Helisel kanatlı ısıtıcı bulunan silindirik kapta faz değiştiren madde ile ısıl enerji depolanmasının deneysel incelenmesi
Experimental investigation of thermal energy storage with phase change material embedded with helical finned heater inside a cylindrical container
Yazar: TOLGA ALTINOLUK
Danışman: PROF. DR. MUSTAFA ÖZDEMİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering

Özet:
Bu tez çalışmasında, faz değiştiren malzeme (FDM) ile ısıl enerjinin depolanması deneysel olarak çalışılmıştır.  FDM olarak Rubitherm RT 64 HC marka parafin kullanılmıştır.  Düz boru ısıtıcı ve kanatlı boru ısıtıcı kullanılarak deneyler yapılmış ve erime süreleri deneysel olarak tespit edilmiştir. 
Deney düzeneğinde, silindirik cam bir tüp FDM kabı olarak kullanılmış ve etrafı cam elyaf ile yalıtılmıştır.  Deney düzeneğinin, ortam şartlarının anlık değişiminden etkilenmesini önlemek için, deney düzeneği büyük bir kutu içine konulmuştur.  Isıtıcı, cam kabın merkezine monte edilmiş olduğundan FDM dolu hacim halkasal bir yapıda olmaktadır. Isıtıcı doğru akımlı güç kaynağı tarafından beslenmektedir.  Elektrik gerilimi ve akımı ölçülerek sisteme verilen enerji belirlenmiştir. Deneyler, güç kaynağının farklı voltaj ve amper değerleri ile tekrarlanarak, FDM'nin farklı ısı akılarındaki erime davranışı araştırılmıştır.
Isıtıcı, cam kap, ısıtıcı kanatları ve yalıtım yüzeylerinden, cam kabın tabanından K tipi termokupllar vasıtası ile zamana bağlı sıcaklık ölçümleri gerçekleştirilmiştir. Aynı zamanda halkasal haznenin merkezinde FDM içindeki sıcaklık değişimi ölçülmüştür.  Bütün sıcaklık ölçümleri eş zamanlı olarak bir veri toplama sistemi (data aquasation system) kaydedilmiş ve bilgisayar ortamına aktarılmıştır. 
Zamana bağlı sıcaklık ölçümleri ile FDM'nin erime sürelerinin analizleri yapılmıştır.  Isıtıcı yüzeyinde, kanatlarda, cam kap yüzeyinde, FDM içindeki noktaların erime süreleri belirlenerek karşılaştırmalar yapılmıştır. 
Aynı ısıl güç ile düz boru ısıtıcıyla yapılan deney ile helisel kanatlı ısıtıcıyla yapılan deney karşılaştırıldığında; ısıtıcı yüzeyinde en üst nokta hariç diğer noktalarda erimeyi geciktirdiği, parafin kabı yüzeyinde ise erimenin %12 ile %25 arasında daha erken gerçekleştiği görülmektedir. Radyal yöndeki sıcaklık dağılımları incelendiğinde, helisel kanatlı ısıtıcılı sistemde ısının radyal yönde daha iyi iletilebildiği görülmüştür. Helisel kanatlı ısıtıcı ile farklı ısıl güçlerde yapılan deneylerde ise, ısıl güç arttıkça doğal taşınımın yükseldiği, bunun da lokal olarak sıcaklıkların yükselmesine sebep olarak, heterojen yüksek sıcaklık bölgeleri oluşturduğu görülmüştür. Isıl güç 11.5 W'tan 20 W'a yükseldiğinde, ısıtıcı yüzeyinde %59 ile %61, parafin kabı yüzeyinde ise %50 ile %56 arasında bir iyileşme görülmektedir. Isıl güç 20 W'tan 30 W'a yükseldiğinde ise, ısıtıcı yüzeyinde %18 ile %25, parafin kabı yüzeyinde ise %11 ile %18 arasında artmıştır.

Abstract:
The energy production methods used today have many harms to nature and human health. For this reason, energy production should be provided from renewable sources to the extent possible. Among the renewable energy sources, solar energy is very promising. But there is a major obstacle to the use of solar energy as a primary source of energy production. Solar energy can not be used in the evening or winter. To overcome this problem, thermal energy storage methods are emerging as a highly effective solution.
In this thesis, thermal energy storage method was investigated, an experimental setup was made and an experimental study was carried out. Latent heat storage is carried out in a storage medium called phase change material (PCM). The advantages and disadvantages of the PCMs presented in the literature have been demonstrated, among which the paraffin heat storage medium, which is an organic PCM, has been picked. Rubitherm RT 64 HC paraffin has been used in the experiments and its thermophysical properties are given according to the manufacturer's instructions. The biggest disadvantage of PCMs is that the heat transfer coefficients are very low, and this is the result of experiments that lead to long charging and discharging times. In order to improve the heat transfer, the integration of a helical finned tubular heater into the heat storage medium has been experimentally investigated. In the test setup, a cylindrical glass tube was used as the PCM container and insulated with glass wool. Since the heater is located at the center of the glass container, the PCM filled volume is in a circular structure.  The experimental setup is also protected by a rectangular box to prevent the experimental setup from being affected by the instantaneous change of ambient conditions. A copper tube heater is located in the center of the glass container and generates heat to be stored by being fed by a direct current power supply. Current and voltage values of direct current power supply are measured and the power that is fed into system is determined by these values. Time-dependent temperature measurements were carried out by means of thermocouples on copper heater. At the same time, temperature measurements were made using thermocouples over the glass containers from the points corresponding to the exact points of the measurement points over the heater, and the times when the melting was performed were determined by these measurements. Temperature measurement was also made at the gap between the heater surface and the container wall for flat heater, and was made on a fin for helical finned heater, so that the PCM's heat transfer character in radial direction can be seen more clearly. In order to find energy losses from system to ambient, temperatures on isolation's surface, container's bottom and from ambient has also been measured Experimental setup with flat heater and helical finned heater was carried out with same conditions. Afterwards, the experiment with helical finned heater was repeated for different power inputs in order to investigate the melting behavior of PCM. All temperature measurements were simultaneously recorded in a data acquisition system and transferred to electronic environment. Experimental results are presented graphically. Time-dependent temperature measurements were used to analyze the melting times of PCM. The melting times of the points in the PCM were determined and compared on the heater surface, fins, and the glass container surface. 
Comparing the experimental results between flat tube heater and helical finned heater's performance under same thermal power, on the surface of the heater, melting delayed at other points except for the uppermost point; on the container's surface, menting time decreased between 12% and 25%. When the temperature distributions in the radial direction are examined, it has been seen that the heat can be transferred better in the radial direction in the helical fins. When the thermal power increases from 11.5 W to 20 W, there is an improvement of melting time about 59% to 61% on the heater surface and 50% to 56% on the container's surface. When the thermal power is increased from 20 W to 30 W, it increases from 18% to 25% on the heater surface and from 11% to 18% on the container's surface. Experiments carried out with a helical finned heater at different thermal powers show that as the thermal power increases, the natural convection rises, which causes the temperature to increase locally, creating heterogeneous high temperature zones. The maximum operating temperature of paraffin used in experiments is 95 oC. Therefore, as a result of locally rising temperature values, the experiments had to be terminated without completely melting the paraffin, when this occurs, the system has not been able to make full use of latent heat storage potential. So, higher the natural convection rate may not mean better latent heat storage performance, some limitations for natural convection rate may be taken into account.
In addition, an energy balance is established. As mentioned above, since the amounts of melted paraffins vary, the mass fraction of melted paraffin must first be determined, since this information is needed to calculate the amount of latent heat storage. For this reason, a proportion was established in order to determine the melted paraffin mass. Then, sensible heat, latent heat, insulation losses, base losses and losses from top were calculated. Thus, the energy balance is established between 1% and 19% error rate. According to these calculations, the most heat was stored at 11.5 W in the helical finned experiment because melted portion of paraffin is significantly higher in that experiment. On the other hand, the biggest heat loss is still at 11.5 W, because the experiment with 11.5 W was the longest continuous experiment.
With the use of helical finned heaters, it has been found that heat can be distributed more homogeneously in paraffin. In this view, improvement of the melting time was observed on the paraffin surface.
As a result of the experiments, it is seen that, energy can be stored more by reaching the maximum operation temperature later. The more uniform the temperature is distributed in the system, the later the maximum operating temperatures are reached. So, heat transfer in the radial direction must also be considered.
Therefore, it seems more convenient to store at 11.5 W if more latent heat energy is required to be stored. But if the aim is to store energy in a limited time, it seems more appropriate to work at higher thermal powers.