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The electrical power system (EPS) encompasses electrical power generation, storage, and distribution. The EPS is a major, fundamental subsystem, and commonly comprises up to one-third of total spacecraft mass. Power generation technologies include photovoltaic cells, panels and arrays, and radioisotope or other thermonuclear power generators. Power storage typically occurs in batteries; either single-use primary batteries, or rechargeable secondary batteries. Power management and distribution (PMAD) systems facilitate power control to spacecraft loads. PMAD takes a variety of forms and is often custom-designed to meet specific mission requirements. EPS engineers often target a high specific power or power-to-mass ratio (W h kg−1) when selecting power generation and storage technologies to minimize system mass impact.
The author would like to highlight that the presented tables are not intended to be exhaustive but to provide an overview of current state-of-the-art technologies and their development status for this small spacecraft subsystem. There was no intention of mentioning certain companies and omitting others based on their technologies.
State of the Art
Solar power generation is the predominant method of power generation on small spacecraft. As of 2010, approximately 85% of all nanosatellite form factor spacecraft were equipped with solar panels and rechargeable batteries. Limitations to solar cell use include diminished efficacy in deep-space applications, no generation during eclipse periods, degradation over mission lifetime, high surface area, mass, and cost. Photovoltaic cells, or solar cells, are made from thin semiconductor wafers that produce electric current when exposed to light. The light available to a spacecraft solar array, also called solar intensity, varies as the inverse square of the distance from the Sun. The projected surface area of the panels exposed to the Sun also affects generation, and varies as a cosine of the angle between said panel and the Sun. Most cells manufactured today for terrestrial applications are single junction cells, using a single material that is responsive to a particular portion of solar radiation spectrum, where the photon’s energy is higher than the band gap of the cell material.
While single junction cells are cheap to manufacture, they carry a relatively low efficiency, usually less than 20%, and are not included in this report. Modern spacecraft designers favor multi-junction solar cells made from multiple layers of light-absorbing materials that efficiently convert specific wavelength regions of the solar spectrum into energy, thereby using a wider spectrum of solar radiation 1.
The theoretical efficiency limit for an infinite-junction cell is 86.6% in concentrated sunlight 2. . However, in the aerospace industry, triple-junction cells are commonly used due to their high efficiency-to-cost ratio compared to other cells. The Figure below illustrates the available technologies plotted by energy efficiency. This section individually covers small-spacecraft targeted cells, fully-integrated panels, and arrays. Table 3-1 itemizes small-spacecraft solar panel efficiency per the available manufacturers.
|Table 3-1: Solar Cell Efficiency|
|Product||Manufacturer||Efficiency||Solar Cells Used||TRL Status|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||28.3%||SpectroLab UTJ||9|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||29.5%||SpectroLab XTJ||9|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||29.6%||AzurSpace 3G30A||9|
|Solar Panel (5 x 5 cm, 1U, 3U, custom)||DHV||29.6%||AzureSpace 3G30C Advanced||8|
|Solar Panel||Endurosat||29.5%||CESI Solar cells CTJ30||9|
|NanoPower (CubeSat and custom)||GomSpace||29.6%||AzurSpace 3G30A||9|
|HAWK||MMA||29.5-30.7%||SolAero XTJ & Prime||7|
|eHAWK||MMA||29.5-30.7%||SolAero XTJ & Prime||9|
|Space Solar Panel||SpectroLab||26.8%||SolAero ITJ||TRL 9|
|Space Solar Panel||SpectroLab||28.3%||SolAero UTJ||TRL 9|
|Space Solar Panel||SpectroLab||29.5%||SolAero XTJ||TRL 9|
|Space Solar Panel||SpectroLab||30.7%||SolAero XTJ Prime||TRL 6|
AzurSpace offers multi-junction solar cells with efficiencies ranging from 28 – 30%. Cells are built from layered GaInP/GaAs/Ge materials, and several dimensional options exist. These cells are used quite often with other solar arrays for space applications. Their 30% efficiency-class, triple-junction cells have a thickness of 80μm and measure 40 x 80 mm ± 0.1 mm with an average voltage of 2350 mV 3.
SpectroLab offers several solar cells in the 26-30% average efficiency range (XJT Prime, XTJ, and UTJ). The most efficient cells are 29.3% and are available in 26.62 cm2, 59.65 cm2 and customizable sizes. All SpectroLabs’ triple-junction cells have been performance validated in orbit to within ±1.5% of ground test results 4. The XTJ Prime cell energy conversion efficiency is 30.7% and can be delivered in scalable sizes (27cm2 through 84 cm2). The XTJ Prime is built on a heritage upright lattice matched XTJ structure 4The 29.5% XJT solar cells have been GEO qualified; wafers are 140 μm thick. The Ultra Triple Junction cells are LEO and GEO qualified, range from of 27.7 – 28.3% efficient, and are performance validated in orbit to 1% of ground test results. The UTJ devices are rated at TRL 9 for small spacecraft applications 5.
Emcore produces two triple-junction solar cells with 28.5% and 29.5% average efficiency that are available in standard and custom sizes. These second and third generation cells have rich flight heritage; ZTJ cells were flown on NASA’s CYGNUS mission 6 7. The 27.7% triple-junction solar cells with a 0.9 W maximum power point were selected for the 3U Phoenix Cubesat, part of the QB50 mission initiative launched in Spring 2017 8 .
Solar cells manufactured by SolAero range from 28 – 30% average efficiency and have extensive flight heritage on both large and small spacecraft. Their latest Z4J cells produce more end-of-life power than the ZTJ cells for GEO missions and are currently being tested to meet AIAA S-111 2014 Standards, making these cells TRL 5. SolAero also manufactures 27% – 29.5% efficiency solar cells (BJT, ATJ, and ATJM) that are fully space qualified for small spacecraft missions 9.
A collaboration between the Air Force Research Laboratory (AFRL) and SolAero has developed Metamorphic Multi-Junction (IMM) solar cells that have been shown to be less costly with increased power efficiency for military space applications 1. The process for developing IMM cells involves growing them upside down, where reversing the growth substrate and the semiconductor materials allows the materials to bond to the mechanical handle, resulting in more effective use of the solar spectrum 1. This also results in a lighter, more flexible product. While further testing and qualification are underway, initial tests show a single IMM cell can leverage up to 32% of captured sunlight into available energy. These IMM cells are expected to reach space by the end of 2018; they are currently TRL 6 1.
Solar Panels & Arrays
This manufacturer provides 1U/2U/3U and custom size GaAs (Triple junction GaInP/GaInAs/Ge epitaxial structure) solar arrays rated to 28.7 % efficiency. These solar arrays have 36.85 mW/cm2 power-generation capacity in LEO and a PCB thickness of <1.7 mm 10.
Innovative Solutions In Space (ISIS)
ISIS provides high-performance, CubeSat compatible solar panels that come in 1 – 6U sizes, for use on applications up to 24U. Panel mass ranges from 0.05 – 0.3 kg. These solar arrays are compatible with Pumpkin structures and the GomSpace NanoPower EPS. The 3U Cubesat MIST will fly with two ISIS 3U solar panels, expected to launch in 2018 11.
Clyde Space solar panels use 28.3% efficient, Spectrolab Ultra Triple Junction (UTJ) cells, mounted to a printed circuit boards (PCB) of Carbon fiber-reinforced plastic (CFRP) substrate, nominally fitting a 7S1P and 9S2P cell configuration per 3U and 6U panel face, respectively. Their spring-loaded hinges and hold-down/release mechanism have been proven on numerous missions 12.
SSolAero manufactures several triple-junction solar cells. Their COBRA and COBRA-1U are designed for small spacecraft applications and use the aforementioned SolAero advanced-triple-junction 29.5% efficiency cells. The COBRA’s stowed power density for launch is upwards of 30 kWm-3 and claims to be the “lowest mass (>7 gW-1) available in a self-contained, plug-and-play design suitable for all orbital environments” 9. The cubesat-specific COBRA-1U can be used on cubesats 1U-3U in size or larger.
EnduroSat sells a variety of space-qualified solar panels with triple-junction (InGaP/GaAs/Ge) cells rated to 29.8% efficiency. Cell thickness is 150 μm ± 20 μm. They offer 1U/1.5U/3U/6U and customized 3U and 6U solar panels, as well as deployable arrays. The 1U and 3U overall panel masses are 0.04 kg and 0.155 kg, respectively. Maximum cell voltages are 2.33 V per cell 13. They also offer 5 configurations (X/Y, X/Y with Magnetorquer, Z, Z with Magnetorquer, X/Y with RBF) that have a mass range of 0.058 – 0.043 kg. The 1U configuration flew on EnduroSat-1 launched in May of 2018.
DHV Technology fabricates 100 x 100 mm 1U solar panels that weigh 39 g and produce 2.24 W. Assemblies with coverglass can reach up to 30% efficiency. DHV also produces 3U (132 g) and 3U-deployable panels producing 8.48 W. In addition to customizable panels, DHV manufactures a 50 x 50 mm “qubesat” panel which weighs 0.023 kg and produces 272 mW 14 . DHV Technology and Spire Global have announced a joint partnership to offer Spire’s double-deployable panels, built and sold through DHV Technology 15. These panels will be available for 3U Cubesats or satellites with larger buses.
GomSpace produces two NanoPower power systems for cubesats, both use 30% efficient cells and include Sun sensors and gyroscopes. The customizable panels have a maximum output of 6.2 W and 7.1 W and include a magnetotorquer. The cubesat panel weighs 26-29 g without an integrated magnetorquer, or 56-65 g with one, and produces 2.3-2.4 W 16.
SpectroLab’s space solar panels have flown on multiple spacecraft in LEO and GEO. They are available in small sizes (30 cm2) and use SpectroLab’s Improved Triple Junction (ITJ), Ultra Triple Junction (UTJ) or NeXt Triple Junction (XTJ) cells 17. Their solar panels were also used on the Juno spacecraft, which reached Jupiter in the Summer of 2016.
MMA Design, LLC
MMA Design’s HaWK (High Watts per Kilogram) solar array is designed for 3U-12U platform spacecraft and is deployable and gimbaled. The HaWK peak power is 36 W with a voltage of 14.2 V 18. The eHaWK solar array is a modular, scalable system designed for 6U cubesats and larger buses. The eHaWK starts at 72 W, uses Spectrolab UTJ 28.3% cells and weighs approximately 0.6 kgThe eHaWK solar array is a modular, scalable system designed for 6U cubesats and larger buses. The eHaWK starts at 72 W, uses Spectrolab UTJ 28.3% cells and weighs approximately 600 g 19. The HaWK is scheduled to launch on NASA’s BioSentinel mission in 2020, and eHaWK is already in deep space onboard the MarCO mission, launched May, 2018.
MMA also has zHaWK solar arrays that are based on HaWK series. The zHaWK consists of two array wings that are mounted on opposite 1U x 3U faces that consist of 6 panels (42 cells total), similarly to the HaWK configuration. The estimated mass of this array is 0.35 kg.
Astro- und Feinwerktechnik
Astro- und Feinwerktechnik have developed an adaptable solar array for minisatellites that is approximately 120 W with a mass of 4.19 kg. The startup-configuration dimensions are 546 x 548 x 620mm. These arrays have successfully flew on the 120 kg microsatellite TET-1 in 2012.
Solar energy is not always available during spacecraft operations; the orbit, mission duration, distance from the Sun, or peak loads may necessitate stored, on-board energy. Primary and secondary batteries are used for power storage and are classified according to their different electrochemistries. As primary-type batteries are not rechargeable, they are used only for short mission durations (around 1 day, up to 1 week). Silver-zinc are typically used as they are easier to handle and discharge at a higher rate, however there is also a variety of lithium-based primary batteries that have a higher energy density including: lithium sulphur dioxide (LiSO2), lithium carbon monofluoride (LiCFx) and lithium thionyl chloride (LiSOCl2) 20.
Secondary-type batteries include nickel-cadmium (NiCd), nickel-hydrogen (NiH2), lithium-ion (Li-ion) and lithium polymer (LiPo), which have been used extensively in the past on small spacecraft. Lithium-based secondary batteries are commonly used in portable electronic devices because of their rechargeability, low weight, and high energy, and have become ubiquitous on spacecraft missions. They are generally connected to a primary energy source (e.g. a solar array) and are able to provide rechargeable power on-demand. Each battery type is associated with certain applications that depend on performance parameters, including energy density, cycle life and reliability 20. A comparison of energy densities can be seen in the Figure above 21 and below. A list of battery energy density per manufacturer is in Table 3-2.
This section will discuss the individual chemical cells as well as pre-assembled batteries of multiple connected cells offered from multiple manufacturers. Due to small spacecraft mass and volume requirements, the batteries and cells in this section will be arranged according to specific energy, or energy per unit mass. There are, however, a number of other factors worth considering, some of which will be discussed below 22.
Due to the extremely short mission durations with primary cells, the current state-of-the-art energy storage systems use lithium ion (Li-ion) or lithium polymer (LiPo) secondary cells, so this subsection will focus only on these electrochemical compositions with some exceptions.
|Table 3‑2: Battery Energy Density|
|Product||Manufacturer||Specific Energy (Whkg-1)||Cells Used||TRL Status|
|40Whr CubeSat Battery||AAC-Clyde||119||Clyde Space Li-Polymer||9|
|BAT-100||Berlin Space Technologies||58.1||Lithium-Ferrite (Li-Fe)||9|
|BCT Battery||Blue Canyon Technologies||Unkn.||Li-ion or LiFePo4||9|
|BP-930s||Canon||132||four 18650 Li-ion cells||9|
|COTS 18650 Li-ion Battery||ABSL||90 – 243||Sony, MoliCell, LG, Sanyo, Samsung||8|
|Li-ion Battery Block VLB-X||Vectronic||Unkn.||SAFT Li-ion||Unkn.|
|NanoPower BP4||GomSpace||143||GomSpace NanoPower Li-ion||9|
|NanoPower BPX||GomSpace||154||GomSpace NanoPower Li-ion||9|
|Rechargeable Space Battery (NPD-002271)||EaglePicher||153.5||EaglePicher Li-ion||7|
Secondary Li-ion and Li-po batteries
Typically, Li-ion cells deliver an average voltage of 3.6 V while the highest specific energy obtained is well in excess of 150 Whkg-1 22.
Eagle Picher produces a number of cells for military and aerospace applications including two advanced Li-ion cells and a rechargeable space battery. Both cells have a high energy density and a TRL of 9. Their integrated space battery has a specific energy of 153.5 Whkg-1 and produces a nominal voltage of 28.8 V but has a slightly lower TRL of 7.
SAFT is another battery manufacturer with a long history of supplying the aerospace industry. Their Li-ion range include energy includes cells ranging from 126-165 Whkg-1 23.
ABSL’s Li-ion 18650 cells have an energy density range of 90-243 Whkg-1. ABSL’s top of the line military and space grade cells (Figure 3.11) have proven long-term reliability and charge life, with safety & protection circuitry built into the battery cells 24. ABSL provides small-spacecraft batteries featuring 4.8-12 Ah capacity, at 23-54W per cell, with a mass of 2-4 kg. ABSL’s Large-Format Li-Ion, 72 Ah Space cell, has recently completed space qualification.
The VLB-16 Li-ion battery pack offered from Vectronic Aerospace is specifically designed for use on small spacecraft and uses small spacecraft-qualified SAFT cells. This battery pack integrates current, voltage and temperature measurement functions and includes dynamic balancing that can be determined through a digital control interface 25.
GomSpace offers a range of cubesat subsystems, including Li-ion batteries. Their NanoPower BP4 Quad-Battery-Pack is designed to integrate seamlessly with their P-series PMADs. It is stackable and available in an International Space Station compliant version. NanoPower BP4 has a TRL of 9, having flown on board the GOMX-1 mission. The BPX series allows a wide range of parallel/series combinations and connections of up to sixteen cells 26. The NanoPower P31u, developed for nanosatellite platforms, is optimal for 1 and 2U platforms. The P31u, rated at 20Wh capacity, can provide up to 30W at 8V 23.
AAC-Clyde has designed Li-polymer batteries specifically for small spacecraft and CubeSats, leveraging a vast investment in Li-polymer technology. The model featured in the table has a specific energy of 119 Whkg-1 and voltage of 6.0-8.4 V. Battery temperature, voltage, current and telemetry can be monitored via an integrated digital interface. The use of Li-polymer cells allows the Clyde Space flat-packed batteries to be mass and volume efficient. Their third generation CubeSat battery line provides 10 – 80 Wh standalone batteries that interface with their Electrical Power System (EPS) offerings built on a standard PC104 interface 24.
There are two battery cells from Ultralife made for small spacecraft applications where primary batteries are an option. The Li-MnO2 and Li-CFx provide an energy density ranging from 350 to 450 Whkg-1. Lithium manganese dioxide cells offer excellent temperature characteristics, a flat discharge curve, and a hermetically sealed, nickel-plated steel container for long-term shelf life. Lithium Carbon Monoflouride cells have the highest energy density and performance characteristics of all lithium based battery chemistries with a strong passivation layer, which allows for long storage periods with minimal loss in cell capacity 25.
Ultralife’s newest hybrid primary cell technology improves upon lithium manganese dioxide chemistry by providing almost a 50% increase in both capacity and shelf-life, whilst also reducing initial suppression of cell voltage that is typical of pure CFx chemistries due to passivation during storage. The Ultralife Hybrid cells come in a variety of sizes (19650, 26500, 26650 and 34610) and are TRL 9 25.
Other 18650 Solutions
LG’s ICR18650 B3 Li-ion cells have a specific energy of 191 Whkg-1 and have flown on NASA’s PhoneSat spacecraft, housed in a 2S2P battery holder from BatterySpace 26. Panasonic produces the 18650B (3400 mAh) Li-ion cells, which have a high energy density of 243 Whkg-1, and flight heritage on small spacecraft missions including NASA’s GeneSat, SporeSat, O/OREOS, and PharmaSat 27. A Molicel offers several different 18650 battery pack modules that are space proven. They manufacture the ICR18650H Li-ion cell with a high specific energy of 182 Whkg-1 which requires pack control circuitry 28.A Li-Ion 18650 Battery Holder (2S2P) flew on NASA’s EDSN mission, in conjunction with LG ICR18650 B3 Li-ion cells. Canon’s BP-930s battery pack is an affordable, flight-proven option for power storage 29. The pack contains four 18650 Li-cells and has flown successfully on NASA’s TechEdSat missions.
Two new 18650-sized products promise improved performance over heritage devices. The Panasonic NCR18650GA, at 3450mAh, provides a specific energy of 258 Whkg-1. The LG MJ1, currently under evaluation at NASA Johnson Space Center (JSC), is rated to 3500mAh and 264 Whkg-1.
Power Management and Distribution
PMAD systems control the flow of power to spacecraft subsystems and instruments and are often custom designed by mission engineers for specific spacecraft power requirements. However, several manufacturers have begun to provide a variety of PMAD devices for inclusion in small spacecraft missions. Several manufacturers supply EPS which typically have a main battery bus voltage of 8.2 V, but can distribute a regulated 5.0 V and 3.3 V to various subsystems. The EPS also protects the electronics and batteries from off-nominal current and voltage conditions. As the community settles on standard bus voltages, PMAD standardization may follow. Well-known producers of PMAD systems that focus on the small spacecraft market include Pumpkin, GomSpace, Stras Space and Clyde Space. However, a number of new producers have begun to enter the PMAD market with a variety of products, some of which are listed below. Table 3-3 lists PMAD system manufacturers; it should be noted that this list is not exhaustive.
|Table 3-3: Power Management and Distribution Systems|
|Product||Manufacturer||Technology Type||TRL Status|
|BCT CubeSat Electrical Power System||Blue Canyon Tech||EPS||*|
|CubeSat Kit EPS 1||Pumpkin, Inc.||EPS||9|
|Cubesat EPS Type I, II and I Plus||Endurosat||EPS||5-7|
|Nanosatellite EPS||Clyde Space||EPS||8|
|PCDU-2100, -2200, -2300||ÅAC Microtec||PMAD||*|
|Power Storage and Distribution||Tyvak||PMAD||*|
|Small Satellite PCDU||Clyde Space||PMAD||9|
|3u cPCI Power Supply||SEAKR||EPS||9|
|Power and Control Unit||Magellan Aerospace||PMAD||9|
ÅAC – Clyde
ÅAC – Clyde provides three Power Conditioning and Distribution Units (PCDU) equipped with different user interfaces (Micro, Mini and Nano) for small spacecraft. They are designed for easy integration of payloads, sensors and sub-systems on advanced small satellites, with a mission lifetime of up to 5 years in LEO 30. The Nano interface has a mass of 0.22 kg, 12 V nominal bus and battery voltage, and an average system power of 20 W 31. There is a PMAD and an EPS targeted specifically at small satellites (The PMAD includes a range of topologies and architectures including DET and PPT, COTS, hybrid, and rad-hard components and has a TRL of 9. Their third-generation (3G FleXU) EPS for 1U-12U cubesats has a TRL of 9 after having flown on the Picasso nanosatellite (launched in January 2018). The 3G FleXU (Figure 3.14) EPS is also planned to fly on the SERB nanosatellite mission (proposed launch in 2020).
Three CubeSat EPS modules are provided by Endurosat. Cubesat Power Module Type 1, 1 Plus, and Type 2 are most suitable for 1U, 1.5U and 2U CubeSat Satellites, and are integrated with one or two Li-Po battery packs. The CubeSat Power Module Type 1 has a 4.2 V battery pack voltage, a total mass of 0.198 kg (one battery pack), and 10.4Wh capacity 32. This EPS has undergone space qualification testing. The CubeSat Power Module I Plus includes two battery packs with a total mass of 0.278 kg, 20.8 Wh battery-pack power, and 4.2 V pack voltage. Qualification tests are pending for this EPS. Finally, the Type II CubeSat EPS can be configured with either one or two battery packs; total mass is 0.28 – 0.42 kg, with 20.7 – 41.1 Wh of battery peak power and 12.6 – 16.8 V maximum pack voltage 32.
The Vasik P1U power supply is optimized for 1U and 2U cubesats. The battery output traverses though redundant converters that can provide 3.3 V, 5 V and 12 V. The supply’s energy rating is 3 Ah (11 Wh), with a mass of 0.08 kg 33. Unregulated 3.7V and regulated buses are also available. This architecture was successfully flight-tested on the ESTCube-1 satellite; this EPS is TRL 9.
GomSpace’s NanoPower P31u PMAD system is designed for small spacecraft requiring power up to 30 W 26.
Modular Devices, Inc.
A 7.5-20 W Hybrid DC-DC power converter specifically designed for cubesat applications in a radiation environment, TID >100 kRad (Si), has been developed and tested at Modular Devices 38.
Stras Space’s Drop-In Power Converter is designed for enclosed spaces and easy mounting. It has a wide input voltage range of 3.3-40 V and operates with a typical efficiency of 90% 39.
Surrey Satellite Technology
Surrey Satellite Technology sells a full PMAD system in the form of their LEO PCDU. It is based on a modular design that is intended to be scalable and customizable. The PCDU system is made up of a battery conditioning module and a power distribution module and has flown on over 30 missions 40.
Vectronic’s PCDU is one of a range of space power systems designed for small spacecraft. The PCDU monitors output from battery and solar power sources, and switches individual subsystems in response to a telecommand or automatically in the event of an overload or short-circuit condition. There are currently at least eight Vectronic PCDUs on orbit.
The Electrical Power System 1 is an efficient, high power option for all nanosatellite platforms developed at Pumpkin. Shown in, this low-mass system has a total mass of <0.3 kg, features up to 3 W, and a 60 V power ring topology that has been space proven on multiple missions 34. This board has flown on several small spacecraft and CubeSat form factors.
The Power Supply System EPSL is a low-power, 23 Wh configuration containing two 7.4V, 3200mAh cells. The EPSH high-power (46 Wh) configuration measures 92.9 x 89.3 x 25 mm, contains four 7.4 V cells (6400 mAh total), and weighs 0.3 kg 35. This system is TRL 9.
Blue Canyon Technologies
Blue Canyon Tech’s BCT CubeSat Electrical Power System includes functionality for solar array input power, on-board or external batteries, charge control, power regulation and distribution, and data acquisition 43.
Design Net Engineering
Design Net Engineering makes aPMAD system and an Energy Storage Module (ESM). The PMAD system is designed to have highly configurable energy storage, battery chemistry and number of panels. The ESM converts battery power into a locally managed brick of energy that can accept charge from any number of power sources and provide power to spacecraft subsystems.
On the Horizon
New technologies continue to be developed for space qualified power generation. Promising technologies applicable to small spacecraft include advanced multi-junction, flexible and organic solar cells, hydrogen fuel cells and a variety of thermo-nuclear and atomic battery power sources.
Multi-junction Solar Cells
Fraunhofer Institute for Solar Energy Systems have developed different four-junction solar cell architectures that currently reach up to 38% efficiency under laboratory conditions, although some designs have only been analyzed in terrestrial applications and have not yet been optimized 36. Fraunhofer ISE and EV have achieved 33.3% efficiency of a 0.002 mm thin silicon based multi-junction solar cell, and future investigations are needed to solve current challenges of the complex inner structure of the subcells 37. Additionally, Boeing Spectrolabs has been experimenting with 5- and 6-junction cells with a theoretical efficiency as high as 70% 38.
Flexible Solar Cells
Flexible and thin-film solar cells have an extremely thin layer of photovoltaic material placed on a substrate of glass or plastic. Traditional photovoltaic layers are around 350 microns thick, while thin-film solar cells use layers just one micron thick. This allows the cells to be flexible and lightweight and, because they use less raw material, are cheap to manufacture. In 2014, FirstSolar announced a flexible solar cell design with an efficiency of 20.4%, closing the gap on single-junction solar cells 39. Flexible solar cells designed specifically for space applications are available from United Solar and have an efficiency of 8% on 1 mil polymer giving them a specific power of 750-1100 Wkg-1.
Additionally, MIT researchers have developed a solar cell material that can be printed onto paper and folded multiple times without loss of function. While still in its infancy, this technology has the ability to massively reduce the cost of solar cell production while increasing the durability of cells 40,41.
A June 2017 International Space Station (ISS) demonstration mission rolled up a solar array to form a compact cylinder for launch 42. ROSA (Roll-Out-Solar-Array), is made of a center wing with flexible material containing photovoltaics.
Organic Solar Cells
Another on-the-horizon photovoltaic technology uses organic or “plastic” solar cells. These use organic electronics or organic polymers and molecules that absorb light and create a corresponding charge. A small quantity of these materials can absorb a large amount of light making them cheap, flexible and lightweight. Currently they are limited by an efficiency of less than 4% 43.
In October 2016, the Optical Sensors based on CARbon materials (OSCAR) stratospheric-balloon flight test demonstrated organic-based solar cells for the first time in a stratospheric environment. While more analysis is needed for terrestrial or space applications, it was concluded that organic solar energy has the potential to disrupt “conventional” photovoltaic technology 44. Since then, studies have shown that ex-situ climate chamber testing is a viable method for simulating aerospace environments. While no standardized stability tests are yet available for organic-based solar cell technology and challenges remain on creating simultaneous environmental influences that would permit in-depth understanding of organic photovoltaic behavior, these achievements are enabling progress in organic-based solar cell use.
In 2018, Chinese researchers in organic photovoltaics were able to reach 17% power conversion energy using a tandem cell strategy. This method uses different layers of material that can absorb different wavelengths of sunlight, which enable the cells to use more of the sunlight spectrum, which has limited the performance of organic cells 45.
Hydrogen fuel cells are appealing due to their small, light and reliable qualities and have a high energy conversion efficiency. They also allow missions to launch with a safe, storable, low pressure and non-toxic fuel source. An experimental fuel cell from the University of Illinois that is based on hydrogen peroxide rather than water has demonstrated an energy density of over 1000 Whkg-1 with a theoretical limit of over 2580 Whkg-1 54. This makes them more appealing for interplanetary missions and during eclipse periods, however unlike chemical cells, they cannot be recharged on orbit. Regenerative fuel cells are currently being researched for spacecraft application. Today, fuel cells are primarily being proposed for small spacecraft propulsion systems rather than for power sub-systems s 55.
Another source of spacecraft power comes from harnessing the energy released during radioactive decay. Radioisotope Thermoelectric Generators (RTGs) are associated with longer lifetimes, high reliability and predictable power production, and are more appealing than relying on batteries and solar panels beyond Mars orbit (>3 AU). A full-sized RTG, such as on New Horizons, has a mass of 56 kg and can supply 300 W (6.3% efficiency) at the beginning of its life 56.
Although a radioisotope power system has not yet been integrated on a small spacecraft, they might be considered for small spacecraft missions that traverse interplanetary space. This concept would require substantial testing and modified fabrication techniques to facilitate use on smaller platforms.
A thermo-photovoltaic (TPV) battery consists of a heat source or thermal emitter and a photovoltaic cell which transforms photons into electrical energy. Thermophotovoltaic power converters are similar to high TRL thermoelectric converters, with the difference that the latter uses thermocouples and the former uses infrared-tuned photovoltaic cells.
In a paper given at the Photovoltaic Specialists Conference in 2011, entitled “Soda-can sized thermophotovoltaic battery replacement”, a TPV with a conversion efficiency of 10% was described that would have a specific energy of approximately 1000 Whkg-1. This is approximately 6.5 times higher than the specific energy for a Li-ion battery, making it a very exciting alternative power source (Figure 3.20). The authors have not produced a physical prototype 57.
Thermophotovoltaics are technically challenging as they require radioisotope fuel to have a temperature of more than 1273 K for high infrared emission, while also maintaining temperature suitable for photovoltaic cells (less than 323 K) for efficient electrical conversion.
Alpha- and Beta-voltaics
Alpha- and beta-voltaic power conversion systems use a secondary material to absorb the energetic particles and re-emit them via luminescence. These photons can then be absorbed by photovoltaic cells. Methods for retrieving electrical energy from radioactive sources include beta-voltaic, alpha-voltaic, thermophotovoltaic, piezoelectric and mechanical conversions. This technology is currently in the testing/research phase.
In the area of power storage there are several efforts at improving storage capability. For example, the Rochester Institute of Technology is prepared to demonstrate a nano-enabled power system on a CubeSat platform. The power system integrates carbon nanotubes into lithium-ion batteries that significantly increases available energy density. The energy density has exceeded 300 watt hours per kilogram during testing, a roughly two-fold increase from the current state-of-the-art 46. A collaborative project between the University of Miami and NASA is aiming to develop a multifunctional structural battery system that will use an electrolytic carbon fiber material that acts as both a load bearing structure and a battery system. This project is still in the initial phases at the time of this report, but if successful, this novel battery system will extend mission life, support larger payloads, and significantly reduce mass 47.
Power Management and Distribution
For small spacecraft, traditional EPS architecture is centralized (each subsystem is connected to a single circuit board). This approach provides simplicity, volume efficiency, and inexpensive component cost. However, a centralized EPS is rarely reused for a new mission, as most of the subsystems need to be altered based on new mission requirements. A modular, scalable EPS for small spacecraft was detailed by Timothy Lim and colleagues, where the distributed power system separated into three modules: solar, battery and payload. This allows scalability and reusability from the distributed bus, which provides the required energy to the (interfaced) subsystem 48.
University of Toronto’s Space Flight Laboratory (SFL) has developed an in house, scalable and reusable Modular Power System (MPS) and have flown systems derived from this architecture on several missions: Norsat-1 & 2, and CanX-7 49.
Driven largely by weight and size limitations, small spacecraft are using advanced power generation and storage technology such as >29% efficient solar cells and lithium-ion batteries. The higher risk tolerance of the small spacecraft community has allowed both the early adoption of technologies like flat lithium-polymer cells as well as COTS products not specifically designed for spaceflight. This can dramatically reduce cost and increase mission-design flexibility. In this way, power subsystems are benefiting from the current trend of miniaturization in the commercial electronics market as well as from improvements in photovoltaic and battery technology.
Despite these developments, the small spacecraft community has been unable to use other, more complex technologies. This is largely because the small spacecraft market is not yet large enough to encourage the research and development of technologies like miniaturized nuclear energy sources. Small spacecraft power subsystems would also benefit from greater availability of flexible, standardized power management and distribution systems so that every mission need not be designed from scratch. In short, today’s power systems engineers are eagerly adopting certain innovative Earth-based technology — like lithium polymer batteries — while, at the same time, patiently waiting for important heritage space technology — like fuel cells and RTGs — to be adapted and miniaturized.