A novel enhanced ammonia-water power/cooling cogeneration system with dual level cooling temperature: Thermodynamic and economic assessments

https://doi.org/10.1016/j.enconman.2021.114530Get rights and content

Highlights

  • An enhanced ammonia-water CCP is introduced and analyzed thermoeconmically.

  • Dual level cooling temperature is achieved by utilizing an ejector.

  • Optimized energy and exergy efficiencies are obtained 68.8 % and 37 %, respectively.

  • The simple payback period is approximately 4.8 years.

Abstract

The present study proposed and investigated a novel parallel combined cooling and power (CCP) system using the ammonia–water solution as a working fluid. The major advantage of this system is employing two evaporators to produce cooling in dual level cooling temperatures and capacities. To produce high-temperature refrigeration output, an ejector refrigeration loop is introduced. Moreover, the power generation subsystem is employed between absorber and rectifier to recover of ammonia content in the vapor exiting the turbine for refrigeration. To prediction of ejector performance, a new model is proposed based on the shock circle model. To find the effect of key parameters on the system performance, the system is analyzed thermo-economically. The parametric study results show that high-temperature cooling load has the biggest effect on both thermodynamic and economic performance. Moreover, a parametric optimization is conducted and the optimized system is compared with several ammonia-water CCP systems in the literature. The comparison results show a significant enhancement in thermodynamic performance. In this case, the optimal energy efficiency of 68.8 %, exergy efficiency of 37 % and cooling-to-power ratio of 8.77 are estimated. From economic analysis, the optimal net present value and simple payback period are obtained by 49.2 M$ and 4.8 years, respectively.

Introduction

Discharging a large amount of mid- and low-grade waste heat from industrial plants and processes into the environment results in both thermal pollution and energy waste. For this reason, waste heat recovery and utilization have received widespread attention in the literature to environmental protection and meet the increasing world energy demand.

Most of the heat recovery systems used worldwide are produced for power amplification, with less attention for cooling or heating applications [1]. However, based on absorption refrigeration technology, many cogeneration cooling and power systems have been widely proposed to simultaneously meet electrical energy and cooling demand using mid- and low-grade thermal energy. Using binary working fluid with variable boiling and condensation temperatures improves the thermodynamic performance, mainly due to the good thermal match between the working medium and the heat source, and hence the irreversibility of the heat transfer process reduces.

A well-known environmentally-friendly binary mixture that has excellent thermophysical properties is the ammonia–water mixture. The standalone ammonia-water based absorption refrigeration cycles [2], [3] and power generation systems (called Kalina cycle) [4], [5], [6] have attracted significant attention from many researchers. Also, the waste heat based series connected cooling and power systems using ammonia-water as working fluid have been investigated widely by researchers [7], [8].

However, to decrease the throttling losses, research on the utilization of a turbine instead of the condenser and the throttle valve has been carried out by Goswami et al. [9], [10], [11]. One of the benefits of such systems is that ammonia can be expanded to a very low temperature in the turbine without condensation. Xu et al. [12] proposed a cogeneration power and cooling (CCP) system which combines a high concentration ammonia vapor Rankine cycle and an ammonia-water absorption refrigeration cycle (which was firstly proposed by Goswami [11]). They showed that by employing flat plate solar thermal collectors in their proposed system, the cost of a solar based CCP system could be reduced more than 1500 $ kW−1. Tamm et al. [13] investigated both experimentally theoretically the CCP system proposed by Goswami. They concluded that optimization of the system for exergy efficiency produces no refrigeration at a high-grade heat source. Zheng et al. [14] proposed an ammonia-water CCP system based on the Kalina cycle. To enhance the separation process as well as obtain high ammonia concentration for the cooling process, they proposed to replace the Kalina flash tank with a rectifier. The energy and exergy efficiencies of their system have been estimated as 24.2 % and 37.3 %, respectively. Liu and Zhang [15] presented a novel ammonia-water CCP system by introducing a splitting/absorber unit integrated with an ammonia refrigeration system and an ammonia-water Rankine cycle. Compared with the separate conventional power generation and refrigeration systems, their proposed system has an 18.2 % decrease in thermal energy consumption. Zhang et al. [16] presented an ammonia-water CCP system with interconnection by separation, absorption and heat transfer processes. The results showed that for a maximum cycle temperature of 450 °C, the first- and second-law efficiencies of 25 % and 51 % could be obtained, respectively. Wang et al. [17] presented a novel ammonia-water CCP system by applying a marginal variation in the proposed system by Zhang et al. [16], [18]. They eliminated the condenser and the pump after and before the turbine. Their results showed that by using a heat source temperature of 300 °C, the exergy efficiency of 35.5 % and the cooling-to-power ratio of 0.36 could be obtained. Wang et al. [19] improved their previous system by applying an ejector between the rectifier and the condenser. Compared with their previous work, improvements of 0.2 % and 21.3 kW were observed in exergy efficiency and refrigeration output, respectively. To find out the effect of operating conditions on the Gowsami cycle [11] performance, a parametric study has been conducted by Padilla et al. [20]. They showed that if the system was employed to generate maximum power, it was not necessary to employ a superheater or a rectifier that leads to a decrease in the investment costs. Also, superheating increased the produced power but the difference was not significant at high-pressure ratios. A thermoeconomic analysis of the Goswami cycle [11] has been conducted by Zare et al. [21]. They evaluated the unit product cost of 235 $ GJ−1 for optimum condition. Sun et al. [22] proposed an ammonia-water CCP system with adjustable solution concentration. In their system, the ammonia content in the vapor exiting the turbine is recovered by a rectifier refrigeration loop. Comparing the ammonia-water CCP systems with fixed and adjustable solution concentration indicated the desirable effect of the solution concentration adjustment. Luo et al. [23] analyzed an enhanced ammonia-water CCP system consisting of a topping Brayton cycle used a closed high-temperature gas cooler reactor. Compared to separate conventional refrigeration and power generation systems, their proposed system has a 9.66 × 104 tone year−1 reduction in fossil fuel consumption. Wang et al. [24] proposed an integrated system including a Kalina cycle and an ammonia-water absorption refrigeration system. The exergy analysis results showed that the highest exergy destruction value belongs to the condenser and the vapor generator. Also, an increase in the separator pressure of or the ammonia mass fraction of basic solution, leads to a growing variation in the energy and the exergy efficiencies. A combination of a Kalina power generation cycle and ejector refrigeration cycle with two evaporators has been presented by Barkhordarian et al. [25]. For their system, the values of 19 % and 39 % were evaluated for energy end exergy efficiencies, respectively. Chen et al. [26] thermoeconomically analyzed an ammonia-water CCP system driven by the waste heat of exhaust gas of an internal combustion engine and jacket water. They found that a higher temperature of the exhaust gas and a lower temperature of the cooling water are beneficial to the system performance. Behnam et al. [27] analyzed an integrated CCP system including a Kalina cycle and an ejector refrigeration cycle thermodynamically. To enhance the system performance, they used a two-phase ejector. Compared with the conventional CCP systems, their system exhibited an improvement in exergetic performance. Han et al. [28] presented an ammonia-water CCP system with an adjustable cooling-to-power ratio by variating mass flow rate ratio. The results of the parametric study indicated that the efficiency of the system rises by increasing the turbine inlet temperature and ammonia concentration and decreases by increasing the basic solution concentration. Ayou and Eveloy [1] presented an exergoeconomical investigation of the ultra-low grade waste heat based ammonia-water CCP system using liquefied natural gas cryogenic exergy. They showed that their proposed system yields the cooling-to-power ratio of 7.3 and the first- and second-law efficiencies of 39% and 36%, respectively. Zhou et al. [29] proposed an ammonia-water combined cooling, power and desalination system consisting of ejector refrigeration and Kalina power generation cycles. They employed an ejector for coupling the ejector refrigeration system and Kalina cycle that increased pressure difference across the ammonia-water turbine to produce the higher power output. They showed that over 70 % of the total exergy destruction belongs to spray flash evaporator, condenser and turbine. Javanshir et al. [30] investigated a geothermal-based ammonia-water CCP system employing a modified Kalina cycle to produce power thermoeconomically. They conducted an optimization based on maximizing exergy efficiency and obtained the values of 34.7 % and 15.8 $ GJ−1 for the exergy efficiency and product unit cost, respectively.

A review of the literature shows that the parallel-connected ammonia-water CCP system proposed by Goswami et al. [11] and Zhang et al. [18] have been widely considered as the basic system configurations for the CCP systems. In these cycles, the vapor exiting the turbine contains ammonia at a relatively low temperature. While improving the CCP system performance, there is a great potential to use the turbine exhaust heat efficiently in the cooling system. Moreover, the ammonia-water CCP systems investigated in the literature have some disadvantages that include the low cooling output or complex system configurations, resulting in higher capital investment.

In this work, to improve the disadvantages as mentioned earlier, a new ammonia-water CCP system driven by waste heat is proposed to simultaneously produce both power and cooling outputs in a parallel-connected loop and requires less equipment. One of the most important restrictions of the same previous works is about the evaporator pressure and temperature, which depends on the turbine outlet pressure and cannot be changed independently. In this study, to overcome this restriction, the outlet turbine ammonia-water is sent to a rectifier. Due to the separation of the ammonia and water to a sufficient concentration, using the rectifier improves the system performance. The present system has two evaporators to generate refrigeration capacity in two different temperature levels simultaneously. Such a system can be used in various applications such as the food industry or food production company where raw materials and products must be stored at different temperatures. To enhance the refrigeration process, an ejector is introduced between the rectifier and the condenser, which high-pressure ammonia-rich vapor from the rectifier and low-pressure ammonia-rich vapor from the high-temperature evaporator play primary and entrained flows, respectively. Moreover, the turbine exhaust heat is recovered by the ammonia-water stream entering the heat recovery vapor generator. Thus, the system performance can improve without significantly increasing the complexity of the system. Furthermore, a comprehensive thermodynamic assessment is conducted to evaluate the feasibility and examine the effects of operating parameters on the system performance. Also, to discover the cost-effectiveness of the proposed CCP system, an economic analysis is carried out. Finally, the parametric optimization is performed with energy and exergy efficiencies and total cost as the objective functions and the results are compared with other ammonia-water CCP systems.

Section snippets

Description of the systems

The configuration of the proposed parallel-connected ammonia-water CCP system is plotted in Fig. 1. The rich ammonia vapor (stream 5) flows to the condenser and condenses to a saturated liquid (stream 1) by rejecting heat to the cooling water (streams 0cw and 1cw). Stream 1 is split into two parts, one part (stream 2), after isenthalpic expansion in throttling valve (TV 1) (stream 3), enters the high-temperature evaporator (HTE) and evaporates to produce the refrigeration output (stream 4).

General methodology

Based on a mathematical model, a computational program is implemented by EES software to model the proposed system. For simplicity, the following assumptions invoked in the analysis are made:

  • Steady-state operation.

  • No changes in kinetic and potential energy.

  • Negligible heat loss from each component.

  • 3 % of pressure loss ratio for each component.

  • Isenthalpic process for throttling valves.

The proposed system components as the control volume need to meet mass, energy, and exergy conservation

Results and discussion

The thermodynamic and economic analyses are carried out based on the operating conditions and necessary economic input data (given in Table 6, Table 7, respectively).

It is well known that the lower exergy destruction in the HRVG and heater can be reached by matching the temperature profiles of the ammonia-water solution with those of the heat source as a result of the non-isothermal phase change. The temperature profiles of heat source and ammonia-water stream through the heat recovery vapor

Conclusion

This study deals with the proposal and assessment of a new parallel combined cooling and power system using the ammonia–water solution as the working fluid. The refrigeration subsystem comprises two evaporators to produce cooling in dual level cooling temperatures and capacities which can be used in various applications such as the food industry. To produce high-temperature refrigeration output, an ejector refrigeration loop is introduced and to ejector performance prediction, a new model is

CRediT authorship contribution statement

A.H. Mosaffa: Conceptualization, Methodology, Software, Validation, Investigation, Writing – original draft, Visualization, Supervision, Project administration. L. Garousi Farshi: Conceptualization, Software, Data curation. : . S. Khalili: Software, Data curation. : .

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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