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Thermodynamic and economic analysis of diesel engine based trigeneration systems for an Indian hotel
Sustainable Energy Technologies and Assessments 13 (2016) 60–67
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Original Research Article
Thermodynamic and economic analysis of diesel engine based trigeneration systems for an Indian hotel Mahesh Shelar b,⇑, G.N. Kulkarni a a b
Department of Mechanical Engineering, College of Engineering Pune, Maharastra, India Department of Mechanical Engineering, K.K. Wagh Institute of Engineering, Education and Research, Panchavati, Nashik 422003, Maharastra, India
a r t i c l e
i n f o
Article history: Received 7 May 2015 Revised 24 November 2015 Accepted 25 November 2015
Keywords: Trigeneration Thermodynamic model Hotel Economic analysis
a b s t r a c t The paper proposes an engine based trigeneration system that would aid in harnessing the energy from engine jacket water and engine exhaust gases for cooling loads via an absorption chiller. A thermal stabilizer is proposed for stabilizing the temperature fluctuations at the desorber inlet of absorption chiller. Two systems are considered for meeting variable thermal and power demand. One configuration includes only absorption chiller with an auxiliary water heater while the other has a compression chiller in addition to absorption chiller and the auxiliary water heater. A comparison of both these alternative options is attempted on thermodynamic and annualized life cycle costing basis for a typical hotel. It is seen that trigeneration systems with compression chillers are more efficient than trigeneration systems without the use of compression chillers for a hotel located in inland peninsular region of India. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction Grid electricity from centralized power utilities is the cheapest option in India because of economies of scale and availability of coal. However, grid electricity is often unreliable and comes costly to non-residential and non-agricultural consumers. With electricity act 2002 in place and other reforms in energy sector, it is possible for consumers to opt for captive power generation. It is therefore likely that consumers like hotels and hospitals may opt for engine generators for their needs. Both hotels and hospitals require a simultaneous supply of power, heating and cooling energy which means the engine based trigeneration system could make sense. Considerable literature and some practical case studies on engine based trigeneration are available. Engine based trigeneration systems involve recovering heat from an engine that is used to meet the thermal demand (heating and cooling) while the engine generator meets the power demand. Literature exists on diesel generator based trigeneration systems where waste heat energy is recovered. Huang et al. [1] in their paper proposes a system with a biomass gasifier integrated with engine, absorption chiller and heating system with storage, but does not detail the heat recovery system as well as operating issues associated with thermal storage. The authors, after mathematical modeling and simulation predict an increase in plant efficiency from 22% to ⇑ Corresponding author. Tel.: +91 (253)2513353. E-mail address: [email protected] (M. Shelar). http://dx.doi.org/10.1016/j.seta.2015.11.008 2213-1388/Ó 2015 Elsevier Ltd. All rights reserved.
53% when energy is recovered from exhaust gases and engine jacket water in trigeneration mode. Espirito Santo [2] in his paper simulates an engine based trigeneration recovering engine jacket water heat for hot water absorption chiller and exhaust energy for steam demand. An auxiliary boiler is introduced to take care of the additional steam demand. The author arrives at efficiency values between 65% and 81%, depending upon the loading conditions and operating strategy. Hospital load curve is considered for analysis. Lee et al. [3] reports experimental findings of their efforts for increasing the efficiency of cogeneration systems. The recombustor is installed at the exhaust gas outlet to perform secondary burning of the exhaust gases. The cogeneration efficiency improves to 85% and the emissions are drastically reduced according to the paper. However they have considered cogeneration system. Efficiency of the whole plant after introduction of absorption chiller is more relevant in trigeneration applications. Authors have not considered recovery from engine jacket water which when utilized improves the system efficiency. Pierro Colonna et al. [4] are a study of engine based trigeneration systems involving ammonia absorption chillers. Mathematical modeling and simulation results for different configurations are reported. One configuration considers recovering heat separately at two temperatures from engine exhaust and jacket water and operating steam driven absorption chiller and superheated water driven absorption chiller. The other configuration discussed in the paper is about recovery of heat to produce superheated water which drives a single absorption chiller. Trigeneration efficiency of 48% is reported. Using two
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Nomenclature Ast C0 Cpw Cpg Cv Ce Cce Cae cos / D H k Kp ma mf mfa mg mw ms msr m0sr mh m0h mm PR Pe Phe Ph P0h Pc Po Pca Pcm qstl QL Qa
surface area of the insulated storage tank (m2) capital investment (INR) specific heat of water (kJ/kg K) specific heat of flue gases (kJ/kg K) lower calorific value of fuel (kJ/kg K) total equivalent electrical cooling load (kWe) equivalent cooling demand met by compression chiller (kWe) equivalent electrical cooling load met by absorption chiller (kWe) power factor diameter of storage tank (m) height of storage tank (m) factor accounting for change in efficiency with part loading loading of diesel generator (%) mass flow rate of air supplied to diesel generator (kg/s) fuel consumption rate of diesel generator (kg/s) fuel consumption rate of auxiliary hot water generator (kg/s) mass flow rate of flue gases released by diesel generator (kg/s) mass flow rate of water circulated through engine jacket (kg/s) mass flow rate of water circulated through exhaust gas heat exchanger (kg/s) mass flow rate of water circulated through low temperature desorber of chiller (kg/s) mass flow rate of water circulated through high temperature desorber of chiller (kg/s) mass flow rate of water from mixer to the heating load (kg/s) mass flow rate of water dependent on the heating load (kg/s) makeup water to adiabatic hot water mixer (kg/s) rating of diesel generator (kV A) electrical power output of diesel generator (kWe) total equivalent electrical heating load (kWe) total heating load after load management (kWth) heating load before load management (kWth) total cooling load (kWth) power demand other than cooling demand (kWe) cooling demand met by absorption chiller due to energy recovered from diesel generator (kWth) cooling demand met by compression chiller (kWth) surface heat losses from storage tank engine thermal losses (kWth) heat energy input for an absorption chiller (kWth)
absorption chillers would however be a costly option, especially for lower capacities. Cardona et al. [5] model a reciprocating engine driven trigeneration system involving two heat exchangers, one recovering energy from jacket water (low temperature heat exchanger) and the other recovering energy from exhaust gases. The hot water is used to fire hot water based single effect lithium bromide water vapor absorption unit. Auxiliary boiler and auxiliary compression chiller are also proposed to take care of additional thermal and cooling demand. However, thermal storage is not introduced. The focus of the study was to formulate an exergy economic analysis methodology. Trigeneration efficiency values possible with this arrangement are therefore not specified. Practical Issues like constraints in energy recovery, especially from jacket water when its
Tr (DTr) ðDT r Þ0
DTj Tg Ta Ts T 0g Tw2 Th t Ust V
desired temperature to be maintained at the low temperature desorber inlet temperature drop of hot water across low temperature desorber temperature drop of hot water across high temperature desorber temperature gain across engine jacket temperature of flue gases exiting the diesel generator (°C) temperature of ambient air (°C) temperature of water in low temperature storage tank (°C) temperature of flue gases at exit of exhaust heat recovery unit (°C) temperature of water (°C) temperature of water in high temperature storage tank (°C) time in seconds overall heat transfer coefficient of storage tank (W/m K) thermal stabilizer volume
Greek symbols q density of water (kg/m3) ge electric efficiency of diesel engine generator at rated conditions (%) gte trigeneration efficiency of diesel engine generator on electrical equivalent basis (%) gt efficiency of trigeneration system (%) ga efficiency of auxiliary heater (%) Subscripts st storage tank w water g engine exhaust gases th thermal e electrical Abbreviations ALCC annualized life cycle cost Aux auxiliary diesel fired hot water heater COP coefficient of performance CRF capital recovery factor DG diesel engine generator RC running cost TR tons of refrigeration VAM vapor absorption chiller VCR vapor compression chiller INR Indian national rupees
temperature increases were not considered. Moreover the part load performance of the engine and its implications are not dealt with in the paper. Cardona et al. [6] discusses sizing of trigeneration plant for the hotel sector in the Mediterranean region. Sizing means deciding prime mover and absorption chiller capacity. They highlight the role of trigeneration plant management in sizing plant components. They consider grid connected trigeneration plant where excess electricity produced can be sold to the grid while excess thermal energy has to be discarded. The authors argue that primary energy savings strategy of load management is better. This study is unique in considering load management strategy in details which many researchers have neglected. Ziher [7] suggest a trigeneration system with chilled water storage for
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Slovenia’s biggest hospital. It is driven by a gas turbine and uses compression chiller to supplement absorption chillers. The operating strategy is discussed. The absorption chiller is operated continuously while the compression chiller with chilled water storage caters to the varying peak load. The paper shows that integrating of chilled water storage with trigeneration makes it profitable. Engine based trigeneration systems, especially those which recover energy from exhaust gases for meeting the cooling demand are installed in India, though a detailed analysis of each is rarely reported. Table 1 includes the system capacities of trigeneration components for some of the engine based trigeneration systems installed in India. This study proposes a trigeneration system which uses the recovered energy from both the engine jacket water and exhaust gases along with thermal storage. The proposed system is designed considering the fact that part load engine performance is poor. If the sizing is done to avoid part loading the performance would improve. The thermal and economic analysis found in the literature often overlooks this fact. The paper is organized as follows. The methodology is presented. The physical model of the trigeneration system is then discussed followed by its thermodynamic modeling. The thermodynamic and economic analysis is presented for the considered load profile of a Hotel located in peninsular India. Two configurations are considered to bring forth the significance of hybridization strategy in enhancing the performance by integrating a compression chiller to a combined plant.
Methodology Based on the literature review and the actual review of trigeneration systems authors propose a trigeneration system which can harness energy both from jacket water and engine exhaust for meeting the cooling demand using a combined effect chiller. This physical model is analyzed on the basis of the first law of thermodynamics. A load study of a typical 100 room hotel from India is undertaken to estimate the representative daily load curve for this hotel. The load curve details the daily heating, cooling and electrical power load. Based on the load curve and energy balance model, two configurations are proposed. One configuration involves the use of only absorption chillers and other configuration involves the use of both absorption and compression chillers. Sizing of trigeneration system components is done for both these configurations. The energy balance model is applied to calculate the trigeneration efficiency and fuel consumption in each case. Life cycle costing method on annual basis is then used to arrive at the cost of energy for both these options.
Proposed physical model The proposed trigeneration system would comprise of diesel generator, exhaust heat recovery unit, thermal stabilizers, auxiliary boiler and an absorption chiller with or without compression chil-
Table 1 Typical trigeneration systems installed in India. Location
Engine capacity
Absorption chiller capacity
Compression chiller capacity
Comments
Pushpanjali Crosslay Hospital Ghaziabad, Uttar Pradesh, India Jai Prakash Narayan Apex Trauma Centre, New Delhi, India Tetrapak Pvt Ltd, Pune, India
774 kW 2 Engines 347 kW Engine 2 MWe 3 gas engines
450 TR 8 2 chillers 105 TR
100 TR chiller
Energy from gas engine exhaust recovered for absorption chiller
250 TR chiller
580 TR 3 (1800 TR)
300 TR 2 chiller
Jacket water energy recovered for hot water and engine exhaust heat energy recovered for absorption chiller Energy recovered from gas engine exhaust gases as well as jacket water for absorption cooling
Fig. 1. Proposed engine based trigeneration system (with compression chiller).
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ler. The schematic of such a system without compression chiller is represented in Fig. 1.Thermal stabilizers are hot water storages at low or high temperatures. All these components of trigeneration system are standardized and commercially available. The vapor absorption machine is a combined effect lithium bromide–water machine. It requires an input of low temperature hot water in low temperature desorber and high temperature hot water in high temperature desorber as seen in the figure. Selecting a diesel generator capacity solely on the power demand would mean the cooling demand remains unsatisfied due to limited heat recovery potential for applications like hotels involving much more cooling demand than the power demand. Thus absorption chiller in this case alone cannot meet the cooling demand. Two alternative configurations are possible to overcome this constraint. In the first scheme, the absorption chiller is sized based on total cooling demand and an auxiliary boiler would supplement the exhaust heat recovery unit in converting the preheated water from storage tank to steam for absorption chiller. It is known that such a sort of ‘‘direct firing” of the absorption chiller is, in general, non convenient from an energetic viewpoint due to the very low COPs of these components, compared to vapor compression machines. The second alternative is sharing of cooling load by compression and absorption chiller. In the second case the absorption chiller would be of lower capacities and auxiliary boiler would be unnecessary or just with a supplementary role. The stabilizer water temperature varies depending on the load profile (both heating and cooling load) and the energy recovered from the engine. In the proposed trigeneration, diesel generator produces electricity to meet the necessary power demand. Diesel generator is selected to meet the peak power demand, which may or may not include the compression chiller demand. Depending on the demand, diesel generator would operate at different percentage loadings. Heat energy is recovered from jacket water and exhaust gases. This heat energy is stored as hot water in an insulated storage tank which acts as a low temperature stabilizer. The water temperature in the low temperature insulated storage tank should always be maintained at 90 °C. If the temperature of water in the tank rises above 90 °C because of lower heating or cooling demand, radiator comes into operation and dumping of energy would begin. Depending on the heating demand, the hot water is withdrawn from the insulated storage tank and make up water is added to the storage tank. The quantity of makeup water to be added is therefore decided by heating demand. The hot water recovered from exhaust gases is stored in high temperature insulated tank which would act as a high temperature stabilizer. The temperature of water in this high temperature stabilizer is maintained at 165 °C. Absorption chiller capacity is selected on the basis of maximum cooling demand.
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obtained from the load curve. The fuel consumed by the diesel engine generator is the function of power demand met and this is expressed in terms of electric efficiency of the engine. Part loading of engines would result in change in electric efficiency, which is corrected by introducing the factor k. The fuel consumed by diesel generator is given by the expression (2). Energy balance model of diesel generator is an expression (3) which relates the energy input to the power output, energy carried away by jacket water and that carried away with exhaust gases. Since the proposed trigeneration system has an arrangement for circulation of the water from the recovery tank to the jacket water heat exchanger and then through the exhaust heat recovery unit, the mass flow rates are equal and taken to be mw. The temperature rise experienced by the jacket water in the exhaust heat recovery unit depends on the flue gas flow rate, exhaust gas temperature and the design of heat exchanger. The heat exchangers are designed for known heat duty obtained from the data reported in the literature, both at rated as well as part load operating conditions.
Pe ¼ K p PR cos /
ð1Þ
mf ¼ P e =ðge kC v Þ
ð2Þ
mf C v ¼ mf ge kC v þ mw C pw ðDT j Þ þ mg C pg ðT g T a Þ þ Q L
ð3Þ
Mathematical model of the absorption chiller Refrigeration load is met by the proposed trigeneration system through combined effect hot water based lithium bromide absorption chiller. This chiller requires an input of hot water at a specified temperature Tr from low temperature storage at its low temperature desorber. When the hot water is at a temperature equal to or greater than Tr, it flows at flow rate msr into and out of the desorber section of the absorption chiller by operating a pump .The hot water while flowing through the desorber section of the chiller supplies heat energy to it. As a result it experiences a temperature drop of DTr in the low temperature desorber section of the chiller. Similarly heat is input from high temperature storage at high temperature desorber of chiller. The desorber input (Qa) and met by the absorption chiller is calculated by the expression (4). The corresponding cooling demand met by absorption chiller is obtained from expression (5). If the cooling demand is not satisfied the residual cooling demand is satisfied by an auxiliary heating system.
Q a ¼ msr C pw ðDT r Þ þ m0sr C pw ðDT r Þ0
ð4Þ
C ca ¼ ðCOPÞ Q a
ð5Þ
Thermodynamic model of the proposed system Mathematical model for low temperature storage Modeling the diesel engine generator Diesel generator is rated at some power P expressed in kVA at full load conditions. The electric power produced is partly consumed by three water pumps which circulate water from and to the hot water storage tank. This auxiliary power consumed can be neglected. The power factor should be considered to calculate net power. This net power output Pe is expressed by Eq. (1).The parameter Kp in the expression accounts for the part load operation of diesel engine generator When the power demand is equal to the rated power of the diesel engine generator, the value of Kp will be unity. For all other power demands, this value will be less than unity. Since the diesel generator is sized based on maximum power demand the value of Kp cannot exceed unity. The value of Kp is
Consider the well insulated cylindrical thermal recovery tank with water. It receives energy recovered from the engine and discharges energy to the heating and refrigeration load when needed. The change in internal energy of water in thermal storage per unit time depends on the energy recovered and energy discharged. Energy is recovered by water from the diesel engine body and increases internal energy of water in low temperature storage. Similarly energy is recovered from exhaust gases and transferred to high temperature storage. Energy transferred from storages to the absorption chiller or heating load results in the decrease in internal energy. For example the energy balance for low temperature storage is expressed as (6). For a cylindrical tank, the surface area of the
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tank is related to the storage volume of the tank by the Eq. (7). The surface losses can be calculated from expression (8).
qVC pw
dT ¼ mw C pw ðT w2 T s Þ fmsr C pw ðDT r Þ þ qstl g dt
ð6Þ
2 H V3 Ast ¼ 1:845 2 þ D
ð7Þ
qstl ¼ U st Ast ðT s T a Þ
ð8Þ
Table 2 Daily loading profile of the hotel estimated for the month of April. Parameter (kW)
0– 03
3–6
6–9
9– 12
12– 15
15– 18
18– 21
21– 24
Pe P 0h Pc Ce Ph Phe Pe + Ce + Phe
55 10 175 50 20 25 130
55 10 175 50 20 25 130
95 190 175 50 20 25 170
45 130 130 40 20 25 110
45 10 130 40 20 25 110
55 40 130 40 20 25 120
85 40 175 50 20 25 160
55 40 175 50 20 25 130
System analysis Power Demand(kW)
gte ¼ ðPe þ C e þ Phe Þ=ðmf þ mfa ÞC v gt ¼ ðP e þ Pc þ Ph Þ=ðmf þ mfa ÞC v
ð9Þ ð10Þ
Cooling load(kW) 200 180 160 140 120 100 80 60 40 20 0 0
3
6
9
12
15
18
21
24
Time of the Day (Hr.) Fig. 2. Load curve for the hotel.
Economic model The cost of energy for proposed system is arrived at using Annualized Life cycle costing method given by expressions (11) and (12). The cost depends on the life of the plant expressed in n years, discount rate, and capital investment on each component and the running cost.
ALCC ¼ C 0 ðCRFÞ þ RC
ð11Þ
n
CRF ¼
Heang load(kW)
Load (kW)
Overall performance of the whole plant (trigeneration system including auxiliary boiler) is determined by a parameter trigeneration efficiency which is the first law efficiency. It compares the output of the system namely the production of power, cooling and heating with the energy input to the diesel engine and auxiliary heater. The trigeneration system efficiency considering electricity equivalent is calculated using expression (9). The trigeneration efficiency can be expressed without considering the electrical equivalent too as in expression (10). Both these expressions include the fuel input from the auxiliary boiler and therefore capture the performance of whole plant.
½dð1 þ dÞ n ½ð1 þ dÞ 1
ð12Þ
Load survey findings of the hotel A load study for the 100 room Hotel included interviewing the staff on load use pattern and obtaining data on connected loads [8]. This Hotel is located in the inland peninsular region of India which experiences a tropical wet and dry climate with four seasons. Researchers emphasize the effects of local climatic conditions on the thermal loads and its influence on the design of an energy supply plant from economic and environmental points of view [9]. One of the important research work focuses on the optimal complementary operation of the simultaneously installed ‘‘thermaldriven” and ‘‘electricity-driven” cooling capacities, when variable tariffs are concerned [10]. The said paper shows that an alternate operation of compression chiller and absorption chiller may be an optimum option in situations where time of the day differential electricity tariffs are in place. Selection of typical days of hourly energy demand that would properly represent the whole year is suggested to take care of the variation in climatic conditions [11]. The present study looked into the climatic conditions and the variation of cooling demand per month. Since the trigeneration system is offgrid system the sizing has to be done on peak demand basis. For grid connected systems, the system can be sized using guidelines from reference [11]. The cooling demand is maximum
in the month of April (summer season). Sizing of the trigeneration system is done on the basis of peak load profile obtained for the month of April. A daily load profile for the month of April is given in Table 2 and Fig. 2. Actual data including the part load performance data of diesel generator has been used for the first time to comment on the economic viability of the diesel engine based trigeneration system (see Tables 3 and 5). The hotel load can be visualized as heating, cooling and electrical load. Heating load comprises of the hot water requirements for bathing and kitchen. Hot water needs to be supplied in the morning. Cooling load is mostly air conditioning load. Refrigerators constitute some cooling demand. Seasonal variation of the airconditioning load is neglected. The electrical power demand is also for pumps which constitute a significant connected load in the morning. The other power loads include that for lighting and elevators. Selection of diesel generator and chiller capacities The load curve typical for an Indian hotel indicates that the cooling demand is substantial. The actual data for a diesel generator set is given in Table 3 indicates the energy recovery potential. Selecting a diesel generator capacity solely on the power demand would mean the cooling demand remains unmet due to limited heat recovery potential. Thus absorption chiller in this case would be undersized. Two alternative configurations are possible. In the first case the cooling demand is met by using an auxiliary boiler which would generate steam using preheated water from storage tank to operate a steam fired double effect absorption chiller. The second alternative is sharing of cooling load by compression and absorption chiller. The methodology of sharing of cooling load by compression chiller and absorption chillers is based on first law energy balance model and was discussed elsewhere by the authors [12,13]. The algorithm of this methodology is included in Fig. 3.
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M. Shelar, G.N. Kulkarni / Sustainable Energy Technologies and Assessments 13 (2016) 60–67 Table 3 Engine data assumed for 125 kWe and 95 kWe DG. Percentage engine loading
Specific fuel consumption in g/kW h
Exhaust gas temperature in °C
Power production in kWe
Total recovered energy from exhaust gases in kW
Total recovered energy from jacket water in kW
For 125 kWe DG 100 75 50
224 238 250
540 446 384
125 94 63
50 38 25
30 22 15
For 95 kWe DG 100 75 50
224 238 250
540 446 384
95 56 48
38 28 18
22 17 12
Fig. 3. Algorithm for sharing of cooling load by compression and absorption chiller.
Thus the power demands would include power demand from compression chillers. It is assumed that the absorption chillers are operated for 24 h while the compression chillers supplement the varying cooling load. In the second case the absorption chiller and auxiliary boilers would be of lower capacities. The components would operate at varying loads depending on the load curve. The
heating load is highly variable and one can flatten the heat load curve by incorporating a separate hot water storage tank, not shown in the Fig. 1. The hourly heat load curve after such heat load management shows a load of 20 kWth for the considered case study. Based on the thermodynamic model the capacities of components of trigeneration system are arrived at for both configura-
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Table 4 Selected trigeneration component capacities. Parameter
System without compression chiller
System with compression chiller
DG capacity VAM capacity VCR capacity Aux capacity for peak cooling capacity Aux capacity for peak cooling and heating load after load management Total load handled
95 kWe 175 kWth – 115 kWth
125 kWe 75 kWth 100 kWth 0 kWth
135 kWth
20 kWth
beyond the permissible upper limit. The diesel generator and compression chiller as well as auxiliary heaters may operate at part loads. Part loading of diesel engine with trigeneration without compression chiller is severe unlike in trigeneration system with compression chillers. This might necessitate a higher capacity auxiliary heater and would also adversely affect the diesel engine efficiency. To understand the impact of loading pattern on the trigeneration efficiencies and corresponding fuel consumptions for both configurations were calculated and given in Tables 8 and 9. Sizing of thermal stabilizer
95 kWe,175 kWth cooling, 20 kWth heating
125 kWe, 175 kWth cooling including 100 kWth handled by compression chiller, 20 kWth heating
Table 5 Input Cost Data used in the analysis. Source: Personal Communication with the Indian manufacturers, 2014. Option
Capital cost
Life
Performance parameter
Diesel generator
Rs 6000/ kWe Rs 25,000/ TR Rs 6000/ TR Rs 30/l Rs 3000/ kWth
20 15
Specific fuel consumption 210–250 g/kW h COP = 1.0
15
COP = 3.5
20 20
Ust = 6 W/m2 K, H/D = 1 ga = 85%
LiBr–H2O absorption chiller combined type Compression chiller Hot water storage Auxiliary diesel fired water heater
The thermal storage capacity of hot water in stabilizer depends on the storage mass and the specific heat of water. For a low temperature stabilizer, the storage temperature cannot increase beyond 90 °C and should preferably not be lower than 88 °C. The upper limit is due to engine cooling requirement while the lower limit is due to desirable feed water temperature to minimize fuel input to auxiliary heater. The thermal capacity of 2500 l tank for 15 min would be about 100 kWth. The energy input to the tank is from jacket water as well as from desorber outlet condensate. Considering the heat transfer rates across the tank, a 2500 l tank would give stability of temperature for about an hour. Thus a volume of low temperature insulated storage tank which acts as a thermal stabilizer was chosen to be 2500 l. A high temperature insulated storage volume of a similar volume was selected. It needs to be emphasized that both the insulated tanks are sized for thermal stability and not storage. Analysis
Discount rate = 0.1, diesel price = Rs 60/l, density of diesel = 850 kg/m3, LHV of diesel = 40,600 kJ/kg, 1 USD = 63.6 INR (as on 5/6/2015).
tions (see Table 4). The thermodynamic model with the load curve input calculates the percentage loading of individual components which are listed in Tables 6 and 7 for the two cases respectively. It is seen that the absorption chillers are continuously operating to ensure that the thermal storage tank temperature does not rise
To meet the given demand of the hotel, the diesel generator would be part loaded when operated without compression chiller as seen in Table 6. This part load operation of engine would result in reduced energy rejection in absolute terms though the percentage of energy rejected would increase because of poor part load efficiency. Poor part load efficiency would mean more fuel consumption by auxiliary boiler. The fuel consumption rate of diesel generator and auxiliary boiler for the various time period of the
Table 6 Percentage loading of components of trigeneration without compression chiller. Time of day
0–03 (%)
03–6 (%)
6–9 (%)
9–12 (%)
12–15 (%)
15–18 (%)
18–21 (%)
21–24 (%)
% Loading (95 kWe DG) % Loading (135 kWth Aux) % Loading (175 KWth VAM)
60 100 100
60 100 100
100 100 100
50 100 75
50 100 75
60 100 75
90 100 100
60 100 100
Table 7 Percentage loading of components of trigeneration with compression chiller. Component
00–03 (%)
03–6 (%)
6–9 (%)
9–12 (%)
12–15 (%)
15–18 (%)
18–21 (%)
21–24 (%)
% % % %
100 100 100 100
100 100 100 100
100 100 100 100
80 75 100 100
80 75 100 100
80 75 100 100
100 100 100 100
100 100 100 100
Loading Loading Loading Loading
(125 kWe DG) (100 kWth VCR) (75 KWth VAM) (20 kWth Aux)
Table 8 Fuel consumption rate and efficiency of trigeneration without compression chiller. Component
00–03
03–6
6–9
9–12
12–15
15–18
18–21
21–24
Daily
95 kWe DG 135 kWth Aux Total kW h
43 kg 42 kg 750 79%
43 kg 42 kg 750 79%
64 kg 42 kg 870 74%
43 kg 42 kg 585 62%
43 kg 42 kg 585 62%
43 kg 42 kg 615 65%
58 kg 42 kg 840 76%
43 kg 42 kg 750 79%
380 kg 336 kg 5745 72%
gt
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M. Shelar, G.N. Kulkarni / Sustainable Energy Technologies and Assessments 13 (2016) 60–67 Table 9 Fuel consumption rate and efficiency of trigeneration with compression chiller. Component
00–03
03–6
6–9
9–12
12–15
15–18
18–21
21–24
Daily
DG (125 kWe) Aux (20 kWth) Total kW h
84 kg 6.5 kg 750 75%
84 kg 6.5 kg 750 75%
84 kg 6.5 kg 870 86%
68 kg 6.5 kg 585 70%
68 kg 6.5 kg 585 70%
68 kg 6.5 kg 615 74%
84 kg 6.5 kg 840 84%
84 kg 6.5 kg 750 75%
624 kg 50 kg 5745 77%
gt
Table 10 Thermodynamic and economic performance of two configurations. Parameter
System without compression chiller
System with compression chiller
Annual electrical equivalent of kW h generated Total capital cost Annualized capital cost Daily fuel consumption Annual fuel consumption Annual energy cost Cost/kW h in INR/kW h
11.6 lakh units
11.6 lakh units
23 lakhs 3.7 lakhs 710 kg 259,000 180 lakhs 16 72% 40%
10 lakhs 1.2 lakhs 675 kg 246,000 172 lakhs 15 77% 43%
gt gte
day are tabulated in Table 8. When a trigeneration configuration with compression chiller is used for meeting the demand of the hotel, the part load operation of diesel generator is avoided most of the time as seen in Table 7. It is seen from Tables 8 and 9, that trigeneration configuration with compression chiller consumes less fuel, in this specific case study about 50 kg/day lesser than the one without compression chiller. The trigeneration system efficiency with compression chillers is better than that in a system without compression chiller. As summarized in Table 10, trigeneration systems with compression chiller are recommended for the considered hotel from both thermodynamic as well as economic perspective. Conclusions Engine based trigeneration system with thermal stabilizers integrated with auxiliary or compression chiller system has been proposed. Thermodynamic model for the proposed system has been formulated on the basis of first law of thermodynamics. It has been shown that trigeneration systems with compression chillers are more efficient than trigeneration systems without the use of compression chillers for a hotel located in inland peninsular region of India. The choice of diesel generator and compression chiller capacity for its integration with the trigeneration system when done using the suggested methodology would maximize fuel savings from trigeneration and the cost of energy would be lower as compared to
trigeneration system sized with only auxiliary heater. It is difficult to generalize the conclusion for a nation like India gifted with extraordinary variety of climatic regions. The present analysis assumes the use of diesel fired auxiliary boiler for comparison. The use of low cost fuel like biomass for auxiliary heater would alter the fuel savings and economics for both these options. India is gifted with extraordinary variety of climatic regions. Similar studies for Hotels in other agro climatic zones should reveal more insights about the suggested hybridisation strategy. References [1] Huang Y, Wang YD, Rezvani S, McIlveen-Wright DR, Anderson M, Hewitt NJ. Biomass fuelled trigeneration system in selected buildings. Energy Convers Manage 2011;52(6):2448–54. [2] Espirito Santo DB. Energy and exergy efficiency of a building internal combustion engine trigeneration system under two different operational strategies. Energy Build 2012;53(October):28–38. [3] Lee Dae Hee, Park Jae Suk, Ryu Mi Ra, Park Jeong Ho. Development of a highly efficient low-emission diesel engine-powered co-generation system and its optimization using Taguchi method. Appl Therm Eng 2013;50(1):491–5. [4] Colonna Piero, Gabrielli Sandro. Industrial trigeneration using ammonia–water absorption refrigeration systems. Appl Therm Eng 2003;23(4):381–96. [5] Cardona E, Piacentino A. A new approach to exergoeconomic analysis and design of variable demand energy systems. Energy 2006;31(4):490–515. [6] Cardona E, Piacentino A. A methodology for sizing a trigeneration plant in Mediterranean areas. Appl Therm Eng 2003;23(13):1665–80. [7] Ziher D, Poredos A. Economics of a trigeneration system in a hospital. Appl Therm Eng 2006;26(7):680–7. [8] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. Trigeneration for a typical Indian Hospital: An assessment considering compulsory load management scenario. In: Proceedings of International conference on advances in energy research 2009. IIT Bombay. p. 338–42. [9] Carvalho M, Serra LM, Lozano MA. Geographic evaluation of trigeneration systems in the tertiary sector. Effect of climatic and electricity supply conditions. Energy 2011;36(4):1931–9. [10] Piacentino A, Barbaro C. A comprehensive tool for efficient design and operation of polygeneration based energy mu grids serving a cluster of buildings. Part II: analysis of the applicative potential. Appl Energy 2013;111 (C):1222–38. [11] Ortiga J, Bruno JC, Coronas A. Selection of typical days for the characterisation of energy demand in cogeneration and trigeneration optimisation models for buildings. Energy Convers Manage 2011;52(4):1934–42. [12] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. Waste heat recovery possibilities for a typical Indian hospital. In: Proceedings of international conference on advances in mechanical engineering. SVNIT (Surat); 2009. p. 36–40. [13] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. An approach for selection of diesel based cogeneration and trigeneration system. In: Proceedings of international conference on advances in energy research. IIT Bombay; 2011. p. 338–42.
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Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original Research Article
Thermodynamic and economic analysis of diesel engine based trigeneration systems for an Indian hotel Mahesh Shelar b,⇑, G.N. Kulkarni a a b
Department of Mechanical Engineering, College of Engineering Pune, Maharastra, India Department of Mechanical Engineering, K.K. Wagh Institute of Engineering, Education and Research, Panchavati, Nashik 422003, Maharastra, India
a r t i c l e
i n f o
Article history: Received 7 May 2015 Revised 24 November 2015 Accepted 25 November 2015
Keywords: Trigeneration Thermodynamic model Hotel Economic analysis
a b s t r a c t The paper proposes an engine based trigeneration system that would aid in harnessing the energy from engine jacket water and engine exhaust gases for cooling loads via an absorption chiller. A thermal stabilizer is proposed for stabilizing the temperature fluctuations at the desorber inlet of absorption chiller. Two systems are considered for meeting variable thermal and power demand. One configuration includes only absorption chiller with an auxiliary water heater while the other has a compression chiller in addition to absorption chiller and the auxiliary water heater. A comparison of both these alternative options is attempted on thermodynamic and annualized life cycle costing basis for a typical hotel. It is seen that trigeneration systems with compression chillers are more efficient than trigeneration systems without the use of compression chillers for a hotel located in inland peninsular region of India. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction Grid electricity from centralized power utilities is the cheapest option in India because of economies of scale and availability of coal. However, grid electricity is often unreliable and comes costly to non-residential and non-agricultural consumers. With electricity act 2002 in place and other reforms in energy sector, it is possible for consumers to opt for captive power generation. It is therefore likely that consumers like hotels and hospitals may opt for engine generators for their needs. Both hotels and hospitals require a simultaneous supply of power, heating and cooling energy which means the engine based trigeneration system could make sense. Considerable literature and some practical case studies on engine based trigeneration are available. Engine based trigeneration systems involve recovering heat from an engine that is used to meet the thermal demand (heating and cooling) while the engine generator meets the power demand. Literature exists on diesel generator based trigeneration systems where waste heat energy is recovered. Huang et al. [1] in their paper proposes a system with a biomass gasifier integrated with engine, absorption chiller and heating system with storage, but does not detail the heat recovery system as well as operating issues associated with thermal storage. The authors, after mathematical modeling and simulation predict an increase in plant efficiency from 22% to ⇑ Corresponding author. Tel.: +91 (253)2513353. E-mail address: [email protected] (M. Shelar). http://dx.doi.org/10.1016/j.seta.2015.11.008 2213-1388/Ó 2015 Elsevier Ltd. All rights reserved.
53% when energy is recovered from exhaust gases and engine jacket water in trigeneration mode. Espirito Santo [2] in his paper simulates an engine based trigeneration recovering engine jacket water heat for hot water absorption chiller and exhaust energy for steam demand. An auxiliary boiler is introduced to take care of the additional steam demand. The author arrives at efficiency values between 65% and 81%, depending upon the loading conditions and operating strategy. Hospital load curve is considered for analysis. Lee et al. [3] reports experimental findings of their efforts for increasing the efficiency of cogeneration systems. The recombustor is installed at the exhaust gas outlet to perform secondary burning of the exhaust gases. The cogeneration efficiency improves to 85% and the emissions are drastically reduced according to the paper. However they have considered cogeneration system. Efficiency of the whole plant after introduction of absorption chiller is more relevant in trigeneration applications. Authors have not considered recovery from engine jacket water which when utilized improves the system efficiency. Pierro Colonna et al. [4] are a study of engine based trigeneration systems involving ammonia absorption chillers. Mathematical modeling and simulation results for different configurations are reported. One configuration considers recovering heat separately at two temperatures from engine exhaust and jacket water and operating steam driven absorption chiller and superheated water driven absorption chiller. The other configuration discussed in the paper is about recovery of heat to produce superheated water which drives a single absorption chiller. Trigeneration efficiency of 48% is reported. Using two
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Nomenclature Ast C0 Cpw Cpg Cv Ce Cce Cae cos / D H k Kp ma mf mfa mg mw ms msr m0sr mh m0h mm PR Pe Phe Ph P0h Pc Po Pca Pcm qstl QL Qa
surface area of the insulated storage tank (m2) capital investment (INR) specific heat of water (kJ/kg K) specific heat of flue gases (kJ/kg K) lower calorific value of fuel (kJ/kg K) total equivalent electrical cooling load (kWe) equivalent cooling demand met by compression chiller (kWe) equivalent electrical cooling load met by absorption chiller (kWe) power factor diameter of storage tank (m) height of storage tank (m) factor accounting for change in efficiency with part loading loading of diesel generator (%) mass flow rate of air supplied to diesel generator (kg/s) fuel consumption rate of diesel generator (kg/s) fuel consumption rate of auxiliary hot water generator (kg/s) mass flow rate of flue gases released by diesel generator (kg/s) mass flow rate of water circulated through engine jacket (kg/s) mass flow rate of water circulated through exhaust gas heat exchanger (kg/s) mass flow rate of water circulated through low temperature desorber of chiller (kg/s) mass flow rate of water circulated through high temperature desorber of chiller (kg/s) mass flow rate of water from mixer to the heating load (kg/s) mass flow rate of water dependent on the heating load (kg/s) makeup water to adiabatic hot water mixer (kg/s) rating of diesel generator (kV A) electrical power output of diesel generator (kWe) total equivalent electrical heating load (kWe) total heating load after load management (kWth) heating load before load management (kWth) total cooling load (kWth) power demand other than cooling demand (kWe) cooling demand met by absorption chiller due to energy recovered from diesel generator (kWth) cooling demand met by compression chiller (kWth) surface heat losses from storage tank engine thermal losses (kWth) heat energy input for an absorption chiller (kWth)
absorption chillers would however be a costly option, especially for lower capacities. Cardona et al. [5] model a reciprocating engine driven trigeneration system involving two heat exchangers, one recovering energy from jacket water (low temperature heat exchanger) and the other recovering energy from exhaust gases. The hot water is used to fire hot water based single effect lithium bromide water vapor absorption unit. Auxiliary boiler and auxiliary compression chiller are also proposed to take care of additional thermal and cooling demand. However, thermal storage is not introduced. The focus of the study was to formulate an exergy economic analysis methodology. Trigeneration efficiency values possible with this arrangement are therefore not specified. Practical Issues like constraints in energy recovery, especially from jacket water when its
Tr (DTr) ðDT r Þ0
DTj Tg Ta Ts T 0g Tw2 Th t Ust V
desired temperature to be maintained at the low temperature desorber inlet temperature drop of hot water across low temperature desorber temperature drop of hot water across high temperature desorber temperature gain across engine jacket temperature of flue gases exiting the diesel generator (°C) temperature of ambient air (°C) temperature of water in low temperature storage tank (°C) temperature of flue gases at exit of exhaust heat recovery unit (°C) temperature of water (°C) temperature of water in high temperature storage tank (°C) time in seconds overall heat transfer coefficient of storage tank (W/m K) thermal stabilizer volume
Greek symbols q density of water (kg/m3) ge electric efficiency of diesel engine generator at rated conditions (%) gte trigeneration efficiency of diesel engine generator on electrical equivalent basis (%) gt efficiency of trigeneration system (%) ga efficiency of auxiliary heater (%) Subscripts st storage tank w water g engine exhaust gases th thermal e electrical Abbreviations ALCC annualized life cycle cost Aux auxiliary diesel fired hot water heater COP coefficient of performance CRF capital recovery factor DG diesel engine generator RC running cost TR tons of refrigeration VAM vapor absorption chiller VCR vapor compression chiller INR Indian national rupees
temperature increases were not considered. Moreover the part load performance of the engine and its implications are not dealt with in the paper. Cardona et al. [6] discusses sizing of trigeneration plant for the hotel sector in the Mediterranean region. Sizing means deciding prime mover and absorption chiller capacity. They highlight the role of trigeneration plant management in sizing plant components. They consider grid connected trigeneration plant where excess electricity produced can be sold to the grid while excess thermal energy has to be discarded. The authors argue that primary energy savings strategy of load management is better. This study is unique in considering load management strategy in details which many researchers have neglected. Ziher [7] suggest a trigeneration system with chilled water storage for
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Slovenia’s biggest hospital. It is driven by a gas turbine and uses compression chiller to supplement absorption chillers. The operating strategy is discussed. The absorption chiller is operated continuously while the compression chiller with chilled water storage caters to the varying peak load. The paper shows that integrating of chilled water storage with trigeneration makes it profitable. Engine based trigeneration systems, especially those which recover energy from exhaust gases for meeting the cooling demand are installed in India, though a detailed analysis of each is rarely reported. Table 1 includes the system capacities of trigeneration components for some of the engine based trigeneration systems installed in India. This study proposes a trigeneration system which uses the recovered energy from both the engine jacket water and exhaust gases along with thermal storage. The proposed system is designed considering the fact that part load engine performance is poor. If the sizing is done to avoid part loading the performance would improve. The thermal and economic analysis found in the literature often overlooks this fact. The paper is organized as follows. The methodology is presented. The physical model of the trigeneration system is then discussed followed by its thermodynamic modeling. The thermodynamic and economic analysis is presented for the considered load profile of a Hotel located in peninsular India. Two configurations are considered to bring forth the significance of hybridization strategy in enhancing the performance by integrating a compression chiller to a combined plant.
Methodology Based on the literature review and the actual review of trigeneration systems authors propose a trigeneration system which can harness energy both from jacket water and engine exhaust for meeting the cooling demand using a combined effect chiller. This physical model is analyzed on the basis of the first law of thermodynamics. A load study of a typical 100 room hotel from India is undertaken to estimate the representative daily load curve for this hotel. The load curve details the daily heating, cooling and electrical power load. Based on the load curve and energy balance model, two configurations are proposed. One configuration involves the use of only absorption chillers and other configuration involves the use of both absorption and compression chillers. Sizing of trigeneration system components is done for both these configurations. The energy balance model is applied to calculate the trigeneration efficiency and fuel consumption in each case. Life cycle costing method on annual basis is then used to arrive at the cost of energy for both these options.
Proposed physical model The proposed trigeneration system would comprise of diesel generator, exhaust heat recovery unit, thermal stabilizers, auxiliary boiler and an absorption chiller with or without compression chil-
Table 1 Typical trigeneration systems installed in India. Location
Engine capacity
Absorption chiller capacity
Compression chiller capacity
Comments
Pushpanjali Crosslay Hospital Ghaziabad, Uttar Pradesh, India Jai Prakash Narayan Apex Trauma Centre, New Delhi, India Tetrapak Pvt Ltd, Pune, India
774 kW 2 Engines 347 kW Engine 2 MWe 3 gas engines
450 TR 8 2 chillers 105 TR
100 TR chiller
Energy from gas engine exhaust recovered for absorption chiller
250 TR chiller
580 TR 3 (1800 TR)
300 TR 2 chiller
Jacket water energy recovered for hot water and engine exhaust heat energy recovered for absorption chiller Energy recovered from gas engine exhaust gases as well as jacket water for absorption cooling
Fig. 1. Proposed engine based trigeneration system (with compression chiller).
M. Shelar, G.N. Kulkarni / Sustainable Energy Technologies and Assessments 13 (2016) 60–67
ler. The schematic of such a system without compression chiller is represented in Fig. 1.Thermal stabilizers are hot water storages at low or high temperatures. All these components of trigeneration system are standardized and commercially available. The vapor absorption machine is a combined effect lithium bromide–water machine. It requires an input of low temperature hot water in low temperature desorber and high temperature hot water in high temperature desorber as seen in the figure. Selecting a diesel generator capacity solely on the power demand would mean the cooling demand remains unsatisfied due to limited heat recovery potential for applications like hotels involving much more cooling demand than the power demand. Thus absorption chiller in this case alone cannot meet the cooling demand. Two alternative configurations are possible to overcome this constraint. In the first scheme, the absorption chiller is sized based on total cooling demand and an auxiliary boiler would supplement the exhaust heat recovery unit in converting the preheated water from storage tank to steam for absorption chiller. It is known that such a sort of ‘‘direct firing” of the absorption chiller is, in general, non convenient from an energetic viewpoint due to the very low COPs of these components, compared to vapor compression machines. The second alternative is sharing of cooling load by compression and absorption chiller. In the second case the absorption chiller would be of lower capacities and auxiliary boiler would be unnecessary or just with a supplementary role. The stabilizer water temperature varies depending on the load profile (both heating and cooling load) and the energy recovered from the engine. In the proposed trigeneration, diesel generator produces electricity to meet the necessary power demand. Diesel generator is selected to meet the peak power demand, which may or may not include the compression chiller demand. Depending on the demand, diesel generator would operate at different percentage loadings. Heat energy is recovered from jacket water and exhaust gases. This heat energy is stored as hot water in an insulated storage tank which acts as a low temperature stabilizer. The water temperature in the low temperature insulated storage tank should always be maintained at 90 °C. If the temperature of water in the tank rises above 90 °C because of lower heating or cooling demand, radiator comes into operation and dumping of energy would begin. Depending on the heating demand, the hot water is withdrawn from the insulated storage tank and make up water is added to the storage tank. The quantity of makeup water to be added is therefore decided by heating demand. The hot water recovered from exhaust gases is stored in high temperature insulated tank which would act as a high temperature stabilizer. The temperature of water in this high temperature stabilizer is maintained at 165 °C. Absorption chiller capacity is selected on the basis of maximum cooling demand.
63
obtained from the load curve. The fuel consumed by the diesel engine generator is the function of power demand met and this is expressed in terms of electric efficiency of the engine. Part loading of engines would result in change in electric efficiency, which is corrected by introducing the factor k. The fuel consumed by diesel generator is given by the expression (2). Energy balance model of diesel generator is an expression (3) which relates the energy input to the power output, energy carried away by jacket water and that carried away with exhaust gases. Since the proposed trigeneration system has an arrangement for circulation of the water from the recovery tank to the jacket water heat exchanger and then through the exhaust heat recovery unit, the mass flow rates are equal and taken to be mw. The temperature rise experienced by the jacket water in the exhaust heat recovery unit depends on the flue gas flow rate, exhaust gas temperature and the design of heat exchanger. The heat exchangers are designed for known heat duty obtained from the data reported in the literature, both at rated as well as part load operating conditions.
Pe ¼ K p PR cos /
ð1Þ
mf ¼ P e =ðge kC v Þ
ð2Þ
mf C v ¼ mf ge kC v þ mw C pw ðDT j Þ þ mg C pg ðT g T a Þ þ Q L
ð3Þ
Mathematical model of the absorption chiller Refrigeration load is met by the proposed trigeneration system through combined effect hot water based lithium bromide absorption chiller. This chiller requires an input of hot water at a specified temperature Tr from low temperature storage at its low temperature desorber. When the hot water is at a temperature equal to or greater than Tr, it flows at flow rate msr into and out of the desorber section of the absorption chiller by operating a pump .The hot water while flowing through the desorber section of the chiller supplies heat energy to it. As a result it experiences a temperature drop of DTr in the low temperature desorber section of the chiller. Similarly heat is input from high temperature storage at high temperature desorber of chiller. The desorber input (Qa) and met by the absorption chiller is calculated by the expression (4). The corresponding cooling demand met by absorption chiller is obtained from expression (5). If the cooling demand is not satisfied the residual cooling demand is satisfied by an auxiliary heating system.
Q a ¼ msr C pw ðDT r Þ þ m0sr C pw ðDT r Þ0
ð4Þ
C ca ¼ ðCOPÞ Q a
ð5Þ
Thermodynamic model of the proposed system Mathematical model for low temperature storage Modeling the diesel engine generator Diesel generator is rated at some power P expressed in kVA at full load conditions. The electric power produced is partly consumed by three water pumps which circulate water from and to the hot water storage tank. This auxiliary power consumed can be neglected. The power factor should be considered to calculate net power. This net power output Pe is expressed by Eq. (1).The parameter Kp in the expression accounts for the part load operation of diesel engine generator When the power demand is equal to the rated power of the diesel engine generator, the value of Kp will be unity. For all other power demands, this value will be less than unity. Since the diesel generator is sized based on maximum power demand the value of Kp cannot exceed unity. The value of Kp is
Consider the well insulated cylindrical thermal recovery tank with water. It receives energy recovered from the engine and discharges energy to the heating and refrigeration load when needed. The change in internal energy of water in thermal storage per unit time depends on the energy recovered and energy discharged. Energy is recovered by water from the diesel engine body and increases internal energy of water in low temperature storage. Similarly energy is recovered from exhaust gases and transferred to high temperature storage. Energy transferred from storages to the absorption chiller or heating load results in the decrease in internal energy. For example the energy balance for low temperature storage is expressed as (6). For a cylindrical tank, the surface area of the
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tank is related to the storage volume of the tank by the Eq. (7). The surface losses can be calculated from expression (8).
qVC pw
dT ¼ mw C pw ðT w2 T s Þ fmsr C pw ðDT r Þ þ qstl g dt
ð6Þ
2 H V3 Ast ¼ 1:845 2 þ D
ð7Þ
qstl ¼ U st Ast ðT s T a Þ
ð8Þ
Table 2 Daily loading profile of the hotel estimated for the month of April. Parameter (kW)
0– 03
3–6
6–9
9– 12
12– 15
15– 18
18– 21
21– 24
Pe P 0h Pc Ce Ph Phe Pe + Ce + Phe
55 10 175 50 20 25 130
55 10 175 50 20 25 130
95 190 175 50 20 25 170
45 130 130 40 20 25 110
45 10 130 40 20 25 110
55 40 130 40 20 25 120
85 40 175 50 20 25 160
55 40 175 50 20 25 130
System analysis Power Demand(kW)
gte ¼ ðPe þ C e þ Phe Þ=ðmf þ mfa ÞC v gt ¼ ðP e þ Pc þ Ph Þ=ðmf þ mfa ÞC v
ð9Þ ð10Þ
Cooling load(kW) 200 180 160 140 120 100 80 60 40 20 0 0
3
6
9
12
15
18
21
24
Time of the Day (Hr.) Fig. 2. Load curve for the hotel.
Economic model The cost of energy for proposed system is arrived at using Annualized Life cycle costing method given by expressions (11) and (12). The cost depends on the life of the plant expressed in n years, discount rate, and capital investment on each component and the running cost.
ALCC ¼ C 0 ðCRFÞ þ RC
ð11Þ
n
CRF ¼
Heang load(kW)
Load (kW)
Overall performance of the whole plant (trigeneration system including auxiliary boiler) is determined by a parameter trigeneration efficiency which is the first law efficiency. It compares the output of the system namely the production of power, cooling and heating with the energy input to the diesel engine and auxiliary heater. The trigeneration system efficiency considering electricity equivalent is calculated using expression (9). The trigeneration efficiency can be expressed without considering the electrical equivalent too as in expression (10). Both these expressions include the fuel input from the auxiliary boiler and therefore capture the performance of whole plant.
½dð1 þ dÞ n ½ð1 þ dÞ 1
ð12Þ
Load survey findings of the hotel A load study for the 100 room Hotel included interviewing the staff on load use pattern and obtaining data on connected loads [8]. This Hotel is located in the inland peninsular region of India which experiences a tropical wet and dry climate with four seasons. Researchers emphasize the effects of local climatic conditions on the thermal loads and its influence on the design of an energy supply plant from economic and environmental points of view [9]. One of the important research work focuses on the optimal complementary operation of the simultaneously installed ‘‘thermaldriven” and ‘‘electricity-driven” cooling capacities, when variable tariffs are concerned [10]. The said paper shows that an alternate operation of compression chiller and absorption chiller may be an optimum option in situations where time of the day differential electricity tariffs are in place. Selection of typical days of hourly energy demand that would properly represent the whole year is suggested to take care of the variation in climatic conditions [11]. The present study looked into the climatic conditions and the variation of cooling demand per month. Since the trigeneration system is offgrid system the sizing has to be done on peak demand basis. For grid connected systems, the system can be sized using guidelines from reference [11]. The cooling demand is maximum
in the month of April (summer season). Sizing of the trigeneration system is done on the basis of peak load profile obtained for the month of April. A daily load profile for the month of April is given in Table 2 and Fig. 2. Actual data including the part load performance data of diesel generator has been used for the first time to comment on the economic viability of the diesel engine based trigeneration system (see Tables 3 and 5). The hotel load can be visualized as heating, cooling and electrical load. Heating load comprises of the hot water requirements for bathing and kitchen. Hot water needs to be supplied in the morning. Cooling load is mostly air conditioning load. Refrigerators constitute some cooling demand. Seasonal variation of the airconditioning load is neglected. The electrical power demand is also for pumps which constitute a significant connected load in the morning. The other power loads include that for lighting and elevators. Selection of diesel generator and chiller capacities The load curve typical for an Indian hotel indicates that the cooling demand is substantial. The actual data for a diesel generator set is given in Table 3 indicates the energy recovery potential. Selecting a diesel generator capacity solely on the power demand would mean the cooling demand remains unmet due to limited heat recovery potential. Thus absorption chiller in this case would be undersized. Two alternative configurations are possible. In the first case the cooling demand is met by using an auxiliary boiler which would generate steam using preheated water from storage tank to operate a steam fired double effect absorption chiller. The second alternative is sharing of cooling load by compression and absorption chiller. The methodology of sharing of cooling load by compression chiller and absorption chillers is based on first law energy balance model and was discussed elsewhere by the authors [12,13]. The algorithm of this methodology is included in Fig. 3.
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M. Shelar, G.N. Kulkarni / Sustainable Energy Technologies and Assessments 13 (2016) 60–67 Table 3 Engine data assumed for 125 kWe and 95 kWe DG. Percentage engine loading
Specific fuel consumption in g/kW h
Exhaust gas temperature in °C
Power production in kWe
Total recovered energy from exhaust gases in kW
Total recovered energy from jacket water in kW
For 125 kWe DG 100 75 50
224 238 250
540 446 384
125 94 63
50 38 25
30 22 15
For 95 kWe DG 100 75 50
224 238 250
540 446 384
95 56 48
38 28 18
22 17 12
Fig. 3. Algorithm for sharing of cooling load by compression and absorption chiller.
Thus the power demands would include power demand from compression chillers. It is assumed that the absorption chillers are operated for 24 h while the compression chillers supplement the varying cooling load. In the second case the absorption chiller and auxiliary boilers would be of lower capacities. The components would operate at varying loads depending on the load curve. The
heating load is highly variable and one can flatten the heat load curve by incorporating a separate hot water storage tank, not shown in the Fig. 1. The hourly heat load curve after such heat load management shows a load of 20 kWth for the considered case study. Based on the thermodynamic model the capacities of components of trigeneration system are arrived at for both configura-
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Table 4 Selected trigeneration component capacities. Parameter
System without compression chiller
System with compression chiller
DG capacity VAM capacity VCR capacity Aux capacity for peak cooling capacity Aux capacity for peak cooling and heating load after load management Total load handled
95 kWe 175 kWth – 115 kWth
125 kWe 75 kWth 100 kWth 0 kWth
135 kWth
20 kWth
beyond the permissible upper limit. The diesel generator and compression chiller as well as auxiliary heaters may operate at part loads. Part loading of diesel engine with trigeneration without compression chiller is severe unlike in trigeneration system with compression chillers. This might necessitate a higher capacity auxiliary heater and would also adversely affect the diesel engine efficiency. To understand the impact of loading pattern on the trigeneration efficiencies and corresponding fuel consumptions for both configurations were calculated and given in Tables 8 and 9. Sizing of thermal stabilizer
95 kWe,175 kWth cooling, 20 kWth heating
125 kWe, 175 kWth cooling including 100 kWth handled by compression chiller, 20 kWth heating
Table 5 Input Cost Data used in the analysis. Source: Personal Communication with the Indian manufacturers, 2014. Option
Capital cost
Life
Performance parameter
Diesel generator
Rs 6000/ kWe Rs 25,000/ TR Rs 6000/ TR Rs 30/l Rs 3000/ kWth
20 15
Specific fuel consumption 210–250 g/kW h COP = 1.0
15
COP = 3.5
20 20
Ust = 6 W/m2 K, H/D = 1 ga = 85%
LiBr–H2O absorption chiller combined type Compression chiller Hot water storage Auxiliary diesel fired water heater
The thermal storage capacity of hot water in stabilizer depends on the storage mass and the specific heat of water. For a low temperature stabilizer, the storage temperature cannot increase beyond 90 °C and should preferably not be lower than 88 °C. The upper limit is due to engine cooling requirement while the lower limit is due to desirable feed water temperature to minimize fuel input to auxiliary heater. The thermal capacity of 2500 l tank for 15 min would be about 100 kWth. The energy input to the tank is from jacket water as well as from desorber outlet condensate. Considering the heat transfer rates across the tank, a 2500 l tank would give stability of temperature for about an hour. Thus a volume of low temperature insulated storage tank which acts as a thermal stabilizer was chosen to be 2500 l. A high temperature insulated storage volume of a similar volume was selected. It needs to be emphasized that both the insulated tanks are sized for thermal stability and not storage. Analysis
Discount rate = 0.1, diesel price = Rs 60/l, density of diesel = 850 kg/m3, LHV of diesel = 40,600 kJ/kg, 1 USD = 63.6 INR (as on 5/6/2015).
tions (see Table 4). The thermodynamic model with the load curve input calculates the percentage loading of individual components which are listed in Tables 6 and 7 for the two cases respectively. It is seen that the absorption chillers are continuously operating to ensure that the thermal storage tank temperature does not rise
To meet the given demand of the hotel, the diesel generator would be part loaded when operated without compression chiller as seen in Table 6. This part load operation of engine would result in reduced energy rejection in absolute terms though the percentage of energy rejected would increase because of poor part load efficiency. Poor part load efficiency would mean more fuel consumption by auxiliary boiler. The fuel consumption rate of diesel generator and auxiliary boiler for the various time period of the
Table 6 Percentage loading of components of trigeneration without compression chiller. Time of day
0–03 (%)
03–6 (%)
6–9 (%)
9–12 (%)
12–15 (%)
15–18 (%)
18–21 (%)
21–24 (%)
% Loading (95 kWe DG) % Loading (135 kWth Aux) % Loading (175 KWth VAM)
60 100 100
60 100 100
100 100 100
50 100 75
50 100 75
60 100 75
90 100 100
60 100 100
Table 7 Percentage loading of components of trigeneration with compression chiller. Component
00–03 (%)
03–6 (%)
6–9 (%)
9–12 (%)
12–15 (%)
15–18 (%)
18–21 (%)
21–24 (%)
% % % %
100 100 100 100
100 100 100 100
100 100 100 100
80 75 100 100
80 75 100 100
80 75 100 100
100 100 100 100
100 100 100 100
Loading Loading Loading Loading
(125 kWe DG) (100 kWth VCR) (75 KWth VAM) (20 kWth Aux)
Table 8 Fuel consumption rate and efficiency of trigeneration without compression chiller. Component
00–03
03–6
6–9
9–12
12–15
15–18
18–21
21–24
Daily
95 kWe DG 135 kWth Aux Total kW h
43 kg 42 kg 750 79%
43 kg 42 kg 750 79%
64 kg 42 kg 870 74%
43 kg 42 kg 585 62%
43 kg 42 kg 585 62%
43 kg 42 kg 615 65%
58 kg 42 kg 840 76%
43 kg 42 kg 750 79%
380 kg 336 kg 5745 72%
gt
67
M. Shelar, G.N. Kulkarni / Sustainable Energy Technologies and Assessments 13 (2016) 60–67 Table 9 Fuel consumption rate and efficiency of trigeneration with compression chiller. Component
00–03
03–6
6–9
9–12
12–15
15–18
18–21
21–24
Daily
DG (125 kWe) Aux (20 kWth) Total kW h
84 kg 6.5 kg 750 75%
84 kg 6.5 kg 750 75%
84 kg 6.5 kg 870 86%
68 kg 6.5 kg 585 70%
68 kg 6.5 kg 585 70%
68 kg 6.5 kg 615 74%
84 kg 6.5 kg 840 84%
84 kg 6.5 kg 750 75%
624 kg 50 kg 5745 77%
gt
Table 10 Thermodynamic and economic performance of two configurations. Parameter
System without compression chiller
System with compression chiller
Annual electrical equivalent of kW h generated Total capital cost Annualized capital cost Daily fuel consumption Annual fuel consumption Annual energy cost Cost/kW h in INR/kW h
11.6 lakh units
11.6 lakh units
23 lakhs 3.7 lakhs 710 kg 259,000 180 lakhs 16 72% 40%
10 lakhs 1.2 lakhs 675 kg 246,000 172 lakhs 15 77% 43%
gt gte
day are tabulated in Table 8. When a trigeneration configuration with compression chiller is used for meeting the demand of the hotel, the part load operation of diesel generator is avoided most of the time as seen in Table 7. It is seen from Tables 8 and 9, that trigeneration configuration with compression chiller consumes less fuel, in this specific case study about 50 kg/day lesser than the one without compression chiller. The trigeneration system efficiency with compression chillers is better than that in a system without compression chiller. As summarized in Table 10, trigeneration systems with compression chiller are recommended for the considered hotel from both thermodynamic as well as economic perspective. Conclusions Engine based trigeneration system with thermal stabilizers integrated with auxiliary or compression chiller system has been proposed. Thermodynamic model for the proposed system has been formulated on the basis of first law of thermodynamics. It has been shown that trigeneration systems with compression chillers are more efficient than trigeneration systems without the use of compression chillers for a hotel located in inland peninsular region of India. The choice of diesel generator and compression chiller capacity for its integration with the trigeneration system when done using the suggested methodology would maximize fuel savings from trigeneration and the cost of energy would be lower as compared to
trigeneration system sized with only auxiliary heater. It is difficult to generalize the conclusion for a nation like India gifted with extraordinary variety of climatic regions. The present analysis assumes the use of diesel fired auxiliary boiler for comparison. The use of low cost fuel like biomass for auxiliary heater would alter the fuel savings and economics for both these options. India is gifted with extraordinary variety of climatic regions. Similar studies for Hotels in other agro climatic zones should reveal more insights about the suggested hybridisation strategy. References [1] Huang Y, Wang YD, Rezvani S, McIlveen-Wright DR, Anderson M, Hewitt NJ. Biomass fuelled trigeneration system in selected buildings. Energy Convers Manage 2011;52(6):2448–54. [2] Espirito Santo DB. Energy and exergy efficiency of a building internal combustion engine trigeneration system under two different operational strategies. Energy Build 2012;53(October):28–38. [3] Lee Dae Hee, Park Jae Suk, Ryu Mi Ra, Park Jeong Ho. Development of a highly efficient low-emission diesel engine-powered co-generation system and its optimization using Taguchi method. Appl Therm Eng 2013;50(1):491–5. [4] Colonna Piero, Gabrielli Sandro. Industrial trigeneration using ammonia–water absorption refrigeration systems. Appl Therm Eng 2003;23(4):381–96. [5] Cardona E, Piacentino A. A new approach to exergoeconomic analysis and design of variable demand energy systems. Energy 2006;31(4):490–515. [6] Cardona E, Piacentino A. A methodology for sizing a trigeneration plant in Mediterranean areas. Appl Therm Eng 2003;23(13):1665–80. [7] Ziher D, Poredos A. Economics of a trigeneration system in a hospital. Appl Therm Eng 2006;26(7):680–7. [8] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. Trigeneration for a typical Indian Hospital: An assessment considering compulsory load management scenario. In: Proceedings of International conference on advances in energy research 2009. IIT Bombay. p. 338–42. [9] Carvalho M, Serra LM, Lozano MA. Geographic evaluation of trigeneration systems in the tertiary sector. Effect of climatic and electricity supply conditions. Energy 2011;36(4):1931–9. [10] Piacentino A, Barbaro C. A comprehensive tool for efficient design and operation of polygeneration based energy mu grids serving a cluster of buildings. Part II: analysis of the applicative potential. Appl Energy 2013;111 (C):1222–38. [11] Ortiga J, Bruno JC, Coronas A. Selection of typical days for the characterisation of energy demand in cogeneration and trigeneration optimisation models for buildings. Energy Convers Manage 2011;52(4):1934–42. [12] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. Waste heat recovery possibilities for a typical Indian hospital. In: Proceedings of international conference on advances in mechanical engineering. SVNIT (Surat); 2009. p. 36–40. [13] Mahesh Shelar, Sunil Bagade, Govind Kulkarni. An approach for selection of diesel based cogeneration and trigeneration system. In: Proceedings of international conference on advances in energy research. IIT Bombay; 2011. p. 338–42.