Abstract
This study investigates cold thermal energy storage (CTES) using a helical coil heat exchanger modified with bubble injection. One of the effective methods for increasing the heat transfer rate in heat exchangers is using bubble injection. A helical coil heat exchanger is immersed inside a cylindrical water storage tank, where the helical coil is the evaporator of a vapor compression refrigeration cycle (VCRC) and provides the designed cooling. Experimental studies were carried out to examine the impact of bubble injection on Nusselt number, the temperature differences in the storage tank, exergy degradation in the evaporator, and cycle coefficient of performance (COP). The bubbles were injected from the bottom of the storage tank in four different geometries at airflow rates ranging from 3 to 11 L/min. The experimental results of this study revealed that bubble injection could significantly increase the COP and heat transfer rate from the storage tank, as well as the exergy destruction and Nusselt number (Nu). This increase was highly dependent on the geometry and flow rate of the bubble injection. The results also indicated that the bubble injection has an optimal flow rate value, which was 9 L/min in this study. More specifically, the COP of the refrigeration cycle and the Nu number increased by 124% and 452%, respectively, compared to the non-bubble injection mode. Finally, for calculating the outside Nusselt number of the helical coil, an empirical correlation as a function of bubble flow rate and bubble injection angle was proposed.
Highlights
Energy storage is an important component of modern energy systems and is being pursued in a variety of applications such as food storage and air conditioning systems [1]
The results showed that using bubble injection significantly increased the number of thermal units (NTU) and heat exchanger efficiency, which increased up to 1.5–4.2 times in NTU and 1.36–2.44 times in effectiveness compared to pure water, respectively
An immersed helical coil heat exchanger in a water storage tank was used for cooling, which was the evaporator of the compression refrigeration cycle
Summary
Energy storage is an important component of modern energy systems and is being pursued in a variety of applications such as food storage and air conditioning systems [1]. Cold thermal energy storage (CTES) in energy systems is one method for reducing peak energy consumption [2]. CTES technology can be implemented using an electric refrigerator. Cold thermal energy storage (CTES) is far less expensive than saving electricity for cold production [3]. The following benefits are associated with the use of cold storage technology: a) essential role in “peak load shifting”, b) lowering the cooling system’s energy con sumption, c) lowering the system’s operating costs during peak hours, and d) reducing the environmental pollution [1,2]. Increasing the efficiency and lowering the electricity consumption of cooling systems in CTES can be accomplished through four general approaches:
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