①Solar water heater and solar cooker
Using solar heating, the most typical applications are solar water heaters and solar cookers.
(1). Solar water heater
A solar water heater is a device that uses the heat of solar radiation for heating to provide hot water. Because it can save electricity for heating, solar water heaters are considered one of the most important environmental protection and energy saving measures. Among the various methods of solar thermal utilization, the most complete development and application is the solar water heater. The common solar water heater system is shown in Figure 1
There are many types of solar water heaters, and they are often classified according to the types of collectors, such as common vacuum tube water heaters and flat-plate water heaters.
(2). Solar cooker
A solar cooker is a device that collects solar energy and uses the absorbed heat for cooking. Solar cookers often use spherical point concentrating collectors, as shown in Figure 2.
At present, solar cookers are widely used and the technology is relatively mature.
②Solar air conditioner
Solar air-conditioning is a relatively common way to use solar energy for cooling. Solar refrigeration refers to a refrigeration method that uses the heat energy provided by the sun to directly or indirectly drive the refrigerator.
General solar thermal utilization projects, such as heating, providing hot water, etc., are often not completely consistent with the supply of solar energy in terms of application requirements. The colder the weather and the more people need warmth, the less solar energy is provided. The energy demand of solar air conditioners is more consistent with the supply of solar energy. The stronger the solar radiation during the day and the hotter the weather, the greater the load people need to have air-conditioning. At the peak of the air-conditioning load in summer, when the solar radiation is the strongest, solar air-conditioning has good seasonal adaptability. This is the most favorable objective factor for the application of solar air conditioning. From this point of view, solar air conditioning should be one of the most reasonable solar energy applications.
There are many types of solar air-conditioning technologies, and their maturity is also different, and their industrialization process is relatively slow. However, it is undeniable that with the inclination of energy policies towards clean energy, the prospects for the popularization and popularization of solar air conditioners are infinitely bright.
The current solar air conditioning technology mainly uses solar absorption refrigeration and photoelectric conversion of electric energy to drive refrigeration.
(1). Solar absorption refrigeration
Solar absorption refrigeration is the most common way to directly refrigerate using thermal energy.
The working fluid used by the absorption chiller is a binary solution composed of two substances with a large difference in boiling point. The substance with a low boiling point is a refrigerant, and the substance with a high boiling point is an absorbent, so it is also called a refrigerant-absorbent The working fluid is right. The more mature ones are “lithium bromide-aqueous solution” absorption refrigeration and “ammonia-aqueous solution” absorption refrigeration. Among them, “lithium bromide-aqueous solution” absorption refrigeration uses lithium bromide (boiling point 1265°C) as the absorbent, and water is the refrigerant; In “aqueous solution” absorption refrigeration, water is used as the absorbent, and ammonia (boiling point 33.4°C) is the refrigerant. Figure 3 shows the working principle of the solar air-conditioning host.
The left half of Figure 3 is the refrigerant cycle, which belongs to the reverse cycle and consists of a condenser, a throttling device and an evaporator. The high-pressure gaseous refrigerant releases heat to the cooling medium in the condenser and is condensed into a low-temperature and high-pressure liquid. When it enters the evaporator through a throttle valve (or expansion valve), it rapidly expands and vaporizes, and a large amount of evaporation is absorbed during the vaporization process. The heat of the refrigerant water in the device, the refrigerant water produced by the refrigerator is led to the air conditioning box to achieve the purpose of refrigeration and air conditioning, and at the same time absorb the heat of the cooled medium to produce a refrigeration effect.
The right half of Figure 3 is the absorbent cycle, which is a positive cycle, which is mainly composed of an absorber, a generator and a solution pump. In the absorber, the liquid absorbent is used to continuously absorb the low-pressure gaseous refrigerant produced by the evaporator to achieve the purpose of maintaining the low pressure in the evaporator; the refrigerant-absorbent solution formed by the absorption of refrigerant vapor by the absorbent is pumped up by the solution After being compressed, the solution enters the generator; in the generator, the solution is directly heated and boiled by the heat collecting medium of the solar collector. The refrigerant with low boiling point vaporizes to form a high-pressure gaseous refrigerant, which enters the condenser to liquefy, and the remaining absorbent The concentrated solution returns to the absorber to absorb the low-pressure gaseous refrigerant again.
It is reported that ammonia absorption refrigeration can produce chilled water at -20~20℃, which can meet the temperature requirements from refrigeration to air-conditioning, but requires a rectification device and the system is more complicated; the lithium bromide absorption refrigeration technology is relatively mature and can produce 7- Chilled water at 20°C.
In addition, in winter, solar air conditioners first store the hot water heated by the collector in the water storage tank. When the temperature of the hot water reaches a certain value, the water storage tank directly provides hot water to the air conditioning tank to achieve the purpose of heating and heating. Figure 4 is a schematic diagram of a solar air conditioner designed by Sri Lanka Power Star.
(2) Photoelectric conversion of electric energy to drive refrigeration
Photoelectric conversion of electrical energy to drive refrigeration actually involves first converting solar energy into electrical energy (a photovoltaic power generation method), and then using electricity to drive air conditioners. The key technology of this method lies in photovoltaic power generation, not in air conditioning, and will not be discussed in detail here. Figure 5 shows the 10200 mobile solar air conditioner designed by Greencore.
Due to the volatility and discontinuity of the sun’s radiation on the ground, if you want to achieve a continuous supply of solar energy, it is necessary to collect as much solar energy as possible when the sun is full, and try to store the unused part so that the sun is not enough. It can even be used at night when there is no sunlight.
The solar pool is a way to centrally store solar energy and can be used as a heat source.
The bottom of the solar pool is deep and dark, and it has a strong ability to absorb solar radiation. The pool water is natural salt water or salt water formed by adding sodium chloride and magnesium chloride. It has a certain concentration gradient. Generally, the surface layer is clear water. The deeper the depth, the greater the salinity, and the bottom layer is even saturated. After the light radiation is absorbed by the bottom of the pool and converted into heat energy, except for the limited heat dissipation at the bottom of the pool, there is basically no heat dissipation to the surface of the pool. This is because the stable salt solution cannot be convective, and the thermal conductivity of the water itself is poor, and the upper layer of water actually becomes a thick insulation layer. The sun keeps radiating and the bottom water keeps storing heat, and the water temperature gets higher and higher. By taking out the heat from the bottom of the solar pool, various applications can be carried out, and the heat source is relatively stable.
Both natural oceans and salt lakes have similar heat storage characteristics. The working diagram of the solar pool is shown in Figure 6.
In 1979, the Israelis built a 150kW solar pool power station in the small town of Einbhoek on the south bank of the Dead Sea. In 1981, another 5000kW solar pool power station was put into operation in Israel. It is said that Israel is designing tens of thousands of kilowatts of such power stations, and it is estimated that the cost per kilowatt is roughly equivalent to the investment in hydropower stations.
The United States, Russia, Canada, Australia, India, Iran, Japan and other countries are all conducting research on solar pools.