Geothermal add-ons for heat pumps on the market today collect heat from the air or the ground. A new device from Wake Forest University uses a fluid that flows through a roof-mounted module to collect heat from the sun while an integrated solar cell generates electricity from the sun’s visible light.
A standard, rooftop solar cell will miss about 75 percent of the energy provided by the sun at any given time because it can’t collect the longest wavelengths of light — infrared heat. Such cells miss an even greater amount of the available daily solar power because they collect sunlight most efficiently between 10 a.m. and 2 p.m.
Tests of the solar-thermal device have shown 30 percent efficiency in converting solar energy to power. By comparison, a standard solar cell with a polymer absorber has shown no greater than 8 percent conversion efficiency.
The design of the new solar-thermal device takes advantage of this heat through an integrated array of clear tubes, five millimeters in diameter. They lie flat, and an oil blended with a proprietary dye flows through them. The visible sunlight shines into the clear tube and the oil inside, and is converted to electricity by a spray-on polymer photovoltaic on the back of the tubes. This process superheats the oil, which would then flow into the heat pump, for example, to transfer the heat inside a home.
Unlike the flat solar cells used today, the curve of the tubes inside the new device allows for the collection of both visible light and infrared heat from nearly sunrise to sunset. This means it provides power for a much greater part of the day than does a normal solar cell.
Because of the general structure and the ability to capture light at oblique angles, this is also the first solar-thermal device that can be truly building-integrated — it can be made to look nearly identical to roofing tiles used today.
The research team will build the first square-meter-size solar-thermal cell this summer, a key step in getting the technology ready for market.
Inverted tandem organic solar cells, consisting of two bulk heterojunction sub-cells with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) active layer and a MoO3/Ag/Al/Ca intermediate layer, have been presented and optimized. Indium tin oxide (ITO) modified by Ca acts as a cathode for electron collection and Ag is used as the anode for hole collection for the tandem device. A proper thickness of Ca (3 nm) forms a continuous layer, working as a cathode for the top sub-cell. MoO3 as the anode buffer layer prevents exciton quenching and charge loss at the anode side, which could result in increase in interfacial resistance. The variance of sub-cell thickness adjusts the optical field distribution in the entire device, facilitating light absorption and good current matching in both sub-cells. The optimal inverted tandem device achieves a maximum power conversion efficiency of 2.89% with a short-circuit current density of 4.19 mA/cm2, an open-circuit voltage of 1.17 V, and a fill factor of 59.0% under simulated 100 mW/cm2 (AM 1.5G) solar irradiation
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