Apple Gate

Upper Black Lake, Photo by Richard Stenzel

Section 3: Typical Infrastructure in Colorado

Typical irrigation structures were identified throughout the course of this project. Information was collected from the returned surveys as well as the project canal investigations. Six types of structures are commonly seen on irrigation canals and ditches across the state; diversion structures, concrete lined chutes, vertical drops, checks, pipelines, and reservoir outlets. The potential exists at all of these structures to produce low head hydropower. This section will review the typical structures, and make recommendations on the types of turbines that should be considered with factors that may affect their feasibility at a particular site.

Diversion Structures

Almost all canals utilize a direct diversion off of a river in Colorado, unless the canal is fed by a reservoir. In order to divert directly from the river into a canal headgate at the point of diversion, a diversion structure of some sort is constructed. Diversion structures can range in size and intricacy depending on the size of both the river and the headgate. The simplest diversion structures are made by placing large boulders in the river to raise the water surface slightly and direct water into the headgate. More complex diversion structures consist of a concrete diversion dam across the river like the one pictured below. This diversion structure has an Ogee weir across the river with two sand gates that allow flushing of the sediment that builds up behind the dam. A trashrack prevents debris from entering the headgate, and gates are used to control the flow into the canal.

There are several reasons why a diversion dam makes a good structure to implement low head hydropower. First, the dam extends across the flow of the river, allowing the hydropower plant to utilize the full flow of the river. Second, the existing infrastructure can be used to lower the installation cost of the unit.  Installing hydropower on a rock diversion dam will be more challenging. If the organization is considering upgrading a rock diversion dam to a concrete diversion dam it would be beneficial to investigate incorporating low head hydro into the new structure early in the process.

There are several turbines that may be used at a diversion dam depending on the size, head and expected flow rate; the VLH Turbine, an Archimedean Hydro Screw, the Natel Hydroengine or a traditional Kaplan. The first three turbines are relatively new to the marketplace and are designed to make use of existing infrastructure. Considering the geometry of the structure may dictate which turbine would be most appropriate. The VLH is produced in 5 standard sizes, and considerable cost could be saved if the turbine could “slip” into an opening. The Archimedean Hydro Screw would be best installed in an inclined opening, like a spillway type opening. Installing either a Natel Hydroengine or a traditional Kaplan will require building a wall inside one of the gates with an opening near the bottom to hook up the turbine.  These turbines also require submergence of the draft tube downstream of the turbine. This may require building a stilling pool on the downstream side of the dam.

At the time this report was written there was a large difference between the price of the VLH and Archimedean Hydro Screw as compared to the Natel Hydroengine. The VLH and Archimedean Hydro Screw could be as much as 2 to 3 times more expensive than the Natel. This is for the cost of the turbine itself, not the civil infrastructure required for installation. This cost difference may decrease in the near future, all three turbines are relatively new, and more are being installed each year. The VLH and the Archimedean Hydro Screws are currently manufactured in Europe.

Concrete Lined Chutes

Concrete lined chutes are commonly used to transport water down a gradual hill while preventing erosion of the native ground. There are two basic methods to utilize these drops for hydropower, one would be to make use of the existing concrete chute with an Archimedean Hydro Screw, and the other would be to pipe the drop adjacent to the existing chute and use a more traditional turbine.

Figure 12: Concrete Lined Chute

The length, angle and width of the chute will determine if an Archimedean Hydro Screw could be installed with minimal modification to the existing structure. The chute pictured here is too long and narrow for a Hydro Screw to “slip” into the existing infrastructure. If the infrastructure does not need significant modifications, this may be a cost effective turbine at a concrete lined chute.

The civil infrastructure required to install a traditional turbine at this site may be as costly as the turbine itself. A pipeline will have to be run alongside the chute and a power house constructed at the bottom. The chute would then be used as a bypass if the turbine was shut down for any reason. This is important for irrigation canals with users that depend on water downstream of the turbine.

Vertical Drops

When the topography of the land is more abrupt the canal may fall vertically at a drop. The infrastructure available at a vertical drop is much different than at a gradually sloping chute. In some cases the concrete infrastructure that exists to create the vertical drop may be used to house a turbine. The turbine can easily be added to the existing infrastructure without costly modifications. This picture shows a typical vertical drop that might be seen in a canal system.  Water cascades over the concrete structure, and could be harnessed to produce power. These drops are typically less than 15-20 feet high.

Figure 13: Vertical Drop

The turbines that would be appropriate at a vertical drop are the same as those appropriate at a diversion dam, where the vertical drop also occurs over a short distance. The Mavel Microturbines, and the Natel Hydroengine would likely require the least amount of modification to infrastructure. A Kaplan turbine or other inline type turbine could be installed but would require a penstock. An Archimedean Hydro-Screw or Ossberger Moveable power plant could be used with more intensive modifications to the structure.

Pipelines

Most canal systems have some length of piped sections. Piped sections may be used to cross heavily populated areas, roads, or areas of highly permeable soils for example. Some canal systems are converting large reaches of canals with pipe as a water conservation measure. Others choose to pressurize the canal system to take advantage of the gravity head available and provide pressure to on farm irrigation practices such as sprinklers. There are numerous possible scenarios regarding pipelines and hydropower.

Figure 14: Pipeline with PRV Vault

If the head generated in the pipeline is supplied to a downstream sprinkler system, and all of the head is required, hydropower cannot be added to the system without impacting the sprinklers. The hydropower would “burn” the head that is needed to properly operate the sprinklers. If there is excess head available, a pressure reducing valve (PRV) is typically used to lower the pressure in the pipeline to an acceptable level. Replacing the PRV with a turbine may be an opportunity to add hydropower to the system without impacting current operations. Generally an inline turbine would be added in parallel with the PRV. The PRV acts as a bypass and will allow the system to function properly if the turbine needs to be taken out of service. Other pipelines dissipate the accumulated pressure in an energy dissipation structure at the outlet of the pipeline.  In this case, a bifurcation could be placed at the outlet of the pipeline to provide a leg for the turbine and a leg for a bypass.

There are a number of inline turbines available that can be used in conjunction with an existing pipeline. The choice of turbine will depend on the flow rate and the head available. Examples of such turbines include most of the propeller type turbines, such as the Voith, Canyon Hydro, and Gilkes Kaplan, the Toshiba e-KIDS, and the Turbinator.  

One additional note regarding hydropower in existing pipelines, it is possible to design some turbines and valves to maintain a downstream pressure. Although this is not the norm, occasionally, some downstream pressure is required. A hydropower facility could be located at the elevation required to accumulate the required head downstream of the unit, or the facility can be designed to “leave” some head in the pipeline immediately downstream of the turbine. If an irrigation company is considering piping sections of the system, consider hydropower during the design process to see if modification to the design could allow for the addition of hydropower in the future.

Checks

Check structures are used to raise the water surface in the canal, generally to supply water to an upstream headgate. These structures often result in water surface elevation drops of 5 feet or less. Checks take on various forms, but generally the concept is to raise the bottom of the canal, and possibly constrict the sides. This will tend to increase the velocity of the water over the structure, and may provide an opportunity for a hydrokinetic turbine.

Figure 15: Check Structure

Hydrokinetic turbines produce power based on velocity instead of pressure and flow. The general equation to determine the power available at a hydrokinetic site, is as follows;

Where P= Power (Watts), A = area of the turbine in flow (ft2), V = Velocity (ft/s). Both velocity and the depth of water at the check must be considered to determine the feasibility of using a hydrokinetic turbine. They are generally effective in the velocity range between 5 and 10 ft/sec, with at least 2 feet of depth. Each manufacturer has specific requirements for each model of turbine available.

Reservoir Outlets

There are a few additional considerations that must be made when adding a turbine to a reservoir outlet. In Colorado many small reservoirs are emptied or lowered on an annual basis to meet late season irrigation demands. The duration of flow may have a significant affect on a project’s viability. Also, the variability of the head in the reservoir may influence the turbine selection and the project’s viability. Most low head turbines can operate within 50-125% of the design head. Fully adjustable Kaplan turbines can extend that range to 45-150%[1]. The power will change proportionate to the change in head, and efficiencies will vary over the range, but the turbine will still be able to effectively operate.

Another consideration when adding a turbine to an existing outlet is the condition and capacity of the outlet pipe.  If the dam is considered jurisdictional in Colorado, the Colorado Dam Safety Branch will be involved in any modification to the dam structure, including the addition of a turbine. The FERC will also be involved in the design process and will ultimately take over jurisdiction of the dam’s safety from the State. 

Figure 16: Reservoir Outlet

When considering the feasibility of a reservoir site, one must understand if the outlet was designed to operate under pressure. Most dams have a valve on the upstream side of the outlet pipe, therefore the actual outlet does not run under pressure. If a turbine were added to the downstream side of the outlet pipe, it would cause the entire pipe to be pressurized. Some dams have oversized non-pressurized tunnels that lead to the outlet valve. In this case it may be possible to add a pressurized penstock and possibly the turbine inside of the tunnel, as space allows.

Typically, custom built, traditional turbines are appropriate for reservoir outlets, and the type would depend on the head available. Some of the newer low head technologies may be applicable depending on the conditions. Generally, a custom built turbine would be more efficient and suited for the exact site conditions, but potentially more expensive than a standardized model.



[1] Gulliver, J and Arndt, R (1991) Hydropower Engineering Handbook, McGraw-Hill, p 4.50