Energy Storage: Compressed air Energy
Storage (CAES)
Mihaela, Popescu
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Study-Program: Industrial Engineering
Module: Renewable Energy Systems
Summer Semester 2021
(Exam)
Lecturer: Prof. Dr. Raimund Brotsack
Deadline of submission of the report: latest: 5th of July 2021
Table of Content
1)
Introduction ............................................................................................................................................... 3
2)
Theoretical basics ...................................................................................................................................... 4
3)
Technical description ................................................................................................................................. 5
4)
Typical application ..................................................................................................................................... 7
5)
Contribution to climate protection ........................................................................................................... 7
6)
Current market description worldwide and in your home country .......................................................... 7
7)
Summary and outlook for the near future ................................................................................................ 9
8)
Literature ................................................................................................................................................. 10
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1) Introduction
Energy is a pillar of our civilization. Our today´s society is centered on a fossil-based economy providing the
services that we use every day, as transportation, food production, entertainment, and health care. Therefore,
energy steadiness and constant power are needed for advancement. Nevertheless, the threat from climate
changes from the emissions suggests that we need to make a change. [1]
Renewable sources are known by their variation in time and availability, renewable sources are the essentials
for the energy technology of the future and this change is already started. But the increase of wind and other
renewable energies sources is creating a challenge for the electricity industry. Commons forms of energy storage include battery storage, pump hydroelectric energy storage (PHES), flywheels, ultracapacitors, superconducting magnetic energy storage, and compressed air energy storage (CAES). Batteries are effective but expensive and lack scale. Pump hydro storage is effective and scalable, but they are expensive and limited to
locations suitable for such plants. Compressed air energy storage using compressed air as the storage medium
is a type of energy storage that I choose to explore. Not only because it is a low cost of maintenance and
operation and has high-power efficiency, but also because has a long service period, low cost of energy, and
was also proven as an economical solution for utility-scale energy storage on the hour’s timescale. Another
study that took into consideration the technical parameters (energy and power density, energy capacity,
power rating, discharge, and round-trip efficiency) for different CAES scales and determined that CAES is to be
soon commercialized thanks to the improvement of a new scroll expander [4].
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2) Theoretical basics
The industrial alternatives for storing and managing large volumes of energy (with a power greater than 100
MW) are Pumped Hydro Storage (PHS) and Compressed air energy storage (CAES), which analysing the Fig.1
can be observed [2].
Figure 1. Energy storage system application’s range [2]
CAES is a technology that can be valuable, thanks to its low geographical limitations, and that allows great
flexibility. This type of energy storage has been implemented already in Hantorf, Germany, in 1978 and
McIntosh in Alabama, in 19991 [3]. In a standard CAES system, the energy of a renewable energy source is
employed to compress the air in a storage cavern. This pressurized air is spread from the cavern across an
expander to generate the required power when it is needed. In both present large-scale CAES systems, natural
gas is utilized as the main heat supplier to pre-heat the gas prior to the power generation process in the expander. The compressed air is fed into a reservoir, which for rational purposes often is old salt air. There, the
compressed air gets heated by isentropic compression and is cooled while inserting the high-pressure cavern.
The energy is regenerated by discharging the compressed air in an adiabatic turbine. In this process the air is
cooled is entropically, to prevent impracticalities, the air is first reheated. Therefore, the air goes through four
processes I types in this cycle: compression, cooling, heating, and expansion.
Based on the type of heat use and air storage, there are three categories of CAES: 1) Diabatic CAES, 2) Adiabatic CAES and 3) Isothermal CAES. Usual (Diabatic) CAES uses turbo-compressors to pressurize the air to approximately seventy bars ahead of storage. With no effective intercooling, the air temperature increases to
around 900 K, guiding to the loss of compression heat and forcing the processing and storage of the gas difficult except if the air is cooled down. To increase the outpower, fuel is injected into the expanding air and
ignited into a turbine. In Adiabatic CAES a proposed solution for the two existing plants in Germany and the
USA, because suffer from low efficiency, loss of heat of compression, and environmental pollution since are
using fossil fuels. The Adiabatic CAES has been showed to have a Thermal Energy Reservoir (TES) to absorb
and store the heat of compression during charging processes. This collected thermal energy is utilized to preheat the air before its expansion process. Therefore, fossil fuels are no longer needed [5].
The third type, the Isothermal CAES, tries to support an isothermal process inside the compression and expansion chamber, yielding high round-trip efficiency and lower capital cost, thanks to minimizing the compression
heat [6]. The following characteristics turn isothermal CAES into the most appropriate CAES technology for
small-scale purposes and particularly for future circulated generation systems, nevertheless it is difficult since
it requires uninterrupted heat ejection from the air throughout expansion, to move toward an isothermal
process. The heat transfer rate is proportional to the result of temperature gradient and the contact area,
hence, to transfer heat at a high rate with a small temperature variation, an incredibly significant heat transfer
area is needed, following in costly heat exchangers.
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3) Technical description
As illustrated in Fig. 3.1 In 1-2 the air is compressed adiabatically. Successively, after compression, the gas is
cooled 2-3 whilst staying deposited in the cavern, this process is isobaric and the heat change at constant
pressure is given by the difference in enthalpy. The compressed air is now at rest. Before producing electric
energy from the compressed air, it is reheated, in the heat exchanger 3-4. The last step in the cycle is expanding
the high-pressure high-temperature gas 4-5, which is like the step 1-2, reversible and adiabatic (then isentropic). Depending on how high the temperature is heated after compression, the final temperature at atmospheric pressure is higher or lower than the ambient temperature.
Figure 2 Process scheme for the compressed air energy storage system
Figure 3 Main components of a CAES
The efficiency of a CAES system can be described by two components: energy ratio and heat rate. This energy
ratio, ER is defined as:
𝐸𝑔𝑒𝑛
𝐸𝑅 = 𝐸
(3.1)
𝑐𝑜𝑚𝑝
Where:
𝐸 𝑔𝑒𝑛 is energy out from generation
𝐸𝑐𝑜𝑚𝑝 is the energy consumed by the
compressor.
The heat rate, HR, is the amount of gas consumed per unit of energy generated. Therefore, the turnaround
efficiency of a gas system is:
1
𝜂 = 𝐸𝑅+𝐻𝑅
(3.2)
Turn around efficiency is a measure of the total efficiency of the system which produces power and consumes
gas and off-peak power [1].
From Fig. 3.1 two components were used, compression/ expansion machines where mechanical work is the
main input vector as well as storage vessels, where the mechanical work is equal to zero. Which are represented in Fig. 3.2.
Both systems can be regarded as separate control volumes and respond to the rate of the first law of thermodynamics:
ⅆ𝑈
𝑊̇𝑖 = 𝑄̇𝑒 + 𝑚̇𝑒 ∙ ℎ𝑒 − 𝑚̇𝑖 ⋅ ℎ𝑖 +
ⅆ𝑡
(3.3)
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Where
𝑊̇𝑖 and 𝑄̇𝑒 are the work and heat flows transferred to the gas from the external
𝑚̇𝑖 and 𝑚̇𝑒 are the input and exit mass flows
ℎ𝑖 and ℎ𝑒 are the corresponding specific enthalpies [J/kg]
The description of a compression machine is based on the hypothesis of the air being thought of as an ideal
gas ( 𝑃𝑉 = 𝑚𝑅𝑇) and, further, on the essential relations for work and the heat:
𝑊 = ∫ 𝑝𝑑𝑉
(3.4)
𝑄̇ = 𝐻𝑐 𝐴𝑐 (𝑇 − 𝑇𝑤 )
(3.5)
Where
𝐻𝑐 is the heat transfer coefficient
𝐴𝑐 is the cylinder surface area exposed to convection
𝑇𝑤 is the temperature of the surface area
𝑇 is the instantaneous gas temperature
The energy flow defined as
Ψ = (h − ℎ0 ) − 𝑇0 ( 𝑠 − 𝑠0 )
(3.6)
Where
ℎ is the specific enthalpy
𝑠 is the entropy
And 0 indicates that are at reference temperature and pressure ( 𝑇0 = 20℃, 𝑝0 = 1 𝑏𝑎𝑟)
The exergy flow (usable energy) of the produced airstream is
𝑋̇ = 𝑚[̇ℎ − ℎ0 − 𝑇0 (𝑠 − 𝑠0 )]
(3.7)
The airstream exergy can be split into two parts, as pneumatic and thermal:
𝑋̇ = 𝑋̇(𝑝𝑛) + 𝑋̇(𝑡ℎ)
(3.8)
Where
𝑝
𝑋̇(𝑝𝑛) = 𝑚̇𝑅𝑇 ln 𝑝
(3.9)
0
𝑇
𝑋̇(𝑡ℎ) = 𝑚̇𝑐𝑝 (𝑇 − 𝑇0 − 𝑇0 ln 𝑇 )
(3.10)
0
The exergy of the compressed air in the reservoir can be achieved as
𝑝=𝑝
𝑝=𝑝
𝑝
0
0
0
𝑇
1
𝑝
𝑝
𝑋2 = ∫𝑝=𝑝 2 𝑋̇(𝑝𝑛) 𝑑𝑝 = ∫𝑝=𝑝 2 𝑚̇𝑅𝑇0 ln 𝑝 ( 𝑇0 ) = 𝑝2 𝑉 (𝑝 + ln(𝑝2 ) − 1) = 𝑚1 𝑅𝑇0 [ln (𝑝2 ) + 𝑝0 − 1]
2
0
2
(3.11)
Isothermal compression and expansion represent the best conditions for storage because of the thermal energy exportation toward the external atmosphere allows [7]
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4) Typical application
The upcoming predictions on CAES potential in backing system operation have become brighter, it is expected
that CAES will play and significant role in the energy management of the time of separation between production and the usage of power. The possible uses of CAES are considered dual: suppliers, the transmission and
distribution network operators, and consumers, the users can use CAES to control the electricity management
taken from the suppliers, established on the energy price to minimize their energy bill.
Peak shaving is the process of collecting energy during off-peak periods and balancing for electrical power
generation shortfalls throughout periods of high demand. As the price tariff periods vary depending on the
peak and off-peak durations, users can apply CAES to regulate demand-side energy by collecting at a lower
price and distributing it during higher electricity prices. The owners of CAES design a target incentive mechanism that can draw support at the project development and estimate the potential revenue generation.
Another application is the continuous integration of renewable generation plants into the existing power network to achieve the goal of stable power grids, which can help to solve the inherent intermittence and bring
stability to renewable power generation. CAES can rapidly suppress the micro-grid power fluctuations and
improve power supply quality, a fact that is essential for a reliable operation of micro-grids. Tang et al., showed
a simulation and experimental platform, including energy storage, photovoltaic, asynchronous wind turbines,
and typical load in micro-grids, with results that prove that effective control of energy storage could help micro-grids keep good voltage and frequency stability during the switching process patterns of off-grid/ grid pattern.
In the power supply failure, CAES systems can be used as backup power or uninterrupted power supply, which
can supply enough power to import users, such as banks, data processing centres, hospitals, and other important sectors. CAES systems could supply the capacity of black-start in a power shutdown condition, The
Huntorf plant offers black-start power to the nuclear units found near the North Sea [8].
5) Contribution to climate protection
The electric power grid works based on a delicate balance between supply and demand. One way to balance
fluctuations in electricity supply and demand is to store electricity during periods of high production and low
demand, then release them during periods of lower production and higher demand. Storing electricity can be
environmentally beneficial, for using the stored energy in the CAES plants to help integrate more renewable
energy into the electricity grid. Furthermore, it also can generate facilities at the best level, and reduce the
use of less efficient generating units that would run only at peak times [9].
Environmental protection results from the peak- shaving and other technical benefits showing lower emission
of greenhouse gases and other emissions, energy market support functions delay energy grid expansions
which save natural resources and reduces the usage of natural resources and emissions from power production.
6) Current market description worldwide and in your home country
The two CAES power plants that are in use right now require fossil fuel, which is not desirable for keeping the
clean energy agenda.
Table 1. Information data about the commercial CAES facilities
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High Power Storage started developing a LAES in the last 10 years, the first 350 KW/2.5 MWh LAES pilot plant
with packed bed cold and thermal storage has proved its value by connection with the local distribution network as shown in Fig. 6.1.
Figure 4 350 KW/MWh LAES pilot at
Birmingham
The demonstration plant has been transferred to Birmingham for further research and educational purposes.
It is going to play a significant role in innovative design, such as thermal storage, with the possibility to overcome the undiscovered drawbacks for the next generation of LAES. Morgan’s method was used to conduct
the theoretical performance and the cycle analysis for the project demonstration.
Fig. 6.2 it is represented the 1.5 MW SC-CAES demonstration project from 2013 which was designed by Industrial Park Development CO. LTD and the Institute of Engineering Thermophysics of the Chinese Academy of
Sciences. Currently, the 1.5 MW SC-CAES system has been running for more than 3000 h successfully and the
system efficiency is around 55%. Along with the experience gained from the demonstration project, in September 2016, the first 10 MW AA- CAES project has been assembled Fig. 6.3 and the system operations have
been running since 2017 [11].
Figure 5 1.5MW SC-CAES pilot plant in China
Figure 6 10MW AA-CAES pilot plant in China
Unfortunately, Moldova does not have the necessary technology for developing such projects. Soon increasingly CAES plants will be developed including in Moldova.
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7) Summary and outlook for the near future
This paper has offered a general outlook to what CAES systems stand for, how it is functioning, projects and
efforts in the last years. This should supply a foundation and useful references for the upcoming research.
CAES has proved its exclusive merits and pluses, but it has also shown weakness in conditions of low efficiency
and lower energy density. The current future technology is concentrating on upgrading its efficiency. A two
stages development was proposed to decrease the systems operational costs involving thermal power unit’s
operation costs. In the close future, it is expected many complicated integrated CAES systems, such as fuel
engines, refrigeration systems, to improve the overall efficiency. The integrated system with the fuel engine
primarily offers hybrid power for vehicles to increase their efficiency, which can also boost the output power
for the cars.
Abbreviations
AA-CAES
Advanced adiabatic compressed air energy storage
LAES
Liquid air energy storage
SC-CAES
Supercritical compressed air energy storage
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8) Literature
[1] Odne Stokke Burheim,” Engineering Energy Storage”, Elsevier Science & Technology, 2017
[2] Varin Vongmanee, “The Renewable Energy Applications for Uninterruptible Power Supply Based on Compressed Air Energy Storage System”, IEE Conference,2009
[3] Karishima Kumari, Kumar Hrishab, Dr. Amit Srivastava, “Renewable Energy Storage”, published in International Journal of Trend in Scientific Research and Development, 2019
[4] X. Luo, J. Wang, M. Door, J. Clark, “Overview of current development in electrical energy storage technologies and the application potential in power system operation”, Appl. Energy, 2015
[5] H. Mozayeni, X. Wang, M. Negnevitsky, "Exergy analysis of a one-stage adiabatic compressed air energy
storage system", International Conference on Energy and Power, 2018
[6] M. Heidari, D. Parra, M. K. Patel, “Physical design, techno-economic analysis and optimization of distributed compressed air energy storage for renewable energy integration “
[7] Alfred Rufer, “Energy Storage: Systems and components”, Taylor & Francis Group, 11-2017
[8] J. Taylor, A. Halnes, “Analysis of Compressed air Energy Storage”, 2017
[9] https://www.epa.gov/energy/electricity-storage 07/2021
[10] https://www.ctc-n.org/technologies/compressed-air-energy-storage-caes 07/2021
[11] J. Wang, K. Lu, Lan Ma, Jidai Wang, M. Dooner, S. Miao, J. Li, D. Wang , “Overview of Compressed Air
Energy Storage and Technology Development “, 2017
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