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[en] Highlights: • Mode 4 has the highest exergy efficiency. • Mode 2 has the largest exergy density. • Second heat exchanger has the largest exergy destruction. - Abstract: Advanced adiabatic compressed air energy storage system plays an important role in smoothing out the fluctuated power from renewable energy. Under different operation modes of charge-discharge process, thermodynamic behavior of system will vary. In order to optimize system performance, four operation modes of charge-discharge process are proposed in this paper. The performance difference of four modes is compared with each other based on energy analysis and exergy analysis. The results show that exergy efficiency of mode 4 is the highest, 55.71%, and exergy density of mode 2 is the largest, 8.09 × 106 J m−3, when design parameters of system are identical. The second heat exchanger has the most improvement potential in elevating system performance. In addition, a parametric analysis and multi-objective optimization are also carried out to assess the effects of several key parameters on system performance.
[en] Highlights: • A solar thermoelectric with micro-channel heat pipe system was presented. • Mathematical model of the system was built. • Experiment and the simulation were compared to verify the model. • Performance of the system with different factors was analyzed. - Abstract: Micro-channel heat pipe can convert the low heat flux to the high heat flux by changing the ratio of the evaporator area to the condenser area and has a higher heat transfer performance than the common heat pipe. Combining the solar concentrating thermoelectric generation with micro-channel heat pipe can save the quantity of thermoelectric generation and reduce the cost significantly. In this paper, a solar concentrating thermoelectric generator using the micro-channel heat pipe array was designed, and the mathematical model was built. Furthermore, the comparison of the experiment and the simulation between the solar concentrating thermoelectric generator using the micro-channel heat pipe array and the thermoelectric generations in series was made. In addition, the performance on the different areas of selective absorbing coating, different concentration ratios, different ambient temperatures, different wind speed all were analyzed. The outcomes showed the overall performance of the solar concentrating thermoelectric generator using the micro-channel heat pipe array system.
[en] Highlights: • A technical solution to the power supply of wireless sensor networks is presented. • The low voltage produced by TEG is boosted from less than 1 V to more than 4 V. • An output current and voltage of TEG device is acquired as 21.47 mA and 221 mV. • The device successfully provides output power 4.7 mW in no electricity conditions. • The thermo-economic value of TEG device is demonstrated. - Abstract: Motivated by the limited power supply of wireless sensors used to monitor the natural environment, for example, in forests, this study presents a technical solution by recycling solar irradiation heat using thermoelectric generators. Based on solar irradiation and the earth’s surface-air temperature difference, a new type of thermoelectric power generation device has been devised, the distinguishing features of which include the application of an all-glass heat-tube-type vacuum solar heat collection pipe to absorb and transfer solar energy without a water medium and the use of a thin heat dissipation tube to cool the earth surface air temperature. The effects of key parameters such as solar illumination, air temperature, load resistance, the proportional coefficient, output power and power generation efficiency for thermoelectric energy conversion are analyzed. The results of realistic outdoor experiments show that under a state of regular illumination at 3.75 × 10"4 lx, using one TEG module, the thermoelectric device is able to boost the voltage obtained from the natural solar irradiation from 221 mV to 4.41 V, with an output power of 4.7 mW. This means that the electrical energy generated can provide the power supply for low power consumption components, such as low power wireless sensors, ZigBee modules and other low power loads
[en] Highlights: • We developed a thermoelectric cap (TC) to harvest hydrothermal energy. • The TC was deployed at a hydrothermal vent site near Kueishantao islet, Taiwan. • The TC monitored the temperature of the hydrothermal fluids during the field test. • The TC could make the thermal energy of hydrothermal fluids a viable power source. - Abstract: Long-term in situ monitoring is crucial to seafloor scientific investigations. One of the challenges of operating sensors in seabed is the lifespan of the sensors. Such sensors are commonly powered by batteries when other alternatives, such as tidal or solar energy, are unavailable. However, the batteries have a limited lifespan and must be recharged or replaced periodically, which is costly and impractical. A thermoelectric cap, which harvests the thermal energy of hydrothermal fluids through a conduction pipe and converts the heat to electrical energy by using thermoelectric generators, was developed to avoid these inconveniences. The thermoelectric cap was combined with a power and temperature measurement system that enables the thermoelectric cap to power a light-emitting diode lamp, an electronic load (60 Ω), and 16 thermocouples continuously. The thermoelectric cap was field tested at a shallow hydrothermal vent site near Kueishantao islet, which is located offshore of northeastern Taiwan. By using the thermal gradient between hydrothermal fluids and seawater, the thermoelectric cap obtained a sustained power of 0.2–0.5 W during the field test. The thermoelectric cap successfully powered the 16 thermocouples and recorded the temperature of the hydrothermal fluids during the entire field test. Our results show that the thermal energy of hydrothermal fluids can be an alternative renewable power source for oceanographic research.
[en] Highlights: • The performance of an ejector in an Organic Rankine Cycle and ejector refrigeration cycle (EORC) was evaluated. • The achieved entrainment ratio and COP of an EORC system is affected significantly by the evaporator conditions (such as temperature, pressure and flow rate). • An optimum distance of 6 mm nozzle position was found that ensures a maximum entrainment ratio, the best efficiency and lowest loss in the ejector. • A reduced total pressure loss between the nozzle inlet and exit leads to a lower energy loss, a higher entrainment ratio and better overall ejector performance. - Abstract: Power-generation systems based on organic Rankine cycles (ORCs) are well suited and increasingly employed in the conversion of thermal energy from low temperature heat sources to power. These systems can be driven by waste heat, for example from various industrial processes, as well as solar or geothermal energy. A useful extension of such systems involves a combined ORC and ejector-refrigeration cycle (EORC) that is capable, at low cost and complexity, of producing useful power while having a simultaneous capacity for cooling that is highly desirable in many applications. A significant thermodynamic loss in such a combined energy system takes place in the ejector due to unavoidable losses caused by irreversible mixing in this component. This paper focuses on the flow and transport processes in an ejector, in order to understand and quantify the underlying reasons for these losses, as well as their sensitivity to important design parameters and operational variables. Specifically, the study considers, beyond variations to the geometric design of the ejector, also the role of changing the external conditions across this component and how these affect its performance; this is not only important in helping develop ejector designs in the first instance, but also in evaluating how the performance may shift (in fact, deteriorate) quantitatively when the device (and wider energy system within which it functions) are operated at part load, away from their design/operating points. An appreciation of the loss mechanisms and how these vary can be harnessed to propose new and improved designs leading to more efficient EROC systems, which would greatly enhance this technology’s economic and environmental potential. It is found that some operating conditions, such as a high pressure of the secondary and discharge fluid, lead to higher energy losses inside the ejector and limit the performance of the entire system. Based on the ejector model, an optimal design featuring a smoothed nozzle edge and an improved nozzle position is found to achieve an improved entrainment ratio, significantly better performance and reduced energy losses in the ejector.
[en] Highlights: • Thermal enhancement in a thermoelectric liquid generator is tested. • Thermal enhancement is brought upon by flow impeding inserts. • CFD simulations attribute thermal enhancement to velocity field alterations. • Thermoelectric power enhancement is measured and discussed. • Power enhancement relative to adverse pressure drop is investigated. - Abstract: Thermoelectric power production has many potential applications that range from microelectronics heat management to large scale industrial waste-heat recovery. A low thermoelectric conversion efficiency of the current state of the art prevents wide spread use of thermoelectric modules. The difficulties lie in material conversion efficiency, module design, and thermal system management. The present study investigates thermoelectric power improvement due to heat transfer enhancement at the channel walls of a liquid-to-liquid thermoelectric generator brought upon by flow turbulating inserts. Care is taken to measure the adverse pressure drop due to the presence of flow impeding obstacles in order to measure the net thermoelectric power enhancement relative to an absence of inserts. The results illustrate the power enhancement performance of three different geometric forms fitted into the channels of a thermoelectric generator. Spiral inserts are shown to offer a minimal improvement in thermoelectric power production whereas inserts with protruding panels are shown to be the most effective. Measurements of the thermal enhancement factor which represents the ratio of heat flux into heat flux out of a channel and numerical simulations of the internal flow velocity field attribute the thermal enhancement resulting in the thermoelectric power improvement to thermal and velocity field synergy
[en] Highlights: ► Thermal interferences progress with borehole compaction, 7 × 6 and 21 × 2 grids analyzed. ► Two numerical models used for heat transfer; ASHRAE/Kavanaugh and Lund/Eskilson. ► A/K model uses simplistic borehole interaction to predict heat build up in the ground. ► L/E model more advanced borehole interaction which results in greater loop length. ► Detailed evolution of ground and borehole fluid temperatures given for 30 year period. - Abstract: Properly sized borehole heat exchanger in geothermal heat pump system (widely known also as ground source heat pump system) needs to minimize long-term ground and working fluid temperature changes. These changes occur due to imbalances of heat extracted from the ground during winter and heat rejected into the ground during summer months, as well as thermal interferences of adjacent boreholes in borehole array. Simple calculations and spreadsheet-based analogical solutions of required borehole length for heat transfer run into difficulties when dealing with such large, complex ground source heat pump heating and cooling systems which use compact borehole array. A number of analytical computer programs are available to simulate how ground loop fluid temperature varies in such complex systems, using so-called ‘g-function’ – a mathematical function dependent on the geometry and shape of the borehole array. The type of calculation involves a type of ‘step-function’, where 24 h ‘step’ of peak loading is superimposed on a top of a long-term base load. In analytical simulation models, the base load is typically specified as the heating and cooling load per month of a typical year, and is simulated as the combination of sequential monthly steps. This paper will show, by simulating long-term operation of complex geothermal heat pump system with multiple boreholes in various geometric arrays, how spacing of adjacent boreholes and thermal interferences influence required borehole length for heat transfer.
[en] Highlights: • A novel and unique type of thermoelectric generators is proposed. • Heat source is combined in thermoelectric elements, reducing heat transfer problems. • Embedding radioactive isotopes is proposed as a way to implement the new design. • Conversion efficiency and power density are estimated for the proposed design. - Abstract: A novel and unique design of thermoelectric generators, in which a heat source is combined with thermoelectric elements, is proposed. By placing heat-generating radioactive isotopes inside the thermoelectric elements, the heat transfer limitation between the generator and the heat source can be eliminated, ensuring simplicity. The inner electrode is sandwiched between identical thermoelectric elements, which naturally allows the inner core to act as the hot side. Analysis shows that conversion efficiency and power density increase as the heat density inside the thermoelectric elements increases and as the thermoelectric performance of the material improves. The theoretical maximum efficiency is shown to be 50%. However, realistic performance under practical constraint is much worse. In realistic cases, the efficiency would be about 3% at best. The power density of the proposed design exhibits a much more reasonable value as high as 3000 W/m2. Although the efficiency is low, the simplicity of the proposed design combined with its reasonable power density may result in some, albeit limited, potential applications. Further investigation must be performed in order to realize such potential
[en] Highlights: • Exergy analysis of BTES for heating season is carried out. • Exergy efficiency of BTES is determined to be 41.35% for overall system. • COPHP is determined to be 2.6 for overall system. • Increasing evaporator temperature to 6 °C decreases the exergy destruction rate. • Increasing condenser temperature to 70 °C increases the exergy destruction rate. - Abstract: A comprehensive thermodynamic assessment of a borehole thermal energy storage system (BTES), which helps in meeting the heating and cooling demands of campus buildings of University of Ontario Institute of Technology (UOIT), is presented for the heating case. The BTES located on UOIT campus in Oshawa, Canada is recognized as the world’s second largest BTES system. Energy and exergy analyses of the heating system are performed through the balance equations, and exergy destruction rates are determined for each system component and the overall BTES. In addition, a comparative system performance assessment is carried out. Based on the conducted research for the studied system, COPHP is calculated to be 2.65 for heating applications. Energy and exergy efficiencies of the boilers are determined to be 83.2% and 35.83%, respectively. The results of the exergy analysis show that the boilers are the major contributor to exergy destruction, followed by condenser and evaporator. The effects of condenser and evaporator temperatures of the heat pump systems on energy and exergy efficiencies are also investigated. The overall exergy efficiency of the whole system is calculated to be 41.35%
[en] The process of charging of an encapsulated ice thermal energy storage device (ITES) is thermally modeled here through heat transfer and thermodynamic analyses. In heat transfer analysis, two different temperature profile cases, with negligible radial and/or stream-wise conduction are investigated for comparison, and the temperature profiles for each case are analyzed in an illustrative example. After obtaining temperature profiles through heat transfer analysis, a comprehensive thermodynamic study of the system is conducted. In this regard, energy, thermal exergy and flow exergy efficiencies, internal and external irreversibilities corresponding to flow exergy, as well as charging times are investigated. The energy efficiencies are found to be more than 99%, whereas the thermal exergy efficiencies are found to vary between 40% and 93% for viable charging times. The flow exergy efficiency varies between 48% and 88% for the flows and inlet temperatures selected. For a flow rate of 0.00164 m3/s, the maximum flow exergy efficiency occurs with an inlet temperature of 269.7 K, corresponding to an efficiency of 84.3%. For the case where the flow rate is 0.0033 m3/s, the maximum flow exergy efficiency becomes 87.9% at an inlet temperature of 270.7 K. The results confirm the fact that energy analyses, and even thermal exergy analyses, may lead to some unrealistic efficiency values. This could prove troublesome for designers wishing to optimize performance. For this reason, the flow exergy model provides the most useful information for those wishing to improve performance and reduce losses in such ITES systems