Energy performance of a hybrid space-cooling system in an office building using SSPCM thermal storage and night ventilation

Thermal performance of a hybrid space-cooling system with night ventilation and thermal storage using shape-stabilized phase change material (SSPCM) is investigated numerically. A south-facing room of an office building in Beijing is analyzed, which includes SSPCM plates as the inner linings of walls and the ceiling. Natural cool energy is charged to SSPCM plates by night ventilation with air change per hour (ACH) of 40 h⁻¹ and is discharged to room environment during daytime. Additional cool-supply is provided by an active system during office hours (8:00-18:00) necessary to keep the maximum indoor air temperature below 28 °C. Unsteady simulation is carried out using a verified enthalpy model, with a time period covering the whole summer season. The results indicate that the thermal-storage effect of SSPCM plates combined with night ventilation could improve the indoor thermal-comfort level and save 76% of daytime cooling energy consumption (compared with the case without SSPCM and night ventilation) in summer in Beijing. The electrical COPs of night ventilation (the reduced cooling energy divided by fan power) are 7.5 and 6.5 for cases with and without SSPCM, respectively.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

Thermal performance of phase change material energy storage floor for active solar water-heating system

The conventional active solar water-heating floor system contains a big water tank to store energy in the day time for heating at night, which takes much building space and is very heavy. In order to reduce the water tank volume or even cancel the tank, a novel structure of an integrated water pipe floor heating system using shape-stabilized phase change materials (SSPCM) for thermal energy storage was developed and experimentally studied in this paper. The thermal performances of the floors with and without the SSPCM were compared under the intermittent heating condition. The results show that the Energy Storage Ratio (ESR) of the SSPCM floor is much higher than that of the non-SSPCM floor; the SSPCM floor heating system can provide stable heat flux and prevent a large attenuation of the floor surface temperature. Also, the SSPCM floor heating system dampens the indoor temperature swing by about 50% and increases the minimum indoor air temperature by 2°C–3°C under experimental conditions. The SSPCM floor heating system has a potential of making use of the daytime solar energy for heating at night efficiently.     

Performance of a hybrid heating system with thermal storage using shape-stabilized phase-change material plates

Performance of a hybrid heating-system, combined with thermal storage using shape-stabilized phase-change material (SSPCM) plates, is investigated numerically. A direct gain passive solar house in Beijing is considered, which includes SSPCM plates as the inner linings of walls and the ceiling. Unsteady simulation is performed using a verified enthalpy model, with a time period covering the winter heating-season. Additional heat supply is employed during load hours at late night and early morning (23:00–07:00 in Beijing) or during the whole day necessary to keep the minimum indoor air temperature above 18 °C. The results indicate the thermal storage effect of SSPCM plates, which improves the indoor thermal comfort level and saves about 47% of normal-and-peak-hour energy use and 12% of total energy consumption in winter in Beijing.     

Thermal performance of phase change material energy storage floor for active solar water-heating system

The conventional active solar water-heating floor system contains a big water tank to store energy in the day time for heating at night, which takes much building space and is very heavy. In order to reduce the water tank volume or even cancel the tank, a novel structure of an integrated water pipe floor heating system using shapestabilized phase change materials (SSPCM) for thermal energy storage was developed and experimentally studied in this paper. The thermal performances of the floors with and without the SSPCM were compared under the intermittent heating condition. The results show that the Energy Storage Ratio (ESR) of the SSPCM floor is much higher than that of the non-SSPCM floor; the SSPCM floor heating system can provide stable heat flux and prevent a large attenuation of the floor surface temperature. Also, the SSPCM floor heating system dampens the indoor temperature swing by about 50% and increases the minimum indoor air temperature by 2°C–3°C under experimental conditions. The SSPCM floor heating system has a potential of making use of the daytime solar energy for heating at night efficiently.     

Thermal characteristics of shape-stabilized phase change material wallboard with periodical outside temperature waves

Thermal characteristics of shape-stabilized phase change material (SSPCM) wallboard with sinusoidal temperature wave on the outer surface were investigated numerically and compared with traditional building materials such as brick, foam concrete and expanded polystyrene (EPS). One-dimensional enthalpy equation under convective boundary conditions was solved using fully implicit finite-difference scheme. The simulation results showed that the SSPCM wallboard presents distinct characteristics from other ordinary building materials. Phase transition keeping time of inner surface and decrement factor were applied to analyze the effects of PCM thermophysical properties (melting temperature, heat of fusion, phase transition zone and thermal conductivity), inner surface convective heat transfer coefficient and thickness of SSPCM wallboard. It was found that melting temperature is one important factor which influences both the phase transition keeping time and the decrement factor; for a certain outside temperature wave, there exist critical values of latent heat of fusion and thickness of SSPCM above which the phase transition keeping time or the decrement factor are scarcely influenced; thermal conductivity of PCM and inner surface convective coefficient have little effect on the phase transition keeping time but significantly influence the decrement factor; and the phase transition zone leads to small fluctuations of the original flat segment of inner surface temperature line. The results aim to be useful for the selection of SSPCMs and their applications in passive solar buildings.     

Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall–roof solar chimney

A theoretical investigation of a combined wall–roof solar chimney to improve night time ventilation in buildings is presented. A spreadsheet computer program is used for the parametric study to find out the optimum configuration of the wall–roof chimney. It has been reported that a roof solar chimney alone can induce an air flow rate of 0.81 m³/s when the average incident solar radiation is 850 W/m². The maximum air velocity induced is 1.1 m/s when the 25° inclined chimney plates are 0.25 m apart. The aim of the paper is to predict the induced air flow rate as a result of the combined effect and to find the best height. The wall chimney height is varied from 1.95 to 3.45 m to determine the optimum length in relation to the chimney inlet. The results show that the air flow rate is three times more than that of the roof solar chimney alone (0.81 to 2.3 m³/s). The maximum air flow rate of 2.3 m³/s occurs at 3.45 m wall height. ACTION Psychrometrics Software (Sunshine Technology, USA, 1995) is used to predict the mean cooling load corresponding to the induced ACH. An air change per hour up to 26 could be achieved for a flat volume (321 m³). Such ACH could be utilized to improve night ventilation to reduce indoor air temperature and cool low-rise heavy buildings with large diurnal outdoor temperature variations.     

Performance of shape-stabilized phase change material wallboard with periodical outside heat flux waves

Numerical simulations were carried out to investigate the performance of shape-stabilized phase change material (SSPCM) wallboard with sinusoidal heat flux waves on the outer surface and compared with traditional building materials – brick, foam concrete and expanded polystyrene (EPS). One-dimensional enthalpy equation was solved using control volume-based implicit finite-difference scheme. Time lag (φ), decrement factor (f) and phase transition keeping time (ψ) of inner surface were applied to analyze the effects of PCM thermo-physical properties, inner surface convective heat transfer coefficient and thickness of SSPCM wallboard. The results showed that for SSPCM, there exist two flat segments within one wave length period of inner surface heat flux lines and it has larger time lag and lower decrement factor than those three ordinary building materials. It was found that melting temperature and thermal conductivity of SSPCM have little effects on φ, f and ψ, which is different from the case of temperature waves; for a certain outside heat flux wave, there exist critical values of latent heat of fusion and thickness of SSPCM above which the heat flux wave amplitude can be diminished to zero; inner surface convective heat transfer coefficient is one important factor which significantly influences the decrement factor; and the phase transition zone leads to small fluctuations of the original flat segments of inner surface heat flux line.     

An assessment of mixed type PCM-gypsum and shape-stabilized PCM plates in a building for passive solar heating

Thermal performance of two phase change material (PCM) composites, mixed type PCM-gypsum and shape-stabilized PCM plates, has been numerically evaluated in a passive solar building in Beijing with an enthalpy model. Effects of the melting temperature and phase transition zone of the PCM are analyzed and a comparison between the two types of PCM composites is performed. The results show that: (1) for the present conditions, the optimal melting temperature is about 21 °C; (2) PCM composites with a narrow phase transition zone provide better thermal performance; (3) both mixed type PCM-gypsum and shape-stabilized PCM plates effectively shave the indoor temperature swing by 46% and 56%, respectively; (4) the shape-stabilized phase change material (SSPCM) plates respond more rapidly than the mixed type PCM-gypsum and prove to be thermally more effective in terms of utilizing the latent heat.     

Numerical investigation of free-cooling system using plate type PCM storage

Free cooling night ventilation is the process of storing the coolness in the night time and releasing this coolness in hot day time. In this paper, a numerical study was carried out to simulate and to find out the optimum design for plate type storage filled with phase change material (PCM) which is used in night ventilation systems. The effect of different parameters such as thickness of PCM-plates, inlet air temperature and air mass flow rates on melting front, cooling power, outlet temperature and thermal performance of heat exchanger was studied. The results showed that cooling power can be increased by increasing the mass flow rate. Also, the thickness of the plates in the storage device plays an important role in the thermal performance of the unit and has a linear relation with the melting process duration of PCM for considered configuration.     

The influence of wind speed,terrain and ventilation system on the air change rate of a single-family house

When the heat balance of a building is assessed, the heat needed for the ventilation air is usually calculated according to the intended ventilation rate. However, in order to calculate the air change rate accurately several aspects have to be considered. One important parameter is the ventilation system. Whether the building has a mechanical exhaust-only, supply-only, balanced exhaust–supply or natural ventilation system will influence the air infiltration rate through cracks in the building envelope. High infiltration rates lead to an increase in the heating demand and can result in an inadequate capacity of the designed heating installation. In this paper, computer simulations of the air change rate for a detached single-family house are presented. The house is simulated in different topographical surroundings, equipped with a mechanical exhaust-only, or a balanced exhaust–supply, ventilation system. In addition, the airtightness of the building is varied, from very tight, 1 air change per hour (ACH), to quite leaky, 6 ACH, when pressurized to 50 Pa. Results from the simulations show that the same house has quite different air change rates in different surroundings with different airtightness.     

Sensitivity analysis of a maritime located night ventilated library building

An examination of the role of design and operational parameters in a night ventilated library building located in Ireland, and which is subject to a maritime type climate, is considered. For this investigation, a self-contained space within the library building is analysed. A dynamic model of the space is created using the ESPr simulation package, which is compared with building data obtained from an experimental monitoring programme. Calculation of the mean bias deviation between the predicted and experimental data indicates agreement better than 0.45 °C for the dry bulb temperature and 1.1 °C for the mean radiant temperature. Using ESPr predictions, the role of different building design and operational parameters are examined by means of parametric analysis including building mass, ventilation duration, internal gain and ventilation rates. Compared to a reference base of the existing building, increasing the building mass was found to achieve a significant reduction in internal dry resultant temperatures, with a decrease of between 2 and 3 °C observed when mass was increased from 800 kg/m² to 1600 kg/m². Reducing building internal gains from 40 W/m² to 20 W/m² was observed to have the potential of further reducing dry resultant temperatures by up to 1 °C. Examination of night ventilation rates indicated that increasing night ventilation up to 10 ACH was observed to have a significant affect on reduction of dry resultant temperature, but further increases of air changes were observed to have a negligible effect.     

PV/T蒸发屋面传热性能实验研究

提出一种新型的PV/T蒸发屋面被,该屋面除具有防水、美观等基本功能外,还兼作PV/T热泵系统的蒸发器,集发电、制热于一体。通过实验,详细分析夏季工况下PV/T屋面各层的温度分布、空腔中流体的流动特性,并探讨PV/T屋面的设置对建筑外表面温度及建筑得热量的影响。结果表明;夏季热泵系统运行时,PV/T屋面系统中腔体内流体被加热,较普通屋面而言,其原屋面外表面温度降低,最高降低21℃,但夜间时略高于普通屋面1℃,总体而言,PV/T屋面的设置使建筑得热量降低44%。     

Energy saving in building with PCM cold storage

This article presents an experimental and numerical analysis of cooling buildings using night‐time cold accumulation in phase change material (PCM), otherwise known as the ‘free‐cooling principle’. Experimental and numerical studies of the ceiling and floor free‐cooling principle, as well as passive cooling, are presented. The free‐cooling principle is explained and some of the types of PCMs suitable for summer cooling are listed. An experiment was conducted using paraffin with a melting point of 22°C as the PCM to store cold during the night‐time and to cool hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time and dimensionless cold discharging values are presented for different air velocities. Experimental analysis of one week of measuring under real conditions is presented in order to show how cold storage functions. Copyright © 2007 John Wiley & Sons, Ltd.     

夏季降温的太阳房实验分析

对建造在南宁市西郊的一座被动式太阳房在夏季运行时的实验数据进行了分析，其屋顶集热表面在夏季夜间能产生一定的致冷效果。所制取的冷空气不但在夜间可以对该太阳房起到降温的作用，并且可以储藏起来以供白天降温使用。     

结合太阳能空气集热器的定形相变蓄能地板采暖系统实验研究

提出了一种与太阳能空气集热器结合的定形相变蓄能地板采暖系统充分利用太阳能.白天,由太阳能空气集热器加热后的热空气通过保温管道输送到相变地板夹层,相变材料(PCM)蓄热;夜间,房间的冷空气进入相变地板夹层,被加热后送入房间,相变材料放热.可行性实验研究表明,此采暖系统安全可靠,能显著提高窒内温度,房间各处受热均匀,热舒适性较好,有一定的应用前景.     

Preparation of macro-capsules containing shape-stabilized phase change materials and description of permeation kinetics of its wall

Through in situ polymerization, a kind of macro-capsule was prepared by using silica gel as the shell material and shape-stabilized phase change materials (SSPCM) containing 50 wt% of n-octadecane (OD) of high density polyethylene (HDPE) as the core. The surface and its construction of capsules, the permeability of the capsule wall and the release kinetics parameters of the OD in the system of petroleum ether were experimental investigated by a scanning electron microscope (SEM), thermal cycles and the extraction release kinetics. The results showed that the wall thickness of the macro-capsules was about 20–50 μm under the experimental conditions. The SSPCM surface, modified with chromic acid, is rough, and there are many tiny holes about 3 μm in diameter in it. For these reasons, either the hydrophilicity of the SSPCM surface or the cohesion between the core and the wall have been greatly improved, and then, the weight loss percentage (WLP) of the macro-capsules is decreased by about 1.5 and 2.5 times relative to that of un-modified and modified SSPCM, respectively. Additionally, after the macro-capsules were re-sprayed by using a calcium chloride solution, its wall was more compact. From the fitting parameters of the power exponent, there were two different release mechanisms, quasi-Fickian diffusion and anomalous transport for the un-encapsulated SSPCM and the macro-capsules, respectively.     

Preparation,characterization and permeation kinetics description of calcium alginate macro-capsules containing shape-stabilize phase change materials

Macro-capsules containing shape-stabilize phase change materials (SSPCM) which have 50 wt% of n-octadecane (OD) and 50 wt% of high-density polyethylene (HDPE) were prepared by using a traditional coating pan with calcium alginate (CA) as the shell material. The surface morphologies and construction, wall permeability and the kinetic release parameters of OD in a solvent of petroleum ether along with the thermal properties of the materials were investigated using a scanning electron microscope (SEM), thermal cycles, the extraction release kinetics, and differential scanning calorimeters (DSC), respectively. The results show the wall thickness of the macro-capsules was about 30–50 μm under the experimental conditions. The surface of the SSPCM after the application of chromic acid is rough and littered with numerous, microscopic holes measuring about 3 μm in diameter. From this one may conclude that either the hydrophilicity of the SSPCM surface or the permeability of the prepared macro-capsules was altered during the process and thus differed from the unmodified samples. In addition to this, the weight loss percentage (WLP) of the macro-capsules was approximately 1.5 times in the unmodified capsules, and 3 times in the modified SSPCM. The addition of the plasticizer glycerin into the wall significantly decreased the impermeability of the macro-capsules. From the parameters of the Power exponent, there are two different release mechanisms, Fickian/quasi-Fickian diffusion and anomalous transport for modified and unmodified SSPCM.     

Numerical study of thermal characteristics of double skin facade system with middle shade

Architectural shade is an effective method for improving building energy efficiency. A new shade combined with the double skin façade (DSF) system, called middle shade (MS), was introduced and developed for buildings. In this paper, a 3D dynamic simulation was conducted to analyze the influence of MS combined with DSF on the indoor thermal characteristics. The research on MS for DSF involves the temperature, the ventilation rate, the velocity distribution of the air flow duct, and the indoor temperature. The results show that the angle and position of the shade in the three seasons are different, and different conditions effectively enhance the indoor thermal characteristics. In summer, the appearance of MS in DSF makes the indoor temperature significantly lower. The indoor temperature is obviously lower than that of the air flow duct, and the temperature of the air flow duct is less affected by MS. The influence of the position of blinds on indoor temperature and ventilation rate is greater than the influence of the angle of blinds. According to the climate characteristics of winter and transition season, in winter, early spring, and late autumn, the indoor temperature decreases with the increase of the position of blinds at daytime, but the opposite is true at night. The results found in this paper can provide reference for the design and use of MS combined with DSF in hot summer and cold winter zone.     

Simulation of interaction between high-temperature process and heat emission from electricity system in summer

During the hot summer season, using electricity systems increases the local anthropogenic heat emission, further increasing the temperature. Regarding anthropogenic heat sources, electric energy consumption, heat generation, indoor and outdoor heat transfer, and exchange in buildings play a critical role in the change in the urban thermal environment. Therefore, the Weather Research and Forecasting (WRF) Model was applied in this study to investigate the heat generation from an indoor electricity system and its influence on the outdoor thermal environment. Through the building effect parameterization (BEP) of a multistorey urban canopy scheme, a building energy model (BEM) to increase the influence of indoor air conditioning on the electricity consumption system was proposed. In other words, the BEP+BEM urban canopy parameterization scheme was set. High temperatures and a summer heat wave were simulated as the background weather. The results show that using the BEP+BEM parameterization scheme of indoor and outdoor energy exchange in the WRF model can better simulate the air temperature near the surface layer on a sunny summer. During the day, the turning on the air conditioning and other electrical systems have no obvious effect on the air temperature near the surface layer in the city, whereas at night, the air temperature generally increases by 0.6 ℃, especially in densely populated areas, with a maximum temperature rise of approximately 1.2 ℃ from 22:00 to 23:00. When the indoor air conditioning target temperature is adjusted to 25–27 ℃, the total energy release of the air conditioning system is reduced by 12.66%, and the temperature drops the most from 13:00 to 16:00, with an average of approximately 1 ℃. Further, the denser the building is, the greater the temperature drop.