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Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 9:38 am
by zzsstt
This week I started up our coolroom for the first time in a couple of years. As the price of electricity has almost doubled, I thought it time to inspect the seals and so forth to see if anything needed fixing. This in turn started me thinking about energy efficiency in refrigerators and freezers.

One of the first thinsg we are told is to minimise the time the door is open, and the number of times it is opened, because the cold air escapes and this wastes energy. Well, clearly it does, but (I wondered) just how much?

The specific heat capacity of water is 4.18J/g/C - so to heat or cool 1g of water by 1degree C requires 4.18J of energy. Assuming that most refrigerated food is comprised of 80% water, lets assume or the sake of argument that "food" approximates to the same heat capacity as water, and beer/milk etc. are the same again.

The volumetric heat capacity of air is 0.00121J/cm3/C

In real life, this means that if I open the door of a 370L upright fridge and allows the entire 370L of air (the fridge is empty!) to "fall out", then cooling the room temperature air that replaced it from 25C to 4C requires:

0.00121 * 370000 * 21 = 9401.7J

which equates to about 0.0026kWh, or about 0.07c if power costs 26c/kWh

In reality the air exchange may be more if the fridge door is left open for a while, or less if the fridge is almost full and has less air to "fall out". But you'd probably need to open the fridge door and completely exchange the air 14 times to use 1c of power.

However, putting a litre of water in the fridge to cool down from 25C to 4C uses:

4.18 * 1000 * 21 = 87780J, about ten times as much power.

So what does this mean in real life? Probably that buying beer (or anything else) from the chiller cabinet will save you electricity, whilst putting warm products in your fridge to cool them down will cost money far in excess of opening the door a few times!!

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 10:31 am
by Gordon-Loomberah
I'm glad I'm not the only one who has realised that opening the door has a fairly minor effect on the energy use of a fridge :)
See here from 2 years ago: http://forums.energymatters.com.au/solar-wind-gear/topic272.html#p1064

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 10:40 am
by zzsstt
Indeed, and yet it's still pushed as a major issue. I was reading just this morning about using plastic tubs in a coolroom to "stop all the cool air falling out".......

It makes me wonder how many of the other standard recommendations would stand up to any serious investigation. Obviously all the water saving taps are a money making venture, LED lights still have a way to go, and the Prius is less economical (alledgedly) than various 100% fossil fueled vehicles........

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 10:48 am
by Gordon-Loomberah
zzsstt wrote: ...Obviously all the water saving taps are a money making venture...


"water saving" bath taps are a pet hate of mine- the water comes out so slowly that you need more hot water to compensate for the much longer bath tap run time and the bath water cooling down.... they dont save any water- you still want the same amount of it! they just mean you need more hot water. How is that a saving of anything!?

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 11:05 am
by zzsstt
They save the homeowner from having to admit that renovating the bathroom with expensive fittings was done only to look good, and they save people from having to make any real reductions in their consumption. They save the manufacturer from having to find a better marketing stategy, and they also save the government from having to make any meaningful decisions or measures that might actually help. That's four savings, they must be good!

The same applies to all water saving taps. To be truely "green" just keep the old taps (new washers if they drip!) and don't turn them on full blast if all you need is a trickle! Of course this argument extends to just about every "green" product in existence!

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 12:08 pm
by Smurf1976
zzsstt wrote:The same applies to all water saving taps. To be truely "green" just keep the old taps (new washers if they drip!) and don't turn them on full blast if all you need is a trickle! Of course this argument extends to just about every "green" product in existence!

True, although one of those fittings that adds air to the water stream does result in a real world water saving since the flow seems larger than it really is when washing hands etc, thus leading a typical user to use less wather than they otherwise would.

Agreed that many things are scams though. As a general rule with few exceptions, if it requires you to purchase something new which replaces something you already have that is still working then it will not be a real means of saving resources, money or the environment. There's a few exceptions but not many.

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 6:27 pm
by zzsstt
Smurf1976 wrote:True, although one of those fittings that adds air to the water stream does result in a real world water saving since the flow seems larger than it really is when washing hands etc, thus leading a typical user to use less wather than they otherwise would.


My grandmother had a rubber aerator that pushed on to her kitched taps to prevent splashing. Exactly the same thing, cost pennies and that was 40 years ago...... Memories!

Re: Energy efficiency myths and realities?

PostPosted: Sat Oct 29, 2011 8:30 pm
by bashworth
The one that gets me is the idea that you can buy a system that recirculates hot water around the plumbing system to 'save water' the heat lost by all the hot water running around the pipes will be a bigger environmantal issue than the water, especially as I keep a watering can in the shower to connect the initial 4-5 litres

Re: Energy efficiency myths and realities?

PostPosted: Sun Oct 30, 2011 6:44 am
by zzsstt
Horses for courses! I live on tank (rain) water and have solar water heating. To me that recirculation system saves water each time somebody needs hot water, at the expense of only a few minutes of sunshine!! And the pump is solar powered through the day! Water 9wait until it rains) is a more precious resource to me than either heat (free from the sun) or power (free from the sun in daylight hours, from the grid at night).

There are many similar issues however. The idiots who believe that food comes from supermarkets want farmer to use less water. Flood irrigation uses (and "wastes") large amounts of water, so it is seen as bad. But drip irrigation (either above or below ground), whilst being far more water efficient, uses far higher amounts of power because the water has to be pushed through small pipes and emitters (sprinklers, drippers etc.) whereas flood irrigation uses gravity to flow the water through a series of channels and paddocks.....

Most of the "green" products are gimmicks to make money from people who don't really consider the issues (or who are looking for an excuse, or a status symbol). A very few devices offer across-the-board benefits, and some are capable of providing benefits in certain situations but not universally. And sometimes it's a trade-off of one thing against another!

Re: Energy efficiency myths and realities?

PostPosted: Tue Nov 01, 2011 10:11 am
by lad
Wife got the taps for the house. Took 30 minutes to run a bath, good deterrent to having one.

Pulled out the water "saving" (read delaying) features and it is now realistic to have a bath if so desired. Water saving taps elsewhere are good.

Being off grid, the pump had to run over a longer period before I fixed the bath taps.

Teenage kids in the house, the taps certainly do save water :mrgreen: