Optimisation of a high-efficiency solar-driven organic rankine cycle for applications in the built environment

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 locationsPeer ReviewedPostprint (updated version

Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow rate variations). Therefore maximising the ORC system performance under off-design conditions is of key importance, for the financial viability and wider adoption of these systems. In this paper, the off-design performance of an ORC engine with screw expander and two heat exchanger (HEXs) architectures is investigated, while recovering heat from an internal combustion engine (ICE). Firstly, nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, while the plate HEXs (PHEXs) heat transfer area requirements are 50% lower, than the respective ones for double pipe (DPHEX) design. Next, the ORC engine operation is optimised at part-load (PL) ICE conditions. Although, the HEXs heat transfer coefficients decrease with part-load, the total HEX effectiveness increases, due to higher temperature difference across the working fluids. Findings also reveal that the PHEX performance is less sensitive to the off-design operation. Off-design power output maps indicate that the optimised ORC engine PL reduces to 72%, for ICE PL of 60%, while ORC engines with PHEXs generate slightly more power, for the same heat source conditions. Overall, the modelling tool developed can predict ORC performance over an operating envelope and allows the selection of optimal designs and sizes of ORC HEXs and expanders

Off-design comparison of subcritical and partial evaporating ORCs in quasi-steady state annual simulations

The subcritical ORC (SCORC) is considered the industry standard due to its simple configuration, acceptable efficiency and low costs. 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 evaporating 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 investigation, the PEORC has an increased annually averaged net power output of 9.6% compared to the SCORC. Use of annually averaged input conditions results 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

Optimisation of high-efficiency combined heat and power systems for distributed generation

Distributed combined heat and power (CHP) systems have the potential to cover a significant amount of the global energy requirements for power, and heating. Small-to-medium scale CHP systems, in the built environment and in the industry (up to a few MWs), are typically driven by internal combustion engines (ICE). In CHP-ICE systems, more than 55% of the energy input is transferred as heat in the exhaust-gas stream and the jacket water cooling circuit. Unless these thermal outputs are utilised, the energy will be released to the atmosphere as waste heat, deteriorating the system’s efficiency. Organic Rankine cycle (ORC) engines are a promising heat-to-power technology, for converting waste heat into power. Therefore, coupling ORC engines as bottoming cycles to CHP-ICEs can maximise overall system efficiency, and reduce energy costs. In this thesis, the design of ICE-ORC CHP systems is investigated from thermodynamic, operating and economic perspectives, aiming to fully unlock the potential of such advanced high-efficiency cogeneration systems. An integrated ICE-ORC CHP optimisation tool is developed, which, unlike previous studies, captures the performance trade-offs between the two interacting engines, to optimise the combined system performance. A dynamic ICE model is developed and validated, along with a steady-state model of subcritical recuperative ORC engines. Multiple working fluids are investigated, along with naturally aspirated and turbocharged ICEs. By optimising the combined ICE-ORC CHP system simultaneously: i) the total power output increases by up to 30%, in comparison to the conventional approach where the two engines are optimised separately; ii) the electrical efficiency increases by up to 21%, in comparison to the stand-alone ICE; and iii) in the integrated system the ICE operation is adjusted to promote the ORC power output, which generates up to 15% of the total power, improving fuel efficiency. When focusing on maximising power output only, this comes at the cost of higher fuel consumption. In contrast, when optimising the integrated ICE-ORC CHP system for specific fuel consumption (SFC), the fuel consumption decreases by up to 17%. These findings prove that by taking a holistic approach in the design of ICE-ORC CHP systems, considering the combined system interactions, these can generate more power, with lower fuel consumption and costs. ORC engines in ICE-ORC CHP systems will experience variable heat-source conditions (temperature and mass flow are), while the ICE load fluctuates. To maximise the running hours of ORC engines, and improve their economic proposition, the system should maintain high efficiency, not only at the design point, but also at off-design operation. An off-design optimisation tool is therefore developed to generate optimised off-design operation maps. This work differs from previous studies in that the tool considers explicitly the time-varying operational characteristics and interactions of the ORC engine components in the integrated system. Double-pipe (DPHEX) and plate-frame heat exchanger (PHEX) models are used for sizing the ORC evaporator and condenser, and piston and screw expander models for sizing the expander. The ORC system is first sized for full-load ICE operation (design point). Then, ICE part-load (PL) conditions are obtained and new ORC operating points are optimised. Results reveal that the ORC engine power output is underestimated by up to 17%, when the off-design operational characteristics of the components are not considered. The piston expander efficiency increases by up to 16% at PL operation, while the ORC thermal efficiency increases by up to 7% at off-design operation. Optimised ORC engines with screw expanders operate always with two expansion stages. Although the latter generate slightly more power at their design point than when using piston machines, pistons perform better at off-design conditions. ORC engines with PHEXs generate 5-12% more power than DPHEX designs, while having U-values almost double that of DPHEXs. Although, heat transfer coefficients decrease by 25-30% at off-design, the HEX effectiveness increases, by up to 15%. By considering the time-varying characteristics of the ORC components, as the ICE PL reduces, the optimised ORC engine power output decreases at a lower rate: at ICE PL of 60%, the optimised ORC engine with fluids, such as R1233zd, operates at 77% of its nominal capacity (with piston expanders). An ORC thermoeconomic optimisation tool is then developed. Unlike other studies where the component types are predefined, the tool scans alternative components’ typologies, sizes, and configurations, to obtain the best-performing design. New cost correlations are presented to assist with predicting ORC costs. ORC engines optimised for maximum power output have the highest SICs, falling in the range of 1,000-7,500 GBP/kW for piston expanders, and 1,500-5,500 GBP/kW for screw expanders, due to high HEX areas, and high volumetric flow rates. In contrast, when minimising SIC, power output reduces by 15-50%, but the cost also reduces by up to 35%. In the optimised designs, the evaporator is selected to be a PHEX, the condenser is a DPHEX, whilst ORC engines with screw machines operate with two-stage expansion. Multi-objective optimisation reveals optimal ORC systems with a range of power outputs between 70-100 kW, for which increasing power by 33%, results in an increase of SIC of less than 10%. This indicates a promising range of capacities for next-generation ORC engines. ORC engines optimised for high power output result in high net present value (NPV), but also high discounted payback period (DPP). In contrast, engines optimised for low SIC achieve DPPs of 2.8-6 years, making them an attractive investment. Overall, these findings can be used by ICE and ORC engine manufacturers, and integrators, to inform component design decisions, and by ORC plant operators to maximise their system performance, under variable operating conditions.Open Acces

Optimisation of a high-efficiency solar-driven organic rankine cycle for applications in the built environment

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 locationsPeer Reviewe

Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications

Organic Rankine cycle (ORC) engines in real applications experience variable heat-source conditions. In this paper, the off-design performance of small- to medium-scale ORC engines recovering heat from stationary internal combustion engines (ICEs) is investigated. Of particular interest are the employment of screw vs. piston expanders, and two heat exchanger (HEX) architectures. Unlike previous studies where the performance of the expander and HEX are assumed fixed during off-design operation, here we consider explicitly their varying and interacting characteristics within the overall system. Nominal sizing results reveal indicated isentropic efficiencies > 80% for twin-screw and > 85% for piston expanders. Following nominal design, the ORC engine operation is optimised for ICE part-load (PL) operation. Although the heat transfer coefficients in the evaporator decrease by up to 30% at PL, the effectiveness in this HEX increases by 20% due to the larger temperature differences across the component. The screw expander efficiency reduces by up to 3% at off-design operation, whilst that of the piston expander increases by up to 16%. Optimised off-design maps indicate that the ORC engine power output reduces to 77% (piston) or 68% (screw) of its full-load value when the ICE operates at 60% PL, and that ORC engines with plate HEXs generate 5-11% more power than those with double-pipe HEX designs. Under variable ICE operation, smaller ORC engines with piston expanders generate more power than larger engines with screw expanders, highlighting the resilient off-design operation of piston machines. The modelling tool developed here can predict ORC performance over a wide operating envelope and provides performance maps that can be used by operators to optimise ORC engine operation in variable conditions and by ORC vendors to inform component design decisions

Hybrid photovoltaic-thermal solar systems for combined heating, cooling and power provision in the urban environment

Solar energy can play a leading role in reducing the current reliance on fossil fuels and in increasingrenewable energy integration in the built environment, and its affordable deployment is widely recog-nised as an important global engineering grand challenge. Of particular interest are solar energy systemsbased on hybrid photovoltaic-thermal (PV-T) collectors, which can reach overall efficiencies of 70% orhigher, with electrical efficiencies up to 15–20% and thermal efficiencies in excess of 50%, dependingon the conditions. In most applications, the electrical output of a hybrid PV-T system is the priority, hencethe contacting fluid is used to cool the PV cells and to maximise their electrical performance, whichimposes a limit on the fluid’s downstream use. When optimising the overall output of PV-T systemsfor combined heating and/or cooling provision, this solution can cover more than 60% of the heatingand about 50% of the cooling demands of households in the urban environment. To achieve this, PV-Tsystems can be coupled to heat pumps, or absorption refrigeration systems as viable alternatives tovapour-compression systems. This work considers the techno-economic challenges of such systems,when aiming at a low cost per kW h of combined energy generation (co- or tri-generation) in the housingsector. First, the technical viability and affordability of the proposed systems are studied in ten Europeanlocations, with local weather profiles, using annually and monthly averaged solar-irradiance and energy-demand data relating to homes with a total floor area of 100 m2(4–5 persons) and a rooftop area of50 m2. Based on annual simulations, Seville, Rome, Madrid and Bucharest emerge as the most promisinglocations from those examined, and the most efficient system configuration involves coupling PV-T pan-els to water-to-water heat pumps that use the PV-T thermal output to maximise the system’s COP. Hourlyresolved transient models are then defined in TRNSYS, including thermal energy storage, in order to pro-vide 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 indicatethat PV-T systems have the potential to cover 60% of the combined (space and hot water) heating andalmost 100% of the cooling demands of homes (annually integrated) at all four aforementioned locations.Finally, when accounting for all useful energy outputs from the PV-T systems, the overall levelised cost ofenergy of these systems is found to be in the range of 0.06–0.12€/kW h, which is 30–40% lower than thatof equivalent PV-only systemsPeer Reviewe

Dissecting miRNA–Gene Networks to Map Clinical Utility Roads of Pharmacogenomics-Guided Therapeutic Decisions in Cardiovascular Precision Medicine

MicroRNAs (miRNAs) create systems networks and gene-expression circuits through molecular signaling and cell interactions that contribute to health imbalance and the emergence of cardiovascular disorders (CVDs). Because the clinical phenotypes of CVD patients present a diversity in their pathophysiology and heterogeneity at the molecular level, it is essential to establish genomic signatures to delineate multifactorial correlations, and to unveil the variability seen in therapeutic intervention outcomes. The clinically validated miRNA biomarkers, along with the relevant SNPs identified, have to be suitably implemented in the clinical setting in order to enhance patient stratification capacity, to contribute to a better understanding of the underlying pathophysiological mechanisms, to guide the selection of innovative therapeutic schemes, and to identify innovative drugs and delivery systems. In this article, the miRNA–gene networks and the genomic signatures resulting from the SNPs will be analyzed as a method of highlighting specific gene-signaling circuits as sources of molecular knowledge which is relevant to CVDs. In concordance with this concept, and as a case study, the design of the clinical trial GESS (NCT03150680) is referenced. The latter is presented in a manner to provide a direction for the improvement of the implementation of pharmacogenomics and precision cardiovascular medicine trials