Analysis of the working principle of solar air conditioner

Solar cooling is the use of solar collectors to provide the absorption chiller with the heat medium water needed for its generator. The higher the temperature of the heat medium water, the higher the coefficient of performance (also known as COP) of the refrigerator, so that the cooling efficiency of the air conditioning system is also higher. For example, if the heat medium water temperature is about 60 ° C, the refrigerator COP is about 0 to 40; if the heat medium water temperature is about 90 ° C, the refrigerator COP is about 0 to 70; if the heat medium water temperature is about 120 ° C, the refrigerator The COP can reach 110 or more. Practice has proved that the solar air-conditioning technology scheme combining heat pipe vacuum tube collector and lithium bromide absorption chiller is successful, which opens up a new application field for solar thermal utilization technology.

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1. Basic working principle

The solar absorption air conditioning system is mainly composed of a solar collector and an absorption refrigerating machine.

1.1 Absorption refrigeration working principle

Absorption refrigeration is carried out by using a binary solution composed of two substances as a working medium. These two substances have different boiling points at the same pressure, wherein the high boiling component is called an absorbent and the low boiling component is called a refrigerant. There are two commonly used absorbent-refrigerant combinations: one is lithium bromide-water, which is usually suitable for large central air conditioners, and the other is water-ammonia, which is usually suitable for small air conditioners.

The absorption chiller is mainly composed of a generator, a condenser, an evaporator and an absorber.

This article takes a lithium bromide absorption chiller as an example. During the operation of the refrigerator, when the lithium bromide aqueous solution is heated by the heat medium water in the generator, the water in the solution is continuously vaporized; the water vapor enters the condenser, is cooled by the cooling water, and is condensed; as the water is continuously vaporized, the generator The concentration of the solution is continuously increased and enters the absorber; when the water in the condenser enters the evaporator through the throttle valve, it rapidly expands and vaporizes, and absorbs the heat of the refrigerant water in the evaporator in the vaporization process to thereby cool down. The purpose of refrigeration; in this process, the low-temperature water vapor enters the absorber, is absorbed by the concentrated lithium bromide solution in the absorber, the solution concentration is gradually reduced, and the solution pump is sent back to the generator to complete the entire cycle.

1.2 Solar absorption air conditioning working principle

The so-called solar absorption refrigeration is to use the solar collector to provide the absorption medium of the heat medium required for the generator. The higher the temperature of the heat medium water, the higher the coefficient of performance (also known as COP) of the refrigerator, so that the cooling efficiency of the air conditioning system is also higher. For example, if the heat medium water temperature is about 60 ° C, the refrigerator COP is about 0 to 40; if the heat medium water temperature is about 90 ° C, the refrigerator COP is about 0 to 70; if the heat medium water temperature is about 120 ° C, the refrigerator The COP can be more than 1?10.

Conventional absorption air conditioning systems mainly include absorption chillers, air conditioning boxes (or fan coils), boilers, etc., while solar absorption air conditioning systems add solar collectors, water storage tanks and automatic controls. system.

In the summer, the hot water heated by the collector first enters the water storage tank. When the temperature of the hot water reaches a certain value, the water storage tank supplies the heat medium water to the refrigerator; the hot water that has flowed out of the refrigerator and has cooled down flows back to the water storage tank. And then heated by the collector into high-temperature hot water; the refrigerant water generated by the refrigerator leads to the air-conditioning box to achieve the purpose of refrigeration and air-conditioning. When the solar energy is insufficient to provide high temperature heat medium water, the auxiliary boiler can replenish heat.

In the winter, the hot water heated by the collector is also first introduced into the water storage tank. When the temperature of the hot water reaches a certain value, the water storage tank directly supplies hot water to the air conditioning box to achieve the purpose of heating and heating. When the solar energy is not able to meet the requirements, the auxiliary boiler can also be used to supplement the heat.

In the non-air-conditioning heating season, as long as the hot water heated by the collector is directly led to the heat exchanger in the domestic storage tank, the cold water in the storage tank can be gradually heated for use.

2. Air conditioning and heating integrated demonstration system

In order to put solar absorption air conditioning technology into practical application, according to the “Ninth Five-Year Plan” national scientific and technological research task, Beijing Solar Energy Research Institute built a set of China's largest solar energy absorption air conditioning and heating integrated demonstration system in September 1999. (See the photo of the title).

2.1 Overview of the installation location

The solar air conditioning demonstration system was built in ^^^ Mountain City, Shandong Province. ^^^ The mountain city is located at the southeastern end of the Shandong Peninsula, with Yantai in the north, Qingdao in the west and the Yellow Sea in the south. The area has good solar energy resources, and the annual average solar radiation is 17?3MJ/m2. The highest temperature in the summer is 33?1 °C, and the lowest temperature in winter is -7-8 °C. There are cooling and heating requirements in summer and winter respectively, so it is a suitable place to install solar air conditioning system.

^^^Shanxi Yintan Tourist Resort is making great efforts to develop tourism industry by utilizing the natural conditions of the region, and is preparing to build the “China New Energy Science Park”. Science Park plans to build 8 halls and halls including wind energy museums and solar energy museums. The solar air conditioning system was built in the solar energy museum in the Science Park.

Here, people can not only visit solar science exhibits, increase solar science knowledge, learn about the latest solar technology, but also experience the comfortable environment created by solar air conditioning and heating while visiting and entertainment.

2.2 main technical performance

The newly built solar air conditioning system consists of heat pipe vacuum tube collector, lithium bromide absorption chiller, hot water storage tank, cold storage tank, domestic hot water storage tank, circulation pump, cooling tower, air conditioning box, auxiliary fuel boiler and automatic control. The system is composed of parts. After the system is installed, the operated and tested in winter, spring and summer, and the main technical performance of Table 1 is achieved.

2.3 system design features

(1) Organic combination of solar energy and building

The overall design of the entire solar pavilion not only makes the building beautiful, novel and unique, but also meets the requirements of collector installation. According to this principle, the south façade of the building adopts a large inclined roof structure. The area of ​​one slope is much larger than that of the plane, and more collectors can be arranged. Second, the heat collector is arranged on the slope without considering front and rear occlusion. The problem, and the shape is also very beautiful. The inclination angle of the inclined roof is 35°, which is close to the local latitude, which is beneficial to the full function of the collector.

(2) Heat pipe vacuum tube collector improves cooling and heating efficiency

The heat pipe vacuum tube collector is a major scientific and technological achievement of the Beijing Solar Energy Research Institute. It has many advantages such as high efficiency, freeze resistance, fast start-up, good heat preservation, high pressure, thermal shock resistance, operation and so on. An important component of the performance solar air conditioning system. The heat pipe vacuum tube collector provides 88 ° C of heat medium water for the high-efficiency lithium bromide refrigerator, thereby improving the refrigeration efficiency of the entire system; this collector can also work effectively in the cold winter in the north to heat the building.

(3) Two large and small hot water storage tanks speed up the daily cooling or heating process

According to the inherent characteristics of the change of solar irradiance in one day, the hot water storage tank not only can make the system run stably, but also can store the excess energy at the peak of solar radiation in the form of hot water. The difference between this system and the general solar air conditioning system is that two large and small hot water storage tanks are provided. The small hot water tank is mainly used to ensure the quick start of the system. The test results show that in the morning of sunny days in summer and winter, the water temperature in the small hot water tank can reach 88 ° C and 60 ° C respectively, thus meeting the requirements of refrigeration and heating.

(4) The dedicated cold storage tank reduces the heat loss of the system.

Although the storage tank can store energy, its ability is limited. This system has specially designed a cold storage tank. In the case of abundant solar radiation during the day, the refrigerant water produced by the refrigerator can be stored in the cold storage tank, which has the advantage that the heat loss of the system in this case is obviously stored in the hot water storage tank in the form of heat medium water. The middle is much lower because the temperature difference between the summer ambient temperature and the refrigerant water temperature is significantly smaller than the temperature difference between the heat medium water temperature and the ambient temperature.

(5) Auxiliary auxiliary boiler enables the system to operate 24/7

The operation of all solar systems is inevitably affected by climatic conditions. In order to enable the system to perform air conditioning and heating functions around the clock, auxiliary conventional energy is essential. The solar air conditioning system uses an auxiliary fuel hot water boiler. When the solar radiation is insufficient during the day and the cold or heat is needed at night, the auxiliary boiler can be started immediately to ensure the system continues to operate stably.

(6) Automatic switching between system operation and operating conditions

In systems that use solar energy to partially replace conventional energy sources, system startup, energy storage, and automation of switching between solar energy and conventional energy sources are particularly important; in addition, the system has several water storage tanks, how to work in different jobs. In the case of automatic activation of different water tanks, taking different pipelines is also the key to the normal operation of the system; in addition, the solar system should also be able to solve the problem of automatic anti-overheating and anti-freezing. Therefore, we have designed a safe and functional automatic control system for the solar air conditioning system.

3. Promote application prospects

Practice has proved that the solar air-conditioning technology scheme combining heat pipe vacuum tube collector and lithium bromide absorption chiller is successful, which opens up a new application field for solar thermal utilization technology.

Compared with conventional air conditioners, solar absorption air conditioners have the following three distinct advantages:

(1) The seasonal adaptability of solar air conditioners is good, that is to say, the cooling capacity of the system increases with the increase of solar radiant energy, which coincides with the urgent requirements for air conditioners in summer;

(2) The traditional compression chiller uses Freon as the medium, which has great destructive effect on the atmosphere. The absorption chiller uses non-toxic and harmless lithium bromide as the medium, which is very beneficial to the protection of the environment;

(3) The same solar absorption air conditioning system can combine summer cooling, winter heating and hot water supply in other seasons, which significantly improves the utilization and economy of the solar energy system.

It is true that everything must be divided into two. While emphasizing the advantages of solar air conditioning, we should also see its current limitations, so we should pay attention to solving these problems during the promotion and application process:

(1) Although solar air conditioners have entered the practical stage, users who want to use solar air conditioners are increasing, but most of the products that have been commercialized are large-scale lithium bromide refrigerators, which are only suitable for central air conditioners of units. In this regard, the air-conditioning and refrigeration industry is actively researching and developing various small-scale lithium bromide or ammonia-water absorption chillers to gradually enter the family with solar collectors;

(2) Although solar air conditioners can use solar energy resources for free, due to the low solar irradiance under natural conditions, the ratio of the collector's lighting area to the air-conditioning building area is limited. Currently, it is only applicable to the few layers. building. In this regard, we are stepping up the development of vacuum tube collectors that can generate water vapor in order to combine with vapor-type absorption chillers to further increase the ratio of collectors to air-conditioning building areas;

(3) Although solar air conditioners can greatly reduce the consumption of conventional energy and significantly reduce operating costs, the initial investment in the system is still high, and it is only suitable for limited wealthy users. To this end, we are constantly reducing the cost of existing vacuum tube collectors, so that more and more units and households have the economic affordability of using solar air conditioners.

In recent years, the surface temperature of the earth has increased year by year, people's requirements for air conditioning in summer have become more and more intense, and the installation of air conditioners has become a wave of consumption in most parts of China. We believe that the solar absorption air conditioning system can exert the comprehensive advantages of summer cooling, winter heating and hot water supply throughout the year, and it will surely achieve significant economic, social and environmental benefits, and has broad application prospects.

In theory, there are two ways to realize solar air conditioning. One is to realize the light-to-electric conversion, and then to drive the conventional compression chiller for cooling. The second is to use the solar heat to drive the refrigeration. For the former, due to the high price of high-power solar power technology, the current practicality is poor. Therefore, solar air conditioning technology generally refers to thermal energy driven air conditioning technology. Of course, solar air conditioning technology in a broad sense also includes geothermal drive and underground cold source air conditioning technology.

Due to technology, cost and other reasons, solar air conditioners generally use absorption and adsorption refrigeration technology. Absorption refrigeration technology is a technology that uses the absorption and evaporation characteristics of an absorbent to perform refrigeration. It is classified into ammonia-water absorption refrigeration and lithium bromide-water absorption refrigeration depending on the absorbent. Adsorption refrigeration technology uses solid adsorbent to adsorb refrigerant to cool, commonly used molecular sieve-water, activated carbon-methanol adsorption refrigeration. Both refrigeration technologies do not use Freon, which can avoid the destruction of the ozone layer, which has special significance; and the two use lower-level energy, which has bright prospects in terms of energy conservation and environmental protection. In addition, the adsorption refrigeration system has low operating cost (or no running cost), no moving parts, long life and no noise, especially in special fields such as aviation and aerospace.

For solar refrigeration technology, because of the efficiency of the collector, etc., it is necessary to use a relatively low heat source temperature. Therefore, solar-powered refrigerators have a problem of low efficiency. With the corresponding cost allocation of collectors, chillers, etc., the collection temperature, cold water temperature and cooling water temperature should be different, in order to establish a most economical and rational solar air conditioning system, it is yet to be The problem to be solved. In addition, due to the aging problem of solar energy collection, the thermal storage technology must also be well solved. A better thermal storage system can make up for the incompatibility and discontinuity of solar energy.

4. Advantages of solar air conditioning technology

At present, most of the air-conditioning technology used is a circulation system that uses electric energy as a power to absorb indoor heat and remove it to the outside. This type of air conditioner collects the heat inside the room and releases it into the atmosphere, further increasing the high temperature of the atmosphere. The more the air is installed, the higher the atmospheric temperature of the city, and the stronger the heat island effect will be. In addition, the widespread use of fluorides such as Freon in refrigeration cycle media has led to the destruction of the atmospheric ozone layer, and the deterioration of the ecological environment is also well known. In recent years, new air conditioners that replace the working medium of Freon (whether it pollutes the environment and have to be tested for a long time) have been put on the market. However, the problem of serious energy consumption still exists. In today's increasingly energy-intensive world, the adoption of more energy-efficient air conditioning systems is a common need of mankind.

The air conditioning system that uses solar energy as an energy source is attractive because when solar radiation is strong, the higher the ambient temperature, the more people need air conditioning. At this time, the cooling capacity of solar air conditioners is stronger. This is the ideal state of harmony between man and nature. The result of using solar air conditioning not only creates a pleasant indoor temperature, but also reduces the ambient temperature of the atmosphere, and also weakens the heat island effect in the city. More preferably, it saves energy and does not use harmful substances such as freon that damage the atmosphere. It is a veritable green air conditioner.

5. Application prospects of solar air conditioning technology

Large-scale commercial central air conditioners and home wall-mounted and vertical air conditioners that are currently mass-produced are not suitable for some high-end residences, and a small-sized central air conditioner is urgently needed to fill this gap. From the perspective of the characteristics and technical characteristics of solar air conditioners, solar air conditioners are most suitable for the resolution and application of the above contradictions. Therefore, the current demand of the air conditioner industry has brought rare opportunities for the development and application of solar air conditioner technology.

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