Segmented packed beds for improved thermal energy storage performance

A scheme for bulk electricity storage known as Pumped Thermal Energy Storage (PTES) is described. PTES uses a heat pump during the charging phase to create a hot and a cold storage space. During discharge, these thermal stores are depleted using a heat engine. This version of PTES uses packed beds (or pebble beds) as the energy store. A relatively new design feature which involves segmenting the packed beds is introduced. Various thermodynamic benefits can be achieved by reservoir segmentation, such as reduced pressure losses and increased energy stored per cycle. This report includes modelling of the storage phases, and it is found that segmentation can reduce the thermal equilibration losses that occur. A simple economic model of the PTES system is introduced so that multi-objective optimisation of efficiency and capital costs can be carried out. Sensitivity to the economic factors is briefly explored. The results show that cold packed beds in particular benefit from being segmented.The work described in this paper was undertaken as part of a project funded by the UK Engineering and Physical Sciences Research Council (EPSRC Grant No. EP/J006246/1). The first author was supported by an EPSRC-funded studentship and attended the Off-Shore Energy and Storage Conference (OSES) 2015 with the assistance of the Energy Storage Research Network (ESRN).This is the author accepted manuscript. The final version is available from the Institution of Engineering and Technology via http://dx.doi.org/10.1049/iet-rpg.2016.003

A comparison of radial-flow and axial-flow packed beds for thermal energy storage

Packed-bed thermal reservoirs are an integral component in a number of electrical energy storage technologies. The present paper concentrates on packed beds where the heat transfer fluid travels along the radial co-ordinate. The governing energy equations and various mechanisms that cause exergetic losses are discussed. The radial-flow packed bed is compared to a dimensionally similar axial-flow packed bed. This approach provides a fair assessment of the underlying behaviour of the two designs. Multi-objective optimisation allows a wide range of design variables to be considered, and is employed to compare optimal radial-flow and axial-flow stores. Axial-flow stores that have been segmented into layers are also considered. The results indicate that radial-flow stores have a comparable thermodynamic performance, but that the additional volume required for by-pass flows leads to higher capital costs.The work described in this paper was undertaken as part of a project funded by the UK Engineering and Physical Sciences Research Council (EPSRC Grant No. EP/J006246/1). The first author was supported by an EPSRC-funded studentship. All authors gratefully acknowledge this support

Parametric studies and optimisation of pumped thermal electricity storage

Several of the emerging technologies for electricity storage are based on some form of thermal energy storage (TES). Examples include liquid air energy storage, pumped heat energy storage and, at least in part, advanced adiabatic compressed air energy storage. Compared to other large-scale storage methods, TES benefits from relatively high energy densities, which should translate into a low cost per MW h of storage capacity and a small installation footprint. TES is also free from the geographic constraints that apply to hydro storage schemes. TES concepts for electricity storage rely on either a heat pump or refrigeration cycle during the charging phase to create a hot or a cold storage space (the thermal stores), or in some cases both. During discharge, the thermal stores are depleted by reversing the cycle such that it acts as a heat engine. The present paper is concerned with a form of TES that has both hot and cold packedbed thermal stores, and for which the heat pump and heat engine are based on a reciprocating Joule cycle, with argon as the working fluid. A thermodynamic analysis is presented based on traditional cycle calculations coupled with a Schumann-style model of the packed beds. Particular attention is paid to the various loss-generating mechanisms and their effect on roundtrip efficiency and storage density. A parametric study is first presented that examines the sensitivity of results to assumed values of the various loss factors and demonstrates the rather complex influence of the numerous design variables. Results of an optimisation study are then given in the form of trade-off surfaces for roundtrip efficiency, energy density and power density. The optimised designs show a relatively flat efficiency vs. energy density trade-off, so high storage density can be attained with only a modest efficiency penalty. After optimisation, losses due to pressure drop and irreversible heat transfer in the thermal reservoirs are only a few percent, so roundtrip efficiency is governed mainly by the efficiency of the compression and expansion processes: overall roundtrip efficiencies approaching those for pumped hydro schemes might be achievable whilst simultaneously attaining energy storage densities of around 200 MJ m–3, but this is contingent upon attaining compression and expansion efficiencies for the reciprocating devices that have yet to be proven.This is the accepted manuscript of a paper published in Applied Energy Volume 137, 1 January 2015, Pages 800–811, DOI: 10.1016/j.apenergy.2014.08.03

Performance response of packed-bed thermal storage to cycle duration perturbations

Packed-bed thermal stores are integral components in numerous bulk electricity storage systems and may also be integrated into renewable generation and process heat systems. In such applications, the store may undergo charging and discharging periods of irregular durations. Previous work has typically concentrated on the initial charging cycles, or on steady-state cyclic operation. Understanding the impact of unpredictable charging periods on the storage behavior is necessary to improve design and operation. In this article, the influence of the cycle duration (or ‘partial-charge’ cycles) on the performance of such thermal stores is investigated. The response to perturbations is explained and provides a framework for understanding the response to realistic load cycles. The packed beds considered here have a rock filler material and air as the heat transfer fluid. The thermodynamic model is based on a modified form of the Schumann equations. Major sources of exergy loss are described, and the various irreversibility generating mechanisms are quantified. It is known that repeated charge-discharge cycles lead to steady-state behavior, which exhibits a trade-off between round-trip efficiency and stored exergy, and the underlying reasons for this are described. The steady state is then perturbed by cycles with a different duration. Short duration perturbations lead to a transient decrease in exergy losses, while longer perturbations increase it. The magnitude of the change in losses is related to the perturbation size and initial cycle period, but changes of 1–10 % are typical. The perturbations also affect the time to return to a steady-state, which may take up to 50 cycles. Segmenting the packed bed into layers reduces the effect of the perturbations, particularly short durations. Operational guidelines are developed, and it is found that packed beds are more resilient to changes in available energy if the store is not suddenly over-charged (i.e. longer perturbations), and if the steady-state cycle duration is relatively long. Furthermore, using the gas exit temperature to control cycle duration reduces the impact of perturbations on the performance, and reduces the time to return to steady-state operation

MIMO DWDM system using uncooled DFB lasers with adaptive laser bias control and postphotodetection crosstalk cancellation

A proof-of-principle demonstration of a multiple input-multiple output (MIMO) dense wavelength division multiplexing (DWDM) system is reported. It uses standard uncooled distributed feedback (DFB) lasers with intensity modulation-direction detection (IM-DD), in which the temperature of each laser is allowed to drift independently within a 50°C temperature range. A feedback-based laser bias control algorithm is introduced to guarantee acceptable wavelength spacing and a post-photodetection minimum mean square error (MMSE) decoder is applied to cancel the inter-channel crosstalk. The relative sensitivity of the MIMO receiver in both a random laser temperature drift scenario and a worst case scenario are investigated by simulations in MATLAB. Experimental results for a 40-channel × 12.5 Gb/s DWDM system transmitting over 28 km of single-mode fiber with worst possible wavelength distribution prove the feasibility of the technique.This is the final published version. It's also available from IEEE in the Journal of Lightwave Technology here: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6847154

Uncooled DWDM using orthogonal coding for low-cost datacommunication applications

We describe a novel dense wavelength-division multiplexed systems (DWDM) system where by introducing orthogonality between adjacent channels and by using overlapping arrayed waveguide grating filter profiles, laser transmitters may operate without the need for thermoelecric cooling. Compared with a traditional DWDMsystem, a power consumption saving of up to 68% may be realized using this scheme. Results of a proof-of-principle 100 Gb/s (10×10 Gb/s) experiments that use alternating NRZ and Manchester (CAP-2Q) modulation is reported.The authors would like to thank the Engineering and Physical Science Research Council (EPSRC) and Corning for awarding the author the grand prize best student paper at OFC 2014.This is the final published version. It first appeared at ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7024915

Motivated proteins: a web application for studying small three-dimensional protein motifs

<b>BACKGROUND:</b> Small loop-shaped motifs are common constituents of the three-dimensional structure of proteins. Typically they comprise between three and seven amino acid residues, and are defined by a combination of dihedral angles and hydrogen bonding partners. The most abundant of these are alphabeta-motifs, asx-motifs, asx-turns, beta-bulges, beta-bulge loops, beta-turns, nests, niches, Schellmann loops, ST-motifs, ST-staples and ST-turns.We have constructed a database of such motifs from a range of high-quality protein structures and built a web application as a visual interface to this. <b>DESCRIPTION:</b> The web application, Motivated Proteins, provides access to these 12 motifs (with 48 sub-categories) in a database of over 400 representative proteins. Queries can be made for specific categories or sub-categories of motif, motifs in the vicinity of ligands, motifs which include part of an enzyme active site, overlapping motifs, or motifs which include a particular amino acid sequence. Individual proteins can be specified, or, where appropriate, motifs for all proteins listed. The results of queries are presented in textual form as an (X)HTML table, and may be saved as parsable plain text or XML. Motifs can be viewed and manipulated either individually or in the context of the protein in the Jmol applet structural viewer. Cartoons of the motifs imposed on a linear representation of protein secondary structure are also provided. Summary information for the motifs is available, as are histograms of amino acid distribution, and graphs of dihedral angles at individual positions in the motifs. <b>CONCLUSION:</b> Motivated Proteins is a publicly and freely accessible web application that enables protein scientists to study small three-dimensional motifs without requiring knowledge of either Structured Query Language or the underlying database schem

Analysis and optimisation of packed-bed thermal reservoirs for electricity storage applications

Several emerging electrical energy storage technologies make use of packed-bed reservoirs to store thermal energy for subsequent conversion back to electricity. The present paper describes analysis and optimisation of such reservoirs under transient and steady-state cyclic operation. The focus is on thermodynamic issues, but a simple costing model is also included in order to determine the influence of cost factors on the main design parameters. A major part of the paper is devoted to segmentation (or layering) of the packed beds, which has previously been proposed as a means of simultaneously attaining high storage efficiency and full utilisation of the reservoirs. As illustrative examples, three different reservoirs are modelled, corresponding to the hot and cold thermal stores of a pumped thermal energy storage system, and a larger thermal store suitable for integration with adiabatic compressed air energy storage. Engineering and Physical Sciences Research Council (Grant ID: EP/J006246/1 and a studentship), Isentropic Lt