Solar Collection and Energy Transport.
Solar Collection and Energy Transport.
Fixing of atmospheric Carbon
into the biomass photosynthesis is a form
of solar collection and energy storage.
The Key to efficient use of solar energy lies in efficient modes
of heat transfer. There are three possible modes:
Transfer of energy via the
vibration of atoms in some medium. The best conductors are
silver and copper.
Transfer of energy via
the bulk motion of a medium (e.g. air, water)
a material to thermal equilibrium via radiation losses.
Stored energy re-emitted (Power per unit Area) goes as T4
Thermal Conductivity of some Materials in relative Units
- heat energy flows from regions of high temperature to low
- wide variation among different materials depends on
the density of free electrons in the material
So Clearly metals are required for efficient heat transfer.
- Silver 1.0
- Copper 0.93
- Gold 0.70
- Aluminum 0.48
- Steel 0.12
- Concrete 0.004
- Brick 0.002
- Wood 0.0002
- Air 0.00005
Heat flow through a wall:
Convection: liquids and gases transfer heat this way
motion of the medium between regions of different temperatures.
Fireplaces produce natural convection
warm air rises
and is replaced by cold air
Most space heating systems operate via convective heat transfer
Material absorbs sunlight and heats up and then re-radiates that
as long wavelength infrared radiation (heat radiation).
Was effectively used centuries ago in American Southwest (
Sandstone dwellings and walls would absorb much sunlight during
the day and then re-radiate that as heat at night.
In general, recovering incident solar radiation via subsequent
thermal radiation of materials is not practical and large amounts
of thermal mass are required. This can be a) expensive and b)
- efficiency depends upon specific heat of material and
its thermal conductivity
- Specific Heat measure of how much energy a substance
can store. This is measured on a scale of 0 to 1. Water has a specific
heat of 1.
- Thermal conductivity measure of efficiency of heat
transfer (i.e. getting it back when you want it)
Values for typical materials:
Water is the clear winner followed by concrete. So thermal
mass is most effectively used in the form of large tanks of
water or several tons of concrete in an insulated container.
Solar Energy: Collector Systems
Variation of Incident
Sunshine with location in the US
Flat Plate Collector Systems:
Heat transfer to a circulating liquid (antifreeze) to be
used as supplemental space heating source in the Winter
Basic System Design
On average, a house loses 1 BTU per cubic foot per degree day
A degree day = 65 - average 24 hour
- 1500 sq. feet and 8 ft ceilings 12,000 BTU per degree day to
maintain interior heat at 65 degrees
- Consider a winter day with average 24 hour temperature = 15 F (
50 degree days) need 600,000 BTUs to keep house at 65 degrees
- 100,000 Btus comes from internal sources (lighting, cooking,
families, TV, etc) need 500,000 more BTUs
- Assume incident radiation is 600 Watts per sq. meter on a slanted
collector surface averaged over 8 hours.
- There are 3.41 BTUs per Watt so this is equivalent to 600 x 8hrs x
3.41 = 16,400 BTUs per sq. meter per day (8 hours)
- Assume collector efficiency is 50% (this is not a photovoltaic
device; we are merely heating something up and transferring that
heat to a working circulating fluid)
- The net yield is then 8200 BTU per day per sq. meter
- We need 500,000 BTUs how many sq. meters are required?
- 500,000/8200 = 60 sq meters 648 sq feet (25x25 feet of collectors)
- Water stores heat at the rate of 62.4 BTUs per cubic foot per
- Stones 20 BTUs per cubic foot per degree F (lower efficiency
due to voids in the storage unit)
- Heat water to 130 F and extract heat from it until it cools to
80 F. Total volume of water required is:
500,000/(62.4*50) = 160 cubic feet = 1250 gallons
- For Stones:
500,000/20*50 = 500 cubic feet = 25 tons of stones !
Energy losses in a flat plate collector system:
- Not all solar photons penetrate black collector surface
- Once the collector becomes hotter than the environment heat
loss by conduction, convection (aided by wind) and radiation
- Transmission/reflectance depends on incident angle
- efficiency starts to rapidly go down if the collector becomes
too hot as the thermal radiation (which goes as temperature to
the fourth power) dramatically increases
- Overall effect is that the collector system has an efficiency
which decreases with higher temperatures. But the effect is not
- Temp. difference between collector and surroundings = 20
F efficiency is 74%
- Temp. difference between collector and surroundings = 100
F efficiency is 48%
Single glass cover admits most sunlight but also most re-radiated
infrared radiation. Best at low temp differences
Three layers of glass -> admits fewer photons but also inhibits
radiative losses best at high temperature differences
Can fiddle with number of glass covers on the front of the collector:
Orientation of collector with
respect to sun is crucial part of overall efficiency
Not practical for the homeowner somewhat dangerous due to
Parabolic reflectors (heliostats) are pretty expensive