- Published on Monday, 02 December 2013 10:42
- Written by Nathan Smith
All water distribution systems have one thing in common: It takes energy to move water from one location to another. The energy required to move the water can come from gravity or from pumps, both of which convert the water’s potential energy into kinetic energy. In a typical water distribution system, control valves are used at some point to manage this built-up energy, otherwise known as flow and pressure.
As water travels through a control valve, energy within the water is simply ‘burned off’ in the forms of noise, heat and vibration. Conduit hydropower installations aim to recover this lost energy by converting it into electricity while still regulating flow and pressure in a distribution system.
What is Conduit Hydropower?
Hydroelectric power generation is classified as "conduit hydropower" if the water flowing through the generating equipment has a primary use other than for power generation. For example, any power generation equipment installed on a pipeline or similar man-made water conveyance system that is operated for the distribution of water for agricultural, municipal or industrial consumption and not primarily for the generation of electricity can be classified as a conduit hydropower installation. This classification is significant because it does not require licensing or exemption by the Federal Energy Regulatory Commission (FERC).
Numerous applications for conduit hydropower exist within water systems, including pressure-reducing sites, reservoirs, treatment plants, industrial processes and irrigation. The main benefit of conduit hydropower experienced by water system operators is reduced operational costs. The power generated by a conduit hydroelectric installation can be sold to the power utility at a wholesale rate ranging anywhere from $0.04/kWh up to $0.20/kWh, depending on the prevailing cost of power at the location. If power is used at a site where electricity is consumed by a pump or other equipment, it can be "net metered." This can reduce both demand charges and the monthly electrical bill, which lowers overall operating costs. Conduit hydropower is a continuous source of energy that typically follows the same diurnal curve as power demand thereby helping to level out the power grid supply and demand. Power utilities generally prefer this type of generation over more intermittent forms of generation such as solar or wind.
What Makes a Site Suitable?
To generate power and offset operating costs, a potential site needs to have both a pressure differential and a volume flow rate. The pressure differential available for power generation is typically the difference in height from the intake to the discharge or in the case of a control valve application, it is simply the pressure differential across the valve. If the differential pressures and flow rates are known, a conduit hydroelectric firm can provide an estimate of how much energy and revenue can be generated at the site.
A suitable site for conduit hydroelectric installations are those with high-pressure differentials and high continuous flows because they will generate the most power. However, variable flow, fixed flow and even intermittent flow applications are often cost effective as well. The potential site should have nearby power lines for interconnection. Conduit projects do not need to be new installations; they can utilize existing infrastructure such as existing vaults, piping and valving to reduce installation costs.
For example, a pressure-reducing valve that has a pressure drop of 50psi and an average flow rate of 2,000gpm could generate up to 35kW of power. If this valve operated year round it would generate 275,000kWh per year. If this power was then sold at a wholesale rate of $.10/kWh, approximately $27,500 per year of revenue would be generated. If this generation was installed at a pump station or treatment plant, for example, a net metering arrangement could be set up in which power generated would directly offset power purchased at the retail rate.
Typical System Configuration
Water delivery is always the main priority of a water distribution system; therefore, conduit hydroelectric projects are configured to be a secondary priority to water delivery. When generating equipment is coupled with a custom control system, the hydro turbine functions in the same way as a control valve. Modes of operation can including pressure reducing, pressure sustaining, level control and flow control among others. The hydro turbine is designed similarly to a control valve in that it relies on inputs from the water system or SCADA to function properly. An experienced hydropower firm will take care to thoroughly understand water system operations before designing and installing a conduit hydroelectric system.
A typical hydroelectric system is plumbed parallel to a new or existing control valve. This parallel valve becomes the turbine bypass valve. When the turbine is down for maintenance or emergency, water delivery can continue uninterrupted. An automatic fail-safe valve is usually installed on the high-pressure side of the hydro turbine to operate as the turbine shutoff valve. The turbine shutoff valve allows the generating equipment to be safely shut down in the event of a power outage or emergency situation. The turbine control system automatically and seamlessly transfers the flow of water from the hydro turbine to the bypass control valve and back upon startup and shutdown.
Various types of turbines have been developed over the years to meet site specific conditions. Pelton turbines, for example, perform well at high-head and low-flow conditions while Kaplan turbines perform well at low-head and high-flow conditions. Francis turbines fall in between these two and generally perform well over a large range of head and flow conditions.
Each type of turbine has appropriate applications for optimal performance. A Pelton turbine is an impulse turbine in which water must be discharged to atmospheric pressure. Applications for a Pelton turbine could include a below-grade reservoir where the discharge can be open to atmosphere. Kaplan and Francis turbines are reaction-type turbines in which the discharge can be pressurized. Reaction turbines are more suitable for water distribution systems at pressure-reducing sites such as in cases where the discharge is pressurized.
Specialty conduit hydroelectric equipment that is designed to maximize power generation from water systems is available on the market. Fixed-flow turbines are available but typically operate at a single design point in an on-off configuration and have a fixed rate of flow. Variable flow turbines operate over a wide flow range and can typically recover more energy than a fixed- flow turbine due to higher efficiencies over a wide flow range. Variable flow turbines are available in vertical shaft orientations with in-line flanges to reduce the equipment footprint and installation costs. Maintenance for the hydroelectric equipment is similar to that of a pump, which water system employees are familiar with.
A Portland Project
SOAR Technologies recently completed design and installation of a project in Portland, OR at a reservoir fill site where an existing vault was used for equipment installation. The vault and piping were reconfigured to allow installation of the generation equipment. A weather enclosure was constructed above grade to protect the electrical equipment from the elements. The equipment installed was a fixed flow, variable speed turbine. This installation was designed for 3,250gpm and a variable head of 40ft to 52ft. The project generates an average of 26kW of power and 163,000kWh per year in generation. This installation will save approximately $10,000 per year in operating costs.