# Study on the Thermal Performance of a Hybrid Heat Collecting Facade Used for Passive Solar Buildings in Cold Region

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Principle of HHCF

- In a sunny day, solar energy is absorbed by the exterior surface of the heat collecting wall and converted into thermal energy for heating the air convectively within the heat collecting space. With the inner double-glass window open, the heated air in the heat collecting space rises up and reaches the top of the heat collecting space under the effect of the buoyancy force. Then, the heated air gathers at the top of the heat collecting space and increases the air pressure at the top of the heat collecting space. Finally, under the effect of the pressure difference, the heated air passes through the heat transfer space and enters into the indoor space along the horizontal direction. Meanwhile, at the bottom of the heat collecting space, the low-pressure cavity is supplied by the indoor air along the horizontal direction. In this way, thermal energy is transferred into the room space and the indoor temperature rises. Moreover, sunlight can also penetrate the windows into the indoor space directly and the solar energy is stored in the interior building construction.
- At night or during a cloudy day, the inner double-glass window should be kept closed, and it increases the thermal resistance between the indoor space and the outdoor environment. Consequently, the heat loss from the indoor space to the outdoor environment can be effectively reduced, especially at night.

#### 2.2. Modeling of the Building with HHCF

#### 2.2.1. Assumptions

- (1)
- Thermal properties of the building materials are kept constant.
- (2)
- Heat transfer processes through walls, floor, roof, and windows are considered as one-dimensional.
- (3)
- The heat storage of glass is ignored.
- (4)
- Air in each zone is well-mixed.
- (5)
- Mean air flow rate between the heat collecting space and the heat transfer space is identical with that between the heat transfer space and the indoor space.

#### 2.2.2. Energy Balance Equations

**Heat transfer of HHCF**

**Heat transfer of building construction**

**Heat balance of indoor air**

#### 2.2.3. Validation of Heat Transfer Model

#### 2.2.4. Energy Saving Comparison of HHCF

## 3. Results and Discussion

#### 3.1. Energy Saving Potential of HHCF

^{2}. Compared with the conventional direct solar gain window, the HHCF reduces the total heating need of the room by 19.2 kWh/m

^{2}and the energy-saving efficiency reaches 40.2%. Even in contrast to the conventional Trombe wall, the HHCF also decreases the total heating need by 21.5%. The comparison results show that the HHCF proposed in this paper has very high energy saving potential.

#### 3.2. Parametric Study on the Thermal Performance of a Room with a HHCF

#### 3.2.1. Effects of Window Operational Schedule

#### 3.2.2. Effects of Width of Heat Collecting Wall

#### 3.2.3. Effects of Absorptivity of Heat Collecting Wall

#### 3.2.4. Effects of Thermal Performance of Inner Double-Glass Window

**U-value**

^{2}·K), the mean indoor temperature will have a drop of 0.3 °C.

**Solar heat gain coefficient**

^{2}·K). As depicted in Figure 17, when the U-value is constant, the indoor temperature increases with the solar heat gain coefficient. If the solar heat gain coefficient increases by 0.1, the mean indoor air temperature has a rise of 2.1 °C. In practical applications, to reduce the heating energy use for the building, both a higher solar heat gain coefficient and a lower U-value are necessary for the double-glass window.

#### 3.2.5. Effects of Outer Single-Glass Window

^{2}·K), the mean indoor temperature has a drop of about 0.17 °C. Similarly, if the solar heat gain coefficient increases by 0.1, the mean indoor temperature rises about 0.4 °C.

#### 3.2.6. Effects of Air Gap Thickness

#### 3.3. Summary of the Presented Results and Discussion

## 4. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Nomenclature

A | building envelope area (m^{2}) | Subscripts and superscripts | |

ACH | air change per hour (1/h) | a | air |

C | specific heat capacity (J/(kg·K)) | con | building construction |

D | total thickness of building construction (m) | con, in | interior surface of building construction |

g | gravity acceleration (m/s^{2}) | con, out | exterior surface of building construction |

Gr | Grashof number | clo | closed double-glass window |

h | convective heat transfer coefficient (W/(m^{2}·K)) | cs | horizontal cross section of the heat collecting space |

H | height of HHCF (m) | diffuse | diffuse radiation |

I | intensity of solar radiation (W/m^{2}) | direct | direct radiation |

Ir | incident radiation (W/m^{2}) | doub | double-glass window |

L | width of double-glass window (m) | hcs | heat collecting space |

l | characteristic length (m) | hcs-hts | convection from heat transfer space to heat collecting space |

m | mass flow rate of air in the heat collecting space (kg/s) | hts | heat transfer space |

Pr | Prandtl number | hts-in | convection from heat transfer space to indoor space |

q | heat flux (W/m^{2}) | in | indoor |

q_{ts} | solar radiation absorbed by the interior surface (W/m^{2}) | interheat | indoor heat source of building |

q_{tsf} | transmitted solar radiation through the fenestration (W/m^{2}) | leak | air leak from room |

Q | heat transfer rate (W) | lw | long-wave radiation exchange |

S | thickness of air gap (m) | ope | open double-glass window |

SHGC | solar heat gain coefficient | out | outdoor |

T | temperature (°C) | r | room |

U | U-value of window (W/(m^{2}·K)) | sig | single-glass window |

V | volume (m^{3}) | sig-hcs | convection from single-glass single-glass window to air in the heat collecting space |

V_{m} | average wind velocity (m/s) | sig-hts | convection from single-glass single-glass window to air in the heat transfer space |

W | width of heat collecting wall (m) | tic | conduction through closed double-glass window from heat transfer space to indoor space |

Greek symbols | va | outlet area of the vent | |

α | absorptivity of building surface | wa-hcs | convection from heat collecting wall to air in the gap |

ρ | density (kg/m^{3}) | win | window |

λ | thermal conductivity (W/(m·K)) | ||

τ | time (s) | ||

ν | dynamic viscosity (Pa·s) | ||

ε | emissivity of building surface | ||

ξ_{1} | vent pressure loss coefficient | ||

ξ_{2} | air gap pressure loss coefficient |

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**Figure 1.**Schematic diagram of the hybrid heat collecting facade (HHCF). (

**a**) with double-glass window open; (

**b**) with double-glass window closed.

**Figure 9.**Comparison of Results from experiments and simulations. (

**a**) Indoor air temperature; (

**b**) Air temperature of heat transfer space; (

**c**) Interior surface temperature of west wall.

**Figure 11.**TRNSYS model for Case 1 and Case 2. (

**a**) TRNSYS model for Case 1; (

**b**) TRNSYS model for Case 2.

**Figure 12.**Comparisons of indoor air temperature and heating energy demand on heating design day. (

**a**) Indoor air temperature for studied cases; (

**b**) Heating energy demand for studied cases.

Construction | Material | Thermal Conductivity (W/(m·K)) | Heat Capacity (J/(kg·K)) | Density (kg/m^{3}) |
---|---|---|---|---|

Exterior wall | 80 mm polyurethane | 0.033 | 1380 | 40 |

240 mm brick wall | 0.89 | 1000 | 1800 | |

20 mm Cement mortar | 0.93 | 1050 | 1800 | |

Interior wall | 20 mm Cement mortar | 0.93 | 1050 | 1800 |

240 mm brick wall | 0.89 | 1000 | 1800 | |

20 mm Cement mortar | 0.93 | 1050 | 1800 | |

Ceiling | 20 mm Cement mortar | 0.93 | 1050 | 1800 |

180 mm concrete | 1.74 | 920 | 2500 | |

20 mm Cement mortar | 0.93 | 1050 | 1800 | |

Door | 60 mm wood | 0.15 | 1630 | 608 |

HHCF | 3 mm single-glass window, U = 5.56 W/(m^{2}·K); SHGC = 0.9; 120 mm-thickness air gap; 12 mm double-glass window, U = 2.83 W/(m ^{2}·K); SHGC = 0.76; 0.5 m-width for each heat collecting wall (W = 0.5 m); Materials of the heat collecting wall is the same with the exterior wall Absorptivity of the heat collecting wall is 0.8; 2.3 m-width for double-glass window (L = 2.3 m) |

Indoor Air Temperature (°C) | Total Heating Energy Demand (kWh/m^{2}) | |||
---|---|---|---|---|

Mean | Minimum | Maximum | ||

Case 1 | 17.5 | 10.4 | 21.9 | 47.9 |

Case 2 | 18.2 | 10.5 | 26.7 | 36.6 |

Case 3 | 18.6 | 11.0 | 27.1 | 28.7 |

Schedule A | Schedule B | Schedule C | Schedule D | |
---|---|---|---|---|

Time to open windows | With windows closed all day | 8:00 | 9:00 | 11:00 |

Time to close windows | 18:00 | 17:00 | 15:00 | |

Indoor air temperature (°C) | 6.7~13.2 | 8.2~19.0 | 8.6~19.0 | 7.8~17.5 |

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**MDPI and ACS Style**

Wang, X.; Lei, B.; Bi, H.; Yu, T.
Study on the Thermal Performance of a Hybrid Heat Collecting Facade Used for Passive Solar Buildings in Cold Region. *Energies* **2019**, *12*, 1038.
https://doi.org/10.3390/en12061038

**AMA Style**

Wang X, Lei B, Bi H, Yu T.
Study on the Thermal Performance of a Hybrid Heat Collecting Facade Used for Passive Solar Buildings in Cold Region. *Energies*. 2019; 12(6):1038.
https://doi.org/10.3390/en12061038

**Chicago/Turabian Style**

Wang, Xiaoliang, Bo Lei, Haiquan Bi, and Tao Yu.
2019. "Study on the Thermal Performance of a Hybrid Heat Collecting Facade Used for Passive Solar Buildings in Cold Region" *Energies* 12, no. 6: 1038.
https://doi.org/10.3390/en12061038