POWER GENERATION SYSTEM HAVING THERMAL ENERGY STORAGE

[0001] Embodiments illustrated herein relate to power generation systems, and in particular, to regulating the power output of a power generation system based on energy storage. Specific embodiments herein relate particularly to the storage of renewable energy.

[0002] Renewable energy resources, for example, wind, sun etc., are regarded as environmentally friendly, relatively inexpensive alternative sources of energy. Renewable energy production sources, such as wind turbines, photo-voltaic cells, hydro-electric power generators, among others, are frequently used in conjunction with conventional thermal power plants such as steam or gas turbine power plants supplying electrical power to a utility. However, renewable electric energy has a disadvantage that the energy production is intermittent, and often cannot be dispatched in line with the demand of the consumers.

[0003] The electrical energy output of a renewable energy source, such as a wind turbine, or a photo-voltaic cell largely depends on weather conditions, for example, wind speed and/or sunshine, which, in turn, vary substantially over the course of a day (24 hours). Electric power demand of the consumers also varies substantially based on the weather, which often results in a situation that the power supply from renewable energy source does not perfectly coincide with the power demand of the consumers. As an example, wind energy, particularly from offshore wind turbines, reaches its maximum output generally in the night hours, when the power demand is relatively low. In the afternoon hours, when the power demand is usually very high due to AC systems, the available energy from wind is negligible.

[0004] In scenarios such as above, the unit price for electricity to the consumers may be a function of the time of the today, and possibly even the time of the year. To that end, a time-of-day (TOD) factor is sometimes determined for a given time-interval in a day, which is multiplied by the base unit price of electricity to obtain the price of electricity paid by an electricity distribution company to an electricity provider. In most cases, the

[0005] TOD factor for the peak-demand hours, for example afternoon and evening hours, is several times greater than the TOD factor for low-demand hours in reduced demand months, such as the late night hours of March in the US west coast.

[0006] Given an increasing fraction of renewable sources in power generation sources, the mismatch between energy demand and supply is a significant problem, often resulting in dumping of energy, and close to zero electricity prices for renewable energy outside high-demand hours.

[0007] The present technique provides a significant increase in value of the electricity produced by a power generation system while substantially eliminating the wastage or dumping of electrical energy produced during times of low power demand. A power generation system includes a power generation module, for example in the form of a power plant such as a steam turbine power plant or a combined cycle power plant, among others. The present technique involves the use of a thermal storage system in conjunction with such a power generation module. The thermal storage system is configured to receive an energy input at a first point in time, to store the energy input as thermal energy, and to dispatch the stored energy in a time-shifted manner to the power generation module for electrical power production. The time-shift may, for example, be a function of the power demand (i.e power output demand) from the power generation system.

[0008] The thermal storage system includes a heat storage medium contained in a storage tank. The energy input into the thermal storage system may be in the form of mechanical or electrical energy. The thermal storage system converts the mechanical or electrical energy input into thermal energy and transfers the thermal energy to the heat storage medium to thereby cause an increase in enthalpy of the medium. The thermal energy is thus stored in the heat storage medium in the storage tank, to be recovered at a latter point in time.

[0009] The thermal storage system is coupled to the power generation module via heat transfer connection. The power generation module converts the stored thermal energy, which is recovered from the thermal storage system via the heat transfer connection, into an electrical power output. The recovery or dispatch of thermal energy from the thermal storage system for electricity generation is carried out at a second point in time which is temporally shifted from the first point in time.

[0010] In one embodiment, the power generation system includes a renewable energy source, which supplies the energy input to the thermal storage system. As noted above, renewable energy sources often provide variable power output depending on weather conditions which does not coincide with the variability in the power demand from the consumer/utility. This embodiment provides storage of renewable energy as thermal energy which could then be used to time-shift the dispatch of the thermal energy so as to match the power demand pattern. The renewable energy source could include, for example, a wind turbine, a photo-voltaic cell, among others, or combinations thereof.

[0011] According to one embodiment, in the first point in time, the power demand is lesser than a potential power output of the power generation system, while in the second point in time, the power demand is greater than the potential power output of the power generation system. The “potential” power output of the power generation system refers to the power output of the power generation system if no thermal energy storage was provided. Because of the high TOD factor associated with a higher power demand, any efficiency losses incurred by the conversion of the electrical or mechanical energy input into thermal energy and the subsequent conversion of the thermal energy back into electrical power output at a latter point in time is more than offset by the increase in value of the electrical power output during the high-demand hours.

[0012] In one embodiment, the thermal storage system is part of a concentrated solar power (CSP) facility, and the renewable energy source is located at the site of the CSP facility.

[0013] In one embodiment, the energy input is obtained from a mechanical power output of the wind turbine. In this case, the mechanical power output is converted into thermal energy via a mechanical friction device, which has a heat transfer connection with the heat storage medium.

[0014] In another embodiment, the energy input may be in the form of electrical energy. In this embodiment, the thermal storage system further comprises an electrical heater element for converting the input electrical energy into thermal energy.

[0015] In one embodiment, the electrical energy is obtained from the power output of a wind turbine or a photo-voltaic cell, wherein the power output of the wind turbine or the photo-voltaic cell is directly connected to the electrical heater element. In this embodiment, the wind turbine or photo-voltaic cell is located at the site of the thermal storage system.

[0016] In another embodiment, the electrical energy is obtained from a power grid, the power grid being supplied by at least one renewable energy source. In this case, the at least one renewable energy may be at a remote or offsite location with respect to the thermal storage system.

[0017] In one embodiment, the power generation module includes a steam turbine power plant comprising a steam generator or boiler that utilizes the thermal energy recovered from the thermal storage system to generate steam for driving a steam turbine. Of course, the power generation module may include any other type of power plant instead, for example, a combine cycle power plant including a gas turbine and a steam turbine.

[0018] In one embodiment, the heat storage medium comprises a phase-change material. Since a phase-change material has a high heat of fusion, it would be capable of storing and releasing a large amount of thermal energy. In an exemplary embodiment, the phase-change material is a molten salt.

[0019] In another aspect, a method is provided for regulating the power output of the power generation system. The method involves operating the thermal storage system and the power generation module such that input energy in the form of electrical or mechanical energy is received and stored by the thermal storage system at a first point in time, and then dispatched to the power generation module for power production at a second point in time that is temporally shifted from the first point in time.

[0020] In yet another aspect, a method is provided for retrofitting an existing power plant by coupling a thermal storage system as described above to the power plant. Such a retrofit may be done to augment the power output of the power plant during high power demand, to increase the value of the electrical power output of the power plant.

[0028] Referring now to FIG. 1, a power generation system 10 is illustrated according to an example embodiment. The illustrated power generation system 10 includes a power generation module 12 in the form of a power plant that supplies electrical power E_O to a utility 15, such as a grid. The power plant 12 illustrated herein is a steam turbine power plant, although, it should be appreciated that the present technique applies to any other type of power plant, such as a combined cycle power plant involving a gas turbine and a steam turbine.

[0029] The power demand experienced by the utility 15 varies substantially in course of the day and may further vary seasonally. The illustrated embodiments provide a technique that ensures that the electrical power E_O supplied by the power generation system 10 is able to substantially match such variation in power demands, while minimizing energy wasted. Specifically, the illustrated embodiments provide a mode of operation wherein maximum power is delivered to the utility 15 during the peak-demand hours, which would significantly increase the value of the electrical power produced by the power generation system 10. To this end, the power generation system 10 is provided with a thermal storage system 11 which is connected to the power plant 12 via a heat transfer connection 13. The thermal storage system 11 is operable to store energy, as thermal energy, and dispatch the stored thermal energy to the power plant 12 to produce a power output by the power plant 12, for example as a function of power demand.

[0030] The energy input to the thermal storage system 11 is received in the form of electrical energy E_I and/or mechanical energy M_I, for example, from a renewable energy source 14. The renewable energy source 14 may or may not be a part of the power generation system 10. As used in this discussion, the term “renewable energy source” refers to a physical system that converts energy from a renewable resource (such as sun, wind, ocean current, etc.) into a useful power output. Exemplary illustrated embodiments of the renewable energy source 14 include wind turbines and photo-voltaic cells, although the present technique may also apply to other renewable sources such as wave energy generators, hydro-electric power generators, among others.

[0031] The thermal storage system 11 includes a heat storage medium contained in a storage tank. For effecting energy storage, the thermal storage system converts the input energy into thermal energy, transfers the thermal energy to the heat storage medium via an increase in enthalpy of the medium, and stores the heated medium in the storage tank.

[0032] In the embodiment illustrated herein, the heat storage medium includes a phase-change material. Since a phase-change material has a high heat of fusion, it would be capable of storing and releasing a large amount of thermal energy. An example of a suitable material for this purpose is molten salt. However, other materials could also be configured for this purpose. Examples include thermal oils, liquid/molten metals, pressurized inert gases such as helium, among others.

[0033] The capability to store energy in the form of thermal energy allows to create a temporal shift (i.e., time-shift) between a first point in time when the input energy is received from the renewable energy source 14 by the thermal storage system 11, and a second point in time when the stored energy is dispatched to the power plant 12 via the heat transfer connection 13 for electrical power generation. In the illustrated embodiment, the second point in time is associated with a peak power demand, while the first point in time is associated with a period of peak power output of the renewable energy source 14. In the context of the present discussion, the terms “first point in time” and “second point in time” refer to finite time intervals.

[0034] FIG. 2 is a an exemplary graphical representation showing a curve 21 that represents variation of power output of a wind turbine with time and a curve 22 that represents a variation of power demand of the utility with time. The curve 21 shows a peak 21a associated with a period t1 of peak wind-power output and a trough 21b associated a period of minimum or significantly reduced wind power output. The curve 22 shows a peak 22a associated with a period t2 of peak-demand and a trough 22b associated with a period of minimum power demand. As seen from FIG. 2, the period of peak wind power output t1, which typically corresponds night hours, overlaps with the period of minimum power demand. Again, the period t2 of peak power demand, which typically corresponds to afternoon hours, overlaps with the period of minimum wind power output due to negligible wind speed during this time.

[0035] Still referring to FIG. 2, an exemplary mode of operation may be configured, wherein the power generation system 10 is operated such that the first point in time associated with receiving and storing renewable energy corresponds entirely to the period t1 or at least overlaps with the period t1, while the second point in time associated with dispatch of the stored thermal energy corresponds entirely to the period t2 or at least overlaps with the period t2. In this example, in the first point in time t1, the power demand is lesser than a potential power output of the power generation system. In the second point in time t2, the power demand is greater than the potential power output of the power generation system. The “potential” power output of the power generation system refers to the power output of the power generation system if no thermal energy storage was provided. It is to be noted that the curve 22 in FIG. 2 does not represent the power output E_O of the power generation system 10 to the utility 15, but merely the energy received from the power output of the renewable energy source 14, which is a wind turbine in this example.

[0036] By thus shifting renewable energy from a low-demand period to a peak-demand period via thermal storage, a significant increase is achieved in the value of electrical power produced by the power generation system. The time shift capability also makes it possible to operate the power generation system 10 such that the electrical power output of the power plant 12, as generated by the dispatch of stored thermal energy from the thermal storage system 11 from time to time, closely matches the power demand pattern at all times, without the need to waste or dump renewable energy produced during off-peak demand hours. With the current difference between peak and off-peak (night) electricity price, the value added to renewable energy by the illustrated embodiments could be about 30%. With increasing renewable energy resources in the market, the difference between peak and off-peak (night) electricity prices is getting increasingly bigger, with the prospect of even greater value-improvement in future using the above-described technique.

[0037] The operation of an exemplary thermal storage system is discussed referring to FIG. 3 through FIG. 6. As illustrated, the thermal storage system 11 incorporates two large tanks C and H with transfer pumps P for circulating the heat storage medium H. Large storage tanks are often used in concentrated solar power (CSP) facilities. Accordingly, in one embodiment, the thermal storage system 11 may be incorporated in a CSP facility, and configured to utilize the available infrastructure of the CSP facility. Furthermore, the renewable energy source 14 may also be co-located in such a CSP facility.

[0038] The illustrated thermal storage system 11 is operable in two modes, namely, charging and discharging. Charging refers to the process of conversion of input energy into thermal energy, transferring the thermal energy to the heat storage medium, and storing the heated medium in a tank. Discharging refers to the process of recovering and dispatching the stored thermal energy to the power plant. The charging and discharging modes are respectively associated with the above-described first point in time and second point in time.

[0039] Embodiments of the charging mode will be first described.

[0040] Depending on the nature of the input energy, the energy transfer effected by the thermal storage system may be direct or indirect. In an embodiment where the input energy is in the form of electrical energy, the energy transfer is effected indirectly. In this case, as schematically shown in FIG. 3, the thermal storage system 11 includes an electrical heating element 31 that converts the electrical energy input E_I into heat, for example, resistively and/or inductively. The heat storage medium M is heated in the following manner. A cold medium M is taken from a first storage tank C and pumped through an electrical heat exchanger HE wherein the medium M is heated via heat transfer from the electrical heating element 31. If CSP infrastructure is used, further heating to the medium M may be provided by way of a CSP collector. The heated medium M is then lead into a second storage tank H where it is stored until a dispatch of the stored energy is initiated.

[0041] In the embodiment illustrated in FIG. 3, the electrical energy input E_I is received from an on-site renewable energy source 14, such as a wind turbine or a photo-voltaic cell, which connected directly to the electrical heating element. In the case of a photo-voltaic cell, a direct connection with the heating element obviates the need for AC-DC converters and transformers. In the case of a wind turbine, such a direct connection eliminates the requirement of an inverter to match grid conditions.

[0042] In an alternate embodiment schematically shown in FIG. 4, the electrical energy input is received from one or more renewable energy sources 14 located remotely or offsite from the thermal storage system 11, via a grid 41. The grid 41 supplies electrical power to the heating element 31, which converts the same into heat and transfers the heat to the medium M via the electrical heat exchanger HE, similar to the previously illustrated embodiment.

[0043] In an embodiment where the input energy is in the form of mechanical energy, a more direct energy transfer may be effected, as schematically shown in FIG. 5. Herein, the cold medium is pumped from the first tank C to a heat source S. In the illustrated embodiment, the heat source S is a mechanical-to-thermal energy converter, such as a friction device, mechanically coupled to the shaft of a wind turbine located on site. The device S converts the mechanical power of the wind turbine shaft into heat and transfers the heat to the medium M, which gets heated. The heated medium M is then pumped into the second tank H for storage and subsequent dispatch. In a further embodiment, if CSP infrastructure is used, the heat source S may include a CSP collector.

[0044] The discharge mode, i.e., the dispatch of thermal energy to the power plant may take place in a manner illustrated schematically in FIG. 6. In this example, the power plant is a steam turbine power plant. As noted above, the discharge mode is typically initiated during high power demand. In this mode, the heated medium M from the tank H is pumped through a steam generator or boiler 61, which is part of the steam power plant 12 (FIG. 1). As shown in FIG. 6, a heat transfer interface is provided between the steam generator or boiler 61 and the circulating medium M, via which heat is transferred from the medium M to aid steam production in the steam generator or boiler 61. The medium M gets cooled in the process and is circulated back into the first tank C.

[0045] The steam turbine power plant 12 may involve, for example, a conventional Rankine cycle with or without reheat, with steam being produced in the boiler or steam generator 61 via thermal energy dispatched via the heat storage medium. The steam produced in the boiler is expanded in a high pressure turbine, optionally reheated, and further expanded in intermediate and low pressure turbines. The steam exhaust from the turbines is condensed in a condenser and pumped back to the boiler or steam generator. The turbines produce mechanical energy in the form of rotation of the turbine shaft by expanding the steam. An electrical generator coupled to the turbine shaft converts the rotational energy into an electrical power output.

[0046] An aspect of the present technique is also directed to retrofitting a conventional power plant, such as the one described above, by coupling a thermal storage system to the power plant. The conventional power plant may include a boiler for generating steam via combustion of a fossil fuel, such as coal, wood, oil or natural gas. The steam generated may be expanded to generate power via the Rankine cycle as described above. The retrofit may involve providing a second boiler or steam generator coupled to the thermal storage system via a heat transfer connection, the second boiler or steam generator being operable for generating steam utilizing thermal energy transferred via the heat storage medium (similar to the boiler or steam generator 61 described above). When the thermal storage system 11 is operated in the discharge mode, the thermal energy transferred to the second steam generator or boiler from the heat storage medium increases the overall steam production, thereby augmenting the electrical power output of the plant.

[0047] In particular, the thermal storage system may be configured to store and dispatch renewable energy, for example, from a wind turbine or a photo-voltaic cell. Using the techniques illustrated above, the operation of such a retrofitted power plant may be enhanced by charging the thermal storage system based on availability of renewable energy, and discharging the thermal storage system as a function of power demand from the power plant.

[0048] Although a steam turbine power plant has been illustrated, the teachings herein may also apply to other types of plants, such as a combined cycle power plant involving a steam turbine power plant (operating by the Rankine cycle) powered by the hot exhaust of a gas turbine (operating by the Brayton cycle).

[0049] While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.