# Presentation: 792315

## Comparative Analysis of the Life Cycle Costs of Geothermal Heat Pumps and Three Conventional HVAC Systems for an Elementary School in Lincoln, Nebraska

- John A. Shonder
- Michaela A. Martin
- Patrick J. Hughes
- Oak Ridge National Laboratory

## Project Background

- Four new, identical 69,000 ft2 elementary schools were built in 1995 in Lincoln, Nebraska
- The decision to use geothermal heat pumps based on life cycle cost comparison using another school in the district
- Pre-construction estimates contain a great deal of uncertainty. Would actual performance would bear out the initial assumptions, and confirm the technology selection?

## Objectives of the study

- Use site-monitored energy use data to develop a calibrated simulation model of one of the schools
- Implement GHPs and three alternative HVAC systems to determine annual energy and water use
- Use maintenance data from the school district to estimate annual maintenance cost for each alternative
- Develop estimates of installed costs for each alternative
- Determine life cycle cost of each alternative

## Maxey Elementary School

**Maxey Elementary School**

## Floorplan of the School

**Floorplan of the School**

## GHP System Design

- Total of 54 GHP units
- 200 tons total nominal cooling capacity
- Peak block loads: 150 tons / 940,000 Btu/hr
- Two 15-ton units provide pre-conditioned OA (ASHRAE 16-1989) with HW coil for cold days
- Gymnasium, cafeteria also introduce OA (40-45%)
- Remaining units (1.4 - 4.5 tons) draw return air from corridors, and supply classrooms, offices, etc.
- Unit heaters in corridors with HW coils
- Four 333,000 BTU/hr gas-fired boilers; gas DHW

## Borefield Details

- 120 vertical loops using 1 in. HDPE pipe
- 12 by 10 pattern located under soccer field
- 20 ft. center-to-center spacing between bores
- Bore depth 240 feet (~140 ft/ton)
- Backfilled with fine gravel, bentonite plug top 10 ft.
- Designed for year-round occupation
- 22% (by volume) propylene glycol solution
- Variable speed pump

## Extensive 10-min Interval Data Were Available from EMS/Utility

- Total building electrical use
- HVAC electrical use
- Compressor, reversing valve, and fan status for each of the heat pumps
- Flow rate, supply/return temperatures for borefield
- Zone temperatures, outdoor air temperatures
- Monthly natural gas use
- All in varying stages of completeness

## Maintenance Data Available

- Lincoln school district provided access to complete maintenance database (preventive and unplanned)
- 18 schools selected (all with _>_ 70% cooled floorspace)
- 4 categories of heating/cooling plants
- 4 Geothermal heat pumps (GHP)
- 2 Air-cooled chiller/gas-fired HW boiler (ACC/GHWB)
- 12 Water-cooled chiller/gas-fired HW boiler (WCC/GHWB) of which 9 were VAV
- Ages of cooling plants: 3 to 32 years
- Ages of heating plants: 3 to 70 years
- Analysis of this data presented in two ASHRAE papers

## Study is based on a calibrated simulation

- Developed simulation model beginning with as-built construction plans
- Estimated internal loads from occupancy, fixture counts
- Developed occupancy schedule based on school calendar
- “Tweaked” the model (adjusting infiltration) until performance matched site-monitored data (daily electrical use, monthly gas use)

## Simulated and monitored daily HVAC electrical use

## Simulated and monitored monthly total natural gas use

## Predicted and MonitoredMinimum Monthly EWT

## Predicted and MonitoredMaximum Monthly EWT

## Alternative systems were dictated by available maintenance data

- Air-cooled chiller, VAV air handling system, gas-fired hot water boiler
- Water-cooled chiller, VAV air handling system, gas-fired hot water boiler
- Water-cooled chiller, constant volume air handling system, gas-fired hot water boiler (unlikely to be installed in a new school)

## Comparison of total energy use

## Comparison of HVAC energy use

## Estimating maintenance costs was somewhat problematic

- Wanted to determine first-year maintenance cost and rate of cost increase for each alternative
- Only 18 systems in database (4 GHP, 2 ACC/VAV, 3 WCC/VAV, 9 WCC/CV)
- Not enough data to perform a statistically significant analysis
- Alternative sources generally unreliable

## Data for most numeroussystem type (WCC/CV)

## Values appeared reasonable

## How maintenance costswere estimated

- Assumed maintenance costs for all systems rose by 1.5% per year
- Calculated initial year maintenance cost on this basis
- GHP: 		9.0 ¢/ft2
- ACC/VAV:	9.5 ¢/ft2
- WCC/CV:	16.6 ¢/ft2
- WCC/VAV:	9.7 ¢/ft2

## Developed independent cost estimates for each system

- Capital costs were estimated for each system by staff estimators, without reference to actual installed cost
- Estimates then given a “reality check” by an experienced estimator familiar with school HVAC systems (including geothermal)
- Estimates also compared to published square foot cost estimating guides

## Capital cost estimatesfor the four systems

## Things to note about the capital cost estimates

- Based on published figures for schools, the estimates appear reasonable
- Excluding the constant volume system, geothermal has the lowest first cost
- A cost of $9.45 per square foot was published for the geothermal systems. We were unable to determine what was included in this estimate
- School district’s accounting system does not separate out the cost of HVAC from the entire school cost.

## Inputs for the life cycle cost analysis

- Energy use for each system based on calibrated simulation models
- Local Lincoln, Nebraska rates (gas, electric and water) used to calculate utility costs
- Maintenance costs based on Lincoln School District database  first year + annual escalation
- Capital costs of HVAC systems based on estimates performed by staff cost estimators
- Real discount rate of 3.1%, DOE energy cost projections, 20 year equipment life

## Comparison of life cycle costs

## Results of the analysis

- Lincoln school district made the correct decision in installing GHPs in these four schools
- Over 20 year life, GHPs have the lowest life cycle cost -- about $230,000 less than the next most economical technology
- First cost is the most important factor in the analysis, followed by energy costs and maintenance costs.
- Maintenance costs of GHPs in this district are marginally lower (about 6%) than the cost of maintaining VAV chiller/boiler systems.

## Results of the analysis

- GHPs have the lowest annual operating costs
- Of the system types commonly installed in schools, GHPs also have the lowest first cost
- Reduced energy use reduces emission of greenhouse gases and other pollutants

## Conclusions

- Obviously, these results apply only to the school examined
- They confirm that GHPs can cause significant reductions in energy use (17%) and energy costs (24%)
- They also counter some of the myths about GHPs:
- No “cost premium” in this case. GHPs have lower first cost
- Maintenance costs comparable with other common technologies