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An experimental study in passive cooling systems and investigation of their role in consumption and diminishing environmental pollutants.


The water is a natural material, renewable and has important role both from environmental and strength, permanence and stability in ecosystems viewpoints. Iran is a country by reason of relatively dryness and hard access to ground water in majority of area and categorize in the dry and semi dry countries in the world. The rate of precipitation in Iran is less than 1.3 average precipitations on the world.

The annual average precipitation in northern parts of Iran and coasted area of Caspian Sea is about 2000mm and other parts of the country such as west and northwest most rate of precipitation is about 500mm and in some another area is about 50mm per year [1]. In some parts of Iran especially desert regions, also in some area using surface water and ground water is very difficult, therefore the people who living in these parts have to create particular plans for passive cooling systems. They could extend these systems to provide air conditioning for any undesirable environments condition themselves [2-8].

Saito [9] has made a comprehensive review on more than 130 papers which has been published based on the researches studies on the cold thermal energy storage system (CTES). This review paper explained the concept of CTES, its applications, various kinds of CTES systems and their values. Thermal stratification phenomenon is often utilized in efficient thermal storage systems and hence the main focus in relevant studies was on the influences of various parameters on preserving the thermal stratification. Nelson et al. [10,11] experimentally investigated the effects of storage tank dimensions, wall physical properties, the mixing effects of fluid flow and the tank insulation thickness on the decay of thermal stratification in a thermally stratified vertical cylindrical chilled water storage tank during both charging and discharging cycles. They noticed that increasing length to diameter ratio prevents the degradation of thermal stratification; however, increasing the aspect ratio beyond 3 has marginal effects. Also they have been found that during charging and discharging the established thermal stratification inside the storage tank was degraded. The material of the storage tank was found to have little influence on the development of thermal stratification during both charging and discharging cycles. They showed that thermal stratification improves with increase in thickness of the tank insulation.

Al-Marafie [12] experimentally analyzed thermal stratification in a 7.5[m.sup.3] chilled water storage tanks which was used to store chilled water produced by an air conditioning system during off-peak periods and deliver it on high peak demand for cooling load. Tank extraction efficiency of nearly 90% was reported, using a distributor header with a proper number and size of holes.

Suri et al. [13] conducted an experimental investi- gation on an air-conditioning system assisted with two equal size 7.5[m.sup.3] capacity fiberglass cold storage tanks. They found that using cool storage-assisted system resulted in an excellent economic advantage and the cost of cooling reduced by 25%.

Borst and Aghamir [14] studied the storage of chilled water in a tank about 35[m.sup.3] in volume. They cooled the water in the cistern by blowing cold winter air over the water surface.

The researches was observed that thermal energy reservoir tanks with daily period are one of main parts of domestic solar thermal systems which was existed more attention for numerous studies about thermal impact assessment, development stratification and analyzing about nourish and discharge periods is available [15-18].

In compare with thermal energy reservoir tanks which was accomplished, a few research has been done about passive energy reservoir tanks with daily period which it could be one of the economical parts of modern air conditions systems. Just case Bahadori is about thermal impact assessment for current cisterns in Iran.

In this research reservoir tank of cistern was filled with cool water in winter and remained in tact till beginning the discharge cycle in summer [19]. Distribution of heat temperature was investigated in water and soil around reservoir of cistern through dissolving sold by the piece equations on limit difference. With calculation distribution water temperature in various mounts of year, which was considered, that formed a stable stratification in reservoir tank. Variation of temperature in upper layer and close to surface water follow from alternations of outside ambient temperature, whereas, variation of temperature in bottom layers (that usage water tank from same layer) was so less, as reported rate of temperature about less than 10[degrees]C in all of operational times.

Previously in many cities of Iran especially in hot and arid regions utilized from cisterns for period and reservoir cold usage water, but nowadays utilized from modern systems such as passive cooling systems and etc. The cisterns was a cylindrical or cubic tank with depth 5-15m [1], which was provided its water through different ways such as subterra- nean, rivers, spring water rain and seasonal floodwater in one of coldest day in winter [1,19].

The most canals were uncovered until water could get cold through cooling evaporation and thermal radiation inter- change [6,19] with 1-7 wind towers (BaudGeer)[1] in shape hexagonal, cubic and octagonal for circulation air flow inside tank and a domed shape roof, which was on top of tank. In cisterns existence heat absorbed by water through thermal radiation inter- change with domed shape roof and thermal conduction transmission with bottom and rims of tank and also, convection atmos- phere on top of upper layer and create stratification in direction of depth water. The outside ambient through arrived canals of wind towers enter to inside tank and evaporate in effect to contact with surface of water in tank.

In this process surface layer of water to lose its heat and replace with another layer that are hottest and down layer by reason of more density and, this replacement exist fix temperature in deeper layers, consequently. The tap water prepares in lowest point of tank and rate of outflow from tap water is some how that, exist any turbulence in stratification and keep it up.

Description of Storage Tank

The considered storage tank is under the hot and arid condition of Yazd, located at the center part of Iran. This cistern is a cylindrical underground water reservoir with some both thickness and height 12m and equipped with four cubic wind towers to height 10m segment. Area 1x1 [m.sup.2] and domed shape roof that prepare a window on the central section of top of it (Fig. 1).


The taking tap is about 90cm upper than bottom of tank for preventing from coming out deposits, which is settled in floor of tank. This storage tank could provide usage water for 1000 person in summer season (from first of Jun. to end of Sep.) Which is living in neighborhood of it and with LPCD 5lit for per person.

Explanation of Experiment

Out side ambient and water temperature was measured in vertical direction once per 10 days (from 1 Jun. to 30 Sep. 2005) and twice per day (once 8.00 AM and other 8.00 PM). It is noticeable between temperature data which was registered in 8.00 am and 8.00 PM was any considerable difference. With due attention to primary height of water in the tank was about 9cm. We could calculate rate of discharge time based on the following relations [20]:


where, [h.sub.1], height of water in primary state (m), [h.sub.2], height of water in secondary state (m), [D.sub.t], thickness of tank (m), di, thickness of pipe (in outflow of tank) about 0.0127m, f, coefficient conflict (0.493), [alpha], coefficient correction (about 1 in SI system), and [[SIGMA]L.sub.e], equivalent thickness of balcony pipe (around 25cm). Table 1 shows that Time and height of discharge in cistern.

The figure 2 shows variation of reserved water in tank in length of discharge time. Also figure 3 shows variation of height water in tank with due attention to consumption of pattern which was mentioned previously. Just as, observe in table 1 and figures 2 and 3 take water achieved from first of Jun. (start of hot season) to end of Sep. (to finished of hot seasons)




Figure 4 shows measured temperature variation at different depth for two months of Jun. and Aug. least

Temperature of water related to sections from surface water surface which was prepared tap water with regard to figure 4 all of reserved water divided two districts:

--Bottom region with linear temperature profile and upper region which is temperature variation in shape logarithmical approximately.

Deviation of temperature distribution is from linear state in upper layers to arise from combination of radiation thermal interchange between upper layer with domed shape roof of tank and transmission of mass and thermal evaporation which was created between surface and wind towers.

Figure 5 to shows outside ambient temperature variation, which was measured during 24 hours for 3 selected days. Maximum ambient temperature reported about 42[degrees]C in hottest day of Yazd city also, was considered when ambient temperature is about 42[degrees]C, the out-flow water temperature from tank is around 13[degrees]C. Thus, the outside ambient temperature had a little effect on taking water temperature. Also outflow water from cistern tank is suitable for drink (temperature of drinkable water have to be between 5-15[degrees]C and best temperature is about 8-12[degrees]C) [21]. There is the variation of outflow temperature from bottom of tank and average temperature of outside ambient per month in length of operation time for comparison.

In table 2 shows that, cisterns have important role for cool and maintenance water in hot and arid regions of Iran. Just as, maintained, one of most important advantages of cisterns is providing cool, drinkable and agreeable water in length of operational time. That create by arrive water to cistern in winter and save it until start of operational period.



Energy Analysis and Environmental Investigation of Cisterns

The energy content of the reservoir at every 10 days Calculated based on the following relations:

E = mc ([T.sub.m] - [T.sub.o] (2)

in which E, m and c are energy, mass and specific heat capacity of the storage and [T.sub.o] is ambient temperature.

In equation (2) calculated [T.sub.m] through numerical integration of the measured date based on the following relation:


The energy delivered by out flow on discharged (rate of passive produced) is calculated based on the following equation:

[Q.sub.d] = [m.sub.e] c([T.sub.out] - [T.sub.o]) (4)

in which me and [T.sub.out] are the mass and temperature of out flow during operation time. You could see in the figure 6. The rate of passive or cooling which was created is about 7.25 MJ cooling energy till end of operation time.

In another prospect reserved cooling about 7.25 MJ with out any expense in summer, which is, sever necessity to this.

With due attention to calculate based on the following relation if we want produce this cooling energy through usual refrigeration with coefficient revenue 1.3 needs to equivalent refrigeration.

[Q.sub.tot] = 7253304.745 kJ

[Q.sub.Operating Period] = 124 day

[Q[degrees].sub.ref] = 745.7253304/124 = 4.58494 kJ/day

[W[degrees].sub.ref] = [Q[degrees].sub.ref]/[COP.sub.R] = 0.67702/1.3 = 520.8 W

Typical Refrigerator Wattage = 3.4 hp = 560 kW

The Equivalent Refrigerator Required = 520.8/560 = 0.93 = 1

Saved Electrical Energy = [W[degrees].sub.ref] x Usage Time = 0.5208 x 124 (day) x 24 (hr) = 1550k W - hr

The above calculates show that utilizes from energy reservoir systems to result in saving in usage electrical energy about 1550 kW-hr in during four month.

With due attention to reference [23] which was reported production of N[O.sub.x] S[O.sub.2] and C[O.sub.2] in per kW-hr for produce electrical energy by arsenals in manner is 166.15 gr, 2.773 gr, 0.994 gr, therefore, rate of diminish product above pollutants, as a result of electrical energy saving around 1550 kW-hr is based on the following related.

Reduced C[O.sub.2] Emission = 1550 x 166.157 = 257543 gr = 257.543 kg

Reduced S[O.sub.2] Emission = 1550 x 2.773 = 4298 gr = 4.298 kg

Reduced N[O.sub.x] Emission = 1550 x 0.944 = 1463 gr = 1.463 kg

With due attention to collected information being about 75 cistern in Yazd city at present [1] if consideration per cistern equivalent with a refrigeration with coefficient revenue 1.3, we could both saving about 116250 kW-hr in use electrical energy and decrees amount production of above pollutants in four mounts.

Reduced C[O.sub.2] Emission = 116250 x 166.157 = 19315751 gr = 19315.75 kg

Reduced S[O.sub.2] Emission = 116250 x 2.773 = 322361.25 gr = 322.36 kg

Reduced [NO.sub.x] Emission = 116250 x 0.944 = 109740 gr = 109.79 kg

The greenhouse gases is a natural phenomena that to result in raising the temperature and thus its climates.

These gases are including: Carbon dioxide ([CO.sub.2]), nitrogen oxides ([NO.sub.x]), chlorofluoro- carbons ([CFC.sub.s]), ozone ([O.sub.3]) and methane ([CH.sub.4]). Between of these gasses C[O.sub.2] is more importance than others, become affect about 55% in raising the earth's average temperature (global warming) the main source of production C[O.sub.2] is combustion of fossil fuels in industries.

In this time, reported an increase rate of emission at atmosphere about 65% extra C[O.sub.2] as a result of fossil fuels of industries [24].

With pay attention to results according numerical [19] and excremental measurements, we could say that, the temperature of water in tank is depend on: volume of storage, first temperature of water to arrive at storage, kind of solid, rate of moister and taking water.



The water is renewable natural resources so important both from an environment and strength-permanence and stability in ecosystems. Previously in most cities of Iran specially in hot-arid regions utilized from cistern for provided and reserved fresh, agreeable and cold usage water, but nowadays utilize from modern air conditioning systems such as, passive cooling systems and etc.

So that, as in hottest months of season when the temperature of extracted water was in the range of 12.7-14.9[degrees]C during the entire summer period whilst the outside ambient temperature was reached up to 37.5[degrees]C, also, observed which was existed a permanence stratification in storage tank.

These stratification divided two districts:

Bottom region with is temperature profile and upper region which is temperature variation in shape logarithmical approximately. When we use from passive cooling systems, in this case will be decreasing various pollutants such as, C[O.sub.2] (to from 55% of greenhouse gases) S[O.sub.2] and NOx which is utilize fossil fuels for produce electrical energy.

Nowadays, cooling energy specially to mix with common air condition systems is developing and to result in transfer usage electrical energy to short-use times and creates cooling reserved energy in peak of necessity to cooling energy.


[1] Dehghani AR. Consideration of history and process of delivery of water by cisterns. J. of Air-Conditioning and Refrigerating, Vol., 2006; 21:4-13.

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[9] Saito A. Recent advances in research on cold thermal energy storage. Int. J. of Refrigeration, 2002; 25:177-189.

[10] Nelson JEB, Balakrishnan AR, Murthy SS. Parametric studies on thermally stratified chilled water storage systems. Applied Thermal Engineering, 1999; 19: 89-115.

[11] Nelson JEB, Balakrishnan AR, Murthy SS. Experiments on stratified chilledwater tanks. Int. J. of Refrigeration, 1999; 22: 216-234.

[12] Al-Marafie AMR. Stratification behavior in chilled water storage tank. Int. J. of Refrigeration, 1987; 10: 364-366.

[13] Suri RK, Al-Marafie AMR, Maheshwari, GP, Al-Juwayhel F, Al-Jandal S, Al-Madani K, Aburshid, H. Experimental investigation of chilled water storage technique for peak power shaving. Int. J. of Refrigeration, 1989; 12: 213-219.

[14] Borst WL, Aghamir F. Long-term Storage of Coolness. Proceedings of the 1980 AS/ISES Annual Meeting, Phoenix, Arizona, 1980; 908-912.

[15] Alizadeh Sh. An experimental and numerical study of thermal stratification in a horizontal cylindrical solar storage tank. Solar Energy, Sept. 1999; 66(6): 409-421.

[16] Gursu S, Sherif SA, Veziroglut TN, Sheffield J W. Analysis and optimization of thermal stratification and self-pressurization effects in liquid hydrogen storage systems. I: Model development. J. energy resour. technol., 1993; 115(3): 221-227.

[17] Pietrzyk JR, Hepworth HK. Analysis of thermal stratification in cryogen storage tanks under quiescent. micro-gravity conditions, AIAA, SAE, ASME, and ASEE, Joint Propulsion Conference, 27th, Sacramento, CA, June 24-26, 1991; 12.

[18] Kerkeni C, BenJemaa F, Kooli S, Farhat, A, Belghith A. Solar domestic hot water: numerical and experimental study of the thermal stratification in a storage tank. Environment and Solar, 2000 Mediterranean Conference, Beirut, Lebanon, 2000; 195-199.

[19] Bahadori M N, Haghighat F. Long-Term Storage of chilled water in Cisterns in Hot, Arid Regions. Building and Environment, effect on the stratification degradation. Energy, 1988; 23: 29-37.

[20] Metcalf & Eddy. Wastewater Engineer-ing: Treatment. Disposal & Revse, USA: Mc Graw Hill publishing company; 2003.

[21] Dehghan A. Energy and Exergy Analysis of a Seasonal Underground Cold Water Storage. 14th Annual (International) Mechanical Engineering Conference, Isfahan University of Technology, Isfahan, Iran, May 2006.

[22] Kumargarg s. Environmental Engineering: Water supply engineering. Vol.1, Delhi: Khanna publishers; 2002.

[23] Balance energy 2004, Power Ministry-energy assistance; winter 2004.

M.R. Khani (1), K. Yaghmaeian (2) and A.R. Dehghani (3), *

(1) Assistant Professor, Department of Environmental Health, School of Public Health, Islamic Azad University of Medical Sciences, Tehran, Iran

(2) Assistant Professor, Department of Environmental Health, School of Public Health, Semnan University of Medical Sciences, Semnan, Iran

(3) M.Sc Graduated, Department of Mechanical Engineering, Islamic Azad University, Science &Research Campus, Tehran, Iran

* E-mail:
Table 1: Time and height of discharge in cistern.

            The time of discharge       The height of
                 water in per       discharge water in per
Month             month(hr)                month(m)

June                 40                      1.37
July                 40                      1.37
August               40                      1.37
September            40                      1.37

Table 2: Temperature of outflow from tank and average of outside
ambient in length of operational time.

Temp          [T.sub.1]      [T.sub.2]      [DELTA]T
            ([degrees]C)   ([degrees]C)   ([degrees]C)


June            12.7           36.1           23.4
July            13.1           38.9           25.8
August          13.3           40.2           26.9
September       13.8           35.3           21.5


[T.sub.1], temperature of taking water from bottom of tank

[T.sub.2], average ambient temperature

[DELTA]T, difference between taking water temperature from bottom of
tank and average ambient temperature
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Author:Khani, M.R.; Yaghmaeian, K.; Dehghani, A.R.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
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