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Modelling of high temperature storage systems for latent heat
There is a huge demand for heat storages for evaporation
applications. Thermal storage systems are
used to increase the efficiency of thermal systems by
an improved adaption of energy availability and energy
demand.
In this paper a possible solution for modular storage
systems from 200-600 °C and pressures up to 100
bar is presented. The application of steam as a working
medium requires the availability of isothermal
storage if charging/discharging should take place at
almost constant pressure. The saturation temperature
range is between 200°C and 320°C. Therefore nitrate
salts are used as phase change material (PCM). The
solution developed at DLR is characterized by a
modular concept of tube register storages surrounded
by both sensible and latent heat storage material.
The focus in this paper is on modelling of the PCM
storage. A model is introduced for melting and freezing
of the PCM. To perform with an acceptable heat
transfer rate inside the PCM, fins are used to increase
the overall thermal conductivity. Instead introducing
mean storage material parameters, like thermal conductivity
or specific heat capacity, the geometry of
the finned tube is modelled by using discrete elements.
Therefore the model allows detailed studies
on heat transfer during space and time. The fin design
can be varied in order to find an optimal configuration.
A set of partial differential equations is
created and solved. When considering a stand alone
system, that means tube, fin and PCM, without a
connection to other components, investigation is
quite effective. In case of the PCM storage there is
the big advantage, compared with a sensible regenerator,
that the almost constant fluid temperature,
when evaporating or condensing, leads to a uniform
temperature distribution in fluid flow direction.
Therefore only a very rough discretisation in axial
direction is needed, what even allows bonding with
other components e.g. from the Modelica Fluid Library.
Sensible storages as they are used for preheating and
superheating have a characteristic temperature gradient
in axial direction. To describe their thermal behaviour
concentrated models, using dimensionless
numbers, are used
Comment on "Searchinger et al. (2018), Assessing the efficiency of changes in land use for mitigating climate change (Nature 564: 249)"
Some days ago, the article from Searchinger et al. 2018 has been published and caused quite some discussion in the media, basically being reported to show that organic agriculture is bad for the climate.
In the following, Adrian Müller (FiBL) present some fastly written compilation of thoughts on this article and this claim from the media that reported on it
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