High performance Carnot Batteries based on hybrid cycles

Pumped thermal energy storage (PTES) has seen a rapid increase in research interest and private investment during the last few years. A range of different concepts has been proposed, based on different thermodynamic cycles, and the most promising ones are already being turned into demonstration projects or small-scale storage plants. These include PTES systems based on the Joule-Brayton cycle, the Rankine cycle and the Liquid Air cycle, among others. This presentation will explore how hybridising some of these concepts can result in systems that are more flexible, cheaper, or have superior performance compared with the original cycles. More specifically, two examples will be shown where the Joule-Brayton cycle can be effectively used to support a Rankine battery and a Liquid Air battery. One general advantage of Brayton-PTES systems is that they can use molten salts as liquid storage media. Molten salts are cheap, safe and abundant, and have been used for concentrated solar power (CSP) applications in a large number of commercial plants. Employing the same storage material at similar temperature levels opens the possibility of hybrid “solar-PTES” systems that would require less capital investment than two separate plants. Such a hybrid system could charge the same hot stores using either solar energy or off-peak electricity, becoming both a power plant and an energy storage plant, therefore increasing the capacity factor while employing a single heat engine during discharge. A numerical model has been implemented to study a solar-PTES system where an existing CSP plant (based on the Rankine cycle) is retrofitted with a Brayton heat pump, and several strategies are explored to boost the overall performance. Similar configurations could be employed to transform other kinds of thermal power plant (such as coal power plants) into Brayton-Rankine batteries. In contrast to most PTES systems, liquid air energy storage (LAES) stores most of the available energy cryogenically, by liquefying atmospheric air and storing it at very low temperatures. This is advantageous because liquid air has a very high energy density - and is free. However, the difficulties in reaching full liquefaction during the charge process have a significant impact on the round-trip efficiency of the cycle. It has been found that these difficulties can be greatly minimised by employing the support of a Brayton cycle. A hybrid system was designed where a Brayton-PTES plant operates as a topping cycle and an LAES plant operates as a bottoming cycle. The cooling provided by the Brayton cycle allows the LAES side to achieve full air liquefaction, which translates into a significant boost in performance. Furthermore, the cold thermal reservoirs that would be required by the two separate cycles are replaced by a single heat exchanger that acts between them, therefore saving significant amounts of storage media per unit of energy stored. Results from a numerical study indicate that the hybrid cycle can increase round-trip efficiency by 5-10 percent points compared with the separate cycles, and achieve an even larger increase in terms of energy density

Bubble-Driven Inertial Micropump

The fundamental action of the bubble-driven inertial micropump is investigated. The pump has no moving parts and consists of a thermal resistor placed asymmetrically within a straight channel connecting two reservoirs. Using numerical simulations, the net flow is studied as a function of channel geometry, resistor location, vapor bubble strength, fluid viscosity, and surface tension. Two major regimes of behavior are identified: axial and non-axial. In the axial regime, the drive bubble either remains inside the channel or continues to grow axially when it reaches the reservoir. In the non-axial regime the bubble grows out of the channel and in all three dimensions while inside the reservoir. The net flow in the axial regime is parabolic with respect to the hydraulic diameter of the channel cross-section but in the non-axial regime it is not. From numerical modeling, it is determined that the net flow is maximal when the axial regime crosses over to the non-axial regime. To elucidate the basic physical principles of the pump, a phenomenological one-dimensional model is developed and solved. A linear array of micropumps has been built using silicon-SU8 fabrication technology, and semi-continuous pumping across a 2 mm-wide channel has been demonstrated experimentally. Measured variation of the net flow with fluid viscosity is in excellent agreement with simulation results.Comment: 18 pages, 18 figures, single colum

Predicting the cosmological constant with the scale-factor cutoff measure

It is well known that anthropic selection from a landscape with a flat prior distribution of cosmological constant Lambda gives a reasonable fit to observation. However, a realistic model of the multiverse has a physical volume that diverges with time, and the predicted distribution of Lambda depends on how the spacetime volume is regulated. We study a simple model of the multiverse with probabilities regulated by a scale-factor cutoff, and calculate the resulting distribution, considering both positive and negative values of Lambda. The results are in good agreement with observation. In particular, the scale-factor cutoff strongly suppresses the probability for values of Lambda that are more than about ten times the observed value. We also discuss several qualitative features of the scale-factor cutoff, including aspects of the distributions of the curvature parameter Omega and the primordial density contrast Q.Comment: 16 pages, 6 figures, 2 appendice

Calculation of nonzero-temperature Casimir forces in the time domain

We show how to compute Casimir forces at nonzero temperatures with time-domain electromagnetic simulations, for example using a finite-difference time-domain (FDTD) method. Compared to our previous zero-temperature time-domain method, only a small modification is required, but we explain that some care is required to properly capture the zero-frequency contribution. We validate the method against analytical and numerical frequency-domain calculations, and show a surprising high-temperature disappearance of a non-monotonic behavior previously demonstrated in a piston-like geometry.Comment: 5 pages, 2 figures, submitted to Physical Review A Rapid Communicatio

The broad band spectral properties of binary X-ray pulsars

The X-ray telescopes on board BeppoSAX are an optimal set of instruments to observe bright galactic binary pulsars. These sources emit very hard and quite complex X-ray spectra that can be accurately measured with BeppoSAX between 0.1 and 200 keV. A prototype of this complexity, the source Her X-1, shows at least seven different components in its spectrum. A broad band measure is therefore of paramount importance to have a thorough insight into the physics of the emitting region. Moreover the detection of cyclotron features, when present, allows a direct and highly significant measure of the magnetic field intensity in the emission region. In this paper we briefly report the results obtained with BeppoSAX on this class of sources, with emphasis on the detection and on the measured properties of the cyclotron lines.Comment: 10 Latex pages, 4 figures, uses psfig.sty. Accepted for publication in Advances in Space Research, in Proceedings of 32nd Scientific Assembly of COSPAR - Symposium E1.1: "Broad-Band X-ray Spectroscopy of Cosmic Sources

A simple scheme for allocating capital in a foreign exchange proprietary trading firm

We present a model of capital allocation in a foreign exchange proprietary trading firm. The owner allocates capital to individual traders, who operate within strict risk limits. Traders specialize in individual currencies, but are given discretion over their choice of trading rule. The owner provides the simple formula that determines position sizes – a formula that does not require estimation of the firm-level covariance matrix. We provide supporting empirical evidence of excess risk-adjusted returns to the firm-level portfolio, and we discuss a modification of the model in which the owner dictates the choice of trading rule

Tunable vector beam decoder by inverse design for high-dimensional quantum key distribution with 3D polarized spatial modes

Spatial modes of light have become highly attractive to increase the dimension and, thereby, security and information capacity in quantum key distribution (QKD). So far, only transverse electric field components have been considered, while longitudinal polarization components have remained neglected. Here, we present an approach to include all three spatial dimensions of electric field oscillation in QKD by implementing our tunable, on-a-chip vector beam decoder (VBD). This inversely designed device pioneers the "preparation" and "measurement" of three-dimensionally polarized mutually unbiased basis states for high-dimensional (HD) QKD and paves the way for the integration of HD QKD with spatial modes in multifunctional on-a-chip photonics platforms.Comment: 10 pages, 3 figure