Wind/Solar Systems


Ain't the weather a wonderful thing. Sure it's not always the case but, it is often calm when the sun is shining and windy when it's not.

What was amazing is when this principle was presented to a supplier of both solar panels and wind generators the idea of combined use was decidedly frowned upon. However, one year later, they now supply a box that accepts a solar array, a wind generator, battery, and load (names have been withheld to protect the ignorant!).

I cannot claim to be the first to have implemented such combined use (although I will claim to have steered the above mentioned company to better profits) but it does appear there are many who are not aware of the benefits of wind/solar as an alternative energy source.

Solar panels have long been a favourite with radio engineers for powering mountain top repeaters, especially where mains power is too distant to prove economically viable. Two challenges face one when installing solar sites being the time the system is to remain operational during periods of little or no sun, and then to recharge with what sun is likely to be available. These two criteria are clearly shown in the section on the "Solar Calculator" with an Excel spreadsheet available for download.

The amount of sun is made up of two parts being the physical useful sun hours per day being modified by the amount of cloud cover. Useful sun hours is an easy calculation based on the line of latitude and rough geographical terrain. Being on a mountain top will offer more sun hours than being in a deep valley as the mountains will block the available sun in the latter case. Cloud cover is the real problem especially if your site is located in a winter rainfall region. Not only is there limited sun but what is available being drastically reduced by almost daily cloud cover - but.......

cloud cover usually brings wind!

windspeed vs outputYip!, I agree, not everywhere is suitable for wind chargers but there are only a few places that don't get wind for at least some of the time. Taking a closer look at the accompanying graph it will seen that it does not take much wind to produce at least half power, and what's more, it's not restricted to the time of day. If wind should blow at midnight it will have as much charging effect as if the same wind were to blow at midday.

If this is the case why bother with solar panels? The answer is as logical as the previous statement in that what will happen to your system should you be subject to a long period of e.g. a high pressure weather system? There are places in the world where there is nothing more than a breeze for weeks on end, decidedly quiet enough to not keep a wind charged site fully operational.

Not only is there a scientific reason to consider wind/solar (it is assumed at this point you are not taking sides with the company that frowned!) but also a very strong economic one. There are distinct savings on two fronts; one the solar panels, and two, the battery.

If you've been adventurous and have played with the solar calculator you will have discovered that even a relatively small system requires a large number of panels to be a reliable solar site (the downfall of many a solar system having been in the economy of the number of solar panels). What raises the number of panels with reliable installations is any lack of sun hours be it distance from the equator, weather, or both.

Not only does the cost skyrocket with the number of solar panels but so does the battery price as the capacity needs to be increased to stay within the C/3 recommended maximum charging current.

Adding a wind charger, at about 50% the cost of a solar array with similar current output, will drastically increase the daily charge time. This impacts the battery size required as the site autonomy may now be reduced to only what is needed (and a little spare) to have failures of the power generation repaired (remember, it takes time to get a technician to site!).

simple wind/solar site

The above image shows a simple wind/solar diagram supplied by a local supplier. Here the solar panels are parallelled with a wind generator and the system regulated by a suitable shunt regulator.

Notes: The blocking diode in the solar panel downlead so as to ensure no back-feed into the panel from the wind generator. Wind generators usually have such blocking diodes and so none is shown, however, do check before installing. The shunt regulator must be able to handle the current from the complete solar array and the maximum current as supplied by the wind generator. Failure to do this will destroy the regulator. A personal preference is to double the required capacity as a minimum. The fuses are extremely important as lead-acid batteries are capable of many amps which, if uncontrolled, could cause fire. Please ensure fuses as well as the correct currents are installed.

If it sounds this good there's got to be a catch!? The short answer is "Yip". But don't despair, it's not as bad as you think. The catch is there is a lot more planning and research time with a wind/solar site than with a solar only site. If asking any of the locals near an intended site about how much sun they get chances are you'll get a pretty accurate answer, but when asking about wind the chances are you'll only get to hear about the bad ones, those that blow with rain. What about all the lovely cooling breezes they had during the hot sweltering sun etc. (and what blows at night)?

Two options are open to you, the former not always being available, the second being fairly sure-fire way of knowing your wind/solar site will work. The first option is to contact the local environment or weather agency. They have monitoring sites in the most unthought of places and maybe there is one near your intended site. One of the elements they measure is wind.

If this is not open to you it is then suggested that you simply install your wind/solar site with some means of monitoring the charge data. The current monitoring shunt resistors should be placed in the negative leads, as indicated in the above diagram, to ensure the common mode voltage capabilities of the monitoring equipment is not exceeded (also see an Elpro based "wind/solar repeater system"). These current measurements are fed into a package that will allow you to collect a year's worth of data. A year is the minimum you should collect, two years is more ideal.

Not everyone can wait this amount of time which is precisely why it is suggested that the site just simply be installed and then carefully monitored. Going to all the trouble of installing a 'computerised weather station' that will capture the data for a year and to then only find out it is suitable is a year wasted wherein your site could already be running. Sure it's a bit of a risk but this method is a lot less costly than installing all the equipment needed for the research and then having to redo all the work installing the site itself.

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