Energy storage air conditioner

1.Basic concepts of energy storage air conditioning

Air conditioning energy storage technology is a practical comprehensive technology that has emerged in China since the 1990s. Due to its ability to shift the peak and fill the valley of the power grid, it is beneficial for optimizing resource allocation throughout society; At the same time, due to the difference in peak valley electricity prices, the operating electricity bills of users have significantly decreased, making it a win-win measure for both the country and the people.

Energy storage air conditioning is the use of energy storage devices to store energy during periods when the air conditioning system does not require energy or uses less energy, and to release this energy during periods of high demand in the air conditioning system. According to the usage object and storage temperature, it can be divided into cold storage and heat storage. Based on the time of use electricity pricing policy of the power system, taking the ice storage system as an example, during the low electricity consumption period at night, an electric refrigeration machine is used to cool the system. The cold produced is stored in the form of ice (or other phase change materials), and the ice is melted and released during the peak air conditioning load (electricity price) period during the day to partially or fully meet the cooling demand.

Latent heat storage is the process of storing the absorbed or released thermal energy through a phase change in a substance, while sensible heat storage is the process of storing the thermal energy absorbed or released when a substance undergoes a temperature change. For example, for every 1 kilogram of water, a temperature change of 1 ℃ will absorb or release 1 kilocalorie of heat to the outside world, which is called sensible heat storage; And for every 1 kilogram of 0 ℃ ice that undergoes a phase change and melts into 0 ℃ water, it needs to absorb 80 kcal of heat to store latent heat energy. It is obvious that the latent heat storage energy (phase change temperature) of the same substance is much higher than the sensible heat storage energy (1 ℃ temperature difference), so using latent heat storage method will greatly reduce the amount of medium and the volume of equipment.

2、 The origin and development of energy storage air conditioning

As early as thousands of years ago, there was a record in China’s “Book of Songs” of “chiseling and flushing ice, and storing it in Lingyin”. At that time, there was no mechanical refrigeration, and our ancestors used nature’s creation to store winter ice for use in summer. This should be the oldest ice storage project.

The use of mechanical refrigeration for energy storage air conditioning in foreign countries first appeared in churches in the 1930s. Due to the small number of people and low load demand on weekdays, and the large number of people and high load demand on Sundays, the refrigeration capacity of the refrigeration machines was relatively small due to manufacturing limitations. Therefore, on weekdays, refrigeration and ice storage were carried out, and on Sundays, refrigeration and ice melting were used simultaneously to provide cooling capacity. It fully embodies the advantages of energy storage systems, which can reduce equipment capacity and improve equipment utilization. At that time, it was mainly used in places with concentrated loads and intermittent cooling, such as theaters and dairy factories. With the advancement of the mechanical manufacturing industry, the development of cold storage technology quickly came to a halt.

In the mid-1970s, with the emergence of the global energy crisis, the development of cold storage technology received new and stronger impetus. Southern California Edison Electric Company was the first to develop a time of use electricity pricing structure in 1978. In 1979, they compiled and published the “Guidelines for Off peak Cooling of Buildings”. After 1981, they promoted the application of cold storage technology and issued relevant incentive measures. By the 1990s, more than 40 power companies in the United States had established time of use electricity pricing, and dozens of companies were engaged in the development of cold storage systems and specialized refrigeration machines for ice storage.

Economically developed countries such as Europe and Japan, as well as Taiwan in China, also began researching the application of cold storage technology in the 1980s. Due to the economic recession and financial constraints caused by Japan’s defeat in the war, before the 1990s, the main focus was on developing water storage systems with lower initial investment. In recent years, Japan has shifted to developing a large number of ice storage systems; In 1990, there were only about 200 ice storage systems, but by 2002, they had developed to over 10000 storage air conditioning systems; About 45% of the low valley electricity in the power grid is utilized, which is characterized by the use of energy storage in small and medium-sized air conditioning systems. More than 600 buildings in Taiwan have adopted energy storage air conditioning systems.

South Korea has legislated in 1999 that public buildings over 3000 square meters must use energy storage air conditioning systems。

Since the 1970s, China has adopted water storage air conditioning systems in many places in sports stadium buildings. Since the construction of ice storage air conditioning systems began in 1994, as of the end of 2002, a total of 259 water storage and ice storage air conditioning systems have been built or are currently under construction. According to the engineering operation survey conducted by Tsinghua University on behalf of State Power Corporation in 1997, the vast majority of ice storage systems are operating normally with a high success rate (systems using dynamic ice storage equipment are difficult to operate and debug). Moreover, all projects after 1996 have achieved automatic control, and now they can basically operate automatically without human supervision.

3、 Energy storage medium and equipment

At present, there are mainly the following types of air conditioning cold storage media:

1. Water

Sensible thermal storage, with a storage temperature of 4-6 ℃. The main feature is that it is easy to utilize conventional chillers for existing air conditioners. The volume and efficiency of the cold storage tank depend on the temperature difference between the supply and return water of the cold storage tank, and are affected by the degree of stratification maintained between the cold storage water and the return water. To reduce and fully utilize the volume of the cold storage water tank, the return water temperature of the air conditioner should be increased as much as possible. For most building air conditioning systems, the temperature difference between the supply and return water and the cold storage tank water can be 8-11 ℃, and the volume of the cold storage tank is 0.303-0.417m3/RTH (0.086-0.118m3/KWH).

2. Ice

Latent heat storage, due to the freezing point of water being 0 ℃, the storage temperature is -3~-9 ℃. The water supply temperature of the chiller unit is significantly lower than that of the chiller unit used in conventional air conditioning, resulting in a decrease in COP and requiring a heat exchange fluid – refrigerant. The volume of the ice storage tank is generally 0.068-0.085m3/RTH (0.02~0.025m3/KWH), which is only about 1/6 of the tank, greatly reducing the equipment’s occupied volume. The ice storage device can provide lower chilled water for the air conditioning system, which is beneficial for improving the temperature difference between the supply and return water of the air conditioning system. At the same time, it can be combined with low-temperature air supply technology to further reduce the piping size and power consumption of the air conditioning system.

3. Eutectic salt

Latent heat storage is a mixture of inorganic salts and water, and the storage temperature varies depending on the composition of the salt. By configuring different proportions of hydrated salts, phase change materials with the desired phase change temperature can be obtained. However, the latent heat of phase transition of such materials is generally smaller than that of ice, and the melting process is prone to delamination, requiring the thickness of the packaging container not to be too large. Currently, price and stability have become obstacles to its widespread use.

4. Gas hydrates

Latent heat storage is a type of enveloping crystal in which foreign gas molecules are firmly surrounded by a crystal network formed by water molecules. Due to the fact that most refrigerants can form gas hydrates at 5-12 ℃, they are more suitable for air conditioning conditions and are easily melted and generated. When the hydrates crystallize, they release phase change heat equivalent to water freezing, with good heat transfer effect, good chemical stability, low corrosiveness, and good safety. Therefore, they are considered a more effective cold storage technology than ice storage. However, gas hydrates have high requirements for the cold storage tank, and the structure, sealing, pressure bearing capacity, and internal non condensable gas content of the cold storage tank all have an impact on the cold storage effect.

This article is provided by ansi-sensor.com, which specializes in producing air conditioning pressure switches and refrigerant pressure switches. Suitable refrigerants include r134a, r22, and r410a. Please feel free to inquire.