Y. Tripanagnostopoulos1, M. Souliotis1, R. Battisti2 and A. Corrado2

1Physics Department, University of Patras, Patra 26500, Greece
Tel/Fax: +30 2610 997472, e-mail:1 yiantrip@physics. upatras. gr,

2Dept of Mechanics and Aeronautics, Univ. of Rome “La Sapienza”, Rome 00184, Italy
Tel:+39 06 44585271, Fax:+39 06 4881759, e-mail: 2riccardo. battisti@uniroma1.it

Hybrid PV/T systems with air heat extraction are an alternative and cost effective solution to building integrated PV systems, because of their easier construction and operation. These systems are usually consisted of PV modules with air channel at their rear surface, where ambient air is circulating in the channel for PV cooling and the extracted heat can be used for building thermal needs. To increase the system thermal efficiency, an additional glazing is necessary, but it has as result the decrease of the PV module electrical output from the additional optical losses of the solar radiation. An extensive study on air cooled PV/T solar systems has been conducted at the University of Patras, where hybrid PVT/AIR prototypes have been experimentally studied in their standard form and also with a low cost modification. The methodology of Life Cycle Assessment (LCA) has been used to do an energetic and environmental assessment of the heat recovery system by the University of Rome “La Sapienza”, implementing a specific software for LCA, SimaPro 5.1. In this paper we provide electrical and thermal energy output results for PV and PVT/AIR systems, analyzing them with respect to their performance improvements and environmental impact, considering their construction and operation requirements.

PVT/AIR system concept

The temperature of PV modules increases due to the absorbed solar radiation that is not converted into electricity causing a decrease in their efficiency. This undesirable effect can be partially avoided by applying a heat recovery unit with a fluid circulation. In hybrid Photovoltaic/Thermal (PV/T) solar systems the reduction of PV module temperature can be combined with a useful fluid heating. Hybrid PV/T systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the absorbed solar radiation. These systems consist of PV modules coupled to heat extraction devices, in which air or water of lower temperature than that of PV modules is heated whilst at the same time the PV module temperature is reduced.

In PV/T system application electricity is of priority and therefore the operation of the PV modules at low temperatures keeps cell electrical efficiency at a sufficient level. This demand limits the effective operation range of PV/T system thermal unit in low temperatures and the extracted heat can be mainly used for low temperature thermal needs (space heating and natural ventilation of buildings, air or water preheating, etc).

Hybrid PV/T systems with air heat extraction (for simplicity PVT/AIR) are more extensively studied, mainly as an alternative and cost effective solution to building integrated PV systems, because of their easier construction and operation. In typical BIPV applications the increase of PV module temperature results to the increase of undesirable heat transfer to the building, mainly during summer. Air cooled hybrid PV/T systems are usually consisted of PV modules with air channel at their rear surface and usually ambient air is circulating in the channel for achieving both PV cooling and thermal energy output, which
can be used for building thermal needs. In PVT/AIR systems the thermal unit for the heat extraction, the necessary pump and the external pipes for air circulation constitute the complete system that extracts the heat from PV module and brings it to the final use. To increase the system operating temperature, an additional glazing is necessary, but it has as result the decrease of the PV module electrical output from the additional optical losses of the solar radiation.

Theoretical and experimental studies are referred to hybrid PV/T systems, with most of them including work on air heat extraction from the PV modules. Among the recent works we can notice the papers of Brinkworth et al (1997), on design and performance of building integrated hybrid PVT/AIR systems and of Hegazy (2000), who compares four PV/T air collectors. We also could refer the work of Eicker et al (2000), with the monitoring results from a BIPV PV/T system that operates during winter for space heating and during summer for active cooling and of Bazilian et al (2001) for the practical use of several PV/T systems with air heat extraction in the built environment. The building integrated PVs is going to be a sector of a wider PV module application and Lee et al (2001), Ito and Miura (2003) and Chow et al (2003), give interesting results on air cooled BIPV modules.

University of Patras has been involved in the research of PV/T systems with work on water and air cooled photovoltaics towards the increase of electrical and thermal output of BIPV PVT/AIR systems. The work aims to air heat extraction improvements with modifications in PVT/AIR systems (Tripanagnostopoulos et al, 2000, 2001a). In addition, improved PV/T systems with dual (air or water) heat extraction operation (Tripanagnostopoulos et al, 2001b) and modeling results confirming the improvements of a modified air cooled PV/T model (Tripanagnostopoulos et al, 2002a) are recently presented.

The electrical output of PV/T systems is of priority, as the cost of PV modules is some times higher than the thermal unit. The different performance of the two subsystems regarding temperature affects system cost and optimised modifications for both electrical and thermal efficient operation must be considered. The consideration of the environmental impact of PV modules by using Life Cycle Assessment (LCA) methodology has been presented for typical photovoltaic systems by several authors. LCA has been extensively used at University of Rome "La Sapienza”, starting with the PhD Thesis of Frankl (1996) on LCA for photovoltaic systems and followed by the study on the simplified Life-Cycle analysis in buildings (Frankl et al,1998), on the overview and future outlook of LCA for photovoltaics (Frankl, 2002) and also on the comparison of PV/T systems with standard PV and thermal systems confirming the environmental advantage of PV/T systems (Frankl et al, 2000).

In the present paper we give results for system energy performance and environmental aspects by the LCA method for standard PV and air-cooled PV/T systems. The work is based on the combined evaluation, for PVT/AIR systems, of both the "energy assessment”, that is experimentally investigated at the University of Patras, and the "environmental assessment”, in terms of LCA results, performed at the University of Rome with the aid of a specific software (SimaPro 5.1). These results are referred to typical PV modules and to glazed and unglazed PV/T solar systems for horizontal and tilted building roof installation, including also modified systems. The use of a booster diffuse reflector between the parallel rows for the horizontal installations is also suggested to increase the solar input to the PV and PVT/AIR systems and the corresponding results are presented. The calculated energy performance and the LCA results can be considered useful as guidelines for the application of the studied standard PV and PV/T systems as well as the modified ones.