• Energy Research

Abstract A new kind of shape-stabilized phase change materials (PCMs) with positive temperature coefficient (PTC) effect was prepared in this paper. The materials were prepared by adding graphite powder (GP) to the paraffin/low density polyethylene (LDPE) composite and the PTC characteristic was found by adjusting the component ratio of the material. Then the physical structures and thermal properties of the materials were investigated and the effect of various GP mass fractions and paraffin/LDPE mass proportions on the PTC behavior of the materials was studied experimentally. The results showed that the switching temperature of the materials was about 25 °C (room temperature) which approached to the first phase change temperature of paraffin dispersed in the materials. The PTC behavior of the materials was the best when the GP mass fraction and the mass proportion of LDPE/paraffin were 40 wt% and 30:70, respectively. Furthermore, the negative temperature coefficient (NTC) effect of the materials could be eliminated effectively with heat treatment. This new kind of materials is different from the former PTC materials which the switching temperatures focus on high temperature ranges. It makes up for the defect of previous materials that the switching temperatures only range in high temperature rather than room temperature and provides a potential means for the thermal control of the electronic devices or other room temperature thermal control applications.

Abstract The accurate assessment of thermal properties is crucial for charring materials simulation and design. In this work, a new inversion method for estimating temperature-dependent thermal conductivity and specific heat capacity from temperature data is presented for charring ablators. Firstly, a one-dimensional thermal response model with surface recession is developed to simulate the thermal behavior of charring ablator. Then based on the developed model, sensitivity analysis is conducted to investigate the correlation between thermal parameters and temperature for determining inversion sequence and find that virgin thermal conductivity has the biggest influence on temperature which should be estimated at first. Finally, the temperature-dependent thermal conductivities and specific heat capacities are obtained by the inversion method from temperature data. The inversion values from simulation temperature data coincide with the reference values, which the average inversion error of thermal conductivities is 4.3% and the average inversion error of specific heat capacities is 3.1%. And the calculated thermal response temperature employing the inversion thermal properties shows a good agreement with the arc jet test data, which the average error is 8.5%. This inversion method can accurately determine unknown temperature-dependent thermal properties such as thermal conductivity and specific heat capacity, which provides an insight into the analysis and design of thermal protection materials for spacecraft.

Abstract The development trend of spray cooling system is adapting to strict working conditions such as high heat flux, large heating surface and complex fluid management. Therefore, for the purpose of cooling large heating surface in a compact space, this paper designed a novel multi-nozzle array and set up a test rig of spray cooling loop. The spray characteristics of the nozzle were tested by Phase Doppler Anemometer system, and the effects of temperature uniformity, flow rate, spray height and surfactant of 2-ethyl-hexanol on heat transfer performance of the spray cooling system were studied. According to the spray cooling curves obtained, the heat flux on the heating surface (30 mm × 30 mm) can reach 102.6 W/cm2 at least using surfactant and 91.5 W/cm2 using pour water. The temperature non-uniformity was expanded with increasing heat flux. The results simultaneously indicate that the heat transfer performance is closely associated with volume flow rate and spray height, while the performance is improved by increasing volume flow rate and optimizing spray height. In addition, trace amounts of 2-ethyl-hexanol surfactant added into the working fluid can enhance the heat transfer performance of spray cooling which is increased by 15% with 200 ppm 2-ethyl-hexanol surfactant compared with water added nothing.

Abstract Positive temperature coefficient (PTC) material is widely used in the field of thermal control due to its unique temperature sensitive characteristics. However, there is no general criterion about PTC material thermal control at present due to the lack of theoretical analysis in the previous works. Thus, in this paper, the dimensionless method is used to study PTC material adaptive thermal control and finally, a fitting formula is obtained to estimate the equilibrium temperature of controlled device. First, a one-dimensional and steady-state heat transfer model was proposed to calculate the equilibrium temperature of controlled device under thermal control of PTC materials, and it was verified by experiment. Then, the model has been nondimensionalized and four dimensionless numbers are introduced by Buckingham theorem, in which the Wr number is presented to represent the heating characteristics of PTC materials. Finally, by fitting all the data, a form of fitting equation of equilibrium temperature of controlled device is obtained. The results show that this form proposed in this paper can be applied to the widely used ceramic PTC materials. The criterion equation fitted by this form can effectively estimate the equilibrium temperature of the controlled devices with good accuracy and wide application.

Abstract The density variation of supercritical carbon dioxide exerts significant effects on heat transfer and the adaptive flow path is verified to be effective in reducing pressure loss. Meanwhile, the effects differ from flow directions of supercritical carbon dioxide. Previous studies focused on heat transfer in vertical tubes rather than vertical heat transfer exchangers. In this paper, an experimental study of supercritical carbon dioxide in a vertical adaptive flow path heat exchanger is conducted to investigate the heat transfer and flow characteristics. The pressure of cold side varies from 14.2 MPa to 16.7 MPa and that of hot side varies from 8 MPa to 10 MPa. The inlet temperature of cold side is 328 K and that of hot side varies from 398 K to 418 K. When mass flow rate ratio increases, the minimum heat transfer effectiveness occurs at 0.6 and maximum overall heat transfer coefficient occurs at 0.65. Due to the effect of flow direction, the overall heat transfer coefficient of horizontal flow is approximately 9% higher than that of upward flow for cold side and 6% lower than that of downward flow for cold side. Hence, the downward flow (cold side) is suggested for installation of heat exchanger in Brayton cycle.

Abstract Positive temperature coefficient (PTC) material is widely used in thermal control due to its unique characteristics. In this paper, a novel heat transfer model of PTC material is proposed with consideration given to the internal heat transfer characteristics of the model which was ignored in the previous model, and it is validated by conducting experiment. Based on this novel heat transfer model, the effects exerted by thermal conductivity, heat capacity and thermal contact resistance on the thermal control of PTC material that have never been studied in previous work are investigated. In this paper, the PTC material with large heat capacity will extend the equilibrium time of the controlled device significantly. Besides, the dynamic thermal control performance of PTC material is verified by carrying out simulation. Herein, thermal control of PTC material under three different ambient temperature conditions is studied. As indicated by the results, PTC material has better thermal control performance under variable ambient temperatures. When the ambient temperature varies periodically with 5 °C amplitude and for a period of more than 300 s, the temperature of controlled device barely changes.

Abstract Application of thermal energy storage technology in practical engineering has been inhibited due to the constant strong rigidity of phase change materials (PCMs) at any given temperature. To solve this problem, a novel kind of thermal sensitive flexible PCMs was developed by using difunctional olefin blockcopolymer (OBC) replacing conventional supporting materials. Due to phase separation morphology, OBC presents physical crosslinked network and elasticity which is essential for macroscopical elastic deformation of composite. The developed PCMs based on OBC not only exhibit the same high latent capacity and stable shape as shape-stabilized PCMs, but also present thermal sensitive flexibility with the melting point of paraffin T paraffin,m as the stimulus. Below the melting point of OBC- T OBC,m , the transformation from rigid to flexibility is reversible. The above excellent flexibility of the developed PCMs contributes to reducing thermal contact resistance and improving poor installation for conventional PCMs in energy storage field, such as in energy-storage air-conditioning and compact electron device thermal control, especially in spacecraft thermal control system. Furthermore, thermal conductivity was improved to a breakthrough level (479% of original rate) by adding 3 wt.% of the maximum size of expanded graphite, which is based on the guidance of the novel calculation model on account of thermal conductivity for thermal sensitive flexible PCMs.

Abstract A three-dimensional thermal-hydrologic model considering water losses was developed to simulate heat extraction from an enhanced geothermal system (EGS), consisting of an injection well, a production well, a reservoir and the surrounding formation. The model was verified by application to Fenton Hill EGS. Based on the model, the influences of some factors on heat extraction were analyzed. As surrounding formation permeability increases, there are more water losses from the reservoir to the surrounding formation, leading to decreased heat extraction rate. When the two wells approach the edge of the reservoir, heat extraction is restrained, indicating that the two wells should be located about 20–25 m from the edge of the reservoir. Enlarged reservoir volume would not efficiently enhance heat extraction with fixed well layout, suggesting that well layout should be further optimized. Optimized open-hole length is equal to vertical reservoir dimension. Lower production pressure enhances heat extraction, resulting from increased production flow rate and decreased water loss rate. For each factor, it is concluded that thermal breakthrough time mainly depends on production flow rate and water loss rate, showing the important effect of water losses. The results of this paper can offer some suggestions to optimize the EGS performance.

Abstract The instability and intermittency feature of low grade heat sources and renewable energy brought about utilization problems, and in most cases, energy source site locates far away from energy demand site, how to implement long distance transmission of energy has been a challenge. To solve above problems, a novel combined power, cooling with solution energy storage and long distance heating/cooling system without heat preservation is proposed. The system integrates Kalina cycle with solution energy storage cycle, which is based on concentration difference of ammonia-strong, ammonia-weak solutions and liquid ammonia, and capable of changing operation modes flexibly according to heat sources and client needs. An analytical model is established and exergy destruction analysis is conducted to indicate exergy losses distribution of components. After optimization of proposed system, exergy efficiency is 0.57, and maximum solution energy storage density reaches 523 MJ/m3. Furthermore, thermal energy is storaged in the form of latent heat, and the working fluids are transported under ambient temperature, therefore, thermal insulation are unnecessary, the maximum heat supply distance is 143 km, 15.7 times that of typical hot water transmission system, and pipe diameter is reduced to 3/20, pump work consumption and construction costs can be reduced dramatically as a consequence.