Comparative performance analysis of four different combined power and cooling systems integrated with a topping gas turbine plant

Highlights

• Performances of four new combined power and cooling systems are compared.

• Energy and exergy analyses are done for comparing the systems.

• Suitable steam pressure in the heat recovery steam generator is identified for each system.

• The system that gives higher energy output and minimum irreversibility is identified.

• The gas turbine plant which is common in all contributes more than 95

Abstract

In this paper, the performances of four combined power and cooling systems are compared in which the exhaust heat of a topping gas turbine plant is utilized for further power and cooling generation. Steam turbines (STs) and organic Rankine cycles (ORCs) are used for power generation by integrating those in a completely different arrangement in the first two systems while the third and fourth systems use ST based power cycles. The first and the fourth configuration uses two absorption cooling systems (ACSs) driven respectively by steam and exhaust heat. In the second and third systems, however, only one ACS is used. Energy and exergy based parametric analyses are done, showing the performance variations with HRSG steam pressure from 89 to 94 bar for comparing the CPC systems. The fourth system was found to be the most appropriate from the energetic and exergetic performance viewpoint with the first system to follow. Despite an additional ACS, the total irreversibility of the first and the fourth systems was found almost equivalent to that of the second system. Only in the third system, the total plant irreversibility was relatively less compared to the first and the fourth. It was found that the GT plant alone contributes more than 95% irreversibility in all the four systems.

Introduction

The demand for global energy is increasing day by day due to population growth, rapid urbanization and industrialization. Large scale energy consumption all over the globe has escalated the problems of global warming, greenhouse gas emission, and climate change. Continuous efforts are therefore being made by the research community to develop new and efficient energy systems. The combined cycle power plant (CCPP) is one such development in this regard which is known for its high efficiency and low greenhouse gas emission [1]. Gas turbine (GT) plant combined with a heat recovery steam generator (HRSG) and steam turbine (ST) represents the state of the art CCPP technology. GT exhaust heat can also be utilized for driving bottoming power cycle other than ST such as organic Rankine cycle (ORC) and Kalina cycle (KC) etc. A lot of research studies have been carried out in the recent past to propose new GT based CCPPs involving ST, ORC and KC etc.

Cao et al. [2] compared the thermodynamic performance of a combined GT-ORC plant with that of a GT-ST plant for the same input parameters. They observed higher thermal efficiency in the GT-ORC plant while the net power output was little more in the GT-ST plant. Anvari et al. [3] made a comparative performance analysis between a combined GT-regenerative ORC (RORC) and a GT-reheat ORC where they observed higher thermal efficiency and lower irreversibility with the GT-RORC. Li et al. [4] compared thermodynamic and economic performances of two combined GT-ORC plants, one with a single stage and the other with a two-stage series ORC (TSORC) where they found higher net power and exergy efficiency with the TSORC integrated CCPP. Due to the installation of second-stage evaporation, the investment cost and the cost of electricity production of the TSORC was however more. Oko et al. [5] performed an exergy analysis of a CCPP integrated with a subcritical ORC. They found 1.95% and 1.93% increase in the energy and the exergy efficiencies when the GT exhaust heat was utilized to drive an ORC for additional power generation. Singh et al. [6] presented an exergy analysis of a CCPP integrating a Brayton-Rankine cycle to a Kalina cycle. With the addition of the Kalina cycle, they observed 1.27% increase in the net power along with 0.54% and 0.51% increase in the thermal and exergy efficiency, respectively.

Apart from power and electricity, a whole lot of global energy is consumed by the air-conditioning and refrigeration industries. Therefore, in recent times, combined power and cooling (CPC) systems are receiving significant research interest. In many industries, power and cooling are produced simultaneously from a single plant through efficient utilization of energy resources and use of CPC systems. Absorption cooling system (ACS) can be integrated with GT plant/CCPP either for power and efficiency enhancement through compressor inlet air cooling or to produce cooling simultaneously in the cogeneration mode. The following are some research articles related to the thermodynamic analysis of combined GT-ACS/GT-ORC-ACS/GT based CCPP-ACS/ORC-ACS plants etc.

Shukla and Singh [7] studied the effect of inlet air cooling on the performance of steam injected GT plant. They observed 6.91% increment in the thermal efficiency due to inlet air cooling which was accomplished through the use of a GT exhaust-driven ACS. Ameri and Hejazi [8] also observed 11.3% increase in power with inlet air cooling achieved through the use of a double effect ACS driven by GT exhaust heat. Mohapatra and Sanjay [9] performed an exergy analysis of a CCPP integrated with an ACS which was used for compressor inlet air cooling. The GT exhaust heat was first used for steam production in the HRSG and next to drive the ACS. The steam produced in the HRSG was used for power production in the ST plant. They found higher exergy efficiency at lower compressor inlet temperature. They also observed that except in the CC, the irreversibility at all other GT plant components increases while the same at the bottoming ST cycle components reduces with increase in compressor pressure ratio. Mone et al. [10] investigated three different CPC systems combining a commercial GT with the single, double and triple effect ACS configurations separately. They calculated the available thermal energy for the chiller application based on the GT size; exhaust gas flow rate and temperature and obtained approximately 300 MW of cooling from a large turbine. The triple effect system showed the highest cooling capacity among the three ACSs. Khaliq [11] performed energy and exergy analysis of a GT plant combined with an HRSG and a single effect ACS for evaluating its energetic and exergetic performance as functions of compressor pressure ratio, gas turbine inlet temperature (GTIT), HRSG pressure and ACS evaporator temperature. He reported that more than 80% of the total exergy destruction is accounted by the combustion chamber (CC) and the HRSG. Mohammedi et al. [12] considered a combined GT-ORC-ACS based system where the GT exhaust was used first to drive a toluene operated ORC and next an ammonia-water based ACS. Under the design conditions, the plant could produce 30 kW of net power, 8 kW of cooling and 7.2 ton hot water with 67.6% efficiency. Anvari et al. [13] proposed a combined GT-ORC-ACS to produce a net power of 30.606 MW, heating of 40.78 MW in the HRSG and 1 MW cooling in the ACS. In the proposed system, the GT exhaust was first passed through the HRSG, then through the RORC and finally through the water-LiBr based single effect ACS. Choosing R123 as a working fluid for the RORC, they observed 2.5% and 0.75% increase in energy and exergy efficiency with ORC addition in the system. Sun et al. [14] analyzed the thermodynamic performance of an R113 operated ORC combined separately with an ACS and an Ejector Refrigeration Cycle (ERC). They found that the power output, cooling capacity and the exergy efficiency of the combined ORC-ACS are all higher than those of the ORC-ERC at ORC evaporation temperature above 153 °C. Sayyaadi et al. [15] recently examined the use of Kalina, organic Rankine, Goswami, and trilateral flash cycles as second bottoming cycles in a GT plant in which an absorption chiller was used as the first bottoming cycle. They optimized the entire secondary bottoming cycles using genetic algorithm in finding out the Goswami cycle as the best alternative among the four with an additional 4.26 MW of power and 0.45 MW of auxiliary cooling. They also identified the Kalina cycle as the least desired one.

Thus, from the above review, it was seen that in energy research, often new CCPPs and GT based CPC systems are proposed and thermodynamic performances are evaluated on the basis of energy and exergy. This is done to fully utilize a given energy resource and to investigate the system performance under a given set of operating conditions. Sometimes it is also done to evaluate the effects of operating parameters and to find out suitable parameters for minimizing the irreversible losses and maximizing energy output and efficiencies. Further, the system schematics which were considered in the above studies were all different in some way or the other. The energy outputs obtained from the system schematics in the form of power and cooling etc. were also not same due to the selected operating parameters and the working fluids, particularly in the ORC and ACS. Thus, the performance of a thermal system may vary depending on how the system is configured and the chosen operating parameters.

As far as the use of GT exhaust heat is concerned, in combined GT-ST based power cycle, it is mainly used for producing steam and driving ST. However, if cooling is also required from the same system in CPC mode, then several options may be considered. Certainly, the exhaust gas heat at HRSG exit could be one option for cooling production through ACS integration. Additionally, another heat source option for cooling could be the steam from a back-pressure ST. In this case, however, the power output from the ST plant will be less. Without compromising much on ST power output, alternately, depending upon the cooling load, a certain fixed amount of steam from the ST can be extracted at some desired pressure and temperature to provide the necessary heat required for vapour generation in the ACS generator. Similarly, as found in some previous studies, a given amount of power in a CCPP can be generated either by integrating an ST plant or an ORC or the combination of both ST and ORC can also be explored. Particularly for the ORC, since they can be operated with a low-temperature heat source, either the steam from a back-pressure ST or alternately the extracted steam from the ST can be used to drive the ORC.

To explore all the above possibilities, in the present study, four new GT based CPC system configurations are considered for comparison. The CPC systems are configured based on the integration of the GT plant with the bottoming power cycles and the ACSs through different integration schemes. The topping GT plant is the same in all system configurations and the bottoming cycles are considered with different arrangements. In the first configuration, for driving an R245fa operated ORC, extracted steam from the ST is used while in the second configuration; the entire steam from a back pressure ST is used as a heat source. Similarly, for the ACS, various heat source options such as HRSG exhaust heat, steam from back pressure ST and extracted steam from ST are explored. As such, the first and second configurations are GT-ST-ORC-ACS systems with two ACSs in the first and one ACS in the second configuration. The other two configurations are GT-ST-ACS systems, the ACS either driven by HRSG exhaust heat or by extracted steam from ST. Each system is unique in terms of the system configuration and is entirely different from some of the previous similar GT-ORC-ACS [12], [13] and GT-ST-ACS systems [7], [9] cited earlier. To the best of the knowledge of the present authors, the systems that are considered in this study for comparison were not analyzed earlier in any previous study. Therefore, this research study is conducted with the following objectives.

• 1. To carry out a parametric study through HRSG steam pressure variation for identification of suitable HRSG steam pressure for each configuration.

• 2. To compare the energetic and exergetic performances of the four CPC system configurations and finally to identify the most suitable configuration based on total energy output and system irreversibility.

The CPC system configurations are described in the following section.