Design Optimization of a Thermally Integrated Solid Oxide Fuel Cell with Triple Effect Absorption Chiller
Authors: Lavernia Alejandro Carlos , Asghari Maryam , Mastropasqua Luca , Brouwer Jack 
Published: July 2021

To reduce emissions and energy use in data centers, a novel integration between a solid-oxide fuel cell (SOFC) and a triple-effect absorption chiller (AC) is proposed to meet both the electrical and cooling requirements of server computers. An installation of SOFC units at a data center can operate using natural gas infrastructure that today supplies natural gas or directed renewable fuel and which can increasingly introduce biogas, renewable hydrogen, and/or synthetic fuels in the future. In fact, SOFC systems have already been employed to produce primary power for data centers[i]with high fuel-to-electricity conversion efficiencies and these efficiencies can be further improved by integration with AC waste heat recovery technology. Utilizing the waste heat in the exhaust stream of the SOFC, a triple-effect AC system, using aqueous lithium bromide as a working fluid, can produce the required cooling for the servers and eliminate the requirement for a backup mechanical cooling system. A 200 kW SOFC system is selected for row-level discretization of power production and is coupled to an equivalent 200 kW AC system to provide the necessary cooling. An additional benefit of this discretized installation is the inherent reliability of the distributed power and cooling generation with limited failure domain. Using this system and an n+1 installation strategy, it is possible to eliminate the requirement for backup power systems as well.

A numerical simulation investigates the design of the integrated 200 kW SOFC and AC system. The 200 kW SOFC is optimized to produce the required electricity at conversion efficiencies up to 60%[ii], while still producing enough thermal energy to run the 200 kW triple-effect AC system. The numerical simulation indicates that a 200 kW SOFC can produce anywhere from 80 – 130 kW of useful heat in the exhaust, as shown in Figure 1. The AC system is optimized to harness this thermal energy from the SOFC exhaust to meet the cooling demand of the servers and can operate at a coefficient of performance (COP) of up to 1.89. A control strategy for the integrated system is developed based on the power demand of the servers and the ambient conditions of the data center to provide the required power and cooling.

Additionally, the numerical study addresses the multiple possible cooling configurations that could be adopted by a data center. Liquid cooled servers utilize higher temperature cooling medium, which allows the AC system to operate at a higher COP. The improvement in AC efficiency when cooling to a higher temperature is explored to further improve the chilling capacity.

In conjunction with the numerical simulation, an experimental system has been constructed to demonstrate the efficacy of an integrated SOFC and AC system for data center applications. The experimental system consists of a single server rack, a 12-kW array of SOFC systems for power, and an 18-kW single-effect absorption chiller for cooling. The experimental setup is shown in Figure 2. The server computers are operated at a simulated computational load, which exercises the SOFC array and absorption chiller, testing the system’s ability to produce power and chilling. The experimental system has demonstrated the ability to operate at a COP of up to 0.77 with SOFC exhaust gasses at 235°C, producing 12.5 kW of chilling, equivalent to 47% of the cooling demand of the server rack at full capacity.

The experimental system is also used to validate the thermo-physical models used in the numerical simulations. This includes the desorber physics at elevated temperatures created by the utilization of fuel cell exhaust heat, which is much hotter than typical AC heat sources. Additionally, by altering the control strategy of the AC system, it is possible to explore elevated cooling temperatures in the evaporator and absorber, pursuing improved COPs that are possible in a liquid-cooled server configuration.

Finally, the financial viability of a large-scale installation of such a system is addressed. The high capital costs associated with SOFC and triple-effect AC systems typically hinder the implementation of such systems. However, our results demonstrate that the integrated system is competitive on a levelized cost basis mainly because of primary energy savings (up to 34% compared to a conventional data center) and ability to displace the installation of backup electrical and chilling systems.

[i] eBay turns on first fuel cell powered data centre, has Bloom tech. (2013). Fuel Cells Bulletin, 2013(10), 3–4. https://doi.org/10.1016/s1464-2859(13)70341-9

[ii] Williams, M. C., Vora, S. D., & Jesionowski, G. (2020). Worldwide Status of Solid Oxide Fuel Cell Technology. ECS Transactions, 96(1), 1–10. https://doi.org/10.1149/09601.0001ecst

Figure 1