Ph.D Thesis

Ph.D StudentKruger Nimrod
SubjectPhoto-Luminescence and Thermal Devices for Solar
Energy Harvesting
DepartmentDepartment of Energy
Supervisor ASSOCIATE PROF. Carmel Rotschild
Full Thesis textFull thesis text - English Version


Solar energy, as the fastest growing industry in the energy economy, has far reaching effects on the power grid in many countries. This development of the last few years is owed to the drastic reduction in manufacturing cost of PV cells. But with the growth of solar energy production, the challenges it poses surface. Among these are the typical low conversion efficiencies, leading to excessive land usage by utility scale PV field, and the intermittent nature of solar power, posing a challenge when fitting energy supply and demand.

In this work we present two novel power conversion concepts. The first, thermally enhanced photoluminescence (TEPL), offers a device with higher conversion efficiencies than commonly used PV cells. We learn that photon rate in photoluminescence (PL) is conserved with temperature increase, while the emission spectrum is blue-shifted. Such endothermic-PL is an ideal optical heat-pump that generates orders of magnitude more energetic photons than thermal emission at similar temperatures. This effect is used in designing a PL absorber coupled to a PV cell where the solar heat is used to up-covert the PL emission. The up-conversion enables operating a high band-gap PV cell with higher operating voltage; extracting more power compared to a single junction PV cell exposed to the direct sunlight. We show here theoretical efficiencies can reach up to 70%, while practical estimations reach 45%.

The second device presented in this work is the luminescent solar power (LSP). Here a photoluminescent (PL) absorber under concentrated sunlight excitation enables the spatial separation between heat and free energy, allowing for simultaneous steam powered and PV powered electricity generation. With the fitting PL absorber-emitter material we tailor the transmitted spectrum toward a PV cell, maximizing electric power output while minimizing parasitic heat in the PV cell. Ideally, the excessive heat of the high energy photons and the unusable infra-red spectrum energy are both absorbed in the PL material and the accumulated heat is conveyed for powering a heat engine cycle. We detail the concept for LSP energy production and the requirements for application, and point-out the benefits of this idea, both in engineering and economical aspects. Remarkably, LSP technology has the potential to reduce the cost of storable utility-scale solar power from $0.06/kWh to below $0.04/kWh.

The PL-absorber material is the heart of both these concepts. Here we solve the issue of identifying the right class of materials for these applications by exploring the optical properties of YAG doped with different optically active elements, such as Chromium, Cerium, Neodymium and Ytterbium. We find their luminescent efficiency is not strongly dependent on temperature and that no adverse effects on PL are evident at temperatures of up-to 600oC. These finding pave the way for further research on both TEPL materials, and LSP absorbers. For the latter we predict fast adoption by the solar energy market as cheap and storable solar power may have disruptive implications and usher the era of cheap and abundant solar power.