18 District Energy / Fourth Quarter 2018 © 2018 International District Energy Association. ALL RIGHTS RESERVED.
which blowdown is controlled to maintain
five “cycles of concentration,” the volume
of which corresponds to 25 percent of the
water consumed in evaporation.
AS REGIONAL FRESH WATER BECOMES
SCARCE, THE BALANCE OF TRADEOFFS
BE T WEEN COOLING ENERGY AND WATER
CONSUMP TION BECOMES AN AREA OF
Although an efficient water-cooled
district cooling plant may consume as
little as half the energy that building air-cooled alternatives do, it comes at the
cost of water consumption on the order
of 2 gal/ton-hr. In many situations in the
past, this has not been a particularly high
price to pay. In a mixed-use urban district in a region with extended high temperature and humidity, however, average
per capita demand for cooling can be on
the order of 0. 5-1.0 ton-hr. The volume
of water evaporated for air conditioning
in this situation approaches the volume
of water used in routine human activity
( 50 gal/day/person). As regional scarcity of fresh water becomes a concern
and overall strategies are developed to
better utilize available resources, the
balance of these fundamental tradeoffs
between cooling energy and water consumption becomes an area of significant
Reliable supply of fresh water is accomplished through policy, urban planning and investment in infrastructure.
Water cost rates, demonstrated to be
effective tools in regulating human behavior, are seen to be increasing, often at
rates exceeding the rate of rise in energy
costs. Strategic development plans and
tightened building codes related to water
use are being adopted by many municipalities. Consumption and production
technologies, ranging from improvements
in household plumbing fixtures to seawater desalination are being adopted.
District energy will play a large role
in the reduction of urban water consumption. Intriguing alternatives to fresh water
consumption for cooling are developing.
Use of treated sewage effluent as a cooling tower makeup source is becoming
a mature technology in the Middle East
and is being applied elsewhere. Hybrid
air-water systems, capable of switching
from air-cooled to water-cooled in response to electric and water price signals
have been deployed. District-scale heat
pump systems, often with ground-cou-pled thermal storage, are well-proven in
areas in locations with both annual heating and cooling loads. Further reductions
in costs of renewable energy production
and storage are beginning to make the
economics of a combination of photovoltaic electricity-powered, air-cooled chilling and thermal (and/or battery) energy
storage a financially viable alternative.
While fundamental physics still prevail,
innovative ideas, such as a cooling tower
plume water recovery scheme recently
published by a team of engineers at MIT,
may be on the horizon.
Consideration of the energy-water
nexus is becoming an essential element
in the planning and design of district
cooling systems intended to operate over
30-to-40-year lives. With water costs on
the rise relative to energy costs, a particularly important metric in considering regional alternatives is the ratio of
the unit cost of water to the unit cost of
energy, the water-to-energy cost ratio.
Planners and designers of district cooling
systems need to include this parameter
when considering lifecycle economics of
plant investment decisions. Continued
investment in research and development
District cooling systems are well-positioned to take a lead role in managing energy and water in the urban environment.
Henry W. Johnstone, PE, is president
and director of mechanical engineering
at GLHN Architects & Engineers Inc. and
an architecture and planning consultant.
Based in Tucson, Ariz., GLHN is a multidisciplinary firm housing integrated services in
architecture and mechanical, electrical and
civil engineering. Johnstone is a member of
the IDEA Board of Directors.
WHO SHOULD ATTEND:
• Campus, hospital, airport and military
Feb. 26 - March 1, 2019 I New Orleans, La.
Pre-Conference Workshops: Feb. 25 & 26
Distribution Workshop & Microgrid Workshop
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