Research on Storage Capacity of Compressed Air Pumped Hydro Energy Storage Equipment

doi:10.4236/epe.2013.54B005

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Energy and Power Engineering, 2013, 5, 26-30

doi:10.4236/epe.2013.54B005 Published Online July 2013 (http://www.scirp.org/journal/epe)

Research on Storage Capacity of Compressed Air Pumped

Hydro Energy Storage Equipment*

Jingtian Bi, Tong Jiang, Weili Chen, Xian Ma

State Key Laboratory of Alternate Electrical Power, System with Renewable Energy Sources,

North China Electric Power University, Beijing, China

Email: [email protected]

Received January, 2013

ABSTRACT

Compressed air pumped hydro energy storage equipment combines compressed air energy storage technology and

pumped storage technology. The water is pumped to a vessel to compress air for energy storage, and the compressed air

expanses pushing water to drive the hydro turbine for power generation. The novel storage equipment saves natural gas

resources, reduces carbon emission, and improves the controllability and reliability. The principle of compressed air

pumped hydro energy storage is introduced and its mathematical model is built. The storage and generation process of

the novel equipment is analyzed using the model. The calculation formula of the storage power is deduced in theory in

different situations of isothermal and adiabatic compression. The optimal storage scheme is given when the capacity

and withstand pressure of the vessel is definitive, and the max available capacity and the equipment utilization effi-

ciency evaluation of the scheme is given.

Keywords: Power Storage; Compressed Air Energy Storage; Hydraulic Equipment; Optimal Operation; Isothermal

Process; Adiabatic Process; Equipment Utilization Efficiency

1. Introduction

In recent years, as the problems of resources shortage

and environment pollution become serious, the new en-

ergy generation increases rapidly, especially the wind

power[1]. However, compared with the traditional gen-

eration, the wind power generation is characteristic of

intermittence, volatility and randomness. The large scale

integration of wind power will aggravate the unbalance

of the supply and the demand of power grid, and the peak

regulation ability of power grid is a serious obstacle to

the development of the wind power[2]. Energy storage

technology will provide a very effective way to solve

these problems.

In various energy storage technologies, the practical

large scale storage only includes pumped storage and

compressed air energy storage. The pumped station re-

quires high geographical conditions, the long construc-

tion period, the large initial investment and damages to

the ecological environment. Compressed air energy stor-

age (CAES) technology is a very promising energy stor-

age system and attracts great attentions in the world, as it

does not require strict geographical conditions and is

economic[3].

Traditional CAES power station is divided into energy

storage subsystem and power generation subsystem[4].

The energy storage subsystem, comprised of compressor,

motor and gas chambers, converts the low-cost electrical

energy to the compressed air stored in gas chambers such

as caves and abandoned mines. During the peak load

period, the power generation subsystem, comprised of

the gas turbine, combustion chamber as well as the heater,

generates power energy by the gas turbine driven by the

compressed gas combustion[5].

Compare with other energy storage technologies, ad-

vantages of compressed air energy storage system in-

clude[6]:

(1) It is suitable for the construction of large power

plants (>100MW). Its storage capacity is just less than

the pumped storage power station. CAES has long

working time, and it can continue working for a few

hours or even a few days.

(2) The unit construction costs and operating costs of

large-scale CAES stations are lower than the pumped

storage power station. CAES has good economic effi-

ciency.

(3) CAES has a long lifespan, and it can store/release

energy tens of thousands of times. Its efficiency can

reach about 70%, which is close to the pumped storage

power station.

*Supported by the National High Technology Research and Develop-

ment of China 863 Program (2012AA050208).

J. T. BI ET AL. 27

However, every storage technology has shortcomings

[7,8]. The disadvantages of compressed air energy stor-

age system include:

(1) Traditional compressed air energy storage system

must work with gas turbine power station, which con-

sumes a lot of natural gas when generation. The tradi-

tional scheme consumes a large amount of non- renew-

able energy and releases a large amount of CO2, mis-

matching the development requirements of reducing

carbon emission and preventing of global warming[9,10].

(2) Compressed air energy storage systems require

conditions of high temperature and high pressure. It

needs high requirements and high maintenance costs. In

addition, the failure rate of the turbine is high and its

service life is short.

2. Compressed Air Pumped Hydro Energy

Storage

The compressed air storage technology combined hydro

equipment attracts great attentions of technicians. The

essence is that the energy is stored in the compressed air,

and is extracted by the hydro turbines driven by high

pressure. Therefore the equipment can be called com-

pressed air pumped hydro energy storage equipment.

Many delightful progresses in this area appear in recent

years.

An implementation of compressed air pumped hydro

energy storage is put forward in [11]. Its block diagram is

shown in Figure 1. The system includes an air compres-

sor, a reservoir, a tank of gas-water and a pump/turbine-

motor/generator. Before the first storage process, air is

compressed into the tank of gas-water by the compressor,

reaching a preset pressure in the tank. Since then the

compressor would not work anymore. During valley de-

mand periods, water in the reservoir is pumped to the

tank by the water pump. With the increase of the water

level, water squeezes gas in the tank, and the gas is com-

pressed. The result is the increase of the pressure. During

peak demand periods, the high-pressure water promotes

the hydraulic turbine to generate power.

A similar compressed air pumped hydro energy stor-

age system is proposed in [12]. Its block diagram is

shown in Figure 2. It is different from the previous en-

ergy storage system in a tank of high-pressure gas, which

stores only compressed air. The tank of high-pressure gas

is connected with gas compressor and gas turbine gen-

erator. Before storage, compressor compresses air to the

tank of high-pressure gas. The tank of high-pressure gas

supplies preset pressure for the tank of gas-water. During

the peak load, the high-pressure gas in the tank of gas-

water push the water, driving the hydraulic turbine to

generate power. During the valley load, the motor-pump

pumps the water in the reservoir to the tank of gas-water

using the surplus power of the grid. The gas pressure

increases in the tank of gas-water, and the pressured gas

is stored in the tank of high-pressure gas at last. During

the peak load, the gas turbine generator can be used to

generate power as a supplement. When the air pressure is

insufficient, the gas compressor will work again.

Compared with the traditional CAES system, the two

compressed air pumped hydro energy storage systems

don’t consume natural gas, and don’t need the air turbo-

expander, nor the gas turbine or auxiliary heating sys-

tems, solving the two problems of traditional CAES sys-

tems mentioned above. In addition, generating by the

hydro turbine improves the controllability of the system

and the reliability of the equipment operation, and sim-

plifies the energy storage system. The novel system com-

bines pumped storage and compressed air energy storage

technology by the tank of gas-water which is buried in

the rock layer deep underground. It can be used widely in

areas short of water and heating fuel.

3. The Principle Analysis of Compressed Air

Pumped Hydro Energy Storage

The mentioned two compressed air pumped hydro energy

storage systems above can be described by the following

model.

The vessel is shown in Figure 3. Its volume is V1, and

the initial pressure of inner air is an atmospheric pressure

(shown in Figure 4(a)). Before storage, compressed air

of certain pressure is preset by the gas compressor. P1 is

the preset pressure (shown in Figure 4(b)). During valley

load period, external water is pumped into the vessel

consuming the surplus power of the grid. With the in-

crease of the amount of water, the inner air is compressed

and the pressure increases. The power energy is con-

verted to the potential energy of the compressed air. The

Figure 1. The block diagram of an implementation pro-

posed in [11].

Figure 2. The block diagram of an implementation pro-

posed in [12].

J. T. BI ET AL.

Figure 3 . The model of compressed air pumped hydro en-

ergy storage.

Figure 4. The relationship between E and V2 in isothermal

process.

max pressure of the compressed air is P2, which is the

withstand pressure value of the vessel, and the volume of

the air is V2 at the moment (shown in Figure 4(c)).

When generation, the high pressure water is pushed out,

driving the hydro turbine to generate electrical power

energy. When all the water from the vessel is discharged,

all the energy which can be emitted is released.

Obviously, in order to store the most energy in the

storage system, the air in the vessel should be com-

pressed to the withstand pressure value of the vessel P2.

Besides, the amount of energy is related to the volume of

the compressed air V2. By setting the volume ratio of the

gas and water reasonably, the largest amount of energy

can be stored, maximizing the utilization of storage de-

vices. Because compression and expansion are isother-

mal or adiabatic process, the following formula of the

relationship between the volume of the compressed air

V2 and the energy E is deduced individually in two dif-

ferent situations. And then get the volume of air when the

energy maximizes.

3.1. Isothermal Process

In the process of power generation, the volume of the air

in the vessel is Vx, and the pressure is Px. In the isother-

mal process, the relationship between the volume and the

pressure is as following:

22 x

VPVP

The process of power generation is over after all the

water in the vessel is discharged. The converted energy

in this process is:

22 1

22 22

Eln|

xxx x

PV V

PdVdVPV VPVln

 



(2)

For a given vessel, its capacity V1 and withstand pres-

sure value P2 are fixed values. Form the formula (2), the

amount of released energy is just determined by the vo-

lume of the compressed air when pressure maximizes.

The preset pressure is 1221

/PPVV



. Thus, the other

conclusion is that the amount of released energy is just

determined by the preset pressure.

Get the derivative to V2 from (2), and the equation is

as follows:

dE Pln P

dVV 2



 (3)

When the derivative equals 0, the volume of com-

pressed air is 21

/VVe



. At the moment, the preset pres-

sure is 12

/ePP



, and the energy is 21 . The

relationship curve between E and V2 is shown in Figure

EPV e/

It can be concluded, in the isothermal process, in order

to release the most energy from a vessel whose volume is

V1 and withstand pressure value is P2, the initial pressure

should be preset to 12

/PPe



. When the air is com-

pressed to the max pressure P2, the volume of the air is

/VVe



. The max energy which can be released in the

process of power generation is .

E/PV e

3.2. Adiabatic Process

In the adiabatic process, the relationship between the

volume and the pressure is as following:

22 x

PV V





 (4)

where



is the heat capacity ratio of the air. The heat

capacity ratio of the atmosphere is 1.4. And then, the

converted energy in this process can be computed:

22 22

xxx x

PV PV

PdVdVV V







 





(

PV VV



)







 (5)

Get the derivative to V2 from (5), and the equation is

as follows:

[γ() 1]

dV V







 (6)

When the derivative equals 0, the volume of com-

(1) pressed air is 1

/VV





. At the moment, the preset

J. T. BI ET AL. 29

pressure is P/1





, and the energy is E



/PV





. hip curve between E and

igure 5.

It can be concluded

The relations

, in th

order

is shown in

e adiabatic process, in

release the most energy from a vessel whose volume is

V1 and withstand pressure value is P2, the initial pressure

should be preset to /1

/PP





. When the air is com-

pressed to the max pressue volume of the air is re P2, th

/VV





. The max energy which can be released in

power generation is /1

E/PV





the process of



 . In

the process of adiabatic compressture

increase of the gas can be computed using the following

equation:

ion, the







V1 is 10m3, an

tempera

(7)

with-

e sys-

TV

ple Calculation

3.3. Exam

d the

Assume the volume of vessel

stand pressure value P2 is 10MPa. By (2)(3)(5)(6), the

max extracted energy E and preset pressure P1 can be

calculated in isothermal and adiabatic processes.

In compressed air pumped hydro energy stora

ms, the preset pressure of the vessel needs to consume

power energy to supply, which cannot be extracted by

the hydro turbine. The consumed power needed to preset

pressure is E0. In single storage-generation process, the

ratio of extracted energy to the total stored energy is:

100%



ed into

erature of preset air

η

the preset gas is compress

e initial temp

Assume the vessel i

hin th

(8)

e ves-

cesses are

isothermal way. By the formula (2), the consumed

energy E0 can be calculated, and then the ratio η can be

calculated.

Assume t

l is t0 = 0℃. The temperature rise in the adiabatic com-

pression can be calculated by the formula (7).

The results in isothermal and adiabatic pro

own in Table 1.

Figure 5. The in adiabatic

process. relationship between E and V2

Table 1. The results in isothermal and adiabatic processes.

Index Isothermal process Adiabatic process

P0(MPa)3.68 3.08

E(×106 J)36.8 30.77

E0(×106 J)1

32.68 105.57

η 21.7 22.57

t0( )℃0

t( )℃ 0 9.27

As can be seen fro the above data, in the adiabatic

ped hydro energy storage has good

pocess, the gas pressure will be affected by the tem-

perature of the gas, which will promote the increase of

the gas pressure when the air is compressed. Thus the

stored energy in the adiabatic process is less than that in

the isothermal process. The compressed air pumped hy-

dro energy storage systems mentioned above have no

adiabatic treatment, nor isothermal measures. So the

practical systems will work at a state between the adia-

batic and isothermal results. When the extracted energy

reaches maximum, the preset pressure will be a value

between 3.08 MPa and 3.68 Mpa, and the max power

energy released will be a value between 30.77 × 106 J and

36.8 × 106 J, and the ratio η will be a value between

21.7% and 22.57%. Based on these data, we can draw the

following conclusions. Compared with traditional CAES

systems, the compressed air pumped hydro energy stor-

age systems store less “effective” energy, that is, the en-

ergy which can be released when peak load period. Con-

sidering the construction costs in the engineering projects

accounted for a large proportion, for the same equipment,

the unit electricity cost in the compressed air pumped

hydro energy storage systems is higher than the tradi-

tional systems.

4. Conclusion

Compressed air pum

prospects for development. The generation unit is con-

structed by hydraulic equipment, breaking through the

shackles of traditional mechanical power generation by

turbines and avoiding the problems of excessive increase

of carbon emissions. The principle of compressed air

pumped hydro energy storage is analyzed. The calcula-

tion formula of the storage power is deduced in theory in

different situations of isothermal and adiabatic compres-

sion. The relationship curve between the storage power

and the initial state is obtained. The optimal storage

scheme is given when the capacity and withstand pres-

sure of the vessel is definitive, and the max available

capacity and the equipment utilization efficiency evalua-

J. T. BI ET AL.

ENCES

[1] L. Y. Hu, “Aichnology,” Shang-

hai Electric Po8-160.

ower, Vol.

tric Power, Vol. 30, No. 9,

d On Compressed Air Energy Storage,

Zhejiang Univer-

d Air Energy Storage Sy

ang and Z. P. Song, “Optimization and

, H. S. Chen, et al., “A Novel Energy

ce Analy-

tion of the scheme is given.

REFER

r Storage Generation Te

wer, Vol. 2, 2006, pp. 15

[2] L. Ye, “Study on Sustainable Development Strategy of

Electric Power in China in 2020,” Electric P

Sto

36, No. 10, 2003, pp. 1-7.

[3] W. D. Song, “Survey of Compressed Air Storage Power

Generation Abroad,” Elec

1997, pp. 53-54.

[4] Y. X. Zhou and S. D. Deng, “New Method of Small Scale

Wind Power Base”

Solar Energy, Vol. 4, 2008, pp. 60-61.

[5] X. Zhai, X. L. Yu and Z. M. Liu, “Research on Hybrid of

Compressed-air and Fuel,” Journal of

sity(Engineering Science), Vol. 40, No. 4, 2006, pp.

610-614.

[6] H. S. Chen, “Characteristic and Development Trend of

Compresse

stem,”

High-Technology & Industrialization, Vol. 17, No. 6,

2011, pp. 55-56.

[7] W. Y. Liu, Y. P. Y

Performance Simulation of Different CAES Systems,”

Journal of Engineering Thermophysics, Vol. 26, No. 1,

2005, pp. 25-28.

[8] J. Liu, H. D. Xia

rage Technology Based on Liquid Air and ITS Appli-

cation in Wind Power,” Journal of Engineering Thermo-

physics, Vol. 31, No. 12, 2010, pp. 1993-1996.

[9] Y. S. H. Najjar and M. S. Zaamout, “Performan

sis of Compressed Air Energy Storage (CAES) Plant for

Dry Regions,” Energy Conversion and Management, Vol.

39, No. 15, 1998, pp. 1503-1511.

doi:10.1016/S0196-8904(98)00034-X

[10] I. Glendenning, “Long-term Prospects for compressed air

storage,” Applied Energy, Vol. 2, No. 1, 1976, pp.

39-56. doi:10.1016/0306-2619(76)90038-6

[11] H. R. Wang, “Water-gas Ecompassing Electric Power,”

essed Air Pumped Hydro En-

Energy Storage System: CN 102434362 A [P/OL].

2012-05-02[2013-01-23].

[12] EBO Group, Inc., Compr

ergy Storage and Distribution System: US 7,281,371 B1

[P/OL]. 2007-10-16[201-01-23].