The Science of Solar Energy Harvesting
Solar energy, a very abundant source of renewable energy is widely available all over the world. As the other sources of energy like water, wind, coal, nuclear fuel, petrol, diesel, natural gas etc. are being used widely and hence the reserves are depleting at pace. So there is a strong need to find more renewable sources of energy like solar energy. In this article, we will have a glance on the basic principle and science of solar energy harvesting.
A brief about Solar Source
Solar energy in terms of availability and cleanliness, is an economical and sustainable source of energy. It is cost effective and easy to install. It is the most widely implemented method today in many countries and the concept of smart cities with smart grids has developed due to the small solar setups. Due to unavailability of solar source at night storage methods have been developed.
A solar panel or photo-voltaic panel works on the principle of photo-electric effect i.e. creation of electron flow from exposure to light and hence generation of electric current. A solar panel comprises of numerous small solar cells working to attain the same purpose. A solar panel is a constant current source hence having a variating voltage that changes with respect to load.
Basic Science of Working
To generate electrical energy of desired frequency the way that is common is to first convert it into DC power with a static voltage level and then make desired AC for utility purpose. For conversion to AC, an inverter is used at the output of a DC source. The solar power is first stored in DC source like a battery but in a controlled way taking into account the level of availability. To create a load independent solar harvesting setup, there is a need to create an electrical circuit that works on the principle of impedance matching between the source (solar panel) and the load (DC source like a battery). This circuit also helps to achieve maximum power extraction from the solar panel as the maximum power theorem states that maximum will be delivered to load if the internal impedance is equal to load impedance.
This electrical circuit consists of a DC-DC converter and an intelligent controller.
The converter must be with buck and boost action as well. In lay language, buck means to lower i.e. when sun is more than requirement then the converter lowers the resulting electrical power. Boost means to increase i.e. when the sunlight has less intensity the converter will boost the electrical power to meet the requirement. For buck and boost operation there are two converters: buck-boost converter and CUK converter. The former one is feasible in terms of cost and less power dissipation due to less hardware but has discontinuous current conduction. The later one is expensive as it involves more hardware and hence more power dissipation but has continuous current conduction. In continuous conduction the current through inductor of converter never goes down to zero and in discontinuous conduction current through inductor goes down to zero at least once during a single conduction cycle. In normal solar setup, buck-boost converter is enough.
The other component is an intelligent controller that controls the operation of DC-DC converter taking into account the power of solar panel and voltage of load (battery). The operating entity of a DC-DC converter is the duty cycle that is controlled by the controller. The controller tunes the duty cycle in such a way that it can draw maximum power at the present time from the panel. In other words the controller tracks the maximum power point of the panel. There are many algorithms that can be implemented in the controller to achieve the maximum power point. These algorithms are called Maximum Power Point tracking (MMPT) Algorithms that mainly include: Perturb & observe (P & O) Algorithm, Incremental Conductance Algorithm, Pulse Width Modulation (PWM) Algorithm and Constant Current Algorithm. The most easiest to implement and cost efficient is Perturb & observe (P & O) Algorithm. In this algorithm the controller starts tuning the duty cycle from a random value and then compares the power of the current instant to the power of the previous instant of the solar panel. If the current power is greater than the previous instant’s power then it keeps on tuning the duty cycle in the same direction and vice versa. The disadvantage is that it does not tempers off charging slowly. But it is of little concern in case of lead acid batteries and other rough batteries. For the choice of controller Arduino AT mega can be used which is coded in C++ language.
At the output of the panel the converter is connected. At the output of converter the battery or load is connected. The switch or a FET (field effect transistor) in the hardware of the DC-DC converter is controlled by controller through the gate signal. The controller will draw maximum power according to the rating of panel. The components of DC-DC converter must be according to the power rating of the panel.
Now the battery can be used as a supply to an inverter that creates AC current of desired frequency for the utility purpose. As solar energy is a renewable source hence the system only requires one time installation. According to the concept of a smart city, the small solar setup along with metering equipment installed in houses contributes to the grid during day and draws power when the solar source is absent or the demand is higher than the generation capacity of solar setup.