Dam-Free Micro-Hydro: Design 1 – Requirements

Jan Borchert, Current Hydro

Dam-free microhydro is an alternative to upgrading existing dams to generate hydropower. The concept is based on the most fundamental requirements of hydropower generation; all that is needed is an elevation change and sufficient water flow. Dam-free hydropower can offer the benefit of renewable energy generation in a run-of-river setting, without the dam-associated downsides, risks or concerns. Let’s talk about the physical requirements and the necessary components for the design of dam-free microhydro.

Physical Requirements

Generating electricity from water requires two things: flow and head (a vertical change in elevation). The stream provides the flow, the head is typically supplied by a dam – but in a dam-free setting, there is no dam. Without a dam, hydropower development is dependent on certain natural circumstances or physical requirements to allow for a proper intake structure (to make use of the flow) and to provide the necessary head.

In other words, we need a waterfall to provide a very steep descent of the water to provide the necessary elevation change (head). The steeper the descent, the shorter the bypass reach – minimizing the impact on the waterfall habitat, which is home to mosses and sparse vascular plants on rocks.

In the natural setting of waterfall, picking the best location for the intake and outlet is key: In a dam-free scenario, the design team will locate the outlet location somewhere near the bottom of the waterfall. But the intake location could be at any elevation along the waterfall, depending on the site-specific parameters. The intake should be located as high as possible to maximize power output, but, depending on the local topography, the shape of the waterfall and the flow direction, might require the design team to locate the intake further downstream. In general we are looking for a minimum head of about 13 feet, even though there are turbine technologies that can work with less.

The turbine technology of choice also defines the flow requirements of our dam-free hydropower system. It is important to ensure that energy is generated as often as possible, ideally continuously throughout the year. This is particularly important in the situation of “micro” hydropower to make a hydropower installation financially viable. Otherwise, if a turbine is designed too large, the stream might not provide enough flow during the dryer summer months to operate the turbine.

Hydro engineers use a flow duration curve (FDC) to estimate flow availability over the course of a year. To create a flow duration curve, they use daily flow data from previous years, sort the data points from large to small, and then display the result as a percentage on a time axis. These percentages identify expected minimum flows, e.g., a flow of X or more occurs 50% of the time (at the 50%-mark). In other words, the FDC allows the reader to see the frequency of occurrence of the average flow exceedance value / the minimum expected flow availability.

Adequate flow estimates are only the first step, as in a natural setting, not all of the available flow is necessarily usable in a dam-free microhydro scheme. Depending on the design of the intake structure and the topography of the stream near the intake location, the actual usable flow might be significantly less than the available stream flow. This is a challenge that does not exist in the dam-setting, since the relationship between usable flow and available flow can be regulated using a simple valve or gate.

Note, turbine technologies can be installed in parallel and in series to match single units’ operating parameters with the existing site parameters to maximize power output.

But besides head and flow, there are other site-specific requirements to a dam-free micro-hydropower system.

Other Site-Specific Requirements

While sufficient head and flow are the obvious two requirements for a micro-hydropower system, other site-specific requirements need attention: the site needs allowance for intake structure installation, it needs space for a penstock and/or stilling chamber (forebay), a location for the turbine, a generator, and electrical equipment.  Some sites might require a powerhouse enclosure as well.

Components of a small hydropower system. Source: Alteration of Micro Hydro Electric Power Diagram

These requirements might limit the usable head of the site because either intake or outlet need to be located at energetically less favorable locations. They might also limit the amount of flow that can be used for energy generation by restricting the intake structure. In the worst-case scenario they might prevent the installation of a dam-free micro-hydropower system all together.

So if one wanted to define the ideal topographic setting for a dam-free micro-hydropower project, it would be a steep waterfall with a natural lake at the top as well as at the bottom of it. The lake at the top side is providing penstock inlet submergence similar to the dam/impoundment setting; the lake at the bottom allows bypassing the waterfall most directly, without creating a bypass reach or aesthetic visual impacts (visible intake structure, penstock). Such a setting also guaranteed enough space for intake and outlet structure on either end of the penstock, while the system can be located closer to a possible interconnection point with the utility grid.

But if you want to design your own dam-free micro-hydropower system or at least get a sense of the overall feasibility at your waterfall, you will have to know a bit more about the design and key aspects of the necessary components of a dam-free micro-hydropower system. So read on: Part 2, Design Components .