Exergetic comparison of wind energy storage with ice making cycle versus mini-hydrogen economy cycle in off-grid district cooling

Abstract

For the feasible and continuous utilization of intermittent wind and solar energy sources for electricity generation in district energy systems in hot-climates, where cooling loads are dominant, ice storage may be an option. In this study, the rationality of the ice storage system for wind energy was investigated using the Rational Exergy Management Model, REMM for two options and compared with a base scenario, which comprises a wind turbine system, grid connection, conventional chillers, and the district cooling system. The main objective is to minimize exergy destructions and thus to improve the exergy performance. The first ice storage option is composed of wind turbines, deep chillers for ice making, ice storage tanks, and the district cooling system. The second option is similar to the first option but it also includes a ground-source heat pump upstream the deep chiller. These options were also compared against a mini-hydrogen economy (District size) alternative, which encompasses a hydrogen-water cycle with excess renewable energy-powered PEM electrolysis unit, hydrogen tank, fuel cell, absorption chiller, gas compression chiller, and the district cooling system. These two options and the hydrogen-water cycle alternative were compared in terms of their REMM efficiency, First and Second-law efficiencies, and the primary energy ratio. A new Sustainability Performance Index, namely SPI was also defined. SPI is the product of the REMM efficiency, First-Law Efficiency, and the load coincidence factor, CF of wind energy. In order to establish a realistic application background for the comparisons, first a nearly-net-zero exergy farmland (nZEXF) utilizing biogas, cogeneration, solar photovoltaics, heat, absorption cycle, ground-source heat pump, Organic Rankine Cycle, and wind turbines was introduced as a model. The primary objective of this study is to determine the best option with the least avoidable CO2 emissions responsibility of the systems considered in terms of the REMM efficiency in thermal or hydrogen storage systems. Results have been compared in terms of SPI with the base scenario and it has been concluded that the second option (SPI = 0.88) is better than the first option (SPI = 0.38). However, hydrogen storage is an even better alternative with an SPI value of 1.06. These figures according to REMM with the coincidence factor being considered, mean that the avoidable CO2 emissions may be reduced by up to 54% compared to the base case. Hydrogen cycle option may also be used with the same effectiveness in district heating, while ice storage options are limited to district cooling only. This paper provides the relevant theory, shows the fundamental calculations about the option rankings based on a unit cooling load, makes recommendations for future district energy systems, and refers to a conceptual hydrogen economy driven city.