Diseño óptimo e instalación de intercambiadores de calor de serpentines

Renewable Energy 35 (2010) 1741–1750

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Optimal design and placement of serpentine heat exchangers for indirect heat withdrawal, inside flat plate integrated collector storage solar water heaters (ICSSWH)
K.P. Gertzos, Y.G. Caouris*, Th. Panidis
Dept. of Mechanical Engineering andAeronautics, University of Patras, 265 00 Patras, Greece

a r t i c l e i n f o
Article history: Received 10 August 2009 Accepted 24 December 2009 Available online 7 February 2010 Keywords: ICS ICSSWH Integrated collector storage Tube heat exchanger CFD FLUENT

a b s t r a c t
Parameters that affect the temperature at which service hot water (SHW) is offered by an immersed tube heat exchanger(HX), inside a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), are examined numerically, by means of Computational Fluid Dynamics (CFD) analysis. The storage water is not refreshed and serves for heat accumulation. Service hot water is drawn off indirectly, through an immersed serpentine heat exchanger. For the intensification of the heat transfer process, the storage water isagitated by recirculation through a pump, which goes on only when service water flows inside the heat exchanger. Three main factors, which influence the performance, are optimized: The position of the HX relative to tank walls, the HX length and the tube diameter. All three factors are explored so that to maximize the service water outlet temperature. The settling time of the optimum configuration isalso computed. Various 3-D CFD models were developed using the FLUENT package. The heat transfer rate between the two circuits of the optimum configuration is maintained at high levels, leading to service water outlet temperatures by 1–7  C lower than tank water temperatures, for the examined SHW flow rates. The settling time is retained at sufficient law values, such as 20 s. The optimal positionwas found to lay the HX in contact with the front and back walls of the tank, with an optimum inner tube diameter of 16 mm, while an acceptable HX length was found to be about 21.5 m. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Solar water heaters are the most accepted solar systems for Domestic Hot Water production. A great portion of them corresponds to the so called IntegratedCollector Storage Solar Water Heaters (ICSSWH), which they use part of the hot water storage as solar collector. They have reduced cost because they integrate collector and heat storage in the same construction. They can be broken down into two basic categories. These of direct configuration in which the storage medium serves also as the energy transfer medium and those of indirect configuration, inwhich the energy storage medium and the energy transfer medium are separated, usually by a heat exchanger (HX), so that the storage fluid is maintained in an unpressurized state. The cost and life cycle savings are the more important factors for solar systems penetration. In conventional indirect circuit solar thermosiphons, most of the cost (around 60%) concerns the storage tank, due to highcorrosion resistance and pressure durability

* Corresponding author. Tel.: þ30 2610 969 410; fax: þ30 2610 996 129. E-mail address: caouris@mech.upatras.gr (Y.G. Caouris). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.12.014

requirements. On the other hand, Integrated Collector Storage Solar Water Heaters (ICSSWH) have reduced cost becausethey integrate collector and heat storage tank in a compound construction. This work aims to investigate such a system with reduced cost and increased reliability too, having as target to approach and to decrease the 4.5 years of payback time (for oil substitution) as concluded by Kalogirou [1]. As the system examined by Kalogirou is of direct type thermosiphon, the proposed configuration of...