Parametric analysis of space cooling systems based on night ceiling cooling with PCM‐embedded piping

A parametric analysis is conducted for space cooling systems based on cold water flowing, during the night, within regularly arranged pipes embedded in a layer of phase change material (PCM), located among the structural layers of the ceiling. The introduced PCM layer in conjunction with night cooling add to the usual ceiling cooling systems offers the advantages of low energy consumption, high cool storage capacity, operation under reduced night electricity price, smoothing of electricity consumption by eliminating daily peak loads, improved thermal comfort and elimination of ceiling dripping. Our parametric analysis is based on a transient three‐dimensional finite‐difference solution of the related heat‐transfer problem for various values of all the main system parameters. PCM phase change process is simulated by using the effective thermal capacity function, which is determined experimentally for PCM suitable for air‐conditioning applications. Our tests showed that the main parameters of the system are pipe spacing, PCM layer thickness, pipe depth within the ceiling, cooling water inlet temperature, night cooling duration and PCM properties (thermal conductivity, phase change heat and ends of phase change temperature range). The effect of all the above parameters is analysed and suggestions are made for selecting the proper combinations of their values in order to obtain the lowest energy consumption in conjunction with the highest level of thermal comfort. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance analysis of phase-change material storage unit for both heating and cooling of buildings

Utilisation of solar energy and the night ambient (cool) temperatures are the passive ways of heating and cooling of buildings. Intermittent and time-dependent nature of these sources makes thermal energy storage vital for efficient and continuous operation of these heating and cooling techniques. Latent heat thermal energy storage by phase-change materials (PCMs) is preferred over other storage techniques due to its high-energy storage density and isothermal storage process. The current study was aimed to evaluate the performance of the air-based PCM storage unit utilising solar energy and cool ambient night temperatures for comfort heating and cooling of a building in dry-cold and dry-hot climates. The performance of the studied PCM storage unit was maximised when the melting point of the PCM was ～29°C in summer and 21°C during winter season. The appropriate melting point was ～27.5°C for all-the-year-round performance. At lower melting points than 27.5°C, declination in the cooling capacity of the storage unit was more profound as compared to the improvement in the heating capacity. Also, it was concluded that the melting point of the PCM that provided maximum cooling during summer season could be used for winter heating also but not vice versa.     

Thermal characteristics of manganese (II) nitrate hexahydrate as a phase change material for cooling systems

The imbalance of electrical demand in summer due to cooling system demand is a big problem in many countries. One promising solution is shifting peak demand from early afternoon to night by utilizing natural cold energy resources such as cool outside air during night or running a refrigerator driven by midnight power. In these cases, using the thermal energy storage (TES) of phase change material (PCM) which has a melting point from 15 to 25 °C is one of the most effective ideas. However, few suitable PCMs for this temperature range are at present commercially available. This study aims to evaluate the potential of Mn(NO₃)₂ · 6H₂O (manganese (II) nitrate hexahydrate) as a new PCM for the TES of cooling systems. First, experiments on the modulation of the melting point of Mn(NO₃)₂ · 6H₂O and reduction of supercooling were made by dissolving small amounts of salts in the material. Consequently, MnCl₂ · 4H₂O was found to have good performance with regard to both modulation of the melting temperature and the heat of fusion. Next, a thermal response test was carried out by using a small cylindrical vessel. Results showed that the required temperature levels for charging and discharging the heat of this mixture were clarified. In addition, the price and safety of this material as a PCM are discussed.     

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.     

Flow investigation of phase change material (PCM) slurry as a heat transfer fluid in a closed loop system

Aqueous phase change material (PCM) particles are dispersed in an organic phase to constitute a slurry for using as a cold heat transfer medium for district cooling in refrigeration and air conditioning industry. The PCM contains 90% of water stabilized by a three‐dimensional network of polymer. The flow behaviour of the slurry is investigated in a small‐scale loop circuit with transparent pipes to allow observation of flow patterns. Data show that pressure drop increases with velocity and decreases with temperature, which can be explained by heterogeneities in flow for temperature higher than 0°C and for Reynolds number (based on the properties of the liquid phase) lower than 7000. A homogeneous particle field is observed for Reynolds number up to 7000, which guarantees a safe operation of the system without the occurrence of clogging in ducts. For this range of flow, the flow rate and the pump consumption for the PCM slurry decrease notably for the same heat transportation quantity compared with chilled water. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical and experimental analysis of the thermal behaviour of a green roof system installed in two residential buildings in Athens,Greece

Measurements of the thermal behaviour of two residential buildings equipped with a green roof system have been performed in Athens, Greece. Experimental data have been used to calibrate detailed simulation tools and the specific energy and environmental performance of the planted roofs system has been estimated in detail. Simulations have been performed for free‐floating and thermostatically controlled conditions. The expected energy benefits as well as the possible improvements of the indoor thermal comfort have been assessed. It is found that green roofs have a limited contribution to the heating demand of insulated buildings operating under the Mediterranean climate. On the contrary, the green roof system is found to contribute highly to reduce the cooling load of thermostatically controlled buildings. For the considered residential buildings, a cooling load decrease of about 11% has been calculated. In parallel, it is found that green roofs contribute to improve thermal comfort in free‐floating buildings during the summer period. The expected maximum decrease of the indoor air and roof surface temperatures is close to 0.6°C. Such a decrease contributes to reduce by 0.1 the summer absolute Predicted Mean Vote Comfort Index levels in the building. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Night ventilation for building cooling in summer

This paper presents a two-step analysis of night ventilation as a way of cooling office buildings and providing comfort in summer. Experimental data first allows us to discuss some factors which affect the performance of the technique, to show that significant comfort improvement may be obtained in “well-designed” rooms, and to investigate the energy removal from the building by defining a potential energy efficiency index. Through the use of numerical simulations, we then deal with the useful cooling energy which is offered by night ventilation in the experimental configuration and show that much care has to be taken when the technique is intended to be used in the frame of a mixed-mode cooling system.     

Effect of pre‐cooling of inlet air to condensers of air‐conditioning units

Energy conservation and increase in performance of air‐conditioning systems could be achieved by pre‐cooling the air intake of the condensers. This paper experiments three different methods of pre‐cooling the condenser air; the cooling pad (CP) setup, the cooling mesh (CM) setup and the shading setup. The CP and CM setups are two different methods of evaporatively cooling the air. The three methods have been applied to three identical, 2.8 tons, split air‐conditioning units during the peak summer time period in Kuwait, under ambient temperatures ranging from 39 to 45°C. The results yielded a drop in the power consumption ranging from 8.1 to 20.5% and an increase in the cooling load ranging from 6.4 to 7.8% by using the CP and CM setups, which, in turn, resulted in an increase in the coefficient of performance (COP) of the units by 36–59%. The shading setup has resulted in an increase of power consumption due to air trapped below the shaded area, which resulted in heat being generated. Copyright © 2005 John Wiley & Sons, Ltd.     

Applications of Thermal Energy Storage in Saudi Arabia

In Saudi Arabia, the heating, ventilating and air conditioning (HVAC) system typically accounts for 65% of the total electrical energy consumption in buildings. This is due to a very high ambient temperature which persists for a long period of time in a summer season. Moreover, gas turbines efficiency decrease also with the high ambient temperatures. In the HVAC industry cool storage, or commonly known as Thermal Energy Storage (TES) is the most preferred demand side management (DSM) technology for shifting cooling electrical demand from peak daytime periods to off‐peak night‐time. The most popular and well‐suited TES concept for Saudi Arabia is either chilled water or ice storage system, depending upon the applications and the required storage capacity. This paper shows how TES offers a means of reducing the electrical demand in large commercial buildings. Additionally, it is seen that efficiencies of the air cooled chillers are increased if they run overnight. Similarly efficiencies of gas turbine is also increased when a TES based pre‐cooled air is used as an inlet to the turbine. This paper also discusses favouring conditions and other aspects of cool storage applications in Saudi Arabia. TES economics are considered and a cost analysis is presented to illustrate the potential savings that can be achieved by the use of TES in Saudi Arabia. Copyright © 1999 John Wiley & Sons, Ltd.     

Development and thermal performance of wood-HPDE- PCM nanocapsule floor for passive cooling in building

Cooling demand in the building sector is growing rapidly; thermal energy storage systems using phase change materials (PCM) can be a very useful way to improve the building thermal performance. This work shows the benefits of PCM when incorporated in wood fiber-polymer composite as floor cooling system using nano-encapsulated PCMs. The wood-plastic-NPCM composites were produced using compression molding process and its mechanical and thermal properties were investigated. Two dynamic simulators were employed to investigate synthesized composites thermal performance. Increasing NPCM content in WPC showed that the fluctuations of the simulator temperature was decreased while the heat fluxes through the floor was increased. The variations of ambient maximum temperature have little effect on the air temperature of the simulator with 40% PCM which indicates that the amount of PCM was enough for studied environmental condition. Field experiments were performed using two medium-scale test houses located on Tehran-Iran. It can be concluded that using NPCM helps to reduce heating and cooling demand. Moreover, the natural night ventilation by opening windows reduced the number of hours that the temperature is above 23°C from 499 h/year in case1 (without opening) to 255 h/year in case 2(with opening). This means that natural night ventilation could help reduce the overheating period to about 50% with the use of NPCM.     

Novel ventilation cooling system for reducing air conditioning in buildings.: Part I: testing and theoretical modelling

A latent heat storage unit incorporating heat pipes embedded in phase change material (PCM) is developed and tested for a novel application in low energy cooling of buildings. A one-dimensional mathematical model of the heat transfer from air to PCM is presented to allow sizing of a test unit. Details of the construction and testing of one heat pipe/PCM unit in a controlled environment are described, and measurements of heat transfer rate and melting times are presented. When the difference between air and PCM temperature was 5°C, the heat transfer rate was approximately 40 W over a melt period of 19 h. The heat transfer rate could be improved, and the phase change time shortened, with an alternative design for finning of the heat pipe inside the PCM.     

Numerical investigation of the performance and soil temperature recovery of an EATHE system under intermittent operations

One of the problems in operating earth air tunnel heat exchangers (EATHE) for cooling in summer, with soil having high specific heat and low moisture content is accumulation of heat around the pipe. The low rate of heat dissipation due to conduction restricts the performance of EATHE over subsequent years. In the present paper, numerical simulations have been performed to investigate the thermal performance and soil temperature during summer operation in Jaipur to estimate extent of soil degradation. The simulation result indicates that by the end of summer, the soil leads to thermal saturation which in turn, may render it unusable for next summer. This scenario demands for heat removal through force convection. Three strategies namely, night purging during summer operation, day operation during winter and night operation during winter were attempted to estimate extent of soil recovery. Simulation results show that the average COPs for summer, summer with night purging, winter day and winter night operation mode are 4.23, 3.68, 5.01, and 6.65 respectively. It was found that advantage of night purging is less than energy required to run blower for night purging. However, winter day/night operation offers space heating and better soil for next summer.     

Dynamics of energy storage in phase change drywall systems

Experimental evaluations of manufactured samples of laminated and randomly mixed phase change material (PCM) drywalls have been carried out and compared with numerical results. The analysis showed that the laminated PCM drywall performed thermally better. Even though there was a maximum 3% deviation of the average experimental result from the numerical values, the laminated PCM board achieved about 55% of the phase change process as against 48% for the randomly distributed drywall sample. The laminated board sample also released about 27% more latent heat than the randomly distributed type at the optimum time of 90 min thus validating previous simulation study. Given the experimental conditions and assumptions the experiment has proved that it is feasible to develop the laminated PCM technique for enhancing and minimising multi‐dimensional heat transfers in drywall systems. Further practical developments are however encouraged to improve the overall level of heat transfer. Copyright © 2005 John Wiley & Sons, Ltd.     

On the use of sunspace for space heating/cooling in Europe

The heating/cooling potential of a typical sunspace has been investigated in the present study as a function of different climatic conditions in several locations throughout Europe. Passive solar design is mainly concentrated on providing space heating during winter as well as on avoiding overheating during the summer period. In the present paper the feasibility of a sunspace as a heating system was studied taking into account the climatic conditions. Simultaneously, various passive cooling techniques are proposed and examined in order to avoid overheating during summer. For this reason, a representative case study is presented and simulated for several climatic conditions. The simulated results showed that sunspaces can be an appropriate and effective system all over Europe during the cold period of the year. Thermal mass within buildings, shading devices, buried pipes and night ventilation techniques can be essential and effective methods to control overheating during summer.     

Field thermal performance of naturally ventilated solar roof with PCM heat sink

For decades, residential and commercial roofs have been considered a prime location for installation of building integrated solar systems. In climatic conditions of East Tennessee, USA, an experimental solar roof was tested during 2009/2010, by a research team representing Metal Construction Association (MCA), and a consortium of building insulation companies, photovoltaic (PV) manufacturers, and energy research centers. The main objective was to thermally evaluate a new roofing technology utilizing amorphous silicon PV laminates integrated with the metal roof panels. In order to mitigate thermal bridging and reduce roof-generated thermal loads, this novel roof/attic assembly contained a phase change material (PCM) heat sink, a ventilated air cavity over the roof deck, and thermal insulation with an integrated reflective surface. During winter, the experimental roof was expected to work as a passive solar collector storing solar heat absorbed during the day, and increasing overall attic air temperature during the night. During summer, the PCM was expected to act as a heat sink, reducing the heat gained by the attic and consequently, lowering the building cooling-loads.In this paper, field thermal performance data of the experimental PV-PCM roof/attic system are presented and discussed. Performance of the PV-PCM roof/attic is evaluated by comparing it to a control asphalt shingle roof. The test results showed about 30% heating and 50% cooling load reductions are possible with the experimental roof configuration.     

Optimal roof structure with multilayer cooling function materials for building energy saving

Both cool roof and phase change thermal storage are promising technologies in decreasing building energy consumption. Combining these two technologies is likely to further enhance the thermal comfort of the building as well as reduce air condition loads. In this paper, the cooling performance and energy-saving effects of four types of roof (normal roof, phase change material [PCM] roof, cool roof, and cool PCM roof [cool roof coupled with PCM]) were investigated under a simulated sunlight. Experimental results indicate that compared with normal roof, the other three roofs are able to narrow the indoor temperature fluctuation and decrease the heat flow entering into the room. Among them, cool PCM roof gave the best energy-saving effect that can lower the indoor temperature and heat entering into rooms by 6.6°C and 52.9%, respectively. Besides, the PCM location, PCM thickness, and insulation thickness exerted great impacts on the cooling performance of the roof. Placing the PCM on the internal layer beneath the extruded polystyrene (XPS) insulation board can make the indoor temperature 1.2°C lower than that on the middle layer. Although thicker PCM panels or insulation boards can provide a better thermal insulation, 5 mm in PCM thickness and 20 mm in insulation thickness are enough to guarantee the indoor temperature of cool PCM roof system at a comfortable range (22°C-28°C) for a whole day. These findings will give guidance in designing buildings with a light and compact roof structure to decrease energy consumption and improve comfort level.     

Night ventilation control strategies in office buildings

In moderate climates night ventilation is an effective and energy-efficient approach to improve the indoor thermal environment for office buildings during the summer months, especially for heavyweight construction. However, is night ventilation a suitable strategy for office buildings with lightweight construction located in cold climates? In order to answer this question, the whole energy-consumption analysis software EnergyPlus was used to simulate the indoor thermal environment and energy consumption in typical office buildings with night mechanical ventilation in three cities in northern China. The summer outdoor climate data was analyzed, and three typical design days were chosen. The most important factors influencing night ventilation performance such as ventilation rates, ventilation duration, building mass and climatic conditions were evaluated. When night ventilation operation time is closer to active cooling time, the efficiency of night ventilation is higher. With night ventilation rate of 10 ach, the mean radiant temperature of the indoor surface decreased by up to 3.9 °C. The longer the duration of operation, the more efficient the night ventilation strategy becomes. The control strategies for three locations are given in the paper. Based on the optimized strategies, the operation consumption and fees are calculated. The results show that more energy is saved in office buildings cooled by a night ventilation system in northern China than ones that do not employ this strategy.     

On the efficiency of night ventilation techniques for thermostatically controlled buildings

A new, integrated method to calculate the energy contribution of night ventilation techniques to the cooling load of a building is presented in this paper. The method is based on the principle of “Balance Point Temperature” and permits the calculation of the energy required to cool a building to acceptable comfort conditions when night ventilation techniques are used. It also permits the calculation of the energy contribution of night ventilated buildings compared to conventional air conditioned buildings. The proposed method is successfully validated with data from an extended and detailed simulation procedure using the TRNSYS simulation programme to calculate dynamically the thermal performance of buildings using night ventilation techniques. It is found that the method is of sufficient accuracy and can be used during the predesign as well as the design phase of a building to access the performance of night ventilated buildings.     

Free cooling of a building using PCM heat storage integrated into the ventilation system

This article presents a study of the free cooling of a low-energy building using a latent-heat thermal energy storage (LHTES) device integrated into a mechanical ventilation system. The cylindrical LHTES device was filled with spheres of encapsulated RT20 paraffin, a phase-change material (PCM). A numerical model of the LHTES was developed to identify the parameters that have an influence on the LHTES’s thermal response, to determine the optimum phase-change temperature and to form the LHTES’s temperature-response function. The last of these defines the LHTES’s outlet-air temperature for a periodic variation of the inlet ambient-air temperature and the defined operating conditions. The temperature-response function was then integrated into the TRNSYS building thermal response model. Numerical simulations showed that a PCM with a melting temperature between 20 and 22 °C is the most suitable for free cooling in the case of a continental climate. The analyses of the temperatures in a low-energy building showed that free cooling with an LHTES is an effective cooling technique. Suitable thermal comfort conditions in the presented case-study building could be achieved using an LHTES with 6.4 kg of PCM per square metre of floor area.