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Development and Validation of a Thermo-Economic Model for Design Optimisation and Off-Design Performance Evaluation of a Pure Solar Microturbine
The aim of this paper is to present a thermo-economic model of a microturbine for solar dish applications, which demonstrates the applicability and accuracy of the model for off-design performance evaluation and techno-economic optimisation purposes. The model is built using an object-oriented programming approach. Each component is represented using a class made of functions that perform a one-dimensional physical design, off-design performance analysis and the component cost evaluation. Compressor, recuperator, receiver and turbine models are presented and validated against experimental data available in literature, and each demonstrated good accuracy for a wide range of operating conditions. A 7-kWe microturbine and solar irradiation data available for Rome between 2004 and 2005 were considered as a case study, and the thermo-economic analysis of the plant was performed to estimate the levelised cost of electricity based on the annual performance of the plant. The overall energy produced by the plant is 10,682 kWh, the capital cost has been estimated to be EUR 27,051 and, consequently, the specific cost of the plant, defined as the ratio between the cost of components and output power in design condition, has been estimated to be around EUR 3980/kWe. Results from the levelised cost of electricity (LCOE) analysis demonstrate a levelised cost of electricity of EUR 22.81/kWh considering a plant lifetime of 25 years. The results of the present case study have been compared with the results from IPSEpro 7 where the same component characteristic maps and operational strategy were considered. This comparison was aimed to verify the component matching procedure adopted for the present model. A plant sizing optimisation was then performed to determine the plant size which minimises the levelised cost of electricity. The design space of the optimisation variable is limited to the values 0.07–0.16 kg/s. Results of the optimisation demonstrate a minimum LCOE of 21.5 [EUR/kWh] for a design point mass flow rate of about 0.11 kg/s. This corresponds to an overall cost of the plant of around EUR 32,600, with a dish diameter of 9.4 m and an annual electricity production of 13,700 [kWh]
A complete X-ray spectral coverage of the 2010 May-June outbursts of Circinus X-1
Circinus X-1 is a neutron-star-accreting X-ray binary in a wide (P = 16.6 d), eccentric orbit. After two years of relatively low X-ray
luminosity, in May 2010 Circinus X-1 went into outburst, reaching 0.4 Crab
flux. This outburst lasted for about two orbital cycles and was followed by
another shorter and fainter outburst in June. We focus here on the broadband
X-ray spectral evolution of the source as it spans about three order of
magnitudes in flux. We attempt to relate luminosity, spectral shape, local
absorption, and orbital phase. We use multiple Rossi-XTE/PCA (3.0--25 keV) and
Swift/XRT (1.0--9.0 keV) observations and a 20 ks long Chandra/HETGS
observation (1.0--9.0 keV), to comprehensively track the spectral evolution of
the source during all the outbursting phases. These observations were taken
every two/three days and cover about four orbital cycles. The PCA data mostly
cover the major outburst, the XRT data monitor the declining phase of the major
outburst and all the phases of the minor outburst, and Chandra data provide an
essential snapshot of the end of this overall outbursting phase. The X-ray
spectrum can be satisfactorily described by a thermal Comptonization model with
variable neutral local absorption in all phases of the outburst. No other
additive component is statistically required. The first outburst decays
linearly, with an ankle in the light curve as the flux decreases below
\,5 10 erg cm s. At the same time, the
source shows a clear spectral state transition from an optically thick to an
optically thin state. While the characteristics of the first, bright, outburst
can be interpreted within the disk-instability scenario, the following, minor,
outburst shows peculiarities that cannot be easily reconciled in this
framework.Comment: Accepted for publication in Astronomy and Astrophysic
Discovery of periodic dips in the light curve of GX 13+1: the X-ray orbital ephemeris of the source
The bright low-mass X-ray binary (LMXB) GX 13+1 is one of the most peculiar
Galactic binary systems. A periodicity of 24.27 d with a formal statistical
error of 0.03 d was observed in its power spectrum density obtained with RXTE
All Sky Monitor (ASM) data spanning 14 years. Starting from a recent study,
indicating GX 13+1 as a possible dipping source candidate, we systematically
searched for periodic dips in the X-ray light curves of GX 13+1 from 1996 up to
2013 using RXTE/ASM, and MAXI data to determine for the first time the X-ray
orbital ephemeris of GX 13+1. We searched for a periodic signal in the ASM and
MAXI light curves, finding a common periodicity of 24.53 d. We folded the 1.3-5
keV and 5-12.1 keV ASM light curves and the 2-4 and 4-10 keV MAXI light curves
at the period of 24.53 d finding a periodic dip. To refine the value of the
period we used the timing technique dividing the ASM light curve in eight
intervals and the MAXI light curve in two intervals, obtaining four and two dip
arrival times from the ASM and MAXI light curves, respectively. We improved the
X-ray position of GX 13+1 using a recent Chandra observation. The new X-ray
position is discrepant by \sim 7\arcsec from the previous one, while it is
compatible with the infrared and radio counterpart positions. We detected an
X-ray dip, that is totally covered by the Chandra observation, in the light
curve of GX 13+1 and showed, a-posteriori, that it is a periodic dip. We
obtained seven dip arrival times from ASM, MAXI, and Chandra light curves. We
calculated the delays of the detected dip arrival times with respect to the
expected times for a 24.52 d periodicity. Fitting the delays with a linear
function we find that the orbital period and the epoch of reference of GX 13+1
are 24.5274(2) days and 50,086.79(3) MJD, respectively.(Abridged)Comment: 12 pages, including 16 figures. Accepted for publication in A&
A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Ser~X-1
Context: Ser X-1 is a well studied LMXB which clearly shows a broad iron
line. Recently, Miller et al. (2103) have presented broad-band, high quality
NuSTAR data of SerX-1.Using relativistically smeared self-consistent reflection
models, they find a value of R_in close to 1.0 R_ISCO (corresponding to 6 R_g),
and a low inclination angle, less than 10 deg. Aims: The aim of this paper is
to probe to what extent the choice of reflection and continuum models (and
uncertainties therein) can affect the conclusions about the disk parameters
inferred from the reflection component. To this aim we re-analyze all the
available public NuSTAR and XMM-Newton. Ser X-1 is a well studied source, its
spectrum has been observed by several instruments, and is therefore one of the
best sources for this study. Methods: We use slightly different continuum and
reflection models with respect to those adopted in literature for this source.
In particular we fit the iron line and other reflection features with
self-consistent reflection models as reflionx (with a power-law illuminating
continuum modified with a high energy cutoff to mimic the shape of the incident
Comptonization spectrum) and rfxconv. With these models we fit NuSTAR and
XMM-Newton spectra yielding consistent spectral results. Results: Our results
are in line with those already found by Miller et al. (2013) but less extreme.
In particular, we find the inner disk radius at about 13 R_g and an inclination
angle with respect to the line of sight of about 27 deg. We conclude that,
while the choice of the reflection model has little impact on the disk
parameters, as soon as a self-consistent model is used, the choice of the
continuum model can be important in the precise determination of the disk
parameters from the reflection component. Hence broad-band X-ray spectra are
highly preferable to constrain the continuum and disk parameters.Comment: 13 pages including 8 figures. Accepted for publication in A&
Chandra X-ray spectroscopy of a clear dip in GX 13+1
The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an
accreting neutron star. It shows highly ionized absorption features, with a
blueshift of 400 km s and an outflow-mass rate similar to the
accretion rate. Many other X-ray sources exhibit warm absorption features, and
they all show periodic dipping behavior at the same time. Recently, a dipping
periodicity has also been determined for GX 13+1 using long-term X-ray folded
light-curves, leading to a clear identification of one of such periodic dips in
an archival Chandra observation. We give the first spectral characterization of
the periodic dip of GX 13+1 found in this archival Chandra observation
performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and
contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray
spectrum. We adopted different spectral models to describe the continuum
emission and used the XSTAR-derived warm absorber component to constrain the
highly ionized absorption features. The 1.0-25 keV continuum emission is
consistent with a model of soft accretion-disk emission and an optically thick,
harder Comptonized component. The dip event, lasting 450 s, is
spectrally resolved with an increase in the column density of the neutral
absorber, while we do not find significant variations in the column density and
ionization parameter of the warm absorber with respect to the out-of-dip
spectrum. We argue that the very low dipping duty-cycle with respect to other
sources of the same class can be ascribed to its long orbital period and the
mostly neutral bulge, that is relatively small compared with the dimensions of
the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Evidence of a non-conservative mass transfer for XTE J0929-314
Context. In 1998 the first accreting millisecond pulsar, SAX J1808.4-3658,
was discovered and to date 18 systems showing coherent, high frequency (> 100
Hz) pulsations in low mass X-ray binaries are known. Since their discovery,
this class of sources has shown interesting and sometimes puzzling behaviours.
In particular, apart from a few exceptions, they are all transient with very
long X-ray quiescent periods implying a quite low averaged mass accretion rate
onto the neutron star. Among these sources, XTE J0929-314 has been detected in
outburst just once in about 15 years of continuous monitoring of the X-ray sky.
Aims. We aim to demonstrate that a conservative mass transfer in this system
will result in an X-ray luminosity that is higher than the observed, long-term
averaged X-ray luminosity. Methods. Under the hypothesis of a conservative mass
transfer driven by gravitational radiation, as expected for this system given
the short orbital period of about 43.6 min and the low mass of the companion
implied by the mass function derived from timing techniques, we calculate the
expected mass transfer rate in this system and predict the long-term averaged
X-ray luminosity. This is compared with the averaged, over 15 years, X-ray flux
observed from the system, and a lower limit of the distance to the source is
inferred. Results. This distance is shown to be > 7.4 kpc in the direction of
the Galactic anticentre, implying a large height, > 1.8 kpc, of the source with
respect to the Galactic plane, placing the source in an empty region of the
Galaxy. We suggest that the inferred value of the distance is unlikely.
(abridged)Comment: 6 pages, 2 figures, accepted for publication in Astronomy &
Astrophysics (A&A
Broadband observations of the X-ray burster 4U 1705-44 with BeppoSAX
4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources.
This source represents a perfect candidate to test different models proposed to
self-consistently track the physical changes occurring between different
spectral states because it shows clear spectral state transitions. The
broadband coverage, the sensitivity and energy resolution of the BeppoSAX
satellite offers the opportunity to disentangle the components that form the
total X-ray spectrum and to study their changes according to the spectral
state. Using two BeppoSAX observations carried out in August and October 2000,
respectively, for a total effective exposure time of about 100 ks, we study the
spectral evolution of the source from a soft to hard state. Energy spectra are
selected according to the source position in the color-color diagram (CCD)
Results. We succeeded in modeling the spectra of the source using a physical
self-consistent scenario for both the island and banana branches (the double
Comptonization scenario). The components observed are the soft Comptonization
and hard Comptonization, the blackbody, and a reflection component with a broad
iron line. When the source moves from the banana state to the island state, the
parameters of the two Comptonization components change significantly and the
blackbody component becomes too weak to be detected. We interpret the soft
Comptonization component as emission from the hot plasma surrounding the
neutron star, hard Comptonization as emission from the disk region, and the
blackbody component as emission from the inner accretion disk. The broad
feature in the iron line region is compatible with reflection from the inner
accretion disk.Comment: 8 pages, 10 figures, accepted for publication by A&
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
We analyze a large collection of RXTE archive data from April 1997 to August
2003 of the bright X-ray source Scorpius X-1 in order to study the broadband
spectral evolution of the source for different values of the inferred mass
accretion rate by studying energy spectra from selected regions in the Z-track
of its Color-Color Diagram. A two-component model, consisting of a soft thermal
component interpreted as thermal emission from an accretion disk and a thermal
Comptonization component, is unable to fit the whole 3--200 keV energy spectrum
at low accretion rates. Strong residuals in the highest energy band of the
spectrum require the addition of a third component that can be fitted with a
power-law component, that could represent a second thermal Comptonization from
a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We
discuss the physical implications derived from the results of our analysis,
with a particular emphasis on the hardest part of the X-ray emission and its
possible origins.Comment: 18 pages. Accepted for publication in Ap
X-ray spectroscopy of the ADC source X1822-371 with Chandra and XMM-Newton
The eclipsing low-mass X-ray binary X1822-371 is the prototype of the
accretion disc corona (ADC) sources. We analyse two Chandra observations and
one XMM-Newton observation to study the discrete features and their variation
as a function of the orbital phase, deriving constraints on the temperature,
density, and location of the plasma responsible for emission lines. The HETGS
and XMM/Epic-pn observed X1822-371 for 140 and 50 ks, respectively. We
extracted an averaged spectrum and five spectra from five selected
orbital-phase intervals that are 0.04-0.25, 0.25-0.50, 0.50-0.75, 0.75-0.95,
and, finally, 0.95-1.04; the orbital phase zero corresponds to the eclipse
time. All spectra cover the energy band between 0.35 and 12 keV. We confirm the
presence of local neutral matter that partially covers the X-ray emitting
region; the equivalent hydrogen column is cm and the
covered fraction is about 60-65%. We identify emission lines from highly
ionised elements, and a prominent fluorescence iron line associated with a
blending of FeI-FeXV resonant transitions. The transitions of He-like ions show
that the intercombination dominates over the forbidden and resonance lines. The
line fluxes are the highest during the orbital phases between 0.04 and 0.75. We
discuss the presence of an extended, optically thin corona with optical depth
of about 0.01 that scatters the X-ray photons from the innermost region into
the line of sight. The photoionised plasma producing most of the observed lines
is placed in the bulge at the outer radius of the disc distant from the central
source of cm. The OVII and the fluorescence iron line are
probably produced in the photoionised surface of the disc at inner radii.
(Abridged)Comment: 18 pages including 12 figures. Accepted for publication in A&
Updating the orbital ephemeris of the dipping source XB 1254-690 and the distance to the source
XB 1254-690 is a dipping low mass X-ray binary system hosting a neutron star
and showing type I X-ray bursts. We aim at obtaining more accurate orbital
ephemeris and at constraining the orbital period derivative of the system for
the first time. In addition, we want to better constrain the distance to the
source in order to locate the system in a well defined evolutive scenario. We
apply for the first time an orbital timing technique to XB 1254-690, using the
arrival times of the dips present in the light curves that have been collected
during 26 years of X-ray pointed observations performed from different space
missions. We estimate the dip arrival times using a statistical method that
weights the count-rate inside the dip with respect to the level of the
persistent emission outside the dip. We fit the obtained delays as a function
of the orbital cycles both with a linear and a quadratic function. We infer the
orbital ephemeris of XB 1254-690 improving the accuracy of the orbital period
with respect to previous estimates. We infer a mass of M
M for the donor star, in agreement with the estimations already
present in literature, assuming that the star is in thermal equilibrium while
it transfers part of its mass via the inner Lagrangian point, and assuming a
neutron star mass of 1.4 M. Using these assumptions, we also
constrain the distance to the source, finding a value of 7.6 kpc.
Finally, we discuss the evolution of the system suggesting that it is
compatible with a conservative mass transfer driven by magnetic braking.Comment: 13 pages, 5 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
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