Elsevier

Energy Conversion and Management

Volume 159, 1 March 2018, Pages 353-370
Energy Conversion and Management

Energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations

https://doi.org/10.1016/j.enconman.2017.12.093Get rights and content

Highlights

  • Systematic optimization and comparison of regenerative and recuperative ORCs.

  • Two novel configurations have been examined with closed-type regenerative preheaters.

  • Highest energetic efficiency by CF-ORC, followed by O-ORC and CB-ORC.

  • Recuperative standard ORCs perform better than non-recuperative regenerative ORCs.

Abstract

The present study focuses on the energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations. More specifically, three regenerative ORCs are examined. The first includes an open preheater, in which the bleed stream is mixed with the working fluid exiting the pump of the cycle (O-ORC). The other two configurations include a closed preheater. In the second configuration, the bleed stream is throttled and conveyed to the condenser (CB-ORC), while in the third one, it is repressurized via a secondary pump and recirculates into the evaporator of the cycle (CF-ORC). The systems are optimized for different working fluids, and their energetic efficiencies are estimated and compared to that of a standard ORC (S-ORC). In all cases, the inclusion of a recuperator has also been investigated. In principle, recuperative and regenerative ORCs are mostly suitable for dry fluids, while the critical temperature can also have a positive influence on the performance improvement. Furthermore, it is estimated that while the recuperative S-ORC has a higher efficiency than the non-recuperative regenerative cycles, recuperative O-ORC and CF-ORC exhibit a relative efficiency gain ranging from 4.98% to 8.05% and 6.22% to 9.29%, respectively. The highest efficiency improvement achieved by the CB-ORC, however, is minimal.

Introduction

The Organic Rankine Cycle (ORC) is one of the commonly considered technologies aimed at the production of electricity from low grade heat, which can be usually derived from biomass fuels [1], [2], [3], [4], solar radiation [3], [5], [6], geothermal energy [7], [8], [9] and industrial waste streams [10], [11], [12]). Its advantages mostly include smaller equipment component sizes, high modularity and simplicity of construction and operation [13], [14].

Most typical ORCs are based on the implementation of a simple Rankine cycle, which includes four basic steps: the saturated liquid refrigerant is firstly pressurized with a pump and then superheated by the heat source. It is subsequently expanded to produce work and then condensed to return to its original state and repeat the cycle. One of the main drawbacks of the standard ORC is its relatively low efficiency, which can be for the most part attributed to the operation under lower temperatures, which usually range from 80 to 300 °C.

As such, several alternative system architectures have been proposed as enhancements to the standard cycle. A review of the various different configurations was carried out by Lecompte et al. [15]. Some of the efficiency improvement designs aim at optimizing the temperature profile matching between the external heat source of the system and the working fluid, thus minimizing the exergy destruction in the evaporator and increasing the overall exergetic efficiency. On the other hand, other designs target at increasing the inherent energetic (first law) efficiency of the cycle.

Among the most commonly investigated options that fall onto the second category is the addition of a recuperator that is used for utilizing the heat of the superheated vapor at the expander outlet to preheat the working fluid before it enters the evaporator. In this way, the external heat input to the cycle is reduced, while the power output is maintained at the same level, resulting in an increase of the energetic efficiency. The implementation of a recuperator is more favourable for dry working fluids, for which there is more sensible heat to recover [16], [17], [18], [19], [20]. Nonetheless, recuperative ORCs are not recommended when there is no limitation on the evaporator outlet temperature of the heat source stream. If this is the case, using a recuperator may lead to reduction of the heat absorbed by the system and thus to a drop of the power output and the exergetic efficiency, since the heat source stream may exit the evaporator at a higher temperature [16], [17].

Another concept that has been proposed is the double stage ORC. The double stage ORC (DS ORC), often called double evaporation or dual loop ORC, operates at two evaporation temperatures. Usually, the first stage (topping ORC) operates at high temperature (HT) and is positioned before the low temperature (LT) stage (bottoming ORC). After exiting the HT evaporator, the heat source stream successively enters the evaporator of the LT stage. One of the earliest studies on multi stage ORCs was carried out by Gnutek and Bryszewska-Mazurek [21], who simulated a system operating with R123 and considered two and four serial heat exchangers for recovering heat with fixed temperature drops in each one. A multi-segment sliding vane expander was considered, consisting of multiple expansion machines attached to a common drive shaft. The authors estimated that the four level ORC exhibited the highest power output and exergetic efficiency. In another study, Ayachi et al. [22] investigated the exergetic optimization of single and multiple stage ORCs for waste heat recovery from almost dry and moist heat source streams. The authors examined different combinations of working fluids, such as R1234yf, R245fa (topping cycle) and R245fa, R125, R41 and CO2 (bottoming cycle) and estimated that the addition of a LT bottoming cycle for recovering the heat during the condensing process offers an efficiency increase potential of about 33%. Shokati et al. [9] performed an exergoeconomic optimization of standard, double stage as well as dual fluid (DF) ORC and Kalina power plants for geothermal applications. They concluded that the double stage ORC reached the maximum value of power output (15.22%, 35.09% and 43.48% higher than the power output of the standard, DF ORC and Kalina cycle respectively). Nevertheless, the Kalina cycle was found to be the optimal in terms of thermoeconomics, since it was associated with the minimum specific electricity generation cost. Zhang et al. [23] carried out a performance analysis of a small scale DS ORC to recover waste heat from a light-duty diesel engine and pointed out that the dual stage ORC is a promising technology for waste heat recovery from vehicular diesel engines. In addition to this, a double stage ORC for high temperature small scale waste heat recovery was investigated by Chengyu et al. [24], who compared the DS ORC to a Brayton cycle and a thermoelectric generator, each being coupled to a bottoming ORC. The authors concluded that the DS ORC was superior to the other technologies in terms of energy utilization. Mosaffa et al. [25] conducted a thermoeconomic analysis of different ORC configurations applied for geothermal heat and liquefied natural gas (LNG) cold energy utilization. He compared the performance of double stage ORCs operating with different working fluids to that of a standard, recuperative and regenerative ORC. The ORC systems operated along with a gas turbine, powered by natural gas. The authors concluded that although the recuperative cycle exhibited the best economic performance, the DS ORC system produced the highest power output. In-Hwan Choi et al. [26] also investigated the implementation of multiple stage (including two and three stages) ORCs for LNG energy utilization. The authors compared these cycles with conventional ones from a thermodynamic and an economic standpoint and concluded that the multi stage systems are very promising despite the increased costs. Soffiato et al. [27] focused on the optimization of ORC cycles for waste heat recovery on board an LNG carrier. The authors investigated the performance of a standard, a regenerative and a two-stage ORC. According to their findings, the highest power output was exhibited by the two-stage configuration, being almost two times higher than that of the standard and the regenerative systems. Yang et al. [28] investigated a dual loop ORC for diesel engine waste heat recovery. It was estimated that the maximum efficiency of the system reached 5.4%, with a maximum net power output of 27.85 kW. The overall efficiency increase of the system was estimated equal to 13%. Sciubba et al. [29] compared the performance of a double stage ORCs for marine engine waste heat recovery, while also exploring the benefits that can occur by the addition of a recuperator in the low temperature stage. The results indicated that the addition of the second stage led to an increase of the electricity production of up to 8.11% and 2.67% in small and large scale systems, respectively. Despite resulting in increasing the thermal efficiency of the ORC, the recuperator did not improve the waste heat recovery efficiency, since it was associated with higher exhaust gas temperatures at the system outlet.

Meanwhile, another efficiency improvement option is the application of regenerative ORCs. Similarly to the case of recuperative ORCs, in regenerative cycles the working fluid is preheated before entering the evaporator. However, in contrast to recuperative ORCs, the heat is derived from bleed vapor that is extracted at an intermediate expansion stage. In the most commonly investigated variation of the regenerative ORC, the preheating occurs in an open-type heat exchanger, in which the subcooled liquid and vapor streams are mixed. So far, a rather limited number of works have focused on regenerative ORCs. Mosaffa et al. [30] investigated regenerative ORCs among other configurations for geothermal energy utilization. The authors concluded that the highest energetic and exergetic efficiencies are obtained for regenerative and recuperative cycles, respectively. Mehrpooya et al. [31] performed an exergoeconomic analysis and optimization of a regenerative solar ORC combined with a recuperator. The system was also coupled with an LNG tank in order to attain very low condensation temperatures and pressures. Mago et al. [32] compared the energetic and exergetic performance an open-type regenerative ORC with that of the standard cycle. He concluded that the regenerative ORC can lead to improved energetic and exergetic performance while pointing out the positive correlation between the boiling point temperature of the working fluids and the thermal efficiency that can be achieved by the system. In another study, Meinel et al. [20] also compared an open-type regenerative ORC with a standard and a recuperative ORC under different boundary conditions, corresponding to a constant heat source outlet temperature and a constant pinch point in the evaporator. The authors reported that the thermal efficiency for regenerative ORCs is maximized at intermediate bleed pressures. An overall efficiency improvement in the range of 1–3% compared to the standard cycle was calculated for different types of fluids. Desai and Bandyopadhyay [33] also examined an open-type regenerative ORC combined with a recuperator. They reported that the combination of a recuperator with a regenerative preheater had a better performance than simple regenerative or recuperative cycles, leading to an average energetic efficiency improvement by 16.5%, reaching a maximum of 34.3% for n-perfluropentane compared to the standard ORC. Gang et al. [34] investigated an open-type regenerative ORC with a recuperator, coupled with solar collectors and PCMs. Similarly to other studies, the authors observed that there is an intermediate optimal regenerative pressure/temperature which maximizes the efficiency of the ORC. The authors accordingly reported an efficiency improvement by 9.2% compared to the standard cycle. Yari [35] performed an exergetic evaluation of combined open-type regenerative and recuperative ORCs, among other configurations. He concluded that the maximum exergetic efficiency was exhibited by the recuperative ORC using R123 and was calculated equal to 7.65%. As far as the regenerative recuperative configuration is considered, the author concluded that the increased exergy losses of the geothermal fluid stream, which leaves the system at a higher temperature, offset the decrease of the exergy destruction in the evaporator and the condenser. Rashidi et al. [36] researched a more complex system consisting of two open regenerative preheaters and three turbines, implementing the artificial bees colony and artificial neural network optimization methods. In accordance with other studies, the authors found that there are interdependent optimal intermediate bleed pressures that optimize the energetic efficiency. Xi et al. [37] performed a parametric optimization of standard and open-type, single and double stage regenerative ORCs. The integration of a recuperator was not considered. The authors found that the double stage regenerative ORC exhibited the highest exergetic efficiency, equal to 56.87% compared to 55.01% (standard ORC) and 50.61% (single stage regenerative ORC). They also estimated that the regenerative ORCs had higher energetic efficiencies. Liu et al. [38] focused on the thermo-economic optimization of different ORC system configurations for low temperature geothermal plants. The examined configurations included among others a recuperative cycle and a regenerative cycle with an open-type regenerative preheater. Interestingly, the authors concluded that despite the slightly higher energetic performance of recuperative and regenerative systems, their higher capital costs inhibited their economic competitiveness and suggested that the standard cycles are more cost-efficient. Imran et al. [39] carried out a thermoeconomic optimization of different regenerative ORCs aimed at waste heat recovery. The regenerative cycles included a single and a double stage open-type regenerative cycle. The optimization variables were the evaporation pressure, the superheating degree, the pinch point values in the evaporator and the condenser as well as the bleed fraction (i.e. the percentage of the mass flow of the total vapor at the expander inlet that is extracted). The authors concluded that the average thermal efficiency increase of the single and the double stage regenerative cycles was equal to 1.01% and 1.45%, respectively. However, an additional specific investment cost of 187 $/kW and 297 $/kW was estimated for these two cases. Furthermore, the authors reported that increasing the superheat degree bears no significant efficiency gains, while it is associated with high costs. On the other hand, they found that increasing the pinch point led to a slight increase of the specific investment cost and a drop of the thermal efficiency and that the evaporation pressure exhibits the highest influence on both of these parameters. Zare [40] applied an exergoeconomic analysis to compare the performance of different regenerative ORCs for binary geothermal power plants. In total, three configurations were examined by the author; a standard ORC, a recuperative ORC, as well as an ORC with an open-type regenerative preheater. Similarly to other thermoeconomic studies, costing correlations were used for the estimation of the costs of all individual components and the total installation. From the optimization results, the author concluded that the highest power output and the lowest total product cost, total capital investment cost and shortest payback period was exhibited by the standard ORC. Meanwhile, the highest energetic and exergetic efficiency was observed for the recuperative ORC. Safarian and Ramoun [41] performed assessments of a standard and a regenerative ORC (with open-type preheater) while also examining the inclusion of a recuperator in each case. However, the design of the cycles was not optimized and the calculation of their performance was carried out only for one case. The authors concluded that the regenerative-recuperative ORC had the best energetic efficiency, equal to 22.8% and 35.5%, respectively. Bina et al. [42] evaluated various ORC configurations, including a standard and a recuperative ORC, along with a regenerative cycle including an open-type preheater and a double stage system. The authors considered the utilization of the geothermal outlet of the Sabalan flash cycle plant, located in Iran. Five different criteria were used for the optimization of the systems; the energetic efficiency, the exergetic efficiency, the net power output, the production cost and the total cost. In respect to the energetic and exergetic efficiency, the recuperative ORC had the best performance. However, when considering the energy production cost and the total energy cost, the regenerative and the standard ORC were the optimal cycles. Lastly, based on the results of a multi-criteria analysis undertaken by using the method of Shannon’s Entropy, the recuperative ORC was the best system, followed by the standard, the regenerative and the double stage system.

Despite the large number of studies on the energetic analysis of ORCs, so far most have focused on either standard or recuperative systems, while relatively few have investigated the potential of regenerative configurations. Furthermore, even considering the limited studies on regenerative systems, in most cases these focus on one configuration, including an open-type preheater, while alternative systems have not been researched. Additionally, the performance of combined regenerative and recuperative systems has not been sufficiently examined, while the energetic optimization aspects of these architectures regarding key design variables has not been explored.

The present study aims to provide a systematic comparison of various regenerative cycles (non-recuperative and recuperative) in respect of their energetic efficiency. Apart from the regenerative ORCs including an open-type preheater (O-ORC), two additional configurations are investigated, which, to the best knowledge of the authors, have not been previously examined. The first one is the closed-type preheater regenerative ORC with backwards bleed condensate circulation (CB-ORC). In this configuration, the bleed stream exiting the preheater is throttled until its pressure matches the condensation pressure of the cycle and then flows into the condenser. Although a portion of its heat content is not utilized, this system does not require the application of a secondary pump, with potential economic benefits. The second one is the closed-type preheater regenerative ORC with forward bleed condensate circulation (CF-ORC). In this system, the bleed stream is pressurized after exiting the regenerative preheater and recirculates into the evaporator. This system is similar to the O-ORC. However, it enables the combined variation of the bleed pressure and the bleed fraction, adding one additional degree of freedom for optimizing its overall performance, as will be subsequently presented. In all cases, the regenerative ORCs are simulated with and without recuperators and compared to the standard ORC. As a result, a total of 8 configurations are simulated and evaluated. Furthermore, an exhaustive search optimization methodology has been applied taking into account the critical variables of each cycle to maximize the energetic efficiency. The optimization results are accompanied by parametric analyses in order to explore the relation between the optimization variables and their impact on the system performance. The ultimate goal of the present work is the systematic analysis and evaluation of the different variations of non-recuperative and recuperative regenerative ORC, the comparison of their performance and an examination of their optimization principles.

Section snippets

Modelling

For modelling the systems, steady state simulation is assumed, while pressure drops and heat losses through pipes and other equipment components are considered negligible. CoolProp software [43] is used for the calculation of the thermophysical properties of the working fluids.

Overview

In Fig. 7, the maximum energetic efficiency for each optimized configuration is presented. The values corresponding to the data of Fig. 7 are also summarized in Table 7.

By comparing the performance of the different configurations, it can be seen that the CF-ORC exhibits the highest efficiency for all fluids. The second highest performance is exhibited by the O-ORC, followed by that of the CB-ORC configuration. In all cases, the S-ORC has the lowest efficiency.

Τhe relative efficiency difference

Conclusions

In the present study, three regenerative ORC configurations, the O-ORC, the CB-ORC and the CF-ORC, were investigated, optimized and compared to the standard cycle, with and without the inclusion of a recuperator. The highest efficiency improvement was achieved by the CF-ORC, followed by the O-ORC and lastly the CB-ORC. The maximum efficiencies for the non-recuperative systems were equal to 18.02% (S-ORC), 20.53% (O-ORC), 19.94% (CB-ORC) and 20.90% (CF-ORC). The maximum efficiencies for the

References (49)

  • B.F. Tchanche et al.

    Low-grade heat conversion into power using organic Rankine cycles – a review of various applications

    Renew Sustain Energy Rev

    (2011)
  • S. Lecompte et al.

    Review of organic Rankine cycle (ORC) architectures for waste heat recovery

    Renew Sustain Energy Rev

    (2015)
  • S. Lecompte et al.

    Review of organic Rankine cycle (ORC) architectures for waste heat recovery

    Renew Sustain Energy Rev

    (2015)
  • D. Maraver et al.

    Systematic optimization of subcritical and transcritical organic Rankine cycles (ORCs) constrained by technical parameters in multiple applications

    Appl Energy

    (2014)
  • I.H. Aljundi

    Effect of dry hydrocarbons and critical point temperature on the efficiencies of organic Rankine cycle

    Renewable Energy

    (2011)
  • T.C. Hung et al.

    A study of organic working fluids on system efficiency of an ORC using low-grade energy sources

    Energy

    (2010)
  • D. Meinel et al.

    Effect and comparison of different working fluids on a two-stage organic rankine cycle (ORC) concept

    Appl Therm Eng

    (2014)
  • Z. Gnutek et al.

    The thermodynamic analysis of multicycle ORC engine

    Energy

    (2001)
  • F. Ayachi et al.

    ORC optimization for medium grade heat recovery

    Energy

    (2014)
  • H.G. Zhang et al.

    A performance analysis of a novel system of a dual loop bottoming organic Rankine cycle (ORC) with a light-duty diesel engine

    Appl Energy

    (2013)
  • C. Zhang et al.

    Comparative study of alternative ORC-based combined power systems to exploit high temperature waste heat

    Energy Convers Manage

    (2015)
  • A.H. Mosaffa et al.

    Thermo-economic analysis of combined different ORCs geothermal power plants and LNG cold energy

    Geothermics

    (2017)
  • I.-H. Choi et al.

    Analysis and optimization of cascade Rankine cycle for liquefied natural gas cold energy recovery

    Energy

    (2013)
  • M. Soffiato et al.

    Design optimization of ORC systems for waste heat recovery on board a LNG carrier

    Energy Convers Manage

    (2015)
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