Daniel Feuermann

«I always turn it on super» : user decisions about when and how to operate room air conditioners

Abstract Room air-conditioner operation was studied in order to understand how energy consumption and peak power demand are determined by user needs, concepts, and behavior. In a multi-family building in New Jersey, thirteen room air conditioners were instrumented in eight apartments, and the residents were interviewed about their cooling needs, decisions about when to turn on their air-conditioning, and their conceptions and operationsof the units. Residents were not billed separately for electricity. They nevertheless limited their use of air-conditioning on the basis of many non-economic factors, including: daily schedule, folk theories about how air conditioners function and the bodys heat tolerance, personal strategies for dealing with all machines, and beliefs and preferences concerning health, thermal comport, and alternative cooling strategies. Across physically similar apartments, seasonal air-conditioner energy consumption varied by two to three orders of magnitude while interior temperature varied by only 2.4 °C to 3.7 °C (4.3–6.7 °F). The least-frequent users were effectively achieving comport at greatly reduced energy consumption, but they were not reducing peak demand since they ran their units only on peak hours of the hottest days of the summer. Three-quarters of the residents did not use their thermostats, controlling cooling instead by switching their units on and off manually. Only one resident consistently let his air conditioner operate thermostatically, and many were not aware that their units had thermostats. The prevailing non-thermostatic mode was initially thought to indicate a need for user education. Further investigation suggests that the cause is in fact a startling mismatch of existing room air-conditioner controls to user needs, with a corresponding opportunity for fundamental redesign of controls.

Toward ultrahigh-flux photovoltaic concentration

We report experimental results with a miniature fiber-optic photovoltaic concentrator with (a) deliverable power density up to 104 suns (10 W/mm2), (b) solar cell efficiencies above 30%, (c) completely passive cooling, (d) uniform and individualized cell illumination, and (e) assembly from readily available components. Measurements include the sensitivity of the conversion efficiency of tandem III–V cells to (1) power input, (2) flux distribution, and (3) the modified spectrum from the fiber-optic concentrators. Our results augur favorably for the feasibility of such designs at concentration levels as high as thousands of suns.

SOLAR FIBER-OPTIC MINI-DISHES: A NEW APPROACH TO THE EFFICIENT COLLECTION OF SUNLIGHT

Abstract A new concept for efficient solar energy concentration and power delivery is proposed — one that offers substantial advantages in efficiency, compactness, reduced mechanical loads, and ease of fabrication and installation relative to conventional solar designs. The design exploits the availability of low-attenuation optical fibers, as well as the practical advantages of mass producing highly accurate very small parabolic dishes. The systems building block is a miniature (e.g. 0.2 m diameter) solar dish which concentrates sunlight into a single optical fiber. The fiber transports power to a remote receiver. A second-stage concentrator can boost flux levels to those approaching the thermodynamic limit and can be performed either in each individual dish or collectively in one or more larger devices at the entrance to the remote receiver. Collector modules, close-packed with mini-dishes, are mounted on individual trackers close to the ground. Systems are modular and can be employed in central power generation ranging from a few kilowatts to tens of megawatts. Designs for maximum efficiency attaining collection efficiencies as high as 80%, and maximum-concentration designs realizing flux levels of 30 000 suns, are achievable.

Photovoltaic characterization of concentrator solar cells by localized irradiation

The ability to determine the macroscopic parameters that characterize photovoltaic performance, including their spatial dependence, especially at high flux, is demonstrated with extensive solar measurements on high-efficiency concentrator solar cells. Two case studies explore (a) the impact of inhomogeneous flux distribution on photovoltaic behavior, (b) establishing how solar cell parameters vary across the cell surface (of particular interest for deployment in high-concentration optical systems), and (c), the sensitivity of photovoltaic parameters to the spatial variation of series resistance that stems from nonuniform cell metallization. In the process, we elucidate current-voltage trends unique to strongly inhomogeneous illumination and to series resistance losses at high flux.

Optical performance at the thermodynamic limit with tailored imaging designs

Ultracompact concentrators and illuminators that approach the thermodynamic limit to optical performance can be realized with purely imaging strategies. We explore two-stage reflector systems where each optical surface is tailored to eliminate one order of aberration--the so-called aplanatic designs. The contours are monotonic functions that can be expressed analytically, which are important for the facilitation of optimization studies and practical fabrication. The radiative performance of the devices presented is competitive with, and even superior to, that of high-flux nonimaging systems. Sample results of practical value in solar concentration and light collimation are presented for systems that cover a wide range of numerical aperture.

Modeling and Experimental Evaluation of Passive Heat Sinks for Miniature High-Flux Photovoltaic Concentrators

An important consideration in the practical realization of high-concentration photovoltaic devices is the heat rejection at high power densities to the environment. Recently, optical designs for generating solar flux in excess of 1000 suns on advanced solar cells-while respecting flux homogeneity and system compactness-were suggested with the introduction of solar fiber-optic mini-dish concentrators, tailored specifically to high-flux photovoltaic devices [1]. At the core of the design is the miniaturization of the smallest building block in the system-the concentrator and the cell-permitting low-cost mass production and reliance on passive heat rejection of solar energy that is not converted to electricity First, this paper proposes a relatively simple 1-D axi-symmetric model for predicting the thermal and electrical performance of such mini-dish high-flux concentrators. Experimental measurements were performed with a real-sun solar simulator, indoors under controllable conditions, at flux levels up to 5,000 suns. A CFD (Computational Fluid Dynamics) model was also developed for model-validation. Both the modeling approaches predict heat sink temperatures within experimental uncertainty of a couple of degrees. Next, the 1-D axi-symmetric model is used to evaluate the sensitivity of different solar cell model assumptions, environmental effects (such as outdoor temperature, and the wind speed), heat sink size and geometry, thermal contact resistance, etc. It was confirmed that the miniaturization of the solar cell module permits passive heat rejection, such that solar cell temperatures should not reach more than 80 °C at peak insolation and stagnation conditions. Though the cell rated efficiency degrades by only 1-2% in absolute terms, higher cell temperatures may compromise the integrity of the cell circuitry and of the encapsulation. The 1-D axi-symmetric model also allows optimization of the heat sink geometric dimensions for a given volume. Hour-by-hour performance simulation results for such an optimized design configuration were performed for one month in summer and one month in winter for two locations namely Philadelphia, PA and Phoenix, AZ. The insight gained from this study is important for the proper design of the various components and materials to be used in PV mini-dishes. Equally important is that it allows similar types of analyses to be performed and well-informed design choices to be made for mini-dishes that have to operate under different climatic conditions with cells of different performance and concentration ratios.

MoS2 Hybrid Nanostructures: From Octahedral to Quasi‐Spherical Shells within Individual Nanoparticles

MoS2, a layered compound with tribological and catalytic applications, is known to form a range of hollow closed nanostructures and nanoparticles, including graphene-like structures. These have been demonstrated experimentally through high-temperature synthesis and pulsed laser ablation (PLA), and theoretically with quantum chemical calculations. The smallest allowed structures are nanooctahedra of 3 to 8 nm size. Nicknamed the “true inorganic fullerene” in analogy to carbon fullerenes, they differ from larger multiwalled MoS2 fullerene-like nanoparticles both in their morphology and predicted electronic properties. The larger fullerene-like particles are quasi-spherical (polyhedral) or nanotubular, typically with diameters of 20 to 150 nm. Above a few hundred nm in size, these nanoparticles transform into 2H-MoS2 platelets. Fullerene-like particles have been recognized as superior solid lubricants with numerous commercial applications, and MoS2 nanooctahedra may have catalytic applications. Understanding the fundamental commonality of these two morphologies might prove essential in the development of new materials. The research on hollow MoS2 nanostructures of minimal size (< 10 nm in diameter) was initiated in 1993 upon the first independent proposal of the formation of nanooctahedra of MoS2 [3,5] (and BN) with six rhombi in their corners. In 1999, it was demonstrated that twoto four-walled MoS2 nanooctahedra, 3–5 nm in size and up to ca. 10 atoms, could be obtained by PLA. Similar results were subsequently reported in Ref. [1, 2] as illustrated in Figure 1a. Recent studies of high energy density methods such as laser ablation and arc– discharge resulted in small structures with only a limited number of atoms: Mo–S clusters or double-walled nanooctahedra.

Analysis of a Two-Stage Linear Fresnel Reflector Solar Concentrator

The two-stage linear Fresnel reflector solar concentrator is analyzed via an in-depth study of an installed, nominally 220 KW{sub t} system. The concentrator includes: a primary linear Fresnel reflector comprised of curved mirrors and a secondary nonimaging CPC-type trough with a tubular receiver. The principal practical design options for the secondary concentrator are evaluated. In this paper, via a computer simulation which includes ray-tracing of the primary reflector, the authors evaluate the sensitivity of energy output to: concentrator optical errors, system geometry, tracking mode, and the option of using flat versus curved primary mirrors. The two-stage Fresnel concentrator can be considerably less expensive than the corresponding parabolic trough collector, but is found to deliver about one-fourth less yearly energy. However much of this difference could be eliminated through the use of higher-quality CPC reflectors.

Validation of models for global irradiance on inclined planes

Abstract The accuracy of models to estimate irradiance on inclined planes is tested by comparing the predictions to measurements taken with four instruments of various tilt and azimuth angles in Sede Boqer, Israel. The three models investigated are: the Perez model, Hays anisotropic model, and the isotropic model. The Perez model is found to perform significantly better than the other two, with residual errors that are comparable to the accuracy of the measuring instruments themselves. The same data are also used to evaluate the precision of empirical correlations to estimate the direct component from global horizontal radiation, and to assess the sensitivity of the predicted irradiance on tilted surfaces to the errors associated with these correlations.

Reversible low solar heat gain windows for energy savings

Abstract Summer cooling loads in buildings can be reduced with windows of low solar heat gain coefficients (SHGC). Such windows are often double glazed with the exterior pane tinted or selectively absorbing. They reject part of the absorbed solar radiation to the environment, reducing the solar heat gain. This effect is undesirable in the cold season. However, the same window installed in reverse, i.e. turned by 180°, has a significantly higher SHGC. Thus, windows that can be reversed according to the season will both reduce summer heat gains and collect much of the beneficial solar radiation in winter. This paper investigates the energy savings achievable by reversing equator-facing windows for the duration of the cold season as opposed to leaving them in the “summer position”. Candidate climates in which these savings may be significant are identified. By means of a computer simulation, seasonal energy savings are predicted for a model room with reversible, low SHGC, windows. The results indicate that for suitable climates, significant savings are achievable.