Design, Evaluation and Optimization of Concentrated Solar Power-Desalination Hybrid Tri-generation Systems

Abstract

Water scarcity and energy security are two global challenges that are of high concern. In order to address the water shortage problem, seawater desalination was found to be a feasible solution that can provide the required amount of water for communities, specifically in the regions where seawater is the only viable source of water. However, compared to conventional water treatment processes, seawater desalination is often considered energy intensive. Therefore, the high amount of energy required for desalination processes entails high greenhouse gas (GHG) emission levels as a result of associated fossil fuel combustion activities, creating significant environmental impacts. Thus, renewable energy driven desalination processes are getting much attention due to their capability of addressing both water and energy problems.

The abundantly available direct normal irradiance (DNI) in specific regions can potentially allow concentrated solar power (CSP) systems to become major energy contributors in the desalination market. Since CSPs can generate both thermal and electrical energy, they have been found to be excellent candidates for sustainable operation of large scale desalination systems, in the long term. This work presents a design of a novel CSP-desalination tri-generation system that is capable of the simultaneous production of three different pressure levels of steam, power and freshwater. A Mixed Integer Nonlinear Program (MINLP) is used as a tool to solve for the optimal configuration of the system specific to each investigated scenario. Due to the complexity of the tri-generation process, it was found imperative to investigate different factors that would improve the overall performance of the system, and ultimately decrease associated costs, such as feedwater salinity and desalination capacity. According to the results obtained, a large scale tri-generation system with an overall production capacity of 100,000 m3/d of freshwater resulted in a 60% reduction of the attained WPC value, when compared against a small scale system with a production capacity of 10,000 m3/d of freshwater. The option of exporting electrical energy to the grid using the proposed tri-generation system was also investigated, and a sensitivity analysis was conducted by varying the price of electrical energy. The attained breakeven energy prices were 7.4, 7.9 and 8.2 cents/kWh at 25, 35 and 45 g/L of feedwater salinity, respectively.

Although CSP systems present great advantages, they are seemingly associated with several challenges: high capital investment costs, daily and seasonal solar intermittency variations, and substantial land footprint requirements. All those factors can greatly affect the net water production cost associated with desalination that rely on CSP systems for energy. In an attempt to overcome such challenges, shifting towards hybrid energy systems for operating desalination schemes offer a great solution. Fossil fuels, biomass and municipal solid waste are potential energy sources that could be integrated with desalination systems, but their combustion is always associated with GHG emissions. Therefore, the proposed design of the tri-generation system was adjusted to include various energy streams that could be coupled with CSP for energy production. Variations in solar energy availability have also been accounted for, by using DNI as the primary variation indicator, as well as the available solar field area, and other parameters. Moreover, carbon taxation on emissions was used to assess the environmental damage caused by the system. The results indicate that despite the high cost of CSP, it was still found to be the best choice in the presence of a carbon tax value. The water production cost of a hybrid natural gas-CSP tri-generation system was estimated at 1.277 USD/m3. This value could be 16% higher in the presence of carbon taxation. Additionally, the high purchasing costs of biomass and municipal solid waste were the main reasons behind the preference of natural gas and CSP options. However, both biomass and solid waste options were found to be very promising energy sources for desalination, when their respective costs were reduced, especially in seasons with the lowest DNI values, which allows the integration of desalination processes with fully local energy independent systems.