Showing papers on "Thermal energy published in 2017"

Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

Abstract: Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface requires materials that strongly emit thermal energy and barely absorb sunlight. We embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window. When backed with a silver coating, the metamaterial shows a noontime radiative cooling power of 93 watts per square meter under direct sunshine. More critically, we demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, which is vital for promoting radiative cooling as a viable energy technology.

Recent Progress in Energy-Driven Water Splitting

Abstract: Hydrogen is readily obtained from renewable and non-renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non-renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost-effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic-integrated solar-driven water electrolysis.

A review on current status and challenges of inorganic phase change materials for thermal energy storage systems

Abstract: Latent heat energy storage system is one of the promising solutions for efficient way of storing excess thermal energy during low consumption periods. One of the challenges for latent heat storage systems is the proper selection of the phase change materials (PCMs) for the targeted applications. As compared to organic PCMs, inorganic PCMs have some drawbacks, such as corrosion potential and phase separation; however, there are available techniques to overcome or minimize these drawbacks. On the other hand, inorganic PCMs are found to have higher thermal conductivity and storage capacity over organic PCMs. As a result inorganic PCMs have a great potential in thermal energy storage field, especially in medium to high temperature applications where organic PCMs are not a viable option. In this study, a detailed review of research outcomes and recent technological advancements in the field of inorganic phase change materials is presented while focusing on providing solutions to the associated disadvantages of this class of PCMs. Long term stability, thermal cycling performance, and heat transfer enhancements are also discussed in the context of this review.

Review of ocean tidal, wave and thermal energy technologies

Abstract: Ocean tidal currents, water waves and thermal gradients are a great source of renewable energy. Ocean tidal, osmotic, wave and thermal sources have annual potentials of 800, 2,000, 8000–80,000 and 10,000–87,600 TWh, which are more than global 16,000 TWh/y electricity demand. Ocean wave generators produce relatively lower output, however, four to eleven meters tidal range stations have large power generation capacities. Abundant ocean heat energy potentially harvested using ocean thermal energy conversion (OTEC) devices and ocean thermo-electric generators (OTEG). Tidal stations may be tidal range or current types, but a wave energy converter (WEC) may be an oscillating water column (OWC), overtopping, heaving, pitching and surging devices. Ocean thermal energy can be harnessed by open, close Rankine cycles, thermo-electric generators and osmotic power plants. Large bays like Turnagain (USA), Annapolis/Minas Passage (Canada), Seven Barrages/Pentland Firth (UK), La Rance (France), Garorim (South Korea) and Mezen/Penzhin (Russia) have huge tidal current power generation capacities. Power Potential from tidal current stations is more than WEC devices which in turn is more than osmotic, OTEC and OTEG technologies. This paper reviews the current state-of-the-art of tidal, wave, OTEC and OTEG ocean energy technologies.

Fe3O4-functionalized graphene nanosheet embedded phase change material composites: efficient magnetic- and sunlight-driven energy conversion and storage

Abstract: As an important energy utilization mode, thermal energy is closely related to human life and social production. Phase change materials have been widely adopted to store thermal energy to improve its utilization efficiency. However, the inherent low energy conversion ability of these materials is one of the key problems to be resolved urgently. In this paper, we report novel magnetic- and sunlight-driven energy conversion and storage nanocomposites based on Fe3O4-functionalized graphene nanosheet (Fe3O4–GNS) embedded form-stable polymer phase change materials. Owing to the excellent magnetocaloric performance of Fe3O4 and the universal photoabsorption and photothermal conversion of graphene, the nanocomposites can effectively convert magnetic or light energy into thermal energy under an alternating magnetic field or solar illumination. The energy is stored by phase change materials during the phase transition process. The obtained hybrid nanocomposites exhibit excellent thermal stability with high melting–freezing enthalpy and excellent reversibility. Furthermore, the novel nanocomposites show the characteristics of form-stable phase transformation. The Fe3O4–GNS embedded phase change material composites for energy conversion and storage are expected to open up a rich field of energy materials.

Rational design of a bi-layered reduced graphene oxide film on polystyrene foam for solar-driven interfacial water evaporation

Abstract: Solar-driven water evaporation has been emerging as a highly efficient way for utilizing solar energy for clean water production and wastewater treatment. Here we rationally designed and fabricated a bi-layered photothermal membrane with a porous film of reduced graphene oxide (rGO) on the top and polystyrene (PS) foam at the bottom. The top porous rGO layer acts as a light absorber to harvest and convert light efficiently to thermal energy and the bottom PS layer, which purposefully disintegrates water transport channels, acts as an excellent thermal barrier to minimize heat transfer to the nonevaporative bulk water. The optimized bi-layered membrane was able to produce water evaporation rate as high as 1.31 kg m−2 h−1 with light to evaporation conversion efficiency as high as 83%, which makes it a promising photothermal material in the literature. Furthermore, the experiments and theoretical simulation were both conducted to examine the relationship between the overall energy efficiency and the depth of the photothermal material underwater and the experimental and simulations results coincided with each other. Therefore, this work provides systematic evidence in support of the concept of the interfacial heating and shines important light on practical applications of solar-driven processes for clean water production.

Optically-controlled long-term storage and release of thermal energy in phase-change materials

Abstract: Thermal energy storage offers enormous potential for a wide range of energy technologies. Phase-change materials offer state-of-the-art thermal storage due to high latent heat. However, spontaneous heat loss from thermally charged phase-change materials to cooler surroundings occurs due to the absence of a significant energy barrier for the liquid–solid transition. This prevents control over the thermal storage, and developing effective methods to address this problem has remained an elusive goal. Herein, we report a combination of photo-switching dopants and organic phase-change materials as a way to introduce an activation energy barrier for phase-change materials solidification and to conserve thermal energy in the materials, allowing them to be triggered optically to release their stored latent heat. This approach enables the retention of thermal energy (about 200 J g−1) in the materials for at least 10 h at temperatures lower than the original crystallization point, unlocking opportunities for portable thermal energy storage systems. Phase-change materials offer excellent thermal storage due to their high latent heat; however, they suffer from spontaneous heat loss. Han et al., use organic photo-switching dopants to introduce an activation energy barrier which enables controllable thermal energy release and retention.

A review for phase change materials (PCMs) in solar absorption refrigeration systems

Abstract: Energy storage has become an important part in renewable energy technology systems. Solar thermal systems, unlike photovoltaic systems with striving efficiencies, are industrially matured, and utilize major part of sun's thermal energy during the day. Yet, it does not have enough (thermal) backup to keep operating during the low or no solar radiation hours. New materials are selected, characterized, and enhanced in their thermo-physical properties to serve the purpose of a 24 h operation in an efficient thermal energy storage system (TESS). Solar absorption refrigeration system requires a continuous operation in many of its applications (food storage, space cooling etc), which in turn requires an efficient TES system utilizing material with high heat of fusion, eg. phase change materials (PCMs). This review is a comprehensive evaluation of suitable PCM selection, methodologies of integration, enhancements and challenges for operating temperatures of each component in a single-effect solar absorption system affecting its performance. Observations and lessons from previous studies are discussed in detail. Recommendations based on investigation results, advantages and drawback of PCMs, PCM enhancement options, energy, exergy and cost analysis are made for the future research direction.

Dynamic tuning of optical absorbers for accelerated solar-thermal energy storage

Abstract: Currently, solar-thermal energy storage within phase-change materials relies on adding high thermal-conductivity fillers to improve the thermal-diffusion-based charging rate, which often leads to limited enhancement of charging speed and sacrificed energy storage capacity. Here we report the exploration of a magnetically enhanced photon-transport-based charging approach, which enables the dynamic tuning of the distribution of optical absorbers dispersed within phase-change materials, to simultaneously achieve fast charging rates, large phase-change enthalpy, and high solar-thermal energy conversion efficiency. Compared with conventional thermal charging, the optical charging strategy improves the charging rate by more than 270% and triples the amount of overall stored thermal energy. This superior performance results from the distinct step-by-step photon-transport charging mechanism and the increased latent heat storage through magnetic manipulation of the dynamic distribution of optical absorbers.

Recent Progress on Integrated Energy Conversion and Storage Systems.

Abstract: Over the last few decades, there has been increasing interest in the design and construction of integrated energy conversion and storage systems (IECSSs) that can simultaneously capture and store various forms of energies from nature A large number of IECSSs have been developed with different combination of energy conversion technologies such as solar cells, mechanical generators and thermoelectric generators and energy storage devices such as rechargeable batteries and supercapacitors This review summarizes the recent advancements to date of IECSSs based on different energy sources including solar, mechanical, thermal as well as multiple types of energies, with a special focus on the system configuration and working mechanism With the rapid development of new energy conversion and storage technologies, innovative high performance IECSSs are of high expectation to be realised for diverse practical applications in the near future

Squeezed thermal reservoirs as a resource for a nano-mechanical engine beyond the Carnot limit

Abstract: The efficient conversion of thermal energy to mechanical work by a heat engine is an ongoing technological challenge. Since the pioneering work of Carnot, it is known that the efficiency of heat engines is bounded by a fundamental upper limit, the Carnot limit. Theoretical studies suggest that heat engines may be operated beyond the Carnot limit by exploiting stationary, non-equilibrium reservoirs that are characterized by a temperature as well as further parameters. In a proof-of-principle experiment, we demonstrate that the efficiency of a nano-beam heat engine coupled to squeezed thermal noise is not bounded by the standard Carnot limit. Remarkably, we also show that it is possible to design a cyclic process that allows for extraction of mechanical work from a single squeezed thermal reservoir. Our results demonstrate a qualitatively new regime of non-equilibrium thermodynamics at small scales and provide a new perspective on the design of efficient, highly miniaturized engines.

Recent progress in solar thermal energy storage using nanomaterials

Abstract: Use of thermal energy storage (TES) materials in solar collectors is known to be the most effective way of storing thermal energy. The most conventional and traditional heat storage element is water. However, due to low thermal conductivity (TC) in vapor state its applications as a heat storage medium are limited. An alternative option is to utilize organic and inorganic TES materials as they both operate at low and medium temperature ranges. Organic TES materials such as paraffins are non-corrosive and possess high latent heat capacity. On the contrary, inorganic TES materials possess high density and appreciable specific heat capacity (SHC). Due to rapid progress and advancement in nanotechnology, varieties of nanomaterials were dispersed in various base fluid(s) to enhance thermo-physical properties. This review paper presents the current status and future development trends of TES materials. Furthermore, an extensive research on enhancement of TC and SHC of various TES material doped with nanomaterials has been discussed.

Energy efficiency of membrane distillation up to high salinity: Evaluating critical system size and optimal membrane thickness

Abstract: This study presents a comprehensive analytical framework to design efficient single-stage membrane distillation (MD) systems for the desalination of feed streams up to high salinity. MD performance is quantified in terms of energy efficiency (represented as a gained output ratio, or GOR) and vapor flux, both of which together affect the specific cost of pure water production. Irrespective of the feed salinity, permeate or conductive gap MD (P/CGMD) perform better than direct contact MD (DCMD) when the heat transfer resistance of the gap (in P/CGMD) is lower than that of the external heat exchanger in DCMD. Air gap MD’s (AGMD) better performance relative to the other configurations at high salinity and large system area can be explained in terms of its thicker ‘effective membrane’, which includes the air-gap region. CGMD and DCMD employing a thick membrane are also resilient to high salinity, similar to AGMD, while not being susceptible to the gap flooding that can harm AGMD’s performance. A method is described to simultaneously determine the cost-optimal membrane thickness and system size as a function of the ratio of specific costs of heat energy and module area. At low salinity and small system size, GOR rises and flux declines with an increase in membrane area. For salty feed solutions, there exists a critical system size beyond which GOR also begins to decline. Since both GOR and flux are lower, no economic rationale favors operation above this critical size, irrespective of the costs of thermal energy and system area. A closed-form analytical expression for this critical system area is derived as a function of the feed salinity and two dimensionless ratios of heat transfer resistances within the MD module.