The generation of fuel through solar energy is a technique based on generating chemical reactions using solar radiation. These chemical processes allow the generation of energy that would otherwise come from a source of fossil fuel or nuclear energy.
A great advantage of the generation of solar fuels is that they can be transported and stored easily. However, adding a step in the generation of electric power (fuel generation) implies a reduction in efficiency: adding an additional step between the storage of energy and the production of electricity drastically decreases the efficiency of the overall process.
Types of solar chemical reactions
Photochemical reactions generate interactions between atoms, small molecules and light. In the photochemistry there are two very important physical laws The first law of photochemistry says that light must be absorbed by a chemical substance to give rise to a photochemical reaction. The second law of photochemistry states that, for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction.
What is solar chemistry? Solar chemistry refers to a series of possible processes that take advantage of solar energy by absorbing sunlight in a chemical reaction. The idea is conceptually similar to photosynthesis in plants, which converts solar energy into the chemical bonds of glucose molecules, but without using living organisms, which is why it is also called artificial photosynthesis.
A promising approach is to use focused sunlight to provide the energy needed to separate hydrogen and oxygen from water in the presence of a metallic catalyst such as zinc. This process is normally carried out in two steps so that hydrogen and oxygen do not occur in the same chamber, to avoid a risk of explosion.
Another approach is to take the hydrogen created in this process and combine it with carbon dioxide to create methane. The benefit of this approach is that there is an established infrastructure to transport and burn methane for power generation, which is not true for hydrogen.
The main drawback of these two approaches is common to most energy storage methods: adding an additional step between energy storage and electricity production drastically decreases the efficiency of the overall process.
Artificial photosynthesis is a chemical process that mimics the natural process of photosynthesis, which converts sunlight, water and carbon dioxide into carbohydrates and oxygen. The term generally refers to any system to capture and store the energy of sunlight in the chemical bonds of a fuel (solar fuel). The photocatalytic dissociation of water converts water into protons (and finally into hydrogen) and oxygen and is one of the main research areas in artificial photosynthesis. The photochemical reduction of carbon dioxide is another process under study and reproduces the natural fixation of carbon.
The research developed in this field includes the design and construction of devices (and their components) for the direct production of solar fuels, photoelectric chemistry and its applications in fuel cells and the engineering of enzymes and photoautotrophic microorganisms for microbial biofuels and the production of biohydrogen from sunlight. Many, if not most, of the research lines are inspired by the biological world, that is, they are based on biomimetics.
Perspectives for the future of solar fuel
One of the current challenges is the development of multielectronic catalytic chemistry involved in the manufacture of carbon-based fuels (such as methanol) from the reduction of carbon dioxide. A viable alternative is hydrogen. The production of protons, although the use of water as a source of electrons (as plants do in photosynthesis) requires mastering the multielectron oxidation of two molecules of water to molecular oxygen.
In certain sectors, it is planned to work with solar fuel plants in metropolitan coastal areas by 2050: the separation of seawater that supplies hydrogen through the adjacent fuel cell electric power plants and the pure water by-product that enters directly to the municipal water system. Another vision involves all the human structures that cover the surface of the earth (ie roads, vehicles and buildings) making photosynthesis in a way even more efficiently than plants.
Hydrogen production technologies have been an important area of solar chemical research since the 1970s. Apart from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. One of these routes uses concentrators to separate water into oxygen and hydrogen at high temperatures (2,300-2,600 ° C or 4,200-4,700 ° F).
Another approach uses the heat from solar concentrators to boost the steam reforming of natural gas, which increases the overall hydrogen yield compared to conventional reforming methods. The thermochemical cycles characterized by the decomposition and regeneration of the reactants present another way for the production of hydrogen. The Solzinc process under development at the Weizmann Institute of Science uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1,200 ° C (2,200 ° F). This initial reaction produces pure zinc, which can subsequently react with water to produce hydrogen.