Conference Paper by liwenting


									                             Conference Paper
                            8 HVACR EXPO &
                           February 23 – 25, 2001
                          EPB Expo Centre, Karachi

        Cogeneration System for Industrial Airconditioning
             with Gas Engine Waste Heat Recovery.

                                                                     Ainul abedin P.E.
                                                                     Fellow, ASHRAE
                                                     Ainul Abedin Consulting Engineers


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

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

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

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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