Cold-Energy Battery

A cold-energy battery based on Phase Change Materials (PCMs) is a type of thermal energy storage system that leverages the properties of PCMs to store and release energy in the form of cold. PCMs are substances that absorb and release thermal energy during the process of melting and freezing (phase change).

In the context of a cold-energy battery, these materials are used to store 'cold' energy, which can be particularly useful in cooling and refrigeration applications.

Published: November 30, 2023.

Endothermic vs. Exothermic Processes

Before discussing further about cold-energy battery, it is important to note the difference between endothermic and exothermic processes.

• Endothermic Process: This is a process in which a system absorbs energy from its surroundings in the form of heat. Common examples include the melting of ice or the evaporation of water.
• Exothermic Process: In this process, energy is released by a system into its surroundings in the form of heat. Examples include the freezing of water or the combustion of fuels. Another example is the recharging of the cold-energy batteries...

When discussing a "cold-energy battery" using Phase Change Materials (PCMs), particularly in the context of absorbing 'cold', it's more accurate to think in terms of the heat transfer rather than 'cold transfer,' as cold is technically the absence of heat.

How Cold-Energy Battery Works

• Charging Phase: During the charging phase, the PCM is exposed to a cold source. This could be during times when electricity is cheaper or when renewable energy availability is high (like wind energy at night). The PCM absorbs this cold, changing its phase from liquid to solid. This process is exothermic, meaning the PCM releases heat (or, in this case, absorbs cold) in the environment.
• Energy Storage: Once the PCM has changed to its solid form, it retains this state until it is exposed to temperatures above its melting point. The energy is stored in the form of latent heat (in this case, latent 'cold'). The ability of PCMs to store large amounts of energy in a relatively small volume and with little temperature variation during the phase change makes them efficient for this purpose.
• Discharging Phase: When cooling is needed, the PCM is allowed to return to its liquid state. This process is endothermic, meaning the PCM-based cold-battery energy releases the stored 'cold' energy - it absorbs heat. This released cold can be used for various applications, such as air conditioning systems, refrigeration, or any other process that requires cooling.
• Reusability: After releasing its stored energy, the PCM can be recharged by exposing it again to a cold source, making it a reusable and sustainable energy storage method.

Simple Example of Cold-Energy Battery

Here is a very simple example of a cold-energy battery.

The battery consists of 1000kg of water ice frozen at 0°C and is used to keep the area cool up to 25°C, with the maximum allowed temperature of water/ice of 20°C.

The basic question is: how much J/Wh of 'cold' energy is stored in such a battery?

Cold Energy (E) is the sum of energy required to melt the ice and to heat the water from 0°C to 20°C.

E (J) = 1000 kg * 333.55 kJ/kg + 1000 kg * 4.18 kJ/kg°C * 20°C = 333.55 MJ + 83.6 MJ = 417.15 MJ

Note: one can see that most cold energy comes from melting the ice - this is the main feature of all PCMs. Adding various salts alters melting point, stored energy, etc. But, more on that later in the Cold-Energy Battery Phase Change Materials (PCMs) section.

Since one Watt-hour equals 3600 Joules, 417.15MJ equals 115.875 kWh. That means that for 10 hours, this battery can provide 11.5875 kW of cold energy, i.e., it can cool the area with 11.5875 kW for 10 hours.

Now, imagine the price of a lithium battery pack that can provide such energy and compare it with a ton of water.

Cold-Energy Battery Applications

Cold-energy batteries can be used in all walks of life, for example:

• Building Cooling: PCMs can be integrated into building materials or used in HVAC systems to reduce the load on conventional air conditioning, especially during peak hours.
• Refrigeration: In refrigeration, PCMs can help maintain temperatures in cold storage facilities, especially in cases of power outages, or reduce energy consumption during peak demand times.
• Transportation of Perishable Goods: PCM-based cold-energy batteries can be used in containers for transporting temperature-sensitive goods, maintaining a stable temperature without continuous energy input.

Cold-Energy Battery Advantages

Cold-energy batteries feature many benefits, including:

• Energy Efficiency: They can shift energy usage from peak to off-peak hours, enhancing grid efficiency.
• Reduced Operational Costs: By utilizing off-peak electricity, operational costs can be reduced.
• Sustainability: They offer an environmentally friendly solution, especially when combined with renewable energy sources.
• Stability: PCMs can maintain a relatively constant temperature while undergoing phase changes, which is beneficial for temperature-sensitive applications.

Cold-Energy Battery Phase Change Materials (PCMs)

Phase Change Materials (PCMs) used in cold-energy batteries, particularly for applications in cooling and refrigeration, vary based on their melting points, thermal conductivity, heat of fusion, and other physical and chemical properties.

Here's a list of some common types of PCMs used in these applications:

• Paraffin Waxes: These are hydrocarbons derived from petroleum. They have a wide range of melting points, are chemically stable, have a high latent heat of fusion, and are non-corrosive. However, their thermal conductivity is relatively low.
• Salt Hydrates: These are inorganic PCMs and include materials like calcium chloride hexahydrate, sodium sulfate decahydrate, and magnesium chloride hexahydrate. They have higher thermal conductivities and latent heat of fusion than paraffins but can suffer from phase separation and supercooling.
• Fatty Acids and Ester: These organic PCMs, such as lauric acid, myristic acid, and palmitic acid, are known for their congruent melting, chemical stability, and minimal supercooling. However, they often have lower thermal conductivity.
• Eutectic Salts: Eutectics are mixtures of chemicals that have a unique melting point lower than that of the individual components. They can be made from various organic and inorganic substances, offering a wide range of melting temperatures.
• Polyethylene Glycol (PEG): PEGs are polymers with a high latent heat of fusion and good thermal stability. They can be used across a range of temperatures depending on the molecular weight.
• Clathrates/Hydrates: These are cage-like structures of water molecules that trap gas molecules inside. They have high latent heat values and are useful in cold storage applications.
• Metallic PCMs: These include low melting point metals and alloys, which can be used for high-temperature applications but are less common in cold-energy storage due to their high melting points.
• Bio-based PCMs: These are derived from natural sources and are gaining interest due to their environmental friendliness. Examples include capric acid, lauric acid, and stearic acid.

Each PCM has its own set of advantages and challenges, such as cost, availability, thermal conductivity, melting point, latent heat capacity, and chemical stability.

The choice of PCM for a particular application depends on the specific thermal management requirements, including the desired operating temperature range and the thermal energy storage capacity needed.

Challenges of Cold-Energy Batteries

• Material Selection: The choice of PCM is crucial and depends on its melting point, thermal conductivity, and heat of fusion.
• Cost: The initial setup cost can be high, depending on the scale and the type of PCM used.
• Integration: Integrating PCM systems into existing infrastructure requires careful design and engineering.

A cold-energy battery using PCMs is a promising technology for efficient and sustainable thermal energy storage, particularly useful in applications where maintaining a consistent, cool temperature is important.

As with any emerging technology, ongoing research and development are expected to further enhance its efficiency, cost-effectiveness, and applicability.