A year ago
I wrote an article about
residual load and flexibility
People constantly ask me
these concepts
They told me that
the content was too hard
and wished that I introduce them
in a more simple way
Thus in this episode
we shall discuss
the paradigm of the new power system:
its logic and its implications.
Title: Residual Load and Flexibility
Before introducing
the new mindset
we must make it clear
what types of services
the generators provide
Although they can provide
many types of services;
at the end of the day
it breaks down to two main function:
the first is
whether a generator can provide
abundant energy with low costs;
the second is
whether it can supply
the flexibility needed
because of load variations.
With this knowledge,
we can see how
the conventional mindset of
base-mid-peak load is formed.
Coal, gas, and hydro:
these conventional plants
are classified into
"baseload plants"
that give low cost electricity,
and "peak load plants"
that gives flexibility
to meet the load variations,
according to their unique technological
and economical properties.
Because "baseload plants"
have lowest annual costs
if they are run
all day long.
So in the past
they were run
24-7
around the clock.
The problem for
wind and solar is that
when they are fed
abundantly onto the grid
this base-mid-peak load model
will gradually lose its merit.
The main reason is that
wind and solar cannot
hold constant power output
as "baseload plants";
neither can they
fully dispatch
according to
the load variation
and provide
flexibility as "peakload plants".
So there exists one theory
Since wind and solar,
those variable renewables,
they do not belong to
base, mid, nor peak load,
they therefore should not be
wildely fed onto the grid and utilized.
However such claim
falls under what
Australian scientist Mark Diesendorf
proposed in 2010:
"Baseload Fallacy"
We introduced the
base-mid-peak load model
in order to explain
how conventional plants provide
the two main services of generators
But the dispatchment model
is not the service itself.
So when a new generation technology comes out
it makes more sense
to ask whether
the services can still be met
with this technology being applied
than to ask whether
it satisfies
the original model
And it has been proven
that power systems can work fine
after large amount of VRE is fed onto grid
But under such circumstances
it would be more meaningful
to use another model
instead of the conventional
base-mid-peak load model.
This model is to
differentiate the load
into the output of variable renewable energy
and residual load
The rationale behind this mindset is
we should first feed VRE onto the grid
as much as possible
then we decide how we can supply
the rest of the demand load
Under this model
the two services of the generators
can still be met.
VRE with zero marginal costs
such as wind and solar
they will provide
abundant and cheap bulk energy;
the other
dispatchable power plants
such as hydro, geo, bio, and gas
will satisfy
the flexibility requirement from residual load variations
Naturally this new model
will bring some paradigm shift
on the power system
Here we first show
two most obvious changes
The first is the change of the power output curve
which will cause an increase
for flexibility requirements
of the power system.
Such thing occurs
mainly because
when VRE is fed onto the grid
in very large scale
the variations of residual load
will be more obvious than the past.
The most famous example
is the change of the power output curves
of Germany these years.
We can see that
in the first week of 2018
VRE (mostly wind)
has been constantly lowering
the traditional "baseload plants"
such as brown coal or nuclear.
What is more important
is that you can see
the variations of residual load
is greater than that of the demand load.
To cope with this
new norm on the market
conventional plants in Germany
enhanced their flexibility a lot in recent years.
The second is
the change of residual load duration curve
which leads to structural changes of conventional plants
To explain this
we might need the graph below
to make it easier to understand
Under different
operation hours
conventional plants will have
different unit costs
These operation hours
will correspond to
the best allocation of capacity
between "base" and "peak" load plants
That's because "baseload plants"
will have cost efficiency
only when running for enough time
therefore we can see
on the residual load duration curve
the optimized capacity for
"baseload plants"
and also the optimized capacity
for "peak load plants".
When variable renewables
are fed in a large scale
they will lead to
a downshift of RLDC
This will lead to
a reduced optimized capacity of "baseload"
while an increase of "peak load".
This is because
though the optimized running time does not change
the capacity corresponded to that time
on the RLDC
would be sharply reduced due to
VRE installation.
From these two examples we see that
when VRE are installed
onto the grid largely
the roles of
"baseload plants"
in the power system
will gradually disappear
The replacement will be
dispatchable plants
with faster response rate
and more flexibility
that could satisfy the remaining
residual load variations
The term "residual load"
was first used in Germany
at around 2009
Over the past decade
literature in Europe has widely used it
On the other hand
in US or Australia
"net load" is more often used
in their literature.
But the concept is the same.
What we need to do is to first discuss
how much power do
wind and solar produce
then we discuss
how to satisfy the rest of the demand
If we want to discuss
how popular this "VRE first"
mindset currently is,
the most famous example
may be the "global status report"
published by REN21 last year (2017)
It dedicated a section
to discuss
the "deconstruction of baseload"
Of course REN21
is an organization dedicated
for promotion of energy transition
But IEA (which historically closely related to fossil fuel industries)
also talks similarly in the report
Status of Power System Transformation this year (2018)
It called for a discussion
returning to the original
two services of generators:
energy volume (bulk energy) and energy option (flexibility);
the base-mid-peak load model
is not necessary in such discussion
From these examples we can see
such way of discussing power systems
is gradually becoming the mainstream.
We also hope that
the discussions of
transition of power systems in Taiwan
would also start to use
this kind of mindset
to analyze the issues to come.
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