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Experimental and Theoretical Investigation on the Performance of Trombe Wall in Summer Season

M.F. AbdRabbo, S.M. Elshamy

Abstract

Solar energy is non negligible energy source, but in Middle East solar heat is very high especially in summer, but it can be useful in other application. The major problem in summer that cooling load is very high especially in building faces the south direction. So using technics of passive solar design to minimize cooling loads in this direction of building. Trombe wall design is used to create buoyancy force for air to move from down to up dragging hot air to outside replacing with cold air. Two different designs are investigated, one with space between trombe wall and ground floor equal 10% of building height allows air to flow from outside to the trombe gap, and the other when the trombe wall is adjacent to the ground allow only the air flow from inside building to trombe gap. Experiments are done with different spaces of trombe gap from building.

Introduction

Every day, a great amount of solar energy arrives the earth’s atmosphere. Most of this is reflected back into space by clouds before it reaches the planet’s surface. Ninety-nine percent of the sunlight, which does reach the ground, is converted into heat (the other 1 % is captured by plants through photosynthesis) and radiated back into space, Fig. 1 illustrates the dissipation of sun rays .The capture of solar energy by passive solar technologies has almost no negative impact on the environment

Fig. 1 Dissipation of sun rays

Solar energy is a radiant heat source that causes natural processes some of the natural processes can be managed through building design in a manner that helps heat and cool the building. When sunlight strikes a building, the building materials can reflect, transmit, or absorb the solar radiation as shown in fig.2. We can use solar heat, to design elements, material choices and placements that can provide heating or cooling effects in a building.

Fig.2 Passive Solar Design

In passive solar building design windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.  The key to design a passive solar building is to best take advantage of the local climate performing an accurate site analysis. Passive solar design techniques can be applied most easily to new buildings, but existing buildings can be adapted.

Passive cooling is the use of the same design principles to reduce summer cooling requirements. Some passive systems use a small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, and use, and reduce undesirable heat transfer.

A Trombe wall is a passive solar building design where a wall is built on the south sun side of a building with a glass external layer. Trombe walls are commonly used to absorb heat during sunlit hours of winter then slowly release the heat over night. The essential idea was first explored by Edward S. Morse and patented by him in 1881. In the 1960s it was fully developed as an architectural element by French engineer Félix Trombe and architect Jacques Michel.  A typical wall cross section is shown in Fig.3 .

Fig.3 Details of Trombe Wall

   

The basic idea of Trombe wall is control the entrance of sunlight and air flows into the building at appropriate times and to store and distribute the heat and cool air so it is available when needed.  Fig.4. illustrate operation of trombe due to summer and winter.

Fig.4 Trombe wall in winter and summer

Operation of Trombe Wall in Winter

During winter daytime operation when heat is desired in the building, the high and low vents through the mass wall are opened. As the sun strikes the dark surface of the mass wall, the air between the glazing and mass wall heats up, expands, rises to the top of the air space and escapes through the upper vents into the building. As warm air leaves the Trombe collector area, cooler air is drawn into the collector through the low vents from the building. This cool air, in turn, is heated, expands, rises, and passes back into the building, thus setting up a natural air circulation pattern. On winter nights it is important that all vents be closed. If they are left open, a reverse convective loop can be set up. Because heat can readily escape through uninsulated Trombe wall glazing, the collector air space cools off until that air is cooler than air inside the building. At this point, if the mass wall vents are open, warm air from the building would enter the Trombe wall collector area through the upper vents, cool off, and return to the building.

Operation of a Trombe Wall during winter (day and night).

Operation of Trombe Wall in Summer

In the summer months when heating is not desired. The upper mass wall vent should be closed and the lower mass wall vents and upper outside (glazing) vents should be open. A window on the north side of the building should also ideally be open to allow relatively cooler air to flow freely through the building and into the Trombe wall collector. As air in the Trombe collector is heated, it expands, rises, and flows out the vents to the outside drawing air behind it from the building (this is called chimney effect). This constant flow of relatively cool air will prevent the collector air and mass wall from heating up much and thus keep the building cool. With these vents open, heat will escape from the building both during the day and night.

Operation of a Trombe Wall during summer (day and night).

Many theoretical and experimental studies have shown that indoor comfort is improved due to well-designed Trombe walls Jie et al. [7]. Most of the studies on Trombe wall are concerned with its winter heating.

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Francesca Stazi et al. [1] experimentally study 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 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.

Tamara Bajc et al. [3] developed a three-dimensional numerical CFD analysis of temperature fields in Trombe wall and in the adjacent room for several days of a typical meteorological year (TMY) for a moderate continental climate. There Simulations have shown a big impact of Trombe wall on the temperature field inside a model house for Belgrade weather data. The contribution of Trombe wall in winter conditions is huge. However, in summer conditions, Trombe wall is an additional source of heat loads. It is possible to achieve acceptable temperature in the room (around 29.8 ◦C) with PV stripes as covered system on the outside of the Trombe wall, but it is still necessary to cool down the house till the temperature reaches 26 ◦C. Nevertheless, it is important to emphasize that the contribution of PV strips in electricity production needed for cooling devices operation is significant.

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The optimum opening and closing modes in the management of air vents has been obtained by testing and analyzing thermal performance parameters of a trombe wall installation by Yanfeng Liu et al. [6]. The parameters considered include air vent velocity, air vent temperature, the temperature distribution of air layer and indoor air temperature. The temperature and velocity distribution of the air layer has been obtained by numerical calculation, together with experimental data including the heat storage and release characteristics and the influencing factors of the trombe wall. The research results show: the optimum time to open the air vent of the trombe wall is 2–3 h after sunrise with closure 1 h before sunset. On the condition of optimum air vent management mode, surface and average temperature of heat storage wall computer simulations were run with CFD to analyze the heat storage and release performance of trombe wall. Then, we can find out the heat storage capacity of the trombe wall reaches its maximum value at 4 pm, its minimum value at 7–8 am.

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Fan and Li [9] compared the Lattice Passive Solar Heating Walls with traditional Trombe wall. Some parameters such as the optical features of surface and transparent cover, the thickness of wall materials, the thermal conductivity and the distribution of ventilation holes were examined. In consequence of the experimental and theoretical study, thermal efficiency on Trombe wall was found out to be 22.6 and 30.2 % in solar wall.

Smolec and Thomas [8] have investigate theoretical model for the temperature distribution of a Trombe wall by using a thermal network and compared the results with the experimental data.

Fan and Li [9] In their study, some parameters such as the optical features of surface and transparent cover, the thickness of wall materials, the thermal conductivity and the distribution of ventilation holes were examined. Compared the Lattice Passive Solar Heating Walls with traditional Trombe wall. In consequence of the experimental and theoretical study, thermal efficiency on Trombe wall was found out to be 22.6 and 30.2 % in solar wall.

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The aim of this investigation is to study experimentally and theoretically using of passive solar systems (trombe wall) which offer to its users a thermal comfortable inside space, aiming to reduce as much as possible the supplementary energy consumption.

Mathematical background

the problems of indoor airflow and convective and radiative heat transfer are governed by the conservation equations for mass, momentum in each flow direction and energy. In addition, the airflow is considered predominantly turbulent [4]. Mass conservation equation for 3D airflow:

∂/∂t+(∂(u))/∂x+(∂(v))/∂y+(∂(w))/∂z=0                                           (1)

Where  [kg/m3] is air density, and u,v,w are velocity components in x, y and z direction respectively. For the incompressible airflow, the air density is constant (the local variations in density are negligible to the local velocity variation), so Eq. (1) can be written as:

∂u/∂x+∂v/∂y+∂w/∂z=0                                                               (2)

Navier–Stokes equation for incompressible 3D airflow with constant viscosity (momentum conservation equations):

(∂u/∂t+u ∂u/∂x+∂u/∂y+w ∂u/∂z)=-∂P/∂x+µ((∂^2 u)/(∂x^2 )+(∂^2 u)/(∂y^2 )+(∂^2 u)/(∂z^2 ))+X             (3)

(∂v/∂t+u ∂v/∂x+∂v/∂y+w ∂v/∂z)=-∂P/∂y+µ((∂^2 v)/(∂x^2 )+(∂^2 v)/(∂y^2 )+(∂^2 v)/(∂z^2 ))+Y     (4)

(∂w/∂t+u ∂w/∂x+∂w/∂y+w ∂w/∂z)=-∂P/∂z+µ((∂^2 w)/(∂x^2 )+(∂^2 w)/(∂y^2 )+(∂^2 w)/(∂z^2 ))+Z        (5)

Where X (X = 0), Y (Y = -g), and Z (Z = 0) are projections of body force F in x, y and z direction, and P is pressure.

EXPERIMENTAL MODEL

The basic operation of a Trombe wall in summer time operation, solar radiation that passes through the glazing is largely absorbed at the wall surface. The air in the gap between wall and glazing is then heated by convection from the absorber and glazing. The decrease in density experienced by the air causes it to rise, whereupon it is replaced by the room’s air from below. The rate at which air is drawn through the room depends upon the buoyancy force experienced, (i.e. the temperature differential), the resistance to flow through the gap, and the resistance to the entry fresh air induced to the room. In this paper A Wooden room was built, and glass wall was mounted in the front of south wall of building leaving air gap (50-100-200 mm) as shown in fig.7. The overall dimensions of the wooden room were: 700 mm (length) x 400 mm (width) x 1500 mm (height), and 3 mm glass wall thickness. The experiments were carried out in the month May 2016 at 6 October city, Egypt

(Latitude:29.945953°  Longitude:30.915754°)

Measurements of temperatures using digital thermometer (k-type thermocouple) were recorded at different positions (12 point on the south wall, 2 points inside the room and one point on the glass wall).

There was window of dimensions 50 x 15 cm in the upper north wall, and a similar one was done in the opposite direction but at the bottom of the wall which allows the entry of air into the room. There is an opening at the top of the air gap channel to allow the air to escape out of the channel to the outside. Tow experiments were run, first one when the trombe wall was rises from the floor by space of 10 % height building (this case was called open), second when the trombe wall was adjacent to the ground (this case was called closed).

Fig. 7 photo of wooden room

10 RESULTS

10 Variation of temperature with time for closed building

Fig.10.5.1 Variation of temperature with time for closed building at space 5cm, 10cm &15cm of glass wall

10 Variation of temperature with time for opened building

 

Fig.10.5.2 Variation of temperature with time for opened building at space 5cm, 10cm &15cm of glass wall

10 Variation of temperature with time for closed and open building

Fig.  Variation of temperature with time for closed and open building                 at space 10 cm of glass wall

Through readings, we find that the best position for glass at 10 cm from the wall ,With two holes integrated under the south wall and the top of the north wall, Where this helps dramatically reduce the degree of heat the southern face

References

[1]       \"Passive Solar Energy\", Energy Educators of Ontario, Energy Fact Sheet, 1993.

[2]       \"The Analysis of the Modified Trombe Wall and its Application to the Heating and Cooling of the Buildings\", Ali R. Dalal, University of Strathclyde.

[3]        \"Passive Solar Design in Architecture\", Faculty of Engineering,  University of Hong Kong.

[4] 4 - ”Energy and Architecture: The Solar and Conservation Potential”, Christoper, Energy and Architecture, Wordwatch Institute, 1980.

[5] ”Solar Energy in Architecture and Urban Planning”, Presel Verlag. 1996.

[6] 6 - “Solar Power a Place in the Sun”, L. Sanford, The Heating and Air Conditioning Journal, Volume 49 No. 570, July 1979, Page 22-38.

[7] 7 – Jie J, Hua Y, Wei H, Gang P, Jianping L, Bin J (2007). Modeling of a Novel Trombe Wall with PV Cells, Building and Environment, 42(3): 1544-1552.

[8] 8 – Smolec W, Thomas A (1991). Some aspects of Trombe wall heat transfer models. Energy Conversion Manage., 32-33: 269-277.

[9] 9 – Fang X, Li Y (2000). Numerical Simulation and Sensitivity Analysis of Lattice Passive Solar Heating Walls, Solar Energy, 69(1): 55-66.

[3] ANSYS FLUENT Theory Guide, ANSYS, 2012.

[4] 2009 ASHRAE Handbook – Fundamentals, ASHRAE, Atlanta, GA, 2009.

[5] P. Nielsen, F. Allard, H. Awbi, L. Davidson, A. Schälin (Eds.), Computational Fluid Dynamics in Ventilation Design, Rehva, Forssa Finalnd, 2007.

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