Laboratory Testing of Variable Speed Compressor and Fan Controls for RTU Optimization

Lab assessment measures the optimal compressor and supply fan speeds in a packaged air-cooled AC unit, for different combinations of cooling load and outside air temperatures. Research provides insight into right-sizing AC units for intended loads.

This report describes a laboratory evaluation to optimize the performance and efficiency of a packaged air-conditioning and heating roof top unit (RTU) with a variable speed electrically communicated motor (ECM) controlling the blower speed and a retrofitted variable speed drive controlling the compressor speed. This technology is being considered as a method to save energy associated with air conditioning at part load conditions. It also may have potential to reduce peak electricity demand in buildings with over-sized equipment or when combined with condenser-air evaporative pre-cooling technology. Equipment capable of modulating delivered air conditioning capacity to a building is important because the building load changes based upon use and outdoor air temperature. Furthermore, engineers specify air conditioning equipment to meet the air conditioning load on a “design day”, meaning the summer peak temperature that will only be exceeded 1% of the time. The result is the other 99% of the hours in the year the equipment will be oversized, a problem exacerbated by the fact that the capacity of the air conditioner increases at the outdoor temperature decreases, resulting in compressor cycling. Variable fan and compressor controls allow the unit to operate at reduced capacity when possible with reduced refrigerant flow and air flow, while benefiting from using the heat exchangers sized for the full capacity system. This technology has the potential to save large amounts of energy, but could also reduce peak demand in cases where the RTU was oversized to meet the demand of a peak day. In a study of 145 RTUs in Northern California, Felts and Bailey, found that “In at least 40% of the cases, the unit size could be dropped by 50% or more” while still meeting peak demand [1]. Adding a variable speed drive and RTU optimization controls could “right size” the unit for the building and significantly increase its efficiency. The technology could also reduce peak demand when combined with condenserair evaporative pre-cooling technology, which drops the inlet air temperature to the condenser so the RTU observes a part-load condition. This project seeks to conduct laboratory testing and analysis of RTU retrofit control strategies. While multiple manufacturers make controllers that fit into this category, Western Cooling Efficiency Center (WCEC) evaluated the potential energy and demand savings that could be achieved from these products if the control strategy were optimized. WCEC did not evaluate a specific commercial product. The control strategies investigated here could be implemented by any retrofit controller manufacturer or even the existing RTU controller or building management system. WCEC retrofitted a 4-Ton packaged RTU with a variable frequency drive (VFD) on the compressor. The VFD allowed control of the compressor to operate between 30-72 Hz (50- 120% of its normal operating speed). The RTU also contained a selectable speed electronically-commuted (ECM) evaporator fan motor. Testing was performed at each of the five available speed settings. A total of 63 tests were performed across a range of compressor speeds, evaporator fan speeds, and outside air dry bulb temperatures. Return air conditions were held constant throughout the testing. Instrumentation and ducting were added to the RTU but no other physical changes were made. For each steady-state test, total capacity, sensible capacity, power, and coefficient of performance were recorded and analyzed. The results show that the part-load energy saving potential of this technology is significant. The baseline RTU tested in the WCEC laboratory consumed 4.16kW of power and had an operating COP of 3.06 at typical rating conditions (Table 1Table 5). When the outdoor air temperature drops to 75°F, the baseline RTU would continue to operate at the same compressor and fan speed. The capacity would increase 15%, which is likely to be completely unnecessary because the thermal load on the building would decrease simultaneously, causing additional cycling of the air conditioner. The power would decrease 16% and the COP would increase 37%. With the advanced controller, the compressor and fan speed could be reduced at the part load condition (Table 1). The scenario shown reduces the power over the baseline operation at 75°F by 25% and increases the COP by 11%. This technology would be extremely beneficial when combined with an evaporative condenser-air pre-cooler technology. In dry California climates, evaporative condenser-air pre-coolers reduce the effective outdoor temperature seen by the condensing unit. At a peak condition, it is possible to reduce the outdoor air temperature 20°F or more. Using a combination of an evaporative condenser-air pre-cooler and fan and compressor speed controls could reduce peak electricity demand by more than 35%. Achieving these results would require retrofitting the existing RTU with evaporative pre-cooler, variable speed drive(s), and controller. A package installation of these technologies would increases savings in comparison to installing a single technology and reduce the transaction cost associated with the RTU retrofit. However, there are challenges associated with this technology that result in market barriers. A significant barrier is that no known manufacturer is manufacturing a comprehensive pre-cooler and RTU optimizer solution. WCEC hopes to reduce this barrier by identifying potential manufacturers to participate in technology development and deployment. Secondly, installation of the system would require a licensed electrician, which increases installation costs. Lastly, installation of a pre-cooler faces the barriers associated with evaporative pre-coolers including access to water and sewer lines and regular maintenance requirements. WCEC recommends further development of this technology, including engaging with potential manufacturing partners. Furthermore, WCEC recommends conducting laboratory and field tests of the technology in combination with an evaporative pre-cooler technology that has already been tested with positive results.