Indoor Heat Gain Research Articles

Investigating the thermal performance of a pipe-encapsulated movable PCM wall and its impact on reducing indoor heat gain during summer

The utilization of phase change materials (PCM) in building envelopes is considered to be an effective technology for reducing energy consumption during building operations. In this study, a novel approach is presented for integrating PCM into walls by encapsulating it using pipes, allowing for its mobility through the use of compressed air. Four experimentally validated thermal models, pipe-encapsulated movable PCM (PEM-PCM) wall (Model 1), wall with PCM fixed to the exterior (Model 2), wall with PCM fixed to the interior (Model 3), and wall without PCM (Model 4), were proposed to compare their energy efficiency in summer. Different upper threshold limits (Ttr,up) and lower threshold limits (Ttr,low) were employed to explore the impact of various threshold combinations control strategies on the thermal performance of the wall. The results show that the most effective operating scheme is Ttr,up of 27 °C and Ttr,low of 25 °C. In terms of long-term operation, cumulative summer indoor heat gain of Model 1 is 10.65 %, 10.85 % and 11.4% lower than that of Model 2,3 and 4. Notably, September demonstrates the greatest potential for energy savings, with Model 1 reducing the monthly cumulative indoor heat gain by 54.12% compared to Model 4.

Advancing energy recovery ventilators with phase change materials for cooling load reduction and heat exchange efficiency improvement

Efforts to reduce building energy emissions and achieve net-zero carbon scenarios globally have increasingly focused the building sector on improving energy efficiency. Heating, ventilation, and air conditioning (HVAC) systems play a vital role in providing comfortable indoor environments but significantly contribute to energy consumption and greenhouse gas emissions. Energy recovery ventilators are efficient active systems that prevent unnecessary indoor heat loss and gain, regardless of outdoor climate, making them excellent technologies for achieving zero energy. In a hot and humid summer climate, such as in Korea, it is crucial to enhance the efficiency of energy recovery ventilators and reduce cooling energy consumption. A Green HVAC design aimed at reducing the energy consumption of active systems to near zero was proposed by applying phase change material-based modules with high heat storage performance to energy recovery ventilators. The phase change material module is stabilized with high thermal conductivity aluminum packing and is configured as a mesh-type structure to amplify heat exchange efficiency with air. Compared to the energy recovery ventilator, the application of the module demonstrated a reduction in supply air temperature by an average of 2.6 °C in high-temperature outdoor. During the effective operating period, a continuous indoor intake temperature reduction effect was observed, and the sensible heat exchange efficiency showed a 21.32 % improvement with the module using n-octadecane. Phase change material modules that can enhance ventilation and improve energy efficiency in high-temperature environments can be proposed as a technology to achieve Green HVAC.

Solar PV vacuum glazing (SVG) insulated building facades: Thermal and electrical performances

As fire emergencies and energy saving demand of buildings have grown around the globe, concerns have been raised about the flammability of conventional external insulation materials in cold weather areas. In this paper, solar PV vacuum glazing (SVG) was proposed as a promising alternative to traditional external insulation layers of buildings due to its incombustible nature and superior thermal insulation performance. To assess the thermal and electrical performances of SVG-insulated facades, a heat transfer model and an effective absorptance calculation model for SVG-insulated façades were developed and validated against experimental data. Results showed that SVG-insulated facades exhibited significantly lower U values, ranging from 44.1% to 47.5% less than those of traditional concrete walls (without insulation layer). Additionally, the application of Low-emissivity (Low-e) coatings on the glazing could further reduce the U value from 2.05 W/(m2·K) to 0.647 W/(m2·K), making SVG-insulated facades competitive with traditional insulation walls. The secondary heat transfer factor (SHTF), defined as the ratio of indoor heat gain from solar radiation absorbed by building facades to incident solar radiation, was also reported for SVG-insulated facades with different solar cells (C-si, A-si, and CdTe), along with their respective efficiencies. Parametric analyses of eight parameters subsequently highlighted that their influence on the thermal performance of SVG-insulated façades was much greater than on the solar cell efficiency. Furthermore, considering the combined effect of optimal value of key influencing parameters, the lowest U value of 0.153 W/(m2·K) could be achieved, which represents approximately 24.6% of the U value of traditional insulation walls. This study provides compelling evidence for the adoption of SVG-insulated facades as replacements for traditional insulation walls and offers insights into optimizing their thermal performance.

Thermal performance of phase-change wall of a hotel building under hot-summer and cold-winter climate

ABSTRACT In China, the energy consumption in the building sector has increased tremendously in the past decades. It is important to investigate advanced passive energy-saving technologies for buildings. By applying phase-change materials in walls, the peak cooling/heating load can be significantly reduced. In the present study, a numerical model of one-dimensional heat transfer within a phase-change wall was developed and solved by Matlab, which was successfully validated with published data. Subsequently, the model was used to simulate the thermal performance of a phase-change wall in a hotel building in Shanghai, China. Simulation results demonstrate that the performance of the phase-change wall is optimized when the phase-change material is applied to both sides of the wall. Year-round heat transfer is minimized using layers of paraffin (C16-C28) and capric acid (2 cm), with phase-change temperatures of 43℃ and 16.3℃, respectively. The PCM wall reduces inwardthe inward heat transfer by 26.9% compared with a common wall without phase-change material. The outward heat loss is also reduced by 15.7% during the heating season. This study demonstrates that the PCM wall can effectively reduce indoor heat gain in summerin the summer and heat loss in winterin the winter. The numerical model has proven to be feasible for optimizing construction design.

Enhancing energy efficiency of PCM wall in winter with novel PCM-DLCB coupling design

In order to enhance the thermal performance of the phase change material (PCM) wall during winter, a novel PCM and double-layer capillary bundles (PCM-DLCB) coupling wall is proposed. The PCM-DLCB wall utilizes the capillary bundle near the outer surface to capture solar energy during sunny winter days. The heat is then transferred to the capillary bundle near the inner surface through a circulating pump, promoting the melting of the PCM. Optimization and long-term operation comparison research were conducted using numerical simulation to investigate the potential of the PCM-DLCB wall to improve energy efficiency during winter. Five structural parameters, including the type of PCM, the position of the capillary bundle, the position and thickness of the PCM layer, the spacing of the capillary bundles, and the flow velocity in the capillary bundle, were selected as variables for structural and operation optimization. The optimization target was the accumulated heat gain from the inner surface of the wall. The research results indicate that: (1) The most suitable PCM is RT-18HC; (2) Placing the PCM layer between the double-layer capillary bundle yields better results compared to other arrangements; (3) Optimal energy saving is achieved when the outer capillary bundle is positioned 6 mm away from the outer surface and the inner capillary bundle is positioned 8 mm away from the inner surface; (4) The optimal thickness of the PCM layer is 10 mm; (5) The optimal flow velocity in the capillary bundle is approximately 0.04 m/s. Furthermore, long-term operation results based on a typical hot summer and cold winter climate zone reveal that the optimized PCM-DLCB wall provides a 7.80 % increase in indoor heat gain compared to an ordinary PCM wall during winter, and an 18.4 % increase compared to a wall without PCM.

A numerical study on summer operation performance of PCM wallboard cooling tower coupling system in hot-summer and cold-winter regions of China

The use of phase change materials in building envelopes shows promise in reducing overall building energy consumption. Researchers are focusing on designing optimal wall structures based on varying climate characteristics to enhance building energy efficiency. This study presents a novel system that couples phase change material wallboard with cooling tower. In the summer months, the system efficiently utilizes cooling water to dissipate heat stored in the phase change material during the day, thereby maximizing the utilization of the phase change material. The research includes the design and optimization of a southward prefabricated phase change material wallboard and evaluates the long-term performance of the phase change material wallboard cooling tower coupling system through a validated numerical model. The optimization findings indicate that the system achieves maximum energy savings with a cooling water flow velocity of 0.0125 m/s. Moreover, double-level control shows a substantial improvement in system performance compared to single-level control. Long-term operation results demonstrate that the phase change material wallboard cooling tower coupling system effectively reduces indoor heat gain during summer, with a cumulative indoor heat gain of −16.29 kWh/m2, representing a 343.5 % reduction compared to the common wallboard. The system has the potential to significantly decrease power consumption in air-conditioning systems, particularly in regions with hot summers and cold winters.

Enhancing indoor thermal comfort and energy efficiency: A comparative study of RC-PCM Trombe wall performance

In response to the issue of excessive heating during summer in traditional Trombe walls, this study proposes a novel system called RC-PCM Trombe walls, which utilizes radiation cooling and phase change materials (PCMs) to traditional Trombe walls. The thermal performances of RC-PCM Trombe wall, conventional Trombe wall, and PCM Trombe wall were compared and studied under summer conditions. The experimental results demonstrate that the addition of heat pipes and radiant cooling devices to the RC-PCM Trombe wall led to a reduction of peak temperatures on the inner surface of the south wall by 3 °C and 1.2 °C, respectively. Furthermore, the time taken to reach the peak temperature was delayed by 43 min and 38 min, respectively. The standard deviations of indoor air temperature were reduced by 0.04 °C and 0.29 °C, while the indoor heat gain was reduced by 26.5 % and 55.2 %, respectively. These findings indicate that the RC-PCM Trombe wall effectively mitigates indoor overheating and enhances the indoor thermal environment during the summer.

Thermal performance of a solar heat collection and storage wall with night insulation in winter: A numerical study

The application of phase change material (PCM) in Trombe walls (TW) can improve all-day indoor thermal comfort. However, the heat loss via the glass is relatively large, resulting in low thermal efficiency at night. This paper designs a double glass and a thermal insulation curtain used to increase the thermal efficiency of a TW with PCM at night. In addition, a mathematical model based on the implicit difference method was established to study the thermal efficiency and heat loss of the proposed wall in Beijing. The findings indicate that the thermal efficiency at night is improved by 16.23 % compared to a traditional TW, while heat loss at night drops by 7.75 %. The heat gain at night first increased and then decreased with the augment of phase change temperature. As the latent heat or thermal conductivity of PCM increases, the heat gain declines during the day and increases during the night. The most suitable thickness of PCM is 0.02 m. The farther the insulation curtain is from the inner glass, the greater the heat loss and the smaller the indoor heat gain at night. And the later the opening time of the air vents during the day, the smaller the heat gain during the day and the greater the heat loss. But the resulting increase in nighttime heat gain is not significant. This study can offer design suggestions for enhancing indoor thermal comfort and nighttime insulation performance.

Shading effect and energy-saving potential of rooftop photovoltaic on the top-floor room

Rooftop photovoltaic panels can serve as external shading devices on buildings, effectively reducing indoor heat gain caused by sunlight. This paper uses a numerical model to analyze rooftop photovoltaic panels' thermal conduction, convection, and radiation in hot summer areas as shading devices. The researcher builds an experimental platform to verify the model, exploring the potential for energy savings of photovoltaic rooftop units in the Wuhan area. The results show that after installing photovoltaic panels, the delay performance of the roof increases by 0.5 h, the roof heat flux is reduced by 41.7%, the peak temperature of the roof is reduced by 22.9 °C, and the daily heat gain is reduced by 74.84%. Finally, this paper discusses the impact of high-reflectance and low-reflectance roofs on the shading effect. The study finds that low-reflectance roofs are more energy-efficient in the hot summer, as high-reflectance roofs lead to a 10.8% increase in indoor heat gain when the photovoltaic panel is installed. Finally, a quantitative method for evaluating the comprehensive potential for energy savings is proposed, considering the electricity generation gain of photovoltaic panels and the comprehensive energy-saving efficiency of photovoltaic roofs, which generates a total potential for energy savings rate of 61.06%. The model presented in this paper provides theoretical guidance for analyzing the comprehensive energy-saving effects of photovoltaic rooftop systems and reveals the potential for energy savings of rooftop photovoltaic panels as external shading devices.

Experimental study on thermal performance of finned tube water flow window

Water flow window is an advanced energy-saving building façade, and the design of the heat exchanger affects the thermal performance directly. A novel finned tube water flow window (FTWFW) applying a finned tube heat exchanger is proposed. Its thermal performance, with regard to energy savings and the thermal environment created under various operating conditions, is studied experimentally. The reduction in indoor heat gain reached 88.1% compared with insulated glazing unit, and the average daily water heat gain efficiency was 65.2% during the summer testing. The inner glazing surface temperature of FTWFW was more stable and closer to the comfort room temperature, especially under non-air-conditioning condition in winter, resulting in enhanced thermal comfort. Moreover, increasing the flow rate or decreasing the temperature of feed water can improve the thermal performance of FTWFW in summer. In winter, compared to increasing the feed water flow rate, raising the feed water temperature was more effective in increasing the room temperature. The advantages of FTWFW in cooling/heating energy saving, thermal collection, and thermal comfort enhancement were well demonstrated. Its large-scale application is expected to make contributions in green and low-carbon building development. The findings of this study can helpful in decision making of real projects.

Large‐Scaled and Solar‐Reflective Kirigami‐Based Building Envelopes for Shading and Occupants’ Thermal Comfort

AbstractConventional air conditioning in buildings consumes substantial energy contributing to an increase in greenhouse gas emissions, which aggravates global warming. Kirigami envelopes with reflective surfaces offer a promising option to reduce indoor heat gain. So far none has performed outdoor testing of kirigami envelopes nor numerical simulation to understand how they interact with the environment. To understand the impact of the geometric design of the kirigami shading system on both daylighting and temperature variation in the indoor environment, a series of prototypes and simulations is devised to investigate the real‐time shading behavior in an outdoor environment. Large‐scaled and silver‐coated kirigami‐based envelopes of different cut patterns and stretching ratios are fabricated and placed in front of custom‐designed chambers equipped with six thermo‐sensors. Simulation is performed to investigate the effect of light modulation and hence temperature inside the chamber, which corroborates with the experiments. The indoor daylighting pattern and temperature are significantly influenced by the geometry of the stretched kirigami envelope, and overall, both the indoor temperature and the spatial distribution of illuminance are more uniform compared with that without the kirigami envelope. These results indicate that the installation of kirigami envelopes can improve building energy saving and occupants’ comfort.

Experimental observations on electricity generation and thermal characteristics of TEG façades

The incorporation of thermoelectric generators (TEGs) into building systems has considerable potential for energy harvesting and merits further exploration. In this study, the electricity generation and thermal characteristics of the TEG façades comprised of TEG cells and feasible wall constructions under different solar heating conditions (peak heat gain: 400–1200 W/m2) were experimentally investigated. The results showed that applying TEGs to a typical structural façade using a 25 cm-thick reinforced concrete (RC), the peak electricity generation was 100.0 mW/m2. For TEG nonstructural façades, if the indoor heat gain was considered, the TEG attached to the 2.5 cm-thick microencapsulated phase change material (mPCM) honeycomb board showed the highest energy ratio. Correlation equations of the power generated by TEG façades per unit area with time during the daytime are provided, which designers can use to estimate the power generation for a TEG façade with a given peak solar heat gain.

The visual comfort, economic feasibility, and overall energy consumption of tubular daylighting device system configurations in deep plan office buildings in Saudi Arabia

Despite the abundance of daylight in hot desert climates, deep-plan office buildings still lack the provision of sufficient daylight. Although tubular daylighting devices (TDDs) offer a potential solution, they may have visual and economic consequences. Therefore, the present study evaluated the visual comfort, overall energy consumption, and economic feasibility of multiple TDD configurations. Typical deep-plan office space has been simulated based on a field-validated simulation process to evaluate visual comfort and energy consumption. The WLCC method was used to assess the economic feasibility. The reference case was illuminated using LED and a dimming control system with photosensors placed in cardinal orientations. Supplementary daylight was provided using different configurations of vertical and horizontal TDDs. Window shading devices were used to enhance visual comfort. The study found that vertical TDDs perform better than horizontal TDDs in terms of illumination uniformity and economic feasibility. They reduced the electric lighting load by up to 21%. However, the utilization of the TDDs in the hot desert climate has the potential to significantly increase indoor heat gain, which, in turn, increases the cooling energy. It is found that the additional energy required for cooling far outweighs the energy-saving benefits gained by TDDs. Accordingly, thermal issues associated with the utilization of TDDs should be investigated to allow widespread use of them in arid regions where high solar radiation is available throughout the year. Moreover, other daylighting-associated benefits, such as users’ productivity enhancement and health and mood improvement, should be considered to promote the usage of TDDs and boost their economic returns.

Modeling, Simulation, and Performance Analysis of a Liquid-Infill Tunable Window

Solar shading is important in buildings for better indoor thermal/light environment and energy conservation, especially in the tropical region. Compared with conventional windows with additional fixed shading devices, windows with adaptive self-shading functions take up less space and require less management labor. The present investigation focuses on a compact liquid-infill tunable window, which can provide adaptive shading with colored liquid-infill according to the surrounding environment. The numerical model of the liquid-infill tunable window was established on the basis of the law of energy and mass conservation, which enabled prediction of the adaptive response of the window under different boundary conditions. Then the thermal performance of this innovative window was analyzed in comparison with triple-layered clear glass windows. Influences of solar radiation level, incident angle, and ambient temperature were taken into consideration. The window was proven to be efficient in reducing indoor heat gain in the cooling season under strong solar radiation. With an 60° incident angle, the total indoor heat gain through window can be reduced by 1.60–8.33%. In the future, the established numerical model may be inserted into existing building simulation software as an energy-efficient window module to evaluate its energy and economic performance. The present study may inspire architectures and engineers in the design of near-zero energy and/or carbon neutral buildings.

Experimental and numerical study of a reversible radiative sky cooling PV window

Transparent envelopes, such as windows, are usually the weak points of building thermal insulation and responsible for the tremendous cooling/heating energy consumption in the building sector. An innovative reversible radiative cooling PV (RRC-PV) window was proposed, which combined the radiative sky cooling and photovoltaic window technologies. It was capable of generating electricity from incident solar energy and reducing the indoor cooling load in the daytime whilst providing natural cooling at night by dissipating heat to outer space through atmospheric window. The thermal and electrical performances were tested. Simulation program was developed with MATLAB and validated successfully.Further, year-round energy performance was evaluated based on the TMY dataset of Shenzhen. The indoor heat gain was effectively reduced by utilizing RRC-PV window instead of common clear glazing window. The total reduction was 208.16 MJ/m2 per cooling season. Meanwhile, the beneficial indoor heat gain during heating season was unfavorably reduced. With the electricity generation taken into consideration, the annual comprehensive energy saving potential was as large as 264.23 MJ/m2 over common clear glazing window under hot summer and warm winter climate of Shenzhen, China. Thermal and energy performances of RRC-PV window could be favorably improved in regions with plentiful solar irradiance and cleaner atmosphere. The local climate and comprehensive energy performance should be evaluated before practical application of RRC-PV window with the proposed methodology. Future research and development of radiative cooling materials would enhance the building energy saving and contribute to the neutral carbon cause.

Preparation and thermal performance of phase change material (PCM) foamed cement used for the roof

PCM foamed cement is to mix PCM with foamed cement. On the one hand, this new type of material can improve the thermal storage capacity of foamed cement, on the other hand, it could improve the thermal insulation performance of PCM when it completely melting or solidification. In this paper, the PCM foamed cement with different foaming rate were first prepared by physical method. Then, thermal conductivity of the PCM foamed cement was tested under different temperatures. Influence of foaming rate to the thermal conductivity was analyzed. Finally, the PCM foamed cement was placed on the external surface of the roofs of real buildings to form the PCM foamed cement roof (PCM FCR). In order to study its thermal performance, two cases were designed, case 1 was with high-reflectivity film on the external surface of the roof, case 2 was without high-reflectivity film. For the two cases, thermal performance of the PCM FCR was compared with foamed cement roof (FCR) and ordinary roof (OR). The results show that, PCM FCR and FCR can reduce the roof internal surface temperature by up to 2 °C and 1.1 °C in case 1, the values were 2.9 °C and 2.5 °C in case 2. The heat gain of PCM FCR and FCR can be reduced by 48.5% and 19.4% on average in case1. In case 2, the values were 59.0% and 51.2% on average. The PCM FCR performed best in reducing internal surface temperature and indoor heat gain among the three roofs.

상변화물질 벽체의 복사열저항 제어를 통한 여름철 축열 및 방열성능 개선

Purpose: The phase change materials are being studied as building heat storage materials to reduce indoor heat gain and indoor cooling loads in summer. However, in order to use phase change materials as heat storage material for walls in the summer, have to solve the problem that the phase change materials absorb and discharge heat relying on surrounding temperature. In this study, a system of improving the thermal performance of phase change materials to solve the problem by controlling radiant heat resistance was proposed. Method: To evaluate the proposed system, a numerical analysis simulation about 1D transient heat transfer was used based on energy balance equation. The proposed system was compared with conventional systems about thermal performance of phase change materials and indoor heat gain in the simulation. The simulation was performed with Matlab 2021a software. Result: The results indicated that the proposed system reduced indoor heat gain by 27.5% ~ 35.9% compared to the conventional systems, which means the thermal performance of the phase change materials can enhance by controlling the radiant heat resistance.

Thermal design method of building wall considering solar energy utilization and thermal insulation

This study makes a breakthrough out of the original thermal design idea which is mainly aim to improve thermal insulation performance of building wall, putting forward thermal design method of building wall to consider solar energy utilization and insulation simultaneously. Firstly, a day is divided into daytime and nighttime, and the expressions of indoor heat gain through the wall during such two periods are derived respectively according to the harmonic response method. The weighted sum of daytime and nighttime heat gain, then, could be used as the evaluation index of wall thermal performance. Taking this index as objective function and wall thermal resistance as well as heat capacity per unit wall area having the corresponding thickness of the thermal resistance as variables, a method for determining the optimum wall thermal resistance and heat capacity is proposed. Finally, for the room without heating measures in Lhasa area, using the typical weather data in winter as input conditions, the optimal matching curve of wall thermal resistance and heat capacity taking into account both the effect of solar energy utilization and insulation is calculated, so are the optimal value of them under different indoor temperatures (0–12 °C, interval of 2 °C). The results show that, in Lhasa area, the south wall has great potential to improve indoor thermal environment by using solar energy. From the perspectives of wall’s “carrier” role of utilizable climatic resources, the higher thermal insulation performance doesn’t always bring better results.

Experimental study on control strategies of radiant floor cooling system with direct-ground cooling source and displacement ventilation system: A case study in an office building

Radiant cooling systems need optimized control strategies to provide superior comfort while maximizing energy-savings. The field measurement method was used to study the operational control of radiant floor cooling (RFC) with a direct-ground cooling source and displacement ventilation (DV) systems. The control methods for the composite system were proposed based on three factors including floor surface temperature relative to the indoor air dew point temperature, the range of indoor/outdoor air temperature and humidity, and the indoor thermal and humidity loads to be countered. These factors were considered in three typical scenarios: intermittent operation, variable initial temperature and humidity conditions, and sudden increases in indoor heat gain, so that maintaining the operative temperature within 26–27 °C. The system required that precooling time of the RFC was 2.5–3 times the time to achieve 63% of the temperature change for intermittent operation on weekends, while the DV system was started 1–1.5 h before work time when initial indoor air humidity was higher than 75% and indoor air temperature was higher than 26 °C, and supply air flow rate was increased to maximum value under sudden increases in indoor heat gain. The results concluded that dynamic optimal control of the radiant cooling system was achieved.

Passive building cooling achieved with a new class of thermal retrofit: The liquid vapour phase change material

For a period covering more than half of the last century and until now, the term “phase change material” (PCM) in context of passive building coolingunanimously referred to a class of materials undergoing solid–liquid phase transition in cyclic manner. In contrast to solid–liquid PCMs (SLPCMs), the liquid-vapour phase transition has not been capitalized in a similar way to cool buildings by passive means despite the clear notion that the latent heat of evaporation exceeds the latent heat of fusion by a significant margin. The present work is an endeavour towards offering a new passive cooling system for buildings in hot climate with the help of liquid–vapour phase transition. Unlike existing active evaporative cooling systems, the liquid–vapour PCM (LVPCM) system does not require any external power supply to induce cooling effect. In the hot climate of Delhi, India, a peak temperature dip of 2.22℃ was achieved with the LVPCM system. An average indoor heat gain reduction of 13.56% was realized which reached a maximum value of 20.51% during the experiment. This novel system could overcome the night time heating issue of SLPCM. Furthermore, several additional benefits (such as the use of environment friendly and low-cost materials, ease of installation etc) could make LVPCM a competitive alternative to the prevailing SLPCM technology.