Thermophotovoltaic efficiency of 40.

Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect, and can enable approaches to energy storage1,2 and conversion3-9 that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref. 10), TPV fabrication and performance have improved11,12. However, despite predictions that TPV efficiencies can exceed 50% (refs. 11,13,14), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs. 13-15). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III-V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900-2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm-2 and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm-2 and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid.

Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance.

Atmospheric water harvesting (AWH) is the capture and collection of water that is present in the air either as vapor or small water droplets. AWH has been recognized as a method for decentralized water production, especially in areas where liquid water is physically scarce, or the infrastructure required to bring water from other locations is unreliable or infeasible. The main methods of AWH are fog harvesting, dewing, and utilizing sorbent materials to collect vapor from the air. In this paper, we first distinguish between the geographic/climatic operating regimes of fog harvesting, dewing, and sorbent-based approaches based on temperature and relative humidity (RH). Because utilizing sorbents has the potential to be more widely applicable to areas which are also facing water scarcity, we focus our discussion on this approach. We discuss sorbent materials which have been developed for AWH and the material properties which affect system-level performance. Much of the recent materials development has focused on a single material metric, equilibrium vapor uptake in the material (kg of water uptake per kg of dry adsorbent), as found from the adsorption isotherm. This equilibrium property alone, however, is not a good indicator of the actual performance of the AWH system. Understanding material properties which affect heat and mass transport are equally important in the development of materials and components for AWH, because resistances associated with heat and mass transport in the bulk material dramatically change the system performance. We focus our discussion on modeling a solar thermal-driven system. Performance of a solar-driven AWH system can be characterized by different metrics, including L of water per m2 device per day or L of water per kg adsorbent per day. The former metric is especially important for systems driven by low-grade heat sources because the low power density of these sources makes this technology land area intensive. In either case, it is important to include rates in the performance metric to capture the effects of heat and mass transport in the system. We discuss our previously developed modeling framework which can predict the performance of a sorbent material packed into a porous matrix. This model connects mass transport across length scales, considering diffusion both inside a single crystal as well as macroscale geometric parameters, such as the thickness of a composite adsorbent layer. For a simple solar thermal-driven adsorption-based AWH system, we show how this model can be used to optimize the system. Finally, we discuss strategies which have been used to improve heat and mass transport in the design of adsorption systems and the potential for adsorption-based AWH systems for decentralized water supplies.

Tunable Metal-Organic Frameworks Enable High-Efficiency Cascaded Adsorption Heat Pumps.

Rising global standards of living coupled to the recent agreement to eliminate hydrofluorocarbon refrigerants are creating intense pressure to develop more sustainable climate control systems. In this vein, the use of water as the refrigerant in adsorption heat pumps is highly attractive, but such adsorption systems are constrained to large size and poor efficiency by the characteristics of currently employed water sorbents. Here we demonstrate control of the relative humidity of water uptake by modulating the pore size in a family of isoreticular triazolate metal-organic frameworks. Using this method, we identify a pair of materials with stepped, nonoverlapping water isotherms that can function in tandem to provide continuous cooling with a record ideal coefficient of performance of 1.63. Additionally, when used in a single-stage heat pump, the microporous Ni2Cl2BBTA has the largest working capacity of any material capable of generating a 25 °C difference between ambient and chiller output.