ADVANCEMENTS IN SOLAR THERMAL WALLS

Srikanth Reddy(M-Tech)

MNIT JAIPUR

Contents

Zigzag Trombe wall Fluidized Trombe wall Trombe wall with phase-change material Composite Trombe wall Photovoltaic (PV) Trombe wall

ZIGZAG TROMBE WALLS Purpose: To reduce excessive heat gain and glare in sunny days.

Construction & working[1]:

It consists of three sections

One facing south other two sections forms a inward “V” shaped wall

One section of V shaped wall faces south east provides light and heat during morning times through windows when immediate heating is required.

The opposite is a classical trombe wall which stores heat during afternoon time and supply heat in night times.

Fluidized Trombe wall[2]: It is a classic Trombe wall but in which the gap between the Trombe wall and glazing is filled with a highly

absorbent, low-density fluid[2].

Fan forces the air through the fluidized bed there by

Facilitating effective heat transfer.

Two filters, which are located at the top and bottom of

the air channel, prevent the fluidized particles from

entering the room[7].

This system has effective heat transfer due to direct

contact between air and fluid particles[8] since the

surface area is increased compared to wall in the

conventional trombe wall.

TROMBE WALL WITH PHASE CHANGE MATERIAL(PCM)

The PCM absorbs the solar energy and changes its phase there by storing heat(latent) energy, which can be released by reversing phase change in night times.

Commonly used PCM’s are phase eutectic salts( NaCl, potassium nitrate) or salt hydrates (Calcium chloride, Sodium sulphate) and paraffin wax[2].

Paraffin wax: Most widely used PCM

Cheap with moderate thermal storage densities (200 kJ/kg or 150 MJ/) and a wide range of melting temperatures.

However, they have low thermal conductivity (0.2 W/ C), which limits their applications. Metallic fillers, metal matrix structures, finned tubes and aluminum shavings were used to improve their thermal conductivity.

TROMBE WALL WITH PCM (cntd.)

Advantages: These PCM’s store more energy in a smaller volume and in materials those are lighter than normal building

materials there by reducing size and weight of storage medium.

A 15 cm concrete wall can be replaced by a 3.5 cm wall of PCM and perform similarly[3].

The time of energy release can be altered by altering the initiation given for phase change at night times.

Disadvantages : Performance of PCM is strongly dependent on thermal stability, repetitive cycling, corrosion between PCM and container[4]

(concrete wall).

Salts have high thermal stability but often suffers from high corrosion, Paraffin wax provides a good option as paraffin's have excellent thermal stability as neither the cycles nor contact with metals degrades their thermal behavior[5].

Reactivity of PCM can be decreased by encapsulating it by non reactive materials like high density polyethylene sheets or plastic pipes[6].

COMPOSITE TROMBE WALL Also called as Trombe-Michel wall, consists of: Semi transparent cover, a mass heating wall, a closed cavity,

a ventilated air cavity and an insulating panel.

Composite Trombe walls are considered a remedy[11] for two deficiencies of Trombe walls:

Heat loss during cloudy winter days and

Undesired heat inputs during hot weather

Both these effects were due to insulation on inner wall

surface.

Unlike the conventional Trombe walls percentage of heat

Transferred through conduction or radiation is very less due

To the presence of insulation on inner side of wall.

COMPOSITE TROMBE WALL(Cntd.)

Advantages:

Users can control the rate of heating by controlling the airflow through the ventilated channel.

The composite Trombe wall’s thermal resistance is extremely high because the wall is insulated on the inner side.

Disadvantages:

This type of wall requires a mechanism to prevent reverse thermo-circulation, which occurs when the storage wall becomes colder than the ambient air of the building’s internal space.

The reverse thermo-circulation can be avoided by using plastic film insertion in the vents which allows the flow of air only in one direction[11].

BIPV trombe wall:

Building integrated photovoltaic thermal (BIPV/T) systems are either opaque or semi-transparent type PV on roof top or facade.

Principle:

The system removes the heat behind the PV panels and cools them.

The decrease in the PV surface temperature provides the increase in electrical efficiency.

The air heated in the air duct/gap is heated up and taken into the building’s HVAC system.

The use of pre-heated air in the HVAC system provides the decrease in the heating and the ventilation loads.

Application:

Production and availability of semi transparent PV modules makes it viable for Trans wall systems also, while opaque PV modules are limited to trombe wall only.

BIPV Trombe Walls

CONCLUSIONS:

The experiments conducted using a-Si BIPV/T has given an

Increase of 2%[9] electrical efficiency and temperature difference

out door and outlet air is 16.8 C thermal performance is reduced

by 17%[10]

Double glazing thermal walls are also getting popular, This

will have less heat loss during night times due to increased

Thermal resistance. but reduction of transmittance is a problem.

In single glass system the solar gain during day time is more

due to more transmittivity compared to double glass

Thus single glass system with shutters in the night is better than

double glass system.

REFERANCES

1. NREL. Building a better Trombe wall, NREL researchers improve passive solar technology. National Renewable Energy Laboratory; 2005.

2. K. Sopian, C.H. Lim, Nilofar Asim, M.Y. Sulaiman,Trombe walls: A review of opportunities and challenges in research and development, Omidreza Saadatian n, Renewable and Sustainable Energy Reviews 16 (2012) 6340–6351

3. Bourdeau LE. Study of two passive solar systems containing phase change materials for thermal storage. Fifth Natl passive solar conference. Amherst, Mass: Smithsonian Astrophysical Observatory; 1980.

4. Zalba B, Marin J, Cabeza L, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251–83.

5. Gibbs B, Hasnain S. DSC study of technical grade phase change heat storage materials for solar heating applications. In: Proceedings of the 1995 ASME/JSME/JSEJ International Solar Energy Conference, Part 2, 1995.

6. Hong Y, Xin-shi G. Preparation of polyethylene–paraffin compounds as a form-stable solid–liquid phase change material. Solar Energy Mater Solar Sells 2000;64:37–44.

REFERANCES

7. Sadineni SB, Madala S, Boehm RF. Passive building energy savings: a review of building envelope components. Renewable and Sustainable Energy Reviews 2011;15:3617–31.

8. Tunc M, Uysal M. Passive solar heating of buildings using a fluidized bed plus Trombe wall system. Applied Energy 1991;38:199–213.

9. panels,Basak Kundakci Koyunbaba Zerrin Yilmaz b, The comparison of Trombe wall systems with single glass, double glass and PV

10. Sun W, Ji J, Luo C, He W. Performance of PV-Trombe wall in winter correlated with south facade design. Applied Energy 2011;88:224–31.

11. Zalewski L, Chantant M, Lassue S, Duthoit B. Experimental thermal study of a solar wall of composite type. Energy and Buildings 1997