The use of renewable energy sources promises many "mouth-watering perks": significant resource savings, improved environmental conditions and even social changes in some regions of the planet. However, in order for these advantages to be fully utilized, it is necessary to learn how to effectively store the collected but unused energy. At the moment, underground storage is a very common method. With their help, for example, it is possible to use the surplus solar energy collected in summer during the winter months. Scientists from Halle-Wittenberg University. Martin Luther (Germany) decided to test whether the use of paraffin wax in the construction of underground thermal energy storage facilities can make them more reliable, durable and efficient. What experiments were carried out to test this idea, what did they show,and is wax as good as scientists thought about it? We learn about this from the report of the researchers. Go.
Research basis
Obviously, not in all regions of our beautiful planet, the same sources of renewable energy will produce the same output all year round. Solar energy is a prime example of this.
There are several methods for storing excess accumulated energy (in this case, in the form of heat): latent, chemical, mechanical, etc.
While latent heat accumulators use phase transition effects (eg water / ice), thermochemical accumulators are based on reversible endo- and exothermic reactions such as salt hydration. These specific methods are quite effective, but are rarely used due to the high initial material costs.
Another common technology is the storage of thermal energy in large artificial ground pools. As a heat carrier in such structures, water or water-filled gravel with a volume of several thousand cubic meters is used.
There are many storage methods, they all work to one degree or another, but there are also problems, some of which are common to all methods. The most obvious problem is heat loss.
To avoid leaks, the pool where the heat carrier is located (water, for example) must be sealed and have low thermal conductivity. The solution to this problem at the moment is a thin plastic shell. However, the materials used for this shell are not ideal, and therefore there are still leaks. The reason for this may be the poor quality or fragility of the insulating material, which leads to contact between the coolant and the environment, from which the efficiency of the entire system decreases.
Given the problems described above, the scientists decided to test the possibility of using wax as an insulating material to prevent thermal leaks in storage.
Paraffin wax is a mixture of hydrocarbon molecules with different numbers of carbon atoms. C-chain lengths range from 20 to 60 for soft and hard paraffin waxes, and this value controls both the melting point and solidification points of the material. For example, at a solidification temperature of 42 ˚C and a melting point of 40 ˚C, the molecules have a chain length of about 21 carbon atoms. The popularity of paraffin in the storage area is also explained by a fairly good indicator of the specific heat of fusion (from 150 kJ / kg to 220 kJ / kg) and a rather low thermal conductivity (from 0.15 W / m K to 0.30 W / m K, which is an order of magnitude lower than that of water-saturated gravel - about 2.4 W / mK). In addition, paraffin is a hydrophobic and non-toxic material.
It is one thing to express beautiful theories, it is quite another to have factual evidence of its reliability. To do this, the scientists conducted a series of experiments in which various combinations of conditions were implemented (temperature conditions, thickness of the tested paraffin membrane, etc.).
Preparing for the experiment
In the first phase of the study, the scientists measured the energy loss when using paraffin inside two sections of the sealing layers of the PTES (for pit thermal energy storage) structure.
Image # 1: Diagram of an experimental setup (top view) for testing thermal performance, showing the location of temperature sensors and materials used (PVC - polyvinyl chloride film; PS - polystyrene glass plates).
Image # 2: photo of an experimental setup with a black PVC film (a) and (bd) PS as a sealing layer. Legend: 1 - surrounding material; 2 - insulating layer of paraffin; 3 - PVC-film; 4 - water; 5 - sealing PS-plates; 6, 7 - temperature sensors in paraffin / water; 8 - heating device; 9 - camera.
An acrylic glass container with dimensions of 1000 x 300 x 600 mm (length, width, height) was used as an external fence. Inside was a small heat storage device with deionized water as the carrier material. The accumulator itself (600 x 200 x 400 mm) was additionally enclosed in an inner sealing shell.
In the first series of experiments, sealing was performed using rigid polystyrene glass (PS) plates 5 mm thick. In the second series of experiments, the PS plates were replaced with a polyvinyl chloride (PVC or PVC) film with a thickness of 0.5 mm, which is usually used to seal the existing tanks.
The scientists note that the comparison between PS and PVC sheets allows them to focus on the potential mechanical deformation when paraffin is included in the insulation system, which has been embedded between the layers of the sealing membrane on one of the short sides of the container ( 2a and 2b ).
Pure paraffin wax was used in the experiments. Inside the sealing membrane, it was distributed over the entire surface without voids (pores), which would not be the case with paraffinic composites.
In a series of experiments with PS plates, the thickness of the paraffin layer was 20 mm ( 2b ), and the volume was 1600 ml. In a series of experiments with PVC, the parameters were the same ( 2a). The wax used has a relatively low solidification point at 42 C and a melting point at about 40 C.
The top lid of the container has been made of clear plastic film to minimize the effects of evaporation. Foamed glass granulate was used to further protect the experiment from environmental influences and to simulate the granular properties of the soil surrounding the tank under real conditions. Considering that this material is recyclable and has granule sizes of no more than 5–8 mm, it also works as an external heat insulator (thermal conductivity λ = 0.084 W / mK).
To heat the medium, a laboratory thermostat with an electric power of 2 kW ( 2c and 2d), while a heating element with a circulation pump was installed in the center of the water column. Thus, an imitation of the direct loading procedure was created without thermal stratification in the basin and a uniform temperature distribution was achieved in all areas of the environment. Two Keysight 34901A 20-channel multiplexers and one Keysight 34972A were used for temperature measurement and data logging. A total of 15 temperature sensors ( 2d ) Pt100 were connected (characteristics: stainless steel, waterproof, 4 wires, length 500 mm, measuring tip 20 mm, accuracy 1/10 DIN).
The accuracy of the sensors is directly dependent on temperature. In the temperature range for all experiments, it ranged from ± 0.04 ˚C (at 20 ˚C) to ± 0.06 ˚C (at 60 ˚C). Three sensors were directly embedded in the wax itself at different heights.
The experiments were visually monitored using an installed HD camera.
Image No. 3: a - diagram of the process of experiments to determine thermal characteristics; b - phases of the experiment (pink - heating / cooling delay due to phase randomness effects; lines: blue - water, green - paraffin, yellow - surrounding material).
The second stage of the study consisted in checking the heat loss in the case of using paraffin.
Leak tests have confirmed the desired self-healing mechanism when using paraffin wax in waterproofing storage membranes. Since the wax is used in its pure form, it has a direct thermal transition with the interfaces of the inner and outer layers and therefore must first melt in the heating phase. Subsequently, it should be in the form of a hydrophobic mobile liquid to close the paths to the colder surrounding material in case of leaks.
Image # 4: Diagram of an experimental setup for checking leaks (green - paraffin, blue - water, red - PVC layer, yellow - surrounding material. Dots indicate the position of the sensors.
Image No. 5: a - photo of the experimental setup; b - a crack in a PVC film with escaping paraffin; c - sand with paraffin; d - impermeable connection of the surrounding material with the pore spaces filled with paraffin.
Operating and measuring equipment (sensors, heating, etc.) were the same as in the previous experimental setup. The differences were only in some dimensions: the outer polystyrene casing was smaller (400 x 200 x 200 mm), and the surrounding material was installed only on one side of the container ( 5a ). Paraffin layer 20 mm thick (800 cm 3) was applied in direct contact with the internal deionized water fill (280 mm x 200 mm x 200 mm). In the outer PS plate, a 50 x 50 mm window was covered with PVC film to simulate various types of sealant leaks such as cracks, large holes and perforated areas ( 5b ).
The area of the material surrounding the container was ultimately 100 x 200 x 200 mm, which made it possible to clearly observe and reasonably accurately measure the paraffin wax yield and dispersion ( 5c and 5b ).
The surrounding material was two substances, each of which was used in a separate series of experiments: fine sand (grain size: 0.063 to 2 mm) was used to simulate real conditions; glass balls with a diameter of 3 mm to simulate the ideal grain structure and to check the behavior of molten wax in environments with a highly porous space ( 5a ).
Image No. 6: a - diagram of the process of experiments on leakage; b is a top view of wax regions formed after a leak.
Experimental results
The graphs below (# 7 and # 8) show the results of the thermal performance tests in the heating and cooling phases for the six selected experimental settings.
Image No. 7: a - delayed heating of the laboratory heat storage due to the melting of paraffin wax; b - additional accumulated heat in the paraffin wax during the heating phase.
Image No. 8: a - delayed cooling of the laboratory heat storage due to paraffin hardening; b - additional heat released by paraffin, measured in the cooling phase.
Scientists note that the first positive results of the experiments could be seen already when evaluating time-lapse photography, since the liquid components could be observed even at low temperatures. Therefore, even experiments where the target temperatures are below the melting point of the used paraffin wax show significant retardation and heat storage / reuse effects.
This may be due to the composition of the paraffin wax, since the paraffin used in the experiments is not a highly purified material. Since it contains hydrocarbon molecules of different lengths, fractionation occurs when heated or cooled, and different partial regions melt and solidify at different temperature ranges.
It should be noted that this applies to all induced phase changes, leading not to clear and sharp, but to soft and slow transitions.
Further, we analyzed the deformations of the paraffin layer during melting when using a PVC film. Displacement of the paraffin wax due to the pressure of the filler towards the surrounding material resulted in a wedge-shaped bulge. As a result, the thickness of the insulating paraffin layer has become non-uniform along the vertical (thicker at the top, lower already due to displacement). However, such side effects can be mitigated by using an additional polystyrene insulating film.
After analyzing the visual data (camera recordings), the scientists proceeded to analyze the temperature data, starting with the heating phase (image # 7). The analysis showed significant delays due to wax melting in all six test options. This is remarkable in that this phase is relatively short with a linear increase in temperature from 0.49 to 0.71 K / min.
Delay period value range ( 7a) of various experimental settings is large, from 360 s to 1600 s (the average melting delay is about 1000 s). This figure is 80% higher than in the case of using conventional PVC film. Consequently, the results of all tests confirm that the desired effect of the use of paraffin has been achieved: the rapid charging of the storage can be effectively delayed by the wax melting process. In addition, these tests further indicate a decrease in lateral heat loss.
Figure 3b shows that there is a close correlation between the delay time and the thermal energy accumulated during the heating phase ( 7b). Consequently, the energy values also show large fluctuations, in the range from 4.21 to 12.44 kJ / kg with an average value of 6.55 kJ / kg. These values are quite small, however, the detection of slower melting processes can be aggravated by rapid heating.
As for the sealing material, its effect is quite insignificant. The difference between PVC and PS at the same temperature is not large, and the value for PS, equal to 5.78 kJ / kg, is slightly higher than the average value of 6.71 kJ / kg for all experiments with PVC.
Based on the most common thermal energy storage systems (PTES), with a storage volume of 50,000 m 3, the thickness of the paraffin layer should be about 0.1 m with a volume of 1000 m 3 .
The results ultimately show an increase in storage capacity from about 3.16 · 106 MJ (0.88 MWh) to 9.33 · 106 MJ (2.59 MWh). In other words, using paraffin will slightly increase the amount of stored energy. Although the difference is not very large, it can be regarded as a pleasant bonus, given that the essence of paraffin is not to increase the volume, but to preserve it (in the fight against leaks).
Further, calculations and assessment of the dynamics and influence of wax on the system during the cooling phase were carried out (image No. 8).
As you might expect, the cooling phase is reflected not by a linear temperature and energy gradient, but by an exponential decrease, converging to the ambient temperature. As a result, this stage covers much longer periods of time until the system temperature is equal to the ambient temperature ( 8a ; average 95 hours, maximum 144 hours).
The first results of the analysis of the cooling phase already show significant differences, since the periods of slowing down caused by the hardening of paraffin wax are several orders of magnitude higher ( 8a). They vary from 8500 s (~ 2.5 h) to about 17000 s (~ 4.7 h), with an average value of 14000 s (~ 3.9 h). In addition, the marked difference between PS and PVC values at the same temperature (34 ˚C) indicates a significant effect of the sealing material, as more paraffin wax can be used to prevent deformation processes. However, at higher operating temperatures, there is no clear tendency for delay time to increase.
In general, the results of delays in the cooling phase demonstrate a more efficient applicability of paraffin in the context of energy storage. As a result, the steepness of thermal gradients towards the environment can be reduced and energy losses are minimized.
Although the natural cooling curve used in the experiments does not adequately reflect the intermittent energy storage and discharge conditions in the particular paraffin application, the results prove that cooling is delayed by the energy recovered when the paraffin wax solidifies. Thus, short-term discharge processes can be buffered and compensated for over a longer period, which leads to a slower decrease in temperature in the storage housing and, therefore, to a lesser effect on the structure of the sealing material (and, as a consequence, on its durability).
If we translate the laboratory results into real conditions, they show that a wax volume of 1000 m3 will provide additional storage capacity from 12.01 MWh to 40.70 MWh (average 28.77 MWh).
Image No. 9: measurements of paraffin formations and surrounding material with different variants of container deformation.
As we already know, in the concept we are considering today, paraffin can serve as a "plugging" of the formed deformations of the outer walls of the storage container.
Since the shapes of the different types of leaks (cracks, round holes, etc.) vary greatly, it would be inappropriate to take into account their length or diameter. Therefore, it was decided to use the total deformation area as an auxiliary parameter for comparing dimensions (“A” in image # 9).
Despite the different dynamics of deformations due to their overall and geometric features, the technique of self-healing of the walls due to paraffin showed excellent results. The principle is really simple: in the event of a crack (or any other deformation), the paraffin comes into contact with the surrounding material, the temperature of which is low enough to cause it to harden, which leads to a blockage of the hole.
To understand how much paraffin will be lost from the total volume in the case of "repair" of deformation, a comparative analysis of the mass and volume of bodies formed in this process was carried out.
Image No. 10: mass (a) and volume (b) of bodies formed after induced leakage, consisting of paraffin wax and surrounding material.
The analysis showed that the proportion of paraffin in the formed bodies is from 36% to 67%. It follows from this that the paraffin wall loses from 5 cm3 to 80 cm3 of its volume. Taking into account the total volume of 800 m3, the losses of paraffin wax are small and range from 1.5% to 17%.
These results prove that the self-healing properties of paraffin can be applied without significant consumption of the material used and that the proposed approach works quite effectively.
For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.
Epilogue
Many things that people have been using for centuries have properties and potential uses that no one previously thought about. Paraffin is a prime example of this.
The resources of our planet are not unlimited, but we consume them oh so much. Therefore, the development of renewable energy technologies should be given maximum attention. While some scientists are concerned with collecting green energy, others are trying to create the perfect method for storing it.
This study described not so much a new method as a modification of the existing one. In currently applicable underground energy storage, the main problem is its leakage. The authors of this work suggested that paraffin wax could be a cheap and effective way to solve this problem. And this is not surprising, because paraffin has a number of useful properties: from hydrophobicity to low melting point.
Experimental results have shown that the use of a small volume of paraffin as an additional shell for energy storage significantly reduces leaks and increases the system's ability to store heat.
In the future, scientists intend to figure out how to translate such inspiring laboratory results into an industrial scale, since with a banal increase in the size of the system, its dynamics change.
However, no matter what difficulties stand in the way of this research, scientists do not doubt its importance, because any new data, new technologies and developments are of great importance for the entire renewable energy industry, which mankind so desperately needs.
Thanks for your attention, stay curious and have a great weekend, guys!
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