| Organic Rankine cycle (ORC) is a potential technology in recovering heat energy from low temperature heat source. The integration of the system and the operation optimization of the cycle are still underdeveloped. Focused on the integration technology and corresponding essential problems in ORC driven by low temperature heat source, experiments under different working conditions were carried out. The system performances as well as the operation characteristic and mechanism of key components were achieved. A parameter that can be treated as an index of the level of operation perfection was proposed. An example ORC work bench which can be driven by unstable low grade heat source was established.A perfect ORC experimental work bench driven by 140～160℃ heat source was established to investigate the cycle operation characteristic under variable working conditions, with R123 as working fluid and scroll expander as heat-to-work converter. Focusing on the real operation characteristic of working fluid pump, it was observed that the tri-plunger pump has relative outstanding real operation efficiency, which was 1.5-2.4 times of those reported in literature. There were coupling between working fluid pump and expander. The cavitation happened in pump would cause the obvious deviation and violent fluctuations. The cavitation can be avoided in a system in which a pump installed in frontier of the working fluid pump. Adjustment strategies with constant fluid flow rate and that with constant torque were compared, it was found that the flow was more stable with the latter strategy, while more sensitive to variations with the former one. The strategy with constant flow rate was better in adjusting the evaporation pressure, while the strategy with constant torque was better in adjusting the degree of working fluid superheating at inlet of expander. A maximum expander shaft work output of 3.45 kW was achieved in steady state working conditions, while the maximum thermal efficiency was 6.29%. The degree of working fluid superheating at expander inlet had an impact on the real efficiency of the expander. There was an optimal range (20℃) in which the expander supplied larger work output and the system exhibited higher thermal and exergy efficiencies. Experiments were carried out on ORC work bench using single screw expander. The maximum expander shaft work output and the highest thermal efficiency were 5.092 kW and 5.05%,respectively. Compared with the scroll expander, the screw expander showed merits such as large output and suitable for low operation pressure, therefore it was more appropriate to be used in ORC driven by low grade temperature heat source.The dimensionless integrated temperature difference ("DITD") was proposed as a new index of the cycle performance. With different heat source temperature and operation conditions, an apparent linear relationship between the first type DITD and the exergy destruction in evaporator was observed. As to the second type DITD there was an optimum range 0.282±0.01, in which the work output, thermal efficiency and exergy efficiency all achieved the maximum no matter what the heat source temperature was.An integrated ORC system with a rated work output of 10 kW was designed and established. A single screw expander was adopted in this cycle. The lubrication plan was improved, and the remote supervision and control system was integrated in the cycle. R245fa was chosen to be the working fluid. The system can operate steadily when driven by 80～120℃ unstable heat source. The tested expander shaft work output and system thermal efficiency were 7.764 kW and 6.11% respectively. The experimental results could supply performance database and operation experience for the ORC system application. |
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