- Email: [email protected]
Energy efficient PCM-based variable air volume air conditioning system for modern buildings
Energy and Buildings 42 (2010) 1353–1360
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Energy efficient PCM-based variable air volume air conditioning system for modern buildings R. Parameshwaran a , S. Harikrishnan a , S. Kalaiselvam b,∗ a b
Department of Mechanical Engineering, Anna University Chennai, Chennai, India Centre for Nanoscience and Technology, Anna University Chennai, Chennai, India
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 19 February 2010 Accepted 8 March 2010 Keywords: Energy savings potential Phase change material Thermal energy storage Variable air volume Ventilation
a b s t r a c t This work aims at achieving enhanced energy conservation for space conditioning with the application of a new combined variable air volume (VAV)-based chilled water air conditioning (A/C) system and thermal energy storage (TES) system. The phase change material (PCM) used in this system exhibited good charging and discharging characteristics that directly helped in conserving the overall energy spent on cooling and ventilation. The present system was experimentally investigated for summer and winter climatic conditions under demand controlled ventilation (DCV) and DCV combined with the economizer cycle ventilation (ECV) to substantiate its energy savings capability. Based on the results, in the DCV and combined DCV–ECV modes, this system achieved 28% and 47% of per day average energy conservative potential, respectively, while compared to the conventional chilled water-based A/C system. Similarly, the VAV–TES system yielded an on-peak total energy savings of 38% and 42%, respectively, for the same operating conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, architects, engineers and consultants related to building services engineering have shown great interest in planning, designing, developing and constructing energy efficient buildings. The role of heating, ventilation and air conditioning (HVAC) systems in such buildings is very significant in terms of providing thermal comfort and acceptable ventilation requirements for occupants without sacrificing energy efficiency. The continuous value-added engineering design and optimization of the HVAC systems being done by engineers for modern buildings, indubitably contribute to conserving the overall building and HVAC system energies as well. In order to reduce the overall building energy consumption, several energy conservative technologies especially applicable to the HVAC systems are being developed. One such technology that brings the attention of building owners, architects and design engineers is the thermal energy storage system. Thermal energy storage or cool storage technology has gained momentum in HVAC applications because of its energy efficient operation during on-peak load conditions, thereby reducing the building energy costs to the maximum possible extent. Dincer [1] dealt with the
∗ Corresponding author. Tel.: +91 4422359214; fax: +91 4422301656. E-mail addresses: [email protected] (R. Parameshwaran), [email protected] (S. Harikrishnan), [email protected] (S. Kalaiselvam). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.03.004
methods and applications of describing and assessing thermal energy storage (TES) systems in buildings. Various technical aspects and criteria for thermal energy storage systems and applications were discussed and energy saving techniques and environmental impacts of these systems were pointed out with illustrative examples. Thermal energy could be stored in the form of sensible, latent and thermo-chemical heat. Apparently, latent heat storage is mostly preferred because of higher energy storage densities, narrow operating temperature, compactness, uniform energy storage and supply. Latent heat storage is also advantageous in terms of capacity, wherein it cumulates the sensible heat matching to the temperature differential between the charge and discharge processes. In latent heat storage, the energy is stored by virtue of a substance changing its phase during melting or solidifying. Voelker et al. [2] carried out an investigation on temperature reduction in many modern buildings using PCMs. El-Dessouky and Al-Juwayhel [3] mentioned the model developed on the effectiveness of a thermal energy storage system using PCMs. Here, the storage material considered was either paraffin wax or calcium chloride hexahydrate (CaCl2 ·6H2 O), whereas the heat transfer fluid used was air or water. Banaszek et al. [4] demonstrated an experiment on the use of a vertical spiral heat exchanger in a latent heat thermal energy storage system. Butala and Stritih [5] conducted an experiment of phase change material (PCM) cold storage using paraffin with a melting point of 22 ◦ C as the PCM to store cold during nighttime and cool
1354
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time were obtained for different air velocities and inlet temperatures. The development of latent thermal energy storage systems that combine building materials with PCM has generated a great interest in the HVAC community for saving energy in buildings. On the other hand, PCMs integrated with chilled water air conditioning systems use cylindrical or spherical encapsulations to store and release energy in conjunction with the building cooling load demand. For this, the PCM encapsulations are enclosed inside a typical storage tank. Bédécarrats et al. [6] investigated the study of an industrial process of energy storage usable for air conditioning or refrigeration, investigating a test plant which is a tank of a reduced size, filled with randomly dispersed commercial nodules, placed in a refrigeration loop. Khudhair and Farid [7] described the investigation and analysis of thermal energy storage systems incorporating PCMs for use in building applications. In this paper, the problems associated with the application of PCMs with regard to the selection of materials and the methods used to contain them were also discussed. Zalba et al. [8] reviewed the history of thermal energy storage with a solid–liquid phase change. Three aspects were the focus of this review: materials, heat transfer and applications. Zhang et al. [9] described the preparation for shape-stabilized PCM and its thermophysical properties were measured. Halford and Boehm [10] revealed potential peak air conditioning load shifting strategies, using an encapsulated PCM. The materials being considered here, were installed within the ceiling or wall insulation to assist in delaying the peak air conditioning demand time until later in the evening. Vakilaltojjar and Saman [11] recommended a phase change energy storage system consisting of sections of different materials with different melting temperatures for air conditioning applications. Halawa et al. [12] analyzed the numerical analysis of the melting and freezing of a PCM thermal storage unit (TSU) with varying wall temperatures. Since the past two decades, greater importance is being given worldwide, for developing systems using renewable energy technologies, to meet the growing energy demand as well as to reduce the carbon emissions. Of these, systems functioning with solar energy sources are highly preferred for building applications that include space conditioning, domestic water heating, drying, electrical power generation, etc. Many research studies have been done on the latent thermal energy storage system blended with solar thermal technologies for air conditioning applications. Kenisarin and Mahkamov [13] conducted research in the largescale utilization of direct solar radiation, with the main focus being on the assessment of the thermal properties of various PCMs, methods of heat transfer enhancement and design configurations of heat storage facilities to be used as a part of solar passive and active space heating systems, greenhouses and solar cooking. Hammou and Lacroix [14] considered a new hybrid thermal energy storage system (HTESS) using phase change materials for managing simultaneously the storage of heat from solar and electric energy. Veerappan et al. [15] had analyzed the phase change behavior of 65 mol% capric acid and 35 mol% lauric acid, calcium chloride hexahydrate, n-octadecane, n-hexadecane, and n-eicosane inside spherical enclosures to identify a suitable heat storage material. Also, they presented analytical models for the solidification and melting of the spheres with conduction, natural convection, and heat generation. Khalifa and Abbas [16] developed a numerical model which determines the suitability of three different storage materials, namely, concrete, hydrated salt (CaCl2 ·6H2 O) and paraffin wax (N-eicosane) as thermal storage mediums for the actual weather conditions of Iraq. The carbon dioxide (CO2 ) contaminant concentration generated mainly due to the occupants inside the conditioned space
has adverse effects on the ventilation air requirements as well as the level of comfort experienced in indoor environments. Hence, the concept of demand controlled and economizer cycle ventilation schemes are suggested for buildings rather than the constant air volume (CAV) fixed ventilation principle. The DCV and ECV techniques would actually conserve energy in terms of cooling and ventilation. The HVAC systems functioning using the DCV and ECV techniques would have an improved energy performance and achieve the ventilation air requirements as well. Chowdhury et al. [17] analyzed the building thermal performances and predicted the occupants’ comfort using the predicted mean-vote (PMV) index. Low-energy cooling technologies such as chilled ceiling (CC), economizer and pre-cooling techniques have been considered for building simulation being performed for summer and winter seasons. Based on the simulation results, chilled ceiling technology seemed to offer better thermal comfort to the occupants in terms of temperature and relative humidity, and these results were validated using the measured data obtained on typical design days in both the seasonal conditions. The well-mixed model [18] is the most popular model for describing the relation between the CO2 concentration and the room, and this model can be applied to any contaminant. Lawrence and Braun [19] discussed the methodology for estimating CO2 source generation rates and flow parameters using a model to predict the return air CO2 concentrations inside a commercial building environment. They have analyzed two distinct techniques for parameter estimation using the simulated data. The two methods presented, differed by the parameters identified, but both methods predicted the CO2 concentrations well. The DCV technique implemented in commercial buildings equipped with packaged HVAC system is reported, wherein a suitable model was identified to predict the indoor CO2 levels [20]. Nassif and Moujaes [21] introduced a split-signal control strategy that gives the required outdoor air control having a minimum pressure drop in the economizer damper and conserves energy on supply and return fans. The strategy calls for two dampers to be fully open during most of the occupied period, thereby controls the outdoor air through only one damper resulting in the pressure drop in economizer dampers and reduced power consumption by both return and supply fans. Simulation was performed using a model that was developed to analyze the airflow rates and pressures in the air handling unit (AHU) as a function of the damper positions. Validation of the control strategy and the model developed was done using the annual data collected on an existing VAV system. Wang and Xu [22] developed a robust control strategy to overcome the control issues, when DCV control is combined with the economizer control. Digital controllers equipped in a building AHU were used to analyze the control strategies and they are simulated using dynamic and realistic models previously developed. The control strategies related to fixed ventilation and the DCV with the economizer ventilation mode were tested for four different weather conditions in Hong Kong. The control strategies achieved better IAQ and energy conservation under different load conditions. In the present work, a new variable air volume (VAV) chilled water-based air conditioning system combined with the TES system was experimentally investigated for summer and winter design conditions under two modes of ventilation techniques, namely, DCV and combined DCV–EC ventilation cycle. The combined effect produced by the TES system and the VAV-based ventilation techniques in both on-peak and part load conditions made the VAV–TES system to exhibit enhanced energy efficiency, compared to the conventional CAV-based chilled water A/C system. This system makes itself distinct from the conventional A/C systems as referred in the past literatures and could be considered to be beneficial for modern HVAC applications.
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1355
Fig. 1. Schematic representation of the VAV–TES air conditioning system.
2. System description and experimentation 2.1. Description of test facility Experimental work was carried out on the VAV–TES air conditioning system to investigate the operational features of the system for a year-round air conditioning application. The schematic representation of the VAV–TES system is shown in Fig. 1. A multizone variable air volume (VAV) air conditioning software/IT park building situated in Chennai was considered for the analysis. The construction and design inputs for performing the cooling load analysis are summarized in Table 1. The key equipments present in the experiment were water cooled screw chillers, primary and secondary pumping units, cooling towers, energy transfer station (ETS), thermal energy storage tank filled with PCM encapsulations, thermally insulated air conditioned space, air handling unit (AHU) equipped with a cooling coil, supply and return air distribution ductwork, VAV unit for air volume control, variable speed supply air fan and return air fan, ventilation air damper (VAD), return air damper (RAD), and exhaust air damper (EAD). The temperature, velocity, CO2 contaminant concentration, chilled water flow and water level measurement sensors were placed at the necessary locations to sense and measure the corresponding variables. The experimental set up has been fully experimented. All the sensors/transducers were interlinked to the centralized controlling unit (CCU) which actually takes care of the entire system operation under varying load conditions. The damper control unit interfaced
to the CCU actuates and controls the damper opening and closing positions effectively in the DCV and DCV–ECV modes. The variable frequency drive (VFD) pumps and motorized valves were operated through the control network offered by the CCU based on the sensor/transducer signals. The sensed signals and the corresponding
Table 1 Construction and design inputs for cooling load analysis. Input parameter
Value
Outdoor design conditions Summer DBT [◦ C] WBT [◦ C] Ventilation airflow rate (l/s/p)
40 30 10
Indoor design conditions Summer DBT [◦ C] RH [%] Winter DBT [◦ C] Internal heat gains Lighting load [W/m2 ] Small power load [W/m2 ] Occupancy Sensible [W/person] Latent [W/person] Overall heat transfer coefficient [W/m2 K] Walls and partitions Floor and ceiling Glazing
24 ± 2 50–70 22 ± 2 25 35 70 50 1.31 1.1 2.2
1356
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
Fig. 2. Variation of cooling load and ambient temperature for (a) summer and (b) winter.
control outputs are easily visualized using the display unit attached to the CCU. 2.2. Experimental methodology The objective of the present research work is to establish the inherent operational characteristics of the new VAV–TES variable air volume (VAV) chilled water-based A/C system equipped with the TES system for a year-round HVAC application based on seasonal variations. The building under investigation actually comprised of a conventional chilled water A/C system for space conditioning and the ventilation requirements were satisfied by the CAV system. In order to improve the energy efficiency, the new chilled water-based VAV–TES A/C system that operates using the DCV and combined DCV–ECV techniques were proposed for this building. The existing HVAC system in the building was operated on the selected design days in summer and winter. The experiment was performed for both summer and winter weather conditions and the outdoor temperature variations were selected according to the ASHRAE standards for Chennai, India. For the design summer and winter design days, the cooling load calculations have been done for the building based on the field measured data. The cooling load profile is shown in Fig. 2. Field measurements related to the chilled water (CHW) flow and return temperatures, chilled water flow rate, differential pressures in the chilled water and ETS loops, indoor air temperature, relative humidity (RH), ventilation and supply airflow rates, etc., were measured and recorded. Using the site measurements, the new VAV–TES A/C system was compared with the existing system for improved performance in terms of energy efficiency. For the analysis, the ventilation air requirements have been selected as per
the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 62 [23] rather than following the value as designed for the existing CAV system. The construction materials and properties were selected as recommended by the ASHRAE handbook [24]. The proposed variable flow hydronic cooling system being investigated is intended to cater to the fluctuating cooling load demand existing in the building. Basically, the flow through the hydronic circuit of the cooling system was divided in two folds: (1) a portion of the chilled water flows through the ETS and (2) a portion of the chilled water flows through the TES tank containing PCM encapsulations. The purpose of dividing the water flow was that, the cooling system would be able to offset the fluctuating cooling loads from the building through the ETS interface, as well as charge and discharge the PCM simultaneously. In order to cater to the building cooling load requirements, the chilled water from the building side was cooled to 4 ◦ C through a series of chiller combinations, which is then allowed to flow through the ETS and the cycle is repeated. Meanwhile, a portion of the chilled water at 4 ◦ C flows into the TES tank to charge the PCM. The PCM used in this experiment exhibited good characteristics to store and release energy as per the cooling load requirements. In the present system, the chilled water was supplied at 4 ◦ C to the TES tank during the charging mode. During the charging mode of the PCM, the chilled water was allowed to flow through the motorized valves 1 and 4 with the motorized valves 2 and 3 remaining closed. As the temperature of the chilled water was below the transition temperature of the PCM, the energy was stored and the PCM got solidified at a faster rate. During the discharge mode, a portion of the warm chilled water from the ETS loop was tapped; and flowed into the TES. As the temperature of the warm chilled water coming from the ETS loop was higher than the inversion temperature of the PCM, discharging of PCM occurs and the energy stored in the PCM was effectively utilized to meet the cooling load demand. In the discharge mode, the chilled water that gets cooled by the PCM was mixed with the chilled water produced by the chiller and this chilled water trapped the cooling load from the building through the ETS interface. The heat energy carried by this chilled water was transferred from the ETS circuit to the building circuit through the heat exchanger interface provided in the ETS; thereby the cooling load demand was effectively trapped out from the building. By this, the temperature of the chilled water flowing into the building side AHU cooling coil would be at 7 ◦ C, and that cools and dehumidifies the air flowing over the cooling coil to the required supply and room design conditions. The supply air temperature was maintained between 13 ◦ C and 17 ◦ C during summer and winter conditions. The room design conditions were maintained at 24 ± 2 ◦ C and 22 ± 2 ◦ C in summer and winter conditions, respectively. Moreover, the chillers were operated at the average actual capacity to meet the design day cooling load demand. By including the effect of the PCM at on-peak load conditions, the energy consumed by the chillers would be reduced substantially. For this, the chillers were operated at 90% their nominal capacity in the on-peak load mode. The rate of energy transfer occurring in the ETS was kept as a reference and using this, the amount of energy to be stored and discharged by the PCM to meet the thermal load demand was determined. It is obvious that, the charging and discharging of the PCM depends mainly on the building cooling energy demand. As stated in the earlier section, during summer, the concept of DCV was implemented, wherein the ventilation air quantity was adjusted based on the CO2 contaminant concentration generated by the occupants on an hourly basis. The opening and closing of the fresh air damper was suitably controlled with respect to the response signal received by the controller from the CO2 and outdoor air temperature sensors. Thus, the ventilation air required to dilute the
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1357
contaminant concentration was brought into the space through the ventilation air damper operation. By this the overall building cooling demand was reduced considerably. Similarly, the control strategy implemented in the supply air fan was to modulate its speed based on the pressure variation observed in the air distribution system sensed by the static pressure sensor and the supply air velocity required to maintain the indoor thermal comfort conditions. The VAV unit incorporated in the supply air duct controlled the volume flow rate of supply air for varying cooling loads and delivered the required quantity of supply air into the conditioned spaces. In winter, the concept of the combined DCV–ECV technique was implemented, since the outdoor air (OA) temperature was found to match with the indoor design requirements. During the economizer cycle, the chiller plant was operated at 40% of its nominal capacity and the fresh air damper was set at its fully open position. Complete fresh air was drawn into the conditioned space to simultaneously achieve energy saving and ventilation air requirements as well. The recirculation air damper was maintained in a fully closed position that enables all the stale air to exit from the occupied space to the atmosphere through the exhaust air damper. By executing the DCV and combined DCV–ECV techniques in the present system, the chilled water flow rate would get reduced and that would enable the chiller to consume lesser energy while compared to a conventional chiller operating under the fixed ventilation mode. 3. Results and discussion In order to exhibit the energy efficiency of the system being investigated, it was compared with the conventional chilled waterbased constant air volume (CAV) air conditioning system and the results are projected. The direct mixing of the recirculated air from the occupied spaces with the ventilation air would also result in cooling load fluctuations, since the outdoor air would be having a higher sensible and latent heat load while compared to the return air from the conditioned space. In applying the DCV and DCV with economizer cycle schemes, as the fresh air and recirculated air proportions were properly altered, the energy efficiency of the VAV–TES system could be increased significantly. 3.1. Effect of the chiller and TES system operation in summer and winter
Fig. 3. Operational characteristics of chiller and TES for (a) summer DCV mode, (b) winter DCV mode, and (c) winter DCV–ECV mode.
The operational characteristics of the chiller and TES for summer and winter design conditions are represented in Fig. 3. Based on the experimental results, it is observed that the capacity of the chiller in the direct cooling mode during on-peak conditions tends to decrease from 166 kW to 106 kW; while the VAV–TES system was operated under the DCV and combined DCV–ECV modes, respectively. This occurs because of the reduction in the total cooling load experienced by the VAV–TES system compared to that of the conventional system. As the ventilation airflow rate in the DCV and DCV–ECV modes was varied proportionately, the amount of energy spent on cooling and ventilation was reduced significantly while compared to that in the fixed ventilation mode. In summer and winter, during the on-peak conditions the load shared by the VAV–TES system also played a significant role. The nominal storage capacity of TES was effectively utilized resulting in a lesser cooling energy consumption by the chiller. Based on the test results obtained for charging and discharging the PCM, the nominal storage capacity of the VAV–TES system varied from 740 kW to 528 kW in summer and winter conditions. The response of the PCM to charge and discharge was good enough, such that the VAV–TES system bridged
the gap between the total cooling load and the cooling capacity of the chiller resulting in substantial energy savings. 3.2. On-peak power consumption of VAV–TES and conventional A/C systems The on-peak energy consumption characteristics of the conventional and VAV–TES A/C systems is presented in Fig. 4. The test result infers that the energy consumed by the conventional system was more than that by the VAV–TES system. The reason is that, the conventional system operated on a constant speed compressor, and the constant speed supply air fan arrangement delivered a critical air volume at all times for the same thermal load fluctuations observed in the building. In summer and winter, on a per day basis, the conventional system operated in the fixed ventilation mode consumed 71.6 kW and 58.5 kW of combined power (summation of compressor power and fan power), respectively. Hence, the energy savings obtained was less on both the cooling and ventilation sides for the conventional system.
1358
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
Fig. 4. Variation of power consumption and TES output for summer and winter.
3.3. Combined power consumption and coefficient of performance of the VAV–TES A/C system
Fig. 6. Variation of fan power consumption and energy savings for on-peak and part load conditions.
The variation of the combined power consumption and coefficient of performance of the VAV–TES system for summer and winter conditions is depicted in Fig. 5. The VAV–TES system operated under the DCV and combined DCV–ECV modes consumed 42.1 kW and 31 kW of combined power, respectively, on a per day basis. Similarly, during the on-peak load conditions observed in summer and winter, the conventional chilled water system consumed 98.4 kW and 80 kW of combined power under the fixed ventilation mode. The VAV–TES system actually consumed a lesser combined power of 50 kW and 46 kW in the DCV and combined DCV–ECV schemes, respectively. The combined coefficient of performance of the system in summer and winter was expected to achieve 3.9 and 4.3, respectively. This is because, by implementing the DCV and combined DCV–ECV techniques, the overall cooling load demand was considerably reduced and that results in reduced power consumption by the chillers.
consumed power ranging from 7 kW to 23 kW and 6 kW to 19 kW, respectively. Similarly, the combined power varied from 49 kW to 70 kW and 36 kW to 53 kW, respectively, for the same operating conditions. Thus, the VAV–TES A/C system conserved an average combined energy of 45% and 51% in the DCV and DCV–ECV modes, respectively, while compared to that of the conventional CAV A/C system.
3.4. Variation of fan power and combined power consumption The power consumed by the supply air fan and the refrigerant compressor from part load to on-peak load condition is represented in Figs. 6 and 7. The results suggest that, as the cooling load varies from part load to on-peak condition, the volume flow rate of air supplied by the fan increases from 335 cmm to 1085 cmm in summer and 285 cmm to 925 cmm in winter, respectively. The supply air fan equipped with VAV control varies the airflow rate according to the cooling load fluctuations; thereby the power consumed by the fan also varied considerably. In summer and winter, using the DCV and DCV combined with ECV modes, the supply air fan
Fig. 5. Variation of combined power consumption and coefficient of performance of VAV–TES system for summer and winter.
3.5. Energy conservative potential In summer and winter conditions, the VAV–TES system operated under the DCV and combined DCV–ECV modes achieved 28% and 47% of per day average energy conservative potential. Likewise, the system yielded an on-peak total energy savings of 38% and 42%, respectively, for the same operating conditions. The energy conserved by the VAV–TES system under the DCV–EC was substantial, since the free cooling option enabled the system to meet both cooling and ventilation strategies, especially under the part load conditions of the system. Thus, it could be seen that, the VAV–TES system equipped with TES, collectively produced enhanced energy savings in the DCV and combined DCV–ECV schemes. The major results from a variety of reviewed papers are summarized in Table 2. Most studies have described the significant effects of the PCMs located in building fabrics (passive systems) in terms of the building thermal and energy performance pertaining to the space conditioning applications. The concept of the free cooling principle was also investigated in these studies in order to reduce
Fig. 7. Variation of combined power consumption and energy savings for on-peak and part load conditions.
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1359
Table 2 Summary of major results from various research studies. S. no.
References
PCMs considered
PCMs location
Objectives of the study
Major results
Methodology
1
Weinläder et al. [25]
RT25 (12 mm) and S27 (8.6 mm)
PCMs located in transparent plastic containers placed behind a double glazing with an air gap of 10 mm
Investigated on a south facade panel in Würzburg, Germany
Experiment and simulation
2
Ismail and Castro [26]
Mixture of commercial Glycol wax
Incorporated in walls and roofs
An existing building in Campinas, SP, Brazil was considered
3
Zhang et al. [27]
Highly crystalline, paraffin-based PCM
PCM frame walls
A full instrumented test house of 1.83 m × 1.83 m × 1.22 m in Lawrence, KS, USA
4
Nagano et al. [28]
PCM composed of foamed glass beads and paraffin waxes
PCM was embedded directly below OA floor boards in the form of granules
A small experimental system with a floor area of 0.5 m2 was investigated
5
Kissock and Limas [29]
K18 with an average melting temperature of 25.6 ◦ C
Integrated in walls and roofs
6
Halford and Boehm [10]
Salt type PCM that is held in stasis by a perlite matrix
Between two layers of insulation in a configuration known as resistive, capacitive, resistive
Study includes concrete sandwich walls, low-mass steel walls under the typical meteorological weather data in Dayton, Ohio A geometry in which the wall/ceiling structure was assumed as a three-layer plane wall having the PCM in the center layer
7
Medina et al. [30]
Paraffin-based PCM
PCM is in thin-walled copper pipes and inserted into horizontal slotscut into the polystyrene foam
Fully instrumented test house of 1.83 m × 1.83 m × 1.22 m was analyzed
8
Stetiu and Feustel [31]
Paraffin-based PCM
The phase change wallboard containing 20% by paraffin mass
A prototype IEA building located in California climate condition was selected
25% of energy gains can be reduced in summer; likewise 30% heat losses and 50% solar heat gains can be reduced in winter Save 19% and 31% energy for cases using window and central AC units The space cooling load and the average wall peak heat flux were found to be reduced about 8.6% and 15%, respectively 89% daily cooling load can be stored in night using a 30 mm thick packed bed of the granular PCM The peak loads can be reduced by 19%, 30%, and 16% in concrete sandwich walls, steel roofs and gypsum wallboards, respectively The results suggest that 19–57% of maximum reduction in peak load as compared to a purely resistive R-19 wall can be achieved The peak heat flux can be reduced by 37% and 62% using a PCMSIP with 10% and 20% PCM concentrations 28% of the peak cooling load was expected to be reduced
the overall building energy consumption. On comparing these studies, the new active VAV–TES A/C system demonstrated in this paper has effectively applied the concept of the potential DCV scheme in conjunction with the ECV technique that in total has contributed towards improved building energy efficiency. 4. Conclusions In the present work, the chilled water A/C system combined with the TES system has been analyzed and tested for summer and winter conditions under the DCV and combined DCV–ECV schemes. The experimental investigations have led to the following conclusions: • The VAV–TES A/C system conserved much energy during the combined DCV–ECV modes without compromising the thermal comfort and ventilation requirements. The implementation of the ECV technique enabled the ambient air to be delivered into the conditioned space to satisfy the cooling and ventilation requirements at part load conditions. The role of PCM-based TES system was significant in terms of cooling capacities shared by the TES during on-peak conditions that collectively contributed to achieving enhanced energy savings. • The test results infer that, the VAV–TES A/C system operated using the DCV and DCV–ECV modes yielded 45% and 51% of average combined energy savings potential in part load to onpeak load conditions. Furthermore, by implementing the DCV and
Simulation and experiment
Experiment
Experiment
Simulation
Simulation
Experiment
Simulation
ECV techniques, the energy spent on cooling and ventilation was reduced; thereby, this system exhibited a combined coefficient of performance of 3.9 and 4.3 in summer and winter conditions, respectively. • For varying occupancy levels, the test results project that the system was capable of addressing the ventilation requirements better in summer and winter design conditions. Activating the ventilation air damper, return air damper and exhaust air damper properly under both the DCV and combined DCV–ECV modes helped the VAV–TES system to deliver the required ventilation air volume into the occupied space based on the CO2 levels detected in the conditioned space. • On comparing the energy savings of the VAV–TES A/C system with that of the conventional chilled water-based A/C system, it was found that the former achieved 28% and 47% of per day average energy conservative potential under the DCV and combined DCV–ECV modes, respectively. Likewise, the VAV–TES system yielded an on-peak total energy savings of 28%, 38% and 42%, respectively, for the same operating conditions.
Acknowledgements The authors would like to acknowledge and thank the CPDE, Anna University, for providing excellent support to this research project.
1360
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
References [1] I. Dincer, On thermal energy storage systems and applications in buildings, Energy and Buildings 34 (4) (2002) 377–388. [2] C. Voelker, O. Kornadt, M. Ostry, Temperature reduction due to the application of phase change materials, Energy and Buildings 40 (5) (2008) 937–944. [3] H. El-Dessouky, F. Al-Juwayhel, Effectiveness of a thermal energy storage system using phase-change materials, Energy Conversion and Management 38 (6) (1997) 601–617. ˜ [4] J. Banaszek, R. Domanski, M. Rebow, F. El-Sagier, Experimental study of solid–liquid phase change in a spiral thermal energy storage unit, Applied Thermal Engineering 19 (12) (1999) 1253–1277. [5] V. Butala, U. Stritih, Experimental investigation of PCM cold storage, Energy and Buildings 41 (3) (2009) 354–359. [6] J.P. Bédécarrats, F. Strub, B. Falcon, J.P. Dumas, Phase-change thermal energy storage using spherical capsules: performance of a test plant, International Journal of Refrigeration 19 (3) (1996) 187–196. [7] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conversion and Management 45 (2) (2004) 263–275. [8] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (3) (2003) 251–283. [9] Y.P. Zhang, K.P. Lin, R. Yang, H.F. Di, Y. Jiang, Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings, Energy and Buildings 38 (10) (2006) 1262–1269. [10] C.K. Halford, R.F. Boehm, Modeling of phase change material peak load shifting, Energy and Buildings 39 (3) (2007) 298–305. [11] S.M. Vakilaltojjar, W. Saman, Analysis and modelling of a phase change storage system for air conditioning applications, Applied Thermal Engineering 21 (3, 1) (2001) 249–263. [12] E. Halawa, F. Bruno, W. Saman, Numerical analysis of a PCM thermal storage system with varying wall temperature, Energy Conversion and Management 46 (15–16) (2005) 2592–2604. [13] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy Reviews 11 (9) (2007) 1913–1965. [14] Z.A. Hammou, M. Lacroix, A new PCM storage system for managing simultaneously solar and electric energy, Energy and Buildings 38 (3) (2006) 258–265. [15] M. Veerappan, S. Kalaiselvam, S. Iniyan, R. Goic, Phase change characteristic study of spherical PCMs in solar energy storage, Solar Energy 83 (8) (2009) 1245–1252.
[16] A.J.N. Khalifa, E.F. Abbas, A comparative performance study of some thermal storage materials used for solar space heating, Energy and Buildings 41 (4) (2009) 407–415. [17] A.A. Chowdhury, M.G. Rasul, M.M.K. Khan, Thermal-comfort analysis and simulation for various low-energy cooling-technologies applied to an office building in a subtropical climate, Applied Energy 85 (6) (2008) 449–462. [18] Y.-P. Ke, S.A. Mumma, Using carbon dioxide measurements to determine occupancy for ventilation controls, ASHRAE Transactions (1997), BN-97-1-1. [19] T.M. Lawrence, J.E. Braun, A methodology for estimating occupant CO2 source generation rates from measurements in small commercial buildings, Building and Environment 42 (2) (2007) 623–639. [20] T.M. Lawrence, J.E. Braun, Evaluation of simplified models for predicting CO2 concentrations in small commercial buildings, Building and Environment 41 (2) (2006) 184–194. [21] N. Nassif, S. Moujaes, A new operating strategy for economizer dampers of VAV system, Energy and Buildings 40 (3) (2008) 289–299. [22] S. Wang, X. Xu, Optimal and robust control of outdoor ventilation airflow rate for improving energy efficiency and IAQ, Building and Environment 39 (7) (2004) 763–773. [23] ASHRAE, ANSI/ASHRAE Standard 62, 2007. [24] ASHRAE, ASHRAE Handbook Fundamentals, ASHRAE Inc., Atlanta, GA, 1990. [25] H. Weinläder, A. Beck, J. Fricke, PCM-facade-panel for daylighting and room heating, Solar Energy 78 (2) (2005) 177–186. [26] K.A.R. Ismail, J.N.C. Castro, PCM thermal insulation in buildings, International Journal of Energy Research 21 (14) (1997) 1281–1296. [27] M. Zhang, M.A. Medina, J.B. King, Development of a thermally enhanced frame wall with phase-change materials for on-peak air conditioning demand reduction and energy savings in residential buildings, International Journal of Energy Research 29 (9) (2005) 795–809. [28] K. Nagano, S. Takeda, T. Mochida, K. Shimakura, T. Nakamura, Study of a floor supply air conditioning system using granular phase change material to augment building mass thermal storage-heat response in small scale experiments, Energy and Buildings 38 (5) (2006) 436–446. [29] J.K. Kissock, S. Limas, Diurnal load reduction through phase-change building components, ASHRAE Transactions 112 (1) (2006) 509–517. [30] M.A. Medina, J.B. King, M. Zhang, On the heat transfer rate reduction of structural insulated panels (SIPs) outfitted with phase change materials (PCMs), Energy 33 (4) (2008) 667–678. [31] C. Stetiu, H.E. Feustel, Phase-change Wallboard and Mechanical Night Ventilation in Commercial Buildings, Lawrence Berkeley National Laboratory, University of California, 1998, http://epb.lbl.gov/thermal/docs/pcm2.pdf.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Energy efficient PCM-based variable air volume air conditioning system for modern buildings R. Parameshwaran a , S. Harikrishnan a , S. Kalaiselvam b,∗ a b
Department of Mechanical Engineering, Anna University Chennai, Chennai, India Centre for Nanoscience and Technology, Anna University Chennai, Chennai, India
a r t i c l e
i n f o
Article history: Received 28 October 2009 Received in revised form 19 February 2010 Accepted 8 March 2010 Keywords: Energy savings potential Phase change material Thermal energy storage Variable air volume Ventilation
a b s t r a c t This work aims at achieving enhanced energy conservation for space conditioning with the application of a new combined variable air volume (VAV)-based chilled water air conditioning (A/C) system and thermal energy storage (TES) system. The phase change material (PCM) used in this system exhibited good charging and discharging characteristics that directly helped in conserving the overall energy spent on cooling and ventilation. The present system was experimentally investigated for summer and winter climatic conditions under demand controlled ventilation (DCV) and DCV combined with the economizer cycle ventilation (ECV) to substantiate its energy savings capability. Based on the results, in the DCV and combined DCV–ECV modes, this system achieved 28% and 47% of per day average energy conservative potential, respectively, while compared to the conventional chilled water-based A/C system. Similarly, the VAV–TES system yielded an on-peak total energy savings of 38% and 42%, respectively, for the same operating conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, architects, engineers and consultants related to building services engineering have shown great interest in planning, designing, developing and constructing energy efficient buildings. The role of heating, ventilation and air conditioning (HVAC) systems in such buildings is very significant in terms of providing thermal comfort and acceptable ventilation requirements for occupants without sacrificing energy efficiency. The continuous value-added engineering design and optimization of the HVAC systems being done by engineers for modern buildings, indubitably contribute to conserving the overall building and HVAC system energies as well. In order to reduce the overall building energy consumption, several energy conservative technologies especially applicable to the HVAC systems are being developed. One such technology that brings the attention of building owners, architects and design engineers is the thermal energy storage system. Thermal energy storage or cool storage technology has gained momentum in HVAC applications because of its energy efficient operation during on-peak load conditions, thereby reducing the building energy costs to the maximum possible extent. Dincer [1] dealt with the
∗ Corresponding author. Tel.: +91 4422359214; fax: +91 4422301656. E-mail addresses: [email protected] (R. Parameshwaran), [email protected] (S. Harikrishnan), [email protected] (S. Kalaiselvam). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.03.004
methods and applications of describing and assessing thermal energy storage (TES) systems in buildings. Various technical aspects and criteria for thermal energy storage systems and applications were discussed and energy saving techniques and environmental impacts of these systems were pointed out with illustrative examples. Thermal energy could be stored in the form of sensible, latent and thermo-chemical heat. Apparently, latent heat storage is mostly preferred because of higher energy storage densities, narrow operating temperature, compactness, uniform energy storage and supply. Latent heat storage is also advantageous in terms of capacity, wherein it cumulates the sensible heat matching to the temperature differential between the charge and discharge processes. In latent heat storage, the energy is stored by virtue of a substance changing its phase during melting or solidifying. Voelker et al. [2] carried out an investigation on temperature reduction in many modern buildings using PCMs. El-Dessouky and Al-Juwayhel [3] mentioned the model developed on the effectiveness of a thermal energy storage system using PCMs. Here, the storage material considered was either paraffin wax or calcium chloride hexahydrate (CaCl2 ·6H2 O), whereas the heat transfer fluid used was air or water. Banaszek et al. [4] demonstrated an experiment on the use of a vertical spiral heat exchanger in a latent heat thermal energy storage system. Butala and Stritih [5] conducted an experiment of phase change material (PCM) cold storage using paraffin with a melting point of 22 ◦ C as the PCM to store cold during nighttime and cool
1354
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time were obtained for different air velocities and inlet temperatures. The development of latent thermal energy storage systems that combine building materials with PCM has generated a great interest in the HVAC community for saving energy in buildings. On the other hand, PCMs integrated with chilled water air conditioning systems use cylindrical or spherical encapsulations to store and release energy in conjunction with the building cooling load demand. For this, the PCM encapsulations are enclosed inside a typical storage tank. Bédécarrats et al. [6] investigated the study of an industrial process of energy storage usable for air conditioning or refrigeration, investigating a test plant which is a tank of a reduced size, filled with randomly dispersed commercial nodules, placed in a refrigeration loop. Khudhair and Farid [7] described the investigation and analysis of thermal energy storage systems incorporating PCMs for use in building applications. In this paper, the problems associated with the application of PCMs with regard to the selection of materials and the methods used to contain them were also discussed. Zalba et al. [8] reviewed the history of thermal energy storage with a solid–liquid phase change. Three aspects were the focus of this review: materials, heat transfer and applications. Zhang et al. [9] described the preparation for shape-stabilized PCM and its thermophysical properties were measured. Halford and Boehm [10] revealed potential peak air conditioning load shifting strategies, using an encapsulated PCM. The materials being considered here, were installed within the ceiling or wall insulation to assist in delaying the peak air conditioning demand time until later in the evening. Vakilaltojjar and Saman [11] recommended a phase change energy storage system consisting of sections of different materials with different melting temperatures for air conditioning applications. Halawa et al. [12] analyzed the numerical analysis of the melting and freezing of a PCM thermal storage unit (TSU) with varying wall temperatures. Since the past two decades, greater importance is being given worldwide, for developing systems using renewable energy technologies, to meet the growing energy demand as well as to reduce the carbon emissions. Of these, systems functioning with solar energy sources are highly preferred for building applications that include space conditioning, domestic water heating, drying, electrical power generation, etc. Many research studies have been done on the latent thermal energy storage system blended with solar thermal technologies for air conditioning applications. Kenisarin and Mahkamov [13] conducted research in the largescale utilization of direct solar radiation, with the main focus being on the assessment of the thermal properties of various PCMs, methods of heat transfer enhancement and design configurations of heat storage facilities to be used as a part of solar passive and active space heating systems, greenhouses and solar cooking. Hammou and Lacroix [14] considered a new hybrid thermal energy storage system (HTESS) using phase change materials for managing simultaneously the storage of heat from solar and electric energy. Veerappan et al. [15] had analyzed the phase change behavior of 65 mol% capric acid and 35 mol% lauric acid, calcium chloride hexahydrate, n-octadecane, n-hexadecane, and n-eicosane inside spherical enclosures to identify a suitable heat storage material. Also, they presented analytical models for the solidification and melting of the spheres with conduction, natural convection, and heat generation. Khalifa and Abbas [16] developed a numerical model which determines the suitability of three different storage materials, namely, concrete, hydrated salt (CaCl2 ·6H2 O) and paraffin wax (N-eicosane) as thermal storage mediums for the actual weather conditions of Iraq. The carbon dioxide (CO2 ) contaminant concentration generated mainly due to the occupants inside the conditioned space
has adverse effects on the ventilation air requirements as well as the level of comfort experienced in indoor environments. Hence, the concept of demand controlled and economizer cycle ventilation schemes are suggested for buildings rather than the constant air volume (CAV) fixed ventilation principle. The DCV and ECV techniques would actually conserve energy in terms of cooling and ventilation. The HVAC systems functioning using the DCV and ECV techniques would have an improved energy performance and achieve the ventilation air requirements as well. Chowdhury et al. [17] analyzed the building thermal performances and predicted the occupants’ comfort using the predicted mean-vote (PMV) index. Low-energy cooling technologies such as chilled ceiling (CC), economizer and pre-cooling techniques have been considered for building simulation being performed for summer and winter seasons. Based on the simulation results, chilled ceiling technology seemed to offer better thermal comfort to the occupants in terms of temperature and relative humidity, and these results were validated using the measured data obtained on typical design days in both the seasonal conditions. The well-mixed model [18] is the most popular model for describing the relation between the CO2 concentration and the room, and this model can be applied to any contaminant. Lawrence and Braun [19] discussed the methodology for estimating CO2 source generation rates and flow parameters using a model to predict the return air CO2 concentrations inside a commercial building environment. They have analyzed two distinct techniques for parameter estimation using the simulated data. The two methods presented, differed by the parameters identified, but both methods predicted the CO2 concentrations well. The DCV technique implemented in commercial buildings equipped with packaged HVAC system is reported, wherein a suitable model was identified to predict the indoor CO2 levels [20]. Nassif and Moujaes [21] introduced a split-signal control strategy that gives the required outdoor air control having a minimum pressure drop in the economizer damper and conserves energy on supply and return fans. The strategy calls for two dampers to be fully open during most of the occupied period, thereby controls the outdoor air through only one damper resulting in the pressure drop in economizer dampers and reduced power consumption by both return and supply fans. Simulation was performed using a model that was developed to analyze the airflow rates and pressures in the air handling unit (AHU) as a function of the damper positions. Validation of the control strategy and the model developed was done using the annual data collected on an existing VAV system. Wang and Xu [22] developed a robust control strategy to overcome the control issues, when DCV control is combined with the economizer control. Digital controllers equipped in a building AHU were used to analyze the control strategies and they are simulated using dynamic and realistic models previously developed. The control strategies related to fixed ventilation and the DCV with the economizer ventilation mode were tested for four different weather conditions in Hong Kong. The control strategies achieved better IAQ and energy conservation under different load conditions. In the present work, a new variable air volume (VAV) chilled water-based air conditioning system combined with the TES system was experimentally investigated for summer and winter design conditions under two modes of ventilation techniques, namely, DCV and combined DCV–EC ventilation cycle. The combined effect produced by the TES system and the VAV-based ventilation techniques in both on-peak and part load conditions made the VAV–TES system to exhibit enhanced energy efficiency, compared to the conventional CAV-based chilled water A/C system. This system makes itself distinct from the conventional A/C systems as referred in the past literatures and could be considered to be beneficial for modern HVAC applications.
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1355
Fig. 1. Schematic representation of the VAV–TES air conditioning system.
2. System description and experimentation 2.1. Description of test facility Experimental work was carried out on the VAV–TES air conditioning system to investigate the operational features of the system for a year-round air conditioning application. The schematic representation of the VAV–TES system is shown in Fig. 1. A multizone variable air volume (VAV) air conditioning software/IT park building situated in Chennai was considered for the analysis. The construction and design inputs for performing the cooling load analysis are summarized in Table 1. The key equipments present in the experiment were water cooled screw chillers, primary and secondary pumping units, cooling towers, energy transfer station (ETS), thermal energy storage tank filled with PCM encapsulations, thermally insulated air conditioned space, air handling unit (AHU) equipped with a cooling coil, supply and return air distribution ductwork, VAV unit for air volume control, variable speed supply air fan and return air fan, ventilation air damper (VAD), return air damper (RAD), and exhaust air damper (EAD). The temperature, velocity, CO2 contaminant concentration, chilled water flow and water level measurement sensors were placed at the necessary locations to sense and measure the corresponding variables. The experimental set up has been fully experimented. All the sensors/transducers were interlinked to the centralized controlling unit (CCU) which actually takes care of the entire system operation under varying load conditions. The damper control unit interfaced
to the CCU actuates and controls the damper opening and closing positions effectively in the DCV and DCV–ECV modes. The variable frequency drive (VFD) pumps and motorized valves were operated through the control network offered by the CCU based on the sensor/transducer signals. The sensed signals and the corresponding
Table 1 Construction and design inputs for cooling load analysis. Input parameter
Value
Outdoor design conditions Summer DBT [◦ C] WBT [◦ C] Ventilation airflow rate (l/s/p)
40 30 10
Indoor design conditions Summer DBT [◦ C] RH [%] Winter DBT [◦ C] Internal heat gains Lighting load [W/m2 ] Small power load [W/m2 ] Occupancy Sensible [W/person] Latent [W/person] Overall heat transfer coefficient [W/m2 K] Walls and partitions Floor and ceiling Glazing
24 ± 2 50–70 22 ± 2 25 35 70 50 1.31 1.1 2.2
1356
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
Fig. 2. Variation of cooling load and ambient temperature for (a) summer and (b) winter.
control outputs are easily visualized using the display unit attached to the CCU. 2.2. Experimental methodology The objective of the present research work is to establish the inherent operational characteristics of the new VAV–TES variable air volume (VAV) chilled water-based A/C system equipped with the TES system for a year-round HVAC application based on seasonal variations. The building under investigation actually comprised of a conventional chilled water A/C system for space conditioning and the ventilation requirements were satisfied by the CAV system. In order to improve the energy efficiency, the new chilled water-based VAV–TES A/C system that operates using the DCV and combined DCV–ECV techniques were proposed for this building. The existing HVAC system in the building was operated on the selected design days in summer and winter. The experiment was performed for both summer and winter weather conditions and the outdoor temperature variations were selected according to the ASHRAE standards for Chennai, India. For the design summer and winter design days, the cooling load calculations have been done for the building based on the field measured data. The cooling load profile is shown in Fig. 2. Field measurements related to the chilled water (CHW) flow and return temperatures, chilled water flow rate, differential pressures in the chilled water and ETS loops, indoor air temperature, relative humidity (RH), ventilation and supply airflow rates, etc., were measured and recorded. Using the site measurements, the new VAV–TES A/C system was compared with the existing system for improved performance in terms of energy efficiency. For the analysis, the ventilation air requirements have been selected as per
the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 62 [23] rather than following the value as designed for the existing CAV system. The construction materials and properties were selected as recommended by the ASHRAE handbook [24]. The proposed variable flow hydronic cooling system being investigated is intended to cater to the fluctuating cooling load demand existing in the building. Basically, the flow through the hydronic circuit of the cooling system was divided in two folds: (1) a portion of the chilled water flows through the ETS and (2) a portion of the chilled water flows through the TES tank containing PCM encapsulations. The purpose of dividing the water flow was that, the cooling system would be able to offset the fluctuating cooling loads from the building through the ETS interface, as well as charge and discharge the PCM simultaneously. In order to cater to the building cooling load requirements, the chilled water from the building side was cooled to 4 ◦ C through a series of chiller combinations, which is then allowed to flow through the ETS and the cycle is repeated. Meanwhile, a portion of the chilled water at 4 ◦ C flows into the TES tank to charge the PCM. The PCM used in this experiment exhibited good characteristics to store and release energy as per the cooling load requirements. In the present system, the chilled water was supplied at 4 ◦ C to the TES tank during the charging mode. During the charging mode of the PCM, the chilled water was allowed to flow through the motorized valves 1 and 4 with the motorized valves 2 and 3 remaining closed. As the temperature of the chilled water was below the transition temperature of the PCM, the energy was stored and the PCM got solidified at a faster rate. During the discharge mode, a portion of the warm chilled water from the ETS loop was tapped; and flowed into the TES. As the temperature of the warm chilled water coming from the ETS loop was higher than the inversion temperature of the PCM, discharging of PCM occurs and the energy stored in the PCM was effectively utilized to meet the cooling load demand. In the discharge mode, the chilled water that gets cooled by the PCM was mixed with the chilled water produced by the chiller and this chilled water trapped the cooling load from the building through the ETS interface. The heat energy carried by this chilled water was transferred from the ETS circuit to the building circuit through the heat exchanger interface provided in the ETS; thereby the cooling load demand was effectively trapped out from the building. By this, the temperature of the chilled water flowing into the building side AHU cooling coil would be at 7 ◦ C, and that cools and dehumidifies the air flowing over the cooling coil to the required supply and room design conditions. The supply air temperature was maintained between 13 ◦ C and 17 ◦ C during summer and winter conditions. The room design conditions were maintained at 24 ± 2 ◦ C and 22 ± 2 ◦ C in summer and winter conditions, respectively. Moreover, the chillers were operated at the average actual capacity to meet the design day cooling load demand. By including the effect of the PCM at on-peak load conditions, the energy consumed by the chillers would be reduced substantially. For this, the chillers were operated at 90% their nominal capacity in the on-peak load mode. The rate of energy transfer occurring in the ETS was kept as a reference and using this, the amount of energy to be stored and discharged by the PCM to meet the thermal load demand was determined. It is obvious that, the charging and discharging of the PCM depends mainly on the building cooling energy demand. As stated in the earlier section, during summer, the concept of DCV was implemented, wherein the ventilation air quantity was adjusted based on the CO2 contaminant concentration generated by the occupants on an hourly basis. The opening and closing of the fresh air damper was suitably controlled with respect to the response signal received by the controller from the CO2 and outdoor air temperature sensors. Thus, the ventilation air required to dilute the
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1357
contaminant concentration was brought into the space through the ventilation air damper operation. By this the overall building cooling demand was reduced considerably. Similarly, the control strategy implemented in the supply air fan was to modulate its speed based on the pressure variation observed in the air distribution system sensed by the static pressure sensor and the supply air velocity required to maintain the indoor thermal comfort conditions. The VAV unit incorporated in the supply air duct controlled the volume flow rate of supply air for varying cooling loads and delivered the required quantity of supply air into the conditioned spaces. In winter, the concept of the combined DCV–ECV technique was implemented, since the outdoor air (OA) temperature was found to match with the indoor design requirements. During the economizer cycle, the chiller plant was operated at 40% of its nominal capacity and the fresh air damper was set at its fully open position. Complete fresh air was drawn into the conditioned space to simultaneously achieve energy saving and ventilation air requirements as well. The recirculation air damper was maintained in a fully closed position that enables all the stale air to exit from the occupied space to the atmosphere through the exhaust air damper. By executing the DCV and combined DCV–ECV techniques in the present system, the chilled water flow rate would get reduced and that would enable the chiller to consume lesser energy while compared to a conventional chiller operating under the fixed ventilation mode. 3. Results and discussion In order to exhibit the energy efficiency of the system being investigated, it was compared with the conventional chilled waterbased constant air volume (CAV) air conditioning system and the results are projected. The direct mixing of the recirculated air from the occupied spaces with the ventilation air would also result in cooling load fluctuations, since the outdoor air would be having a higher sensible and latent heat load while compared to the return air from the conditioned space. In applying the DCV and DCV with economizer cycle schemes, as the fresh air and recirculated air proportions were properly altered, the energy efficiency of the VAV–TES system could be increased significantly. 3.1. Effect of the chiller and TES system operation in summer and winter
Fig. 3. Operational characteristics of chiller and TES for (a) summer DCV mode, (b) winter DCV mode, and (c) winter DCV–ECV mode.
The operational characteristics of the chiller and TES for summer and winter design conditions are represented in Fig. 3. Based on the experimental results, it is observed that the capacity of the chiller in the direct cooling mode during on-peak conditions tends to decrease from 166 kW to 106 kW; while the VAV–TES system was operated under the DCV and combined DCV–ECV modes, respectively. This occurs because of the reduction in the total cooling load experienced by the VAV–TES system compared to that of the conventional system. As the ventilation airflow rate in the DCV and DCV–ECV modes was varied proportionately, the amount of energy spent on cooling and ventilation was reduced significantly while compared to that in the fixed ventilation mode. In summer and winter, during the on-peak conditions the load shared by the VAV–TES system also played a significant role. The nominal storage capacity of TES was effectively utilized resulting in a lesser cooling energy consumption by the chiller. Based on the test results obtained for charging and discharging the PCM, the nominal storage capacity of the VAV–TES system varied from 740 kW to 528 kW in summer and winter conditions. The response of the PCM to charge and discharge was good enough, such that the VAV–TES system bridged
the gap between the total cooling load and the cooling capacity of the chiller resulting in substantial energy savings. 3.2. On-peak power consumption of VAV–TES and conventional A/C systems The on-peak energy consumption characteristics of the conventional and VAV–TES A/C systems is presented in Fig. 4. The test result infers that the energy consumed by the conventional system was more than that by the VAV–TES system. The reason is that, the conventional system operated on a constant speed compressor, and the constant speed supply air fan arrangement delivered a critical air volume at all times for the same thermal load fluctuations observed in the building. In summer and winter, on a per day basis, the conventional system operated in the fixed ventilation mode consumed 71.6 kW and 58.5 kW of combined power (summation of compressor power and fan power), respectively. Hence, the energy savings obtained was less on both the cooling and ventilation sides for the conventional system.
1358
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
Fig. 4. Variation of power consumption and TES output for summer and winter.
3.3. Combined power consumption and coefficient of performance of the VAV–TES A/C system
Fig. 6. Variation of fan power consumption and energy savings for on-peak and part load conditions.
The variation of the combined power consumption and coefficient of performance of the VAV–TES system for summer and winter conditions is depicted in Fig. 5. The VAV–TES system operated under the DCV and combined DCV–ECV modes consumed 42.1 kW and 31 kW of combined power, respectively, on a per day basis. Similarly, during the on-peak load conditions observed in summer and winter, the conventional chilled water system consumed 98.4 kW and 80 kW of combined power under the fixed ventilation mode. The VAV–TES system actually consumed a lesser combined power of 50 kW and 46 kW in the DCV and combined DCV–ECV schemes, respectively. The combined coefficient of performance of the system in summer and winter was expected to achieve 3.9 and 4.3, respectively. This is because, by implementing the DCV and combined DCV–ECV techniques, the overall cooling load demand was considerably reduced and that results in reduced power consumption by the chillers.
consumed power ranging from 7 kW to 23 kW and 6 kW to 19 kW, respectively. Similarly, the combined power varied from 49 kW to 70 kW and 36 kW to 53 kW, respectively, for the same operating conditions. Thus, the VAV–TES A/C system conserved an average combined energy of 45% and 51% in the DCV and DCV–ECV modes, respectively, while compared to that of the conventional CAV A/C system.
3.4. Variation of fan power and combined power consumption The power consumed by the supply air fan and the refrigerant compressor from part load to on-peak load condition is represented in Figs. 6 and 7. The results suggest that, as the cooling load varies from part load to on-peak condition, the volume flow rate of air supplied by the fan increases from 335 cmm to 1085 cmm in summer and 285 cmm to 925 cmm in winter, respectively. The supply air fan equipped with VAV control varies the airflow rate according to the cooling load fluctuations; thereby the power consumed by the fan also varied considerably. In summer and winter, using the DCV and DCV combined with ECV modes, the supply air fan
Fig. 5. Variation of combined power consumption and coefficient of performance of VAV–TES system for summer and winter.
3.5. Energy conservative potential In summer and winter conditions, the VAV–TES system operated under the DCV and combined DCV–ECV modes achieved 28% and 47% of per day average energy conservative potential. Likewise, the system yielded an on-peak total energy savings of 38% and 42%, respectively, for the same operating conditions. The energy conserved by the VAV–TES system under the DCV–EC was substantial, since the free cooling option enabled the system to meet both cooling and ventilation strategies, especially under the part load conditions of the system. Thus, it could be seen that, the VAV–TES system equipped with TES, collectively produced enhanced energy savings in the DCV and combined DCV–ECV schemes. The major results from a variety of reviewed papers are summarized in Table 2. Most studies have described the significant effects of the PCMs located in building fabrics (passive systems) in terms of the building thermal and energy performance pertaining to the space conditioning applications. The concept of the free cooling principle was also investigated in these studies in order to reduce
Fig. 7. Variation of combined power consumption and energy savings for on-peak and part load conditions.
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
1359
Table 2 Summary of major results from various research studies. S. no.
References
PCMs considered
PCMs location
Objectives of the study
Major results
Methodology
1
Weinläder et al. [25]
RT25 (12 mm) and S27 (8.6 mm)
PCMs located in transparent plastic containers placed behind a double glazing with an air gap of 10 mm
Investigated on a south facade panel in Würzburg, Germany
Experiment and simulation
2
Ismail and Castro [26]
Mixture of commercial Glycol wax
Incorporated in walls and roofs
An existing building in Campinas, SP, Brazil was considered
3
Zhang et al. [27]
Highly crystalline, paraffin-based PCM
PCM frame walls
A full instrumented test house of 1.83 m × 1.83 m × 1.22 m in Lawrence, KS, USA
4
Nagano et al. [28]
PCM composed of foamed glass beads and paraffin waxes
PCM was embedded directly below OA floor boards in the form of granules
A small experimental system with a floor area of 0.5 m2 was investigated
5
Kissock and Limas [29]
K18 with an average melting temperature of 25.6 ◦ C
Integrated in walls and roofs
6
Halford and Boehm [10]
Salt type PCM that is held in stasis by a perlite matrix
Between two layers of insulation in a configuration known as resistive, capacitive, resistive
Study includes concrete sandwich walls, low-mass steel walls under the typical meteorological weather data in Dayton, Ohio A geometry in which the wall/ceiling structure was assumed as a three-layer plane wall having the PCM in the center layer
7
Medina et al. [30]
Paraffin-based PCM
PCM is in thin-walled copper pipes and inserted into horizontal slotscut into the polystyrene foam
Fully instrumented test house of 1.83 m × 1.83 m × 1.22 m was analyzed
8
Stetiu and Feustel [31]
Paraffin-based PCM
The phase change wallboard containing 20% by paraffin mass
A prototype IEA building located in California climate condition was selected
25% of energy gains can be reduced in summer; likewise 30% heat losses and 50% solar heat gains can be reduced in winter Save 19% and 31% energy for cases using window and central AC units The space cooling load and the average wall peak heat flux were found to be reduced about 8.6% and 15%, respectively 89% daily cooling load can be stored in night using a 30 mm thick packed bed of the granular PCM The peak loads can be reduced by 19%, 30%, and 16% in concrete sandwich walls, steel roofs and gypsum wallboards, respectively The results suggest that 19–57% of maximum reduction in peak load as compared to a purely resistive R-19 wall can be achieved The peak heat flux can be reduced by 37% and 62% using a PCMSIP with 10% and 20% PCM concentrations 28% of the peak cooling load was expected to be reduced
the overall building energy consumption. On comparing these studies, the new active VAV–TES A/C system demonstrated in this paper has effectively applied the concept of the potential DCV scheme in conjunction with the ECV technique that in total has contributed towards improved building energy efficiency. 4. Conclusions In the present work, the chilled water A/C system combined with the TES system has been analyzed and tested for summer and winter conditions under the DCV and combined DCV–ECV schemes. The experimental investigations have led to the following conclusions: • The VAV–TES A/C system conserved much energy during the combined DCV–ECV modes without compromising the thermal comfort and ventilation requirements. The implementation of the ECV technique enabled the ambient air to be delivered into the conditioned space to satisfy the cooling and ventilation requirements at part load conditions. The role of PCM-based TES system was significant in terms of cooling capacities shared by the TES during on-peak conditions that collectively contributed to achieving enhanced energy savings. • The test results infer that, the VAV–TES A/C system operated using the DCV and DCV–ECV modes yielded 45% and 51% of average combined energy savings potential in part load to onpeak load conditions. Furthermore, by implementing the DCV and
Simulation and experiment
Experiment
Experiment
Simulation
Simulation
Experiment
Simulation
ECV techniques, the energy spent on cooling and ventilation was reduced; thereby, this system exhibited a combined coefficient of performance of 3.9 and 4.3 in summer and winter conditions, respectively. • For varying occupancy levels, the test results project that the system was capable of addressing the ventilation requirements better in summer and winter design conditions. Activating the ventilation air damper, return air damper and exhaust air damper properly under both the DCV and combined DCV–ECV modes helped the VAV–TES system to deliver the required ventilation air volume into the occupied space based on the CO2 levels detected in the conditioned space. • On comparing the energy savings of the VAV–TES A/C system with that of the conventional chilled water-based A/C system, it was found that the former achieved 28% and 47% of per day average energy conservative potential under the DCV and combined DCV–ECV modes, respectively. Likewise, the VAV–TES system yielded an on-peak total energy savings of 28%, 38% and 42%, respectively, for the same operating conditions.
Acknowledgements The authors would like to acknowledge and thank the CPDE, Anna University, for providing excellent support to this research project.
1360
R. Parameshwaran et al. / Energy and Buildings 42 (2010) 1353–1360
References [1] I. Dincer, On thermal energy storage systems and applications in buildings, Energy and Buildings 34 (4) (2002) 377–388. [2] C. Voelker, O. Kornadt, M. Ostry, Temperature reduction due to the application of phase change materials, Energy and Buildings 40 (5) (2008) 937–944. [3] H. El-Dessouky, F. Al-Juwayhel, Effectiveness of a thermal energy storage system using phase-change materials, Energy Conversion and Management 38 (6) (1997) 601–617. ˜ [4] J. Banaszek, R. Domanski, M. Rebow, F. El-Sagier, Experimental study of solid–liquid phase change in a spiral thermal energy storage unit, Applied Thermal Engineering 19 (12) (1999) 1253–1277. [5] V. Butala, U. Stritih, Experimental investigation of PCM cold storage, Energy and Buildings 41 (3) (2009) 354–359. [6] J.P. Bédécarrats, F. Strub, B. Falcon, J.P. Dumas, Phase-change thermal energy storage using spherical capsules: performance of a test plant, International Journal of Refrigeration 19 (3) (1996) 187–196. [7] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conversion and Management 45 (2) (2004) 263–275. [8] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (3) (2003) 251–283. [9] Y.P. Zhang, K.P. Lin, R. Yang, H.F. Di, Y. Jiang, Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings, Energy and Buildings 38 (10) (2006) 1262–1269. [10] C.K. Halford, R.F. Boehm, Modeling of phase change material peak load shifting, Energy and Buildings 39 (3) (2007) 298–305. [11] S.M. Vakilaltojjar, W. Saman, Analysis and modelling of a phase change storage system for air conditioning applications, Applied Thermal Engineering 21 (3, 1) (2001) 249–263. [12] E. Halawa, F. Bruno, W. Saman, Numerical analysis of a PCM thermal storage system with varying wall temperature, Energy Conversion and Management 46 (15–16) (2005) 2592–2604. [13] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy Reviews 11 (9) (2007) 1913–1965. [14] Z.A. Hammou, M. Lacroix, A new PCM storage system for managing simultaneously solar and electric energy, Energy and Buildings 38 (3) (2006) 258–265. [15] M. Veerappan, S. Kalaiselvam, S. Iniyan, R. Goic, Phase change characteristic study of spherical PCMs in solar energy storage, Solar Energy 83 (8) (2009) 1245–1252.
[16] A.J.N. Khalifa, E.F. Abbas, A comparative performance study of some thermal storage materials used for solar space heating, Energy and Buildings 41 (4) (2009) 407–415. [17] A.A. Chowdhury, M.G. Rasul, M.M.K. Khan, Thermal-comfort analysis and simulation for various low-energy cooling-technologies applied to an office building in a subtropical climate, Applied Energy 85 (6) (2008) 449–462. [18] Y.-P. Ke, S.A. Mumma, Using carbon dioxide measurements to determine occupancy for ventilation controls, ASHRAE Transactions (1997), BN-97-1-1. [19] T.M. Lawrence, J.E. Braun, A methodology for estimating occupant CO2 source generation rates from measurements in small commercial buildings, Building and Environment 42 (2) (2007) 623–639. [20] T.M. Lawrence, J.E. Braun, Evaluation of simplified models for predicting CO2 concentrations in small commercial buildings, Building and Environment 41 (2) (2006) 184–194. [21] N. Nassif, S. Moujaes, A new operating strategy for economizer dampers of VAV system, Energy and Buildings 40 (3) (2008) 289–299. [22] S. Wang, X. Xu, Optimal and robust control of outdoor ventilation airflow rate for improving energy efficiency and IAQ, Building and Environment 39 (7) (2004) 763–773. [23] ASHRAE, ANSI/ASHRAE Standard 62, 2007. [24] ASHRAE, ASHRAE Handbook Fundamentals, ASHRAE Inc., Atlanta, GA, 1990. [25] H. Weinläder, A. Beck, J. Fricke, PCM-facade-panel for daylighting and room heating, Solar Energy 78 (2) (2005) 177–186. [26] K.A.R. Ismail, J.N.C. Castro, PCM thermal insulation in buildings, International Journal of Energy Research 21 (14) (1997) 1281–1296. [27] M. Zhang, M.A. Medina, J.B. King, Development of a thermally enhanced frame wall with phase-change materials for on-peak air conditioning demand reduction and energy savings in residential buildings, International Journal of Energy Research 29 (9) (2005) 795–809. [28] K. Nagano, S. Takeda, T. Mochida, K. Shimakura, T. Nakamura, Study of a floor supply air conditioning system using granular phase change material to augment building mass thermal storage-heat response in small scale experiments, Energy and Buildings 38 (5) (2006) 436–446. [29] J.K. Kissock, S. Limas, Diurnal load reduction through phase-change building components, ASHRAE Transactions 112 (1) (2006) 509–517. [30] M.A. Medina, J.B. King, M. Zhang, On the heat transfer rate reduction of structural insulated panels (SIPs) outfitted with phase change materials (PCMs), Energy 33 (4) (2008) 667–678. [31] C. Stetiu, H.E. Feustel, Phase-change Wallboard and Mechanical Night Ventilation in Commercial Buildings, Lawrence Berkeley National Laboratory, University of California, 1998, http://epb.lbl.gov/thermal/docs/pcm2.pdf.