Harbin "Ice Sugar Factory" is open for business! How can 100,000 cubic meters of ice survive the hot summer?

If you ask which city is the hottest recently, Harbin is the most popular. The Ice and Snow World has attracted countless "Southern Little Potatoes" to flock to it. The fancy games such as "frozen pears on a plate", "sweet potatoes with spoons", "tigers acting cute" and enthusiastic services have made tourists reluctant to leave and made locals feel unfamiliar.

Image source: CCTV News

But did you know that the amount of ice used in this year's Ice and Snow World is about 250,000 cubic meters, of which 100,000 cubic meters of ice was collected the year before! And the ice for the next Ice and Snow World has also been mined!

100,000 cubic meters is about the volume of 6-7 standard gymnasiums. How does such a huge volume of ice survive the hot summer? Is it placed in a super large cold storage or a super large ice cellar? Let's take a look~

Image source: CCTV News

01 How is temperature transferred?

Before we understand the refrigeration technology of ice, let's first understand how temperature is transferred. From a microscopic perspective, temperature is the intensity of the molecular/atomic level, representing a kind of energy. According to the principles of thermodynamics, this energy can be transferred, mainly through heat conduction, heat radiation and heat convection.

Three ways of heat transfer

Image source: https://www.nuclear-power.com/nuclear-engineering/heat-transfer

Heat conduction is the transfer of heat from a high temperature section to a low temperature section along an object, which is related to the temperature difference and the thermal conductivity of the material; heat convection is caused by the movement of a fluid under the heat of the fluid, which is related to the temperature difference and the heat transfer coefficient; thermal radiation is the heat transferred by radiation, which is related to the surface temperature, Stokes constant, surface reflectivity, etc. If you want to control the transfer of heat, you need to start from these three perspectives.

02 How to keep the “open-air” Harbin ice rink warm?

The 100,000 cubic meters of ice mined from the Songhua River were stored in the open air without any special storage containers. From the CCTV video footage, we can see that the outermost layer of straw on the behemoth has fallen off, and inside the straw layer is a blue covering, which is a "big thick blanket" made of thermal insulation cotton, rock wool and other materials for insulation. The straw layer and the "big thick blanket" layer are the insulation layers of the ice. In addition, the ice is not placed directly on the ground, and there is also an insulation layer between the ice and the surface. Overall, the ice is wrapped in all directions by the thermal insulation layer before being placed on the ground.

Ice stored “outdoors”

Image source: CCTV

From the stacking of ice cubes, we can see that there is basically no space between them. After the stacked ice cubes are wrapped in the insulation layer, they are basically in a sealed state. Therefore, there is almost no heat convection in the internal space. In addition, there is no high-temperature heat source inside the "big thick blanket", so there is almost no heat radiation.

The only thing that needs to be dealt with is heat conduction, trying to prevent the transfer of heat from the outside to the inside, and the straw layer and the "big thick blanket" made of insulation cotton and rock wool play this role. The thermal conductivity of the material of the "big thick blanket" is very low, and according to the heat conduction formula, the amount of heat transferred is very limited.

The ice inside fits tightly, leaving less room for air to flow

Image source: CCTV

However, since there is no completely insulating material, after a summer, the accumulated heat will still melt some ice. About 20% of the ice will be unusable due to melting, and most of the lost ice is located in the corners of the surface.

03 Did the art of storing ice already exist in ancient times?

In fact, there are records of ice storage in my country since the Zhou Dynasty. "On the second day, ice is cut and shaved, and on the third day, ice is stored in the shade" in "The Book of Songs·Youfeng·July" is about ice harvesting and storage in the Western Zhou Dynasty. "Stored in the shade" means that the key to storing ice is to put it in a cool place, away from direct sunlight, that is, to basically cut off the heat radiation caused by the sun or other high-temperature objects.

However, in ancient times, there were no "big thick blankets" made of thermal insulation cotton and rock wool with lower thermal conductivity. Even if it was kept in a cool place, it was impossible to avoid heat exchange caused by thermal convection. Therefore, special ice cellars were built in ancient times to seal the collected ice cubes in the ice cellars. The thick ice cellar blocked the invasion of heat and minimized heat conduction.

icehouse

Image source: Reference [1]

Most ancient ice cellars were semi-underground buildings, with a side wall for entry and steps down. Ice cellars used by royal nobles were paved with stone strips and then built with bricks. They were about 8 meters wide, 7 meters high, and 20 meters long, with an arched roof. In order to isolate heat, the above-ground part of the ice cellar had to be very thick.

The appearance of the ice cellar after modern renovation

Image source: Reference [2]

The thermal insulation performance of the brick and stone ice cellar is much weaker than the "big thick blanket" made by modern technology, mainly due to heat conduction. The thermal conductivity of clay bricks is between 0.2-0.6 W/(m·K), the thermal conductivity of the stone strips (medical stone) used in the ice cellar is in the range of 0.6-0.8 W/(m·K), and the thermal conductivity of insulation wool and rock wool is usually in the range of 0.03-0.045W/(m·K), which is an order of magnitude different from the previous one. In other words, for the same thickness of insulation layer, the thermal insulation capacity of modern technology is more than ten times that of ancient technology.

The key to the ice from the Songhua River being able to survive the hot summer after being harvested is that the insulation layer is made of a material with a very low thermal conductivity. Even in an open-air environment, it can basically "isolate" the invasion of external heat, and reduce heat radiation, heat convection and heat conduction to a minimum. So the seemingly rough way of storing ice is actually not that simple~

References

[1] Liu Yang, Talking about Ice Cellars in Summer, Encyclopedia Knowledge, 2012, 47-48

[2] Xi Yu, Reconstruction of the Forbidden City Ice House and Improvement and Reuse of Museum Space, Chinese Museums, 2023, 111-115.

[3] Li Jia, Research on ice storage technology in Beijing ice cellars during the Qing Dynasty, Shaanxi: Shaanxi Normal University, 2010

Author: Wang Yongjian, Associate Professor of Nanjing Agricultural University, Deputy Secretary-General of the Science Popularization Committee of Jiangsu Mechanics Society, Chief Science and Technology Communication Expert of Jiangsu Province

Editor: Tinker Bell