The viability of ultra low temperature waste heat recovery using organic Rankine cycle in dual loop data center applications

Abstract

The Organic Rankine Cycle (ORC) is a promising technology for waste heat recovery in data centers due to its low-cost production of electricity, which can be used elsewhere in the data center. In this paper, an analysis is presented which assesses the effectiveness of ORC systems in repurposing data center waste heat energy from both thermodynamic and economic points of view. The starting point for the analysis is the development of a steady-state thermodynamic model for an ORC system consisting of an evaporator, expander, condenser, pump, and superheater. The model includes both first and second law analyses and is verified against existing data. The effect of evaporator temperature and working fluid type (dry, wet, and isotropic) on the efficiency of the system is examined. The ORC thermodynamic model is then integrated into a full server cooling system model. The efficiency of this system is determined for different operating fluids and thermodynamic conditions. Finally, a first-order economic analysis is performed using the results of thermal analysis and the information for price, installation and maintenance cost, provided by an ORC manufacturer. This work is unique in that the use of ORC in extremely low temperature applications has previously been considered to be of questionable benefit due to its low efficiency. This work highlights that there are clear benefits to be gained in ultra-low temperature applications, and is one of the first examples of ORC benefits in data center designs.For data center operating conditions, R134a and R245fa are identified as the optimum selections for the server coolant and ORC working fluid, respectively, but other environmentally friendly options are considered in the case that a data center operator is forced to swap out a fluid due to changes in environmental regulations. This gives data center operators greater insight into the performance of the system in the face of possible constraints. The analysis also considers the use of superheaters to increase the temperature at which the waste heat is recovered. This is often considered to be beneficial in waste heat recovery systems. However, the results show that while increasing the temperature at which the waste heat is recovered improves ORC system performance, the addition of superheaters in either the ORC loop or the server cooling loop results in an overall performance decrease due to the additional power required for their operation. A payback period of between 4–8 years for the use of ORC in data center environments is estimated.