Potential of Utilizing Different Natural Cooling Sources to Reduce the ...

energies Article

Potential of Utilizing Different Natural Cooling Sources to Reduce the Building Cooling Load and Cooling Energy Consumption: A Case Study in Urumqi Chong Shen and Xianting Li * Department of Building Science, Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Tsinghua University, Beijing 100084, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-6278-5860 Academic Editor: Kamel Hooman Received: 5 January 2017; Accepted: 9 March 2017; Published: 15 March 2017

Abstract: Generally, Central Asia is typical for regions with strong solar radiation and various natural cooling sources. The heat gain from the building envelope accounts for a large part of the cooling load there. Thus, the pipe-embedded envelope is receiving attention as a semi-active system of utilizing natural energy for cooling. In this study, the performance of the pipe-embedded envelope used in Urumqi is numerically investigated. The energy saving potential regarding evaporative cooling and a ground-source heat exchanger (GSHE) is evaluated over a complete summer. The results show that the built-in pipes can reduce 80% of the solar heat gain through windows, with an effectiveness of around 60%. External windows rather than internal windows should be insulated because the air cavity is cool. With respect to the pipe-embedded wall, it becomes a radiant cooling panel absorbing the heat from the room, with an effectiveness around 83%. The seasonal cooling energy is decreased by 25%–50% in a typical office with a pipe-embedded envelope. Offices with a large window-to-wall ratio are acceptable because natural cooling is employed. GSHE performs the best among the selected sources. The effectiveness of evaporative cooling is also satisfactory, with an energy saving rate of 27%. Overall, the pipe-embedded system is suitable for climatic regions like Urumqi. Keywords: energy efficiency; building envelope; evaporative cooling; ground-source heat exchanger (GSHE); pipe-embedded envelope

1. Introduction The Silk Road was an ancient network through regions of the Asian continent connecting China to the Mediterranean Sea. Generally, Central Asia is one of the most important components of the Silk Road. The climate of this region has a distinct feature. In the daytime during summer, the solar radiation is strong and the air temperature is high. Thus, the energy consumption of traditional air-conditioning in office buildings here is very high. The cost of dehumidification is low due to the dry climate. The sensible heat transferred through the building envelope accounts for a significant proportion of the total cooling load. It is energy-efficient to reduce the cooling load in this region, especially the load gain from the building envelope. The building envelope comprises of the walls, windows, and roof. Conventionally, thermal insulation is applied to the walls to enhance the thermal resistance [1]. Ojanen et al. [2] reviewed the current situation of building thermal insulation. With decades of development, thermal insulation became one of the most efficient approaches for reducing the heat transfer through the wall. However, large thermal resistance may also influence the heat dissipation of the wall when the ambient environment is cool [3]. For example, the air temperature during summer nights and transition seasons is low in the Central Energies 2017, 10, 366; doi:10.3390/en10030366

www.mdpi.com/journal/energies

Energies 2017, 10, 366

2 of 17

Asian region. Intensive insulation will reduce the heat dissipation effect and, consequently, most indoor heat gain has to be handled by air-conditioning. In addition, thermal insulation faces the limitations of thickness [4], renovation [5], and fire safety [6]. Thus, it is wise to find other alternative solutions together with thermal insulation. With respect to windows, numerous studies have been conducted to improve their insulation and shading performance. Glazing technologies, such as gas-filled glazing [7], multilayer glazing and vacuum glazing [8], have significantly promoted the thermal insulation of windows. Manz et al. [9] even developed a window with a heat transfer coefficient of 0.2 W/(m2 ·K). Coating technologies, such as low emissivity (low-e) coatings [10], have minimized the incident solar radiation to the indoor space. However, the cost of high-performance coating is very expensive. Intensive shading will also have a poor influence for heating in winter. Double skin façade (DSF) is a type of advanced window system. The venetian blinds of DSF can block solar radiation and the air cavity between the two façades can enhance the thermal resistance. Hong et al. [11] investigated the seasonal energy efficiency strategies of a DSF used in Korea. The largest reduction rate of cooling energy consumption in that case was 13%. Cetiner and Özkan [12] assessed the performance of a DSF used in Istanbul. The energy saving rate in that case was 23%. Chan et al. [13] evaluated the technique-economic efficiency of a DSF used in Hong Kong, the cooling load was reduced by 26%. Though DSFs have shown the potential for energy savings, the heat gain through DSFs is still considerable [14]. Parra et al. [15] demonstrated that the venetian blinds have a notable effect on thermal performance of the DSF, and the temperature of venetian blinds could be higher than 50 ◦ C under exposure to solar radiation [16]. Gratia and De Herde [17] indicated that DSFs might cause overheating and a greenhouse effect in the air cavity. Especially under the severe summer climate in the Central Asian region, the traditional DSFs are not adequate for significantly reducing the cooling energy consumption. Except for the mentioned passive technologies, embedding pipes into the building envelope and utilizing natural energy to cool the windows and walls, has been proven very promising. Xu et al. [18] presented the concept of the pipe-embedded wall and reviewed its practical applications. The results showed that the active building envelope has the potential for energy conservation. Shen and Li [19] investigated the dynamic thermal performance of a pipe-embedded wall utilizing evaporative cooling water. It indicated that the reduction rate of electricity consumption for cooling could be higher than 50%. A ground-source heat exchanger (GSHE) also could be used as the cooling source of a pipe-embedded wall [20]. With regard to the pipe-embedded window, its average solar energy transmittance is only 13% and its window temperature is decreased by 10 ◦ C under typical summer conditions [14,21]. Overall, the effectiveness of the pipe-embedded envelope is high because of the direct use of natural cooling sources which does not consume extra electricity. Generally speaking, the Central Asian region has various natural cooling sources. The evaporative cooling there is efficient, and the temperature of ground soil is low. Thus, it may be suitable to adopt the pipe-embedded envelope and utilize natural energy in these regions. However, the investigation of the pipe-embedded envelope applied to these regions is inadequate. In addition, the current studies on the pipe-embedded envelope are separated to either windows or walls, respectively, and only the envelope is considered rather than the entire room. The comparison of the performance among different natural cooling sources is also lacking. Thus, a typical office with a synthetic pipe-embedded envelope utilizing natural cooling is presented in this study. Urumqi (latitude and longitude: 43◦ 490 N, 87◦ 360 E) is employed in the investigation as it is one of the largest cities in the Central Asian region. The hourly heat transfer of the novel envelope is numerically investigated over a complete summer. The effects of window, wall, and indoor heat sources are all taken into account. The energy saving performance of three natural cooling sources are considered: direct evaporative cooling (DEC), indirect evaporative cooling (IEC), and GSHE.

Energies 2017, 10, 366 Energies 2017, 10, 366

3 of 17 3 of 17

2. Methodology 2.1. Description of 2.1. Description of the the Synthetic Synthetic Pipe-Embedded Pipe-Embedded Envelope Envelope The schematic diagram diagramof ofthe thepipe-embedded pipe-embeddedenvelope envelopeis is shown Figure 1. Cooling pipes The schematic shown in in Figure 1. Cooling pipes are are embedded into the wall and the double pane window. Cooling water flows inside to take away embedded into the wall and the double pane window. Cooling water flows inside to take away the the from envelope. transfer of the pipe-embedded structure is more efficient heatheat from the the envelope. TheThe heatheat transfer of the pipe-embedded structure is more efficient thanthan the the traditional indoor fan coil unit. Firstly, the heat transfer area of the pipe-embedded system traditional indoor fan coil unit. Firstly, the heat transfer area of the pipe-embedded system is is sufficient envelope cancan be considered as a heat Secondly, the heatthe transfer sufficientbecause becausethe thewhole whole envelope be considered as aexchanger. heat exchanger. Secondly, heat coefficient of the pipe-embedded system is high. In the wall,In thethe pipes directly contact the solid surface. transfer coefficient of the pipe-embedded system is high. wall, the pipes directly contact the In the window, the pipes directly absorb the incident solar radiation. Finally, natural energy sources can solid surface. In the window, the pipes directly absorb the incident solar radiation. Finally, natural be employed directly cooling regarding thecooling high temperature the window and wall. to energy sources can beforemployed directly for regarding of the high temperature ofAccording the window the feature of DEC, IEC,ofand GSHEDEC, are applied the natural cooling sources in andclimatic wall. According to Urumqi, the climatic feature Urumqi, IEC, andasGSHE are applied as the natural this study. cooling sources in this study.

Figure 1. Sketch of the pipe-embedded envelope connected with natural sources. Figure 1. Sketch of the pipe-embedded envelope connected with natural sources.

2.2. Physical Model 2.2. Physical Model A typical office is adopted to evaluate the performance of the synthetic pipe-embedded A typical office is adopted to evaluate the performance of the synthetic pipe-embedded envelope, envelope, as shown in Figure 2. The dimensions of the office are 6.0 m (length) × 4.0 m (width) × 3.5 as shown in Figure 2. The dimensions of the office are 6.0 m (length) × 4.0 m (width) × 3.5 m (height). m (height). The external envelope of the office consists of window and wall. Window-to-wall ratio The external envelope of the office consists of window and wall. Window-to-wall ratio (WWR) varies in (WWR) varies in different cases. The heat gain from envelope is simulated using a computational different cases. The heat gain from envelope is simulated using a computational fluid dynamics (CFD) fluid dynamics (CFD) program. The indoor heat gain consists of lights (7 W/m22), equipment (15 2 ), equipment program. The indoor heat gain consists of lights (7 W/m (15 W/m ), and occupants W/m2), and occupants (8 W/m2) [22]. The working schedule of the office is from 08:00 to 18:00. All of (8 W/m2 ) [22]. The working schedule of the office is from 08:00 to 18:00. All of the other settings are the other settings are the same in the office with traditional and pipe-embedded envelopes except for the same in the office with traditional and pipe-embedded envelopes except for the pipes. the pipes. The heat transfers of the window unit and wall unit are numerically simulated, respectively. As illustrated in Figure 3, a typical double pane window with venetian blinds and a typical wall with bricks and an insulation layer are employed in the study. Cooling pipes are built in the venetian blinds of the window and the interlayer of the wall. The materials in the window are selected based on [23,24]. The glass has different characteristics in long-wave and short-wave bands. The materials in the wall are selected based on [25,26]. The expanded perlite is employed as the insulation material. The detailed

Energies 2017, 10, 366

4 of 17

Energies 2017, 10, 366

4 of 17

information of the materials in the window and wall is listed in Tables 1 and 2. The insulations of the window are selected complying with the thermal insulation standard in Urumqi [22]. Energiesand 2017,wall 10, 366 4 of 17

(a)

(b)

Figure 2. Configuration of the simulated office: (a) Office with traditional envelope; and (b) Synthetic pipe-embedded envelope.

The heat transfers of the window unit and wall unit are numerically simulated, respectively. As illustrated in Figure 3, a typical double pane window with venetian blinds and a typical wall with bricks and an insulation layer are employed in the study. Cooling pipes are built in the venetian blinds of the window and the interlayer of the wall. The materials in the window are selected based on [23,24]. The glass has different characteristics in long-wave and short-wave bands. The materials (a)[25,26]. The expanded perlite is employed as the insulation (b) material. in the wall are selected based on The Figure detailed information of materials office: in the(a) window and traditional wall is listed in Tables 1 and 2. The 2. Configuration of the the simulated Office with envelope; and (b) Synthetic Figure 2. Configuration of the simulated office: (a) Office with traditional envelope and (b) Synthetic insulations of the window and wall are selected complying with the thermal insulation standard in pipe-embedded envelope. pipe-embedded Urumqi [22]. envelope.

The heat transfers of the window unit and wall unit are numerically simulated, respectively. As illustrated in Figure 3, a typical double pane window with venetian blinds and a typical wall with bricks and an insulation layer are employed in the study. Cooling pipes are built in the venetian blinds of the window and the interlayer of the wall. The materials in the window are selected based on [23,24]. The glass has different characteristics in long-wave and short-wave bands. The materials in the wall are selected based on [25,26]. The expanded perlite is employed as the insulation material. The detailed information of the materials in the window and wall is listed in Tables 1 and 2. The insulations of the window and wall are selected complying with the thermal insulation standard in Urumqi [22].

(a)

(b)

Figure 3. Structures and dimensions of the simulated envelope: (a) Pipe-embedded window; and (b) Figure 3. Structures and dimensions of the simulated envelope: (a) Pipe-embedded window Pipe-embedded wall. and (b) Pipe-embedded wall. Table 1. Physical parameters of the window materials. Table 1. Physical parameters of the window materials. Components Material Components Thickness (mm) Specific Material heat (J/kg∙K) 3) Density (kg/m(mm) Thickness

Internal Skin Double glazing Internal Skin 6 + 12 (air) + 6 100 glazing Double 6 1000 + 12 (air) + 6

External Skin Stalinite External Skin 9 840 Stalinite 2200 9

Venetian Blinds Aluminum alloy Venetian Blinds 2 880alloy Aluminum 2 2700

Specific heat (J/kg·K) 100 840 880 (b) 1000 2200 2700 Density (kg/m3 ) (a) Heat conductivity coefficient (W/(m·K)) 0.09 0.28 180 Figure 3. Structures and1dimensions of the simulated envelope: (a) Pipe-embedded window; and (b) 65 86 0 Transmissivity-SW (%) Pipe-embedded wall. (%) Absorptivity-SW 17 7 80

Table 1. Physical parameters of the window materials. Components Material Thickness (mm) Specific heat (J/kg∙K) Density (kg/m3)

Internal Skin Double glazing 6 + 12 (air) + 6 100 1000

External Skin Stalinite 9 840 2200

Venetian Blinds Aluminum alloy 2 880 2700

Energies 2017, 10, 366

5 of 17

Table 1. Cont. Components

Internal Skin

Reflectivity-SW (%) Transmissivity-LW 2 (%) Absorptivity-LW (%) Heat conductivity coefficient (W/(m∙K)) Reflectivity-LW (%) 1 (%) Transmissivity-SW

External Skin

18 34 48 18

Energies 2017, 10, 366

Venetian Blinds

7 50 40 0.28 86 10

0.09 65 1 SW: (%) Absorptivity-SW 17 2 LW: long wave band, 7 2.7–1000 µm. short wave band, 0–2.7 µm. Reflectivity-SW (%) 18 7 34 50 Transmissivity-LW 2 (%) Table 2. Physical properties of the wall materials. Absorptivity-LW (%) 48 40 Reflectivity-LW (%) 18 10 1

Crushed Stone Expanded SW: short wave band, 0–2.7 μm. 2 LW: long wave band, 2.7–1000 μm.

Materials

Cement Plaster

Concrete

Perlite

Table properties of the wall materials. Thermal conductivity (W/(m ·K))2. Physical0.93 1.51 0.065 3 1800 2400 670 Density (kg/m ) Crushed Stone Expanded Materials Cement Plaster Concrete Perlite Specific heat (J/(kg·K)) 837 920 250

2.3.

Thermal conductivity (W/(m∙K)) Density (kg/m3) Numerical Method and Validation Specific heat (J/(kg∙K))

0.93 1800 837

1.51 2400 920

0.065 670 250

20 0 90 180 010

5 of 17

80 20 0 90 10

Clay Brick 0.81 1800 Clay Brick 1050 0.81 1800 1050

Heat flux on internal side (W/m2)

A model is built to simulate the heat transfer process through the 2.3.comprehensive Numerical Methodnumerical and Validation envelope illustrated in Figure 3. A two-dimensional simplification is adopted in the calculation, A comprehensive numerical model is built to simulate the heat transfer process through the corresponding to similar studies [16,23,27]. The simulation is unsteady, and the time step is one hour in envelope illustrated in Figure 3. A two-dimensional simplification is adopted in the calculation, each case. There are a total of 1728 time steps. To consider the effect of thermal inertia, the simulation corresponding to similar studies [16,23,27]. The simulation is unsteady, and the time step is one hour is conducted 10 days before beginning thesteps. summer period. The computation domain consists in each case. There are athe total of 1728 of time To consider the effect of thermal inertia, the of a solid field andisfluid field. 10 Navier–Stokes equations areofemployed forperiod. the fluid A partial simulation conducted days before the beginning the summer Thedomain. computation differential heat field conduction applied for the solid zone.areEnhanced function is domainequation consists ofof a solid and fluidisfield. Navier–Stokes equations employed wall for the fluid employed forAthe boundary layerequation betweenofsolid fluid. is applied for the solid zone. Enhanced domain. partial differential heat and conduction wall function is employed the boundaryin layer solid and fluid. Structured quadratic cellsfor are generated the between main domain except for a part near the round pipes Structured quadratic cells are generated in the main domain except for a part near the round and venetian blinds. The mesh independence for the wall has been checked by developing 54,000, pipes and venetian blinds. The mesh independence for the wall has been checked 8800, and 1900 cells. The heat transfer through the wall in a day is simulated, and by thedeveloping comparison of 54,000, 8800, and 1900 cells. The heat transfer through the wall in a day is simulated, and the heat flux on the internal surface of wall is illustrated in Figure 4. This indicates that the results from comparison of heat flux on the internal surface of wall is illustrated in Figure 4. This indicates that 54,000 to 8800 cells are very close, while the results from coarse cells have a relative error larger than the results from 54,000 to 8800 cells are very close, while the results from coarse cells have a relative 10%. error Thus,larger the mid-density mesh employed, mesh whose length of theedge cellslength is 6 mm, approximately. than 10%. Thus, theismid-density is edge employed, whose of the cells is 6 Similarly, the mesh independence for the window has been checked by developing 300,000, 140,000 mm, approximately. Similarly, the mesh independence for the window has been checked by and 60,000 cells, and the mesh with 140,000 cells is eventually employed, whose cell edge is about 10 mm. developing 300,000, 140,000 and 60,000 cells, and the mesh with 140,000 cells is eventually employed, whose cell edgeofisthe about 10 mm.mesh The configuration the adopted mesh is shown in Figure 5. The configuration adopted is shown in of Figure 5. 6 5 4 3 8800 cells

2

1900 cells

54000 cells

1 0 0

2

4

6

8

10

12

14

16

18

20

Time (hour) Figure Resultsfrom from different different mesh Figure 4.4.Results meshdensities. densities.

22

Energies 2017, 10, 366

6 of 17

Energies 2017, 10, 366

6 of 17

(a)

(b)

Figure 5. Mesh configurations: (a) Pipe-embedded window; and (b) Pipe-embedded wall.

Figure 5. Mesh configurations: (a) Pipe-embedded window and (b) Pipe-embedded wall. All of the equations are solved using the commercial CFD software ANSYS Fluent (Version 14.5, ANSYS PA,solved USA). The turbulence effect is computed by the κ-ε model.Fluent Velocity All ofInc., the Canonsburg, equations are using the commercial CFD software ANSYS (Version 14.5, and pressure values are coupled by the SIMPLE scheme. Pressure is discretized ANSYS Inc., Canonsburg, PA, USA). The turbulence effect is computed by by thethe κ-εbody-force model. Velocity and weighted algorithm. Radiation is calculated by the discrete ordinate (DO) model. The validation of pressure values are coupled by the SIMPLE scheme. Pressure is discretized by the body-force weighted the numerical model and the details of mathematical model are included in our previous studies algorithm. Radiation is calculated by the discrete ordinate (DO) model. The validation of the numerical [14,19,21,28]. model and theinvestigation, details of mathematical are included inenvelope our previous [14,19,21,28]. In this both the indoormodel and outdoor sides of an are set studies as the boundary In this investigation, the indoorheat and outdoor sides ofofan as the boundary condition of the third type.both The convective transfer coefficients theenvelope surfaces ofare the set envelope 2∙K) (outdoor side) and 6 W/(m2∙K) (room side). The indoor temperature is fixed to 26 °C. are 18 W/(m condition of the third type. The convective heat transfer coefficients of the surfaces of the envelope 2 ·temperature ambient solarand radiation are 2determined referring the climatic database is fixed to ·K) (roomby are The 18 W/(m K) (outdoorand side) 6 W/(m side). Thetoindoor temperature [29]. The angle of the incident solar ray also changes with time. The velocity of water is 0.5 m/s. The ◦ 26 C. The ambient temperature and solar radiation are determined by referring to the climatic temperature of the water is related to the employed natural cooling source. In the system with DEC database [29]. The angle of are theequal incident rayand also changes with time. Theoutdoor velocity and IEC, water temperatures to thesolar wet-bulb dew point temperature of the air of water is 0.5 m/s. The temperature of the water is related to the employed natural cooling source. In the system plus 4 °C. The additional 4 °C is given as a temperature difference for heat exchange to consider the withperformance DEC and of IEC, water temperatures are equal to the wet-bulb and point temperature of the DEC [30] and IEC [31]. In GSHE, water temperature is set as the dew soil temperature plus ◦ C.temperature 4 °C. The is 6.6 °C throughout the whole year [32]. Theexchange to outdoor air surface plus 4 soil The additional 4 ◦and C isalmost givenconstant as a temperature difference for heat coolingthe season of Urumqi isof from 21 June 31 August. consider performance DEC [30] to and IEC [31]. In GSHE, water temperature is set as the soil

temperature plus 4 ◦ C. The surface soil temperature is 6.6 ◦ C and almost constant throughout the 2.4. Evaluation Index whole year [32]. The cooling season of Urumqi is from 21 June to 31 August. The heat flux transferred into the room per square meter of envelope can be obtained directly from the CFD simulation. Based on this, the total indoor cooling load Q (W) of the office can be 2.4. Evaluation Index calculated as:

The heat flux transferredQinto room per meter of envelope can be obtained(1) directly from = qwallthe ∙Awall + qwindow ∙Asquare window + qindoor∙Aindoor + qfresh the CFD simulation. Based on this, the total indoor cooling load Q (W) of the office can be calculated as:

where qwall is the heat gain through the wall, W/m2; qwindow is the heat gain through the window, W/m2; qindoor is the heat gain from the indoor space, 30 W/m2; Awall is the area of the wall, m2; Awindow is the area Q = q ·Awall + qwindow ·Awindow2 + qindoor ·Aindoor + qfresh (1) of the window, m2; Aindoor is thewall floor area of the office, 24 m ; qfresh is the heat gain from the fresh air, 3 W. Awall and Awindow vary with the WWR in different cases. The amount of fresh air is 90 m /h for the 2 where qwall office [22].is the heat gain through the wall, W/m ; qwindow is the heat gain through the window, 2 2; A 2 W/m ; The qindoor is the heat gain from thefor indoor space, is the area of the cooling electricity consumption the office with30theW/m traditional envelope Etrad (W) is wall, m ; wall calculated Awindow is theas: area of the window, m2 ; Aindoor is the floor area of the office, 24 m2 ; qfresh is the heat gain

from the fresh air, W. Awall and Awindow vary in different cases. The amount Etrad with = Qtradthe /EERWWR hp (2) of fresh air is 90 m3 /h for the office [22]. The cooling electricity consumption for the office with the traditional envelope Etrad (W) is calculated as: Etrad = Qtrad /EERhp (2) where EERhp is the energy efficiency ratio of a typical cooling system, which is around 4.0 [33]; Qtrad (W) is the indoor cooling load for the traditional office.

Energies 2017, 10, 366

7 of 17

The cooling electricity consumption for the office with the pipe-embedded envelope Epipe (W) consists of indoor side, envelope side, and the natural cooling source. Epipe is defined as: Epipe = Qpipe /EERhp + Qwater /WTFpipe + Esource

(3)

where Qpipe is the indoor cooling load for the office with pipes, W; Qwater is the heat flux of the water pipes, which is obtained from CFD simulation, W; WTFpipe is the water transport factor of the water pipes (ratio of transferred heat quantity to the electricity consumption of the pump) [34]; (Qwater /WTFpipe ) represents the electricity consumption of transporting the water through the envelope; Esource is the electricity consumption of the natural cooling source. For DEC and IEC, Esource is the electricity consumption of the evaporative cooling machine, which is calculated by: Esource = Qwater /EERsource

(4)

where EERsource is the energy efficiency ratio of the evaporative cooling machine [34,35]. For GSHE, Esource is the electricity consumption of the GSHE pump, which is calculated by: Esource =

ρ·g·L·V 3600 · η

(5)

where ρ is the density of water, kg/m3 ; g is the acceleration of gravity, 9.8 m/s2 ; η is the pump efficiency; L is the pump lift, m; and V is the flow rate of circulating water, m3 /h, which is calculated according to Qwater : 3600 · Qwater V= (6) c · ρ · ∆t where ρ is the density of water, kg/m3 ; c is the specific heat of water, J/(kg·K); and ∆t is the temperature difference between the outlet and inlet of GSHE, 3.5 ◦ C. The electricity reduction rate of the pipe-embedded envelope (ε) is defined by: ε=

Etrad − E pipe Etrad

(7)

The effectiveness of pipes (η) is defined by: η=

Qtrad − Q pipe Qwater

(8)

where η represents how much cooling energy of the pipes is used on the room side. The cooling source with pipes almost does not consume extra energy if η is close to 100%. Since the cooling tower connected with the traditional air-conditioning also consumes energy, only the increased heat flux on the external surface of the pipe-embedded envelope needs the extra energy consumption of the cooling tower. 3. Results and Discussion The temperature distribution of the pipe-embedded envelope under typical conditions is illustrated first. Then, the hourly heat transfer process of the envelope in a complete cooling season is investigated regarding the diverse cooling sources in Urumqi. Finally, the accumulated electricity consumption of a typical office is evaluated with different orientations and WWRs. 3.1. Temperature Distribution in the Envelope According to the summer design parameters in Urumqi [29], a typical weather condition is selected to investigate the performance of the cooling pipes. The outdoor temperature is 33 ◦ C, the water temperature is 24.5 ◦ C, and the solar radiation on the external glass is 600 W/m2 . The temperature and velocity fields of windows with or without pipes are shown in Figure 6. In the traditional

Energies 2017, 10, 366

8 of 17

Energies 2017, 10, 366

8 of 17

double window without pipes, the blinds are significantly heated because of the strong solar radiation. Thermal stratification occurs in the cavity ofwith the double window, theshown highest of the air temperature and velocity fields of windows or without pipes are in temperature Figure 6. In the ◦ ◦ traditional double window the blinds are significantly heatedisbecause in the top area is even morewithout than 50pipes, C. The temperature of the blinds as highofasthe 60strong C, which is solar radiation. Thermal stratification occurs in the thedue highest corresponding to the experimental results in [16], andcavity a partofofthe thedouble bottomwindow, is also hot to the slant temperature the air in the top is even than 50 by °C.the Theshort-wave temperaturesolar of the blinds isisasreduced, solar radiation.ofThus, although thearea direct heat more gain caused radiation 60 °C, which is corresponding to the experimental results in [16], and a part of the bottom is thehigh heatasconduction of the internal glass and the long-wave solar radiation from the shading device also hot due to the slant solar radiation. Thus, although the direct heat gain caused by the short-wave are still considerable. According to the simulation, 37.3% of the solar energy will transfer into the solar radiation is reduced, the heat conduction of the internal glass and the long-wave solar radiation room. However, with the cooling effect of the embedded pipes, the overall temperature of the novel from the shading device are still considerable. According to the simulation, 37.3% of the solar energy double windowinto is greatly decreased. highest air temperature less than pipes, 30 ◦ C.the Theoverall temperature will transfer the room. However,The with the cooling effect of theisembedded ◦ of blinds is only about 26 C, and the blinds no longer transfer heat to the indoor space. The neighbor temperature of the novel double window is greatly decreased. The highest air temperature is less of the device even becomes a only cool about region. In this condition, the heat thanshading 30 °C. The temperature of blinds is 26 °C, and the blinds no60.1% longerof transfer heatgain to from theradiation indoor space. The neighbor the shading device even becomes a cool12.2% region. this condition, solar is directly takenof away by the cooling pipes, and only ofInthe incident radiation 60.1% ofinto the the heatroom gain from solar radiation is directlythe taken away by the cooling pipes, and only 12.2% transfers eventually. Nevertheless, heat dissipation to the ambient environment in of the incident radiation transfers into the room eventually. Nevertheless, the heat dissipation to the the pipe-embedded double window is less than that of the traditional double window because the ambient environment in the pipe-embedded double window is less than that of the traditional double temperature of the pipe-embedded double window is much lower. Thus, the cooling pipes need to window because the temperature of the pipe-embedded double window is much lower. Thus, the deal with more heat. However, its energy-saving potential may still be promising considering the cooling pipes need to deal with more heat. However, its energy-saving potential may still be production of cooling water is very efficient compared with the chiller. promising considering the production of cooling water is very efficient compared with the chiller.

(a)

(b)

Figure 6. Cross section of the window: (a) Temperature distribution; and (b) Velocity distribution.

Figure 6. Cross section of the window: (a) Temperature distribution and (b) Velocity distribution.

The velocity profile in Figure 6b indicates that the overall thermal convection in the traditional window is strongprofile due to in itsFigure large temperature difference the cavity. The air isin heated up The velocity 6b indicates that thethroughout overall thermal convection the traditional by the is bottom surface blinds, and difference rises alongside the blinds. Then the air falls the up by window strong due toand itsvenetian large temperature throughout the cavity. The airalong is heated as it surface is cooledand down by the glass. Theand thermal is the intense especially In theglass bottom venetian blinds, risesconvection alongside blinds. Thennear the the air glass. falls along the the pipe-embedded window, the buoyancy-driven flow rises in the region near the external glass glass as it is cooled down by the glass. The thermal convection is intense especially near the glass. because it is heated up by the glass. The thermal convection near the venetian blinds and internal In the pipe-embedded window, the buoyancy-driven flow rises in the region near the external glass glass is weak due to the small temperature difference. Thus, the heat conduction through the internal because it is heated up by the glass. The thermal convection near the venetian blinds and internal glass glass is reduced in this condition. is weakAdue to the temperature difference. the heat conduction through the typical daysmall in Urumqi is selected to analyze Thus, the effectiveness of the embedded pipes in internal detail. glass is reduced in thisconditions condition. The boundary are selected from the summer design day in [29]. Dynamic heat transfer is A typicaldue daytointhe Urumqi selected to wall. analyze effectiveness of the embedded pipes in detail. considered thermalisinertia of the It isthe a sunny day and the highest horizontal solar 2. The is nearly 800 W/m highest air temperature is design 35.4 °C, day which at 15:00. Cooling Theradiation boundary conditions are selected from the summer inoccurs [29]. Dynamic heat transfer

is considered due to the thermal inertia of the wall. It is a sunny day and the highest horizontal solar radiation is nearly 800 W/m2 . The highest air temperature is 35.4 ◦ C, which occurs at 15:00.

Energies 2017, 10, 366

9 of 17

Energies 2017, 10, 366

9 of 17

Cooling water is produced from DEC. The wet-bulb temperature varies between 15 ◦ C and 20 ◦ C, water is produced from DEC. The wet-bulb temperature varies between 15 °C and 20 °C, which is which is suitable for DEC. Energies 2017, 10, 366 9 of 17 suitable for DEC. The temperature field in the envelope at different times during the conditions with or without The is temperature field in at different during the conditions pipes pipes is illustrated in Figure 7.the In envelope the walltimes without pipes, the temperature of the external water produced from DEC. The traditional wet-bulb temperature varies between 15 °Cwith andor20without °C, which is is illustrated Figure 7. Innoon the traditional without pipes, the16:00, temperature of the external surfaceof is the suitable forinDEC. surface is heated up from to night.wall Especially around the highest temperature heatedisup from noon night. around 16:00, the highest temperature of the envelope 38 the ◦ C.to The temperature field inEspecially the envelope different times during conditions with or without pipes envelope over 38 The temperature of at the internal surface isthe always more than 26 ◦isC.over With °C.isThe temperature of the internal surface iswall always morepipes, than 26 °C. With the embedded cooling pipes, illustrated in Figure 7. In the traditional without the temperature of the external surface is in embedded cooling pipes, the temperature of the envelope is significantly reduced. As shown theheated temperature ofnoon the envelope is significantly reduced. As highest shown in Figure 7, the heat transferred to the up from to night. Especially around 16:00, the temperature of the envelope is over 38 Figure 7, the heat transferred to the room is effectively reduced by the pipes, and the temperature of the room effectively reduced the pipes, andisthe temperature of the internal is aboutcooling 20 °C, which °C. is The temperature of theby internal surface always more than 26 °C. Withsurface the embedded pipes, internal surface islower aboutthan 20 ◦the C, which is even much lower than thepipes roomnot temperature. Thus, the cooling is even much room temperature. Thus, the cooling only decrease the external the temperature of the envelope is significantly reduced. As shown in Figure 7, the heat transferred to the pipes not only decrease the external heat gain through the wall but also absorb the internal heat. heat gain thereduced wall butbyalso the internal heat. room isthrough effectively theabsorb pipes, and the temperature of the internal surface is about 20 °C, which is even much lower than the room temperature. Thus, the cooling pipes not only decrease the external heat gain through the wall but also absorb the internal heat.

Figure 7. Temperature field of a wall during typical times of a day.

Figure 7. Temperature field of a wall during typical times of a day. Figure 7. Temperature field ofSources a wall during typical times of a day. 3.2. Dynamic Heat Transfer with Different Cooling

3.2. Dynamic Heat Transfer with Different Cooling Sources

climatic data of Urumqi in summer is Sources illustrated in Figure 8. Urumqi is a typical city in 3.2.The Dynamic Heat Transfer with Different Cooling

The climatic data of Urumqi in temperature summer is is illustrated Urumqi is a typical city in Central Asia. The ambient dry-bulb around 30 in °CFigure and the8. highest temperature is close The climatic data of Urumqi in summer is illustrated in Figure 8. Urumqi is a typical city ◦ to 40Asia. °C. The radiation here is very intense,isand the highest horizontal solar radiation is more Central Thesolar ambient dry-bulb temperature around 30 C and the highest temperature isinclose Central Asia. The ambient dry-bulb temperature is around 30 °C and the highest temperature is close 2 ◦ 1000solar W/mradiation . Evaporative is efficient here wet-bulb temperature dew-than to 40than C. The here cooling is very intense, and thebecause highest the horizontal solar radiationand is more to 40 2°C. The solar radiation here is very intense, and the highest horizontal solar radiation is more temperature are low. The average soil here temperature isthe only 6.6 °C. Thus, DEC, IEC,and anddew-point GSHE 1000point W/m . Evaporative cooling is efficient because wet-bulb temperature than 1000 W/m2. Evaporative cooling is efficient here because the wet-bulb temperature and deware considered as three alternative cooling issources in ◦this study.DEC, The cooling effects of are temperature are low. The average soilnatural temperature only 6.6 C. °C. Thus, IEC, point temperature are low. The average soil temperature is only 6.6 Thus, DEC, IEC,and andGSHE GSHE different sources in a complete summer are simulated and compared. considered as three alternative natural cooling sourcessources in this in study. The cooling effectseffects of different are considered as three alternative natural cooling this study. The cooling of sources in a complete summer are summer simulated compared. different sources in a complete areand simulated and compared. 40

Dry-bulb Dew-point Dry-bulb Dew-point

Temperature (oC) Temperature (oC)

40

30

Wet-bulb Ground soil Wet-bulb Ground soil

30

20 20

10 10

0 6/21 0 6/21

7/1 7/1

7/11

7/21

7/11

7/31

7/21Time7/31

(a) Time (a)

Figure 8. Cont.

8/10 8/10

8/20 8/20

8/30 8/30

Energies 2017, 10, 366

10 of 17

Energies 2017, 10, 366

10 of 17

Solar radiation (W/m2)

1,000 800 600 400 200 0 6/21

7/1

7/11

7/21

7/31

8/10

8/20

8/30

Time (b) Figure 8. Climatic data of Urumqi: (a) Temperature of different sources; and (b) Horizontal solar radiation.

Figure 8. Climatic data of Urumqi: (a) Temperature of different sources and (b) Horizontal solar radiation.

The dynamic performance of the system appliedto toaa west orientation analyzed as an The dynamic performance of the system applied orientationisis analyzed as example. an example. The average temperature of the cavity demonstrated in in Figure Figure 9a. pipes, thethe average The average temperature of the air air cavity is is demonstrated 9a.Without Without pipes, average temperature of the traditional window is usually higher than 40 °C in the daytime. With the cooling ◦ temperature of the traditional window is usually higher than 40 C in the daytime. With the cooling pipes, the temperature of the double window is significantly reduced throughout the complete pipes, the temperature of the double window is significantly reduced throughout the complete cooling cooling season. The variation of temperature is much more stable. When DEC is employed, the season. The variation of temperature is much more stable. When DEC is employed, the seasonal seasonal average temperature of the window is 22.2 °C, which is 8.5 °C lower than the traditional ◦ C lower than the traditional double average temperature ofaddition, the window is 22.2 ◦ C, which is 8.5 than double window. In the temperature is already lower the room temperature, indicating window. temperature is already lower than of the roomheat. temperature, indicating thatIn theaddition, room will the dissipate heat through the window instead gaining When IEC is employed,that the room will dissipate heat through the window instead of gaining heat. When IEC is effect employed, the seasonal average temperature of the double window is only 17.6 °C, and the cooling is ◦ perfect. average The temperature is evenof lower °C) when adopting the17.6 GSHE.C, and the cooling effect is the seasonal temperature the (16.4 double window is only ◦ C) when The hourly heat is flux on the internal is illustrated in Figure 9b. In the traditional window, perfect. The temperature even lower (16.4glass adopting the GSHE. 2 owing to the large solar radiation and hot the heat gain in the daytime varies from 50 to 300 W/m The hourly heat flux on the internal glass is illustrated in Figure 9b. In the traditional window, 2 due to the cooling window. regard to the window with DEC, heat gain2 varies from 0 to large 100 W/m the heat gain inInthe daytime varies from 50 to its 300 W/m owing to the solar radiation and hot effect of pipes. The accumulative seasonal heat transfer is reduced by 56.4% and the peak value is window. In regard to the window with DEC, its heat gain varies from 0 to 1002 W/m2 due to the cooling reduced by 62.2%. In regards to IEC, the heat gain varies from –20 to 60 W/m . The accumulative heat effect of pipes. The accumulative seasonal heat transfer is reduced by 56.4% and the peak value is gain is reduced by 81.9% and the peak value is reduced by 70.9%. The window will absorb the internal 2 . The accumulative reduced byfor 62.2%. regards to IEC, the gain varies from –20intothe 60GSHE W/mcondition, heat nearlyIn 20% of the daytime. The heat performance is even better in which heat gain is reduced byheat 81.9% the peak value is by 70.9%. The window will absorb the accumulative gainand is reduced by 88.2%. In reduced this condition, the double window almost stops the internal heat for nearly of the daytime. performance is even better thewith GSHE condition, transferring heat to 20% the room. However, theThe cooler natural cooling source has toindeal more heat because the window isheat cooler. The seasonal of thethe pipe are 97.4, 142.5, and in which the accumulative gain is accumulative reduced by 88.2%. Inheat this fluxes condition, double window almost 2 in the condition of DEC, IEC, and GSHE respectively. 153.1 kWh/m stops transferring heat to the room. However, the cooler natural cooling source has to deal with more heat because the window is cooler. The accumulative seasonal heat fluxes of the pipe are 97.4, 142.5, and 153.1 kWh/m2 in the condition of DEC, IEC, and GSHE respectively. The seasonal effectiveness of pipes (η) is around 42.5% in the window, and the values in DEC, IEC, and GSHE conditions are very close to each other, which indicates that η represents the effectiveness of the structure of pipes regardless of the water temperature. The results show that 42.5% of the cooling energy from pipes is used for cooling the indoor space directly. The rest of the cooling energy is used to deal with the increased heat dissipation on the external skin of the window. The temperature of the internal surface of the wall is shown in Figure 10a. The temperature of the traditional wall is more than 26 ◦ C, but the temperature of the pipe-embedded wall is only 21 to 24 ◦ C. The wall panel is also able to cool the indoor environment through radiative heat transfer. The thermal comfort of occupants can be improved because the mean radiant temperature is reduced. The hourly heat flux on the internal surface of the wall is illustrated in Figure 10b. With respect to the traditional wall without pipes, the heat flux varies from 0 to 10 W/m2 . The heat flux is small compared with window because the insulation of the wall is better. In addition, the effect of radiation on the wall is relatively slight. However, with the built-in pipes, the heat flux becomes a negative

Energies 2017, 10, 366

11 of 17

value. The external heat cannot transfer into the room, while the internal heat is taken away by the cooling water. The cooling pipes have a similar effect as a radiant cooling panel in these conditions. The average seasonal heat flux is around −21.7 W/m2 when utilizing DEC, and the heat flux is even less when adopting IEC, which is only −36.9 W/m2 . The effect of GSHE is close to that of IEC, whose average heat flux is −43.2 W/m2 . In addition, due to the water temperature from GSHE being nearly independent of weather, the heat flux of the wall is very stable. The cooling capacity of these pipe-embedded walls is satisfactory. The seasonal effectiveness of pipes (η) is around 83.0% for the pipe-embedded wall, and the values are close in the conditions with different cooling sources. η is very high for the wall because the pipes are arranged on the indoor side of the insulation layer and the insulation of the wall is better than that2017, of glass. Energies 10, 366 11 of 17 60

Temperature (oC)

50 40 30 20 10

Without pipes With IEC

With DEC With GSHE

0 6/21

7/1

7/12

7/23

8/3

8/14

8/25

Time (a) 350


Heat flux (W/m2)

300

With DEC With GSHE

250 200 150 100 50 0 -50 6/21

6/29

7/7

7/15

7/23

7/31

8/8

Time (b) Figure 9. Heat transfer in different windows: (a) Temperature variation; and (b) Heat flux variation. Figure 9. Heat transfer in different windows: (a) Temperature variation and (b) Heat flux variation.

The seasonal effectiveness of pipes (η) is around 42.5% in the window, and the values in DEC, IEC, and GSHE conditions are very close to each other, which indicates that η represents the effectiveness of the structure of pipes regardless of the water temperature. The results show that 42.5% of the cooling energy from pipes is used for cooling the indoor space directly. The rest of the cooling energy is used to deal with the increased heat dissipation on the external skin of the window. The temperature of the internal surface of the wall is shown in Figure 10a. The temperature of the traditional wall is more than 26 °C, but the temperature of the pipe-embedded wall is only 21 to 24 °C. The wall panel is also able to cool the indoor environment through radiative heat transfer. The thermal comfort of occupants can be improved because the mean radiant temperature is reduced. The hourly heat flux on the internal surface of the wall is illustrated in Figure 10b. With respect to the

independent of weather, the heat flux of the wall is very stable. The cooling capacity of these pipeembedded walls is satisfactory. The seasonal effectiveness of pipes (η) is around 83.0% for the pipe-embedded wall, and the values are close in the conditions with different cooling sources. η is very high for the wall because the pipes arranged on the indoor side of the insulation layer and the insulation of the wall is12better Energies 2017,are 10, 366 of 17 than that of glass. 30 Without pipes With IEC

Temperature (oC)

28

With DEC With GSHE

26 24 22 20 18 16 6/21

7/12

8/3

8/25

Time (a) 20


With DEC With GSHE

Heat flux (W/m2)

0 -20 -40 -60 -80 6/21

7/12

8/3

8/25

Time (b) Figure 10. Heat transfer in different walls: (a) Temperature variation; and (b) Heat flux variation. Figure 10. Heat transfer in different walls: (a) Temperature variation and (b) Heat flux variation.

3.3. Seasonal Electricity Consumption under Different WWRs and Orientations 3.3. Seasonal Electricity Consumption under Different WWRs and Orientations Traditionally, glass with good insulation (e.g., double glazing) is employed as the internal skin Traditionally, glass with good insulation (e.g., double glazing) is employed as the internal skin of of a double pane window to reduce the heat conduction from the hot air cavity to the room. However, a double pane window to reduce the heat conduction from the hot air cavity to the room. However, with the cooling effect of pipes, the air cavity is no longer hot. Thus, if we adopt the insulated glass with the cooling effect of pipes, the air cavity is no longer hot. Thus, if we adopt the insulated glass to the external skin of a double pane window, the heat dissipation through the external skin can be to the external skin of a double pane window, the heat dissipation through the external skin can be reduced while the heat flux through the internal skin is almost constant. The seasonal electricity reduced while the heat flux through the internal skin is almost constant. The seasonal electricity consumption for a window unit is shown in Figure 11. The insulated glass is placed in the internal or consumption for a window unit is shown in Figure 11. The insulated glass is placed in the internal external skin of a window. The electricity consumption of the window with internal insulation is or external skin of a window. The electricity consumption of the window with internal insulation is always higher irrespective of the cooling source. Especially for IEC and GSHE, the cooling water is always higher irrespective of the cooling source. Especially for IEC and GSHE, the cooling water is cold, and the heat dissipation of the external skin is considerable. Hence, it is suggested to insulate cold, and the heat dissipation of the external skin is considerable. Hence, it is suggested to insulate external skin. In addition, by this method, the effectiveness of pipes (η) is increased to 60% because external skin. In addition, by this method, the effectiveness of pipes (η) is increased to 60% because the the cooling energy from pipes is decreased. cooling energy from pipes is decreased.

Energies 2017, 10, 366 Energies 2017, 10, 366 Energies 2017, 10, 366

13 of 17 13 of 17 13 of 17

Figure 11. Accumulated electricity consumption for window with different insulation layouts. Accumulated electricity electricity consumption for window with different insulation layouts. Figure 11. Accumulated

The seasonal electricity consumption of the pipe system and the indoor air-conditioning system in a The typical officeelectricity is shown consumption in Figure 12.of The WWR of the and office isindoor 0.5. The results are thesystem average seasonal electricity consumption ofthe the pipesystem system andthe the indoor air-conditioning system The seasonal pipe air-conditioning in values of four orientations E, S, N, W. The accumulated heat gain of the envelope without pipes is in a typical office is shown in Figure 12. WWR The WWR the office 0.5.results The results the average a typical office is shown in Figure 12. The of theofoffice is 0.5. isThe are theare average values 499.1 which takes upW. aW. half the heat totalgain cooling load. This the importance of values of four orientations E,nearly S, N,accumulated Theofaccumulated heat gain of theindicates envelope without pipes is of fourkWh, orientations E, S, N, The of the envelope without pipes is 499.1 kWh, improving the performance of the envelope in this region. The total electricity consumption in DEC 499.1 kWh, which takes up nearly a half of the total cooling load. This indicates the importance of which takes up nearly a half of the total cooling load. This indicates the importance of improving the and IEC arethe almost the same, a reduction of 27% compared to the traditional system. However, improving ofwith the region. envelope intotal this region. The total electricity consumption inalmost DEC performance of performance the envelope in this The electricity consumption in DEC and IEC are their proportions of the components of electricity consumption are different. The consumption of and IEC are almost the same, with a reduction of 27% compared to the traditional system. However, the same, with a reduction of 27% compared to the traditional system. However, their proportions of indoor air-conditioning in IEC is much less than that in DEC because the water temperature in IEC their proportions of the components of electricity consumption are different. The consumption of the components of electricity consumption are different. The consumption of indoor air-conditioning is IEC much lower. However, theinelectricity consumption producing water in IECHowever, isinmuch indoor air-conditioning IEC is much less than that infor DEC because the temperature IEC in is much less than in that DEC because the water temperature incooling IECwater is much lower. higher because its EER is lower. Overall, DEC is more suitable, considering its lower investment. The is much lower. However, the electricity consumption for producing cooling water in IEC is much the electricity consumption for producing cooling water in IEC is much higher because its EER is lower. performance of GSHE is the best, where, ε is larger than 50%. The consumption of indoor airhigher because EERsuitable, is lower. considering Overall, DECitsislower more suitable, considering its lower investment. Overall, DEC isitsmore investment. The performance of GSHE isThe the conditioning in the GSHE condition is reduced by 81%, and its electricity consumption for the pipe performance of GSHE is the best, where, ε is larger than 50%. The consumption of indoor airbest, where, ε is larger than 50%. The consumption of indoor air-conditioning in the GSHE condition system is not asthe high asitsthat of IEC. In addition, GSHE does not consume extra water, which is conditioning GSHE condition isconsumption reduced by 81%, and its electricity consumption the is reduced byin81%, and electricity for the pipe system is not as high asfor that of pipe IEC. beneficial forGSHE dryhigh regions like Thewater, high GSHE efficiency ofnot GSHE is dry because of like the which low soil system is not as as not that ofUrumqi. IEC. In addition, does consume extra water, is In addition, does consume extra which is beneficial for regions Urumqi. temperature in these regions. beneficial for dry regions like Urumqi. The high efficiency of GSHE is because of the low soil The high efficiency of GSHE is because of the low soil temperature in these regions. temperature in these regions.

(a) (a) Figure 12. Cont.


14 of 17 14 of 17

Energies 2017, 10, 366

14 of 17

(b) (b)

Figure consumptionofofthe theoffice: office:(a)(a)Accumulated Accumulated amount; (b) Reduction Figure12. 12.Seasonal Seasonal electricity electricity consumption amount andand (b) Reduction rate. rate. Figure 12. Seasonal electricity consumption of the office: (a) Accumulated amount; and (b) Reduction rate.

Thesolar solarradiation radiationvaries variesin indifferent different orientations. orientations. The Theaccumulative accumulativeradiations radiationsin inthe theworking working The 2 for E, The solar radiation varies inand different orientations. The accumulative radiations inrespectively. the working It is time in summer are 163, 160, 167, 36 kWh/m S, W, and N orientations, 2 time in summer are 163, 160,160, 167,167, and 3636 kWh/m S, W, and andNNorientations, orientations, respectively. It is 2for E, time in summer are 163, andE, kWh/m E, S, W, respectively. It is obvious that the sunshine isisstronger for S, and W,for whereas it is weak for a Nfor orientation. In addition, obvious that the sunshine stronger for E, S, and W, whereas it is weak a N orientation. obvious that the sunshine is stronger for E, S, and W, whereas it is weak for a N orientation. In In radiation is not the is only factor that affects the heat gain. The average ambient air temperature is addition, radiation thethe only factor heat gain.The Theaverage average ambient radiation not is not factorthat that affects affects the the heat gain. ambient air air ◦addition, ◦only 21.7 C from 8:00 to 11:00 and 28.0 C from 15:00 to 18:00 in summer. Thus, it is very beneficial for the temperature is 21.7 °C from 8:008:00 to 11:00 and to18:00 18:00ininsummer. summer. Thus, is very temperature is 21.7 °C from to 11:00 and28.0 28.0°C °Cfrom from 15:00 15:00 to Thus, it isitvery heat dissipation of east orientation because it receives radiation mainly in the morning. The seasonal beneficial for the heat dissipation of east orientation because it receives radiation mainly in the beneficial for the heat dissipation of east orientation because it receives radiation mainly in the electricity consumption of electricity the office in differentoforientations illustrated in Figure The WWR of morning. The seasonal consumption ofthe the office office in orientations is 13. illustrated in in morning. The seasonal electricity consumption inisdifferent different orientations is illustrated the office is 0.5. The results indicate that cooling pipes are effective in each orientation. The stronger Figure 13. The WWR of the office is 0.5. The results indicate that cooling pipes are effective in each Figure 13. The WWR of the office is 0.5. The results indicate that cooling pipes are effective in each orientation. The solar radiation is, the greater the reduction of reduction However, the the solar radiation is,stronger the greater the reduction electricity. However, the rates of electricity orientation. The stronger the the solar radiation is,ofthe greater the reduction ofelectricity. electricity. However, the reduction rates of electricity consumption are close in different orientations. It is not necessary consumption in different orientations. It is in notdifferent necessary to embed the for thetonorth reduction ratesare of close electricity consumption are close orientations. It ispipes not necessary to embed the pipes for theradiation north window because the solar radiation there is weak. window because thethe solar there is weak. embed the pipes for north window because the solar radiation there is weak.

Figure 13. Seasonal electricityconsumption consumption of orientations. Figure 13. Seasonal electricity of the theoffices officesinindifferent different orientations.

WWR is an important factor for the heat gain through the building envelope. The electricity

WWR isFigure an important factor for the heat gain of through theinbuilding envelope. The electricity electricity consumption the offices different orientations. consumption of13. theSeasonal offices with different WWRs is shown in Figure 14. The results are the average consumption of the offices with different WWRs is shown in Figure 14. The results are the average values of four orientations, E, S, N, and W. The electricity consumption is increased with WWR in values of four orientations, E, S, N, and W. The electricity consumption is increased WWR WWR is an important factor for the heat but gain the different. buildingInenvelope. Thewith electricity both traditional and pipe-embedded offices, thethrough reasons are the traditional office, in both traditional and pipe-embedded offices,through the different. Inare the traditional consumption of themeans offices withheat different WWRs isbut shown inwindow, Figureare 14. The the average larger WWR more will transfer thereasons and the results electricity increases office, larger means more willW. transfer through consumption the window, and the electricity increases values of fourWWR orientations, E, S, heat N, and The electricity is increased with WWR in both traditional and pipe-embedded offices, but the reasons are different. In the traditional office, larger WWR means more heat will transfer through the window, and the electricity increases


15 of 17 15 of 17

correspondingly. In In thethepipe-embedded theheat heatflux flux window not grow correspondingly. pipe-embedded envelope, envelope, the of of thethe window will will not grow significantly with WWR. However, the cooling surface of the wall is reduced accordingly, which significantly with WWR. However, the cooling surface of the wall is reduced accordingly, which will will influence the cooling effect of of thethewall the embedded embeddedpipes, pipes, relatively large WWR is influence the cooling effect wallpanel. panel. With With the relatively large WWR is acceptable. example, electricityconsumption consumption of GSHE (WWR = 0.75) is only acceptable. For For example, thethe electricity ofthe theoffice officewith with GSHE (WWR = 0.75) is only than that the traditionaloffice office with 238.8238.8 kWh,kWh, eveneven lessless than that of of the traditional with aaWWR WWRofof0.25. 0.25.

Figure Seasonal electricityconsumption consumption of different WWRs. Figure 14. 14. Seasonal electricity ofthe theoffices officeswith with different WWRs.

4. Conclusions

4. Conclusions

The energy performance of a typical office with pipe-embedded window and wall is numerically

The energy performance of a typical office with pipe-embedded window and wall is numerically investigated in this study. The effectiveness of three natural cooling sources (DEC, IEC, GSHE) is investigated study.The The effectiveness of three natural cooling sources (DEC, IEC, GSHE) is evaluatedininthis Urumqi. influences of insulated glass, orientation, and WWR are considered by evaluated in Urumqi. The influences of insulated glass, orientation, and WWR are considered by simulating a complete summer. The main conclusions drawn are as follows: simulating a complete summer. The main conclusions drawn are as follows: 1.

1.

2.

3. 4.

In windows, cooling pipes can remove 60% of the solar radiation directly and only less than 15% of the radiation can transfer into the room. The insulated glass should be installed to the external In windows, cooling pipes can remove 60% of the solar radiation directly and only less than 15% skin of a pipe-embedded double window to reduce the heat dissipation. The pipe effectiveness of the radiation can transfer into the room. The insulated glass should be installed to the external η is around 60%. skin In ofwalls, a pipe-embedded double window to reduce the heat dissipation. η 2. cooling pipes can reduce the internal surface temperature by moreThe thanpipe 4 °C.effectiveness The pipeis around 60%.wall becomes an efficient radiant cooling panel absorbing the heat from the room. embedded In walls, cooling pipesηcan reduce the internal surface temperature by more than 4 ◦ C. The pipe effectiveness is around 83%. 3. seasonal cooling consumption of radiant the officecooling with pipes is absorbing reduced bythe 25%–50%. A the The The pipe-embedded wallenergy becomes an efficient panel heat from large WWR is acceptable with the cooling effect of pipes. room. The pipe effectiveness η is around 83%. 4. The performance of GSHE is the best among the three sources in Urumqi. The effectiveness of The seasonal cooling energy consumption of the office with pipes is reduced by 25%–50%. A large DEC and IEC is also satisfactory, with an average energy saving rate of 27%. IEC can reduce WWR is acceptable with the cooling effect of pipes. more heat flux through the envelope, but it consumes more energy to provide the cooling water.

The performance of GSHE is the best among the three sources in Urumqi. The effectiveness of the pipe-embedded system is very suitable for utilizing natural energy and dealing with DECOverall, and IEC is also satisfactory, with an average energy saving rate of 27%. IEC can reduce more the climate in regions like Urumqi. heat flux through the envelope, but it consumes more energy to provide the cooling water.

Acknowledgments: This study has been supported by the National Natural Science Foundation of China (Grant

Overall, theand pipe-embedded system is very suitable for utilizing natural energy and dealing with No. 51638010 51578306). the climate in regions like Urumqi. Author Contributions: Chong Shen conducted the simulation and drafted the article. Xianting Li supervised the investigation, reviewed the manuscript and provided comments for revision of the article.

Acknowledgments: This study has been supported by the National Natural Science Foundation of China (GrantConflicts No. 51638010 and The 51578306). of Interest: authors declare no conflict of interest. The funding sponsors had no role in the design of Contributions: the study; in the Chong collection, analyses, or interpretation of data; the writing of the manuscript, in the the Author Shen conducted the simulation andin drafted the article. Xianting Li and supervised decision to publish the investigation, reviewed theresults. manuscript and provided comments for revision of the article.

Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Energies 2017, 10, 366

16 of 17

Nomenclature A E q Q ε η

area, m2 electricity consumption, W/m2 heat gain, W/m2 cooling load, W/m2 electricity reduction rate of pipe-embedded envelope effectiveness of pipes

Abbreviations DEC direct evaporative cooling DSF double skin façade EER energy efficiency ratio GSHE ground-source heat exchanger IEC indirect evaporative cooling WWR window-to-wall ratio

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Bisegna, F.; Mattoni, B.; Gori, P.; Asdrubali, F.; Guattari, C.; Evangelisti, L.; Sambuco, S.; Bianchi, F. Influence of insulating materials on green building rating system results. Energies 2016, 9, 712. [CrossRef] Ojanen, T.; Seppä, I.P.; Nykänen, E. Thermal insulation products and applications—Future road maps. Energy Procedia 2015, 78, 309–314. [CrossRef] Ji, Y.; Fitton, R.; Swan, W.; Webster, P. Assessing overheating of the UK existing dwellings—A case study of replica Victorian end terrace house. Build. Environ. 2014, 77, 1–11. [CrossRef] Sadineni, S.B.; Madala, S.; Boehm, R.F. Passive building energy savings: A review of building envelope components. Renew. Sustain. Energy Rev. 2011, 15, 3617–3631. [CrossRef] Pargana, N.; Pinheiro, M.D.; Silvestre, J.D.; de Brito, J. Comparative environmental life cycle assessment of thermal insulation materials of buildings. Energy Build. 2014, 82, 466–481. [CrossRef] Hidalgo, J.P.; Welch, S.; Torero, J.L. Performance criteria for the fire safe use of thermal insulation in buildings. Constr. Build. Mater. 2015, 100, 285–297. [CrossRef] Bahaj, A.S.; James, P.A.B.; Jentsch, M.F. Potential of emerging glazing technologies for highly glazed buildings in hot arid climates. Energy Build. 2008, 40, 720–731. [CrossRef] Garrison, J.D.; Collins, R.E. Manufacture and cost of vacuum glazing. Sol. Energy 1995, 55, 151–161. [CrossRef] Manz, H.; Brunner, S.; Wullschleger, L. Triple vacuum glazing: Heat transfer and basic mechanical design constraints. Sol. Energy 2006, 80, 1632–1642. [CrossRef] Mohelnikova, J. Materials for reflective coatings of window glass applications. Constr. Build. Mater. 2009, 23, 1993–1998. [CrossRef] Hong, T.; Kim, J.; Lee, J.; Koo, C.; Park, H. Assessment of seasonal energy efficiency strategies of a double skin façade in a monsoon climate region. Energies 2013, 6, 4352–4376. [CrossRef] Cetiner, I.; Özkan, E. An approach for the evaluation of energy and cost efficiency of glass façades. Energy Build. 2005, 37, 673–684. [CrossRef] Chan, A.L.S.; Chow, T.T.; Fong, K.F.; Lin, Z. Investigation on energy performance of double skin façade in Hong Kong. Energy Build. 2009, 41, 1135–1142. [CrossRef] Shen, C.; Li, X. Thermal performance of double skin façade with built-in pipes utilizing evaporative cooling water in cooling season. Sol. Energy 2016, 137, 55–65. [CrossRef] Parra, J.; Guardo, A.; Egusquiza, E.; Alavedra, P. Thermal performance of ventilated double skin façades with venetian blinds. Energies 2015, 8, 4882–4898. [CrossRef] Manz, H. Total solar energy transmittance of glass double façades with free convection. Energy Build. 2004, 36, 127–136. [CrossRef] Gratia, E.; De Herde, A. Greenhouse effect in double-skin facade. Energy Build. 2007, 39, 199–211. [CrossRef] Xu, X.; Wang, S.; Wang, J.; Xiao, F. Active pipe-embedded structures in buildings for utilizing low-grade energy sources: A review. Energy Build. 2010, 42, 1567–1581. [CrossRef]

Energies 2017, 10, 366

19. 20.

21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

32. 33.

34. 35.

17 of 17

Shen, C.; Li, X. Dynamic thermal performance of pipe-embedded building envelope utilizing evaporative cooling water in the cooling season. Appl. Therm. Eng. 2016, 106, 1103–1113. [CrossRef] Li, A.; Xu, X.; Sun, Y. A study on pipe-embedded wall integrated with ground source-coupled heat exchanger for enhanced building energy efficiency in diverse climate regions. Energy Build. 2016, 121, 139–151. [CrossRef] Shen, C.; Li, X. Solar heat gain reduction of double glazing window with cooling pipes embedded in venetian blinds by utilizing natural cooling. Energy Build. 2016, 112, 173–183. [CrossRef] Ministry of Housing Urban-Rural Development. National Standard GB 50736-2012-Code for Design of Heating Ventilation and Air Conditioning; Ministry of Housing Urban-Rural Development: Beijing, China, 2012. Zeng, Z.; Li, X.; Li, C.; Zhu, Y. Modeling ventilation in naturally ventilated double-skin façade with a venetian blind. Build. Environ. 2012, 57, 1–6. [CrossRef] Iyi, D.; Hasan, R.; Penlington, R.; Underwood, C. Double skin façade: Modelling technique and influence of venetian blinds on the airflow and heat transfer. Appl. Therm. Eng. 2014, 71, 219–229. [CrossRef] Xie, J.; Zhu, Q.; Xu, X. An active pipe-embedded building envelope for utilizing low-grade energy sources. J. Cent. South Univ. 2012, 19, 1663–1667. [CrossRef] Lu, Y. Practical Design Handbook for HVAC, 2nd ed.; China Building Industry Press: Beijing, China, 2008. Saelens, D.; Roels, S.; Hens, H. Strategies to improve the energy performance of multiple-skin facades. Build. Environ. 2008, 43, 638–650. [CrossRef] Shen, C.; Li, X. Energy saving potential of pipe-embedded building envelope utilizing low-temperature hot water in the heating season. Energy Build. 2017, 138, 318–331. [CrossRef] China Meteorological Administration. Weather Database for Analyzing the Building Thermal Environment in China, 1st ed.China Architectural & Building Press: Beijing, China, 2005. Klimanek, A.; Cedzich, M.; Białecki, R. 3D CFD modeling of natural draft wet-cooling tower with flue gas injection. Appl. Therm. Eng. 2015, 91, 824–833. [CrossRef] Duan, Z.; Zhan, C.; Zhang, X.; Mustafa, M.; Zhao, X.; Alimohammadisagvand, B.; Hasan, A. Indirect evaporative cooling: Past, present and future potentials. Renew. Sustain. Energy Rev. 2012, 16, 6823–6850. [CrossRef] Zhu, Y. Built Environment, 3rd ed.; China Architectural & Building Press: Beijing, China, 2010. Wu, W.; Wang, B.; You, T.; Shi, W.; Li, X. A potential solution for thermal imbalance of ground source heat pump systems in cold regions: Ground source absorption heat pump. Renew. Energy 2013, 59, 39–48. [CrossRef] National Development and Reform Commission. National Standard GB/T 17981-2007-Economic Operation of Air Conditioning Systems; National Development and Reform Commission: Beijing, China, 2008. Duan, Z. Investigation of a Novel Dew Point Indirect Evaporative Air Conditioning System for Buildings. Ph.D. Thesis, The University of Nottingham, Nottingham, UK, September 2011. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).