- Email: [email protected]
Comparative Investigation of the Performances of Subcritical and Transcritical Biomass-Fired ORC Systems for Micro-scale CHP Applications
Available online at www.sciencedirect.com
ScienceDirect Procedia Computer Science 83 (2016) 855 – 862
The 6th International Conference on Sustainable Energy Information Technology (SEIT 2016)
Comparative Investigation of the Performances of Subcritical and Transcritical Biomass-Fired ORC Systems for Micro-Scale CHP Applications Angelo Algieri* Department of Mechanical, Energy and Management Engineering, University of Calabria Via P. Bucci Cubo 46C, 87036 Arcavacata di Rende, Italy
Abstract The paper aims to analyse the energetic performances of Organic Rankine Cycles (ORCs) for biomass micro-scale CHP applications. Subcritical and transcritical cycles have been compared and the influence of internal regeneration on system behaviour has been evaluated. The investigation illustrates the noticeable influence of the operating conditions and ORC configuration on the main CHP performances. Furthermore, results reveal that the proper choice of the organic working fluid is fundamental to assure proper operations and optimise the system behaviour. The analysis shows that the transcritical cycle with internal regeneration offers the maximum electric performances, whereas the saturated cycle in simple configuration guarantees the highest thermal efficiency. Furthermore, all the investigated systems guarantee positive energy saving capabilities. 2016The TheAuthors. Authors.Published Published Elsevier © 2016 byby Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Conference Program Chairs. Peer-review under responsibility of the Conference Program Chairs Keywords: Organic Rankine cycle; CHP; biomass; transcritical cycle; regeneration; micro-scale.
1. Introduction Nowadays, combined heat and power production (CHP) represents an effective alternative to traditional systems that are characterised by separate electric and thermal generation. Cogenerative systems, in fact, are able to increase
* Corresponding author. Tel.: +39-0984-494665; fax: +39-0984-494673. E-mail address: [email protected]
1877-0509 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Conference Program Chairs doi:10.1016/j.procs.2016.04.176
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Angelo Algieri / Procedia Computer Science 83 (2016) 855 – 862
Nomenclature EUF h η P PES
Q th
T
energy utilisation factor specific enthalpy efficiency power primary energy saving index biomass thermal power temperature
energy saving capabilities and reduce pollutant emissions, compared with traditional units1,2. Specifically, Organic Rankine Cycles (ORCs) appear an interesting option for sustainable and reliable energy supply in micro-scale CHP applications (Pel < 50 kWel), where conventional apparatus are also economically and technologically unfeasible1,3,4. ORC units offer several advantages owing to good partial load performances, fast start-up and stop procedures, low maintenance requirements, and high flexibility and safety5,6. The selection of the most appropriate configurations and operating conditions is fundamental for the optimisation of ORC performances7,8. It is noteworthy to notice that the attention of researchers community and manufacturers has been mainly focused on saturated ORC cycles in simple configuration. On the other hand, transcritical cycles and the adoption of internal regeneration appear of great relevance, owing to the possible increase in the system performances9,10. The selection of suitable working fluids is another fundamental key to maximise the system efficiency, especially in biomass applications, where the large part of organic fluids cannot be adopted due to the high operating temperatures and the consequent chemical instability. The present work aims at investigating the energetic performances of biomass-fired Organic Rankine Cycles for micro-scale combined heat and power applications. To this purpose, a comparative analysis of subcritical and supercritical cycles has been performed and different organic fluids have been adopted. 2. Methodology The main components of an Organic Rankine Cycle are a pump system, an evaporator, a turbine/expander, and a condenser (Figure 1a). The organic working fluid is fed by the pump system to the evaporator (1-2 process), where the fluid is preheated (2-3) and then vaporised (3-4). The fluid flows into the turbine where it is expanded to the condensing pressure (5-6) and, finally, it is condensed to saturated liquid (6-1). An internal heat exchanger (IHE) can be used to recover the thermal energy at the turbine outlet (6-7) and preheat the compressed liquid before the evaporation process occurs (2-9) in order to improve the system efficiency. Figure 1b shows the cycle in the T-s diagram for a typical dry organic fluid with saturated conditions at the turbine inlet. A thermodynamic model has been developed to evaluate the performances of biomass-fired Organic Rankine Cycles11-13. The model is integrated with the REFPROP database14 to define the thermodynamic properties of the organic fluid. A steady state condition has been assumed, while pressure drops and heat losses in the system components have been neglected. The performance parameters used in the analysis are the electric and thermal efficiencies, the energy utilisation factor, and the primary energy saving index. The electric efficiency is defined as
P η el = el Qth where Q th is the thermal input of the biomass boiler and Pel is the ORC electric power, defined as follows:
(1)
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a)
b)
Fig. 1. (a) Typical layout and (b) T-s diagram for an Organic Rankine Cycle with internal heat exchange. Saturated cycle. C: Condenser, Ec: Economyser, Ev: Evaporator, T: Turbine, G: Electrical generator, IHE: Internal heat exchanger.
(2)
Pel = ηem Pu where
Pu ηem
is the net power output; takes into account the mechanical and electric losses.
The thermal efficiency ηth and the energy utilisation factor EUF have been evaluated as follows15:
Q η th = cog Qth EUF =
(3)
Pel + Q cog = η el + η th Q
(4)
th
where Q cog is the thermal power from the condensation process used for cogeneration. Furthermore, the primary energy saving index PES is defined as16:
PES = 1 −
Q th Pel
ηel ,ref where
+
Q cog
(5)
ηth ,ref
ηel, ref
is the reference efficiency for the separate electrical power production in a conventional energy system;
ηth, ref
is the reference efficiency of a conventional boiler that should be used to produce Qcog separately.
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3. Operating Conditions For the CHP system cyclohexane, decane, and toluene have been selected as organic working fluid, due to their high operating temperatures, consistent with the requirements of biomass systems17,18. Both subcritical and transcritical cycles with saturated and superheated conditions at the expander inlet have been investigated. Table 1 highlights the critical conditions of the three organic fluids and the operating conditions adopted during the energetic analysis. In particular, the condensation temperature has been set to 100 °C to satisfy the thermal demand5. The maximum evaporation temperature has been defined to avoid the presence of liquid during the expansion phase, while the minimum evaporation temperature has been set to 200 °C. Furthermore, for ORC transcritical cycles, the supercritical pressures have been set to 1.03 pcrit (the critical pressure), as suggested in literature19. Table 1. ORC operating conditions and investigated configurations. Cyclohexane
Decane
Toluene
Critical conditions Critical temperature
[°C]
280.49
344.55
318.6
Critical pressure
[bar]
40.75
21.03
41.26
Saturated cycle Condensation temperature
[°C]
100
100
100
Condensation pressure
[bar]
1.75
0.10
0.74
Evaporation temperature
[°C]
200 - 269
200 - 337
200 - 300
Evaporation pressure
[bar]
13.44 - 35.30
1.87 - 18.96
7.51 - 32.76
Transcritical cycle Condensation temperature
[°C]
100
100
100
Condensation pressure
[bar]
1.75
0.10
0.74
Maximum temperature
[°C]
290 - 400
350 - 400
330 - 400
Maximum pressure*
[bar]
41.97
21.66
42.50
*Supercritical pressure
According to the literature, the ORC expander and pump efficiencies have been imposed equal to 0.70 and 0.60, respectively, the internal heat exchanger efficiency has been set to 0.95 and the global efficiency of the heating process (from biomass to the organic fluid) is 0.85, whereas the temperature of the vapour at the exit of the internal heat exchanger (T7) has been assumed to be 10 °C higher than the condensation temperature5. 4. Results An energetic analysis of the performances of biomass-fired ORC systems for CHP applications has been carried out. Cyclohexane, decane, and toluene have been selected as working fluids due to their high operating temperatures, consistent with the requirements of biomass systems17,18. Specifically, the influence of the main operating conditions on the system behaviour has been investigated. Figure 2 compares the performances of saturated ORC cycles with and without internal regeneration in terms of electric efficiency (a), thermal efficiency (b), primary energy saving index (c), and energy utilisation factor (d). The condensation temperature has been fixed to 100 °C while the evaporation temperature is 200 °C. The plot illustrates the significant influence of the internal regeneration and working fluids on the ORC efficiencies. The higher electric performances are obtained adopting the CHP system with internal heat exchange (IHE) and toluene: the electric efficiency is equal to 12.4% (Figure 2a) and the PES index is 14.0% (Figure 2c). On the other hand, Figure 2b highlights that the configuration with the better electric behaviour offers the lower thermal performances (ηth = 68.5%) whereas the simple cycle with decane guarantees the higher thermal efficiency (70.9%).
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a)
b)
c)
d)
Fig. 2. Influence of the working fluid and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d) EUF value. Saturated cycle – Evaporation temperature Te = 200 °C.
Furthermore, the analysis put in evidence that the energy utilisation factor maintains similar values (~80%) for all the investigated conditions (Figure 2d), with slightly higher values when the internal exchange is absent. The effect of the evaporation temperature on the performances of saturated ORCs is shown in Figure 3. The analysis has been done considering 250 °C and the maximum value Te,max, that depends on the selected fluid (269 °C for cyclohexane, 337 °C for decane, and 300 °C for toluene), according to Table 1. Results highlight a progressive increase in the electric efficiency and PES index with the evaporation temperature according to the literature20,21. At 250 °C the electric effectiveness of the simple cycle is 11.0% for decane, 12.0% for cyclohexane and 13.2% for toluene, whereas the efficiencies range between 12.1% (decane) to 14.6% (toluene) at the maximum evaporation temperatures, The corresponding saving capabilities are between 12.0% and 16.8%. It is interesting to observe that decane offers the lower electric performances nevertheless this fluid is characterised by higher evaporation temperature. A noticeable upsurge in the electric performances is obtained when the internal heat exchange is used and the effect is amplified with the evaporation temperature. In this condition, the largest efficiency (20.1%) and PES value (23.1%) are found with decane at 337 °C. The significant increase in system effectiveness is due to the possibility to exploit efficiently the higher fluid energy content at the expander exit. As already observed, the EUF index presents similar values for all the investigated configurations. Consequently, the higher the electric performances, the lower the thermal efficiency and vice versa.
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a)
b)
c)
d)
Fig. 3 – Influence of the evaporation temperature and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d) EUF value. Saturated cycle.
Moreover, the effect of the transcritical conditions on the performances of the CHP system has been investigated. Results refer to the same condensation temperature (100 °C) adopted for saturated configurations while the maximum pressures have been set equal to p = 1.03 pcrit, as suggested in literature19. A twofold trend in the system behaviour is found depending on the presence of internal regenerator (Figure 4). When the IHE is absent, a progressive decrease in electric effectiveness and PES index with the maximum temperature is found. As an example, moving from 350 °C to 400 °C, the electric efficiency reduction ranges from 2.4 (toluene) to 5.5 (decane) percentage points and the corresponding decrease in energy saving capability are between 2.1% and 4.9%. On the other hand, the increase in the maximum temperature produces a raise in the thermal power useful for cogeneration purpose and, as a consequence, in the thermal efficiency. Specifically, the maximum value is found with decane (72.3%) at 400 °C. A significant increase in the PES index and electric efficiency with the maximum temperature is observed when the internal heat exchange is adopted, due to the possibility to exploit efficiently the higher fluid energy content at the expander exit. The investigation reveals, in fact, that the better electric performances are offered by decane (ηel = 22.7% and PES = 27.7%) and similar values are obtained adopting toluene (ηel = 22.5%, PES = 27.5%) and cyclohexane (ηel = 21.5%, PES = 26.6%).
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As already observed for the saturated configuration, the influence of the operating conditions and working fluids on the energy utilisation factor is negligible, with values that are always higher than 79.5%. It is noteworthy that the triangular shape of the supercritical cycle generates a more efficient heat exchange between the organic fluid and the thermal oil and a decrease in the irreversibility and energy destruction due to the heat transfer and losses.
a)
b)
c)
d)
Fig. 4 – Influence of the maximum temperature and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d), EUF value. Transcritical cycle.
5. Conclusions The energetic performances of Organic Rankine Cycles for micro-scale CHP applications have been investigated. To this purpose, both subcritical and transcritical configurations have been considered, with saturated and superheated conditions at the expander inlet. Furthermore, the influence of the internal regeneration on the ORC performances has been analysed. The investigation highlighted that the operating conditions significantly affects the system behaviour. For saturated configurations, the electric efficiency and the primary energy saving (PES) index increase with the evaporation temperature, and this behaviour is amplified when the internal heat exchange is adopted. Conversely, the results put in evidence that the thermal power useful for cogeneration purposes and, as a consequence, the thermal efficiency
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reduce with the evaporation temperature and the use of the internal regeneration. All the configurations offer similar EUF values (~80%), nevertheless the significant differences in the operating conditions and working fluids. A twofold behaviour in the system performances has been detected for transcritical arrangements. In particular, a negative impact of the maximum temperature on the electric efficiency and PES index is observed for the simple cycle, while the two dimensionless parameters exhibit a progressive raise with the presence of the internal heat exchange. The opposite trend is found for the thermal efficiency, whereas the energy utilisation factor maintains similar values for all the investigated configurations. The comparison between the different micro-ORC configurations reveals that the highest electric efficiencies and primary energy saving indexes are obtained with the transcritical cycles. Specifically the investigation suggests the adoption of decane (ηel = 22.7% and PES = 25.8%) or toluene (ηel = 22.5% and PES = 25.6%). When the purpose is to maximise the thermal performances of the CHP units, the saturated cycles in simple configuration have to be preferred adopting the lower evaporation temperature. In these conditions similar results are obtained for the three fluids (ηth = 70.1%, 70.9, and 69.4 for cyclohexane, decane, and toluene, respectively). Acknowledgements The present work has been developed within the framework of the “Scienza” Project, supported by Consorzio NET (Nature, Energy and Land) - PON-FESR 2007/2013 of Calabria Region. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Dong L, Liu H, Riffat S. Development of small-scale and micro-scale biomass-fuelled CHP systems - A literature review. Applied Thermal Engineering 2009; 29: 2119-2126. Ren H, Gao W, Ruan Y. Optimal sizing for residential CHP system. Applied Thermal Engineering 2008; 28: 514-523. Liu H, Shao Y, Li J. A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) - Thermodynamic modelling studies. Biomass and Bioenergy 2011; 35: 3985-3994. Preißinger M, Heberle F, Brüggemann D. Thermodynamic analysis of double-stage biomass fired Organic Rankine Cycle for microcogeneration. International Journal of Energy Research 2012; 36: 944-952. Drescher U, Bruggemann D. Fluid selection for the Organic Rankine Cycle (ORC) in biomass power and heat plants. Applied Thermal Engineering 2007; 27: 223-228. Schuster A, Karellas S, Kakaras E, Spliethoff H. Energetic and economic investigation of Organic Rankine Cycle applications. Applied Thermal Engineering 2009; 29: 1809-1817. Qiu G. Selection of working fluids for micro-CHP systems with ORC. Renewable Energy 2012; 48: 565-570. Qiu G, Shao Y, Li J, Liu H, Riffat SB. Experimental investigation of a biomass-fired ORC-based micro-CHP for domestic applications. Fuel 2012; 9: 374-382. Mikielewicz D, Mikielewicz JA. Thermodynamic criterion for selection of working fluid for subcritical and supercritical domestic micro CHP. Applied Thermal Engineering 2010; 30: 2357-2362. Shengjun Z, Huaixin W, Tao G. Performance comparison and parametric optimization of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation. Applied Energy 2011; 88: 2740-2754. Algieri A, Morrone P. Comparative energetic analysis of high-temperature subcritical and transcritical Organic Rankine Cycle (ORC). A biomass application in the Sibari district. Applied Thermal Engineering 2012; 36: 236-244. Algieri A, Morrone P. Techno-economic analysis of biomass-fired ORC systems for single-family combined heat and power (CHP) applications. Energy Procedia 2014; 45: 1285-1294. Algieri A, Morrone P. Energetic analysis of biomass-fired ORC systems for micro-scale combined heat and power (CHP) generation. A possible application to the Italian residential sector. Applied Thermal Engineering 2014; 71: 751-759. Lemmon EW, Huber ML, McLinden MO. REFPROP Reference Fluid Thermodynamic and Transport, NIST Online Databases, 2008. Sanaye S, Meybodi MA, Shokrollahi S. Selecting the prime movers and nominal powers in combined heat and power systems. Applied Thermal Engineering 2008; 28: 1177-1188. AEEG, Italian Authority of Energy and Gas. Available from: http://www.autorita.energia.it/it/index.htm, 2015. Chacartegui R, Sánchez D, Muñoz JM, Sánchez T. Alternative ORC bottoming cycles for combined cycle power plants. Applied Energy 2009; 86: 2162-2170. Wiśniewski S, Borsukiewicz-Gozdur A. The influence of vapor superheating on the level of heat regeneration in a subcritical ORC coupled with gas power plant. Archives of Thermodynamics 2010; 31: 185-199. Schuster A, Karellas S, Aumann R. Efficiency optimization potential in supercritical Organic Rankine Cycles. Energy 35 (2010) 1033-1039. Algieri A, Morrone P. Energy analysis of Organic Rankine cycle for biomass applications. Thermal Science 2015; 19: 193-205. Borsukiewicz-Gozdur A, Nowak W. Comparative analysis of natural and synthetic refrigerants in application to low temperature Clausius– Rankine cycle. Energy 2007; 32: 344-352.
ScienceDirect Procedia Computer Science 83 (2016) 855 – 862
The 6th International Conference on Sustainable Energy Information Technology (SEIT 2016)
Comparative Investigation of the Performances of Subcritical and Transcritical Biomass-Fired ORC Systems for Micro-Scale CHP Applications Angelo Algieri* Department of Mechanical, Energy and Management Engineering, University of Calabria Via P. Bucci Cubo 46C, 87036 Arcavacata di Rende, Italy
Abstract The paper aims to analyse the energetic performances of Organic Rankine Cycles (ORCs) for biomass micro-scale CHP applications. Subcritical and transcritical cycles have been compared and the influence of internal regeneration on system behaviour has been evaluated. The investigation illustrates the noticeable influence of the operating conditions and ORC configuration on the main CHP performances. Furthermore, results reveal that the proper choice of the organic working fluid is fundamental to assure proper operations and optimise the system behaviour. The analysis shows that the transcritical cycle with internal regeneration offers the maximum electric performances, whereas the saturated cycle in simple configuration guarantees the highest thermal efficiency. Furthermore, all the investigated systems guarantee positive energy saving capabilities. 2016The TheAuthors. Authors.Published Published Elsevier © 2016 byby Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Conference Program Chairs. Peer-review under responsibility of the Conference Program Chairs Keywords: Organic Rankine cycle; CHP; biomass; transcritical cycle; regeneration; micro-scale.
1. Introduction Nowadays, combined heat and power production (CHP) represents an effective alternative to traditional systems that are characterised by separate electric and thermal generation. Cogenerative systems, in fact, are able to increase
* Corresponding author. Tel.: +39-0984-494665; fax: +39-0984-494673. E-mail address: [email protected]
1877-0509 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Conference Program Chairs doi:10.1016/j.procs.2016.04.176
856
Angelo Algieri / Procedia Computer Science 83 (2016) 855 – 862
Nomenclature EUF h η P PES
Q th
T
energy utilisation factor specific enthalpy efficiency power primary energy saving index biomass thermal power temperature
energy saving capabilities and reduce pollutant emissions, compared with traditional units1,2. Specifically, Organic Rankine Cycles (ORCs) appear an interesting option for sustainable and reliable energy supply in micro-scale CHP applications (Pel < 50 kWel), where conventional apparatus are also economically and technologically unfeasible1,3,4. ORC units offer several advantages owing to good partial load performances, fast start-up and stop procedures, low maintenance requirements, and high flexibility and safety5,6. The selection of the most appropriate configurations and operating conditions is fundamental for the optimisation of ORC performances7,8. It is noteworthy to notice that the attention of researchers community and manufacturers has been mainly focused on saturated ORC cycles in simple configuration. On the other hand, transcritical cycles and the adoption of internal regeneration appear of great relevance, owing to the possible increase in the system performances9,10. The selection of suitable working fluids is another fundamental key to maximise the system efficiency, especially in biomass applications, where the large part of organic fluids cannot be adopted due to the high operating temperatures and the consequent chemical instability. The present work aims at investigating the energetic performances of biomass-fired Organic Rankine Cycles for micro-scale combined heat and power applications. To this purpose, a comparative analysis of subcritical and supercritical cycles has been performed and different organic fluids have been adopted. 2. Methodology The main components of an Organic Rankine Cycle are a pump system, an evaporator, a turbine/expander, and a condenser (Figure 1a). The organic working fluid is fed by the pump system to the evaporator (1-2 process), where the fluid is preheated (2-3) and then vaporised (3-4). The fluid flows into the turbine where it is expanded to the condensing pressure (5-6) and, finally, it is condensed to saturated liquid (6-1). An internal heat exchanger (IHE) can be used to recover the thermal energy at the turbine outlet (6-7) and preheat the compressed liquid before the evaporation process occurs (2-9) in order to improve the system efficiency. Figure 1b shows the cycle in the T-s diagram for a typical dry organic fluid with saturated conditions at the turbine inlet. A thermodynamic model has been developed to evaluate the performances of biomass-fired Organic Rankine Cycles11-13. The model is integrated with the REFPROP database14 to define the thermodynamic properties of the organic fluid. A steady state condition has been assumed, while pressure drops and heat losses in the system components have been neglected. The performance parameters used in the analysis are the electric and thermal efficiencies, the energy utilisation factor, and the primary energy saving index. The electric efficiency is defined as
P η el = el Qth where Q th is the thermal input of the biomass boiler and Pel is the ORC electric power, defined as follows:
(1)
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a)
b)
Fig. 1. (a) Typical layout and (b) T-s diagram for an Organic Rankine Cycle with internal heat exchange. Saturated cycle. C: Condenser, Ec: Economyser, Ev: Evaporator, T: Turbine, G: Electrical generator, IHE: Internal heat exchanger.
(2)
Pel = ηem Pu where
Pu ηem
is the net power output; takes into account the mechanical and electric losses.
The thermal efficiency ηth and the energy utilisation factor EUF have been evaluated as follows15:
Q η th = cog Qth EUF =
(3)
Pel + Q cog = η el + η th Q
(4)
th
where Q cog is the thermal power from the condensation process used for cogeneration. Furthermore, the primary energy saving index PES is defined as16:
PES = 1 −
Q th Pel
ηel ,ref where
+
Q cog
(5)
ηth ,ref
ηel, ref
is the reference efficiency for the separate electrical power production in a conventional energy system;
ηth, ref
is the reference efficiency of a conventional boiler that should be used to produce Qcog separately.
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3. Operating Conditions For the CHP system cyclohexane, decane, and toluene have been selected as organic working fluid, due to their high operating temperatures, consistent with the requirements of biomass systems17,18. Both subcritical and transcritical cycles with saturated and superheated conditions at the expander inlet have been investigated. Table 1 highlights the critical conditions of the three organic fluids and the operating conditions adopted during the energetic analysis. In particular, the condensation temperature has been set to 100 °C to satisfy the thermal demand5. The maximum evaporation temperature has been defined to avoid the presence of liquid during the expansion phase, while the minimum evaporation temperature has been set to 200 °C. Furthermore, for ORC transcritical cycles, the supercritical pressures have been set to 1.03 pcrit (the critical pressure), as suggested in literature19. Table 1. ORC operating conditions and investigated configurations. Cyclohexane
Decane
Toluene
Critical conditions Critical temperature
[°C]
280.49
344.55
318.6
Critical pressure
[bar]
40.75
21.03
41.26
Saturated cycle Condensation temperature
[°C]
100
100
100
Condensation pressure
[bar]
1.75
0.10
0.74
Evaporation temperature
[°C]
200 - 269
200 - 337
200 - 300
Evaporation pressure
[bar]
13.44 - 35.30
1.87 - 18.96
7.51 - 32.76
Transcritical cycle Condensation temperature
[°C]
100
100
100
Condensation pressure
[bar]
1.75
0.10
0.74
Maximum temperature
[°C]
290 - 400
350 - 400
330 - 400
Maximum pressure*
[bar]
41.97
21.66
42.50
*Supercritical pressure
According to the literature, the ORC expander and pump efficiencies have been imposed equal to 0.70 and 0.60, respectively, the internal heat exchanger efficiency has been set to 0.95 and the global efficiency of the heating process (from biomass to the organic fluid) is 0.85, whereas the temperature of the vapour at the exit of the internal heat exchanger (T7) has been assumed to be 10 °C higher than the condensation temperature5. 4. Results An energetic analysis of the performances of biomass-fired ORC systems for CHP applications has been carried out. Cyclohexane, decane, and toluene have been selected as working fluids due to their high operating temperatures, consistent with the requirements of biomass systems17,18. Specifically, the influence of the main operating conditions on the system behaviour has been investigated. Figure 2 compares the performances of saturated ORC cycles with and without internal regeneration in terms of electric efficiency (a), thermal efficiency (b), primary energy saving index (c), and energy utilisation factor (d). The condensation temperature has been fixed to 100 °C while the evaporation temperature is 200 °C. The plot illustrates the significant influence of the internal regeneration and working fluids on the ORC efficiencies. The higher electric performances are obtained adopting the CHP system with internal heat exchange (IHE) and toluene: the electric efficiency is equal to 12.4% (Figure 2a) and the PES index is 14.0% (Figure 2c). On the other hand, Figure 2b highlights that the configuration with the better electric behaviour offers the lower thermal performances (ηth = 68.5%) whereas the simple cycle with decane guarantees the higher thermal efficiency (70.9%).
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a)
b)
c)
d)
Fig. 2. Influence of the working fluid and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d) EUF value. Saturated cycle – Evaporation temperature Te = 200 °C.
Furthermore, the analysis put in evidence that the energy utilisation factor maintains similar values (~80%) for all the investigated conditions (Figure 2d), with slightly higher values when the internal exchange is absent. The effect of the evaporation temperature on the performances of saturated ORCs is shown in Figure 3. The analysis has been done considering 250 °C and the maximum value Te,max, that depends on the selected fluid (269 °C for cyclohexane, 337 °C for decane, and 300 °C for toluene), according to Table 1. Results highlight a progressive increase in the electric efficiency and PES index with the evaporation temperature according to the literature20,21. At 250 °C the electric effectiveness of the simple cycle is 11.0% for decane, 12.0% for cyclohexane and 13.2% for toluene, whereas the efficiencies range between 12.1% (decane) to 14.6% (toluene) at the maximum evaporation temperatures, The corresponding saving capabilities are between 12.0% and 16.8%. It is interesting to observe that decane offers the lower electric performances nevertheless this fluid is characterised by higher evaporation temperature. A noticeable upsurge in the electric performances is obtained when the internal heat exchange is used and the effect is amplified with the evaporation temperature. In this condition, the largest efficiency (20.1%) and PES value (23.1%) are found with decane at 337 °C. The significant increase in system effectiveness is due to the possibility to exploit efficiently the higher fluid energy content at the expander exit. As already observed, the EUF index presents similar values for all the investigated configurations. Consequently, the higher the electric performances, the lower the thermal efficiency and vice versa.
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a)
b)
c)
d)
Fig. 3 – Influence of the evaporation temperature and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d) EUF value. Saturated cycle.
Moreover, the effect of the transcritical conditions on the performances of the CHP system has been investigated. Results refer to the same condensation temperature (100 °C) adopted for saturated configurations while the maximum pressures have been set equal to p = 1.03 pcrit, as suggested in literature19. A twofold trend in the system behaviour is found depending on the presence of internal regenerator (Figure 4). When the IHE is absent, a progressive decrease in electric effectiveness and PES index with the maximum temperature is found. As an example, moving from 350 °C to 400 °C, the electric efficiency reduction ranges from 2.4 (toluene) to 5.5 (decane) percentage points and the corresponding decrease in energy saving capability are between 2.1% and 4.9%. On the other hand, the increase in the maximum temperature produces a raise in the thermal power useful for cogeneration purpose and, as a consequence, in the thermal efficiency. Specifically, the maximum value is found with decane (72.3%) at 400 °C. A significant increase in the PES index and electric efficiency with the maximum temperature is observed when the internal heat exchange is adopted, due to the possibility to exploit efficiently the higher fluid energy content at the expander exit. The investigation reveals, in fact, that the better electric performances are offered by decane (ηel = 22.7% and PES = 27.7%) and similar values are obtained adopting toluene (ηel = 22.5%, PES = 27.5%) and cyclohexane (ηel = 21.5%, PES = 26.6%).
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As already observed for the saturated configuration, the influence of the operating conditions and working fluids on the energy utilisation factor is negligible, with values that are always higher than 79.5%. It is noteworthy that the triangular shape of the supercritical cycle generates a more efficient heat exchange between the organic fluid and the thermal oil and a decrease in the irreversibility and energy destruction due to the heat transfer and losses.
a)
b)
c)
d)
Fig. 4 – Influence of the maximum temperature and ORC configuration on the (a) electric efficiency, (b) thermal efficiency, (c) PES index, and (d), EUF value. Transcritical cycle.
5. Conclusions The energetic performances of Organic Rankine Cycles for micro-scale CHP applications have been investigated. To this purpose, both subcritical and transcritical configurations have been considered, with saturated and superheated conditions at the expander inlet. Furthermore, the influence of the internal regeneration on the ORC performances has been analysed. The investigation highlighted that the operating conditions significantly affects the system behaviour. For saturated configurations, the electric efficiency and the primary energy saving (PES) index increase with the evaporation temperature, and this behaviour is amplified when the internal heat exchange is adopted. Conversely, the results put in evidence that the thermal power useful for cogeneration purposes and, as a consequence, the thermal efficiency
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reduce with the evaporation temperature and the use of the internal regeneration. All the configurations offer similar EUF values (~80%), nevertheless the significant differences in the operating conditions and working fluids. A twofold behaviour in the system performances has been detected for transcritical arrangements. In particular, a negative impact of the maximum temperature on the electric efficiency and PES index is observed for the simple cycle, while the two dimensionless parameters exhibit a progressive raise with the presence of the internal heat exchange. The opposite trend is found for the thermal efficiency, whereas the energy utilisation factor maintains similar values for all the investigated configurations. The comparison between the different micro-ORC configurations reveals that the highest electric efficiencies and primary energy saving indexes are obtained with the transcritical cycles. Specifically the investigation suggests the adoption of decane (ηel = 22.7% and PES = 25.8%) or toluene (ηel = 22.5% and PES = 25.6%). When the purpose is to maximise the thermal performances of the CHP units, the saturated cycles in simple configuration have to be preferred adopting the lower evaporation temperature. In these conditions similar results are obtained for the three fluids (ηth = 70.1%, 70.9, and 69.4 for cyclohexane, decane, and toluene, respectively). Acknowledgements The present work has been developed within the framework of the “Scienza” Project, supported by Consorzio NET (Nature, Energy and Land) - PON-FESR 2007/2013 of Calabria Region. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
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