Showing papers on "Latent heat published in 2018"

Review of latent heat thermal energy storage for improved material stability and effective load management

Abstract: Thermal energy storage is important to counter balance demand and supply of energy and maintain balance in the system and boost the use of intermittent renewable energy source. Phase change material-based thermal energy storage has massive potential to substitute large-scale energy demand and assist both economic and environmental benefits. This paper reviews functional principle, thermophysical properties and other material characteristics of different phase change materials for thermal energy storage system. Long-term stability of phase change material and its interaction with storage container have been discussed. Various heat transfer and thermal conductivity enhancement technique to enhance latent thermal energy storage system have been discussed. The paper also examines the schematics of some of the proposed & tested systems and describes the results of prototype setup for thermal load management and application in water heating system and buildings. The paper also summarizes energy and exergy analysis of some thermal energy storage systems.

Global lake evaporation accelerated by changes in surface energy allocation in a warmer climate

Abstract: Lake evaporation is a sensitive indicator of the hydrological response to climate change. Variability in annual lake evaporation has been assumed to be controlled primarily by the incoming surface solar radiation. Here we report simulations with a numerical model of lake surface fluxes, with input data based on a high-emissions climate change scenario (Representative Concentration Pathway 8.5). In our simulations, the global annual lake evaporation increases by 16% by the end of the century, despite little change in incoming solar radiation at the surface. We attribute about half of this projected increase to two effects: periods of ice cover are shorter in a warmer climate and the ratio of sensible to latent heat flux decreases, thus channelling more energy into evaporation. At low latitudes, annual lake evaporation is further enhanced because the lake surface warms more slowly than the air, leading to more long-wave radiation energy available for evaporation. We suggest that an analogous change in the ratio of sensible to latent heat fluxes in the open ocean can help to explain some of the spread among climate models in terms of their sensitivity of precipitation to warming. We conclude that an accurate prediction of the energy balance at the Earth’s surface is crucial for evaluating the hydrological response to climate change.

Influence of operational and design parameters on the performance of a PCM based heat exchanger for thermal energy storage – A review

Abstract: Thermal energy storage using phase change materials (PCM) proved to be a promising technology because of its relative advantages over the other types of energy storage methods. Along with thermophysical properties of PCM, the performance of latent heat based thermal energy storage system depends on the design of the heat exchanger. Although extensive research is being carried out over the past few years, an integrated study on the design of PCM heat exchanger is scarce. This review presents the in-depth analysis of various operating conditions and design parameters that need to be considered in the design of a PCM based heat exchanger. Shell and tube type, triple concentric tube type heat exchangers are discussed along with the various heat transfer techniques employed in both the types of heat exchangers. In each enhancement technique, the influencing geometric parameters are summarized, and the recommended values of those parameters are provided. The present article is expected to be a helpful reference for the researchers working in the field of thermal energy storage.

Increased Heat Transport in Ultra-hot Jupiter Atmospheres through H 2 Dissociation and Recombination

Abstract: A new class of exoplanets is beginning to emerge: planets with dayside atmospheres that resemble stellar atmospheres as most of their molecular constituents dissociate. The effects of the dissociation of these species will be varied and must be carefully accounted for. Here we take the first steps toward understanding the consequences of dissociation and recombination of molecular hydrogen (H2) on atmospheric heat recirculation. Using a simple energy balance model with eastward winds, we demonstrate that H2 dissociation/recombination can significantly increase the day–night heat transport on ultra-hot Jupiters (UHJs): gas giant exoplanets where significant H2 dissociation occurs. The atomic hydrogen from the highly irradiated daysides of UHJs will transport some of the energy deposited on the dayside toward the nightside of the planet where the H atoms recombine into H2; this mechanism bears similarities to latent heat. Given a fixed wind speed, this will act to increase the heat recirculation efficiency; alternatively, a measured heat recirculation efficiency will require slower wind speeds after accounting for H2 dissociation/recombination.

Highly Efficient Water Harvesting with Optimized Solar Thermal Membrane Distillation Device

Abstract: Water distillation with solar thermal technology could be one of the most promising way to address the global freshwater scarcity, with its low cost and minimum environmental impacts. However, the low liquid water productivity, which is caused by the heat loss and inadequate heat utilization in solar-thermal conversion process, hinders its practical application. Here, a compact solar-thermal membrane distillation system with three structure features: highly localized solar-thermal heating, effective cooling strategy, and recycling the latent heat, is proposed. The steam generation rate is 0.98 kg m-2 h-1 under solar illumination of 1 kW m-2 in the open system, while the liquid water productivity could be 1.02 kg m-2 h-1 with the solar efficiency up to 72% with a two-level device. The outdoor experiments show a water productivity of 3.67 kg m-2 with salt rejection over 99.75% in one cloudy day. These results demonstrate an easy and high-efficiency way for water distillation, especially suitable for household solar water purification.

How to improve the thermal performance of pulsating heat pipes: A review on working fluid

Abstract: Pulsating Heat Pipes (PHPs) are cooling devices that are compact in size and have an ability to transfer heat in low temperature differences. Working fluids strongly affect the thermal performance of PHPs. In this paper, effects of some thermophysical parameters relating to working fluids, such as boiling point, latent heat of vaporization, surface tension, thermal conductivity and dynamic viscosity, are presented based on experimental and numerical studies done in recent years. Addition of nanoparticles to fluids, or making nanofuild, is a new method of improving thermophysical properties of fluids. Recently, many studies are carried out on thermophysical properties of nano-fuild. Results indicate that using nanofuild could improve thermal performance of heat pips. Finally, in this review, flow regimes of some working fluids are represented under different conditions to obtain a better insight into the effect of input heat on working fluid flow pattern. It is concluded that lower dynamic viscosity and surface tension and higher thermal conductivity improve thermal performance of PHP. For lower heat inputs, lower boiling point of working fluid is more favorable due to faster start-up onset; however, at higher heat loads it causes some problems, such as dry-out.

Concrete as a thermal mass material for building applications - A review

Abstract: Identifying new energy saving methods in the building sector is essential due to limited natural energy sources and the rising population. Thermal mass materials have the ability to absorb and store heat before releasing it later on when necessary. They act as heat sinks during the daytime and as heat sources during the nighttime. Thermal performance is evaluated according to the specific heat capacity and specific latent heat. Applying thermal mass materials such as concrete is deemed a suitable strategy to reduce the energy consumption of buildings. Concrete with low thermal conductivity and high specific heat capacity is desirable in building construction. The aim of this study is to review factors affecting the heat storage capacity of concrete. In addition, common measurement methods of cement-based materials’ thermal conductivity, thermal diffusivity and specific heat capacity are reviewed. Various studies reveal that temperature, humidity, aggregate type, cementitious material type as well as phase change material (PCM) used influence the thermal properties of concrete. The advantages and limitations of PCM-concrete are also summarized in this study.

A New Research Approach for Observing and Characterizing Land-Atmosphere Feedback

Abstract: Forecast errors with respect to wind, temperature, moisture, clouds, and precipitation largely correspond to the limited capability of current earth system models to capture and simulate land-atmosphere feedback To facilitate its realistic simulation in next generation models, an improved process understanding of the related complex interactions is essential To this end, accurate 3D observations of key variables in the land-atmosphere (L-A) system with high vertical and temporal resolution from the surface to the free troposphere are indispensable Recently, we developed a synergy of innovative ground-based, scanning active remote sensing systems for 2D to 3D measurements of wind, temperature, and water vapor from the surface to the lower troposphere that is able to provide comprehensive data sets for characterizing L-A feedback independently of any model input Several new applications are introduced such as the mapping of surface momentum, sensible heat, and latent heat fluxes in heterogeneous terrain, the testing of Monin-Obukhov similarity theory and turbulence parameterizations, the direct measurement of entrainment fluxes, and the development of new flux-gradient relationships An experimental design taking advantage of the sensors' synergy and advanced capabilities was realized for the first time during the Land Atmosphere Feedback Experiment (LAFE), conducted at the Atmospheric Radiation Measurement Program Southern Great Plains site in August 2017 The scientific goals and the strategy of achieving them with the LAFE data set are introduced We envision the initiation of innovative L-A feedback studies in different climate regions to improve weather forecast, climate, and earth system models worldwide

Thermal Conductivity Enhancement of Phase Change Materials for Low-Temperature Thermal Energy Storage Applications

Abstract: Low thermal conductivity is the main drawback of phase change materials (PCMs) that is yet to be fully addressed. This paper studies several efficient, cost-effective, and easy-to-use experimental techniques to enhance thermal conductivity of an organic phase change material used for low-temperature thermal energy storage applications. In such applications, the challenges associated with low thermal conductivity of such organic PCMs are even more pronounced. In this investigation, polyethylene glycol (PEG-1000) is used as PCM. To improve the thermal conductivity of the selected PCM, three techniques including addition of carbon powder, and application of aluminum and graphite fins, are utilized. For measurement of thermal conductivity, two experimental methods—including flat and cylindrical configurations—are devised and increments in thermal conductivity are calculated. Melting and solidification processes are analyzed to evaluate melting and solidification zones, and temperature ranges for melting and solidification processes respectively. Furthermore, latent heat of melting is computed under constant values of heat load. Ultimately, specific heat of the PCM in solid state is measured by calorimetry method considering water and methanol as calorimeter fluids. Based on the results, the fin stack can enhance the effective thermal conductivity by more than 40 times with aluminum fins and 33 times with carbon fins. For pure PCM sample, Initiation of melting takes place around 37 °C and continues to above 40 °C depending on input heat load; and solidification temperature range was found to be 33.6–34.9 °C. The investigation will provide a twofold pathway, one to enhance thermal conductivity of PCMs, and secondly ‘relatively easy to set-up’ methods to measure properties of pure and enhanced PCMs.

The importance of snow sublimation on a Himalayan glacier

Abstract: Snow sublimation is a loss of water from the snowpack to the atmosphere. So far, snow sublimation has remained unquantified in the Himalaya, prohibiting a full understanding of the water balance and glacier mass balance. Hence, we measured surface latent heat fluxes with an eddy covariance system on Yala Glacier (5350 m a.s.l) in the Nepalese Himalaya to quantify the role snow sublimation plays in the water and glacier mass budget. Observations reveal that cumulative sublimation is 32 mm for a 32-day period from October to November 2016, which is high compared to observations in other regions in the world. Multiple turbulent flux parameterizations were subsequently tested against this observed sublimation. The bulk-aerodynamic method offered the best performance, and we subsequently used this method to estimate cumulative sublimation and evaporation at the location of the eddy covariance system for the 2016-2017 winter season, which is 125 and 9 mm respectively. This is equivalent to 21% of the annual snowfall. In addition, the spatial variation of total daily sublimation over Yala Glacier was simulated with the bulk-aerodynamic method for a humid and non-humid day. Required spatial fields of meteorological variables were obtained from high-resolution WRF simulations of the region in combination with field observations. The cumulative daily sublimation at the location of the eddy covariance system equals the simulated sublimation averaged over the entire glacier. Therefore, this location appears to be representative for Yala Glacier sublimation. The spatial distribution of sublimation is primarily controlled by wind speed. Close to the ridge of Yala Glacier cumulative daily sublimation is a factor 1.7 higher than at the location of the eddy covariance system, whereas it is a factor 0.8 lower at the snout of the glacier. This illustrates that the fraction of snowfall returned to the atmosphere may be much higher than 21% at wind-exposed locations. This is a considerable loss of water and illustrates the importance and need to account for sublimation in future hydrological and mass balance studies in the Himalaya.

Novel segregated-structure phase change materials composed of paraffin@graphene microencapsules with high latent heat and thermal conductivity

Abstract: Paraffin, due to its linear chain and saturated hydrocarbons with low thermal conductivity, is difficult to transfer energy effectively. Using amphiphilicity of graphene oxide (GO), Pickering emulsion of paraffin@GO was obtained and then paraffin@graphene microencapsulated phase change materials (MEPCMs) were achieved by chemical reduction through adding hydrazine hydrate. Thermally conductive PCMs with segregated structure were constructed by hot compression of paraffin@graphene microencapsules. Scanning electronic microscopy, differential scanning calorimetry and thermal conductivity test were used to characterize microstructure and thermal properties of MEPCMs. Meanwhile, the effect of graphene on the phase change latent heat and phase transition temperature was investigated. Results indicated that GO did not react with paraffin during the process of chemical reduction and the obtained MEPCMs were regular spheres. These MEPCMs had paraffin content of 99% or even more. Graphene working as shell materials increased the phase change latent heat of paraffin from 227.6 to 232.4 J/g, without affecting the phase transition temperature. The hot-compress molding makes graphene shell form segregated structure with more thermal pathways, further enhancing thermal conductivity. The segregated-structure PCMs with high latent heat and thermal conductivity can be applied in energy storage field.