International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 2 ||Issue|| 4 ||Pages|| 01-06 || April 2019 || ISSN: 2581-4540
Thermodynamic Analysis of Solar assisted vapour absorption air conditioning System 1.
Aakash Ali Khoso, 2. Abdul Fattah Abassi, 3.Samiullah Qureshi 1.
Student M.E, (Energy System Engineering) PGS, Mehran UET Jamshoro 2. Professor Mechanical Engg: Department Mehran UET Jamshoro 3. Lecturer Mechanical Engg: Department Mehran UET Jamshoro
--------------------------------------------------ABSTRACT-------------------------------------------------------In This study thermodynamic modeling is presented for an air conditioning system by using waste heat of CNG station, also assisted by solar collector. The continuous compression of Natural gas at CNG station results huge amount of heat energy, which is wasted without any utilization. This heat has great potential of recovery and can be utilized for useful purpose. This research work represents the thermodynamic model development of waste heat recovery from the Compressed Natural gas (CNG) station. Therefore, a thermodynamic model has been developed for the Vapour absorption cooling system along with adjustable solar energy. The thermodynamic model has been developed in engineering equation solver (EES) software. For the analysis of developed model, a parametric study is carried out by considering various parameters. The results of developed model reveal that the Coefficient of performance (COP) of cooling system increases with increase in generator temperature. Higher COP is achieved with adjustable solar collector efficiency. The overall results reveal that about 140KW of cooling load can be saved through waste heat and solar energy.
KEY WORDS: Thermodynamic Modeling, waste heat, solar integration, vapour Absorption cooling system ----------------------------------------------------------------------------------------------------------------------------- ---------Date of Submission: Date, 21 March 2019 Date of publication: 09. April 2019 ----------------------------------------------------------------------------------------------------------------------------- ----------
I. T = Temperature (C) �̇ = Heat (Kw) �̇ = mass flow rate (kg/sec) h = enthalpy (kj/kg)
NOMENCLATURE Abbreviations TTD: Terminal Temperature difference COP: Coefficient of performance HTF: Heat transfer Fluid EES: Engineering Equation Solver VACS: Vapour Absorption cooling system
abs = Absorber gen = Generator cond = condenser evap = evaporator Éł = efficiency
I.
INTRODUCTION
Energy is a basic step of a nation development because the life standard is raised by energy utilization and its role in socio-economic development. The prosperity and future economic growth of a nation is energy dependent. Economic growth is related to availability of those energy sources, which are easy to afford and environment friendly [1]. Continuous energy supply is main focus. Energy use in transportation is one of main concern. Transport section consumes one third of world’s total energy [2]. Energy consumption in transport results growth in economy, which is about 20 to 30 % [3]. Natural gas is on of main transport fuel. Natural gas is making strong position in world energy mix, due to its efficiency and reliability. Natural gas full fills 16% of world energy demand [4]. With increasing environmental crises of fossil fuels Natural gas is considered as less polluting and environment friendly fuel. Therefore, there is a recordable increase in natural gas vehicles. Due to rapid increase of Natural gas vehicles there is also a increase in CNG stations. Natural gas is compressed through the compressors in CNG stations. The electricity demand and its cost for the operation of a CNG station has an influence on overall operating cost of a CNG station. On other hand the increase in oil crises has affected the overall energy generation in world. Increase in the energy demand causes a considerable raise in Greenhouse gas emissions and energy cost. Developing countries are facing these challenges due to economic conditions. Pakistan is developing country, and is focusing an environment friendly policy. Serious measures have been taken to develop CNG industry in country as a result of that CNG was introduced in country in 1992. Appreciable increase in CNG stations has been recorded
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Thermodynamic Analysis of Solar assisted vapour‌ in last ten years within country. The country aims to reduce oil import bills and Green House gas emissions. At present Natural Gas has 43% share of total energy consumption in country; due to use of CNG the oil consumption has been decreased up to 20% in last few years [5]. In CNG stations the gas is compressed in several stages. Cooling towers are used to reduce gas temperature. Natural gas compressor has a great potential for waste heat [6]. The water after cooling the gas is recycled to cooling tower without any proper utilization. In this research work due to increasing environmental effects for power generation a thermodynamic model has been developed for an environment friendly vapour absorption cooling system. This newly designed VACS is being operated on the waste heat of CNG station along with Solar energy. The model is developed in EES software and then it is analyzed using various parameters, which play vital role in performance of VACS. The purpose of analysis was to achieve desirable cooling for a building to save energy.
II.
SITE DESCRIPTION:
Taj CNG station Moro is located about 5km away from Moro city on the Nationl High way leading to District Naushahro Feroze, sindh province. The CNG station provides compressed natural Gas to the vehicles. The CNG station operates without any shut down. Beside Taj CNG there is Royal Taj Restaurant. It is centrally air conditioned. The total waste heat from compressor can be used for Vapor Absorption cooling system. Vapour absorption helps to minimize heat losses which are escaped to environment without any proper usage [7]. With integration of solar the performance of this cooling system can be increased. Solar integrated cooling systems are efficient [8]. Vapor absorption cooling system derived by solar heat and waste heat has no environmental effects [9].
III.
DESCRIPTION OF SOLAR INTEGRATED COOLING SYSTEM
The Fig:1 shows the schematic diagram of solar integrated Vapour Absorption Cooling system which has been designed in engineering equation solver software (EES). The water coming from compressor is passed through the heat exchanger, where the heat is exchanged between the water and heat transferring fluid (HTF). The HTF considered in this system is therminol. After gaining the heat of compressor in the heat exchanger the HTF is passed through the solar collector. In solar collector heat is further enhanced. Vapour absorption cooling system (VACS) is used to utilize this heat further for cooling. The high temperature HTF is passed from the generator of VACS where the liBrH20 mixture is pumped from the absorber. In generator the solution of LiBrH2O gain heat from the HTF. After generator the HTF is directed back to Heat exchanger while the refrigerant moves to condenser and remaining LiBrH2O mixture moves to absorber. In condenser heat from vapour refrigerant is extracted and it is changed into liquid. This liquid refrigerant is further expanded in expansion valve. Then this refrigerant enters the evaporator and the cooling is achieved from evaporator. The vapour leaving the evaporator comes in absorber. In absorber the LiBr and H2O solution is formed and it is pumped to generator. In between absorber and generator another heat exchanger is used. This heat exchanger exchanges the heat from hot mixture of LiBrH2O leaving the generator to the mixture entering the generator. The schematic diagram of solar integrated Vapour Absorption Cooling system has been designed in engineering equation solver software.
Fig: 1 Schematic Diagram of proposed system
IV. THERMODYNAMC MOEDL DEVELOPMENT A thermodynamic model of solar integrated VACS has been developed in accordance with basic equations and laws of thermodynamics. The mass and energy balance along with necessary equations, applied to each component of the cooling system are expressed bellow. The main input parameters considered in energy analysis are,
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Thermodynamic Analysis of Solar assisted vapour‌ generator temperature and Terminal temperature difference at condenser. The energy balance and mass balance equations are used to calculate COP of cooling system, Area of solar collector and efficiency of solar collector. A. ASSUMPTIONS: • The system is in steady state condition. • At condenser outlet refrigerant is considered as saturated liquid. • Refrigerant at evaporator outlet is considered as saturated vapour. • Ambient temperature is considered as +5 C to Condenser temperature. • Terminal temperature difference for generator is assumed 10C. • Terminal temperature difference for condenser is assumed 10C. • Absorber and condenser temperatures are assumed to be equal. • Expansion valves operate under adiabatic conditions. • Efficiency of Pump is assumed 0.85 • Effectiveness of heat exchanger is taken as 0.7 B. MATHEMAICAL MODEL Solar Collector: đ?‘„̇ solar =Acoll .Éłc.G (1) Where A is area of solar Collecotr, Éłc is efficiency of collector and G is solar irradiance. Value of G for Nawabshah region sindh Pakistan [10 -11] 0.75 – 5.(T4 – T3) Éłc = [12] (1.1) đ??ş .1000 ̇ đ?‘„ HTF.solar = đ?‘šĚ‡HTF . Cp.HTF . (T4-T3) (2) đ?‘„̇ solar = QHTF.solar (3) Solution Heat Exchanger 1: đ?‘„̇ water = đ?‘šĚ‡w . Cp.w . (T1- T2) (4) đ?‘„̇ HTF = đ?‘šĚ‡HTF. Cp.HTF. (T3 – T5) (5) đ?‘„̇ HTF = đ?‘„̇ water (6) Energy Balances Given bellow energy balance is applied to each component of system ̇ đ?‘œđ?‘˘đ?‘Ą ∑đ?‘„̇đ?‘–đ?‘› + ∑đ?‘ŠĚ‡đ?‘–đ?‘› + ∑đ?‘šĚ‡â„Žđ?‘–đ?‘› = ∑đ?‘„̇đ?‘œđ?‘˘đ?‘Ą + ∑đ?‘ŠĚ‡đ?‘œđ?‘˘đ?‘Ą + ∑đ?‘šâ„Ž (7) Where đ?‘„̇đ?‘–đ?‘› is heat in and đ?‘„̇đ?‘œđ?‘˘đ?‘Ą is heat out. đ?‘ŠĚ‡đ?‘–đ?‘› is work in and đ?‘ŠĚ‡đ?‘œđ?‘˘đ?‘Ą is work out Solution Heat exchanger 1: đ?‘„̇ H.E.1 = đ?‘šĚ‡1.(h1-h2) (8) Generator: đ?‘„̇đ?‘”đ?‘’đ?‘› = đ?‘šĚ‡6 â„Ž6 + đ?‘šĚ‡13 â„Ž13 - đ?‘šĚ‡12 â„Ž12 (9) đ?‘„̇đ?‘”đ?‘’đ?‘› = m4(h4-h5) Condenser: đ?‘„̇đ?‘?đ?‘œđ?‘›đ?‘‘ = đ?‘šĚ‡r (â„Ž6 - â„Ž7 ) (10) Evaporator: đ?‘„̇đ?‘’đ?‘Łđ?‘Žđ?‘? = đ?‘šĚ‡r (â„Ž9 - â„Ž8 ) (11) Absorber: đ?‘šĚ‡r â„Ž9 +đ?‘šĚ‡gen â„Ž15 = đ?‘„̇đ?‘Žđ?‘?đ?‘ + đ?‘šĚ‡abs â„Ž10 (12) Solution Heat Exchanger 2: M11(h11-h12)=m14(h14-h13) (13) T14 = Éłhex .T11+(1- Éłhex)T13 (14) Work done by pump: Wp = Wp.specific . đ?‘šĚ‡đ?‘Žđ?‘?đ?‘ (15) Performance of cooling system: The calculation of the performance of cooling system COP is found with following formula COP =
đ?‘„̇đ?‘’đ?‘Łđ?‘Žđ?‘? đ?‘„̇đ?‘”đ?‘’đ?‘›
C Input Data from CNG station: Tcomp.in = 26 0C Tcomp,out= 36 0C
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(16)
�̇� = 1.84 kg/sec
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đ?‘„̇đ?‘’đ?‘Łđ?‘Žđ?‘? = 140Kw
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Thermodynamic Analysis of Solar assisted vapour… V. RESULTS AND DISCUSSIONS: The parametric study is performed on various conditions. The performance of cooling system is evaluated by COP. The efficiency of solar collector on different conditions is also estimated. A Variation of parameters for results: • Temperature of generator (Tgen) = 75 – 100 0C • Terminal Temperature difference over condenser TTDcond = 3-12 0C B. Effect of Generator temperature on Coefficient of performance: The fig: 2 shows the effect of generator temperature on the coefficient of performance of the cooling system. The graph indicates that performance of cooling system is increased with increase in the generator temperature. It is attributed that at higher temperature of generator the COP of cooling system reaches to the higher level.
Fig: 2 Effect of generator temperature on Coefficient of performance C. Effect of Generator temperature on efficiency of Solar Collector: The Fig: 3 shows the relation between the generator temperature and the efficiency of Solar collector. The efficiency of Solar collector decreases with the increase in the generator temperature. Hence for the better performance of cooling system there is no need to select more efficient solar collector.
Fig: 3 Effect of generator temperature on efficiency of solar collector
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Thermodynamic Analysis of Solar assisted vapour… D. Effect of Terminal temperature difference of condenser on COP: In fig:4 the relation between the terminal temperature differences of condenser is shown with the COP of cooling system. As the terminal temperature difference over the condenser increases the COP of cooling system decreases. It is because if condenser will eject higher amount of heat it will influence on the performance of cooling system.
Fig: 4 Effect of terminal temperature difference on COP E. Effect of Terminal temperature difference of condenser on efficiency of solar collector: Fig: 5 indicates the behaviour of terminal temperature difference of condenser over the solar collector efficiency. As the terminal temperature difference of condenser increases the efficiency of solar collector is also increased. It shows that if condenser releases more amount of heat, the more efficient solar collectors are required.
Fig: 5 Effect of terminal temperature difference of condenser on solar collector efficiency
VI CONCLUSION: A thermodynamic model of solar integrated vapor absorption cooling system has been developed in EES, Using the waste heat of CNG station. The results are summarized as • The higher generator temperature plays an important role in enhancing performance of cooling system. • The generator temperature has reverse effect on efficiency of solar collector. • Increase in Terminal temperature difference of condenser reduces the performance of cooling system • For higher Terminal temperature difference of condenser more efficient solar collectors are required.
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Thermodynamic Analysis of Solar assisted vapour… VII. ACKNOWLEDGEMENT: Authors are thankful to Mehran University of engineering & technology jamshoro for providing resources for this research work.
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