Available onlineKeywords: Activation energy dc conductivity ac impedance and diffusion coefficient A B S T R A C T Cathode materials in nano size improve the performance of batteries due to the increased reaction rate and short diffusion lengths. Lithium Iron Phosphate (LiFePO4) is a promising cathode material for Li-ion batteries. However, it has its own limitations such as low conductivity and low diffusion coefficient which lead to high impedance due to which its application is restricted in batteries. In the present work, increase of conductivity with decreasing particle size of LiFePO4/C is studied. Also, the dependence of conductivity and activation energy for hopping of small polaron in LiFePO4/C on variation of particle size is investigated. The micro sized cathode material is ball milled for different durations to reduce the particle size to nano level. The material is characterized for its structure and particle size. The resistivities/ dc conductivities of the pellets are measured using four probe technique at different temperatures, up to 150°C. The activation energies corresponding to different particle sizes are calculated using Arrhenius equation. CR2032 cells are fabricated and electrochemical characteristics, namely, ac impedance and diffusion coefficients, are studied.

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Optimization of closed-loop cooling water systems (CCWS) is essential to reduce operating and capital costs. These systems are generally designed with heat exchangers in parallel arrangement. Although this arrangement is easy to operate, it results in inefficient use of cooling tower capacity and incurs more capital and operating costs. Existing CCWS can be retrofitted by changing heat exchangers from parallel to series or series/parallel arrangement. Previous research on cooling water retrofit design is mainly focused on methods to reduce cooling water flow rate without considering cooling tower operation, pressure drops, and capital costs. With pressure drop and capital cost considerations, minimizing cooling water flow rate does not necessarily give the optimum savings. In the present paper, a holistic approach for retrofit design of CCWS involving multiple cooling towers and heat exchangers is presented by considering exchanger network pressure drops, cooling tower operation, and piping costs besides operating costs. A mixed-integer linear programming model is formulated to maximize the profit for retrofit design, by linearization of equations for piping and heat-exchanger pressure drop, piping cost, and operating cost of pumps and fans. In addition, a mixed-integer nonlinear programming model is presented for the retrofit design of a cooling water system, involving multiple cooling towers and heat exchangers, using one intermediate header. One detailed case study from a chemical plant is presented to demonstrate the potential benefits of using the proposed approach and models.

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