For an energy engineer, renewable energy and energy efficiency are becoming inseparable. With the falling cost of solar photovoltaics (PV) and wind turbines, renewable energy is cost neutral or cost efficient in many cities across the U.S. This is especially true when comparing Levelized Cost of Energy (LCOE) metrics of fossil fuels (LCOE by NREL). Coupled with an increased desire to be carbon-neutral or consume net-zero energy, efficiency measures are not always the first choice for capital projects among many domestic building owners. But no matter the means to save energy, the end-result is nearly the same: to control energy costs.
To truly control energy costs, facility owners are beginning to evaluate the cost effectiveness of battery technology and much more dynamic renewable energy systems such as concentrating solar and solar-tracking arrays. These systems allow for a higher annual energy production per square-foot and reasonable paybacks in high energy-cost jurisdictions. But because these technologies can be difficult to implement in dense urban areas, energy efficiency must be incorporated in order to reduce the dependency on renewables.
While each facility’s energy profile is different from one to the next, the approach for optimization is generally the same: efficiency before renewables.
As energy engineers we strive to create building additions without increasing operating costs and present efficiency measures with two or three-fold benefits. First, when tasked with a building expansion facility owners should always look to engineers to incorporate renewables during construction, typically easing first-cost to a small fraction of the overall structure. In doing so, sophisticated energy models can be used to evaluate various combinations of constructions and building systems, in some cases creating a plan to effectively eliminate utility costs for the expansion. Energy models should also be used to evaluate potential renewable energy systems based on localized climate data in effort to forecast net-consumption and create bankable predictions for utility costs.
Second, engineering analyses should take care to evaluate secondary effects of conservation measures, such as those inherent in LED lighting and central-plant optimization. While LEDs are becoming more life-cycle cost effective than high output fluorescent lighting systems (EERE Lighting Report), lighting power reductions alone often dominate the conversation. But when considering the overall impact lighting has on a facility, secondary effects such as decreased cooling load must also be considered. In some cases heat-loads from lighting retrofits may decrease by 30-50%, potentially extending the useful life of chillers, pumps, fans, and cooling-towers used to cool these buildings. A properly automated facility recognizes these changes in load and adjusts cooling-system performance.
Energy efficiency through central plant optimization has similar layered benefits, and through retro-commissioning or continual commissioning can create layered opportunities for energy efficiency in an automated building. Small measures such as dynamically increasing a chilled water set-point based on load or outdoor ambient temperature may allow chillers to operate at a more efficient part-load, while reducing pumping and cooling-tower loads, and reducing the need for simultaneous heating and cooling through zone-reheat.
When evaluating combinations of efficiency measures such as these before contemplating renewables, these decisions can dramatically reduce a facility’s overall need for renewables, thus reducing size and first cost of renewable energy systems. But optimization does not end by agreeing to implement a PV array after reducing a building’s load. Positioning arrays and sub-arrays based on peak-usage or facility type becomes essential; further reducing peak loads and on-peak energy charges.
Becoming a net-zero consumer or reaching a benchmarked usage index is a complex ambition for controlling energy use, one fraught with many obstacles. Obstacles such as primary heating fuel-type selection, reliability of historical weather patterns for PV and wind turbines, and managing process energy consumption from plug-loads. Because the majority of U.S. building stock is in buildings two decades or older (IEA Building Stock Report), incremental decisions to become a net-zero building (NZEB) are popping up in the existing building sector as well as in new construction.
When considering fuel-type in a NZEB, offsetting the cost of natural gas is commonly compared to the cost of electricity, especially in heating-intensive climates. This evaluation is often conducted by engineers electing to use dedicated outside air systems (DOAS) or energy recovery ventilators (ERV) to de-couple ventilation air-conditioning from space conditioning systems. This de-coupling strategy is becoming commonplace among energy efficient facilities, and is an area of significant R&D. Where natural gas is abundant and low-cost, factory-installed fuel switching technology may be an acceptable alternative to DOAS or ERVs with only one fuel source. (Example Systems) Dual-fuel or hybrid systems also offer the advantage of potentially benefiting from both PV and lower cost solar thermal systems, while historically only domestic hot-water systems and condenser water loops appeared ideal for solar thermal applications.
Hybrid fuel systems rely heavily on building automation and allow facility owners to invest in a more diverse selection of renewables, allowing solar thermal systems to play a larger role in offsetting annual consumption. But because of the still limited amount of options for integration, solar thermal energy is restricted to pre-heating hot-water systems and alone does not provide a viable path to achieving net-zero consumption. Unlike converting from electricity to natural gas to optimize solar thermal collection, net-zero consumption almost begs the need for an all-electric infrastructure. This decision can be difficult for facility owners in very hot or very cold climates, where natural gas may provide an economical benefit from absorption cooling or gas heating. Where net-zero is a tangible goal and all-electric infrastructures are possible, energy efficient heat-pump technology can bridge the gap between a costly electric boilers or reheat systems.
Ground-coupled heat-pumps (GCHP) are sometimes considered renewable energy systems, as compared to true geothermal systems, and do offer a renewable source of heat-transfer without the use of boilers and cooling towers. These two geo systems differ based upon the use of a distributed array of 250-foot deep bore-holes and a refrigeration cycle (ground-coupled), versus the use of deeper earth-boring to generate steam-turbine energy (geothermal). (Ground-Coupled Assoc.) While quasi-renewable GCHPs are typically more cost effective than controlling superheated subterranean temperatures, they tend to be land-hungry and may eventually require the assistance of a boiler or cooling-tower. Much like PV and solar thermal, should a facility be optimized prior to system selection, GCHPs may prove cost comparative against traditional HVAC systems. In a very limited basis and through careful automation, GCHPs may also integrate solar thermal collectors, especially when a bore-field is limited to the amount of heating it can provide.
Historically energy efficiency was driven by the attempt to reduce peak usage and consequent peak demand charges, driving the implementation of thermal-storage load-shifting. By generating chilled-water or ice during low-cost off-peak periods, substantial load reductions were possible when night-time cooling was used during on-peak daytime cooling periods. While these measures still provide tangible energy cost savings, new automation technologies such as smart-metering and grid-scale batteries are beginning to dominate cost efficiency and reliability discussions. Smart-metering and smart-grid technology allow for a strategic allocation of low-cost power resources; from power stored off-peak or renewably generated to be used when charges are excessive or grids are stressed. While these technologies are often a result of large-scale institutional research projects (Smart-Grid NM Labs), smaller micro-grid automation projects offer a building-wide or campus-wide ability to capitalize on similar technologies. These “smart” technologies are slowly becoming a third tier to the one-two of efficiency and renewables.
All this being said, an energy engineer’s analysis and performance predictions rely heavily upon building automation systems and responsible building management. Without proper programing, equipment sequencing, and systems automation an energy master-plan or benchmarked usage-index is simply not possible. While climate-appropriate renewable energy systems can be considered as constants in the performance equation, right-sizing a PV array does no good in a sunny climate when a facility’s consumption escalates – who is in control then?
ABOUT THE AUTHOR
Matthew Higgins, Founder & Chief Analyst, CEM, HBDP, LEED AP (BD+C), MBA
Consulting for energy projects Mr. Higgins has conducted energy incentives research, energy audits, energy and daylight simulations, he has sized and planned renewable energy systems, developed site specific combinations for energy efficient glazing, shading, and construction assemblies, and assisted with building equipment and control decisions. Mr. Higgins has worked on over 300 new and existing building energy modeling projects, over 80 of which had an associated LEED certification goal. His expertise also includes extensive energy measurement and verification studies, Energy Star building certifications, life cycle cost analysis, creation of specialize analysis tools, and a breadth of public speaking experience throughout the southwest.