Electrification of farm plots using small hydro technology

Thu, 24 Nov 2016 12:32:29 +0000

In these times of power deficit, farmers who are fortunate enough to have a stream flowing through their farm or a nearby stream, can generate their own electricity using hydropower.

Hydropower as a source of energy has been used for over two thousand years. Indeed, this natural and recurring energy is a form of solar energy since the sun drives the hydrological cycle, evaporating water, which condenses in the clouds and comes down as rain.

This energy solution is for those farms that are very far from the national grid or for farmers who simply want to cut down on their ZESCO bill to take advantage of the hydropower resource found on their farm and is not limited to large scale hydro like Kafue Gorge (900, 000 kW) but even to very small scale hydro generating as little as 300W.

After all, powering an off-grid house’s lights, refrigerator, well pump and a washing machine can be done with as little as 300W continuous power output. The amount of electricity that can be produced will depend on the amount of water in the stream and the head (the drop in elevation from the intake to the turbine/generator), which is given by the following equation:

 (kW) where Q is water flow in m3/second and H is head in m

This head and water flow combination means that:

If the stream does not have much head, it will need a lot of water

If the stream does not have much flow, it will need a lot of head

Small hydro is a proven technology that can be connected to the main grid, isolated mini grids or as an off-grid stand alone option in the form of pico, micro, mini, and small hydro. In the UK, for example, a Feed-In Tariffs scheme was introduced to encourage the uptake of a range of small-scale renewable and low-carbon technologies. Early this year, in Zambia also, the ERB conducted a workshop on Renewable Energy Feed-In Tariffs. The introduction of these tariffs is, therefore, imminent. It can be combined with irrigation and water supply systems.

According to an article in the National Farmers Union magazine of the UK, Miles and Gail Fursdon created a micro-hydro system on their farm in 1995, using the power of the falling water to generate electricity.

The electricity is used by the farm and the surplus exported to the National Grid. In high winter flow, the larger 90kW turbine produces enough electricity for the surrounding villages (approximately 80 homes). When the flow is lower in the summer, a smaller 20kW turbine is used, allowing the system to still produce electricity.

The benefits of this system are not just felt by the Fursdons; it has produced advantages for the environment and for educating others. The Fursdons claim that the payback period of 80,000 pounds was only 5 years because they did most of the works themselves but could have been 200,000 pounds if done by contractors.

Although there is no internationally agreed upon definition of “small” hydro, the upper limit is usually taken as 10 MW of installed capacity. Within the range of small hydro, distinction can be made between mini hydro (between 100 kW and 1 MW), micro hydro (5 kW and 100 kW) and pico hydro (below 5 kW), each with its own specific technical characteristics.

The high capital cost of installing a small hydropower scheme is often inappropriately compared to the low capital cost of a diesel/petrol generating set because over time, the low operating costs of a small hydro can offset its high capital cost through avoided fuel expenses. For example, in 2008 when I visited the Lwawu Mission generating plant, it was in good working order.

During the 24 years, that the plant had been in use, only the belt that drives the shaft had to be replaced. Even the bearings had not been replaced during this time, but greased each month.

The robust nature of the generating machinery resulted in very little repair. Brother Joseph used to follow the maintenance schedules religiously. In addition, the cost of the diesel engine polluting the environment including noise pollution is often ignored. It is therefore in the society’s interest for government to subsidise renewable energy technologies because of environmental concerns as is done in some countries.

With the emergence of pico-hydro with less water and head requirement a larger area of Zambia is suitable for at least pico hydropower generation at most farms with streams, see the accompanying map of Zambia showing location of meteorological and river flow gauging stations. The abundance of water in Zambia, the current drought notwithstanding, reminds the author of Coleridge’s “Ancient Mariner” which talks of “Water, water, everywhere, nor any drop to drink”, which could be rephrased to “Water, water, everywhere, nor any drop to generate pico hydropower electricity.”

A study on electrification technologies by the World Bank Energy Unit shows that, of the options currently available for off-grid generation, pico hydro is likely to have the lowest cost of generation, see accompanying figure on electrification cost of various technologies.

Pico hydro units are extensively used in South–East Asia and are increasingly being used in other parts of the world such as Kenya, Uganda, Ecuador etc. Unlike small hydro, pico hydro units where they are used are purchased without government subsidy.

There are basically two types of layouts for pico hydro, namely the low head pico hydro and the medium to high head pico hydro.

There are three basic locations that are suitable for installing a low head pico hydro unit. They are:

  1. Waterfall

This is the simplest method when the farm house to be electrified is near a stream with a small waterfall of at least 1.5m high. This can then be used as a platform. Minor modifications to channel the flow may be required.

  1. Dam

If the stream is flat a dam might be necessary. It can be constructed from clay and river boulders, clay and bamboo (or other wood) or even concrete. The dam wall should be 1.75 meters high to allow for a head of 1.5m.

  1. Side Channel

A trench is dug along the bank parallel to the stream, starting at the upstream end where water can enter. The trench follows the contour, i.e. is almost horizontal. The trench should be long enough so that when it is finished the head is around 1.5 m or more above the water surface in the stream below. This area is excavated so that the water can enter the trench, flow along it and exit back into the stream below.

  (kW) where Q is water flow in m3/second and H is head in m

From this power output equation a higher head than the 1.5m shown in the table will result in reduced flow.

This is the layout for a medium to high head pico hydro. The source of water is a stream or sometimes an irrigation canal. Small amounts of water can also be diverted from larger flows such as rivers. The water is fed into a forebay tank. The water flows from the forebay tank or reservoir down a long pipe called the penstock to a turbine to produce shaft power which can be used to mill grain directly or can be attached to a generator to produce electricity.

The missionary secondary school in Mwinilunga district near Kalene hill has harnessed the Sakeji stream producing power at 240 V enough to supply electricity to the boarding school and staff houses by diverting the water into a canal to spin a crossflow turbine, locally manufactured, connected to a generator.

Some of the water in the canal goes to spin a water wheel. A car alternator run by the water wheel generates DC electricity. This electricity is used to charge a bank of batteries in the battery room. The inverter converts the DC voltage to AC voltage at 110 V to run computers. The choice of 110 V is because the computers used are from Canada.

An example of a farm with a hydropower resource is Mulembo Farm named after the Mulembo stream some 70 km away from the national grid in Serenje District. This is a perennial stream with a 4m waterfall which the farmer wanted to exploit for hydropower generation.

The author was hired to carry out a feasibility study which came up with a 75 kW micro hydropower scheme at a cost of about $US 150,000 some 8 years ago to meet the farm’s electrical demand for agro processing such as oil processing, milling, mixing of stock feed, carpentry and workshop.

The project fell through because of lack of funding. The source of power at the time was a diesel generator supplemented with solar panels.

The farmer could have afforded a pico hydropower unit without a loan facility. In Vietnam the pico hydro units imported from China costs less than US$100 whereas the much improved versions under the Powerpal trade mark the price of a 200W unit is US$ 340 and a 2kW double valve turgo is US$ 2,980.

Pico hydro is superior to solar PV because it can provide continuous power for agro-processing and household-based income generating activities such as irrigation/water pumping, grain milling, dehulling cereals, edible oil extraction, food preservation (cooling/freezing facility) space heating for poultry. It is also a versatile power source. AC electricity can be produced enabling standard electrical appliances to be used and the electricity can be distributed to a whole farm or village. Common examples of devices which can be powered by pico hydro are light bulbs, radios, televisions, refrigerators and food processors. Mechanical power can be utilized with some designs. This is useful for direct drive of machinery such as workshop tools, grain mills and other agro-processing equipment.

Solar PV competes well with pico/micro when it comes to lighting and water heating (solar geysers).The wind speeds in Zambia can only be used for water pumping because they are too low to generate electricity.

However pico hydro is a new technology in Zambia and its introduction is likely to be viewed with skepticism compared to diesel generators and solar PV. It is argued that the PV technology has been favoured by donors because solar PVs are not locally produced and have to be bought from them.

This is not so with pico hydropower units which can be locally manufactured. Lack of information on the pico hydro technology can only be overcome through the establishment of demonstration and pico pilot projects to illustrate their economic viability and benefits.  In countries where the pico hydro power units have been used farmers have been able to buy them without government subsidy.

Micro hydropower, on the other hand, is not new in Zambia. Micro-hydro power plants have been installed on some church missions in North Western Province.

The average cost per kW is some US $3000 per installed kW (thus a 50 kW micro hydro project would cost US$ 150,000), which is quite high without government subsidy or donations as has been the case at church missions.

In other countries like Nepal farmers construct, own and manage the micro-hydro plant primarily for agro processing purposes and sell electricity to community households.

The agricultural development bank in Nepal provides 5 to 7 years loans at a 16 percent rate of interest up to 80 percent of the cost of the micro-hydro project.

The only problem is that even when revenues are sufficient to cover the cost of amortisation the bank experiences problems with overdue loans in its loan portfolio on micro-hydro projects because the relatively richer and politically well connected farmers have the worst repayment records.

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