Hybrid and Fully Thermoelectric Solar Harvesting

The main topics covered in this book will be introduced. An overview of the historical trend of energy consumption over the last one hundred years will show the crucial need for renewable sources progressively replacing fossil and nuclear power supply. Among renewables, solar harvesting is surely the most promising technology, already playing a significant role in the global power landscape. Demand for higher efficiencies and lower power costs may open yet partially unexplored paths where PV modules are paired to ancillary harvesters to improve the usability of solar power, which will be the main focus of this book.

This chapter is devoted to an analysis of the physics behind the conversion efficiency of thermoelectric generators. After recalling the basic theory of linear irreversible thermodynamics of thermoelectricity, we will focus on the materials and device factors ruling the conversion efficiency of thermoelectric generators. Moving from the well–known Ioffe–Altenkirch formula, the efficiency in the constant–property limit will be comparatively analyzed under Dirichlet and Neumann boundary conditions. Efficiency will be then reconsidered when large temperatire differences are applied, using both Snyder’s concept of compatibility and Ren’s engineering figure of merit. Perfect thermoelectric generators as instances of exo– and endo–reversible engines will also be briefly reviewed along with the yet widely unsolved problem of thermoelectric efficiency under transient conditions.

In this chapter we will present the full-thermal approach to thermoelectric solar harvesting. Analysing the state of the art of this field we will report on its historical development, showing its advantages. Technical and technological issues solved and yet to be solved will be addressed as well. Starting from a description of the main system components we will analyse the literature and the strategies reported so far. Then we will discuss how a solar thermoelectric genenerator (STEG) may be modeled, quantitatively predicting their final efficiency. This analysis will show which are the main parameters influencing STEG performances, suggesting which are the best solutions to achieve efficiencies competitive with other solar strategies.

The most common and efficient way to covert solar power into useful work is by photovoltaic generation. Photovoltaic cells are devices that convert radiative energy into electric energy. This chapter outlines the mechanism of photovoltaic conversion. The physical principles are introduced and described, and their implementation in real devices (cells and modules) is discussed with reference to the so called three solar cell generations, namely bulk cells, thin film cells, and cells based on dye sensitization. The role played by materials in each cell generation is also examined.

This chapter is devoted to provide the general theory describing the hybridization of solar cells with thermoelectric generators. Moving from a description of the system, its main components will be introduced and analysed. Their characteristics and their impact on the final system efficiency will be scrutinised. Specifically, the heat generation within solar cells will be detailed considering the main losses occurring in a PV cell. This will bring to an evaluation of the temperature sensitivity of solar cells, which is one of the most important parameter to be considered when pairing PV cells and TEGs. In addition, we will introduce the concept of fully hybridized systems, where the thermoelectric and PV devices are both thermally and electrically connected to each other.

This chapter is dedicated to present the state of the art of hybrid photovoltaic–thermoelectric generators based on either organic or inorganic photovoltaic cells. Present challenges and future perspectives of this approach to energy harvesting will be discussed with a special emphasis on materials issues. It will be seen that both classes of PV materials deserve attention in view of applications in hybridized converters, although absorber stability and degradation of its PV efficiency with increasing temperatures sets limitations to currently achievable efficiencies, also in view of the still low efficiency of thermoelectric stages.

In this chapter, we will describe triple cogeneration technologies for solar conversion. The costs of solar conversion technologies are determined by the efficiency of power conversion, the lifetime and reliability of its components, the cost of the raw materials, potentially including storage, and any fabrication or construction required. Recently, photovoltaics and solar thermal have emerged as viable candidates for low cost power production; they each have losses that vary across the solar spectrum, with realized and theoretical efficiencies that are well below fundamental thermodynamic limits. Thus, it is desirable to split the solar spectrum to utilize both technologies in parallel over their respective optimal wavelength ranges. This chapter will present promising triple co-generation solutions that have been developed and implemented to provide electric power generation by a combination of photovoltaic and thermal generation. In particular, we show that splitting the solar spectrum, and then using high-energy solar photons for photovoltaics and medium-energy solar photons for thermoelectrics with a bottoming Rankine cycle has potential to achieve 50% solar-to-electricity conversion using existing materials. Also, over 50% of the harvested energy goes to thermal storage for generation after sunset, which could enable highly efficient baseload solar electricity and heat generation at all hours of the day.

A summary of the main issues covered in the previous chapters will serve a comparative analysis of the current and perspective possibilities that the hybridization of thermoelectric and photovoltaic generators provides. Materials demand, technological open questions, and market-related issues will be discussed. Also concerning the competition with alternate hybridization strategies, an analysis of HTEPV cost-effectiveness will be outlined. It will be shown that HTEPV may have a key role in the development of renewable energy sources, provided that a careful selection of photovoltaic materials is made. The importance of rethinking the layout of thermoelectric generators will be stressed, along with the merits of hybridization in concentrated solar generators. As an overall conclusion, pairing thermoelectric generators to photovoltaic cells will be proved to be profitable for third-generation PV materials, where hybridization might support the differentiation of the solar module market, currently pinned to silicon-based technology.