This paper presents exergetic sustainability evaluation of a Recirculating Aquaculture System (RAS). Some environmental and sustainability aspects of the RAS are investigated in terms of exergetic sustainability parameters. In this regard, the exergetic parameters, such as exergetic efficiency, waste exergy ratio, exergy recoverability ratio, exergy destruction ratio, environmental impact factor, and exergetic sustainability index are studied. The results show that increasing waste exergy ratio increases the exergetic efficiency and decreases exergetic sustainability index. However, the environmental impact of the RAS increases as the waste exergy ratio increases. Thus, it can be said that, the RAS requires much more improvement because of the higher environmental impact factor and lower exergetic sustainability index, resulting from the heat gained from the environment, the back water from the components in the system, the increasing quantity of the unused waste, and the high capacity of the pumps based on the fish production capacity of the system.

Ocean stores energy in the form of currents, waves, tides, heat and salinity that can help to alleviate worldwide demand for power and the global climate change threat. Exergy analysis can be applied to evaluate the work potential (extractable) that a resource contains. This paper focuses on the exergy content that can be extracted from ocean reservoirs and provides a review of existing the technology installed for harvesting ocean energy.

A comprehensive thermodynamic modelling is reported of a trigeneration system for cooling, heating, electricity generation and hot water production. This trigeneration system consists of a gas turbine cycle, an organic Rankine cycle (ORC), a single-effect absorption chiller and a domestic water heater. Energy and exergy analyses, environmental impact assessments and related parametric studies are carried out, and parameters that measure environmental impact and sustainability are evaluated. The exergy efficiency of the trigeneration system is found to be higher than that of typical combined heat and power systems or gas turbine cycles. The results also indicate that carbon dioxide emissions for the trigeneration system are less than for the aforementioned systems. The exergy results show that combustion chamber has the largest exergy destruction of the cycle components, due to the irreversible nature of its chemical reactions and the high temperature difference between the working fluid and flame temperature. The parametric investigations show that the compressor pressure ratio, the gas turbine inlet temperature and the gas turbine isentropic efficiency significantly affect the exergy efficiency and environmental impact of the trigeneration system. Also, increasing the turbine inlet temperature decreases the cost of environmental impact, primarily by reducing the combustion chamber mass flow rate.

Growing energy demands and the desire to reduce pollution have increased interest in and research on unconventional power plant technologies. Geothermal power plant technology is an important area being explored as a renewable and environmentally benign alternative to fossil fuel technologies. Geothermal power plants have sources of emissions associated with use, including the use of evaporation ponds. An example of a geothermal power generation unit that utilizes an evaporation pond to manage spent geothermal fluids during its operation is the Cerro Prieto I plant in Mexico. A theoretical model is developed to retrofit the plant with geothermal fluid re-injection. Energy and exergy analyses are performed for the standard plant, using the evaporation pond, as well as a hypothetical system utilizing fluid re-injection. The plant without re-injection is found to have an energy efficiency of 12.6% and an exergy efficiency of 47.5%. With re-injection the energy efficiency is 16.5% and the exergy efficiency is 51.5%. The greatest loss in the standard system is through direct discharge of the geothermal fluids to the evaporation pond.

In this paper, a comprehensive thermodynamic analysis of Organic Rankine Cycle (ORC) using different working fluids driven by renewable/waste low-grade heat sources is conducted, and the performance and environmental characteristics of cycle, especially the potential CO₂ emission, are investigated. The comparative evaluation of cycle using a combined energy and exergy analysis is performed by varying certain system operating parameters such as efficiencies, mass flow rate, cycle irreversibility and heat input at various temperatures and pressures. Moreover the toxicity, flammability, ODP and GWP of different working fluids besides utilizing renewable heat sources are studied as a safety and environmental assessment. The results from this analysis provide valuable insight into selection of the most suitable fluids for power generating applications using low-temperature heat sources..