Hybrid system combining CPV, ionic thermocells achieves 49.63% energy efficiency

A group of scientists from the North China Electric Power University developed a hybrid energy system combining concentrated photovoltaics (CVP) and liquid ionic thermocells (iTECs).

“The application of ionic thermocells in the full-spectrum solar cascade utilization of concentrated photovoltaic systems is proved to be feasible with significant advantages of low cost, high performance, and flexible operation,” the research's lead author, Zhuo Liu, told pv magazine, noting that iTECs were used in alternative to commonly utilized thermoelectric generators (TEGs). “During comparison, it is clear that the iTEC with the dual capabilities of heat-to-electricity conversion and cooling PV can significantly simplify the overall structure of the hybrid system and save cost by eliminating the expensive TEG, which is promising for practical application.”

TEGs can convert heat into electricity through the “Seebeck effect,” which occurs when a temperature difference between two different semiconductors produces a voltage difference between two substances. The devices are commonly used for industrial applications to convert excess heat into electricity. However, their high costs and limited performance have thus far limited their adoption on a broader scale.

iTECs can also convert low-grade heat into electricity and, according to the researchers, are less expensive than TEGs. They also have an ionic Seebeck coefficient that is high enough to ensure efficient harvesting of waste heat and cooling PV in CPV systems. “Even though the thermoelectric properties of iTEC may decrease due to the lower temperature difference as heat transfer increases, the overall electric and thermal performance of CPV-iTEC hybrid system would be improved due to the higher power of PV with lower temperature,” they explained.

The CPV-iTEC hybrid system consists of a Fresnel lens, a photovoltaic cell, and an iTEC with electrolyte flow. It uses the waste heat generated by the PV unit to produce additional power via the thermogalvanic effect of iTECs, which in turn increases the overall system's output. The iTEC is attached to the back of the PV cell by thermal grease with high thermal conductivity

“The total power of CPV-iTEC hybrid system is mainly composed of the electric power of PV and iTEC, and thermal power of flowing electrolytes in iTEC,” the academics stressed. “The output power of iTEC is dependent on the channel structure and electrolytes/electrodes properties.”

The group compared the performance of the hybrid system with a reference system using a TEG generator instead of iTECs. The latter used, for the cooling of the PV cell, a water heat sink attached via thermal grease to its backside.

During experimental and numerical analyses, the scientists found that the presence of iTEC can significantly decrease the photovoltaic panel temperature from 351.30 K to 325.14 K, and thus broaden the available concentration ratio from 1~21 to 1~39 compared with typical CPV-TEG, reaching a higher output power of 16.61 W.

“During the optimization of advanced redox electrolytes/electrodes, the energy efficiency can reach 49.63% at 21 concentration ratios, which is 5.06% higher than that of CPV-TEG, and show a 22.65%lower overall cost,” Liu stated. “It can overcome, to some extent, the technical bottlenecks of high cost and low efficiency in current CPV-TEG hybrid systems. It can be extensively applied in energy supply stations such as distributed cogeneration, and concentrated photovoltaic power stations.”

The system was introduced in the paper “A novel concentrated photovoltaic and ionic thermocells hybrid system for full-spectrum solar cascade utilization,” which was recently published in Applied Energy.