Low Temperature Waste Heat Recovery Research Articles

Design of Experimental Rig for Validation of Absorption Power Cycle Concept

Absorption power cycles (APC) are an interesting concept of heat engines especially in the domain of low temperature waste heat recovery. It uses a multicomponent working fluid which enables boiling and condensation to take place with a temperature glide instead of isothermal process for single-component fluid, which can serve to increase the exergy efficiency of heat exchangers. Absorption process has also an effect of lowering exhaust pressure of a turbine. Among absorption power cycles nearly all of commercial, experimental and theoretical concepts use water-ammonia mixture as a working fluid. Only very few and mostly recent works are starting to look at other working fluids with specific interest and potential in using LiBr aqueous solution known from absorption cooling.Concept of such cycle has both thermodynamic and environmental benefits; however it brings many challenges that need to be resolved. Among them stands out an experimental validation of actual temperature profile in variable-temperature phase-change heat exchangers. Another major issue for a LiBr system is also purity of the steam going to the expander. Therefore in this work is proposed an experimental rig for validation of these assumptions. Particularly the temperature profile will be investigated within an experimental desorber, purity of steam leaving two phase separator will be analysed to set up design limits for the expander. Next phase will add experimental absorber and expander emulator. Focus will be given on modular approach and possibility to test different equipment designs.

Characterization of Thermal, Mechanical and Tribological Properties of Fluoropolymer Composite Coatings

Perfluoroalkoxy (PFA) is a potential polymer coating material for low-temperature waste heat recovery in heat exchangers. Nonetheless, poor thermal conductivity, low strength and susceptibility to surface degradation by erosion/wear pose restrictions in its application. In this study, four types of fillers, namely graphite, silicon carbide, alumina and boron nitride, were introduced to enhance the thermal, mechanical and tribological properties in PFA coatings. The thermal diffusivity and specific heat capacity of the composites (reinforced with 20 wt.% filler) were also measured using laser flash and differential scanning calorimetry techniques, respectively. The results indicated that the addition of graphite or boron nitride increased the thermal conductivity of PFA by at least 2.8 orders of magnitude, while the composites with the same weight fraction of alumina or silicon carbide showed 20-80% rise in thermal conductivity. The micromechanical deformation and tribological behavior of composite coatings, electrostatically sprayed on steel substrates, were investigated by means of instrumented indentation and scratch tests. The deformation response and friction characteristics were investigated, and the failure mechanisms were identified. Surface hardness, roughness and structure of fillers influenced the sliding performance of the composite coatings. PFA coatings filled with Al2O3 or SiC particles showed high load-bearing capacity under sliding conditions. Conversely, BN- and graphite-filled PFA coatings exhibited lower interfacial adhesion to steel substrate and were prone to failure at relatively lower applied loads.

Dynamic Simulation of an Organic Rankine Cycle—Detailed Model of a Kettle Boiler

Organic Rankine Cycles (ORCs) are nowadays a valuable technology to produce electricity from low and medium temperature heat sources, e.g., in geothermal, biomass and waste heat recovery applications. Dynamic simulations can help improve the flexibility and operation of such plants, and guarantee a better economic performance. In this work, a dynamic model for a multi-pass kettle evaporator of a geothermal ORC power plant has been developed and its dynamics have been validated against measured data. The model combines the finite volume approach on the tube side and a two-volume cavity on the shell side. To validate the dynamic model, a positive and a negative step function in heat source flow rate is applied. The simulation model performed well in both cases. The liquid level appeared the most challenging quantity to simulate. A better agreement in temperature was achieved by increasing the volume flow rate of the geothermal brine by 2% over the entire simulation. Measurement errors, discrepancies in working fluid and thermal brine properties and uncertainties in heat transfer correlations can account for this. In the future, the entire geothermal power plant will be simulated, and suggestions to improve its dynamics and control by means of simulations will be provided.

Studies on the process of recovering low-temperature waste heat from a flue gas in a pilot-scale plant

Abstract This paper presents studies carried out in a pilot-scale plant for recovery of waste heat from a flue gas which has been built in a lignite-fired power plant. The purpose of the studies was to check the operation of the heat recovery system in a pilot scale, while the purpose of the plant was recovery of waste heat from the flue gas in the form of hot water with a temperature of approx. 90 °C. The main part of the test rig was a condensing heat exchanger designed and built on the basis of laboratory tests conducted by the authors of this paper. Tests conducted on the pilot-scale plant concerned the thermal and flow parameters of the condensing heat exchanger as well as the impact of the volumetric flow rate of the flue gas and the cooling water on the heat flux recovered. Results show that the system with a condensing heat exchanger for recovery of low-temperature waste heat from the flue gas enables the recovery of much higher heat flux as compared with conventional systems without a condensing heat exchanger.

Research on Low-temperature Waste Heat Power Generation Device Control System based on Embedded Technology

Industrial waste heat is one of the most widely distributed and used potential of conventional recyclable energy in industrial production .However, for the low concentration and less energy of low quality waste heat resources, the mature traditional waste heat recovery technology is not suitable for this kind of the recycling of waste heat resources due to its recycling economy and feasibility of the restrictions. In this paper, the secondary utilization of low quality waste heat resources is studied, especially the control system research of low-temperature waste heat recovery power generation equipment by heat pipe waste heat recovery device matching with roots-type steam engine. Control system is managed by system hardware which with C8051F040 single-chip microcomputer as CPU, and fuses the signals of speed encoder and temperature sensor to realize real-time input, and realizes real time communication between touch screen and single chip microcomputer through RS232 bus. By testing, system works stably. Finally it is resulted that the recovery of low-temperature waste heat and the conversion of electric energy can be achieved by the test.

Fabrication of thermoelectric modules and heat transfer analysis on internal plate fin structures of a thermoelectric generator

Thermoelectric modules (TEMs) are fabricated for a low-temperature waste heat recovery application. Each module has a surface area of 44×44mm and a thickness of 3.6mm, including a 1-mm-thick ceramic substrate on each side. Prior to fabrication of the system, a series of numerical simulations are conducted to optimize the design of the internal finned structures of a thermoelectric generator. To reduce the difficulty of designing the numerical models, a thermal resistance model is employed to determine the thermal conductivity of the TEM. The optimal number and thickness of the fin structures are determined with respect to the maximum allowable module temperature and the pressure drop characteristics. The accuracy of the numerical model is validated using an existing friction factor correlation and experimental results. The numerical results show that having six 2-mm-thick plate fins on the hot surface of each TEM would provide the most effective temperature fields for TE power generation while keeping the surface temperature of the TEM from exceeding the allowable maximum of ∼473K. The pressure drop across the fins is found to increase with increasing number and thickness of fins. However, the module-level pressure drop is in the range of several pascals, which has a negligible effect on the combustion characteristics of the engine. A thermal resistance equation is proposed to predict the heat transfer characteristics of plate fins employed for thermoelectric generators for heat absorption.

Thermodynamic modelling of a recompression CO2 power cycle for low temperature waste heat recovery

Due to the rising prices of conventional fossil fuels, increasing the overall thermal efficiency of a power plant is essential. One way of doing this is waste heat recovery. This recovery is most difficult for low temperature waste heat, below 240°C, which also covers majority of the waste heat source. Carbon dioxide, with its low critical temperature and pressure, offers an advantage over ozone-depleting refrigerants used in Organic Rankine Cycles (ORCs) and hence is most suitable for the purpose. This paper introduces parametric optimization of a transcritical carbon dioxide (T-CO2) power cycle which recompresses part of the total mass flow of working fluid before entering the precooler, thereby showing potential for higher cycle efficiency. Thermodynamic model for a recompression T-CO2 power cycle has been developed with waste heat source of 2000kW and at a temperature of 200°C. Results obtained from this model are analysed to estimate effects on energetic and exergetic performances of the power cycle with varying pressure and mass recompression ratio. Higher pressure ratio always improves thermodynamic performance of the cycle – both energetic and exergetic. Higher recompression ratio also increases exergetic efficiency of the cycle. However, it increases energy efficiency, only if precooler inlet temperature remains constant. Maximum thermal efficiency of the T-CO2 cycle with a recompression ratio of 0.26 has been found to be 13.6%. To minimize total irreversibility of the cycle, an optimum ratio of 0.48 was found to be suitable.

Dynamic modelling of a low-concentration solar power plant: A control strategy to improve flexibility

This paper deals with a dynamic analysis on a low concentration solar power plants coupled with Organic Rankine Cycles (ORC), which can be an alternative to PV systems because of their capability of providing a smoother electricity production due to their thermal inertia. At least within certain restraints, moreover they are able to exploit diffused solar radiation.The dynamic model of a plant with static Compound Parabolic Collectors and an ORC system, using a rotary volumetric expander, was developed using the simulation tool AMESim. All the main components of the plant are modelled: solar collectors field, heat transfer fluid circuit, heat exchangers and the ORC system. The plant response to the radiation of different days was analyzed to quantify the daily production and the trend of various plant parameters. Real ambient conditions were employed for the simulations by using data obtained by historical series.The results showed that the employment of a volumetric expansion device with variable rotating speed allows the plant to operate at different radiations and ambient temperatures without the need of any storage system or external heat sources. Results can be extended to other applications, such as low temperature waste heat recovery or geothermal systems.

Experimental comparison of organic fluids for low temperature ORC (organic Rankine cycle) systems for waste heat recovery applications

This contribution experimentally evaluates and compares the performance of an ORC (organic Rankine cycle) system for stationary bottoming WHR (waste heat recovery) application operating with two different working fluids, SES36 and R245fa. The test rig is a regenerative cycle equipped with a single screw expander modified from a standard compressor characterized by a nominal shaft power of 11 kW. A total of 36 and 43 steady-state points are collected for SES36 and R245fa respectively, over a wide range of operating conditions by changing the expander rotational speed, the pump frequency and the cooling condenser flow rate. The performances of the ORC components are individually evaluated. A maximum expander isentropic efficiency of 60% is reached using SES36 at 3000 rpm, and a value of 52% is reached with R245fa at 3000 rpm. However, for a given pressure ratio the expander output power is higher with R245fa than with SES36. The overall performance of the ORC unit are investigated in terms of first and second law efficiencies and net output power for the two fluids. The results experimentally demonstrate the correlation between the working fluid critical temperature and the ORC unit working characteristics for low temperature waste heat recovery applications. Open experimental data are provided for both fluids.

Thermoeconomic comparison between pure and mixture working fluids of organic Rankine cycles (ORCs) for low temperature waste heat recovery

Based on the thermoeconomic multi-objective optimization, simultaneously considering exergy efficiency and levelized energy cost (LEC), the thermoeconomic comparisons between pure and mixture working fluids of organic Rankine cycles (ORCs) have been investigated. Four models are proposed based on the different location of evaporating bubble point temperature or condensing dew point temperature for mixture working fluids. The effects of mass fraction and four key parameters (evaporator temperature, condenser temperature, pinch point temperature difference and degree of superheat) on exergy efficiency and levelized energy cost (LEC) are examined. Pareto-optimal solutions of four models using 0.7R245fa/0.3R227ea are obtained and compared. Taking mass fraction into account, the thermoeconomic comparisons between pure and mixture working fluids have been studied. Research demonstrates that the mixtures don’t always present better thermodynamic performance and economic performance than pure working fluids. Model 2 (T7=TE,T3=TC) is the favorable operation condition for its highest thermodynamic performance and relatively low economic factor. Taking mass fraction as decision variable, Pareto-optimal solutions for models 1, 2, 3 and 4 in pairs of (exergy efficiency (%), LEC ($/kWh)) are (56.71, 0.188), (57.67, 0.192), (57.11, 0.194), and (56.91, 0.192), respectively. Compared with pure working fluids, the mixture working fluids present better exergy efficiency but worse LEC except model 1.

Thermosiphon loop thermal collector for low-temperature waste heat recovery

This paper describes the thermal collector type loop thermosiphon for low-temperature waste heat recovery. Water is used as the working fluid for heat transport at temperatures 100 °C and higher. The loop thermosiphon comprises of a thermal receiver, a condenser, and riser and downcomer tubes. The thermal receiver is made of a cupper plate brazed by meandering heat transfer tube. This receiver collects the thermal radiation from the electric heater at the heat transfer area of 1000 cm2 (40 cm × 25 cm), and transports the heat by vaporization of water to the condenser having heat transfer area of 62 cm2. In the no inclination mode, the thermosiphon is upright so that the thermal receiver and the condenser are placed vertically. In this mode, the effective thermal conductivity exceeds 60 kW/(m K) when the thermal receiver temperature was higher than 125 °C for the water filling ratio α = 30–70%. Although the effective thermal conductivity is deteriorated for the higher filling ratio α = 80% and 90%, the effects from the filling ratio are tiny for α = 30–70%. The experimental tests were also made for the negatively inclined mode where the inlet and exit ports of the receiver were directed downward and for the positively inclined modes where they were directed upward. The tests revealed that the negative inclination almost halted the heat transport. However, the tests also indicated that the positive inclination showed the performance comparable to the no inclination mode for the cases up to the inclination angle 90°.

Application of PTFE Heat Exchanger in Low Temperature Waste Heat of Big Coal-Fired Power Plants

With the rapid development of the national economy, the use of low-temperature heat in thermal power plant boiler can not be ignored.Although low temperature economizer is widely used in low-temperature waste heat recovery of thermal power plant boiler, the problems of corrosion and fouling are very significant.New type PTFE heat exchanger filled with high thermal conductivity properties can replace the existing metal heat exchanger, fundamentally solve the problems of corrosion and fouling, meet the future development of the thermal power plant, and realize the energy recycling to maximize the benefit of energy.

CO2 based power cycle with multi-stage compression and intercooling for low temperature waste heat recovery

For low temperature waste heat recovery, CO2 is preferred as the working fluid due to its low critical temperature and easy availability. However, a major limitation of CO2 based power plant with low temperature waste heat recovery is temperature of heat rejection. In the present work, a study has been made to explore possible improved performance of a CO2 power cycle using low temperature waste heat through multi-stage compression and intercooling. A thermodynamic model has been developed to analyze effects of various operating parameters on the performance of a CO2 power cycle with two or more stages of compression and intercooling. Most significant observation is the existence of an optimum combination of the lowest cycle pressure and the intermediate pressure for either maximum specific power output or 2nd law efficiency of the CO2 power cycle with two-stage compression and intercooling.

Numerical Simulation Study of Three-Dimensional Flow Field on Roots- Type Power Machine Based on Dynamic Mesh Technique

Expander which drives an electromotor to generate electricity is the core of low-temperature waste heat recovery equipment. At present, domestic expanders on waste heat recovery system mostly arise from exploration, research and improvement on the existing models of screw expanders and scroll expanders, which have complex structures and high costs. For overcoming the shortcomings mentioned above, a new roots-type power machine is researched and designed. In this paper, the working fluid of low pressure waste heat is stimulated according to both the different operating conditions and the different intake and exhaust pressure and flow, and the changing process of internal flow field is simulated over the time period using dynamic mesh technique when the power machine is in rotating work. Besides, the pressure field, velocity field and graph of mass flow rate are analyzed with the simulation results, thus obtaining the conclusion of optimum operating conditions of the roots-type power machine to guide selection method in practice, that is, selecting appropriate roots-type power machines according to different types of waste heat in industrial production. These efforts can therefore provide strong theoretical guidance and foundation for the subsequent engineering practice of waste heat recovery system on roots-type power machine, and can have a profound impact on further recovery and utilization of low-grade energy.

A new pinch based method for simultaneous selection of working fluid and operating conditions in an ORC (Organic Rankine Cycle) recovering waste heat

ORC (Organic Rankine Cycles) are widely used in low temperature waste heat recovery which can lead to considerable energy saving. Huge amounts of waste heat need to be cooled to the ambient temperature in refineries. The target temperature of this type of waste heat can be as low as the ambient temperature. The selection of working fluid and operating conditions exert significant influence on the performance of an ORC system. In this paper, a new method is proposed to simultaneously determine the working fluid and operating conditions in an ORC system, which recovers this type of waste heat in refineries. PPP (Preheating Pinch Point) and VPP (Vaporization Pinch Point) are newly introduced. The method is based on a newly defined parameter (PREDICTOR) that can predict the Pinch position between the waste heat carrier and the working fluid accurately, calculate the mass flow rate of working fluid and the amount of heat recovered easily, and determine the optimum working fluid and corresponding operating conditions simultaneously. Two illustrative examples are used to demonstrate the effectiveness of the proposed method. The waste heat can be recovered completely and the power output reaches the maximum with the following conditions: (1) An appropriate positive temperature difference between the waste heat inlet temperature and the working fluid critical temperature exists. (2) Working fluid evaporates in its near critical region. However, if the working fluid critical temperature is close to the waste heat inlet temperature, the working fluid evaporating in the near critical region cannot provide maximum heat recovery and power output.

Sensitivity analysis and thermoeconomic comparison of ORCs (organic Rankine cycles) for low temperature waste heat recovery

The sensitivity analysis for low temperature ORCs (organic Rankine cycles), as well as the thermoeconomic comparison between the basic ORC and regenerative ORC using Non-dominated sorting genetic algorithm-II (NSGA-II), are conducted in this paper. The derivatives of five system parameters on system performance are used to evaluate the parametric sensitiveness. The exergy efficiency and the APR (heat exchanger area per unit net power output) are selected as the objective functions for multi-objective optimization using R123 under the low temperature heat source of 423 K. The Pareto frontier solution with bi-objective for maximizing exergy efficiency and minimizing APR is obtained and compared with the corresponding single-objective solutions. The results indicate that the prior consideration of improving thermal efficiency and exergy efficiency is to increase the evaporator outlet temperature. A fitting curve can be yielded from the Pareto frontier between the thermodynamic performance and economic factor. The optimum exergy efficiency and APR of the regenerative ORC obtained from the Pareto-optimal solution are 59.93% and 3.07 m2/kW, which are 8.10% higher and 15.89% lower than that of the basic ORC, respectively. The Pareto optimization compromises the thermodynamic performance and economic factor, therefore being more suitable for decision making.

Modeling analysis of longitudinal thermoelectric energy harvester in low temperature waste heat recovery applications

The worldwide interest in thermoelectric waste heat recovery is constantly growing, with a wide range of applications ranging from small harvesters integrated into wireless sensor networks all the way to larger harvesters such as the ones that can potentially be integrated into cars. The wide range of applications makes a requirement for studying the dynamic response of TEGs. The aim of this work is to develop a mathematical model to accurately simulate the thermal and electrical behaviours of a longitudinal thermoelectric energy harvester (LTEH). In order to implement the theoretical analysis, a new TRNSYS component has been developed so this new model can also be used as a design tool.The LTEH model presented in this paper is validated through the comparison of results between theoretical analysis and experimental data. Testing results and discussion show the reasonability of this new model and also their use as a simulation tool.

Performance analysis of a waste heat recovery thermoelectric generation system for automotive application

Thermoelectric power generators are one of the promising green energy sources. In this case study, an energy-harvesting system which extracts heat from an automotive exhaust pipe and turns the heat into electricity by using thermoelectric power generators (TEGs) has been constructed. The test bench is developed to analysis the performance of TEG system characteristics, which are undertaken to assess the feasibility of automotive applications. Based on the test bench, a new system called “four-TEGs” system is designed and assembled into prototype vehicle called “Warrior”, through the road test and revolving drum test table, characteristics of the system such as hot-side temperature, cold-side temperature, open circuit voltage and power output are studied, and a maximum power of 944W was obtained, which completely meets the automotive application. The present study shows the promising potential of using this kind of thermoelectric generator for low-temperature waste heat recovery vehicle.

Research on Waste Heat Recovery and Utilization of Hydrogen Production Industry Based on Heat Pipe Heat Exchanger Technology

For the low temperature waste heat recovery problem, waste heat recycling system was designed based on the heat pipe heat transfer technology of hydrogen in industry. The system is mainly in the process of hydrogen production by methanol steam reforming, using heat pipe excellent thermal conductivity, isothermal and thermal response characteristics, it will be in the low temperature waste heat of flue gas as heat source, in the heat pipe under the action of high efficiency heat transfer, through technology improvement, structure optimization design, achieved high efficiency, energy saved heat transfer to the methanol in the reaction system of hydrogen production, achieved the goal of remaining heat to be recycled.

Parametric Optimization of Organic Rankine Cycle by Genetic Algorithm

Organic Rankine Cycle (ORC) is widely used in the field of low temperature waste heat recovery, including solar, biomass and geothermal energy, among others. Based on the thermodynamic model of ORC system built up in Matlab, this study employ Genetic Algorithm (GA) on ORC system for parametric optimization and select a ratio of heat transfer area to total net power output as the performance evaluation criterion to predict the economy of system. R11, R113, R123 and isopentane are choosed as the working medium. The results show that the ORC system with isopentane has the minimum objective function value of 0.429m2/kw. The corresponding condensing temperature and degree of supercooling are generally located at lower boundary over their parametric design ranges, and the corresponding pinch point temperature difference are located at upper boundary. For different working fluids, there exist an optimum evaporating temperature and degree of superheat.