The current production and use of air source heat pump type central air conditioners have some urgent problems to be solved. The research and development of ground source heat pump type ice storage central air conditioners has very important significance and development prospects for energy saving, reducing user operating costs and power grid peak shaving . In order to speed up the development and application of ground-source heat pump units with ice storage central air conditioners, it is recommended that the power sector establish and improve preferential policies to encourage low-valley power consumption as soon as possible, such as increasing the peak-to-valley power price ratio as much as possible, and granting subsidies for the development and use of ice storage air-conditioning equipment. At the same time, it is also recommended that relevant manufacturers strengthen the development and research of ground-source heat pump house-type ice-storage central air conditioners, reduce the cost, improve the overall efficiency, and open up a broader market for house-type ice-storage central air conditioners.

1. Development of central air-conditioning units

Residential central air conditioners, which are residential central air conditioners, have been developing rapidly since they entered the Chinese market in the 1990s. For the reasons, first of all, our country has always regarded the housing of urban and rural residents as a top priority.

In recent years, the per capita housing area has been greatly improved, and the housing has also developed into large-scale, multi-bedroom villas, multi-storey and small high-rise buildings. Second, the people’s living standards have improved, and the interior decoration of the houses of wealthy urban and rural residents has reached At the level of "well-off", room air conditioners can no longer meet their requirements, and more people are investing in residential central air conditioners; third, the mature production technology and fierce market competition make the cost of residential central air conditioners gradually accepted by the working class. Fourth, the urban architectural landscape and environmental restrictions have also caused some small-scale commercial users in the city to switch to small-scale centralized air conditioners. It can be seen from the above points that paying attention to and discussing house-type central air-conditioning is not advanced, and house-type central air-conditioning will be a new consumption hotspot in the 21st century.

2. Current problems and solutions for central air-conditioning units

2.1 Current problems with central air-conditioning units

After investigating and understanding the current central air-conditioning units, we found that there are the following problems:

1) The domestically produced central air conditioners are mostly heat pump units with air as the heat source. Although they are convenient for use and installation, in hot summer areas, the unit condensing temperature is higher, the COP value is lower, and the unit consumption Electricity is large; in areas with low temperature and high humidity in winter, the unit needs to defrost, which causes large fluctuations in room temperature and increases power consumption of the unit.

2) Heat pump units that use air as the heat source are greatly affected by outdoor air. With the change of outdoor air temperature, the cooling (heating) volume of the heat pump unit changes in the opposite direction to the cooling (heating) volume of the building, which is difficult to match.

3) Currently domestically produced central air conditioners have no real energy regulation. Due to the large change in outdoor air temperature, during the entire cooling or heating season, the heat pump unit is mostly in a condition of low COP value under partial load. Therefore, there is currently a saying that "house-type central air conditioners are affordable but not affordable." It is understood that in Chongqing, a central air conditioner with a construction area of about 250 square meters costs more than 1,000 yuan per month.

4) Residential central air conditioners with a cooling capacity of 6kW or more require the use of three-phase power supply, but many users do not have the conditions to use three-phase power supply. When users need large cooling capacity, two single-camera groups are often used in parallel. Not only increases the investment, but also brings inconvenience to the installation.

5) The development of household air conditioners, especially central air conditioners with large electricity consumption, has led to a rapid increase in urban peak load, which has exacerbated the contradiction between supply and demand in the urban power grid. For example, the economically developed Shanghai City has a power supply of 9,600MW, and residential air-conditioning power consumption accounts for about 30%. The continuous high temperature in August 1998 and all residential air-conditioning were turned on, causing the air-conditioning load in Shanghai to soar to about 3000MW. On the 13th of the same month, Shanghai’s high electricity load reached 9018MW, a net increase of 1000MW over the same period in 1997. There was a power gap of about 700MW. The peak load and the trough load differ by 3700MW.

2.2 Ground source heat pump unit type ice storage central air conditioner

In view of the problem of air source heat pump type central air conditioner, the author proposes the ground source heat pump type ice storage central air conditioner. Its advantages are:

1) The soil is used as the heat source. As the soil temperature fluctuates little throughout the year, as the depth of the soil increases, the change in soil temperature is relatively stable. The soil temperature is higher than air temperature in winter, and lower than air temperature in summer, so the COP value of heat pump for heating and cooling is high. According to the research group of Professor Liu Xianying of Chongqing University, compared with air source heat pumps, the COP value of ground source heat pumps is increased by about 30% on average [4], which can greatly reduce the power consumption of central air-conditioning units and save users operating costs. .

2) When the outdoor temperature is in a degree state, the user's demand for cold (heat) is at its peak. Due to the delay of the soil temperature, its temperature is not in the end state at this time, and it can provide a smaller condensation temperature and The higher evaporating temperature improves the cooling (heating) capacity of the unit to meet user requirements as much as possible.

3) The buried coil of the ground source heat pump does not need to be defrosted, reducing the loss of frosting and defrosting and complicated defrosting control, thereby reducing the cost of residential central air-conditioning units.

4) The ground source heat pump does not require a fan, which can reduce noise and hot air pollution, and the operation is better than that of the air source heat pump, which has higher reliability. It brings great convenience to users.

5) The main engine of the ground source heat pump can be installed in the storage room or garage without affecting the appearance of the building at all.

6) It is precisely because of the delayed effect of soil temperature, which can increase the cooling capacity of the single unit of the central air-conditioning unit. Coupled with ice storage at night, the cooling capacity of the unit during the day can be reduced, the compressor capacity of the unit can be reduced, and the cost of the unit can be reduced. At the same time, it can also meet the needs of more single-phase electricity users.

7) In summer, even at night, the condensing temperature of the ground source heat pump is lower than that of the air source heat pump, which can reduce the compression ratio of the refrigeration system during operation, which creates extremely favorable conditions for the residential central air-conditioning to use low-valley electric ice storage .

3. Application prospects of ground-source heat pump house-type ice storage central air-conditioning

The ground-source heat pump unit ice storage central air conditioner requires buried coils, which brings certain restrictions to its application, but the author still believes that it has a greater market prospect:

1) Small villas are increasing year by year. Ground source heat pump type ice storage central air conditioner is the type of air conditioner for wealthy urban and rural residents. This part of the population accounts for a small proportion of our country's population, but the number is not small.

2) The expansion of urban green area also provides some low-rise residents, small businesses and office users with the conditions for using ground-source heat pump units with ice storage central air conditioners.

3) The auxiliary rooms of industrial and mining enterprises, such as office, metering, laboratory, and testing, also have the conditions to use the ground source heat pump house-type ice storage central air conditioner.

4) According to foreign data, under constant load operation, the annual energy saving rate of cold storage air conditioning is 13% compared with non-cold storage air conditioning. Therefore, the cold storage air conditioner will be valued and promoted by the state, and it is possible to obtain state subsidies to reduce user investment.

5) In order to solve the problem of uneven power load, my country will further increase the peak-to-valley power price ratio and move closer to the internationally popular peak-to-valley power price ratio to encourage the use of low-valley power. At the same time, "peak and valley electricity prices" are also about to enter households.

Shanghai has taken the lead in implementing "peak-to-valley electricity prices" for households, and it has been implemented on a trial basis in some households.

In order to encourage rational use of electricity by urban and rural residents, Sichuan Electric Power Company has introduced a series of preferential policies for peak and valley electricity prices. A resident user who installed a household with a one-meter time-sharing meter paid only RMB 0.1 per kilowatt-hour of electricity during the late night and low period of the rainy season from June to November. In the rest of the season, the low-night electricity consumption per kilowatt-hour is 0.1875 yuan, while the conventional electricity consumption is 0.3878 yuan per kilowatt-hour. With the full implementation of the “peak and valley electricity price” policy, ground-source heat pump units with ice storage central air conditioners will have a broader market prospect.

6) In recent years, foreign small ice storage air conditioners have developed rapidly. Five companies including BAC in Japan have jointly developed several economical small ice storage air conditioners below 10 horsepower, including internal melting ice, variable frequency and fixed frequency. The model can store cold for 5 hours. Japan's Mitsubishi Corporation has developed a cold storage air conditioning unit with a power of 0.75kW. After foreign products enter the Chinese market, they will inevitably arouse the attention and selection of consumers, and will also promote the development of domestic ice storage central air conditioners.

 4. Discussion on the structure of ground-source heat pump house-type ice storage central air-conditioning unit

For ground-source heat pump units with ice-storage central air conditioners, due to the domestic research on ground-source heat pumps in recent years, the technology has become more mature, so the development of such products should mainly focus on small-scale ice storage. The author believes that the following issues should be considered for ground-source heat pump units with ice storage central air conditioners:

1) Double thermal expansion valve

The thermal expansion valve is a commonly used throttling element. It relies on the superheat of the refrigerant at the outlet of the evaporator to adjust the opening of the valve to automatically adjust the cooling capacity of the unit to meet the needs of external thermal load changes.

The capacity of the thermal expansion valve is related to the refrigeration conditions such as the mass flow of the refrigerant and the pressure difference between the front and rear of the valve. Due to the large differences in refrigerant flow, pressure difference between the front and rear of the valve, and operating characteristics between air-conditioning conditions and ice storage conditions, it is obviously unreasonable to use the same expansion valve for the two operating conditions. Especially due to the limitation of the thermal expansion valve itself, its applicable temperature and adjustment range are small; in addition, the liquid-filled thermal expansion valve operates under ice storage conditions, and its evaporator outlet superheat is often greater than that under air conditioning conditions. Many. If the capacity of the expansion valve is too small, the heat transfer area of the evaporator will not be fully utilized and the cooling capacity will decrease; if the capacity of the expansion valve is too large, it will affect its regulation performance and increase the fluctuation of the evaporator outlet temperature and the degree of superheat. The efficiency of the refrigeration system drops, and severe liquid hammer will occur.

Since the refrigeration host operates under air conditioning conditions or ice storage conditions, generally working under rated load, its operating conditions are relatively stable, and it is more suitable to use dual expansion valves, that is, according to air conditioning conditions and ice storage conditions Select the thermal expansion valve separately. The unit runs under air conditioning conditions and uses the expansion valve for air conditioning; when operating under ice storage conditions, use the expansion valve for ice storage.

In order to meet the requirements of modern control levels, it is better to use electronic expansion valves, which have a large refrigerant flow adjustment range and allow large fluctuations in system load. According to the introduction and engineering practice, under low load, the operation efficiency of the chiller with electronic expansion valve is 28% higher than that with thermal expansion valve, and the cold water temperature can be controlled within ±0.1℃.

2) Double evaporator

There is a big difference in the evaporation temperature between the air conditioning condition and the ice storage condition, and the cooling capacity of the same refrigeration compressor is also quite different. In order to improve the heat transfer efficiency of the evaporator and ensure the good oil return of the refrigeration compressor, the unit should be equipped with two evaporators, one for ice storage in the low power trough, and one for the cooling capacity in the peak power period when the cooling capacity is insufficient. Fetch chilled water for air conditioning.

Using dual evaporators, if the matching is reasonable, a slight improvement can also store ice and produce chilled water at the same time to meet the cooling capacity required by the air conditioning in the low power valley.

3) Melting ice outside the coil

As a household ice storage central air-conditioning system, the process should be as simple as possible, and the unit size should not be too large.

The cold storage adopts the method of melting ice outside the coil. The coil becomes the evaporator of the direct evaporative refrigeration system. The water in the cold storage tank forms a certain thickness of ice on the outer surface of the coil. This not only reduces the volume of the cold storage tank, but also reduces the glycol The complex process and cold loss of the aqueous solution can also increase the evaporation temperature of the refrigeration system, increase the refrigeration capacity of the unit, and reduce the cost of the unit.

The external ice melting method allows the return water of the air conditioner to exchange heat directly with the ice in the cold storage tank. The ice melting speed is fast, and the release temperature can be greater than or equal to 1 to 2°C. After mixing with the return water of the air conditioner, it can directly provide chilled water at 7°C .

5. Conclusions and recommendations

To sum up, the current production and use of air source heat pump type central air conditioners have some urgent problems to be solved. The research and development of ground source heat pump type ice storage central air conditioners is very important for energy saving, reducing user operating costs and power grid peak shaving. Significance and development prospects. In order to speed up the development and application of ground-source heat pump unit-type ice storage central air conditioners, it is recommended that the power sector establish and improve preferential policies to encourage low-valley power consumption as soon as possible, such as increasing the peak-to-valley power price ratio as much as possible, and granting subsidies for the development and use of ice storage air-conditioning equipment. At the same time, it is also recommended that relevant manufacturers strengthen the development and research of ground-source heat pump house-type ice-storage central air conditioners, reduce the cost, improve the overall efficiency, and open up a broader market for house-type ice-storage central air conditioners.