Space P

RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%. Thermoelectric materials in space missions to date have included silicon–germanium alloys, lead telluride and tellurides of antimony, germanium and silver (TAGS). Studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.

A thermionic converter—an energy conversion device which relies on the principle of thermionic emission—can achieve efficiencies between 10–20%, but requires higher temperatures than those at which standard RTGs run. Some prototype ²¹⁰Po RTGs have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these unfeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes.

Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light emitted by a hot surface rather than visible light into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric modules (TEMs) and can be overlaid on top of themselves, potentially doubling efficiency. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%,^[25] but have not yet been tested with radioisotopes. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells and silicon TEMs degrade faster than metal TEMs, especially in the presence of ionizing radiation.

"Energielücke (energy gap)" zum nächsten erlaubten Leitungsband so groß, dass Valenzelektron keine Energie aus einem angeregten Feld aufnehmen kann, um sie zu überwinden. Z.B. Diamant: Eg = 5,3 eV Þ Bei Raumtemperatur kein Strom.

"Einbau" von Fremdatomen in Halbleiterkristall

5-wertige Fremdatome in 4-wertigen Halbleiter (z.B. As, Sb in Si, Ge) Þ geben leicht "überzähliges" Elektron an das Leitungsband des Halbleiters ab (Donatoren Þ n-Typ).

führt zu frei beweglichen Elektronen im Leitungsband und zu positiven unbeweglichen Donatorionen Þ Überschusshalbleitung

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3-wertige Fremdatome in 4-wertigen Halbleiter (z.B. B, Ga in Si, Ge) Þ nehmen leicht ein Elektron aus dem Valenzband auf (Akzeptoren Þ p-Typ) Þ bewegliche Löcher im Valenzband (und unbewegliche, einfach negativ geladene Akzeptorionen) Þ Mangelhalbleitung

Reagierende Stoffe sind chemisch an die Elektrode gebunden

 d.h. die Elektrode wird verbraucht Ø Batterien

Reagierender Stoff wird zur Elektrode geführt, d.h. kein Abbau der Elektrode

Brennstoffzellen

110 kW ISS

50 kW Payload

general PL~0.45 + Power_total

ca 4-5 W array power for 1 kg

Primary Batteries

Solar

RTG

Fuel Cells

Nuclear

Solar Dynamic

Secondary

Batteries

Super Caps

Reversible Fuel Cells

Regenerative Fuel Cells

EnergyWheels

-Battery charge control

-Battery discharge control

- Voltage regulator

Regulate the bus voltage to the required voltage level

- Regulate the power output (voltage and current) from solar cells and energy storage systems

- Regulate the voltages during charging of energy storage systems

Provide direct current (DC)or alternating current (AC)

DC

Low power demands

Low transmission distances

AC

High power demands

High transmission distances

Batteries, fuel cells, solar cellsproduce direct current(DC)

PPT - explanation

When a load is directly connected to the solar panel, the operating point of the panel will rarely be at peak power. The impedance seen by the panel determines the operating point of the solar panel. Thus by varying the impedance seen by the panel, the operating point can be moved towards peak power point. Since panels are DC devices, DC-DC converters must be utilized to transform the impedance of one circuit (source) to the other circuit (load). Changing the duty ratio of the DC-DC converter results in an impedance change as seen by the panel. At a particular impedance (i.e. duty ratio) the operating point will be at the peak power transfer point. The I-V curve of the panel can vary considerably with variation in atmospheric conditions such as irradiance and temperature. Therefore, it is not feasible to fix the duty ratio with such dynamically changing operating conditions.

3000 m² - 250 kW

Power <1kW

+

-Independence of power production from S/C orientation

-Independence of the distance to the Sun -> deep space missions are possible

-Suitable for mission with long eclipse period (lunar landers, etc.)

-No storage required for shadow eclipse

-

-Adversely effect the radiation environment of the S/C -> RTG needs to be deployed on a boom away from the S/C bus

-Careful handling procedures during S/C integration (radioactive source!)

-Potential of dispersing in the atmosphere in case of launch failure

“The Safety analysis report prepared for the US energy department came to the conclusion that the risks are extremely small and well understood. It‘s much more likely someone will be injured driving a car to protest a RTG launch than in an RTG dispersion accident.”