Trout

iTrader: (1)

I came across this fact today on the BBC website (can't you tell I am busy today!).

Now that just sounds wrong (e.g. it implies that a diesel generator would run for five years generating electricity for a house on ten gallons of fuel?!)

If it is true then there is a serious imbalance in how we are using up a finite resource!

Coffin Dodger

That doesn't sound right? Maybe there is enough calorific content in 10 gallons of fuel that if converted into electricity with something nearing 100% efficiency it could run an energy efficient house for five years but I doubt it.

Certainly producing electricity on a massive scale such as that done in power stations must be significantly more efficient than a simple petrol generator, but not to that level. What we ultimately is a massive investment in nuclear power, there is no denying that this is the future is so far as meeting our energy needs in an extremely clean fashion, and a cheap, practical, electric car that can be fast charged (i.e. >5 mins) while out and about. Then we can keep our nasty petrol drinking monsters for the weekend / leisure purposes etc.

kingofturds

iTrader: (1)

450kWh of energy divided by 5 years equals 90 kwh a year so that will power your kettle for about 30 hours a year and that's about it. I guess the rest of this energy will come from the bbc journo spouting this bollox.

ALi-B

iTrader: (1)

It is wrong: Assuming a gas powered power station (god knows what coal is rated at), that figure is way off. I think the average petrol engine is about 25% efficient vs diesel is about 40% (these are school figues, so I guess its a little better these days, but not by much, say 5% at the most). The world's most efficient diesel engines are found in container ships and thats rated at 50% efficiency. Anyway, fossil fueled power station run up to 50% efficiency (Skćrbćk in Denmark runs at 49% efficiency) without CHP (who on earth uses CHP? ).

Part of the issue is the energy contained in the fuel; Calorific value by volume, weight and how much bang per litre (Trout will remember this ).

Calorific value is confusing as depending on measuring method, it varies based on the fuel's density: by weight Petrol has more energy than Diesel (about 45MJ/kg for diesel vs 46MJ/kg for petrol). BUT as diesel is denser than petrol it has more energy by volume so actually diesel has more energy (37MJ/litre for diesel vs 34MJ/litre for petrol). So in effect we get more energy in our tank with diesel.

Of course, both are alot denser than natural gas so have more energy by volume (unless massively compressed), but by weight, gas actually has more energy per kilo. So yes gas is better, but you still need more of it- alot more! Natural gas is about 55.5MJ/kg but is only 21MJ/litre in liquid form.


ANYHOO!

So back to the claimed fact the energy in 10gallons of petrol will run a house for 5 years ? Well lets just say we used a diesel generator rated at 40% efficiency and the average anual energy usage for a 3 bedroom house is about 4200KWh = 15120MJ (1KW/hr = 3.6MJ) (source: Strathclyde Uni - don't ask, I googled it).

Of that amount of energy needed, 60% extra is wasted assuming perfect operation conditions), bringing the total needed to 24192MJ. Diesel has 37MJ per litre so lets divide that = 653.83litres which is 143.82 UK gallons of diesel per year.

Even if it was a power station supplying the house, with it running 50% efficient and running on liquified natural gas (LNG) it would need alot more than 10gallons of LNG (21MJ/litre), to be precise, it would need 1080litres or 237.56uk gallons per year!

So how much energy is in 10gallons of petrol? 10gal = 45.46litres. Petrol at worst is about 34MJ/litre so that is 1545.61MJ. So thats about 37 days worth.

BUSTED


But before people go out and buy a diesel+battery genset (A Yanmar; not a cheapy from machine mart). Consider that the fuel is taxed, so unless you can get your hands on red diesel, use a biofuel or veggy oil (I think Yanmar may not warranty that ), a genny+battery set in your back garden will be more expensive to run and maintain (and your neighbours will love you ). So its still cheaper to buy from the national grip. Although I'm not sure by how much.....

Trout

iTrader: (1)

It was a factoid on the QI website and was on the BBC homepage as 'FACT' of the day.

As I thought, that didn't make sense