A modern computer generates a great amount of heat while working. In order to secure appropriate working conditions by extracting the heat, a specific mechanism should be used. This research paper presents the effect of nanofluids on the microchannel heat sink performance of computer cooling systems experimentally. CeO₂, Al₂O₃ and ZrO₂ nanoparticles suspended in 20% ethylene glycol and 80% distilled water are used as working fluids in the experiment. The concentration of the nanoparticles ranges from 0.5% to 2%, mass flow rate ranges from 0.028 kg/s to 0.084 kg/s, and the ambient temperature ranges from 25 °C to 40 °C. Regarding the thermal component, parameters such as thermophysical properties of the nanofluids and base fluids, central processing unit (CPU) temperature, heat transfer coefficient, pressure drop, and pumping power have been experimentally investigated. The results show that CeO₂-EG/DW, at a concentration of 2% and a mass flow rate of 0.084 kg/s, has with 8% a lower temperature than the other nanofluids and with 29% a higher heat transfer coefficient compared with the base fluid. The Al₂O₃-EG/DW shows the lowest pressure drop and pumping power, while the CeO₂-EG/DW and ZrO₂-EG/DW show the highest. However, a slight increase of pumping power and pressure drop can be accepted, considering the high improvement that the nanofluid brings in computer cooling performance compared to the base fluid.

The paper presents a literature review regarding defrosting methods for household refrigeration and heat pumps. The literature review covers the period between 2008 and 2020. A number of 61 papers have been studied. The literature review was conducted by dividing the defrost methods into passive and active. Passive methods involve surface coating and off-cycle. The active defrost methods involve reverse cycle, ultrasonic vibration, hot–gas defrost, electric heating and hot fluid. Surface coating and ultrasonic vibration are recent defrost methods proposed in the literature. Also, recently, phase change materials have been used in household refrigerators to improve their efficiency. This is a topic that needs further investigation and is a subject of interest as it can lead to lower energy consumption during defrost cycles. From the present study, the research direction regarding the use of PCMs in defrost cycles can be highlighted.

Wastewater treatment plants traditionally dispose of sludge using the method of landfilling and incineration, with both being carbon-intensive and environmentally harmful. Converting sludge into energy or reusable materials avoids landfills or incineration, helping reduce the volume of waste and associated pollution. Sludge treatment with energy recovery can offset fossil fuel use, further reducing the carbon footprint of sewage treatment processes. This research explores ways to recover energy from sewage sludge, a byproduct of wastewater treatment that is often considered waste. Transforming sludge into valuable resources aligns with the principles of the circular economy, where waste streams are repurposed, minimizing environmental impact and enhancing resource efficiency. In this paper, a method is presented to reduce the volume of wastewater sludge by drying it in a hot flue gas stream at 700 °C. The energy of the exhaust gas is recovered in an organic Rankine cycle system, which powers the wastewater treatment facilities themselves, making them more self-sustaining.

The paper aims to investigate how to improve the performance of a refrigeration system (Rs) that equips a cold room, by incorporating phase change materials (Phase Change Materials - PCMs) in these systems, a study that has not yet been extended experimentally. The study is carried out on a cold room within the National University of Science and Technology Politehnica Bucharest - Faculty of Mechanical and Mechatronics Engineering, Department of Thermodynamics, Engines, Thermal and Refrigeration Equipment. This room is equipped with a refrigeration system with mechanical vapour compression (VCRs), which uses R404A as refrigerant. Mechanical vapour compression refrigeration system (VCRs) with an evaporation temperature below 0 oC causes ice to form on the evaporator leading to reduced performance. Currently, the widely used methods for defrosting are the standard methods, the most used being the electric one, which of course consumes energy. This paper aims to evaluate the availability of heat that could be used in the defrosting process by means of PCMs. The study was made using the Engineering Equation Solver software, several types of PCMs and also different refrigerants (R600a, R600, R1234yf, R1234ze, R152a, R290, R32) and in this way it was intended to identify the right agent to be used for a particular type of PCM. Faculty of Mechanical and Mechatronics Engineering, Department of Thermodynamics, Engines, Thermal and Refrigeration Equipment

To improve the efficiency of a diesel internal combustion engine (ICE), the waste heat carried out by the combustion gases can be recovered with an organic Rankine cycle (ORC) that further drives a vapor compression refrigeration cycle (VCRC). This work offers an exergoeconomic optimization methodology of the VCRC-ORC group. The exergetic analysis highlights the changes that can be made to the system structure to reduce the exergy destruction associated with internal irreversibilities. Thus, the preheating of the ORC fluid with the help of an internal heat exchanger leads to a decrease in the share of exergy destruction in the ORC boiler by 4.19% and, finally, to an increase in the global exergetic yield by 2.03% and, implicitly, in the COP of the ORC-VCRC installation. Exergoeconomic correlations are built for each individual piece of equipment. The mathematical model for calculating the monetary costs for each flow of substance and energy in the system is presented. Following the evolution of the exergoeconomic performance parameters, the optimization strategy is developed to reduce the exergy consumption in the system by choosing larger or higher-performance equipment. When reducing the temperature differences in the system heat exchangers (ORC boiler, condenser, and VCRC evaporator), the unitary cost of the refrigeration drops by 44%. The increase in the isentropic efficiency of the ORC expander and VCRC compressor further reduces the unitary cost of refrigeration by another 15%. Following the optimization procedure, the cost of the cooling unit drops by half. The cost of diesel fuel has a major influence on the unit cost of cooling. A doubling of the cost of diesel fuel leads to an 80% increase in the cost of the cold unit. The original merit of the work is to present a detailed and comprehensive model of optimization based on exergoeconomic principles that can serve as an example for any thermal system optimization.

The paper identifies and describes the possibilities for heat and mechanical energy recovery within refrigeration systems using CO₂ as a working fluid, employed in commercial and industrial applications. The heat and mechanical energy recovery methods that can be utilized for beneficial purposes are taken into consideration. These methods could increase the energy efficiency of the refrigeration system or the building in which it operates. This paper summarizes various configurations and recovery methods and critically compares and evaluates them (COP improvements, exergy performance, and system integration complexity) based on the data available in the literature. As a result, the internal heat exchangers can be used as a superheater, in which case the COP can increase to 35%. If the internal heat exchanger is used as a subcooler, it could lead to a COP increase of 17% compared to a CO₂ refrigeration system without subcooling for an evaporating temperature of −10 °C and the temperature of the gas cooler outlet of 30 °C. The heat and mechanical energy recovery possibilities are presented using the available scientific literature.