Technical introduction
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Ice-storage air conditioning uses electricity to store ice in the ice storage device at night when the load is low, and releases the stored cold energy by melting ice during the day, which reduces the power load of air conditioning and the installed capacity of air conditioning system during peak hours of power grid. It represents the development direction of central air conditioning in the world today.
1. Load shedding and power load balancing.
2. Improve the efficiency of the generator set and reduce environmental pollution.
3. Reduce the installed capacity of the unit and save the electricity fee of air-conditioning users.
4. Improve the operation efficiency of refrigeration unit.
5. The cold storage air conditioning system is especially suitable for places where the load is concentrated and changes greatly, such as gymnasiums, theaters, concert halls, etc.
6. The application of cold storage air conditioning technology can expand the use area of air conditioning area.
7. Suitable for the environment where emergency equipment is located, such as computer room, military facilities, telephone room, inflammable and explosive goods warehouse, etc.
superiority
(1) Save power.
(2) It saves the cost of power equipment and the trouble of using electricity.
(3) The efficiency of cold storage air conditioning is high.
(4) Save the cost of cold water equipment.
(5) Save the cost of air-conditioning box dumping equipment.
(6) Good dehumidification effect.
(7) When the power is cut off, the general generator can still keep the indoor air conditioner running.
(8) The cooling effect can be achieved quickly.
(9) Save the maintenance cost of air conditioners and electrical equipment.
(10) Reduce the noisy cold water flow and circulating air, that is, reduce the vibration and noise of water pumps and air conditioning units.
(1 1) Long service life.
disadvantaged
(1) For the ice storage system, its operating efficiency will decrease.
(2) The cost of refrigeration equipment and its occupied space increase.
(3) Increase the insulation cost of water pipes and air pipes.
(4) The coefficient of performance (COP) of refrigeration unit in ice storage air conditioning system will decrease.
Operation strategy
The so-called operation strategy refers to the optimal operation arrangement of the cold storage system according to the load and characteristics of the design cycle (such as design day or design week) and the conditions such as the structure of electricity charges. Generally, it can be summarized as full refrigeration strategy and partial refrigeration strategy.
work pattern
The working mode of the cold storage system refers to whether the system is charged with cold or supplied with cold, and whether the cold storage device and the refrigeration unit work independently or jointly during the cold supply. The cold storage system needs to operate in the specified mode to meet the requirements of cooling load. The common working modes are as follows:
(1) Ice making mode of the unit
(2) simultaneous ice making and refrigeration mode
(3) single refrigerator refrigeration mode
(4) Single melting and cooling mode
(5) Refrigerator and ice melting provide cooling at the same time.
Cooling supply
In this working mode, the refrigerator and the ice storage device run at the same time to meet the refrigeration demand. According to the strategy of partial cold storage operation, this working mode is needed to meet the requirements of cooling supply in hot season. The working mode is divided into two situations: unit priority and ice melting priority.
Unit priority
After being precooled by a refrigerator, the returned hot glycol solution flows through an ice storage device to be melted and cooled to a set temperature.
Ice melting priority
The hot glycol solution refluxed from the load end of the air conditioner is cooled to a certain intermediate temperature by the ice storage device, and then cooled to the set temperature by the refrigerator.
product classification
The ice storage air conditioning ice maker is composed of ice hockey refrigeration, steel coil internal (external) melting ice, ice slurry, ice core and other ice making methods.
Process selection
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When the ice storage air conditioning system is running, the refrigerator can have two operating conditions, namely, ice storage condition and refrigeration condition. Under the ice storage condition, the low-temperature glycol solution cooled by the refrigerator enters the ice storage heat exchanger of the ice storage tank, and the static water in the ice storage tank is cooled and frozen into ice. When the ice storage process is completed, the water of the whole ice storage equipment will be basically completely frozen. During ice melting, the warm glycol solution returned by the system enters the ice storage heat exchanger after heat exchange by the plate heat exchanger to reduce the temperature of the glycol solution, and then is sent back to the load end to meet the needs of air conditioning cooling load.
There are two processes in ethylene glycol solution system: parallel process and series process.
Parallel process
In this process, the refrigerator and the ice storage tank are connected in parallel in the system, and they can jointly provide cooling when the load is maximum. At the same time, the process can be used for cold storage, cold storage and cooling, single solution ice cooling and direct cooling by cooler.
Serial stream
That is, the refrigerator and the ice storage tank are connected in series, and a set of circulating pump is used to maintain the flow and pressure in the system and supply the basic load required by the air conditioner. The serial flow configuration is properly controlled, and various working conditions can also be switched.
The parallel process has a good balance in giving full play to the refrigeration capacity of refrigerator and ice storage tank. When storing cold at night, you only need to turn on the low-power primary pump, which is more energy-saving and flexible to operate. The series flow system is simple and the cooling capacity is constant, which is suitable for small projects and large temperature difference cooling systems.
Selection mode
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Except for air conditioning and refrigeration, all the rest time is used for refrigeration, which can minimize the capacity of the main engine.
Determining the proportion of cold storage is a very important link. Generally a few typical values (such as 30%, etc.). ) make a preliminary selection in the scheme design, and finally choose a better proportion value according to the local power policy, initial investment and operating expenses.
Ice storage tank
Capacity of ice storage tank
q′= N2 * q * T2
Selection of plate heat exchanger
f = Q/(K×δTM)
Where q is the total heat exchange capacity; K is the heat transfer coefficient; δTM is the logarithmic mean temperature difference;
water pump
In the ice storage system, due to the high price of ethylene glycol, the sealing performance of water pump is required. It is generally recommended to use a water pump with mechanical seal, which can reduce or hardly leak liquid.
Pump selection: according to the process, determine the maximum resistance and flow rate under various working conditions; In order to save energy, try to choose multiple pumps.
Parallel flow is adopted in this project, and the flow rate of main pump = q/c× Δ t.
Head p (estimated) =P main engine +P thermal storage tank +P pipeline +P valve.
Head P=P heat exchanger +P cold storage tank +P pipeline +P valve.
After pump selection, it is necessary to cooperate with automatic control specialty to check the flow and resistance distribution under various working conditions, and whether the adjustment ability of three-way valve can meet the requirements of working conditions.
Consideration factor
A) The upstream series system of the main engine is adopted, and the upstream backwater of the main engine flows through the main engine first, so that the main engine runs at a higher temperature, which improves the efficiency of the compressor and reduces the energy consumption.
B) FAFCO standard ice storage tank. FAFCO standard ice storage tank has the following advantages: ensuring thermal conductivity, completely eliminating corrosion hazards and light weight; Using incomplete freezing method can provide stable low-temperature refrigerant, reduce the flow rate of circulating water pump and the diameter of corresponding pipeline, and reduce the initial investment; External icing, no internal stress, long service life; Large heat transfer area and stable freezing and thawing speed; The icing thickness is thin, and the operation efficiency of the refrigeration main machine is high.
C〕 The design day adopts the main engine priority mode, and the main engine always runs at full load, with high unit utilization rate, minimum capacity of main engine and cold storage coil, and the most economical investment.
D) All pumps are imported high-quality products, with variable frequency operation. During the whole refrigeration period, most of the time is part load, and the pump has obvious energy-saving effect through stepless speed regulation and frequency conversion.
System index
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evaporation temperature
Cold storage air conditioning systems, especially ice storage air conditioning systems, generally reduce the evaporation temperature of refrigeration units during cold storage. Theoretically, the average power consumption rate of refrigeration unit will increase by 3% when the evaporation temperature decreases by 1℃. Therefore, when configuring the system and selecting the cold storage equipment, the evaporation temperature of the refrigeration unit should be increased as much as possible. For ice storage system, the main factor affecting the evaporation temperature of refrigeration unit is the thickness of ice. The thinner the ice thickness, the higher the evaporation temperature of the refrigeration unit and the less power consumption. However, if the ice thickness is too thin, the heat exchange area of the coil of ice storage equipment will increase, and the volume of the water tank will also increase. Therefore, the economic thickness should be considered to control the evaporation temperature of refrigeration system.
Cold storage capacity
Nominal storage capacity
Nominal storage capacity refers to the theoretical storage capacity of refrigeration equipment defined by the manufacturer of refrigeration equipment (generally greater than the net available storage capacity). The net available cold storage capacity refers to the maximum actual cold storage capacity that can be provided by the cold storage equipment when it is equal to or less than the available cold source temperature in a given cold storage and release cycle.
Available storage capacity
The percentage of available net storage capacity to nominal storage capacity is an important index to measure refrigeration equipment. The greater the ratio, the higher the utilization rate of the cold storage equipment. Of course, this value is influenced by many factors of the cold storage system, such as the configuration of the cold storage system and the inlet and outlet temperatures of the equipment. For ice storage system, this value can be approximated as the melting rate of ice.
Ice making/melting speed
There are two definitions of ice making rate (IPF), one is the proportion of ice in the water in the ice storage container when a cold storage cycle is completed in the ice storage system, and the other is the ratio of the ice making volume in the ice storage tank to the ice storage tank volume. The ice melting rate refers to the percentage of ice melting in the ice storage container after completing a cycle of melting ice and releasing cold. Ice making rate and melting rate are two very important values for evaluating ice storage equipment in ice storage system. The melting speed of ice is related to the configuration of the system. For the system downstream of the series refrigeration unit, the ice melting rate of the ice storage equipment is higher. On the contrary, it is even lower. The melting speed of parallel system is between the two.
characteristic
Usually, the cold storage temperature of the cold storage system depends on the cold storage rate and the state characteristics of the cold storage tank at this time. For the external ice melting system, it refers to the amount of ice on the inner pipe wall. For ice storage systems with short storage time, generally, a higher storage rate is needed, that is, a lower (average) storage temperature is used for storage; On the contrary, the cold storage speed is slow and the cold storage temperature is high. Generally speaking, refrigeration equipment manufacturers can provide the lowest refrigeration temperature at various refrigeration rates. For cold storage equipment, such as container type, excellent salt type, etc., supercooling will occur in the early stage of cold storage process, which will only happen when the cold storage equipment has been cooled and there is no residual ice inside, and the result is to reduce the heat transfer rate in the early stage of cold storage. Adding nucleating agent can reduce supercooling. According to foreign data, a patented nucleating agent can limit the supercooling to -3℃ ~-2℃.
For the ice storage system, during the cooling period, if the cooling temperature remains unchanged, the cooling will gradually decrease; Or when the cooling rate remains unchanged, the cooling temperature will gradually increase. This is especially obvious for completely frozen and container-type refrigeration equipment, because most of the ice outside the coil and inside the ice hockey is melted by a layer of water for heat exchange, and the heat exchange area is dynamic; For ice-making sliding type and refrigerant coil type cold storage equipment, warm water and ice directly contact to melt ice, and the cooling temperature remains relatively stable.
In fact, cold storage equipment rarely keeps the cooling rate constant, and the actual cooling rate depends on the air conditioning load curve, especially the air conditioning load value in the last few hours is the most important, which determines the maximum cooling temperature value in the cooling cycle. Therefore, for the same type of cold storage equipment, the longer the cooling temperature is kept at the actual cooling speed, the better the performance of the equipment.
Take up space
The space occupied by cold storage equipment is an important consideration for owners and designers, especially in urban areas with many high-rise buildings. Sometimes in order to increase parking spaces, the cold storage air conditioning system will be abandoned. Therefore, the volume or area per available storage capacity of cold storage equipment is an important index to measure cold storage equipment, and priority should be given to cold storage equipment with less space occupation and flexible layout.
thermal losses
When designing a cold storage tank, it should be noted that the tank must have sufficient strength to overcome the static pressure of water, ice-water mixture or other cold media, and the tank should be treated with corrosion protection and waterproof to prevent water evaporation. For the buried cold storage tank, the tank must also bear the pressure of soil and surface water around the tank. Generally, the daily energy loss of a cold storage tank is L-5%, and its value depends on the tank area, heat transfer coefficient and temperature difference between inside and outside the tank. The cooling loss must be considered in the design of the buried cold storage tank, and the heat transfer coefficient is usually 0.58 ~1.9w/m2.k. The material of the tank can be steel structure, concrete, glass fiber reinforced plastic or plastic.
safe
Cold storage air conditioning system is mainly used in commercial buildings, especially in densely populated areas. First of all, consider the security of the system. Usually, the maintenance of refrigeration equipment is very small, such as internal melting ice, container, excellent salt and so on. However, for the refrigerant coil system, because the refrigerant directly evaporates in the cold storage equipment, the evaporation area is large and the demand for refrigerant is large, so the safety and reliability of the cold storage equipment are very important. Attention should be paid to the mechanical maintenance of ice-making slide and ice crystal refrigeration equipment.
life
The service life of general air conditioning system is 15-25 years, and the service life of cold storage equipment should also be limited, at least 15 years to ensure the reliability of equipment. For example, for the advanced salt system, its service life cycle should be more than 3000 times, and the original nominal refrigeration capacity and net available refrigeration capacity of the system should still be maintained.
economy
The initial investment of cold storage air conditioning system is usually higher than that of conventional air conditioning system, which requires designers to correctly grasp the time variation characteristics of building air conditioning load, determine reasonable cold storage equipment and its system configuration, formulate the operation strategy of the system, and make accurate economic analysis, so that investors can recover the redundant investment in a short time in the form of saving electricity. Generally speaking, in a designed refrigeration system, it can be measured by the cost per unit of available storage capacity. In addition, the configuration of refrigeration system also affects the size of refrigeration equipment.
10, on the selection of refrigerants in ice storage; 1) requires the coolant to be in a liquid state at the working temperature without phase change. 2) The freezing temperature of the refrigerant is required to be at least 4 ~ 8℃ lower than the evaporation temperature of the refrigerant, and the standard evaporation temperature is higher than the highest temperature that the refrigeration system can reach. The specific heat is large, and when a certain amount of heat is transferred, it can reduce the circulating amount of coolant, reduce the power consumption of pumps conveying coolant and reduce the consumption of pipelines, thus improving the economy of circulation. In addition, when a certain amount of fluid carries a certain amount of heat, larger specific heat can reduce the temperature difference of heat transfer. 3) Higher thermal conductivity can increase the heat transfer effect and reduce the heat transfer area of heat exchange equipment. 4) viscosity should be small to reduce flow resistance and pump power. 5) chemical stability is required. Coolant will not decompose at working temperature; Do not mix with oxides in the air, and do not corrode equipment and pipelines. Thanks to the teachers of Donghua University School of Environment and Engineering for providing information.
Development state
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In developed countries, more than 60% of buildings have adopted ice storage technology. An urban cooling system in Chicago, USA, has four cooling stations in a building of more than 6 million square meters, and the city provides centralized cooling. Among them, when the ice storage capacity of No.3 cold station for urban cooling in Chicago is 6.5438+0.25 million tons, the power load is 438 MW and the daily ice production capacity is 4,700 tons. From the application of the United States, Japan, South Korea and other countries, ice storage technology can be widely used in areas with concentrated air conditioning load, large peak-valley difference and relatively concentrated buildings. At present, China's new building area is about 2 billion square meters every year, of which 800-900 million square meters are new residential and public buildings in cities, which provides a huge market for the popularization and application of ice storage technology. The area of newly built public buildings in China is about 300 million square meters every year. For example, 30% of newly-built public buildings adopt ice storage air conditioning system, which can save electricity by 654.38+0.5 billion kWh per year.