Theoretical Study Of Wind-Compressed Air -Diesel Hybrid Power System

As the electricity demands increases year by year in marginal region, people usually adopts wind-diesel hybrid power system with battery storage as power source on the basis of conventional diesel power system, aiming to improve the wind power penetration rate and reduce the fuel consumption. Furtherly, the application of battery storage system is limited due to problems of highly cost and secondary pollution. In order to overcome these shortages of battery storage system, a new wind-compressed air-diesel hybrid power system was proposed, where compressed air storage system was applied and redundant wind power was stored as compressed air energy. Thus, it should be a priority for all to improve the energy conversion efficiency of wind-compressed air-diesel hybrid power system during the energy storage and utilization processes.In this thesis, three main subsystems of wind-compressed air-diesel hybrid power system, namely, the wind-compressed air energy storage system, compressed air expanding power system and compressed air-diesel hybrid power system were studied respectively from two aspects, containing of ideal thermodynamic cycle optimization and actual working process simulation. Then, the energy conservation potential of hybrid power system was evaluated in a village context. The main work and conclusions of this thesis are as follows:1) Energy flow analysis of conventional wind-diesel hybrid power system. The numerical simulation model of conventional wind-diesel hybrid power system with battery energy storage was built and validated. Then, the energy flow state of hybrid power system was analyzed. Results show that about 27% of total wind energy can be converted to load power. In weak wind speed range, whole wind energy is wasted and the ratio is about 3%; while in working wind speed range, partial wind energy is wasted. About 18% of total diesel exergy is wasted through exhaust gas. Exergy of free exhaust accounts for 40%～60% to total exhaust exergy; while pressure exergy of free exhaust accounts for 8%～10%.2) Study of wind-compressed air storage system energy conversion process. First, the efficiency analysis model of pistion compressor was built and the influences of operating parameters on energy conversion efficiency were analyzed. Results show that there is a coupling relation among the influences of rotate speed, exhaust pressure and intake pressure on energy conversion process. Decreasing exhaust pressure or rotate speed can improve energy conversion ratio. Then, the efficiency analysis model of wind-compressed air energy storage system was built and the influences of main parameters on wind energy conversion efficiency were studied. Results show that lower wind speed, larger tank volume and lower tank pressure can improve wind energy conversion efficiency. However, the latter two will downgrade enegy quality. When the wind speed is high, the tank volume should be expanded and the air compressor speed should be lowered to increase the energy conversion efficiency. If wind speed is lower than 4 m/s, the energy conversion efficiency of wind-compressed air energy storage system is higher than wind-battery energy storage system and it is quite the opposite when wind speed is higher than 4 m/s. The annual average cost of wind-compressed air energy storage system only accounts for 10%～16% of wind-battery energy storage system.3) Study of compressed air expanding power cycle. The efficiency analysis model of compressed air engine was built and validated. Then, the energy loss mechanism and influences of parameters on compressed air engine performances were analyzed. Results show that intake loss and exhaust loss are two largest loss of compressed air engine, which result from intake throttling and high pressure, low temperature exhausting, respectively. Adding together, intake loss and exhaust loss account for 30%～40% of the total intake exergy. Rising of engine rotating speed or falling of intake temperature both lead to an increase of exergy loss in intake and exhaust processes, resulting in a decrease of exergy efficiency. Rising of the intake pressure leads to an increase of exergy loss in exhaust process and a decrease of exergy loss in intake process, resulting in an increase of exergy efficiency. At last, a test bench of compressed air engine was built, and the influences of parameters on engine performance were verified.4) Study of diesel engine exhaust energy recovery by compressed air. The exhaust exergy recovery efficiency of diesel engine through heat transfer and medium mixing were calculated, respectively. Results show that the method of exhaust energy recovery by compressed air is depended on exhaust pressure level. It is appropriate to adopt combining method to recover exhaust energy. Then, the efficiency analysis model of compressed air-diesel hybrid engine was built, which adopted combining method to recover exhaust energy, and the influences of operating parameters on hybrid engine energy conversion efficiency were analyzed. Results show that the efficiency of diesel engine in hybrid mode almost keeps unchanged, while partial of exhaust exergy is forced into compressed air engine cylinder, which increase exergy efficiency of the latter. The total energy conversion efficiency of hybrid engine is affected by exhausted advancing angle and load of diesel engine, intake advanced angle of compressed air engine and compressed air pressure. Compared to hybrid engine only with heat transfer, the new hybrid engine adopting exhaust energy combining recovery method can recover exhaust pressure energy more efficiently, and the improvement range is 1%～2%. At last, the principle testing bench was built, and the feasibility of combining method to recover exhaust energy was verified.5) Simulation of wind-compressed air-diesel hybrid power system. Based on the wind source and electrical load of a marginal village, every subcomponents of hybrid power system were matched reasonably. The efficiency analysis model of hybrid power system was built, where the exhaust and cooling water energy of diesel engine was recovered and the power of cooling fan was reduced. The corresponding control strategy was applied to utilize energy efficiently. Results show that compared to diesel power system, the fuel consumption of new hybrid power system reduces 32%, where wind energy utilization accounts for 22%; compared to conventional hybrid power system without wasted energy recovery, the fuel consumption of new hybrid power system reduces 10%, where exhaust energy recovery, cooling water energy recovergy and cooling fan power reduction accounts for 7%,2% and 1%, respectively; compared to conventional hybrid power system with battery storage system, the fuel consumption of new hybrid power system increases 8%, while the annual average cost of storage system reduces about 84%.