Pacific Gas and Electric Company Technology Assessment of Emerging Advanced Building

© 2009, Pacific Gas and Electric Company. All rights reserved.

Pacific Gas and Electric Company Emerging Technologies Program

Application Assessment Report # 0725

Technology Assessment of Emerging Advanced

Building Control Strategies

Issued: 12 April 2009 Prepared for: Ken Gillespie Wayne Krill Pacific Gas & Electric Company Prepared by: Greg Risko Peter Salmon Vernon Smith Architectural Energy Corporation

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Style Definition: TOC 2

Technology Assessment for Advanced Building Controls

Architectural Energy Corporation 12 April 2009 ii

Acknowledgements The authors spoke with many design engineers during the course of preparing this briefing paper. We thank the following individuals for sharing their expertise and experiences and especially their time. James Bryan GE/Arden Realty Gary R. Davies ThermalTech Engineering Alain Descoins RFMacDonald Ray Dodd Architectural Energy Corporation Scot Duncan Retrofit Originality, Inc. Ben Erpelding Optimum Energy, Seattle Cliff Federspiel Federspiel Controls Mark Hydeman Taylor Engineering Edward C. Spivey Cogent Energy, Inc. Chris Miller P2S Engineering Richard Miller Armstrong Pump Company Nathan Rothman Optimum Energy, Seattle Ben Sun Flack & Kurtz Mr. Marian Vidovic Armstrong Pump Company We also thank Ken Gillespie and Wayne Krill for their support, and especially for their patience, during the project.


Table of Contents 1. SUMMARY................................ ................................ ................................ ............................. 4

2. INTRODUCTION................................ ................................ ................................ .................... 6 2.1 TRADITIONAL HVAC CENTRAL PLANT CONTROL 6 2.2 ADVANCED CONTROLS FOR COMPONENTS AND SUBSYSTEMS 6

3. ADVANCED STRATEGIES WITH SEQUENCES................................ ................................ ... 7 3.1 PUMP VARIABLE-SPEED DRIVES 7 3.2 CHILLED WATER PUMPING PRESSURE RESET 7 3.3 CHILLED WATER SUPPLY TEMPERATURE RESET 8 3.4 VARIABLE-SPEED CHILLERS 9 3.5 OVERSIZED CHILLERS 9 3.6 CONDENSER WATER TEMPERATURE RESET USING TWO-SPEED OR VARIABLE SPEED COOLING TOWER FAN 9 3.7 CONDENSER WATER TEMPERATURE RESET USING VARIABLE-SPEED CONDENSER WATER PUMPS AND COOLING TOWER FANS 10 3.8 SUPPLY FAN VARIABLE-SPEED DRIVES 10 3.9 DUCT STATIC PRESSURE RESET 11 3.10 SUPPLY AIR TEMPERATURE RESET 12 3.11 CO2-BASED DEMAND-CONTROLLED VENTILATION 13 3.12 SUPPLY AIR CO2 CONTROL 14

4. OPTIMIZATION CONTROL CONCEPTS................................ ................................ ............. 15 4.1 OPTIMIZED CONTROLS USING SIMULATIONS 15 4.2 HARTMAN LOOP AND TRAV 15 4.3 LOBOS 16 4.4 DART™ AND SAV WITH INCITE™ 16 4.5 PATENTED CONCEPTS 17

5. COMPARISON OF OPTIMIZATION CONTROLS................................ ................................ 17 5.1 APPROPRIATE APPLICATIONS 17 5.2 ENERGY SAVINGS 17 5.3 COMMERCIAL AVAILABILITY AND MARKET ADOPTION 18 5.4 COST CONSIDERATIONS 18 5.5 MAINTENANCE 19

6. IMPLEMENTATION STRATEGIES AND CONSIDERATIONS................................ ............. 19

7. MARKET BARRIERS................................ ................................ ................................ ........... 20

8. CONCLUSIONS................................ ................................ ................................ ................... 21

APPENDIX A – PATENTS ................................ ................................ ................................ .......... 22

APPENDIX B – ANNOTATED BIBLIOGRAPHY................................ ................................ ......... 24


Architectural Energy Corporation 12 April 2009 4

1. Summary Advanced based building control technologies (also known as demand-based strategies) are relatively new methods for controlling central plant systems that have demonstrated energy savings of 20 to 50% over traditional methods. These methods take advantage of increased computing power in controller systems and variable-speed motors. Strategies at the component level, or subsystem level, were developed as better controllers and/or variable-speed motors became available from manufacturers. These strategies include, but are not limited to:

• Supply Fan Variable-Speed Drives • Duct Static Reset • Supply Air Temperature Reset • Supply Air CO2 Control • CO2-Based Demand Controlled Ventilation • Pump Variable-Speed Drives • Chilled Water Pumping Pressure Reset • Chilled Water Supply Temperature Reset • Variable Speed Chillers • Condenser Water Temperature Reset

o Using Two-Speed or Variable Speed Cooling Tower Fans o Variable Speed Condenser Water Pumps and Cooling Tower Fans

As variations on these strategies were tested, problems arose that made design engineers cautious. Integrating these strategies across an entire HVAC system was not easy because of unintended consequences—such as increased energy use in subsystems that were not modified, or cascading alarm events that were not foreseen. There are several design firms that have developed sophisticated methods for integrating and optimizing control strategies based on hourly energy simulations and spreadsheets. Some aspects of these methods are published while others are proprietary. The optimized strategies are usually implemented by programming the supervisory plant software to take advantage of the hardware and software features for the selected control system. The engineering studies to implement these strategies require more time, and therefore more budget, than typical designs. Case studies for these strategies show entire plant operation at 0.6 - 0.8 kW/ton, which is significantly better than 1.0 - 1.2 kW/ton for conventionally-controlled plants. A few firms recognized that embedding advanced control strategies in software and/or hardware would save labor and overcome mistakes in programming existing central plant control software. There are four vendors marketing three technologies that have documented case studies available covering water-side and/or air-side controls concepts in hardware/software products. These new technologies offer a dramatic increase in performance—with entire plants operating at an efficient 0.6 kW/ton or less, and with chillers running at 0.3 kW/ton. For most plants this is an energy savings of 20 - 50% over current performance. These large savings are accomplished by using variable-speed drives on all chillers, pumps, and fans, and running the system at partial capacity—as opposed to the traditional strategy of shedding units as demand drops in order to run the remaining units closer to design capacity. Providing demand information from the air-side systems enhances the effectiveness of these technologies and can reduce tenant complaints. They can be applied to any comfort conditioning or variable-load applications, and some process-load applications.



Advanced control strategies are applied to large facilities with a central plant, usually more than 75,000 square feet in conditioned space. Budgets for implementing them are large, running from the tens of thousands into the millions of dollars, depending on what has to be upgraded. In addition to hardware and component upgrades, engineering analyses and technology licensing fees are part of the costs. Due to the complexity of the strategies and the time required for operations staff to learn how to support them, contracts for these products may include services for as long as a year after implementation. Despite many articles and case studies on advanced control strategies, market barriers are creating long sales cycles. Perceived high first cost, distrust of claimed savings, concerns about complexity overwhelming controls contractors and operations staff, and the anticipated additional time and cost of correctly implementing the advanced strategies are contributing to slower adoption.



2. Introduction

2.1 Traditional HVAC Central Plant Control In traditional control systems, components operate independently of each other. For example, chillers operate at fixed chilled water temperature setpoints, regardless of load in the conditioned space. As space temperature changes (due to occupancy or ambient conditions) relative to the space setpoint, the cooling (or heating) coil valves open or close in the air handlers. In turn, this changes the returning water temperature to the chillers (or boilers). On the heat-rejection side, condenser water pumps and cooling tower fans are operated at fixed speeds. To control the entering condenser water temperature, the number of cooling tower cells in operation is changed as the load on the chillers changes. As the demand for conditioned load changes, the chillers and cooling tower cells (or boilers) are staged on or off. On the air-distribution side, traditional controls for large facilities usually include one or more constant-volume or variable-volume air handling units with terminal units. These are controlled using scheduled air-temperature or air-pressure setpoints. As computing technology has become more powerful, some innovative practicing engineers have used simulations to develop optimized control sequences. These techniques have been applied to fixed-speed and variable-speed systems. In most cases, these simulations have focused on some aspect of central plant operation, such as optimizing the condenser side operations. In others, chiller supply side and air distribution are included.

2.2 Advanced Controls for Components and Subsystems With the advent of variable-speed drives, a secondary cooling loop is no longer needed to isolate the chiller so that it operates at near-design conditions. Instead, at part-load conditions, entering and exiting water temperatures for the chillers and cooling towers can be controlled by varying chiller refrigerant flow, chilled water flow, and condenser water flow. These are adjusted in response to changes in the returning chilled water temperature due to changes in loads in the conditioned spaces. Demand-based control can be implemented by monitoring the chilled water return temperature, or by monitoring temperatures and pressures in the distribution system. Some water-side strategies include, but are not limited to the following.

• Pump Variable-Speed Drives • Chilled Water Pumping Pressure Reset • Chilled Water Supply Temperature Reset • Variable-Speed Chillers • Condenser Water Temperature Reset

o Two-Speed or Variable-Speed Cooling Tower Fans o Variable Speed Condenser Water Pumps and Cooling Tower Fans

Air-side controls can be either integrated with water-side controls or used independently. Strategies for optimizing air-distribution energy use are based on temperature and/or air pressure control. Some air-side strategies include, but are not limited to the following.

• Supply Fan Variable-Speed Drives • Duct Static Reset • Supply Air Temperature Reset • Supply Air CO2 Control • CO2-Based Demand Controlled Ventilation



Section 3 provides details and examples of these advanced methods applied to one or more components in the primary and secondary HVAC systems. Integration of these strategies to optimize overall plant performance, in products or services, is discussed in Section 4.

3. Advanced Strategies with Sequences

3.1 Pump Variable-Speed Drives Retrofitting secondary pumping systems with variable-speed drives (VSD) reduces pump energy consumption by reducing the flow-to-building system coils when the building load is less than the design condition. Conventional hot and chilled water piping systems include three-way valves at each coil. These valves allow the flow to bypass the coil during part-load conditions. Replacing three-way valves with pressure-independent, two-way valves, where appropriate, will force the pressure in the system to increase and decrease depending on the building load. A differential pressure transmitter in the pipe is then necessary to sense pressure in the system. A controls contractor must program the pump to modulate its speed to maintain a differential pressure setpoint. A critical step in converting a constant-volume system to a variable-flow system is to ensure the differential pressure sensor is placed in the proper location. The placement of the differential pressure transmitter must account for the variable-friction head. To achieve this, the sensor should be located at the far end of the piping system. Equally important are the two-way valves; the designer must ensure any existing and new two-way valve can accommodate the high pressures in the system without breaking or leaking; they must provide consistent flow through the coil regardless of pressure differential changes. Primary-only systems—piping systems where only one set of variable-speed pumps distribute water through the chillers/boilers and to the coils—require system pressures 20 - 100% greater than primary-secondary systems. Primary-secondary systems typically have two sets of pumps: constant-volume pumps to create flow through the chillers/boilers, and variable-speed pumps servicing all secondary equipment. If the system is primary-only, the designer must review the valve specifications to ensure they can meet the high-pressure requirements. Studies show that systems with valves that cannot tolerate the pressures will allow leakage, lowering the plant efficiency. Pressure-independent control valves are an alternative to the typical two-way globe valve with balance-valve combination. These valves are designed to maintain flow through the valve regardless of pressure changes, whereas pressure-dependent valves cause the flow through the valve to modulate as the differential pressure changes. Pressure-independent valves have been proven to provide better flow through coils than pressure-dependent valves, increasing the change in temperature across the coil.

3.2 Chilled Water Pumping Pressure Reset Controls programming can be added to variable-flow chilled water systems to reset the chilled water supply pressure based on demand. For systems that have direct digital control (DDC) over modulating two-way chilled water valves, one strategy is to vary the pressure between high and low values based on the position of each valve. Each valve position can be multiplied by a weight factor, depending on the flow, and totaled to determine a ”total valve position.” The linear reset can then calculate a differential pressure setpoint in response to the total valve position. Measurement and verification would be required to conclusively determine proper weight factors for each valve in the system. A balancing contractor can determine both the minimum and maximum differential pressure values for the reset strategy.



The “Trim and Respond” control methodology is another strategy for resetting the differential pressure setpoint. (This method involves trimming the static pressure setpoint when all VAV boxes are satisfied, and increasing the setpoint by a select quantity when one or more boxes issue a request for more supply air). A sample strategy using this method may be like the following.

• Establish the minimum and maximum differential pressure at 15 feet and 25 feet w.c. • When the pump is proven on, reduce the static pressure setpoint by 0.5 w.c. (trim) every 2

minutes, as long as there are no pressure requests. A pressure request occurs for a control valve where the actuator position is greater than 90% open.

• If there are more than two (adjustable) pressure requests, increase the setpoint by 0.5 w.c. (respond).

Rogue zones—those with airflow or temperature requirements that cannot be met during any condition—must also be identified and addressed. Rogue zones may be corrected through redesign and renovation, or in the trim-and-respond control method, rogue zones may be ignored.

3.3 Chilled Water Supply Temperature Reset Chilled water supply temperature reset is a control strategy to reduce power consumption of the chilled water plant during conditions where the building load is below its design condition. Traditional chilled water plants maintain a constant chilled water supply temperature regardless of building load. Experience from facility operators and chiller manufacturers show that although increasing the chilled water temperature setpoint may increase pumping power and air-handling fan power, the benefits of reducing chiller power outweigh the negatives in reducing overall building energy costs. Resetting chilled water temperatures may also produce other benefits for the operator and the chilled water plant—including better chilled water valve control and the prevention of chiller surge. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) offers a specific control strategy for resetting chilled water temperature, but only for plants with constant-speed pumps and air-handling fans. In summary, the recommendation is to calculate a time-averaged chilled water load and then calculate a new chilled water supply temperature based on how the load has changed. If the load remains the same as it did during the last sample then the strategy is to use valve positions to determine if the chilled water setpoint should be adjusted. Upper and lower limits are imposed to maintain the setpoint within the chiller’s limits and to ensure comfortable temperatures. Specific strategies also exist for various chiller plant types, including parallel and series operation. Several other strategies are simpler to program, but may not account for interactions on systems downstream of the chiller, depending on how they are programmed. One example is a reset based on space temperature or humidity. If a zone’s space temperature or humidity is above setpoint for a defined period of time, the control system increases the chilled water temperature setpoint by one increment every defined time period until the space setpoint is met. For a system with variable-speed fans, the exchange for chiller energy savings is fan energy—the fans will be consistently at higher airflow if the space temperature is consistently above setpoint, whereas if the chilled water supply temperature is not reset the fans would be operating at lower speed. Other strategies include: reset based on return chilled water temperature, reset based on chilled water bypass valve position, and reset based on primary versus secondary chilled water flow. Considerations must be made for each strategy. For example, an effective reset based on zone temperatures will require zones that are stable—as opposed to rogue zones that cannot maintain their setpoint under any condition (due to changes in space loads from the design intent, or terminal undersizing due to errors in the design). Chilled water temperature reset is typically employed in combination with a supply air temperature reset strategy. When a supply air temperature setpoint is reset to a warmer setpoint, the need for chilled water decreases. As the need for chilled water decreases, chilled water valves begin to



close. Instead of closing off the chilled water valve to reduce flow through the coil, the chilled water supply temperature can be reset to maintain the most-open chilled water valve at 90% open. Various sources cite an improvement in chiller performance on the order of 0.5 - 3.5% per degree of chilled water reset upwards1. Savings in chiller plant power must be weighed against the increase in pumping power required to provide proper chilled water flow.

3.4 Variable-Speed Chillers Since HVAC cooling loads are widely variable over the course of a year, and since equipment is typically sized for design conditions that may only occur 1% of the time, improved efficiency at off-peak conditions contributes greatly to improved energy performance. Variable-speed chillers can meet these off-peak conditions and reduce energy consumption if applied properly. The most-important consideration when using variable-speed chillers is that condenser water reset strategies are also employed. Part-load performance of water cooled chillers with and without VFD’s does not vary significantly with a fixed-condenser water setpoint. Therefore the benefit of the chiller VFD is minimal. However, much higher efficiencies can be obtained at lower part-load conditions when both variable-frequency drives for chiller compressors are combined with condenser water reset strategies.

3.5 Oversized Chillers Oversizing chillers has also been demonstrated to be a viable energy-saving advanced strategy. Most chillers have their highest demand at peak design conditions. In utility areas where reducing peak demand is important, oversizing chillers can produce significant demand savings during peak periods. A lifecycle cost analysis should be performed to assure that the increased capital costs from oversizing will be offset by utility cost savings.

3.6 Condenser Water Temperature Reset using Two-Speed or Variable Speed Cooling Tower Fan Resetting the condenser water temperature setpoint leaving the cooling tower can improve chilled water plant efficiency, if properly controlled. Most chiller refrigeration cycles operate at a higher efficiency when the water entering the condenser is closest to the minimum temperature recommended by the manufacturer, as opposed to a temperature closer to the higher end of the manufacturer’s entering condenser water scale. Traditional condenser water loops are controlled to maintain the same cooling tower leaving water temperature regardless of building load and environmental conditions. Cooling towers typically have bypass valves and either multi-speed fans or variable-speed drives to control the water temperature exiting the tower. Understanding how the chiller and cooling tower fans interact with each other is critical to implementing a condenser water reset strategy. Cooling towers operate under the fan law principles: the energy consumption of the fan is proportional to the cube of airflow. Operating the fan at higher speeds to reduce the condenser water temperature leaving the tower will consume more fan power; however, with a lower condenser water temperature, the chiller kW/ton efficiency improves. An optimal reset control strategy will take into account the combined power of the chillers and cooling towers. Various sources cite an improvement in chiller performance on the order of 0.5 - 2.8% per degree of condenser water reset downwards2. For typical plant controls, the cooling tower fans cycle

1 Koran, B. and K. Stum, “Techniques and Tips for Retrocommissioning Energy Calculations”, 2007 Fall Training Series, Portland Energy Conservation, Inc.



between off/low/high in concert with one or more cooling tower cells to maintain a fixed condenser water setpoint (typically 85°F). The same approach can be used with variable-speed drives on cooling tower fans, modulating them to maintain a fixed condenser water supply temperature setpoint. Since improved chiller performance can be achieved by maintaining lower condenser water temperatures, a trade-off with fan speed can be made. In this exchange, the increased cooling tower fan power needed to create lower condenser water supply temperature is offset by the increased efficiency of the chiller. A popular strategy is to control the cooling tower fans to maintain the design approach. In this scenario, as ambient wet-bulb temperature drops from design to lower temperatures, fan speed is controlled to maintain the design approach, which provides for lower condenser water temperature. There is usually a low limit to this strategy that is set by the chiller manufacturer and/or the refrigerant used. For older R-11 machines, the low limit is around 75°F, for newer machines the low setpoint is around 65°F. Siemens’ Cooling Plant Optimization Guide recommends a reset strategy that does not reset the condenser water setpoint, but rather controls the tower fan speed as a function of chiller load. The controls logic first compares the temperature of the water entering the chiller condenser with minimum and maximum limits provided by the chiller manufacturer and takes necessary action to prevent the water temperature from leaving that range. Then the algorithm calculates the chiller load and compares it to the design load. If a significant change has occurred, the system will compute a new tower airflow to match the load.

3.7 Condenser Water Temperature Reset using Variable-Speed Condenser Water Pumps and Cooling Tower Fans In addition to controlling cooling tower fans as described above, condenser water pump speed may also be controlled. The most typical sequence allows for condenser water pumps to slow down while maintaining the design condenser water temperature difference. Minimum speeds need to be established that maintain minimum: motor cooling, chiller barrel flow, and cooling tower flow. Typical flow adjustments for cooling towers range to 50 - 100% of design flow. If a system incorporates multiple chillers and cooling towers, isolation valves may also be required to maintain minimum flows.

3.8 Supply Fan Variable-Speed Drives Retrofitting fans with variable-speed drives (VSD) reduces fan energy consumption by matching the supply fan airflow with the building load, or in the case of exhaust/return fans, controlling the precise airflow to ensure a properly-pressurized building. Traditional air-handling systems consist of constant-volume fans with zone reheat coils to manage zone temperatures. VSDs reduce the supply airflow when building loads are below design conditions—reducing fan energy consumption and reheat energy. While VSD installations are necessary to convert constant volume systems serving multiple zones into variable-air-volume (VAV) systems, opportunities exist to retrofit single-zone air-handling units. Energy savings is the primary benefit for purchasing a VSD. However the technology provides additional benefits such as: reduced stress on motor windings and bearings and decreased wear and tear on the fan itself (extending equipment life). The conversion from a constant-volume system to a VAV system requires a variable-speed fan motor. VAV boxes include a damper to modulate airflow based on demand. As demand for cooling or heating decreases, the damper in the VAV box closes to restrict airflow to the space. As air is 2 Koran, B. and K. Stum, “Techniques and Tips for Retrocommissioning Energy Calculations”, 2007 Fall Training Series, Portland Energy Conservation, Inc.



restricted, the static pressure in the supply duct increases. VSD supply fans maintain a duct static pressure setpoint—if the reading surpasses the setpoint the fan speed will decrease, and vice-versa. VSDs on return/exhaust fans allow the system to modulate in response to changes in airflow on the supply side. As the supply fan modulates to maintain the building load, a VSD on a return or exhaust fan will modulate to maintain positive pressure in the space, or negative pressure if required. Retrofitting supply-air fans serving single-zone systems require different controls programming than VAV systems. Depending on the type of system and design load requirements, the fan can be either programmed to modulate to maintain space temperature setpoint or programmed to maintain a set speed depending on whether the space is calling for heating or cooling. For example, a building in a climate with hot summers and mild winters will require less airflow in winter than summer. This allows the programmer to modulate fan speed depending on whether the system is heating or cooling the space. However, for either type of air-handling system, minimum outside air fractions must be programmed to maintain the proper amount of outside air regardless of fan speed.

3.9 Duct Static Pressure Reset Duct static reset is a control strategy to reduce the supply air duct static pressure setpoint based on demand. This strategy applies to variable-air-volume (VAV) air-handling systems where the controls system determines the supply fan speed in response to a duct static setpoint. In a typical VAV system, the operations personnel have the ability to modify a setpoint that will ensure each VAV box is satisfied (i.e., the most-open VAV box damper position is 90 - 100%, then all zones are satisfied and an increase in airflow is not required). During periods of low load, this strategy reduces the duct static setpoint while maintaining airflow requirements for each box; this reduces energy consumption of the supply fan. For variable-flow fan systems the first energy-efficiency strategy employed should allow for resetting the duct static pressure to the lowest-possible value required for maintaining design airflow. For systems that have direct digital control (DDC) over VAV terminals, a popular strategy is to vary the duct static pressure by modulating the air handler variable-frequency drive in response to VAV box position. The control strategy modulates duct static pressure in order to maintain the worst-case VAV box with a 90% open damper position. Several successful strategies have been well-documented. One method called “Trim and Respond,” involves trimming the static pressure setpoint when all VAV boxes are satisfied, and increasing the setpoint by a select quantity when one or more boxes issue a request for more supply air. An example of when a box will issue a request for more air is the damper position is greater than 95% for a set period of time; the request is removed when the damper position decreases below 80%. Trim and Respond programming can also be written in a manner to account for how far a temperature is from its setpoint. Another method using proportional-integral-derivative (PID) control loops also polls the box damper positions; however the PID control loop determines the duct static pressure setpoint that will maintain the most open box damper near 90 - 95%. Both types of reset strategies will only be successful after proper measurement, verification and fine-tuning has occurred after implementation. Commissioning the strategy involves adjusting control loop parameters and trim/respond increments to ensure the box dampers are not cycling, which can lead to uncomfortable occupants and increase wear and tear on HVAC equipment. Rogue zones—zones with airflow or temperature requirements that cannot be met during any condition—must be determined during the commissioning period. The person tuning the system



must either create a control sequence that minimizes the rogue zone’s impact or deletes it from the polling sample. With the help of a balancing contractor both minimum and maximum values for duct static pressure can be established as the operating range for the reset strategy. Minimum fan speed for both motor cooling and stable fan operation above the surge line should also be established with the help of the balancing contractor. Trim-and-respond control methodology is preferred over PID control methodology for these applications, since PID loop tuning in the field can be difficult. Rogue zones (those with design deficiencies, such as undersized coils) must also be identified and accommodated. Rogue zones may be corrected through design and construction, or in the trim-and-respond control method, rogue zones may be ignored. A sample strategy for duct static pressure reset using trim and response may be like the following:

• Establish minimum and maximum static pressure at 0.5” and 1.5” w.c. • While the fan is proven on, reduce the static pressure setpoint by 0.04” (trim) every 2

minutes, as long as there are no pressure requests. A pressure request occurs when a VAV box damper position is greater than 90% open—whether by directly reading the damper position, or by reading the actual airflow as a fraction of the total design airflow.

• If there are more than two (adjustable) pressure requests, increase the setpoint by 0.04” (respond).

Common variations on the strategy include the following:

• a damper position of 80% open counts as one pressure request and a position of 95% counts as two requests; and

• the response rate is a function of the number of requests (e.g. response = 0.04” times the number of requests).

In any event, it is important that the control logic be slow acting to avoid hunting. Hunting occurs when the control loop is not satisfied, which causes the control output to oscillate out of control.

3.10 Supply Air Temperature Reset Supply air temperature (SAT) reset is a control strategy to match the supply air temperature to the load. The objective is to reduce simultaneous heating and cooling. It can be employed at any type of air-handling system with reheat downstream of the supply fan. The classic method of selecting reheat coils is to calculate the space load assuming a constant entering air temperature, typically 55°F. Simultaneous heating and cooling occurs when air is first cooled by an air handling unit, either by the cooling coil or free economizer cooling, and then is reheated by duct reheat coils or reheat terminal units. For example, the outside air temperature is 60°F and the space temperature is equal to the setpoint of in a hypothetical office building. The air handling unit’s (AHU) sequence of operation is to economize to maintain an AHU SAT of 55°F—the economizer damper is full open and the cooling coil valve is slightly open. Terminal units serving the space are at their minimum airflow, allowing 55°F air to enter the space. As soon as the space temperature drops below its setpoint due to a loss of internal gain, the terminal box reheat is activated. Therefore, energy is consumed to create 55°F SAT at the AHU and reheat at the terminal box. To help reduce simultaneous heating and cooling, an SAT reset strategy would reset the AHU SAT setpoint to a value greater than 55°F, reducing the need for chilled water flow through the cooling coil and hot water through the reheat terminal. Multiple strategies are available to determine how and when the SAT should be reset. Methods used today include resetting based on: return air temperature, outside air temperature, and the differential between the space temperature and its setpoint. For example, if SAT is reset based on



outside air temperature, when the outside air temperature is 60°F the SAT setpoint shall be 55°F, and when the outside air temperature is 30°F the SAT setpoint shall be 62°F. All three types of reset strategies employ limits to maintain the SAT setpoint within a target range. Pitfalls exist for each type of SAT strategy. Of utmost importance is to ensure the reset strategy is tailored to the specific building, which may require measurement and verification after the space has been occupied. One difficulty with SAT reset based on return air temperature is the return air temperature is not always a reliable indicator of load. If occupants have control of the temperature in individual spaces, they may reset the space temperature setpoint which will inherently affect the return temperature. SAT reset based on the differential between the space temperature and its setpoint may require modifications if rogue zones are present. One method to mitigate this is to use an average differential to reset the SAT setpoint, or a weighted average of the temperature furthest from setpoint and the average of all other zones. Wei et al recommend employing a strategy that is a hybrid of several others—first resetting the SAT setpoint based on the outside air temperature and then fine-tuning the new setpoint based on the return air temperature conditions.3 In general, supply air temperature reset is used to avoid turning on the chiller until the highest-possible outside air temperature. Raising the supply air temperature setpoint when in the economizer mode will allow for the longest use of economizer before reverting to mechanical cooling. Raising the supply air temperature reset during low load or low ambient conditions reduces the amount of simultaneous heating and cooling that must be done when reheating at the zone level. For implementation of this strategy, both the minimum and maximum supply air temperature setpoint must be established. The minimum is usually defined by the design conditions and the maximum is typically 10°F higher. The trim and response method is also preferred here. A sample strategy for supply air temperature reset using trim and response may be like the following.

• Establish minimum and maximum supply air temperature setpoints at 55 - 65°F. • Minimum supply air temperature is required when units are coming out of economizer

mode, typically above 70°F (adjustable), and can be raised proportionally to the maximum supply air temperature as outside air temperature drops to 65°F.

• While the fan is proven on, increase the setpoint by 0.2°F (trim) every 2 minutes, if there are no requests for zone cooling (a cooling request is a VAV box where the damper position exceeds 90% open).

• If there are more than 2 (adjustable) cooling requests, decrease the setpoint by 0.2°F (respond).

Again, it is important that the control logic be slow-acting to avoid hunting. Practical limitations also exist for this measure since raising the SAT increases fan energy, but tends to lower total plant kW. Strategies must carefully ensure proper limits are placed on the implementation; there may be cases where the increased fan penalty begins to outweigh the savings at the plant.

3.11 CO2-Based Demand-Controlled Ventilation Demand-controlled ventilation (DCV) using carbon dioxide (CO2) sensors is a control strategy that matches the quantity of outside ventilation air entering a building with the building occupant load. DCV will reduce energy consumption associated with heating, cooling and dehumidifying the ventilation air by reducing ventilation airflow below the design quantity. Traditionally, engineers calculate ventilation quantities to match the worst-case scenario—a building that is occupied by 3 Wei, G., Turner, D., Liu, M., Claridge, D. (2003). Single-Duct Constant Air Volume System Supply Air Temperature Reset: Using Return Air Temperature or Outside Air Temperature? Architectural Engineering 2003 Conference, Austin, Texas, September.



the maximum number of occupants, regardless of whether the building is fully occupied only a fraction of the occupied period. Although this quantity of air is often called the ”minimum outdoor air quantity” or ”minimum damper position,” when designing a DCV strategy, this is referred to as the ”upper minimum airflow.” The ”lower minimum airflow” is the minimum amount of ventilation air required when the building is not occupied. A DCV strategy will modulate the volume of ventilation air between the upper and lower minimum quantities in response to CO2. (Calculations for determining the upper and lower minimums can be found in ASHRAE Standard 62.1-2004 and Title 24-2008.) Occupant density and occupant variability are characteristics that are critical in determining which building spaces are suitable for DCV. Spaces with higher-than-average occupant densities such as assembly halls and conference rooms have large ventilation requirements, which can be greatly reduced when occupancy is below design. Spaces such as theaters, where the number of occupants can vary dramatically on weekdays versus weekends, make better candidates than manufacturing facilities or warehouses that have the same number of occupants throughout the occupied period. When selecting spaces to implement a DCV strategy, the designer should also consider factors such as space pressurization requirements and the generation of indoor pollutants. A designer can implement DCV at any air-handling system with a motorized outside air damper, and multiple strategies exist for controlling each type of system. Single-zone, constant-volume systems with economizers require just one CO2 sensor placed in the space or return air duct. Multi-zone and variable-air-volume systems covering large areas and multiple spaces with different occupancy patterns and occupant densities will require multiple sensors and more complex controls programming. A sample sequence of operation for DCV implemented on a single zone system may look like the following.4 The outside air damper position shall reset based on space CO2, according to the following schedule:

Space CO2 Outside Airflow 100 ppm above ambient Lower minimum airflow 700 ppm above ambient Upper minimum airflow

The Test, Adjust and Balance contractor shall determine the outside air damper position necessary for the lower and upper minimum airflows.

3.12 Supply Air CO2 Control Demand-controlled ventilation (DCV) using supply air CO2 is a method of controlling an air-handling unit’s outdoor air damper to ensure each space contains the appropriate amount of ventilation air, regardless of occupancy. It is similar to conventional demand-controlled ventilation strategies in that it requires a CO2 sensor to reset the quantity of ventilation air. However, instead of controlling ventilation airflow to maintain proper CO2 concentrations in the space, the strategy ensures the supply air has enough ventilation air to meet the needs of any space. This difference offers multiple benefits over traditional DCV strategies, including reduced cost and better ventilation control over traditional DCV strategies. The strategy’s proponents say it is better than traditional DCV for spaces served by multi-zone and variable-air-volume air handling units because it serves the space with the largest ventilation requirement using only one sensor. Traditional DCV for these types of systems requires multiple CO2 sensors to accomplish the same strategy. Requiring just one sensor reduces both cost and

4 “Demand-Controlled Ventilation Design Brief,” Energy Design Resources (September 2007) from www.energydesignresources.com.



maintenance requirements. The sensor can be located at the unit, with air tubing connecting it to ductwork upstream and downstream of the supply fan; this allows fan suction to draw air to the sensor. A three-way valve to the intake of the sensor will allow the system to sample both supply air and outside air. Proper implementation requires the design engineer to calculate the CO2-concentration setpoint of the supply air. This setpoint is equal to the rise in CO2 concentration of re-circulated air if all the ventilation air originally entering the space were to be consumed, which requires the engineer to determine the CO2 generation rate per person, relative metabolic rate of the occupant and the design outdoor ventilation airflow rates per occupant. (All parameters can be determined from ASHRAE Standard 62.1 and Title 24-2008.)

4. Optimization Control Concepts Demand-based controls are relatively new methods for controlling central plant systems. They are implemented with variable-speed controls on the chillers, water pumps, cooling tower fans, and, in some cases, air-handlers. Variable-speed control has become widely available in the past 10 years and it has been a key factor in changing central plant design. Strategies at the component level, or subsystem level, were developed as better controllers and/or variable-speed motors became available from manufacturers.. The next step—which has become available either as a service or as combination of products and services—is integration and optimization of demand-based controls across the entire system.

4.1 Optimized Controls Using Simulations Taylor Engineering, Cogent Energy, and P2S Engineering use energy simulations and spreadsheet analysis to formulate control sequences that are implemented in supervisory control software of the plant control system. Taylor Engineering has published articles that describe its methods in some detail. Based on interviews with Cogent Energy and P2S Engineering, they appear to use similar methods.

4.2 Hartman LOOP and TRAV The Hartman Company (THC) introduced the concept of “equal marginal performance” principle. With variable-speed control, running multiple units at a fraction of design capacity is more efficient than taking units off line. The Hartman LOOP is based on three patents covering variable-flow controls, sequencing chillers in variable-speed plants, and variable-speed controls for condenser fans, cooling tower fans, and pumps. THC has licensed its technologies to a number of design firms over the past 10 years, and more recently, has established licensing agreements with Optimum Energy and S.A. Armstrong, Limited. Optimum Energy’s version of the Hartman LOOP technology requires integrating a controller based on the Tridium Niagra AX platform to the building automation system (BAS). The Niagara platform is designed to interface with a number of legacy building automation systems, including certain versions of Johnson Metasys, Siemens Apogee, and Honeywell5 control products. The Niagara platform supports BACnet, LON, and ModBus communications protocols. Plant information from the BAS—such as sensor inputs, equipment statuses and drive speeds—are accessed by the controller and processed using the patented LOOP algorithms. The output is a series of operating conditions to select setpoints and manage equipment with the intent of maximizing plant efficiency. Optimum Energy’s product for chiller plants employs multiple strategies to optimize plant performance. For example, the controller will calculate the optimum chilled water supply 5 Tridium became a wholly-owned, independently-operated, subsidiary of Honeywell in 2006.



temperature setpoint once a host of equipment and operating data have been analyzed to determine the building load. The controller also determines the quantity and speeds of operating equipment—such as pumps, cooling tower fans and chillers—in its efforts to operate the entire plant as efficiently as possible. The BAS retains control of equipment alarms, lead/lag changeover, and plant startup and shutdown. Terminal Regulated Air Volume (TRAV), another of THC’s licensed technologies offered by Optimum Energy, is an alternate to traditional variable-air-volume (VAV) air-handling systems controlled by proportional-integral-derivative (PID) control algorithms. PID-controlled VAV systems typically modulate the supply fan to maintain a duct static pressure setpoint—whether fixed or variable. Conversely, TRAV modulates the fan speed to meet terminal airflow requirements to improve occupant comfort and reduce air-side energy consumption. When the air-handling unit is in cooling mode, TRAV adjusts the supply air temperature and airflow setpoint in response to changes in space load. While in heating mode, the system disables the TRAV controller and operates per the BAS sequence. TRAV also includes a self-balancing algorithm to adjust the minimum and maximum airflow setpoints of each terminal over time, within operator limits. S.A. Armstrong Limited has incorporated the Hartman technologies in its central plant equipment controllers. Armstrong’s license with THC covers all skid-mounted chiller applications.

4.3 LOBOS Retrofit Originality, LCC, has developed the Load Based Optimization System (LOBOS). LOBOS uses data from the cooling end uses (thermal zones) to reset controls for the central plant components and air distribution system. It relies on variable-speed control of chillers, water pumps, cooling tower fans, and air handling unit (AHU) fans. On the air distribution side, the LOBOS technology uses the following four strategies for air handling units.

• Reset the discharge static pressure setpoint based on the load of the AHU (or zones). • Reset the supply air temperature setpoint based on the load of the AHU, using the AHU

fan speed direction (increasing or decreasing) and the rate of change as the indicator of load.

• Implement demand-limiting sequences of operation, to allow reductions in power and energy waste on unit startup, during occupied hours, and as controlled by operations staff.

• Implement a cooling load-based coasting cycle routine for central plant and AHUs to allow use of the “flywheel” effect and reducing cooling demands prior to shutting off the AHUs.

For central plant control, the LOBOS technology uses four strategies:

• reset the chilled water differential pressure setpoint; • reset the chilled water supply temperature setpoint; • reset the condenser water supply temperature setpoint; and • reset the condenser water flow rate setpoint.

4.4 DART™ and SAV with INCITe™ Federspiel Controls, LLC has developed two air-side technologies for saving energy: Discharge Air Regulation Technique (DART) and SAV with InCITe™. Discharge Air Regulation Technique (DART) is an application for converting constant-air-volume HVAC systems, particularly those that serve multiple zones, to variable air volume operation. Federspiel Controls offers the DART software in combination with the Federspiel Advanced Control System (FACS). The FACS system is a wireless mesh network using sensing and control modules, which allows the system to be converted to VAV with no mechanical changes to the HVAC system, except addition of variable-speed drives (if needed).



DART modulates the fan speed so that either the highest discharge air temperature is close to a high-temperature setpoint, or the lowest discharge air temperature is close to a low-temperature setpoint. The system uses wireless temperature sensors so that DART setpoints can track a hot deck or cold deck reset schedule. DART achieves energy savings by reducing the speed of supply and return fans. Reducing the supply airflow rate decreases the amount of cold supply air produced by the system and reduces the amount of reheating or hot air produced by the hot deck of a dual-duct system. SAV with InCITe™ is a process for controlling variable-air-volume (VAV) supply fans; it saves energy by reducing supply fan pressure at part-load conditions. This process uses InCITe to: test the supply air system, develop the “optimal” reset strategy from the test data, and then implement Static pressure Adjustment for Volume flow (SAV) by programming new controls sequences into the energy management system. The speed of the supply fan on most VAV systems is controlled to maintain a constant static pressure in the supply duct. Significant energy can be saved during part-load conditions by controlling the fans to supply just the amount of pressure needed to allow the terminal boxes to remain in control. This technique creates energy savings primarily by reducing the supply static pressure and to a lesser extent by decreasing the supply airflow. Heating and cooling energy savings are achieved without compromising thermal comfort or indoor air quality. The amount that can be saved depends on the type of systems in a building, their capacity compared to peak load, and other conditions that vary from one system to another.

4.5 Patented Concepts There a number of patents and published patent applications on optimizing aspects of central plant control. The Hartman Company has at least three patents and there are a number of patent applications underlying the LOBOS, DART, and SAV with INCITe technologies. Appendix A lists issued patents and published patent applications. Most of the assignees listed in Appendix A are equipment manufacturers. Some of the patented or disclosed concepts are likely incorporated into the company’s products.

5. Comparison of Optimization Controls

5.1 Appropriate Applications The advanced technologies discussed above are applicable to larger facilities, generally greater than 75,000 square feet, with central plants and multiple air handlers. The Hartman LOOP is specifically designed for central chilled water plants with variable-speed centrifugal chillers that serve variable loads. Applications are commercial buildings with more than 500 tons of chiller capacity; the return on investment typically increases with larger buildings and larger plants. TRAV is designed for new and existing VAV air-handling systems. Both LOOP and TRAV strategies require the equipment be controlled by a BAS. ROI’s LOBOS can be implemented for comfort or process loads served by constant or variable-speed chillers. It controls water-side and air-side systems based on direct inputs from the air-side systems. Federspiel Controls’ products can be applied to existing constant volume air-handlers and are not dependent on the type of HVAC system.

5.2 Energy Savings



Both the Hartman LOOP and the LOBOS technologies have demonstrated significant energy savings, in the range of 20 - 50% compared to traditional strategies. Chiller plants designed to meet building code will perform in theory at 0.8 kW per ton of cooling.6 Highly-efficient optimized chiller plants can operate at 0.7 kW per ton. However, design efficiency and actual performance are rarely the same, especially in older plants. According to sources cited by THC, most systems operate in the 1.0 – 1.2 kW per ton range. Hartman LOOP chiller plants are operating at 0.6 kW per ton or lower. Case studies from ROI show performance under 0.5 kW per ton.

5.3 Commercial Availability and Market Adoption Taylor Engineering, Cogent Energy, and P2S Engineering offer advanced controls as part of their respective design services. Configuring and running simulations with a spreadsheet optimization program adds between two to eight person-weeks to a project’s budget. The Hartman Company, Retrofit Originality, and Federspiel Controls are primarily engineering or R&D firms that have created software and hardware products as platforms to implement advanced controls. The Hartman Company licenses its technology on a project by project basis. In the past few years, The Hartman Company has licensed its technology to Optimum Energy (Seattle, WA) and the S.A. Armstrong Limited (Toronto, Ontario, Canada). Optimum Energy LLC is a privately-held company that was formed in 2004 to market advanced control solutions for central plant applications. It has over 40 installations under contract and about 120 in the sales pipeline (of these, 50 are new construction). Within 2 years, the firm expects to have over 200 projects under contract. It had early success in the San Diego area and has about 7 million square feet under control. Its market has been 90% retrofit. The plant sizes for its existing projects run from 60 to 4800 tons. Its market is becoming international, with upcoming projects in Hong Kong and Malaysia. Armstrong’s market is skid-mounted chillers; Optimum Energy’s market is everything else. Armstrong is a privately-held international company that has been in business since 1934 and manufactures a wide variety of central plant and packaged HVAC components. Its market for THC technologies is skid-mounted centrifugal chillers and is oriented to new construction more than Optimum Energy. Retrofit Originality provides its product and services on a project-by-project basis. It started working with optimizing performance in central plants in 1985 and started applying its LOBOS (aka “Variable Speed Everything”) technology in 1999. It has developed a supervisory controller that processes sensor data from the water-side and air-side systems and provides commands to appropriate controllers. It has an unpublished patent pending on its process and has applied its technology to over 20 million square feet of commercial and industrial space conditioned by over 50,000 refrigeration tons. Over 90% of its projects are retrofit work. Federspiel Controls has several case studies available for its DART and SAV with INCITe products, but has not had any commercial sales to date.

5.4 Cost Considerations The cost to implement these technologies include: variable-speed control upgrades (if needed), engineering services, support services after implementation, and technology licensing fees. Simple payback figures range from about two to five years, depending on the amount of plant

6 The energy efficiency metric for central plants and chillers is expressed as a “wire-to-water” efficiency in units of kiloWatts per cooling ton.



upgrading or renovating that is required. Total retrofit costs can range from several hundred thousand dollars to several million dollars. THC literature7 states that the cost to construct new all-variable speed chiller plants is not necessarily higher than the costs for the equivalent conventional plants. Using variable-speed motors on all major chiller plant components allows designing with a primary loop only; this eliminates the first cost and second costs associated with secondary loops used in traditional constant flow rate systems. Even greater efficiency can be achieved by installing chillers with 20% more capacity than the anticipated peak load. This can be accomplished by using less-expensive variable-speed chillers with high efficiency below 80% capacity and less efficient above 80%. This design will achieve higher efficiencies throughout the range and still cost the same as a conventional design. THC originally licensed its LOOP technologies for a site license fee of $5 per installed cooling ton. For additional support it offered a “Support License” for $10 per installed ton, which included a design guide, online resources and operational specialists to assist in troubleshooting. For $20 per installed ton it offered the “Engineering Agreement”. This option included the full support of THC engineers throughout design and implementation phases as well as everything included by the other licenses. (These prices may not be current.) THC has licensed Optimum Energy and S.A. Armstrong Limited, apparently on a non-exclusive basis (to be further clarified), to provide LOOP technologies as part of their product and service offerings. The fees charged by these firms are higher due to added hardware, software, and service contracts. Federspiel Controls claims that DART installations are significantly less expensive that conventional CAV to VAV retrofits, and that simple payback is typically less than two years before rebates are applied. SAV with INCITE case studies show simple payback at less than 1.5 years. Note that control upgrades may be needed in facilities with a progression of updates. Advanced strategies can usually be applied in systems less than 15 years old. The network capabilities required for advanced control strategies are very common, but rarely fully implemented in conventional plants.

5.5 Maintenance The Hartman LOOP process will change the run schedule of the equipment significantly. Instead of units cycling on and off to meet demand, they will run continuously at lower power to meet demand. While the result is much longer runtimes, this does not imply more maintenance will be required. THC literature states that during the development of the Hartman LOOP process, this issue was discussed with manufacturers and others. THC claims that maintenance will likely decrease as a result of these three factors: fewer starts, softer starts, and lower average loading on each machine. Currently, there is insufficient field data to say conclusively the LOOP process lowers maintenance. THC states that there is enough data, however, to show wear and tear is not increased in LOOP plants. ROI literature claims 80 - 90% reduction in tenant complaints using its LOBOS technology. It does not comment directly on plant maintenance, but it is likely similar to that required for the Hartman LOOP.

6. Implementation Strategies and Considerations The Hartman LOOP and ROI LOBOS technologies are modular. The water-generation and water-distribution components can be separately converted to advanced controls. For example, if the 7 The Hartman Loop, All-Variable Speed Chiller Plant Design and Operating Technologies: Frequently Asked Questions. Sep 2001. The Hartman Company. www.hartmanco.com



cost of replacing the constant speed chillers cannot be justified, the advanced technologies can be applied to just the cooling towers. Similarly, the Federspiel Controls products can be applied to a few air handlers in a building. While modularity allows a partial implementation, savings are unlikely to be maximized. When considering retrofit implementations, the direct digital controller (DDC) should be 15 years or less in age and needs to meet three major requirements:

• it employs a functional and flexible programming language using floating point math and allows multiply layers of custom math and logic statements;

• it has an automatic networking management capable of using point and variable data from any other controller; and

• it employs standard or gateway protocol features in order to communicate with chiller and variable-speed drive equipment.

The systems from Optimum Energy and ROI deal with DDC integration issues as well as programming the strategies into the supervisory controller. Federspiel Controls’ use of a wireless system for data acquisition mitigates getting information from older building automation systems that may not have the throughput required for the advanced control strategy.

7. Market Barriers Perceived market barriers discussed during the interviews to date were consistent within the engineering practitioners and the vendors. A few end-users have been interviewed, and more interviews will be needed to complete this Technology Assessment. Some engineering practitioners expressed doubt about the claimed savings, even with case studies. They believe that the energy savings from the advanced control strategies developed using simulations were as good as those from the vendors. (There appears to be an unstated bias against patenting as an intellectual property protection vehicle.) From a commissioning perspective, getting traditional controls contractors to implement strategies as designed may be difficult. Controls contractors do not like to implement complex strategies, in part due to: uncertainty as to whether they will work, errors introduced in the controller programming due to complexity, and the potential for callbacks. They are concerned that if something goes wrong it will be difficult to trace the root cause due to the added complexity of the control scheme. Vendors said that engineers tended to distrust the data and case studies presented to support the products. The cost to implement the products was also cited—although paybacks in the two to five year range are often acceptable for other major building improvements. The need to follow up (for at least a year) after the implementation, was noted as a very real need that added cost to projects. The lack of experience and education of building operating staff was cited as a significant barrier to persistence of the strategies and savings. To overcome this issue, the vendors were improving the robustness of software interfaces. The need to build in some degree of fault detection and diagnosis was also recognized.



8. Conclusions All of the example sequences outlined in Section 3 may be added to existing systems since they are largely controls changes; however in many cases variable-speed drives and/or upgraded controllers will be required. All of these measures can have energy-savings benefits, but must be analyzed on a case-by-case basis. All the measures described above—with the exceptions of reducing supply air pressure and chilled water pressure—have energy trade-offs with other equipment. Measures must be implemented carefully, to be applied only for the conditions that provide a net positive effect of overall central plant efficiency. Using optimization engineering services or products, described in Section 4, will also provide significant energy savings. These services and products are backed by valuable experience in integrating advanced controls and sequences, and provide a measure of assurance for new designs or plant upgrades.


Appendix A – Patents This list is not intended to be comprehensive. The reader may use these patent numbers and information about the inventors to search for other patents and patent applications. Issue Date

Description Inventor(s) Assignee Patent No.

08/02/1994 Method and Apparatus for Efficiently Controlling Refrigeration and Air Conditioning Systems

Hullar, Justice EnviroSystems Corporation

5,333,469 and 5,230,233 (issued 07/27/1993)

02/11/1997 Near optimization of cooling tower condenser water

Schwedler, Hage, Dorman, Stiyer

American Standard International

5,600,960

09/23/1997 Variable Speed Control of a Centrifugal Chiller Using Fuzzy Logic

Beaverson, Wueschinski, Shores, Hansen

York International Corp.

5,669,225

09/07/1999 Variable Flow Chilled Fluid Cooling System

Hartman None Listed (The Hartman Company?)

5,946,926

10/02/1999 Digital Controller for a Cooling and Heating Plant Having Near-Optimal Global Set Point Control Strategy

Cascia Siemens Building Technologies, Inc.

5,963,458

02/13/2001 System for Sequencing Chillers in a Loop Cooling Plant and Other Systems that Employ All Variable-Speed Units


6,185,946

7/10/2001 Method of Control of Cooling System Condenser Fans and Cooling Tower Fans and Pumps


6,257,007

08/21/2001 Chiller Capacity Control with Variable Chilled Water Flow Compensation

Sibik American Standard International

6,276,152

04/13/2004 Method and apparatus for controlling variable air volume supply fans in heating, ventilating, and air-conditioning systems

Federspiel None Stated in Patent

6,719,625

Applications Pending Issue Date Description Inventor(s) Assignee Patent No. 10/31/2002 Method of Optimizing and

Rating a Variable Speed Chiller for Operation at Part Load

Haley, Dorman None stated in Patent Application Publication

Pub. No. US 2002/0157405

01/01/2004 Sequencing of Variable Primary Flow Chiller System

Cline, Schwedler

None stated in Patent Application Publication

Pub. No. US 2004/0000155



(likely to be Trane)

09/01/2005 System and Method for Optimizing Global Set Points in a Building Environmental Management System

Cascia, Ahmed None stated in Patent Application Publication (likely to be Siemens)

Pub. No. US 2005/0192680


Appendix B – Annotated Bibliography

Advanced Control Strategies Bernier, M., Bourret, B. (1999). Pumping Energy and Variable Frequency Drives. ASHRAE Journal, December, pp 37-40. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=683

This article describes a study performed to compare two methods of calculating pump power. One method is a simple analysis using only the pump law; power is equal to the cube of speed. The second method uses pump VFD and motor efficiency curves. Results show that actual pump and VFD energy consumption can be significantly higher than predicted values calculated with just the pump law. If the pump is oversized, causing the VFD to operate more hours at lower speeds, the reduction in VFD and motor efficiency at lower speeds will significantly increase total pump power consumption.

Jeannette, E., Philips, T. (2006). Designing and Testing Demand Controlled Ventilation Strategies. National Conference on Building Commissioning, San Francisco, California, April. http://www.peci.org/ncbc/proceedings/2006/23_Jeannette_NCBC2006.pdf

This technical paper provides background information on ASHRAE and Title 24 ventilation standards, and how they apply to demand-control ventilation strategies. The authors provide guidelines on sensor locations and controls sequences. Energy modeling was used to determine the impact of DCV on different types of educational buildings throughout Colorado. Savings ranged from $0.04-$0.34 per square foot, per year.

Strake, D. (2006). System Operation: Dynamic Reset Options. ASHRAE Journal, December, pp 18-32. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=132

This article discusses strategies to satisfy ASHRAE 62.1-2004 section 6.2.7 Dynamic Reset, which gives mechanical designers the option of including a reset strategy to reduce outdoor airflow intake in response to changing operating conditions. The author provides several approaches to accomplishing this, including: reset based on space CO2 concentrations, time-of-day scheduling and use of people-sensing devices.

Taylor, S. (2007). Increasing Efficiency with VAV System Static Pressure Setpoint Reset. ASHRAE Journal, June, pp 24-32. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=32

This article compares the PID-based strategy to the trim-and-respond method for static pressure setpoint reset for VAV systems. The author prefers trim-and-respond because it’s easier to tune, easier to ignore rogue zones and does not require the analog damper position be known—making it available for more types of systems.

Tillack, L., Rishel, J. (1998). Proper Control of HVAC Variable Speed Pumps. ASHRAE Journal, November, pp 41-47. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=623

The authors examine the control requirements necessary for a variable-speed pumping system, including location and quantity of differential pressure sensors and

http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=683

http://www.peci.org/ncbc/proceedings/2006






PID loop tuning, as well as the potential pitfalls of using valve position to determine pump speed.

Warden, D. (2004). Supply Air CO2 Control of Minimum Outdoor Air for Multiple Space Systems. ASHRAE Journal, December, 26-30. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=283

This article describes a method to control the quantity of ventilation airflow entering a space based on the concentration of CO2 in the supply air. Calculations for determining the setpoint are included in the article. Reducing excess ventilation airflow reduces costs associated with heating, cooling and dehumidifying the air. This strategy has benefits over space or return-air CO2 strategies for VAV systems, including reduced first cost and maintenance, and increased reliability.

Wei, G., Turner, D., Liu, M., Claridge, D. (2003). Single-Duct Constant Air Volume System Supply Air Temperature Reset: Using Return Air Temperature or Outside Air Temperature? Architectural Engineering 2003 Conference, Austin, Texas, September. http://cedb.asce.org/cgi/WWWdisplay.cgi?0304580

This conference paper compares two methods of resetting supply air temperature setpoint for constant-speed systems. The researchers conclude that resetting the supply air temperature based on the outside air temperature—as opposed to return air temperature—can save significant energy costs. An outdoor air temperature reset strategy was implemented in a hospital in Minneapolis, resulting in 19% savings on reheat energy savings.

Case Studies Baker, M., Roe, D., Schwedler, M. (June 2006). Prescription for Chiller Plants. ASHRAE Journal, pp H4-H10. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=80

A design team for an all variable-speed chilled water plant at a hospital in Virginia discusses their methodology for selecting the plant and controls. The team eventually selected VFDs on the chillers, condenser water pumps and cooling towers. The plant experiences efficiencies of 0.5 - 0.7 kW/ton during summer months.

San Diego Regional Energy Office. (January 2006). Retrofitting A Water-Cooled Chiller with Turbocor’s Newest Oil-less Compressor: County of San Diego Juvenile Detention Facility. http://www.turbocor.com/literature/pdfs/casestudies/juvenile_hall.pdf

The County of San Diego retrofitted two centrifugal chillers with Turbocor compressors and updated the controls systems to include LOOP strategies. Savings from upgrading the chillers was estimated to be $67,000 per year with a payback of 4.3 years. Upgrading the controls provided an estimate of $42,000 per year in additional energy savings with a 3.2 year payback. The combined measures provide $109,000 energy savings per year with a 3.9 year payback.

Turpin, J (2006). Night Shift. Engineered Systems, July. http://listserv.energy.wsu.edu/read/messages?id=7631

This case study used nighttime chilled water storage and Thomas Hartman’s demand based controls to reduce the electricity used at the Lanterman Developmental Center in Pomona, CA. The end result was an overall savings of half a million dollars a year.


http://cedb.asce.org/cgi/WWWdisplay.cgi?0304580

http://bookstore.ashrae.biz/journal/journal_s_arti

http://www.turbocor.com/literature/pdfs/casestudies/juvenile_hall.pdf

http://list



Products and Services Armstong. (July 2007). IPC 11550: Ultra-Efficient Chilled Water Integrated Plant Control. http://www.armstrongpumps.com/Data/pdfbrochures/Links/02_11_005/5-4_IPC_11550_brochure.pdf

This is a product brochure of Armstrong’s prefabricated plant controller that features Hartman LOOP Nature Curve sequencing strategy. While it briefly describes how the controller can be integrated into a system, it does not detail the strategies the product will employ.

Duncan, Scot M. (2006). Variable Speed Everything: Chiller Plant Design and Load Based Optimization Control Strategy. Retrofit Originality Incorporated, November. www.roi-engineering.com

This publication describes the control strategies and examples from ROI’s projects. The control strategies involve raising chilled water temperatures and lowering the differential pressure setpoint using variable-speed drives. The strategy is to run these systems as energy efficient as possible and still meet the demand load. Savings of 30 - 50% are realized in the examples where these techniques are used.

http://www.roi-engineering.com/New_Articles/VariableSpeedEverythingchillerplants.pdf Hartman, T. (2001) Ultra-Efficient Cooling with Demand Based Control. HPAC Engineering, December. http://www.hartmanco.com/pdf/a39.pdf

In this article Hartman explains how inefficient running a chiller plant without coupled controls can be. Running a chiller plant with demand based controls, variable-speed drives and direct coupled controls will normally increase energy savings 30 - 50%. Hartman substantiates that with a case study of a building already outfitted with variable-speed drives and an early version of demand control. By using the new control setup with Hartman LOOP technologies for coupled demand control, the plant realized another 20% in savings.

Hartman, T. (2001) The Hartman Loop: All-Variable Speed Chiller Plant Design And Operating Technologies. http://www.hartmanco.com/pdf/a33.pdf

This is an informational paper is setup in a Q & A format geared toward Hartman Company customers. The paper is useful for specifics on equipment requirements, energy savings and pricing. It is also a useful resource for quickly finding simple explanations of the concepts and principles.

The Hartman Company. (2001). All-Variable Speed Chiller Plant Design and Operating Technologies: Frequently Asked Questions. September. http://www.hartmanco.com/pdf/a33.pdf

The Hartman Company explains what the LOOP technology is, how it can be incorporated into new and existing chiller plant designs, how the plant will operate and what the requirements will be. Some of the data may be dated now that several have developed their own products based on LOOP technology. US Patents 5946926; 6185946; 6257007 are the basis of the Hartman LOOP technology.

Relational Control and Equal Marginal Performance Principle Hartman, T. (2005). Relational Control. HPAC Engineering, October. http://www.hartmanco.com/pdf/a47.pdf

Most HVAC systems today use PID control loops to govern equipment. Modern PID controllers have the capability to network, though this is rarely used. Each component

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is normally operated on a closed loop separate from everything else. Relational control is a method used to optimize performance by networking all the equipment and setting output levels based on their relationship to one another.

Hartman, T. (2006). New Vistas with Relational Control, A three Part Series, automatedbuildings.com, March, April, May. http://www.automatedbuildings.com/news/mar06/articles/hartman/060227054447hartman.htm http://www.automatedbuildings.com/news/apr06/articles/hrtmn/060327030702hrtmn.htm http://www.automatedbuildings.com/news/may06/articles/thtmn/060427045651hartman.htm

These links contain more on the concept of relational control and Equal Marginal Performance Principle. These ideas are a replacement for the current methods being used and there are many hurdles to overcome before they are widely accepted. The third part link discusses the obstacles and how the author believes they need to be addressed.

Hartman, T. (2005). Designing Efficient Systems with the Equal Marginal Performance Principle. ASHRAE Journal, July, pp 64-70. http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=25929&view=item&page=1&loginid=4867398&priority=cat103egory&words=Hartman&method=and&

This is a technical article focused on EMPP and how it is used to optimize HVAC systems. EMPP states that for any system with multiple modulating components their energy is used most efficiently when they are all operating with the same marginal system output. (Marginal system output is defined as system output per unit energy system input.) Most of the paper is spent deriving and proving the principle mathematically.

Hartman, T. (2001). All Variable Speed Chilled Water Plants. ASHRAE Journal, September, pp 43-51. http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=5981&view=item&page=1&loginid=4867398&priority=cat103egory&words=Hartman&method=and&

This article compares the most common practice at the time—constant speed chillers, to a properly-designed variable-speed centrifugal chiller. The underlying principles are described as well as a look at expected performance improvements. The article goes on to apply the principles to the remaining chiller plant equipment, chillers, pumps and tower fans, further increasing the performance.

Hartman, T. (1999). “LOOP” Chiller Plant Dramatically Lowers Chilled Water Costs. ASME International Conference, Lahaina, Hawaii, April. http://www.hartmanco.com/pdf/pre11.pdf

This paper is based on a presentation given at the American Association of Mechanical Engineers. It contains one the easiest to understand explanations of Hartman LOOP technologies. The paper goes on to discuss methods to maximize the benefits of EMPP, along with cost and performance benefits.

Erpelding, B. (2006) Ultra-Efficient All-Variable Speed Chiller Plants. HPAC Engineering, March. http://www.sdenergy.org/uploads/case_mag_HPAC_Eng_032006.pdf

This study reviewed demand-based optimization, EMPP and Hartman LOOP technologies; it was conducted by Ben Erpelding of the San Diego Region Energy Offices, a state funded non-profit organization. Erpelding reached the conclusion that significant savings can be realized using these strategies and by focusing the plant design on optimizing equipment for partial-load instead of full-load conditions.

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Hartman, T. (2002). Out With the Old, In With the New. HPAC Engineering, August. http://www.hartmanco.com/pdf/a38.pdf

In this short article Hartman compares proportional integral derivative (PID) controls used in many HVAC systems to demand based controls. He discusses how new plants built using demand control are more efficient and less costly to build due to the simplified design. He covers many different components of the HVAC system and describes the improvements and controls involved with demand control.

Other Publications Avery, G. (June 2003). Operating Chillers in the Max-Cap Range. Engineered Systems. http://findarticles.com/p/articles/mi_m0BPR/is_6_20/ai_102862287/pg_1?tag=artBody;col1

This article defines the max-cap range of a chiller and how to apply it in retrofitting primary/secondary chiller water systems. The retrofit process involves installing a check valve in the bypass line and making revisions to the chiller control programming.

Avery, G. (May 2001). Improving the Efficiency of Chilled Water Plants. ASHRAE Journal, pp 14-18.

The author describes why primary-secondary pumping arrangements for chilled water plants are inefficient. The article includes a design strategy to make them more efficient by staging the chillers based on load as opposed to flow requirements.

Braun, D., Diderrich, G. (1990). Near Optimal Control of Cooling Towers for Chilled Water Systems. ASHRAE Transactions, vol 96, part 2, pp 806-813.

This paper discusses a strategy for controlling cooling towers, taking into account trade-offs with chiller power. According to the authors, the algorithm is easy to understand and can be implemented in existing systems.

Crowther, H., Furlong, J. (July 2004). Optimizing Chillers and Towers. ASHRAE Journal, pp 34-40. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=254

This article presents a parametric analysis of cooling tower energy consumption for a 160,000 square foot office building in three climates. The authors analyzed fixed setpoint, fixed approach and an optimized controls methods to control the fan. For each control method, three types of fans were analyzed, including on-off, two-speed and variable-speed. Depending on the climate, the optimized strategy with a VFD tower saves 4 - 9% of chiller/tower energy consumption when compared to the fixed setpoint, on-off strategy.

Hartman, T. (1996). Design Issues of Variable Chilled-Water Flow Through Chillers. ASHRAE Transactions, vol 102, part 2, pp 679-683.

This paper highlights the pros and cons of single-circuit, variable chilled water plants. The author provides a methodology for controlling chilled water plants to satisfy HVAC systems in part load conditions.

Pacific Gas & Electric Co. (October 2008). CoolTools Chilled Water Plant Design and Specification Guide Schwedler, M. (July 1998). Take It to The Limit…Or Just Halfway? ASHRAE Journal, pp 32-39. http://bookstore.ashrae.biz/journal/journal_s_article.php?articleID=602


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This article discusses a method of comparing energy consumption of chiller/cooling tower combinations using manufacturer’s selection software. The method only works with two-speed fans as opposed to fans with VFDs. The evaluation techniques prove that the lowest-possible leaving tower water temperature does not ensure the most efficient chiller/tower system.

Siemens Building Technologies, Inc. (March 2004). Cooling Plant Optimization: Application Guide

This comprehensive guide describes: optimization strategies for all equipment used in chilled water plants, design strategies for plants, fundamental rules that govern pumping systems and how to calculate energy savings from the optimization strategies.

Pacific Gas and Electric Company Technology Assessment of Emerging Advanced Building

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