Small-Scale Cogneration - CADDET 1
Author Info	Centre for the Analysis and Dissemination of Demonstrated Energy Technologies
Details	CADDET Analyses Series 1, Revised November, 1995 1. Introduction Cogeneration, or combined heat and power (CHP), Systems provide electricity and heat or, in some cases, shaft power and heat from a single unit. Overall efficiencies of 85% are normally achieved giving primary fuel savings of 35% compared to separate generation. This directly reduces the users fuel costs and benefits the environment through reduced combustion of fossil fuels. They are suitable for a wide range of applications in both buildings and industry, provided there is a substantial base-load demand for heat. Such applications include hospitals, education centres, sports facilities, hotels, office buildings, residential buildings, various industries, waste water treatment and district heating. Cogeneration units can be designed in a range of sizes. This Analysis Report has focused on units in the 30-1000 kWe range and its purpose is to provide an understanding of the different types of small-scale system available and to give examples of practical applications. 2. Types of System Small-scale cogeneration systems can be classified according to the type of engine or prime mover that they use. • Reciprocating engine systems are the most commonly applied in the 30-1000 kWe size range. They are widely available as compact, fully packaged, skid-mounted units that are easy to install. Furthermore, each unit can readily be tailored to the customer's specific requirements and is assembled and tested by the supplier, prior to installation. Reciprocating engine systems usually use turbo-charged, intercooled industrial engines. These are commonly derived from standard diesel engine blocks and are normally fitted with spark ignition systems. The main fuel used is natural gas, although diesel, LPG, propane and biogas can also be used. Most engines operate at 1500 rpm, and turbo-charging boosts power output by about 40%. The electrical generation efficiency achieved is, typically, 35-40%. Up to 90% of the engine's waste heat is recovered in the form of hot water or low pressure steam from the engine's jacket cooling water and lubrication oil systems and from the exhaust gases. This is achieved using a series of shell and tube heat exchangers and, where there is a demand for low-temperature water, condensing heat exchangers. • Gas turbines are widely used in cogeneration projects larger than 3-4 MWe where there is a demand for high pressure steam. Systems are also available in the 600-1000 kWe range, but the electrical efficiency achieved at this scale is reduced from 30% to around 25%. Despite this. overall efficiency is 80-90% with high grade heat recovery which can be used for medium and high pressure steam and for direct heating or drying applications. • Steam turbines may he appropriate where a site has steam surplus to demand. There are several alternatives to the conventional use of cogeneration systems for generating electricity and heat. For instance. instead of producing electricity, the shaft power can be used to drive compressors for air-conditioning chillers and industrial refrigeration plant or to supply compressed air. The recovered heat, instead of being used for water heating, can be used either for cooling purposes, via an absorption chiller, or for heating air for space- or dryer heating. It is also possible to combine a cogeneration systern with a heat pump to utilise a low temperature heat source in a highly efficient system. 3. System Performance Cogeneration system efficiencies vary with the type uf prime mover. Both reciprocating engines and gas turbines achieve overall efficiencies of 80-90% although, at the scale of operation considered in this report, reciprocating engines achieve the higher electrical efficiency (35-40% compared with 20-25% for gas turbines). Back pressure steam turbine systems achieve a lower electrical efficiency (7-20%) and a lower overall efficiency of 75-85%). Many of these figures were confirmed by equipment suppliers. Overall design efficiencies were in the 85-90% range and showed little variation with system size. Equivalent electrical efficiencies ranged from 28-39% but with a clear improvement with system size. Overall efficiencies of this order are equivalent to primary fuel savings of around 35% when compared with the generation of equivalent amounts of power and heat by conventional means. To be viable, cogeneration systems must operate almost continuously for extended periods of time. In buildings applications a typical minimum utilisation level is 4500 hours/year. In industrial applications utilisation is normally much higher. Unscheduled stoppages result in increased maintenance costs, the need to purchase energy at unfavourable tariffs and the inconvenience associated with switching supply systems and arranging for a service engineer to investigate and correct the fault. Systems must therefore achieve high levels of availability and reliability. Data provided by equipment suppliers indicate that reciprocating engines normally achieve availabilities of 90-95%, while the equivalent figure for gas turbine systems is more than 95%. A number of factors contribute to the unscheduled outages that reduce system availability. They include over-complex systems or controls, poor design or manufacturing quality, operator error or inadequate operator training, poor maintenance scheduling, insufficient protection against extreme conditions, and exceeding the service life of the unit. Servicing and maintenance is frequently contracted to the equipment supplier and remote condition monitoring is installed to aid maintenance scheduling and fault diagnosis. 4. Environmental Considerations Because cogeneration systems burn fossil fuel, they give rise to various products that are damaging to the environment. Noise can also be a source of concern. The most important product of the combustion process is carbon dioxide, well known for its contribution to the greenhouse effect and climatic change. However, where cogeneration replaces the separate fossil fuel generation of electricity and heat, it reduces primary fuel consumption by about 35%. This means a similar reduction in CO2 emissions. Emissions of sulphur dioxide vary directly with the sulphur content of the fuel. In the case of natural gas this is negligible, and condensing heat exchangers can be used to maximise heat recovery wherever appropriate. Diesel fuel and biogas, however, do contain sulphur and, where the sulphur content exceeds the limit set by the manufacturer, some form of fuel cleaning is necessary prior to use. Furthermore, the cost of installing a stainless steel heat exchanger and exhaust flue to counter the corrosive nature of the condensate usually precludes the use of condensing heat recovery systems with these fuels. Oxides of nitrogen (NOx) are produced by burning any fuel in air. The level of NOx emissions, however, is dependent on combustion conditions and particularly on temperature, pressure, combustion chamber geometry and the air/fuel mixture. However, modern cogeneration engines typically achieve emissions levels of 140g/GJ or less. Many of these engines are of lean-burn design incorporating air/fuel ratio control. Others are stoichiometric engines with a three-way catalytic converter to remove NOx, carbon monoxide and unburnt hydrocarbons. The NOx content of exhaust gases can also be reduced using selective catalytic reduction techniques based on ammonia or urea. Carbon monoxide. unburnt hydrocarbons and particulates are rarely a problem unless air/fuel ratios and combustion conditions are inadequately controlled. The use of acoustic enclosures can easily reduce noise levels from their normal value of around 100 dB(a) at I m to 65-75 dB(a), well below the typical legislative limit of 85 dB(a). 5. System Design Any decision to implement cogeneration must be based on adequate knowledge of the site's electrical and thermal load profiles, both throughout the week and seasonally. With this information it is possible to select the best type of prime mover and determine an appropriate system size. Maximum system efficiency is achieved when a unit is operating at full load, and systems that are oversized will either have to run for long periods at part load or operate for a reduced number of hours per year. The correct sizing of a cogeneration system involves identifying the potential for energy conservation and energy efficiency projects and assessing likely changes in the site's future energy demand. The response to any remaining uncertainty in the level of demand should be to undersize units and ensure high utilisation levels. Units should normally be sized at around 50% of the site's maximum thermal demand, with the additional heat demand being met by conventional boilers. It may also be appropriate to incorporate an element of thermal storage into the design or to install more than one cogeneration unit. There are several configurations for connecting the cogeneration system to the existing heating and electrical systems. Most systems are connected in series with existing heating systems. although parallel connection may be selected for new installations, particularly when cogeneration supplies a large proportion of the heat load. Electrical connection is normally to the low-voltage system, although units rated at more than 500 kWe and with a high load factor may be linked directly to the high voltage system and export power back to the grid during periods of low on-site demand. Most cogeneration systems use synchronous generators. These, although connected to the public electricity supply, operate independently of the grid and can therefore continue to supply power in the event of grid failure. 6. Economics and Financing Investment costs vary with the type of prime mover, the degree of sophistication of the automatic monitoring and control system, the need for additional pollution abatement equipment or acoustic protection, and the costs of site preparation, grid connection etc. ln practice, the installed costs of cogeneration systems vary widely, with equipment suppliers quoting from USD 1300/kWe for the smallest gas engine systems to less than USD 800/kWe for units larger than 500 kWe. Systems fueled on digester gas systems may cost as much as USD l700/kWe, while gas turbine systems were found to cost USD I 600~2000/kWe Maintenance may be carried out by site staff but is often the subject of a contract between the host company and the equipment supplier. Costs quoted by equipment suppliers were mainly in the USD 0.008~0.0l3/kWhe range for reciprocating engines and USD 0.003-0.030/kWhe for one of the reciprocating engine unit. The simple payback achieved by the demonstration projects varied from 1.6 to 10 years. with an average of 4.5 years. Although rather long for the tight investment criteria currently being set by industry, such paybacks might be more acceptable within the building services environment. There are three main financing options. • Capital purchase this traditional option involves the host company raising the necessary capital and incurring all the associated risk. However, the host also benefits from all of the savings achieved. • Equipment supplier leasing - the cogeneration unit is installed and owned by the equipment supplier. The host agrees to purchase heat and/or electricity at a discounted rate for a fixed contract period. thereby incurring less risk but also receiving a smaller proportion of the benefits. The host may choose to purchase the unit after a few years of operation. •Contract energy management - a specialist contract energy management company owns, operates and maintains the cogeneration unit on the host's premises. The host purchases services at a discount price and, at the end of the contract period, the plant usually passes into the host's ownership. 7. Demonstration Projects The Report has identified and analysed thirty demonstration projects from seven countries. These projects have been selected to illustrate important technical features and the breadth of possible application. Within the buildings examples are hospitals, hotels, an office building, a college, leisure centres, apartment blocks and district heating. Industrial projects are drawn from the manufacturing and food industries, a brewery, wool processing plant and sewage treatment facilities. Data from all these projects has been analysed to indicate levels of cost and performance that are practically achieved, whilst seventeen projects have been described in detail.
Other Info	129 pp., 1995, $45.00, cad1, ISBN 90-72647-22-X
Publication Price	$ 45.00 each
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