Waste Heat Recovery From Distributed Rack-based Fuel Cells Using Thermoelectric Generators

Off-grid power generation has been demonstrated in data centers through the deployment of site-specific centralized power plants utiLizing gas turbine or fuel cell-based power generation. Because power is centrally generated, power distribution requires a high voltage power grid within the data center with its ancillary storage and conditioning requirements and equipment. An alternative approach is a completely decentralized distributed power generation system in which fuel cells deployed within individual server racks provide power localized to that rack only. Among other advantages, such an approach also greatly increases the ability to modulate and control power to individual rack units. Because the Solid Oxide Fuel Cells (SOFC) proposed in this approach are air-cooled and have extremely high air exhaust temperatures, of order 800 °C, the optimal energy efficient design of an overall localized fuel-cell power generation system should also consider the opportunities to recover and re-use the waste heat. This paper reports on the development of a coupled thermal-electrical model of a thermoelectric generator (TEG) based energy recovery system operating between the fuel cell hot exhaust air temperature and a warm water cooling system deployed within the rack for server cooling. The power generation system consisted of a TEG module sandwiched between a hot air heat exchanger and a colder water based heat exchanger. The design of the TEG module for maximum power generation is heavily coupled with thermal and electrical conditions. Distribution of temperature on generator surfaces change the optimum design, hence the optimization of generator system required co-optimization with the design of the heat exchangers. This paper presents the results of electro-thermal co-optimization, which considers the impact of the flow rates and temperatures of exhaust gas and cooling water, parallel and counter-flow arrangements, as well as the TEG packaging design including junctions, fractional area coverage, and substrate thickness.