Thermal Energy Storage Myths
Jun 05 - Energy Engineering
ABSTRACT
This article attempts to set the record straight on the myths and reality of
this technology by demonstrating how TES is well- positioned to help the move
towards more energy-efficient and environment-friendly air-conditioning systems.
The obvious reason for installing TES is to reduce energy costs. Although
deregulation of the electric industry has created localized anomalies in energy
costs, the basic reality of supply and demand is that on-peak power is more
expensive than off-peak power.1 One consistently proven aspect of TES is that it
saves energy costs, which has more significance now that ANSI/ASHRAE/IESNA
Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential
Buildings, and the LEED rating system are based on energy cost savings. Several
TES projects that have won ASHRAE's Technology Award2,3,4 detail the cost-saving
aspect. However, less emphasis has been given to the reductions of equipment
size and infrastructure that normally occurs.
The basic TES cooling systems that I base most of my analysis on are:
Chiller-based systems. Throughout the adolescent years of TES, a variety of
systems, including site-built liquid overfeed refrigeration systems,
ice-harvesting equipment and others, were used successfully in other
applications. However, 99 percent of commercial air-conditioning TES systems
installed use a standard chiller to produce the cooling. Chillers are familiar,
reliable, capacity rated, and competitively priced. They cool water or a
glycol-water solution.
Ice-based storage. For projects where space is not as much of a
consideration, chilled water storage is becoming widely used.5 However, since so
much HVAC work involves retrofits where space is a concern, ice is the likely
choice.
Closed system. Large district cooling systems use either water and/or ice as
the storage media and the heat transfer fluid. These "open" systems
create added hydraulic complications that need to be carefully addressed.
However, most TES systems now separate the storage media from the heat transfer
fluid, so the systems are the same hydraulically as most chiller systems.
THE MYTHS OF TES
Myth 1-Uncommon and Risky
If I said TES has 100 percent market penetration and that you use it today,
you probably would say I was crazy. Well, I would be right because a domestic
hot water heater is the best example for understanding the value of cool thermal
energy storage. (When applied to commercial air-conditioning applications, TES
often is referred to as the more descriptive term "off-peak cooling.")
To instantaneously heat water for a low-flow showerhead, a simple calculation
shows that 18 kW of power is required (Equation 1) or 36 kW for two simultaneous
showers.
Even a large capacity water heater (electric for simplicity) has a 4.5 kW
heater, at most. So the reduction in infrastructure for wiring and electrical
power associated with it is a minimum of 4 to 1.
Although no one sizes a domestic heating element to handle instantaneous
load, this is done regularly in the HVAC world. Why install a chiller system
that safely (with our understandable use of "safety factors") meets a
load that occurs a couple of hours per year? A simple partial storage system
reduces the chiller size to something safely above the average peak daily load,
which normally reduces the chiller plant size by about 40 to 50 percent, with
the proportional savings of infrastructure that are similar to the water heater.
Myth 2-Too Much Space
Does the water heater in your house take up too much space? In Equation 2(6),
a quick calculation shows the space required for a full storage system.
Storage systems in retrofit applications are usually "partial
storage" and are normally sized to handle about one-third of the peak load,
yielding 0.23 percent of occupied space needed for storage. A 1,000,000 ft^sup
2^ (93 000 m^sup 2^) building needs only about 2,300 ft^sup 2^ (214 m^sup 2^).
For a 2,000 ft^sup 2^ (186 m^sup 2^) home, the water heater takes up 5 ft^sup 2^
(0.5 m^sup 2^) or 0.25 percent. So for off-peak cooling (OPC) using 33 percent
partial storage, the space needed is about as much proportionately as the water
heater in a house.
Myth 3-Too Complicated
Let's go back to the water heater. Is it complicated? No. It has a reliable,
undersized heating element that creates heat whenever the inventory drops below
95 percent.
In a partial storage OPC system, a reliable undersized cooling element
(chiller) runs whenever the inventory drops below 95 percent. TES tanks are
simply thermal capacitors with no moving parts. With partial storage, no on-peak
control malfunction can occur because there is no full-size chiller to
accidentally set a massive electrical peak. What can complicate the system is
mismatching the control complexity with the aptitude of the eventual operator.
Installing a 50 percent sized chiller creates a major advantage in demand cost
savings, which is a good goal for a small application (such as a school with a
janitor). A large OPC system can handle a more complex control strategy, but
that is where problems always occur. A design engineer spends hours figuring out
the precise, logical way to save the most money while working with a complicated
rate structure. Think how incomprehensible the strategy will be to a third-shift
operator when some sensor fails (see appendix "Designing for
Success").
Myth 4-Lack of Redundancy (Risk) with Partial Storage
Almost any OPC system can meet the same redundancy criteria as a conventional
system at a comparable cost. Obviously, in a conventional system with one
chiller where the chiller is inoperable, you are out of luck. It is the same
with a one chiller storage system. However, let's look at a conventional system
with a calculated design day load of 1,000 tons (3500 kW). A reasonable
conventional design would be three 400-ton (1400 kW) chillers, and an equivalent
partial storage system could be two 400-ton (1400 kW) chillers and 3,500 ton-hrs
(12 300 kWh) of ice storage. Figure 1 shows the maximum capacity for both
systems as compared to the design day load profile. Though the 1,200-ton (4220
kW) system could create more ton-hrs in the 24-hour period, it is clear that the
storage system is more than what is required even on a design day.
The next concern is equipment failure. Figure 2 shows if a chiller failed on
the conventional system, or storage was unavailable in the storage system, both
would have two 400-ton (1400 kW) chillers to handle the load, and for six of the
11 hours, the system would be short some capacity. If the component that failed
on the storage system were a chiller, the remaining chiller and storage would be
able to meet the full load for eight of 11 hours. Therefore, the systems are
quite similar and both systems would need 500-ton (1760 kW) chillers (three or
two), instead of the 400-ton (1400 kW) machines, to have "n+1"
redundancy.7
Myth 5-Too Expensive to Install
Figure 1: Excess capacity on design day.
Figure 2: Capacity on design day with one chiller failure.
Specific applications and locations will vary the installed costs, but the
point is that the cost is essentially the same when experienced OPC contractors
compete on new construction projects (more than 400 tons [1400 kW]). In retrofit
projects, as with almost all energy efficiency upgrades (excluding lighting),
there should be another reason to go forward for the paybacks to be reasonable,
i.e., aging chiller plant, CFC replacement, building expansion, strained
electrical supply, etc.
Myth 6-Does Not Save Energy
When analyzing energy savings with OPC, you must consider both energy used at
the building and energy used at the source of generation at the power plant. The
reason is simple. Most energy- efficient products reduce energy use but do not
change when energy is used. As an industry, we have done a poor job of relaying
the energy saving benefits of OPC beyond the meter. Site energy savings may or
may not occur. Source energy savings almost always occur.
Site Energy Savings
Is the goal to save the most energy or energy costs? Clearly the owner's
answer is the latter. However, energy-efficiency funding from most states is
based on kWh saved. With thermal storage, optimizing for energy savings can be
done, but often is not the same as maximizing energy cost reduction. So let's
review a design maximizing energy savings for air-cooled and water-cooled
applications.
First, an air-cooled chiller operating at ARI design conditions, 95F/45F
(35C/7C) (Point A in Figure 3), uses the same kW/ton at ice- making conditions
of 78F/25F (26C/-4C) (Point B, Figure 3). Therefore, a 17F (9C) change in dry
bulb gives equal efficiency for ice making. In \much of the country, the ambient
day-to-night swing is 20F (11C). Because the swing is sinusoidal, the average
for the on-peak hours versus the ice-making hours make the average temperature
swing more like 12F to 14F (7C to 8C). If you then factor in:
1. Undersized chillers are fully loaded for a majority of the hours of
operation, normally their most efficient condition.
2. Chillers in a partial storage system normally operate upstream of ice
storage. Therefore, the chillers cool the upper half of the delta T, and have
higher on-peak efficiencies than if they were producing 45F (7C) liquid (Point C
in Figure 3).
3. Extreme part-load conditions can be met fully with ice to avoid short
cycling of chiller equipment (0 to 20 percent), which is clearly very
inefficient.
For water-cooled OPC applications, the argument is less clear initially for
site energy savings. Ambient wet-bulb temperature only decreases about 5F to 7F
(3C to 4C) from day to night. Therefore, this decrease does not make up for the
lower evaporator temperatures required for ice making, yielding about a 15
percent "penalty" (Figure 4, Point A to B). However, the most
important point is the amount of ton-hrs per year that are actually met with ice
in a design focused on energy savings. In a standard chiller priority, partial
storage system, where a 50 percent sized chiller(s) would work, but a 60 percent
sized chiller is installed, a simple bin analysis shows that the amount of
ton-hrs per year in an office building or school above 60 percent is only about
20 percent. So with the ice-making penalty for air-cooled chillers arguably at O
percent, and 15 percent for water-cooled, the total ice-making penalty for
water-cooled is 20 percent of 15 percent or about 3 percent. Even with the extra
pumping required to put cooling into storage, when the points made earlier for
air-cooled are factored in, it is arguable that the water-cooled difference
drops to nil.
Figure 3. EER of air-cooled chillers (includes heat rejection).
Figure 4. Water-cooled chiller (comp. only).
Routine oversizing of chillers causes related components to be oversized,
including condenser pumps, and cooling towers and transformers, which likely
will never run at full load for the life of the system. Right-sizing chiller
capacity is capable of saving lots of energy, as discussed by Tom Hicks.8 The
best way to conceptualize the energy advantages of "right-sizing" a
system with storage is to compare it to the value gained by using variable
frequency drives (VFD) on motors. VFDs vary the speed to match part- load
conditions: storage allows varying the time at full load of a smaller cooling
plant (which is like having VFDs on the chiller, condenser pump, and cooling
tower fan). Major advantages can be captured here that are yet to be quantified
by accurate simulations.
Source Energy Savings
The California Energy Commission released a report9 in 1996 that clearly
concluded that, for two of the major California utilities, it is 8 to 30 percent
more efficient to create and deliver a kWh during off-peak hours than during
on-peak hours. The combined use of more efficient base load generation plants,
lower transmission and distribution line losses, and cooler nighttime
temperatures creates more efficient nighttime generation. Therefore, if we
assume that a building uses the same amount of kWh before and after an OPC
system is installed, major "source energy" savings exist for each kWh
shifted to off-peak. Also, there are environmental benefits. Regarding an OPC
installation in Manhattan, Ashok Gupta of the Natural Resources Defense Council
stated, "Peak shaving results in lower emissions, because some of the
plants used to meet demand peaks are among the dirtiest in the city."10 In
response to these findings, California's 2005 release of the Title 24 energy
code will value the relative cost of energy for every hour of the year (instead
of a flat rate as allowed in 90.1), otherwise known as "time dependent
valuation." With relative costs of three to four times as high on summer
afternoons, the code will surely drive designers to use more efficient, off-peak
power and OPC.
Myth 7-Electric Rates May Change and Negate Savings
Electric rates will change. The realities of supply and demand will not. In
the past, essentially all monopolistic utilities with decades of experience had
rates that were dependent on time. Demand charges make peak power more
expensive, albeit only for commercial and industrial customers. On a commercial
electric bill, the demand portion of the bill can often equal 50 percent of the
total, so when you use power is almost as important as how much. Even monopolies
realized the dramatic cost of peak power and the cost advantages of raising load
factor (a measure of effective use of installed generating capacity). In the
future's (more) competitive environment, the economics of unregulated generation
will be driven even more by supply and demand.
Demand response programs, which call for reductions of 10 percent of building
loads for four hours on short notice for a given financial incentive, are clear
evidence of this and are tailor made for partial storage OPC systems. Until a
substantial oversupply of generation exists (which is not cost effective), or
the use of power becomes relatively even for day and night (not in my lifetime),
a large difference in the cost of on-and off-peak power will exist. Short-term
anomalies such as flat rates may occur temporarily, but they will pass (even the
flat rates often take into account building load factor, i.e., lower flat rate
for better load factor). Also remember, in new construction there is little or
no first-cost premium, which further reduces the critical nature of exact energy
costs.
Myth 8-Modeling Doesn't Show Energy Savings
Often that is true, and the reason is simply because many modeling programs,
including DOE-II, were never really designed to model all the advantages of
storage. DOE's newest program, E-Plus, will soon have the capabilities to model
storage well and the true energy picture should be clearer.
CONCLUSION
Off-peak cooling uses low-cost electricity that is efficient to generate and
cleaner to make, clearly qualifying it as a "green" technology. TES is
a technology that has grown up. A lot of lessons have been learned, and it is up
to manufacturers to disseminate information on best practices. Only the most
advanced and committed operators require an optimal control system to save every
possible dollar. The remaining users require control complexity to be about that
of an electric water heater.
Maybe the best way to conceptualize and justify the real-world application of
this technology is this: instead of adding a safety factor of 20 percent to the
cooling plant on every job, and paying the price of oversizing for the life of
the building, downsize the actual size by 20 percent (instead of 40 to 50
percent) and add storage. With no loss of redundancy and good gains in energy
cost reduction and full-load operation, the OPC system gains operational
flexibility and reduces the load on the electric grid. The investment is in a
usable asset (storage) instead of a dormant one, namely a backup chiller. OPC
accomplishes its goals at a fraction of the cost of other more "sexy"
technologies (fuel cells, microturbines), which have far go on the learning
curve. TES used for OPC is a proven, simple, and practical solution to rising
energy costs.
* Copyright 2003, American Society of Heating, Refrigerating and
Air-Conditioning Engineers, Inc. (www.ashrae.org). Reprinted by permission from
ASHRAE Journal, September, 2003. This article may not be copied nor distributed
in either paper or digital form without ASHRAE's permission.
References
1. Audin, L. 2003. "Central plant savings." Engineered Systems (5).
2. Evans, W. 1998. "Ice storage cooling for campus expansion."
ASHRAE Journal 40(4).
3. Hersh, D. 1994. "DDC and ice thermal storage systems provide comfort
and energy efficiency." ASHRAE Journal 36(3).
4. O'Neal, E. 1996. "Thermal storage system achieves operating and
first-cost savings." ASHRAE Journal 38(4).
5. Bahnfleth, W. 2002. "Cool thermal storage: is it still cool?"
HPAC Engineering (4).
6. CALMAC. 2003. IceBank Performance Manual.
7. Silvetti, B. 2002. "Application fundamentals of ice-based thermal
storage." ASHRAE Journal 44(2).
8. Hicks, T. 1999. "Small steps bring giant leaps." Building
Operating Management Sept.
9. California Energy Commission. 1996. Source Energy and Environmental
Impacts of Thermal Energy Storage, Report #500-95-005 www.energy.ca.gov/reports/reports_500.html.
10. Gupta, A. 2002. "On Avenue of the Americas, the iceman cometh."
New York Times March 17.
Mark M. MacCracken, P.E., Member ASHRAE
ABOUT THE AUTHOR
Mark M. MacCracken, P.E., is president and CEO of CALMAC Manufacturing in
Englewood, N.J.
APPENDIX
The 'KISS' Principle: Designing for Success
The best example of a well-designed controller for an operator (school
janitor) is a three-position switch, located next to the ice inventory meter,
labeled "cool day-warm day-humid hot day." The controller simply
changes the leaving chilled water temperature on the upstream chiller setting
from 55F to 50F to 45F (13C to 10C to 7C), which changes the control strategy
from full storage to partial storage-ice priority to partial storage-chiller
priority (Figures 5). Because no penalty exists for starting with a full charge
of storage every morning (as with other TES technologies outside of the scope
stated previously), there is no fear of guessing wrong, just change the setting
that morning.
Figure 5
The main goal for this project was to take advantage of a large difference in
energy charges ($0.12 on-peak/$0.06 off-peak). Figures 1b, 1c and 1d illustrate
the success of the strategy. Although the strategy wasn't "optimal,"
it was close. And it worked like a charm.
I can't stress enough the "Ke\ep It Simple Stupid" (KISS)
principle. My advice is to keep the controls as simple as possible but to
install simple real-time feedback on energy use information for the operators
(for example, real-time total building kW). (The real-time game of "what is
causing that electric peak?" is fun. Getting a call from the boss about an
outrageous electric bill from two months ago is not).
With feedback, operators stay interested and tune up a simple system. OPC
systems are different, but not necessarily more complex. Owners and operators
who are aware of the differences can save money.
The above is an article I wrote for the ASHRAE Journal where I separate the
myths from the reality of modem off-peak cooling systems that use ice-based
thermal energy storage. There are two reasons for submitting it to Energy
Engineering:
1. "Green Design" is becoming more mainstream, mainly because of
the USGBC's LEED standard, which in turn puts more pressure on designers to
create more energy efficient buildings. Getting LEED "points" for
energy is based on ASHRAE's 90.1 Energy Standard, which is quite justifiably
based on energy COST reduction, not energy reduction. Based on the LEED
standard, TES is very green because it can get you lots of points by reducing
energy costs. However, the real reason TES is "Green" is that it saves
energy and normally reduces power plant emissions, detailed in a California
Energy Commission report and covered in the my article.
2. In Energy Engineering, Vol. 100, No 5, 2003, there was an article entitled
"Knowledge Based Algorithms for Daily Load Prediction of a Building"
which was mainly based on yet another one of the TES myths which I didn't cover
in the article. Though the research the author presents certainly has value in
the A/C field, the author stated that "Regardless of which type of cold
storage used, the most important thing is to have an accurate thermal load
profile prediction to determine the amount of storage required in the following
day." Thirty years ago that statement was true; however, for 95% of the
current TES designs, in which there is no penalty for making a complete charge
of storage each night, it is incorrect.
The only "Letter to the Editor" on the original publication of the
article pointed out that I had omitted another benefit of using TES, which is a
nice letter to get. (See the ASHRAE Journal, Nov. 2003, for answer). Another
private e-mail said I didn't mention how it helps when installing distributed
generation, or backup generators, on a building. Thermal storage has numerous
benefits to the owner, the grid, and the environment. Clearing up the myths
surrounding TES demonstrates that TES is a valuable asset for energy engineers
to use to help our society move towards a more sustainable future, while saving
money for the owner.
Copyright Fairmont Press, Incorporated 2004