Increasing energy costs and Utility deregulation has encouraged customers to implement various Energy Management and Conservation Measures (EMC’s).
Over the years control companies have lead the way in the implementation of these measures, but more recently, utilities are providing rebates to consumers and contractors for EMC implementation. Balancing the need for increased ventilation and occupant comfort.
EMCs and their savings have been documented by industry organizations such as ASHREA and BOMA – the savings are real.
Understanding the terms and devices used in implementing EMC strategies helps contractors provide additional valued added services as they service existing control systems and consider recommending EMCs to customers .
Basic energy management measures that should be easily implemented are:
• Time Of Day Scheduling
• Temperature Setback
• Temperature/Time Optimization
• Supply Temperature Reset
• Economizer Cycle
• Demand Limiting
• Duty Cycling
Time Of Day Scheduling (TOD)
Time of day scheduling is defined as automatically turning equipment off
when it is not needed, on a predetermined time schedule.
Routines must allow for holidays, weekends, daylight savings time, etc. Override features allow equipment operation when the building is occupied outside of normal hours. The override feature should not interfere with normal operation.
Savings are achieved from not running equipment unnecessarily.
Typical equipment that is usually controlled by Time Of Day :
• ventilation fans
• exhaust fans
• interior / exterior lighting
Devices that may implement Time Of Day Scheduling are:
• electromechanical timers
• electronic timers
• stand alone controllers (electronic or Direct Digital)
• multifunction computer based systems
• lighting controls
• programmable logic controllers
Temperature Setback/Setup
Temperature setback/ setup is the adjustment of the building temperature when it is unoccupied, to minimize heating or cooling requirements.
When the building has a wide variance (irregular) building occupancy schedules, this strategy may present some problems as the override feature may not bring building temperature to normal levels quickly enough.
Evening setback/setup can be started before business hours are over to take advantage of the energy stored in the building mass. However this differs for each building type. The time required for morning startup is dependent on the
ability of the HVAC equipment to heat or cool the building in a worst case situation.
Savings are achieved through the reduction in equipment operating time and the reduction in heat loss or gain through exterior walls and dependent on the length of the unoccupied time and the extent of temperature setback.
Typical equipment that is usually controlled are:
• ventilation fans
• chillers
• heaters/boilers
• air handlers
Devices that implement setback/setup are:
• time controllers • programmable thermostats • stand alone controllers • multifunction computer based systems (Networked Systems) • programmable logic controllers.
Temperature/Time Optimization (Optimized Start-Stop)
Temperature/time optimization is a more accurate control of
temperature setback/setback. The strategy monitors the temperatures
of the outside air and building mass to adjust the startup or setback of
the HYAC system. The savings are achieved by the warm-up or cool-down cycle using the least amount of energy to produce comfort conditions at time
of occupancy.
Modern day control systems have learning and tuning algorithms to take the guessing out of the optimization parameters.
|
|
The optimized stop feature permits programming of selected heating and air conditioning loads to stop at a point in time before the end of the occupied period The occupancy period begins at the time programmed for the Optimized Start set point. The occupancy period ends at the start of the next programmed set point. |
For example, assume that an Optimized Start event is programmed for 8:00 a.m. and a Sensor Control event is programmed for 5:00 p.m. This sets the
occupancy period as 8:00 a.m. - 5:00 p.m. the setback period would be active from 5:00 p.m. - 8:00 a.m. (or until optimized start-up time).
Supply Temperature Reset
Supply temperature reset is the adjustment of the temperature of supply air or water according to the building’s actual heating or cooling needs.
A building’s HVAC system must be designed for the worst case conditions and since these conditions only occur a portion of the time, the HVAC equipment is
designed for a much higher capacity than, the building needs on average.
The outside temperature is measured and the supply air and water temperature is adjusted accordingly.
The supply temperature reset allows the HVAC system to more closely match the building’s actual heating or cooling needs based on outside air temperature
Normally separate control schemes are necessary for boiler water reset and chiller water reset with the savings achieved by maintaining better temperature
control of the building space, therefore avoiding overheating or overcooling the building. In boiler systems outdoor reset controls that have a warm weather
shut down function will prevent overheating of a building in spring and fall by automatically shutting down the boilers and system pump whenever heat is not needed.
Typical equipment with reset strategies are:
• Boiler hot water supply
• Chilled water supply
• Air handler supply air temperature
Economizer Cycle
The economizer cycle uses the outside air for free cooling whenever possible. The strategy is a cooling strategy most commonly found and applied to rooftop equipment.
Savings occur with the economizer cycle by avoiding the use of mechanical cooling equipment when the outdoor air can be used in its place.
|
|
Outside air dampers are controlled to admit additional outside air for cooling when the outdoor air temperature is below the return air temperature of the system. |
When the outside air temperature is below 13deg C (typical) the outdoor air temperature is mixed with the return air to achieve the desired temperature.
When the outside air temperature is above 24 deg C(typical) a minimum of outside air is used.
Enthalpy control (temperature and relative humidity) is a refinement of the economizer cycle where the moisture content of the outside air and return air temperatures are monitored.-and the air mix that will impose the lowest cooling load on the system is selected.
In the past the extra costs and complexity of enthalpy control were not normally justified over the simple economizer cycle unless the HVAC system is complex and has proper maintenance. The reduction in the cost of enthalpy sensors has lead to increased enthalpy control in economizer systems.
Savings occur with the economizer cycle by avoiding the use of mechanical cooling equipment when the outdoor air can be used in its place.
Besides packaged air handler systems, economizer cycles are found in:
• Control of chillers
• Ventilation fans
• Dampers
Demand Limiting
Demand limiting is the control of the monthly electrical peak demand to a preset practical level. Utility companies usually charge extra charges over the base on for peak demands.
A controller is used to disconnect or shed selected electrical loads whenthe peak demand of the building is approaching a preset level.
To obtain full savings on this system the controller must insure that the demand is not exceeded at any point in time throughout the entire month.
As certain loads cannot be turned on and off rapidly, the minimum off and on times of the loads should be taken into account when setting a demand control system. A building may not have any controllable loads.
Savings can be calculated by multiplying the kilowatts saved by the monthly demand charge. Typical applications are:
• water heating
• snow melting
• space heating
• any electrical load that is nonessential or deferrable
Duty Cycling
Duty cycling is the cycling of electrical and mechanical equipment on and off to save energy while maintaining comfort conditions.
HVAC equipment is normally designed to handle worst case conditions which may allow equipment to be controlled intermittently during lower load
conditions.
Some equipment, such as electric motors, may have reduced life if cycled on and off frequently. Temperature compensation can be used on duty cycling to adjust the cycling rate to ensure that the comfort conditions are maintained.
Savings are achieved in the reduction of electrical energy and in the reduction of demand charges.