Performance evaluation of organic Rankine cycle architectures : application to waste heat valorisation

In our society, there is an ever increasing need for electricity. However, today most of the electricity is generated by burning fossil fuels in a thermal power plant. A proposed alternative is to make use of low temperature heat from renewable sources (geothermal and solar) or waste heat (excess heat that is dumped into the atmosphere) in an organic Rankine cycle (ORC) to generate electricity. The purpose of the presented work is to support further adoption of ORC technology. To achieve this, two main challenges need to be resolved. First, sound criteria should be devised to compare and size ORCs and secondly the performance of the ORC should be increased further. From literature it is clear that novel ORC architectures have the opportunity to increase the performance of the basic subcritical ORC. However these studies are not cross comparable. As such, a new screening approach is created which rigorously compares and quantifies the potential of three different ORC architectures. Secondly, the sizing and the financial appraisal of the ORC is tackled by introducing a multi-objective optimization which combines financial and thermodynamic criteria in the optimization objectives. Finally, experimentally validated part-load models of the ORC were developed. These part-load models are crucial to predict the actual power output of time varying heat sources like waste heat streams. In addition, the models permit to investigate the concept of retrofitting existing subcritical ORCs to work under the more optimal working regime of partial evaporation

Thermodynamic analysis of the partially evaporating trilateral cycle

The potential of Organic Rankine Cycles (ORC) to recover low grade waste heat is well known. The high heat recovery potential is partially attributed to a good match of the temperature profiles between working fluid and waste heat stream in the evaporator. This preferable characteristic is mainly induced by the selection of an appropriate working fluid. However, because of the constant temperature evaporation of the working fluid, the heat recovery potential is restricted. In order to overcome this limitation the trilateral cycle (TLC) has been investigated. A Trilateral cycle (also called Triangular cycle) is a modified Rankine cycle. The main difference is that the working fluid is not evaporated but only heated to the saturation temperature. Compared to the ORC, the heat carrier stream can be cooled further and the thermal efficiency is lower. In this study the effect of partial evaporation of the working fluid is investigated

Performance potential of ORC architectures for waste heat recovery taking into account design and environmental constraints

The subcritical ORC (SCORC), sometimes with addition of a recuperator, is the de facto state of the art technology in the current market. However architectural changes and operational modifications have the potential to improve the base system. The ORC architectures investigated in this work are: the transcritical ORC (TCORC), the triangular cycle (TLC) and the partial evaporation ORC (PEORC). Assessing the potential of these cycles is a challenging topic and is brought down to two steps. First, the expected thermodynamic improvement is quantified by optimizing the second law efficiency. Secondly, the influences of technical constraints concerning volumetric expanders are investigated. In the first step, simple regression models are formulated based on an extensive set of boundary conditions. In addition a subset of environmentally friendly working fluids is separately analysed. In the second step, two cases are investigated with the help of a multi-objective optimization technique. The results of this optimization are compared with the first step. As such the effect of each design decision is quantified and analysed, making the results of this work especially interesting for manufacturers of ORC systems

Improving intermittent waste heat recovery with ORC systems by integrating thermal energy storage

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Experimental investigation of a forced convection heat transfer of the organic fluid R-125 at supercritical pressures and under organic rankine cycle conditions

The organic Rankine cycle (ORC) is a suitable technology for utilizing low-grade temperature heat sources of ~100 °C from various industry processes. In the ORC cycle an organic fluid with a lower boiling point is used as a working medium. The performance of the ORCs has advanced significantly in the last decades. However, there is still a possibility of improving the efficiency of this cycle. The supercritical heat transfer in the heat exchanger ensures better thermal match between the heating and working fluids temperatures glides in the heat exchanger. Hence, better understanding of the heat transfer phenomena to a fluid at supercritical state in a horizontal flow and in a large diameter tube is of great importance. Therefore, the tests are performed in a counter-current tube-in-tube test section positioned horizontally with a total length of 4 m and a tube diameter of 0.0286 m. R-125 is used as a working fluid in the experiments. During the measurements the temperature of the heating fluid was 90 °C, the mass flow rate and the pressure of the working fluid R-125 was in the range of 0.2–0.3 kg/s and 38–55 bar respectively. Furthermore, results from the pressure and temperature measurements obtained at the inlet and at the outlet of the test section are reported. The results show that the overall heat transfer coefficient is influenced by the mass flow rate of the organic fluid. At pressures close to the critical pressure of R-125 higher values of the overall heat transfer coefficients are determined. Deteriorated heat transfer is not likely to occur at these operating conditions because the critical heat flux is higher than the one obtained from the measurements. A comparison between the experimental Nusselt number with heat transfer (Nusselt) correlations from the literature is done and the measurement points fall within the uncertainty ranges of both heat transfer correlations.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Design sensitivity analysis of using various flow boiling correlations for a direct evaporator in high-temperature waste heat recovery ORCs

High-temperature waste heat (250°C-400°C) sources being created by industrial operations such as metallurgical industry, incinerators, combustion engines, annealing furnaces, drying, baking, cement production etc. are being utilized in Organic Rankine cycle (ORC) waste heat recovery systems. Alongside indirect ORC evaporators having intermediate heat carrier loops, ORC waste heat recovery can also be done through a direct evaporator (e.g. tube bundles) applied on a heat source. In an evaporator design problem, the accuracy of the design method has a significant impact on the end result. In that manner, for revealing the design accuracy error margin of using various flow boiling heat transfer methods, a design sensitivity analysis is performed by means of using 13 different flow boiling heat transfer correlations. All correlations are implemented separately into an iterative evaporator calculation and the resulting sizing solutions are compared for a representative high-temperature waste heat recovery evaporator case. The volumetric flow rate of the waste heat is 80000 Nm³/h and the inlet temperature is 375°C. The considered working fluid is cyclopentane and the deduced optimal evaporation temperature (OET) is 227°C. The minimum corresponding total transferred heat in the evaporator is at least 3,5 MW in all calculations

An analysis of the retrofitting potential of a subcritical ORC into a partial evaporating ORC under off-design operation

Many applications where ORCs are installed have time varying waste heat streams. To account for this, flexible and robust off-design models are necessary. In addition, alternative operating conditions and cycle architectures show potential for increased performance. This work investigates how operation as a partial evaporating cycle (PEORC) affects the performance of the ORC. More specifically, the question is raised if current subcritical ORC (SCORC) installations provide potential to be retrofitted into PEORC operation by analysing their off-design operation. The initial results show that under these conditions the PEORC shows a potential net power output increase between 2% to 12% relative to the base SCORC depending on the off-design conditions. The optimal expander inlet vapour fraction that maximises the net power output ranges between 0.5 and 1. With roughly the same maximum pressures, the same working fluid, and an adapted measuring and control strategy, significant net power improvements could thus be achieved. It is important to note that for PEORC operation the performance of the pump becomes determinative. Installing a pump with a better efficiency would further benefit operation as PEORC. Based on these results, there are clearly opportunities in retrofitting existing SCORC systems

Experimental assessment of a helical coil heat exchanger operating at subcritical and supercritical conditions in a small-scale solar organic rankine cycle

In this study, the performance of a helical coil heat exchanger operating at subcritical and supercritical conditions is analysed. The counter-current heat exchanger was specially designed to operate at a maximal pressure and temperature of 42 bar and 200 °C, respectively. The small-scale solar organic Rankine cycle (ORC) installation has a net power output of 3 kWe. The first tests were done in a laboratory where an electrical heater was used instead of the concentrated photovoltaic/thermal (CPV/T) collectors. The inlet heating fluid temperature of the water was 95 °C. The effects of different parameters on the heat transfer rate in the heat exchanger were investigated. Particularly, the performance analysis was elaborated considering the changes of the mass flow rate of the working fluid (R-404A) in the range of 0.20–0.33 kg/s and the inlet pressure varying from 18 bar up to 41 bar. Hence, the variation of the heat flux was in the range of 5–9 kW/m2. The results show that the working fluid’s mass flow rate has significant influence on the heat transfer rate rather than the operational pressure. Furthermore, from the comparison between the experimental results with the heat transfer correlations from the literature, the experimental results fall within the uncertainty range for the supercritical analysis but there is a deviation of the investigated subcritical correlations

Simultaneous capacitive probe and planar laser-induced fluorescence measurements in downwards gas-liquid annular flow

Various experimental techniques are available to analyse two-phase flows. The measurement concept and the applicability can however vary greatly. Prime examples from the opposite spectrum are planar laser-induced measurements (PLIF) versus capacitive probes. PLIF is an optical technique, it is non-intrusive but optical access is necessary. PLIF based measurements are known for their high temporal and spatial resolution but require a costly set-up. In contrast, the capacitive probe is another non-intrusive technique but doesn’t require optical access. It is fairly easy to set up, robust, and is cheap to construct. To rigorously compare both techniques, simultaneous PLIF and capacitive probe measurements are made in this work. As the void fraction is one of the key parameters to classify flow regimes, both techniques are compared on the determination of the void fraction. This is done for a limited set of six annular flows. The experiments were performed in a downward annular-flow facility with demineralized water - air as working medium. The first results indicate that both techniques give similar volume averaged void fractions. The mean absolute percentage error and the maximum relative error between both techniques are 0.30% and 0.54%, respectively. The PLIF measurements confirm however to have a better spatial resolution