# Future Renewable Energy Communities Based Flexible Power Systems
## 9 Attainability: Welcome, Non-Welcome, and Ambiguous Energy Sources  
The networks taking advantages of alternative sources are known as smart grids and virtual power plants (VPP). These sources can manage the generation of energy according to their origin, use and location and they can be welcome, not welcome or ambiguous. Regarding the origins of renewable generation sources, small hydraulics are obtained from the potential or kinetic energy of running waters. Wind power comes from the kinetic energy of the winds. The direct solar energy obtained from sunlight can be converted into either, or both, photovoltaic (electrical energy) or heat (thermosolar). It should be noticed, therefore, that the term \"solar\" referring only to photovoltaic energy is inaccurate since \"solar\" could be any source that has the sun as its source. Direct solar energy can locally generate electricity in PV panels as much as taking advantage of the heat from panels exposed to the sun. This is the case of heat water heating, which results in a much higher total level of energy efficiency. Another interesting observation is that, in absolute terms, the PV plant's performance is comparable to that of a hydroelectric plant if only the origin and the final product are considered, that is, the Sun as a source and the resulting electricity. Biomass comes from burning vegetables or decomposing organic matter (biodigestion). A diesel can move combustion and explosion engines. Fuel cells come from electrochemical energy. Geothermal comes from underground thermal energy (superficial or deep). Burning gases or the kinetic energy of gases drives gas turbines and microturbines. Other welcome sources can be supplied with hydrogen, magneto-hydrodynamic, piezoelectric effect, thermocouples, ethanol, methanol, LPG, biodiesel, tides, ocean waves, and thermal gradient, among others. The storage of welcome, non-welcome, and ambiguous sources can be done using electrical, thermal and/or the AC \"battery\" phenomena, which is the public network. A special aspect of a clean energy source is the natural movement of ocean waters. This movement of water depends on the winds and their density, which, in turn, depend on the saline values and temperatures. Such factors create horizontal (H) and/or vertical (V) movements in the water. The movement due to water density usually increases with the depth of the oceans by the weight and temperature of the water itself. With the sun heat and moon proximity, the density on the surface decreases or increases and colder water sinks gravitationally. At the poles, the density of surface water increases by direct cooling or by the formation of ice that constricts the water and increases both salinity and density. Therefore, in Polar Regions the movement of sea water is due to the increase in the density on the surface and the weight of the water causing its sinking. At lower latitudes, in the colder waters, therefore denser and saline, the movement is due to increased evaporation and the intensity of strong winds (see Table 2). The Ocean Thermal Energy Conversion system, known as OTEC, takes advantage of this thermal and density differences to generate electrical energy. Regarding the ambiguity of the sources, it is necessary to take into account the scenarios in which they can be used. Among these, the most common fuel cell, the Proton Exchange Membrane (PEM), works with hydrogen and oxygen. If the source of the hydrogen used by them comes from petroleum or carbonaceous gases, they are not welcome, because by separating the hydrogen from these materials, carbon monoxide is released into nature, which is a polluting material. However, if hydrogen is obtained directly from the dissociation of water by solar heating using parabolic mirrors, the fuel cell will generate an absolutely clean energy and can be obtained anywhere on the planet. The limit for them today is the unavailability of materials that can withstand the high temperatures necessary to dissociate water in obtaining hydrogen (3200 \({}^{\circ}\)C).