Power and Cooling Systems

Overview 

The lander will experience extreme temperatures and pressures while on the surface of Venus. Ambient temperatures will be approximately 500 degrees Celsius, while atmospheric pressures on the surface will be 90 degrees. [1] At these temperatures, conventional silicon semiconductor electronics fail to operate. In order to circumvent this fact the lander will consist of sophisticated insulation and structural materials and utilize active cooling. The key to the lander's design will be two Sterling engines. The first Sterling engine will be powered by a General Purpose Heath Source (GPHS). This engine will be mechanically coupled with a two stage Sterling cooler. The Sterling cooler will lower the temperature within the electronics bay to a manageable 30 degrees Celsius. An alternator will be installed around the Sterling cooler in order to generate electrical power at a rate of 100 W. The following document outlines these systems.

Heat Source

The General Purpose Heat Source (GPHS) will contain radioactive plutonium-238 and will generate temperatures of 1,200 degrees Celsius. [2] The choice of radioactive isotope took into account several factors. These factors were particle emission, half-life, and cost. Plutonium-238 is an alpha emitter, meaning it emits alpha particles (2 protons and 2 neutrons) while not emitting other more harmful radiation particles. It has a half-life of 87.7 years which allows small pellets to emit energy for a long period of time. Other isotopes have considerably shorter half-lives. For example, polonium-210 has a half-life of only 138 days. The cost of plutonium-238 is very high. The US has stopped producing Pu-238 in 1988, and has since been purchasing pellets from Russia. [3] It will cost an estimated 80 million dollars over 5 years to restart domestic production. [4] The DoE and NASA have jointly requested that US Congress provide funding to restart plutonium production, but so far have been denied three years in a row. Politics and availability aside, Pu-238 stands out as the best option for a fuel source in the GPHS. The GPHS cells are depicted in Figure 1. This housing type has been designed proposed by previous proposed missions to Venus. [2] The power requirements of this mission require us to use approximately 5 lbs of Plutonium 238. 

Figure 1: Exploded Diagram of GPHS [2]

Sterling Engine

The Sterling engine will utilize the temperate differential between the GPHS and the Venusian atmosphere to drive a piston. The Figure 2 shows the PV diagram of a sterling engine. The heat from the GPHS increases the temperature and therefore pressure moving from point 1 to 2. This increase in pressure drives a piston to do mechanical work and moves from point 2 to 3. The cycle is reset as the ambient Venus atmosphere cools the helium gas and causes the chamber to compress back to point 1.

The engine will be a Beta model, or rhombic-drive engine. This engine causes a piston to move up and down. Instead of coupling this piston to a crankshaft the piston will be directly connected to the piston of a two stage Sterling cooler. Both this Sterling engine and the Sterling cooler will use a free-piston design that eliminates wear mechanisms and undergoes non-contact operation. This ensures motion will not limit the life of either system. [5]

Figure 2: Sterling Engine PV Diagram Source

Sterling Cooler

The Sterling cooler works like the Sterling engine in reverse. Instead of using a temperature gradient to perform mechanical work, a Sterling cooler uses mechanical work to produce a temperature gradient. The mechanical work for the Sterling cooler is provided by The PV diagram for the Sterling cooler is seen in Figure 3. Mechanical work expands the helium and moves from 1 to 2 in the diagram. The low pressure helium absorbs heat (2 to 3) from the internal chamber and thereby works to lower the temperature. The system resets as the piston compresses the gas and expels the heat on the hot side of the sterling cooler.

A two stage cooler will be employed in order to reduce the electronics bay temperature down to 30 degrees Celsius. The first cooler absorbs heat from the electronics chamber and passes it to the second cooler. This first cooler works to maintain the inner chamber at a temperature of 30 degrees C. The second cooler passes the heat energy from the first cooler out into the ambient environment. The interface between the two coolers will be at a temperature approximately half way between the 500 degree C ambient and the 30 degree C internal. By breaking the cooling into two stages the power requirements will drop.

Figure 3: Sterling Cooler PV Diagram Source

Lander Electrical Generation

In order to generate electrical energy, a linear alternator will be employed. This alternator will be driven by the Sterling engine. The alternator could employ either a permanent magnet, or an electromagnet in the stator. The difficulty is the outer stator will not be able to be mounted within the cooled electronics vessel. Magnets, such as SmCo magnets, can only operate at below 300 degrees C. Wire insulation for an electromagnet must be able to survive external conditions as well. If the stator was placed in the intermediate zone between the first and second cooling stages, the alternator could expect to experience temperatures between 200 and 300 degrees. This would allow for permanent magnets to be successfully implemented within this range.

Orbiter Power System

The orbiter power system design is trivial compared to the lander. The solar arrays will be composed of two triple junction GaAs panels, with a sum of 5.7 square meters of area. These should be able to generate 800 Watts of power near earth, and 1,100 Watts of power at Venus [8]. For power storage, three lithium-ion batteries will be used. This is the same setup as the Venus Express and Mars Express missions. Thermal control must be present on the orbiter as well. The Venus Express has tried and true methods for this as well. Radiators will be installed on the spacecraft surface that are able to keep the orbiter electronics cool while exposed to sunlight.

[1] Williams, David R. (15 April 2005). "Venus Fact Sheet". NASA. Retrieved 2013-7-13.

[2] Colozza, Anthony J. (August 2012). “Radioisotope Stirling Engine Powered Airship for Low Altitude Operation on Venus.

[3] "Economical Production of Pu - 238: Feasibility Study". Center for Space Nuclear Research. Retrieved 20 July 2013.

[4] Wall, Mike (6 April 2012). "Plutonium Production May Avert Spacecraft Fuel Shortage". Space.com. Retrieved 20 July 2013.

[5] “The free-piston Stirling cooling system”. Global Cooling BV. Sunpower, Inc. Greenpeace Netherlands. August 21 1995.

[6] Dyson, Rodger W.; Bruder, Geoffery A. “Progress Towards the Development of a Long-Lived Venus Lander Duplex System”. NASA Glenn Research Center Thermal Energy Conversion Branch. 8th AIAA EICEC Session 126-APS-4. July 27, 2010.

[7] “Satellite Power Systems” European Space Agency Technology Programmes. May 2003. http://www.esa.int/esapub/br/br202/br202.pdf

[8] “About Venus Express” European Space Agency. Accessed July 25, 2013.  http://www.esa.int/Our_Activities/Space_Science/Venus_Express/The_spacecraft