ENERGY EFFICIENT THERMAL ENERGY STORAGE FOR DX AIR CONDITIONING
Paul Kuhlman ([email protected])
Ramachadran Narayanamurthy ([email protected])
Ice Energy Inc.
9351 Eastman Park Dr.; Suite B
Windsor CO 80550
970-545-3630
ice-energy.com
Introduction
The growing demand for peak electrical power and increasingly congested electrical transmission lines is
the primary driver of excessive emissions caused by peak electrical energy generation. Thermal energy
storage(TES) systems are now available for direct expansion air conditioning, which not only shift peak
energy, but are also energy efficient.
This paper addresses:
• Energy efficient Thermal Energy Storage (TES) for DX air conditioning systems
• Peak energy use and its impact on power plant emissions
Technology Description
Ice Energy® Inc.’s Ice Bear® system is designed to alleviate the electrical grid loading during the peaking
hours by attacking the primary source of the peaking, namely, air-conditioning equipment. There are two
distinct air-conditioning markets – large buildings and facilities cooled by water (or a water-glycol
mixture), and smaller buildings and residences cooled directly by refrigerant. In all cases, the ultimate heat
sink is refrigerant, but in the first case, a secondary heat exchange is used to provide cold water to cool
buildings. Of the entire floor space in the United States, 48% is cooled by refrigerant-based systems and
32% by water-based systems. There is already a well established market with proven economics for energy
storage in water/glycol based systems. But due to the complexity of refrigerant management, there are no
known commercially available refrigerant-based TES systems other than Ice Energy’s Ice Bear.
The Ice Bear system utilizes unique technology for efficient energy storage for refrigerant-based airconditioning systems. The heart of the Ice Bear System is the Refrigerant Management System (RMS).
There are three main subcomponents to the Ice Bear System; the Tank, the Heat Exchanger, and the RMS.
The Tank is doubly insulated with an inner and outer skin of High Density Cross-linked Polyethylene.
Between the two layers is a 4”-6” thick foam insulation made of BASF Type AF-0306, injected to fill the
entire space between the two layers. The shell material is designed for high UV resistance, expected to last
for more than 70 years, and the foam provides enough insulation to keep a tank of ice at 120 F outside
temperature for more than 30 days. The heat exchanger is composed of helical coils of copper tubing from
the premier tubing supplier in the country, Wolverine Tube Inc. It is designed to minimize the amount of
copper (and hence cost and refrigerant) while covering the entire volume with equal ice thickness for
maximum efficiency.
The Refrigerant Management System (RMS) is designed with a liquid overfeed system that is widely
recognized within the refrigeration industry as a very efficient technology. The functions accomplished by
the RMS include:
1. Provide refrigerant metering (flow regulation) which is matched to the cooling load (ice formation)
required. This is accomplished by a patented metering device.
2. Provide oil return through a patented oil distillation heat exchanger
3. Feed refrigerant liquid to both the ice tank heat exchanger during the ice make process and to the pump
during cooling
4. Circulate refrigerant with a refrigerant pump (another unique component not found in standard
refrigeration systems) to the evaporator coils
The unique combination of these various components provides the high efficiency of the Ice Bear System
that cannot be replicated with other technologies, such as glycol-water systems.
The Ice Bear System’s integrated controller uses a sophisticated microprocessor-based design, thus
increasing functionality at reduced cost. The refrigerant pump being used for the production models is a
115 V pump and consumes less than 100 W.
The Ice Bear System is designed to replace units up to 7.5 Tons during on-peak hours which may extend
for up to 6 hours. The design is flexible with regards to loading, and there are almost no part-load losses as
with standard air-conditioning systems. The Ice Bear System consumes a maximum of 300 W during
cooling, which is 20 times less power consumption that standard air-conditioners. It also has the option of
stepping down the power consumption during low load time by turning off some components.
Conventional
Outdoor
Condensing Unit
Evaporator
Coil
Heating
Coi/Furnace
Filter
The Ice Bear System can be operated in various configurations based on the building load requirements.
For instance, in buildings requiring larger amounts of cooling (10 or 12.5 Tons), the Ice Bear System can
be operated in combination with a 5 Ton condensing unit to deliver up to 12.5 Tons, saving both peak and
total energy. In situations requiring greater dehumidification, the Ice Bear System can provide greater
energy efficiency using the cold water in the tank. The most popular configuration for peak shifting moves
more than 90% of air-conditioner power consumption off-peak. The Ice Bear System can be applied to a
wide variety of different commercial building types, including office buildings, shops, restaurants, and
sports centers, as well as larger residential buildings.
Conventional
Air Handler
Fan
Refrigeration
Management
System
Ice
Storage
Tank
Ice Storage Unit
Figure 1 Ice Bear Line Diagram
Figure 1 shows a simple line diagram of an Ice Bear System installation with a split system, where it is
installed between the air-conditioning unit and the indoor coil. Figure 2 compares the operating conditions
of an Ice Bear with a standard DX cooling system. During Ice Make, the condensing unit (compressor and
condenser) operates at a lower suction and discharge pressure than conventional cooling, thereby reducing
demand and total energy consumption. During Ice Melt, there is no significant pressure differential across
the loop as the motive force is a small pump with no expansion device.
Standard
Cooling
45 F
Ice Melt
46 F
Standard
Cooling
115 F
Ice Make
25 F
Ice Melt
48 F, 96 PSI
Ice Make
90 F
Figure 2 Ice Bear Connection Drawing
The Ice Bear System has three main modes of operation:
• Ice Melt: This is the cooling mode when the refrigerant pump is operated to provide building cooling.
The condensing unit is turned off in this mode. This mode is normally operated during the peak hours
of the day.
• Ice Make: The condensing unit is operated to freeze the water in the Ice Bear System to store cooling
capacity. This mode is normally operated at night.
• Direct Cooling: This mode allows usage of the Ice Make condensing unit to provide cooling instead of
using stored ice. In the majority of cases, direct cooling is applied when the Ice Bear System is the
only cooling system for a space, but at a time when it not desirable or possible to use the ice (see submodes below). During direct cooling, the energy consumption for the Ice Bear System includes the
power consumption of the Ice Bear System plus the power consumption of the condensing unit. In this
configuration, the user avoids the first cost of purchasing and installing an additional condensing unit.
There are 3 sub-modes for direct cooling:
• Ice Make Cooling: This occurs in cases where there is a requirement for cooling during the
preferred time for Ice Make, and there is no other system to accommodate the load. In this mode,
the condensing unit stays on continuously for the duration of Ice Make, but whenever there is a
call for cooling the refrigerant pump is turned on providing refrigerant to the cooling coil. When
the call for cooling stops, so does the refrigerant pump, and the cold refrigerant provided by the
condensing unit reverts to cooling the water and making ice.
• Ice Save Cooling: This mode is used usually for the mid-morning hours, between the end of ice
make and the beginning of ice melt. Ice Melt is not used in order to save the ice for the peak hours
of the day. In this mode, when there is no call for cooling, all systems are turned off. When the
thermostat calls for cooling, both the refrigerant pump and the condensing unit are turned on at the
same time. The pump re-routes the cold refrigerant generated by the condensing unit to the
cooling coil for room cooling. Both systems turn off when the call for cooling stops.
• Ice Exhausted Cooling: This mode serves to cool the building at the end of the day when all the
ice melt is completed but the room still requires cooling. Proper design ensures that this mode is
not needed during the peak hours of the day. This mode operates similar to Ice Save Cooling
mode with regards to the condensing unit and the Ice Bear System, except that there is no ice or
cooling capacity left in the tank. This mode also serves as a backup cooling mode in case of an
equipment malfunction that results in insufficient ice being produced on the previous night.
Performance Data
Electrical Data:
Voltage: 115 V and 208/230 V
Current: Up to 5 A at 115 V during cooling and up to 25 A at 208/230 V during ice-make
Cooling Performance:
Maximum Capacity: 50 Ton-Hours
Net Usable Latent Capacity: 42 Ton-Hours
Max. Load Capacity: 7.5 Tons
Estimated Power Consumption: Maximum of about 50 KWH over 12-14 hours for a complete ice make on
a peak summer day, and 35 KWH on a typical summer day.
Ice Bear and Emissions Reduction
Ice Energy’s storage technology is applied to off-the-shelf residential split and commercial rooftop
refrigerant-based air conditioning systems. The use of energy storage has the potential to reduce NOx air
emissions by shifting power generation from peak daytime hours to off-peak nighttime hours. Additionally
overall energy savings are realized by running the condensing unit continuously at its maximum heat
transfer rate, eliminating inefficient start-stop cycle losses and by running the condensing unit to store
energy during the relatively cooler evening hours. During the day when a standard thermostat calls for
cooling, a fractional horsepower 300 watt refrigerant pump circulates liquid refrigerant to a standard
evaporator coil. Shifting the demand for powering air conditioner compressors from daytime to nighttime
hours shifts the electricity needed from higher-emitting power plant generating units that are typically used
to meet peak loads to lower-emitting generating units that are used to meet off-peak, or nighttime, loads.
Furthermore, this shift (reduction) in generation and emissions occurs at the particular time — hot, summer
daytime hours — when ozone levels are likely to be highest.
Ice Energy Inc. asked the environmental and energy consulting firm E3 Ventures to investigate and
quantify Ice Bear related emissions reductions in a specific air district. The study1 investigated the time
related energy use of DX air-conditioning systems and the emissions characteristics at the generating
sources serving a specific Air Quality Management District, in this case Sacramento California,
(SMAQMD) and the primary electrical generation utility for the district, Sacramento Metropolitan Utility
District (SMUD). The study then compared the emissions changes that would occur if the Ice Energy Ice
Bear were used.
More precisely, the study established SMUD energy needs during summer peak energy demand periods
with the emissions characteristics of generating sources used to serve SMUD energy needs during off-peak
(nighttime) periods.
For generation sources in the Sacramento area, the study determined and compared the emissions
characteristics of sources that are used during peak hours (in particular, the summer peak load period of
11:00am to 7:00pm) and those that are used during non-peak hours (10:00pm to 6:00am). And finally, the
study calculated the reduction in NOx emissions resulting from equipping an air conditioning unit with the
Ice Bear load-shifting technology.
1 The Air Quality Benefits of Ice Energy’s Energy Storage Technology In Sacramento, California; E3
Ventures, Inc. 780 Simms Street, Suite 210 Golden, Colorado 80401; September 1, 2005
The starting point for evaluating Ice Bear’s potential air quality impacts in the Sacramento area is
understanding historical generation patterns associated with the area’s electricity demand. Data provided by
SMUD give a basic understanding of the general types of generation sources supplying power to the area in
recent years. Figure 3 shows the historic importance of contract power and market purchases to meet
energy demands in Sacramento.
60%
50%
40%
30%
20%
10%
Contract purchases
Market purchases
SMUD hydro
SMUD cogen
SMUD CT peaker
SMUD PV/wind
04
04
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04
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04
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04
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0%
Figure 3: SMUD power sources as percent of total power supply (2003-2004)
In the 2003-2004 period, SMUD facilities accounted for 33 percent of the agency’s power supplies, and
purchased power (both market and contract) accounted for 67 percent. The figure also illustrates the
importance of cogeneration facilities. SMUD-operated cogeneration facilities, which include Campbell
Soup, Carson Ice and Proctor & Gamble SCA and Carson, supplied 20 percent of SMUD’s power during
this period. 2
Importantly, in 2006 the new 500MW Cosumnes Power Plant will begin generating electricity. The
capacity of the new Cosumnes power plant is equal to about 40 percent of SMUD’s off-peak load (typically
at or below 1,200 MW) and will significantly reduce the amount of power purchased by SMUD during offpeak periods. In addition, Cosumnes could be expanded in the future with another similar size unit — i.e.,
another 500 MW unit.
The methodology to evaluate the emissions savings of Ice Energy’s technology focuses on developing
estimates of 1) the emissions characteristics of generating sources used primarily to serve SMUD energy
needs during summer peak energy demand periods (daytime periods associated with high cooling demand)
and 2) the emissions characteristics of generating sources used to serve SMUD energy needs during offpeak (nighttime) periods. The emissions savings associated with Ice Energy’s shifting of energy use from
peak to off-peak times can then be estimated by comparing the difference in emissions characteristics
between the generation sources that provide power during these different time periods.
2
Note that Figure 3 is based on total generation across all hours of the day. This information is useful for
illustrating the historic importance of power purchased by SMUD and the role of cogen facilities. However,
in order to evaluate the benefits of the Ice Bear TES application, it is necessary to understand how
generation patterns vary between peak and off-peak periods of the day.
The geographic region selected for analysis includes Sacramento and adjacent counties, which include
Amador, Contra Costa, El Dorado, Placer, San Joaquin, Solano, Sutter, and Yolo counties. Figure 4
illustrates the power plants in this nine county region. 3 Facilities are classified by their average capacity
factor during the six month ozone seasons (April through September) of 2001, 2002 and 2003. Capacity
factors play a key role in the analysis methodology, as discussed below.
P la c e r
S u tte r
E l D o ra d o
Yo lo
S a c ra m e n to
Am ador
C a p a c ity fa c to r
<40%
S o la n o
>40% , <70%
>70%
S a n J o a q u in
C o n tra C o s ta
Figure 4 Electric generating units in Sacramento region3
The rationale for selecting this multi-county region is three-fold. First, emissions from power generation
sources in this nine county region are likely to directly impact air quality in Sacramento. Secondly, the
region contains a significant number of power plants (approximately 70) that have a combined generating
capacity far in excess of SMUD’s peak demand. Finally, although sources outside these counties may
provide power to Sacramento during both peak and off-peak periods, purchases from distant sources are
likely to occur predominantly during off-peak times because sources used to meet peak loads tend to be
sources that are located near demand pockets. Therefore, the selection of this relatively constrained region
provides a conservative estimate of emissions impacts during peak and off-peak periods.
3
The available latitude/longitude information for almost all of these units referred to the closest major city
or some other “central location” rather than the actual location of the unit. Therefore, a number of units
were “collocated” at the same point. For illustration purposes, many of the generating units have been
manually dispersed in the counties.
Critical Considerations
The methodology for estimating the emissions characteristics of the two categories of generating sources
(those serving load primarily during peak periods and those serving baseload needs) recognizes four key
factors in the Sacramento area:
1.
Gas-fired power is expected to continue to be critical in satisfying future peak electricity demand
(i.e., summertime air-conditioning peaks) and, specifically, to meet changing energy needs at the
margin during peak periods.
2.
SMUD’s new Cosumnes power plant will begin generating power in 2006 and will significantly
reduce the amount of power SMUD purchases.
3.
With the exception of “green” power purchases, SMUD’s power purchase contracts are not tagged
to specific generating facilities. Therefore, precise NOx emissions rate estimates for SMUD’s
purchased power (either during peak or off-peak periods) cannot be developed.
4.
To account for purchased power, estimates of the emissions performance of that portion of
SMUDs energy portfolio can be developed using reasonable assumptions regarding the types and
locations of facilities likely to be supplying power purchased by SMUD during peak and off-peak
periods.
Given the considerations above, the methodology focuses on identifying and calculating an average NOx
emissions rate for natural gas-fired units whose capacity factors indicate that they are likely to be used
during periods of peak demand, and comparing that rate to the average NOx emissions rate of units whose
capacity factors suggest that they are likely sources of nighttime power.4
Calculated capacity factors and emissions rates are based on monthly generation and emissions data for
2001, 2002 and 2003 as reported in CEC’s Power Plant database. For each reported unit, an average
capacity factor is calculated based on that unit’s total generation during the six-month period – April
through September (corresponding to the increased cooling demand) – of 2001, 2002 and 2003. This multiyear averaging approach minimizes the impact of any unusual disruptions in power generation that may
have occurred in a single month. (A similar analysis of a three month ozone season– June through August–
has yielded results similar to the six month analysis.)
For this analysis, gas-fired units with a capacity factor below 40 percent during the six-month period are
assumed to be called on in meeting peak (daytime) demands in the warm summertime months. All units
with a capacity factor of 70 percent or higher during the six-month period are assumed to be used in
meeting off-peak (nighttime) demand during the summertime months. Based on information provided by
SMUD, the new Cosumnes plant is assumed to be operated in a baseload manner. (Attachment A provides
a list of units included in the analysis.)
Results
The analysis finds that generating sources used primarily during peak periods have an average NOx
emissions rate of 0.603 lb/MWh, while sources used to serve baseload energy needs at night have an
4
Hydroelectric facilities that provide power during peak periods were not included in the analysis of the
emissions rate of facilities serving peak load. These facilities were excluded because, according to SMUD,
their operations are unlikely to be changed by shaving peak period demand with TES technologies such as
Ice Bear. TES is unlikely to affect their operations because they operate at low variable costs and therefore
are called on first when available. On the other hand, the operating pattern of natural gas-fired facilities
(from which SMUD purchases considerable amounts of power during peak periods) with higher variable
costs are most likely to be affected from demand reductions that might be associated with TES technology.
This assumption is consistent with assumptions made in the CEC report analyzing TES technologies, which
focused strictly on changes in natural gas generation patterns.
average emissions rate of 0.264 lb/MWh. The emissions savings of 0.339 lbs/MWh (a 56% reduction in the
NOx emissions rate) is multiplied by the amount of energy shifted by a typical Ice Bear unit to estimate the
emissions savings. Additional emissions benefits are also calculated based on the overall energy
conservation savings, which is 4 percent in the Sacramento area according to other analyses conducted for
Ice Energy. In combination, the analysis estimates an overall NOx emissions savings from a typical Ice
Bear installation of about 6 g/day.
Generating sources used to meet demand for power
emissions rate*
(lbs/MWh)
Off-peak demand (10:00pm – 6:00am)
0.264
Peak demand (11:00am – 7:00pm)
0.603
* weighted average emissions rate for generating sources used to meet demand for the period
Calculation of Ice Energy’s air quality benefit
The difference between the daytime NOx emissions rate (i.e., the calculated peak generating source
emissions rate) and the nighttime rate (i.e., the baseload generating source emissions rate) can be multiplied
by the amount of energy shifted from peak to off-peak using Ice Energy’s energy storage technology to
estimate the emissions reduction attributable to the technology. Assuming that the air conditioning unit
without an Ice Bear unit requires 5 kW of energy on average during the peak time of the day (e.g., 11 a.m.
to 7 p.m.) but only 300 W of power with an Ice Bear energy storage unit then the difference, 4.7 kW, is
multiplied by the time period (e.g., 8 hours) to determine, in this case, 37.6 kWh, which is then multiplied
by the daytime-nighttime emissions rate difference (0.603-0.264=0.339 lb/MWh) to determine the NOx
emissions reductions.5
(5 kW – 300 W) x (8 hrs) x (0.603 – 0.264 lbs/MWh) = 4.7 kW x 8 hrs x 0.339 lb/MWh = 0.013 lb/day, or
5.8 g/day*
[*The conversion to g/day is to facilitate comparisons with typical emissions information, such as the daily
emissions of a motor vehicle in this area.]
There is also an “absolute” energy savings (and emissions savings) associated with Ice Energy’s energy
storage technology that can be added to the above estimate. In the Sacramento area, the addition of an Ice
Bear energy storage module is expected to improve overall energy efficiency by 4 percent when compared
to a air conditioner alone6. Thus, the additional emissions reduction would be the nighttime rate
(0.264 lbs/MWh) times the load and time period (5 kW x 8 hour period) times the percent savings in
efficiency (4 percent). The calculation is as follows:
(0.264 lbs/MWh) x (5 kW x 8 hrs) x (4%) = 0.19 grams/day
The total emissions “savings” attributable to an Ice Bear energy storage module based on the methodology
used herein is about 6 g/day.
In addition, the air quality benefit might actually be larger since the nighttime NOx emissions would be
expected to have minimal contribution to ozone smog. Thus, the air quality benefit of this example
application could be as high as about 11 g/day if the nighttime NOx emissions are assumed to have no role
in ozone formation.
Ice Bear & Site Energy Savings
5
The 5 kW load during the daytime period represents a simplified pattern of electricity use during this
period. More precise data on the hourly shift in load (kW’s) can result in a more precise estimate of the
emissions reduction when applying an Ice Bear energy storage module.
6
“Preliminary Energy Analysis of the Ice Bear System: Comparisons with Conventional Package DX
Systems,” Architectural Energy Corporation, June 12, 2004
The Ice Bear system utilizes a very efficient patented liquid overfeed refrigerant management system for
heat transfer during ice-make and ice-melt operations. Condensing unit efficiency gains due to nighttime
versus daytime ambient operating temperatures and the elimination of cycling losses result in a net energy
efficiency of the Ice Bear system over conventional DX. Laboratory tests have indicated that in areas of
the world where the average (cooling season) diurnal ambient temperature swing is 15F or greater, net site
energy consumption is reduced. Improved dehumidification, typical DX over-sizing, DX capacity
degradation with temperature, condensing unit aging, and roof top temperature factors further accentuate
the site energy savings provided by Ice Bear technology.