# Energy-Efficient Wireless Sensor Networks for Precision Agriculture: A Review
## 3 Agriculture-Based Energy-Efficient Schemes in Literature
### Agriculture-Based Energy-Harvesting Techniques
#### 3.2.1 Solar Energy  
_Solar energy_ based on photovoltaic system and methods can be used in agricultural applications based on WSNs [113]. Therefore, solar cells provide a good solution [114] to ensure the survivability of the agriculture monitoring system. Solar cell energy has been used by several studies to provide long-term energy to sensor nodes in agriculture applications. [PERSON] et al. [40] developed an irrigation system based on the ZigBee wireless protocol. The system was developed to improve water usage for agricultural crops. Temperature and soil moisture sensors in WSN were placed in the root of the plants to send sensor information to a web application via a gateway. The proposed algorithm controls the water quantity based on threshold values of soil moisture and temperature that are programmed in a microcontroller-based gateway. The WSN is powered by solar cell panels and rechargeable batteries. Water savings of up to 90% were achieved relative to the conventional irrigation method. Considering that the system is cost effective and autonomous, it can be utilized in geographically isolated regions where water sources are limited. [PERSON] et al. [68] improved the battery lifetime of a WSN based on harvested energy (i.e., solar cell) utilizing shadow detection. With this strategy, sensor nodes can accordingly modify their scheduling to optimize their power production and remaining battery levels. Furthermore, data transmission is optimized through routing and clustering mechanisms. A Bayesian network was employed to provide warning reports of bottlenecks along the path. The system was deployed in real time based on the Texas Instruments CC2530 platform that includes the ZigBee wireless protocol. The battery (lithium type with a storage capacity of 2500 mAh) of the sensor node was charged from the solar cell panel when the harvested power was adequate. The experimental results showed that these techniques allow network activities to occur in a continuous and effective manner. In [115], a micro-irrigation system that relies on a small solar-powered cell was developed. The energy input, energy output, greenhouse gas productions, and solar cell installation for rural areas were investigated with the proposed co-located system. The researchers compared their system with another commonly used method (i.e., alo vera cultivation) in terms of life cycle. The life cycleevaluation showed that the proposed system is economical for several rural areas and may provide opportunities for agriculture electrification and motivate economic growth. [PERSON] [116] presented an irrigation system relying on a solar cell for rural areas where sunlight is often available. The irrigation system includes pumps working on a solar cell instead of an electricity grid and/or a diesel generator. The study showed that the irrigation system based on a solar cell is better than systems that depend on other energy resources because the electricity grid is unavailable at all times, and the operating cost of diesel pumps is high and relies mainly on the diesel price. Although the solar-powered system was relatively expensive, it became a free source of energy after installation in the rural area and proved to be a worthwhile investment in the long term. A similar irrigation system was proposed in [117]. The system adopts photovoltaic panels to power a water pump that converts the continuous drift of water into a drop drift to achieve increased water usage efficiency. [PERSON] et al. [118] suggested a novel prediction technique for energy harvesting with a solar cell within three hours. Moreover, the packet transmissions of the sensor nodes can be controlled depending on the estimated energy. Packet transmissions and energy harvesting are integrated to improve the WSN performance. The integration of these schemes enhances the prediction accuracy and improves the WSN throughput and power consumption. The throughput of the sensor node can be increased by shortening the packet transmission period.