The improvement of photovoltaic and galvanic cell efficiency is a topical issue in the field. This article presents a method to evaluate photovoltaic and galvanic cell efficiency with greenTEG’s heat flux sensors.
Galvanic vs. Photovoltaic cell efficiency
Definitions and working principles
Thermophotovoltaics convert infrared wavelength light to electricity via the photovoltaic effect, and allows the development of new techniques to energy storage and conversion that employ higher temperature heat sources than the turbines that are ubiquitous in electricity production today. [1]
Thermogalvanic cells are devices that typically comprise of two electrodes at different temperatures in contact with a solution or gel that contains both oxidation states of the same redox couple. [2]
With the existence of a temperature gradient (ΔT) across the two electrodes, a difference of potential (ΔV) between the electrodes is generated. As a result, redox processes are triggered, a flow of electrical current is obtained, and the conversion of a temperature gradient into electricity. [2] For both, thermophotovoltaics and thermogalvanic cells, the efficiency remains the biggest challenge for the field. Unlike a solar cell, a thermophotovoltaic cell can preserve and later convert the energy in sub-bandgap photons. Indeed, sub-bandgap photons can be reflected back to the emitter by the cell, the energy is preserved through reabsorption by the emitter. [2]
With both reflected and reabsorbed lights, the emitter remains hot. Consequently, the energy input required to heat the emitter is drastically reduced. For thermophotovoltaics cells, the efficiency can be defined as follow:
With:
- Pout , the electric power generated by the cell.
- Qc, The total heat absorbed and generated in the cell.
- The net energy received by the cell is equivalent to Pout+Qc and can also be expressed as Pinc−Pref, where Pinc is the incident energy and Pref is the reflected energy.
- ηTPV, the efficiency which is the conventional metric used to describe the performance of a cell–emitter pair.
For thermogalvanic cells, the electricity production is driven by entropy, and the magnitude of the driving force is characterized by the thermogalvanic Seebeck coefficient, Se.
With:
- ΔSrc is the difference in entropy between the two redox states.
- n is the number of electrons transferred.
- F is the Faraday constant.
- Se represents the possible potential difference.
Thus, for both type of cells, temperature gradient and resulting heat flux play an important role in the efficiency.
The importance of heat flux for photovoltaic cell efficiency evaluation
Heat flux measurement to perform cell efficiency evaluation
For thermophotovoltaic cell efficiency evaluation, it is required to directly measure the power output Pout and the heat generated in the cell, Qc .
For thermogalvanic cell efficiency calculation, the absolute efficiency, η can be defined as the proportion of thermogalvanic electrical power generated by the cell (pmax) from the corresponding heat flux passing into the cell (q): [2]
Do you want to determine cell efficiency with our heat flux sensors?
Our experts will be happy to guide you through the choice of your sensor.
Materials & Methods
This section presents the heat flux sensor required to perform cell efficiency are described.
The Heat Flux Sensor – gSKIN®-XP is suited for a broad range of application from R&D, thermal optimization, energy efficiency, industrial monitoring of thermal properties.
Advantages
- High-sensitivity to size ratio
- Short response time
- Long thermoresistance
- Reliability and reproducibility
Example 1: Thermophotovoltaic efficiency of 40%
In this first example, Lapotin et al., (2022), broke the current World Record for Thermophotovoltaic cell efficiency, reaching 40%! To measure this, a heat flux sensor, model gSKIN XP, was placed between the cell and the heat sink to measure Qabs.
Thermally conductive adhesive tape kept the sensor on the heat sink, and thermal paste provided thermal contact between the cell and the sensor.
An example of the results obtained is shown in the figure below:
Example 2: Direct measurement of the genuine efficiency of thermogalvanic heat-to-electricity conversion in thermocells
Trosheva et al. (2022) employed a dedicated thermogalvanic cell to interface with the heat flux sensor gSKIN-XP26 9C calibrated. [2]
Reproducible and reliable measurement were obtained thanks to the following experimental setup:
Do you want to measure reliably cell efficiency with heat flux sensor?
Our experts will be happy to guide you through the choice of your sensor.
References
[1] LaPotin, A., Schulte, K. L., Steiner, M. A., Buznitsky, K., Kelsall, C. C., Friedman, D. J., … & Henry, A. (2022). Thermophotovoltaic efficiency of 40%. Nature, 604(7905), 287-291.
[2] Trosheva, M. A., Buckingham, M. A., & Aldous, L. (2022). Direct measurement of the genuine efficiency of thermogalvanic heat-to-electricity conversion in thermocells. Chemical Science, 13(17), 4984-4998.
