Deep Space Travel Energy Sources

Henry Oman Editor Emeritus

Exploration of the planets beyond Mars and their surroundings is already planned. Astronomy researchers are citing important information that can be obtained with instrumented spacecraft that fly beyond the planets of our solar system. Spacecraft tlying these missions need power for performing their functions and communicating with Earth stations. Sunlight i n these zones is so weak that alternative energy sources are needed. Also, rocket thrusters for getting to these regions would be so huge that ion-propulsion looks better. Progress in the search for sources of energy for deep-space missions was reported at the 37”‘ Intersociety Energy Conversion Engineering Conference (IECEC) July 29-3 I , 2002, in Washington, DC.

Solar power, although weak, is available on Mars. The MARS 2003 Lander will carry lithium-ion batteries that accumulate energy collected by solar-cell panels for powering short tours on the surface of the planet. An alternative power source for deep-space missions is radioisotope heated energy converters. The choice of heat-to-electric power conversion is narrowing to: 1) the Stirling engine; and 2) a combined cycle with thermionic and alkali-metal thermoelectric (AMTEC) heat-todectricity conversion. For propulsion into deep s,pace, a nuclear-reactor-heated M T E C energy converter that powers ion engines can become the best alternative to hoisting tons of rockets into Earth orbit.

STIRLING ENGINE FOR POWERING MARS LANDER

In 1816, twenty-six-year-old Reverend Robert Stirling, one year out of divinity school, invented the Stirling engine which is one of the few heat-to-mechanical-power converters limited in efficiency only by the Camot Cycle. Thirty years later, Sadi Camot was able to derive an equation that established the efficiency limit of heat engines. Stirling engines powered a few warships, living-room fans in the early 1920s, and vacuum-tube radios in Africa for a few years after World War I. However, a Stirling engine could not respond to an automobile driver’s demand for sudden response from pushing the accelerator, so they were never used in high-production automobiles.

The operation of a Stirling engine is explained in Figure I. As piston p-2 rises, it compresses the cold gas. Then follows the power stroke in which both pistons travel downward together, pushed by the expanding gas which is preheated by

lrator

Fig. 1. No fuel is burned inside the cylinder of a Stirling-cycle engine

the “regenerator,” and further heated by the “heater.” At the end of the power stroke, piston p-l moves upward, pushing the gas through the regenerator where it deposits its higher-temperature heat before being cooled by the cooler. Then the cycle starts again with piston p-2 recompressing the

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GAS MANAGEMENT

USER MOUNTING INTERFACE

P 1 1 7 I - l l I f

IIP I f / U \ / I I

q w 9 q

I H E i T HEAT S ~ U R C E ELECTRICAL SUPPORT

\ I FLANGE

I I RADIATOR

FIN INSULATION ‘OLD END FEEDTHROUGH SOURCE

Fig. 2. Each Stirling engine in this space-power package has its own radioisotope heat source

160

140

120

60

40

20

Fig. 3. The stoke control, by keeping the Stirling engine’s input-temperature constant, reduces the engine’s power loss as its radioisotope heat source decays during a 100,000-hour mission

gas. The crankshaft drives each piston with an individual arm to get the get the required piston motions.

Previously reported developments have evolved the Stirling engine into a configuration that has the reliability required for long space missions. The movement of each piston has been reduced to a few hundredths of an inch, and all gas spaces are permanently sealed to permit operation in a vacuum. The pistons are precisely supported by radial springs, and there. is

no rubbing contact in the few micrometers of gap that separates each piston from its cylinder. The engine is started with back-and-forth piston motion produced by the permanent-magnet starter-generator.

Robert Cockfield described the configuration of a pair of Stirling engines that convert heat from two standard radioisotope heat sources into electricity [l]. The ground-demonstration “generator” shown in Figure 2, has a

.- -- SOLAR

THERMIONIC CONVERTERS

rlNPYT ,

INFLATABLE CONCENTRATOR

Fig. 4. An inflatable reflector concentrates sunlight into the thermionic heat-to-electric power converters in the HPALM solar system

radioisotope heat source on each end. Each heat source is in contact with the heat-input plate of an engine which pushes and pulls its permanent-magnet converter. The controller in the center between two converters serves to both convert and regulate the power output of each engine. The controller also issues the signals that cause an engine’s converter to kick-start the engine. The controller has the ability to disconnect one converter in the event of a failure. It also has available stroke control that can provide an additional I I watts after 100,000 hours of operation.

The performance of this two-engine power source during a 100,000-hour mission is plotted in Figure 3. A full loading of radioisotope fuel releases 500 watts at the beginning of the mission when the initial heat source temperature is 6 5 0 (2 . This heat release decays as the isotope ages, and the hot-end temperature of the converter falls. The dc output, likewise, decays, but can be partly restored by adjusting the stroke control of the Stirling-cycle engine. The 100,000-hour lifetime corresponds to 11.4 years not available with many alternatives.

At the beginning of the mission, the Stirling radioisotope generator would have a 114-watt output, based on 85% thermal efficiency, 36% engine conversion efficiency, and 93% controller efficiency. Stroke control allows the hot-side temperature to be maintained at 650°C. so that at the end of the mission, the electrical output is 106 watts with stroke control, compared to 95 watts without stroke control. The mass of the assembly is 27.03 kg. An equivalent radioisotope thermoelectric generator would weight 31.2 kg.

SOLAR THERMIONICS FOR GENERATING HIGH-POWER IN SPACE

Thermionic converters for generating power in space have been evaluated many times. Their main problem has been the huge parabolic Sun-pointing mirror that would have to be precisely oriented to maintain the 1800 K temperature required at the converter’s emitter surface to get reasonable efficiency. The power required by communication satellites is growing;

future space-based Air Force missions and planetary missions will require more power than today’s photovoltaic solar arrays can deliver.

New developments described by Lester L. Begg showed that the solar thermionic power source is worth developing for high-power applications [2]. Solarenergy can now becollected with an intlatable structure, such as a parabolic retlector, or a Fresnel lens, or array of reflectors. This captured solar flux can be further concentrated with a secondary concentrator before it enters a thermal receiver. The heat is then converted into electricity by cylindrical multi-cell thermionic converters. Initial estimates yielded a preliminary mass of 80 W per kg and a preliminary stowed volume of 40 kW per cubic meter.

The major components of Begg’s “High-Performance Advanced Low-Mass (HPALM)” solar thermionic systems are shown in Figure 4. The thermionic converters are in cylinders, with 8 cells in each cylinder. The solar flux is concentrated into the heat-receiver where the walls will be at 2000 K so that the emitter in che thermionic converter will operate at 1800 K. The outer walls of the heat-receiver are well insulated to minimize the thermal losses from its exterior. Each thermionic cylinder dissipates its unconverted heat into space from its end that sticks out of the back of the heat-receiver.

The energy balance in a 50 kW solar thermionic system is shown in Figure 5. Of the 553 kW of heat that enters the secondary concentrator, only 465 kW goes into the receiver and 55 kW is lost through receiver insulation. This leaves 410 kW entering the thermionic converter. Operating at 12% efficiency, the converter delivers 50 KW of electric power, leaving 360 kW of heat to be radiated into the space environment.

The graphite receiver can be designed to store heat for periods in which solar radiation is occulted, such as during an eclipse period in the Earth’s orbit. This would significantly reduce the mass of batteries that have to be carried for supplying eclipse-period power. As shown in Figure 6, the batteries in a 50-kW solar-photovoltaic Earth-orbit power source built with today’s technology, would have a mass twice that of the whole solar-thermionic power source.

JO

TI OCTTPIJT 50 kWe i i i i i

I HEATINTO

T I REJECT HEAT INSULATON LOSS: mkw 55kw

HEATINTO CAVlTy 466 kw

Fig. 5. From a 553 kW input, the solar-thermionic space-power source delivers 50 kW of electric power

3500

3000

2000 r I

MRECENER

DCONCENTRATOR

SOURPVTODAY SOLAR PV FUTURE S O U R TI DESIGN

Fig. 6. Mass comparisons show that for generating 50-kW continuously in an Earth-orbiting spacecraft, the mass of the advanced solar-thermionic power source

with thermal heat storage will be a fraction of the m a s of just the batteries in today’s solar-photovoltaic power plant

The thermionic converters reject heat at 1000 K. This reject-temperature is sufficient for a bottoming cycle that can use a thermoelectric, Brayton, Stirling, or AMTEC converter Deep-space probes would explore the environment and for generating additional electric power. Overall conversion structure of the outer planets of our solar system, and the space efficiencies as high as 40% could be achieved with such a outside our solar system. Hoisting into Earth orbit the fuel combined cycle. In contrast, the highest photovoltaic required for rocket-powered propulsion would be very costly. conversion efficiency being achieved today with experimental The intensity of sunlight is inversely proportional to the square solar cells is 34%. of the Sun-to-spacecraft distance. Nuclear power is then the

NUCLEAR ELECTRIC PROPULSION

Reactor Thermal Power (kWt) Reactor SuppIyiRelurn Temperatures (K) Cycle Temperature Ratio Cycle Efficiency HeXe Flow Rate (kgkec) 1.42 1.61

Radiator SupplyiRelurn 345 348

HPR LMCR 395 492 1150 1 150 895 870

/Temperatures (K) I 491 I 557 I

. -. Reactor Subsys;em Power Conv. Subsystem Heat Rejeclion Subsyslem

Total Radiator Area (m2) 1 217 I 180 Total Svstem Mass (ks) 1 3202 I 2918

1535 1345 881 BY9 650 539

PMAD Subsystem I 135 I 135 Total Specific Mass (kglkWe) I 32.0 I 29.2

Fig. 7. For supplying heat-energy to a 100-kW Brayton-cycle turbine, the heat-pipe nuclear-reactor

(HPR) has nearly the same specific mass as the liquid-metal-cooled reactor (LMCR)

and its heat exchanger

only practical propulsion-energy source, and a nuclear reactor would be needed for any spacecraft other than a tiny probe.

In nuclear electric propulsion a reactor is the source of the energy that goes into electric thrusters that ionize and accelerate a propellant to produce thrust. Nuclear-reactor concepts for space power, and even for supplying power to an inhabited base on the Moon were evaluated during the 1953-1990 Cold War period. Lee S . Mason reviewed twelve major programs in which nuclear reactors for space use were developed in this period [3]. For example, during the ten-year SNAP-8 program which started in 1960, nuclear reactors and subsystems were tested. One was a sodium-cooled reactor that delivered mercury vapor to a Rankine-cycle turbine that drove an electric generator. Its flight and ground tests were conducted on the SNAP-IOA program. Fuels and materials were tested on the SNAP-50 program. The SNAP-IO0 program produced by 1993, detailed system designs, their fuel development, and the results from various component tests. This reactor was cooled by liquid lithium. Heat-to-electric-power converters evaluated included silicon-germanium thermoelectric, free-piston Stirling, and helium-xenon Brayton.

Mason used the performance of today’s heat-to-electricity converters to update the conclusions from previous studies that evaluated the performance of nuclear-reactor space power. He analyzed Brayton-cycle, Rankine-cycle, Stirling-engines, thermoelectric-junction, and thermionic converters. In-core thermionic, closed-cycle gas-turbine, Brayton, and free-piston Stirling cycles were evaluated during the early trade studies. Thermionic converters were discounted due to materials issues

Fig. 8. In this base-tube AMTEC cell, the sodium, evaportaed and ionized at the hot end, flows into the base

tube where the positive ions pass through the solid electrolyte to reunite with the collected electrons that had

to flow through the external load (R,). At the cold-end, the now-neutral sodium vapor condenses into a liquid

that is wicked to the high-pressure hot end.

and uncertain life. Stirling-cycle engines were at a low technology readiness level, requiring a focused engine development effort outside the scope of the SP-100 program. Early SP-I00 designs promised a specific mass of 30 kilograms per kilowatt electric (kGkWe). Later it grew to 45 kGkWe. A Greene-Newhouse design review committee selected Brayton-cycle conversion as the lowest risk approach for the SP-100 design; budget reductions ended the program in 1993.

The results from Mason’s analysis of a 100-kW space-power source based on today’s technology, are summarized in Figure 7. The “HPR” reactor contains a high-pressure heat-pipe that delivers hot gas to the Brayton-cycle turbine. In the liquid-metal-cooled reactor (LMCR) system, a heat exchanger delivers the reactor-produced heat as hot vapor to the Brayton-cycle turbine.

AMTEC CONVERTER FOR NUCLEAR ELECTRIC PROPULSION

Alkali Metal Thermal to Electric Conversion (AMTEC) is a very simple new energy conversion technique that utilizes the ionic conducting characteristic of the “beta-alumina solid electrolyte” (BASE) ceramic, and the ionization of an alkali metal when vaporized. The current technology uses sodium as the working fluid in the cell and sodium BASE ceramic as the ion selective membrane.

.z2

Primary -

Fig. 9. Direct coupling an AMTEC generator to a nuclear reactor requires boiling the alkali metal within the reactor

Primary n

Fig. 10. Heating the AMTEC generator's working fluid in a heat exchanger within the reactor

avoids the need to build the AMTEC into a nuclear-containment structure

AMTEC is best described as a regenerating thermodynamic cycle where the nearly isothermal expansion of sodium through the BASE generates high-currenthow-voltage power with high efficiency. With its heat input at 700 to 800" C, AMTEC heat-to-electric efficiencies of 15 to 20% are being achieved.

AMTEC operation can be explained in Figure 8. It shows two pressure chambers separated by the BASE-tube electrolyte, which is a ceramic membrane. When heat (QJ is applied to the bottom of the cell, the sodium working-fluid on it vaporizes and ionizes. The ions and free electrons then drift up into the BASE tubes, through which the positive-ionized sodium atoms pass. The electrons, which are captured on the inner surfaces of the BASE tubes, are gathered by the current

"r..rl s tor

f l h l Na4sK c

Fig. 11. The heat-exchanger within the nuclear reactor's containment shield could deliver liquid sodium

or potassium to a "vapor separator" that produces the hot vapor for the AMTEC's solid-electrolyte tubes

0.3 1 j- 0.3 L

0.28 -- > 0 c a, 0.26

- 0.24 a, z

-- .- E --

0.22 -

4

Direct coupling with K Direct coupling

vnlh - Indirect with Boilers 1 - Indiredwlh Expanders

0.2 0.2 20 25 30 35 40 45 50 55 60 65 70

RPV Mass [ton]

Fig. 12. The highest energy-conversion efficiency is obtained by transferring the nuclear reactor's

heat energy to the AMTEC generator indirectly with boilers

collectors and sent through the electric load (RJ, to the outer surfaces of the BASE tubes. There. they reunite with the sodium ions, forming sodium atoms that drift to the condenser at the top of the AMTEC cell. An exposed wick in this cooler-space, pumps the condensed sodium atoms back into the heated zone where they again vaporize to continue generation of power.

LIQUID-METAL COOLED REACTOR COUPLED WITH AMTEC CONVERTER

In searching for the options for generating electric power in space, Pablo R. Rubiolo evaluated AMTEC conversion of heat from a nuclear reactor into electric power 141. This concept is

3J

COMPONENTS (37)

CANISTER (I.D.=30mm, L =i'Smm)

NEGATIVE

REJECTION

Fig. 13. An AMTEC cell assembled from 3-mm outside-diameter sodium-alumina tubes can capture

waste heat from a thermionic converter in the zero-gravity environment of space

already under study for addressing the power requirements of developing countries that need to supply power at remote locations. The main feature of the concept is the generation of an alkali metal vapor with heat from a nuclear reactor that delivers hot liquid-metal (LMR). Another concept is a fast breeder reactor that boils potassium and delivers the potassium vapor to a turbine. R.E. English and R.N. Weltmann described i t in 1967 [5 ] .

Boiling potassium in the core of a fast reactor has r,afety drawbacks that make it hardly acceptable. These drawbacks can be eliminated if the vapor is generated outside of the core by using expanders. Within the expander, the fluid is routted through a nozzle where vapor is generated by flash vaporization of the alkali metal. This concept is illustraled in Figure 9, where the liquid-metal-cooled-reactor and AMTEC units use the same working fluid. The AMTEC and the core pumps are placed outside of the reactor vessel.

This concept is simple because it eliminates heat exchangers. However, its features complicate the operation and maintenance of the plant. Other drawbacks of this design are the additional shielding requirements, plus the corrosion and material problems that arise from the presence of activation products in the AMTEC. Furthermore, activation of the coolant requires placing the AMTEC unit inside the nuclear-reactor containment, thus increasing the containment's size. Therefore, Rubiolo evaluated two alternatives in which the reactor vessel and the AMTEC generators are indirectly coupled.

The first indirect coupled configuration, shown in Figure 10, has a heat exchanger that separates the AMTEC unit from the core of the reactor. The reactor activation products are retained in the primary circuit. Then the AMTEC unit and secondary pipes do not require shielding and can be placed outside of the containment structure. As the alkali metal heat exchanger is very compact, its added capital cost might be compensated by the reduced shielding and reactor containment

HEAT

TIC EMllTER EVAPORATOR WCK

AMTEC CATHODE

ELECTRODE

AMTEC ANODE

ELECTRODE

BASE TUBE

CAPILLARY TUBE

Fig. 14. The thermionic converter with a n AMTEC bottoming converter forms a combined

cycle that can convert radioisotope heat to electric power with 33% efficiency

size. Also, different working fluids can be used in the primary and secondary circuits. These fluids can he selected to obtain the best neutronic and thermal-to-electric performances. Also, the corrosion products are confined in each loop, allowing selection of the best materials.

The second altemative is replacing the heat exchanger with an alkali-metal boiler (Figure 1 I). This boiler, by avoiding the use of expanders, reduces the number of components and has better corrosion behavior. It has many drawbacks, one of which is the difficulty to produce nucleation of vapor bubbles. The second drawback is the low saturation pressure of alkali metals, which requires keeping pressure losses to very low values to avoid an excessive temperature drop in the two-pressure region. This problem can he solved by increasing the boiler's flow cross-section or by using a high saturation pressure alkali metal, such as potassium or cesium.

The efficiencies optimized plants with an outlet core-outlet temperatures of 1050 K, as a function of plant mass in tons, are compared in Figure 12. Rubiolo cited 12 published references to work in this technology.

CASCADED AMTEC AND THERMIONIC CONVERTER

Kotaro Tanaka described a new approach for an AMTEC cell [6]. It has 37 tubes, each 3 mm in outer diameter, assembled in a standing capillary structure (Figure 13). This capillary structure avoids the need of gravity for making the liquid metal flow to the heat source. The cell, with a base diameter of 1.5 sq cm, produces a power output of 5 W at a relatively low operating temperature of 900 K. This output is

31

almost equal to that of high-temperature cells operating at 1 100 K.

Tanaka also described acascaded energy converter in which the input high-temperature heat goes first into a thermionic converter (Figure 14). The AMTEC converter absorbs the heat rejected by the thermionic converter's collector. The thermionic converter would have an emitter temperature of 1600 K and a collector temperature of 1000 K. The cesium reservoir of the AMTEC cell would reject heat at 577 K. The calculated combined cycle efficiency is 33970, which is close to the efficiency of today's best solar cells. Many technical problems need to be solved before this concept can be realized. Tanaka predicted that this converter could be useful in portable power generators, as well as in deep-space probes.

REFERENCES

The following are published in the Proceedings ofthe 3 7 Intersociety Energy Cotrversion Engineering Conference, held July 29-31, 2002. The Proceedings carries IEEE Catalog Number 02CH37298, and Library of Congress Number 2001096634. Copies of the Proceedings or individual papers can be procured from the Institute of Electrical and Electronic

Engineers, IEEE Operations Center, 445 Hoes Lane, PO Box 1331, Piscataway, NJ 08855-1331, USA.

[I] Cockfield. Roben D. and Chan, Tak S.. 2002. Stirling Radioisotope Generator for Man Surface and Deep Space Missions,

iECEC 2W2 paper20188.

[2l Begg. Lester L. and Associares. 2002, Conceptual Design of High Power Advanced Low Mass (HPALM) Solar Thermionic Power System,

iECEC 2002 paper 20177.

[31 Mason, Lee S., 2002, Power Technology Options for Nuclear Electric Propulsion,

IECEC 2002 paper20159.

141 Rubiolo, Pabla R.. 2002, Design Options for B Liquid Metal Reactor Coupled with an AMTEC Unit.

IECEC 2002 paper 20062. [SI English. R.E. and Weltmann. R.N.,

Experience in investigation of Components of Alkali-Metal-Vapor Space Power Systems,

Proceedings of the Symposium on Alkali Metal Coolant, IAEA Vienna. pp. 71 1-725.

[6] Tanaka. Kawm and Associates, Thermal Designing on a Small-Sized AMTEC Cell,

IECEC 2002 paper20021 m

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