Computational Water, Energy, and Environmental Engineering, 2013, 2, 21-29 Published Online January 2013 (
Refrigerator Coupling to a Water-Heater and Heating
Floor to Save Energy and to Reduce Carbon Emissions
Romdhane Ben Slama
ISSAT Gabes Rue Omar Ibn Khattab, Gabes, Tunisia
Received August 24, 2012; revised September 28, 2012; accepted October 15, 2012
With an aim of rationing use of energy, energy safety, and to reduce carbon emission, our interest was geared towards
the refrigerators and all the refrigerating machines. Indeed the heat yielded by the exchanger condenser can be devel-
oped for the water heating, floors heating etc. After an encouraging theoretical study, two prototypes were produced in
order to validate the theoretical results. A first refrigerator was coupled with a water-heater and another with a heating
floor. The water temperature reached, in one day, is of 60˚C; which makes it possible to predict better results with a
continuously used refrigerator. In the same way for the heating floor coupled with the second refrigerator, the tempera-
ture reached high values because the surface is reduced; however for the heating floors the standard fixes the tempera-
ture between 28˚C and 30˚C.
Keywords: Refrigerator; Recuperator; Heating Water; Heating Floor; Heat Pump
1. Introduction
Energy consumption in the world is significant and in
continuous growth. It is fundamentally linked to the life
level. It promotes the thermal comfort of the citizen,
through the heating and cooling systems (air-conditioners,
water-heaters, refrigerators… ). With the aim of saving
energy and to reduce carbon emission, it is necessary to
find some innovating solutions in this field and to create
For this reason, we propose to recover the waste heat
on the refrigerator condenser level, and this by two me-
The first method is to enhance the heat lost during
condensation of Freon, and this, recovering it in a
water cumulus to heat it and to use in the domestics
or other needs.
the second method consists in heating a floor to re-
place the traditional heating radiators.
Finally, the heat recovery system does not have any
modification on the basic refrigeration cycle. The tests
and experiments show the effectiveness of this heat re-
covery project [1-6].
2. Bibliographical Study
The publications evoking the use of heat pumps for water
heating are relatively very few. Let us quote Grazini,
Skrivan, Huang, Jie [7-10] which extracts heat from the
ambient air like cold source of the heat pump. Figure 1
is given as an example.
Others authors use solar energy like evaporator such as
Hawlader, Li, Chyng, Borges, Huang [11-16].
Among the few publication jointly using the condenser
and the evaporator of a heat pump (or refrigerator), we
even quote our work concerning a solar sea water still
developed by our self, at the National School Engineers
of Gabes, Tunisia [17-23].
Figure 2 shows our system of solar desalination pro-
vided with the heat pump.
The condenser (6) heats water to be evaporated and the
evaporator (2) makes it possible to condense instantane-
ously the vapour and thus to form the condensate col-
lected in the gutter (3).
For the principle of refrigerator operation (Figure 3),
this system can be considered as an insulated cupboard
whose interior temperature is lower than the ambient
temperature. To obtain cold inside the refrigerator, heat
is extracted with the air and food and then is rejected by
the condenser in the kitchen.
3. Theoretical Considerations of the
Coupling of Refrigerator to a
Water-Heater and a Heating Floor
We will begin by determining the intervals of real oper-
ating time of the refrigerator and the heat transfer be-
tween the condenser and water or floor to be heated.
3.1. Preliminary Study of Dimensioning
The tracing of the histogram of Figure 4 allows to know
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Figure 1. Water heating by heat pump.
Figure 2. Solar distiller provided with heat pump.
Figure 3. Refrigerator operation cycle [2].
the operating time of refrigerator’ compressor during 24
hours. The operating time varies from one moment to
another as required by the refrigerator.
Operation of a refrigerator during 24 hours.
t9 h 43 minday
Figure 4. Time operation life of a refrigerator lasting 24 h.
Compressor power of 140 W and using R13a freon. Room
temperature = 19˚C (Tozeur, Tunisia, on March 20, 2009).
3.2. Theoretical Study of a Refrigerator to a
Water-Heater Coupling
The energy yielded by the condenser is transferred to
water to be heated. However, when water heats in the
storage tank, thermals loss will appear (coefficient U =
4.92 W/˚C).
3.2.1. Modeling
The refrigerating machine can be modeled by consider-
ing energies on the level of the compressor, the con-
denser and the evaporator, as follows:
comp condenser evaporator
comp compthcompfr
th fr
totalth fr
The total COP can thus reach raised values, which
confers on our system a great energetic effectiveness,
because, usually the refrigerating machine is used either
like refrigerator to produce the cold, or like heat pump to
3.2.2. Thermal Transfer
For the coupling of a refrigerator to a water cumulus
study, we make the calculation of the water mass which
can be heated with different temperatures. The efficiency
is then evaluated.
It is supposed that the heat yielded by the condenser is
Copyright © 2013 SciRes. CWEEE
R. B. SLAMA 23
recovered by water to heat.
condenser water
comp water
  (1)
ΔT is the water temperature increase from initial to fi-
nal state.
It is possible to determine the variation of ΔT accord-
ing to time, in the same way for the efficiency.
Determination of heat water storage efficiency
useful absorbed
absorbed comp
usefulabs loss
Pcomp: Compressor power.
t: summon operating time of compressor.
U: thermal loss ratio.
Cp V
Therefore we have:
Temperature variation
We replace (4) in (1), then:
Ut MCp
  (5)
Mass of water to heat
Replacing (5) in (4), so:
Tln tC
 
The calculation of the water mass to heat, during 24
hours, can be carried out for different water temperature
According to expression (6) we give Table 1.
1) ΔT variation and efficiency according to time for
water capacity of 50 liters which is the experimentally
capacity used.
Thermal loss coefficient
Cp4185 JkgC
water 1000 kgm
t11 hours; 18:005:00 
V0.05 m
T35C; T22C
U4.92 W/C
According to the expressions (2) and (3) we calculated
the temperature rise, the efficiency, the useful and ab-
sorptive energies, gathered in the following Table 2 and
Figure 5.
comp water
P140 W; COP3; M50 Kg
With time, water warms up by the heat yielded by the
condenser; then, the quantity of heat contained in water
increases. However, the energetic efficiency decreases
following the increase in the losses by convection with
the ambient air (Figure 5).
2) Checking length of the condenser immersed in the
hot water cumulus
In our case, the flow of the refrigerant (R134a freon)
in the condenser immersed in the cumulus is done from
top to bottom, i.e. in against current with the circulation
of the water to be heated which is done upwards.
Table 1. Value of water mass for different rise in tempera-
ΔT (˚C) 20 25 30 35 40
M (kg) 90 72 60 51 45
Figure 5. Rise variation in water temperature according to
Table 2. Variation of ΔT and efficiency according to time.
(hours) T (˚C) efficiency Quseful (J) Qabs (J)
6 14.20 0.83 2972654 3565800
12 24.35 0.71 509736 7131600
18 32 0.62 6691162 10697400
24 38 0.55 7930746 14263200
48 52.50 0.38 10986005 28526400
72 60.22 0.29 12604349 42789600
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Therefore the difference in logarithmic average tem-
perature curve will be carried out in an against current
cycle (Figure 6).
According to Table 2 of comparison during 12 hours
and according to measurements:
Hot source: (condenser)
Te = 62˚C and Ts = 43˚C
Cold source: (water to be heated); with (T =
te = 20˚C and ts = 44.35˚C
Determination of logarithmic average temperature
difference TmL
The expression of TmL is:
T0Tets6244.3517.65 C
 
Heat exchange coefficient:
The condenser exchanger contains freon which passes
from vapor state to liquid state by yielding its heat to
water to be heated (Figure 7).
copper water
ln ln
h freon
 
Figure 6. Cycle of against current.
Figure 7. Heat exchanger (water condenser).
h water50 W/mC; h freon4200 W/mC;
di0.004 m; de0.006 m
Ui50 W/mC
3.3. Theoretical Study of the Coupling of a
Refrigerator to a Heating Floor
In this section we will make the study of a refrigerator to
a heating floor coupling, and the determination of the
temperature variation for each surface for different peri-
ods and for different surfaces to be heated.
Calculation is carried out on concrete surfaces height h
= 0.1 m.
There is a thermal contact between two bodies thus
there is a heat transfer between the two bodies in contact
 (9)
T: variation between the final and the initial tem- pe-
rature of the floor.
Determination of the temperature variation T
We have:
useful absorbedabsorbedloss absorbed
usefulabsorbed loss
The initial temperature is equal to the ambient tem-
We replace (10) in (11):
 (11)
 
Table 3 and Figure 8 indicate the variations of ΔT
according to time and the floor surface.
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R. B. SLAMA 25
The reached temperature on the floor level is a func-
tion of the exchange surface with the condenser; in fact
also with the compressor power.
In practice, a prototype of heating sand floor of surface
S = 0.43 m2 was produced and in which a condenser of 6
m length is immersed there.
4. Experimental Study
First, we make the water cumulus then its coupling with
the refrigerator and the heating floor.
4.1. Water Cumulus Realization
We have a cylindrical tank in plastic, diameter 0.38 m
and height 0.48 m, which is isolated by five cm glass
woo. The unit is then covered by a thin stainless steel
jacket (Figure 9).
4.2. Coupling between the Refrigerator and the
Water Cumulus
We replaced the refrigerator condenser by a copper spiral
serpentine placed in the water cumulus, thus the freon
condensation will allow to yield the latent heat to water
and thus to constitute a hot water storage (Figures 10,
Figure 8. Floor temperature rise according to surface and a
number of day.
Figure 9. Dimensions of the copper condenser immersed in
the water cumulus.
Table 3. Variation of ΔT according to time and of surface.
T (˚C)
Surface (m²)
1 day 2 days 3 days 7 days
1 26 32 35 38
3 8.77 10 11 13
5 5.26 6 7 7
10 2.63 3.2 3.5 3.8
15 1.75 2.15 2.33 2.6
20 1.31 1.61 1.75 2
Figure 10. Refrigerating circuit binding the refrigerator
and the water-heater.
Figure 11. Coupling of the refrigerator to a water-heater
4.3. Heating Floor Realization
The goal for the moment is to construct a transportable
prototype. Thus the floor is carried out by some sand
filling of a wood box of dimensions 0.66 m × 0.66 m,
that is to say a surface of 0.43 m². As the selected height
is 0.1 m, then the sand mass is:
MSH66.6 Kg
 
The condenser of refrigerator is then placed in the
wood box before filling this latter by dry sand (Figure 12).
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4.4. Refrigerator and Heating Floor Coupling
For this system, the refrigerator’ condenser is placed in
the box filled with sand, thus, the foran condensation
does not directly heat the air but the sand, and stores its
heat in the heating floor (Figures 13-15).
It is also possible to couple the water cumulus and the
floor heating with only one refrigerator using an electro-
magnetic sluice gate which makes it possible to ensure
the commutation of the refrigerating circuit of the refrig-
erator either towards the hot water cumulus, or towards
Figure 12. Dimensions of the refrigerator condenser im-
mersed in the sand floor.
Figure 13. Installation circuit of refrigerator/heating floor
Figure 14. Small-scale model of the heating sand floor pho-
Figure 15. Photographs of the two refrigerators: One cou-
pled with the water-heater, the other with the heating floor.
the heating floor.
It is also to be noted that the floor can be heated in a
“conventional” way by the water from big tank storage,
heated by the condenser of fridges which has restaurants
and hotels or refrigeration rooms of foods conservation,
or other.
4.5. Experimental Results
The experiments make it possible to bind the refrigerator
to the system to be heated (water and heating floor) and
to record the heating, refrigeration and ambient tempera-
4.5.1. Evolution of Water-Heater Temperature
We study here the coupling of the refrigerator to a water
heating cumulus.
1) First test
Figure 16 shows that in the morning, with the refrig-
erator starting, the rise in the storage water temperature is
slower than that of the interior of refrigerator (approxi-
mately 8 h 30 against 2 h), this is because of the differ-
ence in heat mass capacity between water and air (4185
against 1000 J/kg˚C).
With an ambient temperature of 26˚C, water tempera-
ture reached is 59˚C, that is to say a rise in water tem-
perature of 33˚C.
2) Second test
At the end of the one day operation, the refrigerator is
voluntarily stopped for reasons of night safety. The fol-
lowing day at 5 am, the tests began again by the refrig-
erator restarting. The water temperature was 35˚C, that is
to say 15˚C higher than for the previous day; what shows
that the cumulus stores well heat thanks to its heat insu-
lation with glass wool.
According to Figure 17, the temperature increases in
the same manner than the first test, it reaches the perma-
nent mode (59˚C) at the end of eight hours. A racking of
ten liters hot water was carried out from the cumulus,
Copyright © 2013 SciRes. CWEEE
R. B. SLAMA 27
followed by a filling of the same cold water capacity.
Quickly the water temperature decreased up to 45˚C then
it increased gradually to reach again the permanent mode
at the end of four hours, against five hours and half for
the preceding heating and the same variation in tempera-
ture. This is to show that hot water can be used several
times per day.
4.5.2. Heating Floor Temperature Evolution
Is studied here the coupling of the refrigerator to a heated
floor consisting of a layer of sand.
For the input/output curves of temperatures in the floor
(Figure 18), we distinguish three zones: Zones 1 and 2
without insulation and zone 3 with insulation.
Zone 1 [5 h – 10 h]: Fast rise in floor temperatures
according to time.
Figure 16. Variation of the temperatures according to time.
Water heating case.
Figure 17. Variation of water, evaporator and ambient
temperatures before and after draining of 10 liters water.
Figure 18. Temperature variation according to time. Heat-
ing floor case.
Zone 2 [10 h - 14 h]: The temperatures reach their
maximum values (permanent mode with 45˚C to
Zone 3 [14 h - 20 h]: Floor temperatures rise com-
pared to Zone 2 up to 54˚C/52˚C thanks to the heat
If the heating floor is insulated, then the heat quantity
stored in sand increases in a significant way.
5. Conclusions
By this project, we tried to widen the use of a refrigerat-
ing system for the water heating, spaces or buildings, and
this by the exploitation of the energy previously rejected
by the condenser. With this manner the refrigerator can
contribute to heat water and/or floor-heating; while keep-
ing its principal function to cool.
The temperatures reached respectively, by water and
floor, are approximately 60˚C and 50˚C, without modi-
fying the temperature of the evaporator, which is at ap-
proximately 20˚C.
The elimination of the grid of the usual condenser lo-
cated at the back of the refrigerator creates an advan-
tage: to avoid its heating and consequently up time of the
compressor and thus its electric consumption.
Finally, the results obtained as well theoretically as in
experiments show clearly that the heat withdraws coming
from the condenser immersed in water or sand, is a reli-
able source of heat, being able to be useful at least for
pre-heating of water or room. The recovered heat enters
in the négawatts production concept.
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R. B. SLAMA 29
COP coefficient of performance
Cp heat mass (J·kg1˚C1)
h heat exchange cefficient (Wm2·K1)
M Mass (kg)
Pcomp power of the compressor (W)
Q Heat quantity (J)
S surface (m2)
t time (h)
U convection losses coefficient (Wm2·K1)
S surface (m2)
T Temperature (˚C)
ΔT Temperature variation (˚C)
Δt time variation (s)
ŋ efficiency
ρ density (kg/m3)
λ conductivity transfer coefficient (Wm1·K1)
amb ambient
i initial
f final
th thermal
fr refrigerator
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