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AIR CONDITIONING – ENERGY CONSUMPTION AND ENVIRONMENTAL QUALITY – Co-Generation - I.P.Koronakis

CO-GENERATION I.P.Koronakis Department of Building Applications, Center for Renewable Energy Sources, Athens, Greece Keywords: Steam turbine systems, gas turbine systems, thermal process, electricity production. Contents 1. General aspects 2. The present and the future of co-generation 3. Modern co-generation techniques 4. Operation modes of co-generation systems 5. Applications of co-generation 6. Advantages and disadvantages of co-generation Glossary Bibliography Biographical Sketch To cite this chapter Summary Co-generation is defined as the combined generation of electric (or mechanical) and thermal energy from the same initial energy source. The conventional way of meeting power and heat loads of one or more consumer(s) is purchasing power from the national network and burning fuel (in a boiler or furnace) to generate heat. However, the total fuel consumption is significantly reduced when “co-generation” or “combined heat and power” (CHP) is applied. Apart from the advantages of co-generation use that are given in this chapter, its disadvantages are also described while the wide application as well as the operational codes are analyzed in detail. 1. General Aspects Thermal energy may be used for heating as well as for cooling or air conditioning purposes. Cooling or air-conditioning is achieved by heat absorption machinery operating on steam or hot water. During the operation of a conventional thermal power station, large amounts of heat are rejected to the environment either through the cooling circuits (steam condensers, cooling towers, diesel engine radiators etc.) or through the exhaust gases (gas turbines, diesel engines, Otto engines etc.). The largest portion of this heat may be recovered and utilized. In particular, conventional power stations have an efficiency of 30-45% whereas co-generation systems have an efficiency of 80-85%.

©Encyclopedia of Life Support Systems (EOLSS)

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Co-generation was initially introduced in Europe and the USA around 1890. During the first decades of the 20th century, most industries had their own power generation units with a steam furnace-turbine, operating on coal. Many of those units were co-generation units. It is worth mentioning that a good 58% of power generated by various industries in the USA was actually generated by co-generation units. Later, a period of decline followed. Industrial co-generation dropped to 15% of the total power generation potential until 1950 and, after that, continued its descending course to as low as 5% in 1974. This course has now been reversed not only in the USA but also in Europe, Japan etc., mainly due to the abrupt rise of fuel prices since 1973, and the energy policy motives provided at a National level. The decrease in the need for co-generation plants may be attributed to two major reasons: a) The development of power transfer and distribution networks which provided

inexpensive and reliable power, and b) The availability of liquid fuels and natural gas which rendered furnace operation

feasible. 2. The Present and the Future of Co-generation On June 4, 1993, ministers from all twenty three member states in the International Energy Committee of the Organization for Economic Cooperation and Development (OECD) met in Paris. In an attempt to create the conditions in which the energy sectors would be able to contribute as much as possible to a financial development and prosperity of the nations, protecting the environment at the same time, they put the following common objectives:

1. diversity, efficiency and flexibility in the energy sector, these being the basic conditions for a long term energy safety

2. capability for a timely and flexible response in case of emergency energy needs 3. environmentally accepted (viable) disposal and usage of energy 4. encouragement and development of more environmentally acceptable energy

sources 5. improvement of energy efficiency, contributing to environmental protection and

energy safety in a more efficient manner 6. continuous research, development and marketing of new and improved energy

technologies 7. energy prices enabling market to function in a more efficient manner 8. free and open commerce and a safe framework for investments contributing to the

energy efficiency and safety 9. co-operation of all members of the energy market aiming to improve

communication and understanding and to encourage the development of efficient, more environmentally acceptable and flexible energy systems and markets.

It is worth noting that the spread of co-generation in the countries of Organization for Economic Cooperation and Development is not only compatible but also contributes to the achievement of all the above objectives. Therefore, the majority of the countries have


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taken measures to encourage co-generation, even if such measures differ from one country to another regarding the efficiency. The spread of co-generation in countries outside the Organization for Economic Cooperation and Development is also significant, especially in Central and Eastern Europe, but the prerequisites for energy and financial efficiency are not always ensured. 2.1 Contribution of Co-generation to the Power Generation and District Heating The average portion of power generated at co-generation plants in all 12 countries of the European Union in 1985 was 8.13% with tendency for a small increase in the recent years. This percentage is rather low and allows significant room for development. It is worth reporting certain particularly interesting cases in detail, because they present either high percentages of co-generated energy forms or exhibit acute growth rates. In Spain, the contribution of co-generation in power generation increased by 56% during 1987-1989 (from 2.0% to 3.12%). In the late 1994, when the ongoing works were completed, co-generated power reached 6.1%. In UK, in 1993, 700 co-generation units with a total capacity of 3,000MWe generated approx. 3-4% of the total public power (compared to 2% in 1990). In Denmark, in 1985, a good 40% of the heating needs of the country were covered by district heating. A 50% portion of the heat consumed in district heating was generated at co-generation plants. During 1990-1991, these figures increased to 63% and 60% respectively, while 29% of the total public power was generated at co-generation plants. In Germany during 1986-1987, 3.9% of the annually generated power was generated at co-generation plants leading to an annual saving of 2.5 billion liters of liquid fuel (IEA, 1988). In 1990, this figure increased to 9.5 %. During 1986-1990, the annual contribution of co-generation to district heating ranged between 67 and 75%. There are 500 district heating networks throughout the country, 80% of which belong to the power corporations. In Finland, 30% of the total power and 45% of the total heat are generated at co-generation plants. A good 71% of the heat consumed in district heating was generated at co-generation plants. In Helsinki, 90% of the buildings are heated by a district heating network. In Holland, co-generation contributes by 15% to the annual public power generation and by 90% in the heat consumed in district heating. In addition to other applications, the spread of co-generation in greenhouses is also significant. In the early 1990, the electric capacity of such systems reached 250MW. During 1990-1991, several systems with electric capacities of 200MW were installed. In Greece, unlike the above mentioned countries, co-generated power is a mere 2.5-3.0% and has been maintained constant during the last years.


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2.2 Co-generation Perspectives Nowadays, co-generation is considered as one of the most important techniques for achieving a more efficient usage of fuels, natural and financial resources savings and environmental protection. Many countries make efforts to overcome obstacles and to facilitate its spreading. The motivations implemented include the relatively high cost of purchase of surplus power from the power corporations as well as the subsidy of investments. Other measures include communication, energy recording and analyses, research and development support etc. Certain assessments for the status of co-generation in the year 2000 have already been made. Among new projects, large stations with capacities above 100MWe are included, many of which will be operating on natural gas. In certain countries, the installed power co-generation capacity has increased by more than 100% in the period between 1990 and 2000. The picture is completed by the following information known for certain countries:

• Austria: District heating has exhibited an annual growth rate of 9% since 1970. The amount of heat consumed in district heating is anticipated to increase by 56% before the year 2000. A good 65% of the loads of the new networks will be covered by co-generation.

• Denmark: It is anticipated that, by the end of this century, 50% of the thermal needs will be covered by district heating, to which co-generation will contribute at a percentage of 70% (IEA, 1988). The contribution of co-generation to district heating increased from 50% in 1985 to 60% in 1990; this means that the objective of an increase of 70% before the year 2000 is achievable.

• Switzerland: The spreading potential for small co-generation units (of medium capacities of the order of 150kW) is estimated to be around 100 units per year.

• Italy: This country exhibits the highest co-generation potential among all EU countries: 44% of the total potential.

• UK: The government aims both to install new units with capacities of 5000MW as well as to achieve a co-generated power percentage of 25% before the year 2000.

• Holland: Remarkable as it may sound, the power percentage generated at the installed co-generation systems with respect to the total power generated has been increased from 13.8% in 1990 to 17% in 1992. Furthermore, it is anticipated that it will reach 31.8% in the year 2000, i.e. a figure much higher than any other country. It is worth mentioning that the objective of 17% for 1995 was already achieved by the year of 1992.

• Finland: An increase of district heating is anticipated, with an annual growth rate of approx. 2%. For the year 2000, it is anticipated that 31.6% of the public power needs will be covered by co-generation systems.

According to assessments made for the industrial sector of EU countries, there is a possibility to install co-generation systems with capacities of 9,000-22,000MW during the next 10 years, corresponding to approx. 5% of the installed power generation systems in these countries.


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In the commercial-building sector, where typically there is no permanently employed technical personnel, the spreading of co-generation requires good technical support by collaborating groups for regular maintenance operations and repairs (or even prevention) of emergency unit break-downs. This problem seems to be successfully dealt with in UK and other countries through the following method. Specialized companies undertake the maintenance and repair operations of co-generation units, where they install instruments and devices for the continuous measurement of critical operational parameters. Locally installed microprocessors collect the measurement data, perform an initial data processing and then send the results to the central computer of the company via an exclusive telephone line. The central computer completes the analysis of the measurements, which points out oncoming faults. In such case, the engineering team is immediately informed on a 24-hour basis and the necessary maintenance and repair operations are performed, even before the outbreak of the fault. Consumer service is not interrupted during the performance of such operations, since the installation is connected to the regional public power network and the backup heat source in the building. Each company of this kind may support tens or hundreds of co-generation units. 3. Modern Co-generation Techniques Most co-generation systems may be labeled either as “topping systems” or as “bottoming systems”. In topping systems, a high-temperature fluid is used for power generation, while the low-temperature rejected heat is used in thermal processes, space heating or even in further power generation. In bottoming systems, high-temperature thermal energy is first generated (in steel mill furnaces, glass furnaces, cement plants etc.) and, then, hot gases are usually conveyed to a heat recovery furnace, producing steam which drives a steam turbine generator. Hot gases may also be conveyed to a steam turbine, which drives a power generator without any intermediate furnace. Figure 1 provides indicative temperature values for both system categories.


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Figure 1: Indicative temperature levels for co-generation systems 3.1 Steam Turbine Systems These are the most commonly used co-generation systems, suitable for capacities of 500 kW - 100 MW or higher. They may operate on any kind of fuel. Even solid wastes may be burned in special furnaces equipped with systems for retaining or neutralization of pollutants and toxic substances produced during combustion. Efficiencies achieved reach 60-85%. For comparison purposes, it is worth mentioning that the efficiency of a conventional steam power station is around 35%. Steam turbine systems feature high reliability [as reliability here is considered the possibility of satisfactory operation for a system for a specific time interval and under predetermined conditions.] which may reach 95%, high availability[availability is the probability for a satisfactory operation of the system at a random time. The mean annual availability is equal to the time percentage (e. g. 8760 hours per year) during which a system may operate satisfactorily (taking into account the preventive maintenance and the emergency faults] (90-95%) and high longevity (25-35 years). However, installation time is 12-18 months for small units and may increase to three years for larger systems. The three basic system arrangements in this category are described below in detail. 3.1.1 Co-generation Systems with Back Pressure Steam Turbine High pressure (20-100 bars) and high temperature (480-540°C) steam is produced in a boiler operating on fossil fuel. This steam is used to drive the steam turbine which is shaft-coupled with the power generator (Figure 2). Steam exits the turbine at a temperature and pressure suitable for thermal processes. The term “back pressure” is used


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to indicate that this pressure is higher than the atmospheric pressure (3-20 bars). The extraction of a portion of the steam from intermediate turbine stages at the desirable pressures is also possible. The back pressure system has the following advantages over the extraction system: – simple form – lower cost – reduced or even elimination of the need for cooling water – higher efficiency (approx. 85%), mainly because heat is not rejected to the

environment through a cooling component.

Figure 2: Co-generation system using a back pressure steam turbine However, an important disadvantage is that the produced electric power is closely related to the required heat. Thus: a) It is not possible to operate the steam power station separately from the heating

network, and b) a bi-directional connection to the national power network is required to cover the

additional needs or to transmit the potential excess of power. 3.12 Co-generation Systems with Extraction Steam Turbine A portion of the steam is extracted from one or more intermediate stage(s) of the turbine at the desirable pressures, while the remaining steam is depressurized down to the pressure in the condenser (Figure 3). The extraction systems are more expensive and exhibit a lower efficiency (approx. 80%) when compared to back pressure systems. However, they allow for independent (within certain limits) adjustment of electric and


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thermal power. This is achieved through the adjustment of the total steam flow rate and, consequently, of the steam supply rate to the condenser.

Figure 3: Co-generation system using an extraction steam turbine 3.2 Gas Turbine Systems There are two basic types of gas turbine systems, “open cycle” type and “closed cycle” type. 3.2.1 Open-cycle Gas Turbine Systems Most gas turbine systems are of the open type, where air is sucked from the atmosphere, compressed and then conveyed to the combustion chamber. The exhaust gases enter the gas turbine driving the power generator and then, exit at temperatures ranging from 300°C to 600°C. The power required to drive the compressor as well as the high temperature of the exiting exhaust gases are the main causes for the low efficiencies observed in such power generation systems (25%-35% and, in modern advanced units, 40%). The high temperature of the exhaust gases renders such units ideal for co-generation increasing the efficiencies achieved to 60%-80%. There are two basic ways to utilize the heat of the exhaust gases: – Through direct use in thermal processes (heating, drying etc.)


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– Leading the exhaust gases to a heat recovery furnace (also known as “stack gas furnace”) where high-quality steam is produced suitable not only for thermal processes but also for driving a steam turbine (coupled with a power generator or other equipment). In this latter case, the system is called “combined-cycle system” and is described later in detail. In both ways, it is possible to increase the heat content (i.e. the temperature) of the exhaust gases and, consequently, the heat delivered, if required. This is due to the high oxygen content of the exhaust gases. When burners are installed downstream the gas turbine, they may use the exhaust gases for additional fuel combustion. Co-generation systems with open-cycle gas turbine exhibit capacities ranging between 100 kW and 100 MW. They usually operate on natural gas or light oil distillates (e.g. diesel), while favorable perspectives are presented for the use of pitcoal in gasified form. They may also operate on gas fuels produced, for example, during catalytic cracking of hydrocarbons in oil refineries. Because of the exposure of the gas turbine blades to combustion products, attention should be paid so that these products would not contain corrosive components (sodium, potassium, calcium, vanadium, sulfur etc.), while solid particles should have a size small enough not to induce wear during impact with the blades. If exhaust gases contain such components, they should be cleaned by means of special arrangements before conveyed to the gas turbine. Cleaning the fuel before entering the combustion chamber may also be required. Installation of gas turbine co-generation systems lasts 9-14 months for capacities up to 7 MW and up to two years for larger units. The reliability and mean annual availability of gas turbine systems, which operate on natural gas, are comparable to those of the steam turbine systems. The units operating on liquid fuel require more frequent maintenance, consequently, featuring a lower availability. The effective service life is 15-20 years and may be reduced considerably when using a poor quality fuel or insufficient maintenance. 3.2.2 Closed-cycle Gas Turbine Systems In closed-cycle gas turbine systems, the working medium (typically, helium or air) circulates in a closed circuit. It is heated up in a heat exchanger to the proper temperature before entering the gas turbine and, after exiting the gas turbine, it is cooled. As the working fluid does not take part in the combustion, it stays clean and, thus, the mechanical and chemical corrosion of the gas turbine caused by the combustion products is avoided. With external combustion, any type of fuel may be used in these systems: coal, industrial or urban wastes, biomass, liquid or gas fuels produced from biomass etc. Nuclear or solar energy may also be used as a heat source. Such systems are currently in use in Europe and Japan, with capacities of 2-50 MW; however, their number is low. After gaining enough experience, the reliability of closed-cycle systems is anticipated to be at least equal to that of open-type systems, while their availability will be higher because of their lower maintenance requirements due to the cleanliness of the working fluid.


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3.3 Systems with Reciprocating Internal Combustion Engine They may be classified separated in three categories: a) small scale units with gas engine (15-1000 kW) or diesel engine (75-1000 kW); b) medium-capacity systems (1000-6000 kW) with gas engine or diesel engine, and c) high-capacity systems (over 6000 kW) with diesel engine. Gas engines are defined as the reciprocating internal combustion engines operating on gas fuel e.g. natural gas, biogas etc. The following types of gas engines are commercially available: – Gasoline automobile engines converted to gas engines. These are typically

low-capacity engines (15-30kW) of high power concentration. The conversion has a minimal effect over efficiency, while it reduces the capacity by approx. 18%. Due to their mass production, their prices are low and so is their service life (10000-30000 hours).

– Diesel automobile engines converted to gas engines. They are available with capacities up to 200kW. Conversion is achieved by modifications of the pistons, the head and the valve mechanism, which are mandatory because ignition is no longer carried out by simple compression but by means of spark plugs. Typically, such conversion does not reduce the engine capacity as there is for surplus air reduction.

– Fixed engines not converted to gas engines or originally designed as gas engines. These engines are heavy and robust. They are constructed for industrial and marine applications. Their capacities reach 3000kW. Their robust construction reduces maintenance requirements, but increases their cost of purchase. They are suitable for continuous operation under heavy load.

– Fixed double-fuel engines. These are diesel engines with capacities of max. 6000 kW. They operate on fuel with a content of 90% in natural gas, which is ignited not by means of spark plugs but through liquid diesel fuel injection (comprising the remaining 10% of the supplied energy). They are advantageous in that they may operate either on natural gas or on diesel fuel, which of course increases the cost of purchase and maintenance. Diesel engines may be categorized in “high speed”, “medium speed” and “low speed” types. All oil distillates are suitable fuels (the heavier ones are to be used on larger motors). The large low-speed motors may also operate even on oil distillation residuals.

3.4 Combined-cycle Systems The term “combined cycle” refers to systems with two thermodynamic cycles connected to each other via a working fluid and operating at different temperatures. The high temperature cycle (“topping”) rejects heats which is recovered and used by the low temperature cycle (bottoming) for additional electric or mechanical power generation, thus, increasing efficiency. The most commonly used combined-cycle systems use a gas/steam turbine combination (Joule/Rankine cycles). Figure 4 illustrates the basic elements of such systems. Steam


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production at two or three separate pressures renders the installation more complex, but increases efficiency. This method is implemented in large units. The high content of the gas turbine exhaust gases in oxygen (approx. 17%) allows the combustion of complementary fuel in the stack gas furnace, if such action is considered necessary in order to increase the capacity of the system. Such complementary combustion increases the efficiency of the system during its operation under partial load, but renders installations (particularly, regulation and control arrangements) more complex. Typically, the combined-cycle systems capacity varies between 20 and 400MW, whereas smaller units are also manufactured with capacities ranging between 4 and 11MW. Power concentration (power per unit volume) of such systems is higher than that of a single-cycle gas turbine (Joule type) or steam turbine (Rankine type). Installation lasts 2-3 years. Installation may be completed in two stages: first, the gas turbine unit is installed, which may be ready to operate within 12-18 months. While the above unit is in service, the system is completed with the steam turbine unit. The reliability of the combined-cycle systems is 80-85%, the mean annual availability is 77-85% and the economically feasible service life is 15-25 years.

Figure 4: Combined-cycle co-generation system with back-pressure steam turbine 3.5 Rankine Bottoming Cycles with Organic Fluids The capacity of these systems varies between 2kW and 10MW. Efficiency is low, around 10-30%, but the important fact is that such systems provide additional power with no extra fuel consumption. Installation of small systems (up to 50kW) and, particularly, of those suitable for use in the commercial-building sector, lasts 4-8 months, while larger


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units may take 1-2 years. As this technology is relatively new, there is no adequate information regarding the reliability of such systems. It is estimated that their mean annual availability is 80-90%. Their expected service life is approx. 20 years. 3.6 Standardized Co-generation Units (“Packages”) The mass production of standardized co-generation units in “package” form at power capacities ranging between 10 and 100MW is expected to greatly broaden the use of co-generation. These units have the following advantages: – low cost – small volume – easy installation (connection to the hydraulic and power networks is all that is

required) – automated operation without the need for continuous monitoring by specialized

personnel. These units usually comprise a diesel engine. For capacities lower than 100kW, an Otto engine may be used, whereas for capacities higher than 600kW a gas turbine may be used. These units may operate on liquid or gas fuel. Natural gas is a particularly suitable fuel for such units due to its cleanliness, no storage requirements and low price. The co-generation packages with diesel engines are particularly suitable for the applications in the commercial-building sector. They are also known as “small-scale co-generation systems”. As much as 27-35% of the fuel energy is converted to power and 50-55% to heat. As the spread of such units has only recently started to grow, there are not many published data regarding their reliability and availability. A related study conducted on 46 such units installed in California, USA showed an average availability of 79% with a standard deviation of 22.9%. The availability of units with careful construction and maintenance may reach 90%. Automatic control of unit operations significantly contributes to that figure. 3.7 Fuel Cells A fuel cell is an electrochemical device, which converts the chemical energy of fuel into electricity with no combustion. It operates as follows: hydrogen and oxygen react in the presence of an electrolyte and produce water, while an electrochemical potential is developed inducing an electric current flow in the external circuit (load). As the reaction is exothermic, heat is produced which may be utilized. The required quantity of hydrogen is produced from fossil fuels, typically from methane (CH4) which is the main component of natural gas. Some types of fuel cells may operate also on carbon monoxide or hydrocarbons. This technology is still at the stage of research and development and is not widely known yet, but shows very good perspectives for implementation in the co-generation sector. Among the various types of fuel cells, only the phosphoric acid cells have been developed to an extend that renders them already suitable for power generation plants. Demo units with capacities of 25kW − 11MW have been constructed in various European countries, USA and Japan. Their operation


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temperature (approx. 200°C) restricts the temperature of recovered heat. Today, there exist co-generation units with heat available at a temperature of 80-90°C. Fuel cells are suitable for co-generation applications in the industrial and commercial-building sector (especially in combination with natural gas). Their main advantages are: – modular structure, which facilitates the construction of units with the desirable

capacity; – maintenance of high efficiencies even under partial load (i.e. lower than the nominal

load); – easiness of automated control; – low pollutant emissions; – low noise level. Due to the high efficiencies achieved and the clean fuels used, CO2 and SO2 emissions are 10-100 times lower compared to other systems. In particular, since the temperatures being developed are significantly lower than those during combustion, NOx emissions are lower by one order of size compared to systems based on combustion. 3.8 Stirling Engines Power and heat co-generation is also feasible with Stirling engines. This technique is not yet fully developed and spread, but the industry showed concern for its development due to its advantages compared to systems with diesel engines and gas or steam turbines. There was potential for higher efficiency, greater fuel flexibility, good performance under partial load, low pollutant emissions, and low noise and vibration levels. In the beginning, automobile engines with capacities of 3-100kW were under the scope of R&D. Next, efforts were focused on engines with capacities of up to 1-1.5MW with expected service life of some 20 years. As the systems are in the development stage, no accumulated data exist regarding their availability and reliability; however, they are expected to be comparable to diesel engines. Thanks to external combustion and closed operation cycle, the moving parts of the engine are not exposed to the combustion products and, consequently, wear is limited. External combustion in Stirling engines allows the use of various fuels: liquid or gas fuels, coal, liquid or gas products of coal, fuel products of biomass. Furthermore, the fuel may be switched without having to stop the engine or modify its settings. Due to their flexibility, Stirling engines may be used in solar or nuclear power generation or co-generation plants. 4. Operation Modes of Co-generation Systems The most important operation modes of a co-generation system, i.e. the modes for setting power and heat capacities at any time, are as follows:

• Heat generation equal to the heat load (“heat match”).


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• If power is generated in excess of the load, the surplus power is sold to the national power network. If power is generated in short of the load, then the power deficit is supplemented by the network.

• Power production equal to the power load (“electricity match”). • An auxiliary furnace supplements the additional heat required, if necessary. Also,

coolers may be installed capable of rejecting the excess heat, if necessary. • Mixed mode. • Either monitoring the heat load [a) mode] or monitoring the power load [b)

mode]. • Full coverage of the heat and power load at any time with no connection to the

national network. This mode of operation requires enough backup capacity and, consequently, a complex co-generation system. This is the most expensive solution, at least regarding the initial cost. As a rule, subject to any exceptions, the first of the above mentioned modes provides the highest energy and economical efficiency for systems in the industrial and commercial-building sector. For co-generation plants of the public power system of the country, the operation mode to be selected depends upon the extensive needs of the network, the available units and the obligations towards the consumers of heat and power. 5. Applications of Co-generation Co-generation may be implemented in four main sectors:

1. public power system 2. industrial sector 3. commercial-building sector 4. agricultural sector.

Data for each of these sectors are presented in detail below. 5.1 Public Power System Power generation plants may be converted into co-generation stations in order to cover heat needs of cities or settlements, industries, water desalination plants, greenhouses, fisheries etc. located in their region. Ensuring proper distance and dispersion of heat consumers around the plant is essential for the feasibility of the overall installation. With district heating, in particular, apart from distance and dispersion, the annual number of degree-days and the required thermal power also play an important role. 5.2 Industrial Sector In the industrial sector, many processes require heat and power at the same time. Depending upon the temperature required the following classification applies:


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a) Low temperature processes (below 100°C) e.g. drying of agricultural products, heating or cooling of areas, utility hot water. b) Medium temperature processes (100-300°C) e.g. processes in paper industry, in spinning mills, in sugar industry, in some chemical industries etc. Typically, these processes require heat in the form of steam. c) High temperature processes (300-700°C) e.g. in some chemical industries. d) Very high temperature processes (above 700°C) e.g. in cement plants, metal industries, glass industries etc. The following industrial branches exhibit a significant co-generation potential: – food and beverages industries – spinning mills – paper industries – chemical industries – crude oil refineries – cement plants – basic metal industries (steel mills, aluminum production plants etc.). Glass industries, ceramic materials industries, wood processing industries etc. exhibit a smaller but perhaps not negligible potential. The majority of the industries with a significant co-generation potential include certain production processes, which generate or reject heat in satisfactory quantity and quality (i.e. temperature level). Recovery of this heat and its subsequent addition to the heat generated by the co-generation system is considered advisable. Certain chemical processes produce fuel gases which may be consumed at the furnaces or the co-generation system itself. 5.3 Commercial- Building Sector This sector includes hotels, hospitals, shopping malls, schools, office buildings, residences etc. Co-generation covers power and heating needs of the buildings (heating or cooling of areas, utility hot water, furnaces etc.) The commercial- building sector may be divided in three main sub-sectors: a) hospitals and hotels; b) multi-apartment complexes; and c) office buildings. Each one of the above sub-sectors features a special form of load curve. Other kind of buildings (e.g. universities, stores etc.) exhibit load curves, which are combinations of those of the three main subsectors. In such applications, co-generation serves not only heating and power needs, but also cooling needs. Load curves should be taken into account in the feasibility study as well as in the final design of the co-generation system.


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5.4 Agricultural Sector Co-generation is not widely spread in the agricultural sector; however, its implementation may lead to fuel savings and positive financial effect on rural communities. Residues produced by agricultural processes may be used as fuel. Ethanol produced from renewable biomass (e.g. grains, sugar cane etc.) may be used as a fuel suitable for Otto engines. The recovered heat may be used for drying of agricultural products, heating of residences and greenhouses etc. 6. Advantages and Disadvantages of Co-generation Through co-generation systems, efficiencies may be boosted to 70-80%. In contrast, power generation plants have an average efficiency of 31%, while combined-cycle plants new with gas turbines may achieve efficiencies of 40-50% (Figure 5). The inefficiency of our power generation infrastructure is due not to poor industry performance but to the lack of policies necessary to extensively implement co-generation.

Figure 5: Co-generation plant efficiency compared to conventional power plants. 6.1 Fuel Consumption Effects All co-generation systems save fuel because they exhibit higher efficiencies compared to separate power and heat generation. For example, using a co-generation system with steam turbines, fuel consumption is reduced by 15% (compared to the separate power and heat generation using a steam turbine unit and a boiler, respectively), whereas using a co-generation system with diesel engine, fuel consumption is reduced by 25% (compared to the separate power and heat generation using a diesel engine driven power generator and a boiler, respectively). The co-generation systems to be installed and the fuels they operate on should be selected in accordance with the national energy policies. 6.2 Public Electricity System Effects In order to deal with the future increase of demand for electric power in each country, the construction of new power generation plants is required. The spread of co-generation


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increases the potential for power generation and reduces the need of construction of new central plants, thus achieving capital savings for the public power company. Since co-generation systems are of smaller size and their installation lasts less than a large central plant, they provide a larger flexibility and adaptability to unexpected power demand variations in the future. Furthermore, the short installation time for the co-generation systems contributes to the reduction of the financial cost, which in turn contributes to the reduction of the unit cost of power generation. Many small co-generation units operating in parallel to the central power generation plants, increase the reliability of power supply, although they may cause stability problems of the network. These problems are limited or even eliminated when the co-generation system and its connection to the network conform to certain specifications. Consultation with the competent local Public Power Corporation services is required for that purpose. The spread of the co-generation could cause negative financial effects to the national power system, if the latter is capable of power generation exceeding the demand or if the rate of capacity rise by constructing new plants is higher than the rate of demand rise. In such cases, the capital cost is allocated to a smaller quantity of power generated, resulting in the increase of unit cost. Central stations are typically located outside the urban centers and the high stacks contribute to a satisfactory diffusion of the pollutants. On the contrary, small co-generation units featuring relatively shorter stacks are typically installed inside or near residential areas, polluting their environment. Among the available co-generation technologies, Diesel and Otto engines exhibit the highest emissions of pollutants. Due to their size, these engines are the most suitable for co-generation applications in the commercial-building sector, thus presenting a greater hazard with their emissions. Fuel cells are more suitable than Diesel or Otto engines for such kind of applications, because they exhibit significantly less emissions of pollutants, as already mentioned. Fuel handling as well as removal of solid combustion residuals may lead to soil and water pollution in the area. The noise produced during the operation of the co-generation system and during the traffic developed for its service, increases sound pollution. Consequently, before installing a co-generation system in a residential area, the following prerequisites should be met: – selection of a technology with low pollutants emission – careful selection of the installation site and – installation of equipment, which controls the reduction of emitted pollutants. 6.3 Environmental Effects Thanks to the efficient fuel usage, co-generation contributes to the direct reduction of the emitted pollutants. The reduction of fuel consumption is accompanied by a further


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indirect reduction of the pollutants produced during the remaining fuel cycle: mining, treatment, transportation, storage. Quantification of this cost is hard to achieve and depends upon various factors e.g. technology, fuel, local conditions etc. When many small and widely dispersed co-generation units replace large central plants with high stacks, then the following are necessary: – elastic seating and sound insulation of the system – construction of a stack higher than the adjacent buildings and – installation of systems for collecting and removing solid and liquid residuals. Glossary Co-generation: The combined generation of electric (or mechanical) and thermal

energy from the same energy source. Steam turbine: It is a form of heat engine in which two distinct changes of energy

take place. The available heat energy of the steam first is converted into kinetic energy by the expansion of the steam in a suitably shaped passage, from it issues as a jet.

Gas turbine: It is a turbine element in which the motive fluid is a heated permanent gas. A gas turbine power plant is usually comprised by one or more compressors, combustors (or heaters) and turbines.

Bibliography Haywood R.W. 1991. Analysis of Engineering Cycles. Fourth edition. Pergamon Press, Oxford, UK. [This book analyses a great variety of thermodynamic cycles used in engineering processes].

Marecki J. 1988. Combined Heat and Power. IEE Energy Series 3, Short Run Press Ltd., Peter Peregrinus Ltd., London, UK. [The central theme of this report is to give specific information on co-generation processes].

Culp A. 1991. Principles of Energy Conversion. Second Edition. McGraw Hill, USA.[The main aspect of this book is to analyze the energy conversion processes].

Fragopoulos C. 1988. Combined Heat and Power. Elkepa. Athens. Greece. [This is a student’s edition book about the main processes of co-generation systems]. Biographical Sketch Name Irene P. Koronakis Date of birth September 27, 1972 Place of birth Illinois,, USA Marital status Married, two sons Address 64 Argous st., 14564 N.Kifissia, Athens, GREECE Profession and current job position: Chartered Mechanical Engineer, Dr. Mechanical Engineer, Researcher of the Department of Building Applications in the Centre for Renewable Energy Sources, CRES Higher Education and Degrees: M.Sc in Energy Engineering,Mechanical Engineering Department, National Technical University of Athens, 1996 Ph.D in Building Applications, Mechanical Engineering Department, National Technical University of Athens, 2000


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Further Education: IKY Fellow in Greece for the PhD thesis (1997) IKY Fellow in Greece for the Post Doctorate thesis (2002) Languages: Greek, English. Understand French Memberships: • Technical Chamber of Greece (TEE)

• ASHRAE, Americal Society of Heating Refrigerating and Air Conditiong Engineers

Subject of Specialization: Energy studies: Renewable Energy Sources Academic Experience: Two years of Air Conditioning and Buildings’ Heat Transfer in the National Technical University of Athens, Mechanical Engineering Department Participation in Energy Projects:

• JOINED RESEARCH FUNDING PROGRAM ’92 • Design and construction of metalic roof panels in conjunction with solar absorption devices for

cooling domestic and industrial buildings. G.S.R.T 93, 61/639. • PENED 1994 • Development of analysis and determination of Thermal characteristics of existing buildings –

Applications, G.S.R.T 318 • JOULE II CEC, DG XII, JOR3-CT96-0034 OFFICE-Passive retrofitting of office buildings to

improve their energy performance and indoor working conditions, (1996-1998). • JOINED RESEARCH FUNDING PROGRAM ’96 Absorption Solar Cooling Engines,

case-studies G.S.R.T 96PS1-51, 61/1070. • THERMIE II, DIS-1283-97, European Commission, Directorate General for Science, Research

and Technology, Efficient Ventilation Systems for Buildings, 1998-1999. • JOULE II JOR3-CT95-0024, European Commission, Directorate General for Science, Research

and Technology. POLIS Project, Urban canyon experiments, (1997-1998). • TEMPUS Joint European Project, Contract 09015-95. • CRAFT-JOULE PROJECT, JOR3-CT98-7038, Improvement of Energy power of Solar Roof by

ventilation with linear static exhauster, (1998-2001). • THERMIE PROGRAMME, European Commission, Directorate General XVII. Energy,

BU/163/98/IT, Technologically Innovative Building with Energy Rational Use, 1999-2000. • SAVE, (NNE5-1999-00771), SSHORT : Sustainable Social Housing Refurbishment

Technologies, (2000-2003) • "Energy Design of the new Police Administration building of Akarnania” (TEGEA S.A.) –

Heating and Cooling Energy Analysis, Project Manager (1999) • ARGOS ENERGY SYSTEMS - "Examination of the thermal perfor-mance of the radiant barrier

THERMO-BRITE and its contribution to energy saving in buildings in Greece” – Heating and Cooling Energy Analysis Project Manager (1999)

• Ministry of Environment, Planning and Public Works - "Compilation of the new Regulation for Rational Use of Energy and Energy Saving in Buildings” – Scientific Committee (2000)

• "Energy Design of the new building of the Technical Museum of Thessaloniki” (T.M.Thes., ELLINOTECHNIKI S.A.) – Heating and Cooling Energy Analysis Project Manager (2000)

• "Bioclimatic Design and Energy Analysis of the new Archaeological Museum of Ancient Helida” (ATOMON S.A.) – Heating and Cooling Energy Analysis Project Manager (2001)

• "Energy analysis of the new Chemical Department Building at the University of Ioannina” (Competition with Doxiadis S.A.) – Heating and Cooling Energy Analysis Project Manager (2001)

• "Bioclimatic Design of the new Archaeological Museum of Akropolis” (Competition with D+L Potiropoulos – 2nd prize) – Heating and Cooling Energy Analysis Project Manager (2001)

• "Energy Analysis the new National Airport of Thessaloniki-“Makedonia” (Competition with Doxiadis S.A.) – Heating and Cooling Energy Analysis Project Manager (2002)

Energy Studies presented in international conferences: Nine such works Energy Studies presented in national conferences: Seven such works Publications: Eight papers regarding buildings’ heat transfer Author of: • One published book on Fluid Mechanics, theory and problem solving.

• Notes on numerous renewable energy courses taught at seminars for higher education graduates


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To cite this chapter I.P.Koronakis, (2004), CO-GENERATION, in Air Conditioning - Energy Consumption and Environmental Quality., [Ed. Matheos Santamouris], in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK, [http://www.eolss.net]


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