The development of perpetually powered sensor networks for environment monitoring to avoid periodic battery replacement\nand to ensure the network never goes offline due to power is one of the primary goals in sensor network design. In many\nenvironment-monitoring applications, the sensor network is internet-connected, making the energy budget high because data\nmust be transmitted regularly to a server through an uplink device. Determining the optimal solar panel size that will deliver\nsufficient energy to the sensor network in a given period is therefore of primary importance. The traditional technique of sizing\nsolar photovoltaic (PV) panels is based on balancing the solar panel power rating and expected hours of radiation in a given area\nwith the load wattage and hours of use. However, factors like the azimuth and tilt angles of alignment, operating temperature, dust\naccumulation, intermittent sunshine and seasonal effects influencing the duration of maximum radiation in a day all reduce the\nexpected power output and cause this technique to greatly underestimate the required solar panel size.Themajority of these factors\nare outside the scope of human control and must be therefore be budgeted for using an error factor. Determining of the magnitude\nof the error factor to use is crucial to prevent not only undersizing the panel, but also to prevent oversizing which will increase the\ncost of operationalizing the sensor network. But modeling error factors when there are many parameters to consider is not trivial.\nEqually importantly, the concept of microclimate may cause any two nodes of similar specifications to have very different power\nperformance when located in the same climatological zone. There is then a need to change the solar panel sizing philosophy for\nthese systems. This paper proposed the use of actual observed solar radiation and battery state of charge data in a realistic WSNbased\nautomatic weather station in an outdoor uncontrolled environment.We then develop two mathematical models that can be\nused to determine the required minimum solar PV wattage that will ensure that the battery stays above a given threshold given\nthe weather patterns of the area. The predicted and observed battery state of charge values have correlations of 0.844 and 0.935\nand exhibit Root Mean Square Errors of 9.2% and 1.7% for the discrete calculus model and the transfer function estimation (TFE)\nmodel respectively.The results show that the models perform very well in state of charge prediction and subsequent determination\nof ideal solar panel rating for sensor networks used in environment monitoring applications.
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