Medium Temperature Systems Research Articles

Quantifying thermophysical properties, characterization, and thermal cycle testing of nano-enhanced organic eutectic phase change materials for thermal energy storage applications

Dispersion of highly conductive nanoparticles in Phase Change Materials (PCMs) tends to improve the thermophysical properties of nanocomposites. The current research condenses the synthesis, chemical, physical, and thermal characterization of novel nano-enhanced eutectic phase change materials (NeUPCMs) dispersed with TiO 2 nanofillers for thermal management applications. The base matrix primarily comprises of a eutectic of paraffin wax and palmitic acid. Detailed analysis of the uncertainty of each thermophysical property measured was performed. The synthesized nanocomposite logged a maximal thermal conductivity of 0.59 W/mK (2.3-fold as compared with the base-0.25 W/mK) with 0.5% nanofillers. The composites displayed excellent solar transmissivity (82%) as they were doped with nanofillers having a high refractive index. The latent heat of the NeUPCMs got enhanced by 17% whereas the melting point showed a slight decrement in nanocomposites. Further, zero phase segregation, no subcooling, stable phase transition temperature, and good chemical, and thermal stability were noted from digital scanning calorimetry results with NeUPCMs. The composites exhibited good thermal reliability beyond 500 thermal cycles. It could be potentially deployed in the thermal management of medium-temperature systems like PVT and LCPVT systems. • Novel nanocomposites were synthesized using paraffin wax, palmitic acid, and TiO 2 . • Nanocomposite had a maximal enhancement of 131.5% in thermal conductivity. • 38% rise in solar transmittance when compared with the eutectic base. • Thermophysical properties remained stable even after 500 thermal cycles.

Thermal performance of nanomaterial in solar collector: State-of-play for graphene

Several studies have performed various nanomaterials for the dispersion of working fluids, including copper oxide, alumina, and silica, and so forth in solar collectors. In this present study thermal performance of graphene nanofluid and hybrid, nanofluid has been considered mostly. The thermo-physical properties of the working fluid are significantly enhanced and the performance of solar collectors is therefore improved. This study summarizes the performance of different solar collectors using carbon-based nanofluids especially graphene. The influence of nanofluid concentration, temperature, and flow rate on the collector efficiency of the solar collectors was highlighted. Using graphene thermal conductivity increase by 17% sharp and using graphene derivatives develop almost a 40% increase. Viscosity enhances almost 50% by using graphene nanoparticles. By the way, graphene has stable properties, cheap in price, using a fluid flow mechanism in thermal collector graphene nanoparticles gives an excellent result. The most important and highlighted inherent properties of graphene are its contact angle which is responsible for conductivity. Higher surface area to volume production improves the heat transfer capacity of graphene nanofluids. In this review, the article elaborates on the graphene and graphene-based nanofluid superiority in the thermal solar collector. Graphene nanofluids perform a crucial role in the development of newer technologies ideal for thermal solar systems. This advanced type of graphene nano liquid has a strong potential for use in medium-temperature systems, for example, solar collectors, as innovative heat transfer fluids.

Superior room-temperature power factor in GeTe systems via multiple valence band convergence to a narrow energy range

GeTe systems are excellent p-type thermoelectric materials at medium temperatures (500–800 K). Recently, their electronic properties were enhanced by band convergence of L and Σ valence bands, which increased the power factor at 500–800 K. However, there are only a few thermoelectric materials suitable for lower-temperature waste-heat recovery applications, and their demand is rapidly increasing. Herein, we demonstrated that the well-known L and Σ bands of GeTe and the additional Δ band converge at a desirable energy level to a very narrow energy range as a result of fine-tuning the crystal symmetry and composition by forming a GeTe-Sb2Te3 alloy. Alloying additionally facilitates a decrease in the bandgap, thereby increasing the near-room-temperature power factor. The GeTe-Sb2Te3 alloy achieved very high room-temperature and maximum power factors of 2.6 mW m−1 K−2 at 300 K and 3.3 mW m−1 K−2 at 423 K, respectively, despite GeTe being a medium-temperature thermoelectric material. Thus, we confirmed the applicability of GeTe as a thermoelectric material at room temperature. This study facilitates the development of high-performance near-room-temperature thermoelectric materials based on existing medium-temperature systems.

Energy and exergy analysis on hydrofluoroolefin/ hydrofluorocarbon (HFO/HFC) refrigerant mixtures in low and medium temperature small-scale refrigeration systems

The new refrigerants such as HFO-1234yf and HFO-1234ze have been considered as long-term replacements for HFC-134a to comply with the Kyoto protocol. In small size refrigeration systems, these refrigerants have poor performance than HFC-134a besides their minor flammability. Previous studies reveal that the addition of HFC-134a in small quantity can improve the performance of a system without exceeding the global warming potential limit (150) prescribed by the European Union. However, this concept is not studied in small-scale refrigerators. Therefore, the performance of low and medium temperature systems of capacity 190 and 285 litre, respectively, working with different hydrofluoroolefin/ hydrofluorocarbon (HFO/HFC) mixtures, has been studied by mathematical simulation. The software tool ‘MATLab’ and the refrigerant property database ‘REFPROP’ have been used in the simulation. The results showed that the coefficient of performance (COP) of Hydrofluoroolefin (HFO) refrigerants and its mixtures decreases from 2.4 to 15.7% than that of HFC-134a. Among the mixtures, HFO-1234ze/HFC-134a (90/10) shows better performance and its predicted coefficient of performance (COP) and exergetic efficiency are 4–8.3% and 5.1–10.5%, respectively, higher than that of other mixtures considered in this study.

Preparation, characterization, viscosity, and thermal conductivity of nitrogen-doped graphene aqueous nanofluids

Nanofluids perform a crucial role in the development of newer technologies ideal for industrial purposes. In this study, Nitrogen-doped graphene (NDG) nanofluids, with varying concentrations of nanoparticles (0.01, 0.02, 0.04, and 0.06 wt%) were prepared using the two-step method in a 0.025 wt% Triton X-100 (as a surfactant) aqueous solution as a base. Stability, zeta potential, thermal conductivity, viscosity, specific heat, and electrical conductivity of nanofluids containing NDG particles were studied. The stability of the nanofluids was investigated by UV–vis over a time span of 6 months and concentrations remain relatively constant while the maximum relative concentration reduction was 20 %. The thermal conductivity of nanofluids was increased with the particle concentration and temperature, while the maximum enhancement was about 36.78 % for a nanoparticle loading of 0.06 wt%. These experimental results compared with some theoretical models including Maxwell and Nan’s models and observed a good agreement between Nan’s model and the experimental results. Study of the rheological properties of NDG nanofluids reveals that it followed the Newtonian behaviors, where viscosity decreased linearly with the rise of temperature. It has been observed that the specific heat of NDG nanofluid reduced gradually with the increase of concentration of nanoparticles and temperature. The electrical conductivity of the NDG nanofluids enhanced significantly due to the dispersion of NDG in the base fluid. This novel type of fluids demonstrates an outstanding potential for use as innovative heat transfer fluids in medium-temperature systems such as solar collectors.

Energy performance of supermarket refrigeration and air conditioning integrated systems working with natural refrigerants

The current trends in commercial refrigeration aim at reducing the synthetic refrigerant charge, either by minimising the internal volume of the circuit or by utilising natural refrigerants, and at energy saving.The energy efficiency of supermarkets can be improved by optimising components design, recovering thermal and refrigerating energy, adopting innovative technology solutions, integrating the HVAC system with medium temperature and low temperature refrigeration plants and, finally, reducing thermal loads on refrigerated cases. This study aims at investigating the performance of different lay-out and technological solutions where only natural refrigerants are used and at finding the potential for improving energy efficiency over the traditional systems in different climates. In the analysis, chillers and heat pumps working with ammonia or propane, medium temperature systems working with ammonia or propane and carbon dioxide as heat transfer fluid or with carbon dioxide as the refrigerant and low temperature systems working with carbon dioxide are considered and benchmarked with a state-of-the-art HFCs based plant. The most efficient investigated solution enables an annual energy saving higher than 15% with respect to the baseline solution for all the considered climates.

COMPARATIVE ANALYSIS OF SECONDARY CO2 SYSTEMS AND WATER-BASED BRINES IN INDUSTRIAL AND COMMERCIAL REFRIGERATION APPLICATIONS

In this paper, a comparative study of CO2 and water-based brine secondary cooling systems for refrigeration applications is presented. CO2 secondary systems are simple and could prove to be attractive for high temperature (HT) and medium temperature (MT) in industrial and commercial refrigeration applications regardless of climate. Calculations show that CO2 secondary systems give a possibility of energy savings in the range of 10–20% for HT systems and more than 20–30% for MT systems. The design and particular challenges of CO2 systems are discussed. The analysis is illustrated by case studies. Further possibilities to optimized secondary CO2 systems by modulating pressure or applying frequency converters are discussed. The use of secondary CO2 systems in commercial refrigeration installations (supermarket systems) is briefly discussed.

Energy performance of supermarket refrigeration and air conditioning integrated systems

The electricity consumption for air conditioning and refrigerated cases in large supermarkets represents a substantial share of the total electricity consumption. The energy efficiency of supermarkets can be improved by optimising components design, recovering thermal and refrigerating energy, adopting innovative technology solutions, integrating the HVAC system with medium temperature and low-temperature refrigeration plants and, finally, reducing thermal loads on refrigerated cases. This study is aimed at investigating the performance of different lay-out and technological solutions and at finding the potential for improving energy efficiency over the traditional systems in different climates. In the analysis chillers and heat pumps working with R410A, medium temperature systems working with R404A and low-temperature systems working both with R404A and R744 were considered. The investigated solutions enable an annual energy saving higher than 15% with respect to the baseline solution for the considered climates.

Analysis on energy saving potential of integrated supermarket HVAC and refrigeration systems using multiple subcoolers

The paper presents a model-based analysis on the energy saving potential of supermarket HVAC (heating, ventilating, and air-conditioning) and refrigeration systems using multiple subcoolers among the high-temperature HVAC system, the medium-temperature refrigeration system, and the low-temperature refrigeration system. The principle of energy reduction is to have the higher COP (coefficient of performance) system generate more cooling capacity to increase the cooling capacity or reduce the power consumption of the lower COP system. The subcooler could be placed between the medium-temperature and low-temperature systems, between the high-temperature and medium-temperature systems, and between the high-temperature and low-temperature systems. All integration scenarios of adding one, two and three subcoolers have been investigated. The energy saving potential varies with the load ratio between high-, medium- and low-temperature systems, COP of three systems, and the “on-off” duty time of HVAC system. The optimal sequence of adding subcoolers is also proposed.

HFC134a/HC600a/HC290 mixture a retrofit for CFC12 systems

The environmental concerns with the impact of refrigerant emissions lead to the importance in identifying a long-term alternative to meet all requirements in respect of system performance and service. Even though HFC134a and HC blend (containing 55.2% HC600a and 44.8% HC290 by weight) have been reported to be substitutes for CFC12, they have their own drawbacks in respect of energy efficiency/flammability/serviceability aspects of the system. In this present work, experimental investigation has been carried out on the performance of an ozone friendly refrigerant mixture (containing HFC134a/HC blend) in two low temperature systems (a 165 l domestic refrigerator and a 400 l deep freezer) and two medium temperature systems [a 165 l vending machine (visi cooler) and a 3.5 kW walk-in cooler]. The oil miscibility of the new mixture with mineral oil was also studied and found to be good. The HFC134a/HC blend mixture that contains 9% HC blend (by weight) has better performance resulting in 10–30% and 5–15% less energy consumption (than CFC12) in medium and low temperature system, respectively.