China Optics Valley 6-car photovoltaic car shed, with a total installed capacity of 15360W, is equipped with 320 watt high-efficiency single crystal solar panels, 15 kW grid connected solar inverters, and the car shed bracket is made of galvanized steel with strong wind resistance, seismic resistance, and pressure resistance. The grid connection point is located indoors within 100 meters.

After the construction of the photovoltaic car shed, a charging pile project can be applied on top of the photovoltaic car shed project through secondary configuration.

The charging pile energy storage and power supply system is divided into three parts: PCS, lithium battery energy storage system, and charging pile system.

The lithium battery energy storage system is composed of retired electric vehicle batteries. The project site is equipped with two PCS energy conversion systems with a power of 8kW and a capacity of 15.36kW · h lithium battery energy storage system.

According to the owner's peak valley time of use electricity price: 0.96 yuan from 8am to 12pm

12:00 noon -6:00 pm 0.62 yuan

6pm -10pm 1.07 yuan

10pm -8am 0.33 yuan

1) Utilization of photovoltaic power generation in car sheds:

During the daytime photovoltaic power generation period, it belongs to the peak period of electricity prices or electricity consumption. Photovoltaic power generation is directly used by local loads, and spontaneous self use is achieved to maximize the benefits of photovoltaic power generation.

2) PCS lithium battery energy storage system charging:

During the low electricity price period, it is charged by the power grid (from 10pm to 8am, 16kW power PCS can charge the lithium battery energy storage system for 7.5 hours to fully charge the 15.36kW · h battery capacity).

3) PCS lithium battery energy storage system discharge:

During the day, the energy storage system outputs electrical energy to the charging pile, or during electricity surplus situations or peak electricity price periods, the energy storage system releases electrical energy for local loads to use.

Summary: By utilizing the bidirectional flow of electrical energy and peak valley time of use electricity prices of PCS lithium battery energy storage system, which means "low price storage and high price use" of electrical energy, on the one hand, it can achieve low cost of charging station power supply, and on the other hand, it can reduce local load electricity consumption during peak electricity prices, saving electricity bills.

There are two types of charging station systems: DC charging station and AC charging station.

The DC charging station directly converts the direct current of the lithium battery energy storage system through DC-DC to obtain electrical energy output. Its characteristic is high output power and fast charging speed of electric vehicles;

The AC charging station converts the direct current of the lithium battery energy storage system through DC-AC to output AC power. Its characteristic is that it is cheap in cost, but the output power is small, and the charging speed of electric vehicles is slow.

3.1 Selection of grid connected inverters

1. Selection of grid connected inverters

Grid connected inverters are the core conversion equipment of photovoltaic grid connected power generation systems, which are connected to the DC and AC sides and need to have complete protection functions and high-quality electrical energy output. The selection of inverters should meet the following requirements:

(1) High conversion efficiency

The higher the conversion efficiency of the inverter, the higher the conversion efficiency of the photovoltaic power generation system, the smaller the total power loss of the system, and the higher the system economy. Therefore, when a single unit has the same rated capacity, an inverter with high efficiency should be selected. The conversion efficiency of inverters includes maximum efficiency and European efficiency. European efficiency is the weighted efficiency of different power points, which better reflects the comprehensive efficiency characteristics of inverters. The output power of photovoltaic power generation systems is constantly changing with the intensity of sunlight, therefore, in the selection process, European inverters with high efficiency should be selected.

(2) Wide DC input voltage range

The terminal voltage of the solar cell module varies with the intensity of sunlight and environmental temperature, and the DC input voltage range of the inverter is wide, which can utilize the power generation during periods of low solar irradiance before sunrise and after sunset, thereby extending the power generation time and increasing the power generation.

(3) High quality electrical energy output

The inverter should have a high-performance filtering circuit, so that the AC output of the inverter has high power quality and will not cause pollution to the power grid quality. When the output power is ≥ 50% of the rated power and the grid fluctuation is less than 5%, the total harmonic distortion (THD) of the AC output current of the inverter is less than 3%.

During the operation of grid connected inverters, it is necessary to collect real-time voltage signals from the AC power grid. Through closed-loop control, the AC output current of the inverter is kept consistent with the phase of the grid voltage, so that the power factor can be maintained around 1.0.

(4) Effective "islanding effect" protection measures

Adopting various "islanding effect" detection methods to ensure accurate tracking and detection of parameters such as voltage, frequency, and phase when the power grid loses power, timely judging the power supply status of the power grid, enabling the inverter to operate accurately and ensuring the safety of the power grid.

(5) System frequency abnormal response

The "Technical Regulations for Connecting Photovoltaic Power Stations to the Power Grid of the State Grid Corporation of China" require large and medium-sized photovoltaic power stations to have a certain ability to withstand system frequency anomalies.

(6) Communication function

The photovoltaic grid connected inverter must provide a communication interface that can upload real-time operation data, fault information, alarm information, etc. of the inverter to the power station monitoring system.

Based on the actual situation on site and the roof conditions in the photovoltaic module laying area are limited, it is recommended to use a series inverter. Compared to centralized inverters, the advantages of series inverters are as follows.

High conversion efficiency, with a European efficiency of 97.5%;

Multi channel MPPT ultimately ensures high system conversion efficiency;

The power generation revenue is significantly higher than that of centralized inverters;

No DC convergence required;

Easy installation, tailored to local conditions, and space saving;

Easy maintenance, reducing average maintenance time;

Wide input range, longer power generation efficiency;

Overall consideration:

This distributed garage photovoltaic project is equipped with one 15kW series connected photovoltaic inverter.

3.2.1 Selection of photovoltaic modules

Selection principles

According to the letter issued by the Comprehensive Department of the National Energy Administration on February 5, 2015, on soliciting opinions on promoting the progress of photovoltaic technology and industrial upgrading by playing a market role (National Energy Comprehensive New Energy [2015] No. 51), it is stipulated that:

Strictly implement market access standards for photovoltaic products. Since 2015, the photovoltaic modules and grid connected inverter products used in photovoltaic power generation projects that enjoy national subsidies should meet the relevant indicator requirements of the "Photovoltaic Manufacturing Industry Specification Conditions". Among them, the conversion efficiency of polycrystalline silicon battery modules shall not be less than 15.5%, and the conversion efficiency of monocrystalline silicon battery modules shall not be less than 16%. The attenuation rates of polycrystalline silicon, monocrystalline silicon, and thin film battery modules shall not exceed 2.5%, 3%, and 5% respectively within one year from the date of production and operation. The weighted efficiency of grid connected inverters in China should meet the following requirements: the type with transformer should not be less than 96%, and the type without transformer should not be less than 98%.

So this project uses 320W single crystal photovoltaic modules, which can smoothly obtain national photovoltaic subsidies.

The photovoltaic modules selected for this power station have the following characteristics:

1) Single crystal silicon photovoltaic modules have been certified by a nationally approved certification agency;

2) The peak power error of the component is ± 2%;

3) The component efficiency is 19.2%;

4) Component stable power attenuation: 1-3 years total attenuation ≤ 5%, 1-10 years total attenuation ≤ 10%, 1-25 years total attenuation ≤ 20%.;

Taking into account module efficiency, technological maturity, and market share, this project selects 320Wp monocrystalline silicon photovoltaic modules,

2) Design principles of photovoltaic arrays

The project adopts a 15kW grid connected inverter, with a DC operating voltage range of 500Vdc~850Vdc. Considering the number of solar photovoltaic modules connected in series, N:

Calculation formula:

In the formula, the open circuit voltage temperature coefficient of the photovoltaic module;

The working voltage temperature coefficient of photovoltaic modules;

Extreme low temperature (℃) under working conditions of photovoltaic modules;

Extreme high temperature (℃) under working conditions of photovoltaic modules;

The maximum allowable DC input voltage of the inverter (V);

Maximum MPPT voltage of inverter (V);

Minimum value of inverter MPPT voltage (V);

Open circuit voltage of photovoltaic modules (V);

Working voltage of photovoltaic modules (V);

Number of battery components connected in series (rounded to an integer).

After calculation, the number of series photovoltaic cells N is 19 ≤ N ≤ 24. Based on the climate and environment of the site area, combined with the temperature correction parameters of the battery module and the optimal input voltage of the inverter, the series number of solar cell modules is calculated after correction.

The number of parallel components should be determined based on system capacity, number of inverters, component capacity, and number of series connections

3.2.2 Component Support Design

(1) Support selection

The bracket shall be made of Q235B cold-rolled steel plate or aluminum profile, and the material selection and bracket design shall comply with the provisions of the national standard "Code for Design of Steel Structures" GB50017. The anti-corrosion of the bracket should meet the following requirements:

1) The crossbeam, colored steel tile fixtures, and crossbeam connectors are all processed first and then hot-dip galvanized. The zinc layer should comply with GB/T13912-2002, and the zinc layer thickness should not be less than 65um. The aluminum alloy surface should have an anodized primary color of AA15.

2) All bolts in this project should comply with the current national standard "Hexagon Bolt-C Grade" (GB5780) and meet on-site anti-corrosion requirements.

3) The edge pressure block and medium pressure block are made of aluminum alloy material;

4) According to the "Code for Seismic Design of Buildings" (GB50011-2010), the seismic intensity of the support system is 7 degrees, the seismic peak acceleration in the engineering area is 0.1g, and the characteristic period of the seismic response spectrum is 0.40S.

5) The fixed support adopts anti-corrosion steel profiles, and all connections (welding points) should be reliably connected to avoid loosening. It is required to be able to withstand outdoor corrosion such as wind, frost, rain, and snow.

6) The fixed bracket can meet the requirements of installation inclination, wind resistance, snow pressure resistance, seismic resistance, corrosion resistance, safety, universality, and rapid installation.

(2) Inclination design

In order to receive more solar energy on the surface of the photovoltaic array, it is best to install the array surface facing the equator (with an azimuth of 0 degrees) according to the laws of solar and terrestrial operation. In order to maximize the utilization of the shed area in this project, the components are installed in a flat manner.

3.3 Cable selection

(1) Selection principles

Environmental condition verification:

ambient temperature

sunshine

wind speed

Dirtiness

Altitude

The selection and laying design of wires and cables for photovoltaic power stations should comply with the provisions of the "Code for Design of Electric Power Engineering Cables" GB50217. The cross-section of wires and cables should be determined after technical and economic comparison.

Cables laid in trenches and boxes should be flame retardant cables of Class C or above.

The wires and cables between photovoltaic modules and between modules and combiner boxes should have fixing measures and sun protection measures.

Cable laying can be done through direct burial, cable trenches, cable trays, cable trays, and other methods. Power cables and control cables should be arranged separately and meet the minimum spacing requirements.

Cable trenches are strictly prohibited as drainage channels.

Fiber optic cables should be used as network cables for long-distance transmission.

Selection of cable rated voltage

The rated voltage between phases of the power cable core in the AC system shall not be lower than the voltage of the working line of the circuit used.

The selection of the rated voltage between the power cable core and the insulation shield or metal sleeve in the AC system shall comply with the following regulations:

① When the neutral point is directly grounded or grounded through low impedance in a system, when the grounding protection action does not exceed 1 minute to remove the fault, the working phase voltage of the circuit should be 100% used.

② For power supply systems outside of item a, the operating phase voltage of the operating circuit should not be less than 133%; In situations where a single-phase ground fault may last for more than 8 hours or where safety requirements for generator circuits are high, it is advisable to use 173% of the working phase voltage of the circuit.

3) The impulse withstand voltage level of cables in the AC system should meet the insulation coordination requirements of the system.

4) The insulation level of DC transmission cables should take into account load variation factors and meet the requirements of internal overvoltage.

5) The selection of the rated voltage of the control cable should not be lower than the working voltage of the circuit and meet the requirements for transient and power frequency overvoltage effects that may be experienced. And it should comply with the following regulations:

① For control cables (guide cables) laid in parallel along longer high-voltage cables, choose a suitable rated voltage.

② The control cables laid in high-voltage distribution devices of 220kV and above should be 600/1000V, or 450/750V can be selected when there is good shielding.

③ Except for items ① and ②, it is generally recommended to choose 450/750V; When external electrical interference is minimal, a lower rated voltage can be chosen.

Selection of cable cross-sectional area;

The cable cross-section should meet the requirements of continuous allowable current, short-circuit thermal stability, allowable voltage loss, etc. For long-distance high current circuits, it is also advisable to choose according to the economic current density.

(2) Cable model

According to the selection conditions, the cable models and specifications selected for this project are as follows:

1) PV 1-F 1 cable selected for output from photovoltaic array string to inverter × 4mm2;

2) The output cable of the 20KW inverter is ZC-YJV-0.6/1kV-4 * 16 mm2.

3) The grounding cable is BVR-450/750V 10mm2.

3.4 Energy Storage System and PCS Design

As a power conversion device between the power grid and batteries in the energy storage system, energy storage devices can achieve bidirectional energy exchange between the power grid and battery packs, used for "peak shaving and valley filling" of the power grid, regulating the continuity and stability of renewable energy power generation system power supply, and serving as emergency and backup power sources for important departments and facilities. This device can be used in new energy power stations, electric vehicle charging and swapping power stations, urban energy storage power stations, and microgrid energy storage, and has good application prospects.

Performance characteristics:

High safety performance: Adopting a power frequency transformer to securely isolate the battery from the power grid

High conversion efficiency: adopting a one-stage conversion, with a simple structure, convenient control, and high energy conversion effect

There are multiple operating modes: constant current charging and discharging, constant power charging and discharging, float charging, constant voltage charging, etc

Rich communication interfaces: equipped with various communication interfaces such as CAN2.0, RS485, LAN, etc., facilitating the implementation of various communication methods

Advanced control technology: internationally leading control and protection technologies such as soft phase-locked loop technology, voltage feedforward technology, vector control, etc

Complete protection functions: module level, device level, and system level three-layer protection

Easy maintenance: modular design, high reliability, and easy maintenance

4. Power generation and benefit analysis

4.1 Theoretical power generation

The monthly and annual peak sunshine hours of this project can be calculated based on the average total solar radiation in each month of the project location. Peak sunshine hours: Converts the amount of solar radiation that can be received during a certain period of time on the plane where the solar cell module is located into the equivalent hours under standard operating conditions with an irradiation intensity of 1000W/m2, known as peak sunshine hours.

If the solar cell module receives solar radiation of 1 kWh/m2. a within 1 hour, defined by the above peak sunshine hours, its peak sunshine hours t can be obtained:

T=(1 kWh/m2. a)/(1000W/m ²）= 1 (h/a)

Due to the peak power of solar cell modules being 1000W/m ² Under certain conditions, the maximum theoretical power generation of the photovoltaic power plant is determined by multiplying the peak sunshine hours by the installed capacity of the photovoltaic power plant.

A total of 48 single crystal silicon photovoltaic modules with a standard capacity of 320Wp are installed in this plan, with a total installed capacity of 15.36KWp. The size of the selected photovoltaic module is 1640mm * 992mm * 35mm, and the installation method is fixed and inclined bracket installation.

The annual effective lighting time in Huanggang area is 1913.5-2161.5 hours. After calculation, the theoretical annual power generation of the photovoltaic array in this project is 29391.36 kW. h

4.2 Theoretical power generation year by year

The first year theoretical power generation of a photovoltaic power plant is the maximum theoretical power generation of the photovoltaic power plant multiplied by the attenuation coefficient of the solar cell module in the first year. The attenuation coefficient of the single crystal silicon solar cell module selected for this project in the first year is 8 ‰, so the theoretical power generation of the photovoltaic power station in the first year is the annual theoretical power generation multiplied by the module attenuation coefficient.

4.3 Efficiency Analysis of Photovoltaic Power Generation System

The efficiency of solar photovoltaic power generation systems includes: solar cell aging efficiency, AC and DC low-voltage system losses

And other equipment aging efficiency, inverter efficiency, transformer and power grid loss efficiency; Combining domestic and foreign related work

The actual power generation situation and empirical coefficients of Cheng, and the values of each efficiency coefficient are as follows:

(1) DC cable loss: 2%;

(2) Anti reflection diode and cable joint loss: 1.5%;

(3) Loss caused by mismatched battery panels: 4%;

(4) Dust shielding loss: 2%;

(5) AC line loss: 0.8%;

(6) Inverter loss: 2%;

(7) Unavailable solar radiation loss: 1.2%;

(8) System failure and maintenance loss: 1%;

(9) Transformer loss: 3%;

(10) Temperature impact loss: 4%;

After calculation and analysis, the overall efficiency of the system is 81%.

4.4 Economic benefits

Based on the above analysis, the total installed capacity of the photovoltaic power station in this project is 15.36kWp, and the cumulative power generation during 25 years of continuous operation is 595175KW · h.

Photovoltaic power generation saves electricity costs: Due to the relatively small installed scale of photovoltaic power compared to the power consumption load, most of the energy generated by photovoltaic power can be used by oneself. With a design life of 25 years for photovoltaic power stations, calculated based on a electricity price of 0.8 yuan, it is expected to save electricity costs by 476140 yuan.