Chemical storage of solar energy

Chemical storage of solar energy

Heat storage at higher temperatures can only be stored through chemical reactions. So far, the chemical storage of solar energy is still a new subject that scientists are constantly exploring. It mainly uses reversible endothermic and exothermic chemical reactions to achieve thermal energy storage of solar radiant energy. It is called thermochemical storage. For example: limestone (calcium carbonate), which is common in nature, is baked at a high temperature in a solar focusing furnace to generate calcium oxide and release carbon dioxide; Adding water can recover a large amount of heat stored in chemical reactions while generating calcium hydroxide:

Chemical storage of solar energy
Thermochemical storage formula

The main advantages of this type of typical solar energy (reversible) and thermochemical reaction storage are: (1) High energy storage density, the general thermochemical reaction energy storage density is 2-10 times higher than the sensible heat or latent heat storage density; (2) The chemical reaction heat substance can be stored for a long time at ambient temperature without special heat insulation measures and will not cause adverse effects on the environment; (3) Long-term heat storage; (4) Long-distance transportation; ( 5) The cost related to energy is very low. The shortcoming, or the technical problem that the storage method needs to be further overcome, is that the energy output of the heat storage material is far less than the input energy, and the operation and maintenance costs of the heat storage system are high, resulting in a lack of economic profitability.

In short, the energy storage device must have the characteristics of being able to receive energy at the maximum rate, release energy at the maximum expected speed, minimize energy loss during the storage process, and have a great storage-release cycle capability, and The cost of the storage device must not be too high, too expensive, and not have a negative impact on the ecological environment, otherwise the value of storing solar radiation energy for practical use will be lost.

In the solar thermal utilization, the working fluid heated by the solar collector is water, and it is reasonable to use the heat preservation hot water storage tank as the energy storage device. The output of solar air collector is hot air, so the choice of energy storage device needs to be carefully considered. Whether to choose energy storage medium with sensible heat effect or latent heat effect, can heat energy be stored more effectively, economically and reasonably. When using a trough or tower focused heat collection system for high temperature heat collection, it is necessary to consider the possibility and necessity of using molten salt as a common heat transfer and heat storage medium.

For the field of solar photovoltaic utilization, since photovoltaic power generation is currently mainly used in small and micro independent power systems; therefore, in a sense, in addition to large-scale grid-connected power generation, all other solar cells, namely crystalline silicon The electrical energy generated by the solar-electric conversion module must be stored in acid or alkaline storage batteries through the conversion of electrical-chemical energy to ensure the normal operation of the electrical system. In this sense, the storage battery of electro-chemical energy conversion is also one of the important methods of solar energy storage. When the battery is discharged, the chemical energy is converted into electrical energy output; when the battery is charged, the electrical energy produced by the solar photovoltaic cell is converted into chemical energy and stored.

In 1859, Prandtl used corroded lead foil to form the active material as the negative electrode material, lead oxide as the positive electrode material, and sulfuric acid and water as the electrolyte, and invented the lead-acid battery. It has been more than 150 years. In the following one and a half centuries, scientists have invented alkaline storage batteries such as cadmium nickel (Ni/Cd) and iron nickel (Ni/Fe), as well as zinc-silver, nickel-hydrogen, lithium-ion and other button-type batteries. Secondary batteries, but in terms of output and application areas, lead-acid batteries still occupy the mainstream position in all types of batteries.

The electro-chemical reaction formula of lead-acid batteries is as follows:

Chemical storage of solar energy
The electro-chemical reaction formula of lead-acid battery

The root of the longevity of lead-acid batteries lies in the fact that among the commonly used batteries, the floating charge of lead-acid batteries has no “memory” effect; it has good performance with increasing temperature in the range of -40°C to 50°C; Inexpensive; in addition to lithium batteries, the voltage is the highest among storage batteries; electrical energy efficiency is as high as 60%; high-rate discharge performance is good. The disadvantage is that the service life is shorter than that of alkaline batteries; the size is larger than that of alkaline batteries and button batteries; a large amount of gas will be released during overcharging, causing the positive electrode active material to fall off and shorten the battery life; deep over-discharge also It will cause damage to the battery plate and affect the normal service life of the battery. However, under modern technical conditions, photovoltaic cell control systems can generally solve the problems of overcharging and overdischarging lead-acid batteries. For example: In recent years, Yunnan Jingneng has carried out a series of innovative technology development on the strict control of overcharge and overdischarge of lead-acid batteries; they changed the original set of batteries in the solar photovoltaic system to two sets of batteries in parallel. , And then use the patented product with independent intellectual property rights: “Photovoltaic system intelligent charge and discharge complementary controller”, based on the principle of high battery pack priority output, alternately charge and discharge the two sets of batteries; thus, it can greatly Reduce the polarization and vulcanization of the battery plate caused by the long-term discharge of a single battery. Long-term charging produces gas that causes the positive active material to fall off; the development of intelligent charge and discharge complementary control battery pulse charge and discharge technology has improved the maximum utilization of photovoltaic power generation, and also improved the conversion efficiency of the battery, so that the service life of the battery The average lengthening is 1~2 times, saving the cost of the photovoltaic system, improving the reliability and stability of the equipment, and reducing the labor intensity of manual maintenance. In the foreseeable future, lead-acid batteries will continue to perform “new veterans” in solar photovoltaic applications in the continuous technological advancement; however, driven by the continuous innovation of photovoltaic energy storage technology, it is clear that they will not let them Lead-acid batteries “only one, no other branch” will sing “one-man show” again.