(408d) Thermoeconomic Optimization of Reverse Brayton Cycle Based Cryocooler for HTS Power Transmission Cable

In the era of the global sustainable energy crisis, an efficient
power transmission system is required to meet the increasing power demand. After the discovery of superconductivity in 1911, many
materials with zero resistance at a temperature below 25 K, known as Low-Temperature Superconductor (LTS), were
discovered and a possible alternative for
efficient power transmission. However, the cost of these materials and cooling
system to maintain them in the superconducting
state was very high. Thus, it is practically not
feasible to use such materials for the transmission
system. In 1986, the discovery of High Temperature Superconductors (HTS), which
can be maintained in the superconducting
state using liquid nitrogen, made the practical and economically feasible for use HTS power cable. These cables have to
be maintained in a temperature range of
65 to 80 K with a cooling load of 2 to 5 kW for 1 km long transmission cable.
Reverse Brayton cycle based Cryocooler (RBC) is the most suitable cooling system because of a wide range of cooling capacity at different temperature level.
However, the capital and operational costs of RBC is high compared to the cryogenic roadmap (Gouge,
2004). The current cost of refrigeration
for HTS devices is five to ten times of cryogenic roadmap ($25 per watt of
refrigeration at 65-80 K). Hence, a thermoeconomic
optimisation of RBC has been carried out to enhance the performance of the
system from the thermodynamic and economic
point of view.

Fig.
1 Reverse Brayton Cycle based cryocooler for HTS power transmission cable.

In the present study, a
reverse Brayton cryocooler consists of a compressor, main
heat exchanger, turboexpander and load heat exchange as shown in Fig.1, has
been investigated and optimised for 10 kW cooling load at 65 K.  A
commercial process simulator Aspen HYSYS V8.6 has been used for the study.
Helium, more efficient than neon (Dhillon, 2017) has been used as working fluid
for the cryocooler. The thermodynamic and economic models have been developed
for analysing of the cryocooler. It is observed that the exergy efficiency of the system has maxima at
1.9 pressure ratio whereas cost per hour has manima
at 2.3 pressure ratio as shown in Fig. 2. The inlet pressure and pressure ratio
of the compressor have been varied to optimise
the system process parameter. The effect of design parameters of equipment on the thermoeconomic
performance of the system has been
studied. The results obtained for RBC are presented in the present work and may
be helpful for engineers and scientist to reduce the cost of RBC cryocooler
without compromising the performance of the system.

Fig.
2 Variation of cost per hour and exergy
efficiency of RBC with different pressure
ratios

1.      Gouge,
M. Cryogenic cooling technology for HTS electric machinery, 2004. IEEE Power
Engineering Society General Meeting, 1-3, DOI: 10.1109/PES.2004.1373239.

2.      Dhillon,
A. K., Dutta R. and Ghosh P. Exergetic analysis of reverse Brayton cryocooler
for different cooling loads at 65 k for HTS cables, 2017. 14^th
Cryogenics 2017 IIR Intermational Conference, DOI: 10.18462/iir.cryo.2017.027