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Abstract A new air-cooled gas-steam combined cycle cogeneration system with absorption heat pump for recovering waste heat from exhausted steam of the steam turbine to achieve double effects of waste heat recovery and water saving is proposed based on a conventional water-cooled gas-steam combined cycle cogeneration system in the paper. The property criteria variation is analyzed before and after modification. In addition, the exergy analyses of primary equipments are carried out based upon the exergy analysis theory. The results demonstrate that the net generating power is approximately increased by 11,082 kW, equivalent coal consumption is reduced by 2.71 g/kWh, the net overall thermal efficiency is improved by 0.91% with 334,245 kW heating load at 100% load of the gas turbine in the modified system. Besides, the overall exergy loss is decreased by 6448 kW and exergy efficiency is improved by 0.98%. The overall property of the whole system is improved. The results show that the property reduction caused by air-cooling modification can be made up by the property improvement due to waste heat recovery. Moreover, the cooling circulating water can be saved by 1196.34 kg/s. The presented measure can not only improve performance of the system but also simultaneously achieve energy and water saving on the premise of satisfying user needs, which has a wide application potential in the water-shortage regions.

Abstract Organic solvent upgrading Indonesian lignite was performed in a 1 L autoclave under moderate temperature. The chemical structure and functional groups transformation of lignite upgraded by two organic solvents (ethanol and n-hexane) were analyzed to explore the upgrading mechanism of solvent thermal treatment by using Fourier transform infrared (FTIR) and 13 C nuclear magnetic resonance (NMR). In addition, the characteristics of pyrolysis of treated samples were investigated using thermo gravimetric (TG) to clarify the variance of pyrolysis reactivity. Results showed that the carbon content and calorific value of upgraded lignite were significantly improved, and H/C and O/C ratios of treated samples were significantly reduced with the temperature increasing. The relative percentage of carbonyl and carboxyl carbon, oxygenated aliphatic carbon and methoxyl carbon of lignite upgraded at 300 °C decreased by 20–30%. However, the carbon-substituted and protonated aromatic carbon at 120–135 ppm and protonated aromatic carbon at 90–120 ppm were significantly increased after lignite was upgraded by the two solvents at above 200 °C. These transformations indicated that oxygen-containing functional group was substituted by hydrogen or carbon-substituent as temperature increased, and were intensified at above 200 °C. In addition, oxygen-loss in the treated samples was attributed to the loss of carbonyl group at 175 ppm, dihydric phenol at 147 ppm, and methoxyl group at 55 ppm. The activation energy of upgraded lignite at 300 °C were higher than those of raw lignite and upgraded lignite at 100 and 200 °C, indicating the low reactivity of pyrolysis of the treated lignite with the temperature increasing.

Abstract Direct de-ionized (DI) water immersion cooling has been verified to be an effective method of managing the operating temperature of silicon solar cells under concentration. However, the stable electrical performance is difficult to be achieved. Possible factors from bare cell self, materials for tabbing cells were investigated in this study for understanding the degradation mechanism. Long term immersion results showed that no significant degradation on bare cells operated in DI water at 65 °C. When cells were tabbed using lead-based solder and flux, the short circuit current ( I sc ) of cells decreased with exposure time, notably under sunlight, but it was not observed for cell open circuit voltage ( V oc ). The epoxy tabbed cells test also demonstrated that the tabbed cells without lead-based solder and flux involved were also found drop in I sc , but with slower rate. The presence of lead and tin black oxides on the lead based-soldered tabbed cells and red deposition on the epoxy tabbed cells confirmed the occurrence of galvanic corrosion. However, particular cleaning recovers the I – V towards its initial values for the former tabbed cells, and partial recovery for the latter tabbed cells, which indicates that the cells are not damaged after long-time DI water immersion.

Abstract A novel combined power and cooling system based on the organic Rankine cycle and the ejector refrigeration cycle for highly efficient utilization of low-grade heat is presented, in which the endothermic process adopts a dual-pressure evaporation approach and the two vapor generators are connected in series. A mathematical model is developed to evaluate the system thermodynamic and exergoeconomic characteristics. The effects of key parameters on system performance are evaluated. Results show that a higher low-pressure evaporation temperature and a higher vapor fraction at the low-pressure vapor generator are conducive to increasing the system cooling output. An optimal high-pressure evaporation temperature exists that gives the maximum exergy efficiency and the minimum sum unit cost of product. Compared with the net power output of the system, the cooling output is more sensitive to the variation of condensation temperature. Among the system components, the ejector has the highest exergy destruction rate and the lowest exergy efficiency. Furthermore, optimization of the system’s performance and working fluid selection for fixed cooling outputs was conducted. The results show that reducing the exergy destruction in the endothermic process is the key to improving system performance, while perfluoropropane was found to be the most suitable working fluid for the proposed system. In the cooling output range of 300–700 kW, a minimum sum unit cost of product of 45.79–58.87 $/MWh can be achieved, and corresponding ranges of net power output, energy efficiency and exergy efficiency are 614.93–430.58 kW, 14.32–19.25%, and 32.3–22.62%, respectively. Finally, the performance of the proposed system is compared with two typical systems for a cooling output range of 300–700 kW. The results show that the sum unit cost of product is reduced by 7.9–11.1%, and the net power output increased by 23.6–40.6% compared with the system with parallel vapor generators. Compared to the system with the ejector installed after the turbine, the sum unit cost of product is increased by 9.16–13.28%, and the net power output is increased by 129.73–118.38%.

Abstract This paper investigated a combined solar/air dual source heat pump water heater system for domestic water heating application. In the dual source system, an additional air source evaporator is introduced in parallel way based on a conventional direct expansion solar-assisted heat pump water heaters (DX-SHPWH) system, which can improve the performance of the DX-SHPWH system at a low solar radiation. In the present study, a dynamic mathematical model based on zoned lump parameter approach is developed to simulate the performance of the system (i.e. a modified DX-SHPWH (M-DX-SHPWH) system). Using the model, the performance of M-DX-SHPWH system is evaluated and then compared with that of the conventional DX-SHPWH system. The simulation results show the M-DX-SHPWH system has a better performance than that of the conventional DX-SHPWH system. At a low solar radiation of 100 W/m2, the heating time of the M-DX-SHPWH decreases by 19.8% compared to the DX-SHPWH when water temperature reaches 55 °C. Meanwhile, the COP on average increases by 14.1%. In addition, the refrigerant mass flow rate distribution in the air source evaporator and the solar collector of the system, the allocation between the air source evaporator and the solar collector areas and effects of solar radiation and ambient air temperature on the system performance are discussed.

Abstract Advanced energy conversion systems on biomass-based are essential for high-efficient utilization of biomass energy. This work proposed a conceptual biomass liquefaction system with supercritical water for combined bio-oil, power and heating trigeneration. Both thermodynamic and life cycle environmental assessment are performed to verify the potential benefits of this novel system. Mass flow in the overall process is configured using Aspen Plus, with detailed energy and exergy flow modelling for the whole system. At the optimal operation parameters, where reactor temperature and pressure are around 390°C and 25 MPa, feedstock concentration is ∼33.3 wt%, water recycle ratio is ∼50%, and gas combustor temperature is about 1000°C, system energy and exergy efficiency reach their highest values, i.e. 58.53% and 50.65%, which are comparable to those of other biomass-based energy conversion system. Effects of those key parameters on exergy and energy efficiency are also discussed. The maximum exergy loss is induced by chemical exergy loss in liquefaction reactor and energy loss is mainly caused by heat transfer. From environmental assessment, GWP, AP, EP and TP values at the optimal operation parameters are lower than those of other biomass-based liquefaction system. When CCS and wastewater treatment units are applied, GWP, AP and EP values can be reduced by 50.0%, 33.2% and 61.5%, respectively. The comparison of thermodynamic and environmental performance among different biomass-based energy conversion systems shows that biomass liquefaction with SCW is a relatively efficient and clean technology to produce carbon-neutral bio-oil.

This paper presents a new scheme of an inverter air cooling heat pump system with domestic hot water. A water reheater is placed between the compressor outlet and the four way valve inlet to utilize the sensible heat of the superheated gas exhausted from the compressor, and a water preheater is placed between the condenser and the throttling device to use the sensible heat of the subcooled liquid flowing out of the condenser. With these two parts of heat, the domestic hot water can be heated to a temperature high enough for domestic use. In order to maintain the system efficiency in the period of part load, an inverter compressor is adopted as the substitute for the constant speed one used in the conventional heat pump system. A hot water storage tank with a circulation pump is placed in the system to reduce the peak load of the system. Compared with the traditional system, this new design is able to reduce energy consumption by 31.1% and decrease thermal pollution to the environment.

Abstract Geothermal energy is one of the most promising renewable energy. In order to improve the overall utilization degree of medium-and-high temperature geothermal water by flash cycle, a new combined heating and power system is proposed to provide electricity and domestic hot water at the same time. This new system is integrated by single-stage flash cycle and ammonia-water absorption heat pump cycle, of which the heat pump cycle recovers two streams of waste heat of the flash cycle, including the waste heat of the turbine exhaust steam and the geofluid drained from the flasher. The mathematical models of the new system is established at length in this article, and a numerical simulation is conducted to present the preliminary design condition. The simulation results show that the thermal efficiency and exergy efficiency of the system could reach 81.8% and 46.99% respectively under the condition of 170℃ geothermal water. Then a thermodynamic parameter analysis is carried out to explore the impact of five key thermodynamic parameters on the system performance. The results show that an optimal flash pressure happens to make the system exergy efficiency maximal, and there is a similar case for the generator temperature. Within some certain value ranges, a higher rectification column pressure brings about a sharp drop in the exergy efficiency, whereas a higher ammonia concentration of ammonia-strong solution leads to the exergy efficiency rising. Additionally a higher evaporation pressure just has slightly positive effect on the exergy efficiency. Finally an optimization and comparison study is implemented between the proposed system and four flash cycle based combined systems, and the optimization results indicate that the proposed system has a better performance no matter from the viewpoint of the first law or the second law of thermodynamics.

Abstract A novel supercritical water system was proposed to treat sewage sludge harmlessly in this work, with its core to produce syngas by supercritical water gasification (SCWG) of sewage sludge and dispose of the residual liquid organics by supercritical water oxidation (SCWO) for heat release and harmless disposal. Moreover, a cool wall reactor was employed to solve corrosion and salt deposition, while the reaction heat was recovered for power generation, and the heating exchange net was optimized. The effects of SCWG temperature (400–550 °C), moisture content of sewage sludge (87–95 wt%), pressure (23–29 MPa), and oxidation coefficient (0–0.5) on the characteristics of the SCWG-SCWO combined system, and the influence of heating method for supercritical water on the exergy destruction and system efficiency, were investigated and discussed. The results showed that an increase in SCWG temperature, the decrease in moisture content and oxidation coefficient, except for the effect of pressure, could significantly improve the net exergy efficiency of the system. Under the condition of SCWG temperature being 450 °C, moisture content of 87 wt%, 25 MPa and oxidation coefficient being 0, the electricity self-sufficient rate, cold gas efficiency, exergy efficiency and net exergy efficiency were 18.08%, 13.09%, 15.94%, 15.41% and 10.06%, respectively. Inputting external energy could tremendously impact the whole system. When the temperature of heat source for heating water was higher than the reaction temperature of SCWO, the net exergy efficiency of the system could reach 27.73%. The thermodynamic efficiency of the SCWG-SCWO combined system outperforms the single SCWO case. This study provided an industrial basis for the hierarchical treatment of sewage sludge, and offered a new direction for pollutant treatment by the combined supercritical water technologies.