Cost effectiveness• Assume a $5000 system• Pays itself off in 27 years if replacing a natural gas

or oil hot water heating system• 14 years if replacing or supplementing electric

hot water heating• Between 1980 and 1985 there were tax credits

for installing these systems. You could install one up to $10,000 at no personal cost.

• Similar credits have been reinstated in 2005 and in the stimulus package

Passive Solar

• Makes use of natural solar heating• Requires buildings be designed to maximize

the suns heating• Most important element: face south (toward

the sun)!• Requires 3 design elements: insulation,

collection, storage• Passive because it does not involve pumps,

fans, fuel, electricity etc.

Insulation

• Keep the heat in!• Walls, floors, ceilings must make use of

materials that help hold in the heat.• Doors and windows must also be designed to

maximize heat retention in the building• Most modern buildings ignore these ideas

Collection

• Need a way to collect the sun’s energy• One way is large windows on the south face of

the building• Another way is a passive solar collector on the

south wall– In the collector, the heated air rises and flows into

the structure, while the cool air from inside sinks and flows back into the collector. No need for fans, this air flow sets itself up naturally

Storage• Need a thermal mass inside the house• Thermal mass-any material that can absorb solar

energy then cool down later giving its energy back to its surroundings.– Example – Why is it always warmer in cities than in

the country in the summer, especially at night?– Buildings and roads act as a thermal mass, heating up

during the day and releasing that heat at night• In our building the material has to hold enough

heat to keep the temperature constant at night or over a cloudy day(s).

Storage• The heat stored in the thermal mass is not much greater than the usual

temperature of the structure, thus a lot of it is needed.• Water is an excellent thermal mass. Tanks of water could be stored just inside the

windows, but that’s not very aesthetic. Another way to use water is a roof pond (yes a pond on your roof!) or green roof (yes your garden on your roof).

• Example-Since Chicago installed a 20,000 square foot "green roof" atop City Hall five years ago(2006 report), the city has saved about $25,000 in energy costs.

• Trombe Wall: A massive concrete wall on the south side of the structure, with a space between it and the windows. The concrete wall acts as the thermal mass. Not only does the wall heat the air in the space and convection sets up a natural flow to warm the room on the other side, but the concrete itself will radiate into the room.

Chicago City Hall green roof

Storage

• Direct Gain method– South facing windows with thermal mass in the

floor and opposite wall to regulate and store heat.

Potential

• Based on sun angle, this figure shows the potential for passive solar use across the US

Solar Electric Power generation

• Two types:– Thermal -use sun’s ability to heat (usually water)

to create electricity– Photovoltaic devices- a device which directly

converts the Suns energy to electricity

Solar Thermal

• Obvious idea would be to use sunlight to boil water and provide steam to drive a turbine

• But what happens when you place a container of water in the sun-it typically does not boil!

• Need to concentrate or focus the sun’s energy to achieve this goal

• How do we focus sunlight?

Basic properties of light

• To answer this question, lets look at some basic properties of light in the wave description of light.

• Refraction-light is bent at the interface between two media.

• Snell’s law relates the angle of incidence and the index

of refraction of medium 1 tothe angle of refraction and index of refraction of medium

2.

Focusing light

• If the interface is flat, the light is not focused.

• Example-pencil in a glass of water

• If it is curved in the correct fashion, i.e. the surface of a convex lens, the light can be brought to a focus

convex

concave

Fresnel Lens• For the most part, lens

are very heavy, suffer from reflection at the surfaces, and are expensive to construct to the sizes needed to achieve the desired heating.

• There is one type of lens, a Fresnel lens that can be inexpensively constructed from plastic

Fresnel Lens

• Seen in lighthouses-used to form a concentrated beam of light.

Fresnel Lens at work

• Fresnel lens melting brick

• International Automated Systems Fresnel system

Reflection

• When light is incident on a surface, it can be reflected

• An interesting result is that the angle of incidence (incoming angle) equals the angle of reflection (outgoing angle.

Reflection from a curved surface

• When the surface doing the reflecting is curved, the light can be brought to a focus.

• The curved surface can be parabolic or spherical.

• Spherical surfaces are cheaper and easier to construct.