Task 1　The Energy Conversion of Solar Cells

Ⅰ.Knowledge of solar cell energy conversion

Solar energy is a kind of radiant energy，which must be converted into electrical energy with the help of energy converter. Energy converter which transforms solar energy into electricity is called solar cell module.

Fig.1-1-1 is a p-type silicon material made of n+/p type mechanism of conventional solar cells.

1.Solar cell module structure

①p layer is the base layer.

②n layer is called top layer，which is above p layer. It is made by high temperature diffusion on the surface of the same material，so it is called diffusion layer. Because it is often heavily doped（semiconductor material-mix），it is often marked as n+. The diffusion layer is on the front side of the cell，which is the sun-light surface，so it is also called light face.

③The interface between p layer and n layer is p-n junctions.

④The diffusion layer has an upper electrode which has ohm contact with it. It consists of a main grating line and a plurality of sub grating lines，which are connected by the main.

⑤Under the base body，there is a lower electrode which has ohm contact with it.

⑥The upper and lower electrodes are made of metal materials and their function is outputting the electrical energy.

⑦There is a layer of antireflection film on the upper face. Its function is to reduce the reflection of light and make solar cell receive more light.

If base body is made only by n type Silicon material，then it becomes p+/n type silicon solar cells. It's structure is the same with the above n+/p type solar cells，but the silicon material of base body is n-type and the diffusion layer material is p-type.

2.Energy conversion through solar cell

Photovoltaic effect of semiconductor p-n junction is the basic theory of energy conversion through solar cell. Photovoltaic effect is a kind of effect that happens when the object is exposed to light and its charge distribution changes to produce electromotive force and current.

The process of silicon solar cell is summarized as follows：

①The first step is to collect sunlight and other light and make it irradiate on the surface of solar cell.

②Solar cells absorb photons with certain energy and excites out unbalanced carriers（Photogenerated carriers），which are also called electron and hole pair. These electrons and holes should have sufficient lifetime and they will not disappear until they are separated.

③In the solar cell p-n junction internal electric field，these photogenerated carriers with oposite electrical signs are separated electron and hole pairs，so that electrons are concentrated on one side while holes on the other. Electrons and holes accumulate opposite charge at the ends of p-n junctions and then it comes photoelectric electromotive force，which is photogenerated voltage.

④Lead out electrodes at two ends of solar battery's p-n junction and connect the loads，photovoltaic current passes through external circuit，and then power is output. In this way，solar cells directly convert solar energy（or other light energy）into electrical energy. Training device consists of Monocrystalline silicon solar cells.

When type p and n semiconductor are combined together，a p-n junction is formed at their interface and inner electric field is built on both sides，which is also called barrier electric field. When irradiated by sunshine，electrons in semiconductor obtain solar energy and become free electrons. Then electron hole pairs appear. Under the influence of electric field，electrons are driven to the N region while holes are driven to the P region. So that N area has surplus electrons and P area has surplus holes. Near the p-n junction，sunlight induced electric field which is opposite to electric field is formed. A part of sun light induced electric field counteract barrier electric field and the rest part makes P area becomes positive and N area negative. Then electromotive force，which is photovoltaic electromotive force，is produced in the thin layer between N and P area. When external circuit is switched on，electric energy can be output.

This is the basic electricity generation principle of p-n junction solar silicon cell，as is shown in Fig.1-1-2. If dozens or hundreds of single solar cells are connected in series or parallel and packaged to solar cell modules，a certain power can be obtained under the irradiation of sun.

Ⅱ.Introduction of training system

1.Training equipment

THEISP-1 solar power technology and application devices（Fig.1-1-3）are composed of two parts：battery plate and main console.

2.Technical parameters of console

①Input voltage：three-phase four-wire system 380V±10％ 50Hz.

②Working environment：temperature -5～40℃，relative humidity <85％（25℃），altitude <4000m.

③Device capacity：≤2000W.

3.Preparation before training

①Please be familiar with relevant components of the platform.

②Please ensure that the training device is powered off.

③Study relevant training content on the book before training.

④The connection between components should be in strict accordance with training steps.

4.Notes

①Strictly follow correct operation when powering on and off training devices.

②Connect parts of solar energy in accordance with requirements，or the service life will be reduced.

③Please read the relevant instructions before operating solar intelligent controller.

④Be careful and do not scratch the surface when operating solar cell components.

⑤When battery is being charged，solar battery charging circuit must be disconnected. All load switches also should be disconnected to prevent the charger from directly supplying power to the load and damage it.

5.Training process

（1）Grouping and reasonable division of labor

Each training proceeds in groups，each of which consisting of 3 to 5 persons. Every group should have a clear division of labor in terms of wiring，loading，regulating voltage or current，recording data and other works to ensure coordination and make recorded data accurate and reliable.

（2）Wiring requirements

Select components，instruments and wiring according to training circuit diagram. Wiring should be simple and clear.

（3）Operation

After wiring，turn on simulating light sources，then solar module receives light and generates electricity. Electricity is sent to main console over air cable，charge the battery through solar intelligent controller and power corresponding load at the same time.

（4）Reading data

Connect corresponding ammeter and voltmeter to circuit，measure current and voltage values and write down，then draw relevant curve.

This is the final stage of training. Training data should be collected with waveform and charts drawn，training phenomena analyzed and exercises written. Each training participant should complete a training exercise independently in a serious and realistic manner. If training result deviates far from the theory，data and results should not be modified at will. Analyze the data and results with the application of theory，explain training phenom enon and find out the cause of error.

6.Training exercises

Training exercises are summary and experience which are summarized from measured data and problems in the training. It should be concise with clean charts and clear conclusions.

The following contents should be included in report：

①Training name，class，group members' name and room number.

②Purposes，circuits and contents.

③Training equipment，instrument model，specifications，nameplate and device number.

④Data collation，list，calculation，and calculation formula used.

⑤Curve of experimental data and waveform.

⑥Analyze and summarize the results according to theoretical knowledge and draw correct conclusion.

⑦Analyze phenomena and problems encountered in the training，write experience and put forward recommendations and improvement measures.

⑧Practical exercises should be written on the specified paper. Keep it clean.

⑨Each one should finish a report independently and send it to the instructor on time.

7.Safety operation rules

In order to complete the training on time and ensure the safety of training and equipment，the following rules must be strictly observed.

①Human body mustn't touch live lines during experiments.

②Wires must be connected or disconnected after powered off.

③Students have to ask their instructor to check and give approval，and inform other members to pay attention，then they can connect or change wires and power the device. If the device breaks down in the experiment，the power should be cut off immediately. And the experiment can only be continued after the problem has been handled.

④Before connecting training device，check whether there is a short circuit in training circuit to avoid damaging training instrument or power supply.

⑤The main power of laboratory or power supply on the control screen of test bench should be controlled by the instructor. And other persons can operate only with the permission of instructor.

Ⅲ.Training steps of solar cell module application

①Switch on air switch of the main control screen.

②Turn on three simulating light source lamps by switching on “SD1，SD2，SD3” on the “Simulating light source” of main control panel and don't forget to open all the cooling fans before that.

③On the analysis of solar panels at this time due to accept the “light” will produce“electricity.”

④Disconnect K1，K2 and connect DC voltmeter with the V1 port. Measure and record the voltage produced by solar panel at the moment.

⑤Disconnect K1，K2，short out the V1 port with wire and connect DC ammeter with A1. Then measure current and record it's value.

⑥Move solar panels to change the distance between light source and solar panels. Repeat step ④ and ⑤，and then record voltage and current value.

【Exercise】

（1）Describe generation process p/n junction. Which movement plays a major role in the process? why?

（2）What is the standard of PV module measurement? What is the influence of the changing of distance between light source and components?

（3）Finish training exercises according to the requirements. Fill in Appendix table 1-1-1 to Appendix table 1-1-6 in Appendix 1.

（4）Draw seven test curves and analyze changing law of each curve.