NASA Technical Reports Server (NTRS) 19930007720: Thermal management in space

13 Pages·0.55 MB·English

by  NASA Technical Reports Server (NTRS)

N93-1 909
Thermal Management systems, as well as the capability
of existing technologies to
in Space
dissipate this heat into the airless
Abe Hertzberg environment of space.
The vehicles and habitats associated It should be pointed out that in a
with space industrialization and vacuum environment, convection is
the exploitation of nonterrestrial no longer available and the only
resources will inevitably require mechanism of rejecting heat is
energy systems far exceeding the radiation. Radiation follows the
current requirements of scientific Stefan-Boltzmann Law
and exploratory missions. Because
of the extended duration of E =oT 4
these missions, it is not possible where
to consider systems involving
E = the energy rejected
expendables such as non- 0, the Stefan-Boltzmann constant,
regeneratable fuel cells. Therefore, = 5.67 W m-2 K-4
these missions become hostages to T = the temperature at which the
the capability of continuous-power heat is radiated
energy systems. These systems
will need to provide hundreds of That is, the total amount of heat
kilowatts to tens of megawatts radiated is proportional to the
of electrical power to a product surface area of the radiator.
fabrication system, whether it uses And the lower the radiation
terrestrial or nonterrestrial raw temperature, the larger the radiator
materials. area (and thus the radiator mass,
for a given design) must be.
Because the power system will be
located in an essentially airless The radiator can only reject heat
environment, rejecting waste heat when the temperature is higher
becomes a limiting aspect of it. In than that of the environment. In
the following paragraphs, Iwill space, the optimum radiation
review space-based or asteroidal efficiency is gained by aiming the
and lunar based power generating radiator at free space. Radiating
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:!
4
toward an illuminatedsurface is reactors adapted for the space
less effective, and the radiator environment appear to offer a
must be shielded from direct logical alternative. In this paper,
sunlight. I deal only with the burdens these
three types of power system will
The rejection of heat at !ow place on the heat management
temperatures, such as would be system.
the case in environmental Control
and in the thermal management of Solar photovoltaics themselves will
amaterials processing unit, iS not burden the power generating
particularly difficult. Therefore, the system with adirect heat rejection
design and operation of the heat requirement, since the low energy
rejection system is crucial for an density of the system requires
efficient space-based energy such a great collection area that it
system. allows rejection of waste radiant
energy. However, if these
Space-Based Power systems are to be employed inlow
Generating Systems Earth orbit or on a nonterrestrial
surface, then a large amount of |
l=
Ina previous paper, space-based energy storage equipment will be |
power generating systems have required to ensure a Continuous
been described in detail. Solar supply of power (as the devices do
photovoltaic systems have a not collect energy at night). And
generating capability of up to the round-trip inefficiencies of even
several hundred kilowatts. The the best energy storage system
power output range of solar today will require that a large
thermal systems is expected to be fraction--perhaps 25 percent--of
one hundred to perhaps several the electrical power generated
hundred kilowatts. While in must be dissipated as waste heat
principle these power systems can and at low temperatures.
be expanded into the megawatt
region, the prohibitive demands Solar thermal systems, which
for collection area and lift capacity include a solar concentrator and
would appear to rule out such adynamic energy conversion
expansion. Megawatt and system, are presumed to operate
multimegawatt nuclear power at relatively high temperatures
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(between 1000 and 2000 K). The In all of these systems the output
efficiencies of the energy power used by the production
conversion system will lie in the system in environmental control
range of 15 to perhaps 30 percent. and manufacturing (except for a
Therefore we must consider small fraction which might be
rejecting between 70 and stored as endothermic heat in the
85 percent of the energy collected. manufactured product) will have to
In general, the lower the thermal be rejected at temperatures
efficiency, the higher the rejection approaching 300 K.
temperature and the smaller the
radiating area required. As with Ithink it fair to state that, in many
solar photovoltaic systems, the of the sketches of space industrial
inefficiencies of the energy storage plants I have seen, the power
system will have to be faced by the system is little more than a
heat rejection system, unless high cartoon because it lacks sufficient
temperature thermal storage is detail to address the problem of
elected. thermal management. We must
learn to maintain an acceptable
The current concepts for nuclear thermal environment, because it is
power generating systems involve expected to become a dominant
reactors working with relatively low- engineering consideration in a
efficiency energy conversion complex factory and habitat
systems which reject virtually all of infrastructure.
the usable heat of the reactor but
at a relatively high temperature. As an example of the severity of
Despite the burdens that this low this problem, let us examine the
efficiency places on nuclear fuel case of a simple nuclear power
use, the energy density of nuclear plant whose energy conversion
systems is so high that the fuel use efficiency from thermal to electric
factor is not expected to be is approximately 10 percent. The
significant. plant is to generate 100 kW of
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usefulelectricity.Thereactor 300K).Assuminganevenbetter,
operatesatapproximate8ly00K, aluminumradiatoorfabout5 kg/m2,
andaradiatorwithemissivity withagainanemissivityof0.85,in
equalto0.85wouldweighabout thiscasewefindthattheareaof
10kg/m2.Thethermapl owerto thelowtemperaturheeatrejection
bedissipatedfromthereactor componenist 256m2,withamass
wouldbeabout1MW.Fromthe approachin1g300kg.*Therefore,
Stefan-BoltzmaLnanw,theareaof wecanseethatthedominanhteat
theradiatowr ouldbeabout50m2 rejectionproblemisnotthatof
andthemassapproximately theprimarypowerplantbutthat
500kg. Thisseemsquite oftheenergythatisusedinlife
reasonable. supporatndmanufacturinwg,hich
mustberejectedatlow
Howeverw,emustassumethat temperaturesU. singthewaste
theelectricitygeneratebdythe heatfromthenuclearpowerplant
powerplant,whichgoesintolife forprocessingmaybeeffective.
supporstystemsandsmall-scale Butlironicallyd,oingsowillin
manufacturinwg,ouldeventually turnrequiremoreradiatorsurface
havetobedissipatedalso,butat toradiatethelowertemperature
amuchlowertemperatur(earound wasteheat.
"Using the Stefan-Boltzmann Law,
E1 = 5.67 x 10-8W m-2 K-4(800 K)4
= 5.67 x 10-Swm -2 K-4x4096x 108K 4
= 5.67 W m-2x4,10 x103
E1 = 23.3 kW m-2
900 kW + 23.3 kW m2 = 38.6 m2
and 38.6 m2 ÷ 0.85 = 45.4 m2
E2 = 5.67 x t0 .8W m-z K-4(300 K)4
= 5.67x 10.8Wm 2 K-4x81 x 108 K4
= 5.67Wm -2x81
E2 =459 W m-2
100 kW - 459 W m-2 = 0.2179 x 103 mz = 218 m2
and 218 m2 + 0.85 = 256 m2
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Heat Rejection Systems For the convenience of the
reader, Iwill briefly describe the
In this section Iwill deal with operational mechanism of the
systems designed to meet the heat basic heat pipe. (See figure 36.)
rejection requirements of power The heat pipe is a thin, hollow
generation and utilization. These tube filled with a fluid specific to
heat rejection systems may be the temperature range at which it
broadly classified as passive or is to operate. At the hot end, the
active, armored or unarmored. fluid is in the vapor phase and
Each is expected to play a role in attempts to fill the tube, passing
future space systems. through the tube toward the cold
end, where it gradually condenses
Heat pipes: The first of these, into the liquid phase. The walls of
called the "heat pipe," is the tube, or appropriate channels
conventionally considered the base grooved into the tube, are filled
system against which all others are with a wick-like material which
judged. It has the significant returns the fluid by surface
advantage of being completely tension to the hot end, where it is
passive, with no moving parts, revaporized and recirculated.
which makes it exceptionally
suitable for use in the space
environment.
Figure 36
Components and Principle of
Operation of aConventional Heat Pipe
Aconventionalheatpipe consistsofa
Container /--Wick /_ Liquid /--Vapor
/ sealed container with aworking fluid, a
passageway for vapor, and acapillary
wick for fiquid transport. During
operation, theheat pipe is exposed to
external heat at one end (the evaporator
IHeitiiput[ /, / / ]Heitoitpui section). Thisheat causes theworking
fluid in thecapillary wick to vaporize,
removing heat equal to theheat of
vaporization of the fluid, Thevapor is
forced down the center of thepipe by
pressure from the newly forming vapor.
I
Whenthe vapor reaches thecool end of
thepipe (the condenser section), it
condenses to afiquid. Thefiquid soaks
into thecapillary wick, through which it
travels back to theevaporator section. As
the fluid condenses, itgives up the heat
sEVcatPo°nrat,°r, Aedic_ibatic ,, C°c_idoensen.,._r of vaporization, which is then conducted
outside the end of thepipe.
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Essentialltyhesystemisasmall forthermaml anagemenintspace
vaporcyclewhichusesthe systems.Forexamplea,tmodest
temperaturdeifferencebetween temperaturesth,eheatpipecould
thehotandcoldendsofthetube bemadeofaluminumb,ecauseof
asapumptotransporhteat, its relatively low density and high
takingfulladvantagoeftheheatof strength. Fins could be added to
vaporizatioonftheparticulafrluid. the heat pipe to increase its heat
dissipation area. The aluminum, in
Thefluidmustbecarefully order to be useful, must be thin
selectedtomatchthetemperature
enough to reduce the mass carried
rangeofoperation.Forexample, into space yet thick enough to
atveryhightemperatureas offer reasonable resistance to
metallicsubstancewitharelatively meteoroid strikes.
highvaporlzatiotnemperature,
suchassodiumorpotassium, A very carefully designed solid
maybeused.Howeverth, is surface radiator made out of
choiceputsaconstrainotnthe aluminum has the following
lowtemperatureendsince,ifthe
capabilities in principle: The
fluidfreezesintoasolidatthelow
mass is approximately 5 kg/m2
temperatureend,operationwould with an emissivity of 0.85; the
ceaseuntiltherelativelyinefficient usable temperature range is
conductioonfheatalongthewalls
limited by the softening point
couldmeltit. Atlowtemperatures
of aluminum (about 700 K). At
afluidwithalowvaporization
higher temperatures, where
temperatures,uchasammonia, refractory metals are needed,
!
mightwellbeused,withsimilar
it would be necessary to multiply
constraintsT. hetemperaturmeay the mass of the radiator per
notbesohighastodissociatethe
square meter by at least a factor
ammoniaatthehotendorsolow
of 3. Nevertheless, from 700 K
astofreezetheammoniaatthe
up to perhaps 900 K, the heat
coldend.
pipe radiator is still a very
efficient method of rejecting
Withproperdesign,heatpipesare heat.
anappropriataendconvenientotol
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Afurtheradvantagiesthateach
Pump loop systems have a unique
heatpipeunitisaself-contained
advantage in that the thermal
machine.Thus,thepunctureof
control system can easily be
oneunitdoesnotconstitute
a integrated into a spacecraft or
single-point failure that would affect space factory. The heat is
the performance of the whole picked up by conventional heat
system. Failures tend to be slow exchangers within the spacecraft,
and graceful, provided sufficient the carrier fluid is pumped through
redundancy. a complex system of pipes
(extended by fins when deemed
Pump loop system: The pump effective), and finally the carrier is
loop system has many of the same returned in liquid phase through the
advantages and is bounded by spacecraft. In the case of the
many of the same limitations Shuttle, where the missions are
associated with the heat pipe short, additional thermal control is
radiator. Here heat is collected obtained by deliberately dumping
through a system of fluid fluid.
loops and pumped into a radiator
system similar to conventional Since the system is designed to
radiators used on Earth. It should operate at low temperatures, a low
be pointed out that in the Earth density fluid, such as ammonia,
environment the radiator actually may on occasion, depending on
radiates very little heat; it is heat loading, undergo a phase
designed to convect its heat. The change. Boiling heat transfer in a
best known examples of the pump low gravity environment is a
loop system currently used in complex phenomenon, which is not
space are the heat rejection well understood at the present
radiators used in the Shuttle. time. Because the system is
These are the inner structure of the subjected to meteoroid impact, the
clamshell doors which are deployed basic primary pump loops must be
when the doors are opened strongly protected.
(fig. 37).
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Right-hand
aft panel --_
Right-hand \
mid-aft \
panel
Figure 37 Rmiigdh-fto-hrwanadrd i
panel
Right+hand
Pump Loop Radiators on the Space forward
Shuttle Payload Bay Doors panel
a. The space radiators, which consist of
two deployable and two fixed panels on
each payload bay door, are designed to
reject waste heat during ascent (doors
closed) and in orbit (doors open). Each
panel contains parallel tubes through
which the Freon in the heat loops can
pass, bringing waste heat from other • Aftpanels are removable.
parts of the orbiter. The total length of Cavity
Freon tubing in these panels is 1.5 kin.
Door
t
c.......
i
b+ The panels have aheat rejection
capacity of 5480 kJ/hr (5400 Btu/hr)
during ascent through the atmosphere
with the doors closed and 23 kJ/hr
i
(21.5 Btu/hr) during orbital operations
with the doors open.
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Z
Despite these drawbacks, pump pipe can be made by clever
loop systems will probably be used modifications to the return wick
in conjunction with heat pipe loop. Looking further downline at
systems as thermal control the problem Of cleployability, people
engineers create a viable space are exploring flexible heat pipes
environment. These armored and using innovative thinking. For
(closed) systems are rather highly example, a recent design has the
developed and amenable to heat pipes collapsing into a sheet
engineering analysis. They have as they are rolled up, the same
already found application on Earth way a toothpaste tube does. Thus,
and in space. A strong technology the whole ensemble may be rolled
base has been built up, and there up into a relatively tight bundle for
exists a rich literature for the storing and deploying. However,
scientist-engineer to draw on in because the thin-walled pipes
deriving new concepts. are relatively fragile and easily
punctured by meteoroids, more
Advanced Radiator Concepts redundancy must be provided.
The same principles, of course,
The very nature of the problems can be applied to a pump loop
just discussed has led to increased system and may be of particular
efforts on the part of the thermal importance when storage limits
management community to must be considered. These are
examine innovative approaches only examples of the various
which offer the potential of approaches taken, and we may
increased performance and, in confidently expect a steady
many cases, relative invulnerability improvement in the capability of
to meteoroid strikes. Although I conventional thermal management
cannot discuss all of these new systems.
approaches, Iwill briefly describe
some of the approaches under The liquid droplet radiator: The
study as examples of the direction basic concept of the liquid droplet
of current thinking. radiator is to replace a solid surface
radiator by a controlled stream of
Improved conventional approaches: droplets. The droplets are sprayed
The continuing search for ways to across a region in which they
improve the performance of heat radiate their heat; then they are
pipes has already shown that recycled to the hotter part of the
significant improvements in the heat system. (See figure 38.)
pumping capacity of the heat
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i
i
i
i
Figure 38
Spacecraft
Two Concepts for a Liquid Droplet
Radiator
In one concept (top), droplets are
generated atthe base of a cone which
contains the source of the waste heat (a
nuclear reactor, for example), and the
molten droplets are sprayed to a six-
i
armed collector array, where they are
caught and then pumped back through a
central pipe to the reactor• Ina somewhat
similar concept (bottom), a deployable
boom has the droplet generator atone Droplet
end and the droplet collector atthe other, generator
with a fluid feed fine between. Here the
droplets are sprayed in a single planar
pattern.
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