Research on Powering a Personal Heating Garment with a Hybrid Power Supply

Due to the extensive contribution of the fossil fuels’ combustion to the global warming, increasing attention has been drawn to reduce the carbon footprint of all sectors by moving toward green technologies. Accordingly, personal comfort systems (PCSs) are considered as a green substitute for the conventional heating, ventilation, and air conditioning (HVAC) systems in the buildings. Precisely, PCSs can address the thermal comfort of each individual in shared spaces, such as office buildings, with remarkably lower energy consumption than the HVAC systems. To clarify, they save energy by conditioning only the air surrounding each individual, leaving the unoccupied points of the buildings unconditioned. However, their main drawback is that they are mostly powered either by a fossil fuel based power supply (i.e. public electricity) or batteries, which to some extent offsets their eco-friendly feature. Regarding batteries, although they are not considered as a fossil fuel based power supply, they can cause negative environmental impact due to their constitutive elements (e.g. cobalt).

Accordingly, in this thesis, it was proposed to develop a hybrid power supply comprising a renewable power source called a thermoelectric generator (TEG) and a rechargeable battery to reduce the carbon footprint of the PCSs. To illustrate, a TEG is considered as a promising thermal energy harvester, which captures the waste ambient heat and converts it into electricity. To put it another way, it employs the temperature difference between a hot and a cold object to generate electricity. In this research, this temperature difference was provided by a candle and the ambient air as the heat and cold sources, respectively. The main reason for using a candle as the heat source was that at room temperature none of the existing internal heat sources (e.g. radiators, hot pipe works) could provide the required temperature difference along the thermoelectric (TE) legs. As a result, the TE legs could not generate enough electricity to power a PCS. Regarding the rechargeable battery, it mainly served as a storage system to store the generated electricity by the TEG to release it on demand. In fact, this storage system performed a pivotal role between the TEG (as a green energy supply) and the electricity demand. To clarify, the generated electricity by a TEG is variable, depending on the temperature difference along its legs. Thus, to provide a reliable power for the studied PCS, usage of a storage system was inevitable. In addition, due to the low output current of the TEGs at the room temperature, the storage system also performed as a backup power supply to provide a constant current for the PCS.

To develop the TEG part of the hybrid power supply, initially the life cycle impact of 14 thermoelectric materials were numerically studied from cradle to gate (i.e. raw material’s extraction to manufacturing the final material). The main aim of this numerical study was to select the most environmental friendly material out of the three TE types of inorganic, organic, and hybrid. The employed software for this life cycle impact assessment (LCIA) was GaBi v.4.4, and the results proved the greatest environmental damage of the inorganic type compared with the other two types. However, Bi2Te3 was an exceptional inorganic TE material that its life cycle impact was not only less than that of the other studied inorganic TE materials, but also far less than that of the studied organic and hybrid ones.

According, an off-the-shelf TEG consisted of Bi2Te3 was selected for this research, and different integrating patterns were determined to couple six of that together. These patterns were considered based on providing enough temperature difference along the TE legs. Accordingly, three different patterns were developed namely, coating either the top half, or the bottom half, or fully the legs with the integrating substrate. To further improve the temperature difference along the legs, the thermal conductivity of the integrating substrate was manipulated by adding different fillers to it. Then, the thermoelectric effects of all of the developed samples were studied both numerically (i.e. computational modelling by COMSOL) and experimentally (i.e. laboratory tests). Based on the outcomes of both studies, coating only the top half of the legs with a low thermal conductivity substrate called polydimethylsiloxane (PDMS) resulted in the highest output power compared with the other two patterns. Accordingly, the TEG part of the hybrid power supply was fabricated with respect to the achieved optimal integrating substrate. Then, to further improve the temperature difference along the TE legs, two different heatsinks (pin-shaped and bumpy-shaped) were developed, and their performances were studied in the laboratory under three air flow conditions (0,1.2, and 2.4 m/s). The results proved the superiority of the pin-shaped heatsink to the bumpy-shaped one after being attached to the PDMS-based integrating substrate.

Next, six of the considered off-the-shelf TEG were coupled together from the top half of their legs using the optimal developed PDMS-based integrating substrate. After that, the pre-developed pin-shaped heatsink was attached to the integrating substrate. Then, the integrated TEGs were coupled with a rechargeable battery to heat a pair of pre-developed heating armbands (i.e. the studied PCS). To test the heating performance of the armbands, they were tested in-field on eight subjects served as office workers in the heating season of March in the UK. The results revealed that at roughly 15 °C to 16 °C ambient temperature, 75% of the subjects felt warmer after 30 min of wearing the armbands. To specify, the armbands improved their average comfort level from slightly uncomfortable to slightly comfortable after 30 min of wearing them. Thus, not only this thesis developed a PCS concerning its conditioning performance, but also it addressed the sustainability aspect of its power supply. Accordingly, this thesis paves the way for further research on developing different types of renewable power supplies for any green technologies, including PCSs.