(375d) Optimization of a Solar-Thermal Cogeneration Plant

Conceptual design, system-level models and optimization of a concentrated solar-thermal plant used for combined power generation and seawater desalination are presented.

Seawater desalination is being increasingly considered as a viable method to address the global shortage of potable water. Conventional large-scale desalination methods (thermal or membrane based) are, however, energy intensive and add considerably to the carbon-dioxide emissions (e.g., 18-25 kg of CO2 per m3 of water produced by fossil-fueled thermal desalination [3]). Desalting seawater using renewable energy sources is a promising alternative, particularly for islands and remote areas, and has recently received considerable attention [2, 1, 4]. Utilization of concentrating solar power (CSP), in particular, for large-scale seawater desalination has been considered by several investigators, most notably, at the German Aerospace Center (DLR) [6], and the Plataforma Solar de Almería [1] in Spain (the AQUASOL project). In CSP, solar radiation intercepted by reflectors (e.g., heliostats and parabolic mirrors) is concentrated onto a receiver, which can have a volumetric or tubular design. The exergy collected at the receiver is used in a power cycle (e.g., Stirling dishes or steam cycle) or used for recharging an exergy storage system.

Here, a new conceptual design is considered for combined power generation and seawater desalination using CSP. In this concept, heliostats strategically positioned on a hillside reflect light directly into a salt pond which acts as combined receiver and storage [5]. The energy collected in the pond is used downstream for seawater desalination and electricity generation. Physics-based system-level dynamic models are developed using JACOBIAN, an equation oriented modeling software. Major sub-models developed are the energy storage system (i.e., the salt pond), a regenerative Rankine cycle using accurate and well-developed physical properties of steam and water, and a hybrid desalination system. An extraction turbine is considered, allowing for extracting steam at various pressures for an open feedwater heater and a multi-effect distillation (MED) system, which is the considered desalination technology in parallel with reverse osmosis (RO). The models are detailed enough to allow for optimization of operation and design under various weather, location and operating conditions (e.g., time-variable operation). Model predictions over a wide range of operating conditions are validated using available measurement and/or commercial software packages. Operation of the plant is systematically optimized (i.e., through mathematical programming) considering both time-variable and time-invariable operating conditions. The optimization problem is formulated as a nonlinear program (NLP) and a heuristic global optimization approach is used. The sequential method of optimization is employed, decoupling the simulation from the optimization. The system considered for the energy collection and storage provides heat at different temperatures, hence different exergies. The optimal use of the thermal energy collected and stored is found considering several possible options. The income from the optimal time-invariant conditions is further increased considering time-variation operating conditions and a time-dependent feed-in tariff (FIT). Optimal operation can be even further optimized considering shutting down RO during high electricity prices. Hence, time-dependent optimization of RO is formulated as a mixed-integer nonlinear program (MINLP), allowing for periods of shut-down. Time-dependent global optimal solution for short-term operation of the RO is determined using BARON, a commercial optimization software package. The results show that operating the RO for minimum energy consumption does not necessarily yield the maximum profit.

References

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[5] A. Slocum and D. S. Codd. Solar energy concentrator system with energy storage. US. Provisional Patent No. APN:61/243763, 2009.

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