International Journal on Architectural Science, Volume 6, Number 4, p.168-172, 2005
STUDY ON HYBRID SYSTEM OF SOLAR POWERED WATER HEATER
AND ADSORPTION ICE MAKER
Zhaohui Qi
School of Energy Science and Engineering, Centralsouth University, Changsha, China, 410075
(Received 23 October 2003; Accepted 14 February 2006)
ABSTRACT
Based on the vacuum tube type solar collector, a system of hybrid solar powered water heater and adsorption ice
maker was designed and a prototype was built. Energy analysis of the hybrid system was presented, and the
prototype was experimentally tested in different season. The results of experiment show that, compared with
traditional solar powered water heater and cooling machine, the hybrid system is more suitable for household
usage, and the utilization of solar energy is more effective.
1.
INTRODUCTION
The utilization of solar energy is a major feature of
modern “green building”, it is concerned by
scientists around the world and some progress is
made. There exist two types of solar collector [1]:
photovoltaic cells and purely thermal collector, the
latter is more common in the application of solar
energy. Based on the thermal collector, solar
powered heating or cooling systems of building
have been developed, some products are used for
residential applications.
The technology of solar powered heating system
has been well developed and successfully used in
solar powered water heater. Solar powered water
heaters are popularly used in China, more than
15,000,000 water heaters in China are powered or
partly powered by solar energy, and the number is
still increasing quickly. Solar powered water heater
is advantageous for environmental friendliness and
energy saving, but it has some disadvantages. The
main disadvantage is that the supply of solar energy
and the need for heated water does not match with
each other in the same season. In summer, the
supply of solar energy reaches maximum level, but
the need for heated water is minimum; in winter, it
is in the opposite situation. So the utilization of
solar energy in water heater is not sufficient.
The technology of solar powered cooling system
has been developed also. It is an excellent
application of solar energy, because the supply of
solar energy and the need for refrigeration both
reach maximum levels in the same season
(summer). Liquid absorption system has been
proposed and tested at first [2], solid adsorption
system has been discussed and tested later. One
characteristic of solid adsorption system is that
there is no need of rectifying column, so it has
advantages of simple construction, no moving parts
and no-noise. On the other hand, non-freon
refrigerant is used in adsorption refrigeration, so it
is environmental friendly. Worsoe-Schmidt has
developed adsorption solar refrigerator which has
been used in areas of the Third World [3]. The main
drawback of adsorption system is that its operation
is limited by season. So the solar powered cooling
machine could only be used seasonal, that is to say,
it is expensive compared with the cooling machine
which can be used for the whole year. Requirement
for cooling and heating of household is dissimilar
in different season. Single solar powered heating or
cooling machine could not meet the need of
household in a year.
The aim of the work described below is to combine
the solar powered water heater and adsorption
refrigeration machine, to meet different
requirement of household in different season, and
to improve the efficiency of utilization of solar
energy.
2.
HYBRID
SOLAR
POWERED
WATER
HEATER
AND
ADSORPTION ICE MAKER
Glass vacuum tube has been widely used as
collector in solar powered water heater. The
technology of vacuum tube type collector is mature,
absorption rate of glass vacuum tubes is more than
93%, and thermal radiation rate at 100 oC is less
than 6%, water temperature in it can reach 98 oC
easily by adjusting the quantity of water. Water at
that temperature can be used as heat source of
adsorption refrigeration system.
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Fig. 1: Schematic of hybrid solar powered water heater and ice maker
According to the above discussion, a hybrid solar
powered water heater and ice maker was designed.
The schematic of system is shown in Fig. 1. The
working principle of the system is just a
combination of a solar water heater and adsorption
ice maker. Heating of the vacuum tube type solar
collector is started in the morning, water in the
solar collector is heated slowly. When temperature
of water in the collector is 5~7 oC higher than
storage tank’s water, the water circulation pump
works to pump up tank’s cold water into the
collector, warm water in the collector flows
through the adsorber, goes back into storage tank.
When the warm water flows through, the adsorber
is heated, the temperature of adsorbent in it rises. In
a ideal process, the adsorbent temperature could be
very close to the maximum temperature of water in
the tank at last. When the temperature in the
adsorbent rises up to the reaction temperature of
desorption, the refrigerant is desorbed from the
adsorbent, and the vapor pressure in the adsorbent
rises too. When the pressure reaches the
condensing pressure, the desorbed refrigerant vapor
is condensed in the condenser and collected in the
receiver. At night, with the reduction of ambient
temperature, the adsorber is cooled by nature
convection, the adsorbent in it is cooled too, and
the pressure in the adsorber drops to value below
evaporation pressure, evaporation begins, ice is
made slowly in the ice box. Refrigeration will
continue for the whole night until the next morning.
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The main difference between the hybrid system
designed in this paper and other hybrid system of
solar powered water heater and adsorption ice
maker is that the adsorber of hybrid system
designed in this paper is separated from the hot
water storage tank, and adsorber of other system is
placed in the hot water storage tank, the desorption
and evaporation of refrigerant depends on the
consumption of hot water. So it is difficult for
them to meet different requirements of hot water
and cooling. The hybrid system designed in this
paper can meet different requirement of hot water
and cooling without any difficulty.
3.
ENERGY ANALYSIS
Solar energy absorbed by the collector will be used
in three way [4]: (1) Qu, energy to heat the water in
the storage tank and adsorber. (2) Qs, energy
storage in the collector. (3) Ql, energy lost due to
various losses. The energy conservation equation
is:
AeG(τα) = Qu + Qs + Ql
(1)
where G is the solar flux density to the collector, τ
is the transmittance coefficient of solar radiation
through the cover of the collector, α is the collector
efficiency, and Ae(m2) is the area of the collector.
International Journal on Architectural Science
In a solar collector, the heat quantity Qu(kJ) is used
to heat the water and adsorber, which is mainly
determined by the efficiency of collector; the heat
quantity Qs(kJ) is determined by the sensible heat
of collector and depends on the collector material;
Ql (kJ)is the heat losses composed of the face loss,
the bottom loss, the four sides loss of collector and
the loss of heat exchange in the adsorber when
warm water flows through the adsorber. Qs and Ql
of vacuum tube collector are less than 10% of the
total solar energy absorbed by the collector.
The useful heat from the collector Qu contributes
both to the heating of the water in the tank and to
the heating of the adsorber which will cause the
desorption of refrigerant from the adsorbent bed,
etc. The energy equation is:
Qu = Qwater+ Qadsorber + Qrefrigerant + Qreaction
(2)
In which Qwater(kJ) represents the heat added to the
water in the storage tank.
Qwater = MwaterCwater( Th - Tl )
(3)
where Mwater is the mass of water in the tank, kg;
Cwater is the specific heat of water, kJkg-1K-1; Th is
the last temperature of water after heating, K; and
Tl is the temperature of city water, K.
Qadsorber(kJ) represents the sensible heat of material
and adsorbent in the adsorber.
’
Qadsorber = (MmCm + MaCa)(Th -T0)
(4)
Mm is the mass of adsorber material, kg; Cm is the
specific heat of material, kJkg-1K-1; Ma is the mass
of adsorbent, kg; Ca is the specific heat of
adsorbent, kJkg-1K-1; Th’ is the last temperature of
adsorber after heating, K; and T0 is the ambient
temperature, K.
Qrefrigerant(kJ) represents the sensible heat of
refrigerant in adsorber.
Qrefrigerant = MRCR (Te -T0)
(5)
MR is the mass of refrigerant in adsorber, kg; CR is
the specific heat of refrigerant, kJkg-1K-1; and Te is
the critical temperature of reaction, K.
Qreaction(kJ) is the heat of desorption.
Qreaction = HRMR
(6)
where HR is the heat of desorption per kilogram,
kJkg-1.
The desorbed refrigerant is condensed in the
condenser and flows into the evaporator. When the
adsorbent bed pressure is lower than the
evaporation pressure, the refrigerant liquid in the
evaporator evaporates which causes the
refrigeration effect in the ice box. The refrigeration
quantity is :
Qref = MiceLe
(7)
In which Le is the latent heat of water vaporization
(kJkg-1), and Mice is the weight of removed ice (kg).
The hybrid system has two useful output, one is
refrigeration, its system COP (Coefficient of
Performance) is:
COPsystem =
Qref
Ae G (τα )
(8)
Another is heating the water in the storage tank, its
system efficiency is:
η system =
Q water
Ae G (τα )
(9)
Chloride strontium (SrCl2) – ammonia (NH3) is
used as working pair of adsorption refrigeration,
SrCl2 is used as the adsorbent, and NH3 is used as
refrigerant, it is desorbed and adsorbed according
to the following equation [5]:
SrCl2·8NH3<＝ > SrCl2·NH3+7 NH3
(10)
Ammonia is not a suitable refrigerant for
residential applications if it is used in air
conditioning, but in the Third World, however, air
conditioning is a luxury most people cannot afford.
In those countries, refrigeration is primarily needed
at the village level, for preservation of foodstuffs
and medical supplies only. Ice maker with box can
meet this need easily, ammonia is acceptable if the
refrigeration system is separated from other
systems [3,5].
4.
EXPERIMENTAL RESULTS
A prototype hybrid system for water heating and
ice-making was developed. The solar energy
collector consisted of 30 φ47×1×1200 glass
vacuum tubes whose collect area of solar energy
was 4.2 m2, structure of adsorber adopted by this
paper is shown in Fig 2. It was made up of unit
generating tubes. Configuration of the unit
generating tube is shown in Fig 3. Unit generating
tube was made up of aluminum fin in which SrCl2
was filled, ammonia-conducted tube and stainless
steel tube. Ammonia-conducted tube was the
passage of ammonia vapor flowing into/out the
generating tube, and stainless steel tube was an heat
exchanger in effect where heat exchange was
conducted when warm water of collector flowed.
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International Journal on Architectural Science
to the collector. Temperatures of water in tank, inlet
and outlet of adsorber and temperature of
evaporator were measured by thermocouple,
ambient temperature, temperatures of city water
and adsorber were measured also. The
measurements and their accuracy are compiled in
Table 2. Weight of the removed ice can be
determined as the last quantity of refrigeration. The
sun began shining normally at 7:00 am, the initial
temperature of water was 18 oC, initial temperature
of adsorber was 32 oC (the same temperature as
environment). After 6 hours, the 180 kg water tank
temperature reached 85 oC, temperature of the
adsorber reached 80 oC, water temperature inlet
adsorber was 91 oC, the desorption process began
obviously. After 5 hours of continuous heating, the
temperature of water tank reached a maximum
value, 93 oC, water temperature inlet adsorber was
98 oC, temperature of the adsorber was nearly 90oC.
Heating and desorption stopped at that time (6:00
pm ). Total solar heat density was 20 MJm-2, that is
to say, about 84 MJ solar heat was collected. At
night, the temperature of adsorber reduced slowly.
At about 11:00 pm, the temperature reduced to 29
o
C, adsorption took place. One hour later, the
temperature of evaporator reduced to -15 oC, ice
was made obviously in the ice box. Refrigeration
continued in the whole night until the next morning.
11.6 kg ice was made at last. Fig. 4 shows the
temperature variation of different parts of the
hybrid system in the experiment.
Fig. 2: Schematic of adsorbor (cross-section)
Fig. 3: Schematic of unit generating tube
Values of the parameters mentioned in equations (3)
to (7) are compiled in Table 1.
The prototype was built in Changsha, a city in
South China, A typical experiment was
demonstrated at the end of August, 2000. Overall
insolation was measured by a pyranometer parallel
Table 1: Relevant parameters in energy analysis
Parameter
value
Mwater
180
Mm
40
Ma
22
MR
15
Mice
11.6
Cwater
4180
Cm
905
Ca
1780
CR
4870
H
1400
Ae
4.2
Table 2: Measurements and their accuracy in experiment
Measurement
Thermocouple (NiCr-Ni)
Alcohol thermometer
Pyranometer
Accuracy
0.1 K
0.5 K
1.0 Jm-2
Temperature (oC)
Measured quantity
Temperature of water, evaporator
Temperature of city water, environment
Solar flux density
Time
Fig. 4: Temperature histories of water, adsorber and environment
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International Journal on Architectural Science
Table 3: Experimental results of the hybrid system
Date
Ice out
/ kg
COPsystem
ηsystem
ηsystem
without ice maker
93 180
11.6
0.069
0.67
0.72
92 120
8.0
0.064
0.61
0.70
Energy
collected / MJ
Hot water
output / oC kg
84
63
Aug. 23-24,
2000
Oct. 15-16,
2000
Here, COPsystem and ηsystem defined by equations (8)
and (9) were 0.069 and 0.67, ηsystem of the same
solar powered water heater system without ice
maker was 0.72. COPsystem of other kinds of solar
power adsorption ice maker varied from 0.05 –
0.12 [1].
Table 3 shows the experimental result discussed
above and another experimental result of hot water
and ice output in a typical season (autumn).
Obviously, with the reduction of collected solar
energy, system efficiency of heating and cooling
became smaller. The value of refrigeration
efficiency (COP) reduced more relatively. On the
other hand, system efficiency without adsorption
ice maker was almost not changed. It was due to
poor heat transfer in the adsorber. Technology of
heat transfer enhancement should be adapted in the
adsorber in order to improve the efficiency of
refrigeration. But generally, the results of
refrigeration are acceptable. The refrigeration
quantity produced after one day operation can be
used to keep a 200 liter cold box for more than 24
hours with temperature less than 5 oC.
5.
CONCLUSION
•
The hybrid system of solar powered water
heater and ice maker uses one solar collector
as heating and refrigeration source, it is
suitable for household application.
•
Adsorber is separated from collector, thus
high efficiency glass vacuum tube can be used
for solar energy collection. Adsorber is also
separated from the water tank [4], thus the
refrigeration quantity is not influenced by the
consumption of hot water. It can meet
different household requirement for heating
and cooling in different season.
•
Compared with single solar powered water
heater and ice maker, system efficiency of
heating and cooling is lower [1], but the
hybrid system has two useful outputs, the
utilization of solar energy is more sufficient
and comprehensive.
REFERENCES
1.
M. Pons and J.J. Guilleminot, “Design of an
experimental solar-powered, solid-adsorption ice
maker”, Journal of Solar Energy Engineering, Vol.
108, pp. 332-337 (1986).
2.
J.C.V. Chinnappa, “Performance of an intermittent
refrigerator operated by a flat – Plate collector”,
Solar Energy, Vol. 6, pp. 143-150 (1962).
3.
P. Worsoe-Schmidt, “Solar refrigeration for
developing countries using a solid-absorption
cycle”, International Journal of Ambient Energy,
Vol. 4, pp. 115-124 (1983).
4.
R.Z. Wang, M. Li, Y.X. Xu and J.Y. Wu, “A new
hybrid system of solar powered heater and
adsorption ice maker”, Solar Energy, Vol. 68, pp.
189-195 (2000).
5.
A. Erhard, K. Spindler and E. Hahne, “Test and
simulation of a solar powered solid sorption
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