Free cooling refers to any technique used to reduce the energy consumed by cooling systems, or the time that the cooling units run, by using the outside temperature of air or water to cool the facility. Generally, it comes from the use of air-side and water-side economizers. But how does it save energy?

What is Free Cooling & How Does it Save Energy

For facilities with water- or air-cooled chilled water plants, free cooling is an economical method of using low external air temperatures to assist in chilling water, which can then be used for industrial processes, or air conditioning systems.  Used in the cooler months of the year, such systems can be made for single buildings or cooling networks. Free cooling can also extend the working life of installed cooling systems, lowering the energy and maintenance costs for facility owners.

The cooling provided, of course, is not completely “free” because the tower, chilled water pumps, and tower fans still must be operated. Nonetheless, it allows cost conscious building or process owners and operators to take advantage of naturally occurring climate conditions to save system operating costs.

Free Cooling & Economizers

Free cooling strategies rely on economizers. Economization is accomplished by taking advantage of the temperature difference between indoor and outdoor ambient conditions rather than running compressors to provide the cooling. The effectiveness of an economizer depends on loads characteristics of the building, type of HVAC system and the local climate.

Economizers draw in outdoor air and mix it with return air from indoors.  Airside economization can be accomplished by pulling cool dry air straight into the building, which is the simplest and most efficient option in many cases. Waterside economization, in contrast, uses an indirect method of economization and pulls cool water from a cooling tower or dry cooler that is cooled by outdoor air and runs the water through coils inside the HVAC units in the building.

Waterside Economizers

A water-side economizer eliminates the need for cooling via compressors and is an effective way to maintain temperature and humidity requirements while reducing or eliminating chiller use.

Water-side economizers are best suited in climates where the wet bulb temperature is lower than 55°F for 3,000 hours or more. This describes the majority of the United States, barring areas in the extreme Southwest and portions of the Southeast. They are commonly used in data centers, which produce a near-constant internal cooling load, and provide an extra level of cooling redundancy in the event of chiller failure.

Water-side economizers are preferable to their air-side counterparts for applications in which specific minimum humidity levels are called for, such as laboratories and hospitals. The waterside economizer requires a winterized cooling tower in many climates. Since the tower is expected to operate when it’s cold outside, it cannot be seasonally drained.

Airside Economizers

Airside economizers are a duct and damper arrangement that allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather. At outside air temperatures below 55 F, the compressors are not required to run, which conserves energy. In turn, cooler outside air is used to cool the space, hence the term “free cooling.”

Common Free Cooling Strategies

Determining the most effective free cooling strategy varies by facility. Below are three of the most common strategies.

Strainer cycle: In this system, the condenser and chilled-water systems are connected. When the outdoor wetbulb temperature is low enough, cold water from the cooling tower is routed directly into the chilled-water loop. Although the strainer cycle is the most efficient water economizer option, it greatly increases the risk of fouling in the chilled-water system and cooling coils with the same type of contamination that is common in open cooling-tower systems. A strainer or filter can be used to minimize this contamination, but the potential for fouling prevents its widespread use.

Refrigerant Migration: In this system, valves are open between the condenser and evaporator of the chiller when the compressor is off. This allows free migration of refrigerant vapor from the evaporator to the compressor and of liquid refrigerant from the condenser to the evaporator.

Plate-and-Frame Heat Exchanger (HX):  Another way to reduce the energy consumption of a chilled-water plant is to precool the water in the chilled-water loop before it enters the evaporator. This can be accomplished by piping a plate-and-frame heat exchanger into the chilled-water and condenser-water loops. When the ambient wet-bulb temperature is low enough, the heat exchanger transfers heat from the chilled water returning to the evaporator to the condenser water returning from the cooling tower. Precooling the chilled water before it enters the evaporator lessens the cooling burden, reducing the energy that the chiller uses.

Free Cooling & Proper Controls

Energy savings from free cooling rely on the appropriate design and controls to work optimally. Economizers add another level to the cooling scheme and must be engineered into the air handling system and controls. Interactions between the economizer and mechanical cooling (condenser pump, tower fan, chilled water pumps, fans, etc. ) must also be seamless and well-defined to ensure energy savings are achieved.

Coupling a free cooling system with an optimization solution like Xpress® allows seamless monitoring and management of all cooling equipment. Along with live and historical data capture, Xpress® combines real-time algorithms and extensive HVAC design experience to automatically optimize operating settings for equipment, maximizing free cooling and reducing energy costs.

Ground Source Heat Pump Basics

Ground source heat pump systems are highly efficient because they move heat from place to place, instead of generating it from a fuel source like oil, natural gas or electricity.

A commercial geothermal heat pump operates on a simple principle: it moves heat from one place to another via a refrigeration process. In a commercial building, a series of heat pumps removes heat from an energy supply source in the ground. The heat pump concentrates this low-grade heat, raising its temperature and then transfers it to the building’s energy distribution system via a heat exchanger.

In the summer, the process is reversed. The heat pumps collect heat from the building and deposit it into the ground loop, providing cooling.

Since the ground maintains a constant temperature of 55­­-70°F depending on location, a geothermal HVAC system can save 25­­-50 percent on HVAC costs compared to conventional systems using air­ source condensing units for cooling and fossil fuels for heating.

Types of Ground Source Heat Pump Systems

There are three basic types of loop systems and their uses depends on the climate, soil conditions, available land, and other site-specific considerations.

1. Closed-Loop Systems

Closed-loop systems are those in which heat-transfer fluid continuously circulates in a closed loop that’s filled just once and used again; no fluid can escape, and no outside materials can enter. Because of this, most closed-loop geothermal heat pumps circulate an antifreeze solution through a closed loop — usually made of plastic tubing — that is buried in the ground or submerged in water. A heat exchanger transfers heat between the refrigerant in the heat pump and the antifreeze solution in the closed loop.

Closed loop geothermal ground loops can last 50+ years with little to no maintenance. Once installed, the buried ground loop will be a permanent fixture on the property for as long as there is a building to heat and cool.

For commercial and industrial applications, the loop can be in two types of configurations:

• Vertical GSHP Configuration

Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, and they minimize the disturbance to existing landscaping. Vertical loops are connected with horizontal pipe, placed in trenches, and connected to the heat pump in the building.

• Pond/Lake GSHP Configuration

A pond / lake ground loop is a series of plastic pipes filled with heat-transfer fluid and submerged in a nearby pond or lake with adequate size, depth, and flow. The loop connects to an indoor geothermal heat pump and uses the pond or lake water as a heat source or heat sink.

2. Open-Loop Systems

An open-loop geothermal system pipes clean ground water directly from a nearby aquifer to an indoor geothermal heat pump. After the water leaves the building, it is expelled back through a discharge well. The water may also be directed into a local pond or approved drainage ditch. This option is practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.

The performance of an-open loop system may degrade over time if water quality issues like silt, sediment or high mineral content are present or if the water supply diminishes for any reason.

3. Hybrid Systems

Hybrid systems use several different geothermal resources, or a combination of a geothermal resource with outdoor air. Hybrid approaches are particularly effective where cooling needs are significantly larger than heating needs. A hybrid system uses conventional technology such as a cooling tower or boiler to meet a portion of the peak heating or cooling load. This innovation allows for a smaller, less expensive heat exchanger.

Maximizing Ground Source Heat Pump Efficiency

Ground Source Heat Pump systems are the leading technology chosen by building owners and entities seeking to achieve LEED, Living Building and Net-Zero Energy Certifications. They are also recognized as building environments that are healthier, more comfortable and more profitable. GSHP heating and cooling systems allow a facility owner to invest in their business on things such as product R&D, new equipment, or increasing personnel.

The design of reliable, cost-effective, energy-efficient GSHPs requires a system approach, as well as HVAC wisdom. tekWorx Approachable Experts® can help your facility incorporate existing plant equipment, including geothermal systems, into a comprehensive cooling optimization strategy.

The most common chiller failures are caused by compressor, electrical, or motor problems.

Compressor Maintenance to Optimize Chiller Performance

Compressor failure can be the result of any number of factors and is often attributable to problems elsewhere in the system. One simple routine task to prevent compressor failure is checking strainers and filters for dirt and debris and ensuring the compressor and pumps are appropriately sized for the load.

Eddy current testing, at a suggested interval of every three years, reduces the risk of chiller failure caused by condenser evaporator tube leaks or failures. Chiller tubes undergo daily stress as part of their normal operation which can allow rust and corrosion to take hold. Additionally, these rust particles can trickle into the evaporator tubes of the chillers, leading to corrosion that damages the compressor itself.  An eddy test determines the wall thickness of the chiller tubes and can detect possible pitting, cracks and bulges that can precipitate tube leaks. Identifying issues before a major problem occurs prevents downtime and expensive replacement costs.

Chiller Electrical Maintenance to Optimize Chiller Performance

Electrical issues in the chiller plant can be caused by a host of issues including wires rubbing equipment frames or condenser fans not working appropriately.  Electrical overload conditions will cause motors to draw in more current to maintain torque and can lead to overheating and damage to winding insulation.  Regular attention to amperage draw, voltage, and contactors can avoid such issues.

Chiller Motor Maintenance to Optimize Chiller Performance

Chillers, pumps and tower fans use motors to move water and air or compress refrigerant. The failure of any of these motors threatens the operation of the entire chiller system. There are many reasons why motors fail, but three fairly simple, non-damaging tests can avoid a world of headaches when performed consistently.

• Oil Analysis: Regularly scheduled analysis of your chiller’s oil is a valuable aid in assessing internal mechanical condition. Oil comes in contact with many important internal components and can therefore hold valuable information about chiller health. An oil analysis will indicate whether there is moisture, acid, corrosion, bearing wear, impeller rubbing, or other equipment problems present. When an oil analysis reveals the presence of wear, a possible bearing or motor failure can be imminent, and a vibration analysis is recommended. The combination of these assessments will typically identify the failing component.
• Vibration Analysis: Every piece of HVAC equipment with rotating components has its own vibration signature. Any change in this signature can be used as an accurate means of identifying developing problems with chiller bearings, impeller imbalance, or open rotor bars in the motor. Vibration analysis should be performed on a regular basis to build a baseline and trend which can significantly aid in avoiding unplanned downtime and replacement costs
• Motor Insulation Resistance: Insulation problems on motors and drives are typically caused by improper installation, environmental contamination, mechanical stress, or age. Insulation tests should be performed on all chilled water system motors to monitor motor health. These assessments measuring the winding resistance. A low resistance indicates that the winding is deteriorating and indicates potential failure. Insulation-resistance trending can ensure that any changes are readily addressed.

Monitoring Chiller Performance

Selecting quality equipment and performing regular maintenance specific to the needs of each unit ensure a long and efficient life cycle. To protect such investments, the remote monitoring of chillers and cooling system can significantly aid maintenance and service efforts.

Chilled water system monitoring, like that of the Xpress® Energy Optimization Dashboard, provides facilities teams with real-time energy data for all chilled water equipment. Understanding how energy is used can help quickly identify energy waste and equipment problems, as well as overcharges and errors on energy bills. Xpress® also acts as an early warning system, sending emails or texts to staff when equipment such as a fan, pump or chiller is operating outside expected parameters.

Analytics packages seem to have it all. At first glance, integrated dashboards provide a snapshot of operational processes, metrics and key performance metrics (KPIs).

Unfortunately, analytics as an actual cost reduction tool in chilled water systems are limited unless a human takes action on this information to adjust operations. Optimizing the efficiency of dynamic systems, such as a chilled water plant requires action, ideally continuous action, that does not rely on manual intervention.

The optimization spectrum

The various levels of efficiency improvements

Of the four basic levels of efficiency improvements in use today, optimization and analytics are the most discussed. While they are complementary, they are not the same. Analytic and model-based strategies simply suggest edits, not action. Without automation, analytic software cannot optimize dynamic settings like a chilled water plant.

tekWorx automated optimization

The Approachable Experts™ at tekWorx can help your team go beyond analytics to optimize the efficiency of your dynamic system.

Our automated optimization solutions maximize the use of your existing mechanical equipment and controls to significantly improve plant performance. Using adaptive algorithms to continuously analyze real-time process variables, key control parameters are automatically adjusted to minimize plant kW/ton without any human intervention.