Increasing electrical power output and fuel efficiency of gas engines in integrated energy system by absorption chiller scavenge air cooling on the base of monitoring data treatment

An advanced scavenge air cooling system for reciprocating gas engines of integrated energy system for combined electricity, heat and refrigeration generation has been developed. New method of deep scavenge air cooling and stabilizing its temperature at increased ambient air temperatures and three-circuit scavenge air cooling system with absorption lithium-bromide chiller and wet-type cooling tower was proposed. Such cooling method does not require essential constructive changes in the existing scavenge air cooling system but only an addition heat exchanger for chilling scavenge air cooling water of scavenge air low-temperature intercooler closed contour by absorption chiller. A chilled water from absorption chiller is used as a coolant. To evaluate the effect of gas engine scavenge air deeper cooling compared with its typical radiator cooling, data on the dependence of fuel consumption and power output of gas engine on ambient air temperature at the inlet of the radiator are analized. The efficiency of engine scavenge air deep cooling at increased ambient air temperatures was estimated by reducing the gas fuel consumption compared with radiator cooling.


Introduction
Reciprocating gas engines (RGE) found a widespread application in integrated energy systems (IES) or so called trigeneration systems for combined electricity, heat and refrigeration generation. They are manufactured as cogeneration modules equipped with heat exchangers for producing hot water used also as a heat source for absorption chiller [1][2][3].
The specific fuel consumption of gas engines increases with arising turbocharger (TC) suction air and scavenge gas-air mixture temperatures at the inlet of engine cylinders. The cooling problem arises especially actually for scavenge air, which heat is rejected traditionally to surroundings by radiator (cooling tower of dry type) through intermediate water cooling contour of engine low-temperature intercooler. The efficiency of such heat rejecting system drops sharp with increasing ambient air temperature above 25…30 °С, when it becomes impossible to cool scavenge air to appropriate temperature of about 40 °С. If scavenge air temperature exceeds the maximum temperature restriction of 50 °С the engine efficiency drops sharp. For protection of engine from negative impact of high scavenge air temperature on the thermal conditions in the combustion cylinders the scavenge air temperature of about 40 °С is maintained automatically by reducing gas supply to engine and, accordingly, engine load.
At high ambient air temperature the exceeding scavenge air temperature causes operation of gas engine on increasing specific gas fuel consumption compared with rated load mode. So, for maintenance of gas engine on rated load or close modes it is necessary more decrease in temperature of cooling water in intermediate cooling contour of low-temperature intercooler. For this purpose a chilled water received in absorption chiller might be used.
To evaluate the effect of gas engine scavenge air deeper cooling compared with its typical radiator cooling, data on the dependence of fuel consumption and power output of gas engine on ambient air temperature at the inlet of the radiator are to be analized. A method for treatment of the monitoring data on fuel consumption and power output of gas engine was presented in [4].
The goal of the research is improving the fuel efficiency of IES on the base of reciprocating gas engines (RGE) while operation at increased ambient air temperatures through using an absorption lithiumbromide chiller (ABCh) for deeper scavenge air cooling in comparison with radiator cooling.

Investigating the efficiency of typical gas engine scavenge air cooling system
The efficiency of cooling gas engine scavenge air is investigated for IES of combined energy supply at the factory "Sandora"-"PepsiCо Ukraine" (Nikolaev, Ukraine). The integrated energy system is equipped with 2 cogeneration Jenbacher gas engines JMS 420 GS-N.LC (rated electric power P eISO = 1400 kW, heat power P h =1500 kW) and absorption lithium-bromide chiller, designed and assembled by "Sinapse"-"GE Energy" (Kiev) and "Khladotechnika" (Nikolaev). The heat of engine scavenge air is rejected to the environment by radiator as cooling tower of dry type.
The absorption chiller utilizes the heat of engine exhaust gas, high temperature scavenge air, engine jacket and lubricant oil cooling water to produce a chilled water with temperature of about 7 °С for technology process cooling and conditioning of air in engine room by central conditioners.
The scheme of conventional integrated energy system of factory "Sandora"-"PepsiCо Ukraine" is presented in Fig. 1. The engine room intake air is cooled in the central conditioner by chilled water from absorption lithium-bromide chiller and the engine turbochargers take air from engine room.  Fig. 1. The scheme of integrated energy system (trigeneration system) of factory "Sandora"-"PepsiCо Ukraine": GE -gas engine; OC -oil cooler; JC -jacket cooler; ExhB -exhaust gas boiler; IC LT and IC HT -low-and high-temperature intercoolers of charged gas-air mixture; F -intake air filter; AC -intake air cooler; ACh -absorption chiller; P -pump.
Intercooler of scavenge air consists of two stages: high-temperature stage (IC HT ), in which cooling of scavenge air-gas mixture is followed by heating water for absorption chiller, and low-temperature stage (IC LT ), from which the heat is rejected to the environment by radiator-cooling tower of dry type.
Some results of monitoring daily variation of volume gas supply B е and electric power output P е of gas engine JMS 420 GS-N.LC automatically changed to maintain a charged gas-air mixture temperature at the level of about 40 °С are presented in Fig. 2 [5].
In summer hot day time interval τ = 9…20 h the ambient air temperatures are rather high: t amb = 30…35 °С, that makes impossible reliable cooling of scavenge gas-air mixture by radiator to appropriate temperature rate of 40 °С. This results in automatically reducing gas supply to engine followed by decreasing load to maintain gas-air mixture temperature at the level of 40 °С (Fig. 2).
A performance of gas engine on part load modes (τ = 9…20 h) is followed by increase in specific gas consumption b е by (20…30)•10 -3 m 3 /(kW•h) (Fig. 3  A reduction of time duration of engine performance at part load modes (τ = 9…21 h, Fig. 3) and decrease of specific gas consumption b е is possible by addition decreasing temperature of scavenge air cooling water by absorption chiller before it entering the engine lowtemperature intercooler IC LT to maintain a scavenge gasair mixture temperature at the appropriate level of 40 °С with variation of ambient temperatures t amb . 3 Investigating the efficiency of improved gas engine scavenge air cooling system A reduction of time duration of engine performance at part load modes (τ = 9…21 h, Fig. 3) and decrease of specific gas consumption b е is possible by addition decreasing temperature of scavenge air cooling water by absorption chiller before it entering the engine lowtemperature intercooler IC LT to maintain a scavenge gasair mixture temperature at the appropriate level of 40 °С with variation of ambient temperatures t amb .
A principal scheme of improved three-circuit scavenge air cooling system with absorption lithiumbromide chiller and wet-type cooling tower is presented in Fig. 4. Such cooling system does not require constructive changes in the scavenge air channel of gas engine and needs only an addition heat exhanger for heat transfer between the circuits connected to the closed water circuit of a traditional scavenge air cooler.
Thus chilled water with temperature t c = 7 ºС is directed from absorption chiller to addition plate water cooler, inserted into the current closed water cooling contour, to cool a cooling water at the inlet of the engine low-temperature intercooler IC LT (Fig. 4).   Fig. 4. The scheme of modified trigeneration system with addition cooling of cooling water for low-temperature intercooler of scavenge gas-air mixture: GE -gas engine; OCoil cooler; JC -jacket cooler; ExhB -exhaust gas boiler; IC LT and IC HT -low-and high-temperature intercoolers of scavenge gas-air mixture; F -intake air filter; ACh -absorption chiller; P -pump; Heat Exh -heat exchanger.
The results of monitoring of JMS 420 GS-N.L engine fuel efficiency were presented in the form of data sets on dependence of fuel consumption B e = f(t amb ) and engine power output P e =f(t amb ) upon the ambient air temperature t amb at the entrance to the radiator of scavenge air cooling for the various air temperatures t in at the inlet of the turbocharger. The goal of monitoring data sets P e = f(t amb ) and B e = f(t amb ) treatment is to calculate the magnitude of the change in power ∆P e and fuel consumption ∆B e , caused by the change in the ambient air temperature t amb at the entrance to the radiator of scavenge air cooling by 1 °С, that is ∆P e /∆t amb and ∆B e /∆t amb , to evaluate the effect of the application of developed scavenge air cooling system.
The results of treatment of data for variation of electrical power output in absolute values P е and relative values P е /P еISO compared to ISO value with changing ambient air temperature t amb at the inlet of radiator for scavenge air cooling and also power output drops ∆P е /∆t /P еISO for 1 °С increment of ambient air temperature ∆t amb = 1 °С at the inlet of radiator for scavenge air cooling at various air temperatures at the inlet of turbochanger t in = 20…30 °С for Jenbacher gas engine JMS 420 GS-N.LC at about rated electrical power output P е = 1250…1400 kW (P еISO = 1400 kW) and at part loads P е = 850…1000 kW are presented below in Fig.5-8.  . 7. Increase of electrical power output drop ∆P е /∆t for 1 °С increment of ambient air temperature ∆t amb = 1 °С at the inlet of radiator for scavenge air cooling versus increasing ambient air temperature t amb at the inlet of radiator at various air temperatures at the inlet of turbochanger t in = 20…30 °С for Jenbacher gas engine JMS 420 GS-N.LC at about rated electrical power output P е = 1250…1400 kW (a) and at part loads P е = 850…1000 kW (b). a) b) Fig. 8. Increase of electrical power output drop for 1 °С increment ∆t of ambient air temperature ∆t amb = 1 °С at the inlet of radiator for scavenge air cooling in relative values ∆P е /∆t /P еISO compared to ISO value versus increasing ambient air temperature t amb at the inlet of radiator at various air temperatures at the inlet of turbochanger t in = 20…30 °С for Jenbacher gas engine JMS 420 GS-N.LC at about rated electrical power output P е = 1250…1400 kW (a) and at part loads P е = 850…1000 kW (b).
The results of treatment of monitoring data on volume gas fuel consumption in absolute values B е and relative values B е /B еISO compared to ISO value and also increase in volume gas fuel consumption B е /∆t and increase in engine specific volume gas fuel consumption ∆b е /∆t for 1 °С increment ∆t of ambient air temperature ∆t = 1 °С at the inlet of radiator for scavenge air cooling with changing ambient air temperature t amb at the inlet of radiator are presented in Fig. 9- Thus, from Fig. 12,a, at operation of JMS 420 GS-N.L at about rated power (1200…1400 kW) and high ambient air temperatures t amb = 30...35°С at the inlet of radiator for scavenge air cooling and at the TC intake air temperatures t in = 25...30 °С the engine specific volume gas fuel consumption increase ∆b е /∆t for 1°С increment of ambient air temperature ∆t = 1°С at the inlet of radiator may be assumed of about (0.5…1.0)⋅ ⋅10 -3 m 3 /(kW•h). A three-circuit scavenge air cooling system with ABCh (Fig.4) allows to maintain practically unchangeable scavenge air cooling water temperature of about t cw = 10...12 °С with the temperature of cooled water from ABCh of 7 °С and ambient air temperatures t amb = 30...35 °С. So such system can provide scavenge air cooling water temperature decrease of about 20...25 °С compared with conventional scavenge air cooling system with radiator, that results in reduction of engine specific fuel consumption by about ∆b е = (10…12)•10 -3 m 3 /(kW•h), i.e. about 4…5% decrease in specific fuel consumption at increased ambient air temperatures t amb = 30...35 °С. At part loads 850…1000 kW a reduction of specific fuel consumption is nearly twice higher (Fig. 12,b).
The efficiency of deep scavenge air cooling by absorption chiller and stabilization of its temperature for JMS 420 GS-N.L results in reduction of the cost of 1 kW generated electricity by 5…7 % compared with traditional radiator cooling.

Conclusions
A treatment of monitoring data on electrical power output and fuel consumption of gas engine JMS 420 GS-N.L in integrated energy system for combined electricity, heat and refrigeration generation has proved non-effective performance of conventional scavenge air cooling by radiator at increased ambient temperatures. The improved gas engine scavenge air cooling system with absorption chiller conversing waste heat of engine into refrigeration was proposed.
An advanced scavenge air cooling system allows to maintain practically stable scavenge air cooling water temperature of about 10...12 °С with the temperature of cheeled water from absorption chiller of 7 °С at high ambient air temperatures. This system provides an increase of engine electrical power output by 2…3 % and decrease of specific fuel consumption by about 4…5% as compared with conventional radiator scavenge air cooling at increased ambient air temperatures 30...35 °С. At part loads the effect is much more.
For modernization of the existing traditional scavenge air cooling system for gas engine of integrated energy system for combined electricity, heat and refrigeration generation such cooling method does not require essential constructive changes but only an addition heat exchanger for chilling scavenge air cooling water of scavenge air low-temperature intercooler closed contour by absorption chiller.