At the APEC 2022 Plenary Session, John H. Scott, Principal Technologist, Power and Energy Storage, NASA Space Technology Mission Directorate presented a very interesting topic: 'On the Moon to Stay', covering the various aspects of power electronics that would be required to make that statement feasible. Space exploration has not only been a dream and a source of imagination, but also an amazing research area seeking to break 'unbreakable' limits, and in the processing providing benefits to many applications we are now using daily on planet Earth.
Figure 01 - O’Neill Cylinder interior - Painting by Rick Guidice (Source: PRBX/NASA)
Taking humans first to the moon, later to Mars and who knows where to next, is far from being an easy job; making life possible and sustainable in such hostile environments is much more than just 'a challenge'. One example is how to feed the space explorers when they are so remote from planet Earth? When considering Mars, it would take 210 days and a significant cost and risk for a re-supply rocket to arrive, which is clearly not an optimum solution. Space farming has been part of that dream and we all remember the O'Neill Cylinder, designed by Princeton physicist Gerard K. O'Neill who published in 1974 an article in Physic Today: 'The Colonization of Space'. O'Neill's article and research fueled a number of sci-fi movies showing the huge rotating cylinder, hosting farms and lit by an artificial sun (Figure 01). We are not there yet, but on that basis the first humans to inhabit Mars may be considered farmers more so than astronauts! So how will power electronics contribute to make the dream a reality?
From Earth indoor farming to Space: Feeding 10 billion people on Earth
Let's start by looking at Earth. If we consider the latest estimation, Earth's population is expected to reach 10 billion by 2050. Simultaneously we are facing climate changes that could impact the complete food ecosystem and require significant modifications to the ways in which we produce and consume food.
Considering all the parameters and requirement to produce food with the highest respect for the environment, in 1999, Dr. Dickson Despommier with his students developed the idea of modern indoor farming, revitalizing the terms coined in 1915 by the American geologist Gilbert Ellis Bailey: "Vertical farming." We have all heard about it and even read articles about industrial buildings that were converted into vertical farms, but from the early days using fluorescent or halogen lighting to Solid State Lighting (SSL), there have been an amazing number of technological innovations that contribute to the effort to optimize the energy delivered to the plants for optimal growth. With these advances, the benefits of indoor farming multiplies. If we consider space utilization, 100 times more food could be produced per square meter compared to traditional agriculture, reducing water utilization by 90% and hazardous chemicals to none. Indoor farming is very attractive but to be really efficient such agriculture requires a very efficient lighting system (Figure 02).
Figure 02 - Solid State Lighting to grow vegetables in indoor farming (source PRBX / asharkyu-Shutterstock)
Not all vegetables can grow with limited soil and nutrition by impregnation but for the ones compatible with this farming method, the results are impressive and can be further improved by using modern lighting technologies that are computer-controlled. This is very interesting area which for power designers to explore, combining advanced power electronics and modern agriculture, while keeping software in mind.
Since its introduction, indoor farming engineers conducted research to determine the spectrum and energy required by different plants to grow efficiently. From wide spectrum fluorescent or halogen lamps to narrower spectrum, the conventional lighting industry innovated a lot but these technologies are not flexible nor efficient enough to respond to the demand.
Figure 03 - The light spectrum to grow plants and vegetables typically starts at 450 nm (blue light) and goes through 730 nm (far red) (source PRBX)
In Japan from 2005-2008, agronomical researchers experimented with different lighting methods to adjust spectrum and energy to specific plants. Researchers concluded that the optimal light spectrum to grow plants and vegetables typically starts at 450 nm (blue light) and goes through 730 nm (far red) (Figure 03). 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 (Figure 04).
Figure 04 - Light energy required ranges from 50 micromoles (µmol) for mushrooms up to 2.000 micromoles for light intensive plants (source PRBX)
Agricultural experts advise that for optimal results different plants require different light spectra as well as differing light balances and intensities at different stages of growth, from seedling through harvest. This often results in the need for the artificial light to have a number of different spectra channels that are individually adjustable for intensity. Some crop growing practices combine different sources of lighting, including the use of UV flashes to prevent the development of parasites, requiring a power supply able to switch from constant voltage to constant current within a range from almost zero to the maximum (Figure 05). This specification for a power supply is very much what will be required for Space Farming, in addition to a power electronics architecture able to combat the effects of space radiation.
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, requiring regular resupply deliveries. Now, while it is feasible for the ISS to be resupplied by cargo spacecraft, clearly it would be much more complicated and expensive to use this method on Mars, which is at an average distance of 220 million km (140 million miles) and more than 200 days traveling.
Figure 05 – COSEL power supply with multi-modes for voltage or current constant from max to near zero (Source PRBX/COSEL)
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 for autonomous space-farming. After a series of experiments which took into consideration the full development cycle, the variety of plants that were selected for further research included lettuces, mustard varieties, and radishes. These crops were first grown in a controlled lab on Earth, then in the ISS to study how plants are affected by the micro-gravity and other factors (Figure 06).
Figure 06 - NASA astronaut Peggy Whitson looks at the Advanced Astroculture Soybean plant growth experiment (Source: PRBX/NASA)(Source PRBX/NASA)
The 'Veggie' project included a large number of experimental factors such as "Pick-and-Eat Salad-Crop Productivity, Nutritional Value, and Acceptability to Supplement the ISS Food System (Veg-04A)" and included 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 including '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 (Figure 07).
https://www.psma.com/HTML/newsletter/pics/prbxa_047_figure_07_space-far…" width="90%" />Figure 07 – Examples of Kennedy Space Center 8KSC) prior, current and future space crop production platforms selected and designed to lead to crop production units destined for the Moon or Mars (Source PRBX/NASA)
Considering the different varieties of plants that will be grown, and 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 must also be energy efficient and small in size. That's in addition to specific constraints related to space, including 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. For all applications, from low orbit satellites to out-of-space exploration, power supplies have been developed with very advanced technologies to make them smaller and energy efficient.
Wide Band gap (WBG) 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. The recent announcement about the newly funded national collaboration led by Penn State to better predict and mitigate radiation-induced damage of WBG semiconductors is 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 and their contribution to the next step.
In parallel, the semiconductor industry is moving forward. 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 08).
Figure 08 – Efficient Power Conversion (EPC) ruggedized GaN FET for space applications and DC/DC converter (Source: PRBX/EPC)
Although one of the biggest challenges to in-spaceship farming is sourcing enough water and nutrients and then cycling them as efficiently as possible, there are many other obstacles we don't have to grapple with on Earth that will need to be considered too, such as cosmic radiation, lack of an atmosphere, and low levels of light. From the 2015 'Growing Beyond Earth' project to current advances in 2022, a lot of progress has been made, contributing to a better understanding of space farming, as well as in power electronics. We are in the early stages of a whole new era in which Wide Bandgap semiconductors in power electronics will play an important role.
Exciting time for power designers, isn't it?!
The Colonization of Space – Gerard K. O'Neill, Physics Today, 1974
Fairchild Botanical Garden
NASA / RADEC 2018
Jean-Marie Lauenstein – NASA GSFC, Greenbelt, MD, USA
Wide-Bandgap Semiconductors in Space: Appreciating the Benefits but Understanding the Risks
Frontiers in Astronomy and Space Science
Large-Scale Crop Production for the Moon and Mars: Current Gaps and Future Perspectives
Published 04 February 2022 / doi: 10.3389/fspas.2021.733944
Efficient Power Conversion (EPC)
Applied Power Electronics Conference (APEC)
Patrick Le Fèvre
Chief Marketing and Communications Officer, Powerbox