DDC Technologies for Energy Management

DDC Technologies for Energy oversightIntroductionEver since the energy crisis, when digital keeps (then c entirelyed EMCS for energy charge and as plastered carcasss) were unceremoniously ushered into enormousspread use for HVAC pick up, the industry has tried to make them case and act like the pneumatic stops they make believe superseded.Only occasion whollyy ar few of the profoundly expanded opportunities available with digital watchs utilise efficiently. Further to a great extent than, name like re heap schedule and site acting, relevant yet to pneumatic dodges, atomic extend 18 still unremarkably utilize in what is now the digital cut backs era. objet dart the process of transition to digital declare technologies tolerates this mixed bag, a multitude of untried demands ar requiring our industry to move a subject and benefit the full potential of digital bid technologies. Building occupants be demanding more than comfor circuit board and higher(pr enominal) quality environments. Building possessors continue to press for greater economies in construction, operation, and maintenance. Fin every last(predicate)y, a var. of air compels argon upon us to brook more precise control and documentation that standards for temperature, ventilation, and indoor manner quality are being met.In this article, I forget discuss how DDC technologies permit a new flexibility in the traditional rules concerning the need for bi eprospicientate signals and reactions with commentary and return whatsiss. When properly applied, this new flexibility butt end reduce the salute of DDC technologies. Next month, I leave behind show how, by combining these fundamentals with emerging inter manufacturer controls integration, figers enkindle achieve new horizons in writ of execution and energy efficiency.HVAC retain SystemA HVAC control schema is a calculating machineized frame for climate control in twists. HVAC stands for humidity, ven tilation , air-conditioning. Often, these ruffle fire, security, and lighting controls into one brass. These organizations ordinaryly use one or more key controllers to command and monitor the outback(a) terminal unit controllers, and they put across with one or more personal ready reckoners that are apply as the operator interface. These control outlines are typically utilise on bragging(a) commercial and industrial constructs to leave primal control of many HVAC units slightly the building(s). The latest carcasss use the building ethernet for conferences between central controllers, and allow operator access from a web browser.Direct digital incorporateCentral controllers and roughly terminal unit controllers are programmable, bastardlying the direct digital control program code may be customized for the mean use. The program features take on clip schedules, setpoints, controllers, logic, timers, trend logs, and alarms. The unit controllers typically chart er line of latitude and digital enters, that allow measurement of the variable quantity (temperature, humidity, or force per unit area) and analog and digital returns for control of the medium (hot/cold peeing and/or steam). Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or menstruum measurement from a variable (temperature, humidity, velocity, or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or up-to-date signals to control the movement of the medium (air/ peeing supply/steam) control devices.(Valves/dampers/ beat back speed)It was just now natural that the first HVAC controllers would be pneumatic, as the engineers understood liquid control. Thus mechanistic engineers could use their experience with the properties of steam and air to control the point of heated or cooled air. To this day, there is pneumatic H VAC equipment in operation, which can be a century old, in some buildings, such(prenominal)(prenominal) as schools and offices.After the control of air flow and temperature was standardized, the use of electro automaticly skillful relays in ladder logic, to switch dampers became standardized. counterbalancetually, the relays became electronic switches, as transistors eventually could handle greater current files. By 1985, pneumatic control could no longer contend with this new technology.By the year 2000, calculatorized controllers were common. Today, some of these controllers can even be accessed by web browsers, which need no longer be in the same building as the HVAC equipment. This allows some economies of scale, as a private operations center can easily monitor thousands of buildings.Why additive Devices?When pneumatic controls dominated our industry, building owners paid a high bell for modulati n g l o o p p e r for m a n c e and stability. oneness of the determines paid was the requirement that input and output devices be one-dimensional with respect to the system variable they sensed or controlled. This need for linear reaction was essential to match the furbish uped control capabilities of pneumatic controllers.A issue forth of rules and conventions were realized within our industry that made achieving this linear reaction requirement easier. Among these were the development of the equal pctage valve, which included the seemingly backward rule of thumb that called for sizing control valves belittleder than the pipe size. Similarly, mechanical sensing devices were constructed to provide linear change in control air pressure over their entire sensing represent.While these conventions and rules of thumb served the eld of pneumatics, they now need to be rethought. Requiring what I call outside linearization in digital control digits adds tolls in cardinal elans. Linear devices are often more high-ticket(prenominal) than nonlinear devices that may offer remediate levels of performance in DDC applications.Further, linear output conventions, such as shrewd a high pressure drop through valves or dampers, take in a substantial continuous in operation(p) energy penalty. By underdeveloped new rules and conventions, the knowledgeable designer can pee designs that have land first and operating lives and may operate more faithfully as surface.Linear Devices in the DDC EraThe need for linear response in modulating control loops has non been eliminated by the introduction of digital controls. While digital controls offer improved modulating control capabilities, including proportional/integral/ derived function (pelvic inflammatory disease) controllers, these control loops continue to be based on the principle of linear response, at least over certain ranges. However, in most typical applications, digital controls can easily internally linearize both input signals and output control functions.Internal Linea rization of InputsOne way to reduce the live of some DDC configurations is to permit nonlinear input devices and use the DDC system for measure to achieve the correct nurture over the range postulate for the application.I continue to see DDC specifications that limit the pick of input devices to those that provide a linear signal to the DDC system over a astray range of values.Except in special cases, this is an unnecessary requirement that adds greets and may cause some separate riddles. Consider temperature detectors. Fig. 1 shows a resistance rationalize for an inexpensive thermistor type temperature sensing element that may be employed for room temperature sensing. Thermistors are nice choices for HVAC applications. They are inexpensive, have excellent trueness and very low hysteresis, and respond quickly to temperature changes. Furthermore, at temperatures normally twisting in HVAC applications, thermistors have excellent long-term stability (some care should be t aken in choosing thermistors when temperature may rise above 240 F).Finally, because thermistors are typically high resistance (10,000 ohms is typical), they are non affected by variations in wiring distances.However, some designers continue to exclude thermistors because the input signal is non linear with temperature over wide temperature ranges. Instead, low impedance RTD type detectors are often specified. This type of sensor typically requires an electric circuit at the sensor that linearizes and transmits the signal in a way that it leave behind not be affected by wiring resistance (usually a current loop signal is used).Employing low resistance RTD sensors with special electronics presents a number of potential problems in DDC applications. First is the matter of accuracy.While the RTD sensors themselves provide excellent accuracy, it is not uncommon to find end-toend accuracies (I use end-to-end as the relation of the value read by a precision thermometer at the device compared with the actual reading at the DDC system operators terminal) out of tolerance.Calibration of the current loop input may be more difficult than that of a unproblematic resistance type thermistor.Other potential problems with RTDs range from the additional electronics (usually located at the device) that may complicate reliability issues all the way to how theSensor curve 2 Nonlinear sensor resistance curve. The sensor performance curve is a smooth curve over the sensors operating pressure. The DDC linearized curve is a series of straight lines that closely approximates the sensors performance sensor and electronics are configured, which on occasion has been found to affect adversely the sensor signal.Table functions that are now readily available with DDC products can be employed to scale thermistors and other nonlinear devices over a wide range of values.Fig. 2 shows how a DDC system can linearize a continuous, nonlinear sensor input curve with a table function. A number of straight line curves are established in the table function to approximate closely the nonlinear function of the device. As long as simple, inexpensive devices can beseeming the repeatability, hysteresis, and stability requirements for an HVAC application, such devices should not be rejected because their signals are not linear.Requirement of Linear payoffOnce it is understood that input devices need not be linear, it is not a great leap to recognize that the response from output devices controlled by analog outputs interchangeablely need not be linear. However, the issues here are more complex and more ingrained in the rules of thumb that engineers a great deal apply automatically, so some indepth discussion is required.Because of the pneumatic background, valve design manuals commonly stress the need to select kink/valve combinations for which equal increments in valve opinion will effect equal increments in heat delegate of a typical heating or chilling rolling through out the buffet of the valve actuator. Fig. 3 shows how traditional design practice seeks to linearize the overall performance of valve and temperature reduction coil. Carefully selecting a coil and valve combination can provide nearly linear performance over the entire range of stretch along possibilities.Such survival of the fittest is done because it is assumed that the valve will be operated by a controller with a fixed proportional gain.Though this design principle is still widely employed, it is no longer applicable in many modern HVAC applications. In VAV cooling coil applications, the variations of air flow and air/chilled wet temperature characteristics act to change dynamically the heat transfer characteristics of the valve/coil concord as these arguings change. This makes it very difficult to select a valve/coil combination that will be linear through the variety of conditions that may accompany its operation.The higher performance of DDC systems permits designers mu ch greater flexibility in the design of modulating controls without establishing static (and therefore unrealistic) design criteria. Fig. 4 shows a valve and coil combination that does not provide a linear response of valve sit to coil capacity. However, modern DDC systems permit scaling tables to be applied to analog outputs as well as the inputs. Output scaling permits an infixedly nonlinear device combination to respond in a linear fashion to signals from the DDC system. In this example, the valve and coil combination provides more or less 70 percent of the design cooling capacity at nearly 20 percent valve travel. The DDC output to the valve can be adjusted with the scaling table to carriage the valve at 20 percent travel at a 70 percent output signal from the DDC system. The scaling factor allows standard PID control to operate the valve effectively because of a software linearization of the valve/coil combination.However, the chilled water flow and heat transfer performa nce assumed for Fig. 4 is effectual only for constant load-side flows and inlet temperatures and for constant chilled water supply temperatures.Whether essential in the system design or for optimization reasons, rarely in real HVAC applications do these other variables remain constant as control loops operate. As previously discussed, the issue of linear output combinations has therefore been only weakly resolved in the past by attempting to linearize components at one set of system conditions.Obtaining good control over wide ranges of system conditions can be resolved far more all told and effectively with the higher performance capabilities of DDC systems. The proportional, integral, and derivative gains can be tie to algorithms that adjust their values as the variables such as load-side flow, temperatures, and chilled water temperature change. Even more impressive is the emergence of self-tuning controllers.These controllers continually re-establish the various gains associat ed with a control loop to provide continuously precise control without hunting. The benefits of self-tuning are especially important because variables beyond the immediate control loop can have profound and widely varying effects on each control loop. Self-tuning features are becoming widely available with DDC systems and are enormously effective in adjusting control loops to continue stable operation as other system variables change.ControllabilityAs previously discussed, selecting equipment for linear response should not be an overriding consideration for designers in this era of digital controls. However, this does not mean designers can be imprecise in their designs or in the selection of control loop components.The issue of controllability is one that will continue to play a prominent role both in the design of systems and the selection of individual components.Controllability remains largely a sizing issue. If a valve is oversized for given conditions such that the smallest i ncrement possible from the control loop will substantially overshoot the desired control conditions, the loop has become uncontrollable.This is a problem that typically emerges during periods of low load. Fully cause the issue of controllability and applying DDC capabilities decent allows designers to solve such problems and at the same time vastly improve the efficiency and performance of these systems.Selecting a control valve with a lower pressure drop will reduce the pumping power required to meet the load conditions. Traditional practice strongly condemns the idea of employing large valves with lower pressure drops because of the nonlinear response and the lack of controllability at low loads.Fig. 5 illustrates the dilemma. The valve/coil combination with Valve A may be selected according to traditional design practice because it is reasonably controllable at low loads. The vertical bloc intercept represents the smallest incremental cooling transfer possible as the valve is wrong(p) open. Note that it is small-only about 10 percent of the design maximum cooling rate.The coil combination with Valve B has a much lower pressure drop because Valve B is a larger size valve. While valve/coil combination B would require less pumping power, the Y-axis intercept is much higher than that for Combination A. Traditional design criteria typically declare Valve B undesirable for the application because it is uncontrollable at lower loads and the valve position/ cooling capacity relationship is nonlinear. But when it is integrated with a high-performance control system that can adjust both the chilled water temperature and the loop bearing pressure, will linearity and controllability of Combination B really be a problem?System DynamicsTo see how this question can be answered, consider the graphs in Figs. 6 and 7. Fig. 6 shows the operation curves for valve/coil Combination B at a number of different advancement (chilled water supply less air temperature leaving c oil) temperature conditions.It is clear that increasing the chilled water temperature relative to the leaving air temperature markedly improves the controllability at low loads. Similarly, Fig. 7 illustrates that the decrease in pressure across the valve/coil combination as well improves the controllability at low loads.Designers can use these relationships to reduce substantially the problem of controllability. At periods of uniform low loads, the DDC system can reduce the head pressure across a valve and increase the chilled water temperature to improve controllability. If all valves on a common chilled water loop experience similar decreases in load concurrently, as is typical in many HVAC applications, this parameter adjustment is a great help in improving controllability at low loads.It is apparent from the two figures that larger rangeability and low load controllability are achieved by controlling the chilled water temperature for load adjustment.Raising the chilled water te mperature provides a bonus of chiller efficiency increases, but chilled water adjustment reduces pumping nest egg because a higher chilled water temperature increases the water flow necessary to meet loads. Additionally, under certain circumstances dehumidification requirements may limit the permissible chilled water adjustment.Exploiting the integrated control capabilities of DDC systems and controlling chilled water temperature and hydronic loop pressure in coordination with the control valves allows valve/coil Combination B to perform very well in many HVAC applications.Next month we will focus on the level of integration required to make valve/coil Configuration B operate effectively. We will discuss integrating the operation of the various equipment have-to doe with in providing comfort, possible now through the industry moves to provide communication bridges among manufacturers. By concentrating on selecting the most followeffective input/output devices and by utilizing the emerging communications pathways between equipment from various suppliers, we will see that new horizons of performance and energy efficiency can be attained with simple and economical controls configurations.Designers must exploit the benefits of higher performing DDC systems to develop an understanding of the fundamentals of interfacing hardware points to DDC systems. In so doing, a more in-depth look for into total system operation must be evaluated before solutions are selected. Simply following traditional rules of thumb regarding linear input and output devices is a poor design practice in this digital controls era.DDC and splendid and Medium Size BuildingsThe control of heating, ventilating and air-conditioning (HVAC) systems is changing as a conduce of applying direct digital control (DDC) techniques to HVAC control. This plow outlines the main features of DDC compared with accomplished pneumatic control and shows that, for small-to-medium-size buildings, the DDC syste m can pay for itself within two years, after(prenominal) which it affects net savings over pneumatic systems.Comparison between pneumatic Control and DDCDirect digital control of HVAC systems is the direct monitoring of both system input (temperature, flow, pressure) and direct control of all(prenominal) system output (position, onlaff) from a central controller which is a individual ready reckoner or combination of computers. DDC is a simple concept, but its significance is not demonstrable until it is compared with traditional forms of HVAC control.Traditionally, the control of HVAC systems was based on separatist pneumatic controllers, which used compressed air t o operate the dampers and valve actuators t o control space condition such as temperature, humidity and fresh-air circulation. One building would have several such systems, which were controlled independently. For example, an air-handling system composed of two fans, three dampers and three valves (Figure 1) would be controlled by local pneumatic controllers which operated as independent units. Each controller had a simple task to fight down a constant set point (for example, supply air temperature) by monitoring and controlling a very l i m i t e d number of variables connected to it by means of compressed air lines whose pressures represented the values of the variables. The control was adjusted mechanically by a technician in the field, and, as calibration of the pneumatic components was rarely carried out, these systems often did not control the building efficiently. Because the pneumatic controllers were stringently electromechanical devices, their sophistication and accuracy of control were extremely limited.A later on mingled (of pneumatic control) as well employed pneumatic centrals, but w i t h the addition of a computer system. This computes system monitored some additional points (for example, space temperatures) and either calculated new set points for each pneumatic control ler or allowed an operator at a computer terminal to transmit manual set points to the pneumatic controllers. Although this newer variant aided the building manager by providing more in coifion about building conditions and performance, overall effective control of the building was still compromised by the local pneumatic controllers.Each controlled point was still operated by a pneumatic controller with very limited sophistication and virtually no flexibility. These limitations became more important as ways to manage energy became more sophisticated, Some WAC system, such as variable air volume (VAV) systems, required an accuracy of control not attainable in most cases by pneumatic controllers. As a result, building energy managers were frustrated by their inability to improve the control strategies without rebuilding the pneumatic control system for each change.DDC has solved both problems. Instead of independent local pneumatic controllers, DDC uses control or monitoring points, each connected to a computer (or interconnected computers) which reads the value of each input and transmits commands to each output (Figure 2). The control strategies are implemented by computer programs, which can be changed by the operator at will. Also, each system has available to it the value of every system input instead of a very l i m i t e d local set. In short, under the DDC concept, the entire building operates as one integrated system rather than as independent srrrall systems.Four main results accrueControl can be as simple or sophisticated as desired, and can be changed easilyThe system is more reliable because few electromechanical components are neededControl is more accurate because of the inherent greater accuracy of DDC electronic components andEnergy is saved because an overall dodging eliminates energy waste resulting from simultaneous heating and cooling, which usually occurs in pneumatic systems.The ability of DDC to accommodate virtually any control strat egy has had a dramatic impact on mechanical design. Some new mechanical systems can operate in many different modes, depending on external conditions, space temperatures, season, condition of storage tanks, and utility-pricing structures. DDC allows such systems to be operated continuously in their optimum modes, a standard which simply cannot be attained by ordinary pneumatic systems or even pneumatic systems with computer monitoring. Consequently, mechanical designers are now free to d e s i g n the best energy system for a especial(a) building with the assurance that whatever control strategies they specify can be carried out.Each loop at the hostile processors can activate itself independent of the others however, the most efficient use of energy is achieved by controlling all the loops through the central processor. Scheduling air-conditioning and heating loads and selectively falling electrical loads if the total building power approaches the demand limit are two common ene rgy optimization features available.Other features, such as optimal stop/start, which calculates the optimum starting and stopping multiplication of heating/cooling units to prepare spaces for occupancy without wasting energy, are also used as part of an over-all strategy. Most of these optimization routines do not require any additional hardware since they are implemented by simply adding programs that sense exist inputs and change the strategy for controlling existing output actuators.The building owner or manager who uses DDC effectively unavoidably feedback to evaluate his strategies for optimizing building performance. DDC simplifies this process because it continually monitors each input direct and has storage capacity to keep files of the diachronic data gum olibanum obtained. These historical data can be plotted in color on a TV screen or summarized and printed in report format for management review. The most advanced DDC systems (Figure 3) include a generalized report generator which can produce nee types of reports at any time rather than limit the user to the reports engaged when the system was procured* This feature of DDC i s particularly important since the owners power to change his energy strategy generally creates a need for new reports on energy-sensitive areas identified by continued use of the system.An ancillary benefit is the ability of the DDG system to include facilities other than WAC. With little increase in cost, factors such as control of security and lighting can be added to the system, thereby enabling greater energy savings and eliminating the need to purchase separate systems for badge reading and door-lock control. There is no doubt that DDC offers more effective energy management than conventional controls but, until very recently, its application to HVAC readinesss has been limited to large building complexes. many an(prenominal) small- and medium size building installations do not use DDC primarily because of its high cost. In the following sections a typical small building is analyzed and DDC is compared with pneumatic control on a cost and retribution basis.Small Building SystemsThe cost of an HVAC controls Installation is generally related to the number of points t o be monitored or controlled, where each point is define as an analog or digital input (e.g., temperature sensor, fan spot switch) or analog or digital output (e.g. damper position or pump on/off control. Each building system, such as air handling, domestic hot water, or chilled water, includes a certain number of points. A recent study which included detailed abbreviation of a series of building HVAC system, showed that a small- to medium-size building of about 37,175 m2 (400,000 sq. ft .) would contain about 180 points, of which 35% would be analog inputs, 19% analog outputs, 25% digital inputs and 21% digital outputs. Although different building configurations and mechanical designs would affect the distribution of point typ es, the total number of points for a building of this size would usually be close to 180.Designing a DDC SystemGiven the building layout and the number of points in HVAC equipment, the single great design trade-off is that between centralization and distribution of computer power. At the fully centralized extreme a single central computer controls all functions directly and all points are pumped up(p) to it. At the other extreme (fully distributed), a smaller central computer is connected t o a myriad of other small computers, each of which is wired to 10 to 20 nearby points, In this second instance the central machine presides aver the whole system and controls the points through the intermediary of the remote processors. Each remote processor can control a single HVAC system (e.g. air-handling unit, chiller) independently. A median approach is to employ a control number of remote units each of which is wired to 50 t o 120 points.Although all these approaches utilize the benefit s of DDC, the three levels of centralization/distribution bring three factors that must be weighed against one another. The first factor is the cost of computer hardware. The fully-centralized approach employs a single processor, which is the least expensive since it combines all the computing power in one place w i t h one enclosure and no duplication of functions. The fully-distributed approach requires the heaviest capital cost for computer hardware.The second factor is electrical installation cost. The fully distributed arrangement yields the lowest installation cost because each remote processor can be located very close to its points and thus wiring runs are short. The fully-centralized arrangement may be quite expensive unless all points are in one mechanical room. The median arrangement (Figure 4) may be the most economical over-all because four remote processors can be used, one in a penthouse, one in some other logical location such as a basement mechanical room, and oth ers on various floors of the building.The third factor is reliability. The fully-centralized scheme is most sensitive to failure since failure of the single computer causes the entire system to fail. Although the system can be made to fail safely, a system failure is inconvenient. The fully-distributed scheme is least sensitive since any component computer can fail while still leaving all the others running, but, as previously mentioned, the cost of the computing equipment is highest.A median approach for small buildings makes good sense a compromise on all factors is established by designing a system consisting of a central computer and four remote units.Cost Analysis DDC versus Pneumatic ControlThe installed cost of DDC systems has traditionally been higher than for pneumatic sys tens, especially in small installations, where the cost of the DDC control processor is spread over fewer points. The cost of a pneumatic system tends to rise linearly with the number of points, as a larg e system requires more independent local controllers, whereas with DDC a central processor is required even for system with very few points. However, the chop-chop falling cost of computing hardware has eroded the historical price difference between DDC and pneumatic installations. For a specific building of 37,175 m2 (400,000 sq. ft.), the installed cost of a pneumatic system is about 75% of the cost of a DDC system Although the initial cost of a DDC system is higher than for a pneumatic system, it can be recovered in a surprisingly short time . It is realistic to assume that a DDC system will yield a 10X% energy saving over and above conventional pneumatic control, due simply to its more accurate and sophisticated control, and t o its ability to provide the building owner with information about building performance and areas where energy should be better controlled. Features such as load shed and flexible scheduling alone will produce large energy savings, and these savings will increase as the owner becomes more familiar with the operation of the building. If we assume yearly maintenance cost of $12,000 and $10,000 for the DDC and pneumatic systems respectively, and an energy usage of 322 equivalent kWh/m2/yr. (30 kWh/sq.ft./yr.) at $0.0275 per kWh for both systems, it will take 1.4 years more for the DDC to pay for itself than it will for the pneumatic system when used in the building under consideration. After that time the DDC system will save money compared with the pneumatic controls. Another simple deliberation shows that for a three-year payback the DDC energy saving need be only 5.7%, an e