What Does It Take to Achieve Net-Zero Energy?
Is it possible to achieve net-zero energy in the health care setting, more specifically, in a full-service hospital? HKS and TLC Engineering for Architecture analyzed this very question in review of two completed greenfield hospital projects.
Both projects were sustainably successful in their own right, having achieved LEED Silver and LEED Gold certification, respectively. The first project was a 103-bed hospital (opened 2010) in Flower Mound, Texas; the second was a 90- bed hospital (opened 2013) in Port St. Lucie, Fla. Both facilities were approximately 200,000 square feet. The supposition considered was “what if the owner of each facility had charged the design team with achieving net-zero energy, rather than attaining the highest possible level of LEED certification?”
To this end, the following questions were considered:
- How could different “passive” design decisions reduce energy consumption?
- How could different “active” design decisions reduce energy consumption?
- What renewable energy strategies would be needed to achieve net-zero/regenerative energy use?
- And of primary importance, what is the approximate net financial impact of the above to the project?
First considered was the current-state energy consumption/energy use intensity (EUI) for each facility. The Texas hospital had an EUI of 238 and the Florida hospital an EUI of 149. Although the projects were located in different climate zones, heating was the primary energy consumer for both facilities, since both were in hot, fairly humid climates that required significant reheat to overcome the very cool air being introduced, as required to meet health care’s stringent air change rates.
What does it take to get to net-zero energy (NZE)? First and foremost is to reduce demand — and the most effective strategies are passive ones, such as:
- Sunshades (horizontal and/or vertical)
- Additional wall and/or roof insulation
- Optimized building form
- Optimized building orientation
- Optimized fenestration/ glazing type
- Reduced internal loads such as lighting and equipment
- Passive solar heating
Only after passive strategies have been exhausted and demand reduced, should active strategies be considered, and these are:
- Right-sized HVAC systems
- Reduced friction losses (pumps and fans)
- Increased equipment efficiency
- Recovered waste heat (air or water)
- Water cooled air delivery, such as chilled beams
- Lower lighting power densities
- Lighting and occupancy sensors
- Harvest-free energy (daylight harvesting, ventilation cooling)
After exhausting all reasonable passive and active strategies (lean planning principles were instrumental in the design of each facility, so the building form was not modified), the EUI for the Texas facility was reduced to 95 and the Florida facility to 111. Although significant reductions, there remained an obvious short fall in achieving net-zero energy, and therein the challenge of how this is accomplished for each facility. The final step to achieving net-zero energy was to explore viable renewable energy strategies. First considered was roof-top photovoltaic panels. The Texas facility had a larger footprint (94,000 square feet) than the Florida facility (52,000 square feet useable), as it contained a low roof on the north side, saving an additional 26 percent, compared to 11 percent for the Florida hospital. Covering the surface parking areas with PV carports was also considered. However, this proved cost prohibitive. Next considered was district sharing opportunities with proximate symbiotic partners, specifically wind energy, geothermal and biogas alternatives, with the latter found to be the most viable practically and financially. In exploring local opportunities for each facility, the most viable — practically and financially — was a cattle feed operation for the Texas facility and a sugar cane bagasse digester for the Florida facility.
Obviously, these strategies come with a price tag. For both facilities, the chilled beams translated to an initial added cost of approximately $25,000, but because they used pipes rather than ductwork, the floor-to-floor height was reduced about one foot per floor, resulting in a net savings of approximately $60,000. Building-mounted photovoltaic panels resulted in a net add of $4.5 million in the Texas facility and $2.5 million in Florida. In both cases, the costs associated with a combined heating and power unit and an anaerobic digester added approximately $8.2 million to overall cost.
Bottom line, the payback for the Texas facility to achieve net-zero energy was about 13.8 years, and for Florida it was 9.9 years. Although the ROI tolerance of most hospital owners is five years, it is not totally out of the question for an owner to consider a 10-14-year payback, as the lifespan of a typical hospital is usually 40-50 years. The good news is that technology is advancing every day, and prices will continue to drop as it becomes more mainstream.
Conclusions and recommendations:
- Driving down the EUI requires a major shift away from “all-air” HVA systems that depend on reheat for temperature control: Consider water-side heat recovery
- Use hydronic systems and radiant heating and cooling systems to take advantage of the greater ability of water to transport energy: Consider using chilled beams
- Because inpatient health care facilities are 24/7/365 operations, their EUI will always be greater than most commercial buildings. The practical limit on EUI may only be about 75-80 kBtu/sf/yr: Use process energy control
- Because their energy intensity is so high, it is usually impractical to use building-mounted renewable energy sources to provide enough energy generation: Roof-mounted PV is typically less than 25% of energy needs
- District or cooperative distributed energy generation is a possible solution to the need for greater generation capacity than the building mounted can provide: Look at the hospital as a community resource
- Partnerships with the community are a logical way to develop these district energy systems: Consider early planning and outreach