Practical course. Fundamentals of software and topology solar power plants.

Fundamentals of software and topology solar power plants.

This course is allowed to be used for educational purposes.

Performance Modeling

Sophisticated simulation software is used to predict the output of a solar power plant. Forecasting reports should take into account factors that negatively affect plant performance.

Calculation of energy production. Software Review. 

To simulate a solar power plant, engineers should use only licensed software, which makes it possible to work with the most relevant databases on climatic conditions and equipment parameters. Using licensed software ensures that the results obtained will accurately predict the results of both industrial solar power plants and home use. 

Uncertainty in predicting energy production for modeling the generated energy depends at each stage of modeling on the uncertainty of the input parameters. Software modeling in itself may allow uncertainties of 2% to 3%.

The use of solar cells coincides with the use of other sources of electricity, but unlike them, solar panels depend on the amount of light that falls on their surface. For example, in cloudy weather, clouds can significantly reduce the output power of the photovoltaic panel, up to 50%. Also, even a small defect in solar cells can reduce the efficiency even for panels from the same batch. Therefore, to ensure the desired power, it is necessary to sort the elements by the output current. An example is the following: if you try to insert a pipe with a smaller diameter into a water pipe with a rather large diameter, it is natural that the watercourse becomes smaller. The same thing happens in chains of solar cells if their parameters are heterogeneous.

Silicon solar cells cannot be described by Ohm’s simple law, since it is a nonlinear cell. Instead, to explain the characteristics of an element, you can use several simple curves – current-voltage characteristics (CVC).

The open circuit voltage generated by one element changes slightly when switching from one element to another in one batch and from one manufacturer to another and is about 0.6 V. This value does not depend on the size of the element. The situation is different with current. It depends on the light intensity and the size of the element, which means its surface area. An element with a size of 100 * 100 mm is 100 times larger than an element with a size of 10 * 10 mm and, therefore, it will give a current 100 times larger at the same illumination. 

Peak power corresponds to a voltage of about 0.47 V. Thus, in order to correctly assess the quality of the solar cell, and also for comparing the cells with each other under the same conditions, it is necessary to load it so that the output voltage is 0.47 V. After the solar Elements are selected for work, it is necessary to solder them. Serial elements are equipped with current collecting grids, which are designed to solder conductors to them.

One of the important aspects of the solar cell is its temperature. Thus,by heating one element only one degree higher than normal (25^C)it may lose a voltage of about 0,002 V, i.e.,°  0.4% / degree. Figure 5.3 shows the IV characteristic curve for temperatures25^ofC and 60C.

In a sunny day sufficiently different elements can be heated up to 60-70^°Cand thus save 0.07-0.09 each. This reason is one of the main ones in the case of a voltage drop, and as a consequence, and a drop in the efficiency of the solar cell.

The efficiency of a conventional solar cell currently ranges from 10-16%. This means that an element with a size of 100 * 100 mm under standard conditions can generate 1-1.6 watts.

All photovoltaic systems can be divided into two types: autonomous and connected to the electric network.

An autonomous system in the general case consists of a set of solar modules located on a supporting structure or on a roof, a storage battery (battery), a discharge controller — a battery charge, connecting cables. Solar modules are the main component for building photovoltaic systems. They can be made with any output voltage.

For ground use, they are usually used to charge rechargeable batteries (batteries) with a nominal voltage of 12 V. In this case, as a rule, 36 solar cells are connected in series and sealed by lamination on glass, textolite, and aluminum. The elements are located between the two layers of the sealing film, without air gap. The technology of vacuum lamination allows to fulfill this requirement. In the case of an air gap between the protective glass and the element, the reflection and absorption losses would reach 20-30% compared to 12% without the air gap.

The electrical parameters of the solar cell are represented as a single solar cell and a current-voltage curve under standard conditions (Standart Test Conditions), i.e., when solar radiation is 1000 W / m^2,a temperature of – 25^°Cand at a latitude of the solar spectrum 45^of( AM1.5). 

The mean value for the operating voltage of the module, which consists of 36 elements, will be from about 16 to 17 V (this is approximately 0.45 … .0,47 In one member) at a standard temperature  25^of C.

Thus, when heated in real operating conditions, the modules are heated to a temperature of 60-70^°C,which corresponds to the displacement of the working point voltage, for example, for a module with the operating voltage 17 V – with values of 17 V to 13,7-14,4 V (0,38-0, 4 V per cell).

Based on the foregoing, we must approach the calculation of the number of module elements connected in series. If the consumer needs to have an alternating voltage, then an inverter-converter of direct voltage to alternating voltage is added to this kit.

Under the calculation of FES refers to the determination of the rated power of the modules, their number, connection diagrams; choice of type, operating conditions and battery capacity; inverter and charge-discharge controller capacities; definition of parameters of connecting cables.

First you need to calculate the total power consumption for all possible consumers. The consumer power can be found in the product passport. At this stage, you can choose an inverter, for this it is enough to increase the power by 1.3 times at least. But it must be borne in mind that some consumer devices, such as a refrigerator, at the time of start-up consume power 2-3 times more than the nameplate. For powerful stations (more than 3 kW), the inverter voltage must be at least 48 V, because inverters cope well with large voltages, thereby reducing losses.

Calculation of power consumed by consumers. When calculating a solar power plant, the first thing to do is to make a simple list of electricity consumers. Calculate how much power each consumer consumes, how much voltage, and accordingly add to the table.

Consumers of alternating voltage (in our case, consumers from No. 1 to No. 4, and No. 6 from the table) are connected to the inverter, and constant consumers (lighting No. 5 and some other consumers No. 7 from the table) directly through the charge controller. In this system under consideration, a bus with a voltage of 24 V corresponding to the voltage of the ASE battery is taken as the main bus.

After that, you need to find out how much time in hours a particular consumer works per day. Then, multiplying the consumer’s power by the entire time of its operation, we will determine how much this load of electricity consumes daily. Thus, build a table of energy consumption per day.

Daily energy consumption table

Load	Power, W	Voltage, V	Operating time, h / day	Consumption, VT · h / day
1 Kettle	1000	220	0.5	500
2 Microwave	1300	220	0.25	325
3 Refrigerator	250	220	12	3000
4 TV	100	220	6	600
5 Lighting	100	24	6	600
6 Laptop	70	220	5	350
7 Other consumers	100	24	3	300
Total	2920			5675 A

solar power plant can supply many energy consumers with the condition that the total energy of the consumers will not exceed the power of the solar power plants. The list of consumers contains loads operating either continuously (lighting) or intermittently (kettle, television). But also, loads that work inconsistently are divided into two categories, some work with a fixed interval, while others work with a floating interval (for example, like a refrigerator from the table). 

Therefore, it is important to correctly determine the total output power of a solar power plant. To reduce the cost of solar cells, it is also necessary to build a schedule of changes in load consumption per day, that is, compile a table by time and enter the time of the load. It is important to ensure that several consumers with high power or a large number of consumers with low power are not simultaneously turned on. When constructing such a schedule, it is very difficult to understand exactly when a consumer with a floating load is turned on (refrigerator, table). To be sure to protect yourself, and not to miss, we assume that such consumers are constantly working.