Internally heated porous convection: an idealised model for Enceladus' hydrothermal activity

Recent planetary data and geophysical modelling suggest that hydrothermal activity is ongoing under the ice crust of Enceladus, one of Saturn's moons. According to these models, hydrothermal flow in the porous, rocky core of the satellite is driven by tidal deformation that induces dissipation and volumetric internal heating. Despite the effort in the modelling of Enceladus' interior, systematic understanding---and even basic scaling laws---of internally-heated porous convection and hydrothermal activity are still lacking. In this article, using an idealised model of an internally-heated porous medium, we explore numerically and theoretically the flows that develop close and far from the onset of convection. In particular, we quantify heat-transport efficiency by convective flows as well as the typical extent and intensity of heat-flux anomalies created at the top of the porous layer. With our idealised model, we derive simple and general laws governing the temperature and hydrothermal velocity that can be driven in the oceans of icy moons. In the future, these laws could help better constraining models of the interior of Enceladus and other icy satellites.Comment: 23 pages, 13 figure

High-Rayleigh-number convection in porous-fluid layers

We present a numerical study of convection in a horizontal layer comprising a fluid-saturated porous bed overlain by an unconfined fluid layer. Convection is driven by a vertical, destabilising temperature difference applied across the whole system, as in the canonical Rayleigh-B\'enard problem. Numerical simulations are carried out using a single-domain formulation of the two-layer problem based on the Darcy-Brinkman equations. We explore the dynamics and heat flux through the system in the limit of large Rayleigh number, but small Darcy number, such that the flow exhibits vigorous convection in both the porous and the unconfined fluid regions, while the porous flow still remains strongly confined and governed by Darcy's law. We demonstrate that the heat flux and average thermal structure of the system can be predicted using previous results of convection in individual fluid or porous layers. We revisit a controversy about the role of subcritical "penetrative convection" in the porous medium, and confirm that such induced flow does not contribute to the heat flux through the system. Lastly, we briefly study the temporal coupling between the two layers and find that the turbulent fluid convection above acts as a low-pass filter on the longer-timescale variability of convection in the porous layer.Comment: Accepted for publication in Journal of Fluid Mechanics, 25 pages, 13 figure

Domestic High Temperature Air Source Heat Pump: Performance Analysis Using TRNSYS Simulations

The aim of this study is to analyze the annual performances of a variable capacity high temperature air source heat pump when retrofitted into a mid-terraced house in different aspects combining control, property ages and locations across the UK. TRNSYS simulations are used to predict the performances of the retrofit high temperature air source heat pump. Firstly, the heat pump model and the whole building model are developed and validated against field trial results. Then, a series of reference dwellings combining three climate conditions (Belfast, Aviemore and Camborne), three dwelling ages (the ‘00s, ‘70s and ‘90s) and two control strategies (fixed water flow temperature and weather compensation) is simulated. The annual simulation results indicate that the heat pump efficiency is highly affected by all three factors. The heat pump’s COPs, energy consumption, running cost and carbon emissions have been discussed in this paper, which can provide good information for further studies assessing the retrofit potential of this kind of heat pump

Tackling coolant freezing in generation-IV molten salt reactors

In this study we describe an experimental system designed to simulate the conditions of transient freezing which can occur in abnormal behaviour of molten salt reactors (MSRs). Freezing of coolant is indeed one of the main technical challenges preventing the deployment of MSR. First a novel experimental technique is presented by which it is possible to accurately track the growth of the solidified layer of fluid near a cold surface in an internal flow of liquid. This scenario simulates the possible solidification of a molten salt coolant over a cold wall inside the piping system of the MSR. Specifically, we conducted measurements using water as a simulant for the molten salt, and liquid nitrogen to achieve high heat removal rate at the wall. Particle image velocimetry and planar induced fluorescence were used as diagnostic techniques to track the growth of the solid layer. In addition this study describes a thermo-hydraulic model which has been used to characterise transient freezing in internal flow and compares the said model with the experiments. The numerical simulations were shown to be able to capture qualitatively and quantitatively all the essential processes involved in internal flow transient freezing. Accurate numerical predictive tools such the one presented in this work are essential in simulating the behaviour of MSR under accident conditions

Polyhydroxybutyrate accumulation by a Serratia sp

A strain of Serratia sp. showed intracellular electron-transparent inclusion bodies when incubated in the presence of citrate and glycerol 2-phosphate without nitrogen source following pregrowth under carbon-limitation in continuous culture. About 1.3 mmol citrate were consumed per 450 mg\ud biomass, giving a calculated yield of maximally 55% of stored material per g of biomass dry wt. The inclusion bodies were stained with Sudan Black and Nile Red (NR), suggesting a lipid material, which was confirmed as polyhydroxybutyrate (PHB) by analysis of molecular fragments by GC and by FTIR spectroscopy of isolated bio-PHB in comparison with reference material. Multi-parameter flow cytometry in conjunction with NR fluorescence, and electron microscopy, showed that not all cells contained heavy PHB bodies, suggesting the potential for increasing\ud the overall yield. The economic attractiveness is\ud enhanced by the co-production of nanoscale hydroxyapatite\ud (HA), a possible high-value precursor for bone replacement materials

Gene surfing

Spatially resolved genetic data is increasingly used to reconstruct the migrational history of species. To assist such inference, we study, by means of simulations and analytical methods, the dynamics of neutral gene frequencies in a population undergoing a continual range expansion in one dimension. During such a colonization period, lineages can fix at the wave front by means of a ``surfing'' mechanism [Edmonds C.A., Lillie A.S. & Cavalli-Sforza L.L. (2004) Proc Natl Acad Sci USA 101: 975-979]. We quantify this phenomenon in terms of (i) the spatial distribution of lineages that reach fixation and, closely related, (ii) the continual loss of genetic diversity (heterozygosity) at the wave front, characterizing the approach to fixation. Our simulations show that an effective population size can be assigned to the wave that controls the (observable) gradient in heterozygosity left behind the colonization process. This effective population size is markedly higher in pushed waves than in pulled waves, and increases only sub-linearly with deme size. To explain these and other findings, we develop a versatile analytical approach, based on the physics of reaction-diffusion systems, that yields simple predictions for any deterministic population dynamics

Tariff-based load shifting for domestic cascade heat pump with enhanced system energy efficiency and reduced wind power curtailment

Cascade air-to-water heat pumps may have good potential for retrofitting UK domestic buildings because they can directly replace existing fossil-fuel boilers without the requirement of considerable modifications to heat distribution systems. A widespread uptake of these heat pumps, however, would pose challenges to the grid. Furthermore, wind power generation has increased in the UK to achieve the target of decreasing CO2 emissions by 2050, but there are high levels of wind curtailment due to the mismatch between electricity supply and demand. In this paper, a load shifting study for cascade heat pumps coupled with thermal energy storage addressing these issues is presented. The main objective is to find the best tariff-based schedule load shifting for cascade heat pumps, which can help to avoid peak demand periods while obtaining enhanced system energy efficiency with minimised running costs and reduced wind energy curtailment. How the retrofit performance of the cascade heat pumps with load shifting is further investigated. TRNSYS was used to simulate the system performance validated against experimental results. Northern Ireland (UK) was selected as the evaluated scenario. Simulation results showed that the tank temperature set point of 75 °C and the storage size of 1.2 m3 could wholly shift the cascade heat pumps’ operation to off-peak periods. The best times to start the cascade heat pumps to charge the storage were at 3 am and 2 pm for the morning and afternoon heating demands, respectively. Compared to oil boilers, the cascade heat pumps with load shifting could obtain lower running costs (16–34%) and carbon emissions (20–37%)