Experimental study of CPC type ICS solar systems
Introduction
Domestic hot water (DHW) solar systems that can satisfactorily cover the needs of about 100–200 l per day of hot water in the low temperature range of 40–70 °C are the flat plate thermosiphonic units (FPTU) and the integrated collector storage (ICS) systems. Flat plate, CPC and evacuated tube collectors can be also used for the same purpose. Although ICS systems have simple construction, installation and operation, they are less applied than FPTU systems because the storage tank presents higher thermal losses during night or during the periods that solar radiation is not sufficient enough. This happens because thermal protection of their water storage tank is difficult, as total or significant part of the external surface of the tank is used for the absorption of solar radiation, contrary to that of FPTU systems, which are fully thermally insulated. Single or double-glazing, selective absorbing surface with evacuated tube (Mason and Davidson, 1995) and transparent insulating materials (Schmidt and Goetzberger, 1990) are used for the thermal protection of the exposed surface of the storage tank to solar rays of the ICS systems, while opaque thermal insulation can be placed only on their non-illuminated parts. Other techniques such as the use of tubular absorbers with baffle plate inside them (Kaptan and Kilic, 1996) or the use of vessel with inner sleeves (Smyth et al., 1999) improve the thermal performance of the ICS systems. On the other hand, ICS systems can be considered more aesthetically approved than FPTU systems because of their lower height and better harmonization to the surrounding architecture. In Fig. 1 we present the front side view of the usual FPTU system and an ICS system to show the difference in size and appearance, considering that the systems have the same storage volume and aperture area.
Thermal performance of ICS systems during both day and night depends on the design principles and the used materials, as well as the weather conditions. The improvement of the thermal performance of ICS systems is a long time research activity of our laboratory and we have proposed several new systems, based on specific design principles. The work carried out includes design, construction and test results of ICS solar devices that consist of horizontal cylindrical storage tanks combined with CPC type reflectors, aiming at achieving effective operation during the day and night. In these models, asymmetric CPC reflectors combined with storage tank of inverted absorbing surface reduce storage thermal losses and preserve the water temperature during night (Tripanagnostopoulos and Yianoulis, 1992). ICS systems with symmetric CPC reflectors combining with non-uniform distribution of solar radiation on its absorbing surface resulted to effective water temperature rise in the storage tank (Tripanagnostopoulos et al., 2002). In the same work ICS systems with asymmetric CPC reflectors and double horizontal cylindrical storage tank achieved satisfactory mean daily efficiency and effective water temperature stratification inside storage tank. Recently, several designs of symmetric and asymmetric CPC type ICS systems with improved thermal performance in heat preservation during the night have been also suggested (Tripanagnostopoulos and Souliotis, 2004a, Tripanagnostopoulos and Souliotis, 2004b, Tripanagnostopoulos and Souliotis, 2004c).
Regarding ICS solar systems consisting of a horizontal storage tank placed in symmetrical CPC reflector trough, we analytically studied the non-uniform distribution of the absorbed solar radiation on the cylindrical storage tank surface during annual operation of the device (Souliotis and Tripanagnostopoulos, submitted for publication). The results from a computational model that we developed and the experimental study of the prototype showed that the greater amount of the total incoming solar radiation on the aperture area is distributed on the upper part of the absorbing surface. This results to a satisfactory daily thermal performance of the ICS system, as the water temperature rise is correlated in a way to this distribution. In Fig. 2 we present the cross-section of the studied ICS system indicating the division of the total absorbing surface in three parts, where each of them contains an equal (1/3) volume of the total stored water.
In the present work we aim to improve the water temperature rise during daily operation and the water temperature preservation during night of ICS systems with similar geometry and effective operation with that presented in a previous work (Tripanagnostopoulos et al., 2002). We investigate the parameters of the ratio of the stored water volume per aperture area and that of the stored water volume per total surface area, taking into account the total water storage volume and the length of the ICS devices. For this purpose we experimentally compared three ICS systems with single horizontal cylindrical storage tank of different diameter inside symmetrical CPC type reflector trough. Temperature variation profiles of the mean storage water temperature, the mean daily efficiency and the thermal losses during night of all systems were compared, in order to find out the optimum system size. Considering the above we constructed two new ICS devices of different absorbing surfaces, reflectors and transparent covers, but of the same storage tank diameter. The extended experimental study of these ICS models includes temperature variation profiles during 24 h and four days operation and calculation of their mean daily efficiencies and also thermal loss coefficients during night. The derived results are estimated and compared to the performance of two FPTU systems (with mat black and selective absorbing surfaces), to confirm the improvement of the suggested ICS systems.
Section snippets
Design and construction of ICS systems
For specific design of ICS systems, among the main parameters are the stored water volume, VT (l), the ratio of the stored water volume per aperture area, VT/Aα (l m−2) and the ratio of the total stored water volume per total (including aperture) system external surface, VT/AS (l m−2). The water temperature rise during day is correlated to the amount of the incoming solar radiation that depends on the aperture area Aα and in a degree to the jacket area of the ICS system AS, which affects its
Construction details of the systems
From the above experimental results of the tested ICS systems (including diagrams of Fig. 4), we estimate that system ICS-2 behaves better than the rest regarding its thermal performance during daily and night operation. The size of the storage tank of this system results to satisfactory increase of the mean water temperature during day and to sufficient preservation of the hot water during night and also contributes to a moderate system depth. Taking into account these results we attempt to
Conclusions
In this paper we present the design principles and the construction details of ICS solar systems, which consist of single storage tank, properly placed in symmetric CPC type reflector trough. We investigated the thermal behavior of three ICS systems for storage tanks of different diameters with regard to the total stored water, VT, the ratios of the stored water volume per aperture area, VT/Aα and that of the stored water volume per total surface area, VT/AS. We experimentally studied these ICS
Acknowledgements
The authors would like to thank the Greek solar collector Company, SAMMLER, supplier of the ICS storage tanks.
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