Payback Time

Letter from Reader

Good day, Nick.
I am a South African electronics engineer who has retired. Having done a bit of research I established that a typical 4 person household (with a standard geyser) uses 18KW/Hr, daily. In this country that translates into R1,000-00 per month or roughly $100-00, US.

If recent trends are followed then in ten years this will be $351, very approximately. Using solar power and batteries known for long life (note, a duty cycle of 25%) the payback point will be achieved after eight years.

In that household the regular consumption is 2KW (all figures are in KW/hour) with short term peaks going up to 12KW. Thus the inverter must cope with surges up to 15 KW. As mentioned (and this is well researched) a typical 4 person household uses 18KW. This is the house. If one is on a farm or not, the household will use the same amount.

Wind power is cheap and much more efficient than solar power but one has the maintenance issue. At least once, every two years the wind turbine will need servicing. This might be costly and will always happen when you most need power, so a small backup unit (around 600 Watts) is very useful to have.

Wind turbines have a number of problems, the most unmentioned one is ball bearing chatter. This happens in two instances, (a) when the wind is not quite enough and (b) when the wind is so strong that the blades are feathered. In this instance a slight but noticeable flat forms on the bearings.

Another problem many people do not take into account is the tower. This must be (a) strong enough to hold the pressure against it and (b) must not be affected by vibration, in fact it must be able to damp vibration (or the ball bearing will wear faster).

All the above negative features of a wind turbine, however, do not cancel out the fact that wind power is initially cheaper to set up, indeed, if the power does not need storing then the cost of a 2.5KW unit can be as low as $1,800-00. Just remember that the devil is in the detail. Wind is not predictable, thus, for a proper installation a means of storage is needed and in this day and age that means batteries, big ones. For the above hyper theoretical household the batteries for a 48 Volt inverter needs to be 930 amp/hour units and this means more money plus the battery needs charging.

There is a theoretical alternative and this is to use the turbine to pump water up a hill and then into a tank. When the wind drops then the water is allowed to flow down and turn a turbine. In South Africa there is one such large scale unit, it pumps water from one dam into another.As an electronics engineer I hate mechanical things, they wear out, naturally enough because they move and there is friction and abrasion. Sand powder can come in extremely fine form and unlike carbon powder, silicon is an abrasive.
The best possible non mechanical system would be the solar system, in both its forms. That is, (a) A heat converter for the geyser and (b) A photon converter for the electricity. It is true that such a system has only an efficiency of 5%, overall. This will rise somewhat but that is not the point. Current systems are rated to last 20 years with an efficiency loss of 20% during the last few years.
Here is a price I was quoted for a seven KVA solar panel array and batteries rated to last three days in Zero output days(total darkness and full autonomy). It is extremely unlikely to happen but with the grids not keeping up with consumption, it is not beyond the realms of imagination.
Below is an actual quote I received from one company, note that it does not include installation, but that is a variable. Divide the below by 10 and add a bit for installation and you have a system with Canadian solar panels (not some Chinese items) which I fully expect will be going strong ten years from now.

I have left out the company name and the quote is not current, what with the Rand not performing well, but the basic idea is there.

The items which need to be purchased would be :-

1) Solar panels R 42,693
2) Sunny Boy R 21,680
3) Sunny Island R 44,623
4) Batteries R 73,416
Total (Ex Vat) R 182,412
Total (Incl Vat) R 207,950

I hope I have contributed a little to the general pool.
Peter Bruyns.

2 Responses

  1. Thanks for the info. I have a situation where I look after elderly ladies on a Plot. The Eskom electrical supplier is rediculous with there costs. The network cost before any electricity is used is RSA R1,000.00 per month. I need to install some power supply that will cater for lights, refrigerator and TV + radio. Any suggestions would be appreciated.

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