Dual-pressure Organic Rankine Cycle Research Articles

Parametric optimization and thermodynamic performance comparison of single-pressure and dual-pressure evaporation organic Rankine cycles

Dual-pressure evaporation organic Rankine cycle (ORC) involves two evaporation processes with different pressures, and can significantly reduce the exergy loss in the heat absorption process compared with conventional single-pressure evaporation ORCs. However, the applicable heat source temperatures of dual-pressure evaporation ORCs and the effects of the working fluid thermophysical properties on the applicable conditions remain indeterminate. Optimal cycle parameters for various heat source temperatures also need to be studied. Solving these questions is crucial for the application and promotion of dual-pressure evaporation ORCs. This study focuses on a typical dual-pressure evaporation ORC driven by the 100–200 °C heat sources without a limit on the outlet temperature. Nine pure organic fluids were selected as working fluids. Evaporation pressures and evaporator outlet temperatures of the single-pressure and dual-pressure evaporation ORCs were optimized, and their optimized system thermodynamic performance was compared. Results show that the applicable heat source temperature range of the dual-pressure evaporation ORC (Wnet,dual>Wnet,single) generally increases as the working fluid critical temperature increases. The upper limit of the applicable heat source temperatures (THS,in_TP), working fluid critical temperature and pinch point temperature difference generally conform to a linear relation. For the heat source temperature below THS,in_TP, the maximized net power output of the dual-pressure evaporation ORC is larger than that of the single-pressure evaporation ORC. Furthermore, the increment generally increases as the heat source temperature decreases, and the maximum increments are 21.4–26.7% for nine working fluids. For the heat source temperature above THS,in_TP, the dual-pressure evaporation ORC is unbefitting.

Energy, Exergy and Economic Evaluation Comparison of Small-Scale Single and Dual Pressure Organic Rankine Cycles Integrated with Low-Grade Heat Sources

Low-grade heat sources such as solar thermal, geothermal, exhaust gases and industrial waste heat are suitable alternatives for power generation which can be exploited by means of small-scale Organic Rankine Cycle (ORC). This paper combines thermodynamic optimization and economic analysis to assess the performance of single and dual pressure ORC operating with different organic fluids and targeting small-scale applications. Maximum power output is lower than 45 KW while the temperature of the heat source varies in the range 100–200 °C. The studied working fluids, namely R1234yf, R1234ze(E) and R1234ze(Z), are selected based on environmental, safety and thermal performance criteria. Levelized Cost of Electricity (LCOE) and Specific Investment Cost (SIC) for two operation conditions are presented: maximum power output and maximum thermal efficiency. Results showed that R1234ze(Z) achieves the highest net power output (up to 44 kW) when net power output is optimized. Regenerative ORC achieves the highest performance when thermal efficiency is optimized (up to 18%). Simple ORC is the most cost-effective among the studied cycle configurations, requiring a selling price of energy of 0.3 USD/kWh to obtain a payback period of 8 years. According to SIC results, the working fluid R1234ze(Z) exhibits great potential for simple ORC when compared to conventional R245fa.

Increasing energy efficiency in passenger ships by novel energy conservation measures

ABSTRACTTo achieve increasing emission requirements, the cruise ship industry is working to develop higher efficiency ships. Cruise ships are different from other ship types in their relatively higher consumption of electrical power, steam and hot water. Several novel high-efficiency system concepts are possible for on-board electrical power generation and other utility services, each with differing impacts and first costs. Low-emission concepts novel to the cruise ship industry include combinations of exhaust gas heat recovery, heat pumps, steam turbines and organic Rankine cycles (ORCs). Yet, evaluation of these concepts is difficult given the different operating modes of cruises, and overall efficiency is dependent on the dynamic operational sequences. In this paper, we compare alternative energy efficiency concepts for cruise ships through simulation studies of the ship operations when equipped with different novel power generation systems. We find that the dual pressure steam systems and ORC offer the greatest potential for energy efficiency improvements in the cruise ship industry. We also find that relatively conventional technologies enable cruise ships to comply with planned upcoming higher ship energy efficiency requirements.

Exergy analysis and optimisation of waste heat recovery systems for cement plants

ABSTRACTIn the last decades, heat recovery systems have received much attention due to the increase in fuel cost and the increase in environmental issues. In this study, different heat recovery systems for a cement plant are compared in terms of electricity generation and exergy analysis. The heat sources are available in high temperature (HT) and low temperature (LT). For the HT section a dual pressure Rankine cycle, a simple dual pressure Organic Rankine Cycle (ORC) and a regenerative dual pressure ORC are compared. Also, for the LT section, a simple ORC is compared with transcritical carbon dioxide cycle. To find the best system, an optimisation algorithm is applied to all proposed cycles. The results show that for the HT section, regenerative ORC has the highest exergy efficiency and has the capability of producing nearly 7 MW electricity for a cement factory with the capacity of 3400 ton per day. The main reason for this is introducing the regenerative heat exchanger to the cycle. For the LT section, ORC showed a better performance than the CO2 cycle. It is worth mentioning that the generated power in this section is far lower than that of the HT section and is equal to nearly 300 kW.

An innovative Organic Rankine Cycle system for integrated cooling and heat recovery

Converting a portion of the waste heat into usable power by implementing Rankine and Organic Rankine Cycles (ORC) on long-haul trucks is seen as a potential way to improve the overall system efficiency. To identify techno-economical heat sources across the drive cycle of a Heavy Duty Diesel Engine (HDDE), an energy and exergy analysis was performed on all the available heat streams. As a result, to recover the combined exhaust gases and coolant heat, a reference cascade system was analysed. Owing to the nature of this application, a size vs. performance optimisation was performed for the cascade system utilising water and R245fa fluid combination. Despite a 1.8% Brake Thermal Efficiency (BTE) improvement, the key consideration in the research and development efforts for ORC systems was identified as the investigation of technical paths that may improve the practicality of such a heat-to-power conversion concept. For this, simple holistic solutions were considered vital to meet the impending CO2 regulations. To provide a potential solution, an innovative dual-pressure ORC system is therefore proposed to partially address the shortcomings of the cascade system. This innovative system is a function of new working fluids (i.e. water blends), its associated cycle operating mode and a novel architecture (i.e. direct engine block heat recovery). A screening and evaluation methodology applied to water–organic blends is presented. Simulations conducted in Aspen HYSYS V8 showed that, compared to the reference cascade system, the proposed dual-pressure system has the potential to deliver an average of 20% improvement in the system power, a 50% reduction in the total heat exchanger footprint, and a reduced system complexity. These advantages bode well for an integrated and relatively compact engine cooling and exhaust heat recovery solution for future automotive HDDEs. Implementation of the proposed system at mid-speed high-load engine operating condition increased the overall BTE from 41.4% to a maximum of 43.6%.

The comparision of a basic and a dual-pressure ORC (Organic Rankine Cycle): Geothermal Power Plant Velika Ciglena case study

In the Republic of Croatia there is some medium temperature geothermal fields (between 100 and 180 °C) by means of which it is possible to produce electricity. However, only recently concrete initiatives for the construction of geothermal power plants have been started. In previous papers, the possible cycles for geothermal fields in the Republic of Croatia are proposed: ORC (Organic Rankine Cycle) and Kalina cycle. Also for the most prospective geothermal fields, energy and exergy analysis for the proposed cycles are performed, on the basis of which the most suitable cycle is proposed. It is ORC which in all cases has better both the thermal efficiency (the First Law efficiency) and the exergy efficiency (the Second Law efficiency). With aim to further improving of geothermal energy utilization in this paper the replacement of a basic ORC with a dual-pressure ORC is analysed. A dual-pressure cycle reduces the thermodynamic losses incurred in the geothermal water-working fluid heat exchangers of the basic ORC, which arise through the heat transfer process across a large temperature difference. The dual-pressure cycle maintains a closer match between the geothermal water cooling curve and the working fluid heating/boiling curve and these losses can be reduced. Now, on the example of the most prospective geothermal field, Velika Ciglena (175 °C), energy and exergy analysis for the proposed the dual-pressure cycle are performed. As a conclusion, in case of Geothermal Power Plant Velika Ciglena, a dual-pressure ORC has slightly lower thermal efficiency (13.96% vs. 14.1%) but considerably higher both exergy efficiency (65% vs. 52%) and net power (6371 kW vs. 5270 kW).

Energies of the future

Dual-pressure organic Rankine cycle (ORC) has great advantages in low-temperature waste heat utilization systems. There are few studies on optimizing the performance and structure of the dual-pressure turbine, which is a key part of the dual-pressure ORC. The constructal design is performed for a dual-pressure axial-flow turbine (AFT) in a dual-pressure ORC to improve the turbine and cycle performances. Total power is taken as optimization objective function, volume ratio of the high-pressure turbine (HPT) and inlet pressure of the low-pressure turbine (LPT) are chosen as optimization variables, and total volume is employed as a constraint. After obtaining the optimization results, the impacts of some flow and structure parameters are analyzed. Furthermore, performances of the dual- and single-pressure AFTs are contrasted. The results demonstrate that the total powers of the dual-pressure AFT after the primary and twice optimizations are increased by 0.97% and 1.69% compared with the initial value, respectively. After the twice optimization, the optimal construct of the dual-pressure AFT is that the volume ratio of the HPT and inlet pressure of the LPT are 0.072 and 695 kPa, respectively. The nozzle inlet hub to tip radius ratios of the two turbines and the working-fluid mass flow rate ratio positively impact the total power, while the total volume hurts it. Besides, the dual-pressure AFT has a better performance advantage than the single-pressure AFT, and the total power of the former is 16.59% higher than that of the latter. The results confirm that the dual pressure is more suitable for low-temperature waste heat utilization systems than the single pressure and provide strategies for improving the turbine performance.