Being in a dilemma: Experiencing birth in Zambia

Numerous publications investigating childbirth in sub-Saharan Africa have overlooked the psychological and emotional elements that women experience, in favour of physical dimensions, such as maternal mortality. The aim of this study was to explore childbirth experiences, in order to better understand how women in Zambia experience and give meaning to the phenomenon. An interpretive phenomenological approach was utilised. Through purposive sampling methods, fifty birthing women, aged between 16 and 38 years, from all the nine provinces of the country were recruited. Unstructured in-depth interviews were conducted. Analysis uncovered six structures. The main focus of this paper is 'Being in a dilemma'. The selection of this structure reflects its general interest and predominance in data analysis. It entailed experiencing the phenomenon without knowledge of whom or what one was going to encounter. The key themes were: 1) choosing where to birth, and 2) choosing the advice to adhere to. The findings illuminated a need for an attitudinal change in maternity care professionals, and a parallel need to build agency and autonomy in women. It is this intrinsic level that is undermining attempts to reduce high maternal mortality in Zambia.Department of HE and Training approved lis

Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results

Reciprocating-piston devices can be used as high-efficiency compressors and/or expanders. With an optimal valve design and by carefully adjusting valve timing, pressure losses during intake and exhaust can be largely reduced. The main loss mechanism in reciprocating devices is then the thermal irreversibility due to the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. In this paper, pressure, volume and temperature measurements in a piston-cylinder crankshaft driven gas spring are compared to numerical results. The experimental apparatus experiences mass leakage while the CFD code predicts heat transfer in an ideal closed gas spring. Comparison of experimental and numerical results allows one to better understand the loss mechanisms in play. Heat and mass losses in the experiment are decoupled and the system losses are calculated over a range of frequencies. As expected, compression and expansion approach adiabatic processes for higher frequencies, resulting in higher efficiency. The objective of this study is to observe and explain the discrepancies obtained between the computational and experimental results and to propose further steps to improve the analysis of the loss mechanisms

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

An investigation of heat transfer losses in reciprocating devices

The paper presents a detailed computational-fluid-dynamic study of the thermodynamic losses associated with heat transfer in gas springs. This forms part of an on-going investigation into high-efficiency compression and expansion devices for energy conversion applications. Axisymmetric calculations for simple gas springs with different compression ratios and using different gases are first presented, covering Peclet numbers that range from near-isothermal to near-adiabatic conditions. These show good agreement with experimental data from the literature for pressure variations, wall heat fluxes and the so-called hysteresis loss. The integrity of the results is also supported by comparison with simplified models. In order to mimic the effect of the eddying motions generated by valve flows, non-axisymmetric computations have also been carried out for a gas spring with a grid (or perforated plate) of 30% open area located within the dead space. These show significantly increased hysteresis loss at high Peclet number which may be attributed to the enhanced heat transfer associated with grid-generated motions. Finally, the implications for compressor and expander performance are discussed.This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant EP/J006246/1. It was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service, provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council

Thermodynamic analysis of pumped thermal electricity storage

The increasing use of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature, will inevitably lead to a greater need for electricity storage. Although there are many existing and emerging storage technologies, most have limitations in terms of geographical constraints, high capital cost or low cycle life, and few are of sufficient scale (in terms of both power and storage capacity) for integration at the transmission and distribution levels. This paper is concerned with a relatively new concept which will be referred to here as Pumped Thermal Electricity Storage (PTES), and which may be able to make a significant contribution towards future storage needs. During charge, PTES makes use of a high temperature ratio heat pump to convert electrical energy into thermal energy which is stored as ‘sensible heat’ in two thermal reservoirs, one hot and one cold. When required, the thermal energy is then converted back to electricity by effectively running the heat pump backwards as a heat engine. The paper focuses on thermodynamic aspects of PTES, including energy and power density, and the various sources of irreversibility and their impact on round-trip efficiency. It is shown that, for given compression and expansion efficiencies, the cycle performance is controlled chiefly by the ratio between the highest and lowest temperatures in each reservoir rather than by the cycle pressure ratio. The sensitivity of round-trip efficiency to various loss parameters has been analysed and indicates particular susceptibility to compression and expansion irreversibility

Uplift histories of Africa and Australia from linear inverse modeling of drainage inventories

We describe and apply a linear inverse model which calculates spatial and temporal patterns of uplift rate by minimizing the misfit between inventories of observed and predicted longitudinal river profiles. Our approach builds upon a more general, non-linear, optimization model, which suggests that shapes of river profiles are dominantly controlled by upstream advec- tion of kinematic waves of incision produced by spatial and temporal changes in regional uplift rate. Here, we use the method of characteristics to solve a version of this problem. A damped, non-negative, least squares approach is developed that permits river profiles to be inverted as a function of up- lift rate. An important benefit of a linearized treatment is low computational cost. We have tested our algorithm by inverting 957 river profiles from both Africa and Australia. For each continent, the drainage network was constructed from a digital elevation model. The fidelity of river profiles extracted from this network was carefully checked using satellite imagery. River profiles were inverted many times to systematically investigate the trade-off between model misfit and smoothness. Spatial and temporal patterns of both uplift rate and cumulative uplift were calibrated using independent geologic and geophys- ical observations. Uplift patterns suggest that the topography of Africa and Australia grew in Cenozoic times. Inverse modeling of large inventories of river profiles demonstrates that drainage networks contain coherent signals that record the regional growth of elevation.This is the final version. It first appeared at http://onlinelibrary.wiley.com/wol1/doi/10.1002/2014JF003297/abstract

Lumped dynamic analysis and design of a high-performance reciprocating-piston expander

A spatially - lumped dynamic model of a reciprocating - piston expander is presented in this paper. The model accounts for the three main loss mechanisms in realistic piston machines, namely: pressure losses through the intake and exhaust valves, heat transfer between the gas and the surrounding cylinder walls, and the mass leakage between the compression/expansion chamber and the crankcase throu gh the piston rings. The model also accounts for real - gas effects with the fluid properties calculated from t he NIST database using REFPROP. The numerical calculations are first compared with experimental pressure - volume - temperature data obtained on a cust om reciprocating - piston gas spring over a r ange of oscillation frequencies. The comparison between numerical and experimental results shows good agreement. It also allows the most accurate heat transfer correlation to be selected for calculating the gas - to - wall in - cylinder heat transfer. The semi - heuristic modelling tool is then used to design an expander for specific pressure ratio s and mass flowrate, and to predict the thermodynamic performance of the piston device over a range of part - load conditions

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

Simulation of thermally induced thermodynamic losses in reciprocating compressors and expanders: Influence of real-gas effects

The efficiency of positive-displacement components is of prime importance in determining the overall performance of a variety of thermodynamic systems. Losses due to the unsteady thermal-energy exchange between the working fluid and the solid walls of the device are an important loss mechanism. In this work, heat transfer in gas-spring devices is investigated numerically in order to focus explicitly on these thermodynamic losses. The specific aim of the study is to investigate the behaviour of real gases in gas springs and compare this to that of ideal gases in order to understand the impact of real-gas effects on the thermally induced losses in reciprocating expanders and compressors. This work relates these losses to the fluid properties and quantifies the influence of the thermophysical models applied. A CFD-model of a gas spring is developed in OpenFOAM. Four different fluid models are compared: (i) a perfect-gas model (i.e., an ideal-gas model with constant thermodynamic and transport properties); (ii) an ideal-gas model with temperature-dependent properties; (iii) a real-gas model using the Peng-Robinson equation-of-state with temperature and density-dependent properties; and (iv) a real-gas model using gas-property tables to interpolate values of thermodynamic and transport properties as functions of temperature and pressure. Results indicate that for simple, mono- and diatomic gases, like helium or nitrogen, there is a negligible difference in the pressure and temperature oscillations over a cycle between the ideal and real-gas models. However, when considering heavier (organic) molecules, such as propane, the ideal-gas model tends to overestimate the temperature and pressure (by as much as 20%) compared to the real-gas model. A real-gas model that uses the Peng-Robinson equation of state underestimates the pressure relative to the more accurate model based on look-up tables by as much as 10%. Furthermore, both ideal-gas and Peng-Robinson models underestimate the thermally induced loss compared to the table-based model for heavier gases. Different alkanes and alkane mixtures are also compared. It is found that, for a fixed volume ratio, pure and mixed alkanes that exhibit a higher heat capacity incur lower losses due to the lower temperature amplitudes, and thus, lower heat transfer occurring in the gas spring. For example, propane, which has a heat capacity only half of hexane, exhibits a loss of 5.1% (defined as the ratio of the net cyclic heat loss to the compression work), while the loss with hexane amounts to 3.6% (in both cases for a volume ratio of 6.63). Real-gas effects play an increasing role for heavier alkanes because the critical temperature and pressure are lower. The thermodynamic state of the gas is close to the critical point where real-gas effects are very prevalent. Finally, mixtures exhibit losses which lie between the value of their respective pure fluids, whereby increasing the proportion of the pure substance with the higher loss also leads to a higher loss for the mixture