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Engineering Laboratory / Building Energy & Environment Division

Mechanical Systems and Controls Group

Sample Experimental Thermal Storage Data

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IBAL Project

The Intelligent Building Agents Laboratory (IBAL) was constructed as a testbed to demonstrate the potential for distributed, intelligent software agents to automatically operate building HVAC systems to reduce energy consumption and operating costs. Details of the design of the laboratory and information about the sensors deployed in the lab can be found in the listed publications.

Charging the Thermal Storage Tank Data Set

This data set contains the results from two days of operation. During those two days the thermal storage tank, an ice on coil design, was charged from 0 % to 100 % ice using the larger of the two chillers in the laboratory, Chiller 2. The experimentalTags.JSON file contains general information about the test. The MetaData.csv file contains information about all the sensors and control signals currently in use in the IBAL. Many of the sensors were not used in this test, but the most relevant measurements for this data set are listed in Table 1 and Table 2.

The ProcessData.csv file contains information about the controls, such as the chiller set point temperature and the PID gains and parameters used to set the temperature in the Condensing Loop. The RawData.csv and ScaledData.csv files contain the raw measurement data and the data scaled using the coefficients in the metadata file, where a0, a1, and a2 are the coefficients of the zeroth, first, and second order terms of a polynomial fit.

The chiller in this test has a capacity of 36 kW (10.25 tons) at design conditions and uses a scroll compressor with R410A. The working fluid in the system is 30 % propylene glycol. The chiller is water cooled using the Condensing Loop in the lab, which is supplied by chilled water from the campus that enters the lab at approximately 42 °F. The chiller set point is - 6.7 °C (20 °F). The specifications for the thermal storage tank are:

  • 274 kWh (78 ton-hours)
  • 3104 L (820 gallons)
  • 1.873 m (73.75 in) diameter
  • 2.08 m (82 in) high

 

Temperature profiles as the thermal storage tank is charged.
Figure 1: Temperature profiles as the thermal storage tank is charged.

The data are collected at a 0.10 Hz rate. Figure 1 shows an example of how this data set can be used. Two of the most important measurements when charging an ice tank are the temperature of the propylene glycol entering and leaving the tank. This figure shows how those temperatures change as the ice making process proceeds. At first, the water in the tank is cooled down to below the freezing point of water, a process known as supercooling. Supercooling occurs when water is cooled but ice does not form due to a lack of nucleation sites. Once ice begins to form, the temperature increases as is seen around time step 1750. After that point the inlet and outlet temperatures slowly decrease, with the exception of the spike around time step 3200, which is the transition from the first to the second day of charging (i.e., the system was shut down in between these days). The charging process is complete when the outlet temperature reaches 28 °F.

Table 1 Key measurements in the RawData and ScaledData files

Measurement ID

Description

ch2_c_out_rtd

Measurement of the temperature of the water at the discharge of the condenser of Chiller 2

ch1_e_in_rtd

Measurement of the temperature of the propylene glycol entering the evaporator of Chiller 2

ch2_e_out_rtd

Measurement of the temperature of the propylene glycol at the discharge of the evaporator of Chiller 2

ch2_f_c

Flow rate of the water in the condenser of Chiller 2

ch2_f_e

Flow rate of the propylene glycol in the evaporator of Chiller 2

ch2_on

Control signal that allows the Chiller 2 to turn on

ch2_power

Power consumption of Chiller 2

ch2_t_sp

Chiller 2 temperature set point

cl_f

Flow rate of the condensing water

cw_in_rtd

Temperature of the chilled water from the campus

pl_f

Flow rate in the primary loop

pl_out_rtd

Temperature at the outlet of the Primary Loop

pump2_on

Control signal to turn on the propylene glycol pump

pump2_p_down

Pressure downstream of Pump 2

pump2_p_up

Pressure upstream of Pump 2

pump2_power

Power consumption of Pump 2

pump2_vfd

Operating frequency of Pump 2

pump4_on

Control signal to turn on the Condensing Loop pump

pump4_out_rtd

Temperature entering the condenser

pump4_p_down

Pressure downstream of Pump 4

pump4_p_up

Pressure upstream of Pump 4

pump4_power

Power consumption of Pump 4

pump4_vfd

Operating frequency of Pump 4

ts_f

Flow rate of the propylene glycol through the ice thermal storage tank

ts_in_rtd

Temperature of the propylene glycol entering the ice thermal storage tank

ts_meter

Percentage of ice in the thermal storage tank

ts_out_rtd

Temperature of the propylene glycol leaving the ice thermal storage tank

v3_pos_c

Control signal for the mixing valve that allows campus chilled water into the Condensing Loop (10 V = fully open)

v3_pos_fb

Feedback signal from the mixing valve that allows campus chilled water into the Condensing Loop

v8_pos_c

Control signal for the valve that allows flow through the ice thermal storage tank (10 V = charge, fully open)

v8_pos_fb

Feedback signal from the valve that allows flow through the ice thermal storage tank

vch2_pos_fb

Feedback from the Chilled Water Regulating Valve that controls the flow of water in the condenser
Table 2 - Key process variables in the ProcessData file

Measurement ID

Description

chiller_2_temperature_sp_f

Set point temperature of Chiller 2 in °F

cl_pid_int_term

Magnitude of the integral term of the Condensing Loop PID controller

cl_pid_prop_term

Magnitude of the proportional term of the Condensing Loop PID controller

cl_temperature_accum_error_f

Accumulated error between the current Condensing Loop temperature and the set point

cl_temperature_cv_v

Control signal for the mixing valve, V3

cl_temperature_error_f

Error between the current Condensing Loop temperature and the set point

cl_temperature_ki

Integral gain for the PID loop that controls the temperature of the water in the Condensing Loop by varying the position of the mixing valve, V3

cl_temperature_kp

Proportional gain for the PID loop that controls the temperature of the water in the Condensing Loop by varying the position of the mixing valve, V3

cl_temperature_sp_f

Set point temperature of the Condensing Loop

cl_temperature_sp_high

High value of the Condensing Loop set point temperature; this is used with the low value to normalize the error term in the PID controller

cl_temperature_sp_low

Low value of the Condensing Loop set point temperature; this is used with the high value to normalize the error term in the PID controller

cl_temperature_ubias

Bias term for the PID loop that controls the temperature of the water in the Condensing Loop by varying the position of the mixing valve, V3

Related Publications:

Related Programs/Projects:

The IBAL is part of the Embedded Intelligence in Buildings Program at NIST. This program includes the Automated Fault Detection and Diagnostics for the Mechanical Services in Commercial Buildings Project and the Smart Building Automation and Control Testbed and Standards Project.

amanda.pertzborn [at] nist.gov (Provide Feedback Here)

The IBAL will produce a large set of data. Please let us know if you find the type of data published here useful or if you have suggestions for other types/formats of data that would be useful to you.

Buildings and Construction and Heating, ventilation and air conditioning equipment

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Created November 2, 2017, Updated August 12, 2021