Türkiye iklim koşullarındaki veri merkezlerinin soğutulmasında ekonomizer kullanımının enerji tasarrufu ve ekonomik potansiyel değerlendirmesi
Energy savings and economic potential assessment of economizer use for cooling data centers in Turkey's climate conditions
Yazar: OZAN GÖZCÜ
Danışman: PROF. DR. İLKER MURAT KOÇ ; DR. ÖĞR. ÜYESİ HAMZA SALİH ERDEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Sistem Dinamiği ve Kontrol Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Veri merkezleri gibi yüksek yoğunlukta elektriksel yük ve soğutma ihtiyacı bulunan tesislerde mekanik soğutma altyapısının enerji tüketimi, toplam tüketimin önemli bir kısmını oluşturmaktadır. Serbest soğutma sistemleri, belirli koşullar altındaki dış hava şartlarını kullanarak mekanik soğutma ihtiyacını tamamen karşılayabilmekte veya azaltabilmektedir. Bu çalışma veri merkezi soğutma altyapısının kısmi yük performansı ve değişken iklim şartlarına duyarlı termodinamik modelleri hakkında genel bir bakış sunmaktadır. Bu modeller yaygın olarak kullanılan Doğrudan Hava Ekonomizeri (ASE), Dolaylı Hava Ekonomizeri (IASE), Dolaylı Evaporatif Soğutmalı (IEC) ve Dolaylı Su Ekonomizeri (WSE) gibi dört ayrı serbest soğutma sistemini kapsamaktadır. Referans olarak bilişim teknolojisi (BT) teçhizatının 1 MW elektriksel yüke sahip olduğu tipik hava soğutmalı bir veri merkezinin termodinamik modeli (BL) kabul edilmiştir. Türkiye'nin 10 farklı şehrindeki iklim şartlarında gerçekleştirilen yıllık enerji hesaplamalarına göre, farklı serbest soğutma yöntemlerinin BL veri merkezine olası iyileştirme uygulamaları olarak etkileri elde edilmiştir. Şehirler belirlenirken şehrin nüfus yoğunluğu, ülke ekonomisine katılım etkisinin yanı sıra iklim çeşitliliğinin sağlanması gibi kıstaslar da dikkate alınmıştır. Enerji tüketim değerlerinin yanı sıra sistemlerin 15 yıllık ekonomik kullanım ömrü gözetilerek toplam sahip olma analizi (TSO) çalışmaları yapılmıştır. Bu çalışmada ekonomik analiz yöntemleri ile hangi şehirde hangi sistemin seçiminin iyileştirme çalışmaları kapsamında daha uygun olacağı belirlenmiştir. Sadece enerji tasarrufu değerleri irdelendiğinde açık koridorlu sistemler için ASE en başarılı sonuçları verirken, daha yüksek sıcaklıklarda çalışma imkanı sağlayan kapalı koridorlu sistemlerde ibre IEC lehine dönmüştür. Sonuçlara toplam sahip olma (TSO) analiz yöntemleri ile elde edilen çıktılar eklendiğinde ise ilk yatırım maliyetinin diğer sistemlere göre çok aşağılarda kalmasının avantajı ile WSE değiştirilmiş iç kârlılık oranı (DİKO) en yüksek yöntem olarak ön plana çıkmıştır. Net bugünkü değer (NBD) yöntemine göre yapılan değerlendirmede ise 15 yıllık ekonomik kullanım ömrü için ASE ve IEC en değerli yatırım opsiyonları olarak görülmektedir. Sonuçlar göstermiştir ki toplam sahip olma (TSO) analizi olmaksızın salt enerji tasarrufu sonuçlarına bakarak alınacak kararların işletmelerin beklenmedik sonuçlarla karşı karşıya kalmalarına neden olması muhtemeldir.

Abstract:
By technological developments, collecting, protecting, processing, and accessing data are getting more critical. Data centers are the places where valuable data are kept for the actions mentioned above. Data center sector has an annual growth rate of about 10% with a global energy consumption share of about 3%. Considering the limited energy sources and increasing costs in data centers, energy efficiency has been an increasing concern for the industry. Mechanical cooling infrastructure energy consumption constitutes a significant fraction of the total energy consumption in facilities like data centers with high-density electrical load and cooling demand. Free cooling systems can eliminate or reduce the need for mechanical cooling by utilizing ambient conditions under certain circumstances. Thus, by free cooling methods (economizers), we aim to reduce the use of compressor-based cooling, which is the primary consumer of energy in the data center cooling infrastructure. Due to the increased concerns about the energy efficiency of data centers, economizers are becoming an integral part of the data center cooling infrastructure. While utilizing economizers in data centers, it is essential to maintain reliable and sustainable operations. Thermodynamic models that capture the off-design performance of key equipment are required to simulate the energy performance of data center cooling infrastructure and various economizer modes. This study presents an overview of thermodynamic models for data center cooling infrastructure that are sensitive to the part-load performance and dynamic climate conditions. The thermodynamic model of a typical air-cooled data center with 1 MW electrical load from IT equipment, is assumed to be the baseline (BL) data center. Based on annual energy calculations executed with the climate conditions of 10 different cities in Turkey, the energy and economic impacts of various free cooling methods on the BL data center as retrofit application have been obtained. These models include widely used free cooling systems such as Direct Air-side Economizer (ASE), Indirect Air-side Economizer (IASE), Indirect Evaporative Cooling (IEC) and Indirect Water-side Economizer (WSE). Climate conditions affect the performance of free cooling modes. It also affects the performance of the main components of the cooling infrastructure indirectly due to inefficiencies caused by the part load operation during free cooling modes. In addition to the energy consumption amounts, total cost of ownership (TCO) analysis of systems are performed considering a 15-year life cycle. For the total cost of ownership analysis of systems, various metrics such as net present value (NPV), modified internal rate of return (MIRR) and discounted payback period methods are used.  With the help of economic analysis methods, for each city, the most proper systems as a retrofit application are determined. Energy savings are not sufficient to make an investment decision. Both economic aspects and energy saving potential results should be considered for a proper decision. In a conventional air-cooled data center, server cabinets (i.e., racks) are arranged to form hot and cold aisles. Computer Room Air Handling Unit (CRAH) fans supply cold air into the pressurized raised floor plenum. Cold air emanates from the perforated tiles in the cold aisle mixes with some recirculated room air and enters racks. The air temperature at the rack exit is higher due to the dissipated heat by the IT equipment. Hot exhaust is collected in hot aisles before returning to CRAH units to reject heat into a chilled water stream supplied by a chiller via an air-to-water cross-flow heat exchanger (HX). Baseline (BL) data center equipped with a water-cooled chiller that is coupled with a cooling tower on the condenser side to reject the heat to the environment. ASE allows outside air in, when the ambient environmental conditions permit. On the other hand, ASE poses an increased risk of contamination for the IT equipment. ASHRAE recommends filtering outside air to reduce gaseous contaminants with gas-phase filtration systems and particulates with MERV 11 or 13 filters, especially for data centers with ASE. Use of additional filters is a source of increased fan power. Indirect air-side economizer (IASE) mitigates most of the abovementioned contamination risks ASE poses by isolating outside air and data center return air. Warm return air exchanges heat with the outside air across the surface of an air-to-air cross-flow HX when it is sufficiently cold outside. Inefficiencies in the IASE configuration primarily stem from the limiting heat transfer effectiveness less than unity and increased fan power to overcome the high flow resistance of the HX. Similar to IASE, IEC also isolates indoor and outdoor air streams. IEC allows more efficient heat transfer than that in IASE by continuously keeping outside surface of IEC HX wet to enhance heat transfer through adiabatic cooling. The indirect WSE configuration includes a plate-frame HX separating the condenser water and chilled water loops. The HX takes advantage of cold water supplied by the cooling tower to indirectly cool the chilled water. A WSE system can be configured to operate in both parallel and series with the existing chiller. Enclosing the aisle is a common approach to improve the efficiency of air-cooled data centers. Enclosed-aisle (EA) data centers decrease the temperature non-uniformity across the servers and allow higher temperature operation that leads to more number of hours economizer operate. 
Results indicate that operating the DC at a higher temperature (BL-H) leads to roughly 15% energy savings. Overall, higher temperature operation provides favorable results in all cases. Current results for ASE-L indicate the reduced need for humidification due to the relaxed lower limit of recommended humidity by the ASHRAE. This modification increased the energy efficiency of the ASE in cold cities like Erzurum (ERZ). IEC configuration can lead to the elimination of mechanical cooling entirely in most of the climates except hot climates considering that data center operators allow higher temperature operation via aisle containment strategies. IEC-H is the best performer among the ten cities. Greater than 50% cooling energy savings are observed with IEC even in warm climates. Energy-saving potential by IEC leading to less than 10% of annual chiller hours across Turkey and less than 1% in half of the 10 cities studied, especially in enclosed aisle DCs. Warmer locations in the southern coastal areas like ANT and ADA depend on the chiller operation for at least 60% of the time for all OA configurations. ERZ is the only city to achieve chiller operation of less than 10% of the time with ASE and OA configurations. On the other hand, higher temperature operation via EA configuration dramatically improves the potential of energy savings, leading to as much as 54% additional annual hours without the need for a chiller. Except cities in the warm regions in the southern coastal areas (e.g., ADA and ANT) and humid northern coastal areas, all cities resulted in less than 10% of annual chiller operating hours with the IEC method in the EA configuration. Other free cooling methods may still require mechanical cooling. Unlike the IEC, where evaporative cooling removes heat directly from the DC air, the process in the WSE produces cold water at the cooling tower that absorbs heat from the chilled water stream. Due to the indirect nature of the evaporative cooling process, results indicate relatively lower performance by the WSE compared to other methods for OA configurations, which is in line with the literature. Relaxed humidity requirements significantly helped ASE configuration to reduce humidification energy consumption, especially in cold climates. However, upper limits on humidity exhibit an energy efficiency bottleneck for ASE. Therefore, systems that indirectly utilize ambient conditions show more improvement in energy efficiency at higher operating temperatures. ASE gave the most successful results for open aisle case, whereas indirect evaporative cooling (IEC) systems forged ahead in enclosed aisle systems that allow higher temperature operation. The variations in the ambient dry bulb temperatures have a significant impact on the performance of IASE-L due to the sensible heat exchanging process. The additional fans to overcome the flow resistances of IASE HX and filters lead to a considerable amount of fan power beyond the existing CRAH fans. Since DC and ambient are isolated, there is no humidification requirement in IASE. The IEC performs better than IASE due to the enhanced heat transfer via evaporative cooling. Water consumption can also be a significant cost factor. CTs, IEC HX, and humidifiers consume water. The WSE method stands out as the most aggressive consumer of water due to the heavy use of CTs. That is why the availability of water is also worth considering for WSE applications. Even though the IEC depends on an evaporative cooling process, the associated increase in water consumption is negligible because of the reduced number of chiller hours. ASE, IASE, and IEC require air-handling units integrated with CRAH units. Contrarily, WSE requires modifications of the chilled and condenser water circuits and installation of the WSE HX in between, which allows for both a cost-effective and operationally less disruptive investment. If DC operators are reluctant to see higher temperature operation (EA), and they can take measures to mitigate contamination risks, ASE may be a decent free cooling approach. IEC is the better option among indirect air-side economizers (IASE and IEC), primarily due to more efficient HX with evaporative cooling. The operational cost reduction is far more significant than the slight increase in the capital and maintenance cost for the IEC configuration.
After adding the outputs of TCO calculations to the energy saving results, WSE came forward as the system with the highest internal rate of return (IRR) due to the relatively low initial investment cost. However, despite greater energy-saving potential, IEC has more extended payback periods of 1,4 to 3,7 years due to the high capital investment compared with those of ASE and WSE with less than 1,4 years. In the assessment based on the criterion of net present value (NPV), ASE and IEC appear to be the most valuable options for the 15-year of life cycle. All cases in IST lead to payback periods of less than four years and NPVs greater than 2,5 million TL in 15 years of lifetime. The results indicate similar cash flows for ASE-L and ASE-H, which is due to the higher cost savings by ASE-L with respect to BL-L than that by ASE-H with respect to BL-H. NPV values of IEC are the best in all cities for EA cases. For all cities except ERZ and VAN (where WSE-L is comparable to IEC-L), ASE-L and IEC-L, present higher NPV values among OA configurations. The impact of regional water consumption on the NPV can be seen especially in cities like ANK where water costs are high. To illustrate, even though ANK is not the best regarding energy consumption, NPV is better than that of ERZ in some cases, especially for EA, due to the high price of water in ANK. Depending on the company policies, discount rates may vary and lead to different NPVs. MIRR values give hints about the effect of the discount rates on the feasibility of applications. For instance, a slight increase of up to 10%–15% of discount rate may lead to unfavorable NPV for the retrofit applications of IASE in ANT and ADA. Similarly, IEC is an unfavorable option for companies seeking high rates of return with payback times of less than a year. 
Overall results show that companies may end up with unexpected results eventually, if decisions are made by considering energy saving results solely without TCO analysis. It is important to note that the economic analysis of enclosing the aisle is not within the scope of this study. Future work may include the analysis of other free cooling methods applicable to legacy and other types of data centers as well as waste heat recovery methods for emerging liquid-cooled data centers.