Thermodynamic Study Regarding the Use of Dimethylether as Eco-Refrigerant

Transcription

1 Thermodynamic Study Regarding the Use of Dimethylether as Eco-Refrigerant VALENTIN APOSTOL 1, GHEORGHE POPESCU 1 *, HORAÞIU POP 1, ELENA EUGENIA VASILESCU 1, CÃTÃLINA MARINESCU 2, CRISTIAN-GABRIEL ALIONTE 1 1 University Politehnica of Bucharest, Department of Applied Thermodynamics, 313 Splaiul Independenþei, , Bucharest, Romania 2 ZECASIN SA, Splaiul Independenþei 202,060208, Bucharest, Romania The paper presents a thermodynamic analysis of dimethylether (DME) as an environmentally friendly substitution refrigerant for CFCs, HFCs and HCFCs. A comparative thermodynamic study has been carried out considering a single-stage vapor-compression refrigeration system (VCRS) using as working fluids DME, R717, R12, R134A, R22 (pure substances) and R404A, R407C (zeotrope mixtures), respectively. The analysis takes into consideration two situations: (i) the cooling load of the refrigeration system is imposed, aiming to establish the possibility of using DME as refrigerant, and (ii) the compressor theoretical volumic flow rate is imposed, to confirm that DME is suitable as a substitution refrigerant and also, to establish which refrigerants could be replaced by DME. The result of this study is that DME could be used as a refrigerant and, more, that DME could be a good substitution alternative for R12 and R134A. Keywords: thermodynamic analysis; vapor compression refrigeration systems; substitution of non-ecological refrigerants; dimethylether The paper presents the results of a thermodynamic theoretical study carried out for a single-stage vaporcompression refrigeration system (VCRS) that uses dimethylether (DME) as working fluid. DME was produced for the first time in 1864 by Tellier, being one of the first refrigerants ever used since the beginnings of artificial refrigeration [1]. But, because of its drawbacks (flammable and explosive) it was gradually abandoned. In the present, severe measures have been taken to gradually remove refrigerants as CFCs, HCFCs and HFCs, synthetic substances which nature cannot dissociate rapidly and become by accumulation pollutants. The scientific community is bringing into attention the possibility of using at large scale, as replacements for pollutant refrigerants, natural substances such as carbon dioxide ammonia and hydrocarbons [2-4]. Thus, the subject of the paper is within the context, and furthermore, it aims to bring back into attention the possibility to use a former refrigerant, namely dimethylether. In recent years, in order to promote the use of DME as an eco-refrigerant, many studies have been carried out with DME as pure refrigerant [5, 6], or as a component in mixture with ammonia [7, 8], or with R404A [9-11]. In the paper, the performances of an VCRS, obtained when using DME as a working fluid, are compared with those obtained when using classic refrigerants such as R717, R12, R134A, R22 (which are pure substances) and R404A, R407C (which are zeotrope mixtures). Theoretical calculations were made considering two distinct cases. The first case involves an imposed cooling load and aims to demonstrate that DME can be used as refrigerant. This first case corresponds to the situation in which the refrigeration system is designed from the very beginning to work on DME. The second case involves the situation in which DME is used as a substitution refrigerant for already existing refrigeration systems. Thus, in this second case the compressor volume flow rate is imposed aiming to give the answer for the question: can DME be used as a substitution refrigerant in existing refrigeration systems, and if yes, for which refrigerants?. The performance calculation was conducted on the basis of thermodynamic properties provided by the EES software [12], in the case of classic refrigerants and, respectively, the p - h diagram for DME [1]. Calculation methodology To demonstrate, from the thermodynamics point of view, that DME can be used in general as refrigerant and in particular as substitution refrigerant in existing refrigeration systems, a comparative analysis based on the theoretical cycle of a single-stage vapor-compression refrigeration system (fig.1) has been carried out. Both classic refrigerants and DME have been used. Computations for the theoretical thermodynamic cycle of the refrigeration system [13] have been done for both previous cases. The following parameters have been considered during the analysis: - evaporating ( t 0 - overheating degree (Δt si - condensing ( t C - subcooling degree (Δt sr [ o C]). Input values were adopted for all these parameters and the next quantities were computed: - subcooling : - compressor suction : Moreover, the specific mass and volume cooling loads have been computed with: - the evaporator mass cooling load: (1) (2) * gpopescu@theta.termo.pub.ro 714

2 the coefficient of cooling performance: (10) To estimate the values of the state parameters associated to the real compressor discharge state (2), the efficiency of the real compression process η c was approximated as the ratio of the absolute saturation s corresponding to the discharge pressure and, respectively, the suction pressure [13]: (11) Further, the calculation methodology is presented, for each of the two cases in part. For the case where the system cooling load is imposed (Q o ), were calculated: - the mass and volume flow rates at the compressor inlet: (12) - the compressor power consumption: (13) For the case where the same volume flow rate at the compressor inlet has been considered (V s ), were calculated: - the cooling load: - the refrigerant mass flow rate: (14) Fig.1. Constructive scheme (a) and theoretical thermodynamic cycle (b) in p-h diagram for single stage vapor compression refrigeration system - the evaporator volume cooling load: where ν 1 [m 3 /kg] represents the refrigerant specific mass volume at the compressor inlet. - the specific mass mechanical work input: - the condenser specific mass heat load: - the overheating specific mass heat load: - the subcooling specific mass heat load: (3) (4) (5) (6) (7) (8) (15) where: λ [-] is the overall flow rate reduction coefficient that can be determined as follows: (16) In equation (16), the overall flow rate reduction coefficient components are: flow rate reduction factor due to existent clearance volume (λ o ), flow rate reduction factor due to suction pressure drop (λ p ), flow rate reduction factor due to refrigerant overheating (λ T ) and flow rate reduction coefficient due leakages (λ e ). The flow rate reduction factor due to existent clearance volume (λ o ) can be written as: (17) where: ε o is the compressor clearance volume coefficient; k - the adiabatic coefficent; H c is the compression ratio. Function of suction (ψ a ) and discharge (ψ r ) pressure drops ratios, H c can be writen: (18) - verifying the energy balances: - (9) Results and discussions The following values were used, respectively: (i) in case the cooling load of the system is imposed: Q o =30 kw, and (ii) in case when the same volume flow rate at the compressor inlet has been considered: V s =22 m 3 /h. 715

3 The following study parameters have been adopted during calculations: t c = +40 o C; Δt sr =9 o C; Δt si =20 o C, ε o =0.05, ψ a = 0.06, ψ r = 0.1, λ T =0.96, λ e = The calculations were carried out for different evaporating s t o =-25 o C +10 o C, at step of 5 degrees centigrade. Values of state parameters (pressure p, t, enthalpy h, entropy s and volume v) in the characteristic points of the theoretical cycle [13] (thermodynamic states 1"-1-2s fig.1) were found using computer software developed in EES for the classic refrigerants [12] and, accordingly, p-h chart, for DME [1]. For ammonia those cases in which the discharge s are higher than 140 o C, were not considered, due to high compression ratios, the discharge s are higher than 140 o C. The results obtained are presented separately as follows: (i) for both analyzed cases, in figures 2 4, 7 and 8; (ii) for the case where the cooling load (Q o ) is imposed, in figures 5, 6 and 9 and (iii) for the case where the theoretical volume flow rate (V s ) is imposed, in figures 10 and 11. Thus, in figures 2 is presented the variation of the evaporation pressure p o (t o ) depending on the evaporating for all studied refrigerants. It is noted that DME has the lowest saturation pressure, but still larger than atmospheric pressure, meaning that the infiltrations of atmospheric air are avoided. Low values of the saturation pressure for the same pairs of evaporating-condensing s in case of DME leads to a lower working pressure in the system. This represents an important first advantage of using DME as refrigerant in vaporcompression systems. Fig. 3. Specific mechanical work consumption variation depending on evaporating Fig. 4. Specific mass heat load variation depending on evaporating q 0 Fig. 2. Evaporation pressure depending on evaporating Although the operating pressures are lower, the compression ratios are approximately equal to each other for the same pairs of evaporating-condensing. In the case of DME, the consumed mechanical work, l C during the compression process, is larger than in case of classic refrigerants, but lower than in case of ammonia, as shown in figure 3. This is due to the adiabatic exponent of the DME, which is higher than those of CFCs, HCFCs and HFCs, but lower than that of ammonia (k CFC =1.1 < k DME =1.142 < k R717 = 1.335). Figure 4 shows the variation of the specific mass heat load,, depending on evaporating, t o. In case of using DME, the mass heat load has values approximately two times bigger than those of CFCs, HCFCs and HFCs, and, accordingly, values about three times lower in comparison to ammonia. Therefore, as shown in figure 5, mass flow (m) of DME is much lower than in the case of using CFCs, HCFCs and HFCs (two times lower compared with R134A, R407C and R22, respectively, three times lower in comparison with R12 and R404A) and nearly double than the case of ammonia. This represents a second Fig. 5. Mass flow rate variation depending on evaporating Fig. 6. Compressor power consumption depending on evaporating 716

4 Fig. 7. COP variation depending on evaporating Fig. 9. Volume flow rate at the compressor inlet depending on evaporating Fig. 8. Specific volume heat load variation depending on evaporating important advantage of using DME in the VCRS refrigeration systems as a substitute for CFCs, HCFCs and HFCs, because it provides a smaller mass flow rate, leading to lower mechanical power consumption (fig.6). Figure 7 shows the variation of the COP variation of the refrigeration system, depending on the evaporating. In case of using DME, it is found that, for all considered evaporating s t o, due to the very important increase of the specific mass heat load, though the specific mechanical work consumption is higher, the COP is approximately two times bigger compared to the other analyzed refrigerants. This is a third important advantage obtained when using DME as refrigerant. In figure 8 is presented the variation of the specific volume heat load, q vo, depending on evaporating, t o. It is shown that DME has the lowest volume heat load values which is explained by the fact that this refrigerant has a very high specific volume at the compressor inlet, and, accordingly, as it is shown in figure 9, the highest volume flow rate at the compressor inlet, V a. This leads to the necessity of using larger compressor inlet pipes and larger reciprocating compressors having higher volume flow rates compared to the other considered refrigerants. This is one of the main disadvantages using DME as a refrigerant. In case the same refrigeration compressor is considered, having a theoretical volume flow rate V s = 22 m 3 /h, the results are presented in figures 10 and 11. In this case, the previous observations regarding the saturation pressure, specific mechanical work, specific mass heat load and coefficient of performance are maintained (figs. 2 4 and 7). Figure 10 presents the variation of the compressor power consumption depending on the evaporating. It is noted that, for s between 0 10 C there is a maximum power consumption corresponding to each Fig. 10. Compressor power consumption depending on evaporating Fig. 11. Cooling load variation depending on evaporating of the analyzed refrigerants. Those regimes of maximum power consumption should be avoided, of course. A disadvantage in case of using DME can be seen in figure 11. Thus, when using the same compressor, the cooling load of the DME is the lowest among all refrigerants due to low mass flow rate and high specific volume at the compressor inlet. Also, it can be observed that DME has a cooling load comparable to R12 and to R134a. One concludes that, from a thermodynamic point of view, the DME can be used as a replacement for R12 and R134A in existing refrigeration systems. Conclusions The paper presents the results of a thermodynamic theoretical study carried out for a single-stage vaporcompression refrigeration system that uses dimethylether (DME) as working fluid. Also, the paper aims to bring back into attention the possibility to use this former refrigerant, 717

5 namely dimethylether, as a substitution refrigerant for CFCs, HCFCs and HFCs. The performances of an vapor compression refrigeration system (VCRS), obtained when using DME as a working fluid, are compared to those obtained when using the classic refrigerants such as R717, R12, R134A, R22, pure substances, and R404A, R407C, zeotrope mixtures. The theoretical calculations were made considering two different cases. The first case involves an imposed cooling load and aims to demonstrate that DME can be used as refrigerant. The second case involves the situation in which DME is used as a substitution refrigerant for already designed refrigeration systems. By comparing the results obtained for the thermodynamic performance of the VCRS (operating under the same imposed conditions and for the same cooling load) for the various studied refrigerants, in the case of using DME, the following advantages emerged: operating at lower pressures, without danger of entering into vacuum; higher specific mass heat load and, therefore, lower mass flow rates; lower energy consumption for compression; higher coefficient of performance. Also, the results obtained show that the use of DME as a replacement for CFCs, HCFCs and HFCs involves a main disadvantage, namely the need to use larger reciprocating compressors. For the case when DME is used as a substitution refrigerant for already designed refrigeration systems, the following conclusions emerged: for s between 0 10 C there is a maximum amount of power consumption. These regimes should be avoided; the cooling load of the DME is the lowest due to the low mass flow rate to and the high specific volume at the compressor inlet; the DME can be used as a replacement for R12 and R134a in existing refrigeration systems; As an overall conclusion, DME can be used as refrigerant, having good thermodynamic properties and some major advantages, which fully qualify DME as a solution to replace synthetic pollutant refrigerants such as R12 and R134A. But, to actually demonstrate that DME is a viable practical solution for the replacement of polluting refrigerants, experimental investigation is required, along with endurance and reliability tests [14]. References 1. KUPRIANOFF, J., PLANK, R., STEINLE, H., Handbuch der kältetechnik Die kältemittel, Springer - Verlag., Berlin LORENTZEN, G., PETTERSEN, J., New possibilities for non-cfc refrigeration, in Pettersen J. Editor, Paper of International Symposium on Refrigeration (IIR), Energy and Environment, p , Trondheim, Norway, LORENTZEN, G., Revival of carbon dioxide as a refrigerant, Int. J. Refrig., 17, nr. 5, 1993, p LORENTZEN, G., The use of natural refrigerants: a complete solution to the CFC/HCFC predicament, Int. J. Refrig., 18, nr. 3, 1995, p TARLEA G,.M., APOSTOL, V., POPESCU, G., MARINESCU, C., POP, H., Romanian line up to the environment European legislation - new eco- refrigerant, Proceedings of 17 th Air-Conditioning and Ventilation Conference, Edited by J. Schwarzer, M. Lain, pp , Society of Environmental Engineering, Prague, Czech republic, May, APOSTOL, V., POPESCU, G., SOLOIU, V., POP, H., TARLEA, G.M., VASILESCU E,.E., MARINESCU, C., Study on efficiency and the ecological value of the DME in refrigeration and air conditioning systems, Scientific Bulletin of Politehnica University of Timisoara, Tome 52(65), Fascicule 1, 2006, p POPESCU, G., POP,H., APOSTOL, V., FEIDT, M., Analiza termodinamicã a unor noi agenþi frigorifici ecologici amestecuri de amoniac cu dimetileter, Proceeding of the 14 th National Conference of SRT, Petroleum - Gas University of Ploiesti, 1, p. 210, 31 May 1 June APOSTOL, V., POPESCU, G., POP, H., FEIDT, M., POPESCU, T., Thermodynamic analysis of new eco-refrigerants ammonia and dimethylether blends, The 4 th French - Romanian Colloquium Energy Environment Economy and Thermodynamics - COFRET 08, pp , Nantes, France, June, APOSTOL, V., POPESCU, G., POP, H., PRODAN, M., POPESCU, T., Thermodynamic analysis of a new eco-refrigerant - R404A and dimethylether blend, Termotehnica Journal, ISSN , Year 11 (1-2), 2007, p POP, H., POPESCU, G., TARLEA G,.M., APOSTOL, V., R404A & DME eco-refrigerant blend as a new solution to limit the global warming effect, Proceeding of CeEx 2007, ARO-PALACE Hotel, Brasov, October POPESCU, G., APOSTOL, V., POP, H., POPESCU, T., Theoretical study of a new eco-refrigerant proposal R404a & dimethylether blend, 8 th IIR Gustav Lorentzen Conference, Copenhagen, Denmark, September 7-10, KLEIN, S.A., ALVARADO, F.L., Engineering Equation Solver V6.271W User Manual, POPESCU, G., APOSTOL,V., PORNEALÃ, S. et al, Refrigeration Equipment and Plants (in Romanian), PRINTECH publishing, Bucharest, MARINESCU, C., POPESCU, G., APOSTOL, V., POP, H. et al, A New Eco-Refrigerants Family, Research report Contract no. 1915/ , National Program RELANSIN 04, beneficiary AMCSIT UPB, Bucharest, Manuscript received: