Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K3/00—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K25/00—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for
  • F01K25/08—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for using special vapours

Definitions

• the present invention relates to the field of energy storage, in particular to a system for storing energy and a method of storing energy using thermal storage.
• the efficiency of the thermal power plant is limited by the Carnot efficiency which is defined by the work divided by the heat put into the system or a relation between the absolute temperature of the cold and hot reser ⁇ voir. This is a theoretical process and its efficiency cannot be surpassed physically. In the real application, additional mechanical (e.g. flow irreversibility in the turbine, pump irreversibility) , thermal, electrical and exergy losses lead to a reduced efficiency. Furthermore, in the Rankine cycle, which is the thermodynamic process applied in steam power plants, the heat addition takes place at several temperature levels in contrast to the Carnot cycle. This further reduces the efficiency.
• HRSG heat recovery steam generator
• a type of power plant in which this happens is a combined cy ⁇ cle power plant (CCPP) , in which hot flue gas that still con ⁇ tains a significant amount of thermal energy leaves the sys- tern and is lost.
• CCPP combined cy ⁇ cle power plant
• a thermal energy storage plant could be configured with ther ⁇ mal storages (also referred to as heat storages) , in which thermal energy is stored, and a HRSG with a water-steam cycle for re-electrification.
• the thermal energy or heat is extracted from the storage, air is guided through the storage arrangement and leaves the storage as heated air. This heated air is then used to generate steam in the HRSG.
• the problem occurs that the hot air cannot be cooled completely down to ambient temperature in the HRSG, so that unused ther ⁇ mal energy leaves the system and reduces the efficiency of the system.
• the problem may be partially solved by building more effi ⁇ cient and more expensive HRSGs with multiple steam pressure levels, such as three pressure levels.
• thermodynamic efficiency of the internal steam cycles with medium and low pressure is rather low in comparison to the high pressure cycle.
• to- tal efficiency of the entire system adds up to a higher value when compared with a single pressure HRSG since the low tem ⁇ perature heat losses are reduced due to the reduction of the HRSG outlet temperature to e.g. 90°C instead of e.g. 200°C. Further reduction of this temperature is economically not feasible due to the required immense enlargement of heat ex ⁇ changer surface in the relevant HRSG.
• thermal storage systems are among the sys ⁇ tems considered for temporarily storing extra power during periods of high production (e.g. strong wind, sunny days) in order to compensate for subsequent periods of low production (e.g. weak wind, cloudy days) .
• high production e.g. strong wind, sunny days
• low production e.g. weak wind, cloudy days
• the steam generator is adapted to heat a steam turbine fluid using the working fluid received at the fluid input and to output the used working fluid through the fluid output for transportation to the second opening of the thermal storage, such that thermal energy re ⁇ maining in the output working fluid is injected into the thermal storage through the second opening.
• thermal storage may in particular denote a device, arrangement or structure that is capable of storing received thermal energy and of releasing (outputting) stored thermal energy.
• the thermal storage may be a sensible thermal storage or a latent thermal storage.
• the thermal storage may be filled with a thermal storage mate ⁇ rial, which has the ability to be heated up, keep the tem ⁇ perature and thus store thermal energy over a predetermined period of time.
• a thermal storage mate ⁇ rial which has the ability to be heated up, keep the tem ⁇ perature and thus store thermal energy over a predetermined period of time.
• water may be used as the thermal storage material.
• the ther ⁇ mal storage material may be heated up (charged) and cooled down (discharged) by a fluid, i.e. the working fluid.
• the first opening may be an opening in the thermal storage through which fluid may be exchanged with one or more exter- nal devices, e.g. a thermal energy source, such as a heating device, and the steam generator.
• a thermal energy source such as a heating device
• the first opening and the device (s) may be connected by a pipe or a duct.
• the second opening may be a further opening in the thermal storage through which the thermal storage is able to exchange fluid with some further external device (s), such as the steam generator, the heating device, or a pump.
• the second opening and the further device (s) may be connected by a pipe or a duct.
• the first opening and the second opening may respectively be denoted the hot opening (or hot end) and the cold opening (or cold end) of the thermal storage, and vice versa.
• the first opening and the second opening are similarly or identically formed. It should however be un ⁇ derstood that the first opening and the second opening may also be formed to have different shapes.
• the first opening and the second opening may be formed in the same side surface of the thermal stor ⁇ age. This configuration may be advantageous when geometric restraints of the overall configuration of the system do not permit another geometrical set-up.
• the first opening and the second opening may be formed in differ- ent side surfaces, in particular in opposing side surfaces, of the thermal storage.
• the working fluid may be a medium, such as for example a gas or a liquid, which flows through the system, in particular from a thermal energy source to the first opening of the thermal storage during charging and from the first opening of the thermal storage to the second opening of the thermal storage via the steam generator during discharging.
• a part of the thermal energy transported away from the ther ⁇ mal storage by the working fluid during discharging can be used by the steam generator while remaining thermal energy is returned to the second thermal storage and thus stored.
• the medium may be a mixture consisting of a main component, such as water, with added supplements for influencing physical characteristics of the mixture, such as for example the evaporation point or the condensation point.
• Boilers are closed devices in which a medium is heated up and boiled, respectively.
• the heated or vaporized medium leaves the boiler and may be used in the steam turbine system.
• Evaporators are devices which are used to turn a medium from its liquid form into its gaseous form. The evaporated medium may as well be used in the steam turbine system.
• the design of the steam generator may be kept simple and thereby cheap.
• the work ⁇ ing fluid output from the steam generator is transported back to the second opening of the thermal storage, such that re ⁇ maining thermal energy is not simply lost, complex steam generator structures and features aiming at extracting as much thermal energy as possible from the working fluid are not necessary.
• even existing steam turbine sys- terns, for example of closed fossil fuel power plants, may ad ⁇ vantageously be reused.
• the steam turbine fluid is a fluid that may be used to drive a steam turbine system.
• the steam turbine fluid may be steam, water vapour or vapour with a high mass fraction of water in it, respectively.
• water vapour may be saturated but also unsaturated.
• supplements to the water vapour to influ- ence physical characteristics of the water vapour, such as for example the evaporation point or the condensation point.
• the working fluid may further be an organic liquid which may be used in an Organic Rankine Cycle (ORC) .
• ORC Organic Rankine Cycle
• the steam turbine comprises blades connected to a shaft.
• the energy of the steam turbine fluid flowing through the steam turbine is transmitted by the blades of the steam turbine to the shaft.
• the generator is connected to the shaft of the steam turbine and converts the rotational energy of the shaft of the steam turbine into electrical energy.
• the generated electrical energy may then be transferred to a power supply network or any other end-user.
• the steam turbine fluid flows through the condenser in which the steam turbine fluid is condensed into its liquid state.
• the first pump and the sec- ond pump drive the steam turbine fluid through the different components of the steam turbine system and through the steam generation device.
• the second pump may be of importance in order to reach the design pressure of the steam in the water steam cycle.
• the system further comprises a controller adapted to end the charging phase when the temperature at the second opening of the thermal storage reaches a predetermined temperature value.
• a similar mechanism may be used to detect when to end the discharging phase. In this case, it may be detected when the temperature of the working fluid that leaves the thermal storage through the first opening (and is guided towards the fluid input of the steam generator) has decreased to a given temperature value.
• the pre- determined temperature value is equal to or larger than the temperature of the used working fluid output by the steam generator through the fluid output.
• the temperature at the second opening (cold end) of the thermal storage is equal to or larger than the tempera ⁇ ture of the working fluid that will be output by the steam generator during a subsequent discharging phase.
• the lowest temperature within the system is that of the output fluid from the steam generator, such that losses are minimized.
• the corresponding temperature difference may be at least 10%, in particular at least 20%, and further in particular at least 30% of the total temperature differ ⁇ ence within the system, i.e. the difference between the high- est and the lowest temperature inside the thermal storage.
• the above mentioned 10% may corre ⁇ spond to around 50 °K
• the above mentioned 20% may correspond to around 100°K
• the above mentioned 30% may correspond to around 150°K.
• the system further comprises a heating device in fluid communication with the first opening of the thermal storage, wherein the heating device is adapted to heat the working fluid during the charging phase.
• the heating device heats the working fluid which is then supplied to the thermal storage through the first opening thereof.
• the heat ⁇ ing device is adapted to transform electric or magnetic ener ⁇ gy into heat.
• the heating device may comprise a resis ⁇ tance heater, an electrical heater or an inductive heater.
• the system may be used to store energy received in the form of electric or magnetic energy as thermal energy in the thermal storage.
• the received energy may e.g. be excess energy produced by a power plant utilizing a renewable energy source, such as wind power or solar power.
• the sys- tern further comprises a first pumping device for transporting heated working fluid from the heating device to the first opening of the thermal storage.
• the first pumping device may be a blower arranged to
• the heat ⁇ ing device is in fluid communication with the second opening of the thermal storage, the system further comprising a se ⁇ cond pumping device for transporting the working fluid from the second opening of the thermal storage to the heating de ⁇ vice .
• the fluid communication between the second opening of the thermal storage and the heating device may be constituted by a pipe or a duct.
• the second pumping device may e.g. be a blower arranged to transport air from the second opening of the thermal storage to the heating device.
• the second pumping device may replace the first pumping device in the sense that the second pumping device also performs the function of the first pumping de ⁇ vice, such that it is capable of transporting the charging fluid from the second opening of the thermal storage to the first opening of the thermal storage via the heating device.
• the first pumping device may also un ⁇ dertake the function of the second pumping device, such that also in these embodiments only a single pumping device is needed in order to perform the functions of both the first pumping device and the second pumping device.
• the first and second pumping devices may be implemented as two physically separate pumping devices, e.g. one (the first pumping device) between the heating de ⁇ vice and the first opening of the thermal storage and another one (the second pumping device) between the second opening of the thermal storage and the heating device.
• the second pump ⁇ ing device serves to extract thermal energy from the thermal storage through the second opening in order to provide a pre ⁇ heated working fluid to the heating device. Accordingly, the heating device will need less energy to heat the working flu ⁇ id to the desired temperature for storing in the (hot part of the) thermal storage.
• the remaining thermal energy of the working fluid that has been stored or buffered in the (cold part of the) thermal storage is re-introduced into the thermal storage through the first opening, such that the losses within the system are minimized independently of the efficiency of the steam generator.
• the sys- tern further comprises a third pumping device for transporting the working fluid from the first opening of the thermal stor ⁇ age through the steam generator and on to the second opening of the thermal storage during the discharging phase.
• the third pumping device serves to transport the working fluid from the first opening of the thermal storage to the second opening of the thermal storage via the steam generator.
• the working fluid leaves the thermal storage through the first opening with a rela ⁇ tively high content of thermal energy and is transported through the steam generator where a certain amount of the thermal energy is used to heat the steam turbine fluid.
• the working fluid then leaves the steam generator with a lower content of thermal energy. This remaining thermal energy is, except for unavoidable losses, re-stored in the thermal stor ⁇ age through the second opening for later use.
• the third pumping device comprises a single pumping unit arranged either between the first opening of the thermal storage and the steam generator or between the steam generator and the second opening of the thermal storage. In other embodiments, the third pumping device comprises two or more pumping units, of which at least one is arranged between the first opening of the thermal storage and the steam generator while another one is arranged between the steam generator and the second opening of the thermal stor- age .
• the working fluid is air.
• the thermal storage comprises a bulk thermal storage material.
• the thermal storage material may be a solid or bulk material, such as sand or gravel, rubble, split, clinker, pebble, slag, stones, bricks, ceramics or other bulk materials, such as ba ⁇ salt or iron silicate slag, which have the ability to be heated up, keep the temperature and thus store thermal energy over a predetermined period of time.
• the steam generator is a heat recovery steam generator (HRSG) .
• a HRSG may be an energy recovery heat exchanger which recovers energy from a fluid stream.
• the HRSG may e.g. comprise the following four principal components: an economizer, an evaporator, a super-heater and a re-heater. The four
• the HRSG may be a vertical type HRSG, a horizontal type HRSG, a single pressure HRSG or a multi pressure HRSG.
• the steam generator is adapted to receive the working fluid at the flu ⁇ id input at a temperature between 550°C and 1000°C, such as around 600 °C, and to output the working fluid through the fluid output at a temperature between 100°C and 400°C, in particular between 150°C and 350°C, in particular between 200°C and 300°C, in particular around 250°C.
• the pre- sent invention can provide a highly efficient system for storing thermal energy, even if the steam generator as such is not particularly effective. Thereby, significant savings can be obtained by using simple and/or existing (old) steam generators .
• the system according to various embodiments of the present invention as set forth above is capable of providing a highly efficient system for storing energy as thermal energy at a relatively low price.
• a power plant comprising (a) a power generator for producing electrical energy based on a renewable energy source, and (b) a system according to the first aspect or any of the above embodiments thereof, wherein the system is adapted to store excess energy from the power generator during overproduction by charging the thermal storage, and wherein the sys ⁇ tem is adapted to release stored energy during insufficient production by discharging the thermal storage.
• This second aspect of the invention is based on essentially the same idea as the first aspect described above. More spe ⁇ cifically, the second aspect uses a system according to the first aspect as part of a power plant that relies upon a re- newable energy source, such as wind power or solar power, which by nature is prone to fluctuations. Accordingly, the power plant according to the second aspect is capable of providing a more stable level of output by buffering excess energy during overproduction for later use to compensate for insufficient production.
• a re- newable energy source such as wind power or solar power
• a method of storing energy comprising (a) feeding a heated working fluid to a first opening of a thermal storage to store thermal energy in the thermal storage during a charging phase, (b) feeding the working fluid from the first opening of the thermal storage to a fluid input of a steam generator during a discharging phase, wherein the steam generator is adapted to heat a steam turbine fluid us ⁇ ing the working fluid, and (c) conducting the used working fluid from a fluid output of the steam generator to a second opening of the thermal storage to store thermal energy re ⁇ maining in the working fluid.
• This third aspect of the invention is based on essentially the same idea as the first and second aspects described above. More specifically, the third aspect relates to a meth ⁇ od of using or operating the system according to the first aspect or a power plant according to the second aspect. That is, thermal energy is stored in a thermal storage during a charging phase and then, during a discharging phase, the thermal energy is extracted from the thermal storage by a working fluid and used to heat a steam turbine fluid. Remain ⁇ ing thermal energy in the working fluid output by the steam generator is returned to the thermal storage through the se- cond opening and thus stored.
• the charging phase is ended when the temperature at the second opening of the thermal storage reaches a predetermined temperature val- ue .
• the heated working fluid flows through the thermal stor- age from the first opening to the second opening, such that a high temperature front moves from the first opening towards the second opening.
• the temperature of the working fluid that leaves the thermal storage through the second opening starts to rise.
• the predetermined temperature value is equal to or larger than the temperature of the used working fluid output by the steam generator through the fluid output.
• the temperature of the second opening (cold end) of the thermal storage is equal to or larger than the tempera- ture of the working fluid that will be output by the steam generator during a subsequent discharging phase.
• the lowest temperature within the system is that of the output fluid from the steam generator, such that losses are minimized.
• Figure 1 shows a schematic diagram of a system according to an embodiment of the present invention.
• Figure 1 shows a schematic diagram of a system 100 according to an embodiment of the present invention. More specifically, the system 100 comprises a thermal storage 110, a steam gen ⁇ erator 120, a heating device 150 and pumps 140 and 142.
• the thermal storage 110 comprises a first opening 112 and a second opening 114 that allow a fluid, such as a gaseous or liquid working fluid, to enter and exit the thermal storage 110, whereby the fluid gets into direct or indirect contact with a thermal storage material disposed within the thermal storage 110.
• the thermal storage material may e.g. comprise stone, bricks, ceramics or another solid material which is capable of storing thermal energy over a predetermined period of time, such as 12 hours or more.
• the steam generator 120 comprises a fluid input 122 in fluid communication with the first opening 112 of the thermal storage 110 via fluid connection 130.
• the steam generator 120 further comprises a fluid output 124 in fluid connection with the second opening 114 of the thermal storage 110 via fluid connection 134.
• the pump 142 e.g.
• a blower is arranged in the fluid path between the fluid output 124 of the steam gen ⁇ erator 120 and the second opening 114 of the thermal storage 110.
• the steam generator 120 receives water via water supply 126 and outputs generated steam via steam output 128.
• the steam output 128 may be connected to a steam turbine (not shown) for producing electricity.
• the fluid path formed between the first opening 112 and se- cond opening 114 by fluid connections 130 and 134, steam gen ⁇ erator 120 and pump 142 is used to discharge the thermal storage 110.
• the system comprises a further fluid path between the open- ings 112 and 114 of the thermal storage 110.
• the heating de ⁇ vice 150 is arranged within this fluid path which is used to charge the thermal storage 110. More specifically, fluid con ⁇ nection 132 connects the heating device 150 and the first opening 112 of the thermal storage, while fluid connection 136 and pump 140, e.g. a blower, connect the heating device 150 and the second opening 114 of the thermal storage 110.
• the steam generator 120 uses a hot working fluid received from the thermal storage 110 at the fluid input 122 to heat water received via water supply
• the hot working fluid prefer ⁇ ably has a temperature around 600 °C.
• the working fluid loses thermal energy and thus leaves the steam generator 120 through fluid output 124 at a lower temperature, such as between 100°C and 400°C, prefera ⁇ bly around 200°C, depending on the efficiency of the steam generator 120.
• the output working fluid is transported to the second opening 114 of the thermal storage 110 via fluid con- nection 134. Thereby, the remaining thermal energy of the working fluid is returned to and stored in the thermal stor ⁇ age 110 such that it can be used when needed at a later stage .
• Charging the system 100 with energy is done by guiding a pre- heated working fluid (e.g. having a temperature around 200°C) from the second opening 114 of the thermal storage 110 to ⁇ wards the heating device 150 by means of blower 140.
• the heating device 150 adds thermal energy to the working fluid and the fully heated working fluid (e.g. having a temperature of around 600°C) is injected into the thermal storage 110 through the first opening 112 to store the corresponding thermal energy therein.
• Figure 1 further shows two schematic temperature profiles 116 and 117 within the thermal storage 110. More specifically, the temperature profile 116 corresponds to an almost dis ⁇ charged state of the thermal storage 110 in which only the left-most part (in the orientation of the figure) of the thermal storage 110 has a high temperature while the major right-hand part of the thermal storage 110 has a lower tem ⁇ perature corresponding to the output temperature from the steam generator 120.
• the temperature profile 117 corresponds to an almost fully charged state of the thermal storage 110 in which only the right-most part (in the orien ⁇ tation of the figure) of the thermal storage 110 has a low temperature while the major left-hand part of the thermal storage 110 has a higher temperature corresponding to the output temperature from the heating device 150 (and input temperature at the steam generator 120) .
• the temperature profile will wander to the right as indicated by arrow 118 and after a certain amount of charging time the temperature profile 117 will be reached.
• the tempera ⁇ ture profile will wander to the left as indicated by arrow 119 and after a certain amount of discharging time the temperature profile 116 will be reached.
• the charging and discharging operations are controlled by a system controller (not shown) .
• the system con- troller ends a charging operation when it detects that the temperature at the second opening 114 of the thermal storage 110 has risen to a predetermined temperature value (equal to or larger than the output temperature of the steam generator 120) .
• a predetermined temperature value equal to or larger than the output temperature of the steam generator 120
• the system controller ends a discharging operation when it detects that the temperature at the first opening 112 of the thermal stor ⁇ age has decreased to a predetermined value (equal to or below the input temperature of the steam generator 120) .
• the hot part of the thermal storage 110 is maintained at the specified input temperature level of the steam generator 120.
• the waste of thermal energy that leaves the HRSG at medium temperature of e.g. 120°C is avoided by storing the thermal energy in the same thermal storage 110, from which the high temperature heat was extracted.
• the thermal storage 110 storage system can be a single stor ⁇ age or comprise an arrangement of two or more thermal energy storages which can be flown through in parallel.
• the storage of medium temperature thermal energy that remains in the exhaust flow of the HRSG e.g. 120°C
• the lower temperature level of the system 100 is not the ambient temperature, but rather the temperature of the working fluid (e.g. air) leaving the HRSG. This temperature is higher than ambient air tempera ⁇ ture .
• the working fluid flows through the system 100 in different directions, depending on whether it is in charging or discharging mode.
• a temperature front travels through the thermal storage 110 and thermal energy is stored.
• the charging cy ⁇ cle is complete when the temperature front reaches the
• This fluid is used in the HRSG 120 to gener ⁇ ate steam and to produce electrical power via a steam
• the fluid still contains thermal energy that is fed back into the thermal storage 110 through the second opening 114. This temperature defines the lower temperature level in the system 100.
• the storage system 100 has two certain states.
• the charge cy- cle of the storage system 100 is completed when the outlet temperature of the thermal storage 110 starts to rise.
• the discharging cycle is finished when the outlet temperature of the thermal storage 110 decreases.
• the recovery of the thermal energy in the air downstream of the HRSG 120 and the increase of its temperature have a posi ⁇ tive impact on the HRSG design and costs.
• the pinch point of the HRSG can furthermore be larger compared to conventional CCPPs with on- ly minor effect on the cycle efficiency. This is due to the fact that in CCPP thermal energy remaining in the flue gas is discharged to the environment. Hence the pinch point is cho ⁇ sen as small as techno-economically feasible.
• the heat ex ⁇ changer surface area in the HRSG 120 strongly increases when reducing the pinch point.
• the energy in the air downstream of the HRSG is recovered and reduces the temperature difference between the air upstream and downstream of the charging equipment.
• the thermal energy that is not transferred to the steam in the HRSG 120 remains in the system 100 and is not lost. Therefore the HRSG 120 can be designed with a larger pinch point and with significant less heat exchanger surface and consequently costs compared to a conventional CCPP.
• the HRSG exit temperature increases which is not desired in CCPPs because the thermal losses then increase.
• the systems according to the present invention can achieve higher steam cycle efficiency because the steam pres ⁇ sure and the HRSG exit temperature can be increased due to the recovery of the thermal energy in the storage system.
• the steam pressure can be increased regardless of the HRSG outlet temperature.
• the steam pressure in one pressure HRSGs is usually relatively low (compared to fired steam generators) to reduce the saturation temperature of the water and conse- quently the HRSG exit temperature.
• the steam generator layout may be especially designed for thermal storag ⁇ es. This allows for higher steam pressure and higher outlet temperature at the HRSG 120than in conventional CCPPs and hence these systems achieve higher steam cycle efficiency.
• the recovery of the thermal energy in the steam generator 120 outlet air reduces the required charge power for the storage and improves the total system efficiency.
• the thermal energy remaining in the exhaust gas is not discharged in the envi ⁇ ronment but stored in the system.
• the pinch point can be de ⁇ signed larger with minor effects on the cycle efficiency and the costs of the steam generator reduce significantly com ⁇ pared to conventional CCPPs.

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Citations (6)

• 2015
  • 2015-04-08 EP EP15162830.2A patent/EP3078817A1/de not_active Withdrawn
• 2016
  • 2016-03-22 WO PCT/EP2016/056281 patent/WO2016162205A1/en active Application Filing

Patent Citations (6)

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