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Article Determination of Optimum Hot-Water Temperatures for PCM Radiant -Heating Systems Based on the Wet Construction Method

Sanghoon Baek 1 and Sangchul Kim 2,* 1 Industry Academic Cooperation Foundation, Hankyong National University, 327, Jungang-ro, Anseong-si, Gyeonggi-do 17579, ; [email protected] 2 School of Architecture, Hankyong National University, 327, Jungang-ro, Anseong-si, Gyeonggi-do 17579, Korea * Correspondence: [email protected]; Tel.: +82-31-670-5277

 Received: 6 September 2018; Accepted: 27 October 2018; Published: 1 November 2018 

Abstract: Owing to use of mortar, which demonstrates low heat storage and discharge performance, conventional radiant floor-heating systems, based on the wet construction method and hot-water circulation, consume large amounts of energy. This study proposes a new type of radiant floor- that is capable of reducing via use of the of a phase change material (PCM), whereby the phase change, which occurs within, is induced by the thermal energy supplied by hot water. Simulation analyses revealed that hot-water supply temperatures, required to maintain the floor-surface and indoor-air temperatures at the set point using PCM latent heat, were in the range 40–41 ◦C. At supply water temperatures measuring less than 39 ◦C or exceeding 42 ◦C, the latent-heat effect of the phase change of the PCM tended to fail, and the corresponding floor-surface temperature assumed a value different from that corresponding to the set point. By contrast, supply temperatures in the range 40–41 ◦C resulted in return temperatures measuring approximately 27.4–27.5 ◦C, which in turn corresponded to an indoor air temperature of 21.6–22.6 ◦C that was stably maintained within ±0.6 ◦C of the 22 ◦C set-point temperature.

Keywords: PCM radiant floor-heating system; wet construction method; hot-water temperature; apartment housing

1. Introduction Nearly all materials undergo transition from one state (solid, liquid, or gas) to another via absorption or discharge of heat, and depending upon which state they lie in, with respect to their particular phase-change points (e.g., melting and boiling points), most substances undergo sensible or latent heat exchange [1]. During exchange, which for liquid water occurs in the temperature range 0–100 ◦C, only the temperature of a substance changes, and no change occurs in its state. In contrast, during latent heat exchange, such as the transition from ice to water at 0 ◦C, the material state of a substance changes isothermally (at the same temperature). Most materials absorb and discharge more heat during latent heat exchange compared to sensible heat exchange, and if a material continues to demonstrate latent heat exchange for a sufficient length of time, it can be used as a system for storing and supplying large quantities of heat. Representative materials that demonstrate rather long latent heat exchange durations are referred to as phase change materials (PCMs). PCMs not only demonstrate extended durations of latent heat exchange during phase change, they are also capable of storing and discharging large quantities of heat in an isothermal manner. These materials are now being used in buildings as heat storage and discharge reservoirs for radiant floor-heating systems. Replacing conventional floor-heat-storage

Sustainability 2018, 10, 4004; doi:10.3390/su10114004 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 4004 2 of 19 materials, which only demonstrate sensible heat exchange, with PCM enables radiant floor-heating systems to maintain indoor-air and floor-surface temperatures at a pre-set value without activating the heat-source system, thereby significantly reducing the energy consumed by the overall system. Many studies concerning PCM radiant floor-heating systems have been performed in recent times to explore appropriate PCM container shapes for floor insertion, and it has been demonstrated that use of a PCM can result in significant benefits in terms of reduced indoor-air and floor-surface temperature maintenance and increased energy savings. Lin et al. proposed an underfloor system with SSPCM (shape-stabilized PCM) based on a dry construction method, and analyzed its thermal storage and heating energy performance [2]. In addition, utilizing a SSPCM, they developed a floor heating system using a conductive method to decrease heating energy usage in an office space, and evaluated its energy saving performance [3]. Jin et al. proposed a floor radiant cooling and heating system based on a structure with both low-temperature and high-temperature PCMs, and analyzed its thermal storage and energy saving via experimental tests [4]. Huang et al. proposed a hybrid PCM floor heating system, which integrated a PCM floor structure and a solar hot water system, and evaluated its energy performance through experimental tests [5]. Barzin et al. developed a PCM wallboard for underfloor heating and analyzed its energy saving performance via experimental tests [6]. Cheng et al. fabricated SSPCMs with various thermal conductivities for an underfloor heating system, and found, experimentally, that conductivities influence the thermal storage and energy performances of PCM [7]. Zhou et al. analyzed changes in thermal storage and energy performance by types of thermal storage masses and electric supply systems in a PCM radiant floor heating system [8]. Maria et al. proposed an underfloor heating system integrating PCM and a solar system with a , and evaluated its effects for reducing heating loads and energy consumptions [9]. Extant studies have focused on the use of PCM systems with dry-constructed floor structures that use as a heat source. However, there has been a complete lack of research concerning the integration of PCMs with concrete-based wet-constructed floor structures that employ hot water as the heat source. Although wet-constructed structures provide high floor strength and serve to improve structural stability and the comfort of residents, owing to the lack of air cavities that generate noise or induce dew condensation [10], such structures consume large amounts of hot water and energy to maintain indoor-air and floor-surface temperatures at a pre-set value during the heating period. This may be attributed to their use of mortar, which only demonstrates sensible heat exchange, as the heat-storage material [11]. Using PCMs with long latent-heat exchange durations can, therefore, reduce the energy consumption of wet-constructed structures whilst also maintaining indoor-air and floor-surface temperatures at a pre-set value. The authors have previously proposed a PCM radiant floor-heating system—built using the wet construction method and employing hot-water circulation—which could potentially form the middle-story of an apartment house. In the proposed and extant studies, the authors propose a design and system structure and describe methods for constructing a PCM container. In addition, the range of PCM melting-point temperatures required to maintain comfortable indoor-air and floor-surface temperatures has also been calculated [12–14]. However, because floors between stories in an apartment housing must meet basic, legally prescribed performance requirements in terms of noise transmission between stories, waterproofing, and structural safety [15,16], it is, in practice, nearly impossible to completely change an existing floor structure to insert PCMs. For this reason, it is more effective to replace part of the existing floor material and subsequently employ the reheating storage method, wherein the heat storage and discharge cycles of PCMs are repeated via circulation of hot water during heating periods. In this case, the hot water for floor heating must have appropriate supply and return temperatures, to maintain the floor-surface and indoor-air temperatures at the pre-set value, while PCM undergoes phase change during the heating period. If the temperature of the supplied hot water is too high, the PCM would enter a superheated state and liquefy, thereby resulting in an increase in the temperature of the PCM Sustainability 2018, 10, 4004 3 of 19

Sustainability 2018, 10, x FOR PEER REVIEW 3 of 19 as well as the floor surface and indoor air, which in turn would lead to occupant discomfort and hot water is too low, the PCM would remain in the supercooled solid state and, if the PCM does not increased energy consumption. On the other hand, if the temperature of the supplied hot water is too undergo a phase change, the pre-set temperature will not be attained and the reduction in energy low, the PCM would remain in the supercooled solid state and, if the PCM does not undergo a phase consumption through the utilization of latent heat will not be realized. To solve these problems whilst change, the pre-set temperature will not be attained and the reduction in energy consumption through efficiently running the PCM radiant floor-heating system, it is necessary to derive optimum values the utilization of latent heat will not be realized. To solve these problems whilst efficiently running the of hot-water supply and return temperatures. PCM radiant floor-heating system, it is necessary to derive optimum values of hot-water supply and Accordingly, the goal of this study was to determine appropriate hot-water supply and return return temperatures. temperatures, for the maintenance of floor-surface and indoor-air temperatures at the pre-set value, Accordingly, the goal of this study was to determine appropriate hot-water supply and return by use of a PCM radiant floor-heating system fabricated using the wet construction method. These temperatures, for the maintenance of floor-surface and indoor-air temperatures at the pre-set parameters were derived using computer simulation of a residential building under pre-set indoor value, by use of a PCM radiant floor-heating system fabricated using the wet construction method. and outdoor environmental conditions. These parameters were derived using computer simulation of a residential building under pre-set indoor2. Materials and outdoor and Methods environmental conditions. 2. Materials and Methods 2.1. Materials 2.1. Materials 2.1.1. System Structure and Characteristics 2.1.1. System Structure and Characteristics Because the PCM radiant floor-heating system proposed in this study was designed to be used in intermediateBecause the PCM of radiant domestic floor-heating apartment-ty systempe housing proposed structures, in this study domestic was designedfloor-structure to be usedstandards in intermediate were applied floors in ofits domesticconstruction apartment-type [15,17]. Thehousing design and structures, structure domestic of the floor-structuresystem could; standardshowever, be were changed applied depending in its construction on the place [15 ,of17 residence]. The design to reflect and structurevarying architectural of the system cultures could; however,and structural be changed standards. depending on the place of residence to reflect varying architectural cultures and structuralThe (a) standards. panel in Figure 1 depicts the floor structure commonly used in domestic apartment-type housing,The (a)which panel is constructed in Figure1 depicts in the following the floor structuresequence commonlyof structural used layers in domesticstacked over apartment-type one another: housing,210 mm concrete which is slab; constructed 20 mm ininsu thelating following material; sequence 40 mm of autoclaved structurallayers lightweight stacked concrete over one (ALC); another: 40 210mm mmmortar, concrete including slab; a 20 hot-water mm insulating pipe with material; a typical 40 inner mm autoclaveddiameter measuring lightweight 15–20 concrete mm [18]; (ALC); and 40finally, mm mortar,floor-finishing including material. a hot-water Because pipe the with floor a typical-finishing inner material diameter is measuringexcluded 15–20from domestic mm [18]; andstructural finally, standards, floor-finishing a prospective material. client Because may theselect floor-finishing it in accordance material with desired is excluded floor fromcharacteristics. domestic structuralIn such systems, standards, mortar a prospective is used to facilitate client may heat selecting via it in storage accordance and discharge with desired of thermal floor characteristics. energy from Inhot such water. systems, However, mortar because is used mortar to facilitate only demonstr heatingates via storage sensible and heat discharge exchange, of wherein thermal temperature energy from hotchanges water. occur However, without because any phase mortar change, only demonstrates its surface temperature sensible heat decreases exchange, rapidly wherein once temperature hot-water changessupply is occur terminated. without Accordingly, any phase change, conventional its surface systems temperature require a decreases continuous rapidly supply once of hot-waterhot water supplyto maintain is terminated. the floor-surface Accordingly, temperature conventional at the pre-set systems value, require whilst a continuous consuming supply large quantities of hot water of toenergy maintain owing the to floor-surface long operation temperature times of the at heat-source the pre-set value,system. whilst consuming large quantities of energyThe owing (b) panel to long in operationFigure 1 timesdepicts of a the schematic heat-source of the system. proposed radiant floor-heating system, whereinThe a (b) PCM panel is ininserted Figure1 into depicts an existing a schematic floor of system the proposed that demonstrates radiant floor-heating sensible system,as well whereinas latent aheat PCM exchanges is inserted [13]. into Because an existing this system floor can system mainta thatin the demonstrates floor-surface sensible and indoor-air as well astemperatures latent heat exchangesat a pre-set [ 13value]. Because via the this supply system of isothermal can maintain energy the floor-surface from PCM, without and indoor-air the need temperatures for continuous at a pre-sethot-water value supply, via the energy supply consumptio of isothermaln can energy be reduced from PCM, through without intermittent the need for operation continuous of hot-waterthe heat- supply,source system. energy consumption can be reduced through intermittent operation of the heat-source system.

Figure 1. Structures of conventional (a) and phasephase changechange material (PCM) (b) radiantradiant floor-heatingfloor-heating systems. ALC: Autoclaved lightweight concrete.

PCM can be inserted into the ALC and mortar layers of an existing floor structure without manipulating the , which performs a structural and insulating function, material for preventing heat loss from the floor. Insertion of PCM into mortar can cause several problems. First, Sustainability 2018, 10, 4004 4 of 19

PCM can be inserted into the ALC and mortar layers of an existing floor structure without manipulatingSustainability 2018, the 10, x concrete FOR PEER slab, REVIEW which performs a structural and insulating function, material4 of for 19 preventing heat loss from the floor. Insertion of PCM into mortar can cause several problems. First,as depicted as depicted in Figure in 2a, Figure the mortar2a, the layer mortar includ layeres a includes 15–20 mm a 15–20diameter mm hot-water diameter pipe, hot-water above which pipe, abovePCM must which be PCM inserted. must If, be for inserted. example, If, for a 10 example, mm thic ak 10 PCM mm is thick to be PCM inserted, is to be the inserted, thickness the of thickness the top ofmortar the top layer mortar would layer be wouldreduced be to reduced 10–15 mm. to 10–15 Since mm. such Since thinning suchthinning of the mortar of the layer mortar would layer lead would to leadcrack to formation crack formation on the onfloor the surface, floor surface, additional additional mortar mortar would would have to have be toapplied, be applied, which which in turn in turnwould would reduce reduce the ceiling the ceiling height height as well as as well the as volume the volume of the ofindoor the indoor space, space, thereby thereby lowering lowering the value the valueand selling and selling price of price the ofbuilding. the building. This effect This effectwould would be particularly be particularly seen in seen construction in construction markets, markets, such suchas South as South Korea, Korea, wherein wherein the ceiling the ceiling height height and in anddoor indoor volume volume are considered are considered sensitive sensitive factors. factors. Although methods exist for reducing ceiling space—for and electric works—which could be used to restore thethe originaloriginal ceilingceiling height,height, followingfollowing mortar thickening to accommodate a PCM (panel (b) of FigureFigure2 2),), reducingreducing thethe ceilingceiling workingworking spacespace cancan havehave significantsignificant effectseffects onon bothboth ductduct and electric works. This, in turn, could lead to cascading effects, wherein all plans related to building heating, ventilation, ventilation, and and air-conditioning air-conditioning (HVAC) (HVAC) would would have have to tobe beamended. amended. For Forthese these reasons, reasons, the theproposed proposed method method avoids avoids PCM PCM insertion insertion into into the themortar mortar layer. layer.

Figure 2. Indoor ( a) and ceiling ( b) space reduction owing to PCM insertion.

Although ALC prevents heat loss from a floor, its practical role is to mitigate noise transmission Although ALC prevents heat loss from a floor, its practical role is to mitigate noise transmission between stories. According to recent studies; however, commonly used ALC materials demonstrate a between stories. According to recent studies; however, commonly used ALC materials demonstrate rather poor noise abatement performance owing to their low density, and new systems to replace it a rather poor noise abatement performance owing to their low density, and new systems to replace are, therefore, being actively developed [19–22]. As an added advantage, PCMs can absorb noise more it are, therefore, being actively developed [19–22]. As an added advantage, PCMs can absorb noise effectively, compared to conventional ALC materials, owing to their very high densities and the fact more effectively, compared to conventional ALC materials, owing to their very high densities and that they enter an intermediate state, between solid and liquid, during phase change. The possibility the fact that they enter an intermediate state, between solid and liquid, during phase change. The of using PCM as a floor material to replace ALC inspired our placement of PCM into the ALC layer in possibility of using PCM as a floor material to replace ALC inspired our placement of PCM into the the proposed system (see Figure1). ALC layer in the proposed system (see Figure 1). PCM used in the proposed system measured 10 mm thick. Because heat storage and discharge PCM used in the proposed system measured 10 mm thick. Because heat storage and discharge capability increased with the amount of PCM inserted, it was advantageous, from the point of view of capability increased with the amount of PCM inserted, it was advantageous, from the point of view energy saving, to insert as much PCM as possible into the floor, thereby replacing as much ALC as of energy saving, to insert as much PCM as possible into the floor, thereby replacing as much ALC possible with PCM. Since preventing noise transmission is also a critical function of intermediate floors as possible with PCM. Since preventing noise transmission is also a critical function of intermediate in an apartment-type housing, further experimental research to obtain quantitative data concerning floors in an apartment-type housing, further experimental research to obtain quantitative data the soundproofing effect of PCM must be performed. However, as this study primarily focused on the concerning the soundproofing effect of PCM must be performed. However, as this study primarily reduction of energy consumption in indoor environments, a set PCM thickness of 10 mm was used in focused on the reduction of energy consumption in indoor environments, a set PCM thickness of 10 anticipation of future research concerning the soundproofing effects of the proposed system. mm was used in anticipation of future research concerning the soundproofing effects of the proposed 2.1.2.system. PCM Aluminum Container

2.1.2.Prior PCM to Aluminum PCM insertion Container into a floor structure, it must be contained within a structure inside which it could safely undergo phase change from solid to liquid. Figure3 depicts a PCM container fabricated Prior to PCM insertion into a floor structure, it must be contained within a structure inside which it could safely undergo phase change from solid to liquid. Figure 3 depicts a PCM container fabricated for use in an extant study and its construction process [13]. Given the high and durability of the floor, the container was fabricated using a thin aluminum sheet to accommodate the 10 mm thick PCM, specified in Section 2.1.1. The aluminum container used was slightly irregular in Sustainability 2018, 10, x FOR PEER REVIEW 5 of 19

terms of thickness, owing to it being constructed using vacuum equipment and the application of a superheated hot wire for use in a previous floor-heating-module investigation. Such irregularities could be avoided in future construction of the container through use of an automated system for its fabrication [23].

Sustainability 2018, 10, 4004 5 of 19

for use in an extant study and its construction process [13]. Given the high thermal conductivity and Sustainability 2018, 10, x FOR PEER REVIEW 5 of 19 durability of the floor, the container was fabricated using a thin aluminum sheet to accommodate the 10 mm thick PCM, specified in Section 2.1.1. The aluminum container used was slightly irregular in terms of thickness, owing to it being constructed using vacuum equipment and the application of a terms of thickness, owing to it being constructed using vacuum equipment and the application of a superheated hot wire for use in a previous floor-heating-module investigation. Such irregularities superheated hot wire for use in a previous floor-heating-module investigation. Such irregularities Figure 3. Fabrication and construction of the aluminum container for the PCM. couldcould be avoided be avoided in future in future construction construction of of the the container container throughthrough use use of of an an automated automated system system for its for its fabricationfabrication [23]. [23 ]. 2.1.3. Surface Temperature and Energy-Consumption Pattern Figures 4 and 5, respectively, demonstrate indicative patterns of floor-surface temperature change in the use of conventional and PCM radiant floor-heating systems, installed in apartment- type housing facilities. Following preheating, to increase the natural floor-surface temperature from its value at the beginning of the heating period to its pre-set value, the surface temperature of the conventional system follows a sawtooth sinewave pattern, between the top (+) and bottom (−) of the error range of the pre-set floor temperature. If the surface temperature attains the value corresponding to the top or bottom of the error range, the hot-water supply must be stopped or restarted, respectively. This is problematic from the perspective of energy utilization, because the time taken to transit from the top to the bottom Figureof theFigure range3. Fabrication 3. Fabrication is very andshort, and construction construction thereby resulting of thethe aluminumaluminum in repeated container container cycles for for the of the PCM.ho PCM.t-water supply and increased energy consumption. 2.1.3. Surface Temperature and Energy-Consumption Pattern 2.1.3. SurfaceIn contrast, Temperature owing to the and thermal Energy-Consumption energy discharge Pattern facilitated by the PCM’s latent heat exchange, the floor-surfaceFigures4 and temperature5, respectively, during demonstrate use of indicative the PCM patterns radiant of floor-surface floor-heating temperature system change could be Figures 4 and 5, respectively, demonstrate indicative patterns of floor-surface temperature maintainedin the use at the of conventional pre-set for a andsufficient PCM time radiant afte floor-heatingr the hot-water systems, supply installed has been in turned apartment-type off. Through changethishousing time-lag in the facilities. usephenomenon, of conventional surface-temperature and PCM radiant decr floor-heatingease is delayed systems, relative installed to the inconventional apartment- Following preheating, to increase the natural floor-surface temperature from its value at the typesystem, housing and facilities.the reoperation time is, therefore, extended via the latent heat exchange of the PCM. beginning of the heating period to its pre-set value, the surface temperature of the conventional system Furthermore,Following aspreheating, demonstrated to increase by previous the natural small-module floor-surface experiments, temperature PCM from also its demonstrates value at the follows a sawtooth sinewave pattern, between the top (+) and bottom (−) of the error range of the beginningbetter sensible-heat of the heating storage period and discharge to its pre-set performanc value, ethe compared surface to temperature mortar [13], ofand the therefore, conventional even systempre-set follows floor a temperature.sawtooth sinewave If the surface pattern, temperature between attainsthe top the (+) valueand bottom corresponding (−) of the to error the top range or of afterbottom the latent of the heat error has range, been the discharged, hot-water supply the surface must be temperature stopped or restarted, decreases respectively. more slowly, This when is the pre-set floor temperature. If the surface temperature attains the value corresponding to the top or comparedproblematic to a fromconventional the perspective system, of energythrough utilization, loss of sensible because theheat. time These taken features to transit allow from the the top PCM bottom of the error range, the hot-water supply must be stopped or restarted, respectively. This is radiantto the floor-heating bottom of the system range is to very be operated short, thereby intermittently resulting in whilst repeated suppressing cycles of hot-water the rapid supply reduction and in problematicfloor-surfaceincreased from energytemperature, the consumption. perspective thereby of reducing energy utilizatio hot watern, asbecause well as the energy time consumption.taken to transit from the top to the bottom of the range is very short, thereby resulting in repeated cycles of hot-water supply and increased energy consumption. In contrast, owing to the thermal energy discharge facilitated by the PCM’s latent heat exchange, the floor-surface temperature during use of the PCM radiant floor-heating system could be maintained at the pre-set for a sufficient time after the hot-water supply has been turned off. Through this time-lag phenomenon, surface-temperature decrease is delayed relative to the conventional system, and the reoperation time is, therefore, extended via the latent heat exchange of the PCM. Furthermore, as demonstrated by previous small-module experiments, PCM also demonstrates better sensible-heat storage and discharge performance compared to mortar [13], and therefore, even after the latent heat has been discharged, the surface temperature decreases more slowly, when compared to a conventional system, through loss of sensible heat. These features allow the PCM Figure 4. Surface temperature (top) and hot-water supply (bottom) patterns corresponding to the radiantFigure floor-heating 4. Surface systemtemperature to be ( topoperated) and hot-water intermittently supply whilst(bottom suppressing) patterns corresponding the rapid reduction to the in conventional radiant floor-heating system. floor-surfaceconventional temperature, radiant floor-heating thereby reducing system. hot water as well as energy consumption. In contrast, owing to the thermal energy discharge facilitated by the PCM’s latent heat exchange, the floor-surface temperature during use of the PCM radiant floor-heating system could be maintained at the pre-set for a sufficient time after the hot-water supply has been turned off. Through this time-lag phenomenon, surface-temperature decrease is delayed relative to the conventional system, and the reoperation time is, therefore, extended via the latent heat exchange of the PCM. Furthermore,

Figure 4. Surface temperature (top) and hot-water supply (bottom) patterns corresponding to the conventional radiant floor-heating system. Sustainability 2018, 10, 4004 6 of 19

as demonstrated by previous small-module experiments, PCM also demonstrates better sensible-heat storage and discharge performance compared to mortar [13], and therefore, even after the latent heat has been discharged, the surface temperature decreases more slowly, when compared to a conventional system, through loss of sensible heat. These features allow the PCM radiant floor-heating system to be operated intermittently whilst suppressing the rapid reduction in floor-surface temperature, Sustainabilitythereby 2018 reducing, 10, x FOR hot waterPEER REVIEW as well as energy consumption. 6 of 19

FigureFigure 5. 5.SurfaceSurface temperature temperature ((toptop)) and and hot-water hot-water supply supply (bottom (bottom) patterns) patterns corresponding corresponding to the PCM to the PCMradiant radiant floor-heating floor-heating system. system. 2.2. Methods 2.2. Methods Supply and return temperatures of hot water, used as the heat source in PCM radiant floor-heating systems,Supply must and be return set to maintaintemperatures the floor-surface of hot water, and indoor-air used as temperaturesthe heat source at the in pre-set PCM value radiant during floor- heatingtimes systems, of minimal must annual be set outdoor to maintain temperature the floor-s [24,25urface]. To derive and indoor-air appropriate temperatures hot water temperatures, at the pre-set valuethe during EnergyPlus times (ver. of minimal 8.7) computer annual simulationoutdoor temperature package was [24,25]. used [ 26To–28 derive]. System appropriate modeling hot was water temperatures,performed inthe the EnergyPlus following sequence:(ver. 8.7) (1)computer Application simulation of weather package data; was (2) buildingused [26–28]. modeling; System modeling(3) composition was performed of envelope in andthe windows;following (4) sequence: PCM application; (1) Application (5) input of indoorweather heat data; gain (2) and building loss modeling;elements; (3) (6) composition application of of heating envelope system; and andwindows; (7) input (4) of PCM heating application; operation conditions. (5) input of indoor heat gain2.2.1. and Weather loss elements; Data (6) application of heating system; and (7) input of heating operation conditions. Table1 lists details concerning the siting and weather corresponding to the simulated 2.2.1.apartment-type Weather Data housing, which was assumed to be located in Seoul, South Korea. Using standard weather data for Seoul distributed by the government [29], the period from January 26 to 28—the coldestTable days1 lists of the details year—was concerning simulated. the siting and weather corresponding to the simulated apartment-type housing, which was assumed to be located in Seoul, South Korea. Using standard weather dataTable for 1.SeoulLocation distributed of simulation by building the governme and weathernt [29], data type.the period EPW: EnergyPlus from January Weather. 26 to 28—the coldest days of the year—wasField simulated. Input Value Site location Seoul, Korea Table 1. Location of simulation buildingLatitude and weather data type. EPW: 37.34 EnergyPlus Weather. Building location Longitude 126.57 RunField period From January 26 toInput January Value 28 WeatherSite location date Seoul standard weatherSeoul, data (EPW Korea File) Latitude 37.34 Standard weatherBuilding data, location with regards to dry bulb temperature, absolute , wind speed,

and solar radiation (Figure6), were used to calculate theLongitude building heating load. For126.57 compatibility with EnergyPlus, data wereRun input period into the program in the formFrom of January a text-based 26 to EnergyPlus January 28 Weather (EPW) file [27]. Weather date Seoul standard weather data (EPW File)

Standard weather data, with regards to dry bulb temperature, absolute humidity, wind speed, and solar radiation (Figure 6), were used to calculate the building heating load. For compatibility with EnergyPlus, data were input into the program in the form of a text-based EnergyPlus Weather (EPW) file [27]. Sustainability 2018, 10, 4004 7 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 7 of 19

FigureFigure 6.6. HourlyHourly outdooroutdoor weatherweather data.data. 2.2.2. Building Modeling 2.2.2. Building Modeling Figure7 shows plan and front views of the apartment-type housing and the elevation view modeledFigure via 7 simulation. shows plan The and building front views was modeledof the ap asartment-type a three-story housing structure and with the a totalelevation height view of 8400modeled mm, via including simulation. the lowest, The building middle (standard),was modeled and as top a three-story stories. Each structure story measured with a total 10,290 height mm in of length,8400 mm, 10,530 including mm in the width, lowest, and middle 2800 mm (standard), in height. and Because top stories. the PCM Each radiant story floor-heatingmeasured 10,290 system, mm proposedin length, 10,530 for use mm inthis in width, study, and is to 2800 be installedmm in height. in the Because middle the story PCM of aradiant building, floor-heating the second system, story ofproposed the modeled for use building in this study, was consideredis to be installed as the in analysis the middle target. story Indoor of a building, space within the second the building story of wasthe modeled divided intobuilding eight was zones, considered with each as zone the assumedanalysis target. to be heated Indoor simultaneously space within the under building identical was temperaturedivided into conditions. eight zones, Public with spaces, each suchzone asassumed elevators to and be stairs,heated were simultaneously excluded from under the modeling, identical andtemperature windows conditions. of different sizesPublic were spaces, installed such on as the elevators south wall. and stairs, were excluded from the modeling, and windows of different sizes were installed on the south wall. 2.2.3. Building Structure The apartment housing envelope comprised an exterior wall, lowest-story floor structure, middle-story floor structure, and top-story roof. Structural components listed in Table2 were constructed using eight different types of materials, including the PCM. Material names and their corresponding thermal-property values, including heat conductivity, density, and specific heat, were provided as input into EnergyPlus.

Table 2. Materials comprising the [30].

Field Input Value Conductivity Density Specific Heat No. Structural Element (W/m·◦C) (kg/m3) (J/kg·◦C) 1 Reinforced concrete 1.400 2300 880 2 Autoclaved lightweight concrete (ACL) 0.150 710 1130 3 Mortar 0.720 1860 780 4 Insulating material 0.025 29 1213 5 Gypsum board 0.170 800 100 6 Air cavity 0.020 1.2 100 7 Phase change material (PCM) 0.200 860 2000 8 Finishing material 0.160 780 1255

Figure 7. Plan, front, and elevation views of the modeled building. Sustainability 2018, 10, x FOR PEER REVIEW 7 of 19

Figure 6. Hourly outdoor weather data.

2.2.2. Building Modeling Figure 7 shows plan and front views of the apartment-type housing and the elevation view modeled via simulation. The building was modeled as a three-story structure with a total height of 8400 mm, including the lowest, middle (standard), and top stories. Each story measured 10,290 mm in length, 10,530 mm in width, and 2800 mm in height. Because the PCM radiant floor-heating system, proposed for use in this study, is to be installed in the middle story of a building, the second story of the modeled building was considered as the analysis target. Indoor space within the building was Sustainabilitydivided into 2018 eight, 10, x FORzones, PEER with REVIEW each zone assumed to be heated simultaneously under identical8 of 19 Sustainabilitytemperature2018 conditions., 10, 4004 Public spaces, such as elevators and stairs, were excluded from8 ofthe 19 2.2.3.modeling, Building and Structurewindows of different sizes were installed on the south wall. The apartment housing envelope comprised an exterior wall, lowest-story floor structure, middle-story floor structure, and top-story roof. Structural components listed in Table 2 were constructed using eight different types of materials, including the PCM. Material names and their corresponding thermal-property values, including heat conductivity, density, and specific heat, were provided as input into EnergyPlus.

Table 2. Materials comprising the building envelope [30].

Field Input Value Conductivity Density Specific Heat No. Structural Element (W/m·°C) (kg/m3) (J/kg·°C) 1 Reinforced concrete 1.400 2300 880 2 Autoclaved lightweight concrete (ACL) 0.150 710 1130 3 Mortar 0.720 1860 780 4 Insulating material 0.025 29 1213 5 Gypsum board 0.170 800 100 6 Air cavity 0.020 1.2 100 7 Phase change material (PCM) 0.200 860 2000

8 Finishing material 0.160 780 1255 Figure 7. Plan, front, and elevation views of the modeled building. Figure 88 depicts depicts thethe envelopeenvelope structuresstructures forfor thethe first,first, second,second, andand thirdthird floors,floors, thethe roof,roof, andand thethe outer wall wall of of the the building. building. The The second second floor floor (middle (middle story) story) was wasequipped equipped with withthe PCM the PCMradiant radiant floor- floor-heatingheating system, system, which, which, along alongwith the with third-story the third-story floor, included floor, included a space a for space duct for and duct electrical and electrical works worksbelow the below concrete the concrete slab. The slab. heat Thetransmittance heat transmittance (U-value) (U-value)of each envelope of each (Table envelope 3) was (Table modeled3) was to modeledmeet domestic to meet standards domestic [31]. standards [31].

Figure 8. Structures of the building envelopes. Sustainability 2018, 10, 4004 9 of 19

Table 3. Heat transmittance by envelope type (U-value).

Heat Transmittance Domestic Standard Envelope Type (U-Value) (U-Value) (W/m2·◦C) (W/m2·◦C) Floor structure of 1st story 0.1741 Below 0.1800 Floor structure of 2nd story (PCM) 0.1534 Below 0.8100 Floor structure of 3rd story 0.1530 Below 0.8100 Roof structure of 3rd story 0.1105 Below 0.1500 Exterior wall structure 0.2005 Below 0.2100

To model building windows, data concerning window materials distributed by the Lawrence Berkeley National Laboratory were used along with the Therm and Window simulation program [32,33]. To meet domestic standards for U-values, windows were constructed comprising three panes. Table4 lists types and properties of panes, insulating gases, and frames comprising the triple-pane windows considered in this study. The panes measured 6 mm thick and possessed identical solar-radiation properties on their respective front and back surfaces. The insulating-gas used was 100% pure argon gas confined to a 12 mm thickness. The 300 mm thick frame was made of polyvinyl chloride (PVC) and was injected with urethane to minimize heat loss.

Table 4. Properties of the pane, insulating-gas, and frame for the window system.

Window-Pane Properties Field Input Value Thickness (mm) 6.000 Conductivity (W/m·◦C) 1.000 Transmittance 0.246 Entire waves Reflectivity 0.218 Exterior surface Transmittance 0.313 Visible waves Reflectivity 0.106 Surface 0.340 Transmittance 0.246 Entire waves Reflectivity 0.218 Interior surface Transmittance 0.313 Visible waves Reflectivity 0.106 Surface emissivity 0.340 Insulating-Gas Properties Field Input Value Type Argon gas (pure 100%) Conductivity 0.016348 (W/m·◦C) Viscosity 0.000021 (kg/m·s) Specific heat 521.9290 (J/kg·◦C) Density 1.782282 (kg/m3) Prandtl number 0.6705 Window-Frame Properties Field Input Value Frame material Polyvinyl chloride (PVC) Thickness 300 (mm) Outside protruded length 129 (mm) Inside protruded length 129 (mm) Filled insulating material Urethane Sustainability 2018, 10, x FOR PEER REVIEW 10 of 19

U-values of the said triple-pane windows were obtained based on properties of the above-listed constituent elements, as given in Table 5. The total pane and gas-layer thicknesses measured 18 and 24 mm, respectively. The thickness of the spacer, which was composed of aluminum infused with a desiccant, installed between the panes measured 12 mm—identical to that of the gas layer. The total Sustainability 2018, 10, 4004 10 of 19 thickness of the glazing system, including the panes, gas layers, and spacers, measured 42 mm, and the total thickness of the frame equaled 300 mm. To calculate the overall U-value, surface heat- transfer coefficientsU-values of theof said23.3 triple-paneand 9.1 W/m windows2·°C were were applied obtained to based reflect on propertiesdomestic ofstandards the above-listed for indoor and constituentoutdoor temperatures elements, as givenof 20 inand Table 0 °C,5. The respectively total pane and[31,34]. gas-layer The thicknessesresulting U-value measured of 18 the and triple- pane24 window mm, respectively. under these The conditions thickness of was the determined spacer, which to was be composed1.101 W/m of2·°C. aluminum infused with a desiccant, installed between the panes measured 12 mm—identical to that of the gas layer. The total thickness of the glazing system, including the panes, gas layers, and spacers, measured 42 mm, and the Table 5. Heat transmittance of the triple-pane window (U-value). total thickness of the frame equaled 300 mm. To calculate the overall U-value, surface heat-transfer ◦ coefficientsType of 23.3of System and 9.1 W/m2· C were applied to reflect Field domestic standards for indoorInput and Value outdoor temperatures of 20 and 0 ◦C, respectively [31,34]. The resulting U-value of the triple-pane window Total pane thickness under these conditions was determined to be 1.101 W/m2·◦C. 18 (mm) (6 + 6 mm) Table 5. Heat transmittanceTotal of the gas triple-pane layers thickness window (U-value). 24 (mm) GlazingType of system System Field(12 + 12 mm) Input Value Total pane thickness Spacer type 18 (mm)Aluminum spacer (6 + 6 mm)(12 + 12 mm) with desiccant Total gas layers thickness 24 (mm) Glazing system (12 + 12Total mm) glazing thickness 42 (mm) Frame system Spacer Total type frame thickness 300 (mm) Aluminum spacer with desiccant (12 + 12 mm) Thermal transmittance (U-Value) 1.101 (W/m2·°C) Triple pane window Total glazing thickness 42 (mm) Domestic standard 1.200 (W/m2·°C) Frame system Total frame thickness 300 (mm) Thermal transmittance (U-Value) 1.101 (W/m2·◦C) Triple pane window 2.2.4. Phase Change Material Domestic standard 1.200 (W/m2·◦C) The PCM inserted into the floor structure of the second story was of the solid–liquid type that undergoes2.2.4. Phase phase Change change Material via latent heat exchange, which occurs within the temperature range 39–42 °C [35]. AsThe depicted PCM inserted in Figure into the9, modeling floor structure of th ofe inserted the second PCM story in wasEnergyPlus of the solid–liquid required the type heat- storagethat capacities undergoes to phase be input change corresponding via latent heat exchange,to 15 temperatures, which occurs including within the those temperature lying within range the ◦ sensible-39–42 andC[ latent-heat-exchange35]. As depicted in Figure temperature9, modeling rang of thees of inserted the PCM. PCM Within in EnergyPlus the 34–38°C required range, the the PCMheat-storage demonstrated capacities solid-state to be input sensible corresponding heat ex tochange, 15 temperatures, wherein includingthe temperature those lying changes within the were ◦ observed.sensible- The and corresponding latent-heat-exchange heat-storage temperature capacity ranges within of the this PCM. temperature Within the 34–38 rangeC range,equaled the 3–9 PCM kJ/kg. demonstrated solid-state sensible heat exchange, wherein the temperature changes were observed. In contrast, the heat storage capacity in the 39–42 °C temperature range, wherein the PCM The corresponding heat-storage capacity within this temperature range equaled 3–9 kJ/kg. In contrast, demonstrated phase change from solid to liquid, assumed values in the range of 22–37 kJ/kg. The the heat storage capacity in the 39–42 ◦C temperature range, wherein the PCM demonstrated phase temperaturechange from range solid to towhich liquid, the assumed PCM demonstrated values in the range maximum of 22–37 heat kJ/kg. storage The temperaturewas observed range to tobe 41– 42 °C,which above the which PCM demonstrated the heat-storage maximum capacity heat storageplummeted was observed to only to2 bekJ/kg, 41–42 and◦C, abovethe system which became the stableheat-storage in the fully capacity liquid plummetedstate. to only 2 kJ/kg, and the system became stable in the fully liquid state.

FigureFigure 9. Variation 9. Variation in inthe the heat-storage heat-storage capacity capacity ofof thethe PCM PCM with with change change in temperature.in temperature. Sustainability 2018, 10, 4004 11 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 11 of 19

2.2.5.2.2.5. IndoorIndoor Heat-GainHeat-Gain andand Heat-LossHeat-Loss ElementsElements FigureFigure 1010 depictsdepicts schedulesschedules ofof indoorindoor heatheat gaingain andand lossloss elementselements employedemployed inin simulationssimulations performedperformed inin this this study. study. These These schedules schedules corresponded corresponded to a resident to a resident family offamily four—the of four—the most common most numbercommon of number family members of family in members domestic Koreanin domestic households Korean [36 households]. The occupancy [36]. The time occupancy considered time was betweenconsidered 18:00 was h between and 08:00 18:00 h the h and following 08:00 h day, the andfollowing it was day, assumed and it thatwas noassumed occupants that wereno occupants present withinwere present the house within at other the house times. at other times. AA ventilationventilation raterate of of 0.5 0.5 air air change change per per hour hour was was used used based based on on the the domestic domestic standard standard “Rules “Rules for Buildingfor Building Equipment Equipment Standards” Standards” [16]. In[16]. accordance In accordance with “Building with “Building Airtightness Airtightness Criteria,” domesticCriteria,” buildings,domestic buildings, based on theirbased infiltration on their rates, are classifiedrates, are asclassified general as buildings general buildings (5.0 air change (5.0 air per change hour orper lower), hour or energy-saving lower), energy-saving buildings buildings (3.0 air change (3.0 air per change hour orper lower), hour or and lower), zero-energy and zero-energy buildings (1.0buildings air change (1.0 air per change hour or per lower) hour [or37 ].lower) The simulated [37]. The simulated apartment apartment housing was housing assumed was toassumed be of the to energy-savingbe of the energy-saving building typebuilding with type a target with ventilation a target ventilation rate of 3.0 rate air changeof 3.0 air per change hour. per hour. EnergyPlusEnergyPlus modelsmodels lightlight operation using a number between 0 and 1 to reflect reflect occupant activity. TheThe maximummaximum valuevalue ofof 11 waswas appliedapplied betweenbetween 18:0018:00 hh andand 22:0022:00 hh asas wellwell asas betweenbetween 05:0005:00 hh andand 08:0008:00 h,h, assumingassuming thesethese toto bebe timestimes ofof maximummaximum occupantoccupant activity.activity. AA minimumminimum valuevalue ofof 0.10.1 waswas appliedapplied betweenbetween 22:0022:00 hh andand 05:0005:00 hh toto reflectreflect sleepsleep periods.periods. NoNo lightinglighting waswas usedused betweenbetween 08:0008:00 hh andand 18:0018:00 h,h, asas nono occupantsoccupants werewere assumedassumed toto bebe presentpresent duringduring thisthis period.period. TheThe heatheat gains gains caused caused by humanby human presence presence and theand operation the operation of electric of equipmentelectric equipment were excluded were fromexcluded the simulation,from the simulation, as these doas these not cast do anot significant cast a significant effect on effect the heating on the heating load in load a four-person in a four- occupiedperson occupied apartment. apartment.

FigureFigure 10.10. DailyDaily schedulesschedules ofof heatheat lossloss andand gaingain factors.factors. 2.2.6. 2.2.6. District Heating District heating was employed as the heat-source supply system for the proposed PCM radiant District heating was employed as the heat-source supply system for the proposed PCM radiant floor-heating system. Figure 11 depicts a conceptual diagram of hot-water circulation via direct supply floor-heating system. Figure 11 depicts a conceptual diagram of hot-water circulation via direct method, as applied during simulations performed in this study. supply method, as applied during simulations performed in this study. When supplied to the building, high-temperature hot water produced by a district heating plant When supplied to the building, high-temperature hot water produced by a district heating plant is converted into mid-temperature hot water by means of a heat exchange unit and subsequently is converted into mid-temperature hot water by means of a heat exchange unit and subsequently returned to the plant. The unit also converts low-temperature hot water obtained from the building to returned to the plant. The unit also converts low-temperature hot water obtained from the building to mid-temperature hot water, and resupplies it to the building. During the heating period, hot water is circulated via repeated transfers between the district heating plant and apartment building. Sustainability 2018, 10, 4004 12 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 12 of 19

mid-temperature hot water, and resupplies it to the building. During the heating period, hot water is In accordance with domestic standards, supply and return temperatures of hot water from and circulated via repeated transfers between the district heating plant and apartment building. to the district heating plant (supply side) equal 115 and 55 °C, respectively. Corresponding supply In accordance with domestic standards, supply and return temperatures of hot water from and to and thereturn district temperatures heating plant for (supply an apartment side) equal building 115 and (demand 55 ◦C, respectively. side) equal Corresponding 60 and 45 °C, supply respectively and [24,25].return However, temperatures because for an the apartment second-floor building PCM (demand radiant side) floor-heating equal 60 and 45system◦C, respectively possesses [ 24structural,25]. and However,thermal becausecharacteristics the second-floor that differ PCM radiant from floor-heatingthose of conventional system possesses systems, structural it and was thermal deemed inappropriatecharacteristics to consider that differ the from said those standard of conventional supply and systems, return it was temperatures deemed inappropriate for use in this to consider study, and therefore,the said new standard PCM floor-heating supply and return hot-water temperatures temperatures for use had in this to be study, investigated. and therefore, new PCM floor-heatingTo accomplish hot-water this objective, temperatures results had concerning to be investigated. various values of building hot-water supply and return temperaturesTo accomplish were this objective, modeled results against concerning constant various district values heating of building plant hot-watersupply and supply return temperaturesand return of temperatures 115 and 55 °C, were respectively. modeled against The supply constant temp districterature heating was plant sequentially supply and decreased return in temperatures of 115 and 55 ◦C, respectively. The supply temperature was sequentially decreased in steps of 1 °C, from the conventional temperature of 60 °C, until the floor-surface and indoor-air steps of 1 ◦C, from the conventional temperature of 60 ◦C, until the floor-surface and indoor-air temperatures attained the pre-set value; the return temperature was then calculated by the flow temperatures attained the pre-set value; the return temperature was then calculated by the controller,flow controller, considering considering changes changes in the floor-surface in the floor-surface temperature temperature as well as as well the as indoor the indoor and andoutdoor temperatures.outdoor temperatures.

Figure 11. Conceptual schematic of the district heating hot-water circulation. Figure 11. Conceptual schematic of the district heating hot-water circulation. Table6 lists the components used to connect the apartment housing and district heating plant inTable the EnergyPlus6 lists the components model. The district used heatingto connect plant th usede apartment hot water housing as a heat and source, district and possessedheating plant in thea EnergyPlus maximum plant model. capacity The district of 1 MW heating to ensure plant the used supply hot water of a sufficient as a heat amount source, ofand hot possessed water. a maximumAn insulation plant conditioncapacity withof 1 noMW heat to loss ensure from the surfacesupply was of considereda sufficient for amount all pipes. of hot water. An insulationDevices condition used with for district no heat heating loss from within the the surface apartment was includedconsidered a hydronic for all pipes. pump, a hot-water pipe,Devices and used a bypass. for district The pipe heating connected within to the the hydronic apartment pump included measured a 15 hydronic mm in internal pump, diameter, a hot-water pipe,and and a a pump bypass. head The pressure pipe connected of 200 kPa wasto the used hydr toonic ensure pump that themeasured hot water 15 reachedmm in internal the third diameter, story and ofa pump the building. head pressure A maximum of 200 motor kPa was efficiency used ofto 90%ensure was that considered the hot alongwater withreached an intermittent the third story of thepump building. control A assembly, maximum to automatically motor efficiency control of the90% flow was rate considered of hot water along in accordance with an withintermittent the floor-surface and indoor-air temperatures. The hot-water pipe was assumed to incur no heat loss. pump control assembly, to automatically control the flow rate of hot water in accordance with the The internal diameter and heat loss of the bypass were set to be identical to those of the hot-water pipe. floor-surface and indoor-air temperatures. The hot-water pipe was assumed to incur no heat loss. The 2.2.7.internal Heating diameter Operation and heat Conditions loss of the bypass were set to be identical to those of the hot-water pipe. Based on domestic regulations and the results reported in extant studies [31,38], floor-surface and indoor-air temperatures of the apartment housing were set to 22 and 30 ◦C, respectively. As described in Section 2.2.5, heating hours corresponded to the duration between 18:00 h to 08:00 h the following day, during which no occupants were assumed present inside the house, whereas non-heating hours corresponded to 08:00 h to 18:00 h (Figure 12). The non-heating hours Sustainability 2018, 10, x FOR PEER REVIEW 13 of 19

Table 6. District heating components.

Loop Type Field Input Value Heating source Hot water District heating plant Heating capacity 1 (MW) loop Maximum flow rate Autosize (m3/s) Pipe inside diameter 0.015 (m) Maximum flow rate Autosize (m3/s) Hydronic Head pressure 200 (kPa) Sustainability 2018, 10, 4004 Pump 13 of 19 Motor efficiency 0.9 Apartment housing were set such that the floor-surface and indoor-airControl temperatures type would not drop Intermittentbelow 5 and ◦ loop 10 C, respectively. Hot-water Inside diameter 0.015 (m)

pipeTable 6. District heating components.Heat loss Adiabatic Inside diameter 0.015 (m) Loop TypeBypass Field Input Value Heating sourceHeat loss Hot water Adiabatic District heating plant loop Heating capacity 1 (MW) 2.2.7. Heating Operation Conditions Maximum flow rate Autosize (m3/s) Based on domestic regulations and the results repoPiperted inside in diameter extant studies 0.015 [31,38], (m) floor-surface Maximum flow rate Autosize (m3/s) and indoor-air temperatures of the apartmentHydronic Pump housing wereHead set pressure to 22 and 30 °C, 200 respectively. (kPa) As described in Section 2.2.5, heating hours correspondedMotor efficiency to the duration 0.9between 18:00 h to Control type Intermittent 08:00 h theApartment following housing day, loop during which no occupants were assumed present inside the house, Inside diameter 0.015 (m) Hot-water pipe whereas non-heating hours corresponded to 08:00 h to 18:00Heat h loss (Figure 12). The Adiabatic non-heating hours were set such that the floor-surface and indoor-air temperaturesInside diameter would not drop 0.015 below (m) 5 and 10 °C, Bypass respectively. Heat loss Adiabatic

FigureFigure 12. 12.Floor-surfaceFloor-surface and and indoor-air temperature temperature schedules. schedules. 3. Results and Discussion 3. Results and Discussion The scope of the analysis was limited to the second story, wherein the proposed PCM Theradiant scope floor-heating of the analysis system was was limited installed. to Simulationsthe second werestory, performed wherein forthe the proposed period between PCM radiant floor-heating26 and 28 system January, was during installed. which theSimulations outdoor temperature were performed typically for attains the itsperiod lowest between annual value. 26 and 28 All conditions, including the building structure as well as the indoor and outdoor environment, January, during which the outdoor temperature typically attains its lowest annual value. All were kept constant while reducing the hot-water supply temperature from 60 ◦C in steps of 1 ◦C. conditions, including the building structure as well as the indoor and outdoor environment, were kept constant3.1. Hot-Water while Supply reducing Temperature the hot-water supply temperature from 60 °C in steps of 1 °C. Figure 13 depicts the hourly variation in floor-surface temperatures, at various hot-water supply temperatures, lying within the range 38–43 ◦C. In each case, following heating at 18:00 h, the floor-surface temperature attained a stable state (represented by nearly straight-line segments in the diagram) at approximately 21:00 h. The period between 18:00 h and 21:00 h can, therefore, be considered as the preheating time during which the floor-surface temperature remains unstable. Consequently, the actual period of surface-temperature analysis was from 21:00 h on 26 January to Sustainability 2018, 10, 4004 14 of 19

08:00 h on 27 January and from 21:00 h on 27 January to 08:00 h on 28 January, during which the floor-surface temperature remained nearly constant. At a hot-water supply temperature of 38 ◦C, the floor-surface temperature was observed to be in the range 28–28.5 ◦C during the period from 21:00 h on 26 January to 08:00 h on 27 January. Correspondingly, between 21:00 h on 27 January and 08:00 h on 28 January, the floor-surface temperature measured in the range 28.1–28.4 ◦C. During the entire simulation period at the said hot-water temperature, the floor-surface temperature was observed to measure less than the pre-set value of 30 ◦C by 1.5–2.0 ◦C. At a hot-water supply temperature of 39 ◦C, the floor-surface temperature measured 28.2–29.2 ◦C during the period from 21:00 h on 26 January to 08:00 h on 27 January. Correspondingly, between 21:00 h on 27 January and 08:00 h on 28 January, the floor-surface temperature measured in the range 28.3–29.1 ◦C. Consequently, during the entire simulation period the floor-surface temperature measured 0.8–1.8 ◦C lower than the corresponding pre-set value. At a hot-water supply temperature of 40 ◦C, the floor-surface temperature measured 28.8–29.9 ◦C during the period from 21:00 h on 26 January to 08:00 h on 27 January. Correspondingly, between 21:00 h on 27 January and 08:00 h on 28 January, the floor-surface temperature measured in the range 28.8–29.8 ◦C. In particular, the floor-surface temperature measured in the range 28.8–29.3 ◦C—slightly lower than the pre-set value—between 21:00 h and 23:00 h. However, after 23:00 h, the floor-surface temperature measured 29.6–29.9 ◦C, which was very close to (within 0.1–0.4 ◦C of) the pre-set temperature. At a hot water supply temperature of 41 ◦C, the floor-surface temperature measured in the range 29.2–30.6 ◦C between 21:00 h on 26 January and 08:00 h on 27 January; correspondingly, the floor-surface temperature measured 29.4–30.5 ◦C between 21:00 h on 27 January and 08:00 h on 28 January. Although during certain hours the floor-surface temperature exceeded the pre-set value (by roughly 0.5–0.6 ◦C), the floor-surface temperature, for the most part, measured very close to the pre-set value (within 0.5 ◦C). At a hot-water supply temperature of 42 ◦C, the floor-surface temperature measured in the range 29.8–31.3 ◦C between 21:00 h on 26 January and 08:00 h on 27 January; correspondingly, the floor-surface temperature measured 29.9–31.2 ◦C between 08:00 h on 27 January to 08:00 h on 28 January. In general, after 21:00 h, the floor-surface temperature was observed to exceed its pre-set value by 0.5–1.3 ◦C. At a hot-water supply temperature of 43 ◦C, the floor-surface temperature measured in the range 30.5–32.0 ◦C between 21:00 h on 26 January and 08:00 h on 27 January; correspondingly, the floor-surface temperature measured 30.5–31.9 ◦C between 18:00 h on 27 January and 08:00 h on 28 January. In this case, the floor-surface temperature was observed to be quite different from its pre-set value at all times. In terms of the average temperature, at all heating times excluding the preheating hours, the average temperature of the floor surface measured 28.2, 28.9, 29.6, 30.2, 30.9, and 31.6 ◦C corresponding to hot-water supply temperatures of 38, 39, 40, 41, 42, and 43 ◦C, respectively. At supply temperatures of 38 and 39 ◦C, the floor-surface temperature did not attain its pre-set value, whereas at supply temperatures of 42 and 43 ◦C, the floor-surface temperature measured higher compared to its pre-set value. However, at supply temperatures of 40 and 41 ◦C, the floor-surface temperature consistently measured close to its pre-set value. In summary, supplying hot water at or below 39 ◦C to the radiant floor-heating system resulted in a floor-surface temperature below it pre-set value of 30 ◦C, corresponding to the PCM that undergoes phase change above 39 ◦C. This causes the PCM to either remain solid in the cooling state or to fail to undergo full liquefaction. Under this condition, occupants may feel discomfort owing to a low floor-surface temperature, and the energy saving effect might not be realized as the latent heat of the PCM can no longer be utilized. On the other hand, supplying hot water at 42 ◦C or higher leads to situations wherein the floor surface could overheat, thereby resulting in supply of unnecessary thermal energy to the indoor space and potential liquefaction of PCM beyond latent heat exchange. In addition to increasing the beyond an unnecessary level, such overheating could lead to structural problems caused Sustainability 2018, 10, x FOR PEER REVIEW 15 of 19 undergoes phase change above 39 °C. This causes the PCM to either remain solid in the cooling state or to fail to undergo full liquefaction. Under this condition, occupants may feel discomfort owing to a low floor-surface temperature, and the energy saving effect might not be realized as the latent heat of the PCM can no longer be utilized. On the other hand, supplying hot water at 42 °C or higher leads to situations wherein the floor surface could overheat, thereby resulting in supply of unnecessary thermal energy to the indoor space and potential liquefaction of PCM beyond latent heat exchange. In addition to increasing the room temperatureSustainability beyond2018 ,an10, 4004unnecessary level, such overheating could lead to structural problems15 of 19 caused by the generation of fine cracks within the ALC and mortar surrounding the PCM container, owing to increasedby the pressure generation and of finevolume cracks within within the the ALC container. and mortar surrounding the PCM container, owing to Theincreased above results pressure suggest and volume that within the most the container. appropriate hot-water supply temperature for radiant floor-heatingThe systems, above results installed suggest with that thea 10 most mm appropriate thick PCM hot-water layer, supply lies temperaturein the range for radiant40–41 °C. By ◦ maintainingfloor-heating the supply systems, temperature installed withwithin a 10this mm rang thicke, the PCM latent layer, heat lies exchange in the range of 40–41the PCMC. can be By maintaining the supply temperature within this range, the latent heat exchange of the PCM appropriatelycan be appropriatelyutilized, and utilized, the floor-surface and the floor-surface temperature temperature can canbe ma be maintainedintained at at its pre-setpre-set value. value.

Figure 13. Hourly variations in the floor-surface temperature with differing supply temperatures. Figure 13. Hourly variations in the floor-surface temperature with differing supply temperatures. 3.2. Return Temperature of Hot Water 3.2. Return TemperatureFigure 14 depicts of Hot hourly Water variations in the return temperatures of hot water supplied at 40 and Sustainability41 ◦ 2018C to, the10, x PCM FOR radiantPEER REVIEW floor-heating system between 26 and 28 January. 16 of 19 Figure 14 depicts hourly variations in the return temperatures of hot water supplied at 40 and 41 °C to the PCM radiant floor-heating system between 26 and 28 January. At the hot-water supply temperature of 40 °C, the corresponding return temperature measured in the range 27.4–35.4 °C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 27.6–35.3 °C between 18:00 h on 27 January and 08:00 h on 28 January. The return temperature measured the lowest between 18:00 h and 21:00 h because a large quantity of thermal energy had to be supplied to raise the temperature of the floor—which had cooled during the non-heating hours-up to the pre-set value. In contrast, after 21:00 h, the return temperature remained approximately constant, as the floor-surface temperature had nearly attained the pre-set temperature. This result suggests an optimum return-temperature value of 27.4 °C, the lowest value measured over the course of the simulation. At a hot-water supply temperature of approximately 41 °C, the return temperature measured in the range 27.5–36.2 °C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 27.8–36.2 °C between 18:00 h on 27 January and 08:00 h on 28 January. Thus, a pattern similar to that observed at a hot-water supply temperature of 40 °C was observed.

That is, the return temperature remained nearly constant in the range 27.5–36.2 °C across heating hours. ThisFigure suggests 14.Figure Hourly 14.an Hourlyoptimal variations variations return-temperature in the in the return return temperat temperatures valueures with withof differing27.5 differing °C supply for supply the temperatures. hot-water temperatures. supply at 41 °C. At the hot-water supply temperature of 40 ◦C, the corresponding return temperature measured 3.3. Indoor-Airin the range Temperature 27.4–35.4 ◦C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, Figure 15 depicts hourly variations in the indoor-air temperatures corresponding to hot-water supply and return temperatures in the range 40–41 °C and 27.4–27.5 °C, respectively, in the PCM radiant floor-heating system during the period from 26–28 January. At hot-water supply and return temperatures of 40 and 27.4 °C, respectively, the indoor-air temperature measured in the range 18.7–22 °C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 18.8–21.9 °C between 18:00 h on 27 January and 08:00 h on 28 January. Excluding preheating hours from 18:00 to 22:00 h, based on above results, the indoor-air temperature was observed to fall within the range 21.6–22.1 °C (i.e., within 0.1– 0.6 °C of its pre-set value of 22 °C). The indoor-air temperature was stabilized approximately one hour after the floor-surface temperature, thereby apparently reflecting the time needed for heat to be transferred from the floor-surface to the indoor space. At supply and return temperatures of 41 and 27.5 °C, respectively, the indoor-air temperature outside of preheating hours measured in the range 22.0–22.6 °C between 22:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 22.1–22.4 °C between 22:00 h on 27 January and 08:00 h on 28 January. Under these hot-water conditions, the indoor-air temperature followed a pattern similar to that observed for supply and return temperatures of 40 and 27.4 °C, respectively, and remained within 0.1–0.6 °C of the 22 °C pre-set value. In summary, outside of the preheating hours (18:00–22:00 h), setting the hot-water supply and return temperatures in the range 40–41 °C and 27.4–27.5 °C, respectively, resulted in steady maintenance of the indoor-air temperature within 0.1–0.6 °C of the 22 °C pre-set value. Sustainability 2018, 10, 4004 16 of 19

the return temperature measured 27.6–35.3 ◦C between 18:00 h on 27 January and 08:00 h on 28 January. The return temperature measured the lowest between 18:00 h and 21:00 h because a large quantity of thermal energy had to be supplied to raise the temperature of the floor—which had cooled during the non-heating hours-up to the pre-set value. In contrast, after 21:00 h, the return temperature remained approximately constant, as the floor-surface temperature had nearly attained the pre-set temperature. This result suggests an optimum return-temperature value of 27.4 ◦C, the lowest value measured over the course of the simulation. At a hot-water supply temperature of approximately 41 ◦C, the return temperature measured in the range 27.5–36.2 ◦C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 27.8–36.2 ◦C between 18:00 h on 27 January and 08:00 h on 28 January. Thus, a pattern similar to that observed at a hot-water supply temperature of 40 ◦C was observed. That is, the return temperature remained nearly constant in the range 27.5–36.2 ◦C across heating hours. This suggests an optimal return-temperature value of 27.5 ◦C for the hot-water supply at 41 ◦C.

3.3. Indoor-Air Temperature Figure 15 depicts hourly variations in the indoor-air temperatures corresponding to hot-water supply and return temperatures in the range 40–41 ◦C and 27.4–27.5 ◦C, respectively, in the PCM radiant floor-heating system during the period from 26–28 January. At hot-water supply and return temperatures of 40 and 27.4 ◦C, respectively, the indoor-air temperature measured in the range 18.7–22 ◦C between 18:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 18.8–21.9 ◦C between 18:00 h on 27 January and 08:00 h on 28 January. Excluding preheating hours from 18:00 to 22:00 h, based on above results, the indoor-air temperature was observed to fall within the range 21.6–22.1 ◦C (i.e., within 0.1–0.6 ◦C of its pre-set value of 22 ◦C). The indoor-air temperature was stabilized approximately one hour after the floor-surface temperature, thereby apparently reflecting the time needed for heat to be transferred Sustainabilityfrom 2018 the, 10 floor-surface, x FOR PEER to REVIEW the indoor space. 17 of 19

Figure 15. Hourly variations in the indoor-air temperature with differing supply temperatures. Figure 15. Hourly variations in the indoor-air temperature with differing supply temperatures. At supply and return temperatures of 41 and 27.5 ◦C, respectively, the indoor-air temperature 4. Conclusionsoutside of preheating hours measured in the range 22.0–22.6 ◦C between 22:00 h on 26 January and 08:00 h on 27 January; correspondingly, the return temperature measured 22.1–22.4 ◦C between 22:00 h Theon proposed 27 January study and 08:00 determines h on 28 January. optimum Under hot-wa these hot-waterter supply conditions, and return the indoor-air temperatures temperature for a PCM radiant floor-heating system. To this end, computer simulations were performed to derive hot-water supply and return temperatures that could stably maintain floor-surface and indoor-air temperatures, close to their respective pre-set values, through use of the latent heat of the PCM during the coldest period of outdoor temperatures. Results of this study are summarized hereunder. (1) The optimum hot-water supply temperature, for maintaining floor-surface and indoor-air temperatures close to their pre-set values using a wet-constructed PCM radiant floor-heating system containing a 10 mm thick PCM layer, lies in the range 40–41 °C. If hot water is supplied at or below 39 °C or at or above 42 °C, the floor-surface temperature cannot be maintained at its pre-set value, and the latent heat of the PCM cannot be effectively utilized, thereby resulting in the compromised energy-saving capability of the proposed system. (2) The optimum hot-water return temperature under very cold conditions (i.e., at minimum annual temperatures) was determined to be the lowest temperature of hot-water returned from the floor during heating hours. Simulations were performed corresponding to hot-water supply and return temperatures in the range 40–41 °C and 27.4–27.5 °C, respectively. However, it should be noted that the difference between the two hot-water supply temperatures was only 1 °C and, therefore, a minimum difference of 0.1 °C in return temperatures should not be surprising. (3) Under the supply and return temperature conditions described in (2), the system was analyzed to determine whether the indoor-air temperature attained its pre-set value of 22 °C. As observed, the indoor-air temperature measured in the range 18.7–22.6 °C during heating hours, and that when preheating hours were exchanged, the above range was narrowed down to ±0.6 °C around the pre-set value (i.e., the indoor-air temperature measured in the range 21.4–22.6 °C). (4) The proposed radiant floor-heating system comprised a 10 mm thick PCM layer, concrete slabs, insulating material, ALC, and mortar. For floor structures and PCM thicknesses differing from these, processes and analysis techniques described in this study could be used as reference for determining corresponding optimum hot-water supply and return temperatures. This study has been built upon extant work performed with regards to deriving an optimum design of a PCM radiant floor-heating system, with appropriate hot-water supply and return temperatures for maintaining the floor-surface and indoor-air temperatures at their respective pre- set values. When employing the actual system within the floor structure of an apartment building under such conditions, it will be necessary to examine how much energy saving can be realized relative to conventional systems. To do so, a follow-up study will be performed to compare the Sustainability 2018, 10, 4004 17 of 19 followed a pattern similar to that observed for supply and return temperatures of 40 and 27.4 ◦C, respectively, and remained within 0.1–0.6 ◦C of the 22 ◦C pre-set value. In summary, outside of the preheating hours (18:00–22:00 h), setting the hot-water supply and return temperatures in the range 40–41 ◦C and 27.4–27.5 ◦C, respectively, resulted in steady maintenance of the indoor-air temperature within 0.1–0.6 ◦C of the 22 ◦C pre-set value.

4. Conclusions The proposed study determines optimum hot-water supply and return temperatures for a PCM radiant floor-heating system. To this end, computer simulations were performed to derive hot-water supply and return temperatures that could stably maintain floor-surface and indoor-air temperatures, close to their respective pre-set values, through use of the latent heat of the PCM during the coldest period of outdoor temperatures. Results of this study are summarized hereunder.

(1) The optimum hot-water supply temperature, for maintaining floor-surface and indoor-air temperatures close to their pre-set values using a wet-constructed PCM radiant floor-heating system containing a 10 mm thick PCM layer, lies in the range 40–41 ◦C. If hot water is supplied at or below 39 ◦C or at or above 42 ◦C, the floor-surface temperature cannot be maintained at its pre-set value, and the latent heat of the PCM cannot be effectively utilized, thereby resulting in the compromised energy-saving capability of the proposed system. (2) The optimum hot-water return temperature under very cold conditions (i.e., at minimum annual temperatures) was determined to be the lowest temperature of hot-water returned from the floor during heating hours. Simulations were performed corresponding to hot-water supply and return temperatures in the range 40–41 ◦C and 27.4–27.5 ◦C, respectively. However, it should be noted that the difference between the two hot-water supply temperatures was only 1 ◦C and, therefore, a minimum difference of 0.1 ◦C in return temperatures should not be surprising. (3) Under the supply and return temperature conditions described in (2), the system was analyzed to determine whether the indoor-air temperature attained its pre-set value of 22 ◦C. As observed, the indoor-air temperature measured in the range 18.7–22.6 ◦C during heating hours, and that when preheating hours were exchanged, the above range was narrowed down to ±0.6 ◦C around the pre-set value (i.e., the indoor-air temperature measured in the range 21.4–22.6 ◦C). (4) The proposed radiant floor-heating system comprised a 10 mm thick PCM layer, concrete slabs, insulating material, ALC, and mortar. For floor structures and PCM thicknesses differing from these, processes and analysis techniques described in this study could be used as reference for determining corresponding optimum hot-water supply and return temperatures.

This study has been built upon extant work performed with regards to deriving an optimum design of a PCM radiant floor-heating system, with appropriate hot-water supply and return temperatures for maintaining the floor-surface and indoor-air temperatures at their respective pre-set values. When employing the actual system within the floor structure of an apartment building under such conditions, it will be necessary to examine how much energy saving can be realized relative to conventional systems. To do so, a follow-up study will be performed to compare the energy consumption of a conventional system against that of the proposed PCM radiant floor-heating system built in accordance with the wet construction method.

Author Contributions: Funding acquisition, S.K.; project administration, S.K.; writing—original draft, S.B.; writing—review & editing, S.B. and S.K. Funding: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number: 2018R1D1A1B07048848). Conflicts of Interest: The authors declare no conflict of interest. Sustainability 2018, 10, 4004 18 of 19

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