Thermal coupling effects of nanofluids and phase change materials in concentrated solar collectors: a review

Performance of compound parabolic concentrator solar air flat plate collector using phase change material

Flexible engineering of advanced phase change materials

In order to optimize the phase-change energy storage materials for asphalt pavement and analyze the feasibility and applicability of phase-change energy storage materials for asphalt pavement, the experimental methods of thermogravimetric analysis (TG) and Fourier infrared spectroscopy (FT-IR) were adopted. The thermal stability, chemical stability and chemical compatibility of various phase change energy storage materials were tested and analyzed. The results show that the fatty acids, stearic acids and fatty alcohols showed significant weight loss at 200°C. PEG and DTC phase change materials showed no mass loss at 200°C and had good thermal stability. PEG2000 and DTC have no chemical changes after high temperature treatment, and PEG2000, DTC and SBS modified asphalt are physical blend, and the chemical compatibility with asphalt is good. Crystalline hydrated salts, paraffin, fatty acid, stearic acid, fatty alcohol phase change materials are not suitable for asphalt pavement, PEG organic phase change materials and DTC composite phase change materials can be used for intelligent temperature control asphalt pavement research.

Recent studies [1-3] have demonstrated that nanotechnology, in the form of nanoparticles suspended in water and organic liquids, can be employed to enhance solar collection via direct volumetric absorbers. However, current nanofluid solar collector experimental studies are either relevant to low-temperature flat plate solar collectors ( 100 °C) indoor laboratory-scale concentrating solar collectors [1, 5]. Moreover, many of these studies involve in thermal properties of nanofluid (such as thermal conductivity) enhancement in solar collectors by using conventional selective coated steel/copper tube receivers [6], and no full-scale concentrating collector has been tested at outdoor condition by employing nanofluid absorber [2, 6]. Thus, there is a need of experimental researches to evaluate the exact performance of full-scale concentrating solar collector by employing nanofluids absorber at outdoor condition. As reported previously [7-9], a low profile ( Thermal experimentation reveals that while the collector efficiency reduced from 73% to 54% when operating temperature increased from ambient to 80 °C by employing a MWCNT nanofluid receiver, the efficiency decreased from 85% to 68% with same operating temperature range by employing black chrome-coated copper tube receiver. This difference can mainly be explained by the reflection optical loss off and higher thermal emission heat loss the front surface of the glass tube, yielding a 90% of transmittance to the MWCNT fluid and a 0.9 emissivity of glass pipe. Overall, an experimental investigation of the performance of a low profile solar collector with a direct volumetric absorber and conventional surface absorber is presented. In order to bring nanotechnology into industrial and commercial heating applications,

Increasing concerns surrounding the rising global energy demand has forced humans to look for alternative energy sources such as the development and utilization of natural gas and nuclear energy, or to increase the efficiency of energy use, thereby optimizing the use of energy. Improving energy efficiency is an effective method that can quickly and efficiently reduce the energy demand and supply gap. Furthermore, developing new technologies for energy storage and energy saving is an effective way to solve the energy crisis, which is of great significance for the sustainable development of energy. Latent heat storage has become a popular research topic owing to its large energy storage density and small temperature changes during energy storage and its excellent thermal stability and high safety. Currently, phase-change materials have been widely used in solar heating systems, air conditioning systems, thermally regulated textiles, energy-efficient building construction, temperature-controlled greenhouses and other fields. The development of phase-change energy storage technology is significant for promoting the development of alternative energy sources and improving energy efficiency. However, phase change materials are prone to liquid leakage during the solid-liquid phase transition, which limits their application. To solve this problem, researchers have started introducing porous support materials to phase-change materials. Porous support materials have attracted extensive research attention in recent years owing to their outstanding properties such as high specific surface area, large pore volume, and low density. Porous support materials can absorb phase-change materials in their pores through the physical adsorption phenomena such as capillary action and interfacial tension, and thereby gradually develop into important substrates for phase-change material encapsulation. Inorganic materials are used as carriers for phase change energy storage materials. Compared with organic carrier materials, inorganic carrier materials have higher mechanical strength, flame retardancy and thermal conductivity, which can reduce the production cost of phase-change energy storage materials, and have a high research value. Mesoporous silica materials have good physical and chemical stability, biocompatibility, flame retardancy, low toxicity, corrosion resistance, controllable size, adjustable surface morphology and high specific surface area. They can comprehensively improve the performance of various aspects of phase change composites and broaden the application space of phase change energy storage materials. In this review, the effects of pore size, pore structure, and pore surface properties on the crystallization behavior of phase change materials in mesoporous silica carriers developed in recent years were comprehensively analyzed, and prospects of research methods for heat storage efficiency were explored.

To achieve affordable housing in a carbon-neutral society, new buildings require a dual-purpose approach that comprises efficient construction and a green energy supply. Energy screw piles [1, 7] meet this demand as they combine the agility of screw pile drilling with the capability of extracting clean shallow geothermal energy. Moreover, the screw piles can be filled with phase change materials (PCM) to provide latent thermal storage. Studies regarding the use of PCMs as borehole backfill in a ground source heat pump system (GSHP) conclude that PCM implementation can improve GSHP performance. However, most commercially available PCMs have low thermal conductivities (λ) which undermine heat exchange rates [5, 10]. Besides, using PCMs as a composition of a concrete energy pile can reduce the pile structural performance since PCMs usually have a low mechanical capacity [2, 8]. The authors recently introduced PCM-sand mixtures as a core in the central hollow part of an energy screw pile, which does not impact the pile structural capacity [6]. The mixtures with a higher PCM content benefit the pile heat exchange through its latent heat, but only when the PCM does not reduce the mixture’s λ. To overcome this problem, this work tests a new underground heat exchange system where instead of mixing the PCM in the energy screw pile filling material, regular screw piles (i.e., without the heat exchange tubes) are filled with pure PCM, acting as thermal storage piles. A numerical model built via COMSOL [4] is used to evaluate how this combination of screw piles performs thermally when supplying/rejecting heat for a GSHP system operating for a whole year.
 This work uses a validated numerical model [3, 9] to simulate a grid of evenly distributed screw piles, where Energy Piles (EP) and Thermal Storage Piles (TSP) are positioned interspersed, evenly spaced 0.7 m apart. Inside the EPs, an U-loop pipe is inserted in the pile steel case and the remaining is filled with grout. In contrast, the steel case of the TSPs is filled with only PCM [11]. All other material properties and the screw pile geometry are based on [1]. The thermal load is based on the design of a building located in Melbourne, Australia (Figure 1(a)). The model considers the hourly operation of a GSHP for one year, calculating the GSHP Coefficient of performance (COP) at every time step and adjusting the ground thermal load. For comparison, two simulations (one where the TSPs are filled with PCM and a reference where they are filled with only air) are undertaken. Besides the energy stored, the COPs of both cases are compared as a relative increase/decrease percentage (COPchange = (COPPCM / COPReference) – 1).
 Figure 1(b) presents the energy stored by the PCM and its corresponding percentage in a solid state over the simulation. As the EPs reject heat underground due to GSHP summer operation for cooling, the TSPs temperature rise and store sensible heat, until they reach their phase change temperature (Tpc), when the PCM melts to store significantly more heat energy in a shorter time window due to latent heat. Conversely, the PCM solidifies when EPs extract more heat than reject due to GSHP winter operation (heating) for a certain period. The PCM melts again on the following summer, restarting the whole process. The peak energy stored in the TSPs is 190.3 MJ/m3. Even though the thermal load varies hourly, the PCM phase change process happens without immediate responses to the load variation. Figure 1(c) presents the resulting change in the hourly COP considering cooling (CCOP) and heating (HCOP) from implementing PCM in the TSPs, compared to the reference scenario (empty TSPs). The extra heat stored by the PCM lowers the circulating fluid temperature while the latent heat is engaged (months 2 to 7 and 12), which results in a higher CCOP and a lower HCOP. By plotting a monthly average of the COPchange (grey markers), it is clear that the impact of the PCM on the COP is positive when cooling is dominant and slightly negative when heating becomes dominant while the latent heat is engaged. When the PCM is implemented in the EP [6, 10], the heat exchanged by the EP drops once the phase change process is over due to the low λ of the PCM, but by positioning it outside the EP the higher CCOP is sustained even after the phase change ends. However, lowering the fluid temperature increases CCOP while lowering HCOP at the same time, which can harm thermal performance if not designed properly. The results underlie the thermal storage potential available not only in screw piles, but also in any hollow pile foundation, by simply implementing PCM in the hollow case.

Liquefied natural gas (LNG) is a preferred distribution method for natural gas, particularly for long distance transport. LNG is regasified before distributed to the final consumers, releasing vast amount of cold energy. Majority studies on recovery of LNG cold energy involve conversion of cold energy into electricity. Usage of other energy carriers such as working fluids and thermal energy storage (TES) are seldom considered. When cold demands exist, cold-to-cold transfer from LNG terminal to demands using electricity as energy carrier can be unfeasible due to extra conversion process required. A study is carried out to compare the performances of different energy carriers, including electricity, working fluids, solid-liquid type phase change materials (PCMs) storage and liquid air energy storage (LAES), to transport LNG cold energy to different cold demands.
\nFour cold demands (air separation, dry ice production, deep-freezing warehouse and district cooling system) are considered, with their setups altered to adapt with different energy carriers. Layouts and performances of different cold demands subjected to different cold carriers are investigated in detail. On overall, using working fluids and PCM TES energy carriers result in 38.0% and 37.0% reduction in carbon emissions, which can be attributed to the replacement of conventional cooling systems in most cold demands supported by these energy carriers. Using LAES yields only 6.0% reduction in carbon emissions, due to the energy-intensive liquid air production process. Considering the costs and feasibility between working fluids and TES for cold distribution, TES is chosen as the next subject of study due to its lower capital and operational costs.
\nTES systems have been widely investigated for heat applications, but less for cold storage, while majority of them focus on low-grade cold storage such as space cooling. Study of TES systems for storage of LNG cold is the first-of-its-kind during the point the study was carried out. A numerical study is carried out using Galden HT-55 as liquid-state heat transfer fluid (HTF) and has operating temperature as low as -80°C. Cascaded PCM in packed bed configuration (different PCM arranged in series), known with higher operational efficiencies, is chosen with three PCMs with different melting temperatures (-49°C, -19.5°C and 6.5°C). The study serves two objectives: (1) to find out the performance of PCM storage for low-temperature cold storage, especially in the real life where PCM’s thermophysical properties deteriorates with decrease of phase change temperature, and (2) to minimize the amount of PCMs for optimum performances. Such optimization study is important as a lot of the time, TES systems act as auxiliary systems to other thermal systems, and their parameters are seldom optimized. The performances of TES system are evaluated based on the operation time and cyclic efficiencies on different TES capacity for each PCM. Cyclic efficiency is found to be the highest when ratio of TES capacity for high-medium-low grade PCMs is 10:7:5. Besides, with all the simulated cases, a generalized map is presented. The map allows a user to predict the performance of a cascaded PCM system with pre-determined storage capacity for each PCM. Besides, if the user has desired performance for the system, the map can be used to determine the suitable allocation of TES capacity for each PCM.
\nLiquid HTF based TES system has higher operating temperature and is unable to fully utilize all exergy of LNG cold, despite the high energy density of the HTF. Gaseous type HTF is thus investigated for charging an in-house-developed PCM. Due to lack of reported experiment for cold storage at temperature as low as -160°C, a packed bed TES experimental setup using in-house-developed PCM is thus built to achieve two purposes: (1) to obtain experimental results for more accurate validation of 3-D simulation models due to clearer understanding on the operating conditions and (2) to investigate a high-grade cold TES system using gaseous type HTF. This TES for such high-grade cold storage (PCM with melting temperature of -118°C) is the first-of-its-kind.
\nFrom the experiments, the large void space in between the encapsulated PCMs which contributed to zero TES capacity and low efficiency of the system is identified as a key improvement area. Granular quartzite pebbles are inserted into the void space to reduce the void fraction. Experiments are then carried out to validate a new numerical model which considers two TES materials in a same numerical grid. Consideration of homogenous mixture of granular materials with PCM is never considered to improve performance of a packed bed PCM TES system. Thus, numerical studies are carried out to identify the influence of different granular materials under systems with different PCMs, encapsulation size and HTF flow rate. Insertion of granular materials improves the TES efficiency by up to 25%, particularly when the PCMs have ultra-low phase change temperature, due to materials’ limitations with thermo-physical properties. Granular materials allow for increase in encapsulation size by up to 60% and decreases encapsulation cost by up to 75% without deteriorating performance of the system. Alumina particles are found to be the best granular material due to the extra storage capacity it provides due to its high density. Using micro-encapsulated PCM (n-decane) brings the highest improvement in cyclic efficiency, however, it is found that if micro-encapsulated PCM with lower phase change temperature is developed, the system performance can be further improved.
\nThus far, 1-dimensional model has been used for numerical studies. However, some parameters of the TES system cannot be studied with 1-dimensional model, such as development of radial temperature profile due to energy loss via the tank wall, as well as additional of inlet from the side where radial flow component needs to be studied. A 3-dimensional COMSOL simulation model is built and validated with the experimental results to study the development of temperature profile in both axial and radial directions of a TES tank, as well as change in performance of system under influence of auxiliary input with different HTF flow direction. Due to the thermal energy via tank wall, it is found that the temperature gradient along HTF axial-flow direction increases, which elongates the charge time of a TES system as PCMs near the tank wall is subjected to reduced thermal power. Side injection inlets are thus proposed to supply HTF with high-grade cold energy directly, to increase the thermal power in the region near the tank wall. However, usage of side inlet alone is unable to solve the problem directly, as mixing occurs between diverted flow from the side inlet and the flow from the main inlet, resulting in great exergy losses, especially if the resultant mixture has temperature higher than the PCM phase change temperature. Thus, operational parameters are also considered where at the beginning of the charging stage, only main inlet is used. After a certain amount of time, part of the flow entering the main inlet is redirected to the side inlet, while maintaining the total flow rate into the system. Such configuration is found to able to decrease the charge time by up to 13.4%.
\nThe abovementioned study identified three aspects on how a TES system designed for cryogenic cold storage can be designed and improved. When all the aspects are considered during design stage, there is no doubt that an optimized TES system, not only for storage of LNG cold, but also for a generic storage of thermal energy can be designed.

This work introduces a novel hybrid system which uses the cooling water of a concentrated PV/T as a preheated water for lithium bromide-water-based Kalina cycle. The PV/T is a combination between solar photovoltaic cells and solar thermal collector. The solar cells convert the sunlight into electricity, and the remaining heat gets absorbed by the collector for additional power generation. As well as, extracting the excess heat from PV cells enhances its electrical efficiency. The Kalina power cycle is mainly driven by concentrated solar parabolic collectors (CSP). The advantage of combining these 2 systems is to employ the heat rejected from the concentrated solar-based PV cells for further electricity generation, thus increasing the system overall efficiency. The system of concentrated solar collectors is divided into 2 parts to avoid getting very high temperature for the PV cells: concentrated solar collectors with PV cells and without PV cells. Due to the high temperature of water which returns from the Kalina cycle, a cooling tower is employed to cool the water before entering the concentrated PV/T system. The mathematical model of the proposed system has been presented and simulated to investigate the effect of the main controlling variables on the proposed system performance: generator temperature, solar radiation, wind speed, and the ambient temperature. The results show that the amount of solar radiation is the primary variable for the investigated system energy productivity, while the design temperature of generator is found to be the most parameter that affects the system overall efficiency. While the overall efficiency is around 18% to 23%, combining PV/T with the proposed solar Kalina cycle increases the system overall efficiency by 22% to 27%. The electrical conversion efficiency of the proposed system is 40% to 68% much more compared to PV/T alone system.

Employment of compound parabolic concentrator (CPC) solar collector with evacuated tubes enables effective collection of solar energy. The main objective of the present experimental work is to investigate thermal performance of CPC solar collector coupled with sensible and latent heat thermal energy storage (TES) system. Depending on obtained thermal stratification in sensible TES system, a commercial phase change material (PCM)—OM 65 has been selected and packed only at top quarter section of the TES system. Results showed an appreciable cumulative energy storage of 21 MJ d−1 due to existence of desired temperature driving potential during charging. A higher thermal stratification has been achieved in latent heat TES system, reducing the “MIX” number and supplying the heat transfer fluid at lower temperature to the collector. Due to enhanced useful heat energy gained, thermal efficiency of CPC collector reaches a maximum of 49% while coupling with latent heat TES system. The outcome of the study highlights the potential role of CPC and PCMs in solar thermal system, particularly for water heating applications.

Latent TES systems utilize phase change materials (PCMs) which at a suitable temperature range can be melted and thus store thermal energy. The stored energy is removed during the reverse process which solidifies the PCM. Due to the superiority of high latent heat compared to sensible heat, PCMs can contribute to the reduction of the storage systems’ size offering a promising solution especially when coupled with solar collectors. Despite the aforementioned advantages, the relatively low thermal conductivity of PCM hinders their wide utilization. In the present study, a thermal analysis of phase change materials is carried out. Different types of phase change materials (PCMs) are analyzed at various temperature ranges. The energy equation for the PCM is solved by implementing a 1D explicit finite difference scheme in Matlab and the results are compared with corresponding results deriving from Comsol. The properties of the utilized PCMs are altered accordingly so as to take into account their variation during phase change. In this analysis, only the thermal behavior of a PCM is investigated while the gravitational effect is neglected. The results of the analysis regard the temperature variations within the phase change material, the stored energy in the PCM per volume unit, the process speed and the effect of thermal conductivity on phase change, especially on the melting front displacement. Primary results have shown that the stored energy depends on the heat source and on the utilized PCM. In order to tackle the problem of PCM low conductivity, nanoparticles are added so as to enhance the stored energy due to the higher thermal conductivity. Upon the addition of two types of nanoparticles, the enhancement of melting fraction and stored heat are determined.

Chalcogenide Phase Change Materials (PCM) and metal insulator transition (MIT) materials are a group of materials that are capable of switching between low resistance and high resistance states. These emerging materials have been widely used in optical storage media and memory devices. Over the past recent years, there have been interests in exploiting the PCM and MIT materials, especially germanium antimony telluride (GST) alloys and vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), for radio frequency (RF) applications. The PCM and MIT-based RF devices are expected to bridge the gap between semiconductor switches and microelectromechanical system (MEMS) switches as they combine the low insertion loss performance of MEMS technology and the small size and reliability performance of semiconductor technology. This article presents an overview of the PCM and MIT materials for RF circuits and discusses the recent advancements in reconfigurable millimeter-wave (mmWave) devices based on PCM and MIT materials in depth.

This work involves an experimental investigation of a Compound Parabolic Concentrator (CPC) solar air flat plate collector with adding Phase Change Material (PCM). To explore the best model performance, the structure of CPC has two symmetric giant parabolic mirror reflectors, a concentration ratio of 1.7, similar flat plate receiver with 12 tubes filled with paraffin wax PCM. The tests were performed in April and May 2023 in Mosul City/Iraq under standard conditions for around 11 hours during the day. The outcomes indicated that a rise in air mass flowrate leads to a rise in the receiver performance. The findings confirm the thermalefficiency of 64.3% for 0.0174kg/s. For constant air flowrate, the performance of involute shape by A significant inlet temperature change has been found in CPC. It is demonstrated that phase change material and the position of the receiver have considerable influence on the model performance. The current work provides important information for evaluating the CPC model performance for Mosul/city.

Advancements in phase change materials for energy-efficient building construction: A comprehensive review

Solar- (Thermoelectric Generator) TEG hybrid system has recently become the most widely studied and developed system. This paper focuses on the creation of solar-TEG hybrid systems and thoroughly assesses recent developments in solar-TEG hybrid energy technology. The study investigates the integration of concentrated solar thermal systems and photovoltaic (PV) cells with thermoelectric generators (TEGs). Studies on hybrid systems integrating concentrated solar units with TEG units have demonstrated encouraging outcomes in increasing energy conversion efficiency. Furthermore, it has been noted that phase change materials (PCM) and appropriate cooling systems are crucial for optimizing the performance of concentrated solar collector (CSC)-TEG or CSC-PVT-TEG systems. The potential of concentrated solar power (CSP) technology in areas with high solar irradiation is also covered in the study, focusing on the significance of connectivity opportunities and efficient energy generation. The current study considered the most recent development in solar-TEG hybrid systems, the results highlight how important new ideas and cutting-edge technology are to expanding the field of solar energy use for sustainable energy solutions.

Building heating applications with phase change material: A comprehensive review