At the August 31, 2017 ARC (RTUG) Subcommittee meeting, we proposed developing a kWh/ton savings estimate for the ARC-lite (fan only savings) and full ARC measures.  We specifically proposed not developing a kWh/ton/hour savings estimate based on the following graph, which shows little to no correlation between savings per ton and RTU annual operating hours.

Feedback at the subcommittee meeting was mixed, with some agreeing with our proposal, based on the data, and others disagreeing, stating hours of RTU operation should be included in the savings metric and if we look at the data on a site-level, we might get a more appropriate resolution in the data.

We took another look at the data, this time on a per site basis, too, and the data seem to show a relationship between savings/ton and operating hours.

Filtering out the non-PNW sites, which the subcommittee generally agreed is a good idea, gives us the following points.  We fit a trendline (through the origin) to the fan-only savings (blue points) whose slope suggests approximately 116 kWh/ton/1000hrs.  This is the basis for our revised proposed savings estimate for the ARC-lite measure (fan-only savings).   

On the same graph, notice the full ARC savings (red points) for PNW sites do not seem to support a strong relationship between savings and hours of operation.  We hypothesize that the additional savings achieved from the full-ARC measure, which we call “compressor savings” here (green points) are much less sensitive to RTU annual operating hours.  As a reminder, these are the savings represented by the additional economizer control and DCV requirements of the full-ARC measure.  So our proposal is to estimate the compressor savings as a kWh/ton metric, independent of operating hours, and add that to the fan-only savings (kWh/ton/1000hrs) to estimate savings for the full-ARC measure.

Rather than using the slope from the regression, which was useful for illustrating the method, we propose to just use the average kWh/ton/1000 hrs, which comes out to a very similar answer: about 120 kWh/ton/1000 hrs. (Note that on a per unit basis, the average fan savings is similar: 129 kWh/ton/1000 hrs.) 

The average compressor savings for the PNW sites comes out to about 400 kWh/ton.  (Note that on a per unit basis, the average compressor savings is similar: 373 kWh/ton.)

So, here’s our new proposal for savings estimates for these measures.  Note we now propose breaking the measures into 1000 hour bins and we’ve calculated fan savings based on the average hours within the bin.