ELK ASIA PACIFIC JOURNAL OF MECHANICAL ENGINEERING RESEARCH. ISSN (Online); EAPJMER/issn /2016; Volume 4 Issue 1 (2018)

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1 A SURVEY ON THERMO MECHANICAL ENERGY STORAGE USING BASIC CONCEPTS OF THERMO MECHANICS Shreyas Najrekar Department of Mechanical Engineering. Shri Ram Murti Smarak College of Engineering & Technology Bareilly, India Abstract The energy storage systems and its technologies are diverse and is providing different storage services from past many years. These systems contribute to the stability and reliability and is considered as an expensive solution for balancing the production. The application of the thermal energy storage technology is used in medium and high temperature. The life expectancies, low costs, vast storage have made the thermal mechanical energy storage system as a challenging concept. This paper provides an overview of the basic concepts of the thermo mechanical energy storage and explains the implementation process in brief. The usage of the heat in charging and discharging process and the integration of the storage modules into power plants is described. Keywords Thermo-mechanical energy storage, temperature. 1. INTRODUCTION During the development of the electricity networks there was more scope for electricity storage due to the limited flexibility of the thermal power plants. This has caused the inefficient partial load which is reduced by the availability of the storage option in thermal cycles. The efficient baseload power load plants are used and the expensive ones is limited. The increased use of the electricity which is produced from the wind or solar irradiation has changed its basis from different application. The effective integration of the energy is provided by the grid scale electricity storage by the renewable energy sources [1]. The storage of energy has vast market roles which helps in the power transmission and distribution. There storage technologies will help in converting the excess electrical energy from the power plants to electromagnetic, thermal or mechanical forms etc. The capacity to determine the appropriate market for storage with the charge and discharge time is enabled [2]. The system present in the storage of the grid scale should attain power of MW and the discharging power should be in the range of 4 12h. The electricity storage systems used in day to day life is Pumped 1

2 hydro energy storage which has the highest capacity [3]. This has the properties of strong off-peak electricity by moving the water from a lower to an upper reservoir. Hydroelectric turbine is used during the discharging of water which is released from an upper reservoir to a lower reservoir. Further it helps in converting potential energy into kinetic which generates electricity. The elevation difference caused between the two reservoirs is termed as stored potential energy and has an ideal capacity of 1 kwh/m 3 with a height difference of 360 m. The survey conducted shows that the total global cumulative generating capacity of pumped hydro energy storage is 127 MW which represents more than 99% of the total bulk storage capacity for electrical energy. The maximum size of single facility of the storage is in the range of 4000 MW and the life expectancy is years. The Pumped hydro energy storage costs are based solely on the location and the cost estimation for a 1000 MW unit ranges from 2000 $/kw to 4000 $/kw [4]. The disadvantages of the Pumped hydro energy storage are environmental impact and geographic dependence. Large scale thermal storage systems are a vital option for concentrating solar thermal power plants in Spain [5]. The numerous solar thermal energy power plants have been built. The commercial storage systems for the CSP applications uses the molten salt as the material for storage [6]. The thermal energy storage systems excellence is used in the large-scale storage of the electricity. The characteristics of these systems are having life term in the range of years, the effect of the environment is low and has a capacity specific cost of 15 $kw [7]. These are based on the transformations which happens between mechanical and thermal energy. The function of thermal energy storage combined with the mechanical energy storage occurs. Later the standard components such as heat exchangers, compressors are combined with the storage components of the thermo mechanical storage systems. The total thermo mechanical energy storage system depends on power and capacity. There are three major basic principles for thermo-mechanical energy storage which are described as follows, Compressed air energy storage (CAES) -This type is having a volume which is charged with pressurized air. Later this pressurized air is used during discharging in the expander process and this process requires the addition of heat. 2

3 Power to Heat to Power (PHP) In this the electrical energy is used as the storage unit. The phase of discharging is operated by a thermal cycle where the heat is delivered by the thermal storage energy unit. Pumped Thermal energy storage (PTES) This uses electrical energy to power a heat pump during the process of charging and creates the temperature difference between the two heat reservoirs. These systems have the advantage of storing heat at temperatures above the ambient temperature or even storing the cold energy below the ambient temperatures and few of the systems can use both kinds of storage [8] Figure 1. Overview of the concepts used in the thermo mechanical energy storage There are numerous concepts used in thermo mechanical storage where not only electric energy is used for discharging but also thermal energy might be used for heating. 2. RELATED WORK 2.1 Compressed air energy storage (CAES) This technology concept was designed for storing the compressed air in underground caverns. The concept is that the air is compressed, stored and is released through the conventional gas turbines. These are based on a two stage of Barton cycle [Prateek]. The off-peak electricity is used as a tool to compress the air and later is stored in a volume. The compressed air which is present is not passed directly into the combustion chamber. This plant was particularly designed to decouple the compression and expansion process of a gas turbine. Currently there are only two CAES plant which exists in the world. The author always looks at the financial benefits to the storage of the hydro pumped and compressed air energy storage underground [9] Hybrid CAES In this the air is compressed in an intercooled compression train which composed of an axial low-pressure compressor and also a centrifugal highpressure compressor with a power of 68MW. The air is cooled before storing. The discharging phase occurs and the air from the caverns flows with a reduced constant pressure of 42 bar into a gas turbine having the combustion chamber [10] Adiabatic CAES This concept helps in eliminating the need for fossil fuel by using the heat released during the compression to reheat the cavern 3

4 air prior to the expansion process. This technology requires a thermal energy system that helps in storing the heat during the charging. The storage efficiency is 100% in an ideal adiabatic CAES. The analysis of an adiabatic CAES system is with the packed thermal energy storage is explained [11]. The advanced adiabatic compressed air energy storage was also developed between 2003 and 2007 [12]. The maximum temperature in the plant can be reduced by increasing the number of stages [13] Isothermal CAES In this concept the air is compressed and expanded at a constant temperature. During compression the cooling of the gas takes place and heat must be transferred to the gas during the expansion process. The 100% efficiency is achieved in an ideal isothermal CAES system and does not require either the thermal energy storage or the fossil fuel. These are operated close to the ambient temperature. By reducing the speed of the processes, the isothermal compression and expansion can be realized. 2.2 Power to Heat to Power (PHP) In this concept the thermo mechanical storage system is charging a storage unit by the joule heating and the discharging process has the storage unit which provides the heat to power a thermal cycle. These kinds of systems can be incorporated using the off-the-shelf components from present CSP applications [14]. The PHP requires a system which is able to provide the heat in the temperature range demanded by the thermal cycle. The process of charging has the molten salt which is pumped out of the cold storage tank to the electrical heating unit and the temperature is increased from 290 degrees to 560 degrees. Figure. 2 Basic concept of pumped thermal energy storage. 2.3 Pumped thermal energy storage (PTES) This is the third basic concept of the thermo mechanical storage system where the excess electricity is used to create the temperature difference between two heat reservoirs. The thermal cycle is operated during the discharging phase. This is also denoted as the pumped heat energy storage [15]. This is termed as pumped thermal energy storage (PTES) to avoid the confusion which is created with the pumped hydro energy storage [16]. The PTES is explained by an early patent in 1922 [17] but till yet it has not been demonstrated. The concept of pumped thermal energy storage is used in recent years for the successful implementation of the thermal energy storage technology. The roundtrip efficiency of the ideal pumped thermal 4

5 energy storage is not dependent on the ratio between the minimum capacity of the hot thermal energy storage and work delivered during the discharging. The minimization of the irreversibility is required for the successful implementation of a pumped thermal energy storage system. The requirement of high efficiency, temperature differences in storage units and heat exchangers is observed in the devices used for the compression and expansion of the working fluid. The efficient implementations of the Pumped thermal energy storage concept are observed in four groups of concepts and can be described PTES based on the Brayton cycle The heat is transferred to the cold gas in the reservoir with low temperature and after the compression the gas transfers the energy to the high temperature storage. The process is that the gas is the charging cycle is closed by expanding the gas and the discharging process is similar to the closed baryton cycle. At first the gas is heated in the high temperature and the turbine is expanded and the transfer o the energy takes place in the cold reservoir. The discharging cycle is closed by the compressing the gas. The PTES system is developed using a reciprocating engine [18] PTES based on CO2 cycle. In this approach the water is used as a storage medium and the salt water ice slurry is used for the cold storage which has a minimum temperature of -21 degrees. This also requires thermal cycle which can be operated with low temperature heat using water as a storage medium PTES based on water steam cycle (CHEST) The CHEST (compressed heat energy storage) concept is based on the conventional steam cycle technology [19]. In this system there is no cold storage system but the environment is used as cold reservoir. The saturated steam is produced while charging from low pressure water by heat form the environment. Later it is compressed using the electrical energy. The thermal energy which is provided by the energy storage system is used to operate in the medium temperature Rankine cycle. [20]. In charging phase, the major phase of the heat is transferred from the condensing steam to the storage system. The discharging phase has evaporation process which requires thermal energy provided by the storage. The discharging process heat is used by the storage system in Rankine cycle with pressure of 80 bar and a two-stage expansion. The main characteristics of the CHEST plant is combined with the results 5

6 of the PTES systems based on a Brayton cycle or CO2 cycle PTES based on the cryogen stage The temperature difference which is available for the operation of thermal cycle is created by removing the heat from a cold reservoir below ambient temperature. In discharging process, the waste heat resources are used are used as the hot reservoirs to operate the turbine. The application of the cryogens such as liquid air is used as working fluids for the successful implementation. The enormous electricity is sued for the production of cryogens in charging process. The cryogens are transformed into a high-pressure gas by absorbing the heat and t8he gas drive the cryogenic turbine generating electricity. 3. CONCLUSION The enormous number of concepts for the thermo mechanical energy bulk storage is explained. The recent progress which is made in the large scale of thermal energy storage systems for different temperatures is achieved. The future of the thermal energy storage systems lies in the lifetime range of years and the advantage of low capacity specific costs makes these systems a promising candidate solution. The concepts used vary in terms of cost, efficiency, development potential, processing method. The weighting of the evaluation criteria is used for choosing the optimal concept. The CAEs plants helps in the improvements of the application of the gas turbine. Different types of CAES plants are briefed and the process is explained. The advanced concept such as pumped thermal energy storage is reviewed along with the different types of implementations methods. The application of the conventional components, thermal mechanical systems are considered in the thermal energy storage technology as the vital option for the future improvement of the bulk energy storage. Furthermore, the specific geological conditions and the impact of environment is not essential for thermo mechanical energy storage. REFERENCES 1. Kempener, R., & de Vivero, G. (2015). Renewables and electricity storage: A technology roadmap for REmap International Renewable Energy Agency. 2. Johnson, P. M. (2014). Assessment of compressed air energy storage system (CAES). 3. N.N. Technology Roadmap Energy Storage IEA International Energy Agency; N.N. Challenges and Opportunities for new pumped storage development. White paper, 6

7 National HydroPower Association; Cabeza, L. F., Sole, C., Castell, A., Oro, E., & Gil, A. (2012). Review of solar thermal storage techniques and associated heat transfer technologies. Proceedings of the IEEE, 100(2), Gil A, et al. State of the art on high temperature thermal energy storage for power generation Part 1- Concepts. Mater modelization Renew Sustain Energy Rev 2010; 14: Stekli, J. (2011). Thermal Energy Storage and the United States Department of Energy s SunShot Initiative. SolarPACES. 8. Steinmann, W. D. (2017). Thermomechanical concepts for bulk energy storage. Renewable and Sustainable Energy Reviews, 75, Chiu, H. H., Rodgers, L. W., Saleem, Z. A., Ahluwalia, R. K., Kartsounes, G. T., & Ahrens, F. W. (1979). Mechanical energy storage systems: compressed air and underground pumped hydro. Journal of Energy, 3(3), Succar, S., & Williams, R. H. (2008). Compressed air energy storage: theory, resources, and applications for wind power. Princeton environmental institute report, Barbour, E., Mignard, D., Ding, Y., & Li, Y. (2015). Adiabatic compressed air energy storage with packed bed thermal energy storage. Applied energy, 155, Jakiel, C., Zunft, S., & Nowi, A. (2007). Adiabatic compressed air energy storage plants for efficient peak load power supply from wind energy: the European project AA- CAES. International Journal of Energy Technology and Policy, 5(3), Grazzini, G., & Milazzo, A. (2012). A thermodynamic analysis of multistage adiabatic CAES. Proceedings of the IEEE, 100(2), Ruer, J. (2013). U.S. Patent No. 8,443,605. Washington, DC: U.S. Patent and Trademark Office. 15. Ma, Z., Glatzmaier, G. C., & Kutscher, C. F. (2011, January). Thermal energy storage and its potential applications in solar thermal power plants and electricity storage. In ASME th International Conference on Energy 7

8 Sustainability (pp ). American Society of Mechanical Engineers. 16. Marguerre, F. (1922). Verfahren zur Aufspeicherung von Energie. Patentschrift Nr, 388(122). 17. Howes, J. (2012). Concept and development of a pumped heat electricity storage device. Proceedings of the IEEE, 100(2), Steinmann, W. D. (2014). The CHEST (Compressed Heat Energy STorage) concept for facility scale LIST OF FIGURES thermo mechanical energy storage. Energy, 69, Marguerre, F. (1933). Das thermodynamische speicherverfahren von marguerre. Escher-Wyss Mitteilungen, 6(3), 67e Bauer, T., Pfleger, N., Breidenbach, N., Eck, M., Laing, D., & Kaesche, S. (2013). Material aspects of Solar Salt for sensible heat storage. Applied energy, 111, Figure- 1 Hybrid Adiabatic Compressed air energy storage Isothermal Thermo mechanical energy storage Power to heat to power Output electricity PCHP Pumped thermal energy storage Brayton cycle CO2 cycle CHEST: RANKINE 8

9 Figure. 2 Basic concept of pumped thermal energy storage. 9