Environmental Engineering Reference
In-Depth Information
Single-Tank Thermocline Storage
A single-tank thermocline storage system stores thermal energy in a solid
medium—usually silica sand—located in a single tank. At any time during
operation, a portion of the medium is at high temperature and a portion is
at low temperature. The hot and cold temperature regions are separated by
a temperature gradient or thermocline. High temperature heat transfer fluid
flows into the top of the thermocline and exits the bottom at low temperature.
This process moves the thermocline downward and adds thermal energy to
the system for storage. Reversing the flow moves the thermocline upward and
removes thermal energy to generate steam and electricity. Buoyancy effects
create thermal stratification of the fluid in the tank, which helps stabilize and
maintain the thermocline. Figure 7.3 shows the basic thermocline storage tank
concept in which hot and cold materials are stored inside the tank.
The thermocline technology has proven advantageous because the reduc-
tion of materials used for constructing the tank and storing heat decreases
cost and energy input. Using a solid storage medium for only one tank
reduces the cost relative to the two-tank systems. The thermocline system
was demonstrated at the Solar One power tower, where steam was used as
the heat transfer fluid and mineral oil was used as the storage fluid. However,
this technology is still undergoing development and requires more research
before it is economically and technically viable.
StorageVesselDesign
Tank Geometry
The cylinder is the most practical and common geometry for a storage
tank. Spherical storage tanks are also common for limited applications. For
instance, spherical vessels are typically used underground or supported by
columns for gravity pressured supply; above-ground tanks are most often
cylindrical because of construction practicalities. These geometries will be
compared in this section. Figure 7.4 indicates the parameters used to char-
acterize them.
For a given volume, spherical geometry presents the least surface area—a
desirable factor for minimizing the materials and area for heat transfer to
the surrounding environment. The surface area-to-volume ratio for a sphere
is a constant equal to 3/
r
, so the size of a spherical vessel is determined by
the volume of storage material needed. The cylindrical shape with the least
surface area is one whose height,
h
, is equivalent to its diameter; the surface
area-to-volume ratio is 3/2
r
. In the best case where
h
is twice the radius, a cyl-
inder has 1.5 times the surface area of the sphere and just 2/3 of its volume.
Table 7.5 shows how these two geometries compare.
