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Roadmap To A Successful Central Plant Optimization
Central plant equipment can present great opportunities for generating energy savings, not only in the plant, but also in the connected buildings as well as improve comfort conditions of building occupants.
Climatec – Building Technologies Group
Market Conditions & Challenges
Today, building owners, managers and operators are being asked to improve the performance of their assets by lowering operating costs, improving tenant satisfaction and implementing sustainability efforts while being good stewards of the environment. They are being asked to do this in an economic climate that offers limited, or no access to capital for improvements, and with limited staff and systems capabilities.
Central plant equipment can present great opportunities for generating energy savings, not only in the plant, but also in the connected buildings as well as improve comfort conditions of building occupants. This article shall discuss primary discussion points required to deliver a successful and optimized central plant.
HVAC System optimization starts at the selection of mechanical equipment. Equipment must be selected to operate at the lowest kW/Ton energy efficiency where the system operates most of the time, usually during part load conditions. Primary variable pumping and AHU variable air volume systems are preferred and all motors must be installed with variable speed drives. Few exceptions should be made.
Variable speed drives must have an open protocol Building Energy Management System (BEMS) interface to report drive speeds and kW usage. Chillers and boilers must be selected to allow turndown of pump speed. If a reduction of at least 40-50% speed is not achievable because of equipment limitation, a primary-secondary pumping strategy should be considered, using smaller primary pumps so larger secondary pumps can be fully modulated. The focus should be to size and select pumps to where they can modulate down together as the whole building load is reduced so any given pump does not have to run exceptionally higher than another pump. Small changes in pressure and flow can exponentially reduce power see Figure 1.
Figure 1 – Illustrates the increase in
flow pressure and power relative
to the increase of speed. Note that doubling speed and flow results in
an 8x increase in power.
All electrical equipment’s power usage must be monitored and quality flow/BTU meters must be in place for any high level of optimization to take place. Piping design must also be planned to allow for proper and accurate measurement. Chillers need to be equipped with open protocol BEMS interfaces in which real time kW monitoring can take place. It is also very important the refrigerant pressures and temperatures are part of the interface so the chiller operational efficiency can be monitored and optimized.
BEMS And Contractor
When selecting a control system it must be a certified open protocol system that includes programmable controllers at all levels. The preferred open protocol for HVAC building automations systems should be 100% top to bottom native BACnet. Hybrid automation systems, BAS using gateways, and LON flat architecture cause difficult integration with supervisory controllers and other 3rd party appliances such as data collection systems or dashboards could be added in the future.
Once a native BACnet control system is chosen, the controls contractor needs to be selected by the owner and engineering team based on experience and ability to implement optimization strategies. The controls contractor is a key part of the project design team from the beginning thru the life of the project. The controls interfaces and complexity of equipment is not part of the day-to-day expertise of the mechanical contractors or engineers. Since the controls technologies in the HVAC industry are changing rapidly it becomes a full-time job for the control contractors to keep up. The experienced controls contractor is an invaluable resource for a successful project and to ensure system compatibility. This will ensure all the subsystems and equipment will work together as intended by the design. This broad knowledge base includes the BEMS equipment as well as the electrical, mechanical, and plumbing equipment from both a technical and engineering perspective.
Base Sequence of Operations
The initial mechanical engineering process must set the design conditions and clearly establish the environmental conditions that the system maintains. Additionally, a base level sequence of operations is established. This base defines the amount of redundancy of the equipment (e.g. lead lag, lead standby, n +1, etc) and the type of plant (variable primary, primary secondary, etc.). The base level sequences should focus on full load operation, the staging up and down of mechanical equipment, and establishing a recovery strategy sequence when an equipment failure occurs. This base level design is determined by overall plant budgets and redundancy requirements of the plant.
Once a base sequence of operations is established, the part load operation for the building can be determined. The minimum operating conditions need to be identified such as minimum flows for chillers, boilers, cooling towers and etc. Equipment should be selected to maximize the amount of flow turndown such as specifying cooling towers with low flow nozzles and ensuring chillers can turn down to at least 40-50% flow. The impact of lowering the flow on the cooling/heating loads must also be taken into consideration. If there are isolated loads that cannot have a reduction in flow then booster pumps should be considered to prevent the flow from having to be provided by speeding up the large central plant pumps.
The range for the chilled water and hot water temperatures resets need to be determined. If there is dehumidification that is required, it may limit how much optimization can take place and some type of DX dehumidification might be considered. Process loads or refrigeration loads that might be put on the central plant systems have to be evaluated as this may also drive how much optimization can take place. It is crucial to analyze very low load conditions, minimum loading of the primary mechanical equipment, and if the mechanical equipment can cycle down properly to determine if an additional piece of “pony” equipment is required to allow for a more efficient system and minimize excessive cycling.
Figure 2 – Depicts the major components of
what an optimization plant
designer should take into consideration when developing optimization
mechanical equipment and building automation systems are
selected, control optimization strategies can be developed. There are
varying levels of optimization which depend on the size of the building
and equipment, operating budgets and staff, and limited construction
costs. The most basic optimized plant can establish temperature resets,
pump speed resets, etc. and with no further optimization can be
considered in an small optimized plant as it will be running more
efficiently than a constant volume system. The level of optimization
can extend to monitoring chiller refrigerant pressures and retrieving
information such as kW and valve positions from all building loads
regardless of the level of the initial optimized control sequences. At
this point the optimization process has only begun.
It is imperative a qualified testing and balance contractor is brought on board that fully understands the testing, adjustment, and balancing of variable volume systems. It also must be specified that the TAB contractor is to test, adjust, and balance the system not only at design conditions but also at part load and minimum conditions.
Plant/System Overviews – Easy to
understand status and conditions of
Systems Graphics – Control and monitor all system equipment (shown with optional touchscreen control interface)
Efficiency – Single screen shows you how efficient systems are
operating and where you are using your energy
both part of the initial construction process and a
continual process that lives with the facility indefinitely. The
initial construction documents must set a “contractual obligation” and
define what initial base level of optimization exists. The construction
contract must come to closure immediately following occupancy. After
the construction contract sign off, a continuous monitoring and
optimization program must be put into place.
The first two years are spent analyzing the initial installation and tuning the system for the most optimal operation. An example would be the balance of pump speeds vs. supply temperature setpoints. Optimization must take into account all of the systems in the building or campus and not just the central plant. For example, raising the CHW supply temperature in the central plant may help chiller efficiency but will increase fan energy at the AHU. The most efficient balance for the HVAC system must be found without compromising comfort, reliability, and building function.
Optimization must take into consideration the operational requirements of the building and requires constant communication with the operations staff and occupants. Running a facility in the most efficient manner means finding the limits of which that building can properly run, and operating at the edge of these limits. Optimization must also consider the reliability requirements of the facility. As systems are optimized the equipment is run closer to its operational limits and leaves less room for error which can cause equipment shutdowns.
Optimization is not limited to just the control system as mechanical equipment or design changes may need to be modified to allow for the most efficient operation. There are no black boxes, or any predetermined engineering practices that can consider all of these variables.
At the end of the
first two years, several seasonal cycles have taken
place and the facility should be settled into its post construction
operation. At this point, any modifications to sequences and
optimization strategies have taken place with the operational
requirements being met. In some cases mechanical systems may have been
changed or modified as to not “drive” the system into inefficient
operation. Even if at any moment the facility is being operated in the
most efficient manner, it will not stay there. Facility operations
change, equipment malfunctions, and mechanical systems wear, get dirty,
and fall out of calibration. Unfortunately, all things will deteriorate
over time. This is what is called ‘Building Drift.’
Optimization and building efficiency are no exception. In order to maintain minimal energy usage, continual monitoring must take place with the focus on optimization. As optimization is not required for a building to meet its mission it is often overlooked or de-prioritized. A continuous optimization program should be in place at all times for preventative maintenance and corrective maintenance, as well as facility improvements and modification. This must be done by a qualified individual or company that can identify inefficient operation as well opportunities for improvements. It is highly recommended that monitoring, optimization and analysis is the primary task of the individual or company assigned to these duties in order to preserve the priority and focus of optimization.
with the philosophies introduced in our original
“Introduction to Axcess” article previously
published in April of 2012,
Axcess for Central Plants (CP for short) specifically focuses on
the optimization of central plants. Axcess is intended to deliver
either standalone optimization of systems or be integrated into the
existing Building Energy Management Systems (BEMS), and supports
all major open protocols, as well as connectivity to proprietary
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About the Author
David Adams – firstname.lastname@example.org
David has over 25 years in the controls industry with control design, programming, and optimization experience of simple and complex central plants from the early 90’s. This included cogeneration plants of that era all the way to the all variable speed plant with magnetic bearing chillers of today. He has worked as a consultant for specifying and integration of open protocols.
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