March 30th 2023

A Deep Dive into the Water-Energy Nexus

A Deep Dive into the Water-Energy Nexus

The Water-Energy Nexus considers the interrelationship between water and energy. E.g. Power plants consume water for cooling systems, and Water plants consume power for treatment and distribution. Calculating the water usage for a building requires accountability for all water that is used and that is recovered, both inside and outside the building. As well as water for DHW purposes such as sinks/showers/dishwashers etc., HVAC systems can use and can reclaim water from various HVAC processes. In fact, HVAC systems can be responsible for up to ~30% of a commercial building’s total annual water use.

Approximately 1% of the earth’s water can be considered useful for human consumption. Of that 1%, about 14% is used by buildings, with a certain amount of this 14% being used for various HVAC applications, such as:  

a) Makeup water for the Cooling Tower to reject heat

b) Direct/indirect Evaporative Cooling system (spray chamber)

c) Steam humidification

In addition, some water can be captured (or reclaimed) from cooling coils in humid locations, when moisture in the air condenses. This reclaimed water can be repurposed for other applications such as irrigation.

Green building rating systems, like LEED, give credit for reduction of indoor water use and outdoor water use. One such credit is the Cycle of Concentration (CoC)1 for the cooling tower. Based on CoC1, the cooling tower will need to remove (or blowdown) and replace water with makeup water.

The IESVE Water-Energy Nexus Calculator in VistaPro will calculate the amount of makeup water required for these HVAC components and also calculate the recovered condensate water from the cooling coils.

  1. Cooling Tower Water Losses: Evaporation, Drift and Blowdown

a) Evaporative loss is a water loss due to evaporation as ambient air moves counter to gravitational water flow.

b) Drift loss is small water droplets that escape the cooling tower within the ambient air stream.

c) Blowdown loss is water loss to reduce the concentration of minerals in the condenser water loop. This is calculated based on the Cycle of Concentration.

Figure 1: Cooling Tower Water Losses

  1. Evaporative/Spray Cooler Water Losses

Water Losses from the Spray Chamber is caused when fine water droplets are sprayed into the air stream. The air temperature decreases and is humidified, with an adiabatic (constant enthalpy) process. The Spray Chamber may be used for direct evaporative cooling and with an air-to-air enthalpy exchanger for indirect evaporative cooling.

  1. Steam Humidifier Water Losses

Steam may also be used for humidification by adding a mist to the air by boiling or heating the water, along with increasing the humidity levels in the air. The warm vapors lost to the air requires makeup water. Steam is commonly used where for infection control is a concern.

Spray Chamber:  Steam Humidifier:

Figure 2: Water usage by Spray Chamber and Steam Humidifier

  1. Water Reclaim from Cooling Coil Condensate:

Latent heat is transferred by moisture in the air to the coil via the process of condensing on the surface of the cooling coil. This condensate may drip from the cooling coil and be collected for reuse.

Figure 3: Cooling Coil Water Condensation

Water-Energy Nexus Model Calculator in IESVE

The “Water-Energy Nexus” feature is a post-processing tool in IESVE Software and can be accessed through the VistaPro Application. Instructions include:

a) Click the “Water Energy Nexus” icon in VistaPro (water droplet)

b) Select the .aps simulation results file and assign the monthly “Cycle of Concentration1” for the cooling tower, assuming a cooling tower exists in the model’s heat rejection setup. Provide a new “.app” file name, then click “Process”.

c) The new files will be available in the “Post-processed” Category. If it doesn’t appear, you may need to click the refresh button to the right of the Simulation dropdown.

Figure 4: Accessing the Water Energy Nexus Tool

Water Analysis Examples

Below is the Cooling tower water loss calculation from a Building Energy Model (BEM) in Phoenix, Arizona (ASHRAE Climate Zone 1B – Very Hot/Dry). Note that the majority of the water loss is occurring due to Evaporation, followed by Blowdown loss and then the Drift loss.

Figure 5: Cooling Tower Water Losses (SI)

The identical Building Energy Model (BEM) was relocated to Orlando, FL (ASHRAE CZ 2A – Hot, Humid) to calculate and compare the condensate reclaimed from the cooling coil. The reclaimed condensate from a cooling coil in Orlando (humid) was significantly higher than that in Phoenix (dry).

Note that the values are negative as this represents that the cooling coil is not using water, but adding water to the project by condensed moisture from the air. This water can be recycled and repurposed for other applications in the building or on the site.

Figure 6: Cooling Coil Condensate Reclaim (IP)

We expect the interrelationship between water and energy will continue to become much more of a critical design consideration in the future. Integrated analysis tools like IESVE Software covers the broad spectrum of performance analyses types, including energy, carbon, water, costs, comfort, daylight, airflow and HVAC system design.

1 Cycles of Concentration (CoC) represents the number of times that a volume of water can circulate through a cooling tower system before dissolved minerals become concentrated. To dilute the concentration, the cooling tower must remove some water and replace it with fresh/mains water.  A municipality’s mains water can provide data bout the varying dissolved minerals in the water throughout the year. An IESVE user can edit the CoC on a monthly basis in the Water-Energy Nexus tool.