I will try to give you some technical information
about the parameters involved when designing solar collectors.
TECHNICAL INFORMATION
This page is intended to give some answers about solar energy,
and the reason it has'nt been used more extensively.
The sun is radiating the earth globe at an effect equal to 170,000 TW (TeraWatt),
some claims 200,000 TW, perheps depending of the calculate method.
I, myself estimate that we receive 100.000 TW at the ground.
Anyway,
1 TW = 1,000,000 MW (MegaWatt), 1 MW = 1,000,000 W (Watt), making the total
effect a fairly strong.....
To comapre with the total effekt of all the energy we are using, including
oil, coal, gas, nuclear and so on equals 12 TW.
Having the sun at a medium elevation, we are receiveing 1000 W/m2 (Watts per
squaremeter) a sunny day. 1100 W/m2 has been measured at Mount Everest heights.
A pequliar thing is that snow or buildings are able to increase the radiation
significantly.
On a snowcovered lawn, I measured 1300 W, a sunny winterday.
On a dual split solar collector, we metered far better effect on the side
that was least covered by the sun. We found out that an opposite mounted roof
acted as a reflector.
Sun radiation is composed of electromagnetic waves within a spectre of
3,000 to 24,000 Angstrom (wavelength) 1 A = 0,0000001 millimeter.
Instant radiation has the strongest wavelength at 4000 A, whilest indirect
radiation (cloudy weather), has strongest wavelength at 5000 - 6000 A.
Ultraviolet 3000 - 4000 A are also significant at instant radiation.
Black object radiation (heat radiation) is the spectre between
20,000 - 200,000 A. This is what is radiating away from your solar collector
when it is beginning to heat up.
X-rays 0.1 - 3000 A are defined electromagnetic waves as well as radar,
radio or common alternate current (AC).
The scientist world has not yet decided whether to define light as waves or
particles (fotons), but state it could be both. As a matter of fact, it has
been proven both.
This rises new questions about what is beyond light emissions.
News from Chandra X-ray observatory implicate X-rays to be a form of light,
as they can use x-rays to look at far distant dark objects in the space.
When we shall choose matter for the solar collector, we want it to be able
to receive all sun radiating spectre.
About thermal solar collectors.
MAXORB adhesive sheet has absorbing rate of 97 - 99 %, and black chrome on
cupper the same. In fact, we have come almost all the way.
Now, when radiation has turned into heat inside the absorber, we want the
heat to stay put, therefore the absorber shall have the quality of low heat
emission (black object radiation). We have developed good material for this
purpose also, and a common value is 10 %, ie. the matter radiates 10 % of
the heat.
I think even better material are on the way.
The heat, now landed on the absorber must be prevented from leaking out to
surrounding air, and this is where insulation comes in handy.
Insulation is mainly used for plane thermal collectors.
Reflector collectors has tiny absorber area, so insulation has little
significance.
Lambda, (specific heat loss), or U-value lately named, is the ability of
a matter to hold different temperatures apart.
Also named specific heat conductance.
Insulation is very important to do when we accumulate or transfer
heat or cold.
Most common matter is spinned glass (Gullfiber) or spinned rock (Rockwool).
The values are 0.04 - 0.05 W/m3*C (Watts per cubicmeter and C degree).
The heat leakage from one cubicmeter of Rockwool is 0.05 Watt whith one
degree temperaure difference between the sides.
1 W leakage with 20 degree difference.
Common air specific heat conductance is 0.0242 W/m3*C.
Better performance is difficult to acheve as long as air act as medium.
Cellular plastic has better values due to small cavities filled with a low
conductive gas, such as Diclordiflourmetan (freon).
But the cavities has a small leakage, so the gas is eventually replaced with
air, and the freon is loose in the atmosphere.
Our old fridges and freezers are insulated whith cellular plastic.
This is the reason our frigdes are blamed to be pollutive to the stratosphere.
Development of new insulation materials (AEROGEL) has eliminated these
disadvantages.
AEROGEL is a highly hygroscopic glass material compsed of millions of
microcavities filled with a gas. The freon is replaced with different
mixtures, most common hydrocarbon and ethanol. AEROGEL is leakproof but
fragile against impact damages or breakage.
It has also better performance due to thinner cavity walls.
If you are to replace your fridge, be sure to buy this kind of equippment.
AEROGEL is not yet used to solar collectors, maybe due to the fragility of
the material. Solar collectors are faced to extreme conditions such as gails
and heavy snow.
All solar collectors has to be light transprent at the front.
Plane thermal collectors are insulated at rear and sides.
Transparency is a look thru capability and is measured in percent.
Common window glass value is 87 %. Specially composed lowironglass has 91 %.
Glass has also a highly reflective surface, making the transparence
angledependent. Efforts to solve this problem has lead to structured surfaces,
but to me it has resulted in severe cleaning difficulties.
Recently a Swede has developed a process to add a thin film of silicon to
the surface, improving transparency from 87 % to 92 % for common window glass.
The long term reliability is to be proven.
There is a small space between glass and absorber (glass distance), and this
distance is very important, as it determine both insulating and thermal air
movement (convection). Troublesome is that they go opposite ways. When you
increase the distance, insulation increases, but faster air movement makes
greater heat loss anyway.
Merging the two diagrams gives a dip (best distance) at different
mounting angles,
upright mounting (90 degree) says 25mm distance,
and ground level mounting (0 degree) says 50mm distance.
As your collector will be mounted somewhere in between, your best distance
will also be somewhere in between.
Increasing the front insulation (konvection obstacle) has been made using a
thin plastic film (TEFLON) beween absorber and glass, no doubt increasing
net collected energy.
Difficulties to secure the film between absorber and glass makes the long term
benefit uncertain. As a result, price rises also.
Efficiency is a major aim, when you shall choose material, but you must take
into account both strength and corrosion stability.
The collector shall whithstand gails, heavy snow, sulfur air, air humidity,
ultraviolet radiation, extreme temperature differences and variations inside
the collector, waterpressure, exhaustion and freezing damages.
Among a few things.....
To mix everything to the best isn't as easy as that, or ?
Now, the collector shall be cheap also, as the last and most significant
parameter.
Now, you know some of the reasons, solar collectors has not yet overflowed
the market.
Be happy
SOLARENERGY (INSTANT RADIATION)
Solarheating (hydraulic systems)
There are three branches being developed.
1. low temperature plane thermic solarcell. The task is to capture a wide
square of sunradiation, lock it and transfer to water flowing thru. They
are difficult to insulate to a resonable price, but they are solid and
firm mounted, so they are well suited to whithstand the element of nature.
Reaching 100 C (212 F) degrees.
2. High temperature concentrating solarcell. Works as a refractor or a
reflector an must trace the sun to work. They can reach up to 300 C degrees.
There is a variant model named "cylindric parabolic reflector" used by
LUZ intl ltd. It works as a semi concentrating solarcell.
3. Vaccumsolarcell. Works as a transparent thermos bottle, is firm mounted
and reach 300 C degrees. They are fragile and need a solid frame to be
mounted at. Named high temperature.
Electric energy (steam engines, turbines)
There is a company in California "LUZ International ltd" who has
constructed a plant for electric energy with a combination of 2. and 3.
This way they have reached 450 C degrees and are able to power turbines
for electric generation.
Solar towers. is another method to reach high temperature, thow they
are heavy to service due to an endless number of mirrors, which all has
to be tracing the sun.
A wide area, filled with mirrors, a boiling tower
at the south end of the field is a rough description of a solar tower.
There is a variety of combinations and variations on the market, but
these are the three branches to separate.
Electric solarcell. They are even more expensive, and isīnt for heating
purposes but more likely to power small electric household equippment.
