Ultra low Head Pico Hydro power generation: A cost effective solution to the power problem for remote areas

Ultra low Head Pico Hydro power generation: A cost effective solution to the power problem.

The aim is to provide an uninterrupted, cheap and sustainable power source to remote areas in long run. In today’s era of depleting natural resources and rising fuel and power costs, a cost effective solution to the power problem is a key area to be looked upon. Following the analysis, a survey of the available renewable energy options is to be made that can be utilized as an uninterrupted power source. The main sources of renewable energy are solar power, Wind power and Hydro power.

The wind type power projects need an average wind speed of around 4m/sec in the region, failing which the project cannot be viable. The solar type projects are good for many regions across globe where there is ample of sunlight throughout the year.  The hydro power generation projects are the ones that have very less maintenance and running cost compared to others. Hydro power projects require two main things for power generation i.e. the water discharge (Q) and the net head (H). The project will be classified in the canal based ultra low head pico hydro power project category.

Options for power supply: The most readily available cheap power that can be produced in the plant is hydro power and the other options which also can be considered are solar power and a new technology is chemical energy (solid oxide fuel cell i.e. SOFC.  Both of the options listed above are from the group of sources that produce green energy without any pollution and environmental hazards.

A brief study report on installation of  Ultra low Head Pico Hydro power generation project done by two graduate engineers Baldev and Aditya is as follows:-

Self survey of site at selected place in the length of 660cm

     

Sr. No.	Length(cm)	Time(sec)	FLOW(m/s)	Depth(m)	Width(m)
1	660	38	0.173684211	0.15	3.04
2	660	24	0.275	0.24	3.21
3	660	27	0.244444444	0.17	3.5
4	660	22	0.3	0.16	3.13
5	660	28	0.235714286	0.46	3.2
6	660	28	0.235714286	0.34	0
7	660	34	0.194117647	0.55	0
8	660	23	0.286956522	0.64	0
9	660	24	0.275	0	0
10	660	24	0.275	0	0
AVG.	660	27.2	0.24956314	0.33875	3.216
Discharge (m^3/s)	0.271879075

Site analysis

Considering the amount of flow in the river, it really has a potential of generating good amount of power. The limitation of the site is head. The point where the river enters the plant and where it exits the plant has an elevation difference of around 5 metres. But the gradient is very low, and hence the velocity head required generally for small hydro production is not achieved. Considering an ultra low head Pico hydro project is the option available with us. Looking forward to generate 2kW power directly from the stream seems to be a costly affair, as this would require a large dam construction to increase the head of the site. We can move ahead with an aim to produce 1 kW either in a single stage or produce 2kW in two stages of 1kW projects each. 

Flow rate

This is defined as the volume of water passing in the stream per unit time. River flowing inside the plant has a potential of generating good amount of power. The flow rate of the river varies from 600m³/sec to 407816.482m³/sec. Now considering the minimum flow rate we can design the project, or we can design it by taking a dependability factor of the flow available and design it for more flow than minimum flow. But the later option would require a dam to be constructed to hold the right amount of water required to generate the power. This would mean more civil costs and ultimately the project cost would be very high.

Head selection:

The amount of gross head available for generation is the total elevation difference of the site of water inlet and the site where turbine is placed. A little bit of dam construction will make the head of the site at about 1.5 m. That is what we are looking for. The gross head of 1.5 m would mean a minimum net head of 1.2 m. Power generation: the amount of power available at the turbine is calculated as follows:

P = Ƞ*Q*H*g

Where, Ƞ= the overall efficiency of the turbine generator set

             H= the net head available.

             Q= the designed discharge/ flow rate.

             G= acceleration due to gravity= 9.8 m/sec.

For power output P= 1 kW

  H = P / (Ƞ*Q*g) = 1 / (.5*.167*9.8) = 1.22 m

So head needed for the 1 kW output power is 1.22 m. We have considered only 50% efficiency here, but Pico hydro turbines give efficiencies up to 80% also. Options available to generate power:

1)       Propeller Turbine

Moving ahead with the products available in the market, a propeller turbine with shaft directly coupled to the generator is the best suited product that can be utilised for a site with very low head. This kind of turbines is robust and if installed properly can have a life of around 15-20 years. Also the elimination of the requirement to construct a penstock is added advantage.

Turbine-Generator Structure: The typical low head Pico-hydro turbine is a single composite unit combining a reaction turbine with an AC generator (see Fig. below). The turbine is an axial flow propeller turbine with fixed guide vanes (wicker gate assembly) to direct water flow axially onto the propeller blades. The generator is typically a single-phase, permanent magnet type alternator, with a rated voltage of 230-240 VAC.

Fig. Turbine Setup and Casing

Types of installation methods: There are three main types of installation methods that are commonly used for low head Pico-hydro in the Lao PDR: 1) Standing method, 2) Lying method, and 3) Angled method.

Standing method

The standing installation is the method usually recommended by manufacturers. In this method, the Pico-hydro turbine is installed in an upright position, with the generator directly above the propeller and at a gross head of 1.5-3 m (see figure below). Two accessories, a draft channel and draft tube, are also recommended to improve the operation of the turbine. The draft channel is used as an intake for the Pico-hydro turbine and a properly designed draft channel ends in an open-spiral volute, which helps to create a free vortex flow into the horizontal drain at the bottom. The Pico-hydro turbine is positioned over the drain and the conical draft tube is connected to the volute directly beneath the turbine. The draft tube acts to draw water flow more efficiently through the drain. Figure below provides typical specifications for the draft channel and draft tube.

Fig: Typical specifications for standing installation accessories: turbine (A), draft channel (B) and draft tube (C).

π*(d/2)²*Velocity(m/s)=Discharge

π*(d/2)²*0.246=0.27187

ð   d=1.18 m

Rest of the calculations can be done as per diagram

Lying method:

In the lying method, the Pico-hydro turbine is installed in-stream of a river and is positioned so that it lies at a slight angle to the horizontal with the propeller facing the water flow of the river (see Fig. 5). Store bought Pico-hydro turbines first need to be adapted for a lying installation. A larger sized propeller (similar to a boat propeller) replaces the normal reaction propeller, normally with an extension to the shaft allowing it to be placed at a greater angle. There is no head associated with a lying installation, and the energy is delivered entirely through the horizontal water flow.

Fig.: Lying installation method.

Angled method:

The angled method can be used when a river’s water flow encounters a drop in elevation and the pico-hydro turbine is installed at an angle over the drop as shown in Fig. 7. This drop can be either natural (e.g. a waterfall) or man-made (e.g. dam or weir). Like the lying method, a store bought turbine needs to be adapted to include a shaft extension and larger propeller.

Fig.: Angled installation method.

Examples of pico-hydro implementations

There are numerous examples demonstrating how the three basic types of installation methods can be implemented. These implementations are largely influenced by the local, natural resources available, in particular the topographical characteristics of the water source. In this section, examples of standing, lying and angled installations that were observed during field trips around the country are presented.

Examples of standing installations

In practice, there are various ways to implement the standing method, some of which conform closely to the recommended installation method (i.e. complete with a draft channel and draft tube). Several types of standing method implementations common in the Lao PDR are discussed below: 1) Diverted channel, 2) Dam or weir and 3) Waterfalls.

1. Diverted channel: The diverted channel offers, in most cases, the ideal configuration. A portion of the streams flow is channeled off to the side of the main stream until its slope has provided the sufficient gross head. The standing installation is then located at the end of the diverted channel where the water is returned to the main stream (see the example shown in Fig.).

This installation method offers several significant benefits.

Firstly the diverted channel allows the turbine to be located above (or away) from flood areas, helping to avoid damage from flooding. Many other installations need to be moved or rebuilt annually when water levels are too high or if they are washed away.

Secondly, a diverted channel offers greater control over the flow, with the ability to utilise overflows and easily block the flow when required. This installation method also has minimal impact to downstream users as it does not require the entire stream’s water flow to pass through the turbine and the water is returned to its original source after a short diversion. However, digging a diverted channel requires the stream’s banks to be of soft soil and largely unaffected by flooding.

2. Dam or weir: The required gross head for a standing installation can be achieved through man-made constructions such as a dam or weir. Local materials (such as rocks, wood and earth native to the area) are often used to create such dams for Pico-hydro installations. In other instances, pre-constructed community dams or weirs are used. Standing installations using a dam or weir are typically implemented in two ways: either the dam is used to direct water flow into a draft channel or alternatively, the dam is used to store water in cases where there is not sufficient flow to run a turbine 24 h a day. Fig. depicts the two ways the standing method can be implemented using a dam or weir.

Dam to direct flow into a draft channel:

When there is a natural constriction of water flow, for instance due to the shape of a rocky river bed, it may be preferable to construct a dam and redirect the flow into a draft channel. The draft channel can protrude over the edge of the dam and the recommended standing installation can be employed (see the example in Fig.). A simple wooden board acts as a flow control gate at the draft channel entrance. This style of dam construction better suits 24 h operation but can also be used as a storage dam and provide intermittent use if the level of flow is too low.

Community dams and weirs that are constructed for other purposes (e.g. water storage, irrigation, etc.) can also be used to achieve the necessary gross head, with minor civil works required to position the draft channel and draft tube against the dam or weir wall.

Fig: Alternative ways to implement the standing method using a dam or weir.

Storage dam:

Preferably a Pico-hydro installation would be run 24 h a day, but in many areas, for significant periods of the year there is not enough water flow to maintain continuous operation. As the primary use of the electricity generated is lighting, 24 h supply is not always essential. In such cases, storage dams can be designed to allow the controlled release of flow, with water stored in a reservoir and released to the Pico-hydro turbine when required. The positioning of the turbine with respect to the dam varies when a storage dam is used. In this configuration, the draft tube and turbine are located on the high water side of the dam. The dam design allows the draft tube to be deployed vertically beneath the turbine, with its intake in the high water side’s river bed. When not in operation an easily removable cover seals the top of the draft tube. The typical wood and earth storage dam (see the example in Fig.) is likely to be washed away under the large flows of the wet season, but the use of locally foraged materials means that it can be annually rebuilt at no financial cost, although not without significant labor.            

Waterfalls can be used for Pico-hydro turbine installation by taking advantage of the natural head that they provide. The restriction is that the head needs to be small enough to be suitable for Pico-hydro turbines, which therefore precludes high waterfalls (see the example shown in Fig.). The topographical characteristics of waterfalls can often make civil works more complicated and less efficient, especially due to water loss between the waterfall and the draft channel. The support structures for the draft channel and draft tube also need to conform to the natural topography of the waterfall.

Examples of lying installations

Lying installations are most commonly used in deeper water on a rivers fast moving main stream. Supports are usually made from A-frame wooden structures or even anchored bamboo rafts. This makes the installation very simple albeit vulnerable to being washed away and difficult to access. Two examples of in-stream lying installations are shown in Figs.

Examples of angled installations

The angled method is most commonly found in sites with shallow streams or small natural waterfalls.

Dam: In smaller, shallower streams a simple dam can be used to direct the flow through the turbines propeller. This can be as simple as some carefully placed rocks and a wooden chute used to better direct the flow. Although a dam requires more in terms of labor and materials for civil works, it does provide good accessibility and a secure structure. However, it is only possible in smaller and shallower streams (see the example in Fig.).

terfall: Small natural waterfalls can be used for angled installations with simple wooden supports for the civil works (see the example in Fig.). However waterfalls need to have appropriate geographical characteristics for such an installation to be feasible and the availability of suitable sites is low.