Sustainable desalination : a case of renewable energy

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.A new energy-efficient and sustainable desalination system has been developed in this research. This system operates under near-vacuum conditions created by exploiting natural means of gravity and barometric pressure head. The system can be driven by low grade heat sources such as solar energy or waste heat streams. Theoretical and experimental studies were conducted to evaluate and demonstrate the feasibility of the proposed process. Theoretical studies included thermodynamic analysis and process modelling to evaluate the performance of the process driven by the following alternate energy sources: solar thermal energy, solar photovoltaic/thermal energy, geothermal energy, and process waste heat emissions. Experimental studies included prototype scale demonstration of the process using direct solar and a combination of solar photovoltaic/thermal sources. In the tests using direct solar energy, freshwater production of 5 L/d was achieved using direct solar energy alone, at efficiencies ranging from 65 to 75%. In the tests using solar photovoltaic/thermal energy, freshwater production of 10 L/d was achieved, at efficiencies ranging from 65 to 90%. Specific energy required for this process to produce 1 kg of freshwater was 2926 kJ, all of which was derived from solar energy.vk201

Improving combined cycle power plant performance in arid regions

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.In arid regions, where cooling water supplies are limited and highly regulated, combined cycle power plants (CCPPs) are turning to air-cooled condensers instead of water-cooled condensers. However, performance of air-cooled condensers can decline as ambient temperatures increase and result in loss of steam turbine power output. At the same time, as ambient temperature rises, net output of the gas turbine also can decline due to increased power consumption by the compressor and reduced power output by the gas turbine. In this paper, it is proposed to remedy these problems by pre-cooling the inlet air to the air-cooled condenser as well as the compressor, using a low-temperature thermal energy storage (TES) system. The TES is maintained around 5ºC by an absorption refrigeration system driven by the waste heat in the stack gases. A thermodynamic analysis of a 500-MW CCPP incorporating the above concept is presented.vk201

Energetic Performance of Photobioreactors for Algal Cultivation

Microalgae are currently being investigated as a potential fuel crop. For algae to be an energy-efficient fuel crop, cultivation systems and operating conditions that have been optimized for biomass productivity have to be refocused toward energy production. In this work, data from the literature on a variety of algal photobioreactors (PBRs) were compiled to reassess them in terms of biomass productivity per unit energy input to the cultivation process. This assessment showed that PBRs that have been optimized for biomass productivity without considering the energy input did not necessarily perform well in terms of energy efficiency. This review recommends that selection of algal PBRs for energy production should consider optimal geometry for efficient utilization of incident light, and their operation should be based on the optimal sparging rate for efficient mixing to maximize energy efficiency