• Energy Research
• 6. Clean water close

Most of the storage systems available on the market use water as storage medium. Enhancing the storage performance is necessary to increase the performance of most systems. The stratification phenomenon is employed to improve the efficiency of storage tanks. Heat at an intermediate temperature, not high enough to heat up the top layer, can still be used to heat the lower, colder layers. There are a lot of parameters to study the stratification in a water tank such as the Mix Number and the Richardson Number among others. The idea studied here was to use these stratification parameters to compare two tanks with the same dimensions during charging and discharging processes. One of them is a traditional water tank and the other is a PCM-water (a water tank with a Phase Change Material). A PCM is good because it has high energy density if there is a small temperature change, since then the latent heat is much larger than the sensible heat. On the other hand, the temperature change in the top layer of a hot water store with stratification is usually small as it is held as close as possible at or above the temperature for usage. In the system studied the Phase Change Material is placed at the top of the tank, therefore the advantages of the stratification still remain. The aim of this work is to demonstrate that the use of PCM in the upper part of a water tank holds or improves the benefit of the stratification phenomenon.

The effect of using external vertical fins in Phase Change Materials (PCM) modules to improve the natural convection coefficient in water is studied in this paper. The use of PCM in a water tank working with a solar system is able to store a lot of energy, but it is necessary to transfer this energy to the water during demand. Optimization of the natural convection to the water is of great importance for the heat transfer. External fins increase the heat transfer surface and the heat transfer coefficient changes. Experimental work was carried out to determine natural convection heat transfer coefficients for PCM modules with two different external vertical fin geometries. Results were presented as a temperature variation over time of the PCM and the water, and the heat transfer coefficient as a function of the temperature difference for different fins. These results were used to validate a simulation code to determine the behaviour of the system using finned PCM modules. The results prove the technical potential of external fins for heat storage systems using PCM. The experimental results were used to determine different correlations for Nusselt number as a function of different dimensionless numbers.

Although water is a cheap and effective medium for thermal energy storage, other options are currently being studied, to increase the storage density or to reduce the cost of the storage. The authors have been developing a system which combines the advantages of stratified sensible heat storage and latent, phase change heat storage; i.e. a hot water storage tank with stratification where a phase change material (PCM) is included into a spiral tube installed in the top of the tank. The PCM used was a granular PCM–graphite compound of about 90 vol:% of sodium acetate trihydrate and 10 vol:% graphite. This paper presents the results of an experimental investigation of the performance of the new storage concept, and of a conventional hot water storage tank for comparison. The data are further analysed with respect to the energetic and exergetic performance of the two systems.

Abstract The efficiency of thermal energy storage and solar collector systems is improved if the water tank is stratified. There are many parameters to characterize stratification but no work compares their suitability. This paper identifies the most used dimensionless numbers to characterize stratification in water tanks and studies their suitability. Experiments with different flow rates were done and the dimensionless numbers were determined. Richardson is the best number to define stratification in a water tank, while Mix number presents some problems and a bad behaviour. The other numbers do not clearly characterize stratification but can be useful combined with Richardson.

Abstract In air-cooled water–LiBr absorption chillers the working conditions in the absorber and condenser are shifted to higher temperatures and concentrations, thereby increasing the risk of crystallisation. To develop this technology, two main problems are to be addressed: the availability of new salt mixtures with wider range of solubility than water–LiBr, and advanced absorber configurations that enable to carry out simultaneously an appropriate absorption process and an effective air-cooling. One way of improving the solubility of LiBr aqueous solutions is to add other salts to create multicomponent salt solutions. The aqueous solution of the quaternary salt system (LiBr + LiI + LiNO3 + LiCl) presents favourable properties required for air-cooled absorption systems: less corrosive and crystallisation temperature about 35 K lower than that of water–LiBr. This paper presents an experimental study on the absorption of water vapour over a wavy laminar falling film of an aqueous solution of (LiBr + LiI + LiNO3 + LiCl) on the inner wall of a water-cooled smooth vertical tube. Cooling water temperatures in the range 30–45 °C were selected to simulate air-cooling thermal conditions. The results are compared with those obtained in the same experimental set-up with water–LiBr solutions. The control variables for the experimental study were: absorber pressure, solution Reynolds number, solution concentration and cooling water temperature. The parameters considered to assess the absorber performance were: absorber thermal load, mass absorption flux, degree of subcooling of the solution leaving the absorber, and the falling film heat transfer coefficient. The higher solubility of the multicomponent salt solution makes possible the operation of the absorber at higher salt concentration than with the conventional working fluid water–LiBr. The absorption fluxes achieved with water–(LiBr + LiI + LiNO3 + LiCl) at a concentration of 64.2 wt% are around 60 % higher than those of water–LiBr at a concentration of 57.9 wt%.