Photovoltaic Cells and Batteries

The photovoltaic (PV) cell is different from any electrochemical cell. It’s also known as a solar cell. This device converts visible light, infrared (IR), and/or ultraviolet (UV) directly into electric current.

Solar Panels

Several, or many, photovoltaic cells can be combined in series-parallel to make a solar panel. An example is shown in following figure. Although this shows a 3 × 3 series-parallel array, the matrix does not have to be symmetrical. And it’s often very large. It might consist of, say, 50 parallel sets of 20 seriesconnected cells. The series scheme boosts the voltage to the desired level, and the parallel scheme increases the current-delivering ability of the panel. It’s not unusual to see hundreds of solar cells combined in this way to make a large panel.

Connection of cells in series-parallel.

Construction and Performance

The construction of a photovoltaic cell is shown in following figure. The device is a flat semiconductor P-N junction, and the assembly is made transparent so that light can fall directly on the P-type silicon. The metal ribbing, forming the positive electrode, is interconnected by means of tiny wires. The negative electrode is a metal backing or substrate, placed in contact with the N-type silicon. Most solar cells provide about 0.5 V. If there is very low current demand, dim light will result in the full-output voltage from a solar cell. As the current demand increases, brighter light is needed to produce the full-output voltage. There is a maximum limit to the current that can be provided from a solar cell, no matter how bright the light. This limit is increased by connecting solar cells in parallel.

Construction of a silicon photovoltaic cell.

Practical Applications

Solar cells have become cheaper and more efficient in recent years, as researchers have looked to them as an alternative energy source. Solar panels are used in satellites. They can be used in conjunction with rechargeable batteries, such as the lead-acid or nickel-cadmium types, to provide power independent of the commercial utilities.

A completely independent solar/battery power system is called a stand-alone system. It uses large solar panels, large-capacity lead-acid batteries, power converters to convert the dc into ac, and a sophisticated charging circuit. These systems are best suited to environments where there is sunshine a high percentage of the time.