The first (almost full month) of data is now available for the Green Building Press's 750 watt array on a solar tracker. I'm really excited about how things are working out. Just take a look at the results below bearing in mind it is still only April. I hope to help prove that the anti-PV lobby are wrong and it is one of the most significant technologies available to us.


First of all. Here is the tracker that the data is collected from.

There are six 125w BP polycrystalline solar panels all wired up as a single group at 72volts but with a variable peak of up to about 150volts. this gets converted back to battery voltage (nominal 24 volts) by the MPPT controller just before being added to the bank.

Here is an extract from part of the next update of Green building Bible 4th edition that shows payback times are falling fast because of rising cost of grid electricity. the sums are simple enough and tracker technology combined with MPPT would probably (based on my early monitoring) improve this perhaps by 30-40% more:

"The installed capital cost of a PV array is approximately £400m2 for commercial scale and £600m2 for domestic buildings (the lower cost of commercial systems arises due to economies of scale). 8m2 of PV cells would give an approximate peak output of 1kW and when using a normal charge controller could produce approximately 900kWh of electricity each year at an installed cost of £3,200 and £4,800 respectively (depending on installation method and solar access). On the other hand, if the same amount of electricity was purchased from the grid at 15 pence per kilowatt hour then the cost would be £135 per year offering a financial payback of 23 and 35.5 years respectively. Of course we are currently witnessing regular large rises in the unit cost of electricity from the grid. If this pattern continues then the payback equation will continue to improve in favour of PV’s. "

Here are some install costs on this particular kit.

MPPT Controller = £425
Our particular tracker is hard to cost because I bought is second-hand but the new cost would be roughly £1,200 + delivery cost to UK
The wattsun prices are here: http://www.wattsun.com/prices/Wattsun_Tracker_Prices.pdf

Six BP solar 125 watt panels (6.1 m2)= £2,800
Installer = between >£800< (this will vary on how much work you can do yourself)
Pole and concrete = £200
Cable terminals and fuses - £600

Total roughly = £6025

That comes out at about £980m2

After 1 year of monitoring we will be able to see how it compares against a static array for payback.

Thanks Stephen. I' have now added a comparison chart that compares the two arrays per sq metre (see above). On your other point, I have plans to add an MPPT to the static array but to confuse it even more I have two static arrays and both will be put on the same controller.

Gawd damn! :) That's a spectacular difference. On the sunny days (22/4) the tracker + MPPT does even more than 100%. You've made me rethink my off-grid design now.... Any chance you have the same data for the autumn/winter months? I'm definitely sold on the MPPT, the value for money proposition is just too good - but I've been hesitant about a mechanical tracker as I thought it wouldn't be as effective since the position of the sun doesn't change that much in winter - which is when we really need the power.

(What's going on on the 11th and 12th? Looks like the values don't correlate...)

There's a discussion about your data on the navitron forums: http://www.navitron.org.uk/forum/index.php/topic,3300.0.html
A comment that came up was that kwh/m2 might not be an accurate comparison if the PV panels aren't identical. Maybe a kWh/kWp (total rated W of the panels) would be better?

May was a little disappointing sun wise but here are the results. Again the static array is included for comparison.

And June data ah/m2

July 2008 data with the static array shown for comparison. Luckily there was a moderate amount of wind again.

August data. Disappointing sunshine (almost non-existant) this month removed some of the advantages of the tracker over a static system.

Keith, did you ever add that MPPT to the static panel? Also, what happened when you tested it for ageing?

If these figs really are normalised for the above, I'm going to forget about static panels (in the UK) and jump straight to the tracker. Terrific stuff!

JackyR, last time I compared the cost of a tracker with the cost of additional panels on a static array - the additional panels won. You can compare output via: http://sunbird.jrc.it/pvgis/apps/pvest.php?europe=

If you're off-grid then that changes things too - since the tracker would be less effective in winter, but the additional static panels more so. And in an off-grid system where you have a constant consumption throughout the year, it's the winter months that really determine the size of the system.

Posted By: PaulTIf you use 1 kwh of electricity and generate 1 Kwh of electricity you get a credit of 0.146 Kg CO2!

This means that there is a bias in SAP for using PV as apposed to using less electricity (just an observation)

That might be a feature, not a bug. If the PV tends to generate electricity at times of day when high emission power stations (coal) are running but domestic use is typically at times when the grid is running mostly on gas, nuclear and a bit of wind then that could plausibly reflect reality.

A major issue is that SAP assumes all types of pv behave the same - that ther is a fixed relationship between the KwPeak ratign and the annualised output.

Do you have any data on how much difference the different types of PV make?

Here is the October data form the tracker and the static array. I have normalised the vertical axis to match the best available output for the year. I will adjust previous charts to all show the same vertical axis when I have time. I should point out (as i have before) that the tracker spends all windy days (over 17mph) in non-tracking mode and anchored.

Thanks Keith, this is really useful data.

Here's my take on your data normalized to hours of nominal panel output per day. In most cases I took the your average Ah for the month, multiplied by 25 volts (guestimated battery charge voltage) then divided by the panel nominal output (480 or 750). For the May tracker/MPPT data point you didn't give the Ah number so I averaged the daily kWh figures and divided that by 750. This seems to give a suspiciously out-out-of-line result.

Month	Static	Tracker/MPPT	Gain
March	1.65
April	2.2	4.08	1.85
May	2.41	5	2.07
June	2.77	5.06	1.83
July	2.43	4.17	1.72
August	1.72	2.82	1.64
Sept	1.73	3.07	1.77
Oct	1.29	2.15	1.67
Nov	0.69	1.07	1.55
Average	-	-	1.76

Gain is the multiplication of nominal hours which the tracker/MPPT combination gives.