Integration Of Low/mid Temperature Solar Heat-Fossil Fuel Hybrid System And Study On Dual-purpose System

The integrated energy systems (IES) with reasonable and high-efficiency utilization of energy have been developed and promoted in recent years. In the context of sustainable energy development strategy of the world, the IES with cascaded utilization of energy and in harmony with environment has become the essential aspect and frontier of power system research and development in 21st century. Supported by the National Natural Science Foundation of China and the National Basic Research Program of China ("973" Program), the studies on system integration of low/mid temperature solar thermal energy-fossil fuel and the dual-purpose system were carried out. Research efforts have been put into the aspects of the energy cascaded utilization mechanism, system features and performances, and the novel system integration proposals. A low/mid temperature solar heat hybrid system with CO₂ capture and a novel dual-purpose system were proposed, and both their thermal and economic performances were analyzed. Last but not least, new evaluation criteria for integration effects of the hybrid system were put forward.For guiding the system integration, the mechanisms of the indirect upgrading of the low/mid temperature solar heat and the cascaded utilization of both the chemiacal and physical energy were investigated. The energy-level correlation formulas of the solar heat indirect upgrading process and both the two ways of combustion (direct and indirect combustion) were established; the relationship of the indirect solar heat upgrading and fossil fuel degrading was revealed and the exergy destruction of two combustion ways was calculated and compared. During the steam-methane reforming and syngas combustion (fuel indirect combustion) processes, the energy level of low/mid temperature solar heat has been upgraded by 1.2 times indirectly, and the exergy destruction in combustion decreased by 8.4%. Comparing the SOLRGT hybrid system (integrating solar heated steam generation, steam methane reforming and intercooling chemically recuperated gas turbine cycle) with SSPRE (integrating solar heated steam generation with steam Rankine cycle) and Solar CC (integrating solar heated methanol decomposition with combined cycle), the system integration of low/mid temperature solar heat and fossil energy hybrid systems was studied.For SOLRGT, the influences of the solar heat input share Xsol were evaluated and the thermal and economic performances of the hybrid system were compared with two single-input reference systems. The advantages of the system integration were embodyed by more reasonable and economical utilization of the solar thermal energy. When Xsol=20.3%, with same energy input, compared with two single-input reference systems (solar parabolic troughs and combined cycle), the relative saving ratio of electricity cost of SOLRGT is 16.6%; the bigger the Xsol is, the higher the saving ratio will be.To demonstrate the benefits of the hybridization, comparing each hybrid system with its corresponding single-input reference systems, all having the same solar and fossil fuel inputs, the relative power gain (RGp) and reduction of CO₂ emission (RDc) of hybrid system were taken as the new evaluation criteria. Analyses show that the RGp and RDc of SOLRGT are in ranges of 5～13%and 5～11%respectively, close to those of Solar CC, and better than those of SSPRE.In this thesis, a novel dual-purpose system (water and electricity cogeneration system) was proposed by integrating chemically recuperated gas turbine cycle (CRGT) with low temperature multi-effect desalination (MED) system. The differences of configurations of CRGT+MED with fuel of methane and methanol were noticed. And the cogeneration system was compared with some typical dual-purpose systems and corresponding separate generation systems. It was found that the integration superiority is that the MED bottom cycle does not decrease the efficiency of the CRGT top cycle; it recovers only the surplus exhaust gas heat downstream the reformer, which avoids a great waste of exergy if motive steam low-grade thermal energy were provided directly by burning fuel in a boiler; what is more, in this way the exhaust heat can be recoverd more efficiently and the water consumption of CRGT can be satisfied by MED. The water cost of methane CRGT+MED is only 1.37$/t, and the integrated system payback period of is only 4.0 years. The relative energy saving ratio of methanol CRGT+MED comparing with its separate generation systems with same output is 8.9%.Based on the princinple of "cascaded utilization of chemiacal energy and physical energy", and to improve the SOLRGT system to a zero CO₂ emission system, a low/mid temperature solar heat hybrid system with CO₂ capture was proposed and named as ZE-SOLRGT. To decrease the exergy destruction of the combustion process and the flue gas emission in SOLRGT, the oxy-fuel combustion was adopted and the new hybrid system was configured to be a quasi-combined cycle, including a high temperature Brayton-like top cycle and a high pressure ratio Rankine-like bottom cycle, with a branch of the exhaust gas re-cycled and injected back into the combustor. Compared to SOLRGT, the exergy destruction of the combustion process and flue gas emission in ZE-SOLRGT decreases by 4.4%and 50.5%respectively; the thermal efficiency and exergy efficiency increase by 3.6%-points and 2.0%-points respectively.