Solar Cells versus Solar Module
Let’s get started by understanding ‘why’ solar cells are interconnected to form a solar module. Then, the ‘how’ to connect solar cells to produce a solar module will be discussed in another post. A single solar cell (e.g. crystalline silicon type) can only produce a small open-circuit voltage Voc of about 0.55 to 0.72 V at a cell temperature of 25°C and depending on the cell area, the short-circuit current Isc ranges from 3 to 15 A (Häberlin, 2012:127). But for optimal power yield, a standard 156×156mm crystalline silicon PV cell is designed to reach its peak watts of about 4 Wp of DC power (Archer & Green, 2015:1) and its voltage at Maximum Power Point (MPP) of between 0.45 to 0.58 V (Häberlin, 2012:127; Hankins, 2010:31).
Most appliances can hardly be operated with such a low voltage produced by a solar cell with the present technology. Hence for solar electricity to be useful for practical purposes, it is necessary to step up the voltage using multiple solar cells wired in series (Häberlin, 2012:127). Other methods of generating higher voltage involve using bigger cells, more efficient cells, or exposing the cells to more intense sunlight. However, there are practical limits to the size, efficiency, and the intensity of sunlight a cell can tolerate (Schaeffer, 2005:77). Therefore, it is more feasible to “wire” several cells in series to produce “decent” voltages that are suitable for a given practical application (Mertens, 2014:135). Join the next stream of our Solar Energy Training in Nigeria
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How Photovoltaic Cells Produce Electricity
The words ‘photo’ and ‘voltaic’ literally translate to ‘light’ and ‘current’ respectively. Sunlight is made of little packets of energy called ‘photons’ and the photovoltaic (PV) cells of a solar module are made of semiconductor materials Figure 1.
Figure 1: From Semi-conductor to Solar Cells to Solar Module
Thus, a solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. When the photons strike a typical photovoltaic cell, the energy from the light is absorbed by the semiconductor material in the solar cell and this releases electrons in the process. It is this flow of electrons that creates electric current.
Hence, the process of converting sunlight to electricity usually involves two basic stages. The first stage requires a material in which the absorption of light raises an electron to a higher energy state. In the second stage, there is a movement of the higher energy electron from the solar cell into an external circuit (e.g. the load). The electron then dissipates its energy in the external circuit and returns to the solar cell. This creates a complete path for electron movement and by extension current flow.
Mechanical Characteristics of Solar Modules
To keep a module’s weight to a size that can be readily handled by one person and minimize damage during installation, many commercial solar modules are built to house between 32 to 72 solar cells in a single enclosure to protect them against the environment (Häberlin, 2012:127) with a typical PV 60-cell and 72-cell modules generating about 240Wp and 300Wp respectively (Archer & Green, 2015:1). The cells are hermetically packed in a transparent plastic material such as ethyl vinyl acetate (EVA) (Häberlin, 2012:127).
Thus, some of the mechanical properties to pay attention to when making a choice of a brand of solar module are number of cells, module weight, module dimensions, module frame (usually anodised aluminium alloy), etc.
Electrical Characteristics of Solar Modules
The electrical properties of solar modules are perhaps the most important parameters that determine the power generating capacity of PV array. They are also the sensitive aspects of the PV array.
If not properly designed and selectect, it may lead to poor performance of the entire solar system and in some cases overall system failure or early degrading.
Some of the important electrical properties include: Module Efficiency, Module Fill Factor, Maximum Power Point (MPP) Voltage (Vmpp), Maximum Power Point Current (Impp), Open Circuit Voltage (Voc), Short Circuit Current (Isc), I-V curves of PV Module, and P-V curves of PV module.
Connecting Solar Modules
In many applications, a single photovoltaic module may not have enough power to operate the appliances connected to it due to some physical constraints which include its weight which must be kept below a maximum limit to permit easy handling at elevated height. Consequently, several identical modules are often connected in three different forms to produce power sufficient enough to run the connected appliances. They include series connection, parallel connection and series-parallel connection. Learn more about Solar Module Mount and Rack Systems for optimal performance of your next solar project.
In conclusion, you should understand that the electrical characteristics of a module are as important as well as sensitive. Hence, an expert in solar system application will be in a better position to determine which module electrical parameters are more suitable for a given application. Let’s be smart to involve a solar PV expert in our next solar project.