CAES system with synchronous reserve power requirements

A CAES system (10) includes an air storage (18), a compressor (20) supplying compressed air to the air storage, a power generating structure (11, 102), a heat exchanger (24), an auxiliary combustor (27), an air expander (30), and an electric generator (32). The system operates in one of modes a) a main power production mode wherein the auxiliary combustor is inoperable and the power generating structure is operable, to produce power by the air expander, fed by the heated compressed air received from the air storage, in addition to power produced by the power generating structure, or b) a synchronous reserve power mode wherein the auxiliary combustor is operable and the power generating structure is inoperable, with compressed air withdrawn from the air storage being preheated by the auxiliary combustor that feeds the air expander, with the air expander expanding the heated air and the generator providing immediate start-up power.

TECHNICAL FIELD

This invention relates to a Compressed Air Energy Storage (CAES) system and, more particularly, to a CAES system that can provide substantially instantaneous, synchronous reserve power.

BACKGROUND

U.S. Pat. Nos. 7,389,644 and 7,406,828 disclose a CAES plants where a compressor supplies compressed air to an air storage during off-peak hours and, during peak hours, the stored compressed air is withdrawn from the storage, is preheated by utilizing the combustion turbine exhaust gas heat, and then is directed into an expander that generates the preheated compressed air power in addition to combustion turbine power. Conventional CAES plant operations are effective in achieving the prime goal of storing off-peak energy in the form of the compressed air and then using the preheated, stored compressed air for generation of the more needed and higher priced energy during peak hours, i.e., management of renewable and base power resources.

Still, electric grids require a number of additional very important functions such as grid regulation and emergency synchronous reserve. The grid regulation is easily provided by U.S. Pat. Nos. 7,389,644 and 7,406,828 that disclose CAES plants with practically instant load following operation of the CAES plants. The emergency synchronous reserve function requires very quick start-up and power delivery. The start-up time of the CAES plants described in each of U.S. Pat. Nos. 7,389,644 and 7,406,828, the contents of which are hereby incorporated by reference into this specification, is dependent on the startup time of combustion turbines (that can take approximately 20-30 minutes) to utilize the combustion turbine exhaust gas heat.

Thus, in a CAES system, there is a need to provide practically instant synchronous reserve power independent of the combustion turbine or other power generation structure.

SUMMARY

An object of the invention is to fulfill the need referred to above. In accordance with the principles of an aspect of the present invention, this objective is achieved by providing a CAES system including a compressor for supplying compressed air to the air storage, an air storage for storing compressed air, a power generating structure, a heat exchanger constructed and arranged to receive heat from the power generating structure and to receive compressed air from the air storage, at least one auxiliary combustor for burning fuel and constructed and arranged to receive compressed air from the air storage, an air expander constructed and arranged to be fed with heated air from one of the heat exchanger or the at least one auxiliary combustor and to expand the heated air, and an electric generator, associated with the expander, for producing electric power. The system is constructed and arranged to selectively operate in at least one of the following power production modes of operation:a) a main power production mode wherein the power generating structure is operable and the at least one auxiliary combustor is inoperable, with the heat exchanger receiving heat from the power generating structure and receiving the compressed air from the air storage so as to heat the compressed air received from the air storage, with the heat exchanger feeding the heated compressed air to the air expander, with the air expander expanding the heated compressed air and the generator providing the electric power in addition to power produced by the power generating structure, orb) a synchronous reserve power mode wherein the at least one auxiliary combustor is operable and the power generating structure is inoperable, with compressed air withdrawn from the air storage being preheated by the at least one auxiliary combustor feeding the heated compressed air to the air expander, with the air expander expanding the heated air and the generator providing substantially immediate start-up power.

In accordance with another aspect of the invention, a method of operating a CAES system is provided. The CAES system includes a compressor for supplying compressed air to the air storage, an air storage for storing compressed air, a power generating structure, a heat exchanger constructed and arranged to receive heat from the power generating structure and to receive compressed air from the air storage, at least one auxiliary combustor for burning fuel and constructed and arranged to receive compressed air from the air storage, an air expander constructed and arranged to be fed with heated air from one of the heat exchanger or the at least one auxiliary combustor and to expand the heated air, and an electric generator, associated with the expander, for producing electric power. The method includes selectively operating the CAES system in at least one of following power production modes:a) a main power production mode by:ensuring that the power generating structure is operable and the at least one auxiliary combustor is inoperable and,providing heat from the power generating structure to the heat exchanger and providing compressed air from the air storage to the heat exchanger so that the compressed air received from the air storage is heated in the heat exchanger, with the heat exchanger feeding the heated compressed air to the air expander,expanding the heated compressed air in the expander, andproviding the electric power via the generator in addition to power produced by the power generating structure, orb) a synchronous reserve power mode by:ensuring that the power generating structure is inoperable and the at least one auxiliary combustor is operable,withdrawing compressed air from the air storage,preheating the withdrawn compressed air in the at least one auxiliary combustor that feeds the air expander,expanding heated air received from the at least one combustor in the air expander andproviding substantially immediate start-up power via the generator.

In accordance with another aspect of the invention, a CAES system includes a compressor for supplying compressed air to the air storage, an air storage for storing compressed air, a source of heat, a heat exchanger constructed and arranged to receive heat from the source of heat and to receive compressed air from the air storage, at least one auxiliary combustor for burning fuel and constructed and arranged to receive compressed air from the air storage, an air expander constructed and arranged to be fed with heated air from one of the heat exchanger or the at least one auxiliary combustor and to expand the heated air, and an electric generator, associated with the expander, for producing electric power. The system is constructed and arranged to selectively operate in at least one of the following power production modes of operation:a) a first power production mode wherein the at least one auxiliary combustor is inoperable, with the heat exchanger receiving heat from the source of heat and receiving compressed air from the air storage so as to heat the compressed air received from the air storage, with the heat exchanger feeding the heated compressed air to the air expander, with the air expander expanding the heated compressed air and the generator providing the electric power, orb) a second, synchronous reserve power production mode wherein the at least one auxiliary combustor is operable and the source of heat is not received by the heat exchanger, with compressed air withdrawn from the air storage being preheated by the at least one auxiliary combustor that feeds the air expander, with the air expander expanding the heated air and the generator providing substantially immediate start-up power.

In accordance with another aspect of the invention, a method of a CAES plant operation includes compressing air and storing wind energy in the form of the compressed air in a supporting tower of a wind power plant. The compressed air released from the supporting tower is preheated to produce heated air. The heated air is supplied to an air expander. The air expander expands the heated air and connected to electric generator for producing power.

DESCRIPTION OF EXAMPLE EMBODIMENTS

With reference toFIG. 1, a CAES system is shown, generally indicated as10, in accordance with an embodiment. The system10includes a power generating structure, generally indicated at11, in the form of a conventional combustion turbine assembly having a main compressor12receiving, at inlet13, a source of inlet air at ambient temperature and feeding at least one main combustor16with the compressed air, a main expansion turbine14operatively associated with the main compressor12, with the at least one main combustor16feeding the main expansion turbine14, and an electric generator15for generating electric power.

As shown inFIG. 1, the system10also includes an air storage18that during off-peak hours stores air that is compressed preferably by at least one auxiliary compressor20. In the embodiment, the auxiliary compressor20is driven by a motor23, but can be driven by an expander or any other source. The auxiliary compressor20supplies compressed air to the air storage18preferably during off-peak hours. Although a single compressor20is shown, the air storage18can be supplied by multiple compressors or with compressed air from any source of air compression.

An outlet22of the storage18is preferably connected with a recuperator or heat exchanger24. The heat exchanger24receives the exhaust gas25from the main expansion turbine14. Thus, the combustion turbine assembly11, in addition to generating the electric power, provides a source of heat. Instead, or in addition to the exhaust gas25from the expansion turbine14of the combustion turbine assembly11, the heat exchanger24can receive any externally available source of heat, as will be explained more fully below. An outlet26of the heat exchanger24is connected to at least one auxiliary combustor27, with an outlet28of the combustor27being connected to an air expander30that is connected to an electric generator32.

In accordance with the embodiment, in a main power production mode of operation of the system10, preferably during peak hours, and with the auxiliary combustor27inoperable, compressed air from the storage18is directed to the heat exchanger24that receives heat from the source of heat (e.g., exhaust of turbine14). The heated air is expanded through the expander30that is connected to the electric generator32and produces the electric power generated by the compressed air in addition to the combustion turbine assembly power. The airflow of expander30is a subject for optimization and driven by the required compressed air generated power. The expander30has a provision for an extracted airflow flow with parameters consistent with the requirements of the air injection power augmentation technology determined by combustion turbine assembly limitations and can be a subject of optimization. In other words, the injection flow parameters of the injected airflow are consistent with flow parameters of the main compressor12at an injection point. Thus, injection can be limited or restricted under certain conditions. For example, based on combustion turbine manufacturer published data, injection at low ambient temperatures may not be permitted or possible, or injection may not be permitted or possible due to accessibility to injection points, or injection may not occur due to operational judgments. The extracted airflow is injected via structure36into the combustion turbine assembly11preferably upstream of the at least one main combustor16with a combustion turbine maximum power augmentation of approximately up to 20-25%. The remaining airflow in the expander30is expanded though low pressure stages to atmospheric pressure. Thus, when injection is possible or desired, not all airflow from the expander30is exhausted to atmospheric pressure.

FIG. 2shows a synchronous reserve power mode of operation of the system10ofFIG. 1. In this mode, the at least one combustor27is operable and the combustion turbine assembly11is not operable. Furthermore, since the combustion turbine assembly11is not operable, the heat exchanger24is not receiving exhaust heat in this mode of operation. Thus, compressed air is withdrawn from the storage18and is preheated by the at least one auxiliary combustor27, for burning fuel, that feeds the expander30. The heated air is expanded though the expander30that is connected to the electric generator32for substantially immediate start-up for synchronous reserve power operation, independent of the combustion turbine assembly11operation.

With reference toFIG. 3, a CAES system which is shown, generally indicated as10′, in accordance with another embodiment. The system10′ includes the same components as inFIG. 1. In a main power production mode of operation of the system10′, preferably during peak hours, and with the auxiliary combustor27inoperable, compressed air is withdrawn from the storage18and directed to the heat exchanger24that receives heat from the source of heat (e.g., exhaust of turbine14). The heated air is expanded through the expander30that is connected to the electric generator32and produces the electric power generated by the compressed air in addition to combustion turbine assembly power. Since the expander30reduces the pressure of the compressed air, the temperature of the compressed air is reduced. Thus, cold (lower than ambient temperature) air of the expander30exhaust is mixed, via structure36, with the ambient air at inlet13, reducing the overall temperature of the inlet air prior to being received by the main compressor12. In the embodiment, the structure36is piping connected between an exhaust of the expander30and the inlet13to the main compressor12. The airflow of expander30is a subject for optimization and driven by the required compressed air generated power. It can be appreciated that instead of all exhaust of the expander30being mixed with ambient inlet air, only a portion of the exhaust of the expander30can be mixed with the ambient inlet air, by connection the piping36to a stage of the expander30, with the remainder being exhausted to atmosphere.

FIG. 4shows a synchronous reserve power mode of operation of the system10′ ofFIG. 3. In this mode, the at least one auxiliary combustor27is operable and the combustion turbine assembly11is not operable. Furthermore, since the combustion turbine assembly11is not operable, the heat exchanger24is not receiving exhaust heat in this mode of operation. Thus, compressed air is withdrawn from the storage18and is preheated by the at least one auxiliary combustor27that feeds the expander30. The heated air is expanded though the expander30that is connected to the electric generator32for immediate start-up for synchronous reserve power requirements independent of the combustion turbine assembly11operation.

With reference toFIG. 5, a CAES system which is shown, generally indicated as100, in accordance with another embodiment of the present invention. The system100is similar to the systems ofFIGS. 1 and 3, but, for distributed power generation applications, the combustion turbine assembly11is replaced with a diesel generator102or any other power producing structure that provides a heat source, or any source of heat. The exhaust25of the power producing structure102, or any heat source is received by the heat exchanger24. Thus, in a main power producing mode of operation, the stored compressed air withdrawn from the storage18, is preheated in the heat exchanger24by utilizing the diesel generator102exhaust gas heat (or the heat from another power producing heat source or heat from any heat source) and is then directed into the expander30that generates the compressed air power in addition to power provided by the diesel generator102. In this mode, the combustor27is not operable.

The system100′ can also operate in a synchronous reserve power mode of operation when the diesel generator102or other power producing structure is not operable and with the at least one combustor27operable, in a manner similar to that discussed above with regard toFIGS. 2 and 4.

The air storage18can be a below ground storage in various geological formations or above ground storage in pressure vessels/piping that are significantly more expansive than underground storages. Since one of the prime functions of a CAES plant is associated with load management of wind power plants, in accordance with an embodiment ofFIG. 6, the air storage18′ can be the supporting tower104of a wind power plant106. Wind power plants are typically installed on the top of the supporting concrete towers104with significant diameter e.g., 10-20 feet and wall thickness 2-3 feet to support the weight and stresses of the wind power plant and to provide an internal chamber108for maintenance and support operations. To store compressed air, the chamber108shall be slightly modified to provide appropriate seals110especially at a top thereof. Thus, the supporting towers104can be utilized for the compressed air storage replacing typical underground storage or the storage of the compressed air in the above ground pressure vessels/piping. Preferably during the off-peak power hours without usage requirements, the wind energy, in the form of the compressed air, will be sent to inlet112/114of the supporting tower104and be stored inside the tower104. If maintenance is required, the compressed air can be removed from the tower104. During peak power hours, the stored compressed air can be directed from exit112/114of the supporting tower, be preheated and sent to expanders for generation of the more needed and higher price energy for example, in the manner discussed above with regard toFIGS. 1 and 3.