Geothermal energy concept applied to All-Air HVAC system

The novel method of combining geothermal energy with an all-air heating, ventilation, and air conditioning (HVAC) system is presented in this work. A building in Lebanon serves as the case study, which aims to determine the necessary mass flow rate for the conditioned space. Rather than permitting ambient air to enter the Air Handling Unit (AHU) directly, the plan calls for a 2-meter-deep geothermal duct to be buried. By adjusting the air temperature prior to it entering the AHU, this geothermal duct helps to improve the efficiency of the HVAC system and lowers the AHU’s yearly energy usage. Furthermore, the idea guarantees that there will always be 100% fresh air available, which means that the all-air HVAC system won’t need to recycle treated air from the conditioned space—also known as return air. According to the findings, the heat rate of the geothermal duct may reach 210 kW, which would result in a large decrease in CO2 emissions and cost savings for the HVAC system.


Introduction
As a vital component of modern life, energy powers a wide variety of everyday uses, as several studies have shown [1][2][3][4][5][6][7][8].A noteworthy development in the field of energy research is the group effort to lower energy usage and carbon dioxide emissions [9][10][11].Using renewable energy sources in conjunction with the skillful energy management techniques described in [12][13] presents a workable way to develop an environmentally sustainable energy landscape without sacrificing necessary services.
It is vital to expend energy in order to achieve thermal comfort in daily living.The well-being and productivity of building occupants are greatly impacted by thermal comfort, underscoring its significance in building design and HVAC system performance.Residential HVAC systems are estimated to account for 40% to 60% of household energy consumption in residential buildings.Given their influence on the environment in the residential sector, this emphasizes the necessity of energyefficient HVAC solutions [14][15][16].The energy footprint of residential HVAC systems varies according to building size, user habits, insulation quality, climate, and system efficiency [17][18].Climate Change News (2022) reports that 39% of the world's greenhouse gas emissions from energy consumption may be traced to the construction industry, with almost half of those emissions coming from HVAC system needs.Investigating all-air HVAC systems requires navigating a terrain of environmental and economic disparities.These systems have advantages in ventilation, heating, and cooling, but they also have drawbacks.Budgets are greatly impacted by the initial expenditures of equipment, installation, and maintenance, which can be high from an economic standpoint.These systems have the potential to be heavy energy users, which would increase greenhouse gas emissions and accelerate climate change.Therefore, a careful blend of study and creativity is needed to achieve a harmonious balance of indoor comfort, financial responsibility, and environmental responsibility.Comino et al. [23], who promoted a hybrid air management system for high latent load buildings and showed greater energy efficiency in a variety of climates, are among the many studies in this topic.A novel thermoelectric-energy recovery system was shown by Ramadan et al. [24], highlighting the potential of thermoelectric generators (TEGs) in the generation of green energy.Additional studies looked at solar-assisted HVAC systems [25] and HVAC system design layout optimization [26], providing information about cutting-edge methods and their drawbacks.Furthermore, it has been noted that centralized model predictive control (MPC) techniques have promise for managing comfort and energy in homes powered by solar energy [27].
The present study, demonstrates a novel method by fusing geothermal energy with a centralized all-air system.With 100% fresh air supplied, this blend guarantees the highest possible indoor air quality.Additionally, geothermal energy bestows ideal conditioning on the incoming air prior to its treatment by the Air Handling Unit (AHU).The combination of these technologies promises significant energy savings while improving energy efficiency and indoor air quality at the same time.

Materials and Methods
An in-depth examination of a particular structure in southern Lebanon is selected in order to meet the objectives of this research.Utilizing the Hourly Analysis Program, the necessary mass flow rate for the case study is determined.The program takes into account a number of building attributes, such as the overall heat transfer coefficient of each structural element (windows, doors, orientation, occupants, electrical equipment, etc.), and it integrates weather data.An overview of the information needed to calculate the required mass flow rate and the cooling and heating loads is given in Table 1.For the purpose of analyzing and developing the geothermal component-which consists of a circular geothermal duct buried two meters below the surface-this mass flow rate is essential.throughout this setup, the soil temperature is consistently between 15 and 16°C throughout all seasons.The geothermal system operates on a constant wall temperature, much like a heat exchanger.Finding the duct length and the corresponding geothermal heat rate is the main goal.In this section, considerations are focused solely on heat transfer through the geothermal duct.The underground duct is conceptualized as a heat exchanger with a consistent temperature applied across its heat exchange surface.Figure 1 demonstrates the interaction between the geothermal duct and the surrounding soil using the Log Mean Temperature Difference.The outside air properties are obtained from the local weather data.In accordance with the classical Log Mean Temperature Difference method, the calculation of the geothermal duct length can be determined using the equation provided below [27]: Where, Qgeothermal is the heat rate from geothermal exchange between the airflow and the soil.A is the area of heat exchange, and for the purposes of this example, it is a circular duct with A = πDL.U is the overall heat transfer coefficient.Since the thermal resistance resulting from the thickness of the circular duct is negligible (about 50 W/mK for galvanized steel), U is the same as the convection coefficient from internal flow (U = hconv).The thermal resistance due to the duct walls are neglected due to its high thermal conductivity.
The equation below presents the length of pipe that depends on the other parameters.
Moreover, the , is the Log Mean Temperature Difference using the below equation [27].Ta,i is inlet air temperature into the geothermal duct.Its value varies according to weather conditions.The Tin-AHU is the desired temperature to be entered into the AHU.In the present study, it is fixed at 24 0 C for cooling processes and 16 0 C for heating processes.These desired temperatures are the main parameters in order to estimate the length of geothermal duct presented in Equation 2. The convection coefficient hconv, is the convection coefficient due to the internal flow that could be calculated using the equation below [27]: And n is a value = 0.3 in cooling and 0.4 in heating.The D is the diameter of duct and k is the thermal conductivity of air taken at the film temperature.Moreover, the Re is the Reynolds number and the Pr is the Prandtl number calculated by the following equations [27]: In this case, the variables ρ (air density), V (duct flow velocity), μ (dynamic viscosity of air), γ (kinematic viscosity of air), and α (air diffusivity) are used.It is necessary to make assumptions about the duct width and the target temperature Tin-AHU in order to integrate these equations.It is also necessary to calculate the mass flow rate of air needed for heating and cooling.In this approach, the convection coefficient, hconv, and Qgeothermal are estimated.It is also crucial to understand the thermophysical characteristics of air at the film temperature.The pipe's length is then approximated in accordance with that.

Results and Analysis
When 90% of the air in the centralized all-air HVAC system is recirculated, the cooling and heating loads for the baseline scenario are calculated.Table 2 presents the main results that were acquired using the HAP software in a concise manner.The mass flow rate of supplied air (11.33 kg/s) and the coil loads for both heating and cooling are the important factors to take into account in this scenario.In the geothermal scenario, this predetermined mass flow rate will be consistently applied and maintained.This study focuses on determining the parameters for a 0.6 m diameter geothermal duct with a constant mass flow rate of 11.33 kg/s and a soil temperature (Ts) of 16 °C.The target temperature (Tin-AHU) varies depending on heating (11-15 °C) or cooling (16-22 °C).Duct length, heat rate, convection coefficient is discussed.The mass flow rate of 11.3 kg/s was determined by the HAP program.For yearround considerations, ambient temperatures of 5°C and 33°C during heating and cooling scenarios are essential.The impact of geothermal energy is highlighted by Figure 2, which illustrates the behavior of heat rate, which can reach up to 195 kW during cooling with notable temperature changes.In addition, for the present case, it is notable that heat rate obtained from the geothermal duct varies between 120 and 200 kW.Otherwise, for heating seasons, the heat rate varies between 70 and 120 kW.These obtained heat rates could be reduced from the consumed wattage that should be provided by the AHU.This could clearly reduce the yearly energy consumed by the AHU.
Based on heat rate data, Figure 3 shows the progression of the geothermal duct's length.For the ambient temperature to change from 33°C at the entry to 16°C at the outlet, a duct length of 311 meters is required.The duct length shrinks as the target temperature rises.When both heating and cooling are required, a balanced solution of 219 meters works best, with cooling aims of 17°C and heating targets of 15°C.

Conclusions
This study establishes the thermal modeling to calculate the length of the geothermal duct required to reach the desired target temperature based on a case study in Lebanon.Compared to a traditional all- IOP Publishing doi:10.1088/1742-6596/2754/1/0120027 air HVAC system, the main findings show that the combination of geothermal energy with the All-Air centralized could be a great solution to decrease the yearly energy consumption.It was shown in the present study that the heat rate obtained from the geothermal duct is relatively high and can reach minimum 60 kW and maximum 200 kW depending on heating and cooling processes.

Figure 1 :
Figure 1: Presents the geothermal duct that is considered as a heat exchanger between air and soil.

Figure 2 :
Figure 2: Geothermal heat rate variation with respect to target temperature at a depth of 2 meters, soil temperature 16 °C, and mass flow rate through the geothermal duct of 11.3 kg/s.

Figure 3 :
Figure 3: Evolution of the length of the geothermal duct with respect to target temperature at a depth of 2m, mass flow rate 11.3 kg/s and soil temperature of 16 °C.

Table 1 :
The structure properties, U value, areas, people and lighting.

Table 2 :
The main results obtained by HAP for the reference case.