There are two major tends driving the U.S. power sector. First, a large number of new technologies are becoming commercially
available at different levels. Renewable power generation technologies,
such as solar and wind, are becoming cheaper and gaining market share.
Fracking has transformed the supply and economics of natural gas and is
making gas-fired generation appear a more attractive option in many
areas.
Affordable energy storage technologies may be capable of shifting
the boundaries of generation, transmission and distribution, and
facilitate integration of renewable energy resources. The second trend has to do with new public health, national security
and climate change policies and regulations that are being tailored to
to promote resource diversity and environmental considerations. Some of
the most notable initiatives for the latter are renewable portfolio
standards (RPS) and the U.S. Environmental Protection Agency’s Clean
Power Plan (CPP).
RPS and various economic incentives – the federal production tax
credit for wind and investment tax credit for solar as well as many
state policies – have helped to promote renewables in many states, and
their development has enabled drastic cost reductions. The positive
feedback combined with tighter environmental regulations has accelerated
their implementation. In small doses, the repercussion to the grid is
small. However, because most renewable generating resources are not
dispatchable, larger penetrations of non-dispatchable generation can
cause difficulties for transmission system operators to manage the grid.
As years pass, this problem is becoming more prominent and some
reliability concerns are increasing.
The CPP represents an important milestone that is likely to
accelerate the transformation of the power industry. It imposes
important regulatory constraints to generation by seeking 32% reduction
of carbon dioxide (CO2) emissions by 2030. The plan intends to reach
such goals by increasing the fuel efficiency of existing power plants,
fuel shifting to lower carbon fuels, such as natural gas or nuclear
power, in place of coal, expanding renewable generation and promoting
demand-side energy efficiency to reduce energy consumption.
In order to accomplish this major overhaul to the electric power
sector, operational and technical upgrades to the power system are
likely needed to accommodate high levels of variable, non-dispatchable
renewable energy generation. The CPP requires every state to formulate a
plan by the fall of 2016 to meet these challenges.
Role of Thermal Units in Stability and Reliability of the Grid
Conventional thermal electric power plants not only provide power to
the grid, because the generation is heavily based on large thermal power
plants, they also have a profound impact on how the system operates. In
order to have a useful grid, it has to function within certain
parameters of frequency, voltage and responsiveness.
For example, the system operator must ensure that power supply very
accurately matches the power demand on the system. If load exceeds
supply frequency, voltage can start to drop and damage equipment
connected to the grid. The system operator must make sure that it has
reserve generation available to supply increases in demand or be able to
shed load if necessary.
Reserve requirements must be sized to replace the largest potential
loss on the system – either the loss of a transmission line or the
largest power plant on the system. Additionally, the transmission system
must supply both real and reactive power. Reactive power can be
produced by rotating generators or capacitors and needs to be produced
locally to support the magnetic feeds of motors, transformers and
compressors. Large thermal generating units are able to provide reactive
power and many of the reserve or ancillary services required to
maintain the grid.
The kinetic energy in the large spinning mass of turbine generators
provides rotational inertia to the system. That function helps enable
the transmission system to ride through momentary upsets that occur due
to transmission line failures or power plant trips.
The bottom line is that the hardware (spinning mass) gives valuable
extra reaction time to operators and existing automatic control that
otherwise would need far more complex and expensive upgrades to the grid
in order to maintain reliability. Furthermore, the resultant sinusoidal
profile of the generating output is as smooth as it can possibly be,
and does not introduce harmful harmonics that might have to be filtered.
Wind and PV do not generally provide such benefits, although they
certainly provide others. Their main contribution is to save fuel,
increase resource diversity and provide CO2-free electricity as cheap as
possible. Although their intermittency implies that other flexible
assets are required to ensure a balanced grid and that, at very large
penetrations, the added value decreases notably, their role in the
future of a healthy U.S. grid cannot be overstated.
Solar Thermal Electric Plants
Solar thermal electric (STE) power plants are similar to conventional
thermal power plants except for the source of energy to run the plant.
In conventional thermal power plants the thermal input comes from
burning fossil fuel or heat from nuclear fission. In a solar thermal
plant, concentrated sunlight is used to provide the energy to power the
plant. Among the different configurations possible, one of the most
efficient, cost-effective, and flexible is a molten salt central
receiver power plant (also referred to as a Molten Salt Power Tower).
Figure 1 shows a schematic of this type of plant.
Fig. 1: Molten salt central receiver STE plant
Molten salt is the working fluid in the solar receiver. The salt used
in these plants is a mixture of sodium nitrate and potassium nitrate,
and is commonly known as Solar Salt. The salt is operated at
temperatures where it stays in a liquid or “molten” state. It is a
nontoxic material that has good thermal conductivities and specific heat
with low viscosity; it is suitable as a heat transport fluid and
storage medium. The solar field is composed of a field of heliostats,
which are large mirrors capable of changing their orientation such that
sunlight is constantly reflected on a solar receiver (or heat exchanger)
located at the top of a tall tower.
Molten salt is taken from the cold storage tank, flows through the
illuminated receiver, heated from a cold temperature of 290ºC (554ºF),
and heated to a hot temperature of 565ºC (1049ºF). Once the salt is
heated, it is stored in the hot storage tank. To produce power, hot salt
is taken from the hot storage tank and run through a heat exchanger to
produce steam and run a conventional steam cycle thermal power plant. In
the process, the salt is cooled back to the cold temperature and
returned to the cold salt storage tank.
The ability to defer the production of energy to a different time
from its collection may be very valuable. Energy storage systems may be
categorized according to a number of characteristics such as capacity,
maximum charge and discharge rates, efficiency and lifetime or the
maximum set of operational cycles until its capacity deteriorates beyond
a certain threshold. STE with storage may efficiently store thermal
energy for periods up to several days. Therefore, it is possible to
size the storage system such that power generation can be decoupled from
the solar resource. Because molten salt thermal energy storage can
undergo tens of thousands of cycles without loss of capacity and the
marginal cost per unit of energy stored is several times lower than
batteries, it is a good match for utility-scale storage applications.
Role of STE in the grid
A system is said to be 100% decoupled from its resource when it is
able to operate at maximum output anytime independently from the
charging profile of the resource. In contrast, a system is 0% decoupled
when it has to transform and deliver everything it receives from the
resource (a PV plant receives radiation from the sun and transforms it
to electricity; a wind farm transforms the wind momentum onto
electricity, etc.).
Fig.
2: When fully decoupled through thermal storage, STE plants may
generate power when they are most valuable while offering capacity
This flexibility enables STE to collect solar during the day and
produce electricity whenever is more valued, even at night. Its
configuration may be tuned to serve as peaking units where flexible
capacity is sought or as baseload plants where stable output 24/7 is
required and still retains flexibility. In contrast to intermittent generation, where larger overall
generation decrements its value, the generated output of STE with
storage retains its value. Figure 2 shows an example of how STE plants
with thermal energy storage can be used to supply energy when they
areneeded to meet the system peak electricity demands.
In summary, STE and thermal energy storage makes a good pair because
the STE plant retains the reliability and flexibility advantages of
conventional power plants. All things being equal, transmission system
operators might prefer to run a dispatchable renewable power plant to
displace fossil-fuel peaking generation whenever possible. The promotion of renewables is starting to result in large quantities
of intermittent generation that can be difficult for the transmission
system operator to manage. STE may alleviate such problems and provide
flexible capacity at the same time.
Furthermore, with the advent of the CPP, many coal-fired power plants
may have to be decommissioned or upgraded, and new natural gas plants
are needed. STE may be valuable for planners since no infrastructure for
transporting fuel is needed and existing conventional power plants that
have a good solar resource may be retrofit with a solar field,
maximizing the value of existing assets in the process and supporting
the transformation of a low carbon grid more cost-effectively.
http://www.renewableenergyworld.com/articles/2015/08/changing-power-market-dynamics-open-up-new-opportunities-for-ste-with-tes.html
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