Earth Notes: London Home Wind Power Pilot: MotorWind (2007)

As of 2007 I already had a solar PV system to power my office lighting and take my main Internet server/laptop off-grid some of the time. (More than 10% of each day on average in Oct/Nov 2007).

The 8-turbine MotorWind I ordered 2007-09-15 arrived 2007-11-26 from Hong Kong. (By sea, to save a little CO2!) Gross weight was 7.8kg (net 6.8kg). The box was 1.15m x 0.28m x 0.19m (ie a little under 4 foot long) and not heavy. My order number was 535. So I'm guessing that ~500 have gone out before mine.

Each turbine is approximately the same swept area as my existing tiny turbines. With them I am mainly attempting to measure availability. As each of those is estimated to generate 1W (typical) to 4W (max) then I might expect the entire MotorWind assembly to generate 8 times that. Which, when there is wind, is ~50% or more of my laptop/server load. (The generator is rated at 50W officially, for brisker wind then I'll have.) As of November 2007 my stats suggest that wind is available ~2% of the time. If so, this turbine might only get me ~15mins/day extra off-grid time. But this experiment is to actually find out what is achievable. This must be 'nanogeneration' I think, since 'microgeneration' is 100s of watts or kW or 10s of kW!

Assembly Instructions from Outer Space

Included in the box are assembly instructions which, as ever with these things, are blindingly obvious to the supplier but somewhat opaque to me! However, the unit is mechanically very simple and looks robust. So I'm feeling fairly confident that I can erect it without damaging me or it.

I have now assembled the frame on which the turbines sit. That required doing up one screw, tightening a couple of bolts, and sliding on a couple of rubber feet (mainly to protect fingers I think).

I have test-spun the generator by hand, and it seems to generate over 12Voc with little resistance and at a believable RPM. (My 23-month toddler was able to turn it fast enough to make a 12V LED unit flash quite brightly, which seemed to impress her.)

Update 2007-11-26 4pm: it's now dark outside (and cold!) so I'll put up the frame and slide on the 8 injection-moulded turbine rotors my next available morning. (I chose green to be easiest on the eye.) Initially I'll just be measuring open-circuit voltage in a decent breeze before I connect anything up electrically. (A MotorWind unit has survived a typhoon, so I'm not expecting running unloaded to represent any kind of mechanical danger to the assembly.) Then I can decide what sort of dump load to use to best protect the battery from overcharging on a windy day, especially when also sunny. I'm going to treat the wind source as very much secondary to the solar controller, ie as a top-up source only. So I'll probably disconnect the wind generator somehow when the battery gets much over 13V, perhaps @14V before a blocking diode. I'll let the solar controller do any clever multi-stage charging.

Assembled, Mounted and Flying

Update 2007-12-01: I partially inserted the mounting bolts a few days ago, but had to buy a bigger hammer to drive them in far enough. I'm not at all convinced about my handiwork and may need to remount the frame and repair some brickwork at some point.

However, the 8 rotors are up and turning, and with a start-up speed similar to, or probably a little lower than, my existing tiny turbines. But in any case they so far seem to spend more time turning than the tiny ones. They make a little bit more noise than I'd hoped for (the tiny turbines are essentially completely silent but I can hear these from inside the house). So I'll have to be alert to annoying my neighbours (and possible remedies). Part of the purpose of this pilot is to gauge acceptability.

I measured ~20Voc with my multimeter in one reasonable gust, soon after putting up the MotorWind. Initially I did not attempt to wire the MotorWind into my system, since I did not have a dump load constructed. I intended to let the MotorWind handle some decent rain and wind for a few days to make sure that it seemed to be robust before I connected up and rearranged my system for it.

However, in strong gusts of wind the rattling is quite loud. Possibly enough to be an irritation to my neighbours. So by connecting the turbine (via my monitoring and 'safety' diode box) I am aiming to dampen turbine speed-up once it passes cut-in since additional energy will be drawn off as electricity. This seems to be working. I believe that it's running a little quieter. (MotorWind already found my remarks on 2007-12-01 and said that they have developed noise reduction rubber rings which when wrapped around the turbines take care of the noise problem to a large extent without affecting performance.) I'll have to manually monitor the system for safety (to avoid overcharging) "riding shotgun" for now.

I'd only seen a few gusts in the first hour or so after installation. I was sure that the MotorWind was charging the battery ("cut-in"). I could be confident because the monitoring point voltage stuck at ~14V. I had not yet attempted to measure charging power (eg with my E-flite).

Incidentally, I think one small part (a rubber washer) was not shipped. And a couple of the rotors show some minor non-critical shipping damage. (Some slight chipping near the hub.) But the way that the MotorWind is constructed it would be possible even if one rotor got damaged to rearrange all the others to work without it. So only losing 1/8th power, which is neat. Spares are available for the turbines, generator, etc.

Update 2007-12-02: by 4pm stats are showing wind power available ~40%, which is a record. Local windspeeds were forecast around the 20mph level. Though on the ground amongst the houses the best we get is good gusts, not steady strong wind.

Turbine Down!

Update 2007-12-03: overnight one of the bolts undid itself from vibration and the MotorWind fell. Amazingly, as it fell into soft earth, nothing is broken. I need to find a sensible way to tighten and hold the bolt(s) affected. I'm not quite ready to glue/fix things permanently until I'm fairly sure that everything is set correctly.

Everything is up and running again. Winds are lighter today.

Update 2007-12-05: the MotorWind people are being very helpful. I am getting emails from HK and India advising on how I might deal with the vibration/nut issue, This is good customer service! In the interim I have secured the bolts with both a little glue and with locknuts. I'll keep checking them regularly for now.

Update 2007-12-19: the various fixes seem to be holding the bolts well for now.

Update 2008-07-08: the MotorWind has been working well, so far as I can tell. No one seems to be bothered about the noise that was worrying me. I'm in a rotten area for wind (in a dip, in the suburbs, amongst trees). But I seem to be getting some output very roughly 10% of the time. In fact, more when the weather (and solar PV output) is poor. And there is at least some energy gathered at night. (Look at the relevant months in the historical data.) I'm not sure whether to go for more wind or more solar this winter. MotorWind has some interesting new designs if it's wind that I decide on.

Update 2009-09-25: I built and installed the dump controller kit I have. With a full battery and bright sunshine, I think I just saw my SCR-based emergency brake come into play in a gust of wind on my turbine. Someone described it as "jamming a stick through the blades" and it certainly had a flavour of that! It's a bit brutal and a proper dump load would be kinder all round.

Overvoltage / Overcharge / Overspeed Protection

Since in my system wind is a secondary charge source (solar PV is primary) I do not need the complexity of a full dump controller. Instead I can use a crowbar to protect against overvoltages caused by an overcharging or disconnected battery or other transients, and/or an overspeeding turbine in high winds.

This crowbar* should stop the turbines supplying charge to the battery, and slow or stop the turbines, when activated. The dissipated energy (as heat) will be shared between the crowbar SCR, the upstream power cables, the generator windings and bridge rectifier.

*This kind of drastic short on an errant power source to protect something valuable (eg a computer) connected to it is known as a 'crowbar'. See Wikipedia. Generally a crowbar, once triggered, remains active until power is removed. In this case braking the turbine until the turbine stops.

Stopping a turbine this way is unkind. But it really is only expected to be a rarely-used emergency mechanism when other regulation has essentially failed.

Given that the MotorWind generator is rated at 50W (~4A) the 25A-rated SCR is generously overrated.

The ~14.8V trigger voltage is similar to the nominal 15V of the power Zener that I was previously using with the two tiny turbines. Allowing for the power rectifier voltage drop (0.7V+) before the battery implies that the crowbar will trigger if the battery voltage gets over 14V. This indicates a high charge but probably before my gel SLA starts gassing. Gassing would be permanently damaging and potentially dangerous.

This crowbar circuit only draws a few milliamps. But it draws no power at all from the battery and thus cannot run it down. It uses no power except from the wind.

Update 2010-10-11: moved the MotorWind turbine over-voltage crowbar circuit to outside junction box. Plumbed output via blocking diode to secondary/LiFePO4 array circuit. Any wind power now charges the LiFePO4 battery unless already full.

Update 2015-03-30: with 45--60mph winds forecast tonight and for the next day, and with the bolts pulling themselves from the wall even in gentler winds, I have taken the MotorWind turbine down for the moment. (A few days before I started monitoring the turbine... Murphy's Law dictated initially that the wind died then that a major storm would turn up, necessitating taking the whole lot down. Grrrr!)

Update 2018-04-16: the MotorWind has never been put back up. It lies mouldering on the garden path, looking a little worn. The rubber washers and the plastic blades had begun to break up.

Update 2021-11-17: MotorWind went today for recycling.