Climate change is upon us, even though there are different schools of thought on how much is the real impact and what are the causes. One aspect of the climate exchange debate that everyone however agrees on is that the emission of greenhouse gas (GHG) is increasing and the earth is getting hotter. The key reason for the increase of GHG is the exponential rate of increase in energy use across the world and buildings are a large consumer of the energy that is generated – upwards of 40 per cent in some markets. As we all know, HVAC systems consume the major portion of energy in a building, anywhere from 30 – 50 per cent depending on the type of systems and use of the building.
HVAC systems have come a long way from the ice cooled systems used in the later part of the 19th century. While air conditioning has enabled humans to work in extreme environments and conditions, the negative impact due to the energy use is an undesirable by product. The HVAC industry has been at the forefront of finding ways to make the systems more efficient and consequently minimise the impact on the environment. This article reviews the developments of the last 3 – 4 decades in HVAC systems that have helped improve the efficiency of the system and what lies in store for this sector as we start the new decade.
HVAC System Energy Use
When one assesses building HVAC systems, the two most common types of systems are the water-cooled plants and the air-cooled plants. For smaller requirement, VRFs or Dx units are also used but this article will focus on the water cooled and air-cooled chillers as they form the bulk of the HVAC tonnage in commercial and retail buildings in the country.
Measurement of Efficiency: The most common approach to quantify the efficiency of a chiller system is to assess the Indicted Kw (iKw). This value represents the amount of energy needed to generate 1 Tr of refrigeration capacity. Thus, if the iKw is 1.2, it indicates that 1.2 units of energy are required to produce one Tr of refrigerant capacity. The lower the iKw, the more efficiency the system is. While iKw gives a broad idea of the efficiency of the chiller, a more accurate assessment is by using the Integrated Part Load Value (IPLV) method, which is essentially the energy efficiency derived based on the time that the chiller would run at designed loads (100%, 75%, 50% and 25%). The IPLV the AHRI standard that the HVAC industry has adopted.
HVAC system energy map: Figure 1 shows the percentage of energy that is consumed by various components of a water cooled and air-cooled chiller plant. As can be seen, the chiller consumes the largest share of energy and hence, is the main target for system optimisation. However, the other auxiliary components such as cooling towers and the primary and secondary pumps also can be optimised to improve overall system efficiency.
Chiller efficiency over the ages
The earliest chillers had reciprocating compressors which consumed substantial amounts of energy to generate the desired cooling capacity. The system capacity control was achieved by bypassing cylinders of the compressor which meant that the system still had to overcome friction losses in the isolated cylinders. Reciprocating chillers had the iKw in the range of 1.2 to 1.5, depending on the capacity and technology used. With the advances in the development of centrifugal fans, the chiller development moved towards centrifugal chillers that leveraged the flow properties of the refrigerant, allowing for larger capacities to be manufactured. The main advantage that centrifugal chillers had was the ability to vary system capacity by varying the speed of the compressor. Of course, the off-design efficiency of the centrifugal chillers was impacted due to the nature of the design of centrifugal fans, but the overall system efficiency remained higher than the reciprocating chillers. Modern centrifugal chillers have iKw values in the range of 0.7 to 1 which is a significant improvement (over 30 per cent more efficiency) over the reciprocating plants.
In the last 5 – 10 years, oil free, magnetic bearing chillers are pushing the envelope of chiller efficiency. These chillers do not have oil bearings like conventional chillers. Instead, the bearings are magnetically levitated so that the losses due to friction in the old filled bearings is eliminated. These chillers are quitter as well as easier to start due to the magnetic bearings. The iKw of the latest magnetic bearing chillers is sub 0.5 and there are some chillers that have achieved iKw as low as 0.3.
Air cooled chillers typically have iKw values in the range of 1.05 to – 1.2 and hence, consume more energy than water cooled chillers. However, when the energy used by auxiliary systems is added, the iKw of water-cooled systems also increases whereas air cooled chillers have very limited auxiliaries and the added energy use increases the iKw only marginally.
The Future of Chiller Efficiency
Manufactures are pushing for ever lower iKw values of the magnetic bearing centrifugal chillers. However, there will be a limit reached due to the mechanical design and thermodynamic limitations of the system and the gains in efficiency will be marginal over the next few years. Some of the ways in which HVAC system efficiency will be improved further during this decade are:
Hybrid systems: These chillers will use renewal energies such as solar power to supplement mechanical power of the compressor thus, improving overall system efficiency.
Integrated chillers: Many chiller manufactures are looking at designs that have heating and cooling capabilities or are integrated with absorption systems to lower the overall energy utilisation.
Connected chillers: With the advent of IoT (Internet of Things) and more advanced data collection systems, manufactures are getting more insight to how the chillers operate under various conditions. Using big data analytics and Artificial Intelligence, manufactures are developing the capability to remotely monitor the operations of the chiller and give real time inputs to operate the system more efficiently. This approach promises to bring in the most improvements in both the chiller efficiency as well as the system efficiency.
New refrigerants: Studies are ongoing across the world on alternative refrigerants that have lower Green House Gas Potential (GHP) as well as allow higher refrigeration capacities. This approach to hold tremendous potential to lower energy use in chillers.
Chiller consume the most amount of energy in a building and the aim of manufactures is to improve the efficiency of this system so that the cost of the HVAC system operation can be lowered. There have been substantial gains in the last 3 – 4 decades in this effort, with modern magnetic bearing chiller working at nearly 60 per cent of the energy that chiller used just 15 years back. There are, however, limitations, on how much efficiency can be achieved through component or equipment level improvements and hence, there is now a greater thrust to use IoT and advances in refrigerant technology to improve the overall system efficiencies. The future is moving towards a ‘Net Zero’ chiller system.
CBRE South Asia Pvt Ltd