Abstract: Node energy supply and management technology is one of the key technologies of wireless sensor network. In view of the special requirements of wireless sensor network for environmental monitoring application, the basic principles of node energy design are proposed. On this basis, the relevant performance of available power supply batteries is compared. Energy supplementation technology suitable for environmental monitoring is studied, a practical solar energy supplementation system is designed, and the power supply capacity is compared with that of a simple battery power supply system. Finally, the relevant technologies for reducing node energy consumption are discussed. Keywords: wireless sensor network; environmental monitoring; solar energy 1. Introduction Wireless sensor network usually includes sensor nodes, aggregation nodes and management nodes. A large number of sensor nodes are randomly deployed in the monitoring area and form a network through self-organization. The monitoring data of each node is transmitted hop by hop along other nodes and then sent to the aggregation node, and finally reaches the management node through long-distance data transmission technology. Users configure and manage the sensor network, publish monitoring tasks and collect data through the management node. Wireless sensor network can be widely used in military security, environmental monitoring, traffic management, disaster forecasting, medical and health fields. The American Technology Review believes that wireless sensor network is one of the ten emerging technologies that will have a huge impact in the 21st century[1]-[2]. The composition of sensor network nodes varies across different applications, but they generally consist of a sensor module, a processing module, a wireless communication module, and a power supply module. The power supply module is crucial for maximizing the node's lifespan. Wireless sensor network nodes are mostly randomly deployed, and due to harsh environments or node movement, each node typically uses a disposable self-contained power supply, which cannot be replaced once deployed. For these reasons, power supply and management technologies for sensor network nodes have always been a key research focus in wireless sensor network technology. This paper will discuss power supply technologies, energy replenishment technologies, and energy consumption reduction technologies for wireless sensor network nodes applied to environmental monitoring. 2. Power Supply Technologies for Environmental Monitoring Sensor Nodes Sensor networks applied to environmental monitoring have changed the previous model of fixed-point, timed measurements using single sensors. They enable multi-angle (multi-type sensor), synchronous, and continuous measurements of the observed object. Therefore, the obtained data is more comprehensive and representative, facilitating the description of the spatial and temporal changes of the observed object and the discovery of its inherent relationships. 2.1 Requirements for Sensor Node Power Supply Technology in Environmental Monitoring ● Small Size: Sensor network nodes should be small enough to ensure they do not affect the target system itself. In some cases, nodes even need to be small enough to be undetectable to complete specific tasks. Therefore, the power supply modules for each node must also be small. ● Strong Environmental Adaptability: Sensor nodes used in environmental monitoring are often placed in harsh, sparsely populated areas. Therefore, the node power supply modules need to have strong environmental adaptability, primarily meeting requirements for operating temperature and water tightness. ● Stable Discharge Characteristics: With the development of chip manufacturing and sensor technology, the power consumption of node sensors, processors, and wireless transceiver modules has been greatly reduced. Therefore, the instantaneous discharge capability of the power supply system is not very high, but the stability of continuous discharge has a significant impact on system use and lifespan. ● Low Cost: Low cost is a basic requirement for sensor nodes. Only with low cost can they be deployed in large quantities. Therefore, expensive technologies cannot be used for power supply modules. ● Pollution-Free: Due to the harsh environments and large numbers of nodes used in environmental monitoring, the power modules must adopt pollution-free or low-pollution technologies. 2.2 Comparison and Selection of Battery Characteristics for Environmental Monitoring Sensor Nodes Based on the technical requirements of environmental monitoring for the energy modules of sensor nodes, the feasible and basically satisfactory solution is to use high-performance batteries as the energy supply module. The characteristics of several main types of batteries are analyzed and compared below. (1) Zinc-manganese alkaline batteries Disposable zinc-manganese alkaline dry batteries are the most common type of battery. These batteries are inexpensive, have good power, long storage time, and good temperature adaptability. The nominal voltage is 1.5V, which is suitable for small and medium current density discharge. The disadvantage is that the internal resistance is generally large. When the discharge current is too large, the electrochemical polarization increases, the working voltage drops rapidly, and the battery output capacity decreases. (2) Nickel-cadmium (Ni-Cd) rechargeable batteries The positive electrode of nickel-cadmium rechargeable batteries is nickel oxide, the negative electrode is spongy metallic cadmium, and the electrolyte is mostly potassium hydroxide and sodium hydroxide alkaline aqueous solution. Small sealed nickel-cadmium batteries have a compact, robust structure and are resistant to impact and vibration. The finished batteries have low self-discharge, are suitable for high current discharge, and have an applicable temperature range of -40℃ to 60℃. They have a long cycle life, theoretically reaching 2000-4000 cycles. (3) Nickel-metal hydride (NiMH) batteries greatly reduce the "memory effect" of NiCd batteries, making them more convenient to use and longer-lasting. NiMH batteries also have advantages such as high capacity, large depth of discharge, resistance to overcharge and over-discharge, and short charging time. Most importantly, NiMH batteries no longer use toxic heavy metals as materials, thus eliminating environmental pollution. However, NiMH batteries have poor high-temperature characteristics and cannot function properly in environments above 45°C. In addition, this type of battery also has a relatively high self-discharge rate. (4) Lithium-ion batteries lithium-ion batteries have the advantages of being lightweight, having large capacity, and high energy density, but they are more expensive. Compared with NiMH batteries, lithium-ion batteries are lighter and have a 60% higher energy density. In addition, lithium-ion batteries have almost no "memory effect" and do not contain toxic substances, which has led to their production and sales gradually surpassing those of NiMH batteries. From the above types of batteries, representative products were selected for comparison of relevant parameters. Among them, the zinc-manganese alkaline battery is selected from Nanfu LR6AA type, the nickel-cadmium battery is selected from Aoneng D-4/3AA1300, the nickel-metal hydride battery is selected from Aoneng H-AA2100A, the lithium battery is selected from Lixing CR2450, and the lithium-ion battery is selected from JJJLC16340. Table 1 Comparison of main parameters of representative models of several types of batteries As can be seen from the table above, although the performance of several types of batteries is different, they can meet the energy supply requirements of wireless sensor network nodes for environmental monitoring in terms of volume, environmental adaptability, discharge stability, cost, etc. Among them, lithium-ion batteries have obvious environmental advantages. In actual use, the selection of different types of batteries is optimized according to the power consumption of each node, the power supply voltage requirements, etc. In the world's most successful wireless sensor network environmental monitoring experiment to date - the Great Duck Island experiment in the United States, the Wec node uses CR2450 batteries, and the Renee, Mica2 and Mica2dot nodes use 2 AA batteries [3]. 3. Energy replenishment technology for environmental monitoring sensor nodes Node energy replenishment technology utilizes available energy resources in the environment where the node is deployed to provide energy for the node's operation or to charge the node's battery, thereby extending the node's lifespan. Sensor nodes used in environmental monitoring can generally be replenished using light and wind energy. Professor Shashank Priya of the University of Texas has created a small generator using piezoelectric devices. This generator can be driven by wind at speeds of 8 to 16 km/h and can provide up to 50 mW of power to independent nodes in a wireless sensor network. This small wind turbine uses mature piezoelectric and mechanical technologies, making the cost of the prototype device less than $20, thus changing the previous problem of wind energy replenishment technology being too expensive and technically demanding[4]. The development of optoelectronic materials has made light collection another possible solution for the energy source of wireless sensor networks[5]. Under normal circumstances, it is not difficult for sensor nodes used in environmental monitoring to obtain sufficient light. A practical energy replenishment system for environmental monitoring sensor nodes was designed using the MAX1555 battery charging control chip. The solar cell is 55 mm × 50 mm in size and is installed on the node's shell or sealing cover. The maximum output voltage is 4.5V, the maximum current is 50mA, and the maximum power is approximately 0.2W. The MAX1555 external circuit is simple, greatly reducing the power consumption, cost, and size of the charging control system. Nanfu LR6AA zinc-manganese alkaline batteries were selected for rechargeability. Based on the battery storage and discharge principle, alkaline zinc-manganese batteries are rechargeable, but the charging current cannot be too high. Only by ensuring sufficient time for the internal chemical reaction of the battery can various polarization phenomena be reduced. The system also incorporates over-temperature and over-voltage protection circuits. The designed power module with solar charging function and a general battery power module were tested under the same load. The experiment took place in winter, with temperatures between 5 and 10℃ and mostly cloudy/overcast weather. The conditions for sunlight and temperature were among the worst in the year. [align=center]Figure 1: Voltage variation of the two power modules under the same load on the first day[/align] As shown in Figure 1, the initial voltages of the two power modules were similar. After the load was connected, the voltage of power module 2 (without charging function) continuously decreased. The voltage of power module 1, equipped with solar charging, varies with the intensity of sunlight, peaking around 2 PM. After three days of discharge testing, the voltage of power module 2 continuously decreased from 3.2V to 3.1V. The voltage of power module 1, except for peaking between 12 PM and 2 PM daily, showed no overall decreasing trend. For example, with an average daily power consumption of 8.148mAh for the sensor node (88J daily), using two LR6AA zinc-manganese alkaline batteries (2500mAh) without solar charging, it could theoretically operate for 300 days. Using the same battery power supply system with solar charging, taking an average annual sunshine duration of 2550.7 hours (Qingdao) as an example, the average daily output of the selected photovoltaic cells is greater than 500J, exceeding the daily energy consumption of the node; therefore, the service life depends on the battery's charging life. 4. Energy Consumption Reduction Technology for Environmental Monitoring Sensor Nodes The energy problem of wireless sensor nodes includes both energy replenishment and energy-saving technologies. The main power saving and consumption reduction measures include several aspects: 1. Node communication mode Sensor nodes generally work in four modes: transmit, receive, idle and sleep. Studies have shown that the power consumption of transmitting, receiving and idle is on the same order of magnitude, while the power consumption of sleep is 1 to 2 orders of magnitude lower than that of other modes; switching from sleep mode to other modes requires more energy consumption (and startup time); the power consumption of the communication module is closely related to factors such as transmission distance and modulation strategy. Therefore, in order to save communication energy consumption, the communication module should be kept in sleep mode as much as possible; the switching of node communication modes should be reduced as much as possible; and multi-hop routing should be appropriately considered to replace single-hop routing. 2. Communication protocol energy efficiency strategy Due to the broadcast characteristics of wireless channels, the research on communication protocols mainly revolves around the MAC protocol to ensure that adjacent nodes can share the channel, reduce or avoid collisions, and improve energy efficiency and throughput. Taking advantage of the characteristic that the processing power consumption is much lower than the communication power consumption, the redundancy in the data is reduced through the processing of intermediate nodes to reduce the amount of communication and improve the effectiveness of communication [6]. 3. Energy efficiency standard of communication routing algorithm [7] The routing protocol of the network layer determines the transmission path of monitoring information [8]. In the selection and design of routing protocols, not only should indicators such as time, congestion, and reliability be considered, but also indicators such as energy consumption of individual nodes and the overall energy distribution balance of the network. 4. Energy Efficiency Optimization of Network Applications The communication and functional design of wireless sensor networks should fully consider the characteristics of environmental monitoring applications, optimize network actions and control behaviors, and avoid unnecessary communication and measurement to ensure the network's lifespan is extended while completing monitoring tasks. 5. Conclusion The application of wireless sensor networks in environmental monitoring is a new way of acquiring environmental information, providing a new perspective and platform. Node energy supply and management is one of the key technologies of wireless sensor networks. 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