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Roof-integrated radiative air-cooling system to achieve cooler attic for building energy saving
Energy & Buildings 203 (2019) 109453
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Roof-integrated radiative air-cooling system to achieve cooler attic for building energy saving Dongliang Zhao a, Ablimit Aili a, Xiaobo Yin a,b, Gang Tan c,∗, Ronggui Yang a,b,∗ a
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, USA Materials Science and Engineering Program, University of Colorado, Boulder, Colorado 80309, USA c Department of Civil and Architectural Engineering, University of Wyoming, Laramie, Wyoming 82071, USA b
a r t i c l e
i n f o
Article history: Received 20 April 2019 Revised 7 August 2019 Accepted 22 September 2019 Available online 23 September 2019 Keywords: Radiative sky cooling Attic cooling Attic ventilation Building energy saving
a b s t r a c t The building attic usually subjects to substantial solar heat gain and has much higher temperature compared to the conditioned living space during the day, especially in summer and in hot areas. Reducing attic temperature can reduce cooling energy consumption in buildings. However, conventional techniques such as cool roof or attic ventilation, suffer from either heating penalty in winter or limited attic temperature reduction. In this work, a new roof-integrated radiative air-cooling system is introduced, which couples radiative sky cooling with attic ventilation to reduce attic temperature. A radiative air cooler with 1.08 m2 surface area is constructed using a recently developed daytime radiative sky cooling metamaterial [Zhai et al., Science 355, pp. 1062–1066, 2017]. Experimental tests show that sub-ambient air cooling is achieved throughout 24-h day-and-night cycle in a summer day with clear sky conditions. Depending on air flow rates, measured sub-ambient temperature reductions of air are 5–8 °C at night and 3–5 °C at noon under direct sunlight, respectively. An in-house model is first developed for the radiative aircooling system, the model is then coupled with EnergyPlus to study annual energy saving of buildings. The performance of the radiative air-cooling system is compared with three reference systems: shingle roof, attic ventilation, and cool roof. Results show that for a single-family house, attic temperature can be substantially reduced by 15.5–21.0 °C, varying with attic insulation level, compared to shingle roof on typical summer days. Compared to a shingle roof (solar reflectance 0.25, thermal emittance 0.9) residential building with attic insulations of R-30 (RSI-5.28), R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roof-integrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7–11.8 kWh/m2 (26.5–76.1%) respectively. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Space cooling is a major part of energy consumption in residential buildings. In 2015, air conditioning accounts for about 12% of the total energy consumption in a typical US home [1]. Globally, using air conditioners and electric fans for cooling purposes accounts for about 10% of all electricity consumption today [2]. More importantly, due to global warming, population growth, and rapid development of living standard in developing countries, especially those in hot climates, such as Brazil, Malaysia, Mexico, and Indonesia, global energy demand for cooling is expected to triple by 2050, according to a report by International Energy Agency [3]. There is a strong need to reduce energy consumption for space cooling. The cooling load of a residential building comes from various aspects, including solar heat gain from the building envelope (e.g., ∗
Corresponding authors. E-mail addresses: [email protected] (G. Tan), [email protected] (R. Yang).
https://doi.org/10.1016/j.enbuild.2019.109453 0378-7788/© 2019 Elsevier B.V. All rights reserved.
roof, windows, and walls), lighting, and heat generation of indoor facilities and occupants. Most residential building roofs are under substantial solar radiation during the day, which results in much higher temperatures (as high as 60 °C) in the attics as compared to the conditioned living space underneath. The heat is then transferred from the attic to the living space through the ceiling, which results in higher cooling load for the air conditioner. Also, many residential buildings have duct system in the attic, and the heat transfer from the attic air to duct causes even larger cooling load to the air conditioner [4]. In the US, it is estimated that roofs contribute ∼14% of the net residential cooling load [5]. Current techniques for reducing heat gain through the roof include the installation of very thick attic insulation [6] and radiant barrier [7], and using attic ventilation [8–10] and cool roof [11–17]. However, each of the techniques suffer from their own drawbacks. Attic ventilation draws ambient air into the attic through natural ventilation (e.g., turbine ventilation [18]) or mechanical ventilation by using a self-supporting fan (e.g., solar cell powered [9]).
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Attic ventilation can reduce attic temperature, but the effectiveness in reducing air conditioner’s energy consumption is limited due to the limited attic temperature reduction. Ismail and Abdul Rahman [18] conducted a full-scale field test to investigate a hybrid turbine ventilator that integrates wind-force turbine ventilator with solar cell powered inner fan in Malaysia. It was found that the turbine ventilator can reduce attic air temperature by ∼1.0 °C in average compared to the case without the turbine ventilator. Yu and Moore [9] experimentally studied attic temperature reduction by using a solar powered attic fan. The average peak attic temperatures were 45.3 °C (Fan OFF) and 42.2 °C (Fan ON), yielding an average attic temperature reduction of 3.1 °C. The other issue with attic ventilation is the pressure imbalance between the conditioned space and the attic. In cases where there are not adequate openings in the soffit vents or the attic floor is not sealed adequately, attic ventilation may not be effective because cool air in the conditioned space can be pulled into the attic through the attic floor [9]. Cool roof refers to those roofing materials that have been designed to reflect more sunlight and thus absorb less heat than a standard roof (e.g., asphalt shingle roof). According to the ASHRAE standard 90.1 [19], cool roofs should have a minimum solar reflectance index (SRI) of 82, which is calculated based on the material solar reflectance and thermal emittance. Synnefa et al. [12] studied the impact of using cool roofs on the cooling loads of residential buildings for 27 different climate conditions. For roof with a U-value of 0.39 (RSI-2.56, or R-15), they found that increasing the solar reflectance from 0.2 to 0.85 can reduce annual cooling load by 7.1–22.1%, depending on the climate conditions. Rosado et al. [13] introduced a “cool” tile roof in a single-family, singlestory home in Fresno, California. The cool tile roof (initial albedo 0.51) has much higher solar reflectance than dark asphalt shingle roof (initial albedo 0.07) and the attic floor is insulated with R19 (RSI-3.3) blown cellulose insulation. The annual cooling energy savings per unit ceiling area was 2.82 kWh/m2 (26% savings of total cooling energy consumption) compared to a “standard” home with dark asphalt shingle roof. However, the use of cool roof could subject to heating penalty in winter season [20], and this is especially true for cold climates. As a “free” cooling source, radiative sky cooling has been explored for building applications for many decades, in particular nighttime cooling. A few radiative sky cooling systems have been built. Meir et al. [21] constructed a polymer-based radiative cooling system that consists of a 5.3 m2 radiator and a 280-liter water tank. The system has achieved an average cooling flux of 55 W/m2 at night in Norway. Eicker and Dalibard [22] developed a photovoltaic–thermal (PVT) system in Germany that can produce both electricity and cooling energy. The measured radiative cooling power levels were between 40 and 65 W/m2 . Zhang and Niu [23] combined nighttime radiative sky cooling with microencapsulated phase change material slurry storage. Modeling results showed that the energy saving potential in Lanzhou and Urumqi (China) can reach 77% and 62% respectively for low-rise buildings. Zhao et al. [24] developed a building-integrated photovoltaicradiative cooling system in Eastern China. Compared to a building integrated photovoltaic (BIPV) system, the total electricity production and cooling energy gain of this system were 97% higher. In summary, cooling potential of nighttime radiative sky cooling system are generally within the range of 40–87 W/m2 [25]. Recent advances in daytime radiative sky cooling technology show great promise for its building applications [26]. Daytime radiative sky cooling materials have been developed to achieve subambient temperatures under direct sunlight [27–30]. In particular, Zhai et al. [29] developed a low-cost polymer-glass hybrid radiative sky cooling metamaterial (hereafter named as RadiCold metafilm) which garnered strong interests. With an averaged infrared emissivity > 0.93 and a solar reflectivity of approximately 0.96, the
RadiCold metafilm has demonstrated an average of 93 W/m2 cooling power at noon. More importantly, the RadiCold metafilm can be scalably-manufactured by the roll-to-roll extrusion and web coating systems, which makes large-scale applications of radiative sky cooling technology possible [25,31]. If the RadiCold metafilm were deployed on building roofs, the attic temperature can be expected to be much lower than using current cool roof products. In this work, we propose a novel roof-integrated sub-ambient radiative air-cooling system to reduce attic temperature by employing the RadiCold metafilm. Advantages of the roof-integrated radiative air-cooling system include: 1) lower (sub-ambient) inlet air temperature into the soffit vents as compared to attic ventilation; 2) higher solar reflectance (at around 0.96) as compared to cool roof products, which usually have initial solar reflectance between 0.6– 0.9 [32]; 3) on/off control of the system gives operational flexibility to avoid possible over cooling (i.e., heating penalty) in winter. A prototype day-night radiative air cooler with 1.08 m2 surface area is built to demonstrate sub-ambient cooling of air under different flow rates. Measured sub-ambient temperature reductions are between 5–8 °C at night and between 3–5 °C at noon respectively. A modeling tool is developed to predict the performance of the radiative air cooler, which shows good agreement with experimental results. The modeling tool is then used to create a customized model for the roof-integrated radiative air-cooling system in building simulation software EnergyPlus. Energy performance of the radiative air-cooling system is compared with three reference systems: shingle roof, attic ventilation, and cool roof. Simulation results show that attic temperature can be substantially reduced by 15.5–21.0 °C (varies with attic insulation level) compared to shingle roof on typical summer days. Annual energy saving analysis with EnergyPlus has been carried out for four different climates in the United States. Compared to a shingle roof (solar reflectance 0.25, thermal emittance 0.9) residential building with attic insulations of R-30 (RSI-5.28), R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roofintegrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7–11.8 kWh/m2 (26.5–76.1%) respectively. 2. System description Fig. 1(a) shows a schematic of the roof-integrated radiative air-cooling system in residential building. The system consists of multiple radiative air coolers connected in parallel, as shown in Fig. 1(b). The system draws air from the ambient, and the air is being cooled to sub-ambient temperature due to significant solar reflection and infrared thermal emission to the sky [29]. The cooled air is then supplied to the attic and mix with attic air to reduce the temperature of the latter one. In this way, air temperature in the attic can be greatly reduced during the day, which reduces heat transfer from the attic to the conditioned space. When cooling is not needed in the conditioned space, for example, in winter, the fan employed in the radiative air-cooling system can be turned off to avoid heating penalty. Radiative air cooler is developed as modules that can be easily scaled up. Fig. 2 shows the schematics and a photograph of such a radiative air cooler (module). Surface area of the radiative air cooler is 1.8 m × 0.6 m (1.08 m2 ). It is built using wood as framework and aluminum plate as baffles [Fig. 2(a)]. Four sides and the bottom are insulated with 5-cm-thick polyisocyanurate rigid foam insulation board. The RadiCold metafilm [Fig. 2(e)] is installed on top of an aluminum sheet (0.5-mm-thick) to provide cooling by using pressure sensitive adhesive. A 15-μm-thick polyethylene (PE) film is then placed 10 mm above the RadiCold metafilm to suppress thermal loss from the top at sub-ambient operating conditions [Fig. 2(b)]. The baffled design enhances the heat transfer between air and top cooling surface by creating air turbulence
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of 18°, which mimics a common 4:12 roof pitch [Fig. 2(d)]. The air flow rate is calculated by using the air velocity measured by an anemometer (MVA-02 mini-vane thermo-anemometer, ±2% uncertainty) and cross-sectional area of the air duct. K-type thermocouples (±0.3 °C uncertainty) are installed at the inlet and outlet of the radiative air cooler for temperature measurement. The thermocouples are connected to a NI-9213 data acquisition module which is read by a LabVIEW program in 10 s time interval. Solar radiation is measured by a CMP 11 pyranometer (±2% uncertainty). Local relative humidity is measured by Ambient Weather WS-1002 weather station (±5% uncertainty). 2.1. Modeling of the radiative air cooler A transient heat transfer model is developed to study the performance of the radiative air cooler. The heat transfer processes involved are shown in Fig. 2(b). The net cooling power, which equals to the rate of cold gain of the flowing air, can be expressed as:
ca m˙ a (Tinlet − Toutlet ) = Prad − Patm − Psolar − Pnon−radiative
Fig. 1. Roof-integrated radiative air-cooling system for attic temperature reduction. (a) Schematic of a radiative air-cooling system integrated with residential building. (b) The radiative air-cooling system consists of multiple radiative air coolers connected in parallel.
where ca and m˙ a are specific heat and mass flow rate of air respectively, Tinlet and Toutlet are the temperatures of air at inlet and outlet. The radiative cooling power of the RadiCold metafilm can be obtained by integrating the spectral radiance over the entire spectrum and the hemisphere [26]:
Prad T f ilm = A and increasing the heat transfer coefficient [Fig. 2(c)]. The length and spacing of the baffles were optimized to balance heat transfer and pressure loss across the radiative air cooler [33]. A small direct current (DC) fan is installed at the inlet to draw ambient air. Input power to the DC fan is adjustable to achieve different air flow rates. The radiative air cooler is tested with a tilt angle
(1)
cosθ d
∞ 0
Ibb
λ, T f ilm ε f ilm (, λ )dλ
(2)
π /2 2π where d = 0 dθ sinθ 0 dφ is the angular integral over a hemisphere, ɛfilm (, λ) is the emissivity of the RadiCold metafilm as a function of direction and wavelength, Ibb (λ, T f ilm ) = 2hc 2
1 is the spectral radiance of blackbody at temλ5 exp(hc/(λkB T f ilm )−1)
perature Tfilm , λ is wavelength, and A is the radiative cooling surface area.
Fig. 2. Prototype radiative air cooler. (a) Exploded view of the radiative air cooler. (b) Cross-sectional view of the radiative air cooler showing heat transfer processes. (c) Dimensions of the radiative air cooler. (d) A photograph of the radiative air cooler with a tilt angle of 18°, which mimics a typical 4:12 roof pitch. (e) A photograph of the low-cost lab-produced RadiCold metafilm roll.
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Fig. 3. Test results of the radiative air cooler under three different air flow rates at Boulder, Colorado, USA. Inlet temperature of the radiative air cooler tracks ambient temperature. (a) The variation of inlet (gray curve) and outlet [blue curve (measured), red curve (calculated)] temperatures with time under flow rate 2.4 L/(m2 ·s). (b) The variation of cooling power and relative humidity with time under flow rate 2.4 L/(m2 ·s). (c) The variation of inlet and outlet temperatures with time under flow rate 5.9 L/(m2 ·s). (d) The variation of cooling power and relative humidity with time under flow rate 5.9 L/(m2 ·s). (e) The variation of inlet and outlet temperatures with time under flow rate 7.9 L/(m2 ·s). (f) The variation of cooling power and relative humidity with time under flow rate 7.9 L/(m2 ·s).
Similarly, the absorbed atmospheric radiation, depending on the emissivities of both the RadiCold metafilm and the atmosphere, is given by:
Patm (Tamb ) = A
cosθ d
∞ 0
Ibb (λ, Tamb )ε f ilm (, λ )εatm (, λ )dλ (3)
where Tamb is the ambient temperature, ɛatm (λ, ) is the atmospheric emissivity, which is a function of the precipitable water [34]. Precipitable water is defined as the depth of water in a column of the atmosphere if all the water vapor in that column were precipitated as rain, and it can be calculated from ambient temperature and relative humidity [30]. In this work, with calculated precipitable water, ɛatm (λ, ) is obtained by using ATRAN – a tool for computing earth’s atmospheric transmission of near- and farinfrared radiation [34].
The absorbed solar irradiation on the RadiCold metafilm surface is calculated by:
Psolar = Aα¯ f ilm Isolar
(4)
where Isolar is the measured solar irradiance at the same tilt angle as the radiative cooling surface, α¯ f ilm is the effective solar absorptance of the RadiCold metafilm, 0.04. The non-radiative (i.e., convection and conduction) heat transfer between the RadiCold metafilm and the ambient is given by:
Pnon−radiative = hair A T f ilm − Tamb
(5)
where Tfilm is the average temperature of the RadiCold metafilm, hair is the overall heat transfer coefficient between the RadiCold metafilm and the ambient air. Earlier experimental studies suggest that the overall heat transfer coefficient from a flat surface can be
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Fig. 4. Sub-ambient temperature reduction (Tinlet - Toutlet ) and cooling power of the radiative air cooler as functions of inlet temperatures (from 0 to 40 °C) and air flow rates [from 2 to 10 L/(m2 ·s)]. The local wind speed is assumed as 1 m/s. Solar irradiation at daytime is assumed to be 800 W/m2 . Precipitable water is assumed to be constant at 10 mm.
quantified by a linear form of correlation [35–37].
hair = a + bv
(6)
where v is the local wind speed, and a and b are fitted parameters. This is a generally accepted correlation that is used to obtain convective heat transfer coefficient on a flat plate, such as solar thermal collectors [38] and solar cells [39]. From our previous study [30], the heat transfer coefficient of the radiative cooling surface varying with wind speed is expressed as h = 2.5 + 2v. As mentioned above, the roof-integrated radiative air-cooling system is composed of multiple radiative air coolers connected in parallel. Therefore, under the same air flow rate per surface area, the internal air flow characteristics of the radiative air-cooling system should be the same as a single radiative air cooler. The outlet air from the radiative air-cooling system is supplied to attic and well-mixed with attic air at each time step. When attic air temperature is lower than indoor air setting temperature, the system is turned off (i.e., fan turned off). Net cooling power of the radiative air-cooling system is the net cooling power per unit area multiply by the total system RadiCold metafilm surface area. 3. Results and discussion 3.1. Performance of the radiative air cooler The prototype radiative air cooler has been tested on summer days under clear sky conditions on the rooftop of the Engineering Center at the University of Colorado Boulder. Inlet and outlet temperatures of the radiative air cooler under three different air flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) were measured, as shown in Fig. 3(a), (c), and (e), while the inlet temperature of the radiative air cooler is the ambient temperature. Fan power consumptions for the three air flow rates were 6 W, 9 W, and 12 W, respectively. The changes of relative humidity during the tests are plotted as red curves in Fig. 3(b), (d), and (f). For all three different air flow rates, sub-ambient cooling has been achieved at noon under direct sunlight. Sub-ambient temperature reductions (temperature difference between inlet and outlet) were between 3–5 °C at noon and between 5–8 °C at night, respectively. Predicted outlet air temperatures from the transient model [red curves in Fig. 3(a), (c), and (e)] are also plotted for comparison with experimental data. For all three cases, the measured outlet temperatures match well with predicted outlet temperatures, which suggests that the model developed for the radiative air cooler is valid. Net cooling power is calculated from the inlet/outlet temperature difference by using
the equation Q = cm˙ T , as shown in Fig. 3(b), (d), and (f). The RSS (root-sum-squares) method described by Moffat [40] was used to analyze the uncertainty for the net cooling power. The relative uncertainty of net cooling power was estimated to be within 3%. The nighttime average cooling powers of the radiative air cooler are 23.5, 57.9, and 64.9 W/m2 for flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) respectively. Noon time (12–2pm) average cooling powers are 11.0, 35.0, and 36.1 W/m2 for flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) respectively. Note that it is possible, though not common, for temperature of the radiative cooling surface to be lower than ambient dew point at night, which could induce condensation on the radiative cooling surface and thus reduces radiative cooling performance. To avoid condensation, slightly higher air flow rate at night is recommended. Experimental results show that larger air flow rates can provide larger cooling powers at both daytime and nighttime. Modeling study was conducted to investigate the sub-ambient temperature reduction and cooling power as functions of inlet (ambient) air temperature (from 0 to 40 °C) and flow rate [from 2 to 10 L/(m2 ·s)], as shown in Fig. 4. The daytime cooling results were obtained with an assumed solar irradiation of 800 W/m2 . It is observed that a larger air flow rate and a higher inlet temperature result in a larger net cooling power, and thus a greater subambient temperature drop. Daytime cooling power and temperature drop are relatively smaller due to the absorption of solar irradiation on the radiative cooling surface. 3.2. Modeling of the roof-integrated radiative air-cooling system for energy saving in residential buildings With the validated radiative air cooler model, net cooling power of the radiative air-cooling system is the net cooling power per unit area multiply by the total radiative cooling surface area. The model developed for the radiative air-cooling system is then coupled with EnergyPlus to evaluate the cooling energy saving for a single-family house. The energy management system (EMS) module was used to create a customized operation model for the radiative air-cooling system in EnergyPlus. Since Eqs. (2) and (3) cannot be directly used in EnergyPlus due to the requirement of integration computation, the regression method [41] was used to reproduce the radiative cooling power. The EnergyPlus house model is originated from the U.S. Department of Energy’s building energy codes program 2012 IECC (ASHRAE standard 90.1 2013 version). The two-story house has two thermal zones: a conditioned living zone and an un-conditioned attic zone. More information on
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D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453 Table 1 Detailed information of the single-family house. Parameters
Value
Length (m) Width (m) Roof area (m2 ) Roof tile angle (degree) Living zone area (m2 ) Living zone volume (m3 ) Attic zone area (m2 ) Attic zone volume (m3 ) Indoor temperature setpoint ( °C) No. of occupants
12 9 117.6 18.4 (4/12 pitch) 223.1 481.7 111.5 85.1 24 3
the single-family house is available in Table 1. Residential buildings have different attic insulation levels according to local climate conditions and economic conditions. Buildings located in cold climates usually have higher attic insulation, while buildings located in hot areas tend to have lower attic insulation. Here, three different attic insulation levels are considered, R-0.8 (0.141 m2 K/W), R-10 (1.76 m2 K/W), and R-30 (5.28 m2 K/W), which correspond to ∼0 cm, ∼7 cm, and ∼21 cm fiberglass insulation, respectively. Similar to mounting solar panels on roof, the roof-integrated radiative air-cooling system cannot fully occupy the roof area because there must exist some spaces for installation and maintenance. Here, we assume the roof-integrated radiative air-cooling system covers ∼50% of the total roof area, that is, 60 m2 . Note that the EnergyPlus house model selected in this work has a tilted roof with an angle of 18.4°. For flat roofs, performance of the roof-integrated radiative air-cooling system could be even better due to a larger view factor from the radiative cooling surface towards the sky. Three reference systems are established to quantify the energy savings from the radiative air-cooling system. The first reference system is a shingle roof, which has a solar reflectance (0.3–2.5 μm) of 0.25 and a thermal infrared (2.5–50 μm) emittance of 0.9. The second reference system is the first reference system (i.e., regular shingle roof) integrated with a solar-powered attic ventilation system that provides an attic ventilation rate at 15 air changes per hour (ACH) [10]. The third reference system is a cool roof system that has a solar reflectance (0.3–2.5 μm) of 0.7 and a thermal infrared (2.5–50 μm) emittance of 0.91, corresponding to a solar reflectance index (SRI) of 86. Hereafter, the radiative air-cooling system and the other three reference systems are named as: radiative air-cooling roof, shingle roof, attic ventilation, and cool roof respectively. Fig. 5 shows the attic air temperature change on a typical summer day under four different roof structures (shingle roof, attic ventilation, cool roof, and radiative air-cooling roof) and three different attic insulation levels (R-0.8, R-10, and R-30). The air flow rate in the radiative air cooler is 5.9 L/(m2 ·s), which corresponds to ∼15 air changes per hour (ACH) in the attic [10]. At night, all roof structures can achieve sub-ambient temperatures in the attic due to their high thermal emittances. During the day, it is observed that attic air temperature is much higher with the shingle roof. Both attic ventilation and cool roof can reduce attic temperatures to a certain level. However, it is only with the radiative aircooling roof that attic temperatures can be reduced to sub-ambient throughout the day. It is also observed that the higher the insulation level, the higher the attic air temperature because attic insulation reduces heat transfer from attic to conditioned living space. Peak attic temperature differences between shingle roof and radiative air-cooling roof are 15.5 °C, 20.6 °C, 21.0 °C for R-0.8, R-10, and R-30, respectively. The lowest insulation (R-0.8) gives lowest attic temperature, which is partly due to heat transfer from attic to conditioned living space since the living space temperature is always
Fig. 5. Attic air temperature change for four different roof structures with three different attic insulation levels on a typical summer day at Phoenix, Arizona. Ambient air temperature is also plotted. The reduction of attic air temperatures has been achieved for all cases.
Fig. 6. Comparison of monthly average cooling energy consumption for four different roof structures at Phoenix, Arizona. Three different attic insulation levels, R-0.8, R-10, and R-30 are investigated, which corresponding to ∼0 cm, ∼7 cm, and ∼21 cm fiberglass insulation respectively.
24 °C. Earlier study on attic ventilation gives peak attic temperature reduction of 8.1 °C in a typical one-story ranch-style house [9], and cool roof technology can provide peak attic temperature reductions of 12.8 °C [11] and 13 °C [6]. Results of the earlier studies match well with our modeling work here. In summary, the radiative air-cooling roof can provide largest attic temperature reduction as compared to the three reference systems. Therefore, largest cooling energy saving in buildings can be expected. Annual EnergyPlus modeling was performed to evaluate cooling energy consumption of the radiative air-cooling roof and the three reference systems for Phoenix, Arizona (hot and dry climate), as shown in Fig. 6. The results confirm that both cool roof and attic ventilation can achieve cooling energy savings as compared to the
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Table 2 Residential attic and wall insulation requirements according to ASHRAE Standard 90.1[19]. Attic insulation min. R-value Climate zones 1–3 (hot areas) Climate zone 4 Climate zones 5–7 (cold areas)
R-38 R-38 R-38
Wall insulation min. R-value (wood-framed) R-13 R-13 + R-3.8 (continuous insulation) R-13 + R-7.5 (continuous insulation)
Fig. 7. Annual cooling energy consumption analysis for four different roof structures at four different locations in the United States, including (a) San Francisco, California (marine climate); (b) Denver, Colorado (cold climate); (c) Miami, Florida (hot-humid climate); and (d) Phoenix, Arizona (hot-dry climate).
shingle roof, but the radiative air-cooling roof has the largest energy saving. Compared to the shingle roof, annual cooling energy savings of the radiative air-cooling roof are 2624 kWh, 804 kWh, and 332 kWh for R-0.8, R-10, and R-30 respectively, corresponding to 11.8 kWh/m2 (30.2%), 3.6 kWh/m2 (12.3%), and 1.5 kWh/m2 (5.5%) savings per square meter of floor area. It is clear that for better attic insulation, the benefit from using the radiative aircooling system becomes marginal. Nevertheless, the currently recommended attic insulation levels in the US are generally high, as shown in Table 2. The main objective for employing such high insulation levels is primarily to block heat transfer from the attic to the living space in summer, and to minimize heat transfer from the living space to the attic in winter. However, it is observed that attic insulation stays at the same level over climate zones 1–7 but wall insulation gradually increases when climate changes from hot to cold areas, as shown in Table 2, which indicate that attic solar heat gain in hot climate is as important as attic heat loss in cold climate. If the attic temperature can be significantly reduced in summer by employing the radiative air-cooling roof, then there is no need to use high attic insulation in hot climates. It is possible to use less attic insulation for residential buildings located in climate zones 1–3.
Furthermore, annual energy saving analysis has been performed for four locations under different climate conditions in the USA, including Miami, Florida (hot-humid climate), Phoenix, Arizona (hotdry climate), San Francisco, California (marine climate), and Denver, Colorado (cold climate). The annual cooling energy consumptions of the radiative air-cooling roof, shingle roof, attic ventilation, and cool roof are plotted in Fig. 7 and summarized in Table A1. Annual cooling energy savings of the radiative air-cooling roof compared to shingle roof, attic ventilation, cool roof are given in Table A2. In general, larger cooling energy savings are associated with less attic insulation and hot climate. Total energy savings of the radiative air-cooling roof in Phoenix and Miami are significant as compared to shingle roof. Note that one should pay attention to both the percentages of cooling energy saving and the absolute numbers of cooling energy saving. For example, even though San Francisco has the largest percentage of annual cooling energy saving, the absolute numbers of annual cooling energy saving are the smallest. Fig. 7 also shows that in most cases, higher attic insulation level results in lower cooling energy consumption. However, this is not the case for radiative air-cooling roof at San Francisco and Denver. The reason is because the radiative air-cooling roof has annual overall cooling effect on the building, and a higher
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attic insulation level blocks cooling energy from the attic and thus increases the cooling load on building air conditioner. It is worth noting that when roof-integrated radiative aircooling system is implemented, annual cooling energy consumption tends to be independent of attic insulation level for all different climate conditions. For example, in Miami, the radiative aircooling roof with R-0.8 and R-30 insulation consume 5456 kWh and 5427 kWh annual cooling energy respectively, which suggest that heavy attic insulation is unnecessary. This further confirms that for hot climates, the radiative air-cooling roof not only can save cooling energy, but also has the potential to save money for attic insulation. Compared to cooling energy saving, it would be appealing to further explore the savings for insulation, which might involve the change of the current building codes to adopt the new roof-integrated radiative air-cooling system.
R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roof-integrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7– 11.8 kWh/m2 (26.5–76.1%) respectively. Modeling results also indicate that the effectiveness of this system depends highly on attic insulation level and climate conditions, with low insulation level and hot-dry climate preferred. It is observed that when the system is implemented, annual cooling energy consumption tends to be independent of attic insulation level for all four different climate conditions. Therefore, substantial capital savings from reducing attic insulation level can be expected in hot climates. It is possible to study on modifying current building code to adapt the new roof-integrated radiative air-cooling system for cooling energy and building cost savings. Declaration of competing interest
4. Conclusions None. In this work, we presented a roof-integrated radiative aircooling system that couples radiative sky cooling with attic ventilation for efficient attic temperature reduction. A prototype radiative air cooler with 1.08 m2 surface area has been built and tested. 24h continuous sub-ambient cooling of air has been achieved under three different flow rates on summer days, which has never been demonstrated before. Measured sub-ambient temperature reductions are between 5–8 °C and between 3–5 °C at night and at noon, respectively. The model established for the radiative air cooler has achieved good agreement with experimental data. Further modeling work using EnergyPlus show that the roof-integrated radiative air-cooling system can provide 15.5–21.0 °C attic air temperature reduction (depending on attic insulation level) as compared to shingle roof (solar reflectance 0.25, thermal emittance 0.9). Compared to shingle roof with attic insulations of R-30 (RSI-5.28),
Acknowledgment The authors acknowledge the financial support of this work from the U.S. Department of Energy’s Advanced Research Projects Agency – Energy(ARPA-E) under contract no. DE-AR0 0 0 0580. D. Z. would like to thank Mr. Dillon Kidd, Mr. Nicolas Seitz, and Mr. Alex Savage for their help in experimental work. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2019.109453. Appendix A
Table A1 Annual cooling energy consumption of the radiative air-cooling roof, shingle roof, attic ventilation, and cool roof at three different attic insulation levels. Annual cooling energy consumption (kWh) Shingle roof Attic ventilation Cool roof Radiative air-cooling roof (this work)
San Francisco
Denver
Miami
Phoenix
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
1092 966 591 261
632 588 488 370
484 471 436 393
2557 2460 1888 1331
1770 1726 1573 1389
1552 1535 1475 1401
7423 7290 6395 5456
6042 5970 5729 5428
5689 5657 5557 5427
8696 8561 7321 6072
6540 6460 6138 5736
5987 5952 5822 5655
Table A2 Annual cooling energy savings of the radiative air-cooling roof compared to shingle roof, attic ventilation, and cool roof at three different attic insulation levels. Annual cooling energy saving [kWh (%)] Compared to shingle roof Compared to attic ventilation Compared to cool roof
San Francisco
Denver
Miami
Phoenix
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
831 (76.1%) 705 (73.0%) 330 (55.9%)
262 (41.4%) 218 (37.1%) 118 (24.1%)
91 (18.8%) 78 (16.6%) 42 (9.7%)
1226 (47.9%) 1129 (45.9%) 557 (29.5%)
381 (21.5%) 336 (19.5%) 183 (11.7%)
151 (9.8%) 134 (8.7%) 75 (5.1%)
1967 (26.5%) 1834 (25.2%) 939 (14.7%)
613 (10.2%) 541 (9.1%) 300 (5.2%)
262 (4.6%) 230 (4.1%) 130 (2.3%)
2624 (30.2%) 2489 (29.1%) 1249 (17.1%)
804 (12.3%) 724 (11.2%) 402 (6.5%)
332 (5.5%) 297 (5.0%) 168 (2.9%)
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Roof-integrated radiative air-cooling system to achieve cooler attic for building energy saving Dongliang Zhao a, Ablimit Aili a, Xiaobo Yin a,b, Gang Tan c,∗, Ronggui Yang a,b,∗ a
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, USA Materials Science and Engineering Program, University of Colorado, Boulder, Colorado 80309, USA c Department of Civil and Architectural Engineering, University of Wyoming, Laramie, Wyoming 82071, USA b
a r t i c l e
i n f o
Article history: Received 20 April 2019 Revised 7 August 2019 Accepted 22 September 2019 Available online 23 September 2019 Keywords: Radiative sky cooling Attic cooling Attic ventilation Building energy saving
a b s t r a c t The building attic usually subjects to substantial solar heat gain and has much higher temperature compared to the conditioned living space during the day, especially in summer and in hot areas. Reducing attic temperature can reduce cooling energy consumption in buildings. However, conventional techniques such as cool roof or attic ventilation, suffer from either heating penalty in winter or limited attic temperature reduction. In this work, a new roof-integrated radiative air-cooling system is introduced, which couples radiative sky cooling with attic ventilation to reduce attic temperature. A radiative air cooler with 1.08 m2 surface area is constructed using a recently developed daytime radiative sky cooling metamaterial [Zhai et al., Science 355, pp. 1062–1066, 2017]. Experimental tests show that sub-ambient air cooling is achieved throughout 24-h day-and-night cycle in a summer day with clear sky conditions. Depending on air flow rates, measured sub-ambient temperature reductions of air are 5–8 °C at night and 3–5 °C at noon under direct sunlight, respectively. An in-house model is first developed for the radiative aircooling system, the model is then coupled with EnergyPlus to study annual energy saving of buildings. The performance of the radiative air-cooling system is compared with three reference systems: shingle roof, attic ventilation, and cool roof. Results show that for a single-family house, attic temperature can be substantially reduced by 15.5–21.0 °C, varying with attic insulation level, compared to shingle roof on typical summer days. Compared to a shingle roof (solar reflectance 0.25, thermal emittance 0.9) residential building with attic insulations of R-30 (RSI-5.28), R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roof-integrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7–11.8 kWh/m2 (26.5–76.1%) respectively. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Space cooling is a major part of energy consumption in residential buildings. In 2015, air conditioning accounts for about 12% of the total energy consumption in a typical US home [1]. Globally, using air conditioners and electric fans for cooling purposes accounts for about 10% of all electricity consumption today [2]. More importantly, due to global warming, population growth, and rapid development of living standard in developing countries, especially those in hot climates, such as Brazil, Malaysia, Mexico, and Indonesia, global energy demand for cooling is expected to triple by 2050, according to a report by International Energy Agency [3]. There is a strong need to reduce energy consumption for space cooling. The cooling load of a residential building comes from various aspects, including solar heat gain from the building envelope (e.g., ∗
Corresponding authors. E-mail addresses: [email protected] (G. Tan), [email protected] (R. Yang).
https://doi.org/10.1016/j.enbuild.2019.109453 0378-7788/© 2019 Elsevier B.V. All rights reserved.
roof, windows, and walls), lighting, and heat generation of indoor facilities and occupants. Most residential building roofs are under substantial solar radiation during the day, which results in much higher temperatures (as high as 60 °C) in the attics as compared to the conditioned living space underneath. The heat is then transferred from the attic to the living space through the ceiling, which results in higher cooling load for the air conditioner. Also, many residential buildings have duct system in the attic, and the heat transfer from the attic air to duct causes even larger cooling load to the air conditioner [4]. In the US, it is estimated that roofs contribute ∼14% of the net residential cooling load [5]. Current techniques for reducing heat gain through the roof include the installation of very thick attic insulation [6] and radiant barrier [7], and using attic ventilation [8–10] and cool roof [11–17]. However, each of the techniques suffer from their own drawbacks. Attic ventilation draws ambient air into the attic through natural ventilation (e.g., turbine ventilation [18]) or mechanical ventilation by using a self-supporting fan (e.g., solar cell powered [9]).
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Attic ventilation can reduce attic temperature, but the effectiveness in reducing air conditioner’s energy consumption is limited due to the limited attic temperature reduction. Ismail and Abdul Rahman [18] conducted a full-scale field test to investigate a hybrid turbine ventilator that integrates wind-force turbine ventilator with solar cell powered inner fan in Malaysia. It was found that the turbine ventilator can reduce attic air temperature by ∼1.0 °C in average compared to the case without the turbine ventilator. Yu and Moore [9] experimentally studied attic temperature reduction by using a solar powered attic fan. The average peak attic temperatures were 45.3 °C (Fan OFF) and 42.2 °C (Fan ON), yielding an average attic temperature reduction of 3.1 °C. The other issue with attic ventilation is the pressure imbalance between the conditioned space and the attic. In cases where there are not adequate openings in the soffit vents or the attic floor is not sealed adequately, attic ventilation may not be effective because cool air in the conditioned space can be pulled into the attic through the attic floor [9]. Cool roof refers to those roofing materials that have been designed to reflect more sunlight and thus absorb less heat than a standard roof (e.g., asphalt shingle roof). According to the ASHRAE standard 90.1 [19], cool roofs should have a minimum solar reflectance index (SRI) of 82, which is calculated based on the material solar reflectance and thermal emittance. Synnefa et al. [12] studied the impact of using cool roofs on the cooling loads of residential buildings for 27 different climate conditions. For roof with a U-value of 0.39 (RSI-2.56, or R-15), they found that increasing the solar reflectance from 0.2 to 0.85 can reduce annual cooling load by 7.1–22.1%, depending on the climate conditions. Rosado et al. [13] introduced a “cool” tile roof in a single-family, singlestory home in Fresno, California. The cool tile roof (initial albedo 0.51) has much higher solar reflectance than dark asphalt shingle roof (initial albedo 0.07) and the attic floor is insulated with R19 (RSI-3.3) blown cellulose insulation. The annual cooling energy savings per unit ceiling area was 2.82 kWh/m2 (26% savings of total cooling energy consumption) compared to a “standard” home with dark asphalt shingle roof. However, the use of cool roof could subject to heating penalty in winter season [20], and this is especially true for cold climates. As a “free” cooling source, radiative sky cooling has been explored for building applications for many decades, in particular nighttime cooling. A few radiative sky cooling systems have been built. Meir et al. [21] constructed a polymer-based radiative cooling system that consists of a 5.3 m2 radiator and a 280-liter water tank. The system has achieved an average cooling flux of 55 W/m2 at night in Norway. Eicker and Dalibard [22] developed a photovoltaic–thermal (PVT) system in Germany that can produce both electricity and cooling energy. The measured radiative cooling power levels were between 40 and 65 W/m2 . Zhang and Niu [23] combined nighttime radiative sky cooling with microencapsulated phase change material slurry storage. Modeling results showed that the energy saving potential in Lanzhou and Urumqi (China) can reach 77% and 62% respectively for low-rise buildings. Zhao et al. [24] developed a building-integrated photovoltaicradiative cooling system in Eastern China. Compared to a building integrated photovoltaic (BIPV) system, the total electricity production and cooling energy gain of this system were 97% higher. In summary, cooling potential of nighttime radiative sky cooling system are generally within the range of 40–87 W/m2 [25]. Recent advances in daytime radiative sky cooling technology show great promise for its building applications [26]. Daytime radiative sky cooling materials have been developed to achieve subambient temperatures under direct sunlight [27–30]. In particular, Zhai et al. [29] developed a low-cost polymer-glass hybrid radiative sky cooling metamaterial (hereafter named as RadiCold metafilm) which garnered strong interests. With an averaged infrared emissivity > 0.93 and a solar reflectivity of approximately 0.96, the
RadiCold metafilm has demonstrated an average of 93 W/m2 cooling power at noon. More importantly, the RadiCold metafilm can be scalably-manufactured by the roll-to-roll extrusion and web coating systems, which makes large-scale applications of radiative sky cooling technology possible [25,31]. If the RadiCold metafilm were deployed on building roofs, the attic temperature can be expected to be much lower than using current cool roof products. In this work, we propose a novel roof-integrated sub-ambient radiative air-cooling system to reduce attic temperature by employing the RadiCold metafilm. Advantages of the roof-integrated radiative air-cooling system include: 1) lower (sub-ambient) inlet air temperature into the soffit vents as compared to attic ventilation; 2) higher solar reflectance (at around 0.96) as compared to cool roof products, which usually have initial solar reflectance between 0.6– 0.9 [32]; 3) on/off control of the system gives operational flexibility to avoid possible over cooling (i.e., heating penalty) in winter. A prototype day-night radiative air cooler with 1.08 m2 surface area is built to demonstrate sub-ambient cooling of air under different flow rates. Measured sub-ambient temperature reductions are between 5–8 °C at night and between 3–5 °C at noon respectively. A modeling tool is developed to predict the performance of the radiative air cooler, which shows good agreement with experimental results. The modeling tool is then used to create a customized model for the roof-integrated radiative air-cooling system in building simulation software EnergyPlus. Energy performance of the radiative air-cooling system is compared with three reference systems: shingle roof, attic ventilation, and cool roof. Simulation results show that attic temperature can be substantially reduced by 15.5–21.0 °C (varies with attic insulation level) compared to shingle roof on typical summer days. Annual energy saving analysis with EnergyPlus has been carried out for four different climates in the United States. Compared to a shingle roof (solar reflectance 0.25, thermal emittance 0.9) residential building with attic insulations of R-30 (RSI-5.28), R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roofintegrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7–11.8 kWh/m2 (26.5–76.1%) respectively. 2. System description Fig. 1(a) shows a schematic of the roof-integrated radiative air-cooling system in residential building. The system consists of multiple radiative air coolers connected in parallel, as shown in Fig. 1(b). The system draws air from the ambient, and the air is being cooled to sub-ambient temperature due to significant solar reflection and infrared thermal emission to the sky [29]. The cooled air is then supplied to the attic and mix with attic air to reduce the temperature of the latter one. In this way, air temperature in the attic can be greatly reduced during the day, which reduces heat transfer from the attic to the conditioned space. When cooling is not needed in the conditioned space, for example, in winter, the fan employed in the radiative air-cooling system can be turned off to avoid heating penalty. Radiative air cooler is developed as modules that can be easily scaled up. Fig. 2 shows the schematics and a photograph of such a radiative air cooler (module). Surface area of the radiative air cooler is 1.8 m × 0.6 m (1.08 m2 ). It is built using wood as framework and aluminum plate as baffles [Fig. 2(a)]. Four sides and the bottom are insulated with 5-cm-thick polyisocyanurate rigid foam insulation board. The RadiCold metafilm [Fig. 2(e)] is installed on top of an aluminum sheet (0.5-mm-thick) to provide cooling by using pressure sensitive adhesive. A 15-μm-thick polyethylene (PE) film is then placed 10 mm above the RadiCold metafilm to suppress thermal loss from the top at sub-ambient operating conditions [Fig. 2(b)]. The baffled design enhances the heat transfer between air and top cooling surface by creating air turbulence
D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453
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of 18°, which mimics a common 4:12 roof pitch [Fig. 2(d)]. The air flow rate is calculated by using the air velocity measured by an anemometer (MVA-02 mini-vane thermo-anemometer, ±2% uncertainty) and cross-sectional area of the air duct. K-type thermocouples (±0.3 °C uncertainty) are installed at the inlet and outlet of the radiative air cooler for temperature measurement. The thermocouples are connected to a NI-9213 data acquisition module which is read by a LabVIEW program in 10 s time interval. Solar radiation is measured by a CMP 11 pyranometer (±2% uncertainty). Local relative humidity is measured by Ambient Weather WS-1002 weather station (±5% uncertainty). 2.1. Modeling of the radiative air cooler A transient heat transfer model is developed to study the performance of the radiative air cooler. The heat transfer processes involved are shown in Fig. 2(b). The net cooling power, which equals to the rate of cold gain of the flowing air, can be expressed as:
ca m˙ a (Tinlet − Toutlet ) = Prad − Patm − Psolar − Pnon−radiative
Fig. 1. Roof-integrated radiative air-cooling system for attic temperature reduction. (a) Schematic of a radiative air-cooling system integrated with residential building. (b) The radiative air-cooling system consists of multiple radiative air coolers connected in parallel.
where ca and m˙ a are specific heat and mass flow rate of air respectively, Tinlet and Toutlet are the temperatures of air at inlet and outlet. The radiative cooling power of the RadiCold metafilm can be obtained by integrating the spectral radiance over the entire spectrum and the hemisphere [26]:
Prad T f ilm = A and increasing the heat transfer coefficient [Fig. 2(c)]. The length and spacing of the baffles were optimized to balance heat transfer and pressure loss across the radiative air cooler [33]. A small direct current (DC) fan is installed at the inlet to draw ambient air. Input power to the DC fan is adjustable to achieve different air flow rates. The radiative air cooler is tested with a tilt angle
(1)
cosθ d
∞ 0
Ibb
λ, T f ilm ε f ilm (, λ )dλ
(2)
π /2 2π where d = 0 dθ sinθ 0 dφ is the angular integral over a hemisphere, ɛfilm (, λ) is the emissivity of the RadiCold metafilm as a function of direction and wavelength, Ibb (λ, T f ilm ) = 2hc 2
1 is the spectral radiance of blackbody at temλ5 exp(hc/(λkB T f ilm )−1)
perature Tfilm , λ is wavelength, and A is the radiative cooling surface area.
Fig. 2. Prototype radiative air cooler. (a) Exploded view of the radiative air cooler. (b) Cross-sectional view of the radiative air cooler showing heat transfer processes. (c) Dimensions of the radiative air cooler. (d) A photograph of the radiative air cooler with a tilt angle of 18°, which mimics a typical 4:12 roof pitch. (e) A photograph of the low-cost lab-produced RadiCold metafilm roll.
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Fig. 3. Test results of the radiative air cooler under three different air flow rates at Boulder, Colorado, USA. Inlet temperature of the radiative air cooler tracks ambient temperature. (a) The variation of inlet (gray curve) and outlet [blue curve (measured), red curve (calculated)] temperatures with time under flow rate 2.4 L/(m2 ·s). (b) The variation of cooling power and relative humidity with time under flow rate 2.4 L/(m2 ·s). (c) The variation of inlet and outlet temperatures with time under flow rate 5.9 L/(m2 ·s). (d) The variation of cooling power and relative humidity with time under flow rate 5.9 L/(m2 ·s). (e) The variation of inlet and outlet temperatures with time under flow rate 7.9 L/(m2 ·s). (f) The variation of cooling power and relative humidity with time under flow rate 7.9 L/(m2 ·s).
Similarly, the absorbed atmospheric radiation, depending on the emissivities of both the RadiCold metafilm and the atmosphere, is given by:
Patm (Tamb ) = A
cosθ d
∞ 0
Ibb (λ, Tamb )ε f ilm (, λ )εatm (, λ )dλ (3)
where Tamb is the ambient temperature, ɛatm (λ, ) is the atmospheric emissivity, which is a function of the precipitable water [34]. Precipitable water is defined as the depth of water in a column of the atmosphere if all the water vapor in that column were precipitated as rain, and it can be calculated from ambient temperature and relative humidity [30]. In this work, with calculated precipitable water, ɛatm (λ, ) is obtained by using ATRAN – a tool for computing earth’s atmospheric transmission of near- and farinfrared radiation [34].
The absorbed solar irradiation on the RadiCold metafilm surface is calculated by:
Psolar = Aα¯ f ilm Isolar
(4)
where Isolar is the measured solar irradiance at the same tilt angle as the radiative cooling surface, α¯ f ilm is the effective solar absorptance of the RadiCold metafilm, 0.04. The non-radiative (i.e., convection and conduction) heat transfer between the RadiCold metafilm and the ambient is given by:
Pnon−radiative = hair A T f ilm − Tamb
(5)
where Tfilm is the average temperature of the RadiCold metafilm, hair is the overall heat transfer coefficient between the RadiCold metafilm and the ambient air. Earlier experimental studies suggest that the overall heat transfer coefficient from a flat surface can be
D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453
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Fig. 4. Sub-ambient temperature reduction (Tinlet - Toutlet ) and cooling power of the radiative air cooler as functions of inlet temperatures (from 0 to 40 °C) and air flow rates [from 2 to 10 L/(m2 ·s)]. The local wind speed is assumed as 1 m/s. Solar irradiation at daytime is assumed to be 800 W/m2 . Precipitable water is assumed to be constant at 10 mm.
quantified by a linear form of correlation [35–37].
hair = a + bv
(6)
where v is the local wind speed, and a and b are fitted parameters. This is a generally accepted correlation that is used to obtain convective heat transfer coefficient on a flat plate, such as solar thermal collectors [38] and solar cells [39]. From our previous study [30], the heat transfer coefficient of the radiative cooling surface varying with wind speed is expressed as h = 2.5 + 2v. As mentioned above, the roof-integrated radiative air-cooling system is composed of multiple radiative air coolers connected in parallel. Therefore, under the same air flow rate per surface area, the internal air flow characteristics of the radiative air-cooling system should be the same as a single radiative air cooler. The outlet air from the radiative air-cooling system is supplied to attic and well-mixed with attic air at each time step. When attic air temperature is lower than indoor air setting temperature, the system is turned off (i.e., fan turned off). Net cooling power of the radiative air-cooling system is the net cooling power per unit area multiply by the total system RadiCold metafilm surface area. 3. Results and discussion 3.1. Performance of the radiative air cooler The prototype radiative air cooler has been tested on summer days under clear sky conditions on the rooftop of the Engineering Center at the University of Colorado Boulder. Inlet and outlet temperatures of the radiative air cooler under three different air flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) were measured, as shown in Fig. 3(a), (c), and (e), while the inlet temperature of the radiative air cooler is the ambient temperature. Fan power consumptions for the three air flow rates were 6 W, 9 W, and 12 W, respectively. The changes of relative humidity during the tests are plotted as red curves in Fig. 3(b), (d), and (f). For all three different air flow rates, sub-ambient cooling has been achieved at noon under direct sunlight. Sub-ambient temperature reductions (temperature difference between inlet and outlet) were between 3–5 °C at noon and between 5–8 °C at night, respectively. Predicted outlet air temperatures from the transient model [red curves in Fig. 3(a), (c), and (e)] are also plotted for comparison with experimental data. For all three cases, the measured outlet temperatures match well with predicted outlet temperatures, which suggests that the model developed for the radiative air cooler is valid. Net cooling power is calculated from the inlet/outlet temperature difference by using
the equation Q = cm˙ T , as shown in Fig. 3(b), (d), and (f). The RSS (root-sum-squares) method described by Moffat [40] was used to analyze the uncertainty for the net cooling power. The relative uncertainty of net cooling power was estimated to be within 3%. The nighttime average cooling powers of the radiative air cooler are 23.5, 57.9, and 64.9 W/m2 for flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) respectively. Noon time (12–2pm) average cooling powers are 11.0, 35.0, and 36.1 W/m2 for flow rates 2.4, 5.9, and 7.9 L/(m2 ·s) respectively. Note that it is possible, though not common, for temperature of the radiative cooling surface to be lower than ambient dew point at night, which could induce condensation on the radiative cooling surface and thus reduces radiative cooling performance. To avoid condensation, slightly higher air flow rate at night is recommended. Experimental results show that larger air flow rates can provide larger cooling powers at both daytime and nighttime. Modeling study was conducted to investigate the sub-ambient temperature reduction and cooling power as functions of inlet (ambient) air temperature (from 0 to 40 °C) and flow rate [from 2 to 10 L/(m2 ·s)], as shown in Fig. 4. The daytime cooling results were obtained with an assumed solar irradiation of 800 W/m2 . It is observed that a larger air flow rate and a higher inlet temperature result in a larger net cooling power, and thus a greater subambient temperature drop. Daytime cooling power and temperature drop are relatively smaller due to the absorption of solar irradiation on the radiative cooling surface. 3.2. Modeling of the roof-integrated radiative air-cooling system for energy saving in residential buildings With the validated radiative air cooler model, net cooling power of the radiative air-cooling system is the net cooling power per unit area multiply by the total radiative cooling surface area. The model developed for the radiative air-cooling system is then coupled with EnergyPlus to evaluate the cooling energy saving for a single-family house. The energy management system (EMS) module was used to create a customized operation model for the radiative air-cooling system in EnergyPlus. Since Eqs. (2) and (3) cannot be directly used in EnergyPlus due to the requirement of integration computation, the regression method [41] was used to reproduce the radiative cooling power. The EnergyPlus house model is originated from the U.S. Department of Energy’s building energy codes program 2012 IECC (ASHRAE standard 90.1 2013 version). The two-story house has two thermal zones: a conditioned living zone and an un-conditioned attic zone. More information on
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D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453 Table 1 Detailed information of the single-family house. Parameters
Value
Length (m) Width (m) Roof area (m2 ) Roof tile angle (degree) Living zone area (m2 ) Living zone volume (m3 ) Attic zone area (m2 ) Attic zone volume (m3 ) Indoor temperature setpoint ( °C) No. of occupants
12 9 117.6 18.4 (4/12 pitch) 223.1 481.7 111.5 85.1 24 3
the single-family house is available in Table 1. Residential buildings have different attic insulation levels according to local climate conditions and economic conditions. Buildings located in cold climates usually have higher attic insulation, while buildings located in hot areas tend to have lower attic insulation. Here, three different attic insulation levels are considered, R-0.8 (0.141 m2 K/W), R-10 (1.76 m2 K/W), and R-30 (5.28 m2 K/W), which correspond to ∼0 cm, ∼7 cm, and ∼21 cm fiberglass insulation, respectively. Similar to mounting solar panels on roof, the roof-integrated radiative air-cooling system cannot fully occupy the roof area because there must exist some spaces for installation and maintenance. Here, we assume the roof-integrated radiative air-cooling system covers ∼50% of the total roof area, that is, 60 m2 . Note that the EnergyPlus house model selected in this work has a tilted roof with an angle of 18.4°. For flat roofs, performance of the roof-integrated radiative air-cooling system could be even better due to a larger view factor from the radiative cooling surface towards the sky. Three reference systems are established to quantify the energy savings from the radiative air-cooling system. The first reference system is a shingle roof, which has a solar reflectance (0.3–2.5 μm) of 0.25 and a thermal infrared (2.5–50 μm) emittance of 0.9. The second reference system is the first reference system (i.e., regular shingle roof) integrated with a solar-powered attic ventilation system that provides an attic ventilation rate at 15 air changes per hour (ACH) [10]. The third reference system is a cool roof system that has a solar reflectance (0.3–2.5 μm) of 0.7 and a thermal infrared (2.5–50 μm) emittance of 0.91, corresponding to a solar reflectance index (SRI) of 86. Hereafter, the radiative air-cooling system and the other three reference systems are named as: radiative air-cooling roof, shingle roof, attic ventilation, and cool roof respectively. Fig. 5 shows the attic air temperature change on a typical summer day under four different roof structures (shingle roof, attic ventilation, cool roof, and radiative air-cooling roof) and three different attic insulation levels (R-0.8, R-10, and R-30). The air flow rate in the radiative air cooler is 5.9 L/(m2 ·s), which corresponds to ∼15 air changes per hour (ACH) in the attic [10]. At night, all roof structures can achieve sub-ambient temperatures in the attic due to their high thermal emittances. During the day, it is observed that attic air temperature is much higher with the shingle roof. Both attic ventilation and cool roof can reduce attic temperatures to a certain level. However, it is only with the radiative aircooling roof that attic temperatures can be reduced to sub-ambient throughout the day. It is also observed that the higher the insulation level, the higher the attic air temperature because attic insulation reduces heat transfer from attic to conditioned living space. Peak attic temperature differences between shingle roof and radiative air-cooling roof are 15.5 °C, 20.6 °C, 21.0 °C for R-0.8, R-10, and R-30, respectively. The lowest insulation (R-0.8) gives lowest attic temperature, which is partly due to heat transfer from attic to conditioned living space since the living space temperature is always
Fig. 5. Attic air temperature change for four different roof structures with three different attic insulation levels on a typical summer day at Phoenix, Arizona. Ambient air temperature is also plotted. The reduction of attic air temperatures has been achieved for all cases.
Fig. 6. Comparison of monthly average cooling energy consumption for four different roof structures at Phoenix, Arizona. Three different attic insulation levels, R-0.8, R-10, and R-30 are investigated, which corresponding to ∼0 cm, ∼7 cm, and ∼21 cm fiberglass insulation respectively.
24 °C. Earlier study on attic ventilation gives peak attic temperature reduction of 8.1 °C in a typical one-story ranch-style house [9], and cool roof technology can provide peak attic temperature reductions of 12.8 °C [11] and 13 °C [6]. Results of the earlier studies match well with our modeling work here. In summary, the radiative air-cooling roof can provide largest attic temperature reduction as compared to the three reference systems. Therefore, largest cooling energy saving in buildings can be expected. Annual EnergyPlus modeling was performed to evaluate cooling energy consumption of the radiative air-cooling roof and the three reference systems for Phoenix, Arizona (hot and dry climate), as shown in Fig. 6. The results confirm that both cool roof and attic ventilation can achieve cooling energy savings as compared to the
D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453
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Table 2 Residential attic and wall insulation requirements according to ASHRAE Standard 90.1[19]. Attic insulation min. R-value Climate zones 1–3 (hot areas) Climate zone 4 Climate zones 5–7 (cold areas)
R-38 R-38 R-38
Wall insulation min. R-value (wood-framed) R-13 R-13 + R-3.8 (continuous insulation) R-13 + R-7.5 (continuous insulation)
Fig. 7. Annual cooling energy consumption analysis for four different roof structures at four different locations in the United States, including (a) San Francisco, California (marine climate); (b) Denver, Colorado (cold climate); (c) Miami, Florida (hot-humid climate); and (d) Phoenix, Arizona (hot-dry climate).
shingle roof, but the radiative air-cooling roof has the largest energy saving. Compared to the shingle roof, annual cooling energy savings of the radiative air-cooling roof are 2624 kWh, 804 kWh, and 332 kWh for R-0.8, R-10, and R-30 respectively, corresponding to 11.8 kWh/m2 (30.2%), 3.6 kWh/m2 (12.3%), and 1.5 kWh/m2 (5.5%) savings per square meter of floor area. It is clear that for better attic insulation, the benefit from using the radiative aircooling system becomes marginal. Nevertheless, the currently recommended attic insulation levels in the US are generally high, as shown in Table 2. The main objective for employing such high insulation levels is primarily to block heat transfer from the attic to the living space in summer, and to minimize heat transfer from the living space to the attic in winter. However, it is observed that attic insulation stays at the same level over climate zones 1–7 but wall insulation gradually increases when climate changes from hot to cold areas, as shown in Table 2, which indicate that attic solar heat gain in hot climate is as important as attic heat loss in cold climate. If the attic temperature can be significantly reduced in summer by employing the radiative air-cooling roof, then there is no need to use high attic insulation in hot climates. It is possible to use less attic insulation for residential buildings located in climate zones 1–3.
Furthermore, annual energy saving analysis has been performed for four locations under different climate conditions in the USA, including Miami, Florida (hot-humid climate), Phoenix, Arizona (hotdry climate), San Francisco, California (marine climate), and Denver, Colorado (cold climate). The annual cooling energy consumptions of the radiative air-cooling roof, shingle roof, attic ventilation, and cool roof are plotted in Fig. 7 and summarized in Table A1. Annual cooling energy savings of the radiative air-cooling roof compared to shingle roof, attic ventilation, cool roof are given in Table A2. In general, larger cooling energy savings are associated with less attic insulation and hot climate. Total energy savings of the radiative air-cooling roof in Phoenix and Miami are significant as compared to shingle roof. Note that one should pay attention to both the percentages of cooling energy saving and the absolute numbers of cooling energy saving. For example, even though San Francisco has the largest percentage of annual cooling energy saving, the absolute numbers of annual cooling energy saving are the smallest. Fig. 7 also shows that in most cases, higher attic insulation level results in lower cooling energy consumption. However, this is not the case for radiative air-cooling roof at San Francisco and Denver. The reason is because the radiative air-cooling roof has annual overall cooling effect on the building, and a higher
8
D. Zhao, A. Aili and X. Yin et al. / Energy & Buildings 203 (2019) 109453
attic insulation level blocks cooling energy from the attic and thus increases the cooling load on building air conditioner. It is worth noting that when roof-integrated radiative aircooling system is implemented, annual cooling energy consumption tends to be independent of attic insulation level for all different climate conditions. For example, in Miami, the radiative aircooling roof with R-0.8 and R-30 insulation consume 5456 kWh and 5427 kWh annual cooling energy respectively, which suggest that heavy attic insulation is unnecessary. This further confirms that for hot climates, the radiative air-cooling roof not only can save cooling energy, but also has the potential to save money for attic insulation. Compared to cooling energy saving, it would be appealing to further explore the savings for insulation, which might involve the change of the current building codes to adopt the new roof-integrated radiative air-cooling system.
R-10 (RSI-1.76), and R-0.8 (RSI-0.14), the roof-integrated radiative air-cooling system can achieve annual cooling energy savings of 0.4–1.5 kWh/m2 (4.6–18.8%), 1.2–3.6 kWh/m2 (10.2–41.4%), and 3.7– 11.8 kWh/m2 (26.5–76.1%) respectively. Modeling results also indicate that the effectiveness of this system depends highly on attic insulation level and climate conditions, with low insulation level and hot-dry climate preferred. It is observed that when the system is implemented, annual cooling energy consumption tends to be independent of attic insulation level for all four different climate conditions. Therefore, substantial capital savings from reducing attic insulation level can be expected in hot climates. It is possible to study on modifying current building code to adapt the new roof-integrated radiative air-cooling system for cooling energy and building cost savings. Declaration of competing interest
4. Conclusions None. In this work, we presented a roof-integrated radiative aircooling system that couples radiative sky cooling with attic ventilation for efficient attic temperature reduction. A prototype radiative air cooler with 1.08 m2 surface area has been built and tested. 24h continuous sub-ambient cooling of air has been achieved under three different flow rates on summer days, which has never been demonstrated before. Measured sub-ambient temperature reductions are between 5–8 °C and between 3–5 °C at night and at noon, respectively. The model established for the radiative air cooler has achieved good agreement with experimental data. Further modeling work using EnergyPlus show that the roof-integrated radiative air-cooling system can provide 15.5–21.0 °C attic air temperature reduction (depending on attic insulation level) as compared to shingle roof (solar reflectance 0.25, thermal emittance 0.9). Compared to shingle roof with attic insulations of R-30 (RSI-5.28),
Acknowledgment The authors acknowledge the financial support of this work from the U.S. Department of Energy’s Advanced Research Projects Agency – Energy(ARPA-E) under contract no. DE-AR0 0 0 0580. D. Z. would like to thank Mr. Dillon Kidd, Mr. Nicolas Seitz, and Mr. Alex Savage for their help in experimental work. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2019.109453. Appendix A
Table A1 Annual cooling energy consumption of the radiative air-cooling roof, shingle roof, attic ventilation, and cool roof at three different attic insulation levels. Annual cooling energy consumption (kWh) Shingle roof Attic ventilation Cool roof Radiative air-cooling roof (this work)
San Francisco
Denver
Miami
Phoenix
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
1092 966 591 261
632 588 488 370
484 471 436 393
2557 2460 1888 1331
1770 1726 1573 1389
1552 1535 1475 1401
7423 7290 6395 5456
6042 5970 5729 5428
5689 5657 5557 5427
8696 8561 7321 6072
6540 6460 6138 5736
5987 5952 5822 5655
Table A2 Annual cooling energy savings of the radiative air-cooling roof compared to shingle roof, attic ventilation, and cool roof at three different attic insulation levels. Annual cooling energy saving [kWh (%)] Compared to shingle roof Compared to attic ventilation Compared to cool roof
San Francisco
Denver
Miami
Phoenix
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
R-0.8
R-10
R-30
831 (76.1%) 705 (73.0%) 330 (55.9%)
262 (41.4%) 218 (37.1%) 118 (24.1%)
91 (18.8%) 78 (16.6%) 42 (9.7%)
1226 (47.9%) 1129 (45.9%) 557 (29.5%)
381 (21.5%) 336 (19.5%) 183 (11.7%)
151 (9.8%) 134 (8.7%) 75 (5.1%)
1967 (26.5%) 1834 (25.2%) 939 (14.7%)
613 (10.2%) 541 (9.1%) 300 (5.2%)
262 (4.6%) 230 (4.1%) 130 (2.3%)
2624 (30.2%) 2489 (29.1%) 1249 (17.1%)
804 (12.3%) 724 (11.2%) 402 (6.5%)
332 (5.5%) 297 (5.0%) 168 (2.9%)
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