7 common solar energy storage methods introduction

In recent years, the application of solar energy cross seasonal thermal technology in energy storage industry is more and more extensive. Solar energy storage methods of cross seasonal thermal storage can effectively solve the mismatch between energy supply and demand in time and space, especially with solar energy system.

It can effectively avoid the intermittent disadvantage of solar energy, and provide a new technical route for rural heating, coal to clean energy, and regional energy supply. This paper will introduce several common solar energy storage methods of cross seasonal thermal storage.

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Types of solar energy storage methods

The solar energy storage methods of cross seasonal thermal storage is committed to solving the contradiction between the seasonal distribution of solar energy and the demand for energy consumption. In short, cross seasonal heat storage is to store heat energy such as solar energy in summer in the heat storage medium, and release the heat for heating in winter. 

According to different heat storage media, there are three solar energy storage methods:

● Sensible heat storage
● Latent heat storage
● Thermochemical heat storage

Sensible heat storage, as the first solar energy storage method, can be divided into hot water storage, gravel water storage, buried pipe storage, aquifer storage, etc. The second types of solar energy storage methods, latent heat storage, is also called phase change storage.

Phase transition is the transformation of the same substance between solid, liquid and gas states. In this process, matter will absorb and release heat. Latent heat storage is a technology that uses heat storage materials to absorb and release heat in the process of phase change.

Thermochemical heat storage uses the heat absorption and release in the process of reversible chemical reaction to store and release heat. Common thermochemical heat storage systems include decomposition of metal hydrides, decomposition of inorganic hydroxides, and reforming of methane and carbon dioxide.

Compared with the other two solar energy storage methods, thermochemical heat storage has higher heat storage density, and can realize long-term heat storage without heat loss at near ambient temperature.

The thermochemical adsorption and absorption reaction temperature zone is consistent with the low and medium temperature heat utilization temperature zone of solar energy, especially suitable for building heating and cross seasonal heat storage with compact structure.

Seven common solar energy storage methods of cross seasonal thermal storage

Since the end of the 1970s, countries in Europe and North America have tried to promote various types of solar energy cross seasonal heat storage technologies. At present, China also has practical projects of the energy storage technologies. Next, several common cross seasonal solar energy storage methods and heating technologies will be introduced.

Cross season heat storage of large heat storage tank

Hot water heat storage is one of the solar energy storage methods. The hot water heat storage generally uses the underground pool or sump for heat storage. The heat exchange can be completed through the direct flow of water into and out. It has the advantages of flexible location, large heat capacity, etc.

Compared with other solar energy storage methods, hot water heat storage has the characteristics of large heat capacity per unit volume, good fluidity, and fast heat storage and retrieval. Therefore, water tank heat storage has been applied in many large-scale seasonal heat storage demonstration projects.

In the cross seasonal solar heat storage system, the hot water heat storage device is generally cylindrical, which helps to reduce the shape coefficient and reduce heat loss, The temperature stratification of the water tank and the outer thermal insulation materials are the focus of the research on the heat storage of the water tank.

Inter seasonal heat storage of underground aquifer

Inter seasonal heat storage of underground aquifer is one of the solar energy storage methods. Aquifer in aquifer heat storage refers to the layered rock stratum that can penetrate a large amount of groundwater. It has the advantages of good economy, large scale, etc., and has high application value in areas with rich shallow groundwater resources.

Aquifer mainly includes underground sandy soil, gravel and limestone layers of groundwater, and the upper and lower layers are impermeable layers. The cost of the underground aquifer inter seasonal heat storage system is relatively low, but it requires high geological conditions, so it is one of the research hotspots of the inter seasonal heat solar energy storage methods.

In the aquifer heat storage device, it is required to install a cold water well and a hot water well respectively. When the solar energy is sufficient in summer, the solar heat energy obtained will be stored in the hot water well. In winter, the hot water in the hot water well is extracted to heat the buildings and domestic hot water, and then the water after the heat extraction is poured into the cold water well.

Cross seasonal heat storage of buried pipes

As one of the solar energy storage methods, buried pipe heat storage generally uses the underground soil to store heat. The buried pipe heat storage device is to set single U-shaped pipe or double U-shaped pipe in the shaft 30-100m below the ground.

During the heat storage process, the solar heat is stored in the soil and rock through water and other media. When heating in winter, the heat in the soil and rock near the shaft is exchanged through water and other media.

Due to the low heat storage density of soil, the soil volume of buried pipe heat storage system is about 3~5 times that of water tank heat storage system. Compared with other solar energy storage methods, buried pipe heat storage has higher geological requirements, and rock and saturated water soil geological types are more appropriate.

In addition, the buried pipe system can be set underground in the parking lot and other places without occupying the surface space.

Cross seasonal thermal storage of rocks

Being one of the solar energy storage methods, the thermal insulation of rock heat storage is similar to that of water tank, and thermal insulation materials need to be installed on the top and around the heat storage area. The commonly used media in rock heat storage are cobbles, gravels, sandstones, bricks, etc.

The rock bed exchanges heat with heat transfer fluid (water or air) to realize heat storage and release. Due to the low energy density, the volume of rock type heat storage system is about 3 times that of water tank heat storage system.

Heat storage in artificial aquifers across seasons

Gravel water heat storage is one of the solar energy storage methods. It is also called artificial aquifer heat storage. The system is generally located underground, and the heat storage medium is composed of gravel and water.

When storing solar heat energy, the heat in the heat collection device is exchanged with the water and gravel in the gravel water heat storage device through the heat exchange tube embedded in the gravel.

Since gravel can disperse the force around and at the bottom of the heat storage device, it only needs a simple load bearing device to stabilize the system, which has a higher economy compared with other solar energy storage methods. The heat storage device does not need to build a load-bearing structure, and the cost is lower than that of the hot water heat storage device.

Cross season heat storage of phase change materials

Being one of the solar energy storage methods, phase change heat storage uses the heat absorbed and released by materials in the phase change process to realize thermal energy storage.

For the cross seasonal large-scale heat storage mode, heat transfer fluid can be used to store the heat collected by the solar heat collection system in the phase change heat storage system. In the global research, researchers have tried to apply the trans seasonal phase change heat storage system to agricultural greenhouses and heat pump systems.

Thermochemical cross seasonal heat storage

Thermochemical heat storage is one of the solar energy storage methods. It has a high heat storage density, and can realize long-term heat storage without heat loss at near ambient temperature.

Among them, the thermochemical adsorption and absorption reaction temperature zone is consistent with the low and medium temperature heat utilization temperature zone of solar energy, which is especially suitable for building heating and cross season heat storage with compact structure compared with other solar energy storage methods.

Similar to the cross seasonal heat storage of phase change materials, the thermochemical cross seasonal heat storage is mainly in the research stage. At present, the global scholars have carried out extensive research on the adsorption/absorption thermochemical cross seasonal heat storage system.

At present, solar thermal storage has great development potential and engineering value. We should actively promote the application of solar energy storage methods in heating, and devote ourselves to exerting the deep potential of cross seasonal heat storage in the field of low-carbon heating.