The energy-water nexus in district cooling
Henry W. Johnstone, PE, President and Director of Mechanical Engineering, GLHN Architects & Engineers Inc.
District cooling has become a funda- mental service in many high-density urban environments, particularly in
cooling-dominated regions with high
annual temperature and humidity. It is
difficult to imagine future growth and
densification of cities like Dubai or Singapore without continuing expansion of
their already-substantial plant and distribution infrastructure. Aggregation of
building thermal loads into consolidated,
energy-intense cooling nodes enables
effective utilization of efficiency, sustainability and resiliency strategies that are
often not financially viable on smaller
scales. These include combined heat and
power, thermal energy storage and microgrid integration. An interesting nexus
of interconnections and tradeoffs exists
between consumption of water and energy in urban air conditioning, enabling
district cooling to assume a leadership
role in more sustainable management of
both resources.
The engineering and economics of
mechanical cooling in a hot climate rely
on basic heat engine physics. The magnitude of electrical energy to drive refrigeration is in proportion to the difference
between the temperature at which heat
is extracted from buildings and the temperature at which the heat is rejected to
the outdoors. The greater this temperature difference, the greater the energy input. Sankey diagrams, in which the width
of the flow lines represents the magnitude of energy expenditure, depict this
situation. In an example shown in figure
1, electric energy is used in an air-cooled
system operating at an efficiency of 1.0
k W/ton (which translates to a coefficient
of performance, COP, of 3. 5) to reject
individual unit or building heat into an
ambient temperature of 120 degrees F.
Compare this to the combination of energy and water used in a water-cooled
system example, shown in figure 2, which
rejects heat into an 80 F evaporatively
cooled water stream, enabling operation
at 0.5 k W/ton (or COP of 7.0). Under
FIGURE 1. Air-cooled refrigeration.
Source: GLHN Architects & Engineers Inc.
FIGURE 2. Water-cooled refrigeration.
Source: GLHN Architects & Engineers Inc.
these conditions, an air-cooling refrigeration system requires twice the energy of
the water-cooled alternative.
Taking advantage of this energy
benefit, many district cooling plants have
historically been designed with large
evaporative cooling towers that recirculate through the refrigeration condensers.
Notice in the diagram in figure 2 that the
units of water flow have been converted
into units of latent energy (Btu/hr) of
evaporation at the rate of 1,000 Btu/lb.
Water supplied to a cooling tower is rarely
pure but includes minerals and salts that
concentrate as evaporation occurs. High
concentration of minerals recirculating
through the refrigeration heat exchangers can drastically erode the efficiency
advantage. A common method of managing mineral concentration is to drain off,
or “blow down,” a portion of the recirculating flow. Figure 2 shows a situation in
