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The success of solar thermal systems for electricity production hinges very crucially on the selection, mechanical design and optimal operation of an energy storage system which can enable the continuous operation of a power plant. The energy storage systems being investigated include solid graphite, encapsulated Phase Change Materials (PCMs) and liquids such as water and ammonia. A storage option being investigated for large solar-thermal systems is liquid ammonia which by its endothermic dissociation converts to nitrogen and hydrogen gases, and can be synthesized exothermically to recover heat when required. Ammonia is an abundantly produced chemical, globally and in Pakistan. The synthesis of ammonia with carbon dioxide results in the formation of urea fertilizer, or carbamide (NH2)2 CO at pressures and temperatures of the order of 150atm and 600K respectively. In Pakistan there are eight large urea fertilizer plants based on the reforming and synthesis of natural gas mainly from the Sui and Marri gas fields. This work considers the potential of liquid ammonia as a storage medium especially with regards to its integration into a urea fertilizer Chemical Process Industry (CPI) infrastructure. We discuss essential thermodynamic and reaction kinetic features underlying the process and then focus on the energy balance of a solar thermal plant which requires compressed syngas for an ammonia reactor. A mathematical model, based on the material and energy conservation of constituent gases is used to obtain information on the process dynamics in a synthesis convertor. From this, the energy production from synthesis is estimated and compared with the compression energy requirement. We are able to demonstrate that compression power is a major concern for future thermal storage systems and may well be the single determining factor in the viability of such renewable energy systems.
S. Siddiq, S. Khushnood, Z. U. Koreshi, M. T. Shah. (2010) Solar Thermal Energy Storage using Liquid Ammonia Systems in Industry, NUST Journal of Engineering Sciences , Volume 3, Issue 1.
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