沙漠研究 19-1, 21- 24 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

New Solar Energy System to Provide Domestic Hot Water in Rural Housing in Tunisia
Majdi HAZAMI1, Sami KOOLI1, Mariem LAZAAR1, Abdel hamid FARHAT1, Ali BELGHITH2
Abstract: This paper deals with an experimental investigation of a low-cost solar storage
collector, with a total aperture area of 5 m², used for providing domestic hot water in rural zone.
The solar storage collector is mainly made up of a concrete absorber with a thickness of 50 mm
covered with a double layer of transparent PVC. The matrix absorber of the solar storage
collector allows the storage of the excess of solar heat. Inside the concrete absorber isincorporated a network of a capillary heat exchanger made of polypropylene used to carry heatfrom the concrete absorber. To evaluate efficiency of the solar collector, outdoor experiments
were carried out under varied environmental conditions for several days. The results indicatethat the solar storage collector is suited to the supply of heated water at a moderate temperature(above 318.15 K) during a typical day of high solar radiation and the water temperature at theoutlet of the solar storage collector was kept over 313.15 K during the whole night in coldseasons. The results showed also that the instantaneous thermal efficiency of the solar collector
was between 15 % and 50 %. Moreover it was found that the solar collector provides anacceptable heating rate by supplying approximately 90% of domestic hot water requirements
for a rural household composed of 5-6 persons.
Keywords: A low cost solar storage collector, Domestic hot water, Rural zone

1. Introduction
Hot water was used in rural households predominantly for taking baths, washing clothes and hygiene
purposes. For heating domestic water, Tunisian rural population exploits excessively traditional sources ofenergy such as firewood and fuel which has become scarce and expensive. Furthermore, the over-use of
firewood and fuel has lead to environmental degradation in heavily populated areas. Thus, actually wide
efforts have been undertaken to alleviate global warming of earth caused by the emission of carbon dioxide
in atmosphere generated by intensive burning of fossil fuels. One way to reduce consumptions of theconventional energy sources is to employ solar energy through solar water heating systems. From theenvironmental point of view the use of solar water heating systems can play an important role to save theenvironment. Indeed, Tunisia is blessed with abundant sunshine thus making it appropriate to select solar
energy for heating domestic water by means of solar collectors. The widely commercialized systems of thesolar collector in Tunisia are the conventional forced circulation type or of the thermo-siphon type basedon flat plate collectors and water storage tanks (Kerkeni et al., 1993). Unfortunately, the investment costs
for the commercialized solar water heating systems in Tunisia are still elevated. To overcome this problem,
we have conceived and developed in our Laboratory (Laboratoire de Maîtrise des Technologies del’Energie, LMTE) an affordable-cost solar collector system which does not require a storage tank. This
environmentally friendly solar collector, having a total surface area of 5 m2 is tilted at 37˚ from horizontal.
This new type of solar collector provides a promising technology. It needs easy and simple maintenanceand operation. This solar storage collector can be produced locally from locally available materials(Hazami et al., 2005). Outdoor experiments were carried out under varied environmental conditions forseveral days using Huang and Taguchi experimental method (Lu et al., 2003; Tsui, 1992) which allows
predicting a performance of the solar storage collector. The main objective of this paper is to elucidate thepotential of the conceived solar collector in supplying heated water at an acceptable temperature (45 ˚C)until evening on most days of a year, especially in cold seasons for the southern rural Tunisian houses.
2. Materials and Methods
The external casing of the collector was made of highly corrosion-resistant and galvanized steel sheet,
1
2Centre de Recherche et des Technologies de l’Energie; Technopole de Borj Cedria Hammam-Lif BP 95.
Faculté des Sciences de Tunis ; Campus, El belvédère Tunis-1060.
Fax no 71430934, Tel no 71430044 / 71430215, E-mail: Hazamdi321@yahoo.fr

22sprayed with rust preventative paint. The casing was covered with a double layer of 4 mm-thicktransparent PVC, followed by a 20 mm-air gap. The absorber plate was made of a 50 mm-thick concreteslab. High radiation absorption was achieved by the use of a matt black paint on the concrete surface. The
undersides of the absorber plate and the side casing were well insulated with a 30 mm-thick polyurethane
foam to reduce conduction losses. A capillary polypropylene heat exchanger, with an exchange surface
equal to 5 m², was embedded inside the concrete absorber. Outdoor experiments were carried out in Borj
Cedria (North of Tunisia): Latitude 36˚ 50’ N, Longitude 10˚ 44’ E. Figure 1 illustrates a schematic
diagram of the experimental loop for the solar storage collector tests. The water supplied by the heat pump
replenish inside the polypropylene capillary heat exchanger. During the experiments, the following
conditions have been taken into account (i) a constant water mass flow rate was maintained by using aflow-meter integrated at the experimental device, (ii) a total solar energy higher than 18 MJm-2; (iii) theinlet water temperature and mass flow in the system was respectively fixed at 295.16 ± 2K and 0.0416 kg s1
by the use of an electric heat pump. To measure the incident solar radiation, a pyranometre (Type Kippand Zonen) was also placed inside the experimental device. Copper-constantan thermocouples were placedat concrete surface and at inlet and outlet of the solar storage collector. These thermocouples wereconnected to a multi-channel digital data acquisition. A meteorology station installed in (LMTE, Tunisia)
supplies the information about relative humidity of the ambient air and wind speed during the experiments.
The test began at 9:00 (solar time) when the data logger system was switched on and all of theseparameters were measured at an interval of 5 min with a total accuracy assumed to be about 3 %.

Fig. 1. View of the experimental device.

3. Energy analysis
According to the INPUT/OUTPUT standard (Hikmet, 2008), the energy gain (daily heat output) of thesolar collector, Qu (W), can be represented by the following empirical equation:(1)
Q u = α 1 H + α 2 (Ta , av − Te, av ) + α 3
where H is the daily solar irradiation at the collector aperture (W.m-2), Ta,av is the average ambient airtemperature (K), Te,av is the average cold-water inlet temperature (K) and α1, α2 and α3 are constants for asystem, determined from the test results;α1,=30 m-2, α2 = 0.35 W.K-1 and α3= -12.7.
The overnight heat loss coefficient, Uc (W/K.m²), of the hot water storage system is determined bymeasuring the temperature loss of the water during a 12 h nocturnal period (Souliotis et al., 2004). The
formula used is:
UC =M c C p, cAc Δ tlnTi − Ta , av
Tf − TAbs, av(2)where Ac is the surface area of the collector (m2), Cp,c is the specific heat of concrete absorber (J/(kg K)), Δt
is the test period (s), Ti is the initial temperature of the water in the tank (K), Tf is the final temperature of
the water at the solar storage exit after 12 h (K), Ta,av is the average ambient temperature (K), TAbs,av is theaverage concrete absorber temperature (K) and Mc is the concrete mass (kg).
The daily thermal efficiency of the solar collector η j is given by the expression:
t2

∫

t2

∫

η j = Q u ( t ) dt / A c .H ( t ) dtt1t1(3)23

The daily thermal efficiency of the solar collector η j is also given by the expression:η j = ηo − U cTAbs,av − Ta ,av
H av−

M c Cp,c (TAbs, t 2 − TAbs, t1 )A c .H av(4)

where Hav is the average solar radiation (W.m-2), TAbs, t1 and TAbs, t2 are the concrete absorber temperatures atthe instants t1 and t2 respectively (K) and η0 is the optical yield. It represents the fraction of the solar
radiation absorbed by the concrete matrix. Eqs.3, 4 permits the experimental determination of η0 (η0 = 0.7).

100040800306002010To
H0-250H (W m )TO (°C)

4. Results and Discussion
The daily thermal efficiency of the solar collector is shown, in Figure 2, as a function of time. The
thermal efficiency of the solar collector was increased with the increase in inlet water temperature. The net
energy efficiency changed between 15 % and 50 %. The efficiency reaches the higher value at 12:30, andthen, the temperature decreases continually. Figure 2 shows that the efficiency reaches its maximum point forsolar energy radiation about 850 W/m2 and for the environment temperature about 295.15 K. To evaluate thesolar storage collector heat loss coefficient, Uc, another test was done. The test consists of covering the solar
storage collector by a 30 mm-thick polyurethane foam while the circulation of water inside the heat exchanger
is stopped. Then we followed the decrease in temperature of the concrete absorber during about 12 h. The
overnight heat loss coefficient, Uc, is experimentally determined by equation 2. It is assumed to be equal to
14 W/K.m2. In order to illustrate the thermal behavior of the solar storage collector, a winter day (cloudy day
characterized by a harshly solar radiation oscillation) has primarily been chosen for the experimental study.
The solar radiation and the water temperature at the outlet of the solar storage collector increases arerepresented in Figure 3. As expected, the outlet water temperature (To) of the solar storage collector depends
on solar radiation. It increases to a maximum value of 318.15 K at 12:00 pm in the noon and remains almost
constant during 4 h before it starts to decrease later in the afternoon. We noted also that the water temperature
at the outlet of the solar collector was not affected by solar radiation oscillation. Indeed, while the solar
storage collector was functioning, the absorbed solar energy was stored in the concrete matrix at a notnegligible rate. The solar heat stored inside the concrete absorber is delivered to the heat exchanger whilst theinsulation starts to decrease. To evaluate this amount of energy we have represented in Figure 4, thevariations of stored energy as a function of time.
We noted that the stored energy inside the absorber matrix was almost constant during morning and it
increased until noon at morning. A maximum peak of stored energy, about 2 kW, was obtained at 13:00 hand remained almost constant during 3 h. Then, the stored energy decreased as the solar radiation
decreased. To improve the stored thermal energy conservation, we opted for covering the collector surface
with an insulation blanket before the solar radiation remarkably decreased (around 16:00). After 18:00, anightly withdrawal of water from the covered solar storage collector was performed (Figure 5). The
results showed that the temperature of the extracted water decreased during the withdrawal process. In fact,
the stocked thermal power inside the concrete absorber decreased continually according to the time. We
noted that the solar storage collector supplied about 150 liters at a temperature of 318.15 K. This quantity

400200
09:00 10:0011:0012:0013:0013:4515:0016:00

Local time (hh:mm)

Fig. 2. The variation of the solar storage collector
efficiencies versus time.

Fig. 3. Outlet water temperature increase versus time
for a mass flow rate of 0.0416 kg s-1 and averageambient temperature of 295.15 K.

24

Fig. 4. Time change in energy stored in the solar
storage collector.

Fig. 5. The outlet water temperature as a function ofwithdrawal volume of water.

Table 1. Comparison between a conventional solar collector and solar storage collector.

Thermal efficiency
System cost
The annual energy gain
Cost payback

Conventional solar collector
38 %800 $1450 kWh
8 yearsSolar storage collector
32 %
250 $
1300 kWh
3 yearsof water extracted can provide about 90 % of family's needs in hot water during one day and about 75 % offamily's needs in hot water during one year. The test result obtained for the concrete solar collector that
does not need a storage tank were compared with tests carried out for the most popular water heating
system in Tunisia (Hasan, 2003) that consisted of a flat plate collector (2 m2 aperture area) and a water
storage tank (capacity 200 l) (Table 1). Results showed that for a rural housing the concrete solar collector
produced from locally available materials (Galvanized iron, (80 $), PVC transparent cover (100 $),concrete absorber (15 $), polypropylene heat exchanger (20 $) and polyurethane (35 $)) is a promisingsolution, which can save a lot of fuel or firewood, especially for economical modest family.
5. Conclusion
The performance of a newly designed solar storage collector that does not need a storage tank has beenpresented. Results showed that the solar storage collector can be the most appropriate solution fordomestic use especially for modest rural families. In fact, this system provides a substantial cost saving.
When compared to the traditional solar system design, it is obvious that the use of this system is promising.
It attained an efficiency of 32 %. The use of a solar storage collector can supply at minimum 90 % of thebuilding’s hot water requirements throughout the year saving the homeowner 75 % of his annual heatingbill and reducing the need for non-renewable energy. In winter, water can be heated to 318.15 K so that itis still at a comfortable bathing temperature of at least 313.15 K in the evening. To reduce its installation
costs in rural buildings, the cement concrete solar collector can be integrated with the roof structure.
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