Chapter 8 Carnot, Rankine, Brayton, and Stirling Cycle Generators

For more powerful Space Power Systems, we can learn from Terrestrial systems, namely,
Brayton, Rankine, and Stirling. These all are limited in efficiency by the Carnot cycle,
T
ηC = 1− C
TH

Carnot Cycle

Figure 8.1 shows the ideal or conceptual Carnot cycle engine.

Turbine

W out

Heat
Heat in
Exchanger
T hot Heat Exchanger

Heat out
T low
W in

Pump
Figure 8.1 Carnot cycle (conceptual)

Heat is added to the Heat Exchanger which heats the working fluid and increases its
pressure. The hot fluid moves into the turbine, rotating it, which generates the Work (out) and
decreases the pressure. The fluid goes to the second Heat Exchanger and is cooled. It then goes
to a pump, which increases the fluids pressure and send it back to the heat exchanger.

Rankine Cycle

Figure 8.2 shows the Rankine cycle.

Turbine Generator
Steam
Electrical
Power Out
W out
Steam
Heat in Boiler

T hot

Condenser

Heat out
T low
W in
Water
Water

Pump
Figure 8.2 Rankine Cycle (ideal)

This is the most common method on Earth to produce power from heat. It is used by
power companies and ships at sea. Water is heated in a boiler and turned into steam. The steam
is very dry, that is, it is molecular water. It is not clumps of many water molecules, it is H2O in
single molecules. If the steam were in droplet form when it was put into the turbines, the energy
contained in the drops would tear the turbine blades off the turbine shaft. This would cause the
destruction of the turbine. The steam hits the turbine blades and turn the turbine. The turbines
are connected to a generator, which turns producing electricity. The steam is then condensed
back into water in the Condenser, and then sent back into the boiler via the water pump. How
does the condenser condense the steam back into water? Usually in terrestrial applications a
river or a stream supplies cooling water. On a ship, the ocean supplies cooling water. In space
the condenser would be a radiator as well, radiating the heat into space and making the steam
condense into water. There are ways to make this cycle more efficient using reheat, regeneration,
and superheat, among others, but we won’t get into these.

As an example of the temperatures and pressures involved, the author was on an aircraft
carrier that had boilers. The steam leaving the boilers was at 363 C, 686 F at 41.37 Bar, 600 Psi.
That means that the water pump returning the water to the boiler must pressurize to greater than
600 Psi or the water won’t go back into the boiler. In order to reach these pressures, the pump
may be driven by steam (more efficient), or may be electric (emergency backup). All of this
lowers the overall efficiency. There are some problems in practical systems that complicate the
engineering design. No system is completely closed, so water must be replaced. A problem is
that air can be introduced into the system. Both of these are not good for a Space System, and
must be greatly reduced in the design, thus driving up cost of design and testing. The oxygen
component of air at these temperatures and pressures can severely corrode the pipes in the boiler.
So de-aerating systems are used to remove the air from the water before it enters the boiler (not
pictured). Finally the water level in the boiler must be maintained at just the right level. Too low
a level and the pipes in the boiler are not cooled by the water and melt, causing an explosion.
Too high a level and moisture is carried over into the turbine causing turbine blades to fly
everywhere. Both can ruin the system and make a space power design difficult. This is why
Rankine systems with water are not designed for space power systems.

Brayton Cycle

Figure 8.3 shows the Brayton cycle.

Heat in

T hot Heat
Exchanger

Turbine
Generator

Compressor Electrical
Shaft Power Out
W in W out

Radiator

Heat out
T low

Figure 8.3 Brayton Cycle (ideal)

This is similar to Rankine except a gas is heated, sent through a turbine which turns a
generator as is done in a Rankine Cycle. The gas then goes to a radiator to cool, then to a
compressor (powered by a shaft hooked to the turbine) which builds up enough pressure to send
it back into the Heat exchanger. Again, to make it more efficient you can have regeneration,
reheat, inter-cooling or other methods. The advantage in using this in Space Power systems is
that there is no phase change. The problems with having a liquid impinge upon turbines and
tearing the turbine blades off the turbine shaft are eliminated. Also, water has a property that
when it freezes it expands. Expanding water breaks pipes on Earth, and breaking pipes in outer
space would be disastrous. (On the Space Station, electric heaters are used to ensure that the
water side of the cooling loop never freezes). Using a gas eliminates this problem as well.
Typical gases that are used are inert, such as He or Ne or mixtures of the two to avoid corrosion
of the pipes and other components.

Stirling Cycle

Stirling is really weird, it uses pistons to move up and down. No rotation, like the above
systems, more like the up and down motion of an automobile engine being converted to rotation.
However, there is much research on using free piston Stirling engines for space power. They
usually use two pistons on opposite sides to reduce vibration. Here is a slide from the Glenn
Research Center at NASA that shows the cycle of the free piston Stirling. These generators are
very likely to be used in space.

Figure 8.4 Stirling Generator (Courtesy NASA) http://www.grc.nasa.gov/WWW/tmsb/stirling/

The piston moving back and forth would be connected to a generator to produce power. To
eliminate vibration, most designs have two generators connected together as shown below. The
advantage over Brayton and Rankine is that there are no turbines, compressors, or pumps. The
disadvantage is that it still has moving parts. Lubrication in all three systems is an engineering
problem to be solved.
 Figure 8.5 Dual Stirling Generator (Courtesy NASA)

Efficiencies

As we said in the first chapter, the efficiencies of converting heat into electrical power for
these systems are roughly 30%. This is a sizable efficiency and is why for the Space Station
there have been proposals for a Solar Brayton Cycle generator. More on that in the Space Station
chapter.

DIPS (Dynamic Isotope Power Systems)

Dynamic Isotope Power Systems, which is basically an easy concept. For power of up to
about 1 Kwe, you can use a radioisotope to power a Rankine, Brayton, or Stirling device. This
obviously has the advantage of 3 times the efficiency (roughly) of a TEG. NASA is testing
Stirling power systems for use in DIPS at the Glenn Research Center.
Temperature Control

Temperature control is a textbook all by itself. Temperature control on any Spacecraft, be
that a satellite, rover on a planet, or the Space Station, is a very complex topic. The biggest thing
to note is that Space is neither hot nor cold. In Earth orbit, the spacecraft is really hot when
exposed to the Sun, and really cold when in the Earth’s shadow. Every electrical device on the
spacecraft produces heat. Power in equals power out plus heat out. When designing Space
Power Systems, heat producing devices (electronics, power converters, power supplies, pumps,
etc.) must be cooled. Also, keeping the Sun’s heat out is important. If you send a spacecraft to
Mercury, your big problem is keeping the Sun’s heat out, while cooling your heat generators. If
you send a craft to Pluto, your problem is to keep the heat in to keep electronics, valves, fuel, etc.
within normal operating limits. That’s why the Galileo probe and the Mars Spirit and
Opportunity rovers had Radioisotope Heating Units (RHU’s). RHUs are small devices (2.54 cm
by 3.3 cm--1 by 1.3 inches) that supply one Watt of heat (thermal). They use the heat of
radioactive decay of Plutonium Dioxide to provide the heat and only weigh about 40 grams (1.4
ounces). The Plutonium is about the size of a pencil eraser.

Figure 8.6 RHU (Courtesy the Department of Energy (DOE))

Heat Pipes

Heat pipes can transfer a great deal of thermal energy from one point to another.

Figure 8.8 shows a heat pipe.

Wick

T hot T cold
Boiler section Condenser
section

Figure 8.8 Heat Pipe

Here's how it works, Heat enters the boiler section, the working fluid evaporates and
travels to the condenser, where it gives up its latent heat. In the wick, surface tension forces
return the condensate back to the evaporator section through capillary channels. Using the latent
heat of evaporation/condensation is extremely efficient--90% or so. Heat pipes can transfer up to
500 times as much heat per unit weight as can a solid thermal conductor of the same cross
section. In theory, the heat pipe operates almost at a single temperature, in reality a small
temperature gradient exists, because a small vapor pressure gradient is generated between the
boiler and condenser sections. They can be designed to operate with a temperature difference of
only a few degrees. In practice, 5 kW thermal have been driven down a 2 ft. molybdenum heat
pipe at 1700 K with a temperature drop of 6 K. The heat pipes don’t have to be cylindrical, they
can be rectangular, square, they can start on one end as square, and the other end as rectangular.
Whatever is needed to fit the device to be cooled. They do not have to be straight, they can curve
around corners. You will see heat pipes all over the place, several experiments have flown on the
shuttle, a candy company uses them to make candy canes more efficiently, and home air
conditioners are being built with heat pipes to be more efficient. On the Space Station, the
computers for Node 1 are external and are on the Primary Mating Adapter #1. Both computers
have Heat Pipe Radiators to cool them. On the P6 segment of the Space Station (a solar array
section), the DC to DC Conversion Unit (DDCU) is mounted on a heat pipe radiator as well,
which are exterior to the station, exposed to space. Multiple pipes are used for meteor
protection--if one pipe is hit and all the fluid leaks out, other pipes can still dissipate heat.