An Efficient Off-grid Express Cabinet Based on Wind-solar Hybrid Power Generation System

In order to effectively solve the shortcomings of traditional express cabinets such as limited service places and seasonal power supply obstacles, this paper studies an off-grid express cabinet using wind-solar complementary principle, which is mainly composed of near-ground and low-speed wind power generation device, solar photovoltaic battery pack, controller, battery pack and DC load. The double-axis tracking solar panels or fixed photovoltaic panels can be used for different regions. At the same time, it can be combined with a near-ground and low-speed wind power generation device to provide a stable power supply for the express cabinets. By programming the control, the power generated by wind-solar hybrid power generation is provided to the load as a priority. The remaining electric energy is stored in the battery pack. The system effectively overcomes the disadvantages of limited-service locations and unstable power supply caused by seasonal barriers in traditional express cabinets. It has the advantages of energy-saving, environmental protection, short payback period, and wide range of supply.


Introduction
Energy shortage and environmental pollution have become worldwide problems.Energy saving and emission reduction, energy consumption reduction and comprehensive use of energy are the fundamental ways to solve the problems.In recent years, with the rapid development of the express delivery industry, the express cabinet has been accepted by people because it alleviates the distribution pressure of the last 100 meters [2].The rapid development of e-commerce has led to an increasing demand for express storage [3], and the future development of smart cities and intelligent society will also rely heavily on the smart courier cabinets [4].However, the traditional intelligent express cabinets have some drawbacks, such as limited delivery locations, high electricity costs, and unstable power supply only by solar or wind energy, which lead to disputes among express delivery companies, community properties and owners [5].China is vast and resource-rich, and the solar and wind energy resources are unbalanced in different regions.Wind-solar hybridization is the best way to make up for the lack of resources in wind power and photovoltaic stand-alone systems.For the reasons above, our team designed an off-grid express cabinet based on wind-solar hybrid power generation.

System Composition And Designing Principle
The express cabinet system consists of photovoltaic intelligent capture system, near-ground and low-speed wind power generation device, intelligent express cabinet and battery pack.The photovoltaic intelligent capture system adopts double-axis tracking solar device to make the solar panel always perpendicular to the sun rays.The near-ground and low-speed wind power generation device uses micro-tubes, which can be adjusted up and down by spring control to form a "jet effect" in front of the blades of wind power generator, collecting wind and improving the wind flow speed.The controller controls the power generated by the generator to be supplied to the load at any time and consumed in real time, and when the power supply is insufficient, the remaining electric energy is stored for use.The valve-controlled sealed lead-acid battery that meets the requirements of YD/T799-2002 shall be used as the battery for storing the excess power.

Photovoltaic intelligent capture system
Solar energy is converted into fluctuating DC power through photovoltaic module, and then converted into stable DC power by solar controller to supply power to communication load.As a kind of clean energy source, it is pollution-free, inexhaustible, and can be intermittent at will.During the day, the sun generates a lot of electricity, but at night, it cannot generate electricity without light.Meanwhile, due to seasonal effects, the power generation in summer is 1.5 to 2.7 times higher than in winter.The overall design frame of the system device is divided into four modules: power actuator module, mechanical body module, detection sensor module and electronic control module.The specific analysis is as follows.5) Complete the processing of four-quadrant sensor signals and the simulation calculation of the sun's motion trajectory with the SCM control system, and realizes the SCM control of the motor and the display of system status through C language program.(6) The control part firstly amplifies the signal sent by the SCM, and then sends it to the motor, which receives the signal and executes the action to realize automatic capture.(7) The mechanical device adopts worm gear transmission to achieve the purpose of decelerating and changing the direction of movement.
Double-axis tracking is adopted, meaning that the solar panel moves independently around the vertical and horizontal axis to change the directional and pitch angle of the system.Because the power generation of solar photovoltaic module array is related to the incident angle of sunlight, when the light is perpendicular to the module plane, the solar radiation received by the photovoltaic module array is the largest, and its power generation is also the largest.The mechanical part is composed of a solar panel (with a photoelectric sensor in the middle), a T-bracket, a connecting rod, a cam, a worm gear reducer and two stepper motors.As shown in Figure 2, the stepper motor drives the T-bracket (equipped with a vertical shaft) after being decelerated by the worm gear reducer.
The cost of wind-solar hybrid power generation device is much lower than that of photovoltaic system, and the cost performance ratio of wind-solar hybrid system is higher than that of pure photovoltaic system.According to the abundance of solar energy resources, the photovoltaic part of wind-solar hybrid system determines its installation mode.In areas with poor solar energy resources, a fixed installation angle will not capture a significant amount of solar energy resources.The solar cell array is kept perpendicular to the sun by using double-axis tracking and automatic tracking, and the received solar radiation energy is maximized, which improves the conversion efficiency of solar energy and ensures the maximum utilization rate of solar energy.Areas with abundant solar energy resources: set the best installation angle.According to the total amount of radiation, scattered radiation and total radiation of the lunar horizontal plane, the amount of solar radiation received by the installation slope of the solar panel is calculated by theoretical calculation and the program written in C language.By calculating the maximum solar radiation received by the solar panel in a certain period of time, the optimal mounting angle of the solar panel can be obtained.

Near-ground and low-speed wind power generation device
The wind is used as a power source to drive the generator to rotate and thus generate electricity.Wind energy is converted into mechanical energy by the wind turbine, and then the mechanical energy is converted into electrical energy.As a kind of clean energy source, it is pollution-free, inexhaustible, and can be intermittent at will.The wind speed is the lowest at noon and gradually increases at night, resulting in an increase in power generation.Due to the seasonal influence, the wind speed is minimum in summer and maximum in winter, and the power generation capacity also increases correspondingly.

Figure 3. Near-ground and low-speed wind power generation device
The self-developed near-ground and low-speed wind power generation device can form a "jet effect" in front of the blades of the wind turbine, gathering wind power to improve the speed of wind flow, so that the power generation efficiency of the near-ground wind power generation device can be increased by 10%-15%, thus realizing the low wind speed power generation with more remarkable energy-saving effect.At the same time, the "mask type" is adopted to install the blades at the rear, which greatly improves installation safety, reduces maintenance costs, and reduces the risk of the blades to birds flying near the ground.The micro-tubes of the wind turbine are adjustable, and the blades of the wind turbine are protected by the deformation of the upper and lower springs, which greatly enhances its stability in bad weather and reduces the probability of damage of the wind turbine.

Battery pack
The valve-controlled sealed lead-acid batteries meeting the requirements of YD/T799-2002 is used as the power storage part, and 4 batteries are selected for every 12 V.This part is an important energy storage device of wind-solar hybrid system, and the original equipment is fully utilized without increasing investment.The wind-solar hybrid power supply system controls the output power of the switching power supply module through the fan controller, thus adjusting the balance between power supply and power consumption.By controlling the output characteristics of the switching power supply module, the electric energy generated by wind-solar hybrid power generation is provided to the load in priority, and the surplus electric energy is transmitted to the battery pack, so as to continuously supply power to the express cabinet in the absence of wind and light, and ensure the smooth operation.Maximize the use of new energy to generate electricity and maximize the effect of energy saving and emission reduction.

Connection method
Series or parallel Series/parallel voltage Series connection: the sum of each load voltage Parallel: system working voltage Battery pack capacity  Battery capacity:

The design of solar cell array power in photovoltaic power supply system
A is the safety factor, between 1.1 and 1.4, and QL is the daily average power consumption of the load, which is the working current × the number of working hours per day; NL is the longest number of consecutive cloudy and rainy days, and TO is the temperature correction coefficient, which is generally 1 above 0. Take 1.1 for temperature above -10°C and 1.2 for temperature below -10°C; CC is the battery depth of discharge, generally 0.75 for lead-acid batteries and 0.85 for alkaline nickel-cadmium batteries.
Number of solar cell modules in series: (2) UR is the minimum output voltage of the solar cell array.UOC is the optimal operating voltage of the solar battery pack.Uf is the floating voltage of the battery.UD is the voltage drop of the diode, generally 0.7V.UC is the voltage drop caused by other factors.
Daily power generation of solar cell modules: (3) IOC is the optimal operating current of the solar cell module.KOP is the slope correction coefficient, and the slope daily radiation correction coefficient KOP is 1.09 ~1.14 according to the installation latitude of solar cell module.CZ is the correction coefficient, which is mainly for the combination, attenuation, dust, charging efficiency and other factors.For the loss of other factors, it is generally 0.8.H refers to the daily solar radiation at the installation site of the solar cell policy.The average daily radiation hours at a standard intensity of Ht. (4) Among them, 2.778/10000 ( h.m/kJ ) is the coefficient to convert the daily radiation into the average daily radiation hours under the standard light intensity (1000 W/m²).
The battery capacity that needs to be replenished during the period: NW is the shortest number of days between the two groups of longest continuous rainy days.Solar cell array power: ) PO is the rated power of a single standard module of the solar cell module.Theoretical calculation of the optimal installation angle in areas with rich solar energy resources [7].

Main parameters of near-ground and low-speed wind power generation device
In the formula: ρ-ground reflectivity, fixed value in the process of 0.22.H-total solar radiation on the horizontal plane.HT-total solar radiation.HbT-direct solar radiation.HdT-sky scattered radiation.HρT-ground Reflected radiation.Hb-direct radiation.Hd-scattered radiation.Hb and Hd are obtained from statistical meteorological data.
According to the formula: Where n is the date serial number within one year.Call the sine (sin) function in the math library to calculate the formula, convert the angle value to a radian value, and enter the date serial number to determine the value of the solar declination angle φ during the period.
Disregarding the influence of atmospheric refraction and landmark twists on solar radiation, according to the formula: Call the arc cosine function (Acos), tangent function (tan) and minimum function in the math library, and input the panel inclination S, local latitude δ and solar declination φ to determine the values of ωsl and ωst.
According to the formula: Use the ωsl and ωst values calculated in the second step, call the π in the math library to calculate the values of Rb, Rd, and Rρ respectively.
According to the model put forward by Nijmeh S [8], the calculation and expression of the above-mentioned sky scattered radiation HdT is as follows: The calculation process of H0 is as follows:  In the above calculation process, GSC takes a fixed value which is 1367 W/m 2 .In this process, the scattered radiation caused by ground reflection follows the law of homogeneous distribution [9], so the calculation of Rρ is as follows: Enter the total monthly radiation, scattered radiation and total radiation to get the total solar radiation received by the solar panel.Through the above calculation method, after multiple cycles of calculation, the relationship of the total amount of solar radiation received by a group of solar panel inclination angle is obtained, and the optimal installation angle is determined.After the calculation is completed, the Ecotect simulation analysis [10] can be used to verify the solar thermal radiation value at the best installation angle.Solar and wind energy are highly complementary in terms of time and region.During the day, when the sun is at its strongest, the wind is very weak, while at night, after the sun goes down, the light is very weak.However, due to the great change in the surface temperature, the wind energy increases.There is little sunshine and strong wind in summer, and weak sunshine and strong wind in winter.The time complementarity of solar and wind energy makes the wind-solar hybrid power generation system have the best matching in resources, and the wind-solar hybrid power generation system is an independent power generation system with the best resource conditions.Northern Ningxia, northern Gansu, eastern Xinjiang, western Qinghai and western Tibet are most suitable for wind-solar hybrid power generation.Northwest Hebei, northwest Shanxi, southern Inner Mongolia, southern Ningxia, central Gansu, eastern Qinghai, southeast Tibet and southern Xinjiang are the second most suitable places.Although the southeast coast is most suitable for wind power generation, the typhoon is so destructive that they may destroy wind-solar hybrid power generation equipment at one time.Therefore, according to the actual location, it is necessary to consider whether to install wind and solar hybrid power generation devices.China's east coast, the three eastern provinces, Inner Mongolia and Tibet are rich in wind power resources and can use near-ground, low-speed wind power generation device.

Benefit analysis of energy saving and emission reduction
Taking the Xining area as an example, the power generation capacity of solar panels is about 0.14-0.15kWh per m² and about 4.2-4.5 kW per h, respectively.According to Xining area is located at longitude 101°77′E and latitude 36°62′N, the best inclination angle is determined to be 34°a nd the average peak sunshine duration is 4.67 h.The average annual wind speed is 4.0~4.9m/s, the 450 W wind generator, the starting wind speed is 1.93 m/s, the rated wind speed is 11 m/s, and the working hours per day is 12 h.The average daily power generation is 5.4 kW, which means that the daily net power output of a wind-solar hybrid intelligent express cabinet is about 10.014-11.415kW, and the annual net power output is about 3650 kW.A Feng Chao Express Cabinet system consumes about 10 kW per day.The wind-solar hybrid power generation system can meet the requirements completely.
The use of clean energy has reduced the dependence on external energy.In addition, every kilowatt-hour saved is equivalent to saving 0.4 kg of standard coal and 4 liters of water, and reducing 0.997 kg of carbon dioxide emissions, 0.03 kg of sulfur dioxide emissions and 0.015 kg of nitrogen oxides.As a result, the annual net output is 9,300 kW, which is equivalent to saving 3720 kg of standard coal and 37,200 kg of water, and reducing 9272.1 kg of carbon dioxide emissions, 279 kg of sulfur dioxide and 139.5 kg of nitrogen oxides emissions per year for each wind-solar hybrid intelligent express cabinet.

Social and economic benefit analysis.
Cost budget: The production cost of a near-ground and low-speed wind power generation device is about 5,000-6,000 yuan, and the cost of a complete wind-solar hybrid power generation system is about 30,000 yuan.
Benefit analysis: With the reduction of cost and the acceleration of product upgrading, China's new generation industry is developing rapidly.The cost of commercial electricity is 1.25 yuan/kWh, and each wind and solar hybrid system can save 4562 yuan per year.The return period on investment is 6.5 years.Compared to the 25-year lifetime of solar photovoltaic panels, the payback period is very attractive.

Conclusions
In this paper, an off-grid courier cabinet using the principle of wind-scenery complementarity is studied, the system composition is analyzed, a mathematical model is established, and the suitable installation method and equipment selection are derived through many cycles of calculations, which are combined with the actual climatic conditions in different regions.The main conclusions are as follows: (1) Taking Xining area as an example, the daily net power generation capacity of wind-solar complementary smart express cabinet is about 10.014-11.415kW, and the yearly net power generation capacity is about 3,650 kW.The wind-solar hybrid power generation system can fully satisfy the daily power consumption requirement of the courier cabinet system.
(2) The wind-solar complementary intelligent express locker uses a large amount of clean energy, reduces the consumption of external energy, and lowers pollutant emissions.Compared with near-earth low-speed wind power generation devices, each wind-solar hybrid system can save $4,562 per year.In terms of energy efficiency and economy, wind-solar hybrid power generation system is more energy efficient and environmentally friendly than traditional power generation system.
(3) The time complementarity of solar and wind energy makes the wind-solar hybrid power generation system have the best resource matching.However, for different regions, it is necessary to consider whether their resource environment is suitable for installing wind-solar hybrid power generation equipment.
The utilization of renewable energy has become an important topic of future energy research.With the advent of the intelligent era, the technology of wind-solar hybrid power generation will become more mature, and realize digitalization and miniaturization [11], and the use of express cabinets will gradually spread to thousands of households.With the continuous improvement and innovation of technology, the wind-solar complementary express cabinets will be more suitable for commercial production, become intelligent and life-oriented, and will shine brilliantly in the future infrastructure construction.

Figure 1 .
Figure 1.The 3D model of equipment

Figure 2 .
Figure 2. Photovoltaic automatic capture system installation (1) Take AT89C51 single-chip microcomputer as the core of the system.(2)Use sensors to identify day/night and weather conditions.(3) Design the photoelectric detection part connected with a single chip microcomputer for effective control.(4) Use LED display and the clock circuit can use serial clock chip DS1302.(5) Complete the processing of four-quadrant sensor signals and the simulation calculation of the sun's motion trajectory with the SCM control system, and realizes the

T 1 -
Photovoltaic cell model 100wpHYT100D-24( Number of photovoltaic cells PV module area( 2 ) A Installation angle Follow-up regulation

Figure 4 .
Figure 4. Distribution map of wind energy and solar energy resources

Table 1 .
Selection and Calculation of Battery For Photovoltaic Power Supply System

Table 2 .
The Design of Solar Cell Array Power In Photovoltaic Power Supply System 5) Load daily average power consumption QL = Working current × Working hours per day.Number of solar modules in parallel:

Table 3 .
Main parameters of near-ground and low-speed wind power generation device