Two types of ICS solar water heaters designed, constructed and tested. The systems consist of two cylindrical storage tanks, which are connected in series and ...
PERGAMON
Renewable Energy 16 (1999)665668
SOLAR ICS SYSTEMS WITH TWO CYLINDRICAL
STORAGE TANKS.
Y. Tripanagnostopoulos, M. Souliotis and Th. Nousia Department of Physics, University of Patras, Patra 26500, Greece Tel/Fax: +30 61 997472
ABSTRACT Two types of ICS solar water heaters designed, constructed and tested. The systems consist of two cylindrical storage tanks, which are connected in series and are horizontally incorporated in a stationary asymmetric CPC type mirror. The efficient operation of the systems is due to the thermal losses suppression of the two inverted cylindrical surfaces and the effective use of the two tanks during sunshine period. Low cost and durable materials are used to construct the systems. The mean daily efficiency and the thermal performance of the hot water storage during night are calculated from outdoor experimental data. The results show that the proposed KS systems are efficient and suitable for practical use as DHW systems. Q 1998 Elsevier Science Ltd. All rights reserved.
KEYWORDS Solar collectors; ICS systems; CPC collectors; DHW systems; Water heating; Hot water storage.
INTRODUCTION Low temperature water heating can be satisfied by using flat plate solar collectors, thermosiphonic systems and integrated collector storage (KS) units. Solar water heaters for domestic use is widespread in many counties and most of these systems are of thermosiphonic type. The development of efficient ICS systems is mainly focused to the sufficient heat preservation by using selective absotbers and transparent insulating materials (Schmidt et al, 1988, Schmidt and Geotzberger, 1990). In addition, most of the ICS units consist of cylindrical tanks and curved mirrors in order to collect greater ammount of solar radiation. In our laboratory we have studied ICS systems with one cylindrical absorber incorporated in a stationary curved mirror of CPC type to suppress thermal losses (TripanagnostoPoulos and Yianoulis, 1992). In this paper we present two ICS systems, that consist of two cylindrical tanks each. These systems are baaed on the following design principles: (i) use of two storage tanks for a sufficient water temperature stratification, (ii) achievement of thermal losses suppression by forming a hot air ttap space between the inverted cylindrical absorber surfaces and the curved mirror, (iii)&ective use of the non uniform distribution of solar radiation on the absorber surfaces and (iv) use of low cost materials.
0960-1481/99/!I--see front matter Q 1998 Elsevier Science Ltd. All rights reserved. PII: SO960-1481(98)00248-l
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Fig. 1
Cross section of the proposed KS systems DTS 1and DTS II
EXPERIMENTAL MODELS The cross section of each ICS system is shown in Fig 1 Both systems consist of two horizontal cylindrical tanks, Al and A2, a stationary asymmetric curved mirror, a transparent cover, thermal insulation at the external reflector surface and two side mirrors with external insulation. Each system has different optical efficiency and their tanks also have different thermal losses protection. The main mirror (ABCD) is of the same geometry for both systems and consists of a parabolic part (CD) and a circular part (BC), with focal length equal to f = [OC] = [OB] = R (circle radius). The mirror parts (EA), (EG) are of parabolic geometry and the parts (AB) are circular with radius R. In system DTS I (Double Tank System) the parabolic mirror (AE) contributes to the collection of a substantial part of the incoming solar radiation by the tank A2 In system DTS II, tank A2 is thermally insulated at the upper part and the parabolic mirror (EG) concentrates solar radiation on the upper absorber surface of tank Al. In order to obtain experimental ICS systems of practical size, the diameter and the length of each cylindrical tank were selected to be 0.26 m and 1.Ol m, respectively. Based on these dimensions, both units are of 107.24 It total water storage volume, 1.0 m hight and 0.69 m depth (including thermal insulation). The aperture area is I .04 m* for DTS I and 0.77 m2 for DTS II and the total water volume per aperture area is 103.11 It/m2 for DTS I and 139.27 It/m2 for DTS II. Possible commercial units could be longer (up to 2.0m) to enlarge the water volume up to 2 I5 It, which is an upper limit for usual DHW units The proposed ICS systems are designed to be cost effective for application in southern countries, by using low cost and durable materials, such as single glazing, polished stainless steel sheet for the mirrors and black paint for the absorbers. The design improvements balance the low optical and thermal efficiencies of the used materials A sufficient water temperature increase can be achieved by the effective use of the non uniform distribution of solar radiation on the tank A2 of DTS I, as almost 70% of the incoming solar radiation is concentrated on it. The water tanks are connected with a pipe, tank Al operates as preheater and tank A2 as main heater. The system DTS I collects greater part of the incoming solar radiation almost alI the year, while the system DTS 11 collects less because of the difference in their aperture area. Despite of this, a longer time of heat preservation is achieved by DTS II, because tank A2 is better insulated. During draw off operation, cold water enters the tank Al, the warmer water of Al enters the tank A2 and the hot water of A2 exits for use. The efficient operation of model DTS I is based on the higher water temperature increase during sunshine with a significant water stratification from the top of A2 to the bottom of Al. In model DTS II the increase of water temperature in both tanks is not like DTS I, but we can achieve an efficient heat storage by tank A2. That can be indused by a draw off operation where warm water from Al enters the tank A2
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RESULTS
The constructed experimental models DTS I and DTS II were tested in outdoor conditions for 24 hours operation without water draining As solar noon at Patras is at about 12:30, we considered daily operation from 6:30 to 18:30 and night operation from 18:30 to 6:30 of the next morning These twelve hours intervals were used to determine the thermal energy gain and the storage heat preservation ofthe systems by the parameters: mean daily efficiency nd and heat losses coeflicient during night Us (WX). The mean daily efficiency nd is obtained from the equation nd = Qu / Qs The useful thermal energy during daily operation of each system is Qu = MnCw(Tf - Ti), where Mw is the total water mass, Cw the specific heat of water and Ti, Tf are the initial (6:30) and final (I8 30) water mean temperature, respectively. The temperatures TI and Tf are derived from the water mean temperatures TI and T2 of the two tanks. The incoming solar energy Qs is calculated during the same I2 hours interval by using the equation Qs = ImAaAt, where Aa (m*) is the aperture area of each system and Im (W/m*) the mean solar radiation intensity in the I2 hours time interval At. The experimental values of nd can be determined as a lizrction of AT / lm (oCW_‘m2),where AT = (Ti + Tf)/2 - Ta.m In order to calculate the coefficient of thermal losses during night we use the equation: Us = (MwCn / At) In[(Ti - Tam) / (Tf - Ta.m)], where At is the I2 hours time interval from 18.30 to 6:30 next morning Ti and Tf are the water mean temperatures at the initial (18.30) and final time (6:30) and Ta.m the mean ambient temperature during the mentioned time period. The variation of the mean water temperatures Tl and T2 for the two storage tanks of each model during a 24 hours operation without water draining are shown in the diagram of Fig 2. These are calculated from 3 points inside the tanks (up, middle, down). In these diagrams are also shown the variation of mean total water temperature Tm, solar radiation intensity I, ambient temperature Ta and wind speed v. Comparing to the test results of the two systems (Fig. 2), we can see that DTS I has higher efficiency during day than DTS II. with faster increase in water temperature over the level of 40°C. We can also notice that during the daily operation the water temperature of the upper part of tank A2 is about 8 “C higher than the presented mean value T2 in Fig 2 and in case of water volume draining, the outlet water temperature is at a more effective level. Both systems were also tested with draw off operation, extracting 26 It of warm water from A2 at 9.30, 12:30, 15:30 and 18:30, entering the Al an equal volume of 20 “C cold water. The results from these experiments showed that the mean daily efficiency is increased, because tank Al has higher efficiency as it operates at lower temperature and the warmer water enters the better thermal protected tank A2. The diagrams of Fig.3 show the experimental results, which refer to the mean daily efficiency nd and the variation of thermal losses coefficient during night Us for both systems. From these diagrams we can see that the model DTS I has higher efficiency nd compared to DTS II, while DTS II keeps a better thermal preservation of hot water in the storage tanks. For both models, the water temperature increase is not hight during daily operation, because of the great ratio of water volume to the aperture area, mostly in DTS II. For a usual night operation without water draining, the values of the thermal losses coefficients for the same temperature difference AT and of the whole system are. DTS I: Us = 6.3 W/‘C (AT=28.3 “C), Us.1 = 3.1 WPC (AT]=23 9 “C); Us.2 = 3.2 (ATz=32.7 “C), DTS Il. Us = 5.6 W/Y (AT=28.9 “C); Us.1 = 3.4 W/Y (AT1=33 4 “C); Us.2 = 2 1 WPC (ATz=24 4 “C), where Us is the total thermal losses coefficient of each system, Us.1 and Us.z, are the thermal losses coetlicient of each tank (for ATI and AT2) and AT = Ti.m - Ta.m, the difference of the initial water temperature and the mean ambient temperature during night. From these values we can see that the tanks of DTS I have a similar thermal performance, but in DTS 11 tank A2 presents a better thermal preservation than tank Al at about 35%. A second glazing could result to a better performance and an electrical heater inside tank A2 could be useti~l in caSe of cloudy days or greater demand of hot water. The proposed ICS systems can be.improved by using mirrors of higher reflectivity and selective absorbers for a efficient operation under less favorable weather conditions.
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CONCLUSIONS Solar ICS systems consisted of an asymmetric CPC mirror and two cylindrical water storage tanks are presented. Experimental results showed that the proposed systems are efficient, regarding to the low cost used materials for the mirror and the absorber surface. In addition, they are suitable for practical use as DHW systems, mainly for applications in southern countries. ACKNOWLEDGMENT Financial support from the Ministry of Development of Greece is gratefully acknowledged REFERENCES Schmidt Ch., A. Goetzberger and J. Schmidt (1988). Test results and evaluation of integrated collector storage systems with transparent insulation. Solar Energy, Q, pp. 487-494. Schmidt C. and A. Goetzberger (1990). Single-tube integrated collector storage systems with transparent insulation and involute reflector. Solar Energy, g, pp. 93- 100. Tripanagnostopoulos Y. and P. Yianoulis (1992). Integrated collector storage systems with suppressed thermal losses. mg 48, pp. 3 I-43.
