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 heat transfer from warmer to cooler areas within or between objects by direct
contact of particles within the objects.
B .Natural convection transfers heat between two objects through a moving fluid such as air or water.
C .Radiation 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
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:
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:
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.
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
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
Advantages: 1.Excellent transmittivity (above 90%)
2.Superior thermal stability ( Upto 400 F)
3.Low thermal contraction/Expansion
5.Resistant to abrasions(wear and tear)
Advantages: 1.Reasonable transmittivity (above 85%)
2.Superior weather conditions
3.Light weight (compared to glass)
5.High impact resistance
Disadvantage: 1.Low impact resistance
Disadvantages: 1.Susceptible to abrasions
2.High thermal contraction/absorption
3.Slight Embrittlement with age
4.Relatively low operating
Examples: Polycarbonates, fluorocarbons, and polyvinyl fluorides
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: 3 ½ inch between wall and glazing
EFFECT ON FLOW RATE:
The flow rate is almost irrespective of air gap, this is because pressure loss is mainly caused by entrance and exit
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 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 ).
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.
If vent area is more then % heat storage will reduce and if it is very less convective heat transfer during day time
Several upper and lower vents with uniform spacing will be effective because of more even convective air flow .
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 (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
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
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
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.
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 may help in fading that misconception and improve the acceptability of trans wall deployment.
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
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.)
• 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
• Sharp increase in heat flux during
daytime(partly due to direct heat
gain and partly due to convective
• 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
Advancements in thermal walls: 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
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
Advancements in thermal walls(contd.)
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.
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
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.
1. ASHRAE 1977, Fundamentals 33.4.
3. Solar Glazing: 1979 Topical Conference. Mid-Atlantic Sea, 2233 Gray's Ferry Avenue,
4. Modem Plastics Encyclopedia. Vol. 54, No. 104. McGraw Hill, Inc., 1221 Avenue of the Americas, New
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.
10. A parametric study of trombe walls for passive heating of buildings, Guohui Gan,Energy and buildings,
11. The comparison of Trombe wall systems with single glass, double glass and PV panels,Basak Kundakci
Koyunbaba Zerrin Yilmaz b