A '''hydro energy conversion system''' (HECS),<ref>[http://www.routledge.com/books/Energy-Harvesting-isbn9781439815083 Hydro and Ocean energy conversion system terms]</ref> or '''hydro energy harvester''' is the term for the machine used to generate this mechanical or electrical energy from the captured energy of moving water.
== Mechanical power ==
===Categorization of freshwater power plants===
Category: plants can also be classified as “along the waterstream” or “other”. The first, "along the waterstream" receives water from the river or stream without accumulation infrastructure (dam, pool, pond ...). The volume of water available for the turbine depends on the instantaneous waterflow of the river.
===Analyzing the available hydropower===
====amount of waterflow and drop height; remoteness of the consumers====
When a need of power emerges, we should seek one site or several sites that could host a hydroelectric powerplant. We understand that the research begins by identifying all sources of energy such as wind, solar, biomass and so on. Our preposition considers the conclusion of this inventory as a preceeding to the decision to install a hydroelectric plant, perhaps in addition to wind turbines or solar energy collectors.
A power plant demands a drop height and a waterflow. Terrain reconnaissance on the field or on topographic map pinpoints the locations of both waterflow and a slope of the terrain. These spots are easily identified on the ground where the river flows torrentially but other locations may also be appropriate. The extent of the drop is easily to determine for a surveyor by means of a theodolite. However, the estimation of the available waterflow is much more difficult because of its variation depending on the season (see average waterflow). It is wise to underestimate the rate available because the powerplant should also not take off all water from the network. For example, in complete removal, the aquatic biotope will be severely disrupted by lack of water and make the spawning of fish impossible. Therefore, a reserve of waterflow should be left in the river to avoid biological depletion. It is possible that other -already authorized- waterusages impose a higher reserve waterflow.
When several sites are identified, the remoteness of the consumer is another criteria for inclusion in the feasibility analysis. Remoteness means a longer power line and makes monitoring of the plant more difficult. An electrical power line represents a important expense and is a source of powerloss.
The amount of instantaneous waterflow of a river system depends on the rains, which is dependant on the season. The instantaneous waterflow varies from day to day with a minimum therof, located usually at the end of the dry season if it is marked. The concept of average flow has no interest in powerplants "along the waterstream", however, it does allow to better estimate the potential energyoutput of an infrastructure if an accumulation is envisaged. Low water flow, ie the minimum flow of the river during 24h states the minimal poweroutput potential of an installation. If the hydrological observations (measures of the flow of the river) are done for several years, it is possible to know the average minimum waterflow attained annually, or it is possible to observe it every 5 years, or -even more rare-, every 10 years. Indeed, the severity of the drought is variable depending on the year. A flow measure during 365 days can not indicate whether the observed minimum is an exceptional speed (either low or high) or rather an average minimum.
The hydrological data may be essential for the design of the proposed small hydroelectric plant. A lack of flow and thus availability of water will lead to disillusionment when the installation is working due to the large gap between the expected power output and true available power. There is of course no need to seek accurate hydrological data if the power output of the proposed installation is well below the maximum power of the site chosen for the project. Given that the turbine is to be placed near the river, it is highly desirable to know the variations of water level, to avoid seeing water invading the facilities during floods.