Photovoltaic–thermal solar energy collectors based on optical tubes

Abstract

In this work, we examine the use of oil filled tubular optical concentrators coupled with a model organic bulk heterojunction photovoltaic: poly-3-hexathiophene-[6,6]-phenyl-C61-butyric-acid methyl-ester (P3HT:PCBM) to create a photovoltaic–photothermal hybrid solar collector. The organic photovoltaic cells were fabricated onto one half of a tubular light pipe and then silicone oil was flowed inside the pipe. This allows solar energy in the visible wavelengths to be effectively converted into electricity by photocell while simultaneously; the silicone oil captures the infrared radiation (IR) part of the spectrum as heat energy. The VIS–IR power conversion efficiency for this model organic system, under normally incident AM1.5G illumination was found to be: PCE ∼ 28%, which is combined by the photovoltaic efficiency (PCE ∼ 2%) and the photothermal efficiency (PCE ∼ 26%). We further show that the oil filled tube, acts as a passive optical element that concentrates the light onto the photovoltaic and thereby increases its overall efficiency but also the range of incident angles in which the light is efficiently captured.

Introduction

The total number of photons that can be captured by a photovoltaic, and thus its resulting power conversion efficiency (PCE) depends on the threshold photon energy below which electricity conversion does not take place, i.e. the bandgap of the photoabsorber. So long wavelength photons typically dissipate their energy as heat in photocells instead of generating power. This brings other undesirable consequences to the solar cells as well such as: a drop in the cell efficiency due to the added heat and even permanent structural damage of the cells (Garg et al., 1994). To better harness the solar energy, it is necessary to collect the solar energy in the long-wave band of the sun’s spectrum. However, trying to use more and more narrow bandgap semiconductors to absorb longer and longer wavelength light results in diminishing voltage output for the solar cell. Thus, the most obvious way to harness this IR solar energy is to transform this part of the spectrum into heat and collect that heat, i.e. photo-thermal conversion (Davies and Luque, 1994). At present, there are a number of efforts which have demonstrated combined photovoltaic–thermal (PV/T) solar energy collectors, but these have been based primarily on planar geometries and silicon absorbers (Charalambous et al., 2007, Chow, 2010, Garg et al., 1994, Kumar and Tiwari, 2008, Singh and Othman, 2009, Davies and Luque, 1994).

These first generation hybrid photovoltaic–thermal solar energy collectors have resulted in some limited success, but their architectures have not been optimized for either solar or thermal collection of energy. That is, such PV/T devices are typically designed in a planar geometry, yielding relatively low combined photovoltaic/thermal conversion performance. In this work, we examine the use of optical tube concentrators, around which a photovoltaic cell has been added. The resulting hybrid photovoltaic–thermal solar energy collector geometry allows for optimized heating of a thermal fluid added to the tube and for concentration of the light onto the photovoltaic which yields great photovoltaic efficiencies. Our demonstration utilizes a solution-processed bulk heterojunction (BHJ) solar cell fabricated on one half of the tubes circumference and silicone oil contained inside the tubes as the photothermal collector. To fabricate these devices, we used a common absorber system: poly-3-hexyl-thiophene and phenyl-C61-butyric-acid methyl-ester (P3HT:PCBM) as a model absorber, which has been successfully applied to the fiber-based and tube-based solar cells recently (Liu et al., 2007, Li et al., 2010a, Li et al., 2010b). Thus, in these organic-PV/T (OPV/T) devices we utilize the advantages of organic photovoltaic (OPV) materials such as low cost, conformal flexibility, and abundant availability (Lewis, 2007). In order to better harness the utilization of the thermal fluid, we use a relatively smaller tube (1.5 mm I.D.) to get a higher ratio of cross-sectional area to volume. So in this case we build the OPV device directly onto the tube rather than adding a roll-to-roll (R2R) producible flexible OPV with an adhesive (Krebs, 2009, Krebs et al., 2009, Krebs et al., 2010a, Krebs et al., 2010b). However, we find that this architecture further exhibits an increase in the performance of the P3HT:PCBM cell due to the concentrator effect of the tube acting as a focusing lens. Therefore, the combined power collection for this system; which is essentially an organic photovoltaic, is surprisingly high (PCE ∼ 28%) compared to around 5% for the P3HT:PCBM system (Kim et al., 2007). We suggest that this approach may provide a more rapid approach to commercial viability of organic photovoltaics.