Earth Air Tunnel Heat Exchanger Research Articles

Numerical analysis of heat and mass transfer in separated ventilation of deeply buried long air intake tunnels

Using earth air tunnel heat exchangers (EATHE) to pre-treat air entering underground space is an effective measure for energy-saving. This paper addresses the scarcity of numerical model for heat and mass transfer in long, separated air intake tunnels by establishing a three-dimensional model that considering condensation, specifically analyzing the heat transfer process in tunnels with separated ventilation earth air tunnel heat exchanger (SV-EATHE). The accuracy of the model was verified by engineering measured data. The heat transfer performance of EATHE and SV-EATHE is compared and analyzed by using the model. The results indicate that the annual heat exchange of SV-EATHE is approximately 80 % of that of EATHE, with the latent heat exchange from March to September accounting for 77.2 % of EATHE's. SV-EATHE significantly reduces condensation in traffic tunnels. Increasing ventilation will correspondingly increase the heat exchange and reduce dehumidification, but the rate of change will gradually decrease. The change in partition wall thickness has an approximately linear relationship with the amount of heat exchange. For every 0.1 m increase in diaphragm wall thickness, the heat exchange in the intake duct in August is reduced by approximately 1500 kWh and the dehumidification is reduced by approximately 0.6 kg/h.

An Investigation of the Thermal Performance of a Tropical Greenhouse Constructed With an Earth Air Heat Exchanger

Abstract To maintain optimum plant growth temperature (i.e., 25–35 °C), the thermal performance of an earth air tunnel heat exchanger (EATHE)-integrated single-span saw-tooth greenhouse (GH) was assessed in peak summer. With the side and roof vents opened, natural ventilation due to wind and stack effect controlled the air movement and temperature inside the GH. In this configuration, the average temperature inside the GH remained higher than the ambient temperature by 1.5 °C for the entire period of the experiment. For EATHE (installed at a depth of 3.2 m) assisted GH with polyethylene (PE) cover, the air from the EATHE outlet entered inside the GH at 33 °C, and the average temperature within the GH was maintained at 4 °C lower than the ambient temperature. When the shading net was installed over the PE cover with the EATHE, the transmitted radiations into the GH were reduced from the roof, and the inside temperature was maintained 7 °C below the average ambient temperature (i.e., 45 °C). The measured temperatures along the length of the EATHE were compared with the indigenously developed code named PEAT (performance analysis of earth–air tunnel) and found to be in good agreement within ±4.5% deviation. The temperature distribution inside the GH was predicted using a computational fluid dynamics (CFD) model in ansys fluent with ±5% deviation from experimental results. With parametric analysis from the PEAT code and CFD model, the desired depth of the EATHE and the mass flowrate of air required to bring down the GH indoor temperatures to the optimum plant growth range were determined.

Numerical modelling of an earth-air- tunnel assisted single span saw-tooth greenhouse for tropical climate

In tropical conditions the greenhouse temperature rise above the advised range at crop height is a major issue. In the present research work, thermal performance of Earth Air tunnel Heat Exchanger (EATHE) assisted single span saw-tooth greenhouse was assessed using indigenously developed code and commercial computer program (ANSYS-Fluent). The numerical model was first validated with published and own experimental data. The effectiveness of coupling EATHE to control inside temperature of the greenhouse was assessed for different side vent opening scenario. The air movement caused by wind and stack effect in greenhouse was an important factor that affected the uniformity of greenhouse temperature. Maximum temperature was observed near the ground because solar radiations transmitted through the greenhouse cover were absorbed by the soil. For greenhouse without EATHE, the inside average temperature remained above 45 °C (ambient temperature) as no cooling mechanism was deployed. On the other hand for EATHE assisted greenhouse, optimum plants growth temperature (between 25 and 35 °C) was maintained at crop height because the annual variation in the underground soil temperature was much smaller compared to the ambient air temperature. Among various configurations, the best thermal performance for optimum plant growth was obtained when both side vents of greenhouse were connected to EATHE.

Computational Studies of Earth Air Heat Exchanger using CFD: Parametric Analysis

Parametric studies of earth air tunnel heat exchanger (EATHE) using FLUENT 6.3 Computational fluid dynamics (CFD) software have been carried out in this paper. From the parametric analysis it is found that economic analysis of the system is required for the optimization of length, diameter, and depth of buried pipe. For a long earth air tunnel more than 60 m, soil thermal conductivity doesn’t give any significant effect. The higher mass flow rate also increases the overall energy conservation potential of the system, but it reduces thermal comfort. So, the mass flow rate also takes to optimize according to the requirement. This study is beneficial to the design and energy researchers of this field.

Estimating the reduction in HVAC load in multipurpose hall with the use of earth air tunnel heat exchanger (at Kathmandu University)

Earth Air Tunnel (EAT) is a renewable form of Air Conditioning system applicable for both heating and cooling purposes with the utilization of Earth’s undisturbed temperature. The Multi-purpose hall located in Dhulikhel, Kavre has a warm and humid climate. The total cooling requirement for the hall is estimated to be 763.6 kW and the corresponding ventilation requirement is 6.03 m3/s (6.97kg/s at 32°C/55% RH). The Earth Air Tunnel is designed for pre-conditioning the ventilated air only. Considering the optimum length of 75m, the designed earth air tunnel has 10 units parallel tube of 75 m length, 400 mm diameter, and flow velocity of 4.8 m/s. The tunnel produces the cooling effect of 132 kW when the inlet and outlet temperatures are 32°C (55% RH) and 19°C (100% RH) respectively. Cooling during humid climate the output air has higher chances to be saturated and even condenses in the tube underground. Dehumidifying the air before entering the EAT can solve this problem and thus enhance the cooling effect during the humid season to some extent. With the assumed 1000 hours per year of operation, the estimated payback time at the rate of 4% per annum (assumed subsidized case) will be 15 years. Earth Air Tunnel must be given greater care for the hot and humid zone because the cost of Earth Air Tunnel will be very high in comparison to the heat pump and also the tunnel does not guarantee to give the intended performance throughout the year which does not seem to make the project feasible for the University hall.

Performance analysis of clean energy using geothermal earth to air heat exchanger (GEAHE) in Lower Himalayan Region – Case study scenario

Energy demand in India at present is fulfilled by non-renewable sources despite the prime geographical location of some states. Geothermal energy is the renewable energy within the earth’s interior will be used for various purposes. In this paper, an attempt has been made to use the renewable source of energy for sustainable development. The thermal performance of earth to air tunnel heat exchanger for passive winter preheating and summer cooling has been evaluated. The effect of an operating parameter such as pipe/tube material, soil thermal conductivity and air velocity on the thermal performance of earth to air heat exchanger (GEAHE) to be used in Lower Himalayan Region has been analyzed. The results indicate that the exit temperature drops with an increase in underground pipe/tube length during summers and decreases during winters. However, the mean efficiency increases with increase in underground pipe/tube length. Maximum efficiency of 85.6% and 87.7% is achieved during summer and winter respectively at an air velocity of 0.5 m/s for an underground pipe of 60 m length. Also, the maximum average cooling potential is 14.81 kWh during summer for earth air tunnel heat exchanger at underground pipe length of 60 m and at a velocity of 0.5 m/s. Similarly, the maximum average heating potential is 27.7 kWh during winter for same operating parameters; which is higher as compared to summers. Thus the results indicate that earth air tunnel heat exchanger is more effective during winter in Lower Himalayan Region. The results demonstrate that geothermal energy stored in a Lower Himalayan Region of India can be a driving force for sustainable development.

Thermal performance and comfort assessment of U-shape and helical shape earth-air heat exchanger in India

Earth Air Tunnel Heat Exchanger (EATHE) systems are not so much popular in India because of restricted accessibility of ground space in majority of residential and commercial sector. In the present article, thermal performance of newly designed helical shaped EATHE system has been compared with conventional U–shaped EATHE system by keeping similar geometrical and operational configuration for the summer (April–June) and winter (December–February), respectively. In this regard, therefore, numerical model of both EATHE systems have been designed and experimentally validated. The thermal performance of both EATHE systems has been compared in terms of outlet air temperature and effectiveness. The present numerical study suggested that the newly designed helical EATHE system exhibits better thermal performance with lower ground requirement as compared to conventional U-shaped EATHE system. To evaluate the thermal comfort potential of EATHE system in office buildings of India, composite climate specific adaptive thermal comfort boundaries has been used. The detailed thermal performance behaviour of the EATHE system suggests that an additional 80% of the time; ambient air conditions became thermally comfortable during peak summer season. In addition, analysis of the results revealed that EATHE system has more potential during heating operation mode to maintain indoor comfort in winter season of composite climate in India.

A numerical method for cooling and dehumidifying process of air flowing through a deeply buried underground tunnel with unsaturated condensation model

Earth-air tunnel heat exchanger (EATHE) system is a potential passive cooling/heating air conditioning technology to pre-treat the ventilation air. Predicting the heat and mass transfer is critically important to ensure sufficient ventilation for occupational safety, while the existing method calculates it without the unsaturated condensation process. In this paper, the description failure of dehumidifying process of unsaturated air is attempted by proposing an advanced model for a deeply buried underground tunnel. Based on the experimental results, the minimum relative humidity of condensation was fitted with air flow rate, equivalent radius of tunnel, inner wall temperature, air temperature and relative humidity. Then, the advanced model was validated with the measured data and good agreement was achieved. Finally, comparisons between new model and existing model were conducted. The results show that the advanced model can calculate the unsaturated condensation process. Besides, the new model has higher calculation accuracy. Specifically, the relative error of temperature and relative humidity of condensation period is reduced by 50% when compared to the existing model.

Computational Fluid Dynamics Simulation Based Comparison of Different Pipe Layouts in an EATHE System for Cooling Operation

Earth air tunnel heat exchanger (EATHE) is a capable and quite simple passive technique which may be utilized for space cooling/heating using the constant temperature of underground subsoil. However, it could not gain much attraction as a heating/cooling system as it requires larger trench lengths to accommodate longer pipes. Larger trench lengths involve huge excavation cost and a sufficiently large piece of land. The length of the trench needed can be reduced substantially by adopting a proper pipe layout. In the present study, the performance of U-shaped, slinky-coil, and helical-coil pipe layouts of an EATHE system is compared numerically using ANSYS FLUENT 15.0. Results reveal that the temperature drop and heat transfer rate per unit trench length are higher in the slinky-coil pipe layout than in U-shaped and helical-coil pipe layouts. After 12 h of continuous operation, the effectiveness of the EATHE system with U-shaped, slinky-coil, and helical-coil pipe layouts is obtained as 0.60, 0.80, and 0.78, respectively. The study reveals that the selection of pipe layout for the EATHE system mainly depends on temperature drop EATHE is capable of giving, heat transfer rate, pumping power required, and ease of fabrication and installation as all these factors directly affect the initial and recurring capital investment for the EATHE system.

Optimization of operating parameters of earth air tunnel heat exchanger for space cooling: Taguchi method approach

In the present study, CFD-based parametric analysis is carried out to optimise the parameters affecting the temperature drop and heat transfer rate achieved from earth air tunnel heat exchanger (EATHE) system. ANSYS FLUENT 15.0 is used for CFD analysis, and k-ε model and energy equation were considered to define the turbulence and heat transfer phenomena. For a straight EATHE system configuration, four design and operating parameters, i.e., diameter of the pipe (A), length of pipe (B), inlet air velocity (C), and inlet air temperature (D), are considered at four different levels in Taguchi method. The Taguchi method is used to obtain maximum air temperature drop and heat transfer rate. The best combination of parameters for achieving a maximum drop in air temperature is A1B4C1D4 and that for obtaining maximum total heat transfer rate is A4B4C4D4. Statistical analysis reveals the percentage contribution of different factors for air temperature drop in the following order: inlet air temperature (57.80%), diameter of pipe (20.66%), length of pipe (12.03%), and air velocity (9.51%), while, for heat transfer rate, pipe diameter (53.28%), inlet air temperature (30.87%), air velocity (9.40%), and length of pipe (6.45%).

Field investigations to determine the thermal performance of earth air tunnel heat exchanger with dry and wet soil: Energy and exergetic analysis

Earth air tunnel heat exchanger (EATHE) systems are inadequate to meet the thermal comfort requirements in semi-arid regions as they require large pipe lengths. The effectiveness and performance of such system can be increased by increasing soil moisture content, making as wet soil configuration. In the present work, the thermal performance of the EATHE system with dry and wet soil has been evaluated during the peak summer season. Further, the thermal behavior of these two EATHE systems has been compared on the basis of temperature of air flowing through buried pipes at different sections along the length. For the same a theoretical model is developed and validated with experimental results and found in good agreement with an error ranging from 2.44% to 10.85%. Moreover, the exergetic analysis has been carried outto investigate the work potential and scope of maximum utilization of energy. Results reveal that thepipe length can be reduced by 12–14 m with wet EATHE system as compared to dry EATHE system for the same cooling performance. The second law analysis shows that the maximum exergetic efficiency is 52.25% and 53.18% for dry and wet soil EATHE systems respectively.

Experimental study to investigate the effect of water impregnation on thermal performance of earth air tunnel heat exchanger for summer cooling in hot and arid climate

Abstract An EATHE system performs well for short periods but its thermal performance gets deteriorated during long continuous operation due to sub-soil thermal saturation in the vicinity of buried pipe. Effect of soil thermal saturation can be reduced to some extent in longer EATHE pipe but it is uneconomical. In the present study sub-soil moisture content is increased to enhance soil thermal properties and its effect on EATHE thermal performance and pipe length requirement for certain temperature drop has been investigated experimentally for summer cooling in hot and arid climate. In the study, two identical experimental set-ups have been developed at Ajmer city (India). A water impregnation system has been introduced to maintain different soil moisture contents in the close proximity of EATHE pipe. Knee point is obtained at a length of 29 m, 28 m, 27 m and 26 m from pipe inlet section with 5%, 10%, 15% and 20% moisture in sub-soil respectively, as compared to 41 m length in dry soil, after 10 h of continuous operation. The average heat transfer rate and COP increased by 24.1% and 24.0% respectively for 20% moisture content at 30 m EATHE pipe length as compared to dry system.

Comparative study of straight and spiral earth air tunnel heat exchanger system operated in cooling and heating modes

Thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) system is found to be highly dependent on thermal properties of soil and soil with poor thermal properties require longer buried pipe for desired heat exchange between air and sub-soil. In densely populated country like India, EATHE systems are rarely used due to the limited ground space (higher aspect ratio in terms of length) in most of the residential and commercial applications. Therefore, in this paper, problem of space limitation for the installation of straight EATHE system is attempted by proposing new spiral shaped EATHE system. The developed EATHE systems were used to determine cooling and heating potential individually and COP were 5.94 and 6.24 in summer; 1.92 and 2.11 in winter for straight and spiral respectively. A transient numerical model has been developed to investigate the performance of both systems. The COP were 4.23 and 4.48 in summer; 5.0 and 5.16 in winter for straight and spiral respectively. The experimental and numerical results suggested that the thermal performance of these two systems are comparable can be interchanged based on available site conditions.

CFD Simulation Studies on Integrated Approach of Solar Chimney and Earth Air Tunnel Heat Exchanger for Building Space Conditioning

Individual studies on solar chimney and earth air tunnel heat exchanger have been carried out by various researchers but individual systems are not fulfilling the space heating and cooling demand of buildings. So, integrated approach of solar chimney and earth air heat exchanger has been studying in this communication. Computational fluid dynamics software is used for the modelling and simulation studies. The cooling effect is produced by 5.30-6.72 kW at 40°C ambient temperature and space heating is evaluated as 10.28-14.71 kW at 5°C ambient temperature and 400-1000 W/m2 16 solar radiations. And it is sufficient heating and cooling for buildings at average solar irradiance. The SC-EATHE integrated system approach produced 18 37% higher heating and cooling effect than the EATHE alone system.

Transient effect of soil thermal diffusivity on performance of EATHE system

Abstract This paper presents effect of thermo-physical properties of soil on performance of an Earth Air Tunnel Heat Exchanger (EATHE). The analysis has been carried out using a validated three-dimensional, transient numerical model for three different types of soil. The governing equations, based on the k – e model and energy equation were used to describe the turbulence and heat transfer phenomena, are solved by using finite volume method. Comparisons were made in terms of temperature drop, heat transfer rate and COP of the EATHE system by operating it continuously for 12 h duration. The study reveals that each soil exhibits different rate of heat dissipation and thermal saturation over a period of continuous operation, which adversely affects the performance of EATHE. Dissipation of heat from the EATHE pipes to its surrounding soil and subsequently to the outer subsoil region is mainly found to be depending upon the thermal conductivity of soil; even of their thermal diffusivity is of different order.

Thermal performance investigation of earth air tunnel heat exchanger coupled with a solar air heating duct for northwestern India

In the present research thermal performance of earth air tunnel heat exchanger (EATHE) coupled with a solar air heating duct has been experimentally evaluated for arid climate of Ajmer city of northwestern India, during winter season. An attempt has been made to enhance the heating capacity of EATHE system by coupling it with a solar air heating duct at the exit end. Results show that the air which comes out of coupled EATHE system is relatively hotter than the air supplied by the stand alone EATHE system. It was found that the heating capacity of EATHE system got increased by 1217.625–1280.753kWh when it was coupled with solar air heating duct with a substantial increase in room temperature by 1.1–3.5°C. The COP of the system also increased up to 4.57 when assisted with solar air heating duct. Therefore, the heating capacity of EATHE can be significantly increased by coupling it with solar air heating duct.

CFD analysis of EATHE system under transient conditions for intermittent operation

Thermal performance of earth air tunnel heat exchanger has been investigated under transient conditions for three different soil conditions considering three operating modes. In first operating mode EATHE works continuously for 12h, in second mode it works for 60min and then it is off for 20min and in the last mode EATHE runs for 60min and remains off for 40min. In second and third mode EATHE is operated for 12h intermittently. CFD model was developed in GAMBIT (version 2.2.3), simulated in FLUENT (version 6.3) and then validated with experimental data. CFD analysis has been carried out using a three-dimensional transient numerical model. Air temperature drop and heat transfer between air and soil is calculated considering different thermal properties of soils. Higher thermal conductivity (1.28Wm−1K−1) soil should be used in first mode of 12h continuous operation as it improves the performance by 5.04% as compared to least thermal conductivity soil and soil with poor thermal conductivity (0.52Wm−1K−1) shows performance improvement of 1.81% when operated in intermittent operation as compared to continuous running.

Thermal performance prediction and energy conservation potential of earth air tunnel heat exchanger for thermal comfort in building

In this communication, thermal performance of building integrated “Earth Air Tunnel Heat Exchanger” (EATHE) has been studied for a composite climate of Delhi. Mathematical model and building thermal simulation software (Energy Plus) are used for prediction of indoor air temperatures. The predicted results are validated through experimental results. Energy conservation potential both for heating and cooling of EATHE has been evaluated for winter and summer months in a specific year. Quantitative assessment of CO2 mitigation has also been computed. It is observed that Coefficient of performance (COP) of EATHE is found between 6.3–13 during summer and 6.8–9.5 during winter season. The maximum drop in air temperature across the earth air tunnel is found to be 8.7 °C with air velocity of 2.69 m/s during peak summer and during winter season the room air temperature is raised by 5–7 °C. On the other hand, the system is capable of maintaining room temperature in the range of 27–30 °C during peak summer when outdoor temperature are in the range of 33–41 °C. EATHE integration with building can significantly reduce the electrical power consumption by 96.63 MWh annually, needed for space heating and cooling load. It is discovered from the study that this EATHE has potential of reducing global warming 8 by substantially mitigating CO2 to the tune of 95 tonnes annually.

Earth–air tunnel heat exchanger for building space conditioning: a critical review

India is one of the largest developing countries across the world, and currently, real estate sector (in residential and commercial building’s area) is growing approximately by 10% annually. The energy demand is increasing in space heating and cooling field because of not only on the growing real estate but also due to high variation between peak seasonal temperatures. The passive heating and cooling system play an important role to meet such energy demand. Earth–air tunnel heat exchanger (EATHE) is one of the passive systems, which utilizes the undisturbed earth temperature to heat and cool the buildings. Authors feel its importance for space heating and cooling of the building, and rigorously reviewed the energy conservation aspect of EATHE. The mathematical modeling, optimization and economic viability are presented in this article. The review also focuses on the design and layout of EATHE for a residential building.

Evaluating Thermal Performance and Energy Conservation Potential of Hybrid Earth Air Tunnel Heat Exchanger in Hot and Dry Climate—In Situ Measurement

Earth air tunnel heat exchanger is a passive cooling device with advantageous feature to reduce energy consumption in buildings. Curtailing the electricity consumption of conventional vapor compression system based air-conditioner is a major concern especially in area with hot and dry weather conditions. The performance of conventional air-conditioners can substantially be enhanced by coupling these active cooling systems with passive cooling systems. In the present research, the thermal performance and energy conservation potential of hybrid cooling system has been investigated experimentally. An attempt has been made to enhance the thermal performance of active cooling system by coupling it with earth air tunnel heat exchanger (EATHE) in two different hybrid modes. The air which comes out of EATHE is relatively cooler than the ambient air and therefore can be used either for cooling the condenser tubes of a conventional window type air-conditioner or supplying it directly to the room being conditioned. The energy consumption of conventional 1.5TR window type air conditioner is found to be reduced by 16.11% when cold air from EATHE is completely used for condenser cooling.