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Households can supplement network

Feb 19 2012 16:30
Francois Williams
Cape Town – Solar power could for most of the year provide the bulk of an ordinary household’s electricity requirements, but this would currently prove expensive.

Municipalities are still resisting the practice – already commonly adopted in Europe – by means of which home owners can connect their renewable energy systems to the power grid and be compensated for the excess electricity they upload to the network.

Three Cape Town homeowners now form part of a one-year test project, together with Cape Town municipality, which will compensate them on a one-to-one basis for any electricity they generate.

This means that when the households' photovoltaic solar panels generate more electricity than the household can use at that point, the electricity is sold to the municipality at the price the households pay for electricity when their solar panels generate insufficient power.

Dr Anthony Keen, a retired academic from the University of Cape Town’s medical school, is one of these homeowners who has been experimenting with domestic solar energy for more than three years.

Between 2007 and 2009 Keen spent R262 329 on constructing his system. Based on this outlay, the electricity that he generates himself costs R2.17/kWh.

But, as Keen says, in 25 years' time it will still cost R2.17/kWh – and what will Eskom charge then?

Moreover, photovoltaic solar panels are becoming cheaper as sales increase and technology improves.

A tariff incentive for selling excess green power to municipalities would make this type of long-term investment even more attractive.

Keen has mounted 20 solar panels with a total generation capacity of 3.8kW on his roof. A bank of 24 2V batteries and a single-phase 6kVA inverter, which converts direct current into alternating current, have been connected to the panels.

In his experiment he simulated two scenarios. In one he aimed to manage, as far as possible, without power from the grid. That was by using battery power when solar power did not suffice. Electricity from the grid was used only when the batteries produced too little power.

In the other scenario net-metering was simulated and the sequence for using electricity was first from the solar panels, then from the power grid and then from the batteries.

Keen said net-metering is easier because everything is automated, but this does not use solar power as efficiently as in the first scenario.

From May 2010 to April 2011 Keen’s household bought an average of only 86kWh/month from the grid.

The remainder came from solar power. Electricity consumption is managed so that most of the power is used when the solar panels are generating most of the electricity.

In the year from March 2009 to April 2010, Keen generated 6 431kWh from his solar panels and bought 2 001kWh from the grid.

According to Keen, the plumbic acid batteries used in his system are his weakest link – they are only 54% efficient.

He wants to invest in lithium batteries. These cost five times as much, but in the long run seem economically attractive.

Another alternative for the future provision of energy is to use surplus electricity to manufacture hydrogen to deliver electricity through a fuel cell.


According to Keen, people have various reasons for tackling a project such as this.

Some want to wean themselves from dependence on Eskom owing to the risk of power disruptions, some want to help Eskom by reducing the burden on the electricity grid, some want to adopt a greener lifestyle and help the planet, and some are simply fascinated by the technical challenges.

It's often a combination of all of the above. Keen decided on solar power because he reckoned it was the most practical and easiest application in an urban environment. He looks upon wind power in such a residential area as being too risky and unpredictable.
solar power  |  green energy



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