Analysis and evaluation of a new renewable energy based integrated system for residential applications

Energy and Buildings 128 (2016) 900–910

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Analysis and evaluation of a new renewable energy based integrated system for residential applications Satyam Panchal ∗ , Ibrahim Dincer, Martin Agelin-Chaab Department of Automotive, Mechanical and Manufacturing Engineering, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada

a r t i c l e

i n f o

Article history: Received 17 May 2016 Received in revised form 11 July 2016 Accepted 13 July 2016 Available online 22 July 2016 Keywords: Buildings Solar energy Efficiency Energy Exergy Renewable energy Multigeneration

a b s t r a c t In this study, a solar based Rankine cycle and geothermal based system for multigeneration is designed and developed. It consists of a Rankine cycle with reheating for power generation, an absorption chiller cycle for cooling, a drying process to dry wet products, useful heat from the condenser of the Rankine cycle, and other useful heat out from heat exchangers. The overall energy and exergy efficiencies of the single generation and multigeneration systems are studied, and it is observed that the energy efficiency of the multigeneration system is higher than the single generation system. The energy efficiency of the single and multigeneration systems are 7% and 37%, respectively. Similarly, the overall exergy efficiencies for the single generation and multigeneration systems are also studied and presented in this paper. In addition to this, parametric studies are performed to observe the effects of different substantial parameters, namely inlet pressure and temperature of the turbine, and reference environment temperature in order to investigate the variations in the system performance in terms of the energy and exergy efficiencies. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The energy and environmental crises and challenges around the globe in the last few decades are a major concern to all countries. Natural resources such as wind, rain, tides, waves, sunlight, geothermal heat and biomass are considered to be excellent renewable energy resources. These can be restored naturally after use [1,2]. Vast portions of renewable electricity production are encouraged to address global warming issues and the growing insufficiency of hydrocarbon fuels [3]. Since it is both available and environmental friendly, solar energy is nowadays a widely used technology [4,5]. The solar energy is the most significant renewable-based replacement for fossil fuels; however, solar energy presents some challenges due to its fluctuating nature during day time as well as locations [6]. These kinds of fluctuations

Abbreviations: COP, coefficient of performance; EBE, energy balance equation; EES, engineering equation solver; EnBE, entropy balance equation; ExBE, exergy balance equation; HPT, high pressure turbine; HTF, heat transfer fluid; kW, kilowatt; kg, kilo gram; kPa, kilo pascal; kg/s, kilo gram per second; kJ, kilo joule; LPT, low pressure turbine; Li-Br, lithium-bromide; MBE, mass balance equation; MG, multigeneration; SG, single generation; TES, thermal energy storage; s, second. ∗ Corresponding author. E-mail addresses: [email protected] (S. Panchal), [email protected] (I. Dincer), [email protected] (M. Agelin-Chaab). http://dx.doi.org/10.1016/j.enbuild.2016.07.038 0378-7788/© 2016 Elsevier B.V. All rights reserved.

are an issue that requires potential solutions. Geothermal energy is also considered a reliable and environmental energy source which uses heat energy from the earth. There are a variety of applications for geothermal energy sources depending upon the amount of heat supplied to the plant, such as electricity production, space heating, water heating, cooling, agricultural drying, desalination, and industrial process heating [7]. One way to use geothermal energy more efficiently is by combining it with another renewable energy source such as solar energy. By combining two energy sources such as solar and geothermal, the deficiency of one source can be overcome by coupling with the other, which leads to a more efficient system. Another way to increase the efficiency of a system is by reducing the losses and using the waste heat to produce a useful commodity which leads to a cogeneration, trigeneration or multigeneration system [7]. In thermodynamic analyses of energy systems, exergy analysis, also called second law analysis, is a powerful tool. In other words, it is widely used in the design, simulation and performance evaluation of energy systems [4]. Dincer at el. [8] described the relationships between energy and exergy, exergy and the environment, energy and sustainable development, and energy policy in detail. The exergy analysis method is employed to detect and to evaluate quantitatively the causes of the thermodynamic imperfection of the process under consideration. Therefore, it can indicate the possibilities of thermodynamic improvement of the process under

S. Panchal et al. / Energy and Buildings 128 (2016) 900–910

Nomenclature ˙ Ex ex h ˙ m P Q˙ s ˙ W ◦C

or K

Exergy rate (kW) Specific exergy (kJ/kg) Specific enthalpy (kJ/kg) Mass flow rate (kg/s) Pressure (kPa) Heat rate (kW) Specific entropy (kJ/kg K) Work rate (kW) Degree celsius or kelvin

Greek symbols  Energy efficiency Exergy efficiency Subscripts Absorber A avg Average Boiler B C Condenser D Destruction E Evaporator en Energy Exergy ex FC Flash chamber G Generator geo Geothermal Heat exchanger HE in Input net Net output Output out P Pump prod Product RC Rankine cycle Source S T Turbine 1, 2. . .46 State numbers 0 Ambient (or reference environment) condition

901

and gas turbine inlet temperature on system performance. They also found the energy and exergy efficiencies to be 54% and 58%, respectively. Coskun et al. [13] also examined mulitgeneration geothermal base systems and they found that the overall system energy and exergy efficiency increased by 3.40 and 1.12 times for the cooling season and about 4.25 and 1.25 times for the heating season, compared to the single power generating option. In another study, Al Zaharani et al. [14] also developed a multigeneration system by cascading a supercritical carbon dioxide (CO2 ), Rankine cycle with Organic (R600) for power generation, hydrogen production and space heating. In their study, they found that the multigeneration system with geothermal energy has higher overall energy and exergy efficiency in the system. Ratlamwala et al. [15] proposed another geothermal base mulitgeneration system and found that increasing the geothermal source temperature, pressure and mass flow rate results in an increase in power and rate of hydrogen production. Lastly, Ozlu et al. [16] conducted a study of the exergy analysis of a solar thermal power system. They basically performed an energy and exergy analysis for the energy-based multigeneration system. Dincer at el. [17] confirmed that a multigeneration renewable energy base system offers better efficiency, cost, sustainability and environment. The literature suggests that multigeneration is advantageous to reduce greenhouse gas emissions and to help increase efficiency. This paper aims to develop, analyze and assess a new solar and geothermal energy based integrated system for multigeneration. In this regard, this study primarily consists of: • Development, design and analysis of a solar and geothermal based system integrated system for multigeneration. • Determination of energy and exergy efficiencies of all subunits, subsystems and the overall system for performance assessment and evaluation and possible improvements. • Calculation of energy losses and exergy destructions of all major system components. • Undertaking parametric studies to investigate the effects of varying various operating conditions and state properties on the system performance. 2. System description

consideration. The concepts of exergy, available energy, and availability are fundamentally alike. The concepts of exergy destruction, exergy consumption, irreversibility, and lost work are also essentially similar. Exergy is a measure of the maximum useful work that can be done by a system interacting with an environment that is at a constant pressure P0 and a temperature T0 [9]. A multigeneration system is one which gives several outputs such as electricity, heating, cooling, and drying. Multigeneration utilises the waste heat of a power plant to improve overall thermal performance, basically consuming the “free” energy available via the waste energy [10]. A number of single and integrated systems have been analyzed by many researchers. Al-Sulaiman et al. [6] studied a multigeneration with trigeneration and showed that the maximum electrical energy efficiency was 14%, while with trigeneration alone the energy efficiency increased to 94%. Ahmadi et al. [11] developed a biomass based integrated multigeneration system in which he studied both the thermoeconomic and multiobjective optimization. In that system, he used exhaust gases from an organic rankine cycle (ORC) turbine for a heating process and a double-effect absorption chiller for a cooling effect. Ozturk et al. [12] proposed an integrated solar power tower and coal gasification system for multi-generation purposes. They conducted parametric studies to show the effects of environment temperature, compressor pressure ratio, nitrogen supply ratio for a combustion chamber

The system primarily uses two renewable sources, i.e., solar and geothermal energies. The integrated system can be subdivided into three main cycles/sub-systems, namely: Rankine cycle, geothermal water cycle, and finally a vapour absorption cycle. Fig. 1 shows a schematic diagram of an integrated solar-geothermal system for multi-generation purposes. This multigeneration system comprises a solar system to raise the temperature of the heat transfer fluid (HTF), Rankine cycle with reheating to produce electricity, a geothermal source to run the turbine, an absorption chiller for cooling, and a heat exchanger to produce hot dry air for drying purposes. 2.1. Rankine cycle Here, we use a solar energy system, which consists of two subsystems: the collector—receiver subsystem and the heat engine subsystem. The collector–receiver circuit consists of a number of parabolic collectors, organized in units that operate in tracking mode so that the working fluid goes through them. The heat engine circuit, which is basically a Rankine cycle, consists of a boiler, two-stage turbine (HPT and LPT or turbine 1 and 2), pump, and a condenser. The hot fluid enters the boiler heat exchanger at 600 ◦ C (state 19) where it heats up the working fluid of the heat engine.

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Fig. 1. Schematic diagram of the system.

The working fluid in the heat engine circuit enters (state 8) at 200 ◦ C and exits (state 1) at 450 ◦ C. The working fluid of the Rankine cycle changes phase in the steam generator and produces power in the turbines, which produce electricity. The pressure at the inlet of the high pressure turbine (state 1) is 5000 kPa at 450 ◦ C. The working fluid is reheated in the boiler (process 2–3) and the temperature and pressure at the inlet to the low pressure turbine (state 3) is now 450 ◦ C and 5000 kPa, respectively. The temperature of the condenser is 120 ◦ C and the ambient temperature is 25 ◦ C. Out of this cycle, two outputs are taken, such as: (1) power output for city, (2) condenser output for the absorption chiller. 2.2. Absorption chiller cycle The heat loss from the condenser is basically used for heating. The steam leaving the Rankine cycle (state 5) is utilized to run the absorption system by supplying the heat energy to the generator. This is called a process heater. In other words, an absorption chiller is a heat functioned refrigeration device that operates on one of the earliest known principles of refrigeration [4]. The cycle uses a refrigerant or primary fluid (water in this case) and an absorbent also known as a secondary fluid (LiBr in this case). The refrigerant is absorbed by the absorbent for the purpose of transferring heat. Usually, there are two types of absorption systems available, based on the refrigerant and absorber combination. These are (1) ammonia/water system, and (2) lithium bromide/water (LiBr-H2 O) combination system. Lithium bromide/water chillers are more appropriate for space cooling while ammonia/water systems offer industrial cooling to as low as −50 ◦ C [18]. The evaporation of the

primary fluid removes heat, thus providing the refrigeration effect. The absorption cycle uses a heat operated generator, heat rejection absorber and a liquid solution pump. As explained in the above paragraph, an absorption chiller cycle uses the waste heat from the reheating Rankine cycle to provide cooling. Instead of using a conventional vapour-compression cycle, an absorption refrigeration system was used to avoid the large amount of electrical power required in order to drive the compressor in the vapour-compression cycle. Conversely, the coefficient of performance (COP) of vapour-compression is higher than absorption chillers. For instance, an absorption chiller usually has a COP within 0.5 and 1.5, whereas modern vapour compression cycles have COPs in excess of 3.0 [18–20]. 2.3. Geothermal water cycle The geothermal water is taken out from ground (state 29) at 500 kPa which also contains some impurities. Therefore, it has to go through a flash chamber and a flash separator which separates the liquid and gas. The gas, at high temperature, is used for space/room heating, thus maintaining the room temperature. Here, the brine is used as a working fluid. Brine at high temperature is also considered as a working fluid in order for the turbine to produce the electricity. In the geothermal cycle, the heat energy from turbine 3 (state 36) is utilized in heat exchanger 5 (HE5) to heat the cold water (state 38) that is then supplied to the building. The heat energy (state 33) from heat exchanger 3 (HE3) is also used to heat the air (state 40) in heat exchanger 4 (HE4) and this heated air is supplied to the building (state 41). The power output from turbine 3 is also supplied to the

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Table 1 Thermodynamic properties at each state of the system. S.No.

Temperature (◦ C)

Pressure (kPa)

Mass Flow (kg/s)

Specific Enthalpy (kJ/kg)

Specific Entropy (kJ/kg K)

Specific Exergy (kJ/kg)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

25 450 176.6 450 – 120 70 92.16 200 71.64 80 41.36 36.06 34 34 78.2 40.11 1.5 1.5 600 264.8 224.7 165 – 120 200 200 151.9 151.9 151.9 90 – 89.96 71.96 25 65 25 90 25 60 110 35 25

101.3 5000 500 500 75 75 75 5000 5000 7.424 7.424 7.424 0.6812 0.6812 7.424 7.424 7.424 0.6812 0.6812 101.3 101.3 101.3 101.3 101.3 101.3 500 1800 500 500 500 500 500 70 70 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

– 40 40 40 40 40 40 40 40 55.81 55 55 55 55.81 55.81 0.8116 0.8116 0.8116 0.8116 372 372 372 372 372 372 200 200 200 179.9 179.9 179.9 20.12 20.12 20.12 100.6 100.6 2819 2819 0.65 0.1 0.1 0.1 65

104.8 3316 2804 3377 2916 – 384.4 389.8 853.8 166 185.6 109 109 90.43 90.43 2646 168 168 2503 661.6 335.2 285.3 98.52 98.52 66.02 2855 852.5 852.5 640.4 1027 1.192 2749 2463 1518 104.8 174.4 298.6 364 – – 238.9 90.25 52.9

0.3669 6.819 6.949 7.945 8.051 – 1.213 1.214 2.325 0.4249 0.4665 0.2378 0.2048 0.1952 0.1952 8.471 0.5737 0.6116 9.114 1.147 0.519 0.4207 0.2387 0.2387 – 7.059 2.33 2.36 1.861 2.77 1.192 6.821 6.939 4.335 0.3669 0.5939 5.695 5.894 – – 6.378 5.922 –

– 1289 738.1 1014 521.4 – 27.43 32.58 165.3 66.68 73.93 65.45 75.28 59.59 59.6 126 1.486 −9.786 −208.3 324.2 185.1 164.5 31.88 31.88 – 756 162.5 153.7 90.3 205.8 −349.6 720.3 400.2 230.7 0 1.904 −1394 −1388 – – 14.08 0.318 –

building. To make the system environmentally friendly and more efficient, geothermal brine is re-injected into the well.

3. Thermodynamics analysis and assessment A comprehensive energy and exergy analyses is accomplished for the proposed multi-generation system. This would provide substantial information about the performance, efficiency and emissions of the cycle. For this study the following assumptions were made and calculations were performed for the inlet and outlet enthalpies, entropies, exergies, mass flow rates, pressures and temperatures. Exergy destructions were also calculated for the system irreversibilities. The following assumptions are made: The ambient temperature and pressure are T0 = 25 ◦ C and P0 = 101.325 kPa. • Air is treated as an ideal gas. • The changes in the kinetic and gravitational terms in the energy and exergy balances are negligible. • The turbines and pumps are adiabatic. • No heat losses take place in the expansion valve in absorption chiller system. • The condenser outlet state is saturated liquid and evaporator exit in a saturated vapor in the absorption chiller cycle.

• The pressure losses in all the heat exchangers and the pipelines are neglected. • There is no chemical reaction between the refrigerant and absorbent in absorption refrigeration cycle. Therefore, chemical exergy is neglected and only physical exergy is taken into account. The related energy and exergy balances for the important sections of the system are discussed in the following sections: 3.1. Rankine cycle In Fig. 1, the high temperature working fluid enters the high pressure turbine at state 1 and after expansion leaves the turbine at state 2. Then there is a reheating between state 2 to state 3. Process 3–4 is an expansion in low pressure turbine (turbine 2). To determine the enthalpies and work output of the turbines, the following energy balance equation can be used: 3.1.1. High pressure turbine (turbine 1) ˙ 1=m ˙2 MBE : m

(1)

˙ T1 ˙ 1 h1 = m ˙ 2 h2 + W EBE : m

(2)

˙ ˙ 1 s1 + Sgen ˙ 2 s2 EnBE : m T1 = m

(3)

˙ T 1 + Ex ˙ D,T 1 ˙ 1 ex1 = m ˙ 2 ex2 + W ExBE : m

(4)

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3.1.2. Low pressure turbine (turbine 2)

3.2.2. Absorber

˙ 3=m ˙4 MBE : m

(5)

˙ 12 + m ˙ 18 = m ˙ 13 MBE : m

(29)

˙ T2 ˙ 3 h3 = m ˙ 4 h4 + W EBE : m

(6)

˙ 12 h12 + m ˙ 18 h18 = m ˙ 13 h13 + Q˙ loss,A EBE : m

(30)

˙ ˙ 3 s3 + Sgen ˙ 4 s4 EnBE : m T2 = m

(7)

˙ ˙ 12 s12 + m ˙ 18 s18 + Sgen ˙ 13 s13 + Q˙ loss,A /TSA EnBE : m A =m

(31)

˙ T 2 + Ex ˙ D,T 2 ˙ 3 ex3 = m ˙ 4 ex4 + W ExBE : m

(8) ˙ 12 ex12 + m ˙ 18 ex18 = m ˙ 13 ex13 + Q˙ loss,A (1 − ExBE : m

3.1.3. Condenser (C 1) ˙ 4=m ˙5 MBE : m

T0 ˙ D,A ) + Ex TSA (32)

(9)

˙ 4 h4 = m ˙ 5 h5 + Q˙ out,C1 EBE : m

(10)

˙ ˙ 4 s4 + Sgen ˙ 5 s5 + Q˙ out,C1 /T0 EnBE : m C1 = m

(11)

˙ 4 ex4 = m ˙ 5 ex5 + Q˙ out,C1 (1 − ExBE : m

T0 ˙ D,C1 ) + Ex TS

(12)

3.1.4. Pump (P 1) ˙ 6=m ˙7 MBE : m

(13)

˙ P1 = m ˙ 6 h6 + W ˙ 7 h7 EBE : m

(14)

˙ ˙ 6 s6 + Sgen ˙ 7 s7 EnBE : m P1 = m

(15)

˙ P1 = m ˙ D,P1 ˙ 6 ex6 + W ˙ 7 ex7 + Ex ExBE : m

(16)

3.2.3. Heat exchanger 2 (HE2) ˙ 14 = m ˙ 9 & Weak solution:m ˙ 10 = m ˙ 11 MBE : Refrigerant (strong solution):m

(33)

˙ 14 h14 + m ˙ 10 h10 = m ˙ 9 h9 + m ˙ 11 h11 + Q˙ loss,HE2 EBE : m

(34)

˙ ˙ 14 s14 + m ˙ 10 s10 + Sgen ˙ 9 s9 + m ˙ 11 s11 + Q˙ loss,HE2 /T0 EnBE : m HE2 = m (35)

˙ 14 ex14 + m ˙ 10 ex10 = m ˙ 9 ex9 + m ˙ 11 ex11 + Q˙ loss,HE2 (1 − ExBE : m

3.1.5. Heat exchanger-1 (HE 1)

T0 ˙ D,HE2 ) + Ex TS,HE2

(36)

˙ 20 = m ˙ 21 and m ˙ 7=m ˙8 MBE : m

(17)

3.2.4. Condenser 2 (C2)

˙ 20 h20 + m ˙ 7 h7 = m ˙ 21 h21 + m ˙ 8 h8 + Q˙ loss,HE1 EBE : m

(18)

˙ 15 = m ˙ 16 MBE : m

(37)

˙ 15 h15 = m ˙ 16 h16 + Q˙ out,C2 EBE : m

(38)

˙ ˙ 20 s20 + m ˙ 7 s7 + Sgen ˙ 21 s21 + m ˙ 8 s8 + Q˙ loss,HE1 /T0 EnBE : m HE1 = m

˙ 20 ex20 + m ˙ 7 ex7 = m ˙ 21 ex21 + m ˙ 8 ex8 + Q˙ loss,HE1 (1 − ExBE : m

T0 ˙ D,HE1 ) + Ex TS

(19)

˙ 15 s15 + S˙ gen,C2 = m ˙ 16 s16 + EnBE : m

(20)

˙ 15 ex15 = m ˙ 16 ex16 + (1 − ExBE : m

3.1.6. Steam generator (boiler)

Q˙ out,C2 T0

T0 ˙ ˙ D,C2 )Qout,C2 + Ex Ts

(39) (40)

3.2.5. Evaporator (E)

˙ 1=m ˙ 8 and m ˙ 2=m ˙3 MBE : m

(21)

˙ 8 h8 + m ˙ 2 h2 + Q˙ in,B = m ˙ 1 h1 + m ˙ 3 h3 EBE : m

(22)

˙ ˙ ˙ 8 s8 + m ˙ 2 s2 + Sgen ˙ 2 s2 = m ˙ 1 s1 + m ˙ 3 s3 (23) EnBE : m B + QinB /TB + m T0 ˙ D,B ˙ 8 ex8 + m ˙ 2 ex2 + Q˙ in,B (1 − ˙ 1 ex1 + m ˙ 3 ex3 + Ex ExBE : m )=m TB (24)

˙ 17 = m ˙ 18 MBE : m

(41)

˙ 17 h17 + Q˙ in,E = m ˙ 18 h18 EBE : m

(42)

˙ 17 s17 + S˙ gen,E + EnBE : m ˙ 17 ex17 + (1 − ExBE : m

Q˙ in,E ˙ 18 s18 =m T0

T0 ˙ ˙ D,E ˙ 18 ex18 + Ex )Q =m Ts in,E

(43) (44)

3.2.6. Pump 2 (P2) 3.2. Lithium-bromide water absorption chiller cycle

˙ 13 = m ˙ 14 MBE : m

(45)

In this cycle, a working fluid (LiBr-water) is heated in the generator as shown in Fig. 1. Li-Br and water separation takes place in the generator. The evaporator provides a cooling effect and the following are the energy and exergy equations used in a thermodynamics analysis of the absorption chiller cycle.

˙ P2 = m ˙ 13 h13 + W ˙ 14 h14 EBE : m

(46)

˙ 13 s13 + S˙ gen,P2 = m ˙ 14 s14 EnBE : m

(47)

˙ P2 = m ˙ D,P2 ˙ 13 ex13 + W ˙ 14 ex14 + Ex ExBE : m

(48)

3.2.1. Generator

3.3. Geothermal cycle

˙ 9=m ˙ 10 + m ˙ 15 MBE : m

(25)

˙ 9 h9 + Q˙ in,G = m ˙ 10 h10 + m ˙ 15 h15 EBE : m

(26)

˙ ˙ ˙ 9 s9 + Sgen ˙ 10 s10 + m ˙ 15 s15 EnBE : m G + Qin,G /TG = m

(27)

˙ 9 ex9 + Q˙ in,G (1 − ExBE : m

T0 ˙ D,G ˙ 10 ex10 + m ˙ 15 ex15 + Ex )=m TG

(28)

The geothermal cycle is shown in bottom half of Fig. 1. In this cycle, there are three outputs: (1) electricity for the building, (2) hot water supplied to the building, and (3) the hot air to the building. To make the system environmentally friendly and more efficient, geothermal brine is re-injected back into the well. The following Equations are used for the geothermal cycle.

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3.3.1. Geothermal pump 4 (P4) ˙ 29 = m ˙ 30 MBE : m

(49)

˙ P4 = m ˙ 29 h29 + W ˙ 30 h30 EBE : m

(50)

˙ 29 s29 + S˙ gen,P4 = m ˙ 30 s30 EnBE : m

(51)

˙ P4 = m ˙ D,P4 ˙ 29 ex29 + W ˙ 30 ex30 + Ex ExBE : m

(52)

˙ 40 ex40 + m ˙ 33 ex33 = m ˙ 41 ex41 + m ˙ 34 ex34 + (1 − ExBE : m

T0 ˙ D,HE4 )Q˙ loss,HE4 + Ex Ts,HE4

(72)

3.3.7. Heat exchanger 5 (HE5) ˙ 38 = m ˙ 39 , & m ˙ 36 = m ˙ 37 MBE : m

3.3.2. Flash chamber (FC1)

˙ 36 h36 + m ˙ 38 h38 EBE : m

˙ 30 = m ˙ 31 MBE : m

(53)

˙ 30 h30 = m ˙ 31 h31 EBE : m

(54)

˙ 30 s30 + S˙ gen,FC1 = m ˙ 31 s31 EnBE : m

(55)

˙ D,FC1 ˙ 30 ex30 = m ˙ 31 ex31 + Ex ExBE : m

(56)

˙ 36 s36 + m ˙ 38 s38 + S˙ gen,HE5 = m ˙ 37 s37 + m ˙ 39 s39 + EnBE : m

˙ D,HE5 + (1 − ˙ 36 ex36 + m ˙ 38 ex38 = m ˙ 37 ex37 + m ˙ 39 ex39 + Ex ExBE : m

3.3.3. Separator 1 (S1) ˙ 31 = m ˙ 32 + m ˙ 35 MBE : m

(57)

˙ 31 h31 = m ˙ 32 h32 + m ˙ 35 h35 + Q˙ loss,S1 EBE : m

(58)

˙ 31 s31 + S˙ gen,S1 = m ˙ 32 s32 + m ˙ 35 h35 + EnBE : m

˙ 31 ex31 = m ˙ 32 ex32 + m ˙ 35 ex35 + (1 − ExBE : m

(73)

˙ 37 h37 + m ˙ 39 h39 + Q˙ loss,HE5 =m

(74) Q˙ loss,HE5 T0 (75)

T0 )Q˙ loss,HE5 Ts,HE5

(76)

3.4. Drying process

Q˙ loss,S1 T0

(59)

T0 ˙ ˙ D,S1 )Q + Ex Ts loss,S1 (60)

The hot and dry air is heated by HTF in heat exchanger 6 to dry a wet product. The hot air enters the dryer with a temperature of 110 ◦ C and a relative humidity of 5% while the incoming wet product from room temperature has a relative humidity of 60%.The enthalpies and exergy destruction are defined by the following balanced equations: ˙ air hair )in + (m ˙ prod hprod )in + (m ˙ water hwater )in = (m ˙ air hair )out (m

3.3.4. Turbine 3 (T3) ˙ 35 = m ˙ 36 MBE : m

(61)

˙ T3 ˙ 35 h35 = m ˙ 36 h36 + W EBE : m

(62)

˙ 35 s35 + S˙ gen,T 3 = m ˙ 36 s36 EnBE : m

(63)

˙ T 3 + Ex ˙ D,T 3 ˙ 35 ex35 = m ˙ 36 ex36 + W ExBE : m

(64)

˙ prod hprod )out + (m ˙ water hwater )out + Q˙ loss,Dryer +(m



˙ air exair )in + m ˙ prod exprod (m



˙ prod exprod + m

 out

 in

(77)

˙ water exwater )in = (m ˙ air exair )out + (m

˙ Dest + Q˙ loss,Dryer (1 − ˙ water exwater )out + Ex + (m dryer

T0 ) Tavg

(78)

3.3.5. Heat exchanger 3 (HE3) ˙ 32 = m ˙ 33 , & m ˙ 21 = m ˙ 22 MBE : m

(65)

˙ 32 h32 + m ˙ 21 h21 = m ˙ 33 h33 + m ˙ 22 h22 + Q˙ loss,HE3 EBE : m

(66)

˙ 32 s32 + m ˙ 21 s21 + S˙ gen,HE3 = m ˙ 33 s33 + m ˙ 22 s22 + EnBE : m

Q˙ loss,HE3 T0 (67)

3.5. Energy and exergy efficiencies The performance of any thermodynamic system or process or application is defined in terms of useful output divided by the total input. In this regard, for each component of the system, the energy efficiency can easily be defined as the ratio of the useful energy output from the system to the total energy input and is given by Eq. (79). =

˙ 32 ex32 + m ˙ 21 ex21 = m ˙ 33 ex33 + m ˙ 22 ex22 + (1 − ExBE : m

T0 Ts,HE1

˙ D,HE3 )Q˙ loss,HE3 + Ex

(68)

(69)

˙ 40 h40 + m ˙ 33 h33 = m ˙ 41 h41 + m ˙ 34 h34 + Q˙ loss,HE4 EBE : m

(70)

˙ 40 s40 + m ˙ 33 s33 + S˙ gen,HE4 = m ˙ 41 s41 + m ˙ 34 s34 + EnBE : m

Useful Exergy Output Total Exergy Destruction =1− Useful Exergy Input Total Exergy Input



˙ 40 = m ˙ 41 , & m ˙ 33 = m ˙ 34 MBE : m

Q˙ loss,HE4 T0 (71)

(79)

Correspondingly, the exergy efficiency is also defined as the ratio of the useful exergy output from the system to the total exergy input and is given by Eq. (80).  =

3.3.6. Heat exchanger 4 (HE4)

Useful Energy Output Useful Energy Input

(80)



Exergy inlet − Exergy outlet = where Total exergy destruction. Now, we can define both energy and exergy efficiencies for the developed system as well as its subsystems. In this regard, for the absorption cooling cycle the energetic coefficient of performance (COPen ) is also given by Eq. (81). COPen =

Q˙ E E Chilled Water = ˙QG + W E Heat Source ˙ P

(81)

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Similarly, the exergetic coefficient of performance (COPex ) is defined as the ratio of the useful exergy obtained from a system to the exergy that is supplied to the system. Thus, (COPex ) of the absorption chiller for the cooling is the ratio of the chilled water exergy at the evaporator to exergy of heat source at the generator and is given by Eq. (82). COPex =

˙ 17 (ex18 − ex17 ) m ˙ 5 (ex6 − ex5 ) m

(82)

The energy efficiency of the Rankine cycle is defined as the ratio of the network output to the total energy input and is given by Eq. (83). RankineCycle =

˙ net W ˙ RC En

(83)

in

Likewise, the exergy efficiency of the Rankine cycle is defined as the ratio of the network output to the total exergy input and is given by Eq. (84). RankineCycle

=

˙ net W ˙ RC in Ex

(84)

For the geothermal power cycle, the energy and exergy efficiencies are defined exactly the way we define them for the steam power plant and are given by Eqs. (85) and (86): geo

geo

˙ net W = ˙ geo in En =

˙ net W ˙ geo Ex

(85)

(86)

in

Finally, the energy and exergy efficiency of the overall system is determined by Eqs. (87) and (88) respectively.

Fig. 2. Effect of ambient temperature on the overall energy and exergy efficiencies of the system with and without multigeneration.

varied. These design parameters played a significant role in the system output. Variations of these parameters are discussed in detail below. 4.1. Effect of ambient temperature on the overall energy and exergy efficiencies The ambient temperature has a great effect on the energy and exergy of the overall system. All subunits and variations in ambient conditions can increase or decrease the performance. Fig. 2 shows the effects of ambient temperature on the overall energy and exergy

overall =

˙ P1 − W ˙ T −W ˙ P4 + Q˙ e + (m ˙ 44 h44 − m ˙ 45 h45 ) + (m ˙ 41 h41 − m ˙ 40 h40 ) + (m ˙ 39 h39 − m ˙ 38 h38 ) W (87) ˙ 29 h29 − m ˙ 37 h37 − m ˙ 34 h34 Q˙ in,B + m

overall =

˙ P1 − W ˙ P4 + Q˙ ex + (m ˙ T −W ˙ 44 ex44 − m ˙ 45 ex45 ) + (m ˙ 41 ex41 − m ˙ 40 ex40 ) + (m ˙ 39 ex39 − m ˙ 38 ex38 ) W (88) ˙ 37 ex37 − m ˙ 34 ex34 ˙ 29 ex29 − m m

˙ T1 + W ˙ T2 + W ˙ T3. ˙ T =W where W 4. Results and discussion A number of performance evaluation and comparisons are made and are presented in Tables 1 and 2. In addition how the system behaves under different operating and environmental conditions was investigated and plotted in Figs. 2–13. All the thermodynamic properties at each state point, i.e., mass flow rate (kg/s), temperature (◦ C), pressure (kPa), specific enthalpy (kJ/kg), specific entropy (kJ/kg K) and specific exergy (kJ/kg), are tabulated in Table 1. These values are calculated using EES software through its built-in databases. Table 2 contains the important outputs of each cycle, including the COPs of the absorption chiller, the work output of turbines, exergy destructions at each subunit of the cycle, and entropy generation at each component in the cycle. Using the assumptions stated above where T0 = 25 ◦ C and P0 = 101.325 kPa, the energy efficiencies of turbines 1, 2 and 3 are found to be 16%, 14%, and 11% while the exergy efficiencies of turbines 1, 2, and 3 are 97%, 97%, and 96%, respectively. This is remarkable mainly because the turbines are assumed to be adiabatic. The exergy destroyed in the turbines (HPT and LPT) is also calculated, it is found that the HPT has higher exergy destruction (1552 kW) than the LPT (1271 kW). To investigate the performance variation of the multigeneration system in terms of energy and exergy efficiencies, certain design parameters, such as ambient temperature, inlet pressure and temperature of the turbine, were

efficiencies of the system with and without multigeneration (i.e single and multigeneration). To determine the effect of changes in ambient temperature on energy and exergy efficiency, as well as on the exergy destruction rate of the major components in the system, the ambient temperature was varied from 15 ◦ C to 30 ◦ C. The results show that the exergy efficiency of single (SG ) and multigeneration (MG ) systems increases when the ambient temperature increases. It is also observed that there is not much change in the energy efficiency of the single and multigeneration system while changing the ambient temperature. The efficiency of the pumps and valves is mainly unaffected by the change in environment temperatures. 4.2. Effect of turbine inlet temperature on the overall energy and exergy efficiencies The inlet temperature of turbines is another important parameter for any power generation. Normally, incoming high temperature and pressure fluids produce more power output at the expense of pressure energy into kinetic energy which moves the turbine blade. Fig. 3 explains the effects of the turbine inlet temperature on the overall energy and exergy efficiencies of the single and multigeneration system. In order to study the effect, the turbine inlet temperature varies between 300 ◦ C to 600 ◦ C, and the results show that the energy efficiency of the single generation system (SG )

S. Panchal et al. / Energy and Buildings 128 (2016) 900–910

Fig. 3. Effect of turbine inlet temperature on the overall energy and exergy efficiencies of the system with and without multigeneration.

Fig. 4. Effect of turbine inlet pressure on energy efficiency of high and low pressure turbine.

increases from 5% to 6%, while the multigeneration system (MG ) decreases from 40% to 35%. Similarly, the exergy efficiency of a single generation system ( SG ) increases from 17% to 25%, and for the multigeneration system ( MG ), it decreases from 27% to 34% with an increase in the turbine inlet temperature. 4.3. Effect of turbine inlet pressure on the turbine energy and exergy efficiencies The power output and efficiency of turbines generally depends on the turbine inlet pressure and temperature. Fig. 4 shows the effect of turbine inlet pressure on the energy efficiency of the turbines (high and low pressure turbines). Here, the inlet pressure varies between 3000 kPa to 6000 kPa in order to study the effect of changes in the turbine inlet pressure on the energy efficiency of the high pressure turbine. It is observed that the energy efficiency of the high pressure turbine (T 1 ) increases between 13% to 17% when the turbine inlet pressure increases. Not much change in the energy efficiency of the low pressure turbine (T 2 ) is observed with an increase in inlet pressure. A similar study was also conducted on the exergy efficiency for HPT and LPT. Fig. 5 shows the effect of turbine inlet pressure on the exergy efficiency of turbines 1 and 2 (high and low pressure turbines). Here, the inlet pressure also varies between 3000 kPa to 6000 kPa, and it is found that the exergy efficiency of the high pressure turbine ( T 1 ) decreases from 98% to 97% when the turbine inlet pressure increases. The exergy efficiency of

907

Fig. 5. Effect of turbine inlet pressure on exergy efficiency of high and low pressure turbine.

Fig. 6. Effect of inlet pressure of the geothermal cycle on the geothermal turbine efficiency.

the low pressure turbine ( T 2 ) is almost constant during changes in the inlet pressure of the turbine. 4.4. Effect of geothermal inlet pressure on the turbine efficiencies Fig. 6 shows the effects of inlet pressure of the geothermal cycle on the energy and exergy efficiencies of turbine 3 within the geothermal plant. To determine the effect of changes in inlet pressure on energy and exergy efficiency, as well as the exergy destruction rate for the major components in the geothermal cycle, the inlet pressure varies between 1000 kPa to 3000 kPa, which shows that the energy efficiency of turbine 3 (T 3 ) increases between 15% to 20% when the inlet pressure increases. The exergy efficiency of turbine 3 ( T 3 ) is almost constant and not much change is observed with the variation in inlet pressure. 4.5. Effect of mass flow rate on the overall energy and exergy efficiencies Fig. 7 shows the effect of the inlet mass flow rate on the overall energy and exergy efficiency of single and multigeneration systems. Here, the mass flow rate varies between 5 kg/s to 90 kg/s and the results show that the energy efficiency (MG ) of the multigeneration system decreases from 60% to 20% with the increase in mass flow rate. For the single generation system, the energy efficiency (SG ) increases between 2% to 11% with an increase in mass flow rate. Similarly, Fig. 8 shows the overall exergy efficiency of the multigen-

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Fig. 7. Effect of mass flow rate of steam on the overall system efficiency.

Fig. 10. Effect of ambient temperature on the exergy destroyed in the turbines (T1, T2, and T3).

Exergy Destruction Rate (kW)

40000 35000 30000 ExD

B

25000

ExD

HE1

20000

ExD

T1

15000 10000 5000 0 3000

3500

4000

4500

5000

5500

6000

P1 (kPa) Fig. 8. Effect of mass flow rate of steam on the overall exergy efficiency of the system.

Fig. 11. Effect of turbine inlet pressure on the exergy destroyed in the boiler, high pressure turbine, and heat exchanger.

ine cycle increases with an increase in mass flow rate. For turbine ˙ D,T 1 increases from 400 kW to 3500 kW and for the turbine 1, the Ex ˙ D,T 2 rises from 300 kW to 2800 kW when the mass flow rate 2 the Ex increases from 10 kg/s to 90 kg/s. ˙ D) 4.6. Exergy destruction (Ex

Fig. 9. Effect of mass flow rate of steam on the exergy destroyed in the turbine (HPT and LTP).

eration and single generation system, with the variation in the mass flow rate. As the mass flow rate parameter increases from 10 kg/s to 90 kg/s, the exergy efficiency of the multigeneration system ( MG ) increases from 21% to 45%, while for the single generation system, exergy efficiency ( SG ) increases between 8% to 44%. Also, the effect of mass flow rate has greater influence on the exergy destruction rate of turbines and the effect is shown in Fig. 9. Here, the exergy destruction rate for turbine 1 (HPT) and turbine 2 (LPT) of the Rank-

Exergy destruction rate is also another important parametric study which was considered in this paper. The exergy destruction rate and corresponding entropy generation for all major components used in the cycle are presented in Table 2. Fig. 10 shows the exergy destruction rate in kW in all three turbines used in this system versus ambient temperature (T0 ). The ambient temperature varies from 10 ◦ C to 30 ◦ C and the effect can be seen. It is observed that the exergy destruction rate increases in all three turbines when the ambient temperature increases. Fig. 11 gives us an insight into the Rankine cycle, i.e. the effect of turbine inlet pressure on the exergy destruction rate in the turbine, the boiler, and heat exchanger 1. It is found that when the inlet pressure of the turbine increases from 3000 kPa to 6000 kPa, the exergy destruction rate also increases in all three components of the system. The higher exergy destruction is in the boiler compared to the turbine and other components, as shown in Fig. 12 and corresponding values are presented in Table 2. Fig. 12 shows the exergy destruction rate at each major subunit of the cycle, such as turbine 1 (HPT), turbine 2 (LPT), turbine 3, boiler, condenser, and heat exchanger 1, all of which values are presented

S. Panchal et al. / Energy and Buildings 128 (2016) 900–910

909

Table 2 Some important values obtained from the developed system. Parameter

Value

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Q˙ E Q˙ A

1895 (kW) 2978 (kW) 0.7017 0.1175 38,918 (kW) 44,656 (kW) 11% 32% 37% 7% 96% 71% 31% 22% 1552 (kW) 1271 (kW) 703.7 (kW) 2343 (kW) 4255 (kW) 35,029 (kW) 51,492 (kW) 8.764 (kW) 7.862 (kW) 14.28 (kW) 5.207 (kW) 4.265 (kW) 2.361 (kW) 8.49 (kW) 66.3 (kW) 0.0294 (kW)

COPEn COPEX ˙ T,RC W ˙ Total W Geo,T 3 RC MG SG Geo,T 3 RC,T 3 MG SG ˙ D,T 1 Ex ˙ D,T 2 Ex ˙ D,T 3 Ex ˙ D,HE1 Ex ˙ D,HE5 Ex ˙ D,B Ex ˙ D,Dryer Ex ˙ D,P1 Ex ˙ Sgen HE1 ˙ Sgen HE5 ˙ Sgen T1 ˙ Sgen T2 ˙SgenT 3 ˙ Sgen B ˙ Sgen C1 ˙ Sgen P1

Fig. 12. Exergy destruction rates of the major components of the system. 1

4.7. Energetic and exergetic COP (COPEn and COPEX ) For the absorption chiller cycle, the energetic and exergetic COP (COPEn and COPEX ) is calculated by using Eqs. (81) and (82), and the corresponding values are 0.7017 and 0.1175, respectively. The energetic COP of the absorption chiller cycle is much greater than the exergetic COP. Moreover, the heat rate for the absorber (Q˙ A ) and evaporator (Q˙ E ) is 2978 kW and 1895 kW, respectively. It was also found that the absorption chiller has higher energy efficiency and lower exergy efficiency. This is because the heat causes irreversible changes in the chiller components and the primary feature for the reduction in exergy efficiency of the absorption chiller is due to the mass transfer processes. The main reason behind the exergy destruction in the generator which constitutes a major part of the total exergy destruction is the temperature difference between the heat source and the working fluid. Consequently, the highly crucial component in the absorption chiller cycle is the generator, which has a significant effect on COPEn and COPEX of the system. The exergy destruction within the chiller can be reduced if one can operate the generator with a lower temperature source. Moreover, the subsequent essential part of the absorption chiller is the absorber due to the fact that it generates 20–25% of the total exergy destruction. If an absorber is efficient, then the exergy production can be minimized. Also, in the condenser and evaporator, the exergy destruction is very low compared to the generator and evaporator.

Ψ

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

in Table 2. In the boiler, the exergy destruction rate becomes maximum due to the temperature difference between the two fluids in the heat exchanger. In the condenser, the exergy destruction rate is also found to be higher. This is mainly because the phase change that takes place in the condenser gives out a significant amount of heat. The exergy destruction rate can be reduced by implementing suitable assumptions, temperature and pressure parameters. In addition, selecting a better working fluid and reference condition also has a significant effect on the efficiency of the cycle.

η

0.9

Efficiency

S.No.

Geo

RC

MG

SG

Fig. 13. Energy and exergy efficiencies of the major components of the system.

The losses in the condenser and evaporator are due to the heat generated by the mixture of the solution that is absent in the pure fluid. This also applies to the single refrigerant in a vapor compression cycle. The effects of exergy destruction on the pump, expansion valve, and the solution are very small; thus can be ignored. The effect of the operating temperatures on the performance of the absorption chiller is significant. These outcomes are crucial for the improvement of the performance of the absorption chiller. 4.8. Overall energy and exergy efficiency Fig. 13 shows the energy and the exergy efficiencies of each subunit and system such as the Rankine cycle, geothermal cycle, and single and multigeneration systems. By using Eqs. (87) and (88), the overall energy and exergy efficiency of the system are calculated as 37% and 31% respectively. Similarly, for the single generation, the overall energy and exergy efficiency of the system are calculated as 7% and 22%, respectively. Furthermore, by using Eqs. (83) and (84), the energy and exergy efficiency of the Rankine cycle is calculated and found to be 32% and 71%, respectively. Finally, for the geothermal cycle, Eqs. (85) and (86) were used and the energy and exergy efficiency were calculated. It is observed that a multigeneration system is advantageous as compared to a single generation. 5. Conclusions In this study, a novel energy based integrated system, using two main renewable energy sources, namely, solar and geothermal energies, is proposed to produce multiple outputs such as power, heating, cooling, drying, and hot water. This integrated system is then thermodynamically analyzed and assessed. Parametric studies are conducted to investigate how energy and exergy efficiencies of

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the integrated system and its subsystems change by varying operating and reference conditions. As expected, these analyses generated some compelling results because of the use of green and environmentally friendly energy sources. The results of this study showed that the overall energetic efficiency of the system without multigeneration is 7%, and with multigeneration the energetic efficiency increased to 37%. The energetic COP of the absorption chiller cycle is 0.70 whereas the exergetic COP is 0.11. The entropy generation and exergy destruction were also carried out at all major components of the system such as turbine 1 (HPT), turbine 2 (LPT), turbine 3, boiler, condenser, heat exchanger 1, pump 1, and dryer. In the parametric study, different variations, such as the ambient temperature, the inlet temperature of the turbine, inlet pressure of the turbine, geothermal cycle inlet temperature and pressure, effect of mass flow rate, were varied and results were plotted. Hence, the benefits of multigeneration in terms of additional products are important and have a significant effect on efficiencies. These studies are necessary for improving the efficiencies of energy systems. References [1] P. Ahmadi, I. Dincer, M. Rosen, Development and assessment of an integrated biomass-based multi-generation energy system, Energy (2013) 155–166. [2] S. Proietti, P. Sdringola, U. Desideri, F. Zepparelli, F. Masciarelli, F. Castellani, Life cycle assessment of a passive house in a seismic temperate zone, Energy Build. 64 (2013) 463–472. [3] F. Sarhaddi, S. Farahat, H. Ajam, A. Behzadmehr, Exergetic optimization of a solar photovoltaic thermal (PV/T) air collector, Int. J. Energy Res. 35 (2011) 813–827. [4] S. Ghosh, I. Dincer, Development and analysis of a new integrated solar-wind-geothermal energy system, Sol. Energy 107 (2014) 728–745. [5] T. Ramesh, R. Prakash, K.K. Shulka, Life cycle energy analysis of buildings: an overview, Energy Build. 42 (10) (2010) 1592–1600. [6] F. Al-Sulaiman, I. Dincer, F. Hamdullahpur, Exergy modeling of a new solar driven trigeneration system, Sol. Energy 85 (9) (2011) 2228–2243.

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