~ Pergamon Renewable Energy, Vol. 13, No. 2, pp. 261 268, 1998 (C 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII : S0960-1481 (97)00049-9 0960-1481/98 $19.00 + 0.00 TECHNICAL NOTE Thermodynamic cycle of a liquid piston pump ROGERIO P. KLUPPEL and JOSI~ MAUR[CIO M. GURGEL Laboratdrio de Energia Solar da UPFB, Cidade Universitfiria, 58059-900, J. Pessoa, PB, Brazil (Received 30 April 1997 ; accepted 28 May 1997) Abstract--This paper presents a contribution to the solution of the irrigation problems of rural areas in developing regions, where conventional energy solutions are often too expensive. The work presents the operational principles, the theoretical analysis and experimental results of a pumping device, for irrigation use, that works based on a cyclical variation of pressure exerted on the water by a confined mass of gas. The gas alternately contacts a hot or a cold plate, by the movement of an insulating displacer, presenting therefore an oscillation in temperature. The movement of the displacer is connected by a buoy to the movement of the liquid surface. Presented here is a description of the experimental prototype and of the mechanical pumping cycle with its connection to the thermodynamic cycle experienced by the gas inside the device. A classical thermodynamics analysis of the idealized cycle is made, shown on equilibrium diagrams. Experiments were conducted with a prototype and the results are presented and discussed. The presented experimental data confirm the initial hypothesis, and suggest the technical feasibility of the device. The final com- ments discuss some technological drawbacks that need yet to be removed in order to arrive at a practical prototype. © 1998 Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION The need for irrigation in rural semi-arid tropical regions of developing countries poses a technological challenge as usually those regions lack electricity distribution networks and the local price of fossil fuels is inflated by transportation costs. The power, therefore, must be provided by locally produced combustibles or by solar energy that happens to be plentiful in such regions. During the 1970s, the technology of Rankine cycle engines using steam produced by concentrators became commercially available [1], but in sizes that put them beyond the reach of individual farmers. Other technologies also became available, e.g. photocell powered centrifugal pumps, that present no scale production problems and in addition are compact and reliable. Nevertheless photocell pumps are still expensive and their maintenance, however low, is a high technology task. The conditions prevailing in developing countries require simple, low cost, low maintenance devices that could be constructed locally and operated by people without sophisticated technical skills [2], like liquid piston units. During a stay at Federal University of Paraiba the late Rudolf Sizmann [3] proposed the study of a device that could produce pumping work using the oscillation in pressure exerted by a variation of temperature in a confined mass of gas exposed alternately to contact a hot or a cold source. The device, after technological development, could be used to pump water for irrigation. A prototype was constructed and preliminary results were published [4], but the early model was far from ready to be produced in commercial scale. In an attempt to make a technically liable system it was decided to restart from the beginning and this paper presents a revised version of an equilibrium thermodynamic analysis of the cycle and of experimental results. 261