Where the resource exists, micro-hydropower offers a proven and reliable source of electrical or mechanical power on demand, usually at a lower life cycle cost than diesel engine, wind, or PV systems. Using the energy of falling water, micro-hydropower systems supply mechanical energy that can be used directly or be converted to electrical energy through a generator, for use in lighting, refrigeration, information and communications technologies (ICT) or to run electric motors.
Micro-hydro systems typically produce more energy per rated kilowatt on a daily bases than wind energy or solar PV systems. This is possible since micro-hydro systems operate round the clock whereas solar and wind power systems generate power for only a few hours each day. For example, a 100 peak Watt solar PV system might produce 100 W * 4 hours = 400 Wh on a sunny day; a 100 W wind energy system might produce 100 W * 8 hours = 800 Wh per day; whereas a micro-hydro system of the same size can, in theory, produce 100 W * 24 hours = 2,400 Wh per day.
Hydropower plants of less than 100 kW capacity are generally categorized as ‘micro-hydro’(definition). Plants in the 1–100 kW range generally supply power through a mini-grid to a rural community. Such plants mostly produce alternating current (AC) and as such the supply is not much different from the supply of electricity from the national grid. This section focuses on very small micro-hydro systems of less than 1 kW – sometimes referred to as ‘pico-hydro’ – which a user might consider installing specifically to power ICTs in isolated rural areas.
Power from falling water comes from its two primary components:
Flow: the rate of water flow, measured in liters per second (lps);
Head: the water’s vertical drop, measured in meters.
The power potential of a site is proportional to the product of these two components. In metric units,
P = g * e * Q *H
P = power (Watts)
g = 9.8 m/s2 (acceleration due to gravity)
e = efficiency (%)
Q = flow (liters/second)
H = head (meters).
The efficiency term ‘e’ indicates that not all the potential of the site is available to the user due to losses in the equipment and structures. Average efficiency for small micro-hydro systems is around 50%.
For example, a flow of 10 liters per second falling through a head of 10 meters would yield power of 490 Watts, assuming a system efficiency of 50%. Because of the product relationship between head and flow, this same power could also be produced by a flow of 20 lps falling through a head of 5 m, or alternatively by a flow of 5 lps falling through 20 m of head.
The bucket method is a simple way of measuring flow in very small streams. The entire flow is diverted into a bucket and the time for the container to fill is recorded. The flow rate is obtained simply by dividing the volume of the container by the filling time (see figure below).
Flows of up to 20 lps can be measured using a 200-liter oil barrel. The rate of flow that is measured during the driest season is used for the system design so that constant power is available throughout the year. Alternatively, a larger system can be installed and a small reservoir built to collect water during the dry season to generate full power for some critical hours of the day.
The head can be roughly measured using a simple eye-level and tape measure or more accurately with a theodolite. The details of different head measuring techniques can be found at: http://www.microhydropower.net/head.html.
The basic components of a micro-hydro hydro system include:
- An intake where the water is diverted from a stream or a spring;
- a desilting tank which allows the sand and silt to settle out before the water is conveyed toward the turbine;
- a head race to transport the water to a forebay tank just above the power plant through a canal or a pipe;
- a forebay tank to collect the water and screen out debris before it is transported to the turbine;
- a penstock pipe to transport the water from the forebay tank to the power plant;
- a turbine to convert the power of the falling water into mechanical rotational power;
- a generator to convert the mechanical power to electricity;
- a controller to maintain a constant load on the generator;
- a tailrace to return the used water to the stream; and
- transmission and distribution lines to the users.
In some cases, the headrace and penstock pipes are the same and there is no forebay tank, as in graphic at the beginning of this section.
Since these systems are small, they can easily be transported or carried to rural areas even where there is no road access. The turbine, generator, control systems and other major components usually come in one unit, simplifying installation. A simple water flow diversion mechanism can be used to divert the water to the penstock pipe. Locally available plastic pipe (HDPE or PVC) can be used for penstock pipe for low cost and ease of installation.
The same precautions apply when operating ICTs off a mini-grid as with electricity supply from the national grid, such as the use of an uninterrupted power supply (UPS) or relay-based protection from fluctuating voltage. To be assured of high quality supply, users of ICTs may wish to confirm that the system is regulated by a load controller.
Available micro-hydro hydro technologies can be divided into two broad categories – those that produce direct current (DC) and are used to run DC loads or charge batteries, and those that produce alternating current (AC). Micro-hydro systems that produce DC power have many similarities to solar PV or wind energy systems. The energy they produce is used to charge batteries, which store the energy for use in applications such as lighting or powering ICT equipment. Often an inverter will be used to convert the DC output to AC.
Micro-hydro systems can also produce AC directly to be used with conventional appliances. Water supply from a stream or a spring is much less intermittent than other renewable energy sources like solar radiation or wind speed. This makes it possible for homes to be supplied with AC power as it is generated from a micro-hydro system without having to go through batteries. This reduces costs compared to solar PV or wind energy systems, which must use batteries for most electrification (non-water pumping) applications.
While micro-hydro systems that produce AC power are less expensive than those that produce DC power, they also pose some challenges for ICT applications. One major challenge is that the systems have difficulty supplying brief spikes in power demand from the loads. The starting current of certain appliances, such as CRT monitors, can be extremely high. Laser jet printers also draw high current spikes. A large current draw, even of momentary duration, can sometimes kill the excitation process, particularly of systems which use induction generators. When this happens, the voltage of the system will be reduced to zero even as the turbine is running at overspeed. Battery-based systems generally don’t have this problem since the batteries are able to provide the momentary high power requirement.
There are a number of solutions to these problems. One of them is to use laptop computers or desktop computers with LCD screens to avoid the problem of high current draw. In case these are not available, a negative thermal coefficient (NTC) resistor can be used in series with the appliance to limit the maximum current draw.
Due to losses in transporting energy over distances, hydropower systems should be located within a reasonable distance of the point of energy use.
Operation & Maintenance
The operation of a very small micro-hydro system is relatively simple compared to larger micro-hydro systems. There is no need for extensive training for the operator. The operation manual and/or manufacturer's guidance are often sufficient to enable operation of the system. Nevertheless, the involvement of a professional is strongly recommended in light of safety considerations, particularly for systems producing AC power.
Micro-hydropower is a clean renewable source of energy that produces negligible pollution during normal operations. There can be an effect on the riparian area both at the settling pool and along the length of river from which the water has been diverted from the increase or decrease in soil moisture. For larger projects, clearing areas for construction and transmission lines may have environmental impacts. All of these factors can be mitigated with good planning and construction practices.
Micro-hydro systems generally cost between $1,000 and $4,000 per installed kW (see Micro-hydro Costs). The fact that many micro-hydro components can be fabricated in developing countries helps to keep costs low. Cash outlays for labor and materials for civil works may be reduced by contributions from the community.
Micro-hydro systems under 1 kW can provide power to rural communities located near a source of running water. Micro-hydro systems are usually less expensive than PV or small wind systems in terms of initial investment and lifetime cost of electricity. For larger projects, community involvement can both reduce the cost of civil works and improve the long-term viability of the project.
A variety of circumstances can reduce the feasibility of a micro-hydro project. Micro-hydro systems take longer to plan and install, and require substantially more civil construction works than solar PV systems. Reliable water sources and appropriate sites with a sizeable vertical drop may not be available at a location close to the community. Available water flow might also diminish during severe dry spells or from competing usage from other applications such as irrigation. Floods, landslides, or other natural calamities can also destroy the systems and can interrupt power generation unexpectedly and for long periods.
Related Web-based Resources