Cool technologies
02-MAY-2006
integrating district cooling and thermal energy storage with on-site power
District cooling is a well established
technology in North America and is growing fast in certain other parts of the
world – notably the Middle East. DC is often enhanced with thermal energy storage,
which acts to smooth energy loads. Here, John Andrepont
summarizes the benefits of these two technologies and some market trends.
District cooling (DC) applications are growing worldwide. The application of
district cooling can often benefit the economics and performance of CHP and
on-site power generation by providing a means of aggregating thermal loads.
Its thermal loads can combine with and supplement seasonally variable heating
loads to achieve a flatter demand profile for the thermal output of a CHP system.
CHP systems are increasingly finding application within combined district heating
and cooling systems.
District cooling is well established throughout North America. This includes
many hundreds, if not thousands, of DC systems among single owner-user systems,
such as university and college campuses, hospital/medical facilities, and other
government and private commercial/industrial facilities. There are also many
dozens of large (mostly urban) DC utility systems that have been developed increasingly
over the past two decades. Growth continues to occur via the development of
new systems as well as by the expansion of existing systems.
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The application of district cooling can often benefit
the economics and performance of CHP and on-site power
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Internationally, DC is growing in many other parts of the world, offering
additional opportunities for CHP applications. The largest and fastest growing
DC market is in the United Arab Emirates and the surrounding Gulf region. East
and south-east Asian markets are also very significant. Many major new real-estate
developments or re-developments there are utilizing DC. Also, parts of southern
Europe and the developed economies of northern and western Europe – long-time
users of district heating – are increasingly developing DC systems as well,
as airconditioning becomes a more common requirement of building operation.
Less mature DC markets, including Australia and South America, have increasing
interest and activity underway.
Thermal energy storage (TES) is often integrated with DC systems, and is a
common enhancement to DC systems at times of new construction and system expansions.
TES not only enhances the operating economics of DC systems, it can reduce the
necessary installed chiller plant capacity and thereby often reduces the net
capital cost. TES also serves to level the 24-hour demand for cooling (and power),
thus improving the economics for CHP or on-site power generation.
ADVANTAGES AND SYNERGIES
There are significant advantages that often proceed from employing DC and/or
TES. And there are often meaningful synergies when integrating DC and/or TES
with CHP or on-site power generation.
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Seasonably variable thermal loads can be combined to
achieve a relatively constant demand for thermal output year-round
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District cooling versus individual chiller systems
The advantages of DC include economies of scale, higher energy efficiencies,
greater labour efficiencies, concentrated and reduced maintenance, better ability
to provide desired extra thermal back-up, better ability to provide sophisticated
controls and operator training, overall space savings, and very significantly,
a better ability to incorporate complementary technologies for economic benefit,
such as TES, hybrid chiller plants, integrated heating and cooling, and combined
cooling, heat and power (CCHP).
Thermal energy storage
The advantages of TES used in large cooling systems, such as DC, include management
of peak cooling demand, reduction of peak electric power demand (and its associated)
cost, capturing savings from time-of-day or real-time energy rates, reduction
in the amount of required installed chiller capacity (and its associated cost)
often achieving a net capital cost savings, and enhanced redundancy and/or emergency
reserve cooling capacity. The secondary benefits include dual-use of TES as
fire-protection water storage, or the enhancement of cooling distribution by
siting TES at a satellite location remote from the chiller plant. Integrating
DC with CHP When integrating DC with CHP, beneficial synergies ensue. Seasonably
variable weather-dependent thermal loads for heating and cooling can be combined
to achieve a relatively constant demand for thermal output year-round. Thermal
revenues are spread more evenly throughout the year. On-site power generation
can more easily be matched to a useful thermal output year round, and thus can
often be more readily justified economically. Also, the cooling system can be
used to provide turbine inlet cooling for any gas turbines that may be employed
in the on-site generation system, thus dramatically enhancing the hot weather
power output of the combustion turbines, typically by 20%–30% or more.
Integrating TES with CHP and on-site power
When integrating TES with CHP or other on-site generation, beneficial synergies
also result. Daily variable demands on electricity and cooling can be balanced
to achieve a relatively constant demand for power and thermal output around-the-clock.
TES can reduce installed chiller plant capacity and often reduce net capital
cost. Once again, on-site power generation can more easily be matched to a useful
thermal output around-the-clock, and thus can often be more readily justified
economically.
System operation principles
District cooling (DC)
A system or application in which two or more buildings or facilities
are provided with cooling from a centralized cooling system connected
to the cooling loads by a piping network that delivers the cooling. Cooling
is generally delivered by water or an aqueous solution (or occasionally
by a refrigerant). Common examples include:
- Single supplier-user DC systems, such as those operated by university/college/school
campuses, hospital/medical complexes, airports, military facilities
and other government facilities
- DC thermal utility systems, in which a DC owneroperator sells cooling
to one or more independent customers, such as urban DC networks or commercial/
industrial applications of outsourced utilities.
Thermal energy storage (TES)
A means of storing heat from cooling loads during high load or ‘on-peak’
periods (known as discharging TES), and subsequently removing the heat
from storage and rejecting it to the ambient environment during low-load
or ‘off-peak’ periods (known as recharging TES). The most common cool
TES technologies employ either:
- latent heat storage, in which energy is stored as a phase change in
the storage medium, usually between liquid and solid states (generally
freezing water to ice during off-peak periods, and melting the ice during
on-peak periods), or
- sensible heat storage, in which energy is stored as a temperature
change in the storage medium (generally cooling water or an aqueous
solution during off-peak periods, and re-heating it during on-peak periods).
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CHILLER AND TES EQUIPMENT AND CONFIGURATIONS
DC systems employ many types of chillers and drives. Chillers can be mechanical
compression chillers (usually centrifugal or screw types) or absorption machines
(usually steam- or hotwater- driven, or directly fired by gas, or by hot exhaust).
Drivers for mechanical chillers can be electric motors, steam turbines, gas
turbines, or gas or diesel engines. Notably, DC systems often employ ‘hybrid’
chiller plants, with a combination of chiller and driver types; this provides
flexibility of operation to manage fuels and costs, and to more readily integrate
with CHP systems. Increasingly, DC plants are designed in a series– parallel
chiller configuration, to achieve larger system supplyto- return temperature
differentials at high energy efficiencies.
Latent heat (ice) TES systems provide the benefits of modularity, high energy
density (compactness), and capability for low supply temperature. Sensible heat
(chilled water and lowtemperature fluid) TES systems deliver a large economy-ofscale,
the flexibility to be sited remotely from chillers, and ease of retrofit to
existing chillers. Chilled water TES footprint is minimized by using relatively
tall storage and/or by employing a large supply-to-return temperature differential.
Volume can be further reduced by use of a low-temperature fluid storage medium,
which can also provide low supply temperatures in the system, even below those
available from ice TES. Although there are exceptions, ice TES dominates in
smaller individual building applications, while sensible heat TES dominates
in large DC applications.
EXISTING APPLICATIONS OF DC WITH TES
Recently, the author compiled extensive databases of the use of TES within
two major families of DC systems: DC utility systems and university/college
campus DC systems. The highlights are reproduced here.
Metropolitan Pier & Exposition Authority at McCormick
Place – Chicago, Illinois, US
A district energy system serves over 5 million gross square feet (500,000
m2) of exposition centre facilities plus surrounding public and private
buildings with heating and cooling, and incorporating on-site generation.
The system has:1
- 3.3 MW of on-site power generation/CHP (specifically ‘trigeneration’
of cooling, heat and power), using a simple cycle with three 1.1 MW
Turbomeca Makila TI combustion turbines
- 370 million BTU/hour of heat production capacity (20 million BTU/hour
from the heat-recovery steam generator (HRSG)
- 16,800 tonnes of chillers (electric centrifugal, steam absorption,
and gas turbine/electric-driven screw chillers)
- 123,000 tonne-hours of SoCool® low-temperature fluid TES at –1ºC
(30ºF) supply temperature and
12ºC (54ºF) return temperature (–1º/+12ºC), with a peak demand reduction
capability of 25,000 tonnes or 20 MWe
- turbine inlet cooling, to 10ºC (50ºF), of the inlet air to the three
1.1 MWe combustion turbines, for a hot weather power increase of 35%
(0.9 MWe).
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TES in DC utility systems
106 examples of TES installations were identified in DC thermal utility systems.
They totalled more than 2.6 million tonne-hours of TES capacity (1 refrigeration
tonne = 3.516 kW thermal), with 75% using sensible heat (chilled water or lowtemperature
fluid) TES and 25% using latent heat (ice) TES. Total peak electrical load management
is 373,000 tonnes (or 280 MWe). On average, they represent 24,630 tonne-hours
of TES and 3500 tonnes (or 2.6 MWe) of load management per installation. US
installations represented 72% of the total capacity and were located in 20 states
and Washington, DC; installations outside the US represented 28% of the total
capacity and were in 16 countries on four continents. The rate of installations
grew rapidly throughout the 1980s and early 1990s; over the past 13 years, the
rate has remained high and averaged 170,000 tonne-hours per year. There are
many repeat users: 12 DC utility systems have installed TES capacity in multiple
phases, while 13 DC system owners have 63 TES installations on 56 of their separate
DC systems. One DC utility developer executed 15 TES installations at 13 DC
systems, with one installation alone storing 160,000 tonne-hours as chilled
water TES and expandable to 250,000 tonne-hours as low-temperature fluid TES.
Reedy Creek at Walt Disney World Lake Buena Vista, near
Orlando, Florida, US
A district energy system serves the world-famous entertainment/ resort
complex with heating and cooling, and incorporating on-site generation.
The system includes:2
- 40 MW of on-site power generation/combined cooling, heat and power
(CCHP), using a combustionturbine combined cycle with one 32 MW GE LM5000
combustion turbine and an 8.5 MW back-pressure steam turbine generator
- 90 million BTU/hour from the HRSG
- 14,400 tonnes of chillers (electric centrifugal and steam absorption
chillers)
- 57,000 tonne-hours of stratified chilled water TES at a supply temperature
of 4ºC (40ºF) and a return temperature of 13ºC (55ºF) from the DC loads
or 21ºC (70ºF) from the turbine inlet cooling load, with a peak demand
reduction capability of 6000 tonnes (4.8 MWe)
- turbine inlet cooling, to 10ºC (50ºF), of the inlet air to the combustion
turbine, for a hot weather power increase of 31% (8 MWe).
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TES on campus
159 examples of TES installations were identified on 124 university or college
campus DC systems. They totalled more than 1.8 million tonne-hours of TES capacity,
with 78% from sensible heat (chilled water or low-temperature fluid) TES and
22% from latent heat (ice) TES. Total peak load management is over 258,000 tonnes
(or 194 MWe). On average, they represent 14,584 tonne-hours of TES and 2083
tonnes (1.6 MWe) of load management per campus. US installations represented
93% of the total capacity and were located in 24 states and Washington, DC;
installations outside the US represented 7% of the total capacity and were in
six countries on four continents.
Princeton University Princeton, New Jersey, US
A district energy system serving Princeton University’s campus facilities
with heating and cooling, and incorporating on-site generation. It has:3
- 14.6 MW of on-site power generation/CCHP, using a combustion-turbine
simple cycle with one GE LM1600 combustion turbine
- 322 million BTU/hour of heat production capacity (182 million BTU/hour
from the HRSG)
- 20,000 tonnes of chillers (electric and steam-turbinedriven centrifugal
chillers)
- 40,000 tonne-hours of SoCool® low-temperature fluid TES at 0º/+13ºC
(32º/56ºF) supply/return temperatures, with a peak demand reduction
capability of 10,000 tonnes or 7.5 MWe
- turbine inlet cooling, to 5º–10ºC (41º–50ºF), of the inlet air to
the 14.6 MW combustion turbine.
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The rate of installations grew rapidly throughout the 1980s; over the past
15 years, the rate has remained high and averaged over 100,000 tonne-hours per
year. There are many repeat users: 20 campuses have executed 55 TES installations
in phases, while 10 statewide campus systems have TES installations on 37 of
their campuses. One campus system has 16 thermally stratified chilled water
TES systems on 14 campuses, totalling 278,000 tonne-hours of TES capacity, and
representing a peak load management of 40,000 tonnes and 30 MWe.
DRAMATIC BENEFITS
DC appears to be applicable and to have taken root in all types of climates
with large air-conditioning demand, including hothumid and hot-arid climates,
and including places with hot year-round climates and very short summer seasons.
Similarly, TES has found broad application within district cooling in all those
climates, and in all parts of the world. TES plays an increasing role in both
new and expanding DC systems.
Existing and new district cooling systems are often attractive candidates for
CHP or on-site generation. DC can often significantly improve the economics
of CHP. Furthermore, TES can further enhance the economics of DC system development
or expansion, as well as the economics of CHP or on-site generation. Specifically,
DC can level the annual demands for the thermal output of a CHP system, while
TES can reduce operating and even capital costs of a DC system and level the
daily demands for both the thermal and electrical outputs of a CHP plant.
The use of DC (or TES) capacity for turbine inlet cooling is yet another enhancement
to on-site power output and economics. An integrated approach incorporating
district cooling, thermal energy storage and on-site generation can, and
often does, reap dramatic benefits.
John S. Andrepont is the Founder and President
of The Cool Solutions Company, providing consulting services in the areas of
thermal energy storage, district cooling and turbine inlet cooling, based in
Lisle, Illinois, US.
Fax: +1 630 353 9691
e-mail: coolsolutionsco@aol.com
NOTES
- Andrepont, J.S., ‘Stratified Low-Temperature Fluid Thermal Energy Storage
(TES) in a Major Convention District – Aging Gracefully as Fine Wine’, ASHRAE
Transactions, Volume 112, Part 1, 2006.
- Clark, K.M., et al., ‘The Application of Thermal Energy Storage for District
Cooling and Combustion Turbine Inlet Air Cooling’, Proceedings of the International
District Energy Association (IDEA) 89th Annual Conference, June 1998.
- Borer, E. and Schwartz, J., ‘High Marks for Chilled-Water System: Princeton
upgrades and expands’, District Energy (magazine of the IDEA), First Quarter
2005, pp. 14–18.
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