pwr_july-2024 - 26

COMBINED HEAT & POWER
The Heating Sector: A CO2
Headache or a Hidden Treasure?
The combination of combined heat and power plants with thermal energy storage
and industrial heat pumps provides a sustainable and efficient heating sector option.
Anders Stuxberg
n recent years there has been a growing
awareness that the heating sector
is the elephant in the room because it
consumes more energy than the electricity
and transport sectors combined, and
most of that energy is also derived from
fossil fuels. Two main technology options
can be identified as efficient ways to decarbonize
the heating sector: combined
heat and power (CHP), and electrification
(largely via heat pumps).
Many argue for electrification as the
I
first choice, with the implication that
CHP should even be phased out. The
challenge, however, is not to choose one
of these technologies, but rather to find
how they can be combined optimally.
These two options either consume or
supply electric power, and since we will
have a variable supply of renewable energy,
we will see fluctuations between
oversupply and undersupply. If you have
both a CHP and an electrical means of
supplying heat, then you will be able to
adapt flexibly.
Electrification of the heat supply raises
several issues. First, it requires a supply of
renewable energy that will only be available
part of the time. Second, it adds to
the required residual load at times when
renewable energy is scarce, and finally it
adds to the issues of congested transmission
capacity. Ideally, we should favor an
electrified solution that can be modulated
or stopped to adapt to supply situations.
However, if you are stuck in a " must supply "
situation because of the heat demand,
then something needs to be done
to allow for decoupling, either through the
introduction of heat storage or by switching
to a different heating solution.
If you choose CHP to supply heat when
the electrified solutions must stop, you are
also supporting the grid with the residual
load that is needed at the same time.
The need for residual load will also grow
significantly with the expected dramatic
increase in electricity demand. In these
cases, a combination of capacity payment
and energy price will be required to compensate
for the provision of electricity.
26
Since other alternatives for providing
residual power will be expensive in a fossil-free
society, the market price for these
services will be high, and CHP will have
the lowest production cost because the
heat provided has a value. It can also be
argued that the heat supply has a higherthan-average
value at these times due to
higher alternative production costs when
renewable power is not available. Even
with very limited dispatch hours of CHP,
this will always be a favorable business
case because the alternatives are costlier
for society. If the dispatch of the residual
load were very low, then the capacity
payment would have to be high enough
to compensate entirely, and this would
be the same for all assets, whether electricity
only or CHP. This means that there
should always be a business argument in
favor of CHP.
How Downstream Thermal Heat
Storage Can Help
Electricity storage for most technologies
is limited to a full-load capacity of a few
hours, maximum one day for both economic
and environmental reasons. Longduration
storage is also on the agenda,
but then the storage media must have
either a high energy density or a very
low cost. This means that except in cases
when pumped hydro is possible, you
can store energy as synthetic fuel or
high-temperature heat. Remember that
re-electrification from fuel or high-temperature
heat means a conventional thermal
power plant (or fuel cells), and that brings
us back to the CHP plant as the most efficient
means for re-electrification.
Focus on Heat Supply Misleading.
Many who discuss the decarbonization
of the heating sector make the mistake
of focusing too much on that sector and
looking only at the electricity sector as
a source of renewable energy. From this
perspective, it would be easy to argue
that heat should be drawn from heat
storage during periods of insufficient
wind and solar generation. The German
language has a term that describes this
period when wind and solar generation is
very low: " Dunkelflaute, " meaning " dark
doldrums, " and it typically lasts anywhere
from a few days to a few weeks.
If you focus first on the electricity sector
and how to provide power during a
dark doldrum, you may find that you
need to operate thermal power plants
and that waste heat recovery from such
plants then also solves the heat supply
issue. Given the very high power demand
and the limited demand for heat,
you would then have to store the heat
and distribute it later. This implies that
you would fill the heat storage instead
of discharging it during a dark doldrum.
That is the opposite of what would be
suggested if only the heating sector was
taken into consideration.
Tapping into a Huge New Potential
for CHP Capacity. By using thermal
storage, you can significantly increase
the installed CHP capacity for the same
heat demand. For example, if you operate
only 25% of the time, then you can
1. Industrial heat pumps are an efficient and cost-effective solution for the generation of heat
and cold. They lift the temperature by absorbing thermal energy from an existing low-temperature
heat source and releasing it to a warmer space. Courtesy: Siemens Energy
www.powermag.com
POWER | July 2024

http://www.powermag.com

pwr_july-2024

Table of Contents for the Digital Edition of pwr_july-2024