Economy 7 Trick

Ben84

Time is the important factor often overlooked when considering energy consumption and I suspect this is because people are sometimes confused by how we use time in relation to energy.

Energy as electric or gas however is commonly measured in kWh, a value that includes time and we need to consider time as much as the watts because it only makes sense in relation to cost and fuel consumption as a combined unit. Watts are not in common understanding a stand alone unit. You cannot have 60 watts in the same way you can have 60 pens. The watt is defined as one joule per second, hence the need to add a time unit to make it meaningful. A measurement that's only watts is more a snapshot of a moment in time, for example you could state that when turned on a 60w light bulb is consuming 60 joules per second. You cannot however from this statement say how many joules it consumes in total any more than you can say how far a car travelling at 70 kilometres per hour has travelled. Adding time makes it possible to answer these questions.

It may confuse people that a 3 kWh electric fire can consume three kilowatt hours in one hour, but time is just a unit like any other that can be used to measure something.

We may need to return to the concept of what is a measurement or a unit to understand it. Measurements not only allow us to compare things but are only meaningful because they are comparisons. For example a metre is only meaningful to someone who knows what a metre is by owning or having seen a metre long ruler. Without a defined relationship to something there is no unit.

Interestingly however, if there were no metre rulers in the world, only 10 centimetre rulers, the metre would still be a valid measurement defined as ten 10 cm rules end to end. The same is entirely true for time as a unit, we can have something consume 3 kWh in one hour by accepting time as a unit of change or progress which is entirely relative to something else.

To return to the pens, while we might not think too much about measuring there being 60 as it seems innately obvious, this measurement too is a measurement of difference. To suggest there are 60 we are of course accepting the possibility of having zero, sixty or even six billion pens. That is a measure of difference. Going deeper in to what this idea means, by measuring 60 pens on your desk you're measuring a difference in mass of maybe 800g of plastic, ink and metal in this specific location to another location. Seems basic, but think carefully about it and you'll realise *everything* is relative and so are all units and measurements to one another.

Just as adding time answers how many kilometres the car has travelled, time is a relative measurement used in the case of energy to answer the question of how much coal has been burnt at the power plant. It is also convenient for us to look at things this way as it allows simple calculations about how much it costs to use an appliance for a set amount of time.

So, time matters. it represents in this case the difference in the amount of coal/oil/uranium/whatever consumed at the power plant. Turning off your freezer or heating for any amount of time means less energy is consumed than if you had left it on. Where there's no energy loss from turning the appliance off time and the kW rating answers the question of how much energy is used. Take for example a light bulb, if you turn it off for ten minutes then back on again it does not consume energy for that time period and turning it back on does not cause it to retrospectively consume the energy it would have used had it been left on. There is no catch up, that time period has passed without energy consumption. Despite having insulation, your freezer isn't so different. For any time period when it's at a higher temperature than it would otherwise have been if it was still connected to the power, less energy is being used because time is passing without energy being used. Of course it will have to lower the temperature to the intended setting when it switches on again, but it does not have to put in the same energy as it would have if left running because the compressor still runs for less time in total when you compare it.

Cardew

Ben84 wrote: »

Take for example a light bulb, if you turn it off for ten minutes then back on again it does not consume energy for that time period and turning it back on does not cause it to retrospectively consume the energy it would have used had it been left on. There is no catch up, that time period has passed without energy consumption. Despite having insulation, your freezer isn't so different.

I like the light bulb analogy! It is probably better than the 'kettle kept simmering'

zeupater

Ben84 wrote: »

..... Of course it will have to lower the temperature to the intended setting when it switches on again, but it does not have to put in the same energy as it would have if left running because the compressor still runs for less time in total when you compare it.

However, running the compressor continuously or for long periods will consume more energy to cool the contents than has yet been considered in this discussion.

Compression generates heat, decompression requires heat. Extended work cycles on the compressor unit will raise the temperature of the casing which in turn will raise the temperature of the compressed refrigerant, which in turn reduces the capacity for the refrigerant to cool the evaporator when decompressed, which both effects the time to pull down the freezers internal temperature and increases the return temperature of the refrigerant gas, which increases the temperature of the compressor ...... this cycle continues until the temperature of the condenser reaches a point where the convected airflow and radiant heat transfer reaches equilibrium with the rate of heat input at which point the system runs hot until the freezer's internal temperature reaches it's thermostatic set point and the compressor is switched off ......

Obviously the above describes a less efficient system which simply creates more external heat to provide the same cooling duty, so it's not quite as simple as only considering the internal heat gain when normal thermostatic cycling and 'long period' cycling.

The real issue which should be considered, and which I have touched on previously, is system failure. Let's assume that the mean temperature of the mechanical compressor when operating for long periods is 20C higher than normal cycling, which would not be atypical (depending on unit design), then you would expect that a unit designed for 15 years of operation would likely fail in 4 years ..... factor the replacement cost into the equation and any possible energy savings suddenly become insignificant.

HTH
Z

veggieblob

MFW_10YRS wrote: »

I'm interested too. We have underfloor heating in part of our property and it is set up to keep the room at a constant 19C, which we were told is more energy efficient than turning it on and off and reheating the entire slab.

I could see the reasoning about re-heating the slab, especially in Winter where you don't want to sit around for hours waiting for the slab to warm up, which in turn then warms up the property. In summer time the property is above 19C from solar gain and so the heating stays off anyway.

I know this is a little off topic, but I've just bought a low carbon "eco house" with hemcrete walls and ufh. Have you actually had a winter there to decide the best way to run the heating in the winter? Mine also has a heat exchange unit which seems to use a lot of energy just sitting there. I only heat my water for an hour a day at night.

HappyMJ

veggieblob wrote: »

I know this is a little off topic, but I've just bought a low carbon "eco house" with limecrete walls and ufh. Have you actually had a winter there to decide the best way to run the heating in the winter? Mine also has a heat exchange unit which seems to use a lot of energy just sitting there. I only heat my water for an hour a day at night.

The big problem with UHF is that it takes a long time to heat up. But it also a long time ages to cool down. So turn it on 2 hours before you get home and time it to turn off 2 hours before you go to bed. If you want heat in the morning then do the same. It's much trickier to time the morning as most poeple are only up and about for less than an hour so a normal convector heater may just be cheaper for a short period of time.

John_Pierpoint

MFW_10YRS wrote: »

Thanks for your post, it has given me something to think about. I'll research this as I really can see the logic on both sides.

If you have a box of megnetite bricks and heat it up overnight in the early hours of the morning using 4p a unit electricity its is called a night storage heater.

If you have a very thick walled building shaded with trees and you filled it up with ice from the adjacent lake during the winter, you were a very wealthy land owner and you had a seasonal ice house and could show off to you guests by making a frozen sillibub in mid summer.

If you have a box full of bricks made of ice and you cool it down during the early hours of the morning you could call it a night storage freezer.

If you have a big compressor and keep on cycling it on and off you will be unlikely to achieve its mean time to failure of 21 years.

[My chest freezer is 35 years old, but it was a high insulation one for its time - but watch out for the ice cream - it is at the top of the freezer and the first thing to notice the temperature is not cold enough.]

John_Pierpoint

veggieblob wrote: »

I know this is a little off topic, but I've just bought a low carbon "eco house" with limecrete walls and ufh. Have you actually had a winter there to decide the best way to run the heating in the winter? Mine also has a heat exchange unit which seems to use a lot of energy just sitting there. I only heat my water for an hour a day at night.

What fuel & method are you using to create the heat?

What make and model is this heat exchanger thing?

The mantra is "build tight ventilate right" - how has this been achieved in you new home?

veggieblob

John_Pierpoint wrote: »

What fuel & method are you using to create the heat?

What make and model is this heat exchanger thing?

The mantra is "build tight ventilate right" - how has this been achieved in you new home?

Hi John, bearing in mind I am a user this is the information that I think you are asking for:

The house is all electric

Heat thing is Ecodan Aerocyl-me Air Source Heat Pump and copper cylinder (Mitsubishi Electric Heating Systems)

Mechanical ventilation and heat recovery system in loft

300mm thick hemcrete walls

Triple glazed windows with no airflow bar

I find the whole thing uses a lot of power when it is just "sitting there". All I need to do at the moment is heat the water for an hour a day (single working occupant)

Any hints, tips or comments gratefully received.

Thanks

grahamc2003

veggieblob wrote: »

Hi John, bearing in mind I am a user this is the information that I think you are asking for:

The house is all electric

Heat thing is Ecodan Aerocyl-me Air Source Heat Pump and copper cylinder (Mitsubishi Electric Heating Systems)

Mechanical ventilation and heat recovery system in loft

300mm thick hemcrete walls

Triple glazed windows with no airflow bar

I find the whole thing uses a lot of power when it is just "sitting there". All I need to do at the moment is heat the water for an hour a day (single working occupant)

Any hints, tips or comments gratefully received.

Thanks

From what I can gather, the efficiency of these things drops off quite quickly with increasing water temperature demand. If you can control the output water temperature, try setting it lower. Or try turning the whole thing off and if you just need hot water, use the immersion. Could be that a 15kw (or whatever) heatpump may not be too great when just dumping 2 or 3kw. (I'm just thinking aloud really - no real basis for those thoughts, just some things I'd try if I were in your siuation).

When you say it uses a lot of power when just 'sitting there' - could you explain a bit further? What is the consumption when it's doing no heating? (I'd expect it to be zero!).

veggieblob

Thanks for your thoughts. I have actually turned the outside unit off as it was pulling about 180w ph 24/7! I know it is supposed to produce 3kw of power per 1kw used, but I don't need that sort of cost for a shower and a bowl of washing up a day! I now just put it on with the immersion (which doesn't seem to work without it!) for an hour in the morning on economy 7 night rate about 4 times a week and that does me.
I am an extremely light user of energy (my average in the past week was 4.06kw per day), but I just hope I'm not harming the unit by having it turned off so much!
I've never had underfloor heating before and I'll be interested to see how economical the whole setup is when winter sets in. I can see me needing "plan b" for part of autumn and spring though. Interesting journey learning and finding out though.
The developers who build these "eco houses" don't seem to understand the system so I will find my own way round it, hopefully with some help from you guys on this site who know a lot more about it than my "I'll check what effect it has if ........" approach!