Civil Engineering Reference
In-Depth Information
Table 1 Thermal properties of various earth materials (Yumrutas and Ünsal
2012
)
Earth type
Thermal conductivity
(W/mK)
Thermal diffusivity
(m
2
/s)
Specific
heat (J/kg K)
Heat capacity
(kJ/m
3
K)
1.39 9 10
-7
Coarse gravelled
0.519
1,842
3,772
5.75 9 10
-7
Limestone
1.3
900
2,250
14.00 9 10
-7
Granite
3.0
820
2,164.8
summer and winter seasons, respectively. By integrating the water tank with the
tubular heat exchanger in the building, the stored energy can be retrieved and
redistributed to the zones requiring cooling or heating.
On the other hand, the gravel water storage makes use of the capillary and
thermal conductivity aspects of rocks or pebbles for achieving the desired thermal
storage in buildings. In general, cavern TES systems are comparatively expensive
than the other type of seasonal storage systems, which is due to their site selection,
constructional and operational aspects on long-term basis.
However, the facility installed in Chemnitz, DE, has an expected solar heat cost
of 240
=
=
MWH for 8,000 m
3
of energy storage capacity (Schmidt et al.
2004
). The
thermal properties of various earth materials and the comparison of various heat
storage systems are listed in Tables
1
and
2
, respectively.
4.7 Rock Thermal Energy Storage
As the terminology infers the utilization of rocks for TES, the thermal capacity,
density and temperature of the rocks play a vital role in capturing and releasing the
cold or heat energy depending upon the fluctuating load conditions in buildings
during seasonal variations. The schematic diagram of the rock TES system is
represented in Fig.
8
.
The impermeable, buckle-free, strong rock structures capable of withstanding
heat or cold energy on a long run are much preferred in this regard. The extraction
of thermal energy from the rock structures is made possible by passing the heat
transfer medium (air or fluid) through the long drilled holes in the rocks.
The absorbed heat energy is then transferred to the room spaces through the
circulation of heat transfer medium in the room heat exchanger. In recent years,
solar collectors are also used for charging (storing) the heat energy at elevated
temperatures in the rock structures with the help of heat transfer fluid. The dem-
onstration facility of a rock storage system with 10,000 kl has been installed in
Sweden.
The rock TES systems are developed for catering the thermal energy redistri-
bution needs for more than 1,000 dwellings on one stretch. The cost involved in
drilling holes through the rocks, rock-site location, strength and reliability of rock
structures subjected to frequent thermal cycles are the limiting factors to be
addressed for their practical implementation.
