## 4 Conclusions  
The research on energy harvesting and transmission continues strong. The idea of a self-powered sensor node is more interesting than ever. At the very same time, the technology is expected to provide new business models through monitoring, tracking and automation capabilities on a large scale. The revolution can be seen everywhere--from connected farms and agriculture to smart cities buildings and homes. Today, the energy harvesting technology enables the gain in the restricted number of applications due to the lack of a universal solution which would provide enough energy to displace batteries and the mains in low power applications. The principal obstacles are the size of the implementation, the high cost and the maturity of the technology. In the current paper, we have developed a practical IoT application for the industrial rotating environment, powered solely by RF-based wireless power transfer. On this way, we first proposed and discussed the methodology to be utilized when developing such an application. Then, we described in details the complete procedure of developing and evaluating the different components of our solution, and the application as a whole. On the one hand, our results confirm that the proposed development methodology is valid. On the other hand, our results show that, even with the state-of-the-art technology, the practical WPT applications are feasible. This may enable the new and interesting use cases--like sensing in a rotating environment, which we have addressed in this paper. At the very same time, there are many various challenges associated with the development of WPT-based applications. First, today, the development of an RF-based WPT-based application is still quite a challenging task, requiring diverse expertise. One of the main reasons for this is the fact that the number of the available today commercial components for WPT is scarce and their price is rather high--thus, many system components have to be developed from scratch. In this respect, the recipes contained in this paper may be especially valuable. Second, there is an obvious gap with respect to the WPT-focused frequency spectrum regulations, which introduces some uncertainty also to the process of designing and using the WPT applications. Third, there are still many open research questions regarding the environmental effects (both the effects on nature and health, and the effect of electromagnetism on the machines and materials), which can be potentially associated with WPT. In addition, when done in-band with the traditional communication, the WPT may cause interferences to the latter. Finally, the business prospects of the WPT solutions are still not very clear--does the long-distance WPT where only some percents of the total power consumed by the WPT transmitter are effectively delivered to the receiver make sense? If it does, for which applications? All of these issues still need to be investigated further. Meanwhile, the efficiency of the design reported in this paper can be further improved in several ways. First, a more directive antenna can be used. Second, the design of the antenna may be changed to make it circularly polarized. Alternatively, the duty-cycling of the WPT transmitter may be synchronized with the rotation of the sensor, thus minimizing the energy losses due to polarization mismatch. Third, some performance improvement can also be achieved by optimizing further the number of the stages in the pump and its matching. Replacing the sensors with the most recently developed ones can also enable some energy savings. Finally, the multi-band RF energy harvesting(e.g., collecting the energy from both bands g3 and h1.7) may also be feasible. We plan to focus on some of these issues in the future. Conceptualization, J.J., K.M. Validation, J.J., J.P. and M.S. ; Formal Analysis, J.J., K.M. ; Investigation, J.J., K.M., J.P. and M.S. ; Writing--Original Draft Preparation, J.J.; Writing--Review and Editing, J.J. and K.M.