Cost effectiveness

1
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

2
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.

3
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

4
Collection

• Need a way to collect the suns 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

5
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).

6
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 thats 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.

7
Chicago City Hall green roof
8
Storage

• Direct Gain method
• South facing windows with thermal mass in the
  floor and opposite wall to regulate and store
  heat.

9
Potential

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

10
Solar Electric Power generation

• Two types
• Thermal -use suns ability to heat (usually
  water) to create electricity
• Photovoltaic devices- a device which directly
  converts the Suns energy to electricity

11
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 suns energy to
  achieve this goal
• How do we focus sunlight?

12
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.
• Snells law relates the angle
• of incidence and the index
• of refraction of medium 1 to
• the angle of refraction and
• index of refraction of medium
• 2.

13
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
14
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

15
Fresnel Lens

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

16
Fresnel Lens at work

• Fresnel lens melting brick

• International Automated Systems Fresnel system

17
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.

18
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.