## 1 Introduction
(2016) also looked at the impact of spreading in a PCM module, combining it with flat heat pipes [17]; the additional thermal spreading reduced the module's temperature. While promising, these studies were done with a simple experimental apparatus that does not fully capture the complex internal heat transfer of a handheld electronic device. [PERSON] et. al. (2019) [18] used a numerical model of a tablet computer to investigate the impact of the transition temperature of a PCM on the performance of the LHTES module. That study used a comparative analysis to show that a lower transition temperature is most beneficial for this application. Additionally, that study investigated how heat is transferred within the tablet. However, there were aspects of this model that could not replicate the physics of a real tablet computer. Predominate among these were simulation of contact resistance between the components within the tablet and accurately modeling the convection from the surface of the tablet. That study showed that experimental investigations of LHTES modules within an experimental apparatus representative of actual handheld electronic devices are required. Such studies have been attempted with older, larger handheld electronic devices. [PERSON] et al. (2002) [19] experimentally tested the use of a LHTES module in a telephone handset. [PERSON] and [PERSON] (2003) [15] investigated a LHTES module in a mock wearable computer. However, the LHTESs used in both studies were thicker than an entire modern mobile device. [PERSON] et al. (2016) [20] studied the integration of a LHTES unit into the temperature control system of a mock handheld computing device that was close to the size of a modern device (122 mm \(\times\) 62 mm \(\times\) 14 mm). This mock device consisted of a case made from 1 mm sheets of acrylic (rear) and a 1 mm polycarbonate sheet (front and side). An electric ceramic heater was mounted to simulate a printed circuit board (PCB). The LHTES unit consisted of a microencapsulated paraffin powder (\(T_{m}\) = 32 \({}^{\circ}\)C) that was mixed (50% by weight) with polyethylene (PE). This mixture of microencapsulated PCM and PE was cast into sheets that were 25 mm \(\times\) 25 mm \(\times\) 4 mm and 50 mm \(\times\) 50 mm \(\times\) 1 mm. Melting experiments were completed with a steady power input of 2 W. They found that the sheetsdelayed the rise in the system's temperature and reduced the system's steady-state temperature. The use of a mock handheld device and LHTES module that could fit into a modern handheld device makes this study particularly relevant. However, the mock device was only a basic representation of a modern device and did not contain a battery or display. Additionally, when the microencapsulation and shape stabilization materials are accounted for, the total thermal energy storage of the LHTES used was small. [PERSON] et al. [5; 6] presented the first published work employing a real mock-up tablet PC, an experimental rig built on a modified Dell Venue tablet computer. The main PCB of the tablet was removed and replaced with a PCB constructed of FR4 copper-clad board. A polyimide heater was used to simulate the heat generated by the processor. Several LHTES storage modules consisting of PCM sealed in pouches manufactured from heat-sealable aluminized film were tested. Both \(n\)-eicosane (\(T_{m}=35.6\)\({}^{\circ}\)C) and a commercial PCM called PT-37 (\(T_{m}=36.4\)\({}^{\circ}\)C) were tested. Analysis of the tablet computer's temperature history showed that the LHTES modules delayed the temperature increase in both the heater and the cover of the tablet computer. However, it was also shown that many of the modules did not melt completely during the experiment and that the lack of a heat spreader was detrimental to their performance. While promising, the lack of a heat spreader makes it difficult to truly judge the utility of the LHTES module from that study. Additionally, the manufacturing technique used to create the LHTES modules resulted in uneven thicknesses.