1. Introduction
Co-generation and tri-generation systems are proved to be smart tools for increasing energy and cost efficiency of supply via the recovery of an energy flow (mainly heat) which is otherwise wasted [
1]. Apart from this fact, the importance of such multi-generation plants is increasing today because of the growing need for integrated energy systems where all the energy demands such as electricity, cold, and heat have synergies with each other [
2]. These synergies are to create the opportunity of transferring cheap-excess energy from one sector to another one while increasing efficiency and decreasing the cost of supply at the same time [
3]. Having said this, one may feel the importance of not only revising the sole-production energy plants for a possible co-/tri-generation design but also introducing new effective tools for multi-generation [
4].
Aghbashlo et al. [
5] presented a comprehensive exergoeconomic analysis for the combination of biogas genset and municipal solid waste digestion plant. Obtaining the cost structure of the combined plant was the main purpose of this study. The unit cost of products was determined to be 2.27 and 26.27 USD/GJ for the biofertilizer and bioelectricity, respectively. Also, an economic investigation showed that the most important component is the genset followed by digester with the cost rates of 101.27 and 68.41 USD/h, respectively. Thermodynamic and economic performance of a dual-fuel cogeneration system operating with natural gas (NG) and biomass as an energy supplier for a hotel was investigated by Yang et al. [
6]. They claimed that dual-fuel CCHP system can be proposed as an interesting solution from the cost-effective operation point of view. It was shown that the system exergetic efficiency during the winter and summer is 12.23 and 8.06%, respectively. They concluded also that the gasifier could result in a significant value of exergy destruction (almost 60% of entire exergy destruction). In addition, it was revealed that the product cost could be lower in summer than in winter. A 4E analysis of solar boosted CCHP system operating based on the natural gas fired Brayton cycle was proposed by Wang et al. [
7]. Solar energy was utilized to increase the air temperature feeding the combustion chamber and exhaust gases exiting gas turbine were used to generate chilled/hot water. It was reported that the energetic and second law efficiencies could reach 66.0/83.6% and 25.7/24.9%, respectively, in the heating/cooling mode. Besides, it was shown that utilizing solar energy could reduce carbon emission of 41%, approximately. Owebor et al. [
8] designed an integrated thermal power plant operating with municipal waste to energy. This system was a combination of gasifier, fuel cell, steam and gas turbine power cycles, ORC, and an absorption chiller. The thermodynamic analysis revealed that the energetic and exergetic efficiencies of the integrated system corresponding to the stack temperature of 54 °C were 62.3 and 55.5%. System exergy performance revealed that the major value of destroyed exergy refers to the incinerator unit (37%). Moreover, the cost of supplied power was found to be 1.8 cent/kWh. A solar-assisted biomass-driven CCHP was analyzed by Wu et al. [
9] using exergy and exergoeconomic methodologies. In fact, a dish collector was employed to collect the solar heat in contribution to biomass and steam gasification, while the product gas was utilized to run an internal combustion engine, first, and then to drive an absorption chiller. The unit cost of products was reported to be 16.4, 85.2, 58.8, and 96.1 cent/kWh for the electricity, space cooling (delivered via chilled water), space heating (delivered via warm water), and domestic hot water (delivered via pressurized warm water), respectively. A comprehensive techno-economic evaluation of a CHP running with organic division of municipal solid waste was examined by Yang et al. [
10]. Results associated with thermodynamic analysis showed that the CHP could operate with an efficiency of almost 60%. Furthermore, economic results revealed that a plant with a capacity of 5 ton per hour requires 27.64 million British pounds as capital investment cost. Yari et al. [
11] compared employing gasifier and digester in a fuel cell-based cogeneration system fed by municipal solid waste. Also, a parametric study was done to examine the effects of stack temperature and current density on the system’s performance. According to the reported results, a cogeneration system operating with digester was more favorable than that of operating with a gasifier. Jack and Oko [
12] proposed a reheat steam cycle to produce electricity from municipal solid waste for the case of Port Harcourt city. They used exergy and exergoeconomic methodologies to evaluate the proposed system from the thermodynamic and economic perspectives. It was found that
$326.4 million is needed to spend as the capital expenditure of a power plant with 117 MW capacity and a payback period of around six years. Nami et al. [
13] compared two different small-scale CCHP systems, one operating with steam Rankine cycle and the next with ORC cycle. Both were designed to harvest the waste heat of the cement industry. They demonstrated that energy, exergy, and sustainability principles would select the ORC-based CCHP as the best solution, while the Rankine-based CCHP could be preferable economically.
In this study, a feasibility study of a municipal waste-fired CCHP is presented and the proposed system is examined in detail using technical, economic, and environmental principles. The energetic and exergetic efficiencies are considered as the decisive tool of the thermodynamic analysis, while the exergoeconomic technique is adopted as the economic evaluator implement. Besides, the sustainability index is taken into consideration to show the relationship between the exergetic performance of the system and its environmental impact. In fact, the presented cogeneration system is a combination of waste incineration unit, a steam Rankine cycle, a large-scale LiBr/H2O (lithium bromide-water) absorption chiller, and some auxiliary heat exchangers. The main aim is to design a local energy system feeding the required energy for the neighborhood in terms of power, district heating, and cooling.
In the ref. [
14], the authors published on this hybrid plant was simply a thermodynamic analysis of the system explaining the energy and exergy efficiency aspects of the system as compared to a conventional waste-driven CHP plant. While the present study digs into the very detailed thermodynamics (rates of exergy and energy losses and flows, the costs associated with these losses, etc.), economic, and environmental (such as CO
2 and NOx emission levels) aspects of the system. This information is informative and essential to know for novel energy systems when are proposed for real-life applications. This results in knowing the most serious drawbacks and loss points/processes of the system and their effects on the economic effectiveness and emission levels of the system and then, proposing suggestions for overcoming these losses/irreversibilities/drawbacks.
The authors believe that reading the present article gives a very clear understanding of the methodology of the research on this work. There is a waste-driven trigeneration plant that is to be investigated in terms of technical, economic, and environmental impacts. Therefore, the performance/operation of the system should be modeled in a software environment or by developing the code of that in a programming environment. In this work, programming in EES has been used for which the detailed mathematical models and specifications of the power plant are presented. Then, the results in a classified and easy-to-follow manner have been presented. Finally, relying on the driven results from the simulations, suggestions are given for improving the performance of the system to increase the technical, economic, and environmental effects of the plant.
2. The Hybrid System and Specifications
Figure 1 depicts the schematic diagram of the presented municipal waste-fired CCHP. As the figure shows, the system consists of a waste incineration unit equipped with a high-pressure steam boiler, a power producing unit operating based on Rankine cycle, a single effect LiBr-H
2O absorption chiller, and some auxiliary heat exchangers. In the conventional municipal waste-driven cogeneration systems, electricity is generated via a steam turbine and high-pressure steam is produced in the steam generator. This is while rejected heat in the condensation process is usually considered as the byproduct and the most common use of this byproduct is to be fed to district heating networks. In some cases, more than one turbine or steam generator is employed, as the plant operates in Bergen, Norway [
15]. Such power sectors are typically used to cover the main grid base-load. In fact, employed condenser in the power block is a kind of heat exchanger attached to the district heating networks. As can be seen from
Figure 1, in the present study, part of the withdrawn heat from the condenser is utilized in a single effect absorption chiller to produce chilled water and the rest is supplied to a special heat exchanger to be delivered as district heating. Besides, waste heat of the exhaust gases is harvested for efficiency improvement. It has been shown that waste heat recovery from exhaust gases has economic benefits even at the expense of increasing capital investment due to the purchasing cost of added heat exchangers [
16]. Also, utilizing the waste heat recovery system decreases the low pressure turbine outlet pressure at the existing design, which results in higher values of electricity generation [
17].
As mentioned before, part of the absorbed heat from the condenser is consumed to run the generator (Gen) to drive the chiller. LiBr and H
2O were supposed to be as the absorbent and the refrigerant in the chiller unit, respectively [
18]. Within the employed absorber, a solution of H
2O and lithium bromide absorbs the refrigerant coming from the evaporator. The watery solution is pressurized and heated up in the SHE (solution heat exchanger) to reach the considered Gen. Vapor content of the solution is vaporized in the Gen and the dense solution flows back to the SHE. Vaporized water enters the condenser and acts as a refrigerant in the evaporator after a pressure drop in the throttling valve. In the end, the Gen exiting relatively warm water flows to the feed water to be combined with the heat exchanger 2 (HE2) exiting flow and complete the cycle. A final notable point about the waste heat recovery system is that a flue gas cleaning unit is supposed in this procedure to remove the pollution of the combustion products to almost zero [
19].
Within the incineration unit, the chemical composition of the waste and its lower heating value is the main influential parameter. Here, data reported in the [
12] are utilized.
Table 1 outlines the input data supposed in municipal waste and waste incineration unit modeling. The following assumptions were made to simplify the proposed cogeneration system modeling:
The whole proposed cogeneration system operated under the steady state situation [
20].
A fixed mass flow rate of 1 kg/s was considered for the municipal waste [
16,
17]. Although, the dynamic model of these systems is of course very important but having the transient model of each component of the system one could simply drive the dynamic performance model of the plant without any significant difficulty. However, waste driven power plants are mainly for base-load coverage with a fixed load operation and seldom come to lower operation loads.
Steam turbine and pumps isentropic efficiencies were set to be 90 and 75%, respectively [
21].
The coolant water temperature of 283 K was supposed.
District heating (pressurized hot water) supply and return temperature were set to be 353 and 313 K, respectively [
22].
District heating (chilled water) supply and return temperature were set to be 278 and 285 K, respectively [
23].
Heat losses and pressure drop from the pipelines were neglected [
24].
The generator temperature was supposed to be 353 K.
Maximum heat exchanger effectiveness of 85% was considered.
Minimum pinch temperature difference of 10 K is supposed.
Minimum stack temperature of 318 K was supposed [
16].
Unit costs of the municipal solid waste, coolant water and air were supposed to be zero, based on Aghbashlo et al. [
5].
Under the base condition, half of the pressurized hot water (point 10 in
Figure 1) is utilized to drive the chiller, while the rest is used to heat the water and provide district heating.
4. Results and Discussions
The results associated with the system simulation are presented and discussed in this section. Proposed municipal waste-driven CCHP is simulated via developing a computer program utilizing EES (engineering equation solver) software [
46]. Thermophysical properties, mass flow rate, and stream composition of each flow are listed in
Table 3. This table is of significant importance to check the applied thermodynamic principles and check the performance of the different heat exchangers employed in the proposed CCHP. State 3 in this table refers to the combustion products within the incinerator and was not shown in
Figure 1.
Exergy flow diagram of the cogeneration plant is shown in
Figure 2. This figure points out the exergy values of different streams under the base condition (half of the pressurized heated water is used to cooling production unit and the residue is utilized to deliver heating). As it was predictable, a considerable value of exergy destruction occurs in the incinerator mainly because of the combustion procedure. In this figure, the absorption chiller is considered as a unit component and causes the second highest exergy destruction. The exergy rates associated with the products are depicted in this figure in terms of electricity (2937 kW), district heating supplied via HE4 (86.2-10.9 kW), district heating supplied via HE2 (721.1-91 kW), and district cooling supplied via absorption chiller (83.2 kW).
The portion of each main section in the total required purchasing cost as the capital investment is shown in
Figure 3. Definitely, the most expensive components will affect the cost effectiveness of the plant and the cost of products as well. According to this figure, the costliest unit is the employed incinerator equipped with a steam boiler, which causes almost 79% of the total required expenditures. The second important unit from the capital investment cost point of view is the steam turbine and 11% of the total needed cost refers to this component. However, it should be noted that the cost related to the waste furnace, cost of the steam generator, and the cost of the emission control unit are taken into account in determining the cost of the incinerator unit [
12].
The technical specifications obtained from the proposed waste-fired modeling resulting from both thermodynamic and economic assessments are listed in
Table 4. Results reported in this table are gained under the base condition. It is worth mentioning that the technical parameters related to the system performance are not a function of the system maximum capacity. Therefore, the electrical capacity equal to the produced power from incineration 1 kg/s of the municipal waste (resulting in 2904 kWe) is considered. Furthermore, off-design analysis was not performed in this study. Though this causes uncertainty in the modeling compared to a real CCHP, it still might be sensible for the presented municipal waste-fired CCHP plant. The first reason to support this idea is that these kinds of multi-generation systems are utilized for base-load coverage, working under the full load condition [
47]. The second reason is that the operation load does not affect the performance degradation of a power plant linearly [
48]. Thermodynamic analysis revealed that incinerating 1 kg/s of municipal solid waste resulted in thermal, exergetic, fuel-to-power, fuel-to-heat, and fuel-to-cold efficiencies of 83.28, 25.69, 23.49, 47.41, and 12.38%, respectively. As can be seen, fuel-to-power efficiency has a value close to the second law efficiency. This is because the exergy rates associated with thermal loads (supplied heating and cooling) are much lower than the electrical load transferred to the main grid. The total capital investment cost of 6.501 million USD was estimated to be for the whole designed CCHP system. Maintenance cost was also included in this value. However, the reported cost refers to the basic operating condition and changing operating conditions would change the capital cost, mainly because of variation in the size of the employed components. Besides, the unit cost of supplied power is estimated to be 1.129 cent/kWh (3.135
$/GJ) via applying exergoeconomic principle which is completely comparable with that of reported in the literature [
12]. In addition, the unit cost of produced heating and cooling are obtained to be 1.407, 3.374 cent/kWh, respectively. Furthermore, the cost rate related to the environmental impact was 158.9
$ per hour, considering environmental degradation associated with CO
2 and NO
x as the main emissions. Moreover, the sustainability index of 1.346 confirms the high value of environmental degradation cost.
One of the main economic indices to show the cost effectiveness of the newly developed energy systems is the payback period. To determine the payback period of the proposed CCHP system NPV method is utilized.
Figure 4 represents the net present value for the first 10 years of the system operation. According to this figure, the payback period of almost 6 years would recover the costs associated with the whole system. Cost of fuel for the presented system was the cost of municipal solid waste and was hypothesized to be zero based on Aghbashloo et al. [
5].
The main exergetic and economic parameters calculated for the proposed cogeneration plant are sketched in
Table 5. In this table, components are ranked in descending order of cost importance using the amount of the capital investment cost plus cost of destroyed exergy (
) based on Bejan et al. [
37]. As was expected, the maximum value of
fits to the incinerator equipped with a boiler zone which has an exergoeconomic factor of 33.38%. This value of
clarifies the exergoeconomic importance of this unit and much attention should be paid on this component to enhance the total economic performance of the system. Destroyed exergy cost of 84.05
$/h and low value of exergoeconomic factor states that the cost rate associated with destroyed exergy is higher than the cost rate associated with the capital investment and maintenance. However, as it is clear, no considerable effort can be made to improve the incinerator performance as all the irreversibility sources, i.e., combustion, temperature difference, and mixing are present within this unit [
49]. Nevertheless, suggesting some limited enhancements like preheating the inlet air can be found in the literature. The second main unit from the exergoeconomic point of view is the employed steam turbine with the
f value of 72.74% and exergy efficiency of 91.81%. This
f value indicates that the purchasing cost of this component is higher than the cost of the destroyed exergy within this unit. Hence, replacement with a cheaper turbine is advised at the expense of the second law efficiency. The involved absorber in the chiller unit is the third important unit with the lowest
f value, among the all. Exergoeconomic factor of 2.88% indicates that the cost of destroyed exergy is thoroughly dominant compared to the capital investment cost, and low value of exergy efficiency (6.91%) confirms this fact. It can be stated that improving the exergetic performance of the absorber at the expense of higher purchasing cost enhances the CCHP economic performance significantly. Opposite to the absorber, heat exchanger 4 operates with the highest value of exergoeconomic factor. In fact, 80.42% of the cost rate related to this component is in conjunction with the capital cost and less than 20% is due to exergy destruction within this unit.
Meanwhile, it should be pointed out that the exergetic and exergoeconomic performance of each component may be deteriorated by improving the performance of each individual component. Then, the recommendations prepared to improve the economic performance of each component does not essentially mean an enhanced performance for the entire proposed waste-fired CCHP system.
To make the obtained outcomes more generalized, the effect of chiller supply on the system performance is examined.
Figure 5 illustrates changes in the cost of products and sustainability index with varying chiller supply. Though the unit cost of electricity and district heating are not impacted significantly, increasing chiller supply decreases the unit cost of district cooling. Power production via steam turbine is not affected by the change in the chiller supply rate and harvested heat from power producing block is fed to generate heating and cooling. This is also the same for heat production through waste heat recovery procedure.
As can be seen, in lover values of , change in the cooling cost is intense. The unit cost of supplied cooling is a complex function of the chiller unit capital cost and cost of harvested heat from the steam cycle. However, to explain this, it should be pointed out that in lover values of chiller supply, the rate of delivering district cooling is low, while the capital cost of the chiller unit is considered. On the other hand, increasing chiller supply causes a reduction in the sustainability index of the whole CCHP. This is because the exergy rate associated with the delivered heat (warm pressurized water) is much higher than that of delivered cool (chilled water). Therefore, supplying higher values of district heating rather than cooling is encouraging from the sustainability point of view. Changing chiller supply rate from 0.01 to 0.99 results in decrease in the sustainability index from 1.369 to 1.291 and decreases the unit cost of cooling from 21.93 to 2.795 cent/kWh.