ASHRAE JOURNAL Letter of July 2014 by Kirby Nelson
In the July 2014 ASHRAE Journal under letters; Pete Menconi P.E. makes the point; “I used my first building simulation in 1973 and have used a variety of methods and on-site comparisons. In general, I’ve found typical +/- 50% variance between simulation and actual energy.”
Peter Rumsey P.E. responds; “In my experience I have found the same thing.” “My experience after having modeled and measured dozens of buildings is that models do not accurately capture system performance.”
I was modeling buildings of Texas Instruments Inc. in 1974 using DOE and ECUBE. These two programs could rack up the total electrical use for a year if you input the schedule and could also do a fair job at simulating the loads due to weather. However, the models could not accurately simulate the air side system or the plant performance. Sure, the models had inputs for the air side and plant but system models were not.
In the late 1970’s some of us made the argument for ASHRAE models based on the equations of thermodynamics, but the decision was made to stick with the DOE approach and that is where we are today. DOE is the basis of eQUEST and Energy Pro as stated by Rumsey.
The reason these DOE based models have not, cannot, and will not in the future model building energy is given on page 70 of the July 2014 ASHRAE Journal. The article (Improving Infiltration in Energy Modeling) makes the obvious but not stated point; 40 years after the oil embargo and an air side model is still not in the ASHRAE and Pacific Northwest National Laboratory (PNNL) accepted energy models. The article also clearly illustrates why; empirical equations with several coefficients that have no relation to the laws of thermodynamics can never accurately model a real system. A real system operates according to the laws of thermodynamics and therefore so must our models.
The hypothesis of energy conservation (first law) was established by J.P. Joule in the 1840’s. For the past 170 years the first law has provided easy to understand relations that define the performance of thermodynamic systems. Is it not time for all to admit we have been on the wrong modeling path for more than 40 years?
Kirby Nelson P.E. ASHRAE life member
