IEEE Power & Energy Magazine - January/February 2017 - 28

14%
34%

12%
21%

20%

Space Heating and
Hot Water <100 °C
Process Heat <100 °C
Process Heat 100-500 °C
Process Heat 500-1,000 °C
Process Heat >1,000 °C

figure 3. The use of different grades of heat in EU-28
industries (based on Naegler et al., 2015).

District Heating and Cooling
District heating pipes carry hot water from centralized heat
plants to buildings with heat exchangers. After heat has
been transferred to the building's heating system, cooled
water flows back to the plants through an adjacent pipe.
District heating is mainly used in more densely populated
areas in northern latitudes (although it is not widespread in
North America).
In addition to economizing with large fuel boilers, district heating offers the possibility to use CHP plants. In
some countries (e.g., Germany and Denmark), even small
district heating systems often have CHP units, while in others (e.g., Russia and Finland) CHP units are found mainly
in bigger systems that can accommodate larger, more
economic plants. When used alongside CHP plants, fuel
boilers cover heating peaks and back up the CHP units.
Combining CHP plants and fuel boilers enables sensitivity
to power prices. Some CHP units can also change the ratio
between heat and electricity production, which increases
their flexibility.
The flexibility of a district heating system can be further increased with heat storages (accumulators) that offer
a very low-cost form of energy storage at district heating
scale (thousands of cubic meters in insulated steel tanks or
caverns). When power prices are sporadically very low [e.g.,
high levels of wind or solar photovoltaic (PV)] and there
are no regulatory hurdles, it can become feasible to install
heat pumps and electric resistance heaters in district heating
systems. Electric heaters offer a low-cost solution to utilize
cheap power, while heat pumps give considerably more heat
per unit of electricity for a higher investment cost.
It is costly and inconvenient to install district heating
pipelines into existing cities. However, new neighborhoods
are a potential target for small-scale networks. In comparison to building-level heating, they decrease the relative cost
of heat generation units with a limited investment in heat
pipelines. But, more importantly, from a flexibility viewpoint, they offer considerable economies of scale for heat
storage, the specific cost of which decreases nearly logarithmically with increasing size.
District cooling is much less common than district heating. The challenge has been that economies of scale are more
28

ieee power & energy magazine

difficult to achieve in cooling units than in heating units.
However, because most people live in climates where cooling is an aspiration, district cooling may have a more important role in the future. District cooling can provide better
access to more efficient ambient heat sinks (e.g., sea water)
than heat pumps located in buildings. This would also help
to keep the cities themselves cooler because waste heat is
transferred away from the city. Cooled fluid, typically water,
could also be stored in accumulators to gain more flexibility
toward the power system.

Heat Use in Industries
Figure 3 shows six grades of industrial heat use, dominated
by process heat, which makes up roughly 85% of the total
energy demand for industrial heat in Europe. The remaining 15% is due to space heating. Heat pumps can serve lowtemperature loads, while CHP units can serve somewhat
higher-temperature levels and still be able to produce
electricity. A large fraction of industrial heat loads, currently dominated by natural gas burners, requires higher
temperatures. However, electric heating technologies such
as resistance heating, electric arc heating, induction heating, and dielectric (radio-frequency) heating can provide
temperature levels above 500 °C and so can replace, e.g.,
natural gas burners. These alternatives can achieve a high
range of temperatures and offer accurate temperature control. They could provide system flexibility if combined
with a heat storage or used in a hybrid configuration with
fuel burners. The costs of energy, equipment, and grid connection have so far limited the use of electric heating as
compared to combustion.
Also, the type of electric heating capable of replacing or
supplementing an industrial gas burner strongly depends on
the process. Quite a few industrial processes also use the
fuel as a raw material. For example, steel production in blast
furnaces requires coke not only as an energy carrier but also
as a reducing agent that takes part in the chemical reaction in
the blast furnace. Thus, the electrification potential of industrial process heat has to be analyzed carefully for each type
of process and will strongly differ across countries.

Characteristics of Heat Demands
and Thermal Storages
Heat Consumption Profiles
Heat demand profiles are determined by the weather,
building characteristics related to thermal losses, occupant
behavior, and occupant expectations about indoor temperature. Consequently, typical daily demand profiles can be
quite diverse across countries (Figure 4).
In Finland, buildings are typically well insulated, and
thermal losses are relatively small even though outside temperatures can get very cold. In district heating systems, heat
is stored in the pipelines and also in the building envelopes,
resulting in a daily profile with little variation-mainly
january/february 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2017