International Journal of Hydrogen Energy 33 (2008) 463 – 469 www.elsevier.com/locate/ijhydene Performance tests on a thermally operated hydrogen compressor P. Muthukumar a , ∗ , M. Prakash Maiya b , S. Srinivasa Murthy b a Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India b Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India Received 19 June 2007; accepted 9 July 2007 Available online 4 September 2007 Abstract A metal hydride based hydrogen compressor using MmNi 4.6 Al 0.4 is tested at both constant and variable delivery pressures. The effects of operating parameters such as supply pressure and heat source temperature on the compressor performance are investigated. The compressor yields a maximum isentropic efﬁciency of 7.3% at 95 ◦ C heat source temperature at variable delivery pressure operation mode, and 14.2% at 85 ◦ C heat source temperature at constant delivery pressure operation mode. At both the operation modes, the volumetric efﬁciency of the compressor is found to decrease at high heat source temperatures.  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Metal hydride; Hydrogen compressor; Volumetric efﬁciency; Isentropic efﬁciency 1. Introduction Mechanical compression of hydrogen gas poses many tech- nical problems related to compressor construction, balancing, lubrication, maintenance, etc. In addition, such compression consumes electrical energy in contrast to thermal compression, which can be accomplished by waste heat. Moreover, it is de- sirable to adopt engineering solutions, which eliminate moving parts working under a hydrogen medium. The metal hydride based hydrogen compressor is one of the potential applications of hydride technology which can be tailored to cover a wide range of operating pressures and pressure ratios by selecting suitable alloys [1–3]. These compressors also permit noise- less and vibration free operation, in addition to delivering pure hydrogen. Recently the authors [4,5] investigated the inﬂuence of op- erational and reactor bed related parameters on the compressor performance by analyzing the heat and mass transfer aspects of the hydride bed. In an experimental study using MmNi 4.6 Al 0.4 , the inﬂuences of supply pressure and hot ﬂuid temperature on the compressor performance were investigated for variable ∗ Corresponding author. Fax: +91 361 258 2699. E-mail address: pmkumar@iitg.ernet.in (P. Muthukumar). 0360-3199/$ - see front matter  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.019 delivery pressure using a constant volume storage cylinder of 1 l capacity. This paper presents the performance studies of a hydrogen compressor using MmNi 4.6 Al 0.4 at both variable and constant delivery pressure modes, and also studies the inﬂuence of dif- ferent storage volumes. 2. Experimental studies 2.1. Operating principle The operating principle of the compressor is explained in an earlier publication by the authors [5]. The operation of the hydride compressor consists of four processes namely: Process AB: Absorption of hydrogen at low temperature (T c ) and low pressure (P s ). Process BC: Sensible heating accomplished by compression from cold ﬂuid temperature (T c ) to hot ﬂuid temperature (T h ). Process CD: Desorption of compressed hydrogen at hot ﬂuid temperature (T h ) and high pressure (P d ). Process DA: Sensible cooling from T h to T c . Process CD can be of two cases: In Case 1, compressor starts to discharge hydrogen when the hydride bed reaches the speciﬁed hot ﬂuid (heat source) temperature. Storage pressure increases with time and becomes equal to hydride equilib- rium pressure at the end of the desorption process. In Case 2,