Source：pv-magazine

Scientists have used stearic acid and a combination of stearic acid and carbon black to increase thermal conductivity in a photovoltaic-thermal system linked to a heliostat field concentrator. From a temperature of 30 C at the inlet, they were able to heat the water to up to 59 C.

A heliostat experimental station in France

Image: David66, Wikimedia Commons

Researchers from Bangladesh’s Rajshahi University of Engineering & Technology have constructed and tested a heliostat field-integrated photovoltaic-thermal (PVT) system supported by two different organic phase change materials (PCMs) to enhance its thermal conductivity.

PCMs are able to absorb, store and release large amounts of latent heat over a defined temperature range and have often been used, at the research level, for PV module cooling.

“Organic PCMs are well suited for solar energy storage because of their small freezing and melting temperature range, and their congruent melting. Most organic PCMs are also readily available, relatively cheap and nontoxic,” explained the group. “In the context of a solar domestic hot water system, careful PCM selection is crucial to ensure desired water temperatures and address safety concerns like potential PCM leakage into the building’s water supply.”

The experimental setup was built on a roof in the Rajshahi district of Bangladesh, which has an average daily irradiance of 6.39 kWh/m2 and a wind speed of 2 m/s. A leakage-free PVT collector was constructed, and four PV panels were attached to it. Each panel had a power output of 5 W, an open circuit voltage of 21.5 V, and a short-circuit current of 0.32 A. A copper tube was used to pass the water through the collector from a storage tank equipped with a centrifugal pump.

“To increase the concentration ratio, the PVT system is integrated with a heliostat field concentrator,” explained the academics. “In the designed heliostat field, there are 20 heliostats made of plane mirrors, each having an area of 0.12637 square meters. The height of the central tower is 1.8288 meters from the ground. The heliostats are placed in three different rows.”

As for the PCMs, the scientists analyzed two options. One was stearic acid (SA) only, at a mass of 16.5 kg; and the other was a mixture of 16.5 kg of SA, with 165 grams of carbon black (0.01% of the entire composition). Both compositions were analyzed on different days, from 10:00 to 18:00, each with a water flow rate of 0.0025 kg/s and 0.0035 kg/s. 0.0045 kg/s and 0.0055 kg/s.

“Employing PCM with or without carbon black has little influence on electrical efficiency, which ranges from 10.50% to 13%,” the academics explained. “The maximum electrical output power was found to be 17.77 W at the water flow rate of 0.0045 Kg/s for the system PVT-PCM with carbon black. The increased water flow rate reduced the cell temperature by transferring more heat from the system. Which ultimately increased the electrical output temperature of the PV cell as well as the electrical efficiency.”

As for thermal efficiency, the highest level without carbon black was 45.45%, while 46.56% was achieved with the compound. The highest outlet temperatures reached a flow of 0.0035 kg/s and the water reached 59 C with the compound, and 58 C without it. “Compared to the other flow rates studied, the average thermal efficiency shows a significant increase of 6.83 % when integrating carbon black with PCM at a flow rate of 0.0035 kg/s,” highlighted the group.

The system was introduced in the study “Performance test of a heliostat field integrated PVT solar collector using organic phase change material and carbon black additives,” published on Energy Reports.