By combining energy harvester with rechargeable battery, we can create battery-based power source of infinite lifetime ideally.
Figure 2. Block diagram of solar energy system.
The prototype system designed for this project is shown in Fig.2. It consists of solar panel, charge regulator, energy store device which is a 1.2 volts rechargeable battery, DC-DC converter and solar tracker. Solar cells can be either connected in series or parallel to provide a desired value of output voltage or current. The solar panel for this project is designed to provide an output voltage of 4 volts (200mA). It can be achieved by using four 2V (100mA) solar cells. Four solar cells are divided into two groups. Each group consists of two solar cells which are connected in series providing an output voltage of 4 volts. Since the current will drop as the voltage later being boosted to a higher level, the two groups of solar cells are therefore connected in parallel to increase the output current to 200 mA. Since the instantaneously available energy collected by solar panel is usually not enough to power the electronic device therefore it is desirable to be accumulated and stored in the battery until the energy is enough to do something practical. The energy stored in the battery later can be boosted to a higher voltage level by DC-DC converter in order to power the DC loads, such as the motor of solar tracker.
Figure 3. How the silicon solar cell works.
As one method that has been widely used for electricity generation from solar energy is photovoltaic generation. Photovoltaics is the term used to describe the electricity from solar energy by giving off electrons when the atoms of photosensitive material are struck by light, which is also known as the photovoltaic effect. Silicon solar cell is basic generator for small amount of electric power since silicon is the most commonly used material in the electronic industry. As the semiconductor technology advanced, the properties of such element become fairly well known and pure silicon is therefore doped or contaminated with other elements in order to create a field, also known as potential difference, allowing the valence electrons ejected from silicon atoms to wander through the crystal lattice forming a steady electron flow, or current. Such semiconductor is capable of maintaining a much greater electron flow than would be possible with the metal in its pure state. The material used for solar cells is called polycrystalline silicon or polysilicon. The single-crystal silicon is produced based on polysilicon in order to improve the electrical properties of silicon. Numerous thin-film technologies, such as crystalline silicon (c-Si) or multicrystalline silicon (mc-Si), as well as thin film CdTe and copper indium gallium diselenide and related materials are currently being developed to reach an energy conversion efficiency of at least 10%. Due to the unused areas between cells, the working efficiency of actual solar modules is only about 10 to 12 percent even the mass-produced single-crystal silicon solar cells can have an efficiency higher to 14 or 15 percent. Since crystalline solar cells are quite common on the market and relative cheap, it is therefore chosen to be the energy collector of this project. The Fig. 1 has fully represented the action of a silicon solar cell that trillions free electrons are ejected from silicon atoms due to photon bombardment. These free electrons are accelerated across the P-N junction of the cell to the front face where they are collected by metallic grids and enter a path formed by grid lines to the external circuit and provide current to the load. Electrons then return to the back of the cells to complete the circuit by fill the waiting holes in the P-silicon. The process is clean since there is either no noise or pollution, only a small amount of heat dissipated to the ambient surroundings [4, 5].
Figure 4. Equivalent circuit of a solar cell.
The internal resistance Rs could affect the performance of solar cell by causing the internal voltage drop according to the Fig. 2 which shows the equivalent circuit for a solar cell. The Eq. (1-3) below show that the net electrical current through the Rs and load RL is the difference between the light-generated current and the current through diode junction.
Due to proper design of the metallization and the conductivity of the material, Rs is often assumed to be equal to zero and therefore the internal voltage drop in a cell can be minimized .
Since only a 1.2V rechargeable battery is selected to be the energy store device, it is not necessary to add a charge controller. Instead, a voltage regulator which is capable of preventing the high voltage that could damage the battery or reduce its lifetime is used to provide a stable voltage of 1.45V to charge the battery safely. As shown in Fig. 5, the is 1.45 volts and the voltage drop between base and emitter of a silicon transistor is always 0.6 volts if the transistor is "on". Therefore, the voltage clamped by the zener diode is 2.05 volts. Since the input voltage is 4 volts, the voltage across is 1.95 volts and therefore the resistance of is equal to be
Fig 6. Boost converter using MAX756.
A dc-dc converter is also needed to provide an output voltage that is higher than the input for certain power needs. The investigation undertaken into dc-dc converter has revealed the fact that a step-up (boost) converter which converts the lower input voltage to a higher voltage level is more suitable for the circuit . The advantage of using this component is that the output voltage can be increased to a higher level from a single battery thereby saving space instead of using multiple batteries to accomplish the same task. Furthermore, it can avoid the situation that the voltage provided by solar panel is not high enough to charge the battery if two or more batteries are used in series to store the electricity. Furthermore, adopting this component can reduce the size and cost of the circuit. A type of circuit diagram has been analyzed as shown in Fig. 2. It has a MAX756 CMOS step-up DC-DC switching regulators for small, low input voltage or battery-powered systems, converting a positive input voltage down to 0.7V to a higher pin-selectable output voltage of 3.3V or 5V .
Figure 6. Block diagram of solar tracker.
The solar tracker is used to track the sun in order to allow the cells face towards to the incoming sunlight, resulting in much-improved efficiency. There are two types of sun-tracking systems. The two-axis tracking system can allow up to 50 percent more energy collected by solar panel. However, the mounting and mechanism of this type are quite costly. Single-axis tracking system has emerged as it offers a good compromise between capital costs and energy delivered . Therefore, solar tracker is of vital importance in the performance of solar energy harvesting system. The design of solar tracker of this project is inspired from an online video . As shown in Fig.6, the motor is powered by a 4AA rechargeable battery which is 4.8 volts. Two photo resistors are soldered to the corresponding pins of potentiometer in the servo motor. The principle is simple that the resistance of photo resistors will change as they sense the change of direction of incoming sun light. The servo motor will therefore rotate according to the potentiometer which is a three-pin resistor acting as a voltage divider.