A CubeSat is a miniature satellite originally designed for space science. Conceived as an educational tool, they have managed to challenge traditional satellite standards and are recognized for their potential utility by space and research agencies around the world . These projects are primarily led primarily by universities and non-US space groups. Government agencies have sponsored the development of these projects through organizations such as; National Aeronautics and Space Administration (NASA), National Science Foundation (NSF), and through the Department of Energy (DoE) . CubeSats are mainly composed of several units; an on-board computer, control and communication systems, among other measurement devices.
In order for these units to operate it is essential that their power is supplied by an EPS. Fig. 2 shows a standard CubeSat design, illustrating some of the most vital parts of the Electronic Power Supply (EPS).
Fig 2: Standard CubeSat Diagram
The goal of the project is to help develop an aerospace engineering experience for students in Puerto Rico and to provide a system engineering experience for the students. The Space Plasma Ionic Charge Analyzer (SPICA) CubeSats main objective is to acquire relevant space weather data in order to understand the effects the sun has on our planet. The UPRM is tasked with designing the EPS system. The Power Electronic courses in the university cover a wide range of power supply designs using DC/DC converters with a variety of applications, including aerospace. A CubeSat is powered with solar energy; solar panels located on each one of its sides. The six solar panels are constructed with 24 Triangular Advanced, Improved triple-junction Gallium Arsenide (GaAs) solar cells. Individually each solar cell has a Voc and the Isc of 2.52V and 0.031A respectively. When the series and parallel connections are made between the solar cells, the end results are six 10cm x 10cm solar panel possessing a Voc and the Isc of 5.04V and 0.372A respectively. The solar panel supplies the power to the necessary loads as well as to power the microcontroller while at the same time charging the battery source of the CubeSat. It is essential to maximize the available electrical energy gained from the minimal solar cell area available. In order to accomplish this, the maximum power point tracking (MPPT) methods is used . The considered MPPT technique to maximize power output is the optimal duty cycle method. The proposed EPS design for the CubeSat is illustrated in Fig. 3.
Fig 3: CubeSat EPS Design
The EPS must provide power to the CubeSats peripherals so it can maintain its functionality. The power supply design will accommodate an onboard computer, ADS Sensor, ACS actuator, several scientific instruments, as well as the communications system. The design consists of a primary DC/DC converter that will perform the MPPT, in order to obtain the maximum power from the solar cell array. In order to regulate voltage at the loads, two voltage regulators also constructed with DC/DC converters are used. For these DC/DC converters, the chosen topology is the Single-Ended Primary-Inductor Converter (SEPIC). A Lithium (Li) battery is used due to its compact size and light weight . The battery is connected to the output of the SEPIC as well as to the input of two DC/DC converters used for voltage regulators.
The analysis of solar panel behavior is performed using methods developed by professors from the UPRM . These models are taught and discussed in ECE power electronics and renewable energy courses. This mathematical model makes use of a series of equations that take into consideration the useful data given by the manufacturer’s data sheet that are given at standard test conditions. The developed mathematical model that represents the output current of the photovoltaic module is shown in equation (1).
In these equations, V is the output voltage of the panel and the variable b is the characteristic constant of the photovoltaic model that describes the I-V relationship. The expression used to obtain the parameter Vx is shown in equation (2).
This expression uses the maximum and minimum voltage values, where Vmax is the open-circuit voltage at 25°C and more than 1,200W/m2, Vmin is the open-circuit voltage at 25°C and less than 200W/m2. Ei is the effective solar irradiation in W/m2. TCV is the temperature coefficient of Voc in V/°C. The variable T is the solar panel temperature in °C. TN is 25°C and the nominal effective solar irradiation EiN is 1,000W/m2. Voc is the open-circuit voltage at 25C and 1000W/m2. The variable Ix in equation (1) can be calculated by using equation (3).
The variable Isc is the short circuit current measured at standard test conditions. In this equation, TCi is the temperature coefficient of Isc in A/C. These are just examples of how mathematical models developed by universities are being implemented in order to aid in the development process.