Air-cooled chillers

EnergyPlus models air-cooled chiller plants through a combination of chiller objects, plant loops for chilled water distribution, variable speed pumps, and when heating is needed, a separate hot water loop served by a fossil fuel boiler. This isn't one tidy object. It's a collection of interconnected components that mirror how these systems actually get built: separate equipment for cooling and heating, each with its own distribution network, all tied together through control logic and shared air-side terminals.

The Chiller Side

The core cooling component is Chiller:Electric:EIR, which uses an energy input ratio formulation to model performance. EIR chillers are more flexible than the older constant or variable COP models because they account for part-load performance and varying condenser conditions through a set of curves.

Air-cooled means the condenser rejects heat directly to outdoor air rather than to a cooling tower and condenser water loop. This simplifies the plant configuration (no cooling tower, no condenser pumps, no water treatment) but makes chiller performance more sensitive to outdoor dry-bulb temperature. When it's 35°C outside, your condenser is working against that, and efficiency drops accordingly.

The chiller object needs rated capacity, rated COP, and three performance curves. The capacity modifier curve adjusts available cooling based on chilled water supply temperature and outdoor air temperature entering the condenser. The EIR modifier curve does the same thing for power consumption. Finally, the EIR part-load curve accounts for efficiency degradation when the chiller runs below full capacity.

Getting manufacturer data for these curves can be tedious. AHRI certification gives you a few operating points, but EnergyPlus wants performance across the full range. You'll often end up curve-fitting from limited data or using generic curves from equipment libraries, which introduces uncertainty. Air-cooled chillers vary more between manufacturers than water-cooled units do, so generic curves are a rougher approximation than you'd get with cooling towers.

Variable Volume Chilled Water Distribution

The chilled water loop connects the chiller to the building's cooling coils through a PlantLoop object. Variable volume means the pump speed modulates based on demand rather than running at constant flow all the time.

You'll specify a Pump:VariableSpeed on the loop, which adjusts flow to maintain a differential pressure setpoint or to meet the flow requirements of active coils. This saves pumping energy compared to constant volume with bypass or three-way valve control. Real variable speed drives accomplish this with VFDs on the pump motors. EnergyPlus models the power reduction using a part-load curve that relates pump power to flow fraction.

The plant loop itself needs a sizing basis (design temperature difference, design flow rate), a setpoint manager to control chilled water supply temperature, and availability schedules. The setpoint manager is where things get interesting. You can use a fixed setpoint (always supply 7°C water, for instance), an outdoor air reset schedule (warmer supply water when outdoor conditions are mild), or more sophisticated strategies that optimise based on system load.

Outdoor air reset makes sense for air-cooled chillers because you're balancing two competing effects. Warmer chilled water reduces the temperature lift the chiller works against, improving efficiency. But warmer water means you need more flow to deliver the same cooling, which increases pump energy and might compromise dehumidification at the coils. The optimum moves around depending on conditions.

The loop configuration matters. Primary-only variable flow is simplest: one pump, variable speed, serving all the coils. Primary-secondary (or now more commonly called dedicated primary/common secondary) uses a decoupled arrangement where the chiller runs on a primary loop at relatively constant flow and a secondary variable speed loop serves the building. EnergyPlus can model either arrangement, though primary-only is becoming more common in modern designs and is simpler to configure.

Hot Water Boiler Plant

Heating comes from a separate PlantLoop with a Boiler:HotWater object. The boiler burns fossil fuel (natural gas, oil, propane) to generate hot water for heating coils throughout the building.

The boiler object needs fuel type, nominal capacity, and a thermal efficiency curve. Unlike chillers where efficiency varies significantly with load and conditions, boiler efficiency curves are often relatively flat, especially for condensing boilers which maintain high efficiency across a wide operating range. Non-condensing boilers show more degradation at part load, and return water temperature affects efficiency for condensing units because you need low return temps to extract the latent heat from flue gases.

Hot water distribution also uses a Pump:VariableSpeed for the same reasons as chilled water: energy savings when not all zones need heating. The pump modulates to maintain flow to active heating coils whilst keeping pressure differential within acceptable limits.

Hot water setpoint control typically uses outdoor air reset in the opposite direction from chilled water. When it's brutally cold outside, you supply hotter water (maybe 80°C). When it's just cool, you drop the supply temperature (perhaps to 40°C). This improves comfort, reduces distribution losses, and for condensing boilers, allows more condensing operation and better efficiency.

Air-Side Integration

The chilled water and hot water loops connect to the building through coils in air handling units or terminal units. For variable volume air distribution, you're typically looking at VAV terminal boxes with hot water reheat coils and central air handlers with chilled water cooling coils.

The central AHU contains a Coil:Cooling:Water supplied by the chilled water loop. This coil cools and dehumidifies the supply air to some setpoint, often 13°C to 15°C. From there, air distributes to VAV boxes which throttle the flow to each zone based on cooling load.

If a zone needs heating, its VAV box contains a Coil:Heating:Water supplied by the hot water loop. The box delivers minimum airflow for ventilation whilst the heating coil warms that air to maintain zone temperature. This is energy-intensive (cooling air centrally then reheating it locally) but common in buildings with simultaneous heating and cooling loads or strict humidity control requirements.

Alternatively, you might have perimeter heating from separate hot water units (radiators, baseboard, fan coils) whilst VAV boxes handle cooling only. This is more efficient but requires coordination between systems to avoid fighting each other.

Plant Operation and Sequencing

Both the chiller and boiler need operating schemes that define when they run and how they respond to load. The PlantEquipmentOperationSchemes object manages this.

For the chiller, you'll typically have a load-based scheme that stages chillers based on cooling demand. If you have multiple chillers (common in larger buildings), the scheme determines which ones run and in what order. Lead-lag rotation, optimal loading algorithms, or simple sequencing are all options.

Chillers don't instantly start and stop. They have minimum runtime limits, startup delays, and sometimes minimum part-load ratios below which they can't operate. EnergyPlus can model these constraints. If your chiller can't run below 20% capacity, the simulation honours that, which affects cycling losses and part-load efficiency.

Boilers similarly have staging logic, though they tend to be more tolerant of low loads and rapid cycling than chillers. Still, you should define realistic constraints. Most boilers have minimum firing rates, and cycling them on and off too frequently reduces efficiency and equipment life even if the simulation doesn't care about the latter.

Pump Control Strategies

Variable speed pumps save energy but only if controlled intelligently. EnergyPlus offers several pump control strategies.

Intermittent operation turns the pump on when flow is needed and off otherwise. This maximises energy savings but can cause pressure transients in real systems, so it's not always used even though it's theoretically optimal.

Continuous operation runs the pump whenever the loop is available, modulating speed to meet demand but never turning off completely. This is more common in practice because it's gentler on the system and maintains stable pressure.

The setpoint determines how the VFD modulates speed. Differential pressure setpoints are common: maintain a certain pressure difference across the loop, and flow follows naturally. More sophisticated strategies reset the differential pressure setpoint based on valve positions (if all zone valves are wide open, you need more pressure; if they're barely cracked, you can reduce it).

EnergyPlus handles this through SetpointManager objects attached to the loop. Getting the setpoint strategy right affects both energy consumption and whether the simulation achieves stable control. Overly aggressive setpoints can cause hunting. Conservative setpoints waste pumping energy.

Condenser Air Flow

Air-cooled chillers need condenser fans to move outdoor air across the condenser coils. These fans consume power (a lot of it, sometimes 5% to 10% of chiller power) and that energy shows up in your plant energy consumption.

The chiller object includes inputs for condenser fan power as a function of capacity or you can specify a simple power fraction. More detailed chiller models account for variable speed condenser fans that modulate based on head pressure or outdoor temperature, saving energy during mild conditions.

This is one place where air-cooled chillers differ significantly from water-cooled units. You're trading condenser pump and cooling tower fan energy for larger condenser fans on the chiller itself. The net energy comparison depends on climate and operating hours, but generally water-cooled systems have an efficiency advantage in hot climates whilst air-cooled systems avoid the water consumption and maintenance complexity.

Sizing and Autosizing

EnergyPlus can autosize chillers, boilers, and pumps based on design day conditions. For chillers, this means sizing to meet peak cooling load at design outdoor conditions. For boilers, it's peak heating load at design winter conditions.

The tricky bit is coordination between equipment capacity and loop flow rates. If you autosize everything independently, you can end up with mismatched capacities where the chiller is larger than the loop can support or the pump is oversized for the actual flow requirements. EnergyPlus generally handles this reasonably, but it's worth checking the sizing outputs to confirm everything makes sense.

You also need to decide whether to size for coincident or non-coincident loads. Building loads rarely peak simultaneously in all zones, so coincident sizing (based on when total building load peaks) gives smaller equipment than non-coincident sizing (sum of individual zone peaks). For central plants serving multiple air handlers, coincident sizing is usually appropriate and is what the autosizing algorithm does.

Performance Expectations

A properly configured model should show chiller COPs ranging from about 2.5 to 3.5 for air-cooled units in typical operation, degrading in hot weather when the condenser is working harder. Water-cooled chillers would show COPs more like 4 to 6, which is why air-cooled units tend to be used where water isn't available or where the installation cost premium for towers and condenser water systems doesn't justify the energy savings.

Boiler efficiency should be 80% to 85% for standard non-condensing units, up to 90% to 95% for condensing boilers when return water temperatures are low enough to allow condensing operation. If your simulation shows significantly different numbers, either your efficiency curves are wrong, your operating conditions are unusual, or something in the configuration isn't realistic.

Variable speed pumping should reduce pump energy by 30% to 50% compared to constant volume with three-way valve control, depending on the building load profile and control strategy. If you're not seeing substantial savings from VFD operation, your minimum flow setpoints might be too high or your control strategy isn't allowing enough turndown.

Control Coordination

The trickiest aspect of these systems isn't the individual components but how they interact. Chilled water reset affects chiller efficiency but also dehumidification performance. Hot water reset improves boiler efficiency but might compromise comfort during rapid weather changes. VAV box minimums affect ventilation, energy, and simultaneous heating-cooling waste.

EnergyPlus simulates all this, but only if you configure the controls properly. The setpoint managers, availability schedules, and equipment operation schemes all need to work together. It's entirely possible to create a model that runs but represents controls that would never work in reality, delivering weird results that technically aren't wrong from the simulation's perspective but don't reflect competent design.

This is where experience matters. The software will happily model a system that supplies 5°C chilled water year-round whilst simultaneously running hot water reheat in every zone. That's thermodynamically valid. It's also stupid. Getting the control logic to reflect actual practice is as important as getting the equipment performance curves right.