18kW Solar Carport System With Solar Battery Storage

I. Introduction

1. Background

With the increasing demand for electricity and growing environmental awareness, PV power is attracting more and more attention. Solar-powered green carports can be built as distributed solar power stations, perfectly combining PV power generation technology with the carport architecture itself, offering significant economic and social benefits. Building solar carports will bring more convenience to people's lives.

2. Characteristics of Photovoltaic Carports

A photovoltaic power generation system is a system that directly converts solar energy into electrical energy using solar panels. Its main components are photovoltaic modules and inverters. Generally, it uses a steel structure support, which is simple, elegant, stylish, and aesthetically pleasing. Its practical features include high reliability, long service life, no environmental pollution, the ability to generate electricity independently or connect to the grid, and it is a clean and environmentally friendly new energy source, effectively alleviating social environmental and energy pressures. It is favored by many investors and has broad development prospects.

3. Advantages of Installing Solar Power Generation Systems

(1) Solar energy is inexhaustible. The solar radiation received by the Earth's surface can meet 10,000 times the global energy demand. Installing solar energy systems on just 4% of the world's deserts can generate enough electricity to meet global needs. Solar power generation is safe and reliable, and will not be affected by energy crises or unstable fuel markets.

(2) Solar energy is readily available and can be supplied locally, eliminating the need for long-distance transmission and avoiding losses from long-distance power lines.

(3) Solar power generation has no moving parts, is not easily damaged, and is simple to maintain, making it particularly suitable for unattended use.

(4) Solar power generation does not produce any pollution, noise, or other public hazards, and has no adverse impact on the environment, making it an ideal clean energy source.

(5) Solar power generation systems have a short construction cycle, are convenient and flexible, and can be adjusted according to load changes, allowing for easy addition or reduction of solar panels to avoid waste.

4. Three Major Benefits of Building Photovoltaic Carports

(1) Building solar carports can reduce urban electricity pressure. Utilizing the idle space of carports, photovoltaic carports generate electricity that can be used by vehicles, and any surplus electricity can be sold back to the state, thus alleviating urban electricity pressure.

(2) Building solar carports can save energy and generate income. Photovoltaic carports not only provide shade and shelter for vehicles but also generate electricity, achieving a win-win situation for both social and environmental benefits.

(3) Building photovoltaic carports embodies the concept of urban ecological protection. Building photovoltaic carports actively responds to the national call for energy conservation and emission reduction, reducing carbon emissions and thus creating a low-carbon and environmentally friendly modern city.

II. Project Overview 

The project site is located in a humid subtropical climate zone with four distinct seasons and abundant rainfall. The average annual rainfall is 117 days, with an average annual precipitation of 1106.5 mm, a relative humidity of 76%, and a frost-free period of 237 days. The plum rain season lasts from late June to early July each year. The average annual temperature is 16.5℃, with the highest annual extreme temperature reaching 39.7℃ and the lowest -13.1℃.

III. Design Basis

GB50217-2007 *Code for Design of Cables for Power Engineering*

GB/T19939-2005 *Technical Requirements for Grid Connection of Photovoltaic Systems*

IEEE1547:2003 *Standard for Interconnection of Distributed Power Sources with Power Systems*

IEEE1547.1:2005 *Test Procedures for Interface Equipment between Distributed Power Sources and Power Systems*

IEC62116 *Test Methods for Anti-Islanding of Inverters for Grid-Connected Photovoltaic Systems*

JGL/T16-92 *Code for Electrical Design of Civil Buildings*

GB50057-94 *Code for Lightning Protection Design of Buildings*

IV. System Design

1. Overall Design

The project is a photovoltaic power station system for a 6-space parking shed with a total installed capacity of 17600W. It adopts 550Wp high-efficiency monocrystalline solar panels and a 15kW hybrid inverter. The shed support is made of galvanized steel with strong wind resistance, shock resistance and pressure resistance. The grid connection point is indoors within 100 meters.

(1) Photovoltaic Power Generation from Carports:

During the daytime photovoltaic power generation period, which coincides with peak electricity price or consumption periods, the photovoltaic power generated directly supplies local loads, maximizing the benefits of self-generation and self-consumption.

(2) Lithium-ionSolar Battery Storage: During off-peak hours, the grid charges the system (from 10 PM to 8 AM, a 15kW inverter charges the lithium-ion solar battery storage system for 3 hours, fully charging its 16.07 kWh capacity).

During peak hours, the solar battery storage system releases electricity to supply local loads.

The bidirectional flow of electricity and time-of-use pricing of the lithium-ion solar battery storage system, i.e., "low-price storage, high-price use," achieves low-cost power supply from charging piles and reduces local load consumption during peak hours, saving on electricity costs.

2. Selection of Hybrid Inverters: 

The inverter is the core equipment of the PV power generation system. It connects the DC and AC sides and needs to have comprehensive protection functions and high-quality power output. The selection of inverters must meet the following requirements:

(1) High conversion efficiency:

The higher the inverter's conversion efficiency, the higher the conversion efficiency of the photovoltaic power generation system, the smaller the total power generation loss, and the higher the system's economic efficiency.

(2) Wide DC input voltage range:

The terminal voltage of solar cell modules varies with solar irradiance and ambient temperature. A wide DC input voltage range for the inverter allows for the utilization of power generation during periods of lower solar irradiance before sunrise and after sunset, thereby extending the power generation time and increasing power output.

(3) Effective "islanding effect" protection:

Employing multiple "islanding effect" detection methods ensures accurate tracking and detection of parameters such as voltage, frequency, and phase when the grid loses power, promptly determining the grid's power supply status, enabling the inverter to operate accurately, and ensuring grid safety.

(4) Communication function:

The inverter must provide a communication interface to upload real-time operating data, fault information, alarm information, etc., to the power station monitoring system.

This distributed carport photovoltaic project is equipped with one 15kW hybrid inverter.

3. Solar Panels

This project plans to use 550Wp high-efficiency monocrystalline silicon PV modules. The module parameters are shown in the table below:

4. Solar Mounting  System Design

Solar mounting system uses Q235B cold-rolled steel plate or aluminum profiles. The material selection and support design should comply with the national standard "Code for Design of Steel Structures" GB50017. The corrosion protection of the support system should meet the following requirements: Crossbeams, corrugated steel sheet clamps, and crossbeam connectors should be pre-processed and then hot-dip galvanized. The zinc layer should meet the requirements of GB/T13912-2002, with a zinc layer thickness of not less than 65µm. The aluminum alloy surface should be anodized to AA15 grade.

All bolts in this project should comply with the current national standard "Hexagonal Bolts - Grade C" (GB5780) and meet on-site corrosion protection requirements.

Side pressure blocks and center pressure blocks are made of aluminum alloy.

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

The fixed bracket is made of corrosion-resistant steel profiles. All connections (welded joints) should be reliably connected to prevent loosening, and it must be resistant to corrosion from outdoor wind, frost, rain, and snow. The fixed bracket must meet the requirements for installation tilt angle, wind resistance, snow pressure resistance, seismic resistance, corrosion resistance, safety, versatility, and rapid installation.

Tilt Design

To maximize the solar energy received by the photovoltaic array surface, based on the Earth-Sun orbital relationship, the array surface should ideally be installed facing the equator (azimuth angle of 0 degrees). In this project, to maximize the use of the carport area, the modules are installed in a flat-lay configuration.

5. Cable Selection

(1) Selection Principles

• Environmental Condition Verification 

• Ambient Temperature 

• Sunlight 

• Wind Speed 

• Pollution 

• 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 Cables for Power Engineering" GB50217. The cross-section of wires and cables should be selected and determined after technical and economic comparison.

• Cables laid in trenches and cable trays should preferably be Class C or higher flame-retardant cables.

• 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 by direct burial, cable trenches, cable trays, cable troughs, etc. Power cables and control cables should preferably be arranged separately and meet the minimum spacing requirements.

• Cable trenches are strictly prohibited from being used as drainage channels.

• For long-distance transmission, fiber optic cables should preferably be used for network cables.

• Selection of Cable Rated Voltage

• The phase-to-phase rated voltage of the power cable core in AC systems should not be lower than the working line voltage of the circuit in use. The selection of the rated voltage between the cable core and the insulation shield or metal sheath of power cables in AC systems shall comply with the following provisions:

① For systems with a directly grounded neutral point or grounded through low impedance, when the grounding protection operation clears the fault within 1 minute, the rated voltage shall be 100% of the operating phase voltage of the circuit.

② For power supply systems other than those specified in item a, the rated voltage should not be lower than 133% of the operating phase voltage of the circuit; in cases where a single-phase ground fault may last for more than 8 hours, or where safety requirements are high, such as in generator circuits, 173% of the operating phase voltage of the circuit should be adopted.

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

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

5) The selection of the rated voltage of control cables shall not be lower than the operating voltage of the circuit and shall meet the requirements for transient and power frequency overvoltage that may be withstood. It should also comply with the following provisions:

① For control cables (guide cables) laid parallel to long high-voltage cables, a suitable rated voltage shall be selected.

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

③ Except for the cases in ① and ②, 450/750V should generally be selected; when the influence of external electrical interference is very small, a lower rated voltage can be selected.

· Selection of cable cross-sectional area; The cable cross-section should meet the requirements of continuous allowable current, short-circuit thermal stability, allowable voltage drop, etc. For long-distance high-current circuits, it is also advisable to select according to the economic current density.

(2) Cable type

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

1) The cable from the photovoltaic array string output to the inverter is PV1-F1×4mm²;

2) The output cable of the 15kW inverter is ZC-YJV-0.6/1kV-4*16mm².

3) The grounding cable is BVR-450/750V10mm².

6. Lithium-ion Solar Battery Storage 

Solar battery are used  used for peak shaving and valley filling of the power grid, regulating the continuity and stability of power supply from renewable energy generation systems, and serving as emergency and backup power for important departments and facilities. 

High Safety Performance: Uses high-quality lithium iron phosphate cells, ensuring no fire or explosion. 

Long Cycle Life: Up to 6000 cycles, with a service life of up to 10 years. 

Rich Communication Interfaces: Equipped with multiple communication interfaces such as CAN2.0 and RS485, facilitating various communication methods. 

Good Charge and Discharge Performance: Supports high-current discharge, stable voltage platform, smooth discharge curve, high conversion efficiency, and better energy utilization. 

Convenient Maintenance: Modular design, high reliability, and easy maintenance.

7. Lightning Protection and Grounding

The lightning protection and grounding of this steel structure carport photovoltaic project primarily adhere to GB50057 "Code for Design of Lightning Protection of Buildings".

The lightning intrusion path of a photovoltaic power generation system includes not only solar panels but also the power distribution lines, grounding wires, and combinations thereof. To ensure the safe operation of the power system and the safety of PV power generation and power facilities, grid-connected photovoltaic power stations must have effective lightning protection, lightning protection, and grounding protection devices. Since the outdoor equipment installation location in this project is not the tallest building in the environment, all steel structures are connected and combined with newly added grounding piles to form a lightning protection network to achieve the purpose of lightning protection.

The following lightning protection and grounding measures are adopted in this project: The aluminum alloy frames and metal supports of solar panels are reliably welded to the lightning protection flat steel strip via grounding flat steel. Reliable grounding is crucial for system lightning protection and safe power use. In this design, the supports, photovoltaic module frames, and connectors are all metal products. PV array naturally forms an equipotential body and is reliably connected to the grounding grid nearby. The grounding resistance at each connection point should be less than 4 ohms. The inverter's AC output is connected to the power grid via an AC combiner box (containing a lightning protection device), which can effectively prevent damage to the equipment caused by lightning strikes and power grid surges. All cabinets must have good grounding, and the grounding resistance of each connection point should be less than 4 ohms.

V. Power Generation and Benefit Analysis

1. Theoretical Power Generation

Based on the average monthly total solar radiation at the project site, the monthly and annual peak sunshine hours for this project can be calculated.

Peak Sunshine Hours: The peak sunshine hours are the equivalent number of hours under standard operating conditions (1000 W/m² irradiance) on the plane where the solar cell modules are located within a certain time period.

If the solar radiation received by the solar cell modules in 1 hour is 1 kWh/m², then according to the definition of peak sunshine hours, the peak sunshine hours t can be obtained as follows:

t = (1 kWh/m².a) / (1000 W/m²) = 1 (h/a)

Since the peak power of the solar cell modules is calibrated under conditions of 1000 W/m², the maximum theoretical power generation of the photovoltaic power station is obtained by multiplying the peak sunshine hours by the installed capacity of the photovoltaic power station.

This project involves the installation of 32 standard 550Wp monocrystalline silicon photovoltaic modules, with a total installed capacity of 17600KWp. The selected photovoltaic modules measure 2274mm*1134mm*35mm and are installed using a bracket-fixed, tilted mounting method.

The annual effective sunshine duration is 1913.5-2161.5 hours. Calculations show that the theoretical annual power generation of the photovoltaic array for this project is 33677 kWh.

2. Annual Theoretical Power Generation: The theoretical power generation of the photovoltaic power station in the first year is the maximum theoretical power generation of the photovoltaic power station multiplied by the degradation coefficient of the solar cell modules in the first year. The degradation coefficient of the selected monocrystalline silicon solar cell modules in this project is 8% in the first year. Therefore, the theoretical power generation of the photovoltaic power station in the first year is the annual theoretical power generation multiplied by the module degradation coefficient.

3. Photovoltaic Power Generation System Efficiency Analysis

The efficiency of a solar photovoltaic power generation system includes: solar cell aging efficiency, AC/DC low-voltage system losses and other equipment aging efficiency, inverter efficiency, and transformer and grid loss efficiency. Based on the actual power generation situation and experience coefficients of relevant domestic and foreign projects, the values of each efficiency coefficient are as follows:

(1) DC cable loss: 2%;

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

(3) Loss caused by panel mismatch: 4%;

(4) Dust shading loss: 2%;

(5) AC line loss: 0.8%;

(6) Inverter loss: 2%;

(7) Unusable solar radiation loss: 1.2%;

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

(9) Transformer loss: 3%;

(10) Temperature effect loss: 4%; After calculation and analysis, the overall efficiency of the system is 81%.

VI. Economic and Social Benefits

1. Economic Benefits

Based on the above analysis, the total installed capacity of this photovoltaic power station is 17,600 kWp, and the cumulative power generation over 25 years of continuous operation is 841,929 kWh.

Photovoltaic power generation saves electricity costs: Because the installed capacity of photovoltaic power is relatively small compared to the power load, most of the electricity generated by photovoltaic power can be used for self-consumption. With a design life of 25 years for the photovoltaic power station, calculated at an electricity price of 0.12 USD, the estimated electricity cost savings are 76,582USD

2. Comprehensive Environmental Benefits

Photovoltaic power generation does not pollute the environment and emits no greenhouse gases during the power generation process, which is a huge advantage of solar photovoltaic power generation. Currently, my country's main power supply still comes from coal-fired power generation. During coal combustion, large amounts of harmful gases such as sulfur dioxide are emitted, causing environmental pollution, and large amounts of carbon dioxide are also emitted. As we all know, carbon dioxide is a greenhouse gas, and its excessive emissions are a major factor in global warming. After the photovoltaic power generation system is in operation, it will have completely "zero" emissions. Based on a stable 25-year operating period for the photovoltaic power generation system, the theoretical cumulative power generation of this system could reach 841,929 kWh.

Note: Saving 1 kWh of electricity corresponds to saving 0.36 kg of standard coal, while simultaneously reducing pollution emissions by 0.272 kg of carbon dust, 0.997 kg of carbon dioxide (CO2), 0.03 kg of sulfur dioxide (SO2), and 0.015 kg of nitrogen oxides (NOx).