ORC cogeneration systems in waste-heat recovery applications

The performance of organic Rankine cycle (ORC) systems operating in combined heat and power (CHP) mode is investigated. The ORC-CHP systems recover heat from selected industrial waste-heat fluid streams with temperatures in the range 150°C-330°C. An electrical power output is provided by the expanding working fluid in the ORC turbine, while a thermal output is provided by the cooling water exiting the ORC condenser and also by a second heat-exchanger that recovers additional thermal energy from the heat-source stream downstream of the evaporator. The electrical and thermal energy outputs emerge as competing objectives, with the latter favoured at higher hot-water outlet temperatures and vice versa. Pentane, hexane and R245fa result in ORC-CHP systems with the highest exergy efficiencies over the range of waste-heat temperatures considered in this work. When maximizing the exergy efficiency, the second heat-exchanger is effective (and advantageous) only in cases with lower heat-source temperatures (< 250°C) and high heat-delivery/demand temperatures (> 60°C) giving a fuel energy savings ratio (FESR) of over 40%. When maximizing the FESR, this heat exchanger is essential to the system, satisfying 100% of the heat demand in all cases, achieving FESRs between 46% and 86%

Case study of an Organic Rankine Cycle (ORC) for waste heat recovery from an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology for the conversion of waste heat to electricity. Although many energy intensive industries could benefit significantly from the integration of ORC technology, its current adoption rate is limited. One important reason for this arises from the difficulty of prospective investors and end-users to recognize and, ultimately, realise the potential energy savings from such deployment. In recent years, electric arc furnaces (EAF) have been identified as particularly interesting candidates for the implementation of waste heat recovery projects. Therefore, in this work, the integration of an ORC system into a 100 MWe EAF is investigated. The effect of evaluations based on averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC system is found capable of generating a net electrical output of 752 kWe with a steam buffer working at 25 bar. If combined heating is considered, the ORC system can be optimized to generate 521 kWe of electricity, while also delivering 4.52 MW of heat. Finally, an increased power output (by 26% with combined heating, and by 39% without combined heating) can be achieved by using high temperature thermal oil for buffering instead of a steam loop; however, the use of thermal oil in these applications has been until now typically discouraged due to flammability concerns

Potential of Organic Rankine Cycles (ORC) for waste heat recovery on an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a 100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC (SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam

Heat recovery and conversion technologies with organic fluid cycles: optimal working fluid and system design

Heat recovery and conversion technologies have become increasingly important for a variety of reasons relating to growing global concerns on energy security, rising electricity costs, CO2 emissions and global warming. Although many energy intensive industries could benefit significantly from the integration of these technologies, the current adoption rate is limited due to the high investment costs involved and the difficulty of prospective investors and end-users to recognise and, ultimately, realise the potential energy savings from such deployment. Thus, the wider adoption of these technologies can be facilitated by improved performance and reduced investment costs. In this context, integrated thermoeconomic optimisation of such power systems through the optimal design of working fluids and operating parameters is invaluable in improving economic viability. The performance of single-component working fluids is limited by the large exergy destruction associated with evaporation and condensation. Thus, working-fluid mixtures promise reduced exergy losses due to their non-isothermal phase-change behaviour, and improved cycle efficiencies and power outputs over their constituent pure fluids. The optimisation of organic Rankine cycle (ORC) systems revealed that mixtures do indeed show a thermodynamic improvement (in terms of the system power output and efficiency) over the pure-fluids. Although systems with fluid mixtures could see up to a 30% increase in power output over those with pure ones, they require larger expansion devices and heat exchangers (evaporators and condensers) due to their deterioration in heat transfer during the phase-change processes; thus, the resulting ORC systems are also associated with higher costs. Hence, ORC systems with pure working fluids have lower plant costs per unit power output, up to 14% lower than those with mixtures, highlighting the importance of considering system cost minimisation in designing ORC plants. The Up-THERM heat converter is a two-phase unsteady heat engine; it contains fewer moving parts than conventional ones and represents an attractive alternative for remote or off-grid power generation. With the aid of a validated first-order lumped dynamic model, its performance with respect to working-fluid selection for its prospective applications is investigated. With the engine's power output and efficiency being conflicting objectives, fluids with low critical temperatures (and high critical pressures, reduced pressures and temperatures) resulted in designs with high power outputs and correspondingly low efficiencies. For a nominal Up-THERM design corresponding to a target application with a heat-source temperature of 360 °C, R113 was identified as the optimal fluid, followed by i-hexane in maximising the power output. The Up-THERM heat converter was also seen to be effective over a range of heat-source temperatures delivering in excess of 10 kW (about four times higher than with water as working fluid in the nominal design) when utilising thermal energy at temperatures above 200 °C. Of all the working fluids considered, ammonia, R245ca, R32, propene and butane feature prominently as optimal and versatile fluids on the developed optimal working-fluid selection maps, delivering high power over a wide range of heat-source temperatures. To facilitate simultaneous optimal working fluid and process design of waste-heat recovery systems, a mixed-integer non-linear programming (MINLP) optimisation framework for the computer aided molecular and process design of heat engines was developed. The components of the framework, consisting of thermodynamic process models, heat-exchanger sizing models, component cost correlations, economic evaluation models, transport property group-contribution correlations and the SAFT-γ Mie group-contribution equation of state are individually and collectively validated to acceptable degrees of accuracy against experimental and available commercial data. Following validation, the framework is used to identify optimal working-fluids for ORC systems with three different industrial waste-heat sources (150, 250 and 350 °C), with different objective functions. Although slightly related, maximising the power output and minimising the specific investment costs (SIC) are not equivalent objectives. From NLP optimisations, n-propane, 2-pentene and 2-hexene were the optimal working fluids when maximising power output, while n-propane, 2-butane and 2-heptene were optimal when SIC is minimised. With MINLP optimisations minimising the SIC, 1,3-butadiene and 4-methyl-2-pentene were the best performing working fluids for the 150 °C and 250 °C heat sources respectively; these novel working fluids do not belong to the common hydrocarbon families assessed in the NLP optimisations. Similarly, with multi-objective cost-power optimisation, the same molecules are identified, striking a delicate balance between investment costs and system performance. Ultimately, the results demonstrate the potential of this framework to drive the search for the next generation of organic Rankine cycle and waste-heat recovery systems, and to provide meaningful insights into identifying the working fluids that represent the optimal choices for targeted applications.Open Acces

Performance of working fluid mixtures in an ORC-CHP system for different heat demand segments

Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery and conversion to power in several industrial settings. In the present paper, we investigate the deployment of working-fluid mixtures in ORCs operating in combined heat and power mode (ORC-CHP) with shaft power provided by the expanding working fluid and heating provided by the cooling-water exiting the ORC condenser. Using the flue gas from a refinery boiler as the waste-heat source and with working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, the ORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to 15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component working fluids such as pentane appear to be optimal at low hot-water supply temperatures. Working-fluid mixtures become optimal at higher temperatures, with the working-fluid mixture combination of octane and pentane giving an ORCCHP system design with the highest efficiency. However, in most CHP applications, the fluctuation of heat demand can determinate a discharge of heating, in particular when a waste-heat source makes profitable the system operation also in only electricity mode, and if thermal storage options are not considered. For this reason, the influence of heat demand intensity on the global system conversion efficiency and optimal working fluid selection is also explored

Thermodynamic and economic evaluation of trigeneration systems in energy-intensive buildings

Within the building sector, supermarkets are responsible for 3- 5% of the electricity consumed in developed countries. To mitigate the associated environmental impact of this consumption, a growing interest has been developed in local combined heat and power (CHP) systems, due to their higher total efficiencies. However, CHP efficiency is highly dependent on the thermal output utilisation. In food retail buildings, where refrigeration dominates the building energy use, a promising means for utilising the thermal output is by using this to operate absorption chillers. This paper reports on a technical feasibility and financial viability study of an ammoniawater absorption chiller, coupled to a CHP unit, that is also compared to a conventional electrically-driven vapour-compression equivalent. A typical distribution centre located in the UK is selected as a case-study. Three alternative systems are considered: i) a conventional grid connected system; ii) a CHP system; and iii) a trigeneration system. Typical daily cooling, heating and hot-water demand data are provided on an hourly basis, and the system’s ability to cover these loads is assessed. The results indicate that the trigeneration system can reduce the electricity demand by 16% compared to the baseline system, while offering a 48% annual energy cost saving. The system’s primary energy utilisation rate exceeds 60%, while the power-to-heat ratio of the building demand improves from 7.0 to 0.9, thereby more closely matching the CHP system generation profile. Furthermore, the trigeneration system achieves CHPQA rating of 106, and it is qualified for enhanced capital allowance for the CHP plant. The results highlight the great energy and cost savings potentials of integrating trigeneration systems in energy-intensive buildings

Preliminary experimental results of an 11 kWe organic Rankine cycle

The organic Rankine cycle (ORC) is considered a viable technology for converting low- and medium-temperature heat to electricity. However, many of ORC systems in practical applications operate in off-design conditions. In order to characterize this operation, experimental data is needed. In this paper, the commissioning of an 11 kWe ORC is described with special attention to the processing of the data. A filtering algorithm is introduced to isolate steadystate working points. This filter is then applied to the raw experimental data. In addition, the reliability of the experimental data is evaluated by investigating the heat balances over the heat exchangers and error propagation of the measurement uncertainties. The result of this work is a test-setup which is fully ready for high-accuracy and reliable measurements, including the post-processing steps. In the future, off-design models will be validated with the acquired experimental data and especially two-phase expansion will be further investigated