Direct energy conversion efficiency

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

The utilization principles are shown in Figure 6, where the typical examples are enumerated. Hydrogen turbine has been studied by Japanese WE-NET project and the achieved energy efficiency was as high as about 60 %, which can be competitive with fuel cell system. One of the typical direct energy conversion systems, which have no movable parts and no noise, is fuel cell. Today topics of clean cars have been focused to the cars with PEMFC as was mentioned previously. 

Both direct and indirect biophotolysis routes suffer from a number of challenges that currently prohibit them from becoming commercially viable processes [180] these include, but are not limited to, solar energy conversion efficiency and the difficulty of bioreactor design. In particular the low solar energy to hydrogen conversion efficiency is a matter of major concern in the field of biophotolysis. The efficiency is calculated as [180,172] ... 

Biodiesel to Fuel a Large Power Plant. Researchers at ASU s Center for Bioenergy and Photosynthesis have calculated that a 25 x 25 km field of bioreactors using cyanobacteria to fix carbon could uptake all of the carbon dioxide produced by a 1.6 GW power plant and subsequently provide the biomass as lipid to fuel the power plant. The parameters necessary to achieve this goal are a seven percent power conversion efficiency for photosynthesis, 40 percent conversion efficiency of biomass to fuel, 50 percent conversion efficiency of fuel to electricity, and 80 percent conversion efficiency of land area covered by the bioreactors. This system would then be carbon neutral in operation and produce about 1.6 GW of electrical power. The key to making this feasible is to achieve a seven percent power conversion efficiency for cyanobacteria. Moore noted that the area required to produce a specified amount of energy scales directly with the energy conversion efficiency of the system or device. 

PV systems using polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) cells have been evaluated [17]. The PV systems considered here are large-scale, centralized systems directly connected to the utility grid and include the balance of system (BOS) with supporting structure, inverter, and DC control device and installed in Tokyo. Assumption for energy conversion efficiencies and PV cell production rates are shown in Table 4. 

Because of the direct energy conversion, fuel cells work at a higher efficiency than energy conversion processes, which... 

The Polymer Electrolyte and Direct Methanol Fuel Cells Power Output and Energy-conversion Efficiency... 

With areal power outputs only 20-30% that of a PEFC and an energy-conversion efficiency of 30% near peak power versus 50% in the case of the PEFC, the DMFC remains of great interest because of the attractive properties of methanol fuel, a liquid of high energy density under ambient conditions, and because the DMFC enables direct conversion of this liquid carbonaceous fuel to electric power. Particularly in portable applications, these features help minimize the overall dimensions of the power system (fuel + fuel cell + auxiliaries) and achieve high system energy density. 

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