Flying on thin solar cells
Professor Craig Underwood and Dr Dan Lamb look at the development of an ultra lightweight solar cell capable
of surviving in space.
Emerging applications in space require large area solar arrays to provide greater power (kWpeak) than is currently used today. These include solar electric propulsion, space-based solar power and lunar and martian bases. To achieve this, new solar cell technology has been designed to meet the emerging demands of new space applications.
Currently, triple junction gallium arsenide-based photovoltaics (PV) dominate space applications, and a cerium-doped radiation hard cover glass is laminated to all solar cells to protect them from damaging radiation and ultraviolet rays (UV). Emerging space applications will need a lower mass and lower cost than current space solar technology and the flexibility to take advantage of new methods of stowage and deployment.
The Centre for Solar Energy Research (CSER) at Swansea University, UK, in collaboration with the Surrey Space Centre and Centre for Engineering Materials at the University of Surrey, UK, has developed a thin film cadmium telluride (CdTe) PV solar cell technology for use in space. Working with industrial partners Qioptiq Space Technology (QST) and Surrey Satellite Technology, they are undertaking research into depositing thin film CdTe solar cells onto QST’s ultra-thin (100μm) cerium-doped cover glass. The thin film CdTe directly deposited onto cover glass can produce a high power-to-weight ratio, cheaper production and flexible ultra-thin cover glass.
An atmospheric-pressure metal organic chemical vapour deposition (MOCVD) process was used to produce all of the semiconductor layers. The PV device structure was achieved by depositing an aluminium-doped zinc oxide transparent conducting oxide and thin film CdTe layers directly onto the cover glass. The research found that when the CdTe is applied to cover glass, it possesses adhesion, bend radius capability, radiation hardness and demonstrates a desirable power-to-weight ratio. Development of the process and materials led to the production of an optimised solar cell with ground-based air mass (AM1.5), with a cell efficiency of around 15% and open-circuit voltages (per cell) above 800mV.
AlSat Nano is a joint satellite mission between the UK Space Agency and Algerian Space Agency (ASAL). In March 2014, both signed a Memorandum of Understanding, whereby the two parties agreed to enhance collaboration in space programmes. A specific action identified was a joint educational CubeSat development programme to be delivered by Surrey Space Centre for Algerian graduate students. The design, construction and verification of the AlSat-1N spacecraft was undertaken by the students to demonstrate the practical elements of low-cost space technology to help Algeria strengthen its domestic space applications.
ASAL provided the launch and during the spacecraft’s commissioning, the operators used the ground station in Guildford, UK, with full operations transferring to ASAL’s ground station in Oran. The team was one of only three successful competitors to be offered the opportunity to work up a payload for the satellite mission – the Thin Film Solar Cell (TFSC) experiment.
For the TFSC payload to be included in the AlSat-Nano mission, three fully working models were delivered to the satellite manufacturers within nine months (one engineering model, one flight model and one flight spare) by designing solar cell encapsulation, measurement electronics and software.
Internal and external
The TFSC is designed to measure the current-voltage (I-V) and temperature response of four experimental thin-film solar cells when illuminated by the sun at a known angle of one degree or more determined by the spacecraft’s altitude determination and control system. The TFSC payload consists of a single board computer (PC104), a compact and expandable embedded computing standard used for developing embedded systems in space where equipment failure is not an option. The internal 10 x 10cm board houses the processing electronics, power and controller area network and bus data interface to the spacecraft. An external board is used to house the four test cells and a temperature sensor.
The internal board comprises an MSP430 microcontroller with controller area network interfaces and integrated analogue-to-digital converters, which measure temperature, current and voltage signals from four test solar cells. The solar cells are each controlled by an eight-bit digital programmable precision current sink, which is switched to each cell in turn.
The current demand is swept from zero (in open circuit conditions), to a maximum beyond the short circuit capability of the cells, meaning that the voltage across the cell moves from open circuit voltage (VOC) to zero at a short circuit current, generating the I-V curve. These solar cells are then protected against reverse bias by a diode strap. The current, voltage and temperature signals are measured via precision instrumentation amplifiers made to 12-bit precision. The maximum VOC the circuit can measure is set to 1V and the maximum current sink available is 40mA, with no more than ~30mA from the 1cm2 area cells under full solar illumination.
The external board comprises a solar cell test structure (four x 1cm2 area test cells), mounted on a printed circuit board (PCB) substrate with an embedded precision temperature sensor glued directly to the back of the cells through the printed board, configured to measure from -56–148oC. The internal board is connected to the external board via a wire harness and PCB tracks on the rear of the CubeSat’s solar panel.
The cell encapsulation and electrical connections were achieved by laminating two pieces of cover glass together using an adhesive polyimide film. Evaporated gold tracks on the second piece of cover glass allowed external contacts to be made to the four 1cm2 solar cells. This laminated structure was then placed onto a further piece of polyimide and the printed circuit board before being heated under vacuum. Indium and gallium metallic pads were then soldered onto the exposed ends of the gold tracks and gold wire connections enabled the current voltage of the four cells to be measured using specialised electronics and software.
Once triggered, the system measures the I-V curve of each cell at 256 measurements per cell with the entire survey of four cells complete in a second. The data is stored locally, and transferred by request as telemetry over the controlled area network bus to the onboard computer. It is stored for downloading, and the TFSC payload is switched off.
AlSat Nano was launched from southern India on 26 September 2016 and deployed into a 690km polar orbit. After a series of spacecraft health checks and in-orbit commissioning procedures by Surrey Space Centre, operations were then transitioned to Algeria’s newly developed AlSat Nano ground station in Oran. A few weeks later, the first data was received from the CdTe solar cell experiment signifying a world first of a solar cell deposited onto cover glass for aerospace.
AlSat-Nano is predicted to operate in Earth orbit for at least a year, providing real-time data from the solar cell throughout the mission lifetime. The research team will be able to look over a series of measurements, to build up a picture of the solar cells performance in space and its robustness to the space environment.
The solar cell technology has the capability to be upscaled using CSER’s new inline MOCVD deposition system to sequentially deposit each of the layers in a thin film solar cell onto 10 x 20cm2 ultra-thin cover glass substrates. Now the test will be to develop large, low-mass solar arrays, suited to high power applications, for example in an Earth observation to support synthetic aperture radar imaging on small satellites.
Professor Craig Underwood is Head of the Planetary Environments Group at Surrey Space Centre, UK, developing the concepts, instruments and techniques to investigate the Earth and other planetary environments from space.
Dr Dan Lamb is SPARC II Project Manager at the Centre for Solar Energy Research at Swansea University, UK, currently researching high-power, low-weight, flexible thin film photovoltaics for space application.