Introduction
With the rapid development of expressways in China, the Ministry of Transport recently proposed the standard requirements for green highways. Great changes are taking place in the construction of expressway projects in China. Especially after the upgrading of expressway service facilities, the expressway service facilities are continuously expanded and reconstructed, resulting in the gradual increase of daily power consumption. In order to solve the problem of increasing power demand and effectively improving the utilization rate of land resources in the expressway field, the promotion and application of solar photovoltaic power generation system in the expressway field is a long-term strategic significance that not only ensures energy supply and improves the ecological environment, but also has the practical economic significance of saving natural resources and reducing energy consumption costs. In this paper, the application of solar photovoltaic power generation system in the field of highways will be studied.
1 Principle of solar photovoltaic power generation system
Solar photovoltaic power generation uses the “photovoltaic effect” to convert the light energy received from solar radiation into electrical energy. Photovoltaic power generation system is divided into independent power generation system and grid connected power generation system.
1.1 Independent power generation system
The independent power generation system is not connected with the power supply network of the power system. The system is composed of photovoltaic array (solar cells are packaged in series and parallel), controller, battery pack and DC/AC inverter, as shown in Figure 1.
The DC power generated by photovoltaic modules in the system can be directly supplied to DC loads or converted into AC power via inverters to supply AC loads. It can also be stored by energy storage devices such as batteries and released for use at any time as needed.
1.2 Grid connected power generation system
The grid connection mode can be divided into centralized mode and distributed mode according to the installation mode. At present, the distributed grid connected power generation system is used in the expressway field in China. According to the specific distribution of service facilities along the expressway, photovoltaic arrays with different specifications and orientations are designed and arranged, and then photovoltaic arrays with the same orientation and specifications are converged through single energy inverters, and then a distributed grid connected power generation mode is formed after multiple inverters are centralized. Specific implementation scheme (see Figure 2).
In addition to supplying the load in the system, the generated energy of the system can also be transferred to the public grid; If the power is insufficient, it can also provide power to the load through the public grid. The power supply is flexible and reliable, while reducing the investment and maintenance costs of the battery, which is an economical and applicable way.
It can be seen from the above analysis that, compared with the traditional power supply mode, the solar photovoltaic power generation technology has made innovations in design concept, operation mode and other aspects. The new concepts, technologies and materials adopted by the solar photovoltaic power generation system are closer to the national requirements for green energy conservation.
2 Application status of solar photovoltaic power generation system in domestic and foreign highways
The rapid development of photovoltaic power generation system has been widely used in the highway field. Here are some examples of the application of photovoltaic power generation systems in expressways:
2.1 Solar Light Highway in Italy
Figure 1 Stand alone photovoltaic power generation system
Fig. 2 Distributed grid connected power generation scheme
On January 1, 2011, the world’s first highway using solar cells was opened to the public. This expressway is the A18 expressway connecting Catania Siracusa, Italy (a new 30km expressway in Sicily’s 600km Sicily expressway network). It is the first electrified expressway in the world that is completely powered by solar cells (as shown in Figure 3). The annual power generation density is estimated to be about 12 million kW · h, which can fully meet the power demand of the whole expressway.
Fig. 3 Photovoltaic Power Generation System of Catania Siracusa A18 Expressway in Italy
2.2 Chatiaoling Tunnel Photovoltaic Power Generation System
In January 2014, the grid connection project of the 110KWp distributed photovoltaic power generation system project built by Shaanxi Expressway Group on the basis of the Chatiaoling Tunnel in the east section of Hanzhong of the Shitian Expressway was successfully completed. The design sketch after completion is shown in Figure 4. After the PV power generation system grid connection project is completed and connected to the grid, the maximum average annual power generation capacity can reach 50400 k W · h, and the power generated is mainly used for fan and fluorescent lighting in the Chatiaoling Tunnel.
Fig. 4 Chatiaoling Tunnel Photovoltaic Power Generation System
2.3 Photovoltaic power generation system of Yushe East Expressway Toll Station
On May 10, 2018, the distributed photovoltaic power generation system of Yushe East Expressway Toll Station of Yushe Heshun Expressway in Shanxi Province was successfully put into trial operation (see Figure 5). The project is a breakthrough of Shanxi Expressway in this field. The power generated by the system is mainly used for the office area and site area of the station. The project is expected to save 54000 tons of standard coal annually.
Fig. 5 Photovoltaic Power Generation System of Yushe East Expressway Toll Station
It can be seen from the above cases that, with the gradual improvement of solar photovoltaic power generation technology, solar photovoltaic power generation systems are increasingly applicable to the highway field, mainly in the following aspects: (1) emergency rescue telephone system; (2) Road detection system; (3) Monitoring system; (4) Traffic signs and warning facilities; (5) Lighting of main line, ramp, toll plaza and overpass tunnel; (6) Highway billboards; (7) Power supply system for electronic sign service area, etc.
3 Application prospect of solar photovoltaic power generation in expressway field
3.1 Analysis of solar energy resources in China
According to the standard determined in the Assessment Method of Solar Energy Resources (QXT 89-2008), China is one of the countries with abundant solar energy resources, with annual total radiation between 3000 and 7500MJ/m2 and annual sunshine hours between 870 and 3570h. The abundant solar energy resources are conducive to the promotion and application of solar photovoltaic power generation systems.
3.2 Analysis from the perspective of development strategy of China’s transportation industry
The Ministry of Transport has put forward the standard of green road, which requires the field of highway development to further optimize the energy structure and reduce energy consumption. The application of new energy is imperative; With the development of intelligent transportation, a large number of equipment, intelligent devices and automatic control devices will be added, as well as the upgrading of high-speed service areas; The characteristics of solar photovoltaic power generation system determine that it is more suitable for application in the highway field.
3.3 Analysis from the current situation and planning level of China’s expressway network
At present, China’s expressway network has formed a scale, the demand for electricity is basically stable, and there are a large number of idle roofs and land resources around service facilities and traffic projects. Therefore, with reference to the existing experience and practices at home and abroad, we can vigorously build solar photovoltaic power generation systems in China’s expressway field, thereby promoting the construction and development of environmental protection and conservation oriented highways in China.
According to the highway network planning in China, for a long time to come, the focus of highway construction will be shifted to mountainous and hilly areas. Most of the areas along the project are economically underdeveloped. The social power supply resources are scarce, and the quality of power supply is poor. However, when power is transmitted to places where power consumption is required for a long distance, the power consumption will be large, and the investment in transmission lines will need to be increased, Therefore, the application of solar photovoltaic power generation system can make good use of local natural resources to meet the power demand of expressway facilities.
4 Technical problems to be solved in the application of solar photovoltaic power generation system in expressway field
Based on the analysis and summary of the application of solar photovoltaic power generation system in the highway field at home and abroad, it is concluded that in order to better promote the application of solar photovoltaic power generation system in the highway field, we should pay attention to the following technical problems:; (2) Safe and reliable grid connection and multi power switching; (3) The load bearing of building roof and the stability of solar panel installation need to be considered; (4) The fixing and waterproof problems to be solved by installing solar cells on the existing inclined roof surface; (5) The cost of series and parallel connection between solar arrays of different buildings.
5 Conclusion
To sum up, solar photovoltaic power generation is a kind of very good clean energy with environmental and economic benefits. The wide application of solar photovoltaic power generation system in the field of highways is in line with the clean and low-carbon regional economic development ideas of “transportation+energy” and “high-speed+photovoltaic”, which will certainly play a great role in promoting the construction of green highways.
