Quick savings solutions

Design considerations like variable frequency drives are approaching the cost of motor starters, which allow for choices that weren't possible until now.

11/19/2012


There are some simple design considerations that will have an impact on energy or operating cost savings with minimal or no increase in first cost. The cost of variable frequency drives (VFDs) is approaching the cost of traditional motor starters. Originally the drives provided a means of saving energy because they would maintain a high level of efficiency while reducing motor speeds and using less power. These are traditionally used in variable air and water flow systems. However, use in some constant flow applications has proven this cost-effective in some circumstances: by using a VFD in lieu of a motor starter, you can save air balancing costs due to sheave and belt changes. This translates to low first cost, less labor, and maintained system efficiency.

Hot and chilled water system pump configurations provide opportunities for quick investment returns. Most pumping configurations will use dual pumps with one unit being used as standby; each pump provides 100% redundancy. In a variable flow configuration, rather than selecting two pumps at 100% redundancy, evaluate the option of operating two pumps in parallel. There is an immediate savings due to a reduction in pump size. Proper selection of the pumps would result in the ability of a single pump to provide as much as 75% total system flow should there be a failure.

Selecting chillers and cooling towers presents several opportunities to keep operating costs at a minimum with few additional costs. Chiller capacity and efficiency is a function of condenser water temperature—colder condenser water temperature will provide a slight increase in unit capacity. The water temperature is a function of the cooling tower size; as with most equipment, there are nominal unit casing sizes. The actual tower performance is a function of the amount of surface area or fill and fan size. Maximizing the amount of surface area within a given tower casing will reduce the condenser water temperature, reduce the tower fan operation, and increase chiller efficiency. This additional first cost becomes negligible within the total system cost and provides maximum benefits.

There are rooftop applications that present almost immediate investment returns. Most rooftop units struggle with large quantities of outside air. This holds true for the heating and cooling side when large quantities of ventilation are required. The choice is either to split the load with multiple units or to move to a larger single unit. In either case, there is an increase in the first cost and usually a loss of unit efficiency.

Installing an air-to-air heat recovery unit between the building exhaust and ventilation air is a simple means of recovering expended building energy and avoiding higher unit first cost and operating costs. Certainly equipment efficiencies and added installation costs are the most obvious, but there are also opportunities when selecting a primary fuel source. The best type of energy is free energy, although there is a premium to be paid for harvesting solar, wind, and in some instances geothermal energy. In most cases there are programs that will help fund the higher first cost through grants or tax incentives. Even with offset dollars, these systems usually have a payback period of 10 to 15 years, which is close to the useful life of the system. If the decision is based purely on economic factors, the design and installation of these systems would never occur.

Besides free energy, there is another opportunity to save operating costs. The comparison of natural gas, fuel oil, and electrical costs is commonly overlooked. Rate structures vary by utility and region, but do provide a method of adding value without increasing first cost. As an example, natural gas or fuel oil would be the most logical heating source in cold climates; however, in some regions this service may be unstable or volatile with respect to availability and cost. Electricity may be a viable solution especially if there is ample capacity as provided by either a nuclear plant or hydropower. The simplest method to compare the values is to break down the units into dollars per Btu.


Peter D. Zak is a principal with Graef-USA Inc., where he manages the MEP group. He is a member of NCEES and is on the editorial advisory board of Consulting-Specifying Engineer. He was an adjunct assistant professor at the Milwaukee School of Engineering for 20 years and is a registered professional engineer in 24 states.



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