Abstract
Commercial paraffin wax used by Bolsius Nederland B.V. for manufacturing various kinds of candles was applied as a phase-change material (PCM) for cooling a 28 W high-power light emitting diode (LED) panel during its operation. The main problem arising during operation of an LED is thermal management. According to the manufacturer’s datasheet specifications (BioSolution Ltd. www.biosolution.pl), the operating temperature range for the LED street lamp UL28W is (-30~{^{circ }}hbox {C}) to (+40~{^{circ }}hbox {C}). The object of the present study was an LED panel containing 28 pieces of high-power 1W LEDs connected in series (4 LEDs in each of the 7 rows) mounted on an aluminum plate of dimensions 80 mm by 135 mm. The tested aluminum plate was placed in a block made of aluminum with a hollow compartment containing Bolsius paraffin wax of density 914~hbox {kg}cdot hbox {m}^{-3} at room temperature. Temperatures were recorded using K-type thermocouples at selected locations of the tested LED panel for several values of the power supplied to it, while utilizing PCM and without it. As the manufacturer of Bolsius wax candles does not provide any data on the thermal properties of the material used, it was necessary to carry out micro-calorimetric research. Thermophysical properties of the paraffin wax such as the apparent specific heat, enthalpy of phase transition and temperature of phase change transition during heating and cooling were determined using the Netzsch DSC 214 Polyma. The Netzsch TG 209F3 Tarsus was used for TG/DTG measurements. DSC investigations revealed the following thermal transitions taking place during the first heating: solid–solid transition (onset 30.4~{^{circ }}hbox {C}, peak at 40.9~{^{circ }}hbox {C}), solid–liquid transition (onset 47.7~{^{circ }}hbox {C}, peak at 54.9~{^{circ }}hbox {C}, end at 58.3~{^{circ }}hbox {C}), latent heat of energy storage 201~hbox {J}cdot hbox {g}^{-1}, apparent specific heat corresponding to peak at 41.5~{^{circ }}hbox {C}(5.498~hbox {J}cdot hbox {g}^{-1}cdot hbox {K}^{-1}). DTG investigations revealed that the decomposition of paraffin wax is a two-step process. At 283~{^{circ }}hbox {C} there was observed a slightly slower decomposition (9.43,%cdot hbox {min}^{-1}) than that at 323~{^{circ }}hbox {C} (12.5,%cdot hbox {min}^{-1}). The experimental results obtained upon cooling the high-power LED lamp during its operation can be applied to verify results of numerical modeling of the heat transfer problems with phase-change transitions. An attempt at modeling such a problem based on 1D fixed grid with variable time step approach was undertaken in this work.
Highlights
The light emitting diode is a semiconductor device that converts electrical energy into light as a result of the radiative recombination of carriers in the p–n junction polarized in the forward direction
Evaporation under HR = 10 K · min−1 of Bolsius paraffin wax shown in Fig. 3 proves that its decomposition is a two-step process
The objective of the undertaken experiments was to investigate a possibility of utilizing a cheap paraffin wax used by Bolsius to manufacture various kinds of candles as a potential PCM for cooling high-power LED lamps
Summary
The light emitting diode is a semiconductor device that converts electrical energy into light as a result of the radiative recombination of carriers (electron and hole) in the p–n junction polarized in the forward direction. LEDs have great potential to replace traditional illumination sources like sodium vapor lamps, incandescent light bulbs or compact fluorescents This is because of their higher luminous efficacy (20– 150 lm · W−1) than that of incandescent bulbs (10–20 lm · W−1) [1], small size, slow failure, shock resistance, relatively long useful lifetime of 35 000 h–50 000 h or more [2]. Their main drawbacks are low thermal conductivity values (k ∼ 0.2 W · m−1 · K−1) [6], which may reduce the rate of heat storage, and extraction during the melting/solidification cycles and a potential risk of material ignition These drawbacks, can be limited by using metallic fillers, a finned tube metal matrix [6], nano-silicon dioxide [7] and the application of a fire-barrier-rated container
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