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Experimental method
and theoretical calculations to
quantify savings
through the use of water-efficient showerheads
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Total savings through the
sale and installation of Oxygenics showerheads
since 2008 have been calculated by multiplying
the number of units sold per month by the
estimated monthly savings per unit (as per
methodology below). We have been
careful to ensure a very conservative total
estimate.
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Experimental method (on-site testing)
Aim:
To determine the probable financial, power and
environmental savings through the replacement of
a water-wasting showerhead with a
water-efficient unit.
Apparatus:
• water-wasting showerhead– Cobra Overhead
Shower (Cobra code # 065) (25 LPM at 3 bar), as
it is widely
obtainable, has been in use for
decades in South Africa and is still installed
in hundreds of thousands of South
African homes.
• water-efficient showerhead – Oxygenics
Skincare (8LPM at 3 bar), as it is a quality
unit widely sold in the USA
and is ideally
suited to South African conditions.
• geyser – Kwikot Dual 400kPa 100 litre, in an
average domestic installation, without geyser
blanket or lagged
pipework
• generic kWatt hour meter
• Cobra pressure regulator 400 kPa
• in-line generic flow control valve
• stopwatch
• bucket
• graduated measuring jug
Method:
In order to verify the accuracy of the
calculations and the assumptions made, a field
test was performed.
The scenario assumed was a family of four, each
showering once a day for six minutes.
• A standard 100 litre 2 kW domestic hot water
geyser was used, without a geyser blanket or
plumbing
insulation. The only draw of hot water
from the geyser was for the timed showers
• Water pressure was approximately 320kPa and
balanced between hot and cold
• The set point of the geyser was 65
°C.
• Energy consumption by the geyser was measured
over a number of 24 hour periods using a
dedicated kWhr
meter on the geyser electrical
supply
• Water temperature at the showerhead was
measured for both the high-flow standard
showerhead and the
water-efficient showerhead.
The temperatures were 44
°C and 46
°C
respectively, using a mix of hot and cold to
result in a comfortable shower experience.
• The flow rate was established by measuring the
volume of water issuing from the showerhead over
20 seconds
(multiplied by 3 to get a LPM
figure).
The electrical energy (kWhrs) used over
successive 24 hours periods was measured
• with no water drawn from the geyser over the
24 hour period, to establish geyser standby
losses
• with two 6 minute showers morning and evening
(4 showers in total, amounting to 24 min of
showering per
day), using the water-wasting
showerhead with a flow of 17.5 LPM. This flow
rate could be achieved only by
installing an
in-line flow controller upstream of the
showerhead, given that the unrestricted flow was
over 25
LPM, well over what can be taken as a
South African average shower flow rate
• with two 6 minute showers morning and evening
at approximately 7.5 LPM (total 4 showers and 24
min of
showering per 24 hrs), using the
water-efficient showerhead, rated 9.5 LPM at 550
kPa.
These tests were repeated for a number of
successive 24 hr periods to ensure consistency
of the results.
Results:
The figures obtained averaged to
• 17 kWhrs per 24 hours for the standard
showerhead
• 9.5 kWhrs per 24 hours for the water efficient
showerhead
Thus the saving was 7.5 kWhrs over the 24 hour
period.
This is a very similar figure to that arrived at
through calculation (8kWhrs).
Any differences between the calculated figures
and the test results may be due to variations in
the cold water intake temperature and ambient
air temperature and therefore standing heat
loss. The pipework heat losses were
approximately 2kWatt hours per 24 hours in
standby mode with no hot water used, and about
3kWatt hours per 24 hrs during the shower test
periods, the extra 1kW of energy being that lost
as the pipes were unlagged.
The figure given for the total number of kWhrs
saved per year has been corrected for
transmission line losses (approx 20%) between
generating source and user.
The electrical energy used to purify water, pipe
it to the home and back to the water-treatment
plant, and to treat it at the plant, has not
been included. This figure can range from 1-3
kWhrs per kL of water, depending on the
municipality and plant (an average figure can be
assumed to be 1.5 kWhr per kL). Hence the energy
savings are even higher than calculated and
measured.
For the water, carbon emission and other
savings, please refer to Appendix B. These
figures can be derived from calculations alone,
rather than needing verification through
experimental method.
Conclusion:
It is clear that even in this conservative
scenario of a family of 4, with a total of 24
minutes of showering per day, the savings of
power, water and waste-water are truly dramatic.
And the comparative savings and relative
cost-effectiveness over other sustainable
interventions are also remarkable.
The savings per household per showerhead are
approximately
• R 4 000 annually on utility bills at current
rates, with a return on showerhead investment
(R200) of 18 days
• 97 000 litres of water (and therefore
wastewater), enough to fill 3 medium sized
domestic swimming pools
• 2 950 kW hrs of power, enough to supply the
average South African home for approximately 7
months per year
• 4 tons of carbon emissions, equivalent to not
driving ones car for ten months of the year
• 1.8 tons of coal with a reduction in all the
associated localised ground and air pollution
Theoretical calculations to support the accuracy
of test results from experimental method
Power
The savings listed below are based on purely
theoretical calculations using the following
assumptions. The calculations are essentially
the same as those used by the Eternally Solar
Power and Water Savings Calculator.
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Parameter |
Equation identifier |
Quantity |
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Water saving showerhead flow rate
(L/M) |
L |
7.5 |
| Assumed water-wasting current shower flow-rate (L/M) |
A |
18 |
| Assumed average shower duration (Min) |
B |
6 |
| Number of showers per household taken per day |
C |
4 |
| Total water used for water wasting showers (L) |
A*B*C=TWW |
432 |
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Total water used for water efficient showers (L) |
L*B*C=TWS |
180 |
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Temp of shower water assumed – water wasting (°C ) |
E |
44 |
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Temp of shower water assumed – aerated water efficient (°C) |
S |
46 |
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Temp of hot water in geyser assumed (°C) |
F |
65 |
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Temp of cold water from mains assumed (
°C ) |
G |
15 |
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Temp elevation mains to geyser 20-60 (
°C ) |
H |
50 |
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Energy required to raise temperature of 1 litre water by 1°C (kWhr) |
I |
0.001162 |
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Flow rate of hot water for water wasting showerhead (L/M) |
((E*A)-(G*A))/
(F-G)=J |
10.44 |
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Flow rate of hot water for water saving showerhead (L/M) |
((S*L)-(G*L))/
(F-G)=K |
4.65 |
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Assumed hot water requirement (L) (for water wasting showers at 44
°C) |
J*A*4=HWW |
250 |
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Assumed hot water requirement (L) (for water saving showers at 46
°C) |
K*L*4=HWS |
112 |
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Electrical energy required at water wasting flow rate (kWhr) |
HWW*I*H=EWW |
14.52 |
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Plus energy lost through unlagged pipes (kWhr) |
EWW+2=EWWc |
16.52 |
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Electrical energy required at water saving flow rate (kWhr) |
HWS*I*H=EWS |
6.5 |
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Plus energy lost through unlagged pipes (kWhr) |
EWS+2=EWSc |
8.5 |
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Energy saved per day (kWhr) |
EWWc-EWSc=N |
8.0 |
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Electrical energy saved per year (kWhr) |
P*365=Q |
2
928 |
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Transmission line losses generation source to user (%) |
R |
20 |
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Total annual electrical energy saved at generation source (mWhr) |
Q+(Q*R)/1000=S |
3.7 |
Some influencing factors
• Use of
heat pumps
in hotels
and gyms to
heat water
can reduce
power costs
by about 60% by about 60%
• Temperature and length of showers varies
widely, and we have used acceptable averages.
• Heat losses from pipework (as determined by
field tests) have been factored in.
• Energy required for water purification,
pumping, and waste-water treatment has not been
factored into our
calculator (it may be in an
updated version), nor have the associated carbon
emissions. Hence another reason
why savings will
be greater than calculated.
Water and Waste-water
The calculations for fresh and waste-water are
straightforward in terms of volume, but the
method of calculating rateable volumes and unit
cost, of both fresh and waste-water varies from
one municipality to another. The cost of
waste-water also varies widely, and the default
figures are based on Cape Town rates which
represent an average.
Water
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Parameter |
Equation identifier |
Quantity
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Current
water-wasting shower flowrate (L/min)
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A |
18
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Average Shower
duration assumed (min) |
B |
6
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Number of
showers per household taken per day |
C |
4
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Total water
used for showers (L) |
A*B*C=D |
432
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| Water
saving showerhead flow rate (L/min) |
E |
7.5
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| Water
used at water saving flowrate (L) |
B*C*E=F |
180
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Water saved
per day (kL) |
(D-F)/1000=G |
0.25
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Water saved
per year (kL) |
G*365=H |
92 |
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Waste-water
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Parameter |
Equation identifier |
Quantity |
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Fresh water
daily savings (kL) |
A |
0.25
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Average
proportion of fresh water volume that
discharges to sewerage system (%)
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B |
90 |
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Daily
waste-water volume saved (kL)
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A*B=C |
0.23 |
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Rating used by
municipality to calculate chargeable
vol. (%)
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D |
70 |
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Daily
waste-water chargeable volume (kL) |
A*D=E |
0.175 |
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In the Eternally Solar Savings Calculator, the
bulk water savings include an additional figure,
representing the
1.3 kL that Eskom uses to
generate 1 mWatt of power. This gives a
saving closer to 90 kL in the household scenario
we have used
Cost Savings
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Parameter |
Units saved |
Current avg. unit
cost (R/kWhr
or kL) |
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Cost
saving |
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Daily |
Annually |
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Power (kWhr) |
6.7 |
R1.20 |
R8.04 |
R2
935 |
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Water (kL) |
0.25 |
R12 |
R3.00 |
R1
095 |
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Waste-water (kL) |
0.175 |
R10 |
R1.75 |
R639 |
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Sub-total |
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R10.45 |
R4
669 |
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| VAT |
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R1.46 |
R654 |
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Total |
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R11.91 |
R5
323
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Cost of average
eco-showerhead |
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R200
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Payback period (days)
(R200/R4 094*365d) |
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14 |
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Carbon and Coal
Waste-water
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Parameter |
Equation identifier |
Quantity |
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Electrical
power saved annually (mWhrs) A 3.0 |
A |
3.0
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CO2
emissions per mWhr generated (metric
ton/mWhr) B 1.1
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B |
1.1 |
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Total CO2 emissions
saved (metric tons) A*B=C 2.6
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A*B=C |
2.6 |
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Coal consumption
(metric ton/mWhr) D 0.5
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D |
0.5 |
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Total coal volume saved (metric
tons) A*D=E 1.45 |
A*D=E |
1.45 |
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• Total energy savings are estimated at source,
by multiplying savings by user by a transmission
line
loss factor (20%)
• Emissions figure used is 1.1 ton CO2 emitted
per megawatt hour of electricity generated. The
emissions may be
as high as 1.1 ton per mWhr
(source: Eskom), depending on the information
source.
• Coal use is approximately 0.5 ton per mWhr of
electricity produced (source: Eskom)
• Cost savings to South Africa would be very
significant and could easily fund the wholesale
installation of water-
efficient showerheads –
especially given that, apart from the
infrastructural savings, the proposed tax
penalty
for emitting excess carbon imposed
internationally would be reduced.
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