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Assessment of community energy supply systems using energy, exergy and exergoeconomic analysis
Energy 45 (2012) 247e255
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Energy journal homepage: www.elsevier.com/locate/energy
Assessment of community energy supply systems using energy, exergy and exergoeconomic analysis Audrius Bagdanavicius a, *, Nick Jenkins a, Geoffrey P. Hammond b a b
Institute of Energy, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, Wales UK Institute for Sustainable Energy and the Environment, Department of Mechanical Engineering, University of Bath, Bath, England, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2011 Received in revised form 19 January 2012 Accepted 22 January 2012 Available online 20 February 2012
Energy, exergy and exergoeconomic analysis are often used for assessing large energy conversion systems. However exergy and exergoeconomic analysis are rarely used when small or medium scale energy generation systems, such as community CHP/CCHP plants or microcogeneration systems are evaluated. In this study energy, exergy and exergoeconomic analysis of four Community Energy Supply (CES) systems has been carried out. Biomass Steam Turbine CHP (BST), Gas Turbine CHP (GT), Biomass Integrated Gasification Gas Turbine CHP (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP (BIGCC) systems have been modelled. Modelling and energy/exergy analysis have been conducted using the computer programme Cycle-Tempo. Exergoeconomic evaluation of CESS has been performed using the Specific Exergy Costing (SPECO) approach. Exergy costs of the main products: heat and electricity, have been calculated. The analysis shows that gasification of biomass reduces overall system efficiency due to the exergy destruction in the thermo-chemical conversion process when air is used as an oxidizer. A GT using natural gas as a fuel and BIGCC are the most exergy efficient systems in this study with the lowest exergy cost of electricity and heat produced. The exergy cost of electricity generated in BST is the highest. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Combined heat and power Community energy supply system District heating Exergy Exergoeconomics SPECO
1. Introduction Increasing concern over climate change is leading to action to reduce CO2 emissions from power plants in a number of countries. One possible way to reduce the impact of electricity generation on the environment is to use cogeneration e the simultaneous generation of electricity and heat in Combined Heat and Power (CHP) plants. However due to their location it is sometimes impractical for large power plants to use this approach while smaller units can more easily use heat from generation that would otherwise be wasted. With such Community Energy Supply (CES) systems electricity is supplied to the local community or exported to the grid and heat is supplied to the community using a District Heating (DH) network. CES systems offer a number of advantages over large central electricity generators and the use of individual natural gas domestic boilers, the energy supply system presently used in the UK. The benefits of CES systems include that: various
* Corresponding author. Tel.: þ44 29 2087 0674; fax: þ44 29 2087 4939. E-mail addresses: [email protected] (A. Bagdanavicius), jenkinsn6@ cardiff.ac.uk (N. Jenkins), [email protected] (G.P. Hammond). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.01.058
biomass fuels can be used, fuel handling is easier and CO2 emissions can be monitored and controlled more easily than from small individual heating systems. Conventional energy analysis and economic evaluation are commonly used when making a decision on the suitability of a CHP scheme. In some cases this analysis is not sufficient and exergy analysis along with thermoeconomic analysis can then be used. Thermoeconomics or exergoeconomics, if a combination of exergy analysis and economics [1] is applied, has been used extensively over the last 30 years [2]. Two main groups of thermoeconomic methods have been developed: cost accounting methods and optimisation methods. Cost accounting methods, such as: Exergy Cost Theory [3], Average Cost (AVCO) approach [4], Last-in-First-out (LIFO) method [5] or the Specific Exergy Costing (SPECO) [6e8] method have been used to calculate costs of exergy streams in energy conversion systems. Other methods, such as: Thermoeconomic Functional Analysis (TFA) [9] or Engineering Functional Analysis (EFA) [10] have been used as optimisation tools of complex energy systems. Several attempts have been made to compare the different thermoeconomic methods [11,12] and to facilitate unification of nomenclature and methodology [13,14]. Despite all these efforts,
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a variety of different methods continue to be used to analyse energy conversion technologies and new methods continue to emerge [15]. One of the best developed and comprehensive methods is the SPECO methodology presented by Lazzaretto and Tsatsaronis [6]. This tool provides simple and unambiguous procedures for evaluating energy conversion systems and uses a matrix formulation which facilitates fast problem solving. Exergoeconomic analysis is rarely applied to CHP schemes in CES systems due to its complexity. This complexity makes it difficult to gather all the necessary information on various alternatives of a small scheme and encumbers decision making. The objective of this study was to apply the exergoeconomic method to a CES system and to perform a comparative study of different CHP plants. The SPECO method [6] was used in this study. 2. Community energy supply systems A CES system, based on a real project in Ebbw Vale (Wales, UK) was used as a case study. The required maximum heating energy demand of the system is 11 MW, electricity demand is 3.5 MW. The CES system was designed to supply the energy required for heating. Electricity, generated by the CHP unit, can be exported or imported from the grid depending on the demand. Four CES systems were studied: Biomass Steam Turbine CHP plant (BST), Gas Turbine CHP plant using natural gas (GT), Biomass Integrated Gasification Gas Turbine CHP plant (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP plant (BIGCC). CHP plants were modelled using the Cycle-Tempo modelling tool [16] developed at Delft University. An ambient temperature T0 ¼ 288.15 K and pressure P0 ¼ 101.3 kPa were assumed for the exergy calculations. Exergies of streams and fuels were calculated using Cycle-Tempo. Total exergy was used in this study, because the use of separate forms of exergy has little effect on the final results [6]. Biomass fuel of a spruce-pine-fir mixture [17] was used for the BST, BIGGT and BIGCC systems, and natural gas was used for GT. Data used for modelling are presented in Table 1. 2.1. Biomass steam turbine CHP system (BST) A Biomass Steam Turbine CHP system (BST) is shown in Fig. 1. The BST system consists of a steam boiler (1), steam turbine (2), district heating (DH) heat exchanger (3), deaerator (4), DH pump (6), pumps (5, 7, and 12), DH network (8), condenser (11), cooling tower (13), and cooling system pump (14). The design steam pressure is 4.5 MPa, steam temperature is 450 C. The design supply temperature of the DH is 90 C and the return temperature is 50 C. All other design values are shown in Table 1. The system is designed to supply the required energy of
11 MW for heating. Biomass fuel is used for combustion in the steam boiler. The steam turbine is extraction type. During the heating season part of the steam at higher temperature is delivered to the heat exchanger (3). The system is designed so that during the peak heating demand all the steam is extracted. During the periods of low heat demand, the steam is delivered to the condenser (11), thus increasing the electrical power output. 2.2. Gas turbine CHP system (GT) The gas turbine CHP system (GT) is shown in Fig. 2. The GT system consists of compressor (1), combustor (2), turbine (3), heat exchanger (4), DH circulation pump (5), DH network (6), and flue gas stack (7). Natural gas is used as fuel in the combustor. The compressed products of combustion expand in the gas turbine generating power and then pass to the heat exchanger (4). Exhaust gas temperatures after the turbine exceed 500 C. The design flue gas temperature after the heat exchanger is 120 C. Other parameters used in modelling are shown in Table 1. 2.3. Biomass integrated gasification gas turbine CHP system (BIGGT) The Biomass Integrated Gasification Gas Turbine CHP system (BIGGT) consists of three subsystems: biomass gasification system, gas turbine and DH network (Fig. 3). A pressurised circulating fluidized bed (CFB) type gasifier using air as oxidant, operating at 18 bar pressure and 850 C temperature was simulated. The gasification process was modelled based on the data taken from an existing gasification combined cycle CHP plant in Värnamo, Sweden [18]. Biomass gasification modelling using Cycle-Tempo is not trivial and very often more complicated models need to be considered in order to obtain syngas composition close to experimental results [19]. In this study a simple biomass gasification model was applied. Syngas composition simulated in CycleTempo is slightly different compared with the real data, but this simplification has little effect on energy and exergy analysis. Biomass is supplied to the gasifier (1). The main gas turbine compressor (5) is used to deliver air to the combustion chamber (4). About 10% of total air supplied by the compressor (5) is used in the gasification process. The auxiliary compressor (2) pressurises air to the required 18 bar pressure. Syngas leaves the gasifier and is cooled down from 850 C to 400 C in the heat exchanger (3). Cooled high pressure gas is supplied to the combustor (4). Hot flue gas expands in the gas turbine (6) generating electricity. Then hot flue gas is cooled in the heat exchanger (7). Heat exchangers (3 and 7) are used to heat DH water. The design DH supply temperature is 90 C, return temperature is 50 C. 2.4. Biomass integrated gasification combined cycle CHP system (BIGCC)
Table 1 Data used for modelling. Parameter
Value
Steam turbine isentropic efficiency, % Gas turbine isentropic efficiency (natural gas), % Gas turbine/compressor isentropic efficiency (syngas), % Gasification compressor isentropic efficiency, % Mechanical efficiency of compressor, GT and ST, % Lower heating value (biomass), kJ/kg Lower heating value (natural gas), kJ/kg Turbine Inlet Temperature, C GT operating pressure (natural gas), MPa GT operating pressure (syngas), MPa Steam cycle operating temperature, C Steam cycle operating pressure BST/BIGCC, MPa DH network supply/return temperature, C
0.80 0.90 0.85 0.80 0.97 16,700 37,999 z1130 1.5 1.7 450 4.5/4.0 90/50
The Biomass Integrated Gasification Combined Cycle CHP system (BIGCC) consists of a number of subsystems: gasifier system, two heat recovery steam generator (HRSG) systems, gas turbine system, steam turbine system and DH network (Fig. 4). The BIGCC model was designed based on a biomass gasification CHP plant in Värnamo [18]. The gasification process is identical to that of the BIGGT system. However in the BIGCC hot syngas is cooled from 850 C to 400 C in the HRSG (3), where steam is produced. Cooled gas is delivered to the combustor (4), expands in the turbine (6) and cools to 120 C in the HRSG (7, 71, 72) and heat exchanger (12). Steam produced in HRSG (3 and 7, 71, 72) at 40 bar pressure and 450 C temperature is supplied to the steam turbine (8). Exhaust steam at 1.1 bar pressure is delivered to the heat exchanger
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Fig. 1. BST system.
(13) where it condenses. District heating water is heated by the exhaust the flue gas from the gas turbine in the heat exchanger (12) and by the condensing steam in the heat exchanger (13).
The SPECO method consists of three steps: - Identification of exergy streams; - Definition of products and fuels; - Construction of cost equations.
3. Methodology The Specific Exergy Costing (SPECO) method was applied in this study. It is a systematic methodology for calculating exergy related costs in thermal systems [6].
The first step was carried out using Cycle-Tempo software. Energy and exergy streams in each model were calculated. Total exergy was used in this study, because the use of separate forms of exergy, such as: thermal, mechanical or chemical, only marginally improves calculation accuracy [6]. Fuels and products of each system were found using the definitions proposed by Tsatsaronis [1,20]. Finally the construction of cost equations was carried out based on the SPECO method [6]. This consists of two parts. First the exergy cost equation for each component of the system is composed. The general form of this equation for the k-th component is:
X X ce E_ e k þ cw;k Wk ¼ cq;k E_ q;k þ ci E_ i k þ Z_ k e
(1)
i
Here ce, ci, cq, cw are the average costs per unit of exergy; E_ e, E_ i, E_ q, Wk are the exergy streams and Z_ k is the sum of capital investments and operating and maintenance expenses. Cost streams C_ associated with the corresponding exergy streams are calculated using equations:
C_ e ¼ ce E_ e ;
Fig. 2. GT system.
C_ i ¼ ci E_ i
(2)
However there are more streams than devices, therefore auxiliary equations have to be formulated. F and P principles [6] are used to compose auxiliary equations. According to the F principle, the specific cost (cost per exergy unit) associated with the removal of exergy from the fuel stream is equal to the average specific cost at which the removed exergy is supplied to the same stream in the upstream components [6]. In this way one auxiliary equation is created for each removal of exergy. The P principle assigns the same unit cost for the added exergy to every exergy stream belonging to the product [6]. Equations associated with the exergy streams
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Fig. 3. BIGGT system.
Fig. 4. BIGCC system.
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supplied to the systems from outside and auxiliary equations formulated based on F and P principles provide the required number of equations in order to find the specific costs of the streams. The external exergy streams are fuel, air or electricity. The cost flow rate of the fuel exergy stream is calculated from the fuel energy price. Biomass fuel exergy was calculated from the Lower Heating Value (LHV) using the equation provided in [21]. The exergy of natural gas was calculated within Cycle-Tempo. The cost flow rate of the air stream was set equal to zero. In CES systems, electricity is used for pumps and auxiliary devices. In this study it was assumed that electricity generated in the CHP plant was used for the pumps and auxiliary compressors, therefore electricity exergy streams were considered as internal streams. Auxiliary equations for the different CES systems are presented in Table 2. To solve the system of linear equations the capital investments and operating and maintenance expenses (Z_ k) for each device were calculated as shown by Kotas [21]. Purchased Equipment Costs (PEC) were calculated using functions presented by Silveira and Tuna [22] and interpolating data from the estimating charts provided in [23]. The PECs of gasifiers were calculated using the cost functions provided by Bridgwater et al. [24]. The Chemical Engineering Plant Cost Index (CEPCI) was used to calculate equipment costs at the reference year (2009, CEPCI ¼ 521.9). All costs were converted to Euro using a fixed currency conversion rate. The total costs of CES systems are presented Table 3. It was assumed that the life-time of CHP is 20 years; interest rate on the capital e 10%; operation time of plant e 7000 h/year. The energy price 0.0205 EUR/kWh of natural gas for industrial consumers in UK in 2009 was taken from EU Energy Portal [25]. A biomass fuel price 90 GBP/t was taken from the E4Tech report [26].
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Table 3 Total capital costs of CES systems. BST
GT
BIGGT
BIGCC
5.14 MEUR
3.46 MEUR
17.01 MEUR
21.57 MEUR
efficiency shows the percentage of product exergy that is found in the fuel exergy of a component. In order to find which components are responsible for the exergy destruction in the system an exergy destruction ratio was calculated using Equation (4) [1]:
yD;k ¼ E_ D;k =E_ D;tot
(4)
The exergy destruction ratio shows the share of the exergy destroyed in the k-th component compared with the total exergy destruction in the system. _ were Using the SPECO method cost rates of exergy streams (C) calculated. From the cost rates associated with exergy stream, cost rates associated with the fuel (C_ F) and the product (C_ P) were calculated. The average cost per exergy unit of fuel and product for the component k was defined using the Equation (5) [1]:
cF;k ¼ C_ F;k =E_ F;k ;
cP;k ¼ C_ P;k =E_ P;k ;
(5)
To conduct exergoeconomic analysis exergoeconomic variables of the component k, such as: cost rate of exergy destruction C_ D,k, relative cost difference rk and exergoeconomic factor fk were calculated. The cost rate of exergy destruction C_ D,k were calculated using Equation (6) [1]:
C_ D;k ¼ cF;k E_ D;k
(6)
is the unit cost of fuel and E_ D,k is the exergy destruction
4. System analysis First exergy analysis was carried out using the Cycle-Tempo software. The exergy of the product and fuel of the CES system components were calculated. Exergy efficiency was calculated using Equation (3) [1]:
ε ¼ E_ P =E_ F ¼ 1 E_ D þ E_ L E_ F
(3)
Here E_ P is the exergy of the product of the component; E_ F is the exergy of the fuel of the component; E_ D is the exergy destruction rate of the component and E_ L is the exergy loss rate of the component. In this study it was assumed that E_ L ¼ 0. Exergy Table 2 Auxiliary equations for different CES systems. BST C_ 14/E_ 14 ¼ C_ 14/E_ 14 C_ 15/E_ 15 ¼ C_ 13/E_ 13 C_ 16/E_ 16 ¼ C_ 13/E_ 13 C_ 1/E_ 1 ¼ C_ 2/E_ 2 C_ 1/E_ 1 ¼ C_ 3/E_ 3 C_ 4/E_ 4 ¼ C_ 5/E_ 5 C_ 10/E_ 10 ¼ C_ 11/E_ 11
GT C_ 12/E_ 12 ¼ C_ 11/E_ 11 C_ 10/E_ 10 ¼ C_ 11/E_ 11 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 9/E_ 9 ¼ C_ 3/E_ 3 C_ 5/E_ 5 ¼ C_ 6/E_ 6
BIGGT C_ 19/E_ 19 ¼ C_ 23/E_ 23 C_ 20/E_ 20 ¼ C_ 23/E_ 23 C_ 21/E_ 21 ¼ C_ 23/E_ 23 C_ 7/E_ 7 ¼ C_ 8/E_ 8 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 8/E_ 8 ¼ C_ 16/E_ 16 C_ 9/E_ 9 ¼ C_ 15/E_ 15 C_ 5/E_ 5 ¼ C_ 6/E_ 6 C_ 11/E_ 11 ¼ C_ 12/E_ 12
BIGCC C_ 26/E_ 26 ¼ C_ 31/E_ 31 C_ 30/E_ 30 ¼ C_ 31/E_ 31 C_ 27/E_ 27 ¼ C_ 32/E_ 32 C_ 28/E_ 28 ¼ C_ 32/E_ 32 C_ 29/E_ 29 ¼ C_ 32/E_ 32 C_ 7/E_ 7 ¼ C_ 8/E_ 8 C_ 10/E_ 10 ¼ C_ 11/E_ 11 C_ 10/E_ 10 ¼ C_ 12/E_ 12 C_ 9/E_ 9 ¼ C_ 25/E_ 25 C_ 12/E_ 12 ¼ C_ 13/E_ 13 C_ 8/E_ 8 ¼ C_ 9/E_ 9 C_ 18/E_ 18 ¼ C_ 17/E_ 17 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 5/E_ 5 ¼ C_ 6/E_ 6 C_ 22/E_ 22 ¼ C_ 23/E_ 23
Here cF,k rate. The relative cost difference rk indicates the relative increase in the average cost per exergy unit between the fuel and product of the component. It was calculated using Equation (7) [1]:
rk ¼ cP;k cF;k cP;k
(7)
Here cF,k is the unit cost of fuel and cP,k is the unit cost of product. The exergoeconomic factor fk combines non-exergy costs (capital investment and operating and maintenance costs Z_ k) with exergy destruction costs C_ D,k. It was calculated using the Equation (8) [1]:
fk ¼ Z_ k = Z_ k þ C_ D;k
(8)
The exergoeconomic factor shows the contribution of nonexergy related cost (capital cost) to the total cost increase. A low value indicates that the larger cost of the component would be acceptable if the exergy destruction were reduced. A high value of fk indicates the cost of the component should be reduced, even if the exergy efficiency of the component decreases.
4.1. BST system The BST exergy efficiency and exergoeconomic parameters are shown in Fig. 5. The largest exergy destruction (white column of Fig. 5a) is observed in the boiler. The combustion process is the largest contributor to exergy destruction. The boiler is responsible for more than 85% of all exergy destruction in the system. The exergy destruction in the other components, such as: turbine and
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Fig. 5. BST system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
heat exchanger (3) is below 10%. The exergy destruction in the deaerator and pumps is insignificant (Fig. 5a). As anticipated the largest relative cost difference and the sum of destruction and capital cost rate (Z_ k þ C_ D,k) are observed in the boiler (Fig. 5b). The low exergoeconomic factor suggests that destruction cost rate is much higher than investment (capital) cost rate. However improvement in the boiler efficiency would not reduce exergy destruction appreciably. The relative cost difference of pumps (5) and (6) is large however the sum of capital cost flow rate and exergy destruction rate (Z_ k þ C_ D,k) is negligibly small, which means that pumps do not contribute to the total exergy destruction in the system. Exergy destruction and capital cost rate (Z_ k þ C_ D,k) in the turbine and heat exchanger are considerably lower than in the boiler. 4.2. GT system GT exergy efficiency and exergoeconomic parameters are shown in Fig. 6. Four main components in the GT system contribute to exergy destruction (Fig. 6a). The combustor and heat exchanger (4) are the main components where about 85% of total exergy destruction occurs. The combustion process causes large exergy destruction. Exergy efficiency of the turbine and compressor are relatively high (z90%) and the exergy destruction ratio of these components is below 10%. Exergy efficiency of the heat exchanger (4) is low (<30%). This is due to the large temperature difference in the heat exchanger, where high temperature exhaust gas is used to
Fig. 6. GT system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
heat low temperature district heating water. The efficiency of the heat exchanger can be improved by changing temperatures in the system. The largest sum of exergy destruction and capital cost rate (Z_ k þ C_ D,k) are observed in the combustor and heat exchanger (4) (Fig. 6b). It is related to the large exergy destruction in these components. The high cost rate (Z_ k þ C_ D,k) and relative cost difference (rk > 2) of the heat exchanger (4) suggest that the performance of this component should be improved. One way to do that is to use the potential of the high temperature flue gas after the gas turbine, by implementing an additional Rankine cycle, for instance. Preheating of the air before combustion may improve the performance of the combustor. Large relative cost difference is also observed in the pump (5) (Fig. 6b). However the sum of exergy destruction and capital cost rate in the pump (5) is very low. The contribution of the pump to the total cost of the system is insignificant. The low exergoeconomic factor of the combustor and heat exchanger (4) indicates that the increase of capital costs of these components would be justified if better efficiency were achieved.
4.3. BIGGT system BIGGT exergy efficiency and exergoeconomic parameters are shown in Fig. 7. Six main components in the BIGGT system contribute to the exergy destruction in the plant (Fig. 7a). Gasifier, heat exchanger (7) and combustor are the main components where
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However, only the heat exchanger (7) plays an important role in the product exergy cost formation process. High exergoeconomic factors of the compressor (2), gasifier (z0.75) and pump (8) indicate that the capital costs of these components should be reduced in order to decrease the cost rate (Z_ k þ C_ D,k), even if the exergy efficiency were reduced. On the contrary very low exergoeconomic factors of heat exchanger (7) and combustor suggest that the efficiency of these components may be improved. It can be done by modifying the plant design by implementing an additional Rankine cycle as suggested for the GT plant. 4.4. BIGCC system BIGCC exergy efficiency and exergoeconomic parameters are presented in Fig. 8. Similar results are observed for the BIGCC as for BIGGT. Gasifier and combustor are those components where exergy destruction ratio is the largest (Fig. 8a). These devices are responsible for about 60% of the total exergy destruction. It is important to note that the effect of heat recovery steam generators (HRSG3 and HRSG7) and heat exchangers (12 and 13) on exergy destruction ratio is considerably less compared with the heat exchangers in the BIGGT system. The reason is that much lower temperature difference of working fluids (syngas e steam, flue gas e steam, flue gas e water, steam e water) is used in the BIGCC. It increases the exergy efficiency of the heat exchangers. The exergy destruction ratio in
Fig. 7. BIGGT system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
about 80% of total exergy destruction occurs. As it is anticipated combustion and gasification are the processes where exergy destruction ratio is high. The exergy efficiency of the turbine and compressor are relatively high (z90%) and the exergy destruction ratio of these components is below 10% (Fig. 7a). The exergy efficiency of the heat exchanger (3) is lower (z25%) than that of the heat exchanger (7). However, the exergy destruction ratio in this component is lower (<10%) than that of the heat exchanger (7) (z24%). The reason for low exergy efficiency is the large temperature difference of fluids in the heat exchangers (3) and (7). High temperature syngas at a temperature of above 850 C in the heat exchanger (3) and flue gas above 550 C in the heat exchanger (7) are used to increase the temperature of the district heating water from 50 to 90 . Therefore, a large quantity of exergy is destroyed. Much larger heat energy (z8800 kW) is transmitted from the exhaust gas to the district heating water in the heat exchanger (7) compared with the heat transmitted from the syngas in heat exchanger (3) (z2100 kW). Therefore, the exergy destruction ratio in the heat exchanger (7) is higher than that of the heat exchanger (3). Exergoeconomic analysis shows that the gasifier, heat exchanger (7), combustor and gas turbine are the main components where the cost rate (Z_ k þ C_ D,k) is high (Fig. 7b). The cost rates (Z_ k þ C_ D,k) of compressor (5), heat exchanger (3), compressor (2) and pump (8) are much smaller. The relative cost difference of the compressor (2), heat exchanger (3) and heat exchanger (7) is high.
Fig. 8. BIGCC system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
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the gas turbine (6) is larger than in the steam turbine (8), although the exergy efficiency of the steam turbine (8) is lower. This is due to the fact that about 70% of total electrical energy is generated in the gas turbine. The effect of the gas turbine compressor (5) on the total exergy destruction is very small, and the effect of auxiliary compressor (2) is negligible. Exergoeconomic analysis shows that the gasifier, combustor, gas turbine (6), compressor (5) and steam turbine (8) are the main components where the cost rate (Z_ k þ C_ D,k) is high (Fig. 8b). The contribution of the heat recovery steam generators (HRSG3 and HRSG7) and heat exchangers (12 and 13) to the increase of the cost rate (Z_ k þ C_ D,k) is smaller in the BIGCC (z17%) compared with the BIGGT system (z31%). It infers that the use of heat transfer devices is more efficient in the BIGCC due to the lower temperature differences between the working fluids. The relative cost difference in the devices, which largely contributes to the increase of exergy destruction cost flow rate, such as: gasifier, combustor, turbine (6), compressor (5) and turbine (8) is relatively small compared with compressor (2) and heat exchanger (12). However these components play a pivotal role in the process of the formation of the product exergy cost. As in the BIGGT case high exergoeconomic factor of the gasifier (z0.75) is observed, which suggest that the capital cost of this component should be reduced. In contrast a very low exergoeconomic factor of the combustor and the heat transfer devices indicates that capital cost flow rate is much less than the exergy destruction cost flow rate. 5. Comparison of CES systems The exergy and exegoeconomic analysis of four CES systems shows the differences between the chosen energy conversion technologies (Fig. 9). The five columns in Fig. 9a represent fuel energy input, fuel exergy, electricity generation, heat energy output and heat exergy output from CHP plant. The solid line indicates energy efficiency and the broken line represents exergy efficiency of the plants. It is seen that the energy efficiency is almost the same for all four CES systems (Fig. 9a). The exergy efficiency of GT and BIGCC systems is highest (z40%) and BIGGT efficiency is lower at z 32%. The exergy efficiency of the BST system is the lowest at z 24%. The gasification process leads to large exergy destruction. Therefore, the total exergy efficiency reduces in the BIGGT and BIGCC compared with the GT system. However, this reduction is compensated by the use of combined cycle in BIGCC. Generally speaking the GT system is the most energy and exergy efficient system. However, its impact on the environment is undoubtedly the highest. Exergy costs of the fuel and products: electricity and heating of CESS are shown in Fig. 9b. It is seen from the graph that the exergy cost of biomass fuel and natural gas are almost the same. The exergy cost of electricity and heat are almost identical in the BST system. Electricity exergy cost is considerably lower than heat exergy cost in the GT, BIGGT and BIGCC systems with the gas turbines. The lowest electrical exergy cost is observed in the GT system. The increase of exergy costs in BIGGT and BIGCC is mainly related to the additional exergy destruction in the gasifier and large capital investments. In the BIGCC system the electricity is generated in the gas turbine and steam turbine. Therefore, two electricity exergy costs have been calculated. Electricity exergy cost generated in the gas turbine is 0.0649 EUR/kWh and the exergy cost generated in the steam turbine is 0.1002 EUR/kWh. The weighted average value is presented in Fig. 9b. Large heat exergy cost in BIGGT system is associated with the large exergy destruction in heat exchangers, where high temperature syngas and exhaust gas are used to heat water in DH system.
Fig. 9. Comparison of CESS: (a) energy consumption, generation and efficiency and (b) exergy costs of electricity and heating.
This issue is circumvented when steam Rankine cycle is implemented in the BIGCC system. The heat exergy cost in the BIGCC is similar to that of in BST system. In this study steady state energy, exergy and exergoeconomic analysis has been carried out assuming fixed heat consumption. In order to understand CESS behaviour under varying energy demand conditions more detailed transient analysis is required. Additionally an environmental analysis would be beneficial. 6. Conclusion Four different CES systems: BST, GT, BIGGT and BIGCC were studied using energy, exergy and exergoeconomic analysis. The results are summarised below. 1. Energy analysis shows that the energy efficiency of all the CES systems is almost the same. Energy analysis alone is not capable of revealing all the attributes of energy conversion systems. It is a necessary but insufficient tool for performing overall plant analysis. 2. Exergy analysis shows that gas turbine technology has a higher exergy efficiency compared to the steam turbine. Exergy analysis is capable of indicating the elements of the system where the exergy destruction is largest. Large exergy destruction rates are observed in the gasification and combustion processes. In a GT system where the fluids with large temperature difference are used in the heat transfer devices, large exergy destruction
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rates are observed. A reduction of temperature difference across elements decreases exergy destruction rate. 3. Exergoeconomic analysis using the SPECO method shows that gasification, combustion and heat transfer processes contribute most to formation of the exergy cost. The lowest product cost are observed in GT and BIGCC systems, which shows that the advantage of gas turbine technology against steam turbine. Exergy costs of electricity generated in steam turbine in BST and BIGCC are considerably higher compared with those generated in systems with gas turbines. This study has shown that exergy and exergoeconomic analysis provides much useful information which can be used to assess the suitability of energy conversion technology. The application of these techniques along with traditional energy and economical analyses is necessary to facilitate the development and utilisation of more sustainable energy conversion technologies. Acknowledgments This research formed part of the programme of the UK Energy Research Centre and was supported by the UK Research Councils under Natural Environment Research Council award (NE/G007748/ 1) (Phase II) and by the Higher Education Funding Council for Wales (HEFCW). Nomenclature c Average cost per unit of exergy (EUR/kJ) C_ Cost rate (EUR/s) Exergoeconomic factor of the k-th component fk E_ Exergy transfer rate (kJ/s or kW) r Relative cost difference W Exergy transfer rate associated with power (kJ/s or kW) Exergy destruction ratio yD Z_ Capital cost rate (EUR/s) ε Exergy efficiency of the component Subscripts D Destruction e Stream exiting the component F Fuel i Stream entering the component k The k-th component L Loss P Product q Stream associated with heat transfer tot Total w Stream associated with work Abbreviations BST Biomass steam turbine BIGCC Biomass integrated gasification combined cycle BIGGT Biomass integrated gasification gas turbine CES Community energy supply CHP Combined heat and power DH District heating GT Gas turbine
HEX HHV HRSG LHV PEC SPECO
255
Heat exchanger Higher heating value Heat recovery steam generator Lower heating value Purchased equipment costs Specific exergy costing
References [1] Bejan A, Tsatsaronis G, Moran M. Thermal design and optimization. New York: A Wiley-Interscience Publication; 1996. [2] Sciubba E, Wall G. A brief Commented History of exergy from the Beginnings to 2004. International Journal of Thermodynamics 2007;10(1):1e26. [3] Lozano MA, Valero A. Theory of exergetic cost. Energy 1993;18(3):939e60. [4] Lazzaretto A, Tsatsaronis G. A general process-based methodology for exergy costing. In: Duncan AB, Fiszdon J, O’Neal D, Braven KD, editors. Proceedings of the ASME advanced energy systems division. New York: ASME; 1996. p. 413e28. [5] Tsatsaronis G, Lin L. On exergy costing in exergoeconomics. In: Tsatsaronis G, Bajura RA, Kenney WF, Reistad GM, editors. Computer-aided energy systems analysis. New York: ASME; 1990. p. 1e11. [6] Lazzaretto A, Tsatsaronis G. SPECO: a systematic and general methodology for calculating efficiencies and costs in thermal systems. Energy 2006;31: 1257e89. [7] Lazzaretto A, Tsatsaronis G. On the quest for objective equations in exergy costing. In: Ramalingam ML, Lage JG, Mei VC, Chapman JN, editors. Proceedings of the ASME advanced energy systems division. New York: ASME; 1997. p. 413e28. [8] Lazzaretto A, Tsatsaronis G. On the calculation of efficiencies and costs in thermal systems. In: Aceves SM, Garimella S, Peterson R, editors. Proceedings of the ASME advanced energy systems division. New York: ASME; 1999. p. 421e30. [9] Frangopoulos CA. Thermoeconomic functional analysis and optimization. Energy 1987;12(7):563e71. [10] von Spakovsky MR, Evans RB. Engineering functional analysisdPart I. ASME Journal of Energy Resources Technology 1993;115(2):86e92. [11] Valero A, Correas L, Zaleta A, Lazzaretto A, Verda V, Reini M, et al. On the thermoeconomic approach to the diagnosis of energy system malfunctions Part 1: the TADEUS problem. Energy 2004;29:1875e87. [12] Valero A, Lozano MA, Serra L, Tsatsaronis G, Pisa J, Frangopoulus C, et al. CGAM problem: definition and conventional solution. Energy 1994;19(3):279e86. [13] Erlach B, Serra L, Valero A. Structural theory as standard for thermoeconomics. Energy Conversion and Management 1999;40:1627e49. [14] Tsatsaronis G. Definitions and nomenclature in exergy analysis and exergoeconomics. Energy 2007;32:249e53. [15] Kim DJ. A new thermoeconomic methodology for energy systems. Energy 2010;35:410e22. [16] Cycle-Tempo Release 5. Delft University of Technology; 1980-2007. [17] Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR. Biomas gasification in a circulating fluidized bed. Biomass & Bionergy 2004;26:171e93. [18] Stahl K, Neergaard M. IGCC power plant for biomass utilisation, Varnamo, Sweden. Biomass & Bionergy 1998;15(3):205e11. [19] Altafini CR, Wander PR, Barreto RM. Prediction of the working parameters of a wood waste gasifier through an equilibrium model. Energy Conversion and Management 2003;44:2763e77. [20] Tsatsaronis G. Thermoeconomic analysis and optimization of energy systems. Progress in Energy and Combustion Science 1993;19:227e57. [21] Kotas TJ. The exergy method of thermal plant analysis. Malabar: Krieger Publishing Company; 1995. [22] Silveira JL, Tuna CE. Thermoeconomic analysis method for optimization of combined heat and power systems. Part I. Progress in Energy and Combustion Science 2003;29:479e85. [23] Loh HP, Lyons J, White CW. Process equipment cost estimation. Final Report. DOE/NETL-2002/1169 U.S. Department of Energy, National Energy Technology Laboratory; 2002. [24] Bridgwater AV, Toft AJ, Brammer JG. A techno-economic comparison of power production by biomass fast pyrolysis with gasification and combustion. Renewable & Sustainable Energy Reviews 2002;6:181e248. [25] www.energy.eu [accessed 03.02.2011]. [26] E4Tech. Biomass prices in the heat and electricity sectors in the UK. URN 10D/ 546; 2010. p. 33.
Contents lists available at SciVerse ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Assessment of community energy supply systems using energy, exergy and exergoeconomic analysis Audrius Bagdanavicius a, *, Nick Jenkins a, Geoffrey P. Hammond b a b
Institute of Energy, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, Wales UK Institute for Sustainable Energy and the Environment, Department of Mechanical Engineering, University of Bath, Bath, England, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2011 Received in revised form 19 January 2012 Accepted 22 January 2012 Available online 20 February 2012
Energy, exergy and exergoeconomic analysis are often used for assessing large energy conversion systems. However exergy and exergoeconomic analysis are rarely used when small or medium scale energy generation systems, such as community CHP/CCHP plants or microcogeneration systems are evaluated. In this study energy, exergy and exergoeconomic analysis of four Community Energy Supply (CES) systems has been carried out. Biomass Steam Turbine CHP (BST), Gas Turbine CHP (GT), Biomass Integrated Gasification Gas Turbine CHP (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP (BIGCC) systems have been modelled. Modelling and energy/exergy analysis have been conducted using the computer programme Cycle-Tempo. Exergoeconomic evaluation of CESS has been performed using the Specific Exergy Costing (SPECO) approach. Exergy costs of the main products: heat and electricity, have been calculated. The analysis shows that gasification of biomass reduces overall system efficiency due to the exergy destruction in the thermo-chemical conversion process when air is used as an oxidizer. A GT using natural gas as a fuel and BIGCC are the most exergy efficient systems in this study with the lowest exergy cost of electricity and heat produced. The exergy cost of electricity generated in BST is the highest. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Combined heat and power Community energy supply system District heating Exergy Exergoeconomics SPECO
1. Introduction Increasing concern over climate change is leading to action to reduce CO2 emissions from power plants in a number of countries. One possible way to reduce the impact of electricity generation on the environment is to use cogeneration e the simultaneous generation of electricity and heat in Combined Heat and Power (CHP) plants. However due to their location it is sometimes impractical for large power plants to use this approach while smaller units can more easily use heat from generation that would otherwise be wasted. With such Community Energy Supply (CES) systems electricity is supplied to the local community or exported to the grid and heat is supplied to the community using a District Heating (DH) network. CES systems offer a number of advantages over large central electricity generators and the use of individual natural gas domestic boilers, the energy supply system presently used in the UK. The benefits of CES systems include that: various
* Corresponding author. Tel.: þ44 29 2087 0674; fax: þ44 29 2087 4939. E-mail addresses: [email protected] (A. Bagdanavicius), jenkinsn6@ cardiff.ac.uk (N. Jenkins), [email protected] (G.P. Hammond). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.01.058
biomass fuels can be used, fuel handling is easier and CO2 emissions can be monitored and controlled more easily than from small individual heating systems. Conventional energy analysis and economic evaluation are commonly used when making a decision on the suitability of a CHP scheme. In some cases this analysis is not sufficient and exergy analysis along with thermoeconomic analysis can then be used. Thermoeconomics or exergoeconomics, if a combination of exergy analysis and economics [1] is applied, has been used extensively over the last 30 years [2]. Two main groups of thermoeconomic methods have been developed: cost accounting methods and optimisation methods. Cost accounting methods, such as: Exergy Cost Theory [3], Average Cost (AVCO) approach [4], Last-in-First-out (LIFO) method [5] or the Specific Exergy Costing (SPECO) [6e8] method have been used to calculate costs of exergy streams in energy conversion systems. Other methods, such as: Thermoeconomic Functional Analysis (TFA) [9] or Engineering Functional Analysis (EFA) [10] have been used as optimisation tools of complex energy systems. Several attempts have been made to compare the different thermoeconomic methods [11,12] and to facilitate unification of nomenclature and methodology [13,14]. Despite all these efforts,
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a variety of different methods continue to be used to analyse energy conversion technologies and new methods continue to emerge [15]. One of the best developed and comprehensive methods is the SPECO methodology presented by Lazzaretto and Tsatsaronis [6]. This tool provides simple and unambiguous procedures for evaluating energy conversion systems and uses a matrix formulation which facilitates fast problem solving. Exergoeconomic analysis is rarely applied to CHP schemes in CES systems due to its complexity. This complexity makes it difficult to gather all the necessary information on various alternatives of a small scheme and encumbers decision making. The objective of this study was to apply the exergoeconomic method to a CES system and to perform a comparative study of different CHP plants. The SPECO method [6] was used in this study. 2. Community energy supply systems A CES system, based on a real project in Ebbw Vale (Wales, UK) was used as a case study. The required maximum heating energy demand of the system is 11 MW, electricity demand is 3.5 MW. The CES system was designed to supply the energy required for heating. Electricity, generated by the CHP unit, can be exported or imported from the grid depending on the demand. Four CES systems were studied: Biomass Steam Turbine CHP plant (BST), Gas Turbine CHP plant using natural gas (GT), Biomass Integrated Gasification Gas Turbine CHP plant (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP plant (BIGCC). CHP plants were modelled using the Cycle-Tempo modelling tool [16] developed at Delft University. An ambient temperature T0 ¼ 288.15 K and pressure P0 ¼ 101.3 kPa were assumed for the exergy calculations. Exergies of streams and fuels were calculated using Cycle-Tempo. Total exergy was used in this study, because the use of separate forms of exergy has little effect on the final results [6]. Biomass fuel of a spruce-pine-fir mixture [17] was used for the BST, BIGGT and BIGCC systems, and natural gas was used for GT. Data used for modelling are presented in Table 1. 2.1. Biomass steam turbine CHP system (BST) A Biomass Steam Turbine CHP system (BST) is shown in Fig. 1. The BST system consists of a steam boiler (1), steam turbine (2), district heating (DH) heat exchanger (3), deaerator (4), DH pump (6), pumps (5, 7, and 12), DH network (8), condenser (11), cooling tower (13), and cooling system pump (14). The design steam pressure is 4.5 MPa, steam temperature is 450 C. The design supply temperature of the DH is 90 C and the return temperature is 50 C. All other design values are shown in Table 1. The system is designed to supply the required energy of
11 MW for heating. Biomass fuel is used for combustion in the steam boiler. The steam turbine is extraction type. During the heating season part of the steam at higher temperature is delivered to the heat exchanger (3). The system is designed so that during the peak heating demand all the steam is extracted. During the periods of low heat demand, the steam is delivered to the condenser (11), thus increasing the electrical power output. 2.2. Gas turbine CHP system (GT) The gas turbine CHP system (GT) is shown in Fig. 2. The GT system consists of compressor (1), combustor (2), turbine (3), heat exchanger (4), DH circulation pump (5), DH network (6), and flue gas stack (7). Natural gas is used as fuel in the combustor. The compressed products of combustion expand in the gas turbine generating power and then pass to the heat exchanger (4). Exhaust gas temperatures after the turbine exceed 500 C. The design flue gas temperature after the heat exchanger is 120 C. Other parameters used in modelling are shown in Table 1. 2.3. Biomass integrated gasification gas turbine CHP system (BIGGT) The Biomass Integrated Gasification Gas Turbine CHP system (BIGGT) consists of three subsystems: biomass gasification system, gas turbine and DH network (Fig. 3). A pressurised circulating fluidized bed (CFB) type gasifier using air as oxidant, operating at 18 bar pressure and 850 C temperature was simulated. The gasification process was modelled based on the data taken from an existing gasification combined cycle CHP plant in Värnamo, Sweden [18]. Biomass gasification modelling using Cycle-Tempo is not trivial and very often more complicated models need to be considered in order to obtain syngas composition close to experimental results [19]. In this study a simple biomass gasification model was applied. Syngas composition simulated in CycleTempo is slightly different compared with the real data, but this simplification has little effect on energy and exergy analysis. Biomass is supplied to the gasifier (1). The main gas turbine compressor (5) is used to deliver air to the combustion chamber (4). About 10% of total air supplied by the compressor (5) is used in the gasification process. The auxiliary compressor (2) pressurises air to the required 18 bar pressure. Syngas leaves the gasifier and is cooled down from 850 C to 400 C in the heat exchanger (3). Cooled high pressure gas is supplied to the combustor (4). Hot flue gas expands in the gas turbine (6) generating electricity. Then hot flue gas is cooled in the heat exchanger (7). Heat exchangers (3 and 7) are used to heat DH water. The design DH supply temperature is 90 C, return temperature is 50 C. 2.4. Biomass integrated gasification combined cycle CHP system (BIGCC)
Table 1 Data used for modelling. Parameter
Value
Steam turbine isentropic efficiency, % Gas turbine isentropic efficiency (natural gas), % Gas turbine/compressor isentropic efficiency (syngas), % Gasification compressor isentropic efficiency, % Mechanical efficiency of compressor, GT and ST, % Lower heating value (biomass), kJ/kg Lower heating value (natural gas), kJ/kg Turbine Inlet Temperature, C GT operating pressure (natural gas), MPa GT operating pressure (syngas), MPa Steam cycle operating temperature, C Steam cycle operating pressure BST/BIGCC, MPa DH network supply/return temperature, C
0.80 0.90 0.85 0.80 0.97 16,700 37,999 z1130 1.5 1.7 450 4.5/4.0 90/50
The Biomass Integrated Gasification Combined Cycle CHP system (BIGCC) consists of a number of subsystems: gasifier system, two heat recovery steam generator (HRSG) systems, gas turbine system, steam turbine system and DH network (Fig. 4). The BIGCC model was designed based on a biomass gasification CHP plant in Värnamo [18]. The gasification process is identical to that of the BIGGT system. However in the BIGCC hot syngas is cooled from 850 C to 400 C in the HRSG (3), where steam is produced. Cooled gas is delivered to the combustor (4), expands in the turbine (6) and cools to 120 C in the HRSG (7, 71, 72) and heat exchanger (12). Steam produced in HRSG (3 and 7, 71, 72) at 40 bar pressure and 450 C temperature is supplied to the steam turbine (8). Exhaust steam at 1.1 bar pressure is delivered to the heat exchanger
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249
Fig. 1. BST system.
(13) where it condenses. District heating water is heated by the exhaust the flue gas from the gas turbine in the heat exchanger (12) and by the condensing steam in the heat exchanger (13).
The SPECO method consists of three steps: - Identification of exergy streams; - Definition of products and fuels; - Construction of cost equations.
3. Methodology The Specific Exergy Costing (SPECO) method was applied in this study. It is a systematic methodology for calculating exergy related costs in thermal systems [6].
The first step was carried out using Cycle-Tempo software. Energy and exergy streams in each model were calculated. Total exergy was used in this study, because the use of separate forms of exergy, such as: thermal, mechanical or chemical, only marginally improves calculation accuracy [6]. Fuels and products of each system were found using the definitions proposed by Tsatsaronis [1,20]. Finally the construction of cost equations was carried out based on the SPECO method [6]. This consists of two parts. First the exergy cost equation for each component of the system is composed. The general form of this equation for the k-th component is:
X X ce E_ e k þ cw;k Wk ¼ cq;k E_ q;k þ ci E_ i k þ Z_ k e
(1)
i
Here ce, ci, cq, cw are the average costs per unit of exergy; E_ e, E_ i, E_ q, Wk are the exergy streams and Z_ k is the sum of capital investments and operating and maintenance expenses. Cost streams C_ associated with the corresponding exergy streams are calculated using equations:
C_ e ¼ ce E_ e ;
Fig. 2. GT system.
C_ i ¼ ci E_ i
(2)
However there are more streams than devices, therefore auxiliary equations have to be formulated. F and P principles [6] are used to compose auxiliary equations. According to the F principle, the specific cost (cost per exergy unit) associated with the removal of exergy from the fuel stream is equal to the average specific cost at which the removed exergy is supplied to the same stream in the upstream components [6]. In this way one auxiliary equation is created for each removal of exergy. The P principle assigns the same unit cost for the added exergy to every exergy stream belonging to the product [6]. Equations associated with the exergy streams
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Fig. 3. BIGGT system.
Fig. 4. BIGCC system.
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supplied to the systems from outside and auxiliary equations formulated based on F and P principles provide the required number of equations in order to find the specific costs of the streams. The external exergy streams are fuel, air or electricity. The cost flow rate of the fuel exergy stream is calculated from the fuel energy price. Biomass fuel exergy was calculated from the Lower Heating Value (LHV) using the equation provided in [21]. The exergy of natural gas was calculated within Cycle-Tempo. The cost flow rate of the air stream was set equal to zero. In CES systems, electricity is used for pumps and auxiliary devices. In this study it was assumed that electricity generated in the CHP plant was used for the pumps and auxiliary compressors, therefore electricity exergy streams were considered as internal streams. Auxiliary equations for the different CES systems are presented in Table 2. To solve the system of linear equations the capital investments and operating and maintenance expenses (Z_ k) for each device were calculated as shown by Kotas [21]. Purchased Equipment Costs (PEC) were calculated using functions presented by Silveira and Tuna [22] and interpolating data from the estimating charts provided in [23]. The PECs of gasifiers were calculated using the cost functions provided by Bridgwater et al. [24]. The Chemical Engineering Plant Cost Index (CEPCI) was used to calculate equipment costs at the reference year (2009, CEPCI ¼ 521.9). All costs were converted to Euro using a fixed currency conversion rate. The total costs of CES systems are presented Table 3. It was assumed that the life-time of CHP is 20 years; interest rate on the capital e 10%; operation time of plant e 7000 h/year. The energy price 0.0205 EUR/kWh of natural gas for industrial consumers in UK in 2009 was taken from EU Energy Portal [25]. A biomass fuel price 90 GBP/t was taken from the E4Tech report [26].
251
Table 3 Total capital costs of CES systems. BST
GT
BIGGT
BIGCC
5.14 MEUR
3.46 MEUR
17.01 MEUR
21.57 MEUR
efficiency shows the percentage of product exergy that is found in the fuel exergy of a component. In order to find which components are responsible for the exergy destruction in the system an exergy destruction ratio was calculated using Equation (4) [1]:
yD;k ¼ E_ D;k =E_ D;tot
(4)
The exergy destruction ratio shows the share of the exergy destroyed in the k-th component compared with the total exergy destruction in the system. _ were Using the SPECO method cost rates of exergy streams (C) calculated. From the cost rates associated with exergy stream, cost rates associated with the fuel (C_ F) and the product (C_ P) were calculated. The average cost per exergy unit of fuel and product for the component k was defined using the Equation (5) [1]:
cF;k ¼ C_ F;k =E_ F;k ;
cP;k ¼ C_ P;k =E_ P;k ;
(5)
To conduct exergoeconomic analysis exergoeconomic variables of the component k, such as: cost rate of exergy destruction C_ D,k, relative cost difference rk and exergoeconomic factor fk were calculated. The cost rate of exergy destruction C_ D,k were calculated using Equation (6) [1]:
C_ D;k ¼ cF;k E_ D;k
(6)
is the unit cost of fuel and E_ D,k is the exergy destruction
4. System analysis First exergy analysis was carried out using the Cycle-Tempo software. The exergy of the product and fuel of the CES system components were calculated. Exergy efficiency was calculated using Equation (3) [1]:
ε ¼ E_ P =E_ F ¼ 1 E_ D þ E_ L E_ F
(3)
Here E_ P is the exergy of the product of the component; E_ F is the exergy of the fuel of the component; E_ D is the exergy destruction rate of the component and E_ L is the exergy loss rate of the component. In this study it was assumed that E_ L ¼ 0. Exergy Table 2 Auxiliary equations for different CES systems. BST C_ 14/E_ 14 ¼ C_ 14/E_ 14 C_ 15/E_ 15 ¼ C_ 13/E_ 13 C_ 16/E_ 16 ¼ C_ 13/E_ 13 C_ 1/E_ 1 ¼ C_ 2/E_ 2 C_ 1/E_ 1 ¼ C_ 3/E_ 3 C_ 4/E_ 4 ¼ C_ 5/E_ 5 C_ 10/E_ 10 ¼ C_ 11/E_ 11
GT C_ 12/E_ 12 ¼ C_ 11/E_ 11 C_ 10/E_ 10 ¼ C_ 11/E_ 11 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 9/E_ 9 ¼ C_ 3/E_ 3 C_ 5/E_ 5 ¼ C_ 6/E_ 6
BIGGT C_ 19/E_ 19 ¼ C_ 23/E_ 23 C_ 20/E_ 20 ¼ C_ 23/E_ 23 C_ 21/E_ 21 ¼ C_ 23/E_ 23 C_ 7/E_ 7 ¼ C_ 8/E_ 8 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 8/E_ 8 ¼ C_ 16/E_ 16 C_ 9/E_ 9 ¼ C_ 15/E_ 15 C_ 5/E_ 5 ¼ C_ 6/E_ 6 C_ 11/E_ 11 ¼ C_ 12/E_ 12
BIGCC C_ 26/E_ 26 ¼ C_ 31/E_ 31 C_ 30/E_ 30 ¼ C_ 31/E_ 31 C_ 27/E_ 27 ¼ C_ 32/E_ 32 C_ 28/E_ 28 ¼ C_ 32/E_ 32 C_ 29/E_ 29 ¼ C_ 32/E_ 32 C_ 7/E_ 7 ¼ C_ 8/E_ 8 C_ 10/E_ 10 ¼ C_ 11/E_ 11 C_ 10/E_ 10 ¼ C_ 12/E_ 12 C_ 9/E_ 9 ¼ C_ 25/E_ 25 C_ 12/E_ 12 ¼ C_ 13/E_ 13 C_ 8/E_ 8 ¼ C_ 9/E_ 9 C_ 18/E_ 18 ¼ C_ 17/E_ 17 C_ 2/E_ 2 ¼ C_ 3/E_ 3 C_ 5/E_ 5 ¼ C_ 6/E_ 6 C_ 22/E_ 22 ¼ C_ 23/E_ 23
Here cF,k rate. The relative cost difference rk indicates the relative increase in the average cost per exergy unit between the fuel and product of the component. It was calculated using Equation (7) [1]:
rk ¼ cP;k cF;k cP;k
(7)
Here cF,k is the unit cost of fuel and cP,k is the unit cost of product. The exergoeconomic factor fk combines non-exergy costs (capital investment and operating and maintenance costs Z_ k) with exergy destruction costs C_ D,k. It was calculated using the Equation (8) [1]:
fk ¼ Z_ k = Z_ k þ C_ D;k
(8)
The exergoeconomic factor shows the contribution of nonexergy related cost (capital cost) to the total cost increase. A low value indicates that the larger cost of the component would be acceptable if the exergy destruction were reduced. A high value of fk indicates the cost of the component should be reduced, even if the exergy efficiency of the component decreases.
4.1. BST system The BST exergy efficiency and exergoeconomic parameters are shown in Fig. 5. The largest exergy destruction (white column of Fig. 5a) is observed in the boiler. The combustion process is the largest contributor to exergy destruction. The boiler is responsible for more than 85% of all exergy destruction in the system. The exergy destruction in the other components, such as: turbine and
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Fig. 5. BST system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
heat exchanger (3) is below 10%. The exergy destruction in the deaerator and pumps is insignificant (Fig. 5a). As anticipated the largest relative cost difference and the sum of destruction and capital cost rate (Z_ k þ C_ D,k) are observed in the boiler (Fig. 5b). The low exergoeconomic factor suggests that destruction cost rate is much higher than investment (capital) cost rate. However improvement in the boiler efficiency would not reduce exergy destruction appreciably. The relative cost difference of pumps (5) and (6) is large however the sum of capital cost flow rate and exergy destruction rate (Z_ k þ C_ D,k) is negligibly small, which means that pumps do not contribute to the total exergy destruction in the system. Exergy destruction and capital cost rate (Z_ k þ C_ D,k) in the turbine and heat exchanger are considerably lower than in the boiler. 4.2. GT system GT exergy efficiency and exergoeconomic parameters are shown in Fig. 6. Four main components in the GT system contribute to exergy destruction (Fig. 6a). The combustor and heat exchanger (4) are the main components where about 85% of total exergy destruction occurs. The combustion process causes large exergy destruction. Exergy efficiency of the turbine and compressor are relatively high (z90%) and the exergy destruction ratio of these components is below 10%. Exergy efficiency of the heat exchanger (4) is low (<30%). This is due to the large temperature difference in the heat exchanger, where high temperature exhaust gas is used to
Fig. 6. GT system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
heat low temperature district heating water. The efficiency of the heat exchanger can be improved by changing temperatures in the system. The largest sum of exergy destruction and capital cost rate (Z_ k þ C_ D,k) are observed in the combustor and heat exchanger (4) (Fig. 6b). It is related to the large exergy destruction in these components. The high cost rate (Z_ k þ C_ D,k) and relative cost difference (rk > 2) of the heat exchanger (4) suggest that the performance of this component should be improved. One way to do that is to use the potential of the high temperature flue gas after the gas turbine, by implementing an additional Rankine cycle, for instance. Preheating of the air before combustion may improve the performance of the combustor. Large relative cost difference is also observed in the pump (5) (Fig. 6b). However the sum of exergy destruction and capital cost rate in the pump (5) is very low. The contribution of the pump to the total cost of the system is insignificant. The low exergoeconomic factor of the combustor and heat exchanger (4) indicates that the increase of capital costs of these components would be justified if better efficiency were achieved.
4.3. BIGGT system BIGGT exergy efficiency and exergoeconomic parameters are shown in Fig. 7. Six main components in the BIGGT system contribute to the exergy destruction in the plant (Fig. 7a). Gasifier, heat exchanger (7) and combustor are the main components where
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However, only the heat exchanger (7) plays an important role in the product exergy cost formation process. High exergoeconomic factors of the compressor (2), gasifier (z0.75) and pump (8) indicate that the capital costs of these components should be reduced in order to decrease the cost rate (Z_ k þ C_ D,k), even if the exergy efficiency were reduced. On the contrary very low exergoeconomic factors of heat exchanger (7) and combustor suggest that the efficiency of these components may be improved. It can be done by modifying the plant design by implementing an additional Rankine cycle as suggested for the GT plant. 4.4. BIGCC system BIGCC exergy efficiency and exergoeconomic parameters are presented in Fig. 8. Similar results are observed for the BIGCC as for BIGGT. Gasifier and combustor are those components where exergy destruction ratio is the largest (Fig. 8a). These devices are responsible for about 60% of the total exergy destruction. It is important to note that the effect of heat recovery steam generators (HRSG3 and HRSG7) and heat exchangers (12 and 13) on exergy destruction ratio is considerably less compared with the heat exchangers in the BIGGT system. The reason is that much lower temperature difference of working fluids (syngas e steam, flue gas e steam, flue gas e water, steam e water) is used in the BIGCC. It increases the exergy efficiency of the heat exchangers. The exergy destruction ratio in
Fig. 7. BIGGT system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
about 80% of total exergy destruction occurs. As it is anticipated combustion and gasification are the processes where exergy destruction ratio is high. The exergy efficiency of the turbine and compressor are relatively high (z90%) and the exergy destruction ratio of these components is below 10% (Fig. 7a). The exergy efficiency of the heat exchanger (3) is lower (z25%) than that of the heat exchanger (7). However, the exergy destruction ratio in this component is lower (<10%) than that of the heat exchanger (7) (z24%). The reason for low exergy efficiency is the large temperature difference of fluids in the heat exchangers (3) and (7). High temperature syngas at a temperature of above 850 C in the heat exchanger (3) and flue gas above 550 C in the heat exchanger (7) are used to increase the temperature of the district heating water from 50 to 90 . Therefore, a large quantity of exergy is destroyed. Much larger heat energy (z8800 kW) is transmitted from the exhaust gas to the district heating water in the heat exchanger (7) compared with the heat transmitted from the syngas in heat exchanger (3) (z2100 kW). Therefore, the exergy destruction ratio in the heat exchanger (7) is higher than that of the heat exchanger (3). Exergoeconomic analysis shows that the gasifier, heat exchanger (7), combustor and gas turbine are the main components where the cost rate (Z_ k þ C_ D,k) is high (Fig. 7b). The cost rates (Z_ k þ C_ D,k) of compressor (5), heat exchanger (3), compressor (2) and pump (8) are much smaller. The relative cost difference of the compressor (2), heat exchanger (3) and heat exchanger (7) is high.
Fig. 8. BIGCC system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.
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the gas turbine (6) is larger than in the steam turbine (8), although the exergy efficiency of the steam turbine (8) is lower. This is due to the fact that about 70% of total electrical energy is generated in the gas turbine. The effect of the gas turbine compressor (5) on the total exergy destruction is very small, and the effect of auxiliary compressor (2) is negligible. Exergoeconomic analysis shows that the gasifier, combustor, gas turbine (6), compressor (5) and steam turbine (8) are the main components where the cost rate (Z_ k þ C_ D,k) is high (Fig. 8b). The contribution of the heat recovery steam generators (HRSG3 and HRSG7) and heat exchangers (12 and 13) to the increase of the cost rate (Z_ k þ C_ D,k) is smaller in the BIGCC (z17%) compared with the BIGGT system (z31%). It infers that the use of heat transfer devices is more efficient in the BIGCC due to the lower temperature differences between the working fluids. The relative cost difference in the devices, which largely contributes to the increase of exergy destruction cost flow rate, such as: gasifier, combustor, turbine (6), compressor (5) and turbine (8) is relatively small compared with compressor (2) and heat exchanger (12). However these components play a pivotal role in the process of the formation of the product exergy cost. As in the BIGGT case high exergoeconomic factor of the gasifier (z0.75) is observed, which suggest that the capital cost of this component should be reduced. In contrast a very low exergoeconomic factor of the combustor and the heat transfer devices indicates that capital cost flow rate is much less than the exergy destruction cost flow rate. 5. Comparison of CES systems The exergy and exegoeconomic analysis of four CES systems shows the differences between the chosen energy conversion technologies (Fig. 9). The five columns in Fig. 9a represent fuel energy input, fuel exergy, electricity generation, heat energy output and heat exergy output from CHP plant. The solid line indicates energy efficiency and the broken line represents exergy efficiency of the plants. It is seen that the energy efficiency is almost the same for all four CES systems (Fig. 9a). The exergy efficiency of GT and BIGCC systems is highest (z40%) and BIGGT efficiency is lower at z 32%. The exergy efficiency of the BST system is the lowest at z 24%. The gasification process leads to large exergy destruction. Therefore, the total exergy efficiency reduces in the BIGGT and BIGCC compared with the GT system. However, this reduction is compensated by the use of combined cycle in BIGCC. Generally speaking the GT system is the most energy and exergy efficient system. However, its impact on the environment is undoubtedly the highest. Exergy costs of the fuel and products: electricity and heating of CESS are shown in Fig. 9b. It is seen from the graph that the exergy cost of biomass fuel and natural gas are almost the same. The exergy cost of electricity and heat are almost identical in the BST system. Electricity exergy cost is considerably lower than heat exergy cost in the GT, BIGGT and BIGCC systems with the gas turbines. The lowest electrical exergy cost is observed in the GT system. The increase of exergy costs in BIGGT and BIGCC is mainly related to the additional exergy destruction in the gasifier and large capital investments. In the BIGCC system the electricity is generated in the gas turbine and steam turbine. Therefore, two electricity exergy costs have been calculated. Electricity exergy cost generated in the gas turbine is 0.0649 EUR/kWh and the exergy cost generated in the steam turbine is 0.1002 EUR/kWh. The weighted average value is presented in Fig. 9b. Large heat exergy cost in BIGGT system is associated with the large exergy destruction in heat exchangers, where high temperature syngas and exhaust gas are used to heat water in DH system.
Fig. 9. Comparison of CESS: (a) energy consumption, generation and efficiency and (b) exergy costs of electricity and heating.
This issue is circumvented when steam Rankine cycle is implemented in the BIGCC system. The heat exergy cost in the BIGCC is similar to that of in BST system. In this study steady state energy, exergy and exergoeconomic analysis has been carried out assuming fixed heat consumption. In order to understand CESS behaviour under varying energy demand conditions more detailed transient analysis is required. Additionally an environmental analysis would be beneficial. 6. Conclusion Four different CES systems: BST, GT, BIGGT and BIGCC were studied using energy, exergy and exergoeconomic analysis. The results are summarised below. 1. Energy analysis shows that the energy efficiency of all the CES systems is almost the same. Energy analysis alone is not capable of revealing all the attributes of energy conversion systems. It is a necessary but insufficient tool for performing overall plant analysis. 2. Exergy analysis shows that gas turbine technology has a higher exergy efficiency compared to the steam turbine. Exergy analysis is capable of indicating the elements of the system where the exergy destruction is largest. Large exergy destruction rates are observed in the gasification and combustion processes. In a GT system where the fluids with large temperature difference are used in the heat transfer devices, large exergy destruction
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rates are observed. A reduction of temperature difference across elements decreases exergy destruction rate. 3. Exergoeconomic analysis using the SPECO method shows that gasification, combustion and heat transfer processes contribute most to formation of the exergy cost. The lowest product cost are observed in GT and BIGCC systems, which shows that the advantage of gas turbine technology against steam turbine. Exergy costs of electricity generated in steam turbine in BST and BIGCC are considerably higher compared with those generated in systems with gas turbines. This study has shown that exergy and exergoeconomic analysis provides much useful information which can be used to assess the suitability of energy conversion technology. The application of these techniques along with traditional energy and economical analyses is necessary to facilitate the development and utilisation of more sustainable energy conversion technologies. Acknowledgments This research formed part of the programme of the UK Energy Research Centre and was supported by the UK Research Councils under Natural Environment Research Council award (NE/G007748/ 1) (Phase II) and by the Higher Education Funding Council for Wales (HEFCW). Nomenclature c Average cost per unit of exergy (EUR/kJ) C_ Cost rate (EUR/s) Exergoeconomic factor of the k-th component fk E_ Exergy transfer rate (kJ/s or kW) r Relative cost difference W Exergy transfer rate associated with power (kJ/s or kW) Exergy destruction ratio yD Z_ Capital cost rate (EUR/s) ε Exergy efficiency of the component Subscripts D Destruction e Stream exiting the component F Fuel i Stream entering the component k The k-th component L Loss P Product q Stream associated with heat transfer tot Total w Stream associated with work Abbreviations BST Biomass steam turbine BIGCC Biomass integrated gasification combined cycle BIGGT Biomass integrated gasification gas turbine CES Community energy supply CHP Combined heat and power DH District heating GT Gas turbine
HEX HHV HRSG LHV PEC SPECO
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Heat exchanger Higher heating value Heat recovery steam generator Lower heating value Purchased equipment costs Specific exergy costing
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