ASHRAE Journal - June 2019 - 70

COLUMN ENGINEER'S NOTEBOOK

economizer and low availability of
waterside economizer, for example,
San Francisco, would suggest an allair system, with central air-handling
units and outdoor air access, as
opposed to a water or refrigerant fan
coil system with dedicated outdoor air
system sized only for the airflow ventilation rate.

FIGURE 4 Availability of free cooling for office occupancy schedule in New York City (percent of hours).

Full Cooling Airside Economizer
Full Cooling Airside Economizer SAT Reset

HVAC Condensate Recovery

33.9%
48.3%

Waterside Economizer Part Cooling
Natural Ventilation Operable Windows

J U N E 2 0 19

75.5%

Waterside Full Free Cooling

18.0%

FIGURE 5 Monthly recovered condensate from a DOAS in New York City, assuming office ventilation schedule and 106,120 cfm (50,083 L/s) and 50°F (10°C) supply air temperature.

140,000
130,227

132,959

120,000
Condensate Recovery (Liters)

Renderings of solar data in a location often
include a sun angle diagram as shown in Figure 6.
These diagrams are readily available from a variety
of sources, but require significant processing to
yield any information concerning the design.
Figure 7, on the other hand, shows the results of a
processing an entire year of sun angle data to determine the effectiveness of a simple overhang sunshade for windows on the south, east and west sides
of the building, compared with no overhangs. This
diagram was generated by calculating, on an hourly
basis, for each month of the year, the depth of the
shadow on a window of a certain height immediately below the overhang. The percentage of the
window above the shadow line was then mapped
onto a gray scale, to show shading percentage pattern over the entire year. This diagram shows that,
near the middle of the day, in the summer months,
when cooling loads are likely to be highest, the
horizontal overhang does provide some benefit for
shading the window. More comprehensive studies
using this tool have demonstrated that the impact
of the horizontal sunshade on the east and west is
more profound at lower latitudes.
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60.8%

Part Cooling Airside Economizer

Solar Shading Effectiveness

ASHRAE JOURNAL

52.6%

Full Cooling Airside Economizer
Evaporative Cool

Another issue that can be assessed
prior to building design is the
feasibility of harvest condensate from a DOAS system. Figure 5 was created in a spreadsheet using a
TMY3 weather file for New York City. Inputs consisted only of a ventilation flow rate, ventilation
schedule, supply air temperature (it is presumed
that supply air temperature will be constant whenever outdoor air requires dehumidification), and
latent efficiency of the energy recovery device.

70

47.5%

96,908

100,000
80,000
56,612

60,000
40,000
20,000
0

18,188
9,374
0
0
0
0
201 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

FIGURE 6 Sun angle chart for 40 degree north latitude.
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ASHRAE Journal - June 2019

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