Optimization of multiple fuel utilization options in Tri-generation systems

This work investigates the design of optimal tri-generation systems for heat, power and water production via multiple fuel selections, thus aiming to reduce the reliance on fossil fuel consumption. Generally speaking, tri-generation systems are associated with high levels of carbon dioxide emissions to meet energy and water production requirements. Hence, a shift towards more renewable energy sources can assist in partially reducing the environmental damage associated with standard tri-generation operations. Since the switch from fossil fuels to renewable energy is very costly, hybrid energy systems were found an appealing solution that could allow a gradual reduction of carbon emissions. Hence, the novelty aspect of this work is the ability to generate cost-effective tri-generation systems that incorporate optimal hybrid energy selections and utility generation routes, subject to specific net carbon reduction targets (NCRT). As such, four different energy sources (natural gas, biomass, municipal solid waste (MSW) and Concentrated Solar Power (CSP) were investigated, together with five different routes for steam expansion and electricity production using a Mixed Integer Nonlinear Program (MINLP), including technical, economic and environmental constraints. In order to study the effect of different fuel selections, energy production operations, and water production routes on the performance of tri-generation systems, data from three different desalination plants (located in USA, Cyprus and Qatar) were used. The results obtained show that energy requirements for desalination greatly affects the order of selection of energy sources. In general, biomass was identified as the best alternative to replace natural gas at NCRT values below 40%. On the other hand, MSW incineration using grate-fired and fluidized bed boilers became more desirable for steam production when higher NCRT values were utilized. The water production costs (WPC) of a standalone CSP system integrated with each of the studied plants, having a feedwater salinity of 33.5, 41.8 and 45 g/L, were estimated at 1.739, 2.233 and 2.67 USD/m3, respectively. In addition, an average incremental increase of 5.5% in the WPC has been observed during seasons that provide the lowest solar availability values.