Saturday, 9 February 2013

Current Understanding

The less technically minded of us look away now - after twelve months of drawing
power from the sun we can dodge the issue no longer - and will now attempt to explain
how our off-grid system works.

Being off-grid is a funny old thing. Most people own a TV but I’d hazard a guess that only a small fraction of us could convincingly explain how one works. Similarly, we know that our household electricity comes from sunlight, collected by panels on our roof and stored in batteries in our power shed, but when interested people ask for more detail we begin to feel a responsibility to be better informed. Another good reason for increasing our understanding is the other side of no longer being beholden to a power company: we are now responsible for our own system’s monitoring and maintenance.
So, without using circuit diagrams or scale models, here is what we know (don’t worry, to our shame this shouldn’t take long).

 In the beginning, there is light, - harvested and converted into an electrical
current by our solar panels.
 Our solar panels are photovoltaic, which means ‘direct conversion of light to electricity’. They consist of an array of solar cells containing a semiconductor wafer (usually silicon). This has a positive charge on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose and captured in the form of an electric current. Now I’m going to strain for credibility by invoking the name of history’s greatest pop scientist: in 1905 Albert Einstein described the photo-electric effect which photovoltaic technology is based on, for which he won a Nobel Prize in physics.

The charge from our panels is stored in these twelve batteries –
the heart of our power system.
Electricity in the form of direct current is then transferred to a bank of twelve deep cycle batteries in our custom-built power shed.
(Before reaching these, the current first passes through the charge controller, which does exactly as the name suggests and prevents the batteries overcharging. It also logs system performance data).
Deep cycle means the batteries are designed to be able to discharge a significant (deep) amount of their capacity and then recover it again as they receive new charge (cycle).
The batteries are the lead acid variety which store electrical power (893 ‘Amp-hours’ each - one Amp delivered for one hour equals one Amp-hour) in chemical form.
Sulphuric acid reacting with compounds of lead on positive plates inside results in the liberation of electrons – in other words, the current which flows into our home. We’re almost there – but not quite.

 Not much to look at, but without the charge controller (centre) and inverter (right)
we’d never be able to use the electricity stored in the batteries
As mentioned before, the electricity we receive from our panels is direct current (DC) and needs to be converted to alternating current (AC) for household usage. This is performed by our inverter/charger. The first part of its function gives us high voltage AC from low voltage DC by using an oscillator which causes the stored current from our batteries to repeatedly switch direction, and an amplifier to increase its voltage.
And so the transformation from sunlight to reading light is completed!
The ‘charger’ part of its function comes into play whenever we need to charge our batteries using ‘Little Red’ - our small petrol generator (Honda EU30 3kW). Once our only source of power before we moved into the house, Little Red produces an AC current, which then passes through a wave rectifier to be converted back to DC for storage in our batteries.

To make sure that our batteries enjoy a long healthy life they need to be constantly monitored to ensure they never discharge more than half their capacity. So in winter, when sunlight can be scarce, we need to charge them with our generator if the gauge display approaches -300 that is: 300 amp hours removed.
Once a month, we also equalise the batteries. This means running the generator for a couple of hours to standardise their performance and ability receive charge equally. If we look after our batteries, they’ll look after us (for ten years or so, anyway).
This technology combines to meet our ‘power budget’ of 4.5 kilowatts per day: less than half that of an average grid-dependant home but still easily achievable with electricity-saving lights and appliances, and a general power-conserving outlook.

To re-cap, here's a diagram:

Sacrifices are minor and gains have been considerable in terms of independence and an opportunity to lessen our personal impact on the environment. One of the cardinal rules of this blog has been to never preach or gloat, but I can’t resist mentioning the fact that Greytown suffered a major power outage lasting most of a day at the end of last year, and we were utterly oblivious to it!

Like Superman, we draw our mighty power from Earth’s yellow Sun.