Putting the heat in power
17 March 2016 Satellites need electrical power. Fortunately in space, the Sun provides a readily available source of energy. However, the Sun's intensity is not the same everywhere. Beyond Jupiter, the intensity decreases to a level such that it becomes highly impractical to generate sufficient power. Near the Sun however things are also complicated as the high intensity easily heats up a satellite to several hundreds of degrees. This has been one of the challenging requirements for ESA's upcoming Solar Orbiter mission.
However, this also raises an interesting question. As the incoming intensity increases and the spacecraft heats up, could we use this power to our advantage? Actually, generating electricity from heat directly is already dong using thermionic energy conversion (used in radioisotope generators). A practical TEC however requires several thousands of degrees to operate efficiently. A temperature way beyond satellite operating conditions. "A near-Sun mission around 0.3AU would experience temperatures above 1000 degrees Celsius", said Thijs Versloot, energy researcher at the ACT, "which places it right between photovoltaic and thermionic systems. Definitely a challenging window for producing a working device."
Based on recent academic research performed Schwede et ala new concept was explored in a GSP Ariadna study on 'photon-enhanced thermionic emission' (PETE). Combining a thermionic with a photovoltaic systems could theoretically offer similar efficiencies, but now with the benefit that it can cope with much higher temperatures. "It would be ideal for close Sun missions", according to Thijs Versloot.
Together with the Instituto de Microelectronica de Madrid (IMM) and the Instituto Italiano di Technologia (IIT), the ACT investigated the feasibility of using the PETE for a near Sun mission. IMM focused on the photon absorption and electron transport in the cathode, while IIT developed a 3D model of the cathode-anode interface using the calculated cathode electron emission as input parameter. "We had to overcome multiple complications in combining these different types of devices", said Thijs Versloot, "but in the end we managed to grasp all the physical processes to get a working model of the device."
Using a combination of nanostructuring to enhance light-trapping and different gate voltages and geometries to identify the optimal structure, the operational power could maximized. One of the main parameters influencing the efficiency turned out to be the vacuum gap between the electrodes. "But interestingly, the device could also be operated isothermally, meaning without a temperature difference between both electrodes", said Thijs Versloot. "This could significantly reduce the complexity of the design, thus providing an interesting angle for future study."