Principle

A fluid must be superheated above its saturation temperature for nucleate boiling to occur. The amount of liquid superheat sustainable is influenced by the presence (or absence) of nucleation sites.

Object

To measure the superheat required to boil water for containers having different surface characteristics.

Background

At one atmosphere pressure, the saturation temperature of water is 100 °C. When a pan of water is heated to 100 °C on an electric heating element, you will observe columns of steam bubbles rising from the base surface of the container. This is nucleate boiling. However, the water in the thin thermal boundary layer near the heating surface is superheated to a temperature greater than 100 °C. Thus, its temperature is greater than the 100 °C saturation temperature.

This experiment demonstrates the degree to which water may be superheated, and the effect of “nucleation sites” in the container wall on the amount of superheating that occurs.

A microwave oven is used for the experiment, because the microwave supplies approximately uniform heat per unit volume to the water.

Apparatus

One litre of degassed water

A clear glass beaker

A microwave oven

A thermocouple to measure the water temperature

A handful of clean sand, or other insoluble granular material

Procedure

The water is first “degassed” by vigorously boiling it in a pan, using a surface heating unit.

The optimum configuration and operating strategy for a wind-diesel system is highly dependent upon the peculiarities of the wind regime and load pattern at the host location. Thus it is highly desirable before deciding upon a specific system to obtain an insight into performance by using a mathematical model.

In this chapter various methods are outlined whereby wind-diesel systems can be simulated and the aim and state of development of each approach is given. Originally it had been the intention of the authors to identify the most accurate and cost effective models to emerge from the International Energy Agency programme on Decentralised Applications of Wind Energy. This has not proven to be possible due to both the specific nature of most of the current models being developed and to the difficulty and cost of obtaining a comprehensive data set with which to validate them. Full details have therefore only been included on the simpler approaches. To guide future researchers an overview is given of the IEA model validation exercise, and recommended common reporting formats are suggested for future intercomparison of the models.

The purpose of developing models of wind-diesel systems is primarily to fulfil the need for cost effective planning and design tools. Depending upon the level of decision being taken on the planning of the system, different types and complexity of model are required. There are two main approaches to modelling: Time Series and Statistical.

The following persons assisted greatly in the creation of this book, either by participating in the parent co-operative technical programme, or by providing text for the various chapters. Without their help and that of their organisations, this book would not have been possible.

1. Dr J. Walmsley, Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4, CANADA

2. Mr R. Rangi, Energy, Mines and Resources, 580 Booth Street, Ottawa, KlA 0EY, CANAD

3. Mr M. A. Lodge, Island Technologies Inc, Charlottetown, Prince Edward Island, CIA 719, CANADA

4. Mr P. Lundsager, Test Station for Windturbines, RISø National Laboratory, PO Box 49, DK 4000, Roskilde, DENMARK

5. Mr F. Van Hulle, Mr J. Pierik, ECN, PO Box 1, 1755 ZG Petten, NETHERLANDS

6. Dr S. J. Reid, Ministry of Transport, New Zealand Meteorological Service, PO Box 722, Wellington 1, NEW ZEALAND

7. Dr ø. Skarstein, Mr K. Uhlen, Mr T. Toftevaag, Norwegian Electric Power Research Institute (EFI), Sem Saelandsvei No 11, N-7034, Trondheim, NORWAY

8. Mr F. Martin, Instituto de Energias Renovables/CIEMAT, Avda Complutense 22, DP 28040, Madrid, SPAIN

9. Mr G. Svensson, Swedish State Power Board, Dept UP, S-162 87, Vallingby, SWEDEN

10. Dr O. Carlson, Department of Electrical Machines and Power Electronics, Chalmers University of Technology, S-41296, Göteborg, SWEDEN

11. Mr R. Horbaty, Oekozentrum Langenbruck, Schwengistrasse 12, CH-4438, Langenbruck, SWITZERLAND

12. […]

In this chapter we will look at a few real wind-diesel schemes. Three systems have been chosen for examination.

The first is an experimental/demonstration system which has been developed in Norway. The system is a very good example of a self-contained, ‘go-anywhere’ package. The storage device is coupled to the system electrically. The system is now commercially available.

The second scheme is entirely different from most architectures considered in the book insofar as it relies upon consumer load management principles. It is a real commercial, working system and has been installed on the Scottish island of Foula by Windharvester Ltd of the UK. A comprehensive description is given to illustrate to the reader the level of sophistication and complexity which may be required to make best use of cheap renewable energy inputs.

The third system is the wind-diesel research facility at the UK Rutherford Appleton Laboratory. It is a good example of a laboratory scale experimental facility. The scheme was the first to use flywheel energy storage coupled mechanically to the diesel gen-set.

CASE STUDY 1 - THE FROEYA WIND-DIESEL DEMONSTRATION

Design aims and background

Although Norway has abundant, cheap hydro power, interest in wind-diesel systems is growing, particularly since they could form attractive alternatives for island communities currently served either by decentralised diesels, or by ageing submarine cables.

In recognition of this, EB-Energy has developed a commercial wind-diesel package, partly funded by the Norwegian Water Resources and Energy Administration (NVE).

REMOTE POWER

Historically, until the advent of electricity, all power systems were decentralised, ie power was produced at the location where it was required. In the case of wind power, milling, pumping, and irrigation were popular applications. However, electricity brought about the development of grid networks with centralised generating capacity, and the demise of many decentralised power systems.

Today, we tend to forget that there are still many locations in the world which do not have an electrical connection to a central utility network. Furthermore, in many places, due to remoteness and cost, it is unlikely that a main grid connection will ever be established. However the need for power still exists. Power systems which can generate and supply electricity to such remote locations are variously termed ‘remote, decentralised, autonomous, or stand-alone’. The purpose of this book is to show that in many locations wind power can usefully be incorporated into, or form the basis of, such systems.

Broadly speaking, there are three types of application for remote electrical power, these being:

• Power for specialised applications in remote areas, eg, communications, irrigation.

• Power to remote communities in industrialised countries, and on islands.

• Community power generation in developing countries.

INTRODUCTION

The main purpose of this chapter is to provide advice on choosing a decentralised wind energy site on the basis of an assessment of its wind resource alone. (Other factors are considered elsewhere in this book.) A secondary purpose is to provide guidance on estimating the wind resource. The end product, which may apply for various time periods (e.g., month, season, year), may be in one or more of the following forms:

1. a Mean wind speed.

2. b Wind speed as a function of wind direction.

3. c Wind speed frequency distribution (a function of wind speed class).

4. d Joint wind speed frequency distribution (function of direction and class).

5. e Weibull parameters of wind speed frequency distribution.

6. f Weibull parameters as a function of wind direction.

7. g Wind speed time series (sampled typically at 1 Hz) for periods of hours to days.

Items a to f may be derived by a combination of modelling and measurement whilst item g can only be obtained by measurement. Wind speed information may be converted to a similar description of available wind power which can then be used to select a suitable Wind Turbine Generator (WTG) or evaluate the economics of one that has been previously selected, say to fit the needs of the community, as discussed in Chapter 2.

An attempt will be made to take account of factors peculiar to such sites (e.g., limited funding and local turbulence effects). The emphasis, therefore, will be on small WTG's.

Wind-diesel systems by virtue of their capital, operational, and maintenance costs are most often considered for use in remote, isolated and poorly accessible regions of the world where power from conventional large or national electricity grids is not available. In most instances where wind-diesel electrical generation systems are contemplated there are few human, technical, and physical resources available to design, build, install, operate, and maintain the system. When designing the system and planning its subsequent installation, operation, and maintenance, it is therefore vital to take due regard of the characteristics of the available resources.

Some of the principal matters or issues that must be considered and resolved by potential installers and operators are outlined in the sections that follow. The subjects have been organized more or less in the order in which events will proceed during realisation of a project. Because of the very wide potential range of project sizes, configurations and site conditions, variable weighting, based on experience and solicited advice of experts, should be given by the developer to each of the items.

THE IMPORTANCE OF VERIFICATION TESTS

Developers and purchasers of wind-diesel systems for installation in remote and isolated locations must have a high level of confidence that the system will have high reliability and perform as claimed or specified.

To assess the viability of a proposed decentralised wind energy system, it is essential to obtain knowledge both about the present and anticipated pattern of power and energy use, and also about the meteorological regime and other environmental and community aspects that may be specific to the area and need to be considered.

ASSESSING THE CONSUMER LOAD DEMAND

What data are needed?

Consumer demand can best be described on two timescales - long-term (up to 1 year) and short-term (less than 1 minute). Both can have a major impact on the design, viability and operation of decentralised wind-diesel systems.

The long-term data will show if the demand changes on a daily, weekly or seasonal basis. This is most important as it is vital to match the available wind energy to the consumer demand requirements. A simple comparison of annual totals will fail to identify this. Often it may be possible to delay particular consumer demands (for example heating and refrigeration loads) by a few hours to obtain a better match between supply and demand. Sometimes there may be a correlation between available wind energy and electricity use. This is particularly true where there is a high heating component to the demand. Long-term data are also important when deciding on the relative ratings of the components of the wind-diesel system. Ideally, the information described in Table 2.1 should be obtained when assessing a site.

In Chapter 1 a description was given of what constitutes a typical wind-diesel system, and sample configurations were outlined. The purpose of this chapter is to expand upon this basic information by describing in detail the design constraints and considerations which apply to a wind-diesel system and to its various components.

To be effective and economically viable, wind-diesel systems must be optimised for each individual application. In particular both the characteristics of the host wind regime dealt with in Chapter 3 and the consumer load (Chapter 2) must be considered since both affect performance and method of operation.

SYSTEM OPERATION

Two major methods of system operation are possible, these involving running the diesel either continuously or intermittently. There are advantages and disadvantages to both methods.

Continuous diesel operation

This primary case has the advantage of technical simplicity and reliability since there is little difficulty in maintaining the continuity of supply, although care must be taken not to overload other generator sets in multiple diesel systems when some of the units are switched off. In general the operating diesel set(s), together with a dump load where required, maintain system voltage and frequency. The main limitations of this approach are low utilisation of wind energy, and correspondingly moderate diesel fuel savings. The former is especially noticeable for systems with a large wind component.

It is poor part-load performance of diesel engines, especially of smaller sets, which limits potential fuel savings.

The International Energy Agency (IEA) was formed as a result of the 1973 oil crisis to provide a means whereby member countries could co-ordinate their energy policies to ensure long term mutual safeguards to continuity of supply.

Wind has been recognised for some time as one of the most promising ‘renewable’ sources of energy, and considerable development and commercialisation of the technology has taken place in recent years. There is in addition growing concern about carbon dioxide emissions and the detrimental effects which result from burning fossil fuels on our planet.

The IEA has been active in encouraging interaction within wind technology, and has sponsored two major programmes, one (IEA LS WECS) has supported development of large machines, whilst the other (IEA R&D WECS) has encouraged more generic research. As part of this latter programme, a project was set in motion in 1985 to support research into the problems of integrating wind into small decentralised systems.

In June 1985, a group of experts, gathering under the auspices of the IEA, met at the National Engineering Laboratory, UK, to discuss their common interest in the development of wind-diesel technology.