On-off cyclic testing of a micro-cogeneration Stirling unit

Stirling engines are a promising candidate for micro-cogeneration in residential and small-scale tertiary applications. Due to the variability of energy demand profiles and electricity tariffs, real applications often require to operate the cogeneration unit with multiple daily starts and stops, especially during summer and intermediate seasons. This work focuses on the experimental analysis of a commercial 1 kWel Stirling unit, burning natural gas and generating 8 kWth of useful heat through hot water and up to 12 kWth with an auxiliary burner, when subjected to cyclic on-off operation. The scope is collecting useful data about energy balances and emissions during on-off transients, which can be later used to optimize the management of the cogeneration unit when coupled with real users. Different cyclic tests are experimented (with intermediate stops and operation of either one or two burners), keeping the temperature of the cogeneration water at the unit inlet at 50°C and its mass flow rate at the nominal value of 0.194 kg/s. The Stirling unit has shown an electrical efficiency of 8.9%, based on Lower Heating Value (LHV), in the most favorable cyclic test and 8.2% in the worst case, while thermal efficiency ranges between 91.0 and 92.6%. For comparison, the steady state electrical efficiency is 10.8% (LHV) while the thermal is 90.1% with only one burner running in full cogeneration mode. Steady state efficiencies become 7.2% and 92.0% (LHV), respectively, with the auxiliary burner running. The significant reduction of average electrical efficiency suggests the necessity to limit the frequency of starts and stops in real operation. Emissions show modest peaks in NOx and CO, which do not compromise the environmental impact, confirming the low emission combustion features of the Stirling unit

Characterization of ion-beam-sputtered AlF3_3 thin films for gravitational-wave interferometers

Thermal noise in amorphous coatings is a limitation for a wide range of precision experiments such as gravitational-wave detectors (GWDs). Mirrors for GWDs are composed of multiple thin layers of dielectric materials deposited on a substrate: the stack is made of layers with a high refractive index interleaved with layers of a low refractive index. The goal is to obtain high reflectivity and low thermal noise. In this paper we report on the optical and mechanical properties of ion-beamsputtered aluminium fluoride (AlF3) thin films which have one of the lowest refractive index among the known coating materials and we discuss their application in current and future GWDs

knowledge spillovers congestion effects and long run location patterns

We introduce an evolutionary two-country model to characterize long run location patterns of the manufacturing activities of competing multinational enterprises. Firms located in country 1 can decide to offshore their manufacturing activities to country 2. The profitability of production in a country depends on several factors: unitary costs of production, the number of firms that are located in each country, within-country spillovers, and cross-border spillovers. Furthermore, profits in country 2 are influenced by congestion costs. Country 1 is assumed to be technologically advanced and has an advantage in terms of internal spillovers. In contrast, country 2 offers lower production unit cost which, however, may be offset by congestion costs. The firms' (re)location choices are based on a simple comparison of current production costs obtained in the two countries and the dynamics of switching is modeled by a simple replicator dynamics. The global analysis of the resulting one-dimensional dynamical system reveals that a large advantage in terms of unitary production costs encourages the firms to off-shore manufacturing activities to country 2. This off-shoring process stops when congestion costs offset this advantage of country 2, even though congestion costs do not cause all manufacturing activities to be re-shored to country 1. The re-shoring process can be accelerated by an increase of within-country spillovers in country 1, while cross-border spillovers tend to favor a geographic dispersion of manufacturing activities and make location patterns that lead to suboptimal long run outcomes less likely

All-sky search for short gravitational-wave bursts in the second Advanced LIGO and Advanced Virgo run

We present the results of a search for short-duration gravitational-wave transients in the data from the second observing run of Advanced LIGO and Advanced Virgo. We search for gravitational-wave transients with a duration of milliseconds to approximately one second in the 32-4096 Hz frequency band with minimal assumptions about the signal properties, thus targeting a wide variety of sources. We also perform a matched-filter search for gravitational-wave transients from cosmic string cusps for which the waveform is well modeled. The unmodeled search detected gravitational waves from several binary black hole mergers which have been identified by previous analyses. No other significant events have been found by either the unmodeled search or the cosmic string search. We thus present the search sensitivities for a variety of signal waveforms and report upper limits on the source rate density as a function of the characteristic frequency of the signal. These upper limits are a factor of 3 lower than the first observing run, with a 50% detection probability for gravitational-wave emissions with energies of ∼10-9 Mc2 at 153 Hz. For the search dedicated to cosmic string cusps we consider several loop distribution models, and present updated constraints from the same search done in the first observing run

Search for Eccentric Binary Black Hole Mergers with Advanced LIGO and Advanced Virgo during Their First and Second Observing Runs

When formed through dynamical interactions, stellar-mass binary black holes (BBHs) may retain eccentric orbits (e > 0.1 at 10 Hz) detectable by ground-based gravitational-wave detectors. Eccentricity can therefore be used to differentiate dynamically formed binaries from isolated BBH mergers. Current template-based gravitational-wave searches do not use waveform models associated with eccentric orbits, rendering the search less efficient for eccentric binary systems. Here we present the results of a search for BBH mergers that inspiral in eccentric orbits using data from the first and second observing runs (O1 and O2) of Advanced LIGO and Advanced Virgo. We carried out the search with the coherent WaveBurst algorithm, which uses minimal assumptions on the signal morphology and does not rely on binary waveform templates. We show that it is sensitive to binary mergers with a detection range that is weakly dependent on eccentricity for all bound systems. Our search did not identify any new binary merger candidates. We interpret these results in light of eccentric binary formation models. We rule out formation channels with rates ⪆100 Gpc-3 yr-1 for e > 0.1, assuming a black hole mass spectrum with a power-law index ≲2

Modeling house price dynamics with heterogeneous speculators

This paper investigates the impact of speculative behavior on house price dynamics. Speculative demand for housing is modeled using a heterogeneous agent approach, whereas ‘real’ demand and housing supply are represented in a standard way. Together, real and speculative forces determine excess demand in each period and house price adjustments. Three alternative models are proposed, capturing in different ways the interplay between fundamental trading rules and extrapolative trading rules, resulting in a 2D, a 3D, and a 4D nonlinear discretetime dynamical system, respectively. While the destabilizing effect of speculative behavior on the model’s steady state is proven in general, the three specific cases illustrate a variety of situations that can bring about endogenous dynamics, with lasting and significant price swings around the ‘fundamental ’ price, as we have seen in many real markets