Y. Tripanagnostopoulos

Hybrid photovoltaic/thermal solar systems

We present test results on hybrid solar systems, consisting of photovoltaic modules and thermal collectors (hybrid PV/T systems). The solar radiation increases the temperature of PV modules, resulting in a drop of their electrical efficiency. By proper circulation of a fluid with low inlet temperature, heat is extracted from the PV modules keeping the electrical efficiency at satisfactory values. The extracted thermal energy can be used in several ways, increasing the total energy output of the system. Hybrid PV/T systems can be applied mainly in buildings for the production of electricity and heat and are suitable for PV applications under high values of solar radiation and ambient temperature. Hybrid PV/T experimental models based on commercial PV modules of typical size are described and outdoor test results of the systems are presented and discussed. The results showed that PV cooling can increase the electrical efficiency of PV modules, increasing the total efficiency of the systems. Improvement of the system performance can be achieved by the use of an additional glazing to increase thermal output, a booster diffuse reflector to increase electrical and thermal output, or both, giving flexibility in system design.

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.

Integrated collector-storage systems with suppressed thermal losses

Abstract Stationary concentrating solar devices, of the integrated collector and storage (ICS) type, that consist of a cylindrical tank placed horizontally in a properly shaped, curved mirror envelope, were designed. Prototype models were constructed, tested, and compared with conventional solar water heaters. In this article we describe the design of the ICS solar units with respect to the minimization of thermal losses from the absorber to the ambient. Experimental results were derived from the constructed models, a conventional concentrating stationary ICS prototype, and flat plate collectors of the thermosiphon type. Finally some conclusions regarding the practical application of the integrated units are presented.

CPC type integrated collector storage systems

Integrated collector storage (ICS) solar water heaters with stationary compound parabolic concentrating (CPC) reflectors are designed and test results are presented. The systems consist of single and double cylindrical horizontal tanks properly placed in truncated symmetric and asymmetric CPC reflector troughs. The suggested designs aim to achieve low cost systems with improved performance by the reduction of their thermal losses and the increase of water temperature rise by using the non-uniform distribution of solar radiation on the absorber surface. Four experimental models were constructed and tested outdoors to determine their mean daily efficiency and thermal losses during the night. Test results showed that asymmetric CPC reflectors contribute to lower thermal losses and the two connected in series cylindrical storage tanks result in effective water temperature stratification. The system with the single cylindrical storage tank and the symmetric CPC reflector performs satisfactorily during the day as well as during the night and regarding its simpler design it could be considered cost effective among the studied ICS systems. A typical thermosiphonic system with flat plate collector was tested for performance comparison, by which the improved daily efficiency of ICS systems and also their moderate water storage heat preservation during the night were confirmed.

CPC Solar Collectors With Flat Bifacial Absorbers

Abstract The design, construction and test results of non-evacuated stationary CPC solar collectors with flat absorbers are presented and discussed. The proposed collector design is based on a truncated asymmetric CPC reflector, consisting of a parabolic and a circular part. A flat bifacial absorber is installed at the upper part of the collector, parallel to the glazing to form a thermal trap space between the reverse absorber surface and the circular part of the mirror. Two prototypes based on the same collector geometry were constructed and tested. The first model consists of one mirror–absorber unit and the second of three smaller units integrated in one collector device. The truncated CPC mirror and the installation of the absorber parallel to the glazing keep the optical efficiency at a satisfactory level. The reduction of radiative thermal losses by using selective absorbers and the suppression of convection thermal losses from the reverse absorber surface to the collector cover result to a significant decrease of the total collector thermal losses. The experimental results showed that the proposed CPC collector could achieve a maximum efficiency of 0.71 and a stagnation temperature of about 180°C, with the multiunit collector device being more efficient and practical.

ICS solar systems with horizontal (E-W) and vertical (N-S) cylindrical water storage tank

Cylindrical type storage tanks are used in most commercial ICS systems, as they resist to the pressure of water mains. ICS systems with one or two cylindrical storage tanks combined with curved reflectors are cost effective devices and can be easily used in domestic solar applications. In this paper we compare ICS solar systems with cylindrical water storage tank and different mountings of it in a symmetric CPC or involute reflector trough. The design, construction and experimental results from four ICS systems regarding water temperature profile, mean daily efficiency and thermal losses during night are presented and discussed. The experimental models are tested outdoors under the same weather conditions and without water drainage for a sequence of 3 days, starting the first day with cold water. During tests we have recorded solar radiation, ambient temperature, water temperature in different locations inside the storage tank and also wind speed. The results show that during the day the ICS systems with a horizontal tank present a satisfactory performance in water temperature rise and heat preservation in water storage tank during night. Regarding reflector types, the ICS models with CPC reflectors achieve higher mean daily efficiency, while models with involute reflectors present high efficiency only in low operating temperatures.

Integrated collector storage solar systems with asymmetric CPC reflectors

New types of ICS solar systems were designed and outdoor tests of experimental models were performed. The systems consist of single cylindrical horizontal water storage tanks placed inside stationary truncated asymmetric CPC reflector troughs of different design. We used high emittance absorber surface, low cost curved reflectors, iron oxide glazing and thermal insulation at the non illuminated tank surfaces, aiming towards cost effective ICS systems with satisfactory heat preservation during the night. Four experimental models of different designs were constructed and tested to determine their performance regarding their mean daily efficiency and thermal losses during the night. The new ICS systems were compared to an ICS system with symmetric CPC reflectors of similar construction and dimensions and also to a typical Flat Plate Thermosiphonic Unit (FPTU). Test results showed that the ICS systems with asymmetric CPC reflectors present almost the same mean daily efficiency and better preservation of hot water temperature during the night, compared to the ICS system with the symmetric CPC reflectors. The comparison with the FPTU system confirmed the satisfied daily operation of all ICS systems and their moderate storage heat preservation during the night. Theoretical results showed acceptable thermal performance of all ICS systems regarding annual operation.

Cost effective asymmetric CPC solar collectors

Low cost CPC solar collectors were designed, constructed and tested. The collectors consist of two separate absorbers, which are horizontally incorporated in a stationary asymmetric CPC mirror. The efficient operation of the proposed collectors is due to the direct absorption of a large part of the incoming solar radiation and to the thermal losses suppression by the inverted surface of both absorbers. Two collector types with the same basic design are presented. The first type has tubular absorbers which are used for direct water heating and the second has flat fin type absorbers with pipe. Test results showed that the proposed collectors operate efficiently and are suitable for hot water applications.

Solar ICS systems with two cylindrical storage tanks

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 system 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 ICS systems are efficient and suitable for practical use as DHW systems.

CPC solar collectors with multichannel absorber

Abstract Stationary CPC solar collectors with symmetric and asymmetric mirrors have been studied and tested. In these collectors the distribution of solar radiation on the absorber surface is not uniform, with areas of low and high intensity, variable and depending on the position of the Sun relative to the collector. This effect can be used to improve the efficiency of the collector if the absorber is flat and divided in several sections, which form the channels of fluid flow. By selecting the channels with the higher radiation density, the working fluid can be heated to higher operating temperatures. Here we present work on the optical analysis and experimental tests on models consisting of four channel absorbers. The results show that efficient operation at temperatures in the range of 100–200°C can be achieved with relatively simple types of concentrating solar collectors.