In today's world, the efficient use of energy is known to be playing an ever greater role. Our world has made a habit of having sufficient energy “on demand” from finite resources (oil, coal, etc.) or from increasingly obsolete models (nuclear power). But this luxury is a burden upon the environment – it requires unnecessarily large power generation reserves, increases CO2 emissions, and results in additional environmental burdens caused by the generation, transport and disposal of these energy sources.
This provides us with access to sufficient energy, but has the problem that supply and demand are often not aligned timewise. Such differences can be measured in just a few minutes (e.g. in production) up to several hours. For example, solar heat energy, which is supplied during the day but is barely consumed during that time, has demand that literally heats up overnight. Other major phase-change applications are present in combined heat & power plants, heat pumps, fuel cells and more. This is where the topic of storage comes into play. The many volatile energy generation sources (electrical, thermal) that have come into play as a result of the energy reform make storage essential. The perspective below considers the storage of thermal energy in particular.
Latent storage is based on the principle of phase changes of a suitable material (PCM => phase change material). If this material is in a solid state (crystalline) and it is supplied with energy in the form of heat, the temperature of the PCM rises until it begins to melt. This is the point at which phase change begins. During this process, the intermolecular bonds are thermally broken, causing the PCM to liquefy. From this point, the PCM begins to absorb thermal energy, without changing the temperature (latent). Only when the PCM is fully melted does the temperature rise (sensible) with a constant supply of energy.
With water storage, for instance (above 0 °C), the relatively high sensible heat is used, which water can absorb. This is 1.16 Wh/kg*K. This means that the higher the temperature differential, the more thermal energy is stored. At a temperature differential of 50 K, for instance, 1 kg of water would absorb 50 * 1.16 = 58 Wh.
The fundamental requirement here, however, is that such major temperature differentials are available in the first place! If they are not, as is required for latent storage applications, such major temperatures have no place in capacity specifications or similar in PCM technologies.
However, a good PCM can absorb as much as 50 Wh/kg in a temperature range of around just 10 K, which is why the use of latent storage is limited to low temperature differentials. The smaller the temperature differential, the more effective the latent storage, making a calculation of PCM storage capacity at large temperature differentials complete nonsense. Where larger temperature differentials are available from around 20 K, a capacity/cost comparison with water-based storage media is always advisable. There is little purpose to calculating latent storage with such large temperature differentials, because the actual function, the phase change, has been completed and with a temperature differential of 50 K, around 40 K of this is already accounted for as sensible heat. Example: 1 kg PCM (paraffin base) has a sensible heat of around 0.6 Wh/kg*K (half that of water!). 40 K * 0.6 Wh/kg*K = 24 Wh. The storage heat is low in respect of the great expense for the temperature differential, but distorts the specified fusion enthalpy of the PCM by a (useless) 50%.
At present, large water tanks are mainly used for thermal storage. This is an advisable technique when handling large temperature differentials. If, for instance, 90 °C water is stored in a water tank and the consumer uses it till 30 °C by way of example, this is a very large temperature differential of 60 K. With such large temperature variations, there is no reasonable alternative replacement for a sensible (water) storage medium. However, systems with large temperature differentials are becoming ever rarer (legacy devices), because these are prime wasters of energy. Modern energy-based systems operate with low energy differentials, putting the temperatures of the generator and consumer very close together and only requiring a bare minimum of energy to return the consumed energy to circulation by raising the temperature slightly. Such efficient systems operate with just a fraction of the primary energy that used to be consumed. The lower the temperature differential, however, the less energy sensible storage media can store. If, for instance, the usable temperature differential is just 5 K (e.g. from 5 to 10 °C), a water-based storage medium will store around 5.8 Wh per kilogram of water. With every extraction of energy, the working temperature differential also declines, thereby also reducing performance at the same time.
This is where latent storage, containing “phase change materials” (PCMs), reveals its key benefits. PCMs have the property of liquefying or solidifying at defined temperatures – a process known as changing phases (liquid-to-gas and similar systems will not be considered here).
1Under thermodynamic principles, the only thing that is exists is “heat”, but to simplify the description, the terms “heat” and “cold” will be used.
In such a phase change, a great deal of thermal energy is absorbed (solid=>liquid) or dispensed (liquid=>solid) while at a constant temperature.
For example, the heat capacity of water is 1.16 Wh/kg*K (heated by 1°C). At 5 K, this is therefore (5 K*1.16 Wh =) 5.8 Wh per kg of water. If, on the other hand, we consider the main melting point of a typical PCM, such as the paraffin-based ATP 62 from Axiotherm, we get a storage capacity of 53 Wh per kg of PCM at a temperature differential of 5 K, so a little more than nine times the storage per kilogram compared to water. The benefit declines for higher temperature differentials, coming to just 5.2 times the storage capacity with a temperature differential of 10 K, for instance. While this data would have to be adjusted in terms of density, depending on the application (paraffin PCMs can have a density of between 0.7 and 0.85 g/cm³), these values illustrate the clear advantage that latent storage has over sensible storage at relatively lower temperature differentials.
The poor heat conductivity of PCMs makes it pointless to fill a container completely with a PCM and, for instance, use a spiral tube with water flowing through to it in the hope of being able to extract and storage heat and cold this way. There have been plenty of such approaches, which more or less failed to meet expectations due to physical conditions. In such structures, the poor heat conductivity of the PCM prevents continuous energy feed-in (storage) and prevents energy discharge (extraction) even more so. This can be compared to a boiled egg – the solid layers that arise as a result of the cooking process cause a decline in the feed-in of heat into the core of the egg, and it is only due to the large temperature differentials that a relatively low heat flow is generated. For latent storage, this means that, due to the small temperature differentials, a very long time is needed to store and extract the thermal energy and the temperature gradients are very large (the core of the egg is still hot, the shell is warm at most) or smaller or bigger parts of the PCM haven't even changed phase at all.
Axiotherm latent storage on the other hand is a hybrid storage medium, with macroencapsulated PCMs in a container filled with water.
These macrocapsules are designed to keep PCM layer thicknesses as low as possible while maintaining a large surface, enabling the entire PCM mass to participate in the phase change process while also allowing for packages that provide full surface exposure and ease of flow (by means of the macrocapsule stacked above one another). The water contained in the storage medium is also used to manage the energy balance and dynamics. The mass is used to store appropriate amounts of energy in all temperature ranges in the water component, depending on temperature differential. The use of plate heat exchangers to separate the Axiotherm latent storage from the secondary circuit enables this storage water to be pumped into the circuit, preserving the energy stored in the storage system.
The water is used not only to carry thermal energy for the purpose of thermal feed-in and extraction, but also as a sensible storage medium.
Such designs operate similarly to water-based designs, only with air used as the heat carrier in this case. In principle, the idea of storage PCMs in a kind of cartridge and using them as a storage element is not entirely new. The key aspect here is also to unify the thermodynamic, production and economic needs. These can operate under the principle of direct or indirect systems, meaning that the air to be heated or cooled can flow through heat exchanger systems, be blown directly into the chamber, or be circulated. Using a PCM, it is possible for instance to cool the PCM using cold night air, and then using this “cold energy” to balance out the performance spikes in the air conditioning systems during the day. Conversely, heat from during the day can be made available at night.
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