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Interview - November 2001
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EMAIL INTERVIEW  Tom Hartman & Ken Sinclair 

The Hartman Company was founded in 1972 as a high technology engineering firm, specializing in applying computer technology to commercial and industrial building control and energy management. Hartman has played an important role in pioneering the use of advanced computer based energy management control strategies. He continues to place a strong emphasis on the use of modeling for evaluating potential improvements, and has developed a number of in-house programs to model a variety of energy and financial improvement scenarios. Today THC is utilizing dynamic control concepts with networks, TRAV and advanced chiller design. 

 The Effect of New Control Technologies on Sensor Selections


Sinclair: This month's focus is on sensors. I was surprised to see in your press release and article regarding the application of the Hartman LOOP chiller plant technologies in a Bellevue office building that the chiller plant is not controlled to a chilled water temperature setpoint, nor is the distribution pumping controlled to a differential pressure setpoint. Do these recently developed ultra-efficient control technologies envision a change in the type of sensors employed for HVAC control generally, and if they do, can you outline for me what those changes are likely to be?

Distech Controls

Hartman: Yes. Our work in developing "Demand Based Control" technologies, which is the basis for LOOP chiller plant control technologies, point to a need to radically revise what is monitored to provide more effective control of building comfort systems. In the past we have controlled tower fans to provide a specific leaving tower water temperature, chillers for a chilled water temperature setpoint, distribution pumps for a particular differential pressure and so on. This type of control "decouples" equipment which allows independent operation. For example, we know that if a specific tower water temperature is maintained, the chiller can operate entirely independently at all load conditions, the same is true of the other equipment down the line. Independent operation was necessary in pre-DDC days because pneumatic controls could not integrate systems together effectively. LOOP Demand Based Control technologies use the DDC network to integrate the operation of all components together in order to optimize the entire system at all loads. In this new method of control independent operation is no longer employed and pressure, temperature and flow setpoints are almost never required. So from a control standpoint, the need for these sensors is all but eliminated in demand based control which is applicable to all elements of building comfort systems.

Sinclair: Well, you are right in your characterization of this as a potentially "radical" revision in the operation of HVAC systems. I wonder if a shift from the type of sensing and control with which engineers and operators have been trained and are most familiar is really necessary. Can't the present optimization approaches really accomplish the same results?

Reliable ControlsHartman: No. To reach easily achievable comfort and efficiency improvements, the changes I have suggested are necessary. The problem with conventional control is that it can never operate HVAC equipment as efficiently as possible. When optimization is implemented a layer of fixed setpoint controls is typically overlaid with a new layer of routines which try to continuously reset the setpoints so that adjacent equipment operates marginally more efficiently. These constantly changing setpoints makes a system more complicated for the operator than is really necessary. Furthermore, to maintain stable control, equipment is designed and selected such that it is more complex and operates much less efficiently than it otherwise could. For example, a rule of thumb to select control valves is to have about one-quarter or more of the total distribution system pressure drop occur across the valve. In this way, extra energy is consumed to afford stable temperature setpoint control. With Demand Based Control this extra energy use is unnecessary because valves are not required to control against large pressure or temperature differentials. Also, the whole ingrained philosophy of decoupling HVAC equipment using "primary-secondary" circuits, or "decoupling lines" works counter to efficiencies available with new demand based control technologies. If we were to apply current HVAC system operations to automobiles, we would teach drivers to operate the accelerator to maintain a fixed engine RPM, the clutch to provide a specific torque, and the brake to control the vehicle's speed. A radical change in operating strategy would be needed to improve automobile efficiency. But the change would allow cars to be built with smaller engines and simpler clutches and braking systems than they have today. This is roughly analogous to the situation in our industry. Demand based control is a lot simpler than conventional control and can reduce the size and complexity of the HVAC systems, but a basic change in how the system is operated has to be made first to achieve this simplicity of configuration and operation.

Sinclair: O.K., outline for me what the sensors are that need to be replaced in our industry to achieve more efficient "Demand Based Control" and what types of sensors are required to replace them.

Hartman: For a number or reasons, HVAC sensing is woefully out of date and we need to consider changing every type of sensor employed in HVAC systems today. Let's start with space temperature sensors. For effective control, it is absolutely essential that the HVAC system knows how well space conditions are being met. Studies we and others have done show that common space temperature sensors do a much better job of measuring the temperature of the wall on which they are mounted than the temperature of the room air. Many are also affected by drafts within the walls, and very few space temperature sensors are located near enough to people to be effective even if they did work. Our recent studies show ceiling mounted radiant sensors are far better than conventional space temperature sensors for sensing space comfort conditions for building occupants.

In terms of primary equipment, while conventional temperature, flow and pressure sensors can be helpful for operator information, and they are often useful in limiting component operation under certain circumstances, most are not useful from a control standpoint with new ultra-efficient demand based controls. However, there are some notable exceptions. For example, CO2 sensors and outdoor airflow sensors are very important because modified demand ventilation is an effective means of maintaining adequate outside airflow. Also enthalpy instrumentation required to determine when an economizer cycle is effective is essential for systems with airside economizers.

The most important new type of sensing in demand based control systems is power sensing. The old saying holds here that you can't control something you don't measure. Demand based control seeks at all times to maintain space conditions by operating the entire cooling system most efficiently. To achieve optimal operation, the elements of a closely coupled HVAC system are operated according to relative energy demands rather than according to temperature or pressure setpoints. Demand based control optimizes the overall HVAC system by operating equipment at specific relative power ratios. The basic control is simple and very stable and the instrumentation is focused on measuring power demand. Since demand based control anticipates the use of variable speed drives on all equipment, power sensing that is incorporated into the drives is the best choice for this instrumentation.

Sinclair: I think a great many designers share a high level of frustration about space temperature sensing. Tell me a little more about radiant temperature sensing. Is it really viable?

Hartman: Yes, it is viable without question. Currently, radiant temperature sensors are hard to find and are prohibitively expensive for most applications, but there is no reason they need be. Intelligent integration and marketing is the key to the success of this superior temperature sensing technology. We have completed studies that show very clearly the comfort benefits of radiant space temperature control when it is properly applied, and we have developed and tested modules that integrate radiant space temperature sensing with occupancy sensing and other environmental quality sensing and adjustment features. Such devices can be very cost effective and attractive to building occupants. Unfortunately building occupants have almost no say in building comfort matters as the industry is now structured, and manufacturers with whom we have discussed ideas for new products are incredibly risk adverse. It may take some time to sort this out, but those who really wish to improve the quality of building environments should join and work together to press for this important change in space sensing technology. It will be worth the effort.

Sinclair: I appreciate you sharing your thoughts on the impact this new control technology will have on sensors. One final question. What is a reasonable expectation for the energy efficiency improvement that can be had by applying demand based technologies and do they require new types of control systems to be applied effectively?

Hartman: The potential electric energy reductions for HVAC systems from the application of demand based control strategies are so significant that they deserve to be considered for inclusion in National energy policies. When demand based control is installed throughout most typical HVAC systems, one can expect a 25% to 45% annual electrical energy reduction. The existing equipment and configuration requires only slight modifications to be operated in accordance with demand based control in place of conventional optimized control. For most systems the savings will actually be higher because few are really well optimized. Furthermore, when specifically configured for demand based control, the resulting configuration should be less expensive to design, install and maintain than a conventional system, and the resulting buildings will be far more comfortable.

In so far as controls are concerned, many existing DDC systems are suitable for applying demand based control. I recommend selecting a DDC system that provides full floating point math capabilities, automatic point and data network capabilities, and a good programming language that meets our firm's Operators' Control Language (OCL) guidelines. Probably the majority of the systems available today can meet these requirements sufficiently well to apply demand based control effectively. The real limitation to a wider use of the technology today is the design and implementation processes. The key to success in applying demand based control effectively is developing an effective method of support for this new technology through the processes of designing, constructing and operating building comfort systems. That may seem simple at first glance, but in truth it is a real challenge to owners who desire greater performance and efficiency from their buildings. Our major focus is on these issues. We know they can be solved, but to do so requires new types of teamwork throughout the design and construction processes.

Sinclair: I'd like to discuss those issues. Perhaps we can do so another day. Thanks for your participation in our sensor focused issue.

Hartman: You are very welcome!


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