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Exergoeconomic analyses of an energy supply chain for space heating in a building
Energy and Buildings 62 (2013) 343–349
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Exergoeconomic analyses of an energy supply chain for space heating in a building Cem Tahsin Yucer a , Arif Hepbasli b,∗ a b
Department of Mechanical Engineering, Graduate School of Natural and Applied Sciences, Ege University, 35100 Bornova, Izmir, Turkey Department of Energy Systems Engineering, Faculty of Engineering, Yas¸ar University, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 13 January 2013 Received in revised form 10 March 2013 Accepted 11 March 2013 Keywords: Exergy analysis Exergoeconomic analysis EXCEM Exergetic cost effectiveness Buildings
a b s t r a c t This study evaluates both exergetically and exergoeconomically a building along with its heating system, which is examined from the generation stage to the envelope of the building. The energy and exergy flows between all the stages are determined using a predesign tool, which has been recently used to optimize various building designs. The findings based on applying the proposed new term, the lowexergoeco (a combination of the low exergy and exergoeconomics) analysis method is utilized to investigate the system performance. A steam boiler, a heat exchanger and a radiator in a room are considered to analyze the heating system. The ratio of thermodynamic loss rate over cost (R˙ ex ) is calculated to be 4.52 W/US$ for the generation stage while it is 19.77 W/US$ for the steam boiler. A new indicator, exergetic cost effectiveness, defined as the multiplication of the components’ contribution to the total cost and contribution to the total exergy destruction in the system is also proposed. This parameter gives designer or researcher the possibility to decide which components of the system to be improved. First two high values are found to be 0.278 and 0.063 for the generation and the building envelope, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Energy prices are mostly changing according to the demand from industrial production and consumption. Prices based on short sighted political events may result in wrong decisions. On the other hand, prices based on exergy values can easily meet the resource saving and efficient technology requirements. Prices of physical resources are more dependent on their physical values like exergy. Exergy is an effective tool for combining the conservation of mass and energy principles to design and analyze energy systems. It assesses the vital efficiencies and enables to determine the real magnitudes of wastes and losses. It reveals how to design more efficient energy systems by reducing inefficiencies in the systems and finally it helps achieve sustainable development. The objective of thermoeconomics is to minimize exergy costs. Exergetic cost investigates the relation between the system and environment as well as the irreversibilities in the system. These irreversibilities increase the production costs. Thus irreversibilities taking place in the system and their quantities have to be determined. Then their contribution to the product cost is found through thermoeconomic analysis. The system can have one or more
∗ Corresponding author. Tel.: +90 232 411 5492; fax: +90 232 374 8562. E-mail addresses: [email protected], [email protected] (A. Hepbasli). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.03.005
products. The contributions of irreversibilities in each product cost are calculated separately. In systems where the energy transfer is achieved by lighting the fuel, irreversibilities are more substantial. They reduce the energy produced and hence the efficiency. To evaluate and compare thermal systems, the costs, which are calculated according to the themoeconomic analysis, should be considered. In the first law of thermodynamics, it is stated that energy can never be lost. Exergy can be lost. This loss is caused due to the irreversibilities, which can be minimized using low exergy requiring or exergy efficient systems. As far as recent studies on low exergy systems are concerned, Balta [1] made energy, exergy, exergy cost analyses and sustainability assessment of a low exergy heating system. He considered an indoor sports hall with a floor area of 2366 m2 as a case study and investigated three different heating options including a conventional boiler, a condensing boiler and an air heat pump. Overall exergetic efficiencies of the heating systems were determined to be 2.10%, 2.33% and 2.42%, respectively. These heating systems were also compared with each other based on the exergy costs of the considered systems. Hepbasli [2] presented a comprehensive review on related works for lowex analyses. He explained lowex heating and cooling systems, which allow low energy values for energy sources. He reported that low energy valued systems are the ones that provide heating and cooling energy at a temperature close to room temperature. He also highlighted that lowex systems have lower
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temperature levels when compared with high valued energy systems while indoor characteristics are more comfortable and more homogeneous. Thermoeconomics is a combined analysis of economic and thermodynamic methods. In this approach, efficiencies are calculated via an exergy analysis and also non exergetic expenditures like financial and labor costs. Exergy possess a strong and direct correlation with economic values. Likewise extended exergy accounting (EEA) method investigates this correlation when developing a new form of method with exergetic and monetary metric values. The so-called EXCEM analysis is also another methodology, which incorporates four key parameters, namely exergy, cost, energy and mass. This system requires an examination of the flows of the quantities represented by itself into, out of and at all points within a system. Of all the quantities only mass and energy are subject to conservation laws. Exergy decreases or remains constant while cost increases or remains constant. Rosen and Dincer [3] used illustrative examples to clarify the general concepts of EXCEM analysis. They applied this analysis to three equipment, namely pump, steam turbine, coal fired electrical generating station. For calculating the cost generation rate, the capital cost was multiplied by the amortization factor and divided by the load factor. The amortization value took the time value of money into account. Exergy lost rates were also calculated to be 48.5 MW and 10.2 MW for boiler and the turbine generator, respectively. Kalinci et al. [4] studied on the exergoeconomic analysis of hydrogen production from biomass gasification. They used the EXCEM method to obtain energy and exergy streams of the system, exergetic efficiency values of all equipment. The costs of equipment with their thermodynamic loss rates and ratio of thermodynamic loss rate to capital cost were calculated. According to the findings, the unit power cost changed from 0.04 to 0.15 $/kWh and unit hydrogen cost increased to 1.802 $/kg. The main objectives of this contribution are to (i) apply the exergoeconomic analyses to a building heating system, (ii) assess its performance through efficiencies, and (iii) define a parameter named exergetic cost effectiveness (ECE), which was proposed in this study for the first time to the best of the authors’ knowledge. In the open literature, there are some papers using lowex method to calculate heat losses in exergy values. In this paper, the EXCEM analysis is applied to the results of the lowex predesign tool. The combination of these two analyses is called lowexergoeco analysis in this study. For the first time, exergy loss rates of the heating system stages calculated by the lowex analysis are evaluated with the cost figures of the equipment in that system. Costs are based on both the equipment and the building components in the overall system.
2. System description 2.1. Heating system The building selected to apply the analysis is heated by a steam boiler. The type of the fuel consumed is fuel oil. The chemical energy contained in fuel oil turns into thermal energy in the steam boiler. The energy carrier is then goes through the pipes to the distribution system. The steam gives its energy to the water in the heat exchanger where the heat transfer takes place. There are heat exchangers before the water heaters and heating zones. The emission system consists of radiators, which supply the required heat to the room air. Finally, the remaining energy is conducted to the building envelope. A schematic view of the heating system explaining sub systems in the overall heating system is shown in Fig. 1.
Exergy Flow
Heat Exchanger
Steam boiler
Radiator
Building Envelope
Fig. 1. Exergy flow in the heating system.
Exergy is the portion of energy, which can be translated to other forms of usable energy. The largest exergy loss takes place in the primary energy transformation phase. Chemical energy contained in fuel oil turns out to be thermal energy. During this transformation process, exergy losses occur because of the deviation of the chemical composition of the fuel. These are also investigated in the analysis of the heating system equipment part.
2.2. Building characteristics The building is located in Izmir, Turkey and serves as a house of accommodation. The construction materials are locally manufactured. Exterior wall consists of aerated concrete, plaster for exterior and interior surfaces and polystyrene hard foam plate as insulation material. Thermal transmittance value of exterior wall is found to be 0.41 W/m2 K. Floor to the ground consists of ceramic, grading concrete, concrete, foam plate and plaster for interior surface. Thermal transmittance value of the floor to the ground is calculated to be 0.47 W/m2 K. The ceiling (roof) consists of thermal insulation material, concrete and plaster for interior surface. Thermal transmittance value of the roof is 0.32 W/m2 K. According to the Turkish Building Standard, TS 825 [5], the insulation in walls, on floor to the ground and on the roof is appropriate. Table 1 presents the project data on the building, which has double glazing windows (PVC material) facing all directions and the thermal transmittance value for a window is 2.55 W/m2 K while the distance between two glasses in this double glazing windows is 6 mm. The indoor and outdoor design temperatures for the city of Izmir in Turkey are 20 ◦ C and 0 ◦ C, respectively.
3. Analysis 3.1. Determination of the building construction materials’ costs Building materials are selected consciously since their strength and insulation properties for heat, water and sound are very important. In this study, the investigated building has three floors (including basement) with a height of 9 m. Total calculated wall surface area of the building is 3630 m2 . The components in the exterior wall are aerated concrete, interior and exterior plasters and hard polystyrene foam. The materials used in the exterior wall are presented in Table 2.
Table 1 Boundary conditions for the building. Volume (inside) (m3 )
V = 37,350
Net floor area (m2 ) Indoor air temperature (◦ C) Exterior temperature (◦ C)
AN = 4150 Ti = 20 T0 = 0
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Table 2 Components that form the building and the heating system. Component
Quantity
Unit cost
Aerated concrete Interior plaster Exterior plaster Hard polystyrene foam (3 cm thickness) 2 in. pipe 2½ in. pipe 5 in. pipe Radiator Steam boiler Heat exchanger
3630 m2 3630 m2 3630 m2 111.8 m3 56 m 57 m 5m 500 mm × 750 mm (135 units) 1 piece 1 piece
7.15 $/m2 0.45 $/m2 0.45 $/m2 45 $/m3 2.79 $/m 6 $/m 15.68 $/m 46.5 $/m2 16,807.9 $/piece 960.45 $/piece
Total cost (US$) 25,954.5 1633.5 1633.5 5031 156.24 342 78.4 2353.67 16,807.9 960.45
Based on the exchange rate of 1.77 Turkish Lira (TL)/US$ dated January 9, 2013 by the Central Bank of Turkey Taken from Ministry of Environment and Urban Planning’s Bill of Quantities
3.2. Determining the costs of the heating equipment
Ventilation heat loss is calculated using
The heating system considered consists of generation, distribution and emission stages. In the generation stage, there is a steam boiler, which is used to produce steam to heat the water stream in the pipeline for heating rooms and to meet domestic hot water requirements. The steam boiler has a heating capacity rate of 1450 kW. Operating pressure is 3 bars. In the distribution stage, the energy carrier is carried by pipes. Here, the pipes between the heating center and the heating zone are taken into account. Pipe lines are insulated to prevent heat losses to the environment. Existing pipe type is screwed pipe. The diameters of the pipes used are 2” or larger. The diameter, length and cost information of the pipes are given in Table 2. Cost information is taken from Ministry of Environment and Urban Planning’s Bill of Quantities. Steam is a high quality energy carrier while it is dangerous and not economic for emission stage. Thus energy transfer equipment (heat exchangers) is used to transfer the heat from hot stream to the cold stream. At the emission stage, there are 135 panel radiators to heat the rooms. The height, length and cost information of panel radiators and the cost figures of heating system equipment are presented in Table 2.
v (cp × × V × nd × (1 − v ) × (Ti − T0 )
(2)
In this study, nd , air exchange rate, is assumed as 0.4/h because of the double glazing windows [8]. cp is the specific heat of air, is the density of air and v stands for the heat exchanger efficiency, if there is mechanical ventilation with heat recovery. The subscript “v” indicates ventilation. In this paper, internal heat gains are neglected and solar heat gains are taken into account. Transmission and ventilation heat losses caused by the building envelope and the solar heat gains are used to obtain the total heating demand of the heating system. Heating demand for the examined building is shown in the following equation. H = T + v − S
(3)
To compare different types of buildings, the specific heating demand is calculated as
H =
H AN
(4)
3.4. EXCEM analysis 3.3. Determination of the energy demand Steady-state energy and exergy analyses are performed using an Excel tool based on that developed within the framework of IEA ECBCS Annex 49 [6]. Heat losses are calculated as transmission heat loss and ventilation heat loss. Sum of all losses from all surfaces are calculated to obtain transmission heat losses. Transmission heat losses for each of the building part are presented in Fig. 2. T =
(Ui × Ai × Fxi ) × (Ti − T0 )
(1)
i
where the Fxi values, temperature correction factors, are taken from [7]. For the exterior walls, windows and doors, the selected Fxi values are the same and equal to 1. For the floors to the ground, it is taken to be 0.6.
EXCEM analysis dictates mass, energy, exergy and cost balance equations. A mass flow rate balance for a system may be written as ˙ 0=m ˙a ˙ i −m m
(5)
where indices “i”, “o” and “a” denote input, output and accumulation, respectively. The energy and exergy balances (Eqs. (6)–(7)) can be written as E˙ i − E˙ 0 = E˙ a
(6)
˙ 0 − L˙ ex = Ex ˙ a ˙ i − Ex Ex
(7)
L˙ ex stands for thermodynamic loss rate. Cost is an increasing, nonconserved quantity. The cost balance can be written as, Ki + Kg − K0 = Ka
(8)
where index “g” represents the generation term. Cost generation corresponds to the appropriate capital and other costs associated with the creation and maintenance of a system. In this study, we used the capital costs to express the cost generations. Exergy losses can be identified from Eq. (7), and are of two types: external (the loss associated with exergy that is emitted from the system, or waste exergy output) and internal (the exergy losses within the system due to process irreversibilities, or exergy consumption). Sum of these two types of exergy losses forms total exergy loss. Hence, exergy loss rate, L˙ ex is defined as follows: Fig. 2. Transmission losses for the building parts.
L˙ ex = Exergy consumption rate + Waste exergy output rate
(9)
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In the above equations, the exergy rate entering the control ˙ in ) is equal to the exergy rate exiting from the control volume (Ex ˙ out ) and the exergy destruction rate (Ex ˙ des. ). In Eq. (14) volume (Ex ˙ f and Ex ˙ st denote exergy rate of fuel and exergy change rate of Ex
P-8
Effluents
steam. Exergy rate of the fuel is calculated by
Fuel and air
Water inlet
Steam outlet
P-6
P-5
P-4
E-3
˙ is defined as the ratio of thermodynamic loss Another parameter, R, rate L˙ to cost K as follows, L˙ K
(10)
Here, the value of R˙ generally depends on whether it is based on energy loss rate (R˙ en ) or exergy loss rate (R˙ ex ), while in this study R˙ ex values were used. R˙ ex =
L˙ ex K
(11)
The value of R˙ ex varies in different situations (e.g., technology, time, location, resource costs, knowledge). Also, during periods when energy-resource costs increase, the value of R˙ ex is likely to decrease (i.e., greater capital is invested to reduce losses). 3.5. Exergetic and exergoeconomic analyses of the heating system equipment 3.5.1. Steam boiler Steam boiler is equipment, which generates steam at the required pressure, temperature and quantity. It is assumed as an open system thermodynamically. The steam boiler in the heating center is going to be examined in terms of the exergy issue. The heating capacity rate of the steam boiler is 1450 kW while supply/return temperatures are measured to be 113/75 ◦ C. The operating pressure is 1.6 bars in this study. Fuel oil is used as a fuel. The fuel is the low sulfur containing fuel oil type with a lower heating value of 44,600 kJ/kg. The control volume of the steam boiler is shown in Fig. 3. According to the first law of thermodynamics, the efficiency of the steam boiler is calculated as follows, =
˙ st × cp (Tst,in − Tst,out ) m ˙ f × Hu m
(15)
εf = Hu × ϕ
(16)
˙ f is the mass flow rate of fuel consumption and εf denotes where m the specific exergy of fuel while ϕ represents the chemical exergy factor. According to Ref. [9], the chemical exergy factor for fuel oil is found to be 1.04. Exergy rate gained by steam in the steam boiler can be determined from ˙ st = m ˙ st × [(hout − hin ) − T0 (Sout − Sin )] Ex
Fig. 3. Control volume of the steam boiler.
R˙ =
˙ f =m ˙ f ×εf Ex
(12)
The ratio between the energy transferred to the water and the ˙ st energy obtained by firing the fuel gives the efficiency value. m ˙ f denotes the mass flow represents the mass flow rate of steam, m rate of fuel and Hu is the lower heating value of the fuel. Exergy analysis is carried out by balancing the exergy input rates and exergy output rates in the control volume. In this study exergy of air to burn the fuel is neglected. The exergy rates of the flue gas and the heat loss from the surface are included in the exergy destruction terms. The exergy balance can be written as follows: ˙ in − Ex ˙ out ˙ des. = Ex Ex
(13)
˙ f = Ex ˙ st + Ex ˙ ef + Ex ˙ l,pet + Ex ˙ d,b Ex
(14)
(17)
where hout and hin represent outlet and inlet specific enthalpies of the steam while Sout and sin stand for the outlet and inlet specific entropies of the steam. The exergy loss resulting from the energy conversion between chemical energy and heat energy can be formulated as ˙ l,pet = m ˙ st (hout − hin ) Ex
To Tad.
(18)
˙ l,pet indicates exergy loss rate at the primary energy transwhere Ex formation and Tad. stands for the adiabatic flame temperature of fuel oil [10]. The exergy loss rate resulting from energy conversion between effluents and steam flow is formulated as
˙ ef = m ˙ st T0 (sout − sin ) − Ex
(Hout − hin ) Tad.
(19)
Finally, the exergetic efficiency can be obtained from =
˙ st Ex ˙Exf
(20)
Sustainability assessment requires that the resources should be utilized efficiently, and also it may be performed with sustainability index (SI) method, which is related to exergy efficiency, as proposed by Rosen et al. [11]. In this regard, exergy methods are essential in improving efficiency that allows society to maximize the benefits it derives from its resources while minimizing the negative impacts such as environmental damage. So, sustainability index (SI) is a factor, which is directly related to the exergetic efficiency while it is written [11]. =1−
1 SI
(21)
3.5.2. Heat exchanger Heat exchanger is equipment, which is designed to transfer heat from the hot stream to the cold stream. The existing heat exchanger is a plate type one. The hot water stream enters the equipment at 113 ◦ C and leaves at a temperature of 92 ◦ C. Inlet and outlet temperatures of the cold stream are 65 ◦ C and 85 ◦ C, respectively. Exergy balance equation is formed according to the entering and exiting streams, as shown in Fig. 4. ˙ 2 − Ex ˙ 1 + Ex ˙ D = Ex ˙ 3 − Ex ˙ 4 Ex
(22)
where indices (Eqs. (1)–(2)) belong to the cold streams and (Eq. (3)–(4)) belong to the hot streams. If Eq. (22) is expressed more explicitly, it becomes ˙ d,h = m ˙ cw × [(h2 − h1 ) − T0 (s2 − s1 ] + Ex ˙ hw m × [(h3 − h4 ) − T0 (s3 − s4 )]
(23)
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Entering hot stream
347
Heat transfer to the room
Water inlet Entering cold stream
Radiator
Water outlet
Exing cold stream Fig. 5. Control volume of the radiator. E-2
The exergy balance of a radiator is obtained from ˙ rad,in = Ex ˙ rad,out + Ex ˙ rad + Ex ˙ d,rad Ex
Exing hot stream
Exergetic efficiency is calculated by the ratio of exergy rate obtained over exergy rate consumed (Fig. 5).
Fig. 4. Control volume of the heat exchanger.
˙ cw and m ˙ hw denote mass flow rates of the cold and where m ˙ d,h is the exergy destruction rate for the heat hot water while Ex exchanger. Exergetic efficiency of the heat exchanger can be expressed by h
=
˙ 2 − Ex ˙ 1 Ex ˙Ex3 − Ex ˙ 4
(24)
3.5.3. Pipes Pipes carry the heat source hot water to the heating zone. Heat load cannot be totally transmitted to the emission system. Since there is heat loss from the surface of pipes, the pipes are insulated. 2 and higher diameter sized pipes are considered compared to lower sizes. The surface area of a pipe can be calculated by A= ×D×l
(25)
The heat loss on the pipes is mainly caused by convection. For flat areas, the heat convection coefficient (˛) is dependent on the speed of air. If the velocity of air (v) is lower than 5 m/s, the heat convection coefficient can be calculated [12] as follows ˛ = 5.8 + 3.9v
(31)
(26)
rad
=
˙ rad Ex ˙ rad,out ˙ rad,in − Ex Ex
(32)
3.5.5. Exergetic cost effectiveness (ECE) To the best of the authors’ knowledge, the definition “Exergetic Cost Effectiveness” was proposed for the first time to include both the effects of equipment’s exergy destruction and cost figure to the overall system. In the literature, no other term explaining the combination of cost contribution and exergy destruction contribution in a system with one parameter has been originally proposed by the authors. The parameter does not have any units. The evaluation of each stage or component in a system can be examined using this parameter. The word stage means the generation, the distribution, the emission, the building envelope etc. Components in a system may be boiler, heat exchanger, pipes, radiator etc. The contribution of a stage or a component to the total cost (a) is calculated as follows: a=
cost of a stage or a component total cost of the system
(33)
The contribution of a stage or a component to the total exergy destruction (b) is expressed as
In this study, the air velocity is ignored and the heat convection coefficient is applied as
b=
˛ = 5.8
The ECE parameter is obtained by applying the following relation.
W m2 K
(27)
where Ts and T0 indicate the surface and outdoor temperatures, respectively. Exergy loss rate can be applied after determining the heat loss rate. ˙ l,p = Ex
Ts − T0 × Q˙ l,p Ts
(28)
3.5.4. Radiators The emission system has radiators to heat the rooms in the building. Indoor temperature is assumed to be 20 ◦ C. Heat load rate (Q˙ rad ) of a radiator is calculated as follows: ˙ w,rad × (h85 − h65 ) Q˙ rad = m
(29)
˙ w,rad , h85 and h65 are mass flow rate of the water in the where m radiator and enthalpies of the water at 85 ◦ C and 65 ◦ C, respectively. Exergy rate of the radiator is calculated by
˙ rad = Q˙ rad 1 − Ex
T0 Troom
(34)
ECE = a × b
Heat loss rate occurred on the surface of pipes is calculated as Q˙ l,p = ˛ × A × (Ts − T0 )
exergy destruction of a stage or a component total exergy destruction of the system
(30)
The low ECE value dictates more cost effective, more appropriate and more efficient component or stage. The ECE parameter takes values between zero and one. Zero value shows that the equipment has no influence on the cost or exergy destruction terms. The value one means that other equipment has no influence on cost and exergy destruction terms. 4. Results and discussion 4.1. Exergetic results The predesign tool Annex 49 includes energy and exergy results of the chosen building and its heating system. According to the project data used in predesign tool, transmission and ventilation heat loss rates are found to be 100.7 kW and 100.1 kW, respectively. Solar heat gain rates are calculated to be 23.6 kW. By taking these values into account, the heat demand and specific heat demand are found to be 177.2 kW and 42.7 W/m2 , respectively. The heat loss caused by each building component is shown in Fig. 2. The energy,
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Table 3 System rates in kW. Stages Input After prim.en. trans. After generation After distribution After emission After room
Energy load rate
Exergy load rate
Exergy loss rate
312.04 240.02 213.63 186.53 177.21 177.21
293.32 225.63 48.05 42.22 39.93 12.09
– 67.69 177.58 5.83 2.29 27.84
Table 5 Exergoeconomic results of the stages in the lowex analysis and the equipment in the heating system. L˙ ex (kW)
Stages Generation Distribution Emission Building envelope Steam boiler Heat exchanger Pipes Radiators
245.26 5.83 2.29 27.84 1039.92 10.62 4.43 9.59
K ($)
R˙ ex (W/$)
16,807 696.6 2353.67 34,289 16,807 960.45 696.6 2353.67
4.52 2.82 0.28 0.26 19.77 3.39 2.26 1.13
Table 4 Exergetic rate results for the heating equipment in kW.
4.2. Exergoeconomic results
Equipment
Exergetic figures
Steam boiler
˙ f Ex 1289.48 ˙ hw Ex
˙ st Ex 249.56 ˙ cw Ex
E˙ x˙ ef 509.53 ˙ d,h Ex
16.91 ˙ w Ex 103
6.29 ˙ rad Ex 32
10.62 ˙ d,rad Ex 71
Heat exchanger Radiator
E˙ x˙ i,pet 98.50
E˙ x˙ d,b 431.89
exergy loads and exergy losses obtained by applying predesign tool are listed in Table 3. After calculating the pre-design tool results, the energy and exergy input rates to the system are approximately 312 kW and 293 kW, respectively. After exiting from each stage, the load values decrease. The decrease is more noticeable in the exergy terms. An energy rate of 177.2 kW is transferred to the room. As exergy rate, 40 kW is obtained from the emission system. On the other hand, the maximum exergy loss rate took place at the generation stage in the steam boiler as 177.6 kW. Other high exergy loss rates occurred in the primary energy transformation and distribution stages. The main equipment in the heating system is the steam boiler, the heat exchanger and the radiator. The findings after calculating exergy flows affecting the control volume of each equipment are shown in Table 4. Exergetic efficiencies of the steam boiler, the heat exchanger and the radiator in the heating system are 19.35%, 37% and 31%, respectively. The exergetic efficiency of the overall system is calculated to be 3.18%. If the exergetic efficiency of any equipment is high, it will have a high sustainability index. The sustainability index for the steam boiler in this study is found to be 1.24. Since the exergetic efficiencies of the heat exchanger and the radiator are higher than that of the steam boiler, their sustainability indices are higher. It is determined to be 1.59 for the heat exchanger and 1.45 for the radiator. It can be considered that in the heating systems, the selection of the equipment and building materials play an important role. More expensive, but high exergy efficient components may lead to a system with more sustainable conditions.
The EXCEM analysis is applied to the stages in the heating system and also to the selected equipment in that system. The thermodynamic loss rates, the capital costs and the exergoeconomic results are presented in Table 5. It is obvious from this table that the maximum thermodynamic loss rate belongs to the generation stage, being 245.26 kW. The minimum thermodynamic loss rate is calculated at the emission stage as 2.29 kW. By taking the capital cost into account, the EXCEM figure R˙ is obtained. This ratio takes its maximum value at the generation stage as 4.52 W/$. Since the capital cost spent for the materials form the building envelope is the highest, the minimum ratio value is found to be 0.26 W/$ for the building envelope. The findings related to the selected equipment based on the EXCEM analysis are listed in Table 5. The capital costs for each equipment are taken from the Ministry of Environment and Urban Planning’s Bill of Quantities. When one compares the equipment, the maximum and minimum thermodynamic loss rates belong to the steam boiler and pipes, respectively. In this paper, the cost and the exergy loss rates for the outdoor pipes are taken into consideration. Exergy loss rates of the steam boiler and the pipes are 1039.92 kW and 4.43 kW, respectively. It is not surprising that the steam boiler has the maximum EXCEM ratio as 19.77 W/$. The emission equipment radiators have the minimum EXCEM ratio as 1.13 W/$, because their capital cost is higher than that of pipes. Higher ECE value means higher cost and higher exergy destruction in the system. It helps to determine the most inefficient equipment in the overall system. After calculating the ECE values of all equipments, they can be put in an ascending order. The maximum value means that the equipment should be examined firstly. The findings related to the parameter ECE to compare stages are presented in Table 6. Each stage or equipment can be evaluated by comparing its ECE value with that of other stages or equipment. It can be seen that the maximum ECE value represents the generation among all stages. It is found to be 0.278. This result means that the generation stage needs to be firstly improved before other stages. Secondly, in the building envelope, the materials with less thermal transmittance values can be selected. The ECE value for the building envelope is 0.063.
Table 6 ECE results of the stages in the lowex analysis and the equipment in the heating system. Stages
˙ d (kW) Ex
Generation Distribution Emission Building envelope Steam boiler Heat exchanger Pipes Radiators
245.26 5.83 2.29 27.84 1039.92 10.62 4.43 9.59
K ($) 16,807 696.6 2353.67 34,289 16,807 960.45 696.6 2353.67
a
b
ECE (a × b)
0.32 0.01 0.04 0.63 0.81 0.05 0.03 0.11
0.87 0.02 0.01 0.10 0.976 0.01 0.005 0.009
0.278 0.0002 0.0004 0.063 0.79 0.0005 0.00015 0.00099
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It is obvious from Table 6 that the steam boiler has the maximum ECE value. It is calculated to be 0.79. The choice for the type of the steam boiler can be considered again. After that, the radiators at the emission stage can be examined if there is a way to improve their exergetic efficiencies. Its ECE value is approximately 0.001.
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The component with the highest ECE value should be preferred to make analysis for improvements. According to the findings, the generation stage (0.278) and the steam boiler (0.79) had the maximum ECE values among other ones. (g) For a future work, exergoenvironmental analysis is recommended to take environmental values into account.
5. Conclusions Acknowledgement In this study, we have applied exergy and exergoeconomic analyses to a building and its heating system. We have calculated energy and exergy flows from the first stage, the primary energy source to the last stage building envelope through the predesign tool in Annex 49. We have also implemented exergoeconomic analysis at the stages after determining the exergetic results in the lowex predesign tool defined in Annex 49. Some concluding remarks obtained from the results of the present study may be listed as follows: (a) Total exergy input rate was determined to be 293.32 kW while the largest exergy loss rate was calculated as 177.58 kW. (b) According to the exergy analyses, exergetic efficiencies of the steam boiler, the heat exchanger and the radiator were calculated to be 19.35%, 37% and 31%, respectively. (c) The overall system exergy efficiency was determined to be 3.18%. (d) By examining the R˙ ex results, the generation stage had a maximum value of 4.52 W/$. The distribution, the emission and the building envelope stages had R˙ ex values of 2.82 W/$, 0.28 W/$ and 0.26 W/$, respectively. (e) When the equipment in the heating system was compared, the maximum R˙ ex was for the steam boiler, with a value of 19.77 W/$. The heat exchanger, the pipes and the radiators followed the steam boiler with R˙ ex ratios of 3.39 W/$, 2.26 W/$ and 1.13 W/$, respectively. (f) The authors expect that the newly proposed parameter, the ECE, will help researchers put the components in a precedence order.
The authors would like to thank the three reviewers due to their valuable comments, which have been utilized to improve the quality of the paper. References [1] M.T. Balta, Exergetic cost analysis and sustainability assessment of various low exergy heating systems, Energy and Buildings 55 (2012) 721–727. [2] A. Hepbasli, Low exergy heating and cooling systems for sustainable buildings and societies, Renewable and Sustainable Energy Reviews 16 (2012) 73–104. [3] M.A. Rosen, I. Dincer, Exergy-cost-energy-mass analysis of thermal systems and processes, Energy Conversion and Management 44 (2003) 1633–1651. [4] Y. Kalinci, A. Hepbasli, I. Dincer, Exergoeconomic analysis of hydrogen production from biomass gasification, International Journal of Hydrogen Energy 37 (2012) 16402–16411. [5] TS 825, Turkish Standard, Heating Insulation Rules in Buildings (in Turkish), 1998. [6] ECBCS Annex 49. Low Exergy Systems for High-Performance Buildings and Communities, http://www.annex49.com (accessed 10.03.13). [7] D. Schmidt, Design of low exergy buildings-method and a pre-design tool, International Journal of Low Exergy and Sustainable Buildings 3 (2004). [8] M. Shukuya, Exergy concept and its application to the built environment, Building and Environment 44 (2009) 1545–1550. [9] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, Anchor Brendan Ltd., Tiptree, Essex Great Britain, 1985. (accessed [10] http://en.wikipedia.org/wiki/Adiabatic flame temperature 2.6.2013). [11] M.A. Rosen, I. Dincer, M. Kanoglu, Role of exergy in increasing efficiency and sustainability and reducing environmental impact, Energy Policy 36 (2008) 128–137. [12] Wall G, Exergetics, http://exergy.se/ftp/exergetics.pdf, 2009 (accessed 25.12.2012).
Contents lists available at SciVerse ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Exergoeconomic analyses of an energy supply chain for space heating in a building Cem Tahsin Yucer a , Arif Hepbasli b,∗ a b
Department of Mechanical Engineering, Graduate School of Natural and Applied Sciences, Ege University, 35100 Bornova, Izmir, Turkey Department of Energy Systems Engineering, Faculty of Engineering, Yas¸ar University, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 13 January 2013 Received in revised form 10 March 2013 Accepted 11 March 2013 Keywords: Exergy analysis Exergoeconomic analysis EXCEM Exergetic cost effectiveness Buildings
a b s t r a c t This study evaluates both exergetically and exergoeconomically a building along with its heating system, which is examined from the generation stage to the envelope of the building. The energy and exergy flows between all the stages are determined using a predesign tool, which has been recently used to optimize various building designs. The findings based on applying the proposed new term, the lowexergoeco (a combination of the low exergy and exergoeconomics) analysis method is utilized to investigate the system performance. A steam boiler, a heat exchanger and a radiator in a room are considered to analyze the heating system. The ratio of thermodynamic loss rate over cost (R˙ ex ) is calculated to be 4.52 W/US$ for the generation stage while it is 19.77 W/US$ for the steam boiler. A new indicator, exergetic cost effectiveness, defined as the multiplication of the components’ contribution to the total cost and contribution to the total exergy destruction in the system is also proposed. This parameter gives designer or researcher the possibility to decide which components of the system to be improved. First two high values are found to be 0.278 and 0.063 for the generation and the building envelope, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Energy prices are mostly changing according to the demand from industrial production and consumption. Prices based on short sighted political events may result in wrong decisions. On the other hand, prices based on exergy values can easily meet the resource saving and efficient technology requirements. Prices of physical resources are more dependent on their physical values like exergy. Exergy is an effective tool for combining the conservation of mass and energy principles to design and analyze energy systems. It assesses the vital efficiencies and enables to determine the real magnitudes of wastes and losses. It reveals how to design more efficient energy systems by reducing inefficiencies in the systems and finally it helps achieve sustainable development. The objective of thermoeconomics is to minimize exergy costs. Exergetic cost investigates the relation between the system and environment as well as the irreversibilities in the system. These irreversibilities increase the production costs. Thus irreversibilities taking place in the system and their quantities have to be determined. Then their contribution to the product cost is found through thermoeconomic analysis. The system can have one or more
∗ Corresponding author. Tel.: +90 232 411 5492; fax: +90 232 374 8562. E-mail addresses: [email protected], [email protected] (A. Hepbasli). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.03.005
products. The contributions of irreversibilities in each product cost are calculated separately. In systems where the energy transfer is achieved by lighting the fuel, irreversibilities are more substantial. They reduce the energy produced and hence the efficiency. To evaluate and compare thermal systems, the costs, which are calculated according to the themoeconomic analysis, should be considered. In the first law of thermodynamics, it is stated that energy can never be lost. Exergy can be lost. This loss is caused due to the irreversibilities, which can be minimized using low exergy requiring or exergy efficient systems. As far as recent studies on low exergy systems are concerned, Balta [1] made energy, exergy, exergy cost analyses and sustainability assessment of a low exergy heating system. He considered an indoor sports hall with a floor area of 2366 m2 as a case study and investigated three different heating options including a conventional boiler, a condensing boiler and an air heat pump. Overall exergetic efficiencies of the heating systems were determined to be 2.10%, 2.33% and 2.42%, respectively. These heating systems were also compared with each other based on the exergy costs of the considered systems. Hepbasli [2] presented a comprehensive review on related works for lowex analyses. He explained lowex heating and cooling systems, which allow low energy values for energy sources. He reported that low energy valued systems are the ones that provide heating and cooling energy at a temperature close to room temperature. He also highlighted that lowex systems have lower
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temperature levels when compared with high valued energy systems while indoor characteristics are more comfortable and more homogeneous. Thermoeconomics is a combined analysis of economic and thermodynamic methods. In this approach, efficiencies are calculated via an exergy analysis and also non exergetic expenditures like financial and labor costs. Exergy possess a strong and direct correlation with economic values. Likewise extended exergy accounting (EEA) method investigates this correlation when developing a new form of method with exergetic and monetary metric values. The so-called EXCEM analysis is also another methodology, which incorporates four key parameters, namely exergy, cost, energy and mass. This system requires an examination of the flows of the quantities represented by itself into, out of and at all points within a system. Of all the quantities only mass and energy are subject to conservation laws. Exergy decreases or remains constant while cost increases or remains constant. Rosen and Dincer [3] used illustrative examples to clarify the general concepts of EXCEM analysis. They applied this analysis to three equipment, namely pump, steam turbine, coal fired electrical generating station. For calculating the cost generation rate, the capital cost was multiplied by the amortization factor and divided by the load factor. The amortization value took the time value of money into account. Exergy lost rates were also calculated to be 48.5 MW and 10.2 MW for boiler and the turbine generator, respectively. Kalinci et al. [4] studied on the exergoeconomic analysis of hydrogen production from biomass gasification. They used the EXCEM method to obtain energy and exergy streams of the system, exergetic efficiency values of all equipment. The costs of equipment with their thermodynamic loss rates and ratio of thermodynamic loss rate to capital cost were calculated. According to the findings, the unit power cost changed from 0.04 to 0.15 $/kWh and unit hydrogen cost increased to 1.802 $/kg. The main objectives of this contribution are to (i) apply the exergoeconomic analyses to a building heating system, (ii) assess its performance through efficiencies, and (iii) define a parameter named exergetic cost effectiveness (ECE), which was proposed in this study for the first time to the best of the authors’ knowledge. In the open literature, there are some papers using lowex method to calculate heat losses in exergy values. In this paper, the EXCEM analysis is applied to the results of the lowex predesign tool. The combination of these two analyses is called lowexergoeco analysis in this study. For the first time, exergy loss rates of the heating system stages calculated by the lowex analysis are evaluated with the cost figures of the equipment in that system. Costs are based on both the equipment and the building components in the overall system.
2. System description 2.1. Heating system The building selected to apply the analysis is heated by a steam boiler. The type of the fuel consumed is fuel oil. The chemical energy contained in fuel oil turns into thermal energy in the steam boiler. The energy carrier is then goes through the pipes to the distribution system. The steam gives its energy to the water in the heat exchanger where the heat transfer takes place. There are heat exchangers before the water heaters and heating zones. The emission system consists of radiators, which supply the required heat to the room air. Finally, the remaining energy is conducted to the building envelope. A schematic view of the heating system explaining sub systems in the overall heating system is shown in Fig. 1.
Exergy Flow
Heat Exchanger
Steam boiler
Radiator
Building Envelope
Fig. 1. Exergy flow in the heating system.
Exergy is the portion of energy, which can be translated to other forms of usable energy. The largest exergy loss takes place in the primary energy transformation phase. Chemical energy contained in fuel oil turns out to be thermal energy. During this transformation process, exergy losses occur because of the deviation of the chemical composition of the fuel. These are also investigated in the analysis of the heating system equipment part.
2.2. Building characteristics The building is located in Izmir, Turkey and serves as a house of accommodation. The construction materials are locally manufactured. Exterior wall consists of aerated concrete, plaster for exterior and interior surfaces and polystyrene hard foam plate as insulation material. Thermal transmittance value of exterior wall is found to be 0.41 W/m2 K. Floor to the ground consists of ceramic, grading concrete, concrete, foam plate and plaster for interior surface. Thermal transmittance value of the floor to the ground is calculated to be 0.47 W/m2 K. The ceiling (roof) consists of thermal insulation material, concrete and plaster for interior surface. Thermal transmittance value of the roof is 0.32 W/m2 K. According to the Turkish Building Standard, TS 825 [5], the insulation in walls, on floor to the ground and on the roof is appropriate. Table 1 presents the project data on the building, which has double glazing windows (PVC material) facing all directions and the thermal transmittance value for a window is 2.55 W/m2 K while the distance between two glasses in this double glazing windows is 6 mm. The indoor and outdoor design temperatures for the city of Izmir in Turkey are 20 ◦ C and 0 ◦ C, respectively.
3. Analysis 3.1. Determination of the building construction materials’ costs Building materials are selected consciously since their strength and insulation properties for heat, water and sound are very important. In this study, the investigated building has three floors (including basement) with a height of 9 m. Total calculated wall surface area of the building is 3630 m2 . The components in the exterior wall are aerated concrete, interior and exterior plasters and hard polystyrene foam. The materials used in the exterior wall are presented in Table 2.
Table 1 Boundary conditions for the building. Volume (inside) (m3 )
V = 37,350
Net floor area (m2 ) Indoor air temperature (◦ C) Exterior temperature (◦ C)
AN = 4150 Ti = 20 T0 = 0
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Table 2 Components that form the building and the heating system. Component
Quantity
Unit cost
Aerated concrete Interior plaster Exterior plaster Hard polystyrene foam (3 cm thickness) 2 in. pipe 2½ in. pipe 5 in. pipe Radiator Steam boiler Heat exchanger
3630 m2 3630 m2 3630 m2 111.8 m3 56 m 57 m 5m 500 mm × 750 mm (135 units) 1 piece 1 piece
7.15 $/m2 0.45 $/m2 0.45 $/m2 45 $/m3 2.79 $/m 6 $/m 15.68 $/m 46.5 $/m2 16,807.9 $/piece 960.45 $/piece
Total cost (US$) 25,954.5 1633.5 1633.5 5031 156.24 342 78.4 2353.67 16,807.9 960.45
Based on the exchange rate of 1.77 Turkish Lira (TL)/US$ dated January 9, 2013 by the Central Bank of Turkey Taken from Ministry of Environment and Urban Planning’s Bill of Quantities
3.2. Determining the costs of the heating equipment
Ventilation heat loss is calculated using
The heating system considered consists of generation, distribution and emission stages. In the generation stage, there is a steam boiler, which is used to produce steam to heat the water stream in the pipeline for heating rooms and to meet domestic hot water requirements. The steam boiler has a heating capacity rate of 1450 kW. Operating pressure is 3 bars. In the distribution stage, the energy carrier is carried by pipes. Here, the pipes between the heating center and the heating zone are taken into account. Pipe lines are insulated to prevent heat losses to the environment. Existing pipe type is screwed pipe. The diameters of the pipes used are 2” or larger. The diameter, length and cost information of the pipes are given in Table 2. Cost information is taken from Ministry of Environment and Urban Planning’s Bill of Quantities. Steam is a high quality energy carrier while it is dangerous and not economic for emission stage. Thus energy transfer equipment (heat exchangers) is used to transfer the heat from hot stream to the cold stream. At the emission stage, there are 135 panel radiators to heat the rooms. The height, length and cost information of panel radiators and the cost figures of heating system equipment are presented in Table 2.
v (cp × × V × nd × (1 − v ) × (Ti − T0 )
(2)
In this study, nd , air exchange rate, is assumed as 0.4/h because of the double glazing windows [8]. cp is the specific heat of air, is the density of air and v stands for the heat exchanger efficiency, if there is mechanical ventilation with heat recovery. The subscript “v” indicates ventilation. In this paper, internal heat gains are neglected and solar heat gains are taken into account. Transmission and ventilation heat losses caused by the building envelope and the solar heat gains are used to obtain the total heating demand of the heating system. Heating demand for the examined building is shown in the following equation. H = T + v − S
(3)
To compare different types of buildings, the specific heating demand is calculated as
H =
H AN
(4)
3.4. EXCEM analysis 3.3. Determination of the energy demand Steady-state energy and exergy analyses are performed using an Excel tool based on that developed within the framework of IEA ECBCS Annex 49 [6]. Heat losses are calculated as transmission heat loss and ventilation heat loss. Sum of all losses from all surfaces are calculated to obtain transmission heat losses. Transmission heat losses for each of the building part are presented in Fig. 2. T =
(Ui × Ai × Fxi ) × (Ti − T0 )
(1)
i
where the Fxi values, temperature correction factors, are taken from [7]. For the exterior walls, windows and doors, the selected Fxi values are the same and equal to 1. For the floors to the ground, it is taken to be 0.6.
EXCEM analysis dictates mass, energy, exergy and cost balance equations. A mass flow rate balance for a system may be written as ˙ 0=m ˙a ˙ i −m m
(5)
where indices “i”, “o” and “a” denote input, output and accumulation, respectively. The energy and exergy balances (Eqs. (6)–(7)) can be written as E˙ i − E˙ 0 = E˙ a
(6)
˙ 0 − L˙ ex = Ex ˙ a ˙ i − Ex Ex
(7)
L˙ ex stands for thermodynamic loss rate. Cost is an increasing, nonconserved quantity. The cost balance can be written as, Ki + Kg − K0 = Ka
(8)
where index “g” represents the generation term. Cost generation corresponds to the appropriate capital and other costs associated with the creation and maintenance of a system. In this study, we used the capital costs to express the cost generations. Exergy losses can be identified from Eq. (7), and are of two types: external (the loss associated with exergy that is emitted from the system, or waste exergy output) and internal (the exergy losses within the system due to process irreversibilities, or exergy consumption). Sum of these two types of exergy losses forms total exergy loss. Hence, exergy loss rate, L˙ ex is defined as follows: Fig. 2. Transmission losses for the building parts.
L˙ ex = Exergy consumption rate + Waste exergy output rate
(9)
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In the above equations, the exergy rate entering the control ˙ in ) is equal to the exergy rate exiting from the control volume (Ex ˙ out ) and the exergy destruction rate (Ex ˙ des. ). In Eq. (14) volume (Ex ˙ f and Ex ˙ st denote exergy rate of fuel and exergy change rate of Ex
P-8
Effluents
steam. Exergy rate of the fuel is calculated by
Fuel and air
Water inlet
Steam outlet
P-6
P-5
P-4
E-3
˙ is defined as the ratio of thermodynamic loss Another parameter, R, rate L˙ to cost K as follows, L˙ K
(10)
Here, the value of R˙ generally depends on whether it is based on energy loss rate (R˙ en ) or exergy loss rate (R˙ ex ), while in this study R˙ ex values were used. R˙ ex =
L˙ ex K
(11)
The value of R˙ ex varies in different situations (e.g., technology, time, location, resource costs, knowledge). Also, during periods when energy-resource costs increase, the value of R˙ ex is likely to decrease (i.e., greater capital is invested to reduce losses). 3.5. Exergetic and exergoeconomic analyses of the heating system equipment 3.5.1. Steam boiler Steam boiler is equipment, which generates steam at the required pressure, temperature and quantity. It is assumed as an open system thermodynamically. The steam boiler in the heating center is going to be examined in terms of the exergy issue. The heating capacity rate of the steam boiler is 1450 kW while supply/return temperatures are measured to be 113/75 ◦ C. The operating pressure is 1.6 bars in this study. Fuel oil is used as a fuel. The fuel is the low sulfur containing fuel oil type with a lower heating value of 44,600 kJ/kg. The control volume of the steam boiler is shown in Fig. 3. According to the first law of thermodynamics, the efficiency of the steam boiler is calculated as follows, =
˙ st × cp (Tst,in − Tst,out ) m ˙ f × Hu m
(15)
εf = Hu × ϕ
(16)
˙ f is the mass flow rate of fuel consumption and εf denotes where m the specific exergy of fuel while ϕ represents the chemical exergy factor. According to Ref. [9], the chemical exergy factor for fuel oil is found to be 1.04. Exergy rate gained by steam in the steam boiler can be determined from ˙ st = m ˙ st × [(hout − hin ) − T0 (Sout − Sin )] Ex
Fig. 3. Control volume of the steam boiler.
R˙ =
˙ f =m ˙ f ×εf Ex
(12)
The ratio between the energy transferred to the water and the ˙ st energy obtained by firing the fuel gives the efficiency value. m ˙ f denotes the mass flow represents the mass flow rate of steam, m rate of fuel and Hu is the lower heating value of the fuel. Exergy analysis is carried out by balancing the exergy input rates and exergy output rates in the control volume. In this study exergy of air to burn the fuel is neglected. The exergy rates of the flue gas and the heat loss from the surface are included in the exergy destruction terms. The exergy balance can be written as follows: ˙ in − Ex ˙ out ˙ des. = Ex Ex
(13)
˙ f = Ex ˙ st + Ex ˙ ef + Ex ˙ l,pet + Ex ˙ d,b Ex
(14)
(17)
where hout and hin represent outlet and inlet specific enthalpies of the steam while Sout and sin stand for the outlet and inlet specific entropies of the steam. The exergy loss resulting from the energy conversion between chemical energy and heat energy can be formulated as ˙ l,pet = m ˙ st (hout − hin ) Ex
To Tad.
(18)
˙ l,pet indicates exergy loss rate at the primary energy transwhere Ex formation and Tad. stands for the adiabatic flame temperature of fuel oil [10]. The exergy loss rate resulting from energy conversion between effluents and steam flow is formulated as
˙ ef = m ˙ st T0 (sout − sin ) − Ex
(Hout − hin ) Tad.
(19)
Finally, the exergetic efficiency can be obtained from =
˙ st Ex ˙Exf
(20)
Sustainability assessment requires that the resources should be utilized efficiently, and also it may be performed with sustainability index (SI) method, which is related to exergy efficiency, as proposed by Rosen et al. [11]. In this regard, exergy methods are essential in improving efficiency that allows society to maximize the benefits it derives from its resources while minimizing the negative impacts such as environmental damage. So, sustainability index (SI) is a factor, which is directly related to the exergetic efficiency while it is written [11]. =1−
1 SI
(21)
3.5.2. Heat exchanger Heat exchanger is equipment, which is designed to transfer heat from the hot stream to the cold stream. The existing heat exchanger is a plate type one. The hot water stream enters the equipment at 113 ◦ C and leaves at a temperature of 92 ◦ C. Inlet and outlet temperatures of the cold stream are 65 ◦ C and 85 ◦ C, respectively. Exergy balance equation is formed according to the entering and exiting streams, as shown in Fig. 4. ˙ 2 − Ex ˙ 1 + Ex ˙ D = Ex ˙ 3 − Ex ˙ 4 Ex
(22)
where indices (Eqs. (1)–(2)) belong to the cold streams and (Eq. (3)–(4)) belong to the hot streams. If Eq. (22) is expressed more explicitly, it becomes ˙ d,h = m ˙ cw × [(h2 − h1 ) − T0 (s2 − s1 ] + Ex ˙ hw m × [(h3 − h4 ) − T0 (s3 − s4 )]
(23)
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Entering hot stream
347
Heat transfer to the room
Water inlet Entering cold stream
Radiator
Water outlet
Exing cold stream Fig. 5. Control volume of the radiator. E-2
The exergy balance of a radiator is obtained from ˙ rad,in = Ex ˙ rad,out + Ex ˙ rad + Ex ˙ d,rad Ex
Exing hot stream
Exergetic efficiency is calculated by the ratio of exergy rate obtained over exergy rate consumed (Fig. 5).
Fig. 4. Control volume of the heat exchanger.
˙ cw and m ˙ hw denote mass flow rates of the cold and where m ˙ d,h is the exergy destruction rate for the heat hot water while Ex exchanger. Exergetic efficiency of the heat exchanger can be expressed by h
=
˙ 2 − Ex ˙ 1 Ex ˙Ex3 − Ex ˙ 4
(24)
3.5.3. Pipes Pipes carry the heat source hot water to the heating zone. Heat load cannot be totally transmitted to the emission system. Since there is heat loss from the surface of pipes, the pipes are insulated. 2 and higher diameter sized pipes are considered compared to lower sizes. The surface area of a pipe can be calculated by A= ×D×l
(25)
The heat loss on the pipes is mainly caused by convection. For flat areas, the heat convection coefficient (˛) is dependent on the speed of air. If the velocity of air (v) is lower than 5 m/s, the heat convection coefficient can be calculated [12] as follows ˛ = 5.8 + 3.9v
(31)
(26)
rad
=
˙ rad Ex ˙ rad,out ˙ rad,in − Ex Ex
(32)
3.5.5. Exergetic cost effectiveness (ECE) To the best of the authors’ knowledge, the definition “Exergetic Cost Effectiveness” was proposed for the first time to include both the effects of equipment’s exergy destruction and cost figure to the overall system. In the literature, no other term explaining the combination of cost contribution and exergy destruction contribution in a system with one parameter has been originally proposed by the authors. The parameter does not have any units. The evaluation of each stage or component in a system can be examined using this parameter. The word stage means the generation, the distribution, the emission, the building envelope etc. Components in a system may be boiler, heat exchanger, pipes, radiator etc. The contribution of a stage or a component to the total cost (a) is calculated as follows: a=
cost of a stage or a component total cost of the system
(33)
The contribution of a stage or a component to the total exergy destruction (b) is expressed as
In this study, the air velocity is ignored and the heat convection coefficient is applied as
b=
˛ = 5.8
The ECE parameter is obtained by applying the following relation.
W m2 K
(27)
where Ts and T0 indicate the surface and outdoor temperatures, respectively. Exergy loss rate can be applied after determining the heat loss rate. ˙ l,p = Ex
Ts − T0 × Q˙ l,p Ts
(28)
3.5.4. Radiators The emission system has radiators to heat the rooms in the building. Indoor temperature is assumed to be 20 ◦ C. Heat load rate (Q˙ rad ) of a radiator is calculated as follows: ˙ w,rad × (h85 − h65 ) Q˙ rad = m
(29)
˙ w,rad , h85 and h65 are mass flow rate of the water in the where m radiator and enthalpies of the water at 85 ◦ C and 65 ◦ C, respectively. Exergy rate of the radiator is calculated by
˙ rad = Q˙ rad 1 − Ex
T0 Troom
(34)
ECE = a × b
Heat loss rate occurred on the surface of pipes is calculated as Q˙ l,p = ˛ × A × (Ts − T0 )
exergy destruction of a stage or a component total exergy destruction of the system
(30)
The low ECE value dictates more cost effective, more appropriate and more efficient component or stage. The ECE parameter takes values between zero and one. Zero value shows that the equipment has no influence on the cost or exergy destruction terms. The value one means that other equipment has no influence on cost and exergy destruction terms. 4. Results and discussion 4.1. Exergetic results The predesign tool Annex 49 includes energy and exergy results of the chosen building and its heating system. According to the project data used in predesign tool, transmission and ventilation heat loss rates are found to be 100.7 kW and 100.1 kW, respectively. Solar heat gain rates are calculated to be 23.6 kW. By taking these values into account, the heat demand and specific heat demand are found to be 177.2 kW and 42.7 W/m2 , respectively. The heat loss caused by each building component is shown in Fig. 2. The energy,
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Table 3 System rates in kW. Stages Input After prim.en. trans. After generation After distribution After emission After room
Energy load rate
Exergy load rate
Exergy loss rate
312.04 240.02 213.63 186.53 177.21 177.21
293.32 225.63 48.05 42.22 39.93 12.09
– 67.69 177.58 5.83 2.29 27.84
Table 5 Exergoeconomic results of the stages in the lowex analysis and the equipment in the heating system. L˙ ex (kW)
Stages Generation Distribution Emission Building envelope Steam boiler Heat exchanger Pipes Radiators
245.26 5.83 2.29 27.84 1039.92 10.62 4.43 9.59
K ($)
R˙ ex (W/$)
16,807 696.6 2353.67 34,289 16,807 960.45 696.6 2353.67
4.52 2.82 0.28 0.26 19.77 3.39 2.26 1.13
Table 4 Exergetic rate results for the heating equipment in kW.
4.2. Exergoeconomic results
Equipment
Exergetic figures
Steam boiler
˙ f Ex 1289.48 ˙ hw Ex
˙ st Ex 249.56 ˙ cw Ex
E˙ x˙ ef 509.53 ˙ d,h Ex
16.91 ˙ w Ex 103
6.29 ˙ rad Ex 32
10.62 ˙ d,rad Ex 71
Heat exchanger Radiator
E˙ x˙ i,pet 98.50
E˙ x˙ d,b 431.89
exergy loads and exergy losses obtained by applying predesign tool are listed in Table 3. After calculating the pre-design tool results, the energy and exergy input rates to the system are approximately 312 kW and 293 kW, respectively. After exiting from each stage, the load values decrease. The decrease is more noticeable in the exergy terms. An energy rate of 177.2 kW is transferred to the room. As exergy rate, 40 kW is obtained from the emission system. On the other hand, the maximum exergy loss rate took place at the generation stage in the steam boiler as 177.6 kW. Other high exergy loss rates occurred in the primary energy transformation and distribution stages. The main equipment in the heating system is the steam boiler, the heat exchanger and the radiator. The findings after calculating exergy flows affecting the control volume of each equipment are shown in Table 4. Exergetic efficiencies of the steam boiler, the heat exchanger and the radiator in the heating system are 19.35%, 37% and 31%, respectively. The exergetic efficiency of the overall system is calculated to be 3.18%. If the exergetic efficiency of any equipment is high, it will have a high sustainability index. The sustainability index for the steam boiler in this study is found to be 1.24. Since the exergetic efficiencies of the heat exchanger and the radiator are higher than that of the steam boiler, their sustainability indices are higher. It is determined to be 1.59 for the heat exchanger and 1.45 for the radiator. It can be considered that in the heating systems, the selection of the equipment and building materials play an important role. More expensive, but high exergy efficient components may lead to a system with more sustainable conditions.
The EXCEM analysis is applied to the stages in the heating system and also to the selected equipment in that system. The thermodynamic loss rates, the capital costs and the exergoeconomic results are presented in Table 5. It is obvious from this table that the maximum thermodynamic loss rate belongs to the generation stage, being 245.26 kW. The minimum thermodynamic loss rate is calculated at the emission stage as 2.29 kW. By taking the capital cost into account, the EXCEM figure R˙ is obtained. This ratio takes its maximum value at the generation stage as 4.52 W/$. Since the capital cost spent for the materials form the building envelope is the highest, the minimum ratio value is found to be 0.26 W/$ for the building envelope. The findings related to the selected equipment based on the EXCEM analysis are listed in Table 5. The capital costs for each equipment are taken from the Ministry of Environment and Urban Planning’s Bill of Quantities. When one compares the equipment, the maximum and minimum thermodynamic loss rates belong to the steam boiler and pipes, respectively. In this paper, the cost and the exergy loss rates for the outdoor pipes are taken into consideration. Exergy loss rates of the steam boiler and the pipes are 1039.92 kW and 4.43 kW, respectively. It is not surprising that the steam boiler has the maximum EXCEM ratio as 19.77 W/$. The emission equipment radiators have the minimum EXCEM ratio as 1.13 W/$, because their capital cost is higher than that of pipes. Higher ECE value means higher cost and higher exergy destruction in the system. It helps to determine the most inefficient equipment in the overall system. After calculating the ECE values of all equipments, they can be put in an ascending order. The maximum value means that the equipment should be examined firstly. The findings related to the parameter ECE to compare stages are presented in Table 6. Each stage or equipment can be evaluated by comparing its ECE value with that of other stages or equipment. It can be seen that the maximum ECE value represents the generation among all stages. It is found to be 0.278. This result means that the generation stage needs to be firstly improved before other stages. Secondly, in the building envelope, the materials with less thermal transmittance values can be selected. The ECE value for the building envelope is 0.063.
Table 6 ECE results of the stages in the lowex analysis and the equipment in the heating system. Stages
˙ d (kW) Ex
Generation Distribution Emission Building envelope Steam boiler Heat exchanger Pipes Radiators
245.26 5.83 2.29 27.84 1039.92 10.62 4.43 9.59
K ($) 16,807 696.6 2353.67 34,289 16,807 960.45 696.6 2353.67
a
b
ECE (a × b)
0.32 0.01 0.04 0.63 0.81 0.05 0.03 0.11
0.87 0.02 0.01 0.10 0.976 0.01 0.005 0.009
0.278 0.0002 0.0004 0.063 0.79 0.0005 0.00015 0.00099
C.T. Yucer, A. Hepbasli / Energy and Buildings 62 (2013) 343–349
It is obvious from Table 6 that the steam boiler has the maximum ECE value. It is calculated to be 0.79. The choice for the type of the steam boiler can be considered again. After that, the radiators at the emission stage can be examined if there is a way to improve their exergetic efficiencies. Its ECE value is approximately 0.001.
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The component with the highest ECE value should be preferred to make analysis for improvements. According to the findings, the generation stage (0.278) and the steam boiler (0.79) had the maximum ECE values among other ones. (g) For a future work, exergoenvironmental analysis is recommended to take environmental values into account.
5. Conclusions Acknowledgement In this study, we have applied exergy and exergoeconomic analyses to a building and its heating system. We have calculated energy and exergy flows from the first stage, the primary energy source to the last stage building envelope through the predesign tool in Annex 49. We have also implemented exergoeconomic analysis at the stages after determining the exergetic results in the lowex predesign tool defined in Annex 49. Some concluding remarks obtained from the results of the present study may be listed as follows: (a) Total exergy input rate was determined to be 293.32 kW while the largest exergy loss rate was calculated as 177.58 kW. (b) According to the exergy analyses, exergetic efficiencies of the steam boiler, the heat exchanger and the radiator were calculated to be 19.35%, 37% and 31%, respectively. (c) The overall system exergy efficiency was determined to be 3.18%. (d) By examining the R˙ ex results, the generation stage had a maximum value of 4.52 W/$. The distribution, the emission and the building envelope stages had R˙ ex values of 2.82 W/$, 0.28 W/$ and 0.26 W/$, respectively. (e) When the equipment in the heating system was compared, the maximum R˙ ex was for the steam boiler, with a value of 19.77 W/$. The heat exchanger, the pipes and the radiators followed the steam boiler with R˙ ex ratios of 3.39 W/$, 2.26 W/$ and 1.13 W/$, respectively. (f) The authors expect that the newly proposed parameter, the ECE, will help researchers put the components in a precedence order.
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