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Performance and thermo-sustainability analysis of non-hybrid organic Rankine cycles (ORCs) at varying heat source and evaporator conditions.

1. Introduction

The concern regarding the sustainability of energy resources in addition to the efficiency of energy conversion systems have been a topic of great interest to both government and private organisations. In recent times this situation is disturbing following the dwindling rate of conventional energy resources. Conversely, sustainability denotes the supply of energy in an accessible and reasonable cost with petite or probably no effect on the ecosystem. Besides, recent studies have shown that the exergy approaches are well flexible tools in evaluating the sustainability level of energy systems in addition to material exchanges with the environment (Aydin 2013; Midilli, Kucuk, and Dincer 2012; Rosen, Dincer, and Kanoglu 2007; Thawonngamyingsakul and Kiatsiriroat 2012). Additionally, studies have proposed cleaner methods of producing electricity for low CO2 emissions. These include the use of low-temperature heat energy (Chen, Goswami, and Stefanakos 2010; Abam et al. 2017). Similarly, the thermodynamic cycles used for converting the low-grade energy into electricity include the following the ORC (organic Rankine cycle), SRC (supercritical Rankine cycle), Kalina cycle, trilateral flash cycle and Goswami cycle (Kang 2012; Pei et al. 2011; Wang et al. 2010). An ORC is analogous to the conventional Rankine system but uses a biological or organic fluid as working fluid instead of water. The ORC is progressively embraced as one of the pronounced technology in transforming low-temperature heat energy sources into electrical energy. Moreover, the ORCs are characterised based on their applications and heat sources as, geothermal, low-grade energy recovery and solar energy utilisation (Marin et al. 2014). For instance, the studies of Hettiarachchi et al. (2007) projected the optimal performance of ORC for a geothermal plant based on different refrigerants, R123, PF 5050 and n-pentane. The influence of condensation and evaporation temperatures were evaluated for varying cooling water velocities. On the same vein, Saleh et al. (2007) examined 31 refrigerants for both sub-critical and supercritical ORCs for geothermal power plants, and the performance of the different refrigerants was established. Wei et al.(2007) measured the effects of exhaust flow rate, the initial temperature of the exhaust, the mass flow of air and the environmental temperature on the overall output power and rate of exergy destruction rate of an ORC for waste energy recovery. It was inferred: system efficiency and total output power could be enhanced by choosing an appropriate nominal state. Roy and Misra (2012) investigated an ORC performance using R-123 refrigerant considering transitional temperature heat source. The results indicated the best ORC performance at 2.7 MPa turbine pressure. Further works on ORCs efficiencies for different waste heat are contained in (Hung, Shai, and Wang 1997; Liu, Chien, and Wang 2004; Maizza and Maizza 2001). However, most published literature are encircled on ORC performance, working fluid suitability, optimum performance and second law evaluation. The work of Safarian and Aramoun (2015) delivered an all-inclusive energy and exergy analysis of modified ORCs at selected operating conditions. Their study did not include thermo-sustainability indicators (TSI) of these ORCs. Furthermore, the present study intends to deliver a theoretical analysis of TSI and performance of the adapted ORCs. Accordingly, compared performance of the ORCs and TSI at different EVP and HST.

2. ORCs structures and exergy balancing

The different ORCs structures are depicted in Figure 1 (Safarian and Aramoun 2015). Figure 1(a) denotes the generic ORC. Four processes exist (1-2) pumping process, (2-3) constant pressure heat addition, (3-4) heat rejection adiabatic expansion process and (4-1) heat rejection constant pressure process. Figure 1(b) describes a modified ORC with an internal heat exchanger (IHE). The addition of IHE in Figure 1(b) is important since the variations between the turbine and the condenser exit temperatures are significant. In Figure 1 (c), (ORC-turbine bleeding) is built-in with a feedwater heater (FWH). The extracted steam from the turbine mixes with the FWH and exit as a saturated liquid (3-4), while Figure 1(d) is built-in with turbine bleeding and a regeneration system.

2.1. Assumptions

The following conditions were considered for the analysis: (1) the system was assumed to operate in a steady-state condition, (2) the pressure drop and heat losses in the ORC components are negligible (3) the study considered two working fluids (a) R113 and (b) R141b, (4) the condenser inlet temperature and evaporator pressure were set at 298 K and 2.5 MPa, respectively while the isentropic efficiencies of the pump and turbine set at 85 and 90%, in that order (5) the evaporator heat input ([Q.sub.in]) is a hot-stream of gas obtained at 252 kW for all ORCs at operating conditions of 0.1 MPa and 573 K.

Additionally, to evaluate the TSI based on the second law of thermodynamics, an all-inclusive exergy balance of the ORCs is carried out. For any thermodynamic steady-state flow system, the exergy balance is presented as (Tchanche et al. 2010):

[??] = [summation over in][[??].sub.e] - [summation over out][[??].sub.e] - [[??].sup.Q.sub.in] - [[??].sup.W.sub.out] = [T.sub.0][[??].sub.gen] (1)

[??] = exergy destruction rate

[[??].sub.e] = exergy flow of the working fluid

[[??].sup.Q.sub.in], [[??].sup.W.sub.out] = exergy of heat input and work output

[[??].sub.gen] = entropy generation rate.

The thermomechanical exergy flow is expressed in Equation (2)

[e.sub.x] = h - [h.sub.0] - [T.sub.0](s - [s.sub.0]) (2)

where [h.sub.0], [s.sub.0] and, ([P.sub.0], [T.sub.0]) denotes specific enthalpy, entropy, temperature and pressure at dead state, respectively.

The equation for entropy generation applied generally for a steady-state thermodynamic process is given by (Cengel and Boles 2007):

[summation] [[Q.sub.k]/[T.sub.k]] + [summation][[??].sub.e][s.sub.e] + [summation][[??].sub.i][s.sub.i][[??].sub.gen] = [d[s.sub.cv]/dt] (3)

The component [d[s.sub.cv]/dt] diminishes for steady-state condition, reducing Equation (3) as:

[[??].sub.gen] =[summation][[??].sub.e][s.sub.e] + [summation][[??].sub.i][s.sub.i] - [summation][[[??].sub.k]/[T.sub.k]] (4)

where

[[??].sub.k], [T.sub.k], and [??] are, the rate of heat transfer, temperature and mass flow rate, respectively. Equation (4) is used to develop the entropy generation for the ORC components. Furthermore, the chemical exergy of refrigerants can be expressed as in Equation (5) (Safarian and Aramoun 2015):

[e.sub.ch] = [[e.sup.0.sub.ch]/M][[[T.sub.0]/298.15]] + [[DELTA][H.sub.0]/M][[[T.sub.0] - 298.15/298.15]] (5)

where [DELTA][H.sub.0] and [e.sup.0.sub.ch] denotes standard enthalpy of devaluation and exergy of organic substance (Safarian and Aramoun 2015).

The exergy balances for all the components of the ORCs in (Figure 1) is developed and presented using Equations (1) and (2).

2.2. Basic ORC

Evaporator(2-3), (1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in] + [??][x.sub.2] = [??][x.sub.3] + [[??].sub.Deva] (6)

Turbine (3-4), [??][x.sub.3] = [??][x.sub.4] + [??][x.sub.t] + [[??].sub.Dt] (7)

Pump (1-2), [??][w.sub.p] + [??][x.sub.1] = [??][x.sub.2] + [[??].sub.Dp] (8)

Condenser (4-1), [??][x.sub.4] = [??][x.sub.5] + [[??].sub.Dc] (9)

Exergy efficiency, [psi] = [[??][W.sub.t] - [??][W.sub.p]/(1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in]] (10)

2.3. ORC-internal heat exchanger

Pump (1-2), [??][x.sub.1] + [??][w.sub.p] = [??][x.sub.2] + [[??].sub.Dp] (11)

Evaporator (3-4), [??][x.sub.3] + (1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in] = [??][x.sub.4] + [[??].sub.Deva] (12)

Turbine (4-5), [??][x.sub.4] = [[??].sub.t] + [??][x.sub.5] + [[??].sub.Dt] (13)

Condense (6-1), [??][x.sub.6] = [??][x.sub.1] + [[??].sub.Dc] (14)

Heat exchanger (5-6), [??][x.sub.5] + [??][x.sub.2] = [??][x.sub.3] + [??][x.sub.6] + [[??].sub.Dht] (15)

Exergy efficiency, [psi] = [[??][W.sub.t] - [??][W.sub.p]/(1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in]] (16)

2.4. ORC-turbine bleeding

Pump 1 (1-2), [??][x.sub.1] + [[??].sub.p1] = [??][x.sub.2] + [[??].sub.Dp1] (17)

Pump 2 (3-4), [??][x.sub.3] + [[??].sub.p2] = [??][x.sub.4] + [[??].sub.Dp2] (18)

Evaporator(4-5)[??][x.sub.4] + (1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in] = [??][x.sub.5] + [[??].sub.Deva] (19)

Turbine (4-5 & 5-7), [??][x.sub.5] = [[??].sub.t] + [??][x.sub.6] + [??][x.sub.7] + [[??].sub.I] (20)

Feedwater heater (6-3-2), [??][x.sub.8] + [??][x.sub.2] = [??][x.sub.9] + [??][x.sub.3] + [E.sub.Dhtr] (21)

Condenser (7-1), [??][x.sub.7] = [??][x.sub.1] + [[??].sub.Dc] (22)

Exergy efficiency, [psi] = [[[??].sub.t] - ([[??].sub.p1] + [[??].sub.p2])/(1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in]] (23)

2.5. ORC-turbine bleeding and regeneration

Pump 1 (1-2), [??][x.sub.1] + [[??].sub.p1] = [??][x.sub.2] + [[??].sub.Dp1] (24)

Pump 2 (4-5), [??][x.sub.4] + [[??].sub.p2] = [??][x.sub.5] + [[??].sub.Dp2] (25)

Evaporator(4-5)[??][x.sub.5] + (1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in] = [??][x.sub.6] + [[??].sub.Deva] (26)

Turbine (6-7&6-8), [??][x.sub.6] = [??][x.sub.8] + [??][x.sub.9] + [[??].sub.t] + [[??].sub.Dt] (27)

Heat exchanger (2-3&8-9), [??][x.sub.8] + [??][x.sub.2] = [??][x.sub.9] + [??][x.sub.3] + [[??].sub.Dht] (28)

Feedwater heater (7-4-3), [??][x.sub.7] + [??][x.sub.3] = [??][x.sub.4] + [[??].sub.Dht] (29)

Condenser (9-1), [??][x.sub.9] = [??][x.sub.1] + [[??].sub.Dc] (30)

Exergy efficiency, [psi] = [[[??].sub.t] - ([[??].sub.p1] + [[??].sub.p2])/(1 - [[T.sub.0]/[T.sub.in]])[Q.sub.in]] (31)

3. Thermo-sustainability indicators

The TSI (exergetic-sustainability indicators) are delivered for all the ORCs obtained from the thermodynamic equations of the different ORCs structures (Figure 1).

3.1. Waste exergy ratio (WER)

The waste exergy (WER) represented a summation of the destroyed and lost exergy evaluated using Equation (32), whereas the WER is expressed as the ratio of the total WE to the overall exergy input Equation (33) (Aydin 2013).

[summation]E[x.sub.we,out] = [summation][??][x.sub.dest,out] + [summation][??][x.sub.loss,out] (32)

WER = [Overall exergy waste/overall exergy input] (33)

3.2. Environmental effect factor (EEF)

The EEF is a significant TSI since it specifies whether there occur a loss to the environment owing to the waste exergy destruction. The EEF as defined in Equation (34) (Aydin 2013).

EEF = [Waste exergy ratio/Exergy efficiency] (34)

3.3. Exergy efficiency

The overall exergy efficiency (OEE) ([[psi].sub.overall]) is the ratio of the total output exergy to the exergy input (Equation 35). Additionally, Equations (10), (16), (23) and (31) expresses the OEE for the different ORCs (Tchanche et al. 2010).

([[psi].sub.overall]) = ([??][x.sub.out]/[??][x.sub.in]) (35)

3.4. Exergetic sustainability index (ESI)

The ESI is likewise an important parameter among other TSIs. It defined the extent of sustainability and calculated as a reciprocal of the EEF (Equation 36) (Midilli, Kucuk, and Dincer 2012)

ESI = [1/Environmental effect factor] (36)

4. Results and discussion

4.1. Exergy performance of the ORCs

The ORCs performance is ascertained using refrigerants 113 and 141b. The operating conditions for the R113 and R141b at 298 K were estimated at 0.046 and 0.079 MPa, respectively. The thermodynamic flow characteristics for the two refrigerants are depicted in Table 1. The results indicate the components efficiencies were high in ORC-turbine bleeding/regeneration (Figure 2(a)) whose values ranged from 80.02 to 80.12%. Also, for the ORC-basic, the components exergy efficiencies vary from 54.87 to 79.77% given a proportion improvement of approximately 0.3%. Conversely, the highest component exergy destruction (ED) of 54. 27 kW was obtained in the evaporator with the ORC-basic (Figure 2(b)), while for the overall cycle; the ED was least with ORC-turbine bleeding/regeneration. The percentage components exergy destruction in all the cycles for R113 and R141b refrigerants is shown in Figure 3. The results indicate that the largest exergy destruction for all the ORCs occur in the evaporator indicating the evaporator is a critical component in the ORC. Additionally, ORC-turbine bleeding and ORC-turbine bleeding/regeneration show less exergy destruction. Further performance comparison of the ORCs was observed at varying evaporator pressure (EVP) and heat source temperature (HST) in the subsequent sections.

4.2. Performance of the ORCs at EVP and HST

The effect of evaporator and HST on the OEE and the overall exergy destruction (OED) and power output (POT) is presented in Figure 4. The OEE for evaporator pressure ranged betw een 35.45 [less than or equal to] OEF [less than or equal to] 41.76%, 38.57 [less than or equal to] OEF [less than or equal to] 47.23% , 35.92 [less than or equal to] OEF [less than or equal to] 43.67% and 37.11 [less than or equal to] OEF [less than or equal to] 46.54% for ORC-basic, ORC-IHE, ORC-turbine bleeding and ORC-turbine bleeding/regeneration (Figure 4(a)). Similarly, the OEE varies from 32.07 to 62.75% across the cycles for a HST range between 500 and 650 K
Fig. 3. Percentage of components exergy destruction for R113 and R141b
refrigerants.

(a) ORC-basic with R113

Evaporator  80%
Pump         2%
Turbine     18%

(b) ORC-basic with R141b

Evaporator  84%
Pump         2%
Turbine     14%

(c) ORC-IHE with R113

Evaporator  76%
Pump         2%
Turbine     22%

(d) ORC-IHE with R141b

Evaporator  78%
Pump         2%
Turbine     20%

(e) ORC-Turbine bleeding with R113

Evaporator  67%
Pump 1       1%
pump 2       1%
Turbine     31%

(f) ORC-Turbine bleeding with R141b

Evaporator  44%
Pump 1       2%
pump 2       4%
Turbine     50%

(g) ORC-Turbine bleeding/reg with R113

Evaporator  65%
Pump 1       1%
pump 2       2%
Turbine     32%

(h) ORC-Turbine bleeding/reg with R141b

Evaporator  43%
Pump 1       2%
pump 2       4%
Turbine     51%

Note: Table made from pie chart.


(Figure 4(b)). Improved cycle efficiencies were achieved at high evaporator pressures and low HST with R141b. The cycle overall exergy destruction ranged between 70.14 [less than or equal to] OED [less than or equal to] 77.71 kW, 63.57 [less than or equal to] OED [less than or equal to] 73.95 kW, 67.55 [less than or equal to] OED [less than or equal to] 77.9 kW and 63.0 [less than or equal to] OED [less than or equal to] 76.48 kW (Figure 4(c)) for ORC-basic, ORC-IHE, ORC-turbine bleeding and ORC-turbine bleeding/regeneration, respectively. For a fixed evaporator pressure and at a HST range between 500 [less than or equal to] HST [less than or equal to] 650K the overall exergy destruction varies from 53.5to 92.64 kW across the ORCs (Figure 4(d)). However, the overall exergy destruction increases for increasing HST. The reduction in the overall exergy destruction at increasing evaporator pressure noticed in this study is ascribed to the equivalent decrease in temperature between the inlet temperature of the heat source stream and the temperature of the evaporator, causing a less entropy generation and hence, a reduction in exergy destruction. The ORCs power output is depicted in Figure 4(e) and (f). The maximum power output of 54. 74, 54.18 and 58.18 kW (using R113) was achieved at evaporator pressure of 3 MPa for ORC-IHE, ORC-turbine bleeding and ORC-turbine bleeding/reg while power output of 51.69, 68.14 and 69.58 kW was attained with R141b for the same cycle (Figure 4(e)). Similarly, at varying HST and fixed evaporator pressure the power output was almost constant for all the cycles (Figure 4(f)) with a maximum value of 65.64 kW obtained using ORC-turbine bleeding/regeneration.

4.3. Effect of evaporator pressure and HST on TSIs

4.3.1. Exergetic efficiency

The exergy efficiency is an important thermo-sustain-ability indicator. Though, the influence of evaporator pressure on the total exergy efficiency for the cycles was partially discussed in Section 4.2. Moreover, the cycle performance with R141b gave high values of OEE with increasing evaporator pressure and low overall exergy destruction with decreasing HST. The latter showed good performance in ORC-turbine bleeding and ORC-turbine bleeding/regeneration with improved efficiency not greater than 0.95% from the generic cycle.

4.3.2. Waste exergy ratio

Figure 5 depicts the waste exergy ratio (WER) for the ORCs at different evaporator pressure. The results indicate that WER decreases for all increasing evaporator pressure (Figure 5(a)) while low values of WER were obtained with ORC-IHE for all increasing evaporator pressures. Furthermore, the lowest WER of 0.253 was got at maximum EVP of 3 MPa with ORC-IHE (Figure 5(a)) whereas at same EVP the basic ORC had the highest WER approximated at 0.288. Considering the performance of R141b on the ORCs (Figure 5(b)), WER was found to rise for all increasing evaporator pressures as noticed in ORC-turbine bleeding and ORC-turbine bleeding/regeneration cycles. Consequently, for increased HST the WER was found to increase in all the ORCs with minimum environmental impact obtained with the ORC-IHE for both working fluids

4.3.3. Exergetic sustainability index (ESI)

The variation in exergetic sustainability index (ESI) at different evaporator pressure and HST is presented in Figure 6. The results show that maximum values of ESI exist at evaporator pressure of 3 MPa and HST of 500 K with R131. Similarly, maximum ESI occurred at low evaporator pressure and HST of 2 MPa and 500 K, respectively, with R141b. Furthermore, for all ORCs ESI decreases with increasing HST, whereas cycle performance was different for varying EVP. The highest ESI was achieved with ORC-IHE and ORC-turbine bleeding/regeneration. The previous may be ascribed to operating conditions of the cycles, psychometric properties of the working fluids and the variance between the condensing and critical temperatures of the refrigerants.

4.3.4. Environmental effect factor (EEF)

The environmental effect factor (EEF) connotes the degree of environmental damage owing to waste exergy destruction. The EEF is high for the generic cycle for both evaporator and HST variations (Figure 7). Nonetheless, minimum EEF values are achieved with R113 for the ORCs (Figure 7(a)) while for the same working fluid (Figure 7(b)) the EEF increases for increasing HST. The EEF values fluctuated between 0.5342 [less than or equal to] EEF [less than or equal to] 0.6905 and 0.1498 [less than or equal to] EEF [less than or equal to] 0.3683 for evaporator pressure and HST variations (Figure 7(a) and (b)), respectively. Additionally, maximum EEF values were got with R141b ranging between 0.2394 and 1.148 for EVP and HST variations (Figure 7(c) and (d)) while the least EEF values were observed in ORC- turbine bleeding/regeneration and the ORC-IHE. Moreover, the study reveals that operational data, working fluid and system configuration plays an important role in influencing the system performance. Consequently, obtaining optimum operational values may help reduce ecological complications.
Figure 7. EEF of the QRCS at varying EVP and HST for (aMb) run and
(c)-(d) R141b refrigerants.

(a) ORC with R113
Evaporator pressure (MPa)

                      2       2.111   2.222   2.333   2.444   2.556

ORC-Turbine bleeding  0.6503  0.6344  0.62    0.6069  0.595   0.584
and Regeneration
ORC-Turbine bleeding  0.7305  0.7123  0.6961  0.6312  0.6675  0.6549
ORC-Heat exchanger    0.5966  0.5867  0.5778  0.5698  0.5626  0.5557
ORC-Basic cycle       0.7222  0.7127  0.7041  0.6971  0.6905  0.6345

                      2.667   2.778   2.889   3

ORC-Turbine bleeding  0.5739  0.5644  0.5556  0.5475
and Regeneration
ORC-Turbine bleeding  0.6434  0.6328  0.623   0.6139
ORC-Heat exchanger    0.5496  0.5439  0.5388  0.5342
ORC-Basic cycle       0.6792  0.6745  0.6702  0.6664

(b) ORC with R113
Heat source temperature (kW)

                      500       516.7     533.3     550       566.7

ORC-Turbine bleeding    0.1498    0.169     0.1871    0.204     0.2199
and Regeneration
ORC-Turbine bleeding    0.2301    0.2493    0.2673    0.2843    0.3002
ORC-Heat exchanger      0.1915    0.2108    0.2288    0.2457    0.2616
ORC-BASIC cycle         0.2308    0.25      0.268     0.285     0.3009

                      583.3

ORC-Turbine bleeding    0.235
and Regeneration
ORC-Turbine bleeding    0.3152
ORC-Heat exchanger      0.2767
ORC-BASIC cycle         1.3159

                      600       616.7      633.3     650

ORC-Turbine bleeding    0.2491     0.2626    0.2753    0.2873
and Regeneration
ORC-Turbine bleeding    0.3294     0.3429    0.3556    0.3676
ORC-Heat exchanger      0.2909     0.3043    0.317     0.329
ORC-BASIC cycle         0.3301     0.3435    0.3562    0.3683

(c) ORC with R141b

Evaporator pressure (MPa)
       ORC-Basic  ORC-Turbine  ORC-Heat   ORC-Turbine
       cycle      bleeding     exchanger  bleeding and
                                          Regeneration

2      0.87        0.761        0.8209      0.7299
2.111  0.8625      0.7472       0.8177      0.7224
2.222  0.8527      0.7352       0.8271      0.7269
2.333  0.8455      0.7243       0.8154      0.7282
2.444  0.8395      0.7141       0.8217      0.781
2.556  0.8342      0.705        0.8263      0.7857
2.667  0.8293      0.6969       0.8327      0.7917
2.778  0.8255      0.6891       0.8457      0.7991
2.889  0.8216      0.682        0.5499      0.8979
3      0.8186      0.656        0.5605      0.8278

(d) ORC with R141b

       ORC-BASIC  ORC-Turbine  ORC-Heat   ORC-turbine
       cycle      bleeding     exchanger  bleeding and
                                          Regeneration

500    0.5369     0.3633       0.5302      0.2394
516.7  0.6092     0.419        0.6017      0.2829
533.3  0.6809     0.4741       0.6726      0.3266
550    0.7518     0.5287       0.7429      0.3698
566.7  0.8215     0.5825       0.8118      0.4124
583.3  0.8896     0.6357       0.8792      0.4547
600    0.9565     0.6877       0.9455      0.496
616.7  1.022      0.7384       1.011       0.5368
633.3  1.086      0.7882       1.074       0.5767
650    1.148      0.8367       1.135       0.6155

Note: Table made from bar graph.


5. Conclusion

Thermo-sustainability and performance of organic Rankine cycles (ORCs) at EVP and HST are considered with different working fluids. The findings include:

* The ORC-turbine bleeding, ORC-turbine bleeding/regeneration and the ORC-IHE had the highest OEE and less exergy destruction among the considered ORCs.

* For all the configurations and at EVP and HST the exergy destruction dominates in the evaporator. However, for the ORC-basic, the evaporator contributes approximately 70% of the exergy loss while ORC-turbine bleeding/regeneration, ORC-turbine bleeding and the ORC-IHE the evaporator contributes not greater than 42% to the exergy destruction. The reduction in the exergy destruction is attributed to the integration of the feedwater heater. The latter raises the temperature of the refrigerants before entering the evaporator thus reducing the heat transfer across a finite temperature difference.

* For all configurations, the overall exergy efficiencies increase with increasing evaporator and decrease with increasing HST. Similarly, the overall exergy destruction increases with increasing HST. These findings are consistent since a reduction in exergy destruction results to increase efficiency. The reason is that for increasing evaporator pressure the temperature of the working fluid is closer to that of the hot gas inflowing the evaporator, thereby facilitating heat addition across a low-temperature difference. Similarly, for an increasing HST, this may lead to the large temperature difference between the hot gas temperature and that in the evaporator creating large entropy generation and thus large ED.

* The WER and EEF were found to decrease with increasing EVP and increases for increasing HST. The reason is obvious since at fixed evaporator pressure and varying HST the difference between the hot gas temperature and that of the evaporator is largely resulting in large entropy generation leading to high WER and EEF.

* The ESI was approximated at0.1498 [less than or equal to] ESI [less than or equal to] 1.148 for the ORCs. The ESI increases for all increasing evaporator pressure and decreases for increased HST.

* For the considered ORCs, ORC-turbine bleeding/regeneration and ORC-IHE cycles are more sustainable at some operating conditions. Nonetheless, choice of refrigerants and system configurations contribute largely to performance enhancement. Optimisation will enhance performance and reduce environmental impact.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Fidelis I. Abam [iD] http://orcid.org/0000-0001-6794-0118

Samuel O. Effiom [iD] http://orcid.org/0000-0003-4248-9871

Notes on contributors

Fidelis I. Abam was born in Ikom, Nigeria, in 1972. He obtained a BEng degree in mechanical engineering from Federal University of Agriculture, Makurdi erstwhile University of Jos Makurdi Campus, Nigeria, in 1998, an MSc degree in mechanical engineering from university of Lagos, Nigeria, in 2007, and PhD degree in energy and power technology from University of Nigeria, Nsukka in 2011. He is currently an associate professor in the Department of Mechanical Engineering Michael Okpara University of Agriculture, Umudike, Nigeria. His area of research interests includes exergy and environment, thermal power plants and renewable energy systems.

Ekwe B. Ekwe was born in Nko, Nigeria, in 1984. He received BEng degree from Cross River University of Technology Calabar, Nigeria in 2012 and MEng degree in energy and power engineering from Michael Okpara University of Agriculture Umudike, Nigeria in 2015. He is currently a PhD research scholar in Michael Okpara University of Agriculture, Umudike, principally engaged in research poly-generation systems.

Samuel O. Effiom was born in Calabar, Nigeria, in 1989. He obtained BEng degree from Cross River University of Technology Calabar, Nigeria in 2011 and MSc degree in thermal power (Gas turbine/aerospace propulsion) Cranfield University UK in 2015, and Ph.D degree in Energy and power Engineering from Michael Okpara University of Agriculture Umudike, Nigeria in 2018. Currently, he is working in the department of mechanical engineering, Cross River University of Technology Calabar, Nigeria. His research interest includes thermal power plants for aero propulsion systems, energy and environment, Heat transfer and CFD.

Christopher B. Afangideh was born in Eket, Nigeria, in 1971. He received the bachelor's degree in (BEng) in mechanical engineering from University of Nigeria Nsukka (UNN), Nigeria in 1997. The MSc and PhD degrees were obtained in water resources and environmental engineering from UNN in 2004 and 2010, respectively. He is currently a lecturer in the mechanical engineering department in Akwa Ibom State University Ikot Akpaden, Nigeria. His research areas include environmental engineering, fluid mechanics and renewable energy systems.

References

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Fidelis I. Abam (a) [iD], Ekwe B. Ekwe (b), Samuel O. Effiom (c) [iD] and Christopher B. Afangideh (d)

(a) Energy, Exergy, and Environment Research Group (EEERG), Department of Mechanical Engineering, Michael Okpara University of Agriculture, umudike, umuhia, Nigeria; (b) Department of Mechanical Engineering, gregory university, uturu, Nigeria; (c) Department of Mechanical Engineering, cross river university of Technology, calabar, Mgeria; (d) Department of Mechanical Engineering, Akwa Ibom state university, lkot Akpaden, Mgeria

KEYWORDS

thermo-sustainability; exergy efficiency; ORQ regeneration

ARTICLE HISTORY

received 25 June 2017

Accepted 27 August 2017

https://doi.org/10.1080/14484846.2017.1373585
Table 1. thermodynamic flow parameters at different state points for
the ORCS with R113 and R141b refrigerants.

                       R113
Points                 T ([degree]C)  P (MPa)  e (kJ/kg)  E(kW)

ORC-Basic
1                       25.00         0.046      0.000      0.000
2                       26.72         2.500      0.006      0.064
3                      193.70         2.500     62.310     67.109
4                       65.63         0.046      2.247      2.381
ORC-Internal heat
exchanger
1                       25.00         0.046      0.000      0.000
2                       26.72         2.500      0.006      0.007
3                       55.00         2.500      2.999      3.179
4                      213.60         2.500     72.810     77.178
5                       94.99         0.046      5.532      5.532
6                       66.71         0.046      2.339      2.479
ORC-turbine bleeding
1                       25.00         0.046      0.000      0.000
2                       25.67         1.000      0.001      0.001
3                      138.00         1.000     18.180     19.271
4                      139.10         2.500     18.530     19.642
5                      298.60         2.500    109.400    115.965
6                      266.90         1.000     89.330     32.114
7                      181.20         0.046     23.460     16.434
ORc-turbine
bleeding/regeneration
1                       25.00         0.046      0.000      0.000
2                       25.67         0.046      0.0009     0.0007
3                       40.00         1.000      0.398      0.292
4                      138.00         1.000     19.270     14.150
5                      139.10         2.500     18.180     13.300
6                      298.60         2.500     18.530     13.540
7                      266.90         1.000    109.400     79.860
8                      181.20         0.046     89.330     65.120
9                      163.60         0.046     23.460     17.070

                       R141b
Points                 T ([degree]C)  P (MPa)  e(kJ/kg)  E(kW)

ORC-Basic
1                       25.00         0.079      0.000     0.000
2                       27.35         3.420      0.017     0.068
3                      190.5          3.420     56.300    59.678
4                       25.28         0.079      0.269     0.285
ORC-Internal heat
exchanger
1                       25.00         0.079      0.000     0.000
2                       27.13         3.100      0.016     0.0167
3                       55.00         3.100      4.294     4.552
4                      184.00         3.100     68.21     72.303
5                       25.28         0.079      0.304     0.322
6                       25.28         0.793      0.251     0.266
ORC-turbine bleeding
1                       25.00         0.079      0.000     0.000
2                       25.98         1.470      0.004     0.0025
3                      138.00         1.470     23.980    25.418
4                      139.50         3.420     24.570    26.044
5                      238.50         3.420    110.400   117.024
6                      196.00         1.470     86.640    91.838
7                       92.02         0.079      5.800     3.678
ORc-turbine
bleeding/regeneration
1                       25.00         0.079      0.000     0.000
2                       25.98         1.470      0.004     0.003
3                       40.00         1.470      0.540     0.397
4                      138.00         1.470     23.980    27.289
5                      139.50         3.420     24.570    17.985
6                      238.50         3.420    110.400   80.769
7                      196.00         1.470     86.640    63.247
8                       92.02         0.079      5.801     4.230
9                       71.32         0.079      2.994     2.181
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Article Details
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Author:Abam, Fidelis I.; Ekwe, Ekwe B.; Effiom, Samuel O.; Afangideh, Christopher B.
Publication:Australian Journal of Mechanical Engineering
Geographic Code:6NIGR
Date:Oct 1, 2018
Words:5504
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