Comparative analysis of solid heat storage, water heat storage and molten salt heat storage

Brief Introduction of 1. Solid Heat Storage Technology Principle

Low valley electric solid heat storage equipment is an advanced and efficient clean heating product. The principle of low valley electric solid heat storage is to convert the low valley electric energy delayed by the power grid into heat energy and store it for heating or heating water during peak hours during the day, or use wind power to store unstable wind energy and turn it into a stable heat source for output, which is a clean and pollution-free product.

The solid heat storage medium is high purity magnesium oxide brick.

Working principle of solid heat storage equipment: The work of solid heat storage equipment is divided into 3 processes, and the working process is as follows:

the first process -heating process, in which the heating wire is energized and heated in the heat storage body, the electric energy is converted into heat energy, and the heat energy is stored in the solid heat storage body through heat exchange. It mainly uses trough electricity or abandoned wind power to heat the heat storage pool to meet the heat demand during peak hours during the day.

the second process -heat storage process, the heat generated by the heating wire is continuously absorbed by the solid magnesium oxide brick, and the temperature of the heat storage brick continues to rise. the temperature can reach above 750 ℃ from normal temperature, and the heat storage process is completed. The outer layer of the heat storage tank adopts high-grade thermal insulation materials, so that the high-temperature heat storage tank and the external environment reach a thermal insulation state, and ensure the high efficiency and energy saving of the heat storage system.

the third process -heat release process. according to the heat demand of the user side, the equipment can realize air-water heat exchange through frequency conversion fan and water pump according to the preset program, and gradually release the heat of the heat storage tank.

Brief Introduction of 2. Molten Salt Heat Storage Technology

Common molten salts mainly include carbonate, chloride salt, nitrate and fluoride salt. Among them, the price of carbonate and its mixture is not high, the heat of solution is large, the corrosiveness is small, and the density is high, but the melting point of carbonate is high and the viscosity of liquid carbonate is high, and some carbonates are easy to decompose. The price of chloride salt is generally very cheap, and it can be made into mixed salts with different melting points. The disadvantage is that it is highly corrosive. Fluoride salt has a high melting point and a large latent heat of melting, but the volume shrinkage of fluoride salt liquid in solid phase change is large, and the thermal conductivity is low. The advantages of nitrate are low price, low corrosiveness and will not decompose below 500 ℃, while the disadvantages are low heat of dissolution and low thermal conductivity.

Molten salt heat storage is a phase change heat storage technology. The molten salt currently used is mostly crystals such as chloride salt and nitrate. When heated, it changes from solid to liquid and absorbs heat. This is a heat storage process. When exothermic, it changes from liquid to solid, releasing solidification heat. The heat storage of molten salt mainly depends on heat absorption / heat release (latent heat of the phase) when the phase changes to complete heat storage / heat release.

The characteristics of molten salt heat storage are:

    1、The phase change heat of molten salt is relatively small, the melting point is relatively low, and the heat storage density is relatively small.

    2、The heat storage medium is crystalline salt. During the use process, the phase is converted between solid - liquid. At present, the unfavorable factors of phase separation in the use process have not been overcome. The phase change heat of molten salt will gradually decay, and the medium needs to be replaced regularly, which makes the operation cost higher.

    3、In the exothermic process, molten salt is easy to agglomerate, solid molten salt will have large voids, and the heat storage density and thermal conductivity will decrease.

4, liquid molten salt is corrosive to the copper pipe and steel pipe of the heat exchanger, and the heat exchanger must use corrosion-resistant materials, so the efficiency of the heat exchanger is low.

    5、Molten salt heat storage is suitable for low-grade heat sources such as steam, not high-grade electric energy heat sources.

Comparative Analysis of 3. Solid Heat Storage, Water Heat Storage and Molten Salt Heat Storage

Remarks:

? The basic water temperature is calculated according to 65℃。

? Density of magnesia bricks3.58g/cm³，The specific heat capacity of magnesia bricks is1.46kJ/（kg·K）。

? The density of molten salt is as follows1.8g/cm³Calculation, specific heat capacity1.4kJ/（kg·K）。