PCM-based hybrid thermal management system for photovoltaic modules: A comparative analysis

Proper temperature regulation of photovoltaic (PV) modules increases their performance. Among various cooling techniques, phase change materials (PCMs) represent an effective thermal management route, thanks to their large latent heat at constant temperatures. Radiative cooling (RC) is also recently explored as a passive option for PV temperature regulation. In this paper, a heat sink (HS), phase change materials, and radiative cooling are integrated with photovoltaic modules to achieve low and uniform temperature distribution along the PV module and improved performance. Eight different combinations are considered for the proposed system, including HS, PCM, and RC, and their various combinations. The PCM is selected according to the environmental conditions of the selected location. A comprehensive 2-D model is developed and analyzed in COMSOL-Multiphysics software by solving the governing equations using the finite element method. The performance analysis is carried out for the climatic conditions of the Atacama Desert, having high solar radiation and ambient temperature. The effects of PCM height, ambient temperature, wind velocity, and solar radiation on the performance of the proposed system are studied. The performance of eight different configurations is also compared. The maximum reductions in PV temperature, maximum PV power, and a minimum drop in PV conversion efficiency are observed to be 22 ^oC, 152 W, and 14% using a combined heat sink and radiative cooling systems, among all other configurations. The findings of this study can be used to select the best PV cooling method among different configurations.

Application of heat sinks inside the pyramid solar distiller: experimental study on distiller performance under various operating conditions

Solar still is a cheap decentralized mode for obtaining potable water from saline water using solar energy, but it has low productivity. Previous studies showed that using pin fins in the absorber can increase the solar still efficiency and yield. The heat sink has better heat transfer properties than those of the pin fin because it has a higher surface area to volume ratio than that of the pin fin. The current study investigates the effect of heat sinks in passive pyramid solar still under two water depths (11 and 30 mm) on the hourly yield, accumulated yield, and efficiency of the distiller. Three cases were compared: conventional solar still (CSS), regular distribution of heat sinks (DHS), and grouped heat sinks in the middle of the solar still basin (GHS). In addition, the effect of atmospheric parameters such as solar radiation, UV index, humidity, dry bulb temperature, and ambient temperature on the solar still parameters such as water, vapor, cover, and feedwater temperature is investigated. Unexpectedly, heat sinks weakly affect the performance of solar still. Therefore, the conclusion of the previous studies that the finned absorber has a favorable impact on the solar still performance is not general and needs further investigation. The highest thermal efficiency was observed for the DHS and GHS at 30-mm water depth, where the efficiency was 35%.

Numerical Analysis of Microchannels Designed for Heat Sinks

Conjugate heat transfer is numerically investigated using a three-dimensional computational fluid dynamics approach in various microchannel geometries to identify a high-performance cooling method for piezoelectric ceramic stacks and spindle units in high-precision machines. Straight microchannels with rectangular cross sections are first considered, showing the performance limitations of decreasing the size of the microchannels, so other solutions are needed for high applied heat fluxes. Next, many microchannel designs, focusing on streamwise geometric variation, are compared to straight channels to assess their performances. Sinusoidally varying channels produce the highest heat transfer rates of those studied. Thus, their optimization is considered at a channel width and height of 35 and 100 μm, respectively. Heat transfer increases as the amplitude and spatial frequencies of the channels increase due to increased interfacial surface area and enhanced Dean flow. The highest performance efficiencies are observed at intermediate levels of amplitude and frequency, with efficiency decreasing as these geometric parameters are increased further at the onset of flow separation. The sinusoidal channel geometries are then optimized with respect to minimizing the system's pressure drop for all applied heat fluxes between 5690 and 6510 kW/m². Doing so created an optimal geometry curve and showed that all geometries in this region had amplitudes close to 40 μm. Therefore, imposing a fixed heat flux requirement for a case study of cooling piezoelectric ceramics, the optimized sinusoidal geometry decreases the system pressure drop by 79% relative to a straight channel while maintaining a larger minimum feature size.

Heat sink effect of underwater polypectomy in a porcine colon model

BACKGROUND

Underwater polypectomy without the need for submucosal injection has been reported. A heat-sink effect by immersing the polyp in water was proposed but no such experiment has been performed to support the claim. We compared the temperature rise on the serosal side during polypectomy between air- and water-filled colon.

METHOD

Freshly harvested porcine colons were placed in a metal tray with cautery electrode pad attached to its bottom. An upper endoscope was used with a cap and a rubber band mounted to the distal end. A mucosal site was randomly selected and identified on its serosal surface with a marker while suction was applied. Suction was applied again and a ligation band was applied to create a polyp. A cautery snare grasped the artificial polyp just below the band. An assistant placed the tip of a thermometer at the marked site on the serosal surface to record the baseline temperature before cautery and the highest temperature during polypectomy. Seven polypectomies in air and underwater were performed.

RESULTS

Mean (standard deviation) baseline temperature were 23.3 (0.6) °C and 23.4 (0.6) °C in the air and water groups, respectively. The maximum rise in temperature during polypectomy was 6.1 (4.5) °C and 1.4 (1.0) °C in the air and water groups, respectively (P = 0.004).

CONCLUSIONS

The maximum temperature rise during polypectomy was significantly less when polypectomy was performed underwater, supporting the hypothesis that a heat-sink effect does exist during underwater polypectomy.

Accessory to dissipate heat from transcranial magnetic stimulation coils

BACKGROUND

Transcranial magnetic stimulation (TMS) produces magnetic pulses by passing a strong electrical current through coils of wire. Repeated stimulation accumulates heat, which places practical constraints on experimental design. New method: We designed a condensation-free pre-chilled heat sink to extend the operational duration of transcranial magnetic stimulation coils.

RESULTS

The application of a pre-chilled heat sink reduced the rate of heating across all tests and extended the duration of stimulation before coil overheating, particularly in conditions where heat management was problematic. Comparison with existing method: Applying an external heat sink had the practical effect of extending the operational time of TMS coils by 5.8-19.3 minutes compared to standard operating procedures.

CONCLUSION

Applying an external heat sink increases the quantity of data that can be collected within a single experimental session.

Evaluation of the Heat Sink Effect After Transarterial Embolization When Performed in Combination with Thermal Ablation of the Liver in a Rabbit Model

PURPOSE

To assess the contribution of the heat sink effect when combining thermal ablation with transarterial embolization (TAE).

MATERIALS AND METHODS

Radiofrequency ablation (RFA) or microwave ablation (MWA) were performed in the liver of non-tumor bearing rabbits. Three perfusion groups were used: rabbits that were killed then immediately ablated (non-perfused liver group to simulate embolized tumor with no heat sink), rabbits that underwent hepatic TAE followed by ablation (embolized liver group), and rabbits that underwent ablation while alive (normally perfused liver control group). For each perfusion group, 8 RFAs and 8 MWAs were performed. Probes were inserted using ultrasound guidance to avoid areas with major blood vessels. During ablation, temperatures were obtained from a thermocouple located 1 cm away from the ablation probe to assess heat conduction. With MWA, temperatures were also measured from the antennae tip.

RESULTS

For RFA, embolization of normal liver did not increase temperature conduction when compared to the control group. However, temperature conduction was significantly increased in the non-perfused group (simulating embolized tumor) compared to controls (p = 0.007). For MWA, neither embolization nor non-perfusion increased temperature conduction compared to controls. With MWA, the probe tip temperature was significantly higher in the non-perfused group compared to the control and embolized group.

CONCLUSIONS

In non-perfused tissue simulating tumor, RFA demonstrated modest enhancement of temperature conduction, whereas MWA did not. Embolization of normal liver did not affect RFA or MWA. Findings suggest that heat sink mitigation plays a limited role with combination embolization-ablation therapies, albeit more with RFA than MWA.

Minimal vascular flows cause strong heat sink effects in hepatic radiofrequency ablation ex vivo

BACKGROUND

The present paper aims to assess the lower threshold of vascular flow rate on the heat sink effect in bipolar radiofrequency ablation (RFA) ex vivo.

METHODS

Glass tubes (vessels) of 3.4 mm inner diameter were introduced in parallel to bipolar RFA applicators into porcine liver ex vivo. Vessels were perfused with flow rates of 0 to 1,500 ml/min. RFA (30 W power, 15 kJ energy input) was carried out at room temperature and 37°C. Heat sink effects were assessed in RFA cross sections by the decrease in ablation radius, area and by a high-resolution sector planimetry.

RESULTS

Flow rates of 1 ml/min already caused a significant cooling effect (P ≤ 0.001). The heat sink effect reached a maximum at 10 ml/min (18.4 mm/s) and remained stable for flow rates up to 1,500 ml/min.

CONCLUSIONS

Minimal vascular flows of ≥1 ml/min cause a significant heat sink effect in hepatic RFA ex vivo. A lower limit for volumetric flow rate was not found. The maximum of the heat sink effect was reached at a flow rate of 10 ml/min and remained stable for flow rates up to 1,500 ml/min. Hepatic inflow occlusion should be considered in RFA close to hepatic vessels.