Low-grade Heat Recovery Research Articles

Study of the Process of Generation of Thermoelectricity in Low Grade Heat Recovery of Waste Gases

Purpose of research.The purpose of the study is to develop an experimental design of an integrated air heater, conduct and analyze experiments as well as to determine the main characteristics of a thermoelectric generator in low-grade heat recovery of waste gases.Methods. The experimental unit consists of two blocks – a thermoelectric generator operating on the principle of cross heat exchange for heat recovery with concurrent heating of the incoming air, which is supplied as a mixture to the burner device of the boiler unit; and an adsorber block filled with blast furnace slag for cleaning the waste gases from nitrogen, sulphur and carbon oxides. To achieve these goals, the work is based on the proposed thermoelectricity effect, which works on the principle of converting thermal energy into electricity at a temperature difference between hot and cold junctions in thermoelectric sections consisting of two metals different in their electronegativity. At the same time, in an integrated air heater, the intensification of the process of adsorption of harmful components by granulated blast furnace slag takes place, which reduces the heat content and temperature of waste gases, reduces emissions of flue gases and, as a result, increases environmental safety of the area adjacent to the boiler. In particular, the study of countercurrent heat exchange in the design of the thermoelectric generator was carried out.Results.The main result of the research work is the development of an experimental design of an integrated air heater, an experimental technique and the determination of the main characteristics of the thermoelectricity generation process.Conclusion.The use of such an integrated air heater can improve the efficiency of heat-generating units of low, medium and high power installed in the central heating stations, multi-family and single-family houses. The gained electric power with the subsequent transformation can be used for power supply of automatiс equipment of boilers of low, medium and high power, and for power supply of the stations of cathodic protection against electrochemical corrosion of back-end surfaces formed as a result of the presence of water vapors in waste gases.

Parameter analysis of an ammonia-water power cycle with a gravity assisted thermal driven “pump” for low-grade heat recovery

The diaphragm pump is commonly utilized in the small-scale ammonia-water power cycle for pumping the liquid from absorber to evaporator. The electricity consumption and possible leakage of such a pump influence the system efficiency and reliability significantly. In order to find an alternative “pump” with high reliability and low cost, a gravity assisted thermal driven “pump” (GTP), which is consisted of three top-down organized units connecting absorber and evaporator separately, is designed. With the charging and discharging phases, the pressure in each unit fluctuates, and the level of the liquid increases and decreases alternately by the function of gravity. The results of the system show that the net work and thermal efficiency are 10.68 kW and 9.9%, respectively, when the evaporator and absorber are at 140 °C/4000 kPa and 25 °C/800 kPa separately. The optimal net work, thermal efficiency and exergy efficiency are improved by 4.87%, 3.62% and 10.06% respectively compared with the conventional cycle. An application of the GTP power cycle with the capacity of 10 kW driven by the biomass boiler is analyzed, and the results show that the electricity produced by 645 kg biomass pellets can support more than 12 households per day.

Numerical and experimental study of a looped travelling‐wave thermoacoustic electric generator for low‐grade heat recovery

Thermoacoustic technology has drawn increasing attention due to its advantages such as reliability and environmental benignity. Aiming at low-grade heat recovery, we developed a travelling-wave thermoacoustic electric generator consisting of a looped travelling-wave thermoacoustic engine and a linear alternator. In order to explore the operating characteristics of the electric generator, we numerically analyzed the acoustic field characteristics with a modified model. The analysis shows that high acoustic impedance appears in all three stages, and the travelling-wave component dominates the acoustic field of the loop, which is significant for both thermoacoustic conversion and acoustic power propagation. Furthermore, we also investigated the effects of external electric compliance, resistance, and hot end temperature on the output electric power, thermal-electric efficiency, and other related parameters. In the experiments, a thermal-electric efficiency of 3.7% with an output electric power of 24 W has been achieved, when the hot end temperature is 120°C. The relative Carnot efficiency can exceed 14% when the hot end temperature is between 120°C and 190°C. The promising results demonstrate the significant potential of thermoacoustic electric generation in low-grade heat recovery.

Two-stage cascading desorption cycle for sorption thermal energy storage

Abstract Sorption thermal energy storage is an effective technology for low-grade heat recovery and it could solve the problem of mismatching between thermal energy demand and supply. However, the conventional single-stage cycle couldn't adapt well to the unstable heat source with variable temperature from solar energy or industry. To overcome this problem, two-stage cascading desorption cycles are proposed and compared with conventional single-stage sorption cycle. Under the condition of small temperature variation of heat source, which is divided into 5 stages between 68.2 °C and 152.8 °C, results show that various working pairs for two-stage cascading desorption cycles adapted to the heat source better than that for single-stage cycle, considering the effect of hysteresis. Under large temperature variation of heat source conditions such as larger than 50 °C, the concept of modularization thermal cell is proposed and the guideline for optimal combination between various halides and cycles is summarized. Considering both thermal energy density and grade, the combined two-stage cascading desorption cycle with three halides of optimal filled mass proportion is recommended, with system energy storage density of 879 kJ/kg and the product of temperature increment and thermal energy storage density of 81.1 MJ K/kg.

Performance analysis of a looped travelling-wave thermoacoustic engine with phase-change

Traditional thermoacoustic engines using conduction-driven, sensible heat transfer are unable to utilize low-grade thermal energy efficiently. Recently, it has been demonstrated that a phase-change thermoacoustic engine can initiate oscillations at a very low temperature difference; however, the steady-stage performance has yet to be extensively studied. In this work, a phase-change thermoacoustic engine with a looped resonator was simulated, based on a linear phase-change thermoacoustic theory. A binary gas mixture consisting of an inert gas and a condensable component was adopted as the working fluid. We numerically investigated the distributions of pressure, volumetric velocity, and acoustic power in the system. We also examined the steady-state performance (characterized by the pressure amplitude, acoustic power, and thermal efficiency) of the system under different mean pressures and temperature differences. The results show that the phase-change thermoacoustic engine can be driven by very low temperature differences (< 50 K) much more efficiently than its dry equivalent (i.e., the same system but without the condensable component). The findings demonstrate the promising potential of generating acoustic power through low-grade heat recovery, which can then be converted into electricity and cooling.

Pressure-retarded membrane distillation for low-grade heat recovery: The critical roles of pressure-induced membrane deformation

Pressure-retarded membrane distillation (PRMD) is an emerging membrane process to recover energy from low-grade heat sources. The applied hydraulic pressure on the cold-water side in PRMD may strongly affect both energy conversion efficiency and membrane performance. Here, we report the first systematic study on this critical issue. A commercial nanoporous polytetrafluoroethylene membrane was evaluated as a general membrane sample over a range of applied pressures from 0 to 10 bar at a temperature difference of 40 °C. Our results show that the theoretically projected constant water vapor flux decreases significantly with the increase of the applied pressures, which can be attributed to the severe membrane deformation induced by pressures. The membrane in the active-layer-facing-hot-solution orientation is mechanically unstable with the complete loss of water vapor flux under 2 bar. In contrast, the membrane in the active-layer-facing-cold-solution orientation can still work under 10 bar. Combining theoretical analysis and detailed characterization of membrane physical structures, we show that the properties of membrane active layers (i.e., pore size, porosity, and thickness) deteriorate under elevated pressures. Deformed membranes have lower permeability and higher temperature polarization in PRMD, resulting in the observed lower water vapor fluxes. Our results suggest that improving the mechanical stability of membranes would be the first critical step in realizing practical applications of PRMD for low-grade heat recovery. Potential research directions for developing novel PRMD membranes are also proposed.

Recovery of Low-Grade Heat (Heat Waste) from a Cogeneration Unit for Woodchips Drying: Energy and Economic Analyses

The aim of this paper is to improve the operating share of a biomass cogeneration unit by using unavoidable heat waste heat recovered from a district network heating used for drying woody biomass’ return water (law-grade temperature heat). The optimal operating conditions of a drying unit added to the system were estimated from an energy and a financial point of view, applying four objective functions (drying time, energy consumption, energy balance, and financial performance of the cogeneration unit). An experimental design methodology used heat for the implementation of these functions and to obtain an operating chart. Numerical modelling was performed to develop a simulation tool able to illustrate the unsteady operations able to take into account the available waste heat. Surprisingly, the model shows that the right strategy to increase the financial gain is to produce more warm water than necessary and to consequently dispose higher quantities of unavoidable heat in the network’s return water, which heat up the drying air at a higher temperature. This result contrasts with the current approaches of setting-up cogeneration units that are based on the minimization of the heat production.

Thermodynamic Performance and Optimization Analysis of a Modified Organic Flash Cycle for the Recovery of Low-Grade Heat

The recently proposed organic flash cycle (OFC) has the potential for the efficient recovery of low-grade heat, mainly due to the reduction of irreversibilities in the heat input unit. In the present study, a modified OFC (OFCM) employing a two-phase expander (TPE) and regeneration is proposed and thermodynamic and optimization analysis on this cycle is conducted compared with the basic OFC (OFCB). Six substances are considered as the working fluids. Influences of flash temperature, source temperature, and working fluid are systemically investigated on the system performance. Results showed that OFCM is superior to OFCB in the aspects of power production, thermal, and second-law efficiencies.

Innovative model of trigeneration system generating desalinated water, hot and cold by using low grade heat recovery from nuclear reactor set in cascade of sorption devices

The paper introduces an innovative conceptual model of a trigeneration system based on implementation of sorption devices in cascade configuration: absorption heat pumps and adsorption chillers connected with thermal energy storage, for recovering useless heat from secondary cooling circuit of a research nuclear reactor. Proposed trigeneration source provides building with useful heat for the purposes of heating system with thermal energy storage and cold for air-conditioning purposes. Also, desalinated water covering technological demand is produced. Useful heat is produced by an absorption heat pump, cold and desalinated water by adsorption chiller/desalinator. For the described trigeneration system calculations based on commercially available equipment (lithium-bromate absorption heat pumps and silica-gel adsorption chillers with desalination option) and required heat/cold/desalinate demand have been carried out. Operational data collected from an existing installation extended by introducing thermal energy storage to the system was used to simulate the heat demand during the year. 5-year operational data from the “MARIA” research nuclear reactor located at the National Center for Nuclear Research in Świerk, Poland was used to simulate low source variations for the absorption heat pump operation. The results of model implementation demonstrate a series of promising effects on many levels of system operation, including production of desalinated water on a large scale and significant reduction of: (I) energy usage (by 40% when considering only heating scenario), (II) nuclear fuel consumption, (III) heat delivery losses.

Experimental analysis of a micro-ORC driven by piston expander for low-grade heat recovery

In this paper, a full experimental characterization of a micro-scale ORC system is presented. The facility under investigation is driven by a piston expander prototype, made of three cylinders arranged radially around the drive shaft. The system is rated for a thermal input around 30 kW, being suitable for residential, tertiary sector or small industry applications. It is conceived for exploiting low temperature heat sources, such as solar collectors, biomass boilers, geothermal energy or waste heat streams. The facility was provided with an electric boiler as heat source, which warms water up to 90 °C, and cold water at ambient temperature as heat sink. A test campaign was performed varying the hot source temperature and the organic fluid feed pump velocity, in order to characterize the system behavior at different off-design working conditions. The electric consumption of the ORC feed pump was measured, in order to quantify the actual impact of the auxiliaries on the overall efficiency. Moreover, the number of electric loads connected to the generator was varied, changing the equivalent phase impedance value, for evaluating the effect on the expander rotating speed and power output.The experimental analysis demonstrated that small reciprocating expander is suitable for exploiting low enthalpy heat sources, with quite good performances compared to other architectures like scroll and screw expanders, more applied within low temperature sources. The results show that the gross electric power output varied between 250 W and 1150 W, depending on the expander speed and on the number of electric loads activated. The expander total efficiency showed a barely constant trend around 40%. The pump total efficiency varied between 10% and 20%, increasing with the pump rotational speed. The maximum ORC gross and net efficiency were 4.5% and 2.2% respectively, confirming that the auxiliaries impact cannot be considered negligible in such type of systems.

Performance of a looped thermoacoustic engine with multiple loads capable of utilizing heat source below 200 °C

Thermoacoustic engine is a promising approach to low-grade heat recovery with the outstanding characteristics of high reliability and environmental friendliness. In this work, systematic experiments were conducted on a three-stage looped thermoacoustic engine to drive one to five loads, with the hot temperatures below 200 °C. The R-C load method was adopted to measure the output acoustic power. In the experiments, the highest thermal efficiency achieved is 9.6% at the hot temperature of 195 °C when five loads are installed, and the corresponding relative Carnot efficiency is 25.6%. The highest relative Carnot efficiency is 26.4%, achieved at the hot temperature of 173 °C in the case with four loads. The thermoacoustic engine was also modeled to discuss the mechanism for the efficiency gain by adding the quantity of loads. The results show that the increased quantity of loads can reduce the proportion of acoustic power loss and increase the proportion of output acoustic power. This work will be instructive for increasing the thermal efficiency of the low-grade heat driven thermoacoustic engine systems.

Small scale Organic Rankine Cycle testing for low grade heat recovery by using refrigerants as working fluids

Small scale Organic Rankine Cycle testing for low grade heat recovery by using refrigerants as working fluids

Experimental analysis of binary salt mixture of NaNO3:KNO3 for low-grade waste heat recovery system

Objective: To find out how energy storage capacity and energy holding capacity changes at different concentration and the effects of each salt on these factors. Method/Statistical Analysis: This system utilises a mixture of nitrate salts in water as primary medium for storing sensible heat.A mixture (300 gm) of Sodium Nitrate (NaNO3 ) and Potassium Nitrate (KNO3 ) at different wt% (60/40, 50/50, 40/60) in 1 litre of distilled water were used for experiments. Each sample was heated until its boiling point was reached and time was noted for each 1-degree rise. Findings: In this study a Thermal Energy Storage system for low-grade heat recovery is being proposed. As the atomic size of salt increases calorific value of mixture decreases & overall energy storage capacity decreases. But temperature increment of system is faster. But, for energy holding capacity KNO3 has shown better results than NaNO3 . Its reason could be the atomic weight of the salt molecule. Among all 50 : 50 solutions have the maximum amount of energy stored and 60 : 40 solution has the slowest rate of heat loss among all. Applications/Improvements: This system can be used for applications like space heating, water heating etc. and for power generation also in conjunction with Organic Rankine Cycle (ORC) turbines which can operate at temperatures as low as 70°C. Keywords: Heat Capacity, Heat Loss Rate, Low-Grade Heat, Specific Heat Capacity, Waste Heat Recovery, KNO3 ,NaNO3

A study of clearance height on the performance of single-screw expanders in small-scale organic Rankine cycles

A single-screw expander (SSE) is a type of volumetric expander that can be applied to low-grade heat and waste heat recovery utilizing the organic Rankine cycle system. In this paper, a new thermodynamic modeling of SSEs considering the friction power consumption is carried out to examine the performance. The mathematical model was verified by experimental results, and the calculation results have a good agreement with the experimental results. The clearance height has great influence on the expander performance. The influence of clearance height on the leakage flow rate, friction power consumption, volumetric and isentropic efficiency were investigated. Results show that path L8, L7 and L2+4 are the three main leakage paths, and path L8, L7 and L9 are the three main friction power consumption. Compared with the meshing clearance height, the fitting clearance has a more significant effect on the performance of SSEs. Setting a higher fitting clearance and meshing clearance is an effective measure to reduce friction power consumption and prevent gaterotor wear, respectively. It would be favorable if the fitting and meshing clearance height is no less than 0.04 mm and 0.02 mm, respectively.

Low grade heat recovery for power generation through electrochemical route: Vanadium Redox Flow Battery, a case study

Solar heat collection efficiency reduces with increase in temperature of the heating media owing to heat loss. Thermal energy capture is easily carried out using heat exchangers, but there are challenges in converting this to usable power, especially at lower temperatures. An electrochemical system which has reasonable variation in free energy with temperature can be used to capture this low grade energy by charging at a higher temperature. Vanadium Redox Flow Battery (VRFB) is considered as a case study to see the feasibility of this concept. For this purpose, a lumped parameter model of the VRFB system is developed to study the high temperature operation. It is observed that high temperature operation enables charging at lower potentials and achieving higher current density. Based on this study a method for capturing low grade energy using electrochemical systems is investigated.

Experimental study of a new ejector-absorption refrigeration cycle driven by multi-heat sources

This paper proposes a new ejector-absorption refrigeration cycle driven by two heat sources at different temperature levels to realize the economic optimization in low-grade heat recovery application. The cycle uses the exergy of steam from a high-pressure generator and entrains the steam out of a low-pressure generator. The quantity of the steam that comes out of the low-pressure generator for the new cycle is greater than that of a conventional double-effect cycle, which means that more low-temperature-level heat, such as non-focused solar thermal heat, can be used. Therefore, the heat required to input into the system and the temperature level required to drive the cycle declines. In this study, water-lithium bromide as the working pair and a hot air temperature level ranging from 185 °C to 215 °C is used as the high level driving heat source, while the hot water heated by a solar energy collector is the low level driving heat source. In some conditions of the new cycle’s COP, 0.95 can be achieved for refrigeration, which is about 20% higher than that of the single-effect cycle. Besides this, the proposed chiller requires lower driven temperature than that of a conventional double-effect cycle.

Low grade heat recovery system for woodfuel cogeneration plant using water vapour regeneration

The paper analyses low grade heat recovery problem for modern woodfuel cogeneration plant. The woodfuel flue gas, behind the condensing economizer, still contains a considerable amount of heat, main part of which is the latent one. To recover this low grade heat, the heat pump technology can be used, which is related with additional consumption of energy (electric, mechanical or heat). Another technique that could be applied is a heat regeneration when flue gas heat, mostly latent, is transmitted to air blown towards burning chamber. Therefore, the analysed heat recovery system operates mainly like mass regenerator which contains only blowers that use some electric energy. The regenerator consists of two cyclically operating columns with packing material. Energetic analysis demonstrates that 13% of additional heat can be produced utilizing this low grade heat. The economic valuation shows that investment in a heat recovery system is quite effective; the payback time is about four years.

Affordable Process for the Production of Strontium Bromide Used in Low Grade Heat Recovery Applications

Strontium bromide, SrBr2, has a strong potential for the recovery, storage and controlled release of low grade heat (<100°C). SrBr2 is currently only produced at lab scale, making it unaffordable for large scale application. In this paper, a route for cost-effective industrial scale production of SrBr2, of a potentially lower purity but still fit-for-purpose, is sought by modifying and simplifying existing processes. Based on literature, one such modified process is proposed and experimentally investigated. It is based on the Soda Ash method of producing strontium carbonate, SrCO3, from celestine ores (SrSO4) available in Spain. The process generates a valuable by-product, ammonium sulfate. It is found that SrBr2 of up to 97.6% purity could be produced using HBr. If NaBr brine is available, the use of expensive HBr can be avoided, thanks to the high solubility of SrBr2 in ethanol. Projected prices lie in the range 0.09-0.28 €/kWh of heat stored based on purified NaBr brines. A simplified LCA based on the SrBr2 production indicates a climate change contribution of SrBr2 to the heat storage of ∼0.052kg CO2 eq./kWh stored. These encouraging results show that hygroscopic salts are a promising sustainable solution for the recovery of solar and/or excess industrial heat.

Optimization-based Investigations of a Thermofluidic Engine for Low-grade Heat Recovery

This paper presents an analysis of the non-inertive-feedback thermofluidic engine (NIFTE) under cyclic steady-state conditions. The analysis is based on a nonlinear model of NIFTE that had previously been validated experimentally, and applies an optimization-based approach to detect the cyclic steady states (CSS). The stability of the CSS is furthermore determined by analyzing their monodromy matrix. It is found that NIFTE can exhibit multiple CSS for certain values of the design parameters, which may be either stable or unstable, a result that had not been reported before. Subsequently, a parametric study is conducted by varying key design parameters, revealing that higher efficiencies could be achieved by controlling the engine at different CSS, including unstable ones. Lastly, the paper investigates the trade-offs between efficiency and work output in NIFTE.

Investigation of Support Vector Machine and Back Propagation Artificial Neural Network for performance prediction of the organic Rankine cycle system

Low temperature power generation system based on organic Rankine cycle (ORC) has been a popular candidate for low grade heat utilization and recovery. To find a way to predict the performance of the ORC system, the exploration and analyses of the Support Vector Machine (SVM) and Back Propagation Artificial Neural Network (BP-ANN) were carried out. For comparison, both Gauss Radial Basis kernel function (SVM-RBF) and linear function (SVM-LF) have been employed in SVM. Additionally, for the sake of comprehensiveness, two division methods for data set called “random division method” and “blocked division method” were studied. Finally, SVM-LF and BP-ANN demonstrated better stability and higher accuracy for both two division methods and for different testing sets while SVM-RBF showed good results for random division method and disappointing results for blocked division method.