1.Energy Saving Technologies in generation, conversion of electrical energy

Ion V. Ion, associate professor, mechanical engineering: “Dunarea de Jos” University of Galati, Faculty of Mechanical Engineering, Thermal Systems and Environmental Engineering Department, 111, Domneasca St., off. G102, Galati, 800201, Romania, tel. +40 740566214, e-mail: ion.ion@ugal.ro

1.1.Cogeneration

1.1.1.Introduction

According to the Directive 2004/8/EC of the European Parliament and of the Council of February 11, 2004 on the promotion of cogeneration based on a useful heat demand in the internal energy market and amending Directive 92/42/EEC cogeneration means the simultaneous generation in one process of thermal energy and electrical and/or mechanical energy [1]. In literature the following definitions are often used:

Cogeneration is the combined production of electrical (or mechanical) and useful thermal energy from the same primary energy source [2];

Cogeneration is the sequential production of thermal and electric energy from a single fuel source [3];

Cogeneration is on-site generation and utilisation of energy in different forms simultaneously by utilising fuel energy at optimum efficiency in a cost-effective and environmentally responsible way [4].

The mechanical energy produced by cogeneration can be used to drive auxiliary equipment as well. The thermal energy can be used either for heating or for cooling. Cooling can be obtained by thermally driven chillers (usually adsorption or absorption chillers).

In a conventional thermal power plant, large amount of heat (50-70%) is wasted with exhaust gases and cooling agent. A large portion of the waste heat can be recovered and used by combining the electrical generation and heat production processes, increasing in this way the overall efficiency to (80-90)%. This combination of the electrical generation and heat production processes represents the combined heat and power (CHP) generation or cogeneration concept.

In order to see the overall fuel utilization efficiency, a comparison of individual generation of heat and electricity and cogeneration is shown in Fig. 1.

The main advantages of cogeneration systems are the following:

• improve energy efficiency at national level leading to conservation of fossil energy resources;

• enable locally generation of electricity and reduce the heat losses;

• enable the use of different fuels;

• can be used in remote areas;

• reduce the environmental impact due to higher efficiency of fuel conversion.

The main disadvantages of cogeneration systems are:

• have high investment and operation costs;

• require utilisation of the generated heat in the case the generated electricity is fully utilised;

• require back-up system in order to ensure supply security of electricity and heat, increasing the investment cost.

a) b)

Figure 1. Comparison between individual generation of electricity and heat (a) and cogeneration (b).

The main components of a cogeneration system are (Fig. 2):

• a prime mover;

• an electrical generator;

• a heat recovery exchanger;

• operating control systems.

The prime mover is a thermal engine (Rankine, Brayton, Diesel, Otto, Stirling) or a combination of thermal engines which converts chemical energy of fuel into mechanical energy transmitted to electrical generator. A special system, which converts fuel chemical energy directly into electricity, is the system that uses fuel cell as prime mover. The heat recover maybe a heat exchanger or a network of heat exchangers which transfers the heat from exhaust gases or engine cooling agent to the heating agent or to water (domestic hot water).

Figure 2. Typical cogeneration system.