Ground-air Heat Exchanger Research Articles

Assessing the viability of using ground-air heat exchangers during winter months in different climatic conditions of Iran

ABSTRACT Ground combined with subsurface tubes as heat exchangers, known as Ground-Air Heat Exchangers (GAHEs), have recently gained attention for heating and cooling. However, several simplifications were used to capture complex airflow characteristics in previous studies which potentiate inaccuracy in their results. This gap is addressed in this study by developing a 3D transient model incorporating realistic heat dynamics and turbulent airflow simulation using the k-epsilon model over a complete temperature cycle. The experimental result serves to corroborate the model’s accuracy. Further, the model is utilised to assess energy-conversion potential of GAHEs for heating a generic building during January in four Iranian cities with different climates. Findings show distinct differences between the GAHE’s energy-conservation capacity in these cities. The GAHE can conserve up to 714.6 kWh of energy monthly in the coldest climate and reduce peak heating loads by up to 36.0% in the hottest climate, with these averages observed across all soil types. GAHE energy saving is increased up to 27.7% when the soil around it is silt with a higher conductivity. Additionally, an adverse impact on building’s heating is created by the GAHE in some cases, highlighting the significance of hourly management of the system. Highlights A 3D transient numerical scheme was created based on fluid mechanics principles to simulate a multitube GAHE, and the solution was verified using empirical data. The effectiveness of the GAHE and its ability to reduce the maximum heating demand of a particular building for different climates within Iran in January were examined. The impact of soil characteristics on the energy-saving capability of the GAHE was found to be significant in the four cities under study, and a distinct difference in heat flux to the working fluid was discovered in different climates.

Experimental investigation of earth-air heat exchanger using porous clay vessels for eco-friendly buildings

This study introduces an experimental investigation of a novel direct trend evaporative cooler based on a ground-air heat exchanger (GAHE) using porous clay vessels as an evaporation media under a variety of operational conditions, including air flow rate, inlet air temperature, temperature of inlet water, and in air humidity. The evaluation of the GAHE performance was based on the air-cooling effect, wet-bulb and dew-point efficiencies, energy efficiency ratio, water evaporation rate, specific water evaporation, specific cooling capacity, specific total cost, and CO2 emission rate. The influences of dry-bulb temperature, the incoming air's relative humidity (RH), and six air flow rates ranging from 11 to 25 L/s on the performance are investigated and discussed. Results indicated that increasing the air flow rate leads to an increase in the cooling capacity. Energy efficiency ratio (EER) reaches the highest value of about 25.5 recorded at 3:00 PM with air flow rate = 11 L/s. The lowest EER value is approximately 7.2 when the measured inlet and outlet temperatures are the closest at 7:00 PM, with a flow rate of 25 L/s. Increasing the air flow rate from 11 to 17 L/s increased the wet bulb efficiency, and the airflow rate was inversely proportional to wet-bulb efficiency. The maximum and minimum average dew-point efficiencies are 64% and 58% at 17 L/s and 22 L/s respectively. The water evaporation rate increases by 182.1%, increasing the air flow rate from 11 to 25 L/s.

A new theoretical approach to determine the air outlet temperature of an air-to-ground heat exchanger

In this study, the control volume method is used to determine the air temperature at the outlet of an air-to-ground heat exchanger. Its implementation consists in dividing the duct of the ground-air heat exchanger into micro-volumes of identical size. An energy balance is then established for each micro-volume. The input parameters used to implement this model are related to the city of Yaoundé in the equatorial zone. The results show that when the total length of the air-to-ground heat exchanger duct varies between 0 and 100 m, the air temperature at the outlet also varies between 34.5 and 24 °C. The air-to-ground heat exchanger operates in cooling mode. As the length of the air-to-ground heat exchanger duct increases, the temperature of the air at the outlet of the air-to-ground heat exchanger decreases, approaching that of the ground. Based on the results obtained using the control volume model, the minimum total length of air-to-ground heat exchanger duct recommended for this zone is 40 m. Admittedly, air pressure drops, air humidity and the geometry of the air-to-ground heat exchanger are aspects that have not yet been taken into account in the implementation of this model. Nevertheless, the control volume method can be used to optimise the parameters influencing the thermal performance of an air-to-ground heat exchanger.•The control volume method is implemented here by dividing the air-to-ground heat exchanger duct into identical micro-volumes and then establishing an energy balance for each micro-volume;•In this work, the control volume method was used to optimise the total length of the duct of a ground air heat exchanger installed in an equatorial zone;•Some important aspects such as air pressure drops, air humidity, and the geometry of the air-to-ground heat exchanger are not yet taken into account in the implementation of the control volume method.

Thermal Performance of Earth Air Heat Exchanger for Geothermal Energy Application in Hot Climate using CFD Simulation

The Ground Air Heat Exchanger (GAHE) is a sustainable, environment friendly, and efficient device that can be used for both heating and cooling applications. Careful design of GAHE enables efficient exploit of the earth interior energy. The design of a GAHE relies on the constant temperature of the earth interior which allows consistent and reliable source of geothermal energy. By harnessing this renewable energy, a sustainable solution for heating and cooling needs is attained while minimizing the impact on environment. In this study, the performance of GAHE was examined using ANSYS Fluent 19 R1 and SOLID WORK 16.0 software. The efficiency and Coefficient of Performance (COP) of the ETHE have been investigated. The effect of air flow rate and operation conditions on the outlet air temperature have been studied. GAHE is made of Polyvinyl Chloride (PVC) pipe of 0.1 m diameter, 0.005 m thickness and 18 m horizontal length. Computer simulations were carried out for five different air velocities (1, 2, 3, 4, and 5 m/s) at various operation conditions. Results show that the 18 m pipe length is adequate to attain useful air outlet temperature giving COP values between 0.5 and 1.3. The length of the horizontal part of GAHE can be further increased for air velocities between 3 and 5 m/s. Comparison between the results obtained by the CFD model and experimental work demonstrated that the CFD model is capable of producing results with acceptable accuracy. This suggests that the CFD software can accurately model the performance of the GAHE under different operation conditions. Increasing the length of the horizontal part of the GAHE can improve its COP when higher air velocities are used.

Numerical modeling based on CFD of the thermal potential of a ground-air heat exchanger for 2 climates of the Mexican Republic

Numerical modeling based on CFD of the thermal potential of a ground-air heat exchanger for 2 climates of the Mexican Republic

Thermal performance of helical ground air heat exchanger under Saharan climate conditions: an experimental study

Thermal performance of helical ground air heat exchanger under Saharan climate conditions: an experimental study

Ground-Air Heat Exchanger as Passive Cooling In Building Ventilation

Energy consumption for air conditioning systems and thermal comfort in a building in Indonesia is quite high. The solution that can be developed is to use the thermal inertia stored in the soil. This research aims to design and test the effectiveness of heat transfer between soil-air as a passive coolant in building ventilation systems. This research was conducted for 48 hours, the incoming air velocity was 4.68 m/s, the measurement results showed various weather and temperature conditions including average light intensity of 1822.6 Lux, average ambient temperature of 28.4 °C, average humidity of 99.4%, average room temperature of 30 °C, average water temperature of 28 °C and average soil temperature of 28.1 °C and maximum effectiveness value of 2.46 and minimum air value of 0.9. Experimental results show that groundwater-air heat transfer used as passive cooling in building ventilation systems can provide good performance with temperature reductions reaching 2-10°C.
 Konsumsi energi untuk sistem pengkondisian udara dan kenyamanan termal bangunan di Indonesia cukup tinggi. Salahsatu solusi yang dikembangkan ialah menggunakan inersia termal yang tersimpan di dalam tanah. Penelitian ini bertujuan untuk merancang-bangun serta menguji efektivitas perpindahan kalor antara tanah-udara sebagai pendingin pasif pada sistem ventilasi bangunan. Penelitian dilakukan selama 48 jam dan kecepatan udara masuk 4,68 m/s, hasil pengukuran menunjukan berbagai kondisi cuaca maupun suhu meliputi intensitas cahaya rata-rata 1822,6 Lux, temperature lingkungan rata-rata 28,4 °C, kelembapan rata-rata 99,4 %, temperature ruangan rata-rata 30 °C, temperatur air rata-rata 28 °C dan temperatur tanah rata-rata 28,1 °C dan nilai efektivitas maksimum sebesar 2,46 dan nilai minimum udara sebesar 0,9. Hasil penelitian menunjukkan bahwa perpindahan kalor air tanah-udara yang digunakan sebagai pendingin pasif pada sistem ventilasi bangunan memiliki kinerja yang cukup baik dengan pengurangan temperatur mencapai 2-10°C.

Parametric Study of the Ground-Air Heat Exchanger (GAHE): Effect of Burial Depth and Insulation Length

A parametric study of a ground-to-air heat exchanger (GAHE) using numerical models based on computational fluid dynamics with the finite volume method to evaluate the thermal potential of GAHE is presented. After the validation of the numerical code developed with published experimental data, it is proceeded to the study of the geometric parameters to define those that have the greatest impact on the application potential of GAHE. Climatological variables such as relative humidity, air flow velocity, and inlet air temperature are analyzed, as well as the increase in the thermal conductivity of the soil due to its humidity content. In addition, a study of the optimal installation depth as well as the length of the thermal insulation in the outlet pipe of the GAHE is presented. The results reveal that there is a higher heat exchange potential in the GAHE for an optimal burial depth of 4 m and a length of pipe of 15 m, 30% soil moisture content for heating and 32% for cooling, and a pipe diameter of 0.15 m. The use of thermal insulating is recommended only for the last 2 m of length in the outlet pipe of the GAHE.

Numerical investigation of using earth-air heat exchangers to reduce building cooling and heating loads in southern Iraq

The Ground Air Heat Exchanger (EAHE) is an integrated passive heating and cooling system for many greenhouses, industrial and residential building applications. Reduces heating and cooling loads by transferring heat between ventilated air and the ground. In this paper, the performance of EAHE was studied numerically using the ESP-r program under weather and soil conditions in Al-Diwaniyah city (southern Iraq), connecting the pipe to a building model and testing it during the summer and winter months. In the case of summer, the results indicated a decrease in the temperature of the air supplied to the air-conditioned space, where the temperature reached 37.42 ° C, 40.09 ° C and 39.93 ° C, while the temperature of the outside air was 47.09 ° C, 47.45 ° C and 48.48 ° C during the summer. July, August and September respectively. In the case of the winter months, the air leaving the tube is heated to a temperature of 16.38°C, 13.99°C, 16.42°C, and 16.42°C, when the outside air temperature is 3.33°C, -0.01°C and 5.37° percentage during December, January and February, respectively. The use of this type of heat exchanger achieved a reduction of more than 10 ° C during the summer and an increase of more than 13 ° C in the winter.

Renewable energy sources and other energy technologies as a measure for mitigating the impact of urban heat islands

Urban heat islands (UHI) represent increase of temperature inside of cities in regard to their rural environment. Causes of their formation are diverse and multiple: reduced amount of vegetation and waterproofing of surfaces, changed radiation and thermal characteristics of materials, urban geometry. Consequences are also diverse and significant: increasing consumption of energy for cooling, negative impact on human health, deterioration of air quality. Conventional methods for mitigating their impact include reflective and green roofs and introduction of green and water surfaces in cities. Different renewable energy technologies can also have positive impact on urban heat islands, but at the same time they contribute to greater energy independency of cities what is goal of future urban development. Proposed and described technologies are solar cooling, groundair heat exchanger, passive cooling, solar pond.

On the thermal performance enhancement of spiral-coil energy piles with a thermal recovery system

The pile foundation designed to ensure building stability when equipped with heat exchanger pipes to harvest geothermal energy is called an energy pile. Ground Source Heat Pump (GSHP) systems combined with energy piles have been used and developed as sustainable and efficient HVAC systems. Energy piles suffer from cold or heat accumulation in and around the pile, degrading their long-term performance. The current study seeks to alleviate this problem by proposing a thermal recovery system. The proposed system circulates ambient air in the pile foundation to extract the accumulated heat. A three-dimensional transient computational fluid dynamics model of the GSHP system coupled with the energy pile and the suggested recovery system is developed. This model is then used to investigate the effect of the various design parameters on the performance of the system and to determine the optimum system parameters. It is demonstrated that the maximum provided cooling load of the system with the proposed recovery system is 50% higher than the conventional system without the recovery. Moreover, the fifteen-year average COPannual of the system with double Ground-Air Heat Exchanger (GAHE) pipes and the system with one GAHE pipe is 30% and 16% more than the system without recovery.

Thermal performance analysis of helical ground-air heat exchanger under hot climate: In situ measurement and numerical simulation

The ground air heat exchanger (GAHE) is a promising passive approach for cooling and heating buildings. In this study, thermal performance of helical ground air heat exchanger (HGAHE) has been experimentally assessed in summer season for arid climate. The findings revealed that, the outlet air temperature of the HGAHE is strongly dependent on the inlet air temperature. Furthermore, the air temperature drop and the heat exchange rate as high as 13.3 °C and 463.4 W respectively, are attained at the highest inlet temperature of 41.0 °C. Besides, a transient numerical model was established and validated through the experimental data to investigate the heat penetration into the borehole. The results acknowledge that, the borehole temperature distribution in axial direction is higher at the upper surface and then decreases with the HGAHE length. In the other hand, the borehole temperature distribution in the radial direction reduces rapidly with the distance away from the pipe surface. Moreover, when the ambient air temperature during the night shift is lower than the borehole temperature, the forced convection which helps to take the heat away via purge air circulating into the HGAHE pipe allowed the borehole to restore its cooling ability.

Multi-Objective Optimization of Hybrid Renewable Tri-Generation System Performance for Buildings

Hybrid renewable energy systems are subject to extensive research around the world and different designs have found their way to the market and have been commercialized. These systems usually employ multiple components, both renewable and conventional, combined in a way to increase the system’s overall efficiency and resilience and to lower GHG emissions. In this paper, a hybrid renewable energy system was designed for residential use and its annual energy performance was investigated and optimized. The multi-module hybrid system consists of a Ground-Air Heat Exchanger (GAHX), Photovoltaic Thermal (PVT) panels and Air to Water Heat Pump (AWHP). The developed system’s annual performance was simulated in the TRaNsient SYStem (TRNSYS) environment and optimized using the General Algebraic Modelling System (GAMS) platform. Multi-objective non-linear optimization algorithms were developed and applied to define optimal system design and performance parameters while reducing cost and GHG emissions. The results revealed that the designed system was able to satisfy building thermal heating/cooling loads throughout the year. The ground source heat exchanger contributed 21.3% and 26.3% of the energy during heating and cooling seasons, respectively. The initial design was optimized in terms of key performance parameters and module sizes. The annual simulation analysis showed that the system was able to self-generate and meet nearly 29.4% of the total HVAC electricity needs, with the rest being supplied by the grid. The annual system module performance efficiencies were 13.4% for the PVT electric and 5.5% for the PVT thermal, with an AWHP COP of 4.0.

THERMAL STATE OF THE AIR - GROUND HEAT EXCHANGER PREDICTION BASED ON ARTIFICIAL NEURAL NETWORK

The work aim is to predict the thermal state of the air-ground heat exchanger based on an artificial neural network. Training, testing and validation of the proposed model were made on experimental data obtained in the thermophysical laboratory of the Institute of Engineering Thermophysics of the National Academy of Sciences of Ukraine. A simple neural network is used in this work. The air temperature at the inlet to the heat exchanger, and its relative humidity are selected as input parameters for the neural network. The MATLAB (R2016a) and Levenberg-Markwatt model were used in this article's calculations. One hidden layer and 10 neurons were presented in the model. The array of analysed data was divided into ratios of 70%, 15%, 15% for neural network training, validation and testing, respectively. As a result, it is obtained that the forecasting takes place with acceptable accuracy in all models. The root mean square error for the whole data set for different models varies from 0.105 to 2.323°С. The maximum mean absolute percentage error was the largest for CFD model and was 11.2%. The minimum mean bias error of the predicted data from the experimentally measured ones was found in the model using temperature, humidity, and air temperature at the outlet of the air-ground heat exchanger for the previous hour and was 0.02%. The training and testing of the proposed models based on an artificial neural network are satisfactory enough to predict the temperature taking into account the influence of weather conditions. Artificial neural networks can be used to predict the thermal state of the air-ground heat exchanger. Data representing the description of a real system are required for forecasting the parameters based on the ANN.

Experimental house EB2020 – Research and experimental measurements of an energy roof

The paper focuses on the experience with research of combined building-energy systems such as energy roofs, ground heat storage and active thermal protection. The EB2020 experimental house was largely designed in accordance with the patented ISOMAX system (SK 284 751, author: Dipl.-Ing., Phys. Edmond D. KRECKÉ), which represents a high potential for the use of renewable energy sources. When designing the experimental house EB2020, we eliminated the problem of this system by designing top heat sources, whose heat sources are exclusively solar and geothermic energy. The investigated building is designed with the following heat sources and energy systems: energy roof, low-temperature gas boiler, fireplace with hot water exchanger, ground heat storage, storage water heat storage with electric reheating, active thermal protection, large-area radiant low-temperature heating / high-temperature cooling, liquid cooling circuit in the ground outside the building, recuperative air handling unit and air ground heat exchanger. In this paper we describe the theoretical procedure for calculating the efficiency of the energy roof, comparison with a classic solar collector, experimental measurements of the energy roof during one season and evaluation of the measured data. The application of an energy roof requires lower investment costs than conventional solar collectors, but experimental measurements have shown that the energy gain and the achieved temperatures of the working substance at the outlet are significantly lower. For higher efficiency energy roof, it is worth considering installing a dark roofing and installing more circuits with a suitable orientation according to cardinal directions. This may be the subject of further research.

Energy potential of a ground-air heat exchanger – measurements and computational models

The construction industry is responsible for 36% of the European Union's greenhouse gas emissions. The main goal of the EU in the last decade has been to boost energy efficiency and expand the use of renewable energy in buildings. Ground-to-air heat exchangers may be a solution to reduce the use of primary energy in buildings from nonrenewable sources. The study in this paper deals with an evaluation of the heating and cooling energy potential of a ground-to-air heat exchanger (GAHE), serving a pilot energy-efficient building in Romania. The research follows a comparison between the experimental behavior of GAHE, evaluated based on measured outlet temperatures for two full years, and theoretical behavior, assessed using two computational models following EN 15241:2007 (Model 1) and EN 16798-5-1:2016 (Model 2). A comparison is made between the energy potential calculated using temperature measurements and the energy potential determined based on calculated outlet temperatures using the two models. The assessment is performed as well for conventional climate corresponding to the building location.

Thermal Performance of Helical Ground Air Heat Exchanger Under Saharan Climate Conditions: An Experimental Study

Thermal Performance of Helical Ground Air Heat Exchanger Under Saharan Climate Conditions: An Experimental Study

CFD simulation study to evaluate the economic feasibility of backfilling materials for ground-air heat exchanger system

The ground-air heat exchanger (GAHE) system is an energy-efficient system for building cooling/ heating, but its initial cost is higher due to the requirement of longer pipes to achieve the desired heat exchange between air and soil. To reduce required pipe length, secondary soil (i.e. soil of high thermal conductivity) can be used as backfilling material in the vicinity of GAHE pipe. However, due to the use of secondary soil, the installation cost of the GAHE system may increase. Therefore, the key objective of this study is to explore the economic feasibility of using backfilling materials in the vicinity of GAHE pipe. For this, a three-dimensional CFD simulation model of GAHE system with backfilling material has been developed using FLUENT 15.0, and the required length of GAHE pipe to achieve a specified drop in air temperature with backfilling materials has been determined. Based on this, the initial installation cost of the GAHE system (i.e. pipe cost, trench excavation cost, backfilling material cost) has been estimated. The study revealed that the initial installation cost of a GAHE system could be reduced by 25.7%, 32%, and 38% using dry sand-bentonite, wet sand-bentonite, and wet native soil respectively, as backfilling materials in place of dry native soil.

A novel ground thermal recovery system for horizontal ground heat exchangers in a hot climate

Ground Source Heat Pumps (GSHP), as a renewable source heating, ventilating, and air conditioning (HVAC) technology, has the highest energy efficiency among different heat pump types. One of the major drawbacks of GSHPs is the long-term ground temperature variations as a result of heat accumulation or depletion in the ground. This contribution puts forward a novel ground thermal recovery system for horizontal ground heat exchangers in a hot climate. The proposed recovery system consists of open-loop horizontal Ground-Air Heat Exchangers (GAHEs) that are buried between the horizontal Ground-Water Heat Exchangers (GWHEs). A fan supplies ambient air to the GAHE pipes when the soil around GWHE pipes is warmer than the ambient air, to remove the accumulatedheatfrom the ground. A three-dimensional quasi-steady Computational Fluid Dynamics (CFD) model of the system is developed. This model is then used to evaluate the five years performance of the GSHP system with the proposed thermal recovery system for space cooling of a residential house in a hot desert climate. Moreover, the effect of various geometrical parameters, i.e., GWHE length, GWHE pipe spacing, GWHE buried depth, and GAHE pipe diameter on the system performance, are investigated. It is demonstrated that, by deploying the proposed recovery system, the five-year average annual Coefficient of Performance (COP) of the system can be increased by up to 20.2%. Moreover, applying the proposed recovery system not only enhances the ground thermal recovery but also improves the maximum cooling load that can be supplied to the building per unit land area. Furthermore, it is demonstrated that by applying the proposed recovery system, the average annual COP of the system gradually increases; however, it decreases for the GSHP without a recovery system.

Reducing energy consumption for electrical gas preheating processes

Gas preheating is a key process to prevent the formation of hydrates in gas stations. In order to ensure safe operation of nearly 900 pressure reduction stations in Poland, it is necessary to provide a significant amount of energy, estimated up to 700 TJ per year. Gas can be preheated by means of gas and electrical technologies, both of which have their advantages and disadvantages. This paper focuses on electrical technologies based on renewable energy sources (RES): air source electric heat pump and ground source heat pump, ground air heat exchangers, horizontal and vertical heat exchangers. The calculation methodology of natural gas parameters for the proposed preheating systems, based on the composition and basic properties of chemical compounds of the fuel, was presented. The analyses were performed for three climate types (cold, average and warm) and for the two assumed modes of operation of pressure reduction station (PRS). The results showed that the electrical preheating solution by RES can save more than half of the primary energy in an environmentally friendly way and reduce the harmful gas emissions. The high coefficient of performance (COP) from 2.1 to 5.7 enables the energy demand to be reduced from 45 kW to 8.1–14.9 kW.