The electrical power system (EPS) manages electrical power generation, storage and management and commonly makes up one-third of total spacecraft mass. Power generation technologies include photovoltaic cells, solar panels and arrays, and radioisotope or other thermonuclear power generators. Power storage takes place in batteries, which can either be primary batteries charged once before launch, or rechargeable secondary batteries. Power management and distribution (PMAD) systems allow operators to control the flow of power to the spacecraft instruments and subsystems. PMAD systems take a variety of forms and are often custom designed to meet specific mission requirements. Engineers often focus on power generation and storage technologies that have a high specific power or power-to-mass ratio (Whkg-1) to ensure launch mass is minimized.
State of the Art
The majority of small spacecraft missions exploit the photoelectric effect to generate electrical current during their mission. Photovoltaic cells, or solar cells, are made out of thin wafers of semiconductors that produce electric current when exposed to light. Solar intensity varies as the inverse square of the distance from the Sun and the projected surface area of the panels exposed to the Sun varies as a cosine of the angle between the 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 are associated with a relatively low efficiency, usually less than 20%. To increase the efficiency of solar cells, multiple layers of materials with different band gaps are combined in multi-junction cells, which can use a wider spectrum of solar radiation. The theoretical efficiency limit for an infinite-junction cell is 86.6% in concentrated sunlight . However, in the aerospace industry, triple junction cells are commonly used due to their high efficiency-to-cost ratio compared to other cells. While solar cells are utilized on most small spacecraft missions, limitations include diminished efficacy as a function of distance from the Sun, inability to function during eclipse periods, high surface area and mass, degradation over time and high cost. Figure 3.1 illustrates the available technologies plotted by energy efficiency. This section will discuss individual solar cells and fully integrated solar panels and arrays separately that are applicable on small spacecraft. Table 3.1 describes small spacecraft solar panel efficiency for different available manufacturers.
|Product||Manufacturer||Efficiency||Solar Cells Used||Status|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||28.3%||SpectroLab UTJ||TRL 9|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||29.5%||SpectroLab XTJ||TRL 9|
|Solar Panel (0.5-12U); Deployable Solar Panel (1U, 3U)||Clyde Space||30.0%||AzurSpace 3G30A||TRL 9|
|Solar Panel (5 x 5 cm, 1U, 3U, custom)||DHV||30.0%||Unkn.||TRL 8|
|NanoPower (CubeSat and custom)||GomSpace||30.0%||AzurSpace 3G30A||TRL 9|
|HAWK||MMA||28.3%||SolAero ZTJ||TRL 7|
|eHAWK||MMA||28.3%||SolAero ZTJ||TRL 7|
|Space Solar Panel||SpectroLab||26.8%||SolAero ITJ||TRL 9|
|Space Solar Panel||SpectroLab||28.3%||SolAero UTJ||TRL 9|
|Space Solar Panel||SpectroLab||289.5%||SolAero XTJ||TRL 9|
AzurSpace’s single-junction Silicon Solar Space Cell S32 has unremarkable energy efficiency at only 16.9% and the mass per surface area ratio is less than half of anything else listed at only 32 mgcm-2. Additionally, AzurSpace is offering a number of other cells ranging in efficiency from 28-30%. The solar cells are equipped with an integrated bypass diode .
SpectroLab offers several solar cells in the 26-30% efficiency range. The most efficient cells are 29.3% and are available in 26.62 cm2, 59.65 cm2 and customizable. All of SpectroLabs triple-junction cells have had their on-orbit performance validated to ±1.5% of ground test results .
Emcore produces two triple-junction solar cells with 28.5% and 29.5% efficiency that are available in standard and custom sizes. These second and third generation cells have rich flight heritage and the ZTJ cells were flown on NASA’s CYGNUS mission ,.
One of the highest-efficiency cells on the market today is manufactured from SolAero at 33% efficiency. The cell mass is only 49 mgcm-2, which is around 40% less than traditional multi-junction solar cells. However, its high cost and low TRL (space qualification is still in process) make it less appealing for small spacecraft designers today. SolAero also manufactures 29% and 29.5% efficiency solar cells both of which are fully space qualified for small spacecraft missions .
Solar Panels & Arrays
SolAero’s COBRA and COBRA-1U are designed for small spacecraft applications and use SolAero’s advanced triple junction 29.5 % efficiency cells mentioned above. 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” . The cubesat-specific COBRA-1U can be used on cubesats 1U-3U in size or larger, see Figure 3.2.
DHV Technology fabricates 100 x 100 mm 1U solar panels that weigh 39 g and produce 2.24 W that can be seen in Figure 3.3. Assemblies with coverglass can reach up to 30% efficiency if required. 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 23 g and produces 272 mW .
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 .
Clyde Space produces 0.5U-12U solar panels, as well as deployable solar panels for 1U and 3U cubesats where alternative solar cells previously mentioned are used. Both the mounted and deployable panels have flown on small spacecraft .
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 .
MMA Design’s HaWK (High Watts per Kilogram) solar array is designed for 3U-12U platform spacecraft, shown in Figure 3.4, and is deployable and gimbaled with peak power of 36 W and voltage of 14.2 V . 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 600 g . Both of these technologies are currently around TRL~7 with the HaWK scheduled to launch on the NASA’s BioSentinel mission and JPL’s MarCO mission in 2016 and the eHAWK currently undergoing environmental testing, see Figure 3.4.
Solar power generation is not always available for spaceflight operations; the orbit, mission duration, distance from the Sun or required peak instrument power may need stored on-board energy. Primary and secondary batteries are used for power storage and classified according to their different electrochemistries. As primary type batteries are not rechargeable, they are used only for real 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) .
Secondary type batteries include nickel-cadmium (NiCd), nickel-hydrogen (NiH2), lithium-ion (Li-ion) and lithium polymer (Li-po) and 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 provider such as a solar array and are able to provide power on demand and recharge. Each battery type are associated with certain applications that depend on performance parameters, including energy density, cycle life and reliability . A comparison of energy densities can be seen in Figure 3.5  and Figure 3.6 and 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 energy density. There are, however, a number of other factors worth considering, some of which will be discussed below .
Due to the extreme short mission duration with primary cells, the current state of the art energy storage systems use lithium ion (Li-ion) or lithium polymer (Li-po) secondary cells, and this subsequent section will focus only on those electrochemical composition.
|Product||Manufacturer||Energy Density (Whkg-1)||Cells Used||Status|
|COTS 1865 Li-ion Battery||ABSL||90 – 243||Sony, MoliCell, LG, Sanyo, Samsung||TRL 8|
|BP-930s||Canon||132||four 18650 Li-ion cells||TRL 9|
|Li-Polymer, 8.2 V, 1.25 – 20 Ah||Clyde Space||150||Clyde Space Li-Polymer||TRL 9|
|Li-Polymer, 32 V, 6.25 Ah||Clyde Space||150||Clyde Space Li-Polymer||TRL 8|
|Rechargeable Space Battery (NPD-002271)||EaglePicher||153.5||EaglePicher Li-ion||TRL 7|
|NanoPower BP4||GomSpace||160||GomSpace NanoPower Li-ion||TRL 9|
|NanoPower BPX||GomSpace||157-171||GomSpace NanoPower Li-ion||TRL 9|
|Li-ion Battery Block VLB-X||Vectronic||Unkn.||SAFT Li-ion||Unkn.|
Secondary Li-ion and Li-po batteries
Typically, Li-ion cells delivery an average voltage of 3.6 V while the highest specific energy obtained is well in excess of 150 Whkg-1 .
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 an energy density 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 .
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 have proven long-term reliability and charging, safety & protection circuit built into the battery cells , see Figure 3.7.
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 , see Figure 3.8.
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 .
LG’s ICR18650 B3 Li-ion cells have an energy 191 Whkg-1 and have flown on NASA’s PhoneSat spacecraft housed in a 2S2P battery holder from BatterySpace . Panasonic produces the 18650B (3400 mAh) Li-ion cells have a high energy density of 243 Whkg-1 that has a flight heritage on small spacecraft missions including NASA’s GeneSat, SporeSat, O/OREOS and PharmaSat .
Molicel manufacturers the ICR18650H Li-ion cell with a high energy density of 182 Whkg-1 which require pack control circuitry . BatterySpace.com sells a Li-Ion 18650 Battery Holder (2S2P) that was used on NASA’s EDSN mission in conjunction with LG ICR18650 B3 Li-ion cells. Canon’s BP-930s battery pack, see Figure 3.9, is an affordable, flight-proven option for power storage . The pack contains four 18650 Li-cells and has flown successfully on NASA’s TechEdSat missions.
Clyde Space has designed two Li-polymer batteries specifically for small spacecraft and cubesats. With an energy density of up to 150 Whkg-1 and voltage of 8.2-32 V, battery temperature, voltage, current and telemetry can be monitored via integrated digital interface. They also have an integrated heater which maintains battery temperatures above 0°C. The use of Li-polymer cells allows the Clyde Space flat-packed batteries to be mass and volume efficient. According to the manufacturer, Clyde Space batteries are used on more cubesat missions than any other provider’s.
Power Management and Distribution
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 Electrical Power Systems (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 non-nominal current and voltage conditions. As electronics vendors settle on standard voltages, PMAD will become more standardized. 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 manufactures and it should be noted that this list is not exhaustive.
|DPCU-2100, -2200, -2300||ÅAC Microtec||PMAD||Unkn.|
|BCT CubeSat Electrical Power System||Blue Canyon Tech||EPS||Unkn.|
|Small Satellite PCDU||Clyde Space||PMAD||TRL 9|
|Nanosatellite EPS||Clyde Space||EPS||TRL 8|
|P1U “Vasik”||Crystalspace||EPS||TRL 8|
|DNE Energy Storage Module||Design Net Engineering||ESM||Unkn.|
|Series 3699 DC-DC Converter||Modular Devices Inc.||Power Converter||Unkn.|
|Drop-In Power Converter||Stras Space||Power Converter||TRl 9|
|LEO PCDU||Surrey||PMAD||TRl 9|
ÅAC Microtec provides three Distributed Power and Control Units equipped with different user interfaces (I2C, USB, SpaceWire), see Figure 3.10. They are designed for easy integration of payloads, sensors and sub-systems on advanced small satellites .
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 .
Clyde Space produces a PMAD and an EPS targeted specifically at small satellites, see Figure 3.11. The PMAD includes a range of topologies and architectures including DET and PPT, COTS, hybrid, and rad-hard components and has at TRL of 9. Their third-generation EPS for 1U-12U cubesats has a TRL of 8 while the second generation EPS is a veteran of many small spacecraft missions .
Crystalspace manufactures a P1U power supply that is optimized for 1U and 2U cubesats. The battery output travels though duplicated converters that can provide 3.3 V, 5 V and 12 V .
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.
GomSpace’s NanoPower P31us PMAD system is designed for small spacecraft requiring power between one and 30 W, see Figure 3.12 . Modular Devices, Inc. makes a 7.5-20 W Hybrid DC-DC power converter specifically designed for cubesat applications in a radiation environment, TID >100 kRad (Si) . 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% .
Surrey Satellite Technology sells a full PMAD system in the form of their LEO PCDU, see Figure 3.13. 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 .
Vectronic’s Power Control and Distribution Unit 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 atomically in the event of an overload or short-cut condition. There are currently eight Vectronic PCAD units on orbit.
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
A four-junction solar cell, developed by Fraunhofer Society, is currently reaching 46% efficiency under laboratory conditions and concentrated sunlight, although it is unclear whether the power-to-weight ratio remains the same as current triple-junction cells . Additionally, Boeing Spectrolabs has been experimenting with 5- and 6-junction cells with a theoretical efficiency as high as 70% .
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, 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 , shown in Figure 3.14. A flexible solar cell designed specifically for space applications is available from United Solar and has 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 , .
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% .
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 and has a theoretical limit of over 2580 Whkg-1 . 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 .
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 when surpassing Mars orbit (>3 AU). A full size 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 .
Although a radioisotope power system has not yet been integrated on a small spacecraft, they can still be considered when small spacecraft missions traverse interplanetary space. Additional testing and fabrication may be required for 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, see Figure 3.15. The authors have not produced a physical prototype . 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 the energy through luminescence. These photons can then be absorbed via photovoltaic cells. Methods for retrieving electrical energy out of radioactive sources include beta-voltaic, alpha-voltaic, thermophotovoltaic, piezoelectric and mechanical conversions. This technology is currently in the testing/research phase.
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 commercial-off-the-shelf products not specifically designed for spaceflight. This dramatically reduces cost and increases flexibility of mission design. 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 utilize 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.