Flexible Heat Research Articles

Operating characteristics of a flat evaporator loop heat pipe having a flexible heat transport path

Operating characteristics of a flat evaporator loop heat pipe having a flexible heat transport path

Dynamic optimization with flexible heat integration of a solar parabolic trough collector plant with thermal energy storage used for industrial process heat

There is an increasing need to reduce fossil fuel consumption used for industrial process heat to slow the effects of climate change. Using solar thermal heat is a viable way to replace fossil fuel use, but solar industrial process heat plants have limited implementation due to large upfront costs and inefficiencies from the inherent variability from solar energy. Having more flexible and optimized control over these plants can enable them to be more efficient. In this work, a solar industrial process heat plant with thermal energy storage that can flexibly collect and deliver heat to two industrial processes (flexible heat integration) is dynamically modeled with control setpoints found by a dynamic optimization method. The optimized case can increase the solar efficiency of the plant by 7.5% on average relative to a base case. Results show that it is best to collect heat at lower temperatures for all but ideal solar conditions, only medium to high quality heat should be stored in large quantities, and exchanging heat with a lower temperature heat sink is generally more efficient. The optimized case is able to reduce the levelized cost of heat of the plant to $31.83/MWhth compared to a base case value of $34.10/MWhth. The optimized case can reduce emissions by 22.2% compared to a plant which uses only natural gas. This work shows that dynamic optimization with flexible heat integration can be a cost-effective way to increase efficiency so that more of these types of plants can be implemented.

Wide‐temperature range thermoregulating e‐skin design through a hybrid structure of flexible thermoelectric devices and phase change materials heat sink

AbstractThe realization of the thermoregulating function of electronic skin (e‐skin) by simulating the human temperature perception system can greatly improve the intelligence of the e‐skin. Here, we report a thermoregulating e‐skin that fits on the surface of a prosthetic limb based on a hybrid structure consisting of a flexible thermoelectric device and a phase‐change heat sink. The hybrid e‐skin possesses outstanding temperature adaptability similar to that of the human body; it can maintain the surface temperature at 35°C in environmental temperatures ranging from 10 to 45°C. The power expenditure of the e‐skin is essentially the same as the energy required by the human body to regulate temperature and is only 14.22 mW cm−2 in the thermoneutral zone. Thermoregulation based on this e‐skin can greatly improve the temperature distribution of the target surface, providing a promising solution for the biomimetic thermoregulation of robots and the next generation of intelligent prostheses.image

A Liquid Metal-Enhanced Wearable Thermoelectric Generator.

It is a key challenge to continuously power personal wearable health monitoring systems. This paper reports a novel liquid metal-enhanced wearable thermoelectric generator (LM-WTEG that directly converts body heat into electricity for powering the wearable sensor system. The gallium-based liquid metal alloys with room-temperature melting point (24~30 °C) and high latent heat density (about 500 MJ/m3) are used to design a new flexible finned heat sink, which not only absorbs the heat through the solid-liquid phase change of the LM and enhances the heat release to the ambient air due to its high thermal conduction. The LM finned is integrated with WTEG to present high biaxial flexibility, which could be tightly in contact with the skin. The LM-WTEG could achieve a super high output power density of 275 μW/cm2 for the simulated heat source (37 °C) with the natural convective heat transfer condition. The energy management unit, the multi-parameter sensors (including temperature, humidity, and accelerometer), and Bluetooth module with a total energy consumption of about 65 μW are designed, which are fully powered from LM-WTEG through harvesting body heat.

Textile Heat Flux Sensor Used in Stress Detection of Children with CP

This work is part of the European project MOTION (Interreg 2 Seas Mers Zeeën), which aims to develop an exoskeleton for children with cerebral palsy (CP). The developed exoskeleton is equipped with a smart garment in order to detect the stress (e.g. physical, physiological) during the rehabilitation. Five different sensors, i.e. electrocardiogram (ECG), respiratory rate (RR), pressure, galvanic skin response (GSR) and textile heat fluxmeter (THF), are integrated into this smart garment for stress detection. This paper focuses on the development of the textile heat fluxmeter. Several researchers used heat fluxmeters in physiological studies to measure the body heat exchanges with the environment. However, the non-permeability of such fluxmeter gives inaccurate measurements in wet condition. Innovative flexible textile heat fluxmeter may detect, analyze, and monitor the heat and mass transfers with minimum disturbance due to its porosity. Moreover, it is desirable to have flexible sensors when they need to be in contact with the human body, in which the flexibility and non-irritability requirements are of utmost importance.

Shape-stable and smart polyrotaxane-based phase change materials with enhanced flexibility and fire-safety

With the high performance of intelligent electronic machines and the popularity of flexible and foldable devices and high-speed/large capacity 5G communication, the market demand for flexible heat storage and smart temperature control materials are gradually rising. Therefore, phase change materials (PCMs) have received much attention in the related energy storage and temperature control field. As the amount of PCM increases, so do the fire safety concerns associated with their applications. Therefore, the development of improved the flexibility and fire safety for advanced PCMs is imperative. In this work, we reported a pentaerythritol phosphate modified sustainable polyrotaxane as PCMs for thermal energy storage and thermal management. The structure of pentaerythritol phosphate was adequately confirmed by 13C NMR, 31P NMR and ATR-FTIR. Subsequently, the tensile properties, fire safety, phase change performances and shape memory properties of the PCMs were fully analyzed. The results showed that all the flexibility, fire safety and thermal conductivity of modified polyrotaxanes were enhanced remarkably compared to those of pristine polyrotaxane. Further with the high form stability, and excellent cycle stability, the modified polyrotaxanes are therefore promising sustainable and fire safe shape-stable PCMs for thermal energy storage. Notably, its ultra-high flexibility (with elongation higher than 1000%), processing ability, excellent form-stability, enhanced fire-safety (with UL94 V-0 rating and LOI of 33.0%) and thermal conductivity (0.47 W m−1 K−1) provide a practical way for the safe and intelligent heat treatment for some electronic devices, such as solid-state disk.

Greening of the District Heating Systems—Case Study of Local Systems

The integration of renewable energy resources into district heating systems is gaining momentum across Europe, as heat producers are expected to work towards the EU Directive of Efficient District Heating and Cooling to achieve carbon neutrality by 2050. This paper studies the techno-economic implications of transforming conventional district heating systems of six locations in Poland, generating 8.5 PJ of heat annually, into sustainable and efficient district heating systems. These new systems consist of flat solar collectors integrated with seasonal pit thermal energy storages and gas heating plants, acting as flexible heat sources, covering residual heat demand and/or increasing the parameters of the working medium in the network. Using the IEA-TIMES software, two scenarios were considered, namely STAT and DYN. The results show that reaching a 20% share of heat production by solar thermal would demand extra construction of seasonal heat storage facilities with a total capacity of 197 TJ, which is approximately 4.5 times bigger than the largest seasonal heat storage located in Vojens, Denmark. The projected increase in the prices of natural gas and CO2 emission allowances accelerates the transformation of systems towards greater use of solar heating plants. In the period 2025–2050 the heat generation costs increase by ca. 65%. The contribution of the CAPEX and OPEX costs components are presented.

10 kW rectangular laser beam generation with incoherent space combiner

In this paper, we report an incoherent space laser beam combiner for rectangular beam combination with 18 fiber-transmitted semiconductor lasers at 972 nm wavelength. The coupled thermo-mechanical properties of all the optical lenses in the combiner were analyzed to evaluate the long-term reliability of the combiner subjected to beyond 10 kW high-power laser irradiation. An experiment-based numerical model was developed on the basis of the multi-beam laser volumetric heat source, and the performance of lenses irradiated by 10 kW combined laser for 1000 s was studied with finite element method (FEM). The maximum temperature among all lenses was 427.27 K which is much lower than the softening point temperature of fused silica material made of lenses. And the 0.1 aperture number corresponding to the maximum thermal deformation of 4.53 µm is much smaller than the conventional optical lens processing tolerance. The maximal stress of 12.73 MPa is far less than the yield stress of 4.5 GPa for the material. The manufactured laser beam combiner realized a highest output power of 10.641 kW at a power combination efficiency of 98.5%, a combination length of 200 mm and a rectangular focal beam spot of 30 mm × 10 mm. Our method provides a valuable choice for realizing fast and flexible laser surface heat treatment.

Simulating a solar parabolic trough collector plant used for industrial process heat using an optimized operating scheme that utilizes flexible heat integration

This study focuses on the modeling and simulation of a novel design and operation of a solar industrial process heat (SIPH) plant that uses a parabolic trough collector system for generating process heat. The SIPH plant incorporates flexible heat integration (FHI) by having two options for heat sinks as well as a flexible collection temperature for the heat transfer fluid. Leveraging the degrees of freedom created by FHI allows an optimized operating scheme to be created which maximizes the solar heat delivered to the industrial processes. Yearly results for a 27 MW-t plant located in Salt Lake City, UT are simulated for a base case and a case utilizing optimized FHI. For the main process temperature case presented, FHI increases the total solar share by 13.0% and reduces the levelized cost of heat of the SIPH plant by 10.4% relative to the base case. Three case study days are presented which highlight how FHI maximizes solar share, specifically during mornings, afternoons, and days with intermittent solar conditions. An environmental analysis is also performed showing that the SIPH plant can reduce harmful emissions by 15.4% for the base case and 17.4% for the FHI case compared to a plant that only utilizes a natural gas furnace for process heat. Overall, the work thoroughly explores an effective operating scheme for a SIPH plant, helping to make it more economical and environmentally beneficial.

High-sensitivity self-powered temperature/pressure sensor based on flexible Bi-Te thermoelectric film and porous microconed elastomer

• A self-powered Bi-Te and porous microcone-based temperature/pressure sensor is presented. • A flexible TEG could harvest body heat to power pressure sensor and sense temperature. • Bi-Te based TE thin film with heat sink achieves a high-precision temperature sensor. • The porous microconed architectures greatly enhance the sensitivity of pressure sensor. Electronic skins are artificial skin-type multifunctional sensors, which hold great potentials in intelligent robotics, limb prostheses and human health monitoring. However, it is a great challenge to independently and accurately read various physical signals without power supplies. Here, a self-powered flexible temperature-pressure bimodal sensor based on high-performance thermoelectric films and porous microconed conductive elastic materials is presented. Through introducing flexible heat-sink design and harvesting body heat energy, the thin-film thermoelectric device could not only precisely sense temperature signal but also drive the pressure sensor for detecting external tactile stimulus. The integration of Bi-Te based thermoelectric film with high stability in wide temperature range enables the sensor to sense the ambient temperature with high resolution (<0.1 K) as well as excellent sensitivity (3.77 mV K –1 ). Meanwhile, the porous microconed elastomer responds to pressure variation with low-pressure detection (16 Pa) and a high sensitivity of 37 kPa –1 . Furthermore, the bimodal sensor could accurately and simultaneously monitor human wrist pulse and body temperature in real time, which demonstrates promising applications in self-powered electronic skins for human health monitoring systems.

Facile Approach to Enhance Electrical and Thermal Performance of Conducting Polymer PEDOT:PSS Films via Hot Pressing.

This paper studies the impact of hot pressing on the electrical and thermal performance of thick (thickness >5 μm) conducting polymer poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films after acid treatment. Thick conducting polymer films usually exhibit low electrical and thermal conductivities similar to bulk polymer because charge and heat carriers are easily scattered by the irregular arrangement of crystalline domains inside the polymer. In this work, the in-plane electrical conductivity of thick hot-pressed PEDOT:PSS film exceeded 1500 S/cm, and 50% enhancement was obtained in comparison with its non-hot-pressed counterparts. Its in-plane thermal conductivity reached as high as 1.11 W/mK (improved by almost 100% compared to acid-treated PEDOT:PSS films), which is comparable to that of some commercial thermal pads. Such electrical and thermal enhancement via the hot-pressing process is attributed to the optimized morphology and microstructures, which provide short paths for thermal and electrical transportation. We have also demonstrated that the hot-pressed PEDOT:PSS films could be potentially utilized as a flexible conductor and heat spreader for application in flexible electronics and thermal management, respectively. This study not only offers a new insight into the process-property relationship for conducting polymers but also further enables the use of PEDOT:PSS films with simultaneously improved electrical and thermal performance in practical applications, such as thermal management for organic electrodes in batteries, flexible electronics, soft robotics, and bioelectronics.

Electrical and Thermal Properties of Carbon Nanotube Polymer Composites with Various Aspect Ratios.

In response to the rising need for flexible and lightweight materials capable of efficient heat transport, many studies have been conducted to improve the thermal properties of polymers via nanofillers. Among the various nanofillers, carbon nanotubes (CNTs) are considered as the most promising, owing to their excellent thermal and electrical properties. Accordingly, CNT/polymer composites can be used as flexible and lightweight heat transfer materials, owing to their low density. In this study, we fabricated multi-walled CNT (MWCNT)/polymer composites with different aspect ratios to investigate their effects on electrical and thermal properties. Through a three-roll milling process, CNTs were uniformly dispersed in the polymer matrix to form a conductive network. Enhanced electrical and thermal properties were observed in MWCNT composite with a high aspect ratio as compared to those with a low aspect ratio. The thermal conductivity of composites obtained as a function of the filler content was also compared with the results of a theoretical prediction model.

Design and application of a cooling device based on the phase-change material for heat stroke

Heat stroke is a critical and health-threatening disease, triggered by thermal stimulus and progressing rapidly. It can give rise to multiple organ dysfunction syndrome (MODS), resulting in a high mortality rate. Nearly 30% of survivors will suffer with different sequelae, for instance, the neurological sequelae. Currently, the early rapid cooling is the focus of therapy for heat stroke. Therefore, it is imperative to design a cooling module suitable for the treatment of heat stroke in the field and in the hospital to realize the goal of early rapid cooling and the effective targeted temperature management (TTM). The cooling device is composed of a cooling blanket and a cooling cap. The blanket and cap are made by temperature changeable fabric. The cooling blanket comprises a backing layer, a buffer layer, a flexible heat conduction capsule body, a temperature changing component, a fixed part and a temperature sensor. The cooling cap includes a main body and two side ears, in which the main body is worn on the top of the patient's head, and the front is equipped with a flexible display screen, which is convenient for real-time monitoring the temperature of the temperature change component of the cooling blanket. The lateral ear can wrap the patient's ears and neck, and the tympanic membrane thermometer is designed to monitor the tympanic membrane temperature in real time. The tympanic membrane thermometer is also designed at the ear to monitor the tympanic auditory canal temperatures in real time.Continuous dynamic temperature monitoring can guide the duration of cooling treatment and stop cooling in time. The cooling component is portable, easy to operate, real-time temperature monitoring, excellent cooling effect and reusable. It is used for on-site first aid, transportation and continuous cooling for patients with heat stroke in the ward.

Effect of the Aspect Ratio and Tilt Angle on the Free Convection Heat Transfer Coefficient Inside Al2O3-Water-Filled Square Cuboid Enclosures.

This experimental study provides a comprehensive investigation of natural convection heat transfer inside shallow square cuboid enclosures filled with aluminum oxide–water nanofluid at four different volume concentrations: 0.0%, 0.2%, 0.4%, and 0.8%. Two square cuboid enclosures were used with sizes 30 × 30 × H cm3, where H is the inside thickness of the enclosures. This led to two different enclosure aspect ratios (κ = H/30 = 0.033 and 0.066). Four inclination angles to the horizontal position of the enclosures were used: 0°, 30°, 60°, and 90°. The crucial thermophysical properties of the synthetic nanofluid were obtained. The thermal conductivity of the nanofluid was measured experimentally at various volume concentrations. Furthermore, the viscosity and density were also measured experimentally at temperatures ranging from 15 to 40 °C as a function of the volume concentration. The heat transfer data were generated by heating the lower surface of the enclosure using a uniform flexible heat flux heater. The opposite surface was cooled using an air fan. The results of the experimental physical parameter measurements show that the percent of maximum deviation in thermal conductivity with those in the literature were 6.61% at a 1.0% volume concentration. The deviation of dynamic viscosity was between 0.21% and 16.36% at 0.1% and 1% volume concentrations, respectively, and for density it was 0.29% at 40 °C and a 1% volume concentration. The results showed up to a 27% enhancement in the Nusselt number at an angle of 60° and a 0.4% volume concentration in the largest aspect ratio (κ = 0.066). However, for the low aspect ratio enclosure (κ = 0.033), there was no noticeable improvement in heat transfer at any combination of volume concentration and inclination angle. The results show that the inclination angle is a significant factor in natural convection only for large aspect ratio enclosures. Furthermore, for large aspect ratio, the Nusselt number increased until the angle approached 60°, then it decreased again.

Development Status and Perspective Trend of Flexible Heat Pipe

Development Status and Perspective Trend of Flexible Heat Pipe

Highly flexible cellulose nanofiber/single-crystal nanodiamond flake heat spreader films for heat dissipation

A simple method was proposed to produce a flexible heat spreader with efficient thermal transportation performance.

Polycaprolactone Adsorption and Nucleation onto Graphite Nanoplates for Highly Flexible, Thermally Conductive, and Thermomechanically Stiff Nanopapers.

Free-standing nanopapers based on graphene and its related materials have been widely studied and proposed for flexible heat spreader applications. Given that these materials are typically brittle, this work reports the exploitation of polycaprolactone (PCL) as a polymer binder to enhance resistance and flexibility of nanopapers based on graphite nanoplates (GNP), while maintaining a high thermal conductivity. Properties of nanopapers appear to correlate with the excellent PCL adhesion and strong nucleation of the surface of GNP flakes. Furthermore, different crystalline populations were observed for PCL within the nanopaper and were investigated in detail via differential scanning calorimetry advanced techniques and X-ray diffraction. These demonstrated the coexistence of conventional unoriented PCL crystals, oriented PCL crystals obtained as a consequence of the strong nucleation effect, and highly stable PCL fractions explained by the formation of crystalline pre-freezing layers, the latter having melting temperatures well above the equilibrium melting temperature for pristine PCL. This peculiar crystallization behavior of PCL, reported in this paper for the first time for a tridimensional structure, has a direct impact on material properties. Indeed, the presence of high thermal stability crystals, strongly bound to GNP flakes, coexisting with the highly flexible amorphous fraction, delivers an ideal solution for the strengthening and toughening of GNP nanopapers. Thermomechanical properties of PCL/GNP nanopapers, investigated both on a heating ramp and by creep tests at high temperatures, demonstrated superior stiffness well above the conventional melting temperature of PCL. At the same time, a thermal conductivity > 150 W/m·K was obtained for PCL/GNP nanopapers, representing a viable alternative to traditional metals in terms of heat dissipation, while affording flexibility and light weight, unmatched by conventional thermally conductive metals or ceramics. Besides the obtained performance, the formation of polymer crystals that are stable above the equilibrium melting temperature constitutes a novel approach in the self-assembly of highly ordered nanostructures based on graphene and related materials.

Optimizing the renewable and fossil-fired generation capacities: Case study of interconnected district-level systems

We first comprehensively analyze determining the generating capacities of each type, then discuss the heat distribution across interconnected district-level heat energy systems (“the hubs”) containing both low-scale and centralized heat plants; discussion is presented from the standpoint of primary energy consumption and costs. The applied tool includes a flexible heat generation model and an accurate objective function. The set of constraints and the Jacobian matrix are configured manually. We apply the fixed-point iteration approach to solve an equation system. In order to evaluate the consequences of all possible scenarios, we analyze a novel district heating (DH) system with low-scale solar collectors and heat pumps against central combined heat-and-power (CHP) plant fueled with fossil fuels and biomass. The proposed system is evaluated from the standpoint of the state-of-the-art, future prospects, and environmental impact. For each specific type of capacities, we herein present maximum achievable net generation of heat. System layout sketch is based on the data on the actual districts of Canberra, Australia. All the hubs are studied as a whole with due account of heat redistribution. The configuration made implies construction renewable energy facilities with a heat generation of 6.5+ MW, which will suffice to cover about a fifth of the peak load. This approach is quite novel for Australia. With greater electricity and heat supplies, renewable resources can cover an even greater share of such supplies. To sum up, a significant difference between the costs of a natural gas-based DHS and a coal-fired DH one lies in the fixed costs of operating the CHPPs and in the fuel costs. The self-generation of a hub may be inconsistent with the total energy balance, which is the total of eigen-production, energy inflows and outflows. The total system performance is considerably better when coal-fired CHPPs account for the bulk of net production.

Identification of urban cellular structures for flexible heat and temperature distribution in district heating networks

Heat maps provide static information about spatial distribution of heat and cooling demands and they specify heat load and peak power densities in urban areas as well. But the maps may also include data about temperatures requested by the building heating systems (probably dependent on the building efficiency classes and the type of domestic hot water generation system) or any time series of either heat load or supply and return temperatures, respectively. Based on those data (steady or transient) cellular structures will be identified to be used for a more flexible operation of DH (district heating) networks or for re-designing new network topologies. The aim of these cells is the transition to locally flexible supply temperatures in the DH network: Each cell will be provided with an “individual“ supply temperature meeting its needs. This may open new perspectives to reduce heat losses and to integrate low emission heat sources.Groups of heat consumers having both relatively closed-by positions in the urban heating system and similar supply temperature set points will be merged into a single DH cell. Different clustering methods have been analyzed to identify those DH cells: Single-Linkage, DBSCAN and OPTICS, Affinity Propagation and spectral clustering. The splitting of a DH system into a plenty of cells is demonstrated by means of an urban, unmeshed example network thus evaluating the suitability of the entire process. The programming language Python is used.To create the input data for the algorithms, two approaches have been applied (depending on the type of algorithm): (1) A distance matrix is created by reducing a graph model of the overall network so that only the distance between consumer substations (similar to length of pipeline) remains. Besides this information, the matrix does not contain information about the network topology (“reduced model”). (2) Some algorithms accept graph models as input, so that the model of the overall network can act as input variable. The model does not need to be reduced and thus contains more information about the network topology.A lesson learned is that the utilization of affinity propagation – an algorithm using the reduced model – can lead to implausible cells regarding network operation due to the missing information about the network topology. However, the Single-Linkage algorithm accepts a graph model of the overall network as input as a so called “connectivity constraint”. It leads to a highly reliable DH cell structure at least for unmeshed DH networks. Beside net topology it may be helpful to characterize DH cells not only by fixed supply temperature but also by supply temperature profiles. With respect to practical testing and realization of flexibility measures also the results of an ongoing clustering of a DH network at different points in time should be evaluated.

Weather-induced variability of country-scale space heating demand under different refurbishment scenarios for residential buildings

The decarbonisation of residential heat through integration with the power system and deployment of refurbishment policies is at the core of European energy policies. Yet, heat-electricity integration may be challenged, in practice, by the large variability of heat demand across weather years. Current approaches for residential heat demand simulation fail to provide insights about the extent of such variability across many weather years and about the benefits potentially brought about by nearly zero-energy buildings. To fill this gap, this work develops an open-source space-heating demand simulation workflow that is applicable to any country's building stock. The workflow, based on a well-established lumped-parameter thermodynamic model, allows capturing sub-national weather-year variability and the mitigation effects of refurbishment. For Italy, different weather years lead to variations in heat demand up to 2 TWh/day, lasting for several days. Moreover, some weather regimes produce spatial asymmetries that may further complicate heat-electricity integration. The refurbishment of about 55% of buildings constructed before 1975 could substantially mitigate such oscillations, leading to a 31–37% reduction of yearly heat demand, primarily in colder regions. Intra-day heat demand variations, driven by user behaviour, are not substantially impacted by refurbishment, calling for the simultaneous deployment of flexible heat generating technologies.