Latent heat

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

Augmenting the productivity of solar still using multiple PCMs as heat energy storage

Abstract: In recent years, several research studies were conducted to enhance the productivity of solar stills. One of the approaches was the integration of latent heat energy storage mediums (phase change materials (PCM)) in the conventional solar stills to extend the water yield ability after the sunshine hours. Though several studies were conducted in this research area, none of the studies analyzed and reported the time of day at which the PCM start to release the stored latent heat, especially in the case of use of multiple PCMs. The present study is an experimental investigation that compare the thermal performance of three same-sized passive solar stills, i.e., conventional solar still (Still 1), still with single phase change material (Still II), still with two phase change materials (Solar III). The two PCMs used in still III are chosen in such a way that both the PCMs have almost same latent heat storage capacity, but different phase change temperature range. PCM1 and PCM2 used in solar still III have the phase change temperature (PCT) range of 58.03 °C–64.5 °C and 53.05 °C–62 °C, respectively. The experiments were conducted for the climatic conditions of Chennai, India during the month of February to May. The major inference from the experiments conducted is that, when multiple PCMs are employed in a still, selection of PCM with appropriate PCT range is very important. PCM1 should start discharging the stored latent heat energy stored once the solar radiation gets diminishing and PCM2 should start to release the heat when PCM1 has almost discharged the entire stored energy. By this way, the time of yield can be prolonged. In the results, the thermal performance of Still I, II and III was analyzed in terms of hourly yield per day and exergy efficiency. The yield of Still I was 3.680 L/m2/day while the yield of Still II and III were 4.020 L/m2/day and 4.400 L/m2/day, respectively. The exergy efficiency of still I, II and III were found to be 3.92%, 3.23% and 3.52%, respectively.

Simulating air temperature in an urban street canyon in all weather conditions using measured data at a reference meteorological station

Abstract: A model is proposed that adapts data from a standard meteorological station to provide realistic site-specific air temperature in a city street exposed to the same meso-scale environment. In addition to a rudimentary description of the two sites, the canyon air temperature (CAT) model requires only inputs measured at standard weather stations; yet it is capable of accurately predicting the evolution of air temperature in all weather conditions for extended periods. It simulates the effect of urban geometry on radiant exchange; the effect of moisture availability on latent heat flux; energy stored in the ground and in building surfaces; air flow in the street based on wind above roof height; and the sensible heat flux from individual surfaces and from the street canyon as a whole. The CAT model has been tested on field data measured in a monitoring program carried out in Adelaide, Australia, in 2000–2001. After calibrating the model, predicted air temperature correlated well with measured data in all weather conditions over extended periods. The experimental validation provides additional evidence in support of a number of parameterisation schemes incorporated in the model to account for sensible heat and storage flux. Copyright © 2006 Royal Meteorological Society

Thermal energy storage for solar heating and off-peak air conditioning

Abstract: Latent heat thermal energy storage materials suitable for solar heating and off-peak air conditioning were investigated and evaluated in terms of criteria developed to judge their usefulness. The often recommended sodium sulfate decahydrate and its mixtures were shown to have undesirable melting properties, causing a reduction in storage capacity upon repeated cycling. Materials more suitable at present are paraffin waxes for solar heating and paraffin waxes and tetrahydrofuran hydrate for off-peak air conditioning. Sensible heat storage has the economic advantage of not requiring heat transfer surfaces between storage and collection and between storage and delivery. Latent heat storage provides operational advantages of smaller temperature swings and lower, more efficient solar collector temperature, smaller size and lower weight per unit of storage capacity. The optimization of the thermal energy storage subsystem is dependent on the collection and delivery characteristics of the system. Parametric designs for two latent heat materials (sodium thiosulfate pentahydrate and a paraffin wax) and for a sensible heat material (a 1:1 mixture of water and ethylene glycol) were compared as to cost, performance, and space requirements. The conditions of equal cost for latent heat and sensible heat storage systems were determined as functions of latent heat capacity, thermal conductivity, and the temperature swing allowed in the sensible heat storage tank. The comparative designs include the cost of the heat exchanger required for latent heat storage and the operating penalty due to storage temperature swings inherent in sensible heat storage.

The role of advective heat transport in talik development beneath lakes and ponds in discontinuous permafrost

Abstract: [1] Regions of warm, thin, discontinuous permafrost have been observed to be experiencing rapid changes in lake and pond dynamics in recent decades Even though surface water and groundwater interactions are thought to play a significant role in heat transport in these regions, the effect of these interactions on permafrost remains largely unquantified In order to examine the influence of groundwater flow on permafrost dynamics, we modeled the development of a sub-lake talik under permafrost conditions similar to those observed in the southern-central Seward Peninsula region of Alaska using a numerical solution that couples heat transport and groundwater flow, including the effect of water phase changes on soil permeability and latent heat content A comparison of model simulations, with and without near surface subpermafrost groundwater flow, indicates that stable permafrost thicknesses are 2 to 5 times greater in the absence of groundwater flow Simulations examining the thermal influence of lakes on underlying permafrost suggest that a through-going talik can develop in a matter of decades and that the incorporation of advective heat transport reduces the time to complete loss of ice beneath the lake by half, relative to heat transport by conduction alone This work presents the first quantitative assessment of the rates of sub-lake permafrost response to thermal disturbances, such as talik development, in systems with near-surface groundwater flow The results highlight the importance of coupled thermal and hydrologic processes on discontinuous permafrost dynamics