The impact of component performance on the overall cycle performance of small-scale low temperature organic Rankine cycles

Low temperature organic Rankine cycles offer a promising technology for the generation of power from low temperature heat sources. Small-scale systems (~10kW) are of significant interest, however there is a current lack of commercially viable expanders. For a potential expander to be economically viable for small-scale applications it is reasonable to assume that the same expander must have the ability to be implemented within a number of different ORC applications. It is therefore important to design and optimise the cycle considering the component performance, most notably the expander, both at different thermodynamic conditions, and using alternative organic fluids. This paper demonstrates a novel modelling methodology that combines a previously generated turbine performance map with cycle analysis to establish at what heat source conditions optimal system performance can be achieved using an existing turbine design. The results obtained show that the same turbine can be effectively utilised within a number of different ORC applications by changing the working fluid. By selecting suitable working fluids, this turbine can be used to convert pressurised hot water at temperatures between 360K and 400K, and mass flow rates between 0.45kg/s and 2.7kg/s, into useful power with outputs between 1.5kW and 27kW. This is a significant result since it allows the same turbine to be implemented into a variety of applications, improving the economy of scale. This work has also confirmed the suitability of the candidate turbine for a range of low temperature ORC applications

A barrier and techno-economic analysis of small-scale bCHP (biomass combined heat and power) schemes in the UK

bCHP (Biomass combined heat and power) systems are highly efficient at smaller-scales when a significant proportion of the heat produced can be effectively utilised for hot water, space heating or industrial heating purposes. However, there are many barriers to project development and this has greatly inhibited deployment in the UK. Project viability is highly subjective to changes in policy, regulation, the finance market and the low cost fossil fuel incumbent. The paper reviews the barriers to small-scale bCHP project development in the UK along with a case study of a failed 1.5MWel bCHP scheme. The paper offers possible explanations for the project's failure and suggests adaptations to improve the project resilience. Analysis of the project's: capital structuring contract length and bankability; feedstock type and price uncertainty, and plant oversizing highlight the negative impact of the existing project barriers on project development. The research paper concludes with a discussion on the effects of these barriers on the case study project and this industry more generally. A greater understanding of the techno-economic effects of some barriers for small-scale bCHP schemes is demonstrated within this paper, along with some methods for improving the attractiveness and resilience of projects of this kind

The Application of Similitude Theory for the Performance Prediction of Radial Turbines Within Small-Scale Low-Temperature Organic Rankine Cycles

For small-scale organic Rankine cycles (ORCs) to be a competitive technology, it is reasonable to assume that the same turbine design will be implemented into a range of different applications. It is therefore critical to be able to predict turbine off-design performance over a range of different operating conditions while utilizing different working fluids. Similitude theory can be used for this purpose, and it has been well validated for ideal gases. However, the same cannot be said for its applications to the organic fluids found within ORCs. This paper considers a candidate subsonic turbine design operating with R245fa and the corresponding turbine performance map. Similitude theory is used to predict the performance of the same turbine operating at different inlet conditions using R245fa, R123, and R1234yf. The similitude predictions are compared to computational fluid dynamics (CFD) results obtained using ansys CFX. The original similitude theory using turbine total inlet conditions was found to only apply within a small range of operating conditions, so a modified similitude theory has been suggested that uses the choked flow conditions instead. This modified similitude theory agrees with the CFD predictions to within 2%, right up until the choked mass flow rate. Further studies considering supersonic turbines are required to establish the applicability of similitude for applications beyond the choked pressure ratio

Influence of condenser conditions on organic rankine cycle load characteristics

A 7 MWth combined heat and power plant (CHP) based on an organic Rankine cycle (ORC) with 5.3 MWth and 1 MWel nominal output is analyzed. A district heating system serves as heat sink; the entire system is heat-led. Two examples for winter and summer operation are shown. The observed characteristics of the condenser are compared to results of a theoretical model. Variable mass flows, temperature levels (72 °C–95 °C) and temperature spreads result in varying condensation temperatures and pressure levels in the condenser (90 mbar to 150 mbar). High mass flows on the secondary side and related low temperature spreads improve the heat transfer and increase the condensation rate in the condenser. The monitoring data support the findings of a steady-state condenser model. As a consequence, advantageous load profiles according to the pressure characteristic of the system can be reached. Live steam pressure, pressure difference across the turbine, and flow rate increase. The effect on the electric efficiency is one percentage point in summer and 1.5 percentage points in winter, which translates to a difference in the electric yield of the cycle of about 10%. Furthermore, the data show that the transient sink conditions cause unsteady operation for the entire cycle