Abstract
Sensitivity analysis of molecular design problem for the development of
novel working fluids for power cycles
Jérôme Frutiger, Jens Abildskov, Gürkan Sin
CAPEC-PROCESS, Department of Chemical and Biochemical Engineering, Technical
University of Denmark
In recent years there is a large availability of low-temperature heat sources in different
applications such as waste heat in chemical industries and refrigeration plants as well as
renewable energy sources such as biomass combustion, geothermal and solar heat sources. Power
cycles are an important technology to convert such waste heat sources into usable energy. So far
the low-temperature heat is not utilized efficiently for electricity generation.
To optimize the heat transfer process and the power generation, the influence of the working
fluid, the cycle designs and the operating conditions is vital. Multi-criteria database search and
Computer Aided Molecular Design (CAMD) can be applied to generate, test and evaluate
promising pure component/mixture candidate as process fluids to help optimize cycle design and
performance [1]. The problem formulation for the development of novel working fluids is
a CAMD challenge both in terms of data and computational demand, because includes
process related as well as property related equations.
In CAMD problems the identification of target properties is often based on expert
knowledge. However, sophisticated information concerning the influence of for example
a certain working fluid property parameter on the performance of the power cycle, i.e. the
net power output, can facilitate the identification key properties for working fluids. In
that sense a sensitivity analysis of the different parameters is suggested in this work as a
systematic method to efficiently identify the target properties of the CAMD problem for
working fluids.
In this study the CAMD problem for the development of novel working fluids for organic
Rankine cycles (ORC) is formulated mathematically. It integrates both a system model
for the ORC and property models including the Peng-Robinson equation of state [2], for
the working fluids (enthalpy, entropy, etc) and group contribution (GC) models for pure
component property predictions including critical properties as well as environmental and
safety related properties. In addition to these property models, the system consists of
coupled mass and energy balances for a pump, a condenser, a turbine and an evaporator.
As regards sensitivity analysis method, a global sensitivity analysis is performed based
on Morris screening to determine which change of input parameters have important
effects on the net power output. The screening is composed of individually randomized
one-factor-at-a-time parameter changes over the whole parameter space [3]. This allows
users to identify which properties are important (as well optimal region of property
values) hence can be candidate as target properties for a working fluid design and/or
search. In addition, environmental and safety properties such as the ozone depletion
potential (ODP), the global warming potential (GWP) as well as flammability limits are
taken into account to give a multi-criteria framework.
1
Abstract
Furthermore, a process optimization with respect to the properties is performed to
identify the optimal property values for a given process set-up [4]. This in combination
with the sensitivity analysis to specify systematically the boundaries for the cycle related
target properties. This facilitates an efficient screening for the optimal pure components
or mixture working fluids.
The methodology will be applied in a case study of an Organic Rankine Cycle (ORC)
with a low-temperature heat source. The heat source is a hot water stream from waste
heat of a chemical site. Giving this pre-exquisite the method allows to identify the most
favorable working fluid along with the corresponding optimal process conditions in order
to get the highest possible power output.
The study presents a new approach for the identification of target properties of CAMD
problems based on sensitivity analysis and shows its application for the development of
novel working fluids of organic Rankine cycles for low temperature heat sources.
[1]
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[2]
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[3]
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[4]
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