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'Success? Failure?' Rating: ★★★☆☆
After completing my first indoor solar-powered lighting system, I thought I'd have a go at building a second system using an alternative approach: 'hacked up' solar-powered garden lights (similar to those used to light my solar-heated outdoor bath project). I wanted to see if I could build a useable indoor lighting system using this significantly cheaper approach. I wanted three new lighting installations: a small unit for our laundry; a medium-sized unit for our bathroom; and a large unit for our dining room/kitchen. The latter needed only to provide ambient lighting over our dining table (backed up with candles), but the other two installations would ideally provide all the night-time illumination required in those two rooms (which are not used much at night, other than to brush one's teeth). I selected these three rooms as they all have a louver-style window that facilitates easy wiring (we rent our house, so I wanted to minimise/eliminate any requirements for hole-drilling etc.).
I started with a set of 24 solar-powered garden lights from The Warehouse (on sale, 8 for NZ$20) and began with some experimentation. These lights work by charging a NiCd battery during the day, which then lights a single LED during the night. My electronics knowledge is not great, but I pictured this system working like a see-saw: when the PV panel voltage is higher than the battery voltage (i.e. when the sun is shining on the PV), the LED is switched off and the battery gets charged; when the PV panel voltage drops lower than the battery (i.e. when the sun stops shining), the see-saw tips, the LED is switched on and the battery discharges. In my 'hack up' I wanted to separate the LED from the battery/PV unit so that I could place the former inside whilst the latter remained outside. The first thing I wanted to test was whether I could connect several of the battery/PV units together in series without disrupting the voltage-balanced 'see-saw'. This was desirable for two reasons: 1., several units in series multiplies up the voltage and thus makes for more efficient power transmission; and 2., a single cable can carry the power from the set of battery/PV units to the set of LEDs. Unfortunately when several battery/PV units (connected in series) were hooked up to several LEDs (connected in series) the results were disappointing: the LEDs no longer switched off during the daytime; and the overall LED brightness seemed to be less than expected. The see-saw had been disrupted. At this point I had a choice: build a separate switching system to control the LEDs, plus increase the battery/PV-to-LED ratio (to increase the brightness); or forget about combining the systems and run each as a stand-alone unit. Option 2., whilst less elegant, required less effort - what was a boy supposed to do?
One big issue with treating each unit as a distinct element involves the wiring required. Each battery/PV to LED connection requires a pair of wires, which would mean my largest lighting unit (12 LEDs) would require 12 pairs of wires running in from outside - not pretty. A related issue is that the battery/PV to LED wires would be running at 1.2V (the nominal voltage for an AA NiCd battery), so transmission distances would need to be kept as short as possible (a low voltage leads to high power losses during transmission). In order to mitigate the cabling issue I selected some Cat 6 Solid Core UTP Network Cable to do the job. This cable contains 4 pairs of 23AWG wires - a tidy package of low-loss wire. A downside is that this cable is relatively stiff to work with. I was still going to need three cables for my largest lighting unit, but this was better than 12 separate wire pairs.
First up I built 3 'head units' to house the battery/PV units: a set of 4, 8 and 12 units, respectively. These head units would be secured to the flat roof immediately over each louver-style window. The head unit frame holds the battery/PV units at a 45° angle, just a fraction greater than the latitude of our house (thus the mounting angle slightly favours Winter conditions, when the sun is lower in the sky). Each battery/PV unit has a wire pair running from where the LED was mounted to a D connector socket (female). Soldering the wiring in directly would have been more efficient, but I wanted to be able to swap out each cable should I decide to move the head units in future (if we move house, for example). Each head unit was mounted on the roof with waterproof duct tape (again, not using any form of permanent fixing was desirable). Any doubts about the strength of this set up were laid to rest during a gale (with gusts over 125km/hr [78mph]) that blew in the day after I installed the head units (I think the fact that they a tucked in close to the roof reduces the wind they experience).
A lesson I'd learnt building solar-powered lights 1 was just how directional LEDs are. In 'solar-powered lights 2' I decided to distribute the LEDs along a strip in order to spread out the resulting light. I routed out three wooden strips and drilled them to take a pair of LED leads every 10cm [4"] (or every 20cm [8"] on the strip holding the set of 4 LEDs). I wired each strip with a D connector socket (female). If you decide to copy this method, ensure you double-check where you want the cable to exit from the lighting strip - I placed mine on the wrong side on two of the light strips. Each strip was attached to the ceiling with double-sided, sponge-backed sticking tape.
Each unit required one or more Cat 6 cable to run from the head unit on the roof to the lighting strip on the ceiling. Each was sized to run as directly as possible between the two components. The connection on the underside of each head unit was weather-proofed by sealing it inside an outdoor-grade ABS plastic box, backed up with dollops of silicon sealant. The cabling in the laundry and bathroom was left 'in the raw', but the triple cable required to hook up the 12 LED unit in the dining room/kitchen was sheathed in white duct tape to make it look a bit tidier. Each cable run was between 2.5m and 3.5m [8' and 11'] in length.
The resulting lighting systems are most satisfactory (the photos, below, don't really do the systems justice). They are nowhere near as bright as solar-powered lights 1 - the LEDs are of lower quality/output and the system voltages are lower - but each room now possesses a light source that switches on and off automatically and provides enough light to see by in these rooms that experience little night-time use. We no longer use the mains-powered lighting at all in either the laundry or the bathroom, and we've significantly reduced our usage of the mains-powered lights in our dining room/kitchen (LEDs plus candles makes for a very pleasant light to eat by).
Based on the success of the project above I used 5 more solar-powered garden lights to make a 3 LED light for our front porch (no more fumbling to find the lock) and a 2 LED light that shines in through the window of our second toilet (an easy way to get light from the outside to the inside). Both lights are only 'night lights' (i.e. provide only enough light to act as a guide), but both are very useful extensions to the idea above.
Pros: Very cost effective; modular/relocatable.
Cons: Could be brighter; quantity of cable required to run a pair of wires per LED.
Enhancement Suggestions: An alternative method of solar-powered garden light 'hack up' that occurred to me after I was finished was to extend the wires between the PV and the rest, as opposed to between the LED and the rest as I did above. This would mean more of the kit would be inside (out of the weather) and that the power transmission losses would occur between the PV and battery, rather than the battery and LED. This could be advantageous as the PV to battery wires would run at a slightly higher voltage when charging (1.9V as opposed to 1.2V), i.e. less power loss, and the LED would receive the maximum power provided by the battery (because they are not separated by a long cable). I don't think the battery charging would be too adversely effected by this alternative set up as the PV provides significantly more output than is required to charge a single AA NiCd. One consequence would be a lot more hardware on the ceiling inside, so you'd need to design a very different lighting unit to house it all in a tidy way.
Update: I received the following e-mail from Ramon with a suggestion of how to improve the wiring:
"Chris - stumbled upon 'solar-powered lights 2' while looking for something for the local market here in Yemen where I'm working - looking at a village of 25 families with no mains within 30km. Probably going to go with commercial units though.
Main point, why two wires from each solar cell? One is live (yes can understand keeping them separate) but other is earth/neutral so could just use a single wire linking them all, and one down into the house. Cuts total wiring in half!
Just another Kiwi working overseas!"
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