Energy and Exergy Analyses of Copper–Chlorine (Cu–Cl) Based Integrated Systems for Hydrogen Production

Abstract

Hydrogen production via thermochemical water splitting using the Cu–Cl cycle is a promising alternative to conventional hydrogen production methods. Multi-generation systems are often attractive due to their higher energy and exergy efficiencies than individual cycles (e.g. steam or gas turbine cycles). An increase in efficiency often allows, for the same output, less resources (or exergy) to be used. This, in return, reduces the extraction from the environment of energy resources (e.g. fossil fuels), and decreases the associated environmental impacts. In this study, comprehensive thermodynamic analyses of two Cu–Cl based integrated systems for hydrogen production are reported. The systems considered here are developed to produce hydrogen and provide cooling. Hot water, drying air and oxygen are also by-products of the system. The first system also has the capability of generating electricity using a two-stage steam turbine cycle. The main sub-units of the system are the Cu–Cl thermochemical water splitting cycle and a LiBr–H₂O absorption cooling system. Solar energy drives the first system using a heliostat solar tower, whereas waste/process heat from a Generation IV nuclear supercritical water cooled reactor (SCWR) is the energy source of the second system. The energy efficiency of the first system is found to be approximately 70 %, while the exergy efficiency is approximately 58 %. The second system, on the other hand, has an energy efficiency of 63 % and an exergy efficiency of 41 %.

Nomenclature

ex :

Specific exergy, MJ/kg

Ex _d :

Exergy destruction, MJ

ex _ch :

Specific chemical exergy, MJ/kg

Ex _Q :

Exergy of thermal energy, MJ

h :

Specific enthalpy, kJ/kg

LHV :

Lower heating value, MJ

$\dot{m}$ :

Mass flow rate, kg/s

η _en :

Energy efficiency

η _ex :

Exergy efficiency

Q :

Heat, MJ

S _gen :

Entropy generation, MJ/K

T _i :

System temperature, K

T ₀ :

Reference-environment temperature, K

W :

Work, MJ

${\dot{W}}_{net}$ :

Net power output, MW

ACS:

Absorption cooling system

EES:

Engineering equation solver

HP:

High pressure

HTF:

Heat transfer fluid

LP:

Low pressure

SCWR:

Supercritical water cooled reactor

STC:

Steam turbine cycle