A review of desert photovoltaic power transmission technology

Ruowei Wang1, Yinxuan Li2, Weichun Ge3, Shitan Zhang1, Chuang Liu1, Shuai Chu4

1. School of Electrical Engineering, Northeast Dianli University, Jilin City, Jilin Province, 132012

2. State Grid Tianjin Marketing Service Center (metering; Center, Hexi District, Tianjin 300120, China

3. State Grid Liaoning Electric Power Co., Ltd., Shenyang City, Liaoning Province, 110006

4. School of Electrical Engineering, Shenyang University of Technology, Shenyang 110870, China

Summary of Desert Photovoltaic Power Transmission Technology

Wang Ruwei1, Li Inxuan2, Gay Weichun3, Zhang Shitan1, Liu Chuang1, Chu Shuai4

1.School of Electrical Engineering, Northeast Electric Power University, Jilin 132012, Jilin Province, China

2.State Grid Tianjin Marketing Service Center (Metrology Center), Hexi District, Tianjin 300120, China

3.State Grid Liaoning Electric Power Co. , Ltd. , Shenyang 110006, Liaoning Province, China

4.School of Electrical Engineering, Shenyang University of Technology, Shenyang 110870, Liaoning Province, China

summary

Making full use of renewable energy is an important guarantee for the mainland to achieve the "dual carbon" goal, however, because centralized photovoltaic is located in a desert area, photovoltaic power transmission has become a bottleneck restricting the development of photovoltaic power. As a way to transport electric energy, transportation battery technology can provide a new option for the existing desert electric energy transmission. Firstly, from the perspective of desert centralized photovoltaic power transmission, the actual operation of the existing desert photovoltaic power transmission is discussed, and the working principle of battery transportation technology is analyzed.

Key words: carbon peak ; carbon neutrality ; Desert photovoltaic ; Battery shipping

Abstract

To achieve carbon peaking and carbon neutrality goals and improve energy efficiency, new energy plays a major role in China strategic deployment. However, as centralized photovoltaic is located in desert areas, photovoltaic power transmission has become a bottleneck problem. Battery transportation technology provides new possibilities for the existing problems of desert electric energy transmission. Firstly, from the perspective of desert centralized photovoltaic transmission mode, this paper focused on the actual operation of the existing desert photovoltaic transmission and analysed the working principle of battery transportation technology. Then, the differences such as investment cost, transmission capacity, and ability to cope with desert climate were compared between traditional desert photovoltaic transmission and battery transportation technology. Finally, the advantages of battery transportation technology, such as low construction cost and strong ability to deal with desert climate were summarized and the limitations were also discussed.

Keywords： peak carbon dioxide emissions ; carbon neutrality ; desert photovoltaic ; battery transportation

0 Introduction

Striving to achieve "carbon peak" by 2030 and "carbon neutrality" by 2060 is one of the important strategies of the mainland [1-2]. With the continuous growth of the continent's energy demand, the storage of fossil energy is declining, and the combustion of fossil energy will generate a large number of harmful gases such as NOx and CO2, causing serious environmental pollution and global warming. Vigorously developing renewable clean energy such as solar energy and building a new low-carbon power system is one of the important measures for the mainland to achieve the "double carbon" goal. Compared with other energy sources, solar energy has the advantages of wide distribution, no pollution, and abundant reserves, which makes photovoltaic power generation develop rapidly all over the world. As of the end of November 2021, the cumulative installed capacity of photovoltaic power generation in mainland China has exceeded 290 million kW. However, with the rapid expansion of photovoltaic installed capacity, the problem of photovoltaic power transmission has intensified.

At present, scholars have studied the economic problems of desert photovoltaic power stations based on indicators such as life cycle cost, minimum energy consumption, payback period, and net present value. Among them, the net present value is an economic indicator commonly used in the construction and operation of desert photovoltaic power plants, which takes into account the timeliness of the life and capital cycle of the entire photovoltaic power plant project [3]. In terms of cost, Ref. [4] argues that photovoltaic power generation is superior to traditional power generation by comparing the cost and service life of photovoltaic power generation with traditional power generation. In terms of life-cycle cost, Ref. [5] constructs a life-cycle model of photovoltaic power generation, implements a variety of cost concepts into cost accounting, and takes a photovoltaic power generation project in China as an example to calculate the whole life cycle of photovoltaic power generation. Ref. [6] established an inventory of carbon emissions from different photovoltaic power generation and compared the carbon emissions in different environments. For the problem of new energy access to the power grid, Ref. [7] constructs the scenario of new energy grid-connected DC power grid by analyzing different transmission distances and capacities, and concludes that the DC voltage level with the lowest annual cost and the flexible DC transmission technology are better under different combinations. The above literature focuses on the construction economy of desert photovoltaic power stations and the ability of photovoltaic power to connect to the power grid, analyzes and summarizes the cost composition, influencing factors, accounting methods, and application scope of different grid-connected methods, and improves the economy of desert photovoltaic power generation projects by optimizing the allocation of photovoltaic power plants, improves the competitiveness of photovoltaic power generation, and helps the photovoltaic power generation system to develop in the direction of high stability and low cost. At present, there have been many studies on improving the grid-connected capacity of photovoltaic power transmission, but there are few studies on the economics of photovoltaic transmission in desert areas.

Long-distance transmission requires transmission lines of high voltage levels, so UHV AC transmission and HVDC transmission occupy an important position in China's power transmission [8-10]. At present, scholars have conducted research on high-voltage DC and ultra-high voltage AC transmission. In Ref. [11], a fault ranging method for HVDC transmission lines was studied, and a fault ranging method for HVDC transmission lines based on the frequency difference ratio of double-ended traveling waves was proposed, which eliminated the influence of wave velocity attenuation and realized double-ended ranging without the synchronization of clocks on both sides of the line. In Ref. [12], the protection of the UHV AC line against lightning bypass in actual operation was studied, and the influence of tower wave impedance and grounding current on the protection characteristics of lightning protection device was analyzed, so as to prove the effectiveness of the AC line arrester on lightning bypass protection in the UHV transmission system in practical work. Ref. [13] proposes a hybrid cascade UHVDC transmission scheme of voltage source converter and grid commutation converter for high proportion of new energy transmission, which effectively solves the problems of static voltage stability, transient overvoltage and lack of inertia of the sending system faced by high proportion of new energy transmission, and reduces the capacity requirements of the synchronous condenser and static synchronous compensator of the sending system. In summary, high-voltage transmission can effectively solve the long-distance new energy transmission demand of the mainland, and compared with the traditional transmission lines, it has the advantages of long distance, large capacity and low loss, and is currently widely used in the fields of long-distance power transmission and new energy access to the power grid in the mainland. Due to the long length of the line, the climate and environmental changes of the passing area, and the high probability of line failure, the safety of high-voltage transmission lines still needs to be further studied.

In order to solve the problem of difficult photovoltaic power transmission in desert, this paper proposes a strategy of using transportation batteries for electric energy transportation. By analyzing the working principle of photovoltaic power generation system and battery transportation technology, focusing on the economic transmission distance of equipment, investment cost, operation loss and other economic indicators, and comparing the differences between transportation batteries and other desert photovoltaic transmission modes, it is found that transportation battery technology has the advantages of low operation difficulty and strong desert climate, and at the same time, due to the use of camel transportation mode to transmit electric energy, it can effectively alleviate the employment pressure in the western part of the mainland.

1 Transport battery technology

Desert photovoltaic power generation is characterized by large-scale centralized development, medium and high voltage access, and high-voltage long-distance transmission and consumption. Transportation battery technology does not require high-voltage transmission and offers significant advantages over conventional transmission technologies. The transportation battery technology can store the electric energy output of desert photovoltaic in the battery, and transport the battery to the battery swap station for application through artificial camel transportation, which can effectively avoid the harm of desert wind and sand, high salt and other characteristics to the transmission line, and improve the ability to cope with the desert climate.

1.1 Desert photovoltaic power station

The main components of a photovoltaic power generation system are photovoltaic arrays, inverters, transformers and other grid-connected links, as shown in Figure 1. Among them, a photovoltaic array consists of multiple photovoltaic cells connected in series and parallel, and the electrical energy generated by the photovoltaic array is transmitted to the grid through inverters, filters, and step-up transformers [14].

Figure 1

Figure 1 Photovoltaic cell power station

Fig. 1 Photovoltaic cell power station

In transport battery technology, the battery is charged first by a photovoltaic system, and the battery charging power can be obtained by calculating the power generated by the photovoltaic power plant. According to the light-receiving area of a single solar PV module, calculate the light-receiving area of all solar array modules:

�(�)=∑�=1���(�)

(1)

where: ( ) is the total area of all solar array cell modules at time t, m2;M is the number of solar array cell modules, m2; ( ) is the light receiving area of the mth solar photovoltaic cell module at time t, m2.

According to the photoelectric conversion efficiency of a single solar photovoltaic cell module, the photoelectric conversion efficiency of photovoltaic power station is calculated η:

�=∑�=1���(�)���(�)

(2)

where ηm is the photoelectric conversion efficiency of the mth solar PV module, %.

According to equations (1) and (2), calculate the output power of the photovoltaic power generation system:

s( )= ( )

(3)

where: s( ) is the output power of the photovoltaic power station at time t, kW;r is the radiance, kW/m2.

1.2 Battery state of charge

When the output power of photovoltaic energy fluctuates, the battery capacity and its maximum allowable charge/discharge power limit the charge-discharge power of the energy storage system [15]. To this end, a mathematical model of the charge-discharge system of the photovoltaic energy storage battery (as shown in Fig. 2) was established to study the charge-discharge power and the battery S(t) (state of charge) of the photovoltaic energy storage battery system at each time, so as to meet the requirements of the photovoltaic battery energy storage system for battery charge and discharge control, and to judge the state of charge of the battery through S(t).

Figure 2

Figure 2 State of charge of the battery

Fig. 2 Battery state of charge

The ratio of the remaining capacity of the rechargeable battery of a photovoltaic battery energy storage system after a period of use or long-term shelving to its capacity in a fully charged state is usually expressed as a percentage. The value range is 0~1, when ( )=0, it means that the electric energy in the photovoltaic cell is completely released, and when S(t)=1, it means that the electric energy in the photovoltaic cell reaches the full value.

�(�)=(1-�)�(�-1)+�s(�)�ch��b

(4)

where S(t) and S(t-1) are the battery state of charge at time t and time t-1, respectively. is the self-discharge rate of the battery, %; ch is the charging efficiency of the energy storage battery, %;Δ is the sampling period, h; b is the capacity of the energy storage battery, A⋅h.

1.3 Desert power transmission mode

There are many ways to transmit electricity in the desert, and this article focuses on transportation battery technology, 220 kV transmission lines, UHV AC transmission, and ± 800 kV DC transmission.

1) Transportation battery technology

Transportation battery technology differs from traditional power transmission methods in many ways. First of all, it is necessary to set up a photovoltaic power-to-battery charging device in a centralized photovoltaic power station in the desert to convert the fluctuating photovoltaic power into battery power. Then, the batteries are manually transported on the desert to the nearby battery swap station by camel transport, so as to realize the transmission of photovoltaic power in the desert. The transportation battery technology does not require the construction of transmission lines, which avoids the loss of transmission lines in the desert climate. However, the conveying distance and conveying capacity are relatively low.

2) UHV AC transmission

UHV AC transmission realizes long-distance transmission by increasing the voltage level of traditional transmission lines, increasing the transmission capacity of transmission lines, and increasing the transmission distance of the system, which is basically the same as that of ordinary AC transmission systems [16-18]. Although the transmission distance of UHV AC transmission projects is still relatively short, studies have shown that if the 1 000 kV AC transmission line is segmented every 500 km, the transmission distance can reach 2 000 km and the transmission power can reach 4 000 MW by using the intermediate switch station plus the stationary reactive power compensator and line series capacitance compensation [19-21]. With a total length of about 34 200 m, Sutong is currently the world's highest voltage level, the largest transmission capacity, the longest transmission distance and the most advanced technical level of rigid gas insulated transmission line project, which has been completed and put into operation in September 2019.

3) UHVDC transmission

Continental's UHVDC transmission voltage is typically ± 800 kV or higher. In recent years, with the continuous improvement of the requirements for transmission line transmission capacity in various regions of the mainland, the research on UHVDC transmission technology is gradually deepening in order to rationally develop and utilize the mainland's power resources [22-24]. The basic principle of HVDC transmission is: AC at the sending end becomes DC, and then the power is transmitted to the receiving end through the UHVDC line, and the DC at the receiving end is changed to AC, so as to realize power transmission. There are two voltage levels of UHVDC: DC ± 800 kV and DC ± 1 100 kV. At present, the mainland has realized the construction of UHVDC ± 800 kV transmission projects. The UHV transmission line from Xiangjiaba to Shanghai adopts the transmission mode of UHVDC transmission ± 800 kV, with a transmission distance of 2 000 km and a transmission capacity of 6 400 MW. The ± 800 kV transmission line from Hami to Zhengzhou in Xinjiang is currently under construction, with a transmission distance of 2 200 km and a transmission capacity of 8 000 MW.

2 Comparison of the economy of transportation batteries with other modes of transmission

In order to reflect the economics of various modes of transportation, this paper compares various indicators of desert photovoltaic and battery transportation, including the economic transmission distance of equipment, investment cost, operating loss, unit annual cost, design complexity, transmission capacity, and ability to cope with desert climate. Table 1 provides an overview of the economic indicators of each transmission mode.

Table 1 Comparison of the economy of transportation batteries with other modes of transmission

Tab. 1 Performance comparison of the transportation battery technology with other methods

Comparison items	Transport the battery	220 kV transmission	UHV AC transmission	± 800 kV DC transmission
Economical conveying distance	Less than 100 km	200~300 km	600~800 km	1 200~2 700 km
Investment costs	The investment fee is particularly low, Less than 1 million yuan/km	The upfront investment cost is relatively moderate, It is about 1.2 million ~ 2 million yuan/km	High upfront investment costs, It is about 2.5 million yuan/km	The upfront investment cost is the highest, It is about 3.3 million yuan/km
Operational attrition	The annual operating loss rate is generally greater than 5%	The annual average linear loss rate is 1%~3%	Annual average line loss rate<2%	The average annual line loss rate is mostly > 6.5%
Annual cost per unit	Relatively high	The annual cost is relatively modest	The annual fee is relatively high	transmission distances greater than 1 300 km, The annual fee is relatively low
The complexity of the design	Low design requirements, General personnel can carry out operations	The design difficulty is low	The design requirements are high	High design requirements, Detailed research is required
Transmission capacity	Varies with the size of the shipment	Less than 200 MW	Up to 4 000 MW	More than 6 000 MW
Ability to cope with desert climates	strong	weak	weak	weak
Degree of security	There is no high-risk risk in battery technology, High level of security	A high degree of security	The degree of safety is low, and it can be endangered in the event of a failure Receiver network	Less secure

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1) Economical conveying distance

The economic transmission distance refers to the transmission distance corresponding to the transmission line when the economy of the transmission line is optimal under the corresponding voltage level. The transportation battery technology does not erect transmission lines, and its transmission mode is completely manual to transport photovoltaic energy storage batteries through camels to nearby battery swap stations, and its economic transmission distance is shorter. In the process of transmitting electric energy, the transmission loss of transportation battery technology is larger than that of other traditional transmission methods, and the transmission loss is mainly concentrated in the loss of camels and batteries, and with the increase of distance, labor costs will increase significantly, and its economy will be greatly reduced. The transmission voltage of the 220 kV transmission line is low, and the loss will increase greatly with the long distance, and the economic transmission distance is moderate. For high-voltage transmission lines, its construction cost is much higher than that of traditional transmission lines, but its voltage level is higher than that of traditional transmission methods, the transmission capacity is several times that of traditional transmission modes, and the transmission distance is also greatly increased, and the corridor covers an area of only about half of traditional transmission lines, and the transmission loss is less than half of the traditional transmission loss when transmitting the same capacity of electric energy.

2) Investment costs

Figure 3 is a schematic diagram of the comparison of investment costs of various transmission modes. The investment cost of transportation battery technology transmission is mainly concentrated in the construction of battery equipment and battery swap stations and the purchase of camels and batteries, and the investment cost is low. The cost of 220 kV transmission line includes ordinary steel towers, high-strength steel towers, conductors, ground wires, and other materials, and the adjustment factor of labor, materials, and machinery is adjusted according to the price level of the same period, of which the adjustment coefficient of labor cost is about 45.92%, and the adjustment factor of material and machine cost is about 29.83% [25]. The cost of UHV AC transmission is higher than that of conventional transmission lines, and the price level of conductor erection and installation costs and component installation costs is much higher than that of conventional lines. In the actual cost control of the 1 000 kV Jindongnan-Nanyang-Jingmen UHV AC transmission line project, the cost of these two projects has increased excessively, which is mainly due to the particularity of the construction and management of UHV projects. ± 800 kV DC transmission equipment cost includes converter stations, DC lines, arresters, etc. The cost of HVDC transmission lines is higher than that of transportation battery technology and traditional transmission lines, and lower than that of UHVAC transmission lines, but the cost of HVDC converter stations is more expensive than that of AC transmission line substations. Due to the influence of the electromagnetic environment on the transmission line, the cross-sectional area of the UHV AC transmission line model should exceed the needs of its actual use when designing, which will lead to the high cost of the transmission line project.

Figure 3

Figure 3 Investment costs of various transmission modes

Fig. 3 Investment cost of various transmission modes

3) Running attrition

The operating loss of transportation battery technology mainly considers the degree of loss of camels and batteries, which are relatively low due to the life of camels and batteries, but the loss rate is relatively high. When a large number of camels are used to transport batteries in long queues, the caravans can be approximated as conveyor belts that continuously transport batteries at a certain rate. Excessive wind speed in desert areas will increase the single transportation time of camel caravans, and slow down the transportation efficiency of single camels, but it will have little effect on the transportation efficiency of the overall transportation process. In addition, since the caravan does not need to consume electrical energy during the transportation of the battery, the battery usually does not incur electrical energy loss during transportation. Due to the different design, construction and current density of the 220 kV transmission system, the average annual line loss rate is 1%~3%. UHV AC transmission loss is mainly composed of power loss of transmission lines and power loss of substations. Due to line losses, the power emitted at the transmission end will be reduced when it reaches the receiving end, which will affect the transmission efficiency [26-27]. The power loss of substation mainly includes the power loss of power equipment, such as transformers, reactors, reactive power compensation devices and power loss of power stations. The operating loss of HVDC transmission is mainly divided into two parts: the loss of converter station and the loss of electric energy transmission line. The losses of converter stations are mainly those of converter transformers, converter valves and filters. When the AC transmission system is not disturbed by accidents, the utilization rate of the AC system is much lower than that of the DC system.

4) Annual unit fee

Figure 4 shows the annual unit cost of various transmission modes. Transportation battery technology has a lower annual cost and higher economy when transmitting electricity over short distances. Due to the use of camel transport battery transmission in the transportation battery technology, with the increase of transmission distance, the labor cost and camel loss will increase significantly, and the annual cost will also increase, and the economy will be reduced. The annual unit cost of 220 kV in the range of 0~50 km is slightly higher than that of the transportation battery technology, and the economy is best when the transmission distance is 100~200 km. When the transmission distance is greater than 500 km, the economy of HVDC transmission and UHVAC transmission is higher, and the annual cost gap between high-voltage transmission and transportation battery technology will greatly increase with the significant increase of distance. This is mainly due to the relatively low operating losses and low investment costs of the equipment used to transport battery technology and 220 kV transmission lines when the transmission distances are close. With the increase of transmission distance, the investment cost and maintenance cost of transportation battery technology and 220 kV will increase significantly, while the transmission capacity of high-voltage transmission lines is large, and the economic advantages of DC are gradually emerging. In the DC engineering investment, the investment of converter station does not change with distance, and the higher the voltage level, the faster the cost per unit capacity of the line decreases with the distance. At the same time, with the increase of photovoltaic power transmission distance, the proportion of line investment has increased, and the economy of DC transmission at higher voltage levels has also improved.

Figure 4

Figure 4 Annual unit costs of various transmission modes

Fig. 4 Annual cost of various transmission modes

5) Design complexity

The transportation battery technology has low environmental requirements, does not need to consider the comprehensive effects of wind and sand in the desert, and can design photovoltaic power stations, photovoltaic cell charging stations and lines from photovoltaic power stations to battery swap stations to adapt to the desert climate. The design of the traditional desert power transmission mode is very complex, and it is necessary to consider the comprehensive effects of wind and sand, study the wind-induced vibration characteristics such as the transmission tower-line coupling system and the lightning rod structure, and consider the influence of wind speed, time and corrosion. Therefore, it is necessary to investigate the stability of the transmission tower-line coupling system under wind speed for a long time, establish the monitoring points of the typical regional aeolian sand-deposited foundation, and evaluate the stability of the transmission tower-line coupling system according to the monitoring data. For AC transmission, because the transmission line transmits both active and reactive power, the voltage drop of the line is greater than that of the DC transmission line, and the voltage drop needs to be considered in the design of the transmission mode of the desert photovoltaic power station, which makes it more difficult to expand the transmission line in the future.

6) Transmission capacity

In desert power transmission technology, the transmission capacity of transportation battery technology mainly depends on the scale of transportation batteries, such as the amount of batteries transported by a single camel and the total number of camels transported at a time. But overall, the transmission capacity of transportation battery technology is lower than that of high-voltage transmission capacity. The traditional transmission method has a high voltage level and a large transmission capacity, which is conducive to the transmission of high-power, medium- and long-distance power. UHVDC transmission is mainly used for medium and long-distance large-capacity transmission with stable transmission direction, such as power transmission between some provincial grids. UHV AC transmission is mainly used for large-capacity transmission in close proximity, and at the same time is used to build a power system network with higher voltage levels. However, large-capacity long-distance AC transmission also has the problem of transmission capacity limitations, such as being limited by the maximum voltage drop allowed by the transmission line. Desert photovoltaic power supply, the power generated is affected by the environment, and the transmission power will fluctuate accordingly, so the safety of the transmission line needs to be considered.

7) Ability to cope with desert climates

The high wind speed and long duration in desert areas, as well as the high salinity of sand and dust [28-29], affect the service life of power transmission and transformation infrastructure equipment in desert areas. For the transportation battery technology, the desert climate will mainly affect the camels and labor during a single transportation, and excessive wind speed will reduce the transportation efficiency of a single camel transportation battery. Compared with the vehicle transportation battery, the camel transportation battery is less affected by the weather. If the vehicle is used for battery transportation, the bad weather in the desert will affect the smoothness of the transportation route, improve the difficulty of transportation, and at the same time, the wind and sand weather will reduce the visibility of the road, increase the potential safety hazards in the vehicle transportation process, and reduce the safety guarantee degree of transportation. The camel transport battery is not limited by the established paved road, and the path can be flexibly adjusted to reduce the impact of the weather on the battery transportation, and the continuous supply of battery power can be realized. The desert climate has a great impact on 220 kV transmission lines and UHV. In desert areas, transmission lines will be subjected to desert hazards in varying forms and degrees. Wind erosion, the transmission and accumulation of wind and sand in the desert will cause different degrees of harm to the transmission line project, among which wind erosion will cause the destruction of the foundation of the line tower, and the transmission of wind and sand will cause the wear and tear of the transmission tower, and even the formation of wind and sand electricity, endangering the safety of the transmission and transformation line. The accumulation of wind and sand will shorten the distance between the overhead line and the ground, causing potential safety hazards to the transmission line. In addition, the one-time cost and technical requirements of UHV special equipment are high, and some of the lines that have been put into use have the problem of low ability to resist severe natural disasters.

8) Degree of security

In terms of safety, transport battery technology has the highest degree of safety, and it can be operated by ordinary personnel during the process of charging the battery. At the same time, during transportation, there is no need to consider safety issues such as voltage drop and lightning strike on the transmission line, and the safety factor of battery transportation is relatively high. Although the UHV AC transmission mode is mainly used for short-distance power transmission, it still has the potential safety hazards of power transmission in desert areas. The voltage level of the UHV AC line transmission line is high, and if it uses a single-circuit line to transmit power to the nearby power grid to reach 10%~15% of the transmission line protection rate, it may lead to transmission line failure and tripping, thereby endangering the safe operation of the distribution network. In the event of multiple faults, multiple UHV transmission lines in adjacent corridors will stop working at the same time, which will have a significant impact on the safe operation of local power supply. Therefore, if there are multiple UHV lines fed in or transmitted at the same time in a certain place, these AC transmission lines do not need to be transmitted from individual large power plants, and it is best to divide several different large power plants into multiple UHV AC transmission lines, which is convenient for in-depth control of overvoltage, reduces the level of equipment and line insulation, and saves engineering investment [30-31]. Stability issues also arise when the UHVDC mode is used for long-distance, high-capacity transmission. First of all, due to the high voltage levels of UHVDC transmission, the cost per kilowatt of equipment and per kilometer of line is very high, which can lead to long-term operation of single-circuit lines with very large transmission capacity. When the power supply or line fails, it will have a serious impact on the safe and stable operation of the receiving power grid. When the power supply equipment and lines are faulty, it will pose a huge danger to the safety and smooth operation of the system of power supply at the receiving end.

3 Advantages and Challenges of Transportation Battery Technology

3.1 Advantages of transporting battery technology

With China's increasing emphasis on new energy, the photovoltaic industry in the western region has continued to develop well in recent years. In the future, China's photovoltaic power generation technology will grow steadily, with a lot of room for development, and the supporting transportation battery technology has a very considerable space for development. Transportation battery technology is different from traditional desert transmission lines and has fewer transmission losses, as shown in Figure 5. The battery transmission mode can realize the local use of photovoltaic power, and the application is more flexible. Transportation battery technology has significant advantages in the following two aspects: 1) with its flexible response speed and strong flexibility of the battery, it can effectively participate in the frequency regulation of the power grid and improve the stability of the power grid, and 2) with its low cost and simple structure, it can effectively reduce the loss of desert power transmission and improve the ability to cope with the desert climate.

Figure 5

Figure 5 Transporting battery technology

Fig. 5 Transportation battery technology

3.2 Participate in frequency modulation to improve the stability of the power grid

In recent years, the use of battery energy storage technology to participate in power grid frequency modulation has received extensive attention from the industry. Frequency modulation can charge and discharge cycles in the order of seconds. The use of battery energy storage technology can effectively avoid the fluctuation of the output power of desert photovoltaic power stations with the change of environment, which in turn causes power quality problems in the power grid. At the same time, battery energy storage technology has the following characteristics: 1) fast response speed and can achieve full power output within milliseconds, and 2) stable control and can maintain stable output at any power point within the rated power range [32-34]. Energy storage responds much faster to frequency changes than thermal power units, which can allow energy storage to participate in frequency adjustment before thermal power units, which can effectively reduce the maximum frequency deviation of the system [35]. With the increase of the proportion of new energy power generation, the capacity substitution capacity of energy storage for thermal power units under the same index will continue to increase, and the increase of energy storage in the power system with a high proportion of renewable energy is far greater than the frequency stability of the increase of thermal power units, which provides a reference for the configuration of energy storage capacity.

3.3 High economy and strong ability to cope with desert climate

Traditional desert power transmission methods generally need to erect transmission lines, and the construction of inverter stations and substations for long-distance transmission is even more expensive. Compared with traditional power transmission methods, battery transportation technology has the following advantages: 1) The cost of using battery transportation technology is lower than that of other power transmission methods. The camel transport battery is used to transport photovoltaic power, replacing the traditional transmission mode to build desert transmission lines, which greatly reduces the construction cost, and 2) its process content is low. Only the construction of photovoltaic power stations and battery swap stations can meet the technical needs of transportation batteries, and the operation process does not require professionals; 3) Strong ability to cope with desert climate. The climate in the desert area is changeable, and the nature of wind and sand and high salt seriously endangers the life of the transmission line. The impact of desert climate on the transportation battery technology mainly lies in the impact on camels and artificial labor during transportation, and the excessive wind speed slows down the efficiency of camel transportation, but the harm to the transportation battery itself is not great.

3.4 Existing technical difficulties

At present, there are few studies on desert transmission modes in China, and most of them focus on the construction of desert climate models to reduce their losses on transmission lines, but they fail to theoretically determine and quantify the economic problems of desert transmission modes. As a way to transport electric energy, the transportation battery technology provides a new possibility for the existing desert electric energy transmission problem, but the transportation battery technology transports the battery to the battery swap station through camels, and the transmission distance is less than 100 km, and the transmission distance is short. The transmission capacity of transport battery technology mainly depends on the size of the transport battery, such as the amount of battery transported by a single camel and the total number of camels transported at a time. Overall, however, the transmission capacity of transport battery technology is lower than that of high-voltage transmission.

4 Conclusion

Promote the development of photovoltaic power in the power system, actively respond to national policies, and help achieve the goal of "double carbon". By comparing the different modes of transportation of photovoltaic power in the desert, it is concluded from the perspective of economy that the transportation battery technology has high economy, low process technology content, strong ability to cope with desert climate, and is more suitable for short-distance transmission of photovoltaic power in the desert. At the same time, the transportation battery technology uses energy storage batteries to transport electric energy, and with the advantages of fast response and precise control of energy storage batteries, the transportation battery technology can participate in the frequency regulation of the power grid and effectively improve the ability of the power grid to accept clean energy. Transportation battery technology provides new possibilities for the existing problem of power transmission in deserts.