Water-cooled chillers

EnergyPlus models water-cooled chiller plants as a network of interconnected components: the chiller, a cooling tower to reject heat, plant loops for both chilled water and condenser water, variable speed pumps on multiple loops, and when heating is needed, a separate hot water system with a fossil fuel boiler. It's more complex than air-cooled systems because you've added an entire condenser water loop and tower, but the payoff is better efficiency when outdoor conditions cooperate.

The Chiller Side

Water-cooled chillers use Chiller:Electric:EIR (energy input ratio), the same object type as air-cooled units but with different performance characteristics and condenser arrangement. Instead of rejecting heat directly to outdoor air, these chillers dump heat into condenser water which then flows to a cooling tower.

The chiller object needs rated capacity, rated COP, and the usual three performance curves. The capacity modifier curve adjusts available cooling based on chilled water supply temperature and condenser water entering temperature (not outdoor air temperature, that's the key difference). The EIR modifier curve does the same for power consumption. The part-load curve accounts for efficiency degradation at reduced loads.

Water-cooled chillers typically achieve COPs of 4 to 6, sometimes higher with modern high-efficiency equipment. That's substantially better than air-cooled units running at 2.5 to 3.5, which is why water-cooled dominates in larger installations where the added complexity pays for itself through reduced energy costs. The catch is you need cooling tower infrastructure, condenser water treatment, and you're consuming water through evaporation.

Condenser Water Loop

The condenser water loop sits between the chiller and cooling tower. It's a closed loop that circulates water through the chiller's condenser and then to the tower where heat gets rejected to outdoor air through evaporative cooling.

You'll define a second PlantLoop specifically for condenser water. This loop has its own Pump:VariableSpeed (or constant speed, though variable is becoming standard), its own setpoint manager, and connects the chiller condenser to the cooling tower.

Condenser water temperature matters enormously for chiller efficiency. Lower condenser water temperature means the chiller works against a smaller temperature lift, which directly improves COP. The cooling tower's job is to provide the coldest condenser water it can given outdoor wet-bulb conditions, and the condenser pump circulates that water through the chiller at whatever flow rate the system needs.

Setpoint management on the condenser loop usually targets a fixed approach temperature to outdoor wet-bulb (maybe 3°C to 5°C above wet-bulb) or uses a reset schedule that allows higher condenser temperatures during mild weather to save tower fan energy. The optimum is a moving target: running the tower harder gives better chiller efficiency but costs tower fan power.

Cooling Tower Configuration

The cooling tower object, typically CoolingTower:VariableSpeed or CoolingTower:TwoSpeed, models how effectively the tower rejects heat and how much fan and pump energy that requires.

Towers work through evaporative cooling. Water sprays or flows over fill material whilst fans pull outdoor air through. Some water evaporates, which removes heat from the remaining water. The theoretical limit is outdoor wet-bulb temperature—you can never cool the water below that, though real towers get within a few degrees (that difference is called approach).

Tower performance depends on outdoor wet-bulb, the temperature difference between entering water and outdoor air, and how hard you run the fans. Variable speed tower fans modulate to maintain the condenser water setpoint whilst minimising fan energy. When it's cool and humid (low wet-bulb), the tower barely has to work. When it's hot and dry (high wet-bulb), the fans run hard and you might still not achieve your target approach.

The tower object needs design water flow rate, fan power at design conditions, and performance curves that account for varying wet-bulb, range (entering minus leaving water temperature), and approach. Getting tower performance data is easier than chiller curves because tower manufacturers publish fairly detailed performance tables, but you still need to fit that data to EnergyPlus's curve formats.

Water consumption happens through evaporation, drift (water droplets carried out with the exhaust air), and blowdown (periodically draining concentrated water to prevent scaling). EnergyPlus can track all three if you care about water usage, which you should in water-scarce climates. Evaporation is the big one—roughly 3 litres per hour per tonne of cooling under typical conditions, though it varies with weather.

Chilled Water Distribution

The chilled water loop works identically to the air-cooled chiller setup. Variable volume Pump:VariableSpeed, setpoint managers for supply temperature control, connections to cooling coils throughout the building. The difference is your chiller is now dependent on the condenser water loop performing properly, which adds another control layer to worry about.

Primary-only variable flow is still common, though larger installations might use primary-secondary arrangements. The control logic is the same: modulate pump speed to maintain differential pressure or to meet coil flow requirements, reset chilled water temperature based on outdoor air or system load if you're doing anything sophisticated.

One thing to watch: if your condenser water temperature drifts too high (because the tower can't keep up or your control strategy is aggressive about saving tower fan energy), the chiller efficiency tanks. You can have a beautifully optimised chilled water loop achieving perfect distribution, but if condenser water is running 5°C warmer than it should, you're wasting energy at the source.

Hot Water Boiler Plant

The heating side is identical to the air-cooled chiller system: separate PlantLoop with Boiler:HotWater burning fossil fuel (gas, oil, propane) to heat water for distribution to heating coils.

Same setup requirements: fuel type, nominal capacity, thermal efficiency curve. Condensing boilers still benefit from low return water temperatures to extract latent heat. Non-condensing units show flatter efficiency across operating range. Variable speed pumping saves energy when only some zones need heating.

Hot water setpoint control typically uses outdoor air reset—hotter water when it's cold outside, cooler water during mild weather. This improves distribution efficiency and, for condensing boilers, enables more hours of condensing operation when return temps drop low enough.

The interaction between chilled and hot water systems happens at the air side, where VAV boxes might simultaneously receive cooled air from the central AHU and use hot water reheat coils. That simultaneous heating and cooling is thermodynamically inefficient but sometimes unavoidable in buildings with perimeter heating loads whilst maintaining dehumidification or in spaces with tight temperature control requirements.

Multiple Pump Loops

Water-cooled chiller systems have at least three pumps: chilled water, condenser water, and hot water. Each can be constant or variable speed, each has its own control strategy, and each contributes to total system energy consumption and part-load performance.

Variable speed on all three is becoming standard practice because the energy savings are substantial, but it increases control complexity. You're coordinating pump speeds across multiple loops, managing minimum flows to prevent dead-heading or inadequate heat transfer, and ensuring the controls don't fight each other.

EnergyPlus handles this through individual pump objects and setpoint managers for each loop, but you need to configure them sensibly. If your condenser pump is undersized or poorly controlled, it doesn't matter how good your chiller and tower are—you won't get the performance you're expecting because you can't move enough water through the system.

Plant Sequencing and Staging

The PlantEquipmentOperationSchemes object manages when equipment runs and how it responds to load. For water-cooled systems, you're coordinating chiller operation, tower operation, and pump operation across two separate plant loops.

Chillers need the condenser loop running before they can operate. The control logic has to stage the tower and condenser pump first, establish flow and temperature conditions, then enable the chiller. Similarly, when shutting down, you can't just kill the tower whilst the chiller is still rejecting heat—you'll trip on high head pressure.

EnergyPlus handles these dependencies through availability managers and equipment operation schemes, but you have to set them up properly. The simulation will catch egregious errors (like trying to run a chiller with no condenser water flow) but subtler control sequence problems might run whilst producing unrealistic results.

Multiple chillers introduce additional complexity. Lead-lag rotation, optimal loading to maximise efficiency, staging based on predicted load rather than reacting to current demand—all of these strategies exist in real buildings and can be modelled in EnergyPlus if you're willing to work through the control logic carefully.

Tower Fan Control

Cooling tower fans can operate at constant speed, two-speed, or variable speed. Variable speed is most efficient, modulating fan power to maintain condenser water setpoint whilst minimising energy use.

The control strategy matters. Simple approach control runs the fans to achieve a target approach to wet-bulb. Range control targets a temperature difference between entering and leaving water. More sophisticated strategies might combine both or use condenser water temperature directly as the controlled variable.

Tower fan energy isn't trivial—it can be 2% to 5% of total chiller plant energy consumption. During cool weather when the tower barely has to work, fan energy becomes proportionally larger. Optimising tower control to avoid running fans harder than necessary delivers real savings, but only if you don't compromise chiller efficiency by letting condenser water temperature drift too high.

Sizing Considerations

Water-cooled chiller systems have more equipment to size and more opportunities for mismatches. The chiller, tower, condenser pump, chilled water pump, and boiler all need to be sized compatibly.

Autosizing works, but you should verify the results. Common issues include towers undersized relative to chiller heat rejection (the tower has to handle chiller cooling load plus compressor power input), condenser pumps with inadequate flow for the chiller and tower, or mismatched temperature differences that prevent the system from achieving design conditions.

The chilled water loop design delta-T (typically 5°C to 6°C) affects both chiller and pump sizing. Larger delta-T means less flow for the same cooling capacity, which reduces pump energy but might affect control stability or coil performance. You can't just arbitrarily pick numbers—the equipment and distribution have to work together.

Performance Expectations

Water-cooled chiller plants should show better efficiency than air-cooled systems, typically COPs of 4 to 6 for the chiller itself. Total plant efficiency (including tower fans and all pumps) drops to perhaps 3.5 to 5 depending on configuration and operating conditions. That's still better than air-cooled plants, which is the whole point of the added complexity.

Boiler efficiency remains 80% to 95% depending on whether you've got condensing or non-condensing equipment and whether your control strategy enables condensing operation when possible.

Variable speed pumping should deliver 30% to 50% energy savings compared to constant volume on each loop. If you're not seeing substantial savings, your minimum flow setpoints are too conservative or your control strategy isn't allowing adequate turndown.

Water consumption through the tower matters in some climates. Expect roughly 12 to 15 litres per tonne-hour of cooling for evaporation plus another 20% to 30% for drift and blowdown. That adds up—a 500 tonne chiller running 2,000 hours per year consumes roughly 15 to 20 megalitres of water. If water is expensive or scarce, that becomes a significant operating cost that might swing the economic balance back toward air-cooled systems despite their lower efficiency.

Control Coordination

Water-cooled systems have more control loops interacting than air-cooled plants. Chilled water reset affects chiller efficiency. Condenser water temperature affects chiller efficiency differently. Tower fan control affects condenser water temperature. Hot water reset affects boiler efficiency. VAV minimums affect simultaneous heating-cooling waste.

Getting all of this right in EnergyPlus requires careful attention to setpoint managers, availability schedules, and equipment operation schemes across multiple plant loops. The simulation will run even if your controls make no sense, happily modelling a system that achieves your setpoints whilst consuming ridiculous amounts of energy because the cooling tower is fighting the chiller or the hot water system is battling the chilled water distribution.

This is where modelling experience separates reasonable results from nonsense. The physics are straightforward—chiller efficiency improves with lower condenser water temperature, towers achieve lower temperatures by running fans harder, pumps save energy by slowing down when flow requirements drop. But coordinating all of that into control sequences that actually work requires understanding how real buildings operate, not just how the simulation engine solves equations.