Thursday, February 6, 2014

Great Expectations - Utility Cost Savings Don't Always Mirror Energy Savings

The following is a lessons learned taken from the renovation of elementary school in rural New Mexico. It was part the findings from a post occupancy evaluation (POE) conducted by M.E. GROUP for the New Mexico Public School Facilities Authority. The format it's presented in comes from the Garrison Institute's September 2013 Climate, Buildings and Behavior Symposium.

Intended Goal/Desired Outcome: Incorporate several high performance design strategies in the renovation of a rural New Mexico elementary school to create a school facility that reduces energy and water consumption and utility costs relative to the original school, as well as improves the teaching/learning environment.

Less than Ideal Actual Outcome:  There was a 22.6% decrease in the total annual energy consumed comparing the average of the post renovation school years with the pre renovation year as indicated by the blue line in the figure on the left. However there was a corresponding 168% increase in annual energy costs, as indicated by the blue line in the figure on the right. This decrease in energy consumption is impressive, as the pre-renovation cooling systems consisted of undersized evaporative coolers which consumes significantly less electrical energy than the post-renovation all electric geothermal ground coupled water source heat pump system. And from what was learned, this evaporative cooling system was poorly performing relative to occupant comfort. But the reduction in gas load from pre to post-renovation (green line in the figure on the left) is greater than the corresponding increase in electrical load (red line on the left). The elimination of the old gas heating system and transition to all electric water heaters everywhere but the kitchen, combined with the more energy efficient lighting system incorporating daylight harvesting (despite its problems), the improved roof insulation and the white TPO roof likely account for the overall decrease in energy consumption post-renovation. Plus there may have been a transition to more efficient kitchen equipment as well as potential scheduling changes (possibly less use after hours and over the summer post renovation – though there was conflicting information on this).

What went wrong or appears to have gone wrong? This seeming contradiction in decreased energy consumption but increased utility costs (shown in the figure on the right) is likely due to shifting the school’s load from primarily gas to primarily electric, as the school's average electric utility rate for the 2011-2012 school year was slightly more than twice the average gas utility rate.  A decrease in electric rates from pre-renovation to post-renovation likely explain why the increase in utility costs weren’t more than the 168% increase that did occur (as well as the addition of the PV system to a small degree).

There was also an increase in the estimated annual green house gas (GHG) emissions produced by the school’s energy consumption over this period, as indicated by the green line, going from 306,931 lbs to an average of 533,273.5 lbs. As with the energy costs, this is likely due largely to the shift from a gas heating / evaporative cooling (electric) HVAC system to an electric only HVAC system. Electricity generation in New Mexico is mostly via coal fired power plants and burning coal releases more CO2 and SO2 than burning natural gas. However this analysis is based on limited years of utility data before and after renovation.

What systematic challenges or issues does this example elucidate? In the effort to reduce energy and water consumption through high performance, sustainable design, utility costs are sometimes left out of the analysis and GHG emissions are rarely considered at all. However, there are often significant cost differences between different fuel sources (as this example shows). Saving significant energy does not automatically translate into significant utility cost savings if there is a change in the ratio of fuel sources used. And this can have a significant impact on owner operational budgets, particularly small, rural school districts. And as climate change, mitigation and adaption inevitably become  more of the public discussion, a facility’s greenhouse gas emissions will become a formal part of a project’s life cycle cost analysis (LCCA), energy analysis and energy model.

What are the lessons learned? Two basic lessons:

  • In addition to first cost, maintenance costs, energy consumption and occupant comfort/controllability, the following should also be considered in the selection/specification of major systems and equipment: differences in operational (i.e., utility) costs and green house gas emissions of electric vs. natural gas systems/equipment (and how that may change over the next 20+ years).
  • Utility rates/rate structures have a significant impact on utility costs. Coordinate what these will be with the local utilities as opposed to using national/regional averages and forecast near term changes in these rates if possible.

1 comment:

  1. Really good post! Hope there will be more good post here!Thanks for sharing valuable power