How to reduce energy consumption of a chiller
In this article, we'll look at different ways to improve the efficiency of your existing chiller setup. We will cover compressor retrofit, compressor VFD, chilled water reset, condenser water reset, condenser and evaporator cleaning, expansion valve, air-cooled chiller, condenser fans, cooling Free or Economizer Cooling, Chiller and Plant Sequencing, Controllers and Accessories, Cooling Tower, AHU VFD Pumps and Fans
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In most applications, chillers are the most energy consuming piece of equipment in a building. There is a lot of pressure on engineers as well as building and facility managers to reduce energy consumption and the carbon footprint of buildings, so chillers will naturally be at the center of these concerns. We'll look at low-cost options, as well as some potential projects to achieve this.
How chillers use energy
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| How chillers use energy |
First consider how energy is used in chillers. First we have the compressor, it takes the low pressure refrigerant and compresses it to a higher pressure, to do this work we introduce electrical energy represented by Qein.
As the refrigerant moves, it removes unwanted heat energy from the evaporator buildings, so we have heat energy going into the system represented here by Qthin.
This is then rejected from the capacitor, so we have energy leaving the system represented here by Qthout. We also have fans in air-cooled coolers that work to force unwanted heat dissipation, so we have electrical power coming in here as well represented by the Qein.
The condenser and evaporator are heat exchangers and are exposed to water, dirt, and biological growth that limit heat transfer. This results in reduced performance and increased power consumption to meet design criteria.
Between the evaporator and the condenser we have the expansion valve that maintains the pressure differential between the high and low pressure side, this also controls the flow of refrigerant to the evaporator and finally to the compressor.
Then we have the controls and accessories that dictate how the system works.
So we have many components that play their role in the system to modify and control the movement of heat energy and pressure at different points. These all play a role in how much power the cooler will draw.
Let's walk through the system to consider different ways to reduce the energy consumption of chillers. I just want to point out that the estimated power savings listed for each of the items are for the individual implementation. You can combine the above steps, but the potential savings will not only add up, they will decrease.
The first area we will consider is the part that consumes the most energy, that is, the compressor. In our previous video we looked at chiller types and applications where we covered different types of compressors, applications, and efficiencies. Please refer to this as it will help you understand; Also, I recommend you check out the Essential Chiller Terminology article and video tutorial to help you master the chiller language.
Compressor retrofit:
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| Compressor renewal |
In many cases, a chiller compressor can be retrofitted with newer technology. Check with the cooler manufacturer or your cooler service specialist to see if your cooler can be retrofitted. If so, the refund can be very fast. For example, replacing a fixed speed scroll compressor with a variable speed compressor can result in approximately a 9% reduction in energy consumption. The most important factor affecting savings is the length of the year that the chiller is running at part load. If it is at full load for most of the year, there will be little or no savings, but if the chiller is running at part load for most of the year, as is the case with most chillers, there is great potential for energy savings. Energy. .
Compressor VFD Upgrade:
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| VFD Compressor Upgrade |
If it’s not feasible to replace the compressor, VFD’s or variable frequency drives can If replacing the compressor is not possible, VFDs or variable frequency drives can often be installed on the compressor, allowing the chiller to run more efficiently under part load conditions. However, full load efficiency will be reduced due to inverter losses. Most chillers will run part load for most of the year, so this is an attractive option. Upgrading a VFD can often result in energy savings of around 20%. Check with a local specialist to verify compatibility.
The next part is to look at what the chiller compressor does and how it relates to the operating conditions of the evaporator and condenser. Remember that it takes refrigerant at low pressure and compresses it at high pressure. That's what electricity is for. So let's take a look at some ways to reduce this.
Chilled Water Reset:
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| Chilled Water Reset |
The leaving chilled water temperature has traditionally been fixed, typically around 6°C or 42.8°F, but it is now common practice to apply a chilled water reset strategy. Chilled water reset allows for increased chilled water temperature under part load conditions. This reduces the amount of work done by the chiller compressor.
As the chilled water temperature increases, the refrigerant evaporator pressure increases. The compressor takes in the refrigerant at evaporator pressure, then compresses it and builds its pressure up to condenser pressure. Thus, the higher the evaporator pressure, the less the compressor must work to increase this pressure.
Typically, chiller efficiency can be increased by 1-2% per degree C increase in chilled water temperature.
There are many ways to implement this, you should first check with your chiller manufacturer if this applies to your chiller and ask what temperature or pressure limits should be imposed. If applicable, you can start to decide what the deciding factor for the temperature change will be, as there are many ways to do it.
We won't go into too much detail on this in this article, but some common options include controlling by outside air temperature, controlling by actual load, both sensible and latent heat, or being controlled by average or most demanding. position value in the refrigeration coils.
Before implementing a chilled water reset, it is important to consider
· Chilled water pumps will speed up variable volume systems to meet the cooling load. The sweet spot must be calculated so that the increase in pumping power does not offset the savings from the chiller.
· Cooling coils are often used for humidity control. Chilled water temperature is crucial to this, so check to see if this applies to your system.
· Check to see if the chiller manufacturer recommends temperature and pressure limits for the water or coolant.
Condenser Water Reset:
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| Condenser Water Reset |
This can only be implemented on water-cooled chillers, since air-cooling does not use condenser water. This is very similar to the chilled water reset we just looked at, by lowering the condenser water temperature you will reduce the amount of work the compressor has to do because the lowering of the water temperature will cause the condenser refrigerant pressure to rise. fall.
Again, as a general rule of thumb, you can save 1-2% per degree C reduction. Typically, the condenser return water temperature set point is set to around 27°C or 80 °F, the cooling tower or bypass line is adjusted to control this temperature.
Some parts of the world with temperate climates may reduce the condenser water temperature as the outside wet bulb temperature decreases. This will reduce the condensing pressure of the refrigerant so the compressor has to do less work. However, there are limits to how high you can descend, the cooler will eventually shut down to protect itself, so check with the manufacturer first before doing so.
clean the cooler
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| Cleaning an evaporator and a chiller condenser |
It should be pretty obvious, but you'll find buildings that have never or rarely cleaned their chiller heat exchangers.
Every chiller is designed to have a certain amount of scale, but over time biological growth, dust, foreign particles and internal corrosion particles accumulate and coat the surfaces of piping and heat exchangers, forcing It causes the pumps to work harder but also reduces the ability of the heat exchanger to transfer heat energy between the water and the refrigerant.
Adequate water treatment must be carried out to maintain the system; you should contact a local water treatment specialist to determine the correct chemicals and dosing intervals. Additionally, the condenser and evaporator must be physically cleaned periodically.
For condensers connected to open cooling towers, it is recommended to clean them every year.
For closed loop evaporators and condensers, it is good practice to physically clean every 3 years.
How much you can actually save depends on how much dirt has accumulated on the surfaces. Cleaning a heavily contaminated chiller could result in energy savings of up to 10%, while a lightly contaminated chiller would see a 1-3% improvement. This will depend on the amount of dirt that accumulates, as well as how long the chillers have been running. It is best to keep a daily log of operating conditions to compare performance over time, this will indicate when it is economical to insulate the chiller and clean it.
Don't forget to also clean the condenser fins of an air-cooled chiller. Over time, these can collect dirt, dust, bits of trash, animal nests, and biological matter that collects on surfaces and reduces heat transfer efficiency.
Expansion valve:
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| update regulators |
Many chillers use a thermal expansion valve to control the flow of refrigerant through the evaporator to achieve the desired superheat.
If you want to know how TXV works, we have already made a very detailed animation video, click here to see.
The thermal expansion valve can be replaced with an electronic expansion valve which will provide much more stable superheat, especially at light load, and because it is digital, it can respond to changes and control superheat more quickly and accurately.
To do this, it measures both the pressure and the temperature at the outlet of the evaporator. The precision allows the condensing pressure to be reduced so that the compressor does not have to work as hard. By replacing a thermal expansion valve with an electronic expansion valve, an energy saving of approximately 14% is possible, but this varies depending on the chiller load.
Air cooler, condenser fans
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| EC fan upgrade on an air-cooled chiller |
Air-cooled chillers use condenser fans to reject heat. They are often controlled to work 100% or 100% with no variation between them. Sometimes these fans are grouped together to provide cooling stages, especially in larger units. These fans can be replaced with EC or electronically commutated fans that allow them to vary speed and airflow. This provides much more even airflow over the condenser coils and also saves energy. Replacing the fans in an air-cooled chiller with EC fans can result in an energy reduction of approximately 6%.
Free cooling or economizer cooling
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| Refrigeration without chiller |
It is a very popular choice for data centers but it can also be used for commercial buildings. This involves removing unwanted heat from the cooling system with little or no use of compressors. This is not possible in all locations, it can only be used when the outside air temperature is below the chilled water set point temperature.
Using a free cooling or economizer strategy will shut down the chillers compressor, but the pumps and fans will still run and likely run at high speeds, so some of the savings from compressor shutdown are offset here. Using a free cooling cycle could reduce the annual energy consumption of the chiller by 20-50%; again, it really depends on local environmental conditions and the set temperature.
Air coolers sometimes have a built in free cooling feature that uses the fans without the compressors if conditions are right, but I have come across a few installations where this has not been mandated so check with the manufacturer if this feature is available or if retro mountable.
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| outdoor cooler |
Alternatively, air-cooled condensers can be installed in the system to use only ambient air to remove as much heat as possible before reaching the chiller.
For water chillers, a common approach is to connect the condenser loop and the chilled water loop through a plate heat exchanger to transfer heat directly.
Controls, controllers and accessories
Whenever possible, choose digital sensors and controllers as they can work faster and more accurately. When replacing an accessory, check with manufacturers for more energy efficient and reliable accessories. With fixtures and control gear, manufacturers will eventually stop making parts, which means it will be more difficult and expensive to source third-party replacement parts. Stay on top of this and don't just replace the same, always look to improve.
On the control side, the manufacturer will often release new software updates periodically for drivers and equipment, make sure the service engineer and BMS or maintenance company update their systems where possible, as It will allow drivers to work more efficiently and without errors. If you hire a third party to maintain your chillers, push them to come up with a new energy-saving control strategy, not just for the chiller, but also for how it interacts with the entire cooling system. You can request this as part of your normal service or for contract renewal or just ask them to provide quotes for upgrades.
Plant Cooler and Sequencing
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| Chiller plant sequencer |
Chiller, cooling towers and pumps have different efficiency curves, especially under part load conditions. An installation sequencer can be used to ensure the most efficient combination of installation elements is used to match the current cooling load. The sequencer can be chiller only, or more advanced chillers will include all the associated big factory items.
If you go for the combined sequencer from the factory, then it is possible to reduce the power consumption of the entire system by about 20%, sometimes more, but it depends on the severity of the control strategy above and the efficiency of the elements of your factory. at partial load.
Cooling tower, AHU and VFD pump
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| vfd induction motor |
Cooling tower fans, chilled water and condenser pumps, and AHUs use induction motors. Although they are not part of the chiller, they are part of the cooling system that the chiller serves and their operation will affect the performance of the chiller. These motors can be easily integrated with variable speed drives to produce significant energy savings. You will often find that the pumps and fans provide more flow than the system was designed for; You can check the commissioning data of the building to check it. These factory items are typically 5-20% larger in size, so installing VFDs allows them to be reversed to match the design.
Also, with some sensors and controls, VFDs can be used to match the actual load and vary the speed to precisely match. Using this method can typically lead to a 30% reduction in power consumption per motor.
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