A Singapore–invented micro-thermophotovoltaic generator holds promise as a reliable and versatile portable energy source, particularly when packed modularly into a power plant.

By S K CHOU, Wenming YANG, Zhiwang LI and Jun LI

Demand for clean and renewable energy sources has increased significantly in the past decade. Recent advances in autonomous robots and the proliferation of personal electronic devices have created strong demand for better and more powerful portable power sources.

The present scenario, marking the limitations of the traditional rechargeable battery, offers an enormous opportunity for new power technologies and generation systems. Power microelectromechanical systems (MEMS), a class of micro-power devices, can satisfy the need for small-scale and high-energy-density power sources.

In 2001, a group of researchers at the Department of Mechanical Engineering (ME), National University of Singapore (NUS), initiated a study of a new power-MEMS concept based on a thermophotovoltaic (TPV) system. Figure 1 shows the schematic of the micro-TPV power generator and its working mechanism. The power generator uses photovoltaic (PV) cells to convert heat radiation obtained from the combustion of hydrogen or hydrocarbon fuels into electricity.

Figure 2 illustrates that the energy density stored in hydrogen or hydrocarbon fuels exceeds that of batteries on a unit-mass basis. Micro-TPV applications require high output and uniform temperature along the wall of the micro-combustor.

The micro-TPV power-generator design fully utilises the high surface-to-volume ratio of the micro-combustor, thus maximising the power density of the system. According to the cubic-square law, any decrease in combustor size will bring about a drastic increase in the surface-to-volume ratio; therefore, at any given fuel-consumption rate, reducing combustor size directly increases the energy density of the power plant. The potential gain from miniaturisation makes the development of micro-TPV power generators particularly attractive.

The prototype micro-TPV power generator developed at ME comprises a cylindrical silicon-carbide (SiC) micro-combustor, a nine-layer dielectric filter, and a gallium-antimony (GaSb) photovoltaic cell array. The power generator has a volume of about 3cm3 (Figure 3) without the cooling fins. The micro-SiC combustor, which acts as the radiation emitter, burns a mixture of hydrogen and oxygen. As the emitter heats to a high temperature, it gives off a stream of photons that impinge on the PV cell array, thus producing electricity.

Test results show that the micro-TPV generator holds promise as a reliable and versatile portable power source. The system, packed into a space approximately 3cm3, has no moving parts and the user can fabricate and assemble it relatively easily. As a result, the new energy source offers an attractive alternative for portable power in a range of military, security, lifestyle, and personal applications. Given that the micro-combustor only measures 0.113cm3 in volume, much scope in exploiting its power density for practical deployment exists.

The modularisation of the micro-TPV power plant constitutes an important development. Figure 4 shows the schematic of one such hypothetical power plant — in size a fraction of a conventional power generator using the same principle. It consists of a series of planar micro-combustors emitters, filters, and PV cells. The number of micro-TPV generator units packed into a power plant would depend on the specific power requirement. The proposed package decreases heat loss effectively and facilitates plant cooling. The design also reduces the complexity of fabrication and assembly as it only employs planar PV cells.