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Forced ventilation is a common means to solve the problem of high ambient temperature during the construction of high-ground-temperature tunnels. In forced ventilation, it is very important to prevent the fresh cold airflow from being heated by the high-ground-temperature environment when it reaches the working surface so as to ensure the cooling effect. Based on heat transfer theory, a temperature numerical calculation model is proposed for forced ventilation in high-ground-temperature tunnels that considers the convection heat transfer between the airflow inside the duct, and the airflow inside the tunnel and the surrounding rock, combined with the influence of the duct leakage rate. This model compensates for the lack of considering the surrounding rock temperature and the duct leakage rate in relevant studies. Using the 7# cross tunnel of an actual tunnel as a case study, the effects of ventilation volume, construction distance and thermal insulation duct on the airflow temperature inside the duct and tunnel were investigated. According to the field test results, the developed numerical calculation model is in good agreement with field test data, with a maximum difference of 2 °C between the numerical calculation results and field test data. The thermal insulation duct has good thermal insulation performance. Under different construction distances, the thermal insulation duct can reduce the airflow temperature at the outlet of the duct by 6–14 °C, up to as much as 28–40%. Simply increasing the ventilation volume has no significant effect on preventing the airflow temperature rise inside the duct. At the same construction distance, although the airflow temperature at the duct outlet gradually decreases with the increase in the ventilation volume, the decrease is small: For every 10 m3 s−1 increase in the ventilation volume, the temperature is only reduced by 1 °C. For a tunnel with short construction distance and low surrounding rock temperature, the requirements can be met without thermal insulation treatment of the duct, but for a long-distance tunnel, the thermal insulation duct is an effective and economical auxiliary cooling means. [ABSTRACT FROM AUTHOR]

Railroad transportation has been widely adopted and developed in recent years. In particular, many railroads have been built on complex terrains such as plateaus and mountainous areas, and these tunnels are generally characterized by large inclination angles or very long lengths and are equipped with rescue stations. These factors lead to changes in environmental conditions, tunnel geometry, and the angle of inclination, thus presenting new challenges to ventilation as well as fire safety in railroad tunnels. This review argues that there is a lack of systematic and comprehensive analysis of smoke control in railroad tunnel fires, including horizontal tunnels, inclined tunnels, and rescue stations within long railroad tunnels. Hence, this work attempts to address the key fire safety characteristics from the existing literature covering critical velocity, back-layering length, the maximum temperature under the ceiling of the tunnel, and longitudinal smoke temperature distribution. Important factors affecting smoke movements such as fire source, tunnel geometries, and blockages are also evaluated. Moreover, this study calls attention to the impact of extended blockages and temperature distribution in blocked railroad tunnels, as well as emerging innovative solutions such as intelligent monitoring and disaster prevention technologies for smoke control in railroad tunnels. [ABSTRACT FROM AUTHOR]

Controlling the ambient temperature inside a tunnel to less than 28 °C is the key to ensure safe operation of tunnels that experience high-geothermal temperature. One of the most popular methods to lower the ambient temperature of an extra-long tunnel with high-geothermal temperature is mechanical ventilation. Taking the Layue tunnel as an example, this paper examined the impact of natural wind, piston airflow, and mechanical ventilation on tunnel thermal hazard control. The results show that the cooling effect of natural ventilation is better when natural wind flows from entrance to exit than when it blows from exit to entrance. When there is only natural ventilation, the length of tunnel ambient temperature higher than 28 ℃ is more than 15 km within 5.0 years of natural ventilation. Piston airflow does not significantly reduce the length of high tunnel ambient temperature distribution range. Only relying on natural wind and piston airflow cannot control heat damage in the Layue tunnel. The thermal layer can reduce the length over which high tunnel ambient temperature is distributed, but the maximum reduction is within 2500 m. When the mechanical airflow speed is at least 3.0 m s−1, the heat damage can be effectively controlled within one year. Whether mechanical airflow is started in summer or winter, mechanical airflow shall be carried out at the speed of 3.0 m s−1 at least 200 days in advance to effectively control the heat damage of Layue tunnel. [ABSTRACT FROM AUTHOR]

A novel polygeneration system is designed to provide electricity, heating and cooling capacity on basis of solar energy. Two-stage power cycle of sCO2 Brayton and steam Rankine is propelled to generate electricity with residual heat transferred to drive an efficient heating device and an intercooled absorption cooling system. System performance is comprehensively analyzed from thermal efficiency in terms of energy and exergy, based on the quasi-steady numerical model. Results indicate that built system obtains electricity, heating and cooling capacity of 243.84, 42.05 and 10.746 MW with energy efficiency of 0.5304, 0.0915 and 0.0234 and exergy efficiency of 0.562, 0.0174 and 0.0024, respectively. Mass flows of working fluids and detailed specifications of solar collecting field are also included in this investigation. Parametric analysis exhibits that operating parameters exert remarkable influence on thermal efficiencies. Proposed system can save amounts of energy over three individual systems with identical products by means of a primary energy saving ratio analysis and energy saving performance is more sensitive to turbine inlet temperature of Brayton. Exergoeconomic evaluation reveals that designed system achieves inferior electricity performance and superior heating and cooling performance in comparison with current circumstance. In conclusion, this novel cogeneration system takes full advantage of solar energy to meet various energy consumptions. [ABSTRACT FROM AUTHOR]

A novel micro-combined polygeneration system based on solar energy and fuels is designed with aim to simultaneously satisfy energy demands of electricity, heating and cooling in distributed areas. Integration solar radiation with conventional natural gas-fired power systems is conceived to eliminate existing disadvantages of low efficiency and high cost for fuel-only and solar-only plants. Thermodynamic and economic analyses are evaluated to explore performance of proposed systems with three working fluids. Energetic and exergic efficiencies are individually 0.839 and 0.4536, 0.8721 and 0.5852, and 0.8656 and 0.5825 for N2-, He- and Ar-based micro-combined systems under specified power capacity of 100 kW. Solar collector field, compressor, turbine and combustion chamber exhibit four largest exergy destruction, followed orderly by heat exchanger, generator, condenser, evaporator, throttling valve and pump. Preliminary economic analysis indicates that levelized electricity costs of N2-, He- and Ar-based micro-combined systems are 0.112, 0.103 and 0.101 $ kWh−1 with corresponding payback periods of 4.563, 5.090 and 5.138 years. Parametric analysis reveals that five operating parameters (compression ratio, turbine inlet temperature, solar collector field outlet temperature, generator outlet temperature and split ratio) exert noticeable influence on thermal efficiency of micro-combined systems. In conclusion, proposed micro-combined polygeneration system achieves efficient conversation of thermal energy into electricity, heating and cooling at a rational energy utilization for distributed areas. [ABSTRACT FROM AUTHOR]

In this study, yttrium-stabilized zirconia (YSZ) was firstly used as a nucleating agent and copper foam as the matrix to simultaneously reduce the supercooling degree and improve the thermal conductivity of sodium acetate trihydrate (SAT). The SAT/copper foam/YSZ composite phase change material (PCM) was prepared by ball milling and melting impregnation method. The influences of content of YSZ on supercooling degree of SAT and porosity of copper foam on thermal conductivity of SAT were studied experimentally. The step-cooling curve method results indicated that the supercooling of SAT can be eliminated by 0.8 mass% YSZ without any chemical interaction between SAT and YSZ. The thermal conductivity test and heat transfer test results of SAT/copper foam/YSZ composite PCM indicated that copper foam increases the thermal conductivity of SAT. The thermal conductivity of SAT/copper foam/YSZ composite PCM was 5.9, 6.9 and 9.4 times as that of pure SAT when the porosity of copper foam was 8 ppi, 15 ppi and 40 ppi. Moreover, the volume energy storage density of SAT/copper foam/YSZ composite PCMs was calculated as 448.6–473.0 MJ m−3, which was 2.09–2.20 times as that of water. [ABSTRACT FROM AUTHOR]

In this study, iron oxide nanoparticles (α-Fe2O3) have been firstly used as a nucleating agent, which simultaneously reduces the supercooling degree of sodium acetate trihydrate (SAT) and improves its thermal conductivity. A series of SAT composite phase change materials (PCMs) for potential latent heat thermal energy storage applications were prepared by a ball milling method using carboxymethyl cellulose as a thickening agent and sodium dodecyl sulfonate as a dispersant. In order to investigate the effect of the mass ratio of α-Fe2O3 nanoparticles on the supercooling degree of SAT, various α-Fe2O3 contents (0.2, 0.4, 0.6, 0.8, and 1.0 mass%) were added into the SAT matrix. It was found that the supercooling degree of the SAT composite PCM was reduced to 0 °C at a α-Fe2O3 content of 0.8 mass%. Furthermore, no chemical reaction between SAT and α-Fe2O3 occurred, and the presence of α-Fe2O3 had no effect on the energy storage capability of SAT. The thermal conductivity of the SAT composite PCM was improved by 22.5% due to the addition of 0.8 mass% α-Fe2O3. After 60 melting-freezing cycles, the composite PCMs retained excellent stability with a small reduction in the phase change temperature (0.33 °C) and low latent heat loss rate (0.796%). [ABSTRACT FROM AUTHOR]

Polyethylene glycol (PEG) as phase change materials has been extensively studied. However, it is difficult to uniformly disperse the PEG in the unmodified matrix material owing to the impregnation problem. It is therefore challenging to overcome the shortcoming of low thermal conductivity. In order to solve this problem, carbon microspheres (CMPs) prepared by the hydrothermal method is firstly proposed as the supporting matrix for preparation of PEG composite. The PEG/CMPs composite is prepared via a mutual diffusion methodology in a high-temperature environment. The test result shows the CMPs nano-material has rich oxygen-based functional groups and can be uniformly dispersed in the PEG phase. As the carbon content increases, the CMPs become gradually connected in the PEG/CMPs composite, which can result in a network with good thermal conductivity. Compared with the thermal conductivity of pure PEG, the thermal conductivity of the composite is increased by 65.07%. Owing to the existence of hydrogen bonds in the composite, the crystallinity fraction of the PEG is in the range of 102-105%. It means that the test result is higher than the theoretical value of latent heat for the composite. Finally, its cycling performance was measured. After 500 thermal cycles, the phase transition temperature of the composite remains almost constant, and the latent heat values of the melting and freezing decrease by 1.05 and 1.45%, respectively. The PEG/CMPs composite would be a promising material for thermal energy storage applications and can be used in various engineering disciplines. [ABSTRACT FROM AUTHOR]

Current Al alloys still have shortcomings in their volumetric latent heat (LHV), compatibility and high-temperature inoxidizability, which limit their applications in the field of latent heat energy storage (LHES). The performance of aluminum alloys can be improved by the addition of Cu. The effects of the Cu content on the phase change temperature, mass latent heat (LHM), LHV, supercooling degree and microstructure of Al-Cu alloys were first studied by means of power-compensated differential scanning calorimetry, density, composition analysis and metallographic analysis. The measured values of the latent heat of Al-Cu alloys have been compared with the theoretically predicted values. The results show that for Al-Cu alloys with 7.3-52.8% Cu, the melting/freezing temperature is 540-655 °C/510-637 °C; the LHM and the LHV are 290-340 J g and 877-1224 J cm, respectively; and the degree of supercooling is within 10 °C. The LHM and LHV of Al-Cu alloys decrease with the increase in the Cu content; when the content of Cu is over 16.6%, the difference between the theoretical value of the LHM and the measured average of the Al-Cu alloys is within 5%. The LHES phases in Al-Cu alloys are the α-Al and theta phases. Quantitative relationships of the Cu content and metallurgical microstructure with the LHM and LHV of Al-Cu alloys are established, and both theoretical and empirical equations are obtained for the estimation of the latent heat for Al-Cu alloys. [ABSTRACT FROM AUTHOR]

Nanofluids of LiCO-NaCO-KCO improved by three nano-AlO samples are firstly prepared by means of two-step aqueous method to enhance thermal properties for high-temperature heat transfer, when used as heat transfer fluids and thermal energy systems for concentrating solar power systems. Specific heat of ternary carbonates containing AlO of 0.2, 0.4, 0.8, 1.0, 1.4 and 2.0 mass% is measured, and nanofluids with 1.0 mass% of 20-nm AlO, 1.0 mass% of 50-nm AlO and 0.8 mass% of 80-nm AlO are selected as superior candidates. The maximum enhancement of specific heat is 18.5% in solid and 33.0% in liquid, 17.9% in solid and 22.7% in liquid, 13.2% in solid and 17.5% in liquid for nanofluids containing 20-, 50- and 80-nm AlO. Thermal conductivity is, respectively, improved by 23.3, 28.5 and 30.9% under the addition of AlO. New chemical bonds and crystals are scarcely formed in composites through FT-IR and XRD determination. SEM images certify that nano-AlO are homogeneously mixed into nanofluids and this structure may be a critical incentive for enhancing thermal properties. There are no significant changes with respect to the heat flow, melting/freezing point and latent heat after the 30 circles of determination. Briefly, it can be speculated that these nanofluids will exhibit tremendous potential in the coming applications of heat transfer and thermal storage for concentrating solar power systems. [ABSTRACT FROM AUTHOR]

Both hydroscopic materials and phase change materials represent popular research topics. This paper proposes PEG polymers as endothermal-hydroscopic materials adjusting the temperature and humidity of an environment. A homogeneous PEG mixture with different water contents is prepared via a mutual diffusion methodology in a high-temperature environment. The as-prepared mixture is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and isothermal adsorption. The XRD and SEM results indicate that the crystallinity decreases as the moisture content increases, and the primary particles disappear. According to the DSC results, both the phase transition temperature and latent heat exhibit a negative linear relationship with the water content. Isothermal adsorption curves indicate that water molecules uniformly dispersed in the polymer can optimize its microstructure, providing a mass transfer channel for water vapour. Additionally, PEG2000 exhibits the best endothermal-hydroscopic performance. The PEG2000 phase transition temperature and latent heat decrease by 6.7 °C and 28.9 J g, respectively, when a 10 % liquid is added to the polymer. The D-value (change value) is clearly lower than those of PEG4000 (10.9 °C and 36.9 J g) and PEG8000 (10.1 °C and 55.2 J g). In addition, the PEG2000 water uptake is 198 g kg for 6 h. [ABSTRACT FROM AUTHOR]

Based on the theory calculation, the capric-palmitic-stearic (CA-PA-SA) ternary eutectic mixture was prepared with melting and freezing temperature of 19.93 and 16.84 °C, respectively. Regarding CA-PA-SA as phase change materials (PCMs), and activated carbon (AC) as the additive, the CA-PA-SA/AC composites can be prepared by physical blend method which can be applied to high-temperature cooling application in solar cooling systems, radiant cooling systems, etc. A comprehensive study of the structure, thermal properties, thermal stability, thermal durability and thermal storage-release characteristic of CA-PA-SA/AC composites was conducted through the FT-IR, DSC, TG, accelerated thermal cycling test and melting-freezing experiments. The results show that, with the increase in AC content, the melting temperature of CA-PA-SA/AC composites decreases gradually, whereas the freezing temperature increases. And the addition of AC can enhance the thermal stability, the thermal durability and the thermal storage-release rate of PCM: the thermal stability of PCM1-PCM4 is much greater than that of pure mixture, and its storage time was reduced by 37, 31, 23 and 9 %, respectively, while its release time was reduced by 67, 58, 48 and 43 % respectively. [ABSTRACT FROM AUTHOR]