Journal of Power Sources 158 (2006) 225–244 Design and partial load exergy analysis of hybrid SOFC–GT power plant F. Calise a,∗ , A. Palombo a , L. Vanoli b a DETEC, Universit` a degli Studi di Napoli Federico II, P.le Tecchio80, 80125 Naples, Italy b Dipartimento di Scienza degli Alimenti, Universit` a degli Studi di Napoli Federico II, Via Universit` a 100, Portici, NA 80055, Italy Received 3 June 2005; received in revised form 28 July 2005; accepted 31 July 2005 Available online 19 October 2005 Abstract This paper presents a full and partial load exergy analysis of a hybrid SOFC–GT power plant. The plant basically consists of: an air compressor, a fuel compressor, several heat exchangers, a radial gas turbine, mixers, a catalytic burner, an internal reforming tubular solid oxide fuel cell stack, bypass valves, an electrical generator and an inverter. The model is accurately described. Special attention is paid at the calculation of SOFC overpotentials. Maps are introduced, and properly scaled, in order to evaluate the partial load performance of turbomachineries. The plant is simulated at full-load and part-load operation, showing energy and exergy flows trough all its components and thermodynamic properties at each key-point. At full-load operation a maximum value of 65.4% of electrical efficiency is achieved. Three different part-load strategies are introduced. The off-design operation is achieved handling the following parameters: air mass flow rate, fuel mass flow rate, combustor bypass, gas turbine bypass, avoiding the use of a variable speed control system. Results showed that the most efficient part-load strategy corresponded to a constant value of the fuel to air ratio. On the other hand, a lower value of net electrical power (34% of nominal load) could be achieved reducing fuel flow rate, at constant air flow rate. This strategy produces an electrical efficiency drop that becomes 45%. © 2005 Elsevier B.V. All rights reserved. Keywords: SOFC; Exergy; Modeling 1. Introduction High temperature solid oxide fuel cells (SOFC) are promised to be the most efficient device for direct conversion of fuel chem- ical energy into electricity. Its efficiency can be further increased when coupled with a gas turbine (GT) cycle up to 70% [1]. The perspective of full commercialization of hybrid SOFC–GT plants is very attractive since these devices could represent the most efficient equipment for residential/distributed power/heat generation applications [1–4]. Theoretically, the lower size of a hybrid SOFC–GT power plant is conditioned only by the small- est commercial available microturbine [1]. According to the present technological progress in microturbine research field, it is possible to assume that hybrid plants could be commercial- ized from the size of 100 kW up to 10 MW, perfectly suitable for cogeneration distributed plants [1,2]. ∗ Corresponding author. Tel.: +39 081 7682304; fax: +39 081 2390364. E-mail address: frcalise@unina.it (F. Calise). Nowadays, at least four different types of SOFC are available (tubular, tubular “high power density”, planar, and microtubo- lar) [1,2]. Although these have been studied since 1970s only a few prototypes are presently operating. Siemens Westinghouse is probably the main company involved in the design and com- mercialization of hybrid SOFC–GT power plants [1–4]. This company installed a 220 kW hybrid tubular SOFC–GT system at the National Fuel Cell Research Center in California. Results coming from these experimental tests showed values of the net electrical efficiency lower than the target ones (52% versus 57%) [1]. This experimental campaign demonstrated the huge diffi- culty in integrating SOFC with traditional components [1]. Usually, SOFC–GT hybrid plants are designed only at full- load operation [5–8], taking into account the connection with the electrical network, determining an energetic and economic dependence of the plant on the network. This circumstance can be avoided since SOFC–GT hybrid plants are perfectly suitable also as stand alone devices. In fact, as shown in many previous papers [9–11], conversely from the traditional energy conver- sion devices, these hybrid plants show very slight efficiency 0378-7753/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2005.07.088