Cfd-based Design and Characterization of Hot Water Seasonal Heat Stores

Computational fluid dynamics (CFD) bears great promises for the design and characterization of systems for which an approach based on field or laboratory data is impractical. Due to their large sizes and seasonal operation hot water seasonal storages (HWSHS) are suitable candidates for CFD-based design analysis. However, well-calibrated CFD models that sufficiently account for the turbulence and appropriate characterization schemes to grade heat store performances are still not available. Panthalookaran, et al. (2006) report the development of suitable CFD model for simulating HWSHS. In order to derive design concepts from such CFD models a fitting characterization scheme has to be formed. The current paper is a step in this direction. The existing methods of characterization of thermal energy stores (TES) may be broadly categorized as 1) those based on the general dimensionless numbers of heat transfer and fluid dynamics, 2) those based on the First Law of Thermodynamics (energy-based characterization), and 3) those based on the Second Law of Thermodynamics which includes both entropy and exergy-based characterizations. The First Law based characterization methods account for the heat losses due to the heat transfer interactions between TES and its surrounding. Some of these methods neglect the effects of internal mixing (Wildin and Truman, 1989) and some others consider both the heat losses to the surroundings and the enhanced enthalpy flow out of the TES due to substantial internal mixing. However the actual loss of usefulness of the stored energy as a result of internal mixing is often overlooked. An accurate characterization of a TES requires a strategy which is also based on the Second Law of Thermodynamics (Bejan, 1978; Rosen et al. 2004). In this approach TES is considered as an exergy store rather than an energy store and the major concern is the reduction of the entropy generation during different TES processes. However, the First Law concerns on different TES processes are often overlooked in a purely Second Law based characterization. In the current paper we develop a new method of characterization of different storage processes which integrates the First and Second Law concerns simultaneously. The CFD analysis when combined with the new characterization scheme offers a new way of deriving useful design concepts for HWSHS. Some design parameters such as general shape and boundary structures are analyzed in this paper to show how the new HWSHS design scheme works.