Study on power-saving effects in direct-use of geothermal energy for datacenter cooling systems

Over recent years, against a social background of rising energy costs and a desire to create a “low-carbon society,” the demand to reduce electricity consumption has been growing. As for data centers, cooling of IT equipment (like servers and communication devices) by using means like heat-pump systems consumes a large amount of energy. As a means of reducing the electrical power consumed by cooling data centers, utilizing natural energy is being recommended. An example of such a means is a method utilizing air-side economizer and geothermal energy. The authors have constructed and experimentally validated a system that utilizes naturally existing geothermal energy. In the case that an underground heat source is used as a cooling source, so-called “heat-pump systems” are often used. However, if the characteristics of underground temperature in Japan (namely, all-year-round stability and heat discharge from IT equipment of data centers being higher than the underground temperature) could be utilized, cooling by energy generated by the difference between underground temperature and temperature of IT-equipment-generated heat would be possible, and an underground heat source could be used as a cooling source without the need to use a heat pump. Up until now, few cases of directly using an underground heat source for cooling a data center — without using a heat pump — have been reported. Given those circumstances, the authors have constructed a “geothermal air-conditioning system” at an actual data center and determined its performance. In this study, verifying the cooling effect of the geothermal air-conditioning system on that data center and revealing the effect of transport power and IT-equipment-generated heat density on the efficiency of the system were set as goals. In the case that the IT equipment in the data center generates 63 kW of heat, a geothermal cooling capacity of 27.4 kW is obtained, and “seasonal coefficient of performance” (sCOP) of the geothermal air-conditioning system is 5.1. Compared to a high-efficiency air conditioner we developed [1], the developed geothermal air-conditioning system reduces electrical power consumption by 21%. Utilizing natural energy for cooling data centers in this manner will significantly contribute to saving energy used for air conditioning.