There's No Such Thing as Free Cooling… Is There?

In a perfect world, free cooling would be available all the time to everyone - along with free ice cream and small waistlines. In the real world, free cooling is one of those available for a limited time, small upfront cost type of deals. But, despite its limitations, the upfront investment has a very fast return, saves energy and operational costs and is good for the environment. What free cooling refers to is eliminating the use of an energy-guzzling AC condenser/compressor while still meeting cooling needs. In New England, the most popular options for free cooling are with an economizer on a rooftop unit, or with a cooling tower on a chill water system.

• Rooftop Economizer Option
• Chiller/Cooling Tower Option

Adding an economizer (left hand side of photo) to your rooftop unit can save you between 14-40% each year on your energy bill. -Photo courtesy of Carrier

Rooftop Economizer Option

One of the simplest ways to get free cooling is by installing an economizer on your rooftop unit. An economizer is a damper, or adjustable opening, that is controlled by a sensor which measures one or more of three things: outside air temperature, outside air humidity, and/or CO2 levels of inside air (for air quality purposes). The sensor uses these measurements to determine the most energy-efficient way to condition the air. It compares the temperature and humidity of the outside air to the return air from inside the building and then sends whichever air is cooler to the unit. The cooler air is to begin with, the less energy is needed to condition it. When the outside air hits low enough temps, the AC compressor doesn't need to run at all, hence, free cooling!

Outside air that is at or below 70?F can be used to reduce the cost of operating your air conditioning. The point at which cooling becomes free depends on both temperature and humidity levels, but generally free cooling is at or below 55?F outside temps. For New Bedford, the average temps are at or below "free cooling" temps for at least six months out of the year (source: www.weather.com). If your facility only uses cooling for the other six months out of the year, you may not receive a justifiable cost-benefit. But, many facilities, computer rooms, industrial/processing plants, restaurants, large office buildings, etc. require year-round cooling. For them, free cooling means big savings.

To maximize savings, economizers need to be programmed initially and maintained regularly. Moisture and dirt can cause the damper to stick which can waste energy and even cause damage to equipment. When programmed and maintained correctly, an economizer will save you 14-40% each year on your energy bill.

Other Benefits of Rooftop Economizers

By bringing in outside air to use for cooling, the economizer "cleans" the air inside of a building. Air filters in your HVAC system filter out particles from the air, such as dust and pollen, but one of the biggest culprits for "bad" air in a building can't be filtered because it is a gas and not a particle.

Though some people seem to create more "hot air" than others, all humans exhale CO2 when we breathe. The CO2 can build in an enclosed space until it becomes unhealthy. Using an economizer to reduce cooling costs has the side effect of creating cleaner inside air.

Consistently high CO2 levels are a contributing factor to Sick-Building Syndrome, which can cause discomfort and illness in tenants and employees in the building. If high CO2 levels are an ongoing problem, you can use the economizer to better control building ventilation. By adding a CO2 sensor to the economizer, it will sense when CO2 levels are climbing and automatically open the economizer to let in more fresh air. This is known as Demand Control Ventilation.

Cooling Tower (top left) uses the evaporation of water and the surrounding air temp to cool system water. The cooler the water is before it reaches the chiller condenser (bottom left), the less energy needed to cool it to target temps. -Schematic courtesy of MicroAir Mechanical.

Chiller/Cooling Tower Option

In the same way that the rooftop economizer reduces or eliminates the use of the rooftop compressor to save energy, a cooling tower reduces or eliminates the use of the chiller compressor to save energy. The process is similar in that it uses cooler outdoor air to reduce energy consumption, but the process is less direct in the chiller system. Instead of simply using the cooler outside air to cool the building, it uses the cooler outside air to cool water circulating through the chilled water system.

In typical chiller operation, warm water from the building goes through the chiller condenser. The condenser removes the heat from the water and circulates it back through the building. This process uses a significant amount of energy.

With the addition of a cooling tower, the warm water coming back from the building gets sent to the cooling tower first. The cooling tower cools the water by using the process of evaporation and/or by the presence of colder outside air. If the water is cooled enough by evaporation and outside air, the chiller condenser doesn't need to run, hence, free cooling!

There are three options to set up free cooling with a chiller/cooling tower system. In each option, the cooling tower must be designed for winter operation in order to be effective for free cooling. These options are most effective during the colder six months out of the year and therefore work best for those buildings that require some amount of cooling year-round.

Refrigerant Migration Set-up

When certain conditions are present, the natural chemical and physical properties refrigerant will cool the water in the chill water system without requiring the assistance of a compressor. In typical operation, the chiller compressor is needed to compress liquid refrigerant into a gas as it moves through the system. When the refrigerant changes state, it absorbs heat from the building and then cycles through the system until it releases the heat to the outside.

In the Refrigerant Migration Set-up, the refrigerant moves through the system and changes state without the assistance of the compressor. Though this set-up can be effective - producing as much as 40% of the chiller's cooling capacity without using any energy-consuming mechanical cooling - it only works when outdoor air temps are below 50?F, and thus has limited application.

Drawbacks to this set-up:

1. Only effective when outdoor temps are below 50?F.

Strainer Cycle Set-up (Cooling Tower Only)

In the Strainer Cycle set-up, the water from the chilled water system simply circumvents the chiller (and its expensive cooling process) in favor of the free cooling in the cooling tower. Water sent to the cooling tower is cooled by evaporation and the cold outside air and then circulated directly through the building with no additional cooling needed. In this set-up, the chiller condenser is not used at all.

This system is commonly called the strainer cycle because water from the cooling tower is generally sent through a filter or "strainer" before it enters the building's chilled water system. The strainer helps to remove any particles that may have collected in the water in its travels through the cooling tower.

There are a few drawbacks to this set-up.

1. Water circulated through a cooling tower tends to become more corrosive since it is exposed to the elements. When untreated cooling tower water is circulated through the system, it can accelerate pipe corrosion and cause air handlers to plug up more frequently. This issue can be mitigated by adding chemical water treatment into the system. This adds to the upfront and ongoing maintenance costs of the system.
2. In this set-up, free cooling is only available on an all-or-nothing basis. The water doesn't get any additional cooling after going through the cooling tower, so it must be cool enough on its own to satisfy all of the buildings cooling needs. This limits its availability to only the coolest months.

Plate and Frame Set-up (Add-On Heat Exchanger)

The Plate and Frame set-up eliminates the all-or-nothing limitation of the Strainer Cycle by allowing for some additional mechanical cooling after the cooling tower. The water still circumvents the chiller condenser entirely. The additional cooling is provided by an add-on heat exchanger. When extra cooling is necessary, the water is sent from the cooling tower to the heat exchanger. Since it has been cooled somewhat already by the cooling tower, less energy is used to bring it to the ideal cooling temp.

In order to take advantage of the energy savings, outside air temps must be at least 10 degrees cooler than the return water inside the building. The greater the differences in temperature, the more savings are recognized. When the water from the cooling tower is cool enough, neither the heat exchanger nor the chiller condenser is used, giving you free cooling.

The Plate and Frame set-up allows for reduced energy consumption during those times of year when the outside temp isn't quite low enough to take care of all of the buildings cooling needs. And still allows for free cooling when outside temps are ideal.

Drawbacks to this set-up:

1. Added upfront expense of the add-on heat exchanger.
2. The addition of the heat exchanger causes pressure loss in the system which must be made up by the circulating pump. The expense of operation and additional wear and tear on the pump offsets a small portion of the energy savings.

For more information, or to find out if Economizer & Cooling Tower options can help your business, call Advance Air at 508-763-3738 or send an email to info@advanceair.net.

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