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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]
Timing of the accreting millisecond pulsar IGR~J17511--3057
{Timing analysis of Accretion-powered Millisecond Pulsars (AMPs) is a
powerful tool to probe the physics of compact objects. The recently discovered
\newigrj is the 12 discovered out of the 13 AMPs known. The Rossi XTE satellite
provided an extensive coverage of the 25 days-long observation of the source
outburst.} {Our goal is to investigate the complex interaction between the
neutron star magnetic field and the accretion disk, determining the angular
momentum exchange between them. The presence of a millisecond coherent flux
modulation allows us to investigate such interaction from the study of pulse
arrival times. In order to separate the neutron star proper spin frequency
variations from other effects, a precise set of orbital ephemeris is
mandatory.} {Using timing techniques, we analysed the pulse phase delays
fitting differential corrections to the orbital parameters. To remove the
effects of pulse phase fluctuations we applied the timing technique already
successfully applied to the case of an another AMP, XTE J1807-294.} {We report
a precise set of orbital ephemeris. We demonstrate that the companion star is a
main sequence star. We find pulse phase delays fluctuations on the first
harmonic with a characteristic amplitude of about 0.05, similar to what also
observed in the case of the AMP XTE J1814-338. For the second time an AMP shows
a third harmonic detected during the entire outburst. The first harmonic phase
delays show a puzzling behaviour, while the second harmonic phase delays show a
clear spin-up. Also the third harmonic shows a spin-up, although not highly
significant (3 c.l.). The presence of a fourth harmonic is also
reported. In the hypothesis that the second harmonic is a good tracer of the
spin frequency of the neutron star, we find a mean spin frequency derivative
for this source of \np{1.65(18)}{-13} Hz s.} (continue ...)Comment: 9 pages, 12 figures, A&A accepted on 23/10/201
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&
A broad iron line in the Chandra/HETG spectrum of 4U 1705-44
We present the results of a Chandra 30 ks observation of the low mass X-ray
binary and atoll source 4U 1705-44. Here we concentrate on the study of
discrete features in the energy spectrum at energies below 3 keV, as well as on
the iron Kalpha line, using the HETG spectrometer on board of the Chandra
satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and
2.6 keV. The 1.47 and 2.6 keV are probably identified with Ly-alpha emission
from Mg XII and S XVI, respectively. The identification of the feature at 2.0
keV is uncertain due to the presence of an instrumental feature at the same
energy. The iron Kalpha line at ~6.5 keV is found to be intrinsically broad
(FWHM ~ 1.2 keV); its width can be explained by reflection from a cold
accretion disk extending down to 15 km from the neutron star center or by
Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona.
We finally report here precise X-ray coordinates of the source.Comment: 8 pages including 2 figures. ApJ Letters, in pres
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
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
New orbital ephemerides for the dipping source 4U 1323-619: constraining the distance to the source
4U 1323-619 is a low mass X-ray binary system that shows type I X-ray bursts
and dips. The most accurate estimation of the orbital period is 2.941923(36)
hrs and a distance from the source that is lower than 11 kpc has been proposed.
We aim to obtain the orbital ephemeris, the orbital period of the system, as
well as its derivative to compare the observed luminosity with that predicted
by the theory of secular evolution. We took the advantage of about 26 years of
X-ray data and grouped the selected observations when close in time. We folded
the light curves and used the timing technique, obtaining 12 dip arrival times.
We fit the delays of the dip arrival times both with a linear and a quadratic
function. We locate 4U 1323-619 within a circular area centred at RA (J2000)=
201.6543\degree and DEC (J2000)= -62.1358\degree with an associated error of
0.0002\degree, and confirm the detection of the IR counterpart already
discussed in literature. We estimate an orbital period of P=2.9419156(6) hrs
compatible with the estimations that are present in the literature, but with an
accuracy ten times higher. We also obtain a constraint on the orbital period
derivative for the first time, estimating
s/s. Assuming that the companion star is in thermal equilibrium in the lower
main sequence, and is a neutron star of 1.4 M, we infer a mass of
0.280.03 M for the companion star. Assuming a distance of 10
kpc, we obtained a luminosity of (4.30.5) erg s,
which is not in agreement with what is predicted by the theory of secular
evolution. Using a 3D extinction map of the K radiation in our Galaxy, we
obtain a distance of 4.2 kpc at 68\% confidence level.
(Abridged)Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294
We performed a timing analysis of the 2003 outburst of the accreting X-ray
millisecond pulsar XTE J1807-294 observed by RXTE. Using recently refined
orbital parameters we report for the first time a precise estimate of the spin
frequency and of the spin frequency derivative. The phase delays of the pulse
profile show a strong erratic behavior superposed to what appears as a global
spin-up trend. The erratic behavior of the pulse phases is strongly related to
rapid variations of the light curve, making it very difficult to fit these
phase delays with a simple law. As in previous cases, we have therefore
analyzed separately the phase delays of the first harmonic and of the second
harmonic of the spin frequency, finding that the phases of the second harmonic
are far less affected by the erratic behavior. In the hypothesis that the
second harmonic pulse phase delays are a good tracer of the spin frequency
evolution we give for the first time a estimation of the spin frequency
derivative in this source. The source shows a clear spin-up of Hz sec (1 confidence level). The
largest source of uncertainty in the value of the spin-up rate is given by the
uncertainties on the source position in the sky. We discuss this systematics on
the spin frequency and its derivative.Comment: 17 pages, 4 figures, Accepted by Ap
A possible solution of the puzzling variation of the orbital period of MXB 1659-298
MXB 1659-298 is a transient neutron star Low-Mass X-ray binary system that
shows eclipses with a periodicity of 7.1 hr. The source went to outburst in
August 2015 after 14 years of quiescence. We investigate the orbital properties
of this source with a baseline of 40 years obtained combining the eight eclipse
arrival times present in literature with 51 eclipse arrival times collected
during the last two outbursts. A quadratic ephemeris does not fit the delays
associated with the eclipse arrival times and the addition of a sinusoidal term
with a period of yr is required. We infer a binary orbital
period of hr and an orbital period derivative of
s s. We show that the large orbital
period derivative can be explained with a highly non conservative mass transfer
scenario in which more than 98\% of the mass provided by the companion star
leaves the binary system. We predict an orbital period derivative value of
s s and constrain the companion star
mass between 0.3 and M. Assuming that the
companion star is in thermal equilibrium the periodic modulation can be due to
either a gravitational quadrupole coupling due to variations of the oblateness
of the companion star or with the presence of a third body of mass M
Jovian masses.Comment: 10 pages, 6 figures. Accepted by MNRA
Study of the reflection spectrum of the LMXB 4U 1702-429
The source 4U 1702-429 (Ara X-1) is a low-mass X-ray binary system hosting a
neutron star. Albeit the source is quite bright ( erg s)
its broadband spectrum has never been studied. Neither dips nor eclipses have
been observed in the light curve suggesting that its inclination angle is
smaller than 60.We analysed the broadband spectrum of 4U 1702-429 in
the 0.3-60 keV energy range, using XMM-Newton and INTEGRAL data, to constrain
its Compton reflection component if it is present. After excluding the three
time intervals in which three type-I X-ray bursts occurred, we fitted the joint
XMM-Newton and INTEGRAL spectra obtained from simultaneous observations. A
broad emission line at 6.7 keV and two absorption edges at 0.87 and 8.82 keV
were detected. We found that a self-consistent reflection model fits the 0.3-60
keV spectrum well. The broadband continuum is composed of an emission component
originating from the inner region of the accretion disc, a Comptonised direct
emission coming from a corona with an electron temperature of
keV and an optical depth , and, finally, a reflection
component. The best-fit indicates that the broad emission line and the
absorption edge at 8.82 keV, both associated with the presence of \ion{Fe}{xxv}
ions, are produced by reflection in the region above the disc with a ionisation
parameter of . We have inferred that the inner radius,
where the broad emission line originates, is km, and the inner
radius of the accretion disc is km. (Abridged)Comment: 9 pages, 9 figures, accepted for publication by A&
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