Feed In Tariffs(FIT) Announced.

ruatua

noncom wrote: »

I've tried asking this on the Navitron forum, but so far nobody's replied...... so I'll see if there are any experts here who can advise me quicker!

I had three quotes from PV installers the other day, and there was one fundamental disagreement between them. We have two roofspaces on the back of the house, either or both of which could be used. They both face in exactly the same direction (210deg, so just slightly West of directly South), but the roof on the extension at the back is lower (single storey) and a shallower pitch. So we have a rectangular extension roof at about 30 degree from horizontal pitch, and an irregular shaped main roof above with a more traditional pitch of perhaps 45-50 degrees.

There are no significant shading issues on either roofspace if we site the panels carefully.

Ideally I'd like to get a system as close to 4kW as possible to maximise the return-cost ratio with the FITs, but neither of the two spaces on their own is big enough to do this (probably get about 3kW on the top one if we used the most efficient panels).

However two of the three installers recommended putting panels on both spaces, linked to a single inverter (wired as a single series string). The third said that because the pitches of the roofs are different, then the fact that the blocks of panels will always operate at different efficiencies from each other (due to the different angles to the sun wherever it is), will act as a brake on the whole system, and we'd be better off simply putting a single block onto the larger roof and ignoring the lower one entirely. They weren't trying to sell me an extra inverter, so I don't think it was anything other than a genuine opinion, but I wondered what people here (installers preferably) thought about this? Is the difference in the two pitches really significant enough to make using both areas together (with one inverter) a bad idea?

Hoping someone can help me make a decision.....

I have also been told that any panels on different facing roofs will need a different inverter so you will need to do your sums on whether it is worth the extra cost or whether it is best to try and max out the panels on just one side of the roof. In theory having 2 smaller inverters in place of 1 should not add more than £1000 or so to the cost?

BTW if you have an iphone there is an amazing free app which will tell you how much your pv system will output just by resting the phone on your roof (assuming you have a ladder, please be careful!) download from your phone by searching for "sma solarchecker" or look on the sma.de website

Cardew

zeupater wrote: »

Hi

I agree with Dave's post above (#187). if you look at the variables involved all you have is the distance from the sun, the angle of the panels to the sun, the distance the radiation travels through the atmosphere, the current weather conditions, and the panel temperature.

Hi,

Not sure where we are going with this; albeit it is an interesting discussion.

Presumably you do accept that the maps are correct and irradiation and kWp is generally greater as you go south?

http://sunbird.jrc.it/pvgis/cmaps/eur.htm

The same colour legend represents also potential solar electricity [kWh/kWp] generated by a 1 kWp system per year with photovoltaic modules mounted at an optimum inclination and assuming system performance ratio 0.75.

Interestingly the I Phone app - see post above - can only work by taking your location from GPS and overlaying it on a solar map.

Your calculations conclude there is only 3.7% difference in irradiation(due to atmosphere) between Inverness and Plymouth.

Clearly there is a far greater difference in practice,(around 50%) so assuming your maths and geometry is correct there must be other factors, or flaws in your reasoning.

Daves's point earlier was that if it was purely the distance the sun's rays travelled through the atmosphere, then there would be equal irradiation along each line of latitude.

zeupater

Cardew wrote: »

Hi,

Not sure where we are going with this; albeit it is an interesting discussion.

Presumably you do accept that the maps are correct and irradiation and kWp is generally greater as you go south?

http://sunbird.jrc.it/pvgis/cmaps/eur.htm


Interestingly the I Phone app - see post above - can only work by taking your location from GPS and overlaying it on a solar map.

Your calculations conclude there is only 3.7% difference in irradiation(due to atmosphere) between Inverness and Plymouth.

Clearly there is a far greater difference in practice,(around 50%) so assuming your maths and geometry is correct there must be other factors, or flaws in your reasoning.

Daves's point earlier was that if it was purely the distance the sun's rays travelled through the atmosphere, then there would be equal irradiation along each line of latitude.

Hi

I agree that the maps are correct in terms of total annual kWh production from a 1kWp system situated anywhere within the UK. The legend clearly states that the map shows yearly energy generated by a 1kWp system with a performance ratio of 0.75 (kWh/kWp), which could have been better stated as x kWh/kWp:y at a 75% efficiency. The differences shown in the referenced maps are surely due to a combination of the annual available daylight hours and the average cloud cover/sky clarity across the country as these are the main variables which would account for the differences.

The calculated 3.7% difference in absorption and scattering of the solar radiations due to a longer path through the atmosphere is merely to show that with the same clear sky conditions at either of the two locations, a 1kWp array mounted at an angle to optimise power production at noon on the 21st June will produce approximately the same maximum registered output (kW) with the only known fixed difference being the 3.7%. This was done simply as an exercise to show that it is possible for an array to achive a very similar maximum power production at either location, therefore there would be no effect on the FIT banding at either location if the FIT banding was based on the grid being capable of accepting 4kW per phase without detailed investigation of each installation.

Total production capability measured in kWh is a different story, with much larger differences which you rightly highlight, being not only due to weather conditions, but also because every day is not the 21st June.

Regards
Z

Dave_Fowler

Cardew,

A fascinating map and website. Looking into several of the links on the site sends you to http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php A really interesting application which gives you the opportunity to put in your location, PV array size and ask it to optimize elevations and azimuth. It plots a month by month estimated output.

I fed in the size of the PV array I've had a quote for, put in my location 25 miles north of London (51-52N 0-26 W) and asked for a calculation. For the 3.2kWp array, I was given best inclination of 35 degrees. Allowing for the calculated losses of 26.6% (12% temperature, 14% inverter + cables) the annual output was calculated to be 2600kWh.

I then did the same calculation on the same array pretending I was in Aberdeen 57-09N, 2-16W. Result best inclination 39 degrees, losses 26.2% (slightly less because of lower ambient temperature). Calculated output 2460kWh. This is a reduction of only 5.4%. Nothing like the vast reductions given for solar heating.

Perhaps PV installations are the way to go in Scotland - or perhaps the website has got it wrong.

Cardew

Dave/Zeupater,

A couple of quotes:



Available Sunlight for Solar PV Panels (Photovoltaics)

The amount of available sunlight in the UK varies with the latitude of the site. The south of the country has more available light than the north. The total annual energy available varies from approximately 1,259 kWh per metre squared per year in the South-West of England to 850 kWh per metre squared per year in Scotland. The light energy also varies during different seasons of the year with more daylight available in the summer than in the winter.

This suggests that South West England has a 48% greater output than Scotland which is rather mre than the 5.4% you got for just north of London.

The model algorithm estimates beam, diffuse and reflected components of the clear-sky and real-sky global irradiance/irradiation on horizontal or inclined surfaces. The total daily irradiation [Wh.m-2] is computed by the integration of the irradiance values [W.m-2] calculated at regular time intervals over the day. For each time step during the day the computation accounts for sky obstruction (shadowing) by local terrain features (hills or mountains), calculated from the digital elevation model.

Pasted from <http://www.earth.org.uk/note-on-solar-tools.html>

No comment!

Dave_Fowler

Cardew wrote: »

Dave/Zeupater,

A couple of quotes:



Available Sunlight for Solar PV Panels (Photovoltaics)



This suggests that South West England has a 48% greater output than Scotland which is rather mre than the 5.4% you got for just north of London.




No comment!

What chance have we mere mortals got when the experts from the European Commission and those you quoted can't even agree to a factor of almost 10 to 1 in the reduction in the sun's power between the north and south of the UK!

It would appear the experts need to get together and agree on a single set of figures. We can sit on this forum till the cows come home taking figures from one source and from another and either proving or disproving the facts. I wonder on which set of data the government based its FITs payments.

Perhaps we should all club together and sponsor two people, one in the south and one in the north to have two identical systems installed and get some real actual results rather than reading these 'expert' predictions. PS I'd be happy to host the south's trial!!

John_Pierpoint

I already have a friend with these panels in the South so who wants to find an identical roof in Scotland - probably get it done cheaper up there too!

zeupater

Cardew wrote: »

Dave/Zeupater,

A couple of quotes:

Available Sunlight for Solar PV Panels (Photovoltaics)

This suggests that South West England has a 48% greater output than Scotland which is rather mre than the 5.4% you got for just north of London.

No comment!

Hi Cardew / All

Regarding the above ... this is surely due to the 'real sky conditions' which are included in your quote ....

The model algorithm estimates beam, diffuse and reflected components of the clear-sky and real-sky global irradiance/irradiation on horizontal or inclined surfaces. The total daily irradiation [Wh.m-2] is computed by the integration of the irradiance values [W.m-2] calculated at regular time intervals over the day. For each time step during the day the computation accounts for sky obstruction (shadowing) by local terrain features (hills or mountains), calculated from the digital elevation model.

Pasted from <http://www.earth.org.uk/note-on-solar-tools.html>

The clear sky model allows for the calculation of the maximum power which would be available at a location at a specific point in time, the real-sky allows for the average prevailing sky conditions at that location as measured and built into the the model which is used to compute the annual kWh available at each location, surely it is therefore the real-sky model which is used for the creation of the irradiance maps being referenced.

Referencing details of the model used for the JRC map posted yesterday, further details of the models are available for both clear-sky and real-sky ...
http://re.jrc.ec.europa.eu/pvgis/solres/solmod3.htm#clear-sky radiation
http://re.jrc.ec.europa.eu/pvgis/solres/solmod3.htm#overcast radiation

Looking at it from a logical point of view, any insolation map being used to establish annual power (kWh) available without taking average prevailing weather conditions (overcast) into account wouldn't really be much use, therefore as the map clearly relates to annual kWh availability it must be based on the GRASS GIS Mode2 model (which is a measurement over time), not the GRASS GIS Mode1 model which would show the power (kW) available in clear-sky conditions for a given date/time and therefore the peak power available on 21st June at midday (which is a spot measure).

Referencing the following link to provide further details of GRASS GIS Mode1 & Mode2 ......
http://re.jrc.ec.europa.eu/pvgis/solres/solmod3.htm#Implementation


This really describes why there is confusion about the large discrepancies being discussed. Available 'peak' power (kW) is based on the clear-sky model showing a smaller difference between the south coast and more northerly locations, with average available power (kWh), being based on average sky overcast conditions, showing a greater difference between the two locations than allowance for latitude change alone.

Regards
Z

zeupater

Hi All

Supporting & confirming the logical assumptions contained within the last post, the full methodology for the creation of the JRC PVGIS map in post #193 is available in the following link ....

http://re.jrc.ec.europa.eu/pvgis/solres/solrespvgis.htm

Regards
Z

Cardew

zeupater wrote: »

Hi All

Supporting & confirming the logical assumptions contained within the last post, the full methodology for the creation of the JRC PVGIS map in post #193 is available in the following link ....

http://re.jrc.ec.europa.eu/pvgis/solres/solrespvgis.htm

Regards
Z

Thanks, but kids stuff really - I worked that lot out in my head;)

The bottom line however is that someone in Lands End with a 4kWp array gets a lot more FIT than someone with the same array in Wick.