Journal of Power Sources 125 (2004) 189–198 Solid oxide fuel cell architecture and system design for secure power on an unstable grid Susan Krumdieck a,∗ , Shannon Page a , Simon Round b a Department of Mechanical Engineering, University of Canterbury, PB 4800, Christchurch, New Zealand b Department of Electrical and Computer Engineering, University of Canterbury, Christchurch, New Zealand Received 28 May 2003; received in revised form 28 July 2003; accepted 28 July 2003 Abstract In a power grid with significant components of distributed generation and insufficient spinning reserve, the quality of delivered power may not meet the requirements of advanced manufacturing. A system design for power quality security which uses solid oxide fuel cell (SOFC) technology is described. Critical parameters for system performance are continuous supply voltage at the nominal voltage and frequency. The grid chosen for this study has significant voltage fluctuations and periodic voltage drops and surges, including total power loss. A supply of methane from a sewer sludge digester is scrubbed of CO 2 and used for continuous standby operation, with excess stored to enable 8 h operation of an uninterruptible power supply (UPS). The system employs a modular, thermally coupled, SOFC architecture that includes steam reforming of the methane fuel, a rectifier, power controls, and control system. Continuous operation of a 125 kW tubular SOFC stack maintains operating temperature and steam for fuel reforming in a secondary SOFC stack, by exhausting through it before a gas turbine expands the exhaust to supply the plant air and fuel compression. Modelling of the energy balance of the system demonstrates the standby and full power operating modes. The system is sized at 250kW to supply secure power for a manufacturing facility. © 2003 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Uninterruptible power supply; Sustainable energy system; Power quality; Steam reforming; Unstable grid 1. Introduction The motivation for this study was to investigate the possi- bility of utilizing solid oxide fuel cells (SOFCs) to provide an uninterruptible power supply (UPS). This is a technical study to evaluate the feasibility of an original idea for SOFC architecture conceived at the University of Canterbury. The approach is focused on developing a total system concept (TSC) for a power plant to meet a specific energy demand scenario within the context of a particular environment. Several of the technologies modelled in this study are still in the development stage. The goal is to propose the design space for an innovative combination of components. By investigating the energy system in context, the intention is to provide momentum for technological development for a clearly defined application. The purpose of a UPS system is to keep critical loads under power in the event of an outage in a typical alter- nating current (ac) grid. The present investigation takes the ∗ Corresponding author. Tel.: +64-3-364-2987x7249; fax: +64-4-364-2078. E-mail address: s.krumdieck@mech.canterbury.ac.nz (S. Krumdieck). function of a UPS system one step further in that it also provides power-conditioning in an unstable ac grid that has frequent power dips and surges. Hospitals, police, and mil- itary installations have employed back-up diesel generators for many years. There is now a rapidly growing demand for UPS systems for manufacturing facilities where a power outage could mean heavy losses in productivity or risks of damaging equipment. Modem high-tech manufacturing facilities and financial institutions can suffer heavy losses from disruptions in the voltage or the power factor. UPS loads can range from 5 kW for a critical computer network, to several megawatts for manufacturing. A major motivation for this investigation arises from two factors: the sensitivity of many activities to power loss and power quality, coupled with decreasing spinning reserve in large grid networks as peak demands approach gener- ation capacity. The target application for the design of an SOFC-based UPS system is a manufacturing facility in Singapore. The local grid is based on numerous distributed generators, with no large baseline power plants. Surges, dips, single-phase disruptions, and outages are becoming a serious concern for the economic future of the country’s high-tech and financial industries [1]. 0378-7753/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2003.07.011