In the future, this material will be used to build houses, which will be warm in winter and cool in summer without air conditioning.

Produced by: Science Popularization China

Produced by: Zhiyao Science

Producer: Computer Network Information Center, Chinese Academy of Sciences

There is a doggerel saying:

"Withered vines, old trees, and crows

Air conditioning WiFi watermelon

Ge You style sofa

Sunset

I just leaned forward."

Heating in winter and air conditioning in summer make our lives more comfortable, but at the same time, they increase the burden on the environment.

According to the China Building Energy Conservation Association, in 2018, the country's building operations emitted 2.11 billion tons of carbon dioxide, accounting for 21.9% of the country's total carbon emissions, of which heating and air-conditioning energy consumption in buildings accounted for as much as 50%-70%.

According to the plan, in order to achieve the goal of carbon neutrality by 2060, the total energy consumption of building operations must be reduced by about 90%. Although the country has taken many measures, such as encouraging people to increase the temperature of air conditioners by one degree, this is far from enough.

How to save energy and reduce emissions while ensuring people's quality of life? Researchers start from the building itself and develop new building materials to ensure that the indoor temperature is warm in winter and cool in summer. One of them is "phase change thermal storage material".

What is phase change thermal storage material?

Phase Change Materials (PCM) refers to a type of material that can store energy through the phase change process of matter. This may be a little difficult to understand. "What is phase change?" "What is phase change thermal storage?"

Next, let’s discuss these two issues in detail.

What is phase change?

Phase change is the process in which the physical phase (solid, liquid, gas, etc.) of a substance changes. Take water as an example:

Water has three states: water vapor, water and ice, corresponding to the gas phase, liquid phase and solid phase respectively.

When water at room temperature is gradually heated to 100°C, it will gradually boil and emit a large number of bubbles until it completely evaporates. This process of liquid water turning into gaseous water is called gas-liquid phase transition.

When water at room temperature is placed in the freezer of a refrigerator, the temperature will continue to drop. The water will gradually cool down and begin to freeze at 0°C, turning into a mixture of ice and water until it completely solidifies into a complete ice cube. This process of liquid water turning into solid water is called liquid-solid phase transition.

Water "phase change" (Image source: Veer Library)

The vast majority of various substances in nature exist in the three aggregate states of solid, liquid and gas, and can also undergo corresponding phase change processes.

Strictly speaking, the so-called phase refers to the homogeneous material part with the same physical properties in the material system, which is separated from other parts by a certain interface.

It should be emphasized that matter has only one gas phase, but it does not necessarily have only one solid phase or liquid phase. For example, diamond and graphite are both solid phases of carbon, but their physical and chemical properties are completely different, and they are two different solid phases. The process of graphite turning into diamond is also a solid-solid phase change. The relationship between water and ice is similar. Water has one liquid state, while ice has 7 solid phases. These solid-solid transitions and solid-liquid transitions are also phase transitions.

What is phase change thermal storage?

Similar to the melting of ice and the boiling of water, the phase change process often requires the environment to supply or remove a large amount of heat. This feature is the key to phase change materials being used as heat storage materials.

Still taking water at normal temperature and pressure as an example:

Before 100℃, when we heat water, the temperature will gradually rise;

When it reaches 100°C, water will boil, and the temperature will remain at 100 degrees until it is completely transformed into gas;

After it has all turned into gas, if you continue to heat the water vapor, the temperature will continue to rise;

Schematic diagram of water temperature changing with stored heat (Image source: provided by the author)

It can be found that as the temperature rises, while the phase change occurs, the temperature of the water-water vapor system can remain constant within a fairly large energy storage range. In other words, it can absorb or release a large amount of heat within a constant temperature range. This is the basic principle of phase change heat storage.

With phase change heat storage, we have a heat energy "bank" - when the temperature is high, the excess heat is stored to keep the system cool; when the temperature is low, the heat is released to keep the system warm. In this process, as long as the heat throughput is always within the tolerance range of the "bank", the temperature of the system will remain unchanged, just like the process of water boiling.

What’s even more wonderful is that different substances have different phase transition temperatures. We can choose the right material to maintain the phase change heat storage process at a specific temperature.

In addition to water, typical phase change materials include the following:

Inorganic salts/hydrated inorganic salts: This type of material has a very wide working range and is a more commonly used phase change thermal storage material. The higher aluminum silicate salts have a melting point of about 600°C and a phase change heat of about 500kJ/kg, and are generally used in high-temperature fields; the lower hydrated acetates are around 50 degrees and are generally used for temperature control near room temperature.

Paraffin: When used as a phase change material, the operating temperature is generally in the normal temperature range of 40~70℃, and the phase change heat is about 200kJ/kg.

High-tech nanomaterials: There are also many cutting-edge research works that attempt to more precisely control the temperature and phase change heat of PCM, and have pioneered a series of synthetic preparations of nanoscale PCM, mainly including microcapsules, polymer particles, graphene-based composites, foam metal-graphite composites, etc.

By choosing the right PCM, we can maintain the system at whatever temperature we want it to be at.

At this point, maybe you already have some ideas about how to build a house that is warm in winter and cool in summer. Then, let's see how phase change thermal storage materials can be put into use!

Application of Phase Change Thermal Storage Materials

Before we build a house with PCM, let's practice with a few simple examples:

1. Ancient “Refrigerator”

Suppose you went back to ancient times and without the help of modern technology, but you still wanted to eat cool chilled fruit in the hot summer, what would you do?

Yes, our ancestors had the same idea as you, using the ice stored in the cave in winter as a phase change energy storage material. The heat from the environment is stored in the heat "bank" of ice, thereby maintaining the low temperature of the contents; and in winter, the water can be frozen into ice again, and a new round of cycles will begin. This container is called an "ice container".

Zenghouyi Bronze Ice Mirror (Image source: Reference 6)

The use of ice mirrors can be traced back to the Western Zhou Dynasty. "Zhou Li·Tianguan·Lingren" records: "Ice mirrors are used for sacrifices." Generally, ice mirrors are divided into two layers, the outer layer holds ice cubes, and the inner layer stores food or wine. There are small holes in the outer layer, and when the ice melts into water, it will flow out from the bottom, which can also lower the temperature of the entire room. Yuan Zhen, a famous poet in the Tang Dynasty, also used ice mirrors to describe the moon: "The ice mirror of the Jianghe River is bright, and the yellow road is majestic." ("Thirty Rhymes of the Moon")

2. "Constant temperature" water cup

Although iced tea is nice, you may want to drink some warm tea when the cold winter comes. If you want to drink hot water at any time in the cold winter, we can't do without thermos cups, but "opening the lid is hot and it takes time to cool down" has become a problem we have to face. What should we do if we want to always drink warm water around 50℃?

It is not difficult to figure out that the design idea is exactly the same as that of the ice jar. It only needs to turn the outer layer of ice into a material that undergoes phase change at around 50°C.

Through inquiry, we found that the melting point of sodium acetate trihydrate is about 58°C, so it is perfect to fill it into the outer wall of the thermos cup.

What will happen when we pour 95℃ hot water into such a thermostatic cup? The heat in the hot water will be quickly transferred to the sodium acetate trihydrate on the outer wall and "stored". Since the sodium acetate trihydrate will maintain 58℃ when it undergoes phase change, the water in the cup will also be quickly cooled to this temperature. Not only that, if you don't drink this cup of water for a long time, the heat just stored in the sodium acetate trihydrate will be taken out to maintain the water temperature in the cup. Who doesn't love a cup of water that automatically cools and keeps warm?

Schematic diagram of the constant temperature water cup design (left)

The temperature of hot water in an ordinary water cup (red) and a constant temperature water cup (blue) changes over time. It is not difficult to see that when using a constant temperature cup, you can drink water of the appropriate temperature over a longer period of time (Image source: provided by the author)

3. "Constant Temperature" Room

With the same idea, researchers have also created many phase-change energy storage materials for buildings, trying to create a "constant temperature" room that is like spring all year round. Common material systems such as paraffin, fatty acids, salt hydrates, polymers, foam metals, and graphene-based composite materials have been used in the design and manufacture of "constant temperature" rooms. By injecting the above materials into the wall interlayer, the room's tolerance to temperature fluctuations can be greatly improved.

The University of Darmstadt in Germany has designed many "low-energy" houses using PCM materials. PCM lime panels are used as ceilings. No additional cooling and heating systems are designed indoors. The heat storage capacity of PCM alone can maintain a stable temperature indoors.

You may have realized that as a thermal energy "bank", phase change thermal storage materials cannot store and transport heat indefinitely. The upper limit of what they can store and transport is the total phase change heat of the phase change material.

Imagine if we keep pouring 95℃ hot water into a thermostatic cup. When the heat stored in the cup wall exceeds the phase change heat of sodium acetate trihydrate, the sodium acetate trihydrate in the cup wall will completely turn into liquid. If we continue to add hot water, the cup wall will no longer have a constant temperature effect. At this time, we must let the liquid sodium acetate trihydrate in the cup wall cool down and take out some energy so that it can continue to play its constant temperature role - for example, we can add a cup of cold water, and the cup wall will automatically heat the cold water to 58℃; or let it stand in the air to dissipate heat, both are possible.

The same is true for buildings. If they encounter high temperatures or cold snaps for days or even months, their carrying capacity will be limited. Therefore, in actual applications, they are generally equipped with supplementary heat and heat transfer facilities according to the local temperature conditions to achieve the purpose of complete temperature control - and compared to turning on the air conditioner and heater all day, this amount of heat is much smaller.

Phase change thermal storage buildings and "carbon neutrality"

Traditional fossil energy supply is relatively stable. After all, wind, thunder and rain will not prevent coal from being thrown into the furnace and burned. But this is not the case with new energy.

Most clean energy sources, including photovoltaics and wind power, have significant instability: photovoltaic output depends on sunshine conditions, while wind power output depends on atmospheric flow. This leads to peaks and valleys in their power supply costs, which are sometimes high and sometimes low.

If you use these energy sources as the main method of heating and cooling, you will face a big time allocation problem: for example, now there is enough electricity but the temperature is not high, you don’t want to turn on the air conditioner; when the temperature goes up, there is not enough electricity.

The application of phase change thermal storage materials essentially makes the time allocation of thermal energy a simple matter: when there is sufficient power, the heating/cooling will store the heat in the wall, and release it when the power is insufficient, which greatly improves the utilization rate of new energy and reduces carbon emissions.

Nowadays, some PCM materials have been put into the market and gradually commercialized, and researchers are also concentrating on research, hoping to find a better phase change thermal storage system. With the steady progress of the carbon neutrality plan, we are getting closer and closer to the vision of "turning every household into a low-carbon cabin."

References:

(1) Li Bei, Liu Daoping, Yang Liang. Research progress of composite phase change thermal storage materials[J]. Journal of Refrigeration, 2017, 38(04): 36-43.

(2) Zhu Chuanhui, Li Baoguo. Research status of phase change thermal storage materials applied to solar heating[J]. Chinese Journal of Materials Progress, 2017, 36(03): 236-240.

(3) Chang Zhao, Chen Baoming, Luo Dan. Research progress of phase change energy storage materials[J]. Gas and Heat, 2021, 41(04): 21-27+98-99.

(4) Jiang Yu, Wang Qian, Wang Dong, Zhao Tong. Research progress of high temperature phase change energy storage microcapsules[J]. Journal of Engineering Science, 2021, 43(01): 108-118.

(5) Li Binhong, Zhao Tianyu. Research progress of phase change heat storage technology for passive building energy saving [J]. China Residential Facilities, 2020(10):113-114. (6) Zhou Ran. The ancient refrigerators let the nobles spend the summer leisurely and show off their wealth [J]. National Humanities and History, 2020(15):36-43.