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Design and Economic Analysis of a Heating/Absorption Cooling System Operating with Municipal Solid Waste Digester: A Case Study of Gazi University

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Recovering energy from municipal solid waste (MSW) is one of the most important issues of energy management in developed countries. This raises even more interest as world fossil fuel reserves diminish and fuel prices rise. Being one of main
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   Environmental and Climate Technologies 2013 / 11  _________________________________________________________________________________________________ 12 doi: 10.2478/rtuect-2013-0002 Design and Economic Analysis of a Heating/Absorption Cooling System Operating with Municipal Solid Waste Digester: A Case Study of Gazi University Gökhan Co ş ar  1 , Mirparham Pooyanfar  2 ,   Ehsan Amirabedin 3 ,   Hüseyin Topal 4 , 1-4   Gazi University    Abstract – Recovering energy from municipal solid waste (MSW) is one of the most important issues of energy management in developed countries. This raises even more interest as world fossil fuel reserves diminish and fuel prices rise. Being one of main processes of waste disposal, anaerobic digestion can be used as a means to reduce fossil fuel and electricity consumption as well as reducing emissions. With growing demand for cooling in Turkey, especially during warm seasons and considering the energy costs, utilizing heat-driven absorption cooling systems coupled with an anaerobic digester for local cooling purposes is a potentially interesting alternative for electricity driven compression cooling. The aim of this article is to study the viability of utilizing biogas obtained from MSW anaerobic digestion as the main fuel for heating facilities of Gazi University, Turkey and also the energy source for an absorption cooling system designed for the central library of the aforementioned campus. The results prove that the suggested system is sustainably and financially appealing and has the potential to replace the conventional electricity driven cooling systems with a reasonable net present worth; moreover, it can notably reduce carbon dioxide emissions.  Keywords  – MSW, absorption cooling system, anaerobic digestion, economic analysis, greenhouse gas reduction. I.   I  NTRODUCTION  Unavoidable population growth and modern lifestyle has resulted in a more rapid depletion of natural resources and an increasing rate of environmental issues. In addition, changing consumption patterns has led to rising numbers of solid waste  produced per person. Consequently, in line with global sustainable development, in order to convert wastes from  being a threat to the environment and human health into a source of income for the economy, employing waste management strategies is inevitable. Integrated waste management generally starts with waste reduction at its source, recycling and recovery and ends with the disposal of gathered waste. Municipal solid waste subjected to anaerobic digestion demonstrates a promising potential to produce  biogas [1, 2]. During the process of anaerobic digestion, organic materials are converted to their oxidized and reduced form to yield methane, carbon dioxide and trace gases in absence of air [3, 4]. Igoni et al. [2] have discussed the key  points and characteristics for designing an optimized anaerobic digester. They have noted that MSW size reduction and providing the appropriate digestion environment i.e. temperature, moisture, hydrogen-ion amount and some other factors are necessary in order to obtain an optimal design for anaerobic digester. They have also concluded that batch digestion system is a feasible means by which to convert MSW into useful energy forms. Several other technologies are also employed in waste management strategies, especially with the aim to produce energy via biogas. Murphy and Mckeogh [5] have studied four main methods for energy recovery from MSW: thermal technologies including incineration and gasification; and  biogas technologies for CHP and transportation fuel  production. They have deduced that in almost all cases despite having its own shortfalls, producing fuel for transportation is the most feasible solution both from the economic (gate fee) and environmental (greenhouse gas emissions) point of view. Also, various investigations have been carried out to assess the application of biogas for cogeneration and trigeneration. Mikael Lantz [6] has investigated the feasibility of different technologies for CHP production using manure-based biogas. Bruno et al. [7] studied different integrated arrangements of absorption chillers and biogas-fired micro gas turbine CHP systems. They carried out a case study for a sewage treatment  plant and concluded that the most appropriate configurations are the ones that trigeneration plant, using biogas and supplementary natural gas, which totally replaces the available system. On the other hand, during recent decades there has been an immense increase in cooling demand globally. In warmer regions electricity demand peaks are in part due to electrical cooling devices which lead to higher peak load power generation and subsequently higher costs and carbon dioxide emissions [8-10]. Aside from climate conditions, population growth combined with a modern lifestyle and an expanding use of electric devices like lighting and computers also escalate cooling demand [8]. About 40% of Europe’s  buildings are equipped with cooling systems, a number that rises to 80% in the United States and Japan [11]. Therefore, a major part of the world energy consumption is used for cooling purposes. Almost 40% of the electric consumption in warm regions is spent on cooling while in Turkey 7% of the  produced electric energy is used in air conditioning industries [12]. Replacing the conventional electricity-driven compression cooling (CC) chillers with heat-driven absorption cooling (AC), particularly in areas where industrial excess Unauthenticated | 81 214 143 155Download Date | 11/20/13 3:45 PM   Environmental and Climate Technologies  ________________________________________________________________________________________________ 2013 / 11   13 heat or recovered energy from MSW are available, is a  promising option to reduce the power used for cooling and to decrease the rate of carbon dioxide emissions. However, despite this increasing demand and raising energy costs, there is a dearth of literature about the application of  biogas produced from municipal solid waste for heating and cooling purposes. The general objective of this research is to assess the feasibility of utilizing MSW to meet the heating/cooling demands of Gazi University. Calculations have been carried out to specify both the heating and cooling loads. Absorption cooling chillers are proposed as a sustainable and economically advantageous solution. Net Present Worth and payback period for the system are computed and compared to the existing system. II.   M UNICIPAL S OLID W ASTE  Differing with respect to the urban and rural population characteristics, social and economic structure, consumption  patterns and geographical location, Turkey produces 1,14 kilograms of waste per person each day [kg/person-day]. During the warm seasons of the year, this number rises to 1.15 [kg/person-day] while during cold seasons it is 1.10 [kg/person-day]. Considering the current population of 73 million, the amount of waste produced across Turkey can be estimated at 69.252 tons per day (25.277 million tons per year). The statistical data obtained on waste across Turkey is  presented in Table I [13]. Also, Table 2 presents the physical characteristics of sample MSW of Ankara in the year 2005 [14]. Physical characteristics, ultimate and approximate analyses results of the MSW used in this study is presented in Table III [15]. III.   S YSTEM D ESCRIPTION  Regarding the energy consumption patterns and costs, replacing the currently used natural gas driven heating and electrical cooling with a MSW driven heating/cooling system is a promising alternative. This research plans to planned to assess the technical and economical possibility of replacing the currently used natural gas driven heating and electrical cooling with a thermal system driven by biogas and an AC system, respectively in order to provide 26.3 MW of heating and 1.1 MW of cooling demands of Gazi University main campus (Figure 1). Figure 2 shows a three dimensional diagram of the simulated biogas production, heating and cooling system. As it can be seen, gathered waste is brought to the facility and stored in the balancing ponds, after which it is sent to the  biogas production unit. The produced biogas is led to the  boilers and AC generator in order to provide the energy demands of the heating and cooling systems. The system used in this study mainly consists of an anaerobic digester, heating facilities and an absorption cooling system. Each system is described briefly below: TABLE I S TATISTICAL DATA OBTAINED ON WASTE ACROSS T URKEY (TÜ İ K)  Data Amount  Number of cities providing waste management services 2879 Ratio of waste service offered population to total population (%) 83 Total gathered waste amount (1000 ton/year) 25277 Average waste amount per person per day (kg/person-day) 1.14 Average waste amount during warm seasons (kg/person-day) 1.15 Average waste amount during cold seasons (kg/person-day) 1.10  Number of landfill of wastes facilities in Turkey 52 Capacity of landfill of wastes facilities 1000 ton) 432142 Disposed waste amount (1000 ton/year) 14309  Number of compost facilities 5 Capacity of compost facilities (1000 ton/year) 556 Amount of waste received at compost facilities (1000 ton/year) 216  Number of thermal recovery facilities 2 Capacity of thermal recovery facilities (1000 ton/year) 44 Amount of burnt medical waste (1000 ton/year) 6 Ratio of population receiving disposal and recovery services to total population (%) 47 Fig.1. Gazi University main campus map Fig.2. 3D flow diagram of the simulated biogas-cooling facility Unauthenticated | 81 214 143 155Download Date | 11/20/13 3:45 PM   Environmental and Climate Technologies 2013 / 11  _________________________________________________________________________________________________ 14 TABLE II P HYSICAL CHARACTERISTICS OF MSW  FOR ANKARA  Ingredients Paper Plastic Metal Textile Glass Ruble stone Organic material Ash Percentage (%) 8.4 7.7 1.1 1.3 1.6 1.6 47.8 30.5 Weight (kg) 89 81 12 14 18 18 508 324 Density (kg/m3) 0.27 34.1 240.3 84.1 193.8 104.1 288.3 480 Volume (m3) 1.35 0.33 0.05 0.22 0.07 0.17 1.76 0.68 Compression ratio 0185 0.1 0.225 0.15 0.4 0.2 0.33 0.75 Compressed volume (m3) 0.225 0.038 0.009 0.033 0.028 0.034 0.58 0.810 TABLE III P HYSICAL CHARACTERISTIC ,  ULTIMATE AND APPROXIMATE ANALYSES RESULTS OF THE USED MSW  Physical characteristics Density of organic waste (kg/m 3 ) 288.6 Sludge density (kg/m 3 ) 758.26 Ultimate analysis (%) Moisture 25.2 Ash 21 Volatile matter 25 Fix carbon 28.8 Calorific value (kJ/kg) 10113  Approximate analysis (%) C 28.1 H 3.9  N 0.4 S 0.3 O 2 0.5 Cl 20.6 TABLE IV D ETAILED INFORMATION ON THE BOILERS  Boiler Room Capacity(kW) Quantity Location Boiler room 1 1,162 3 Rectorate building, Kindergarten and Museum 400 1 Boiler room 2 697 1 Medical center and cafeteria Boiler room 3 3,021 3 Technology Faculty and Cultural Center 651 1 Boiler room 4 2,556 2 Education Faculty and Faculty of Science and Literature 1,162 2 Boiler room 5 1,220 2 Sport Center and Graduate School of Physical Education 2,092 1  A.    Heating facility The current heating facilities of Gazi University consists of five natural gas-fired boiler rooms, each with total capacities of 3877 kW, 700 kW, 9700 kW, 7440 kW and 4500 kW, respectively. The detailed information of the boilers is  presented in Table IV. By modifying the current burners,  boilers are capable of operating with biogas instead of natural gas, without any major changes in the system.  B.    Absorption Cooler An AC system operates similar to vapor-compression cooling systems. The difference is that it uses thermal compressors instead of electrically driven compressors. Depending on the number of pressure stages, AC systems can  be divided into two main categories: single-effect and double-effect. A fundamental AC system consists of four main components: a generator, an absorber, a condenser and an evaporator, and it operates with absorption-refrigerant pair rather than a pure refrigerant used in electrically driven cooling systems [16]. In this study, considering the heat source and site conditions, a single effect AC system working with Li-Br pair is selected to provide the required cooling demand. Related technical characteristics of the AC system are presented in Table V. TABLE V T ECHNICAL CHARACTERISTICS OF THE DESIGNED SYSTEM    Parameter Unit Value Outlet Temperature of the evaporator °C 10 Cooling capacity kW 1163 Thermal energy demand of the generator kW 1738 Condenser capacity kW 1224 Absorber capacity kW 1677 COP - 0.669 C.    Anaerobic digester Categorization of anaerobic digester systems mainly depends on the process type, total solid content of the MSW, temperature level and complexity. There are two most widespread kinds of anaerobic digesters which are defined in regard to complexity: single stage and multi stage. To be able to choose an appropriate digester type, all biological, technical, economical and environmental conditions should be taken into consideration. Furthermore, there are several factors which affect the performance of anaerobic digesters. Reactors’ temperature, waiting time, organic loading rate, ratio of the total solid amount to loading ratio, PH and buffering capacity, Carbon/Nitrogen ratio and toxicity are the main factors affecting the performance of the digesters. Considering the aforementioned parameters and requirements of the designed system, a double-stage digester is used. In double-stage digesters, two separate reactors are Unauthenticated | 81 214 143 155Download Date | 11/20/13 3:45 PM   Environmental and Climate Technologies  ________________________________________________________________________________________________ 2013 / 11   15 used for different steps. Generally, acid hydrolysis is produced in the pre-digester (first reactor) and then it is sent to the main digester (second reactor) in order to produce biogas. It should  be stated that according to previous literature and experimental studies, calorific value of the produced biogas varies between 19,950-23,830 kJ/Nm 3  [5 and 17]. Normally wastes do not display stable performance in single-stage digesters. Subsequently, aside from higher reactor speed, the main advantage of double-stage digesters is higher reliability of the waste. Waiting time may or may not be included in the design of the digester, where in the first case the system will  be more reliable against production of nitrogen gas and toxins. In designing the digester, it was decided to use 10% solid content, double-staged, continuous feed and completely mixed digester type and 15 days of waiting time with a mesophilic working condition which are the most proper design  parameters for the system’s application location in Ankara. Also, 50% of volatile solid devolatilization, 0.0975 m 3  of specific gas production per 1 kg of MSW and the produced  biogas with calorific value of 20,900 kJ/m 3  were assumed for the digester. Further technical data of the used digesters are  presented in Table VI. A simple flow diagram of the used double-stage anaerobic digester is presented in Figure 2. TABLE VI M AIN TECHNICAL DATA OF THE ANAEROBIC DIGESTER     Parameters Unit Value Digester volume (one pre-digester and three main digester) m 3  9382.5×4=37530 Digester diameter m 38.6 Digester height m 9 System thermal energy demand MW 64.34 System electrical energy demand MW 13.6 Daily liquid fertilize t/day 2160 IV.   E CONOMIC A  NALYSES  Economic evaluation model provides a means to assess the costs and benefits of investments on anaerobic digestion utilization for heating and cooling purposes. In order to perform an economic assessment of the studied  project, the payback method and net present value method are employed. In the payback period method, the period of time needed for return on an investment in order to refund the initial investment is calculated while the net present value method yields the difference between the present value of the future cash flows and the amount of initial investment. Present worth of the expected cash flows can be calculated  by discounting them at the required rate of return. As Quoilin and Lemort [18] have expressed, the net present worth can be calculated using the equation below:  NPW=PW  benefits - PW  costs (1) Equation (2) can be used to calculate the net present value of benefits or costs. n i F     )1(Pr   (2) Series present worth of benefits and costs can be obtained using equation (3).  nn iii A )1(1)1( Pr   (3) In Equation (2), “Pr” is the present sum of money, “F” is the future sum of money, “I” is the interest rate per interest  period and “n” shows the related year. Also, “A” in the equation (3), symbolizes an end-of-period cash receipt. After carrying out these computations, if the present worth of benefits is higher than present worth of costs, the project can be interpreted as acceptable. Despite the fact that there is not any exact information about the investment costs of establishing an anaerobic digester system in Turkey, it can be estimated that the average investment cost for producing one m 3  of biogas is approximately 150 € [19]. Moreover, with reference to  previously established similar AC systems in Turkey, it is assumed that the total cost per 1 MW of cooling load is about 115,000 € [20]. Further assumed information for economic analysis of the system is presented in Table VII. TABLE VII A SSUMPTIONS MADE IN ECONOMIC ANALYSIS OF THE SYSTEM    Parameters Unit Value Value of disposal of MSW (paid from municipality)  €/t 5 Value of compost €/t 10 Value of liquid fertilizer €/m 3  6  Natural gas price €/m 3  0.4 Electric price €/kWh 0.07 Lifetime of the system Year 20 Interest rate (i) % 8 Salvage value % 5% of primary investment cost O&M (including electric cost, water consumption cost, etc.)  % 15% of total investment cost V.   R  ESULTS AND D ISCUSSION    A.   Technical results Table VIII illustrates the obtained technical results for applying MSW digestion technology for heating/cooling  purposes in a case study of Gazi University, Ankara. Total annual energy demand of the system is determined by adding up the total heat demand of the University during the cold season, cooling demand of the Central library during the warm season and anaerobic digester system’s internal energy consumption which is found to be around 126 MW. Moreover, for providing 126 MW of energy demand, approximately 23,923 m 3 of biogas with calorific value of 20,900 kJ/m 3  should be provided daily. On the other hand, for providing this amount of biogas, 245.4 tons of MSW equivalent to 850 m 3 Unauthenticated | 81 214 143 155Download Date | 11/20/13 3:45 PM   Environmental and Climate Technologies 2013 / 11  _________________________________________________________________________________________________ 16 must be gathered from municipality and campus area and supplied to the system continuously every day. Some of the main byproducts of anaerobic digestion systems are compost and liquid fertilizer which have considerable economic value (related economic data is  presented in Table VII). In this regard, daily compost and liquid fertilizer production of the system is determined. As it is shown in Table VIII, the compost and liquid fertilizer obtained amounts to approximately 21 t/day and 2,160 m 3 /day, respectively. In addition, in order to provide 1.1 MW of cooling demand of central library of Gazi University, the generator of the AC must be fed with 216 m 3 /h of biogas. TABLE VIII E CONOMIC ANALYSES RESULTS    Parameters Unit Value Energy demand MW 126.2 Calorific value of produced biogas kJ/m 3 20,900 Daily demand of biogas m 3 /day 23,923 MSW consumption rate t/day 245.4 MSW consumption volumetric rate m 3 /day 850.2 Compost production rate t/day 21 Liquid fertilizer production rate m 3 /day 2,160 Cooling load of central library kW 1,163 Biogas consumption rate for cooling m 3 /h   216  B.    Economic analyses results The economic evaluation of the MSW driven heating/cooling system is applied using equations and assumptions given in Section IV. The results are summarized in Tables IX and X. Table IX presents the total outcome, computed income and salvage value of system’s first year of operation. The total outcome can be obtained considering the operation and maintenance costs of the system and it is found to be 1,496,720 € for the first year. On the other hand, total income can be calculated by summing up the incomes due to produced compost and liquid fertilizer, waste disposal and natural gas and electric cost savings. Adding up the incomes, a total amount of 2,904,586 € is obtained for the first year. Also, the salvage value is calculated as 498,906 € over the mentioned  period. Table X illustrates the annual present worth of benefits and costs over 20 years of system’s lifetime in order to determine the payback period and net present worth of the project. Also, net annual income and its cumulative sum, used in the calculations, are given in this Table. The present worth of the benefits (which include present sum of income and present sum of salvage value) and present worth of the costs (which includes present sum of outcome and initial investment cost) at the end of economical lifetime of project are calculated as 28,624,690 € and 24,673,132 €, respectively. By using Eq.1, NPW of the project is calculated about 4,605,911 €, which is a positive value and as it was mentioned in the Section IV, the project can be considered economically feasible. Moreover, as it can be seen from Table X, after approximately 11 years of operation, the cumulative sum of net annual income exceeds the initial investment cost; therefore, it can be stated that the payback period of the  project is about 11 years. TABLE IX E CONOMIC ANALYSES RESULTS    Parameters Value (€)  Facility outcome   Total Investment Cost (Anaerobic digester and Absorption cooling systems) 9,323,772 O&M 1,49637,20 Total outcome for the first year 1,49637,20    Facility income  First year compost income 47,922 First year liquid fertilizer income 1,704,400 First year income for disposal of wastes (paid from municipality) 108,110 First year costs due to natural gas consumption before application of MSW anaerobic digester 968,477 First costs due to electric consumption for cooling before application of MSW anaerobic digester 75,680 Total income for the first year 2,904,586 Salvage Value (SV) 498,910 TABLE X E CONOMICAL ANALYSES RESULTS   Year  Annual income [€/Year]  Annual outcome [€/Year]  Net annual income [€/Year] Sum [€/Year] 1 2867504 1477611 1389893 1389893 2 2458423 1266813 1191609 2581503 3 2276317 1172975 1103342 3684845 4 2107701 1086088 1021613 4706458 5 1951575 1005637 945938 5652396 6 1807014 931146 875868 6528265 7 1673161 862172 810990 7339254 8 1549224 798307 750916 8090170 9 1434466 739173 695293 8785463 10 1328209 684420 643790 9429252 11 1229824 633722 596101 1002535 12 1138725 586780 551946 - 13 1054375 543315 511061 - 14 976273 503069 473205 - 15 903957 465805 438152 - 16 836997 431300 405697 - 17 774997 399352 376 - 18 717590 369771 348 - 19 664435 342380 322 - 20 615218 31702 298 - Pr 28153580 14507403 - - Pr of SV 105673 - - - PW 28259253 24358143 - - Unauthenticated | 81 214 143 155Download Date | 11/20/13 3:45 PM
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