Session: 11-10-01 Single/ Two-Phase Heat Transfer in Active and Passive Systems

Paper Number: 66381

Start Time: Tuesday, 07:40 PM

66381 - Simulation-Based Correlation for Saved Energy in Ground Source Heat Exchangers for Middle East Region 

Shallow geothermal energy is a renewable energy source used to reduce electric demand to produce cooling and heating buildings.  The temperature at a specific ground level is constant year-round depends on the geographic region. It can be utilized by exchanging the hot weather (cooling) heat or cold weather (heating) using Ground Source Heat Exchangers GSHE. Many attempts have been proposed to investigate the GSHE controlling factors.  These factors are either geometric factors, physical factors, or operational factors.  Typical geometric factors are; wellbore diameter, and pipe length, and diameter.  Typical thermophysical factors are; wellbore grout thermal conductivity and soil thermal properties.  Typical operational factors are; circulating water flow rate and water input temperature. The accurate impact of these factors' interconnection on the GSHE performance is still needed for further investigation.  This paper proposes a CFD simulation-based nonlinear correlation between the controlling factors and the amount of normalized saved energy, maximum temperature difference, and the GSHE COP.  The study concerns with high-temperature ground sources similar to that in the Middle east.  The current work investigates the interconnection relation of seven factors; three geometrical factors, two thermophysical factors, and two operational and environmental factors.  The studied geometrical factors are the wellbore diameter and length and the tube diameter.  The studied two thermophysical properties are the thermal conductivities of the wellbore grout and soil.  The two studied operational and environmental factors are the circulating fluid flow rate, circulating fluid input temperature difference with the soil temperature.

CFD model is used to investigate the effect of the controlling parameters on the targeted output measures.  The CFD model is verified against published in-situ experiments at different conditions showing excellent agreement.  Then the CFD model is parameterized for the correlation study.  Third-order correlations of the three main outputs are achieved using hybrid Box-Behnken-Central-Composite design of experiments methods DOE.  The Box-Behnken method concerns the mid of extremes, and the Central-Composite method concerns the rotatable variable interconnections.  Although both methods are designed for second-order response surfaces, the proposed hybrid method can accurately predict third-order correlation using the kriging method on 160 design points.    The nonlinear correlations are verified using another 160 random verification points, showing a root mean square error of less than 1%. In contrast, the second-order correlation shows a root mean square error of more than 10%.  

The significance of each parameter and the optimum conditions to achieve the maximum of the three outputs are presented. The tube diameter, grout conductivity, soil conductivity, and temperature difference are the most significant parameters controlling the GSHE performance, while the grout diameter is insignificant.  The water mass flow rate is a critical parameter showing a proportional increase in the GSHE performance up to an optimum flow rate, after which the GSHE performance is inversely decreasing or, at best conditions, is insignificantly affected.  The response surface study has shown high normalized saved energy of 100W/m of the tube length for the investigated domains.
 

Presenting Author: Khaled I. Ahmed King Abdulaziz University

Authors:

Khaled I. Ahmed King Abdulaziz University
Abobakr Almashhor King Abdulaziz University
Mohamed H. Ahmed King Abdulaziz University