New Thermal Architecture For Future Green Data Centres

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1 New Thermal Architecture For Future Green Data Centres S. Le Masson, D. Nörtershäuser France Télécom R&D 2, Av Pierre Marzin Lannion FRANCE ; Abstract - It is now admitted that electrical power devoted to telecommunication industry will grow tremendously with the development of internet services [1] [2]. In a data centre, most of the energy supplied to the IT equipments is converted into heat and diffused in a closed volume. The energy losses through the walls are negligible, even in winter. To compensate the energy balance, an air conditioning system must be used to transfer heat power at least equal to the total equipments power. The use of free cooling can be an easy way to reduce the energy consumption. Furthermore, if the building construction is adapted (thermal inertia, thickness of the walls, flow rate ), an optimization is possible by using a night cool storage directly in the masonry. Previous studies related to small buildings have shown good results [3]. This paper describes an experimental study in order to define new conceptions of optimized telecommunication buildings.. I. INTRODUCTION In a context of growing energy demand, when the energy production has a severe impact on environment, it becomes necessary to estimate the responsibility of each economical sector, particularly telecommunications or IT (information Technology) domain. Indeed, the energy consumption of the different networks and IT equipments is far from being negligible. Even if authors do not agree on the rise [1] [2], it is now admitted that electrical power devoted to telecommunication industry will grow tremendously with the development of internet services. Thus, it is necessary to study solutions to reduce energy consumptions, particularly regarding air conditioning systems [3]. The air conditioning part represents 30 to 50% of the whole energy consumption, particularly in data centres that are strategic rooms. According to its sustainable development policy, France Telecom has developed low-consumption air conditioning systems [4] devoted to existing buildings. This document is about studies that aim at defining rules to build more efficient buildings for telecommunication centres. Actually, as it can be seen on figure 1, the energy consumption of France Telecom data centres represents 17% of the whole energy consumption. Figure 1: Distribution of energy consumption by types of networks [5] Figure 2: Distribution of energy consumption in a data centre II. DATACENTER THERMAL PROBLEM The term data centres includes all buildings, facilities, offices and rooms which contain enterprise servers, server communication equipment, cooling equipment and power equipment, and provide some form of data service (e.g. large scale mission critical facilities all the way down to small server rooms located in office buildings). The surface of these rooms can reach 1000 m² with a growing power density through the years (from 500W/m² to 2kW/m²). The figures 3 and 4 illustrate the common architecture of rooms with various equipments and various /10/$ IEEE

2 thermal dissipations (very different from one equipment to another). Figure 5: Simplified thermal problem Figure 3: Inside view of a data centre In a data centre, most of the energy supplied to the IT equipments is converted into heat and diffused in a closed volume. The energy losses through the walls are negligible, even in winter. To compensate the energy balance, an air conditioning system has to be used to transfer a heat power at least equal to the total equipments power. In summer, the air conditioning system has to remove the energy diffused through the walls plus the energy of the equipments. The use of free cooling can be an easy way to reduce the energy consumption. Furthermore, if the building construction is adapted (thermal inertia, thickness of the walls, flow rate, diffusion ), an optimization is possible by using a night cool storage directly in the masonry. Previous studies related to small buildings have shown good results [6]. Figure 4: Inside view of a data centre Up to now, data centres buildings have been constructed like residential buildings that are not intended to dissipate heat but to keep it inside. The figure 5 is a scheme of the thermal problem with internal dissipation in the building. Compared to the heat dissipated through walls or roof, the power dissipated by the equipments is huge. III NUMERICAL SIMULATIONS In a first time, a numerical model based on nodal method has been developed in order to simulate small telecommunications buildings and to deduce some constructions rules. A diagram of the thermal numerical model is represented on figure 4. The user has to enter the following input: - Temporal evolution of the external temperature - Temporal evolution of the solar heat flux - Internal power dissipated by the equipments. Geometric data (size of the building) and physical properties of each layer of the walls have to be known too. Then, the temporal evolution of the mean internal temperature can be computed. External temperature Solar heat flux Internal power Thermal model (geometrical data, physical properties) Mean internal temperature Figure 6: Numerical model

3 Numerical computations have shown that the power that can be dissipated in the building can be seriously enhanced with walls made of concrete with embedded phase change materials micro-capsules [7], and an external thermal insulation. Such a conception works only if fresh air is introduces when the external temperature remains below a limit (typically, the evening, at night and the morning). IV EXPERIMENTATIONS ON SMALL-SCALE BUILDINGS WITH PHASE CHANGE MATERIALS In order to check the promising numerical results, three small-scale models of buildings have been constructed (figures 7 and 8). 4.1 Reference model M1 The first building model will be our reference. Its conception is classical. The internal volume is 1 m 3. The floor is made of concrete (thickness=10 cm), thermally insulated on the external face with extruded polystyrene (thickness=5 cm). The making up of the concrete is: 2 volumes of sand, 2 volumes of gravels, 1 volume of Portland cement. The melting has been made with an electrical cement mixer. The walls are made of breezeblocks (thickness=20cm), put together with cement mortars.the making up of this cements mortar is: 3 volumes of sand and one volume of cement. The breezeblocks are filled with the same concrete as the floor in order to enhance the thermal mass. A removable ceiling has been used. It has the same shape and properties as the floor. All the faces of the model have been isolated with extruded polystyrene (BASF styrodur ). Figure 8 Finished M1 model Each model has an air input and an air output (Figure 5 and Figure 6) in order to ensure forced air renewal when the external temperature is below a certain level. The main goal is to use the thermal inertia of the building as a cooling storage and to use it during the hottest hours of the day. 4.2 M2 Model The M2 model is very close to M1 model, with the exception of the walls that are not filled with concrete. This modification is made to evaluate the effect of the improved thermal inertia of M M3 model with micro-encapsulate phase change material In order to improve the thermal storage capacity of the walls, micro-encapsulate phase change materials have been used. These products are made by the Microtek Company (MicroPCM product). MicroPCM s consist of an encapsulated paraffin-wax substance which absorbs and releases heat in order to maintain a regulated temperature. The particles have a typical size distribution in range of 5 40 microns (Figure 9). The phase change temperature of the PCM is 28 C for the M3 model. If the capsules are not destroyed during the concrete mixing process, this would normally lead to walls with a high storage capacity around 28 C. Figure 7 Making up of M1 model. Figure 9 Micro-encapsulate phase change material (source Microtek)

4 From a geometrical point of view, the M3 model is the same as M1 and M2. The only differences come from the mortars and concrete composition which are detailed below. The concrete of the floor and ceiling is made as follows: volumes of sand, 1.5 volumes of gravels, 2 volumes of PCM28, 1.2 volumes of cement. The cement mortar used to joint the blocks is made as follows: - 2 volumes of MicroPCM 28, 2 volumes of sand, 1 volume of cement. 4.4 Measurements under artificial climate The three models have been simultaneously tested under an artificial climate in a laboratory (Figure 10) that can recreate both external temperature and solar heat flux [8]. Température ( C) :00:00 04:48:00 09:36:00 14:24:00 19:12:00 00:00:00 04:48:00 Heure Temp Flux Figure 11. Artificial climate laboratory Each model has a ventilation system switched on from 11:30PM to 9AM in order to cool the building structure at night (this extremely simplified regulation can be used as we know a priori the external temperature) Densité de flux solaire (W/m²) Only first results can be presented. In order to simulate the power dissipated by the equipments, modified fan heaters have been used (the modification has been made to ensure a steady dissipated power over time). The temporal evolution of the slab temperatures are represented on figure 12. The ambient temperatures of the models are represented on figure Figure 10. Artificial climate laboratory The solar and temperature simulated are represented on figure 11. The solar heat flux simulation is made with incandescent lamps. The room temperature is controlled with heating units and fresh air inputs. Températures ( C) :00 12:00 0:00 12:00 0:00 12:00 Date Heure M1 Tdalle M2 Tdalle M3 Tdalle Figure 12. Evolution of slab temperatures During the period of concern, the ambient temperature reaches a maximal value of 45 C in the M2 model, in other words in the model characterized by the smaller thermal storage capacity. The M1 model leads to a lower ambient temperature (- 3 C), thanks to the concrete added in thermal blocks. At last, the model made with micro PCM leads to the lower temperature (33.6 C), which represents a lowering of 10 C compare with other models.

5 The analysis of the temperature of each model slab illustrates very clearly the thermal inertia of each structure. The temperature of the slabs of M1 and M2 models are quite similar. On the other hand, the M3 slab does not exceed 27.2 C. Températures ( C) Ventilation M1 Tamb haute M2 Tamb haute M3 Tamb haute Ventilation [2] Koomey, J. G. (2007). Estimating total power consumption by servers in the U.S. and the world, Staff Scientist, Lawrence Berkeley National Laboratory and Consulting Professor, Stanford University: 31. [3] Roth, K., F. Goldstein, et J. Kleinman, (2002) Energy consumption by office and telecommunications equipment in commercial buildings, volume i : Energy consumption baseline. Technical report, ADL. [4] S. Le Masson, J. Gautier, D. Nörtershäuser (2005) La climatisation simplifiée des commutateurs téléphoniques. Congrès SFT 2005 Reims [5] L. Souchon. TIC et Énergétique : Techniques d estimation de consommation sur la hauteur, la structure et l évolution de l impact des TIC en France. Thèse INT [6] D. Nörtershäuser, S. Le Masson Using phase change materials and efficient coldless air conditioning systems to optimize the heat management in telecom shelters IEEE-Intelec 2007 Rome. [7] D. Nörtershäuser, S. Le Masson, Optimisation du conditionnement d'air des locaux de télécommunication par utilisation de produits à changement de phase, Congrès SFT 2008, Toulouse [8] D. Nörtershäuser, S. Le Masson Clim@Lan: A powerful artificial climate facility designed to test telecommunication equipments IEEE- Intelec 2005 Berlin 18:48 23:36 4:24 9:12 Date Heure 18:48 23:36 4:24 Figure 13. Evolution ambient temperatures 9:12 These first results prove that the use of PCM to enhance the storage capacity of a building is an interesting way of lowering the energy consumption of air conditioning systems. V CONCLUSIONS According to its sustainable development policy, France Telecom has developed low energy consumptions air conditioning systems for existing buildings. In order to build more energy efficient data centres, a study has been initiated to define new conceptions of optimized telecommunication buildings. The first results clearly prove the efficiency of the PCM to enhance the storage capacity of the whole building. Actually, by lowering significantly the internal temperature, it will reduce the energy consumption of air conditioning systems.. This study shows also that the building of a data centre must be constructed carefully in order to increase the energy efficiency. Some others works must be done to improve theses results, and to optimise night cooling with an optimise ventilation instead of a simple time switch on ventilation. Tests must be done for a longer period (10 days for example) to be sure that the PCM is regenerated every night. REFERENCES [1] Mills, M., et P. Huber, 1999 Dig more coal - the pcs are coming. Forbes Magazine.