I. Overview of Photovoltaic Power Generation Systems
1. Concept: Photovoltaic power generation technology refers to the technology of converting solar energy into electrical energy. It utilizes the photovoltaic effect at semiconductor interfaces to directly convert light energy into electrical energy.
2. Advantages: Compared with commonly used thermal power generation systems, the advantages of photovoltaic power generation are mainly reflected in:
2.1 No risk of depletion;
2.2 Safe and reliable, noiseless, and pollution-free;
2.3 Not limited by the geographical distribution of resources, and can utilize the advantages of building rooftops;
2.4 Can generate and supply electricity locally without consuming fuel or erecting transmission lines;
2.5High energy quality;
2.6 Short construction period and short time required to obtain energy.
3. Classification: Photovoltaic power generation systems are divided into: off-grid systems and grid-connected systems.
Off-grid system: This refers to a solar system that does not connect to the power grid and directly supplies power to the load. Excess electrical energy is stored in batteries. This system generally includes: solar arrays, inverters, battery , distribution boxes, etc.
Grid-connected system: This refers to a system where the electricity generated by the solar cells is directly transmitted to the power grid through a grid-connected inverter. This system typically includes several components such as a solar panel array, a grid-connected inverter, and a boost control system, thus eliminating the need for a battery bank. While this type of system has a lower unit cost, it requires grid support.
II. Project Location Overview
1. Project Location: The project site is Kathmandu, Nepal.
The Federal Democratic Republic of Nepal, commonly known as Nepal, has its capital Kathmandu located at 27°42'N, 85°19'E, in a landlocked mountainous country in South Asia.
2. Climate and Environment: Nepal experiences a dry season from October to March of the following year, with very little rainfall and significant temperature differences between day and night, ranging from around 10°C in the morning to 25°C at midday. The rainy season (summer) runs from April to September, with April and May being particularly hot and humid, often reaching highs of 36°C. Rainfall beginning in May often precedes the rainy season and continues until the end of September, characterized by abundant rainfall and frequent flooding.
Nepal's geography varies greatly from north to south, resulting in distinct regional climates. It is divided into three climate zones: the northern highlands, the central temperate zone, and the southern subtropical zone. The northern part is a high-altitude, cold mountainous region with year-round snow cover and minimum temperatures reaching -41 degrees Celsius; the central river valleys have a mild climate with spring-like weather year-round; the southern plains are hot year-round, with summer highs reaching 45°C.
3. Solar Energy Resource Analysis: Nepal borders Tibet, China, and is one of the world's richest regions in solar energy resources. Kathmandu has thin air and high transparency, with long hours of sunshine annually, totaling approximately 2500-2550 hours. The radiation intensity is high, with an average annual total radiation of 7000 megajoules per square meter.
III. Off-grid Solar Power Generation System Design Scheme
Based on the information provided by the owner (building area 100m2) and the electrical load, the electrical load of this system is summarized in the following table:
The off-grid solar power generation system is designed as follows: Solar panels are solar power generation devices that directly convert sunlight into direct current (DC) electricity. Depending on the user's different requirements for power and voltage, solar panels can be manufactured for individual use, or several solar panels can be connected in series (to meet voltage requirements) or in parallel (to meet current requirements) to form a power supply array to provide greater electrical power. Solar panels are characterized by high area-to-power ratio, long lifespan, and high reliability; within a 20-year service life, the output power typically decreases by no more than 20%.
3.1 Solar Power Generation Calculation
1. Customer consumes 14.4 kWh of electricity per day.
2. Daily electricity consumption: 14.4 kWh
3. Equivalent sunshine duration: 4.5 h
4. Photovoltaic module: 550 W
5. Off-grid system efficiency: 80%
6. Effective daily power generation per module: 0.55kW × 4.5 × 0.8 = 1.98 kWh
7. Required number of modules: 14.4 ÷ 1.98 ≈ 7.27. 8 Solar panels are needed.
8. 8 panels × 550 W = 3850 kWp. Considering the need for additional energy to charge the battery while minimizing costs, we add 2 more solar panels. Total: 10 solar panels
9. Daily solar power generation is 550 * 10 * 0.8 * 4.5 = 19.8 kWh
10. Solar panel parameters
Maximum power (Pmax/W):550W
Open-circuit voltage (Voc/V):49.8
Short circuit current (Isc/A):13.99
Peak power voltage (Vmp/V):41.95
Peak power current (Imp/A):13.12
3.2 Solar Battery
It stores the electrical energy generated by photovoltaic power generation during the day and provides continuous power to the load at night or during cloudy/rainy weather, ensuring stable system operation. This system uses lithium iron phosphate (LiFePO₄) batteries, which have advantages such as high safety, long cycle life, high discharge efficiency, and low maintenance costs. By rationally configuring the battery capacity and depth of discharge, the daily power needs of users can be effectively met, improving the reliability and continuity of off-grid power supply.
3.2.1 Battery Capacity Calculation
The customer requires backup power for one day, so the battery capacity must be greater than or equal to 14kWh. Here, we assume a battery depth of discharge of 95%, an efficiency of 97%, and an inverter efficiency of 97%.
Capacity = Daily Consumption ÷ (η1*η2*η2) = 14.4 ÷ (0.95*0.97*0.97) = 16.1 kWh
Here, we select a 51.2V 314Ah battery. 51.2V * 314Ah = 16.07kWh. This lithium ion solarbattery supports remote updates via APP, touchscreen, automatic inverter communication protocol selection, virtual address bits, and requires no DIP switches.
3.2.2 Battery parameter
Voltage:51.2V
Capacity:314Ah
Discharge current:200A
Charge current:200A
3.3 Inverter Power Calculation
Solar power generation: 550 * 10 = 4950W. Total power of all appliances: 7kW. Inductive loads draw 3-4 times their rated current at startup. The air conditioner and refrigerator are inductive loads in the customer's load. The air conditioner's startup power is approximately 9kW. Therefore, We selected an 8kW off-grid inverter with a peak power of 12kW.
Inverter Parameter
Nominal power:8kW
Peak power:12kW
MPPT:2
MPPT operating voltage:60V-480V
Output voltage:230V
Battery voltage:48V
Each MPPT is connected to 5 solar panels.
4.Solar Mounting System
Hot-dip galvanized steel supports are used here, featuring high strength, strong wind and snow resistance, corrosion resistance (hot-dip galvanizing ≥80μm), low cost, and a service life of 20–25 years or more.
5.Distribution Box
The distribution box is used for the safe distribution and protection of electrical energy in the off-grid photovoltaic system.
It rationally distributes power to various circuits and, through circuit breakers, residual current devices (RCDs), and surge protectors, provides overload, short-circuit, leakage, and lightning protection, effectively ensuring equipment and personal safety. Simultaneously, the distribution box facilitates daily inspection and maintenance of the system, ensuring the long-term stable and reliable operation of the off-grid system.
SPDs, MCBs, and fuses are added at the photovoltaic end; MCBs are added at the battery end; and RCDs, MCBs, SPDs, and fuses are added at the load end.
IV. System Investment Costs and Power Generation Analysis
1. System Investment Costs
The total construction cost of this system is approximately 8400USD.
2. System Power Generation Analysis (Photovoltaic Array Tilting Angle 29 Degrees)
Based on the solar energy resources at the project site, analysis using relevant software indicates that the system's first-year power generation is approximately 7492 kWh, and the average power generation over 25 years is 6673 kWh.
