Compressed air-brine energy storage

Energy Vol. 13, No. 2, pp. 183-190, 1988 Printed in Great Britain. All rights reserved

COMPRESSED Chemical Engineering

O360-5442/88 $3.00 + 0.00 Copyright @ 1988 Pergamon Journals Ltd

AIR-BRINE

Department,

ENERGY

STORAGE

SIDNEY LOEB Ben Gurion University of the Negev, Beersheva 8410.5, Israel (Received

10 February 1987)

AbstractXompressed air-brine energy storage (CABES) is similar to ordinary compressed air energy storage (CAES). However, in CABES, the heat of compression of the air is stored via a surface-type heat exchanger in water or, preferably, concentrated brine contained in an unpressurized reservoir. Furthermore, the brine is stratified into a hot, lower density, upper layer and a cold, higher density, lower layer, thus eliminating half the needed reservoir volume. In the energy delivery phase the hot brine heats the compressed air prior to its expansion through an expander/generator to recover the stored electric energy. Calculations on a three-stage CABES plant indicate that: (1) the overall electric efhciency is at least 67%; (2) the energy storage density of the brine is 0.016 m’ per electric kWh delivered from storage; (3) the required unit heat transfer surface is 0.27 m’ per electric kWh; (4) the contribution of the reservoir and heat exchanger costs to the cost of electric energy delivered from storage is not excessive.

INTRODUCTION

Large fossil and nuclear-fuelled power plants are at highest efficiency when operated at steady state. However, the power demand varies on a daily, weekly, and seasonal basis. Solar power plants have intermittent or variable supplies of energy. Accordingly, in these plants, arrangements must be provided for storing excess generated energy or otherwise accommodating the disparity between energy supply and energy demand. One of the most promising methods for storing electric energy is CAES, as described by Stys.’ The electric energy to be stored is used to compress air, which is then cooled, compressed to a higher pressure of about 70 bars, cooled again, then stored in an underground cavern. The heat removed during the two cooling stages is discarded. During the energy delivery phase, the stored, compressed air is burned with enough fuel oil to heat the mixture to 823 K. The mixture is partially expanded, fuel oil addition and combustion are repeated, after which the mixture is finally expanded to the atmosphere at 693 K. The expander drives a generator. Because of the fuel oil addition only about half of the electric energy delivered to the load from storage can be attributed to the energy stored in the compressed air. Furthermore, an appreciable fraction of the electric energy to the compressors is lost in the cooling stages. As a result of these drawbacks the adiabatic CAES plant has been proposed.’ Heat removed during the air cooling stages is not discarded but stored. This heat is used to preheat the air to be expanded. The favored thermal storage medium consists of pebble beds in direct contact with the air passing through. Typically, such beds would cycle between 310 K and 750 K at a pressure of 82 bars, requiring an expensive pressure vessel. Furthermore, the thermal stresses produced in the pebbles might cause them to break into dust particles which could be carried out of the bed by the air and would damage rotating machinery. I propose an adiabatic CAES plant incorporating improvements in thermal storage, in particular the use in sensible heating and cooling of an unpressurized, low-boiling, cheap liquid, exemplified by water or brine and with other improvements, as described herein. 2. BASIC

CONCEPT

OF COMPRESSED

AIR-BRINE

ENERGY

STORAGE

((‘ABES)

2.1. Energy storage phase As shown in Fig. 1, the excess electric energy from the power plant is used to compress and heat atmospheric air via a compressor/motor (C/M). The air is then cooled in a surface-type, counter-current, brine-air heat exchanger (BAHX) after which it enters an air container (AC) such as a cavern. The cooling liquid is water or concentrated brine, preferably the latter. The 183

184

SIDNEY LOEB

EVERCY STORAGE

Air I” from Amosphere

PH.~SE Open M atmosphere

Fig. 1. Schematic of the basic concept invobxd in

compressedair-brine energy storage (CABES).

cold brine enters the BAHX from the bottom of a thermal storage reservoir (RES) open to the atmosphere, and therefore relatively cheap to construct. After passage through the BAHX the heated brine is returned to the top of the thermal reservoir. Thus, the reservoir liquid is stratified into a hatter, lower density, upper layer and a cooler, higher density, lower layer. By the use of a temperature-stratified reservoir, the required reservoir volume is cut in half. A quiescent tank, temperature-stratified in this manner, woutd equilibrate only very slowly across the interface between the two layers, since equilibration will occur primarily by thermal conduction. The theoretical rate of temperature equilibration has been essentially obtained in experiments carried out in 200-I. tanks by de Gerloni and Vezzuh.3 The maintenance of stratification becomes easier as tank diameter and height increase. 2.2. Energy delivery phase By means of the phase selector valve (PS) and the reversible pump (RP), the fluid flow directions are reversed in the air container, the BAHX, and the reservoir. An expander/generator (E/G) converts the stored thermal and mechanical (pressure) energy back into electrical energy. The partially cooted brine leaving the BAHX passes through a reject heat exchanger (RHX) cooled by the environment in order that the brine can attain its final cold temperature. The RHX is necessary because the rotating components are not thermodynamically 100% efficient. Thus some mechanical energy is converted to heat. Furthermore, the BAHX must operate with an adequate temperature difference driving force. This necessary heat exchanger inefficiency also contributes heat losses to the environment. 3. A THREE

STAGE

CABES

PLANT

The CABES system of Fig. 1 is not practical for large-scale application largely because the air cannot be sufficiently compressed in one stage. Figures 2 and 3 show schematic drawings of a three stage CABES plant which stores compressed air at 60 bars and 310 K, Figures 4 and 5 show thermodynamic characteristics of the air in this plant. The statepoints of Figs. 4 and 5 are also shown on Figs. 2 and 3.

Compressed

air-brine

energy storage

One kg Air In fronp-

185 (‘Ircled

Nos.on FIN. 4

-.

6 95

6.54

Electric Energy

I

into Storage

A,

BAHX

6.!2

Fig. 2. Schematic of three stage CABES plant during energy storage phase

3.1.

Plant guidelines

3.1.1. General guidelines. (a) All heat exchangers are of the surface type with counterflow. (b) The air flow through each BAHX is in series. The brine flow is in parallel. The minimum driving temperature difference is 5 K. (c) The upper temperature of the storage brine is 400 K, assumed to be about 2 K below its atmospheric boiling point. This requirement can be approximately met by a brine having the composition of calcium chloride hexahydrate.4 (d) The frictional losses in the brine and air transport systems are negligible. 3.1.2. Storage guidelines. (a) The ambient air entering the first stage compressor is at a temperature of 300 K and a pressure of one bar. (b) The isentropic temperature, if attained after compression, would be the same for all the three stages of compression and such that the finally stored air would be at 60 bars pressure and at a temperature of 310 K (the appropriate isentropic temperature, found by trial and error on Fig. 4, is 460 K). (c) The actual temperatures after compression are determined from the pressure attained in the hypothetical isentropic compression and from the actual enthalpy change, given by: (actual enthalpy change) = (isentropic enthalpy change)/0.85 where 0.85 is the assumed fractional isentropic efficiency of each of the three compressor/motor (C/M) pairs.

SIDNEY LOEB

186

One kg Air OUI IO Atmosphere ~

He;rl lrom

Circled Nos.011 Fig 5

KliS

182 kJ/kg Reject Heal lo Environment

Fig. 3. Schematic of three stage CABES plant during energy delivery phase.

3.1.3. Delivery guidelines. (a) The exhaust air leaving the final expander has a pressure of one bar and a temperature in the vicinity of 300 K. (b) The isentropic temperature, if attained after expansion, would be the same for the first two stages of expansion and such that the exhaust air from the third stage meets Guideline 3a. (The appropriate isentropic temperature, found by trial-and-error on Fig. 5, is 245 K). (c) The actual temperatures after expansion are determined from the pressure attained in the hypothetical isentropic expansion and the actual enthalpy change given by: (actual enthalpy change) = (isentropic enthalpy change) (0.90), where 0.90 is the assumed fractional isentropic efficiency of the expander/generator (E/G). (d) The minimum driving temperature difference in the reject heat exchanger (RHX) is assumed to be 10 K. The environmental cooling water is assumed to be at a temperature of 295 K. Therefore the temperature of brine leaving the RHX and entering the reservoir is 295 + 10 = 305 K. 3.2.

Pertinent characteristics of the three stage CABES plant (see Appendices

I and II)

(a) The overall efficiency (electrical energy delivered from storage divided by electrical energy into storage) is 67%. This efficiency can be increased by increasing the upper storage temperature. At 425 K, the efficiency would be 70% (calculations not given herein).

Compressed

air-brine

energy storage

187

I

r

MO-

I.

I

J I

4so

I

-

i

2 M $

a-2 _z

350

B w

-

One kg Cooled Cornmessed

I/

I

Fig. 4. The three stage CABES plant. Thermodynamic phase.

changes are shown for the energy storage

(b) The required unit volumes of stored brine and stored air are respectively O.Ol%t and 0.145 m3 per electric kWh delivered from storage in one cycle. (c) The maximum and minimum air temperatures are considerably below corresponding temperatures in ordinary CAES. (d) The mass ratio of brine rate to air rate is 2.40. (e) The storage brine is cheap, non-deteriorating, and has good heat transfer properties in comparison to solids. (f) The storage reservoir has the following properties: (i) unpressurized, mild steel tank (if air is excluded above brine); (ii) low heat losses and moderate insulation requirements because of the relatively low upper temperature of the storage brine; (iii) stratified storage of hot and cold brine in the same reservoir, thus cutting the required reservoir volume in half. (g) The required total heat transfer surface is 0.27 m’/kWe. (h) Because of the use of a surface-type heat exchanger rather than a direct contact type, there is no possibility of contaminating the air transport system, especially the rotating components, with species other than air itself. tThe value of 0.0158 is of the same order of magnitude as that for pebble storage, as described in Sec. 1.

SIDNEY LOEB

188

Circled Numb1ers on Fig

3

I

htropy 1

1

1

1

1

5.5

1

kJ/kgAir 1

1

6.0

Fig. 5. Three stage CABES plant. Thermodynamic

I

I

K 1

1

1 6.5

1

1

1 MS

changes are shown for the energy delivery phase.

4. COMMENTARY

4.1. On contributions of reservoir and heat exchangers to costs of energy delivered from storage According to order-of-magnitude estimates in Appendices I and II, the costs of the reservoir (including brine), and the heat exchangers will contribute 0.00039 and 0.0014 dollars respectively to each kWh of electricity delivered from storage during the lifetime of the CABES unit. 4.2. On substitution of another storage liquid In the course of a personal discussion on CABES with R. Schainker of EPRI, he suggested that, assuming its technical feasibility, the concept might have a better possibility for near-term use if a high-boiling oil would be used as the thermal storage liquid. By this means, presently designed high temperature compressors and expanders could perhaps be used.5 This may be an appropriate suggestion. However, the use of such a high boiling oil might be accomplished only at the sacrifice of some of the advantages of brine, as enumerated in Sec. 3, items 3.2e and f. On the other hand, it is probable that the overall efficiency would be considerably higher at the higher temperatures obtainable with oil.

Compressed Acknowledgement-J.

concepts described CABES approach.

air-brine

189

energy storage

Pellin of SocittC Electrique de I’Our, Luxembourg, suggested application to CAES of storage to the SocittC in January, 1986. This suggestion provided the impetus for development of the

REFERENCES 1. J. Silverman, Energy Storage. Pergamon Press, New York (1980). 2. L. Marksberry, Thermal energy storage media for advanced compressed air energy storage systems. Proc. 17th Intersoc. Energy Conversion Engng Conf 4, 2000, Los Angeles (August 1982). 3. S. Loeb, M. de Gerloni, and A. Vezzuli, experiments (unpublished) carried out at ENEL-DSR-CRIS, Via Ornato 90/14, Milano, 20162, Italy (Sept. 1985-Jan. 1986). 4. N. Isshiki, Y. Maekawa, M. Takeuchi, I. Nikai, T. Akuta, and J. Kameshida, Energy conservation and storage by CDE (concentration difference energy) engine and system. Proc. 12th Intersoc. Energy Conversion Engng Conf 2,

1117, Washington,

D.C. (Sept. 1977).

5. R. Schainker, EPRI, Palo Alto, California (6 June 1986). 6. I. Kirk and D. Othmer, Encyclopedia of Chemical Technology, 3rd edn., 4, Wiley, New York (1978). I. M. Peters and K. Timmerhaus, Plant Design and Economics for Chemical Engineers, 3rd edn., McGraw-Hill,

New

York (1980). APPENDIX

I

Calculation of Quantities of Interest

(except heat transfer calculations) (A) Brine rate/air rate = (550)/(400 - 305)(2.41) = 2.40 kg brine/kg air, where 550 is the thermal energy (kJ/kg air) to the reservoir during one cycle, and 2.41 is the estimated specific heat (kJ/kgK) of CaC1,.6H,O as brine.6 The specific heat is the arithmetic mean of the specific heats of CaCI, and 6H,O. (B) Electric energy storage density 1. Of the brine = (2.40)(1)(3600)/(364)(1500)

= 0.0158 m3 brine/kWhe delivered in one cycle, where (1) is kJ/kWs, 3600 is s/h, 364 is the electric energy (kJ/kg air) delivered from the expander/generators in one cycle, and 1500 is the approximate density of the brine (kg/m3). 2. Of the air = (0.0147)(1)(3600)/364=0.145 m’ air/kWhe delivered in one cycle, where 0.0147 is the specific volume of the air (m3/kg) at 60 bars and 310 K.

(C) Contribution of brine-filled reservoir to the cost of the total electric energy delivered by the CA BES unit unit cost for an insulated, unpressurized mild steel tank of 50$/m3. For the brine I assume a cost of (1500)(0.507)(0.15) = 114$/m3, where O.l5$/kg is the assumed cost of solid CaCl, and 0.507 is the weight fraction of CaCI, in CaC1,.6H,0.6 The contribution of the brine-filled reservoir is then given by (50+ 114)(0.0158)/(20)(365)(0.9) = O.O00394$/kWhe delivered in the lifetime of the CABES unit, where (20)(365)(0.9) is the number of cycles in an assumed 20 year life with 90% on-stream time. I assume an installed

(D)

RHX temperatures

1. Of entering brine: This temperature equals that of the brine leaving BAHX’s = 400 - [(368)/(2.40)(2.41)] = 336 K where 368 is the thermal energy (kJ/kg air) leaving the reservoir during one cycle. 2. Of the environmental cooling water leaving This temperature is arbitrarily chosen as 310K, i.e., about halfway between the assumed cooling water inlet temperature of 295 K and the inlet temperature of the brine, 336 K. (E)

Overall

electric

compressor/motors (F)

efficiency = (364/547)(100) = 67%, during one cycle.

where

547 is the electric

energy

(kJ/kg

air) to the

Sample calculation of statepoint change

The air at statepoint 1 (H = 300 kJ/kg air) is first assumed to be compressed isentropically to 460 K (see guideline 3.1.2b). At this condition, Fig. 4 shows that the pressure is 4.3 bars and H (isentropic) is 462 kJ/kg air, giving AH (isentropic) = 462 - 300 = 162 kJ/kg air. However, by guideline 3.1.2c, AH (actual) = 162/0.85 = 191 kJ/kg air, giving H (actual) = 300 + 191 = 491 kJ/kg air. The pressure is still 4.3 bars. These two conditions locate statepoint 2 on Fig. 4. from which we see that S = 7.0 kJ/kg air K and T = 488 K.

APPENDIX

II

Heat Transfer Calculations (A) Heat transfer equations and heat transfer coefficients

The thermal power in terms of heat transfer across the exchanger wall is kWth,

= UA,

ATLM,

(II-l)

190

SIDNEY

LOEB

where kWth,

is the thermal power in a heat exchanger (kW), U is the overall heat transfer coefficient (kW/m* K), K. The thermal power in terms of heat added to the air passing through is

AHX is the heat transfer surface (m*), and ATLM is the log mean driving temperature,

kWthf,* = (kJth,/kg

air)(Z)(1)(1/3600),

(II-Z)

where kJth,/kg air is the thermal energy added to the air, Z is the air rate, kg/h (not to be calculated here). Equating of Eqs. (II-l) and (11-2) and solving for A, yields A,

= (kJth,/kg

air)(Z)/(36~)(~)

AT,,,.

(11-3)

The overall heat transfer coefficient is assumed to be 0.29 kW/m2 K for both the BAHX and RHX exchangers.7 (B)

Unit heat transfer surface (m’/kW)

required for heat exchangers.

1. Reject heat exchanger (RHX) By Eq. (11-3) and Fig. 3 A RNx = (182)~2~~,/3~)(ln

26/10)/(0.290)(26

- 10) = (37.5)(2~~,)/(3~),

(11-4)

where Zde, is the air rate in the dehvery phase. The electric power delivered from storage is kWe = (364)(2,,,)(1)/3600.

(11-5)

The power is assumed to be the same in the storage and delivery phases. Dividing Eq. (11-4) by Eq. (H-5) gives the unit area of the RHX as 0.103 m’/kWe. 2. Brine-air

heat exchungers

(BA HX)

Since the delivery phase was found to require a larger heat transfer surface, calculations are shown only for this phase as follows:

BAHX considered High pressure Intermediate pressure Low pressure

kJ,,,,/kg

Heat transfer surface ‘“qi (;?)I m

air

(Fig. 3)

91

12.7

(24.7)(Z,,,)/3600

140

26.8

(IS.O)(Z,,,)/3600

137

26.3

(IS.O)(Z~,)/3~

Total:

Using Eq. (11-5) as before we find that the sum of the unit area of all the BAHX’s is 0.167 m’/kWe, giving a total unit area of (0.103 + 0.167) = 0.27 m*/kWe. (C)

Co~~~b~tio~ of heat exchangers

to co.st of energy delivered from storage during lifetime of CABES

unit

I assume an installed cost of 200$/m* for the heat exchangers. Then the contribution of the heat exchangers is given by (200)(0.27)/(20)(365)(0.9)(6) = O.O0137$/kWhe delivered, where all terms are already known except 6, assumed as the average delivery time, hours, in a cycle.