*> > Jeff B. wrote:*

*> > > Hi there,*

*> > > Looking to a proposal made to us for a 2 kW PV generator (16.000 ), I*

*> > > was wondering whether we could design a cheap 1 kW PV generator using*

*> > > Fresnel lenses in combination with high efficiency multijunction PV*

*> > > cells.*

*> > > What if we put 4 triangular multijunction PV cells (or TASC, assembled*

*> > > into a 3.1cm x 3.18 cm square) close to the focal point of a Fresnel*

*> > > lens ?*

*> > > Could a PV generator comprising 25 Fresnel lenses in a 5 x 5 square*

*> > > (2m x 2m square) with their PV cells approx. 30 cm behind them, the*

*> > > whole thing being mounted onto a passive solar tracker, do it ?*

*> > > From the Spectrolab spec: 27 mW/cm (for 1 sun) are produced by the*

*> > > TASC. The Fresnel lens focuses approx 155 times the solar irradiance*

*> > > reaching the 4 TASC cells (3.1cm x 3.18cm) thus producing*

*> > > 27x155x(3.1x3.18)=41,25 W. If we consider 25 units of this kind*

*> > > (Fresnel lens+4 TASC), this would produce approx. 1 kW for approx.*

*> > > 2250 $ or 1450 (plus framing structure to add). Either there is an*

*> > > error in my rough calculation or this would be a cheap way to produce*

*> > > electricity. Do you think it is feasible ?*

*> > > Jeff*

*> > > 25 large Fresnel lenses from 3dLens or BhLens (http://www.3dlens.com/ *

*> > > shop/largefresnellens.php)=25 x 22.68 $*

*> > > 1 spectrolab box of 100 triangular PV cells (http://www.spectrolab.com/ *

*> > > DataSheets/PV/PV_NM_TASC_ITJ.pdf)=250 $*

*> > > 1 Zomeworks UTRK40 passive solar tracker (http://zomeworks.com/files/ *

*> > > pv-trackers/Track%20Rack%20Matrix%202008.pdf) = 1450 $*

*> > You only need one cell and one lens to test it, ???$0 (you could even*

*> > sell a kit, many world wide would like to try that :)*

*> > - I doubt it's that simple, some must have already tried. I read about*

*> > that kind of cells used with concentrators*

*> > - a problem must be that the light spot power distribution is not*

*> > homogeneous (*)*

*> > - the time life of the cell maybe reduced ?? (*)*

*> > - the cell must saturate at ?some light flux ?? (*)*

*> > - dissipating the energy is not a problem. With a 100 sun*

*> > concentrator, it would give at worse: 1 kW/m2 >> 0.1W/cm2 >> * 100 =*

*> > 10W/cm2. A 4C/W heat sink used in electronic can easily be fit under*

*> > the cell ($.5) and will work without a fan. With this,*

*> > you keep your cell below 70C. With that much power to dissipate, a*

*> > smart design, profiting of convection, would probably deeply boost*

*> > your heat sink efficiency.*

*> > Choose your heat sink at: http://www.mouser.com/catalog/634/1876.pdf *

*> > Usually they are in aluminium.*

*> > If you use circulating water heated at max +20C, you'll need 0.43 L*

*> > of water per cm2 of cell per hour, 10 liters per cm2 of cell per day,*

*> > ~1m3/day for 1 square meter of panel. Not a good idea if water is not*

*> > recycled :)*

*> > If these cells could work at 100C one could grab the heat use it for*

*> > heating ...*

*> > (*) I'd like to have your views about these points ???*

*> > Sholl- Masquer le texte des messages prcdents -*

*> > - Afficher le texte des messages prcdents -*

*> There are two difficult points, the more concentrated:*

*> - the more accurate your tracking system must be*

*> - the more calories you have to dissipate*

*> for the first point, I was in search of cheap/simple passive trackers.*

*> liquid mass transfer (with shadowing apparatus) is attractive by its*

*> simplicity but I have doubt about their accuracy and their reliability*

*> with variable weather conditions. It may require a lot of*

*> experimentations to get worked.*

*> you have to collimate a centimeter radius focused light circle on a cm*

*> square of multijunction PV cells a few centimeters before the focal*

*> point (say at 30 cm from the Fresnel lens (sub degree tracking).*

*> I can't see any Shape Metal Alloy (for solar tracking purposes)*

*> commercially available ?*

*> The simplest tracking system I can see is first to use a square based*

*> pyramid of basic PV cells to ensure a rought differential tracking*

*> (~degree accuracy), orientation of the optical frame by two linear*

*> motors. This is to bring the sunlight closer to the axis of the*

*> Fresnel lens & multijunction cells (this couple form a pyramid, with*

*> mirrors covering the lateral inside faces).*

*> The high quality PV cells are the Spectrolab ones designed to cope*

*> with high concentrations.*

*> There are 9 of them in a 3 x 3 square.*

*> The first tracking system is to ensure that the focussed light spot*

*> intercepts this square (the target is then logged on).*

*> Knowing the amount of light reaching each of the nine cells, you can*

*> finally track in a very accurate manner and ensure maximum electricity*

*> generation.*

*> There are simple circuitry available on the web. The first and second*

*> tracking phases could form a vernier. This system could compensate*

*> analog biases (components in the tracking circuit) increasing with*

*> time.*

*> For the heat dissipation, I am also looking in the direction of high*

*> performance cooling systems available for processors (IBM has its own*

*> concentrator).*

*> There is a liquid metal system (with a magnetic pump) from danamics*

*> that seems very reliable/efficient and with reasonnable appetite (1W),*

*> I don't know its price but you are right, heat dissipation is very*

*> important for the best performance and life time of the cells. If you*

*> expect to get 500 suns foccused on 10 cm and given that your cells*

*> are ~0.3-0.4 efficient, this is a lot of heat of a small solar furnace*

*> to evacuate.*

The Spectrolab 'terrestrial' FAQ states that the cells can work at

100C with little loss.

*> PS) it appears that spectrolab do not sell small quantities of*

too bad :)

*> multijunction cells. no order under $000 (120 cells, enought to make*

*> a 3 kW generator). no possibility to build a small 170W PV generator*

*> (suitable for our roofs). looking to Emcore.*

Thank you for your interesting posts. I can't add much.

I just made some basic computation to evaluate the necessary tracking

accuracy:

500 suns concentration on a 1 cm2 cell, means a 500 cm2 Fresnel lens,

lens size: 23 * 23 cm, with assumed focal distance of 23 cm

for very small tracking error angle:

Focus_Shift(FS) ~= Focal_Distance(FD) * tg (Tracking_Accuracy(TA))

FS(cm) FD(cm) TA(degree) ~loss(%)

1 23 2.489 10

0.1 23 0.249 1

0.01 23 0.0249 0.1

So, these cells, used in these conditions, require a tracking accuracy

greater than 0.5 degree! Such a tracking accuracy is a challenge for a

'solar affordable' outdoor systems (dust, snow, wind, ice, rain,

temperature, ??road traffic vibrations??, animals, ...). It seems to

me that a secondary concentrator, close to the cell - simple mirrors,

compound concentrator, ... -, allowing lower tracking accuracy, is

necessary/mandatory.

A better design may require less tracking accuracy: ?larger cell, ?

shorter focal distance, ?lower concentration, ...

A Fresnel lens model would show what exactly happens to focus when the

light source is not perfectly aligned for a lens ? My experience :)

shows that the spot turns elliptic?? Maybe it's not as bad as the

above numbers predict.

Sholl

Oops ...

*> 500 suns concentration on a 1 cm2 cell, means a 500 cm2 Fresnel lens,*

*> lens size: 23 * 23 cm, with assumed focal distance of 23 cm*

*> for very small tracking error angle:*

*> Focus_Shift(FS) ~= Focal_Distance(FD) * tg (Tracking_Accuracy(TA))*

*> FS(cm) FD(cm) TA(degree) ~loss(%)*

*> 1 23 2.489 10*

*> 0.1 23 0.249 1*

*> 0.01 23 0.0249 0.1*

Focus_Shift(FS) ~= Focal_Distance(FD) * tg (Tracking_Accuracy(TA))

FS(cm) FD(cm) TA(degree) ~loss(%)

1 23 2.489 100

0.1 23 0.249 10

0.01 23 0.0249 1.0

~Loss were 10 times to small

Sholl

Also, do a Google search on "Sunball".

> > Jeff B. wrote:> > > Hi there,> > > Looking to a proposal made to us for a 2 kW PV generator (16.000 ), I> > > was wondering whether we could design a cheap 1 kW PV generator using> > > Fresnel lenses in combination with high efficiency multijunction PV> > > cells.> > > What if we put 4 triangular multijunction PV cells (or TASC, assembled> > > into a 3.1cm x 3.18 cm square) close to the focal point of a Fresnel> > > lens ?> > > Could a PV generator comprising 25 Fresnel lenses in a 5 x 5 square> > > (2m x 2m square) with their PV cells approx. 30 cm behind them, the> > > whole thing being mounted onto a passive solar tracker, do it ?> > > From the Spectrolab spec: 27 mW/cm (for 1 sun) are produced by the> > > TASC. The Fresnel lens focuses approx 155 times the solar irradiance> > > reaching the 4 TASC cells (3.1cm x 3.18cm) thus producing> > > 27x155x(3.1x3.18)=41,25 W. If we consider 25 units of this kind> > > (Fresnel lens+4 TASC), this would produce approx. 1 kW for approx.> > > 2250 $ or 1450 (plus framing structure to add). Either there is an> > > error in my rough calculation or this would be a cheap way to produce> > > electricity. Do you think it is feasible ?> > > Jeff> > > 25 large Fresnel lenses from 3dLens or BhLens (http://www.3dlens.com/> > > shop/largefresnellens.php)=25 x 22.68 $> > > 1 spectrolab box of 100 triangular PV cells (http://www.spectrolab.com/> > > DataSheets/PV/PV_NM_TASC_ITJ.pdf)=250 $> > > 1 Zomeworks UTRK40 passive solar tracker (http://zomeworks.com/files/> > > pv-trackers/Track%20Rack%20Matrix%202008.pdf) = 1450 $> > You only need one cell and one lens to test it, ???$0 (you could even> > sell a kit, many world wide would like to try that :)> > - I doubt it's that simple, some must have already tried. I read about> > that kind of cells used with concentrators> > - a problem must be that the light spot power distribution is not> > homogeneous (*)> > - the time life of the cell maybe reduced ?? (*)> > - the cell must saturate at ?some light flux ?? (*)> > - dissipating the energy is not a problem. With a 100 sun> > concentrator, it would give at worse: 1 kW/m2 >> 0.1W/cm2 >> * 100 => > 10W/cm2. A 4C/W heat sink used in electronic can easily be fit under> > the cell ($.5) and will work without a fan. With this,> > you keep your cell below 70C. With that much power to dissipate, a> > smart design, profiting of convection, would probably deeply boost> > your heat sink efficiency.> > Choose your heat sink at: http://www.mouser.com/catalog/634/1876.pdf> > Usually they are in aluminium.> > If you use circulating water heated at max +20C, you'll need 0.43 L> > of water per cm2 of cell per hour, 10 liters per cm2 of cell per day,> > ~1m3/day for 1 square meter of panel. Not a good idea if water is not> > recycled :)> > If these cells could work at 100C one could grab the heat use it for> > heating ...> > (*) I'd like to have your views about these points ???> > Sholl- Masquer le texte des messages prcdents -> > - Afficher le texte des messages prcdents -> There are two difficult points, the more concentrated:> - the more accurate your tracking system must be> - the more calories you have to dissipate> for the first point, I was in search of cheap/simple passive trackers.> liquid mass transfer (with shadowing apparatus) is attractive by its> simplicity but I have doubt about their accuracy and their reliability> with variable weather conditions. It may require a lot of> experimentations to get worked.> you have to collimate a centimeter radius focused light circle on a cm> square of multijunction PV cells a few centimeters before the focal> point (say at 30 cm from the Fresnel lens (sub degree tracking).> I can't see any Shape Metal Alloy (for solar tracking purposes)> commercially available ?> The simplest tracking system I can see is first to use a square based> pyramid of basic PV cells to ensure a rought differential tracking> (~degree accuracy), orientation of the optical frame by two linear> motors. This is to bring the sunlight closer to the axis of the> Fresnel lens & multijunction cells (this couple form a pyramid, with> mirrors covering the lateral inside faces).> The high quality PV cells are the Spectrolab ones designed to cope> with high concentrations.> There are 9 of them in a 3 x 3 square.> The first tracking system is to ensure that the focussed light spot> intercepts this square (the target is then logged on).> Knowing the amount of light reaching each of the nine cells, you can> finally track in a very accurate manner and ensure maximum electricity> generation.> There are simple circuitry available on the web. The first and second> tracking phases could form a vernier. This system could compensate> analog biases (components in the tracking circuit) increasing with> time.> For the heat dissipation, I am also looking in the direction of high> performance cooling systems available for processors (IBM has its own> concentrator).> There is a liquid metal system (with a magnetic pump) from danamics> that seems very reliable/efficient and with reasonnable appetite (1W),> I don't know its price but you are right, heat dissipation is very> important for the best performance and life time of the cells. If you> expect to get 500 suns foccused on 10 cm and given that your cells> are ~0.3-0.4 efficient, this is a lot of heat of a small solar furnace> to evacuate.