Chemical energy, conversion

Methane reforming reaction is accomplished under the action of heat collected from mirror concentrator of solar light. The mixture of CO and H2 produced in this reaction can be stored and then, when necessary, converted into high-potential heat (with the temperature up to 950 K) in the methanation catalytic reactor. The efficiency of solar-to-chemical energy conversion as high as... 

As shown in [30], the maximum achievable efficiency of solar-to-chemical energy conversion (calculated with respect to Gibbs free energy) is ca. 60% for the thermocatalytic method and ca. 30% for the photocatalytic method. 

CHEMICAL ENERGY CONVERSION TECHNOLOGIES FOR EFFICIENT ENERGY USE... 

Keywords chemical energy conversion energy storage chemical heat pump separation hydrogen production reaction equilibrium... 

Chemical heat pump is a representative of chemical thermal energy conversion and storage systems. This section shows fundamental of chemical heat pump. The knowledge would be applicable for other chemical energy conversion system. 

CO2 in Figure 225(c) induces also non-equilibrium state and enhances CO2 production, then H2 productivity and purity are also enhanced. These separation processes would realize not only high-yield of H2, but also decrease of temperature of the endothermic reforming. It means that the separation process is important methodology for energy media transformation and chemical energy conversion. 

Chemical energy conversion has potential to realize efficient energy usage. It is important to find appropriate reversible chemical reaction for the temperature range of subjected energy source. Every reaction would have possibility to develop every original energy conversion process. Not only to find new reaction system, but also to improve proposed reaction is key point for the... 

To calculate the optical to chemical energy conversion efficiency, Halmann95 used the following equation ... 

The photoelectrochemical reduction of C02 at illuminated p-type semiconductor electrodes is also effective for C02 reduction to highly reduced products. The combination of photocathodes with catalysts for C02 reduction leads to a marked decrease in the apparent overpotential. At present, however, light to chemical energy conversion efficiencies are still very low, and negative in some cases. 

As mentioned in the text, it is clear that C02 reduction to valuable fuels and/or raw chemicals is still at an early stage and, unfortunately, far from having practical application, especially in the case of solar to storable chemical energy conversion by photo-electrochemical means. Many difficulties remain to be overcome. However, it is also true that there has been much progress in C02 reduction in recent years. 

The are two possibilities, the direct solar into electricity conversion (photovoltaic) and the solar into chemical energy conversion (solar fuels), e.g., the production of H2 by water splitting or the photocatalytic conversion of C02 into chemi-... 

Therefore, although research on the use of solar energy, especially for solar to chemical energy conversion, is a long-term objective, it is of paramount relevance to already push research activities in this direction. 

The suitability of a cycle for hydrogen production depends upon the overall thermal efficiency and operational feasibility. A highly endothermic reaction step is required in a cycle to achieve effective heat-to-chemical energy conversion. For efficient mass and momentum transfer a fluid based system is preferred [96] and, ultimately, for large-scale hydrogen production other factors such as environmental effects and cost effectiveness must also be considered. 

These cells operate under illumination in combination with a bias, which serves to either drive electrolytic reactions for which the photon energy is insufficient or to increase the rate of chemical energy conversion by reducing electron-hole recombination in the semiconductor bulk. Most commonly an electrical bias is provided to drive the reactions [36-41]. 

Two electrode measurements were performed to determine the photoconversion, light energy to chemical energy conversion, efficiency. Photoconversion efficiency r is calculated as ... 

Despite extensive studies, the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. The main reason is that it is very difficult to meet all requirements for high efficiency. For example, high catalytic activity and sufficient passivation at the electrode surface are incompatible. It was found, however, that a semiconductor electrode modified with small metal particles can meet all the requirements and thus becomes an ideal type semiconductor electrode. Cu, Ag, and Au were chosen because they were reported to work as efficient electrocatalysts for the C02 reduction. p-Si electrodes modified with these metals in C02-staurated aqueous electrolyte under illumination produce mainly methane and ethylene.178 This is similar to the metal electrodes but the metal-particle-coated electrodes work at approximately 0.5 V more positive potentials, contrary to continuous metal-coated p-Si electrodes. 

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