We present the results of a radio transient and polarization survey towards the Galactic Centre, conducted as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients pilot survey. The survey region consisted of five fields covering similar to 265 deg(2) (350 degrees less than or similar to l less than or similar to 10 degrees, vertical bar b vertical bar less than or similar to 10 degrees). Each field was observed for 12 min, with between 7 and 9 repeats on cadences of between one day and four months. We detected eight highly variable sources and seven highly circularly polarized sources (14 unique sources in total). Seven of these sources are known pulsars including the rotating radio transient PSR J1739-2521 and the eclipsing pulsar PSR J1723-2837. One of them is a low-mass X-ray binary, 4U 1758-25. Three of them are coincident with optical or infrared sources and are likely to be stars. The remaining three may be related to the class of Galactic Centre Radio Transients (including a highly likely one, VAST J173608.2-321634, that has been reported previously), although this class is not yet understood. In the coming years, we expect to detect similar to 40 bursts from this kind of source with the proposed 4-yr VAST survey if the distribution of the source is isotropic over the Galactic fields.

We report the discovery of a highly circularly polarized, variable, steep-spectrum pulsar in the Australian Square Kilometre Array Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey. The pulsar is located about 1° from the center of the Large Magellanic Cloud, and has a significant fractional circular polarization of ∼20%. We discovered pulsations with a period of 322.5 ms, dispersion measure (DM) of 157.5 pc cm−3, and rotation measure (RM) of +456 rad m−2 using observations from the MeerKAT and the Parkes telescopes. This DM firmly places the source, PSR J0523−7125, in the Large Magellanic Cloud (LMC). This RM is extreme compared to other pulsars in the LMC (more than twice that of the largest previously reported one). The average flux density of ∼1 mJy at 1400 MHz and ∼25 mJy at 400 MHz places it among the most luminous radio pulsars known. It likely evaded previous discovery because of its very steep radio spectrum (spectral index α ≈ −3, where S ν ∝ ν α ) and broad pulse profile (duty cycle ≳35%). We discuss implications for searches for unusual radio sources in continuum images, as well as extragalactic pulsars in the Magellanic Clouds and beyond. Our result highlighted the possibility of identifying pulsars, especially extreme pulsars, from radio continuum images. Future large-scale radio surveys will give us an unprecedented opportunity to discover more pulsars and potentially the most distant pulsars beyond the Magellanic Clouds.

We present results from a search for the radio counterpart to the possible neutron star–black hole merger GW190814 with the Australian Square Kilometre Array Pathfinder. We have carried out 10 epochs of observation spanning 2–655 days post-merger at a frequency of 944 MHz. Each observation covered 30 deg2, corresponding to 87% of the posterior distribution of the merger’s sky location. We conducted an untargeted search for radio transients in the field, as well as a targeted search for transients associated with known galaxies. We find one radio transient, ASKAP J005022.3−230349, but conclude that it is unlikely to be associated with the merger. We use our observations to place constraints on the inclination angle of the merger and the density of the surrounding environment by comparing our non-detection to model predictions for radio emission from compact binary coalescences. This survey is also the most comprehensive widefield search (in terms of sensitivity and both areal and temporal coverage) for radio transients to-date and we calculate the radio transient surface density at 944 MHz.

Abstract The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and transient radio sources on timescales from 5 s to $\sim\!5$ yr. In this paper, we present the survey description, observation strategy and initial results from the VAST Phase I Pilot Survey. This pilot survey consists of $\sim\!162$ h of observations conducted at a central frequency of 888 MHz between 2019 August and 2020 August, with a typical rms sensitivity of $0.24\ \mathrm{mJy\ beam}^{-1}$ and angular resolution of $12-20$ arcseconds. There are 113 fields, each of which was observed for 12 min integration time, with between 5 and 13 repeats, with cadences between 1 day and 8 months. The total area of the pilot survey footprint is 5 131 square degrees, covering six distinct regions of the sky. An initial search of two of these regions, totalling 1 646 square degrees, revealed 28 highly variable and/or transient sources. Seven of these are known pulsars, including the millisecond pulsar J2039–5617. Another seven are stars, four of which have no previously reported radio detection (SCR J0533–4257, LEHPM 2-783, UCAC3 89–412162 and 2MASS J22414436–6119311). Of the remaining 14 sources, two are active galactic nuclei, six are associated with galaxies and the other six have no multi-wavelength counterparts and are yet to be identified.

The Australian Square Kilometre Array Pathfinder (ASKAP) collects images of the sky at radio wavelengths with an unprecedented field of view, combined with a high angular resolution and sub-millijansky sensitivities. The large quantity of data produced is used by the ASKAP Variables and Slow Transients (VAST) survey science project to study the dynamic radio sky. Efficient pipelines are vital in such research, where searches often form a ‘needle in a haystack’ type of problem to solve. However, the existing pipelines developed among the radio-transient community are not suitable for the scale of ASKAP datasets. In this paper we provide a technical overview of the new “VAST Pipeline”: a modern and scalable Python-based data pipeline for transient searches, using up-to-date dependencies and methods. The pipeline allows source association to be performed at scale using the Pandas DataFrame interface and the well-known Astropy crossmatch functions. The Dask Python framework is used to parallelise operations as well as scale them both vertically and horizontally, by means of a cluster of workers. A modern web interface for data exploration and querying has also been developed using the latest Django web framework combined with Bootstrap.

We report the discovery of a highly polarized, highly variable, steep-spectrum radio source, ASKAP J173608.2−321635, located ∼4° from the Galactic Center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888 MHz. It exhibited a high degree (∼25%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2–4 week cadence. The source was not detected with MeerKAT before 2021 February 7 when it appeared and reached a peak flux density of 5.6 mJy. The source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from −11.8 ± 0.8 rad m−2 to −64.0 ± 1.5 rad m−2 over three days. No X-ray counterpart was found in follow-up Swift or Chandra observations about a week after the first MeerKAT detection, with upper limits of ∼5.0 × 1031 erg s−1 (0.3–8 keV, assuming a distance ∼10 kpc). No counterpart is seen in new or archival near-infrared observations down to J = 20.8 mag. We discuss possible identifications for ASKAP J173608.2−321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center radio transient: none of these fully explains the observations, which suggests that ASKAP J173608.2−321635 may represent part of a new class of objects being discovered through radio imaging surveys.

This work presents the benefits of using a model predictive control (MPC) approach for controlling a high efficiency absorption chiller-based solar cooling system with thermal energy storage, incorporating perfect solar resource and load forecasting information. A dynamic physics-based model of the solar air-conditioning system has been built for studying the system behavior. A genetic algorithm based predictive controller is utilized to minimize backup energy consumption while satisfying the cooling demand. The simulations have been carried out using the open-source programming language Python. Detailed investigation of the role of the predictive controller and its decision strategy have been carried out using ten and fifty days simulations. Effect of storage tank heat losses has been investigated. For the simulated example case pertaining to a building, results show the model predictive controller usage delivers about 10% reduction in auxiliary energy use in the system. This is achieved through reduction in tank heat losses, better utilization of heat stored in the tank. It is seen that the MPC based controller enables new system operational capabilities by running the solar collector pump in variable flow mode and allowing the simultaneous heat delivery from storage and backup devices. Opportunities to improve the MPC benefits have been identified. The benefits of the MPC are seen to be sensitive to the system parameters and specific constraints. In summary, this paper provides valuable insights into solar cooling system design and control. Crown Copyright (C) 2019 Published by Elsevier B.V. All rights reserved.

The example in chapter 5 examines the potential for using high-efficiency, multi-effect absorption chillers, running at high temperatures (>150 °C). In principle, the high efficiency of the chiller offers the potential for (i) a smaller solar collector field size, (ii) reduced heat rejection and (iii) the option of using gas as a backup (without undue primary energy penalty). This may lead to more cost effective next generation solar cooling, particularly in large scale applications. Modelling showed that concentrating collectors would be effective in climate zones where the direct component of annual solar radiation is greater than around 60%. In climates with more diffuse radiation, high temperature stationary collectors would need to be considered. Realizing a cost-effective and robust high-temperature design can be challenging. An experimental system is described that employs thermal oil as a heat transfer medium, to enable atmospheric pressure operation across most of the system. The control strategy sources heat to the chiller from either 100% solar mode or 100% backup heater mode, in order to maximize solar gain. Robust operation of the system was achieved. However, the small size of the system made it difficult to prevent significant heat losses. The chapter concludes with tips and improvements for maximizing technical performance and economic viability.

Thermal energy storage (TES) plays an important role in ensuring continuous heat supply to solar powered thermal systems such as solar cooling plants. Various sensible and latent heat storage material options are available when designing a solar cooling system. Latent heat materials are known to have higher energy density resulting in lower storage volume. However, it is unclear if there are any energy benefits due to these materials while used in a typical solar cooling application. In this paper we investigate the system performance of different storage materials while delivering cooling to a typical commercial building in Australia. This system uses high efficiency triple effect absorption chiller as the cooling delivery system. Heat requirement for this chiller is provided through parabolic trough collectors delivering heat over 200 degrees C.A suitable approach for storage system design that enables direct comparison of sensible and latent heat storage benefits is described in this paper. In order to simulate the latent heat storage system, a new numerical model has been developed, validated with experimental data and implemented in the simulation environment as an external library. Commercially available liquid sensible storage materials have been compared against latent heat materials with a phase change temperatures suitable for triple effect chiller operation. A parametric analysis of the system design parameters such as the collector area, storage volume has been carried out. Results from annual simulations have been presented for fixed cooling load and variable cooling load scenarios.Latent heat storage systems functioned with high storage efficiency compared to sensible heat storage systems, a reflection of low heat losses due to reduced storage sizes. It is seen that the collectors have higher yield while functioning with a sensible heat storage medium. As a result, for the chosen configuration, the sensible heat storage materials provided higher annual performance than the latent heat material choices. (C) 2016 Elsevier Ltd. All rights reserved.

This study investigates the techno-economic feasibility of solar-powered absorption cooling and heating systems for a large-sized hotel building in Sydney, Australia. The proposed plant primarily consists of evacuated tube solar collectors, a hot water storage tank, a single-effect absorption chiller, and a backup gas burner. Dynamic simulation of the system has been carried out using the TRNSYS environment. Several control strategies have been implemented in the model to increase the overall efficiency of the system. Solar fraction and levelized total cost of the system have been considered as energetic and economic indicators, respectively. The parametric study results reveal that the optimal values of the storage tank volume and specific collector area are 70 L/m2 and 4 m2 per kW cooling capacity of the chiller, corresponding to the solar fraction of ∼72% and levelized total cost of ∼874,000 AUD/year. Finally, the payback period of the solar equipment is calculated to be 30.8 years, reiterating this technology still needs a great deal of subsidy in order to be economically competitive with conventional air-conditioning systems.

Solar-assisted cooling technology has enormous potential for air-conditioning applications since both solar energy supply and cooling energy demand are well correlated. Unfortunately, market uptake of solar cooling technologies has been slow due to the high capital cost and limited design/operational experience. In the present work, different designs and operational modes for solar heating and cooling (SHC) absorption chiller systems are investigated and compared in order to identify the preferred design strategies for these systems. Three control scenarios are proposed for the solar collector loop. The first uses a constant flow pump, while the second and third control schemes employ a variable speed pump, where the solar collector (SC) set-point temperature could be either fixed or adjusted to the required demand. Series and parallel arrangements, between the auxiliary heater and the storage tank, have been examined in detail from an energy efficiency perspective. A simulation model for different system layouts is developed in the transient system simulation environment (TRNSYS, Version 17). Simulation results revealed that the total solar fraction of the plant is increased by up to 11% when a variable speed solar loop pump is used to achieve a collector set-point temperature adjusted according to the building load demand. Another significant finding of this study is that a parallel configuration for the auxiliary heater out-performs a conventional series configuration. The yearly performance of an auxiliary heater in parallel with the storage tank enhances the plant solar fraction, and the average collector efficiency, by up to 13% and 9%, respectively (as compared to the same components in series). Taken together, nearly 20% higher solar fraction (as compared to conventional designs) is possible through the control strategies and operational modes presented here without adding a substantial capital cost to the system. (C) 2015 Elsevier Ltd. All rights reserved.

This paper presents a review of thermal storage media and system design options suitable for solar cooling applications. The review covers solar cooling applications with heat input in the range of 60–250°C. Special attention is given to high temperature (>100°C) high efficiency cooling applications that have been largely ignored in existing reviews. Sensible and latent heat storage materials have been tabulated according to their suitability for double effect and triple effect chillers. A summary of system designs for water storage (sensible heat), and phase change material storage (latent heat) has been provided. The article summarizes literature related to solar thermal air-conditioning systems from a material level as well as plant level considerations. This includes evaluating various control strategies for managing the thermal store, that aid in optimal functioning of a solar air conditioning plant. Modeling approaches are reviewed for sizing the solar thermal store, highlighting the large difference seen in specific storage size when applied in different applications.

High-temperature absorption chillers (double-effect and triple-effect) have a higher coefficient of performance (COP) than single-effect chillers. This can reduce the collector's footprint and cost in a solar-cooling plant. Though single-effect, absorption chiller-based solar-cooling systems have been studied for the past 20 years, very little information is available on the performance benefits of high-temperature solar-cooling systems. The behaviour of a solar-driven, triple-effect absorption chiller with thermal storage, when serving an office building load, have been analysed in this paper. Characteristic equations for the triple effect chiller have been developed from their operating performance data. The effect of different control strategies and design parameters on the annual performance of the system have been captured in this paper. These results indicate that it is possible to achieve very high solar fractions (> 0.9) by choosing the right size of system components and a suitable control strategy.