3352
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Ingenieure 2004). The figures show that the natural water
content of the soils is very important for the thermal parameters.
Figure 2. Typical values of thermal conductivity i
n W/(m∙K)
.
Figure 3. Typical values of volumetric heat capacity in MJ/(m
3
∙K)
.
The range of operational temperature must be as large as
practically possible, as the range will impact on the necessary
volume of storage.
Large seasonal thermal energy storages may be established as
one of the following systems: TTES, PTES, ATES or BTES.
The TTES (Tank Thermal Energy Storage) system consists of
an insulated steel tank filled with water and is widely used in
the short-term regulation of the heat consumption against the
heat production at heating plants. A volume of 1,000
–
5,000 m
3
is often adequate for most Danish medium-sized district heating
systems. Typically operational temperatures are T = 30 - 90°C,
i.e. ΔT ≈ 60°C
.
The PTES (Pit Thermal Energy Storage) system is an
excavated pit, which is lined by a membrane and filled with
water. Operational temperatures are typically 30 - 90°C. The
upper temperature affects the lifetime of the membrane and long
term exposure of the upper temperatures therefore has to be
avoided. The permissible level and exposure time of the upper
temperature is a trade-off between the lifetime of storage and
storage capacity. Usually the storage is not insulated towards
the soil, as the energy loss through these areas is acceptable low
under certain circumstances. The top of the storage
–
the water
surface
–
is exposed to alternating climate conditions, including
cooling by wind, which requires insulation. The insulation may
be floating on the water or carried by a supporting system. For
larger storages a PTES system is very cost-effective. In
Denmark a few pilot PTES plants are in use with volumes up to
10,000 m
3
, but larger plants are under construction, as this
article describes.
The ATES (Aquifer Thermal Energy Storage) system stores
the heat in a groundwater aquifer. The extend and
characteristics of the aquifer must be well-known as the
groundwater is pumped from a number of wells and
–
after
passing a heat exchanger to impact or extract heat energy -
infiltrated into the aquifer in another part of the aquifer.
Typically operational temperatures are 5 - 30°C
with ΔT ≈
25°C, i.e. the volume of water must be larger than for the above
mentioned storages. This type of plant requires a groundwater
aquifer with high permeability. In Denmark most of the
drinking water supply is based on groundwater, and this implies
that large ATES’s will not be allowed in areas with
special
interest of drinking water supply. A
growing number of ATES’s
have though been established, mainly initiated by a need of
cooling during summertime of large building complexes.
The BTES (Borehole Thermal Energy Storage) system
consists of a large number of boreholes with loops of heat pipes
installed. The heat is transferred to the soil by circulating brine
in the heat pipes and vice versa when the heat is to be
consumed. As operational temperatures are 20 - 60 °C
, i.e. ΔT ≈
40°C, and the heat capacity of soil is small compared to water, a
larger soil volume is needed than for storages based on water.
This is compensated as the boreholes usually go to 50
–
100
meters depth. The thermal conductivity of soil is moderate, and
the response of the storage is thus relatively slow. At present
only one pilot plant has been established in Denmark at a
district heating plant.
2 SEASONAL ENERGY STORAGE IN MARSTAL
Marstal is a town with 2400 citizents on the Danish island
Aeroe. For many years the district heating system in the city of
Marstal has been based on oil fuels. During the last decade
Marstal District Heating has turned towards 100 % renewable
energies, so that from 2012 a solar heat system
–
covering
33,000 m
2
solar heat panels on 10 hectares of land
–
will
produce more than 50 % of the heat consumption and the rest
will come from biomass energy. At present the plant is the
largest solar heat plant for district heating in the world, but this
ranking will presumable only be held for a short period.
Marstal has been a Danish pioneer in thermal energy storage.
In 1998 a 3,000 m
3
combined gravel and water pit has been
built, and the plant was followed by a 10,000 m
3
PTES in 2003.
Calculations have shown that the requested large percentage of
solar heat coverage in Marstal needs a volume of 75,000 m
3
water in which case
all
surplus solar energy produced during
summertime can be stored until winter. This volume is
established by a PTES plant. The project is economically
supported by the European Union (EU).
Performing a PTES has some preferred technical conditions in
relation to an economic design in regards of both the
establishment phase and the operation phase as described in the
following:
The pit
must
be performed as an open pit without using e.g. a
framing sheet pile wall which would increase the cost
considerably. To minimize excavation costs the ground must
consist of soils which can be excavated and handled by
traditionally methods and with no significant groundwater
handling.
To reduce heat loss into the air the pit must be covered by
insulation with guaranteed resistance to temperatures up to
90°C for the lifetime of the storage. The top insulation and the
bottom membrane (in this case a 2.5 mm HDPE liner) are some
of the most expensive parts in a PTES and the area of the
insulation must consequently be minimized.
Dry soils insulate better than moist or saturated soils, and
moreover groundwater may introduce unwanted heat loss if
heated groundwater flows across the site. Therefore, the
groundwater level must be at a convenient depth below the
bottom of the pit, alternatively a higher groundwater level is
tolerated, but in that case no significant groundwater flow
across the site is allowed.
The loss of heat is reduced to a theoretical minimum when the
pit has a spherical shape. This is not obtainable in practice and
excavation is often performed as an upturned frustum of
pyramid. The width must be minimized, for which reason the
slopes of the sides of pyramid must be as steep as practical
possible. This reduces moreover the area of the expensive top
insulation.
In order to establish soil balance in the project the excavated
soil is to be used in building up embankments around the
0
1
2
3
4
5
6
Sand, dry
Sand,
saturated
Clay/silt, dry Clay/silt,
saturated
Water
0
1
2
3
4
5
Sand, dry
Sand,
saturated
Clay/silt, dry Clay/silt,
saturated
Water
