I have noticed these two lines on my BEPS report. Which number accurately
reflects the energy consumption of my building? What is the difference
between the two? If all of my energy was generated on site would the
numbers be equal?
TOTAL SITE ENERGY 2517.74 MBTU 29.0 KBTU/SQFT-YR GROSS-AREA
29.0 KBTU/SQFT-YR NET-AREA
TOTAL SOURCE ENERGY 4867.80 MBTU 56.0 KBTU/SQFT-YR GROSS-AREA
56.0 KBTU/SQFT-YR NET-AREA
Thank you,
JAMES F. GEERS
The Site is your building model. Source takes in to account losses and
inefficiencies in delivery of energy from the source "plant", and hence
reflects energy used at that source. If all energy was on site, they
should be equal.
Alan Jackson, LEED AP
In essence Alan is correct. However - for electric energy the largest
losses are not the T&D losses (7.2% according to Wikipedia; 3% according
to the chart below from the DOE) , but the conversion losses at the
plant. So the site-source utilization is around 33%
For an on-site CH&P generator, you would still have some losses - around
25% conversion loss, so the site-source utilization is around 75%, so
there should still be a discrepancy between the two.
Vikram Sami, LEED AP
I think if you modeled an on-site CHP system, the BEPS report should show roughly matching numbers for fuel consumed by a generator.
There may be a difference for source-to-site natural gas based on transportation from the gas well, but it shouldn't include the 25% waste heat from the diagram - that occurs on-site.
A value from the EPA is 4.7% for natural gas - so if eQuest used the same factor, I'd expect a CHP installation (assuming 100% of electricity usage was generated on-site) to show 4.7% difference between site and source in the BEPS report.
The link below will help explain:
http://www.energystar.gov/ia/business/evaluate_performance/site_source.pdf
The 25% losses of unrecoverable waste heat from the second diagram would be included in the on-site usage, and would show up in both lines of the BEPS.
David