EQUiSat solar panels are made in-house with scrap Gallium-Arsenide cells using a documented, repeatable production process. These panels cost 35 times less than the off-the-shelf panel and have 80% the power output. The solar panels double as light-sensors to tell the satellite when the it is in the sun. The current design for a side panel calls for 4 cells in series with 5 in parallel for a total of 20 cells. The top panel design calls for 4 in series and 6 in parallel for a total of 24 cells. This configuration was chosen to best match the nominal voltage of our battery packs.
- Total size must be less than 8.3 cm X 10 cm per side and 10 cm X 10 cm for top and bottom panels
- Must have >15% efficiency
- Must be available “off-the-shelf” without consumer purchasing restrictions
- Must be inexpensive
- Triple Junction Gallium Arsenide TASC cells (Spectrolab)
- 8.76 V nominal per side
- 60% Packing Factor
- 1.2-1.4 W produced per side
- cell efficiency ~27% (typical silicon cells are approx. 16% efficient)
- $2.50 per cell (~$80 per side)
One of the most important parts of construction is achieving such a high packing factor while ensuring that the cells do not touch one another (and result in a short). The placement template is a piece of acrylic which was cut using a Universal Laser Cutter. In the future these templates could potentially be made using rapid-prototyping in order to maintain the desired accuracy while minimizing costs.
During assembly, a solar cell can be placed in the cutout rectangle and pressed against the wall to ensure correct placement. The plans for the template were made by creating an Adobe Illustrator file using measurements taken from the PCB design software. The template is aligned with the PCB by sliding a dowel pin through the mounting holes in the PCB which have matching holes on the top placement template and a corresponding thick solid piece of aluminum.
Printed circuit boards have numerous advantages, the first of which is that they not only become the physical connection between the solar cells and the chassis but they also provide the electrical connection between cells which cut down on excess wiring that would be very difficult without professional processes.
The boards can also be produced in quantity for cheap prices and high precision using online vendors. When placing and arranging these cell rectangles on the PCB, multiple considerations were taken into account. The maximum packing factor was desired to covert the maximum energy possible, but was limited due to a few conditions. Due to the manual construction process, some safety factor had to be maintained between the cells in order to prevent accidental shorting due to human error. Also, the batteries’ maximum voltage is 8.4V so the panels must have solar cells in 5 cell groups in order to produce a high enough voltage. Mounting holes were placed on each of the corners to secure the PCB to the chassis.
The free program PCB Express was used to design our PCBs (seen on the right).
Two easy to use mediums exist to electrically and physically connect the solar cells to the PCB, solder paste and conductive epoxy. Solder paste is placed between two metal surfaces, such as between a solar cell and a PCB pad and then paste is heated with a rework tool to secure the connection. Conductive epoxy on the other hand is an adhesive which usually contains silver. Epoxy comes in two tubes, resin and hardener, and when combined it begins to cure and eventually solidifies. The epoxy found to suit these purposes is Epoxy Technology’s EPO-TEK H20E Electrically Conductive Silver Epoxy.
There are distinct trade-offs with regard to the use of both of these setting materials. After careful consideration, we have chosen to continue producing panels with solder paste and a rework tool or heat gun. This allows for mistakes in placement to be corrected easily, and allows for broken cells during testing or handling to be removed from the panel after completion. This saves a great deal of money and resources for the team.
Trade offs: Solder Paste & Rework Tool over Silver Conductive Epoxy
- 1/8” to 1⁄4” Acrylic
- Screws or metal rods, wooden or metal base
- Solder Paste
- TASC Cells
- Printed Circuit Board
- Rework tool, heat gun
- Soldering iron, ideally with a temperature adjustment
- Solder (Avoid Tin Solder)
- 30 AWG wire, 22 AWG wire
- Wire strippers & cutters
- Handi-Vac suction tool
- Cut a solder guide out of acrylic using a laser cutter and Adobe Illustrator file
- Mount the acrylic on the board using screws or metal rod, taking care to make sure that it is oriented properly
- Put a thin strip of solder paste across each of the pads
- Using a suction tool, place the solar cells in their place using the guide, and heat with rework tool, then allow to cool before checking that it is secure
- Use the rework tool to reapply any of the solar cells that may have come loose or been displaced
- Strip the end of a spool of 30 AWG wire, then cut this end to be approximately 2 inches in length. Do not strip the other end of the wire.
- Repeat step 6 until you have enough wires to complete the panel
- Using an adjustable temperature soldering iron and solder, secure the stripped end of each wire into the through holes
- Once secured, cut each of the wires to be about 3⁄4 of an inch
- Strip the ends of each wire attached to the board, taking care not to damage any cells in the process
- Replace wires that have fallen out or become entirely stripped as needed
- Curve the wires using tweezers so that the stripped end is over the pad of the solar cell
- Tin both the pad and the wire using flux and secure the wire to the pad on the cell
- Check for any loose connections and broken solar cells and replace as necessary
- Cut and strip each end of a 22 AWG wire and secure with solder into the positive and negative terminals of the panel
- Check connections with a multimeter and test in sunlight
- Care must be taken to avoid creating a solder bridge between the positive and negative sides of the Cell while soldering the wire into place which could short the two sides
Lithium Ion batteries, standard in many satellites, are lightweight and small. These batteries offer a low depth of discharge for increased life expectancy and low voltage drop. They are designed redundantly in case of battery failure. Recently, the power subsystem has reevaluated their choice of batteries for the satellite. This decision will be made based upon the needs of the flash system, which has also made some recent changes. The two options for batteries are the lithium ion and lithium iron phosphates, which have a different chemistry which lends itself to a different function. The trade offs of the two different types are shown below.
We have decided to go through with the LiFePO4 battery option for our main battery due to its unique current draw capabilities that will decrease EQUiSat’s reliance on a capacitor bank. For safety reasons, we have chosen to break the battery system into two stand-alone units: this set of 4 LiFePO4 batteries that will manage the flash system power and energy requirements, and another set of 4 rechargeable coin cell batteries that will handle the radio and microprocessor power needs.
- Lightweight and compact
- Limit depth of discharge to 20% to increase longevity
- Avoid large voltage conversion between batteries and subsystems to maximize efficiency
- 4 LiFePO4 18650 Batteries
- 4 Li Ion Coin Cell Batteries
- 2S2P Configuration
- 6.4 V nominal output voltage
- 1.1 Ah capacity
- 80 g total weight
This test will allow us to measure the depth of discharge and the lifetime of our batteries. This can be done using a relatively simple circuit that continually charges and discharges the batteries, using data acquisition and a relay switch. This data would potentially allow us to find a charge and discharge curve for our particular batteries.
One of the more important sequences of testing will be when we test the solar panels and batteries in conjunction with the other subsystems of the satellite specifically the Flash and Electronics teams. Once the circuitry for the battery management system is completed by the Electronics team we will be able to run charge tests where our solar cells can charge both the Lithium Iron Phosphate batteries and the coin cell batteries. We can then run discharge tests with fully charged batteries and the Flash panel. These types of test will be the most important in ultimately determining the validity of our designs and give us important information of the cycling that our batteries will experience in orbit.
Given the series and parallel wiring, the ideal output for the panels is 7.56V with 210mA for the side panels and 240mA for the top panels. When testing at 3pm on April 16, 2013 in Providence, RI (latitude 42º) the panels performed below specifications. The average performance for the full side panels was 7.08V and 136mA and 7.09V and 133mA for the top panels. When one side and one top panel were wired in parallel, as they would be in the satellite, 7.18V and 290mA were produced. The expected values were 7.56V and 450mA.
The lower performance was expected as we were testing under atmospheric conditions in Providence, RI (latitude 42º). However, there may be may be an additional degradation of output due to construction methods.
Through Brown’s School of Engineering, we have access to a Solar Simulator Lamp, provided by Professor Pacifici. This lamp provides the power of a sun (100 W/m^2) that you cannot receive from a normal lightbulb. In addition, this machine can shift the wavelength of light to simulate being in space, which will be used for tests in the future. This lamp will be used to test the efficiency of our cells as well as our panels. The software of the solar simulator outputs our panel’s IV curves, which can be used to test our efficiency under different conditions, such as at an angle or at a higher temperature.
Control Tests: Before doing special cases tests, we ran a control test to set benchmarks on the performance of each panel individually. Each panel under perfect conditions should output 7.56V. However, the sun simulator was outputting the power of light at the equivalent of Earth, not space, so some decrease in voltage was expected.
Results: As you can see in the graph, the two side panels performed close to this standard, while the top panels underperformed at 6.2V and 5.3V. This also gave us a better idea of which panels has issues during construction. Another value you can see on the graph is the voltage at max power output of the panels. This is a lower number, due to the shape of the IV curve of the panels and cells. The max power values pointed us in the direction of considering powerpoint tracking for our panels to optimize their power output.
Conformal Coating Tests
Our solar panels will be coated in a conformal epoxy layer before launch in an attempt to eliminate any physical damage to the solar cells, their connections, and the panels themselves during launch or handling. We will be using a clear silicone epoxy provided by NuSil for our conformal coating. Tests run on the coating need to be performed to ensure that the solar panels still function at a high efficiency after being covered with the epoxy. Some of the challenges with this process include eliminating any air bubbles between the epoxy layer and the panel itself. These air pockets would obviously prove hazardous in orbit because of the vacuum of space. Once application methods have been optimized tests can be run on the panels to compare the efficiencies of the bare cell tests and the coated cell tests. Tests that we have performed so far have only been done on small panels with one or two cells but will eventually be expanded to the larger panels.
Lost Cell Analysis
An additional test case that our panels were analyzed for was that of a ‘Lost Cell’. A lost cell could be the result of mechanical damage to a single cell, wiring or shorts, or a degradation in performance due to localized thermal conditions throughout the panel. For this study, test panels were wired with only two TASC cells in series or parallel and performance was recorded using a Sun Simulator while one of the cells was covered (blacked out).
As can be seen in the included tables, in the parallel configuration ‘eliminating’ one cell resulted in approximately have the current and power, as expected. In the series case, covering a single cell resulted in have the open circuit voltage, as expected, but did not produce consistent maximum voltage or current changes.
Additional testing is required. Coming soon…
According to the Spectrolab data sheet for the TASC cells, the peak power voltage of an individual cell decreases by 6.2mV/ºC increase in temperature. This means, for example, that a 100ºC increase in temperature of the panels would decrease the open circuit voltage from 7.56V to 5.7V for the sets of three TASC cells in series.
Results: Coming soon…
EQUiSat provides verification for our unique power system, designed specifically for affordable, yet power intensive CubeSats. This is broken down into the validation of our in-house fabrication methods for solar panels as well as the use of LiFePO4 batteries in space applications.
The size of EQUiSat’s solar panels is limited by the CubeSat architecture. We have optimized our panels for our own uses by making efficient, affordable panels which produce voltages at our desired range. In accordance with our primary mission of accessibility, it was important for us to minimize the use of pre-built, and often expensive CubeSat components, such as Clyde Space solar panels. We found that by using low-cost, Gallium Arsenide scrap cells from Spectrolab, we can reduce the cost of the solar panels by 35 times while maintaining their efficiency in comparison to prebuilt panels. In order to do so, we have developed, and are continuing to iterate, an intuitive fabrication process to make these solar panels from scratch. This process, which has already been utilized by other university satellite groups (such as the University of New Mexico) through the documentation on our website, can help amateurs effectively produce cost-efficient solar panels.
Our chosen LiFePO4 batteries provide a number of unique benefits, and their success in space could expand the power capabilities of small satellites. A123 Systems LiFePO4 cells have high current draw capabilities, up to 30A continuously and 60A pulsed. This eliminates the need for large, electrolytic capacitors, which can be volatile in a vacuum environment. Their chemistry is generally more stable, and as a result, a safer option in comparison to other lithium-ion batteries. Given the problematic phenomenon of thermal runaway, observed in other batteries, the thermal stability of LiFePO4 batteries is promising. LiFePO4 batteries are stronger candidates for use in a space environment where temperatures are likely to increase to extremes near 70 ̊C. The eventual launch of the satellite requires the batteries to be tolerant to physical abuse caused by heavy vibrational loads. Testing conducted in precedent studies has shown that the batteries can be exposed to high magnitude vibrations and other physical stimulation without any noticeable effects to performance. CubeSat requirements also stipulate a mass budget for the satellite, and LiFePO4 batteries offer a lightweight solution for the power system when compared to other available battery options. Additional testing of these batteries is currently being conducted for applications in space at various NASA centers.
The majority of this validation will take place in orbit as we monitor the power budget of our satellite and the health of our system. We will be able to send down this data via our radio to be processed at our ground stations. However, we have already begun the verification of these components on the ground. Through extensive testing in both normal and extreme conditions, we will reduce failure points and validate each component’s success in the space environment. The success of EQUiSat’s power system will set a precedent for the use of this new battery chemistry and the fabrication of space components, opening new opportunities for small satellites. Furthermore, power system failures are among the biggest causes for systemic failure in small satellites. Our efforts to create a reliable, low cost, and simple power system that is capable of handling high power (over the typical high energy) requirements will be extremely beneficial to the CubeSat community.