Category Archives: Solar

Mid-Summer solar data

Amazingly given that it is mid-summer and Wimbledon has just started the sun has actually been out for a good few days!   Some good data collection has been taking place and my rig has proved itself reliable at last.

Green is the PV, Red is the thermal.

21st of June, longest day!   Some big fluffy cloud banks passing over as you can tell from the big dips.   My fault, had family over for a BBQ.


22nd of June, pretty sunny day.   A little hazy just after lunch and then blue sky all afternoon.


23rd of June, a bit more cloud.   Didn’t mind as it was a Monday so stuck at work.


24th of June, cooler start with little clouds passing over from lunchtime giving the very sharp drops in the PV wattage.   Cooled off in the evening.


These show nicely the larger working angles of the evacuated tube solar thermal collector.   It starts to make heat best part of 2 hours before the PV cells really kick in.   But this makes sense as a cylinder will always present the same surface area no matter the angle you view it from.   A flat sheet doesn’t.

The drop off in the evening is actually the house getting in the way.   I could move the experiment to the house wall but someone might not like the noise of the pump and the potential for the glycol solution leaking again.

The PV panel has a collector area of roughly 800 sq cm. (25x35cm with ~10% removed for corners and gaps)   The solar thermal collector has an area of 290 sq cm.   36% the area of the PV.   But we see equal if not better peak power outputs from the thermal collector.

The average per day is much higher from the thermal collector.   Ranging from ~1.8x higher on a sunny day to ~2.5x higher on a more cloud strewn day.   So per sq cm of collector the efficiency is almost 7x higher than a silicon PV cell.

OK, not the most scientific test and I’m not entirely sure of the efficiency of my PV panel.   But if we assume its in the 12-14% range (its from China via ebay after all) it puts the evacuated tube in the 95% range.   Which is about right from my research.

If I was using hot fluid in the cooling circuit this efficiency would drop.   Most solar thermal systems aim for 70-90 deg C.   The fluid temps I’m dealing with are in the mid 30’s.   Mainly for safety as the experiment is a little Heath Robinson.

Solar Datalogging project is working again!

I’ve been curious about solar energy for quite a long time.   Not really from a ‘renewable energy’ point of view, more the physics and efficiencies.   I recently went to the Mediterranean and saw a LOT of solar thermal and photo voltaic installs.   So here is an experiment to test how they both work in the UK climate.

The experiment uses the following collectors

  • 10W 12V solar cell (I had it spare from a previous project that went nowhere)
  • 58mm diameter x 500mm long evacuated tube heat pipe solar thermal collector.   I love ebay!

Both are mounted to the wall of my garage at the same angle as the roof.   Essentially emulating a system placed on the garage roof.   Why the garage?   Just cos its easier and the roof points the same way as the house.

The test load for the PV is pretty simple.   a 30ohm resistor on a heatsink.   I measure the voltage and power is V^2 * load resistance.

The solar thermal isn’t quite so easy.   First plan involved thermocouples, pumps and flow meters.   But when you actually work out the accuracy of the flow meter and thermocouples vs just how little temperature gain you will get the numbers would have been so fuzzy and worthless.   OK, take two!

Second plan involved a peristaltic dosing pump and some Dallas DS18B20 temp sensors.   Dosing pumps give a known volume of liquid per revolution and the DS12B20 could be put right in the flow.   No analogue wires to pick up noise.

I got a tiny pump from ebay.   Cost only a few quid.   Basically a 360 motor with a 3 rotor pump head and some 2mm ID silicone hose.   I fitted it with an IR reflective sensor to measure the RPM.   Worked well!


The whole setup was run by a Raspberry Pi with a GertDuino addon board.   Not enough IO on the raspberry and I can code Atmels in my sleep.   And the spare IO was connected to the leftover parts of an old Maplin wireless weather station.   Result!


The data was logged with WeeWx.   Its simple and easy to modify, although Python is the work of the devil.

I used copper brake pipe to make a heat exchanger for the solar thermal collector.   Brake pipe can be a mare to work with and making a tight coil is hard.   I have a lathe so I made a jig to bend the pipe around.


The final coil fitted snugly over the top of the heat pipe.


The whole lot would be covered in 1″ thick pipe insulation.   No way heat is leaking out!   30% glycol solution would ensure no freezing!


Small box on the solar cell is the outdoor temp and humidity sensor, it doesn’t live there normally!   The pipe insulation is to stop me catching my arm on the corners.

It worked well for about a month.   Then I started getting really bonkers thermal readings.   Turns out the little pump which was spinning at 100+RPM had split its silicone tube and leaked everywhere!   Couldn’t find any suitable tube to replace it so shelved that part for a while.

The weather station and PV logging worked for another 2 months or so before a power cut claimed the file system on the Raspberry Pi.   My fault for not backing it up!   So 4 month ish of data lost.   Bah!   Take three!

So gone is the ebay dosing pump and the Raspberry Pi is off to a new home on something less 24/7.   OK, what next?   Well as I’d based this on a GertDuino I decided to go whole hog and use all Arduino based hardware.   I had an Uno lying around and ordered up an ethernet shield.   Job done, send the data straight to my server which has a big UPS powering it.

I found a stepper motor based peristaltic pump which would be ideal apart from the price.   This whole project has cost very little as its basically odds and ends and cheap bits from ebay.   the Pi and GertDuino were the largest capital investments so far.

Hmm.. I have a stepper motor and drive spare.. I have a workshop where I can make stuff.. how hard is a pump?   Well not very!   A block of delrin made the base and some aluminium made a rotor with some more delrin for the rollers.   A length of santoprene tube finished it off.   One DIY stepper motor pump.


From left to right:

  • Cooling coil for the fluid returning from the solar collector (yes, its a failed brake pipe coil)
  • 1ltr container for the heat transfer fluid (Tesco mini chocolate rolls, yum!)
  • Pump, 3.5cc per revolution
  • Upper right hand end – Arduino Uno, ethernet shield and some veroboard for signal conditioning and connectors
  • Lower right hand end – PV load and heatsink

So now we are back in business and logging data!   A bit of fine tuning to do but it will be done before June 21st!   Want to get that high sun logged.