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Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow-rate variations). Maximising ORC system performance while accounting for off-design operation in response to such variations is of crucial importance for the financial viability and wider adoption of this technology. In this paper, the off-design performance of an ORC engine is investigated in a heat-recovery application from a stationary internal combustion engine (ICE), employing a screw expander and two heat exchanger (HEX) architectures. Unlike previous studies where the ORC expander and HEX performance are assumed fixed during off-design operation, here we consider explicitly the time-varying characteristics of these system components. Nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, and that the heat transfer area requirements of plate HEXs (PHEXs) are 50% lower than those of double-pipe HEXs (DPHEX). Following nominal system design, the ORC engine operation is optimised at conditions relating to part-load (PL) operation of the ICE. Although, the heat transfer coefficients in the evaporator decrease by 30% at PL, the HEX effectiveness increases by up to 20% due to higher temperature differences between the working fluid and the heat source, and the performance of the PHEX appears less sensitive to off-design operation conditions. The optimum PL screw expander efficiency maps reveal only a minor reduction of the expander efficiency (by 2%) relative to the design conditions. Optimum off-design maps indicate that the power output of the ORC engine reduces to 72% of full-load for an ICE at 60% of full-load, and that ORC engines with PHEXs generate slightly more power for the same heat-source conditions. Overall, the tool developed predicts ORC performance over an operating envelope and allows the selection of optimal HEX and expander designs. The findings can be used by ORC plant operators to optimise the ORC engine power output given varying on-site heat-source conditions, and by ORC vendors to inform HEX and expander design decisions.

The subcritical ORC (SCORC) is considered the industry standard due to its simple configuration, acceptable efficiency and low cost s. However , it is known that alternative ORC configurations have the potential to increase efficiency. A cycle modification which closely resembles the SCORC is the partial evap orati ng ORC (PEORC), where a two -phase mixture of liquid -vapour enters the expander instead of superheated vapour. In theoretical studies at design conditions , higher power outputs are achieved for the PEORC compared to the SCORC. This work aims to go a step further by investigating the performance of the SCORC and PEORC under time -dependent operating conditions. A direct comparison between the SCORC and PEORC is made for identically sized systems using as input the waste heat stream of a waste incinerator plant and the changing ambient conditions . Performance maps of both cycle configurations are compiled and the benefit of an expander operating at variable speed is briefly discussed . The results indicate that for the specific case under investigati on, the PEORC has an increased annual ly averaged net power output of 9.6% compared to the SCORC. Use of annually averaged input conditions result s in an overestimation of the net power output for both the SCORC and PEORC, and furthermore , the relative improvement in power output for the PEORC is reduced to 6.8%. As such, the use of time -averaged conditions when comparing cycle architectures should preferably be avoided.

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising and predicting system performance, which can be used, along with the optimisation tool, by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.

The wider adoption of organic Rankine cycle (ORC) technology for power generation or cogeneration from renewable or recovered waste-heat in many applications can be facilitated by improved thermodynamic performance, but also reduced investment costs. In this context, it is suggested that the further development of ORC power systems should be guided by combined thermoeconomic assessments that can capture directly the trade-offs between performace and cost with the aim of proposing solutions with high resource-use efficiency and, importantly, improved economic viability. This paper couples, for the first time, the computer-aided molecular design (CAMD) of the ORC working-fluid based on the statistical associating fluid theory (SAFT)-γ Mie equation of state with thermodynamic modelling and optimisation, in addition to heat-exchanger sizing models, component cost correlations and thermoeconomic assessments. The resulting CAMD-ORC framework presents a novel and powerful approach with extended capabilities that allows the thermodynamic optimisation of the ORC system and working fluid to be performed in a single step, thus removing subjective and pre-emptive screening criteria that exist in conventional approaches, while also extending to include cost considerations relating to the resulting optimal systems. Following validation, the proposed framework is used to identify optimal cycles and working fluids over a wide range of conditions characterised by three different heat-source cases with temperatures of 150 °C, 250 °C and 350 °C, corresponding to small- to medium-scale applications. In each case, the optimal combination of ORC system design and working fluid is identified, and the corresponding capital costs are evaluated. It is found that fluids with low specific-investment costs (SIC) are different to those that maximise the power output. The fluids with the lowest SIC are isoheptane, 2-pentene and 2-heptene, with SICs of £5620, £2760 and £2070 per kW respectively, and corresponding power outputs of 32.9 kW, 136.6 kW and 213.9 kW.

Organic Rankine cycle (ORC) engines are an efficient means of converting low - to - medium renewable or waste heat to useful power . In practical applications, ORC systems experience varying thermal input profile , due to the dynamic nature of real heat source s . M aximis ing the uptake of this technology requires optim ised ORC design s and sizing to maintain high efficiency and power output, not only at full - load operation, but also under off - design conditions. Key for maintaining the efficient operation of the sys tem is the maximisation of heat extraction from the heat source, in the ORC evaporator. In this paper, the off - design operation of an ICE - ORC combined heat and power (CHP) system is investigated, to optimise the ORC performance under varying ICE load condi tions. First, the ORC engine thermodynamic design is optimised for the 100% load operation of the ICE. Alternative working fluids are inv estigated, including low ODP/ GWP refrigerants and hydrocarbons. The ORC system is then sized using two different heat e xchanger (HEX) architectures; tube - in - tube (DPHEX) and plate (PHEX) design s , at design conditions. The sizing results reveal that the PHEX area requirements are almost 50% lower than the respective ones for DPHEX, while recovering equivalent quantities of heat. Next, the ORC engine operation is optimised at part - load ICE condition s , and the HEX heat transfer coefficients (HTCs) are predicted. R esults indicate that : i) PHEX HTCs are up to 50% higher than DPHEX equivalents ; ii) HTCs decrease at part load f or both HEXs, but because the average temperature difference increases, the overall HEX effectiveness improves; and iii) the ORC system with a PHEX evaporator has slightly higher power output tha n the DPHEX equivalent at off - design operation. Overall, t he modelling tool developed here can predict ORC performance over an operating envelope and allows the selecti on of optimal design s and size s of ORC HEXs.

This thesis aims to address the process synthesis problems of emerging reaction pathways for bio-based polymers and their monomers and provide the optimal design of the process flowsheet with simultaneous heat integration. To solve these problems, mixed integer linear programming (MILP) models and solution approaches are developed along with a decomposition method to handle numerical complications arising from the utilisation of a large number of discretised variables. Regarding the process synthesis problem, an optimisation-based framework is first developed to identify the optimal configuration of a process network that consists of both reaction and separation systems. The problem is formulated as a mixed integer linear programming (MILP) model with the objective to maximise the economic potential. A logic-based formulation using binary variables is designed for the simultaneous synthesis of separation sequences. The solution of the optimisation problem includes the best possible economic performance, identification of active reactions, reaction ordering and separation sequences along with the corresponding flowsheet of the entire process. Secondly, simultaneous heat integration is incorporated into the model without introducing nonlinearity by using a novel variable discretisation approach. The modified model aims to provide additional information of the optimal flowsheet such as the utility cost, the minimum cooling and heating duties required, and energy savings due to heat integration. The process synthesis model consists of a large number of integer variables and becomes difficult to solve. A decomposition method, which utilises the GAMS grid facility, breaks down the original problem into an equivalent set of subproblems by the partitioning of decision variable domains and allows inter-job communications to exchange the best bound of each subproblem. The results indicate that significant improvement in the computational efficiency is achieved when inter-job communication is integrated with the decomposition method. Open Access

The successful commercialisation of organic Rankine cycle (ORC) systems across a range of power outputs and heat-source temperatures demands step-changes in both improved thermodynamic performance and reduced investment costs. The former can be achieved through high-performance components and optimised system architectures operating with novel working-fluids, whilst the latter requires careful component-technology selection, economies of scale, learning curves and a proper selection of materials and cycle configurations. In this context, thermoeconomic optimisation of the whole power-system should be completed aimed at maximising profitability. This paper couples the computer-aided molecular design (CAMD) of the working-fluid with ORC thermodynamic models, including recuperated and other alternative (e.g., partial evaporation or trilateral) cycles, and a thermoeconomic system assessment. The developed CAMD-ORC framework integrates an advanced molecular-based group-contribution equation of state, SAFT-γ Mie, with a thermodynamic description of the system, and is capable of simultaneously optimising the working-fluid structure, and the thermodynamic system. The advantage of the proposed CAMD-ORC methodology is that it removes subjective and pre-emptive screening criteria that would otherwise exist in conventional working-fluid selection studies. The framework is used to optimise hydrocarbon working-fluids for three different heat sources (150, 250 and 350 °C, each with mcp = 4.2 kW/K). In each case, the optimal combination of working-fluid and ORC system architecture is identified, and system investment costs are evaluated through component sizing models. It is observed that optimal working fluids that minimise the specific investment cost (SIC) are not the same as those that maximise power output. For the three heat sources the optimal working-fluids that minimise the SIC are isobutane, 2-pentene and 2-heptene, with SICs of 4.03, 2.22 and 1.84 £/W respectively.

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.

Improvements in the thermal and economic performance of organic Rankine cycle (ORC) systems are required before the technology can be successfully implemented across a range of applications. The integration of computer-aided molecular design (CAMD) with a process model of the ORC facilitates the combined optimisation of the working-fluid and the power system in a single modelling framework, which should enable significant improvements in the thermodynamic performance of the system. However, to investigate the economic performance of ORC systems it is necessary to develop component sizing models. Currently, the group-contribution equations of state used within CAMD, which determine the thermodynamic properties of a working-fluid based on the functional groups from which it is composed, only derive the thermodynamic properties of the working-fluid. Therefore, these do not allow critical components such as the evaporator and condenser to be sized. This paper extends existing CAMD-ORC thermodynamic models by implementing group-contribution methods for the transport properties of hydrocarbon working-fluids into the CAMD-ORC methodology. Not only does this facilitate the sizing of the heat exchangers, but also allows estimates of system costs by using suitable cost correlations. After introducing the CAMD-ORC model, based on the SAFT-γ Mie equation of state, the group-contribution methods for determining transport properties are presented alongside suitable heat exchanger sizing models. Finally, the full CAMD-ORC model incorporating the component models is applied to a relevant case study. Initially a thermodynamic optimisation is completed to optimise the working-fluid and thermodynamic cycle, and then the component models provide meaningful insights into the effect of the working-fluid on the system components.

Energy security, pollution and sustainability are major challenges presently facing the international community, in response to which increasing quantities of renewable energy are to be generated in the urban environment. Consequently, recent years have seen a strong increase in the uptake of solar technologies in the building sector. In this work, the potential of a solar combined heat and power (CHP) system based on an organic Rankine cycle (ORC) engine is investigated in a domestic setting. Unlike previous studies that focus on the optimisation of the ORC subsystem, this study performs a complete system optimisation considering both the design parameters of the solar collector array and the ORC engine simultaneously. Firstly, we present thermodynamic models of different collectors, including flat-plate and evacuated-tube designs, coupled to a non-recuperative sub-critical ORC architecture that delivers power and hot water by using thermal energy rejected from the engine. Optimisation of the complete system is first conducted, aimed at identifying operating conditions for which the power output is maximised. Then, hourly dynamic simulations of the optimised system configurations are performed to complete the system sizing. Results are presented of: (i) dynamic 3-D simulations of the solar collectors together with a thermal energy storage tank, and (ii) of an optimisation analysis to identify the most suitable working fluids for the ORC engine, in which the configuration and operational constraints of the collector array are considered. The best performing working fluids (R245fa and R1233zd) are then chosen for a whole-system annual simulation in a southern European climate. The system configuration combining an evacuated-tube collector array and an ORC engine is found to be best-suited for electricity prioritisation, delivering an electrical output of 3,605¿kWh/year from a 60¿m2 collector array. In addition, the system supplies 13,175¿kWh/year in the form of domestic hot water, which is equivalent to more than 6 times the average annual household demand. A brief cost analysis and comparison with photovoltaic (PV) systems is also performed, where despite the lower PV investment cost per kWel, the levelised energy costs of the different systems are found to be similar if the economic value of the thermal output is taken into account. Finally, a discussion of the modelled solar-CHP systems results shows how these could be used for real applications and extended to other locations

There is great interest in distributed combined heat and power (CHP) generation in the built environment due to the higher overall efficiencies attained in comparison to separate provision of these vectors. Organic Rankine cycle (ORC) systems are capable of generating additional electricity from the thermal outputs of CHP engine s , improving the electri cal conversion efficiency and power - to - heat ratio of such system s . Thermodynamic analysis and technical feasibility are at the core of the development of these systems , while a critical factor for the wider adoption of ORC systems concerns their economic proposition. O btain ing credible estimates of system costs requires correct sizing of individual components. This work focuses on the thermodynamic optimisation, sizing and costin g of ORC units in CHP applications, over a range of heat - source temperatures. The working fluids examined include R245fa, R1233zd, Pentane and Hexane, due to their good performance and favourable environmental character istics. The o ptim al cycles obtained c an increase the power - to - heat ratio of the complete CHP - ORC system by up to 65%. A lternative equipment sizing methods are then applied for each fluid and the resultant component sizes are compared. The cost estimates obtained from t he alternative methods are also compared to real ORC application. Based on this, a hybrid costing method is proposed and applied to an ORC system design , in order to obtain the specific investment cost (SIC ). The results indicate that as the heat source te mperature increases, the power output increases, result ing in larger and more expensive components. Nevertheless, the SIC drops from 1 7 GBP/W for low - power outputs to 1.1 GBP/W for high - temperature/high - power outputs.

Solar energy can play a leading role in reducing the current reliance on fossil fuels and in increasing renewable energy integration in the built environment. Hybrid photovoltaic-thermal (PV-T) systems can reach overall efficiencies in excess of 70%, with electrical fficiencies in the range of 15-20% and thermal efficiencies of 50% or higher. In most applications, the electrical output of a hybrid PV-T system is the priority, hence the contacting fluid is used to cool the PV cells to maximise their electrical performance, which imposes a limit on the fluid's downstream use. When optimising the overall output of PV-T systems for combined heating and cooling provision, this technology can cover more than 60% of the heating and about 50% of the cooling demands of households in the urban environment. To achieve this, PV-T systems can be coupled to heat pumps or absorption refrigeration systems as viable alternatives to vapour-compression systems. This work considers the techno-economic challenges of such systems, when aiming at a low cost per kWh of energy generation of PV-T systems for co- or tri-generation in the housing sector. First, the viability and afordability of the proposed systems are studied in ten European locations, with local weather pro files, using annually and monthly averaged solar-irradiance and energy-demand data. Based on annual simulations, Seville, Rome, Madrid and Bucharest emerge as the most promising locations from those examined, and the most efficient system confi guration involves coupling PV-T panels to water-to-water heat pumps that use the PV-T thermal output to maximise the system's COP. Hourly resolved transient models are then defi ned in TRNSYS in order to provide detailed estimates of system performance, since it is found that the temporal resolution (e.g. hourly, daily, yearly) of the simulations strongly affects their predicted performance. The TRNSYS results indicate that PV-T systems have the potential to cover 60% of the heating and almost 100% of the cooling demands of homes at all four aforementioned locations. Finally, the levelised cost of energy for these systems is found to be in the range of 0.06-0.12 e/kWh, which is 30-40% lower than that for equivalent PV only systems.

For buildings with low heat-to-power demand ratios, the installation of internal combustion engines (ICEs) for the onsite provision of combined heat and power (CHP) results in large amounts of surplus heat. In the UK, such installations risk being ineligible for the CHP Quality Assurance (CHPQA) programme, thereby incurring additional levies. In this work, a technoeconomic optimisation of small-scale organic Rankine cycle (ORC) engines is performed, in which the ORC engines recover heat from the ICE exhaust gases in order to increase the overall efficiency of this combined solution and meet the CHPQA requirements. Two competing system configurations are assessed. In the first, the ORC engine also recovers heat from the CHP-ICE jacket water to generate additional power. In the second, the ORC engine operates at a higher condensing temperature, which prohibits jacket-water heat recovery but allows heat from the condenser to be delivered to the building. When optimised for minimum specific investment cost, the first configuration is initially found to deliver 20% more power (25.8 kW) at design conditions, and a minimum specific investment cost (1600 £/kW) that is 8% lower than the second configuration. However, the first configuration leads to less heat from the CHP-ICE being supplied to the building, increasing the cost of meeting the heat demand. By establishing part-load performance curves for both the CHP-ICE and ORC engines, the economic benefits from realistic operation can be evaluated. The present study goes beyond previous work by testing the configurations against a comprehensive database of real historical electricity and heating demand for thirty energy-intensive buildings at half-hour resolution. The discounted payback period for the second configuration is found to lie between 3.5 and 7.5 years for all of the buildings considered, while the first configuration is seen to recoup capital investment costs for only 23% of the buildings. The broad applicability of the second configuration offers attractive opportunities to increase manufacturing volumes and reduce unit costs. The findings are relevant to a range of buildings with heat-to-power demand ratios from 20% to 100%.

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising and predicting system performance, which can be used, along with the optimisation tool, by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.