## 1 Introduction  
The use of latent heat thermal energy storage (LHTES) modules for temperature control has been proposed for a variety of systems. These include spacecraft, handheld electronics, electric vehicles, and robotic systems [1, 2, 3, 4]. LHTES modules are useful for thermal management of systems that generate heat following an intermittent/variable profile. During periods in which heat generation rates are high, the PCM undergoes a phase transition, absorbing heat and slowing the system's temperature rise. Heat stored in the LHTES is subsequently released to the environment during periods with low heat generation rates. In systems with cyclic heating, this process is similar to peak shaving. Thermal management of handheld electronic devices such as smart phones and tablet computers presents a number of challenges. The temperature of the internal components must be kept within their operating range. Surface temperatures must be kept below the user's comfort threshold. Space availability and noise constraints make traditional active thermal management systems difficult to implement. However, it may be possible to leverage the intermittent operation of these devices with a LHTES thermal management module [5, 6]. Modern handheld electronic devices use thermal throttling to manage their temperature. If a component is approaching its temperature limit, the device reduces its operating speed in order to lower heat generation rates. The major goals of passive thermal management modules are to reduce the rate of temperature rise and avoid or delay performance drops due to thermal throttling. In addition, reducing the mean temperature of internal components can increase reliability and help prevent premature failures. The critical temperatures for internal components and the device surface can vary depending on a number of design choices. In this work, the handheld device will be considered to have overheated if an internal temperature is measured to exceed 80 \({}^{\circ}\)C or if the surface of the device exceeds 40 \({}^{\circ}\)C [7,8]. Much of the research on LHTES-based thermal management systems has been directed towards applications larger than would be suitable for handheld electronics. Many of these LHTESs can be characterized as hybrid heat sinks. In essence, a hybrid heat sink combines a finned heat sink with a LHTES by fully or partially filling the space between the fins with PCM. Systems of this type have been investigated in many different sizes and configurations, including plate fin [9, 10, 11], pin fin [12, 13, 14], and metal foam [15]. However, modern handheld devices are typically less than 1 cm thick, and for a LHTES to be viable in these systems, it would have to be a fraction of that size (only a few mm thick). Only a few research groups around the world have published on work performed on thin LHTESs. Fundamental work on the use of thin LHTES modules for thermal management was carried out by [PERSON] et al. (2019) [16]. Using a simplified setup, LHTES modules (100 mm \(\times\) 100 mm \(\times\) 2 mm) were experimentally tested. These modules were created by encapsulating PCM between two sheets of heat-sealed laminated film. Both \(n\)-eicosane and dodecanoic acid were tested. In the experiments, the LHTES module was placed in a cavity between an insulated Teflon base and a nylon cover. A copper heating block embedded in the base simulated the small discrete heat sources that are present in handheld electronic devices. The heat source was on (constant 5 W) for a period of 60 or 120 min, depending on the study, after which the sources were turned off and the package was allowed to cool to ambient temperature. The measured transient temperature histories showed that the LHTES module slowed the temperature rise of both the heating block and the nylon cover. However, due to its extreme aspect ratio, the LHTES module was much more effective when paired with an aluminum heat spreader, pointing to the fact that heat spreading is poor in such a laminated PCM LHTES. The study of [PERSON] et al.