Exergy–energy analysis for a feasibility trigeneration system at Kocaeli University Umuttepe Campus

1 Introduction

Energy, which is the reason for constantly developing and advancing technological developments and also the most basic need of human beings, has taken its place at the highest level in our lives with many different alternatives. Considering the energy resources and electricity demands in Turkey, Turkey is a dynamic country among the world countries with its speed in industrialization, increasing energy demands, and growing economy. With regards to electricity generation, while Turkey’s installed power was 88,551 MW at the end of 2018, it reached 91,270 MW at the end of 2019. While the total consumption of electrical energy in Turkey was 22630356.02 MW h in February 2019, it increased to 24213523.12 MW h in February 2020 [1]. While the cumulative electricity generation in 2020 was mostly 17% with natural gas, the lowest rate was wastes with 2% [2]. In the light of these data, it can be said that the most important factors affecting energy production are the inseparable link between energy and economy. Additionally, in Turkey, the natural gas-based supply of electricity and heating needs and the ineffective use of existing energy potentials cause us to import a large part of our energy needs. This situation causes us to be economically dependent on foreign sources and moves away from the principle of sustainability in energy supply. For these reasons, it is essential to prioritize energy efficiency and savings.

Cogeneration and trigeneration systems are energy efficient production systems that can be called human and environment friendly working for this purpose. Trigeneration means triple production as the word illustrates and it is an added version of the cooling system as an advanced version of the cogeneration systems. It is the simultaneous production of electricity, heat, and cooling with the same fuel input as in cogeneration systems. In countries with high energy costs, the use of trigeneration systems is a cost-effective approach. However, the correct determination of the system capacity, the correct selection of the system components and the determination of how the selected system can be operated at the optimum level are the most important criteria for the efficient and productive operation of Trigeneration systems, the basic principle of which is efficiency and energy saving. In order to achieve all these, it is important to perform exergetic and energetic analyses of the designed system.

Many academic scientific articles have been published and studies have been conducted on energy and exergy analysis and optimization for Trigeneration and Cogeneration area. Piacentino et al. [3] studied the Reliability of Optimization Results for Trigeneration Systems in Buildings, in the Presence of price uncertainties and erroneous load estimation. This study was very guiding and helpful academic article. In this study an analysis of the influence of erroneous estimation of the uncertain energy loads and prices on the optimal plant design and operation is proposed. With reference to a hotel building, a number of realistic scenarios is developed, exploring all the most frequent errors occurring in the estimation of energy loads and prices. Then, profit-oriented optimizations are performed for the examined scenarios, by means of a deterministic mixed integer linear programming algorithm. Çelik and Kabul [4] carried out energy and exergy analyses by considering the modeling of a solar energy sourced cogeneration system in their study. It was determined that the collector efficiency of the examined system was 21%, and the thermal efficiency of the ORC system was 6%. It was determined that the highest exergy loss in the system occurred in the parabolic solar collector with 44%. Therefore, it has been emphasized that improvement on the parabolic solar collector should be made. The exergy efficiency of the cogeneration system, which should be concentrated and also examined, was calculated as 22%. In the article published by Aras and Balli [5], exergy and exergoeconomic analyses applied in a combined heat and power system with micro gas turbine (MGTCHP) are mentioned. While determining the exergy consumption and cost of the system, the exergy and exergy cost balances were found for each component of the system. According to the results, the exergic efficiency of the MGTCHP system was 123 kW with 35.80%. In the study conducted by Özgöztaşı [6] the system was designed with operating loads of 2,000 kW h electricity, 3,284 kW h heating, and 1,221 kW h cooling. The flow diagrams of the designed system are explained with the basic working principles and the sub-units of the whole system and exergy and energy analyses are examined. According to this study, the thermal efficiency of the system was calculated as 49.3%, the electrical efficiency as 43.68%, and the exergetic efficiency as 47.97%.

Wang et al. [7] carried out performance investigation of a solar-assisted hybrid combined cooling, heating, and power system based on energy, exergy, exergoeconomic and exergo-environmental analyses. In this study, the energy and exergy analyses with the Sankey diagrams indicated that the hybrid system achieves annual energy efficiency of 76.3% and exergy efficiency of 22.4%. Another work by Wang, is investigation of a Mixed Effect Absorption Chiller Powered by Jacket Water and Exhaust Gas Waste Heat of Internal Combustion Engine, Wang and Wu [8]. A mixed effect absorption chiller (AC), which couples together single effect and double effect processes, is investigated to recover these two kinds of waste heat simultaneously in this work.

Although many exergy and energy studies have been carried out in literature, there is no study very different campuses based on a real case, consumption values which are under in-dependent working conditions included in a single trigeneration system. There is no study in the literature in which the planning and control of such a system is questioned with seasonal exergetic and energetic analyses.

Today, it is possible for institutions to maintain their sustainability by producing their own energy, and it is an important indicator of development in institutions that can achieve this. In this study, a trigeneration system’s exergy and energy analyses were made operational at optimum level with natural gas and their feasibility has been studied, in order to meet the electricity, heating, and cooling needs of all existing buildings such as faculties, medical school, cafeteria, library, service buildings, institutes, and the rectorate in Kocaeli University Umuttepe Campus.

In this way, the feasibility of using the trigeneration system in a university campus will be obtained and it is planned to perform exergetic and energetic analyses for the design, size, and sustainability of the system in the campus that produces its own energy in order to facilitate applications. Using this designed system, the electricity needs of the campus and the need for heating in the winter and cooling in the summer will be met by utilizing the electrical energy produced in the system and/or the waste heat generated as a result of the cycle. The system has been dimensioned on the basis of minimizing distribution and transmission losses [9] and data were recorded for possible improvement and solution suggestions [10]. In addition, by determining the working conditions and operating load values suitable for the season [11], mathematical models for summer and winter modes were developed separately [12]. In this study, it is aimed to present the feasibility of a cost-effective, workable, and sustainable energy source within the university by selling the excess electricity produced to the mains at the current electricity unit prices determined by EPDK (Energy Market Regulatory Board in Turkey) [13].

In the light of all this, as a result of the existing and requested energy data and campus explorations, the parameters of the system under consideration were decided, and then the design of the system’s units and equipment were created in accordance with the supply/demand for the needs. The performance analyses of the trigeneration units selected in accordance with the real energy supply/demands were carried out and interpreted, and the decision mechanisms on the energetic and exergic efficiencies created by thermodynamic theories were verified. In this study, final designed and installed system are verified and discussed with the highest exergetic and energetic seasonal efficiencies. It was emphasized that performing exergy and energy analyses and creating mathematical models to determine the feasibility of installation of the trigeneration system are the most effective and closest to the truth method in the decision-making phase of the establishment of the system in this study.

2 Materials and methods

In this study, the independent demand values are calculated with the necessary assumptions in order to meet all the needs at the highest level for the part of Umuttepe Campus where the faculties and the rectorate buildings are located and the other part where the hospital and Faculty of Medicine are located.

In the studies carried out for determining the design and energy feasibility of the trigeneration system application, the possible improvements (revisions) in terms of thermodynamics in order to maximize the system performance and the possibility of increasing the investment cost of the system, and contrarily, if efforts are made to keep the investment cost low, the performance of the system may decrease at the same rate, have also been taken into account. For this reason, the campus (Part 1) with 26 buildings such as Kocaeli University, rectorate, Law, Engineering, etc., and the campus where the hospital and medical faculty are located (Part 2) with different operating conditions are considered independent of each other. All energy analysis values and calculation results obtained are given in Table 1.

While determining the optimal working method especially for use in a trigeneration system, many factors affect the determination of the very dynamic working regime such as electricity trading and unit costs determined by EPDK [13], working conditions and working environment, city conditions, post-investment maintenance and operating costs of the units selected in the installed system, and fuel costs used in the system [14,15].

Thanks to cleverly developed methods, it should be an improvement in the selected equipment and investment amounts [16]. A small amount of electricity will be purchased from the mains thanks to a 2 MW gas engine to be selected, and in the remaining 8 months, excess electricity will be sold to the mains with a 100% fully loaded gas engine [17].

In addition, this will allow us to run the power generator at full load and eliminate any possible loss of cost. On the other hand, working with a higher engine or capacity will not have to manage the budget of the excess electricity that may occur, and it will not increase the investment costs of the equipment operating at higher capacity and will not increase the amortization times more than it is [11]. At the same time, it is useful to draw attention to the fact that the gas motor selected with a higher capacity will not work as efficiently as the one obtained at full load in some months, thus reducing the effectiveness of the system [18]. As a result, it can be clearly said that the electrical operating load for part 2, similar to Umuttepe part 1, can be 2 MW. According to this information, Tables 2 and 3 clearly show how the electrical load should be chosen.

The electrical loads of both parts can be met from the trigeneration system, which we have made feasible, and in the missing parts, 300–500 MW energies can be taken from the mains, and the excess production can be sold to the mains at the unit prices to be determined by EPDK [13]. Thus, the electricity to be produced by the selected gas engine at 2,028 kW h and hourly consumption amounts for Umuttepe Parts 1 and 2 sections were compared. The comparison is shown in Tables 2 and 3.

All the consumption and production, purchases, and sales on a monthly basis are shown in Table 2 (for Part 1) and Table 3 (for Part 2). It is clearly seen from Tables 2 and 3 that the excess production capacities that will be realized, thanks to trigeneration, can also reduce the possible energy costs.

3 System description

The system was designed for Kocaeli University Umuttepe Campus and the piping and instrumentation diagram (P&ID) was prepared with reference to the layout plans of the trigeneration system. According to this P&ID, from line losses to efficiency analyses of all lines and from technical calculations to mathematical modeling of the system has been detailed over a single module, exactly the same for both Umuttepe Parts 1 and 2. As a result, a trigeneration system in 1 + 1 module was designed for installation on the campus, but in a way to keep transmission and distribution leakages at a minimum level in each campus’s own site [9,19]. Accordingly, 1 ea. gas motor, 1 ea. multi ABS chiller, 1 ea. cooling tower, 1 ea. HT heat exchanger, 1 ea. LT heat exchanger, 1 ea. HT radiator, and 1 ea. LT radiator were selected in the facility.

The P&ID performs preliminary explorations and best possible determinations for the facility with maximum effort to minimize transmission and distribution losses of the facility and ensure that the facility can operate effectively (location, noise. level, distance from living area etc…) [9,19].

In this study, since the 1st law efficiency of thermodynamics (conversion efficiency) cannot be taken as the best possible measure, and because it may cause wrong evaluation of the systems, the 1st law efficiency cannot be revealed and/or is insufficient [20], and work potentials, exergy calculations, exergy yields, 2nd law of thermodynamics yields, and coefficient of performance (COP) values were found [21] for 10 ea. units and 26 ea. flows with the most realistic working criteria [22]. The mentioned engineering solutions, thermodynamic equilibrium equations, and mathematical modeling have been made in accordance with thermodynamic assumptions as given below:

1. All the systems are under thermodynamic, thermal and mechanical equilibrium.

2. The fluid circulating is water and the system is closed. Its specific heat c _p is assumed to be constant.

3. Pumps, possible frictions, and the other pressure losses are neglected.

4. The flow is continuous.

5. Dead state temperature and pressure are T ₀ = 25°C and P ₀ = 1 atm (101.325 kPa), respectively.

6. Kinetic, potential, and chemical energies are neglected.

3.1 Exergy losses, exergy, and energy analysis with 2nd law efficiency of thermodynamics

Each of the flow point is taken to be “t”.

Since there are 26 pieces of flow values assigned to the system, there will be as much flow as t ₁, t ₂, t ₃, …. t ₂₆.

Each of the unit is taken to be “i”.

“i” value sometimes forms a unit with only 2 flows, sometimes in conjunction with 4 flows, and since there are 10 units in total in the system, there will be as many as, i ₁, i ₂, i ₃, ….i ₁₀ units.

Accordingly, the total destruction exergy of all “i” values in the system is formulated with the following equations (1) and (2):

(1) Total destruct exergy = ∑ i = 1 10 Ė x ( kW ) ,

(2) ∑ Ė x destruction = Ė x İ 1 + Ė x İ 2 + Ė x İ 3 + Ė x İ 4 + Ė x İ 5 + Ė x İ 6 + Ė x İ 7 + Ė x İ 8 + Ė x İ 9 + Ė x İ 10 ( kW ) ,

(3) ∑ Ė x destruction = Ė x After cooler + Ė x LT Heat exchanger + Ė x Jacket water + Ė x HT Heat exchanger + Ė x LT Cooling radiator + Ė x HT Cooling tower + Ė x ABS,HT Generator + Ė x ABS Chiller,LT generator + Ė x ABS Chiller evaporator + Ė Cooling tower ( kW ) .

Accordingly, each value is defined as given below:

• Ėx _İ1 = Ėx _{After cooler},

• Ėx _İ2 = Ėx _{LT Heat exchanger},

• Ėx _İ3 = Ėx _{Jacket water},

• Ėx _İ4 = Ėx _{HT Heat exchanger,}

• Ėx _İ5 = Ėx _{LT Cooling radiator},

• Ėx _İ6 = Ėx _{HT Cooling tower},

• Ėx _İ7 = Ėx _{ABS,HT generator},

• Ėx _İ8 = Ėx _{ABS Chiller,LT generator,}

• Ėx _İ9 = Ėx _{ABS Chiller,evaporator}

• Ėx _İ10 = Ėx _{Cooling tower}.

All the results obtained by applying the equilibrium equations to all defined points in the system are substituted in equations (2) and (3) for the 26 flow points obtained and the total exergy loss is found as given below:

• Ėx _İ1 = 5.99 kW,

• Ėx _İ2 = 9.88 kW,

• Ėx _İ3 = 200.50 kW,

• Ėx _İ4 = 43.58 kW,

• Ėx _İ5 = 6.62 kW,

• Ėx _İ6 = 208.45 kW,

• Ėx _İ7 = 321.29 kW,

• Ėx _İ8 = 200.50 kW,

• Ėx _İ9 = 95.37 kW,

• Ėx _İ10 = 78.73 kW.

Exergy destruction for the entire feasibility system , ∑ i = 1 10 = 1170 . 96 kW is found.

According to the above information, the total exergy losses, loss percentages, and 2nd law efficiencies are given in Table 4. The total exergy losses and exergy loss percentages are given for each defined point in Figure 1. 2nd law efficiencies are given for each defined point in Figure 2.

Table 4

Exergy analysis results applied to each point of the feasibility system and 2nd law efficiency findings

Unit	Exergy destruction (kJ/h)	Exergy destruction (rate) (kW)	Exergy destruction (%)	2nd law efficiency of thermodynamics (%)(exergy efficiency)
After cooler	21574.08	5.992800	0.51	93.38
LT exchanger	35601.12	9.889200	0.84	89.77
Jacket water	721814.73	200.504092	17.12	61.92
HT exchanger	156914.73	43.587425	3.72	92.16
LT cooling radiator	23845.01	6.623613	0.57	93.38
HT cooling radiator	750451.68	208.458800	17.80	61.89
ABS chiller high temp. generator–HTG	1156674.65	321.298515	27.44	34.52
ABS chiller low temp. generator–LTG	721814.73	200.504092	17.12	61.92
ABS chiller evaporator	343340.68	95.372412	8.14	33.16
ABS chiller cooling tower	283432.00	78.731111	6.72	95.62
Total exergy destruction	4215463.41	1170.96	100%

Figure 1

Total exergy losses and exergy loss percentages of each point of the system.

Figure 2

2nd law efficiencies of system units/flow points (exergic efficiencies).

There are 7 ea. units, assigned to 26 ea. flow points and 10 ea. unit elements in the system [23]. The connection between units, unit elements, and the flow points are described in Table 5.

Table 5

System unit elements and assigned flow points in the feasibility system

System unit elements	Connected flows points	Connected units
Gas motor	t 1, t 2	i 1
	t 7, t 8	i 3
Multi ABS chiller	t 17, t 18, t 19, t 20	i 7
	t 21, t 22	i 8
	t 23, t 24	i 9
Cooling tower	t 25, t 26	i 10
HT exchanger	t 9, t 10, t 11, t 12	i 4
LT exchanger	t 3, t 4, t 5, t 6	i 2
HT cooling radiator	t 15, t 16	i 6
LT cooling radiator	t 13, t 14	i 5

4 Results and discussion

In this study, the control and verification of the design were provided through the exergic and energetic analyses applied for the trigeneration system. This is a method that has never been tried in the literature.

First of all, the realistic operating loads of the designed trigeneration system were clarified for electricity, heating, and cooling, after the technical visits to Umuttepe Campus. Cooling unit designs, efficiency and all engineering calculations, energy and exergy analyses of the system were made by considering the appropriate system design, the compatibility of the selected parameters with the production consumption values, availability, and performance values.

Thanks to the Multi ABS chiller in the system, seasonal heating and cooling can be adjusted. According to this, all flows and units specified in Table 5 and all Ėx values calculated in detail will not be used in all seasons. In other words, it can be said that the system elements that create the cooling energy to be obtained in the summer season by deactivating some flows and units will not be equal to the system elements that should be used in the winter season. Consequently, the Multi ABS Chiller is adjusted in summer mode in order to obtain cooling energy in the summer season by putting.

The Ėx values of the flows that come into and out of the trigeneration system for summer season are shown in Table 6. Thermodynamic poperties, exergy fluxes, and flow rates of each point defined and calculated below are given in Tables 7–10.

There are six different inputs to the system and six different outputs from the system in the summer mode. Total exergy losses are found by applying equation (4).

All the input and output flows in summer season mode are given in Tables 7–10. and the total destruct exergy is found by applying equation (5).

Ėx _destruction = 2296221.98 kJ/h = 637.83 kW.

From here

ƞ II = 1 − 637.83 kW 1701.75 kW = 0 . 62518 or ƞ II = ( Ė x 2 + Ė x 7 + Ė x 24 + Ė x 20 + Ė x 24 + Ė x 22 ) ( Ė x 1 + Ė x 8 + Ė x 23 + Ė x 19 + Ė x 23 + Ė x 21 ) = 0 . 62518 .

In the most general terms, from the data sheets given by the manufacturer of the gas motor and multi ABS chiller, chosen for the trigeneration system, we have

• Jacket temperature of 1,203 kW described in Manufacturer datasheet.

• It has been given that useful heat energy can be obtained from the exhaust gases [24], which are released at 421°C with the operation of the 985 kW gas motor.

• For the multi ABS chiller, it is stated that 10,360 kg/h of waste heat (flue gas-exhaust gases) can circulate in the entire system per hour.

• High concentration LiBr + water is used as mixing liquid in multi ABS chiller. In the light of the information given above, our aim is to obtain useful heat from the harmful and waste heat discharged from the chimney at 421°C, by applying equation (7) given above.

• System is in the adiabatic process

• c _p,exhaust = 1,121 kJ/kg K.

• Q = Expresses the maximum heat energy that can be obtained from the flue gas.

Q = 971.02 kW.

• The spent value of COP is 1,203 kW jacket heat + 971.02 kW flue gas heat,

• If 1,800 kW is the desired value (our aim is the energy).

COP SM = 1 , 800 kW 2 , 174 kW = 0 . 82 .

Cooling energy = ⌊1,800 kW × 0.82 = 1,476 kW⌋ ≈ 1.5 MW.

In order for the trigeneration system to be operated in accordance with the winter season, all the principles described above are the same for the system. It is calculated and read the Ėx values of the flows input and output in the trigeneration system for the winter season from Tables 7–10. The flow points are given in Table 11.

There are seven different inputs and seven different outputs to the system when operated in winter mode. The total exergy (usable) is found by applying equation (9) as given below:

Ėx _{destruction rate} = 2071005.23 kJ/h = 575.27 kW.

• 985 kW flue gas heat, ƞth (thermal efficiency) spent value,

• If 861 kW is ƞth (thermal efficiency) the desired value,

5 Conclusion

In this study, a feasibility of the trigeneration system, which is one of the on-site energy production methods, was prepared for Kocaeli University Umuttepe Campus, and energy and exergy analyses were carried out for each point of the system. The results of the exergy and energy analyses were found by making the assumptions that the designed system should be provided with 24 h uninterrupted electricity, heating in winter (for 6 months) and cooling for at least 8 h a day in summer.

According to the exergy and energy analyses finding at ABS chiller evaporator, it has been observed that a loss of 95.37 kW occurred in this unit, with the 2nd law efficiency increasing to 33.16%. Therefore, the most reasonable improvement is expected to be made here. When going into the details of this unit and possible improvements there, an economizer can be added to the flue gas output of the system.

In order to benefit more from the thermal energy, the efficiency of the system can be increased by drawing some more thermal energy from the waste flue gases at 120°C. In this way, the gases coming out of the system can be economized a little more, and the gases that will come out at 120°C can be passed through the last economizer and then discharged to the atmosphere at a lower temperature of 90°C. This will enable us to benefit from the system at a more environmental and more economical level, to obtain a little more heating and to obtain some more heat from the system with the heating collector.

Kocaeli University trigeneration system was made feasible as a result of exergic and energetic analyses.

• 0.87411 thermal efficiency, 2.05 MW heating capacity with an exergic efficiency of 66% in winter.

• COP of 0.82, 1.5 MW cooling capacity with 63% exergic efficiency in summer.

• 2.02 MWe instantaneous electrical power can be obtained non stop in summer and winter both Parts 1 and 2 of Umuttepe.

Exergic and energic analyses results show that the feasibility of trigeneration system in Umuttepe Campus is a facility that is open to development and worth establishing.