Investigation of cold filling receiver panels and piping in molten-nitrate-salt central-receiver solar power plants Page: 3 of 10
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
1995 ASME International Solar Energy Conference
Lahaina, Maui, HI
March 19-24, 1995
THE NEED FOR COLD FILLING PIPING AND THE
In a molten-salt central-receiver power plant, the parasitic
electrical power consumption can be a significant fraction of the
total power production if it is not properly managed. Good
management also involves careful assessment of operating
strategies to minimize the parasitics. Since the nitrate salt, which
serves as the heat transfer medium between the receiver and the
steam generator, has a freezing point of 430*F (2214C) the
associated piping, valves, instrumentation, and tanks must be kept
above this temperature (typically at 550*F, 288*C) to assure that
the salt will not freeze. During inclement weather and during
nightly shut down, the plant is not operating, but the heat trace is
kept energized to maintain the temperature of the salt lines at
550 F (288*C).
One strategy of reducing the nightly parasitic power consumption
is to drain the system and to turn off the heat trace at night to the
major pipes such as the riser and down comer and receiver tube
headers, allowing the piping to cool down to ambient, then cold
filling the piping at startup the next morning. Cold filling, also
referred to as cold starting, involves flowing molten salt through
piping or the receiver when all or part is below the salt freezing
Cold filling has several advantages in the operation of a plant that
experiences cyclic operation. After scheduled or unplanned
maintenance, it may take several hours to heat up the piping with
heat trace - time that could be used to collect and produce energy
rather than consume it. If the molten salt can be pumped through
part of the system which is below the freezing point, then
parasitics could be reduced, and the operation of the plant could
be more flexible increasing the availability.
Hours before morning startup, the heat trace to the receiver
headers and jumper tubes (the section of tubing that transitions
between the headers and absorber panels) has to be turned on to
ensure their temperatures are at the salt temperature. This
parasitic power could be reduced if the headers and jumper tube
could be filled cold.
In addition, the absorber panels do not have heat trace and must
be heated with heliostats before they are filled with molten salt. It
is difficult to uniformly preheat the absorber panels with
heliostats in the early morning. Some areas will experience much
more heating than others due to non-uniform flux profiles from
heliostats. This is a particular concern for the east side of an
external cylindrical receiver during morning start up. Localized
convection will add to the problem. If the receiver can be filled
with molten salt when some areas of the receiver are below the
salt freezing point, the receiver start up procedure would be much
simpler and start up could occur sooner.
There are two major concerns with cold filling components and
piping: freezing of the molten salt and transient thermal stresses.
There has been very little data collected on cold starting molten-
salt receivers and piping at temperatures below the molten-salt
freezing point. The Molten Salt Electric Experiment receiver in
the external configuration was cold started at temperatures below
the freezing point. In one of three cases, the receiver partially
froze (Bergan, 1986). However, no detailed analysis was done on
the transient freezing phenomenon for this experiment.
Fortunately, there is a large body of literature which describes
experiments and theoretical treatment of transient freezing. This
paper describes experiments and analyses we have performed on
cold starting receiver panels and piping.
DESCRIPTION OF THE FLOW LOOP
We conducted cold fill experiments on two molten-salt receiver
panels that we removed from a salt-in-tube receiver and on a
section of piping in a molten-salt loop. Each panel consists of
two serpentine-flow passes. Each pass has six 1 inch (2.5 cm)
OD 304 stainless steel tubes with 0.065 inch (1.65 mm) thick
walls. The two passes are connected to a common 6 inch (15 cm)
diameter manifold (schedule 80 piping) at the top of the panel.
Each panel vent connects to a common 1 inch vent line. The
experiment is located at the base of the Solar Tower at the
National Solar Thermal Test Facility at Sandia National
Laboratories in Albuquerque, NM. Figure 1 shows a schematic
of the system and a photograph of the receiver panels.
In this flow loop, salt is pumped from the salt sump and can either
return to the sump or can be diverted up the riser. At the top of
the riser is the pressurized accumulator (surge) tank. The salt
flows through the down comer and can either be diverted to the
panel or back to the sump. The outlet of the panel returns to the
sump. The pump can flow salt at 100 gallons per minute (380
liters/min) through 2 inch (5.1 cm) schedule 40 stainless steel
We measured the thermal responses as the panel tubes and system
piping underwent rapid changes in temperature due to filling with
molten salt and estimated the heat transfer coefficients during the
transients. We also derived expressions describing the transient
stresses that a pipe or tube experiences during a thermal shock.
Using a correlation that describes the penetration distance of a
liquid as a function of the fluid properties and flow conditions,
we estimated the distance salt could flow through cold piping
before freezing shut.
RESULTS OF COLD FILL PANEL AND PIPING TESTS
We conducted tests where we varied the initial panel temperature
to determine whether salt could flow through all four passes of
the panel before freezing. The flow velocity was approximately
the same for each test. The purposes of these tests were to 1)
determine if salt flow could be established in "cold" manifolds,
panels, and piping, 2) measure the thermal responses of the tubes
and manifolds undergoing thermal shock, and 3) estimate the
corresponding stresses in the materials.
We conducted a series of tests with decreasing panel preheat
temperatures ranging from 550*F (288*C) to ambient before
initiating salt flow. Then we tried flowing salt through cold (near
ambient) manifolds (heat trace off) with the panels preheated to
550*F. Finally, we tried flowing through cold manifolds and cold
panels. Each one of these scenarios was repeated several times.
James E. Pacheco, et. al.
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Pacheco, J. E.; Ralph, M. E. & Chavez, J. M. Investigation of cold filling receiver panels and piping in molten-nitrate-salt central-receiver solar power plants, article, December 31, 1994; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1280613/m1/3/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.