Latent Heat Research Articles

Effects of segmentation in composite phase change material on melting/solidification performance of triplex‐tube thermal energy storage systems

AbstractIn this work, a numerical evaluation of the melting/solidification performance of phase change material (PCM) filled inside a triplex‐tube latent heat storage unit has been carried out. To enhance the melting/solidification performance, the porous Cu metal foam (MF) was embedded inside PCM (termed as composite PCM). Alternative segments of pure PCM and composite PCM have been allocated in such a way that both the pure PCM and composite PCM occupy the equal annular area (i.e., equal volumes). Influence of increasing number of segments was delineated on the melting/solidification rate, complete melting time, and thermal energy storage/recovery enhancement. The comparisons were drawn with reference to the model having two segments of PCM and composite PCM. The results show that the model containing 64 segments with alternate allocations of PCM and composite PCM has a faster melting/solidification rate than other models. With 32 alternate segments of MF, the full melting/solidification time reduced by 23%/77% with respect to the case with one segment of MF only. The melting/solidification performance gets saturated beyond 32 segments (M‐5) and negligible variation (only ~1%) in the thermal performance was noticed upon further segmentation. Finally, the model M‐5 proved as the best model considering the aspects of augmented melting/solidification rate and associated complexities. Moreover, the heterogeneity of MF applied in 32‐segment model confirmed that the anisotropic MF results in an increased melting rate and leads over other random isotropic distributions of MF.

Thermal conductive enhanced phase change composites with high latent-heat for constant temperature thermal management

Phase change materials (PCMs) can absorb and release heat without the temperature changing to realize the constant temperature thermal management. The low thermal conductivity (K) of PCM molecules leads to slow heat absorbing and releasing. In this study, a polyethylene glycol (PEG) based cross-linked polyurethane network is synthesized to wrap PCMs microcapsules. The composites have a high latent heat (152–164 J g−1) at 38 °C. Carbon fiber (CF) and Al powder with high K is composted to improve the thermal conductivity of the PCMs. After doping with CF, the K increases to two-fold to 0.306 W m−1 K−1. The experimental result reveals that the heat transfer inside the PCMs is improved obviously after doping with CF, which is further verified by a simulation result, and the composites show excellent shape stability and latent heat retention rate (>92 %) under 30 days' cycle test. This thermal conductivity enhanced PCMs composite is also formable which may have the potential applications for the thermal management of electronic devices, such as chips, fast charging, and battery.

The effects of fins number, metal foam, and helical coil on the thermal storage enhancement of the phase change material: An experimental study

Phase change materials (PCMs) are commonly used to store thermal energy as latent heat storage. The low thermal conductivity of PCMs elongates the charging/discharging process time. Using porous media and fins to improve the PCMs’ conductivity are two suggested solutions that have been evaluated in recent research studies. The present work experimentally analyzed the effect of fins usage over a helical tube, the number of fins, and filling the coil internal space with porous media during the energy storage in Paraffin PCM. The thermal conductivity of the PCM is enhanced through the utilization of porous metal foams made of copper, which have a porosity of 0.9 and a pore density of 12 pores per inch. The sole effects of using porous media or fins, in line with their simultaneous effects, and the number of fins on the system’s heat transfer enhancement are investigated. The heat transfer enhancement has been evaluated by examining parameters like the temperature distribution of different points of the heat storage tank, the charging/discharging time, the mean temperature response, the Stefan number, and the obtained discharge power. Results indicate that using either fins or porous media in the storage tank can remarkably improve thermal performance. Furthermore, the best heat transfer enhancement is obtained when using fins and porous media simultaneously. Also, using more fins beside the porous media could further reduce the charge/discharge time. For instance, comparing the cases with only 25 fins, only porous media and the case combined cases reveal respectively about 8.5%, 11%, and 25% decrease in the time of charging process. Also, the best case with 35 fins beside the porous metal foam can reduce the charging/discharging time by about 25%-28%.

Numerical simulation of keyhole-induced pores for TA15 in laser powder bed fusion (L-PBF)

In the complex laser powder bed fusion (L-PBF) process, the occurrence of pores is often triggered when localized temperatures of the materials exceed their boiling point, posing a significant impediment to the overall quality of manufactured parts. In this work, considering the thermophysical properties and latent heat of phase transformation of the material, the formation and evolution of keyholes and pores during the L-PBF single-track processing of TA15 titanium alloy was investigated using X-ray computed tomography (X-CT) and FLOW-3D finite volume analysis software coupled with multiphysics fields. The simulated molten pool characteristics generally agree with the experimental data, including internal pores The results showed that the evolution of keyholes is mainly attributed to surface tension and recoil pressure, the Marangoni flow and surface tension contributed to closed localized cooling regions on the keyhole walls to generate pores. Additionally, the Marangoni flow and buoyancy effects lead to pore motion within the molten pool. Furthermore, the laser energy density is inherently related to the total pore counts and the mean penetration depth of keyholes, respectively. Understanding the underlying physical mechanisms helps to optimize process parameters and reduce pores in L-PBF, ultimately enhancing the quality of the additively manufactured parts.

Effects of hydraulic conductivity on simulating groundwater–land surface interactions over a typical endorheic river basin

Hydraulic conductivity is the key variable in subsurface configuration that is important in modeling groundwater and its interactions with land surface processes. However, an acquisition of quantitatively distributed hydraulic conductivity is challenging due to limitations on data availability, accuracy and resolution, which inhibits an advanced understanding of hydrologic and energy cycles at the basin scale. This study investigated the role of hydraulic conductivity in simulating groundwater-land surface interactions over a typical endorheic river basin in northwestern China. A 3D variably saturated groundwater model coupled to a land surface model was configured to account for interactions between groundwater dynamics and land energy fluxes. Four scenarios of hydraulic conductivity were tested and inter-compared, adopting both homogeneous and heterogeneous parameterization strategies based on Global Soil Dataset for use in Earth System Models (GSDE) and HYdrogeology MaPS (GLHYMPS). The overall objectives are to diagnose effects of hydraulic conductivity on simulating groundwater–land surface interactions, with a particular focus on differences in spatial distributions and statistical characteristics between scenarios, as well as the dynamics and feedback mechanisms between groundwater and land surface energy fluxes. Comprehensive simulations and analyses were conducted to discern spatial distributions and statistical characteristics of key hydrological variables (streamflow, water table depth, latent heat flux, sensible heat flux) under different scenarios. Results showed that the hydraulic conductivity of the saturated zone had a significant control over water table depth, while the combine hydraulic conductivity in the vadose zone and the saturated zone governed the evolution of streamflow. Moreover, the water table critical range—where the land energy budget is highly sensitive to groundwater table—was 0.1 m–5 m and the area might expand approximately 4.5 % after considering the heterogeneity of hydraulic conductivity. Compared to the homogeneous scenario, the spatial heterogeneity of hydraulic conductivity amplified water table depth variability while diminishing land energy fluxes variability.

Level-set topology optimization of heat sinks with phase-change material

Phase-change materials (PCMs) excel in storing significant thermal energy through the latent heat of fusion during phase changes. However, they often suffer from low thermal conductivity, requiring the incorporation of materials with higher thermal conductivity to overcome this limitation. In this study, we employ the level-set method to topologically optimize the distribution of both PCM and high thermal conductivity materials within heat sinks.The heat transfer process is modeled as an unsteady diffusion problem, with PCM integrated using the apparent heat capacity method. The finite element method, implemented using FEniCS, is utilized to compute the temperature distribution at each time step. The optimization problem is solved using ParaLeSTO, a modular topology optimization software written in C++, seamlessly interfacing with FEniCS through its Python interface.To compute boundary point sensitivities for level-set topology optimization, the discrete adjoint method is employed, combining a perturbation scheme with automatic differentiation. The proposed methodology is first applied to a two-dimensional example of temperature oscillation minimization with a heat flux boundary condition, drawn from existing literature. Subsequently, a two-dimensional example with multiple volumetric heat sources is studied, followed by an extension to a three-dimensional design domain.Comparisons with optimized designs without PCM, serving as a reference, highlight the superior thermal performance of heat sink designs with PCM. The results showcase a maximum temperature reduction of up to 42%, a mean temperature reduction of up to 42%, and a temperature variance reduction of up to 94%. This research contributes to heat sink design by offering discrete solutions that significantly improve thermal performance as compared to designs without PCM. Its impact extends to addressing crucial challenges in thermal management across various applications, including automotive cooling systems, aerospace components, and consumer electronics.

Simulation Study on the Autumn Typhoon Mangkhut (2018) in 10.5 Days over the South China Sea: Microphysical Characteristics and latent heat budget

Simulation Study on the Autumn Typhoon Mangkhut (2018) in 10.5 Days over the South China Sea: Microphysical Characteristics and latent heat budget

Study on saline-alkali water distillation system by reflection enhanced solar heating

A saline-alkali water distillation system by reflection enhanced solar heating is proposed to treat the saline-alkali soil washing water. In this system, the heat concentration collector employs heat collecting plate to receive solar radiation energy, and converges to small-area steam generating tube through efficiently heat transfer. Without expensive optical tracking equipment, the system also employs an external reflector to reflect the solar radiation to the surface of the heat concentration collector. Thereby the energy flow density is further increased, and the useful energy and vaporization intensity is promoted. The latent heat of the steam is reused by heat recovery to enhance the energy utilization efficiency. The optimal inclination angles of reflector in different seasons and system performance were theoretically calculated. In the spring equinox, summer solstice, autumn equinox, and winter solstice, the optimal inclination angles of the reflector were 30°, 45°, 30°, and 15°, respectively. The freshwater production, useful energy, and heat recovery efficiency of the system were investigated by experiments. The daily freshwater production of the system was 17.04 kg, and the maximum production rate was 3.44 kg/h. The reflector increased the useful energy and freshwater production of the system by 33.68% and 35%, respectively. With heat recovery, the initial temperature of the saline-alkali water was increased to 53.8 °C and the freshwater production was increased by 13.16%.

Air–sea heat fluxes variations in the Southern Atlantic Ocean: Present‐day and future climate scenarios

AbstractThis study investigates the variations in air–sea heat fluxes and temperatures in two ocean front regions, the Southwestern Atlantic Ocean (SWA) and the Drake Passage, widely recognized as hotspot areas with significant influences on South America weather and climate. We analyse means and trends of latent and sensible heat fluxes (LHF and SHF) and their associated air–sea temperatures (SAT and SST), based on monthly ERA5 (1985–2014) and eight CMIP6 models, for historical and long‐term simulations (2015–2099). The ERA5 trend for all parameters was positive over SWA, contrasting with the negative values observed in the Drake Passage, indicating surface warming and cooling, respectively. In the SWA, the ERA5 and CMIP6 multi‐model ensemble (MME) align in SST and SAT values, with a historical trend of 0.1°C·decade−1 and a significantly increasing trend in warming estimated by 0.4°C·decade−1 up to 2099. The ERA5 LHF and SHF trends were 4 and 0.6 W·m−2·decade−1, respectively. The MME shows historical (SSP5‐8.5) trends of 0.2 (1.3) W·m−2·decade−1 for LHF and −0.2 (−0.1) W·m−2·decade−1 for SHF. In the Drake Passage, the models accurately reproduced the air–sea mean temperatures; however, they failed to simulate negative trends observed in SAT and SST. Under the high emissions scenario, all CMIP6 models predict an increasing warming trends of 0.1–0.4°C·decade−1 and ocean heat gain of −0.2 to −1.2 (−1 to −2) W·m−2·decade−1 for LHF (SHF). For both regions, spatial analyses of SST and SAT highlight the coupling between the ocean and the atmosphere, alongside changes in air–sea heat fluxes. Our findings reveal inhomogeneous patterns of SST warming trends by the end of the 21st century, which are approximately 2–4 times greater than historical trends. The results suggest the persistence and enhancement of these regions as hotspots with significant potential to influence oceanic and atmospheric dynamics.

Realizing rapid cooling and latent heat recovery in the thermoelectric-based battery thermal management system at high temperatures

To realize rapid cooling of the battery at high temperatures and effective latent heat recovery from phase change materials (PCMs), a thermoelectric-based battery thermal management system (BTMS) is proposed. Additionally, a transient multiphysics numerical model is developed to predict the system's thermal performance, and the concept of the latent heat recovery rate of PCMs is introduced. Results show that the introduction of thermoelectric coolers (TECs) significantly enhances the system's efficiency in cooling the battery at high temperatures (Stage 1) and recovering PCM latent heat (Stage 2). The overall thermal performance can be further improved by utilizing PCMs with a higher mass fraction of expanded graphite (EG) or increasing the TEC input current. Moreover, after the end of Stage 2, the power supply for TECs is interrupted, and the system enters Stage 3, which only relies on PCMs to control the battery temperature. The duration of Stage 3 increases with the EG mass fraction, achieving a peak of 3830 s at an EG mass fraction of 12%. Considering the thermal performance and power consumption of the system, the optimal solution is determined as an EG mass fraction of 12% and a TEC input current of 4 A. In this situation, the required time for Stage 1 and Stage 2 is 170 s and 620 s, with the latent heat recovery rate of PCMs up to 361.94 J/kg/s. The findings will provide new insights for the development of the thermoelectric-based BTMS.

Enhanced mechanical and thermal properties of expanded graphite/ice phase change materials through in-situ polymerization of sodium polyacrylate in cold chain logistics

The growing demand for high-quality food and medicine has propelled the expansion of the global cold chain logistics market. Passive cold chain logistics, utilizing phase change materials (PCMs), has emerged as an advanced and environmentally friendly technology. This study addresses current challenges in phase change materials, including leakage and low mechanical strength, by introducing a novel composite phase change material (cPCM) with enhanced thermal conductivity and flexibility. The expanded graphite-sodium polyacrylate (EG-PAAS) dual network framework, formed through vacuum impregnation and in-situ polymerization, results in a cPCM (H2O@EG-PAAS) with high thermal conductivity (5.64 W·m−1·K−1) and substantial latent heat (263.1 J·g−1). Addressed prolonged cooling times in phase change materials, accomplishing rapid cooling and uniform temperature distribution. The 8 L insulation box was maintained below 8 °C for 1124.1 min, rendering it a promising solution for efficient and flexible cold chain transportation.

The cores regulation of paraffin-chitosan phase change microcapsules for constant temperature building

Phase change materials (PCMs) can store and release latent heat under the designed phased change temperature and have received substantial interest for energy conservation and thermal control purposes. The use of PCMs in the construction of constant temperature buildings can improve the comfortable environment and save more energy. However, the leakage of PCMs during phase change process limits the application of PCMs. In this paper, a series of PCMs microcapsules with controllable core numbers is synthesized with paraffin (37 ℃) as the core and cross-linked chitosan as the wall. The single-core phase-change microcapsules (S−PCM) and multicore phase-change microcapsules (M−PCM) were prepared by adjusting the preparation condition. The latent heat of S−PCM and M−PCM are 61.4 mJ mg−1 and 50.1 mJ mg−1, respectively. The S-PCM and M−PCM display good stability without paraffin leakage. In addition, the composite blocks of gypsum and S−PCM (GSCM) and M−PCM (GMCM) were prepared and the thermoregulatory effection was investigated, where the surface temperature of GSCM was 5–10 ℃ lower than that of pure gypsum block. PCMs may also have broad application space in electronics, cold chain, and other industries.

Evapotranspiration estimation using high-resolution aerial imagery and pySEBAL for processing tomatoes

AbstractThe use of high-resolution aerial imagery for assessing actual crop evapotranspiration $$ \left({ET}_{a}\right)$$ holds the potential to optimize the use of limited water resources in agriculture. Despite this potential, there is a shortage of information regarding the effectiveness of energy balance algorithms, initially designed for satellite remote sensing in estimating $$ {ET}_{a}$$ using aerial imagery. This study addresses this gap by employing the remote sensing model pySEBAL (Surface Energy Balance Algorithm for Land) in conjunction with high-resolution aerial imagery to estimate $$ {ET}_{a}$$ for processing tomatoes. Throughout the 2021 growing season, an aircraft captured multispectral and thermal imagery over a processing tomato field near Esparto, California, USA. Simultaneously, an eddy covariance flux tower within the field measured high-frequency turbulent fluxes and low-frequency biometeorology variables essential for evaluating the energy balance. The comprehensive assessment of energy balance components, including $$ {ET}_{a}$$, yielded compelling evidence that pySEBAL accurately estimated $$ {ET}_{a}$$ at high spatial resolution. The root mean square error (RMSE) and normalized RMSE for various energy balance components were as follows: 33 W m− 2 (12%) for latent heat flux, 29 W m− 2 (35%) for sensible heat flux, 24 W m− 2 (4%) for net radiation, and 10 W m− 2 (15%) for soil heat flux. Additionally, $$ {ET}_{a}$$ exhibited an RMSE and NRMSE of 0.26 mm d− 1 (6%). Moreover, the spatial mapping of $$ {ET}_{a}$$ across the processing tomato field visually depicted the spatial variability associated with irrigation scheduling, crop development, areas affected by disease, and soil heterogeneity. This research underscores the value of high resolution spatial aerial imagery and pySEBAL algorithm for estimating $$ {ET}_{a}$$ variability in the field, a crucial aspect for guiding precision irrigation management and ensuring the optimal use of limited water resources in agriculture.

One-step preparation of macropore phase change materials enabled exceptional thermal insulation, thermal energy storage and long-term stability

Phase change materials (PCMs) capable of reversibly storing and releasing thermal energy have been widely used in our daily life to reduce energy consumption. However, traditional PCMs are mainly focused on the promotion of their enthalpy and thermal conductivity yet are rarely noticed on incorporating their thermal energy storage capacity with thermal insulation performance. Herein, a kind of macropore PCMs (MPCMs) was synthesized by directly adding expanded microspheres into polyethylene glycol-based PCMs, in which the microspheres can form an internal closed porous structure in matrix for realizing thermal insulation and polyethylene glycol is used as the phase change component for achieving thermal energy storage. Amazingly, the designed MPCMs have significant latent heat storage capacity reached up to130.2 J g−1 and simultaneously possess extremely low thermal conductivity of 0.08577 W m−1 K−1. The thermal power generation system test further revealed the thermal insulation capability of these MPCMs was eleven times more than that of the commercial wood. Especially, these MPCMs with relatively low density of 0.492 g cm−3 can still maintain the remarkable tensile strength of 11.74 MPa and can also exhibit good toughness via multiple cyclic shape memory analysis. The focus of this work that is to combine the thermal insulation ability of porous materials with the thermal energy storage ability of PCMs, can effectively reduce the heat conduction meanwhile can maintain the stability of internal temperature contributed to reducing energy consumption, applying in food transportation, building energy conservation and other fields.

Hybrid silver-graphene nanoparticles enhanced Lauric Acid phase change material for photovoltaic and thermoelectric generator applications: Experimental and simulation analysis

The global initiatives for renewable energy promote sustainable energy technologies such as photovoltaic thermal (PVT) systems and thermoelectric generators (TEGs). Latent heat energy storage is an effective method for conserving excess thermal energy and can be applied to PVT and TEG technologies. The fatty acid phase change materials (PCMs) are appealing, but their low thermal conductivity limits their practical usage. In this study, silver (Ag) and graphene (Gr) hybrid nanoparticles (NPs) are considered as additives to augment thermo-physical properties of lauric acid (LA) PCM for implementation in PVT and TEG applications. The nanocomposites are prepared by two-step technique. Additionally, implementation of composites in PVT has been evaluated experimentally and through TRNSYS simulation. Besides, composites impact on TEG application was assessed experimentally. As per results, photo-absorption analysis; nanocomposite LAG-3.5 (LA with 3.5 % hybrid Ag and Gr NPs) displayed the highest 76 % decrease in transmissibility. In addition, LAG-3.5 nanocomposite augmented thermal conductivity of LA by 57 % due to integration of high thermally conductive hybrid NPs. Further, LAG-3.5 composite reported an 8 % increment in latent heat enthalpy. As per reliability, after 500 thermal cycles, nanocomposite LAG-3.5 showed chemical stability and thermal durability. Moreover, optimal composite LAG-3.5 exhibited enhanced electrical characteristics while implementing in photovoltaic and thermoelectric generator application. Therefore, the nano-enhanced composite can be significant application prospects in the 45 °C to 50 °C melting temperature range.

An increase in autumn marine heatwaves caused by the Indian Ocean Dipole in the Bay of Bengal

Abstract This study investigates the interannual variability of Marine Heatwaves (MHWs) in the Bay of Bengal (BOB) associated with the Indian Ocean Dipole (IOD) from 1982 to 2021. The results revealed a significant positive correlation at the 95% confidence level between the IOD and MHW days in the central bay at the peak of the IOD in autumn. During positive IOD (pIOD) events, the central bay experienced more MHW days in autumn, with an average increase of 7.4 days. The increased MHW days in the central bay could be primarily attributed to the enhanced net heat flux (TQ), which is 9.7 times the contribution of ocean dynamic processes (horizontal advection + entrainment). The reduced latent heat flux loss and enhanced shortwave radiation due to the anomalous atmospheric low-level high pressure associated with the pIOD account for 63% and 50%, respectively, of the anomalous enhanced TQ, while the longwave radiation and sensible heat flux make smaller contributions of −20% and 7%. In addition, thermocline deepening in the southwestern bay, caused by this anomalous high pressure and associated anticyclonic wind anomalies, favors the occurrence and persistence of MHWs by reducing the mixed-layer cooling rate. In addition to the influence of the IOD, the El Niño-Southern Oscillation mainly affects MHWs from winter to the following summer, which confirms the result of a previous study.

Orogen‐scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention

AbstractGranulite facies metapelitic gneisses collected over a km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K‐feldspar–ilmenite plagioclase magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite‐ or sillimanite‐free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of kbar and °C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re‐equilibration due to minor melt retention after peak metamorphism. Re‐equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H2O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum – conditions in the Kakamas Domain reflect the upper‐ stability of biotite, and also corresponds to the intersection of the well‐known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the – history of exhumed terranes.

Local and Nonlocal Biophysical Effects of Historical Land Use and Land Cover Changes in CMIP6 Models and the Intermodel Uncertainty

AbstractLand use and land cover changes (LULCCs) can influence surface temperature through local and nonlocal biophysical processes, which remain inadequately addressed. In this study, we separate the local and nonlocal effects of historical (1850–2014) LULCCs based on model outputs from the Coupled Model Intercomparison Project Phase 6. We also attempt to explore the sources of intermodel differences in the effects of LULCCs. The multimodel mean shows a cooling effect of −0.05°C (with an intermodel range of −0.24–0.06°C) at the global scale due to cropland and pastureland expansion, consisting of dominant nonlocal cooling of −0.06°C (with an intermodel range of −0.26–0.06°C) and slight local warming of 0.01°C (with an intermodel range of −0.01–0.05°C). The modeling results show some clear consistency in the effects of LULCCs despite considerable intermodel uncertainties. The local effects cause warming at low latitudes and cooling in boreal regions via changes in upward shortwave radiation and sensible and latent heat fluxes. The nonlocal effects mainly cause cooling via decreases in downward longwave radiation and increases in upward shortwave radiation. Intermodel differences in the total effects are dominated by those in the nonlocal effects, which are further attributed to divergent changes in downward longwave radiation and sensible heat flux across the models. This study highlights the importance of the nonlocal effects of LULCCs in terms of strength and intermodel uncertainty, with implications for designing land‐based solutions aimed at climate change mitigation.

Strong Skin Cooling and Its Impacts on Lake Thermal Processes in a Large Lake on the Tibetan Plateau

Abstract Skin cooling, wherein the surface temperature of a water body Tskin is lower than the temperature below the surface, is a widespread phenomenon. Previous studies have almost ignored this effect on the Tibetan Plateau (TP), despite the presence of thousands of lakes on the TP and the fact that extraordinary solar heating leads to very strong energy exchanges on the lake surfaces. This study utilizes in situ observations and MODIS-derived Tskin data at Lake Nam Co, one of the largest lakes on the TP, to quantify the skin cooling effect. The observed nighttime skin cooling is approximately 0.52°C on average, with the maximum of about 1°C, during the lake water turnover period (from October to mid-November), which obviously surpasses reported values for oceans (less than 0.4°C). To understand the impact of the skin cooling on the lake thermal processes, a skin cooling parameterization is validated and incorporated into the WRF-lake model. Simulations with the updated model show that accounting for the skin cooling process systematically lowers sensible and latent heat fluxes by a few watts per square meter, which yields an increase in water temperature by 0.45°C at the end of December and may delay the onset of lake freeze. Finally, we show that the inclusion of the skin cooling process in a lake model needs simultaneous adjustment of the parameterization of heat/water vapor transfer. Significance Statement Skin cooling is a widespread phenomenon for a water surface, and its intensity depends on the energy flux exchange of the water surface. The Tibetan Plateau possesses the presence of thousands of lakes, but early studies have ignored the skin cooling effect. We found that the nighttime skin cooling magnitude during the lake water turnover period in this region obviously exceeds reported values for oceans, due to the strong surface energy exchange in the Tibetan Plateau. Neglecting the skin cooling process may lead to systematic overestimation of turbulent heat fluxes and underestimation of water temperature. We highlight that accounting for this skin cooling process is crucial to select appropriate parameterization schemes for heat/water vapor transfer in lake thermal process modeling.

Parametric analysis of system performance and cost of heating systems with heat pump and latent thermal energy storage

This paper presents a parametric analysis examining the impact of phase change material (PCM) melting temperature, latent thermal energy storage (LTES) volume, and solar collectors’ surface area on the performance and costs of heating systems integrating a heat pump and LTES by using dynamic system simulations. The computational models, implemented in TRNSYS software, and the numerical procedure were validated by comparing the numerical results with experimental measurements. The computational models represent the entire heating systems and thus consider the mutual interaction of heat production, distribution and automatic regulation system as well as a building which represent the heat load, in transient boundary conditions. Two main system designs with different LTES roles were considered – a system in which the role of the LTES is to provide more favorable operating conditions and a system in which the LTES role is to store the heat on a sufficiently high temperature to be used directly for heating. System performance was analyzed in terms of the share of renewable energy in total delivered energy, power consumption, stored energy in LTES, seasonal performance factor, average solar collector efficiency and total annual costs. The results showed that, regarding the system in which LTES was used as a heat source for the heat pump, a better overall performance and lower costs were observed in systems with PCMs with melting temperatures in range between 19 and 29 °C, compared to systems with PCMs with melting temperatures outside this range. For system in which the heat from the LTES was used directly for heating, the highest values of system performance criteria and the lowest cost were achieved with the PCM of 54 °C melting temperature. Almost all of the considered system performance criteria and total costs were higher when the LTES volume and solar collector surface area were larger as well.