The distribution of solar resources is uneven and unstable. Solar energy has the characteristics of periodicity, discontinuity, dispersion, and regional characteristics. The use of energy by humans can only be used according to the actual needs of their own production and life. This demand cannot be completely consistent with the random fluctuations of solar radiation; therefore, if solar energy is to be able to meet the actual needs of human production and life If necessary, two key issues must be solved: one is the efficient collection of solar energy; the second is how to efficiently store the collected solar energy with high density, long time and large capacity. Only after solving these two problems, especially after solving the problem of effective energy storage, can the use of solar energy have a wide range of practical value and universal significance. The storage of solar energy actually involves a huge system project, which includes: solar energy collection equipment, conversion equipment, energy storage technology development and equipment manufacturing, energy consumption (load) devices, auxiliary energy supporting systems, and intelligent control systems, etc. A series of engineering and technical issues. Among them, the optimal capacity of the energy storage device is related to the energy consumption demand, and is also determined by the time-varying solar radiation supply and reliability, as well as the parallel connection mode of auxiliary energy and related technical and economic benefit analysis.
The storage of solar energy can generally be divided into two categories: thermal energy storage and electrical-chemical energy storage. Among them, the thermal energy storage of solar radiation can be divided into three types of storage methods: sensible heat storage, latent heat storage, and chemical storage:
Below we will talk about sensible heat storage
The so-called sensible heat storage is to use the properties of substances to absorb or release heat correspondingly when the temperature rises or falls to achieve the purpose of storing thermal energy. The heat storage capacity (Q) of the sensible heat storage body is equal to the product of the mass of the heat storage body (M), the specific heat of the material (C) and the temperature (ti) change range:
Q=M • C • (t2-t1) (1)
Sensible heat storage usually uses liquid and solid materials, but there are not many sensible heat storage materials that can well meet the solar thermal energy storage conditions in all aspects. Generally, water is the first choice for the heat storage liquid materials in the middle and low temperature section; the solid materials are rocks, pebbles, and sand. Because these materials have large specific heat capacity, abundant sources, low price, non-toxic and harmless, and do not produce large chemical corrosion to the container. The disadvantage of sensible heat storage is that the energy storage density of general sensible heat storage materials is relatively small, so when more heat needs to be stored, the mass and volume of the heat storage medium are relatively large, resulting in a huge heat storage system. The cost is high. In addition, when inputting and outputting heat to the sensible heat storage material, firstly, the time limit is longer; secondly, the temperature change range is larger and the heat flow is unstable; therefore, the complexity and cost of the system operation are increased.
In the utilization of low-temperature solar heat, water is basically used as a sensible heat storage material, because in addition to the stable physical, chemical, and thermodynamic properties of water, water also has a higher boiling point and a lower vapor pressure, which can avoid the accumulation of heat. The instantaneous phase change of gas occurs in the heat exchanger, which reduces the pressure on the heat collection container, and can be used as a heat storage medium and a heat carrier at the same time, avoiding the efficiency loss caused by the secondary heat exchange. Water has a small coefficient of thermal expansion, low viscosity, and good fluidity. It is an ideal medium for natural circulation or forced circulation heat exchange. But everything has advantages and disadvantages. As a sensible heat storage material, water also has disadvantages. As an electrolytic corrosive substance, oxygen dissolved in water can easily cause oxidation and corrosion on containers and pipelines. When the water temperature drops to 0℃ and freezes, the volume of the frozen water expands greatly, which is easy to cause damage to pipes and containers. When the water temperature exceeds 100℃, the steam pressure produced by boiling will increase exponentially with the increase of absolute temperature. Therefore, for the heat-carrying medium-water, the temperature and pressure of heat storage must be strictly controlled within the phase transition criticality. As the water container of sensible heat storage material—heat preservation hot water storage tank, it is required to have anti-oxidation, corrosion resistance, pressure resistance, airtightness, and heat preservation. The volume and installation status of the heat preservation hot water storage tank should be based on heat The specific requirements of the water system are set scientifically. Because each item is closely related to the design goals of the system and the requirements of system operation, there is one that is not considered properly, which will affect the use of the system or cause new safety hazards. The basic function of the heat preservation hot water storage tank is heat preservation. In order to have an objective evaluation of the heat storage effect of the heat preservation water tank, we can objectively evaluate and compare whether the design scheme meets the requirements by calculating the heat conduction value of the flat wall around the heat preservation water tank. According to the “Fourier’s law” of heat conduction, the heat (Q) transferred by the flat wall of the heat preservation hot water storage tank is proportional to the temperature gradient (T1-T2), the heat transfer time (t) and the cross-sectional area (S) perpendicular to the direction of heat transfer. Proportional; Inversely proportional to the thickness (L) of the thermal insulation layer.
Q = λ / L • (TI－T2) • S • t (2)
In the formula: λ——The thermal conductivity of the flat wall material (kCal/m • h• ℃);
T1——The temperature inside the flat wall (℃);
T2——The temperature outside the flat wall (℃);
According to (2) we can know that the heat storage effect of a thermal insulation hot water storage tank is closely related to factors such as the physical properties of the thermal insulation material, the heat dissipation area of the thermal insulation water tank, the ambient temperature of the thermal insulation water tank and the thickness of the thermal insulation layer. .
The use of pebbles or sandstone solid sensible materials to store solar thermal energy is not technically demanding and not only has low cost, but also the storage container system structure has no more special requirements in terms of pressure resistance and corrosion resistance; for areas lacking water resources, almost It is the best choice. But this kind of system generally can only choose to use air as the heat-carrying medium, and use the natural circulation of air or the forced circulation of the fan to promote the heat exchange between the heat-storing medium and the heat-carrying medium; because the air is not corrosive and sealed The performance requirements are not strict, so the piping system structure is simple. The disadvantage is that due to the low air density, the heat exchange efficiency is not high; in addition, the flow direction of the airflow is exactly opposite when storing heat and extracting heat, so one channel cannot be shared at the same time. In addition, although the density of solid heat storage materials is higher than that of water, the specific heat capacity is smaller than that of water, and the gap between particles is larger. Therefore, when the same amount of heat is to be stored, the volume space required is greater than that of an insulating water tank with the same heat storage. Almost more than double the volume.