Temperature Waste Heat Research Articles

Modeling and analysis of an ammonia–water absorption refrigeration system utilizing waste heat with large temperature span

Waste heat sources such as flue gas generate large temperature span during recovery process. In order to improve the temperature match with such waste heat sources, a novel absorption refrigeration system with continuous-temperature-changing generation process is proposed. Internal heat recovery process is introduced to both the generator and absorber, and a premixer is added to take the place of the solution heat exchanger in traditional system. A numerical model of the system is built, in which the continuous-temperature-changing generation process is simulated based on a stripping column with heat input at each stage. New parameters are defined and optimized to strengthen the internal heat recovery process, and thus to improve system efficiency. Simulation results show that the system can operate under a large waste heat temperature span of 67.5 K, almost twice of the conventional system. With the evaporation temperature, waste heat inlet temperature, and cooling water temperature at −15 °C, 150 °C and 20 °C, respectively, a maximum COP of 0.93 and exergy efficiency of 49.7% is acquired while the waste heat temperature span is 60 K, showing that the system can acquire larger utilization rate of waste heat with higher system efficiency.

Dynamic modelling and control of supercritical CO2 power cycle using waste heat from industrial processes

Large amount of waste heat is available for recovery in industrial processes worldwide. However, significant proportion (up to 50%) of this thermal energy is released directly to the environment. Application of waste heat to power (WHP) technologies can increase the energy efficiency and cut CO2 emissions from these facilities. Steam Rankine cycle (SRC) and organic Rankine cycle (ORC) are commonly deployed for this purpose. The main drawback of SRC and ORC is the high irreversibility in the heat exchangers. In addition, ORC has limited temperature range and low efficiency while SRC has a large footprint. Supercritical CO2 (sCO2) power cycle is considered an attractive option, which provides better matching of waste heat temperature in the main heater (i.e. low irreversibility). It offers compact design, improved performance and it is applicable to a wide range of waste heat source temperature. The conditions of industrial waste heat sources are highly variable due to continuous fluctuations in the operation of the process. This is likely to significantly affect the dynamic performance and operation of the sCO2 power cycle. In this work, dynamic model in Matlab/Simulink was developed to assess the dynamic performance and control of the sCO2 power cycle for waste heat recovery from cement industry. The case of waste heat at 380 °C utilized to deliver 5 MWe of power was considered. Steady state simulation was performed to determine the design point values. Open loop simulation was performed to show the inherent dynamic response to step change in the temperature of the waste heat. The dynamic performance and control of the system under varying exhaust gas flow rate between 100% and 50% of the design value were studied. Similar study was done for varying exhaust gas temperature between 380 °C and 300 °C. The results showed that the thermal efficiency of proposed single recuperator recompression sCO2 is about 33%. Stable operation of the system can achieved by using cooling water control and throttle valve to maintain constant precooler outlet condition. Dynamic simulation result showed that it is best to allow the turbine inlet temperature to vary according to the fluctuation in the waste heat source. These findings indicated that dynamic modelling and simulation of WHP system could contribute to understanding of the behaviour and control system development under fluctuating waste heat source conditions.

Optimization model for power generation using thermoelectric generator

In this work, optimization model has been developed to simulate TEG (Thermoelectric generator) based power generation system. Proposed model able to predict the gross power, net power and pumping power for all operating conditions and heat exchanger dimensions. Model also able to choose the optimum operating conditions, dimensions of heat exchanger to obtained maximum net power. TEG module has been chosen from the commercially available modules with efficiency of 1.91% for the targeted low-grade waste heat temperature of Th=90 oC and Tc=15 oC. Predicted maximum net power was 79.02 W from 50 TEG modules and optimum operating conditions were 300 L/min mass flow rate, exchanger dimensions are 0.08 m length, 1 m width and 4 mm gap size.

Pyro-electrolytic water splitting for hydrogen generation

Water splitting by thermal cycling of a pyroelectric element that acts as an external charge source offers an alternative method to produce hydrogen from transient low-grade waste heat or natural temperature changes. In contrast to conventional energy harvesting, where the optimised load resistance is used to maximise the combination of current and voltage, for water splitting applications there is a need to optimise the system to achieve a sufficiently high potential difference for water electrolysis, whilst also maintaining a high current output. For the thermal harvesting system examined here, a high impedance 0.5 M KOH electrolyte with working electrodes connected to a rectified pyroelectric harvester produced the highest voltage of 2.34 V, which was sufficient for H2 generation. In addition to electrolyte concentration, the frequency of the temperature oscillations was examined and reducing the heating-cooling frequency led to a larger change in temperature to generate increased pyroelectric charge and a higher potential difference for pyro-water splitting. Finally, in the absence of sacrificial reagents, cyclic production of H2 (0.654 μmol/h) was demonstrated for the optimised processing parameters of electrolyte and thermal cycling frequency using the external pyroelectric element as a charge source for water splitting.

Performance Analysis of a RED-MED Salinity Gradient Heat Engine

A performance analysis of a salinity gradient heat engine (SGP-HE) is presented for the conversion of low temperature heat into power via a closed-loop Reverse Electrodialysis (RED) coupled with Multi-Effect Distillation (MED). Mathematical models for the RED and MED systems have been purposely developed in order to investigate the performance of both processes and have been then coupled to analyze the efficiency of the overall integrated system. The influence of the main operating conditions (i.e., solutions concentration and velocity) has been quantified, looking at the power density and conversion efficiency of the RED unit, MED Specific Thermal Consumption (STC) and at the overall system exergy efficiency. Results show how the membrane properties (i.e., electrical resistance, permselectivity, water and salt permeability) dramatically affect the performance of the RED process. In particular, the power density achievable using membranes with optimized features (ideal membranes) can be more than three times higher than that obtained with current reference ion exchange membranes. On the other hand, MED STC is strongly influenced by the available waste heat temperature, feed salinity and recovery ratio to be achieved. Lowest values of STC below 25 kWh/m3 can be reached at 100 °C and 27 effects. Increasing the feed salinity also increases the STC, while an increase in the recovery ratio is beneficial for the thermal efficiency of the system. For the integrated system, a more complex influence of operating parameters has been found, leading to the identification of some favorable operating conditions in which exergy efficiency close to 7% (1.4% thermal) can be achieved for the case of current membranes, and up to almost 31% (6.6% thermal) assuming ideal membrane properties.

Cascade energy optimization for waste heat recovery in distributed energy systems

The efficiency of distributed energy systems can be significantly increased through waste heat recovery from industry or power generation. The technologies used for this process are typically dependent on the quality and temperature grades of waste heat. To maximize the efficiency of cascade heat utilization, it is important to optimize the choice of waste heat recovery technologies and their operation. In this paper, a detailed mixed integer linear programming optimization model is proposed for waste heat recovery in a district-scale microgrid. The model can distinguish waste heat quality for planning and operation optimization of distributed energy systems. Heat utilization technologies are formulated in this developed model and categorized in different temperature grades. The developed model is validated using four typical cases under different settings of system operation and business models. It is found that the optimization model, by distinguishing waste heat temperature, can increase energy cost savings by around 5%, compared to models that do not consider waste heat temperature grades. Additionally, the results indicate that the developed model can provide more realistic configuration and technologies dispatch.

Influence of the working fluid properties on optimized power of an irreversible finite dimensions Carnot engine

A Carnot-type engine with a changing phase working fluid is modelled and optimized. The engine is externally heated by a waste heat in order to valorize it. The waste heat is a flue gas at 350 °C, corresponding to a usual temperature of industrial furnace effluents. The objective function is the maximization of the net power output. In the numerical application, the working fluid is water. The optimization variables are the working fluid temperature of vaporization, its temperature of condensation and the allocation of a total thermal transfer area between the boiler and the condenser. This article aims to define an upper bound for waste heat to power conversion, depending on the waste heat temperature and its available mass flow rate, on the working fluid used and on the exchangers’ size. The methodology used can be applied to optimize other externally heated engines. Different control volumes are selected for the analysis: the adiabatic converter, the non-adiabatic converter, the system (the converter and the exchangers) and the system placed in the environment. The impact of the choice of the control volume on optimization results, global efficiency and entropy production is studied. It appears that the optimal heating and cooling temperatures are the same in all sub-systems, but the optimal allocation of the total thermal transfer area differs slightly. However, in all cases the optimal condenser thermal transfer area is larger than the optimal boiler thermal transfer area. This imbalance corresponds to the need to evacuate the mechanical energy degradation into heat. It also appears that the control volume increase implies a decrease of the first law efficiency. The entropy analysis allows an identification and a quantification of the different contributions to entropy production. Then, a sensitivity analysis to the relevant parameters is carried out. The relevant parameters are the heating fluid temperature and mass flow rate, the total thermal transfer area, the compressor efficiency and the turbine efficiency.

Activation of Persulfate at Waste Heat Temperatures for Humic Acid Degradation

Humic acid (HA) as a major constituent of natural organic matter (NOM) in raw water presents major challenges to drinking water production including membrane fouling and serving as a precursor for ...

Power generation from low grade waste heat using thermoelectric generator

Thermoelectric technology is thought to be a great solution in near future for producing electrical power and recovering low grade waste heat to cut the cost of power generation because of its consistency and eco-friendly affability. Though commercial accessibility of TEG is available currently but heat to electricity conversion efficiency is still low and cost of the module is reasonably high. It’s essential to use the modules competently which is strongly depends on suitable heat exchanger design and selection of proper operating conditions. In this work, TEG module has been selected from the commercially available modules with efficiency of 1.91% for the targeted low-grade waste heat temperature of Th=90°C and Tc=15°C which validated by experiment. Mathematical model has been proposed to simulate TEG based power generation system; the model can predict maximum net power, choose optimum operating conditions and dimensions of heat exchanger. Lab scale design with channel length 1 m, width 0.08 m and gap size 9 mm which is suitable for 50 TEG module (4 mm x 4 mm) have been simulated using proposed mathematical model. For above temperature range, predicted optimum net power was 76.45 W with optimum flow rate 0.94 L/s (56.4 L/min). This lab scale setup will be used for experimental validation of the proposed mathematical model. The obtained results from experiments and simulation are closely matched.

Comparative Thermodynamic Assessment of Various Super- and Trans-Critical Working Fluids for Low Temperature Power Generation Applications

Recently, energy need is exponentially increasing in the world while energy sources are decreasing rapidly. Therefore, this issue requires energy sources to be used more efficiently and urges professionals to utilize energy from low temperature energy sources such as waste heat and low temperature renewable sources. In this study, energy and exergy analyses of several clean working fluids are comparatively studied in several organic Rankine cycle configurations. CO 2 , N 2 O, and SF 6 fluids are compared with the conventional R23 in three ORC configurations, namely the basic ORC cycle, regenerative ORC cycle, and reheat and regenerative ORC cycle, respectively. Effects of various selected system and environmental parameters on the system performances are comprehensively investigated. Even though R23 shows the best energy and exergy performances than those of other investigated working fluids at low-temperature applications, N 2 O and CO 2 provide a clean solution to high GWP (global warming potential) R23 with similar performance characteristics at low and high temperature power generation applications.

Simulation research on a variable-lift absorption cycle and its application in waste heat recovery of combined heat and power system

To solve the dilemma between temperature lift capability and coefficient of performance (COP) when absorption heat pump is applied in low grade heat utilization, a variable-lift absorption cycle and an improved one based on it are introduced here. The variable-lift absorption cycles have relatively simple systems and contains enough number of absorbers and condensers to develop plenty of gradually increasing temperature levels to match heating process. For a given heating demand and waste heat temperature level, the results of application of variable-lift absorption cycles in waste heat recovery of combined heat and power(CHP) system show that, the variable-lift cycles can always work efficiently when return water temperature of primary heating network (PHN) varies from 30 °C to 50 °C. The needed heat source temperatures of variable-lift cycles are always over 12 °C lower than single effect cycle. The higher return water temperature of PHN yields more obvious advantage of the improved cycle. When the return water temperature of PHN is 50 °C, variable lift-2C cycle and variable lift-3C cycle can reduce the equivalent power consumption for space heating by 8.5% and 15% respectively when comparing to single effect cycle. The essence of the variable-lift cycles is further clarified from the heat transfer analysis in T-Q diagram.

Experimental Investigation of the Effect of Generator Temperature on the Performance of Solution Transportation Absorption Chiller

It is effective to recover waste heat to reduce primary energy consumption. From this point of view, we proposed and examined a new idea of heat transportation using ammonia–water as the working fluid in the system named the Solution Transportation Absorption chiller (STA). As waste heat sources are not necessarily located close to areas of heat demand, conventionally, absorption chillers are located on heat source side and produce chilled water that is transported to heat demand side through pipelines with an insulation. In contrast, the proposed system STA divides an absorption chiller into two parts. The generator and the condenser are located on heat source side while the evaporator and the absorber are on heat demand side. Both the conventional system and STA system satisfy the same boundary condition of heat recovery and heat supply to the demand side, STA can work for transferring thermal energy as the conventional system does even though the temperature of the media is ambient without an insulation. Our previous studies of the STA were based on the experimental investigation with the STA facility where the cooling power was 90[Formula: see text]kW (25.6 refrigeration ton) at the generator temperature 120[Formula: see text]C from 0[Formula: see text]m (normal absorption chiller) to 1000[Formula: see text]m. Thus, the Coefficient of Performance (COP) of STA was found to have almost the same value of 0.65 with conventional absorption chillers without depending on the transportation distances. The objective of this study is to examine the effect of generator temperature from 100[Formula: see text]C to 120[Formula: see text]C on the performance of solution transportation of ammonia–water solution, because the generator temperature is directly linked to the waste heat temperature, so its effect needs to be investigated. The experimental facility tested the performance with 0[Formula: see text]m (normal absorption chiller), 200[Formula: see text]m and 500[Formula: see text]m distance. The results indicate that the effect of the generator temperature and solution transportation distances showed no significant on the COP.

Technical and economic feasibility of the Isopropanol-Acetone-Hydrogen chemical heat pump based on a lab-scale prototype

Chemical heat pump are promising alternatives in waste heat recovery applications. The present paper focuses on the technical and economic feasibility analysis of the Isopropanol-Acetone-Hydrogen Chemical Heat Pump (IAH-CHP) system. A small scale prototype of the IAH-CHP was established. Coefficient of performance (COP), exergy efficiency and entransy efficiency analysis were adopted to evaluate the performance of the IAH-CHP prototype. The stable operation is given with the waste heat temperature of 90 °C and the high-level output temperature of 160 °C. The COP, exergy efficiency and entransy efficiency of the system are up to 24.3%, 42.3% and 29.1%, respectively. Moreover, based on the detailed experimental results of the lab-scale apparatus, a 100 kWth model was built to evaluate economic feasibility of the IAH-CHP. The exergy cost and the thermoeconomic cost based on the structural theory, as well as the payback period were evaluated. The results indicate that the exergy destruction and investment cost of the distillation column is the highest, and the payback period is 5.6 year for the case of the optimal performance. The unit exergy cost of the final exergetic product is 6.56 W/W. The results proved that the IAH-CHP system is efficient in recovering the low-level waste heat.

Modeling the Effect of Heat Source Temperature on the Performance of Two-stage Air Cooled Silica Gel + Water Adsorption System

Adsorption systems can utilize low grade waste heat (temperature <100 degrees C) or energy from renewable sources like solar for cogeneration i.e. cooling and desalination. The throughput and performance of such systems are sensitive to the available temperature difference between heat source and sink. This paper presents a modelling study of two-stage 2-bed air cooled silica gel + water adsorption system wherein the effect of heat source temperature variation in the range of 65-85 degrees C on system performance parameters viz. SCC, SDWP and COP are investigated for the first time. It is shown that the optimum half cycle time for SCC and SDWP increases from 1800 s to 2700 s as the source temperature decreases from 85 degrees C to 65 degrees C; thereby necessitating the use of adaptive cycle time control for maximum throughput. However, it is observed that the COP is relatively insensitive to such alterations.

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.

Modelling and Optimization of Low-temperature Waste Heat Thermoelectric Generator System

Abstract Thermoelectric generation (TEG) technology has several kinds of merits; waste heat recovery is one of its most promising applications, is becoming a noticeable area of research. The primary challenge of this technology is its relatively low heat to electricity conversion (low conversion efficiency), but low efficiency is not a major issue since this technology used waste heat; costless thermal energy input. System performance/efficiency of TEG technology is depends on module properties and system configurations. In this project, a theoretical model has been developed to analyze the different affecting factors to optimize the electric power from low grade waste heat using TEG technology. From the optimization model it can be found that waste heat temperature, arrangement of TE modules, enhancing heat transfer co-efficient and parasitic loss plays an important role to maximize the net electric power. This project is still under develop, the aim of this project is to find out all possible affecting factors to optimize the output power from low grade waste heat source and also the theoretical analysis will be used to established an experimental setup to validate the optimization technique.

Residential Heat Supply by Waste-Heat Re-Use: Sources, Supply Potential and Demand Coverage—A Case Study

This paper deals with climate change mitigation and addresses waste heat reuse as a measure which is until now considered only to a limited extent. The City of Vienna serves as a case study to explore potentials to improve the urban heat supply using waste heat as an additional energy source. As no observation data about waste heat and detailed heating demand is available, this data is derived from proxy data for estimating waste heat reuse potential and residential heating demand patterns. Heat requirements for manufacturing and service provision is explored and, based on the distribution of the companies within the city, mapped as waste heat sources. Employees per company serves as proxy data to allocate the heat volume. Waste heat share and temperature ranges is reviewed from literature. Heating demand is mapped based on floor space of the buildings by age class and building type. Merging supply and demand maps allows to quantify the residential heating demand coverage through local waste heat in the potential supply areas within different distance ranges and housing density classes. In high density housing areas, only a small share of the demand can be covered by waste heat supply even within 250 m distance from sources due to few companies which could provide waste heat. In medium to low density housing areas in Vienna’s outer districts with more industry, a higher share of residential heating demand near the sources can be covered by waste heat within a 250 m distance. Within a 500 m distance, around half of the residential heating demand can be covered only in low density housing areas near the waste heat sources.

스택 폐열을 이용한 연료전지 자동차용 열펌프 시스템의 성능 특성에 관한 연구

This study was conducted to develop a heating system for a fuel cell-driven electric vehicle. The system consists of a compressor, an expansion device and three heat exchangers. A conventional air source heat exchanger is used as primary heat exchanger of the system, and an additional water source heat exchanger is used as a pre-heater to supply heat to the upstream air of the primary heat exchanger. On the other hand, the third heat exchanger consists of a water-to-refrigerant heat exchanger. The heat source of the pre-heater and the water-refrigerant heat exchanger is the waste heat from the fuel cell's stack. In the experiment, the indoor and the outdoor air temperature were fixed, and the compressor speed, EEV opening and waste heat temperature were varied. The results indicate that the <TEX>$COP_h$</TEX> of the proposed system is 3.01 when the system is operating at a 1,200 rpm compressor speed, 50% EEV opening, and <TEX>$50^{\circ}C$</TEX> waste heat source temperature in air pre-heater operation. However, when the system uses a water-refrigerant heat exchanger, the <TEX>$COP_h$</TEX> increases to up to 9.42 at the same compressor speed and waste heat source temperature with 75% EEV openings.

Thermodynamic analysis and comparison between CO2 transcritical power cycles and R245fa organic Rankine cycles for low grade heat to power energy conversion

In this paper, a theoretical study is conducted to investigate and compare the performance of CO2 transcritical power cycles (T-CO2) and R245fa organic Rankine cycles (ORCs) using low-grade thermal energy to produce useful shaft or electrical power. Each power cycle consists of typical Rankine cycle components, such as a working fluid pump, gas generator or evaporator, turbine with electricity generator, air cooled condenser and recuperator (internal heat exchanger). The thermodynamic models of both cycles have been developed and are applied to calculate and compare the cycle thermal and exergy efficiencies at different operating conditions and control strategies. The simulation results show that the system performances for both cycles vary with different operating conditions. When the heat source (waste heat) temperature increases from 120°C to 260°C and heat sink (cooling air) temperature is reduced from 20°C to 0°C, both thermal efficiencies of R245fa ORC and T-CO2 with recuperator can significantly increase. On the other hand, R245fa ORC and T-CO2 exergy efficiencies increase with lower heat sink temperatures and generally decrease with higher heat source temperatures. In addition, with the same operating conditions and heat transfer assumptions, the thermal and exergy efficiencies of R245fa ORCs are both slightly higher than those of T-CO2. However, the efficiencies of both cycles can be enhanced by installing a recuperator in each system at specified operating conditions. Ultimately, optimal operating states can be predicted, with particular focus on the working fluid expander inlet pressure for both cycles.

Comparative assessment of different categories of absorption heat transformers in water desalination process

During the last decade, there has been a significant amount of research conducted on absorption heat pumps (AHPs). Second type AHPs, namely absorption heat transformers (AHTs) are useful devices, capable of raising the temperature of waste heat to more useful temperature levels. The excluded heat from AHTs can provide the required energy for desalination. In the present work, performances of three main categories of AHT systems comprising single, double and triple absorption heat transformers (SAHT, DAHT, and TAHT) while they are coupled to water purification system are investigated and compared. A simulation model has been developed in EES (Engineering Equation Solver) to evaluate the effects of several key thermodynamic parameters on the performance of the cycle and amount of desalinated water. The results show that the highest gross temperature lift can be achieved with TAHT while it has the lowest COP among the other types of AHTs. Additionally, a performance optimization performed by using EES shows that SAHT, DAHT, and TAHT can provide fresh water for 853, 796 and 697 residential units respectively if they operate nonstop.