When evaluating water-chiller efficiency, facility managers need to understand efficiency ratings and how to apply them in order to provide the best guidance for acquiring new chillers. A clear understanding of two measures of chiller efficiency — the design-efficiency rating and the Non-standard Part Load Value (NPLV) rating — can help organizations obtain the best capital cost and energy efficiency. It also might help facility managers understand why they may not be getting the level of energy efficiency they expect from existing chillers.

Common Misconceptions

Enlarge this picture ECAT = entering-condenser-air temperature ECWT = entering-condenser-water temperature Table 1 — NPLV Operating-Condition Assumptions, ARI.

Many system designers make the mistake of reasoning that “more is better” and therefore use both the design-efficiency rating and the NPLV rating in the specifications they provide to facility managers and building owners. However, this approach is based on several misconceptions.

The first misconception is that the NPLV rating is only concerned with chiller performance at off-design conditions, which are sometimes assumed, erroneously, to be less important than performance at design conditions. By definition, the design-efficiency rating refers to chiller efficiency at design conditions, when design load and design tower-water temperature (or design air temperature for air-cooled chillers) occur simultaneously. However, the NPLV rating already includes both design efficiency and off-design efficiency.

Off-design performance is of paramount importance because empirical observations have shown that chillers, including those in multiple-chiller plants, operate most of their operating hours at partial loads and/or at off-design tower-water temperatures (or off-design air temperatures, if air-cooled). To verify and quantify these observations, the Air-conditioning and Refrigeration Institute (ARI) studied 25 years of weather data collected in 29 cities that represent 80 percent of U.S. chiller purchases during that same time period.

Table 1, which lists the assumptions that go into the calculation of the NPLV rating, shows what the ARI found. Specifically, 99 percent of chiller operating hours are spent at off-design conditions.

A second misconception is that a chiller with good efficiency at design conditions will automatically have a good NPLV rating. In fact, chillers can have the same design efficiency but have NPLV ratings that vary widely, depending on capital cost. That’s because chillers can have different off-design efficiencies.

Comparing NPLV and Design Efficiency

Enlarge this picture Table 2 — 1,000 TR Chiller Comparisons Table 3 — Impact of Specifying Both NPLV and Design Efficiency

Consider an example of what happens when both the design-efficiency and NPLV ratings are applied by comparing two 1,000 TR chillers (see Table 2).

The specified chiller has an NPLV rating of 0.466 kW/TR, a design efficiency of 0.562 kW/TR, and costs $250,000. Option-A chiller, which costs less at $240,000, also has an NPLV rating of 0.466 kW/TR, but a design efficiency of 0.576 kW/TR, which is higher than the specified chiller. Because both chillers have equal NPLV ratings, they will have equal annual energy consumption (remember, the NPLV rating includes both off-design and design efficiency).

If the specification contained only the NPLV rating, option-A chiller might be an attractive choice. However, if the specification requires that a chiller meet both the NPLV rating and the design-efficiency rating, option-A chiller can’t meet both ratings and, therefore, can’t be bid. It may be a function of compressor size, impeller diameter or rotational tip-speed. Regardless, the manufacturer of option-A chiller will usually need to modify it by adding more heat-exchanger surface to meet the design-efficiency rating. The performance of this new chiller is shown in Table 3 as option-B chiller.

Because of the additional heat-exchanger surface, option-B chiller has an improved NPLV rating of 0.448 kW/TR, resulting in annual energy that is 4 percent better than the specified chiller. But in meeting the design rating, it has also become more expensive. In this example, it costs $31,000 more than option-A chiller. Instead of equalizing energy consumption as a basis for comparing costs, now both annual energy consumption and pricing are unequal.

Building owners may also face higher capital costs. Suppose the specified chiller and option-B chiller are bid by two different manufacturers. The manufacturer of the specified chiller has no incentive to lower its price below $270,000, so that’s where the chiller price settles out.

Unfortunately, the owner in this example is likely to end up purchasing the specified chiller, but will pay about $20,000 more and get no additional energy savings, simply because option-A chiller did not satisfy the design-efficiency specification and could not be bid.

Electrical Demand, Wiring Size, Codes and Rebates

Although including the design-efficiency rating may diminish owner value, there is still a tendency to use it. Some designers believe that the design-efficiency rating will impact electric-demand charges. Others are concerned that it may affect power-wiring size. It pays to examine each of these reasons to see if using the design-efficiency rating is really warranted and, if it is, how restrictive it must be.

Consider the aforementioned option-A chiller, which has a design efficiency of 0.576 kW/TR. At first glance, that chiller would appear to cause higher electric-demand charges than the specified chiller, which has a design efficiency of 0.562 kW/TR. But chiller peak kW usually has little impact on building demand because of heat-load timing. The building’s kW and the chiller’s kW typically peak at different times of the day. This phenomenon can be referred to as the “flywheel effect” of the building’s demand versus the chiller’s demand, and is illustrated in Figure 1 (page 54).

Most air-conditioned buildings reach their peak electric demand between 10:00 a.m. and 3:00 p.m. That’s when occupancy is usually at its highest, which maximizes the “people” load. Higher occupancy also translates into more heat generated by lights, elevators, cafeterias, office equipment, etc. When these factors are combined, the building encounters its peak kW draw in late morning to early afternoon. Surprisingly, most chillers reach peak electric demand between 3:00 p.m. and 7:00 p.m. Why so late? At about 12:00 p.m., the sun’s rays strike the ground at the most direct angle. Through convection, the ground then heats the ambient air to its highest dry-bulb temperature at about 2:00 p.m. Once the air temperature is at its maximum, the heat is slowly conducted through the building skin, a process that peaks building heat load around 4:00 p.m. In parallel, the wet-bulb temperature of the ambient air also reaches its maximum later in the day.

The higher wet-bulb temperature raises the entering-condenser-water temperature, which raises the head pressure against which chillers must work, hurting energy efficiency. When these factors are combined, the chiller sees its peak load, peak head, and, therefore, peak kW in late afternoon, hours after the building has passed its peak.

Enlarge this picture Figure 1.  A comparison of building demand and chiller demand profiles.

If the chiller is being used to cool a process, then its power profile will typically be flatter than shown in Figure 1, and its demand will have an impact on total demand. During the one or two peak-cooling months, option-A chiller may have a slightly higher demand charge. However, the fact that both chillers have the same NPLV rating means that option-A chiller must have better off-design efficiency. So during the many months of off-design operation, option-A chiller will likely have lower demand charges. Hence, annual demand charges may actually be less. If a chiller is cooling a process, and demand is ratcheted year-round, then chiller kW could impact building demand. However, the number of applications in this situation is relatively small.

When energy codes or utility rebates require inclusion of the design-efficiency rating in the specification, it is better for the designer to specify the maximum kW or kW/TR required by the code or rebate. That’s because a lower value could result in higher capital costs with no reduction in annual energy costs. This disparity is leading more code-writing agencies to recognize the NPLV rating.
Small differences in the design efficiencies between chillers usually have little impact on demand charges.

Words of Advice

Chiller-efficiency specifications that specify both the NPLV rating and the design-efficiency rating may hinder the designer’s ability to meet the owner’s goals, if the objective of the specification is attaining the lowest capital cost for similar annual energy. That’s because the two ratings can create inequalities in annual energy-consumption comparisons, which also result in higher capital costs passed on to the owner. Also, the design-efficiency rating usually has little practical impact on electrical-demand charges and power wiring size.

Instead of using both ratings, the best chiller-efficiency specification uses the NPLV rating by itself. For power-wire sizing, specifying the maximum full-load amps and the minimum power factor eliminates all ambiguity about actual size requirements. If energy codes or utility rebates require that the specification include the design-efficiency rating, the maximum allowable kW or kW/TR should be specified.