Power is Vital to Long-Term Space Exploration

­Visiting the moon, Mars and who knows where next, is far from being an easy job as sustaining life in such hostile environments is much more than just ‘a challenge’. How would we feed the space explorers when they are so far from Earth? In the case of Mars, it would take 210 days for a re-supply rocket to arrive at a huge cost. Clearly not an optimum solution. The first humans to inhabit Mars may be considered farmers rather than astronauts! So how will power electronics contribute?

First on earth

Earth’s population is expected to reach 10 billion by 2050. Simultaneously we are facing climate changes that could affect the complete food ecosystem and require significant modification to the ways in which we produce and consume food.

In 1999, Dr. Dickson Despommier and his students developed the idea of a modern indoor farming, revitalizing the term coined in 1915 by the American geologist Gilbert Ellis Bailey: “Vertical farming”. We have all read articles about industrial buildings converted into vertical farms using fluorescent or halogen lighting to Solid State Lighting (SSL), there are an amazing number of technology innovations contributing to optimize the energy delivered to the plants for optimal growth and benefits of indoor farming multiples. If we consider floorspace utilization, 100 times more food could be produced by square meter compared to regular agriculture, with 90% less water utilization and reducing the use of hazardous chemicals to zero. Indoor farming is very attractive, though to be really efficient such agriculture requires a very effective lighting system (Figure 1).

Not all vegetables can grow with limited soil and nutrition, but for the ones applicable, the results are impressive and getting even more impressive when using modern computer-controlled lighting technologies – a very interesting area for power designers that combines advanced power electronics, software and modern agriculture.

Since its introduction, indoor farming engineers have conducted research to validate the spectrum and energy required by different plants to grow efficiently. From wide spectrum fluorescent or halogen lamps to more narrow spectrum, the conventional lighting industry innovated a lot but those technology are not flexible nor efficient enough to respond to the demand.

Following experimentation in Japan in 2005-2008, agronomical researchers investigated the different lighting methods to adjust the spectrum and energy to specific plants. Researchers concluded that the specific light spectrum to grow plants and vegetables typically starts at 450 nm (blue light) and goes through 730 nm (far red) (Figure 2). The Photosynthetic Photon Flux Density (PPFD) required ranges from 50 micromoles (µmol) for mushrooms up to 2,000 micromoles for plants like tomatoes and some flowers that thrive in full summer light.

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Figure 2: The light spectrum to grow plants and vegetables typically starts at 450 nm (blue light) and goes through 730 nm (far red) (source PRBX)

Agricultural experts tell us that for optimal results, different plants types may require different light spectra as well as differing light balance and intensities between the seedling to harvesting stages. This often results in a requirement for the artificial light to have a number of different spectra channels that are individually adjustable for intensity. Some crop growing processes combine different sources of lighting, including the use of UV flashes to prevent the development of parasites. This requires a power supply able to switch from constant voltage to constant current within a range from almost zero to the maximum (Figure 3). This specification for a power supply is very much what will be required for Space Farming, in addition to a power electronics architecture that is able to combat the effects of space radiation.

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Figure 3: COSEL power supply with multi-modes for voltage or current constant from max to near zero (Source PRBX/COSEL)

Bringing Earth farming to space

As NASA plans long-duration missions to the Moon and Mars, a key factor is figuring out how to feed crews during their weeks, months, and even years in space. Food for crews aboard the International Space Station (ISS) is primarily prepackaged on Earth, and requiring regular resupply deliveries. While the ISS is able to be resupplied by cargo spacecraft, it would be much more complicated and expensive to do the same with Mars, which is at an average distance of 220 million km (140 million miles) and more than 200 days traveling.

In 2015, NASA in association with the Fairchild Botanical Gardens in Miami began a project called ‘Growing Beyond Earth’ to define what plants would be suitable from autonomous space-farming. After a series of experiments and taking into consideration the full development cycle, it was decided to grow a variety of plants including lettuces, mustard varieties, and radishes. Firstly in a controlled lab on Earth, then in the ISS to study how plants are affected by the micro-gravity and other factors (Figure 4).

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Figure 4: NASA astronaut Peggy Whitson looks at the Advanced Astroculture Soybean plant growth experiment (Source: PRBX/NASA)

The ‘Veggie’ project included a large number of experimental factors e.g., “Pick-and-Eat Salad-Crop Productivity, Nutritional Value, and Acceptability to Supplement the ISS Food System (Veg-04A)” including research on the optimum lighting conditions to grow plants. On the ISS, two light treatments with different red-to-blue ratios were tested for each set of crops to define light colors, levels, and horticultural best practices to achieve high yields of safe, nutritious leafy greens and tomatoes to supplement a space diet of pre-packaged food, and later for Moon or Mars farming. A number of reports have been released e.g., ‘Large-Scale crop production for Moon and Mars: Current gaps and future perspectives’ published in February in ‘Frontiers in Astronomy and Space Sciences’ summarizing seven years of experimentation on Earth and in the ISS.

Considering the different varieties of plants to grow, the distance and cost, the power supplies for space-farming will have to accommodate different power profiles combining constant current or constant voltage, peak power, and to be energy efficient and small in size. That’s in addition to specific constraints related to space in terms of immunity to radiation, operating temperature, shock and vibration.

The importance of optimizing the payload, the weight and size of everything is a big concern for space applications, and from low orbit satellites to outer-space exploration, power supplies have been developed with very advanced technologies to make them smaller and energy efficient.

Wide-bandgap semiconductors in space applications have formed a part of many research projects, and it’s worth mentioning the report presented by NASA, in 2018, at the (RADECS) conference in Gothenburg: ‘Radiation and its Effects on Components and Systems’. This identified the strengths and weaknesses of WBG when exposed to radiation, and the recent announcement about the newly funded national collaboration led by Penn State to better predict and mitigate radiation-induced damage of WBG semiconductors interesting. The U.S. Department of Defense awarded the team a five-year, $7.5 million Defense Multidisciplinary University Research Initiative Award. This clearly shows the high level of importance of WBG in space applications.

In parallel, the semiconductor industry is moving forwards and one example is the new division and products for space applications launched by Efficient Power Conversion (EPC). For power designers, having access to COTS ruggedized GaN for space applications will reduce the development time and cost when developing power supplies for space applications (Figure 5).

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Figure 5: Efficient Power Conversion (EPC) ruggedized GaN FET for space applications and DC/DC converter (Source: PRBX/EPC)

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