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Reflective coatings for interior and exterior of buildings and improving thermal performance
Applied Energy 103 (2013) 562–570
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Reflective coatings for interior and exterior of buildings and improving thermal performance Ali Joudi a,b,⇑, Harald Svedung b, Mathias Cehlin c, Mats Rönnelid a a
Energy and Environmental Technology, Dalarna University, SE-79188 Falun, Sweden SSAB EMEA, SE-78184 Borlänge, Sweden c Building, Energy and Environmental Engineering, University of Gävle, SE-80176 Gävle, Sweden b
h i g h l i g h t s " Increase building energy efficiency by optimizing surface optical properties. " Study different scenarios with both interior and exterior reflective coatings. " Combined thermal effect of both interior and exterior reflective coatings.
a r t i c l e
i n f o
Article history: Received 13 May 2012 Received in revised form 7 September 2012 Accepted 5 October 2012 Available online 26 November 2012 Keywords: Reflective coatings Low energy building Energy simulation Total solar reflectance Interior thermal emittance
a b s t r a c t The importance of reducing building energy usage and thriving for more energy efficient architectures, has nurtured creative solutions and smart choices of materials in the last few decades. Among those are optimizing surface optical properties for both interior and exterior claddings of the building. Development in the coil-coating steel industries has now made it possible to allocate correct optical properties for steel clad buildings with improved thermal performance. Although the importance of the exterior coating and solar gain are thoroughly studied in many literatures, the effect of interior cladding are less tackled, especially when considering a combination of both interior and exterior reflective coatings. This paper contemplates the thermal behavior of small cabins with reflective coatings on both interior and exterior cladding, under different conditions and climates with the aim to clarify and point out to the potential energy saving by smart choices of clad coatings. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A substantial portion of total energy usage and greenhouse gas emissions belongs to building sectors [1]. Improving the energy performance of the building and simultaneously maintaining a desirable indoor climate for the occupants is a precious goal to thrive. In Sweden, for example, aligned with the decision of European Council, there is a target to reduce energy usage in residential and commercial buildings with 20% by 2020 and 50% by 2050 comparing from year 1995 [2,3]. In the frontiers of energy saving studies, reflective coatings are addressed to have substantial potentials to improve thermal performance of the building as cool roof [4–8] and heat island reduction [9,10] for exterior cladding and as radiation barriers for interior claddings [11–13]. Besides that, the energy needed to
⇑ Corresponding author at: Energy and Environmental Technology, Dalarna University, SE-79188 Falun, Sweden. Tel.: +46 23778533; fax: +46 23778701. E-mail address: [email protected] (A. Joudi). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.10.019
maintain a desirable indoor climate during building lifetime is considerable compared to the energy used to fabricate the building materials [14]. Thus the correct choice of materials, including surface optical properties is of great interest. Surface optical properties are categorized based on their functions; those sitting on the outermost surfaces (exterior) can have different solar reflective properties and thermal emissivities and those sitting on the innermost surfaces (interior) can have only different thermal emissivities. Solar energy travels in electromagnetic radiation with the most energy content at the wavelengths of 0.25–3.0 lm [15]. It includes different wavelength regions; ultraviolet, visible and near infrared regions. The near infrared region stands for more than half of the solar radiation. However, surface optical properties embody all wavelength regions, including both visible and near infrared. Therefore it is possible to have surfaces with the same colors but different total solar refection (TSR), i.e. surfaces with same reflective property in visible region but different reflectivity in near infrared region. This fairly new concept has been recently applied to coil-coated steel sheets which are used as the building exterior claddings,
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making it possible to optimize surface radiation properties for different building applications. Furthermore, coil coating development has also made it possible to adjust the longwave radiation properties for the interior surfaces by using reflective pigment in the longwave (thermal) radiation. In this new concept, surface emissivity of 0.9 (common for most materials) can be reduced down to 0.25. These coatings are mainly used for the interior of the buildings [12]. In order to investigate the effect of this modified surface radiation properties, small test cabins with and without reflective coatings have been studied in earlier work [12] with the focus on measurement results and thermal behavior observations of these cabins at Borlänge, Sweden with the latitude of 60°N.
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The optical properties of all cabins are presented in Table 1. It shall be mentioned that although all test cabins have almost same color on exteriors (same reflectivity in the visual spectrum), they have different reflection properties in the near infrared radiation region. One of the pigments used in the exterior coating on one of the cabins increases the TSR by an extremely sharp transition from absorption in the visible spectrum to high transmittance in the near infrared. Test cabins were equipped with heating and cooling utilities to assess the heating and cooling demand of each test cabin. Cabins specifications including the envelope transmission are given in Table 1. 3.2. The coatings
2. Aim of the study
3. The experimental setup
Coil-coating of galvanized steel sheet material can be done using a variety of paint systems differing largely in the polymer used as paint binder and in any cross linker chemistry. In the case of our test cabins thermo-set polyester-melamine coatings as well as thermoplastic PVC-plastisol coatings are used for the exterior surfaces. Normally a non-metallic paint surface has a high thermal emissivity independent of the type of binder used. However the thermal emissivity of the coil-coated surface was in the case of our reflective interior surfaces reduced dramatically from above 90% to 25% by the use of aluminum flakes dispersed in the epoxy-based thermo-set coating. Pigments used in coil-coating formulations differ both in visual color and near infra-red reflectivity to some extent independently. Thus the TSR of the coated surface can be achieved depending on the choice of pigments in the paint formulation within a wide range from 0.5 to around 90% and in smaller ranges independent on the visual color of the surface.
3.1. Cabins
4. Simulations
The three cabins (Fig. 1) include a normal cabin with both interior and exterior conventional coating, an interior reflective cabin with low thermal emissivity coating on interior claddings and a both-side reflective cabin which has both low thermal emissivity (high reflective) coating on interior surfaces and high TSR coating on the exterior roof and wall claddings.
4.1. IDA ICE software
The aim of this study is to highlight the role of different coatings and their contributions to different thermal savings. Starting from measured results in the test cabin under given conditions, this is a theoretical study to understand the thermal behavior of the simulated cabin model under different scenarios. To further investigate the cabins under different conditions and climates, and in order to map the sensitivity of the cabins energy use with different radiation properties, this paper studies the thermal behavior of coatings under various conditions, highlights pros and cons, addresses the counter balancing effect of coating combinations and finally introduces other important interplaying parameters in a dynamic building energy simulation model.
To evaluate the annual energy use of the test cabins (Fig. 1) as specified in Table 1 depending on the surface radiation properties under a controlled set of conditions, the simulation program IDA Indoor Climate and Energy (IDA ICE) [16] is used here.
Fig. 1. Test cabins with windows facing south (the upper drawing show the model used in the simulation of the cabins).
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Table 1 Test cabins specifications and optical properties (dimension in millimeter). Location Latitude U-values
Glazing Cooling system Heating system Ventilation rate Infiltration Internal load Normal cabin
Interior reflective cabin
Both-side reflective cabin
a b
Borlänge, Sweden 60°N Walls 0.247 W m2 °C Roof 0.207 W m2 °C Floor 0.158 W m2 °C Total 0.68 m2 (Double glazing) Split type air conditioner (EER:4.08) Electrical floor heating 70 m3 h1 nominal capacity Negligible Negligible Normal exterior TSRa 0.10 IR-eb 0.92 Normal interior IR-eb 0.91 Normal exterior TSRa 0.10 IR-eb 0.92 Reflective interior IR-eb 0.25 a Reflective exterior TSR 0.39 b 0.92 IR-e b Reflective interior IR-e 0.25
TSR: Total solar reflectance. IR-e: Infrared emissivity.
IDA ICE is a building performance simulations tool to model buildings with wide variety of heating and cooling systems and different controllers. It is an hourly dynamic simulation program which is an equation based model, using either the Modelica language or the Neutral Model Format (NMF) [17–22]. Regarding radiation and optical properties on exterior surfaces, TqFace module inside IDA ICE uses following energy balance Eq. (1) on the surface thermal node. This surface node is thermally coupled with the adjacent thermal node inside the solid part of the building material.
ð1 TSRÞðIdir þ Idiff ;sky þ Idiff ;grnd Þ þ fsky reext ðT 4sky T 4so Þ þ fgrnd reext ðT 4grnd T 4so Þ þ qcond;w þ qconv ¼ 0
ð1Þ
In Eq. (1) optical properties of exterior surfaces can be seen in terms of TSR which is the total solar reflectance and eext is the longwave emissivity of the exterior surface. r is Stefan–Boltzmann constant and Idir, Idiff,sky, Idiff,grnd are direct solar radiation, diffuse solar radiation from the sky and diffuse solar radiation from the ground, respectively in W m2. fsky and fgrnd are surface view factors to sky and ground, respectively. Tso, Tsky and Tgrnd are exterior surface, sky and ground absolute temperatures, respectively, where ground temperature is assumed to be the same as ambient air and sky temperature, 5 °C below ambient air according to Climate Processor of IDA ICE. qcond,w and qconv are conductance to wall and convection to the ambient air, respectively in W m2 where the former depends to the building material and the later relates to ambient air temperature, wall orientation and local wind speed according to [23]. To account for radiation between interior surfaces for a nonhorizontal roof construction, Energy Model inside IDA ICE simulation uses the concept of mean radiation temperature where it comes to the radiative heat exchange from the interior surface, mainly to simplify geometry complexity in view factor calculation. According to CESIMZONE module inside IDA ICE, mean ration temperature relates to surface temperatures, surface longwave emissivity and radiation from internal sources, e.g. lighting, equipment and people. This internal source radiation is assumed to be distributed among internal surfaces based on area ratio [23]. The linearized energy balance for mean radiation temperature is given by:
Q r;s þ
N X hr;i Ai ðT si;k T MRT Þ ¼ 0; k¼1
ð2Þ
where Qr,s is the radiation from internal sources in W and Ai is the surface area in m2. Tsi,k and TMRT are interior surface temperature and mean radiation temperature, respectively in Kelvin for all surfaces related to the zone from k = 1 to N surface. hr,i is the longwave heat transfer coefficient in W m2 K1 which is derived from surface emissivity and power three of the absolute surface temperature [23]. Operative temperature is a weighted average between air and mean radiation temperate dependent to corresponding convective and radiative heat transfer coefficients [24]. In IDA ICE, the operative temperature TOPT is assumed to be an average of air and mean radiation temperature according to:
T OPT ¼ 0:5 ðT air;mixed þ T MRT Þ;
ð3Þ
In this simplification, radiative and convective heat transfer coefficient are assumed to be equal important where detailed indoor climate analysis such as air velocities at the occupant location and view factors from different internal radiative sources are not available in details. In Eq. (3), TOPT ,Tair,mixed and TMRT are operative temperature, well-mixed air temperature and mean radiation temperature. IDA ICE version 4.2 is capable of modeling electrical floor heating with PI controller (as in the test cabins). IDA ICE uses a concept of ‘‘ideal heater’’ and ‘‘ideal cooler’’ where detailed information of the heating or cooling systems is neither available nor motivated. In this concept, the ideal heating or cooling unit has constant performance parameter and does not have a real physical location inside the zone to have, e.g. view factor dependency or conduction to certain wall. For the ideal cooler, COP (Coefficient of performance) is set to 3.0.The ideal heater radiative fraction is set to 40% in this simulation study [25]. 4.2. Cabin model In Section 4.1, simulations are preformed using a detailed model of the test cabins and dynamic boundary conditions close to experimental location in Borlänge, Sweden. From a comparison point of view; average annual saving at three different operative temperature set points of 20 ± 1 °C (heating to 19 °C and cooling to 21 °C), 20 ± 2 °C and 20 ± 3 °C are compared with the measurement results obtained from the test cabins during one or two-week long data logging periods over 3 years. The variable exterior boundary conditions are taken from synthetic weather data, i.e.,
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an available ASHRAE weather file for Stockholm (latitude 59N) which is relatively close to the Borlänge test site. The terms net heating and net cooling herewith relates to heating and cooling demand of the cabins, respectively. In Section 4.2, the combined effect of interior and exterior reflective coatings is presented. For a straight forward comparison, windows and doors are omitted and operative temperature is set to 20 ± 1 °C without internal heat load or ventilation. In Sections 4.3–4.6, simulations are implemented according to flowchart given in Fig. 5, where the three test cabins are evaluated under various conditions and scenarios; with different scales, internal loads, ventilation and geographical location. Ditto, windows and doors are omitted and operative temperature is set to 20 ± 1 °C for all cases with the ideal heater and cooler units as described above. The three test cabins, herewith, are normal cabin (with both interior and exterior normal coating), interior reflective cabin (with reflective interior and normal exterior coating) and both-side reflective cabin (with both interior and exterior reflective coatings) with the same optical properties as given in Table 1.
Table 2 Simulation results for floor heating and air cooling for the test cabins in Stockholm climate.
Interior reflective cabin Both-side reflective cabin
Heating savings (%) (October, November, December, January, February and March)
Cooling savings (%) (June, July and August)
3 0
2 42
For a period of 3 years, experimental heating and cooling test has been performed in the real cabins in up to two-week periods using split type air conditioner and electrical floor heating with different set-point temperatures varying from 17 to 23 °C. Electrical consumption from heating and cooling system has been separately measured during test periods. Fig. 2 shows the reduction in electrical energy consumption measured in the Interior reflective and Both-side reflective cabin, compared to the Normal cabin. The measurement presented consists of totally 83 measurement periods of 1–2 weeks each, of which 55 is during heating period (winter) and 28 is during cooling periods (summer). According to Fig. 2, reflective interior coating contributes to both heating and cooling savings. Heating electrical consumption is reduced as less radiation from the heated floor is absorbed and dissipated by interior surfaces. Cooling electrical consumption is reduced as the reflective low emissivity interior has higher surface resistivity toward the heat flux into the building [12]. In the both-side reflective cabin, high TSR exterior surfaces contribute considerably to the reduction of cooling electrical consumption. However, high TSR coatings introduce an inevitable heating penalty during cold periods due to the decreased absorption of solar radiation. Measurements are based on maximum 2 weeks data logging. However, for longer period of time, IDA ICE simulation is carried
out for the test cabins with results given in Table 2 for different seasons. Simulation results and measurement both agree in terms of relative saving trend, i.e. cabins with different combination of interior and exterior coatings have the thermal behavior as seen in the measurements and simulation. Savings in terms of absolute values are not directly comparable due to different climate input and simulation period. Simulation is based on electrical consumption for electrical floor heating and air cooling device with constant coefficient of performance COP of 3.0. Comparing Interior reflective cabin with both-side reflective cabin, Both short period measurement (Fig. 2) and longer seasonal simulation (Table 2) indicate a negative savings the cabins with high reflective exterior (high TSR values according to Table 1). The heating penalty is limited during the dark winter months but more significant later in the year with longer days and still low ambient temperatures. This undesirable reflection in cold seasons reduces the passive solar gain and inflicts a thermal loss due to use of high TSR exterior surfaces. Snow gathering on the exterior, on the other hand, will pacify the heating penalty inflicted by high TSR exterior in the cold season, which is evaluated more in the Section 5.2. Remaining snow on the exposed parts of the cabins directly affects the optical properties of the surfaces. To investigate the effect of the snow sitting on the exterior surface, Fig. 3 is given as an estimation of the area-weighted amount of roof covered with snow in usually snow covered winter months of January and February. Snow is assumed to have 0.9 TSR in the simulations. The aim of this comparison is to magnitude the reduction of heating penalty of high TSR exterior roof versus snow covering percentage. As seen in Fig. 3, the heating penalty partially reduces where the effect of snow coverage on the roof is taken into account, considering the original TSR values of the walls. The snow layer can, in reality, decrease the total U-value as well as keeping the exterior surface temperature at 0 °C when thawing or freezing. However this fact is simplified so that only the optical properties of snow can be considered in the simulation. There remains a certain amount of heating penalty even in the case that all of the roof is
Fig. 2. Measurement results from the test cabin (heating for winter and cooling for summer weeks), where lower limit relates to 25th percentile (Q1), mid values are median values and upper limit is the 75th percentile (Q3).
Fig. 3. Heating penalty of high TSR roof and snow gathering effect at the coldest months.
5. Results 5.1. Measurements vs. simulation
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covered by snow since the other walls with high TSR values still undesirably reflects back the incident solar radiation in these cold months. Heating penalty reduces by one third when the roof is assumed to be covered with snow. Reflective interior coatings contributes to both cooling and heating load reduction in the test cabins. This fact is shown to a lesser extent by simulation compared to the measurement results. It shall be mentioned that any effect of dust gathering, dirt pickup and eventual condensation on the surfaces are not considered in the current model. 5.2. Exterior–interior coating combinations Simulation based on the ideal heater and cooler concept described in Section 4.1 is made using four different combinations of coatings on both interior and exterior sides. As seen in Fig. 4, interior reflective coating reduces the heating but does not necessarily reduce the cooling. High TSR exterior increases the heating and reduces the cooling. High TSR exterior increases heating larger than decreasing the cooling therefore total energy demand increases (in Sweden climate). Both-side reflective cabin is not better than normal cabin because heating demand is more increased than the cooling saving for this climate and with no internal load. Having reflective interior, as seen in Fig. 4 contributes to total load reduction. High TSR exterior reduces the cooling load but on the other hand, it inflicts a net heating penalty, significant to the total load in Stockholm climate. So only high reflective exterior coating has the net highest thermal demand in the given condition. In the case where both reflective exterior and interior coatings are applied, this heating penalty is close to compensated by saving due to reflective interior coatings in Stockholm cold climate and with cabins assumed to have no internal heat load.
Fig. 5. Map of three test cabins studied within different scenarios and conditions (original condition).
5.3. Larger buildings To investigate if the volume of the cabins has an impact on the savings in term of percentage of total load, the test cabins are scaled three times by a factor of three. Simulations are based on the original conditions marked with in Fig. 5 and scaling factor varying from 1 to 9. The aim herewith is to evaluate merely the scaling effect. Any possible effects of larger stratified air-volumes on the net heat flux is not considered as an assumption of a well mixed interior air volume is inherent in the simulation model. Clad area includes all wall and roof surfaces. Fig. 6 shows the total heating and cooling saving vs. scaling factors, comparing interior reflective cabin with normal cabin. As the
Fig. 4. Four cabins with different combinations of coatings and their annual thermal load with Stockholm climate data.
Fig. 6. Ratio of clad surface area to internal volume and total savings for heating and cooling for Interior reflective cabin vs. scale of the cabin.
cabin enlarges, the ratio between clad areas to internal volume reduces. In consequence, the impact of the surface optical properties on total heating and cooling reduces. The larger the building, the more is the saving in term of kW h per year (apparently, as the total thermal load increases). However the relative saving in term of percentage reduces to almost a constant value. It is an established fact that increased thermal mass can reduce the energy demand due to dynamic effects. Scaling up the building will increase the thermal mass of the enclosed air with a power of three, while the thermal mass of the walls increases with a power of two, and thus the thermal mass per m2 surface area increases while increasing the size of building. This is the explanation to the decreased energy demand per m2 when increasing the scale of the building seen in Fig. 6.
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The dynamic behavior of the surrounding climatic conditions gives a temperature fluctuation on the interior surfaces of the building. Larger buildings give lower temperature fluctuations of the interior air due to the larger thermal mass of the air, and the reduced temperature fluctuations of the air affects that of the surface temperatures which also is reduced. The relative saving by using low emissive interior cladding compared to normal cladding, seen in Fig. 6, is decreased with increased building volume since the former mainly is reacting on the reduced air temperature fluctuations, while the latter also is affected by the reduced surface temperature fluctuations. For larger scales of the building (over 3:1 with the chosen geometry of the building), the surface temperature fluctuations become so small that the relative savings by using a low e interior coating will be almost independent of further increasing of the building scale. 5.4. Internal load In most real cases, heat is generated inside the building by activities, lighting and equipment. Here simulations are made with a range of heat loads with the same 40% radiation fraction as the ideal heater. Simulations are based on original conditions as indicated in Fig. 5. People, lighting and equipments are the three sources for internal load. This load can vary considerably with different application; In a warehouse (goods and materials storage) internal heat gains are in general rather low. On the other hand, in the datacom (data processing and telecommunications) facilities, heat load can have densities of several 100 W m2. In other application such as Retail facilities, lighting heat gains can be about 20–40 W m2 for sale area and 60–85 W m2 for jewelry [26] (with energy efficient lamps and armatures this figure can be probably lower) . In this study, a wide range of internal load has been considered in Fig. 5 ranging 0–500 W m2 (with extreme values acting as control parameters for simulations). As indicated in Fig. 7 for reflective interior cabin, the cooling saving disagrees to increasing internal load. Since 40% of internal load is assumed to dissipate via radiation, reflective interior, undesirably, reflects back this radiation part. This results to net cooling penalty (negative saving) with higher internal loads and reflective interior coatings. With the internal load of less than 20 W m2 , the net heating and cooling savings are positive, meaning a net reduction of total thermal load by use of reflective interior coatings in Stockholm climate. The net heating and cooling saving is almost zero at 20 W m2 and thereafter with larger internal load there is net
Fig. 8. Saving vs. internal load; comparing both-side reflective cabin with normal cabin.
penalty by the use of only low emissive coatings. The reason is that in this point the need for heating starts decreasing until it disappears and cooling penalty becomes more and more dominating. At internal load of 50 W m2 cooling penalty (negative savings) results to a net total loss by the use of reflective interior and large internal load. Savings in energy use for heating and cooling by the combination of high TSR exterior surfaces and highly reflective interior surfaces are shown in Fig. 8. When the internal load increases, cooling saving improves until it reaches a maximum value for cooling savings at internal load of about 20 W m2.Thereafter the cooling saving reduces. The reason for having a maximum point is that there is a combination effect of both interior and exterior reflective coatings. Reflective exterior (high TSR coating) reduces solar gain thus it saves cooing. However the maximum capacity of the exterior reflective coating to save cooling is restricted to a finite maximum solar gain reduction at a given TSR value. On the other hand, reflective interior, as discussed earlier in Fig. 7, inflicts negative savings (penalty) when internal load increases. Above about 100 W m2 the cooling penalty caused by reflective interior overcomes the cooling savings caused by reflective exterior coatings which results to a sum negative cooling after 100 W m2of internal loads. The effect of high TSR exterior is addressed by comparing interior reflective cabin (Fig. 7) with both-side reflective cabin (Fig. 8). Regarding the heating saving, high TSR exterior inflict an inevitable part of heating penalty (heating penalty still exists with even 10 W m2 in this case). At higher internal loads, the heating penalty (resulting from the high TSR exterior) decreases as heating becomes less frequent over time. There is a critical level of internal load for the whole thermal saving to switch from the net loss to a net saving (in this case at about 5 W m2 internal loads). As seen in Fig. 8, at this critical level, the heating penalty is small enough and cooling load is large enough to result in a net positive saving for the both-side reflective coatings. At About 20 W m2 internal load, there is a maximal net savings and solar gain due to reflective exteriors overcomes the negative savings of reflective interiors and there is no heating penalty. At internal load of 50 W m2, heating and cooling savings are still positive (but not maximal) until high internal load of 100 W m2 in which there will be net loss by the use of both interior and exterior coatings.
5.5. Outdoor air ventilation
Fig. 7. Saving vs. internal load; comparing interior reflective cabin with normal cabin.
A series of 1 year building energy simulations were made using the test cabin model with a range of outdoor ventilation rates. The outdoor air ventilation rate was varied from zero to 1.5 ACH (air
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change per hour). Simulations are based on the original conditions marked with in Fig. 5. Total building load (heating and cooling) increases with higher ventilation rates (assuming no heat recovery system). Nevertheless, how different coatings interact with this inflicted load varies with the optical properties of each coating. As the outdoor ventilation rate increases, the more heating is required to compensate for the heat lost by ventilation during the colder season in the Stockholm climate. To satisfy the minimum operative temperature, the auxiliary heating source conveys more energy to the cabin. Here, this heat source has a significant radiation fraction of 0.4. The more energy supplied, the more energy is thus retrieved by the interior reflective coatings. Thus with increasing the ventilation rate the heat recovery role of the interior reflective coatings gain more weight so the heating load saving become larger as seen in Fig. 9. In the summer the main source of surplus heat is the absorbed solar radiation energy conducted from the exterior to the interior side of the cabins. As the building envelope warms up, the radiation heat exchange increases among the interior surfaces. As the cooling mechanism in the simulation model is cooling the air uniformly in the interior volume, the amount of cooling required shows little dependence on the efficiency of radiative heat transfer between interior surfaces. There is however a slight decrease in cooling saving going from 0 to 0.5 ACH as moderate ventilation will compensate for some solar gain. At and above 1 ACH there is a slight increase in cooling saving over the year. The reduction in total energy consumption by the use of high thermal reflectivity interior surfaces of the cabin increases with higher ventilation rate due to the increase of heating savings with higher ventilation rates, even though the cooling saving remain largely unaffected according to Fig. 9. The higher solar reflectivity on exterior surfaces contributes to less cooling demand. However, the amount of cooling savings in the case of exterior reflective coatings reduces as the ventilation rate increases as shown in Fig. 10. This may again be due to the fact that by increasing the ventilation rate, the cabin cools down by incoming outdoor air and that will compensate a part of cooling load demanded from the auxiliary unit. Fig. 10 shows that the significance of the heating penalty is reduced as the ventilation rate increases. The exterior reflective coatings are the main source of the heating penalty. On the other hand, interior reflective coating increases the heating saving with ventilation as discussed above, in the case of only reflective interior. The combined effect of interior and exterior reflective coatings leads in this case to a heating penalty with low ventilation rate, and a heating saving with high ventilation rates. With ventilation rate above
Fig. 9. Saving vs. outdoor air ventilation, comparing interior reflective cabin with normal cabin.
Fig. 10. Saving vs. outdoor air ventilation, comparing both-side reflective cabin with normal cabin.
1.5 ACH the effect of higher thermal reflectivity of the interior surfaces is in this case larger than the heating penalty due to high TSR exterior on a yearly basis, by comparing heating savings of interior reflective cabin (Fig. 9) and both-side reflective cabin (Fig. 10). 5.6. Other climates The response of the model in three locations with different climate is illustrated in Fig. 11 showing separately the heating, cooling and total loads showing the result for cabins with normal coating and interior reflective coatings. Simulations are based on the original conditions marked with in Fig. 5 and three different climates. In warmer climate as in Riyadh (latitude 22N), any increase in TSR value of the exterior coating reduces the thermal load significantly and the heating penalty becomes less important. Hence, the overall thermal performance of the cabin increases with high TSR coating on the exterior. There is an opposite trend in colder climate like Stockholm (latitude 59N) were the heating load and heating penalty by use of very high TSR coatings may be larger than the cooling saving over the year. Regarding milder climate like Paris (latitude 48N), both heating and cooling demands lies between extreme values of Riyadh and Stockholm, indicating a possible existence of optimum optical properties for the coating where the total thermal load is minimal (e.g. with exterior TSR of 0.3 and interior reflective coating). This optimal point depends apparently to individual case with considered simulation conditions, in this case without internal heat load and ventilation. According to Fig. 11 and under the given conditions, reflective interior has almost negligible impact in warmer climate like Riyadh given that the ideal cooler has a zero radiation fraction and the assumption of a well mixed interior air volume is valid, where as in colder climate reflective interior appears to improve the overall thermal performance of the building by improving the efficiency of radiation heating. Similar climate impact analyzing simulations are made on a simple model of a 113 m2 retail building (scale three of the cabins) with the different exterior and interior coating combinations and with an internal heat load of 40 W m2 (lighting 25 W m2, equipment 10 W m2 and people 5 W m2) and 1.5 ACH ventilation with constant setpoints temperature of 20 ± 1 °C in Stockholm, Paris and Riyadh climates. The net heating and cooling in these cases are shown in Table 3 for three separate seasons during the year together with the total annual saving in comparison with the normal coatings cases.
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Fig. 11. Thermal load of the test cabins vs. exterior coating TSR, with and without reflective interior in different climates (R represents cabin with reflective interior coating and N stands for cabin with normal interior coating).
Table 3 Net heating and cooling in a simulated retail house during different seasons at three locations related to cold, mild and warm climates. January–April Net heating (kW h) Stockholm
Paris
Riyadh
a
May–August Net cooling (kW h)
Net heating (kW h)
Net cooling (kW h)
September–December
Annual savingsa
Net heating (kW h)
Total heating and cooling (kW h per year)
Net cooling (kW h)
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
24548 21433
10 13
1300 891
2089 2376
13379 11201
145 173
0 5384
24959
3
1397
1813
13556
118
375
21798
7
964
2097
11344
145
5116
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
10431 8537
154 186
216 116
3894 4285
6875 5555
764 900
0 2755
10703
123
229
3604
6989
690
4
8773
151
124
3996
5651
822
2817
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
285 163
4822 5167
0 0
14461 14871
206 130
7878 8258
0 937
293
4526
0
14121
216
7573
923
168
4875
0
14540
135
7960
26
Positive value for savings and negative for total loss (compared with normal coating).
It can be noted that under the conditions of the simulations in Stockholm climate there is a significant saving in net heating and a slight increase in cooling during the whole year from the use of interior reflective coatings. That is consistent with less absorption of the heat radiation from the internal load. The use of exterior reflective coating in this case results in a reduced net cooling but an even larger increase in the net heating during all three seasons. The total effect of the both-side reflective coatings is dominated by the reduction in net heating by the interior reflection of heat radiation. Using Paris climate the trends are similar. There is however a slightly larger saving in net cooling by the use of exterior reflective coating making the both-side reflective coatings combination the most favorable. Simulating Riyadh climate conditions results in a
significant increase in net cooling by the use of interior reflective coatings and a relatively moderate saving in net cooling by the use of exterior reflective coatings compared to the total amount of cooling needed. The latter finding is consistent with higher ambient exterior air temperatures and the ventilation contributing to the total cooling load.
6. Conclusions The current evaluation of the cabin models is focused on heating and cooling resolved annual energy usage. The effect of exterior and interior surface radiation properties under the different conditions on heating and cooling is shown to be significant in line with
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experimental results and readily explained by the concept inherent in the model. Dependent to the internal heat load, ventilation rate and climate the choice of reflective coating for the interior and exterior of building material varies. Generally, reflective coatings for the interior contribute to net heating savings where it may not necessarily contribute to net cooling saving with higher internal load therefore total savings can vary depending on the simulation case. Reflective coatings on the exterior, however, contribute to net cooing savings and inflict net heating loss (penalty). The total saving for such coatings can be seen in warmer climate and where there is a dominant cooling load, e.g. due to high internal load. Combination effect of both interior and exterior reflective coatings is very much dependent to individual case. In the given example of retail building, simulation show that where reflective interior are good solution for colder climates and reflective exterior best for warmer climate, the combination of both reflective interior and exterior results the best thermal performance for the milder climate. The remarkably small reduction in air cooling by use of highly reflective interior in warmer climate is arguably related to the well mixed assumption. In the earlier work [12] it was shown that there is a measurable increase in the vertical air temperature gradient and a more stable interior air stratification as well as a measurable cooling saving with reflective interior surfaces. Exterior reflective coatings contributes to less cooling demand, while introducing an inevitable heating penalty during the colder seasons. Adding reflective interior to existing reflective exterior coating will both alleviate this heating penalty and contribute to cooling savings. However, as the interior reflective coating acts like a thermal radiative mirror, additional care must be taken when redundant internal load with significant radiation fraction needs to be cooled down. Acknowledgement Financial support from Knowledge Foundation is acknowledged. References [1] 2010 Building Energy Data Book. US Department of Energy – Energy Efficiency and Renewable Energy; March 2011. [2] Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings. Official Journal of the European Union. 153 (13) 2010.
[3] Sweden’s Environmental Objectives – No Time to Lose (appendices to the indepth evaluation). Swedish Environmental Protection Agency; 2008. [4] Pisello AL, Goretti M, Cotana F. A method for assessing buildings’ energy efficiency by dynamic simulation and experimental activity. Appl Energy 2012;97:419–29. [5] Jo JH, Carlson JD, Golden JS, Bryan H. An integrated empirical and modeling methodology for analyzing solar reflective roof technologies on commercial buildings. Build Environ 2010;45(2):453–60. [6] Wray C, Akbari H. The effects of roof reflectance on air temperatures surrounding a rooftop condensing unit. Energy Buildings 2008;40(1):11–28. [7] Wang XX, Kendrick C, Ogden R, Maxted J. Dynamic thermal simulation of a retail shed with solar reflective coatings. Appl Therm Eng 2008;28(8– 9):1066–73. [8] Levinson R, Akbari H, Reilly JC. Cooler tile-roofed buildings with near-infraredreflective non-white coatings. Build Environ 2007;42(7):2591–605. [9] Santamouris M, Gaitani N, Spanou A, Saliari M, Giannopoulou K, Vasilakopoulou K, et al. Using cool paving materials to improve microclimate of urban areas – design realization and results of the flisvos project. Build Environ 2012;53:128–36. [10] Synnefa A, Karlessi T, Gaitani N, Santamouris M, Assimakopoulos DN, Papakatsikas C. Experimental testing of cool colored thin layer asphalt and estimation of its potential to improve the urban microclimate. Build Environ 2011;46(1):38–44. [11] Joudi A, Svedung H, Ronnelid M. Energy efficient surfaces on building sandwich panels – a dynamic simulation model. Energy Buildings 2011;43(9):2462–7. [12] Joudi A, Svedung H, Bales C, Ronnelid M. Highly reflective coatings for interior and exterior steel cladding and the energy efficiency of buildings. Appl Energy 2011;88(12):4655–66. [13] Daoud A, Galanis N, Bellache O. Calculation of refrigeration loads by convection, radiation and condensation in ice rinks using a transient 3D zonal model. Appl Therm Eng 2008;28(14–15):1782–90. [14] Gustavsson L, Sathre R. Variability in energy and carbon dioxide balances of wood and concrete building materials. Build Environ 2006;41(7):940–51. [15] Duffie JA, Beckman WA. Solar engineering of thermal processes. Wiley; 2006. [16] EQUA. IDA Indoor Climate and Energy 4. Version 4.2.2 release year 2012 ed. [17] Validation of IDA indoor climate and energy 4.0 with respect to CEN Standard EN 15265-2007. Equa Simulation AB; 2010. [18] Validation of IDA indoor climate and energy 4.0 build 4 with respect to ANSI/ ASHRAE Standard 140-2004. Equa Simulation AB; 2010. [19] Björsell N, Bring A, Eriksson L, Grozman P, Lindgren M, Sahlin P, Shapovalov A, Vuolle M. IDA indoor climate and energy. In: Proceedings of the 6th international IBPSA conference, Kyoto; 1999. [20] Moosberger S. IDA ICE CIBSE-validation test of IDA indoor climate and energy version 4.0 according to CIBSE TM33, issue 3. HTA LUZRN/ZIG; 2007. [21] Sahlin P, Bring A, Eriksson L. Real controllers in the context of full building, whole-year simulation. Eleventh International IBPSA, Building Simulation, Glasgow; 2009. [22] Sahlin P, Eriksson L, Groman P, Johnsson H, Shapovalov A, Vuolle M. Wholebuilding simulation with symbolic DAE equations and general purpose solvers. Build Environ 2004;39(8):949–58. [23] Bring A, Sahlin P, Vuolle M. IEA SHC Task 22 – Subtask B – Models for building indoor climate and energy simulation, KTH, Stockholm; 1999. [24] 2009 ASHRAE handbook – fundamentals (SI Edition). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc; 2009. [25] IDA Indoor Climate and Energy 4.0 Manual. Equa Simulation AB; 2009. [26] 2011 ASHRAE Handbook – Applications (SI edition). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc; 2011.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Reflective coatings for interior and exterior of buildings and improving thermal performance Ali Joudi a,b,⇑, Harald Svedung b, Mathias Cehlin c, Mats Rönnelid a a
Energy and Environmental Technology, Dalarna University, SE-79188 Falun, Sweden SSAB EMEA, SE-78184 Borlänge, Sweden c Building, Energy and Environmental Engineering, University of Gävle, SE-80176 Gävle, Sweden b
h i g h l i g h t s " Increase building energy efficiency by optimizing surface optical properties. " Study different scenarios with both interior and exterior reflective coatings. " Combined thermal effect of both interior and exterior reflective coatings.
a r t i c l e
i n f o
Article history: Received 13 May 2012 Received in revised form 7 September 2012 Accepted 5 October 2012 Available online 26 November 2012 Keywords: Reflective coatings Low energy building Energy simulation Total solar reflectance Interior thermal emittance
a b s t r a c t The importance of reducing building energy usage and thriving for more energy efficient architectures, has nurtured creative solutions and smart choices of materials in the last few decades. Among those are optimizing surface optical properties for both interior and exterior claddings of the building. Development in the coil-coating steel industries has now made it possible to allocate correct optical properties for steel clad buildings with improved thermal performance. Although the importance of the exterior coating and solar gain are thoroughly studied in many literatures, the effect of interior cladding are less tackled, especially when considering a combination of both interior and exterior reflective coatings. This paper contemplates the thermal behavior of small cabins with reflective coatings on both interior and exterior cladding, under different conditions and climates with the aim to clarify and point out to the potential energy saving by smart choices of clad coatings. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A substantial portion of total energy usage and greenhouse gas emissions belongs to building sectors [1]. Improving the energy performance of the building and simultaneously maintaining a desirable indoor climate for the occupants is a precious goal to thrive. In Sweden, for example, aligned with the decision of European Council, there is a target to reduce energy usage in residential and commercial buildings with 20% by 2020 and 50% by 2050 comparing from year 1995 [2,3]. In the frontiers of energy saving studies, reflective coatings are addressed to have substantial potentials to improve thermal performance of the building as cool roof [4–8] and heat island reduction [9,10] for exterior cladding and as radiation barriers for interior claddings [11–13]. Besides that, the energy needed to
⇑ Corresponding author at: Energy and Environmental Technology, Dalarna University, SE-79188 Falun, Sweden. Tel.: +46 23778533; fax: +46 23778701. E-mail address: [email protected] (A. Joudi). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.10.019
maintain a desirable indoor climate during building lifetime is considerable compared to the energy used to fabricate the building materials [14]. Thus the correct choice of materials, including surface optical properties is of great interest. Surface optical properties are categorized based on their functions; those sitting on the outermost surfaces (exterior) can have different solar reflective properties and thermal emissivities and those sitting on the innermost surfaces (interior) can have only different thermal emissivities. Solar energy travels in electromagnetic radiation with the most energy content at the wavelengths of 0.25–3.0 lm [15]. It includes different wavelength regions; ultraviolet, visible and near infrared regions. The near infrared region stands for more than half of the solar radiation. However, surface optical properties embody all wavelength regions, including both visible and near infrared. Therefore it is possible to have surfaces with the same colors but different total solar refection (TSR), i.e. surfaces with same reflective property in visible region but different reflectivity in near infrared region. This fairly new concept has been recently applied to coil-coated steel sheets which are used as the building exterior claddings,
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making it possible to optimize surface radiation properties for different building applications. Furthermore, coil coating development has also made it possible to adjust the longwave radiation properties for the interior surfaces by using reflective pigment in the longwave (thermal) radiation. In this new concept, surface emissivity of 0.9 (common for most materials) can be reduced down to 0.25. These coatings are mainly used for the interior of the buildings [12]. In order to investigate the effect of this modified surface radiation properties, small test cabins with and without reflective coatings have been studied in earlier work [12] with the focus on measurement results and thermal behavior observations of these cabins at Borlänge, Sweden with the latitude of 60°N.
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The optical properties of all cabins are presented in Table 1. It shall be mentioned that although all test cabins have almost same color on exteriors (same reflectivity in the visual spectrum), they have different reflection properties in the near infrared radiation region. One of the pigments used in the exterior coating on one of the cabins increases the TSR by an extremely sharp transition from absorption in the visible spectrum to high transmittance in the near infrared. Test cabins were equipped with heating and cooling utilities to assess the heating and cooling demand of each test cabin. Cabins specifications including the envelope transmission are given in Table 1. 3.2. The coatings
2. Aim of the study
3. The experimental setup
Coil-coating of galvanized steel sheet material can be done using a variety of paint systems differing largely in the polymer used as paint binder and in any cross linker chemistry. In the case of our test cabins thermo-set polyester-melamine coatings as well as thermoplastic PVC-plastisol coatings are used for the exterior surfaces. Normally a non-metallic paint surface has a high thermal emissivity independent of the type of binder used. However the thermal emissivity of the coil-coated surface was in the case of our reflective interior surfaces reduced dramatically from above 90% to 25% by the use of aluminum flakes dispersed in the epoxy-based thermo-set coating. Pigments used in coil-coating formulations differ both in visual color and near infra-red reflectivity to some extent independently. Thus the TSR of the coated surface can be achieved depending on the choice of pigments in the paint formulation within a wide range from 0.5 to around 90% and in smaller ranges independent on the visual color of the surface.
3.1. Cabins
4. Simulations
The three cabins (Fig. 1) include a normal cabin with both interior and exterior conventional coating, an interior reflective cabin with low thermal emissivity coating on interior claddings and a both-side reflective cabin which has both low thermal emissivity (high reflective) coating on interior surfaces and high TSR coating on the exterior roof and wall claddings.
4.1. IDA ICE software
The aim of this study is to highlight the role of different coatings and their contributions to different thermal savings. Starting from measured results in the test cabin under given conditions, this is a theoretical study to understand the thermal behavior of the simulated cabin model under different scenarios. To further investigate the cabins under different conditions and climates, and in order to map the sensitivity of the cabins energy use with different radiation properties, this paper studies the thermal behavior of coatings under various conditions, highlights pros and cons, addresses the counter balancing effect of coating combinations and finally introduces other important interplaying parameters in a dynamic building energy simulation model.
To evaluate the annual energy use of the test cabins (Fig. 1) as specified in Table 1 depending on the surface radiation properties under a controlled set of conditions, the simulation program IDA Indoor Climate and Energy (IDA ICE) [16] is used here.
Fig. 1. Test cabins with windows facing south (the upper drawing show the model used in the simulation of the cabins).
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Table 1 Test cabins specifications and optical properties (dimension in millimeter). Location Latitude U-values
Glazing Cooling system Heating system Ventilation rate Infiltration Internal load Normal cabin
Interior reflective cabin
Both-side reflective cabin
a b
Borlänge, Sweden 60°N Walls 0.247 W m2 °C Roof 0.207 W m2 °C Floor 0.158 W m2 °C Total 0.68 m2 (Double glazing) Split type air conditioner (EER:4.08) Electrical floor heating 70 m3 h1 nominal capacity Negligible Negligible Normal exterior TSRa 0.10 IR-eb 0.92 Normal interior IR-eb 0.91 Normal exterior TSRa 0.10 IR-eb 0.92 Reflective interior IR-eb 0.25 a Reflective exterior TSR 0.39 b 0.92 IR-e b Reflective interior IR-e 0.25
TSR: Total solar reflectance. IR-e: Infrared emissivity.
IDA ICE is a building performance simulations tool to model buildings with wide variety of heating and cooling systems and different controllers. It is an hourly dynamic simulation program which is an equation based model, using either the Modelica language or the Neutral Model Format (NMF) [17–22]. Regarding radiation and optical properties on exterior surfaces, TqFace module inside IDA ICE uses following energy balance Eq. (1) on the surface thermal node. This surface node is thermally coupled with the adjacent thermal node inside the solid part of the building material.
ð1 TSRÞðIdir þ Idiff ;sky þ Idiff ;grnd Þ þ fsky reext ðT 4sky T 4so Þ þ fgrnd reext ðT 4grnd T 4so Þ þ qcond;w þ qconv ¼ 0
ð1Þ
In Eq. (1) optical properties of exterior surfaces can be seen in terms of TSR which is the total solar reflectance and eext is the longwave emissivity of the exterior surface. r is Stefan–Boltzmann constant and Idir, Idiff,sky, Idiff,grnd are direct solar radiation, diffuse solar radiation from the sky and diffuse solar radiation from the ground, respectively in W m2. fsky and fgrnd are surface view factors to sky and ground, respectively. Tso, Tsky and Tgrnd are exterior surface, sky and ground absolute temperatures, respectively, where ground temperature is assumed to be the same as ambient air and sky temperature, 5 °C below ambient air according to Climate Processor of IDA ICE. qcond,w and qconv are conductance to wall and convection to the ambient air, respectively in W m2 where the former depends to the building material and the later relates to ambient air temperature, wall orientation and local wind speed according to [23]. To account for radiation between interior surfaces for a nonhorizontal roof construction, Energy Model inside IDA ICE simulation uses the concept of mean radiation temperature where it comes to the radiative heat exchange from the interior surface, mainly to simplify geometry complexity in view factor calculation. According to CESIMZONE module inside IDA ICE, mean ration temperature relates to surface temperatures, surface longwave emissivity and radiation from internal sources, e.g. lighting, equipment and people. This internal source radiation is assumed to be distributed among internal surfaces based on area ratio [23]. The linearized energy balance for mean radiation temperature is given by:
Q r;s þ
N X hr;i Ai ðT si;k T MRT Þ ¼ 0; k¼1
ð2Þ
where Qr,s is the radiation from internal sources in W and Ai is the surface area in m2. Tsi,k and TMRT are interior surface temperature and mean radiation temperature, respectively in Kelvin for all surfaces related to the zone from k = 1 to N surface. hr,i is the longwave heat transfer coefficient in W m2 K1 which is derived from surface emissivity and power three of the absolute surface temperature [23]. Operative temperature is a weighted average between air and mean radiation temperate dependent to corresponding convective and radiative heat transfer coefficients [24]. In IDA ICE, the operative temperature TOPT is assumed to be an average of air and mean radiation temperature according to:
T OPT ¼ 0:5 ðT air;mixed þ T MRT Þ;
ð3Þ
In this simplification, radiative and convective heat transfer coefficient are assumed to be equal important where detailed indoor climate analysis such as air velocities at the occupant location and view factors from different internal radiative sources are not available in details. In Eq. (3), TOPT ,Tair,mixed and TMRT are operative temperature, well-mixed air temperature and mean radiation temperature. IDA ICE version 4.2 is capable of modeling electrical floor heating with PI controller (as in the test cabins). IDA ICE uses a concept of ‘‘ideal heater’’ and ‘‘ideal cooler’’ where detailed information of the heating or cooling systems is neither available nor motivated. In this concept, the ideal heating or cooling unit has constant performance parameter and does not have a real physical location inside the zone to have, e.g. view factor dependency or conduction to certain wall. For the ideal cooler, COP (Coefficient of performance) is set to 3.0.The ideal heater radiative fraction is set to 40% in this simulation study [25]. 4.2. Cabin model In Section 4.1, simulations are preformed using a detailed model of the test cabins and dynamic boundary conditions close to experimental location in Borlänge, Sweden. From a comparison point of view; average annual saving at three different operative temperature set points of 20 ± 1 °C (heating to 19 °C and cooling to 21 °C), 20 ± 2 °C and 20 ± 3 °C are compared with the measurement results obtained from the test cabins during one or two-week long data logging periods over 3 years. The variable exterior boundary conditions are taken from synthetic weather data, i.e.,
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an available ASHRAE weather file for Stockholm (latitude 59N) which is relatively close to the Borlänge test site. The terms net heating and net cooling herewith relates to heating and cooling demand of the cabins, respectively. In Section 4.2, the combined effect of interior and exterior reflective coatings is presented. For a straight forward comparison, windows and doors are omitted and operative temperature is set to 20 ± 1 °C without internal heat load or ventilation. In Sections 4.3–4.6, simulations are implemented according to flowchart given in Fig. 5, where the three test cabins are evaluated under various conditions and scenarios; with different scales, internal loads, ventilation and geographical location. Ditto, windows and doors are omitted and operative temperature is set to 20 ± 1 °C for all cases with the ideal heater and cooler units as described above. The three test cabins, herewith, are normal cabin (with both interior and exterior normal coating), interior reflective cabin (with reflective interior and normal exterior coating) and both-side reflective cabin (with both interior and exterior reflective coatings) with the same optical properties as given in Table 1.
Table 2 Simulation results for floor heating and air cooling for the test cabins in Stockholm climate.
Interior reflective cabin Both-side reflective cabin
Heating savings (%) (October, November, December, January, February and March)
Cooling savings (%) (June, July and August)
3 0
2 42
For a period of 3 years, experimental heating and cooling test has been performed in the real cabins in up to two-week periods using split type air conditioner and electrical floor heating with different set-point temperatures varying from 17 to 23 °C. Electrical consumption from heating and cooling system has been separately measured during test periods. Fig. 2 shows the reduction in electrical energy consumption measured in the Interior reflective and Both-side reflective cabin, compared to the Normal cabin. The measurement presented consists of totally 83 measurement periods of 1–2 weeks each, of which 55 is during heating period (winter) and 28 is during cooling periods (summer). According to Fig. 2, reflective interior coating contributes to both heating and cooling savings. Heating electrical consumption is reduced as less radiation from the heated floor is absorbed and dissipated by interior surfaces. Cooling electrical consumption is reduced as the reflective low emissivity interior has higher surface resistivity toward the heat flux into the building [12]. In the both-side reflective cabin, high TSR exterior surfaces contribute considerably to the reduction of cooling electrical consumption. However, high TSR coatings introduce an inevitable heating penalty during cold periods due to the decreased absorption of solar radiation. Measurements are based on maximum 2 weeks data logging. However, for longer period of time, IDA ICE simulation is carried
out for the test cabins with results given in Table 2 for different seasons. Simulation results and measurement both agree in terms of relative saving trend, i.e. cabins with different combination of interior and exterior coatings have the thermal behavior as seen in the measurements and simulation. Savings in terms of absolute values are not directly comparable due to different climate input and simulation period. Simulation is based on electrical consumption for electrical floor heating and air cooling device with constant coefficient of performance COP of 3.0. Comparing Interior reflective cabin with both-side reflective cabin, Both short period measurement (Fig. 2) and longer seasonal simulation (Table 2) indicate a negative savings the cabins with high reflective exterior (high TSR values according to Table 1). The heating penalty is limited during the dark winter months but more significant later in the year with longer days and still low ambient temperatures. This undesirable reflection in cold seasons reduces the passive solar gain and inflicts a thermal loss due to use of high TSR exterior surfaces. Snow gathering on the exterior, on the other hand, will pacify the heating penalty inflicted by high TSR exterior in the cold season, which is evaluated more in the Section 5.2. Remaining snow on the exposed parts of the cabins directly affects the optical properties of the surfaces. To investigate the effect of the snow sitting on the exterior surface, Fig. 3 is given as an estimation of the area-weighted amount of roof covered with snow in usually snow covered winter months of January and February. Snow is assumed to have 0.9 TSR in the simulations. The aim of this comparison is to magnitude the reduction of heating penalty of high TSR exterior roof versus snow covering percentage. As seen in Fig. 3, the heating penalty partially reduces where the effect of snow coverage on the roof is taken into account, considering the original TSR values of the walls. The snow layer can, in reality, decrease the total U-value as well as keeping the exterior surface temperature at 0 °C when thawing or freezing. However this fact is simplified so that only the optical properties of snow can be considered in the simulation. There remains a certain amount of heating penalty even in the case that all of the roof is
Fig. 2. Measurement results from the test cabin (heating for winter and cooling for summer weeks), where lower limit relates to 25th percentile (Q1), mid values are median values and upper limit is the 75th percentile (Q3).
Fig. 3. Heating penalty of high TSR roof and snow gathering effect at the coldest months.
5. Results 5.1. Measurements vs. simulation
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covered by snow since the other walls with high TSR values still undesirably reflects back the incident solar radiation in these cold months. Heating penalty reduces by one third when the roof is assumed to be covered with snow. Reflective interior coatings contributes to both cooling and heating load reduction in the test cabins. This fact is shown to a lesser extent by simulation compared to the measurement results. It shall be mentioned that any effect of dust gathering, dirt pickup and eventual condensation on the surfaces are not considered in the current model. 5.2. Exterior–interior coating combinations Simulation based on the ideal heater and cooler concept described in Section 4.1 is made using four different combinations of coatings on both interior and exterior sides. As seen in Fig. 4, interior reflective coating reduces the heating but does not necessarily reduce the cooling. High TSR exterior increases the heating and reduces the cooling. High TSR exterior increases heating larger than decreasing the cooling therefore total energy demand increases (in Sweden climate). Both-side reflective cabin is not better than normal cabin because heating demand is more increased than the cooling saving for this climate and with no internal load. Having reflective interior, as seen in Fig. 4 contributes to total load reduction. High TSR exterior reduces the cooling load but on the other hand, it inflicts a net heating penalty, significant to the total load in Stockholm climate. So only high reflective exterior coating has the net highest thermal demand in the given condition. In the case where both reflective exterior and interior coatings are applied, this heating penalty is close to compensated by saving due to reflective interior coatings in Stockholm cold climate and with cabins assumed to have no internal heat load.
Fig. 5. Map of three test cabins studied within different scenarios and conditions (original condition).
5.3. Larger buildings To investigate if the volume of the cabins has an impact on the savings in term of percentage of total load, the test cabins are scaled three times by a factor of three. Simulations are based on the original conditions marked with in Fig. 5 and scaling factor varying from 1 to 9. The aim herewith is to evaluate merely the scaling effect. Any possible effects of larger stratified air-volumes on the net heat flux is not considered as an assumption of a well mixed interior air volume is inherent in the simulation model. Clad area includes all wall and roof surfaces. Fig. 6 shows the total heating and cooling saving vs. scaling factors, comparing interior reflective cabin with normal cabin. As the
Fig. 4. Four cabins with different combinations of coatings and their annual thermal load with Stockholm climate data.
Fig. 6. Ratio of clad surface area to internal volume and total savings for heating and cooling for Interior reflective cabin vs. scale of the cabin.
cabin enlarges, the ratio between clad areas to internal volume reduces. In consequence, the impact of the surface optical properties on total heating and cooling reduces. The larger the building, the more is the saving in term of kW h per year (apparently, as the total thermal load increases). However the relative saving in term of percentage reduces to almost a constant value. It is an established fact that increased thermal mass can reduce the energy demand due to dynamic effects. Scaling up the building will increase the thermal mass of the enclosed air with a power of three, while the thermal mass of the walls increases with a power of two, and thus the thermal mass per m2 surface area increases while increasing the size of building. This is the explanation to the decreased energy demand per m2 when increasing the scale of the building seen in Fig. 6.
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The dynamic behavior of the surrounding climatic conditions gives a temperature fluctuation on the interior surfaces of the building. Larger buildings give lower temperature fluctuations of the interior air due to the larger thermal mass of the air, and the reduced temperature fluctuations of the air affects that of the surface temperatures which also is reduced. The relative saving by using low emissive interior cladding compared to normal cladding, seen in Fig. 6, is decreased with increased building volume since the former mainly is reacting on the reduced air temperature fluctuations, while the latter also is affected by the reduced surface temperature fluctuations. For larger scales of the building (over 3:1 with the chosen geometry of the building), the surface temperature fluctuations become so small that the relative savings by using a low e interior coating will be almost independent of further increasing of the building scale. 5.4. Internal load In most real cases, heat is generated inside the building by activities, lighting and equipment. Here simulations are made with a range of heat loads with the same 40% radiation fraction as the ideal heater. Simulations are based on original conditions as indicated in Fig. 5. People, lighting and equipments are the three sources for internal load. This load can vary considerably with different application; In a warehouse (goods and materials storage) internal heat gains are in general rather low. On the other hand, in the datacom (data processing and telecommunications) facilities, heat load can have densities of several 100 W m2. In other application such as Retail facilities, lighting heat gains can be about 20–40 W m2 for sale area and 60–85 W m2 for jewelry [26] (with energy efficient lamps and armatures this figure can be probably lower) . In this study, a wide range of internal load has been considered in Fig. 5 ranging 0–500 W m2 (with extreme values acting as control parameters for simulations). As indicated in Fig. 7 for reflective interior cabin, the cooling saving disagrees to increasing internal load. Since 40% of internal load is assumed to dissipate via radiation, reflective interior, undesirably, reflects back this radiation part. This results to net cooling penalty (negative saving) with higher internal loads and reflective interior coatings. With the internal load of less than 20 W m2 , the net heating and cooling savings are positive, meaning a net reduction of total thermal load by use of reflective interior coatings in Stockholm climate. The net heating and cooling saving is almost zero at 20 W m2 and thereafter with larger internal load there is net
Fig. 8. Saving vs. internal load; comparing both-side reflective cabin with normal cabin.
penalty by the use of only low emissive coatings. The reason is that in this point the need for heating starts decreasing until it disappears and cooling penalty becomes more and more dominating. At internal load of 50 W m2 cooling penalty (negative savings) results to a net total loss by the use of reflective interior and large internal load. Savings in energy use for heating and cooling by the combination of high TSR exterior surfaces and highly reflective interior surfaces are shown in Fig. 8. When the internal load increases, cooling saving improves until it reaches a maximum value for cooling savings at internal load of about 20 W m2.Thereafter the cooling saving reduces. The reason for having a maximum point is that there is a combination effect of both interior and exterior reflective coatings. Reflective exterior (high TSR coating) reduces solar gain thus it saves cooing. However the maximum capacity of the exterior reflective coating to save cooling is restricted to a finite maximum solar gain reduction at a given TSR value. On the other hand, reflective interior, as discussed earlier in Fig. 7, inflicts negative savings (penalty) when internal load increases. Above about 100 W m2 the cooling penalty caused by reflective interior overcomes the cooling savings caused by reflective exterior coatings which results to a sum negative cooling after 100 W m2of internal loads. The effect of high TSR exterior is addressed by comparing interior reflective cabin (Fig. 7) with both-side reflective cabin (Fig. 8). Regarding the heating saving, high TSR exterior inflict an inevitable part of heating penalty (heating penalty still exists with even 10 W m2 in this case). At higher internal loads, the heating penalty (resulting from the high TSR exterior) decreases as heating becomes less frequent over time. There is a critical level of internal load for the whole thermal saving to switch from the net loss to a net saving (in this case at about 5 W m2 internal loads). As seen in Fig. 8, at this critical level, the heating penalty is small enough and cooling load is large enough to result in a net positive saving for the both-side reflective coatings. At About 20 W m2 internal load, there is a maximal net savings and solar gain due to reflective exteriors overcomes the negative savings of reflective interiors and there is no heating penalty. At internal load of 50 W m2, heating and cooling savings are still positive (but not maximal) until high internal load of 100 W m2 in which there will be net loss by the use of both interior and exterior coatings.
5.5. Outdoor air ventilation
Fig. 7. Saving vs. internal load; comparing interior reflective cabin with normal cabin.
A series of 1 year building energy simulations were made using the test cabin model with a range of outdoor ventilation rates. The outdoor air ventilation rate was varied from zero to 1.5 ACH (air
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change per hour). Simulations are based on the original conditions marked with in Fig. 5. Total building load (heating and cooling) increases with higher ventilation rates (assuming no heat recovery system). Nevertheless, how different coatings interact with this inflicted load varies with the optical properties of each coating. As the outdoor ventilation rate increases, the more heating is required to compensate for the heat lost by ventilation during the colder season in the Stockholm climate. To satisfy the minimum operative temperature, the auxiliary heating source conveys more energy to the cabin. Here, this heat source has a significant radiation fraction of 0.4. The more energy supplied, the more energy is thus retrieved by the interior reflective coatings. Thus with increasing the ventilation rate the heat recovery role of the interior reflective coatings gain more weight so the heating load saving become larger as seen in Fig. 9. In the summer the main source of surplus heat is the absorbed solar radiation energy conducted from the exterior to the interior side of the cabins. As the building envelope warms up, the radiation heat exchange increases among the interior surfaces. As the cooling mechanism in the simulation model is cooling the air uniformly in the interior volume, the amount of cooling required shows little dependence on the efficiency of radiative heat transfer between interior surfaces. There is however a slight decrease in cooling saving going from 0 to 0.5 ACH as moderate ventilation will compensate for some solar gain. At and above 1 ACH there is a slight increase in cooling saving over the year. The reduction in total energy consumption by the use of high thermal reflectivity interior surfaces of the cabin increases with higher ventilation rate due to the increase of heating savings with higher ventilation rates, even though the cooling saving remain largely unaffected according to Fig. 9. The higher solar reflectivity on exterior surfaces contributes to less cooling demand. However, the amount of cooling savings in the case of exterior reflective coatings reduces as the ventilation rate increases as shown in Fig. 10. This may again be due to the fact that by increasing the ventilation rate, the cabin cools down by incoming outdoor air and that will compensate a part of cooling load demanded from the auxiliary unit. Fig. 10 shows that the significance of the heating penalty is reduced as the ventilation rate increases. The exterior reflective coatings are the main source of the heating penalty. On the other hand, interior reflective coating increases the heating saving with ventilation as discussed above, in the case of only reflective interior. The combined effect of interior and exterior reflective coatings leads in this case to a heating penalty with low ventilation rate, and a heating saving with high ventilation rates. With ventilation rate above
Fig. 9. Saving vs. outdoor air ventilation, comparing interior reflective cabin with normal cabin.
Fig. 10. Saving vs. outdoor air ventilation, comparing both-side reflective cabin with normal cabin.
1.5 ACH the effect of higher thermal reflectivity of the interior surfaces is in this case larger than the heating penalty due to high TSR exterior on a yearly basis, by comparing heating savings of interior reflective cabin (Fig. 9) and both-side reflective cabin (Fig. 10). 5.6. Other climates The response of the model in three locations with different climate is illustrated in Fig. 11 showing separately the heating, cooling and total loads showing the result for cabins with normal coating and interior reflective coatings. Simulations are based on the original conditions marked with in Fig. 5 and three different climates. In warmer climate as in Riyadh (latitude 22N), any increase in TSR value of the exterior coating reduces the thermal load significantly and the heating penalty becomes less important. Hence, the overall thermal performance of the cabin increases with high TSR coating on the exterior. There is an opposite trend in colder climate like Stockholm (latitude 59N) were the heating load and heating penalty by use of very high TSR coatings may be larger than the cooling saving over the year. Regarding milder climate like Paris (latitude 48N), both heating and cooling demands lies between extreme values of Riyadh and Stockholm, indicating a possible existence of optimum optical properties for the coating where the total thermal load is minimal (e.g. with exterior TSR of 0.3 and interior reflective coating). This optimal point depends apparently to individual case with considered simulation conditions, in this case without internal heat load and ventilation. According to Fig. 11 and under the given conditions, reflective interior has almost negligible impact in warmer climate like Riyadh given that the ideal cooler has a zero radiation fraction and the assumption of a well mixed interior air volume is valid, where as in colder climate reflective interior appears to improve the overall thermal performance of the building by improving the efficiency of radiation heating. Similar climate impact analyzing simulations are made on a simple model of a 113 m2 retail building (scale three of the cabins) with the different exterior and interior coating combinations and with an internal heat load of 40 W m2 (lighting 25 W m2, equipment 10 W m2 and people 5 W m2) and 1.5 ACH ventilation with constant setpoints temperature of 20 ± 1 °C in Stockholm, Paris and Riyadh climates. The net heating and cooling in these cases are shown in Table 3 for three separate seasons during the year together with the total annual saving in comparison with the normal coatings cases.
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Fig. 11. Thermal load of the test cabins vs. exterior coating TSR, with and without reflective interior in different climates (R represents cabin with reflective interior coating and N stands for cabin with normal interior coating).
Table 3 Net heating and cooling in a simulated retail house during different seasons at three locations related to cold, mild and warm climates. January–April Net heating (kW h) Stockholm
Paris
Riyadh
a
May–August Net cooling (kW h)
Net heating (kW h)
Net cooling (kW h)
September–December
Annual savingsa
Net heating (kW h)
Total heating and cooling (kW h per year)
Net cooling (kW h)
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
24548 21433
10 13
1300 891
2089 2376
13379 11201
145 173
0 5384
24959
3
1397
1813
13556
118
375
21798
7
964
2097
11344
145
5116
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
10431 8537
154 186
216 116
3894 4285
6875 5555
764 900
0 2755
10703
123
229
3604
6989
690
4
8773
151
124
3996
5651
822
2817
Normal coatings Interior reflective coatings Exterior reflective coatings Both-side reflective coatings
285 163
4822 5167
0 0
14461 14871
206 130
7878 8258
0 937
293
4526
0
14121
216
7573
923
168
4875
0
14540
135
7960
26
Positive value for savings and negative for total loss (compared with normal coating).
It can be noted that under the conditions of the simulations in Stockholm climate there is a significant saving in net heating and a slight increase in cooling during the whole year from the use of interior reflective coatings. That is consistent with less absorption of the heat radiation from the internal load. The use of exterior reflective coating in this case results in a reduced net cooling but an even larger increase in the net heating during all three seasons. The total effect of the both-side reflective coatings is dominated by the reduction in net heating by the interior reflection of heat radiation. Using Paris climate the trends are similar. There is however a slightly larger saving in net cooling by the use of exterior reflective coating making the both-side reflective coatings combination the most favorable. Simulating Riyadh climate conditions results in a
significant increase in net cooling by the use of interior reflective coatings and a relatively moderate saving in net cooling by the use of exterior reflective coatings compared to the total amount of cooling needed. The latter finding is consistent with higher ambient exterior air temperatures and the ventilation contributing to the total cooling load.
6. Conclusions The current evaluation of the cabin models is focused on heating and cooling resolved annual energy usage. The effect of exterior and interior surface radiation properties under the different conditions on heating and cooling is shown to be significant in line with
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experimental results and readily explained by the concept inherent in the model. Dependent to the internal heat load, ventilation rate and climate the choice of reflective coating for the interior and exterior of building material varies. Generally, reflective coatings for the interior contribute to net heating savings where it may not necessarily contribute to net cooling saving with higher internal load therefore total savings can vary depending on the simulation case. Reflective coatings on the exterior, however, contribute to net cooing savings and inflict net heating loss (penalty). The total saving for such coatings can be seen in warmer climate and where there is a dominant cooling load, e.g. due to high internal load. Combination effect of both interior and exterior reflective coatings is very much dependent to individual case. In the given example of retail building, simulation show that where reflective interior are good solution for colder climates and reflective exterior best for warmer climate, the combination of both reflective interior and exterior results the best thermal performance for the milder climate. The remarkably small reduction in air cooling by use of highly reflective interior in warmer climate is arguably related to the well mixed assumption. In the earlier work [12] it was shown that there is a measurable increase in the vertical air temperature gradient and a more stable interior air stratification as well as a measurable cooling saving with reflective interior surfaces. Exterior reflective coatings contributes to less cooling demand, while introducing an inevitable heating penalty during the colder seasons. Adding reflective interior to existing reflective exterior coating will both alleviate this heating penalty and contribute to cooling savings. However, as the interior reflective coating acts like a thermal radiative mirror, additional care must be taken when redundant internal load with significant radiation fraction needs to be cooled down. Acknowledgement Financial support from Knowledge Foundation is acknowledged. References [1] 2010 Building Energy Data Book. US Department of Energy – Energy Efficiency and Renewable Energy; March 2011. [2] Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings. Official Journal of the European Union. 153 (13) 2010.
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