Active Solar heating

1. Active Solar heating • Used for space and or water heating • Flat plate collector system

2. Elements of a flat plate collector • Cover (also called glazing) protects the system and keeps heat in. • Absorber plate-absorbs solar energy. Usually made of a metal that is a good conductor of heat such as aluminum or copper and painted with a coating that helps absorb and retain the heat (black paint is the lowest order of these types of coatings) • Insulation on the bottom and sides to reduce heat losses. • Flow tubes –air or fluid to be heated flows though these tubes

3. How does this work? • Cover is transparent to sunlight, so the light passes through the cover to the absorber. • The absorber will absorb energy from the sunlight and then try to re-emit it to come into thermal equilibrium with its surroundings. But the absorber re-emits the energy at infrared wavelengths. • Glass allows visible but not infrared radiation to pass through, so the energy emitted by the absorber is absorbed by the glass. • The glass re-emits this energy to the outside air and back into the collector. • The energy trapped in the collector heats the inside of the collector, and this energy is transferred to the air or fluid in the tubes via conduction

4. How does this work? • The energy emitted from a hot surface is described by Stefan’s Law: P/A = εσT4 Where ε is the emissivity (describes the degree to which a source emits radiation, ranges from 0 (no emission) to 1 (a perfect emitter) and σ is the Stephan-Boltzman constant = 5.67 x 10-8 W/m2 K4. P/A is the power emitted per unit area, T is the temperature in Kelvin.

5. How does this work? • The wavelength at which this energy is emitted from the surface is described by the Wien Displacement Law: λmax(μm)= 2898 T(K) This gives the wavelength at which an object emits the maximum amount of energy

6. Types of flat plate collectors • Liquid flat-plate collectors heat liquid as it flows through tubes in or adjacent to the absorber plate. • Often unglazed

7. Types of Flat plate collectors • Air flat-plate collectors – used for solar space heating. • The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. • The air flows past the absorber by using natural convection or a fan. • air conducts heat much less readily than liquid does, less heat is transferred from an air collector's absorber than from a liquid collector's absorber, and air collectors are typically less efficient than liquid collectors

8. Types of Flat Plate Collectors • Evacuated Tube collectors -usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiative heat loss. • Air is removed, or evacuated, from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. • Evacuated-tube collectors can achieve extremely high temperatures (170°F to 350°F), making them more appropriate for cooling applications and commercial and industrial application. However, evacuated-tube collectors are more expensive than flat-plate collectors, with unit area costs about twice that of flat-plate collectors.

9. Limitations • Need a storage system for cloudy days and nights. • Amount of solar energy that is usefully collected is 50%. • To heat 100 gallons of water a day from a temperature of 50° to 120° you need a collector with a surface area of 112 square feet. That is one panel 9 ft x 14 ft. This would fill a good portion of our classroom • Where do you put it? In the back yard, on the roof? • Are there structural, aesthetic considerations? (Al Gore’s troubles with installing solar panels)

10. 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

11. 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.

12. 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

13. 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

14. 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).

15. Storage • The heat stored in the thermal massive 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.

16. Chicago City Hall green roof

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

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