THERMAL STORAGE WALLS

PRESENTATION BY:

SRIKANTH REDDY

PRAKASH SHARMA

LEKHRAJ KUMAWAT

INTRODUCTION The space heating systems performs three actions

Collection of solar energy, Storage, Distribution of stored energy to living space

The systems which can perform these activities can be classified into two types:

1.Active solar heating systems :Use mechanical devices to circulate fluids( water, air) for solar energy collection and distribution of energy(heat) from storage to living space thus become complex and relay on external energy sources to operate.

2.Passive solar heating systems : These systems use no external active energy for heat transfer but natural heat transfer methods

A. Thermal conduction[1] heat transfer from warmer to cooler areas within or between objects by direct contact of particles within the objects.

B .Natural convection[1] transfers heat between two objects through a moving fluid such as air or water.

C .Radiation[1] is the transfer of heat through space by wave motion.

In all three modes, heat moves from warmer to cooler objects. The greater the difference in temperature, the greater the heat flow.

In the following slides main aspects of solar passive heating are explained

OVERVIEW Thermal walls can be categorized into three types:

Trombe wall: Those utilizing a massive wall to store heat

Water wall: Those utilizing a water to store heat

Trans wall: Those utilizing water for storage along with transparent absorber to facilitate visual comfort

Of the three structures Trombe walls are most used.

The main aspects of Trombe wall are:

I. Glazing

II. Air gap between glazing and thermal wall

III. Mass or thermal storage

IV. vents (in some thermal storage walls)

V. roof overhang (especially in warm climates).

In the following slides it is emphasized that how these elements enable a thermal storage wall to function in heating a building.

Principle of operation The Trombe wall can be:

Non-Vented:

The heat energy is stored in thermal during day time

and radiated and conducted into the living space,

no direct convection of air between air gap and living space.

Suitable for homes as thermal storage is required for night heating.

Doesn’t aid ventilation during summer.

Vented:

Vents on the upper and lower side of the wall provides

direct convection between air gap and living space there by

increasing heat transfer in day times.

Most suitable for office buildings as the working hours during day

time will get efficient heating and storing is not a obligation.

Aid’s the natural ventilation during summer.

Ventilation by warm and cool air can be facilitated by using series of flaps/vents to wall and glazing as shown in figure[2]

Thermal Storage Wall Components: Glazing

Purpose : To trap heat from the incoming solar radiation.

A good glazing material Should allow maximum transmission of solar (short wave) radiation And it should keep heat loss to a minimum by preventing long-wave transmission and by serving as a barrier to heat loss.

Additionally, an ideal solar glazing should possess: Good thermal stability, a high resistance to abrasion and weather, low maintenance and purchase costs, high fracture and Impact resistance, and ease of handling.

Glazing Materials: Commonly used materials fall into two broad categories: Glass and plastic

Glass[3] Plastic[4]

Advantages: 1.Excellent transmittivity (above 90%) 2.Superior thermal stability ( Upto 400 F) 3.Low thermal contraction/Expansion 4.Easily available 5.Resistant to abrasions(wear and tear)

Advantages: 1.Reasonable transmittivity (above 85%) 2.Superior weather conditions 3.Light weight (compared to glass) 4.Won’t yellow 5.High impact resistance

Disadvantage: 1.Low impact resistance 2.cost

Disadvantages: 1.Susceptible to abrasions 2.High thermal contraction/absorption 3.Slight Embrittlement with age 4.Relatively low operating temperatures(200F) Examples: Polycarbonates, fluorocarbons, and polyvinyl fluorides

THERMAL MASS Thermal mass in trombe wall is to store heat to : Distribute at night times or to introduce time delay(used for summer conditions)

Main properties of thermal mass:

Heat storage capacity: This depends on the specific heat of the material (Cp) more the Cp more the capacity.

Density: Denser materials will store more heat than the lighter ones ( i.e .

Thermal conductivity : It should be moderate as high value of K will tend to increase cooling loads in summer and low value of K will tend to increase heating loads in winter/night.

Properties[5] some of the commonly used materials are listed below:

NOTE: A natural building material made from sand, clay , water , and some kind of fibrous or organic material.

EFFECT OF AIRGAP:

If Air gap is too small :

Increase in glazing temperature which leads to higher radiative loss.

If Air gap is too large :

Convective loss will increase due to local circulation.

Optimum Air gap[7]: 3 ½ inch between wall and glazing

EFFECT ON FLOW RATE[10]:

The flow rate is almost irrespective of air gap, this is because pressure loss is mainly caused by entrance and exit vents.

Pressure loss due to friction of air gap is very less

compared to that of openings(vents).

How ever if the channel width and vent openings are of same size then flow rate will increase as the area for air flow is increased.

EFFECT OF THICKNESS:

THICKNESS OF WALL:

Apart from the energy/heat stored it effects the temperature swings of indoor environment and time lag. Typical values[7] for concrete wall are given as follows:

VENTING FOR THERMAL WALLS The unvented wall delivers heat to inside by conduction and radiation where as a vented wall will additionally

heats up the building by convection loop ( approx. 30% by convection and 70 % by conduction[7] ).

The temperature swing/variation of inside environment is more in case of vented wall.

May cause higher temperatures in day time and low temperatures in night times.

To avoid the above problem additional storage material can be used in floor, partition walls etc.

SIZING AND PLACEMENT OF VENTS: Generally 2 % of wall area is suggested for combined( upper and lower) vent area[7].

If vent area is more then % heat storage will reduce and if it is very less convective heat transfer during day time will reduce.

Several upper and lower vents with uniform spacing will be effective because of more even convective air flow .

VENT CLOSURES:

Outer vents(for glazing) should be transparent and inner vents can be of any material preferably the one which insulates heat loss when closed ids best. Examples are Styrofoam, thermocol etc.

Advanced vent closures accomplished with heat sensors, thermostats and automatic damper.

OVERHANG, INSULATOR AND REFLECTORS:

Fold down insulator reflector : Maximizes the heat gain during day/winter time and reduces heat loss in night time and reduces heat gain in summers.

Movable insulation will also performs same function except increasing heat gain during day time/winter.

Water wall: In this structure water contained in drums/pipes is used as storage medium.

The outer surface is painted black and inner surface can be in direct contact with living space.

Properties/ Characteristics

Specific heat capacity:

Water (Cp=4.2 kJ/kg K) has more specific heat capacity compared to

normal brick wall (Cp=1kJ/kg K).

This will increase the wall thickness (volume) for the given amount of

storage requirement.

Thermal conductivity:

Water (0.55 w/m K) has lower thermal conductivity compared to brick (0.77 w/m K) .

Density: water has a lower density 1000 kg/ against 1750 kg/ for brick.

Being convective body water will allow rapid heat transfer.

Water wall (Cntd.):

A Drum water wall system (water with a thin concrete

layer behind it) ensures a good load levelling

and significant phase shift.

So this system is attractive when both day and night

performance as well as load levelling are the prime

concern.

Trans wall[8] The trans wall consists of a transparent absorber sandwiched between two water columns which are

contains by glazing on either side.

The trans wall partially absorbs and partially transmits the solar radiation.

It combines features of direct gain and thermal storage.

It allows the visual transmission there by reducing the

lighting load(during day times).

Thermal convective heat transfer reduces the efficiency

but can be reduced by baffles and gelling agent

(eg. Agar-Agar, gelatin) which increases the viscosity

of fluid(water).

Difference between conventional thermal storage wall and trans wall: Most of the solar radiation is absorbed at the centre of the wall but not at the front surface as in case of water

and trombe wall.

Trans wall (cntd.)Properties of absorber:

The semi transparent absorber will have Transmissivity in the range of 0.8-0.9

Absorptivity in the range of 0.1-0.2

Advantages:

Rapid heat transfer due to convective heat transfer through water and direct heat gain.

Reduced heat loss: As most of the solar radiation is absorbed in centre(through absorber) and close to/at the living space(by direct gain) heat loss to ambient air is less.

Disadvantages:

The transmission of light through water may cause glare.

There may be a problem of over heating due to direct gain in day times.

Inefficient for night heating since time/phase shift between heat flux and solar flux.

Trans wall (cntd.) Its been a strong intuition that trans wall will produce glare and visual discomfort to the occupants, the

following[8] may help in fading that misconception and improve the acceptability of trans wall deployment.

Trans wall(cntd.)

Effect of width of inner and outer water columns: As the width of outer water column increases (keeping inner column and absorber width constant) the loss

to ambient increases thus reducing the average heat flux.

This is because as outer column width increases, the quantity of heat stored towards the glazing(outer glass) part of the Trans wall increases.

As the width of inner water column increases better load levelling can be achieved i.e flux fluctuation will reduce.

Effect of absorber width: As the thickness of it increases, the heat loss to the ambient decreases since the resistance to conduction of

heat from the water column increases, But at the same time, it also reduces the transmission of solar flux.

Beyond that if the thick ness is even increased the reduction in transmission will be much more than reduction in heat loss thus reducing overall heat flux.

Comparison of thermal walls

NOTE: The solar heating fraction (SHF), the fraction of the heating requirement supplied by sunlight. More the SHF less the supplementary heating(given by auxiliary heating device) requirement.

Comparison of thermal walls(cntd.)[9]• Trans wall: It can be seen that the trans

wall is useful when immediate heat transfer is required.• Suitable for offices, schools where

heating in day time is significant.• No time shift between heat flux and

solar radiation.• Sharp increase in heat flux during

daytime(partly due to direct heat gain and partly due to convective heat transfer)

• Trombe wall: • On the other hand trombe wall is

suitable for load(heating) levelling through out the day.

• This is due to time/phase shift is significant depending upon thickness.

• Suitable for homes.

Advancements in thermal walls[11]: BIPV/T and Double glass systems

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

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

Advancements in thermal walls(contd.) 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.

CONCLUSIONS:

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

Increase of 2% electrical efficiency and temperature difference

out door and outlet air is 16.89 K thermal performance is reduced

by 17%[]

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.

References 1. ASHRAE 1977, Fundamentals 33.4.

2. http://web2.mendelu.cz/af_291_projekty2/vseo/stranka.php?kod=1071

3. Solar Glazing: 1979 Topical Conference. Mid-Atlantic Sea, 2233 Gray's Ferry Avenue, Philadelphia,Pennsylvania 19146

4. Modem Plastics Encyclopedia. Vol. 54, No. 104. McGraw Hill, Inc., 1221 Avenue of the Americas, New York 10020

5. 1 Handbook of Fundamentals, 1977. American Society of Heating, Refrigerating and Air-Conditioning Engineers, New York, N.Y.

6. Mazria, Baker, Wessling, Predicting the Performance of Passive Solar Heated Buildings,

7. J.D.Balcomb , “designing passive solar buildings to reduce temperature swings” LASL

8.PASSIVE SOLAR HEATING OF BUILDINGS USING A TRANSWALL STRUCTURE'I" R. Fuchs and J. F. MCCLELLAND Ames Laboratory-USDOE and Departments of Physics and Chemistry, Iowa State University, Ames, IA 50011, U.S.A.

9.TRANSWALL VERSUS TROMBE WALL: RELATIVE PERFORMANCE STUDIES ,J. K. NAYAK, Energy Systems Engineering, Mechanical Engineering Department, Indian Institute of Technology, Powai, Bombay 400 076, India.

References

10. A parametric study of trombe walls for passive heating of buildings, Guohui Gan,Energy and buildings, Elsevier.

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