IEEE Power & Energy Magazine - January/February 2017 - 29

Time Constants and Thermal Storages
It is technically possible to store heat from one season to
another, but this has proved economically challenging.
Storing heat becomes more viable when considering time
spans of several days (or shorter). The storage time constants
depend on the storage size or on end-user comfort or needs,
which might be affected by the operation of the heating
device. Here is an approximate list of time constants for different heat uses:
✔✔ domestic refrigerator/freezer: 15 min-1 hour
✔✔ supermarket refrigeration systems: 15 min-3 hours
✔✔ thermal mass of buildings: 2-12 hours
✔✔ buildings with local hot water storage: 2-24 hours
✔✔ district heating pipelines: 1-5 hours
✔✔ district heating storages: hours to several days.
For economic reasons, water is commonly used as a
medium, even though other viable heat storage materials
exist. A cubic meter of water changing 55-95 °C offers
january/february 2017

22:00

20:00

18:00

16:00

14:00

12:00

10:00

8:00

6:00

4:00

2:00

0:00

Demand of Yearly Demand (%)

0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000

Large District Heating
System in Finland
Domestic Gas Demand
in Belgium

Large District Heating
System in China
Electric Space Heating
in Ireland

figure 4. Hourly heat profiles from a winter weekday.

1.00
Demand of Yearly Demand (%)

driven by longer-term ambient temperature variations. Inside
temperatures are kept nearly constant, even when occupants
are not present. By contrast, houses in Ireland leak more,
and the small share of buildings that rely on electric radiators use them mainly when occupants need the extra heat-
for example, in the morning and evening during weekdays.
When occupants are not present or they are sleeping, inside
temperatures are often allowed to drop. Despite the weather
being more moderate in Ireland than in Finland, the average
Irish living room is probably colder than its Finish counterpart due to different occupant expectations.
Annual profiles can also be quite different, although they
follow more closely the inverse of the ambient temperature.
Figure 5 shows that systems where the heat source also provides hot domestic water have some load during the summer.
In China, district heating systems can be shut down outside
the heating period.
While not shown in Figure 5, cooling could complement
the annual space heating profiles. In some climates where
heating and cooling needs are comparable, similar flexibility from cooling could complement flexibility from heating.
Wherever there are interconnected power grids spanning
across warm and cold climate zones, part of the variations,
depending on the relative strength of the interconnections,
can be smoothed at this continental scale. In either case,
heating and cooling could provide a rather stable source of
potential flexibility for the power sector. Furthermore, in
hot and sunny countries, cooling loads and PV generation
may complement one another well.
Industrial heat demand at the country level does not
exhibit strong seasonality and could, therefore, provide
year-round flexibility (e.g., the industrial heat demand from
Finland shown in Figure 5). Also, daily profiles, especially
in heavy industries, are typically relatively flat. In individual
industrial sites, the profiles can have more variation-for
example, lower demand for products can cut work shifts.

0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
1

51

101

151

201

251

301

351

Large District Heating
System in Finland

Large District Heating
System in China

Domestic Gas Demand
in Belgium

Electric Space Heating
Demand in Ireland

Industrial Heat Demand
in Finland

Heat Demand in
the United Kingdom

figure 5. The average daily heat demands over a year.

about 58 kWh of thermal energy storage. In a not-so-well
insulated house on a cold day, this would last about half a
day. It can become quite impractical to install much larger
hot-water tanks inside residential buildings because they
require considerable space and would likely not fit through
door frames. Most existing water tanks are much smaller.
Consequently, for the most part, hot-water tanks offer flexibility constrained by a limited time constant-although the
flexibility can still be valuable for the power system when
aggregated over millions of houses.
ieee power & energy magazine

29



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

IEEE Power & Energy Magazine - January/February 2017 - Cover1
IEEE Power & Energy Magazine - January/February 2017 - Cover2
IEEE Power & Energy Magazine - January/February 2017 - 1
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IEEE Power & Energy Magazine - January/February 2017 - Cover3
IEEE Power & Energy Magazine - January/February 2017 - Cover4
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