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Cogeneration techniques: economically viable alternative.

A cogeneration unit provides continuous power and thermal energy in the face of shortages of generating capacity or other power outages. Small modular-type units, which simultaneously produce electricity and usable energy from the same fuel, can be used in all industries.

The energy demand varies with equipment size, usage, geographic position and weather. Experience all over the world has shown that the use of cogeneration techniques by the industry could be an economically viable alternative to relying on mains electricity with diesel generators as emergency backup power.

A cogeneration unit provides continuous power and thermal energy in the face of shortages of generating capacity or other power outages. Small modular-type units, which simultaneously produce electricity and usable energy from the same fuel, can be used in all industries.

In most of applications there is a steady electrical base load demand, plus an economic use for thermal energy produced. However, the electrical demand may not always be at the same time as the thermal energy load, and this will mean careful analysis in determining the modular unit size required. Only in-depth look at the specific circumstances as evaluation of the current and projected costs, work load fuel costs plus return on investment and pay back expected will show if a project is viable. National fuel policy and a utility's stance on cogeneration techniques must also be taken into account. Most commercial users do not require self-generating capacity of more than 1500 kwh. So packaged modular units or pre-engineered systems in the range of 100 kwh to 1500 kwh are, ideal for smaller end-users where the main supply is unreliable or expensive and where the thermal energy also produced can be put at a good use. A major attraction of the units in the "free" thermal energy they provide is that it can be used as low pressure steam, and with the use of a vapour compressor as high temperature steam. Air-conditioning can be produced by absorption type chillers. Because reciprocating engines run more efficiently at constant speed and the efficiency of diesel engines in particular falls off quite rapidly at slow speed, cogeneration modules are generally sized to provide the base electricity load. A specific use for the surplus thermal energy generated is, therefore, essential to optimise results.

A typical 100 kwh unit can use synchronous or induction type generators. The hot water from the engine cooling system can be used as hot water or flashed-off as low pressure steam with specially designed ebullient cooling systems. Engines for these systems are specially produced for cogeneration packages. Some of the exhaust gas heat can also be recovered by heat exchangers or exhaust gas boilers although the cost of this option may not always justify the investment.

Most modules are complete in themselves and only need to be hooked up to the existing power supply, water and fuel lines. Units with acoustical housings can usually be sited on a concrete pad adjacent to the existing system. This would simplify installation and help keep down costs. Packaged systems are generally limited in weight and sizeto allow for factory assembly and testing, easy shipment and immediate site installation. If the thermal energy produced is not to be continuously used, then forced air cooling radiators will be required on-line.

An analysis of existing costs and operating conditions must be made and then compared with projected costs of cogeneration operation and maintenance. A feasibility study should project the future cost of electricity from existing sources for the next ten years. it should compare this with the projected cost of self-generated energy plus the projected cost of any additional purchase. It should then take into account the capital cost of the unit, maintenance and finance charges.

If the savings after using the cogenerated thermal energy are substantial and those from the power generation are small, there may be a case for sizing the module larger than the electrical base load. Conversely, optimum savings could be achieved by sizing a unit smaller than the base load. In other situations, the optimum economy and efficiency may be achieved with the installation of two modules which would provide two levels of base load.

In this case two units provide considerable flexibility in optimum power and energy output. This is usually regarded as being more efficient than running of diesel engines at half output speed where specific fuel consumption does not fall prorata. Routine maintenance can be performed in the unit when it is off-line, thus allowing a substantially greater portion of the load to be met during mains failure.
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Title Annotation:Pakistan's enery requirements
Author:Ghanghro, Amir Ahmed
Publication:Economic Review
Date:Apr 1, 1992
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