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Add-On-Variable Speed Controls for Roof Top Units

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The US Department of Energy (DOE) with data from AHRI informs us that 60% of commercial floor space in the US is heated and cooled by one speed roof top units (RTUs). This means that every time anyone of these RTUs comes on, a mild spring day or the full heat of summer, all that it knows is one speed, Full Blast!

HVAC systems are designed to meet the heating and cooling loads of buildings, but contractors don't want to be called out on hot days or cold nights so most systems are oversized with a safety margin of 40 to 50%. [1]

In addition to being one-speed and oversized, most RTU's economizers don't work well or at all. Economizers are intended to bring in temperate air from the outside when conditions in spring or fall, even mornings and evening in summer, saving a great deal of energy, especially in Sacramento where the diurnal swing in temperature is 30°F. However, New Buildings Institute's report "Review of Recent Commercial Roof Top Unit Field Studies in the Pacific Northwest and California" reports 2 out of 3 economizers not working properly, if at all.

The other part of this pervasive norm is that no regular communications exist between these RTUs and facility managers, neither users nor electric utilities. They are usually reliable, always inefficient and often run with little or no attention for years until something, like a fan belt or motor breaks. They typically run, Full Blast, with little or no economizer, very inefficiently, to failure. They run on without the inhabitants knowing their state of maintenance and with no abilities to respond to utilities demand reduction programs or the Smart Grid.

The good news is that even when building owners are not in the position to purchase new replacement RTUs an add-on, variable speed control can be added to most RTU's which also provide improved economizer functioning and communications via "the cloud".

The added communications have the ability to adjust controls, turn them on and off and to respond to utility demand response programs, all via "The Cloud". Even installation can be supervised to insure proper installation every time, all via "the cloud"!

Just by varying the speed of the fan alone big energy savings can be achieved because the relationship between fan rpms/volume of air and energy consumption is to the third power. As the speed of the fan is reduced a little, energy consumption goes down significantly. Add to these savings improved economizer performance and the savings are almost hard to believe, and offer a great opportunity for utilities to have a significant opportunity to enhance demand programs by working with this 60% sector which is currently unable to participate.

One commercially available advanced controller (variable speed control, economizer control, fault-detection, M&V and control via the internet) has been tested by Pacific Northwest National Laboratory (PNNL). PNNL monitored 66 RTUs on 8 buildings, in four climate zones and collected data over a 12-month period, in FY12. "The advanced controller reduced the normalized annual RTU energy consumption between 22% and 90%, with an average of 57% for all RTUs." Average savings of 57%, that is huge!

The Sacramento Municipal Utility District (SMUD) Energy R&D Department has conducted a field test with a large customer employing the same advanced controller tested by PNNL. The advanced controller was installed on four 20 ton RTUs of the total of 80 RTUs on the customer's roof. Energy savings ranged from 32% to 58%, with the average energy savings of 39%. This test was conducted in 2014.

Figure 1 Advanced Controls are added in a simple box with attached cellphone for communications the link.

As mentioned before leading companies marketing these products also provide fault detection, measurement of energy savings, adjustment of controls, email alerts, viewing performance and Demand Response by way of the internet or the Smart Grid.

This performance information can revolutionize measurement and verification by utilities and the on-going refinement of technologies by industry. Instead of conducting tests over the period of one or two years and longer, collecting and analyzing data, costing multiple thousands of dollars, only to find the technology has already changed performance can be known from the outset.

With advent of these communications enhancements, performance of each project can be known from day one of installation and operation, and adjustments can be made as time goes along if warranted.

The market success of the leading manufacture of an advanced controller has caught the attention of competitors across the country and numerous companies will soon be offering competing products.

SMUD, PG&E, SCE, SDG&E and Bonneville Power Authority (BPA) currently offer rebates/incentives for advanced controls meeting specified performance parameters.




Bruce Baccei, Author, is responsible for HVAC as part of SMUD's Energy R&D Department. He has been working in the field of energy efficiency and renewable energy for over 30 years.

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Variable speed drives on nearly ANY system that involves fluid flow (gas or liquid), can improve overall system efficiency, and not by just a little bit. Dropping fan speed by 20% cuts power consumption by over half, and air flow is only cut by 20%! Going to half speed cuts consumption by 7/8 - the savings are remarkable.

And yes, many systems are grossly oversized, to cover the worst possible situations, rather than the "normal" ones, say statistically 95% (+,- 2 sigma) of heating and cooling days. So during the typical summer for example, on a couple of blisteringly hot days, your office might get up to 76 or 80 degrees for a few hours, instead of the 74 that is the typical setpoint, IF you set a limit on system size during the design or replacement phase.

Other efficiencies can be had by insuring that zones do not get over-cooled or under-cooled during normal times. Local variable speed fans included in the duct network designs can provide better balance and lower power consumption, assuming one pays the up front expense for this better distribution system and the controls necessary to manage it.


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My company, Transformative Wave, is behind the development and distribution of the product mentioned in this article. We call it the CATALYST (efficiency enhancing controller), which has been subjected to numerous 3rd party field trials, only a few of which are referenced in this great article.

We appreciate the emphasis on our communication and data collection capabilities that we call the eIQ Platform (energy intelligence). Connecting users to the real-time condition of these systems is critical to the perpetuation of the energy savings. Our fault detection and remote diagnostics highlight the most serious issues with these HVAC systems as they occur.

One clarification is that our wireless communications do not rely on a cellular connection. The RTUs communicate via our own WiFi rooftop network to a central access point. This access point can connect to the Internet via the customer LAN, a dedicated wireline circuit, or a cellular modem.

Thank you, Bruce, for such a comprehensive overview of this new area of efficiency opportunity.

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