Analysis of Solar System DC AC Ratio
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Analysis of Solar System DC/AC Ratio White Paper [email protected] www.ginverter.com PV capacity and inverter capacity’s ratio is called the DC/AC ratio. Specific project conditions needs to be taken into consideration for a reasonable DC/AC ratio design. The main factors include irradiation, system loss, inverter efficiency, inverter lifetime, inverter working voltage range, module mounting angle, etc. Take a closer look into the factors: Different Areas Have Different Irradiation In a PV system, the PV panels convert the solar power to DC power, and the power would go through the cables to the combiner box, then to the inverter, and AC cable, AC combiner box, then to the grid. There is power loss in each and every step of the process. Usually the DC loss takes about 7-11%, inverter loss 1-2%, and total loss of the system is 8-13% (not including the transformer and cable loss afterwards). So if the PV panel capacity is same as the inverter capacity, inverter can only produce max 90% of the inverter capacity because of the system loss. Even with the best insolation, inverter can’t work at full load. Read below solar resource map from the Global Solar Atlas, owned by the World Bank Group and provided by Solargis. We can see that the irradiation of different areas varies greatly and the long-term average yearly sum ranges from 365-3,652kWh/m2. The countries with higher irradiation resources are Australia, Mexico, central America, northern Chile, Saharan countries, South Africa, Namibia, the Arabian peninsula and the US Southwest. These countries are most suitable to build photovoltaic or solar thermal plants. To achieve same production of the PV system for different areas with different irradiation, we can change the DC/AC ratio and keep high DC/AC ratio for areas with low irradiation. System Loss The efficiency of the inverter is not constant. There are power switching device losses and magnetic losses. At low output power, the efficiency is relatively low. When the power is between 40% and 60% of the rated output power, efficiency is the highest. When the power is over 60%, the efficiency will be gradually reduced. Therefore, the total photovoltaic power should be controlled between 40% and 60% of the inverter power to obtain the best efficiency. Inverter Efficiency PV inverter is an electronic product, and its reliability is highly related to the operating temperature of the inverter. For components such as capacitors, fans and relays, their failure rate will increase by more than 50% when their temperature increases by 10 °C. The operating temperature, on the other hand, is related to power. According to statistics, on a long-term basis the lifespan of inverter that works at 80-100% of the normal output power is about 20% lower when compared to inverter works at 40-60% of the normal output power. Inverter lifetime When inverter works at its norminal voltage, the efficiency is the highest. For single-phase inverter at 230V AC voltage, the norminal DC voltage is around 360V; and for three-phase inverter at 400V AC voltage, the norminal DC voltage is around 650V. For example, for a 3kW inverter using 260W panel with MPPT voltage at 30.5V, the best configuration is 12 panels per string with string MPPT voltage at around 366V and PV capacity 3.12kW. For a 30kW inverter using 260W panel, the best configuration is 21 panels per string with string MPPT voltage at 640.5V. The total panel quantity is 126pcs and total power 32.76kW. Best Working Voltage Range of Inverter Given all these factors when PV power is between 40-60% of the rated power of the inverter, the system achieves the highest efficiency and the inverter would have the longest lifespan. In order to optimize the performance of the inverter, the PV and inverter capacity should be configured at different ratios according to the solar insolation conditions. Summary For areas where the average irradiation exceed 5 hours, and the production time is calculated as 10 hours per day, the DC/AC ratio can be 1, average power is 50%Pn. For areas where the average sunshine time is around 4 hours, production time as 9 hours per day, the DC/AC ratio can be 1.1, average power is 49%Pn (4*11/9). For areas where the average sunshine time is 3 hours, production time as 8 hours, the DC/AC ratio can be 1.3, average power is 48.75%Pn. For PV plants on hill with different module directions and distributed solar rooftops with complex conditions and different orientations, some of the PV modules will not face south, and the tilt angle of the roof is not the optimal tilt angle. The configuration of the inverter could be done flexibly according to the specific situations. MAX 50-80KTL3 LV IHS Global Shipment Ranking TOP 10
Transcript of Analysis of Solar System DC AC Ratio
Analysis of Solar System DC/AC Ratio
White Paper
[email protected] www.ginverter.com
PV capacity and inverter capacity’s ratio is called the DC/AC ratio. Specific project conditions needs to be taken into consideration for a reasonable DC/AC ratio design. The main factors include irradiation, system loss, inverter efficiency, inverter lifetime, inverter working voltage range, module mounting angle, etc. Take a closer look into the factors:
Different Areas Have Different Irradiation
In a PV system, the PV panels convert the solar power to DC power, and the power would go through the cables to the combiner box, then to the inverter, and AC cable, AC combiner box, then to the grid. There is power loss in each and every step of the process. Usually the DC loss takes about 7-11%, inverter loss 1-2%, and total loss of the system is 8-13% (not including the transformer and cable loss afterwards). So if the PV panel capacity is same as the inverter capacity, inverter can only produce max 90% of the inverter capacity because of the system loss. Even with the best insolation, inverter can’t work at full load.
Read below solar resource map from the Global Solar Atlas, owned by the World Bank Group and provided by Solargis. We can see that the irradiation of different areas varies greatly and the long-term average yearly sum ranges from 365-3,652kWh/m2. The countries with higher irradiation resources are Australia, Mexico, central America, northern Chile, Saharan countries, South Africa, Namibia, the Arabian peninsula and the US Southwest. These countries are most suitable to build photovoltaic or solar thermal plants. To achieve same production of the PV system for different areas with different irradiation, we can change the DC/AC ratio and keep high DC/AC ratio for areas with low irradiation.
System Loss
The efficiency of the inverter is not constant. There are power switching device losses and magnetic losses. At low output power, the efficiency is relatively low. When the power is between 40% and 60% of the rated output power, efficiency is the highest. When the power is over 60%, the efficiency will be gradually reduced. Therefore, the total photovoltaic power should be controlled between 40% and 60% of the inverter power to obtain the best efficiency.
Inverter Efficiency
PV inverter is an electronic product, and its reliability is highly related to the operating temperature of the inverter. For components such as capacitors, fans and relays, their failure rate will increase by more than 50% when their temperature increases by 10 °C. The operating temperature, on the other hand, is related to power. According to statistics, on a long-term basis the lifespan of inverter that works at 80-100% of the normal output power is about 20% lower when compared to inverter works at 40-60% of the normal output power.
Inverter lifetime
When inverter works at its norminal voltage, the efficiency is the highest. For single-phase inverter at 230V AC voltage, the norminal DC voltage is around 360V; and for three-phase inverter at 400V AC voltage, the norminal DC voltage is around 650V. For example, for a 3kW inverter using 260W panel with MPPT voltage at 30.5V, the best configuration is 12 panels per string with string MPPT voltage at around 366V and PV capacity 3.12kW. For a 30kW inverter using 260W panel, the best configuration is 21 panels per string with string MPPT voltage at 640.5V. The total panel quantity is 126pcs and total power 32.76kW.
Best Working Voltage Range of Inverter
Given all these factors when PV power is between 40-60% of the rated power of the inverter, the system achieves the highest efficiency and the inverter would have the longest lifespan. In order to optimize the performance of the inverter, the PV and inverter capacity should be configured at different ratios according to the solar insolation conditions.
Summary
For areas where the average irradiation exceed 5 hours, and the production time is calculated as 10 hours per day, the DC/AC ratio can be 1, average power is 50%Pn. For areas where the average sunshine time is around 4 hours, production time as 9 hours per day, the DC/AC ratio can be 1.1, average power is 49%Pn (4*11/9). For areas where the average sunshine time is 3 hours, production time as 8 hours, the DC/AC ratio can be 1.3, average power is 48.75%Pn.
For PV plants on hill with different module directions and distributed solar rooftops with complex conditions and different orientations, some of the PV modules will not face south, and the tilt angle of the roof is not the optimal tilt angle. The configuration of the inverter could be done flexibly according to the specific situations.
MAX 50-80KTL3 LV
IHS Global Shipment Ranking
TOP 10
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