Characteristics of forced convection heat transfer to R125 at supercritical state in a horizontal tube

In the last 100 years, the energy use has risen significantly in various sectors. Up to 42% of the worldwide energy is used in industry. However, of this share 50% could be recovered as residual heat (i.e. waste heat). Therefore, there is a huge potential for waste heat recovery from industry. However, the temperature of this heat is usually lower than 200°C and cannot be used in the classical thermal processes for electricity production. A promising technology that can convert low-to-medium temperature heat (90-250°C) into electricity is the organic Rankine cycle (ORC). The ORC is a thermodynamic power cycle that resembles the classical Clausius-Rankine cycle but instead of water it uses an alternative working fluid (e.g. refrigerants). The ORC consists of four basic components: a pump, a heat exchanger (an evaporator or a vapour generator), a condenser and an expander. The working fluid is at first pressurized and transported to the evaporator. In the evaporator the working fluid is heated at constant pressure to superheated or saturated vapour. Then there is an expansion process in the expander/turbine to extract the mechanical work. By dissipating the heat, the working fluid is condensed to saturated liquid in the condenser. The condensed liquid is pumped again to the desired pressure with which the cycle closes and the process repeats again. Even though the ORC is well known technology there is still room for improving the efficiency and the performance. One possible way to achieve this is to ensure supercritical heat transfer in the vapour generator. The heat addition in the heat exchanger occurs at a near constant pressure which is above the critical pressure of the working fluid. This means that the two-phase region of the saturation curve is omitted and the heat addition is accompanied by a temperature increase of the working fluid. The benefit lays in a reduced temperature difference between the heat source and the working fluid temperature profile. The supercritical vapour expands in the turbine/expander and generates mechanical work. The working fluid is cooled down in the condenser up to saturated liquid. Then the working fluid is pressurized with the pump and the cycle is closed. Hence, operating with supercritical pressure can lead to improved cycle efficiency. This thermodynamic cycle, with supercritical heat addition and subcritical heat rejection is called a transcritical cycle. Research activities on heat transfer to fluids at supercritical pressures started in the early 20th century. In these early works the focus was mainly on invesxix 1 tigating supercritical heat transfer of water and CO2. Furthermore, the scope of the earlier experimental investigations was limited to vertical flow directions (upward and downward flow) in small tube diameters. There are many heat transfer correlations available in literature. However, their use for practical applications is limited because they are mainly validated with the specific data that they were derived for. Furthermore, there is little information in literature available about supercritical heat transfer to refrigerants circulating in horizontal flow and in large tube diameters. Additionally, the buoyancy has a different effect in horizontal and vertical flow direction. Therefore, experimental investigation of forced convection heat transfer to refrigerants at supercritical pressure is necessary. Particularly for this thesis, a new test facility was built. The aim was performing heat transfer measurements to supercritical refrigerants under organic Rankine cycle conditions. More specifically, this means heat transfer measurements at low temperatures (heat fluxes) of the heating fluid (90-125°C). On a component level, the test facility ’iSCORe’ is similar to an organic Rankine cycle but instead of an expander and expansion valve was used. Furthermore, the configuration of the test section was a horizontal tube-in-tube heat exchanger with a counter-current flow. The working fluid was flowing in the central tube while the heating fluid in the annulus. The central tube has an outer diameter of 28 mm and inner diameter of 24 mm. Moreover, the test facility was equipped with a number of pressure and temperature sensors needed for control and measurements purposes. A number of measurements were performed by varying the inlet parameters (mass flux, heat flux, pressures). The mass flux was in the range of 400-650 kg=(m2s), the heat flux was between 14-28 kW=m2 and the pressure of the working fluid was (1.05-1.15)pcr. A real challenge when working with fluids at supercritical state is closing the heat balance over the test section. In certain operating conditions the deviation of the energy balance can reach up to 20%. This is especially noticeable when the fluid is near the pseudocritical point. Moreover, the reliability of the test facility was verified by repeating several different measurements. Based on the experimental results the supercritical heat transfer is strongly affected by the mass fluxes and the heat fluxes. Higher mass flux and lower heat flux lead to enhanced heat transfer. However, there was one deviation noticed on this trend. The heat transfer shows enhancement at lower mass flux when the fluid is close to the pseudocritical temperature. This could be due to the rapid changes of the thermophysical properties. Furthermore, at pressure closer to the critical pressure of the working fluid enhanced heat transfer is observed. Moreover, it was determined that the buoyancy effect cannot be neglected in horizontal flow. Furthermore, the results from the measurements were compared with heat transfer correlations from literature. Even though the heat transfer correlations have a correction factor in order to account for the drastic property changes, they do not show good agreement with the experimental results in the entire range. Therefore, for deriving general heat transfer correlation that will be xx Summary applicable for wider operating conditions it is important the heat transfer correlation to be validated with extensive experimental data. In conclusion, first set of measurements for supercritical R125 was obtained. The reproducibility tests prove good operation of the test facility. There were proposed suggestions for practical improvements to the test facility and to lower the uncertainties in the measurements

Design sensitivity analysis of a plate-finned air-cooled condenser for waste heat recovery ORCs

The study is related to the design sensitivity analysis of a plate-finned tube bundle V-shaped air-cooled condenser design problem for a range of representative low-temperature waste heat recovery Organic Rankine Cycle (ORC) cases. An iterative design model is implemented which reveals the thermodynamic and geometric design error margins that occur when different in-tube prediction methods are used. 19 condensation heat transfer correlations are used simultaneously within arrays of geometric and thermodynamic variables. Through attained 19 different convective coefficients, a design sensitivity on the calculated overall heat transfer coefficient, total transferred heat, degree of subcooling, required tube and fin material amount, air- and refrigerant-side pressure drops is reported

Design sensitivity analysis of using various in-tube condensation correlations for an air-cooled condenser for ORCs

The study is related to the evaluation of using 19 condensation heat transfer correlations in an annular finned horizontal round tube V-shaped air-cooled condenser design problem for a representative low-temperature waste heat recovery Organic Rankine Cycle (ORC) case. The condensation is realized through cold air provided by the fan suction at a mass flow rate of 90,35 kg/s, whereas the working fluid mass flow rate is 7,8 kg/s. The considered condensation temperature is 40°C which corresponds to a saturation pressure of 1,17 bar. The ambient air is considered to be 15°C. The investigated working fluid is SES36. For a given set of geometrical constraints, an iterative condenser design model is implemented. All considered correlations are applied separately for the same boundary conditions. The design sensitivity on the overall heat transfer coefficient, total transferred heat, required fan power, air- and refrigerant-side pressure drops is assessed. By those means, the engineering error margin of using different calculation tools in designing air-cooled condensers for ORC is reported

Experimental investigation at sub- and supercritical conditions of a helical coil heat exchanger particularly designed for a small-scale solar ORC

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Performance assessment methods for boilers and heat pump systems in residential buildings

According to the European Commission, 40% of the total energy use belongs to the buildings sector. That corresponds to 36% of CO2 emissions in the European Union alone. Currently, HVAC systems are the major energy users in the building sector. Therefore, there is a necessity to assess the performance of different energy/comfort systems in buildings. However, finding a way to mitigate the performance gap between the calculated and real energy use in dwellings is of great importance. In Flanders, the Energy Performance and indoor climate regulation (EPB) dates back to 2006. Since the building context related to energy demand has changed significantly over the past years, investigation on how to evolve building energy assessment method framework in the EPB regulation in Flanders by dealing with the current issues will be indispensable. In 2017, new EN52000 series of standards have been published, containing extensive methods of assessing the overall energy performance of buildings. The main focus of this article is to analyze the assessment methods for the energy performance of boilers and heat pumps for residential appliance by comparing methodology stated in respected Energy performance and indoor climate regulation in Flanders (EPB), EcoDesign regulations and EN52000 standard series. The aim for future research is to determine the parameters that mostly influence the performance and in a next step compare the predicted performance to real energy use

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

Design and rating of an evaporator for waste heat recovery organic rankine cycles using SES36

The paper presents a design and rating study of a 4MW evaporator having plain horizontal carbon steel tubes having diameters of 25,4 mm, 31,8 mm and 38 mm, to be used in waste heat recovery via Organic Rankine cycle (ORC). SES36 is chosen as working fluid due to its low boiling point, which makes it suitable for low-grade waste heat recovery with subcritical ORCs. Waste heat carrier industrial air arrives at the evaporator bundle at 280°C. Inlet temperature of the working fluid is 40°C and the evaporation occurs at 125°C and 1,09 MPa. Furthermore, a design sensitivity analysis is made by means of using 13 different in-tube flow boiling correlations. The resulting design and rating parameters yielded by each correlation are compared to each other. By those means, a design error margin of various thermo-hydraulic heat exchanger parameters is revealed, when different in-tube flow boiling heat transfer calculation methods are used. The change in the error margins are investigated with respect to changing tube outer diameter, tube wall thickness, fin density and tube layout (staggered and inline)

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