Utilization Of Low-grade Heat Research Articles

Experimental and mathematical study of the spray flash evaporation phenomena

Spray flash evaporation is a promising way that potentially enables various renewable energy sources to be employed for desalination. This paper specifically studies the flash evaporation phenomenon of a hot water jet sprayed into a low-pressure chamber. Special considerations are rendered to the flash evaporation process under lower temperature differences which evolves higher thermal efficiency and better low-grade heat utilization for desalination. An experimental setup has been developed and temperature profiles of the sprayed water in the axial direction are mapped. Under low flow velocity conditions, the water jet shatters into droplets due to the flash atomization effect. Such a shattering phenomenon can be captured and quantified by a mathematical model based on droplet analysis. Applying this model, the temperature profiles are translated into the mean spray droplet diameters. The effect of different variables on the flash evaporation process is further investigated. Key results revealed that the mean droplet diameters of the spray are several orders of magnitudes smaller than the nozzle diameter. Such fine droplets allow complete evaporation to be accomplished within 50 cm from the nozzle exit, enabling a compact evaporator design. Additionally, higher initial temperature differences and higher flow velocities reduce the mean droplet diameter to be smaller than 300 μm, and the corresponding vertical distance required to complete the evaporation process is shorter than 10 cm.

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.

A thermosyphon solar water heating system for sub zero temperature areas

Heating without global warming is a key challenge being faced by world community today. In this regard, harvesting solar energy to meet domestic hot water demand of cold regions is attempted as an alternative design solution. Low solar insulation, chilly winds and sub-zero temperatures have been identified as major hindrances in harnessing solar energy in cold regions. A thermosyphon driven supercritical CO2 fluid based solar water heating system is developed for use in subzero temperature areas. The solar collector consists of parallel U-tubes inserted in fins inside the evacuated glass tubes. Special arrangements in manifolds and evacuated glass tubes were applied to make it possible to stop the reverse thermosyphon. Optimal heat transfer refrigerants were studied for this solar heating system. At ambient temperature range of 30 to 35°C range, CO2 refrigerant easily attains 75°C with collector efficiency ranging from 80 to 85%. When the hot refrigerant is passed through helical coil counter flow type heat exchanger, the inlet water temperature increases from 26 to 55°C giving off a temperature gradient of 29°C, resulting in efficient heat transfer of system. The system provides 23°C greatest temperature difference (GTD), 14°C lowest temperature difference (LTD) and 18.13°C log mean temperature difference (LMTD). This innovative solar water heater can perform adequately in subzero temperatures where water based systems fail to perform well due to freezing. The proposed system is a promising alternate for utilization of low grade heat in cold regions.

A numerical study of a looped-tube thermoacoustic engine with a single-stage for utilization of low-grade heat

Multi-stage thermoacoustic engines with a looped structure have become a promising technology to use heat at a low temperature level. This study forwards a single-stage engine with comparable capabilities. A stub is used to suppress acoustic impedance disturbances. This conception shows to be a powerful tool to accurately tune acoustic conditions in the regenerator. Furthermore, a systematic parametric study is carried out numerically with DeltaEC to examine their relations. This could help in a system-wide optimization according to various application requirements. The study comprises geometrical parameters of regenerator, heat exchangers, and feedback loop as well as process parameters like operational temperatures, mean pressure and working gas. Discrepancies between ideal conditions on component level and on system level have been observed and an approach to interpret them is given. Recommendations for parameter settings depending on an efficiency or power driven operation are supplied. Since a proper coupling between acoustic load and acoustic field in the loop is essential for a sufficient overall-performance, joining mechanisms have been examined in terms of acoustic impedance and position of the load. Eventually it is shown that the proposed engine type is able to power an acoustic load with a relative Carnot efficiency greater than 50% when heat is supplied at 150°C and rejected at 15°C.

On low-grade waste heat utilization from a supercritical steam power plant using an ORC-bottoming cycle coupled with two sources of heat

This paper analyzes a waste heat recovery system based on a binary vapor cycle composed of an organic Rankine cycle (ORC) bottoming a supercritical steam cycle. The organic Rankine cycle is supplied by two heat sources. The first one is waste heat from a steam boiler, which condenses flue gases to 200MWt at 90°C and preheats the fluid with a low boiling point. The second one is a steam condenser, which also acts as a low-boiling-point fluid vapor generator. Steam condensation temperatures was tested in the range 55–115°C. Usage of a low-boiling-point fluid instead of steam in range of the low temperature (below 100°C) has several advantages. One advantage is the possibility for the effective utilization of a large amount of low-grade waste heat from a supercritical steam cycle. For the most efficient configuration, 22.92MW of additional electrical energy is generated. The thermal efficiency of the waste heat recovery system is 11.46%, which is 71.75% of the Carnot efficiency. Usage of an organic Rankine cycle for bottoming the supercritical steam cycle also provides cubature reduction of the power plant. For the most efficient case, a steam volume flow at the new steam turbine outlet is reduced by 88% compared to a reference stream turbine. The volume flow at the ORC turbine outflow is reduced by 54%. Numerical analyses of the thermodynamic cycles, before and after modifications, are carried out using computational flow mechanics, mainly, with in-house code.

Thermodynamic mechanism for hybridization of moderate-temperature solar heat with conventional fossil-fired power plant

The objective of this study is to achieve a higher solar-to-electricity conversion efficiency through solar-fossil hybrid thermal power systems compared to a solar-only power plant. The study reveals the thermodynamic details for the improved solar-to-electricity efficiency in a solar hybrid power plant. A correlation was established to describe the main factors influencing the thermodynamic performances, including higher collector efficiency, higher turbine efficiency and upgraded energy level of the moderate-temperature solar heat. This proposed mechanism can be applied to effectively integrate solar and fossil-fired energy in a power system. The studies took typical fossil-fired power plants to hybridize with solar heat in three approaches: preheating the feed water before it entering the boiler for coal-fired system; heating for generation of saturate steam or superheated steam in gas-fired combined cycle. The results indicate that the moderate-temperature solar and fossil hybridization technology can provide a promising direction for efficient utilization of low-grade solar heat.

Investigation on an innovative cascading cycle for power and refrigeration cogeneration

In order to further realize efficient utilization of low grade heat, an innovative cascading cycle for power and refrigeration cogeneration is proposed. Pumpless Organic Rankine Cycle (ORC) acts as the first stage, and the refrigerant R245fa is selected as the working fluid. Sorption refrigeration cycle serves as the second stage in which silica-gel/LiCl composite sorbent is developed for the improved sorption characteristic. The concerning experimental system is established, and different hot water inlet temperatures from 75°C to 95°C are adopted to investigate the cogeneration performance. It is indicated that the highest power and refrigeration output are able to reach 232W and 4.94kW, respectively under the condition of 95°C hot water inlet temperature, 25°C cooling water temperature and 10°C chilled water outlet temperature. For different working conditions, the total energy and exergy efficiency of the cascading system range from 0.236 to 0.277 and 0.101 to 0.132, respectively. For cascading system the exergy efficiency of heat utilization ranges from 30.1% to 41.8%, which is 144% and 60% higher than that of pumpless ORC and sorption chiller when the hot water inlet temperature is 95°C.

Investigation on a small-scale pumpless Organic Rankine Cycle (ORC) system driven by the low temperature heat source

A small-scale pumpless Organic Rankine Cycle (ORC) system driven by the low temperature heat source is established to investigate the overall performance. Hot water temperature from 75°C to 95°C is adopted for performance analysis whereas the environmental temperature is about 25°C. Refrigerant R245fa is selected as working fluid, and scroll expander is employed for power generation. One worth noting fact is that pumpless ORC system shows great potential for low temperature heat recovery. Experimental results indicate that the maximum power output is 232W, which is obtained at 95°C hot water inlet temperature. Correspondingly, the average power output is 204W which is lasted for 6.6min, revealing the high stability for power generation. For different hot water inlet temperature, the highest energy and exergy efficiency of the system are 2.4% and 13.7%. Besides, performance of novel pumpless ORC system is compared with that of our previous prototype. It shows a remarkable improvement in terms of power output, energy efficiency and exergy efficiency. Power generation process is able to keep constant for 95% of the cycle time. As a result, pumpless ORC may become an alternative solution to low grade heat utilization when compared with conventional small-scale ORC.

Durability of thermochemical heat storage demonstrated through long-term repetitive CaCl2/H2O reversible reactions

Thermochemical heat storage is a promising technology for improving thermal energy efficiency. In this study, CaCl2 and H2O were selected as the reactant and working medium, respectively, assuming the utilization of low-grade waste heat. To investigate the durability of the CaCl2/H2O reaction, 1000 repetitive operations were conducted at the same 110 °C charging and discharging temperatures. During the long-term repetitions, a decrease in the average volumetric power density was observed. The peak value in repetitions 901–1000 was 11% lower than the peak value in the initial 50 repetitions. This trend was mainly caused by a decrease in the heat transfer rate; it was observed after the repetitive experiments that reactant particles had moved out of the heat exchanger. Although a decreasing trend in the power density was observed, a peak value of Q = 320 kW/m3 and a conversion ratio of approximately 0.7 were observed in repetitions 901–1000. In addition, approximately 90% of the stored heat was released in all 1000 repetitions. Thus, using long-term repetitions (1000), this study confirmed the durability of the CaCl2/H2O reaction for thermochemical heat storage.

Experimental investigation on an innovative resorption system for energy storage and upgrade

Progress of efficient thermal energy storage (TES) has become a key technology for the development of energy conversion system. Among TES technologies, sorption thermal energy storage (STES) has drawn burgeoning attentions due to its advantages of high energy density, little heat loss and flexible working modes. Based on STES, this paper presents an innovative resorption sorption energy storage (RTES), and the experimental system is established and investigated for energy storage and upgrade. 4.8kg and 3.9kg MnCl2 and CaCl2 composite sorbents are separately filled in the sorption reactor, and expanded natural graphite treated with sulfuric acid (ENG-TSA) is integrated as the matrix for heat transfer intensification. It is indicated that the highest energy storage density are 662kJ/kg and 596kJ/kg when heat input temperature is 125°C and heat release temperature are 130°C and 135°C, respectively. For different heat input and release temperature, the energy efficiency and exergy efficiency range from 27.5% to 40.6% and from 32.5% to 47%, respectively. The novel RTES system verifies the feasibility for energy storage and upgrade, which shows the great potential for low grade heat utilization especially for industrial process.

The effectiveness of heat pumps as part of CCGT-190/220 Tyumen CHP-1

The article considers the possibility of increasing the energy efficiency of CCGT-190/220 Tyumen CHP-1 due to the utilization of low-grade heat given off in the condenser unit of the steam turbine. To assess the effectiveness of the proposed system, the indexes of thermal efficiency are given. As a result of a research the following conclusions are received: The heat-transfer agent heat pump, when heated uses low-grade heat TPP and increases heat output, but consumes the electricity. Using a heat pump is effective for a small temperature difference between the condenser and the evaporator. Good example is heating water before chemical treatment. This method is more efficient than using a replacement boiler and it is used in steam selection.

On the thermodynamic analysis of a novel low-grade heat driven desalination system

Thermally-driven desalination technologies are ideal for mitigating the problem of water scarcity using sustainable and renewable energy sources. This paper presents a novel technology for thermal desalination: a spray assisted low-temperature desalination. It applies direct contact heat and mass transfer mechanism for both evaporation and condensation, which markedly enhances heat and mass transfer while eliminating the need for a metallic surface inside the chambers. To perform an in-depth analysis on this novel technology, a detailed thermodynamic model has been developed. The model is based on the principles of appropriate heat balance, mass balance and salt balance, and judicious heat transfer and mass transfer in the system. Key input parameters that affect system performance are taken into account in formulating the model. Simulations were conducted for top brine temperatures spanning 55–90°C for low-grade heat utilization. Key results from the model revealed that more operating stages and higher top brine temperatures are essential to enable higher production rates and realizing better thermal efficiencies, while a cooling water flowrate close to the feed water flowrate is at the optimal. A system performance ratio of 6.5 can be achieved for a 14-stage system with a top brine temperature of 90°C.

Thermoeconomic analysis of configuration methods for modular Organic Rankine Cycle units in low-temperature applications

Organic Rankine Cycle (ORC) is a promising technology for the utilization of low-grade waste heat. However, due to tailor-made power plants for different heat source temperatures, specific investment costs are still too high to be profitable. This study compares two different methods to configure a modular ORC in the temperature range of 373–463K. The first method assumes a simple adaption of the mass flow rate within the ORC (mass flow method). In the second method, simultaneous adaption of mass flow rate and working pressure (combined method) take place. The common purpose of both methods is the optimization of the net power output for heat source temperatures lower and higher than the reference plant. Analyses are carried out for common fluorinated refrigerants (R227ea and R236ea) as well as for iso-alkanes (isobutane and isopentane). It is shown that within a wide range of temperatures the deviation in net power output between the simpler mass flow method and the more sophisticated combined method is below 10%. However, the deviation strongly depends on the location of the pinch point and on the choice of the working fluid. In general, it is shown that the mass flow method in combination with a working fluid, for which the pinch point is located at the beginning of the preheating, is thermodynamically favorable for plant manufacturers. Economic analyses subsequently compare both methods with respect to payback period and cash flow. Additional investment costs for the combined method are allowed to be up to 10% in order to reach higher profitability than units with mass flow method.

Network Modeling and Design for Low Grade Heat Recovery, Refrigeration, and Utilization in Industrial Parks

Low grade heat still widely exists in energy-intensive industrial parks, although good energy integration has been accomplished for individual processes or plants. Low grade heat is notably large but difficult to utilize because of the limitation of heat transfer and the scarcity of low grade heat sinks. Large scale utilization of low grade heat is very challenging for energy-intensive industries or industrial parks. A large scale low grade heat recovery, refrigeration, and utilization network system is introduced in this study to improve energy performance for industrial parks. In order to model the large scale system, the system is decomposed into three levels: pipe networks, refrigeration stations and absorption chillers. A mixed integer nonlinear programming model is presented that considers mass and energy networks, pipes, refrigeration stations, absorption chillers, and economic performance. The mathematical model is applied to the optimization and economic analysis for the low grade heat utilizatio...

Comparative study of energy saving potential for heavy chemical complex by area-wide approach

The thought of single site approach for energy saving in a site has been evolved to that of area-wide approach which considers multiple sites together as if they were a single entity. In area-wide approach, R-curve analysis plays an important part to evaluate the energy efficiency of the site utility system for utilizing high temperature heat, which is possible to identify energy saving potential and understand the characteristics of the utility system. R-curve analysis was applied to Mizushima complex, one of the ten largest heavy chemical complexes in Japan, and identified the characteristics and the energy saving potential for the whole of the complex. In area-wide approach, low-grade heat utilization for energy saving is also important. Total site profile analysis was used to evaluate the possibility of low-grade heat utilization. Mizushima complex was evaluated for the characterization by comparison of its energy saving potential to that of another heavy chemical complex, Chiba. It was found that the energy saving potential of Mizushima complex was twice in R-curve analysis than that of Chiba complex. This meant that the equipment in the utility system in Mizushima complex performed less efficiency than that in Chiba complex.

Thermochemical Heat Storage Performance in the Gas/Liquid-Solid Reactions of SrCl<sub>2</sub> with NH<sub>3</sub>

Thermochemical heat storage is a promising technology for improving energy efficiency through the utilization of low-grade waste heat. The formation of a SrCl2 ammine complex was selected as the reaction system for the purpose of this study. Discharge characteristics were evaluated in a packed bed reactor for both the gas-solid reaction and the liquid-solid reaction. The average power of the gas-solid reaction was influenced by the pressure of the supplied ammonia gas, with greater powers being recorded at higher ammonia pressure. For the liquid-solid reaction, the obtained average power was comparable to that obtained for the gas-solid reaction at 0.2 MPa. Moreover, the lower heat transfer resistance in the reactor was observed, which was likely caused by the presence of liquid ammonia in the system. Finally, the short-term durability of the liquid-solid reaction system was demonstrated over 10 stable charge/discharge cycles.

Design and optimization of a novel organic Rankine cycle with improved boiling process

In this paper we present a novel organic Rankine cycle layout, named the organic split-cycle, designed for utilization of low grade heat. The cycle is developed by implementing a simplified version of the split evaporation concept from the Kalina split-cycle in the organic Rankine cycle in order to improve the boiling process. Optimizations are carried out for eight hydrocarbon mixtures for hot fluid inlet temperatures at 120 °C and 90 °C, using a genetic algorithm to determine the cycle conditions for which the net power output is maximized. The most promising mixture is an isobutane/pentane mixture which, for the 90 °C hot fluid inlet temperature case, achieves a 14.5% higher net power output than an optimized organic Rankine cycle using the same mixture. Two parameter studies suggest that optimum conditions for the organic split-cycle are when the temperature profile allows the minimum pinch point temperature difference to be reached at two locations in the boiler. Compared to the transcritical organic Rankine cycle, the organic split-cycle improves the boiling process without an entailing increase in the boiler pressure, thus enabling an efficient low grade heat to power conversion at low boiler pressures.

Dynamic modelling and simulation of a heat engine aerobot for atmospheric energy utilization

The low grade heat utilization is not only an inevitable option to solve the energy and environment problems, but also a critical issue for many remote power applications and planetary explorations. In this study, a novel design called a heat engine aerobot which can convert planetary atmospheric energy to electricity is proposed and analysed. A dynamic theoretical model is established and some key issues, such as the thermodynamic performance and conversion efficiency are analysed. It shows that the heat engine aerobot is capable to convert the low grade atmospheric energy to electricity during its self-sustained vertical oscillation movement. Parametric analysis shows that some design parameters, such as the nozzle number, the nozzle outlet diameter, the initial liquid mass and the turbine start height may have significant influence on the energy generation performance.

Research on Improved CCHP System Based on Comprehensive Performance Evaluation Criterion

In view of energy consumption characteristics of a typical industrial park, to further improve the low-grade waste heat utilization of CCHP system, an improved CCHP triple-generation system (ITRI) equipped with thermal storage device (TSD) and electric chiller (EC) is proposed. A comprehensive performance evaluation criterion (CPEC) is established, the capacity of triple-generation system is configured by using nonlinear programming method. A hybrid operating mode optimization strategy is proposed based on the different CPEC values of different operating mode. The results demonstrate that: after adopting ITRI, compared with traditional triple-generation system (TTRI), although annual cost increased by 0.63%, the primary energy consumption rate and lower carbon dioxide emissions decreased by 5.45% and 3.72% respectively, CPEC value increased by 19.7%. This paper has a theoretical guidance for design of triple-generation system in new industrial park and has a certain reference value for operating mode optimization of triple-generation system in the industrial parks which has been built.

A Hierarchical Approach for Evaluation of Waste Heat Utilization Opportunities

This paper presents a ranking criterion for evaluating opportunities that utilize recovered energy from the available waste heat in a process site. The ranking criterion takes into account the energy performance of waste heat recovery technologies associated with each opportunity, their potential to reduce greenhouse gas emissions (namely CO2) and the economics (costs and benefits). Mathematical modelling of the opportunities using the ranking criterion is also developed to allow for systematic evaluation of opportunities, for example within an optimization framework. The methodology is applied to a case study of a petroleum refinery. Hierarchy and performance of waste heat utilization opportunities depends on the temperature of the heat available, amongst other factors. Sensitivity of the ranking to energy prices is studied, to explore the outlook for low-grade heat utilization in the future. The methodology can be applied to the process industries and other facilities producing lowgrade heat.