Hydro Power on a Human Scale
By Michael Camp for Mouser Electronics
If the phrase "Hydroelectric Power" conjures images of towering dams and vast reservoirs, then you might be surprised at the extent to which hydropower technology has evolved over the past few years. No longer the exclusive province of billion-dollar public works projects, hydroelectric has quietly found its place alongside more visible renewable energy technologies and has scaled down to satisfy local, and even personal, applications.
Figure 1: Aerial View of the Hoover Dam. Source: istock
Much of the publicity surrounding alternative energy sources tends to focus on Solar (also referred to as photovoltaic or PV generation) and Turbine (wind) sources, but these are not the only technologies that are growing in adoption. The past five years have shown increased availability of hydroelectric generation systems that range from a few kilowatts down to a few hundred watts. These systems bring low-cost, renewable power to locations that may not be suitable for other generation options.
Figure 2: A pico hydro system made by the Sustainable Vision project from Baylor University (Source: Wikipedia)
Micro Hydropower
Micro Hydropower systems are simply scaled-down versions of their government-sized siblings. Most of these small-scale systems are considered to be a run-of-the-river system, which means that the waterway is not blocked or re-directed to accommodate power generation. Instead, a portion of the flowing water is diverted through a debris screen. The screen prevents floating material and adventurous fish from entering an intake.
Screened water then enters a pipe called a penstock. The size of this pipe limits the amount of water diverted from the main flow and ultimately determines the flow rate available for generation. The difference in elevation between the intake and the turbine is referred to as the head, and is the key parameter in the formula that defines the theoretical maximum power that can be generated:
P(Watts)=Head(m) x Flow Rate(l/s) x 9.81 m/s^2
These small systems have a typical system efficiency of about 50-60%, including the mechanical loss of the turbine and the electrical loss of the generator itself. Efficiency loss due to friction and buildup of minerals in the wet part of the system will increase over time and represent the primary source of maintenance issues in these systems.
The turbine itself is generally one of two kinds. A reaction turbine uses nozzles to direct the flow of water at a runner, or paddle wheel-like device that turns a shaft. An alternative design includes a propeller or squirrel-cage that sits directly in the flow of water to turn the generator shaft. After the water has passed through the turbine, water is directed back to the main waterway through a tailrace.
In some installations, the AC output of the generator is sent to a transformer to be converted into household AC current. Most designs include a battery bank that will provide continuous power, even when the hydro generator is offline for maintenance. These systems require a transformer to convert AC generator output to DC and a charge controller to regulate the flow of electricity to the batteries, and to shunt excess power to a secondary load to prevent overcharging. Highly integrated charge controllers to manage battery packs make it easier to implement, such as the Texas Instruments' bq40Z50-R1 lithium-ion battery pack manager, which manages 1 to 4 cells and has many programmable features and convenient Battery Management Studio software. Charge controllers can also function in the interface between the system and the local grid, directing excess power into the grid when possible.
One key component in a battery-based energy storage design is the inverter, which takes DC power from the battery bank and converts it to the appropriate AC Voltage and frequency. Some inverters also can charge the battery banks from an alternate power source, such as a fuel generator, should the hydro generator fail or be taken offline.
Finally, the AC output of the inverter is passed through a circuit breaker panel to be distributed for use in household or commercial applications. Protection of circuits at the point of use is essential in a system that is continuously producing power. Variants of standard breakers suited for harsh environments, like those from Carling Technologies are recommended due to the high humidity that is typically present in hydro systems.
When is hydropower an option?
Mankind has long understood the power of flowing water and considerable effort has been made to harness that power to do work. From Archimedes' screw to elaborate turbines, humans have been constructing water-powered machines for over two thousand years. These machines required little more than a water source, a vertical drop in which potential energy is converted to kinetic energy, and a wheel or turbine to convert kinetic energy to useful motion.
Figure 3: Archimedes' screw, which looks like an auger, was often used for transferring water for irrigation or for draining water. (Source: Wikipedia)
While the technology and efficiency of hydropower systems has advanced, the need for a water source and a vertical drop remain the same. The vertical drop can be as little as one meter and still produce 200+ Watts, or enough to provide light for a single family in a remote area. In places with year-round flowing water, these systems do not even require expensive batteries for storage, as production is continuous.
Locations with distinct wet and dry seasons benefit from hydropower generation during the annual monsoon when the availability of PV-generated power is minimal. During the dry season, PV takes over as the primary source when flowing water is scarce. In this case, multiple generation sources are connected to a microgrid to ensure continuous availability of electricity.
Geographic restrictions are the primary factor when selecting hydropower as a renewal energy source, but even in cases where water flows year-around there tends to be variability in the volume of water available to drive generators.
Handling varying water flow
Seasonal variations in water volume can be partially compensated for by changing to larger or smaller nozzles that direct the stream of water at the turbine. All turbines have an optimal RPM at which the generator produces the maximum voltage. During a dry season, the intake may not be able to provide a large enough volume of water to maintain sufficient pressure at the turbine. Many small-scale generators allow the user to switch to a smaller diameter nozzle, which accelerates the speed of the water flow, and helps to keep the turbine spinning near its optimal RPM.
Fine tuning of the generator's rotor involves adjusting the position of the rotor relative to the stator and measuring the output current with a high-quality multimeter, such as the Fluke 323 True-rms Clamp Meter. The rotor will want to find a stationary point of equilibrium within the magnetic field and will resist spinning. Moving the rotor further away from the stator will reduce this resistance, but will also reduce the generated current. The optimal position of the rotor will balance the resistance to spin and keeping the rotor within the magnetic field for maximum power generation.
Alternative uses for hydro power
Personal hydropower
Portable might be the last word that comes to mind when someone mentions hydro power, but a new wave of products on the market allow you to take a generator to the water source rather than the other way around.
Designed to provide USB charging for campers and hikers, these devices include a semi-submersible "probe" that houses the runner and generator, a battery for storing the charge, and a USB-compatible cable. They can charge mobile phones, GPS receivers and any other USB-chargeable gizmo that today's connected Lewis or Clark can't live without.
Gravity Storage
If you like the idea of water-powered power, then you'll love the back-to-the-future vibe of ditching expensive batteries for a big container of water in the attic. Off-gridders and survivalists can choose to store a portion of well or stream water into an elevated tank, which is then used to create an artificial head for a hydro generator.
The scenario is particularly efficient when the storage tank is used for supplying water for washing clothes or bathroom use. This water, which is destined for these uses and not specifically diverted for power generation, can be used to reclaim some of the energy that would be otherwise wasted in an off-grid location.
Residential power harvesting
City dwellers that dream about being off-grid can also harvest energy from water supplies. Several companies now make purpose-built generators that are driven by the pressure from civic water sources. Like in the gravity storage example, the water is going to be used for some other purpose after passing through the generator, so there is no net increase in water use for the purpose of generating power.
Michael Camp is an embedded software and IoT consultant with 30 years as a software engineer and technical manager. He previously worked at Freescale Semiconductor, where he was responsible for the Freescale Linux distribution for embedded devices. Michael also worked at Nokia Mobile Phones where he was part of the Bluetooth SIG software working group, and a contributor to the original specifications. Michael has a BS in Computer Science from LSU Shreveport and MBAs from Cornell University and Queen's University of Canada.
