Long-term Thermal Performance Research Articles

Heat exchange performance of an energy-pile group in a hybrid GSHP system and effects of pile spacing

In long-term operations, seasonal imbalance in the thermal load may adversely affect the heat transfer performance of the energy piles, potentially resulting in thermal accumulation within the ground and eventual system failure. The heat transfer performance of energy pile systems during long-term operation under unbalanced thermal loads must be investigated. Moreover, the design parameters of energy piles are usually constrained by the requirements of foundation structural design, resulting in energy piles being densely arranged. Hence, the influence of pile spacing on the heat exchange performence of energy piles must be comprehensively understood. In this study, two- and three-dimensional energy-pile heat transfer models were established and innovatively coupled based on an engineering project currently under design. Numerical simulations were performed to investigate the heat transfer behavior of energy-pile groups subjected to unbalanced thermal loads and the effect of pile spacing on their heat exchange performance. Furthermore, design recommendations regarding the determination of the proportion of thermal loads to be borne by the energy piles in a hybrid GSHP system were provided. The results indicate that the proposed 2D-3D coupled modeling approach is able to simulate the heat exchange performance of large-scale energy pile groups. Pile spacing considerably affects the long-term thermal performance of energy-pile groups, especially in cases with small pile spacings. The influence of pile spacing on the heat exchange capacity of energy piles can be considered in the design phase by incorporating a group effect coefficient η, which are calculated to be 0.165, 0.470, 0.732, and 1 for pile spacings of 2 m, 4 m, 6 m, and 10 m, respectively.

Deep learning-based inversion framework by assimilating hydrogeological and geophysical data for an enhanced geothermal system characterization and thermal performance prediction

Enhanced geothermal systems (EGS), which are developed by creating artificial fractures to enhance the permeability of deep reservoir, show their its advantage in power generation. However, due to the limited wells in deep depth, characterizing fractured geothermal reservoirs with sparse data is difficult and poses challenges for long-term thermal performance prediction. Interpreting observation data through inversion to characterize the fracture aperture and predict EGS long-term thermal performance is a hopeful scheme. Thus, a joint inversion framework, convolutional variational autoencoder-ensemble smoother with multiple data assimilation (CVAE-ESMDA), is proposed with the following aspects: (i) CVAE is trained to parameterize the high-dimensional Non-Gaussian aperture field, (ii) ESMDA is applied to integrate hydrogeological and geophysical data for aperture characterization. Based on the inversion results, the long-term performance is predicted. The normalized root mean square errors (NRMSEs) of inversion results decrease from 27.66 % to 24.52 % after multi-type data are integrated for CVAE-ESMDA. Furthermore, the NRMSEs of long-term thermal prediction of two production wells also decrease to only 2.3 % and 8.2 %. To further exhibit the advantages of CVAE-ESMDA, another joint inversion framework, principal component analysis (PCA)-ESMDA, is also compared. However, PCA is limited in addressing Non-Gaussian aperture field and its long-term thermal prediction performance is unsatisfactory.

Enhancing Thermal Insulation of Poly(β-Hydroxybutyrate) Composites with Charring-Foaming Agent-Coated Date Palm Wood

Date palm fiber (DPF) holds great potential for composite materials, but its flammability limits its practical applications. In this study, DPF was modified using a pad-drying method to impregnate it with a 5 wt.% solution of ammonium dihydrogen phosphate (ADP). These treated fibers were then utilized to fabricate poly(β-hydroxybutyrate)- (PHB-) based composites. The resulting thermal insulators were comprehensively evaluated for their flammability, physical, mechanical, and thermophysical properties, as well as morphological and thermal stability characteristics. The findings revealed a significant reduction in flame spread and smoke suppression; however, the concentration used is not sufficient to achieve the desired rating grades. The thermal insulation capacity of the modified fiber composites was substantially enhanced, particularly with the 40% PHB/DPF-ADP composite displaying the lowest thermal conductivity at 0.0564 W/m.K. Moreover, the presence of gaps and voids at the interface led to a reduction in tensile strength to 4-7 MPa. Additionally, the modified fiber composites exhibited significantly reduced water absorption (~0.76%), attributed to the formation of a highly water-resistant substance containing a furan compound. This work provides a simple and effective approach for achieving durable flame retardancy and long-term thermal insulation performance, offering promising opportunities for the practical application of biobased PHB composites.

In-situ synthesis of ferrocene modified formaldehyde-free phenolic resin and its temperature-resistant microwave absorbing coating

The requirements for long-term stability and thermal performance of resin-based coating materials are gradually increasing with the development of aerospace industry. As a low-toxicity aldehyde, terephthalaldehyde (TPA) with aromatic ring structure and high reactivity is an ideal formaldehyde substitute for synthesizing high thermal stability phenolic resins and coating candidate. In this work we provided a strategy for the synthesis of ferrocenecarboxaldehyde (Fc) in-situ modified TPA-phenol resin (PTPA-Fc), obtaining resin products with high glass transition temperatures at 302 °C. The non-isothermal curing process and activation energy changes of resins with different TPA-phenol ratios were further investigated. It was found that the highest cross-linking density of resin was achieved when TPA/phenol was 0.5, with the initial decomposition temperature (T5%) reached 378 °C. Afterwards, carbonyl iron powder (CIP) filled PTPA-Fc microwave absorbing coating was prepared. Due to the improved oxidation resistance, composite based on PTPA-Fc had an effective bandwidth of 3.1 GHz (14.5–17.6 GHz, thickness 3 mm) after 100 h heat treatment at 250 °C and could still cover the 9.5–18 GHz absorption band (with RL ≤ -5 dB) with 5 mm matching thickness, reflecting a good long-term thermal stability.

RTV silicone rubbers cured by polyhedral oligomeric silsesquioxane and ladder-like polysilsesquioxane: Thermostability, long-term thermal properties, and pyrolysis mechanism

At present, silicone rubber has been widely used in the fields of aerospace and electronic materials, yet its long-term thermal performance still remains a challenge. In this investigation, polyhedral oligomeric silsesquioxane and ladder-like polysilsesquioxanes with multiple silicon-methoxy groups (MSPOSS, MSLPSQ) were synthesized successfully and used as cross-linkers to cure room temperature vulcanized silicone rubber (POSS-SR, LPSQ-SR). The results demonstrate that both MSPOSS and MSLPSQ can effectively improve thermostability, mechanical properties, and long-term thermal properties of RTV silicone rubber. The silicone rubber exhibits a remarkable preservation of strength and flexibility after long-time heat treatment at 400 °C. The silicone rubber demonstrates an impressive retention rate of 84.52% for tensile strength and 69.16% for elongation at break. An analysis of the pyrolysis process of the silicone rubber reveals that the 'back-biting' of polysiloxane chains were inhibited effectively by both MSPOSS and MSLPSQ. As a result of this inhibition, the silicone rubber's terminal chains are delayed from decomposing, resulting in retardation of initial thermal decomposition and significant improvement long-term thermal properties. This work may provide a new prospect for room temperature vulcanized silicone rubber for aerospace applications in extreme environments.

Long-term thermal performance investigation of phase change plates (PCPs) employing tunnel lining ground heat exchangers for cool storage

Combining tunnel lining ground heat exchangers (GHEs) and phase change plates (PCPs) to extract and store the geothermal energy is a novel and environment-friendly energy utilization method, and then PCPs can be used for space cooling. However, existing investigations have not focused on the long-term thermal performances of PCPs employing tunnel lining GHEs for cool storage. In the present work, a coupled finite element model of tunnel lining GHEs and PCPs was built to study long-term operation performances of PCPs during the cold energy charging operation period. Numerical modeling and programming (i.e., co-simulation) were employed to realize the continuous cold energy charging operation mode of PCPs employing tunnel lining GHEs for cool storage. The results indicate that the solidification efficiency (i.e., cold energy charging efficiency) of PCPs and phase interface evolution within the PCPs under the long-term cold energy charging mode are affected by the number of times for cool storage (N) and thermal conductivity of the surrounding rock (ksr) compared with the short-term mode. The drop rate of the liquid fraction and solidification efficiency of PCPs decrease with an increase in the number of times for cool storage. At N = 1, there is no obvious difference for the solidification efficiency of PCPs with increasing ksr. The solidification efficiency of PCPs under N = 10 improves by approximately 35.3% as ksr rises by 3 W/(m K). Additionally, the solidification time of PCPs increases linearly with N under ksr = 1.22 W/(m K). However, the solidification time is related parabolically to N under ksr = 2.22, 3.22, and 4.22 W/(m K). Finally, the long-term thermal response of the surrounding rock is analyzed, which also verifies the heat transfer characteristics of PCPs under the long-term cold energy charging mode.

Numerical investigation of the long-term thermal performance of a novel thermo-active foundation pile coupled with a ground source heat pump in a cold-climate

The potential to drastically reduce the high upfront cost and the additional land requirement of installing the ground heat exchanger of a ground source heat pump (GSHP) system has led to an increasing interest in thermo-active foundation piles. This study numerically investigates the long-term thermal performance of a novel offset pipe thermo-active foundation pile in a GSHP system for residential space heating and cooling. The pile is a fully enclosed helical steel pile that is 18.288 m deep and 0.139 m in diameter with eccentrically placed inlet and outlet pipes. A thoroughly validated numerical model coupled with realistic normalized building load profiles was employed for the analysis. Four installation configurations (pile beneath a basement, pile beneath a surface-level concrete slab, pile installed at the ground surface level, and pile buried underground) were considered. In addition, the potential of the pile to meet three realistic normalized building load profiles (maximum heating load: 0.3 tons, 0.4 tons, and 0.6 tons) was investigated. The long-term study results indicate that the ground thermal imbalance causes an average ground temperature drop of −0.18 K/year, −0.25 K/year, −0.41 K/year for the three load profiles. Moreover, the ground thermal imbalance causes the system heating COP to decrease by 0.006/year, 0.011/year, and 0.021/year for the 0.3-ton, 0.4-ton, and 0.6-ton maximum load profiles, respectively. Results further show that installing the pile under the basement gives the best overall performance regarding heating COPs and heat pump operability. When used as a retrofit solution for an existing building, burying the pile underground gives the best performance compared to installing it at the ground surface level.

Drying of an aerogel-based coating system in Swedish climates: Field tests and simulations

Aerogel-based coating mortars (ACM-systems) introduce new solutions for energy-retrofit of uninsulated building envelopes, preserving their characteristics while minimizing the material thickness. However, when introducing new solutions, the long-term durability needs to be investigated. The hygrothermal (heat and moisture) performance is one aspect that needs to be secured. The aim of this study is to investigate the hygrothermal performance, with specific focus on moisture drying, of an ACM-system for external applications in Swedish climates. A field test was conducted where an ACM-system with 40 mm of ACM was applied on the exterior of a brick masonry wall partition. The temperature and relative humidity in the wall were monitored for 15 months. Furthermore, numerical hygrothermal simulations were used to predict the early stage drying and long-term performance of the ACM-system in four Swedish climates. The field measurements showed that the built-in moisture dried out after approximately 6 months, after which the ACM-system followed the variations in the surrounding climate for the remaining period. The simulations predicted that the early stage drying time ranged from 134 to 336 days, depending on climate and time of application. Furthermore, the elevated relative humidity in the ACM due to rainwater absorption resulted in an average thermal conductivity of up to 9 % above the rated value. Consequently, mitigating water absorption at the exterior is crucial for enhancing the long-term thermal performance in high rain load scenarios.

Effect of groundwater flow on thermal performance of SBTES systems

In this study the influence of different groundwater flow conditions on the short- and long-term thermal performance of SBTES system is studied. Numerical analyses of a SBTES system are performed using COMSOL with four groundwater table depths and with four groundwater velocities. The study evaluates the thermal performance of SBTES in terms of the average temperature, heat stored, and system efficiency over a 10-year operational period. The results indicate that high groundwater velocity (1 × 10−5 m s−1) has a detrimental effect on the thermal performance of SBTES, causing significant reduction in the stored heat. Medium groundwater velocity (1 × 10−7 m s−1) with the groundwater table at half the depth of SBTES has an insignificant effect during the initial years of operation but causes significant reduction in heat stored over the long term. The study also highlights the potential of heat loss resulting from high velocity groundwater flow taking place below the SBTES. The findings suggest that groundwater flow plays a critical role in the thermal performance of SBTES, and long-term analysis is necessary to evaluate the impact.

Effects of the ground surface thermal boundary condition on the long-term thermal performance of energy diaphragm walls

Energy diaphragm walls have attracted increasing attention due to their high heat transfer performance. Thus, the factors that affect the thermal performance of energy walls have been investigated to provide insights for further practical applications. Numerical simulation is widely used in energy wall analysis due to the complexity of conducting field tests. Therefore, the choice of boundary conditions in the simulations is crucial; it refers to whether the model reflects reality and has a significant impact on the results. In most cases, the assumed ground surface thermal boundary is not the same (e.g., thermal insulation, constant temperature), and the ground temperature is also assumed to be constant with depth, which may result in a large loss of thermal energy or inaccurate assessment of thermal performance for the energy wall. In this respect, this paper investigates the long-term differences in thermal performance under these boundaries and considers the ground temperature that varies with depth and time, which has not been comprehensively considered before in the analysis of energy walls. The results show that accounting for air temperature or ground temperature variations with depth and time can result in a great increase in thermal performance, e.g., a 34% increase in the cooling thermal load compared with the thermal insulation boundary. In the performed parametric analysis, the results highlight the significant influence of the thermal load and wall height on the thermal differences between these boundaries. These findings provide a reference for choosing ground surface thermal boundaries and better thermal design of energy diaphragm walls.

Development of Accelerated Test Method to Evaluate the Long-Term Thermal Performance of Fumed-Silica Vacuum Insulation Panels Using Accelerated Conditions.

International standards for vacuum insulation panels (VIPs) include an accelerated test method and a minimum quality standard for evaluating their long-term thermal performance after 25 years. The accelerated test method consists of various tests according to the characteristics of the core material and requires six months (180 days) at minimum. Herein, we propose an accelerated method for determining the long-term thermal performance of fumed-silica VIPs by shortening the required time and simplifying the procedure. Highly accelerated conditions (80 °C and 70% Relative humidity (RH)) were set for the evaluation method, using the maximum temperature (80 °C) cited in international standards and compared with the accelerated test method under accelerated conditions (50 °C and 70% RH). The inner-pressure increase rate of the VIP samples after conditioning for approximately 70 days was similar to that after conditioning for 180 days under highly accelerated and accelerated conditions, respectively. In addition, the estimated long-term thermal conductivities of the fumed-silica VIP were derived as 0.0076 and 0.0054 W/m·K under highly accelerated and accelerated conditions, respectively. These accelerated methods can be used to produce fumed-silica VIPs with similar long-term thermal conductivities. Therefore, the accelerated test method for long-term thermal performance using the highly accelerated conditions can be evaluated using a test that involves conditioning the sample for approximately 70 days under 80 °C and 70% RH.

Influence of extreme fracture flow channels on the thermal performance of open-loop geothermal systems at commercial scale

Adequate stewardship of geothermal resources requires accurate forecasting of long-term thermal performance. In enhanced geothermal systems and other fracture-dominated reservoirs, predictive models commonly assume constant-aperture fractures, although spatial variations in aperture can greatly affect reservoir permeability, fluid flow distribution, and heat transport. Whereas previous authors have investigated the effects of theoretical random aperture distributions on thermal performance, here we further explore the influence of permeability heterogeneity considering field-constrained aperture distributions from a meso-scale field site in northern New York, USA. Using numerical models of coupled fluid flow and heat transport, we conduct thermal–hydraulic simulations for a hypothetical reservoir consisting of a relatively impervious porous matrix and a single, horizontal fracture. Our results indicate that in highly channelized fields, most well design configurations and operating conditions result in extreme rates of thermal drawdown (e.g., 50% drop in production well temperatures in under 2 years). However, some other scenarios that account for the risks of short-circuiting can potentially enhance heat extraction when mass flow rate is not excessively high, and the direction of geothermal extraction is not aligned with the most permeable features in the reservoir. Through a parametric approach, we illustrate that well separation distance and relative positioning play a major role in the long-term performance of highly channelized fields, and both can be used to help mitigate premature thermal breakthrough.

Effect of two organic phase change materials on the thermal performance of asphalt pavements

ABSTRACT Higher temperature is one of the key reasons for thermal distresses in asphalt pavement. It also adds to the Urban Heat Island (UHI) effect as it influences the near-surface air temperature. To design cooler pavements, phase change materials (PCMs) can be incorporated into asphalt pavements. However, considering the negative effects of PCM leakage on the physical and rheological properties of asphalt binder, a cent-percent effective core-shell encapsulated PCM suitable for asphalt pavement applications was developed under this study. With the incorporation of two organic mixture (OM) based PCMs, i.e., OM-35 and OM-42, a peak decrease in the pavement surface temperature of 3.05 °C and 4.36 °C, respectively were observed under the field condition. Furthermore, from the long-term thermal performance assessment, it was found that the magnitude of temperature reduction depends on the phase change temperature and latent heat of PCM. The season of occurrence of the peak temperature reduction depends on the phase change temperature of the PCM. Due to PCM solidification, the increase in night-time pavement surface temperature was observed to be about half of that during day time. Further, the statistical analysis reveals that the decrease in pavement surface temperature due to PCM incorporation is significant and consistent.

Durable flame-retardant, smoke-suppressant, and thermal-insulating biomass polyurethane foam enabled by a green bio-based system

Bio-based polyurethane foam has attracted increasing attentions due to eco-friendliness and fossil feedstock issues. However, the inherent flammability limits its application in different fields. Herein, we demonstrate a green bio-based flame-retardant system to fabricate polyurethane foam composite with durable flame retardancy, smoke suppression, and thermal insulation property. In this system, the green bio-based polyol (VED) with good reactivity and compatibility plays a role of flame retardant and EG acts as a synergistic filler. As a result, the LOI value of foam composite increased to 30.5 vol.% and it achieved a V-0 rating in the UL-94 vertical burning test. Additionally, the peak heat release rate (pHRR) and the total smoke production (TSP) decreased by 66.1% and 63.4%, respectively. Furthermore, the foam composite maintained durable flame retardancy after accelerated thermal aging test, whose thermal-insulating property was maintained even after being treated in high-humidity environment with 85% R.H. for a week. This work provides a facile strategy for durable flame retardancy and long-term thermal insulation performance, and creates opportunities for the practical applications of bio-based foam composites.

Thermal performance analysis of a large scale water pit heat storage

Solar thermal application is an effective approach to achieve carbon neutrality. The large scale water pit heat storage (PTES) is a key component to ensure the stability of large-scale solar thermal plants. Based on the model of type 1535-1301, this paper performs a 2D simulation investigation of the long-term thermal performance of PTES using Trnsys. The accuracy of this model was verified by comparison with measured data from the solar plant in Dronninglund, Denmark. The results show that the average error between the model and the actual measurements is only 3%. In addition, it can be found that the PTES average temperature decreases from 24°C to 12°C before the 80th day due to heat losses. During the charging period from the 80th to 250th days, the PTES average temperature increases to 68°C. In which the PTES formed a temperature stratification with 80°C at the top and 40°C at the bottom. The sloping soil temperature boundary expands and shows a non-uniform stratification of 70-10°C with 10m thickness. The average temperature of the PTES gradually decreases to 16°C during the discharge period after the 250th day. The actual charging and discharging energy are 11,868 MWh and 11,250 MWh per year, respectively, and the total heat loss is 1,157 MWh. The storage efficiency is as high as 90.1% because PTES is used for both long-term and short-term energy storage. This paper reveals the dynamic thermal process of PTES throughout the year to provide a reference for designers.

Long-Term Mechanical and Thermal Performance and Lifetime Estimation of a Carbon Fiber Reinforced Polycarbonate Laminate

Long-Term Mechanical and Thermal Performance and Lifetime Estimation of a Carbon Fiber Reinforced Polycarbonate Laminate

Influencing factors analysis for the long-term thermal performance of medium and deep U-type borehole heat exchanger system

The medium and deep U-type borehole heat exchanger (MDUBHE) system has recently received extensive attention for heating buildings. The MDUBHE system could operate with low energy efficiency or even shut down due to the improper design influencing factors. Therefore, studying the influencing factors is a great need to ensure the stable and efficient operation of MDUBHE system. In this paper, a transient heat transfer model for the MDUBHE system is proposed, and its accuracy is validated using experimental data. The impacts of influencing factors on the performance of MDUBHE system during long-term operations are quantified. The results show that the buried pipe depth and geothermal gradient significantly impact the MDUBHE system's performance, while the pipe specifications have less impact on it. Furthermore, after 15 years of operation, when the inlet water temperature increases from 4.5 °C to 24.5 °C, the heat extraction rate decreases by 43.4%, and the energy consumption decreases by 52.7%. The thermal conductivity of grout increases from 2.2 W/(m ∙ K) to 2.7 W/(m ∙ K), while the heat extraction rate increases by only 0.7%, which is not significant to improve the performance of MDUBHE. This study has significant guidance for the engineering practice of MDUBHE system.

Effects of microencapsulated phase change material on indoor thermal comfort and energy consumption

Phase change materials, PCMS can be also used in thermal energy systems with various building materials in order to increase thermal performance. In this study, artificial marble with microencapsulated PCM (MPCM) was fabricated and its indoor environmental control and energy saving effects were analyzed using field tests in a standard Korean apartment building. The MPCM capsules contained approximately 70% n-octadecane and were fabricated using the in situ polymerization chemical method. The thermal environment was analyzed using internal temperature and the predicted mean cote (PMV) measurements. The results of the wall board with MPCM installed room temperature fluctuation decreased by 1–2 °C that it was possible to maintain a stable indoor temperature due to the thermal buffering effect. The average change of PMV was −0.03 for 12 h that no significant effect on the thermal comfort. The results of convection heating system energy consumption was reduced by 27.7%. Furthermore, we studied the long-term real-scale thermal performance in winter seasons.

Determination of the Long-Term Thermal Performance of Foam Insulation Materials through Heat and Slicing Acceleration.

Foam insulation materials are widely used in the construction industry due to their low thermal conductivity attributable to their microstructures and their low-conductivity blowing agents and affordability. In this study, we evaluate how the thermal performance of foam insulation materials used for the exterior walls of buildings, viz., extruded polystyrene (XPS), polyisocyanurate (PIR), and phenolic foam (PF), age over the life cycle of a building. To compare the aging of thermal performance during the life cycle of a building, each material was tested at 70 and 110 °C and with slicing acceleration according to EN and ISO standards. The thermal conductivity of each foam insulation material was measured using a heat flow meter at an operating temperature of 23 °C and converted into thermal resistance values. Different foam insulation materials have different aging procedures according to material-specific EN standards, while ISO 11561 applies the same procedure to all material classifications. Upon comparing the aged values according to ISO and EN standards to the initial values, the analysis showed a change rate of 23 to 26% in PIR and 18 to 20% in PF. In XPS, a rate of change of 10 to 23.8% was calculated. Our results indicated that the slicing acceleration induced a thermal resistance reduction rate about three times faster than aging at 70 °C. However, the long-term changed thermal resistance values of the foam insulation material applied via the calculating procedure specified in the ISO and EN standards were similar.

A three-dimensional investigation of the thermoelastic effect in an enhanced geothermal system reservoir

During heat extraction in enhanced geothermal system (EGS) reservoirs, thermal contraction is induced by cold fluid injection, resulting in thermoelastic deformation. The induced thermoelasticity can alter rock properties, including their fracture aperture and permeability; therefore, the thermoelastic effect is crucial in understanding EGS reservoir behaviour. Based on coupled thermo-hydro-mechanical (THM) processes extended in COMSOL Multiphysics to include the thermoelastic effect, this paper presents a three-dimensional (3D) numerical model of an EGS reservoir with a multiple planar fracture system to investigate the influence of thermoelasticity on reservoir thermal performance. The model is used to perform an in-depth analysis to determine the rate at which the thermoelastic effect develops during heat extraction in relation to a baseline THM model without thermoelasticity. After demonstrating that thermoelasticity influences the thermal performance of EGS reservoirs, the study is further extended to investigate the effect of injection temperature and Young's modulus on fracture aperture opening, reservoir impedance, thermal front propagation, and flow rate. The results show that thermoelasticity affects the long-term thermal performance of EGS reservoirs by reducing the energy extraction rate due to increased flow pathways. Due to high reservoir impedance, the thermoelastic effect appears to cause thermal short circuits (growth of rapidly cooling paths with high flow rates). The results suggest that thermoelasticity has a significant impact on system thermal performance for deep EGS reservoirs.