WO2026022242A1

WO2026022242A1 - Waste heat recovery system in turbo-compression applications combined with solar energy

Info

Publication number: WO2026022242A1
Application number: PCT/EP2025/071218
Authority: WO
Inventors: Gabriele LIBERATORI; Simone Amidei
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2024-07-24
Filing date: 2025-07-23
Publication date: 2026-01-29
Anticipated expiration: 2027-01-24

Abstract

An integrated solar combined cycle comprising a compression train, preferably a turbo-compression train of a gas storage plant and a waste heat recovery system is disclosed. In particular, the compression train comprises one or more compression stages, wherein heat exchangers are configured to recover heat from the gas between the compression stages by exchanging heat with a thermal fluid circulating in a thermal circuit in a plurality of heat exchangers including higher temperature heat exchangers and lower temperature heat exchangers. Additionally, the waste heat recovery system comprises a thermodynamic cycle system operating a working fluid and including a thermodynamic cycle circuit comprising first heaters configured to exchange heat between the thermal fluid of the thermal circuit, downstream of the higher temperature heat exchangers, on one side and said working fluid on the other side, to heat and evaporate said working fluid, one or more expanders, arranged downstream of said first heaters and configured to expand said working fluid to give a mechanical work, coolers arranged downstream of the expanders and pumps downstream of the coolers and upstream of the first heaters. In particular, the coolers are configured to exchange heat between the working fluid on one side and the thermal fluid of said thermal circuit downstream of the lower temperature heat exchangers, on the other side, to cool and condensate said working fluid. Finally, the waste heat recovery system comprises a solar thermal system, including a solar field, to heat alternatively the thermal fluid or a solar thermal fluid.

Description

Waste Heat Recovery System in Turbo-Compression applications combined with Solar Energy

Description

TECHNICAL FIELD

[0001] The present disclosure pertains to the field of thermal energy recovery, specifically in turbo-compression applications. It involves the use of thermal energy dissipated by low temperature sources, such as interstage exchangers of gas compressors, combined with the thermal energy supplied by solar energy.

BACKGROUND ART

[0002] At present, available heat at temperatures not much higher than ambient temperature is not conveniently recoverable, and then actually wasted in the environment. According to the prior art, an Integrated Solar Combined Cycle (ISCC) is a hybrid technology in which a solar thermal field is integrated within a combined cycle plant, the latter comprising a topping cycle, typically a Brayton cycle and a bottoming cycle, typically a Rankine cycle using water steam or an organic fluid as working fluid. In ISCC plants, solar energy is used as an auxiliary heat supply, supporting the bottoming cycle, which results in increased generation capacity and/or a reduction of fossil fuel consumption. Thermodynamic benefits of the ISCC include that daily startup losses of the turbine of the bottoming cycle are eliminated.

[0003] In fact, it is known that some factors limit the load output of the combined cycle power plant according to the prior art, the major factors being the allowed pressure and temperature transients of the bottoming cycle turbine, the time required to the heat recovery generator of the bottoming cycle to establish required working fluid thermodynamic conditions and the warm-up time needed for the balance of the temperature of the plant and of the main piping system. These limitations also influence the fast start-up capability of the turbine of the bottoming cycle, which must concurrently respect a waiting time, during which the gas turbines continue to consume gas.

[0004] In this respect, the solar component of an ISCC allows, if the plant is started after sunshine, or before, if there is heat storage, to preheat the working fluid of the bottoming cycle to the required conditions. As a consequence, the plant can be started faster and with a reduced consumption of gas before achieving operating conditions. These solutions of the prior art allow to gain also economic benefits, the cost of the solar components of an ISCC being from 25% to 75% lower than those of a Solar Energy Generating Systems plant of the same collector surface.

[0005] However, ISCCs also have limits making their use non-optimal. A limiting feature of ISCC is the use of steam systems and steam turbines. In particular, when the bottoming cycle operates steam as working fluid, since water evaporates and condensate at high thermal conditions (in particular at a higher temperature than that of an organic fluid, such as for example a pentafluoro-propane), there is a need for thermal plants that reach high temperatures and pressures. The possibility to condensate the working fluid under vacuum (i.e. at a pressure lower than the atmospheric pressure), in order to lower the temperature, produces the need for vacuum systems that are difficult to operate and not economically convenient if the plant is operated with daily start-up and shut-down. On the other hand, the condensation of water at ambient pressure and at a temperature of 100°C (to avoid the need for a vacuum system) represents a limit for the use of the heat by the interphase coolers of the turbo-compressors train.

[0006] In summary, the drawbacks of the ISCCs according to the prior art, comprise alternatively:

- condensation at a pressure lower than atmospheric pressure produces the need for “vacuum systems” that are difficult and not economically convenient if the plant is operated with daily start-up and shut-down; or

- the condensation of water at ambient pressure and at 100°C (avoiding vacuum system) represents a limit for the use of the heat by the interphase coolers and of the lube oil of the turbo-compressors train.

[0007] Accordingly, an improved Integrated Solar Combined Cycle (ISCC) system with an optimized configuration to address the issues of the systems of the current art would be beneficial and would be welcomed in the technology. More in general, it would be desirable to provide systems adapted to more efficiently address problems entailed by the recovery of energy at low temperature from the refrigeration of turbo compressor train. [0008] According to the invention, it becomes convenient to recover low temperature heat if: combined in cascade with a solar thermal system, obtaining higher temperatures, converted directly into mechanical energy, rather than passing through the intermediate state of electrical energy, avoiding the loss of conversion step, applicated in turbo-compression trains of gas storage plants because operative just during the summer season, namely when is higher the availability of solar radiation.

SUMMARY

[0009] In one aspect, the subject matter disclosed herein is directed to a waste heat recovery system in an Integrated Solar Combined Cycle (ISCC) system. In particular, the ISCC system comprises a compression train with one or more compression stages, wherein heat exchangers are configured to recover heat from the gas between the compression stages by exchanging heat with a thermal fluid circulating in a thermal circuit in a plurality of heat exchangers including higher temperature heat exchangers and lower temperature heat exchangers. Additionally, the waste heat recovery system comprises a thermodynamic cycle system operating a working fluid and including a thermodynamic cycle circuit comprising first heaters configured to exchange heat between the thermal fluid of the thermal circuit, downstream of the higher temperature heat exchangers, on one side and said working fluid on the other side, to heat and evaporate said working fluid, one or more expanders, arranged downstream of said first heaters and configured to expand said working fluid to give a mechanical work, coolers arranged downstream of the expanders and pumps downstream of the coolers and upstream of the first heaters. In particular, the coolers are configured to exchange heat between the working fluid on one side and the thermal fluid of said thermal circuit downstream of the lower temperature heat exchangers, on the other side, to cool and condensate said working fluid. Finally, the waste heat recovery system comprises a solar thermal system, including a solar field, to heat alternatively the thermal fluid or a solar thermal fluid.

[0010] In particular, when the solar field is configured to heat the thermal fluid, then the solar field is arranged along the thermal circuit upstream of the first heaters of the thermodynamic cycle circuit. Alternatively, when the solar field is configured to heat a solar thermal fluid, then the solar field is arranged along a solar thermal circuit separate from the thermal circuit, the thermodynamic cycle circuit additionally comprising second heaters, configured to exchange heat between the solar thermal fluid of the solar thermal circuit on one side and the working fluid of the thermodynamic cycle circuit on the other side. More in particular, the second heaters are arranged along the thermodynamic cycle circuit downstream of the first heaters and upstream of the expanders.

[0011] The invention has both economic and environmental advantages over the prior art. The economic advantage is a reduction of gas consumed by the gas turbine, while the environmental advantage is a reduction of CO2 footprint.

[0012] As a consequence, the heat at medium-low temperatures of the inter-phase heat exchangers of compression trains can be used in combination with the heat at medium-high temperatures of a small solar thermal plant to obtain free energy to be converted to mechanical work by turbo-expander. The use of solar energy and the optimized recovery of wasted heat according to the present disclosure permits to reduce the gas consumption in the turbine and the CO2 footprint.

[0013] Therefore, the present disclosure is a thermal energy recovery system in turbo-compression applications combined with a solar energy plant. The system of the disclosure provides for the use of thermal energy dissipated by low temperature sources, such as interstage exchangers of gas compressors and even as lube oil heat exchanger, to be combined with the thermal energy supplied by solar energy, to bring a thermal fluid from ambient temperature to medium-high temperature; obtaining availability of mechanical energy to contribute to the compression of gas in a turbocompression train. Optionally, the system of the invention can also be combined with a Brayton gas turbine cycle.

[0014] According to one embodiment of the thermal energy recovery system of the present disclosure, the solar plant can also be used to directly or indirectly provide heat to one or more fluids to be heated. In particular, the solar system of the thermal energy recovery system of the present disclosure can further comprise an auxiliary solar thermal circuit with one or more auxiliary heat exchangers configured to exchange heat with a water/glycol separation system arranged along a glycol recovery circuit of a natural gas de-hydration system

[0015] The invention has many advantages over the prior art, including a better efficiency, as a consequence of direct transformation of thermal energy into mechanical energy, without the intermediate step of transformation into electrical energy (which determines an overall loss of efficiency of approximately 20%). Another advantage is savings due to the elimination of the investment for the purchase of an electric generator, as in other cases of application of an organic Rankine cycle to recover energy from low enthalpy sources. An additional advantage is due to a seasonal climatic opportunity: in fact, in case the compression train is configured for the reinjection of natural gas at storage purpose, then it is required only during the summer season, when the combination with solar thermal energy is ideal thanks to its greater availability in summer season. Additionally, in winter season, the solar thermal energy can be used to provide heat to a water/glycol separation system arranged along a glycol recovery circuit of a natural gas de-hydration system, which is needed to remove water from the natural gas extracted from the storage wells.

[0016] Side advantages include geo-politics contribution due to the ever-increasing political instabilities of the countries with the largest natural gas availability, and the consequent increase of demand for gas injection at storage purpose by natural gas user countries and politics-ecological contribution. In fact, despite the low/medium enthalpy contribution of the organic Rankine cycle application turboexpanders, the reduction of CO2 emissions into the atmosphere resulting from the optimization of thermodynamic cycles and the combined use with renewable energy sources promises to become increasingly convenient, also thanks to possible forms of contribution of the national and international institutions, such as the European Union with an ecologic purpose (short-term) and with the increasing cost of fossil energy source (long term).

[0017] Finally, the thermal energy recovery system in turbo-compression applications combined with solar energy according to the present disclosure, which uses low temperature waste heat from inter-phase heat exchangers of compression trains and combines it with heat from small solar thermal plants to produce mechanical energy directly without the intermediate step of electrical conversion allows for a plurality of advantages, including the ones explained herein above. The system is particularly ad- vantageous for use in turbo-compression trains of gas storage plants, during the summer season and, according to a preferred embodiment, also during the winter season, providing economic and environmental benefits by reducing gas consumption and CO2 emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. l illustrates a block diagram of a waste heat recovery system, according to a first embodiment;

Fig.2 illustrates a block diagram of a waste heat recovery system, according to a second embodiment;

Fig.3 illustrates a block diagram of a waste heat recovery system, according to a third embodiment;

Fig.4 illustrates a block diagram of a waste heat recovery system, according to a fourth embodiment;

Fig.5 illustrates a block diagram of a waste heat recovery system, according to a fifth embodiment;

Fig.6 illustrates a block diagram of a waste heat recovery system, according to a sixth embodiment;

Fig.7 illustrates a block diagram of a natural gas de-hydration system and the solar field of a waste heat recovery system, according to a seventh embodiment; and

Fig.8 illustrates a block diagram of a natural gas de-hydration system and the solar field of a waste heat recovery system, according to an eighth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] According to one aspect, the present subject matter is directed to a waste heat recovery system in a compression train, preferably a turbo-compression train of a gas storage plant, the compression train comprising one or more compression stages mounted on a rotating shaft or integrally geared and configured to compress a gas, each compression stage comprising a gas inlet and a gas outlet, the waste heat recovery system comprising one or more heat exchangers connected downstream of the gas outlets of each compression stage and configured to exchange heat between the gas on one side and a thermal fluid on the other side, the heat exchangers being arranged along a thermal circuit circulating said thermal fluid, said heat exchangers comprising higher temperature heat exchangers and lower temperature heat exchangers; the waste heat recovery system additionally comprising a thermodynamic cycle system operating a working fluid and including a thermodynamic cycle circuit comprising one or more pumps, one or more first heaters arranged downstream of said pumps and configured to exchange heat between said thermal fluid of said thermal circuit, downstream of said higher temperature heat exchangers, on one side and said working fluid on the other side, to heat and evaporate said working fluid, one or more expanders, arranged downstream of said first heaters and configured to expand said working fluid to give a mechanical work, and one or more coolers arranged downstream of said expanders and upstream of said pumps and configured to exchange heat between said working fluid on one side and said thermal fluid of said thermal circuit, downstream of said lower temperature heat exchangers, on the other side, to cool and condensate said working fluid, and a solar thermal system, including a solar field.

In particular, according to alternative embodiments, the solar field can be arranged along the thermal circuit to heat the thermal fluid, upstream of the first heaters of the thermodynamic cycle circuit, or along a solar thermal circuit to heat a solar thermal fluid, the solar thermal circuit being separate from the thermal circuit, and the thermodynamic cycle circuit additionally comprising second heaters, configured to exchange heat between the solar thermal fluid of the solar thermal circuit on one side and the working fluid of the thermodynamic cycle circuit on the other side, the second heaters being arranged along the thermodynamic cycle circuit downstream of the first heaters and upstream of the expanders.

[0020] According to one aspect, the solar field is configured to heat the thermal fluid or the solar thermal fluid alternatively by direct heat exchange or by an intermediate photovoltaic system, powering a heat pump. The waste heat recovery system can further comprise auxiliary heat sources cooperating with the solar field to provide heat to the thermal fluid. In particular, when the solar field is arranged along the thermal circuit, the auxiliary heat sources comprise a heat exchanger configured to exchange heat between the exhaust gas of a gas turbine and the thermal fluid, the heat exchanger being arranged along the thermal circuit in parallel to the solar field and upstream of the first heaters. Differently, when the solar field is arranged along the solar thermal circuit, the auxiliary heat sources comprise a heat exchanger configured to exchange heat between the exhaust gas of a gas turbine and the solar thermal fluid, the heat exchanger being arranged along the solar thermal circuit in parallel to the solar field and upstream of the second heaters.

[0021] In one aspect, when the solar field is arranged along the thermal circuit, the solar thermal system comprises one or more storage vessels, arranged along the thermal circuit, upstream and/or downstream of the solar field, to store the thermal fluid. Differently, when the solar field is arranged along the solar thermal circuit, the solar thermal system comprises storage vessels, arranged along the solar thermal circuit, upstream and/or downstream the solar field, to store the solar thermal fluid.

[0022] In one aspect, the expander is a turbo expander coupled with at least one compression stage of the compression train. In particular, the turbo expander can be coupled with the at least one compression stage of the compression train through a common shaft or through a gear box. Alternatively, the turbo expander can be coupled with the at least one compression stage of the compression train through a clutch.

[0023] In another embodiment, the expander is an expander compressor coupled with a compressor cinematically independent from the compression train. In particular, the compressor can be part of a heat pump configured to operate a cooling fluid circulating in a cooling circuit comprising one or more heat exchangers arranged along the gas inlets of the compression stages of the compression train and configured to exchange heat between the cooling fluid on one side and the gas to be compressed on the other side. Alternatively, the expander compressor can be configured to operate the same gas of the compression stages of the compression train and is arranged upstream of at least some of the impellers of the gas compression stages of the compression train or downstream of the compression train or in an intermediate position between the compression stages of the compression train. [0024] In one aspect, the waste heat recovery system of the present disclosure further comprises one or more auxiliary heat exchangers, arranged along the thermal circuit and configured to exchange heat between a lubrication fluid of a lubrication circuit of the compression train on one side and the thermal fluid of the thermal circuit on the other side.

[0025] In another aspect, the compression train is a turbo-compression train of gas storage plants. In still another aspect, the thermal fluid of the thermal circuit is water and/or the working fluid of the thermodynamic cycle circuit is pentafluoro propane.

[0026] In one aspect, the solar system of the waste heat recovery system further comprises an auxiliary solar thermal circuit sharing the solar field with the solar thermal circuit, to heat an auxiliary solar thermal fluid, and also comprises one or more auxiliary heat exchangers arranged along the auxiliary solar thermal circuit to exchange heat between the auxiliary solar thermal fluid of the auxiliary solar thermal circuit on one side and one or more fluids to be heated on the other side. In one aspect, the one or more auxiliary heat exchangers are part of a water/glycol separation system arranged along a glycol recovery circuit of a natural gas de-hydration system, and the one or more auxiliary heat exchangers are configured to exchange heat between the auxiliary solar thermal fluid of the auxiliary solar thermal circuit on one side and a solution of water and tri-ethylene glycol on the other side. In particular, the one or more auxiliary heat exchangers can comprise

- a first auxiliary heat exchanger to heat the solution of water and tri-ethylene glycol upstream of a degaser, wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from a first combustor up to a temperature allowing separation of gases from the solution, and

- a second auxiliary heat exchanger, configured to heat the solution of water and tri-ethylene glycol upstream of an evaporator, wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from a second combustor up to a temperature allowing separation of steam from the solution.

Optionally, the degaser and/or the second auxiliary heat exchanger further comprise an electrical heater, connected to an electrical power generator coupled with the expander.

[0027] Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0028] When introducing elements of various embodiments the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0029] When making reference to relative terms, such as higher and lower in the definition of higher temperature heat exchangers and lower temperature heat exchangers, the meaning of such relative terms should be intended as referring to the components of the waste heat recovery system of the present disclosure only, any reference with features not forming part of the waste heat recovery system of the present disclosure being expressly excluded.

[0030] Referring now to the drawings, Fig.1 shows a block diagram of an exemplary waste heat recovery system according to the present disclosure. In particular, the waste heat recovery system is configured to recover heat from a compression train 10, the compression train comprising two compression stages 11, 11’ that are mounted on a same rotating shaft 12 with a gas turbine 41 and that are configured to compress a gas, directed to each compression stage 11, 11’ through respective gas inlets, namely a first compression stage gas inlet 111 and a second compression stage gas inlet 111’ and exiting from each compression stage 11, 11’ through respective gas outlets 112, 112’, namely a first compression stage gas outlet 112 and a second compression stage gas outlet 112’. The waste heat recovery system comprises heat exchangers 14, 14’, 14” connected downstream of the gas outlets 112, 112’ to exchange heat between the gas from the compression stages on one side and a thermal fluid on the other side, the thermal fluid flowing inside a thermal circuit 15. Further, the waste heat recovery system comprises an auxiliary heat exchanger 16, arranged along the thermal circuit 15 to exchange heat between a lubrication fluid of a lubrication circuit 13 of the compression train 10 on one side and the thermal fluid of the thermal circuit 15 on the other side. Preferably, the thermal fluid is water, because of its low cost and high safety, allowing for storage of great amounts as thermal accumulation. The thermal fluid is used as an intermediate heat transfer fluid between the gas from the compressors and a working fluid flowing inside a thermodynamic cycle system 20, to convert thermal energy obtained through heat exchange with the thermal fluid of the thermal circuit 15 into kinetic energy. In particular, the thermodynamic cycle system 20 includes a thermodynamic cycle circuit 21 comprising a pump 22, first heaters 23, 23’, 23” arranged downstream of said pump 22 and configured to exchange heat between the thermal fluid of the thermal circuit 15 and the working fluid, to heat and evaporate the working fluid, an expander 24, arranged downstream of the first heaters 23, 23’, 23” and configured to expand the working fluid to give a mechanical work, which is transmitted to the compression train 10 through a clutch 17, and two coolers 25, 25’, arranged downstream of the expander 24 and upstream of the pump 22 and configured to exchange heat between the working fluid on one side and the thermal fluid of the thermal circuit 15 on the other side, to cool and condensate said working fluid. Preferably, the working fluid of said thermodynamic cycle system 20 is an organic compound and the thermodynamic cycle system 20 operates an Organic Rankine Cycle. More preferably, the working fluid is pentafluoro propane. In particular, the thermal circuit 15 includes higher temperature heat exchangers 14, 14’ and lower temperature heat exchangers 14”, the gas from the first compression stage 11, at an exemplary temperature of 90 °C, flowing first in the higher temperature heat exchanger 14, to heat the thermal fluid flowing in a first section of the thermal circuit 15 up to an exemplary temperature of 80°C, the gas downstream of the higher temperature heat exchanger 14, at an exemplary temperature of 60-65°C, flowing subsequently in the lower temperature heat exchanger 14”, to heat the thermal fluid flowing in a second section of the thermal circuit 15 up to an exemplary temperature of 35°C. The gas from the second compression stage 11’, at an exemplary temperature of 110 °C, flows in the higher temperature heat exchanger 14’ of the second compression stage 11’, to heat the thermal fluid flowing in a third section of the thermal circuit 15 up to an exemplary temperature of 95°C. The thermal fluid downstream of the higher temperature heat exchanger 14 of the first compression stage 11 and the thermal fluid downstream of the higher temperature heat exchanger 14’ of the second compression stage 11’ are directed to the first heaters 23, 23’, 23” of the thermodynamic cycle system 20, to heat and evaporate the working fluid at an exemplary pressure of 7bar from an exemplary temperature of 35-40°C up to an exemplary temperature of 90°C. The working fluid is then directed to the expander 24 where its energy is converted into mechanical work while its pressure is lowered down to 2bar and its temperature is lowered down to 65°C. The working fluid is subsequently cooled and condensate inside the coolers 25, 25’, by exchanging heat with the thermal fluid flowing in the second section of the thermal circuit 15, downstream of the lower temperature heat exchanger 14”. Finally, the working fluid is directed to the pump 22, wherein its pressure is increased up to 7 bar. It is intended that the operating temperature and pressure of the waste heat recovery system can vary depending on the fluid used as thermal fluid in the thermal circuit and as working fluid in the thermodynamic cycle system. According to the exemplary embodiment disclosed with reference to Fig.1, the exemplary temperature and pressure are referred respectively to water as the thermal fluid and pentafluoro propane as the working fluid.

[0031] The waste heat recovery system shown in Fig.1 additionally comprises a solar thermal system 30, including a solar field 31 arranged along the thermal circuit 15, to heat the thermal fluid, upstream of the first heaters 23, 23’, 23” of the thermodynamic cycle circuit 21. The solar thermal system operates in parallel to the higher temperature heat exchangers 14, 14’ increasing the temperature of the thermal fluid up to an exemplary value of 95°C. downstream of the solar field 31, the thermal fluid is therefore mixed together with the thermal fluid downstream of the higher temperature heat exchanger 14’ of the second compression stage of the compression train 10. Storage vessels 34, 34’ are arranged along the thermal circuit 15, upstream and downstream of the solar field 31, to store the thermal fluid, during different operating conditions. In particular, during the daytime the storage vessel 34’ downstream of the solar field 31 allows for a large quantity of water to be accumulated at a temperature close to 100°C, to obtain availability during the intermediate evening hours and during any moment of the day with skies covered by clouds. During the nighttime, when the waste heat recovery system is stopped because of unavailability of the additional thermal energy from the solar field 31, the storage vessel 34 upstream of the solar field 31 allows for thermal energy from the compression stages and lube oil heat exchangers to be accumulated, to minimize the work of the solar field 31 in the morning hours and increase the operating times of the expander 24.

[0032] A limit of the waste heat recovery system of Fig.1 is the low enthalpy available because of the use of water as the thermal fluid. In fact, the difference of the maximum water temperature at atmospheric pressure (100°C) and the ambient temperature is relatively low, with the consequence that low power is available with a relatively low thermodynamic efficiency.

[0033] In some embodiments the solar field 31 is configured to heat thermal fluid by an intermediate photovoltaic system, which can also be used for powering a heat pump. While in the schematic of Fig.1 described so far the expander 24 is coupled with the compression stages of the compression train 10 through a clutch 17, in other embodiments the expander 24 can be coupled with one or more of the compression stages of the compression train 10 through a common shaft 12 or through a gear box.

[0034] In a particularly preferred embodiment, the compression train 10 is a turbocompression train of a gas storage plant.

[0035] With continuing reference to Fig. 1, a further embodiment of a waste heat recovery system is show+n in Fig.2. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig.l and described above, and which will not be described again. The waste heat recovery system of Fig.2 differs from the waste heat recovery system of Fig.1 mainly in that the system further comprises an auxiliary heat source 40 cooperating with the solar field 31 to provide heat to said thermal fluid, namely a heat exchanger 40 configured to exchange heat between the exhaust gas of the gas turbine 41 and the thermal fluid. In particular, as shown in Fig.2, the heat exchanger 40 is arranged along the thermal circuit 15 in parallel to the solar field 31 and upstream of the first heaters 23. According to this embodiment, the functionality of the turboexpander 24 is always guaranteed. In fact, in the event of unavailability of insolation in the solar field 31, the supply of heat to the thermal fluid is guaranteed by the exhaust gas of the gas turbine 41. The decrease in efficiency of the Brayton cycle of the gas turbine 41 during the night and during periods of lack of sunlight is compensated by the recovery and continuity of operation of the turboexpander 24 with the Rankine cycle.

[0036] With continuing reference to Figs 1 and 2, a further embodiment of a waste heat recovery system is shown in Fig.3. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig.l and

2 and described above, and which will not be described again. The waste heat recovery system of Fig.3 differs from the the waste heat recovery system of Figs.1 and 2 mainly in that the expander 24 is replaced by an expander compressor 24’, coupled with a compressor 27. In the embodiment of Fig.3, the compressor 27 is cinematically independent from the compression train 10. Nevertheless, the expander compressor 24’ can be configured to operate the same gas of the compression stages 11, 11’ of the compression train 10, and can be alternatively arranged upstream of or downstream of the compression train 10 or in parallel to at least one of the impellers of the compression stages 11, 11’ of the compression train 10. In an alternative embodiment, the compressor 27 is part of a heat pump configured to operate a cooling fluid circulating in a cooling circuit comprising one or more heat exchangers arranged along the gas inlets 111, 111’ of the compression stages 11, 11’ of the compression train 10, to exchange heat between the cooling fluid on one side and the gas to be compressed on the other side. According to this embodiment, when there are solar thermodynamic conditions, or accumulation, or optionally from a combined cycle with a gas turbine to start-up the turboexpander, this is connected to an autonomous single-impeller compressor which contributes directly to the process with a further compression stage. No clutch is required but an additional process valve to allow operation in modes with or without turboexpander-compressor running.

[0037] With continuing reference to Figs 1, 2 and 3, a further embodiment of a waste heat recovery system is shown in Fig.4. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig.l, 2 and 3 and described above, and which will not be described again. The waste heat recovery system of Fig.4 differs from the waste heat recovery system of Figs.1, 2 and

3 mainly in that the solar field 31 is arranged along a solar thermal circuit 32 and is configured to heat a solar thermal fluid, the solar thermal circuit 32 being separate from the thermal circuit 15. According to this embodiment, the thermodynamic cycle circuit 21 additionally comprises second heaters 26, 26’, configured to exchange heat between the solar thermal fluid of the solar thermal circuit 32 on one side and the working fluid of the thermodynamic cycle circuit 21 on the other side, the second heaters 26, 26’ being arranged along the thermodynamic cycle circuit 21 downstream of the first heaters 23, 23’, 23” and upstream of the expander 24. According to this embodiment, the use of a high temperature solar thermal fluid in the solar field 31 is allowed. As a consequence, the working fluid of the thermodynamic cycle can be heated up to 150°C upstream of the expander 24 by heat exchange with the solar thermal fluid of the solar field 31. The thermal fluid of the thermal circuit 15 exchanging heat with the gas from the compression stages 11, 11’ of the compression train 10 is still used to heat the working fluid of the thermodynamic cycle from ambient temperature to 100°C, in parallel to the solar thermal fluid of the solar field 31 for a sizing that guarantees accumulation. This embodiment involves higher investment costs, because the solar thermal cycle must be sized for relatively high pressures and temperatures. On the other hand, according to this embodiment, the higher enthalpy offered by a temperature of about 150°C higher than the ambient temperature, higher power is available with a relatively high thermodynamic efficiency. Moreover, this embodiment also allows for forms of night-time accumulation by the thermal fluid from the compression stages and the lube oil system (between 55°C and 90°C), which can be combined with forms of daytime accumulation in the solar field tanks 34’ (between 65°C and 150°C) by oversizing the solar system field 31. In some aspects, this embodiment could minimize or even zeroize the inoperability of the turbo-expander 24. In fact, given both the higher enthalpy availability and the greater accumulation capacity of the thermal fluid and the solar thermal fluid, reducing the load of the turboexpander 24 can be more convenient than operating it intermittently, when the thermal availability of the solar field 31 drops (example range is between 120°C and 150°C and therefore with a boiling pressure of the pentafluoro propane feed pump between 10 and 25 barg). Additionally, the operative range of the turboexpander 24, at different pressures and different flow rates, allows the optimization of operation in cases of variable weather and extends the functionality of the system even during a portion of the nighttime.

[0038] With continuing reference to Figs 1, 2, 3 and 4, a further embodiment of a waste heat recovery system is shown in Fig.5. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig.l, 2, 3 and 4 and described above, and which will not be described again. The waste heat recovery system of Fig.5 differs from the waste heat recovery system of Fig. 4 mainly in that an auxiliary heat exchanger 40 is arranged along the solar thermal circuit 32 to exchange heat between the exhaust gas of the gas turbine 41 and the solar thermal fluid. In particular, the heat exchanger 40 is arranged along the solar thermal circuit 32 in parallel to the solar field 31 and upstream of the second heaters 26, 26’. According to this embodiment, the functionality of the turboexpander is always guaranteed because, in the event of unavailability of sunlight to the solar field 31, the supply of heat to the high temperature solar thermal fluid is guaranteed by the gas turbine 41. The decrease in efficiency of the Brayton cycle of the gas turbine 41 during the night and during periods of lack of sunlight is compensated by the recovery and continuity of operation of the turboexpander 24 with the Rankine cycle.

[0039] With continuing reference to Figs 1, 2, 3, 4 and 5, a further embodiment of a waste heat recovery system is shown in Fig.6. The same reference numbers designate the same or corresponding parts, elements or components already illustrated in Fig.l, 2, 3, 4 and 5 and described above, and which will not be described again. The waste heat recovery system of Fig.6 differs from the waste heat recovery system of Fig.5 mainly in that the expander 24 is replaced by an expander compressor 24’, coupled with a compressor 27. As for the embodiment shown in Fig.3, according to this embodiment a helper function is realized directly in thermodynamic process through an additive compression stage 27, instead of as mechanical load to the shaft line, it can occur with or without integration in a combined cycle with a gas turbine.

[0040] With continuing reference to Figs 1, 2, 3, 4, 5 and 6, a further embodiment of a waste heat recovery system is shown in Fig.7. The waste heat recovery system of Fig.7 differs from the waste heat recovery system of Figs.1, 2, 3, 4, 5 and 6 in that the solar system 30 further comprises an auxiliary solar thermal circuit 32’ sharing the solar field 31 with the solar thermal circuit 32. In particular, the auxiliary solar thermal circuit 32’ is configured to heat an auxiliary solar thermal fluid, flowing inside the auxiliary solar thermal circuit 32’. Auxiliary heat exchangers 61, 71 are arranged along the auxiliary solar thermal circuit 32’ to exchange heat between the auxiliary solar thermal fluid of the auxiliary solar thermal circuit 32’ on one side and a solution of water and tri-ethylene glycol on the other side, the auxiliary heat exchangers 61, 71 being part of a water/glycol separation system arranged along a glycol recovery circuit of a natural gas de-hydration system.

[0041] In particular, Fig. 7 shows a gas well 50, for natural gas accumulation during the periods of the year with a lower demand for natural gas, in particular in summer. According to a preferred embodiment of the present disclosure, the natural gas is compressed and stored in the gas well by means of the compression train 10. A separator 51 is provided at the wellhead, where the free liquid fraction eventually aspirated is separated by gravity. The water 52 separated by gravity is subsequently automatically discharged from the separators 51 by a control valve. An injector 53 of glycol (namely tri-ethylene glycol) is arranged downstream of the wellhead separator 51 and upstream of a regulation valve 54, which provides for pressure reduction from an exemplary maximum static well pressure of 150 bar-g down to an exemplary gas pipeline pressure of 60 bar-g. The function of glycol is to inhibit the formation of hydrates in natural gas, which could condense during gas transport due to the pressure drop. A horizontal condensate separator 56 (slug-catcher) is arranged downstream of the regulation valve 54, to provide further separation of the condensed liquids transported by the gas (slugs) and part of the glycol injected through the injector 53. Ethylene glycol fixing with water molecules improves separation at the bottom of the condensate separator 56. Subsequently the gas is conveyed to the lower part of a structured filling dehydration column 57, which is a pressure vessels in which the gas flow goes up in countercurrent to drops of tri-ethylene glycol in order to lower its dew point according to the specifications required for transport by gas pipeline. The exhaust tri-ethylene glycol solution exits from the bottom of the condensate separator 56 and the dehydration column 57 and is sent, after passing through the valves 58 and 58’ to pump 59. The exhaust triethylene glycol solution is then heated in a first auxiliary heat exchanger 61, configured to heat the solution of water and tri-ethylene glycol by heat exchange with the auxiliary solar thermal fluid of the auxiliary solar thermal circuit 32’. The amount of exhaust tri-ethylene glycol solution directed to the first auxiliary heat exchanger 61 is controlled by the valves 60, 62 alternatively directing the solution through a by-pass line.

[0042] The exhaust tri-ethylene glycol solution is then heated in a degasser 63, wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from a first combustor 64 up to a temperature allowing separation of gases 65 from the solution 66, which is directed first to a wet glycol tank 67, from where it is then pumped by a pump 68 to a second auxiliary heat exchanger 71, configured to heat the solution of water and tri-ethylene glycol by heat exchange with the auxiliary solar thermal fluid of the auxiliary solar thermal circuit 32’ and subsequently to an evaporator 73, wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from a second combustor 74 up to a temperature allowing separation of steam 75 from the solution, to obtain a stream 76 of regenerated glycol, which is then sent to a storage tank 77 and subsequently to a pump 78 to be reused through injection in the injector 53 and to the dehydration column 57.

[0043] According to the embodiment of Fig.7, the solar field 31 provides the heat necessary for the glycol/water separation process. Depending on: the size of the solar filed 31, the related heat accumulation systems, the intensity of solar radiation during the day or the season, the heat supplied by the solar field 31 can be either in the quantity necessary to support the process, zeroizing the use of the traditional heat source (gas burners 64, 74), or can be used in combination with the traditional heat source, for hybrid operation.

[0044] With continuing reference to Figs 1, 2, 3, 4, 5, 6 and 7, a further embodiment of a waste heat recovery system is shown in Fig.8. The waste heat recovery system of Fig.8 differs from the waste heat recovery system of Fig.7 mainly in that the degaser 63 and the evaporator 73 further comprise an electrical heater, connected to an electrical power generator. According to this embodiment, the expansion of natural gas between the pressure of the well 50 and the pressure of the final dehydration treatment can be exploited by a turbo-expander producing electrical energy thanks to the coupling with a generator. In particular, the natural gas downstream of the injector 53 is directed to a first turbo expander 80 coupled with a first power generator 81. The electric heaters 63’, 73’ can work in combination with the traditional heat source from the combustors 64, 74 in hybrid mode, or exclusively, in the separation process of water and glycol. A first horizontal condensate separator 82 is arranged downstream of the first turbo expander 80, to separate the condensed liquids transported by the gas and part of the glycol injected through the injector 53. Subsequently, the gas is conveyed to a first electric heater 84 and to a second turbo expander 80’, downstream of a second injector 85 of dehydrated glycol from the evaporator 73. A second power generator 81’ is connected to the second turbo expander 80’, to generate additional electrical power. A second horizontal condensate separator 82’ is arranged downstream of the second turbo expander 80’, to separate the condensed liquids transported by the gas and part of the glycol injected through the injector 85. Subsequently the gas is conveyed to a second electric heater 84’ and to a third turbo expander 80”, downstream of a third injector 85’ of dehydrated glycol from the evaporator 73. A third power gener- ator 81” is connected to the third turbo expander 80”, to generate additional electrical power. Finally, the gas from the third turbo expander 80” is directed to the lower part of the dehydration column 57. According to this embodiment, the electrical energy obtained by the first turbo-expander-generators 81, 81’, 81” is used, in whole or in part, to produce heat by an electric heater 63’ of the degaser 63 and an electrical heater 73 ’ of the evaporator 73.

[0045] While aspects of the invention have been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirt and scope of the claims. In addition, unless specified otherwise herein, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.

Claims

1. An integrated solar combined cycle comprising a compression train and a waste heat recovery system, the compression train comprising one or more compression stages (11, 11’) mounted on a rotating shaft (12) or integrally geared and configured to compress a gas, each compression stage (11, 11’) comprising a gas inlet (111, 111’) and a gas outlet (112, 112’), the waste heat recovery system comprising one or more heat exchangers (14, 14’, 14”) connected downstream of the gas outlets (112, 112’) of each compression stage (11) and configured to exchange heat between the gas on one side and a thermal fluid on the other side, the heat exchangers (14, 14’, 14”) being arranged along a thermal circuit (15) circulating said thermal fluid, said heat exchangers (14, 14’, 14”) comprising higher temperature heat exchangers (14, 14’) and lower temperature heat exchangers (14”); the waste heat recovery system additionally comprising

- a thermodynamic cycle system (20) operating a working fluid and including a thermodynamic cycle circuit (21) comprising one or more pumps (22), one or more first heaters (23, 23’, 23”) arranged downstream of said pumps (22) and configured to exchange heat between said thermal fluid of said thermal circuit (15), downstream of said higher temperature heat exchangers (14, 14’), on one side and said working fluid on the other side, to heat and evaporate said working fluid, one or more expanders (24, 24’), arranged downstream of said first heaters (23, 23’, 23”) and configured to expand said working fluid to give a mechanical work, and one or more coolers (25, 25’) arranged downstream of said expanders (24, 24’) and upstream of said pumps (22) and configured to exchange heat between said working fluid on one side and said thermal fluid of said thermal circuit (15), downstream of said lower temperature heat exchangers (14”), on the other side, to cool and condensate said working fluid, and

- a solar thermal system (30), including a solar field (31), and wherein alternatively

- the solar field (31) is arranged along said thermal circuit (15) and is configured to heat said thermal fluid, upstream of said first heaters (23, 23’, 23”) of the thermodynamic cycle circuit (21), or - the solar field (31) is arranged along a solar thermal circuit (32) and is configured to heat a solar thermal fluid, the solar thermal circuit (32) being separate from said thermal circuit (15), and said thermodynamic cycle circuit (21) additionally comprises second heaters (26, 26’), configured to exchange heat between said solar thermal fluid of the solar thermal circuit (32) on one side and said working fluid of the thermodynamic cycle circuit (21) on the other side, said second heaters (26, 26’) being arranged along said thermodynamic cycle circuit (21) downstream of said first heaters (23, 23’, 23”) and upstream of said expanders (24).

2. The integrated solar combined cycle of claim 1, wherein the solar field (31) is configured to heat said thermal fluid or said solar thermal fluid alternatively by direct heat exchange or by an intermediate photovoltaic system, powering a heat pump.

3. The integrated solar combined cycle of claim 1 or 2, wherein the system further comprises auxiliary heat sources (40) cooperating with the solar field (31) to provide heat to said thermal fluid.

4. The integrated solar combined cycle of claim 3, when the solar field (31) is arranged along said thermal circuit (15), wherein the auxiliary heat sources (40) comprise a heat exchanger (40) configured to exchange heat between the exhaust gas of a gas turbine (41) and said thermal fluid, the heat exchanger (40) being arranged along said thermal circuit (15) in parallel to said solar field (31) and upstream of said first heaters (23).

5. The integrated solar combined cycle of claim 3, when the solar field (31) is arranged along said solar thermal circuit (32), wherein the auxiliary heat sources (40) comprise a heat exchanger (40) configured to exchange heat between the exhaust gas of a gas turbine (41) and said solar thermal fluid, the heat exchanger (40) being arranged along said solar thermal circuit (32) in parallel to said solar field (31) and upstream of said second heaters (26, 26’).

6. The integrated solar combined cycle of one or more of claims 1-4, when the solar field (31) is arranged along said thermal circuit (15), wherein the solar thermal system (30) comprises one or more storage vessels (34, 34’), arranged along said thermal circuit (15), upstream and/or downstream of the solar field (31), to store said thermal fluid.

7. The integrated solar combined cycle of one or more of claims 1, 2, 3 and 5, when the solar field (31) is arranged along said solar thermal circuit (32), wherein the solar thermal system (30) comprises storage vessels (34, 34’), arranged along said solar thermal circuit (32), upstream and/or downstream the solar field (31), to store said solar thermal fluid.

8. The integrated solar combined cycle of one or more of claims 1-7, wherein said expander (24, 24’) is a turbo expander (24) coupled with at least one compression stage (11, 11’) of said compression train (10).

9. The integrated solar combined cycle of claim 8, wherein said turbo expander (24) is coupled with said at least one compression stage (11, 11’) of said compression train (10) through a common shaft (12) or through a gear box.

10. The integrated solar combined cycle of claim 8, wherein said turbo expander (24) is coupled with said at least one compression stage (11, 11’) of said compression train (10) through a clutch (17).

11. The integrated solar combined cycle of one or more of claims 1-7, wherein said expander (24, 24’) is an expander compressor (24’) coupled with a compressor (27) cinematically independent from said compression train (10).

12. The integrated solar combined cycle of claim 11, wherein said compressor (27) is part of a heat pump configured to operate a cooling fluid circulating in a cooling circuit comprising one or more heat exchangers arranged along said gas inlets (111, 111’) of the compression stages (11, 11’) of the compression train (10) and configured to exchange heat between the cooling fluid on one side and the gas to be compressed on the other side.

13. The integrated solar combined cycle of claim 11, wherein said expander compressor (24’) is configured to operate the same gas of the compression stages (11, 11’) of the compression train (10) and is arranged upstream of or downstream of the compression train (10) or in parallel to at least one of the impellers of the compression stages (11, 11’) of the compression train (10).

14. The integrated solar combined cycle of one or more of the preceding claims, wherein the system further comprises one or more auxiliary heat exchangers (16), arranged along said thermal circuit (15) and configured to exchange heat between a lubrication fluid of a lubrication circuit (13) of the compression train (10) on one side and the thermal fluid of the thermal circuit (15) on the other side.

15. The integrated solar combined cycle of any of the preceding claims, wherein said compression train (10) is a turbo-compression train of gas storage plants.

16. The integrated solar combined cycle of any of the preceding claims, wherein said thermal fluid of said thermal circuit (15) is water.

17. The integrated solar combined cycle of any of the preceding claims, wherein said working fluid of said thermodynamic cycle circuit (21) is pentafluoro propane.

18. The integrated solar combined cycle of any of the preceding claims, wherein the solar system (30) further comprises an auxiliary solar thermal circuit (32’) sharing the solar field (31) with the solar thermal circuit (32) and being configured to heat an auxiliary solar thermal fluid, one or more auxiliary heat exchangers (61, 71) being arranged along the auxiliary solar thermal circuit (32’) and being configured to exchange heat between said auxiliary solar thermal fluid of the auxiliary solar thermal circuit (32’) on one side and one or more fluids to be heated on the other side.

19. The integrated solar combined cycle of claim 18, wherein the one or more auxiliary heat exchangers (61, 71) are part of a water/glycol separation system arranged along a glycol recovery circuit of a natural gas de-hydration system, the one or more auxiliary heat exchangers (61, 71) being configured to exchange heat between the auxiliary solar thermal fluid of the auxiliary solar thermal circuit (32’) on one side and a solution of water and tri-ethylene glycol on the other side.

20. The integrated solar combined cycle of claim 19, wherein the one or more auxiliary heat exchangers (61, 71) comprise a first auxiliary heat exchanger (61), configured to heat the solution of water and tri-ethylene glycol upstream of a degaser (63), wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from a first combustor (64) up to a temperature allowing separation of gases from the solution and a second auxiliary heat exchanger (71), configured to heat the solution of water and tri-ethylene glycol upstream of an evaporator (73), wherein the solution of water and tri-ethylene glycol is heated by heat exchange with a gas from the same or a different combustor (74) up to a temperature allowing separation of steam from the solution.

21. The integrated solar combined cycle of claim 20, wherein the de- gaser (63) and/or the evaporator (73) further comprise an electrical heater, connected to one or more electrical power generators (81, 81 ’, 81”) coupled with respective expanders (80, 80’, 80”), the expanders being configured to expand the gas from a high pressure storage (50) down to the pressure of the distribution pipeline.