Control of electricity costs and temperature exchange in hydronic cooling systems

Over time, the summer increase in the energy bill becomes just another budget item in the cost of operating facilities with hydronic heating, ventilation and air conditioning (HVAC) systems. Anticipating these costs, most managers keep an eye on energy saving methods to improve efficiency, fully aware that no matter how tight their system is, they are wasting money. Energy saving methods usually involve changes in the behavior of building occupants, new equipment, or both, which come with their own challenges. Often the solutions do not address fundamental system energy efficiency issues such as number of cycles, temperature change (Delta T [∆T]) between feed water and return water, and the temperature transfer necessary to achieve and maintain ambient set points.

IMAGE 1: Commercial primary energy consumption, HVAC, quads / year (2017) (Images courtesy of Pace Solutions)

Cooling Energy Saving Methods and Challenges

Behavior changes are the most difficult to monitor or control, and since comfort is directly related to employee morale, they are often short lived, especially in hot conditions. HVAC engineers recommend a minimum occupied temperature set point of 24 ° C (75 ° F) in summer to maintain comfort. Depending on the system cycle rate and Energy Management System (EMS) settings, maintaining the recommended temperature threshold in moderate to extreme weather can increase energy production and increase costs.

In the 2017 Annual Energy Outlook (AEO), the U.S. Energy Information Administration (EIA) reported that the U.S. commercial building sector consumes approximately 17.83 quadrillion Btu (quads) of primary energy. Of this total, HVAC systems consumed 5.35 quads, which represents 30% of the total energy consumption of commercial buildings. Almost a third of this amount was allocated to cooling.1 (Image 1)

greater return water temperature
IMAGE 2: The higher the temperature of the return water, the lower the energy requirements of the system.

Cooling systems as a whole consume ≥ 1.49 quads per year. Proportional amounts of power consumption are based on both efficiency and the total number of units used, so having a piece of equipment top the list of power consumption can reflect its popularity more than its effectiveness:2

  • rooftop units (0.74)
  • coolers (0.44)
  • central AC split system (0.19)
  • room air conditioning (0.09)
  • gas systems (0.02)
  • geothermal heat pump (

Changing equipment for more energy efficient models is always expensive and should be based on a cost benefit analysis. However, regardless of the efficiency of a single renovation, lower energy costs can be directly linked to a bad T. An inadequate ∆T results in:

  • excessive chilled water pumping requirements to provide the required cooling energy using a lower temperature differential
  • reduced chiller operating capacity
  • more chillers operating at part load, which affects efficiency
  • higher energy requirements at chillers due to additional machines running outside their most efficient operating range (at low loads and high flow rates)
  • increased energy consumption for chilled water pumps
  • additional cooling towers and condenser water pumps
  • higher energy requirements of the condenser water pump and cooling tower to support additional chillers
  • excessive equipment wear and reduced life cycles due to longer run times

consumption on weekends
PICTURE 3: Weekend consumption (blue) is generally lower than weekday consumption due to different control settings in the center.

Flow rates, system loads and T reduction

Typically, chilled water systems will maintain a reasonable T at high loads up to 12 F (6.7 C). However, at lower loads the T can drop to

Higher flow rates spread across multiple chillers at low system loads mean each chiller is operating near the lower end of its load range, dramatically reducing system efficiency. Additionally, any time a new chiller is started, the condenser water pump and cooling tower fan (s) may also need to run. This greatly increases the energy consumption.

To maximize efficiency, flow rates must be properly managed to properly sequence chillers so that each chiller will operate near the top of its load range and at higher efficiency levels for as long as possible. Reducing the chilled water flow by improving the T will also have a direct impact on the energy requirements of the chilled water pump. By maintaining a fixed T of chilled water throughout the year, operators should see a reduction in the annual energy consumption of the pumps (Image 2).

trial period and weekend
IMAGE 4: Yellow represents the testing period (October 5-15, 2019). Green represents weekends.

Water efficiency and its effect on ∆T

Surfactants have been tested for decades to reduce the surface tension of water and improve temperature transfer, but there hasn’t been much success until the last decade. The obstacle was the stability of the temperature in the stream. Relatively recent innovations in nonionic surfactants for heated and chilled water have provided facility managers with a new way to improve efficiency, reduce costs and improve T.

Poured by hand or using a metering machine at a designated inlet point of the system, with a concentration of only 1% in recirculated water, nonionic surfactants improve water temperature transfer refrigerated to the pipe wall. This achieves several energy goals, because the system:

  • reaches room temperature set points faster, reducing energy load
  • reduces fan energy used to maintain set points
  • improves the efficiency of all system cooling technologies
  • reduces the number of cooling cycles
  • increases ∆T

Over 100 case studies of all types and ages of system conducted over a decade across a range of facilities, the average reduction in energy and emissions was 15%. In addition, it has been shown to be safe to use with inhibitors and glycols.

Case study

In the summer of 2019, Allegany College of Maryland added a nonionic surfactant to its hydronic system for cooling and recorded a 13.21% reduction in electricity costs and reduced usage of 14.63 kW by a month.

Founded in 1961, Allegany College of Maryland serves students in Allegany County and the surrounding area. The campus is located approximately 150 miles (240 km) from the coast in the foothills of western Maryland, near the town of Cumberland. The average temperature ranges from 77 to 95 F (25 to 35 C) between mid-May to mid-September.

The main campus is comprised of three one-story facilities made from concrete blocks and brick materials common in 1960s construction. Served by a 220-ton chiller, the goal of introducing a Nonionic surfactant was to reduce energy costs and emissions for cooling buildings when classes started in mid-August when the weather was still quite warm.

Historical data was only found in utility bills, which include non-HVAC consumption, so an electrical sub-meter was installed on the chiller in June 2019 and ran until mid-September. to establish a baseline. The system operated for three months to model pre-installation consumption against cooling degree days (CDD) collected at the Greater Cumberland Regional Airport at a base load of 65 F (Image 3).

During the analysis, a marked increase during the latter part of the summer was detected. This was likely due to the increased use of the facility as the school year resumed. Analysts decided to use data from mid-August to mid-September as the primary benchmark and data from mid-September to mid-October for analysis (Figure 4).

A clear change in system performance was observed during and after installation of the nonionic surfactant. The average power can be converted into consumption to calculate the financial savings: energy consumption (kWh) = power (kW) x time (h).

The 327.12 kW of electricity observed has the equivalent of carbon dioxide (CO2e) 231 kg, the same volume of emissions emitted by a car traveling 566 miles. During a cooling season, the expected reduction in greenhouse gases (GHGs) is 42.1 metric tons, the emissions released by nine passenger vehicles traveling more than 102,500 miles.

Each day, operators saw an energy saving of 327.12 kW, totaling 5,561.04 kW / h over 17 days. By paying $ 0.084 / kWh, the college achieved financial savings of $ 467.12. The cooling period is estimated at six months per year (approximately 182 days). The expected savings are 59,535 kWh or $ 5,000 per year with a return on investment (ROI) of two and a half years.

Minimize costs by improving the T

Regardless of the cost of high efficiency equipment, the conversation is not “equipment versus nonionic surfactants”, but how to maximize system function and reduce costs. By closely monitoring the system, adjusting certain operational aspects, and improving water efficiency, systems of all ages benefit.

The references

EIA. Annual energy outlook 2017. Table: Key indicators and consumption of the commercial sector. Reference case. August 2017. eia.gov/outlooks/aeo/data/browser/#/?id=5-AEO2017&cases=ref2017&sourcekey=0

BTO. 2017. Basic Scout Energy Calculator. Accessed February 2021. trynthink.github.io/scout/calculator.html

WM Group Engineers, “Study of the delta T of chilled water”. November 2008. wmgroupeng.com/sites/wmgroupeng.com/files/CHW%20Delta%20T%20Studynov2008.pdf

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