Basic energy principles without being over technical

SOLAR ENERGY

Is often said that solar hot water panels are not very efficient?
But if they were twice as efficient, then they could be half the size - and i dont think that size is really the issue?
Nor is the question (and answer) so simple.

So really the basic question is;
Are solar hot water panels effective?

Solar panels, even typical domestic can boil water and even produce steam, but only a kettle or three.
Depending on the ratio of solar panel area to water heated - you can either have a very small amount of very hot - OR a very large amount of luke warm = swimming pool.

The first high temp/ steam senario is low efficiency - The second swimming pool senario is highly efficient - Circa 90% - Meaning maximum % conversion of solar energy into heat energy.

For most people, the fitting of a swiming pool in the airing cupboard is not practical.
Nor
However a cylinder store is usually possible and the art is in optimising the solar colector and ratio to storage volume.
The comprimise is between solar efficiency/ space available/ usefull temperatures.
See cylinder sizing

The system size/ ratio's you will find here are balanced to provide domestic hot water. Circa 55'C
Around .75 M2 of flat plate roof panel or 0.5M2 of evacuated tube surface area will provide an average 55 litres per day (one person) at 55'c from Spring - Autaum with neglegable input in winter and an excess mid summer.
See solar sizing (and below) for more info
Ultimately the question should be 'are they economic' ? I suggest you go read the 'eco-nomic' page!

ENERGY FROM THE SUN

Heat and light are two ends of the same spectrum.
Long wave = heat and short wave = light.

You can heat metal from red hot to white hot. Heat = Light! (Infra red to Ultra violet - see our rainbow logo)
Think of the regular Edison Incandescant light bulb.
If you have more of one, light or heat, then you have less of the other, but the energy total remains the same.
This is why an energy efficient light bulb produces less heat and the older incandescant, a 'warmer' glow.
If you think of the 'shimmer' created by the hot sun on a black tarmac road - this is where the amount of heat - long wave re-irradiation is great enough to effect the visable spectrum - distorting the ratio what we otherwise call 'normal' - the appearance of the world we live in which is basically light/ energy.

With a solar (hot water) panel, we work with the principle that light will travel through space (and around 80-83% through glass ) whereas radiant heat, only a very small % (without getting too technical/ complicated)

Black is the colour the eye percieves when visable light has been 'absorbed' - more acuratly stated as not having been reflected.
Regular B&Q matt black paint will absorb around 80-83% - but will 'emit' an equal amount of energy/ heat - which is why motor cooling radiators/ electrical heat sinks are matt black - and not used or reccomended for solar collectors.
Once light has been absorbed, this light energy will degrade (entropy) to heat/ thermal energy.
If the colour black is the surface of a heat conductive material - metal - (tightly packed with molecules/atoms) copper!
The more tightly packed the molecules, the higher a materials 'specific heat capacity'
Because cold is simply the absense of heat just like dark is the absence of light, then each excited molecule/ atom will conduct its excitment to its lesser excited neighbor - conducting energy!
Total energy absorbed = 'thermal mass' whether a little of hot or lots of warm.
Insulation to back and sides reduces thermal losses - insulation = 180% inverse to conduction.
The function if the glass cover is to trap the heat/thermal energy thus eliminating convection losses (heat loss via fluid air movement) - aka greenhouse effect.

Very special 'selective coatings' used on the solar absorber (inside the solar collector or 'panel') are surface colors (not really a coating) only microns thick. A technology advance over regular matt black paint.
The basic idea is that the colour surface thickness be equal to the shorter wave lengths within the light and glass transmitancy range, but not equal to the longer length heat waves. The longer wave lengths then hit the upper surface of the surface at an angle or off square, thus reducing the amount that get back through to the surface, or 'emit'
Effectively, this amounts to a one way energy valve. Very clever.
Also, by working with the surface structure, presenting maximum area - rough, matt to the light, but minimum, smooth, gloss to the surface underside - The selective coatings can inprove the absorbance:emmitance ratio from around 81:81 for B&Q matt black, to 84:13 for typical 'selectives' (meaning 83 in: 13 out (plus/minus a few either side)
Selecive coatings are the tommorows world technology from the 1970's and represent 95% of all modern collector systems.
Do not buy a collector that does not have a selective surface.

The difference between B&Q home made and selective has a significant impact, except in height of summer when outdoor air is not so different to desired the water temperature in panel.
The difference between one manufacturers selective surface and another is not that great and so shouldnt really be a defining issue.
Dont buy a non selective surfaced solar panel.

EFFICIENCY
ABSORBER COLLECTOR SOLAR PANEL

When solar manufacturers quote collector effieiencies of around 80-85% they are refering to the static or 'instantanios efficiency' - which assumes that the environmental/ outdoor air temperature is exactly the same.
This only exists in the laboratory or on paper.
What you will really be interested in is the overall collector effecieny which will include the increasing rate of heat loss as the collected temperature rises above the ambient outdoor air temp.
The is not much between all collectors including DIY painted radiators - when the air temp is not much lower than the heat/ fluid within the collector.
As the delta T or temperature difference increases however, the overall collector effeciency tails off.
This curve is steepest with a swimming pool collector (black hose pipe) and most shallow/ most effecient with an evacuated tube system (basically a selective coated absorber inside a transparent thermos flask)
A typical quality flat plate collector is perfectly adequate to provide hot water for most of the summer. You do not need your hot water to exceed 60'C and so evacuated tubes are not needed.
In the spring autaum however or even mid winter when you may have strong sunlight and cold air temperatures, then you will notice the difference. This will be a factor if you are looking to compliment wood burner heating and are wanting the solar to take over as the wood heating season ends.
Note that 85% of the annual solar energy arrives in the summer - 15% in winter.
There is marginal benifit in chasing winter gains - unless you are looking to space heat (see underfloor section) or compliment solid fuel.
If you have an automatic gas/oil system then the issue is simply of economics/ personal preference.

Do not get hung up on the technical differences within the flat plate OR evacuated categories.
There is not a lot between them.
Having chosen tube or flat plate (integrated or on roof) Pay more attention to life cycle.
There are no moving parts and no reason why your collector should not work almost indefinatly.
20-30-50 years is a very long time - choose a collector of good build quality for long life.
I am still servicing systems installed in the 70's (£125-£250 at the time - installed!)

SYSTEM
OVERALL SYSTEM EFFICIENCY

This is where the secret lies, the art of designing an optimised system.
This where efficiency gains can be gained or maximised, often for free.

You may have a top of the range solar collector, but if the rest of the system is poor it may be like powerfull engine with a badly tuned carbourettor and a 1 gallon fuel tank - or driving with a flat tyre. You wont get very far very fast.

Also If your solar system is not both matched and integrated with your top up and space heating system, it may be like choosing a

See the 'laymans Tech' pages within each section.

When the collector temperature inside is at the same temperature as the outside air (ambient) then we have the 'instantanious efficiency' of around 90-95% and this often quoted fiqure is true for almost all collectors - from black B&Q hose pipe to top end evacuated tubes.
Some solar sales companies quote this fiqure as the solar system efficiency which is very misleading!

As the collected temperature increases above ambient so does the rate of heat loss from the panel or 're irradiation'
The typical collector in real life opporates at around the 55% mark. (If correctly sized etc)
The lower the design temperature, the greater the usefull % of sun energy collected.

This being a comprimise between the
Cheapest and fastest payback but least effective - the B&Q hosepipe.
Expensive and longest payback but most effective - the evacuated tube.
The optimum design/ technology is therefore not neccesarilly the cheapest or fastest payback but one where the design life equals the payback.
The quality 'flat plate' collector (integrated if possible) often best meets (and exceeds) this criteria.