Solar
Energy Collection
Solar energy collection
systems transform buildings from energy consumers to energy producers
Considering solar
access when designing roofs, walls, windows and external shading devices
intended for solar collection offers the best economics and esthetics.
For the least collector area and lowest cost, building elements should
be arranged to optimize tilt and orientation to the sun. If this is
not possible, collector areas should be adjusted accordingly, and
mounts designed with appearance in mind.
Traditional solar
hot water and photovoltaic (PV) collectors are typically rectangular
and designed for rack mounting. However, new PV products designed
to be incorporated into wall and roof assemblies are now available:
- “solar
shingles”;
- panels designed
to integrate into curtain walls; and
- etching techniques
designed to be used as sunshades, with the designers’ choice
of shading factor.
Since these are
intended to replace conventional weather surfaces, they can be more
economical.
Collectors operate
best with unobstructed access to the sun. Shading studies of the building
are essential to ensure solar collection potential. Roof-mounted collectors
are easiest to tilt optimally and are less likely to be shaded than
collectors mounted on walls, though locations towards the top of walls
can help.
For maximum solar
energy potential, collectors should:
- face south
or, better, southwest; and
- be tilted at
34° from the horizontal (Santa Monica’s latitude).
Orientation and
tilt are forgiving; annual energy performance drops ~15% for collectors
oriented up to 45° from due south; and by less than 10% if collectors
are tilted 15° from the latitude angle, i.e., between 19°
and 49°.
A rough rule of
thumb will help designers in allocating solar water collector area
during conceptual design: 1 sq.ft. of flat plate collector area with
optimal slope and tilt provides slightly less than 1 U.S. gallon of
hot water per day, on average over the year. (Collector area should
be confirmed before finalizing form decisions, since performance will
vary with the collectors selected.)
Areas required
for solar electricity are typically much larger than for solar hot
water. Currently available PV panels are unlikely to satisfy a building’s
total electricity consumption without covering the roof and all non-north
elevations with panels and implementing extraordinary electricity
conservation.
For PV collectors
placed on walls, a narrow building oriented on a north-south axis
with the majority of the panels on east and west walls generates the
most power, especially in summer. East-west plans generate approximately
88% of the optimum; square plans approximately 75% relatively consistently
throughout the year.
Tilting south-facing
walls increases both the collection area exposed to the sun and annual
electrical energy generated. A 20° tilt from vertical produces
a 21% increase; 30° increases generation by 46%.
Cautions
- The best building
forms for solar energy collection differ from those for controlling
solar cooling loads.
- Integrating
solar collection into building exteriors requires careful detailing,
coordination between designers, site review and commissioning.
- Solar collectors
require access for periodic cleaning for the best performance.
