During the recent years PUE was the most important Parameter for the energy effectivenes of data centers. Now, scientists and practitioners more and more see the shortcomings of that concept. PUE (Power usage effectiveness)means the relation of energy used for purposes like cooling, climatization etc. to the energy used for IT-work. But PUE has some shortcomings. The following were referred to during Datacloud Europe 2015 in Monaco in June:
1. It conflicts with other Parameters, e.g. the share of reused excess heat: the warmer air or water is leaving the data Center the easier it is to use it e.g. for district heating. The other way round: The less excess heat a datacenter produces the less possible is it to recycle the heat. So, in some cases it may very well be better to have a Little less favorable PUE to reach a higher percentage of reused thermal energy and in total a higher energy efficiency.
2. The utilisation of the IT Equipment is not put into the equation: Average datacenters have today an utilisation of 30 to 40 percent maximum, according to datacenter specialists. 60 to 70 percent of the Equipment uses energy with no use. So some specialists want to use a new term (TCUE, Total Cost of Used Equipment). The term divides the total Leasing cost of IT Equipment and the total power cost by the sum of IT power and the cost of the relly used IT Equipment. This puts utilisation in the Picture and shows that bad usage practise may very well (over)compensate the use of renewable energy reources. One source of overequipping datacenters ist security: The higher the Tier Level, the more redundancy has to be built into the datacenter. But in Monaco specialists said: For most datacenters, Tier II and Tier III (partial redundancy) is well enough and saves tons of Money and energy use. So, have a close look at Your infrastructure and make sensible decisions on how to protect what infrasatructurewise.
3. Research at the Polytechnicum of Turin shows that local hot and cold spots from irregular airflows e.g. between or inside the racks have to be taken into account to find the Optimum ingress temperature for cold air into Servers and other IT equipment. These calculations are performed by advanced fluid dynamics algorithms, but promise interesting results. These effects are not represented in the PUE calculations.
4. The degree of used renewable energy: If the datacenter is operated effectively and with a high utilisation, the most important source of further carbon dioxide savings and in future probabely also money is the use of new renewable energy sources instead of buying certificates or using „conservative“ renewable sources e.g. water power from dams. That is why more and more big cloud Players invest into solar or wind power plants. Latest examples are Yahoo (wind farms), Amazon (Solar power) and Google 8wants to use renewables only in five years).
