A short recent review on geothermal energy piles

This manuscripts presents a short recent review of geothermal energy piles, emphasizing their problems, design elements, heat transfer fluids, and classification. Phase change materials (PCMs) are used as heat transfer fluids, and their beneficial effects on energy pile performance are highlighted. Design factors for the best energy pile performance are examined, including the usage of nanofluids and geometrical optimization. The analysis presented provides brief insightful information about the state of geothermal energy piles heaps now, laying the groundwork for future studies and advancements in this area.


Introduction
Numerous applications depend on energy to function on a daily basis, and it is currently the focus of millions of researchers [1]- [9] Energy consumption and carbon dioxide emission reduction are the realworld trends in energy research [10]- [12].The greatest portion of energy demand is due to HVAC systems.Renewable energy sources RESs and effective energy management can help with this [13], [14].This is why all RES have been the center of attention in the whole world such as wind, solar, marine, hydropower, and geothermal [15].Geothermal energy is the energy stored below the earth's crust that can be used for several purposes such as heating and cooling and power generation.[16], [17].The use of geothermal energy presents a solution to the environmental issues and provides the needed cooling and heating requirements [18], [19].Around 82 countries in the world use geothermal energy directly, whereas 26 countries use it for power generation [20].By the year 2050, it is estimated that geothermal heat supply will reach 611 TWh, and electricity generated by geothermal energy will reach 1400 TWh/year, which accounts for 3.5% of global electricity production [21].Geothermal energy has some advantages over other RESs as it is determinedly available despite all the conditions, and has a low operating cost compared to other systems [15].Several studies have been done in the geothermal energy field.which is affected by energy extraction and injection rates.Additionally, the effect of several factors on the performance of the system was studied such as thermal diffusivity, specific heat capacity, thermal conductivity, pile length, pile diameter, and many more.The author proves that energy piles make a great alternative as an energy source.Sadeghi et al. [22] also presented a paper that evaluates the different design and construction method of precast and driven energy piles.Concrete driven-energy pile is considered a better alternative than cast-in-place energy piles.They have higher quality assurance and quality control and many advantages over cast in place piles in the environmental technical, and economical aspect.Another paper by Faizal et al. [23] aims to find methods to improve thermal properties of geothermal piles.The total pile thermal resistance can be reduced by the number of pipes and their arrangement, the conductive and convective properties of the heat carrier can be enhanced using nanofluids.Highly thermal conductive fillers can be used to reduce total thermal resistance by increasing heat transfer between primary circuit fluid and the ground, and concrete thermal conductivity can be enhanced by adding thermally conductive fillers.Cunha et al. [24] also presented a review that targeted the use of energy piles for enhancement of buildings energy.This paper presents a short recent review of geothermal energy piles.In the first instance, the classification of geothermal energy piles according to many variables is shown such as the construction method, climate conditions, soil type, load conditions, pile spacing, and pile material.After that, different energy piles heat transfer fluids are discussed.Then different design aspects are discussed that effect the energy pile performance.Finally, several problems that arise in the process of heat extraction using energy piles are mentioned.

Geothermal energy piles classification
According to Sadeghi et al. [22] there is a significant increase in the use of energy piles all around the world.Energy piles are reinforced concrete deep foundations that has heat transfer pipes embedded in them; mainly made of high-density polyethylene, were the heat transfer fluid circulates.Heat transfer in energy piles takes place between liquid and pipes, then pipes and concrete, then concrete and soil.The energy is used for heating and cooling after it is transferred to ground surface using a heat pump.Energy pile performance is affected by many variables.Sadeghi et al. [22] compared between piles according to the construction method; cast in place concrete or precast concrete driven.According to the results, precast energy piles are more advantageous than cast in place piles.They have higher quality control, and are easier and faster installed.The author also mentioned the limitations of using precast piles including transportation, bedrock depth, joints between segments, temperature effect on shear strength of soil-pile interface, and lack of design guidelines.On the other hand, Han et al. [25] compared the pile performance according to the climate conditions.Han  System coefficient of performance cost saving, and CO2 emission reduction between different climates.Energy saving increases as the heating to cooling ratio of the city increases.Severe cold zones mostly benefit from energy pile systems, but suffer from some long-term performance degradation due to ground temperature unbalance caused by unbalances heat extraction and injection.The zones that are most favourable to use pile systems in are cold zones and hot-summer/cold-winter zones since performance doesn't show any degradation with time.On the other hand, hot-summer/warm-winter zones are suitable for energy piles but have a longer payback period.The performance is mainly impacted by the climate zone and not the location of coastal or inland area.Finally, mild zones have the least benefits of energy, environmental, and economic, but has a utility cost saving of 21%, energy saving of 19%, and 26% reduction in CO2 emission.The soil type and loads conditions also affect the energy pile performance.Jelusic et al. [26] constructed a model using more than one type of material for the piles and subjected it to geotechnical designs and structural resistance, and settlement constraints based on the Eurocode and requirements of design of geothermal energy piles.Then the author used the optimal design of conventional and geothermal energy piles for investigation under various soil and load conditions.The results showed that the design optimal dimensions vary based on vertical permanent load, young's modulus, and shear strength of a cohesive soil profile.Geothermal energy piles are most economic for small vertical loads on soil with high young's modulus.Finally, Yang et al. [27] investigated the effect of pile spacing, pile material, soil type, and pile configuration on the thermo-mechanical behaviour of the pile group.For the investigation, a 3D model was built with a layout of 3 * 3, shown in fig 1.The results show that the increase in pile spacing enhances the thermal performance of piles and reduces the pile displacement, however, it weakens the pile group effect leading to an increase in axial force of the pile.In addition, the heat exchange rate per unit pile depth is the highest in rock-soil; 66 W/m for spacing 3.5D.As for pile displacement, it is highest in sand; 1.83 mm for 2.5D, then in clay, and lastly in rock-soil.The axial force of pile is largest in rock-soil then in sand and finally in clay is the smallest.Concerning the pile material effect, the increase of pile thermal conductivity from 1.27 to 2.7 W/m/K lead to an increase in the thermal exchange of energy piles by 25%, which also lead to aggravation of heat accumulation and an increase in the rise degree of pile body temperature leading to larger axial force and top displacement of the pile.On the other hand, the increase in concrete compression modulus increases the pile axial force but doesn't affect the pile displacement.The increase in concrete compression modulus is also related to the position of pile in the configuration.The pile arrangement also has an effect on the pile behaviour as the cross arrangement improves the heat exchange rate per unit soil volume.The heat exchange rate of pile group in cross arrangement is 13% more than that of pile group in sequential arrangement.However, the soil temperature variation is larger in cross arrangement than sequential, which leads to a decrease in pile axial force and an increase in pile displacement.This means that for improvement of pile group performance, small pile spacing and cross arrangement can be adopted when soil specific heat capacity is large, and the pile thermal conductivity should match the soil thermal conductivity to assure an efficient heat transfer, and finally the compression modulus of the pile material should be determined according to the thermo-mechanical performance of the pile group.

Geothermal energy piles transfer fluid
As for the heat transfer fluid, PCM is significantly being used and studied in energy piles.Hajaj et al. [28] performed a numerical analysis on geothermal energy piles with and without PCM embedded and compared the performance of both.They also studied the influence of paraffin wax PCM in the pile at different flow rates.The results showed an improvement in the pile performance with PCM compared to pile with no PCM.The energy stored increased by 42.21% for 735 mL/min flowrate, 44.71% for 1470 mL/min, and 46.23% for 2100 mL/min compared to piles with no PCM incorporated.In addition, the extracted energy increased by 45.87% for 735 mL/min, 53.09% for 1470 mL/min, and 59.55% for 2100 mL/min.These results show that the incorporation of PCM in geothermal energy piles has a great impact on pile performance.Fei et al. [29] also studied an energy screw pile that is filled with several PCM materials such as solid grouting and paraffin.To study the performance of piles with PCM material, simulation is done considering several parameters such PCM solid mixtures, phase change temperature, moisture conditions, and operation schemes.The results show that higher effective thermal conductivity of mixture slows the PCM phase transition and leads to lower temperature of the fluid feeding the GSHP leading to a higher COP of building cooling mode.On the other hand, dry mixtures with more PCM content enhances the fluid temperature reduction, and in wet mixtures low PCM content is considered good.Moreover, higher specific heat capacity enhances the absorption of heat and reduces the thermal radius of influence, however over-enhancing the effective thermal conductivity affects the thermal interference negatively.Lastly, Mousa et al. [30] studied the effect of PCM on the storage capacity and thermal radius of energy piles.The effect of melting range and PCM location was also investigated.The results showed that PCM addition increase the COP of heat pump by 5.28%, however the pile performance was negatively affected by PCM after it reached a complete state decreasing the COP by 1.8% compared to conventional pile due to low thermal conductivity.In addition, location of PCM has a great impact on pile performance; if the PCM is located inside the concrete shell, then the performance is higher than if it was located outside the concrete shell, and the performance decreases as the distance between PCM and U-loop increases.As for PCM located in the concrete shell, the optimal location was found to be 40 cm from the center of energy pile.The PCM melting range also has an effect on performance; changing the melting range from 4-6 to 1-2 decreased the COP from 5.28 to 4% during solidification, and increased the negative effect from 1.8 to 2.2% during complete solid-state.Finally, the average daily COP increased by 26% with average enhancement of 9% when PCM was divided into small containers.

Geothermal energy piles design
Energy pile design is also important for the optimal performance.Faizal et al. [23] presented a review of different methods to enhance heat transfer in geothermal energy piles.First, they discussed enhancement by geometrical optimization, and looked at the pile diameter, concrete cover, number of pipes, and pipe configuration as single U-tube, double U-tube, triple U-tube, multi-tubes, W-shaped, helical shapes, and coaxial types as shown in Fig 2. Larger pipe diameters were found to give better heat transfer.Double U-shaped pipes gave better results compared to single U-shaped pipes and multi-tube shapes.However, coaxial heat exchanger pipes gave better results compared to double U-shaped pipes, and helical shaped pipes gave better results than U-shaped pipes.The authors also discussed the usage of nanofluids.It was found that using nanofluids increases heat transfer, but further research is required in this field.The heat transfer between fluid, pipe, pile, and surrounding is shown in Fig 3. Besides, highly thermal conductive fillers can be used to reduce total thermal resistance by increasing heat transfer between primary circuit fluid and the ground.Finally, concrete thermal conductivity can be enhanced by adding thermally conductive fillers such as silane, silica fume, graphite, and many others.Zayed et al. [31] also presented a review on numerical modelling for sizing and design of geothermal systems and for heat extraction purposes.Several methods for heat transfer extraction from geothermal wells, geothermal reservoirs, and geothermal energy piles are discussed such as finite element method, finite difference method, and finite volume method.The review also represents several parameters that affect heat extraction including pipe arrangement, pipe pitch, pipe diameter, pipe length, pipe material, center-to-center distance, and borehole depth.The numerical modeling is crucial since the thermal interaction between boreholes affects the performance of system over long periods of time.In addition, numerical methods are more accurate than analytical methods, and the most used numerical method is the two-region approach which analyzes the energy transfer in the pipe and the ground individually.Changing borehole configuration such as pipe arrangement and center-to-center distance has a great impact on thermal performance, but might be costly.On the other hand, having a borehole pipe diameter that is small compared to the length has a great impact on heat transfer from fluid to soil.Finally, the best pipe material was found to be high-density ethylene pipe.

Geothermal energy piles problems
Energy piles face several problems in the process of energy extraction such as underground thermal imbalance and thermal performance decay.Saaly et al. [32] studied the energy pile performance in cold areas where underground thermal imbalance occurs when using geothermal energy for heating purposes alone in cold regions, and proposed solutions for the problem.Thermal balance of the ground can't be met of the geothermal piles support the total energy demand of the building, and leads to freezing of the pile-soil interface deep in the soil.This doesn't occur at shallow depths since the soil temperature increases due to building heat loss.Thermal balance of soil can be achieved by the reinjection of the heat lost to the ground by changing the operation strategy of the geothermal piles.The operation strategy suggested in this paper is based on simultaneous extraction and injection of the heat from/to the ground.For instance, 50 geothermal piles reject heat from the ground, and 69 piles extract heat from the ground.On the other hand, in summer the greater number of piles is used for heat rejection into the ground.This reduced the soil thermal imbalance and yielded steady temperature at higher depths.Another solution proposed is solar water heater thermosyphone where the heated water is rejected into the soil and increases the underground system.The results show that the use of a 5 m geothermal pile supplies 4-15% and 7-41% of energy demand during winter and summer respectively while maintaining soil thermal balance and preventing pile-soil interface freezing.Furthermore, it was shown that increasing the pile length increases the energy supply.The energy supply during winter increases to 20-41% of the heat demand if the pile length is increased to 12 m.On the other hand, Tiwari et al. [33] studied the pilesoil heat exchange and the pile thermal interaction to see its effect on power output expected from a pile group.Finite element analysis shows the effect of diameter, orientation of fluid circulation, spacing, and thermal operation time of piles on thermal interaction between the group of piles.The author proposed an expression to calculate a power reduction factor for thermal interaction between two active piles.This factor is then used to estimate the power output from a group of piles.The results show that the increase in duration of pile thermal operation leads to a decay in the thermal performance of acting geothermal piles.In addition, thermal interaction between acting piles decreases with the increase of spacing between the piles.The power output from piles at a spacing to diameter ratio less than or equal to 15 is less than that of same number of piles but isolated.Thermal interaction is not affected by increase of pile diameter for a specific spacing.Finally, circulation tube arrangement does not have any effect on the thermal interaction.

Discussion
The use of geothermal energy piles has shown a great significance in the geothermal energy world.First of all, precast energy piles were found to be more advantageous than cast in place piles since they have higher quality control, and are easier and faster installed [22] In addition, the zones that are most favourable to use pile systems in are cold zones and hot-summer/cold-winter zones since performance doesn't show any degradation with time [25] Geothermal energy piles are also most economic for small vertical loads on soil with high young's modulus [26].As for enhancement of pile group performance, pile spacing and cross arrangement can be adopted when soil specific heat capacity is large, and the pile thermal conductivity should match the soil thermal conductivity to assure an efficient heat transfer [27] On the other hand, the incorporation of PCM in geothermal energy piles has a great impact on pile performance [28].Finally, PCM addition increase the COP of heat pump by 5.28%, however the pile performance was negatively affected by PCM after it reached a complete state decreasing the COP by 1.8% compared to conventional pile due to low thermal conductivity [30].

Conclusions and recommendations
To sum up, geothermal energy stacks show promise as a sustainable energy alternative, especially for HVAC systems.Precast energy piles appear to have benefits over other building methods when it comes to overall performance, convenience of installation, and quality control.The performance of energy piles is highly influenced by climate, with cold zones and hot-summer/cold-winter zones showing the best long-term results.If the specific heat capacity of the soil is taken into account, design factors like pile spacing and cross arrangement can significantly improve the performance of pile groups.Phase change materials (PCMs) can also be incorporated to increase energy pile efficiency, although great consideration must be given to the PCM's placement and melting range.Notwithstanding these developments, problems like thermal imbalances beneath the surface and deteriorating thermal performance still exist.Innovative approaches to preserving soil thermal equilibrium show promise, such as simultaneous heat injection and extraction.This analysis highlights the importance of geothermal energy stacks in the context of renewable energy and lays the groundwork for future studies targeted at resolving issues and enhancing performance.

Figure 1
Figure 1 Pile group and single pile top views[27].

Figure 2
Figure 2 Different configurations of energy loops within a GEP[23].
et al. compared the performance in seven different climate zones in China in a high-rise building.Results showed a huge difference in energy saving, utility