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Abstract The objective of this work is to evaluate the surge characteristics of an axial compressor operating under surge conditions, by using a hybrid BDF/harmonic balance method. During a surge event, a large amount of backflow occurs in the compressor. Consequently, the airflow oscillates along the axial direction at a low frequency, which can considerably damage the compressor. In this paper, a numerical investigation is performed to examine the effects of the system volume and average mass flow on the surge characteristics of a transonic high speed single stage axial compressor (stage 35) designed and evaluated at the NASA Glenn Center. The results show that the oscillation frequency of the airflow in the compressor generally ranges from 14.32–26.04 Hz during surge conditions, and both the system volume and average mass flow considerably influence the surge characteristics. For a constant average mass flow, as the system volume decreases, the oscillation frequency of the airflow in the compressor increases slightly, while the oscillation amplitude of the exit static pressure decreases significantly, and the maximum static pressure at the outlet remains nearly constant. In the case of a constant system volume, with the decrease in the average mass flow, the oscillation frequency of the airflow in the compressor gradually decreases, and the oscillation amplitude of the exit pressure increases significantly. With a further decrease in the average mass flow, the oscillation frequency stabilizes at a certain point and does not change thenceforth. Furthermore, the surge frequency obtained using the numerical method shows a same tendency with the Helmholtz frequency, but there is a considerable difference between them.

Abstract The objective of this work is to evaluate the surge characteristics of an axial compressor operating under surge conditions, by using a hybrid BDF/harmonic balance method. During a surge event, a large amount of backflow occurs in the compressor. Consequently, the airflow oscillates along the axial direction at a low frequency, which can considerably damage the compressor. In this paper, a numerical investigation is performed to examine the effects of the system volume and average mass flow on the surge characteristics of a transonic high speed single stage axial compressor (stage 35) designed and evaluated at the NASA Glenn Center. The results show that the oscillation frequency of the airflow in the compressor generally ranges from 14.32–26.04 Hz during surge conditions, and both the system volume and average mass flow considerably influence the surge characteristics. For a constant average mass flow, as the system volume decreases, the oscillation frequency of the airflow in the compressor increases slightly, while the oscillation amplitude of the exit static pressure decreases significantly, and the maximum static pressure at the outlet remains nearly constant. In the case of a constant system volume, with the decrease in the average mass flow, the oscillation frequency of the airflow in the compressor gradually decreases, and the oscillation amplitude of the exit pressure increases significantly. With a further decrease in the average mass flow, the oscillation frequency stabilizes at a certain point and does not change thenceforth. Furthermore, the surge frequency obtained using the numerical method shows a same tendency with the Helmholtz frequency, but there is a considerable difference between them.

Abstract A Triangle Cycle with a piston engine expansion unit is used to convert low temperature heat into electrical energy. In this process, the isentropic efficiency of the expansion unit is considered to be unknown, and a theoretical approach for the calculation of isentropic efficiency is presented. A number of influences are taken into account – dead volume, residual mass, liquid injection performance and wall heat transfer. Various working fluids are investigated in a wide range of temperatures (333K–573K), engine speeds (5 Hz–30 Hz) and stroke volumes (0.1 L–50 L). The isentropic efficiency of water as working fluid is in the range of 0.75–0.88 and drops significantly for high stroke volumes and engine speeds. In general, injection mass has the most impact on isentropic efficiency because it influences dead volume and injection performance. The injection mass increases with vapor density and therefore is significantly influenced by working fluid and temperatures. The Triangle Cycle is compared with Organic Rankine Cycles by using determined isentropic efficiency. The exergetic efficiency of the Triangle Cycle using water is up to 35–70% higher than that of supercritical Organic Rankine Cycles in situations with a heat source temperature of up to 450K.

Abstract A Triangle Cycle with a piston engine expansion unit is used to convert low temperature heat into electrical energy. In this process, the isentropic efficiency of the expansion unit is considered to be unknown, and a theoretical approach for the calculation of isentropic efficiency is presented. A number of influences are taken into account – dead volume, residual mass, liquid injection performance and wall heat transfer. Various working fluids are investigated in a wide range of temperatures (333K–573K), engine speeds (5 Hz–30 Hz) and stroke volumes (0.1 L–50 L). The isentropic efficiency of water as working fluid is in the range of 0.75–0.88 and drops significantly for high stroke volumes and engine speeds. In general, injection mass has the most impact on isentropic efficiency because it influences dead volume and injection performance. The injection mass increases with vapor density and therefore is significantly influenced by working fluid and temperatures. The Triangle Cycle is compared with Organic Rankine Cycles by using determined isentropic efficiency. The exergetic efficiency of the Triangle Cycle using water is up to 35–70% higher than that of supercritical Organic Rankine Cycles in situations with a heat source temperature of up to 450K.

Abstract We propose a novel transparent electrode consisting of silver (Ag)–aluminum doped zinc oxide (AZO)–nickel (Ni) deposited at room temperature on glass and polyethylene terephthalate (PET) for flexible organic photovoltaics. The electrode combines the high electrical conductivity and mechanical ductility of Ag, the transparency and antireflection of AZO and the high work function and stability of Ni. Environmental stability results confirmed that the electrode maintains its optical and electrical properties at both elevated temperature (500 °C) and under damp heat conditions (85 °C and 85% relative humidity). To demonstrate its functional potential, the electrode has been used as the transparent anode in an organic solar cell (OSC), which shows an efficiency of 2.6%, very close to the value (2.9%) obtained in a similar cell using the widely used indium tin oxide (ITO). With respect to ITO, the proposed transparent electrode has several advantages, including mechanical flexibility, room temperature processing and low cost.

Abstract We propose a novel transparent electrode consisting of silver (Ag)–aluminum doped zinc oxide (AZO)–nickel (Ni) deposited at room temperature on glass and polyethylene terephthalate (PET) for flexible organic photovoltaics. The electrode combines the high electrical conductivity and mechanical ductility of Ag, the transparency and antireflection of AZO and the high work function and stability of Ni. Environmental stability results confirmed that the electrode maintains its optical and electrical properties at both elevated temperature (500 °C) and under damp heat conditions (85 °C and 85% relative humidity). To demonstrate its functional potential, the electrode has been used as the transparent anode in an organic solar cell (OSC), which shows an efficiency of 2.6%, very close to the value (2.9%) obtained in a similar cell using the widely used indium tin oxide (ITO). With respect to ITO, the proposed transparent electrode has several advantages, including mechanical flexibility, room temperature processing and low cost.

We present a methodology for applying silicon pho-nonic crystals (Si PnCs) to photovoltaics (PV). One-dimensional PnCs made from ultrathin Si, indium tin oxide, and graphene are designed, as well as two-dimensional Si-only PnCs. The general elastic wave equations are employed to solve the frequency band structures. The obtained bandgap can effectively suppress the carrier relaxation. The potential of Si PnCs for PV applications is theoretically assessed within our model. The calculated upper limit of the thermodynamic efficiency is approximately 58%, which is well beyond the Shockley–Queisser limit of either a Si solar cell or an all-Si tandem solar cell. Very importantly, our calculations show that it is not necessary to fully suppress the carrier relaxation to achieve ultrahigh efficiency. This work offers a strategy to develop ultrahigh-efficiency single-junction Si solar cells using the Si PnCs with ultrathin absorbers at extremely material cost.

We present a methodology for applying silicon pho-nonic crystals (Si PnCs) to photovoltaics (PV). One-dimensional PnCs made from ultrathin Si, indium tin oxide, and graphene are designed, as well as two-dimensional Si-only PnCs. The general elastic wave equations are employed to solve the frequency band structures. The obtained bandgap can effectively suppress the carrier relaxation. The potential of Si PnCs for PV applications is theoretically assessed within our model. The calculated upper limit of the thermodynamic efficiency is approximately 58%, which is well beyond the Shockley–Queisser limit of either a Si solar cell or an all-Si tandem solar cell. Very importantly, our calculations show that it is not necessary to fully suppress the carrier relaxation to achieve ultrahigh efficiency. This work offers a strategy to develop ultrahigh-efficiency single-junction Si solar cells using the Si PnCs with ultrathin absorbers at extremely material cost.

The project between tthe Deutsche Biomasseforschungszentrum (DBFZ) and the Karlsruhe Institute of Technology (KIT) focuses on the pr rovision of alternative fuels by thermochemical conversion. Biogenic residues and wastes which are not used yet or which could be utilised more efficiently are studied. The selection of possible feedstock was supported by a techhnical potential analysis including the competition to th he food industry. The technical suitability of raw materials for the fast pyrolysis (FP) process was of special in nterest. As a possible feedstock following types of biomass were studied: corn stover, corn cobs, biogenic floating re efuse (river Rhine and Baltic Sea), scrap wood, bark, rape s straw, sunflower straw, draff, diverse residues of flour production and hay. A process development unit (PDU) with a biomass feeding rate of 10 kg/h and a twin screw m mixer reactor was used for all experiments. It was found that different types of biomass form different char, condensate e and gas yields due to varying ash levels and lignocellulosic composition. Elemental formulas for feedstock, char, organic condensate and gas were estimated independent on t the feedstock due to similar elemental compositions. Pyrolysis gas analysis during the experiments gave information on the mass yields. A CO/CO2-ratio of 1 (i.e. wood) corresponds to organic condensate yields of about 50 wt.-%%, whereas a ratio of 0.3-0.7 (straw) corresponds to 18-32 wt. .-% respectively. Proceedings of the 20th European Biomass Conference and Exhibition, 18-22 June 2012, Milan, Italy, pp. 973-977