- General
Photovoltaic power generation unit is a basic organization in photovoltaic power station. Usually, each photovoltaic power generation unit is equipped with one transformer and several inverters. In the actual project, the installed capacity of a photovoltaic power generation unit is more flexible, and a photovoltaic power generation unit can be designed according to the scale of 500kwp, 1mwp, 2mwp, etc.
As the photovoltaic modules need to avoid being shaded as much as possible, the inverter chamber equipped with inverter, step-up transformer and supporting switchgear is usually arranged outside the whole photovoltaic power generation unit. At the same time, placing the inverter cells of multiple photovoltaic power generation units on both sides of the main road in the site can reduce the construction of roads in the site and reduce the construction cost. Some studies mentioned that the photovoltaic array should be arranged as a rectangle as far as possible, but did not analyze the reasonable layout of the inverter chamber.
Due to the large floor area of each photovoltaic power generation unit of the photovoltaic power station and the large amount of cables, some researchers have paid attention to the selection of economic section of the confluence cable on the DC side of the photovoltaic power station according to the line loss, but only the DC cables from the confluence box to the inverter distribution cabinet have been analyzed, and all DC cables on the DC side have not been analyzed.
This paper discusses the layout form of setting a photovoltaic power generation unit with 1MW, and comprehensively compares the cable consumption of photovoltaic power generation unit and the transformer and other equipment consumption of photovoltaic power station in the design of a project. Through analysis, it is proved that arranging the inverter chamber outside the photovoltaic power generation unit reduces the road construction cost in the field, but increases the cable consumption and increases the cost more. The layout of the inverter cell in the geometric center of the photovoltaic power generation unit increases the road construction cost in the site, but it reduces the cost by reducing the cable consumption, reduces the power loss on the line and improves the power generation efficiency of the photovoltaic power station.
- Arrangement of components
With the continuous reduction of the price of photovoltaic power station, how to improve the return on investment of power station has become the concern of power station developers, and the fine design of power station has gradually been put on the agenda. In view of this, this section discusses the horizontal or vertical arrangement of components from the perspective of floor area, steel support consumption and maximization of power generation.
2.1 comparison of land occupation and support consumption
(1) Floor area comparison
The floor area of components refers to the floor area of the whole array when the front components do not cover the rear (usually the front does not cover the rear at 9:00-15:00 when the winter solstice sun is true). With an inclination of 30 °, a shadow magnification of 3.0 and a size of 1650mm for 40 pieces × Take the 992mm component and the 0.01M interval between components as an example to calculate the floor area. There are three layout modes, namely, the components are arranged horizontally, 4 in the north-south direction, 10 in the east-west direction, as shown in Figure 1, the components are arranged vertically, 2 in the north-south direction and 20 in the east-west direction, as shown in Figure 2; The components are arranged vertically, with a total of 2 arrays. Each array is vertically arranged with 2 pieces in the north-south direction and 10 pieces in the east-west direction, as shown in Figure 3.
It is calculated that 40 components (4) are arranged according to the arrangement in Figure 1 × 10) The total floor area is 156.925m ²； Using the arrangement of Figure 2, 40 components (2 × 20) The total floor area is 156.875m ²； Using the arrangement of Figure 3, 40 components (2 × two × 10) The total floor area is 157.580m ²。 It can be seen that when the capacity and inclination of components are certain, the floor area of horizontal and vertical components is basically the same.
(2) Comparison of steel quantity for support
The support is a structure that fixes the component on it and supports the self weight of the component, wind and snow load resistance, etc. Under the condition that the wind and snow load in the same area is certain and the inclination angle and number of components are the same, the amount of steel used for supports with different layout methods is the same. In the actual design, if four rows of components are arranged horizontally, five beams are required, and the steel consumption of the horizontal row is a little more, but the amount of formwork in the north-south direction will be less. After optimization by some experts of the Design Institute, the steel consumption of the horizontal row and vertical row of components is almost the same in practical application, and the steel consumption of the support is the same in theory.
(3) Comparison of installation difficulty
Horizontal installation is slightly more difficult. After the components are arranged horizontally, the support height is usually slightly higher than that of the vertical row, and four rows of components need to be installed in the north-south direction, which is slightly more difficult. However, with the development of photovoltaic industry in recent years, the component installation team has become more and more experienced, and has made various component installation auxiliary mechanisms, which can adapt to various support heights and forms. The difficulty of support installation is not a limiting factor that hinders the popularity of horizontal component arrangement. The installation cost of modules accounts for about 1% of the total investment of photovoltaic power station. Even if the installation cost of horizontal row is increased by 10% compared with that of vertical row, it will only increase the cost by 1 / 10000, which is at least one order of magnitude different from the increment of power generation.
(4) Implicit income comparison
1) Array spacing
With the increase of horizontal support, the installation difficulty increases slightly, but in areas with small latitude or support inclination angle, the array spacing is small, and it is difficult for slightly larger vehicles to pass through during component cleaning. In addition, in the power station with the combination of photovoltaic and agriculture, the array spacing is small, which is very inconvenient for agricultural operation or other operations. Under the same shadow magnification, the higher the bracket, the greater the array spacing. When the bracket is slightly higher in horizontal row, the array spacing increases accordingly.
2) Consumption of photovoltaic special cable
Two strings can be installed in four rows of modules. After the U-shaped string of photovoltaic special cable, the DC side is more concentrated, as shown in Figure 6-6. The cable of each string can be completed by using the positive and negative wires of the module, 1 × 4mm ² The cable consumption and line loss will be reduced accordingly. When the components are arranged vertically, two additional cables need to be connected to the combiner box, and the cable consumption and line loss will increase.
It should be noted that in order to ensure that the positive and negative poles of the assembly are on the same side, two rows of assembly U-shaped string wires are usually used, and only one string is connected. Otherwise, each string will use an additional array length of photovoltaic special cable to increase the initial investment and operation line loss.
2.2 comparison of power generation
(1) Theoretical analysis
When it comes to the theoretical basis of component layout, it is first determined by the composition of components. Taking the common 60 cell module as an example, it is composed of 60 cells in series, with one bypass diode added every 20 cells, and the series direction of the cells is basically a U-shaped loop in the east-west direction.
The characteristics of component circuit structure determine the different anti shadow ability of components. Because the shadow is mostly close to the ground, we take two rows of batteries close to the ground as an example to illustrate the impact of shadow on power generation performance.
Block the bottom two rows of battery cells of a component. When arranged horizontally, the bypass diode at the bottom is connected, and the upper two rows of battery cells continue to have power output.
Block the bottom two rows of battery cells of a component. When vertically arranged, each circuit of the component is blocked, the circuit is open, and the three rows of battery cells have no power output.
Because the sun rises and sets from the horizon every day, when the support cannot be arranged infinitely, at least when the sun rises and falls, the shadow occlusion of the components exists, which also proves theoretically that the anti shadow occlusion ability of the horizontal arrangement of components is stronger.
(2) Experimental data
An experimental platform was built in Hohhot, Inner Mongolia to test the influence of horizontal and vertical shielding of four components, s-145d, s-165d, s-165da and s-180c, on the output electrical performance, and verify the difference between theory and practice. It can be seen that the power losses of s-145d and s-165d components are 26.46% and 14.18% respectively when the component is 50% shielded laterally; When the vertical shielding of the same component is 50%, the power loss is 97.60% and 99.23% respectively. When the s-280d module is 50% shielded laterally, the power loss is 21.03% and 18.37% respectively; When the vertical shielding of the same component is 50%, the power loss is 99.62% and 99.67% respectively, and the component current is almost zero. It can be seen that when the components are arranged horizontally, the anti shielding ability is stronger.
2.3 Application Analysis of different arrangement modes
(1) Different applicable conditions
For a relatively flat ground power station, during a period of time when the sun rises and sets, the front row components will block the rear row in parallel. After converting scattering, reflection and other factors, it is estimated that the horizontal row in the morning and evening is less than the vertical row for 10 minutes. Considering that the radiation decreases when the sun rises and sets, it is calculated that the output power is 15% of the maximum output power. For a power station with an annual utilization of 1500 hours, the power generation of horizontal arrangement is 1.2% more than that of vertical arrangement.
It should be noted that the power generation gain is different under different latitudes and radiation conditions, but for shadow occlusion, the anti shadow occlusion ability of the horizontal arrangement of components is stronger than that of the vertical arrangement.
At present, the combination of photovoltaic power stations with agriculture and forestry is increasing, which will inevitably be built in uneven terrain such as mountains and slopes. For the power stations built on the South and north slopes of mountains, similar to the flat land, the horizontal arrangement is better than the vertical arrangement. For the power station built on part of the east slope and west slope, when the sun rises in the morning, the east slope is first illuminated by the sun, and the west slope is shaded. As the sun gradually rises and moves south, the west slope is gradually illuminated by the sun, and the east slope is shaded, and the shading is still roughly parallel to the long side of the component. The sun moves between the Tropic of cancer. In addition, most areas in China where photovoltaic power stations are built are north of the Tropic of cancer. Therefore, from a qualitative point of view, in irregular areas such as mountains and slopes, the ability of horizontal arrangement of modules to resist shadow is greater than that of vertical arrangement.
The distributed photovoltaic power station installed on the roof is the same as the ground power station if it is installed in an open and unobstructed manner with an inclination angle. When there is vertical shielding such as electric pole or antenna and it is impossible to avoid, if there are many shielding objects, vertical installation can be considered. If only a few places have vertical shielding, the power station with inclined installation is more suitable for horizontal installation. Because the installation in horizontal rows can not only increase the power generation, but also the inclination of the general roof distributed power station is small, and the component aisle is very narrow, which is not conducive to maintenance and cleaning. If the components are in horizontal rows, multiple components can be installed, the support becomes larger, and the aisle becomes larger, which is more convenient for maintenance (the floor area does not increase).
(2) When tracking the multi-channel MPPT of series and distributed inverters (2 strings horizontally and 1 string vertically), the horizontal arrangement of components also has its unique advantages for the multi-channel MPPT of series and distributed inverters. Because when the components are arranged in horizontal rows, four rows are usually arranged in the north-south direction, so that two strings can be designed for each support. The uppermost string of each support can be connected to the same MPPT. When the lower row is blocked, the upper row can still generate electricity, which can improve the power generation. When the two components are arranged in series, there is usually no advantage in the same direction of the two vertical rows. In conclusion, for power stations with series and distributed multi-channel MPPT inverters, the advantages of component horizontal arrangement are greater than vertical arrangement.
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