Photovoltaic cells and batteries

Chapter Photovoltaic cells and batteries

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Photovoltaic cells and batteriesThe photovoltaic cell is completely different from any of the electrochemical cells. It’salso known as a solar cell. This device converts visible light, infrared, and/or ultravioletdirectly into electric current.Solar panelsSeveral, or many, photovoltaic cells can be combined in series-parallel to make a solarpanel. An example is shown in Fig. 7-6. Although this shows a 3 × 3 series-parallel ar-ray, the matrix does not have to be symmetrical. And it’s often very large. It might con-sist of, say, 50 parallel sets of 20 series-connected cells. The series scheme boosts thevoltage to the desired level, and the parallel scheme increases the current-deliveringability of the panel. It’s not unusual to see hundreds of solar cells combined in this wayto make a large panel.Construction and performanceThe construction of a photovoltaic cell is shown in Fig. 7-7. The device is a flat semi-conductor P-N junction, and the assembly is made transparent so that light can fall di-rectly on the P-type silicon. The metal ribbing, forming the positive electrode, isinterconnected by means of tiny wires. The negative electrode is a metal backing,placed in contact with the N-type silicon. Most solar cells provide about 0.5 V. If thereis very low current demand, dim light will result in the full output voltage from a solarcell. As the current demand increases, brighter light is needed to produce the full out-put voltage. There is a maximum limit to the current that can be provided from a solarcell, no matter how bright the light. This limit is increased by connecting solar cells inparallel.Practical applicationsSolar cells have become cheaper and more efficient in recent years, as researchershave looked to them as a long-term alternative energy source. Solar panels are usedin satellites. They can be used in conjunction with rechargeable batteries, such asthe lead-acid or nickel-cadmium types, to provide power independent of the com-mercial utilities.A completely independent solar/battery power system is called a stand-alone sys-tem. It generally uses large solar panels, large-capacity lead-acid or NICAD batteries,power converters to convert the dc into ac, and a rather sophisticated charging circuit.These systems are best suited to environments where there is sunshine a high percent-age of the time.Solar cells, either alone or supplemented with rechargeable batteries, can be con-nected into a home electric system in an interactive arrangement with the electric util-ities. When the solar power system can’t provide for the needs of the household all byitself, the utility company can take up the slack. Conversely, when the solar power sys-tem supplies more than enough for the needs of the home, the utility company can buythe excess.Photovoltaic cells and batteries127