Scrutinizing Air Changes
It is important to analyze the air changes and question the requirements and motivation behind these changes. Ventilation has a huge impact on overall operating costs not due just to the energy required to move the air, but also due to the energy required to heat and cool the air, particularly in extreme climates. The ventilation rates are often overestimated, so do not assume the air change rate is always driven by thermal loads. Additional changes do not necessarily equate to increased safety.
“Base optimum minimum ventilation requirements on user needs, health and safety protection, and energy consumption,” says Mathew. “The environmental health and safety officers usually set the rates, so designers need to work with them to optimize the rates. Consider design options, including exhaust alternatives, computational fluid dynamics (CFD) modeling, a panic switch for emergency airflow in case a spill occurs, cascading air use from clean areas to dirty areas, and occupancy sensors or a schedule-driven approach to change the rate when the space is not occupied.”
By performing CFD modeling of indoor airflows, designers can study airflow patterns and optimize the position of supply diffusers, return grilles, and fume hood locations relative to work surfaces and, thereby, improve the effectiveness of the airflow.
The ventilation rate seems to differ among research facilities and correlates with a corresponding difference in energy usage. Owners, architects, and engineers should inquire about what the proper rate should be in order to achieve maximum safety and savings.
Hutchinson Hall at the University of Rochester is an example of a building that had 10 air changes per hour, which represents an original standard set at least four decades ago. The environmental safety and health officer was asked why the rate was set at 10 and whether the potential hazards were reviewed. After reviewing the rate and its subsequent ramifications, the officer defined various hazard levels for different types of laboratories. This approach is called control banding. In each one, there was an occupied setting, as well as an unoccupied setting for the air change rate.
The issue of fume hoods being massive beasts of energy consumption must be considered, as well. A single fume hood consumes as much energy as three average homes, while a lab with 100 hoods utilizes as much energy as a small neighborhood. The LBNL provides an online tool at http://fumehoodcalculator.lbl.gov that enables users to compare fume hoods. Information about the type of hood, the price of electricity and gas, as well as the climate zone, can be entered into the system and the calculator will make comparisons between fume hoods. The calculator can be used to test the energy and cost impacts of improving component efficiencies and comparing options.
“Everything owners and programmers can do to reduce the number and the size of fume hoods is the best way to improve energy efficiency,” says Mathew. “Make sure you allow for easy additions and removals to alleviate concerns from faculty who think if it is not done now, it will never be installed. Consider variable air volume (VAV), two-speed fume hoods, the new generation of high-performance hoods that use a different airflow pattern and provide excellent containment with much lower volumes.”
Reduce the Pressure Drop
Approximately 50 percent of the total heating, ventilation, and air conditioning energy in a lab is related to fans and the electricity used to operate them. There are several ways to reduce that energy usage with the most logical being to improve the efficiency of the fans by ensuring the motors are in top working order. Total airflow can also be reduced through the VAV supply.
“An often overlooked strategy is to reduce system pressure drop throughout the entire ventilation system on both the supply and exhaust sides,” says Mathew. “Low-pressure drop design strategies can be used across the entire air distribution network. This low-pressure drop can result in 75 percent energy savings for fan loads, smaller fans used, longer filter life with less maintenance, and a more quiet operation.”
Reducing the total pressure drop through a ventilation system can be achieved by increasing the area of cooling and heating coils, resulting in the fans requiring less energy to push the air through the coils. Another way to reduce the drop is to use radial ducts instead of square ducts.
A major complaint architects hear from mechanical engineers is that they do not provide them with enough shaft space and ceiling plenum space. Therefore, allowing for larger ducts requires good upfront architectural integration to provide larger spaces.
At the University of California, Davis Tahoe Center for Environmental Sciences, engineers looked at the base case of the air handler, which was 2.2 inches of water gauge (w.g.) and dropped it to 0.68 w.g. The duct work was kept as straight and short as possible with large ducts. As a result of including these strategies, the engineers achieved a pressure drop that resulted in decreased energy usage.
Get Real with Plug Loads
Plug loads are basically the heat loads that result from any lab equipment that requires electricity and generates heat. HVAC systems are often oversized out of fear that a facility might not be able to meet over-estimated plug loads in the future. As a result, chillers and air handlers are often oversized and this leads to unnecessary expense and wasted operating cost in the long term.
Mechanical equipment should be right sized to save capital and operating costs. Study actual loads at comparable facilities to fully understand the real load, using improved estimates of heat gain from plug loads.
“Find out what the plug loads are at a comparable lab and then start your sizing based on that rather than an arbitrarily high number,” suggests Mathew. “You need to design for high part-load efficiency because labs aren’t always going to operate at peak loads. One of the ways you can do this is by using a modular approach.”
For example, two large boilers at the LBNL were replaced with 11 smaller, modular boilers. Each boiler kicks on as needed as the load ramps up. No more than seven of the boilers have operated simultaneously, leaving four completely redundant and demonstrating how oversizing occurs.
UC Davis is concerned about plug loads and is using right-sizing, in part, because of tight construction budgets and the need to minimize the impact of large mechanical equipment. In order to accomplish these objectives, the University began sizing to a 15-minute average peak rather than an instantaneous peak. Electrical systems must be sized accordingly, but mechanical systems should be sized for a lower quantity.
Right sizing is also being used at the Molecular Foundry Laboratory at the Berkeley Lab. The air handlers and electrical generators were downsized, resulting in a multi-million dollar initial cost savings. Some of the money saved was applied toward additional green features that qualify the facility for a Silver-level LEED certification.
