November 2010


Appliances &Solar18 Nov 2010 02:35 pm

The early summer cold-front that has been chilling Johannesburg for the last few days has me scratching my head. I measure my electricity consumption regularly and record it on a spreadsheet, and suddenly found that the household’s daily usage had jumped from about 15 kilowatt hours to about 25. Then we ran out of tea just a few days after we opened a box of 80 bags. Could there be a connection? Four children are at home studying for exams in the cold, and teacups have been piling up on their desks. With a thermometer, a stopwatch and a calculator, I set out to solve this mystery.

To raise a litre of water by one degree Celsius theoretically requires 0.0011 kilowatt hours, so heating my 1.7 litre kettle from tap temperature to boiling, 17 to 97 on my thermometer, should use 0.15 kWh. But since it took 3:48 to boil while using approximately 2750 watts, the actual consumption was more like 0.175.  (My Watts Up meter cannot handle appliances over 2000 watts, so I could not measure directly.) The inefficiency probably comes from the heat lost warming up the stainless-steel kettle itself—a plastic kettle might reduce those losses—and the delay between when the water reaches boiling and the kettle shuts off.

Of course, they don’t all share a pot of tea at once, so I used a spreadsheet to simulate two scenarios. In one, my children have suddenly become green angels, and measure out exactly one mug full of water into an empty kettle to boil. Under these ideal conditions, their additional 24 cups of tea a day would use just 0.59 kWh. The worst-case—and very common—scenario is to fill a kettle and boil it over and over again  until nearly empty before repeating the process. Under these circumstances, those same 24 cups of tea use nearly three time as much electricity, or 1.61 kWh. (Even after taking into account the warm water that is being re-boiled.)

My mystery is not entirely solved. The kettle could account for a sixth of the increase, but the rest must come from addition lights burning at desks, the refrigerator door opening and closing for study snacks, and those electronic devices that seem to take over study breaks.

Still, I’ve learned something useful. Changing kettle habits can make a significant impact on consumption. The Eco-Kettle is designed to make this simple, saving you from running back and forth to the sink to measure another cup of water. The water reservoir at the top of the kettle can release measured cups of water to the element at the bottom. It is available from a few sources in South Africa for R450 and up. Six people drinking 4 cups of tea a day with a full-kettle habit to break could probably save enough to pay for the expense in a couple of years.  I’ve seen a few complaints on the internet about durability, however, and I wonder if a simpler solution wouldn’t suffice. I plan to put a water jug next to the kettle and ask my family to measure out the water in their teacup before pouring it into the kettle. (Our kettle has a flat bottom. If yours has a raised element, it may need extra water to cover the element.) It should save them time, too. My stopwatch tells me that a cup of water boils in 51 seconds. A full kettle takes nearly 4 minutes.

While on the subject of kettles, I sometimes use ours as a back-up geyser. I try to keep the electric elements in my two, 300 litre solar-heated geysers switched off, and most days I don’t need them. But occasionally I have been seen pouring a few kettles full of boiling water into a bath on a winter night to keep my wife from grumbling about my solar fanaticism. This causes her to ask, “wouldn’t it use less electricity just to turn on the element in the tanks for an hour?” My spreadsheet provides the definitive answer. Boiling even five kettles of lukewarm water from the hot tap uses 2/3 of a kilowatt hour. The 4000 watt element on the geyser would use 4 kWh in an hour. There are times when a full kettle is green.

Global Warming &Vehicles01 Nov 2010 01:23 pm

95 right? More likely wrong.

The conversation after Sunday lunch somehow migrated to the topic of octane in petrol. A woman was complaining that her husband makes her buy premium 95 petrol even though it costs more. The husband, who drives a Prius, was defending the extra expense on the grounds that the extra octane makes the engine run more efficiently, reducing carbon dioxide emissions.

I had never heard this before, and any idea that might reduce C02 emissions catches my ear. Was I unwittingly damaging the atmosphere by my frugal habit of buying the cheaper petrol? I had to look into this.

One of the best sources of information available was an Engen FAQ on octane, which confirmed, as I had suspected, that we cheapskates are right. “The great majority of vehicles inland . . .  are satisfied by 93 octane,” the website explains. “The use of octane grades higher than your vehicle actually requires will cost you more, cost the country more and have a negative environmental impact.”

The FAQ explains that this is because:

“There will be no additional benefit to driveability or performance of your vehicle if you use a higher grade than it requires. Petrol with a higher octane requires more severe refining and greater energy use in the production process. If not offset by greater fuel efficiency of the vehicle using the fuel, this extra energy use is wasted energy. This wastage results in higher emissions of greenhouse gas (carbon dioxide), which harm our environment.”

What the Engen website did not explain is the relationship between octane and altitude. Octane is purely a measure of how petrol reacts to pressure. Higher octane fuels will not ignite prematurely under higher pressure, which could cause engine knocking. But the Highveld altitude has a low ambient air pressure, which reduces the pressure inside most engines and thus reduces the need for octane. I spoke to John Fitton, an independent petroleum industry consultant, who said that most engines requiring 95 octane at the coast—as is recommended in the manual for my Honda Jazz—only need 91 octane in the Highveld.

South African regulations allow for three grades: 91, 93 and 95. At low altitude, 95 is the only grade available. Highveld petrol stations offer 93 and 95, but 91 isn’t sold at all in South Africa. Fitton told me that this is only because the petrol retailers are trying to satsify ill-informed customers. “Consumers think octane is power,” he said.

Fitton pointed out that the lower octane requirements at higher altitude do not apply to turbocharged engines and a minority of newer engines with gasoline direct injection. (These engines are sold under a variety of brand names such as Mercedes-Benz’s Charged Gasoline Injection, VW’s FSI and Ford’s EcoBoost.) Owners of these cars should obey the instructions in the car manual whether they are in Lesotho or Lambert’s Bay. For the rest, Fitton said, “There’s no advantage to using the higher octane; you’re just going to spend more money and emit more C02.”