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Microturbines for dispersed generation

S. Barsali, M. Ceraolo, R. Giglioli, P. Pelacchi

University of Pisa - Department of Electric Systems and Automation Tel: +39-050 565305 – Fax: -39-050 565333 – e-mail: ceraolo@dsea.unipi.it

Keywords: Power Systems, Microturbines, Dispersed Generation, Cogeneration.

Abstract

The use of turbines having electric powers below 1 MW, or microturbines, as dispersed generation sources is analysed, with particular attention to the combined heat/electricity generation.

Some technical details of these devices are discussed, along with possible schemes for using them when they are either the only source of electricity or in parallel with an external network.

In some case-studies the economic feasibility of the proposed systems is analysed.

Some experimental results obtained on a 45 kW turbine are also shown.

Introduction

Changes have been recently made in a number of elec- tric power systems all over the world to increase the op- portunity for competition. Also in Italy an electricity mar- ket liberalisation process is in progress: new opportunities rising from this situation will have an important role espe- cially for the generation system; in fact the idea of con- tinuing to build expensive central power plants that take years to build, require kilometres of distribution wires, and take decades of pay off has been virtually abandoned. In this framework small dispersed generation could represent a competitive solution especially when it is associated with thermal energy production (cogeneration). It is the case, for example, of density populated areas where electricity de- mand is associated with heat demand both for winter heat- ing and for summer conditioning.

The small turbine generators, or microturbines, seem to be a good solution; in fact they are quieter and cleaner than the bigger turbines, and relatively free of maintenance. Moreover, they can burn a variety of fuels including natural gas, diesel, gasoline and methane. They are very simple and compact because the gas turbine is directly connected with an high speed turbo generator (about 100.000 rpm). Low emissions and relatively low investments costs are base characteristics of this class of generators.

In order to determine the real economic convenience of these small generation plants, it is very important to define adequate management structures because they could strongly influence the total production costs. A centralised management structure using new signal networks can be adopted.

Small cogeneration plants made by microturbines

Principle scheme of a cogeneration turbine

Very often industrial or tertiary energy customers have both thermal and electrical needs. If their electrical peak load is in the range 0.1-1MW and the ratio thermal/electric powers is in the range 2-6, thermal and electrical loads can be fed by a systems based on cogeneration microturbines as indicated in fig. 1.

The scheme shows that the turbine has two inputs re- lating to the power generation (air and fuel, often natural gas), and tow kinds of control variables. The latter are:

•a variable controlling the position of the bypass-flow

modulation valve. This variable is of great importance, since it allows to tune the microturbine behaviour to the ratio of load thermal and electric powers. Just to give an idea, for the turbine [6], a ratio from 2 to 5 can be ob- tained, depending on the valve position;

•variables controlling the forced-commutated inverter. From the load terminals point of view, the inverter can be seen as an three-phase bipole constituted by an e.m.f. in series with an internal impedance. The e.m.f. can be controlled in amplitude and phase, and the control vari- ables can be assumed as proportional to these two scalar quantities. In alternative, the two control variables can be assumed as proportional to active and reactive load powers.

The block called electric load in the figure is the user’s load, either an industrial or tertiary user. In the following paragraphs some example of use of microturbines to feed some specific load are given. In general, this electrical load can be fed:

• by one or more cogeneration microturbines;

•in addition, possibly, the supply from the utility (indi-

cated in the figure as electricity distributor).

It is to be explicitly noted that the possibility of what is often called islanded supply (i.e., without connection to the electricity distributor) is included in the scheme, and, in fact, appears feasible, as discussed in the following. On the other hand, the use of single turbines or clusters of turbines is also possible, although in case of islanded operation the

use of single turbines is often not acceptable.

This solution appears potentially competitive in com-

parison with feeding the thermal load with a pure-thermal boiler and the electric loads totally from the utility because of two main reasons. The first one is related to the recent improvements in turbine efficiencies, that have lead to mi- croturbines having electric efficiencies of up to 30%. The second is that it not only takes advantage from the com-

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