DIY: PV+LED-BASED 'FAKE' SKYLIGHT

QR code: https://chrismolloy.com/skylightShort-cut URL for this page: https://chrismolloy.com/skylight. This article is featured in ReNew magazine, issue 111 (April - June, 2010), pg 64 - 66, as well as on the Makezine.com blog.

'Success? Failure?' Rating: ★★★★★

I decided my 20W PV panel was not working hard enough. The solar-powered attic-to-basement fan system, plus a separate fan system used to redistribute heat from our lounge to our bedrooms in winter, still left a good deal of spare capacity when it wasn't hot or cold enough to need the fans running. The heat redistribution system also relied on a lead-acid battery to do its job, and I've never had much luck when it comes to lead-acid batteries. It was time to come up with yet another use for my PV panel...

A couple of articles by Lance Turner in ReNew magazine caught my eye: an article to build your own (mains-powered) LED light fitting (Issue 104/July-Sept 2008, pp 70-71); and an article about skylights, including how to build your own 'fake', LED-based one (Issue 108/July-Sept 2009, pp 26-28). What excited me about combining these two articles was the possibility of building a fake, LED-based skylight driven directly by a PV panel - with no electronics in between to complicate the system (and use some of the power being produced). This possibility hinges on the fact that a PV panel is a constant current source and an LED is a constant current consumer - a perfect match! A non-obvious (to me, at least) feature of this system is that the brightness of the fake skylight will vary with the level of insolation falling on the PV panel, and will thus mimic a real skylight amazingly well.

The basic plan is to hook up strings of Cree XR-E LEDs (rated at 3.3V - 3.7V, up to 1000mA) to match the rating of your PV panel. A good match (that allows a safety margin when driving the LEDs) is to hook up strings of five LEDs in series (i.e. with a total forward voltage of 16.5V - 18.5V), and then connect these strings in parallel, one for each 10W of PV panel output (i.e. ≈600mA per string). Based on this plan I needed two strings of five Cree XR-E LEDs in order to match my skylight unit with my 20W PV panel.

One possible addition to this plan concerns fuses. If one of your LED strings were to fail (for whatever reason), then the current from the PV panel will be split between the remaining strings. If this results in more than 1000mA (1.0A) flowing through the remaining strings then you risk damaging the LEDs in those strings. A solution is to put a 1.0A fuse into each string. The prudent approach might be to add this fuse into each string regardless of circumstances, but if it is unlikely to be needed (e.g. the loss of one string will not cause too much current to flow though the remaining strings), then the non-prudent approach might be to save the cost and complexity (not to mention a small drop in power) of putting the fuses in. I've written an LED Calculator to help you with this decision.

One further issue to deal with is one of heat dissipation. Cree XR-E LEDs run hot enough to do themselves damage unless they are properly heat-sinked (is that a word?). The ReNew magazine articles state that a good rule-of-thumb for this is to allow 100cm² [16"²] of 3mm [⅛"] aluminium plating per watt of LED. Ten Cree XR-E LEDs will thus require 3000cm² [465"²] of aluminium (e.g. 100cm x 30cm [39.4" x 11.8"]). Each LED should be securely fastened to your aluminium base plate, and should be bedded on heat-sink paste to ensure good thermal conduction.

In order to make my fake skylight look like a real one I wanted to have it mounted in a frame and behind glass. I decided to use aluminium 'U' channel to make the frame, and to ensure a good thermal bond between it and the aluminium base plate to further aid in keeping the unit cool. The frame was to be left open-ended to ensure good airflow through the unit. The glass was to be textured (unlike most real skylights) in order to blur the look of the LEDs somewhat. I also wanted to be able to tilt my fake skylight, rather than be forced to have it shine straight down, so I built a hinge/tilting mechanism into the frame as well.

LED Calculator

I wrote a wee calculator to help match your PV panel to the Cree XR-E LEDs. The calculator makes a recommendation, for your specific panel (as specified by entering its voltage and current at maximum output), about: the number of LED strings (such that no more than 1.0A flows though each string); the number of LEDs per string (such that there is no more than a 3.7V drop across each LED); and whether or not each string should be fused (i.e. would the loss of a single string result in more than 1.0A flowing though the remaining strings?).

To find the voltage and current at maximum output of your panel you can use the data from the manufacturer (which is unlikely to match your specific panel exactly), or you can measure it by pointing your panel directly at the midday sun on a cold, clear day (which should result in the maximum output you can expect from your panel). Apply your own judgement to the results from this calculator - no liability of any kind for the use, or misuse, of this information will be accepted by the owner of this web site.

V
A
Pmax. 19.9W
Number of LED strings 2 (570mA per LED)
Number of LEDs per string 5 (3.5V per LED)
Current per LED with one string not working 1140mA
1.0A fuse per string recommended? Yes

Materials List

Here's a list of what I used to build my fake skylight. You may want to tailor this list to meet your specific needs:

The total cost to build this unit was around NZ$300, excluding the PV panel. The single most expensive item was the safety glass (NZ$120), which could easily be omitted if cost was more important to you than aesthetic. As a comparison, a single Solatube-type unit (including installation) can be had for around NZ$800.

Building Steps

Here's what I did to put my unit together...

Materials: aluminium, bolts and LEDs
Materials: aluminium, bolts and LEDs
Assemble your materials. Leave the protective plastic on one side of the aluminium sheet - this will be the LED side of the skylight. Rule lines on the protective plastic to indicate where you're going to place the LEDs with the aim of distributing them evenly and maintaining a good separation between them. I ruled lines across every 100mm [4"], beginning 50mm [2"] in from one end, and along the length of the plate, 75mm [3"] in from each side. This resulted in twenty spots, of which I used every second one, in a staggered pattern.
Aluminium base plate drilled
Aluminium base plate drilled
Drill Ø3.2mm [⅛"] holes in the aluminium base plate. Each LED will need two pairs of holes: one pair that matches the body of the LED so it can be secured to the plate; and one pair perpendicular to the first, and 20mm [⅞"] away from the LED on either side to accommodate the wiring. You will also need a row of holes, around 200mm [8"] apart, along each side of the plate to facilitate riveting on the side rails (aluminium 'U' channel). Drill the plate and rails together to ensure a perfect match. You may also wish to drill holes to accommodate your mechanism for connecting your skylight to your ceiling - I used a pair of semi-concealed hinges on one side and tapped a pair of holes in the other side to accommodate a pair of set screws (which are coupled to a pair of hasps on the ceiling during the final install). Ensure all holes are burr-free (especially those for the wiring).
Aluminium base plate: rails attached
Aluminium base plate: rails attached
Cut away the protective plastic along each side of the base plate and rivet on the side rails (aluminium 'U' channel). I ran a bead of heat-sink paste between each rail and the base plate to ensure good thermal conduction. Go easy with the heat-sink paste - a little goes a long way. Cut away the protective plastic where each LED will be mounted.
Aluminium base plate: hinges and hasps
Aluminium base plate: hinges and hasps
Attach any appropriate components for connecting your skylight to your ceiling (in my case the semi-concealed hinges).
LEDs: spare connectors isolated
LEDs: spare connectors isolated
I had not used Cree XR-E LEDs before, and found them to be a little user-unfriendly. You get two pairs of soldering pads on each LED, presumably to provide some level of wiring flexibility. Unfortunately these clash with the notches that seem useful for accommodating the mounting bolts, such that it is very easy to short out each LED by electrically coupling one set of pads to the base plate via the bolts. To get around this I covered one pair of solder pads on each LED with liquid insulation gunk and let this fully set before mounting the LEDs.
LEDs: mounted with bolts and heat-sink paste
LEDs: mounted with bolts and heat-sink paste
Mount the LEDs on top of a blob of heat-sink paste, taking care not to over-tighten the mounting bolts. Ensure the solder pads align with the wiring holes on each side, and ideally ensure the polarity of each LED is consistent (to reduce the risk of wiring things up incorrectly).
LEDs: wired up
LEDs: wired up
Wire the LEDs up in strings of five in series. I grouped my strings by end (i.e. so each string starts and finishes in the middle of the base plate), but you could just as easily wire up one string along each edge (leading to much longer wires to connect the strings together, however).
LEDs: wired up
LEDs: wired up
Connect each string in parallel to one side of a connector block. The other side of the connector block will go to the PV panel. Double-check the polarity of every connection that you've made. Ready to test it out? If you have a (current limited) bench-top power supply, use this to test the unit. Limit the output current to 1.0A (in case one of your strings is not working, you don't want more than 1.0A going through the other string), set the output voltage to 16.5V, connect your skylight up and switch it on. All the LEDs should light up. Disconnect the power immediately if you spot a problem.
Unit installed and running
Unit installed and running
Rear hasp and temperature sensor
Rear hasp and temperature sensor
Unit installed and running
Unit installed and running
Secure the unit to the ceiling (or wherever) and connect it to your PV panel. To help with airflow around the unit I made sure that there was at least 25mm [1"] between the unit and the ceiling. By tilting the unit a little (to throw more light into the centre of our room), the air gap at the back is more like 50mm [2"]. And if/when you want to move your skylight, simply unscrew it from the ceiling (try doing that with a Solatube!).

Success? Failure?

★★★★★

Pros: By far the best PV panel / LED -based thing I've built to date - this unit is extremely functional and pleasantly illuminates an otherwise dim end of our lounge.

Cons: None that I can think of.

Enhancement Suggestions: Add a stick-on strip thermometer (the kind you see on the outside of fishtanks; available from any pet shop) to the outside of the aluminium frame so you can monitor the temperature of the unit. These thermometers typically only go up to 38°C [100°F] or so, but this unit should run well under that.

Update: I replaced the glass with a diffuser (½" grid, mirrored plastic) and the result looks a lot better, and is more effectively diffused. I suspect the ventilation is better, too (not that heat has been a issue at all).