(89d) Thermal Energy Storage System for Cold Engine Start up in Automobiles

Driving with a cold engine increases fuel consumption and greenhouse gas emissions in automobiles. Thermal Energy Storage (TES) devices could be utilized to provide heat during engine warm-up. A TES device could be charged from an engine’s waste heat during normal operation. In this work, a latent heat TES device has been designed, constructed and tested to investigate waste heat recovery and reduction of engine warm-up time by using phase change materials (PCMs) as a thermal energy storage (TES) medium. Two types of paraffin waxes with melting points of 70 and 83 oC, and a 50/50 mixture have been examined to characterize their behaviors under repetitive heating/freezing cycles. The paraffin was placed in a rectangular container and hot or cold water was circulated through thin cooling plates, which were placed in the container at regular spacing. Thin aluminum fins were also placed between the cooling plates to increase heat transfer inside the paraffin. In the experiments, temperature variations in the paraffin and at the water inlet and outlet were recorded for a period of five minutes. From the experimental results, the fins enhanced heat transfer for both narrow and wide plate spacing, but the enhancement was more significant for the wider plate spacing. The efficiency of heat recovery was also positively correlated to the high circulating flow rate of the cold water and the use of a 50/50 mixture of the two paraffin waxes. An overall heat transfer coefficient was evaluated from the heat transfer rate and the temperature difference between the average water and paraffin temperatures. The product of the overall heat transfer coefficient and plate surface area was found to nearly double by inserting the thin aluminum fins. In five minutes, the total heat recovered from 3.6 kg of paraffin ranged from 600 to 1,000 kJ, depending on the water flow rate and plate spacing. Finally, experiments were conducted to heat up an aluminum block weighing 30 kg which simulated a common engine block. The aluminum block was heated up from 0 oC by water circulating through multiple TES containers. In five minutes, the aluminum block’s temperature could be increased by 30, 34 and 42 oC, using 2, 3 and 4 TES containers, respectively. The results of these engine block heat-up simulation experiments demonstrated the potential of the latent heat TES in automobile applications.