modelAllElectricCWStorage

Information

This model represents a combined heating and cooling plant where chilled water is produced by cooling-only chillers and heat recovery chillers, hot water is produced by heat recovery chillers, and a thermal energy storage tank is integrated in the condenser water circuit to maximize heat recovery ("Tank Charge/Discharge" operating mode). Cooling towers allow rejecting excess heat from the condenser loop ("Heat Rejection" operating mode). Air-source heat pumps allow injecting heat into the condenser loop ("Charge Assist" operating mode).

This model has been developed based on the publication by B. Gill (2021) and further discussions with Taylor Engineers.

Abbreviations and naming conventions

The following abbreviations are used in the documentation of this model and of its components.

To clearly distinguish cooling-only chillers from heat recovery chillers, the term "chiller" is used systematically to refer to cooling-only chillers whereas the abbreviation "HRC" is used systematically to refer to heat recovery chillers.

Each HRC can operate under the following modes. In cascading heating mode, the condenser barrel is connected to the HW loop and the evaporator barrel is connected to the CW loop (CWE circuit). The onboard controller controls the HRC to track a HW supply temperature setpoint at condenser outlet. In cascading cooling mode, the condenser barrel is connected to the CW loop (CWC circuit) and the evaporator barrel is connected to the CHW loop. The onboard controller controls the HRC to track a CHW supply temperature setpoint at evaporator outlet. In direct heat recovery mode, the condenser barrel is connected to the HW loop and the evaporator barrel is connected to the CHW loop. The onboard controller controls the HRC to track a HW supply temperature setpoint at condenser outlet while the plant supervisory controller maintains the CHW supply temperature at setpoint by modulating the evaporator flow rate or the condenser entering temperature.

System schematic

The schematic below represents a configuration of the system with two chillers and three HRCs. The equipment tags correspond to the component names in the plant model. The control points used by each control function are represented at the intersection of the gray area that describes the function and the four bus lines corresponding to the different control point categories (AI, DI, AO, DO). For the sake of clarity, control logic that is duplicated between multiple units (for instance the chiller isolation valve control) is only illustrated for one unit. The detailed description of each control function is available in the documentation of Buildings.DHC.Plants.Combined.Controls.Controller. For an overview of the different operating modes and the design principles of such a system, the user may refer to the article by B. Gill (2021).

Control points

Cooling and heating Enable signals

The cooling and heating Enable signals u1Coo and u1Hea shall be computed outside of the plant model, for instance based on a time schedule.

CHW and HW supply temperature and differential pressure setpoint

Those setpoints are provided as control inputs. Ideally, a reset logic based on consumer valve requests should be implemented to adapt those setpoints to the demand.

Details

Sizing considerations

Sizing the TES tank and the heat pumps is a joint optimization problem under the constraint that on a design heating day, heating loads can be met using both the recovered heat and the heat added to the tank by the heat pumps. As stated by B. Gill (2021), increasing the tank capacity generally improves plant efficiency by providing more opportunity for heat recovery. Tank capacity should therefore be maximized under the limit corresponding to the amount of heat that can be recovered over the day.

The model is configured by default with a tank that is sized to store the heat needed to operate the HRCs during 3 h at peak heating load with a ΔT covering the two temperature cycles specified with the parameter TTanSet (heels and thermocline neglected). This default can be overwritten.

TES tank

The tank is assumed to be integrated without pressure separation, i.e., the operating level of the tank sets the system pressure and no pressure sustaining valve or discharge pump is included. The operating level is approximated as equal to the tank height. A default height to diameter ratio of 2 is also taken into account (designers tend to favor a height to diameter ratio above 1.5 in order to minimize the volume of the thermocline which is considered useless). No high limit is considered for the tank mass flow rate.

CHW and HW minimum flow bypass valve

As per standard practice, the bypass valve is sized for the highest chiller minimum flow. The bypass valve model is configured with a pressure drop varying linearly with the flow rate, as opposed to a quadratic dependency usually considered for a turbulent flow regime. This is because the whole plant model contains large nonlinear systems of equations, and this configuration limits the risk of solver failure while reducing the time to solution. This has no significant impact on the operating point of the circulation pumps due to the control loop that modulates the valve opening to generate enough pressure differential at the chiller boundaries to allow for minimum flow circulation. So whatever the modeling assumptions for the bypass valve, the control loop ensures that the valve creates the adequate pressure drop and bypass flow, which will simply be reached at a different valve opening with the above simplification.

Cooling tower circuit

The design heat flow rate used to size the cooling towers and the intermediary heat exchanger corresponds to the heat flow rate rejected by all HRCs operating in cascading cooling mode and all chillers operating at design conditions. The cooling towers are sized with a default approach of 3 K to the design wetbulb temperature. The intermediary heat exchanger is sized with a default approach of 2 K.

Chiller and HRC performance data

The chiller performance data should cover the CW temperature levels reached when the plant is operating in Heat Rejection mode. The parameter TCasConEnt_nominal (set with a final assignment) provides the maximum CW supply (condenser entering) temperature in this operating mode. The HRC performance data should cover the HRC lift envelope, that is when the HRC is operating in direct heat recovery mode, producing CHW and HW at their setpoint value at full load.

References

Brandon Gill, P.E., Taylor Engineers, Alameda, CA, USA.
Solving the large building all-electric heating problem.
ASHRAE Journal, October 2021.

Parameters

Connectors

Components

Contents

Name	Description
MediumAir	Air medium
MediumConWatCoo

Revisions

• February 24, 2023, by Antoine Gautier:
  First implementation.