**4. Conclusions**

There are several related scales for the managemen<sup>t</sup> of the hydraulic energy generation, including local, regional, national and international, when considering water as a resource. However, even today, energy recovery is a very attractive possibility in water networks, with small additional costs for managers and investors. The success of this novel use depends on the experience acquired in hydropower plants with higher installed power.

A deep review has been presented, analyzing the different alternatives for hydropower production from large to pico facilities, according to the production levels, the economic and the environmental points of view, as well as the classification of the hydraulic machines. Considering the evolution of these energy solutions, energy recovery in water distribution networks is an alternative for the development of systems towards more sustainable and efficient solutions. Technological, economic and environmental implications of hydropower systems for energy recovery around the world have to be considered. Despite the large list of references, only a few can be related to agricultural water network energy recovery because this subject has not ye<sup>t</sup> been explored.

Analysis of the references cited in this document establishes that recovery systems in pressurized water networks are:


installation of these machines in water pipe systems to be promoted. The main aspects negative of PATs are related to their low efficiency when operating outside their best efficiency point. Operation with different flows can be solved by the development of new regulation techniques (e.g., variable operation strategies (VOS)) with electronic regulation. The positive resolution of this aspect is a crucial point for expanding use of PATs in water distribution networks. Issues related to the use of the generated energy for self-consumption may include storage in batteries and integrating this renewable energy in a similar manner as other supplementary sources (e.g., solar and wind).

(4) Different case studies have been developed using specific software (e.g., EPANET and WaterGEMS), which have been combined with optimization methodologies to maximize the recovered energy. Future simulations should take into account the integration of VOS as well as the variation of the machine efficiency with the rotational speed. These simulations should consider discretized demand over time to improve the analyzed energy values because the majority of studies only consider the mean demand value or modulation curves. The development of a specific methodology to determine this variation of flow over time in water supply networks is crucial to improve the fit between theoretical and real values of recovered energy. Regarding the software used, it is necessary to implement operation rules for these machines in specific algorithms. This implementation is the key point in the development of optimized techniques, making possible studies similar to those with water pump systems. The primary need is for correct machine selection and establishment of the rotational speed as a function of the flow, maintaining the maximum efficiency at each operation point of the machine.

Therefore, hydraulic recovery in water networks is a real and necessary alternative to improve the energy efficiency of the whole system. By means of implementing energy converters, the energy efficiency will be increased and operating costs will be reduced (i.e., the energy footprint of water). The implementation of these systems will essentially depend on the physical characteristics of the systems. The orography, topology, and volumes of water consumed establish the economic viability of these recovery strategies in water distribution networks. When the investment analyses are developed, recovery systems have acceptable values of economic feasibility indexes (e.g., payback value and internal rate of return). A better understanding of the operation of each recovery system in terms of water-energy managemen<sup>t</sup> is needed that considers the high global volume distributed in pressurized water networks (i.e., drinking and irrigation) each year. This understanding will positively contribute to the sustainability and efficiency of near future recovery system applications.

**Acknowledgments:** No additional funds have been received for this research.

**Author Contributions:** All the authors have participated in any step of this research. Particularly a brief description is attached: Helena M. Ramos has contributed by supervising the state of the art and revision of the whole paper. Francisco Javier Sánchez-Romero developed the study of large and small hydropower. Modesto Pérez-Sánchez wrote and analyzed the state of the art in water networks taking account the energy recovery. P. Amparo López-Jiménez supervised the whole research and she was involved in conclusions determination.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, and in the decision to publish the results.
