Cooling Systems using combined Water plus Phase Change

Abstract: The most promising technique for reducing the weight of cooling systems for burst-mode thermal loads is by reducing the chiller size. Addition of thermal storage provides the means to achieve this weight reduction. The heat reservoir must be designed to absorb high peak thermal powers as well as having a large energy storage capacity. Storing energy as phase-change latent-heat, rather than as temperature-change sensible-heat, results in a significant increase in storage capacity. Distributing the heat through a large volume is achieved by imbedding the phase-change-material (PCM) in a porous, high thermal-conductivity matrix. When the heat load is cyclical, even higher peak power capabilities can be achieved by first absorbing the heat in a small quantity of water and then transferring it to the PCM between heating pulses. We developed high-power, high-energy dual-material heat-reservoirs consisting of graphite-porous plates impregnated with paraffin PCM combined with water in order to cool bursts of thermal pulses. The bursts consisted of multiple, short, high-power heating pulses followed by longer interpulse periods. Bursts were followed by longer recovery period during which the small chiller extracted energy from the Paraffin. Measurements were made to characterize the cooling performance of water-PCM graphite reservoirs under continuous and burst thermal loads, and they were compared to a theoretical model. The main conclusions are that the measurements agree well with the model and that substantial weight reduction of burst-mode cooling-systems can be achieved by using the water plus PCM graphite heat-reservoir. Key-Words: PCM, thermal storage, cooling, graphite, bursts