Energy use calculaton

Additional information.
average shower is 8 min long
standard shower rose 15 litres per minute
120 litres per shower
http://www.polyworld.com.au/calculations.htm

hi-flow shower head: 5-7 (20 - 25 L) gallons/minute. The average shower lasts 7.5 minutes.
http://www.wholywater.com/waterusage.html

Mean daily per capita water use, 12 study sites
Shower: 11.6 Gal per day. 43.9 litres per day
http://www.aquacraft.com/Publications/resident.html

Re: Energy use calculaton
Sponge bath.

The other variable in the equation is what type of water heater is used.

Some water heaters store cold water and only heat it on demand as it is used.

Other water heaters store a large tank of water and keep it heated at a constant level in case it is used.

In all seriousness: the wider the temp differential from ambient, the higher the maintenance energy requirement.

We are only talking about a hotwater storage tank.

Most water from an urban supply would be roughly the same temp I imagine Chumly, underground pipes and all. For the purpose of the excercise we need to assume so anyway.

I am not talking about supply temperatures.

I am talking about the maintenance energy required to keep a specified amount of water at a specified temperature relative to the surrounding ambient temperature.

Supply temperatures are not relative to maintenance energy requirements. Supply temperatures are only relative to heating energy requirements.

I never mentioned heating energy requirements, read my prior post carefully, what I said was accurate.

Of course. I don't think Chumly is referring to the temperature of water from the municipal supply. Just that storing a lower temperature of water results in less energy loss, so it might be more economical to keep the water in the tank at around the temperature it will be used at, rather than keeping is extremely hot, and diluting it when used.

Caveat: I always get in trouble when I try to say what somebody else meant.

Of course, sometimes, I also cross post.

That is what I meant! Of course all other variables must be equal for this proportionality to remain consistent.

energy consumption is directly proportional to the weight of the water times the increase in temperature

specific heat X weight in Kg X rise in temp

Assume we use 120 Litres at 50 c in our shower. we then have to heat 120 L of water to 50 C again

4.186KJ X 120Kg X 50C = 25116 KJ

if the temp of our water is 70 C and we would use less heated water to maintain a shower temp of 50 C

How much cold water (assumed @ 5C) do we need to add to reduce the water to 50C?

Re: Energy use calculaton


First one must know the temperature of the incoming stream (the cold water)--this can be determined from mixing and assumming the shower head temperature is a the same.

50 l*70 DegC=50 l*Tc=100 l*40 DegC
Tc=2*40-1*70)=10 DegC

Energy required for heat 50 l of water to 70 DegC
E= 50 l * 1 Kg/l * 1 Kcal/ (Kg DegC) *(70-10) DegC =50/1*1*60 Kcal= 3000 Kcal

Energy required to heat 100 l of water to 40 DegC
E=100 l * 1 kg/l * 1 Kcal/(Kg DegC) * (40-10) DegC=100/1*1*20 K= 3000 Kcal

Calculation Assumptions-
1) Both shower head temperatures are the same
2) The density of water is constant
3) The water heater scaling factor is not affected by temperature (scaling increases with temperature)

The same amount of energy is used in taking the shower---however; there will be a greater loss of energy in storing water at 70 DegC than 40 DegC. Hot water tank heaters store water in hopes of use. The energy loss of the tank is a function of the insulation, the surface area, and the difference between the water temperature and the ambient (room temperature). Since the insulation and the surface area of the water tank doesn't change, only the Delta T (temperature difference) the hotter water storage will take moer energy to store than the colder water.

Note if you use a tankless (demand) water heater this argument is false, unless there's a great deal of pipe between the water heater and the shower head (this volume of this water is lost, and the inherent energy in generating this lost hot water is also lost).

Aside---100 l (~25 gal) to me is a long shower---I've put a low flow shower head (~1 one half a GPM) and my usual 4 minute shower uses about 2 gal (8 liter). US Naval servicemen use even less as at sea the water is turned off between soap and rinse further reducing the water demand for a shower to less than a gallon, but then nothing will encourage a quick shower better then using cold water.

Rap

Mh is mass of hot water
Mc is mass of cold water
Density of water is 1 kg/l
70 DegC * Mh + 5 DegC * Mc= 50 DegC *120 Kg
Mh
Mh + Mc = 120 kg

Dividing 1st equation by 5, and subtracting the second from the reduced first
11 Mh=1080 Kg
or Mh=1080/11 kg =1080/11 l
& Mc = 120/11 l

Note 5 DegC is a pretty cold ground temperature, to us iggorant yanks thats 41 DegF---the US has a pretty constant ground temperature of about 50 DegF (that's 10 DegC to the rest of the civilized world)

Rap

Thanks Rap

It would appear the most effective way to reduce energy consumption is to use less hot water. Ie reduced shower time and loflow shower head.

I also thought 120 L was excessive but was guided by the websites I looked at. I also know through my measuremnts that water use is very tricky to estimate.

I can accept 10C as pipe temp. My 5C was a guess.

I follow what you and chumly are saying about maintaining temp against ambient.

Did I understand you saying 120L of hot water @70C and 11 L of cold water?
(i dont follow math real well).

If thats the case we only use 120-11=109L of hot water @ 70C.

So we only need to reheat 109 L to 70 C

but in the lower temp experiment still need to heat 120 L of water to 40 C

Sorry I made an arithmetic error in my first calculation----that's why there should be a peer review for every calculation.

for cold water @ 5 DegC its
70Mh+5Mc=120*50
70Mh+5Mc=6000
divide this by 5
14Mh+Mc=1200 (1)

now you're using 120 l total so
Mh+Mc=120 (2)

so
Mc =1200-14Mh from (1)
&
Mc=120-Mh from (2)

since Mc=Mc from (1) & (2) then
1200-14Mh=120-Mh
rearranging
13Mh=1080
& Mh=1080/13 l= 83 l hot
Mc=120-Mh=120-1080/13 l = 37 l

if you use 10 DegC for cold water
70Mh+10Mc=120*50 (divide by 10)_
7Mh+Mc=600 (1)
Mh+Mc=120 (2)
subtract 2 from 1
6Mh=480
Mh=480/6= 80 l
Mc=120-80=40 l

5 DegC difference in cold water temperature doesn't make a great deal of difference in mixing.

Rap

Variables can have a significant influence on this decision. I once had a "house-mate" who thought we would save money in our all-electric house if she opened the windows front and back, and shut off the AC when she came home from work at night (2nd shift at a hospital). We went around and around on this, because she thought it was a simple calculation, and had only to do with the ambient temperature outside. I tried to explain to her that she was ignoring significant variables, and she got her back up (mostly because she was an older, independent woman who didn't like to be contradicted). I finally convinced her to give it a comparison, and we compared the electric rates for July and August--July with her system, and August with mine.

We had an "in-wall" AC unit in the kitchen, next to the utility room. We lived in southern Illinois, which has very high humidity in the summer time, and for those who are not in the United States, temperatures can run very high, easily hitting the 90s F in summer time, day after day. But there are always at least two "air conditioning" units in your house. One is called the air conditioner, and the other is called the refrigerator. When she opened the windows at night, she wasn't simply raising the inside temperature (temperatures at night in the summer in that area of North America easily run into the low 80s, and usually in the 70s even on a cool night), she was also raising the ambient humidity. A raise in temperature means that the refrigerator has to cycle on more often, and it dumps waste heat into the interior air of the house. Additionally, refrigerators remove humidity from the internal air, especially with a "frost-free" freezer. The front door opened four times a day, usually--when i went to work and returned, and when she went to work and returned. The back door usually was opened at least twice a day--when she went out to sun herself before work, and when she came back into the house. The refrigerator door was opened many times a day, when we wanted meals and when we wanted a cold drink. So any notional savings on the AC was being lost with the refrigerator, which was dumping waste heat into the inside atmosphere, and cycling on and off to maintain its interior humidity. Also, with the high humidity of the ambient exterior atmosphere in southern Illinois, when the AC was turned back on (previously, when i went to work in the morning), it cycled on and off more often until it had removed the excess moisture from the air. At the end of August, when we compared the electric bills, the difference was dramatic. Although an all-electric house, it was recently built, and very well insulated--so the more than $20 difference in the electric bills (August lower than July), was pretty good evidence that keeping the AC on constantly, and opening exterior doors as infrequently as possible would be a more efficient use of electricity.

With a water heater, the variables would probably be more problematic. In the central part of the United States and Canada, the exterior temperatures can vary by more than 100 degrees F from the middle of winter to the middle of summer. Where the water heater is located would make a big difference, too. If it is in a basement, then in summer, it probably doesn't require much energy to maintain the water temperature, unless the basement is air conditioned. If it is in a utility closet on the first or second floor (i doubt that many water heaters are on the second floor in a single family dwelling unit), then the means of heating or cooling the living areas would come into play. If it is in the basement in winter time, and the basement is not heated (any more than is necessary to keep the pipes from freezing, at any event--and that usually doesn't require any effort to heat the basement, there being sufficient heat in the house and from the furnace or water heater [for radiator heat] to serve that purpose). In winter time, a water heater in the basement probably would require a significant amount of heat to maintain temperature, and almost none at all in the summer. A water heater in a closet in the living area of a house in the central portions of the United States and Canada would probably not need much energy to maintain temperature in the winter, but might need it summer if the residence is air conditioned. These factors apply to the northeast United States and eastern Canada as well, where the temperature variables are similar to the central portions of the U.S. and Canada. Calculations are going to be different for the southeastern and southwestern U.S. and for western Canada.

So DP's question probably hinges on the temperature variables in Oz (about which i personally don't know squat) from season to season, and where the water heater is located in the house (is there a basement? . . . is that where the water heater is located?). Your mileage may vary.

I agree with Setanta to an extent--as for water heating I put a demand water heater (no tank) in my home. Although the heater was twice the price, and it required a second 220V service (4 110 V 60A circuits), my monthly electric bill dropped $15 (US) per month--the difference in price was paid in less than 14 months.

Also living in the Ohio Valley I know of the 95 95 (95 DegF and 95% relative humidity) days of July and August. But it is possible to minimize the electric use of AC. During the day shut the house tight and use the AC to (more importantly) control the humidity. At night, when the sun goes down, and the temperature drops, you can open the doors and windows. My house even has a solar tower that pulls the air from the basement to the upper floors. Using a little forethought I've reduced my July/August electric demand to less than 600 Kilowatt hours. Refrigeration use as Setanta mention is reduced by minimized by reducing the refrigerator air exchange, consequently through the door ice and cold water features provide energy savings, along with chest vs upright freezers (cold air is more dense than warm air--so when you open a door on an upright unit the cold air falls out the door).

A second thing to consider is auto AC. It is usually more efficient to use car AC than it is to open the windows at interstate speeds. Opening the windows increases the drag coefficient, adversely affecting fuel economy. As for you pickemup drivers, the tailgate debate is pretty much a wash (I've seen pro and cons), but since I usually have something in the back of my pickemup, my tailgate stays up.

Rap

You already have a 120 / 240 three wire service to your house. As such either the ampacity of the service was upgraded of there was sufficient ampacity with the existing service for the additional load.

I seriously doubt you would have "a second 220V service". However my neighbor does, as he gets a reduced rate for electric heat, it was an offer BC Hydro had, alas long gone but grandfathered on the house not the owners.

To have "a second 220V service" would mean another feed from the provider's distribution transformer, and another meter, and another main distribution panel.

Myth: Auto Air Conditioning is more fuel efficient - Busted!

http://mythbusters-wiki.discovery.com/page/Keeping+the+Car+Cool?t=anon

There are so many variables in reference to energy consumption that I have found it best to assume the general relationships of the obvious ratios to be correct but that the net results cannot necessarily be so easily predicted over longer time periods.

Lets take your demand system compared to a storage system as an example.

I do not know, as you did not say, if your demand system was a centralized one, or a multiple zoned one, but in any case the calculations are not as simple as installation costs versus ongoing savings versus original costs.

Why?

Because all systems have life spans and storage hot water tanks are generally considerably cheaper to replace than demand systems. I have been involved in many energy saving schemes over the years and the long term net results are not always as predicted, often enough either due to changing technologies or unpredicted / unaccounted-for maintenance costs.

For example we are on well-water and the minerals reduce the usual lifespan of the hot water tank, and fuc-up some of the water fixtures at times due to mineralization.

However I am a big fan of the upcoming LED lighting revolution, that one seems a no-brainer once costs and technology are base-lined. Compact fluorescents I am not so sure......dubious claims about lifespan among other things. We have a whole swack of them and many have died prematurely.

Don't even get me started about who really benefits from the reduction of parasitic loads, versus other more practical energy savings methodologies.......wow I can actually turn off the light?

I once heard there was something called a "sweater".

Thanks blokes. I was aware i was ignoring important variables but wanted to see the calculations for "ideal" conditions. Also with the effective insulation on my hot water service, heat loss to atmosphere must be very low. but i acknowledge, heat loss must be higher at 70C.

Set:
By basement I assume you mean constructed below the normal ground level. If this is so there are very few houses with a basement.
If by basement you mean room(s) that have its floor level (approx) level with the normal ground level then these are becoming more common as affluence increases.

Most houses are single story with all rooms located at ground level.

We (aussies) also don't do attics. Most houses have space between the ceiling and actual roofing material but it is generally unused and not easily accessible.

Ambient Temp variables in my area.
Average daily winter temp say 9/10C
with lows of -1C common and -6C on a few days each year.

avg summer temps. 30C with highs of 38 common and extremes of 41c a few days each year.
Its been quite a ride this month (autumn) high of 37.7C on Day 15 and Low of 3.5C on Day 29.
http://www.mansfieldweather.com.au/


Average temp Victoria experiences warm to hot summers, mild autumns, cool to cold winters and cool springs. The climate varies across the state, with the north experiencing much drier and warmer weather than the south.

* Summer: December to February
Temperatures: from 21°C to 34°C, average 28°C
* Autumn: March to May
Temperatures: from 11°C to 19°C, average 17°C
* Winter: June to August
Temperatures: from -1°C to 12°C, average 8°C
* Spring: September to November
Temperatures: from 10°C to 20°C, average 17°C
From this site

I've always felt because we have such a mild climate that home was never very environmentally friendly and there is lots we could do to decrease energy use.
I feel we could decrease energy consumption markedly by implementing some of the N. American design features.
Double glazing is one such element.

I read somewhere that Australia has the highest per capita energy use in the world.