Energy Management Control Systems

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ENERGY MANAGEMENT CONTROL SYSTEMS

EVOLUTION PERSPECTIVE OF EMCS

The primary purpose of energy management control systems (EMCS) is to provide healthy and safe operating conditions for building occupants, while minimizing the energy and operating costs of the given building. Aided by technological developments in the areas of electronics, digital computers, and advanced communications, EMCS have been developed to improve indoor quality while saving more energy.

Electromechanical Timers

The earlier devices used in the first half of the twentieth century to control building loads (such as lighting and space conditioning) were electromechanical timers, in which a small motor coupled to a gearbox was able to switch electrical contacts according to a predefined time schedule. Normally the output shaft of the gearbox causes one or more pairs of electrical contacts to open or close as it rotates. These electromechanical devices were simple and reliable, and they are still used to control lights and ventilation in some buildings. However, for lighting and space conditioning, the main drawbacks of this type of timer was inflexibility. Manual intervention is required to change settings, and the operation mode is essentially without feedback (open-loop controllers), since the schedule of operation is not easily influenced by the variables in the controlled process.

Electronic Analog Controllers

The development of electronic circuitry capable of processing sensor signals made possible the appearance of electronic controllers able to respond to variable conditions. For example, to control street lighting a light sensor coupled to a simple electronic amplifier and switch can turn off the lights during the day, avoiding energy waste. In some cases the analog electronic controllers were coupled with electromechanical timers, providing the possibility of controlling the output as a function of time and/or operation conditions.

Although electronic circuitry has been available since the beginning of the twentieth century, the invention of the transistor in 1948 was a key milestone, bringing a decrease in costs, an improvement in reliability, and a reduction in the size of control circuits. During the 1950s and 1960s, electronic analog controllers became widely available for controlling lighting, heating, ventilation, and air conditioning (HVAC). Although these controllers made possible improved control based on information from sensor feedback (closed-loop control), they still suffer from lack of flexibility, since they are able to implement only simple control strategies and require manual change of settings.

Digital Controllers

The invention of the microprocessor in 1970 signaled a radical change in the area of building controls, allowing the development of increasingly powerful EMCS. From the outset, microprocessor-based EMCS could be easily programmed for changeable and variable time schedules (e.g., workday versus weekend operation, public holiday operation). In addition to this flexibility, increasingly powerful microprocessors, along with new energy management hardware and software, allowed for the progressive implementation of sophisticated control algorithms, optimization of building operation, and integration of more functions into building control and supervision.

Most new hardware and software development in the EMCS market now aims at utilizing the full potential of EMCSs and at making the information obtained more usable and accessible. This accessibility has created a secondary benefit with a greater potential for customer/utility communication. In many ways the EMCS evolution and market penetration in the industrial, commercial, and to a certain extent, residential markets, has equaled that of the personal computer.

During the 1970s EMCS digital controllers were mostly electronic time clocks that could be programmed according to a variable schedule. However, increasingly powerful, low-cost microcomputers have rapidly improved computing power and programmability and hence increased the application of EMCS. This rapid improvement has in turn introduced benefits such as the following:

flexible software control, allowing simple modification;
built-in energy management algorithms, such as optimal start of space conditioning, limiting the electricity peak demand; also, control of HVAC and lighting loads in practically all EMCS applications;
integrated process, security, and fire functions;
monitoring capability allowing identification of faulty equipment and analysis of energy performance;
communication with the operator and other EMCS that can be remotely located.

These possibilities make EMCS increasingly attractive for energy monitoring, control, and utility/customer interface. The integration of functions associated with safety and security (e.g., intrusion and fire) as well as those associated with building diagnostics and maintenance are also expanding the potential market of EMCS. The application of advanced EMCS with these features is leading to the appearance of "smart buildings," which can adjust to a wide range of environmental conditions and which offer comfort and security with minimal use of energy.

CURRENT EMCS CHARACTERISTICS

An EMCS is usually a network of microprocessor-based programmable controllers with varying levels of intelligence and networking capabilities. This "typical" EMCS has three components (see Figure 1):

Local control module: directly wired to sensors and actuators (e.g., equipment to control space conditioning, such as pumps and fans);
Command control module: makes control decisions;
Personal computer interface: simplifies operator control of the system through a user-friendly interface.

An important feature of the typical system is its modularity. The most powerful EMCS installations have all three components, but often only a single module is necessary for simple applications, such as controlling a single air-conditioning unit, or for most applications in the residential sector. Thus local and control modules are capable of stand-alone operation without higher-level components. The functions, hardware, software, and communication characteristics of these three components are as follows.

Local Control Modules

A local control module performs the following basic functions in an EMCS:

receives information from sensors;
controls actuators, through relays, to switch equipment on and off or to change its variable output;
converts analog sensor data to a digital form;
performs direct digital control;
communicates with the command module.

Modern EMCS use a variety of sensors, including temperature, humidity, occupancy, light, pressure, air flow, indoor air quality, and electric power (normally pulses from power meters). The actuators are the units that can influence the state of the system, including chillers, pumps, fans, valves, and dampers.

Command Control Module

The command control module, or control module, is the real intelligence of the EMCS; all programming and control software resides here. Command control module software can create reports (e.g., for recording the historical status of variables) and perform various demand-limiting schemes. The common energy management strategies offered at this level include proportional-integral-derivative (PID) control loops, duty cycling, and optimal start/stop of HVAC units. Also available are economizer control (use of free cooling with outside air when the outside temperature is below the target temperature) and programmed start/stop with demand limiting of selected loads.

The command module can be programmed either through a keypad or by downloading programs from the PC host. The information presented in command module reports is easy to change, but it must follow a prearranged format. These reports usually show temperatures, peak demand, whole-building energy, equipment status, maintenance records, and alarm records. Automatic reporting of a system alarm, such as machine failure, by phone or by electronic mail is another typical software feature.

Command modules communicate with other modules through a local area network (LAN). Through this LAN, command modules receive information from the local control modules and store data. These data can be stored from a week to two years, depending on the recording interval and the number of points to be monitored. Unlike host-based systems, which use a central computer to interrogate each command module individually, the computer interface can tap into the network like any other command module.

A recent data communications protocol for Building Automation and Control Networks (BACnet), ASHRAE Standard 135-1995, is an important step to ensure that controllers made by different manufacturers can communicate with each other in a simple way, avoiding the expense of additional interface hardware and communication software.

Personal Computer Interface

The personal computer interface allows for easy operation of an EMCS, but all of the system's control functions can be performed in its absence. This user interface serves three purposes:

storage of a backup of the command module's programs to be used in case of power or system failure;
archiving of trend data for extended periods of time;
simplification of programming and operation of the system through a user-friendly interface.

The software at the user-interface level usually involves block programming, whereby the operator can define control loops for different sensors as well as other specific control strategies in a BASIC-type, easily understood programming format. This block structure allows modification or enabling of different program blocks, avoiding the necessity of rewriting the main program. Programs at this level are menu-driven, with separate sections for trend reports, programming, graphics, etc. Visual displays, such as the schematic of HVAC equipment interconnections, are used to make information easier to comprehend. Building floor plans can be incorporated to display information such as room temperatures.

INTEGRATION OF FUNCTIONS

Because of increasing computation power, EMCSs can integrate other functions and also be coupled to other processes. By helping coordinate plant operation, the integration of functions also makes the acquisition of EMCSs more attractive, since a significant portion of the hardware can be shared by different applications. For tracking the performance of the building operation and carrying out the required maintenance, monitoring and data-logging functions are essential in an integrated system, since operators need to be aware of the operating status of each part of the plant and to have access to reports of previous performance. Also, alarm display and analysis is a very convenient function for carrying out diagnostics that can be performed by an integrated system. Safety and security functions can be easily and economically integrated, although some installations prefer separate systems due to potential litigation problems.

The use of peak electricity demand-reducing technologies, such as thermal storage and peak-shaving involving the use of a standby generator, also can benefit the control and monitoring capabilities of EMCSs. In thermal storage (ice or chilled water for cooling), the EMCS can schedule the charging of the system during off-peak hours to optimize savings in both peak demand and energy costs. Temperatures in the storage system are monitored to minimize energy and demand costs without adversely affecting plant operations or products. Thermal storage, already an attractive option for space cooling in commercial and industrial buildings and in several food industries (dairy, processed meat, fish) and for space cooling, has become an even more effective energy saver thanks to EMCSs. An EMCS also can determine when the use of an existing standby generator during periods of peak demand can reduce costs, taking into account the load profile, demand and energy costs, hour of the day, fuel costs, etc. The standby generator also can be put on-line on request from the utility in times of severe peak demand or loss of generating capacity reserve margins.

EMCS FOR ENERGY SAVINGS

Typical savings in energy and peak demand are in the range of 10 to 15 percent. These savings are achieved by reducing waste (e.g., switching off or reducing the lights and space conditioning in nonoccupied spaces), by optimizing the operation of the lighting (e.g., dimming the lights and integration with natural lighting), and space conditioning (e.g., control of thermal storage).

THE EMCS FOR REAL-TIME PRICING

State-of-the-art systems with flexible software allow for utility interface. These systems are all currently capable of data monitoring and of responding to real-time pricing, depending only on the software installed on the user-interface computer and the number of sensors the customer has installed for end-use monitoring.

To reduce use of electricity when electricity costs are highest, EMCS use a network of sensors to obtain real-time data on building-operating and environmental conditions. Some large electricity consumers are connected with the utility through a phone line for the communication of requests to reduce the peak electricity load and for present and forecasted demand. The building operator traditionally closes the link, instructing the EMCS to respond to the utility's signals. However, this current manual load shedding/shifting response to utility prices is too labor-intensive and operationally inefficient for large-scale implementation.

Energy management systems have been developed that can control loads automatically in response to real-time prices. Real-time prices are sent to the customer, whose EMCS can modulate some of the loads (e.g., air conditioning, ventilation, nonessential lighting). Thus peak demand can be reduced at times of high electricity price, maintaining all essential services. An EMCS could be used to modulate HVAC load by controlling temperature, humidity, volatile organic compounds (VOCs), and carbon dioxide levels within a window of acceptance, the limits of which may be adjusted as a function of the real-time prices. In theory this strategy can save energy and substantially decrease peak demand. The most attractive candidates for ventilation control performed by EMCSs are: large commercial buildings with long thermal time constants (or with thermal storage), buildings with low pollutant emission from building materials, buildings that house furnishings and consumer products, and buildings that require a large volume of ventilation (or circulation) air per occupant.

THE EMCS FOR DATA MONITORING

Parallel to the increase in performance of microcomputers in the late twentieth century been an exponential decrease in the price of semiconductors and memory. This means that data-logging functions can be added at a small extra cost, considerably increasing the usefulness of a given EMCS.

Because monitoring and data-logging facilities add to initial cost, some customers may be hesitant to choose between a simple configuration without those capabilities and a more complex model including them. The benefits associated with monitoring often outweigh the price premium, as they allow the user to:

tune the performance of a system and check energy savings;
record the load profile of the different operations or major pieces of equipment within the plant;
find the potential for improvements;
allocate energy charges to each operation or product line;
submeter electricity end use, allowing more accurate charging of costs to specific processes or divisions;
check worn or faulty equipment (e.g., declining fan pressure in an air-handling unit may mean a clogged filter);
check utility meter readings (potentially a particularly useful feature in plants using power electronics control devices, which can generate errors in conventional power meters).

Figure 2 shows a diagram of the layout of EMCS-based end-use monitoring in which the data collected by the EMCS can be remotely monitored by the utility or by the maintenance staff in the company's central office.

The load data can be disaggregated into main uses by using suitable algorithms that take into consideration sensor information, plant equipment, and plant schedule. Figure 3 shows hourly electricity use of an office building, broken down by major end uses. This type of data analysis can be useful for monitoring building performance and for identifying opportunities to save energy at peak demand.

TRENDS IN THE EVOLUTION OF ENERGY MANAGEMENT AND CONTROL SYSTEMS

Future trends for the evolution of EMCS are likely to involve improvements in user interface, easier access, better controls, and advances in integration, namely including the following:

better access to system information, with more remote diagnosis and maintenance capabilities;
easier installation and programming, with advanced graphics user interface;
smaller, more distributed controllers and unitary control for more advanced energy management optimization;
easier integration of products from different manufacturers, and continuing effort toward communication standardization;
less integration of fire and security functions.

One of the most significant EMCS trends is toward better access to the information gathered by the EMCS. The EMCS industry is developing smaller, distributed units with increased programmability and control capabilities. These smarter local units offer increased system versatility and reliability and allow smaller units to perform functions that would previously have required larger, more expensive systems. Another trend in EMCS hardware development has been the integration of sensors and controllers from various manufacturers. The work performed toward the development of a standard communications protocol (BACnet) is of the greatest importance in ensuring communication compatibility between and among equipment made by various manufacturers.

One system feature of particular relevance is remote troubleshooting. For example, in case of an alarm signal in the building, the user can trace back through the system to find the cause of the alarm. In fact, manufacturer representatives can perform much of the routine troubleshooting over the phone. Whenever an operator has a software problem, the representatives can call up the system to correct programming problems or help develop new applications. Both preventive maintenance (carried out at programmed intervals of operation time) and predictive maintenance management (carried out when the plant sensors detect a deterioration in the equipment performance) can be incorporated in powerful EMCS, including databases containing details (even images) of the spare parts required for maintenance.

The creation of graphics can be menu-driven, often utilizing a building floor plan or system schematic to display the collected data. The floor plan is first drawn by the customer, and then variables, such as current room temperature, are superimposed.Optical scanners also can allow easier graphics creation, and user-selected video frames can be incorporated into the software displays.

Voice communication capabilities, including voice recognition and speech synthesis, are also being increasingly used to provide a simpler user interface. Thus, for example, verbal instructions can be given for resetting set points (temperature, etc.), or to request other actions from the system. Fire and security monitoring is one EMCS feature, which may require its own dedicated system, although this leads to higher costs. This trend is conditioned by insurance and liability issues.

Anibal T. de AlmeidaHashem Akbari

BIBLIOGRAPHY

Akbari, H.; de Almeida, A. T.; Connell, D.; Harris, J.; and Warren, M. (1988). "Use of Energy Management Systems for Performance Monitoring of Industrial Load-Shaping Measures." Energy, The International Journal 13(3):253–263.

Akbari, H.; Heinemeier, K.; Le Coniac, P.; and Flora, D. (1988). "An Algorithm to Disaggregate Commercial Whole-Building Electric Hourly Load into End Uses." Proceeding of the ACEEE 1988 Summer Study on Energy Efficiency in Buildings 10:13–26.

American Society of Heating, Refrigeration, Air-Conditioning Engineers (ASHRAE). (1995). ASHRAE Standard 135-1995: BACnet—Data Communication Protocol for Building Automation and Control Networks. Atlanta: Author.

Electric Power Research Institute (EPRI). (1985). Control Strategies for Load Management. Palo Alto, CA: Author.

Electric Power Research Institute (EPRI). (1986). Energy Management Systems for Commercial Buildings. Palo Alto, CA: Author.

Fisk, B., and de Almeida, A. T. (1998). "Sensor-Based Demand Controlled Ventilation." Energy and Buildings 29(1):35–44.

Heinemeier, K., and Akbari, H. (1992). "Evaluation of the Use of Energy Management and Control Systems for Remote Building Monitoring Performance Monitoring." Proceedings of the ASME International Solar Energy Conference.

Macmillan Encyclopedia of Energy