IEEE Electrification Magazine - September 2015 - 39

Near-Black-Body
Particle Receiver

Electric
Grid

Generator

Power
Turbine

Receiver
Tower

Bucket
Lifter
Heliostats
Solar Field

Heat
Rejection
Hot
Silo
Cold
Silo Thermal
Energy
Storage

Working
Fluid

Particle
Flow Cotrol Room

Fluidized-Bed
Heat Exchanger

Pump/
Compressor

Power
Block

Fluidized-Bed
Thermal System

Figure 11. A schematic of a fluidized-bed CSP system with solid-particle TES being developed at NREL. (Source: Alfred Hicks, NREL.)

provides a unique platform for testing power-convertercoupled technologies for voltage and frequency fault ridethrough performance (e.g., both balanced and unbalanced,
low- and high-voltage conditions, distortions) and for frequency regulation services. Figure 8 shows the main characteristics of the CGI, and Figure 9 provides the conceptual
layout of the NWTC site.
The NWTC has two dynamometers (2.5 MW and 5 MW).
Currently, there are four multi-MW wind turbines (Siemens
2.3 MW, Alstom 3 MW, Gamesa 2 MW, and GE 1.5 MW) and
a 1-MW PV array installed on the site. Additional large wind
generation machines are planned in the future.
The uniqueness of the NWTC site is that energy storage can be tested in ways not possible at other existing
test facilities-i.e., testing of energy storage
xx
as a stand-alone system connected directly to the Xcel
electrical grid
xx
connected to the CGI grid/fault simulator
xx
in combination with NWTC renewable generation
sources such as wind turbines and PV arrays connected to the Xcel grid
xx
in combination with NWTC renewable generation
sources connected to the CGI grid/fault simulator.
Figure 10 is a simplified single-line diagram that demonstrates how the different renewable technologies and
energy storage can be combined on the simulated grid.
This test platform offers unique flexibility to test the

grid-integration aspects of renewable technologies with
energy storage under a controlled grid environment.

Thermal Energy Storage
Concentrating solar power (CSP) plants have used thermal
energy storage (TES) to store excess solar heat and to
increase the value of power generation to hours when
demand is at a peak (Figure 11). Thermal energy, stored in
the form of the internal energy of a material, includes both
sensible and latent heat. TES can enable CSP to be a dispatchable power generation, but it may also extend to EES
for wind and PV generation shifting-where off-peak electricity, having a lower value, is stored as thermal energy and
then dispatched later to produce peak-demand electricity
with a higher value. Heat is typically relatively straightforward and inexpensive to store but is a more indirect conversion to electricity compared to other methods. One CSP
study has examined the potential of TES as a means for
EES. In TES, unlike other EES devices, Joule heating can
cause the conversion of electricity to thermal energy to be
100% efficient. Converting thermal energy back into electricity is limited by the Carnot cycle efficiency, which caps
the thermal-to-electric conversion. To impact electric storage, TES must improve the efficiency of the thermal cycle.
This high efficiency will boost the attractiveness of TES for
storing electricity because of TES's relatively low cost, coupled with its site flexibility and overall reliability.
IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 5

39



Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015

IEEE Electrification Magazine - September 2015 - Cover1
IEEE Electrification Magazine - September 2015 - Cover2
IEEE Electrification Magazine - September 2015 - 1
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IEEE Electrification Magazine - September 2015 - Cover3
IEEE Electrification Magazine - September 2015 - Cover4
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http://www.nxtbook.com/nxtbooks/pes/electrification_september2019
http://www.nxtbook.com/nxtbooks/pes/electrification_june2019
http://www.nxtbook.com/nxtbooks/pes/electrification_march2019
http://www.nxtbook.com/nxtbooks/pes/electrification_december2018
http://www.nxtbook.com/nxtbooks/pes/electrification_september2018
http://www.nxtbook.com/nxtbooks/pes/electrification_june2018
http://www.nxtbook.com/nxtbooks/pes/electrification_december2017
http://www.nxtbook.com/nxtbooks/pes/electrification_september2017
http://www.nxtbook.com/nxtbooks/pes/electrification_march2018
http://www.nxtbook.com/nxtbooks/pes/electrification_june2017
http://www.nxtbook.com/nxtbooks/pes/electrification_march2017
http://www.nxtbook.com/nxtbooks/pes/electrification_june2016
http://www.nxtbook.com/nxtbooks/pes/electrification_december2016
http://www.nxtbook.com/nxtbooks/pes/electrification_september2016
http://www.nxtbook.com/nxtbooks/pes/electrification_december2015
http://www.nxtbook.com/nxtbooks/pes/electrification_march2016
http://www.nxtbook.com/nxtbooks/pes/electrification_march2015
http://www.nxtbook.com/nxtbooks/pes/electrification_june2015
http://www.nxtbook.com/nxtbooks/pes/electrification_september2015
http://www.nxtbook.com/nxtbooks/pes/electrification_march2014
http://www.nxtbook.com/nxtbooks/pes/electrification_june2014
http://www.nxtbook.com/nxtbooks/pes/electrification_september2014
http://www.nxtbook.com/nxtbooks/pes/electrification_december2014
http://www.nxtbook.com/nxtbooks/pes/electrification_december2013
http://www.nxtbook.com/nxtbooks/pes/electrification_september2013
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