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Text/Jia Rui said
Editor/Carry said
preface
In recent years, there has been a growing focus on something called a phase change material (PCM), which is used to store thermal energy, like a "piggy bank" for thermal energy. This kind of thing has a lot of potential in helping buildings save energy and manage it. Among them is a phase change material called paraffin, which is an organic substance that can change from solid to liquid at a specific temperature. This paraffin wax is particularly useful for solar thermal storage.
So, we can use this paraffin wax to solve some problems. For example, we know that solar energy doesn't always exist, sometimes it does, sometimes it doesn't. But if we use something like paraffin to store solar energy, we can continue to use the stored heat energy when the sun does not come out, so that there is no need to use electricity for heating, ventilation and air conditioning all the time, and domestic hot water can also be more energy-efficient. This makes it possible to use solar energy more efficiently and reduce the cost of electricity.
An overview of the built environment
Buildings consume a lot of energy, accounting for about 45% of global energy consumption, and emit about the same proportion of greenhouse gases. Energy use in the construction industry has been increasing due to population growth, urbanization, economic development, and improved quality of life. According to the International Energy Agency, if no measures are taken, the energy demand of buildings could increase by 30% by 2060.
A large part of the energy consumed by buildings is used for building services such as heating, ventilation, air conditioning and domestic hot water. Energy demand for air conditioning is expected to increase by more than 70% from 2010 to 2050. In recent years, it has been widely recognized that the application of renewable energy to buildings, especially the use of solar thermal energy, is one of the effective ways to reduce the energy consumption of air conditioning.
Solar energy is one of the most attractive renewable energy sources, not only to directly provide heat for indoor heating, but also to achieve renewable cooling. However, because solar energy is unstable, solar thermal energy and thermal energy storage need to be combined to achieve reasonable energy management. Among the various heat storage technologies, heat storage technology using phase change materials (PCM) is attracting more and more attention.
A phase change material is a substance capable of absorbing, storing, and releasing large amounts of thermal energy through a phase change over a specific temperature range. During phase change, PCM with solid-liquid phase change is suitable for use in the built environment due to its small volume change. Compared to ordinary heat storage, the use of heat storage technology of phase change materials not only reduces the storage volume, but also releases heat energy at a relatively constant temperature.
Phase change materials are mainly divided into organic, inorganic and eutectic materials, and organic phase change materials can be further divided into paraffin wax and non-paraffin. Paraffin wax has a phase change over a wide temperature range, from minus zero to above 100 degrees Celsius.
Paraffin wax not only has an ideal phase change temperature range, but also has the advantages of consistent phase change, self-nucleation, avoiding supercooling, no corrosion, long-term chemical stability, and moderate cost. However, paraffin wax features include low flammability, low thermal conductivity, and relatively little heat storage per unit volume.
When paraffin is heated to a temperature of about 60°C or higher, it undergoes a phase change. This property, combined with the previously mentioned benefits, makes paraffin wax a good choice for storing solar thermal energy in the construction sector. This can help strengthen solar-assisted heating, ventilation, air conditioning (HVAC) and hot water supply (DHW) power generation systems.
Research on paraffin-based phase change heat storage materials for construction
In the construction sector, there are two main ways to use paraffin wax to improve energy efficiency. The first is the use of paraffin-based phase change materials (PCMs) in combination with solar collectors to enhance the thermal efficiency of the system and act as an energy reservoir when needed. Another approach is to use these paraffin-based PCMs as stand-alone heat storage units, connected to solar thermal collectors to provide a continuous heat supply to the building.
In both methods, paraffin wax is first stored by absorbing heat and then used for specific applications, such as indoor heating or cooling, through a heat transfer fluid (heat transfer medium).
Combining paraffin-based phase change materials with solar collectors can reduce temperature peaks and extend the service life of solar collectors, while improving the thermal efficiency of energy systems and storing energy on-site. For example, the scientists applied paraffin wax with a phase change temperature of about 60°C to water heating by adding nano-copper particles.
Studies have shown that adding paraffin wax to solar collectors can significantly improve the energy efficiency of the system, but the thermal conductivity effect of nanocopper particles is limited. Scientists have also tried to prepare a large number of paraffin-based PCM composites, such as PCM and compressed expanded natural graphite, and applied them to flat plate solar water heaters.
After a large number of evaluations of different materials, it was found that paraffin-based PCM composites (RT 65/CENG) have high thermal stability, thermal conductivity and storage density, and are the most suitable choice. The results show that this material can effectively improve solar energy utilization in summer, but due to the high heat loss of flat plate solar collectors, solar energy utilization is low in winter.
The combination of paraffin-based phase change materials with vacuum tube collectors appears to have more potential than in cases with high heat loss. For example, there is a paraffin wax with a melting point of 67°C, which scientists fill in the manifold of a vacuum tube heat pipe solar collector as a heat storage unit to improve the performance of hot water supply.
The study found that this new system and PCM could make solar water heaters more efficient. This system can operate with a thermal efficiency of 55-60% and is not affected by changes in the amount of water pumped, which conventional systems do not have. The scientists conducted a series of experiments and found that putting a substance called paraffin wax into a vacuum tube could improve the performance of the system, compared to using erythritol.
Compared to ordinary solar hot water heating, the thermal efficiency of this new system is increased by 26%. Paraffin wax melts at 58-62°C and can be used to store heat energy. The experimental results showed that the energy efficiency of using this paraffin wax in the vacuum tube with and without fins increased by 35.8% and 47.7%, respectively.
Although the use of fins hinders the heat transfer of paraffin wax during energy storage, it is beneficial to improve overall energy efficiency in the process of releasing thermal energy.
Paraffin-based phase change materials were used as TES units
When using stand-alone solar collectors, one approach is to combine a phase change material (PCM) with a heat transfer fluid tank to enable thermal energy storage. This fluid tank contains a special substance called paraffin, which melts at a certain temperature. With this design, we are able to store two types of thermal energy at the same time: sensible heat, which is the heat released when the temperature rises, and latent heat, which is the heat released or absorbed when the phase change (melting or solidification) of the material.
This design not only improves the ability to store thermal energy, but also has the added benefit that it can help reduce the loss of heat energy during storage. Specifically, paraffin wax helps the water in the tank to form temperature stratification so that cold and hot water do not mix directly, reducing heat loss.
In this area of research, scientists select appropriate phase change materials, and the phase change temperature and shape of these materials are important. As an example, they encapsulated paraffin in a spherical capsule and then placed it in a tank for domestic hot water applications. The melting temperature of paraffin wax is 60°C, which allows the storage and release of thermal energy within the appropriate temperature range.
The scientists also conducted experiments on different methods of hot water recovery and found that the intermittent discharge method is more suitable when intermittent hot water is demanded. They also tested combinations of other phase change materials, such as spherical capsules containing paraffin, all to find optimal designs.
Overall, this study shows that the use of phase change materials as a way to store thermal energy is feasible in solar water heating systems. Different designs and phase change material choices can affect the performance of the system, but these studies help us better understand how to efficiently store and utilize the thermal energy generated by solar energy.
Unlike the first case, the second solution only uses the phase change material (PCM) heat storage unit as a heat exchanger to store thermal energy. For example, some researchers have conducted numerical studies in which three waxes with different phase change temperatures are packaged in spherical capsules and placed in different parts of the heat storage unit for different heat storage stages. Studies have found that the flexibility of the system can be improved by increasing the rate of heat release and heat absorption.
However, the thermal benefits obtained by increasing the number of layers of the phase change material are limited. Some people put a wax with a melting temperature of about 56-65 ° C into a shell and tube heat exchanger for heat storage in solar water heating systems. They did experiments to analyze the energy efficiency and lifetime cost of the system at different water flows. The results show that a larger water flow can improve energy efficiency and reduce service life costs, but at the same time reduce the efficiency of the stove.
In addition, some researchers have tested air-based phase-change filled beds, and they have verified some possible models of hot storage-filled beds. Through experimental measurements, they found a suitable model, which can describe the heat transfer function in different working fluids to indicate when the heat storage unit of the phase change material works best.
Integrated paraffin-based PCM solar auxiliary heating system
When we want to heat indoors, in addition to the traditional method, there is also a very clever way, which is to use the heat energy of the sun. This method uses a special system with something called a paraffin-based phase change energy storage unit, which can store thermal energy like a battery.
The principle of this system is simple, it consists of several parts: solar collector, paraffin-based phase change energy storage unit, pump, electric heater and piping system. During the day, solar collectors convert solar energy into hot water.
This hot water can be used directly to heat the indoor space through the heating plate under the floor and can also be stored in paraffin energy storage units. At night or when heating is needed, the system circulates the stored hot water and the heat energy in the paraffin energy storage unit through pipes, and releases the heat into the indoor space through the heating plate of the floor.
A feature of this system is that it can adjust the temperature of the heated water as needed, ensuring that the room is always comfortably warm. If solar energy is not enough to provide enough heat, there is also an electric heater that can help ensure that it is not too cold inside. In addition, the system can also control the flow of heating water by controlling the speed of the pump to meet the needs of indoor heating.
In short, this system uses solar energy and special energy storage methods to make keeping the room warm more energy-efficient and efficient.
The rotary desiccant cooling system is a new cooling method that combines dehumidification from humid air with cooling with low-quality thermal energy. This system can replace the traditional air conditioning system, which has many benefits, such as not using environmentally harmful chemicals, using heat energy that is not too hot, and independently controlling humidity and temperature.
Therefore, compared with traditional air conditioning, it is more energy-efficient and more environmentally friendly. In this system, cooling is generated by removing moisture from humid air by a desiccant material, which needs to be regenerated with low-quality thermal energy, and solar thermal energy is a good choice.
A MODEL WAS BUILT IN TRNSYS TO DESCRIBE THE SYSTEM. This system includes the part of solar energy that collects heat and stores thermal energy, and the part that collects heat energy is heated using the same model as the third part. There are also heat exchangers, ventilators for recovering heat, dehumidifiers, indirect evaporative coolers, auxiliary electric heaters and fans, each modeled in different models.
Using the same characteristics of a typical Australian house, we simulated the cooling load required for a building in Sydney summer weather conditions.
The system is a solar-assisted dehumidification and cooling system that demonstrated excellent performance over three consecutive summer days. During this time, the system not only meets the required cooling, but also stores additional energy. As can be seen from the results, the energy of the system comes mainly from the operation of pumps and fans, without the need for auxiliary heaters. Among them, the energy consumption of the fan (30.55 kWh) in the part that supplies dehumidification and cooling is much greater than that of the pump in the part where solar energy collects heat and stores it (2.43 kWh).
Although the fan is the main energy consumer, it consumes much less energy than the energy required to reheat the drying wheel. The performance indicators of the system are also very good, with an average thermal power of 16.55 and a corresponding system effectiveness (COP) of 14.37. It can be seen from the results that when the energy storage material is charged (red background), it can be effectively charged, and when discharging, due to the efficient recovery of energy, the output water temperature can reach more than 68.88 °C. In these three summer test days, the energy storage effect of paraffin energy storage materials fluctuated between 0.52% and 103.85%, showing that the heat capacity performance of energy storage materials has been well utilized.
conclusion
Paraffin wax is a special material that can change from solid to liquid at a specific temperature, a change called a phase change. This phase change temperature is very useful for the storage of solar energy. We can use paraffin phase transitions in buildings to solve the problem of solar energy use, because solar energy is not always available, sometimes intermittently.
We can give two examples to illustrate, one is the use of solar auxiliary heating system, the other is solar auxiliary dehumidification cooling system. Paraffin phase change materials are used in these systems. Studies have found that the use of paraffin phase change materials can improve indoor heating and cooling effects, making energy use more efficient. By using paraffin phase change materials to store solar heat, the heating, ventilation and air conditioning systems of buildings can be greatly improved.