Ice Slurry TES for TIC

Application of GREEN Ice Thermal Storage System for Peaking Gas Turbine Power Plant

Application of GREEN Ice Thermal Storage System for
Peaking Gas Turbine Power Plant.
Mr. Stan (Shlomi) Rott, IDE Technologies; Dr. Ishai Oliker, P.E., Joseph
Technology Corporation, Inc.

Abstract
The latest developments in Thermal Energy Storage (TES) technology have played an
increasingly important role in its use in peaking power plants for Gas Turbine (GT) inlet
cooling applications. The chiller system design utilizes TES to increase hot weather GT
power output and improved performance, while shifting chiller parasitic power
consumption to off-peak periods. Additional benefits can be achieved if Vacuum Ice
Making (VIM) technology producing Ice Slurry (as the TES medium) is utilized,
resulting in low GT compressor inlet temperatures of about 42°F.

The presented study is based on an actual peaking load power station with a single 47
MW (ISO conditions) Stewart & Stevenson LM6000 HP SPRINTTM GT, equipped with a
chiller for inlet cooling. The study was prepared while extensively utilizing analytical
tools for modeling GT performance and economics. VIM TES and cycle simulations
were based on the actual hourly GT compressor inlet temperatures, atmospheric pressure,
gross MW output power and net MW power measurements that were recorded at the
power plant over a one year period. Additionally, the VIM TES charge/discharge
schedule was optimized based on LM6000 HP SPRINTTM compressor mass flow
requirements to reduce chiller parasitic power consumption to the maximum extent
possible.

Use of the VIM TES has been found advantageous in terms of parasitic load shift to off-
peak hours, as well as an increase in net MW power output. The results were based on the
turbine manufacturer’s proposed modifications to the SPRINTTM system, in conjunction
with the low compressor inlet temperatures achievable with VIM TES technology. This
approach resulted in a 25% decrease in chiller parasitic power consumption and a 12%
increase in net MW power output of the turbine.

Key words
• Ice Slurry • Static Ice
• Vacuum Ice Maker (VIM) • Dynamic Ice
• Thermal Energy Storage (TES) • TES Systems
• “On demand” Chillers • Turbine Inlet Cooling (TIC)

Introduction
This paper presents the specialized application of TES for the Combustion Turbine (CT)
inlet cooling of an LM6000 gas turbine equipped with the SPRINTTM system. The main
objective of the study was to evaluate the effects of the lowest allowable compressor inlet

March 3, 2011 Page | 1


temperature on the electric output of the turbine, as well as to quantify annual added
MW-hours.

A variety of cooling approaches are typically considered for turbine cooling applications.
One characteristic common to all approaches is the ability to push closer to (or even
below) the compressor air inlet temperature at ISO conditions, namely 59°F and 14.7psig.
Additional factors that need to be considered during the selection process are the
electrical rate structure or time-variable value of power, the capital cost of the TIC
system, operation and maintenance costs, efficiency of the equipment and modularity and
expandability of the system to accommodate future expansions.

One of the most popular approaches to GT inlet cooling is the use of standard mechanical
chillers. These chillers are brought online simultaneously with the GT, at the time when
load is demanded. Therefore, they are sometimes referred to as “on demand” chillers.
When “on demand” chillers are applied to a variable load, they should be able to respond
by increasing or decreasing their compressor capacity in the most efficient and rapid way.
When an “on demand” chiller is coupled with a turbine GT that operates during peak
demand hours only, the plant owner will realize a significant loss of net power and
revenue due to the chiller’s parasitic power consumption. Most standard chillers are rated
at a 45°F supply water temperature and can experience as much as 30% reduction of
capacity if and when operating at substantially lower than rated temperatures.

Ice-based systems are frequently found to be used in industrial cooling applications. Ice-
based TES systems function independently of the cooling load. During its operation, an
ice-based TES system charges while building ice for use during a subsequent discharge
cycle. Such ice-based systems, in which the ice is formed and later melted in one place
(on a heat transfer surface), are known as “static ice” systems. The ice formation occurs
while a low temperature refrigerant is circulated through the heat exchange surface to
extract the heat of the surrounding water. In order to address the required load, the ice is
melted by circulating a secondary refrigerant through the tank or through the heat
exchange surface. In the former case, the secondary refrigerant melts the external layer of
ice, hence “external melt”; in the latter case, the secondary refrigerant melts the internal
layer of ice, hence “internal melt”.

Ice slurry makers are often referred to as “dynamic ice” systems. Such systems are able to
produce ice with the consistency of slush or snow. In other words, the ice particle is very
small. An additional characteristic of ice slurry is improved heat transfer capability due to
the vastly increased available heat transfer surface. In addition, VIM ice slurry does not
require a defrost cycle in the traditional sense, which allows for a rapid and highly
variable discharge rate to address load fluctuations when required. Finally, with VIM ice
slurry production, there are no adverse insulating effects associated with the thickness of
an ice layer formed on a heat transfer plate, coil or tube.

In general, the benefits of applying any type of TES to the industrial processes include,
but are not limited to, the following: smaller capacity and footprint attributed to the
refrigeration equipment, increased level of redundancy, load shifting, revenue recovery



attributed to minimizing the on-peak “parasitic” power consumption and (especially for
ice slurry TES) lower water supply temperatures even during rapid TES discharge.

Feasibility Analysis
The plant under consideration is equipped with one (1) LM6000 SPRINTTM Gas Turbine,
manufactured by GE's Stewart & Stevenson. The GT combustor temperature is 1,600 °F,
and the heat rate of the GT is 8,900 Btu/kWh. The plant efficiency is estimated at 38%.
The GT is equipped with an HP SPRINTTM, which injects an atomized spray of
demineralized water into the inlet of the high-pressure stage of the compressor in order to
increase the GT’s electric power output and improve its heat rate.

Existing TIC
The plant is located in the humid, continental climate of the North East coast of the
United States and is equipped with a McQuay 2,000 Ton chiller with two compressors.
The chiller uses 134a refrigerant and circulates a water-glycol solution through the GT
compressor air inlet coil. Typical inlet air temperature at the compressor inlet is between
50 and 55 °F; glycol supply temperature is 47 °F with a return temperature of 54 °F. The
estimated parasitic power consumption due to the operation of the “on demand” chiller is
in access of 2 MW. The specific power consumption of the chiller system (chiller, pumps
and condenser fans) is 0.755 kW/Ton.

Plant Operation Mode
The plant operates for 7 to 8 hours a day during peak load demand hours, including
weekends, for the most of spring, summer and early- to mid-fall months.
Operating Hour Statistics
The plant’s operating history is presented below in Figure 1; the data was collected
between the years 2005 and 2010. The total time that the plant was operational during the
year 2009 was 395 hours. This small amount of operational hours is attributed to the
scheduled maintenance that the turbine underwent during that period.

Figure 1: Years 2005 - 2010



Plant Annual Hourly Operation Data
The evaluation of the turbine operation in general, and TIC functionality in particular,
was based on the annual hourly data that was recorded starting from August 1, 2009 to
July 31, 2010. The recorded data included the following parameters: Gross Power Output
[MW], Net Power Output [MW], Compressor Inlet Temperature [°F], Ambient Dry Bulb
Air Temperature, DB [°F], Compressor Inlet Pressure [psia], and Relative Humidity [%].

The analysis of the aforementioned data included filtering out all the operating values
attributed to the turbine operation without inlet cooling. Accordingly, values that were
recorded while the ambient air temperature was below 46 °F were not included in the
analysis. Also, the turbine is equipped with a boiler, which supplies hot water to the inlet
coil when ambient air temperature drops below 42 °F.

Superimposing Gross Power Output with the Compressor Inlet Temperatures in Figure 2
allowed the reconstruction of the turbine performance signature, and, further, gave the
opportunity to extrapolate this to compressor inlet temperatures lower than 46 °F, down
to 42 °F, which is considered the lower value for a safe operating range for the LM6000
(to avoid the chance of icing in the inlet to the compressor).

Figure 2: Turbine Signature

Next, unavoidable parasitic power consumption has to be isolated in order to evaluate the
magnitude of the shifted load that would be acquired due to the use of the TES.
Comparing ambient air temperatures with the delta between the turbine’s Gross and Net
Power Output in Figure 3 allowed estimating avoidable parasitic power consumption.
The average monthly DB temperatures are shown in Figure 4.

LM6000 Cooling Load Requirements
According to ASHRAE guidelines for Turbine Inlet Cooling, the site conditions of TDB =
91 °F, TWB = 74 °F, and optimal inlet air TWB = 46 °F should be used in order to estimate



the TIC required cooling load. Considering GT Inlet Compressor nominal mass flow of
291 lb/s, the required calculated cooling load is:
. lbs sec Btu Btu 1
Q = m(h1 − h2 ) = 291 ] * 3,600[
[ ] * (37.8[ ] − 18.2[ ]) * = 1,711TR
sec hour lbs lbs Btu
12,000[ ]
tonR
For good engineering practices, the TES sizing and operation schedule was based on a
cooling load requirement of 1,750 tonR.

Figure 3: Avoidable Parasitic Power Consumption

90
AVERAGE MONTHLY DRY BULB

80
70
TEMPERATURE, F

60
50
40
30
20
10
0
0 2 4 6 8 10 12 14
Figure 4: Average Monthly DB Temperatures

However, the reduction of the compressor inlet temperature to below 46 °F would require
modification of the SPRINTTM system, as well as turbine control system adjustments.



The manufacturer of the LM6000 confirmed that keeping the compressor inlet air
temperature at 45°F would result in a turbine electric output increase from about 47 MW
(today) to 51.3 MW provided the generator operates at, or above, a Power Factor (PF) of
0.85 (Figure 5).

Figure 5: Generator Curve

Equipment Selection Considerations
Several types of TES systems were considered for this particular application. It is also
important to note that TES offers a better redundancy in comparison to the “on demand”
chiller alone. If the chiller was brought down for maintenance, the TES would have
supplied the necessary cooling load to support turbine operation. Conversely, if the TES
is taken off line, the chiller would have been able to supply the cooling load to support
the turbine operation.

Chilled Water TES
The chilled water storage is the most common and tested approach for inlet cooling. It
offers the desired redundancy at low initial cost and ease of integration into any existing
inlet cooling system. However, when operating at lower discharge temperature, the
chiller power consumption increases and its performance is de-rated. In addition, the
required chilled water TES volume is several times larger than that of an Ice TES of



compatible capacity. With space limitation at the power plant, chilled water TES size
becomes an issue for the implementation. Finally, stratified chilled water TES is limited
to a minimum supply temperature of approximately 39 °F (4 °C), which is the
temperature at which water exhibits its maximum density.

Static Ice TES
Static ice TES is a frequently used solution for shifting load in commercial and industrial
applications. It also offers the desired redundancy; however, it does so at a higher initial
cost in comparison to chilled water TES, when applied to large scale applications.
Additionally, most often, static ice TES requires dedicated chillers and rather
complicated tanks filled with internals and/or moving parts, resulting in extensive
maintenance. Also, due to the adverse insulating effects of the static ice layers, the
refrigerant during the freeze cycle has to be at a very low temperature, typically at 14 to
22 °F. Such low temperatures require, or prefer, the application of ammonia, the use of
which becomes an issue, especially in urban surroundings. Finally, static ice TES
systems, with their inherently limited heat transfer surface area, are incapable of
maintaining a cold supply temperature during the rapid TES discharge periods that are
commonly desired for TIC applications.

Dynamic Ice TES
Dynamic or ice slurry TES systems are commonly used when rapid refrigeration is
required. As with the two previously discussed TES systems, dynamic ice TES offers the
desired redundancy, and does so at a cost comparable to static ice. Due to its dynamic
state, ice slurry can be pumped; and therefore the ice slurry generator can be located
separately from the TES tank. This advantage allows for the construction of simple, low
cost and low maintenance TES tanks, without any internal heat transfer surfaces or
moving parts. Finally, VIM ice slurry is produced using water vapor as the only
refrigerant, making it environmentally friendly.

Operation of the Vacuum Ice Maker (VIM)
In order to reduce the LM6000 compressor inlet air temperature to below 46 °F and avoid
the chiller parasitic losses during daily plant operation, it is proposed to install the VIM
TES system described below in Figure 6.

Inside the VIM freezer, water is at its “triple point” where all three phases (vapor, liquid,
and solid) are in equilibrium, exposed to a deep vacuum at 32 °F. The vacuum forces a
small part of the water to evaporate while another part of the remaining water freezes,
forming a water-ice mixture. The mixture is pumped out of the freezer as ice slurry into a
TES tank until the ice concentration in the tank reaches 50%.

In order to maintain the deep vacuum in the freezer, the water vapor is continuously
evacuated from the freezer, compressed and fed into a condenser by a unique centrifugal
compressor. Condensing the water vapor requires cooling water at 5.5 °C (42 °F), which
is supplied from a conventional new or (in this instance) existing water chiller.



During the TES discharge cycle, chilled water at 32 °F from the bottom of the TES tank
is circulated through a heat exchanger in order to meet the required cooling load demand.

Figure 6: VIM TES Flow Diagram

Proposed Configuration
The integrated approach for the retrofit of the existing TIC system at the LM6000 seems
to be most appropriate. In addition to the immediate capital investment savings,
integrating VIM TES into the existing TIC offers a greater degree of redundancy and,
therefore, ensures continued operation of the Power Plant overall (Figure 7).

Figure 7: Proposed Configuration - Flow Diagram



Proposed Operation Cycle
The proposed operation cycle is based on a weekly cycle charge period that takes place
during off-peak hours on weekdays, as well as during extended hours over the weekend.
The off-peak period is Monday through Friday between 10:00 pm and 8:00 am; therefore,
the proposed daily off-peak charge period is 10 hours. A VIM with a nominal capacity of
1,000 Tons will be able to accumulate 10,000 ton-hrs during each weekday charge
period. In addition, the VIM will continue charging TES during the extended weekend
hours in order to make up for the mismatch of cooling charge and discharge capacities
during weekdays.

8-Hours Daily Discharge Period
The average duration of the LM6000 operation is 8 hours per day, including weekends.
Therefore, the required daily cooling load is:
DailyDemand = 8hours *1,750Ton = 14,000TR − h
In order to optimize the TES tank capacity and minimize capital investment, the weekend
daily discharge period is reduced to 5 hours per day. Therefore, the weekend cooling load
demand for the inlet cooling is:
WeekendDem and = 5hours * 1,750Ton = 8,750TR − h
The estimated minimal TES Tank capacity required to support the operation of the
weekly cycle is 30,000 Ton-hrs. The optimized Charge/Discharge TES cycle, with 8
hrs/day of discharge on weekdays and 5 hrs/day of discharge on Saturday and Sunday,
and 10 hrs/night of charge on weeknights and 18.8 hrs/night of charge on Saturday and
Sunday, is illustrated in Table 1:

[TR-h] Mon Tue Wed Thu Fri Sat Sun
TES Cap 30,000 26,000 22,000 18,000 14,000 10,000 20,000
Discharge 14,000 14,000 14,000 14,000 14,000 8,800 8,800
Residual 16,000 12,000 8,000 4,000 0 1,200 11,200
Charge 10,000 10,000 10,000 10,000 10,000 18,800 18,800
Final Cap 26,000 22,000 18,000 14,000 10,000 20,000 30,000

Table 1: TES Discharge Cycle - 8-Hours

Considering 3 ft3 per Ton-hr of TES, the estimated volume of the ice slurry TES Tank is
90,000 ft3 or about 0.67 million gallons (e.g. a vertical cylindrical tank of 53.5 ft diameter
and 40 ft high).

Power Consumption Estimation
The power consumption of the VIM TES has to be looked at separately during the
Charge and Discharge cycles. During the off-peak Charge cycle, the biggest power
consumers, in descending order, will be the supporting (existing) chiller, the VIM, and
the coolant pump. It is important to note that the use of TES and the optimized



Charge/Discharge schedule will minimize existing chiller power consumption during
peak (high power value) periods. The heat load that the existing chiller is required to
reject includes VIM and its auxiliaries, estimated to be about 1,120 Tons. With a specific
power consumption of 0.755 kW/Ton, the 2,000 Ton-rated chiller will reject at least 25%
more cooling load. However, the system will operate during the night hours at cooler
ambient condensing temperatures in comparison to the day time. This factor will
contribute to an improvement in the chiller’s seasonal efficiency. Table 2 summarizes the
power consumption data during the off-peak TES Charge cycle:

Power
Item Qty Consumption
(kW)
VIM System 1 382
Supporting Chiller 1 868
Coolant Pump 1 55
Total: 1,305

Table 2: Power Consumption – Off-Peak TES Charge Cycle

During the peak TES Discharge cycle the power consumption of the system is limited to
two pumps only, namely the circulation and coolant pumps. Table 3 summarizes the
power consumption data during the peak TES Discharge cycle.

Power
Item Qty Consumption
(kW)
Circulation Pump 1 75
Coolant Pump 1 75
Total: 150

Table 3: Power Consumption – Peak TES Discharge Cycle

Results
The analysis of the original annual data and the performance simulation yielded the
following observations and results. The average electric gross power output of the
LM6000 SPRINTTM with the existing TIC chiller running is about 47 MW (Figure 2).
The average parasitic power consumption of the turbine support systems is about 3.5
MW, with an estimated total unavoidable parasitic power consumption attributed to the
auxiliaries and natural gas supply pump of 1.5 MW. The estimated avoidable parasitic
power consumption attributed to the existing chiller is about 2 MW. Therefore, the output
of the turbine, including unavoidable power consumption, is estimated at 45 MW.

March 3, 2011 Page | 10


According to the simulation results and manufacturer’s data, executing the compressor
inlet temperatures below 46 °F, control adjustments, and HP SPRINTTM conversion to
full SPRINTTM will increase electric power output to 51.3 MW, resulting in a net increase
of about 4.3 MW.

Summarizing the above, the retrofit for the existing turbine inlet cooling systems, in
conjunction with the required modification, will result in shifting 2 MW from peak to off-
peak hours as well as increasing turbine electric output by an additional 4.3 MW. The
results of the simulation are summarized in Table 4:

Operation Data Summary
Discharge Period (weekdays) 8-hours
Annual estimated recharge hours (VIM Operation hours) 2,628
Charge cycle power consumption, off-peak (MW) 1.305
Annual power consumption to recharge TES, off-peak (MW-h) 3,430
Annual estimated discharge hours 1,680
Discharge cycle power consumption, on-peak (MW) 0.15
Annual power consumption to discharge TES, on-peak (MW-h) 252

Avoided parasitic power consumption, on-peak (MW) 2
Annual avoided parasitic power consumption, on-peak (MW-h) 3,360
Estimated added power capacity, on-peak (MW) 4.3
Annual added electric power output (MW-h) 7,224

Total annual increase in net off-peak consumption (MW-h) 3,430
Total annual increase in net on-peak production (MW-h) 10,584

Table 4: Summary of Simulation Results

Conclusions
The study of the VIM TES retrofit for the existing mechanical chiller inlet cooling system
of the LM6000 SPRINTTM has been found plausible and attractive in terms of parasitic
load shift to off-peak hours, as well as in terms of an increase in net MW peak power
output.

When considering the turbine power electric output, the addition of 4.3 MW results in a
9.1% increase in terms of turbine gross electric output. When considering the parasitic
power consumption, the load shift of 2 MW to off-peak hours results in a 3.0% increase
in terms of turbine gross electric output. The total increase of turbine net electric output is
about 12% or 5.7 MW.

The TIC configuration currently installed at the turbine uses an “on-demand” chiller to
address the required cooling load. After the VIM TES retrofit, the required cooling load
is reduced from 1,750 Tons (occurring during peak) to only about 1,100 Tons (occurring

March 3, 2011 Page | 11


off-peak). Considering the chiller’s specific power consumption of 0.755 kW/Ton, the
total parasitic power consumption was reduced by more than 25% and moved from
critical, high-value peak periods to less critical, low cost off-peak periods. In addition,
the use of TES adds valuable redundant capacity to the TIC system.

March 3, 2011 Page | 12


Appendix

Table of Figures
Figure 1: Years 2005 - 2010 ......................................................................................................... 3
Figure 2: Turbine Signature ......................................................................................................... 4
Figure 3: Avoidable Parasitic Power Consumption ....................................................................... 5
Figure 4: Average Monthly DB Temperatures .............................................................................. 5
Figure 5: Generator Curve ........................................................................................................... 6
Figure 6: VIM TES Flow Diagram .................................................................................................. 8
Figure 7: Proposed Configuration - Flow Diagram ........................................................................ 8

List of Tables
Table 1: TES Discharge Cycle - 8-Hours......................................................................................... 9
Table 2: Power Consumption – Off-Peak TES Charge Cycle ......................................................... 10
Table 3: Power Consumption – Peak TES Discharge Cycle........................................................... 10
Table 4: Summary of Simulation Results .................................................................................... 11

March 3, 2011 Page | 13

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