NASA Technical Memorandum 103145 _ .......... _

. .........-_ Energy Storage for a Lunar Base by the

Reversible Chemical Reaction:

CaO +H20 _ Ca(OH)2

Maria E. Perez-Davis

Lewis Research Center ....

Cleveland, Ohio

and

Frank DiFilipo

Case Western Reserve Un_iversi_

Cleveland, Ohio

- (NASA-TM-I03145) ENERGY STORAGE FOR A LUNAR N90-25419 .

6A_r _Y THE REVERSIBLE CHEMICAL RrACTION:

CaO+H20 RFVFRSI_LE REACTION Ca(OH)2 (NASA)

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Unclas

G3/44 02_?147

ENERGY STORAGE FOR A LUNAR BASE BY THE

REVERSIBLE CHEMICAL REACTION: CaO + H20 -Ca(OH)2

Maria E. Perez-Davis

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

Frank DiFilipo*

Case Western Reserve University

Cleveland, Ohio 44106

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Cr_

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Abstract

A thermochemical solar energy storage concept involving

the reversible reaction CaO + H20 _ Ca(OI-I) 2 is proposed as

a power system element for a lunar base. The operation and

components of such a system are described. The CaOAI20

system is capable of generating electric power during both the

day and night. The specific energy (energy to mass ratio) of the

system was estimated to be 155 W-hr/kg. Mass of the required

amount of CaO is neglected since it is obtained from lunar

soil. Potential technical problems, such as reactor design and

lunar soil processing, are reviewed.

Introduction

A permanently manned moon base powered by solar energy

will require a large storage system because of the 14 day long

lunar night. Many types of storage systems have been

proposed, such as regenerative hydrogen/oxygen fuel cells,

NiH batteries, flywheels, and superconducting inductors, in

addition to beamed power (Personal communication from

David J. Bents of NASA Lewis Research Center, Cleveland,

OH, 1990). The choice of storage system will be based on

many factors including but not limited to specific energy

(energy to mass ratio), energy conservation efficiency,

lifetime, maintenance, and cost. Specific energy is likely to be

the most important factor because of the high cost of transporting materials from the earth to the moon. Considerable

improvements in the specific energy might be achieved by

using raw materials that are found on the moon.

Thermochemical storage systems have been examined in

recent years for solar energy storage on earth (refs. 1 to 5). In

these systems, solar energy is stored as the heat of reaction of

a reversible chemical reaction. One example is the reaction of

water with calcium oxide, producing calcium hydroxide and

large amounts of heat:

CaO + H20 _ Ca(OH)2 + heat

The fact that CaO is a major constituent of lunar soil (9 to

16 percent) (ref. 6) singles out this reaction as a possible solar

energy storage system for a moon base.

Process Description

In the proposed system, figure 1, direct solar radiation

incident on a concentrator is directed onto a reactor (heat

receiver). The calcium hydroxide inside the receiver is dissociated at a temperature of 500 to 550 "C to generate steam

and solid calcium oxide. The steam is circulated through a

turbine to generate electrical power. The calcium oxide is

passed through a heat exchanger to reduce its temperature to

ambient and stored at 25 "C. When required, CaO and H20 are

combined, initiating the chemical reaction and the heat of

reaction is liberated.

*Summer Intern at NASA Lewis Research Center.
System Calculation

Chemical Reaction

There are two types of energy storage involved in this

reaction: sensible heat (heating from 25 to 510 "C) and heat of

reaction, The reaction is reversible, and the chemical cycle can

be divided into charging reactions (daytime) and discharging

reactions (nighttime). During the day, solar energy is used to

heat the Ca(OH)2 to the dissociation temperature (510 to

550 °C), producing CaO and steam (which is condensed and

stored as liquid water):

Charging reactions:

Ca(OH)2 + 56.0kJ/mol----Ca(OI-I)2 (I)

(25 "C) (510 °C)

The amount of energy needed to heat calcium hydroxide from

25 to 530 "Cis 56 kJ/mol (ref. 7), known as the sensible heat.

Ca(OH) z + 94.6 kJ/mol ---* CaO + 1-I20 (2)

(530"c) (530"c) (530 "c, 30atm)

The heat of reaction for the above at a temperature range 530

to 800 °C was calculated by Samms & Evans (ref. 12) and was

found to be 94.6 ld/mol.

H20(g) ---" I-I20(g) + 48.4 kJ/mol (3)

(500 "C, 30 atm) (99.6 "C, 1 atm)

The energy released by reaction (3) is used to generate electrical

power by use of a steam turbine (ref. 9).

The water is brought in contact with the CaO, forming

Ca(OH)2 and releasing large amounts of energy, which can be

used for a secondary electrical power generation:

Discharging reactions (ref. 13):

CaO(s)+ H200) "" Ca(OH)20) + 61.5 kJ/mol

(25 "C) (25 "C) (25 to 200 "C)

(4)

Many charge/discharge cycles will be required during the

lifetime of the system; hence side reactions with impurities

and container materials must be minimized to insure high

reversibility of the CaO/H20 reaction. The major side reaction

of concern for CaO/H20 solar energy storage systems on earth

is the reaction of CaO with carbon dioxide:

CaO + C02 --" CaCO3 (5)

forming CaCO3, which has a higher dissociation temperature

(898 "C). On the moon, however, this will not be a problem

because of the lack of CO2. Another potential side reaction is

the formation of noncondensible gases (mainly H2) if a corrosion reaction with a stainless steel container occurs (ref. 1).

This side reaction does not reduce the reversibility by consuming the reactants, but rather by the presence of the noncondensible gases which reduce the partial pressure of the water

vapor. Removing these gases by degassing the apparatus

restores the reversibility of the reaction. This would easily be

accomplished on the moon by venting the reactor because the

moon has negligible atmosphere. Tests have shown that the

degree of reaction completion is 94 to 99 percent after over

1100 hydration/dehydration cycles if the formation of CaCO3

is not taken into account (refs. 1 and 8).

A major concern for CaO/H20 storage systems on earth is

the time required for reaction completion. This can be influenced by reactor design. Several experiments have been performed using the metal fins to improve heat transfer in the

powdered CaO and Ca(OI-I) 2, which have a low thermal conductivity (refs. 5, 7, 10, and 11). Reaction times were typically about 2 hr or less. Reactor pressure is a parameter which

can be controlled for system optimization. The dependence of

the dissociation temperature on pressure is shown in figure 2

(ref. 12). Having the reactor at a higher temperature and pressure would allow higher efficiency, but it would also require an

increased reactor wall thickness (and mass) to withstand higher

pressures. This is a trade-off which would have to be considered in designing the reactor.

Energy and Mass Balance

In order to estimate the solar collector area and flow rate

required to produce 100 KWe power generation system to

operate on the lunar surface, the following was assumed:

(1) A conversion of stored energy to chemical reaction in

the dissociation reactor has an efficiency of 84 percent

(ref. 4);

(2) The solar collector weighs 1.5 kg/m 2

(3) The steam turbine specifications are: 500 "C, 30 bar,

turbine efficiency of 80 percent, steam quality of 0.2;

The energy and mass balance based on the above assumptions

are presented in figure 3.

Reactor Design

The reactor design recommended for this noncatalytic

reaction is a continuous (moving bed) reactor (ref. 13). There

is direct heat transfer between the reacting solids and the gas.

The shrinking core model and the following assumptions were

made in order to estimate the solids residence time and reactor

size.

(1) The rate controlling mechanism is a chemical reaction;

(2) Res_nce time is the same for all solid panicles;

(3) The H20 vapor concentration is constant throughout the

reactor;,

(4) Solid panicles are uniform in size, 1 mm;
Theestimatedsolidsresidencetimebasedon theabove

assumptionsis 1.5min;thereactorsizecalculationswere

madeusingtwicethisestimatedtime.

Specific Energy

For the purpose of calculation of the specific energy, a

system providing 100 kW of electricity during the lunar night

(and 123 kW during the lunar day) will be assumed. The lunar

night is 350 hr; hence 1.26x108 kJ of electrical energy is

needed, which corresponds to 88 125 kg of H20. Adding

10 percent to account for vapor losses during reactor

evaluation, reversibility losses, and extra water needed as the

working fluid means that about 96 938 kg of 1-120 will be

needed. Also, 274 478 kg of Cat will be needed, but it will be

processed from the lunar soil and not brought from earth.

The estimated masses of other components which need to be

brought from earth are listed below.

Component Mass (kg_

Solar concentrator 516

I-I20 tanks 88

Reactors 44 000

Radiator 34 041

Power management and distribution 200

Pumps 600

Heat exchangers 50000

Subtotal 129 445

Water 96 938

Total 226 383

The reactor construction material selected was Inconel

(density = 8.3 g/cm3).

The system produces 105 kW of electricity, the extra 5 kW

are used to run the pumps needed in the system.

The specific energy is calculated as follows

specific energy = ng.0.gf.g_(100 kW)(350 hr_ 155 W-hr

mass 2.26x 105 kg = kg

This number can be compared to the specific energies of more

familiar energy storage systems. Current technology Ni-H

batteries have a specific energy of 32 W-hr/kg, with up to

75 W-hr/kg projected. Regenerative hydrogen/oxygen fuel cells

planned for the lunar surface will have specific energies

ranging from 700 to 1200 W-hr/kg (Personal communication

from David J. Bents of NASA Lewis Research Center,

Cleveland, OH, 1990).

Feasibility

Aside from theoretical considerations there are some

technical barriers which would have to be overcome before the

CaO/H20 system could compete with batteries and fuel cells.

One problem is a reactor design. There may be problems with

the heat transfer within the reactor, but metal fins may

overcome the problem. A corrosion reaction with stainless

steel has been observed and the effect of corrosion on the

lifetime of a reactor made of Inconel is not known. If Inconel

is not suitable, then either another alloy could be used, or a

protective coating could be deposited inside the reactor.

The second major technical barrier is the extraction of Cat

from the lunar soil. The chemical composition of the lunar

soil is listed below (ref. 6):

Maria (seas), Terrae (highlands),

percent percent

SiO-z 45 45.0

FeO 20 4.0

A1203 8 28.0

Cat 9 16.0

MgO 14 6.0

TiOz 2 0.4

The Cat content is highest in the highlands, so highland soil

should be processed to obtain the Cat. A dry-extraction

process has been proposed to recover the "useful" products,

silicon, aluminum, and oxygen, from the mineral amorthite

(CaA12Si2Os) in the lunar soil (ref. 14). The "waste" product

of the process, Cat, is actually the useful product in this case.

The I-IF acid leach process has also been proposed for

separation of lunar soil (ref. 15). This process offers total

separation into the elements (A1, Ca, Fe, Mg, Si, Ti) or their

oxides. It therefore appears that the basic technology for Cat

extraction has been established. Because Cat is a by-product

of lunar soil separation, the mass of a soil separation system

was not included in the specific energy calculation since no

extra equipment is needed to be brought from earth.

The CaO/H20 reaction is simple: no catalysts or special

conditions are necessary. The basic design and technology are

also relatively simple. The most significant foreseeable technical difficulty would be problems associated with a closed

system, such as pressure control and pumping.

Conclusions

The CaO/H20 thermochemical energy storage system has

been proposed as a candidate for lunar energy storage. In the

processing of lunar soil, Cat is considered to be an unwanted

by-product, but it has a potential use in energy storage. In addition to providing energy storage for the lunar night, the

CaO/H20 system also produces electricity during the day: a

feature not offered by other storage systems.

The major disadvantages of the CaO/H20 system are the

complexity of the reactor design and its unknown lifetime and

the processing of the lunar soil to obtain Cat. Because of the

need for lunar soil processing, the CaO/H20 system would not

be an option for an initial moon settlement. Further investigation is needed for a more accurate calculation of specific
energy,evaluationof feasibility, and determination of the

lifetime and cost of the system before comparison can be made

to other systems.

Acknowledgements

The authors would like to thank David J. Bents, Lisa L.

Kohout, and Paul Schmitz from the Power Systems Integration Office at the NASA Lewis Research Center for their

valuable advice and guidance.

References

1. Rosemary, J.K.; Bauerle, G.L.; and Springer, T.H.: Solar

Energy Storage Using Reversible Hydration-Dehydration of

CaO-Ca(OH)2. J. Energy, Vol. 3, No. 6, Nov.-Dec. 1979,

pp. 321-322.

2. Fujii, I.; and Tsuchiya, K.: Experimental Study of Thermal

Energy Storage by Use of Reversible Chemical Reactions.

Alternative Energy Sources, Vol. 9, T.N. Veziroglu, ed.,

Hemisphere Publishing Co., Washington, D.C., 1978,

pp. 4021-4035.

3. Wereko-Brobby, Y.; and Gibbs, B.M.: Calcium Hydroxide

as an Energy Storage Medium for Solar Power Systems.

International Conference on Future Energy Concepts, IEE

CP-171, Institution of Electrical Engineers, London, 1979,

pp. 207-210.

4. Nix, R.G.; Bergeron, P.W.; and West, R.E.: Reversible

Chemical Reactions for Energy Storage in a Large-Scale

Heat Utility. Energy-The Spark and Lifeline of Civilization

(17th IECEC), Vol. 4, IEEE, 1982, pp. 2019-2024.

5. Fujii, I., et al.: Studies of an Energy Storage System by

Use of the Reversible Chemical Reaction:

CaO + I-I20 _ Ca(OH)2, Solar Energy, Vol. 34,

Nos. 4-5, 1985, pp. 367-377.

6. Smith, R.E.; and West, G.S.: Space and Planetary

Environment Criteria Guidelines for Use in Space Vehicle

Development, Vol. 1, 1982 Revision. NASA TM-82478,

1982.

7. Kanzawa, A.; and Arai, Y.: Thermal Energy Storage by the

Chemical Reaction: Augmentation of Heat Transfer and

Thermal Decomposition in the CaO/Ca(OI-I)2 Powder.

Solar Energy, Vol. 27, No. 4, 1981, pp. 289-294.

8. Bauerle, G.L.; Pearlman, H.; Rosemary, J.K.; and Springer, T.H.: Solar Energy Storage by Reversible Chemical

Processes. Rockwell International Corp., Sandia Contract

Fao 92-7671, Jan. 1979.

9. Wark, K.: Thermodynamics, 4th ed., McGraw-Hill, 1983.

10. Schmidt, E.W.; and Lowe, P.A.: Thermochemical Energy

Storage Systems. Eleventh Intersociety Energy Conversion Engineering Conference Proceedings (IECEC),

Vol. 1, AIChE, 1976, pp. 665-672.

11. Abhat, A.; and Huy, T.Q.: Heat and Mass Transfer Considerations in a Thermochemical Energy Storage System

Based on Solid-Gas Reactions. Solar Energy, Vol. 30,

No. 2, 1983, pp. 93-98.

12. Samms, J.A.C.; and Evans, B.E.: Thermal Dissociation of

Ca(OH)2 at Elevated Pressures. J. Appl. Chem., Vol. 18,

No. 1, Jan. 1968, pp. 5-8.

13. Smith, R.D., et al.: Chemical Energy Storage for Solar

Thermal Conversion. Rocket Research Company. SAND79-8198, RRC-80-R-678, Apr. 1979.

14. Keller, R." Silicon, Aluminum, and Oxygen from Lunar

Ore. Space Manufacturing 6: Nontetrestrial Resources, Biosciences, and Space Engineering, B. Faughnan and

G. Maryniak, eds., AIAA, 1987, p. 360.

15. Waldron, R.D.: Total Separation and Refinement of Lunar

Soils by the HF Acid Leach Process. Space Manufacturing 5: Engineering with Lunar and Asteroidal Materials,

B. Faughnan and G. Maryniak, eds., AIAA, 1985, pp. 132-

149.
f

COOLING

WATER CONDENSER

IREGENERAT'ONI--.. CONOENTRATOR

"--1 .EAOTORI- I ..-'_

q REACTOR / \'-J

SOLAR

Figure 1. - Solar power generation system with calcium hydroxide

as storage medium.

.20

f

COOLING

WATER CONDENSER

1000 --

800 -

600 -

4OO

,(_

,_ 200

ui

no9

o9

lOO

0_

8O

6O

4O

2O

4O

2O

n

10

- _ 6

-- l

1

8O0

Ca(OH) 2

EXHAUST STEAM

99.6 °C

1 atm

CaO

[]

I I ] I [__

750 700 650 600 550

TEMPERATURE, °C

Figure 2. - Pressure versus dissociation

temperature for CaO + H20_Ca(OH)2

(ref. 12).

J

5OO

N_ STEAM 188 kJ

I aOatm

H20 _ I STEAM

_1 REGENERATION L._ OH) I .069 kg/s

REACTORI\.. 1

c_of .2e,skg,sI OaiOHl_/ --_

CaO

•2178 kg/s

_ RADIATOR

CaO

Figure 3. - Energy and mass balances.
NationalAeronautiCsand

SpaceAdministration

1. Report No.

NASA TM-103145

Report Documentation Page

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

Energy Storage for a Lunar Base by the Reversible

Chemical Reaction: CaO + H20 _ Ca (OH) 2

7. Author(s)

Maria E. Perez-Davis and Frank DiFilipo

9. Performing Organization Name and Address

5. Report Date

June 1990

6. Performing Organization Code

8. Performing Organization Report No.

E-5497

10. Work Unit No.

506-41-41

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

15. Supplementary Notes

Maria E. Perez-Davis, NASA Lewis Research Center; Frank DiFilipo, Case Western Reserve University,

Cleveland, Ohio 44106 and Summer Intern at NASA Lewis Research Center.

16. Abstract

A thermochemical solar energy storage concept involving the reversible reaction CaO + H20 :-_Ca(OH)2 is

proposed as a power system element for a lunar base. The operation and components of sucli a system are

described. The CaO/H20 system is capable of generating electric power during both the day and night. The

specific energy (energy to mass ratio) of the system was estimated to be I55 W-hr/kg. Mass of the required

amount of CaO is neglected since it is obtained from lunar soil. Potential technical problems, such as reactor

design and lunar soil processing, are reviewed.

17. Key Words (Suggested by Author(s))

Energy storage; Thermochemical; Lunar base;

Lunar night; Calcium oxide

18. Distribution Statement

Unclassified- Unlimited

Subject Category 44

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages

Unclassified Unclassi fled 6

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