Latent heat

About: Latent heat is a research topic. Over the lifetime, 13503 publications have been published within this topic receiving 302811 citations.

High temperature latent heat thermal energy storage: Phase change materials, design considerations and performance enhancement techniques

Abstract: A very common problem in solar power generation plants and various other industrial processes is the existing gap between the period of thermal energy availability and its period of usage. This situation creates the need for an effective method by which excess heat can be stored for later use. Latent heat thermal energy storage is one of the most efficient ways of storing thermal energy through which the disparity between energy production or availability and consumption can be corrected, thus avoiding wastage and increasing the process efficiency. This paper reviews a series of phase change materials, mainly inorganic salt compositions and metallic alloys, which could potentially be used as storage media in a high temperature (above 300 °C) latent heat storage system, seeking to serve the reader as a comprehensive thermophysical properties database to facilitate the material selection task for high temperature applications. Widespread utilization of latent heat storage systems has been held back by the poor thermal conductivity and some other inherent drawbacks of the use of PCMs; this paper reviews several heat transfer and performance enhancement techniques proposed in the literature and discusses a number of design considerations that must be taken into account aiming to provide a broad overview for the design of high temperature latent heat based thermal energy storage systems.

Climate and energy exchange at the snow surface in the Alpine Region of the Sierra Nevada: 2. Snow cover energy balance

Abstract: A detailed evaluation of surface climate and energy exchange at the snow surface in a small alpine watershed, typical of much of the southern Sierra Nevada, is presented for the 1986 water year. Measurements of snowfall, meteorological and snow cover conditions, and snow cover ablation, described in part 1 of this paper (Marks et al., this issue), are used to characterize the climate. Each form of energy transfer, radiation, sensible and latent heat flux, soil heat flux, and heat flux by mass advection, is evaluated separately to determine how its magnitude changes during the snow season. These are then combined to approximate a snow cover energy balance and determine the relative importance of each form of energy transfer in the seasonal energy and mass balance of the snow cover. Radiation and sensible and latent heat flux dominate the snow cover energy balance throughout the snow season. During snowmelt, radiation accounts for between 66 and 90% of the energy available for melt. Sensible and latent heat transfer during this time are of approximately equal magnitude but are usually of opposite sign and therefore cancel. Calculated sublimation during the entire snow season accounted for the loss of about 20% (approximately 50 cm snow water equivalent) of the mass of the snow cover. This experiment shows that energy and mass transfer can be adequately monitored at a remote site using a combination of measured and modeled parameters and that the energy balance of the snow cover in the alpine zone of the Sierra Nevada is dominated by net radiation during snowmelt.

Regionalization of surface flux densities and moisture indicators in composite terrain. A remote sensing approach under clear skies in Mediterranean climates.

Abstract: The growing concern about environment has increased the number of land surface processes studies. Computer simulation models of land surface processes have been developed for a range of scales and with different levels of physical complexity. Because the interactions between soil, vegetation and atmosphere vary both spatially and temporally, regional evaporation in heterogeneous natural landscapes is difficult to predict by means of computer simulation models. Remote sensing measurements of land surface radiative properties offer however a means to indirectly measure land surface state conditions at a range of scales. A straightforward estimation of evaporation from radiative properties of the land surface is hampered by the fact that only a very few parameters of the classical flux-profile relationships can be estimated directly from remote sensing measurements. Moreover, the accuracy of surface temperature measurements necessary to solve flux-profile relationships is still poor. Inclusion of ground measurements is a possible solution, but the absence of such data on large scales and for heterogeneous land surfaces where these parameters are not measured, forms an immediate obstacle for the implementation of remote sensing algorithms. A Surface Energy Balance Algorithm for Land (SEBAL) has been developed in a way that the need for collateral measurements is partly eliminated, a very accurate surface temperature map is no longer required (although it should be as good as possible) and a land use classification to relate surface temperature to evaporation is not needed. Each pixel is characterized by a surface hemisherical reflectance, surface temperature and a vegetation index. The methodology composes of multiple flux-profile relationships for small sub-areas. Although the concept has a physical basis, the parameters are estimated by empirical relationships, for instance a relationship between near-surface vertical air temperature difference and surface temperature forms an essential component in the estimation of the sensible heat flux density. The absolute surface energy balance terms are estimated on an instantaneous time basis. Temporal integration of instantaneous surface flux densities is feasible using the evaporative fraction (latent heat flux density/net available energy): The evaporative fraction remains fairly constant during daytime hours for both homogeneous and heterogeneous areas. A physical explanation for this is given. A bulk surface resistance of a heterogeneous landscape has been related analytically to canopy and bare soil evaporation resistances. Measurements in central Spain have shown that the evaporative fraction and bulk surface resistance are suitable indicators to describe areal patterns of near-surface soil water content. Although the bulk surface resistance has a distinct diurnal variation, it is much less affected by changes in net available energy and therefore preferred to describe the energy partitioning for longer time series (weeks, months). SEBAL has been validated with data available from regional evaporation projects in Egypt and Spain. The error of high resolution evaporative fraction estimations decreases from 20% to 10% at a scale of 1 km to 5 km respectively, The error of low resolution evaporative fraction images at a scale of 100 km is approximately 1 to 5 %. Hence, the error averages out if a larger set of pixels is considered. It is concluded that the uncertainty of evaporation in regional water balances and model studies can be substantially reduced by estimating evaporation with remote sensing measurements using the proposed SEBAL technique.