Another method used a [[trompe]] to produce compressed air from falling water, which could then be used to power other machinery at a distance from the water.
== Hydropower plants ==
Note that the top 4 energy conversion systems are '''ocean energy conversion systems'''' (use in open sea), the osmotic power plant is used where sweetwater and saltwater converges (ie river mouths ending into the sea) and the lower 3 energy conversion systems are used in (freshwater) rivers, streams or on basins/ponds.
To calculate the theoretical maximum energy that can be produced from a microhydro plant, a few pieces of information are needed. Among these is the Hazen-Williams equation, which allows for the calculation of loss due to friction. The equation is as follows: f=0.2083*(100/c)<sup>1.852</sup>*Q<sup>1.852</sup>/d<sup>1.852</sup>, where c is the hazen-williams coefficient and is unique the piping used, Q is the flow in gallons per minute, and d is the diameter of the pipe in inches.
For instance, to find the friction loss of a 500 gpm flow through an 8" PVC pipe with a c value of 150, the Hazen-williams equation provides a loss of 0.395 feet-loss per 100 feet pipe.
To determine the power in such a system, the equation P=Q*H/k is used, where Q is the flow rate, H is the head loss, and k is a constant of 5,310 gal*ft/(min*kW). For a flow of 500 gallons per minute and a head loss of 60 feet, maximum power would be found as: (500 gal/min *60 feet)/5310 gal-ft/min*KW, which comes out to 5.65 kW.
This theoretical maximum is prevented by real world obstacles, such as turbine efficiency, pipe friction, and the conversion from potential to kinetic energy.
===Categorization of freshwater power plants===