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Trombe wall management in summer conditions: An experimental study
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Solar Energy 86 (2012) 2839–2851 www.elsevier.com/locate/solener
Trombe wall management in summer conditions: An experimental study Francesca Stazi a,⇑, Alessio Mastrucci a, Costanzo di Perna b a
Dipartimento di Ingegneria Civile, Edile e Architettura (DICEA), Universita` Politecnica delle Marche, Via Brecce, Bianche, 60131 Ancona, Italy b Dipartimento di Ingegneria Industriale e Scienze Matematiche, Universita` Politecnica delle Marche, Via Brecce, Bianche, 60131 Ancona, Italy Received 30 December 2011; received in revised form 27 April 2012; accepted 24 June 2012 Available online 24 July 2012 Communicated by: Associate Editor Ursula Eicker
Abstract The application of Trombe walls in temperate climates is problematic due to undesired heat gains and overheating phenomena in summer. A proper shading and ventilation of this system can reduce such drawbacks, but the impact of these strategies on the wall’s thermal parameters is yet not widely investigated in quantitative terms. This paper presents an experimental study on the thermal behavior of Trombe walls in summer under Mediterranean climate conditions. The aim of the study is to determine experimentally the thermal parameters of a Trombe wall in summer conditions through the changing of shading, ventilation and operational conditions. In order to do that a series of experimental campaigns were carried out on a case study. A detailed simultaneous monitoring of two Trombe walls made it possible to compare the thermal behavior by varying the screening, ventilation and internal gains conditions. Furthermore, monitoring of indoor thermal comfort conditions and energy simulation using a model in dynamic state were carried out. The results showed that shading, ventilation and occupancy conditions affect significantly the thermal parameters of Trombe wall in summer: screening with roller shutters determines a decrease in internal surface temperature of the wall of 1.4 °C and a decrease in daily heat gains of about 0.5 MJ/m2; the combined use of overhangs, roller shutters and cross ventilation for the Trombe wall can assure a satisfactory thermal comfort level in summer and a reduction of the cooling energy needs respectively of 72.9% and 63.0% for a dwelling with low or highly insulated building envelope in comparison with the case of an unvented Trombe wall without solar protections. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Trombe walls; Experimental study; Shading devices; Natural ventilation; Adaptive thermal comfort
1. Introduction The increase of cooling consumptions of buildings is still an unsolved problem in Mediterranean climates. Current European energy regulations deal mainly with heating energy needs and the optimization of cooling energy performances is not focused properly. As a result, buildings are often characterized by high cooling demand and low indoor thermal comfort in summer. Development and application of passive cooling techniques is encouraged ⇑ Corresponding author. Tel.: +39 071 2204783, mobile: +39 328 3098217; fax: +39 071 2204378. E-mail address: [email protected] (F. Stazi).
0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.06.025
by the European Directive, 2010/31/EU in order to avoid overheating in buildings. The use of passive solar systems for cooling is one of the best solutions to increase summer energy performances of the building. However there are still open problems regarding their application and design for summer. This study focuses on the thermal behavior of Trombe walls in summer and their contribution to indoor thermal comfort and energy performances of residential buildings. Classical Trombe wall is a passive solar system made up of a south-facing massive wall painted black on the external surface, an air layer and glazing on the exterior. The wall is equipped with vents at the top and at the bottom for the air thermo-circulation between the air gap and the
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indoor environment. Dampers on the external side can be opened for summer cross ventilation, using the system as a solar chimney. Shading devices such as overhangs and roller shutters provide solar radiation control. The system collects and stores solar energy by the massive wall. Heat exchange with the indoor environment is partly by transmission through the wall and partly by ventilation through the vents. Trombe wall has been widely studied regarding winter behavior, since this system was originally conceived for cold climates. In summer this system can be the cause of undesired energy transfers and overheating phenomena especially in well insulated buildings (Stazi et al., 2011). Only few studies focus on summer behavior (Fig. 1). Actions to improve summer behavior can be grouped into three categories: ventilation, shadings and insulation. Ventilation of Trombe wall for summer cooling was studied by Gan (1998) with a CFD numerical analysis. It was found that the ventilation rate induced by the buoyancy effect increases with the wall temperature, solar heat gain, wall height, thickness and insulation, distance between wall and glazing. Few experimental studies consider summer ventilation of Trombe walls (Ghrab-Morcos et al., 1993). Solar shading of Trombe wall in summer is an action recommended by several authors (Tasdemiroglu et al., 1993) but only a few determined its influence on Trombe wall thermal behavior. Different types of shading are proposed in literature such as overhangs, shutters and blinds. The behavior of a solar wall screened by overhangs was studied in hot climate conditions (Torcellini and Pless, 2004). The experience proved that solar walls impose an additional cooling load on the building even if the system is screened by overhangs. Jaber and Ajib (2011) recommend roller shutters and insulation curtains between glass and masonry wall layer for the optimum design of Trombe wall system in a Mediterranean region. Blasco Lucas et al. (2000) experimented several passive systems, including a not ventilated solar wall, in comparison with reference systems. In summer the solar wall was screened by a PVC rolling curtain during the daytime and determined small heat gains over the traditional system. Several ways to prevent overheating with Trombe walls were compared by Ghrab-Morcos et al. (1993) under hot climate conditions
in Tunis. It was found that Trombe wall screening is more efficient than using Trombe wall as a solar chimney. It was also shown that a very acceptable comfort level can be reached with the appropriate actions. The effect of a low emissivity shading device in the air gap was studied only in winter condition (Chen et al., 2006). Some other actions to prevent overheating are suggested in literature: insulating the internal surface of Trombe wall and double glazing (Gan, 1998) or using a more complex design of the system known as composite Trombe wall (Zalewski et al., 1997). The aim of this study is to estimate the thermal parameters of Trombe wall in summer conditions, in a Mediterranean climate, under different shading conditions (overhangs and roller shutters) also at the varying of internal loads and ventilation conditions. Summer thermal comfort conditions and cooling energy needs are also investigated. The research consisted of a series of experimental activities on a case study and parametric analysis using a virtual energy model in dynamic state calibrated with experimental data. 2. Methodology Monitoring campaigns were carried out for several years on a case study to investigate summer thermal behavior of Trombe walls. The research included the following phases: – Detailed monitoring of Trombe walls’ thermal parameters to assess the effect of shading, ventilation and room occupation on the behavior of the system. – Thermal monitoring of the indoor environment adjacent to the Trombe walls to evaluate thermal comfort with the system in optimal configuration: screened and ventilated. – Calculation of the cooling energy need at the varying of shading and ventilation condition, using an energy model in dynamic state. 2.1. Description of the case study The case study is a residential building in Ancona, central Italy, and it was built in 1983 as a prototype to test
Fig. 1. Solar walls proposed for summer: (a) not ventilated solar wall with solar screening (Blasco Lucas et al., 2000 [1]); (b) Trombe wall for summer cooling (Gan, 1998, [5]); (c) Trombe wall with cross ventilation and overhangs (Ghrab-Morcos et al., 1993); (d) natural ventilation by metallic solar wall (Hirunlabh et al., 1999 [7]); (e) Trombe wall combined with a roof duct for evaporative cooling (Raman et al., 2001[10]).
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many passive solar systems. The prototype includes nine flats on three floors, each equipped with a different passive solar system on the south-facing wall. The house has a compact shape and it is oriented along the E–W axis in order to maximize solar supply. The research focused on the accommodation with Trombe walls (Fig. 2). The system is made up of a 40 cm concrete wall painted black, a 10 cm air layer and glazing on the exterior. Adjustable vents are placed on the top and on the bottom of the wall to activate thermo-circulation through the air gap. Shading devices are of two types: horizontal overhangs and PVC roller shutters. Horizontal overhangs projects of 1.0 m out of the facßade. The roller shutters and their box are placed on the external side of the wall to minimize heat losses. The belt and slides were placed inside black vertical aluminium bands insulated by means of polyurethane. The thermal resistance of the roller shutter is 0.10 m2K/W and it can shade completely the Trombe wall. Dampers for external ventilation were designed as longitudinal bands at the top of the exterior glazing system. Cross ventilation operation mode shown in Fig. 2 is considered in this study. 2.2. Experimental measuring procedure Experimental data reported in this paper refer to two experimental campaigns in different years. Representative periods were selected from the data collected. During the summer campaign 2011 Trombe walls were monitored to study the influence of screening and occupation. Data selected for this campaign refer to the period
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17th June to 3rd July 2011. Two Trombe walls were monitored simultaneously varying the shading conditions (Fig. 2). Only one of the two walls was screened from direct solar radiation closing completely the external roller shutters. Cross ventilation was activated for both Trombe walls keeping the wall bottom vent and the external damper open, while the wall top vent was sealed up and closed. The windows facing north were kept open in order to have cross ventilation in the house. Indoor environmental conditions changed during the experiments. In the first part (17th June–24th June 2011) the house was not occupied, in the second part (24th June–3rd July 2011) the residents occupied the house. Summer campaign 2005 was dedicated to study the thermal comfort of the accommodation using screened Trombe walls. The period selected for this paper was 30th July to 4th August 2005, characterized by extreme conditions (highest outside temperatures). Thermal comfort analysis was carried out using adaptive method according to EN 15251 (CEN, 2007) assuming category II. Measurements were carried out according to ISO 7726 (ISO, 1998) using data loggers and different types of probes. The following investigations were carried out for both the experimental campaigns: – Outdoor local climate conditions. An external weather station with hydro-thermal probe, wind speed and direction probe, solar radiation probes were used. Outdoor dry bulb temperature, relative humidity, wind speed and direction, solar global horizontal radiation were measured.
Fig. 2. Monitoring scheme: plan of the accommodation (a), external elevation of south facade (b). Internal elevation (c), side section of Trombe wall with roller shutters open (d) and closed (e).
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– Indoor environmental conditions. An indoor microclimate station including hydro-thermal probe was used to measure wet and dry bulb temperature and relative humidity. A black-globe temperature probe was used in addition to the previous probes for 2005 monitoring. – Thermal survey of Trombe walls. A set of thermo-resistances was used to measure internal and external surface temperatures of the wall. In addition to these investigations, summer campaign 2011 included a detailed thermal survey of the two Trombe walls monitored simultaneously. The following probes were used: – Flat thermo-resistances, tolerance according to IEC 751, to measure wall internal and external surface temperatures, glass surface temperature, rolling shutter external surface temperatures (when roller shutter closed). – Thermo-resistances, tolerance according to IEC 751, to measure air temperatures in the middle of the gap and near to the bottom vent. – Heat flux meters, tolerance according to ISO 8302, with a sensitivity of 50 lV/Wm2 to measure wall heat flux densities on the internal side. – Hot-sphere anemometers to measure air velocity in the cavity. – Pyranometer to measure vertical global solar radiation incident on the wall. Temperature probes exposed to direct solar radiation were screened from solar radiation to avoid values alteration. A thermo-graphic survey was carried out using an infrared thermo-camera in order to study temperature distribution on the external surfaces of the two Trombe walls.The accuracy provided by manufacturer for the probes used is indicated below:
– Pyranometers: 0.5% m.v. + 5 W/m2. – Wind direction probe: 5°. – Wind speed probe: 2.5% m.v./reading. Accuracy of data logger is 3% m.v./reading. 2.3. Analytical activities Numerical simulations were performed in dynamic state using software EnergyPlus. Trombe walls were modelled in EnergyPlus using the algorithm “Trombe Wall” validated by Ellis (2003) for unvented Trombe walls. The model was built up starting from the case study and then changing parameters to extend the results. Trombe walls were simulated changing shading conditions (unscreened; overhangs; roller shutters; overhangs and roller shutters) and ventilation condition (unvented; vented with cross ventilation). In order to introduce realistic input of ventilation rates due to Trombe walls in the model, experimental data about air velocity were used as input. Furthermore, the insulation level of every component of the building envelope, except the Trombe walls on the south facßade, was varied according to several standards: as built envelope; conventional envelope complying with current Italian energy regulations; super-insulated envelope characterized by low transmittance values typical of passive houses. For each case, cooling energy needs of the accommodation were calculated running simulations for the whole cooling season. This comparison was made to analyse the effect of introducing a Trombe wall system in buildings with different insulation level. More detailed description of the model, input data and calibration by comparison of measured values and calculations are reported in (Stazi et al., 2011). 3. Experimental results and discussions
– Thermoresistances (surface and air temperatures): 0.15 °C (at 0 °C). – Heat flux meters: 5% m.v./reading. – Black globe temperature probe: 0.15 °C (at 0 °C). – Hydro thermal probe: temperature 0.15 °C (at 0 °C); UR 2% (5–95%, 23 °C). – Hot sphere anemometer: air velocity 0.03 m/s + 5% m.v./reading.
3.1. Thermal parameters of Trombe wall in summer conditions Results showed in this paragraph refer to a period of 15 days selected as representative from 2011 monitoring campaign. Thermal behaviors of the screened and unscreened Trombe walls adjacent to the living room were
Fig. 3. Climatic data: air temperature and global solar radiation.
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compared for the whole period. As inhabitants were present in the house only during the second part of the experiments, influence of occupation was evaluated by comparison of two days with similar weather conditions and different occupation. The weather was characterized mostly by sunny conditions during the experiments, except cloudy conditions on 30th June (Fig. 3). Wind conditions are shown in Fig. 4.
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3.1.1. Surface temperatures Results of thermal monitoring for the unscreened and screened Trombe walls are respectively in Figs. 5 and 6 for the period 17th–23rd July (not occupied house). Internal and external surface of the Trombe walls (Tw int and Tw ext), glass surface temperature (Tglass), shutter surface temperature when present (Tshutter) are shown against room air temperature (Troom), outdoor air temperature
Fig. 4. Climatic data: wind speed.
Fig. 5. Internal and external surface temperatures and glazing temperature of the screened Trombe walls against weather data.
Fig. 6. Internal and external surface temperatures, glazing and shutter temperature of the unscreened Trombe walls against weather data.
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Fig. 7. Internal and external surface temperatures of Trombe walls under different screening conditions.
(Tout) and vertical global solar radiation incident on Trombe wall. Trombe walls were protected from direct solar radiation for the most part of the day by the horizontal overhangs. As a consequence solar radiation incident on the wall is relatively low. In Fig. 5 can be noted that external surface temperature of the unscreened wall rises over 30 °C after the solar incident peak radiation. Internal surface temperature average is 25.8 °C, and it is always higher than room temperature Troom (average 24.5 °C). Fig. 6 shows the results for the screened Trombe wall. Roller shutter external surface temperature is close to outside air temperature. The presence of screening determines glass maximum temperatures of 28–29 °C and external surface temperatures of 24–26 °C. Internal surface temperature of the wall average is 24.5 °C and it is very close to room air temperature. Comparison of internal and external surface temperatures of the two Trombe walls are shown in Fig. 7. Screening solar radiation with roller shutters determines a decrease on surface temperatures of the Trombe wall. On sunny days the external surface temperature peak of the unscreened Trombe wall is about 5–6 °C higher than the screened one. During night time the temperature difference is about 1.5 °C. The internal surface temperature difference between the screened and unscreened walls remains constant around 1.4 °C during the period considered. Internal surface temperature of the screened wall is near to the room air temperature, while internal surface temperature of the unscreened wall is higher all time. One representative day was selected to study temperatures peaks and time lags on the different layers of the system (Fig. 8). Glass surface reach the highest peak of 32.2 °C at 2:30 PM without screening and 28.2 °C 3 h later with screening. External surface temperatures reach daytime peak earlier in the case of unscreened wall (31.3 °C at 5:20 PM) than in the case of screened wall (26.0 °C at 8:00 PM).
Fig. 8. Surface peak temperatures of the two Trombe walls.
Internal surface temperature peak is 26.2 °C without screening and 24.0 °C with shading. Time lag of the massive wall was estimated 9 h for the unscreened Trombe wall and 10 h for the screened Trombe wall. 3.1.2. Transmission heat fluxes Internal heat flux densities (Fint) of the screened and unscreened Trombe walls are shown in Fig. 9. As internal surface temperature of the unscreened Trombe wall is always higher than the screened one, heat fluxes are higher too. Internal heat flux of the unscreened wall is always directed from the wall to the room and higher during night time. Heat flux is lower in the case of screened Trombe wall and directed from the wall to the room during morning and nighttime mainly, from the room to the wall in the afternoon mainly. Daily heat gains and losses of the two Trombe walls are shown in Fig. 10. The unscreened Trombe wall is characterized by daily heat gains between 0.58 MJ/m2 and 0.75 MJ/m2. Daily heat gains of the screened Trombe wall are between 0.03 MJ/m2 and 0.16 MJ/m2, up to 18 times lower than the unscreened wall’s values. Daily heat losses are equal to zero for the unscreened Trombe wall, while they are around 0.01–0.02 MJ/m2 for the screened wall. Results showed that the use of roller shutter has a evident impact in reducing heat gains of Trombe wall in summer.
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Fig. 9. Internal heat flux densities of Trombe walls under different screening conditions.
Fig. 10. Daily heat gains and losses of Trombe walls under different screening conditions.
3.1.3. Air layer temperatures and air velocity Air temperature in the gap (Tair gap) varies significantly according to the screening conditions (Fig. 11). Daytime air temperature peak in the middle of air gap is among 32–34 °C without screening while 27–29 °C with screening. Inlet air temperature (Tair inlet) is higher for the unscreened Trombe wall. Thermal gradient between inlet and middle of the gap in correspondence to the daytime peak is about 4–5 °C with open shutter and less than 1 °C with closed
shutter. Thermal gradient in correspondence to the lowest peak is near to zero in both cases. Air velocity in the gap was compared in the case of unscreened and screened Trombe wall (Fig. 12). Data collected demonstrated that wind condition has an important influence on air velocity in the gap. Air in the gap was characterized by relatively low speeds except on the days when wind speed was high. Two days characterized by different wind conditions were compared (19th–20th June). On the
Fig. 11. Air gap and bottom vent temperatures of Trombe walls under different screening conditions.
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Fig. 12. Air velocity of Trombe wall’s gap under different screening conditions.
windy day average air velocity in the gap is similar in both cases, regardless screening conditions, due to the influence of wind (0.12–0.13 m/s). On the day with low wind conditions, air velocities are lower (0.06 m/s without screening and 0.02 m/s with screening). Average air velocity for the
Fig. 13. Weather comparison between 21st June (unoccupied house) and 27th June (occupied house).
period (17th–23rd June) was 0.06 m/s for the unscreened Trombe wall and 0.04 m/s for the screened one. Screening influences air velocity in the gap reducing stack effect and flow rates in the gap. The effect is higher with low wind conditions. 3.1.4. Thermal parameters of Trombe wall in real occupancy conditions Two days with similar weather conditions (Fig. 13) but different occupation were selected to compare the thermal behavior of Trombe wall (21st June: unoccupied – 27th June: occupied). Temperatures and heat fluxes of Trombe walls were compared for the two days selected with and without occupants (Fig. 14). Air temperature is higher when the occupants are inside the room (9:00–20:00), due to the internal heat gains. The daily average difference in room air temperature is around 1.0 °C. Trombe wall inside surface temperature average difference between the occupied and not occupied day is 0.7 °C for the unscreened Trombe wall and 0.8 °C for the screened Trombe wall. As the tem-
Fig. 14. Influence of occupation on room temperatures, internal surface temperatures and heat fluxes densities of the unscreened and screened Trombe walls (in grey occupied hours).
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Fig. 15. Influence of occupation on air temperatures in the gap, inlet air temperatures and air velocities in the gap of the unscreened and screened Trombe walls.
perature difference between wall surface and room air is lower when the occupants are present, heat flux density is lower in this case. It follows that transmission heat gains from the wall to the room are reduced under the condition of occupied house. The behavior analysis during the day shows that the Trombe wall without screening releases heat to the room for the whole period in both cases. The screened Trombe wall absorbs heat when the room temperature is higher than surface temperature (afternoon) and release it when the room temperature is lower (morning and night time), giving an important contribution in maintaining the room temperature constant. Air temperature in the middle of the cavity, inlet air temperature and air velocity in the cavity are shown in
Fig. 15. Air velocity in the gap is higher during the day with occupation, due to the higher ventilation rates in the house when the occupants are in. In the case of unscreened Trombe wall the average air velocity is 0.06 m/s with occupation and 0.04 m/s without occupation. In the case of screened Trombe wall the average air velocities are very low due to the minor efficiency of stack effect. 3.1.5. Results of thermo-graphic survey Temperature distributions on the two Trombe walls external surfaces were investigated through thermo-graphic survey (Fig. 16 and Table 1). In order to obtain reliable data was necessary to open the Trombe wall glazing and external shutters of the screened wall. The unscreened
Fig. 16. Thermo-graphic image of the unscreened and screened Trombe walls.
Table 1 Results of thermo-graphic survey. Type of wall
Avg temp (°C) Min temp (°C) Max temp (°C) Difference temp (°C) (Max–min temp)
Line 1
Line 2
Line 3
Unscreened Trombe
Screened Trombe
Unscreened Trombe
Screened Trombe
Unscreened Trombe
Screened Trombe
28.2 27.8 29.1 1.3
23.6 23.4 23.8 0.4
27.3 26.9 27.9 1.0
23.5 23.3 23.6 0.3
28.3 27.6 28.9 1.5
23.6 23.4 23.8 0.4
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Trombe wall surface temperatures vary in the range of 1.5 °C both in vertical and horizontal directions. The highest surface temperatures were observed at mid-height in correspondence of the air vents. The screened Trombe wall’s temperature distribution is almost uniform in both horizontal and vertical direction, with temperature differences lower than 0.5 °C. 3.2. Results of thermal comfort analysis Thermal comfort analysis refers to a period of eight days selected from summer 2005 monitoring campaign. The period selected was characterized by summer extreme conditions with maximum temperatures over 35 °C (Fig. 17). The occupants were free to adapt themselves opening windows and closing window screening. Trombe internal surface average temperature was 27.4 °C, slightly higher than room air temperature (Fig. 18). Internal and external temperatures of the aluminium frame were particularly high and highlighted the incidence of the thermal bridge. Results of thermal comfort analysis with adaptive method (Fig. 19) results show that operative temperature in the room is within the comfort range for the whole period. The results demonstrate that Trombe wall, screened from solar radiation, can assure a satisfactory comfort level even in summer conditions.
3.3. Results of cooling energy need calculation Cooling energy need of the dwelling was calculated at the changing of shading and ventilation condition of the Trombe walls. The analysis was extended to several types of building envelopes, insulated according to different standards. 3.3.1. Analysis of unvented Trombe walls The influence of several types of shading on cooling energy need was analyzed in the case of unvented Trombe walls (Table 2): unscreened (case A), overhangs (case B), roller shutters (case C) and the combination of both overhangs and roller shutters (case D). Results for unvented Trombe walls demonstrated that the use of overhangs (case B) determine a reduction in cooling energy needs among 29.7% and 44.4% in comparison with the unscreened case depending on the insulation level of the building envelope. Roller shutters (case C) are more effective than overhangs in reducing cooling loads (among 58.0% and 63.2%), however the best effect is obtained combining both types of shading (case D) with reductions among 59.7% and 72.6%. 3.3.2. Analysis of vented Trombe walls The effect of cross ventilation in combination with solar shading was also studied (Table 3). First, comparison
Fig. 17. Climatic data.
Fig. 18. Room dry bulb and black globe temperatures, internal and external surface temperatures of Trombe wall and aluminium.
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Fig. 19. Results of thermal comfort analysis with adaptive method.
Table 2 Seasonal cooling energy needs of the accommodation with unvented Trombe walls at the changing of type of screening for several types of building envelopes. Insulation level of the building envelope
Type of shading for unvented Trombe walls A Unscreened
B Overhangs
C Roller shutters
D Overhangs and roller shutters
1. As built envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
16.52 –
9.19 44.4%
6.08 63.2%
4.53 72.6%
2. Conventional envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
29.40 –
20.20 31.3%
12.28 58.2%
11.69 60.2%
3. Super-insulated envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
33.15 –
23.31 29.7%
13.92 58.0%
13.37 59.7%
between the case of unvented and vented Trombe walls was made in terms of cooling energy need using equal shading conditions for every insulation level of the building envelope. This comparison highlights the benefit of activating ventilation of Trombe walls. Results showed that activating the ventilation of Trombe walls always gives a further contribute in reducing cooling energy needs in comparison with the unvented Trombe wall. The use of cross ventilation for the case of Trombe walls without solar screening (Table 3 – case A), determines a reduction in cooling energy needs among 13.5% and 15.1% depending on the type of building envelope. This confirms that the single effect of ventilation is less effective than the single effect of every type of solar screening. The reduction in cooling energy needs, due to the ventilation strategy, decreases at the increase of efficiency of shading system. For instance, in the as built envelope case, the reduction is 9.5% using overhangs (case B), 8.2% using roller shutters (case C) and 1.3% using a combination of overhangs and roller shutters (case D). Second, the difference between each case and the case of unscreened – unvented Trombe wall, in terms of cooling energy need, highlights the overall benefit in using ventilation in combination with shading for every insulation level
of the building envelope. The best result is obtained combining overhangs, roller shutters and cross ventilation (Table 3 – case D). The reduction in cooling energy needs using such a set-up is 72.9% in the as built case, 63.4% in the conventional case and 63.0% in the super-insulated case. 3.3.3. Influence of insulation level of the building envelope on the results The influence of the insulation level of the building envelope on the results was analyzed in terms of effectiveness of shading and ventilation of Trombe walls in reducing cooling energy needs. Results showed that the reduction in cooling energy needs determined by Trombe wall’s shading is more effective with a lower level insulation of the other elements of the building envelope. For instance, considering unvented Trombe wall with both overhangs and roller shutters (Table 2 – case D), the reduction in cooling energy needs in comparison with the unscreened case is 72% for the as built envelope, 60.2% for the conventional envelope and 59.7% for the super-insulated envelope. The same trend can be outlined for the vented Trombe walls cases: the reduction in cooling energy needs in comparison with
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Table 3 Seasonal cooling energy needs of the accommodation with vented Trombe walls at the changing of type of screening for several types of building envelopes. Insulation level of the building envelope
1. As built envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case 2. Conventional envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case 3. Super-insulated envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case
Type of shading for vented Trombe walls A Unscreened
B Overhangs
C Roller shutters
D Overhangs and roller shutters
14.29 13.5%
8.32 9.5%
5.81 8.2%
4.48 1.3%
13.5% 25.03 14.9% 14.9% 28.13 15.1% 15.1%
the unscreened-vented case using overhangs and roller shutters (Table 3 – case D) is 68.7% for the as built envelope, 57.0% for the conventional envelope and 56.4% for the super-insulated envelope. On the contrary, the reduction in cooling energy needs due to cross ventilation of Trombe wall is more effective increasing the insulation level of the building envelope. Results in Table 3 results show that the difference between vented and unvented case, using the same type of shading, is higher at the increase of the insulation level. For instance, using both overhangs and roller shutters (Table 3 – case D), the difference is 1.3% for the as built envelope, 8.0% for the conventional envelope and 8.3% for the super-insulated envelope. 4. Conclusions An experimental study on the thermal behavior of Trombe walls in summer under a Mediterranean climate was carried out in a residential building. The effect of screening devices, ventilation and occupancy was studied in detail. It was found that roller shutters have a relevant influence in reducing surface temperatures of Trombe wall of 1.4 °C and daily heat gains toward the room of about 0.5 MJ/m2. Air velocity in the cavity is influenced by both wind conditions and temperatures in the air gap. Air velocity in the gap was found to be low, especially in the case of Trombe wall screened by roller shutters. Thermo-graphic survey was useful to prove that Trombe wall external surface temperature distribution is rather uniform when screened from direct solar radiation. The analysis of Trombe walls in real use conditions showed that the presence of occupants in the house determines an increase in room air temperature, but also higher
49.7% 17.59 12.9% 40.2% 20.18 13.4% 39.1%
64.8% 11.28 8.2% 61.6% 12.66 9.0% 61.8%
72.9% 10.76 8.0% 63.4% 12.26 8.3% 63.0%
ventilation rates due to window opening. In such conditions, heat fluxes from the wall to the room are reduced while air velocity in the gap are higher. Thermal comfort analysis for a selected period characterized by severe summer conditions confirmed that comfort level was satisfactory for the house. Energy simulation in dynamic state showed that: the use of overhangs and roller shutters is more effective than cross ventilation in saving energy for the cooling of a dwelling with Trombe walls; enhancing the insulation level of the building envelope determines decreased efficiency for solar shading and increased efficiency for ventilation of Trombe wall; regardless of the insulation level of the building envelope, the best energy performance was obtained combining overhangs, roller shutters and cross ventilation, with a reduction in cooling energy need up to 72.9% compared to the case of an unvented Trombe wall without solar protections. References Blasco Lucas, I., Hoese´, L., Pontoriero, D., 2000. Experimental study of passive systems thermal performance. Renew. Energy 19, 39–45. CEN, 2007. EN 15251:2007. Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. Chen, B., Chen, X., Ding, Y.H., Jia, X., 2006. Shading effects on the thermal performance of the Trombe wall air gap: an experimental study in Dalian. Renew. Energy 31, 1961–1971. 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. Ellis, P.G., 2003. Development and validation of the unvented Trombe wall model in EnergyPlus. Master’s thesis, University of Illinois at Urbana–Champaign. Gan, G., 1998. A parametric study of Trombe walls for passive cooling of buildings. Energy Build. 27, 37–43.
F. Stazi et al. / Solar Energy 86 (2012) 2839–2851 Ghrab-Morcos, N., Bouden, C., Franchisseur, R., 1993. Overheating caused by passive solar elements in Tunis. Effectiveness of some way to prevent it. Renew. Energy 3 (6/7), 801–811. Hirunlabh, J., Kongduang, W., Namprakai, P., Khedari, J., 1999. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renew. Energy 18, 109–119. ISO, 1998. ISO 7726:1998 ergonomics of the thermal environment – instruments for measuring physical quantities. Jaber, S., Ajib, S., 2011. Optimum design of Trombe wall system in Mediterranean region. Sol. Energy 85, 1891–1898. Raman, P., Mande, S., Kishore, V.V.N., 2001. A passive solar system for thermal comfort conditioning of buildings in composite climates. Sol. Energy 70 (4), 319–329.
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Stazi, F., Mastrucci, A., Di Perna, C., 2011. The behavior of solar walls in residential buildings with different insulation levels: an experimental and numerical study. Energy Build. http://dx.doi.org/10.1016/j. enbuild.2011.11.039. Tasdemiroglu, E., Berjano, F.R., Tinaut, D., 1993. The performance results of Trombe-wall passive systems under Aegean sea climatic conditions. Sol. Energy 30 (2), 181–189. Torcellini, P., Pless, S., 2004. Trombe walls in low-energy buildings: practical experiences. Technical report, NREL Report No. CP-55036277, National Renewable Energy Laboratory. Zalewski, L., Chantant, M., Lassue, S., Duthoit, B., 1997. Experimental thermal study of a solar wall of composite type. Energy Build. 25 (1), 7–18.
Solar Energy 86 (2012) 2839–2851 www.elsevier.com/locate/solener
Trombe wall management in summer conditions: An experimental study Francesca Stazi a,⇑, Alessio Mastrucci a, Costanzo di Perna b a
Dipartimento di Ingegneria Civile, Edile e Architettura (DICEA), Universita` Politecnica delle Marche, Via Brecce, Bianche, 60131 Ancona, Italy b Dipartimento di Ingegneria Industriale e Scienze Matematiche, Universita` Politecnica delle Marche, Via Brecce, Bianche, 60131 Ancona, Italy Received 30 December 2011; received in revised form 27 April 2012; accepted 24 June 2012 Available online 24 July 2012 Communicated by: Associate Editor Ursula Eicker
Abstract The application of Trombe walls in temperate climates is problematic due to undesired heat gains and overheating phenomena in summer. A proper shading and ventilation of this system can reduce such drawbacks, but the impact of these strategies on the wall’s thermal parameters is yet not widely investigated in quantitative terms. This paper presents an experimental study on the thermal behavior of Trombe walls in summer under Mediterranean climate conditions. The aim of the study is to determine experimentally the thermal parameters of a Trombe wall in summer conditions through the changing of shading, ventilation and operational conditions. In order to do that a series of experimental campaigns were carried out on a case study. A detailed simultaneous monitoring of two Trombe walls made it possible to compare the thermal behavior by varying the screening, ventilation and internal gains conditions. Furthermore, monitoring of indoor thermal comfort conditions and energy simulation using a model in dynamic state were carried out. The results showed that shading, ventilation and occupancy conditions affect significantly the thermal parameters of Trombe wall in summer: screening with roller shutters determines a decrease in internal surface temperature of the wall of 1.4 °C and a decrease in daily heat gains of about 0.5 MJ/m2; the combined use of overhangs, roller shutters and cross ventilation for the Trombe wall can assure a satisfactory thermal comfort level in summer and a reduction of the cooling energy needs respectively of 72.9% and 63.0% for a dwelling with low or highly insulated building envelope in comparison with the case of an unvented Trombe wall without solar protections. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Trombe walls; Experimental study; Shading devices; Natural ventilation; Adaptive thermal comfort
1. Introduction The increase of cooling consumptions of buildings is still an unsolved problem in Mediterranean climates. Current European energy regulations deal mainly with heating energy needs and the optimization of cooling energy performances is not focused properly. As a result, buildings are often characterized by high cooling demand and low indoor thermal comfort in summer. Development and application of passive cooling techniques is encouraged ⇑ Corresponding author. Tel.: +39 071 2204783, mobile: +39 328 3098217; fax: +39 071 2204378. E-mail address: [email protected] (F. Stazi).
0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.06.025
by the European Directive, 2010/31/EU in order to avoid overheating in buildings. The use of passive solar systems for cooling is one of the best solutions to increase summer energy performances of the building. However there are still open problems regarding their application and design for summer. This study focuses on the thermal behavior of Trombe walls in summer and their contribution to indoor thermal comfort and energy performances of residential buildings. Classical Trombe wall is a passive solar system made up of a south-facing massive wall painted black on the external surface, an air layer and glazing on the exterior. The wall is equipped with vents at the top and at the bottom for the air thermo-circulation between the air gap and the
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indoor environment. Dampers on the external side can be opened for summer cross ventilation, using the system as a solar chimney. Shading devices such as overhangs and roller shutters provide solar radiation control. The system collects and stores solar energy by the massive wall. Heat exchange with the indoor environment is partly by transmission through the wall and partly by ventilation through the vents. Trombe wall has been widely studied regarding winter behavior, since this system was originally conceived for cold climates. In summer this system can be the cause of undesired energy transfers and overheating phenomena especially in well insulated buildings (Stazi et al., 2011). Only few studies focus on summer behavior (Fig. 1). Actions to improve summer behavior can be grouped into three categories: ventilation, shadings and insulation. Ventilation of Trombe wall for summer cooling was studied by Gan (1998) with a CFD numerical analysis. It was found that the ventilation rate induced by the buoyancy effect increases with the wall temperature, solar heat gain, wall height, thickness and insulation, distance between wall and glazing. Few experimental studies consider summer ventilation of Trombe walls (Ghrab-Morcos et al., 1993). Solar shading of Trombe wall in summer is an action recommended by several authors (Tasdemiroglu et al., 1993) but only a few determined its influence on Trombe wall thermal behavior. Different types of shading are proposed in literature such as overhangs, shutters and blinds. The behavior of a solar wall screened by overhangs was studied in hot climate conditions (Torcellini and Pless, 2004). The experience proved that solar walls impose an additional cooling load on the building even if the system is screened by overhangs. Jaber and Ajib (2011) recommend roller shutters and insulation curtains between glass and masonry wall layer for the optimum design of Trombe wall system in a Mediterranean region. Blasco Lucas et al. (2000) experimented several passive systems, including a not ventilated solar wall, in comparison with reference systems. In summer the solar wall was screened by a PVC rolling curtain during the daytime and determined small heat gains over the traditional system. Several ways to prevent overheating with Trombe walls were compared by Ghrab-Morcos et al. (1993) under hot climate conditions
in Tunis. It was found that Trombe wall screening is more efficient than using Trombe wall as a solar chimney. It was also shown that a very acceptable comfort level can be reached with the appropriate actions. The effect of a low emissivity shading device in the air gap was studied only in winter condition (Chen et al., 2006). Some other actions to prevent overheating are suggested in literature: insulating the internal surface of Trombe wall and double glazing (Gan, 1998) or using a more complex design of the system known as composite Trombe wall (Zalewski et al., 1997). The aim of this study is to estimate the thermal parameters of Trombe wall in summer conditions, in a Mediterranean climate, under different shading conditions (overhangs and roller shutters) also at the varying of internal loads and ventilation conditions. Summer thermal comfort conditions and cooling energy needs are also investigated. The research consisted of a series of experimental activities on a case study and parametric analysis using a virtual energy model in dynamic state calibrated with experimental data. 2. Methodology Monitoring campaigns were carried out for several years on a case study to investigate summer thermal behavior of Trombe walls. The research included the following phases: – Detailed monitoring of Trombe walls’ thermal parameters to assess the effect of shading, ventilation and room occupation on the behavior of the system. – Thermal monitoring of the indoor environment adjacent to the Trombe walls to evaluate thermal comfort with the system in optimal configuration: screened and ventilated. – Calculation of the cooling energy need at the varying of shading and ventilation condition, using an energy model in dynamic state. 2.1. Description of the case study The case study is a residential building in Ancona, central Italy, and it was built in 1983 as a prototype to test
Fig. 1. Solar walls proposed for summer: (a) not ventilated solar wall with solar screening (Blasco Lucas et al., 2000 [1]); (b) Trombe wall for summer cooling (Gan, 1998, [5]); (c) Trombe wall with cross ventilation and overhangs (Ghrab-Morcos et al., 1993); (d) natural ventilation by metallic solar wall (Hirunlabh et al., 1999 [7]); (e) Trombe wall combined with a roof duct for evaporative cooling (Raman et al., 2001[10]).
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many passive solar systems. The prototype includes nine flats on three floors, each equipped with a different passive solar system on the south-facing wall. The house has a compact shape and it is oriented along the E–W axis in order to maximize solar supply. The research focused on the accommodation with Trombe walls (Fig. 2). The system is made up of a 40 cm concrete wall painted black, a 10 cm air layer and glazing on the exterior. Adjustable vents are placed on the top and on the bottom of the wall to activate thermo-circulation through the air gap. Shading devices are of two types: horizontal overhangs and PVC roller shutters. Horizontal overhangs projects of 1.0 m out of the facßade. The roller shutters and their box are placed on the external side of the wall to minimize heat losses. The belt and slides were placed inside black vertical aluminium bands insulated by means of polyurethane. The thermal resistance of the roller shutter is 0.10 m2K/W and it can shade completely the Trombe wall. Dampers for external ventilation were designed as longitudinal bands at the top of the exterior glazing system. Cross ventilation operation mode shown in Fig. 2 is considered in this study. 2.2. Experimental measuring procedure Experimental data reported in this paper refer to two experimental campaigns in different years. Representative periods were selected from the data collected. During the summer campaign 2011 Trombe walls were monitored to study the influence of screening and occupation. Data selected for this campaign refer to the period
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17th June to 3rd July 2011. Two Trombe walls were monitored simultaneously varying the shading conditions (Fig. 2). Only one of the two walls was screened from direct solar radiation closing completely the external roller shutters. Cross ventilation was activated for both Trombe walls keeping the wall bottom vent and the external damper open, while the wall top vent was sealed up and closed. The windows facing north were kept open in order to have cross ventilation in the house. Indoor environmental conditions changed during the experiments. In the first part (17th June–24th June 2011) the house was not occupied, in the second part (24th June–3rd July 2011) the residents occupied the house. Summer campaign 2005 was dedicated to study the thermal comfort of the accommodation using screened Trombe walls. The period selected for this paper was 30th July to 4th August 2005, characterized by extreme conditions (highest outside temperatures). Thermal comfort analysis was carried out using adaptive method according to EN 15251 (CEN, 2007) assuming category II. Measurements were carried out according to ISO 7726 (ISO, 1998) using data loggers and different types of probes. The following investigations were carried out for both the experimental campaigns: – Outdoor local climate conditions. An external weather station with hydro-thermal probe, wind speed and direction probe, solar radiation probes were used. Outdoor dry bulb temperature, relative humidity, wind speed and direction, solar global horizontal radiation were measured.
Fig. 2. Monitoring scheme: plan of the accommodation (a), external elevation of south facade (b). Internal elevation (c), side section of Trombe wall with roller shutters open (d) and closed (e).
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– Indoor environmental conditions. An indoor microclimate station including hydro-thermal probe was used to measure wet and dry bulb temperature and relative humidity. A black-globe temperature probe was used in addition to the previous probes for 2005 monitoring. – Thermal survey of Trombe walls. A set of thermo-resistances was used to measure internal and external surface temperatures of the wall. In addition to these investigations, summer campaign 2011 included a detailed thermal survey of the two Trombe walls monitored simultaneously. The following probes were used: – Flat thermo-resistances, tolerance according to IEC 751, to measure wall internal and external surface temperatures, glass surface temperature, rolling shutter external surface temperatures (when roller shutter closed). – Thermo-resistances, tolerance according to IEC 751, to measure air temperatures in the middle of the gap and near to the bottom vent. – Heat flux meters, tolerance according to ISO 8302, with a sensitivity of 50 lV/Wm2 to measure wall heat flux densities on the internal side. – Hot-sphere anemometers to measure air velocity in the cavity. – Pyranometer to measure vertical global solar radiation incident on the wall. Temperature probes exposed to direct solar radiation were screened from solar radiation to avoid values alteration. A thermo-graphic survey was carried out using an infrared thermo-camera in order to study temperature distribution on the external surfaces of the two Trombe walls.The accuracy provided by manufacturer for the probes used is indicated below:
– Pyranometers: 0.5% m.v. + 5 W/m2. – Wind direction probe: 5°. – Wind speed probe: 2.5% m.v./reading. Accuracy of data logger is 3% m.v./reading. 2.3. Analytical activities Numerical simulations were performed in dynamic state using software EnergyPlus. Trombe walls were modelled in EnergyPlus using the algorithm “Trombe Wall” validated by Ellis (2003) for unvented Trombe walls. The model was built up starting from the case study and then changing parameters to extend the results. Trombe walls were simulated changing shading conditions (unscreened; overhangs; roller shutters; overhangs and roller shutters) and ventilation condition (unvented; vented with cross ventilation). In order to introduce realistic input of ventilation rates due to Trombe walls in the model, experimental data about air velocity were used as input. Furthermore, the insulation level of every component of the building envelope, except the Trombe walls on the south facßade, was varied according to several standards: as built envelope; conventional envelope complying with current Italian energy regulations; super-insulated envelope characterized by low transmittance values typical of passive houses. For each case, cooling energy needs of the accommodation were calculated running simulations for the whole cooling season. This comparison was made to analyse the effect of introducing a Trombe wall system in buildings with different insulation level. More detailed description of the model, input data and calibration by comparison of measured values and calculations are reported in (Stazi et al., 2011). 3. Experimental results and discussions
– Thermoresistances (surface and air temperatures): 0.15 °C (at 0 °C). – Heat flux meters: 5% m.v./reading. – Black globe temperature probe: 0.15 °C (at 0 °C). – Hydro thermal probe: temperature 0.15 °C (at 0 °C); UR 2% (5–95%, 23 °C). – Hot sphere anemometer: air velocity 0.03 m/s + 5% m.v./reading.
3.1. Thermal parameters of Trombe wall in summer conditions Results showed in this paragraph refer to a period of 15 days selected as representative from 2011 monitoring campaign. Thermal behaviors of the screened and unscreened Trombe walls adjacent to the living room were
Fig. 3. Climatic data: air temperature and global solar radiation.
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compared for the whole period. As inhabitants were present in the house only during the second part of the experiments, influence of occupation was evaluated by comparison of two days with similar weather conditions and different occupation. The weather was characterized mostly by sunny conditions during the experiments, except cloudy conditions on 30th June (Fig. 3). Wind conditions are shown in Fig. 4.
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3.1.1. Surface temperatures Results of thermal monitoring for the unscreened and screened Trombe walls are respectively in Figs. 5 and 6 for the period 17th–23rd July (not occupied house). Internal and external surface of the Trombe walls (Tw int and Tw ext), glass surface temperature (Tglass), shutter surface temperature when present (Tshutter) are shown against room air temperature (Troom), outdoor air temperature
Fig. 4. Climatic data: wind speed.
Fig. 5. Internal and external surface temperatures and glazing temperature of the screened Trombe walls against weather data.
Fig. 6. Internal and external surface temperatures, glazing and shutter temperature of the unscreened Trombe walls against weather data.
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Fig. 7. Internal and external surface temperatures of Trombe walls under different screening conditions.
(Tout) and vertical global solar radiation incident on Trombe wall. Trombe walls were protected from direct solar radiation for the most part of the day by the horizontal overhangs. As a consequence solar radiation incident on the wall is relatively low. In Fig. 5 can be noted that external surface temperature of the unscreened wall rises over 30 °C after the solar incident peak radiation. Internal surface temperature average is 25.8 °C, and it is always higher than room temperature Troom (average 24.5 °C). Fig. 6 shows the results for the screened Trombe wall. Roller shutter external surface temperature is close to outside air temperature. The presence of screening determines glass maximum temperatures of 28–29 °C and external surface temperatures of 24–26 °C. Internal surface temperature of the wall average is 24.5 °C and it is very close to room air temperature. Comparison of internal and external surface temperatures of the two Trombe walls are shown in Fig. 7. Screening solar radiation with roller shutters determines a decrease on surface temperatures of the Trombe wall. On sunny days the external surface temperature peak of the unscreened Trombe wall is about 5–6 °C higher than the screened one. During night time the temperature difference is about 1.5 °C. The internal surface temperature difference between the screened and unscreened walls remains constant around 1.4 °C during the period considered. Internal surface temperature of the screened wall is near to the room air temperature, while internal surface temperature of the unscreened wall is higher all time. One representative day was selected to study temperatures peaks and time lags on the different layers of the system (Fig. 8). Glass surface reach the highest peak of 32.2 °C at 2:30 PM without screening and 28.2 °C 3 h later with screening. External surface temperatures reach daytime peak earlier in the case of unscreened wall (31.3 °C at 5:20 PM) than in the case of screened wall (26.0 °C at 8:00 PM).
Fig. 8. Surface peak temperatures of the two Trombe walls.
Internal surface temperature peak is 26.2 °C without screening and 24.0 °C with shading. Time lag of the massive wall was estimated 9 h for the unscreened Trombe wall and 10 h for the screened Trombe wall. 3.1.2. Transmission heat fluxes Internal heat flux densities (Fint) of the screened and unscreened Trombe walls are shown in Fig. 9. As internal surface temperature of the unscreened Trombe wall is always higher than the screened one, heat fluxes are higher too. Internal heat flux of the unscreened wall is always directed from the wall to the room and higher during night time. Heat flux is lower in the case of screened Trombe wall and directed from the wall to the room during morning and nighttime mainly, from the room to the wall in the afternoon mainly. Daily heat gains and losses of the two Trombe walls are shown in Fig. 10. The unscreened Trombe wall is characterized by daily heat gains between 0.58 MJ/m2 and 0.75 MJ/m2. Daily heat gains of the screened Trombe wall are between 0.03 MJ/m2 and 0.16 MJ/m2, up to 18 times lower than the unscreened wall’s values. Daily heat losses are equal to zero for the unscreened Trombe wall, while they are around 0.01–0.02 MJ/m2 for the screened wall. Results showed that the use of roller shutter has a evident impact in reducing heat gains of Trombe wall in summer.
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Fig. 9. Internal heat flux densities of Trombe walls under different screening conditions.
Fig. 10. Daily heat gains and losses of Trombe walls under different screening conditions.
3.1.3. Air layer temperatures and air velocity Air temperature in the gap (Tair gap) varies significantly according to the screening conditions (Fig. 11). Daytime air temperature peak in the middle of air gap is among 32–34 °C without screening while 27–29 °C with screening. Inlet air temperature (Tair inlet) is higher for the unscreened Trombe wall. Thermal gradient between inlet and middle of the gap in correspondence to the daytime peak is about 4–5 °C with open shutter and less than 1 °C with closed
shutter. Thermal gradient in correspondence to the lowest peak is near to zero in both cases. Air velocity in the gap was compared in the case of unscreened and screened Trombe wall (Fig. 12). Data collected demonstrated that wind condition has an important influence on air velocity in the gap. Air in the gap was characterized by relatively low speeds except on the days when wind speed was high. Two days characterized by different wind conditions were compared (19th–20th June). On the
Fig. 11. Air gap and bottom vent temperatures of Trombe walls under different screening conditions.
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Fig. 12. Air velocity of Trombe wall’s gap under different screening conditions.
windy day average air velocity in the gap is similar in both cases, regardless screening conditions, due to the influence of wind (0.12–0.13 m/s). On the day with low wind conditions, air velocities are lower (0.06 m/s without screening and 0.02 m/s with screening). Average air velocity for the
Fig. 13. Weather comparison between 21st June (unoccupied house) and 27th June (occupied house).
period (17th–23rd June) was 0.06 m/s for the unscreened Trombe wall and 0.04 m/s for the screened one. Screening influences air velocity in the gap reducing stack effect and flow rates in the gap. The effect is higher with low wind conditions. 3.1.4. Thermal parameters of Trombe wall in real occupancy conditions Two days with similar weather conditions (Fig. 13) but different occupation were selected to compare the thermal behavior of Trombe wall (21st June: unoccupied – 27th June: occupied). Temperatures and heat fluxes of Trombe walls were compared for the two days selected with and without occupants (Fig. 14). Air temperature is higher when the occupants are inside the room (9:00–20:00), due to the internal heat gains. The daily average difference in room air temperature is around 1.0 °C. Trombe wall inside surface temperature average difference between the occupied and not occupied day is 0.7 °C for the unscreened Trombe wall and 0.8 °C for the screened Trombe wall. As the tem-
Fig. 14. Influence of occupation on room temperatures, internal surface temperatures and heat fluxes densities of the unscreened and screened Trombe walls (in grey occupied hours).
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Fig. 15. Influence of occupation on air temperatures in the gap, inlet air temperatures and air velocities in the gap of the unscreened and screened Trombe walls.
perature difference between wall surface and room air is lower when the occupants are present, heat flux density is lower in this case. It follows that transmission heat gains from the wall to the room are reduced under the condition of occupied house. The behavior analysis during the day shows that the Trombe wall without screening releases heat to the room for the whole period in both cases. The screened Trombe wall absorbs heat when the room temperature is higher than surface temperature (afternoon) and release it when the room temperature is lower (morning and night time), giving an important contribution in maintaining the room temperature constant. Air temperature in the middle of the cavity, inlet air temperature and air velocity in the cavity are shown in
Fig. 15. Air velocity in the gap is higher during the day with occupation, due to the higher ventilation rates in the house when the occupants are in. In the case of unscreened Trombe wall the average air velocity is 0.06 m/s with occupation and 0.04 m/s without occupation. In the case of screened Trombe wall the average air velocities are very low due to the minor efficiency of stack effect. 3.1.5. Results of thermo-graphic survey Temperature distributions on the two Trombe walls external surfaces were investigated through thermo-graphic survey (Fig. 16 and Table 1). In order to obtain reliable data was necessary to open the Trombe wall glazing and external shutters of the screened wall. The unscreened
Fig. 16. Thermo-graphic image of the unscreened and screened Trombe walls.
Table 1 Results of thermo-graphic survey. Type of wall
Avg temp (°C) Min temp (°C) Max temp (°C) Difference temp (°C) (Max–min temp)
Line 1
Line 2
Line 3
Unscreened Trombe
Screened Trombe
Unscreened Trombe
Screened Trombe
Unscreened Trombe
Screened Trombe
28.2 27.8 29.1 1.3
23.6 23.4 23.8 0.4
27.3 26.9 27.9 1.0
23.5 23.3 23.6 0.3
28.3 27.6 28.9 1.5
23.6 23.4 23.8 0.4
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Trombe wall surface temperatures vary in the range of 1.5 °C both in vertical and horizontal directions. The highest surface temperatures were observed at mid-height in correspondence of the air vents. The screened Trombe wall’s temperature distribution is almost uniform in both horizontal and vertical direction, with temperature differences lower than 0.5 °C. 3.2. Results of thermal comfort analysis Thermal comfort analysis refers to a period of eight days selected from summer 2005 monitoring campaign. The period selected was characterized by summer extreme conditions with maximum temperatures over 35 °C (Fig. 17). The occupants were free to adapt themselves opening windows and closing window screening. Trombe internal surface average temperature was 27.4 °C, slightly higher than room air temperature (Fig. 18). Internal and external temperatures of the aluminium frame were particularly high and highlighted the incidence of the thermal bridge. Results of thermal comfort analysis with adaptive method (Fig. 19) results show that operative temperature in the room is within the comfort range for the whole period. The results demonstrate that Trombe wall, screened from solar radiation, can assure a satisfactory comfort level even in summer conditions.
3.3. Results of cooling energy need calculation Cooling energy need of the dwelling was calculated at the changing of shading and ventilation condition of the Trombe walls. The analysis was extended to several types of building envelopes, insulated according to different standards. 3.3.1. Analysis of unvented Trombe walls The influence of several types of shading on cooling energy need was analyzed in the case of unvented Trombe walls (Table 2): unscreened (case A), overhangs (case B), roller shutters (case C) and the combination of both overhangs and roller shutters (case D). Results for unvented Trombe walls demonstrated that the use of overhangs (case B) determine a reduction in cooling energy needs among 29.7% and 44.4% in comparison with the unscreened case depending on the insulation level of the building envelope. Roller shutters (case C) are more effective than overhangs in reducing cooling loads (among 58.0% and 63.2%), however the best effect is obtained combining both types of shading (case D) with reductions among 59.7% and 72.6%. 3.3.2. Analysis of vented Trombe walls The effect of cross ventilation in combination with solar shading was also studied (Table 3). First, comparison
Fig. 17. Climatic data.
Fig. 18. Room dry bulb and black globe temperatures, internal and external surface temperatures of Trombe wall and aluminium.
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Fig. 19. Results of thermal comfort analysis with adaptive method.
Table 2 Seasonal cooling energy needs of the accommodation with unvented Trombe walls at the changing of type of screening for several types of building envelopes. Insulation level of the building envelope
Type of shading for unvented Trombe walls A Unscreened
B Overhangs
C Roller shutters
D Overhangs and roller shutters
1. As built envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
16.52 –
9.19 44.4%
6.08 63.2%
4.53 72.6%
2. Conventional envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
29.40 –
20.20 31.3%
12.28 58.2%
11.69 60.2%
3. Super-insulated envelope Seasonal cooling energy needs (kW h/m2) Difference with the unscreened case
33.15 –
23.31 29.7%
13.92 58.0%
13.37 59.7%
between the case of unvented and vented Trombe walls was made in terms of cooling energy need using equal shading conditions for every insulation level of the building envelope. This comparison highlights the benefit of activating ventilation of Trombe walls. Results showed that activating the ventilation of Trombe walls always gives a further contribute in reducing cooling energy needs in comparison with the unvented Trombe wall. The use of cross ventilation for the case of Trombe walls without solar screening (Table 3 – case A), determines a reduction in cooling energy needs among 13.5% and 15.1% depending on the type of building envelope. This confirms that the single effect of ventilation is less effective than the single effect of every type of solar screening. The reduction in cooling energy needs, due to the ventilation strategy, decreases at the increase of efficiency of shading system. For instance, in the as built envelope case, the reduction is 9.5% using overhangs (case B), 8.2% using roller shutters (case C) and 1.3% using a combination of overhangs and roller shutters (case D). Second, the difference between each case and the case of unscreened – unvented Trombe wall, in terms of cooling energy need, highlights the overall benefit in using ventilation in combination with shading for every insulation level
of the building envelope. The best result is obtained combining overhangs, roller shutters and cross ventilation (Table 3 – case D). The reduction in cooling energy needs using such a set-up is 72.9% in the as built case, 63.4% in the conventional case and 63.0% in the super-insulated case. 3.3.3. Influence of insulation level of the building envelope on the results The influence of the insulation level of the building envelope on the results was analyzed in terms of effectiveness of shading and ventilation of Trombe walls in reducing cooling energy needs. Results showed that the reduction in cooling energy needs determined by Trombe wall’s shading is more effective with a lower level insulation of the other elements of the building envelope. For instance, considering unvented Trombe wall with both overhangs and roller shutters (Table 2 – case D), the reduction in cooling energy needs in comparison with the unscreened case is 72% for the as built envelope, 60.2% for the conventional envelope and 59.7% for the super-insulated envelope. The same trend can be outlined for the vented Trombe walls cases: the reduction in cooling energy needs in comparison with
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Table 3 Seasonal cooling energy needs of the accommodation with vented Trombe walls at the changing of type of screening for several types of building envelopes. Insulation level of the building envelope
1. As built envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case 2. Conventional envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case 3. Super-insulated envelope Seasonal cooling energy needs (kW h/m2) Difference with the unvented case (using the same type of shading) Difference with the unscreened – unvented case
Type of shading for vented Trombe walls A Unscreened
B Overhangs
C Roller shutters
D Overhangs and roller shutters
14.29 13.5%
8.32 9.5%
5.81 8.2%
4.48 1.3%
13.5% 25.03 14.9% 14.9% 28.13 15.1% 15.1%
the unscreened-vented case using overhangs and roller shutters (Table 3 – case D) is 68.7% for the as built envelope, 57.0% for the conventional envelope and 56.4% for the super-insulated envelope. On the contrary, the reduction in cooling energy needs due to cross ventilation of Trombe wall is more effective increasing the insulation level of the building envelope. Results in Table 3 results show that the difference between vented and unvented case, using the same type of shading, is higher at the increase of the insulation level. For instance, using both overhangs and roller shutters (Table 3 – case D), the difference is 1.3% for the as built envelope, 8.0% for the conventional envelope and 8.3% for the super-insulated envelope. 4. Conclusions An experimental study on the thermal behavior of Trombe walls in summer under a Mediterranean climate was carried out in a residential building. The effect of screening devices, ventilation and occupancy was studied in detail. It was found that roller shutters have a relevant influence in reducing surface temperatures of Trombe wall of 1.4 °C and daily heat gains toward the room of about 0.5 MJ/m2. Air velocity in the cavity is influenced by both wind conditions and temperatures in the air gap. Air velocity in the gap was found to be low, especially in the case of Trombe wall screened by roller shutters. Thermo-graphic survey was useful to prove that Trombe wall external surface temperature distribution is rather uniform when screened from direct solar radiation. The analysis of Trombe walls in real use conditions showed that the presence of occupants in the house determines an increase in room air temperature, but also higher
49.7% 17.59 12.9% 40.2% 20.18 13.4% 39.1%
64.8% 11.28 8.2% 61.6% 12.66 9.0% 61.8%
72.9% 10.76 8.0% 63.4% 12.26 8.3% 63.0%
ventilation rates due to window opening. In such conditions, heat fluxes from the wall to the room are reduced while air velocity in the gap are higher. Thermal comfort analysis for a selected period characterized by severe summer conditions confirmed that comfort level was satisfactory for the house. Energy simulation in dynamic state showed that: the use of overhangs and roller shutters is more effective than cross ventilation in saving energy for the cooling of a dwelling with Trombe walls; enhancing the insulation level of the building envelope determines decreased efficiency for solar shading and increased efficiency for ventilation of Trombe wall; regardless of the insulation level of the building envelope, the best energy performance was obtained combining overhangs, roller shutters and cross ventilation, with a reduction in cooling energy need up to 72.9% compared to the case of an unvented Trombe wall without solar protections. References Blasco Lucas, I., Hoese´, L., Pontoriero, D., 2000. Experimental study of passive systems thermal performance. Renew. Energy 19, 39–45. CEN, 2007. EN 15251:2007. Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. Chen, B., Chen, X., Ding, Y.H., Jia, X., 2006. Shading effects on the thermal performance of the Trombe wall air gap: an experimental study in Dalian. Renew. Energy 31, 1961–1971. 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. Ellis, P.G., 2003. Development and validation of the unvented Trombe wall model in EnergyPlus. Master’s thesis, University of Illinois at Urbana–Champaign. Gan, G., 1998. A parametric study of Trombe walls for passive cooling of buildings. Energy Build. 27, 37–43.
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