High-temperature Heat Recovery Research Articles

A novel strategy of inserting radiation shields to enhance the performance of thermoelectric generator systems for industrial high-temperature heat recovery

Static thermoelectric generator (TEG) systems have been widely used for recovering the high-temperature heat from the industrial sector. To control the heat loss through the gap uncovered by the thermoelectric legs for system performance improvement, a novel strategy of inserting the radiation shields in the gap that introduced additional surface and spacing resistances and regulated the gap radiation loss, was proposed. With a constructed high-fidelity theoretical model which used the finite element method to consider the temperature-dependent properties of the thermoelectric materials, comprehensive comparisons were conducted between the systems with/without radiation shields and using three commonly adopted strategies to evaluate the effectiveness and potential of this strategy. Compared to the base case without radiation shields, inserting one radiation shield with an emissivity of 0.5 enhances the TEG efficiency and system power by 15 % and 9.5 % respectively. The system using the proposed strategy outperforms that using the commonly adopted strategies, especially the strategies of lowering the gap pressure and filling the gap with thermal insulation materials, highlighting the potential of the proposed strategy. Besides, the influences of the shield emissivity and number were studied; from it, less than two radiation shields made by the polished copper are recommended for practical applications.

Life cycle assessment of post-combustion carbon capture and storage for the ultra-supercritical pulverized coal power plant

In this paper, different emerging post-combustion technologies, i.e., monoethanolamine (MEA), aqueous ammonia, pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane and calcium looping, were applied to an ultra-supercritical coal-fired power plant for carbon capture. A ‘cradle-to-grave’ life cycle assessment (LCA) was conducted to evaluate the technical performance and environmental impacts of the power plant with six emerging carbon capture technologies. Carbon capture significantly influences the impact categories directly associated with flue gas emission. The application of carbon capture reduced the GWP in the range of 49–75 %. TAP also reduced in the range of 18–51 %. However, the human toxicity potential, eutrophication potential, ecotoxicity potential and particulate matter formation potential increased due to energy and resource consumption in the upstream and downstream processes. For the life cycle water consumption potential, it decreased by 8 % with calcium looping, whereas it increased in the range of 36–75 % with other post-combustion technologies. The highest reduction in GWP and the least reduction in power efficiency was observed in calcium looping because of the high-temperature heat recovery from flue gas and elimination of complex solvent manufacturing. The plant with aqueous ammonia and membrane separation had the second and third highest reductions in GWP. In addition, the lowest values for TAP, FEP, and MEP were obtained in the membrane system. With MEA for CO2 capture, the total GWP value of the plant is slightly higher than these three technologies mentioned above, and the highest HTPc, FETP, and METP can be observed in this case. TSA and PSA have the most significant environmental impacts in most categories due to higher energy requirements.

Experimental and analytical investigations on heat transfer performance of naphthalene heat pipe heat exchanger

The heat transfer performance of a naphthalene heat pipe (HP) heat exchanger (HE) was theoretically analyzed and experimentally confirmed. The tested HE comprised a staggered 9 × 17 array of copper tubes with 144 U-bends, forming a closed loop. The working fluid was naphthalene with a filling ratio of 45 %. The operating temperature range was 80–300 °C, and the airflow rate was 100, 150, 200, or 400 m3/h. The maximum heat transfer rate was discovered to be 3.86 kW, and the HP thermal resistance was approximately 0.00553 K/W for an air flow rate of 100 m3/h and at an operating temperature of 300 °C; the maximum thermal conductivity was 7155.43 W/m·K. The experimental data were compared with the predictions of empirically derived equations; the best predictions were obtained when the Jouhara and Robinson correlation for condensation was combined with the Kutateladze correlation for evaporation. The mean deviations of the predictions were 36.05 % for the overall thermal resistance and 17.34 % for the average temperature difference between the evaporation and the air side. Although the thermal conductivity of the naphthalene HP was much lower than that of a water HP, it improved as the temperature increased; hence, naphthalene HPs are effective for high-temperature heat recovery.

On the Design of Recuperator for Transcritical Cycle Adopting CO2‐Based Mixture as Working Fluid: A Focus on Transport Properties Prediction

Transcritical cycles working with CO2‐based mixtures gain considerable attention due to thermodynamic efficiency gain compared to pure sCO2 in hot environments. Previous literature works prove that the adoption of CO2 mixtures provides a reduction of the levelized cost of electricity in concentrated solar power applications and medium–high temperature heat recovery. However, for techno‐economic analysis and heat exchanger design, proper evaluation of transport properties of the CO2‐based mixtures in power cycle conditions is necessary. Herein, it deals with the analysis of the proper transport properties models for CO2 mixtures to assess their actual thermal behavior. A literature review on transport properties models, and their validation with available experimental data, proves that the friction theory is suitable for CO2 blended with dopants having high molecular complexity. The impact of the different model selection on the recuperator sizing, considering optimized power cycle conditions, is assessed on the CO2 mixtures with hexafluorobenzene and decane: The TRAPP and Chung–Lee–Starling models are imported from Aspen Plus, while the friction theory model is implemented and calibrated in an in‐house MATLAB code. The optimal design of the recuperator for the CO2 + C6F6 mixture in a 100 MWel power block coupled with a solar power plant located in Sevilla is carried out.

Experimental demonstration of high-temperature heat recovery in a solar reactor

The production of drop-in liquid fuels via solar-driven thermochemical processes is a promising pathway for sustainable aviation and maritime sectors. The most important performance indicator is the solar-to-fuel energy efficiency, which is severely limited in a temperature-swing redox cycle without the application of heat recovery between the reduction and oxidation steps. In this work, we report on the successful experimental demonstration of high-temperature heat recovery in a solar reactor using a honeycomb-based thermal energy storage unit. With nitrogen as the heat transfer fluid, a heat recovery effectiveness of 33% is obtained using solar radiation as the sole energy input.

Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels

This study analyzes the technical performance, costs and life-cycle greenhouse gas (GHG) emissions of the production of various fuels using air-captured water and CO2, and concentrated solar energy as the source of high-temperature process heat. The solar thermochemical fuel production pathway utilizes a ceria-based redox cycle for splitting water and CO2 to syngas – a tailored mixture of H2 and CO – which in turn is further converted to liquid hydrocarbon fuels. The cycle is driven by concentrated solar heat and supplemented by a high-temperature thermal energy storage for round-the-clock continuous operation. The study examines three locations with high direct normal irradiation using a baseline heliostat field reflective area of 1 km2 for the production of six fuels, i.e. jet fuel and diesel via Fischer-Tropsch, methanol, gasoline via methanol, dimethyl ether, and hydrogen. Two scenarios are considered: near-term future by the year 2030 and long-term future beyond 2030.In the near-term future in Sierra Gorda (Chile), the minimum fuel selling price is estimated at around 76 €/GJ (2.5 €/L) for jet fuel and diesel, 65 €/GJ for methanol, gasoline (via methanol) and dimethyl ether (DME), and 42 €/GJ for hydrogen (excluding liquefaction). In the long-term future, with advancements in solar receiver, redox reactor, high-temperature heat recovery and direct air capture technologies, the industrial-scale plant could achieve a solar-to-fuel efficiency up to 13–19 %, depending on the target fuel, resulting in a minimum fuel selling price of 16–38 €/GJ (0.6–1.3 €/L) for jet fuel and diesel, and 14–32 €/GJ for methanol, gasoline, and DME, making these fuels synthesized from sunlight and air cost-competitive vis-à-vis e-fuels. To produce the same fuels in Tabernas (Spain) and Ouarzazate (Morocco) as in Sierra Gorda, the production cost would increase by 22–33 %. Greenhouse gas savings can be over 80 % already in the near-term future.

High-temperature heat recovery from a solar reactor for the thermochemical redox splitting of H2O and CO2

The solar splitting of H2O and CO2 via a thermochemical redox cycle offers a viable pathway for producing sustainable drop-in fuels for the transportation sectors. The key performance metric is its solar-to-fuel energy efficiency, which is strongly dependent on the ability to recover heat during the temperature swing between the reduction and oxidation steps. Here we report on the experimental investigation of a novel heat recovery method based on coupling the solar reactor with two thermocline energy storage units made of a packed-bed of alumina spheres. Using N2 as an inert heat transfer fluid, the heat rejected during cooling from the reduction to the oxidation temperature is stored and, following the oxidation step, delivered back to preheat the solar reactor towards the reduction temperature, thus reducing the required solar input and consequently increasing the efficiency. With a first experimental prototype, a heat extraction effectiveness of up to 70% from a 4 kW solar reactor is obtained with measured N2 outlet temperatures exceeding 1250°C. Energy flow modeling of a 50 kW solar reactor predicts a theoretical upper limit value of the energy efficiency of 42% for perfect heat recovery without transient losses, and 14.7% with such losses included. Several improvements and insights into high-temperature heat recovery are detailed.

Direct cooling of a planar magnetic converter using dielectric liquid forced convection enabled by additive manufacturing

Power densification of magnetic converters has encountered a fundamental challenge due to the inability to effectively remove heat produced by power losses. State-of-the-art thermal management technologies for planar printed circuit board (PCB) transformers are limited by the inability to substantially improve the overall thermal conductance from the transformer to the cold plate. Furthermore, a lack of thermal management technologies capable of reducing temperature gradients in these devices exists. Direct liquid cooling reduces the number of thermal resistances from hot spot to coolant, resulting in lower maximum temperatures and reduced thermal gradients. Here, we study the thermal-hydraulic performance of a PCB planar transformer cooled using a polymer additively manufactured (AM) housing and single-phase internal flow with Novec 7300 and pure ethylene glycol. Computational fluid dynamics simulations were used to identify hot spots, quantify temperature gradients, and characterize pressure drop. Experiments in a custom-made setup quantified the maximum temperature and pressure drop with different flow rates (0.4–4 LPM) of Novec 7300 and ethylene glycol. Both working fluids showed better thermal-hydraulic performance when compared to results found in literature which implements cold-plates. Analysis reveals that selection between both fluids is guided by flow rate, pressure drop and pumping power, such that if low pressure drop is a priority, Novec 7300 is a better choice, and if low pumping power and flow rate are a priority, ethylene glycol is better. Our results show that direct liquid cooling can reduce thermal resistance by up to 80% using an 80% lower pressure drop when compared to a cold-plate benchmark. As a result, the developed AM-enabled thermal management technology can potentially improve the reliability of the converter, provide an alternative for high temperature heat recovery, and facilitate further converter power densification.

Decarbonization options for cement production process: A techno-economic and environmental evaluation

The long-term environmental targets require significant cuts of CO2 emissions in fossil fuel-intensive industrial processes. Currently, the cement production sector is responsible for approximate 8% of world anthropogenic CO2 emissions. The present paper assesses from the technical, environmental and economic point of view different decarbonization systems integrated into cement production plants. As CO2 capture technologies, the tail-end post-combustion capture using amine-based chemical scrubbing, membrane and calcium looping were assessed as well as the partial and total oxy-combustion options. The size of decarbonized cement concept has a capacity of 1 Mt/y with 90% CO2 capture rate. The overall techno-economic assessment was based on modelling and simulation to assess the key performance indexes. A non-decarbonized cement plant was also assessed for comparison to calculate the energy and economic penalties for decarbonization. The membrane and oxy-combustion concepts have the lowest cement production cost in comparison to the reactive absorption and adsorption concepts (112–119 vs. 125–145 €/t). The full oxy-combustion plant followed by the membrane concept exhibit the lowest CO2 avoided costs comparable with the current emission tax (60–70 vs. 60–80 €/t). The reactive gas–solid system (calcium looping) exhibits superior indicators than the chemical scrubbing system due to high temperature heat recovery.

Carbon Dioxide Mixtures as Working Fluid for High-Temperature Heat Recovery: A Thermodynamic Comparison with Transcritical Organic Rankine Cycles

This study aims to provide a thermodynamic comparison between supercritical CO2 cycles and ORC cycles utilizing flue gases as waste heat source. Moreover, the possibility of using CO2 mixtures as working fluids in transcritical cycles to enhance the performance of the thermodynamic cycle is explored. ORCs operating with pure working fluids show higher cyclic thermal and total efficiencies compared to supercritical CO2 cycles; thus, they represent a better option for high-temperature waste heat recovery provided that the thermal stability at a higher temperature has been assessed. Based on the improved global thermodynamic performance and good thermal stability of R134a, CO2-R134a is investigated as an illustrative, promising working fluid mixture for transcritical power cycles. The results show that a total efficiency of 0.1476 is obtained for the CO2-R134a mixture (0.3 mole fraction of R134a) at a maximum cycle pressure of 200 bars, which is 15.86% higher than the supercritical carbon dioxide cycle efficiency of 0.1274, obtained at the comparatively high maximum pressure of 300 bars. Steam cycles, owing to their larger number of required turbine stages and lower power output, did not prove to be a suitable option in this application.

A Fast Prediction Model for Heat Transfer of Hot-Wall Heat Exchanger Based on Analytical Solution

The hot-wall heat exchanger (HWHE) has been widely used in thermal engineering fields such as ceiling radiant heating/cooling, refrigerator condenser, solar heat collection, and high-temperature heat recovery. However, the numerical simulation normally used for heat transfer prediction in HWHE is usually not as convenient as the analytic solutions in engineering applications. In this paper, a new heat transfer mathematical model of HWHE-based on analytic solutions was developed, which could be much faster to obtain the heat transfer properties of HWHE. The proposed model was validated under four conditions with literature values, which showed that the deviations of heat flux are 2.53%, 0.99%, 2.12%, and 1.96%, indicating its accuracy is satisfied. The model was then used to analyze the thermal property of HWHE. The results show the thermal resistance caused by panel with heat convection and conduction accounts for 96.54% of HWHE thermal resistance, and the thermal resistance caused by heat convection on the surface of panel is 74.43%. The analyzation results also show that adding aluminum foil around pipes could decrease HWHE thermal resistance by 5.11%. Besides, the influence of pipe diameters, pipe distance, pipe heat conductivity, side wall heat conductivity, and convective heat transfer coefficient on the heat transfer performance of HWHE was analyzed. The research in this paper can be used for fast prediction and optimization of heat transfer in HWHE.

Study on power plants arrangements for integration

The three power cycles viz. gas cycle (Brayton cycle), steam cycle (Rankine cycle) and organic Rankine cycle (ORC) are arranged in two possible methods to form a triple cycle and to increase its power generating performance. In one approach, all the three cycles are arranged in series, the heat rejection of gas cycle is supplied to steam power plant having back pressure turbine and the heat rejection of steam plant is supplied to ORC plant similar to a cascading. In second option, the two bottoming cycles i.e. steam plant and ORC plant are arranged in parallel to gas turbine exhaust. In this connection, steam cycle works with high temperature heat recovery and ORC plant with low temperature heat recovery. Thermal efficiency and specific work of two triple cycles are compared with combined cycle (CC) power plant to draw the relative merits of two choices with a focus on compressor pressure ratio and gas turbine inlet temperature (GTIT). The results showed that the parallel arrangement in triple cycle offers greater benefit over the series arrangement. Therefore the wok is focused on parallel configuration triple cycle results.

Experimental and numerical investigations of a new high temperature heat pump for industrial heat recovery using water as refrigerant

A new high temperature heat pump using water as refrigerant has been designed and built for testing on a laboratory test bench that reproduces the operating conditions of real-case industrial applications. Experimental investigations of this heat pump were carried out in the condensing temperature range of 130–140 °C. A new dynamic model has also been developed using Modelica to take into account the presence of non-condensable gases and the purging mechanism. In addition, a new combined finite-volume and moving boundary method (FV-MB) approach is applied for the plate heat-exchanger models and a moving boundary (MB) approach between phases is implemented for the models of the purging and the flash evaporation systems. The start-up phase of this high-temperature heat recovery and heat enhancement of the heat pump are simulated experimentally and numerically. A comparison between both results is presented and analyzed. Global energy savings and the environmental benefits have been illustrated.

Design of Cascaded Oxide Thermoelectric Generator

This paper describes the design of two- and three-stage cascaded oxide thermoelectric generators (TEGs) for high-temperature heat recovery using reported data to optimize energy conversion efficiency. We used the general intermetallic compounds Bi 2 (Se,Te) 3 and (Bi,Sb) 2 Te 3 for the low-temperature stages and oxides of TiO 1.1 , La-doped SrTiO 3 , Na x Co 2 O 4 , and Al-doped ZnO for the higher-temperature stages. A two-stage TEG with TiO 1.1 as the p-type material and La-doped SrTiO 3 as the n-type material was found to have the highest efficiency at heat-source temperatures below 852 K, while the three-stage TEG was slightly more efficient than the two-stage TEG for heat-source temperatures above 852 K. For the three-stage TEG, the optimal boundary temperature of the second and third stages was calculated to be 698 K; at this temperature, the maximum energy conversion efficiency, 13.5%, was obtained at a heat-source temperature of 1223 K. The results showed that the designed two- and three-stage cascaded oxide TEGs have high potential for heat recovery from high-temperature waste.

Solar energy powered Rankine cycle using supercritical CO 2

A solar energy powered Rankine cycle using supercritical CO 2 for combined production of electricity and thermal energy is proposed. The proposed system consists of evacuated solar collectors, power generating turbine, high-temperature heat recovery system, low-temperature heat recovery system, and feed pump. The system utilizes evacuated solar collectors to convert CO 2 into high-temperature supercritical state, used to drive a turbine and thereby produce mechanical energy and hence electricity. The system also recovers heat (high-temperature heat and low-temperature heat), which could be used for refrigeration, air conditioning, hot water supply, etc. in domestic or commercial buildings. An experimental prototype has been designed and constructed. The prototype system has been tested under typical summer conditions in Kyoto, Japan; It was found that CO 2 is efficiently converted into high-temperature supercritical state, of while electricity and hot water can be generated. The experimental results show that the solar energy powered Rankine cycle using CO 2 works stably in a trans-critical region. The estimated power generation efficiency is 0.25 and heat recovery efficiency is 0.65. This study shows the potential of the application of the solar-powered Rankine cycle using supercritical CO 2.

Experimental study of a burner with high temperature heat recovery system for TPV applications

An experimental investigation to develop and test a burner and a heat recovery system for thermophotovoltaic (TPV) applications is presented. Experimental data have been compared with theoretical calculations and considerations in the pre-design and design phases of the project to find the weakest point of the concept and to validate the expected performance. The TPV generator has been designed as a compact module in order to be used as a range extender in an electric car. The heat recovery system is the key element to increase the efficiency of the system. The heat recovery system presented in this paper is a rotary type regenerator that is very compact and has higher effectiveness in comparison with other types of regenerators with the same number of transfer units (NTU). The experimental data have been used to verify the numerical models used in the calculations for design of the regenerator matrix. A new version of the numerical model has been developed to take into account the variation of the thermal properties of the system with the temperature. Dimensions, weight, efficiency, emissions and high working temperatures have been the most important competitive constraints to observe for design of the system.

Modeling and effect of leakages on heat transfer performance of fixed matrix regenerators

The following undesired leakages are inherent in operation of fixed matrix regenerators: gas pressure leakages due to pressure difference such as leakages through the valves and through cracks in regenerator housing for very high temperature heat recovery, and carryover leakages from the hot gas to cold gas and vice versa. The objective of this paper is to present a methodology for evaluation of the leakages and to determine quantitatively the detrimental influence of pressure leakages on the regenerator heat transfer performance. In this respect, a drop (reduction) in actual regenerator heat transfer effectiveness due to various leakages is presented in the paper. The results clearly suggest that a drop in the effectiveness due to leakages can be significant and depends on the category of leakages. The flow leakages due to the cracks in the regenerator housing have the most impact on reducing the regenerator performance.

Groundwater treatment for high temperature heat recovery

Aquifer thermal energy storage (ATES) systems have been utilized to conserve high temperature waste heat from industrial processes. In the heat recovery phase, whenever groundwater with a high level of hardness is heated to temperatures in the range of 50°C or higher, heat exchanger surfaces are susceptible to scale buildup with subsequent clogging of the heat exchanger. In this study, the performance of CO2 treatment as a means of preventing CaCO3 scaling in heat exchangers was evaluated. The rate of CO2 gas injected into the water was controlled by a proportional control valve to maintain a CaCO3 saturation index of zero. The CO2 treatment method showed a beneficial effect in controlling scaling on the brass heat exchanger tubes.Key words: aquifer thermal energy storage, CO2 treatment, CaCO3 scaling.

New working pairs for medium and high temperature industrial absorption heat pumps

The application of absorption cycles for high temperature heat recovery systems calls for the investigation of new working pairs. To qualify as a potential working pair, a mixture of two substances has to fulfill stringent requirements with respect to thermodynamic properties, corrosion and safety hazards like toxicity and inflammability. Based on a thermodynamic analysis of an absorption heat pump cycle a systematic search for new working pairs has been conducted. The investigation dealt exclusively with organic compounds. To get a first estimate of the properties of mixing, a molecular group contribution model (UNIFAC) was employed. This method has so far been widely used for chemical engineering purposes and renders agreeable estimates for the thermodynamic behavior of organic mixtures. With the help of the UNIFAC method, various pairings of functional molecular groups have been investigated for their possible potential to form effective working pairs for medium and high temperature absorption heat pump cycles. As a consequence of this analysis, ten alternative working pairs are proposed and their respective theoretical performance data and their toxicity characteristics are given.

Advanced industrial ceramic heat pipe recuperators

This paper summarizes the results of an investigation involving the use of ceramic heat pipe recuperators for high-temperature heat recovery from industrial furnaces. The function of the recuperator is to preheat combustion air with furnace exhaust gas. To maximize fuel savings, a very high air preheat temperature is desirable; this necessitates the use of ceramic elements in the recuperator. The heat pipe recuperator comprises a bundle of individual ceramic heat pipes acting in concert, with a partition separating the air and exhaust gas flow streams. The heat pipe fluid is a liquid metal. The ceramic heat pipe recuperator concept offers several advantages as compared to tubular type ceramic recuperators. Because each heat pipe is essentially an independent heat exchanger, the failure of a single tube does not compromise recuperator integrity and has only a minimal effect on overall heat exchanger performance. This independent element characteristic also enables easier replacement of individual heat pipes in the recuperator. In addition, the heat pipe acts as an essentially isothermal heat transfer device, leading to a high thermodynamic efficiency. Cost estimates developed for heat pipe recuperator systems indicate favorable payback periods. Laboratory studies have demonstrated the feasibility of fabricating the required ceramic tubes, coating the inside of the tubes with CVD tungsten (which functions as both a protective layer and a heat pipe fluid wick), and sealing the heat pipe with an electron-beam-welded or vacuum-brazed end cap.