Switching geyser off?

Wiring, circuits, batteries etc.

Switching geyser off?

Postby windgat on Mon May 25, 2009 8:45 pm

Some say switching your geyser off saves power, others say it depends how long you leave it off for, since when its turned on again, the element stays on a long time to reheat the entire geyser. What do you think?

It would great if someone could put together a formula for power consumed based on things like the specific capacity of water, element wattage, thermostat setting (target temperature), and temperature to which the geyser cools (ambient), which would allow some plots to be made of saving vs time a geyser is left off. Any offers? (It would need to be someone really sharp with their formulas, like Greystoke... :P )
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Tue May 26, 2009 6:26 am

The savings are in the thermal losses of the geyser. Once the water is on temperature, the heater will only come on again to compensate for the thermal losses. This can be avoided (to some extent) by leaving the heater off, and only switching it back on again when you need hot water. It's a matter of careful timing and knowledge of your geyser' heat losses.

It would be a nice exercise to work it out. I'll give it a go :!:
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby Greystoke on Tue May 26, 2009 4:03 pm

Image

To determine the heat losses of the geyser you need to be able to measure the temperature of the water while the heaters are switched off (see graph).
Assume that the starting temperature = 60.00°C, and that after one hour of cooling down, the temp has dropped by 0.86° to: 59.14°C.
(In order to get some accuracy, you need to do this a number of times over a few hours, and average the results.)
Basic heat transfer theory, ie: one calorie heats up one ml of water one degree, tells us that the heat loss equals: 100*0.86/3.6/0.24 = ± 100 Watt. This also equals the power required to maintain the water at a steady 60°C.

Suppose the geyser is used once a day, in which on average 50L of hot water is replaced with tap water at ambient temperature. Assume the heater is a 2kW element.
To heat up 50L of water from 20° to 60° takes 1.16 hours. From then onwards we need ±100 Watt to maintain the full geyser at 60°C.
Total energy use =
1.16x2000 + 23hrsx100 = 4620 Watt.hrs per day.

Now let’s assume that we switch the heater off directly after use, and back on again in time to heat the water up before the next use.

It can be seen that the water – diluted with 50L of cold tap water during use - will have cooled down to ± 32°C after about 22hrs. When switched back on, It will take the heater about 1.85 hrs to heat the full geyser up to 60°C.
Total energy use =
1.85x2000 = ± 3700 Watt.hrs per day.

This is 80% of the uninterrupted heater operation!
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sat May 30, 2009 8:20 am

Thanks for that info!

Thinking about it, we need to consider real world factors (you know the joke about how the physicist starts by assuming an elephant is a homogeneous smooth sphere...)

What prompted this question is the plan to remotely turn off geysers, and turn them on in low demand times. So one could anticipate the geyser going off in the mornings and evenings (peak demand) and on perhaps at 10pm (low demand). This in effect saves Eskom money, since they need capacity to match peak demand (lower peak = less capacity required = less capital/infrastructure required). My question is, what about the homeowner?

Keeping the geyser at a high temperature overnight is a waste of energy - to conserve electricity perhaps the best way is to turn the geyser off at around midday. In my experience the water stays plenty hot enough for the evening. The evening hot water usage would then cool the geyser, reducing the thermal gradient, and hence reduces thermal energy loss overnight. Then about an hour or so before hot water is needed in the morning, it should be turned on.

These two requirements (conservation vs peak reduction) are in conflict. What I am proposing from intuition is the theory that remote switching will decrease Eskom's costs, and simultaneously INCREASE the homeowners electricity bill.

What is needed is some real world data - someone with a data logger out there who can connect it to the geyser wires? If so, lets come up with an experimental protocol together!
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby windgat on Sat May 30, 2009 9:42 am

I don't have a data logger (any contributions accepted :) ), but I do have a spreadsheet. So I started with a simulation. Parameters are:
Time step 00:30:00
Max ambient 25
Min ambient 8
Min deg (geyser) 45
Max deg 50
Loss deg/hour 2
Heating rate deg/hour 10
I am trying to keep it simple as possible, e.g. for cooling due to water usage, I just enter the degrees by which the water is cooled within each time slice.
Can you help with a highly simplified formula, which gives degrees lost as a function of (only) geyser temp, ambient temp? Each cell will have the formula:
Temp = previous temp + heating deg/hour (if geyser on) - degrees heat loss (fn of ambient temp) - usage degrees lost
Here is the graph so far - seems to be kind of sensible!
Geysergraph1.png
First draft
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sat May 30, 2009 12:11 pm

OK,
Just give me a chance :D
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sat May 30, 2009 2:29 pm

:impatient:

(Just wanted to try the new icons!)
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sat May 30, 2009 2:37 pm

Patience, patience . . . Go watch the rugby. Image
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sat May 30, 2009 8:20 pm

I don't watch rugby... :shifty:
Instead I did something useful...
Geysergraph2.png
Graph draft 2


I put some guess figures in, like 35 litres hot water used in the morning and 40 in the evening.
Looks realistic to me so far! :thumbup:
Could still add some things, like T[n+1] = T[n] - (T[n] - T[ambient]) x fac maybe?
One surprising result so far in the model is that the bigger the geyser capacity, the less the electricity used! Do you think that's accurate?
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sun May 31, 2009 7:01 am

windgat wrote:I don't watch rugby... :shifty:
Instead I did something useful...
Pity . . you missed a really good game. But . . . some horses are "working" horses, while others are "luxury" horses. :lmao:

windgat wrote:One surprising result so far in the model is that the bigger the geyser capacity, the less the electricity used! Do you think that's accurate?
That would make sense. The geyser insulation relates to the area coverered (m²), and the content to m³. So, if you double the linear sizes of the tank, you need 4X the amount of insulation to cover 8X the amount of water.

Back to the formula:
Geyser heat loss= (T[n] - T[ambient])/R, In which R represents the thermal resistance of the geyser. MIne (100L) is about 0.4°/Watt. You can measure that value by doing a "cooling " exercise. It's not too difficult, but you do need a thermocouple thermometer. (some electronic multimeters come with themocouple probes)

Geyser heat gain= 3.6x0.24xHxL°/hr, In which H= heater element in kW, and L = liters of water
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 8:15 am

I follow re area vs volume ratio - that's why a mouse needs to eat so much more per body weight than an elephant (those grey spheres, remember?).

What was surprising is that I have not yet included this fact in the model! So I think it was purely the greater capacity meant a smaller drop in temp for the same volume of water used, hence less power to get it back to temp. I will incorporate yr formulas and post the results...
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 8:57 am

Hmmm, its not looking right. It cools off FAR too quickly, and heats up too quickly too.

You have given me heat loss in Watts - what I really need is temperature decrease per time unit.

3.6 x 0.24 x H ... lets work with 2kW geyser so that all comes to 1.728 ... x 100 litres... :shock: You better turn your geyser off before it explodes! :lmao:

Perhaps you meant to divide by litres?
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sun May 31, 2009 9:49 am

Yep :!:
It should be /L , and the geyser element should be in Watts (not kW)
sorry :oops:

windgat wrote:You have given me heat loss in Watts - what I really need is temperature decrease per time unit.
Temperature decrease is not constant. It is ruled by the balance of heat gain vs heat loss. You need to work out the heat loss based on the current temperature (ie: (Th-Ta)/R) and then convert that loss to temperature drop via the formula we just corrected (ie: ?T=3.6X0.24XLoss/L)

PS: Note that the heatloss formula: (Th-Ta)/R represents the (absent) amount of power that would otherwise MAINTAIN the temp @Th.
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 11:03 am

Great, working now. Not bad for a luxury horse! :lol:
Below are the parameters so far. Can you check and suggest improvement? I have chosen thermostat hysteresis as 2 degrees - complete guess. The ambient temperature varies as a sine curve peaking around 1pm. As before, 70 litres hot water have been used.
GeyserParams1.png
Guestimated values
GeyserParams1.png (3.93 KB) Viewed 6942 times

The total daily power use comes in at 4.8kW - seems too low. :problem: I would expect 10 - 15 kWh.
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 2:10 pm

Greystoke wrote:You can measure that value by doing a "cooling " exercise. It's not too difficult, but you do need a thermocouple thermometer. (some electronic multimeters come with themocouple probes)

I have one, but how can I get my probe into the hot water geyser? (sounds like a line from a Pam Anderson movie...)
Last edited by windgat on Sun May 31, 2009 2:34 pm, edited 1 time in total.
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sun May 31, 2009 2:32 pm

windgat wrote:The total daily power use comes in at 4.8kW - seems too low. I would expect 10 - 15 kWh.
4.8 kW looks OK to me. :?:

Greystoke wrote:you do need a thermocouple thermometer.
windgat wrote:I have one, but how can I get my probe into the how water geyser? (sounds like a line from a Pam Anderson movie...)
I have faith in your ingeniouty. :D So, kindly tell me how you did it. :mrgreen:
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 4:10 pm

'Normal' suburban household use ~30kWh a day; 'normal' percentage of total bill due to geyser ~40-50%, hence I thought 5kWh too low...

Ok, so here is the graph. Whad'ya think?
Attachments
Geysergraph3.png
Draft 3
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Sun May 31, 2009 4:27 pm

I got a very similar use. I suppose we need to investigate this a bit more closely.
How about putting-in a meter?
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Sun May 31, 2009 9:39 pm

Similar to :?:
Here is the spreadsheet, see if its usable/improvable?
To get a smooth temperature at midnight I manually fiddle the starting temp to about match the ending temp. It may need to be over multiple days for more accuracy.
Attachments
Geysergraph.xls
Geyser power usage model
(180.5 KB) Downloaded 348 times
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Mon Jun 01, 2009 7:19 am

windgat wrote: . . To get a smooth temperature at midnight I manually fiddle the starting temp to about match the ending temp. It may need to be over multiple days for more accuracy.
Did the same thing trying to get an estimate for the thermal resistance of my geyser. I'm sure its the right order of magnitude, but I truly haven't a clue how accurate it is.

I have a spare kWh meter in the granny flat. It's a bit of a mission to install it in-line with the geyser, but if I DO want to make savings, I need accurate data.

Perhaps we're a bit impatient. Let's go back to school, ie: Fit the theory, then check validity with measurements.

BTW: I'll check the spreadsheet today.
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby Greystoke on Mon Jun 01, 2009 9:11 am

Nothing wrong here. :!:
In fact, if I enter my conditions (as above) I get a use of 4.67 kWh as opposed to 4.62 kWh in my calculations. That's ±1% difference. Not bad what :?:
There's a bit of aproblem getting the timing right. Meaning: when to switch on and when to switch off. It doesn't seem to happen exactly at Tmin and Tmax, but it does come right (more or less) on average. (my heatloss= 100 Watt, yours= 97.2 Watt)

I think the time has come to validate our constants, ie: kWhrs, thermal resistance.
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Mon Jun 01, 2009 11:16 am

Fantastic! Looks like a good model :clap:

Validation sounds like a good idea. I am thinking of getting an amp or watt hour meter, but perhaps a more versatile USB logger is a better spend. :? Any suggestions?

Rigging up your meter at the geyser would give great data (if the luxury horse get around to it... :) ). Perhaps also try and estimate litres hot water used per day, and the times used?
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby Greystoke on Tue Jun 02, 2009 6:57 am

Measuring amps will be OK provided Vmains stays constant (which is not the case where I live). The biggest problem will be the installation of a thermal probe. I've been looking at that geyser of mine and still wonder how to do that. Looks like I need to drill a hole through one of the plumbing connectors so that the probe sits in the hot water. MInd you, you don't need the thermometer to check the thermal resistance, the kWhr meter will tell us exactly :!:

My hot water usage is very straight forward . . . I use instant (tankless) heaters on all taps, including the kitchen, so I only use the geyser to bath or shower.

What dose a USB logger do :?:
Regards
Cor
Greystoke
Hyper active member
 
Posts: 180
Joined: Thu Jun 05, 2008 9:10 am
Location: Port Elizabeth, South Africa

Re: Switching geyser off?

Postby windgat on Tue Jun 02, 2009 8:41 am

USB logger - I imagine there are devices out there which record various inputs (like amps, volts etc) which you can later connect to a USB port and download all the data for analysis on a PC.

Probe sounds a hassle - and it needs to be deep in the water to get the core temp. As you say, maybe its less important than the power. We could test a thermostat to find the hysteresis, and then we would know the on and off temperature points, so the current would tell us the temperature at the thermostat on and off points. That would be enough to get the model perfectly tuned.
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Re: Switching geyser off?

Postby windgat on Tue Jun 02, 2009 8:44 am

USB logger - I imagine there are devices out there which record various inputs (like amps, volts etc) which you can later connect to a USB port and download all the data for analysis on a PC.

Probe sounds a hassle - and it needs to be deep in the water to get the core temp. As you say, maybe its less important than the power. We could test a thermostat to find the hysteresis, and then we would know the on and off temperature points, so the current would tell us the temperature at the thermostat on and off points. That would be enough to get the model perfectly tuned.

PS: This forum went down yesterday for about 12 hours - did you notice? The database on the server froze up, and the 110MB.com support staff were completely unhelpful/unresponsive. Apologies for any inconvenience.
windgat
Founding member
 
Posts: 558
Joined: Mon Apr 30, 2007 10:49 am
Location: Cape Town, South Africa

Next

Return to Electrics and controls

Who is online

Users browsing this forum: No registered users and 0 guests

cron