Numerical Evaluation of the Transient Performance of Rock-Pile Seasonal Thermal Energy Storage Systems Coupled with Exhaust Heat Recovery

Abstract

This study seeks to investigate the concept of using large waste rocks from mining operations as waste-heat thermal energy storage for remote arctic communities, both commercial and residential. It holds its novelty in analyzing such systems with an experimentally validated transient three-dimensional computational fluid dynamics and heat transfer model that accounts for interphase energy balance using a local thermal non-equilibrium approach. The system performance is evaluated for a wide range of distinct parameters, such as porosity between 0.2 and 0.5, fluid velocity from 0.01 to 0.07 m/s, and the aspect ratio of the bed between 1 and 1.35. It is demonstrated that the mass flow rate of the heat transfer fluid does not expressively impact the total energy storage capacity of the rock mass, but it does significantly affect the charge/discharge times. Finally, it is shown that porosity has the greatest impact on both fluid flow and heat transfer. The evaluations show that about 540 GJ can be stored on the bed with a porosity of 0.2, and about 350 GJ on the one with 0.35, while the intermediate porosity leads to a total of 450 GJ. Additionally, thermal capacity is deemed to be the most important thermophysical factor in thermal energy storage performance.

Progress on rock thermal energy storage (RTES): A state of the art review

“…Another example is the utilization of RTES in remote communities that instead of letting the summer heat go to waste, RTES can modify the heat to work seasonally. 11,190 The high dependency on fossil fuels, diesel generators, and electric heating in remote communities with arctic climates leads to inefficiency in the process of supplying power. During winter, the generators discarded a significant amount of heat through the exhaust due to the lower efficiency of conventional diesel engines, which is about 35%.…”

“…Among TES, rock thermal energy storage (RTES) has attracted significant attention for implementation in large-scale thermal systems due to its favorable features such as large storage capacity, simple storage mechanism which translates to a simple storage system, low environmental impact, wide range of operating temperatures, and low material costs. 10,11 RTES is considered to be more cost-effective than its water-based counterpart when implemented in solar water heater systems for domestic heating. 12 Typically, an RTES uses air as heat transfer fluid (HTF)-although other HTFs have also been considered and implemented-and a pile of crushed rocks as energy storage medium.…”

“…Numerous studies on RTES have been conducted and reported, experimentally [17][18][19] and numerically. 11,[20][21][22] Despite extensive studies that have been reported, further research and development is necessary to ensure the maturity and wide adoption of this technology. To expedite this process, a compilation of the important findings from previous studies is crucial to direct and guide future investigations.…”

“…It becomes an integral part of various present thermal systems especially in large‐scale applications where the demand fluctuates significantly. Among TES, rock thermal energy storage (RTES) has attracted significant attention for implementation in large‐scale thermal systems due to its favorable features such as large storage capacity, simple storage mechanism which translates to a simple storage system, low environmental impact, wide range of operating temperatures, and low material costs 10,11 …”

Annual Energy Performance of an Air Handling Unit with a Cross-Flow Heat Exchanger

“…Air density (ρ) and specific heat capacity (c p ) at known temperature (t) can be computed from the following formulas [56,57]…”