Energy Efficiency Reference/Refrigeration/Power & Energy
Refrigeration: Power & Energy
This section of the assessment guide presents analysis tools and techniques that will be used later in analyzing different recommendations. Section 6 of this guide: "Recommendations", outlines the required, procedures, and concepts. These equations and calculations are referred to in analyzing the different recommendations.
Motor Power Motor input and output power are not equivalent. Calculate input power from voltage, current, and power factor. Output power requires motor efficiency or direct measurement of work performed.
Calculate input power with line-to-line voltage, line current, and power factor. The cubed root of three in the equation is for balanced three-phase power, and is applicable for wye or delta power systems (pf = power factor)
- Energy = Power times Operating Hours
Condenser Fan Use Factor Estimate a Use Factor (UF) to calculate energy use for motors that cycle on and off. For example, condenser fans with on-off control cycle to maintain refrigerant discharge pressure. UF represents the fraction of time the fan operates for each bin temperature. For variable-speed fans, UF represents the percentage of maximum speed at which the fan operates.
Calculate fan UF for each bin temperature by comparing the condensing minimum approach temperature difference (MATD) to the difference between the minimum condensing (Tm) and the bin temperature (BTi). Fans operate more (UF increases) as ambient (bin) temperatures increase. This is represented below for each bin temperature:
- Use Factor = minimum approach temperature difference / (min condensing temp - bin temp)
The above equations will yield confusing results in a few cases. If a use factor is calculated to be greater than one or less than zero, set UFi to one (100%). Such a situation occurs when ambient (bin) temperature is greater than the minimum condensing temperature minus MATD. Physically the outside temperature is high, the fans run continuously and the discharger pressure rises to maintain MATD to the ambient temperature.
The annual fan use factor is normalized sum of the each bin temperature.
- Total Use Factor = sum of (bin use factor x (operating hours / total bin hours))
Condenser Fan Energy Use 4a. On-off fan energy use Calculate fan energy for each bin temperature with fan power, use factor, and the bin hours. Sum the energy for each bin temperature, normalized with operating hours, for total fan energy.
- Fan Energy = sum of (bin hrs x use factor x fan power_ x (operating hours / total bin hours)
4b. Adjustable speed fan energy use Estimate energy for each temperature with fan power, percent capacity, bun hours, and the following cubic relation. The cubic relation shown is curve fit based on measured fan data that relates the fan percent power to the bin specific use factor. Sum energy for each bin temperature, normalized with operating hours for total fan energy.
- percent power = curve fit based on percent capacity
- fan energy = sum of (total power x % power x bin hrs) x (operating hours / total bin hrs)
The curve fit is based on data collected by a former OSU graduate student.