**1. Introduction**

In the last years there has been a growing interest in intelligent, autonomous devices for household applications. In the near future this technology will be part of our society; sensing and actuating will be integrated in the environment of our houses by means of energy scavengers and wireless microsystems. These systems will be capable of monitoring the environment, communicating with people and among each other, actuating and supplying themselves independently. This concept is now possible thanks to the low power consumption of electronic devices and accurate design of energy scavengers to harvest energy from the surrounding environment.

In principle, an autonomous device comprises three main subsystems: an energy scavenger, an energy storage unit and an operational stage. The energy scavenger is capable of harvesting small amounts of energy from the surroundings and converting it into electrical energy. This energy can be stored in a small unit like a small battery or capacitor, thus being available as a power supply. The operational stage can perform a variety of tasks depending on the application.

Inside its application range, this kind of systems presents several advantages with respect to devices that exploit external energy supplies. They can be simpler to employ and install, as no external connections are needed; they are environmentally friendly and might be economically advantageous in the long term. Furthermore, their autonomous nature permits the use in locations where the local energy grid is not present and allows them to be 'hidden' in the environment, being independent from interaction with humans.

The idea is to make autonomous and more energy efficient processes in some very specific areas, particularly in the management of household heating/cooling systems, and in the

© 2012 Chiaberge et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

environmental monitoring. The basic concept is to convert a fraction of the energy that would be normally dissipated by the process into electrical energy. These "secondary" energy sources are then used as primary sources in micro generators whose electrical current will be used to power the devices distributed along the process. The physical phenomena involved in this energy conversion can be essentially: the piezoelectric, photovoltaic and thermoelectric effects and phenomena related to fluid dynamics. Significant examples of energy harvesting are evident in particular in the following scientific fields: the construction of electric generators coupled to microturbines (Chunyan et al., 2010), (Bansal et al, 2009), (Yan et al., 2011), (Zainuddin, H et al., 2009); the Stirling thermodynamic cycle (Valdes, 2004), the Seebeck effect in thermoelectric generators (Lineykin et al., 2007), (Lu et al., 2010) and an Helmholtz resonator based generator (Kim et al. 2009).

Within this context the objective of this work is to present a trade-off analysis between different types of hydraulic machines, electric generators and energy storage units to reach a good compromise in the design of harvesting devices to be integrated in fluid distribution systems.
