# Field Observation of Cooling Energy Savings Due to High-Reflectance Paints

^{*}

^{†}

## Abstract

**:**

^{−2}·day

^{−1}.

## 1. Introduction

^{−2}·day

^{−1}, respectively. We have conducted similar experiments in Japan.

## 2. Outline of Building and Measurement Case Study

^{2}) two-story research building (with its first floor raised on pilotis) at Kobe University (see Figure 1 and Table 1). The test building is expected to offer significant potential for achieving cooling energy savings by deploying high-reflectance paint, owing to its small window area (15%) and large roof area (25%–30%) ratios to the total envelope surface area.

**Figure 1.**(

**a**) Facade and (

**b**) Plan (second floor) of test building (x: Measurement point in Figure 2).

Room | Roof-to-Envelope Ratio | Wall-to-Envelope Ratio | Window-to-Envelope Ratio | Set Temperature |
---|---|---|---|---|

Room 1 | 20.7% | 38.2% | 5.3% | 26 °C before 15:00, 8 August 2011 25 °C after 15:00, 8 August 2011 25 °C through 2012 |

Room 2 | 24.9% | 36.8% | 4.1% |

^{−2}·day

^{−1}and about 28 to 32 °C, respectively. The albedo of the objective paint is 86.9% (300–2500 nm wavelength according to JIS K 5602 [1]). The albedo measured by the pyranometer (which comprises white and black boards and measures solar radiation) was 16.9% before, 86.9% immediately after, and 76.1% after one year, of painting [8]. These are averaged values of the measurement results at three points on the roof surface (east, middle, and west).

**Figure 3.**Measurement results for temperature under slab, and above and under suspended ceiling in room 1; outdoor air temperature; and solar radiation.

**left**: Before painting;

**right**: After painting.

**Figure 4.**Measurement results for cooling energy consumption and solar radiation.

**left**: Before painting;

**right**: After painting.

## 3. Method of Evaluating Cooling Energy Savings due to High-Reflectance Paint

_{a}+ a × I/α, where T

_{a}is air temperature (°C); a is solar absorptance (-); I is solar radiation (W/m

^{2}); α is convective heat transfer coefficient (W/m

^{2}/K). After painting, the sol–air temperature changes significantly (influenced by solar radiation) but the power consumption does not change as much. The air temperature difference is averaged and the power consumption is integrated over 1 h. Since the temperature under the slab reaches its maximum around the evening, it is difficult to recognize the relationship between the temperature difference of outdoor considering solar radiation and room conditions and power consumption using hourly data in the right of Figure 5.

**Figure 5.**Hourly relationship between outdoor-to-room air temperature difference and cooling power consumption. black: Before painting; white: After painting;

**left**: Air temperature difference without considering solar radiation effect;

**right**: Air temperature difference considering solar radiation effect.

- internal heat generation;
- set temperature of the air conditioner;
- weather condition (air temperature, solar radiation).

^{−1}); I is the daily integrated solar radiation (Wh day

^{−1}); ΔT is the difference between the daily averaged outdoor and room temperatures (in K); A is the coefficient related to absorptivity; B is the coefficient related to thermal conductance; and C is the internal heat generation. The cooling power consumption is directly proportional to these coefficients. As per the method, we should use the difference of enthalpy instead of ΔT since the power consumption includes the latent heat process, and consider the coefficient of performance of the air conditioner. However, in this study, we measured E, I, and ΔT and assumed that B and C do not change after painting. The influence of the set temperature of the air conditioner is reflected in ΔT, and that of the weather condition is reflected in ΔT and I. In the following chapter, we analyze the measurement results derived using Equation (1).

## 4. Examination of Cooling Energy Savings due to High-Reflectance Paint

^{−1}(72 Wh m

^{−2}day

^{−1}).

**Figure 6.**Relationship between (1) daily averaged outdoor-to-room air temperature difference and daily integrated cooling power consumption (

**left**) and (2) daily integrated solar radiation and cooling consumption (

**right**).

^{2}for room 1 and 43.5 m

^{2}for room 2) and expressed in the table. Since the slopes of the regression Equation (B) in the three scenarios are relatively similar in each room, the influence of ΔT on the cooling power consumption is similar in each room. For room 1, the values of B are almost similar for both immediately and one year after painting. However, the difference of energy savings between the immediately and one year after painting is not significant due to a little lower determination coefficient of each regression equation. The cooling energy savings in room 2 are lower than that in room 1. The ratio of internal heat generation to the total cooling load in room 2 is higher than that in room 1 because the former’s floor area is approximately twice the latter’s. Since the values of B for room 2 are lower than those for room 1, room 2 is not heavily influenced by the external weather condition.

**Table 2.**Regression equations for cooling power consumption due to air temperature difference between outdoor and room condition for the scenarios (1) before, (2) immediately after, and (3) one year after painting, in rooms 1 and 2.

Room | Condition | Regression Equation | Energy Savings |
---|---|---|---|

Room 1 | Before painting * | E = 48.6 ΔT + 262.7 (R^{2} = 0.45) | 72 Wh·m^{−2}·day^{−1} 91 Wh·m ^{−2}·day^{−1} |

After painting ** | E = 50.3 ΔT + 190.8 (R^{2} = 0.82)
| ||

One year after painting *** | E = 48.0 ΔT + 171.6 (R^{2} = 0.82) | ||

Room 2 | Before painting * | E = 26.7 ΔT + 151.3 (R^{2} = 0.54 | 35 Wh·m^{−2}·day^{−1}9 Wh·m ^{−2}·day^{−1} |

After painting ** | E= 27.6 ΔT + 116.6 (R^{2} = 0.60)
| ||

One year after painting *** | E = 26.9 ΔT + 142.8 (R^{2} = 0.77) |

## 5. Conclusions

^{−1}(72 Wh m

^{−2}day

^{−1}). Similar results were confirmed one year after painting. However, the difference of energy savings between the immediately and one year after painting is not significant due to a little lower determination coefficient of each regression equation. Although the results measured in a building actually under use were considered, the approximate energy savings could be satisfactorily evaluated. However, when ΔT is relatively higher, the cooling power consumption tends to be significantly higher. Therefore, we may have to consider an improvement in the coefficient of performance of the air conditioner in such a case. In this study, the cooling energy saving effect by high-reflectance paint is mainly discussed. For example, reduction of mean radiation temperature by lowering the ceiling surface temperature should be evaluated in future study.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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## Share and Cite

**MDPI and ACS Style**

Takebayashi, H.; Yamada, C.
Field Observation of Cooling Energy Savings Due to High-Reflectance Paints. *Buildings* **2015**, *5*, 310-317.
https://doi.org/10.3390/buildings5020310

**AMA Style**

Takebayashi H, Yamada C.
Field Observation of Cooling Energy Savings Due to High-Reflectance Paints. *Buildings*. 2015; 5(2):310-317.
https://doi.org/10.3390/buildings5020310

**Chicago/Turabian Style**

Takebayashi, Hideki, and Chihiro Yamada.
2015. "Field Observation of Cooling Energy Savings Due to High-Reflectance Paints" *Buildings* 5, no. 2: 310-317.
https://doi.org/10.3390/buildings5020310