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Exergo-ecological evaluation of adsorption chiller system
Energy xxx (2014) 1e7
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Exergo-ecological evaluation of adsorption chiller system Wojciech Stanek*, Wies1aw Gazda Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2013 Received in revised form 11 February 2014 Accepted 15 February 2014 Available online xxx
Technological processes in food industry require chilling agent. System of chillers can be driven by nonrenewable or renewable sources. In the group of renewable sources solar collectors seem to be an attractive solution both as the basic or the auxiliary power source for adsorption refrigerators. The common tool for the evaluation of the refrigerator systems used in practice is the energy effectiveness expresses as the COP (coefficient of performance). In the case of energy systems with energy fluxes of different parameters or supplied partly with renewable and non-renewable sources such evaluation is not enough. The authors proposed the method of exergetic evaluation. In the paper the comparison of the exergetic effectiveness and thermoecological cost of an example refrigeration system existing in Polish food industry supplied with heat alternatively from boiler house, cogeneration system and heat from solar collectors is presented. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Exergy analysis Food industry Adsorption chillers Thermo-ecological cost
1. Introduction Many technological processes require different cooling and chilling agents. Especially in food industry the demand for chilling agents can be significant and can play important role in the energy management of factory. To cover the demand for cold carrier different techniques are applied [1e4]: compressor refrigerators, absorption and adsorption machines. Compressor systems require the consumption of electricity that is mainly produced basing on non-renewable natural resources. Absorption and adsorption machines, driving with flux of heat, can be supplied from nonrenewable source (e.g. heat from gas boiler) but also can use renewable energy sources (heat from solar collectors). Most often the chilling systems are evaluated only from the point of view of first law of thermodynamics using COP (coefficient of performance). Such analysis is relatively simply and common especially in practice. However, in systems with energy fluxes (heat, electricity, fuels) of different quality such evaluation is not enough [4,5]. COP can be useful only in the case of comparison of chilling systems with similar construction and similar thermodynamic parameters. In general, for systems with different construction and different parameters second law of thermodynamics and exergy analysis has to be additionally applied [4,6e8]. Exergy [6] is the maximum
* Corresponding author. E-mail addresses: [email protected] (W. Stanek), [email protected] (W. Gazda).
ability of energy carrier to perform work in respect to common environment. Comparison of different refrigeration systems should be based on exergetic efficiency and exergy losses [5]. While the direct exergy analysis doesn’t show the exergetic results of interactions between system components the complex energy or any production systems should be additionally analysed with the tools offered by the thermo-economics [9e12]. The potential of this approach in the case of multi-purpose industrial park is efficiently demonstrated in Ref. [11]. The application of thermo-economics for the analysis of tri-generation systems is presented e.g. in Refs. [7,8]. When considering additionally the possibility of application of renewable and non-renewable resources the analysis should be furthermore supplemented with system analysis applying the common indicator of non-renewable natural resource quality and the balance boundary should reach the level of natural resources. Szargut was the first who extended the exergy analysis from a single process to the whole production chain by proposing the important concept of CExC (cumulative exergy consumption) [13] and subsequently he extended this concept to the TEC (thermoecological cost) [6,14,15], which enables the applications of exergy analysis in the field of environmental aspects. The TEC expresses the cumulative consumption of non-renewable exergy of natural resources burdening fabrication of any useful product [6]. The Szargut’s method in comparison with other methods of ecological assessment, can bring all environmental impacts to one metric which is the exergy of consumed natural non-renewable resources. The TEC method fulfils the requirement of LCA (Life Cycle Assessment); moreover, the minimisation of the TEC [14] ensures a
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mitigation of the depletion of non-renewable resources. To compare the ecological effects of the refrigerator system supplied with different sources of heat the authors used in this paper direct exergy analysis and system exergy based on the theory of thermoecological cost proposed by Szargut. Also, some remarks resulting from comparison with the analysis made purely by COP and PES (primary energy savings) are formulated. In the paper comparison of two systems are included: - refrigeration systems equipped with adsorption chiller supplied with heat from CHP (Combined Heat-and-Power) unit, - refrigeration system equipped with compressor refrigerator and additional adsorption chiller supplied with heat from solar collector. 2. Analysed refrigeration systems The first of analysed systems is presented in Fig. 1 CHP plant produces heat to drive adsorption chiller. The CHP plant is fired with natural gas from gas-network. The surplus of electricity is consumed within the factory or is transferred to the grid. The proposed adsorption system equipped with CHP unit replaced old one supplied with heat from boilers fired with gas. The savings in non-renewable resources is the results of application of cogeneration. In this case in the system two products are generated parallel e heat and electricity. To evaluate the performance indices of the system the method of cost division has to be assumed and the results of direct energy evaluation [16] have been applied. In the analysis the authors assumed exergetic cost method [10] to divide consumption of fuel between heat and electricity. The basic parameters of the adsorption system supplied with heat from CHP unit is included in Table 1 [16]. The second analysed system is partly supplied with heat from solar collector. Taking into account the range of ambient temperatures in Polish conditions glycol has been applied as intermediate working fluid in the solar collector. The simplified scheme of this solution is presented in Fig. 2. The more detailed description of the system and its energy analysis are presented in Ref. [16]. The basic parameters of liquid chilling package are summarised in Table 2. Assuming that the (COP) of the LCP (liquid chilling package) module is constant and taking into account data from Table 2 the coefficient of performance of the compressor chiller (LCP) is equal to (COP)LCP ¼ 4.28. In the case of the second system the solar collector has only the supplementary character. The minimum temperature required at inlet of adsorption chiller has been assumed as tgr ¼ 55 C, that is confirmed in Refs. [7] and [22]. The distribution of temperature ranges of collector output temperatures is presented in Fig. 3.
Fig. 1. System 1: adsorption refrigerator powered by the heat from the cogeneration power plant CHP e cogeneration heat-and-power plant, HT e hot water tank, AD e adsorption refrigerator, FT e food technology, CT e cooling tower, EL e electricity.
Table 1 Technical specification data of the adsorption refrigerator. Energy carriers
Temperature ( C)
Chilled water Cooling water Hot water
Capacity (kW)
Inlet
Outlet
12.0 30.0 90.0
7.0 35.0 83.0
251.0 613.0 362.0
It can be easily concluded that the support from solar collector is possible by the period of 1364 h/year. The rest of yearly operation (6760 h/year) is realised only by means of compressor chiller (LCP). 3. Evaluation of systems effectiveness 3.1. System equipped with CHP unit Total consumption of chemical energy in CHP results from energy efficiency:
hE;CHP ¼
Q_ CHP þ NCHP E_
(1)
F;CHP
where: Q_ CHP heat flux produced in CHP to drive AD, kW, NCHP CHP electric power, kW, E_ F;CHP flux of chemical energy of fuel consumed in CHP plant, kW. The energy efficiency of the CHP unit has been assumed due to the data from manufacturer (Table 3). To evaluate the advantageous of the cogeneration or tri-generation most often only the energy efficiency and primary energy savings are taken into account e.g. Ref. [17]. However, to determine the fuel consumption burdening separately heat and electricity production some additional rule has to implemented for energetic cost allocation. One of them is the method proposed by the “Cogeneration directives” but, as it was proved in Ref. [18] this method, in some cases can’t fulfil the first and second law of thermodynamics. Other method that can be used for cost allocation is the method of avoided cost [19]. However, in this method the results are strongly dependent on two factors: 1) definition of main and by-product, and 2) the arbitral assumption of the energy efficiency of replaced process. Another possibility is offered by the exergy analysis and thermoeconomics [6,8,9]. In this method for cost allocation the exergy of products are applied. This cost is defined as the total exergy consumption necessary to obtain
Fig. 2. System 2: adsorption refrigerator powered by the heat from the solar energy source SC e solar collectors, HT e hot water tank, AD e adsorption refrigerator, FT e food technology, CT e cooling tower, LCP e liquid chilling package, EL e electricity.
Please cite this article in press as: Stanek W, Gazda W, Exergo-ecological evaluation of adsorption chiller system, Energy (2014), http:// dx.doi.org/10.1016/j.energy.2014.02.053
W. Stanek, W. Gazda / Energy xxx (2014) 1e7 Table 2 Technical specification data of the liquid chilling package. Energy carriers
Temperature ( C)
Chilled water Cooling water Power
Capacity (kW)
Inlet
Outlet
12.0 30.0 e
7.0 35.0 e
251.0 309.7 58.7
the specific amount of exergy of useful product and in the case of CHP process it can be expressed as follows:
k* ¼
aE_ F;CHP FT ¼ Tm;AD T0 _ PT Q CHP Tm;AD þ NCHP
(2)
where: FT total exergy of fuel feeding the CHP system, PT total exergy of useful products of the CHP system, a the ratio of chemical exergy of fuel per unit of lower heating value (bch,F/LHV), Tm,AD mean thermodynamic temperature of water driving adsorption chiller AD, T0 ambient temperature. Using the specific cost of CHP products e k* the chemical energy of fuel burdening respectively production of heat in CHP can be determined as follows: *
Tm;AD T0 k E_ F;Q ¼ Q_ CHP a Tm;AD
(3)
The partial exergetic efficiency of heat production is defined as:
hB;Q ;CHP ¼
T T0 Q_ CHP m;AD Tm;AD
E_ F;Q a
(4)
The results of calculation of characteristic indices for the investigated CHP plant coupled with cold water production are presented in Table 3 It should be noticed that the exergy efficiency of heat production in CHP unit is relatively high and amounts to 48.9% and is significantly higher than exergetic efficiency of replaced gas boiler that reaches 23.4%. Exergy efficiency of heat production in CHP reaches the level of exergy efficiency in the case of modern steam generators as is confirmed in Refs. [20,21]. Additionally it should be pointed out
3
that the exergetic allocation of fuel costs between heat and electricity eliminates the non-correct from thermodynamic point of view results. E.g. in [#1] the assumed boiler efficiency is 0.92, and the calculated thermal efficiency of CHP is lower amounting to 0.57. These results can bring to misleading conclusion that the perfection of useful heat production in single process (boiler) is much lower than in cogeneration. The obtained exergetic efficiencies for the analysed CHP plant influences the total exergetic efficiency of cold production in system 1 that has been evaluated at the level of 13.8%. 3.2. System equipped supplemented with solar collector The production of heat in solar collector system is strongly dependent on several factors: 1) the atmospheric condition and solar energy flux reaching the collector system, 2) the collector energy efficiency, 3) design parameters as: tube diameter, collector area, distance between tubes, length of tubes. The problem of Thermo-Ecological optimisation of solar collector system has been presented by Szargut and Stanek in Ref. [14]. Additionally in the case of application of solar collector for AD the temperature constrain has to be taken into account. In the presented analysis it has been assumed that the AD input temperature should be at least 55 C, which is confirmed in the results of investigation of solar driven adsorption chiller presented in Refs. [7] and [22]. This constrains results in the constrained amount of working hours of the collector during the year. In Fig. 3 the possible time of solar collector cooperation with AD is presented. Due to data presented at this diagram it can be noticed that the total working time of the collector is constrained to 1366 h/year, which is the 16% of the year. The amount of the useful heat produced in the collector results from the energy efficiency of the solar collector, which is defined:
_ l Tc;out Tc;in mc F I_
hE;c ¼
(5)
b
while the exergetic efficiency of solar collector can be calculated as follows:
"
hB;c ¼ hE;c
Tc;out T0 ln 1 Tc;out Tc;in Tc;in
!#
I_b b_ r
(6)
where: _ mass flow rate of working agent in the collector, kg/s, m cl heat capacity of collector working fluid, kJ/(kgK), Table 3 Input data and results of exergy analysis of system with CHP unit.
Fig. 3. Accessibility of outlet temperature from collector during the year.
No.
Parameter
Symbol
Unit
Value
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Electric power of CHP Thermal power of CHP Energy efficiency of CHP Cogeneration ratio Exergy flux feeding the AD Chemical exergy of fuel Exergetic efficiency of CHP Exergetic cost of CHP operation Fuel exergy consumption e electricity Fuel exergy consumption e heat Exergy efficiency of heat production System exergy efficiency
NCHP Q_
kW kW e e kW kW % kW/kW kW kW % %
337.50 362.00 0.909 0.9324 66.95 815.70 49.59 2.017 680.70 135.00 48.90 13.78
CHP
hE,CHP xCHP BQ,AD BF,CHP
hB,CHP
k*CHP BF,CHP,EL BF,CHP,Q
hB,Q,CHP hB,sys
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W. Stanek, W. Gazda / Energy xxx (2014) 1e7
Using the energy and exergy characteristics of collector presented in Fig. 6 and time curves of solar energy and exergy resources presented in Fig. 5 the actual production of useful heat and useful exergy in collector can be determined q_ F;AD ðsÞ and b_ F;AD ðsÞ. Furthermore the yearly average energy and exergy efficiency of the collector can be calculated from the following formulae:
hE;c ¼
Zsgr _ qF;AD ds I_ 0
(8)
b
Fig. 4. Solar radiation reaching collector.
Tc,out,Tc,in temperature at outlet and inlet of the collector, K, I_ flux of solar radiation reaching the collector at b angle, W/m2, b
F active area of the collector, m2.
0
The ratio of solar radiation exergy to solar energy can be calculated from the formula [6,20]:
b_ r 4T 1 T0 4 ¼ 1 0þ 3 T 3T I_
hB;c ¼
Zsgr _ bF;AD _ I_b be_ r
ds
(9)
r
For the analysed system the following values has been achieved: - yearly average energetic efficiency of solar collector: hE;c ¼ 46.5%, - yearly average exergetic efficiency of solar collector: hB;c ¼ 8.4%.
(7)
Assuming T ¼ 5870 K and T0 ¼ 300 K the ratio b_ r =I_ b ¼ 0.93 and has been assumed to be constant in the presented analysis. The example of calculation of I_b for Polish condition in month July is presented in Fig. 4. In the system with solar collectors the solar energy flux is strongly time dependent and is also constrained through the year. Fig. 5 presents the dependence on time of solar energy and solar exergy flux. The operation of the collector e adsorption system 2 is constrained with the minimum collector output temperature tin,min ¼ 55 C. The presented time dependence of b_ r has been determined using equation (7) and characteristics similar to that presented in Fig. 4. The yearly production of heat in solar collector q_ F;AD that is used for powering the adsorption refrigerator and useful exergy of this amount of heat b_ F;AD have been calculated using equations (5)e(7). These fluxes are also presented in Fig. 5, energetic and exergetic characteristics of solar collector is presented in Fig 6.
It can be noticed that the average exergetic efficiency is relatively low. In the system 1 the exergetic efficiency of heat production in CHP unit reaches 48.4% and exergetic efficiency of gas boiler 23.4%. The value is than lower than the exergetic efficiency of both e heat production in the system equipped with CHP presented in Section 3.1, but from the other hand it should be underlined that the driving force in collector is coming from renewable sources while the CHP unit and gas boiler are supplied with non-renewable exergy of natural gas. The obtained results indicate the necessity of introduction additional criterion to compare systems driven with non-renewable and renewable energy and exergy. For this reason TEC (thermo-ecological cost) will be proposed. TEC analysis is presented in Section 4 of the paper. Other important factor in the analysis that influence the behaviour of the refrigerator system is the active area of solar collector F. The total production of heat is strongly dependent on this factor. Hypothetically there is a hours during the years in which the solar collector can work alone in the system 2 without the production of cold carrier in the compressor refrigerator. For such assumption and taking into account the energetic characteristic of the collector the required area F can be calculated. Results of such calculations are demonstrated in Fig. 7. The accessible area in the investigated food factory is limited to Fmax ¼ 900 m2. For this reason during the whole year the work of
Fig. 5. Heat and useful exergy production in SC during the year.
Fig. 6. Energy and exergy efficiency of collector.
b
where: T ¼ temperature of emitting surface, K, T0 ¼ temperature of environment, K.
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Fig. 7. Required active area of solar collector as dependent on collector inlet temperature and the degree of production of cold based on heat from SC. Fig. 9. Regime of operation of system with solar collector.
pair AD with collector and LCP is necessary. The maximum share of collector in production of cooling agent is about 80%. To calculate the total efficiency of the whole refrigerator system the knowledge of characteristic of coefficient of performance is necessary. In the case of system 2 equipped with LCP e compressor refrigerator e the COP can be assumed to be constant in the investigated range of loads. In the case of system 1 equipped with AD e adsorption chiller e the COP is dependent on the inlet and outlet temperature of hot water delivered from collector or from CHP unit. In the case of solar collector system this temperature is varying during the daily operation. The energy and exergy characteristic of adsorption chiller is presented in Fig. 8. In the range of inlet temperature marked in Fig. 8 as (A) the exergetic efficiency of adsorption chiller hB,AD is higher than that of LCP. In Fig. 9 the regime of component operation of system 2 is presented. It can be noticed that the maximum power is supplied from collector between tout 80e100 C. However, in this range the energetic efficiency of the collector is rather low and the exergetic efficiency of AD chiller is lower than the maximum one. Taking into account the changing regime of system 2 operation which is strongly dependent on solar energy accessibility the average system exergy efficiency can be calculated to compare all of
analysed systems. The results for the investigated refrigerator configuration are as follows: - refrigerator system 2 average exergy efficiency e mode: collector þ LCP operation ¼ 2.30% - refrigerator system 2 average exergy efficiency e mode pure LCP operation ¼ 23.50% - refrigerator system 1 average exergy efficiency e mode AD supplied with CHP ¼ 13.80%. The presented results indicate that the less favourable system is that one supported with collectors and the most favourable system with compressors fed with electricity. If we take into account the energy indicators (COP) the profitability of system 2 with LCP is even more evident. These analyses did not show any advantages of application of RES (Renewable Energy Source) in the case of refrigeration system supported with solar collector. The positive effects of cogeneration are also not so evident. It can be concluded that such comparison as well as that based on COP and PES [18], is not sufficient and additional criterion has to be applied in this case. This criterion has to be able to: - measure all energy fluxes from the point of view of energy quality, - brings the analysis to the common balance boundary which is the level of natural resources. In the next section the authors presented the application of TEC (thermo-ecological cost) for evaluation of effectiveness and sustainability of investigated refrigerator systems. 4. Thermo-ecological analysis
Fig. 8. Energy and exergy effectiveness of AD chiller.
Szargut [6,13,15] defined the TEC (thermo-ecological cost) as cumulative consumption of non-renewable exergy connected with the fabrication of a particular product with inclusion of additional consumption resulting from the necessity of compensation of environmental losses caused by rejection of harmful substances to the environment. TEC of any product results mainly from consumption of non-renewable chemical exergy extracted directly from the nature bs as fuels, mineral ores, nuclear ores or fresh water. However this consumption appear only in the production processes directly connected with some extraction of resources from nature, e.g. in coal mine. Due to interconnections in the production systems, the TEC results also from consumption of domestic semi-finished products aij exchanged between links of system. Additionally, some burden of TEC can results from
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interregional exchange if within the national system there are some imported and exported goods. Besides consumption in some branches by-production can appeared. For example in cogenerated processes we can distinguish main product and byproducts. By products can replace production in other branches and for this reason decrease the final value of TEC. In the TEC balance presented in Eq. (10) by-products are taken into account by coefficient of by-production fij appearing in output of balance of TEC. Thermo-Ecological Cost of useful by-product should be determined using the principles of avoided exergetic cost. Besides consumption or by production described above, the balance of (TEC) for j-th production branch includes also the additional consumption of resources jj0 connected with rejection of wastes to environment. This additional consumption can result from two effects [19]. First, it can be connected with the consumption in abatement installation. Second, it results from the necessity of compensation of losses appearing in environment due to rejection of different aggressive wastes. The idea of exergetic TEC is to take into account only the consumption non-renewable resources (as in the case of considered system 2). Considering processes fed with renewable resources the direct consumption of chemical exergy should not be taken into account in the balances of TEC [14]. Consumption of renewable exergy, e.g. solar energy, doesn’t make any deleterious impact on depletion of deposits of the limited stock of resources. Basing on assumption explained so far the equation of the balance of operational TEC takes the following form [6,15,19]:
rj þ
X i
X X fij aij ri arj rr ¼ bsj þ jj0 r
(10)
s
where: aij coefficient of the consumption of the i-th product per unit of the j-th major product, e.g in kg/kg or kg/MJ, fij coefficient of the consumption and by production of the i-th product per unit of the j-th major product, e.g in kg/kg or kg/MJ, arj coefficient of the consumption of the r-th imported product per unit of the j-th major product, e.g. in kg/kg or kg/MJ, bsj exergy of the s-th non-renewable natural resource immediately consumed in the process under consideration per unit of the j-th product, MJ/kg, ri,rj, rr specific thermo-ecological cost of the i-th, j-th domestic product and the r-th imported good, e.g. in MJ/kg, jj0 requirement for natural resources exergy to compensate or to avoid the environmental losses resulting from operation of jth production process, MJ/kg of k-th waste product. The detailed description of the TEC method can be found for example in Refs. [6,15,19]. In Ref. [14] Szargut and Stanek presented the Thermo-Ecological optimisation of solar collector used for production of hot water. In the presented analysis the following indices of thermoecological cost has been used [19]: electricity (TEC)el ¼ 3.6 MJ/MJ; natural gas (TEC)ng ¼ 1.04 MJ/MJ. Using the results of analysis presented in Section 3 and the methodology for determination of TEC presented in this section the following yearly thermoecological cost for investigated cases of refrigerator systems has been achieved: case 1) adsorption chiller with gas boiler (TEC) ¼ 12,033.51 GJ/a, case 2) compressor chiller system without collector support (TEC) ¼ 5814.29 GJ/a, case 3) compressor chiller system with collector support (TEC) ¼ 5186.27 GJ/a, case 4) adsorption chiller with CHP unit (TEC) ¼ 4094.06 GJ/a.
If we assume the mentioned case 1) (adsorption chiller fed with the existing in the factory gas boiler) as the reference, the relative savings of exergy of primary non-renewable resources can be determined from the formula:
DðTECÞ ¼
ðTECÞ0 ðTECÞ1 ðTECÞ1
(11)
where: (TEC)0 ¼ yearly thermo-ecological cost for the reference system, (TEC)1 ¼ yearly thermo-ecological cost for the combined cooling-and-power system. For the above cases and the considered in the paper cases the following have been obtained: case 2) compressor chiller system without collector support
D(TEC) ¼ 52%, case 3) compressor chiller system with collector support
D(TEC) ¼ 57%,
case 4) adsorption chiller with CHP unit D(TEC) ¼ 66%
If it is assumed that the reference system is that with LCP driven with electricity (case 2) the application of the CHP system to drive the adsorption chiller results in the relative TEC savings amounting to D(TEC) ¼ 29.6%. It is evident that the assumption of the reference system has relatively high influence in the analysis of TEC relative savings. In Ref. [18] it has been reported that with low COP of thermal driven chiller the primary energy savings for CCP (combined cooling-and-power) is almost the same as that of reference system, while with COP ¼ 1 the primary savings reaches the level of 29%, which is in both cases lower than the results obtained in the presented work by means of TEC analysis. This comparison of the results proved the thesis included in Section 3.1 of the presented paper, that firstly the energy analysis is not enough to investigate complex multi-purpose systems. Secondly, that in such systems the boundary should reach the level of non-renewable resources as is assumed in the TEC analysis, and that the objective results are achieved where the total TEC is compared instead of the relative ones. The analysis of (TEC) proved that also application of cogeneration and renewable energy for support refrigeration systems are profitable from the point of view of rational management of nonrenewable resources and from the sustainable point of view. Replacement of gas boilers by CHP production of heat decrease three times the influence of investigated refrigerator system on the depletion of natural resources. Also the support of solar collector is profitable from the point of view of savings of natural resources. However, due to the solar radiation characteristic in Poland the positive effects are lower than that of combination of adsorption chiller with CHP. 5. Summary and conclusions In the paper the authors analysed two systems for cold water production in example Polish food factory. These systems are characterised by fluxes of heat and energy of different quality. The parameters of produced useful heat are often close to the environmental parameters and from other hand they are produced using energy of high quality. The obtained results confirmed that application of only energetic evaluation of these systems by means of COP coefficient is purposeless, because this analysis doesn’t take into account the energy and in general resources quality. The authors proposed than exergetic evaluation which detailed algorithm is discussed in the paper. The results of calculation average system
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exergy efficiency of the investigated refrigerator configuration are as follows: - refrigerator system 2 average exergy efficiency e mode: collector þ LCP operation ¼ 2.30% - refrigerator system 2 average exergy efficiency e mode pure LCP operation ¼ 23.50% - refrigerator system 1 average exergy efficiency e mode AD supplied with CHP ¼ 13.80%.
7
boundary results in different ranking of technical solutions that obtain in the case of global boundary reaching the level of natural resources extractions. In the case of systems fed with renewable energy the second boundary has to be applied. Additionally it has been proved that comparing the systems producing energy fluxes of different quality as well as fed partially by renewable energy sources it is more objective to analyse the total TEC instead of relative savings. References
It is worth to stress that in the presented work, the calculations have been done under assumption that during the year the installation produces constant amount of chilled water however in the case of solar collector the calculations cover day-by-day all the year taking into account the variation of solar radiation (Fig. 3). The accessibility of high outlet temperature from collector is in Polish conditions very constrained. It is the cause while the average yearly exergetic efficiency in the mode: collector þ LCP operation is extremely low. From other hand it should be stressed that part of useful product are generated using free solar energy replacing the electricity from grid necessary to drive the LCP. The discussed direct exergy analysis takes into account the energy quality, but still doesn’t show any advantages of application of RES in the case of collector supported refrigeration system. Also the positive effects of cogeneration are not so evident. For this reason the authors proposed to use the criterion that is able to: - measure all energy fluxes from the point of view of energy quality, - brings the analysis to the common balance boundary which is the level of natural non-renewable resources. The TEC analysis showed the full system effects and proved that both e application of cogeneration and renewable energy for support refrigeration systems are profitable from the point of view of rational management of non-renewable resources and from the point of view of sustainable development. The performed analysis shown that yearly thermoecological cost for investigated refrigerator systems are as follows: - adsorption chiller with gas boiler, (TEC) ¼ 12,033.51 GJ/a, - compressor chiller system without collector support, (TEC) ¼ 5814.29 GJ/a, - compressor chiller system with collector support, (TEC) ¼ 5186.27 GJ/a, - adsorption chiller with CHP unit, (TEC) ¼ 4094.06 GJ/a. Replacement of gas boilers by CHP production of heat decrease three times the influence of investigated refrigerator system on the depletion of natural resources. Also the support of solar collector is profitable from the point of view of savings of natural resources. The presented results also that in the case of production systems supplied with renewable and non-renewable resources it is important to properly define the system boundary. The presented results shown that exergy analysis within the plant
[1] Calise F, Palombo A, Vanoli L. Maximization of primary energy savings of solar heating and cooling systems by transient simulations and computer design of experiments. Appl Energy 2010;87:524e40. [2] Wang JJ, Jing YY, Zhang CF, Zhai Z. Performance comparison of combined cooling heating and power system in different operation modes. Appl Energy 2011;88:4621e31. [3] Saidur R, Hasanuzzaman M, Mahlia TMI, Rahim NA, Mohammedd HA. Chillers energy consumption, energy savings and emission analysis in an institutional buildings. Energy 2011;36:5233e8. Fauchais P., Plasma Theory, Proceedings of ICHMT Seminar, Çesme, July 4-8, 1994, pp. 1e14. [4] Morosuk T, Tsatsaronis G. A new approach to the exergy analysis of absorption refrigeration machines. Energy 2008;33:890e907. [5] Morosuk T, Tsatsaronis G. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy 2009;34:2248e58. [6] Szargut J. Exergy method: technical and ecological applications. Southampton-Boston: WIT Press; 2005. [7] Deng J, Wang R, Wu J, Han G, Wua D, Li S. Exergy cost analysis of a microtrigeneration system based on the structural theory of thermoeconomics. Energy 2008;33:1417e26. [8] Roque Díaz P, Benito YR, Parise JAR. Thermoeconomic assessment of a multiengine, multi-heat-pump CCHP (combined cooling, heating and power generation) system e a case study. Energy 2010;35:3540e50. [9] Valero A, Lozano M, Muñoz M. A general theory of exergy saving. Part I: on the exergetic cost. In: Gaggioli R, editor. Computer aided engineering and energy systems. Second law analysis and modeling, vol. 3. New York, USA: ASME; 1986. pp. 1e8. [10] Usón S, Valero A, Agudelo A. Thermoeconomics and industrial symbiosis. Effect of by-product integration in cost assessment. Energy 2012;35:43e51. [11] Valero A, Usón S, Torres C, Valero Al, Agudelo A, Costa J. Thermoeconomic tools for the analysis of eco-industrial parks. Energy 2013;62:62e72. [12] Valero A. Exergy accounting: capabilities and drawbacks. Energy 2006;31: 164e80. [13] Szargut J. Analysis of cumulative exergy consumption. Energy Res 1987;4: 541e7. [14] Szargut J, Stanek W. Thermo-ecological optimization of a solar collector. Energy 2007;32:584e90. [15] Szargut J. Application of exergy for the determination of ecological costs. Bull Pol Acad Sci Technol 1986;(7e8). [16] Gazda W, Stanek W. Influence of power source type on energy and ecological effectiveness of cooling system with adsorption refrigerator. In: Proc. Conf. SDEWES; 2013. [17] Frangopoulos Ch. Method to determine the power to heat ratio, the cogenerated electricity and the primary energy savings of cogeneration systems after the European Directive. Energy September, 2012;45:52e61. [18] Schicktanz MD, Wapler J, Henning HM. Primary energy and economic analysis of combined heating, cooling and power systems. Energy 2011;36:575e85. [19] Stanek W. Method of evaluation of ecological effects in thermal processes with the application of exergy analysis. Silesian University of Technology Press; 2009 [in Polish]. [20] Szargut J. Exergy. Guide for calculation and application. Silesian University of Technology; 2007 [in Polish]. [21] Szargut J, Ziebik A. Cogenerated production of heat and electricity. Polish Academy of Science; 2007 [in Polish]. [22] Habib K, Choudhury B, Chatterjee PK, Saha BB. Study on a solar heat driven dual-mode adsorption chiller. Energy 2013;63:133e41.
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Exergo-ecological evaluation of adsorption chiller system Wojciech Stanek*, Wies1aw Gazda Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2013 Received in revised form 11 February 2014 Accepted 15 February 2014 Available online xxx
Technological processes in food industry require chilling agent. System of chillers can be driven by nonrenewable or renewable sources. In the group of renewable sources solar collectors seem to be an attractive solution both as the basic or the auxiliary power source for adsorption refrigerators. The common tool for the evaluation of the refrigerator systems used in practice is the energy effectiveness expresses as the COP (coefficient of performance). In the case of energy systems with energy fluxes of different parameters or supplied partly with renewable and non-renewable sources such evaluation is not enough. The authors proposed the method of exergetic evaluation. In the paper the comparison of the exergetic effectiveness and thermoecological cost of an example refrigeration system existing in Polish food industry supplied with heat alternatively from boiler house, cogeneration system and heat from solar collectors is presented. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Exergy analysis Food industry Adsorption chillers Thermo-ecological cost
1. Introduction Many technological processes require different cooling and chilling agents. Especially in food industry the demand for chilling agents can be significant and can play important role in the energy management of factory. To cover the demand for cold carrier different techniques are applied [1e4]: compressor refrigerators, absorption and adsorption machines. Compressor systems require the consumption of electricity that is mainly produced basing on non-renewable natural resources. Absorption and adsorption machines, driving with flux of heat, can be supplied from nonrenewable source (e.g. heat from gas boiler) but also can use renewable energy sources (heat from solar collectors). Most often the chilling systems are evaluated only from the point of view of first law of thermodynamics using COP (coefficient of performance). Such analysis is relatively simply and common especially in practice. However, in systems with energy fluxes (heat, electricity, fuels) of different quality such evaluation is not enough [4,5]. COP can be useful only in the case of comparison of chilling systems with similar construction and similar thermodynamic parameters. In general, for systems with different construction and different parameters second law of thermodynamics and exergy analysis has to be additionally applied [4,6e8]. Exergy [6] is the maximum
* Corresponding author. E-mail addresses: [email protected] (W. Stanek), [email protected] (W. Gazda).
ability of energy carrier to perform work in respect to common environment. Comparison of different refrigeration systems should be based on exergetic efficiency and exergy losses [5]. While the direct exergy analysis doesn’t show the exergetic results of interactions between system components the complex energy or any production systems should be additionally analysed with the tools offered by the thermo-economics [9e12]. The potential of this approach in the case of multi-purpose industrial park is efficiently demonstrated in Ref. [11]. The application of thermo-economics for the analysis of tri-generation systems is presented e.g. in Refs. [7,8]. When considering additionally the possibility of application of renewable and non-renewable resources the analysis should be furthermore supplemented with system analysis applying the common indicator of non-renewable natural resource quality and the balance boundary should reach the level of natural resources. Szargut was the first who extended the exergy analysis from a single process to the whole production chain by proposing the important concept of CExC (cumulative exergy consumption) [13] and subsequently he extended this concept to the TEC (thermoecological cost) [6,14,15], which enables the applications of exergy analysis in the field of environmental aspects. The TEC expresses the cumulative consumption of non-renewable exergy of natural resources burdening fabrication of any useful product [6]. The Szargut’s method in comparison with other methods of ecological assessment, can bring all environmental impacts to one metric which is the exergy of consumed natural non-renewable resources. The TEC method fulfils the requirement of LCA (Life Cycle Assessment); moreover, the minimisation of the TEC [14] ensures a
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mitigation of the depletion of non-renewable resources. To compare the ecological effects of the refrigerator system supplied with different sources of heat the authors used in this paper direct exergy analysis and system exergy based on the theory of thermoecological cost proposed by Szargut. Also, some remarks resulting from comparison with the analysis made purely by COP and PES (primary energy savings) are formulated. In the paper comparison of two systems are included: - refrigeration systems equipped with adsorption chiller supplied with heat from CHP (Combined Heat-and-Power) unit, - refrigeration system equipped with compressor refrigerator and additional adsorption chiller supplied with heat from solar collector. 2. Analysed refrigeration systems The first of analysed systems is presented in Fig. 1 CHP plant produces heat to drive adsorption chiller. The CHP plant is fired with natural gas from gas-network. The surplus of electricity is consumed within the factory or is transferred to the grid. The proposed adsorption system equipped with CHP unit replaced old one supplied with heat from boilers fired with gas. The savings in non-renewable resources is the results of application of cogeneration. In this case in the system two products are generated parallel e heat and electricity. To evaluate the performance indices of the system the method of cost division has to be assumed and the results of direct energy evaluation [16] have been applied. In the analysis the authors assumed exergetic cost method [10] to divide consumption of fuel between heat and electricity. The basic parameters of the adsorption system supplied with heat from CHP unit is included in Table 1 [16]. The second analysed system is partly supplied with heat from solar collector. Taking into account the range of ambient temperatures in Polish conditions glycol has been applied as intermediate working fluid in the solar collector. The simplified scheme of this solution is presented in Fig. 2. The more detailed description of the system and its energy analysis are presented in Ref. [16]. The basic parameters of liquid chilling package are summarised in Table 2. Assuming that the (COP) of the LCP (liquid chilling package) module is constant and taking into account data from Table 2 the coefficient of performance of the compressor chiller (LCP) is equal to (COP)LCP ¼ 4.28. In the case of the second system the solar collector has only the supplementary character. The minimum temperature required at inlet of adsorption chiller has been assumed as tgr ¼ 55 C, that is confirmed in Refs. [7] and [22]. The distribution of temperature ranges of collector output temperatures is presented in Fig. 3.
Fig. 1. System 1: adsorption refrigerator powered by the heat from the cogeneration power plant CHP e cogeneration heat-and-power plant, HT e hot water tank, AD e adsorption refrigerator, FT e food technology, CT e cooling tower, EL e electricity.
Table 1 Technical specification data of the adsorption refrigerator. Energy carriers
Temperature ( C)
Chilled water Cooling water Hot water
Capacity (kW)
Inlet
Outlet
12.0 30.0 90.0
7.0 35.0 83.0
251.0 613.0 362.0
It can be easily concluded that the support from solar collector is possible by the period of 1364 h/year. The rest of yearly operation (6760 h/year) is realised only by means of compressor chiller (LCP). 3. Evaluation of systems effectiveness 3.1. System equipped with CHP unit Total consumption of chemical energy in CHP results from energy efficiency:
hE;CHP ¼
Q_ CHP þ NCHP E_
(1)
F;CHP
where: Q_ CHP heat flux produced in CHP to drive AD, kW, NCHP CHP electric power, kW, E_ F;CHP flux of chemical energy of fuel consumed in CHP plant, kW. The energy efficiency of the CHP unit has been assumed due to the data from manufacturer (Table 3). To evaluate the advantageous of the cogeneration or tri-generation most often only the energy efficiency and primary energy savings are taken into account e.g. Ref. [17]. However, to determine the fuel consumption burdening separately heat and electricity production some additional rule has to implemented for energetic cost allocation. One of them is the method proposed by the “Cogeneration directives” but, as it was proved in Ref. [18] this method, in some cases can’t fulfil the first and second law of thermodynamics. Other method that can be used for cost allocation is the method of avoided cost [19]. However, in this method the results are strongly dependent on two factors: 1) definition of main and by-product, and 2) the arbitral assumption of the energy efficiency of replaced process. Another possibility is offered by the exergy analysis and thermoeconomics [6,8,9]. In this method for cost allocation the exergy of products are applied. This cost is defined as the total exergy consumption necessary to obtain
Fig. 2. System 2: adsorption refrigerator powered by the heat from the solar energy source SC e solar collectors, HT e hot water tank, AD e adsorption refrigerator, FT e food technology, CT e cooling tower, LCP e liquid chilling package, EL e electricity.
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W. Stanek, W. Gazda / Energy xxx (2014) 1e7 Table 2 Technical specification data of the liquid chilling package. Energy carriers
Temperature ( C)
Chilled water Cooling water Power
Capacity (kW)
Inlet
Outlet
12.0 30.0 e
7.0 35.0 e
251.0 309.7 58.7
the specific amount of exergy of useful product and in the case of CHP process it can be expressed as follows:
k* ¼
aE_ F;CHP FT ¼ Tm;AD T0 _ PT Q CHP Tm;AD þ NCHP
(2)
where: FT total exergy of fuel feeding the CHP system, PT total exergy of useful products of the CHP system, a the ratio of chemical exergy of fuel per unit of lower heating value (bch,F/LHV), Tm,AD mean thermodynamic temperature of water driving adsorption chiller AD, T0 ambient temperature. Using the specific cost of CHP products e k* the chemical energy of fuel burdening respectively production of heat in CHP can be determined as follows: *
Tm;AD T0 k E_ F;Q ¼ Q_ CHP a Tm;AD
(3)
The partial exergetic efficiency of heat production is defined as:
hB;Q ;CHP ¼
T T0 Q_ CHP m;AD Tm;AD
E_ F;Q a
(4)
The results of calculation of characteristic indices for the investigated CHP plant coupled with cold water production are presented in Table 3 It should be noticed that the exergy efficiency of heat production in CHP unit is relatively high and amounts to 48.9% and is significantly higher than exergetic efficiency of replaced gas boiler that reaches 23.4%. Exergy efficiency of heat production in CHP reaches the level of exergy efficiency in the case of modern steam generators as is confirmed in Refs. [20,21]. Additionally it should be pointed out
3
that the exergetic allocation of fuel costs between heat and electricity eliminates the non-correct from thermodynamic point of view results. E.g. in [#1] the assumed boiler efficiency is 0.92, and the calculated thermal efficiency of CHP is lower amounting to 0.57. These results can bring to misleading conclusion that the perfection of useful heat production in single process (boiler) is much lower than in cogeneration. The obtained exergetic efficiencies for the analysed CHP plant influences the total exergetic efficiency of cold production in system 1 that has been evaluated at the level of 13.8%. 3.2. System equipped supplemented with solar collector The production of heat in solar collector system is strongly dependent on several factors: 1) the atmospheric condition and solar energy flux reaching the collector system, 2) the collector energy efficiency, 3) design parameters as: tube diameter, collector area, distance between tubes, length of tubes. The problem of Thermo-Ecological optimisation of solar collector system has been presented by Szargut and Stanek in Ref. [14]. Additionally in the case of application of solar collector for AD the temperature constrain has to be taken into account. In the presented analysis it has been assumed that the AD input temperature should be at least 55 C, which is confirmed in the results of investigation of solar driven adsorption chiller presented in Refs. [7] and [22]. This constrains results in the constrained amount of working hours of the collector during the year. In Fig. 3 the possible time of solar collector cooperation with AD is presented. Due to data presented at this diagram it can be noticed that the total working time of the collector is constrained to 1366 h/year, which is the 16% of the year. The amount of the useful heat produced in the collector results from the energy efficiency of the solar collector, which is defined:
_ l Tc;out Tc;in mc F I_
hE;c ¼
(5)
b
while the exergetic efficiency of solar collector can be calculated as follows:
"
hB;c ¼ hE;c
Tc;out T0 ln 1 Tc;out Tc;in Tc;in
!#
I_b b_ r
(6)
where: _ mass flow rate of working agent in the collector, kg/s, m cl heat capacity of collector working fluid, kJ/(kgK), Table 3 Input data and results of exergy analysis of system with CHP unit.
Fig. 3. Accessibility of outlet temperature from collector during the year.
No.
Parameter
Symbol
Unit
Value
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Electric power of CHP Thermal power of CHP Energy efficiency of CHP Cogeneration ratio Exergy flux feeding the AD Chemical exergy of fuel Exergetic efficiency of CHP Exergetic cost of CHP operation Fuel exergy consumption e electricity Fuel exergy consumption e heat Exergy efficiency of heat production System exergy efficiency
NCHP Q_
kW kW e e kW kW % kW/kW kW kW % %
337.50 362.00 0.909 0.9324 66.95 815.70 49.59 2.017 680.70 135.00 48.90 13.78
CHP
hE,CHP xCHP BQ,AD BF,CHP
hB,CHP
k*CHP BF,CHP,EL BF,CHP,Q
hB,Q,CHP hB,sys
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W. Stanek, W. Gazda / Energy xxx (2014) 1e7
Using the energy and exergy characteristics of collector presented in Fig. 6 and time curves of solar energy and exergy resources presented in Fig. 5 the actual production of useful heat and useful exergy in collector can be determined q_ F;AD ðsÞ and b_ F;AD ðsÞ. Furthermore the yearly average energy and exergy efficiency of the collector can be calculated from the following formulae:
hE;c ¼
Zsgr _ qF;AD ds I_ 0
(8)
b
Fig. 4. Solar radiation reaching collector.
Tc,out,Tc,in temperature at outlet and inlet of the collector, K, I_ flux of solar radiation reaching the collector at b angle, W/m2, b
F active area of the collector, m2.
0
The ratio of solar radiation exergy to solar energy can be calculated from the formula [6,20]:
b_ r 4T 1 T0 4 ¼ 1 0þ 3 T 3T I_
hB;c ¼
Zsgr _ bF;AD _ I_b be_ r
ds
(9)
r
For the analysed system the following values has been achieved: - yearly average energetic efficiency of solar collector: hE;c ¼ 46.5%, - yearly average exergetic efficiency of solar collector: hB;c ¼ 8.4%.
(7)
Assuming T ¼ 5870 K and T0 ¼ 300 K the ratio b_ r =I_ b ¼ 0.93 and has been assumed to be constant in the presented analysis. The example of calculation of I_b for Polish condition in month July is presented in Fig. 4. In the system with solar collectors the solar energy flux is strongly time dependent and is also constrained through the year. Fig. 5 presents the dependence on time of solar energy and solar exergy flux. The operation of the collector e adsorption system 2 is constrained with the minimum collector output temperature tin,min ¼ 55 C. The presented time dependence of b_ r has been determined using equation (7) and characteristics similar to that presented in Fig. 4. The yearly production of heat in solar collector q_ F;AD that is used for powering the adsorption refrigerator and useful exergy of this amount of heat b_ F;AD have been calculated using equations (5)e(7). These fluxes are also presented in Fig. 5, energetic and exergetic characteristics of solar collector is presented in Fig 6.
It can be noticed that the average exergetic efficiency is relatively low. In the system 1 the exergetic efficiency of heat production in CHP unit reaches 48.4% and exergetic efficiency of gas boiler 23.4%. The value is than lower than the exergetic efficiency of both e heat production in the system equipped with CHP presented in Section 3.1, but from the other hand it should be underlined that the driving force in collector is coming from renewable sources while the CHP unit and gas boiler are supplied with non-renewable exergy of natural gas. The obtained results indicate the necessity of introduction additional criterion to compare systems driven with non-renewable and renewable energy and exergy. For this reason TEC (thermo-ecological cost) will be proposed. TEC analysis is presented in Section 4 of the paper. Other important factor in the analysis that influence the behaviour of the refrigerator system is the active area of solar collector F. The total production of heat is strongly dependent on this factor. Hypothetically there is a hours during the years in which the solar collector can work alone in the system 2 without the production of cold carrier in the compressor refrigerator. For such assumption and taking into account the energetic characteristic of the collector the required area F can be calculated. Results of such calculations are demonstrated in Fig. 7. The accessible area in the investigated food factory is limited to Fmax ¼ 900 m2. For this reason during the whole year the work of
Fig. 5. Heat and useful exergy production in SC during the year.
Fig. 6. Energy and exergy efficiency of collector.
b
where: T ¼ temperature of emitting surface, K, T0 ¼ temperature of environment, K.
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W. Stanek, W. Gazda / Energy xxx (2014) 1e7
5
Fig. 7. Required active area of solar collector as dependent on collector inlet temperature and the degree of production of cold based on heat from SC. Fig. 9. Regime of operation of system with solar collector.
pair AD with collector and LCP is necessary. The maximum share of collector in production of cooling agent is about 80%. To calculate the total efficiency of the whole refrigerator system the knowledge of characteristic of coefficient of performance is necessary. In the case of system 2 equipped with LCP e compressor refrigerator e the COP can be assumed to be constant in the investigated range of loads. In the case of system 1 equipped with AD e adsorption chiller e the COP is dependent on the inlet and outlet temperature of hot water delivered from collector or from CHP unit. In the case of solar collector system this temperature is varying during the daily operation. The energy and exergy characteristic of adsorption chiller is presented in Fig. 8. In the range of inlet temperature marked in Fig. 8 as (A) the exergetic efficiency of adsorption chiller hB,AD is higher than that of LCP. In Fig. 9 the regime of component operation of system 2 is presented. It can be noticed that the maximum power is supplied from collector between tout 80e100 C. However, in this range the energetic efficiency of the collector is rather low and the exergetic efficiency of AD chiller is lower than the maximum one. Taking into account the changing regime of system 2 operation which is strongly dependent on solar energy accessibility the average system exergy efficiency can be calculated to compare all of
analysed systems. The results for the investigated refrigerator configuration are as follows: - refrigerator system 2 average exergy efficiency e mode: collector þ LCP operation ¼ 2.30% - refrigerator system 2 average exergy efficiency e mode pure LCP operation ¼ 23.50% - refrigerator system 1 average exergy efficiency e mode AD supplied with CHP ¼ 13.80%. The presented results indicate that the less favourable system is that one supported with collectors and the most favourable system with compressors fed with electricity. If we take into account the energy indicators (COP) the profitability of system 2 with LCP is even more evident. These analyses did not show any advantages of application of RES (Renewable Energy Source) in the case of refrigeration system supported with solar collector. The positive effects of cogeneration are also not so evident. It can be concluded that such comparison as well as that based on COP and PES [18], is not sufficient and additional criterion has to be applied in this case. This criterion has to be able to: - measure all energy fluxes from the point of view of energy quality, - brings the analysis to the common balance boundary which is the level of natural resources. In the next section the authors presented the application of TEC (thermo-ecological cost) for evaluation of effectiveness and sustainability of investigated refrigerator systems. 4. Thermo-ecological analysis
Fig. 8. Energy and exergy effectiveness of AD chiller.
Szargut [6,13,15] defined the TEC (thermo-ecological cost) as cumulative consumption of non-renewable exergy connected with the fabrication of a particular product with inclusion of additional consumption resulting from the necessity of compensation of environmental losses caused by rejection of harmful substances to the environment. TEC of any product results mainly from consumption of non-renewable chemical exergy extracted directly from the nature bs as fuels, mineral ores, nuclear ores or fresh water. However this consumption appear only in the production processes directly connected with some extraction of resources from nature, e.g. in coal mine. Due to interconnections in the production systems, the TEC results also from consumption of domestic semi-finished products aij exchanged between links of system. Additionally, some burden of TEC can results from
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interregional exchange if within the national system there are some imported and exported goods. Besides consumption in some branches by-production can appeared. For example in cogenerated processes we can distinguish main product and byproducts. By products can replace production in other branches and for this reason decrease the final value of TEC. In the TEC balance presented in Eq. (10) by-products are taken into account by coefficient of by-production fij appearing in output of balance of TEC. Thermo-Ecological Cost of useful by-product should be determined using the principles of avoided exergetic cost. Besides consumption or by production described above, the balance of (TEC) for j-th production branch includes also the additional consumption of resources jj0 connected with rejection of wastes to environment. This additional consumption can result from two effects [19]. First, it can be connected with the consumption in abatement installation. Second, it results from the necessity of compensation of losses appearing in environment due to rejection of different aggressive wastes. The idea of exergetic TEC is to take into account only the consumption non-renewable resources (as in the case of considered system 2). Considering processes fed with renewable resources the direct consumption of chemical exergy should not be taken into account in the balances of TEC [14]. Consumption of renewable exergy, e.g. solar energy, doesn’t make any deleterious impact on depletion of deposits of the limited stock of resources. Basing on assumption explained so far the equation of the balance of operational TEC takes the following form [6,15,19]:
rj þ
X i
X X fij aij ri arj rr ¼ bsj þ jj0 r
(10)
s
where: aij coefficient of the consumption of the i-th product per unit of the j-th major product, e.g in kg/kg or kg/MJ, fij coefficient of the consumption and by production of the i-th product per unit of the j-th major product, e.g in kg/kg or kg/MJ, arj coefficient of the consumption of the r-th imported product per unit of the j-th major product, e.g. in kg/kg or kg/MJ, bsj exergy of the s-th non-renewable natural resource immediately consumed in the process under consideration per unit of the j-th product, MJ/kg, ri,rj, rr specific thermo-ecological cost of the i-th, j-th domestic product and the r-th imported good, e.g. in MJ/kg, jj0 requirement for natural resources exergy to compensate or to avoid the environmental losses resulting from operation of jth production process, MJ/kg of k-th waste product. The detailed description of the TEC method can be found for example in Refs. [6,15,19]. In Ref. [14] Szargut and Stanek presented the Thermo-Ecological optimisation of solar collector used for production of hot water. In the presented analysis the following indices of thermoecological cost has been used [19]: electricity (TEC)el ¼ 3.6 MJ/MJ; natural gas (TEC)ng ¼ 1.04 MJ/MJ. Using the results of analysis presented in Section 3 and the methodology for determination of TEC presented in this section the following yearly thermoecological cost for investigated cases of refrigerator systems has been achieved: case 1) adsorption chiller with gas boiler (TEC) ¼ 12,033.51 GJ/a, case 2) compressor chiller system without collector support (TEC) ¼ 5814.29 GJ/a, case 3) compressor chiller system with collector support (TEC) ¼ 5186.27 GJ/a, case 4) adsorption chiller with CHP unit (TEC) ¼ 4094.06 GJ/a.
If we assume the mentioned case 1) (adsorption chiller fed with the existing in the factory gas boiler) as the reference, the relative savings of exergy of primary non-renewable resources can be determined from the formula:
DðTECÞ ¼
ðTECÞ0 ðTECÞ1 ðTECÞ1
(11)
where: (TEC)0 ¼ yearly thermo-ecological cost for the reference system, (TEC)1 ¼ yearly thermo-ecological cost for the combined cooling-and-power system. For the above cases and the considered in the paper cases the following have been obtained: case 2) compressor chiller system without collector support
D(TEC) ¼ 52%, case 3) compressor chiller system with collector support
D(TEC) ¼ 57%,
case 4) adsorption chiller with CHP unit D(TEC) ¼ 66%
If it is assumed that the reference system is that with LCP driven with electricity (case 2) the application of the CHP system to drive the adsorption chiller results in the relative TEC savings amounting to D(TEC) ¼ 29.6%. It is evident that the assumption of the reference system has relatively high influence in the analysis of TEC relative savings. In Ref. [18] it has been reported that with low COP of thermal driven chiller the primary energy savings for CCP (combined cooling-and-power) is almost the same as that of reference system, while with COP ¼ 1 the primary savings reaches the level of 29%, which is in both cases lower than the results obtained in the presented work by means of TEC analysis. This comparison of the results proved the thesis included in Section 3.1 of the presented paper, that firstly the energy analysis is not enough to investigate complex multi-purpose systems. Secondly, that in such systems the boundary should reach the level of non-renewable resources as is assumed in the TEC analysis, and that the objective results are achieved where the total TEC is compared instead of the relative ones. The analysis of (TEC) proved that also application of cogeneration and renewable energy for support refrigeration systems are profitable from the point of view of rational management of nonrenewable resources and from the sustainable point of view. Replacement of gas boilers by CHP production of heat decrease three times the influence of investigated refrigerator system on the depletion of natural resources. Also the support of solar collector is profitable from the point of view of savings of natural resources. However, due to the solar radiation characteristic in Poland the positive effects are lower than that of combination of adsorption chiller with CHP. 5. Summary and conclusions In the paper the authors analysed two systems for cold water production in example Polish food factory. These systems are characterised by fluxes of heat and energy of different quality. The parameters of produced useful heat are often close to the environmental parameters and from other hand they are produced using energy of high quality. The obtained results confirmed that application of only energetic evaluation of these systems by means of COP coefficient is purposeless, because this analysis doesn’t take into account the energy and in general resources quality. The authors proposed than exergetic evaluation which detailed algorithm is discussed in the paper. The results of calculation average system
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exergy efficiency of the investigated refrigerator configuration are as follows: - refrigerator system 2 average exergy efficiency e mode: collector þ LCP operation ¼ 2.30% - refrigerator system 2 average exergy efficiency e mode pure LCP operation ¼ 23.50% - refrigerator system 1 average exergy efficiency e mode AD supplied with CHP ¼ 13.80%.
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boundary results in different ranking of technical solutions that obtain in the case of global boundary reaching the level of natural resources extractions. In the case of systems fed with renewable energy the second boundary has to be applied. Additionally it has been proved that comparing the systems producing energy fluxes of different quality as well as fed partially by renewable energy sources it is more objective to analyse the total TEC instead of relative savings. References
It is worth to stress that in the presented work, the calculations have been done under assumption that during the year the installation produces constant amount of chilled water however in the case of solar collector the calculations cover day-by-day all the year taking into account the variation of solar radiation (Fig. 3). The accessibility of high outlet temperature from collector is in Polish conditions very constrained. It is the cause while the average yearly exergetic efficiency in the mode: collector þ LCP operation is extremely low. From other hand it should be stressed that part of useful product are generated using free solar energy replacing the electricity from grid necessary to drive the LCP. The discussed direct exergy analysis takes into account the energy quality, but still doesn’t show any advantages of application of RES in the case of collector supported refrigeration system. Also the positive effects of cogeneration are not so evident. For this reason the authors proposed to use the criterion that is able to: - measure all energy fluxes from the point of view of energy quality, - brings the analysis to the common balance boundary which is the level of natural non-renewable resources. The TEC analysis showed the full system effects and proved that both e application of cogeneration and renewable energy for support refrigeration systems are profitable from the point of view of rational management of non-renewable resources and from the point of view of sustainable development. The performed analysis shown that yearly thermoecological cost for investigated refrigerator systems are as follows: - adsorption chiller with gas boiler, (TEC) ¼ 12,033.51 GJ/a, - compressor chiller system without collector support, (TEC) ¼ 5814.29 GJ/a, - compressor chiller system with collector support, (TEC) ¼ 5186.27 GJ/a, - adsorption chiller with CHP unit, (TEC) ¼ 4094.06 GJ/a. Replacement of gas boilers by CHP production of heat decrease three times the influence of investigated refrigerator system on the depletion of natural resources. Also the support of solar collector is profitable from the point of view of savings of natural resources. The presented results also that in the case of production systems supplied with renewable and non-renewable resources it is important to properly define the system boundary. The presented results shown that exergy analysis within the plant
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Please cite this article in press as: Stanek W, Gazda W, Exergo-ecological evaluation of adsorption chiller system, Energy (2014), http:// dx.doi.org/10.1016/j.energy.2014.02.053