Thermodynamic analysis of a compressed air energy storage system with constant volume storage considering different operating conditions for reservoir walls

Abstract

In the search for alternatives to fossil fuels, renewable energy sources such as wind and solar are gaining increasing importance in the global energy grid. Despite its reserves are abundant, this type of energy is intermittent, which causes some unpredictability in meeting energy demand through these resources. To mitigate this problem, energy storage systems can be used combined with renewable sources. Compressed air energy storage (CAES) systems stand out for their high efficiency and affinity with the environment. In the present article a thermodynamic analysis of an operating cycle of a small scale CAES system with constant volume reservoir is conducted, taking into account three different operating conditions for compressed air storage walls. The conducted analysis of the CAES system considered includes an energetic and an exergetic investigation, aiming to determine and assess which are the most relevant points of exergy destruction in the system, as well as the effects of air storage conditions on the thermodynamic performance of the global system. Among the main results it was observed that the combustion chamber presented the highest rates of exergy destruction, and that the throttle valve installed on the exit of the air storage reservoir was responsible for an average of 9% of the total exergy destroyed in the CAES system over a complete work cycle.

Introduction

The increasing and intensive use of fossil fuels has been contributing directly to the increase of pollutant emissions as carbon dioxide and nitrogen oxides and consequently for the phenomenon of global warming. In addition, there is an increase in world energy demand due to population and economic growth, which requires a search for alternative energy sources, since conventional ones are finite and environmentally harmful. Due to these and other factors, power generation from renewable sources has received great incentives, enabling the development of new technologies that increasingly boosts the participation of these sources in the world energy scenario [1,2].

The production of electric energy from renewable sources has increased considerably in the last decade, especially that produced from wind and solar energy, and is expected that this trend will be further accentuated in the near future [3]. Fig. 1 shows the increase in the generation of electric energy from wind and solar sources, illustrated by the sharp growth in the global installed electricity generation capacity of these sources in recent years, in Megawatts. According to the data presented by the International Renewable Energy Agency (IRENA) [4], between 2010 and 2018 the installed capacities for power generation through wind and solar resources have increased 210% and 1090% respectively.

A major disadvantage associated to electric power generation from renewable energy sources such as wind or solar corresponds to the unpredictability and inconsistency of energy production through these sources, what can cause a large mismatch between supply and demand [5]. In this context, the application of Energy Storage Systems (ESS) combined with power generation through renewable sources can play a key role to ensure continuous and stable electric energy production, in order to balance energy supply and demand [6]. Among the different energy storage methods, Compressed Air Energy Storage (CAES) is one of the most promising, being intensively studied nowadays due to its high efficiency and environmental affinity [7].

A CAES power plant is basically a modification of a conventional gas turbine power plant. In CAES systems, compression and expansion trains are decoupled, and the compressors can be driven using low cost energy from the grid, produced during off-peak periods, or from renewable sources such as wind or solar. A work cycle of a CAES system includes a charging and a discharging process of a compressed air storage reservoir. During the charging process, air from the compression train is stored at high pressure in a cavern or in an artificially constructed reservoir. The discharging process normally occurs during peak hours. In this process the high-pressure air leaves the reservoir and is normally mixed with a small amount of natural gas, which is then burned in a combustion chamber in order to increase the air temperature. Flue gas leaves the combustion chamber at a high temperature and is expanded as it passes through the turbines, thus generating energy [8,9]. So, in a CAES system, the charging and discharging processes are separate and power from turbine no longer needs to be used to drive the compression train [10]. When a CAES system is installed as an autonomous plant in an electrical network, an electrical engine is used to drive the compressor train, while an electric generator is driven by the turbine to deliver power. Very small amounts of energy are lost in the conversions between different forms of energy that occur in the electrical engine and generator. Conversion efficiencies greater than 98% are very common for larger electrical machines [11].

Regarding the type of reservoir used, CAES systems can basically operate using an isochoric reservoir, in which the volume of the reservoir remains constant while the pressure of the stored air varies with time, or an isobaric reservoir, in which air is stored at a constant pressure while the volume of the reservoir varies with time [12]. Normally the reservoir used has a constant volume, as seen in the two CAES plants in operation in the world. The first one is located in Huntorf, in northern Germany, and the other one is located in McIntosh, Alabama, United States. In these two plants the compressed air is stored at high pressure in large underground caves [1,12]. Usually, in projects involving CAES plants with an isochoric reservoir a throttle valve is installed in the exit of the reservoir in order to reduce the pressure of the air leaving the reservoir. This is beneficial for the operation of the turbine as it maintains a constant pressure at the turbine inlet throughout the discharging process of the system. In CAES systems with isochoric storage the minimum operation pressure of the air storage reservoir generally corresponds to the value of the turbine inlet pressure and functions as the operating limit of the system during the discharging process, representing the moment when the compressed air storage is considered empty and a new work cycle of the CAES system can be started [6,13]. Large-scale CAES systems generally depend on the proper choice of a location capable of storing a large amount of compressed air. Small-scale CAES systems are a more flexible option, especially for cases involving distributed generation. These small-scale systems can normally deliver power ranging from a few kilowatts to a few megawatts [14,15].

Several studies involving compressed air energy storage have been developed recently. Venkataramani et al. [16] presented a detailed review on various aspects associated to CAES systems, citing articles related to modeling and simulation analysis, thermodynamic analysis, experimental research and economic evaluation of such systems. Among the conclusions drawn by the authors it is highlighted the greater attention that has been directed by the scientific community to small-scale CAES systems and the perception of the technological potential of integrating CAES systems to Combined Cooling, Heating and Power (CCHP) systems in the near future, aiming to increase the global efficiency of systems that generate energy sustainably from renewable sources. Sciacovelli et al. [17] presented a model that simulates the operation of an Adiabatic CAES (A-CAES) system and evaluated the influence of the performance of the Thermal Energy Storage (TES) system, integrated to the CAES system, on the performance of the overall system. The authors observed that the A-CAES system achieves efficiencies between 60 and 70% when the TES system operates with a storage efficiency greater than 90%. Wang et al. [18] conducted a comparative study aiming to evaluate if the compressed air of an Advanced Adiabatic Compressed Air Energy Storage (AA-CAES) system is better stored underground (at constant volume) or underwater (at constant pressure). The results obtained showed that the underwater CAES presented better performance in terms of round-trip efficiency and exergy density than the underground CAES under the same conditions. Zhang et al. [19] conducted a thermodynamic analysis of the throttle valve and the cavern used as the air storage reservoir of a large-scale CAES system with isochoric reservoir. The authors observed that approximately 18.85% of the total exergy destruction over a complete work cycle of the CAES system is due to these two components, with the throttle valve and the cavern responsible for approximately 5.14% and 13.71% of the total exergy destruction respectively. Minutillo, Lavadera and Jannelli [20] proposed a combination of a small-scale Adiabatic CAES unit with a photovoltaic power system as an interesting option to satisfy the energy demand of a stand-alone radio base station for mobile telecommunications. The adiabatic condition of the CAES system is guaranteed by a TES system used to recover the heat generated in the compression process. The authors performed a sensitivity analysis in order to assess the optimal plant operating parameters such as the average operating pressure range. Szablowski et al. [21] developed a mathematical model of an A-CAES system with an isochoric reservoir with a volume equal to 3.1 × 10⁵ m³. The authors conducted energy and exergetic analyzes of this system. The main exergy destruction points of the system were evaluated, and it was verified that the low-pressure compressor is responsible for the largest share of exergy destruction per work cycle (151.429 MWh/cycle), followed by the high-pressure compressor (124.762 MWh/cycle) and the throttle valve responsible for controlling the compressed air pressure at the outlet of the air storage (103.688 MWh/cycle). The round-trip efficiency calculated for the system was equal to 50%. Mohamad et al. [22] proposed a configuration of a trigenerative compressed air energy storage system, giving priority to electrical energy production and applying it at a micro-scale, in the order of a few kW. A complete thermodynamic model of the system was developed, including the existing technological aspects and the relations between components. A poor performance was observed for the system, what was attributed to the exergy losses in the throttling valve and the low values of the component efficiencies at a micro-scale. Alami et al. [23] built and tested a small-scale, low-pressure CAES system. The maximum storage pressure of the system is equal to 5 bar, and the low-pressure compressed air is stored in small steel cylinders with a volume of seven liters. The system achieves a maximum overall efficiency of 97.6% and a maximum mechanical efficiency of 95.6%. The authors also concluded that the developed system operates safely and requires a low financial investment compared to other energy storage systems. Parkes et al. [24] describe a novel technique used to derive potential storage cavern locations and to estimate physical volumes that might be available for storage purposes for CAES systems. The technique uses Esri's ArcGIS Geographic Information System software and involves defining the spatial distribution, thickness and insoluble content of the halite beds, together with an estimate of the potential physical volumes of solution-mined caverns. Krawczyk et al. [25] performed a comparative thermodynamic analysis between a CAES system and a Liquid Air Energy Storage (LAES) system. A clear advantage of LAES systems compared to CAES systems is related to the considerably smaller energy storage volumes required by LAES technology. Due to the use of a regenerator to preheat the air before the combustion chamber of the LAES system, the round-trip efficiency of the LAES system was approximately 15% higher than that of the CAES system. The authors also registered that without the use of the regenerator in the LAES system, both systems would present roughly the same round-trip efficiency. The study also showed that the components responsible for the highest rates of exergy destruction in both systems are the combustion chambers and heat exchangers. Mohammadi and Mehrpooya [26] proposed a system that integrates a gas microturbine, a compressed air energy storage and a solar dish collector. The authors carried out energetic and exergetic analyzes of this system. It was found that the highest rates of exergy destruction in the system occur in the solar dish collector and in the combustion chamber, and the authors stated that the combustion chamber has a dominant effect on the total exergy destruction of the plant. It is also analyzed how the solar energy absorbed in the solar dish collector influences the fuel consumption in the combustion chamber for different times of the day, considering the variation of the solar incidence throughout a day. The round-trip efficiency and the exergetic efficiency of the system were calculated to be respectively equal to 76.47% and 53.36%.

Considering the importance of optimizing energy consumption in current days, the present study aims to conduct a thermodynamic analysis of a CAES system and thereby contribute to the state-of-the-art in this area of knowledge. The compressed air storage reservoir has a constant volume, and three heat transfer conditions will be considered for the walls of this reservoir: isothermal walls, adiabatic walls and walls that exchange heat by convection with stored air. For each of these scenarios, energetic and exergetic analyzes of each component of the system will be conducted considering a complete work cycle of the CAES system. In this study an adaptation of the CAES system to operate as an Adiabatic CAES was not considered, since this will be the focus of analysis of a subsequent study. In this future study a packed bed TES system will be combined to the CAES system here considered, in order to store and reuse the thermal energy generated during compression stage.