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Conference Paper th 8 HVACR EXPO & Conference February 23 – 25, 2001 EPB Expo Centre, Karachi Cogeneration System for Industrial Airconditioning with Gas Engine Waste Heat Recovery. AUTHOR: Ainul abedin P.E. Fellow, ASHRAE Principal Ainul Abedin Consulting Engineers Karachi. ABSTRACT: Many industries in Pakistan, specially in the textile sector, are faced with the dilemma of meeting the world quality standards of production and yet not able to afford the associated high energy costs. High quality production invariably requires controlled indoor environment and large cooling, and some times heating loads, result in high operating costs. Many industries in Pakistan have installed gas engine-generators for electric power demand and separate gas fired boilers for steam and hot water requirements, since generally neither our industries have adopted correctly designed cogeneration power plants for most economic energy needs nor the government gas policies have encouraged such installations for real-term benefits to the industry and to the country. However, cogeneration system with existing low-efficiency gas engine generators offers an opportunity for economical central chilled water system using absorption chillers based on engine waste heat recovery. Gas engine provides waste heat recovery in both engine exhaust and jacket water circuits. High temperature exhaust allows use of economical 1-stage absorption chiller with low cost and highest possible coefficient of performance (COP). Further chiller capacity can be added with low temperature hot water from engine jacket water system. Though the associated absorption chiller efficiency is much lower then the earlier high temperature hot water chiller (requiring larger heat transfer surfaces), the absorption machine is still economical due to waste heat utilization. Cogeneration systems cover simultaneous production of electrical or mechanical energy (power) and useful thermal energy from a single energy stream such as natural gas, oil, solar, etc. thus, utilizing waste heat which would otherwise be rejected to the environment. Cogeneration power plants can operate at very high efficiencies, when compared to those achieved when power and heat are produced in conventional separate processes. Such high thermal efficiencies correspond to enormous fuel savings and result in very attractive returns on additional investment. Industries requiring process steam and hot water, specially in textile sector, are ideally suited for cogeneration systems since normally natural gas used for steam and hot water production in conventional boilers would supply adequate fuel energy for cogeneration power plant meeting all the steam and hot water requirement and producing “free” electric power for the industry. For example, an industry needing 18 tonnes/ hour steam at 6 bars would require about 60,000 SCFT/Hr gas supply with 80% boiler efficiency. Same 60,000 SCFT/Hr gas, when used in optimum-designed cogeneration power plan can produce full 18 Tonnes/Hr of required steam and also generate about 2,500 kw of electric power free of any fuel cost. This would normally translate to 2-3 years pay-back period, depending on actual load factors. However, ground realities in Pakistan are very different. Due to various factors, the most prominent being lack of proper energy policy at the highest level, our industries have very seldom adopted long-term (4-5 years) basis for investment and have chosen the path of investing in a “visible corridor” of 1-2 years, not knowing what the guiding policies would be after that! The results are obvious. Industrial power generation, normally, is based on reciprocating engines and for steam/hot water production, conventional boilers are installed. Such installations have been openly encouraged by the Gas Cos., justifying the natural gas allocation to export-oriented units, not realizing that such exports with high-cost energy input would not be able to compete in the free-trade world environment of the near future. However, to make best use of available equipment for which the country has invested heavily in foreign exchange, cogeneration opportunities can be created to utilize existing low-efficiency gas engine generators (less than 1/3rd. efficient) for production of thermal energy, either for hot water utilization in the process or for central airconditioning of process areas with absorption chillers for high-quality standards of production. Such cogeneration application would allow waste heat utilization of gas engine generators resulting in high thermal efficiencies of this process; though overall thermal efficiency of the industry would still be very low if major steam requirements were to be met by conventional boilers. Gas engine generators provide opportunities for waste heat recovery in both engine exhaust and jacket water circuits for possible use in absorption chillers for industrial airconditioning systems. Due to high temperature exhaust, normally 500 deg C (932 deg F) to 550 deg C (1022 deg F) range, waste heat recovery is also at higher temperature and both 1 bar g (14.5 psig steam) or 121 deg C (250 deg F) hot water can be produced for efficient waste heat utilization. This steam or high temp hot water is ideally suited for low-cost single-stage absorption chiller with best possible efficiency, coefficient of performance (COP) of about 0.67. Higher efficiency means lesser heat transfer surfaces and thus smaller machine size and associated lower capital cost per cooling ton basis. Jacket water, normally available at 90 deg C (194 deg F) to 95 deg C (203 deg F), without the concepts of ebullient cooling which entails very good, controlled water treatment facilities and highly specialized maintenance & monitoring of the engine, is also a good source of hot water for 1-stage absorption chiller; though with lower efficiency due to lower hot water temp requiring larger heat transfer surfaces (upto 40 % extra). Even though hot water availability is on “free energy” basis, higher heat transfer surface areas result in higher capital costs per cooling ton basis. The engine-generator is normally selected to provide prime power for the required electrical load and thus the relationship to waste heat availability and cogeneration system efficiency is fixed accordingly. A critical factor in economic feasibility is the ability of the system to use maximum available waste heat. Typical distribution of input fuel energy under selective control of the thermal demand for an engine operating at rated load is as follows: Engine Heat Rate 10,700 BTU/kWH (shaft power about 32%) Available heat recovery in jacket water 3,210 BTU/kWH (total waste heat 30%, all available for recovery) Available heat recovery in engine exhaust 1,920 BTU/kWH (total waste heat 30%, upto 60% available for recovery) heat rejected by convection / radiation (8%, not available for recovery) thus, typically thermal – to – electric ratio of 5,130 BTU/kWH can be achieved, which gives 80% cogeneration efficiency under peak load conditions. The above typical heat balance for recip. gas engine-generators show that where higher- grade heat can be used with greater efficiency (as for absorption chillers with higher efficiency, requiring 40 % less surface areas than the same size absorption chiller with lower hot water temp.), the first option of waste heat utilization should be of engine exhaust. For industrial airconditioning applications, maximum capacity should be obtained from available engine exhaust waste heat recovery boiler with high hot water temp absorption chiller and the balance capacity can then be provided, depending on cooling loads, by lower temp. hot water absorption chiller for lowest overall capital cost. To give another example, an application with 2500 kW gas engine generators running at rated load would provide about 4.8 MM BTU/HR engine exhaust recovery with 121 deg C (250 deg F) hot water, enabling peak chiller cooling capacity of about 268 tons in the higher efficiency mode. Additional jacket water heat capacity of 8 MM BTU/HR with 92 deg C / 198 deg F hot water, with lower efficiency, could provide additional absorption chiller capacity of about 310 tons, but with much higher cost per ton of chiller capacity, upto about 40% extra costs due to larger size of chillers to meet the requirement of higher heat transfer surfaces. Engineering design for such cogeneration systems would specially cover the following: 1. Due to pressurized hot water system for 121 deg C (250 deg F) chiller operation, hydronics design must ensure minimum required standing system pressure at all times to avoid flashing of hot water to steam. 2. Detailed study of load profiles should be conducted to check waste heat availability under part – load conditions. 3. Absorption chillers will lose capacity if design hot water temperatures are not achieved. There will be greater loss of capacity for chiller with lower design hot water temperature and it might be a good practice to maintain both high temp. & lower temp. hot water circuits at same operating pressure so that required quantities of high temp. hot water may be bled to lower temp. hot water circuit, if required, with water make-up from the lower temp hot water circuit for system balancing. 4. Absorption chiller of both high temp. and lower temp. hot water circuits should be of similar specifications for flexibility in operation and maintenance so that either chiller could be operated for one or the other circuit, though with different available capacities. Reference: American Society of Heating, Refrigerating & Airconditioning Engineers, ASHRAE, Handbook 2000, Chapter 7 on Cogeneration.
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