Decisions made during the design and planning stages of a chilled water plant system have long lasting effects on the energy demands and usage of the building.
Chilled water plant design and energy usage of the facilities are closely related, as the largest energy consumer is often the HVAC systems.
Rising energy costs and peak demand prices are rising, as are the issues of a buildings carbon footprint. It is important therefore to consider designing the chiller system to run as efficiently as possible, minimizing energy usage and operation costs (OpEx).
The ASHRAE 90.1 “Energy Standard for Buildings Except Low-Rise Residential Buildings” has evolved over the years. The requirements for chilled plants have increased, and provide a solid basis for good practices to be followed during your design stage. For example, the full-load coefficient of performance (COP) of a 500-ton centrifugal machine has increased from 3.8 in the 1975 code to 6.28 in the 2016 code.
As technology has improved, there have been advances made on the manufacturers side which have resulted in higher efficiency and lower energy consumption. However, despite these improvements, an efficient well thought out design continues to provide the most benefit in the long term OpEx of the facilities.
How to Quantify Chiller Efficiency?
The specification for the efficiency of chillers is stated in several ways. Nominal kW/ton, integrated part load value (IPLV), non-standard part load value (NPLV), and COP are all metrics of performance used to evaluate chiller options. Chillers spend most of their time operating at only a fraction of their full load capacity. Therefore he IPLV metric is one of the most useful for evaluating the efficiency and comparing different chiller plants. The ILPV metric makes the assumption that the chiller will spend “x” percentage of it’s ton-hours at “y” percentage of its peak capacity, and also defines the entering condenser water temperature for each condition. For more details on ILPV and it’s modified cousin NPLV click here.
It is important however to remember, the chiller is only one part of the chilled water plant system. There are also water pumps, cooling towers and condenser pumps. Each component in the system contributes to the total energy usage of the plant. Therefore each of these components should also be carefully considered and analyzed during the design phase to determine the most suitable equipment for the operation.
The actual energy usage of the plant will vary month to month, or even hour by hour based on the prevailing weather conditions and the demand loads being placed by the building and other facilities. This is why it is important to perform a full year hourly analysis simulation of the complete chilled water plant system. There are softwares that run these kinds of simulation such as the Trane – Trace 700 Chiller Plant Analyzer.
Maximize Chiller Plant Efficiency
As a general rule, your strategy when designing the Chilled Water Plant is aim to maximize the number of hours the chiller will be operating at partial load, thus minimizing the time spent at maximal load. The Chiller will be at it’s highest efficiency when operating at partial load. The Chilled Water Pumps should be variable speed, controlled by a VFD. This allows them to match building loads reducing energy costs. Cooling Tower Fans should be specified to optimize the inlet temperature at the condenser. This usually means running the tower fans at higher speeds than usual. However, this higher speed fan energy usage is offset by he improved efficiency seen at the Chiller.
For accurate tracking of the energy usage of your Chilled Water Plant, suitable monitoring and sub metering systems can be installed. These sensors will provide feedback to your building management system (BMS). You can then quantify the energy used of the various components in the Chiller Plant. This realtime monitoring provides you the information needed to run the plant most efficiently.
In addition to these general design principles there are more specific parameters that attention should be given.
- Delta-T (∆T). This is a measurement of the difference between the chilled water supply temperature and he chilled water return temperature. Your aim should be to maximize this number, the greater the difference the better as it means there is a higher transfer of heat. Conversely a smaller ∆T means less heat is being removed for the same amount of pumping energy expenditure. Therefore, a maximum ∆T means that for a given amount of pumping energy you are removing a higher amount of heat from the building. Thus meaning a more efficient operation.
- Variable Speed Drives (VFD). The cooling demands of the building will vary throughout the day. As load requirements on the air handling system decrease the system will require less chilled water flow. The use of a VFD means that the pumps can modulate their speed matching the water flow rates o the demands of the system. The cooling coils should have 2 way valves and not 3 way valves installed. A 3 way valve will allow water to bypass the coils. This does not decrease the flow rate, the pump must still provide the same volume of water. Some of this energy is therefore wasted, as it’s spent pumping water that is not removing any heat from the building. This leads to a low ∆T.
- Load Matching. This approach to designing the system matches the chiller size with the modular load in the building. If a building has an annual cooling base load, you would design one chiller that precisely accommodates this load. There may be days during the year or months, where he base load is exceeded. This additional load is taken up by additional callers that can remain off for the period of time that there is lower cooling demand.
- Demand Reduction. Most electric companies have peak demand charges, which can result in large additional costs. Systems that decrease the peak demand can result in big savings. VFD’s are one example of a mechanism that can reduce peak demand. By spooling up pump motors you can avoid surges in power consumption associated with the chiller pumps turning on. Other demand reduction techniques include turning off none essential equipment during the peak demand hours, using battery backup power systems.
Maintaining efficiency throughout building lifecycle
So now we have designed an efficient Chilled Water Plant System, we need to evaluate the system during commissioning to check it performs as expected and on a regular basis and ensure it is still running efficiently.
This process is called Benchmarking. You should track the annual usage costs of the system on a per square foot basis of the building. You can then compare your collected data to the performance of similarly sized buildings. The building should also be in the same climate zone and performing a similar function. Of course it doesn’t make sense to benchmark a data center building in the Arabian Desert to an office building in Alaska. The benchmark metric is cost per square foot per year ($/sqf/year). A database of building performance benchmarks can be found from the U.S Department of Energy.
In some cities benchmarking is required by law. For example, in St. Louis any building over 50,000 sqft must be benchmarked, and it’s annual energy and water consumption must be reported. Chicago has a similar city ordinance. While these benchmark figures that are made public don’t specifically segregate the chiller plant energy usage, it still provides a good indication of your performance in relation to others. Primarily, because one of the largest OpEx costs in a building is the Chilled Plant System. The installation of sub metering for the Chilled Water Plant would allow you to track the efficiency and observe over time. If regulations are amended to include sub metering, you can then benchmark specifically the chiller plant and compare to other facilities.
Sub-metering of the Chilled Water Plant system or components is a great way to identify the biggest energy hogs in your building. ASHRAE has published guidelines “instrumentation for Monitoring Central Chilled-Water Plant Efficiency” (22-2012). These guidelines contain the recommendations and methods or devices that can be used to measure the electrical usage, the water flow rates, temperatures and proper calculations for system efficiency. When sub metering is in place you can make changes to your Chilled Water Plant system and see realtime the effects on the energy. Sub-metering provides quantitative data that can be used to analyze your system efficiency, and identify how changes you make improve or decrease your energy usage. For example, with a sub metered system, you could explore if your power usage is less if you run more chillers at a lower load, or fewer chillers at a higher load. Sub-metering takes the guess work out of your plant operation energy usage calculations.
The Chilled Water Plant is one of the most complex parts of a buildings services. It’s design has a massive impact on the energy usage of the building, carbon footprint and the OpEx. It is therefore critical that the design team evaluate the system throughout its design and commissioning. Ongoing monitoring through sensors and power meters can ensure that efficiency is maintained and aid in diagnosing faults or improvements in energy usage based on your systems program.