Introduction

There is a need in developing countries for a simple inexpensive means of harnessing the seasonal low head energy flow of rivers. The linear turbine which consists of a cascade of vanes moving in a horizontal loop across the river flow, is ideally suited to this application since, due to low flow loadings and simple design, it can be constructed from locally available material and by local manpower.

A small size (1Kw) linear turbine design is developed as a useful prime mover for household power etc. and as a means to develop the skill and experience to build larger units. To design this turbine a computer analysis of the flow conditions is made and several possible configurations are discussed.

The final design uses two loops of chain to support sheet metal vanes which are balanced so that the vane angle can change to suit operating conditions and the vanes can be tripped for speed control. Power take off is by means of a modified bicycle transmission and “V” belt drive to two car alternators. The whole assembly is supported in the river by floats and cables to the river bank.

INTRODUCTION

In developing countries where energy is a basic requirement for development there is an urgent need to harness the vast source of energy that is flowing away daily in rivers and canals. The most common types of water flow energy conversion systems use rotating turbines due to their low maintenance and operating costs. This justifies the high initial cost of design and manufacture including building dams. However the needs for developing countries are vastly different. As well as the need to develop the natural energy resources of rivers etc. there is also a great need to develop the large source of human talent that is lying idle.

A number of systems have been proposed to meet this need. The system being developed by the Intermediate Technology Development Group (1) is one. This uses a four bladed Darrieus turbine mounted either horizontally or vertically in the river flow below the surface. Construction is very simple and a unit built by local labour is now operating on the White Nile in Southern Sudan.

The Salford oscillator(2) being developed by Wilson, Bassett and Jones at Salford University is also a possible system. This is a linear system and hence its size is not limited. It consists of a barrage of vertical vanes that oscillate back and forth across the flow. Presently a tidal power unit is being developed.

The Schneider engine(3) is another alternative. It is also a linear system, however, in its present configuration with horizontal vanes it requires a dam. Also since the vane angles are fixed it requires guide vanes which add to the cost and complexity.

The linear turbine system proposed by Ishida and Service (4) offers the most promise to meet the needs of developing countries. It is similar in layout to the Schneider engine but with the vanes in a horizontal loop instead of vertical loop. Its main difference is that the vanes can swing to take up the optimum angle to the flow and this does away with the need for inlet guide vanes. To date there has been little work done on developing this system and it is the purpose of this paper to develop a feasible design which can be made by interested persons and groups throughout the world, and so develop the necessary operating experience to build larger and more suitable designs to meet their own needs.

DESIGN PARAMETERS

To satisfy the above requirements a small size unit seems most appropriate as this will be simpler and cheaper, however it must be big enough to be useful.

With this in mind the following design parameters were chosen:

1. Approximately one kilowatt capacity from a river flow of 1m/sec.

2. Simple and cheap construction from locally available materials by semi—skilled labour using simple tools.

3. Portable for ease of maintenance and site changing to meet seasonal demands.

4. Simple to operate and versatile.

To meet these design parameters a vane depth of 600mm was chosen since this would result in a compact portable unit and also enable optimum use of sheet steel or plywood which comes in 1800mm (6 foot)length sheets. To determine the required length of the turbine a design analysis using this vane size is made.

THEORETICAL ANALYSIS

The theory of Ishida and Service (4) can be used as a guide to determine the length and operating conditions of the turbine. A summary of this theory is given in the form of a computer program in appendix A. The theory is based upon open channel flow conditions where the linear turbine completely blocks the flow. This is the ideal case but will not be practical in most situations due to river size, river navigation and seasonal changes in river flow.

For open channel flow the energy available to the turbine is dependent upon the difference between the flow energy upstream and the minimum energy required to maintain the volume flow rate. This can be expressed as (5):

where q is the flow per unit width of upstream flow and g is the gravitational constant.

Fig.2 Theoretical power output and loads for

a 0.8m/sec 600mm deep upstream flow.

X- force on vanes transverse to flow

Y- force on vanes in direction of flow

Fig.3 Relative inlet and outlet angles for a vane speed of 0.9m/sec.

The energy available is therefore:

For a fixed vane depth (h), equation (2)shows that the energy available is a function of inlet velocity (C). This function has been calculated by the program in appendix A and is plotted in Fig.1. It shows that the maximum energy available for 600mm inlet flow is 0.96 Kw/meter at a flow velocity of 0.8 m/sec.

At higher flow velocities and hence volume flow rates the energy available decreases down to zero as the upstream flow conditions become critical.

For 600mm deep vanes therefore, the optimum flow will be 0.8m/sec. Hence this value is used in the computer program (appendix A) to determine the theoretical operating conditions. A plot of the results is given in Fig.2 which shows a maximum power output of 0.89 Kw/meter occurring at a vane speed of 0.9m/sec. The flow inlet and outlet angles for this vane speed are given in Fig.3. and this shows that to achieve the theoretical power output of 0.89 Kw/m requires a stage 1 flow deflection of 11 deg.(59.4—48.4) and a stage 2 flow deflection of 39.2 deg. (38.8+0.4). Just how close these conditions can be approached will depend upon the design of the vanes, vane spacing and actual flow conditions.

DESIGN

According to the theory, for an ideal flow of 0.8m/sec with 600mm deep vanes, 0.89Kw power output can be obtained per meter of span. For an initial design analysis therefore a 2 meter wide turbine with 600mm deep vanes will be considered with the hope of obtaining 1Kw power output.

One arrangement using ropes/cables to support the vanes is shown in Fig. 4 and full working drawings for one possible design using chain to support the vanes are given in appendix C.

The vanes can be either curved or symmetrical. However, by choosing symmetrical vanes a balanced vane system can be used eliminating the need to set the vane angles (see balanced vanes)

To determine the optimum design the various options need to be considered in the light of the design parameters.

Vane Support and Drive

The type of vane support depends upon locally available materials and tools. Possible systems are rope, cable or chain.

Rope/cable system. With the rope or cable vane support system shown in Fig.4, simple wheels (i.e. bicycle wheel rims) can be used, making a cheap, lightweight design. The main problem with this system however is that since the top and bottom loop lengths cannot be made identical due to stretch, the top and bottom wheels must be able to rotate independent of each other so that the vanes remain vertical as they move across the flow. Thus either a method of bracing is needed at various intervals along the cascade as in Fig. 4 (a), or the vane attachment to the cable must provide a resistance to skewing as shown in Fig. 4(b).

With either method however the drive from the top and bottom wheels must be independent, necessitating either 2 separate power outputs, or a single power take—off through a differential system. Due to these problems it is felt that a chain system will be more satisfactory.