Analysis of Thermal Performance and Energy Saving Potential by PCM Radiant Floor Heating System Based on Wet Construction Method and Hot Water

energies

Article Analysis of Thermal Performance and Energy Saving Potential by PCM Radiant Floor Heating System based on Wet Construction Method and Hot Water

Sanghoon Baek 1 and Sangchul Kim 2,* 1 Industry Academic Cooperation Foundation, Hankyong National University, 327, Jungang-ro, Anseong-si, Gyeonggi-do 17579, Korea; [email protected] 2 School of Architecture, Hankyong National University, 327, Jungang-ro, Anseong-si, Gyeonggi-do 17579, Korea * Correspondence: [email protected]; Tel.: +031-670-5277

Received: 19 January 2019; Accepted: 22 February 2019; Published: 2 March 2019

Abstract: A phase change material (PCM) is an energy storage mass with high heat storage performance. In buildings, PCMs can be utilized to save energy in radiant floor heating systems. This study aims to analyze the thermal performance and energy saving potential by the PCM radiant floor heating system based on wet construction method and hot water. For such analysis, EnergyPlus program was used. As for the results, it was found that the proposed system almost maintained the set point of indoor air and a floor surface. Moreover, when a 10 mm PCM was applied, it was possible to save 2.4% of heating energy annually compared to existing buildings. In particular, when a 20–50 mm PCM was applied, it was found that 7.3–15.3% of heating energy was reduced annually. If indoor air temperature exceeds the comfort range of the proposed system, this problem can be solved by adjusting the set point of the floor surface or by increasing the temperature of hot water.

Keywords: phase change material; radiant ﬂoor heating system; heat storage and discharge; wet construction method

1. Introduction Studies on thermal energy storage systems that combine phase change materials (PCMs) with building structures have been actively conducted to reduce the heating energy consumption in buildings [1]. In particular, studies have been continuously conducted on PCM radiant floor heating systems, which can decrease heating energy by improving the energy performance of existing radiant floor heating systems [2–11]. Lin, K. et al. developed a shape-stabilized PCM (SSPCM) plate to be applied to a radiant floor heating system and proved that this system could maintain the indoor heating temperature in the comfort range through theories and computer simulation [2]. Moreover, they introduced a new radiant floor heating system that could perform heating by combining an office floor with an air cavity including an SSPCM plate and supplying air into the office through a floor diffuser. It was found through experiments that this system could effectively reduce the electrical energy consumed for office heating during daylight hours and maintain indoor temperature in the comfort range [3]. Jin, X. et al. introduced a PCM floor heating system that could perform cooling and heating by inserting cooling and heating coils and low-temperature and high-temperature PCMs into a floor structure. Through simulation, they found that the annual energy consumption could be reduced by approximately 37.9–41.1% using the proposed system [4]. Huang, K. et al. introduced a PCM radiant floor heating system with a solar hot-water system. This system had a unique structure in which a PCM and a capillary hot-water coil were combined on a floor in a grid form without the use of a PCM

Energies 2019, 12, 828; doi:10.3390/en12050828 www.mdpi.com/journal/energies Energies 2019, 12, 828 2 of 17 case. This system not only maintained the indoor set point in a stable manner but also decreased energy consumption by increasing the solar hot-water fraction [5]. Brazin, R. et al. developed a PCM wallboard for radiant floor heating and analyzed its heat storage and energy saving performances. The radiant floor heating system with the PCM wallboard decreased energy consumption by 18.8% and energy cost by approximately 28.7% compared to existing systems [6]. Cheng, W. et al. analyzed the correlation between the thermal conductivity of a PCM and the thermal performance of the floor in a PCM radiant floor heating system through theoretical and experimental tests. They found that the thermal performance of the floor could be improved by increasing the thermal conductivity of the PCM in the range below 1.0 W/m·K[7]. Zhou, G. et al. compared the heating effects depending on the combination of a PCM, sand, a polyethylene coil, and a capillary mat, which could be utilized for radiant floor heating systems, through theoretical and experimental methods. It was found that the combination of the PCM and capillary mat could most effectively reduce heating energy consumption compared to other combinations [8]. Zhang, Y. et al. analyzed the energy performance of a radiant floor heating system that combined a solar hot-water system with a PCM in an office building, as part of the study by Huang, Kailiang et al. [5]. The system reduced heating load and increased the solar energy utilization fraction by approximately 30%, thereby proving that energy consumption was decreased [9]. Plytaria, M. T. et al. combined a solar hot-water system with a radiant floor heating system operated by a heat pump and analyzed the decrease in the energy consumption of the heat pump due to the PCM. It was found through simulation that the electrical energy consumed by the heat pump was 42–67% lower in the floor structure with the PCM than in the floor structure without the PCM [10]. In summary, as shown in Table1, most existing radiant floor heating systems have the floor structure of the dry construction method or use electricity as a heat source. Even though a few companies applied such systems to actual buildings, these systems also use the dry construction method and electricity [11]. Furthermore, it is impossible to apply the existing systems as they are to buildings that use the wet construction method, including concrete and autoclaved lightweight concrete (ALC), and hot water, such as residential buildings in South Korea. Even though a few previous studies introduced systems that use the wet construction method and hot water, the systems used cooling and heating simultaneously or they did not suggest the optimal hot water temperature [4,5,9,10]. As cold water and hot water are used in one coil, the coil or floor structure can be damaged owing to the repeated contraction and expansion of the coil. Moreover, when the optimal supply and return temperatures for floor heating are not suggested, indoor temperature may exceed the comfort range or unnecessary energy can be consumed.

Table 1. Previous studies on phase change material (PCM) radiant ﬂoor heating systems.

Author and Company Year Type of Floor Energy Resource Lin, K. et al. [2] 2004 Dry construction Electrical energy Lin, K. et al. [3] 2007 Dry construction Electrical energy Hot and cold Jin, X. et al. [4] 2011 Wet construction water Solar energy Huang, K. et al. [5] 2014 Wet construction Study & hot water Barzin, R. et al. [6] 2015 Dry construction Electrical energy Cheng, W. et al. [7] 2015 Dry construction Electrical energy Zhou, G. et al. [8] 2015 Dry construction Hot water Solar energy Zhang, Y. et al. [9] 2016 Dry construction & hot water Solar energy Plytaria, M. T. et al. [10] 2018 Wet construction & hot water Energies 2019, 12, 828 3 of 17

Table 1. Cont.

Author and Company Year Type of Floor Energy Resource Type 1 2018 Dry construction Solar energy Negishi corporation Company Ltd. [11] Type 2 2018 Dry construction Solar energy Type 3 2018 Wet construction Electrical energy

Based on these analyses, the authors proposed a new PCM radiant floor heating system for residential buildings in previous studies [12–14]. The proposed system uses the wet construction method based on ALC and mortar and only hot water as a heat source. Moreover, the system is formed such that heavy-weight impact noise and light-weight impact noise are less than 50 and 58 dB, respectively. Thus, it can be used as the floor structure of middle stories for preventing interlayer noise in residential apartment buildings. In the previous studies, the design and characteristics of the proposed system were explained and the optimal melting point of a PCM for floor heating was presented [12,13]. Furthermore, the improved heat storage performance of the system compared to existing systems was confirmed through module tests. In addition, the optimal hot-water supply and return temperatures of the system for maintaining indoor air and floor surface temperatures in the comfort range were presented [14]. This study aims to analyze the thermal performance and energy saving potential by applying the proposed system to an actual residential building. The thermal performance was analyzed to examine whether the indoor air and floor surface temperatures could be maintained at a set point that is within the comfort range. Energy saving potential was analyzed to examine whether the system could reduce the annual heating energy compared to existing systems. As a test method, indoor temperature and floor surface temperature distributions and the annual heating energy consumption were analyzed after applying the existing system and the PCM radiant floor heating system, respectively, to the middle stories (second stories) of the residential apartment buildings with the same indoor and outdoor environments. EnergyPlus ver. 8.7 (ver. 8.7, National Renewable Energy Laboratory, Denver, CO, USA), which is a software to evaluate building environment and energy, was used for this study.

2. Materials and Methods

2.1. Materials

2.1.1. PCM Radiant Floor Heating System Figure1 compares the existing radiant floor heating system with the PCM radiant floor heating system. Both systems are based on the wet construction method. The existing floor structure consists of ALC and mortar, and a hot-water coil is inserted into the mortar layer. In this structure, ALC prevents the heat loss of the upper layer and interlayer noise [12,15]. The mortar stores thermal energy from the hot-water coil and discharges the energy, thereby practically performing indoor heating. However, as this structure contains only a sensible heat section in which only temperature changes occur, its heat storage performance is extremely low [12]. If hot-water supply is stopped, surface temperature decreases sharply owing to out of the set-point in the mortar surface. As a result, consistent hot-water supply and energy consumption are required to maintain the set points of indoor air and the floor surface. To overcome this shortcoming, a PCM, which is a high-performance latent heat storage material, was combined with the floor structure in this study. As the floor structure can consist of sensible heat and latent heat sections in the proposed PCM radiant floor heating system, constant surface temperature can be maintained by the amount of stored latent heat and gradual temperature decrease can be induced in the sensible heat section. Consequently, indoor and floor surface temperatures can remain constant as the time-lag phenomenon, in which Energies 2019, 12, 828 4 of 17

the surface temperature decrease time is delayed, can be extended. Additionally, heating energy Energiesconsumption 2019, 12, canx be reduced by operating the hot water and heating unit intermittently [12]. 4 of 17

121 122 Figure 1.1. ComparisonComparison between between existing existing and and phase phase change chan materialge material (PCM) (PCM) radiant radiant floor heating floor systems.heating 123 systems. 2.1.2. Characteristics of PCM and Fabrication of PCM Case 124 2.1.2.Based Characteristics on the system of PCM proposed and Fabrication in the of previous PCM Case studies, the melting point of the PCM for 125 maintainingBased on floor the surface system temperature proposed andin the indoor previous air temperature studies, the in themelting comfort point range of wasthe presentedPCM for ◦ ◦ 126 maintainingas 35–45 C[ 12floor]. The surface melting temperature point of the PCMand ind usedoor in air this temperature study was 44 inC. the Tables comfort2 and 3range show thewas 127 presentedthermal characteristics as 35–45 ℃ [12]. and the The heat melting capacity point of theof the PCM PCM at different used in temperaturesthis study was [16 44]. The℃. Tables temperature 2 and ◦ ◦ 128 3at show which the the thermal PCM began characteristics to liquefy wasand 40theC, heat and capa thecity temperature of the PCM of complete at different liquefaction temperatures was 44 [16].C. ◦ 129 TheMoreover, temperature the temperature at which withthe PCM the highest began latentto liquefy heat was storage 40 ℃ capacity, and the was temperature 43–44 C. of complete 130 liquefaction was 44 ℃. Moreover, the temperature with the highest latent heat storage capacity was Table 2. The thermal characteristics of the PCM with the melting point of 44 ◦C. 131 43–44 ℃. PCM Melting Point Specific Heat (Cp) Density (ρ) Conductivity (k) ◦ ◦ ◦ 132 TypeTable 2. The thermal( C) characteristics of (J/kg the· PCMC) with the melting(kg/m3 )point of 44 (W/m℃. · C) Organic material 41–44 2000 800 0.2 Melting Specific heat Density Conductivity PCM Point (𝐶) (ρ (k) type (℃) (J/kg·℃) (kg/m3) (W/m·℃) Organic 41–44 2000 800 0.2 material

133 Table 3. Heat capacity of PCM at different temperatures [16].

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Energies 2019, 12, x 5 of 17 Table 3. Heat capacity of PCM at different temperatures [16].

Temperature (°C)Temperature (◦C) Capacity (J/kg) Capacity (J/kg) 35 2000 35 2000 36 2000 36 2000 37 37 3000 3000 38 38 2000 2000 39 39 3000 3000 40 40 6000 6000 41 41 43,000 43,000 42 42 53,000 53,000 43 43 107,000 107,000 44 44 25,000 25,000 45 2000 45 2000 46 2000 46 2000 47 2000 47 48 2000 2000 48 49 2000 2000 49 50 2000 2000 50 2000

Also, the Figure2 shows the fabricating progress of PCM case and the method to apply the case 134 Also, the Figure 2 shows the fabricating progress of PCM case and the method to apply the case to the ﬂoor structure. The PCM case is a very thin aluminum material, and it is manufactured by 135 to the floor structure. The PCM case is a very thin aluminum material, and it is manufactured by using a vacuum device and a high-temperature heat wire, as the PCM case can have high conductivity, 136 using a vacuum device and a high-temperature heat wire, as the PCM case can have high corrosion-resistant, and superior adhesive property to mortar. 137 conductivity, corrosion-resistant, and superior adhesive property to mortar.

138

139 FigureFigure 2. TheThe fabricating fabricating progress progress and the applic applicationation to the floor ﬂoor of the PCM case.

140 2.1.3. Energy Energy Consumption Patterns 141 Figures 33 andand4 4present present the the patterns patterns for for the the energy energy consumption consumption by by the the changes changes of of the the floor floor 142 surface temperaturestemperatures in in the the existing existing and and PCM PCM radiant radiant floor floor heating heating systems. systems. Since theSince existing the existing systems 143 systemsdo not have do not the latenthave the heat latent sections heat that sections enable that the enable phase changethe phase to occur, change the to floor occur, surface the floor temperature surface 144 temperatureis rapidly decreased is rapidly if thedecreased hot-water if the supply hot-water is stopped supply as showedis stopped in theas showed Figure3 .in In the contrast, Figure PCM 3. In 145 contrast,radiant floor PCM heating radiant system floor heating can constantly system can maintain constantly the floormaintain surface the temperaturefloor surface attemperature the set-point at 146 thefor specificset-point times for specific by releasing times theby releasing latent heat the as latent shown heat in theas shown Figure in4. the Consequently, Figure 4. Consequently, the time-lag 147 thephenomenon, time-lag phenomenon, which the point which in time the re-operatingpoint in time of re-operating heat-resource of unit heat-resource is delayed, occurs.unit is Thus,delayed, the 148 occurs.hot-water Thus, supply the hot-water and the heat-resource supply and the unit heat-resource can be intermittently unit can operated,be intermittently energy consumptionoperated, energy can 149 consumptionbe also reduced. can be also reduced.

Energies 2019, 12, 828 6 of 17 Energies 2019,, 12,, xx 6 of 17

150 151 FigureFigure 3. 3. TheThe patterns patterns of of the the floor ﬂoor surface surface temperature temperature and and the the energy energy consumption consumption in in the the existing existing 152 radiantradiantradiant floorfloor ﬂoor heatingheating system.system.

153 154 FigureFigure 4. TheThe patternspatterns of of the the floor floor surface surface temperature temperature and and the energythe energy consumption consumption in the PCMin the radiant PCM 155 radiantradiantfloor heating floorfloor heatingheating system. system.system. 2.2. Methods 156 2.2. Methods 2.2.1. Apartment Building 157 2.2.1. Apartment Building EnergyPlus ver. 8.7 was used to analyze the energy performance of the proposed PCM radiant 158 EnergyPlus ver. 8.7 was used to analyze the energy performance of the proposed PCM radiant 158 floorEnergyPlus heating system ver. [8.717– 19was]. Figureused to5 showsanalyze the the floor ener plangy performance of the apartment of the and proposed the elevation PCM modeled radiant 159 floor heating system [17–19]. Figure 5 shows the floor plan of the apartment and the elevation 159 floorin the heating simulation. system The [17–19]. building Figure consisted 5 shows of three the storiesfloor plan with of a heightthe apartment of 2.6 m each.and the Even elevation though 160 modeled in the simulation. The building consisted of three stories with a height of 2.6 m each. Even 160 modeledthe inside in was the composedsimulation. of The various building zones, consisted all zones of werethree assumedstories with to be a height heating of targets. 2.6 m each. Two Even types 161 though the inside was composed of various zones, all zones were assumed to be heating targets. Two 161 thoughof buildings the inside were was modeled composed for the of comparisonvarious zones, of all energy zones consumption. were assumed One to be was heating a general targets. building Two 162 types of buildings were modeled for the comparison of energy consumption. One was a general 162 typeswithout of thebuildings PCM, whilewere modeled the other for was the a buildingcomparis withon of the energy PCM consumption. radiant floor heatingOne was system. a general The 163 building without the PCM, while the other was a building with the PCM radiant floor heating system. 163 buildingenvelopes without of the the buildings PCM, while were the reinforced other was concrete a building structures, with the and PCM the radiant details floor of each heating envelope system. are 164 The envelopes of the buildings were reinforced concrete structures, and the details of each envelope 164 Theshown envelopes in Table of4. the All buildings envelopes were were reinforced designed toconcre meette the structures, heat transmittance and the details (U-value) of each of domesticenvelope 165 are shown in Table 4. All envelopes were designed to meet the heat transmittance (U-value) of 165 arestandards shown [20in ].Table Four 4. windows All envelopes of different were sizes designed were installed to meet onthe the heat south transmittance side of each (U-value) building. Asof 166 domestic standards [20]. Four windows of different sizes were installed on the south side of each 166 domesticshown in standards Table5, the [20]. window Four consisted windows of of triple different pane glazing,sizes were argon installed insulating on the gas, south and an side aluminum of each 167 building. As shown in Table 5, the window consisted of triple pane glazing, argon insulating gas, and 167 building. As shown in Table 5, the window consisted of triple2 ·pane glazing, argon insulating gas, and frame. The U-value of the entire window was 1.208 W/m K, which2 was slightly higher than the 168 an aluminum frame. The U-value of the entire window was 1.208 W/m2 ⋅K, which was slightly higher 168 an aluminum frame. The U-value2· of the entire window was 1.208 W/m ⋅K, which was slightly higher domestic criterion (1.200 W/m K) [20].2 In particular, the proposed PCM radiant floor heating system 169 than the domestic criterion (1.200 W/m2⋅⋅K) [20]. In particular, the proposed PCM radiant floor heating 169 thanwas the applied domestic to the criterion second (1.200 stories W/m of theK) buildings[20]. In particular, because the it was proposed designed PCM for radiant the middle floor heating stories 170 system was applied to the second stories of the buildings because it was designed for the middle 170 systemof apartment was applied buildings. to the Therefore, second stories among of thethe simulationbuildings because results ofit was the indoordesigned environment for the middle and 171 stories of apartment buildings. Therefore, among the simulation results of the indoor environment 172 and energy consumption for the buildings, the results obtained for the second stories were the major 173 analysis targets.

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energy consumption for the buildings, the results obtained for the second stories were the major analysis targets. Energies 2019, 12, x 7 of 17

174

175 FigureFigure 5.5.Floor Floor planplan ofof thethe apartmentapartment buildingbuilding ((lefleftt)) andand modelingmodeling byby simulationsimulation ((right).right).

176 TableTable 4.4. StructuresStructures andand characteristicscharacteristics ofof buildingbuilding envelopesenvelopes [20[20].].

Thickness Thickness (c) U-value U-value(c) Section Section MaterialMaterial MaterialMaterial Total Total 2 (mm)(mm) (mm) (mm) (W/m ·K)(W/m2⋅K) ConcreteConcrete slab slab 210 210 Insulating material 120 1st-story Insulating materialALC 40 120 1st-story 415 0.1741 floor structure Hot-waterALC coil 15 40 floor Mortar 40 415 0.1741 Hot-water coil 15 structure Finishing material 5 MortarGypsum board 10 40 FinishingAir material cavity 100 5 Concrete slab 210 GypsumInsulating board material 20 10 2nd-story ALC (a) 15 Air cavity 100 425 0.1534 floor structure PCM (b) 10 Concrete slab 210 ALC 15 InsulatingHot-water material coil 15 20 2nd-story Mortar 40 ALC(a) 15 floor Finishing material 5 425 0.1534 PCM(b) 10 structure Gypsum board 10 ALCAir cavity 100 15 Concrete slab 210 Hot-water coil 15 3rd-story Insulating material 20 425 0.1530 floor structure MortarALC 40 40 FinishingHot-water material coil 15 5 Mortar 40 GypsumFinishing board material 5 10 AirGypsum cavity board 10 100 Air cavity 100 3rd-story Concrete slab 210 Insulating material 90 490 0.1105 3rd-storyroof structure InsulatingConcrete material slab 210 20 floor 425 0.1530 ALCMortar 80 40 structure Hot-water coil 15 Mortar 40 Finishing material 5 3rd-story Gypsum board 10 490 0.1105

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Table 4. Cont.

Thickness U-value (c) Section Material Material Total (mm) (mm) (W/m2·K) Concrete wall 180 Exterior Insulating material 120 310 0.2005 wall Gypsum board 10 (a) Autoclaved Lightweight Concrete. (b) Phase Change Material. (c) Thermal Transmittance.

Table 5. Structure and characteristics of the window.

Glass Manufacturing company Shanghai Yaohua Pilkinton Glass Ltd. Thickness 6 (mm) Thermal conductivity 1.000 (W/m·K) Transmittance 0.246 Total wave Reflectance 0.232 Exterior Transmittance 0.313 surface Visible wave Reflectance 0.302 Surface emissivity 0.840 Transmittance 0.246 Total wave Reflectance 0.218 Interior Transmittance 0.131 surface Visible wave Reflectance 0.106 Surface emissivity 0.540 Insulating gas Gas type Argon (100% pure) Thickness 12 (mm) Thermal conductivity 0.016348 (W/m·K) Specific heat 521.929 (J/kg·◦C) Density 1.782282 (kg/m3) Viscosity 0.000021 (kg/m·s) Prandtl number 0.6705 Triple pane glazing system Total thickness 42 (mm) Total U-value 1.208 (W/m2·K) Frame system Material Aluminum Filled insulating material Urethane Width 300 (mm) Projected length from glazing 129 (mm) Entire window system Exterior surface heat transfer coefficient 23.3 (W/m2·K) Interior surface heat transfer coefficient 9.1 (W/m2·K) Total U-value of window system 1.208 (W/m2·K)

2.2.2. Weather Data As the modeled buildings were assumed to be located in Seoul, South Korea, Seoul standard weather data were used [21]. Figure6 shows the representative weather data constituting the outdoor environment in the simulation. The total period of the weather data was one year, and the time interval was entered every hour. Energies 2019, 12, x 9 of 17

Material Aluminum

Filled insulating material Urethane

Width 300 (mm)

Projected length from glazing 129 (mm)

Entire window system

Exterior surface heat transfer coefficient 23.3 (W/m2⋅K)

Interior surface heat transfer coefficient 9.1 (W/m2⋅K)

Total U-value of window system 1.208 (W/m2⋅K)

181 2.2.2. Weather Data 182 As the modeled buildings were assumed to be located in Seoul, South Korea, Seoul standard 183 weather data were used [21]. Figure 6 shows the representative weather data constituting the outdoor 184 Energiesenvironment2019, 12, 828 in the simulation. The total period of the weather data was one year, and the time9 of 17 185 interval was entered every hour.

186 187 FigureFigure 6.6. AnnualAnnual Seoul standard standard weather weather data. data.

188 2.2.3.2.2.3. Indoor Indoor Heat Heat Gain Gain and and Loss Loss 189 TableTable6 shows6 shows four four elementselements that caused caused indoor indoor heat heat gain gain and andheat heatloss in loss the insimulation. the simulation. Four 190 Fouroccupants occupants were were assumed. assumed. Light Light entered entered at the at ho theurly hourly fraction fraction depending depending on the on occupants. the occupants. In 191 Inaccordance accordance with with the the domestic domestic standards, standards, the the infiltra inﬁltrationtion and and ventilation ventilation rates rates were were entered entered as as1.0 1.0 air air exchange per hour [22] and 0.5 air exchange per hour [20], respectively, for energy-saving buildings. In addition, as the heat gain from indoor devices and occupants does not signiﬁcantly affect the indoor environment and energy consumption in small residential buildings, such elements were excluded from the simulation conditions.

Table 6. Schedules of daily occupants, light, inﬁltration, and ventilation.

Factor Time Interval Unit Value 00:00–08:00 4 Occupant 08:00–18:00person 0 18:00–24:00 4 00:00–05:00 0.1 05:00–08:00 1.0 Light 08:00–18:00Fraction 0.0 18:00–22:00 1.0 22:00–24:00 0.1 Inﬁltration 00:00–24:00 air-change/h 3.0 00:00–08:00 0.5 Ventilation 08:00–18:00air-change/h 0.0 18:00–24:00 0.5 Energies 2019, 12, x 10 of 17

192 exchange per hour [22] and 0.5 air exchange per hour [20], respectively, for energy-saving buildings. 193 In addition, as the heat gain from indoor devices and occupants does not significantly affect the 194 indoor environment and energy consumption in small residential buildings, such elements were 195 excluded from the simulation conditions.

196 Table 6. Schedules of daily occupants, light, infiltration, and ventilation.

Factor Time interval Unit Value

00:00–08:00 4

Occupant 08:00–18:00 person 0

18:00–24:00 4

00:00–05:00 0.1

05:00–08:00 1.0

Light 08:00–18:00 Fraction 0.0

18:00–22:00 1.0

22:00–24:00 0.1

Infiltration 00:00–24:00 air-change/h 3.0

00:00–08:00 0.5

Ventilation 08:00–18:00 air-change/h 0.0

18:00–24:00 0.5 Energies 2019, 12, 828 10 of 17

197 2.2.4. District Heating 2.2.4. District Heating 198 As shown in Figure 7, a district heating system was applied to supply hot water to the modeled 199 building.As shown Under in the Figure domestic7, a district standards, heating the system maximum was appliedsupply toand supply return hot temperatures water to the of modeled the hot 200 waterbuilding. provided Under by the the domestic domestic standards, district heat theing maximum corporation supply for apartment and return buildings temperatures are 115 of the℃ and hot ◦ 201 water55 ℃, providedrespectively by [23]. the domestic Moreover, district the supply heating and corporation return temperatures for apartment of the buildings hot water are provided 115 C and to ◦ 202 apartment55 C, respectively buildings [23 through]. Moreover, a heat theexchange supply unit and are return 60 ℃ temperatures and 45 ℃, respectively of the hot water [24]. However, provided as to ◦ ◦ 203 theapartment floor of buildings the second through story aof heat the exchangeapartment unit building are 60 withC and the 45 PCMC, respectivelyradiant floor [24 heating]. However, system as 204 hasthe ﬂoora different of the second structure story compared of the apartment to existing building buildings, with the PCMnew radianthot-water ﬂoor supply heating and system return has 205 temperaturesa different structure must be compared applied. In to the existing previous buildings, study, we new proposed hot-water that supply the optimal and return supply temperatures and return 206 temperaturesmust be applied. of hot In the water previous for the study, system we proposedare 40–41 that℃ and the optimal27.4–27.5 supply ℃, respectively and return temperatures[14]. Among of hot water for the system are 40–41 ◦C and 27.4–27.5 ◦C, respectively [14]. Among these ranges, 207 these ranges, the supply and return temperatures of 40 ℃ and 27.5 ℃, respectively, were applied to the supply and return temperatures of 40 ◦C and 27.5 ◦C, respectively, were applied to the simulation. 208 the simulation.

209 Energies 2019, 12, x 11 of 17 210 Figure 7. ConceptionConception of of hot-water hot-water circulation circulation between th thee district heating and an apartment building. 211 Figure 8 shows the connections of the heating units for modeling an apartment building that 212 uses Figuredistrict8 showsheating the in connections EnergyPlus. of theA hydronic heating units pump, for modelinghot-water an pipes, apartment cold-water building pipes, that usesand 213 districtsplitters heating are required in EnergyPlus. to supply Ahot hydronic water from pump, the hot-water district heating pipes, cold-waterto the floors pipes, of the and apartment splitters 214 arebuilding. required The to inlet supply and hotoutlet water of each from unit the must district be connected heating to according the ﬂoors to of the the flow apartment direction building. of hot 215 Thewater. inlet Then, and the outlet performances of each unit and must characteristics be connected of according such units to themust ﬂow be directionentered, ofas hotshown water. in Table Then, 216 the7. performances and characteristics of such units must be entered, as shown in Table7.

217

218 Figure 8. Connection loop of each unit for circulation of hot water in EnergyPlus.

219 Table 7. Performances and characteristics of units for hot-water circulation.

Unit Value Hydronic-pump pipe diameter 0.015 (m) Hydronic-pump surface heat loss Adiabatic Hydronic-pump flow rate Autosize Hydronic-pump head pressure 200,000 (Pa) Hydronic-pump motor efficiency 0.9 Hydronic-pump control type Intermittent Hot-water pipe diameter 0.015 (m) Hot-water pipe surface heat loss Adiabatic Cool-water pipe diameter 0.015 (m) Cool-water pipe surface heat loss Adiabatic Splitter flow rate control Autosize

220 2.2.5. Heating Schedule 221 It is necessary to control indoor air and floor surface temperatures simultaneously for indoor 222 heating by the radiant floor heating system. Table 8 shows the set-point schedules of indoor air and 223 the floor surface in EnergyPlus. The set points of indoor air and the floor surface are 20 ℃ and 30 ℃, 224 respectively, in the time interval with occupants and 10 ℃ and 15 ℃, respectively, in the time interval 225 without occupants.

226 Table 8. Schedules of set points for indoor air and the floor surface.

Factor Time interval Value 00:00–08:00 20 ℃ 08:00–18:00 10 ℃ Indoor temperature 18:00–00:00 20 ℃ 18:00-24:00 20 ℃ Floor surface temperature 00:00–08:00 30 ℃

Energies 2019, 12, 828 11 of 17

Table 7. Performances and characteristics of units for hot-water circulation.

Unit Value Hydronic-pump pipe diameter 0.015 (m) Hydronic-pump surface heat loss Adiabatic Hydronic-pump flow rate Autosize Hydronic-pump head pressure 200,000 (Pa) Hydronic-pump motor efficiency 0.9 Hydronic-pump control type Intermittent Hot-water pipe diameter 0.015 (m) Hot-water pipe surface heat loss Adiabatic Cool-water pipe diameter 0.015 (m) Cool-water pipe surface heat loss Adiabatic Splitter flow rate control Autosize

2.2.5. Heating Schedule It is necessary to control indoor air and floor surface temperatures simultaneously for indoor heating by the radiant floor heating system. Table8 shows the set-point schedules of indoor air and the floor surface in EnergyPlus. The set points of indoor air and the floor surface are 20 ◦C and 30 ◦C, respectively, in the time interval with occupants and 10 ◦C and 15 ◦C, respectively, in the time interval without occupants.

Table 8. Schedules of set points for indoor air and the ﬂoor surface.

Factor Time Interval Value 00:00–08:00 20 ◦C 08:00–18:00 10 ◦C Indoor temperature 18:00–00:00 20 ◦C 18:00-24:00 20 ◦C 00:00–08:00 30 ◦C 08:00–18:00 15 ◦C Floor surface temperature 18:00–00:00 30 ◦C 18:00-24:00 30 ◦C

3. Results

3.1. Indoor Air and Floor Surface Temperatures It was analyzed whether indoor and floor surface temperatures were maintained at the set points on the second stories of the apartment buildings with the existing and PCM radiant floor heating systems. The analysis was conducted from 0:00 on January 26th to 1:00 on January 28th (two days), when the outdoor temperature was the lowest throughout the year, to examine detailed temperatures. Figure9 shows the indoor temperature distributions of the existing building and the building with the PCM radiant floor heating system. The indoor temperature of the existing building ranged from 14.7 to 18.5 ◦C for 48 h, including the heating and non-heating periods. The indoor air temperature of the building with the PCM radiant floor heating system ranged from 14.2 to 18.5 ◦C. In the non-heating period, indoor temperature was controlled to remain at 10 ◦C or higher, and the two buildings met this set point. In particular, in the heating period, the two buildings maintained an indoor temperature of 18 ◦C within an error range of approximately ± 1 ◦C. This is approximately 2 ◦C lower than the set point of 20 ◦C. This difference is because of the insufficient thermal energy supply from the radiant floor heating system to the inside. However, as the temperature is not significantly different from the set point, this problem can be addressed by slightly increasing the heat capacities of the radiant floor heating system. There are two methods of increasing the heat capacities of the floor structures for Energies 2019, 12, x 12 of 17

08:00–18:00 15 ℃ 18:00–00:00 30 ℃ 18:00-24:00 30 ℃

227 3. Results

228 3.1. Indoor Air and Floor Surface Temperatures 229 It was analyzed whether indoor and floor surface temperatures were maintained at the set points 230 on the second stories of the apartment buildings with the existing and PCM radiant floor heating 231 systems. The analysis was conducted from 0:00 on January 26th to 1:00 on January 28th (two days), 232 when the outdoor temperature was the lowest throughout the year, to examine detailed temperatures. 233 Figure 9 shows the indoor temperature distributions of the existing building and the building 234 with the PCM radiant floor heating system. The indoor temperature of the existing building ranged 235 from 14.7 to 18.5 ℃ for 48 h, including the heating and non-heating periods. The indoor air 236 temperature of the building with the PCM radiant floor heating system ranged from 14.2 to 18.5 ℃. 237 In the non-heating period, indoor temperature was controlled to remain at 10 ℃ or higher, and the 238 two buildings met this set point. In particular, in the heating period, the two buildings maintained 239 an indoor temperature of 18 ℃ within an error range of approximately ± 1 ℃. This is approximately 240 2 ℃ lower than the set point of 20 ℃. This difference is because of the insufficient thermal energy 241 supply from the radiant floor heating system to the inside. However, as the temperature is not 242 significantly different from the set point, this problem can be addressed by slightly increasing the Energies 2019, 12, 828 12 of 17 243 heat capacities of the radiant floor heating system. There are two methods of increasing the heat 244 capacities of the floor structures for raising low indoor temperature to the set point. The first is to 245 raisingincrease low the indoortemperature temperature of hot water, to the setand point. the second The first is to is increase to increase the theset temperaturepoint of the floor of hot surface. water, 246 andIf insufficient the second heat is to supply increase is improved the set point through of the these floor surface.methods, If it insufficient will be possible heat supply to maintain is improved indoor 247 throughair temperature these methods, in the comfort it will berange. possible to maintain indoor air temperature in the comfort range.

248

249 Figure 9. Indoor air temperatures on the second stories of the apartment buildings.buildings.

250 Figure 10 shows the floorfloor surface temperatures on the second stories of the existing building and 251 the building with the PCM radiant floorfloor heating system.system. The floor floor surface temperature of the existing 252 building was was 27.8–30.5 27.8–30.5 ℃◦C during during the the heating heating and and non-heating non-heating peri periods,ods, while while that thatof the of building the building with 253 withthe PCM the PCM was was27.3–30.1 27.3–30.1 ℃. ◦InC. the In thenon-heating non-heating period, period, the the floor floor surface surface temperatures temperatures of of the the two 254 buildings remained within within 27.3–27.8 27.3–27.8 ℃◦C,, which which were were higher higher than than the the set set point point of of15 15℃.◦ C.Moreover, Moreover, in 255 inthe the heating heating period, period, the the floor floor surface surface temperat temperaturesures of of the the two two buildings buildings remained remained at 30 ◦℃C within an 256 error range ofof ± 11 ℃◦C.. Therefore, Therefore, it it was was confirmed confirmed that that the the proposed proposed PCM PCM radiant radiant floor floor heating system 257 couldEnergies almost 2019, 12 meet, x the set pointspoints ofof indoorindoor air.air. 13 of 17

258

259 Figure 10.10. Floor surface temperatures onon thethe secondsecond storiesstories ofof thethe apartmentapartment buildings.buildings.

260 3.2.3.2. Annual HeatingHeating EnergyEnergy ConsumptionConsumption andand ReductionReduction RatioRatio 261 TheThe analysisanalysis ofof energyenergy consumptionconsumption waswas focusedfocused onon districtdistrict heatingheating andand hot-waterhot-water use byby 262 apartmentapartment buildings. Figure Figures 11 11 and and Figure 12 show 12 sh theow energy the energy consumptions consumptions and reductionand reduction ratios ratios per 263 yearper year on the on second the second stories stories of the of existing the existing building building and the and building the building with the with PCM the radiant PCM floorradiant heating floor 264 system.heating Thesystem. annual The energy annual consumption energy consumption by district by heating district was heating 123.9 was MWh 123.9 for MWh the existing for the building existing 265 andbuilding 121.8 and MWh 121.8 for MWh the building for the withbuilding the PCMwith the radiant PCM floor radiant heating floor system. heating Whensystem. the When existing the 266 floorexisting system floor was system replaced was byreplaced the PCM by radiantthe PCM floor radiant heating floor system, heating district system, heating district was heating reduced was by 267 approximatelyreduced by approximately 1.6%. Moreover, 1.6%. Moreover, the annual the hot-water annual hot-water energy consumption energy consumption was 41.7 was MWh 41.7 for MWh the 268 existingfor the existing building building and 39.7 and MWh 39.7 for MWh the building for the withbuilding the PCMwith radiantthe PCM floor radiant heating floor system. heating Therefore, system. 269 Therefore, the hot-water energy reduction ratio obtained by applying the PCM radiant floor heating 270 system instead of the existing system was approximately 4.8%. Overall, the total annual energy 271 consumption on the second story was 165.6 MWh for the apartment building with the existing floor 272 system and 161.5 MWh for the building with the PCM radiant floor heating system. The total annual 273 heating energy reduction ratio obtained by applying the PCM radiant floor heating system instead 274 of the existing system was 2.4%.

275 276 Figure 11. Annual energy consumptions of the PCM radiant floor heating systems.

Energies 2019, 12, x 13 of 17

258 259 Figure 10. Floor surface temperatures on the second stories of the apartment buildings.

260 3.2. Annual Heating Energy Consumption and Reduction Ratio 261 The analysis of energy consumption was focused on district heating and hot-water use by 262 apartment buildings. Figure 11 and Figure 12 show the energy consumptions and reduction ratios 263 per year on the second stories of the existing building and the building with the PCM radiant floor 264 heating system. The annual energy consumption by district heating was 123.9 MWh for the existing 265 building and 121.8 MWh for the building with the PCM radiant floor heating system. When the 266 existing floor system was replaced by the PCM radiant floor heating system, district heating was Energies 2019, 12, 828 13 of 17 267 reduced by approximately 1.6%. Moreover, the annual hot-water energy consumption was 41.7 MWh 268 for the existing building and 39.7 MWh for the building with the PCM radiant floor heating system. 269 theTherefore, hot-water the energyhot-water reduction energy ratioreduction obtained ratio byobtained applying by applying the PCM the radiant PCM floor radiant heating floor systemheating 270 insteadsystem ofinstead the existing of the system existing was system approximately was approx 4.8%.imately Overall, 4.8%. the totalOverall, annual the energytotal annual consumption energy 271 onconsumption the second on story the wassecond 165.6 story MWh was for 165.6 the apartmentMWh for the building apartment with building the existing with floorthe existing system floor and 272 161.5system MWh and for161.5 the MWh building for the with building the PCM with radiant the PC floorM radiant heating floor system. heating The system. total The annual total heating annual 273 energyheating reduction energy reduction ratio obtained ratio obtained by applying by applyi the PCMng the radiant PCM radiant floor heating floor heating system system instead instead of the 274 existingof the existing system system was 2.4%. was 2.4%.

275

276 Energies 2019, 12FigureFigure, x 11.11. AnnualAnnual energyenergy consumptionsconsumptions ofof thethe PCMPCM radiantradiant ﬂoorfloor heatingheating systems.systems. 14 of 17

277

278 FigureFigure 12.12. Annual energy savingsaving ratioratio ofof thethe PCMPCM radiantradiant ﬂoorfloor heatingheating system.system.

279 4.4. DiscussionDiscussion 280 TheThe aboveabove analysisanalysis showedshowed thatthat thethe annualannual heatingheating energyenergy reductionreduction ratioratio obtainedobtained byby applyingapplying 281 thethe PCMPCM radiantradiant ﬂoorfloor heatingheating systemsystem insteadinstead ofof thethe existingexisting systemsystem waswas 2.4%.2.4%. However,However, thisthis valuevalue 282 isis extremelyextremely lowlow andand unsatisfactoryunsatisfactory inin aspectaspect ofof energyenergy reduction.reduction. Hence,Hence, thethe capacitycapacity ofof thethe PCMPCM 283 waswas increasedincreased graduallygradually toto furtherfurther improveimprove thethe energyenergy savingsaving performance of the proposed system. 284 InIn thisthis instance,instance, thethe PCMPCM cannotcannot exceedexceed thethe maximummaximum thicknessthickness ofof thethe ALCALC becausebecause itit isis designeddesigned toto 285 bebe insertedinserted inin thethe ALC ALC layer, layer, as as shown shown in in Figure Figure 13 13.. Therefore, Therefore, the the thickness thickness of PCMof PCM was was set set as 20,as 30,20, 286 40,30, and40, and 50 mm 50 mm in the in additionalthe additional tests test tos improve to improve system system performance. performance.

287 288 Figure 13. Application method of 10–50 mm PCM. ALC: autoclaved lightweight concrete.

289 Figure 14 compares the annual heating energy consumptions on the second story of the 290 apartment building, where the thickness of the PCM is varied from 10 to 50 mm. According to the 291 analysis results described earlier, the annual heating energy consumptions in the cases with the 292 existing system and 10 mm PCM were 165.6 and 161.5 MWh, respectively. When the thickness of the 293 PCM was increased to 20, 30, 40, and 50 mm, the heating energy consumptions were 153.5, 147.8, 294 143.6, and 140.3 MWh, respectively. As shown in Figure 15, the total heating energy reduction ratio 295 was 2.4% in the building that applied the 10 mm PCM, and it ranged from 7.3% to 15.3% when the 296 thickness of the PCM increased from 20 mm to 50 mm. This is because the discharge of latent heat 297 and sensible heat increased with the capacity of the PCM. In other words, the use of hot water was 298 reduced as the duration in which floor surface temperature remained constant was extended by the 299 PCM. Therefore, it can be judged that the energy saving performance of the floor can be improved as 300 the capacity of PCM increases in the PCM radiant floor heating system. However, as the proposed 301 PCM radiant floor heating system is designed as the floor structure for the middle stories of 302 apartment buildings, it must also prevent interlayer noise [12]. Therefore, when ALC is replaced by 303 PCMs in the future, it is necessary to examine whether the new system can exhibit the same noise 304 performance as the existing system.

Energies 2019, 12, x 14 of 17

277 278 Figure 12. Annual energy saving ratio of the PCM radiant floor heating system.

279 4. Discussion 280 The above analysis showed that the annual heating energy reduction ratio obtained by applying 281 the PCM radiant floor heating system instead of the existing system was 2.4%. However, this value 282 is extremely low and unsatisfactory in aspect of energy reduction. Hence, the capacity of the PCM 283 was increased gradually to further improve the energy saving performance of the proposed system. 284 In this instance, the PCM cannot exceed the maximum thickness of the ALC because it is designed to 285 Energiesbe inserted2019, 12in, 828the ALC layer, as shown in Figure 13. Therefore, the thickness of PCM was set 14as of 20, 17 286 30, 40, and 50 mm in the additional tests to improve system performance.

287

288 Figure 13. Application method of 10–50 mm PCM. AL ALC:C: autoclaved lightweight concrete.

289 Figure 1414 compares compares the the annual annual heating heating energy energy consumptions consumptions on the secondon the storysecond of thestory apartment of the 290 building,apartment where building, the where thickness the ofthickness the PCM of isthe varied PCM fromis varied 10 to from 50 mm. 10 to According 50 mm. According to the analysis to the 291 resultsanalysis described results described earlier, the earlier, annual the heating annual energy heating consumptions energy consumptions in the cases with in the existingcases with system the 292 andexisting 10 mmsystem PCM and were 10 mm 165.6 PCM and were 161.5 165.6 MWh, and respectively.161.5 MWh, respectively. When the thickness When the of thickness the PCM of was the 293 increasedPCM was toincreased 20, 30, 40,to and20, 30, 50 40, mm, and the 50 heating mm, the energy heating consumptions energy consumptions were 153.5, were 147.8, 153.5, 143.6, 147.8, and 294 140.3143.6, MWh,and 140.3 respectively. MWh, respectively. As shown inAs Figure shown 15 in, the Figure total 15, heating the total energy heating reduction energy ratio reduction was 2.4% ratio in 295 thewas building2.4% in the that building applied that the 10applied mm PCM, the 10 and mm it PCM, ranged and from it ranged 7.3% to from 15.3% 7.3% when to 15.3% the thickness when the of 296 thethickness PCM increasedof the PCM from increased 20 mm from to 50 20 mm. mm This to 50 is becausemm. This the is dischargebecause the of latentdischarge heat of and latent sensible heat 297 heatand sensible increased heat with increased the capacity with of the the capacity PCM. In of other the PCM. words, In the other use words, of hot waterthe use was of reducedhot water as was the 298 durationreduced as in the which duration floor surface in which temperature floor surface remained temperature constant remained was extended constant by was the extended PCM. Therefore, by the 299 itPCM. can beTherefore, judged thatit can the be energy judged saving that the performance energy saving of theperformance floor can beof the improved floor can as be the improved capacity as of 300 PCMthe capacity increases of PCM in the increases PCM radiant in the floor PCM heating radiant system. floor heating However, system. as theHowever, proposed as the PCM proposed radiant 301 floorPCM heatingradiant systemfloor heating is designed system as the is floordesigned structure as the for floor the middle structure stories forof the apartment middle buildings,stories of 302 itapartment must also buildings, prevent interlayerit must also noise prevent [12]. interlayer Therefore, noise when [12]. ALC Therefore, is replaced when by PCMsALC is in replaced the future, by 303 itPCMs is necessary in the future, to examine it is necessary whether to the examine new system whether can the exhibit new the system same can noise exhibit performance the same asnoise the 304 Energiesexistingperformance 2019 system., 12, xas the existing system. 15 of 17

305 306 FigureFigure 14. 14. EnergyEnergy consumptions consumptions by by the the application application of of 10–50 10–50 mm mm PCM. PCM.

307 308 Figure 15. Energy reduction ratios by the application of 10–50 mm PCM.

309 5. Conclusion 310 This study aimed to evaluate the indoor environment and energy saving performance for the 311 PCM radiant floor heating system based on the wet construction method and hot water. The results 312 of this study are summarized as follows: 313 1. When the radiant floor heating system with the 10 mm PCM was applied to an apartment 314 building, the indoor air temperature ranged from 14.2 to 18.5 ℃ with a control error of ± 1 ℃ for 315 the simulation days including heating and non-heating periods. However, considering the 316 heating period only, the indoor air temperature was maintained at around 18 ℃. Furthermore, 317 the floor surface temperature by the PCM radiant floor heating system ranged from 27.3 to 30.1 318 ℃ with a control error of ± 1 ℃ in the heating and non-heating periods. On the other hand, 319 considering the heating period only, the floor surface temperature was maintained at 320 approximately 30 ℃. 321 2. Consequently, the indoor air and floor surface temperatures were nearly maintained at each set- 322 point for the heating period by applying to the PCM radiant floor heating system to the 323 apartment building. However, in the case of the indoor air temperature, it was around 2 ℃ lower 324 than the set-point of 20 ℃. As a result, if the indoor air temperature is maintained at lower or

Energies 2019, 12, x 15 of 17

305 Energies 2019, 12, 828 15 of 17 306 Figure 14. Energy consumptions by the application of 10–50 mm PCM.

307 308 FigureFigure 15. 15. EnergyEnergy reduction reduction ratios ratios by by the the application application of of 10–50 10–50 mm mm PCM. PCM. 5. Conclusions 309 5. Conclusion This study aimed to evaluate the indoor environment and energy saving performance for the 310 This study aimed to evaluate the indoor environment and energy saving performance for the PCM radiant floor heating system based on the wet construction method and hot water. The results of 311 PCM radiant floor heating system based on the wet construction method and hot water. The results this study are summarized as follows: 312 of this study are summarized as follows: 313 1.1. WhenWhen the the radiant radiant floor floor heating heating system system with with the the 10 10 mm mm PCM PCM was was applied applied to to an an apartment apartment ◦ ◦ 314 building,building, the the indoor indoor air air temperature temperature ranged ranged from from 14.2 14.2 to to18.5 18.5 ℃ withC with a control a control error error of ± of 1 ℃± 1forC 315 thefor simulation the simulation days days including including heating heating and and non-heating non-heating periods. periods. However, However, considering considering the the ◦ 316 heatingheating period period only, only, the the indoor indoor air air temperature temperature was was maintained maintained at at around around 18 18 ℃C.. Furthermore, Furthermore, 317 thethe floor floor surface surface temperature temperature by bythe thePCM PCM radiant radiant floor floor heating heating system system ranged ranged from from27.3 to 27.3 30.1 to ◦ ◦ 318 ℃30.1 withC a withcontrol a control error of error ± 1 of℃ ±in1 theC heating in the heating and non-heating and non-heating periods. periods. On the Onother the hand, other 319 consideringhand, considering the heating the heating period period only, only,the floor the floor surface surface temperature temperature was was maintained maintained at at ◦ 320 approximatelyapproximately 30 30 ℃.C. 321 2.2. Consequently,Consequently, the the indoor indoor air air and and floor floor surface surface temperatures temperatures were were nearly nearly maintained maintained at each at set- each 322 pointset-point for the for heating the heating period period by applying by applying to the to thePCM PCM radiant radiant floor floor heating heating system system to to the the ◦ 323 apartmentapartment building. building. However, However, in in the the case case of of the the indoor indoor air air temperature, temperature, it it was was around around 2 2 ℃C lower lower ◦ 324 thanthan the the set-point set-point of of 20 20 ℃.C. As As a aresult, result, if ifthe the indoor indoor air air temperature temperature is is maintained maintained at at lower lower or or higher temperatures than the set-point, it can be solved by adjusting the floor surface temperature through the changes of the hot-water temperature or the PCM melting point.

3. District heating plant was applied to supply the hot water to the PCM radiant floor heating system for the heating period, and the annual energy consumption was analyzed by connecting the district heating plant and the PCM radiant floor heating system. While the annual energy consumed by the existing radiant floor heating system was found to be 165.6 MWh, the annual energy consumed by the PCM radiant floor heating system was 161.5 MWh. Therefore, it revealed that the radiant floor heating system with 10 mm PCM can reduce the energy consumption of around 2.4 % annually as compared with the existing system. 4. The thickness of the PCM was increased to 20–50 mm to improve the energy performance of the PCM radiant floor heating system. As a result, the annual energy saving ratio was increased from 7.3 to 15.3 %. This is because the discharge of latent heat and sensible heat increased with the capacity of the PCM. However, when the proposed system is utilized as the floor structure on the middle stories of apartment buildings, the change in noise performance caused by replacing the existing ALC with PCM must be examined. Energies 2019, 12, 828 16 of 17

Along with the previous studies, this study presented the results on the design, structure, optimal hot-water temperature, and energy performance for the PCM radiant ﬂoor heating system based on the wet construction method and hot water. In future, an experimental test will be conducted to analyze the actual heating energy consumption by applying the proposed system to actual apartment buildings or test houses.

Author Contributions: Funding acquisition, S.B. and S.K.; project administration, S.B. and S.K.; writing—original draft, S.B.; writing—review & editing, S.B. Funding: This research was funded 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.

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