July 2005
Interview
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Tom HartmanEMAIL INTERVIEW  Tom Hartman & Ken Sinclair

 Tom Hartman Principal, The Hartman Company

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. 


What is The Equal Marginal Performance Principle?

The Equal Marginal Performance Principle is an entirely new way of looking at systems that are composed of multiple power-modulating components such as fans, chillers, and pumps.

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Sinclair:  You have an article in the July ASHRAE Journal focused entirely on the Equal Marginal Performance Principle. We’ve also seen reference to this principle in some of your earlier articles in automatedbuildings.com http://www.automatedbuildings.com/news/may02/articles/hrtmn/hrtmn.htm and in other articles http://www.hartmanco.com/pdf/a39.pdf. I understand that you believe this principle points to a completely new way to operate HVAC systems.  Can you briefly explain what this principle is all about and why you’ve written this article? 

Hartman:  The Equal Marginal Performance Principle is an entirely new way of looking at systems that are composed of multiple power-modulating components such as fans, chillers, and pumps. It’s of particular value because applying variable frequency drives (VFDs) on all motors in an HVAC system and configuring it as an “all-variable speed” system can improve efficiency and performance enormously. The problem has been how to optimize the design and operation of a configuration such that it is simple, stable, and yet achieves the highest possible efficiencies and improves overall system performance. The Equal Marginal Performance Principle (or EMPP) answers that question by replacing conventional temperature and pressure control loops with a simpler, direct power based control enabled by networked digital controls. From a control design perspective, the Equal Marginal Performance Principle is a new cornerstone of a new overall digital network enabled strategy for control called “relational” control. But beyond control, the Equal Marginal Performance Principle can also help designers select optimum equipment configurations too. Once this new approach is understood, I think it will be seen by many designers as an improved method both for selecting optimum equipment configurations and for developing the most effective control strategies. I’ve written this article to introduce the EMPP and encourage the additional cooperative work and research required to develop the principle into a really effective tool.

Sinclair:  You say that once it is understood you believe designers will find it more effective. What can it do that is not being done today?

Hartman:  There are three basic categories of improvement opportunities offered by the Equal Marginal Performance Principle. First, and the reason I originally began work to develop the EMPP, is the improvement in energy performance it offers. The second major benefit is that designing with the EMPP nearly always results in simpler and easier to maintain mechanical configurations. Finally, EMPP directed control results in more stable operation than conventional setpoint-feedback controls employed today. There are other benefits, but these are quite compelling on their own.

Sinclair:  Sure, these are compelling, but one might be a little skeptical that a single principle can really do all this. And how much can it do? Can you give us some order of magnitude on these listed benefits?

Hartman:  I can, but keep in mind the magnitude will depend on the application. For typical HVAC systems, we can expect that annual electric energy use would be cut by about one quarter to one half from that of a conventionally designed system of the same first cost. From a system simplicity standpoint, the Equal Marginal Performance Principle generally directs designs toward systems with equally sized chillers, towers, pumps, etc, and away from extras such as balancing valves and dampers or circuit setter devices. EMPP inspired Designs employ direct coupled configurations so an entire pumping stage in conventional primary-secondary can be eliminated, as are decoupling or bypass lines and controls for such circuits. From the perspective of operating stability, control based on EMPP (which is called “demand based control”) replaces independently operating PID control loops with direct power based control relationships. This direct control eliminates much of the hunting that occurs today with conventional control loops. Except for emergency actions, control execution cycles need occur only about once every 30 seconds to one minute which can reduce wear and tear on some system components.

Control Solutions, Inc Sinclair:  If this technology is really all that beneficial, why isn’t the industry already flocking to develop and use applications based on the Equal Marginal Performance Principle on their own?

Hartman:  It is a little perplexing, but I think the reaction we are seeing may be for reasons similar to those that led to the slow acceptance of VAV a generation ago. Many in the industry appear not to understand the EMPP well enough to really see the substance in it. There is a lot of smoke and mirrors involved in selling efficiency measures and to some extent the EMPP may be caught up in it. I’ve had discussions and correspondence with some who appear to have trouble extending the world of controls beyond what is already known and being applied today. The Equal Marginal Performance principle does not use setpoint and feedback control in any conventional sense and cannot be visualized without leaving some of those terms and concepts behind. The EMPP does require a leap from current practice.  That is not an attractive option for many.

Sinclair:  OK, lead us through the process a little. First, how would the design process change with the Equal Marginal Performance Principle?

Hartman:  Let’s say we are designing a VAV cooling system for a building. Conventional design methodology has us start with some rules of thumb and size components based on these criteria. For example, we would likely assume a design supply air temperature of 55oF. We would then size the fan system(s) to provide cooling to the spaces served based on that supply air temperature. Similarly, assuming a chiller plant provides the cooling, we would base chiller, distribution system and cooling coils on a specific supply chilled water temperature and a design chilled water delta T. System control would be based on temperature and pressure setpoints developed from this design criteria.

If we decided instead to use the EMPP, the question we need to ask to kick off the design would be: “As we approach full load, what combination of equipment will yield a power relationship such that we could provide a very small amount of additional cooling by adding the same marginal power to any component in the system?”  The Equal Marginal Performance Principle tells us that the configuration will be optimal only if adding the same marginal power to any system component will yield the same increase in cooling output. Once the optimal configuration is determined that meets this design criteria, the system will be operated based on optimal power relationships at all load requirements, not intermediate temperature and pressure setpoint control.

Sinclair:  There is obviously a substantial difference in design and operation as you point out, but seems that setpoint control cannot be eliminated. Isn’t it possible if no attention is paid to temperatures that a totally unrealistic system might result, one that would operate efficiently, but at a very high or low supply air temperature that would not keep occupants comfortable? Furthermore, the purpose of the system is to maintain a temperature in the spaces served. So it would seem temperature control is essential.

Hartman:  With the Equal Marginal Performance Principle, intermediate temperature and pressure limitations are employed as “constraints” to the system. A minimum supply air temperature (to ensure comfort or prevent duct sweating), maximum leaving tower water temperature (as may be required by the chiller), limits on chilled water temperature (to prevent freezing), or a maximum fan static pressure (to protect ductwork) are all constraints that are treated as limits rather than operating setpoints. Optimal system operation is constrained to operate within these prescribed limitations. It should be noted that if the system is correctly designed, deviation from optimal operation by the control system due to such constraints is limited to exceptional circumstances such as a component failure or other extraordinary operating conditions. Anytime such a constraint (or limit) is reached so that control adjustment is required to accommodate a system constraint, the operator is likely automatically notified and an inspection of the system may be requested to determine the cause.

Maintaining space temperature and humidity conditions at the spaces served is the system objective and the system is controlled accordingly as a single entity to meet these requirements. For this control requirement when multiple spaces are involved I recommend another of the new relational control approaches called “intelligent iterative control.”

Reliable Controls Sinclair:  What about control simplicity? You stated earlier that control is simplified, but with everything operating at variable speeds, how can control be simplified?

Hartman:  Control based on the EMPP – called demand based control – is very straightforward and simple. Instead of numerous control loops operating simultaneously, demand based control directly controls equipment speed (or power) to maintain optimized power relationships among the various system components. This is accomplished with a single step for each component control. Any automatic or manual tuning that may be applied is done to fine tune optimization, not to improve stability since the control is inherently stable.

Sinclair:  Earlier you state that the EMPP can result in better performing systems. Can you give us any examples of how operating with the EMPP will make systems perform better?

Hartman:  A simple example is the problem of low delta T that many chiller plants experience. Low delta T limits the capacity of chiller plants because the pump(s) at full flow cannot extract the full capacity for the chiller(s) due to the lower than design delta T. However, consider a control methodology wherein the chiller power and pumping power relationship is controlled instead of temperature. With an EMPP based design and demand based control, the pump will operate at full power (and full flow) only when the chiller is also operating at full power. Because of the power based control instead of temperature setpoint control, it is impossible to have the situation where one component is operating at full capacity and another is not. Therefore problems like low delta T are completely eliminated when design and operation is based on the Equal Marginal Performance Principle.

Sinclair:  It does seem that you are correct in stating that the EMPP is an entirely new approach. But it’s also not hard to see why it may take some time for members of the industry to fully understand its implications, and to be certain it will provide all the benefits you think it will. Perhaps we should revisit this technology with more questions and comments when we have read up on it more.

Hartman:  Thank you. I’d like very much to do that!

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