Heat Sink Conditions Research Articles

A Theoretical Study on the Thermal Performance of an Increasing Pressure Endothermic Cycle for Geothermal Power Generation

In this study, a power cycle (IPEC), with an increasing pressure endothermic process in a downhole heat exchanger (DHE) and a CO2-based working fluid mixture, was developed for geothermal power generation. The increasing pressure endothermic process, which cannot be achieved in a conventional evaporator on the ground, was realized using the gravitational potential energy in the DHE. The parameters of the power cycle and the structural size of the DHE were optimized simultaneously. Using CO2-R32 as the working fluid of the IPEC provides the highest net power output. The net power generated with the IPEC was compared with a single-flash (SF) system, a trans-critical CO2 (t-CO2) system, and an organic Rankine cycle (ORC) under the same heat source and sink conditions. Six selection maps were generated for choosing the optimum power cycle for electricity production, in which four power generation systems (ORC, t-CO2, IPEC, and SF) were included, and two DHE diameters (0.155 m and 0.22 m) were investigated. It was found that the IPEC system had more net power output than the other three systems (ORC, t-CO2, and SF) under the conditions that the geofluid’s mass flow rate was less than 10 kg/s and its temperature was lower than 180 °C.

Heat-sink-free solar-driven thermoelectric device for micro energy scavenging

Harvesting energy from nature, such as mechanical energy, thermal energy, and biomass energy, has become an effective way to supply power for self-driven sensing and Internet of Things (IoT) applications. Among that, solar-driven thermoelectric (STE) device takes advantage of all-solid-state energy conversion, such as no noise, easy access, and free of wear, exhibiting great potential. Here, we propose a solar-driven thermoelectric device for harvesting solar energy, based on Micro Electromechanical System (MEMS) technology. Specifically, the optimization of solar absorptances of structural layers significantly improved the photo-thermo-electric conversion performance. Furthermore, the electrical output strongly depends on temperature difference dominated by the absorptance difference. Therefore, the reported device can be free of traditional heat sink, which greatly simplifies its structure and expands the working conditions, especially in microdevices. Particularly, a continuous output voltage of 0.34 V/cm2 is achieved with concentrated solar radiation, demonstrating its potential under no heat sink conditions. Besides, a fast response (4.2 ms under 660 nm laser) is achieved benefitting from the micron-size solar absorber, showing its significance in sensing applications. Finally, the proposed STE device is utilized to harvest outdoor solar energy, displaying promising potential in solar energy harvesting devices and providing an insightful perspective for thermoelectric applications.

Theoretical investigation of a novel distributed compression cycle for CO2 trans-critical system

Efficient cooling of the trans-critical CO2 vapor compression cycle to improve system performance is a crucial research topic. This paper proposes an innovative approach called distribution compression (DC) to achieve the same effect as subcooling in a trans-critical CO2 thermodynamic cycle. In the DC system, the trans-critical CO2 at the gas cooler outlet is no longer further cooled but subjected to a secondary pressure boost and releases heat in ordinary heat sink conditions as in the gas cooler. Thermodynamics calculated the change in performance of the DC system under different working conditions with different secondary boost ratios. The optimal secondary boost ratio related to evaporation and gas cooler outlet temperatures under optimal discharge pressure based on the baseline system was obtained. The results show that compared with the baseline system, the DC system can effectively improve the system performance, and the maximum refrigeration COP increase is between 8.2 % ∼ 10.76 %, and the maximum heating COP increase is between 5.37 % ∼ 6.34 %. The cooling or heating capacity increase can reach up to about 26 %. The ideal secondary boost ratio in a DC system is not highly demanding, and the increased input power for the second input power for the secondary boost will not exceed 20 % relative to the baseline system. Comparing the COP of the DC system over current systems that only use a single subcooling technology still shows advantages. The DC system provides a new idea to improve the performance of the trans-critical CO2 vapor compression cycle.

Energetic Analysis of Low Global Warming Potential Refrigerants as Substitutes for R410A and R134a in Ground-Source Heat Pumps

The European building sector is responsible for approximately 40% of total energy consumption and for 36% of greenhouse gas emissions. Identifying technological solutions capable of reducing energy consumption and greenhouse gas emissions is one of the main objectives of the European Commission. Ground source heat pumps (GSHPs) are of particular interest for this purpose, promising a considerable reduction in greenhouse gas emissions of HVAC systems. This paper reports the results of the energetic analysis carried out within the EU research project GEO4CIVHIC about the performance of geothermal heat pumps working with low-GWP refrigerants as alternatives for R134a and R410A. The work has been carried out through computer simulations based on base and regenerative reverse cycles. Several heat sink and heat source temperature conditions have been considered in order to evaluate the GSHPs’ performance in the whole range of real conditions that can be found in Europe. Particular attention has been paid to the evaluation of compression isentropic efficiency and its influence on the overall cycle performance when dealing with steady-state heat pump simulations. To do so, five different scenarios of isentropic efficiency calculation have been studied and discussed.

Supercritical CO2 mixture Brayton cycle with floating critical points for concentrating solar power application: Concept and thermodynamic analysis

A novel concept of supercritical Brayton cycle with floating critical points is proposed to cope with the rapid deterioration of cycle performance at high heat sink temperatures. By coupling the power cycle with a distillation based composition tuning subsystem, the circulating mixture composition in the cycle can be altered dynamically to obtain the desired critical points at various ambient temperatures, thereby leading to a better match between the cold-end parameters of power cycle and the heat sink conditions. A thermodynamic model is established for the proposed novel system based on a recompression layout with single stage reheating. Four binary mixtures, including CO2–H2S, CO2–Butane, CO2–Isobutane, and CO2–Butene are employed. Simulation results indicate that an increasingly larger improvement in cycle efficiency compared to the pure CO2 cycle can be obtained as the ambient temperature increases from 20 to 50 °C. When the composition tuning process is not considered, the CO2–H2S cycle shows the best performance among the four mixture cycles, with an improvement between 1.3% and 7.4% being achieved over the entire temperature range. Several feasible schemes for composition tuning subsystem are proposed and compared in terms of energy consumption. Finally, the hourly efficiencies of the integrated system are evaluated based on typical daily weather conditions, with a piecewise control strategy for critical points. It is found that although the use of CO2–H2S causes relatively high energy consumption for distillation, a significant improvement in the daily average thermal efficiency of the integrated system can still be achieved, with efficiency being increased by 3.5%-5.1% in comparison with pure CO2 cycle on hotter days.

Performance improvement of supercritical carbon dioxide power cycle at elevated heat sink temperatures

The supercritical carbon dioxide power cycle has emerged as an advanced power generation system owing to its better efficiency, compactness, and capability to synergize with a broad range of boundary conditions. Its superior performance is due to the liquid-like compression near the critical point. However, this phenomenon deteriorates significantly for elevated heat sink temperatures and is therefore highly detrimental to the cycle performance. Thus, this study proposed and investigated an integrated power and refrigeration cycle scheme to curtail the effects of elevated heat sink temperatures. A theoretical model of the proposed scheme based on industrial flue gases was developed, and the influence of heat sink at a range of heat-source temperatures (200–500 °C) was examined. Subsequently, benchmark cases are established and optimized, and the performance improvement with respect to the benchmark case is reported, with net power as the performance indicator. The optimization of the benchmark case reveals that the simultaneous optimization of low- and high-pressure levels of the cycle is necessary to maximize the net power. The parametric investigation of the proposed configuration shows that the performance is sensitive to the saturation temperature in the evaporator of the refrigeration cycle and to the ratio of low- and high-pressure levels in the power cycle. Therefore, the proposed case was optimized for both influencing parameters for each boundary condition. The results show that for a given heat-source temperature, higher heat sink temperatures favor the proposed design. For the heat-source temperatures of 200 °C, the benchmark case is superior for all heat sink conditions, whereas for 300 °C, 400 °C, and 500 °C, the proposed system results in an increase in power generation of 10.67%, 7.2%, and 5.2%, respectively, at a sink temperature of 50 °C. Finally, a case study with molten salt as the heat source was conducted, and the results indicated that the proposed scheme outperformed the benchmark case by 32.87% in terms of net power. The results are significant in simplifying the design of pressurization components and performance improvement of the supercritical carbon dioxide power cycle at elevated heat sink temperatures.

Design and Development of a PCM-Based Two-Phase Heat Exchanger Manufactured Additively for Spacecraft Thermal Management Systems

For spacecraft thermal management systems, it is crucial to diminish the overall mass of on-board thermal storage system and minimize the temperature fluctuations when the environmental temperature changes drastically. Since there is no atmosphere in outer space, heat can only be rejected to space using radiation (e.g., radiators). The heat sink conditions, and the heating power subjected to be rejected vary continuously at the orbiting stage of the spacecraft. Without thermal storage capability, the radiator is required to be large enough to release the highest power at the hottest of the heat sink. By engaging and integrating phase-change materials (PCMs) into a passive two-phase heat exchanger, the radiator can be designed and sized for the average rather than the maximum power. To meet the NASA needs, the prototype developed and fabricated in the present study is a novel two-phase heat exchanger that integrates PCMs within a vapor chamber through multiple thin drawers loaded with bulk PCM. The entire solid structure of the prototype is manufactured additively using Maraging Steel with the Direct Laser Metal Sintering (DLMS) technique. The chosen working fluid is Methanol to undertake two-phase heat transfer via saturated vapor from the heated surface of evaporator to the condenser surface, and the condensate then returns back to the heated region and keeps recirculated using the capillary pumping pressure performed by the wick structure without any reliance on the gravitational magnitudes. Depending on the heat sink temperature and PCM melting temperature, the developed heat exchanger can either function as a thermal capacitor or a two-phase heat exchanger with thermal storage capability, that is, two modes of operation to be engaged within the spacecraft's launching/landing stages or orbiting stage, respectively.In the present study, the geometry and number of the PCM-loaded drawers enclosed inside the heat exchanger's casing are optimized based on the mass ratio (the ratio of PCM mass to the total heat exchanger mass), additive manufacturing (AM) constraints, and total thermal resistances imposed on the system for either of the operation modes. The AM printed prototype with its all components are also represented along with technical drawings and specifications. Furthermore, the support structure model generated for metal printing of the proposed heat exchanger will be exposed and discussed. The wick structure design procedure is also presented and its main performance characteristics (e.g., effective pore size, effective thermal conductivity, porosity, and permeability) are reported to analytically examine the capillary pumping limit for the proposed heat exchanger.

Comparative study of upgraded CO2 transcritical air source heat pump systems with different heat sinks

For improving the coefficient of performance of system for heating, CO2 transcritical heat pump system is upgraded by using two compressors. In this paper, three CO2 transcritical systems with two-compressors (the CO2 cascade system, the CO2 two-stage compression system and the CO2/CO2 mechanical subcooling system) are discussed and compared with CO2 single-stage compression transcritical heat pump system under three different heat sinks. The three heat sinks are the small temperature difference fan-coil unit, the floor-coil radiator and the traditional designed radiator. And the optimal operation states of three upgraded systems are analyzed. When the thermodynamic performance of CO2 transcritical heat pump system is optimized, the initial investment cost of its equipment often increases and the overall cost will be affected during the life cycle of the equipment. Thus, an objective function that comprehensively considers the initial investment cost and operating cost is defined, and the comparative analysis of four CO2 transcritical heat pump systems is combined meteorological parameters of five typical regions in China for home heating. The results show that under nominal working conditions of three heat sinks, COPs of CO2 cascade system, CO2 two-stage compression system and CO2/CO2 mechanical sub-cooling system are all increased by more than 10% compared with that of single-stage system and the growth extent of COP increases with the rise of return water temperature of heat sink, but based on the economic analysis, it is implied that CO2 cascade system, CO2 two-stage compression system and CO2/CO2 mechanical sub-cooling system are not conducive to their application in home heating compared with single-stage system, and more suitable for the situations with low evaporation temperatures and large capacities. The analysis results show that the economy of single-stage system and the three upgraded systems are related to the evaporation temperature and heating capacity. Under the analysis conditions in this paper, CO2 cascade system is the least economical. Single-stage system and the other two upgraded systems can become the most economical system in different application ranges under three heat sink conditions.

A new method to identify the optimal temperature of latent-heat thermal-energy storage systems for power generation from waste heat

The integration of thermal-energy storage (TES) within waste-heat recovery power generation systems has the potential to improve energy-efficiency in many industrial processes with variable and/or intermittent waste-heat streams. The first objective of this paper is to present a novel model of these systems that can be used at an early design stage to provide fast and accurate estimates of performance. More specifically, the method can identify the optimal temperature of latent-heat TES systems for waste-heat recovery applications based only on the known heat-source and heat-sink conditions (i.e., temperature, mass-flow rate and specific-heat capacity), and can assess both single-stage and cascaded systems. The model has been validated against optimal organic Rankine cycle systems identified from a thermodynamic cycle optimisation. The second objective is to identify the characteristics of optimal systems for different heat-source profiles. The results indicate that, for a given application, there exists an optimal temperature for the latent-heat TES system that depends primarily on the relative size of the heat sink. Moreover, it is found that, for a heat engine operating with TES, the power rating ranges between 25% and 60% of the corresponding power rating for an optimal heat engine, operating without TES, that adapts instantaneously to heat-source fluctuations, whilst the total energy production is reduced by between 45% and 85% respectively. Finally, a small deviation is observed between the results obtained for the different heat sources considered, which suggests that these findings can be extrapolated to other heat sources not considered within this study.

Porous Ni–Al–CGO Cermet for Use in Solid Oxide Fuel Cells

Porous Ni–Al–CGO cermet (CGO = Сe0.9Gd0.1O2) for use in solid oxide fuel cells was fabricated by thermal explosion (volume reaction) in Ni–Al–CGO powder compacts in different heat sink conditions. Temperature profiles of thermal explosion were recorded and analyzed as a function of green composition. Phase composition of resultant porous materials was found to depend on the CGO content of green mixture and temperature of vacuum annealing. Starting and final materials were characterized by XRD, SEM, and EDS. Synthesized uniform cermets with a porosity of 50–60% can be recommended for use as a support for solid oxide fuel cells with Ni/CGO anode.

Energetic and Exergetic Analysis of Low Global Warming Potential Refrigerants as Substitutes for R410A in Ground Source Heat Pumps

In the European Union (EU), buildings are responsible for about 40% of the total final energy consumption, and 36% of the European global CO2 emissions. The European Commission released directives to push for the enhancement of the buildings energy performance and identified, beside the retrofit of the current building stock, Heating, Ventilation, and Air Conditioning (HVAC) systems as the other main way to increase renewable energy sharing and overall building energy efficiency. For this purpose, Ground Source Heat Pumps (GSHPs) represent one of the most interesting technologies to provide energy for heating, cooling, and domestic water production in residential applications, ensuring a significant reduction (e.g., up to 44% compared with air-source heat pumps) of energy consumption and the corresponding emissions. At present, GSHPs mainly employ the refrigerant R410A as the working fluid, which has a Global Warming Potential (GWP) of 2087. However, following the EU Regulation No. 517/2014 on fluorinated greenhouse gases, this high GWP refrigerant will have to be substituted for residential applications in the next years. Thus, to increase the sustainability of GSHPs, it is necessary to identify short time alternative fluids with lower GWP, before finding medium-long term solutions characterized by very low GWP. This is one of the tasks of the UE project "Most Easy, Efficient, and Low-Cost Geothermal Systems for Retrofitting Civil and Historical Buildings" (acronym GEO4CIVHIC). Here, a thorough thermodynamic analysis, based on both energy and exergy analysis, will be presented to perform a comparison between different fluids as substitutes for R410A, considered as the benchmark for GSHP applications. These fluids have been selected considering their lower flammability with respect to hydrocarbons (mainly R290), that is one of the main concerns for the companies. A parametric analysis has been performed, for a reversible GSHP cycle, at various heat source and sink conditions, with the aim to identify the fluid giving the best energetic performance and to evaluate the distribution of the irreversibilities along the cycle. Considering all these factors, R454B turned out to be the most suitable fluid to use in a ground source heat pump, working at given conditions. Special attention has been paid to the compression phase and the heat transfer in evaporator and condenser.

Trilateral Flash Cycle (TFC): a promising thermodynamic cycle for low grade heat to power generation

Energy crisis, fossil fuel depletion and growing environmental concern pushing not only for new source of energy but also developing advance technology for power generation. Besides conventional heat sources, low grade heat source of below 100 °C temperature like solar thermal, geothermal or waste heat has a good potential to become a sustainable source of power. Organic Rankine Cycle is the widely use thermodynamic power cycle for low grade heat to power generation where low boiling point pressurised organic working fluid is vaporised and expands through a turbine fed generator thus outputting usable electrical energy. The used fluid then returns to a reservoir before it is condensed and forced through the same process again via a high-pressure pump. Another thermodynamic cycle namely Trilateral Flash Cycle (TFC) which is nothing but a modified ORC where the organic working fluid is heated up to the saturation temperature under high pressure rather than evaporated. In this paper, an analytical comparative study on ORC and TFC shows that TFC has about at least 50% more power generation potential than that of conventional ORC for the same heat source and heat sink conditions if the heat source temperature is just below 100 °C. TFC even has the potential of power generation from the heat source below 80 °C where ORC is not economically viable.

Dynamic modeling and simulation of a heat pump system for enhancing cycle flexibility

This paper presents a dynamic simulation of a heat pump using butane refrigerant for the purpose of integration with a combined cycle power system and thermal energy storage to enhance flexibility. A dynamic model of a heat pump system is developed using the Amesim software and the simulation result in the case of changing the heat source and heat sink conditions is shown to validate the simulation performance.

An appraisal of proportional integral control strategies for small scale waste heat to power conversion units based on Organic Rankine Cycles

Despite the increasing number of Organic Rankine Cycle (ORC) installations at megawatt scale, the waste heat rejected by industrial processes can vary substantially from a few kWh to many megawatt hours. Hence, ORC units with a power output in the range of tens of kilowatts should be developed to tackle the heat recovery and business opportunities that can arise from this market segment. In the current research activity, a dynamic model of a small scale ORC system was developed using a commercial software platform. The unit is equipped with two plate heat exchangers, a centrifugal pump and a radial turbine designed and optimized using an in-house code and a commercial 1D modelling tool. The off-design behaviour of the ORC system has been characterized by varying the inlet conditions of heat source and sink, and the revolution speed of the turbomachines. Moreover, the response to transient thermal inputs at different time scales has also been investigated. Finally, four control strategies have been compared from the performance and energy efficiency perspectives. The results show that the turbine based regulation strategies achieve better control performance while pump based controls are able to regulate the system by maintaining the net power output closer to the design point.

Experimental results of a small-scale organic Rankine cycle: Steady state identification and application to off-design model validation

This paper presents experimental results from a prototype of an 11 kWe organic Rankine cycle (ORC). Special attention has been paid to detecting reliable steady state data points. This has been achieved by application of a novel steady state detection algorithm. This method also allows evaluating the performance of the sensors in relation to remaining steady state fluctuations. The reliability of the experimental data is evaluated by investigating the heat balances over the heat exchangers and through error propagation of the measurement uncertainties. In addition, the experimental data from the setup is used for validating an off-design ORC model. The proposed simulation model solely requires the pump and expander speed as inputs besides the given heat source and sink conditions while subcooling is assumed to be constant. The important dependent model outputs are the evaporation pressure, the condensation pressure and the working fluid mass flow rate. All three predicted outputs show a maximum deviation of less than ±1% from the measured values. The modelled net power output deviates less than ±2% from the measured values. In general, this is a satisfactory result that gives confidence in using these models in future off-design simulations.

Gasless Reactive Compositions for Materials Joining: An Overview

Some results of recent studies on materials joining by using exothermic reactive compositions that burn without notable gas evolution are overviewed. We also considered some possible routes to controlling wave propagation velocity in conditions of strong heat sink, to lowering ignition temperature for exothermic combustion reactions, and to improving the adhesion strength. The use of exothermic powder compositions opens up new horizons for combustion‐aided joining of materials, including refractory and dissimilar ones. Further advance in the field can be reached through mechanical activation (nanostucturing) of green compositions in ball mills and by cold rolling of reactive mixtures to fabricate reactive tapes. The examples of joining carbon materials, metals, and ceramic–metal pairs were presented. To date, the technique of combustion‐assisted joining of materials has become a promising and rapidly developing branch of modern R & D.

A Novel Pinch Point Design Methodology Based Energy and Economic Analyses of Organic Rankine Cycle

A generalized methodology for pinch point design and optimization of subcritical and transcritical organic Rankine cycles (ORCs) using both wet and dry fluids is adopted in this study. The presented algorithm can predict the pinch point location in evaporator and condenser simultaneously and optimize the evaporator pressure for best performance with various heat source and sink conditions. Effects of pinch point temperature difference (PPTD), isentropic efficiency, subcooling, superheating and regenerator on the energy and economic performances are discussed for selected working fluids. System yields similar optimum design for both maximum power generation and minimum capital cost per unit power. At optimum condition, ammonia is best in terms of higher thermal efficiency and lower component size, R152a is best in terms of higher net power output and heat recovery efficiency (11.1%), and toluene is best in terms of lower capital cost and cost per unit power output (7060 $/kW). Effect of heat source and sink parameters on both energy and economic performances is significant. Contour plots are presented to select the best ORC design parameters for available heat source condition. PPTD and expander isentropic efficiency have significant effect on performances. However, the effect of subcooling, superheating and regenerator depends on working fluid.

Novel pinch point method based exergetic optimisation of subcritical organic Rankine cycle for waste heat recovery

A new pinch point design methodology for subcritical organic Rankine cycle (ORC) is proposed to predict pinch point locations in both evaporator and condenser, and optimise working fluid mass flow rate to get maximum heat recovery and exergetic efficiency for given heat source and heat sink conditions. Selected working fluids have been compared based on different performance parameters for various heat source conditions. The present method seems to be better than previous pinch point design methods as it optimises the cycle by considering both source and sink. At optimum operation, ammonia is best in terms of lower mass flow rate and turbine size, toluene is best in terms of lowest heat exchanger size and cost, whereas, R-1233zd and isopentane are best in terms of higher power output and heat recovery, exergy and entrancy efficiencies. Contour plots are presented as well to select optimum design parameters for available heat source and sink.

Study on off-design performance of transcritical CO2 power cycle for the utilization of geothermal energy

Exploiting renewable energy could greatly alleviate current severe energy situation. Transcritical CO2 (tCO2) cycle system as a type of competitive and promising energy converter could utilize low temperature geothermal energy effectively. In this paper, the methodology for the off-design operation of tCO2 system has been proposed and the quantitative performance analysis is carried out for the utilization of geothermal energy. With the aim at obtaining better thermodynamic and economic performance, the optimal power cycle parameters can be determined by the non-dominated sorting genetic algorithm-II (NSGA-II) in the design stage. The models of exergoeconomic analysis and main system components including turbine, pump and heat exchangers are established. The sliding pressure control strategy is applied to respond to the varieties of heat source and heat sink conditions in the off-design stage. Results show that there exists an optimal value of geothermal resource mass flow rate to maximum the thermal efficiency. The pump rotational speed increases linearly with the increasing geothermal resource temperature and geothermal resource mass flow rate while the pump rotational speed is gradually sensitive to the increase of heat sink temperature. The guideline listed in the table for pump could provide effective reference for practical operation.

Development and application of a 200 kW ORC generator system for energy recovery in chemical processes

Depending on heat source and heat sink conditions, an organic Rankine cycle (ORC) can be used to convert low temperature thermal energy (such as industrial waste heat, geothermal, biomass, and solar thermal energy) into electrical power. This research focuses on the development of an ORC with 200 kW power rating, which is intended to convert thermal energy into electricity for the chemical company in Taiwan. R134a is selected as the working fluid. The main components of the ORC consist of a working fluid pump, a shell-and-tube type evaporator, a shell-and-tube type super heater, a screw expander, an oil-vapor separator, and a shell-and-tube type condenser. This is the first commercially sized ORC (200 kW) operating at a low temperature (82℃) to have been successfully developed in Taiwan, with all of the components also having been manufactured in Taiwan. The test results show that the net power output was 193.75 kW with a thermal efficiency of 4.701% for a water flow based heat source at a temperature of 81.5℃ and at a flow rate of 189 tons per hour. The ORC system has been running for more than 20,600 hours (Nov. 2014 to Apr. 2017) with a total power generation of more than 4,248,000 kWh without the occurrence of any faults.