SUNNY WALLS VS. SUNNIER ROOFS
A study on the advantages of roofs for solar collection

by
Thanos N. STASINOPOULOS
Architect PhD NTUA, AAGradDipl.
Laskou 30, GR-156 69 Papagou, Greece
delaxo@central.ntua.gr

A paper accepted in PLEA 2001 - The 18th Conference on Passive and Low Energy Architecture
Florianopolis - Brazil, 7-9 November 2001
Paper Code PL01-201 

ABSTRACT:
This study focuses on the widely accepted principle that the equatorial sides of a building offer the optimum solar potential for solar space heating. A comparison between the solar irradiation on the south walls & horizontal roofs of buildings in London & Athens highlights the energy benefits of facing the sky rather than the equator. Four building examples exemplify practical ways to utilize the rather neglected potential of roofs as solar collectors for space heating.

1 INTRODUCTION
A common rule of thumb in passive solar design is that the elevation of a building facing the equator offers the highest potential for solar collection; therefore the equatorial side of the building is the favorite area for applying solar heating techniques like direct gain, conservatories or Trombe walls. This approach is based on the assumption that solar irradiance on surfaces directed to the equator is higher than to any other orientation.
But is it truly so? Always & everywhere? When solar data indicates higher incident energy on the equatorial elevation, is this what we really get in practice? And besides the energy per square meter, what about the total amount of irradiation on the entire collector? If one wishes to harvest as much solar energy as possible through the building envelope, is the equatorial side the optimum option? These are questions addressed by the present study, having as examples two locations with differing solar regimes, London & Athens.

 
2 OBSERVATIONS
2.1 Radiation Data
Radiation data from London & Athens (see Table 1) supports the presumed superiority of the equatorial side during some of the winter months (Figure 1) [in this study, 'winter' denotes the period from November to April, when the average ambient temperature is less than 10oC in London and 18oC in Athens -see temperature data in Table 1].
However, if we consider the entire winter period, the sum of incident energy on a vertical south surface is less than on horizontal, the highest values being for a 40o slope (Figure 2). Naturally, the winter sum of irradiance on vertical walls directed away from south decreases -down to 1/3 of the horizontal (Figure 3).
These observations are more distinct in Athens due to higher solar paths, emphasizing the need of a different strategy for solar collection as we move closer to the equator.

Figure 1:

 Monthly irradiance on a south vertical wall in London & Athens as % of horizontal.

Figure 2:

Winter sum of irradiance on south slopes in London & Athens as % of horizontal (slope=0o).

Figure 3:

Winter sum of irradiance on vertical walls directed from south (0o) to north (180o) in London & Athens as % of horizontal.
2.2 Practical drawbacks
In practice, the brief supremacy of equatorial elevations in terms of irradiance, is often hampered by several obstacles:
Size: The area of the collecting surface is confined by the total size of the equatorial wall, which sometimes is too small due to site or functional restrictions.
Orientation: The equatorial side of a building might be unfit for solar use, being for instance a blind party wall next to an adjoining building or to a steep hillside.
Shading: The vertical sides of a building are frequently overshadowed by neighboring obstacles such as buildings, hills or trees, that reduce incident radiation -especially during winter when the sun is low.
Safety: Solar collectors usually consist of glazing which -near ground level- poses a security risk and increases the possibility of vandalism, thus adding to the construction & running cost.
Indoor effects: Applications like direct gain or conservatories can create overheating, glare or privacy problems, sometimes forcing the occupants to constrict or even block solar access, canceling the passive process.
Cloudy sky: Irradiance on the equatorial elevation drops considerably under overcast sky, which usually implies adverse weather conditions that increase energy requirements. In that case, maximum solar input comes from horizontal surfaces that face the entire sky.
Due to these frequent facts, another part of the building envelope can often be more advantageous than the equatorial side: Roofs as solar receivers present none of the above drawbacks; furthermore, they may provide higher total solar input due to their large size which compensates for the lower irradiance during certain periods. The potential of roofs for solar collection is examined below through a simple comparison between the irradiation on an equatorial wall and a flat roof.
 
3 WALL VS. ROOF
3.1 Annual comparison
Let us assume a flat roof sized WxD [width x depth] receiving RH solar radiation and a vertical wall facing the equator sized WxH [width x height] receiving RV. The ratio RH /RV is

RH /RV = (iH .W.D) / (iV .W.H)   or

RH /RV = (iH /iV) . (D/H)           (1)
where iH & iV is the incident radiation on the horizontal & vertical surface per unit area (the effect of width W is the same on both surfaces).
From (1) we conclude that a roof receives more energy than a vertical equatorial wall when

D/H>iV /iH               (2)
Exploring relation (2) in practice, equation (1) has been applied on a number of simple rectangular volumes taking into account radiation data of London & Athens (Table 1). The Depth/Height ratio (D/H) of the volumes is taken as 1/4, 1/2, 1, 2 & 4, as shown in Figure 4.
Figure 4:
 The volumes of the study in section along N-S axis
Figure 5 shows the seasonal variations of the RH /RV ratio for each D/H value. In both locations the irradiation on the south elevation exceeds that on the roof only in the case of 'shallow' buildings, that is those where the depth behind the south elevation is less than its height (D/H<1). In 'deep' buildings (where D/H>1), the roof generally receives more solar load than the south elevation.
Figure 5: Monthly values of the RH /RV  ratio (%) between irradiation on a flat roof & a south wall in London & Athens, according to the D/H proportion of the building assuming unobstructed solar access [logarithmic y-scale].
London            Athens
3.2 Winter period
To compare the potential for space heating applications, we take into account only the winter values of the RH /RV ratio, which are displayed by the continuous curve in Figure 6 as a function of the D/H proportion. At that period, the roof receives more radiation in buildings where D/H>1 (London) or D/H>0.8 (Athens).
Figure 6: Winter values of the ratio RH /RV (%) between irradiation on a flat roof & a south wall in London & Athens, as a function of the D/H proportion; dotted curves refer to 30% overshadowing on wall [logarithmic y-scale].
London Athens

The solar potential of the equatorial elevation is further weakened if we consider overshadowing -a fact occurring frequently on the sides of a building but rarely on its roof. This is illustrated by the dotted curves in Figure 6, showing the winter values of the RH /RV ratio assuming a 30% reduction of the irradiation on the south elevation due to overshadowing. Under such conditions (typical in urban contexts), the south elevation receives more solar energy in winter only if D/H<0.7 (London) or D/H<0.5 (Athens).
If the building is rotated off the N-S axis, solar irradiation on the equatorial elevation decreases accordingly (see Figure 3), but that on the roof remains unaltered. This underlines the fact that the solar capacity of roofs is independent of orientation, a convenient feature in cases where the alignment of the building is dictated by factors other than solar access.
These observations highlight the solar potential offered by roofs due to their large size, free from overshadowing & orientation restrictions. A few examples on how to utilize that potential follow next.
 
4 SOLAR ROOFS
4.1 Current practice
Roofs are regularly used to install solar collectors for domestic hot water or PV arrays, but not much so for space heating applications. Passive solar devices like glazed openings (for direct gain) and conservatories, Trombe walls or air collectors (for ventilation preheat) are typically attached to the equatorial elevation of the building rather than on its top.
In that manner the sun benefits just the equatorial side of the interior, while the rest can only be affected indirectly, especially in deep plans (an exception is the Barra-Constantini system which transfers solar heat through air deeper than the other techniques).
4.2 An alternative approach
But the top of a building can also accommodate most of the usual passive solar means commonly installed on the equatorial side, adding certain advantages:
Roofs do not present the shortcomings of the equatorial walls described earlier (insufficient size, improper orientation, over-shadowing, vandalism etc.).
Total energy input from the roof can be much higher than from the equatorial side, depending on size (D/H ratio) and season.
In several building types like factories, sport halls or schools, roofs are typically large surfaces used only to shield the volume underneath; the additional function of solar collection can transform a usually idle structure into a productive component.
4.3 Summer heat
In most solar applications for space heating, summer overheating is a crucial risk. This is more so if the solar collectors are integrated to the interior like typical windows or conservatories, because they cannot be 'shut down' since they are linked to other functions like view or space use.
The overheating risk is higher in roofs, given the intense radiation they receive from the sun above. But, just like the usual solar collectors for water heating, solar devices on the roof can
- be installed outside the insulation layer, thus preventing the conductive heat flow towards the interior and
- be turned-off, ceasing the convective heat transfer (e.g. shutting-down the fans that draw warm air from a roof conservatory).
Furthermore, they can function as solar chimneys, promoting the extraction of warm air from indoors.
 
5 EXAMPLES
5.1 Roof conservatories
Figure 7 shows two typical apartment blocks on an E-W narrow street in dense downtown Athens, with conservatories added on the roof & upper balconies. Warm air from the top conservatory is driven by fans to the flats below via existing air shafts; in summer, indoor air is expelled outdoors through sliding sections of the glazing at opposite sides.
Besides the energy benefits, the roof sunspace can accommodate various activities by the occupants below, from drying clothes to growing plants, offering also a play area protected from bad weather, thus justifying its extra cost in several manners [3].
Figure 7: Cross-section of typical apartment blocks in Athens with conservatories fitted on the upper floors & roof.
Figures 8 refers to a two-storey school in Central Greece currently under construction. A sunspace over the middle corridor is used for ventilation preheat and to enhance daylight in the classrooms through inner windows. In summer, warm air from the classrooms raises to the sunspace, then it is expelled outdoors. The concept is applied on both legs of the L-shaped plan of the school independently of their orientation.
Figure 8: Cross section of a school in Central Greece with a sunspace over the middle corridor.
5.2 Roofs as air collectors
Another approach -perhaps more cost effective than a roof conservatory- would be to transform the entire roof into a large solar air collector by modifying its design and materials. In the two examples presented below, ventilation preheat is achieved trough the roof at a marginal extra cost.
Figure 9 shows a section of a two-storey detached house east of Athens. The roof is facing south at a 13o slope and it consists of two layers of corrugated metal sheets with an air gap between them. Ambient air enters from the bottom end of the gap and is heated by the solar radiation absorpted by the dark-coloured upper sheet. Warm air gradually raises to the top where a transparent cover enhances its temperature and flow, then it is collected in a well-insulated 'air tank' above the false ceiling. A centrifugal fan with dampers propels the warm air into the interior through the hollow floor for mass storage or alternatively expels it outdoors, according to seasonal & diurnal conditions (for details see [4]).
Construction has not been completed, so the actual performance of this simple scheme has not been tested yet. However, a similar air system commercially available in North America is claimed to yield up to 500W for an air flow of 25m3/h per m2 of opaque collector [5].
Figure 9: Cross section of a house east of Athens featuring an opaque solar roof.
Figure 10 shows the same concept being applied on a detached house at the northern outskirts of Athens, with the roof directed 20o SW at a 15o slope. In this case translucent plastic sheets are used instead of the top "roof-tile looking" corrugated metal, a choice that will boost performance. Construction of this project is on the way.
Figure 10: A similar solar roof design applied on a detached house at the northern outskirts of Athens.
6 CONCLUSIONS
The examples shown here illustrate techniques to utilize the high solar potential of roofs -a standard but hardly used element in all buildings- using normal building components. Solar roofs -opaque or not- can supply preheated air without the restrictions imposed by equatorial walls, while offering extra advantages:

Collection area can be larger than the equatorial side, with minor concern for orientation or overshading and at a low extra cost per kWh.

All sides of the building are available for other functions, since solar energy is collected by a normally idle element located 'out of the way'.

Air as heat transfer fluid imposes fewer technical considerations than the water systems, like caution for leaks, corrosion or frost, consequently reducing building & running cost.

Solar heat is kept outside the inner volume, minimizing the overheating risk.
Table 1: Climatic data for London & Athens; green figures indicate maximum irradiance per month; columns WS give the irradiance sum for the winter period. (data from [1] for average sky conditions, ground albedo 0.2)  

LONDON

 

 

 

 

 

 

Daily irradiance on south slopes, kWh/m2 

 

 

 

 

Tilt

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

WS

0o

0.6

1.1

2.1

3.0

4.1

5.0

4.4

3.6

2.7

1.6

0.8

0.5

242

10o

0.6

1.2

2.3

3.2

4.2

5.1

4.5

3.7

2.9

1.7

0.9

0.6

265

20o

0.7

1.3

2.4

3.2

4.3

5.0

4.4

3.8

3.0

1.9

1.0

0.6

282

30o

0.7

1.4

2.5

3.2

4.2

4.9

4.3

3.7

3.1

1.9

1.1

0.7

292

40o

0.7

1.4

2.6

3.2

4.1

4.7

4.1

3.6

3.1

2.0

1.2

0.7

296

50o

0.7

1.4

2.6

3.0

3.8

4.3

3.9

3.4

3.0

2.0

1.2

0.8

294

60o

0.7

1.4

2.5

2.8

3.5

3.9

3.5

3.2

2.9

2.0

1.2

0.8

285

70o

0.7

1.4

2.4

2.6

3.2

3.5

3.1

2.8

2.7

1.9

1.2

0.8

271

80o

0.7

1.3

2.2

2.3

2.8

3.0

2.7

2.5

2.4

1.8

1.2

0.7

252

90o

0.6

1.2

2.0

2.0

2.3

2.4

2.2

2.1

2.1

1.6

1.1

0.7

229

Mean ambient temperature, oC

 

 

 

 

 

 

 

4.2

4.5

6.6

9.5

12.6

15.8

17.5

17.1

14.9

11.6

7.5

5.3

 

ATHENS

 

 

 

 

 

 

Daily irradiance on south slopes, kWh/m2 

 

 

 

 

Tilt

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

WS

0o

1.8

2.6

3.8

5.1

6.4

6.8

6.9

6.2

4.9

3.4

2.3

1.7

524

10o

2.0

2.8

4.0

5.3

6.4

6.9

7.0

6.4

5.2

3.7

2.6

2.0

565

20o

2.1

3.0

4.2

5.2

6.3

6.7

7.0

6.5

5.3

4.0

2.9

2.2

593

30o

2.3

3.1

4.2

5.1

6.0

6.4

6.7

6.4

5.4

4.2

3.1

2.4

608

40o

2.3

3.1

4.2

4.9

5.6

5.9

6.4

6.1

5.3

4.2

3.2

2.5

608

50o

2.3

3.1

4.0

4.5

5.1

5.4

5.9

5.7

5.1

4.2

3.2

2.6

595

60o

2.3

2.9

3.8

4.1

4.5

4.7

5.2

5.2

4.7

4.0

3.2

2.6

568

70o

2.2

2.8

3.4

3.6

3.8

3.9

4.4

4.6

4.3

3.8

3.1

2.5

529

80o

2.1

2.5

3.0

3.0

3.1

3.1

3.6

3.8

3.7

3.5

2.9

2.4

480

90o

1.9

2.2

2.6

2.4

2.3

2.3

2.7

3.0

3.1

3.1

2.6

2.2

421

Mean ambient temperature, oC

 

 

 

 

 

 

 

9.4

10.3

11.7

15.8

20.6

25.2

27.9

27.8

23.9

18.7

15.0

11.4

 

 
REFERENCES
1 T. N. Stasinopoulos, Geometric Form & Insolation, PhD thesis, National Technical University of Athens,1999, with radiation data on slopes computed according to [2].
2 J. K. Page (ed.), Prediction Of Solar Radiation On Inclined Surfaces vol. 3, Reidel, 1986.
3 A 1985 graduate student project by the author.
4 www.ntua.gr/arch/geometry/tns/ecompetition/.
5 The Canadian Conserval Engineering Inc. has developed a vertical air collector system with opaque metal sheets -see www.solarwall.com.