This project detailed the design of a self-contained energy source for the lighting needs of a schools in Uganda. The system will be used to power basic lighting needs for the schools so that students can study during night time hours when there is not sufficient sunlight available. Some of the factors considered in design are that design must provide atleast 75W of power for 4hrs to power five 15W DC compact fluorescent lightbulbs. The design must also take into account characteristics such as low cost, ease of use/maintenance, use of renewable energy sources, and resistance to harsh weather conditions. After an extensive design selection process,using multiple idea generation and evaluation methods, a solar-powered battery system was selected as a viable option for this problem. The overall design of the setup is shown and the individual components are discussed, along with the constraints and criteria used to evaluate the design. The solar-powered battery system is a simple solution consisting of few parts that can be bought off the shelf and should supply a sufficient amount of power. The system setup consists of a solar panel, a battery, a battery enclosure, and a current control regulator. The solar panel is connected to the battery and acts as the charging source for it. For this setup, a Kyocera 135W 12V solar panel was selected and matched with a Power Sonic 55Ah battery to obtain the necessary amount of electrical power. A Sunsei 10A controller was selected to prevent the battery from overcharging and to prevent power losses in the battery once it has been fully charged. The device enclosure is constructed out of polypropylene plastic and protects both the battery from environmental conditions and the operator from possible injury. Calculations were done using Joule’s Law to ensure that the system could supply enough energy to satisfy the constraints as specified by the customer. Information needed for these calculations was acquired from the manufacturers of the components. These calculations were also used to match a battery with the proper solar panel.
Because the system must be able to store electricity during the day to be used at night, a rechargeable battery is needed. Also, a deep cycle battery is needed because the system will be charging and discharging every day. Deep cycle batteries are designed to not be as susceptible to degradation from constant power cycling . A sealed lead-acid deep cycle battery is chosen as the battery type because it is rechargeable, has a large power-to-weight ratio, and is relatively low cost . The system for this design must be able to supply a minimum of 75W over a 4hr discharge time (300W∙h). Theoretically, a 20A∙h battery would be able to supply the necessary amount of power. However, the electrical charge capacity advertised by manufacturers is rated over a 20hr discharge period. If a battery is discharged at a higher current over a period shorter than 20hrs, it will not be as efficient as its manufacturer amp-hour rating. When selecting the battery, it is important to consider this fact that the charge capacity will be less than what the battery is rated. Keeping this fact in mind, a 55A∙hPower Sonic brand lead-acid battery is chosen for the design setup. Over a 20hr discharge period, this battery should supply 2.75A. However, to determine current output over the four hour period required for this device, data of differing discharge currents was acquired from the manufacturer, which can be seen in Appendix B. A modified plot of the effective charge capacity using this information is shown in figure 3. The manufacturer chart shows that the effective current for this battery over 4 hrs will be 11A. This means that the 55A∙h battery will only have a capacity of 44A∙h for this system. This will translate to 132 W or 528 W∙h:
Solar Panel Design
The solar panel, also known as a photovoltaic panel, this system will be the charging source for the battery. A solar panel consists of a series of interconnected photovoltaic cells. Solar panels are a solar-electric system components, where sunlight is used to make a direct current (DC). They take photons, which are light energy, from the sun and generate electricity using the photovoltaic effect. In the photovoltaic effect, the energy from the photons causes electrons to be transferred between valence and conduction bands in a material, which are silicon cells for the solar panel. This causes a voltage to be built up across two electrodes. The selection of a solar panel for the power system is based on the solar radiation in Uganda and power requirements for the lighting system. Using a solar panel to charge the battery takes advantage of the fact that Uganda has a high number of peak sun hours (5.32 hrs) over the course of a normal day. Through an entire day, peak sun hours reflect the amount of total radiant energy that is falling on the surface in a givenday in terms of how many hours it would have taken to gather that same amount of energy if the sun were directly overhead the entire time. When the sun is directly overhead, or at its peak, the radiation is 1000 W/m 2. For example, Uganda averages 5.32 peak hours during a day. This means that the total amount of energy falling on the surface is equivalent to 5.32 hours of having the sun constantly at its peak. Also, solarpanels are rated under standard test conditions when the sun is at its peak and has an irradiance of 1000 W/m 2. For example, a 135W solar panel should produce 135W when the sun is at its peak. Based off of the capacity of the battery being charged and the amount of peak hours during the day in Uganda, a suitable solar panel was selected. The panel selected is a Kyocera KD125GX-LPU135W solar panel as shown below. The cost of the panel is $342.56 which was the lowest priced panel that still satisfied the power needs for this design. This amounts to $2.54 per Watt of power.
A charge controller is needed to help protect the battery from being overcharged and increase the life of the battery. Most solar panels actually output a higher voltage than their 12V rating (up to 18V). Without any kind of regulation, this could overcharge the battery, which will significantly reduce its life. A charge controller will maintain a constantand correct voltage for the system’s battery. As a battery begins to become fully charged, the controller will start to lower the voltage and current to protect the battery from being damaged. Not only does a controller protect the battery from being overcharged, it protects the solar panel from being damaged as well. When a solar panel is not generating any electricity, electricity will flow back from the battery to the solar panel. This occurs during the night timewhen the battery is full and is known as reverse-current flow. This will drain the battery in addition to damaging the solar panel. When no energy is coming from the solar panel, the controller will close the circuit, stopping any kind ofreverse-current flow. Since the Kyocera solar panel selected can produce a maximum current of 7.63A, the controller needs to be able to handle at least that much. To ensure that the current could be handled, a 10A controller, which can handle any current up to 10A, was selected. The model selected was a Sunsei CC-10000 10A controller. It is a standard controller that has a built in LED indicator that shows when the battery is charging. It has an intelligent Pulse Width Modulation(PWM) charge control that makes sure that the battery reaches its maximum charge. PWM monitors the battery’s condition and charging needs and tapers the current as necessary. It also self-regulates in case of voltage drops or temperature effects in the system. The controller also has integrated screw terminals to connect the solar panel and the battery. The cost of the controller is $32.95.
In order to increase the safety of the design and protect the battery from any kinds of weather conditions, a device enclosure is created. The enclosure will also act as a chassis forthe different components. Theinspiration for the design of the enclosure is based off of a standard batterybox enclosure with slight modifications. The enclosure consists of a large box made out of plastic with a hinged lid. The battery itself goes inside of the box. The lid latches down to close the box and there is a layer of rubber between the lid and the box to seal out any moisture or other contents that may harm or corrode the battery.The box has handles on each side of the box so that the enclosure can be carried and movedby a single person. Polypropylene plastic is selected as the material for the enclosure because it has a low electrical conductivity and resistance to chemical solvents such as battery acid. Polypropylene is a poor conductor of electricity. A material with low electrical conductivity will increase the safety of the design by ensuring that the operator can not beshocked while handling the box with the battery inside of it. Even though a sealed lead-acid battery is being used, which are designed to prevent against most electrolyte spills, it is still important to make sure that the enclosure properly seals in the battery in case an electrolyte spill does happen. A hinged lid that opens and closes allows for easy access to the battery in case maintenance needs to be done, such as cleaning the terminals or adjusting the cable connections. Also, it allows for the battery to be easily replaced when needed. The handles, hinges, screws, and latch can all be bought off the shelf and the enclosure itself requires few assembly steps.