Feasibility study of the application of a cooling energy storage system in a chiller plant of an office building located in Santiago, Chile

Feasibility study of the application of a cooling energy storage system in a chiller plant of an office building located in Santiago, Chile

Accepted Manuscript Feasibility study of the application of a cooling energy storage system in a chiller plant of an office building located in santi...

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Accepted Manuscript

Feasibility study of the application of a cooling energy storage system in a chiller plant of an office building located in santiago, Chile Tomas ´ Venegas-Troncoso , Gaspar Ugarte-Larraguibel , Diego A. Vasco , Fabien Rouault , Rodrigo Perez ´ PII: DOI: Reference:

S0140-7007(19)30044-1 https://doi.org/10.1016/j.ijrefrig.2019.01.028 JIJR 4264

To appear in:

International Journal of Refrigeration

Received date: Revised date: Accepted date:

20 December 2017 7 January 2019 24 January 2019

Please cite this article as: Tomas ´ Venegas-Troncoso , Gaspar Ugarte-Larraguibel , Diego A. Vasco , Fabien Rouault , Rodrigo Perez , Feasibility study of the application of a cooling energy storage sys´ tem in a chiller plant of an office building located in santiago, Chile, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.01.028

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Highlights  

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LHTES system was implemented along with a conventional chiller system of a building Strategies of operation were assessed by computational simulation with EnergyPlus A hybrid operation strategy was evaluated, and the cooling loads were obtained LHTES reduces by 7.8% the cooling energy consumption of the cooling system

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FEASIBILITY STUDY OF THE APPLICATION OF A COOLING ENERGY STORAGE SYSTEM IN A CHILLER PLANT OF AN OFFICE BUILDING LOCATED IN SANTIAGO, CHILE Tomás Venegas-Troncoso, Gaspar Ugarte-Larraguibel, Diego A. Vasco*,

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Departamento de Ingeniería Mecánica, Universidad de Santiago de Chile, Av. Bdo. O’Higgins 3363, Santiago, Chile Fabien Rouault

Escuela de Construcción Civil, Pontificia Universidad Católica de Chile, Vicuña

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Mackena 4860, Santiago, Chile Rodrigo Pérez

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LBN Consulting, Malaga 115, Santiago, Chile

Abstract

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Energy consumption of commercial buildings has increasingly gained attention worldwide, because of its significant electricity consumption and peak power demand.

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Chiller plants are mostly used in commercial buildings to generate cold water for air

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conditioning. However, because the chiller plants and the refrigeration systems tend to be oversized with respect to their critical design load conditions, they certainly lead to

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higher energy consumption than a properly sized chiller plant. One possible way to reduce the power consumption and redistribute energy use is through the integration of latent heat thermal energy storage (LHTES) systems with air-cooling system in buildings. In the present work, a LHTES system based on ice is implemented along with a conventional chiller system of an existing commercial building located in Santiago, *

[email protected] Phone (562) 27183120 2

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Chile. Different strategies of operation in a design day are assessed by computational simulation with EnergyPlus. A hybrid operation strategy is evaluated for the hottest summer week, and the results of the cooling loads are obtained. Finally, we founded that the implementation of a LHTES reduces by 7.8% the cooling energy consumption of the

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cooling system during the analyzed period. Keywords

Latent heat thermal storage; ice tank; building simulation; cooling loads; chiller plant.

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Introduction

The continuous growth of population and the demand of energy have led scientists, engineers, and world leaders to ask themselves how to ensure enough energy resources to satisfy future demands. The current situation of the environment, and the availability of

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economical energy resources have increased the interest in subjects such as energy

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efficiency, sustainability, and renewable sources of energy. In Chile, the main energy supplies are oil (32.9%), coal (24.4%), wood and biomass (23.7%), and hydropower

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(6.4%). In 2014, the generation of electric energy was mainly thermal (coal 41% and gas 11%), and hydropower was the main renewable source (34%). The average power

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generation of the Central Interconnected System (SIC) of Chile was 48,207 Gwh between 2010 and 2014, mainly shared by hydroelectricity (43%), thermoelectricity from coal,

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natural gas, and diesel (52%), and a minor share by solar, biomass, and wind energy (5%). The main consumers of energy in Chile are five economic sectors: commercial, public, and residential (CPR), industry and mining, and transport. In 2014, the industrial sector and mining industry consumed approximately 40% of the country’s total electric energy, followed by the transport sector (33%) and the commercial, public, and

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residential sector (21%). The most widely used energy sources in the former sector are electricity (34%), biomass, mainly wood (32%), liquefied petroleum gas (18%), and natural gas (11%) (Ministerio de Energía 2016).

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For 2035, some of the commitments of the Energy Policy of Chile are to reduce by 30% CO2 emissions per gross domestic product (GDP) unit with respect to 2007. According to this Energy Policy, new buildings should be built according to efficiency construction standards of the OECD, and the main categories of appliances and equipment sold in the

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Chilean market should be energy efficient (Ministerio de Energía 2016). The building sector, which consists of residential and commercial end users, accounts for 20% of the total delivered energy consumed worldwide (U.S. Department of Energy 2016). The

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Technology Roadmap of the International Energy Agency on energy efficient buildings, through the BLUE MAP scenario, propose a new building energy mix to drastically

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reduce energy consumption and greenhouse gas emissions by 2050 (International Energy Agency 2010). One of the main conditions of the BLUE MAP scenario is the

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implementation of heat pump technology for space heating, refrigeration (chillers), and

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service water heating. Moreover, the BLUE MAP scenario highlights the importance of integrating Thermal Energy Storage (TES) based on phase change materials (PCMs) in

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heating and air conditioning systems. TES systems allow storing heat or cold to be used when required. The storage method should be reversible to supply or absorb thermal energy when needed. TES systems are usually classified into three main groups: latent heat storage systems, sensible heat storage systems, and storage systems associated with reversible chemical or physical proccesses. TES systems can reduce the gap between

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energy demand and energy supply (Soares et al. 2017; Nikoofard et al. 2015), and likewise, through TES systems it is possible to shift peak loads to hours of lower consumption and thus avoid high demand loads (Bourne, S. & Novoselac. 2015). The operation of air conditioning systems during the off-peak night hours is more efficient

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due to the lower outside temperature. Under this condition, the system may operate at a constant and optimum power point instead of meeting the user’s load demand, thanks to the support of the TES during the hours of higher energy demand, when the system would operate far from the optimum point. TES works by storing heat that will be later

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supplied as cooling or heating load, according to the application requirement. Therefore, the use of Latent Heat Thermal Energy Storage (LHTES) systems that store and supply thermal energy through absorption and release of heat, respectively, during the phase

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transition of a PCM, coupled with a heat pump system, may be a potential solution to improve the yield of air conditioning systems. Using this technology, it is possible to

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reduce the nominal thermal power of the chillers, and therefore the use of refrigerants, energy consumption, maintenance and operating costs, greenhouse gas (GHG) emissions,

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and noise levels. Moreover, it is possible to improve the cooling capacity of the chiller

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plant, the useful life of the equipment, and the system’s efficiency, reliability, and

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control.

In an IBM building located in Canada, an LHTES system was integrated to a chiller plant working at maximum capacity during 24 hours per day (35000 kW) (Bilodeau & Gagne 2005) to produce cold water between 5 ºC and 8 ºC. When the sum of the thermal loads is lower than the capacity of the air conditioning equipment, the surplus energy is stored,

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and when the sum of the thermal loads is higher than the capacity of the air conditioning equipment, the lack of energy is provided by the thermal storage bank. The chiller plant does not cover directly the thermal load, instead, the storage tank is loaded, and the chiller plant works in steady state near its highest efficiency condition. Pare and Bilodeau

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(2007) studied a system with a natural cooling heat exchanger, a variable frequency drive chiller (5275 kW), two LHTES units (5630 kW-h), and a plate heat exchanger (8800 kW). In order to maximize energy efficiency, a partial storage approach was used. From the point of view of the environmental impact, GHG emissions were reduced by 45%.

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The energy saving potential of a hybrid cooling system made up of a radiant cooling ceiling and two heat exchangers immersed in LHTES systems was proved in five cities of China (Wang et al. 2008). A conventional vapour compression refrigeration system was

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used to provide cold water when the stored energy in the PCM (slurry of Hexadecane) is not enough to be used in the radiant cooling ceiling. On the other hand, when the

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temperature of the water of the cooling tower is lower than the melting temperature of the PCM, the water is pumped to the heat exchanger to cool the PCM. Using the

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ACCURACY energy balance software, the assessment of the system was performed.

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Because of the research, the designed hybrid system showed energy savings between 10% and 80%. In a project developed in Italy (De Falco et al. 2015), a prototype of an

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LHTES system (5 kWh) for a residential air conditioning system was designed. The system can work in three modes: charging, mixed discharging, and pure discharging. During the charging process, all the cooling energy of the chiller is used in the solidification of the PCM. During pure discharge, the LHTES system cools the water for the user while the chiller plant is off. During the mixed discharge, the cold water is

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obtained at the same time from both LHTES and the chiller plant. The proposed solution for the prototype is the optimization of heat exchange between the PCM and the water of the chiller plant circuit.

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Parameshwaran et al. (2010) studied experimentally the integration of an LHTES system with a variable air volume air conditioning system (VAV). The experimental results showed that the VAV-LHTES system can reach daily energy savings and total energy savings in peak hours during summer and winter, respectively. Cui et al. (2016) proposed

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a computationally based method to evaluate the economical savings by using an LHTES along with an air conditioning system to manage the thermal loads of a non-residential building. The results of the simulations show that susbstantial cost savings may be

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reached with a relative small TES. Recently, Al-Aifana et al. (2017) performed an experimental investigation about the operational characteristics of a variable refrigerant

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volume air conditioning system (VRV) and an LHTES device (dimethyl adipate). The study was focused on the effect of the LHTES system in the reduction of the energy

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consumption and the COP of the system. The implementation of this technology allowed

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the reduction of the cooling capacity of the VRV system from 19 kW to 11 kW and from

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15.2 to 8.8 kW under summer and winter desings conditions, respectively.

The present paper studies the technical feasibility of the implementation of an LHTES system in a centralised space cooling system of an existing office building located in Santiago, Chile, through the energy balance simulation tool EnergyPlus. Thanks to its high energy storage density and its melting point, water (ice) is used as a PCM in the

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LHTES. Three different operating strategies are assessed in a design day and then the best strategy is selected, which is simulated for the hottest summer week in Santiago,

Nomenclature : Internal surfaces of conditioned zone [m2]

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Chile.

: Air volume specific heat [J/kg K] : Water specific heat [kJ/kg K] : Sensible heat capacity multiplier

COP: Coefficient of performance

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CFM: Cubic feet per minute

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ASHRAE: American Society of Heating, Refrigerating and Air Conditioning Engineers

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FOM: Full operating mode

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: Heat transfer coefficient from surfaces to air volume [W/m2 K] HVAC: Heating, ventilation and air conditioning

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: Time interval of 3600 [s] LEED: Leadership in Energy & Environmental Design

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LHTES: Latent heat thermal storage system ̇

: Space infiltration flow rate [kg/s]

̇ : Water mass flow rate [kg/s]

POM: Partial operating mode PCM: Phase change material

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̇ : Internal convective loads [W] ̇ ̇

: Ice storage charge/discharge rate [W] : Zone cooling load [W]

: Nominal energy storage capacity [GJ] : Chilled water temperature to ice storage [ºC] : Chilled water temperature from ice storage [ºC]

: Infiltration air temperature [ºC]

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: Inlet water temperature [ºC]

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TES: Thermal energy storage

: Freezing temperature of ice storage [ºC] : Outlet water temperature [ºC]

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: Zone air temperature [ºC]

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: Internal surfaces temperature [ºC]

: Ice storage overall heat transfer coefficient [W/ ºC]

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: Current ice fraction [-]

: Air volume density [kg/m3]

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: Logaritmic mean temperature difference across ice storage [ºC]

Definition of the problem

2.1 Building characteristics The analyzed building is located in Santiago (Las Condes), Chile. This location has 33°25’S and 70°35’W coordinates and is located at 476 meters above sea level. This is a “Csb” climate zone, according to Köppen, which accounts for a mediterranean climate 9

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with hot summers in latitudes between 30° and 45°S. According to ASHRAE (ASHRAE Standard 90.2 2004), it is a “3C” climate type, subtropical with dry summers. The building has 7 floors below ground and 23 floors above ground. The underground floors are used for car parking, control rooms, garbage rooms, and

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common areas. The first floor is retail space, floors 2 to 21 are open plan offices, and floors 22 and 23 are equipment rooms. The building envelope is composed of light weight metal-framed thermally insulated walls with a U-value of 0.368 [W/m2 K], and double-glazing. The glazing area amounts to 69% of the building’s outer surface

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above ground, and it has a U-value of 2.17 [W/m2K] and a solar heat gain coefficient of 0.245. The building roof is also composed of lightweight. metal-framed thermal

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insulating material, and a U-value of 0.273 [W/m2K].

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The cooling system of the building is composed of fan-coil terminal units, with cold water-cooling coils for space cooling and electric resistances for space heating. Cold water is generated in a thermal plant with two screw type chillers. Due to the two main uses of the building (offices and retail), the design comprises two independent chillers,

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one of them meant to supply the retail space cold water loop and the other, of higher thermal power, to supply the office cold water loop. According to the equipments’ datasheet, the retail space cold-water loop has an air-cooled chiller with a thermal power of 117.6 kW and a nominal COP of 2.88. The office cold-water loop has a water-cooled

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chiller with a thermal power of 564 kW and a nominal COP of 6.3. Both chillers use R134a as refrigerant fluid. According to these requirements, a chiller available in the market capable of providing those cooling loads was selected. The selected chiller for the

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office cold water loop was an RTWD 150 SE model, from Trane®. Moreover, the HVAC system considers the energy consumption of the water pumps and the cooling tower.

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LHTES device integration proposal.

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To store energy in an ice tank as an LHTES device, the refrigeration equipment must be able to provide cold water at temperatures between -9 °C and 3 °C. These values are

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below the usual temperature operation range for air conditioning equipment. The low storage temperature of ice allows producing cold air at lower temperatures than usual for

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air conditioning systems. These working conditions of the chiller make it operate far from its nominal COP values, therefore we have used the efficiency data provided by EnergyPlus. The analyzed system includes a TES ice tank system, which is located downstream of the offices cold water loop chiller. Figure 1 shows the generation circuit of cold water

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without the ice tank. The chiller is connected to a flow splitting device (demand splitter), to allow for splitting according to demand, for cold water to be distributed later to each

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coil for zone air conditioning, and finally returning to the chiller.

Figure 1. Schematics of the original HVAC system

The LHTES unit is integrated in parallel with the chiller, to allow the following three

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operating modes: i) Pure load of the LHTES unit, ii) Simultaneous discharge (cooling load supplied by the chiller and the LHTES unit at the same time), and iii) Pure discharge

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of the LHTES. A system developed by De Falco et al., 2015, uses a configuration which

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allows for the three modes of operation. The configurations for the LHTES used in this

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study are shown in Error! Reference source not found. and 3 (De Falco et al. 2015).

During the charging process, the ice tank is connected in series with the chiller, and the

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demand is excluded from the loop. The upper thinner line in Figure shows the cold-water supply, while the lower thinner line shows the return circuit. For the pure LHTES unit discharge (Thicker line in Figure 2), the ice tank supplies the cold-water demand, while the chiller remains off. Only a system with a LHTES can operate under this configuration and maintain a nearly constant temperature since the operation temperatures of the

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thermal energy storage heat exchanger are restricted to a range given by the PCM fusion process. Finally, during the simultaneous discharge, both the chiller and the ice tank work

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in parallel producing cold water (Figure ).

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Figure 2. Pure charge and discharge of the ice tank: Hot water circuit (continuous line)

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and cold-water circuit (dashed line).

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Figure 3. Simultaneous discharge of ice tank and chiller operation mode: Hot water

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circuit (continuous line) and cold-water circuit (dashed line).

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The operation strategies of the cold storage systems are usually classified as full storage

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or partial storage. West and Braun (1999) proposed a validated model to predict the performance of ice storage during partial charging and discharging. Habeebullah (2007)

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performed an economic feasibility analysis of retrofiting an ice storage system for the air conditioning system of a religious building in Makkah, Saudi Arabia. The results showed that with the subsidized electricity rate there is no gain in introducing ice storage system neither for full nor partial load scenarios. Mehling and Cabeza (2008) describe the partial cold storage of 66 MWh that Cristopia implemented at the exhibition center “Fiera di rimini” in Italy. During daytime, two chillers were running supported by the cold storage. 14

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The chiller capacity supplied 57% of the cooling demand and the storage system supplied the remaining 43%, thereby shifted 18.4 MWhel to offpeak electricity period. Sanaye and Hekmatian (2016) compared a traditional air conditioning system with an optimized TES-modified system in full and partial operating modes. They observed a reduction in

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electricity consumption of 11.83% and 10.23% for full and partial operating modes, respectively. Sanaye and Hekmatian (2016) compared an ice-TES system in full (FOM) and partial (POM) operating modes with that for traditional air conditioning system showed reduction in electricity consumption (11.83% for FOM and 10.23% for POM).

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Additionally, switching electricity consumption from on-peak to offpeak hours caused a reduction in electricity consumption cost (32.65% for FOM and 13.45% for POM). The systems that operate under partial storage strategies may be sized to operate according to

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load leveling or demand limitations. These strategies are focused on shifting the required energy during the periods of peak demand to periods of low demand (Yamaha et al.

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2008). In the present work, the use of a partial storage strategy has been implemented, specifically; the use of a hybrid strategy has been analyzed. Under this strategy, both the

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maximum operation cooling load of the chiller and the operation period are limited. To

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limit the operation power and the period, an LHTES unit is incorporated in the cold-water loop, and to limit the cooling load, the LHTES unit starts the operation supplying the

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stored energy at the moment when the chiller reaches its previously set maximum operation cooling load. At this point both cold water sources, the chiller and the LHTES unit, supply the total cooling load required by the building. It is important to mention here that ideally the maximum operation-cooling load of the chiller should be set at its highest efficiency point. In our case this ideal scenario is not possible, since we analysed

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the effect of adding an ice based LHTES to an already designed HVAC system, so we were constrained to modify only its working temperatures.

The hybrid strategy also has an hourly control method, which means that during certain

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defined hours, the building cooling load is supplied exclusively by the LHTES unit, while the chiller is not working. The goal of this approach is to avoid the consumption of electric energy on peak hours, and that during these periods the cooling load is totally provided by the LHTES unit, already charged by the chiller during the off-peak hours.

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The hybrid strategy limits the chiller operation far from its highest efficiency point and avoids the power consumption of the chiller on peak hours. The sizing of the LHTES capacity is based on the building performance simulation carried out during the warmer

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day of the location considering both strategies: load leveling and demand limitation.

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Materials and methods

3.1 Building energy modeling

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The computational analysis proposed in this paper is based on building computational

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simulations using EnergyPlus, which is a well recognized and accepted building energy analysis software tool (Qin and Yang 2016) (Yu et al. 2014). EnergyPlus has been used

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by engineers, architects and researchers to model both energy consumption (heating, cooling, ventilation) and water use in buildings. For the energy simulation the weather file CHL_Santiago-855740_IWEC.epw obtained from the EnergyPlus™ website (EnergyPlus) was used.

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The building simulation comprises the interaction of the zones to be conditioned, the air conditioning system terminal units, and the cold-water generation plant. We divided these zones using nodes according to ASHRAE Standard 90.1. The cold-water loop includes the chiller evaporator circuit and the condenser. The evaporator circuit cools down the

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water to be used for air conditioning in the occupied zones of the building and to charge the ice tank. The energy demand for cooling is calculated from the air circuit, which must be supplied by the cold-water generation plant through the water loop, which also charges

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the ice tank.

The relation between the zones and the air system is made through energy and humidity balances and solving ordinary differential equations using a predictor corrector scheme.

)

̇

(1)



(

)

∑ ̇

(

) ̇

(

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̇



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The formulation using a thermal balance is shown in equation (1):

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The left side term of the equation is the thermal energy store in zone air, which contains the density and heat capacity. The right-side terms of the equation correspond to: i) the

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sum of the convective internal loads, ii) convective heat transfer from the zone surfaces, iii) heat transfer due to infiltration of outside air, iv) heat transfer due to inter-zone air

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mixing, v) air systems output, respectively.

Using equation (1), the energy required by the air system to bring the zone air temperature to a desired level is calculated. With this energy demand, the air system is simulated to determine its cooling capacity at each instant of time. The cooling demands

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obtained from the air circuit allow the calculation of the temperature of water and the mass flow to cover the air conditioning load according to Equation 2: ̇

(2)

̇

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3.2 Occupancy and hourly schedules

Different occupancy rates for offices and retail space are implemented. The offices have an occupancy rate of 1 person per 10 m2, while the retail space has an occupancy rate of 1

Table 1. 9

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12

Offices (%)

30

80

100

100

100

Retail (%)

20

20

40

60

100

13

14

15

16

17

18

19

20

50

50

100

100

100

100

40

20

100

100

60

40

40

80

100

60

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Time (hours) 8

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person per 8 m2. The occupancy rate schedule from Monday through Friday is shown in

Table 1 Weekly Occupancy schedule. Source: produced by the authors based on

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information provided by the owner.

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Table 2 shows the temperature, humidity and ventilation rate requirements for offices,

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retail space, and the building lobby. For the sizing of the ventilation rates, the usual requirements have been used (ASHRAE, Standard 62.1-2007, 2007), but increased by

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30% (offices 20 CFM/person, retail space 30 CFM/person), since the project opts to comply with the requirements of LEED (Leadership in Energy & Environmental Design) certification. The lighting project has been designed to be below the lighting power densities for each space according to ASHRAE, Standard 90.1-2007,2007, using the

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space-by-space method. In the offices, a lower lighting power density of 10 [W/m2] has been considered, which is below the reference.

Offices Bulb

Relative Humidity

Dry Bulb Temperature

Temperature [°C]

[%]

[°C]

Summer

24.0

50

23

Winter

21.1

50 (not controlled)

20

Humidity

[%] 60

50 (not controlled)

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Table 2 Air conditioning design conditions.

Relative

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Dry

Retail space and lobby

The power density of the electric equipment to be used in the different spaces is dependent on the building occupants, information that was not available during the

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design stage. To estimate a value for the power density of the electric equipment, we consulted Table 11 of the ASHRAE, Handbook of Fundamentals, (2013), which for

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offices indicates 15 [W/m2]. For the operation of lighting and office equipment, variable

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schedules have been incorporated, based on the building occupancy schedule, as shown in Table 3. Minimum values have been implemented as fixed during weekends, when the

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building is not occupied.

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Offices Time (hours)

23 to 7 8

Lighting (%)

0

5

9

10

11

12

13

14

15

16

17

18

19

20

21 22

100 100

100 100 100 100 100 100 100 100 100 100 50

50 50

35

35

85

100 100 100 100 100 100 100 100 45

5

9

10

11

Electric

(%) Retail space Time (hours)

23 to 7 8

Lighting (%)

0

15

12

13

14

15

5

20

21 22

100 100

100 100 100 100 100 100 100 100 100 100 50

50 50

60

60

60

100 100 100 100 60

(%)

16

17

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Electric equipment

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equipment

60

60

18

19

100 100 100 60 15

Table 3. Lighting and electric equipment levels, Monday to Friday.

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3.3 Ice tank computational modeling

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There are two methods to perform the ice tank simulation with EnergyPlus. The first one consists of a single ice tank, in which the curves of the processes of charge and discharge

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of the ice tank are defined by default by EnergyPlus. The second one is called detailed ice tank, and allows the incorporation of the coefficient curves, according to the technical

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specifications provided by the manufacturer of the ice tank. The model is based on an

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integrated chiller and storage tank model developed for a special optimization project (Henze and Krarti 2002). For the charging process of the ice bank, the following conditions must be met: 1. If the inlet water temperature is higher than -1°C, the charging rate is annulled. 2. If the inlet water temperature is higher than or equal to the outlet required temperature, the charging rate is annulled. 20

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3. If the solid fraction is equal to 1, the tank is completely charged, and the charging rate is annulled.

According to the proposed model of an air conditioning plant developed by King and

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Potter (1998), the ice based LHTES can be considered a heat exchanger wherein the heat charge/discharge rate is given by: ̇

(3)

is the logarithmic mean temperature difference across ice storage for

charging (4) and discharging (5): (

) (

[( (

)⁄( ) ( )⁄(

)] ) )]

(4) (5)

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[(

)

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where

The ice storage overall heat transfer coefficient for the charging process is represented by

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the following equation:

)

(6)

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(

where

is the actual ice fraction,

is the nominal energy storage capacity,

is the nominal logarithmic mean

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time interval of one hour, and

is a

temperature difference. The ice-on-coil internal-melt charging and discharging coefficients of equation (3) were obtained by King and Potter (1998) fitting manufacturer’s data used by Strand et al. (1994), who claimed that this model can be used for any ice tank geometry. Particularly, ice-on-coil thermal storage tank is a widely

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used ice based LHTES in HVAC systems, where a secondary refrigerant flow through coils, charging or discharging the ice tank.

), that is present in equation (2), is provided by the chiller

( ̇

) or both, depending on the operation mode of the system.

), the ice tank ( ̇

The discharging conditions of the ice tank are the following:

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The total cooling load ( ̇

1. If the inlet water temperature is lower than 1 °C, the discharging rate is annulled.

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2. If the inlet water temperature is lower than or equal to the required outlet temperature, the discharge rate is annulled.

3. If the solid fraction is equal to 0, the tank is completely discharged, and the

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discharging rate is annulled.

Equation (6) is valid during the discharging process of the ice tank, using (

)

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instead of the ice fraction . It is worth to mention here that equation (6) assumes pure

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1994).

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conduction through the melted region and mass flow independent systems (Strand et al.

Results and discussion

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4.1 Sizing of the energy storage system The sizing of the original air conditioning system was made during a previous project and in an independent stage of the present study, using the design day methodology for Santiago, Chile. A company dealing with the design and sizing of air conditioning systems performed the sizing of the HVAC system without the TES, but they did not

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make further analysis of the energy performance of the system. The study regarding the feasibility of the incorporation of the LHTES was carried out later. For the sizing of the LHTES device to be integrated in the chiller of the building, a design day (January 21) was chosen, since it is the most unfavourable day for cooling with the higher

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temperatures for the location. Figure shows the hourly cooling demand for this day. It can be seen that the highest value is 540 kW, which takes place at 15:00 h. The total energy used for cooling during the analyzed day amounts to 4360 kWh.

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500 400 300 200 100

M

Cooling load [kW]

600

0

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time

PT

Figure 4. Cooling load profile for January 21.

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Figure describes the load levelling hybrid operational strategy, which was implemented using EnergyPlus. This strategy was carried out using performance data provided by the

AC

chiller’s manufacturer, which states that its highest COP (7.19) corresponds to 50% thermal load (286.9 kW). According to this, the sizing of the LHTES provides the difference between the instantaneous thermal load of the building and the chiller’s operating at 50% of its maximum power. Using this strategy, we look for reaching the

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ideal scenario in which the chiller operates at its highest efficiency point; it does not work during the peak hours, and during the lower electricity cost hours the ice tank is charged.

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500 400 300 200

100

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Cooling load [kW]

600

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time

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Figure 5. Load levelling hybrid strategy. TES charging (grided), TES discharging (black),

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and cooling load of the building provided by the chiller (hatched).

The amount of energy stored using the hybrid strategy corresponds to the cooling load of

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the building, less the chiller maximum cooling load up to 18:00 h, plus the total cooling

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load of the building during the peak on hours (18:00 h to 22:00 h), which amounts to

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2147.97 kW-h.

Because this calculation method allows to perform the sizing of the TES as needed considering the less favourable day for cooling, it is possible that the TES would be oversized for the rest of the period. For this reason, the studied period has been analized for three cases varying the TES energy storage capacity. The three analysed cases are the calculated capacity of 2148 kW-h and two lower capacities of 1524 kW-h and 914 kW-h, 24

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which are equivalent to ice volumes of 25.3 m3, 17.95 m3, and 10.76 m3, respectively. These values were obtained dividing the capacities by the product of the latent heat (333.55 kJ/kg) and the ice density (916.2 kg/m3). The hourly solid fraction of the TES during the study period was analyzed to determine the lowest storage capacity, which

4.2 Control strategy simulation using

the

energyPlus

modules

PlantEquipmentOperationSchemes,

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By

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allows supplying the cooling energy required for each 24-hour cycle.

PlantEquipmentOperation:ComponentSetpoint and SetpointManager:Scheduled in the outlet nodes of the chiller and the ice tank, the operation modes and cooling rates are defined for both devices. To implement the proposed strategy, hourly schedules of

Chiller

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setpoint temperatures are defined for the chiller and ice tank outlet nodes (Table 5). Ice Tank

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Set point temperature

Time

Set point temperature Time

[°C] -5.0

24:00 a 08:00

7.22

07:00 to 10:00

7.22

08:00 a 11:00

99

10:00 to 15:00

10.5

11:00 a 24:00

7.22

15:00 to 18:00

11.5

18:00 to 22:00

99

22:00 to 23:00

8.0

23:00 to 24:00

-2.0

AC

CE

PT

24:00 to 07:00

[°C]

Table 5. Setpoint temperatures of the outlet nodes of the chiller and ice tank.

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The ice tank charging process begins during nighttime hours. Initially, the chiller is required to gradually reduce the water temperature in the circuit down to 8 °C and then to -2 °C before begining the charging process. If the charging of the ice tank starts at 22:00 hours, an incremental reduction of temperature is required since the difference between

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the chiller outlet node temperature and the TES charging required temperature would force the chiller to operate at its maximum cooling capacity. Therefore, if the incremental temperature reduction is not applied, the chiller will be forced to operate at its maximum capacity, which implies higher electric power consumption during peak hours and to

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operate far from its maximum efficiency point declared by the chiller’s manufacturer (50%). Between 8:00 and 11:00 hours, cooling loads are provided exclusively by the chiller, therefore the ice tank outlet temperature is set at 99 °C and using this setpoint the

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ice tank does not operate. During daytime hours two different operation temperatures are set, since between 15:00 and 18:00 hours higher cooling loads are required than between

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10:00 and 15:00 hours. For this reason, during the hours with higher cooling loads, a higher chiller outlet temperature is set, with the purpose of forcing the ice tank to provide

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a higher fraction of the cooling load than the chiller. During peak hours (18:00 to 22:00

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hours), the chiller outlet temperature is set at 99°C, with the purpose of forcing the chiller

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to remain off and the ice tank to supply the building cooling loads exclusively.

Figure 6 displays the LHTES hourly solid fraction for the analyzed period. It shows that the amplitude of the variation of the solid fraction increases by reducing the storage capacity. For the lowest simulated storage capacity (914 kW-h), the ice tank manages to both fully charge and discharge during every 24-hour period. This situation confirms the

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previous asumption that by sizing the ice tank for the less favourable cooling day, it may result in an oversizing for any other days. Based on this observation, the analysis continues using the lower storage capacity.

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It is seen in Figure 6 that for the case with the lower storage capacity (914 kW-h) the discharging process begins at 11:00 hours and ends with an almost fully discharged ice tank around 20:00 hours. If the ice tank storage capacity were to be any lower, the chiller would be needed to supply the required cooling load when the ice tank is fully

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discharged, which would mean for the chiller to operate beyond its most efficient point or at peak hours.

0.8

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0.6 0.4 0.2

2148 kWh 914 kWh 1524 kWh

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Solid Fraction

1

PT

16 16 16 16 16 16 17 17 17 17 17 17 18 18 18 18 18 18 19 19 19 19 19 19 20 20 20 20 20 20 21 21 21 21 21 21

01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr

0

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Figure 6. Ice tank solid fraction for different storage capacities from January 16 to

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January 21.

Figure 7 shows the cooling loads supplied by the chiller and the ice tank for the period between January 16 and January 21. During the hours between 17:00 and 22:00, the chiller does not operate, and the ice tank exclusively supplies the cooling load. When the ice tank charging period begins, the largest cooling load to be supplied by the chiller is that due to the loop water cooling down from 8 °C to -2 °C. Once the water temperature 27

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reaches -5 °C, the chiller operates at a lower cooling load to only maintain such temperature and to charge the ice tank. During the first hours of daytime, when the lower

500

Chiller TES

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400 300 200 100

01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr

0

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16 16 16 16 16 16 17 17 17 17 17 17 18 18 18 18 18 18 19 19 19 19 19 19 20 20 20 20 20 20 21 21 21 21 21 21

Cooling Load [kW]

cooling load demands occur, only the chiller operates.

Figure 7. Chiller instantaneous cooling load (red) and cooling load supplied by

M

discharging the ice tank (black).

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Figure8 depicts the charging and discharging loads of the ice tank during the days between January 16t to January 21. It is seen that during the times between 10:00 and

PT

18:00 hours the loads vary according to the chiller outlet scheduled setpoint temperatures.

CE

During this period, the cooling load supplied by the ice tank increases as the chiller outlet temperature increases. During the ice tank charging stage, the required chiller load is

AC

reduced after the setpoint at 5 °C has been reached, because as the ice tank solid fraction increases, the required cooling load is reduced.

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TES Discharge Rate [W]

TES Charge Rate [W]

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01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr 01hr 05hr 09hr 13hr 17hr 21hr

300 250 200 150 100 50 0

16 16 16 16 16 16 17 17 17 17 17 17 18 18 18 18 18 18 19 19 19 19 19 19 20 20 20 20 20 20 21 21 21 21 21 21

Cooling Load TES [kW]

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Figure 8. Ice tank charging and discharging loads.

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According to the results shown in Table 6, the implementation of a LHTES reduces by 7.8% the cooling energy consumption during the analyzed period, including the consumption of the chiller, water pumps, and cooling tower (heat rejection). Considering only the chiller, the energy savings amount to 12%. However, the water pumps energy

M

consumption increases 45% due to the night-time LHTES charging operation by the

ED

chiller. Concerning the cooling tower energy consumption, it is reduced by 57%, because the heat rejection period is shifted from high temperature daytime hours to the more

PT

favourable lower temperature night-time hours.

AC

CE

End uses (kW-h)

Without

With

LHTES

LHTES

Chiller

4955

4356

Pumps

469

683

Heat rejection

77

33

Total

5501

5072

Table 6. Cold water generation system end energy use.

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Conclusions The incorporation of an ice tank energy storage device for cold-water generation in an air conditioning system in a building located in Santiago, Chile, has been analyzed. A hybrid operation strategy of the LHTES device has been implemented for sizing the energy

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storage capacity of the ice tank. The incorporation of the ice tank with a hybrid operation strategy integrated to the cold-water generation system for the air conditioning of a building has been simulated using the thermal load calculation of EnergyPlus.

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The results of the simulation show that the implementation of a LHTES with a hybrid operation strategy is feasible, since during the analyzed period the system operates as

M

foreseen during the design day.

The implementation of a LHTES reduces by 7.8% the cooling energy consumption of the

ED

cooling system during the analyzed period, including the consumption of the chiller, water pumps, and cooling tower (heat rejection). However, the water pump energy

PT

consumption increases 45% due to the night-time LHTES charging operation by the

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chiller. The cooling tower energy consumption is reduced by 57%.

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Acknowledgments

The authors acknowledge with thanks the supports through Proyecto Código 051816VC, Direccioón de Investigación, Científica y Tecnológica, Dicyt, Universidad de Santiago de Chile. F. Rouault acknowledges CONICYT under Fondecyt Project 11160690.

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