The Application of Solar Photovoltaic Power Generation Systems to Rail Transit (Part Two)
2.2 Selection of the combination of photovoltaic modules and buildings
The combination of photovoltaic modules and buildings is divided into Building Attached Photovoltaic and Building Integrated Photovoltaic. BAPV(Building Attached Photovoltaic) is the combination of buildings and photovoltaic systems. There is no need to change the building; just fix the components on the building, that is, paving a layer of photovoltaic modules on the building. Special design is conducted for BIPV(Building Integrated Photovoltaic) to achieve the purpose of a good combination of building and photovoltaic modules
The roof of the car depot warehouse generally adopts the steel grid. Factors such as lighting, heat insulation, shading and other factors should be considered for the design. The combination of BAPV and BIPV can be adopted for the car depot. The lighting belt part of the roof adopts the BIPV scheme, while the nonlighting part of the roof adopts the BAPV scheme. The use of solar energy is brought into the overall design of the environment, integrating architecture, technology and aesthetics. Solar energy facilities become a part of the building and are organically integrated to reduce the impact on the shape of the building; using solar energy facilities to replace part of the roof covering can reduce costs and improve efficiency.
3. Grid-connected design of photovoltaic power generation systems
There are two schemes for grid-connected photovoltaic power generation systems in rail transit projects: low-voltage grid-connected and medium-voltage grid-connected.
3.1 Low-voltage grid-connected schemes
Low-voltage grid-connected systems are usually used in places where the installed capacity of the system is small or the installation site of photovoltaic modules is limited. In rail transit projects, elevated stations (ground stations) should adopt low-voltage grid-connected methods; photovoltaic power generation systems are connected to two bus sections on the low-voltage side of substations. Anti-reverse flow devices are equipped to avoid interference to the high-voltage side protection devices.
According to the low-voltage load capacity and grade requirements of rail transit, this plan suggests that the low-voltage grid-connected system mainly supplies power to the secondary and tertiary loads of the station, such as normal lighting, street lighting, advertising lighting and general power loads.
3.2 Medium voltage grid-connected schemes
Medium-voltage grid-connected systems are usually used in places where the installed capacity of the system is large or there is enough space to install photovoltaic modules. In rail transit projects, the car depot and parking lot should adopt the medium-voltage grid-connected method. The grid-connected method matched with the inverter is better, which can improve the conversion efficiency of the inverter. The grid-connected system is connected to the 35kV medium voltage ring network through a step-up transformer, and the photovoltaic power generation system is connected to the two bus sections of the 35kV side of the car depot substation.
4. Benefit analysis
4.1 Energy and economic benefits
In this article, the solar energy resources are in the fourth area. Take a rail transit depot project in Hangzhou as an example for analysis. The roof of the depot warehouse generally provides an installation site area of about 50,000㎡, and the roof utilization rate is calculated at 50%. The average annual total radiation of this area is 4800MJ/m2. According to the common parameters of the existing photovoltaic module market, the module parameters with a rated power of 285Wp and a size of 2100 mm x 1130 mm are selected for calculation and analysis.
Since the installation angle, blocking of the sunlight, dust, inverter efficiency, transmission line loss and other factors will affect the efficiency of the photovoltaic power generation system, the above factors should be considered when the system efficiency is calculated. The overall efficiency of the system in this article is 75%. Table 2 shows the calculation of the system's annual on-grid power and energy saving and emission reduction.
Table 2 Yearly calculations of on-grid electricity consumption, energy saving and emission reduction
The combination of photovoltaic modules and buildings is divided into Building Attached Photovoltaic and Building Integrated Photovoltaic. BAPV(Building Attached Photovoltaic) is the combination of buildings and photovoltaic systems. There is no need to change the building; just fix the components on the building, that is, paving a layer of photovoltaic modules on the building. Special design is conducted for BIPV(Building Integrated Photovoltaic) to achieve the purpose of a good combination of building and photovoltaic modules
The roof of the car depot warehouse generally adopts the steel grid. Factors such as lighting, heat insulation, shading and other factors should be considered for the design. The combination of BAPV and BIPV can be adopted for the car depot. The lighting belt part of the roof adopts the BIPV scheme, while the nonlighting part of the roof adopts the BAPV scheme. The use of solar energy is brought into the overall design of the environment, integrating architecture, technology and aesthetics. Solar energy facilities become a part of the building and are organically integrated to reduce the impact on the shape of the building; using solar energy facilities to replace part of the roof covering can reduce costs and improve efficiency.
3. Grid-connected design of photovoltaic power generation systems
There are two schemes for grid-connected photovoltaic power generation systems in rail transit projects: low-voltage grid-connected and medium-voltage grid-connected.
3.1 Low-voltage grid-connected schemes
Low-voltage grid-connected systems are usually used in places where the installed capacity of the system is small or the installation site of photovoltaic modules is limited. In rail transit projects, elevated stations (ground stations) should adopt low-voltage grid-connected methods; photovoltaic power generation systems are connected to two bus sections on the low-voltage side of substations. Anti-reverse flow devices are equipped to avoid interference to the high-voltage side protection devices.
According to the low-voltage load capacity and grade requirements of rail transit, this plan suggests that the low-voltage grid-connected system mainly supplies power to the secondary and tertiary loads of the station, such as normal lighting, street lighting, advertising lighting and general power loads.
3.2 Medium voltage grid-connected schemes
Medium-voltage grid-connected systems are usually used in places where the installed capacity of the system is large or there is enough space to install photovoltaic modules. In rail transit projects, the car depot and parking lot should adopt the medium-voltage grid-connected method. The grid-connected method matched with the inverter is better, which can improve the conversion efficiency of the inverter. The grid-connected system is connected to the 35kV medium voltage ring network through a step-up transformer, and the photovoltaic power generation system is connected to the two bus sections of the 35kV side of the car depot substation.
4. Benefit analysis
4.1 Energy and economic benefits
In this article, the solar energy resources are in the fourth area. Take a rail transit depot project in Hangzhou as an example for analysis. The roof of the depot warehouse generally provides an installation site area of about 50,000㎡, and the roof utilization rate is calculated at 50%. The average annual total radiation of this area is 4800MJ/m2. According to the common parameters of the existing photovoltaic module market, the module parameters with a rated power of 285Wp and a size of 2100 mm x 1130 mm are selected for calculation and analysis.
Since the installation angle, blocking of the sunlight, dust, inverter efficiency, transmission line loss and other factors will affect the efficiency of the photovoltaic power generation system, the above factors should be considered when the system efficiency is calculated. The overall efficiency of the system in this article is 75%. Table 2 shows the calculation of the system's annual on-grid power and energy saving and emission reduction.
Table 2 Yearly calculations of on-grid electricity consumption, energy saving and emission reduction
| Serial numbers | Names | Numerical values |
| 1 | Numbers of solar cell module panels | 10535 |
| 2 | System installation capacity/kWp | 3002.5 |
| 3 | Panel areas/m2 | 24999.5 |
| 4 | Annual power generation per unit area/ (kWh/m2) | 200 |
| 5 | Annual theoretical power generation/kWh | 4999900 |
| 6 | Annual on-grid power/kWh | 3749925 |
| 7 | Annual effective power generation utilization hours/h | 1248.9 |
| 8 | Annual savings of standard coal/t | 1462.5 |
| 9 | Annual CO2 emission reduction/t | 5249.9 |
According to the above calculation data, the installed capacity of the system can be achieved at 3002.5kWp. The entire system is considered based on a 25-year operating cycle, and the annual component attenuation is calculated at 0.8%. The system is expected to generate approximately 85277MWh of electricity in 25 years, save approximately 33258 tons of standard coal, and reduce total carbon dioxide emissions by approximately 119,388 tons.
The comprehensive cost index of solar photovoltaic power generation systems is 16RMB per watt on the current market. According to this index, the project cost is about 48.04 million RMB. According to the current subway operating electricity price of 0.75RMB in Hangzhou (about an annual increase of 2%), this project's investment recovery is about 16 years, and the economic return in 25 years is about 33.23 million RMB. If policy subsidies are considered, the payback period of the project investment will be shorter and the economic return will be better.
4.2 Social and environmental benefits
Solar photovoltaic power generation is a kind of clean and renewable energy. It will neither produce pollutants nor greenhouse gases that damage the atmospheric environment. It can be seen that the environmental benefits are obvious from saving coal and reducing carbon dioxide. The application of this system can respond to the call for energy saving and actively cooperate to complete the targets of energy saving and emission reduction; making full use of renewable clean energy can not only reduce the demand for fossil energy, but also reduce environmental pollution and protect the human ecological environment. Its social benefits are obvious.
5. Conclusion
With the continuous acceleration of China's urbanization process, the construction of rail transit has also been greatly promoted, and its advantages on the site provide the possibility for the application of solar photovoltaic power generation systems. Rail transit, as a public building with wide coverage and great passenger flow, adopts solar photovoltaic power generation systems, which can play a good role in promoting and demonstrating energy conservation and emission reduction.
