, particularly based on C0₂ gas absorbed in a liquid absorbent used as working- fluid in a system comprising at least one C0₂ gas absorber, at least one heat exchanger and a device for converting gas expansion work to mechanical work. Said working fluid is fed to the device for releasing said CO₂ gas under pressure by heat supplied from an external heating source operating at least at 70°C, preferably at 80 to 90°C or higher, wherein the device being selected from the group consisting a modified 2-stroke engine, a turbocharger and a turbine.

Description

Field of the invention,

Reference is made to PCT/SE/2012/050319 v/ith priority from SE 1100208-6 (filed 22.03.2011}, US application 61/468 474 (filed 28.03.2011} and SE 1100596-4 (filed 16.08.2011}, these

documents are included by way of reference. These documents describe a novel thermodynamic cycle for energy and cold production from heat below 16Q°C, called C3 or "Carbon Carrier Cycle" .

In the Carbon Carrier Cycle, C3, heat is used to chemically desorb CO₂ from a carrier medium or working fluid. After the CO₂ has expanded so as to create electricity, for instance, it is chemically absorbed into the carrier medium during cooling. The chemical absorption is very efficient at creating low pressure, and the CO₂ can be expanded to temperatures as low as -78°C resulting in high power output, despite of low^" heat source te peratures.

All technical solutions for energy conversion are in constant need of improvement , cost reduction, size reduction, increased plant availability, and improved integration into existing infrastructure . As examples, (i) turbines are usually the most expensive parts of thermodynamic cycles, therefore any cheaper solution can be competitive even if efficiency is sacrificed, (ii) equipment size can enable or prevent usage in certain applications, therefore even marginal improvements (most often size reductions} can have significant effects, (iii) systems are rarely stand-alone, but form, rather part of a more complex architecture, therefore smart integration of all material and energy streams is highly desirable. This text describes current problems in detail and discloses various ways of solving said problems.

Background, of the invention

The invention disclosed here solves the problem of the current art, namely that the overall performance of the energy

generation system is too low in terms of footprint, costs per kWh generated, or lack of integration. Various partial

solutions can be used alone or in combination. Various

embodiments are described showing preferred ways of solving said problems .

Suraraary of the invention

One object of the invention is to achieve a method by which the performance of thermodynamic cycles, particularly the;

performance of the C3 (Carbon Carrier Cycle) based on CO₂ gas absorbed in a liquid absorbent used as working fluid in a system is improved.

This object is achieved according to the invention by a method for improving performance of thermodynamic cycles,

particularly the performance of the C3 (Carbon Carrier Cycle) based on CO₂ gas absorbed in a liquid absorbent used as working- fluid in a system comprising at least one CO₂ gas absorber, heat exchangers, at least one pump device and a pipe system for transport of said working fluid through the system, and a device for converting gas expansion work to mechanical work, characteri zed by : - feeding said working fluid to the device for releasing said CO? gas under pressure by heat supplied from an external heating source operating at least at 70°C, preferably at 80 to 90°C or higher, wherein the device being selected from the group consisting a modified 2-stroke engine, a turbocharger and a turbine.

Preferred embodiments are defined in the appending dependent claims . Brief description, of the drawings

The invention is described in more detail below in the form of non-limited examples, reference being made to the appended drawings , i n which

- Fig. 1 represents a generalized view of a Carbon Carrier Cycle from a system perspective,

- Fig. 2 is a schematic view of a piston engine in its

stripping stage for converting energy in the form of

pressurized gas into mechanical rotation of a rotating axis, and

- Fig, 3 is a schematic view of said piston engine in Fig. 2 in its adsorption stage.

Description of preferred embodiments

Fig. 1 illustrates a generalized view of the Carbon Carrier Cycle f om a system perspective having an essentially closed loop comprising a liquid pump 6, a pre-reactor (which can for most practical purposes be considered as heat exchanger and preferably forms a part of a heat exchanger 12, a reactor 7, a booster 19, an expansion system 15, an absorber system 3 and a cooling system. 4. The pump 6 increases the pressure of a CO₂ gas liquid absorbent, where the pre-reactor and the reactor 7 release the C0₂ by thermal heat. The possible booster 19 further heats the high pressure CO? before it enters the expansion device 15 to produce electricity. The expanded gas optionally enters a heat exchanger 35 for extracting low temperature either for external cooling applications or cooling the absorbent before absorption. The expanded CO₂ gas is chemically absorbed or converted to CO? gas liquid absorbent in the absorption system 3 during cooling by the cooling system 4 for the process to repeat.

The invention will be described with reference to the below examples .

Example 1 A piston engine is used as an expansion device for turbine replacement since said piston engine is a suitable means for converting energy in the form of pressurized gas into

mechanical rotation of a rotating axis to which an electrical generator may be coupled. It may be less efficient than turbines, but as piston, engines are mass-produced, they can. be considerably cheaper, and therefore the cost, per kWh generated can be lower .

Fig. 2 shows the working principle of such a piston engine. Only one cylinder is shown, but it. should be understood that a plurality of cylinders can be used in the invention such that an almost continuous mode of operation can be achieved. In the C3 cycle, CO₂-loaded liquid, absorbent, e.g. a mixture of dibutylamine and Carbitol solvent which contains between 1 and 99% of the theroretical maximum of CO₂ (chemically bonded as per the a.m. disclosures), is pumped through an inlet 1 as "rich liquid absorbent" into a cylinder 43 of an. engine, which resembles a 2-stroke engine. The inlet 41 is provided with a valve 40 and an outlet 42 for hot pressurized CO₂ g s, the outlet 42 provided with a valve 49 is placed down streams of the valve 40 as seen in the flow direction of the rich liquid absorbent. The rich liquid absorbent is heated through heat supply 48 and is optionally even pre-heated before entry through inlet 1. CO₂ gas is produced in the cylinder, expanding and driving a piston 44 and a crankshaft 45 connected thereto. An electricity generator (not shown) may be coupled to the crankshaft 5 so as to produce electric energy. After the CO₂ gas has expanded a valve 49 is opened which allows the CO₂ gas to proceed to an absorption stage (see Fig. 2} . The CO?- depleted absorbent ("lean, hot absorbent") is exiting through an outlet 47. Preferably, the outlet 47 is at the bottom, of the cylinder 43 such that the lean, hot absorbent and the CO₂ gas can exit the cylinder 43 simultaneously. Valves 49, 40 and 46 are opened and closed as required. Following the gas evolution, the piston 44 will move to the left, as seen in Fig. 1, the cylinder is essentially emptied of both the liquid absorbent and the C0₂ gas, and the process can start again. A pump is preferably pumping out the lean hot absorbent through outlet 47 as the pressure in the cylinder 43 can be below 1 bar. It is evident, that the control of valve opening times and injection / ejection periods of the liquid absorbent and the CO₂ gas is a balancing and optimization task not un-similar to conventional combustion engine management.

Now referring to Fig. 2, the absorption process proceeds as follows : CO2 gas enters the absorption engine or cylinder 43 through inlet 41 and valve 40. The cylinder volume is close to its maximum at this stage, i.e. the piston 43 is positioned at the right hand side of Fig. 2. Lean, cold absorbent is pumped, preferably sprayed into cylinder 43 through an inlet 52 and valve 53, causing the C0₂ gas to be absorbed in the absorbent and further causing- the pressure in the cylinder 43 to drop to near vacuum. This in turn causes the piston 44 to move the left, as seen in Fig. 2. The absorbent is cooled by a cold supply 54, and exits the cylinder 43 through outlet 47 and valve 46. Thereafter, the cycle starts again. Valve opening times and adjustment to the piston position are optimized carefully. Optionally, a gas expansion turbine is placed between the desorber and the absorber engine, however, the pressurized gas from the desorber is usually more useful for pushing back the piston (to the right) . Both the desorber and the adsorption engines can be connected to the same axis 51, or on a separate axis each. The engines may contain internal devices for agitation and more intensive heat transfer (both heating and cooling) .

Compared with other solutions, such as turbines, the mode described in this example allows a higher degree of discharge of rich loaded absorbent, i.e. the total amount of liquid per kWh generated can be lower. In terms of dimensions, assuming a flow of 5 kg CO 2 per second for a 600 k generator, the

required absorbent flow will be about 100 kg/s. Accordingly, an engine with 10 cylinders of 25 liter each operating at 1 rpm is an appropriate engine size (or 10 cylinders, 12 liter volume per cylinder, 2 rpm) . A similar construction can be rea1 i zed u s ing a rotary Wanke1 engine .

Example 2

In example 2 the use of one or more turbochargers as expansion device is described. Similar to the piston engine

configuration in example 1, turbochargers such as used in trucks, are non-optimized for e.g. CO₂ expansion, however, they are cheap due to mass fabrication. An additional benefit is that a plurality of turbocharaers or expanders can be used such that variable heat supplies can be accommodated, simply by using only one, two, three or as many as technically ideal at a certain load or demand. A. single turbine usually has a very narrow performance regime.

Counter-current absorption of CO? in a spray reactor, and reducing the space requirement of the absorber section.

According to the referenced documents, lean absorbent is sprayed into the absorption reactor in the form of droplets, e.g. between 50 and 200 micrometer in size. These droplets are heated by the exothermic absorption of CO₂, and it turned out to be useful to operate this absorption section in a

countercurrent mode, and also to collect liquid at least once and to re-spray partially loaded, and preferably cooled absorbent again into the CO₂ gas stream. In this way, a cold rich absorbent stream can be collected at the point where the CO₂ gas enters the absorber section. Preferably, the absorber- is placed horizontally such that liquid can be collected at the bottom of this section. Also, agitation devices, e.g. a transport screw, can be installed e.g. at the bottom of the absorber, in order to pump absorbent and improve the heat discharge to a cooling medium. It should be understood that the spray can be generated in various ways, both in a

traditional absorption reactor and in engine configurations as per Example 1 and 2. The absorbent liquid may be pumped through filters or nozzles which generate the spray, or the absorbent liquid may be pumped onto a rotating disc or rotary atomizer, such as generally known from the powder production or painting industry, or similar devices which disintegrates the liquid mechanically into droplets. Other known engineering solutions are conceivable including- wiped-film, arrangements which help to agitate the absorbent and accelerate the uptake of CO₂ as by the absorbent. Cooling elements may be included in the absorber in a way and at adequate spacing such that CO₂ gas can flow unhindered within the absorber volume.

Combinations of the mentioned techniques, such as spray, cooled walls, recycle of absorbent and agitation are useful and within the scope of the invention.

A way of compacting the C3 unit is to employ a Venturi

scrubber design downstream of a turbine . Here, the gas flow is accelerated as the flow channel for CO₂ gas is converging and therefore narrowed. Lean, cold absorbent enters the gas flow near or at this narrow channel. The high gas speed serves to disintegrate the liquid absorbent into the desired small gas droplets. As the channel widens, saturated absorbent droplets will contact the wall of the diverging section, and can be cooled such that CO₂ evaporation is limited. Various Venturi and cyclone designs can be chosen such that turbulence before and after the narrow throat section, residence time of

droplets in the gas stream etc., number of spray nozzles etc., can be reduced or optimized, respective1y . Similarly, a small turbine can be used, partly for compacting the C3, partly for using the high gas speed at the exit of the turbine - which as such means loss of kinetic energy - for disintegrating the liquid, possibly even in combination with the Venturi design mentioned above . It will be understood that the choice of technology is a balance between parameters such as space requirement, desired product ratio (electricity versus cold), turbine or expansion device costs and so forth. This choice depends on the application. Example 4

An amine/solvent liquid absorption medium yields CO₂ gas upon heating and it is practical to use heat exchangers which allow easy escape of gas as soon as said liquid absorbent is heated. Suitable heat exchangers are those known in the art e.g. for evaporation of Rl34a, butane or similar liquids. Said heat exchangers heating up rich, i.e. CO₂ loaded absorbent, may form part of the desorber or stripper section, and their gas outlets may directly be in connection with the pipe; leading to the turbine or expansion device.

A booster section, sometimes referred to as superheating section, as described in a.m. disclosures, may be employed in order to selectively heat the CO₂ gas before it enters the expansion device .

Example 5

A geothermal heat source may be used since heat of about 90°C is available from geothermal sources in 2-3 km depth.

Temperatures above 11Q°C are very difficult to obtain at reasonable costs, except at certain places e.g. with volcanic activity (e.g. Iceland). It is highly useful to couple the production of 90°C water streams, and the electricity

production using the C3 cycle, to district heating networks. In this sense, a highly useful configuration is given by using a 20 °C temperature delta (i.e. 90-70°C) for the C3 cycle, and the residual flow (70-40 or 30°C) for heating of buildings. Maximizing the temperature difference (delta) between hot water production and cold water re- injection into the ground is beneficial for the overall economy. Exaiaple 6

Effluents from power plants may be used as eat source. At power plants, hot water streams between 120 or 110 down to 90°C are available. Where possible, these streams are diluted to a lower temperature, such as 70 or 80°C, for pumping into district heating networks. It is highly efficient to use the C3 cycle for electricity production near the power plant, where the C3 cycle serves as first cooling device for the hot water stream. Ideally, the C3 is used to cool the hot water effluent down to 70 or 80°C.

Exaiaple 7

Preferred cosolvents and amines. Preferred amines as

absorbents are described in the referenced documents. It turned out that certain cosolvents are very beneficial for a) reducing the viscosity of the CO₂-loaded absorbent, b)

preventing crystallization of loaded absorbent, c) providing lubrication for movable parts in pumps etc, d) preventing foaming of absorbent during the stripping stage, and e) optimizing the spraying performance during absorption or during flash evaporation in the stripper section. Particularly useful cosolvents are glycol ethers, such as mono- or dialkyl- substituded oligomers of ethylene or propylene oxides.

"Carbitol solvent", or diethylene glycol monoethylether is one of the preferred cosolvents due to good price/performance ratio. Carbitol and butylcarbitol are also preferred for highly alkaline amines (whereby "highly" means pK_b values below 3,3} such as diaza-bicyclo-undecene, 1-Methylimidazole and other strong amines (see references) . A useful ratio of dibutylamine and Carbitol is 2 weight parts amine and 1 weight part Carbito1 , or preferab1y 1ess Carbito1 ,

It is also noteworthy that certain solvents are capable of reducing the energy required for desorption of CO2 , or the exothermicity of the CO2 absorption reaction. It was noted that the mixing of amine and solvents such as ethylene glycol, Carbitol Solvent, phenol, cresols, glycerol and the like generates heat. This heat (calculated as product of mass, heat capacity and temperature increase) can be expressed in terms of energy per mass CO2 which can be absorbed in the mass of amine used in the specific experiment. It is found that this energy (in the order of 100-300 kj./kg C0₂) is about 50% of the measured reduction of the absorption enthalpy or energy. Also, pure carbamates such as the reaction product of 2 moles dibutylamine and 1 mole CO2 , were dissolved in Carbitol, ethylene glycol, methanol and the like. Surprisingly, the mixture is cooling down (by some 8-12 ⁰ C) . Also this energy (in the order of 100-300 kJ/kg C0₂) can be related to the amount of CO2 which is absorbed in the respective sample, and again the value is in the order of 50 % of the absorption enthalpy. Without wishing to be bound by scientific theory, we assume that the observed energy changes reflect a change in the reaction energetics. In practice, it is found that the absorption enthalpy for the amine + CO2 ~ products reaction is reduced from ca . 1800 kj./kg C0₂ to about 1100 kJ/kg C0₂ (as measured from the slope of a logarithmic pressure versus 1/T diagram (also referred to as "van' t Hoff diagram"}).

Example 8

Mixed carbamates: tertiary amines such as MDEA are known to offer relatively low absorption enthalpies and are therefore LZ

being studied in the carbon, capture field. Typically, they require water as reaction partner for the formation of

bicarbonates or carbonates. Unfortunately,

show low reaction speeds and therefore have to be "activated" e.g. us ing pipe az ine , with 1 imited success. Water-free systems were studied here comprising tertiary and

secondary/primary amines. It was found that such systems absorb significantly more (50% and more} CO2 than given by dialklyamine carbamate stoichiometry . It can thus be

speculated that tertiary amines are protonated and can form counterions for e.g. carbamate anions. Surprisingly, very low absorption enthalpies, in the order of 800-1100 kJ/kg CO2 , are found for such systems, including equimolar mixtures of dibutylamine and trihexylamine, or dibutylamine and MDEA (limethyIdlethanoIamine) , both optionally in solvents as

described above. Also surprisingly, high reaction speeds were measured in such water-free CO2 absorption liquids comprising tertiary and secondary primary amines. Useful tertiary amines include MDEA, trialkylamines with alkyl being methyl, ethyl, propyl, butyl (not preferred due to toxicity), pentyl, hexyl, cyclohexyl, piperidines etc. Especially useful are water-free amines comprising tertiary and primary or secondary amine functions in one mo1ecu1e , e.g. diethy1amino-propy1amine or aminopropyl-imidazole, or amines such as 1-Methylimidazole in combination with primary or secondary amines such as dibutyl- or dihexylamine . Said system offers the additional benefits of low viscosity at high CO2 loading degree and low freezing point of the C02-loaded absorbent mix, and this system does not necessarily require a cosolvent.

The mixtures described here can also be used for gas

separation purposes, e.g. for the removal of C0₂ from biogas, water-shift gas or other industrial gas streams from which C02 has to be removed. Most compositions are hardly water- absorbing, and if they absorb water (e.g. in the case of

DEA) , water can be removed easily by heating / vacuum.

It will be appreciated that a range of parameters can be chosen quite freely for the operation of the C3 process for power generation, including amine chemistry, absorption enthalpy, pressure quote, pressure levels before and after the turbine or expansion device, C0₂ loading degree of the

absorption liquid before and after the absorption section, total CO₂ content, all depending upon the available neat source and cooling source. In some embodiments, volatile tertiary amines can be used with the intention to cause an additional gas flow through the turbine (CO₂ ^'plus tertiary amine, e.g. triethylamine) . In general, however, less volatile components (amines, solvents) are preferred in order to prevent problems with condensing liquids downstream, of the turbine. In general, the presence of water is not desired due to condensation /' icing problems and due to the vapour pressure of water in the absorption section which has a negative impact on the pressure quote. However, in special cases (e.g. thermomorphic systems) the presence of water is acceptable.

Elaborating on the freedom, to choose operation parameters, one important aspect of a power system is to adapt to varying sources of energy. Temperatures can vary between sites and over time. It is advantageous if the system is able to produce as much power as possible under varying circumstances without complete and costly redesign of the system itself.

The Carbon carrier cycle uses carbon dioxide, CO₂, that is absorbed in and is desorbed from a liquid medium. As

explained in the a.m. disclosures, the loading level will vary throughout the cycle centered around an average loading level. The amount of CO₂ then used in the expansion depends on the system configuration itself and also the flow rate of

absorbent that is used in the cycle matched with the heating and cooling source.

The average loading level in the cycle combined with the absorption and desorption amount can be used to control the absolute pressures in the system and the flow rate of

absorbent can be used to control the amount of CO₂. The reason for performing this adaption is to let the power producing expansion device operate at an optimal setpoint, thus

producing a maximum amount of energy output.

In an example, a system designed for 90°C on the hot side and 20°C on the cold side is used in another application where the heat source is at 80°C and the cooling source is at 10°C. In this case, additional 10%-30% loading of C0₂ is added to the system in order to maintain exactly the same pressure ratio or the absolute pressure values as the expansion device was designed for. The CO₂ concentration can even be varied in a system within a few minutes if the heating and/or cooling source is variable, e.g. on. a scale of hours.

Thus, according to the inventive method it is possible to adapt the average gas loading level in the C3 cycle between 20%" 90 % of the maximum loading to the hot and cold source, in order to maximize the power output of the expansion device, and a decreasing of the average gas loading level in case the C3 cycle is operated at higher temperatures. Example 9

Cold may be extracted from the closed loop thermodynamic cycle as disclosed in the references mentioned. The working medium CO₂ gas is expanded in the expansion device to very low pressure and very low temperature, such as minus 60°C. In the simplest embodiment, the gas is led through a heat exchanger allowing heati g of the gas and transfer of cold, e.g. a stream, of minus 20°C for any type of cooling requirement. The arrangement is simple, but will cause a back-pressure to the turbine thereby reducing the electricity production. A more advanced form of cold extraction is realized by pumping a liquid through the space through which the gas moves towards the absorption chamber. This liquid may be pumped in the form of a spray or a falling film or any other configuration which allows good contact between the cold CO₂ gas and the liquid. Preferably, liquids of high boiling- point and low melting point are used. Silicone oils are suitable candidates. There is a risk that droplets of said heat transfer liquid enter the thermodynamic cycle. This may be controlled by elements such as demisters. A preferred way of operating the cold extraction is by using the same amine/solvent system which is sed in the absorber, also for the cold extraction. A certain loss of the cold extraction medium can be compensated by pumping saturated amine/solvent from, the absorption, section, to the cold

extraction section. As the cold extraction medium is fully saturated, depending on the case a certain period after start^¬ up, there will be no heat development in the cold extraction section due to additional CO₂ absorption. This liquid is preferably led through a heat exchanger, from which a. second suitable liquid, such as glycol ethers or the like as known in the art, is carrying the cold energy to the point-of-use .

Claims

*>JL¾

1. A method for improving the performance of thermodynamic cycles, particularly the performance of the C3 (Carbon Carrier Cycle) based on C0₂ gas absorbed in a liquid absorbent used as working fluid in a system comprising at least one CO₂ gas absorber, heat exchangers, at least one pump device and a pipe system for transport of said working fluid through the system, and an expansion device for converting gas expansion work to mechanical work, characterized by:

- feeding said working fluid to the expansion device for releasing said CO₂ gas under pressure by heat supplied from an external heati g source operating- at least at 70°C, preferably at 80 to 90°C or higher, wherein the expansion device being selected from the group consisting a modified 2-stroke engine, a turbocharger and a turbine.

2. The method according to claim 1, characterized by pre^¬ heating the working fluid before feeding the same to the device .

3. The method according- to claim 1, characterized by

- using said modified 2-stroke engine as device to absorb CO? in the absorption stage of the thermodynamic cycle, and

- using said modified 2-stroke engine as device to release CO₂ under pressure by heat supplied from, an external heating source .

4. The method according- to claim 1, characterized by coupling an outgoing rotating shaft from said device to a generator for generation of electricity.

5. The method according to claim 1, characterized by

- using a counter-current gas absorber to absorb the CO₂ gas in which the liquid absorbent is sprayed into the incoming CO₂ gas stream.,

- partially collecting said liquid absorbent, and

- re-spraying said partially collected liquid absorbent into the CO₂ gas stream, preferably at the upstream section of said absorber, i.e. so as to interacting with fresh gas, thus enabling a maximum loading level of the CO₂ gas into said liquid absorbent, and

- enabling intermediate cooling of the liquid absorbent.

6. The method according to claim 1, in case the device being a turbine,

- using a Venturi design for accelerating the gas flow to the turbine and for allowing high gas speed to disintegrate liquid absorbent droplets, and

- optimizing the flow of droplets and their residence time in the CO₂ gas absorber such that liquid absorbent droplets loaded with CO₂ can transfer the heat generated during the absorption of gas to a cooling wall in the CO₂ gas absorber.

7. The method according to claim 1, characterized by

- using said at least one heat exchanger as central heat exchanger for transferring heat from hot lean liquid absorber to cold rich liquid absorbent.

8. The method according to claim 1, characterized by

- supplying heat from a geothermal power unit by a hot water stream having a temperature from about 90 to 110°C,

- cooling the hot water stream to about 80 to 7Q°C, and

- using possible remaining hot water for district heating or heating an underground reservoir for heat storage.

9. The method according to claim 1, characterized by the liquid absorbent comprising- cosolvents such as glycol ethers, preferably diethylene glycol mono-ethyl -ether, and amines such as dialkylamines, and preferably water-free amines comprising tertiary and secondary amines, optionally combined in one molecule, or mixtures of strongly basic amines such as diaza- bicyclo-undecene or 1 -Methylimidazole in combination with secondary or primary amines, in order to adjust the viscosity, provide lubrication, provide heat transfer, prevent foaming and crystallization of the liquid absorbent.

10. The method according to claim 1, characterized by

extracting cold from the thermodynamic cycle by leading cold CO₂ gas after having expanded in the device through a heat exchanger.

11. The method according to claim 10, characterized by the heat exchanger being co structed without separation walls by using a liquid which is in direct contact with the CO₂ gas but will not hinder the gas transport to the C0₂ gas absorber, whereby the liquid absorbent used in the CO₂ gas absorber is also used for cold extraction.

12. The method according to claim 1, characterized by

adaptation of the average gas loading level in the C3 cycle between 20%-90 % of the maximum loading to the hot and cold source, in order to maximize the power output of the expansion device, and a decreasing of the average gas loading level in case the C3 cycle is operated at higher temperatures.

13. The method according to claim 1, characterized by the use of a booster section or super-heater section in which

preferably only the CO2 gas is heated, prior to entering the expansion device, to a higher temperature than the temperature provided by the primary heat source.