Proof: For superior energy efficiency - Water Beats VRF - Case Study
ASHRAE’s Apples to Apples Comparison Proves Hydronics Delivers Superior Energy Efficiency over VRF
For years Variable Refrigerant Flow (VRF) manufacturers have been touting their systems’ superior energy efficiency over hydronics without providing the supporting data. Then ASHRAE decided to showcase both systems in its Atlanta headquarters and meter their performance.
The results aren’t what the VRF manufacturers wanted to see: Water Beats VRF!
ASHRAE Building HVAC Study
An opportunity to compare apples to apples
The ASHRAE building HVAC retrofit
The ASHRAE Headquarters Renewal program serves as a prototype for the organization’s Building Energy Quotient (Building EQ) labeling program on energy efficiency. In 2008 the Atlanta building went through an HVAC retrofit to upgrade its heating and cooling system. It was a unique opportunity to directly compare the performance of two different HVAC systems on its two floors. — Hydronics and VRV. A geothermal ground source heat pump system was installed to serve the second floor, and a VRF system with multiple zones was installed to serve the ground floor. Both systems use no backup heat and rely totally on the electric energy to the compressors to both heat and cool the building, affording an apples to apples comparison.
The data
ASHRAE’s goal was to meter the two installed systems through several winters and summers, to see what the difference in energy consumption would be between the two systems. ASHRAE itself has offered no conclusions on the data but has put the information gained online for industry viewing. That data can be found at at http://images.ashrae.biz/renovation/ or you can snap the QR code above.
The data should be of particular interest to both sides of the HVAC system divide because VRF suppliers persist in making claims of superior energy efficiency without providing the supporting data the industry would like to see. From the VRF standpoint, the ASHRAE building energy study is a good test case to demonstrate their claim that a variable speed driven VRF system is more efficient than a constant volume or constant speed ground source heat pump system.
That has not been affirmed by the metered study, however. During the winter months in Atlanta, which of course is not a real high-heat climate compared to northern tier locales, the VRF system has an electrical energy consumption approaching three times the ground source heat pump system. On an annualized basis, the VRF system had an energy consumption 57% higher in 2010 than the hydronic system, 84% higher in 2011 and 61% higher in 2012.
The VRF problem
What’s the problem for the VRF system in winter? VRF manufacturers claim that their system does not require backup heat. They claim that they can maintain constant heat output from their airsource heat pumps even as the ambient temperature decreases. In fact, though, capacity and efficiency of air-source heat pumps decrease with lower ambient temperatures, in accordance with the 1st and 2nd Laws of Thermodynamics. Yes, VRF air-source heat pumps put out a constant amount of heat; but to accomplish that they need additional energy, 1st Law, which they get through speeding up the compressors from 60 Hz up to as much as 120 Hz at low ambient temperatures. The compressor generates more heat because more fluid is being pumped, but that capacity is at the expense of efficiency, 2nd law which drops even faster at higher RPMs than a typical air-source heat pump. The added heating energy comes from the compressor by virtue of the electricity to run it.
The conclusion
One can think of it this way: if the efficiency of a VRF system increases because we can decrease the speed of the compressor, if we increase the speed of the compressor the efficiency is going to decrease. So the net effect is that in the heating mode, VRF systems are not efficient. In fact, they are very inefficient, and the ASHRAE headquarters’ data clearly shows that. For the majority of the United States, which is heating dominated, this means that geothermal heat pump systems in particular will always outperform a VRF system.
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