West / White / Loughridge Alternative Energy Systems

Electrical Integration and Utilisation
1. Auflage 2013
ISBN: 978-1-4831-5401-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 300 Seiten

Alternative Energy Systems: Electrical Integration and Utilisation covers the proceedings of the conference held at the Coventry (Lanchester) Polytechnic Coventry, England from 10th to 12th of September 1984. The book presents 24 papers that tackle alternative energy systems and their electrical integration and utilization. The text covers different alternative energy sources, such as hydro, solar, wind, wave, and waste materials. The book discusses the practical, technical, and economic aspects of alternatives energy systems and their electrical integration and utilization. The book will be of great use to individuals who are interested in the application of alternative energy systems.

Autoren/Hrsg.

West, Mike

White, Peter

Loughridge, Brian

Leseproben

Energy from Low Head Water Sources

A.M. Peatfield, L.J. Duckers, F.P. Lockett, B.W. Loughridge, P.R.S. White and M.J. West,     Coventry (Lanchester) Polytechnic, Coventry, England

ABSTRACT

The high cost of electrical power from fossil fuels in the developed countries, the lack of indigenous fuels in the developing countries, the world wide abundance of hydro sites with potential capacities in the region of 10 kW and recent UK legislation ensuring reasonable ‘buy-back’ prices for surplus electricity from private generation, has led to a resurgence of interest in micro hydro sites. The Authors describe a modular low head hydro device capable of efficient electrical power generation from available resource heads of as little as 1 metre. The device consists of a chamber which, by use of suitable valve action on the water flow, acts as a water-to-air gearbox and results in air being alternately drawn into and expelled from the chamber. A Wells turbine located in the air ducting is directly coupled to an electrical generator, the type of which depends on user requirements. An analysis of a unit utilising a 10 kW resource with a head of 1 m is presented, as are indications of how production costs may be minimised by using locally available skills and materials.

KEYWORDS

Low head

micro hydro

electrical power generation

Wells turbine

hydroelectric system

INTRODUCTION

The energy from fast flowing or falling water has been used for centuries to provide power for particular applications and more recently for the generation of electricity. Over the years of this century the abundance of cheap power available from oil or coal has caused many of the small dam and run-of-the-river sites to be abandoned as uneconomic or rejected in favour of larger centralised systems. However in recent years the escalating costs of electricity generation has brought about a resurgence of interest in the redevelopment of abandoned sites and the development of new sites.

REVIEW

Many of the developing countries with little indigenous supply of hydrocarbon fuel are unable to afford expensive imports and are rapidly exploring and developing their hydro-power potential. 40% of the electricity used by developing countries is produced by hydro-power. (Flood, 1983). While major multi-megawatt schemes would appear initially to be the most economically attractive, in many cases the huge capital costs and long lead times of the construction work, together with the lack of a suitable distribution system make impossible demands on the overstretched budgets of developing countries. Often, therefore, small schemes serving limited areas and requiring construction effort within the capability of local communities are the ones which stand most chance of success. China, for example, makes extensive use of small hydro, deriving more than 7 GW from nearly 100,000 micro hydro sites.

Even in developed western countries recent legislative policies of ensuring reasonable ‘buy-back’ prices for surplus electricity from private generation, sometimes combined with encouraging tax advantages, has brought the start of a tremendous new growth in the exploitation of small hydro power.

Salford, (1980), has surveyed 565 hydro-power sites in Wales each with potential capacities greater than 25 kW. Of these, 78 have heads of less than 3 m. The number of possible sites of less than 25 kW in Wales is therefore expected to be very large. Indeed if we consider the sites of old mills (typically producing 5 hp of mechanical power, about 4 kW), it is estimated that there were as many as 20,000 in England alone during the 18th century. Rainfall in England and Wales is typically 500–2000 mm per year. In Table 1, we have estimated the percentage of land area of each continent receiving various levels of rainfall and it is interesting to note that a considerable portion of land surface area is subjected to rainfalls in the range 500 – 2000 mm per year.

TABLE 1

World Rainfall

From an extrapolation of the U.K. situation, the World-wide potential for low head hydro power is likely to be vast, provided that the installations can be cost effective.

Turbine technology is well developed and many advances are being made in reducing the cost of construction and in the use of electronics to aid efficient and near automatic control of generation. However, even though in many cases the required construction works would be of a relatively simple nature, the development of the vast number of sites with available heads of less than about 3 metres is usually considered uneconomic and impractical due to the large size and slow rotational speeds of conventional water turbines operating under such small heads.

This paper outlines the design of a modular water-air system capable of operating efficiently with heads as low as 1 m. The characteristics of a typical operational cycle are evaluated and some possible electrical generation equipment and integration strategies are considered for a number of end use situations.

PRINCIPLE OF OPERATION

In its simplest form the system consists of an enclosed chamber into which the available water flow can be controlled by the operation of inlet and outlet valves in such a way that the effective driving pressures for both the filling and the emptying cycles can be a large proportion of the head available at the site chosen.

At the top of the enclosed chamber, above the maximum height of the upstream water level, is an inlet-outlet air duct leading through an air turbine to atmosphere (see Figure 1.). In low head operations the Wells turbine would be well suited to provide a power take-off unit with a high rotational speed suitable for electrical generation, with its ability to operate in reversing flow without the use of rectifying valves being a major advantage.

Fig. 1 Schematic diagram of chamber showing water in/air out phase

The operational cycle commences with the opening of the water inlet valve, allowing water into the empty chamber with the outlet valve closed, thus filling the chamber with water and driving the air out under pressure through the rotating Wells turbine. At a suitable point near the end of the filling cycle the inlet valve is closed and the outlet valve is then opened allowing exit of the water to the downstream side of the water retaining structure. This emptying process causes air to be sucked back into the chamber through the still rotating Wells turbine which is thus used to extract energy at the optimum rate during both parts of the cycle. Then at a suitable point near the bottom of the emptying cycle the outlet valve is closed and the whole cycle recommences with the re-opening of the inlet valve.

CYCLE CONTROL

There are many possible methods of controlling the cycle of operation of the device. The essential requirement is to operate the ‘in’ and ‘out’ water control valves in appropriate sequence and phase. This may be met by either an externally powered system (probably electrically based) or by an internal system deriving its energy from the changing water levels within the chamber.

These systems could be designed to respond to a variety of sensors, for example water height, flow, pressure, air flow, turbine speed etc. For the externally powered system an option might be time control or a combination of sensed signals. Initially, however, we propose a simple, passive, internal system, moving the valves by the forces on floats within the enclosed water. The phasing of the valve operation is determined by triggering floats fitted at preset water levels. The features sought are those of low cost, simplicity, and minimal, easy maintenance.

ANALYSIS OF AN OPERATING CYCLE

Consider a system with an air chamber of uniform cross-sectional area A in which inlet and outlet stroke extremes are symmetric about the head midpoint level. Suppose we wish to exploit a resource with mean water flowrate qw and head h, so the mean water power is qwpgh watts. Ignoring hydraulic losses, the pressure in the air chamber at any instant during the cycle will be proportional to the difference in water levels between reservoir and chamber. Assuming the air is incompressible, the water flowrate into (or out of) the chamber is equal to the air flowrate, which is controlled by the damping rate of the turbine. If the turbine is of the Wells type, run at or near a constant speed, the air flowrate and pressure drop are at all times proportional, q = ?p say and hence

where × is the difference between the instantaneous chamber level and the midpoint level.

Let qo be the air flowrate at the midpoint level of each half cycle, then

Integrating the flow equation then gives

and a is the ratio of chamber stroke to water head h. k(a) is approximately equal to 1 for small a and decreases, tending to 0 as a tends to 1. Also

mean air (or water) power = qwpgh = midpoint airpower.k (a)

and full cycle period T = 2A.stroke/qo.k(a) =...